Iron–iron bonding versus iron–phosphorus bonding in binuclear diphosphacyclobutadiene iron carbonyl complexes

The diphosphacyclobutadiene sandwich compounds (η4-But2C2P2)2Mz (M = Ni, z = 0; M = Co, z = −1, 0; M = Fe, z = −1, 0) as well as the iron carbonyl derivative (η4-But2C2P2)Fe(CO)3 have recently (2008) been synthesized by Lammertsma and co-workers using the dimerization of the phosphaalkyne ButCP: on suitable reactive transition metal sites. The structures and energetics of the closely related dimethyl derivatives (Me2C2P2)Fe(CO)n (n = 3, 2, 1) and (Me2C2P2)2Fe2(CO)n (n = 5, 4, 3, 2) have now been investigated by density functional theory. For (Me2C2P2)2Fe2(CO)5 a structure with a bridging η1,η4-Me2C2P2 ring and no iron–iron bond is energetically preferred by more than 24 kcal/mol over an alternative structure with only terminal η4-Me2C2P2 rings and an iron–iron single bond. The lowest energy singlet and triplet (Me2C2P2)2Fe2(CO)4 structures have at least one bridging η1,η4 ligand, which in one case is in the form of an (η4-Me2C2P2)2Fe(CO) sandwich bidentate ligand chelating to an Fe(CO)3 unit through two ring phosphorus atoms. In contrast to (Me2C2P2)2Fe2(CO)n (n = 5, 4), the lowest energy structures for (Me2C2P2)2Fe2(CO)3 have only terminal η4-Me2C2P2 ligands, three carbonyl groups, and an FeFe triple bond. Interesting structures for the dicarbonyl (Me2C2P2)2Fe2(CO)2 include a structure with a (η4-Me2C2P2)2Fe sandwich ligand chelating to an Fe(CO)2 group and a structure with exclusively terminal η4-Me2C2P2 ligands and a short Fe–Fe distance of ∼2.16 Å, suggesting a formal quadruple bond.

The lowest energy (Me2C2P2)2Fe2(CO)5 structure has a bridging η1,η4-Me2C2P2 ring and no iron–iron bond. The lowest energy singlet and triplet (Me2C2P2)2Fe2(CO)4 structures have at least one bridging η1,η4 ligand. However, the lowest energy structures for (Me2C2P2)2Fe2(CO)3 have only terminal η4-Me2C2P2 ligands, three bridging carbonyl groups, and an FeFe triple bond.Figure optionsDownload as PowerPoint slide