MAGNETIC EXCHANGE IN DINUCLEAR CHROMIUM(II) COMPLEXES - EFFECT OF BRIDGING CHLORIDES AND BRIDGING HYDRIDES IN ANTIFERROMAGNETIC COUPLING

The preparation, solid-state structures, and magnetic properties of two new Cr(II) amido diphosphine complexes are described. Reaction of the lithium salt LiN(SiMe(2)CH(2)PPh(2))(2) with CrCl2.THF results in the formation of the dinuclear chloride-bridged species {[(Ph(2)PCH(2)SiMe(2))(2)N]Cr}(2)(mu-Cl)(2), 1. This complex is paramagnetic at room temperature with a magnetic moment corresponding to four unpaired electrons per chromium; however, as the temperature is lowered, magnetic susceptibility measurements show that 1 exhibits antiferromagnetic behavior with J = -12.4 cm(-1) and g = 1.99. Chloride 1 can be converted to the mononuclear methyl complex CrMe[N(SiMe(2)CH(2)PPh(2))(2)] by reaction with MeLi. Under a hydrogen atmosphere, the methyl complex is converted to the dinuclear hydride derivative {[(Ph(2)PCH(2)SiMe(2))(2)N]Cr}(2)(mu-H)(2), 3. A variable temperature magnetic susceptibility study shows that this species is very strongly antiferromagnetically coupled with J = -139 cm(-1) and g = 1.98. The magnetic coupling correlates with the Cr-Cr distances observed in the solid state for these complexes: for 1, the Cr-Cr distance is 3.64 while for the hydride 3 a much shorter distance of 2.641 Angstrom is found. Crystals of {[(Ph(2)PCH(2)SiMe(2))(2)N]Cr}(2)(mu-Cl)(2), 1, are triclinic, a = 11.508(2) Angstrom, b = 12.267(3) Angstrom, c = 13.096(2) Angstrom, alpha = 90.80(2)degrees, beta = 112.91(1)degrees, gamma = 109.52(2)degrees, Z = 1, and space group P $($) over bar$$ 1; those of {[(Ph(2)PCH(2)SiMe(2))(2)N]Cr}(2)(mu-H)(2), 3, are monoclinic, a = 11.776(1) Angstrom, b = 19.061(2) Angstrom, c = 13.990(2) Angstrom, beta = 103.153(10)degrees, Z = 2, and space group P2(1)/n. The structures were solved by direct (chloride) and Patterson (hydride) methods and were refined by full-matrix least-squares procedures to R = 0.064 and 0.038 (R(w) = 0.037 and 0.039) for 1832 and 3617 reflections with I greater than or equal to 3 sigma(I), respectively.