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Inorganica Chimica Acta 358 (2005) 2909–2920

Reactions of pentaammineruthenium complexes with1,2- and 1,4-dicyanobenzene and cyanobenzamides:evidences of neighboring group participation q

Elia Tfouni a,*, Antonio Marcos de Souza Macedo a, Luciana nunes Cardoso b,Kleber Queiroz Ferreira b, Erica Cristina de Oliveira b, Zenis Novais da Rocha b

a Departamento de Quımica da Faculdade de Filosofia, Ciencias e Letras de Ribeirao Preto, Universidade de Sao Paulo, Av.Bandeirantes,

3900, 14040901 Ribeirao Preto, Brazilb Instituto de Quımica da, Universidade Federal da Bahia, Brazil

Received 30 July 2004; accepted 7 September 2004

Dedicated to the memory of Professor Rex. E. Shepherd

Abstract

The spectral (UV–Vis and IR) and electrochemical behavior of the nitrile bonded complexes [Ru(NH3)5L]2+ (L = 1,4-dicyano-

benzene (1,4-dcb), 1,2-dicyanobenzene (1,2-dcb)), [Ru(NH3)5(NHC(OH)-bz-4-CN)]3+, [Ru(NH3)5(NHC(O)-bz-2-CN)]2+ and

[Ru(NH3)5(NH(C)NHC(O)bz)]3+ (NH(C)NHC(O)-bz = 3-imino-1-oxo-isoindoline) are described. Oxidation of [Ru(NH3)5L]2+,

at 0 6 pH P 6, is followed by hydrolysis of the coordinated nitrile to give amide complexes in which the amide is through the nitro-

gen, with pH-dependent rate constants. The estimated values of the rate constant of hydrolysis (kobs) at 25 �C are 2.9 · 10�3 s�1 for

[Ru(NH3)5(1,4-dcb)]3+ and 5.6 · 10�3 s�1 for [Ru(NH3)5(1,2-dcb)]

3+ at pH 4.65. Reduction of [Ru(NH3)5(NHC(O)-bz-4-CN)]2+ and

[Ru(NH3)5(NHC(O)-bz-2-CN)]2+ is followed by two reactions, one is an aquation forming [Ru(NH3)5(OH2)]2+ and free ligand, and

the other an intramolecular linkage isomerization forming [Ru(NH3)5(NC-bz-4-NH2C(O))]2+ and [Ru(NH3)5(NC-bz-2-NH2C(O))]2+.

The oxidized1,2-cyanobenzamide complex [Ru(NH3)5(NHC(OH)-bz-2-CN)]3+ undergoes an amide to nitrile intramolecular linkage

isomerization, followed by a cyclization reaction resulting in [Ru(NH3)5(NH-(C)(HN-C(O)-2-bz))]3+ ((NH-(C)(HN-C(O)-2-bz)) =

3-imino-1-oxo-isoindoline bonded through the exocyclic nitrogen) (pKa = 4.3). The rates of these reactions, which occur with neigh-

boring group participation, increase with acidity. The reduced form, [Ru(NH3)5(NH-(C)(HN-C(O)-2-bz))]2+, is relatively substitution

inert.

� 2004 Elsevier B.V. All rights reserved.

Keywords: Ruthenium; dicyanobenzene; Cyanobenzamide; Neighboring group participation; Nitrile hydrolysis

0020-1693/$ - see front matter � 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.ica.2004.09.008

q Taken in part from: E.C. Oliveira, M.Sc. Dissertation, Univer-

sidade de Sao Paulo, Ribeirao Preto, Brazil, 1998. A.M.S. Macedo,

M.Sc. Dissertation, Universidade Federal da Bahia, Salvador, Brazil,

2002.* Corresponding author. Tel.: +55166023748; fax: +55166331851.

E-mail address: eltfouni@usp.br (E. Tfouni).

1. Introduction

A general feature, the hydrolysis of nitriles in aqueous

solutions, is greatly enhanced by coordination of RCN in

a metal complex as demonstrated by 106–108-fold in-

crease in the rates of hydrolysis compared to the uncoor-

dinated ligand [1–9]. These rates can be lower, though, in

cases where the metal ion is capable of p back-bonding,such as with ruthenium(II) ammines. This reaction is

2910 E. Tfouni et al. / Inorganica Chimica Acta 358 (2005) 2909–2920

important in fundamental chemistry [10–12], and in

industry [13,14]. Amides are used in a wide variety of

industrial applications as lubricant; detergent additives,

drug stabilizers, and monomers [14], in the industrial

conversion of nitriles to amides, such as acrylonitrile to

acrilamide [13], and in biochemistry, as in nitrile hydra-tase [15–17]. Reactions of coordinated organonitriles

and catalysis aspects to amide formation were reviewed

recently [2].

In general, the hydrolysis results in coordinated

amide complexes. However, the reactivity of the coordi-

nated nitrile is not restricted to hydrolysis, as exempli-

fied by several reactions [2,7,18] of nitrile complexes

and of their products. Oxygen–nitrogen linkage isomeri-zation occurs for amido bonded complexes, which re-

sult, from the hydrolysis of the coordinated nitrile

[19,20]. Linkage isomerizations also occur in bifunc-

tional nitriles, such as 4-cyanopyridine and acrylonitrile

[9,21–23]. Deprotonation reactions leading to further

reactions [18] and oxidative dehydrogenation of a-carbon of 4-pyridylacetonitrile were also reported [24].

A chelation reaction for the products of the hydrolysisof [Ru(NH3)5(2-NCpy)]3+ (2-NCpy = 2-cyanopyridine)

had been reported [25]. For cobalt complexes, other

reactions have been reported. Intramolecular cyclization

of the coordinated 2-cyanobenzamide, resulting in a

complex of 3-imino-1-oxo-isoindoline bonded through

the exocyclic nitrogen and deprotonated at the endocy-

clic nitrogen, was reported for [Co(NH3)5(1,2-dcb)]3+

(1,2-dcb = 1,2-dicyanobenzene) [26]. The same reactionhad been inferred for ruthenium in acidic pH [27].

In our studies related to coordinated nitriles, we re-

cently drew our attention to dinitriles, such as malonit-

rile, fumaronitrile, succinonitrile, 1,4-dicyanobenzene

(1,4-dcb), and 1,2-dicyanobenzene, investigating linkage

isomerization reactions. In view of the reported prelim-

inary and complicated nature of the reactions involved

with the 1,2-dicyanobenzene ruthenium complex [27],we decided to further investigate the behavior of

[Ru(NH3)5(1,2-dcb)]2+ and, for comparison purposes,

also of [Ru(NH3)5(1,4-dcb)]2+. Thus, in this paper the

synthesis and reactions of these two complexes and the

reactions of their products are described.

2. Experimental

2.1. Chemicals and reagents

Ruthenium trichloride (RuCl3 Æ 3H2O) (Strem) was

the starting material for ruthenium complexes syntheses.

Reagent grade 1,4-dicyanobenzene and 1,2-dicyanoben-

zene (Aldrich) were used as supplied. Sodium tetrafluo-

roborate (Aldrich) was recrystallized from hot water.Acetone and ethanol (Aldrich) grade P.A were used as

supplied. Doubly distilled water was used throughout

the work. All other materials were of reagent grade

and were used without further purification.

2.2. Complexes synthesis

2.2.1. RuCl(NH3)5]Cl2This compound was prepared as described [28] with

slight modifications [11]. Average yield: 85%.

2.2.2. Ru(NH3)5(1,2-dcb)](BF4)2 Æ H2O

This compound was synthesized by the method de-

scribed for other pentaammineruthenium(II) complexes

with azine ligands with some modifications [29,30]. 100

mg (0.342 mmol) of [RuCl(NH3)5]Cl2 was suspended indeaerated water (5 mL) and argon was bubbled through

the mixture. After dissolution, Zn(Hg) was added to the

solution, with continuous argon bubbling and protection

from light. After the solution became bright orange, 10

mL of a deaerated solution of 260 mg (2.00 mmol) of

1,2-dicyanobenzene in 1:1 acetone:ethanol was added.

After 2 h, the mixture was filtered over a freshly prepared

and deaerated NaBF4 solution (0.15 g/0.15 mL). The sol-vent was removed under reduced pressure to give an or-

ange solid, which was washed several times with doubly

distilled, cold and deaerated water, to extract the orange

complex. Then, the orange filtrate was evaporated to dry-

ness under reduced pressure to give an orange solid with a

color more intense than the first solid. This solid was

washed with cold and deaerated water/ethanol (1:1 mix-

ture), and finally washed with cold and deaerated ethanoland vacuum dried. Average yield: 83%. Elemental Anal.

Calc. for RuC8H21N7B2F8O: C, 18.99; N, 19.38; H,

4.18. Found: C, 18.55; N, 18.95; H, 4.10%. IR: mmax/

cm�1 (nujol) = 2230 (mCN free); 2180 (mCN coordinated).1H NMR: dH/ppm (dmso-d6) = 8.01 (d); 7.76 (t); 7.68

(t); 7.43 (d).

2.2.3. Ru(NH3)5(1,4-dcb)](BF4)2 Æ 2H2O

This compound was synthesized by the same proce-

dure used for the 1,2-dcb complex, but using 200 mg

(1.54 mmol) of 1,4-dicyanobenzene. Average yield: 65–

75%. Elemental Anal. Calc. for RuC8H23N7B2F8O2: C,

18.34; N, 18.71; H, 4.42. C, 17.91; N, 18.55; H, 4.30%.

IR: mmax/cm�1 (nujol) = 2233 (mCN free); 2182 (mCN

coordinated).

2.2.4. Ru[(NH3)5(NHC(OH)-bz-4-CN)](BF4)350 mg (1.0 mmol) of [Ru(NH3)5(1,4-dcb)](BF4)2 Æ 2-

H2O was dissolved in 1 mol L�1 HCl solution and then

0.5 mL of 30% H2O2 was added. After 24 h, �10 mL of

acetone was added dropwise. The yellow solid formed

was collected by filtration and washed with ethanol

and vacuum dried. Average yield: 62%. Elemental anal-

ysis was consistent with RuC8H21N7OB3F12. Anal.Calc.: C, 16.21; N, 16.54; H, 3.57. Found: C, 15.73; N,

16.92; H, 3.84%. IR: mmax/cm�1 (nujol) = 2233 (mCN

E. Tfouni et al. / Inorganica Chimica Acta 358 (2005) 2909–2920 2911

free); 1630 (mC@O). This product was also prepared in

solution by oxidation with controlled potential electrol-

ysis of [Ru(NH3)5(1,4-dcb)]2+ (10�3 mol L�1 of com-

plex) in 0.1 mol L�1 HCl/KCl solution or 1 mol L�1

CH3CO2H=CH3CO2� (pH 4.65) at +500 mV versus

Ag/AgCl.

2.2.5. Ru(NH3)5(3-imino-1-oxo-isoindoline-exo-N)]

(BF4)3, [Ru(NH3)5(NH-(C)(HN-C(O)-2-bz))](BF4)3This compound was isolated after chemical oxidation

of [Ru(NH3)5(1,2-dcb)]2+. About 50 mg (1.0 mmol) of

[Ru(NH3)5(1,2-dcb)](BF4)2 Æ H2O was dissolved in 1

mol L�1 HCl solution and then 0.5 mL of 30% H2O2

was added. After 24 h, �10 mL of acetone was addeddropwise. The yellow solid formed was collected by fil-

tration and washed with ethanol and vacuum dried.

Average yield: 45%. Elemental Anal. Calc. for RuC8H21-

N7OB3F12: C, 16.21; N, 16.54; H, 3.57. Found: C, 15.97;

N, 16.35; H, 5.33%. IR: mmax/cm�1 (nujol) = 1630

(mC@O); 1610 (mC@N).

This product was also prepared in solution by oxida-

tion with controlled potential electrolysis of [Ru(NH3)5(1,2-dcb)]2+ (10�3 mol L�1 of complex) in 0.1 mol L�1

HCl/KCl solution or 1 mol L�1 CH3CO2H=CH3CO2�

(pH 4.65) at +550 mV versus Ag/AgCl.

2.3. Physical measurement

The electronic absorption spectra were recorded

using Hewlett Packard Models 8452 A and 8453 record-ing spectrophotometers using quartz cells. IR absorp-

tion spectra were recorded in 4000–400 cm�1 range

using a Bomem MB-102 spectrophotometer. Spectra

were taken in KBr discs.

Cyclic voltammetry (CV), differential pulse polarog-

raphy (dpp) and constant potential electrolysis measure-

ments were taken with PARC, Model 273, potentiostat/

galvanostats. All tests were carried out using a conven-tional three electrodes cell. Glassy carbon was used as

working electrode for cyclic voltammetry and differen-

tial pulse polarography, and platinum gauze for coul-

ometry. Ag/AgCl and platinum wire were used as

reference and auxiliary electrodes, respectively. Electro-

chemical data were obtained in aqueous solutions of 1

mol L�1 CH3CO2H=CH3CO2� (pH 4.65) or 0.10 mol

L�1 KCl/HCl. All solutions were deaerated by bubblinghigh purity argon and thermostated using a Haake,

Model FK, ultracryostat. In the CVs and dpps, no ano-

dic or cathodic peaks in the absence of complexes in the

potential range studied were observed. The reported Ef

values (±10 mV) are the arithmetic mean of Epa and Epc

values.

The spectroelectrochemical measurements were car-

ried out using a gold mini-grid working electrode, Ag/AgCl as reference electrode and platinum wire as an

auxiliary electrode and quartz cell with 0.030 cm optical

path. Successive spectra were recorded during the redox

processes of the complexes at 25 �C.

3. Results and discussion

3.1. [Ru(NH3)5(1,4-dcb)]2+ and [Ru(NH3)5(1,2-dcb)]

2+

complexes

The known pentaammineruthenium(II) complex with

1,4-dcb complex and the new complex with 1,2-dcb were

synthesized following the procedure described for other

pentaammine nitrile complexes. The electronic spectrum(Table 1) in aqueous solution of [Ru(NH3)5(1,4-dcb)]

2+

(1A) is similar to that previously reported [31,32], and

the spectrum of [Ru(NH3)5(1,2-dcb)]2+ (1A) is similar

to that of 1,4-dcb and similar to those of other pentaam-

mines [29]. The spectra are dominated by an absorption

band of high absorptivity in the visible range assigned to

a metal to ligand charge transfer (MLCT) transition and

UV bands assigned as internal ligand (IL) in analogy toanalogous nitrile complexes. Compared to the MLCT

band of [Ru(NH3)5(bzn)]2+ (bzn = benzonitrile) [32],

which appears at 376 nm, the MLCT band of

[Ru(NH3)5(1,2-dcb)]2+ is red shifted to 430 nm due to

the electron-withdrawing nature of the nitrile substitu-

ent in the aromatic ring. A further red shift to 442 nm

is observed for the MLCT band of [Ru(NH3)5(1,4-

dcb)]2+. Inspection of Table 1 shows that the energy ofthe MLCT band in the dicyanobenzene complexes de-

pends on the relative ring positions of the cyano groups,

as for cyanopyridine complexes [33,34] and could be ex-

plained likewise.

The redox potentials (Table 1), in aqueous solution of

[Ru(NH3)5(1,4-dcb)]2+ (1A) and [Ru(NH3)5(1,2-dcb)]

2+

(1B) are +360 mV versus Ag/AgCl (+582 mV versus

NHE) and +400 mV versus Ag/AgCl (�+600 mV versusNHE), respectively, and they are similar to those previ-

ously reported [27]. These potentials are assigned to the

Ru(III/II) processes and are larger than those of the

analogous benzonitrile complex (+485 mV versus

NHE) [35] for the presence of a second electron-with-

drawing group in the aromatic group. These reduction

potentials reflect the relative Ru(II) to Ru(III) affinity

ratios for the nitrile ligands, and, as for the MLCTbands energies, can be explained in terms of the elec-

tron-withdrawing ability of the groups and ring

positions.

3.2. Chemical and electrochemical reactivities of

[Ru(NH3)5(1,4-dcb)]2+ (1A) and [Ru(NH3)5(1,2-

dcb)]2+ (1B)

The CVs of [Ru(NH3)5(1,4-dcb)]2+ (1A) and

[Ru(NH3)5(1,2-dcb)]2+ (1B), from +0.6 to �1.1 V versus

Table 1

Electronic absorption data and reduction potentials of some pentaammineruthenium(II) amide or nitrile complexes and related species in aqueous

solutions

Complex or pentaammine ligand kmax/nm (log e) Ef/mV vs. NHE Ref.

MLCT IL

1,4-dcb 442 (4.19) 250 (4.45) 582a,b c

463 [31,32]

1,2-dcb 430 (3.90) 257 (4.19); 236 (4.09) 622a,b c

bzn 376 (3.93); 347 (3.84)(sh) 249 (4.21); 226 (4.17) 485d [32,35]

4-NCpy 425 (3.73) 253 (3.95) 592b [33]

424 (4.00) [36]

422 (4.05) 253 (4.22); 212 (4.09) [12]

2-NCpy 408 (3,97) 256 (4.18); 222 (3.94) 562e [12]

408 (3.82) 256 (4.18); 222 (3.94) [33]

3-NCpy 398 (3.98) 255 (4.29); 218 (4.05) 549e [12]

401 (3.75) 336(sh) (3.58), 255 (4.97), 219 (3.87) [33]

NHCO-4-py 475 (log e = 4.06)g 260 (3.57) [22]

NHCO-1-methyl-4-pyh 695 (3.84)g �130f [22]

NC-bz-4-NH2C(O) (8A)h 421 (3.90), 239 (4.31) 557b,e c

NC-bz-2-NH2C(O) (9B)h 408 (3.96) 240 (4.26) 547b,e c

NH2C(O)-bz-2-CN (4B)h 535 (3.52) 246 (4.26) ��178b,e d

N-(C)(HNC(O)-2-bz) (12B) 533 (3.99)b,c 225 (4.27) 297b,e,h d

trans-[RuCl(cyclam)(NH2CO-4pyH)]2+ h 550 (2.65), 357 (3.02)b 256 (2.84) �28h [19]

trans-[RuCl(cyclam)(OCNH2-4pyH)]2+ h �495 (2.82)sh, 357 (3.31), 312 (3.13)b 256 (3.63), 228 (3.66) �148h [19]

bzn, benzonitrile.a 0.1 mol L�1HCl, pH 1.b 1 mol L�1 CH3CO2H=CH3CO2

� solution, pH 4.65.c This work and Ef values vs. Ag/AgCl converted to NHE by adding +222 mV.d 0.1 mol L�1 p-toluenosulfonic acid/potassium p-toluenosulfonate.e 0.1 mol L�1 KCl.f In aqueous pH 11 0.01 mol L�1 phosphate buffer.g Data obtained from the reaction solution.h 0.1 mol L�1 HCl/KCl.

2912 E. Tfouni et al. / Inorganica Chimica Acta 358 (2005) 2909–2920

Ag/AgCl, in deaerated aqueous solution, in 1 mol L�1

CH3CO2H=CH3CO2� at pH 4,65, and in 0.1 mol L�1

HCl/KCl at pH 1, show one-pH independent pair of

peaks with Ef = +360 and +400 mV versus Ag/AgCl

for Ru3+/2+ in the 1,4-dcb and 1,2-dcb complexes,

respectively (Eq. (1)).

½RuðNH3Þ5ðNCRÞ�nþ ��e�

þe�½RuðNH3Þ5ðNCRÞ�ðnþ1Þþ ð1Þ

The monoelectronic process assigned is further sup-

ported by controlled potential electrolyses of 1 · 10�3

mol L�1 aqueous solutions of [Ru(NH3)5(1,4-dcb)]2+

or [Ru(NH3)5(1,2-dcb)]2+, in the same above conditions,

at E = +550 mV versus Ag/AgCl, which show consump-

tion of 1 F mol�1 and orange to yellow color change

in both cases. However, oxidation of Ru(II) in

[Ru(NH3)5(1,2-dcb)]2+ or [Ru(NH3)5(1,4-dcb)]

2+, inaqueous media, is followed by hydrolysis of the coordi-

nated nitrile to the corresponding amide (Eq. (2)), with

the resulting amide complex coordinated through the

amide nitrogen, as for other nitriles with ruthenium am-

mines [6,9,36,37].

½RuðNH3Þ5ðNCRÞ�nþ

þH2O� ½RuðNH3Þ5ðNHCðOHÞRÞ�nþ ð2Þ

These hydrolysis reactions are slow enough as to have

the pairs of peaks of the 1,2-dcb and 1,4-dcb complexes

with a reversible profile according to the criteria of

Nicholson and Shain [38]. At scan rates smaller than

50 mV s�1, peaks dependent on the more positive anodic

peak start to appear at �380 mV (Epc) and �140 mV

(Epa), for the 1,4-dcb complex, as a result of the coordi-

nated nitrile hydrolysis reaction, to form the corre-sponding Ru(III) amide (Eq. (2)), resulting from the

oxidation of the nitrile complex (Eq. (1)). The cathodic

peak at �380 mV corresponds to the reduction of the

Ru(III) complex with the coordinated amide bonded

through the N atom (Eq. (3)). The anodic peak at

�140 mV is dependent on the �380 mV cathodic peak

and corresponds to the oxidation of [Ru(NH3)5(OH2)]2+

formed from the aquation reaction of the reduced amidecomplex (Eqs. (4) and (5)).

½RuðNH3Þ5ðNHCðOHÞRÞ�3þ

�þe�

�e�½RuðNH3Þ5ðNHCðOHÞRÞ�2þ ð3Þ

½RuðNH3Þ5ðNHCðOHÞRÞ�2þ

þH2O� ½RuðNH3Þ5ðOH2Þ�2þ þNH2COR ð4Þ

E. Tfouni et al. / Inorganica Chimica Acta 358 (2005) 2909–2920 2913

½RuðNH3Þ5ðOH2Þ�2þ ��e�

þe�½RuðNH3Þ5ðOH2Þ�3þ ð5Þ

This behavior is consistent with that reported for

other ruthenium ammine nitriles [5,9,10,37]. The rates

of hydrolysis of the coordinated nitriles are pH depend-

ent, and, accordingly, the cyclic voltammogram profile

is pH dependent. The peaks corresponding to the

formed amide are only seen in the CV at scan rates be-

low 50 mV s�1 for the 1,4-dcb, and not observed for the1,2-dcb complex, preventing a good estimate of the rates

by the method of Nicholson and Shain [38]. Actually,

the pH-dependent reactions undergone by the 1,2-dcb

complex upon oxidation are more complicated than

those of the 1,4-dcb complex (see ahead) and they

should be considered in the interpretations of its CVs.

The small rate of hydrolysis of the coordinated nitrile

allowed to have the UV–Vis spectrum of [Ru(NH3)5(1,4-dcb)]3+ (2A) recorded during electrochemical oxidation

of [Ru(NH3)5(1,4-dcb)]2+ at pH 4.65. This spectrum

shows decrease of the MLCT band at 442 nm and

appearance of a band at 279 nm (Table 2), which can

be assigned as ligand to metal charge transfer (LMCT)

in the formed [Ru(NH3)5(1,4-dcb)]3+ (2A). Oxidation

Table 2

Electronic absorption data of some pentaammineruthenium(III) amide or ni

Ligand kmax/nm (log e)

NHC(OH)-bz-4-NC (3A0) 388 (3.69), 320 (4.0),

388 (3.77), 320 (3.89b384 (3.19); 326 (3.7

NHC(O)-bz-2-CN (3B) 385 (3.03), 327(3.93)

385 (3.56), 315 (3.41

1,4-dcb 279b

NH-(C)(HN-C(O)-2-bz) (12B) 331 (3.89), 321 (3.89

516 (2.88), 376 (3.72

NHC(O)CH3 383 (3.54), 249 (3.36

NHC(OH)CH3 322 (3.19)e

NHCOC6H5 393 (3.61), 314 (3.56

NHC(OH)C6H5 385 (3.17), 320 (3.54

NHCO-4-py 386 (3.51), 306 (3.49

386i

384 (3.57), 262 (3.71

NHCO-4-pyH 358 (3.67), 264 (3.67

358 (3.48)d

NHCO-4-pyCH3 358 (3.70), 268 (3.67

350k

NHCHO 383 (3.58)l

a pH 1.b CH3CO2H/CH3CO2Na solution, pH 4.65.c pH 7.d 1.0 mol L�1 NaClO4.e 1 mol L�1 HClO4.f H2O, pH � 8 (NaHCO3).g 2 M HClO4.h In aqueous, pH 6.8, phosphate buffer.i 0.1 mol L�1 LiClO4.j In 0.1 mol L�1 triflic acid.k 0.1 mol L�1 CF3CO2H/CF3CO2Na.l In HCONH2 acidified with mol L�1 CF3SO3H (1 mol L�1).* This work.

of [Ru(NH3)5(1,4-dcb)]2+ with K2S2O8 also leads to

bleaching of the MLCT band. In both oxidations, the

coordinated nitrile in the 1,4-dcb Ru(III) complex

undergoes hydrolysis to amide forming [Ru(NH3)5(NH-

C(O)-bz-4-CN)]3+ (3A). The other nitrile group does not

undergo hydrolysis, as evidenced by the presence of thenitrile stretching band at 2230 cm�1 in the infrared spec-

tra of this latter complex (3A). The UV–Vis spectrum

(Table 2) of [Ru(NH3)5(NHC(O)-bz-4-CN)]3+ (3A) is

pH dependent, as those of other pentaammineruthe-

nium(III) amide complexes [20]. Amide bonded Ru(III)

ammine complexes display pH-dependent absorption

bands in the 300–400 nm range assigned as LMCT tran-

sitions [6,22,39,40], arising from different modes ofbonding of the amide in these complexes, including N

and O linkage isomers. In general, in acidic medium,

the substitution inert complexes with the amide N coor-

dinated to Ru(III) have absorptions lower in energy

than the corresponding substitution labile O linkage iso-

mers, which can undergo amide aquation. Accordingly,

the absorption bands of [Ru(NH3)5(NHC(O)-bz-4-

CN)]2+ in the 300–400 nm range can be assigned asLMCT as for other related amide complexes with the

trile complexes

Ref.

sh285; 234.a *

), 235 (4.38)b *

0); 236 (4.01)

, 319 (3.93)a *

), 278 (3.43), 227 (4.02)b *

*

), 295 (3.74)a *

), 325 (3.69), 275(sh) (3.72), 220 (4.46)c *

)d [6]

[6]

), 270 (3.58), 228 (3.97)f [6]

), 270 (3.42), 219 (3.97)g [6]

), 270 (3.57)h [22]

[36]

), 228 (3.61)b [9,12]

)j [22]

[36]

), 220 (3.86)h [22]

[37]

[39]

2914 E. Tfouni et al. / Inorganica Chimica Acta 358 (2005) 2909–2920

amide N-bonded to Ru(III), such as acetamide [6]. As

for other amide complexes, these absorptions bands

are shifted to higher energies at lower pHs as a result

of protonation of the coordinated amide. The pKa for

the protonated form of this complex (Eq. (6)),

[RuIII(NH3)5(NHC(OH)-bz-4-CN)]3+, is 1.8, a valuethat is close to those found (�2.5) for other nitrogen

bonded amide ruthenium complexes [20].

½RuðNH3Þ5ðNHCðOHÞ-bz-4-CNÞ�3þ �

½RuðNH3Þ5ðNHCðOÞ-bz-4CNÞ�2þ þHþ ð6Þ

The rate constant of hydrolysis (kobs) of the coordi-

nated nitrile of [Ru(NH3)5(1,4-dcb)]3+ (2A), generated

by chemical oxidation of [Ru(NH3)5(1,4-dcb)]2+ (1A),

was estimated as 2.9 · 10�3 s�1 at pH 4.65. This value

is consistently larger than the value of 2 · 10�4 s�1 re-

ported for this complex in the pH range 0–2 [27]. Thisvalue is also larger than that of the benzonitrile complex

(2 · 10�4 s�1) at pH 5 [12] because of the presence of the

electron-withdrawing nitrile group in position 4 of the

ring, turning the coordinated nitrile more prone to

nucleophilic attack. This is further illustrated by the lar-

ger values with 1-R-4-cyanopyridinium complexes, as

for instance, 0.16 and 0.71 s�1, for 4-mcp and 4-bcp,

respectively [37], which come from the presence of thepositive charge introduced by the substituent in the 4

position.

N(NH3)5Ru

HII

e-

e

-

-

[Ru(NH3)5(OH2)]2+

+

NH2O

CN IINC(NH3)5Ru

O

-

-

e+

e

+

IIC(NH3)5Ru N

-

(1A)

(4A)

(5A)

(7A)

(8A)

NC

NC

kaq

H

O

Scheme 1

3.3. Reactions of [Ru(NH3)5(1,4-cyanobenzamido)]2+

(3A)

The CV of [Ru(NH3)5(NHC(O)-bz-4-CN)]2+ (3A)

formed by controlled potential electrolysis at +500 mV

versus Ag/AgCl of [Ru(NH3)5(1,4-dcb)]2+ (1A) and sub-

sequent hydrolysis of [Ru(NH3)5(1,4-dcb)]2+ (2A), in 0.1

mol L�1 KCl solution at pH 4.8, scanned at 100 mV s�1,

shows one cathodic peak at �290 mV assigned to the

reduction of Ru(III) in [Ru(NH3)5(NHC(O)-bz-4-

CN)]2+. Dependent on this cathodic peak, one anodic

peak at �110 mV, attributed to the oxidation of Ru(II)

in [Ru(NH3)5(OH2)]2+, and a pair of peaks at +370 mV

(anodic) and +300 mV (cathodic), with Ef = +335 mVversus Ag/AgCl, assigned to [Ru(NH3)5(NC-bz-4-

NH2C(O))]2+ (8A) appear.

Reduction of [Ru(NH3)5(NHC(O)-bz-4-CN)]2+ in the

presence of 10-fold excess of 4-acetylpyridine (4-acpy) is

followed by two reactions (Scheme 1). One is aquation

(�45%), resulting in free 1,4-cyanobenzamide and

[Ru(NH3)5(OH2)]2+, which reacts with acpy forming

[Ru(NH3)5(4-acpy)]2+, the amount of which was calcu-

lated by the electronic absorption band at 526 nm [41].

The other reaction is a N-amide to N-nitrile linkage

isomerization reaction (�55%), to give [Ru(NH3)5(NC-

bz-4-NH2C(O))]2+ (8A). These reactions are similar to

those that occur for [Ru(NH3)5(NHC(O)4-py)]2+, ob-

tained after hydrolysis of [(NH3)5Ru(4-NCpy)]3+, and

could be explained likewise. For [Ru(NH3)5(NHC(O)4-

HN

O

(NH3)5Ru

II(NH3)5Ru

++

CO NH2

CNH2

NC(NH3)5Ru

(2A)

(3A)

(6A)

CN

NC

kis

H2O

III

III

C N

H

.

400 600 800-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

F

IAbs

orba

nce

wavelenght (nm)

Fig. 1. Successive spectra during oxidation of [Ru(NH3)5(1,2-dcb)]2+

by controlled potential electrolysis at +550 mV (vs. Ag/AgCl) in

CH3CO2H=CH3CO2� solution, pH 4.65, l = 1 mol L�1 at 25 �C.

[Ru(II)] = 2.93 · 10�3 mol L�1. I, initial spectrum; F, final spectrum.

E. Tfouni et al. / Inorganica Chimica Acta 358 (2005) 2909–2920 2915

py)]2+, reduction to RuII[(NH3)5(NHC(O)-4-py)]+ is fol-

lowed by an amido to pyridinyl linkage isomerization

competitive with aquation, forming [Ru(NH3)5(isn)]2+,

[Ru(NH3)5(OH2)]2+ and free isonicotinamide (isn)

[9,12,22,23]. The linkage isomerization occurs through

a series of stepwise g2-bonded (NH3)5Ru(II) isonicotin-amide complexes [22,23] and presumably, the same type

of mechanism should be occurring for the isomerization

with 1,4-cyanobenzamide. The UV–Vis spectrum of the

nitrile linkage isomer [Ru(NH3)5(NC-bz-4-NH2C(O))]2+

(8A) displays a band at 421 nm (log e = 3.90) and 239 nm

(log e = 4.31) (Table 1), assigned as MLCT and IL,

respectively. Compared with that of [Ru(NH3)5(1,4-

dcb)]2+, the MLCT band is blue shifted, since the amidegroup is less electron-withdrawing than the nitrile

group.

3.4. Reactions of [Ru(NH3)5(1,2-dcb)]2+ after

oxidation

The reactions undergone by [Ru(NH3)5(1,2-dcb)]2+,

after oxidation, are largely pH dependent. As aforesaid,

the CVs of this complex show a reversible pair of peaks

with Ef = +400 mV versus Ag/AgCl in the pH range 1–

5.6 corresponding to the [Ru(NH3)5(1,2-dcb)]3+/2+ proc-

ess. However, unlike [RuII(NH3)5(1,4-dcb)]2+, for which,

at scan rates smaller than 50 mV s�1, peaks dependent

on the more positive anodic peak start to appear, as a

result of the coordinated nitrile hydrolysis reaction, to

form the corresponding Ru(III) amide, the CVs only

show one pair of peaks. On one hand, at first, it could

be imagined that this behavior could be resulting from

a specific rate constant of hydrolysis in the 1,2-dcb com-

plex smaller than that of 1,4-dcb. On the other hand, theCV profile is different from those previously reported in

the pH 0–2 range [27], suggesting that the reactions are

different. As a matter of fact, the difference is arising

from the reactions involved and the different experimen-

tal conditions, as will be seen. In the reported CVs [27],

the 1,2-dcb complex is already in the oxidized form and

the outcome should be resulting from the elapsed time

from dissolution and CV scan, during which time reac-tion products already had been formed. The estimated

rates of reaction of the oxidized 1,2-dcb complex in

the pH 0–2 range were reported as 6 · 10�4 s�1, which

are larger than those of the 1,4-dcb complex, and would

explain the reason for the appearance of more than one

pair of peaks since the 1,2-dcb complex screened was al-

ready in the oxidized form [27]. However, the differential

pulse polarogram (dpp) of [Ru(NH3)5(1,2-dcb)]2+ at pH

5.6 shows one cathodic peak of relatively low intensity

at negative potential assigned to the reduction of

[Ru(NH3)5(NHC(O)-bz-2-CN)]3+ (3A).

The UV–Vis spectra, recorded during electrochemical

oxidation of [Ru(NH3)5(1,2-dcb)]2+, at pH 4.65, show

decrease of the MLCT and IL bands at 430 and 257

nm, respectively, and the appearance of absorptionbands at 385 nm (log e = 3.56), 315 nm (log e = 3.41),

278 nm (log e = 3.43) and 227 nm (loe = 4.02) (see Fig.

1).

As for [Ru(NH3)5(NHC(O)-bz-4-CN)]2+ (3A), the

pH-dependent absorption bands (Table 2) in the 300–

400 nm range can be assigned to the amido nitrogen

bonded [Ru(NH3)5(NHC(O)-bz-2-CN)]2+ (3B) complex.

The pKa of the protonated form, [Ru(NH3)5(NHC(OH)-bz-2-CN)]3+ (3B 0), is 1.5, close to those of other analo-

gous complexes [20,22].

The rate constant value (kobs) for the hydrolysis reac-

tion of the coordinated nitrile of the chemically oxidized

[Ru(NH3)5(1,2-dcb)]3+ (2B) is 5.6 · 10�3 s�1 at pH 4.65

as estimated by spectral monitoring of the formation of

[Ru(NH3)5(NHC(O)-bz-2-CN)]2+. This value is nearly

twice that of the 1,4-dcb complex and consistently oneorder of magnitude larger than those reported for the

pH 0–2 range (2 · 10�4 and 6 · 10�4 s�1) for 1,4-dcb

and 1,2-dcb, respectively [27]. However, it should be

noted that subsequent reactions occur at these lower

pHs.

3.5. Reactions of [Ru(NH3)5(1,2-cyanobenzamido)]2+

(3B) and [Ru(NH3)5(1,2-cyanobenzamide)] (4B) at

pH 4.65

The CV scanned after electrochemical oxidation

(electrolysis) at +550 mV versus Ag/AgCl of

[Ru(NH3)5(1,2-dcb)]2+ (1B) to [Ru(NH3)5(1,2-dcb)]

3+

(2B) and after the hydrolysis of the coordinated nitrile

forming [Ru(NH3)5(NHC(O)-bz-2-CN)]2+ (3B), in

CH3CO2H=CH3CO2� at pH 4.65, shows one cathodic

peak (1c) at �410 mV versus Ag/AgCl, which corre-

sponds to the reduction of [Ru(NH3)5(NHC(O)-bz-2-

CN)]2+ (3B). One anodic peak at �130 mV (2a), with

2916 E. Tfouni et al. / Inorganica Chimica Acta 358 (2005) 2909–2920

a barely seen corresponding cathodic peak, and a pair of

peaks at +360 mV (4a) and +290 mV (4c) versus Ag/

AgCl (Ef = +325 mV versus Ag/AgCl) dependent on 1c

also appear. Repetitive cycles (Fig. 2) result in CVs dis-

playing more peaks. There are cathodic peaks at +40

mV (3c) and at �190 mV (2c), more pronounced inthe second scan and one anodic peak (3a) at +110

mV, with decrease of the 1c peak current intensity.

The pairs of peaks 4a/4c (Ef = +325 mV, DEp = 70

mV) and 2a/2c (Ef = �160 mV, DEp = 60 mV) are

dependent on peak 1c. The pair of peaks 2a/2c can be

assigned to [Ru(NH3)5(OH2)]3+/2+ [35,37] and it indi-

cates that dissociation of coordinated amide in Ru(II)

complex is taking place (Scheme 2). Peaks 4a/4c are as-signed to [Ru(NH3)5(NC-bz-2-NH2C(O))]3+/2+ (9A).

The pair of peaks 3a/3c (Ef = +75 mV; DEp = 70 mV)

is dependent on 4a and is assigned to [Ru(NH3)5(NH-

(C)(HN-C(O)-2-bz))](BF4)3 (12A) (see ahead).

Controlled potential electrolysis of [Ru(NH3)5-

(NHC(O)-bz-2-CN)]2+ (3B) (1 mol L�1 CH3CO2H=CH3CO2

�, pH 4.65) at �500 mV versus Ag/AgCl, with

simultaneous UV–Vis spectrophotometrical monitoring,showed that upon reduction of 3B, the absorption bands

of 3B are bleached and absorption bands at 408 nm

(log e = 3.96) and 246 nm (log e = 4.26) (Table 1) are

developed. The consumption of ca 1 F mol�1 suggests

that monoelectronic charge-transfer occurred. Chemical

reduction of 3B with Zn(Hg) develops a transient purple

color which turns orange. The UV–Vis spectra show

first bands at 535 and 408 nm. The band intensity at535 nm decreases with increase of the intensity of the

408 nm band. As for other similar complexes, the band

at 535 nm (Table 1) can be assigned to a MLCT transi-

tion in the intermediate [Ru(NH3)5(NH2C(O)-bz-2-

CN)]2+ (4B) complex. Conceivably, the final product

could be a cis-tetraammine, since it can be formed by

-600 -400 -200 0 200 400 600-25

-20

-15

-10

-5

0

5

10

2c 3c 4c

4a

3a2a

1c

I (µ

A)

E (mV vs Ag / AgCl)

Fig. 2. Repetitive cyclic voltammogram of 1.29 · 10�3 mol L�1

[Ru(NH3)5(NHC(O)-bz-2-CN)]2+ in CH3CO2H=CH3CO2� aqueous

solution, pH 4.65, m = 100 mV s�1.

reduction of pentaammineruthenium(II) with substitu-

tion of labile ligands, in the presence of ortho chelating

ligands [25,42,43]. For instance, oxidation of the closely

related nitrile-bonded [Ru(NH3)5(2-NCpy)]2+ in aque-

ous solution is followed by hydrolysis to the corre-

sponding amide complex [Ru(NH3)5(NHC(O)2-py)]2+

[9,25]. Reduction of this Ru(III)-amide complex results

in addition to aquation (30%), in a chelation reaction

(70%), to give cis-[(NH3)4RuII(2-pca)] (2-pca = pico-

linamide), with displacement of one coordinated cis-

ammonia [9,25]. Thus, it is reasonable to expect similar

reactions after reduction of [Ru(NH3)5(NHC(O)-bz-

2-CN)]2+ (3B) to [Ru(NH3)5(NH2C(O)-bz-2-CN)](4B).

However, the spectra are not consistent with a cis-tetraammine species [25,42–46] and, thus, by anal-

ogy to [Ru(NH3)5(NHC(O)-bz-4-CN)]2+ (3B) and

[Ru(NH3)5(NHC(O)-4-py)]2+ [9,22,23], reduction of 3B

to 4B should be followed by amide aquation, forming

free 1,2-cyanobenzamide (6B) and [Ru(NH3)5(OH2)]2+

(5B), and by amide-N to nitrile-N bonded linkage

isomerization reaction, forming Ru(NH3)5(NC-bz-

2-NH2C(O))]2+ (9B) (Scheme 1). The UV–Vis spectraand CV results support this assumption. The band at

408 nm (Table 1) can be assigned to a Ru(II)–NC

MLCT transition in [Ru(NH3)5(NC-bz-2-NH2C(O))]2+

(9B). Peaks 4a/4c (Ef = +325 V versus Ag/AgCl) (Figs.

2 and 3), are consistent with [Ru(NH3)5(NC-bz-

2-NH2C(O))]3+/2+ (10B/9B) and peaks 2a/2c with

[Ru(NH3)5(OH2)]3+/2+.

Upon reduction of [Ru(NH3)5(NHC(O)-bz-2-CN)]2+

(3B), the [Ru(NH3)5(NC-bz-2-NH2C(O))]2+ (9B) com-

plex is formed in 53% and the other 47% corresponds

to aquation (5B), as calculated spectrophotometrically

by the spectra of [Ru(NH3)5(4-acpy)]2+ and [Ru(N-

H3)5(isn)]2+, formed in the presence of 10-fold excess

of 4-acetylpyridine or isonicotinamide, from reaction

of the aqua species, [Ru(NH3)5(OH2)]2+, resulting from

the reduction of 3B. The CVs of [Ru(NH3)5(NHC(O)-bz-2-CN)]2+ (3B) during electrolysis at �500 mV (reduc-

tion) are consistent with the reactions outlined and

resemble those of Fig. 2. They show the pairs of peaks

2a/2c ([Ru(NH3)5(OH2)]3+/2+), 4a/4c ([Ru(NH3)5(NC-

bz-2-NH2C(O))]3+/2+), and, in the beginning, the catho-

dic peak 1c, with the corresponding anodic peak at �370

mV for Ru3+/2+ in [Ru(NH3)5(NHC(O)-bz-2-CN)]3+/2+.

Again, dependent on 4a, appears the pair of peaks 3a/3c. The current peaks 2a/2c [Ru(NH3)5(OH2)]

2+/3+ de-

crease with exhaustive electrolysis probably due to

some recombination of the aqua species with the free

1,2-cyanobenzamide to form [Ru(NH3)5(NC-bz-2-

NH2C(O))]2+ to which the pair of peaks 4a/4c are as-

signed, and contributing to increase its current intensity.

At the end, only peaks 4a/4 and 3a/3c are displayed,

with peaks 2a/2c barely seen.However, the I–E profiles for [Ru(NH3)5(NC-bz-2-

NH2C(O))]2+ ion (9B), obtained in solution from reduc-

-

+e-

-e-

H2O

II

13B12B O11B

OO

10B

8B 9B

5B

4B3B

II

2B1B

+ [(NH3)5Ru(OH2)]

-e-

+e-

-e-

+e-

N(NH3)5Ru

HIII

H2O

C

CN(NH3)5RuIIIII

(NH3)5Ru N

H

C

N (NH3)5Ru

C

C

C

NC(NH3)5Ru

ONH2

C

CN

NH2

O

6B7B

OHN

H

(NH3)5Ru

NC

II

II

C

NC(NH3)5Ru

ONH2

III

-e-

+e-

H

N (NH3)5Ru

HIII

CO

N

III

NH

N

O

(NH3)5Ru

H

+ H+

O

H

CONH2

II(NH3)5Ru

CN

(NH3)5Ru

NH

NH

C

II

C

HN

Ru(NH3)5

CNH2

III

O

-

+ H++ OH

-

CC

III

C

H N

HN

Ru(NH3)5

O

N

NN

N

Scheme 2.

E. Tfouni et al. / Inorganica Chimica Acta 358 (2005) 2909–2920 2917

tion of [Ru(NH3)5(NHC(O)-bz-2-CN)]2+ (3B), at pH

4.65 and at scan rates of 100 mV s�1 (Fig. 3) and 500

mV s�1, show, in addition to 4a/4c, a pair of peaks

(3a/3c) with Ef = +75 mV versus Ag/AgCl.

This pair of peaks (3a/3c) is dependent on the oxida-

tion of [Ru(NH3)5(NC-bz-2-NH2C(O))]2+ (9B) (4a) and

is shown also in the I–E profiles of [Ru(NH3)5(NH2-

C(O)-bz-2-CN)]2+ (4B) (at pHs 1 and 4.65) and[Ru(NH3)5(NHC(O)-bz-2-CN)]2+ (3B) (pH 1) ions,

where these complexes result in [Ru(NH3)5(NC-bz-2-

NH2C(O))]2+ (9B) and [Ru(NH3)5(NC-bz-2-

NH2C(O))]3+ (10B). The potential of the pair 3a/3c

agrees with that previously reported, assigned by infer-

ence to [Ru(NH3)5(3-imino-1-oxo-isoindoline-exo-

N)]3+/2+ (12B/13B). Thus, it is reasonable to conclude

that oxidation of 9B to 10B is followed by a reaction

with a rate constant higher than the rate constant ofhydrolysis of [Ru(NH3)5(1,2-dcb)]

3+. This result is not

-600 -400 -200 0 200 400 600-10

-5

0

5

10

15

3c

3a

4c

4a

I

E (mV vs Ag / AgCl)

( µA

)

Fig. 3. Repetitive cyclic voltammograms after chemical reduction of

1.29 · 10�3 mol L�1 [Ru(NH3)5(NHC(O)-bz-2-CN)]2+ in

CH3CO2H=CH3CO2� aqueous solution, pH 4.65, m = 100 mVs�1.

-400 -200 0 200 400-35

-30

-25

-20

-15

-10

-5

0

5

10

15

20

25

30

3c

3a

I (µ

A)

E (mV vs Ag / AgCl)

Fig. 4. Cyclic voltammograms after electrochemical oxidation of

[(NH3)5Ru(NC-bz-2-NH2C(O))]2+ at +500 mV, at pH 4.65.

[(NH3)5Ru(NC-bz-2-NH2C(O))]2+ was obtained in solution by electr-

ochemical oxidation of 1.0 · 10�3 mol L�1 [(NH3)5Ru(1,2-dcb)]2+, in

CH3CO2H=CH3CO2� solution, followed by reduction.

2918 E. Tfouni et al. / Inorganica Chimica Acta 358 (2005) 2909–2920

unexpected, since the rate of hydrolysis of the coordi-

nated nitrile in [Ru(NH3)5(NC-bz-2-NH2C(O))]3+

(10B) should be smaller than for [Ru(NH3)5(1,2-

dcb)]3+ (1B) because of the decrease of r electron-

withdrawing properties of amide compared to thatof the remote nitrile [47]. Thus, formation of

[Ru(NH3)5(NHC(O)-bz-2-NH2C(O))]3+ (11B) would oc-

cur in minor extension compared to [Ru(NH3)5(3-imino-

1-oxo-isoindoline-exo-N)]3+ (12B).

The cyclic voltammogram (Fig. 4) scanned after elec-

trolysis of [Ru(NH3)5(NC-bz-2-NH2C(O))]2+ (9B) at

+500 mV at pH 4.65, with scan rates of 25–200 mV

s�1, showed a behavior different from another penta-ammine amido complexes [9,19,21–23,25,37]. There is

only one pair of peaks, 3a/3c, with Ef = �75 mV ver-

sus Ag/AgCl (DE = 70 mV), assigned to [Ru(NH3)5-

(NH-(HN-C(O)-2-bz))]3+/2+ (12B–13B). Oxidation of

[Ru(NH3)5(NC-bz-2-NH2C(O))]2+ (9B) to Ru[(NH3)5-

(NC-bz-2-NH2C(O))]3+ (10B) is followed by an

intramolecular cyclization of the ligand, forming

[Ru(NH3)5(NH-(HN-C(O)-2-bz))]3+ (11B), with possibleminor hydrolysis of the coordinated nitrile to amide in

[Ru(NH3)5(NC-bz-2-NH2C(O))]3+ (10B), since this rate

should be much smaller than in [Ru(NH3)5(1,2-dcb)]3+

(2B).

Controlled potential electrolysis of [Ru(NH3)5(NC-

bz-2-NH2C(O))]2+ (9B), obtained in 1 mol L�1

CH3CO2H=CH3CO2� solution from reduction of 3B,

at pH 4.65 and at +500 mV, with simultaneous UV–Vis spectrophotometrical monitoring, showed that upon

oxidation of 9B, the absorption bands at 408 and 246

nm (Table 1) are bleached and showed absorption bands

(Table 2) at 516 nm (log e = 2.88), 376 nm (log e = 3.72),

325 nm (log e = 3.69), 275(sh) (log e = 3.72), and 220 nm

(log e = 4.46). These bands are pH dependent (Table 2),

and at pH 1 they appear at 331 nm (log e = 3.89), 321 nm

(log e = 3.89), and 295 nm (log e = 3.74). This pH

dependence allowed the calculation of a pKa of 4.3.

Consistently, the spectra after oxidation match those

of [Ru(NH3)5(NH-(C)(HN-C(O)-2-bz))]3+ (12B), which

result from an intramolecular cyclization of the coordi-

nated 2-cyanobenzamide of [Ru(NH3)5(NC-bz-2-

NH2C(O))]3+ (10B).

The reaction of nitrile bonded pentaammin(2-cyano-benzamide)cobalt(III), in aqueous base, had been stud-

ied [48,49] and it was recently confirmed that the data

are more consistent with an intramolecular cyclization

of the coordinated 2-cyanobenzamide, resulting in a

complex of 3-imino-1-oxo-isoindoline bonded through

the exocyclic nitrogen and deprotonated at the endocy-

clic nitrogen [26]. The same reaction had been inferred

for ruthenium in acidic pH [27]. Thus, in analogy to pen-taammine(2-cyanobenzamide)cobalt(III), in addition to

the described reactions, another reaction has occurred

for the ruthenium complexes, as previously inferred by

Schaffer and Taube [27], presumably an intramolecular

cyclization of the coordinated 2-cyanobenzamide to 3-

imino-1-oxo-isoindoline-exo-N, resulting in [Ru(NH3)5-

(NH-(C)(HN-C(O)-2-bz))]3+ (12B), for which the pair of

peaks 3a/3c in the CVs can be assigned.

3.6. Reactions of [Ru(NH3)5(1,2-cyanobenzamido)]2+

(3B) and [Ru(NH3)5(1,2-cyanobenzamide)] (4B) at

pH 1

Oxidation of [Ru(NH3)5(1,2-dcb)]2+ (1B) at pH 1 re-

sults in an apparent different behavior. The CV scanned

right after controlled potential electrolysis at +550 mVversus Ag/AgCl to oxidize [Ru(NH3)5(1,2-dcb)]

2+ in

HCl/KCl 0.1 mol L�1 (pH 1) is similar to that at 4.65.

There are peaks attributed to [Ru(NH3)5(1,2-dcb)]3+/2+

(1B/2B) due to the slower rate of hydrolysis at this

E. Tfouni et al. / Inorganica Chimica Acta 358 (2005) 2909–2920 2919

pH; to reduction of [Ru(NH3)5(NHC(O)-bz-2-CN)]2+

(3B) to the aqua species [Ru(NH3)5(OH2)]3+/2+; to the

nitrile bonded cyanobenzamide [Ru(NH3)5(NC-bz-

NH2C(O))]3+/2+ (9B–10B) and to the cyclized ligand

complex [Ru(NH3)5(NH-(C)(HN-C(O)-2-bz))]3+/2+

(12B/13B). However, letting the reactions to proceedfor 1 h results in the CV profile similar to that of Fig.

4 and with an UV–Vis spectrum that corresponds to

[Ru(NH3)5(NH-(C)(HN-C(O)-2-bz))]3+ (12B).

As at pH 4.65 (Fig. 2), after oxidation of 1B, results

in [Ru(NH3)5(NHC(O)-bz-2-CN)]2+ (3B). However, at

pH 1, this complex (3B) (protonated at the amide

group, [Ru(NH3)5(NHC(OH)-bz-2-CN)]3+; pKa = 1.5)

rearranges to form the nitrile bonded isomer,[Ru(NH3)5(NC-bz-NH2C(O))]3+ (10B) (Scheme 2),

which in its turn undergoes a reaction to form

[Ru(NH3)5(NH-(C)(HN-C(O)-2-bz))]3+ (12B). The

roughly estimated rate constant values (kobs) for this last

reaction, from the decrease of current intensities of peak

4a as a function of time, are 1.7 · 10�2 s�1 at pH 2.5,

2 · 10�2 s�1 at pH 1.5, and 4.6 · 10�2 s�1 at pH 0.2.

These values indicate that the rate of the cyclizationreaction increases with increasing acidity of the medium,

while this trend is inverse for the analogous Co(III)

complex, for which these values are three order of mag-

nitude smaller, as expected [27]. The rate constant value

(kobs) of the linkage isomerization reaction (3B–10B)

was roughly estimated as 2 · 10�3 s�1 at pH 1.5, a value

that is smaller than that for cyclization, and should in-

crease with increased acidity. Thus, after isomerization,cyclization to 12B occurs relatively fast. At pH 4.65,

these rates are small enough to allow the identification

of [Ru(NH3)5(NHC(O)-bz-2-CN)]2+ (3B). That cycliza-

tion involves a rearrangement similar to that suggested

for the amido-N to nitrile-N bonded linkage isomeriza-

tion reaction in [Co(NH3)5(2-cyanobenzamido)]3+

[26,47] in acidic medium. In aqueous acid, the amido

bonded cobalt complex is protonated at the amidegroup (pKa = 0.4) and rearranges rapidly (kH = 0.3

s�1) to form the nitrile-bonded isomer and then the cyc-

lyzed ligand complex [50]. The reason for the smaller

rate (2 · 10�3 s�1 at pH 1.5) of this reaction for the

Ru complex is unclear. The disappearance of the pair

of peaks 4a/4c is consistent with the hydrolysis of the

oxidized 1,2-dcb complex, forming first the amide

bonded 1,2-cyanobenzamide (peak 1c), followed by theappearance of the pair of peaks at Ef = +325 mV from

the nitrile bonded 1,2-cyanobenzamide complex, result-

ing from the isomerization reaction of the amido bonded

complex, and then results in the final product (12B,

13B).

For [Ru(NH3)5(NHC(OH)-bz-2-CN)]2+ (3B), there

are two important possible reactions to be considered.

One reaction is the isomerization reaction to form[Ru(NH3)5(NC-bz-2-NH2C(O))]3+ (10B/12B) followed

by hydrolysis of the coordinated nitrile group, resulting

in N-bonded diamide complexes, [Ru(NH3)5(NHC(O)-

bz-2-NH2C(O))]2+ (11B). The rate of hydrolysis of the

coordinated nitrile is accelerated by the presence of elec-

tron-withdrawing substituents R on the nitrile (NC-R)

[26]. Compared to the dinitrile complex, the rate of

hydrolysis of coordinated nitrile in [Ru(NH3)5(NC-bz-2-NH2C(O))]3+ should be smaller, since the amide group

is less electron-withdrawing than the nitrile. For

[Ru(NH3)5(1,2-dcb)]3+ (1B), the rate constants (kobs) of

hydrolysis are 5.6 · 10�3 s�1 at pH 4.65 and 2 · 10�4

[27], at pH 0–2, so it is expected that the rate of hydrol-

ysis for [Ru(NH3)5(NC-bz-2-NH2C(O))]3+ (10B) should

be smaller.

Chemical reduction of [(NH3)5Ru(NH-(C)(HN-C(O)-2-bz))]3+ (12B) by Zn(Hg) resulted in a color

change from yellow to purple (13B). The absorptions

change from 516, 371, 325, 275, 220 nm for the oxidized

form at pH 1, to 533 (log e = 3.99), and 225 nm

(log e = 4.27) for the reduced form, which can be as-

signed to [(NH3)5Ru(NH-(C)(HN-C(O)-2-bz))]2+ (13B)

(during reduction a transient band at 285 nm devel-

oped). The reduced form is easily oxidized to regenerate12B in the presence of air. Both oxidized and reduced

forms have the same I–E profiles, with peaks at

Epa = +110 mV and Epc = +40 mV. If [Ru(NH3)5(NH-

C(O)-bz-2-NH2C(O))]2+ (11B) had been formed, it

would have rapidly been aquated upon reduction to re-

sult in [RuII(NH3)5(OH2)]2+ and free amide, as occurs

with (4B) and other amides with ruthenium(II) penta-

ammine. Reduction of 12B in the presence of pyridine(py), isonicotinamide (isn), pyrazine (pz), 2-acetylpyri-

dine (2-acpy), or bipyridine (bpy) resulted in only minor

amounts of [Ru(NH3)5(L)]2+ (L = py, izn, pz) or

[Ru(NH3)4(L)]2+ (L = 2-acpy, bpy). Also, the spectral

features are consistent with a pentaammine, ruling out

possible cis-tetraammines. However, although phtali-

mide could have been formed instead of the exo

bonded ligand (3-imino-1-oxo-isoindoline-exo-N) in[Ru(NH3)5(NH-(C)(HN-C(O)-2-bz))]3+ (12B), the pH

dependence of the spectrum is more consistent with this

latter species.

In an attempt to get structural evidence to further

support the assignments for the reaction products of

[Ru(NH3)5(1,2-dcb)]2+, 1H and 13C NMR spectra were

run in D2O and DMSO. The spectral analyses are com-

plicated by the large dependence on pH. The spectrashow that there are several products whose intensities

and chemical shifts change dramatically with pH. To

avoid the pH dependence, the spectra were run in

DMSO, and preliminary results show an apparent

inconsistency with those in D2O, which could come

from the different nature of the solvent. In addition to

a more complete study of the spectra in D2O, at different

pDs, and DMSO, a chemical and electrochemical studyin DMSO would be required to elucidate the reactivity

in both solvents, since they may vary with solvent.

2920 E. Tfouni et al. / Inorganica Chimica Acta 358 (2005) 2909–2920

4. Conclusions

The reactions undergone by [Ru(NH3)5(1,4-dcb)]2+

(1A) are basically similar to those of [Ru(NH3)5(1,4-

dcb)]2+, i.e., oxidation of this complex in aqueous solu-

tion is followed by nitrile hydrolysis to form the amidonitrogen bonded [Ru(NH3)5(NHC(O)-bz-4-CN)]2+ (3A).

This complex, upon reduction, undergoes two reactions.

One is amide aquation, forming [Ru(NH3)5(OH2)]2+ and

1,4-cyanobenzamide. The other reaction is a linkage

isomerization reaction, forming the nitrile bonded

[Ru(NH3)5(NC-bz-4-NH2C(O))]2+ (8A). Similar reac-

tions occur for [Ru(NH3)5(1,2-dcb)]2+ (1B). The rate

constants (kobs) of the nitrile hydrolysis are pH depend-ent, as for other coordinated nitriles, the higher the pH

the larger the rate. However, in addition to those reac-

tions, the results suggest that [Ru(NH3)5(1,2-dcb)]2+

(1B), after oxidation, forming [Ru(NH3)5(NHC(O)-bz-

2-CN)]2+ (3B), an amide to nitrile linkage isomerization

followed by an intramolecular cyclization reaction of the

coordinated 2-cyanobenzamide occur, resulting in a

complex of 3-imino-1-oxo-isoindoline bonded throughthe exocyclic nitrogen, [Ru(NH3)5(NH-(C)(HN-C(O)-

2-bz))]3+ (12B), as occurs for the similar cobalt complex.

The rates of these reactions are pH dependent and con-

trary to hydrolysis, the more acidic the medium, the lar-

ger the rate constant values. The reduced species

[Ru(NH3)5(NH-(C)(HN-C(O)-2-bz))]2+ (13B) is easily

oxidized and is relatively substitution inert. As for the

cobalt complex, these reactions are consistent withneighboring group participation and illustrate the rich-

ness involved in the chemistry of coordinated nitriles,

including ruthenium.

Acknowledgments

The authors thank FAPESP, CAPES, CNPq, and thePADCT program for grants and fellowships. We thank

Profs. Dr. Bruce McGarvey, Gil Valdo Jose da Silva and

Ms. Virginia Helena Betarello for help in NMR meas-

urements, and Prof. Dr. Angelica M. Lucchese for help-

ful discussions concerning phtalamide and phtalimide.

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