Heterobimetallic Complexes Comprised of Nb and Fe: Isolation of a Coordinatively Unsaturated NbIII/Fe0 Bimetallic Complex Featuring a NbFe 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 C-3-symmetric tris(phosphinoamide) ligand framework is used to construct a Nb/Fe heterobimetallic complex Cl-Nb((PrNPPh2)-Pr-i)(3)Fe-Br (2), which features a Fe -> Nb dative bond with a metal-metal distance of 2.4269(4) angstrom. Reduction of 2 in the presence of PMe3 affords Nb((PrNPPh2)-Pr-i)(3)Fe-PMe3 (6), a compound with an unusual trigonal pyramidal geometry at a Nb-III center, a Nb Fe triple bond, and the shortest bond distance (2.1446(8) angstrom) ever reported between Nb and any other transition metal. Complex 6 is thermally unstable and degrades via P-N bond cleavage to form a Nb-V=NR imide complex, (PrN)-Pr-i=Nb((PrNPPh2)-Pr-i)(3)Fe-PMe3 (9). The heterobimetallic complexes (PrN)-Pr-i=Nb((PrNPPh2)-Pr-i)(3)Fe-Br (8) and 9 are independently synthesized, revealing that the strongly pi-bonding imido functionality prevents significant metal-metal interactions. The Fe-57 Mossbauer spectra of 2, 6, 8, and 9 show a clear trend in isomer shift (delta), with a decrease in delta 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.