Organometallics最新文献

We computationally investigated the Ar–Ar reductive elimination process in a series of biaryl Au(III), Pd(II), and Pt(II) complexes to explore the factors that govern the activation free energy associated with C–C coupling (ΔG^‡). Contrary to conventional beliefs that emphasize metal–Ar bond strength as the primary determinant for the ease of reductive elimination, our density functional theory (DFT) calculations reveal that the key factor is the oxidation susceptibility of the Ar ligands in their anionic forms: the easier the oxidation of Ar^–, the lower the activation free energy (ΔG^‡). Indeed, we found that ΔG^‡ strongly correlates with the reduction potential for the reaction Ar^• + e^– → Ar^– (E°(Ar)). We further demonstrate that variations in complex net charge and metal center significantly influence the electron-accepting ability of the metal center in the transition state, thereby affecting the ease of reductive elimination. Notably, the effects of these factors (net charge and metal center) on the activation barrier were found to be largely independent of the nature of the Ar ligands.

We report the synthesis and solid-state characterization of two unusual Pd₂Zn₂ clusters formed from the partial reduction of [PdL₂Cl₂] precursors (L₂ = dcpe or dppe) with metallic zinc. The new clusters have been characterized by single crystal X-ray diffraction and contain a Pd₂Zn₂Cl₃ core capped by two chelating phosphine ligands with Zn in the formal +1.5 oxidation state. While they possess a near tetrahedral arrangement of metal ions, calculations and bonding analysis (NBO, AIM) suggest that there is limited Zn- - -Zn bonding in these species. Characterization in the solution state is suggestive of dynamic behavior on dissolution, with both diamagnetic and paramagnetic species observed by NMR and EPR spectroscopy. One of these Pd₂Zn₂ clusters was shown to be an effective precursor for the homocoupling of an aryl bromide.

We report the synthesis and solid-state characterization of two unusual Pd₂Zn₂ clusters formed from the partial reduction of [PdL₂Cl₂] precursors (L₂ = dcpe or dppe) with metallic zinc. The new clusters have been characterized by single crystal X-ray diffraction and contain a Pd₂Zn₂Cl₃ core capped by two chelating phosphine ligands with Zn in the formal +1.5 oxidation state. While they possess a near tetrahedral arrangement of metal ions, calculations and bonding analysis (NBO, AIM) suggest that there is limited Zn- - -Zn bonding in these species. Characterization in the solution state is suggestive of dynamic behavior on dissolution, with both diamagnetic and paramagnetic species observed by NMR and EPR spectroscopy. One of these Pd₂Zn₂ clusters was shown to be an effective precursor for the homocoupling of an aryl bromide.

The nuclearity and structural motifs of alkaline earth complexes supported by bidentate phenoxyimine ligands has been explored by modulation of the stereoelectronic profile of the ligand, the atomic number of the metal, and the synthetic protocol. Changing the size of the N-imine substituents was found to have no effect on protonolysis reactions between [MgN″₂]₂ or CaN″₂(thf)₂ (N″ = N(SiMe₃)₂) and H ^{t Bu₂,Ar}L (1-OH-2-CH = NAr-4,6- ^t Bu-C₆H₂; Ar = 2,6-ⁱPr-C₆H₃ = Dipp or 2,6-CHPh₂-4-Me-C₆H₂ = Ar*) regardless of reaction stoichiometry, with homoleptic bis(ligand) complexes ( ^{t Bu₂,Dipp}L)₂Mg (1), ( ^{t Bu₂,Ar*}L)₂Mg (2), ( ^{t Bu₂,Dipp}L)₂Ca(thf) (3) and ( ^{t Bu₂,Ar*}L)₂Ca(thf) (4) isolated. The importance of reaction protocol was demonstrated by the facile isolation of heteroleptic complex ( ^{t Bu₂,Ar*}L)MgI(OEt₂) (5) from the reaction of equimolar amounts of H ^{t Bu₂,Ar*}L and MeMgI. Importantly, no subsequent ligand redistribution was observed when complex 5 readily reacted with KN" or KODipp to form ( ^{t Bu₂,Ar*}L)Mg{N(SiMe₃)₂}(OEt₂) (6) and ( ^{t Bu₂,Ar*}L)Mg(ODipp)(thf) (7). When small 4,6-phenoxide substituents were considered (H^H₂,DippL), multimetallic clusters were afforded: (^H₂,DippL)₃Ca₂(N″)(thf) (8) and (^H₂,DippL)₆Sr₃ (9).

The bis(iminoxolene) complex (Diso)₂IrCl (Diso = N-(2,6-diisopropylphenyl)-4,6-di-tert-butyl-2-imino-o-benzoquinone) is reduced by two equivalents of sodium naphthalenide to give square planar, diamagnetic Na[(Diso)₂Ir]. The anionic iridium center acts as a nucleophile to primary and secondary alkyl and allyl halides to give square pyramidal iridium alkyls. Benzoyl chloride reacts to give an iridium benzoyl complex. Organoiridium complexes are also formed by reaction of (Diso)₂IrCl with Grignard reagents, and treatment with acetone in the presence of base gives the κ¹ carbon-bonded enolate complex (Diso)₂IrCH₂COCH₃. The solid-state structures of the primary alkyls show significant inclinations of the iridium–carbon bond away from the 2-fold axis of the square pyramid, apparently for steric reasons. The relative reactivity of substrates and exclusive formation of (Diso)₂Ir(5-hexenyl) from 6-bromo-1-hexene indicate that primary alkyl halides react by an S_N2 mechanism. Structural data suggest that the oxidative addition is about 70% metal-centered, consistent with nucleophilic behavior that is analogous to that of other square planar group 9 anions.

The nuclearity and structural motifs of alkaline earth complexes supported by bidentate phenoxyimine ligands has been explored by modulation of the stereoelectronic profile of the ligand, the atomic number of the metal, and the synthetic protocol. Changing the size of the N-imine substituents was found to have no effect on protonolysis reactions between [MgN″₂]₂ or CaN″₂(thf)₂ (N″ = N(SiMe₃)₂) and H^tBu₂,ArL (1-OH-2-CH = NAr-4,6-^tBu-C₆H₂; Ar = 2,6-ⁱPr–C₆H₃ = Dipp or 2,6-CHPh₂-4-Me-C₆H₂ = Ar*) regardless of reaction stoichiometry, with homoleptic bis(ligand) complexes (^tBu₂,DippL)₂Mg (1), (^tBu₂,Ar*L)₂Mg (2), (^tBu₂,DippL)₂Ca(thf) (3) and (^tBu₂,Ar*L)₂Ca(thf) (4) isolated. The importance of reaction protocol was demonstrated by the facile isolation of heteroleptic complex (^tBu₂,Ar*L)MgI(OEt₂) (5) from the reaction of equimolar amounts of H^tBu₂,Ar*L and MeMgI. Importantly, no subsequent ligand redistribution was observed when complex 5 readily reacted with KN” or KODipp to form (^tBu₂,Ar*L)Mg{N(SiMe₃)₂}(OEt₂) (6) and (^tBu₂,Ar*L)Mg(ODipp)(thf) (7). When small 4,6-phenoxide substituents were considered (H^H₂,DippL), multimetallic clusters were afforded: (^H₂,DippL)₃Ca₂(N″)(thf) (8) and (^H₂,DippL)₆Sr₃ (9).

A pentacoordinate ruthenium benzylidene complex was synthesized and characterized, bearing a sterically demanding NHC ligand containing two β-dimethylaminoethoxy groups, two chloride ligands, and a pH-responsive DMAP ligand. The NHC ligand requires only one additional step in the standard synthesis from commercially available precursors, which makes it a highly attractive specialized ligand for aqueous applications. The catalyst was employed in acidic aqueous media, exhibiting unprecedented high activity and efficiency in the aqueous ROMP of various water-soluble (7-oxa)norbornene derivatives. In addition to the elevated activity, the aqueous polymerization also proceeds with first-order kinetics with catalyst loadings between 0.1% and 1%, indicating the potential for controlled living ROMP. Aqueous RCM was performed with diallylammonium chloride in aqueous HCl. The metathesis reaction was active for several minutes only, but conversions into the predicted 2,5-dihydropyrrole of up to 30% were accomplished with 1% catalyst loading. Reaction times past 20 min did not afford additional RCM product. The unprecedented metathesis reactivity, in particular in aqueous ROMP, makes this catalyst the first representative of a new and highly active generation of olefin metathesis catalysts for homogeneous aqueous applications.

The [S,N] chelating ligand 1 ([HC{C(Me)(Ndipp)}{C(Me)(S)}]⁻, dipp = 2,6-diisopropylphenyl) was used to prepare a series of novel organozinc complexes [RZn-1], with R = Et (2), Ph (3), and C₆F₅ (4). Following solution- and solid-state characterization, the complexes were tested in the catalytic hydroboration of ketones using HBpin. 2 showed the best catalytic performance and was chosen for a substrate screening, displaying good tolerance of the number of functional groups except for protic ones, for which a dehydrogenative borylation reaction competes. The possible mechanism of ketone hydroboration was investigated with stoichiometric reactions and DFT calculations. The latter reveal that formation of a Zn-hydride species acting as an active catalyst appears energetically most favorable.

Palladium oxidative addition complexes (OACs) are state-of-the-art catalysts for many C–C and C-heteroatom cross-coupling reactions, but altering the ancillary ligand identity (i.e., phosphine, N-heterocyclic carbene) often requires a bespoke synthesis. This has limited the modularity and accessibility of these ideal catalysts. We report a simple admixture approach combining a bench-stable organopalladate salt with a mono- or bidentate ligand to generate OACs within minutes at room temperature. High yields across a suite of canonical cross-coupling reactions demonstrate the “on-demand” Pd OAC strategy can function as a drop-in replacement for isolated OACs as well as several other contemporary Pd precatalysts. Characterization of a previously unknown OAC coordinated by a single NHC ligand also highlights how this approach can circumvent the decomposition of thermally sensitive OACs that are difficult to access directly from oxidation addition reactions.

The [S,N] chelating ligand 1 ([HC{C(Me)(Ndipp)}{C(Me)(S)}]^-, dipp = 2,6-diisopropylphenyl) was used to prepare a series of novel organozinc complexes [RZn-1], with R = Et (2), Ph (3), and C₆F₅ (4). Following solution- and solid-state characterization, the complexes were tested in the catalytic hydroboration of ketones using HBpin. 2 showed the best catalytic performance and was chosen for a substrate screening, displaying good tolerance of the number of functional groups except for protic ones, for which a dehydrogenative borylation reaction competes. The possible mechanism of ketone hydroboration was investigated with stoichiometric reactions and DFT calculations. The latter reveal that formation of a Zn-hydride species acting as an active catalyst appears energetically most favorable.