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)(mu -GaMes) (Cp' = eta -C5Me5 (1a), eta -C5H5 (1b); Mes = 2,4,6-trimethylphenyl) were synthesized by the reactions of M[Cp'Fe(CO)(2)] (M = K, Na) with MesGaCl(2) 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)(mu -CO)(mu -GaMes) (2a: Cp* = eta -C5Me5), in which the two Cp* rings are located on the opposite side with respect to the GaFe2C four-membered ring. In contrast to la, photolysis of a toluene solution of Ib afforded a mixture of the cis and trans isomers of [CpFe(CO)](2)(mu -CO)(mu -GaMes) (2b: Cp = eta -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) Angstrom) and trans-2b (2.3438(5) Angstrom) are much shorter than those of 1a (2.4315(15) Angstrom). A kinetic investigation of the isomerization between cis-ab and trans-2b afforded the activation parameters (trans --> cis: DeltaH(double dagger) = 102(1) kJ mol(-1) DeltaS(double dagger) = 36(4) J K-1 mol(-1), DeltaG(298)(double dagger) = 90(2) kJ mol(-1); cis -->trans: DeltaH(double dagger) =105(2) kJ mol(-1), DeltaS(double dagger) = 52(6) J K-1 mol(-1), DeltaG(298)(double dagger) = 90(3) kJ mol(-1)). A possible mechanism is proposed for the isomerization which contains [CpFe(CO)(2)][CpFe(CO)](mu -GaMes) as a key intermediate.