Synthesis and Structures of Mesitylgallylene-Bridged Diiron Complexes [Cp‘Fe(CO)]₂(μ-CO)(μ-GaMes) (Cp‘ = η-C₅Me₅, η-C₅H₅; Mes = 2,4,6-Trimethylphenyl) and Kinetic Study of the Cis−Trans Isomerization of [(η-C₅H₅)Fe(CO)]₂(μ-CO)(μ-GaMes)

Gallylene-bridged diiron complexes [Cp‘Fe(CO)2]2(μ-GaMes) (Cp‘ = η-C5Me5 (1a), η-C5H5 (1b); Mes = 2,4,6-trimethylphenyl) were synthesized by the reactions of M[Cp‘Fe(CO)2] (M = K, Na) with MesGaCl2 in THF. Photolysis of a toluene solution of complex 1a resulted in the formation of the trans isomer of the gallylene- and carbonyl-bridged diiron complex [Cp*Fe(CO)]2(μ-CO)(μ-GaMes) (2a: Cp* = η-C5Me5), in which the two Cp* rings are located on the opposite side with respect to the GaFe2C four-membered ring. In contrast to 1a, photolysis of a toluene solution of 1b afforded a mixture of the cis and trans isomers of [CpFe(CO)]2(μ-CO)(μ-GaMes) (2b: Cp = η-C5H5). Recrystallization of 2b from a toluene/hexane solution afforded pure trans-2b. Crystal structure analyses of 1a, 2a, and trans-2b revealed that the Fe−Ga distances of 2a (2.3147(9) Å) and trans-2b (2.3438(5) Å) are much shorter than those of 1a (2.4315(15) Å). A kinetic investigation of the isomerization between cis-2b and trans-2b afforded the activation parameters (trans → cis: ΔH⧧ = 102(1) kJ mol-1, ΔS⧧ = 36(4) J K-1 mol-1, ΔG⧧298 = 91(2) kJ mol-1; cis → trans: ΔH⧧ = 105(2) kJ mol-1, ΔS⧧ = 52(5) J K-1 mol-1, ΔG⧧298 = 90(3) kJ mol-1). A possible mechanism is proposed for the isomerization which contains [CpFe(CO)2][CpFe(CO)](μ-GaMes) as a key intermediate.