Preparation and characterization of dialkylcarbamato derivatives of niobium and tantalum

The mononuclear compounds M(O(2)CNR(2))(n) (M = Nb or Ta, R = Et, n = 5; M = Nb, R = Et or Pr-i, n = 4) have been prepared by treating the corresponding metal chlorides or their adducts, i,e. [NbCl4(thf)(2)](thf = tetrahydrofuran), with CO2-NHR(2) in toluene. The molecular structures of Ta(O(2)CNEt(2))(5) and Nb(O(2)CNEt(2))(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-NHR(2)(R = Et or Pr-i) 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 [M(2)(eta(6)-C(6)Me(6))(2)Cl-4] (M = Nb or Ta), which are the precursors to the dialkylcarbamates of niobium(II) and tantalum(II), have been obtained. The [M(2)(eta(6)-C(6)Me(6))(2)Cl-4]-CO2-NHPr2i system (M = Nb or Ta) in toluene gave metal(II) dialkylcarbamato complexes of analytical composition M(eta(6)-C(6)Me(6))(O2CNPr2i)(2): these substantially diamagnetic compounds are suggested to be dinuclear with bridging carbamato groups, a metal-metal bond and axially co-ordinated hexamethylbenzene.