Heterobimetallic Complexes Comprised of Nb and Fe: Isolation of a Coordinatively Unsaturated Nb^III/Fe⁰Bimetallic Complex Featuring a Nb≡Fe Triple Bond

Abstract

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-symmetric tris(phosphinoamide) ligand framework is used to construct a Nb/Fe heterobimetallic complex Cl-Nb(PrNPPh)Fe-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 PMe affords Nb(PrNPPh)Fe-PMe (6), a compound with an unusual trigonal pyramidal geometry at a Nb 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 Nb═NR imide complex, PrN═Nb(PrNPPh)Fe-PMe (9). The heterobimetallic complexes PrN═Nb(PrNPPh)Fe-Br (8) and 9 are independently synthesized, revealing that the strongly π-bonding imido functionality prevents significant metal-metal interactions. The Fe 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.

Nb(iPrNPMe2)3Fe–PMe3: A potential high reactivity heterobimetallic catalyst for acetylene cycloadditions

“…As shown in Table 1, the geometry parameters of Nb( i PrNPPh 2 ) 3 Fe−PMe 3 and Nb( i PrNPMe 2 ) 3 Fe−PMe 3 (denoted as CAT) calculated at the B3LYP/LanL2DZ and OPBE/def2‐tzvp levels are close to each other, and all of them are close to the experimental results from single‐crystal X‐ray diffraction. [ 42 ] In the following discussion, B3LYP/LanL2DZ is used for geometry optimization and the phenyl group on P atom is replaced by a methyl group to reduce the computation cost.The optimized geometry of CAT in its singlet state ( 1 CAT) is shown in Figure 1. The geometry of 3 CAT is similar to that of 1 CAT.…”

“…In other words, the singlet state is the ground state of CAT, which is consistent with the results reported by Thomas et al . [ 42 ]…”

“…It likely to have the high reactivity due to the existence of the low‐valent niobium center. [ 42 ] In this work, a detailed density functional theory study on the catalytic mechanism of Nb( i PrNPPh 2 ) 3 Fe–PMe 3 and its effect on the cycloaddition of acetylenes is performed. The aims of this work are to illustrate the following: (1) Can Nb( i PrNPMe 2 ) 3 Fe–PMe 3 be a potential high reactivity heterobimetallic catalyst in the cycloaddition of acetylenes?…”

Computational prediction on the catalytic activity of heterobimetallic complex featuring MMˊ triple bond in acetylene cyclotrimerization: Mechanistic study

“…Recently, Thomas group synthesized a series of VFe, Nb‐Fe heterobimetallic complexes featuring short metal–metal multiple bonds 44,45 . The short distance permitted the strong interaction between the two metal centers.…”

“…Compared to ZrCo bimetallic complexes, V/Nb heterobimetallic compounds provides a distinct advantage because it has a wider range of accessible oxidation states and can therefore bind substrates and participate in redox processes more easily. 45,46 …”