Metallaboratranes: Bis- and Tris(methimazolyl)borane Complexes of Group 9 Metal Carbonyls and Thiocarbonyls

The iridium poly(methimazolyl)borane complexes [IrH(CE)(PPh3){kappa(3)-B,S,S'-B(mt)(2)R}](Ir -> B)(8) (mt =methimazolyl = 2-mercapto-3-methylimidazol-1-yl; E=O,S; R = mt, H) are described in detail. For R = nit, 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){kappa(4)-B(mt)(3)}]X(Ir -> B)(8) (E = O, S; X = Cl, BF4). The rhodium poly(methimazolyl)borate complexes [Rh(CO)(PPh3){kappa(2)-S,S'-HB(mt)(2)R}] (R = mt, H) are also reported. These compounds are obtained in preference to the respective borane complexes (analogous to iridium); however [Rh(CO)(PPh3){kappa(2)-S,S'-HB(mt)(3)}] is observed to undergo facile solution-phase conversion to [Rh(CO)(PPh3){kappa(4)-B,S,S',S ''-B(mt(3)}]Cl(Rh -> B)(8) in chiorinated solvents. The ramifications of these results, with respect to metallaboratrane formation, are discussed, Substantiating previous mechanistic conjecture. In all attempt to establish an alternative route to iridaboratranes, the first isolable tris(methimazolyl)borate complex of iridium, cis,cis-[IrHCl(PPh3)(2){kappa(2)-S,S'-HB(mt)(3)], is reported and shown not to evolve to the iridaboratrane [IrCl(PPh3){kappa(4)-B(mt)(3)}](Ir -> B)(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.