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 (RCH 2 ) 4 Ta 2 ( CHR) 2 (Cl) 2 (R = SiMe 3, 1 ), prepared by the addition of 2 equiv of HCl to a symmetrically bridging bis(alkylidyne) complex (RCH 2 ) 4 Ta 2 (μ-CR) 2 ( 2a ), decomposes through elimination of SiMe 4 to form an unstable alkylidene−alkylidyne complex (RCH 2 ) 3 Ta 2 ( CHR)(⋮CR)(Cl) 2 ( 3 ). This conversion of an alkylidene to alkylidyne ligand was found to follow first-order kinetics with Δ H 1 ⧧ = 14.1(0.8) kcal/mol and Δ S 1 ⧧ = −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 (RCH 2 ) 3 Ta CHR → (RCH 2 ) 4 Ta 2 (μ-CR) 2 ( 2a ) reported earlier. Substitution of the chlorides by SiR 3 - (R = SiMe 3 ) leads to a preferential elimination of HSiR 3 and formation of the first silyl bis(alkylidyne) complex (RCH 2 )(R 3 Si)Ta(μ-CR) 2 Ta(CH 2 R) 2 ( 4 ). Addition of excess PMe 3 to 1 produces a novel 1,1‘-dimetallacyclobutadiene derivative (Me 3 P) 2 (Cl)Ta(μ-CR) 2 Ta(Cl)(CH 2 R) 2 ( 5a ) with two nonsymmetrically bridging alkylidyne ligands. Both bridging ligands coordinate with C Ta 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 Δ H 2 ⧧ = 13.6(0.4) kcal/mol and Δ S 2 ⧧ = −5(2) eu in 5a, and Δ H 3 ⧧ = 13.0(0.3) kcal/mol and Δ S 3 ⧧ = −5(1) eu in 5b . Addition of 2 equiv of RCH 2 Li to replace the Cl ligands in 5a converts 5a to 2a, completing a reaction cycle 2a → 1 → 5a → 2a . (Me 3 P) 2 (RCH 2 )Ta(μ-CR) 2 Ta(Cl)(CH 2 R) 2 ( 6 ) was identified as an intermediate in the conversion of 5a to 2a .