Silver(I) catalysts have been developed for nitrene transfer reactions such as aziridination and C-H insertion. For some catalysts, structures determined by X-ray crystallography reveal dimers with silver-silver interactions, leading to mechanistic speculation about the potential role of dinuclear silver complexes in catalysis. However, it is often unclear if the silver-silver interactions persist in solution. Here we use EXAFS to directly interrogate the solution-phase structures of several silver(I) nitrene transfer catalysts. Retention or loss of the silver-silver interaction in solution can be clearly observed.

The 2,3-dimethylbutadiene complexes of Group 4 metals with constrained geometry (cg) ligands have been prepared and found to adopt a supine orientation with σ,π bonding. Treatment of cgTi(2,3-dimethylbutadiene) (1-Ti) with BuNC leads to the formation of a titana-aziridine (3) with a coordinated cyclopentenimine that arises from the formal [4+1] addition of the diene to the isonitrile. In contrast, the reactions of cgZr(2,3-dimethylbutadiene) (1-Zr) or cgHf(2,3-dimethylbutadiene) (1-Hf) with 2 equiv ofBuNC or XyNC proceeded in a more sophisticated manner to yield unsymmetrical 2,5-diazametallacyclopentane derivatives (4, 6-Zr, and 6-Hf) or symmetrical 2,5-diazametallacyclopentene complexes (7-Zr and 7-Hf). The unsymmetrical products contain coordinated cyclopropanes; the strength of the interaction is measured by the reduction in the J of the C-C bond that is coordinated. A detailed mechanistic analysis has been possible with the related cgM(Me) (M = Ti and Hf) complexes. The first insertion is too fast to monitor, but allows complete conversion to an alkyl iminoacyl intermediate. The second isonitrile (RNC) may react with that intermediate by either of two different mechanisms, reductive elimination and coordination/insertion. In the first mechanism (Ti), rate-determining C-C coupling gives a titana-aziridine, followed by fast coordination of the isonitrile. In the second mechanism (Hf), coordination is the slow step; insertion to form a bis(iminoacyl) Hf complex is rapid.

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