Amidate-supported two- and three-coordinate Ni complexes were synthesized by reduction of the corresponding Ni precursors. A dramatic change in binding mode is observed upon reduction from Ni to Ni . The Ni derivatives include an unprecedented Ni bis(C-H) agostic complex and a two-coordinate Ni complex.

We report a mechanistic study of C(sp)-H bond activation mediated by nickel. Cyclometalated Ni(II) ureate [(PEt)Ni(κ- C,N,N-(CH)N(Cy)(CO)N((N)-quinolin-8-yl))] was synthesized and isolated from the urea precursor, (Me)(Cy)N(CO)N(H)(quinolin-8-yl), via C(sp)-H activation. We investigated the effects of solvents and base additives on the rate of C-H activation. Kinetic isotope effect experiments showed that C-H activation is rate determining. Through deuterium labeling and protonation studies, we also showed that C-H activation can be reversible. We extended this reaction to a range of ureas with primary and secondary C(sp)-H bonds, which activate readily to form analogous nickelated products. Finally, we showed that carboxylate additives assist with both ligand dissociation and initial N-H bond activation, consistent with a concerted metalation-deprotonation mechanism.

The N-heterocyclic carbene supported three-coordinate Ni­(I) complex (IPr)­Ni­(κ1-N-N­( i Pr)­C­(O) t Bu)­(CNXyl) ([2-CNXyl]; CNXyl = 2,6-dimethylphenyl isocyanide) undergoes disproportionation with 1 equiv of CNXyl to afford an equimolar mixture of Ni(0) and Ni­(II) complexes (IPr)­Ni­(CNXyl)3 ([3-CNXyl]) and (IPr)­Ni­(κ2-N,O-N­( i Pr)­CO­( t Bu))2 ([4]), respectively. Complexes [3-CNXyl] and [4] are also produced when 2 equiv of the isonitrile CNXyl is added to (IPr)­Ni­(κ3-N,bis­(H 2 CMe)-N­( i Pr)­C­(O) t Bu) ([1]). Furthermore, we show that reactions with π-acceptors CO and select alkenes also result in disproportionation. In the case of CO we show that reactions in the solid state afford selectivity for and isolation of the reaction intermediate (IPr)­Ni­(κ1-N-N­( i Pr)­C­(O) t Bu)­(CO) ([2-CO]), which is not isolable by solution methods. Kinetic analysis of the disproportionation reaction with alkenes suggests that the reaction is first order in both nickel and alkene. Preliminary trapping experiments rule out amidyl radicals as intermediates in these reactions. We propose instead that a bimolecular ligand transfer is operative, in part due to the hemilabile nature of amidate ligands.

A scandium-triflate-catalyzed arylative desulfonation of diarylmethyl phenyl sulfones with arenes and heteroarenes was established. A variety of both sulfone and arene substrates were reacted to afford symmetric and nonsymmetric triarylmethanes in good yields. Further transformations of the resulting triarylmethanes and application to the concise synthesis of a bactericidal agent analogue were also demonstrated.

The preparation of four-membered aminophosphine (PN) chelates from common metal precursors has largely evaded realization because of the ring strain associated with these species. We report a straightforward approach to the synthesis of such PN metallacycles using simple α-PN ligands analogous to the popular class of small-bite-angle diphosphinomethane ligands. It is demonstrated that bulky phosphine substituents are important to the formation of these chelates.