Coupling of cationic olefin complexes of platinum(II)with potential ambident nucleophiles

Carmen R. Barone a, Renzo Cini b, Sara de Pinto a, Nicola G. Di Masi a, Luciana Maresca a,*,Giovanni Natile a, Gabriella Tamasi b

a Dipartimento Farmaco-Chimico, Università degli Studi di Bari, Via E. Orabona 4, 70125 Bari, Italyb Dipartimento di Scienze e Tecnologie Chimiche e dei Biosistemi, Università degli Studi di Siena, Via A. Moro 2, 53100 Siena, Italy

a r t i c l e i n f o

Article history:Received 29 July 2009Accepted 21 August 2009Available online 4 September 2009

Keywords:Platinum complexesAnilinesPhenolX-ray crystal structureNMRDFT calculations

a b s t r a c t

The reactivity of [PtCl(g2-CH2@CHR)(tmeda)]+ (R = H, 1a, or Me, 1b; tmeda = N,N,N0,N0-tetramethyl-1,2-diaminoethane) towards some ambident nucleophiles like anilines and phenolate anion has been tested.The reaction of 1a with N-methylaniline gives immediately N-addition to the coordinated ethene (3a),but, in the presence of an inorganic carbonate, a partial rearrangement, with the para carbon of the phe-nyl ring taking the place of nitrogen, is observed (4a and 5a). Reaction with a tertiary aromatic amine,such as N,N-dimethylaniline, leads exclusively to the C-coupled species. The phenolate anion acts initiallyas an oxygen donor, however the resulting species (6a), in contact with free phenol, rearranges to C-bonded species (7a). For free phenol/6a ratios P 5 the rearranged product has an isomeric ortho/pararatio of �3. For lower free phenol/6a ratios (6 1) oligomeric complexes, in which two or three platinumethanide moieties are bound to the same phenol ring, are also formed. In the case of 1b, the abovedescribed reactivity has to compete with the base-induced deprotonation of propene, leading to forma-tion of the allyl-bridged platinum dimer [{PtCl(tmeda)}(l-g1:g3-CHCHCH2){Pt(tmeda)}]+. The X-raycrystal structure of 1b has also been determined; the structural parameters are very similar to those pre-viously reported for 1a. DFT calculations have shown a similar activation of the two complexes towardsnucleophilic addition at the coordinated olefin, although in 1b the electrophilic character of the olefin ismasked by the BrØnsted acidity of the propene methyl protons.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction

Cationic olefin complexes of late transition metals are a topic ofcurrent interest. The coordinated alkene generally becomes veryelectrophilic (and though very reactive towards nucleophiles)and, in addition, can exhibit acidic character or insert into an adja-cent metal–hydrogen or metal–carbon bond [1]. These complexes,apart from being proposed as the active species in catalytic cyclesleading to functionalization or polymerization of alkenes, are oftenisolable also in the pure state [2,3].

The reactivity of the cationic complexes depends upon severalfactors such as the nature of the metallic core and of the ancillaryligands and the overall complex charge. For instance, the electro-philic character of the unsaturated ligand increases sensibly byraising the electric charge of the complex from +1 to +2 [4].

Over the years we have isolated and characterized several mon-ocationic complexes of formula [PtCl(g2-CH2@CHR)(tmeda)]+, 1,

(R = H, alkyl, or aryl; tmeda = N,N,N0,N0-tetramethyl-1,2-diaminoe-thane) having highly electrophilic character [5].

Among them, the most active was that containing ethene, 1a(R = H), the olefin of which can add inorganic anions [5e,f], tertiaryaliphatic amines [5c,d], and a para ring carbon of N,N,N0,N0-tetra-methyl-1,8-diaminonaphthalene (widely known as Proton Sponge)[5j].

On the other hand, when R is other than H, the unsaturated li-gand behaves differently and can be displaced by inorganic anions[6] or deprotonated by tertiary amines [5n,o].

In the present study we have extended the investigation to aro-matic amines and phenolate anion, both types of nucleophiles areamphiphilic and can act as ethero atom or carbon donors [7–9].The reactions have been performed on 1a and on the closest ana-logue, the propene complex 1b.

To fully compare the olefin ground state in the two complexes(1a and 1b), the X-ray crystal structure of 1b has been determined(the crystal structure of 1a, obtained by both X-ray and neutrondiffraction analysis, was already known) [5b,g]. Moreover, sincethe reactivity of a metal coordinated olefin towards nucleophilesappears to be ruled by the energy difference between the LUMO

0020-1693/$ - see front matter � 2009 Elsevier B.V. All rights reserved.doi:10.1016/j.ica.2009.08.035

* Corresponding author. Tel.: +39 080 5442776; fax: +39 080 5442230.E-mail address: maresca@farmchim.uniba.it (L. Maresca).

Inorganica Chimica Acta 363 (2010) 205–212

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of the complex and the p* orbital of the free ligand [10], DFT calcu-lations for 1a and 1b have also been performed.

2. Experimental

2.1. General

Solvents and reagents, unless otherwise stated, were commer-cially available (purchased from Aldrich Chemical Company) andused as received. Chlorinated solvents were dried over activatedmolecular sieves (beads, 4–8 mesh). Elemental analyses were per-formed with a CHN Eurovector EA 3011. 1H, 13C and 195Pt NMRspectra were recorded with a 300 MHz Mercury Varian and aDPX 300 Avance Bruker instruments equipped with probes for in-verse detection and with z gradient for gradient-accelerated spec-troscopy. 1H and 13C NMR spectra were referenced to TMS; theresidual proton signal of the solvent was used as internal standard.1H/13C inversely detected gradient-sensitivity enhanced heterocor-related 2D NMR spectra for normal coupling (INVIEAGSSI) were ac-quired using standard Bruker automation programs and pulsesequences. Each block of data was preceded by eight dummy scans.The data were processed in the phase-sensitive mode. For 195PtNMR spectra, K2PtCl4 was used as external standard (-1643.00ppm). ESI-MS spectra were recorded with an Agilent 1100 SeriesLC-MSD Trap System VL. GC–MS spectra were recorded with anAgilent 6890N-5973N MSD.

2.2. Syntheses

2.2.1. [PtCl{g1-CH2CH2-N(CH3)C6H5}(tmeda)], (3a), and [PtCl{g1-CH2CH2-C6H4-p-NH(CH3)}(tmeda)], (4a)

[PtCl(g2-C2H4)(tmeda)](ClO4), 1a(ClO4) [5a] (237 mg, 0.5 mmol),was suspended in CH2Cl2 (5 mL) in the presence of K2CO3 (276 mg,2 mmol). N-methylaniline (163 lL, 1.5 mmol) was then added andthe mixture was stirred in the dark for 48 h at room temperature.The mother liquor was then filtered and evaporated in vacuo. The ob-tained oily product, after trituration with diethyl ether, gave a solidwhite powder, which was recovered by filtration and pumped in va-cuo. The solid was a mixture of 3a (major) and 4a (minor). Pure 3acould be obtained, as colorless needles, by crystallization of the reac-tion product from CH2Cl2/diethyl ether. Anal. Calc. for C15H28ClN3Pt:C, 37.46; H, 5.87; N, 8.74. Found: C, 37.62; H, 5.87; N, 8.58%. Peak ofgreatest intensity in ESI-MS (both for 3a and its mixture with 4a): m/z = 445.0 = [M�Cl]+. NMR (CDCl3, 294 K, ppm) for 3a: dH 1.55 (t, 2H,2JPt–H ca. 88 Hz, Pt–CH2CH2–N), 2.96 (s, 3H, N-methylaniline N(CH3)),2.4–3.0 (m, 16H, tmeda N(CH3) and N(CH2)), 3.42 (t, 2H, Pt–CH2CH2N), 6.55 (t, 1H, CHpara), 6.75 (d, 2H, 2 CHortho), 7.18 (t, 2H, 2CHmeta); dPt �3456. NMR (CDCl3, 294 K, ppm) for 4a: dH 1.55 (t, 2H,2JPt–H ca. 88 Hz, Pt–CH2CH2C), 2.59 (t, 2H, Pt–CH2CH2C); dPt �3493.

2.2.2. [(tmeda)ClPt{CH2CH2-N(CH3)-C6H4-p-CH2CH2}PtCl(tmeda)],(5a)

[PtCl(g2-C2H4)(tmeda)](ClO4), 1a(ClO4) (237 mg, 0.5 mmol),was suspended in CH2Cl2 (5 mL) in the presence of K2CO3

(276 mg, 2 mmol). A stoichiometric amount, in terms of platinum,of the reaction mixture 3a plus 4a (240 mg, 0.5 mmol) was thenadded and the mixture was stirred in the dark for 24 h at roomtemperature. After filtration, the mother liquor was evaporatedin vacuo and the oily residue triturated with diethyl ether anddried. The obtained white solid was the dinuclear species 5a. Anal.Calc. for C23H47Cl2N5Pt2: C, 32.32; H, 5.54; N, 8.19. Found: C, 32.04;H, 5.27; N, 8.33%. Peak of greatest mass in ESI-MS: m/z =817.8 = [M�Cl]+. NMR (CDCl3, 294 K, ppm): dH 1.55 (t, 4H, 2JPt–H

ca. 88 Hz, Pt–CH2CH2N and Pt–CH2CH2C), 2.60 (t, 2H, Pt-CH2CH2C),2.89 (s, 3H, N-methylaniline N(CH3)), 2.4–3.0 (m, 32H, tmeda

N(CH3) and N(CH2)), 3.37 (t, 2H, Pt–CH2CH2N), 6.68 (d, 2H, 2CHortho), 7.16 (d, 2H, 2 CHmeta); dPt �3461 and �3499.

2.2.3. [PtCl{g1-CH2CH2-C6H4-p-N(CH3)2}(tmeda)], (40a)The synthetic procedure was completely analogous to that re-

ported for 2.2.1, starting from [PtCl(g2-C2H4)(tmeda)](ClO4), 1a(ClO4)(237 mg, 0.5 mmol), K2CO3 (276 mg, 2 mmol), and N,N-dimethyl-aniline (190 lL, 1.5 mmol). The conversion of the starting complexwas 70% after 48 h. Anal. Calc. for C16H30ClN3Pt: C, 38.83; H, 6.11;N, 8.49. Found: C, 39.08; H, 6.02; N, 7.94%. Peak of greatest inten-sity in ESI-MS: m/z = 459.0 = [M�Cl]+. NMR (CDCl3, 294 K, ppm): dH

1.59 (t, 2H, 2JPt–H ca. 87 Hz, Pt–CH2CH2C), 2.51 (t, 2H, Pt–CH2CH2C),2.4–3.0 (m, 16H, tmeda N(CH3) and N(CH2)), 2.88 (s, 6H, N,N-dimethylaniline N(CH3)2), 6.68 (d, 2H, 2 CHortho), 7.20 (d, 2H, 2CHmeta).

2.2.4. Acid hydrolysis of amine addition productsIn a typical experiment, 0.2 mmol of 3a plus 4a, 5a, or 40a were

treated with 2 mL of concentrated aqueous HCl (37%). The mixturewas kept stirring at room temperature for several hours. Themother liquor, separated by filtration from the solid [PtCl2(tme-da)], and divided in two aliquots, was evaporated to dryness in avacuum desiccator (in the presence of NaOH and H2SO4, as dryingagents). The two dried samples were dissolved in basic water(KOH), then one sample was extracted with CDCl3 (for 1H NMR)and the other with Et2O (for GC–MS). The two organic phases werekept for a few hours over Na2SO4 and then analyzed.

2.2.5. [PtCl(g1-CH2CH2-OPh)(tmeda)], (6a)[PtCl(g2-C2H4)(tmeda)](ClO4), 1a(ClO4) (237 mg, 0.5 mmol),

was suspended in CH2Cl2 (5 mL). Sodium phenolate (58 mg, 0.5mmol) was then added and the mixture was stirred for a few min-utes at room temperature. The mother liquor was then filtered andevaporated in vacuo. The obtained oily product, after triturationwith diethyl ether, left a yellowish powder, which was recoveredby filtration and pumped in vacuo. Alternatively, 6a could be pre-pared from 1a(ClO4) (237 mg, 0.5 mmol) and phenol (94 mg,1 mmol), in the presence of K2CO3 (276 mg, 2 mmol). The mixturewas kept stirring for 24 h at room temperature, and the subsequentworking up was performed as previously described. The isolatedyield, referred to platinum, was ca. 95% in both cases. Anal. Calc.for C14H25ClN2OPt: C, 35.94; H, 5.39; N, 5.99. Found: C, 35.61;H, 5.36; N, 5.90%. Peak of greatest intensity in ESI-MS: m/z =490.8 = [M+Na]+. NMR (CDCl3, 294 K, ppm): dH 1.8 (t, 2H, 2JPt–H

ca. 104 Hz, Pt–CH2CH2O), 2.4–3.0 (m, 16H, tmeda N(CH3) and N(CH2)),4.1 (m, 2H, Pt–CH2CH2O), 6.87 (t, 1H, CHpara), 7.04 (d, 2H, 2 CHorto),7.24 (t, 2H, 2 CHmeta).

2.2.6. Rearrangement of 6a in the presence of added phenolIn a typical experiment 6a (117 mg, 0.25 mmol) was dissolved

in CH2Cl2 and mixed with a solution of phenol in the same solvent(phenol quantity ranging from 0.05 to 1.25 mmol). The chlorinatedsolvent was evaporated in vacuo, to leave a sticky solid, which wasleft standing at 278–298 K. After 1–4 days the sticky solid was trit-urated with diethyl ether to give a yellowish powder. The conver-sion of 6a into 7a was quantitative.

2.2.7. [PtCl(g1-CH2CH2-C6H4-OH)(tmeda)], (mononuclear 7a) (seeScheme 2)

The complex was prepared according to the general proceduregiven above, using a phenol/6a ratio close to 5; the reaction tem-perature was kept in the range 293–298 K and the reaction timewas ca. 24 h. Anal. Calc. for C14H25N2ClOPt: C, 35.94; H, 5.39; N,5.99. Found: C, 35.73; H, 5.33; N, 5.88%. Peak of greatest intensityin ESI-MS: m/z = 490.8 = [M+Na]+. NMR (CDCl3, 294 K, ppm): dH

1.58 (t, 2H, 2JPt–H ca. 91 Hz, Pt–CH2CH2C), 2.45 (m, 2H, Pt–

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CH2CH2C), 2.4–3.0 (m, 16H, tmeda N(CH3) and N(CH2)), 6.73 (t, 1H,CHpara), 6.88 (d, 1H, CHortho), 7.04 (m, 2H, 2 CHmeta).

2.2.8. Acid hydrolysis of phenolate addition productsThe acid hydrolysis was performed as previously described for

the amine addition products (see Section 2.2.4). In each experi-ment, the resulting acid solution was divided in two aliquots andextractions with CDCl3 (for 1H NMR) and with Et2O (for GC–MS)were performed. The two organic phases were kept for a few hoursover a mixture of Na2CO3 and Na2SO4 and then analyzed.

2.3. X-ray crystallography

Data collection for 1b(ClO4) was performed through a SiemensP4 four circle diffractometer, located at CIADS (Centro per Analisie Determinazioni Strutturali, University of Siena, Italy) at roomtemperature (293 ± 2 K) (Table S1). Cell constant determinationwas performed through a least-squares analysis from the valuesof 40 high intensity randomly selected reflections in the range10 6 2h 6 41�. The original data sets (2113 reflections, Rint =0.0375, R(sigma) = 0.0217) were corrected for Lorentz-polariza-tion effects through XSCANS computer package [11]. The absorptioneffects were corrected through the w-scan techniques by usingthe XEMP computer software [12].

The structure was solved through direct methods implementedin the SHELXS-97 software [13a,b] and refined via SHELXL-97 program[13b,c], both implemented under WINGX package [14]. All the atomsof the coordination shell and most of the other non-hydrogenatoms were located from the structure solution, whereas theremaining atoms were located from subsequent cycles of Fourier-difference synthesis and least-squares calculations. The hydrogenatoms were located via the HFIX options of SHELXL-97 [13c] and leftriding on the atoms to which they are attached. The non-hydrogenatoms were refined as anisotropic, and the hydrogen atoms weretreated as isotropic with thermal factors equal to 1.2 or 1.5 timesthose of the corresponding heavier atoms. Twenty-four cycles ofleast-squares calculations gave a nice convergence for the model.The conventional R1 and wR2 factors converged to 0.0387 and0.0957. The analysis of molecular geometry parameters was per-

formed through PARST-97 [15] and the molecular graphics wascarried out through ORTEP-32 [16]. All the calculations were per-formed by using Pentium IV personal computers operating throughthe Windows-XP system. CCDC-665826 (1b) contains the Supple-mentary crystallographic data for this paper.

2.4. Computational details

The calculations of complexes 1a and 1b were performed in thegas phase by using GAUSSIAN-98/RevA.11.3 [17], implemented onIBM SP5 machines of CINECA (Consorzio Interuniversitario delNord-Est per i Calcoli Avanzati, Casalecchio di Reno, Bologna) atthe B3LYP level [18,19], using as basis set: Lanl2dz for Pt and 6-31G(d,p) for all other atoms (H, C, N, and Cl) [20]. The geometryoptimizations were performed without any symmetry constraint.Cartesian coordinates for computed structures 1a and 1b are re-ported in the Supporting information Section (Tables S2 and S3).

3. Results and discussion

3.1. Reactions with aromatic amines

The overall reaction pattern of 1a with N-methylaniline is de-picted in Scheme 1. The aminic function adds to the coordinatedethene to give the addition product 2a, which is the dominant spe-cies in the presence of excess amine (amine/Pt ratio P 3). Whenthe reaction is carried out in the presence of solid K2CO3, apartfrom deprotonation of 2a to give 3a, another species, 4a, is alsoformed. 4a, which amounts to ca. 20% of the reaction products inthe used experimental conditions (r.t. and 48 h), has elementalcomposition and ESI-MS molecular peak coincident with that of3a. Its NMR spectra, however, unambigously showed how the pla-timum–ethanide moiety was bound to the para carbon of the phe-nyl ring.

3a and 4a have almost coincident signals for CaH2, but distinctlydifferent ones for CbH2 (at 3.42 and 2.59 ppm for 3a and 4a, respec-tively), reflecting the different nature of the nucleophile donoratom, the aminic nitrogen in the former case and a ring carbonin the latter one (Fig. S1). Moreover, the aromatic proton reso-nances were in accordance with a monosubstituted phenyl ring

+

NHMe

Pt

Cl

N

N

+

Pt

Cl

N

N

NMeH

+

Pt

Cl

N

N

NHMe

Pt

Cl

N

N

NMe

Pt

Cl

N

N

NMe

Pt

Cl

N

N

a4a1

3a

5a

2a

K2CO3

K2CO3

1a, K2CO3

Scheme 1. Reaction pattern of complex [PtCl(g2-CH2@CH2)(tmeda)]+, 1a, with N-methylaniline.

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866

in the case of the major component 3a and with a 1,4-disubstitutedphenyl ring in the case of the minor component 4a. The structureof 4a was also confirmed by analysis of the corresponding aciddemolition products (see later on in this section). Two 195Pt NMRsignals were present at �3456 and �3493 ppm, the higher fieldsignal was the less intense and therefore was assigned to 4a. Themixture of 3a and 4a was further reacted with an equimolaramount of complex 1a, always in the presence of solid K2CO3, togive quantitative formation of dinuclear 5a. 5a was characterizedvia elemental analysis, ESI-MS and NMR. In the 2D (1H, 1H) COSYspectrum (CDCl3, 300 MHz, 294 K) the N- and C-coupled Pt–CaH2CbH2– moieties gave rise to two spin systems, of equal inten-sity, similar to those formed in 3a and 4a, respectively (Fig. S2).Moreover, the aromatic proton signals are consistent with a 1,4-disubstituted phenyl ring. Two signals of equal intensity are pres-ent in the 195Pt NMR spectrum, their frequencies (at �3461 and�3499 ppm) are close to those of the mononuclear species 3aand 4a.

The mixture of 3a and 4a, as obtained from the reaction, washydrolyzed with concentrated aqueous HCl (37%). The GC–MSanalysis of the organic fraction showed a major peak at m/z =107, corresponding to N-methylaniline, and a minor one at m/z =135, corresponding to p-ethyl-N-methylaniline (the presence ofthe ethyl group on the para carbon of the aromatic ring was re-vealed by a 1H NMR spectrum). Hydrolysis of dinuclear 5a pro-duced essentially p-ethyl-N-methylaniline. Thus, in the usedexperimental conditions, the acid (HCl) cleaved the Pt–C r bondonly when the nucleophilic attack to the olefin was promoted bya carbon donor and reversed the reaction in the case of amine addi-tion. In all cases the platinum was recovered as [PtCl2(tmeda)]complex.

When the reaction was carried out with an amine/Pt ratio of 1:1and always in the presence of K2CO3, dinuclear species with twoplatinum ethanide residues bound to the phenyl ring of N-methy-laniline did form. The GC–MS analysis of the organic fraction, afterhydrolysis of the crude reaction products with concentrated HCl,showed an intense peak at m/z = 163, corresponding to a N-methy-laniline having two ethyl substituents on the aromatic ring. Afterthe link of the first Pt–CaH2CbH2– moiety, the phenyl ring is acti-vated towards further electrophilic attack.

In the case of N,N-dimethylaniline, the tertiary aminic nitrogendid not add to the coordinated ethene, but, in anhydrous condi-tions and in the presence of inorganic carbonate, a carbon additionproduct analogous to 4a, namely [PtCl(g1–CH2–CH2–C6H4–p-NMe2)(tmeda)] (40a), formed in nearly quantitative yield. 40a wascharacterized by elemental analysis, ESI-MS, and NMR. Hydrolysiswith concentrated aqueous HCl produced, almost exsclusively, p-ethyl-N-methylaniline (GC–MS and 1H NMR).

The reactivity of complex 1a towards N-methyl-p-anisidine wasalso tested; the N-addition product was the exclusive form andthere was no evidence of C-coupling taking place. The results justdescribed clearly indicate that is the carbon in para position ofthe aromatic ring the preferred site of attack by 1a.

Moving to the case of 1b, we found that neither N-methyl- norN,N-dimethylaniline were able to give C-coupled addition products,but the inorganic carbonate, present in the reaction scene, promotedolefin deprotonation with formation of the already reported dinucle-ar cation [PtCl(tmeda)(l-g1:g3-CHCHCH2)Pt(tmeda)]+ [5n].

As shown in the years by Buncel [7] and Terrier [8] the reactionwith an aniline is a well established procedure for testing thestrength of an electrophile. Aniline binds exclusively through theaminic function when reacted with electron-deficient organic mol-ecules, such as 1,3,5-trinitrobenzene [7] or 4-nitrobenzofuroxane(NBF) [21]. In contrast, the reaction with 4,6-dinitrobenzofuroxane(DNBF) and 4,6-dinitrobenzofurazane (DNBZ) leads, besides to theN-coupling, also to C-coupling with the para carbon of the aromaticamine [7] (Fig. S3).

The nucleophilic attack of an electron-rich phenyl ring carbononto an electron-deficient carbon (C7 of DNBF or DNBZ) producesthe so called Wheland intermediate [22], which can exhibit acidicproperties and give rise to spontaneous deprotonation of the phe-nyl ring carbon. For instance, products of coupling of DNBF andDNBZ with 1,3,5-trimethoxybenzene have been isolated by Terrieras stable salts of alkali metals [8a]. An aminic functionality presenton the electron-rich phenyl ring can capture the outgoing proton;this situation is featured by the behavior of Proton Sponge (pKa =12) towards both polynitroaromatics (DNBF and DNBZ) [8a] andcomplex 1a [5j]. The aromatic amines used in the present investi-gation have lower basicity (pKa of anilines is in the range 5–6),therefore coupling with 1a via a phenyl ring carbon can only be fa-voured by the presence of a stronger base like carbonate. In thiscontext we ought to mention a recent study by Vitagliano et al.,in which the nucleophilic attack of a phenyl carbon of 1,3-dime-thoxybenzene onto a dicationic platinum ethene complex (a stron-ger electrophile than our compounds 1a and 1b) generates aWheland intermediate with a remarkable acidic character. Thereleased proton can cleave the platinum–carbon bond, affordingdirectly the ethylated 1,3-dimethoxybenzene [23].

3.2. Reactions with phenolate anion

1a reacted rapidly and quantitatively with phenolate anion togive the O-addition product [PtCl(g1-CH2-CH2-OPh)(tmeda)], 6a.The same result was obtained reacting 1a with phenol in the pres-ence of solid K2CO3 (Scheme 2), but a longer reaction time was re-quired. The structure of 6a was unambiguously established on thebasis of the 1H NMR spectrum, which clearly showed the presenceof the Pt–CaH2CbH2–OPh moiety [the a and b methylenes resonateat 1.8 (2JPt–H = 104 Hz) and 4.1 ppm, respectively] and an aromaticportion consistent with a preserved monosubstituted phenyl ring(Fig. S4). 6a is rather moisture sensitive and, if not kept in anhy-drous conditions, it undergoes some hydrolysis with an OH� [5h]replacing the phenolate anion which is released as free phenol.

If 6a is kept in contact with phenol, it undergoes a rearrange-ment from O-bonded phenolate to C-bonded phenol. This transfor-mation occurs and it is even faster if performed in the solid phase

Pt

N

N

ClOH

O

Pt

N

N

ClK2CO3

Pt

N

N

Cl

1a a7a6

OH

n

n = 1, 2, 3

OH

+

+

Scheme 2. Reaction pattern of complex [PtCl(g2-CH2@CH2)(tmeda)]+, 1a, with phenolate anion.

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(see Section 2.2.6). We have carried on several experiments, usingdifferent amounts of phenol. Using a phenol/6a ratio P 5, the onlyobserved transformation was the complete isomerization of 6ainto mononuclear 7a (Scheme 2), which was characterized viaemental analysis, ESI-MS, and NMR. In the 1H NMR spectrum theb methylene resonates at 2.45 ppm as compared to 4.1 ppm in 6aand the pattern of aromatic protons is consistent with a dominant1,2-disubstituted phenyl ring (Fig. S5). The acid hydrolysis (carriedout in similar conditions to those used for the amine coupled prod-ucts) performed on 7a produced ortho- and para-ethylphenol, theortho isomer being �70% of the total mixture (GC–MS analysis).It is to be noted that the same acid hydrolysis performed on 6a re-leased unalkylated phenol.

Using a phenol/6a ratio 6 1, polinuclear 7a species were alsoformed (Scheme 2). The GC–MS analysis of the organic fraction,produced by acid hydrolysis of the crude reaction products,showed, besides the presence of ortho- and para-ethylphenol(in the same ratio found in the previous experiment), also thepresence of some di- and tri-ethylphenols. In particular, while aphenol/6a ratio close to 1 afforded only trace amounts of polyalky-lated phenols, using a phenol/6a ratio of 0.2, the polyalkylatedphenols became a relevant percentage of the total mixture.

These findings give further evidence that a Pt–CaH2CbH2 moietyon a phenyl ring acts as an activating substituent. The presence ofexcess phenol disfavors, on a statistical basis, formation of poly-alkylated species.

The transformation 6a ? 7a is reminiscent of the rearrange-ment of alkyl aryl ethers, which has been known for over a century[24]. This rearrangement is catalyzed by Lewis acids. Reactionyields and possible mechanisms depend upon several factors, suchas nature and quantity of catalyst, as well as the reaction condi-tions (homogeneous or heterogeneous). In the case of homoge-neous conditions a relevant role is also played by the solvent

[25]. The activated complex, formed by the ether and the catalyst[25b,c], can evolve intramolecularly, by shifting the alkyl groupon a phenyl carbon of the same ether molecule, or intermolecu-larly, via detachment of the alkyl group and its subsequent electro-philic attack on a phenyl ring belonging to another molecule(Fig. S6) [25c]. In the formation of ortho-alkyl phenols, the intramo-lecular reaction pattern seems to be dominant, while para isomersare essentially formed via the intermolecular path. This dual mech-anism can lead to an ortho/para ratio higher than expected for apurely aromatic electrophilic substitution [26].

For a better understanding of the mechanisms operating in thecase of 6a, the rearrangement was investigated in the presence ofperdeuterated phenol. Using an excess of phenol-d6 (conditionallowing the exclusive formation of mononuclear 7a) and thenperforming the hydrolysis with HCl, the GC–MS analysis of therecovered organic fraction revealed the presence of ortho- andpara-ethylphenol in the usual ratio (close to 3:1), both containingtwo deuterium atoms on the aromatic ring (m/z = 124). This featureimplies that, in the rearrangement, the platinum–ethanide moietyhas moved to a perdeuterated phenol molecule. It is to be notedthat, in strong acidic conditions, the phenol undergoes H/Dexchange at the ortho and para positions and, as a consequence,only the deuterium atoms in positions 3 and 5 are retained [27].

When 6a was treated with stoichiometric or substoichiometricquantities of phenol-d6, the GC–MS spectrum of the organic frac-tion, after hydrolysis of the crude reaction products, did show amixture of mono-, di-, and tri-ethylphenols. For the mono-ethyl-phenols (usual ortho/para ratio close to 3:1) two peaks at m/z = 122 and 124 were found (the former peak corresponded toalkylation of undeuterated phenol and the second peak to alkyl-ation of perdeuterated phenol). The intensity of the higher masspeak decreased by decreasing the quantity of free phenol used(the free phenol was always perdeuterated); using a phenol/6a

O

+

Pt

ClN

N

DD

D

O

D

ortho isomer

D

O

D

D

D

D D

D

Pt

ClN

N Oαβ

.. ..

. . ..

D

..

Pt

ClN

N

D

D

D

D

O

D

..

. . ..

6a

7a

. ...

Scheme 3. Inter-molecular rearrangement of complex [PtCl(g1-CH2CH2-OPh)(tmeda)], 6a, in the presence of free phenol-d6.

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ratio of 0.2, the peak at m/z = 124 almost disappeared. In the lattersituation the obtained di- and tri-ethylphenols were also deprivedof deuterium (only peaks at m/z = 150 and 178, respectively).Therefore, the obtainment of deuterated mono-ethylphenolsin the reaction performed in the presence of an excess of phenol-d6 is indicative of a transposition reaction taking place mainlythrough an intermolecular concerted mechanism as shown inScheme 3. This requires that an external perdeuterated phenolmolecule while donating the hydroxyl proton to the ether oxygenof 6a uses the electron-rich ortho carbons (and to a lesser extentthe para carbon) to perform nucleophilic attack on the b carbonof 6a. If the used phenol-d6 is only equimolar with respect to 6a,while the transposition reaction is progressing, undeuterated phe-nol (initially present in 6a) is liberated in the reaction mixture withthe consequence that the transposed molecules are a mixture ofnon deuterated and perdeuterated species. A further decrease offree perdeuterated phenol (phenol/6a ratio of 0.2) not only practi-cally abolishes the formation of deuterated alkylation products, butalso favors multiple alkylation of the same aromatic nucleus.

A system, closely related to the one presently described, hasbeen recently investigated by Benedetti et al. [28]. The cationicplatinum complex, considered in that study, is an analogous of1a, having 1,10-phenanthroline (1,10-phen) instead of tmeda inthe platinum coordination sphere. The addition product corre-sponding to 6a, namely [PtCl(g1-CH2-CH2-OPh)(1,10-phen)], inthe presence of excess phenol not only rearranged, shifting the al-kyl platinum moiety to an ortho carbon of the aromatic ring, butthe phenolate oxygen also gave a ring closing reaction by substitu-tion of the cis chlorido ligand. The authors proposed for the reac-tion a concerted mechanism similar to that reported in Scheme 3.

The ortho isomer of mononuclear 7a could, in principle, give thering closing reaction, with elimination of HCl and formation of aPt–O bond; differently from the system investigated by Benedettiet al., in our case this step requires the presence of a strong base(KOH).

The reaction with phenolate anion was performed also on com-pound 1b. The formation of the O-coupled species, 6b, was rapidand quantitative, but its reaction with phenol, performed asalready described in the case of 6a, was rather complex. Withtime, a number of products, including those derived from deproto-nation of 1b [5n], were formed and the system was not furtherinvestigated.

3.3. The coordinated olefin in the cationic complexes

Relevant structural and NMR parameters for free and coordi-nated ethene and propene are collected in Tables 1 and 2. Thestructure of 1a(ClO4) had already been reported [5g], whereasthe X-ray crystal structure of 1b(ClO4) is part of the present inves-tigation (Table S1). The crystal structure is drawn in Fig. 1 and se-lected geometrical parameters are reported in Table 1. Thestructures of the cations 1a and 1b are very similar and share anumber of features, such as the elongation of the Pt–N bond transto the olefin with respect to that cis to it (1a, 2.119(2) and2.084(2) Å; 1b, 2.105(9) and 2.074(9) Å, respectively) and the sym-metrical coordination to the metal core with the two Pt–C dis-tances almost identical in both complexes (1a, 2.184(5) and2.166(5) Å; 1b, 2.195(10) and 2.221(11) Å). The main differencelies in the orientation of the alkene, which is perpendicular tothe coordination plane in 1a (deviation of only 2� with respect tothe ideal value of 90�) and slightly canted in 1b (deviation of ca.14�). The just quoted difference could be due to crystal packing ef-fects and is not relevant to the solution behavior of the two com-plexes, since the olefin is free to rotate about the axis connectingthe metal core to the centroid of the carbon–carbon double bond[29].

13C NMR parameters (Table 2) have been acquired for both com-pounds (acetone-d6, 294 K). Upon coordination, the olefinic carbonresonances undergo an upfield shift; moreover, the 1JPt–C values forthe two carbons of 1b are very similar and close to that of 1a, indi-cating that the symmetrical coordination of the olefin is retainedalso in solution.

3.4. DFT calculations

The geometries of 1a and 1b have been optimized at the B3LYPlevel using, as basis set, Lanl2dz for Pt and 6-31G(d,p) for all otheratoms and the atom coordinates of the crystal structures as start-ing point [5g]. The calculated geometric values (Table S4) comparewell to the experimental data [30].

DFT calculations have shown how for both complexes (1a and1b) the LUMO is lower in energy than the p* of the free olefin (en-ergy values: 10.9 kcal/mol for ethene and �132.76 kcal/mol for 1a,17.17 kcal/mol for propene and �130.32 kcal/mol for 1b) (Fig. 2).

Table 1Selected bond lengths (Å) and angles (�) for [PtCl(g2–CH2@CH–CH3)(tmeda)](ClO4),1b(ClO4).

Vector Length Vector Length

Pt–Cl(1) 2.289(3) N(1)–C(1) 1.510(14)Pt–N(1) 2.074(9) N(2)–C(2) 1.491(15)Pt–N(2) 2.105(9) C(1)–C(2) 1.48(2)Pt–C(1o) 2.195(10) C(1o)–C(2o) 1.39(2)Pt–C(2o) 2.221(11) C(2o)–C(3o) 1.48(2)

Vector Angle Vector AngleN(1)–Pt–N(2) 84.5(4) N(1)–Pt–Cl(1) 174.6(3)N(1)–Pt–C(1o) 99.1(4) N(2)–Pt–Cl(1) 90.3(3)N(1)–Pt–C(2o) 90.6(4) C(1o)–Pt–Cl(1) 85.5(4)N(2)–Pt–C(1o) 161.9(4) C(2o)–Pt–Cl(1) 94.8(3)N(2)–Pt–C(2o) 161.4(5)

Table 2Bond distances (Å) and 13C chemical shifts (acetone-d6, 294 K, ppm) for free andcoordinated olefins (1JPt–C in Hz are reported in brackets).

Compound d(C@C) d(Pt�CH2) d(Pt�CHR) d (CH2) d (CHR)

Ethene 1.33 � � 123.3 �1a5g 1.390(2) 2.184(5)

2.166(5)� 80.2

[166]�

Propene 1.33 � � 115.7 133.71b 1.39(2) 2.195(10) 2.221(11) 77.6

[149]104.6[153]

C(1o)

C(2o)

C(3o)

Cl(1)

Pt N(2)

N(1)C(4)

C(3)

C(2)

C(1)

C(5)

C(6)

Fig. 1. ORTEP drawing for [PtCl(g2-CH2@CH–CH3)(tmeda)]+, 1b. Ellipsoids enclose20% probability.

210 C.R. Barone et al. / Inorganica Chimica Acta 363 (2010) 205–212

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This property, shared also by closely related styrene complexes[5r], is in full agreement with the observed reactivity. Low lyingLUMO levels favor the interaction with an incoming nucleophileand a positive charge on the metal fragment is an important aidingfactor for the reaction to take place. Although, in principle, such anactivation towards nucleophilic addition may apply only to transi-tion states, in the present case there is an intrinsic activation al-ready in the ground state. Moreover, in the case of 1b, the LUMOappears to be more localized on the substituted carbon, fully sup-porting the experimentally observed prevalence of the Markovni-kov type of attack in the nucleophilic addition [5l,p,q].

4. Conclusions

DFT calculations have shown how both complexes, 1a and 1b,are intrinsically activated towards nucleophilic attack, since theLUMO lies lower in energy with respect to the p* orbital of the freeolefin. The electrophilic character conferred to the olefin by thebound PtCl(tmeda)+ fragment is particularly evident in complex1a, the olefin of which is able to couple with weak carbon donors.In complex 1b, a subtile balance between steric and electronic fac-tors gives a rationale to the observed regiochemistry of nucleo-philic addition (Markovnikov type). Moreover, in this latter casethe electrophilic character of the coordinated olefin is oftenmasked by the presence of another reaction route: loss of allylicprotons, which become acidic as a consequence of olefin coordina-tion to the platinum moiety.

Acknowledgements

The authors gratefully acknowledge: Dr. Francesco Berrettini(CIADS, Centro di Analisi e Determinazioni Strutturali, Universityof Siena) for the X-ray data collection, Dr. Giovanni Di Pinto (Dipar-timento Farmaco-Chimico, University of Bari) for recording massspectra, and CINECA (Consorzio Interuniversitario per il CalcoloAutomatico dell’Italia Nord-Orientale, Casalecchio di Reno, Casa-lecchio di Reno, Bologna, Italy) for the grant to use IBM SP5 ma-chine for DFT calculations. The authors also thank the financialsupport of the Universities of Siena and Bari.

Appendix A. Supplementary data

CCDC 665826 contains the supplementary crystallographic datafor complex [1b(ClO4)]. These data can be obtained free of chargefrom The Cambridge Crystallographic Data Center via www.ccdc.

cam.ac.uk/data_request/cif. Supplementary data associated withthis article can be found, in the online version, at doi:10.1016/j.ica.2009.08.035.

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[29] There are several intra- and inter-molecular hydrogen bonds [5r] whichinvolve: (i) the tmeda and propene methyl groups and the coordinatedchlorido (C(5). . .Cl(1), C(6). . .Cl(1), and C(3o). . .Cl(1) contact distances of3.430(16), 3.230(19), and 3.388(15) Å, respectively; corresponding C-H. . .Clangles of 118.2(9) , 120(1) , and 132.2(8) , respectively); (ii) the perchlorateanion which is encapsulated by several C-H. . .O hydrogen bonds from tmedamethyl and methylene and even from the olefin.

[30] For the bond lengths, the average deviation is 0.04 Å, while the greatestdeviation is 0.09 Å and involves the Pt–N bond length cis to the olefin in 1b. Forthe bond angles, the average deviation is 1�, while the maximum deviation is3.8� and it involves one CH3–Ntrans–Pt bond angle in 1b. The computed olefinsare symmetrically bound to the metal centre, as experimentally observed.Upon coordination, the C=C bond undergoes a lengthening (by 0.06 Å in 1a and0.09 Å in 1b). The a angle, which is a measure of the bending-back of olefinsubstituents, is close to 32� in both complexes and similar to the experimentalvalue found in Zeise’s anion (for the definition of a angle see J.K. Stalick, J.A.Ibers, J. Am. Chem. Soc. 92 (1970) 5333 and J.S. Ricci, J.A. Ibers, J. Am. Chem.Soc. 93 (1971) 2391).

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