Cyclonickelated complexes featuring a terminal or bridging triazole

Abstract

Cyclometallated complexes can serve as models for studying the mechanistic details of CH functionalization processes. In this context, we have shown that the cyclonickelated complexes {κP,κC-(i-Pr)2PO-Ar}Ni(Br)(NCMe) can be isolated from CH nickelation of aryl phosphinites. Previous studies showed that treating these compounds with PhCH2Br or (i-Pr)2PCl allows CC and CP functionalized products, respectively, whereas reaction with hydroxylamines HO-N(CH2R)2 fails to promote the desired CN functionalization, giving instead the imine adducts {κP,κC-(i-Pr)2PO-Ar}Ni(Br)(κN-N(CH2R)CH = CHR) arising from net dehydration of the hydroxylamine substrate. The present report describes a study on the reactivities of the dimeric complexes [{κP,κC-(i-Pr)2P-OAr}Ni(μ-Br)]2 (Ar = C6H4, 1a;4-OMe-C6H3, 1b; 4-Cl-C6H3, 1c; 1-naphthyl, 1d; 4-OMe,1-naphthyl, 1e) with 4-Amino-4H-1,2,4-triazole featuring a potentially labile NN bond. As was observed previously with hydroxylamines, here too the hoped-for CN functionalization did not materialize, the reactions giving instead the mononuclear triazole adducts {κP,κC-(i-Pr)2P-OAr}Ni(Br)(κN-4-amino-4H-1,2,4-triazole) 2a and 2e and the triazole-bridged dinuclear adducts [{κP,κC-(i-Pr)2PO-Ar}Ni(Br)}2(μ,κN,κN’-4-amino-4H-1,2,4-triazole) 3b, 3c, and 3d. The solid-state structures of these new compounds reveal the following three features: (a) slightly distorted square planar geometries around the Ni center, (b) 50-65° angles between the plane of the triazole ligand and the coordination planes, and (c) significantly shorter Ni-N distances in the mononuclear adducts. Variable temperature NMR monitoring of the reactions between the Ni precursors and the triazole substrate pointed to a dynamic exchange process between the mono- and dinuclear triazole adducts.