Spectroscopic, Structural, Electrochemical, and Kinetic Studies of Ligand Substitution in the 33e Dinuclear Radical Fe2(CO)7(μ-PPh2) and the 34e Analogues - and FeCo(CO)7(μ-PPh2)

Article abstract of DOI:10.1021/ja00233a017

The 33e dinuclear radical Fe2(CO)7(μ-PPh2) undergoes rapid CO ligand substitution with a variety of tertiary phosphorus ligands, L, to give mono- and disubstituted 33e products, which were characterized by elemental analysis and by IR and ESR spectroscopy.While the first susbtitution gives a single product, with L on the six-coordinate Fe center trans to the PPh2 bridge (confirmed by X-ray diffraction for L = P(OMe)3), further substitution (observed for L = PMe3, PEt3, P(OMe)3) is complex, giving two isomeric 33e disubstituted radicals, minor amounts of 35e addition products Fe2(CO)6L2(μ-PPh2), and diamagnetic disproportionation products ^+- (L = PMe3, n = 0; L = P(OMe)3, n = 2), as confirmed by an X-ray diffraction study of the PMe3 derivative.The 34e anion ^-, as the (Et4N)⁺ salt, adds two ligands in THF to give the 36e anions ^- (L = PMe3, PPh3, P(OMe)3), which have one L on each Fe, both trans to the PPh2 bridge (confirmed by X-ray diffraction for L = PPh3).The intermediacy of the monosubstituted 34e anion was ruled out.The 34e heterobimetallic complex FeCo(CO)7(μ-PPh2) reacts with PPh3 to give a 34e kinetic product with L on Co trans to the PPh2 bridge; this product rearranges at 25 deg C to the thermodynamic product with L on Fe.With P(OMe)3, monosubstitution occurs as above and disubstitution gives both 34e and 36e products, both with one L on each metal (confirmed for the 34e product by X-ray diffraction).With PMe3, ligand addition gives 36e FeCo(CO)7(μ-PPh2)>^-, with L on Co.Electrochemical studies show that the 33e unsubstituted and monosubstituted diiron radicals exhibit chemically reversible 1e reductions to the 34e CO-bridged anions.A 1e oxidation of the disubstituted 36e anion ^- leads to the monosubstituted 33e radical, via loss of PPh3.While oxidation of 34e FeCo(CO)7(μ-PPh2) is chemically irreversible, 1e reduction leads to CO loss to give the 33e radical anion ^-, which undergoes a further chemically reversible reduction to the 34e dianion.Similarly, 1e reduction of monosubstituted FeCo(CO)6(PPh3)(μ-PPh2) gives the 33e monosubstituted radical anion via CO loss, while a chemically reversible 1e oxidation gives the 33e radical cation ⁺.Kinetic studies of ligand monosubstitution in the 33e diiron radical Fe2(CO)7(μ-PPh2) using transient electrochemical techniques are consistent with an associative mechanism involving a 35e radical intermediate.Activation parameters obtained support the proposed associative pathway.Comparison of the reactivities of 33e Fe2(CO)7(μ-PPh2) and its 34e analogues ^- and FeCo(CO)7(μ-PPh2) show that the radical complex is about 10⁵ - 10⁶ times more reactive toward PPh3 than the diamagnetic 34e compounds.The mono- and disubstituted 35e radicals have been observed by ESR spectroscopy for various L's and are proposed to have a (CO)2-bridged structure, with two six-coordinate metal cente...