Synthesis and Structural Features of Arduengo Carbene Complexes of Group 4 Metallocene Cations

• Published in: Organometallics 2002-07, Vol.21 (14), p.2905-2911
• Main Authors: Niehues, Martin, Erker, Gerhard, Kehr, Gerald, Schwab, Pia, Fröhlich, Roland, Blacque, Olivier, Berke, Heinz
• Format: Article
• Language: English
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/om020088k

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) Å, d(Ti−CH3) = 2.178(3) Å] was bonded to titanium in an orientation where the imidazol-2-ylidene ring lies in the major σ-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 σ-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 C4HC5H 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).