The construction of well-defined early/late heterobimetallic complexes has attracted increasing attention as a method to promote small molecule activation and other multi-electron transformations. The combination of a Lewis acidic early transition metal with an electron rich late transition metal results in enhanced redox activity, and the two different metals can act in a cooperative way to activate small molecules. In this context, our group has been interested in the design, synthesis and characterization of tris(phosphinoamide)-linked C3-symmetric early-late heterobimetallic complexes to investigate their activity towards small molecule activation and electron transfer processes. In the process of our studies, a number of metal-metal multiple bonds have been uncovered between unprecedented metal-metal combinations, including Zr≡Co, Ti≡Co, and V≡Fe triple bonds. The polar metal-metal bonding in the Zr/Co and Ti/Co systems, in particular, has led to facile multielectron transformations and unusual reactivity profiles with substrates such as hydrazine and CO2. In the context of extending this chemistry to the less oxophilic early metals in group 5, we turned our attention to Nb, a metal that has been quite underexplored in the field of metal-metal bonding. The heterobimetallic NbIV/FeI complex Cl-Nb(iPrNPPh2)3Fe-Br was synthesized, and it was found that two-electron reduction of this compound in the presence of PMe3 affords the NbIII/Fe0 complex Nb(iPrNPPh2)3Fe-PMe3, which features a Nb≡Fe triple bond and an unusual trigonal monopyramidal geometry about the Nb center. Nb(iPrNPPh2)3Fe-PMe3 represents the first example of a heterobimetallic multiple bond including Nb and features the shortest bond between Nb and any other transition metal (including itself). The spectroscopic characterization, redox properties, electronic structure and bonding, and reactivity of these complexes will be presented, along with extensions to other unusual metal-metal combinations.

Constructing multiple bonds between unlikely metal pairs: Niobium-iron triple bonds, and other awkward relationships

254th National Meeting (2017)

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