SYNTHESIS AND SPECTROSCOPIC AND X-RAY STRUCTURAL CHARACTERIZATION OF THE 1ST HOMOLEPTIC TRANSITION-METAL BORYLOXIDES [MN(OBTRIP2)(MU-OBTRIP2)]2 AND [FE(OBMES2)(MU-OBMES2)]2

The reactions of the metal amides Mn[N(SiMe3)2]2 and Fe[N(SiMe3)2]2 with the sterically crowded boronous acids Trip2BOH and Mes2BOH (Trip = 2,4,6-i-Pr3C6H2, Mes = 2,4,6-Me3C6H2) afford the compounds [Mn(OBTrip2)(mu-OBTrip2)]2 (1) and [Fe(OBMes2)(mu-OBMes2)]2 (2), which are the first two examples of homoleptic transition-metal boryloxides. The X-ray crystal structures of compounds 1 and 2 have also been determined. The data show that both 1 and 2 are dimeric with three-coordinate Mn and Fe centers that are bound to one terminal boryloxide ligand and to two bridging boryloxide ligands. The M...M distances (3.094 (5) angstrom for Mn and 3.057 (5) angstrom for Fe) are considerably longer than those found in the amide precursors. Surprisingly, the metric features of 1 and 2 are very close to those observed in the closely related bis(aryloxo) complexes [M(OAr)2]2 (Ar = 2,4,6-t-Bu3C6H2, M = Mn (3), Fe (4)). This suggests that the M-O bonding in 1-4 is similar: furthermore, it is mainly ionic and little evidence for a pi-contribution to the M-O bond could be observed. Compounds 1 and 2 have also been characterized by magnetic measurements. Crystallographic data with Mo K-alpha radiation (lambda = 0.71069 angstrom) at 130 K: 1, a = 33.898 (11) angstrom, b = 16.985 (5) angstrom, c = 30.861 (11) angstrom, beta = 134.65 (2)-degrees, Z = 4, R = 0.088, space group C2/c; 2, a = 15.004 (5) angstrom, b = 14.957 (4) angstrom, c = 16.915 (6) angstrom, beta = 93.86 (3)-degrees, Z = 2, R = 0.074, space group P2(1)/n.