Transition metal imido complex

In coordination chemistry and organometallic chemistry, transition metal imido complexes is a coordination compound containing an imido ligand. Imido ligands can be terminal or bridging ligands. The parent imido ligand has the formula NH, but most imido ligands have alkyl or aryl groups in place of H. The imido ligand is generally viewed as a dianion, akin to oxide.

Structure of a representative imido complex (py = pyridine, CMe₃ = tert-butyl)^[1]

Structural classes

Core of W(NAr)₂(N(H)Ar)₂ (Ar = C₆H₃-2,6-iPr₂).^[2]

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.^[3]

Typical Schrock-style olefin metathesis catalyst features imides as spectator ligands.

Complexes with bridging imido ligands

Imido ligands are observed as doubly and, less often, triply bridging ligands.

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:

L_nMO + H₂NR → L_nMNR + 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).^[4]

L_nMCl₂ + 3 H₂NR → L_nMNR + 2 RNH₃Cl

Aryl isocyanates react with metal oxides concomitant with decarboxylation:

L_nMO + O=C=NR → L_nMNR + CO₂

Structure of OsO₃(N-t-Bu) (multiple bonds are not drawn explicitly). Selected distances: Os-N, 1.689; Os-O, 1.678 Å.^[5]

Alternative routes

Some are generated from the reaction of low-valence metal complexes with azides:

L_nM + N₃R → L_nMNR + N₂

A few imido complexes have been generated by the alkylation of metal nitride complexes:

L_nMN⁻ + RX → L_nMNR + 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.^[6]