Synthesis, Structure, and Ligand Exchange Reactions of Tetramethyleneethane Complexes of Cobalt

The synthesis of (eta(3):eta(3)-TME)[Co(CO)(3)](2) (1) was achieved using 2,3-bis(bromomethyl)-1,3-butadiene (TMEBr2) as the tetramethyleneethane (TME) ligand precursor and Na[Co(CO)(4)]. Solution NMR studies suggested an eta(3):eta(3)-configuration, which has been confirmed in the solid state by single-crystal X-ray diffraction studies. The series of complexes (eta(3):eta(3)-TME)[Co(CO)(2)PR3](2) (R = Me, 2; R = Et, 3; R = n-Bu, 4; R = Ph, 5; R = OPh, 6) were also synthesized by ligand exchange reactions, demonstrating that only one carbonyl may be exchanged for a phosphine group on each metal center. The eta(3):eta(3)-configuration of the tetramethyleneethane ligand in these complexes was determined by crystallographic studies. The effect of the electron-donating properties of PR3 was studied by cyclic voltammetry (CV) and infrared spectroscopy. The greatest degree of electron donation was seen when R = Et (3) and lowest when R = Ph (5) or R = OPh (6). Electronic communication between the metal centers was observed by CV. The chemical oxidation of 1 resulted in a highly unstable species that decomposed to {[(CO)(2)Co]TME[Co(CO)(3)]}(+)[BF4](-) (1(+)d), determined by its crystal structure. The synthesis of (eta(4):eta(4)-TME)[CoCp*](2) (7) has been achieved using a dipotassium 2,3-bis(methylene)-1,3-butanediyl (TMEK2) synthon. NMR studies suggested that 7 adopts an unusual eta(4):eta(4)-configuration, which was confirmed with the aid of crystallographic studies. DFT calculations were performed in order to rationalize the bonding for 1, 7, and hypothetical (eta(4):eta(4)-TME)[CoCp](2) (8). The large energy difference between the two coordination isomers 1 and la confirmed the eta(3):eta(3)-configuration. For isomers 7/7a and 8/8a, the energy difference between the two isomers (ca. 15 kJ mol(-1)) is in favor of the eta(4):eta(4)-configuration. For complexes 1(+) and 8(+), the calculations suggested complete delocalization on the system when one electron was removed.