A pyridine bridged dicarbene ligand and its silver(I) and palladium(II) complexes: synthesis, structures, and catalytic applications

www.elsevier.com/locate/ica Inorganica Chimica Acta 327 (2002) 116– 125

A pyridine bridged dicarbene ligand and its silver(I) and palladium(II) complexes: synthesis, structures, and catalytic applications David J. Nielsen a, Kingsley J. Cavell b,*, Brian W. Skelton c, Allan H. White c a

School of Chemistry, Uni6ersity of Tasmania, G.P.O. Box 252 -75, Hobart, Tasmania 7001, Australia b Department of Chemistry, Cardiff Uni6ersity, P.O. Box 912, Cardiff CF10 3TB, UK c Department of Chemistry, Uni6ersity of Western Australia, Nedlands, WA 6907, Australia Received 17 May 2001; accepted 18 July 2001 Dedicated to Professor Kees Vrieze

Abstract The potentially tridentate ligand 2,6-bis[(3-methylimidazolium-1-yl)methyl]pyridine dibromide reacts readily with silver(I) oxide in dichloromethane or dimethylsulfoxide to give a dinuclear silver(I)– carbene complex that was isolated as the tetrafluoroborate salt. Single crystal X-ray crystallography shows that each silver(I) ion is bridged by two ligands bonding through the carbene donors. Treatment of the silver(I) complex with suitable palladium(II) precursors gave the complexes PdCl[(CNC)]BF4 and [PdMe(CNC)]BF4 (CNC=2,6-bis[(3-methylimidazolin-2-yliden-1-yl)methyl]pyridine), in which the pyridyl and both carbene moieties are coordinated to a single palladium(II). The palladium(II) complexes have been fully characterised, including X-ray crystallography, and exhibit good activities in the Heck coupling reaction of 4-bromoacetophenone and n-butyl acrylate. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Palladium; Silver; Carbene complexes; Catalysis; Heck reaction

1. Introduction N-Heterocyclic carbenes (NHCs) [1] based on the imidazole ring system have been found to make excellent ligands for a variety of transition [2,3] and main group elements [4]. A number of transition metal – NHC complexes have given rise to active catalytic systems for a range of reactions [5 – 9], most notably the Heck and Suzuki CC coupling reactions. Interest in the functionalisation of NHCs with groups bearing suitable hemilabile donor groups for coordination to transition metals has followed in an effort to improve catalyst activity and stability. A number of publications have explored the mono- [10 – 15] and di-functionalisa

A contribution in recognition of the outstanding achievements of Kees Vrieze in the fields of organometallic and coordination chemistry. * Corresponding author. Fax: + 44-29-2087 5899. E-mail address: [email protected] (K.J. Cavell).

tion [11,13,14,16,17] of a single NHC ring, and more recently initial reports on the bridging of two NHCs with a single pyridine functionality, either as a discrete unit [13,18 –20] or in a cyclophane arrangement [21], have appeared. In several cases highly active catalysts for the Heck reaction have resulted from the application of functionalised NHC ligands [10 –12]. It has previously been shown that there exists a facile reductive elimination route to decomposition of Ni- and Pd-hydrocarbyl complexes bearing NHCs as ligands [11,22] which has implications for the application of these complexes in catalysis. This tendency to eliminate 2-hydrocarbylimidazolium salts may be reduced by the presence of stronger donor groups on the NHC [11], that can stabilise the complex through either, or a combination of, the following mechanisms: preventing the NHC and hydrocarbyl groups from attaining a mutually cis geometry; constraining the NHC ring to an orientation in which it cannot interact electronically

0020-1693/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 0 - 1 6 9 3 ( 01 ) 0 0 6 7 7 - 6

D.J. Nielsen et al. / Inorganica Chimica Acta 327 (2002) 116–125

with the hydrocarbyl groups; or possibly by operating in a large excess of the imidazolium salt, i.e. ionic liquid solvents [23]. This report is part of ongoing efforts to create stable, catalytically active transition metal systems based on NHC ligands of the form carbene(donor)carbene in which catalyst decomposition might be minimised by adoption of suitable ligand geometry. A report on the unusual reactivity of Pd(OAc)2 towards a ligand motif of this type has appeared previously [24] which demonstrates the problem of deprotonation h to the functional group that may be associated with the interaction of basic metal salts and functionalised NHCs containing ‘active’ hydrogens. The transfer of an NHC ligand from a Ag(I)– carbene complex to a suitable Pd(II) precursor has been shown to be a preferred method of accessing many Pd– hydrocarbyl complexes of functionalised NHC ligands [10,11], although low temperature free carbene formation has been successful in limited cases [12]. The Ag(I)– carbenes are simply prepared by deprotonation of the imidazolium salt with the mild base Ag2O in a suitable solvent [25]. A handful of structural studies of Ag(I) complexes of functionalised NHC’s have been carried out, revealing mononuclear complexes with one [26] or two carbene [26] groups bound to Ag(I), or halide-bridged dinuclear complexes [26]. The pyridyl-bridged bis-NHC cyclophane Ag(I) complex of Garrison et al. [21] is dinuclear with the functional groups remaining uncoordinated. The absence of coordination of hemilabile functional groups in mono-NHC ligands bound to both Ag(I) [26] and Pd(II) [10,16] has also been observed. The cationic Pd(II) complex [PdBr(CNC%)]Br (I) where CNC% represents the related but more rigid functionalised bis-NHC ligand 2,6-bis(3-methylimidazolin-2-yliden-1-yl)pyridine was found to exhibit high thermal stability and perform satisfactorily in the Heck coupling of aryl bromides and iodides with styrene in refluxing dimethylacetamide (DMA) under air [19]. Pdhydrocarbyl complexes of carbene(donor)carbene ligands have not previously been reported.

2. Experimental

2.1. General All reactions were performed under an atmosphere of dry nitrogen using standard Schlenk techniques, and

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solvents were dried by usual means [27], unless otherwise indicated. 2,6-Bis(bromomethyl)pyridine [28] and PdClMe(cod) [29] (cod= 1,5-cyclooctadiene) were prepared by literature methods. All other reagents were used as received. NMR spectra were run on Varian Gemini 200 or Unity Inova 400 instruments at ambient temperature unless otherwise specified and referenced to residual solvent signals. Melting points were performed under air, and are uncorrected. Elemental analyses (Carlo Erba EA1108) and Mass Spectrometry (Kratos Concept ISQ) using the Liquid Secondary Ion (LSIMS) method were performed by the Central Science Laboratory, University of Tasmania.

2.1.1. 2,6 -Bis[(3 -methylimidazolium-1 -yl)methyl]pyridine dibromide (1) 2,6-Bis(bromomethyl)pyridine (0.953 g, 3.59 mmol) was taken up in THF (25 ml) and 1-methylimidazole (0.64 ml, 0.66 g, 8.0 mmol) added with stirring. A white solid began precipitating after several minutes and after stirring for 5 days at room temperature Et2O (25 ml) was added and the product filtered on a glass sinter and washed with Et2O. Compound 1 (1.41 g, 91.5%) was obtained as a white powder after drying under vacuum. X-ray quality crystals were grown by diffusion of Et2O into a MeCN solution of 1. Required for C15H19Br2N5: C, 41.98; H, 4.46; N, 16.32%. Anal. Found: C, 41.86; H, 4.65; N, 16.16%. MS (LSIMS); m/z: 350.1 [M−Br]+ (30%), 268.2 [M− H− 2Br]+ (100%). HRMS, [M− Br]+: 348.08072 Da (− 4.78 ppm from calc.). 1H NMR (199.98 MHz, DMSO-d6): l 9.22 (s, 2H, imC2H), 7.97 (t, J= 7.7Hz, 1H, pyrC4H), 7.76 & 7.71 (m (× 2), each 2H, imC4&5H), 7.41 (d, J= 7.9Hz, 2H, pyrC3,5H), 5.57 (s, 4H, CH2), 3.92 (s, 6H, NCH3). 13C NMR (100.51 MHz, DMSO-d6): l 153.66 (pyrC2,6), 138.79 (pyrC4), 137.23 & 137.20 (imC2), 123.37 & 123.13 (imC4,5), 122.01 (pyrC3,5), 52.45 (CH2), 35.99 (NCH3). M.p.: 78–80 °C. 2.1.2. [Ag2(CNC)2] ·2BF4 (CNC = C,C%-2,6 -bis[(3 -methylimidazolin-2 -yliden-1 -yl)methyl]pyridine) (2) Imidazolium salt 1 (0.31 g, 0.72mmol) was suspended in 50 ml DCM and 6ml methanol (MeOH) added to effect dissolution. Ag2O (0.167 g, 0.721 mmol) was added and the resulting black suspension was protected from light and stirred at room temperature until only a voluminous light grey precipitate remained. The product was filtered on a glass sinter, washed with MeOH and dried in vacuo giving [Ag(CNC)]n ·n(AgBr2) (0.33 g, 72.1%). The Ag(carbene)·AgBr2 salt (0.31 g, 0.48 mmol) was suspended in hot MeCN (70 ml) and treated with AgBF4 (0.099 g, 0.509 mmol). The precipitated AgBr was removed by filtration and the solvent removed in vacuo, leaving an off-white powder that was washed with MeOH and Et2O and dried in vacuo to give 2 (0.19 g, 57.0% from 1). X-ray quality crystals were grown by diffusion of Et2O into a MeCN solution

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of 2. MS (LSIMS); m/z: 374.0 [(M − 2BF4)/2]+ (100%). HRMS, [(M −2BF4)/2]+: 374.05366 Da (0.48 ppm from calc.). 1H NMR (399.70 MHz, DMSO-d6): l 7.75 (t, J= 7.7Hz, 2H, pyrC4H), 7.49 & 7.45 (d of d, J= 1.8 Hz, 8H, imC4,5H), 7.15 (d, J =7.6 Hz, 4H, pyrC3,5H), 5.29 (s, 8H, CH2), 3.73 (s, 12H, NCH3). 13C NMR (100.51 MHz, DMSO-d6): l 180.94 (imC2), 155.97 (pyrC2,6), 138.69 (pyrC4), 122.93 & 122.86 (imC4,5), 121.31 (pyrC3,5), 55.25 (CH2), 38.12 (NCH3).

2.1.3. [PdCl(CNC)]BF4 (CNC = C,N,C%-2,6 -bis[(3 -methylimidazolin-2 -yliden-1 -yl)methyl]pyridine) (3a) PdCl2(MeCN)2 (0.0474 g, 0.183 mmol) was dissolved in DMSO (4 ml) at room temperature and 2 (0.0804 g, 0.174 mmol) added, giving a cloudy orange solution. After stirring for 5 min the colour of the suspension had lightened considerably and AgCl had precipitated. The suspension was stirred for a further hour with little change, then allowed to settle. The supernatant was decanted from the precipitate and residual DMSO removed in vacuo at 40 °C leaving a yellow/green oil that was triturated with DCM (3 ml) and the residue washed with Et2O and MeOH before drying in vacuo and recrystallisation from MeCN/Et2O to give 3a (0.035 g, 41%) as a pale green powder. The desired complex was more efficiently prepared by a sequential one-pot reaction: 1 (0.107 g, 0.250 mmol) was dissolved in DMSO (5 ml) and Ag2O (0.058 g, 0.250 mmol) added. The resulting suspension was stirred at room temperature until the Ag2O had reacted (approximately 2 days). AgBF4 (0.050 g, 0.257 mmol) was then added, followed by PdCl2(MeCN)2 (0.0652 g, 0.251 mmol). The yellow colour faded after several minutes, and after a further 30 min stirring the reaction was filtered, the DMSO removed in vacuo, and the residue washed with Et2O. The crude product was recrystallised from MeCN/Et2O and dried in vacuo to give 3a (0.10 g, 81%). Required for C15H17N5ClPdBF4: C, 36.32; H, 3.45; N, 14.12%. Anal. Found: C, 36.34; H, 3.46; N, 14.10%. MS (LSIMS); m/z: 410.0 [M −BF4]+ (100%), 373.0 [M− Cl −BF4]+ (17%). HRMS, [M −BF4]+: 406.01944 Da ( −3.46 ppm from calc.). 1H NMR (399.70 MHz, DMSO-d6): l 8.20 (t, J = 7.8 Hz, 1H, pyrC4H), 7.84 (d, J = 8.0 Hz, 2H, pyrC3,5H), 7.57 & 7.36 (d×2, J=2.0 Hz, each 2H, imC4,5H), 5.68 (distorted q, 4H, CH2), 3.93 (s, 6H, NCH3). 13C NMR (100.51 MHz, DMSOd6): l 164.37 (imC2), 155.50 (pyrC2,6), 141.79 (pyrC4), 125.63 & 123.59 & 121.63 (imC4,5 & pyrC3,5), 54.48 (CH2), 37.05 & 37.01 (NCH3). M.p.: 229– 232 °C. 2.1.4. [PdMe(CNC)]BF4 (3b) This complex was made in a one-pot reaction analogous to 3a, utilizing 1 (0.4850 g, 1.130 mmol), Ag2O (0.2621 g, 1.131 mmol), AgBF4 (0.228 g, 1.171 mmol) and PdClMe(cod) (0.3084 g, 1.163 mmol) in DMSO (25 ml) at 35 °C. The crude product was re-

crystallised from MeCN at − 20 °C. Yield: 0.46 g, 86%. Crystals suitable for X-ray crystallography were grown from diffusion of Et2O into a MeCN solution of 3b at room temperature. Required for C16H20N5PdBF4: C, 40.41; H, 4.24; N, 14.73%. Anal. Found: C, 40.41; H, 4.30; N, 14.80%. MS (LSIMS); m/z: 388.1 [M−BF4]+ (100%), 373.1 [M− Me − BF4]+ (30%), 292.0 [M− MeIm− BF4 + H]+ (32%). HRMS, [M−BF4]+: 384.07518 Da (− 4.14 ppm from calc.). 1H NMR (399.70 MHz, DMSO-d6): l 8.08 (t, J= 7.8Hz, 1H, pyrC4H), 7.72 (d, J=7.6 Hz, 2H, pyrC3,5H), 7.53 & 7.33 (d×2, J= 1.6 Hz, each 2H, imC4,5H), 5.45 (m, 4H, CH2), 3.78 (s, 6H, NCH3), 0.69 (s, 3H, PdCH3). 13 C NMR (100.51 MHz, DMSO-d6): l 175.78 (imC2), 154.39 (pyrC2,6), 140.46 (pyrC4), 124.41 & 122.84 & 121.78 (imC4,5 & pyrC3,5), 54.11 (CH2), 37.07 & 37.03 (NCH3), − 15.06 (PdCH3). M.p.: \ 230 °C (slight decomp.).

2.2. Structure determinations Full spheres of CCD area-detector diffractometer data were measured at approximately 153 K (Bruker AXS instrument, monochromatic Mo Ka radiation, u=0.71073 A, , v-scans, 2qmax = 68°) yielding Nt(otal) reflections, these merging to N unique (Rint quoted) after ‘empirical’/multiscan absorption correction, No with F\4|(F%) being considered ‘observed’ and used in the full-matrix least-squares refinements. Anisotropic thermal parameter forms were refined for the non-hydrogen atoms; (x, y, z, Uiso)H in all metal complexes, constrained in the ligand salt. Conventional residuals R, Rw on F are quoted at convergence (weights: (| 2(F%)+ 0.0004 F 2) − 1). Neutral atom complex scattering factors were employed within the context of the XTAL 3.7 program system [30]. Pertinent results are given in the tables and figures, the latter displaying 50% displacement ellipsoids for the non-hydrogen atoms, hydrogen atoms having arbitrary radii of 0.1 A, .

2.2.1. Crystal/refinement data 2.2.1.1. Compound 1 ·H2O. C15H21Br2N5O, M= 447.2. Monoclinic, space group P21/c, (C 52h, No. 14), a= 13.572(1), b=10.986(1), c= 13.921(1) A, , i= 117.591(2)°, V= 1893 A, 3. Dcalc (Z=4)= 1.614 g cm − 3. vMo = 44 cm − 1; specimen: 0.40× 0.35× 0.13 mm, ‘Tmin,max’= 0.40, 0.70. Nt = 27 898, N= 7198 (Rint = 0.044), No = 4944; R= 0.027, Rw = 0.037; Dzmax = 0.61(6) e A, − 3. 2.2.1.2. [Ag2(CNC)2](BF4)2 (2). C30H34Ag2B2F8N10, M= 924.0. Triclinic, space group P1( (C 1i , No. 2), a= 8.293(1), b=10.474(2), c= 11.672(2) A, , h= 63.637(2), i= 74.064(2), k= 83.821(2)°, V=873 A, 3. Dcalc (Z=1 (dimeric f.u.))= 1.757 g cm − 3. vMo = 12.0 cm − 1; speci-

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men: 0.40×0.12×0.03 mm, ‘Tmin,max’= 0.75, 0.89. Nt = 13 591, N =6730 (Rint =0.033), No =5153; R= 0.041, Rw = 0.043; Dzmax =1.7(1) e A, − 3.

2.2.1.3. [PdX(CNC)](BF4), X = Cl (3a), Me (3b). These compounds are isomorphous, monoclinic, space group C2/c (C 62h, No. 15), and Z = 8. Compound 3a: X= Cl. C15H17BClF4N5Pd, M =496.0. a =16.703(2), b= 11.287(1), c=19.887(2) A, , i =106.847(2)°, V =3588 A, 3. Dcalc =1.836 g cm − 3. vMo =12.3 cm − 1; specimen: 0.40× 0.40×0.35 mm, Tmin,max =0.68, 0.80. Nt = 27 338, N= 7081 (Rint =0.022), No =6193; R =0.027, Rw = 0.038; Dzmax = 1.18(5) e A, − 3. Compound (3b): X = CH3. C16H20BF4N5Pd, M = 475.6. a =16.929(1), b= 11.3572(7), c= 19.678(1) A, , i =106.901(1)°, V= 3620 A, 3. Dcalc = 1.745 g cm − 3. vMo =10.8 cm − 1; specimen: 0.38×0.34×0.13 mm, Tmin,max =0.64, 0.79. Nt = 26 581, N =7046 (Rint =0.016), No =6569; R= 0.020, Rw = 0.033; Dzmax =0.75(4) e A, − 3. 2.3. Catalysis Heck reactions were carried out using aryl halide (25 mmol), n-butylacrylate (5.0 ml, 35 mmol), sodium acetate (2.23 g, 28 mmol), and Pr4NBr (1.33 g, 5 mmol) at 120 °C in N,N-dimethylacetamide (DMAc), with di(ethylene glycol) n-butyl ether as internal standard. An appropriate amount of catalyst was injected in DMAc solution when the operating temperature was reached, followed by hydrazine hydrate (10– 70 ml) if appropriate. Coupling product yields were calculated by GC based on residual aryl halide, and product identity confirmed by 1H NMR and GC – MS.

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3. Results and discussion The functionalised bis-imidazolium ligand 1 was prepared in good yield by the reaction of 2,6-bis(bromomethyl)pyridine with 1-methylimidazole in tetrahydrofuran (THF). The dibromide salt of the protonated bis-NHC ligand crystallises as the monohydrate, 1·H2O, the X-ray structure of which is shown in Fig. 1. The water molecule is an integral part of an extended hydrogen-bonded array, with the two hydrogen atoms linking to the two bromide ions (O(1)···Br(1,2)); 3.348(2), 3.277(2) A, ), and the two bromide ions to the C(22,22%) protons of successive glide related ligands (Br(1)···C(22)). (x, 112 − y, z−1/2) = 3.601(2), Br(2)··· C(22%) = 3.502(2) (A, ). The imidazolium rings on each ligand molecule are rotated about their pendant bonds so that the C(22,22%) protons lie directed well away from each other. Pertinent non-hydrogen geometries are listed in Table 1 for subsequent comparison with those of the deprotonated ligand in diverse association with Ag(I) and Pd(II). The free carbene of 1 has previously been prepared by treatment of the imidazolium hexafluorophosphate salt with sodium hydride in liquid ammonia/THF at −78 °C [31], but shows poor stability under even these mild conditions, perhaps due to the inherent acidity of the methylene protons h to both the functional group and imidazolium nitrogens. The complexation of the free carbene of 1 to suitable metals has not previously been reported. Initial trials on the reaction of 1 with Pd(OAc)2 in the classical manner [2] gave a complex mixture of products, and although the absence of signals due to 1 imC2H in the H NMR indicated that carbene com-

Fig. 1. Projection of the bromide ions of 1, in stringwise association, with the water molecule and adjacent cations.

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Table 1 Selected bond distances (A, ) and bond angles (°) for the ligands in 1, 2, 3a, and 3b. The two values in each entry correspond to the unprimed, primed ligand sections, respectively Complex

[H2(CNC)]2+ (1)

[Ag2(CNC)2]2+ (2)

[PdCl(CNC)]+ (3a)

[PdMe(CNC)]+ (3b)

MeIm ring distances (A, ) C(21)N(21) N(21)C(22) C(22)N(23) N(23)C(24) C(24)C(25) C(25)N(21)

1.463(3), 1.334(3), 1.326(3), 1.381(4), 1.355(4), 1.382(3),

1.465(3) 1.336(2) 1.329(3) 1.382(3) 1.356(3) 1.383(3)

1.468(4), 1.350(3), 1.353(4), 1.381(5), 1.347(4), 1.385(5),

1.475(4) 1.354(3) 1.354(4) 1.382(5) 1.344(4) 1.383(5)

1.467(2), 1.355(2), 1.345(2), 1.388(3), 1.348(2), 1.386(3),

1.458(2) 1.355(2) 1.351(3) 1.384(3) 1.347(2) 1.385(3)

1.465(1), 1.358(2), 1.354(1), 1.390(2), 1.348(2), 1.385(2),

118.2(2), 124.7(2), 108.8(3), 108.5(2), 108.9(2), 125.2(2), 107.1(2), 106.7(2),

115.7(2) 124.4(2) 108.9(2) 108.3(2) 109.0(2) 124.8(2) 107.1(2) 106.7(2)

114.9(2), 124.3(3), 111.5(3), 104.3(3), 111.1(2), 124.3(3), 107.0(3), 106.1(3), 125.5(2), 129.4(2),

115.6(2) 123.0(3) 111.2(2) 104.2(3) 111.3(2) 124.8(3) 106.6(3) 106.8(3) 130.2(2) 125.6(2)

117.7(1), 121.5(2), 111.3(1), 104.8(2), 110.8(1), 125.6(2), 107.2(2), 106.0(2), 118.7(1), 135.9(1),

117.8(1) 121.8(2) 111.5(1) 104.5(2) 110.7(1) 125.6(2) 107.4(2) 105.8(2) 117.8(1) 137.5(1)

116.6(2), 117.06(9) 122.6(1), 122.8(1) 111.62(9), 111.8(1) 104.1(1), 103.8(1) 111.1(1), 111.3(1) 125.9(1), 125.6(1) 106.8(1), 107.2(1) 106.3(1), 105.9(1) 118.77(7), 117.58(9) 136.15(9), 138.38(9)

Bond angles (°) N(1)C(2)C(21) C(21)N(21)C(22) C(22)N(21)C(25) N(21)C(22)N(23) C(22)N(23)C(24) C(22)N(23)C(231) N(23)C(24)C(25) C(24)C(25)N(21) MC(22)N(21) MC(22)N(23) Torsion angles (°) C(2)C(21)N(21)C(22), F (°) Interplanar dihedral angles (°) Pyridyl/MeIm MeIm/MeIm% Coord./MeIm Coord./pyridyl a b

1.457(2) 1.359(1) 1.357(2) 1.385(2) 1.348(2) 1.388(2)

104.2(3), 76.6(3)

−81.5(3), 164.7(2)

59.0(2), 59.3(2)

61.2(2), 60.2(1)

66.8(1), 63.7(1) 62.6(1)

74.4(1), 70.5(1) 9.2(1) 77.55(9) a, 75.17(10) a 50.36(9) a

66.14(7), 59.05(7) 85.10(8) 43.86(6) b, 42.95(6) b 41.89(6) b

65.48(5), 60.32(5) 83.03(5) 42.28(4) b, 43.57(4) b 40.63(4) b

For ‘coordination plane’ read the [AgC(22,22%)]2 plane. ‘Coordination plane’ defined by C(H3)/Cl, N(1), C(22,22%).

plexes had formed, this approach was abandoned in favour of the more promising Ag– carbene route. Imidazolium salt 1 reacts readily with Ag2O in dichloromethane (DCM) to give a compound with limited solubility but a 1H NMR spectrum consistent with the formation of a Ag– carbene adduct. Elemental analysis of the product of the reaction indicated probable inorganic salt contamination, as has previously been noted for functionalised NHC complexes of Ag(I) [10]. Treatment of this initial product with AgBF4 in hot acetonitrile (MeCN) gave the more readily soluble tetrafluoroborate salt 2, which was crystallised from MeCN/ether and had a formulation consistent with the association of one silver ion per ligand. A single crystal X-ray investigation showed that the [Ag2(CNC)2]2 + cation of 2 is a (binuclear) centrosymmetric dimer containing two Ag(I) ions bridged by two NHC ligands binding in a C ‚ C mode (Fig. 2(a, b)). One half of this dimer, with one tetrafluoroborate anion, comprises the asymmetric unit of the structure, with each ligand bridging the pair of silver atoms. The ligands coordinate through the carbenic atoms of the two MeIm (MeIm= 3-methylimidazolin-2-yliden-1-yl) rings only, with Ag···N(1) (1−x, 1 −y, z¯ ) =3.039(2) A, . The coordination about each Ag(I) is quasi-linear with a

CAgC bond angle of 176.78(9)°, and AgC(22%,22) (1− x, 1 − y, z¯ ) being 2.088(4), 2.093(4) A, , comparable with other examples of linear Ag(I)bis-NHC complexes [21,25,26]. Ag···Ag is 3.7171(5) A, , slightly shorter than the C(22)···C(22%) distance of 3.807(4) A, , but greater than the sum of the van der Waals radii (3.40 A, [32]) considered necessary for a significant metal–metal interaction. The two independent MeIm rings are appreciably tilted with respect to the central Ag2C4 plane with appreciably different associated torsions in the CC bonds. Each silver atom is essentially coplanar with the two C3N2 MeIm planes to which it is coordinated, with deviations of 0.292(5) A, , 0.024(5) A, (Ag, Ag%). Exocyclic bonds to the pendant metal atom are, unsurprisingly, appreciably different to those found in the palladium complexes 3a and 3b (Table 2). The 1H NMR spectrum of 2 shows the symmetry inherent in the structure, and most signals are shifted slightly upfield from the values observed for 1. The 13C NMR spectrum shows the characteristic carbene signal at l 180.94, moved downfield from l 137.2 in 1, but coupling to 107/109Ag is not observed. A one-pot reaction for the high yield preparation of Pd(II) complexes from imidazolium salt 1, Ag2O, AgBF4, and a suitable Pd(II) source was developed

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Fig. 2. Projections of the [Ag2(CNC)2]2 + cation of 2 (a) through, and (b) normal to the central Ag2C4 ‘plane’. Table 2 Palladium environments in [PdCl(CNC)]BF4 (3a), [PdMe(CNC)]BF4 (3b) Bond distances (A, ) XPd N(1)Pd C(22)Pd C(22%)Pd

2.2978(5), 2.044(1) 2.068(1), 2.1536(9) 2.026(2), 2.030(1) 2.029(2), 2.035(1)

Bond angles (°) XPdN(1) XPdC(22) XPdC(22%) N(1)PdC(22) N(1)PdC(22%) C(22)PdC(22%)

178.19(3), 176.59(4) 93.15(5), 94.17(6) 93.21(5), 93.12(6) 86.41(1), 86.02(4) 87.21(6), 86.65(4) 173.60(7), 172.65(5)

(Scheme 1) to conveniently provide [PdCl(CNC)]BF4 (3a) and [PdMe(CNC)]BF4 (3b) in high yields. Crystals of the Pd(II) complexes suitable for single crystal X-ray crystallography were grown from diffusion of Et2O into MeCN solutions, and the complexes were found to be monomeric with the NHC ligand chelating in the CNC mode. The complexes 3a and 3b are isomorphous, one formula unit comprising the asymmetric unit of the structure in each case (Fig. 3(a) and (b), respectively). Although devoid of crystallographic symmetry, each cation has, overall, putative 2 -symmetry. In each complex Pd deviates only slightly from ideal square planar coordination, with the pyridyl and MeIm rings of the ligands twisted well out of the coordination plane

D.J. Nielsen et al. / Inorganica Chimica Acta 327 (2002) 116–125

122

Scheme 1.

defined by N(1), C(22,22%), and Me/Cl (Table 1). The MeIm rings are inclined to the coordination plane by an average of 43.4° (3a) and 42.9° (3b). These may be compared with monodentate NHC ligands that are usually aligned more nearly perpendicular to the coordination plane, subject to the steric influences of the N-substituents [5,11]. Significant differences are observed between the 3a and 3b in respect of the PdN(1) distances (3a, 2.068(1); 3b, 2.1536(9) A, ), reflecting the increased trans effect due to the methyl group as compared to chloride, but in other respects the ligand environment varies little between the two complexes. The similar C(22)N(1)C(22%) angles (3a, 91.98(7)°; 3b, 90.13(4)°) and MeIm-coordination plane dihedrals in 3a and 3b suggest that the steric requirements of the chloro and methyl substituents are similar and unlikely to influence the ligand/metal association. Complexes 3a and 3b are intrinsically chiral by virtue of the manner of the concerted inclination of the MeIm and pyridyl rings to the Pd coordination plane. Each complex contains two boat-shaped six-membered palladacycles that give rise to a distorted quadruplet in the 1H NMR around l 5.7 (3a) and 5.5 (3b) at room temperature in DMSO-d6 owing to geminal coupling of the protons on the methylene group linking the pyridyl and MeIm rings. A variable temperature NMR study of 3b found that the signal due to the protons on the methylene

bridges gradually coalesces to a sharp singlet at l 5.46 at 70 °C in DMSO-d6, when the ring conformation is able to change freely. The room temperature 13C NMR spectra of 3a and 3b also show two poorly resolved signals around l 37.0 (3a) and l 37.1 (3b) that arise as the change in conformation of the palladacycles is conferred across the MeIm rings to the NMe substituents. The 1H NMR spectrum of 2 exhibits a (somewhat broadened) singlet at room temperature at l 5.29 that suggests that the dinuclear structure observed in the X-ray study exhibits sufficient conformational flexibility in solution at room temperature to make the methylene protons equivalent on the NMR timescale. The other NMR features of 3b are typical for trans bis-NHCPd(II) complexes [11], with the carbenic carbon observed at l 175.78 in the 13C NMR. However, the corresponding 13C NMR signal for 3a, at l 164.37, occurs at a value previously considered indicative of two NHC ligands in cis coordination at Pd(II) [11]. The methyl group of 3b gives rise to a singlet at l 0.69 in the 1H NMR and at l −15.06 in the 13C NMR. The 1 H NMR resonance of the PdMe group of 3b occurs downfield of that of previously reported NHCPd – methyl complexes [5,10–12,22]. Complex 3b exhibits unusually high stability for a cationic NHCPd –hydrocarbyl complex, requiring approximately 15–18 h at 150 °C in DMSO-d6 to completely remove the PdMe signal from the 1H NMR spectrum. Products of the thermal decomposition of 3b included substantial Pd black formation and a mixture of several unidentified carbene species. Methyl group transfer from Pd to imC2 was observed by the growth of characteristic signals in the 1H NMR at l 2.54 and l 2.42 for imC2Me [22], implying at least two different imidazolium products, probably present as the BF4 salts. The imC2Me peak of the bis-2,3-dimethylimidazolium dibromide analogue of 1 occurs at l 2.45 in the

Fig. 3. (a) Projection of the cation of 3a, oblique, showing the ligand conformation, and (b) the cation of 3b, normal to the coordination plane.

D.J. Nielsen et al. / Inorganica Chimica Acta 327 (2002) 116–125

Scheme 2. 1

H NMR, making an exact identification of this species as a decomposition product inconclusive. The initial product of any methyl-NHC reductive coupling in 3b is likely to be a mono-NHCPd0 complex with a pendant cationic 2,3-dimethylimidazolium-1-yl group (Scheme 2). The immediate absence of further groups at Pd suitable for reductive coupling to the remaining NHC moiety suggests that this complex may be a relatively stable species in the decomposition pathway. A further stage in the decomposition of 3b may involve the generation of an as yet unidentified Pd(II) complex with a methyl group and a bidentate NHC ligand derived from the initial reductive elimination shown in Scheme 2. Subsequent methyl-NHC reductive coupling from this species would generate the bis-2,3-dimethylimidazolium analogue of 1 with deposition of Pd black. Further studies to discover the mechanism by which this decomposition occurs are underway. It is likely that the relative thermal robustness of 3b is due to a reduced ability of the rotationally constrained MeIm rings to interact with the Pd– methyl group and undergo reductive elimination. Table 3 lists the results of the Heck coupling reactions in which 3a and 3b were tested as precatalysts.

123

Unexpectedly, since the active catalytic species derived from each precatalyst could reasonably be expected to be identical, 3a was consistently more active than 3b in the Heck coupling of 4-bromoacetophenone (BAP) and n-butyl acrylate (BA) when treated with hydrazine hydrate, in the absence of quaternary ammonium salt. Hydrazine hydrate had no effect on the activity of 3b as shown by the identical activities (within experimental error) of Entries 6a and 7. Unusually, the addition of 5 mmol of tetrapropylammonium bromide, previously reported to improve catalyst performance [11], boosted the catalytic activity of 3b (Entry 8), but had a slight inhibitory effect on that of 3a (Entry 4). The demonstrated activity of 3a and 3b in the coupling of BAP and BA at low catalyst loadings indicates that the CNC binding mode is compatible with catalytic activity in these reactions. The main product of the coupling reactions of BAP and BA was the desired n-butyl-(E) 4-acetylcinnamate, with the Z isomer not detected by 1 H NMR or GC –MS. However, the selectivity for the desired CC coupling product was decreased, with between 5 and 10% (by GC–MS) of other coupling products formed. A major and notable impurity identified by GC–MS is n-butyl 2-bromo-3-(p-acetylphenyl)propanoate, probably formed by reductive elimination from a PdBr(alkene insertion product) complex competing successfully with the b-hydride elimination process that gives the desired product. This is present in elevated quantities when compared to other systems [10], and is probably a reflection of the lack of vacant sites at Pd and a resultant reduced preference for

Table 3 Heck coupling reactions a of aryl halides and n-butyl acrylate catalysed by 3a and 3b

Entry

Aryl halide

Catalyst

Loading (mol%)

Time (h)

Yield b (%)

TON c

TOF d

1 e,g 2 e,g 3e 4 e,f,g 5 e,g

BAP BAP BAP BAP CBA

3a 3a 3a 3a 3a

1.02×10−3 1.03×10−3 9.80×10−3 9.80×10−4 1.01×10−2

20.3 66 21 20 100

41 72 100 35 11

40 160 70 140 10 180 35 400 1130

1980 1060 485 1770 11

6a e 6b e,g 7g 8 f,g

BAP

3b

1.01×10−3

BAP BAP

3b 3b

1.01×10−3 1.01×10−3

20 111 20 20

17 71 17 35

16 550 70 770 16 940 34 330

827 638 850 1720

a

Conditions as described in Section 2. Total yield (GC) of coupled products. c Mol(product)/mol(Pd). d Mol(product)/mol(Pd)/h. e Hydrazine hydrate (10–70 ml) added to run. f Pr4NBr (5 mmol) added to run. g Reactions halted before complete conversion. b

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D.J. Nielsen et al. / Inorganica Chimica Acta 327 (2002) 116–125

b-hydride elimination. The high thermal stability of 3b was reflected in its ongoing activity during extended runs at low catalyst loading in the Heck reaction at 120 °C. The Table 3 data shows that catalyst activity during the final 91 h of an extended run (Entry 6b) was reduced by an average of only 23% compared to the average activity during the initial 20 h (Entry 6a). The activity of precatalyst 3a decreased over time by a somewhat larger degree when Entries 1 and 2, representing independent runs, are compared. Here, the overall activity dropped by almost 50% as the reaction time was extended from 20.3 to 66 h. Complex 3a was also tested in the Heck reaction of 4-chlorobenzaldehyde and BA at the (relatively low) loading of 1.01× 10 − 2 mol%, but showed a conversion of only 11% even after extended reaction times (Entry 5).

4. Conclusions A dinuclear Ag(I) complex and two mononuclear Pd(II) complexes, including the first cationic Pd– hydrocarbyl example, of a pyridine functionalised bis-NHC ligand have been synthesised and fully characterised. The palladium complexes are isomorphous with the ligand binding in the C ‚ N ‚ C mode, while the Ag(I) complex shows the ligand bridging two silver atoms through the carbene donors with the pyridyl nitrogen remaining uncoordinated. The Pd– methyl complex exhibits relatively high thermal stability for a cationic Pd –hydrocarbyl NHC complex, which is attributed at least in part to the MeIm rings being constrained in their rotation to the Pd coordination plane. The Pd(II) complexes have shown good activity in a model Heck coupling reaction using activated aryl bromides, but were less effective when tested with an aryl chloride substrate. Work is continuing in the application of these systems as precatalysts in other CC bond forming reactions. The preparation, chemistry, and catalytic behaviour of a series of complexes of related functionalised bis-NHC ligands with varied hemilabile donor groups and a range of bulky N-substituents will be reported subsequently.

5. Supplementary material Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre, CCDC Nos. 162761– 162764 for 1·H2O, 2, 3a, and 3b, respectively. Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: + 44-

1223-336-033; e-mail: [email protected] or www: http://www.ccdc.cam.ac.uk).

Acknowledgements We thank the Australian Research Council for financial support, the Faculty of Science and Engineering, University of Tasmania for providing a scholarship for D.J.N., and Johnson-Matthey for a generous loan of Pd salts. We also thank the technical staff of Cardiff University, and staff of the Central Science Laboratory, University of Tasmania for valuable assistance and the use of instruments. References [1] A.J. Arduengo III, R.L. Harlow, M. Kline, J. Am. Chem. Soc. 113 (1991) 361. [2] T. Weskamp, V.P.W. Bohm, W.A. Herrmann, J. Organomet. Chem. 600 (2000) 12. [3] W.A. Herrmann, M. Elison, J. Fischer, C. Kocher, G.R.J. Artus, Chem. Eur. J. 2 (1996) 772. [4] C.J. Carmalt, A.H. Cowley, Adv. Inorg. Chem.: Main Group Chem. 50 (2000) 1. [5] D.S. McGuinness, M.J. Green, K.J. Cavell, B.W. Skelton, A.H. White, J. Organomet. Chem. 565 (1998) 165. [6] A. Furstner, O.R. Thiel, L. Ackermann, H.-J. Schanz, S.P. Nolan, J. Org. Chem. 65 (2000) 2204. [7] W.A. Herrmann, C.-P. Reisinger, M. Spiegler, J. Organomet. Chem. 557 (1998) 93. [8] M.G. Gardiner, W.A. Herrmann, C.-P. Reisinger, J. Schwarz, M. Spiegler, J. Organomet. Chem. 572 (1999) 239. [9] T. Weskamp, V.P.W. Bohm, W.A. Herrmann, J. Organomet. Chem. 585 (1999) 348. [10] D.S. McGuinness, K.J. Cavell, Organometallics 19 (2000) 741. [11] A.M. Magill, D.S. McGuinness, K.J. Cavell, G.J.P. Britovsek, V.C. Gibson, A.J.P. White, D.J. Williams, A.H. White, B.W. Skelton, J. Organomet. Chem. 617 – 618 (2001) 546. [12] A.A.D. Tulloch, A.A. Danopoulos, R.P. Tooze, S.M. Cafferkey, S. Kleinhenz, M.B. Hursthouse, Chem. Commun. (2000) 1247. [13] K.-W. Lee, J.C.C. Chen, I.J.B. Lin, J. Organomet. Chem. 617 – 618 (2001) 364. [14] W.A. Herrmann, C. Ko¨ cher, L.J. Goossen, R.J. Artus, Chem. Eur. J. 2 (1996) 1627. [15] W.A. Herrmann, L.J. Goossen, M. Spiegler, Organometallics 17 (1998) 2162. [16] J.C.C. Chen, I.J.B. Lin, Organometallics 19 (2000) 5113. [17] R.-Z. Ku, J.-C. Huang, J.-Y. Cho, F.-M. Kiang, K.J. Reddy, Y.-C. Chen, K.-J. Lee, J.-H. Lee, G.-H. Lee, S.-M. Peng, S.-T. Liu, Organometallics 18 (1999) 2145. [18] A. Caballero, E. Diez-Barra, F.A. Jalon, S. Merino, J. Tejeda, J. Organomet. Chem. 617 – 618 (2001) 395. [19] E. Peris, J.A. Loch, J. Mata, R.H. Crabtree, Chem. Commun. (2001) 201. [20] J.C.C. Chen, I.J.B. Lin, J. Chem. Soc., Dalton Trans. (2000) 839. [21] J.C. Garrison, R.S. Simons, J.M. Talley, C. Wesdemiotis, C.A. Tessier, W.J. Youngs, Organometallics 20 (2001) 1276. [22] D.S. McGuinness, K.J. Cavell, Organometallics 19 (2000) 4918.

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