Interplay among Tetrahedrane, Butterfly Diradical, and Planar Rhombus Structures in the Chemistry of the Binuclear Iron Carbonyl Phosphinidene Complexes Fe_2(CO)_6(PX)_2

摘要

Density functional theory studies on a series of Fe_2(CO)_6(PX)_2 derivatives show the tetrahedrane to be the most stable for the alkyl (X = Me, ~tBu), P-H (X = H), and chloro (X = Cl) derivatives. However, butterfly diradical and planar rhombus structures are found to be more stable than tetrahedranes for the amino (X = NH_2, NMe_2, and N~iPr_2) and aryloxy (R = 2,6-~tBu_2-4-Me-C_6H_2O) derivatives. For the chloro (X = Cl) and methoxy (X = OMe) derivatives energetically accessible bishomotetrahedrane Fe_2(CO)_6P_2(μ -X)_2 isomers are observed in which the X substituents on the phosphorus atoms interact with the iron atom to form two direct Fe-X bonds at the expense of two of the four Fe-P bonds. In addition, the global minimum for the hydroxy (X = OH) derivative is an unusual FeP-butterfly structure with a central Fe-P bond as well as two external Fe-P bonds, one external P-P bond, and one external Fe=Fe double bond. Comparison of calculated with experimental ν(CO) frequencies shows that low-temperature Nujol matrix photolysis of (~iPr_2NP)_2COFe_2(CO)_6 leads to a planar rhombus rather than a tetrahedrane isomer of Fe_2-(CO)_6(PN~iPr_2)_2.