No abstract

A rare Ta bis­(imido) complex, which has unique reactivity, was prepared by manipulating the coordination sphere of a CCC-NHC pincer Ta complex. The reaction of lithium tert-butylamide with complex 1 yielded (1,3-bis­(3′-butylimidazol-2′-yl-1′-idene)-2-phenylene)­bis­(tert-butylimido)­tantalum­(V) (2) as a lithium iodide bridged dimer, as determined by the X-ray structure. Complex 2 catalytically cyclized α,ω-aminoalkenes to effect an oxidative amination of alkenes (dehydrogenation by C–H activation) and produced a cyclic imine, an equivalent of reduced substrate, and varying proportions of hydroamination. Various additives and concentration impact the catalytic results. Computational and experimental observations have led to an initial mechanistic hypothesis. Based upon it, precatalyst 2 appears to be the first example of a bifunctional catalyst (MH-NHR) that is highly selective for nonpolar CC bonds in preference to polar CX bonds for outer-sphere hydrogenation.

Dynamic NMR (13C and 1H) studies of (μ-SCH2XCH2S)[Fe(CO)3]2 complexes (X = CR2, NR) were utilized to examine the fluxional processes that are important in the [FeFe]-hydrogenase active site models, where an open site for proton/hydrogen binding, achieved by configurational mobility of the Fe(CO)3 unit, is required for electrocatalysis of proton reduction. In order to interrogate the effects of fluxional mobility on electrochemical response to added acid, energy barriers for the CO site exchange in Fe(CO)3 rotors were determined for nitrogen- and carbon-based bridgehead complexes. The effect of the methyl substituent in both the NH/NCH3 and CH2/C(CH3)2 cases is to lower the Fe(CO)3 rotational activation barrier relative to the NH or CH2 analogues. Although the C(CH3)2 case has the lowest Fe(CO)3 rotational barrier, its performance as a proton reduction electrocatalyst is 2-fold less than that for the X = NR species, indicating the proton-directing effect of the pendent base on catalytic efficiency.

A CCC-NHC pincer Ni(ii)Cl complex was prepared according to the metallation/transmetallation methodology. It was fully characterized by electrochemical, NMR spectroscopic, theoretical, and X-ray crystallographic methods. The complex and its cation were evaluated for electrocatalytic reduction of CO under a variety of conditions and found to provide some of the fastest catalytic rates and highest substrate selectivities (COvs. H) reported. Rates improved in the presence of water and, significantly, catalysis occurred at the first reduction potential, presumably at the Ni(i) state. Controlled potential electrolysis (CPE) was found to yield CO at 34% and formate at 47% Faradaic efficiency (FE).