Dinitrogen and acetylide complexes of low-valent chromium

Reaction of trans-(dmpe)(2)CrCl2 (dmpe = 1,2-bis(dimethylphosphino)ethane) with one equivalent of LiCCSiMe3 and one equivalent of nBuLi in THF under a dinitrogen atmosphere affords dark orange trans, trans-[(Me3SiCC)(dmpe)(2)Cr](2)(mu N-2) center dot hexane (1). Under similar conditions but in the absence of acteylide ligand, the reaction of trans-(dmpe)(2)CrCl2 with 2 equivalents of nBuLi yields the previously characterized complex trans-(dmpe)2Cr(N2)2 (2), while the reaction of trans-(dmpe)(2)CrCl2 with 2 equivalents of LiCCSiMe3 in THF yields trans-(dmpe)(2)Cr(CCSiMe3)(2) (3). Compound 3 can also be synthesized by irradiating a mixture of trans-(dmpe)(2)CrMe2 and HCCSiMe3 or by reduction of HCCSiMe3 with compound 2. The magnetic properties, electrochemistry, and crystal structure of trans, trans[(Me3SiCC)(dmpe)(2)Cr](2)(mu N-2) are consistent with the complex containing two Cr-I ions bridged by a neutral N-2 moiety, with a 1.178(10) angstrom N N bond distance. For complex 1 redox processes centered at E-1/2 = -1.69 V (Delta E-p = 185 mV) and -1.43 V (Delta E-p = 182 mV) versus Fe(Cp)(2)/Fe(CP)(2)(+) are assigned to the (CrCrI)-Cr-I/(CrCrII)-Cr-I and (CrCrII)-Cr-I/(CrCrII)-Cr-II couples, respectively. For trans-(dmpe)2Cr(CCSiMe3)2 a reversible couple assigned as the Cr-II/III couple was observed at -1.59 V (Delta E-p = 242 mV) versus Fe(CP)(2)/Fe(Cp)(2)(+). The dinuclear Cr(I)-dinitrogen complex 1 has a room temperature magnetic moment of 2.77 mu(B) while compound 3 displays a moment of 2.55 mu(B). Density-functional theory calculations performed on a model compound of 1,,namely, trans, trans-[(HCC)(dpe)(2)Cr](2)(mu-N-2) (dpe = diphospinoethane), indicate that oxidation of the molecule should result in weakening of the dinitrogen triple bond.