Heterobimetallic Complexes Comprised of Nb and Fe: Isolation of a Coordinatively Unsaturated NbIII/Fe0 Bimetallic Complex Featuring a NbFe Triple Bond

Heterometallic multiple bonds between niobium and other transition metals have not been reported to date, likely owing to the highly reactive nature of low-valent niobium centers. Herein, a C3-symmetric tris­(phosphinoamide) ligand framework is used to construct a Nb/Fe heterobimetallic complex Cl–Nb­(iPrNPPh2)3Fe−Br (2), which features a Fe→Nb dative bond with a metal–metal distance of 2.4269(4) Å. Reduction of 2 in the presence of PMe3 affords Nb­(iPrNPPh2)3Fe–PMe3 (6), a compound with an unusual trigonal pyramidal geometry at a NbIII center, a NbFe triple bond, and the shortest bond distance (2.1446(8) Å) ever reported between Nb and any other transition metal. Complex 6 is thermally unstable and degrades via P–N bond cleavage to form a NbVNR imide complex, iPrNNb­(iPrNPPh2)3Fe−PMe3 (9). The heterobimetallic complexes iPrNNb­(iPrNPPh2)3Fe−Br (8) and 9 are independently synthesized, revealing that the strongly π-bonding imido functionality prevents significant metal–metal interactions. The 57Fe Mössbauer spectra of 2, 6, 8, and 9 show a clear trend in isomer shift (δ), with a decrease in δ as metal–metal interactions become stronger and the Fe center is reduced. The electronic structure and metal–metal bonding of 2, 6, 8, and 9 are explored through computational studies, and cyclic voltammetry is used to better understand the effect of metal–metal interaction in early/late heterobimetallic complexes on the redox properties of the two metals involved.