Department of Chemistry, Brandeis University , 415 South Street, Waltham, Massachusetts 02454, United States.
Department of Chemistry and Chemical Biology, Harvard University , 12 Oxford Street, Cambridge, Massachusetts 02138, United States.
J Am Chem Soc. 2017 Jul 19;139(28):9627-9636. doi: 10.1021/jacs.7b04151. Epub 2017 Jul 5.
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.
迄今为止，尚未报道铌与其他过渡金属之间的杂核多重键，这可能是由于低价铌中心具有很高的反应性。在此，使用 C 对称的三（膦酰胺）配体框架来构建 Nb/Fe 杂双金属配合物 Cl-Nb(PrNPPh)Fe-Br（2），其具有 Fe→Nb 配位键，金属-金属距离为 2.4269(4) Å。在 PMe 的存在下还原 2 得到 Nb(PrNPPh)Fe-PMe（6），这是一种具有在 Nb 中心处异常三角锥形几何形状的化合物，具有 Nb≡Fe 三重键，以及在 Nb 和任何其他过渡金属之间报告的最短键距离（2.1446(8) Å）。配合物 6 热不稳定，并通过 P-N 键断裂降解，形成 Nb═NR 亚胺配合物，PrN═Nb(PrNPPh)Fe-PMe（9）。独立合成了杂双金属配合物 PrN═Nb(PrNPPh)Fe-Br（8）和 9，表明强 π 键合亚胺官能团阻止了显著的金属-金属相互作用。2、6、8 和 9 的 Fe Mössbauer 光谱显示出明显的位移（δ）趋势，随着金属-金属相互作用变强和 Fe 中心被还原，δ 值减小。通过计算研究探索了 2、6、8 和 9 的电子结构和金属-金属键合，并用循环伏安法更好地理解早期/晚期杂双金属配合物中金属-金属相互作用对所涉及的两种金属的氧化还原性质的影响。
