Preparation and characterization of dialkylcarbamato derivatives of niobium and tantalum

Abstract

The mononuclear compounds M(O₂CNR₂)_n(M = Nb or Ta, R = Et, n= 5; M = Nb, R = Et or Prⁱ, n= 4) have been prepared by treating the corresponding metal chlorides or their adducts, i.e.[NbCl₄(thf)₂](thf = tetrahydrofuran), with CO₂–NHR₂ in toluene. The molecular structures of Ta(O₂CNEt₂)₅ and Nb(O₂CNEt₂)₄ have been solved by X-ray diffraction methods. The tantalum atom is eight-co-ordinated, being surrounded by three bi- and two mono-dentate diethylcarbamato groups, in a slightly distorted square-antiprismatic arrangement. The niobium compound consists of mononuclear units, where the niobium atom is dodecahedrally co-ordinated to eight oxygen atoms of four bidentate diethylcarbamate ligands. The reaction of [NbCl₃(dme)](dme = 1,2-dimethoxyethane) with CO₂–NHR₂(R = Et or Prⁱ) gave the corresponding dialkylcarbamates; on the basis of spectroscopic and magnetic data these niobium(III) derivatives are suggested to be dinuclear with bridging and terminal carbamato groups. Improved yields of [M₂(η⁶-C₆Me₆)₂Cl₄](M = Nb or Ta), which are the precursors to the dialkylcarbamates of niobium(II) and tantalum(II), have been obtained. The [M₂(η⁶-C₆Me₆)₂Cl₄]–CO₂–NHPr₂ⁱ system (M = Nb or Ta) in toluene gave metal(II) dialkylcarbamato complexes of analytical composition M(η⁶-C₆Me₆)(O₂CNPr₂ⁱ)₂: these substantially diamagnetic compounds are suggested to be dinuclear with bridging carbamato groups, a metal–metal bond and axially co-ordinated hexamethylbenzene.