The crucial role of the diphosphine heteroatom X in the stereochemistry and stabilization of the substitution-inert [M(N)(PXP)]²⁺ metal fragments (M = Tc, Re; PXP = diphosphine ligand)

The nature of the heteroatom X incorporated in the five-member PXP-diphosphine spacer was found to play a primary unit role both in the stereochemical arrangement and overall stability of nitrido containing [M(N)(PXP)]²⁺ metal fragments (M = Tc, Re). Thus, by mixing PXP ligands with labile [Re(N)Cl₄]^- and [Tc(N)Cl₂(PPh₃)₂] nitrido precursors in CH₂Cl₂/MeOH mixtures, a series of neutral [M(N)Cl₂(PXP)] complexes (M = Tc, 1-5; M = Re, 8-9) was collected. In the resulting distorted octahedrons PXP adopted facial or meridional coordination, and combination with halide co-ligands produced three different stereochemical arrangements, i.e. fac,cis mer,cis and mer,trans, depending primarily on the nature of the diphosphine heteroatom X. When X = NH, mer,cis-[Tc(N)Cl₂(PNP1)] 1 was the only isomer formed. Alternatively, when a tertiary amine nitrogen (X = NR; R = CH₃, CH₂CH₂OCH₃) was introduced in the spacer, fac,cis-[M(N)Cl₂(PN(R)P)] complexes (M = Tc, 2, 3; M = Re, 8f) were obtained. Isomerization into the the mer,cis-[Re(N)Cl₂(PN(R)P)], 8m, species was observed only in the case of rhenium when the tertiary amine group carried the less encumbering methyl substituent. Fac,cis-[Tc(N)Cl₂(PSP)], 4f, was isolated in the solid state when X = S, but mixture of fac,cis-[Tc(N)Cl₂(PSP)] and mer,trans-[Tc(N)Cl₂(PSP)], 4m, isomers was found in equilibrium in the solution state. A similar equilibrium between fac,cis-[M(N)Cl₂(POP)] (M = Tc, 5f; M = Re, 9f) and mer,trans-[M(N)Cl₂(POP)] (M = Tc, 5m; M = Re, 9m) species was detected in POP containing complexes. The molecular structure of all of these complexes were assessed by means of conventional physico-chemical techniques including multinuclear NMR spectroscopy and X-ray diffraction analysis of representative mer,cis-[Tc(N)Cl₂(PN(H)P)] 1, fac,cis-[Tc(N)Cl₂(PSP)] 4f, and mer,cis-[Re(N)Cl₂(PN(Me)P)] 8m compounds.