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.

Tertiary and quaternary phosphonium borane catalysts are employed as catalysts for CO2/epoxide copolymerization. Catalyst structures are strategically modified to gain insights into the intricate structure–activity relationship. To quantitatively and rigorously compare these catalysts, the copolymerization reactions were monitored by in situ Raman spectroscopy, allowing the determination of polymerization rate constants. The polymerization rates are very sensitive to perturbations in phosphonium/borane substituents as well as the tether length. To further evaluate catalysts, a nonisothermal kinetic technique has been developed, enabling direct mapping of polymerization rate constant (k p) as a function of polymerization temperatures. By applying this method, key intrinsic attributes governing catalyst performance, such as activation enthalpy (ΔH ‡), entropy (ΔS ‡), and optimal polymerization temperature (T opt), can be extracted in a single continuous temperature sweep experiment. In-depth analyses reveal intricate trends between ΔH ‡, ΔS ‡, and Lewis acidity (as determined using the Gutmann–Beckett method) with respect to structural variations. Collectively, these results are more consistent with the mechanistic proposal in which the resting state is a carbonate species, and the rate-determining step is the ring-opening of epoxide. In agreement with the experimental results, DFT calculations indicate the important contributions of noncovalent stabilizations exerted by the phosphonium moieties. Excitingly, these efforts identify tertiary phosphonium borane analogues, featuring an acidic phosphonium proton, as leading catalysts on the basis of k p and T opt. Mediated by phosphonium borane catalysts, epoxides such as butylene oxide (BO), n-butyl glycidyl ether (BGE), 4-vinyl cyclohexene oxide (VCHO), and cyclohexene oxide (CHO) were copolymerized with CO2 to form polyalkylene carbonate with >95% chemo-selectivity. The tertiary phosphonium catalysts maintain their high activity in the presence of large excess of di-alcohols as chain-transferring agents, affording well-defined telechelic polyols. The results presented herein shed light on the cooperative catalysis between phosphonium and borane.

A one-pot synthetic procedure for a series of bimetallic Nb/M complexes, Cl−Nb( i PrNPPh 2 ) 3 M−X (M = Fe (2), Ni (4), Cu (5)), is described. A similar procedure aimed at synthesizing a Nb/Co analogue instead affords i PrNNb( i PrNPPh 2 ) 2 (μ-PPh 2 )Co−I (3) through cleavage of one phosphinoamide P−N bond under reducing conditions. Complexes 4 and 5 are found to have short Nb-M distances, corresponding to unusual metal−metal bonds between Nb and these first row transition metals. For comparison, a series of heterobimetallic ONb( i PrNPPh 2 ) 3 M−X complexes (M = Fe (7), Co (8), Ni (9), Cu ( 10)) was synthesized. In these complexes, the Nb V center is engaged in sufficient π-bonding to the terminal oxo ligand to remove the driving force for direct metal−metal interactions. A comparison of the cyclic voltammograms of 2 and 4−10 reveals that the presence of a second metal shifts the redox potentials of both Nb and the late metal center anodically, even when direct metal−metal interactions are not present.

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