Metallaboratranes: Bis- and Tris(methimazolyl)borane Complexes of

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Organometallics 2010, 29, 326–336 DOI: 10.1021/om900655k

Metallaboratranes: Bis- and Tris(methimazolyl)borane Complexes of Group 9 Metal Carbonyls and Thiocarbonyls Ian R. Crossley,*,†,‡ Anthony F. Hill,*,† and Anthony C. Willis† †

Research School of Chemistry, Institute of Advanced Studies, Australian National University, Canberra, Australian Capital Territory, Australia and ‡Department of Chemistry, University of Sussex, Brighton, United Kingdom Received July 24, 2009

The iridium poly(methimazolyl)borane complexes [IrH(CE)(PPh3){κ3-B,S,S0 -B(mt)2R}](IrfB)8 (mt = methimazolyl = 2-mercapto-3-methylimidazol-1-yl; E = O, S; R = mt, H) are described in detail. For R = mt, these materials are elucidated as paradigms for the final mechanistic intermediate in metallaboratrane formation, a role illustrated through hydride abstraction to afford the cationic salts [Ir(CE)(PPh3){κ4-B(mt)3}]X(IrfB)8 (E = O, S; X = Cl, BF4). The rhodium poly(methimazolyl)borate complexes [Rh(CO)(PPh3){κ2-S,S0 -HB(mt)2R}] (R = mt, H) are also reported. These compounds are obtained in preference to the respective borane complexes (analogous to iridium); however [Rh(CO)(PPh3){κ2-S,S0 -HB(mt)3}] is observed to undergo facile solution-phase conversion to [Rh(CO)(PPh3){κ4-B,S,S0 ,S00 -B(mt)3}]Cl(RhfB)8 in chlorinated solvents. The ramifications of these results, with respect to metallaboratrane formation, are discussed, substantiating previous mechanistic conjecture. In an attempt to establish an alternative route to iridaboratranes, the first isolable tris(methimazolyl)borate complex of iridium, cis,cis-[IrHCl(PPh3)2{κ2-S,S0 -HB(mt)3], is reported and shown not to evolve to the iridaboratrane [IrCl(PPh3){κ4-B(mt)3}](IrfB)8 under conditions that lead to the corresponding rhodaboratrane. Factors are discussed that may contribute to this fine balance between the formation of methimazolylborate and methimazolylborane complexes.

Introduction The poly(methimazolyl)borate ligands HB(mt)31 and H2B(mt)22 (mt = methimazolyl), purported to serve as “soft analogues” of the ubiquitous poly(pyrazolyl)borates,3 have captured the imaginations of organometallic, bioinorganic, *To whom correspondence should be addressed. E-mail: a.hill@ anu.edu.au. (1) (a) Garner, M.; Reglinski, J.; Spicer, M. D.; Kennedy, A. R. Chem. Commun. 1996, 1975. (b) Reglinski, J.; Carner, M.; Cassidy, I. D.; Slavin, A.; Spicer, M. D.; Armstrong, D. R. J. Chem. Soc., Dalton Trans. 1999, 2119. (c) Spicer, M. D.; Reglinski, J. Eur. J. Inorg. Chem. 2009, 1553. (2) (a) Kimblin, C.; Hascall, T.; Parkin, G. Inorg. Chem. 1997, 36, 5680. (b) Kimblin, C.; Bridgewater, B. M.; Hascall, T.; Parkin, G. J. Chem. Soc., Dalton Trans. 2000, 891. (3) (a) Trofimenko, S. Scorpionates: The Coordination Chemistry of Polypyrazolylborate Ligands; Imperial College Press: London, 1999. For reviews on the organometallic chemistry of poly(pyrazolyl)borate complexes see: (b) Caldwell, L. M. Adv. Organomet. Chem. 2008, 56, 1. (c) Lail, M.; Pittard, K. A.; Gunnoe, T. B. Adv. Organomet. Chem. 2008, 56, 95. (d) Becker, E.; Pavlik, S.; Kirchner, K. Adv. Organomet. Chem. 2008, 56, 155. (e) Crossley, I. R. Adv. Organomet. Chem. 2008, 56, 199. (4) (a) Abernethy, R. J.; Hill, A. F.; Neumann, H.; Willis, A. C. Inorg. Chim. Acta 2005, 358, 1605. (b) Crossley, I. R.; Hill, A. F.; Humphrey, E. R.; Smith, M. K.; Tshabang, N.; Willis, A. C. Chem. Commun. 2004, 1878. (c) Foreman, M. R. St.-J.; Hill, A. F.; White, A. J. P.; Williams, D. J. Organometallics 2003, 22, 3831. (d) Crossley, I. R.; Hill, A. F.; Humphrey, E. R.; Smith, M. K. Organometallics 2006, 25, 2242. (e) Foreman, M. R. St.-J.; Hill, A. F.; Tshabang, N.; White, A. J. P.; Williams, D. J. Organometallics 2003, 22, 5593. (f) Cade, I. A.; Hill, A. F.; Tshabang, N.; Smith, M. K. Organometallics 2009, 28, 1143. (g) Abernethy, R. J.; Hill, A. F.; Tshabang, N.; Willis, A. C.; Young, R. D. Organometallics 2009, 28, 488. (h) Hill, A. F.; Tshabang, N.; Willis, A. C. Eur. J. Inorg. Chem. 2007, 3781. (i) Foreman, M. R. St.-J.; Hill, A. F.; Smith, M. K.; Tshabang, N. Organometallics 2006, 25, 5224. pubs.acs.org/Organometallics

Published on Web 12/22/2009

and coordination chemists alike. As a consequence, the literature contains numerous examples of these sulfur-donor ligands acting as monodentate (κ1-S), bidentate (κ2-S,S0 ), and tridentate (κ3-S,S0 ,S00 or κ3-S,S0 ,H) chelates to mid-late transition metals in low oxidation states,4-11 with more recent reports illustrating a capacity to ligate early (NbV, TaIII-V, TiIII-IV, ZrIV)12,13 and late (PtIV)14 transition metals in higher oxidations states. In an organometallic context, of some note has been the unprecedented capacity for these ligands to undergo B-H activation at metal centers, leading to the installation of a σ(5) (a) Dodds, C. A.; Lehmann, M.-A.; Ojo, J. F.; Reglinski, J.; Spicer, M. D. Inorg. Chem. 2004, 43, 4927. (b) Garner, M.; Lewinski, K.; Pattek-Janczyk, A.; Reglinski, J.; Sieklucka, B.; Spicer, M. D.; Szaleniec, M. Dalton Trans. 2003, 1181. (c) Garner, M.; Lehmann, M.-A.; Reglinski, J.; Spicer, M. D. Organometallics 2001, 20, 5233. (d) Schwalbe, M.; Andrikopoulos, P. C.; Armstrong, D. R.; Reglinski, J.; Spicer, M. D. Eur. J. Inorg. Chem. 2007, 1351. (e) Wallace, D.; Gibson, L. T.; Reglinski, J.; Spicer, M. D. Inorg. Chem. 2007, 46, 3804. (f) Dodds, C. A.; Garner, M.; Reglinski, J.; Spicer, M. D. Inorg. Chem. 2006, 45, 2733. (g) Cassidy, I.; Garner, M.; Kennedy, A. R.; Potts, G. B. S.; Reglinski, J.; Slavin, P. A.; Spicer, M. D. Eur. J. Inorg. Chem. 2002, 1235. (6) Melnick, J. G.; Docrat, A.; Parkin, G. Chem. Commun. 2004, 2870. (b) Morlok, M. M.; Docrat, A.; Janak, K. E.; Tanski, J. M.; Parkin, G. Dalton Trans. 2004, 3448. (c) Docrat, A.; Morlok, M. M.; Bridgewater, B. M.; Churchill, D. G.; Parkin, G. Polyhedron 2004, 23, 481. (d) Bridgewater, B. M.; Fillebeen, T.; Friesner, R. A.; Parkin, G. Dalton Trans. 2000, 4494. (e) Bridgewater, B. M.; Parkin, G. Inorg. Chem. Commun. 2001, 4, 126. (f) Kimblin, C.; Bridgewater, B. M.; Churchill, D. G.; Hascall, T.; Parkin, G. Inorg. Chem. 2000, 39, 4240. (g) Kimblin, C.; Bridgewater, B. M.; Hascall, T.; Parkin, G. Dalton Trans. 2000, 891. (h) Kimblin, C.; Bridgewater, B. M.; Churchill, D. G.; Parkin, G. Chem. Commun. 1999, 2301. (i) Bridgewater, B. M.; Parkin, G. J. Am. Chem. Soc. 2000, 122, 7140. r 2009 American Chemical Society

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Organometallics, Vol. 29, No. 2, 2010 Chart 1. Selected Metallaboratranesa

retrodative metalfboron linkage, the first unequivocally authenticated examples thereof.15-18 These metallaboratrane complexes (Chart 1, M = Ru,15,19 Os20), in which the retrodative MfB linkage is supported by three buttressing methimazolyl units, affording a tricyclo-[3.3.3.0] cage, have (7) (a) Maria, L.; Paulo, A.; Santos, I. C.; Santos, I.; Kurz, P.; Spingler, B.; Alberto, R. J. Am. Chem. Soc. 2006, 128, 14590. (b) Paulo, A.; Correia, J. D. G.; Campello, M. P. C.; Santos, I. Polyhedron 2004, 23, 331. (c) Garcia, R.; Xing, Y.-H.; Paulo, A.; Domingos, A.; Santos, I. Dalton Trans. 2002, 4236. (d) Garcia, R.; Domingos, A.; Paulo, A.; Santos, I.; Alberto, R. Inorg. Chem. 2002, 41, 2422. (e) Garcia, R.; Paulo, A.; Domingos, A.; Santos, I. J. Organomet. Chem. 2001, 632, 41. (f) Garcia, R.; Paulo, A.; Domingos, A.; Santos, I.; Ortner, K.; Alberto, R. J. Am. Chem. Soc. 2000, 122, 11240. (8) (a) Ibrahim, M. M.; Shu, M.; Vahrenkamp, H. Eur. J. Inorg. Chem. 2005, 1388. (b) Ibrahim, M. M.; Shu, M.; Vahrenkamp, H. Eur. J. Inorg. Chem. 2005, 1388. (c) Ji, M.; Benkmil, B.; Vahrenkamp, H. Inorg. Chem. 2005, 44, 3518. (d) Benkmil, B.; Ji, M.; Vahrenkamp, H. Inorg. Chem. 2004, 43, 8212. (e) Schneider, A.; Vahrenkamp, H. Z. Anorg. Allg. Chem. 2004, 630, 1059. (f) Shu, M.; Walz, R; Wu, B.; Seebacher, J.; Vahrenkamp, H. Eur. J. Inorg. Chem. 2003, 2502. (g) Tesmer, M.; Shu, M.; Vahrenkamp, H. Inorg. Chem. 2001, 40, 4022. (9) (a) Patel, D. V.; Kreisel, K. A.; Yap, G. P. A.; Rabinovich, D. Inorg. Chem. Commun. 2006, 9, 748. (b) Graham, L. A.; Fout, A. R.; Kuehne, K. R.; White, J. L.; Mookherji, B.; Marks, F. M.; Yap, G. P. A.; Zakharov, L. N.; Rheingold, A. L.; Rabinovich, D. J. Chem. Soc., Dalton Trans. 2005, 171. (c) Denish, V.; Mihalcik, D. J.; Kreisel, K. A.; Yap, G. P. A.; Zakharov, L. N.; Kassel, W. S.; Rheingold, A. L.; Rabinovich, D. Dalton Trans. 2005, 2410. (d) Alvarez, H. M.; Tanski, J. M.; Rabinovich, D. Polyhedron 2004, 23, 395. (e) Philson, L. A.; Alyounes, D. M.; Zakharov, L. N.; Rheingold, A. L.; Rabinovich, D. Polyhedron 2003, 22, 3461. (f) Alvarez, H. M.; Tran, T. B.; Richter, M. A.; Alyounes, D. M.; Rabinovich, D.; Tanski, J. M.; Krawiec, M. Inorg. Chem. 2003, 42, 2149. (g) White, J. L.; Tanski, J. M.; Rabinovich, D. Dalton Trans. 2002, 2987. (h) Maffett, L. S.; Gunter, K. L.; Kreisel, K. A.; Yap, G. P. A.; Rabinovich, D. Polyhedron 2007, 26, 4758. (10) (a) Bailey, P. J.; Lorono-Gonzales, D. J.; McCormack, C.; Parsons, S.; Price, M. Inorg. Chim. Acta 2003, 354, 61. (b) Bailey, P. J.; McCormack, C.; Parsons, S.; Rudolphi, F.; Perucha, A. S.; Wood, P. Dalton Trans. 2007, 476. (c) Bailey, P. J.; Dawson, A.; McCormack, C.; Moggach, S. A.; Oswald, I. D. H.; Parsons, S.; Rankin, D. W. H.; Turner, A. Inorg. Chem. 2005, 44, 8884. (11) (a) Kuan, S. L.; Leong, W. K.; Goh, L. Y.; Webster, R. D. J. Organomet. Chem. 2006, 691, 907. (b) Kuan, S. L.; Leong, W. K.; Goh, L. Y.; Webster, R. D. Organometallics 2005, 24, 4639. (12) (a) Hill, A. F.; Smith, M. K. Chem. Commun. 2005, 1920. (b) Hill, A. F.; Rae, A. D.; Smith, M. K. Inorg. Chem. 2005, 44, 7316. (c) Hill, A. F.; Smith, M. K.; Wagler, J. Organometallics 2008, 27, 2137. (d) Hill, A. F.; Smith, M. K. Organometallics 2007, 26, 4688. (e) Hill, A. F.; Smith, M. K. Dalton Trans. 2007, 3363. (f) Hill, A. F.; Smith, M. K. Organometallics 2007, 26, 3900. (g) Hill, A. F.; Rae, A. D.; Smith, M. K. Inorg. Chem. 2005, 44, 7316. (h) Hill, A. F.; Smith, M. K. Dalton Trans. 2005, 28. (13) Buccella, D.; Shultz, A.; Melnick, J. G.; Konopka, F.; Parkin, G. Organometallics 2006, 25, 5496. (14) Crossley, I. R.; Hill, A. F.; Willis, A. C. Organometallics 2005, 24, 4889. (15) Hill, A. F.; Owen, G. R.; White, A. J. P.; Williams, D. J. Angew. Chem., Int. Ed. 1999, 38, 2759. (16) Many early examples of compounds formulated as having metal-boron dative bonds17 have subsequently been called into question.18 At present, the metallaboratrane class of complexes constitute the only structurally authenticated examples of MfB dative bonding. (17) Gilbert, K. B.; Boocock, S. K.; Shore, S. G. In Comprehensive Organometallic Chemistry; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.; Pergamon: Oxford, U.K., 1982; Vol. 6, pp 880-886. (18) (a) Braunschweig, H. Angew. Chem., Int. Ed. 1998, 37, 1786. (b) Braunschweig, H.; Kollann, C.; Rais, D. Angew. Chem., Int. Ed. 2006, 45, 5254. (19) (a) Foreman, M. R. St.-J.; Hill, A. F.; Owen, G. R.; White, A. J. P.; Williams, D. J. Organometallics 2003, 22, 4446. (b) Crossley, I. R.; Foreman, M. R. St.-J.; Hill, A. F.; Owen, G. R.; White, A. J. P.; Williams, D. J.; Willis, A. C. Organometallics 2008, 27, 381. (20) Foreman, M. R. St.-J.; Hill, A. F.; White, A. J. P.; Williams, D. J. Organometallics 2004, 23, 913. (21) (a) Crossley, I. R.; Hill, A. F.; Willis, A. C. Organometallics 2006, 25, 289. (b) Crossley, I. R.; Foreman, M. R. St.-J.; Hill, A. F.; White, A. J. P.; Williams, D. J. Chem. Commun. 2005, 221. (c) Crossley, I. R.; Hill, A. F.; Humphrey, E. R.; Willis, A. C. Organometallics 2005, 24, 4083. (d) Crossley, I. R.; Hill, A. F.; Willis, A. C. Organometallics 2007, 26, 3891.

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MLL0 = Ru(CO)(PPh3), Os(CO)(PPh3), Fe(CO)2, RhCl(PPh3), Rh(cod)þ, Rh(PMe3)2þ, Rh(S2CNMe2), RhH(PPh3), Rh(PPh3)(CNC6H2Me3), PtH(PPh3)þ, PtI2; M0 L00 = Pt(PPh3), Co(PPh3)þ NiBr, Pd(PMe3); M00 L00 = Rh(DMAP), Rh(CO), PdCl, PtCl; M000 = Ni, Pd, Pt, CuCl, AgCl, AuCl, Auþ; DMAP = N,N0 -dimethylaminopyridine. a

subsequently become a subject of fascination, with respect to the processes involved in their formation and their structural facets. In seeking to explore these features we have prepared several more examples from groups 9 (Rh,21 Ir22) and 10 (Pt23), including both neutral and cationic, octahedral and trigonal bipyramidal species, thereby demonstrating a degree of generality for the metallaboratrane structural motif. Rabinovich has isolated a rare example of a coordinatively unsaturated cationic cobaltaboratrane, [Co(PPh3){B(mimtBu)3}]þ,24,25 while Tatsumi has described the paramagnetic nickelaboratrane [NiCl{B(mimtBu)3}],26 although the mechanisms by which these two unique examples form remain unknown. Subsequently, Parkin showed that not only could the chloride in Tatsumi’s nickeloboratrane be metathesized with various halides and pseudohalides but the NifB bond could be oxidatively cleaved with iodine, haloforms, and XeF2.27a Parkin has reported mimtBu and mimPh analogues27b of the previously reported complexes (22) Crossley, I. R.; Hill, A. F.; Willis, A. C. Organometallics 2005, 24, 1062. (23) (a) Crossley, I. R.; Hill, A. F. Organometallics 2004, 23, 5656. (b) Crossley, I. R.; Hill, A. F.; Willis, A. C. Organometallics 2008, 27, 312. (c) Crossley, I. R.; Hill, A. F. Dalton Trans. 2008, 201. (24) For non-methimazolyl-derived borates, the abbreviations mimR are taken to indicate 3-R-2-mercaptoimidazolyl. Accordingly methimazolyl would have the abbreviation mimMe; however the simpler “mt” is widely used in the literature, while Bm and Tm are often used to refer to the H2B(mt)2 and HB(mt)3 ligands, respectively. (25) Milhalcik, D. J.; White, J. L.; Tanski, J. M.; Zakharov, L. N.; Yap, G. P. A.; Incarvito, C. D.; Rhengold, A. L.; Rabinovich, D. Dalton Trans. 2004, 1626. (26) Senda, S.; Ohki, Y.; Yasuhiro, H.; Tomoko, T.; Toda, D.; Chen, J.-L.; Matsumoto, T.; Kawaguchi, H.; Tatsumi, K. Inorg. Chem. 2006, 45, 9914. (27) (a) Pang, K.; Tanski, J. M.; Parkin, G. Chem. Commun. 2008, 1008. (b) Landry, V.; Melnick, J. G.; Buccella, D.; Pang, K.; Ulichny, J. C.; Parkin, G. Inorg. Chem. 2006, 45, 2588. (c) Figueroa, J. S.; Melnick, J. G.; Parkin, G. Inorg. Chem. 2006, 45, 7056. (d) Pang, K.; Quan, S. M.; Parkin, G. Chem. Commun. 2006, 5015.

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[RhCl(PPh3){B(mt)3}]21 and [IrH(CO)(PPh3){B(mt)3}] (1),22 in addition to the novel ferra- and palladaboratranes [Fe(CO)2{B(mimtBu)3}]27c and [Pd(PMe3){B(mimtBu)3}],27d the latter being an analogue of the platinaboratrane [Pt(PPh3){B(mt)3}].23a Connelly has demonstrated that a rhodaboratrane may also be constructed in which the methimazolyl buttress is replaced by the 4-ethyl-3-methyl-5-mercapto-1,2,4-triazolylheterocycle.28 While it might appear that metallaboratrane formation is a peculiarity of the geometric features of HB(mt)3 and H2B(mt)2 ligands, in a highly significant breakthrough, Bourissou has shown that metallaboratranes may be prepared via the direct reaction of late transition metal complexes with preformed neutral ω-phosphinoboranes.29 The implications of this are far-reaching and promise a significant broadening of the field, which is already well underway.30 Previously, we presented the first systematic investigation of a series of isoelectronic rhodaboratranes, which differed in their ancillary co-ligands and complex charge.21 In so doing, we gleaned insights into (i) the processes involved in their formation and (ii) their co-ligand substitution chemistry, and we (iii) presented a qualitative bonding model. Subsequently, quantitative theoretical studies of various metallaboratranes have appeared,26,27,29,30 which point to variable degrees of electron transfer from metal to boron with respect to the metal-boron dative bond. Notably, complex charge seemingly exerts little influence over the capacity to maintain the σ-retrodative RhfB linkage; indeed the cationic rhodium(I) systems we have so far isolated appear to be significantly more stable than their neutral counterparts. This is further substantiated by the isolation of both neutral and cationic metallaboratranes based on platinum(0) and platinum(II) (Scheme 1).31 In contrast, the nature of the ancillary ligand set would seem to be of fundamental importance and thus warranted further study. Indeed, we have on several occasions reached this same conclusion, not least in our studies of the interaction of Na[HB(mt)3] with Vaska’s complex, which afforded (28) (a) Blagg, R. J.; Charmant, J. P. H.; Connelly, N. G.; Haddow, M. F.; Orpen, A. G. Chem. Commun. 2006, 2350. (b) Blagg, R. J.; Adams, C. J.; Charmant, J. P. H.; Connelly, N. G.; Haddow, M. F.; Hamilton, A.; Knight, J.; Orpen, A. G.; Ridgway, B. M. Dalton Trans. 2009, 8724. (29) Bontemps, S.; Gornitzka, H.; Bouhadir, G.; Miqueu, K.; Bourissou, D. Angew. Chem., Int. Ed. 2006, 45, 1611. (30) (a) Bontemps, S.; Bouhadir, G.; Miqueu, K.; Bourissou, D. J. Am. Chem. Soc. 2006, 128, 12056. (b) Bontemps, S.; Bouhadir, G.; Apperley, D. C.; Dyer, P. W.; Miqueu, K.; Bourissou, D. Chem. Asian J. 2009, 4, 428. (c) Sircoglou, M.; Bontemps, S.; Bouhadir, G.; Saffon, N.; Miqueu, K.; Gu, W.; Mercy, M.; Chen, C.-H.; Foxman, B. M.; Maron, L.; Ozerov, O. V.; Bourissou, D. J. Am. Chem. Soc. 2008, 130, 16729. (d) Hudnall, T. W.; Kim, Y.-M.; Bebbington, M. W. P.; Bourissou, D.; Gabbai, F. P. J. Am. Chem. Soc. 2008, 130, 10890. (e) Howard, J. A. K.; Dyer, P. W.; Miqueu, K.; Bourissou, D. Chem.;Eur. J. 2008, 14, 731. (f) Vergnaud, J.; Grellier, M.; Bouhadir, G.; Vendier, L.; Sabo-Etienne, S.; Bourissou, D. Organometallics 2008, 27, 1140. (g) Bontemps, S.; Bouhadir, G.; Gu, W.; Mercy, M.; Chen, C.-H.; Foxman, B. M.; Maron, L.; Ozerov, O. V.; Bourissou, D. Angew. Chem., Int. Ed. 2008, 47, 1481. (h) Sircoglou, M.; Bontemps, S.; Mercy, M.; Saffon, N.; Takahashi, M.; Bouhadir, G.; Maron, L.; Bourissou, D. Angew. Chem., Int. Ed. 2007, 46, 8583. (i) Bergnaud, J.; Ayed, T.; Hussein, K.; Vendier, L.; Grellier, M.; Bouhadir, G.; Barthelat, J.-C.; Sabo-Etienne, S.; Bourissou, D. Dalton Trans. 2007, 2370. (31) Our preferred method for assigning oxidation states assumes that the boranes HB(mt)2 and B(mt)3, which are independently isolable,32,33 coordinate as neutral ligands. Alternative perspectives on oxidation state, valency, and dn configurations in metallaboratrane have been presented: (a) Hill, A. F. Organometallics 2006, 25, 4741. (b) For a perspective based on valency rather than oxidation states see: Parkin, G. Organometallics 2006, 25, 4744.

Crossley et al. Scheme 1. Mechanistic Proposal for Rhodaboratrane and Platinaboratrane Formation (L = PPh3)a

a (i) þ Na[HB(mt)3], - NaCl, - L; (ii) B-H activation; (iii) cage closure following phosphine dissociation; (iv) cage closure following chloride dissociation; (v) reductive deprotonation.

Scheme 2. Iridaboratrane Formation (L = PPh3)

the unprecedented dibuttressed metallaboratrane [IrH(CO)(PPh3){κ3-B,S,S0 -B(mt)3}](MfB)8 (1),22 apparently due to the lack of a sufficiently labile ancillary ligand, and hence latent coordination site, to accommodate the third methimazolyl donor. The isolation of the complexes [IrH(CO)(PPh3){κ3-B,S,S0 -BR(mt)2}](MfB)8 (R = mt 1, R = H 2, Scheme 2) was significant in showing that the MfB dative bond is not simply a geometric corollary of the tricicylo-[3.3.3.0] cage topology. Given the parallels between the chemistry of rhodium(I) and iridium(I) and the unique character of the iridaboratranes thus far obtained, the extended investigation of the group 9 carbonyls seemed appropriate for assessing the mutual interplay of metal and ancillary ligand. We thus report herein the reactions of Na[H4-nB(mt)n] (n = 2, 3) with the complexes [MCl(CO)(PPh3)2] (M = Rh, Ir) and [IrCl(CS)(PPh3)2], leading to the isolation of metallaboratrane and poly(methimazolyl)borate

Article


Table 1. Selected Infrared Data for Iridium Complexes (L = PPh3) complex [IrCl(CO)L2]34 [IrH(CO)L(HB(mt)2}] [IrH(CO)L{B(mt)3}] [IrCl(CO)L2(O2)]34 [IrCl(CO)L2(SO2)]35 [IrCl(CO)L2(RNSO)]a,36 [IrCl(CO)L2(CS2)]37 [IrCl(CO)L2(CSe2)]38 [IrCl(CO)L2(F2CdCF2)]39 [IrCl(CO)L2(F3CCtCCF3)]39 [IrD2Cl(CO)L2]b,34 [IrHCl2(CO)L2]34 [IrCl3(CO)L2]34 [IrH(BCat)Cl(CO)L2]40

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Chart 2. Sulfur Dioxide Coordination Modes and Comparisons with Metallaboratranes

νCO/cm-1 1961 1988 1990 2015 2020 2034 2013 2025 2040 2025 2034 2046 2075 211841

a R = SO2C6H4Me-4. b Datum for the D2 adduct provided since the Ir-H and CO modes couple in the H2 adduct (see ref 34).

complexes and the elucidation of processes leading to their interconversion. Aspects of this work have contributed to preliminary reports.21a,d

Results and Discussion Synthesis and Characterization of Iridaboratranes. The dichloromethane-mediated reactions between equimolar amounts of the salts Na[HnB(mt)4-n] (n = 1, 2) and Vaska’s complex [IrCl(CO)(PPh3)2] proceed readily over 3 h to afford exclusively the complexes [IrH(CO)(PPh3){κ3-B,S, S0 -B(mt)3}] (1) and [IrH(CO)(PPh3){κ3-B,S,S0 -BH(mt)2}] (2), respectively (Scheme 2). The formulation of 1 and 2 followed unequivocally from spectroscopic data, based upon the key features and trends that we have previously established.15,19-23 Thus, the 1H NMR spectra clearly indicated the “locked” geometry associated with the metallaboratrane cage, unique environments being observed for each of the methimazolyl groups within both 1 and 2. Additionally, the presence of a single triphenylphosphine ligand is apparent from the aromatic proton resonances, which integrate consistently against those of the methimazolyl groups, and the discrete metal-hydride (δH -11.8 1, -13.6 2), which confirms the occurrence of B-H activation. For 1, this latter feature is further supported by the absence of any resonances associated with either terminal (B-H) or bridging (B-H-M) borohydride functionalities, evidence for which is also conspicuously absent from the infrared spectrum. In the case of 2, a single B-H infrared absorption (νBH = 2384 cm-1) is observed for the retained borohydride function, although its typically broadened resonance could not be unambiguously identified in the 1H NMR spectrum. The 31P{1H} NMR spectra each exhibit a single, heavily broadened, resonance (δP 4.5, halfheight width (hhw) = 163 Hz for 1; δP = 6.2, hhw = 95 Hz for 2), indicative of appreciable interaction with the quadrupolar boron nucleus and consistent with a trans disposition of these two nuclei. While we have, on occasion,23 resolved spin-spin coupling between trans-disposed boron and phosphorus nuclei, this is not the case for either 1 or 2. However, a coupling of magnitude 8 Hz was resolved in the 1H NMR spectrum of 2, which was assigned to a cis 31P-1H interaction, thus further defining the stereochemistry. The final (sixth) coordination site in both 1 and 2 is occupied by the carbonyl ligand, the retention of which is clear from the infrared spectra (νCO 1990 1, 1988 cm-1 2). In keeping with

the Lewis acid boranes HB(mt)2 and B(mt)3 being neutral isolable compounds,32,33 the values of νCO for 1 and 2 are consistent with the description of these compounds as adducts of iridium(I) (Table 1).34-41 Thus coordination of a range of π-acceptor ligands (O2, CS2, CSe2, F2CdCF2, F3CCtCCF3) to Vaska’s complex results in a substantial increase in νCO of 55-80 cm-1, somewhat less than observed for conventional two-fragment oxidative addenda (H2, HCl, Cl2) for which the iridium(III) oxidation state is invoked without debate. Perhaps the most germane entries are however the SO2 and iminooxosulfurane adducts.42 Sulfur dioxide was the first recognized example of what have in the interim come to be termed “Z-type” ligands (or to use the older and perfectly adequate term, Lewis acids). Thus [IrCl(CO)(PPh3)2(SO2)] involves a dative bond from iridium to a pyramidalized sulfur(IV) atom (A, Chart 2), which is distinct from the two alternative coordination modes in which the sulfur (B) or one SdO double bond (C) provides (32) Crossley, I. R.; Hill, A. F. Manuscript in preparation. (33) (a) The conventional Lewis base adduct dmffB(mt)3 has been structurally characterized: Schwalbe, M.; Andrikopoulos, P. C.; Armstrong, D. R.; Reglinski, J.; Spicer, M. D. Eur. J. Inorg. Chem. 2007, 1351. Similarly, Bailey has reported the synthesis of MeImfB(mt)3 (MeIm = N-methylimidazole).10b (34) Vaska, L. Acc. Chem. Res. 1968, 1, 335. (35) (a) Vaska, L.; Bath, S. S. J. Am. Chem. Soc. 1966, 88, 1333. (b) LaPlaca, S. J.; Ibers, J. A. Inorg. Chem. 1966, 5, 405. (c) Blake, A. J.; Ebsworth, E. A. V.; Henderson, S. G. D.; Murdoch, H. M.; Yellowlees, L. J. Z. Kristallogr. 1992, 199, 290. (36) (a) Herberhold, M.; Hill, A. F. J. Chem. Soc., Dalton Trans. 1988, 2027. (b) Herberhold, M.; Hill, A. F. J. Organomet. Chem. 1990, 387, 323. (37) Baird, M. C.; Wilkinson, G. J. Chem. Soc. A 1967, 865. (38) Kawakami, K.; Ozaki, Y.; Tanaka, T. J. Organomet. Chem. 1974, 69, 151. (39) Parshall, G. W.; Jones, F. N. J. Am. Chem. Soc. 1965, 87, 5356. (40) Westcott, S. A.; Marder, T. B.; Baker, M. R.T.; Calabrese, J. C. Can. J. Chem. 1993, 71, 930. (41) Given the low electronegativity of boron and at best modest π-acidity of the BCat ligand, this value would appear anomalously high. However, the trans disposition of hydride and carbonyl ligands results in significant coupling (νIrH = 2007 cm-1) of the νCO and νIrH oscillators. This is not an issue when the hydride and carbonyl ligands are mutrually cis, as in the case of 1 and 2. For a discussion of this phenomenon see: Vaska, L. J. Am. Chem. Soc. 1966, 88, 4100. (42) For reviews on the coordination chemistry of sulfur dioxide and iminooxosulfuranes, respectively, see: (a) Ryan, R. R.; Kubas, G. J.; Moody, D. C.; Eller, P. G. Struct. Bonding (Berlin) 1981, 46, 47. (b) Hill, A. F. Adv. Organomet. Chem. 1994, 36, 131.

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3

0

Figure 1. Molecular structure of [IrH(CO)(PPh3){κ -B,S,S -B (mt)3}] (1) in a crystal of 1 3 (CH2Cl2)1.43. Methimazolyl hydrogen atoms are omitted; phenyl groups simplified for clarity (50% thermal ellipsoids). Selected bond distances (A˚) and angles (deg): Ir1-B1 2.193(6), Ir1-C1 1.811(7), Ir1-P1 2.404 (1), Ir1-S1 2.463(1), Ir1-S2 2.417(1), Ir1-H1 1.60(5), S1-C11 1.707(5), S2-C21 1.711(5), S3-C31 1.699(5), B1-N12 1.583(6), B1-N22 1.553(7), B1-N32 1.567(7), Ir1-B1-N12 108.0(3), Ir1-B1-N22 109.4(3), Ir1-B1-N32 117.4(3), N12-B1N22 106.3(4), S1-Ir1-S2 93.46(4), Ir1-S1-C11 96.0(2), Ir1-S2-C21 97.7(2).

two electrons to the metal center. In both the SO2 and iminooxosulfurane adducts of Vaska’s complex, the Lewis acid interaction is labile and the iridium center not greatly displaced from the almost square coordination plane of the remaining ligands. A similar situation is observed for [Ir(O3SMe)(CO)(PPh3)2(SO2)],43 [Ir(CO)(PPh3)(NHC6H4PPh2)(SO2)],44 and the unusual complex [IrH(CO)(PTol3)2(SO2)] (Tol=C6H4Me-4),45 in which the CO rather than SO2 ligand assumes the axial coordination site.46 This last example (with νCO = 2064, νIrH = 1965 cm-1) and the PPh3 analogue (νCO = 2050, νIrH = 1965 cm-1) are in some respects most akin to 1 and 2 and have been designated by Mingos46b as d8 complexes; that is, the weak coordination of a Lewis acid to the metal center is not taken to constitute a twoelectron oxidation or depletion in the d-configuration. In a similar manner Gilbert has recently reported a DFT study of the complexes [M(SO2)(PPh3)3] that describes these as having a d10 configuration.47 Chart 2 depicts compounds in support of this analogy. Given that the iridium SO2 complexes have significantly higher vCO values than do 1 and 2, it would seem especially appropriate to denote these iridaboratranes as involving monovalent d8-iridium. The complexes 1 and 2 have been structurally characterized and details of 2 discussed in a preliminary report.22 In the interim, structural data for the analogues [IrH(CO) (PPh3){κ3-B,S,S0 -B(mimR)3}] (R = tBu, Ph) have become available.27b The molecular geometries of 1 and 2 are (43) Randall, S. L.; Miller, C. A.; Janik, T. S.; Churchill, M. R.; Atwood, J. D. Organometallics 1994, 13, 141. (44) Dahlenburg, L.; Herbst, K. Chem. Ber. 1997, 130, 1693. (45) Miller, C. A.; Janik, T. S.; Churchill, M. R.; Atwood, J. D. Inorg. Chem. 1996, 35, 3683. (46) (a) Levison, J. J.; Robinson, S. D. J. Chem. Soc., Dalton Trans. 1972, 2013. (b) Bell, L. K.; Mingos, D. M. P. J. Chem. Soc., Dalton Trans. 1982, 673. (47) Brust, D. J.; Gilbert, T. M. Inorg. Chem. 2004, 43, 1116.

Figure 2. Molecular structure of 2 in a crystal of 2 3 CHCl3. Methimazolyl hydrogen atoms are omitted; phenyl groups simplified for clarity (50% displacement ellipsoids). Selected bond distances (A˚) and angles (deg): Ir1-B1 2.210(5), Ir1-C1 1.826(5), Ir1-P1 2.411(1), Ir1-S1 2.485(1), Ir1-S2 2.417(1), Ir1-H1 1.63(3), S1-C11 1.717(5), S2-C21 1.703(5), B1-N12 1.557(6), B1-N22 1.581(6), Ir1-B1-N12 107.8(3), Ir1-B1N22 106.5(3), N22-B1-N12, 107.4(3), S1-Ir1-B1 85.90(13), S2-Ir1-B1 86.01(12), S1-Ir1-S2 91.69(4), B1-Ir1-H1 86.4 (11), Ir1-S1-C11 96.27(16), Ir1-S2-C21 97.18(15).

depicted in Figures 1 and 2, respectively, and confirm the formulations based on spectroscopic and microanalytical data. Although not shown, the chloroform solvate fits comfortably within a cleft provided by the two methimazolyl groups and one phenyl ring of 2, though the CH 3 3 3 S separations (2.78, 2.98 A˚) are beyond those attributable to significant hydrogen-bonding interactions. Geometric features associated with the carbonyl, phosphine, and (located and refined) hydride ligand are unremarkable, leaving us to focus on the metallaboratrane cage of 2 itself. The interligand angles at iridium and boron are essentially octahedral and tetrahedral, respectively. The minor deviations may be readily attributable to the modest steric profile of the hydride ligand (angles to cis ligands being marginally acute) and the geometrical constraints of chelation (S2-Ir1-B1 86.01(12), S1-Ir1-B1 85.90(13) A˚). The typically strong trans influence of the hydride ligand is manifested in a significant (68 esd) lengthening of the iridium sulfur bond to which it is trans coordinated, relative to that trans to the carbonyl ligand. The iridium-boron separation of 2.210(5) A˚ is considerably longer than those between octahedral iridium and three-coordinate boron, which span the range 1.990-2.121 A˚. Notably, despite the inclusion of large N-substituents in the iridaboratranes [IrH(CO)(PPh3) {B(mtPh)3}] and [IrCl(PPh3){B(mtR)3] (R = tBu, Ph)27b these each have shorter Ir-B separations than found in 1. In any event, the Ir(1)-B(1) bond length in 2 is well within the sum of the covalent radii (2.25 A˚). The molecular structure of 1 (Figure 1) is topologically similar to that of 2 with a comparable Ir1-B1 bond length of 2.193(6) A˚. That is, the inclusion of a fourth non-hydrogen substituent on boron does not lead to a sterically induced lengthening of this bond; rather a modest (statistically insignificant, 3 esd) shortening is suggested. Indeed the entire molecular geometry of 1 is remarkably similar to that of 2; that is, all cis interligand angles involving the hydride are marginally acute, a trans influence is demonstrated by the hydride ligand, and the angles about iridium and boron are close to octahedral and tetrahedral, respectively.

Article

The inherent significance of compound 1, in relation to the ongoing study of metallaboratrane formation, is that it serves as a paradigm for the final intermediate of our proposed mechanism for this process (Scheme 1), from which the κ4-B(mt)3M tricyclo-[3.3.3.0] cage can be generated through loss of a labile co-ligand and chelation of the pendant methimazolyl donor. That bicyclo-[3.3.0]-1 can be isolated quantitatively as an appreciably stable material would, therefore, seem to be attributable to the absence of any sufficiently labile ligand within its coordination sphere. However, we have observed that on prolonged standing (several days), NMR samples (CDCl3 solution) of 1 exhibit evidence for the formation of a trace material (50%). However, clean quantitative conversion could not be effected, presumably due to subsequent decomposition, the 1H NMR spectra exhibiting a plethora of intractable side-products that come to predominate over relatively short time frames. Greater success was achieved by treating 1 with 1 equiv of [Ph3C]BF4, which effected in excess of 95% conversion to the presumed cationic material 3 3 BF4 (δH = 3.45 (3H, NCH3); 3.48 (6H, NCH3); 7.84 (2H, CHdCH)). However, separation of the Ph3CH byproduct proved problematic, thus further confounding unequivocal identification of the remaining imidazole resonances within the cluttered aromatic region and indeed the acquisition of informative microanalytical data. In view of these difficulties, it seemed that an alternative hydride precursor might allow us to definitively demonstrate the proposed conversion, to which end the thiocarbonyl analogue of 2 was chosen. The syntheses of [IrH(CS)(PPh3){κ3-B,S,S0 -B(mt)3}] (4) and its bis-methimazolyl analogue [IrH(CS)(PPh3){κ3-B,S, S’-BH(mt)2}] (5) are largely comparable to those of 1 and 2, but employ [IrCl(CS)(PPh3)2]48 in place of Vaska’s complex. Complex 4 is obtained quantitatively and unequivocally (48) (a) Lu, G.-L.; Roper, W. R.; Wright, L. J.; Clark, G. R. J. Organomet. Chem. 2005, 690, 972. (b) Hill, A. F.; Wilton-Ely, J. D. E. T.; Breedlove, B. K.; Kubiak, C. P. Inorg. Synth. 2002, 33, 244. (c) Hill, A. F.; Wilton-Ely, J. D. E. T. Organometallics 1996, 15, 3791.


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identified on the basis of spectroscopic data, which are comparable to those of 1, although the CtS stretching mode (1363 cm-1) coincides with the fingerprint region of the B (mt)3 ligand. It is, however, noted that the yields of complex 5 are poor, and samples typically contain intractable contaminants. Nonetheless its identity is clear from somewhat limited spectroscopic data. Efforts to obtain single crystals of both 4 and 5 have proven unsuccessful, an apparent result of reduced solution-phase stability, relative to the carbonyl analogues; indeed, this results in the complete decomposition of 5 within 48 h in anaerobic solutions. The solution-phase behavior of complex 4 is, however, more interesting, since it serves to demonstrate conclusively the capacity to convert a neutral complex of the type [IrH(CE)(PPh3){κ3-B,S,S0 -B(mt)3}] into a cationic metallaboratrane salt, i.e., [Ir(CE)(PPh3){κ4-B,S,S0 ,S00 -B(mt)3}]Cl (E = O 3 3 Cl, S 6 3 Cl). When solutions of 4 in CDCl3 are left to stand for prolonged periods, the 1H NMR resonances associated with this material (i.e., 3 NCH3; 6 imidazole CH) are observed to diminish, concomitant with the development of signals suggestive of the cationic metallaboratrane salt [Ir(CS)(PPh3){κ4-B,S,S0 S00 -B(mt)3}]Cl (6 3 Cl) (i.e., 2 NCH3 (2:1); 4 imidazolyl CH (2:1:2:1), Figure S1 see Supporting Information). Under ambient conditions, this conversion proceeds slowly over the course of several days, with 6+ ultimately accounting for >95% of the NMR active materials (4 days), although complete conversion is never achieved. Nonetheless, alongside the observations noted for 1 (vide supra) this process clearly defines the final stage of metallaboratrane formation as being that which we had presumed, i.e., dissociation/abstraction of a labile ligand to allow for chelation of the third methimazolyl donor. Hydride ligands are not dissociatively labile per se but are readily abstracted by electrophiles, in this case adventitious acid. It should be noted that these cationic complexes are isoelectronic with the previously reported and structurally characterized neutral group 8 examples [M(CO)(PPh3){κ4-B(mt)3}] (M = Ru,15 Os20) and [Ru(CS)(PPh3){κ4B(mt)3}].19b The validity of these arguments has recently received further support from Parkin’s synthesis of the carbonyl-free metallaboratrane [IrCl(PPh3){κ4-B(mt)3}],27b obtained from [IrCl(PPh3)(η4-C8H12)] in the manner previously described for the rhodium analogue [RhCl(PPh3){κ4-B(mt)3}],21b,c a process that is clearly facilitated by the absence of the somewhat inert carbonyl ligand. Interestingly, we have also obtained the rhodium analogue from Wilkinson’s catalyst, which led us to investigate the reaction between Na[HB(mt)3] and [IrCl(PPh3)3]. Unfortunately, only intractable mixtures were obtained, a situation that was also encountered when exploring the reaction between Na[HB(mt)3] and [IrCl(η4C8H12)2]2, in an attempt to synthesize [Ir(η4-C8H12){κ4-B(mt)3}]Cl, by analogy with the rhodium analogue.21 Unperturbed by these failures we envisioned an alternative route to iridaboratranes, again by analogy with our earlier work,19,20 commencing from an iridum(III) precursor. Our initial ruthenaboratrane syntheses involved the installation of the HB(mt)3 cage on ruthenium(II) precursors that bore either a σ-aryl or hydride ligand that could serve as hydrogen sink through reductive elimination of arene or dihydrogen.15 In the course of other studies we had cause to prepare the iridium(III) complex [IrHCl2(PPh3)3], which we anticipated would react with Na[HB(mt)3] to afford the first example of an isolable tris(methimazolyl)borate complex of iridium. Indeed, this reaction proceeds, in dichloromethane solution,

332


Chart 3. Considered Structures for Complex 7 with Associated Point Groups

over the course of 2 h to afford a single product that we formulate as [IrHCl(PPh3)2{κ2-S,S0 ,-HB(mt)3}] (7), akin to [RuH(CO)(PPh3)2{κ2-S,S0 -HB(mt)3}].19 Although attempts to obtain X-ray quality crystals of this material have thus far proven unsuccessful, the formulation follows with confidence from spectroscopic and elemental microanalytical data. Our preferred formulation for 7 is shown in Chart 3 (A) alongside alternatives that are consistent with the bulk composition but which may be discarded on the basis of spectroscopic data. The 1H NMR spectrum demonstrates the retention of the hydride ligand (δH -18.1 (dd), JPH 20.2, 14.2 Hz), the couplings to which are of magnitudes consistent with a cis disposition to two phosphorus nuclei, a situation confirmed by observation of two distinct resonances in the 31 P NMR spectrum at 6.01 (dd, JPP 14 Hz; JPH 14 Hz) and 0.83 (dd, JPP 14 Hz; JPH 20 Hz) ppm, thereby excluding the Cs-symmetric formulations C and E. From the sharp nature of these two resonances and the observation of an absorption consistent with a terminal B-H functionality in the infrared spectrum (νBH 2433 cm-1) it may be inferred that neither B-H activation nor two-center, two-electron coordination has occurred (excluding C and D). The observation of three unique methimazolyl groups in the 1H NMR spectrum also allows us to exclude entries C and E and while the salts B49,50 and D51 have no element of symmetry (due to the chiral C3 HB(mt)3Ir cage in B), the latter may be discounted on the basis of the small 2JPP coupling, since trans couplings are (49) Tridentate coordination of the HB(mt)3 ligand through three sulfur donors typically results in high barriers to C3-HB(mt)3M cage inversion50,51 rendering ligands L in HB(mt)3MXL2 diastereotopic on NMR time scales: Foreman, M. R. St.-J.; Hill, A. F.; White, A. J. P.; Williams, D. J. Organometallics 2003, 22, 3831. (50) Bailey, P. J.; Dawson, A.; McCormack, C.; Moggach, S. A.; Oswald, I. D. H.; Parsons, S.; Rankin, D. W. H. Inorg. Chem. 2005, 44, 8884. (51) A similar topology was recently established for [RuCl(dmso)2{κ3-H,S,S0 -HB(mt)3}], giving rise to three distinct methimazolyl environments.4g

Crossley et al. Scheme 3. Methimazolylborate and Borane Complexes of Rhodium (L = PPh3)

typically an order of magnitude larger.52 The data fail to unequivocally discriminate between A and B, and it is perhaps noteworthy that the ESI-mass spectrum is devoid of a molecular ion, with the heaviest attributable isotopic envelope corresponding to [M - Cl]þ; however it should be noted that the technique is particularly prone to inducing metal halide ionization. Given our recent observations on hydridoplatinaboratrantes23 we reasoned that the dehydrochlorination of 7 might be effected by the addition of a strong non-nucleophilic base, e.g., DBU, thereby affording an iridium(I)31 metallaboratrane. Unfortunately, while a reaction clearly ensues, as evident from a color change and the complete loss of 7 upon workup, the nature of the product mixture is complex and remains indeterminate but does not appear to contain a metallaboratrane species. Attempted Synthesis of [RhH(CO)(PPh3){K3-B,S,S0 HnB(mt)3-n}] (n=1, 2). We have previously reported, at length, the synthesis of numerous rhodaboratrane complexes, with both neutral and cationic metal centers (Chart 1).21 However, given our isolation of iridium complexes 1 and 2, thus far unique (supplemented by mimR analogues reported by Parkin) in exhibiting a κ3-B,S,S’ binding mode for the B(mt)2R (R = mt, H) ligands, the pursuit of rhodium analogues seemed of value. Somewhat surprisingly, the reactions of [RhCl(CO)(PPh3)2] with Na[HnB(mt)3-n] (n = 1, 2) failed to result in B-H activation, yielding instead the simple poly(methimazolyl)borate complexes [Rh(CO)(PPh3){η2-S,S0 -HB(mt)2R}] (R = mt 8, H 9, Scheme 3), akin to Connelly’s mercaptotriazolyl analogue.28 Once again, formulations follow from spectroscopic data, which reveal retention of the carbonyl ligands (νCO 1973 cm-1 8, 1976 cm-1 9) and B-H functionalities (νBH 2307 cm-1 8, 2380, 2208 cm-1 9), which in the case of 9 seemingly involve a bridging B-H-M mode in the (52) The two phosphine ligands in F are chemically inequivalent, as are the three methimazolyl groups; however trans-1JPP couplings are typically an order of magnitude larger than cis-1JPP couplings. For example, the complex trans-[Os(C6H5)(CO)(PPh3)2{κ2-N,N0 -HB(pz)3}] has 2JPP =301.8 Hz: Burns, I. D.; Hill, A. F.; White, A. J. P.; Williams, D. J.; Wilton-Ely, J. D. E. T. Organometallics 1998, 17, 1552. (53) The concept of oxidation state loses its utility in highly covalent compounds involving bonds between elements of comparable electronegativity; that is, the hypothetical perspective of 100% ionic bonding has little to offer. The term “formal oxidation state” is often used to allude to this artifice; however even this concession can lead to dichotomies, e.g., in describing Hartwig’s complexes54 [RhHx(BO2C2Me4)4-x(C5Me5)] (x = 1, 2) as being rhodium(V) when rhodium is more electronegative than boron or hydrogen. (54) Hartwig, J. F.; Cook, K. S.; Hapke, M.; Incarvito, C. D.; Fan, Y.; Webster, C. E.; Hall, M. B. J. Am. Chem. Soc. 2005, 127, 2538.

Article

solid state, as reported previously for the complexes [RhL2{H2B(mt)2}] (L2 = 1,5-cyclooctadiene or L = CO, CNC6H2Me2-2,6).4d This interaction is lost in solution (CH2Cl2: νBH 2308 cm-1). The 1H NMR spectra of both 8 and 9, though not allowing for resolution of the borohydride functionalities, indicate exchange of the methimazolyl sites, which exhibit a single magnetic environment at ambient temperature. Retention of a single phosphine ligand is clearly evidenced by the observation of both a single resonance in the respective 31P NMR spectra (δP 40.1, d, JRhP 159.7 Hz 8; 44.6, d, JRhP 151 Hz 9) and aromatic resonances in the 1H NMR spectra that integrate consistently with the methimazolyl signals. Both 8 and 9 exhibit limited stability in the solid state, even with the rigorous exclusion of air and moisture, which might seem to suggest a predominance of the square-planar, 16-electron κ2-S,S0 -HB(mt)2R (R=mt, H) forms, despite the inferred involvement of five-coordintate κ3-S,S0 ,S00 and/or κ3-S,S0 H geometries in solution suggested by the equilibration process. Indeed, the solution lifetimes of 8 and 9 are appreciably greater than in the solid state; thus, while ESI mass-spectrometric and 13C{1H} NMR data can be readily obtained, microanalytical data cannot. These difficulties have also precluded the growth of X-ray quality single crystals. Interestingly, in solution, complex 8 is observed to undergo a slow, albeit partial, conversion to another material, the spectroscopic data (1H, 31P NMR) for which are entirely consistent with it being the rhodaboratrane salt [Rh(CO)(PPh3){B(mt)3}]Cl (10 3 Cl). Although its isolation and comprehensive characterization have not been effected, comparison of spectroscopic data with those for the isoelectronic neutral complex [Ru(CO)(PPh3){κ4-B(mt)3}]15 further substantiates the formulation. Given that one would presume the intermediacy of [RhH(CO)(PPh3){κ3-B,S,S0 B(mt)3}] in this process, by analogy with the iridium systems, these observations together would seem to imply that (i) B-H activation at rhodium is less facile than at iridium and (ii) the hydride at rhodium is significantly more reactive than that at iridium. Although the latter of these is undisputed, the former would seem counterintuitive if one draws the analogy between B-H and C-H “oxidative” addition. However, given the relative Pauling electronegativities of boron (2.0), carbon (2.5), hydrogen (2.2), rhodium (2.3), and iridium (2.2), the terms “oxidative addition” and “oxidation state” should be used with cautious cognizance of the axioms upon which they are based. We have recently demonstrated that the B-H activation of chelated poly(methimazolyl)borate ligands can be a reversible process (Scheme 4).23c For the platinum systems studied, the ratelimiting step was found to be dissociation of a labile ligand (typically a phosphine) from the square-planar HB(mt)3 complex salt [Pt(PR3)2{κ2-S,S0 -HB(mt)3}]Cl (R = alkyl, aryl), this apparently being prerequisite to B-H activation or at least to attaining a stable molecular geometry, i.e., the octahedral [PtH(PR3){κ4-B(mt)3}]Cl. This process was impeded when R = Me and Et, due to the reduced lability of these stronger σ-donors (relative to PPh3). Moreover, treatment of the platinaboratrane with an excess of PR3 (R=Me, Et) effects, quantitatively, migration of the hydride back to boron, to afford [Pt(PR3)2{κ2-S,S0 -HB(mt)3}]Cl. In the present discussion, the lability of the ligand set is not an issue, given that it is invariant; rather, it is the intrinsic lability of the metal centers that must be considered. It is


333

Scheme 4. Relative Positions of the B-H Activation Equilibria for Pt, Rh, and Ir

clear that the rhodium-hydride linkage is appreciably more labile than in the iridium analogue, given the ease with which it is lost in chlorinated solvent (vide supra). A further result of this lability would be a tendency to impede the B-H activation step, by virtue of the reverse process (i.e., B-H elimination) being facile; that is, the borate-borane equilibrium is shifted in favor of the borate complex. Thermochemical data associated with M-H, M-C, and M-Cl bond formation point to the process becoming more favorable as one descends group 9.55,56 There are few such data available for metal boryls (M-BR2)57 and none for metal-boron dative bonds. Furthermore, since all examples of MfBR3 bonding to date involve chelated or cage structures, such data would, were they to eventuate, include contributions from ring/cage strain. However, since the covalent radii for rhodium (142 pm) and iridium (141 pm)58 are similar, we might assume that in the present case cage strain and entropic considerations may be factored out. This leaves, at least from a superficial perspective, the simple arithmetic sum of bond dissociation energies (BDE) for the respective MfB and M-H bonds versus the cleaved B-H bond as the determinant factor. Comparative data for bond dissociation energies are not available for metallaboratranes as such, since the MfB bonds are housed within a cage. Bourissou has however investigated in detail, both crystallographically and computationally, the complexes [MCl2{iPr2PC6H4)2BPh}] (M = Pd, Pt),30e [M{iPr2PC6H4)3B}] (M = Ni, Pd, Pt), and [MCl{iPr2PC6H4)3B}] (M = Cu, Ag, Au).30c These studies concluded that in each case the metalfboron interaction was strongest for the 5d metal in each triad. For the group 11 examples from copper to gold there was a smooth increase in (55) Martinho Simoes, J. A.; Beauchamp, J. L. Chem. Rev. 1990, 90, 629. (b) Pearson, R. G. Chem. Rev. 1985, 85, 41. (56) Nolan, S. P.; Hoff, C. D.; Stoutland, P. O.; Newman, L. J.; Buchanan, J. M.; Bergman, R. G.; Yang, G. K.; Peters, K. S. J. Am. Chem. Soc. 1987, 109, 3143. (57) Rablen, P. R.; Hartwig, J. F.; Nolan, S. P. J. Am. Chem. Soc. 1994, 116, 4121. (58) Cordero, B.; G omez, V.; Platero-Prats, A. E.; Reves, M.; Echeverrı´ a, J.; Cremades, E.; Barragan, F.; Alvarez, J. Dalton Trans. 2008, 2832.

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the degree of electron transfer from the metal to boron. For the group 10 examples, the NifB interaction was found to be stronger than the corresponding PdfB interaction, but still weaker than for platinum. This result, which was attributed to better housing of the smaller nickel center within the cage, offset the weaker basicity of the 3d metal relative to its 4d and 5d congeners. Given that each group 9 element is more electropositive than the corresponding elements of groups 10 and 11, it might be presumed that this trend would be reproduced. The B-H bond dissociation enthalpy (BDE) for H-BH2NH3 has been estimated at 430 kJ/mol, and for a range of three-coordinate boranes BDEs span the range 440-470 kJ/mol. With recourse to data for the addition of dihydrogen (H-H) versus catecholborane (H-BCat) to Vaska’s complex, Hartwig and Nolan have concluded that the resulting Ir-B bond is at least equal to or greater than that of the corresponding iridium hydride.56 The addition of HBCat to Vaska’s complex is exothermic by ca. 63 kJ/mol, while the corresponding rhodium complex is unknown.59,60 It might therefore be surmised that the driving force for the spontaneous formation of iridaboratranes arises similarly from the favored formation of IrfB and Ir-H bonds. In the case of rhodaboratrane formation the reaction leading to weaker RhfB and Rh-H bonds does not provide sufficient impetus for the accumulation of (spectroscopically) significant amounts of the rhodaboratrane unless this is compensated for by subsequent exothermic reaction of the transient hydride with HCl or halocarbon solvent.56 Thus, only where the hydride is lost to a more facile process (i.e., solvent exchange) is the RhfB linkage locked in, hence the slow formation of 10 3 Cl, without observation of the intermediate hydride complex [RhH(CO)(PPh3){κ3-B, S,S0 -B(mt)3}], which presumably exists only transiently. These arguments are, on reflection, more widely applicable, in accounting for our ability to prepare a variety of platinaboratranes,22,23 while palladaboratranes remain rare.27d One might thus expect to observe a similar phenomenon with respect to any 4d-5d couplet of metallaboratrane analogues and might thus question the comparable facility with which the original group 8 examples [M(CO)(PPh3){κ4-B(mt)3}] (M = Ru, Os)9-11 were obtained. It should, however, be noted that these neutral materials were obtained from organometallic M(II) precursors, in which a σ-organyl (alkyl, aryl, vinyl) served as hydride acceptor, the elimination of RH thus driving the equilibrium in favor of the borane complex. Furthermore, the mechanistic subtleties by which these group 8 metallaboratranes form remain to be elaborated given that evidence is not yet available to discriminate between addition-elimination or σ-complexassisted61 metathesis pathways. Indeed, this phenomenon is perhaps best illustrated by rhodium, the original rhodaboratrane [RhCl(PPh3){B(mt)3}]21b being obtained in under 1 h from [RhCl2(Ph)(PPh3)2], while other examples generated from 16-electron Rh(I) precursors have proven more difficult to obtain.21 (59) Although not documented in SciFinder (C43H31BClO3P2Rh), the related complex [RhHCl(CO)(PPh3)2(BO2C2Me4)] may play a role in the dehydrogenative borylation of alkenes with HBO2C2Me4, which is mediated by [RhCl(CO)(PPh3)2],60 either within the catalytic cycle or as a tangential catalyst resting state. (60) Mkhalid, I. A. I.; Coapes, R. B.; Edes, S. N.; Coventry, D. N.; Souza, F. E. S.; Thomas, R. L.; Hall, J. J.; Bi, S.-W.; Lin, Z.; Marder, T. B. Dalton Trans. 2008, 1055. (61) Perutz, R. N.; Sabo-Etienne, S. Angew. Chem., Int. Ed. 2007, 46, 2578.

Crossley et al.

Conclusions We have described the synthesis of the bis-methimazolylbuttressed iridium-borane complexes [IrH(CE)(PPh3){κ3B,S,S0 -B(mt)2R}] (E O, S; R = mt, H) and attempts to prepare the rhodium analogues, which have instead afforded the more conventional borate complexes [Rh(CO)(PPh3){κ2-S,S0 -HB(mt)2R}] (R = mt, H). Despite appreciable air sensitivity in the solid state, the complex [Rh(CO)(PPh3){κ2-S,S0 -HB(mt)3}] has been observed to slowly convert in solution to the rhodaboratrane salt [Rh(CO)(PPh3){κ4B(mt)3}]Cl, the iridium analogue of which can be generated only in the presence of a strong hydride acceptor (e.g., trityl tetrafluoroborate). In contrast, the iridium thiocarbonyl analogue [Ir(CS)(PPh3){κ4-B(mt)3}]Cl forms, albeit slowly (4 days) under mild conditions in chlorinated solvent (CDCl3). The differing facility of these conversions has been attributed in part to the thermodynamics of the resultant M-H and inferred MfB bonds. Moreover, this same effect has been implicated in the apparently reduced propensity toward B-H activation observed for the rhodium compounds, as compared with iridium, since the lower dissociation energies of the Rh-H and RhfB bonds precludes the observation of the inferred transient rhodaboratrane [RuH(CO)(PPh3){B(mt)3}]. The equilibrium can be driven to favor the borane, only in the presence of an irreversible hydride acceptor. Circumstantial evidence that the BDE for RhfB is less than that for IrfB is provided by the ready isolation of [IrH(CS)(PPh3){HB(mt)2}], while the corresponding thiocarbonyl rhodaboratrane [RhH(CS)(PPh3){HB(mt)2}] has not been observed but rather evolves spontaneously into the unusual thioketone complex [RhH(PPh3){SdC(PPh3)BH(mt)2}] via insertion of CS into the RhfB bond.21d

Experimental Section General Methods. Conventional Schlenk and vacuum line techniques were routinely employed throughout for the exclusion of air. Unless otherwise stated, products were moderately air-stable; thus analytical samples were typically prepared in air. Solvents were distilled under prepurified nitrogen from appropriate drying agents. The compounds [IrCl(CO)(PPh3)3],62 [IrCl(CS)(PPh3)2],48 [RhCl(CO)(PPh3)2],63 Na[HB(mt)3],15 and Na[H2B(mt)2]4e were prepared according to published procedures. All other materials were obtained from commercial sources and used as supplied after spectroscopic and analytical verification of purity. All NMR spectra were recorded on a Varian Inova 300 instrument (1H, 299.9 MHz, 13C, 75.42 MHz, referenced to external SiMe4; 31P, 121.4 MHz, ref external 85% H3PO4; 11B, 96.23 MHz, ref external BF3 3 OEt2). Carbon-13 NMR assignments were confirmed with recourse to 2-D correlation (HMQC and HMBC) spectra. Elemental microanalytical and mass spectrometric data were provided by the ANU analytical services, the latter confirmed by simulation of isotopic manifolds. Synthesis of [IrH(CO)(PPh3){K3-B,S,S0 -B(mt)3}] (1). A mixture of [IrCl(CO)(PPh3)2] (1.00 g, 1.29 mmol) and Na[HB(mt)3] (0.503 g, 1.34 mmol) in dichloromethane (30 mL) was stirred under a closed atmosphere of N2 for 3 h. After allowing the suspension to settle, the solution was filtered via cannula to a second Schlenk flask and concentrated under reduced pressure. (62) Collman, J. P.; Sears, C. T., Jr.; Kubota, M. Inorg. Synth. 1990, 28, 92. (63) Evans, D.; Osborn, J. A.; Wilkinson, G. Inorg. Synth. 1990, 28, 79.

Article The addition of excess ether effected precipitation of an offwhite solid. The solvent was removed via cannula and the solid washed with diethyl ether and dried in vacuo. The product was recrystallized from a mixture of CH2Cl2 and hexane. Yield: 0.930 g (87%). IR (KBr): νCtO 1990 νIrH 2130 cm-1. NMR (CDCl3, 25 °C), 1H: δH -11.8 (s br, 1 H, IrH), 3.39, 3.54, 3.58 (s 3, 3 H 3, NCH3), 6.54, 6.64 (d 2, 3JHH 2.3 Hz, 1 H 2, NCHdCH), 6.59, 6.61 (d 2, 3JHH 2.0 Hz, 1 H 2, NCHdCH), 6.77, 6.92 (m br 2, 1 H 2, NCHdCH), 7.57-7.48 (m, 6 H, PPh3), 7.41-7.32 (m, 9 H, PPh3). 13C{1H }: δC 33.9, 34.4, 34.5 (CH3 3), 117.3, 118.9 (NCHdCH), 118.0 (br, NCHdCH 2), 122.4, 122.8 (NCHdCH), 128.1 [d, 3JPC 9.5, C3,5(C6H5)], 129.6 [d, 4JPC 2.0, C4(C6H5)], 133.8 [d, 2JPC 12.5, C2,6(C6H5)], 134.7 [d, 1JPC 36.3 Hz, C1(C6H5)], 163.6, 163.9, 166.3, 173.7 (CdS 3, CtO). 31P{1H}: δP 4.5 (m br, hhw 163 Hz). 11B{1H}: δB 3.19 (s br, hhw 149 Hz). Anal. Found: C, 44.47; H, 3.93; N, 9.66; S, 10.74. Calcd for C31H31BIrN6OPS3: C, 44.66; H, 3.75; N, 10.68; S, 11.54. Crystals of a nonstoichiometric dichloromethane solvate 1 3 (CH2Cl2)1.43 suitable for diffractometry were grown by slow diffusion of hexane into a dichloromethane solution of 1. Crystal data for 1 3 (CH2Cl2)1.34: [C31H31BIrN6OPS3] 3 (CH2Cl2)1.34], M = 947.36, monoclinic, C 2/c (No. 15), a = 39.7468(7) A˚, b = 9.6094(1) A˚, c = 20.3913(4) A˚, β = 99.9747(10)°, V = 7670.6(2) A˚3, Z = 8, Dc = 1.641 Mg m-3, μ(Mo KR) = 3.908 mm-1, T = 200(2) K, colorless needles, 6772 independent measured reflections, F refinement, R1 = 0.0288, wR2 = 0.0315 for 4591 independent absorption-corrected reflections [I > 3σ(I); 2θmax = 50°], 437 parameters, CCDC 736053. Synthesis of [IrH(CO)(PPh3){K3-B,S,S0 -BH(mt)2}] (2). In a procedure analogous to that used for the preparation of 1, Vaska’s complex (2.000 g, 2.56 mmol) and Na[H2B(mt)2] (0.726 g, 2.78 mmol) were stirred in dichloromethane (100 cmL) for 3 h and then isolated as for 1 above. The product was recrystallized from CH2Cl2 to yield pure 2 3 (CH2Cl2)0.5 as an offwhite solid. Yield: 1.61 g (87%). IR (CHCl3): νCtO 1988 cm-1, νIrH 2132, νBH 2384 cm-1. NMR (CDCl3, 25 °C), 1H: δH -13.4 (s br, 1 H, IrH), 3.35, 3.47 (s 2, 3 H 2, NCH3 2), 6.69, 6.72 (d 2, 3JHH 2.0, 1 H 2, NCHdCH), 6.74, 6.81 (d 2, 3JHH 1.9 Hz, 1 H 2, NCHdCH), 7.60-7.52 (m, 6 H, PPh3), 7.43-7.35 (m, 9 H, PPh3). 13C{1H}: δC 33.8, 34.4 (NCH3 2), 120.0, 120.3 (NCHdCH), 122.3, 122.7 (NCHdCH), 128.1 [d, 3JPC 9.6, C3,5(C6H5)], 129.8 [d, 4JPC 1.5, C4(C6H5)], 133.8 [d, 2JPC 13.0, C2,6(C6H5)], 135.5 [d, 1JPC 35.0 Hz, C1(C6H5)], 164.6, 164.9 (CdS), 175.6 (CtO). 31P{1H}: δP 6.2 (m br, hhw 95 Hz). 11B{1H}: δB -4.5 (s br, hhw 300 Hz). Anal. Found: C, 43.37; H, 3.88; N, 7.01; S, 7.90. Calcd for C27H27BIrN4OPS2(CH2Cl2)0.5: C, 43.23; H, 3.69; N, 7.33; S, 8.39. Crystals of the chloroform solvate 2 3 CHCl3 suitable for diffractometry were obtained via slow cooling of a saturated solution of 2 in chloroform. Crystal data for 2 3 CHCl3: [C27H27BIrN4OPS2] 3 CHCl3, M=841.05, triclinic, P1 (No. 2), a=9.3509(2) A˚, b=9.7847(2) A˚, c=19.1038(5) A˚, R=92.759(2)°, β=99.661(1)°, γ=110.926 (1)°, V = 1598.35(7) A˚3, Z = 2, Dc = 1.747 g cm-3, μ(Mo KR) = 46.4 cm-1, T = 200(2) K, yellow plates, 7345 independent measured reflections, F refinement, R1 = 0.030, wR2 = 0.035 for 6032 independent absorption-corrected reflections [I > 3σ(I); 2θmax = 48°], 373 parameters, CCDC 252699. Synthesis of [IrH(CS)(PPh3){K3-B,S,S0 -B(mt)3}] (4). A mixture of [IrCl(CS)(PPh3)2] (0.100 g, 0.125 mmol) and Na[HB(mt)3] (0.048 g, 0.128 mmol) in dichloromethane (10 mL) was stirred under a closed atmosphere of N2 for 20 min. After being allowed to settle, the solution was filtered via cannula to a second Schlenk and concentrated at reduced pressure. The addition of excess diethyl ether effected the precipitation of an off-white solid; the solvent was filtered off via cannula and the solid dried in vacuo. Yield: 0.07 g, 66%. IR (KBr): νIrH 2134, νCS 1363 cm-1. NMR (CDCl3, 25 °C), 1H: δH -9.3 (s br, hhw 12 Hz, JPH 6 Hz, 1 H, IrH), 3.32, 3.56, 3.60 (s 3, 3 H 3, NCH3 3), 6.59, 6.60, (s br 2, 1 H 2, NCHdCH); 6.64, 6.66 (d 2, 3JHH


335

2.0, 1 H 2, NCHdCH), 6.72, 6.95 (d 2, 3JHH 2.0 Hz, 1 H 2, NCHdCH), 7.64-7.55 (m, 6 H, PPh3), 7.43-7.33 (m, 9 H, PPh3). 13C{1H}: δC 33.9, 34.5, 34.6 (NCH3 3), 116.7, 118.3, 118.4, 120.5, 121.9, 122.9 (NCHdCH), 128.0 [d, 3JPC 10.0 Hz, C3,5(C6H5)], 129.7 [s, C4(C6H5)], 134.1 [d, 2JPC 12.0 Hz, C2,6(C6H5)], 163.6, 166.9 (CdS), M-CS not unequivocally identified. 31P{1H}: δP 6.8 (s br, hhw 156 Hz). 11B{1H}: δB 4.3 (s br, hhw 271 Hz). Anal. Found: C, 42.84; H, 3.72; N, 9.26; S, 13.76. Calcd for C31H31BIrN6PS4 3 (CH2Cl2)0.5: C, 42.40; H, 3.61; N, 9.42; S, 14.37. ESIþ MS: m/z 850.2 [M]þ, 849.3 [M - H]þ, 805.2 [M - CS - H]þ, 737.2 [M - mt]þ, 586.8 [M - PPh3]þ. Acc. Mass: 849.08659. Calcd for [M - H]þ = C31H30BN6PS4Ir: 849.08749. Synthesis of [IrH(CS)(PPh3){K3-B,S,S0 -BH(mt)2}] (5). A mixture of [IrCl(CS)(PPh3)2] (0.100 g, 0.125 mmol) and Na[HB(mt)3] (0.035 g, 0.134 mmol) in dichloromethane (10 mL) was stirred under a closed atmosphere of N2 for 1.5 h. After being allowed to settle, the solution was filtered via cannula to a second Schlenk and concentrated at reduced pressure. The addition of excess diethyl ether effected the precipitation of an off-white solid; the solvent was filtered off via cannula and the solid dried in vacuo. Yield: 0.04 g, 43%. NMR (CDCl3, 25 °C), 1 H: δH -11.1 (s br, JPH 7.6 Hz, 1 H, IrH), 3.32, 3.50 (s 2, 3 H 2, NCH3 2), 6.73 (m, 1 H 2, NCHdCH); 6.77, 6.84 (m 2, 1 H 2, NCHdCH), 7.67-7.56 (m, 6 H, PPh3), 7.45-7.33 (m, 9 H, PPh3). 31P{1H}: δP 9.6 (s br, hhw 112 Hz). 11B{1H}: δB -0.5 (s br, hhw 231 Hz). ESIþ MS: m/z 737.0 [M]þ, 474.9 [M PPh3]þ. Acc. Mass: 739.1105. Calcd for [HM]þ = C27H28N4S3BPIr: 739.0935. Satisfactory elemental microanalytical data not obtained due to solution instability. Formation of [Ir(CS)(PPh3){K4-B(mt)3}]Cl (6 3 Cl). 64 An NMR sample solution of 4 in CDCl3 was left to stand and monitored periodically by 1H NMR spectroscopy (Figure S1, Supporting Information). Optimal conversion was achieved after 4 days. NMR (CDCl3, 25 °C), 1H: δH 3.40 (s, 3 H, NCH3), 3.48 (s, 6 H, NCH3), 6.85 (m, 1 H, NCHdCH), 7.13 (m, 2 H, NCHdCH), 7.75 (d, 3JHH 2.2 Hz, 1 H, NCHdCH); 8.84 (d, 3 JHH 2.2 Hz, 2 H, NCHdCH), 7.58-7.43 (m, 15 H, PPh3). 13 C{1H}: δC 33.9 (NCH3, 2C), 34.1 (NCH3), 120.2, 125.1 (NCHdCH), 121.6, 124.8 (2C, NCHdCH), 128.5 [d, 3JPC 9.3 Hz, C3,5(C6H5)], 130.7 [s, 4JPC, C4(C6H5)], 133.5 [d, 2JPC 11.6, C2,6(C6H5)], 161.1, 163.3 (CdS), M-CS not resolved. 31 P{1H}: δP -0.3 (s br, hhw 148 Hz). 11B{1H}: δB 6.9 (s br, hhw 216 Hz). Synthesis of [IrHCl2(PPh3)3]. In a modification of a literature procedure65 a mixture of [IrCl6](NH4)2 (1.70 g, 3.85 mmol) and PPh3 (6.00 g, 23.00 mmol) in iPrOH (30 mL) was brought to reflux for 3 days. The resulting yellow suspension was allowed to cool to ambient temperature, then extracted with three successive 30 mL portions of diethyl ether, each of which was decanted from the mixture. These fractions were combined and filtered to recover any remaining suspended material. The bulk sample was collected by filtration, washed with ether, and dried in vacuo. The combined solids were recrystallized from CH2Cl2/hexane to afford the desired product as a pale yellow solid, which was characterized by comparison to literature data as the cis-mer isomer.65 Yield: 1.405 g (35%). NMR (CDCl3, 25 °C), 1H: δH -19.2 (dt, JPH 15, 13 Hz, 1H); 7.35-6.85 (m, 45H). 31P{1H}: δP -2.7 (d, JPP 18, 2P); -7.9 (t, JPP 18, 1P). Synthesis of [IrHCl(PPh3)2{K2-S,S0 -HB(mt)3}] (7). A mixture of [IrHCl2(PPh3)3] (0.500 g, 0.476 mmol) and Na[HB(mt)3] (0.178 g, 0.476 mmol) in dichloromethane (40 mL) was stirred (64) We note that while all other data (spectroscopic and microanalytical) unequivocally confirm the identity and high purity of compounds 6 to 8, carbon analyses have routinely been found to be significantly in error. Given the reproducibility of this error and the lack of deviation for other elemental compositions (H, N, S), we must conclude this to be a peculiarity of rhodaboratranes for which we are currently unable to account. (65) Vaska, L.; DiLuzio, J. W. J. Am. Chem. Soc. 1962, 84, 4989.

336


under a closed N2 atmosphere for 2 h. After allowing the halide salts to settle, the solution was filtered via cannula to a second Schlenk and concentrated under reduced pressure. The addition of excess Et2O resulted in precipitation of the product. The solvent was filtered off by cannula and the solid dried in vacuo. Recrystallization from dichloromethane/hexane yielded 7 as a pale yellow solid. Yield: 0.437 g, 83%. NMR (CDCl3, 25 °C), 1 H: δH -18.1 (dd, JPH 20.2, 14.2 Hz, 1 H), 2.80, 3.38, 3.70 (s 3, 3 H 3, NCH3 3), 6.69 (d, 3JHH 2.0 Hz, 1H, NCHdCH), 6.96 (overlapping doublets (2), 3JHH 1.8, 1 H 2, NCHdCH), 6.98, (d, 3JHH 2.0 Hz, 1H, NCHdCH); the final two imidazole resonances are lost within the aromatic region 7.49-6.99 (m, 32 H, PPh3 þ 2 NCHdCH). 13C{1H}: δC 33.2, 34.2, 35.2 (NCH3 3), 120.5, 121.3, 123.5 (NCHdCH) other imidazole resonances buried under aromatics, complex unresolved aromatics, 152.6, 154.0, 155.6 (CdS). 31P{1H}: δP 6.0 (dd, JPP 14 Hz, JPH 13 Hz, 1P), 0.83 (dd, JPP 14 Hz, JPH 20 Hz, 1P). ESIþ MS: m/z 1069.5 [M - Cl]þ, 805.3 [M - Cl - PPh3 - H]þ. High res: [M - Cl]þ 1069.22, C48H47BN6P2S3Ir. Anal. Found: C, 51.84; H, 4.49; N, 7.87; S, 8.68; Cl, 3.61. Calcd for C27H27BIrN4OPS2: C, 52.20; H, 4.29; N, 7.61; S, 8.71; Cl, 3.21. Synthesis of [Rh(CO)(PPh3){K2-HB(mt)3}] (8).64 A mixture of [RhCl(CO)(PPh3)2] (0.200 g, 0.289 mmol) and Na[HB(mt)3] (0.110 g, 0.294 mmol) in dichloromethane (15 mL) was stirred under a closed atmosphere of N2 for 1.5 h. After being allowed to settle, the solution was filtered via cannula to a second Schlenk and concentrated at reduced pressure. The slow addition of excess diethyl ether effected the precipitation of an offwhite solid; the solvent was filtered off via cannula and the solid dried in vacuo. Yield: 0.144 g, 67%. IR (KBr): νCO 1973, νBH 2307. NMR (CDCl3, 25 °C), 1H: δH 3.53, (9 H, NCH3), 6.66, (6, 3 JHH 1.9 Hz, 3 H, NCHdCH); 7.11 (s br, 3 H, NCHdCH), 7.72-7.61 (m, 6 H, PPh3), 7.41-7.30 (m, 9 H, PPh3). 31P{1H}: δP 40.1 (d, JRhP 159.7 Hz). 11B{1H}: δB -1.76 (s br, hhw 300 Hz). 13 C{HMQC}: δC 35.2 (NCH3), 118.8 (NCHdCH), 122.8 (2C, NCHdCH), 128.1, 130.1, 133.4 (PPh3).

Crossley et al. Synthesis of [Rh(CO)(PPh3){K2-H2B(mt)2}] (9). A mixture of [RhCl(CO)(PPh3)2] (0.200 g, 0.289 mmol) and Na[H2B(mt)2] (0.090 g, 0.343 mmol) in dichloromethane (15 mL) was stirred under a closed atmosphere of N2 for 1.5 h. After being allowed to settle, the solution was filtered via cannula to a second Schlenk and concentrated at reduced pressure. The slow addition of excess diethyl ether effected the precipitation of an off-white solid; the solvent was filtered off via cannula and the solid dried in vacuo. Yield: 0.119 g, 65%. IR (KBr): νCO 1976, νBH 2380, νBHRh 2208 cm-1; (CH2Cl2): νCO 1965, νBH 2350, 2338 cm-1. NMR (CDCl3, 25 °C), 1H: δH 3.32 (9 H, NCH3), 6.48 (d, 3JHH 1.6 Hz, 3 H, NCHdCH); 6.72 (d, 3JHH 1.6 Hz, 3 H, NCHdCH), 7.72-7.63 (m, 6 H, PPh3), 7.34-7.23 (m, 9 H, PPh3). 31P{1H}: δP 44.6 (d, JRhP 151.0 Hz). 11B{1H}: δB -4.7 (s br, hhw 179 Hz). 13 C{1H}(CD2Cl2): δC 35.0 (NCH3), 119.6 (NCHdCH), 122.2 (NCHdCH), 128.0 [d, 3JPC 10 Hz, C3,5(C6H5)], 129.9 [s, 4JPC 1.5, C4(C6H5)], 134.3 [d, 2JPC 11.1, C2,6(C6H5)], 160.9 (CdS). Spectroscopic Elucidation of [Rh(CO)(PPh3){K4-B(mt)3}]Cl (10 3 Cl). NMR (CDCl3, 25 °C), 1H: δH 3.47 (s, 6 H, NCH3), 3.48 (s, 3 H NCH3), 6.74 (d, 3JHH 2.0 Hz, 1 H, NCHdCH), 6.97 (d, 3JHH 2.0 Hz, 2 H, NCHdCH), 8.86 (d, 3JHH 2.2 Hz, 2 H, NCHdCH), 8.90 (d, 3JHH 2.2 Hz, 2 H, NCHdCH); 8.84 (d, 3JHH 2.2 Hz, 2 H, NCHdCH), 7.58-7.43 (m, 15 H, PPh3). 31 P{1H}: δP -5.6 (m br).

Acknowledgment. We thank the Australia Research Council (DP034270, DP0771497) for financial support. I.R.C. gratefully acknowledges the award of a Royal Society Fellowship. Supporting Information Available: Full details of crystallographic structure determination for the solvate of 1 3 (CH2Cl2)1.43 (CCDC 736053) and 2 3 CHCl3 (CCDC 252699) in CIF format; 1H NMR spectra monitoring the progress of the reaction of 4 with CDCl3. This material is available free of charge via the Internet at http://pubs.acs.org.

Metallaboratranes: Bis- and Tris(methimazolyl)borane Complexes of

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