Cationic Carbene Complexes of Platinum(IV): Structure of a

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Organometallics 1995,14, 1030-1038

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Cationic Carbene Complexes of Platinum(IV): Structure of a Secondary Carbene Complex Louis M. Rendina, Jagadese J. Vittal, and Richard J. Puddephatt” Department of Chemistry, The University of Western Ontario, London, Ontario, Canada N6A 5B7 Received September 29, 1994@ Treatment of the organoplatinum(I1) precursors [PtMeCl(tbu2bpy)l Pbu2bpy = 4,4’-di-tertbutyl-2,2’-bipyridine) and [PtMe2(tbu2bpy)lwith chloroiminium (Vilsmeier) salts affords the (l),[PtCLnovel cationic platinum(N)-carbene complexes [PtC12Me(CHNMe2)(tbu2bpy)lC1 (31, and [PtClMez(CClNMez)Me(CC1NMe2)(tbu2bpy)lCl(21, [PtC1Me2(CHNMe2)(tbu2bpy)lCl (tbu2bpy)]C1(4) in good yield. For 1 and 2, the carbene moiety is located trans to one of the chloro ligands, whereas in 3 and 4 it is found trans to the N-donor. The complexes show good stability toward air and moisture, but 1and 2 are thermally unstable and decompose to [PtC12(tbu2bpy)l aRer a few days at room temperature. Complex 4 can also be prepared as a mixture of two geometrical isomers which do not interconvert; it is proposed that the isomers are formed by two independent pathways. The major (4a) and minor (4b)isomers are formed by cis- and trans-oxidative addition, respectively, of [C12C=NMe21C1 to [PtMez(tbu~bpy)l,and likely mechanisms are discussed. Crystals of 1are monoclinic, space group P21/c, with a = 11.064(1) b = 21.310(2) c = 12.277(3) /3 = 91.20(2)”,2 = 4, and R = 0.0718. The structure of 1 consists of discrete [PtC12Me(CHNMe2)(tbu2bpy)l+ cations and chloride anions, with one CHzC12 molecule of crystallization per complex ion. The carbene ligand occupies a position trans to one of the chloro ligands, and the Pt-C,&e,, bond length is significantly shorter than the Pt-Me distance (2.13(2) The C-N bond (1.99(2) length of 1.31(2)8, is indicative of substantialp,-p, bonding. Reaction of nucleophiles with 3 leads to rapid reductive elimination to give a n iminium salt [H(X)C=NMe21Cl (X = nucleophile) and [PtMe2(tbu2bpy)].This work demonstrates the versatility of chloroiminium salts in the preparation of aminocarbene complexes of metals in high oxidation states.

A,

A)

A,

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A).

two traditional types of metal-carbene complexes, i.e., low-oxidation-state “Fischer-type” carbene complexes Transition-metal carbene complexes have played an with heteroatom substituents and high-oxidation-state important role in the development of organometallic “Schrock-type” alkylidene species with alkyl substituchemistry and find applications in many catalytic reacents.l tions as well as in stoichiometric organic synthesis.l Synthetic routes to platinum(IV)-carbene complexes Short-lived, cationic platinum-carbene species may have included (i) oxidative addition of Me1 or Cl2 to be involved in various insertion reactions, such as those various platinum(I1)-carbene c o m p l e ~ e s , ~(ii) ~ ~reac~~g involving diazo compounds.2 Indeed, some authors have tion of the alkyne HC4.XH2CH20H with cationic pointed out the apparent neglect in the study of cationic dimethylplatinum(IV1 species to give (alkoxycarbenelcarbene complexes of platinum and palladium, as those platinum(IV) complexes via intramolecular nucleophilic of the latter have been suggested as intermediates in attack of a reactive, transient z-alkyneplatinum(IV) the copolymerization of carbon monoxide and e t h ~ l e n e . ~ c o m p l e ~ and (iii)oxidative addition of organic , ~ ~finally ,~ carbene compounds, e.g., chloroiminium (Vilsmeier) Most mononuclear platinum-carbene complexes resalt^,^^,^ to platinum(I1) species. Method iii has been ported to date are derivatives of the metal in oxidation states 0 and +II.4 Very few platinum(N)-carbene applied successfully for the synthesis of carbene comcomplexes have been d e ~ c r i b e dand , ~ their chemistry plexes of several transition metals, but in platinum chemistry it is limited to the formation of [PtCl4has not been investigated in detail. Such complexes are of synthetic, structural, and theoretical interest because (CHNMez)PEt31 from [Pt2Cl2~-Cl)2(PEt3)21and [Clhigh oxidation state, late-transition-metal-carbenecomHC=NMe21Cl. The product was sparingly soluble and plexes would represent a subclass that lies between the was characterized by elemental analysis and IR and partial IH NMR spectroscopic data.5b,dThe Pt-CHNMe2 proton was observed a t 6 8.93 but no platinum coupling Abstract published in Advance ACS Abstracts, January 15, 1995. (1)(a)Dotz, K. H.; Fischer, H.; Hofmann, P.; Kreissl, F. R.; Schubert, was resolved. U.; Weiss, K. Transition Metal Carbene Complexes; Verlag Chemie: We now report the synthesis and complete characWeinheim, Germany, 1983.(b)Advances in Metal Curbene Chemistry; Schubert, U., Ed.; NATO AS1 Series C, Mathematical and Physical terization of some cationic platinum(IV1-carbene comSciences 269;Kluwer Academic Publishers: Dordrecht, The Netherplexes by oxidative addition of chloroiminium salts to lands, 1989.(c) Wulff, W. D. Adv. Met.-Org. Chem. 1989,1, 209. (d) the electron-rich organoplatinum(I1)precursor [PtMe&Brown, F.J. Prog. Znorg. Chem. 1980,27, 1. (e) Dotz, K. H.Angew. Chem., Znt. Ed. Engl. 1984,23,587. and bu2bpy)l (tbu2bpy= 4,4’-di-tert-butyl-2,2’-bipyridine) (2)McCrindle, R.;McAlees, A. J. Organometallics 1993,12,2445, its chloro analogue [PtC1Me(tbu2bpy)l. Cationic platiand references therein. (3)Batistini, A.; Consiglio, G. Organometallics 1992,11, 1766. num(IV)-carbene complexes are expected to behave as

Introduction

@

0276-733319512314-1030$09.00/00 1995 American Chemical Society

Cationic Carbene Complexes of Pt(IV)

highly reactive, transition-metal-stabilized carbonium ions. This paper also reports the addition of various nucleophiles to the carbene complexes prepared in this work.

Experimental Section All reactions were performed under a Nz atmosphere using standard Schlenk techniques. All solvents were freshly distilled, dried, and degassed prior to use. NMR spectra were recorded by means of Varian Gemini spectrometers ('H at 300.10 and 200.00 MHz; 13C a t 75.43 and 50.30 MHz). Chemical shifts are reported in ppm with respect to TMS reference. All spectra are referenced to the residual protons of the deuterated solvent. IR spectra (Nujol mull or CHzCl2 solution) were recorded in the range 4000-400 cm-l on a Perkin Elmer 2000 FT-IR instrument. Elemental analyses were determined by Guelph Chemical Laboratories, Guelph, Canada. The compounds tbu2bpy,7 [PtMe~(~buzbpy)l,~ and trans[PtC1Me(SMez)2I9were prepared by the literature methods. The chloroiminium salts, [ClHC=NMezICl and [C12C=NMe21(4)(a)For pre-1982references, see: Hartley, F. R. In Comprehensive Organometallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon: New York, 1982;Vol 6, p 502. (b) Michelin, R. A.; Bertani, R.; Mozzon, M.; Zanotto, L.; Benetello, F.; Bombieri, G. Organometallics 1990,9,1449. (c) Barefield, E. K.; Carrier, A. M.; Sepelak, D. J.; Van Derveer, D. G. Organometallics 1982,1,103. (d) Gaber, B.; Krueger, C.; Marczinke, B.; Mynott, R.; Wilke, G. Angew. Chem., Int. Ed. Engl. 1991,30,1666.(e) Sellmann, D.; Prechtel, W.; Knoch, F.; Moll, M. Znorg. Chem. 1993,32, 538. (f) Michelin, R. A.; Bertani, R.; Mozzon, M.; Bombieri, G.; Benetello, F.; Dasilva, M. D. C. G.; Pombeiro, A. J. L. Organometallics 1993,12,2372.(g) Fehlhammer, W. P.; Bliss, T.; Fucho, J.; Holzmann, G. 2,Naturforsch. B 1992,47, 79.(h) Ardvengo, A. J.,111; Gamper, S. F.; Calabrese, J. C.; Davidson, F. J . Am. Chem. SOC.1994,116,4391. (i) Erker, G.; Menjon, B. Chem. Ber. 1990,123,1327.(i)Hoover, J. F.; Stryker, J. M. J. Am. Chem. SOC.1990,112,464. (k) Pill, T.; Polborn, K; Beck, W. Chem. Ber. 1990, 123,ll.(1) Chen, J. T.; Tzeng, W. H.; Tsai, F. Y.; Cheng, M. C.; Yu, W. Organometallics 1991,10,3954. (m) Michelin, R. A.; Benetollo, F.; Chapuis, G.; Bombieri, G.; Guadalupi, G.; Ros, R. Znorg. Chem. 1989, 28, 840. (n) Cetini, G.; Bandini, A. L.; Banditelli, G.; Minghetti, G.; Operti, L.; Vaglio, G. A. Org. Mass Spectrom. 1989, 24, 479. (0) Nakamura, S.;Morokuma, K Organometallics 1988, 7 , 1904. (p) Ruegg, H. J.; Botteghi, C; Pregosin, P. S.; Scrivanti, A.; Toniolo, L. J. Organomet. Chem. 1986,316,233.(9)Facchin, G.;Campostrini, R.; Michelin, R. A. J.Organomet. Chem. 1985,294,C21.(r)Fehlhammer, W. P.; Volkl, A.; Plaia, U.; Bartel, K.; Liu, A. T. Chem. Ber. 1985,118, 2235. (s) Canziani, F.;Albinati, A.; Galimberti, F.; Ganazzoli, F.; Garlaschelli, L.; Malatesta, M. C. J. Chem. SOC.,Dalton Trans. 1983, 827.(t) Belluco, U.;Michelin, R. A.; Ros, R.; Bertani, R.; Facchin, G.; Mozzon, M.; Zanotto, L. Znorg. Chim. Acta 1992,198-200, 883 and references therein. (u) Crociani, B. In Reactions of Coordinated Ligands; Braterman, P. S., Ed.; Plenum: New York, 1986;Vol. 1, p 553.(v) Belluco, U.; Crociani, B.; Michelin, R. A.; Uguagliati, P. Pure Appl. Chem. 1983,55,47. (w) Lappert, M. F. J. Organomet. Chem. 1988,358,185and references therein. (x) Herdeis, C.; Beck, W. Chem. Ber. 1983,116,3205.(y) Cross, R. J.; Davidson, M. F.; Rocamora, M. J . Chem. SOC.,Dalton Trans. 1988,1147.(z) Bertani, R.; Mozzon, M.; Michelin, R. A.; Benetollo, F.; Bombieri, G.; Castilho, T. J.;Pombeiro, (aa)Michelin, R. A; Zanotto, A. J. L. Inorg. Chim. Acta 1991,189,175. L.; Braga, D.; Sabatino P.; Angelici, R. J. Znorg. Chem. 1988,27,85, 93.(bb) Zanotto, L.;Bertani, R.; Michelin, R. A. Znorg. Chem. 1990, 29,3265. (5)(a)Walker, R.;Muir, K W. J . Chem. SOC.,Dalton Trans. 1975, 272. (b) Hartshorn, A. J.; Lappert, M. F.; Turner, K J. Chem. SOC., Dalton Trans. 1978,348.(c) Chisholm, M. H.; Clark, H. C. J. Chem. Soc., Chem. Commun. 1971,1484.(d) Cetinkaya, B.; Lappert, M. F.; Turner, K.; J. Chem. SOC.,Chem. Commun. 1972,851. (e) Muir, K W.; Walker, R.; Chatt, J.; Richards, R. L.; Royston, G. H. D. J . Organomet. Chem. 1973,56,C30. (0 Balch, A. L. J. Organomet. Chem. 1972,37,C19. (g) Chisholm, M. H.; Clark, H. C.; Johns,W. S.; Ward, J. E. H.; Yasufuku, K. Znorg. Chem. 1975,14,900.(h) Clark, H. C.; Manzer, L. E. Znorg. Chem. 1973,12,362. (6)(a)Zminium Salts in Organic Chemistry, Parts 1 and 2; Bahme, H., Viehe, H. G., Eds.; Advances in Organic Chemistry: Materials and Results; Taylor, E. C., Series Ed.; Wiley: New York, 1979.(b) MethStanforth, S. P. In Comprehensive Organic Synthesis; Trost, Cohn, 0.; B. M., Fleming, I., Eds.; Pergamon: Oxford, U.K., 1991;Vol. 2,p 777. (c) Kantlehner, W. In Comprehensive Organic Synthesis; Trost, B. M., Fleming, I., Eds.; Pergamon: Oxford, U.K., 1991;Vol. 6, p 485. (d) Hartshorn, A. J.; Lappert, M. F.; Turner, K. J . Chem. Soc., Chem. Commun. 1975,929. (7)Badger, G. M.;Sasse, W. H. F. J. Chem. SOC.1956,616.

Organometallics, Vol. 14, No. 2, 1995 1031 C1, were obtained commercially. The salts were dried in uacuo over P z O at ~ room temperature prior to use. Preparation of Complexes. Chloro(4,4-di-tert-butyl-

2,2'-bipyridhe) (methyl)platinum@) ([PtClMe(%u2bpy)l).

To a stirred solution of trans-[PtClMe(SMe2)zl (1.17 g, 3.16 mmol) in diethyl ether (100 mL) was added 'buzbpy (0.850 g, 3.17 "01). A dark-yellow precipitate formed immediately on addition of the ligand, and the mixture was stirred for 16 h a t room temperature. Evaporation of the solvent in uacuo gave a yellow solid which was recrystallized from CHzCldn-hexane at -30 "C to afford bright-yellow microcrystals which were collected by filtration, washed with diethyl ether, and dried in uucuo over PzOs. Yield 1.20 g (74.0%). Anal. Calcd for CleH27ClNzPt: C, 44.40; H, 5.30; N, 5.45. Found: C, 44.01; H, 5.27; N, 5.31. NMR in CDzC12: 6('H) 9.34 [d, lH, 3 J p t ~= @ 13.0 Hz, 'JHW= 5.9 Hz, H61, 8.97 [d, 'H, 3 J = 60.0 ~ Hz; 3 J ~ =6 6.2 ~ Hz, H'], 7.97 [d, lH, 4J~57p = 1.7 Hz, H3' I, 7.94 = 5.9 Hz, [d, l H , 4 J ~ 3 ~=S 2.2 Hz, H31, 7.62 [dd, lH, 3J~.H5, 4Jp7-p,= 2.0 Hz, H'], 7.45 [dd, l H , 3J~5Hg = 6.2 Hz, 4 J ~ 5 = ~ 3 2.2 Hz, H61, 1.42 [s, 18H, 'bu], 0.99 [s, 3H, 2 J p t ~= 78.0 Hz, Pt-Me].

Dichloro(4,4'-di-tertt-butyl-2,2'-bipyridine)platinum(11) ([PtCl2('bu2bpy)l). To a stirred solution of K2[PtCL] (0.200 g, 0.482 mmol) in 6 M HCl(50 mL) was added 'buzbpy (0.130 g, 0.484 mmol). The mixture was refluxed for 6 h, and the bright-yellow precipitate (0.240 g, 93.2%)was isolated by filtration and washed with distilled water, ethanol, and diethyl ether. The solid was dried in uucuo over PzO5 at room temperature for several hours. Anal. Calcd for ClsH~4ClzNzPt: C, 40.46; H, 4.53; N, 5.24. Found: C, 40.55; H, 4.56; N, 5.22. N M R in CDCl3: WH) 9.45 [d, 2H, 3 J ~=66.3 ~ Hz, 3 J ~ = ca. 30 Hz (br), H61, 7.87 [d, 2H, 3 J ~ 3 = ~ 62.0 Hz, H31, 7.46 [dd, 2H, 4 J ~ s = ~ 62.0 HZ, 3 J ~ 6 = ~e 6.3 Hz, H51, 1.44 [S, 18H, "u].

Dichloro(dimethylaminomethy1ene)(I,l'-di-tert-butyl2,2'-bipyridine)(methyl)platinum(lV)chloride ([PtClme(CHNMe2)(%~2bpy)ICl, 1). To a cooled (-10 "C) and stirred solution of [ClHC=NMez]Cl (0.025 g, 0.195 mmol) in CHzCl2 (30 mL) was added [PtC1Me(tbu2bpy)](0.100 g, 0.195 mmol). The color of the solution immediately changed from yellow to pale yellow on addition of the complex. After 1h, the solvent was evaporated in uacuo to afford a pale-yellow powder (0.110 g, 88.0%). Recrystallizationof the solid is usually unnecessary, although it may be recrystallized from CHzCldn-pentane a t -30 "C to afford pale-yellow microcrystals. Microanalyses could not be obtained for the product owing to its facile transformation to [PtC12(tbuzbpy)]under ambient conditions. NMR in CD2C12: WH) 10.43 [br s, Pt-CHNMe21, 9.19 [d, 'H, 3 J ~=65.8 ~Hz, 3 J w = 8.1 Hz, H'], 9.02 [d, l H , 3 J ~ = 66.4 ~ ~ Hz, H6], 8.45 [d, lH, 4J~s7p,= 1.7 Hz, H3'], Hz, 3 J = 38.9 8.43 [d, lH, 4 J ~ 3 ~=6 2.1 Hz, H31, 7.85 [dd, lH, 3 J ~ 6 q p= 5.9 = 1.8 Hz, H"], 7.82 [dd, lH, 3J~6HB= 6.3 Hz, 4 J ~ 6 ~ Hz, 4J~.H3, = 2.1 Hz, H6], 3.77 [br d, 3H, 4 J = ~0.9 Hz, NMe], 3.62 [d, 0.8 Hz, 4Jm= 8.0 Hz, NMe], 2.45 [s, 3H, 2 J m = 3H, 4 J= ~ 64.8 Hz, Pt-Me], 1.48 [s, 9H, tbul, 1.47 [s, 9H, 'bu]; W3C) = 177.86 [VRC= 931.7 Hz, Pt-CHNMezI, 54.56 L3Jptc = 50.5 Hz, NMe], 46.46 [3Jpt~ = 28.1 Hz, NMel, 6.77 ['Jptc = 475.6 Hz, Pt-Me]. IR in CHzC12: v(C-N) 1638 cm-'.

Dichloro(chlorodimethylaminomethy1ene)($4-di-tertbutyl-2,2'-bipyridine)(methyl)platinum(lV)chloride ([PtCl~e(CClNMe~)('bu~bpy)lCl, 2). To a cooled (-10 "C) and uigorously stirred suspension of [Cl~C=NMe21C1(0.032 g, 0.197 mmol) in CHzClz (30 mL) was added [PtClMe('bu2bpy)] (0.100 g, 0.195 mmol). After ca. 1 h, the color of the solution had changed from bright to pale yellow. The solvent was then evaporated in uucuo to give a pale-yellow powder (0.100 g, 76.0%). Recrystallization of the product is usually unnecessary, although it may be recrystallized from CHzCldn-pentane (8) Achar, S.; Scott, J. D.; Vittal, J. J.; Puddephatt, R. J. Organometallics 1993,12,4592. (9)Scott, J. D.;Puddephatt, R. J. Organometallics 1983,2, 1643.

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1032 Organometallics, Vol. 14,No. 2, 1995

immediate color change from pale yellow to deep orange (or deep yellow) ensued, and the mixture was stirred for 1 h at room temperature. Evaporation of the solvent in vacuo gave a solid, which was extracted with an appropriate deuterated solvent (acetone-& or CDzClZ). The 'H NMR spectrum was then recorded immediately. X-ray Crystal Structure Determination. Light-yellow rodlike crystals of 1 were grown from CHzClz solution at room temperature. A long rod was cut into a suitable size (0.4 x 0.2 x 0.2 mm), wedged inside a Lindemann capillary tube, flame sealed, and used in the diffraction experiments. The density of the crystal was measured by the neutral buoyancy method. The diffraction experiments were carried out on a Siemens P4 difiactometer with the XSCANS software packagelo using graphite monochromated Mo Ka radiation at 23 "C. The cell constants were obtained by centering 25 highangle reflections (24.2 s 28 s 25.0"). The Laue symmetryZ/m was determined by merging symmetry-equivalent reflections. A total of 4649 reflections were collected in the 8 range 1.822.5" (-1s h 5 11, -1 s K 5 22, -13 II I13) in 8-28 scan mode at variable scan speeds (2-10 deg/min). Background measurements were made at the ends of the scan range. Three standard reflections were monitored at the end of every 197 reflections collected. Three standard reflections decayed t o 50% during the data collection. The data were corrected for absorption 01 = 5.2 mm-') using an empirical method involving scans of nine reflections (13.3" 28 < 20.8"). The maximum and minimum transmission factors were 0.114 and 0.087, respectively. The space group P21/c was determined from the systematic absences (h01,1 = 2n 1 and OKO, k = 2n 1). The data processing, solution, and the initial refinements were done using SHEIXTL-PC programs.ll The final refinements were performed using SHELXL-93 software programs.lZ Only 17 out of the 34 hydrogen atoms in the cationic molecule were located in the difference Fourier routine. The carbene H(2) atom could not be located. However, all the hydrogen atoms Chloro(chlorodimethylaminomethylene)(4,4-di-tert- were placed in the calculated positions and they were included butyl-2,2'-bipyridine)dimethylplatinum~ chloride ([R- for the purpose of structure factor calculations only. The isotropic thermal parameters of the hydrogen atoms were ClMea(CClNMe~)('buabpy)]Cl,4). To a cooled (-10 "C) and allowed to ride on the appropriate C atoms. Three different vigorously stirred suspension of [Cl~C=NMe~]Cl(0.066 g, 0.406 orientations of the disordered CHzClz solvate were located in mmol) in THF (30 mL) and CHzClz (15 mL) was added the difference Fourier routines (occupancies 0.4,0.35 and 0.25). [PtMe~(~buzbpy)] (0.200 g, 0.405 mmol). Over a period of A common C-C1 distance was refined in the least-squares several minutes, the color of the solution changed from bright cycles. No hydrogen atoms were included for the solvent orange t o pale yellow. After 2 h, the solvents were evaporated molecule. Isotropic thermal parameters were refined for all in uucuo to give a pale-yellow powder (0.210 g, 79.0%). the carbon atoms in the bipyridyl ring. In the least-squares Recrystallization of the product is usually unnecessary, alcycles, the anisotropic refinements of the thermal parameter though it may be recrystallized from CHzCldn-pentane at -30 of the C(1) carbon atom became "non-positive-definite" despite "C to afford very pale-yellow microcrystals. Anal. Calcd for applying a weak restraint, ISOR, and we could not find any Cz3H36C13N3Pt: C, 42.11; H, 5.53; N, 6.41. F o n d : C, 42.37; reason for this anomalous behavior. The thermal H, 5.91; N, 6.27. N M R in CDzClz (4a):6('H) 9.33 [d, 'H, 3 J ~ obvious ~ parameters of C(1) and C(2) atoms were refined isotropically, = 6.0 Hz, 3Jpt~6, = 11.2 Hz, H6'], 8.63 [m, 2H, H3' and H3], 8.61 and the rest in the cationic molecule were refined anisotropi[d, l H , 3 J ~ =6 6.3 ~ Hz, 3Jm6 = 24.1 Hz, H61, 7.89 [dd, lH, cally. In the final least-squares refinement cycles on F ,the 3J~67.p = 6.0 Hz, 4J~6@= 1.5 Hz, H'I, 7.77 [dd, 1H, 3J~5H6= model converged at R1= 0.0718, wR2 = 0.1425, and GOF = 6.0 Hz, 4J~5p = 1.9 Hz, H61, 4.25 [s, 3H, 4Jpt~ = 9.6 Hz, W e ] , 1.068 for 2311 observations with F, 2 40(F0) and 244 param= 8.8 Hz, NMe], 1.67 Is, 3H, ' J p t ~= 64.8 Hz, 4.03 [s, 3H, 4Jpt~ eters, and R1 = 0.1358, wR2 = 0.1723 for all 3776 data. In Pt-Me], 1.47 [s, 9H, tbu], 1.46 [s, 9H, tbu], 1.19 [s, 3H, Vpt~= the final difference Fourier synthesis, the electron density 67.3 Hz, Pt-Me]; 6(13C) 171.27 [lJptc = 1325.3 Hz, Ptfluctuated in the range 1.57-1.71 eA-3; of the top five peaks, CClNMez], 52.81 [3Jpt~ = 19.8 Hz, NMel, 50.66 L3Jptc = 23.7 four peaks were associated with the Pt atoms at distances Hz, NMe], 10.38 = 569.0 Hz, Pt-Me], 4.53 [lJptc= 568.9 1.03-1.12 A. The mean and the maximum shift/ESD in the Hz, Pt-Me]. IR in Nujol: v(C-N) 1575 cm-l. If the above final cycles were 0.000 and 0.008. reaction is performed in CHzClz solution only, the minor isomer (4b)is formed in a ca. 3:l ratio (at lower reaction temperatures (-80 "C), the ratio is ca. 53). NMR in CDzClz (4b): 6Results ('HI 8.87 [d, 2H, 3 J ~ 6=~6.0 6 Hz, 3JptH6 = 12.6 Hz, H6], 8.44 [d, 2H, 4 J ~ 5 = ~ 31.8Hz, H3], 7.79 [dd, 2H, 3 5 ~ 5 @ = 6.0 Hz, 4 J ~ 6 ~ 3 Oxidative addition of chloroiminium salts of the type = 1.8 Hz, H51, 3.73 [s, 3H, 4Jpt~ = 9.0 Hz, NMeI, 3.67 [s, 3H, [Cl(X)C=NMedCl (X = C1, H) to the uncharged plati4JptH = 8.8 Hz, NMeI, 1.81 [s, 6H, 2 J m = 64.5 Hz, Pt-Me], 1.49 [s, 18H, tbu]. (10)XSCANS; SiemensAnalytical X-ray InstrumentsInc.,Madison, WI. -, 1990. ~ -- Reactions with Nucleophiles. In a typical experiment, (11) Sheldrick, G.M. SHELXTL-PC Software; Siemens Analytical ca. 0.005 g of 1-4 was treated with approximately 1.5 mole X-ray Instruments Inc., Madison, WI,1990. equiv of nucleophile (NaBH4, NaOH, NaOMe, or LiMe) in 10 (12) Sheldrick, G.M. SHELXL-93; J. Appl. Crystallogr., in preparamL of an appropriate solvent (acetone, CH2C12, or THF). An tion. at -30 "C to afford pale-yellow microcrystals. Microanalyses could not be obtained for the product owing to its facile transformation to [PtClz('buzbpy)] under ambient conditions. ~ Hz, 3 J ~ = 6 NMR in CDzClz: 6('H) 9.27 [d, 'H, 3 J ~ =6 6.4 39.6 Hz, H6], 9.15 [d, lH, 3 J ~ 5 @= 5.8 Hz, 3Jpt~@ = 9.0 Hz, H6'], 8.49 [d, 1H, 4J~3@ = 1.7 Hz, H3'l, 8.46 [d, 1H, 4 J ~ 3 ~=6 2.3 Hz, H3], 7.82 [m, 2H, H6' and H61, 4.07 [s, 3H, 4Jpt~ = 8.0 Hz, W e ] , 3.79 [s, 3H, 4Jpt~ = 7.7 Hz, NMel, 2.83 [s, 3H, 'JRH = 65.4 Hz, Pt-Me], 1.51 [s, 9H, "u], 1.50 [s, 9H, tbul; 6(13C) 162.58 [lJptc = 1198.4 Hz, Pt-CClNMez], 51.92 L3Jptc = 23.9 Hz,NMe], 51.21 [3Jpt~= 18.4 Hz, NMel, 9.79 [lJptc = 455.8 Hz, Pt-Me]. IR in CHzClZ: v(C-N) 1594 cm-l. Chloro(dimethylaminomethy1ene)(4,4-di-tert-butyl2,2'-bipyridine)dimethylplatinum(Iv)chloride ([RCIMez(CHNMez)(%uzbpy)lCl, 3). To a cooled (-10 "C) and stirred solution of [ClHC=NMe2]Cl (0.026 g, 0.203 mmol) in CHzClz (30 mL) was added [PtMe~(~buzbpy)l (0.100 g, 0.203 "01). The color of the solution immediately changed from bright orange to pale yellow. After 1h, the solvent was evaporated in uucuo to give a pale-yellow powder (0.110 g, 87.3%). Recrystallization of the product is usually unnecessary, although it may be recrystallized from CHzCldn-pentane at -30 "C to afford very pale-yellow microcrystals. Anal. Calcd for C23H37ClzN3Pt. 0.5CHzC12: C, 42.51; H, 5.77; N, 6.33. Found: C, 42.96; H, = 22.5 6.15; N, 6.41. NMR in CDzClZ: WH) 10.81 [s, 'JRH = 6.0 Hz, 3 J ~ =* 12.2 Hz, Pt-CHNMez], 9.47 [d, lH, 3J~5@, ~ Hz, 3Jm6 = 19.1 Hz, H61, Hz, H6'l, 8.61 [d, IH, 3 J ~ =5 6.1 8.26 [d, 1H, 4J~37p=1.9 Hz, H'], 8.25 [d, 1H, 4 5 ~ 3 = ~ 62.0 Hz, ~" 6.0 Hz, 4 J ~ 5 ~ = 3 , 1.9 Hz, H"], 7.74 H31, 7.90 [dd, 1H, 3 J ~ 5 , = [dd, l H , 3JH5H6 = 6.1 Hz, 4 J ~ 6 = ~ 32.0 Hz, H51, 4.11 [br s, 3H, NMe], 3.94 [s, 3H, 4Jpt~ = 8.8 Hz, NMel, 1.47 [s, 9H, tbul, 1.46 [s, 9H, tbu], 1.41 [s, 3H, ' J p t ~= 64.6 Hz, Pt-Me], 1.10 [s, 3H, 2 J m= 67.8 Hz, Pt-Me]; 6(13C)185.02 [lJptc = 1040.2 Hz, PtCHNMez], 54.69 [3Jpt~= 57.7 Hz, NMel, 47.16 L3Jptc = 30.9 Hz, NMe], 2.08 ['Jptc = 581.1 Hz, Pt-Me], -2.18 [lJptc = 583.6 Hz, Pt-Me]. IR in Nujol: v(C-N) 1628 cm-'.

+

+

Cationic Carbene Complexes of Pt(N)

Organometallics, Vol. 14, No. 2,1995 1033

Pt-Meb

11

-Med

Pt-Me"

\

200

-

180

ii

40

Figure 1. l3C(lH}N M R spectrum of 3 in CDZC12. The quintet centered at 6 53.8 is due to the solvent. num(I1) precursors [PtXMe(tbuzbpy)l (X = Me, C1) affords the novel cationic platinum(IV)-carbene complexes 1-4 in good yield. The structures of the com-

r

l+ cl-

L

-I

X-H;Y-CI(l)

-

x = Y CI(2)

- -

X C1; Y Me (4b)

J

L

X = H (3) X = CI (4a)

plexes are presented in Figure 1. The presence of tertbutyl substituents on the 2,2'-bipyridine ligand greatly assists in the solubility of the cationic complexes in common organic solvents such as acetone and CHzClZ. All the complexes are reasonably air- and moisturestable at room temperature, although 1 and 2 are thermally unstable and are transformed to bright yellow [PtCl~(~buzbpy)l on standing for a few days, even in the solid state and in the absence of light. Even though up t o four stable, geometrical isomers are feasible for 1 and 2,only one was observed in each case. For 3, only one of three possible isomers was formed, but for 4,two isomers were often formed. In each case, 4a was the major isomer and the ratios of

4a to 4b when the reaction was carried out in different conditions were 3:l (CHzC12 solvent, -10 "C), 5:3 (CH2Clz solvent, -80 "C), and >9:1 (THF/CHzClz solvent, -10 "C). No interconversion between 4a and the minor isomer 4b occurs. For example, the isomeric composition of the product obtained in THF/CH&lZ does not change when it is isolated and redissolved in CHzClz. The NMR PH and 13C{lH}) spectroscopic data for complexes 1-4 are presented in Table 1. The decrease in magnitude of z J m for the methylplatinum signals of the products 1-4 compared with the platinum(I1) precursors is consistent with oxidation of the platinum center from +I1 to +IV and is a function of the diminished s-character of dzsp3-hybridizedplatinum(IV) compared t o dsp2-hybridized platinum(II).13 Furthermore, the magnitude of z J m depends on the nature of the ligand trans to the methyl group and is a sensitive probe of stereo~hemistry.'~~ For 1 and 2,the Pt-Me resonance is shifted considerand ably t o lower field than that of CPtCIMe(fbu~bpy)l, the magnitude of z J m (ca. 65 Hz) is consistent with the methyl ligand being trans to either the chloro or the N-donor ligand.13 The X-ray structure of 1 (vide infra) shows that the methyl group is trans to the N-donor ligand, and since the NMR spectroscopic data of 1 and 2 are very similar, it is almost certain that they have the same stereochemistry. In the lH NMR spectra of 3 and 4a, there are two distinct Pt-Me signals for each complex. The magnitude of 2 J p t ~decreases significantly from 86.0 Hz in [PtMe~(~buzbpy)l~ to ca. 65 Hz for the low-field signal and ca. 68 Hz for the high field signal; the latter is assigned to the methyl group trans to the chloro ligand.13 The methyl group cannot be trans to the carbene ligand since a significantly lower value of z J p t ~ (ca. 48 Hd5gPhwould be expected for the Pt-Me resonance of such a group. The presence of six distinct ~

~~

~~~

(13) (a) Pregosin, P. S.; Omura, H.; Venanzi, L. M. J. Am. Chem. Soc. 1973,95,2047.(b)Appleton, T. G.;Hall, J. R. Inorg. Chem. 1971, 10, 1717. (e) Anderson, C. M.; Crespo, M.; Jennings, M. C.; Lough, A. J.; Ferguson, G.;hddephatt, R. J. Organometallics 1991, 10, 2672. (14)Chisholm, M. H.; Clark, H. C.; Ward, J. E. H.; Yasufuku, K Inorg. Chem. 1976, 14, 893.

1034 Organometallics, Vol.14,No.2, 1995 aromatic signals in the lH NMR spectra of complexes 3 and 4a shows unequivocally the unsymmetrical nature of the two complexes, and the remaining possible (symmetrical) stereoisomer can be ruled out for these complexes. The IH NMR spectrum of 4b shows three distinct aromatic resonances and only one Pt-Me resonance, thus proving the stereochemistry with the chloro ligand and carbene ligand mutually trans. Crystals of either 3 or 4 could be obtained but they were not suitable for a X-ray structure determination. The lH NMR spectra of 1 - 4 display two sharp N-methyl signals of approximately equal intensity a t 6 ca. 3.6-4.3, attributed to the N-methyl groups, which are nonequivalent due to the restricted rotation about the Ccarbene-N partial double bond. This is a characteristic feature of the room-temperature NMR spectra of many other transition-metal-aminocarbene complexes15 and organic amides.16 Indeed, the X-ray structure of 1 and IR data for complexes 1-4 (vide infra) confirm the high Ccarbene-N bond order. The barrier t o rotation of the N-methyl groups about the Ccarbene-N bond must be high, since the two signals are sharp at room temperature. High-temperature lH NMR spectra of 3 (to 60 "C in acetonitrile-ds) show no appreciable change in the line shape of the two signals. Interestingly, in the rhodium(II1)-aminocmbene species CRhC13(CHNMe2)(PEt3)21, no coalescence or change of line shape was observed up to 150 "C in a-bromonaphthalene.17 The secondary carbene complexes 1 and 3 show a distinct low-field signal a t 6 ca. 10.5 attributed to the hydrogen substituent on the carbene carbon atom. The position of the resonance is consistent with the electrophilic nature of the carbene carbon atom and is similar to that observed for the electrophilic CH in the iminium salt [ClHC=NMezICl (6 10.83 in CDCls), and other secondary-carbene complexes, e.g., trans-[PtCl(CHNHp-tolyl)(PEt&-lCl (6 11.45 in CDC13)l8 and [Rh13(CHNMe2)(PEt3)21 (6 11.09 in CDC13).l7 For 1, the signal is somewhat broadened, indicative of unresolved Ig5Ptsatellite signals. For 3,the resonance is flanked by well-resolved lg5Ptsatellite signals, thus confirming is ca. 0.7 times the assignment. The magnitude of 2Jpt~ that of typical cationic platinum(I1)-secondary carbene complexes, e.g., trans-[Pt(CN-p-tolyl)(CHNH-p-tolyl)(PEt3)21(C104)2C2Jpt~ = 33 Hz),18 demonstrating the diminished s-character of the platinum(IV1 hybrid orbital used in bonding to the carbene ligand. The 13Cj1H}NMR spectra of 1-4 support the structures assigned to the complexes on the basis of lH NMR spectroscopy. The 13C(lH} NMR spectrum of 3 is presented in Figure 1. As expected, the carbene carbon atom appears at low field (6 = 163-185) and is flanked by lg5Ptsatellite signals (IJptc = 932-1325 Hz). The magnitude of Vptc is considerably larger than that of the Pt-Me resonance(s) (l&c = 456-584 Hz), indicating strong Pt-Ccwbene bonding. Furthermore, both the (15)(a) Cardin, D. J.; Cetinkaya, B.; Doyle, M. J.; Lappert, M. F. Chem. SOC.Rev. 1973,2,99.(b) Cotton, F.A.; Lukehart, C. M. h o g . Znorg. Chem. 1972,16,487.(c) Cardin, D. J.; Cetinkaya, B.; Lappert, M. F. Chem. Rev. 1972,72,545. (16)Robin, M. B.; Bovey, F. A.; Basch, H. In The Chemistry of Amides; Zabicky, J., Ed.; Wiley: New York, 1970;p 1. (17)Cetinkaya, B.;Lappert, M. F.; McLaughlin, G. M.; Turner, K. J . Chem. SOC.,Dalton Trans. 1974,1591. (18)Christian, D. F.;Clark, H. C.; Stepaniak, R. F. J . Orgunomet. Chem. 1976,112,227.

Rendina et al. natures of the carbene substituents and the trans ligand have a marked influence on the magnitude of IJptc, as has been observed in other systems.14J9The 13C shielding of the Ccarbne atom in 1-4 is considerably higher than that usually observed for typical "Fischer-type" carbene complexes,15although it is comparable to that observed for the iridium(II1) species [1rCl~(CO)(PPh3)2(CHNMe2)ICl (6 = 192.7 in DMSO-d6)5band the platinum(I1) complexes trans-[PtC12(PnBu3){C(NMeCH2)2}1 (6 = 196.5 in CDC13)1gcand [PtH(C(NCH&HzCH2}NHp-MeOCsH4)(dppe)lBF4(6 = 194.5 in C D ~ C ~ Z ) . ~ ~ For complexes 1-4,the lJptc values associated with the Pt-Cewbnesignals are of similar magnitude to those of cationic platinum(I1)-aminocarbene species, e.g., truns-[PtCl(AsMes)2jC(Me)(NHMe)}lPFs PJptc = 1047 Hz in CD2C12),14trans-[PtC1(~Me,)z(C(Me)(NMe2)}lPFs (lJptc = 1070 Hz in acetone-ds),14and [PtH(C{NCH2CH2CH2}NH-p-MeOC6H4)(dppe)lBF4 (lJptc = 1134 Hz in C D Z C ~ This ~ ) . ~is~unexpected since there is usually a marked decrease in the magnitude of lJptc for platinum(1V) compared to platinum(I1) complexes (vide supra). Complexes 1 and 2 show one Pt-Me resonance at high field in their respective I3Cj1H) NMR spectra, but for complexes 3 and 4a,two distinct Pt-Me signals are evident, again supporting the assigned stereochemistry. Interestingly, the magnitude of lJptc for those signals corresponding to a methyl group trans t o the N-donor (456-581 Hz) is lower than that reported for other cationic platinum(IV)-methyl complexes, e.g., [PtMes(pz3[PtMes(py3CH)IPFs (pz = 1-pyrazolyl; lJptc = 688 HZ),~O and [PtMes{(Me2pz)&Hz}CH)IPFs (lJptc = 688 Hz),~O (py)]PFs (lJptc = 669 and 698 H Z ) . ~ It ~appears that the chloro and carbene ligands in 1-4 play a significant electronic role in lowering the magnitude of lJptc for the Pt-Me signal(s1. Furthermore, increasing the number of chloro ligands about the metal center significantly decreases the value of lJptc. As observed in the IH NMR spectra of 1-4, two distinct N-methyl resonances surrounded by Ig5Ptsatellite signals are also found in each of the 13C{IH} NMR spectra, again showing the magnetic nonequivalence of the N-methyl groups due to restricted rotation about the Ccarbene-N bond. In confor the two N-methyl group resonances, trast to 4Jpt~ differ significantly for each complex, the those of 3Jpt~ larger being assigned t o the N-methyl group that is located syn t o the platinum atom.14 The IR spectra of complexes 1-4 (Table 1) show a characteristic v(C-N) band a t 1630-1575 cm-l, each being lower in frequency compared to that of the precursor iminium salts (1667 (Nujol) and 1663 (CH2Cl2) cm-l for [ClHC=NMe21Cl, and 1630 (Nujol) and 1626 (CHzCld cm-I for [C12C=NMe21Cl). The v(C-N) band in the IR spectra of 1 and 3 appears in the range found for other secondary aminocarbene complexes, e.g., [RhC13(CO)(CHNMe2)(PPh3)1(1615 ~ m - ~ ) and " cis[PtC12(CHNMe2)(PPh3)1(1612 cm-1).4c For complexes 2 and 4,the position of the v(C-N) band is comparable to other chlorocarbene complexes, e.g., [IrC13(CClNMe2)(19)(a) Chisholm, M. H.; Clark, H. C.; Manzer, L. E.; Stothers, J . (b) Chisholm, M. B.; Ward, J. E. H. J . Am. Chem. SOC.1973,95,8574. H.; Clark, H. C.; Manzer, L. E.; Stothers, J. B. J . Chem. SOC.,Chem. Commun. 1971,1627.(c) Cardin, D. J.; Cetinkaya, B.; Cetinkaya, E.; Lappert, M. F.; Randall, E. W.; Rosenberg, E. J . Chem. SOC.,Dalton Trans. 1973,1982. (20)Clark, H.C.; Ferguson, G.; Jain, V. &; Parvez, M. J. Organomet. Chem. 1984,270,365.

Organometallics, Vol. 14, No. 2, 1995 1035

Cationic Carbene Complexes of Pt(N)

c3

Figure 2. Molecular structure of the [PtClzMe(CHNMez)(tbuzbpy)]+ion (1).Only the carbene hydrogen atom H(2), placed in its calculated position, is shown. Thermal ellipsoids are represented at the 50% level. Table 2. Summary of Crystallographic Data for [PtC12Me(CHNMe2)('Buzbpy)Cl~HzClz (1) chem formula fw space group

t% deg

CZZH~~C~~N~FWHZC~~ 726.90 P21/c 11.064(1) 21.310(2) 12.277(3) 2894.0(7) 9 1.20(2)

Z

4

a, A

b, A c, A

v,A3

T, K

Sb

296 1.667 1.65(5) 5.2 1432 0.0718 1.068

R = cllFol

- lFcl)l/EIFol.S = goodness of fit = [cw(Fo2- Fcz)z/

ecalc,g

cm-'

@ob% g

cm-'

IC,

"-'

F(OW Ra

(n - p)]1'2.where n is the number of unique reflections and p the number

of parameters.

(PPh3)21 (1538 cm-1)21 and [RhCL(CClNMez)(PPh&I (1591 cm-1).22 On the basis of IR spectroscopic data, the C=N bond order decreases in the order [ClHC=NMe2lC1 > [Cl2C=NMez]Cl > 1 PZ 3 > 2 PZ 4 and is interaction on consistent with a diminished C-Np,-p, coordination of the iminium fragment to the platinum(IV)center. The X-ray crystal structure of 1 consists of discrete [F'tC12Me(CHNMe2)(tbu2bpy)l+cations and chloride anions, with one CH2C12 molecule of crystallization per complex ion. The molecular structure of the cation is presented in Figure 2 with the atom-labeling scheme. A summary of crystallographic data, important bond geometries, and atomic coordinates of the non-hydrogen atoms for 1 is presented in Tables 2, 3 and 4, respectively. The platinum atom is coordinated to a methyl group, two chlorine atoms, a chelating tbu2bpy ligand, and the (dimethy1amino)methylene moiety in an octahedral geometry. The carbene ligand and one of the chlorine atoms occupy a mutually trans arrangement, with the remaining chlorine atom located trans to one of the N-atoms of the tbu2bpy ligand. The arrangement of ligands about the platinum center is marginally dis(21)Clark, G . R.; Roper, W. R Wright, A. H. J. Organomet. Chem. 1982,236,C7. (22) Hartshorn, A. J.; Lappert, M. F.;Turner, K. J. Chem. Soc., Chem. Commun. 1975, 929.

1036 Organometallics, Vol. 14,No. 2, 1995

Rendina et al.

Table 3. Bond Lengths (A) and Angles (deg) for 1 Pt(l)-Cl(l) Pt(1)-N(l) Pt(1)-C(l) N(l)-C(5) N(2)-C(14) N(3)-C(2) N(3)-C(4) C(6)-C(7) C(7)-C(15) C(9)-C(10)

2.378(5) 2.120(16) 2.129(15) 1.34(2) 1.32(2) 1.31(2) 1.49(2) 1.38(3) 1.48(3) 1.46(2)

C1(2)-Pt(l)-Cl(l) N(2)-Pt(l)-Cl(l) C(2)-Pt(l)-Cl(l) N(2)-Pt(l)-C1(2) C(2)-Pt(l)-C1(2) N(2)-Pt( 1)-N( 1) C(2)-Pt( 1)-C( 1) N(1)-Pt(1)-C(1) C(9)-N(l)-Pt(l) C(14)-N(2)-Pt(l) C(14)-N(2)-C(10) C(2)-N(3)-C(4) N(3)-C(2)-Pt(l) C(7) -C(6) -C( 5 ) C(6)-C(7)-C(15) C(9)-C(8)-C(7) N(l)-C(9)-C(lO) C(ll)-C(lO)-N(2) N(2)-C(lO)-C(9) C(ll)-C(l2)-C(l3) C(13)-C(12)-C(19) N(2)-C(14)-C(13) C(7)-C(15)-C(16) C(7)-C(15)-C(18) C(16)-C(15)-C(18) C( 12)-C( 19)-C(22) C(12)-C(19)-C(21) C(22)-C(19)-C(21)

C(ll)-C(12) C(12)-C(19) C(15)-C(17) C(15)-C(18) C(19)-C(22) Pt(1)-Cl(2) Pt(1)-N(2) Pt(1)-C(2) N(l)-C(9) N(2)-C(10) 89.5(2) 87.7(4) 174.2(6) 175.7(4) 95.0(7) 78.7(6) 87.6(7) 175.8(7) 113.9(11) 126.1(14) 116(2) 120(2) 131(2) 124(2) 123(2) 120(2) 117(2) 120(2) 113(2) 115(2) 124(2) 125(2) 108(2) 115(2) 107.0(14) 113(2) llO(2) 108(2)

1.39(2) 1.47(3) 1.51(2) 1.55(2) 1.54(2) 2.289(5) 2.030(19) 1.991(21) 1.38(2) 1.42(2)

N(3)-C(3) C(5)-C(6) C(7)-C(8) C(8)-C(9) C(lO)-C(ll) C(12)-C(13) C(13)-C(14) C(15)-C(16) C(19)-C(20) C(19)-C(21)

N(1)-Pt(1)-Cl(1) C(1)-Pt(1)-Cl(1) N(1)-Pt(1)-Cl(2) C(l)-Pt(l)-Cl(2) C(2)-Pt( 1)-N(l) C(2)-Pt( 1)-N(2) N(2)-Pt(l)-C(l) C(S)-N(l)-Pt(l) C(5)-N(l)-C(9) C(lO)-N(2)-Pt(l) C(2)-N(3)-C(3) C(3)-N(3)-C(4) N(l)-C(5)-C(6) C(6)- C(7)-C( 8) C(8)-C(7)-C(15) N(l)-C(9)-C(8) C(8)-C(9)-C(lO) C(ll)-C(lO)-C(9) C(lO)-C(ll)-C(l2) C(l l)-C(l2)-C(19) C(14)-C(13)-C(12) C(7)-C(15)-C(17) C(17)-C(15)-C(16) C(17)-C(15)-C(18) C(12)-C(19)-C(20) C(20) -C( 19)-C( 22) C(2O)-C(19)-C(21)

1.45(2) 1.37(3) 1.41(2) 1.39(2) 1.35(2) 1.40(3) 1.35(3) 1.54(2) 1.53(2) 1.54(2)

89.7(4) 89.1(5) 98.0(5) 86.0(5) 93.3(7) 88.0(8) 97.2(6) 127.8(15) 118(2) 117.6(11) 128(2) 112(2) 121(2) 115(2) 122(2) 121(2) 122(2) 127(2) 123(2) 120(2) 120(2) 110.3(14) 108.0(14) 108.0(13) 11l(2) 107.1( 14) 108(2)

torted from an ideal octahedron; the C(B)-Pt-CI(l) angle encompassing the carbene ligand and transchlorine is 174.18(64)'. Otherwise, there are no significant deviations within the coordination sphere. The Pt-Me bond length (2.129(15)A) is close to that of other a-alkylplatinum(IV) complexes containing a trans N-donor ligand, e.g., [PtClMez{Me2NCH2CHzN-CHCsH4Cl)I (2.068(15) All3 and [PtIMez(SiMes)(bpy)l (2.096(18) and 2.120(16) A).23 The two Pt-N bond lengths (2.120(16) and 2.030(19) A) differ significantly and conclusively demonstrate the much greater trans influence exerted by the methyl group over the chloro ligand, the former being close t o Pt-N bond lengths in other a-alkylplatinum(IV)-bpy complexes, e.g., [PtBrMez(CHzC02Me)(bpy)l (2.165(4) and 2.159(4) A>s and [PtIMez(SiMe3)(bpy)l (2.186(2) and 2.16(1) A).23 The two Pt-C1 bond lengths (2.289(5)and 2.378(5) A) also differ significantly and show the greater trans influence of the carbene ligand over the N-donor. The trans influence of the carbene ligand is somewhat less than that observed for a o-alkyl ligand, e.g., [PtCIM ~ ~ { ( ~ Z ) ~ C R C H ~ - N ,(R N '= , CMe, ) I 2.421(3) A; R = CH2C1, 2.443(2) A).24 The Pt-Ccarbene distance is 1.991(21)A and is similar t o the value determined for the only other crystallographically determined example of a platinum(IV)carbene complex, [PtC12{C(NHMe)(NHCsH4Cl))(PEt3)2]clod (1.973(11) A).5a The Pt-Ccarbene bond length is (23) Levy, C.J.; Puddephatt, R. J.; Vittal, J. J. Organometallics

1994, 13, 1559.

(24) Canty, A. J.; Honeyman, R. T.; Skelton, B. W.; White, A. H. J. Organomet. Chem. 1990, 389, 277.

Table 4. Selected Atomic Coordinates ( x lo4) and Equivalent Isotropic Displacement Parameters (A2x 103) for 10 atom

X

Y

Z

Pt( 1) Cl(1) CK2) N(l) N(2) N(3) C(1) C(2) (33) C(4) C(5) C(6) C(7) C(8) C(9) C(10) C(11) C(12) C(13) C( 14) C(15) C(16) C(17) C(18) C(19) C(20) C(21) C(22) Cl(3)

2467.3(9) 3436(5) 830(5) 3457(17) 3997(17) 1181(16) 1594(17) 1727(22) 901(23) 773(23) 3185(20) 3890(20) 4924(20) 5238(19) 4527(18) 4873(18) 5916(18) 6219(20) 5387(20) 4332( 17) 5620(17) 6933(20) 5646(21) 5149(24) 7370(21) 7381(23) 8423(18) 7606(21) 2231(6)

7345.8(3) 795 l(2) 7224(3) 6546(6) 7434(6) 6353(7) 8175(7) 6900(9) 5927(9) 6 108(10) 6145(8) 5629(8) 549 l(8) 5930(7) 6458(7) 6950(7) 6989(7) 7508(7) 8000(9) 7935(8) 4903(8) 5080( 10) 4525(8) 4470( 10) 7523(6) 7025(9) 7386(10) 8159(8) 4425(3)

1204.4(6) 2596(4) 2294(4) 1733(12) 334(12) -138(14) 625(13) -69(17) 741( 16) - 1224(15) 2536( 14) 2746( 15) 2192( 14) 1382(13) 1194(13) 434( 13) - 118(13) -743( 14) -731(15) -210(12) 2366( 12) 2682(17) 1323(13) 3282(16) - 1324(15) -2223 15) -523(16) - 1869(18) 432(6)

35.6(3) 52(1) 63(2) 50(5) 43(5) 530) 33(4) 57(6) 64U) 70) 450) 490) 41(5) 390) 33(4) 33(4) 3x41 39(5) 500) 31(4) 46(3 99( 11) 66(8) 107(12) 43(6) 7x7) 77(7) 81(8) 83(2)

U(eq) is defined as one-third of the trace of the orthogonalized U, tensor.

significantly shorter than the Pt-Me distance, and differences in the covalent radii of sp2-and sp3-hybridized carbon atoms (ca. 0.04 A)25would play a significant role here. Even though d,-p, bonding might also contribute to the short Pt-Ccarbene bond, the high oxidation state of platinum should lead to only weak x-bonding. The atoms Pt, C(2), N(3), C(3), and C(4) lie approximately in the same plane, thus confirming the sp2hybridized nature of both the nitrogen and carbene carbon atoms. The Pt-C(2)-N(3) bond angle (131(2)") is significantly greater than the ideal 120', although it is not as large as the distortion found in the secondary carbene complexes [RhC13(CHNMez)(PEt3)21 (139.6(9)')22 and [RuIz(CO)(CHNMe(p-MeCsH4))(CNpMeCsH4)(PPhdI (141.5(5)")26and is comparable to other mononuclear, transition-metal-secondary aminocarbene complexes, e.g., [Fe(Cp)(CO)({CHNMe)zBH2)1 (124.1(10)"and 130.4(11)0),27 cis-[PtC12(CHNMe2)(PPh3)1 (129.4(8)"),kand mer,trans-[Mn(C0)3(PPh&(CHNHCH2CsHdICF3S03 (131.6(3)0).28The tbu2bpy ligand is slightly twisted; the angle between the mean planes defining the two pyridine-like rings being 11.96'. The very short C(2)-N(3) distance (1.31(2) A) is indicative of a bond order greater than 1 and is attributed to extensive pn-pn bonding between the two atoms. This conclusion is supported by spectroscopic (NMR and IR) data. The Ccarbene-N bond length is (25) Cotton, F. A.; Wilkinson, G.; Gaus, P. L. Basic Inorganic Chemistry, 2nd ed.; Wiley: New York, 1987; p 93. (26) Clark, G. R. J. Organomet. Chem. 1977, 134, 51. (27) Butler, W. M.; Enemark, J. H. J. Organomet. Chem. 1973,49, 233. (28)Gibson, D. H.; Mandal, S. K.; Owens, K.; Richardson, J. F. Organometallics 1990, 9, 1936.

Cationic Carbene Complexes of Pt(N)

Organometallics, Vol. 14, No. 2, 1995 1037

Scheme 1. Proposed Mechanism for the Reductive Elimination Reaction on Treatment of 3 with a Nucleophile MX (M = Na, X = OMe, OH, H; M = Li, X = Me)

..

r

I

rt-cp

,me2 X

(1)

-

0 Pt-c\

//NMe2

X

(11)

-

I.'"

1

Pt=C

\X

(m)

Figure 3. Resonance stabilization of the platinum(IV1carbene bond (X = H, Cl). Other ligands have been omitted for clarity.

Mx

r

I

-Ma

1 5

reactions are accompanied by formation of other unidentified platinum complexes. Complexes 1-4 do not react with nucleophiles such as HzO or with electrophiles such as CF3COOH. Furthermore, no exchange of the carbene H atom in l or 3 by deuterium is observed when the complexes are treated with DzO, and this is reminiscent of the chemistry of other secondary carbene complexes and organic amides.16vz2

Discussion

similar to that found in other transition-metal-secondary carbene complexes, e.g., [RhC13(CHNMez)(PEt&l (1.289(14) AIz2 and [Fe(Cp)(CO)({CHNMe}zBH2)1 (1.293(17) and 1.337(16) A) 27 and in the iminium salt [MezC=NMezIC104 (1.3O2hz9 The N-methyl bond lengths (1.45(2) and 1.49(2)A) are very similar to those found in [RhC4(CHNMez)(PEt3)21 (1.455(17) and 1.497(19)A),22cis-[PtClz(CHNMez)(PPh3)1 (1.42(2) and 1.46(2)A),& and typical organic C-N single bonds, e.g., diethylamine (1.47(2) A).30 Perhaps to alleviate unfavorable steric interactions with atoms in the equatorial coordination plane, the NMez substituent bisects the equatorial plane between Cl(2) and NU), with the dihedral angle between N(3)-C(2)-Pt(l)-N(l) and N(3)-C(2)-Pt(l)-C1(2) being 48.79(2.25)" and -49.59(2.22)", respectively. Addition of various nucleophiles (e.g., Me-, OMe-) to complexes 1-4 leads to reductive elimination, even a t low temperatures (Scheme 1). For example, addition of NaOMe to 3 results in an immediate color change of the solution from pale yellow to deep orange, characteristic of the platinum(I1) precursor [PtMe2(tbu2bpy)l, and the reaction is quantitative as determined by lH NMR spectroscopy. The organic fragment formed in the reaction is [H(OMe)C=NMez]Cl, as determined by lH NMR spectro~copy.~~ The reaction of nucleophiles with 1,2,and 4 gives [PtCl~(~buzbpy)l and [PtMeCl(tbu2bpy)l (for 1 and 2) and [PtMedtbu2bpy)1(for 41, but these

On the basis of the method first described by Lappert et ~ l . , " ,addition ~ of chloroiminium salts to organoplatinum(I1)precursors affords the novel, cationic platinum(IV)-carbene complexes 1-4. Lappert used precursor complexes containing a weakly bonded neutral ligand which could be displaced by chloride, thus yielding neutral carbene complexes by three-fragment oxidative addition. In this work, the reactions are simple twofragment oxidative additions and the strong donor tbu2bpy ligand stabilizes the cationic products. To our knowledge, 1, 3 and 2, 4 are the first examples of secondary carbene and halocarbene complexes of platinum(N), respectively, that do not contain P- or As-donor ligands. In contrast to cationic dimethylplatinum(N)amino- and alkoxycarbene complexes with P- and Asdonor l i g a n d ~ , ~the g dimethyl complexes 3 and 4 show considerably better thermal stability toward reductive elimination. Indeed, 3 and 4 can be stored a t room temperature for indefinite periods of time. The dichloro complexes 1 and 2 do decompose slowly by reductive elimination to give [PtClz(tbu2bpy)land [Me(X)C=NMezIC1 (X = H and C1, respectively). We propose that the lower stability of 1and 2 compared to 3 and 4 is due to the lower electron density at the metal enter,^^,^^ when a strongly a-donating methyl group in 3 or 4 is replaced by an electronegative chloro ligand. The reductive elimination of MeCl (or ethane, for 3 and 41, or the migration of a methyl ligand onto the carbene carbon atom is never observed for any of the complexes prepared in this work. The bonding of the carbene ligand t o the cationic platinum(IV) center in complexes 1-4 can be rationalized by considering the resonance hybrid deriving from the contributing forms depicted in Figure 3. NMR ('H and l3C(lH}) and IR spectroscopy, and the X-ray structure determination of 1 show that canonical form (11) makes a high contribution to the hybrid, with the NMez substituent providing significant stabilization to the electrophilic carbene carbon atom by p,-p, bonding.

(29)Trefonas, L.M.; Flurry Jr., R. L.; Majeste, R.; Meyers, E. A.; Copeland, R. F. J. Am. Chem. SOC.1966,88,2145. (30)Allen, P.W.;Sutton, L. E. Acta Crystallogr. 1960,3 , 46. (31)Smith, T.D.J. Chem. SOC.A 1966,841.

(32)For example, see: (a) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Applications of Organotransition Metal Chemistry; University Science Books: Mill Valley, CA, 1987;p 324.(b) Clark, H.C.; Manzer, L. E. Inorg. Chem. 1973,12, 362.

1038 Organometallics, Vol. 14,No.2, 1995

Rendina et al.

The products 1 and 2 contain a chloro ligand both cis and trans to the carbene ligand, and so it is not possible to determine whether they are formed by cis or trans oxidative addition. However, it is clear that 3 and 4a are formed by cis-oxidative addition and that 4b is formed by trans-oxidative addition. What does this tell us about the mechanism? I t is known that most alkyl halides react with [PtMez(diimine)l complexes by the SN2 mechanism and that trans-oxidative addition occurs, sometimes with subsequent stereochemical change.33 Complexes 4a and 4b do not interconvert readily, so they must be formed by competitive pathways. The cis stereochemistry in the formation of 3 and 4a is most easily rationalized by a concerted mechanism, as proposed for oxidative addition of aryl halides to [PtMez(diimine)l ~omp1exes.l~~ Clearly 4b could be formed by a polar s N 2 mechanism, analogous to that established for alkyl halide addition.34 Addition of nucleophiles to 1-4 leads to their decomposition by reductive elimination. For example, reaction of Mx (M = Na or Li; X = nucleophile) with 3 leads to rapid reductive elimination to give [PtMez(tbuzbpy)l,

MC1 (M = Na, Li) and [H(X)C=NMez]Cl (X = nucleophile). A proposed mechanism is shown in Scheme 1. The nucleophile is expected to add readily to the highly electrophilic carbene carbon atom of 3 to give the aminoalkylplatinum(IV) intermediate 5, but this must undergo rapid reductive elimination, with regeneration of an iminium salt and [PtMez(tbuzbpy)l. Attempts to detect 5 by low-temperature NMR spectroscopy were unsuccessful, and it must be a very short-lived intermediate. This result is similar to the reaction of “Fischer-type” aminocarbene complexes of the type [M{CR(NRR)}(C0)51 (M = Cr, W; R = alkyl, aryl; R = R = H, alkyl) with HX (X = C1, Br) to give the iminium salt [Mx(CO)~1[H(R)C=NRRl.35 In conclusion, this work demonstrates that the electron-rich [PtMez(tbu2bpy)l gives stable aminocarbene complexes of platinum(IV) by the very easy oxidative addition of chloroiminium salts and that the products are very reactive toward nucleophiles.

(33)For example, see: (a) Monaghan, P. K.; Puddephatt, R. J. J. Chem. Soc., Dalton Trans. 1988,595.(b) Crespo, M.; Puddephatt, R. J. Organometallics 1987,6,2548.(c) Jawad, J.; Puddephatt, R. J. J. Organomet. Chem. 1976,117,297. (d) Jawad, J.; Puddephatt, R. J. J. Chem. Soc., Dalton Trans. 1977, 1466.(e) Kuyper, J. Znorg. Chem. 1977,16, 2171. (0 Stille, J. K. In The Nature of the Metal-Carbon Bond; Hartley, F. R., Patai, S., Eds.; Wiley: New York, 1985;Vol. 2, p 625. (34)A systematic study of the product ratio 4a:4b as a function of solvent polarity might provide useful evidence on mechanism but the choice of solvents is very restricted. Thus the chloroiminium salts are insoluble in nonpolar solvents like benzene, and they react with polar solvents like acetone or acetonitrile.

Supplementary Material Available: A summary of crystallographic data and experimental details, and tables of additional interatomic distances and angles, atomic anisotropic displacement parameters, positional parameters for the hydrogen atoms, torsion angles for non-hydrogen atoms, and selected weighted least-squares planes (7 pages). Ordering information is given on any current masthead page.

Acknowledgment. We thank the NSERC (Canada) for financial support to R.J.P. and a Canada International Fellowship to L.M.R.

OM940757X (35)Fischer, E.0.; Schmid, K. R.; Kalbfus, W.; Kreitner, C. G. Chem. Ber. 1973,106,3893.