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

摘要

The diphosphacyclobutadiene sandwich compounds (η~4-Bu ~t _2C_2P_2)_2M~z (M = Ni, z = 0; M = Co, z = -1, 0; M = Fe, z = -1, 0) as well as the iron carbonyl derivative (η~4-Bu~t _2C_2P _2)Fe(CO)_3 have recently (2008) been synthesized by Lammertsma and co-workers using the dimerization of the phosphaalkyne Bu ~tCP: on suitable reactive transition metal sites. The structures and energetics of the closely related dimethyl derivatives (Me_2C _2P_2)Fe(CO)_n (n = 3, 2, 1) and (Me _2C_2P_2)_2Fe_2(CO) _n (n = 5, 4, 3, 2) have now been investigated by density functional theory. For (Me_2C_2P_2)_2Fe _2(CO)_5 a structure with a bridging η~1, η~4-Me_2C_2P_2 ring and no iron-iron bond is energetically preferred by more than 24 kcal/mol over an alternative structure with only terminal η~4-Me_2C_2P _2 rings and an iron-iron single bond. The lowest energy singlet and triplet (Me_2C_2P_2)_2Fe _2(CO)_4 structures have at least one bridging η~1,η~4 ligand, which in one case is in the form of an (η~4-Me_2C_2P_2) ~2Fe(CO) sandwich bidentate ligand chelating to an Fe(CO)_3 unit through two ring phosphorus atoms. In contrast to (Me_2C _2P_2)_2Fe_2(CO)_n (n = 5, 4), the lowest energy structures for (Me_2C_2P _2)_2Fe_2(CO)_3 have only terminal η~4-Me_2C_2P_2 ligands, three carbonyl groups, and an FeFe triple bond. Interesting structures for the dicarbonyl (Me_2C_2P_2)_2Fe _2(CO)_2 include a structure with a (η~4- Me_2C_2P_2)_2Fe sandwich ligand chelating to an Fe(CO)_2 group and a structure with exclusively terminal η~4-Me_2C_2P_2 ligands and a short Fe-Fe distance of ～2.16 ?, suggesting a formal quadruple bond.