Building upon the precedent of catalytically active (NHC)Cu-FeCp(CO)2 complexes, a series of (NHC)Cu-[M] complexes were synthesized via the addition of Na(+)[M](-) reagents to (NHC)CuCl synthons. The different [M](-) anions used span a range of 7 × 10(7) relative nucleophilicity units, allowing for controlled variation of nucleophile/electrophile pairing in the heterobimetallic species. Direct Cu-M bonds (M = Cr, Mn, Co, Mo, Ru, W) formed readily when the bulky IPr carbene was used as a support. Crystallographic characterization and computational examination of these complexes was conducted. For the smaller IMes carbene, structural isomerism was observed when using the weakest [M](-) nucleophiles, with (IMes)Cu-[M] and {(IMes)2Cu}{Cu[M]2} isomers being observed in equilibrium. Collectively, the series of complexes provides a toolbox for catalytic reaction discovery with precise control of structure-function relationships.
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.
The heterobimetallic complex (IPr)Cu-Fp (IPr = N,N′-bis(2,6-diisopropylimidazol-2-ylidene, Fp = FeCp(CO) 2 ) was identified previously as a nonprecious metal catalyst for C− H borylation. To better understand the nature of the bimetallic reaction pathways operative in this system, we have conducted a thorough mechanistic study of alkyl halide activation by the Cu− Fe heterobimetallic reaction center. Use of cyclopropylmethyl halide substrates as radical clocks established that alkyl halide activation occurs by a two-electron mechanism for alkyl bromides and chlorides but not iodides. Eyring analysis of the activation of benzyl chloride allowed for experimental determination of activation parameters, including a large and negative entropy of activation (ΔS ⧧ = −36 eu). A Hammett study with parasubstituted benzyl chlorides revealed a reaction constant of ρ = 1.6, indicating accumulation of negative charge in the transition state on the alkyl halide carbon. The Ru analogue, (IPr)Cu-Rp (Rp = RuCp(CO) 2 ), was found to react approximately 17−25 times more slowly with selected benzyl chlorides than (IPr)Cu-Fp, indicating that the relative nucleophilicities of the free metal carbonyl anions are predictive of the relative reactivities of their heterobimetallic counterparts. Synthesis and characterization of the new Ag and Au analogues, (IPr)Ag-Fp and (IPr)Au-Fp, are reported along with the observation that these more covalent congeners are significantly less reactive toward alkyl halides. DFT calculations were used to model a transition state for the Cu− Fe reaction, which was identified as stereoinvertive at the alkyl halide carbon. NBO calculations indicate crucial roles played by the CO ligands within the Fp group: they both act as redox noninnocent ligands and also provide structural templating to stabilize the transition state as the metal−metal bond breaks. ■ INTRODUCTIONThe mechanisms of oxidative addition (OA) and reductive elimination (RE) at single metal sites have been subject to intense study, in part due to their versatility in a wide range of important catalytic transformations. Analogous reactions of higher nuclearity, for example bimetallic oxidative addition (BOA) and bimetallic reductive elimination (BRE) at binuclear sites, are comparatively underexplored mechanistically, as well as for catalytic applications. 1−4 While many studies on BOA and BRE mechanisms have been reported with precious metals, 5−48 which are also typically capable of conducting single-site OA/RE chemistry, BOA and BRE pathways become particularly intriguing when they feature earth-abundant firstrow transition metals not usually associated with single-site OA/RE independently. 49 Mechanistic studies on such bimetallic pathways are more limited in number. 50−62 Our group recently reported that the heterobimetallic complex (IPr)Cu-Fp (IPr = N,N′-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene, Fp = FeCp(CO) 2 ) is a catalyst for the C−H borylation of arenes, 63 a reaction more typically conducted using single-site OA/RE cycling by Ir...
Structural data pertaining to bimetallic complexes with semibridging carbonyl (SBCO) ligands are analyzed using a comprehensive search of the Cambridge Structural Database (CSD). A brief review of the history of SBCO structures is presented, and this history is evaluated in light of the vastly larger dataset available today from the CSD. Additionally, computational analysis is used to reinforce conclusions from the structural data analysis, as well as to estimate the strength of a typical SBCO interaction. In view of the increasing number of research groups pursuing bimetallic strategies in inorganic/organometallic chemistry, this study adds to the understanding regarding an interaction type that, while weak, is known to play important structural and kinetic roles in certain systems.
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