Formation of Metallocene-Stabilized Planar-Tetracoordinate Carbon Compounds by a Protonation Route

Abstract

Bis(alkynyl)zirconocene and -hafnocene complexes 4 react with (butadiene)zirconocene or -hafnocene, respectively, to yield the binuclear σ,π-acetylide bridged complexes [(Cp2M)2(μ-C≡C−R)2] 7 (M = Zr, Hf; R = CH3, C2H5, or CH2Ph). The complexes 7 are selectively protonated upon treatment with N,N-dimethylanilinium tetraphenylborate or tetrakis(pentafluorophenyl)borate to yield the dinuclear metallocene cation complexes [(Cp2M1)(μ-RCC1−H)(μ-C≡C−R)M2Cp2+] 13 (M1 = M2 = Zr, a−c; M1 = Zr, M2 = Hf, h; M1 = M2 = Hf, i) that contain a planar-tetracoordinate carbon atom (C1-H) bearing a hydrogen substituent. Complex 13a was characterized by X-ray diffraction. It reveals an agostic interaction of the C1-H moiety with the M2Cp2 unit. The planar-tetracoordinate carbon unit C1-H of 13a exhibits 13C/1H NMRchemical shifts of δ = 161.0 and −0.11 and a coupling constant of 1JCH = 103 Hz. Selective protonation of (μ-alkynyl)bis(group 4 metallocene) complexes containing other additional bridging ligands provides a rather general synthetic entry to planar-tetracoordinate carbon containing complexes of this type: [Cp2M1(μ-C≡C−CH3)(μ-Cl)M2Cp2] (10a−c, M1, M2 = Zr, Hf) and[Cp2M1(μ-C≡C−CH3)(μ-CH3)M2Cp2] (11a,b) selectively add a proton from [(HNMe2Ph)+(BAr4)−] at carbon atom C1 of the acetylide ligand to yield the corresponding “anti-van′t Hoff/LeBel complexes” [Cp2M1(μ-CH3CC1-H)(μ-X)M2Cp2+] (13d−g and k, X = Cl, CH3), respectively.