Synthesis and structural features of Arduengo carbene complexes of group 4 metallocene cations

Treatment of [Cp2TiCH3(thf)(+)] (5), with [BPh4-] anion) with 1,3-diisopropylimidazol-2-ylidene (4) at ambient temperature resulted in a rapid displacement of the thf ligand by the stable heterocyclic carbene to yield the Arduengo carbene methyltitanocene cation complex 6a (> 90% isolated). The X-ray crystal structure analysis of 6a showed that the heteroatom-stabilized carbene ligand [d(Ti-C(carbene) = 2.289(2) Angstrom, d(Ti-CH3) = 2.178(3) Angstrom] was bonded to titanium in an orientation where the imidazol-2-ylidene ring lies in the major sigma-ligand plane of the bent metallocene moiety. A DFT calculation of 6a and a series of related model compounds has revealed that the Arduengo carbene serves as a pure sigma-donor ligand to the titanocene moiety. The observed favored "in-plane" orientation of the ligand is steric in origin. Consequently, complex 6a attains an analogous C-s-symmetric structure in solution, featuring symmetry-equivalent Cp rings and a pair of diastereotopic isopropyl substituents as well as chemically differentiated imidazol-2-ylidene (CH)-H-4=(CH)-H-5 groups. The reaction of the ion pair [(Cp2ZrCH3)(+)(CH3B(C6F5)(3)(-)] (7) with 4 gave the analogous Arduengo carbene zirconocene cation complex 6b (> 95% isolated, with [CH3B(C6F5)(3)(-)] anion).