Effect of Aryloxide Ancillary Ligands on the Chemistry of Molybdenum-Molybdenum and Tungsten-Tungsten Multiple Metal-Metal Bonds

Article abstract of DOI:10.1021/ja00298a029

The effect of aryloxide ancillary ligands on the chemistry of Mo2 and W2 dinuclear compounds has been investigated by reaction of the compounds M2(NMe2)6 (M = Mo and W) and Mo2(O-t-Pr)6 with a variety of phenols.The sterically less demanding ligands 4-methylphenol (HOAr-4-Me) and 3,5-dimethylphenol (HOAr-3,5-Me2) recat with Mo2(NMe2)6 to give salt complexes in good yields.A single-crystal diffraction study shows the anion obtained by using 4-methylphenol to contain a confacial bioctahedral arrangement of ligands in the Mo2O7N2 core, with a metal-metal distance of 2.601 Angstroem.Both complexes are slightly paramagnetic in solution at 30 deg C, ν = 0.8-0.9 ν_b by Evans' method.However, relatively sharp contact shifted 1H NMR spectra are present at 30 deg C for both complexes, and in the case of the 4 -methylphenoxo derivative, the temperature dependence of the 4-methyl groups indicates that the electrons on the molybdenum atoms are antiferromagnetically coupled.An estimate of the exchange integral, J gave a value between 230 and 290 cm^-1.Using W2(NMe2)6 instead of Mo2(NMe2)6 in the reaction yields only the simple amine adducts W2(OAr-4-Me)6(HNMe2)2 and W2(OAr-3,5- Me2)6(HNMe2)2.Addition of only 4 equiv of 3,5-dimethylphenol to Mo2(NMe2)6 in the presence of a large excess of HNMe2 leads to the formation of a reduced, Mo⁴⁺-containing species Mo2(OAr-3,5-Me2)4(HNMe2)2 as a blue crystalline solid in good yield.Ligand exchange with trimethylphosphine gives the emerald green Mo2(OAr-3,5-Me2)4(PMe3)4.The pathway of the reduction is uncertain, but from the reaction mixture an oxidized species Mo2(OAr-3,5-Me2)6(NMe2)(HNMe2)2 was isolated and structurally characterized.The Mo2⁷⁺ core is surrounded by a confacial bioctahedral arrangement of ligands with a bridging dimethylamino group.The Mo-Mo distance of 2.41 A is consistent with the presence of a bond order of 2.5.Electrochemical one-electron oxidation of the quadruply bonded Mo2(OAr-3,5-Me2)4(PMe3)4 leads to the monocation containing an Mo2⁺⁵ core with a formal bond order of 3.5.EPR measurements show the resulting unpaired electron coupled to the four PMe3 nuclei.Crystal data for the Mo2(OAr-4-Me)7(HNMe2)2> * n-hexane at -163 deg C are as follows: a = 14.677 (6) A, b = 16.62 (7) A, c = 14.402 (6) A, α = 115.78 (2) deg, β = 107.67 (2) deg, γ = 77.82 (2) deg, Z = 2, d_calcd= 1.281 g cm^-1 in space group P1.Crystal data for Mo2(OAr-3,5-Me2)6(NMe2)(HNMe2) at -140 deg C are as follows: 29.281 (6) A, b = 12.071 (4) A, c = 14.653 (8) A, β = 92.57 (3) deg, Z = 4, d_calcd = 1.350 g cm^-3 in space group P2₁/n.