Coordination chemistry of arylhydrazones of methylene active compounds Contents

R

C

Ka

b

C

a

ARA

KAcMTTC

c

D

0h

Coordination Chemistry Reviews 257 (2013) 1244– 1281

Contents lists available at SciVerse ScienceDirect

Coordination Chemistry Reviews

jo u r n al hom ep age: www.elsev ier .com/ locate /ccr

eview

oordination chemistry of arylhydrazones of methylene active compounds

amran T. Mahmudova,b,∗, Maximilian N. Kopylovicha,∗∗, Armando J.L. Pombeiroa,∗∗

Centro de Química Estrutural, Complexo I, Instituto Superior Técnico, Technical University of Lisbon, Av. Rovisco Pais, 1049-001 Lisbon, PortugalDepartment of Chemistry, Baku State University, Z. Xalilov Str. 23, Az 1148 Baku, Azerbaijan

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12452. Synthesis and physico-chemical properties of AHMAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1245

2.1. Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12452.2. Terminology, tautomeric, isomeric and structural features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12452.3. Acid–base and redox properties of AHBDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1258

3. Complexes of AHMACs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12593.1. Complexation of metals with AHBDs in solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12603.2. Sodium(I) and potassium(I) complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12603.3. Magnesium(II) complex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12613.4. Manganese(II and III) complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12613.5. Iron(III) complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12613.6. Cobalt(II) complexes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12613.7. Nickel(II) complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12623.8. Copper(II) complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12633.9. Zinc(II) complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12673.10. Cadmium(II) complexes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12693.11. Platinum(II) complex. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12693.12. Lead(II) complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12693.13. Heterometallic complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1269

4. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1279Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1279References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1279

r t i c l e i n f o

rticle history:eceived 31 October 2012ccepted 23 December 2012

a b s t r a c t

The methods of synthesis of arylhydrazones of methylene active compounds (AHMACs) as well as thepreparation and properties of their metal complexes are reviewed. Nuclearity, supramolecular arrange-ments and other features (e.g., solubility, redox potentials, catalytic and antibacterial activities) of thecomplexes can be controlled by the introduction of different substituents to the methylene active and/or

eywords:rylhydrazones of methylene activeompoundsetal complexes

automeric equilibriaemplate synthesis

the aromatic fragments of AHMAC and by variation of metal-ions used in the synthesis.Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.

atalysis

Abbreviations: AAC, aromatic azocompound; AHBD, arylhydrazone of �-diketone;

ompound; RAHB, resonance assisted hydrogen bond; TEMPO, 2,2,6,6-tetramethylpiperid∗ Corresponding author at: Centro de Química Estrutural, Complexo I, Instituto Superiorepartment of Chemistry, Baku State University, Z. Xalilov Str. 23, Az 1148 Baku, Azerbai

∗∗ Corresponding authors. Tel.: +351 21 841 9237; fax: +351 21 846 4455.E-mail addresses: kamran [email protected], kamran [email protected] (K.T. Mahmudov

010-8545/$ – see front matter. Crown Copyright © 2013 Published by Elsevier B.V. All rittp://dx.doi.org/10.1016/j.ccr.2012.12.016

AHMAC, arylhydrazone of methylene active compound; MAC, methylene activeine-1-oxyl.

Técnico, Technical University of Lisbon, Av. Rovisco Pais, 1049-001 Lisbon, Portugal;jan. Tel.: +351 93 436 7357/+994702073771; fax: +351 21 846 4455.

), [email protected] (M.N. Kopylovich), [email protected] (A.J.L. Pombeiro).

ghts reserved.

dx.doi.org/10.1016/j.ccr.2012.12.016

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K.T. Mahmudov et al. / Coordination Ch

. Introduction

Aromatic azocompounds (AACs) provide ubiquitous motifs inrganic chemistry and are widely used as organic dyes, indicators,igments, food additives, radical reaction initiators, therapeuticgents, etc. [1,2]. The coordination chemistry of AACs has beenxtensively studied during the last decades on account of possibleombination of interesting functional properties with a diver-ity of molecular geometries of their complexes. Thereby, theseomplexes have attracted attention in diverse fields, such as prepa-ation of conducting and magnetic materials, non-linear optics,upramolecular chemistry, catalysis and bioinorganic chemistry,mong others [1–5].

Methylene active compounds (MACs, e.g., �-diketones, �-initriles, benzoylacetonitriles, cyanothioacetamide, ethylyanoacetate, cyanoanilides, etc.) have also been of consider-ble interest in organic chemistry, for instance, �-diketones aremportant starting materials in many organic reactions [6–10], inarticular in the perfume and cosmetic industries [8,9]. Moreover,any representatives of MACs have been widely used in coordi-

ation chemistry for a long time and have recently been the objectf increasing attention as constituents of polydentate ligands inetallo-supramolecular chemistry. Thus, the rich coordination

hemistry of �-diketones has been reviewed recently [8,9]. Thehemistry of cyano-substituted MACs concerns such organic reac-ions as dimerization, hydrolysis, halogenation, reduction, carbonylondensation, ortho-ester and ylide formation; they are widelypplied in syntheses of, e.g., herbicides, dyes, polymers, washingnd bleaching compositions, lubricants, and optical sensitizers11,12]. Various inorganic template transformations [13] added

further interest on cyano-substituted MAC for metal-mediatedynthesis and coordination chemistry.

A combination of an AAC, in particular an arylhydrazone,ith a MAC in one molecule (Scheme 1) creates multifunctional-

zed arylhydrazones of methylene active compounds (AHMACs).HMACs are versatile starting materials for a number of organicyntheses leading to compounds which are biologically active14], possess liquid crystal properties [15], can be applied asnalytical reagents [16–21], indicators [22], ionophores [23], orydrazone dyes [24]. Moreover, the rich tautomerism and isom-rism of AHMACs together with the intramolecular resonancessisted hydrogen bond (RAHB) system (see below) can be appliedor regulation of tautomerization-isomerization, activation of thearbon in � position to a carbonyl, antiferroelectric–paraelectricransition, regioselective activation of dinitriles, catalysis, ligandiberation, etc. [25–28].

On the other hand, AHMACs constitute a promising type ofigand toward the formation of novel coordination compounds.
or example, a particular representative of the AHMAC fam-ly – arylhydrazones of �-diketones (AHBD) – drastically differn physico-chemical, analytical and coordination properties from
Scheme 1. Arylhydrazones of methylene active compounds (AHMACs).

ry Reviews 257 (2013) 1244– 1281 1245

�-diketones [14–23,29–50]; the possibility to include –OH,–AsO3H2, –SO3H, –COOH or –NO2 groups at the ortho-position ofthe aromatic ring (Scheme 2b and c) increases the stability of AHBDcomplexes in comparison with “simple” AHBD (Scheme 2a) and�-diketones [31,51–55]. Moreover, the coordination compoundsof AHBD possess catalytic [51–55] and biological [14] activities,photoluminescence [56], can also be used in thin films as opticalrecording media [57], in high-density recordable optical storageand spin-coating films [58]. Hence, a review on the coordinationchemistry of AHMACs would be timely.

2. Synthesis and physico-chemical properties of AHMAC

2.1. Synthesis

The first methylene active arylhydrazones (Scheme 1) werereported as early as 1883, by Richter and Müntzer, then designatedas “benzolazoaceton” [59] which was five years later shown by Jappand Klingemann to be a hydrazone [60]. Until now, the synthesis ofthis type of AHMACs consists in the coupling of a MAC with an aro-matic diazonium salt, mostly performed in methanolic or ethanolicsolution containing acetate (Scheme 3) [61,62]. This conversion wasconsidered so important that the term Japp–Klingemann became astandard reaction name [61]. In its course, the respective anilinerequires a preceding diazotization with sodium nitrite in acidicmedium to produce the corresponding diazonium salt [60–62],which is an inevitable part of the full Japp–Klingemann conver-sion. The original procedure underwent many modifications andimprovements, e.g., higher yields of more pure products can beobtained when the coupling is undertaken in a solution of sodiumhydroxide instead of sodium acetate (Scheme 3) [29]. On the otherhand, the acid used in the diazotization process strongly influencesthe yield and stability of diazonium salt [1,2].

The Japp–Klingemann method has given rise to an immensevariety of AHMACs derived from various arylamines and MACs[14–23,29–98]. The AHMAC products can be used as intermedi-ates in further synthesis of organic molecules [14] or as ligands incoordination chemistry [e.g., 51–55,88,90,92,99–102].

Another approach to the synthesis of AHMACs involves the reac-tion of a MAC with n-nonafluorobutylsulfonylbenzotriazole in thepresence of NaH in an inert medium (Scheme 4) [103,104]. How-ever, the synthesis of n-nonafluorobutylsulfonylbenzotriazole isnot trivial, and this preparative procedure has not been widely used.

A new template synthesis of AHMAC ligands has recently beenreported (Scheme 5) [13], where one cyano group of a starting dini-trile 1 undergoes a metal (Mn)-assisted nucleophilic attack withdifferent nucleophiles leading to the corresponding unsymmetricproducts 2–5. Nevertheless, the thus synthesized ligands 2–5 werenot liberated and fully characterized. Later, it was demonstratedthat similar AHMAC ligands can be formed upon copper(II)-mediated synthesis and easily liberated on account of the formationof resonance assisted hydrogen bond (RAHB) (Scheme 6) [105]. TheRAHB can also facilitate the direct organic synthesis of symmet-ric and unsymmetric AHMAC compounds (Scheme 7) [25]. In thiscase the cyano moieties of the starting material 6 are easily con-verted to amidine, carboxamide and iminoester derivatives 12–16depending on the nucleophiles and conditions used.

Most of the known AHMACs are AHBDs, the majority of thembeing combined in Table 1. The meaning of the formula symbols isindicated in Scheme 8.

2.2. Terminology, tautomeric, isomeric and structural features

AHBDs derived from symmetric �-diketones exist in solutionand in solid state as the hydrazone E-isomers, whereas AHBDs

1246 K.T. Mahmudov et al. / Coordination Chemistry Reviews 257 (2013) 1244– 1281

Scheme 2. Possible chelating modes of coordination within the AHBD complexes [52,53].

ann

deb[itabahco2o

tete

S[

Scheme 3. Japp–Klingem

erived from unsymmetric �-diketones can exist as a mixture ofnol-azo and hydrazone tautomers in E and Z isomeric forms ore stabilized as the E-isomers of the hydrazone form (Scheme 9)52,70,129]. The protonation/deprotonation of the MAC moietynduces a rotation around the hydrazone C N double bond, leadingo isomerization [129–132]. Therefore, we believe that gener-lly these compounds are better called arylhydrazones of, e.g.,eta-diketones, rather than hydrazono-derivatives of triketoness was proposed in some publications [128,133]. On the otherand, the designation [14] “2-(het)arylhydrazono-1,3-dicarbonylompounds” is not generally correct, because, e.g., arylhydrazonesf pentane-2,4-dione should be called 3-(arylhydrazone)pentane-,4-diones [88]. Thus, generally one should not indicate the positionf the arylhydrazone group.

AHBDs possess longer conjugated chains with more possibili-ies to form different tautomers and isomers, in comparison with,
.g., �-diketones, the well-known model to study the keto-enolautomerization [7,10,134–136]. Thus, AHBDs potentially mightxist in three tautomeric forms – (E/Z)-enol-azo, keto-azo and
cheme 4. Synthesis of AHMAC with N-nonafluorobutylsulfonylbenzotriazole103,104].

synthesis of AHBD [55].

(E/Z)-hydrazone (Scheme 9), giving rise to their application fordesign of functional materials attributed to smart hydrogen bond-ing and structural switching [63–76]. From another perspective,all the structurally studied AHBDs have a characteristic conjugatedheterodienic system forming a strong N–H· · ·O intramolecularRAHB linking one of the carbonyl groups to the NH-moiety ofthe hydrazone unit (Scheme 9) [15,26,60–98]. This hydrogen bondstrongly influences the properties of AHBD [67] and its strength isessentially determined by the degree of �-delocalization withinthe ketohydrazone hetero-conjugated system and is modulatedby the factors that can affect the degree of conjugation, includ-ing inductive ones and nonbonding intermolecular interactions.Also it was assumed that there is a cooperative effect between theH-bond strengthening and the �-electron delocalization of con-jugated bonds linking the proton-donating and proton-acceptinggroups [63,71]. The stable RAHB system formed hampers the com-plex formation and in many cases must be destroyed (e.g., with abase) before addition of a metal ion [54,113].

As mentioned above (Scheme 9), AHMAC and in particular AHBDcan, in principle, exist in several tautomeric forms. However, twopeaks for the acetyl groups of the diketone moiety of AHBD in theNMR spectra indicate that the resonance of one of these groupsundergoes a shift as a result of hydrogen bonding with the hydra-zone NH moiety. These data support the predominance of thehydrazone form with a six-membered H-bonded ring involvingone of the carbonyl groups and the protonated nitrogen moietyof the hydrazone structure (Scheme 9). On the other hand, the 1H
NMR spectra of the ortho-hydroxy substituted AHBD derived fromunsymmetrical �-diketones consist of two sets of signals, indicatingtheir existence as a mixture of enol-azo and hydrazone tautomericforms (Scheme 9) [52]. The hydrazone form of AHDB is generally

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Table 1Examples of known AHBD.a

Symbols o m p m′ o′ Complexes Ref.

Characterizedby X-rays

Arylhydrazones of pentane-2,4-dione (X = Y = CH3)17 H H H H H Fe3+, Cu2+, UO2

2+, Co2+, Ni2+,Mn2+, Zn2+, Cd2+, Ca2+, Mg2+

– [29,33,37,79,89,95,96,106]

18 OH H H H H Fe3+, Cu2+, UO22+, Co2+, Ni2+,

Mn2+, Zn2+, Cd2+, Ca2+, Mg2+,La3+, Ce3+, Pr3+, Nd3+, Sm3+,Eu3+, Gd3+, Tb3+, Dy3+, Ho3+,Er3+, Yb3+, Lu3+

Cu2+, Zn2+ [16,40,43,52,54]

19 OH SO3H H NO2 H Fe3+, Cu2+, UO22+, Co2+, Ni2+,


K+, Cu2+, Fe3+ [40,41,43,107–109]

20 OH SO3H H Cl H Fe3+, Cu2+, UO22+, Co2+, Ni2+,


K+, Co2+, Cu2+ [40,43,107]

21 OH SO3H H SO3H H Fe3+, Cu2+, UO22+, Co2+, Ni2+,


[40,43,44]

22 OH NO2 H NO2 H Na+, K+, Ni2+ K+, Ni2+ [110]23 OH H NO2 H H Fe3+, Cu2+, UO2

2+, Co2+, Ni2+,Mn2+, Zn2+, Cd2+, Ca2+, Mg2+,La3+, Ce3+, Pr3+, Nd3+, Sm3+,Eu3+, Gd3+, Tb3+, Dy3+, Ho3+,Er3+, Yb3+, Lu3+

Cu2+, Zn2+ [40,43,51,54]

24 OH H COOH H H Cu2+ Cu2+ [111]25 AsO3H2 H H H H Fe3+, Cu2+, UO2

2+, Co2+, Ni2+,Mn2+, Zn2+, Cd2+, Ca2+, Mg2+

Cu2+ [53]

26 SO3H H H H H Cu2+ Cu2+ [53]27 NO2 H H H H – [53,88,89,106]28 COOH H H H H La3+, Ce3+, Pr3+, Nd3+, Sm3+,

Eu3+, Gd3+, Tb3+, Dy3+, Ho3+,Er3+, Yb3+, Lu3+

Na+, Mg2+,Mn2+, Ni2+,Cd2+, Zn2+, Cu2+

[63,70,112,113]

29 COOCH3 H H H H – – [53]30 Cl H H H H – – [53]31 CH3 H H H H – – [89]32 I H H H H – – [106]33 CH3 H H H CH3 – – [88,89,106]34 CH3 H H CH3 H – – [88]35 CH3 H Br H CH3 – – [106]36 NO2 H H H NO2 – – [89]37 CH3 H H H NO2 Ni2+ Ni2+ [88,89]38 CN H H H H – – [47]39 SC2H5 Ni2+, Pd2+, Pt2+ Ni2+, Pt2+ [114]40 C(O)CH3 H H H H La3+, Ce3+, Pr3+, Nd3+, Sm3+,


– [115]

41 H H F H H Fe3+, Cu2+, UO22+, Co2+, Ni2+,

Mn2+, Zn2+, Cd2+, Ca2+, Mg2+– [30,43]

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Table 1 (Continued)



42 H H Cl H H Fe3+, Cu2+, UO22+, Co2+, Ni2+,

Mn2+, Zn2+, Cd2+, Ca2+, Mg2+– [29,33,42,79,116]

43 H H Br H H Fe3+, Cu2+, UO22+, Co2+, Ni2+,

Mn2+, Zn2+, Cd2+, Ca2+, Mg2+– [33,39,43,106]

44 H H I H H Fe3+, Cu2+, UO22+, Co2+, Ni2+,

Mn2+, Zn2+, Cd2+, Ca2+, Mg2+[33,38,91]

45 H H CH3 H H Cu2+ Cu2+ [79,88,116]46 H H OCH3 H H – [79]47 H H C(O)CH3 H H Fe3+, Cu2+, UO2

2+, Co2+, Ni2+,Mn2+, Zn2+, Cd2+, Ca2+, Mg2+

[43,48]

48 H H NO2 H H Fe3+, Cu2+, UO22+, Co2+, Ni2+,

Mn2+, Zn2+, Cd2+, Ca2+, Mg2+[79,94]

49 H H SO3H H H Fe3+, Cu2+, UO22+, Co2+, Ni2+,

Mn2+, Zn2+, Cd2+, Ca2+, Mg2+[36]

50 H H COOH H H Ni2+, Cu2+, Zn2+, Cd2+, Pb2+ Ni2+, Cu2+, Zn2+,Cd2+, Pb2+

[69,99–102]

51 H H COOC2H5 H H – – [43]52 H H N(CH3)2 H H – – [79]53 H H OH H H – – [77]54 H H CN H H – – [47]

55, 56 – – [77]

57 Cu2+, Ni2+, Zn2+ – [117]

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Table 1 (Continued)



58 Fe3+, Cu2+, UO22+, Co2+, Ni2+,

Mn2+, Zn2+, Cd2+, Ca2+, Mg2+– [35]

59 Cu2+, Ni2+, Pd2+, Fe3+ – [118]

60 Mn2+, Co2+, Ni2+, Cu2+, Zn2+ – [119]

61 Ru2+ – [96]

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Table 1 (Continued)



62 Ru2+ – [96]

Arylhydrazones of ethyl 3-oxobutanoate (X = OC2H5 , Y = CH3)63 H H Br H H – – [106]64 COOH H H H H Cu2+, Co2+ – [90]65 OH H H H H Fe3+, Cu2+ Cu2+ [45,52]66 CH3 H H H CH3 – – [106]67 CH3 H Br H CH3 – – [106]68 OH SO3H H NO2 H Fe3+, Cu2+, UO2

2+, Co2+, Ni2+,Mn2+, Zn2+, Cd2+, Ca2+, Mg2+

– [45]

69 OH SO3H H Cl H Fe3+, Cu2+ – [45]70 OH SO3H H SO3H H Fe3+, Cu2+ – [45]71 OH H NO2 H H Fe3+, Cu2+ Cu2+ [45,52]

Arylhydrazones of 1-phenylbutane-1,3-dione (X = CH3 , Y = C6H5)72 H H H H H – – [79]73 H H CH3 H H – – [79]74 H H OCH3 H H – – [79]75 H H Cl H H – – [79]76 H H NO2 H H – – [79]77 H H N(CH3)2 H H – – [79]78 OH H H H H Cu2+ Cu2+ [23,31,49]79 OH SO3H H NO2 H Fe3+, Cu2+, UO2

2+, Co2+, Ni2+,Mn2+, Zn2+, Cd2+, Ca2+, Mg2+

– [21,49]

80 OH SO3H H Cl H Cu2+ – [43,49]81 OH SO3H H SO3H H – – [43,49]82 OH H NO2 H H – [49,50]83 COOH H H H H La3+, Ce3+, Pr3+, Nd3+, Sm3+,


– [112]

84 C(O)CH3 H H H H La3+, Ce3+, Pr3+, Nd3+, Sm3+,Eu3+, Gd3+, Tb3+, Dy3+, Ho3+,Er3+, Yb3+, Lu3+

– [115]

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85 Cu2+, Ni2+, Pd2+, Fe3+ – [118]

86 Cu2+, Ni2+, Zn2+ – [117]

Arylhydrazones of 1,3-diphenylpropane-1,3-dione (X = Y = C6H5 , CH3)87 H H H H H – – [65,98,106]88 OH H H H H – – [49]89 C(O)CH3 H H H H La3+, Ce3+, Pr3+, Nd3+, Sm3+,


– [114]

90 H NO2 H H H – – [65]91 H H CH3 H H – – [65,98]92 H H OCH3 H H – – [65,98]93 H H F H H – – [98]94 H H Br H H – – [98]95 H H I H H – – [98]96 H H NO2 H H – – [98]97 H OCH3 H H H – – [98]98 Cl H Cl H H – – [98]99 Cl H H Cl H – – [9]100 Cl H H H Cl – – [98]101 H Cl Cl H H – – [98]102 H Cl H Cl H – – [98]103 Cl Cl Cl H H – – [98]104 OH SO3H H NO2 H – – [50]105 OH SO3H H Cl H – – [50]106 OH SO3H H SO3H H – – [50]107 OH H NO2 H H – – [50]108 CH3 H H H CH3 – – [106]109 CH3 H Br H CH3 – – [106]110 OCH3 H H H H – – [65]

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111 C(O)NH–NHC(O) H H H Cu2+, Ni2+, Pd2+, Fe3+ – [118]

Arylhydrazones of 5,5-dimethylcyclohexane-1,3-dione [X = Y = C(CH2)2(CH3)2]112 H H H H H Fe3+, Co2+, Ni2+, Cu2+, Zn2+ – [120,121]113 Cl H H H H – – [120]114 NO2 H H H H – – [120]115 H H CH3 H H – – [120]116 H H NO2 H H – – [120]117 H H Br H H – – [122]118 SO3H H H H H Cu2+ Cu2+ [123]119 OH H H H H Cu2+, Zn2+ Cu2+, Zn2+ [46,52,54]120 OH SO3H H NO2 H Fe3+, Cu2+, UO2

2+, Co2+, Ni2+,Mn2+, Zn2+, Cd2+, Ca2+, Mg2+

– [43]

121 OH SO3H H Cl H Fe3+ Fe3+ [108]122 OH SO3H H SO3H H Cu2+ – [18]123 OH H NO2 H H Cu2+, Zn2+ Cu2+, Zn2+ [46,52,54]124 C(O)CH3 H H H H La3+, Ce3+, Pr3+, Nd3+, Sm3+,


– [115]

125, 126 Ni2+, Cu2+ – [82]

Arylhydrazones of barbituric acid [X = Y = C(NH)2(=O)]127 H H Br H H – – [122]128 COOH H H H H Cu2+ Cu2+ [97]

Arylhydrazones of 4,4,4-trifluoro-1-(thiophen-2-yl)butane-1,3-dione [X = CF3 , Y = C(CH)3S]129 OH H H H H – – [43]130 OH SO3H H NO2 H Fe3+, Cu2+, UO2

2+, Co2+, Ni2+,Mn2+, Zn2+, Cd2+, Ca2+, Mg2+

– [19,43]

131 OH SO3H H Cl H – – [43]132 OH SO3H H SO3H H – – [43]133 OH H NO2 H H – – [43]

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Arylhydrazones of other MACs

134, 135 Ni2+, Cu2+ – [124]

136 Co2+, Ni2+, Cu2+, Zn2+ – [125]

137 – – [106]

138–140 Ni2+ – [58]

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141–143 Cu2+ – [57]

144 Co2+, Ni2+, Cu2+, Zn2+ – [126]

145 Cu2+ Cu2+ [90]

146 Na+ Na+ [92]

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147 Ni2+, Cu2+, Zn2+ – [127]

148 – – [15]

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149, 150 – – [91]

151–153 – – [70]

154, 155 Ni2+ Ni2+ [128]

a For the formula symbols, see Scheme 8.

K.T. Mahmudov et al. / Coordination Chemistry Reviews 257 (2013) 1244– 1281 1257

form

tiatt

Scheme 5. Template metal-mediated

he preferred one in non-polar solvents and at low temperature and
n the solid state. With a decrease of temperature, solvent polaritynd of the electron-withdrawing properties of the substituents, theautomeric balance shifts to the hydrazone form [47,50]. The thirdautomeric form, viz., keto-azo, was not detected in solution under
H

N

N

C

CN

C

ClH3N

N

O

H2C

CH2NH3

87

HCl

6

10

SO

Cu

N

N

C

CN

C

H2N

N

O

O

H2C

CH2NH2

OH2

Cu(ac)2

H2N(C H2)2NH2

CN

N

HC

N

CN

C

OCu

N

NHC

C

N

C

H3CO

N

C

OCu

N

NHC

C

N

C

H3CO

N

CO

Cu

N

HN

C

C N

C

CH3O

N

CO

Cu

N

NH

C

CN

C

OCH3

N

O

O

O

O

RAHB...

..

...

Scheme 6. Template synthesis and RAHB-prom

ation of AHMAC ligands from 1 [13].

any experimental conditions, presumably due to its lower stability
in comparison with the others [47,50,52].
From another viewpoint, unsymmetrical AHMACs exist in solu-tion as a pair of intramolecular H-bonded hydrazone (E,Z)-isomers,and a transfer between these isomers can be regulated by pH [132]

S

O

O

9

H2N(CH2)2NH2

C

OH

H

N

O

C

CN

C

ClH2N

N

O

11

HCl

H3C

H2N(CH2)2NH2S

OO

OH

RAHB

RAHBRAHB

...

.

...

SO

H

N

C

CN

N

C

NH

O

O

NH

N

...

...

...H

oted liberation of AHMAC ligands [105].


of AH

oceststp

hmrctirs

cefii

drt[t

Scheme 7. Synthesis

r coordination [131]. All the described intramolecular conversionsan be also induced by solute–solvent interactions leading to inter-sting solvatochromic effects [48,137]. Although the presence ofeveral tautomeric and/or isomeric forms hampers the interpre-ation of the spectroscopic data [31,52], theoretical calculationsometimes allow one to overcome these difficulties by rationalizinghe relation between the tautomeric ratio and the physico-chemicalroperties [29,73–75,138].

In the solid phase, all the AHBDs described exist only in theydrazone form with a six-membered RAHB involving the N–NH–oiety and a carbonyl group (Scheme 9) [14,29,30,52,70,89,94]. X-

ay analyses of some AHBD at different temperatures allows one toonclude [52] that, in the solid state, the temperature does not affecthe tautomeric competition with an intramolecular proton transfer,n contrast to what was found in solution by NMR (see above) andeported in the solid phase for the related ketohydrazone–azoenolystem [73].

Therefore, based on IR, NMR and X-ray structural analyses, onean conclude that the AHBDs derived from the symmetric MACsxist in solution and in the solid phase mainly in the hydrazoneorm, while those derived from the unsymmetric MACs are presentn solution as a mixture of hydrazone and enol-azo forms, althoughn the solid phase the hydrazone form predominates.

The study of the tautomerization and isomerization of AHMACseserves further exploration because it can open up a broadange of opportunities for the design of new switching sys-
ems [73–75,139], molecular rotors [140], optical recording media57,58], solvatochromic colorants [48,137,139], reactions relatedo the photosynthetic pathway [141], cation-diffusion facilitators
′

′

Scheme 8.

MAC via RAHB [25].

[141] and the construction of artificial systems mimicking the bio-logical ones [142].

2.3. Acid–base and redox properties of AHBDs

The acid–base properties of AHMACs influence their complexingability which is reflected in their role as, e.g., analytes in analyticalchemistry [16–23]. For instance, the inclusion of stronger electron-withdrawing groups to AHBD increases the acidity and can facilitatethe complexation with a certain metal ion.

pH-metric titrations of unsubstituted AHBDs show only onejump, while ortho-hydroxy and carboxyl substituted AHBDs behaveas diprotic weak acids (Tables 2 and 3) [16–18,22,29–42,46,51].The data show that with an increase of the electron-withdrawingproperties of a substituent in the aromatic part or the �-diketonefragment, the acidity of AHBD increases (pK decreases), the depro-tonation being endothermic. The introduction of a functional groupin para-position of the aromatic ring of the molecule influencesthe thermodynamic characteristics of the dissociation process of17, 41, 42, 48 and 50: with an increase of the inductive effect ofthat substituent ( H < Cl < COOH < F < NO2) the O H and N Hbonds weaken, �H◦ decreases and �S◦ becomes more negative(see below), i.e., the acidity and the number of ions in the systemincrease [29,30,46]. A large positive value of �G◦ indicates thatthe dissociation process is not spontaneous, i.e., it should be forcedby a base if the deprotonation is needed (e.g., for coordination)

n
[29,30,46]. Trends between Hammett’s �p, normal � p, inductive�I, resonance �R, negative �p
− and positive �p+ polar conjugation

and Taft’s �op substituent constants and some properties of series

of para substituted aryl AHBD were searched for [29]. However,

Table 2Dissociation constants of monoprotic AHBD.

Compound para- �I [143] pK Ref.

45 CH3 −0.05 8.60 ± 0.02 [43]17 H 0 8.54 ± 0.02 [33]47 C(O)CH3 0.28 8.50 ± 0.01 [43]44 I 0.39 8.46 ± 0.02 [38]43 Br 0.45 8.43 ± 0.03 [39]42 Cl 0.47 8.36 ± 0.02 [42]41 F 0.51 8.12 ± 0.01 [30]54 CN 0.52 8.11 ± 0.03 [47]48 NO2 0.64 8.10 ± 0.02 [29]


xy sub

i

lc

isowt

Scheme 9. Possible tautomeric and isomeric transitions in ortho-hydro

n 3-(ortho-substituted phenylhydrazo)pentane-2,4-diones ( Cl,AsO3H2, OH, NO2, COOH, SO3H, and COOCH3) such corre-

ations were not observed, possibly due to the formation of a threeenter RAHB system (Scheme 9) [53].

The electrochemical behavior of 17, 41, 42, 48 and 50 were stud-ed using cyclic voltammetry [29], and it was observed that the
ubstituent at the aromatic ring has a stronger influence on thexidation potential than on the reduction one, which is in accordith the HOMO and LUMO compositions, the former with an impor-
ant contribution of the aromatic component and the latter being

stituted AHBD (R1 is a stronger electron-donor group than R2) [23,31].

essentially localized at the �-diketone fragment. Theoretical stud-ies account for the irreversibility of the redox processes and allowone to propose conceivable mechanisms involving single-electronanodically induced dimerization and two-electron cathodicallyinduced protonation followed by N N bond cleavage [29].

3. Complexes of AHMACs

Many coordination compounds with AHMAC ligands wereprepared recently, but not all of them were structurally fully


Table 3Dissociation constants of diprotic AHBD.

Compound pK1 pK2 Ref. Compound pK1 pK2 Ref.

18 6.50 ± 0.01 10.41 ± 0.01 [16] 80 5.73 ± 0.01 9.33 ± 0.03 [49]19 5.71 ± 0.01 8.84 ± 0.01 [41] 81 5.41 ± 0.04 8.94 ± 0.02 [49]20 6.28 ± 0.01 10.14 ± 0.01 [41] 82 5.90 ± 0.05 9.51 ± 0.02 [49]21 6.03 ± 0.03 9.78 ± 0.06 [40] 119 6.38 ± 0.04 10.22 ± 0.05 [46]23 6.35 ± 0.04 10.22 ± 0.04 [51] 120 5.61 ± 0.03 8.74 ± 0.01 [43]50 3.98 ± 0.03 8.24 ± 0.02 [29] 121 6.17 ± 0.02 9.93 ± 0.02 [43]65 6.03 ± 0.03 9.86 ± 0.05 [46] 122 5.90 ± 0.03 9.67 ± 0.04 [18]68 5.50 ± 0.01 8.63 ± 0.02 [46] 123 6.26 ± 0.04 10.05 ± 0.05 [46]69 5.94 ± 0.04 9.75 ± 0.04 [46] 129 5.29 ± 0.03 9.10 ± 0.03 [43]70 5.78 ± 0.02 9.55 ± 0.03 [46] 130 4.25 ± 0.01 8.20 ± 0.01 [19]71 6.19 ± 0.05 9.98 ± 0.06 [46] 131 4.82 ± 0.01 8.41 ± 0.01 [43]78 5.98 ± 0.04 9.72 ± 0.03 [55] 132 4.51 ± 0.02 8.33 ± 0.03 [43]79 4.98 ± 0.03 8.53 ± 0.01 [21] 133 5.07 ± 0.03 8.63 ± 0.06 [43]

Table 4Stability constants of complexes of AHBD with some metal ions.

Ligand Stability constants FeIII CuII UO2II NiII CoII ZnII CdII MnII MgII CaII

17 [37] log ˇ1 9.10 8.95 8.78 8.65 8.28 7.96 7.74 7.56 7.22 6.92log ˇ2 16.98 16.03 15.64 15.31 15.02 12.78 12.23 11.65 11.06 10.11

19 [32] log ˇ1 10.43 10.08 9.90 7.50 7.40 7.28 7.10 6.93 6.75 6.68log ˇ2 20.54 19.03 17.82 14.70 14.45 14.13 13.67 13.57 13.02 12.83

20 [32] log ˇ1 12.38 11.91 11.22 8.60 8.52 7.60 7.45 7.40 7.25 7.00log ˇ2 24.63 23.13 21.84 15.70 15.62 13.80 13.75 13.38 13.32 13.01

130 [43] log ˇ1 8.67 8.35 8.10 7.60 7.55 7.47 7.41 7.27 7.17 7.03log ˇ2 15.60 15.17 14.72 14.25 14.11 14.03 13.86 13.67 13.52 13.28

131 [43] log ˇ1 9.49 9.33 9.20 9.11 8.80 8.75 8.69 8.51 8.38 8.30log ˇ2 17.56 17.08 16.84 16.68 16.10 16.01 15.89 15.67 15.43 15.31

7.66

4.15

cSX

3

wrw<ogo(

3

aa1aTeoa

ssb

159 consists in the formation of a 3D metal-organic network, drivenby 5-connected potassium and �5-monodeprotonated 22 nodes,the topological analysis of which reveals a uninodal net with therare 5/4/t5 topology.

Pentane-2,4-dione [144] log ˇ1 9.80 8.31

log ˇ2 18.80 15.60 1

haracterized. To avoid ambiguities, most of this section (excludingection 3.1) focus only on the AHMAC complexes characterized by-ray single crystal structural analysis.

.1. Complexation of metals with AHBDs in solution

In solution the reactions of complex formation of metal ionsith AHBDs proceed with a shift in the tautomeric equilib-

ium [19–21,46]. Generally the stabilities of metal complexesith AHBDs increase in the order CaII < MgII < MnII < CdII < ZnII

CoII < NiII < UO2II < CuII < FeIII [32–44] and are higher than those

f metal complexes with simple �-diketones, while the functionalroups introduced into the �-diketone fragment and aromatic partf the molecule affect the selectivity toward certain metal-ionsTable 4) [16,17,19–21,23].

.2. Sodium(I) and potassium(I) complexes

Compound 146 featuring a 2-carboxy group at the aromatic partnd a trifluoromethyl in the pentane-2,4-dione moiety (Scheme 8nd Table 1) forms the binuclear Na+ chelate complexes 156 and57 composed of sodium ions, two carboxylic hydrazone ligandsnd two coordinated alcohol solvent molecules (Scheme 10) [92].he RAHB system within the AHBD ligand is formed at the morelectron-donating side of the dione moiety. The hydrogen bondswing to the alcohol molecules give rise to the stack formation of

supramolecular cluster.
The complex salt 158 (Scheme 11) was prepared by refluxing a
olution of 28 and sodium hydroxide in methanol [113]. As it can beeen, even the application of this strong base does not allow one toreak the RAHB system. A 1D coordination polymer with bridging

6.06 5.40 5.07 3.84 4.24 3.67 –10.77 9.57 9.02 6.70 7.35 6.38 –

water molecules is formed in the solid state. The coordination num-ber of sodium is 6 and it has a distorted octahedral coordinationenvironment. The carboxyl group of 28 is deprotonated and com-pensates the charge of sodium ions.

The first KI–AHBD complex 159 was isolated in the course of thesynthesis of 22 from 23 [110], involving a shift of the p-nitro groupwith phenyl ring nitration (Scheme 12). A noteworthy feature of

Scheme 10. Ref. [92].


3

([chaw

3

l(a(rs1atiwm

wif


.3. Magnesium(II) complex

Structurally similar to 158, the MgII–AHBD complex salt 160Scheme 13) was synthesized by reacting 28 with MgII hydroxide113]. The magnesium coordination number is 6 and its positiveharge is neutralized by the deprotonated carboxyl groups andydroxyl ligands. The MgII ions are combined through hydroxy-nd aqua-bridges in 1D chain, while the RAHB system is preservedithin the AHBD ligand.

.4. Manganese(II and III) complexes

The template one-pot transformations of 1 on a MnII matrixead to three complexes of different nuclearities and topologiesScheme 14) [13]. The complex 161 of MnIII was prepared by themination of 1 with ethylenediamine. The linear, mixed-valenceMnII/MnIII) trinuclear complex 162 was formed in situ upon theeaction of azide with 1. In a reaction involving two different initu ligand formations, a cyclocondensation of two molecules of

and a hydrolysis of one nitrile group of 1, the first example of tetradecanuclear MnII aggregate, 163, was isolated and charac-erized. Magnetic measurements of 162 indicate the presence ofntramolecular and intermolecular antiferromagnetic interactions

hile those on 163 suggest the presence of dominating antiferro-agnetic coupling.The mononuclear hexa-coordinate complex 164 (Scheme 15)

II
as obtained by reaction of Mn acetate with 28 [113]. The multiplentramolecular RAHB systems stabilize the molecule, one of themorming a curious Mn O C O· · ·H O metallacycle.
Scheme 12. Synthesis of


3.5. Iron(III) complexes

The water-soluble FeIII–AHBD complexes 165 and 166(Scheme 16) were isolated in solid state upon reaction of FeIII

chloride with 19 and 121, respectively, in acidic medium (pH2) [108]. Both octahedral type structures are similar, with theeffective charge of FeIII being neutralized by the deprotonatedsulfo group, alkoxy oxygen atoms and hydrazone nitrogen. Themodification of the MAC fragment as well as of the aromaticpart (NO2 replacement by Cl) does not significantly influencethe structure. The coordinated water molecules are labile, whichcan facilitate exchange steps in catalysis. Accordingly, they wereapplied as catalysts for cyclohexane oxidation and good yields andhigh selectivities were observed [108].

3.6. Cobalt(II) complexes

It was demonstrated [32] that CoII ions in solution interact withsome AHBDs, but attempts to prepare complexes in the solid stateby concentration of a solution of CoII nitrate and 20 failed [55].To solve this problem, 20 was first deprotonated by addition ofethylenediamine (en), and then the thus prepared salt 167 was usedas the starting material for further coordination to CoII to afford 168(Scheme 17) [55]. The added ethylenediamine conceivably weak-ens the hydrogen-bonded system in 20 and the reactivity of suchactivated 20 increases, facilitating the N H deprotonation and the
coordination to the metal ion. Alternatively, 168 can be preparedvia 167 formed in situ, upon neutralization of 20 with en in solutionand subsequent addition of the CoII salt.
22 from 23 [110].


S d the complete aggregate viewed along its 3-fold axis is shown for 163) [13].

3

rancs

wstpdro

cheme 14. MnII–AHMAC complexes (structural formula of the asymmetric unit an

.7. Nickel(II) complexes

The NiII complex 169 (Scheme 18) was prepared by a simpleeaction of 33 with a NiII salt [88]. The complex is mononuclearnd bears two six-membered chelate metallacycles involving aearly regular N2O2 square planar coordination geometry. In therystal packing, strong and weak H-bond interactions lead to aupramolecular network structure.

Two other monomers 170 [110] and 171 [113] (Scheme 19)ere synthesized by deprotonation of 22 and 28 with base and

ubsequent addition of NiII salts. Both the hexacoordinate NiII struc-ures bear three coordinated water molecules, two of them in axial
ositions. The AHBD ligands are tridentate, coordinated via theeprotonated OH (or COOH) group in ortho position of the aromaticing, one of the nitrogen atoms of the former hydrazone unit andne oxygen atom of a carbonyl group.



17. R

ratma

Scheme

Complexes 172 and 173 (Scheme 20) are formed by templateeactions of 154 and 155 with o-phenylenediamine or ethylenedi-mine on a NiII matrix [128]. Both 172 and 173 are monomers,
he AHBD ligands coordinate in a N4-chelating mode. Magnetic
easurements and ESR spectroscopic studies revealed the appear-nce of paramagnetism due to a tetrahedral distortion of the


ef. [55].

coordination unit and also an unusual behavior of the effectivemagnetic moment at low temperatures.

The reaction between Ni(OAc)2·4H2O and the ortho–SC2H5 sub-stituted AHBD 39 (1:2 molar ratio) affords 174 (Scheme 21) [114].In this complex, 39 acts as O,N,S tridentate chelating ligand. Coulo-metric oxidation of 174 led to the formation of the NiIII state provenby its EPR spectrum. Moroever, 174 has shown a good anti-bacterialactivity.

3.8. Copper(II) complexes

AHBDs have proven to be especially good and versatile ligandsfor the formation of CuII complexes. In these complexes, the CuII

ions are mainly bound in a chelating fashion creating a CuNNCCOmetallacycle, while additional substituents may influence the over-all geometry. These compounds are promising building blocks forthe design of self-assembled coordination organic-inorganic mate-rials [145] or smart functional materials due to tautomerism [69].

For instance, series of 3-(arylhydrazone)pentane-2,4-diones 33and 45 were investigated with regard to complex formation with
CuII salts, leading to the isolation of complexes 175 and 176(Scheme 22) [88]. Both are mononuclear with a ligand to metal ratioof 2:1. The ligands as a whole bear chelating N2O2 binding sites thatcreate a square-planar environment. In 175, the coordinated water


Scheme 19. Refs. [110,113].

action

m1

ieTi

Scheme 20. Template re

olecules link the complexes by hydrogen bonds creating infiniteD-chains.

The AHBD compound 144 and its CuII complex 177 were stud-
ed in order to expose the effect of a methyl for trifluoromethylxchange in the pentane-2,4-dione fragment (Scheme 23) [89].he replacement of the methyl for trifluoromethyl group has anmpact on the crystalline packing of both the free ligand and the

s of 154 and 155 [128].

corresponding CuII complexes, while the changes of the moleculargeometries are secondary.

The centrosymmetric trinuclear complex 178 (Scheme 24) wasprepared by reaction of CuII sulfate with 19 [41]. In 178, AHBD actsas a tetradentate ONO and O ligand; the central Cu atom has thecoordination number six, and the terminal ones have the coordi-nation number five. The complex exhibits several labile sites andis soluble in water, properties that are potentially important forcatalytic applications.

Reaction of 26 with copper(II) acetate hydrate in the presenceof pyrazine (pz) gives rise to the 1D coordination polymer [Cu2(�-L)2(H2O)2(�-pz)]n (179) (Scheme 25) [109] where the bidentatepz molecules interlink the [Cu2(�-L)2(H2O)2] units. The coppercenters show distorted octahedral coordination, being connectedvia oxygen atoms of the sulfo groups. Hydrogen bonds involv-ing the uncoordinated carbonyl group, coordinated water and pzmolecules give rise to the stack formation of a supramolecular 3Dassociate.

Complexes 180–185 (Scheme 26) were synthesized using theortho-hydroxy substituted AHBDs 18, 23, 64, 70, 118, and 122[51,52]. The substituents in the �-diketone fragment and in thearomatic part of the AHBD ligands strongly influence the overallstructures. Thus, 180 and 182 are dimers, dimeric repeating unitsare also main building blocks of the polymeric grid-type network of
183, while 184 and 185 are monomers associated by intense �· · ·�interactions involving metallacycle rings.
Either the monomer 186 or the dimer 180 can be obtained from23 and CuII nitrate; under reflux in methanol solution 186 converts


Scheme 22. R

teslwass

bc2Cp

tion of 199 with Bu2SnO in dimethylformamide gave 200. CuII inboth 199 and 200 is penta-coordinate, while the doubly deproto-nated 24 acts as a ONOO-tetradentate ligand. Solvent molecules


o 180 (Scheme 27). The salt 187 can be prepared by reaction ofthylenediamine (en) with 23 and acts as an intermediate in theynthesis of 186. The reaction of 23, CuII nitrate and en at pH 2eads to a template Schiff-base condensation with formation of 188,

hile reactions of 180 and 186 with en in neutral conditions ort pH 2, correspondingly, give the Schiff-base polymer 188, whichpreads unidimensionally in ladder type chains, extending to theecond dimension by means of intermolecular hydrogen bonds.

Reactions of CuII with 3-arylhydrazonepentane-2,4-dionesearing a substituent in the ortho-position lead to a variety ofomplexes including the dimeric 190, the polymeric 191 (Scheme
8), often bearing two fused six-membered metallacycles [53].omplexes 192–194 can interconvert, depending on pH and tem-erature, whereas the reactions of CuII with 26 in the presence of

ef. [88].

cyanoguanidine or imidazole afford the monomeric compound 195and the heteroligand polymer 196, respectively (Scheme 29).

Complex 197 (Scheme 30) was synthesized by reaction of 127with CuII nitrate [97]. The copper atom is coordinated by the nitro-gen atom of the former hydrazone group close to the phenyl ring,the oxygen atom of one of the carbonyl groups of the barbituricmoiety and one of the oxygen atoms of the carboxylate, formingtwo six-membered chelate rings. Two water molecules complete atypical 4 + 1 square-based pyramidal coordination.

The CuII complex 198 (Scheme 31) is a coordination polymerwith a binuclear core of a distorted square pyramidal geometry;the coordination sphere is filled by hydrazone nitrogen, alkoxy andcarbonyl oxygen atoms [31]. The polymeric chain is formed via axialcoordination of the non-chelating carbonyl group of the MAC partof 78.

24 exists in the hydrazone tautomeric form in the free state aswell as in the di- and mononuclear CuII complexes 199 and 200(Scheme 32) [111]. Complex 199 was isolated from the mixture ofCuII nitrate with 24 in methanol solution upon refluxing; the reac-



6. Ref

cacd[ap

(cO

t(cmegmo

eaas7ch

Scheme 2

omplete the CuII coordination sphere. 199 and 200 display cat-lytic activity for the peroxidative oxidation of cyclohexane toyclohexanol and cyclohexanone and for the selective aerobic oxi-ation of benzyl alcohols to benzaldehydes in aqueous solution111]. Magnetic susceptibility measurements of 199 reveal strongntiferromagnetic coupling between the CuII ions through the �2-henoxido-O atoms.

Reaction of CuII acetate with 50 gave the polymer complex 201Scheme 33) [102]. The RAHB system stays intact and the CuII

ations are combined through coordination by three carboxylate atoms from one AHBD ligand and two acetates.

The CuII–AHBD complexes 202–206 were prepared by reac-ing 118 with a CuII source in the presence of auxiliary ligandsScheme 34) [123]. In 202, water molecules play the role of bridgesonnecting two dimeric structural units, while a coordination poly-er is formed by ligation of the sulfo group to CuII. 202 is the first

xample in which both nitrogen atoms of the former hydrazoneroup are simultaneously coordinated to CuII. Pyrazine bridges twoonomers forming 206. 202–206 were applied as catalysts in the

xidation of cyclohexane to cyclohexanol and cyclohexanone [123].Upon reactions of CuII with 1 and 6 in the presence of methanol,

thanol, water, ethylamine and ethylenediamine, nucleophilicttacks to the cyano moieties occur leading to a variety of lig-ted amidines, carboxamides and iminoesters depending on the
tarting ligands, nucleophiles and conditions used. Mononuclear, 207–209, tetranuclear 10, 211, 212 and polymeric 208 and 210omplexes were thus synthesized (Scheme 35) [105]. The easyydrogen-bond assisted liberation of ligands 8 and 11 from 7 and
s. [51,52].

10 was demonstrated (Scheme 6). Selective oxidation of primaryand secondary alcohols to the corresponding carbonyl compounds,as well as diastereoselective nitroaldol (Henry) reaction catalyzedby the complexes were studied, affording typical yields of 80–99%[105].

The water-soluble zwitterionic complex 213 was prepared byreaction of CuII nitrate with the ethylenediammonium salt of 20(167) (Schemes 17 and 36) [55]. The zwitterionic nature of com-plex 213 (pI = 4.96) can be used in applications where pH-tunableor buffer properties of the system are crucial for the optimal perfor-mance of a specific function (e.g., in catalysis). The buffer zones ofthe complex lay in the weak acidic region with pH close to pK2 = 6.84(where K2 is the deprotonation constant of 213 concerning theH3N+ group of Hen) and in the acid area with pH close to pK1 = 3.08(where K1 concerns the deprotonation of the SO3H group). Possi-bly due to the formation of the zwitterion, the detection limit forthe analytical determination of CuII with 167 (concerning the for-mation of 213) decreases in comparison to 20 and other similarreagents, with both the sensitivity and selectivity being enhanced[49]. In the formation of 213, 20 provides a chelating ligand, whilea crucial synthetic and structural role is played by en, allowing it toorganize water-soluble assemblies and to influence the overall 3Dsupramolecular arrangement. These features should be accountedfor in further exploration of the still rare intramolecular NH3

+

−O3S interaction and H-bonding abilities of the AHBD–en systemsin aqueous medium crystal engineering. 213 acts as catalyst precur-sor for the aerobic TEMPO-mediated selective oxidation of benzylicalcohols to the corresponding aldehydes, in aqueous medium.


ith 23

tptsCac2u

sdaa

Scheme 27. Reactions of CuII w

3-(2-(Alkylthio)-2-phenylazo)-2,4-pentanedione coordinateso CuII as an O,N,S-donor ligand and generates a Cl−-bridgedolymer (214) or a SCN−-bridged dimer (215) (Scheme 37) andhe monomeric 192–194 ones (Scheme 29) [146]. Electrochemicaltudies reveal a quasi-reversible CuII/CuI redox couple along with auI/Cu0 reduction; the EPR spectra in solution support a metal-to-zoimine charge transfer. The complexes show antiferromagneticoupling with J = −0.5 ± 0.1 cm−1 for 214 and −25.8 ± 0.5 cm−1 for15. The spectra, magnetism and redox properties are explainedsing DFT computation on the optimized geometries.

The copper(II) complexes 216 and 217 (Scheme 38) were
ynthesized reacting CuII acetate hydrate with 50 and N,N-iethylpyridine-3-carboxamide (cardiamine) in the presence andbsence of sodium bicarbonate, respectively [147,148]. The CuII
tom in 216 lies in the plane defined by the O atoms of the


and its modified form 187 [52].

carboxylate ions, the N atom of pyridine moiety and the watermolecule, while the apical coordination to the amido O atom ofan adjacent N-heterocycle leads to the polymeric chain formation.The CuII atom in monomer 217 lies in the center of inversion, and iscoordinated by the O atom of each carboxylate moiety, the pyridyl Natom and a water molecule in an all-trans octahedral geometry. Thewater molecule is hydrogen-bond donor to the carbonyl and pyri-dine of the adjacent molecule thus generating linear supramolec-ular chains running along the a-axis of the triclinic unit cell.

3.9. Zinc(II) complexes

The tetranuclear 218 and trinuclear 219 complexes (Scheme 39)were prepared by treatment of 50 with ZnCl2·2H2O orZn(NO3)2·6H2O, correspondingly [100]. In 218, a 3D solid-state supramolecular structure was constructed by intra- andintermolecular hydrogen bonds and Van der Waals interactions.In 219, the three zinc ions have two kinds of coordination: theterminal Zn1 and Zn2 are four-coordinate, while the central Zn3is six-coordinate. The Zn1 and Zn2 adopt a slightly distortedtetrahedral geometry, in which three oxygen atoms from threebridged monodeprotonated 50, and one oxygen atom belongs tothe coordinated water. Zn3 is coordinated by six oxygen atoms ofsix bridged monodeprotonated 50 ligands. The thermal behaviorand fluorescent properties of 218 and 219 were also studied.

Treatment of sodium salts of 50 with Zn(OAc)2 and Zn(NO3)2
in methanol afforded complexes 220 and 221 (Scheme 40) [99]. In220, the acetate anions bridge the ZnII ions forming an infinite 1Dpolymeric chain with monodeprotonated 50 units acting as mon-odentate ligands. The linear 220 chains are linked by intermolecular


29. R

h5bmm

b4bn

forming infinite zigzag chains. Terminal monodentate monodepro-tonated 50 coordinates to ZnII and hangs from the main chain[Zn(4,4′-bipy)]n. Thermal and fluorescent properties of 222 werealso investigated.

Scheme

ydrogen bonds forming a 2D sheet. In 221, two monodeprotonated0 coordinate in a monodentate fashion. Each ZnII is coordinatedy six oxygen atoms, four of which from coordinated methanololecules at the equatorial plane, and the other two from twoonodeprotonated 50 ligands located at the axial positions.The coordination polymer 222 (Scheme 41) has been prepared

y the reaction of the sodium salt of 50 with ZnII sulfate and
,4′-bipyridine [101]. The ZnII ions are coordinated by two car-oxylato oxygen atoms from two monodeprotonated 50 and twoitrogen atoms from two 4,4′-bipyridines. The metal ions are linked

ef. [53].

by 4,4′-bipyridine and carboxylate monodeprotonated 50 bridges



32. Re

clht(sstfdibtcafcd

laoom

3

pCtma

Scheme

Simple and effective syntheses of the mono- and dinuclear 5-oordinated ZnII complexes 223–226 (Scheme 42) with the AHBDigands 18, 23, 118 and 122 were achieved [54]. The AHBDs studiedave a difficult-to-destroy RAHB, thus hampering the coordina-ion to metal ions. Upon concentration of the solution, the RAHBdestroyed by interaction with the solvent molecules) regains itstructure. To shift the overall process to the complex formation, onehould remove the proton by addition of a base but this can leado hydrolysis of ZnII with its precipitation as a hydroxide. Thus, theollowing procedure was used for the synthesis of the complexes:issolution of the pro-ligand in a methanol–water solution contain-

ng two equivalents of sodium (or potassium) hydroxide, followedy addition of ZnII chloride and concentration of the reaction mix-ure. The structure and the nuclearity of the studied ZnII–AHBDomplexes are dependent on the substituents in the �-diketonend aromatic parts of the AHBD ligands. 223–226 act as catalystsor the addition of nitroethane to various aldehydes producing theorresponding �-nitroalkanols (Henry reaction) in high yields andiastereoselectivities.

The tetranucler ZnII–AHBD complex 227 (Scheme 43) was iso-ated from reaction of ZnII acetate with 28 [113]. In 227 two ZnII

toms bear a tetrahedral coordination while for the other two anctahedral coordination is observed. Two AHBDs behave as mon-dentate and other two as bidentate ligands. Although a basicedium was used, the RAHB system stays intact.

.10. Cadmium(II) complexes

Treatment of 50 with Cd(NO3)2 and Cd(OAc)2 afforded com-lexes 228 and 229, respectively (Scheme 44) [99]. In 228, four

II
d ions are bridged by bridging monodeprotonated 50 leadingo a tetranuclear core [Cd4(�2-monodeprotonated 50)2(�2-�2-
onodeprotonated 50)6(H2O)4(MeOH)2]. In the polymer 229, thecetate anions bridge the CdII ions leading to a 1D chain containing


f. [111].

chelating monodeprotonated 50 units in the side chain. Each CdII

atom is seven-coordinated, and is in a single-cap triangular prismenvironment with three oxygen atoms from two �2-�2-OOCCH3units, two oxygen atoms from one chelating 50 and two oxygenatoms from two water molecules. Thus, in the solid state struc-ture of 229, linear [Cd(�2-50)(�2-�2-OOCCH3)(H2O)2]n chains arelinked by intermolecular hydrogen bonds forming a 2D sheet. These2D sheets pack each other through Van der Waals interactionsforming a 3D supramolecular structure.

Reaction of CdII acetate with 28 affords the mononuclear hepta-coordinate complex 230 (Scheme 45) [113], where the metalpositive charge is neutralized by two carboxylato groups. Thehydrazone moieties of the AHBD ligands are not involved in themetal coordination.

3.11. Platinum(II) complex

39, a potential ONS donor ligand, was used for the synthe-sis of the first PtII-AHBD complex 231 (Scheme 46) [114]. Theligand forms two essentially planar five- and six-member chelaterings. The most prominent intermolecular interaction in the crystalstructure appears to be of the type C H· · ·Cl which results in theformation of linear supramolecular chains aligned along the b-axis.The compound possesses an anti-bacterial activity.

3.12. Lead(II) complexes

The coordination polymer 232 has been prepared by reactionof 50 with Pb(OAc)2·2H2O (Scheme 47) [101]. In 232, PbII ions arebridged by �-�2-CH3COO− moieties to form a linear 1D chain. Thecoordination sphere around each PbII is filled with four oxygenatoms from four bridging-acetates, two oxygen atoms from onechelating �2-monodeprotonated 50, and one oxygen from coordi-nated water. The thermal and fluorescent properties of 232 werealso investigated.

3.13. Heterometallic complexes

The 3D CuII–KI coordination polymers 233 and 234 (Scheme 48)were easily generated by self-assembly from CuII nitrate, potassiumhydroxide and a multi-functionalized AHBD, viz., 20 or 19 [107].233 and 234 have slightly different 3D metal-organic networks. Thetopological analysis of 233 discloses a rare binodal 5,6-connected
underlying net with the unprecedented topology defined by thepoint (Schläfli) symbol of (3.45.53.6)(32.46.53.64). Both networkspossess voids filled by crystallization water molecules, which canreversibly escape and bind back. Besides, 233 act as a catalyst


34. Re

pb

wcanTCio

Scheme

recursor for the aerobic TEMPO-mediated selective oxidation ofenzyl alcohol to benzaldehyde in aqueous medium.

In acidic medium (pH 2) the same AHBD 19, upon interactionith CuII, produces the dimer 235 (Scheme 49) [149]. The positive

harges of CuII ions are neutralized by the deprotonated nitrogentom of the ligated hydrazone derivatives and by the deproto-ated O atom from the ortho-hydroxo group of the phenyl ring.
he 5-coordinate copper atoms are bridged via carbonyl groups.omplex 235 was immobilized on Zn–Al-layered double hydrox-
de and applied as a recyclable heterogeneous catalyst for alkanexidation [149].

f. [123].

A water-soluble CuII–Na coordination polymer,{[Cu(H2O)Na(H2O)(�9-L)]·2H2O}n 237 (Scheme 50), was gen-erated by self-assembly from copper(II) nitrate, sodium hydroxideand 2-hydroxy-5-nitro-3-(2-(2,4,6-trioxotetrahydropyrimidin-5(2H)-ylidene)hydrazinyl)benzene sulfonic acid (236) [150].The crystal structure of 237 is constructed from copper(II) andsodium ions linked by fully deprotonated 236 ligands in an overall
�9-chelating mode, forming 1D chains. These chains have inter-calated crystallization water molecules enabling the expansionof the structure to 3D by means of extensive hydrogen bondinteractions.


Scheme 35. Reactions of 1 and 6 with CuII [105].




Scheme 38. Refs. [147,148].




233 234





Scheme 51. Possible coordination modes of AHBD.

emist

4

heh

acimachaA[mel[tdbcptgIcf[

bspeNboit(hNp

cmeStwcmalbf

a[Hsos


. Conclusions

The rich organic chemistry of AHMACs [14,25] makes themighly promising in the organic synthesis of compounds with inter-sting possible applications [22,23,80,151], namely related to theirigh propensity to tautomerism and isomerism [25–28].

With respect to coordination chemistry, AHMACs are valu-ble ligands for many metal ions allowing one to create differentonfined and overall geometries and structures. Thus, depend-ng on the position of a substituent and its nature and on the

etal ion, different types of coordination (Scheme 51) can bechieved. Many AHBD ligands form trivial mononuclear diligandomplexes (I) [53,88,99,113,123], but the introduction of, e.g., anydroxo substituent at the ortho position of the aromatic partllows one to yield mononuclear complexes with one ligatedHBD or binuclear diligand complexes (III and II, respectively)

31,51,52]. The ortho-OH group particularly favors the dimer for-ation, in comparison to other substituents in this position. For

xample, a carboxy or sulfo group in this position frequentlyeads to other types of coordination and overall structures (VI–XI)41,53,55,99–101,107,111,113,123,149,150]. On the other hand,he non-chelating carbonyl of the MAC fragment can further coor-inate to a metal-ion (IV) [31,52] and a coordination polymer cane formed. The position of the substituent in the aromatic partan drastically influence the geometry of the complexes (e.g., com-are V and VI). In addition, variation of the metal ions used forhe synthesis allows one to prepare complexes of different topolo-ies and coordination modes using the same ligand [113] (e.g., seeI–IV or VI–IX). Both nitrogen atoms of the former hydrazone groupan coordinate in one compound (X) [123], and, by using suitableunctional groups, one can prepare different coordination polymers53,101,105,123], including heterometallic ones (XI) [107].

In many cases the coordination sphere of the metal ions cane completed with labile (e.g., solvent) molecules. Complexes withquare planar, square pyramidal or octahedral geometries and withlanar tridentate ONO-coordination sites are more common, butxamples of other geometries and/or different NNO-, ONS-, OO- orO-coordination motifs also exist. The hydrazone hydrogen atomecomes labile upon complexation of the AHBD, with formationf a mono- or a dianionic deprotonated ligand. Whether a bases required for the deprotonation before coordination depends onhe metal ion used for the complex formation and other conditionscounter-ion, pH, etc.). For instance, FeIII can form complexes in aighly acidic medium (pH 2) [108], while complexes of, e.g., CoII,iII, and ZnII were not isolated even at pH 7. In these cases thero-ligand should first be deprotonated by base [54,55,110].

From another perspective, AHMACs and their complexesan create interesting supramolecular architectures involvingonomeric, oligomeric or polymeric units [31,51–53]. Differ-

nt nucleophiles have participated in this process via templatechiff base condensation or as proton attractors and spacers,hus allowing one to regulate the size of channels or cavitiesithin the H-bonded assemblies [14,52,53,55]. Hence, the AHMAC

omplexes can be used as promising building blocks in variousulti-component supramolecular architectures. Conformational

nd geometrical (tautomeric–isomeric) flexibilities of AHMACigands with protonated functional groups make them elegantuilding components for the construction of metal coordinationrameworks and organo-inorganic hybrid solids.

AHMAC complexes posses various biological activities [96,114],re promising agents for the blue-ray optical storage system152,153] and organic recording medium for the next-generation
D-DVD-R [57]. The CuII and FeIII complexes with ortho-hydroxy
ubstituted AHBD show catalytic activities for the peroxidativexidation of cyclohexane to cyclohexanol and cyclohexanone, forelective aerobic oxidation of benzyl alcohols to benzaldehydes in

ry Reviews 257 (2013) 1244– 1281 1279

aq. solution, mediated by TEMPO radical, under mild conditions,and for the microwave-assisted solvent-free synthesis of ketonesfrom secondary alcohols with tert-butylhydroperoxide as oxidant[51–53,108]. The ZnII-AHBD complexes are effective catalysts forthe addition of nitroethane to various aldehydes producing thecorresponding �-nitroalkanols in high yields and diastereoselec-tivities.

Therefore, AHMAC ligands show a high potential toward thecreation of new materials and multinuclear assemblies includingcoordination polymers, as well as in template synthesis and cataly-sis. However, AHMAC complexes have yet to be reported, or are stillvery rare, for many metal ions, thus further investigation shouldalso be directed to fill these gaps.

Acknowledgements

This work has been partially supported by the Foundationfor Science and Technology (FCT), Portugal, and its PTDC/QUI-QUI/102150/2008 and PEst-OE/QUI/UI0100/2011 projects and“Science 2007” program. K.T.M. and M.N.K. express gratitude to theFCT for a post-doc fellowship and a working contract.

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