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Insertion and cycloaddition reactivity of a transition-metal N-metalloimine

Hevia, E and Perez, J and Riera, V and Miguel, D (2002) Insertion and cycloaddition reactivity of a transition-metal N-metalloimine. Angewandte Chemie, 41 (20). pp. 3858-3860.

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The presence of electron-releasing main-group-metal substituents on the nitrogen atom increases the reactivity of imines A towards dipolar reagents such as isocyanates1 and ketenes.2 We speculated that, in addition to this inductive effect, a low-oxidation transition-metal substituent would contribute a destabilizing interaction between the lone pair on the nitrogen atom and the filled metal d orbitals.3 However, all structurally characterized M(NCR2) complexes have essentially linear M-N-C geometries4 as a result of π donation of the lone pair on the nitrogen atom to empty metal orbitals. Thus, these azavinylidene5 complexes B are more related to alkylideneammonium cations than to imines.6 Herein we report the synthesis, structure, and a preliminary account of the reactivity of an N-metalloimine (type A) which contains a transition-metal fragment. The reaction of KNCPh2 with [Re(OTf)(CO)3(bpy)] (bpy=2,2′-bipyridine) afforded the complex [Re(NCPh2)(CO)3(bpy)] (1), which was characterized spectroscopically and by X-ray diffraction (Figure 1 a). For comparison, the structure of the imine complex [Re(HNCPh2)(CO)3(bpy)]+ (2), synthesized as its triflate salt by reaction of 1 with HOTf, was also determined (Figure 1 b). The Re-N-C angle (133.9(4)°) for the alkylideneamido group of 1, even smaller than the Re-N-C angle (138.9(2)°) of 2, indicates the absence of N-to-Re π donation. The larger deviations from linearity in previously known azavinylidene complexes (152.9(4)° and 157.2(4)° for [OsCl(NCMe2)(CCH3)(PiPr3)2][OTf]7 and [OsCl(NCMe2)(CHPh)(PiPr3)2],8 respectively) can be related to the presence of bulky ancillary ligands, whereas such effects are absent in 1. The ReNCPh2 distance (2.113(4) Å) in 1 is comparable to the ReN(H)CPh2 distance (2.192(3) Å) in 2, which is in agreement with the lack of multiple-bond character of the ReN bond.