Dinitrogen Complexes of Cobalt(-I) Supported by Rare-Earth Metal-Based Metalloligands.

Abstract

Sequential reactions of heptadentate phosphinoamine LH3 with rare-earth metal tris-alkyl precursor (Me3SiCH2)3Ln(THF)2 (Ln = Sc, Lu, Yb, Y, Gd) and a low-valent cobalt complex (Ph3P)3CoI afforded rare-earth metal-supported cobalt iodide complexes. Reduction of these iodide complexes under N2 allowed the isolation of the first series of dinitrogen complexes of Co(-I) featuring dative Co(-I) → Ln (Ln = Sc, Lu, Yb, Y, Gd) bonding interactions. These compounds were characterized by multinuclear NMR spectroscopy, X-ray diffraction analysis, electrochemistry, and computational studies. The correlation of N-N vibrational frequencies with the pKa of [Ln(H2O)6]3+ showed that strongest activation of N2 was achieved with the least Lewis acidic Gd(III) ion. Interestingly, these Ln-Co-N2 complexes catalyzed silylation of N2 in the presence of KC8 and Me3SiCl with turnover numbers (TONs) up to 16, where the lutetium-supported Co(-I) complex showed the highest activity within the series. The role of the Lewis acidic Ln(III) was crucial to achieve catalytic turnovers and tunable reactivity toward N2 functionalization.