Bi- and trimetallic sigma-acetylide complexes connected through a phenyl ring in the Fe(Cp*)(dppe) series

Treatment of 1 equiv of the 1,3-bis(trimethylsilylethynyl)benzene with 2.3 equiv of [Fe(Cp*)(dppe)Cl] [1, Cp* = eta(5)-C5Me5, dppe = eta(2)-bis(diphenylphosphino)ethane], KF and KPF6 salts in a methanol/THF mixture (10:1) produced the bis(iron) alkyne complex [{Fe(Cp*)(eta(2)-dppe)(C=C-)}(2)(1,3-C6H4)] (2) in 80% yield. Similarly, the trinuclear iron derivative [{Fe(Cp*)(eta(2)-dppe)(C=C-)}(3)(1,3,5-C6H3)] (3) was obtained from reaction of 1 with 1,3,5-tris(trimethylsilylethynyl)benzene (80% yield with respect to the organic ligand). The X-ray crystal structure of 3 shows that it crystallizes in the triclinic space group with unit-cell parameters of a = 18.142(6) Angstrom, b = 18.652(8) Angstrom, c = 20.108(6) Angstrom, alpha = 113.72(3), beta = 89.87(3), gamma = 116.07(3)degrees, and Z = 2. The structure was solved and refined (6922 reflections) to the final values R = 0.081 and R-w = 0.069. Cyclic voltammetric analysis of complexes 2 and 3 from -1.0 to +0.5 V displays two and three one-electron reversible oxidation waves, respectively. The redox processes are all separated by 0.130 +/- 0.010 V, indicating significant electronic communication between the metal centers. A theoretical treatment using density functional molecular orbital calculations has been made on compounds 2, 3, and the related compound [{Fe(Cp*)(eta(2)-dppe)(C=C-)}(2)(1,4-C6H4)]. These results verify the experimental structure of 3 and allow an interpretation of its electronic structure.