Spectroscopic, structural, electrochemical, and kinetic studies of ligand substitution in the 33e dinuclear radical Fe₂(CO)₇(μ-PPh₂) and the 34e analogues [Fe₂(CO)₇(μ-PPh₂)]^- and FeCo(CO)₇(μ-PPh₂)

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 substitution 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 [Fe2(CO)5L3(μ-PPh2)] +[Fe2(CO)8-nLn(μ-PPh 2)]- (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 [Fe2(CO)6(μ-CO)(μ-PPh2)]-, as the (Et4N)+ salt, adds two ligands in THF to give the 36e anions [Fe2(CO)6L2(μ-PPh2)]- (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°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)7L(μ-PPh2), with L on Co. Electrochemical studies show that the 33e unsubstituted and monosubstituted diiron radicals exhibit chemically reversible 1e reductions to give the 34e CO-bridged anions. A 1e oxidation of the disubstituted 36e anion [Fe2(CO)6(PPh3)2(μ-PPh 2)]- 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 [FeCo(CO)6(μ-PPh2)]-, 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 [FeCo(CO)6(PPh3)(μ-PPh2)]+. 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 [Fe2(CO)6(μ-CO)(μ-PPh2)]- 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 centers. Analogous 36e intermediates in the [Fe2]- and FeCo systems have the all-terminal-CO structure, with two five-coordinate metal centers. © 1988 American Chemical Society.

Duke Scholars

Author Michael J. Therien Chemistry