Divergent Reactivity of CO2 and CO and Related Substrates at the Nickel Carbon Double Bond of (PCcarbeneP)Ni(II) Pincer Complexes

Abstract

ABSTRACT:The addition of carbon monoxide (CO), carbon dioxide (CO 2 ) and isoelectronic isocyanide and isocyanates to the nickel carbene bond in PC carbene P pincer complexes is reported. For CO and CNR, irreversible group transfer reactions are observed, while for CO 2 , a reversible 2+2 addition to the carbene moiety occurs. The course of the CO and CO 2 reactions are strongly affected by the nature of the PC carbene P pincer ligand framework and a new more rigid ligand based on a 10,10-dimethyl-9,10-dihydroanthracene core was designed in order to study these reactions, illustrating the profound effect of small changes in ligand structure on reaction outcomes.The fundamental reactivity of carbon dioxide at transition metal centers is intensely pursued with the aim of the development of methods utilizing CO 2 as a practical C 1 source. 1 In nature, the interconversion of CO 2 and CO, the primary industrial C 1 source, is efficiently accomplished enzymatically through the action of [NiFe]-carbon monoxide dehydrogenase (CODH). 1 The proposed mechanism of action of (CODH) is believed to proceed via CO 2 binding to a square planar nickel center at the [NiFe 4 S 4 ] active site. 2 As such the coordination chemistry of CO 2 and CO at nickel centers supported by pincer ligands to rigorously enforce a square planar geometry is of particular interest, and has been widely explored. [3][4][5][6][7][8][9][10][11][12][13] Notably, the isolation of key proposed intermediates in selective reduction of CO 2 to CO, 3 and a closed synthetic cycle for the reduction CO 2 to CO have recently been realized using PNP pincer ligand supported nickel complexes. 4,9 Despite the rich coordination chemistry of CO 2 , 14 there is a relative dearth of examples of CO 2 bonding and activation using metal-ligand cooperation. 15 Many of these examples consist of 1,3-activation of CO 2 at dearomatized PNP or PNN pincer complexes of second and third row transition metals, 16-18 though examples have been noted with Fe 19 and Ni, 20 and the formal 1,2-addition has been observed at pincer ligand supported Mo and W amides. 21 In a second mode of activation, the double insertion of CO 2 into a three coordinate Ni carbene and imido complexes complex is proposed to proceed first through the 2+2 addition of CO 2 to the Ni=X double bond, with direct evidence for 2+2 reactivity in the case of Ni-imido complexes. [22][23] We have reported the (PC carbene P)Ni complex 1 L1 , containing an unusual carbene central donor 24 (Scheme 1) which readily participates in metal-ligand cooperation in the activation of H-X (X=NH 2 , H, OH, OMe, CCPh, HSiR 3 ) type bonds. [25][26] In all cases the bond is cleaved across the Ni=C bond in a 1,2 fashion, with regioselectivity favoring addition of the electrophilic end of the polarized H-X bond to the carbene carbon. Closely related Pd complexes are also active in H-X bond activation chemistry, [27][28][29][30] and their redox properties and activity in frustrated-Lewis-pair type chemistry have been explored. 31-34 However, th...

On the Realties of Base Metal Catalysis: An Overview

“…[61][62][63][64][65][66] Piers et al highlight the ability of an electrophilic anthracene-based PCcarbeneP Ni complex to undergo two reductions to first generate a Ni(II) radical and, later, an electrophilic carbene complex (Figure 4A). 66 Compared to the previously explored nucleophilic Ni carbene species, [67][68][69][70] the electrophilic carbene displays unique reactivity with silanes and ammonia. A similar study on cobalt showcased the influence of ligand donicity on the nature of the resulting carbene complex with less donating ligands stabilizing radical carbene species (Figure 4B).…”