A unique cooperative H2 activation reaction by heterobimetallic (NHC)M'-MCp(CO)2 complexes (NHC = N-heterocyclic carbene, M' = Cu or Ag, M = Fe or Ru) has been leveraged to develop a catalytic alkyne semi-hydrogenation transformation. The optimal Ag-Ru catalyst gives high selectivity for converting alkynes to E-alkenes, a rare selectivity mode for reduction reactions with H2. The transformation is tolerant of many reducible functional groups. Computational analysis of H2 activation thermodynamics guided rational catalyst development. Bimetallic alkyne hydrogenation and alkene isomerization mechanisms are proposed.

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.

A catalytic method to prepare highly substituted 1,3-dienes from two different alkenes is described using a directed, palladium(II)-mediated C(alkenyl)-H activation strategy. The transformation exhibits broad scope across three synthetically useful substrate classes masked with suitable bidentate auxiliaries (4-pentenoic acids, allylic alcohols, and bishomoallylic amines) and tolerates internal nonconjugated alkenes, which have traditionally been a challenging class of substrates in this type of chemistry. Catalytic turnover is enabled by either MnO as the stoichiometric oxidant or co-catalytic Co(OAc) and O (1 atm). Experimental and computational studies were performed to elucidate the preference for C(alkenyl)-H activation over other potential pathways. As part of this effort, a structurally unique alkenylpalladium(II) dimer was isolated and characterized.

This Perspective discusses catalytic hydrogenation reactions of alkenes, alkynes, and other unsaturated C−C substrates in which the catalyst design involves cooperative strategies that enable bifunctional H 2 activation and delivery. These approaches complement the more traditional use of single-site precious metal systems for such hydrogenation reactions in homogeneous catalysis. Strategies included in this Perspective are cooperation between (a) a catalytic metal site and a basic ligand residue, (b) a catalytic metal site and an acidic ligand residue, (c) frustrated acid/base pairs, and (d) two cocatalytic metal sites. Unique reactivity and selectivity patterns with nonprecious elements that have emerged from successful implementation of these strategies in catalytic transformations are emphasized.

Complexes of the type (NHC)M-Fp (NHC = N-heterocyclic carbene, M = Cu or ZnCl, Fp = FeCp(CO)2) have been used recently as replacements for noble metal C-H functionalization catalysts and for small molecule activation studies. The promising reactivity of these systems has been linked to the use of the late metal electrophiles Cu and Zn in place of early metal electrophiles, and also to the ability of the M-Fe pairs to cooperate during catalytically relevant multielectron redox processes such as bimetallic oxidative addition and bimetallic reductive elimination. Using Mössbauer spectroscopy and metal K-edge XANES analysis, a detailed electronic structure description of these complexes is presented. One unusual feature of the late-metal M-Fp interactions is the presence of significant M → Fe π-backdonation in addition to Fe → M σ-donation; this π-backdonation is absent in early metal analogues and is apparent from analysis of Mössbauer data and Fe K-edge data. Multi-edge XANES analysis of C-I bimetallic oxidative addition at a Cu-Fe reaction center reveals little change in metal effective nuclear charges during the two-electron redox process. IR spectroscopy indicates that the supporting carbonyl ligands participate to a large extent in the redox process.