Reactivities of a Bis(alkylidene) Complex. Synthesis of a Silyl Bis(alkylidyne) Complex and a Reaction Cycle among Symmetric Bis(alkylidyne), Bis(alkylidene), and Nonsymmetric Bis(alkylidyne) Compounds

Abstract

An unstable bis(alkylidene) complex (RCH2)4Ta2(CHR)2(Cl)2 (R = SiMe3, 1), prepared by the addition of 2 equiv of HCl to a symmetrically bridging bis(alkylidyne) complex (RCH2)4Ta2(μ-CR)2 (2a), decomposes through elimination of SiMe4 to form an unstable alkylidene−alkylidyne complex (RCH2)3Ta2(CHR)(⋮CR)(Cl)2 (3). This conversion of an alkylidene to alkylidyne ligand was found to follow first-order kinetics with ΔH1⧧ = 14.1(0.8) kcal/mol and ΔS1⧧ = −12(3) eu. This is, to our knowledge, the second reported kinetic study of alkylidene → alkylidyne conversion, and the kinetics of this conversion is in contrast to that of (RCH2)3TaCHR → (RCH2)4Ta2(μ-CR)2 (2a) reported earlier. Substitution of the chlorides by SiR3- (R = SiMe3) leads to a preferential elimination of HSiR3 and formation of the first silyl bis(alkylidyne) complex (RCH2)(R3Si)Ta(μ-CR)2Ta(CH2R)2 (4). Addition of excess PMe3 to 1 produces a novel 1,1‘-dimetallacyclobutadiene derivative (Me3P)2(Cl)Ta(μ-CR)2Ta(Cl)(CH2R)2 (5a) with two nonsymmetrically bridging alkylidyne ligands. Both bridging ligands coordinate with CTa double bonds to the same metal atom (and with C−Ta single bonds to the other metal atom). The two bridging alkylidyne ligands in 5a, which are coordinated to one metal atom in the axial and equatorial position, respectively, are involved in an intramolecular ligand exchange. The kinetic barriers for this exchange were determined to be ΔH2⧧ = 13.6(0.4) kcal/mol and ΔS2⧧ = −5(2) eu in 5a, and ΔH3⧧ = 13.0(0.3) kcal/mol and ΔS3⧧ = −5(1) eu in 5b. Addition of 2 equiv of RCH2Li to replace the Cl ligands in 5a converts 5a to 2a, completing a reaction cycle 2a → 1 → 5a → 2a. (Me3P)2(RCH2)Ta(μ-CR)2Ta(Cl)(CH2R)2 (6) was identified as an intermediate in the conversion of 5a to 2a.