Transition metal imido complex
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Through Coordination complex and Ligand
Structural classes
Complexes with terminal imido ligands
In some terminal imido complexes, the M=N−C angle is 180° but often the angle is decidedly bent. Complexes of the type M=NH are assumed to be intermediates in nitrogen fixation by synthetic catalysts.
Complexes with bridging imido ligands
Imido ligands are observed as doubly and, less often, triply bridging ligands.
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Synthesis
From metal oxo complexes
Commonly metal-imido complexes are generated from metal oxo complexes. They arise by condensation of amines and metal oxides and metal halides:
- LnMO + H2NR → LnMNR + H2O
This approach is illustrated by the conversion of MoO2Cl2 to the diimido derivative MoCl2(NAr)2(dimethoxyethane), precursors to the Schrock carbenes of the type Mo(OR)2(NAr)(CH-t-Bu).
- LnMCl2 + 3 H2NR → LnMNR + 2 RNH3Cl
Aryl isocyanates react with metal oxides concomitant with decarboxylation:
- LnMO + O=C=NR → LnMNR + CO2
Alternative routes
Some are generated from the reaction of low-valence metal complexes with azides:
- LnM + N3R → LnMNR + N2
A few imido complexes have been generated by the alkylation of metal nitride complexes:
- LnMN− + RX → LnMNR + X−
Utility
Metal imido complexes are mainly of academic interest. They are however assumed to be intermediates in ammoxidation catalysis, in the Sharpless oxyamination, and in nitrogen fixation.
In nitrogen fixation
A molybdenum imido complex appears in a common nitrogen fixation cycle:
- Mo•N2 → Mo-N=N− → Mo-N=NH (diazenido) → Mo-N=NH2+ → Mo=N-NH2 (hydrazido) → Mo=N-NH3+ (hydrazidium) → Mo≡N (nitrido) + NH3 → Mo≡NH+ → Mo=NH (imido) → Mo=NH2+ → Mo-NH2 (amido) → Mo-NH3+ → Mo•NH3 (ammine);
with the oxidation state of molybdenum varying to accommodate the number bonds from nitrogen.