Macrocyclic complexes of niobium(III):: Synthesis, structure, and magnetic behavior of mononuclear and dinuclear species that incorporate the [P2N2] system

The coordination chemistry of the two mixed donor macrocyclic ligands (R)[P2N2] (where (R)[P2N2] = RP(CH2SiMe2NSiMe2CH2)(2)PR, R = cyclohexyl (Cy) or phenyl (Ph)) with niobium(III) is presented. The reaction of the dilithio precursors (R)[P2N2]Li-2(S) (R = Cy, S = THF; R = Ph, S = 1,4-dioxane) with NbCl3(DME) (DME = 1,2-dimethoxyethane) generates the complexes (R)[P2N2]NbCl (R = Cy, 1; R = Ph, 2). For R = Cy, single-crystal X-ray diffraction studies and variable-temperature magnetic susceptibility measurements indicate that 1 is mononuclear in the solid state; however, analogous variable-temperature magnetic data suggest that 2 is dinuclear in the solid state due to the observation of antiferromagnetic exchange. In solution, 2 is apparently monomeric similar to 1. Adduct formation between these mononuclear complexes is also evident; reaction of 2 with neutral donors and coordinating solvents produces the mononuclear derivatives (Ph)[P2N2]NbCl(L) (L = py, CO, PMe3, THF, MeCN), of which the pyridine adduct, 3e, has been characterized crystallographically. Subsequent replacement of the chlorides can be achieved to generate the paramagnetic alkyl complexes (R)[P2N2]NbR ' (R = Cy, Ph; R ' = CH2SiMe3, CH(SiMe3)(2)). The representative compounds (Cy)[P2N2]NbCH2SiMe3 (4a) and (Ph)[P2N2]NbCH(SiMe3)(2) (5b) have been characterized by X-ray crystallography.