Reactions of a phosphido-bridged unsymmetrical diiron complex (η5-C5Me5)Fe2(CO)4(μ-CO)(μ-PPh2) with various alkynes

A phosphido-bridged unsymmetrical diiron complex (η5-C5Me5)Fe2(CO)4(μ-CO)(μ-PPh2) (1) was synthesized by a new convenient method; photo-dissociation of a CO ligand from (η5-C5Me5)Fe2(CO)6(μ-PPh2) (2) that was prepared by the reaction of Li[Fe(CO)4PPh2] with (η5-C5Me5)Fe(CO)2I. The reactivity of 1 toward various alkynes was studied. The reaction of 1 with tBuCCH gave a 1:1 mixture of two isomeric complexes (η5-C5Me5)Fe2(CO)3(μ-PPh2)[μ-CHC(tBu)C(O)] (3) containing a ketoalkenyl ligand. The reactions of 1 with other terminal alkynes RCCH (R=H, CO2Me, Ph) afforded complexes incorporating one or two molecules of alkynes and a carbonyl group. The principal products were dinuclear complexes bridged by a new phosphinoketoalkenyl ligand, (η5-C5Me5)Fe2(CO)3(μ-CO)[μ-CR1CR2C(O)PPh2] (4a: R1=H, R2=H; 4b: R1=CO2Me, R2=H; 4c: R1=H, R2=Ph). In the cases of alkynes RCCH (R=H, CO2Me), dinuclear complexes having a new ligand composed of two molecules of alkynes, a carbonyl group, and a phosphido group; i.e. (η5-C5Me5)Fe2(CO)3[μ-CRCHCHCRC(O)PPh2] (5a: R=H; 5b: R=CO2Me), were also obtained. In all cases, mononuclear complexes, (η5-C5Me5)Fe(CO)[CR1CR2C(O)PPh2] (6a: R1=H, R2=H; 6b: R1=H, R2=CO2Me; 6c: R1=H, R2=Ph) were isolated in low yields. The structures of 1, 4c, 5b, and 6a were confirmed by X-ray crystallography. The detailed structures of the products and plausible reaction mechanisms are discussed.