CAS: Can 1st Row Transition Metals Express the Covalency Typifying 2nd and 3rd Row Transition Metals: Comparative Studies

Cornell University, Ithaca NY

Investigators

Abstract

With the support of the Chemical Synthesis Program of the Chemistry Division, Professor Peter T. Wolczanski and his research team at Cornell University will investigate whether 1st row transition metals can be induced to express covalency typical of 2nd and 3rd row metals. Most commodity and fine chemicals are synthesized utilizing catalytic chemistry, with 2nd row transition metals being used to catalyze bond formations to carbon most commonly. Many of these metals are in low abundance and expensive; so-called precious metals. This research is directed at developing more sustainable alternative chemistries; namely by establishing ways of utilizing more abundant metals that are modified by ligands to provide the chemical reactivity necessary to supplant the current catalysts. Transformative ligands that expand the scope of applications via unique reactivity are a cornerstone of the program. A number of approaches involving different metal-ligand binding modes, and different electronic capabilities are expected to allow for the effective modulation of catalyst reactivity. One specific application is the synthesis of simple coordination complexes to enable light-initiated processes to occur more sustainably and with greater facility. An internet program/website will be established to provide a forum for Q&A for the community, and to serve as a news interface with the general public. The project will train graduate and undergraduate students in the fields of inorganic and organometallic chemistry in the synthesis and reactivity of transition metal compounds. As an ancillary effort an oral history organometallic chemistry will be assembled and made accessible through the internet. The coordination of strong field and redox non-innocent (RNI) ligands will be utilized with the goal of expanding the redox capability of targeted 1st transition series metals. The goal is the development of catalytic properties of these metals that are typical of 2nd and 3rd row metals. As strong fields impart covalency, bidentate pyrimidine-based chelates featuring anionic or neutral M-C bonds will be employed as inductively withdrawing ligands. As these complexes are predicted to have low-lying pi-antibonding orbitals and corresponding low-lying charge-transfer (CT) excited states, the complexes have potential applications as photoactive materials in photocatalysis. Reversible C-C bond breaking will be employed in low coordinate complexes to restrict RNI to 2e- changes. Chromium tetradentate chelates manifest chemistry that provides proof of this concept. New chelates with sterically/electronically protected components will hamper degradation and stabilize ligand platforms that will enable CH activation through transient imide/nitrene complexation. Non-ancillary ligands based on benzimidazole diamides will be used to reversibly mask alkylidenes that are capable of ring-opening metathesis polymerization and olefin metathesis. Using this ligand system, a reversible cycle involving a net methylene/dihydrogen metathesis will be realized. Additional fundamental information will be gleaned from comparisons of Cr vs Mo or W, Ti vs Zr, or V vs Nb, where comparative chemistry (metalaradical vs concerted) will elicit interpretations based on covalency. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.