Heterobimetallic Cu-Fe and Zn-Fe complexes catalyze C-H borylation, a transformation that previously required noble metal catalysts. The optimal catalyst, (IPr)Cu-FeCp(CO)2, exhibits efficient activity at 5 mol% loading under photochemical conditions, shows only minimal decrease in activity upon reuse, and is able to catalyze borylation of a variety of arene substrates. Stoichiometric reactivity studies are consistent with a proposed mechanism that exploits metal-metal cooperativity and showcases bimetallic versions of the classical organometallic processes, oxidative addition and reductive elimination.

Heterobimetallic complexes of the formulations (NHC)Cu−FeCp(CO) 2 (NHC = IPr, IMes, SIMes), (IPr)Cu− MoCp(CO) 3 , and (IPr)(Cl)Zn−FeCp(CO) 2 were synthesized in high yield from readily available starting materials and characterized crystallographically. The solid-state structures of the Cu−Fe systems reveal close, secondary interactions between Cu and one CO ligand from the [FeCp(CO) 2 ] unit that are absent in the Zn−Fe analogue. The heterobimetallic complexes feature short yet polar Cu−Fe, Cu−Mo, and Zn−Fe bonds in which the electrophilic metal (Cu, Zn) is later in the transition series than the nucleophilic metal (Fe, Mo), thereby subverting the more common early−late heterobimetallic paradigm. DFT analyses were used to assess M−M′ bond polarity and examine effects on M−M′ bonding of systematic modifications to both the nucleophilic and electrophilic fragments. Experimental confirmation of Cu−Fe bond polarity was obtained by analysis of product mixtures resulting from the reactions between (NHC)Cu−FeCp(CO) 2 complexes and MeI, which produced (NHC)Cu−I and Me−FeCp(CO) 2 products.

To understand better how homogeneous catalysts comprised of two base metals can mimic precious metal catalysts, we have elucidated a complete mechanistic pathway for C−H borylation with Cu−Fe catalysts that is consistent with experimental observations as well as first-principles quantum chemistry. The catalytic cycle begins with the B−H bond of the borane inserting into the Cu−Fe bond of the catalyst, followed by bimetallic oxidative B−H activation and release of the NHCbound Cu−H group. After UV irradiation, release of CO permits the inner-sphere Fe coordination of a solvent arene molecule, which then undergoes C−H borylation via a concerted, 4-centered transition state. The resulting iron-hydride can undergo bimetallic reductive elimination with the Cu−H partner to form H 2 , closing the catalytic cycle. Analysis of fragment charges during these processes confirms that the bimetallic reaction pathways resemble oxidative addition and reductive elimination steps. Spectroscopic studies are included to probe the nature of the unsupported Cu−Fe bonds of the catalyst in solution. This extensive experimental and computational investigation provides useful insight into canonical organometallic reaction mechanisms involved in bimetallic catalysts, which are generally less well understood than their monometallic counterparts.

Readily available copper precatalysts, (NHC)CuOtBu (NHC = Nheterocyclic carbene), catalyze dehydrogenative borylation and silylation of styrenes with moderate to high yields, using ketone additives as sacrificial oxidants. This method provides access to trisubstituted vinylboronates and vinylsilanes without requiring noble metal catalysis.

New CO-free iron boryl complexes, CpFe(PR3)2(Bpin), are described. The CpFe(PEt3)2(Bpin) derivative is uniquely capable of UV-free arene borylation at 70 °C via a dissociative pathway. Catalytic C-H borylation does not proceed using either monometallic or heterobimetallic schemes, and this failure is rationalized through analysis of relative pKa values for the corresponding iron hydride species.

Bimetallic effects on stoichiometric β-hydride elimination and migratory insertion reactions were examined. Bimetallic reaction conditions drove β-hydride elimination at Cu, while bimetallic C-B elimination occurred in the absence of β-hydrogens. The inherent migratory insertion chemistry of alkynes at Ni was diverted under bimetallic reaction conditions to favor C-H deprotonation.

This article presents a retrospective account of our group’s heterobinuclear (NHC)Cu-[MCO] catalyst design concept (NHC = N-heterocyclic carbene, [MCO] = metal carbonyl anion), the discovery of its application towards UV-light-induced dehydrogenative borylation of unactivated arenes, and the subsequent pursuit of thermal reaction conditions through structural modifications of the catalysts. The account highlights advantages of using a hypothesis-driven catalyst design approach that, while often fruitless with regard to the target transformation in this case, nonetheless vastly expanded the set of heterobinuclear catalysts available for other applications. In other words, curiosity-driven research conducted in a rational manner often provides valuable products with unanticipated applications, even if the primary objective is viewed to have failed.1 Introduction to Heterobinuclear Catalysts for C–H Borylation2 Pursuit of Thermal Borylation Conditions3 Catalysts beyond Copper Carbenes4 Conclusions

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