Activation of Nitriles by Metal Ligand Cooperation. Reversible Formation of Ketimido- and Enamido-Rhenium PNP Pincer Complexes and Relevance to Catalytic Design

Abstract

The dearomatized complex cis-[Re(PNP(tBu)*)(CO)2] (4) undergoes cooperative activation of C≡N triple bonds of nitriles via [1,3]-addition. Reversible C-C and Re-N bond formation in 4 was investigated in a combined experimental and computational study. The reversible formation of the ketimido complexes (5-7) was observed. When nitriles bearing an alpha methylene group are used, reversible formation of the enamido complexes (8 and 9) takes place. The reversibility of the activation of the nitriles in the resulting ketimido compounds was demonstrated by the displacement of p-CF3-benzonitrile from cis-[Re(PNP(tBu)-N═CPh(pCF3))(CO)2] (6) upon addition of an excess of benzonitrile and by the temperature-dependent [1,3]-addition of pivalonitrile to complex 4. The reversible binding of the nitrile in the enamido compound cis-[Re(PNP(tBu)-HNC═CHPh)(CO)2] (9) was demonstrated via the displacement of benzyl cyanide from 9 by CO. Computational studies suggest a stepwise activation of the nitriles by 4, with remarkably low activation barriers, involving precoordination of the nitrile group to the Re(I) center. The enamido complex 9 reacts via β-carbon methylation to give the primary imino complex cis-[Re(PNP(tBu)-HN═CC(Me)Ph)(CO)2]OTf 11. Upon deprotonation of 11 and subsequent addition of benzyl cyanide, complex 9 is regenerated and the monomethylation product 2-phenylpropanenitrile is released. Complexes 4 and 9 were found to catalyze the Michael addition of benzyl cyanide derivatives to α,β-unsaturated esters and carbonyls.

Reactivity and Properties of Metal Complexes Enabled by Flexible and Redox-Active Ligands with a Ferrocene Backbone

“…[9][10][11][12] For example, Milstein and coworkers demonstrated the role of PCP and PNP pincer ligands in catalysis and small molecule activation and proposed mechanisms in which metal-ligand cooperation plays a major role (Chart 1c). [13][14][15][16][17][18][19][20][21] In addition, Chirik and coworkers exploited the potential of base metals in catalytic transformations by using flexible and redox non-innocent ligands to overcome the limitation of first row transition metals that usually undergo single electron processes (Chart 1d). [22][23][24][25] Besides traditional phosphine and nitrogen donors, an arene is suitable for metal-ligand cooperation as well: first, the coordination mode of the arene can vary from  6 to  2 according to the degree of delocalization.…”

Non-order–disorder allotwinning of the rhenium pincer complexcis-Re[(PNPCH2-iPr)(CO)2Cl]

“…Molecular structure Complex (1) adopts an octahedral coordination (Fig. 3), which is characteristic of this class of compounds (Vogt et al, 2013) and will not be expanded upon. Including the disordered Cl and CO ligands, the complex has twofold rotational pseudosymmetry [ Fig.…”

“…It is shown that, once the nature of the diffraction pattern is understood and proper intensity data are derived, structure solution and refinement can be surprisingly trouble-free. The t Bu analogue ISSN 2052-5206 of (1), cis-Re[(PNP CH 2 -tBu)(CO) 2 Cl], has been described previously (Vogt et al, 2013) and does not feature polytypism.…”

Benzo annulated cycloheptatriene PCP pincer iridium complexes

“…In recent years cooperation of some pincer ligands with the coordination centre have been reported. [9][10][11][12] In the course of this process the ligands undergo reversible structural changes during the activation of a substrate and while forming the products. In this way the scope of pincer complex chemistry has been expanded.…”