Preparation and characterization of dialkylcarbamato derivatives of niobium and tantalum
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.