Gold(I) Complexes Bearing Sterically Imposing, Saturated Six- and Seven-Membered Expanded Ring N-Heterocyclic Carbene Ligands

Abstract

The synthesis and characterization of novel six-and sevenmembered expanded ring N-heterocyclic carbene (NHC) complexes of the general formula [Au(NHC)Cl] are described. The key structural parameters of complexes 1, 2, and 4 have been evaluated by solid-state analysis and by means of the percent buried volume (%V bur ) analysis. Complex 4 is found to demonstrate the largest %V bur value of any gold(I) NHC complex reported to date, with a value of 52.6. All complexes 1−4 have been evaluated in a preliminary catalytic survey of the hydration of internal alkynes.

Expanded‐Ring N‐Heterocyclic Carbenes for the Stabilization of Highly Electrophilic Gold(I) Cations

“…The structure reveals two [(6‐Dipp)Au] + fragments bridged by a single chloride ligand, with no significant interaction with the [BAr f 4 ] − counterion. Marked elongation of the AuCl bonds over the parent [(6‐Dipp)AuCl] system is observed (2.310(1), 2.316(1) vs. 2.281(12) Å, respectively),13f consistent with the bridging nature of the chloride ligand. It might be expected that this elongation would be accompanied by a shortening of the AuC NHC bonds as the effective trans influence of the chloride is decreased; however, this appears not to be a statistically significant effect (1.989(4), 1.988(4) vs. 2.009(7) Å).…”

“…It was hypothesized that alternative supporting ligands possessing further increased steric bulk might be less amenable to such redistribution processes, due to the highly crowded nature of a putative 2:1 complex. With this in mind, 6‐Dipp and 7‐Dipp were considered to be viable candidates, since each has a percentage buried volume (% V bur ) of >50 (50.8 and 52.6, respectively),13f, 23 and the former has been shown explicitly to be incapable of forming a 2:1 complex with Au + through conventional C2 ligation 12a…”

“…The known complex [(6‐Dipp)AuCl]13f was therefore treated with one equivalent of Na[BAr f 4 ] in CD 2 Cl 2 at −78 °C and the reaction monitored in situ by 1 H VT‐NMR spectroscopy (Figure 7). Gradual warming of the sample reveals little reaction from −78 °C to −20 °C.…”

The Role of Low Valent Transition Metal Complexes in Homogeneous Catalysis: An EPR Investigation

“…Five-membered ring carbenes with bulky N-substituents are most commonly employed, as these appear to aid in the stabilisation of low-coordinate Ni(0) species, which have been proposed to be active in the catalytic cycle. In a number of examples, ring-expanded 6-, 7-and 8-membered N-aryl-substituted heterocyclic carbenes (RE-NHCs) have been found to enhance the catalytic efficiency of Ir, Pt, Cu and Au complexes [17][18][19][20][21]. Whittlesey et al [22] therefore decided to explore the possibility that a similar catalytic enhancement could be observed using low valent Ni(I)-based analogues.…”

Synthesis of New C2-Symmetric Six-Membered NHCs and Their Application for the Asymmetric Diethylzinc Addition of Arylaldehydes

“…To date, most of the research has employed five-membered NHCs based on imidazole or imidazoline. The so-called "expanded-ring" NHCs with six- [22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37] and seven- [38][39][40][41][42][43][44][45][46][47][48][49] heterocyclic rings have recently attracted attention, as these "non-standard" NHCs show quite different properties, such as stronger basicity (nucleophilicity) and greater steric demand [50]. Structurally, the larger ring sizes of these unusual NHCs will lead to a comparatively large N-CNHC-N angle and consequential smaller C NHC -N-C R angle, which in turn results in better protection of metal centers and subsequently better performance in catalysis.…”