Asymmetric niobium guanidinates as intermediates in the catalytic guanylation of amines

Abstract

The molecular structure of the guanidinate complex {NbBz₂(N^tBu)[(4-BrC₆H₄)NC(NⁱPr)(NHⁱPr)]}, previously obtained by reaction of [NbBz₃(N^tBu)] and the corresponding guanidine proligand, has been established by X-ray diffraction. The series of complexes {NbBz₂(N^tBu)[(Ar)NC(NⁱPr)(NHⁱPr)]} (Ar = 4-BrC₆H₄, 4-^tBuC₆H₄, 4-MeOC₆H₄) and {[NbBz₂(N^tBu)]₂[(C₆H₄)(NC(NⁱPr)(NHⁱPr))₂]} show a preferred asymmetric coordination of the guanidinate ligand by means of one alkylamino nitrogen and the arylimino nitrogen atom. Computational studies confirm this preference and the results suggest that electronic factors prevail over steric factors. In addition, reaction of complex [NbBz₃(N^tBu)] with {2-(ⁿbutyl)-1,3-diisopropylguanidine} did not give rise to the regioselective asymmetrical guanidinate. Instead, the complex {NbBz₂(N^tBu)[(ⁿBu)NC(NⁱPr)(NHⁱPr)]} was obtained as a mixture of three isomers with symmetrical and asymmetrical coordination modes. Finally, the complex [NbBz₃(N^tBu)] was shown to be a suitable precatalyst for the guanylation reaction of a wide range of amines under mild conditions. Guanidinates are proposed as intermediates in the mechanism of this reaction. The molecular structure of the biguanidine {2,2′-(1,4-phenylene)bis(2′,3-diisopropylguanidine)} was also established by X-ray diffraction studies.