Dinitrogen and Acetylide Complexes of Low-Valent Chromium

Reaction of trans-(dmpe)2CrCl2 (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)2Cr]2(µ-N2)·hexane (1). Under similar conditions but in the absence of acteylide ligand, the reaction of trans-(dmpe)2CrCl2 with 2 equivalents of nBuLi yields the previously characterized complex trans-(dmpe)2Cr(N2)2 (2), while the reaction of trans-(dmpe)2CrCl2 with 2 equivalents of LiCCSiMe3 in THF yields trans-(dmpe)2Cr(CCSiMe3)2 (3). Compound 3 can also be synthesized by irradiating a mixture of trans-(dmpe)2CrMe2 and HCCSiMe3 or by reduction of HCCSiMe3 with compound 2. The magnetic properties, electrochemistry, and crystal structure of trans,trans-[(Me3SiCC)(dmpe)2Cr]2(µ-N2) are consistent with the complex containing two CrI ions bridged by a neutral N2 moiety, with a 1.178(10) Å NN bond distance. For complex 1 redox processes centered at E1/2 = –1.69 V (ΔEp = 185 mV) and –1.43 V (ΔEp = 182 mV) versus Fe(Cp)2/Fe(Cp)2+ are assigned to the CrICrI/CrICrII and CrICrII/CrIICrII couples, respectively. For trans-(dmpe)2Cr(CCSiMe3)2 a reversible couple assigned as the CrII/III couple was observed at –1.59 V (ΔEp = 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 µB while compound 3 displays a moment of 2.55 µB. Density-functional theory calculations performed on a model compound of 1, namely, trans,trans-[(HCC)(dpe)2Cr]2(µ-N2) (dpe = diphospinoethane), indicate that oxidation of the molecule should result in weakening of the dinitrogen triple bond.