Keying Ding

Three-Coordinate and Four-Coordinate Cobalt Hydride Complexes That React with Dinitrogen

We report the formation and N(2) reactivity of novel cobalt hydride complexes supported by bulky beta-diketiminate ligands (L). Addition of KHBEt(3) to LCoCl gives [LCo(mu-H)](2) (1) or K(2)[LCoH](2) (2), depending on the amount of borohydride used. Compound 2 is the first example of a crystallographically characterized hydride complex in which a transition metal is three-coordinate. Both 1 and 2 react with N(2) at room temperature to give dinuclear N(2) complexes with loss of H(2).

Cobalt-Dinitrogen Complexes with Weakened N-N Bonds

Reported N(2) complexes of cobalt do not have substantial weakening of the N-N bond. Using diketiminate ligands to enforce three-coordinate geometries, we have synthesized several novel CoNNCo complexes. In formally univalent complexes, cobalt is poorer than iron at weakening the N-N bond, but in formally zerovalent complexes, cobalt and iron give similar N-N weakening. The weakening is due to cobalt-to-N(2) pi-backbonding, and potassium cations pull more electron density into N(2). These results show that the low coordination number of a trigonal-planar geometry is impetus enough to make even the electronegative cobalt weaken the N-N bond of N(2).

First-Row Transition-Metal Chloride Complexes of the Wide Bite-Angle Diphosphine iPrDPDBFphos and Reactivity Studies of Monovalent Nickel

The diphosphine 4,6-bis(3-diisopropylphosphinophenyl)dibenzofuran (abbreviated as (iPr)DPDBFphos) has been metalated with transition metal dichlorides of zinc, cobalt, and nickel to yield ((iPr)DPDBFphos)MCl(2) complexes. Within these compounds, the diphosphine (iPr)DPDBFphos adapts a wide range of bite angles (115 to 180°) as determined by X-ray crystallography. A three-coordinate planar Ni(I) species was isolated from the reduction of ((iPr)DPDBFphos)NiCl(2) with KC(8). Low-temperature electron paramagnetic resonance (EPR) measurements of ((iPr)DPDBFphos)NiCl allow the determination of g values (2.09, 2.14, 2.37) and hyperfine coupling constants to two (31)P nuclei, A(iso) = 46 × 10(-4) cm(-1), and one (37)Cl/(35)Cl nucleus, A = (12, 0.7, 35) × 10(-4) cm(-1). Density functional theory (DFT) studies reveal the nature of the magnetic orbital to be d(xy), which has σ-antibonding and π(∥)-antibonding interactions with the phosphorus and chloride atoms, respectively. The monovalent nickel complex reacts with substrates containing C-X bonds; and in the case of vinyl chloride, a Ni(II) vinyl species ((iPr)DPDBFphos)Ni(CH═CH(2))Cl is generated along with the Ni(II) dichloride complex. The monovalent Ni(I) chloride is an active catalyst in the Kumada cross-coupling reaction of vinyl chloride and phenyl Grignard reagent.

Mixed-Valent Dicobalt and Iron-Cobalt Complexes with High-Spin Configurations and Short Metal-Metal Bonds

Cobalt-cobalt and iron-cobalt bonds are investigated in coordination complexes with formally mixed-valent [M2](3+) cores. The trigonal dicobalt tris(diphenylformamidinate) compound, Co2(DPhF)3, which was previously reported by Cotton, Murillo, and co-workers (Inorg. Chim. Acta 1996, 249, 9), is shown to have an energetically isolated, high-spin sextet ground-state by magnetic susceptibility and electron paramagnetic resonance (EPR) spectroscopy. A new tris(amidinato)amine ligand platform is introduced. By tethering three amidinate donors to an apical amine, this platform offers two distinct metal-binding sites. Using the phenyl-substituted variant (abbreviated as L(Ph)), the isolation of a dicobalt homobimetallic and an iron-cobalt heterobimetallic are demonstrated. The new [Co2](3+) and [FeCo](3+) cores have high-spin sextet and septet ground states, respectively. Their solid-state structures reveal short metal-metal bond distances of 2.29 Å for Co-Co and 2.18 Å for Fe-Co; the latter is the shortest distance for an iron-cobalt bond to date. To assign the positions of iron and cobalt atoms as well as to determine if Fe/Co mixing is occurring, X-ray anomalous scattering experiments were performed, spanning the Fe and Co absorption energies. These studies show only a minor amount of metal-site mixing in this complex, and that FeCoL(Ph) is more precisely described as (Fe0.94(1)Co0.06(1))(Co0.95(1)Fe0.05(1))L(Ph). The iron-cobalt heterobimetallic has been further characterized by Mössbauer spectroscopy. Its isomer shift of 0.65 mm/s and quadrupole splitting of 0.64 mm/s are comparable to the related diiron complex, Fe2(DPhF)3. On the basis of spectroscopic data and theoretical calculations, it is proposed that the formal [M2](3+) cores are fully delocalized.

Characterization of the Fe-H bond in a three-coordinate terminal hydride complex of iron(I).

Hydride complexes of transition metals play a central role in organometallic chemistry.[1, 2] They are also implicated in biological inorganic chemistry, where hydrides are known or thought to be present in key intermediates in H2 utilization by hydrogenases[3, 4] and in N2 reduction by iron-molybdenum nitrogenases.[5, 6] In both cases, trapped intermediates exhibit large 1H hyperfine couplings from hydrides bonded to paramagnetic iron ion(s).[7-11] As these biological hydrides arise in catalytic intermediates that are not amenable to crystallographic characterization, it is essential to identify the spectroscopic signatures of crystallographically characterized transition-metal hydride complexes.[12]

Study of the conformationally flexible, wide bite-angle diphosphine 4,6-bis(3-diisopropylphosphinophenyl)dibenzofuran in rhodium(I) and palladium(II) coordination complexes.

The diphosphine 4,6-bis(3-diisopropylphosphinophenyl)dibenzofuran (abbreviated as (iPr)DPDBFphos) was prepared and studied for its potential as a trans-chelating ligand in transition-metal coordination complexes. In the rhodium norbornadiene complex [((iPr)DPDBFphos)Rh(NBD)]BF(4), which has been characterized with multinuclear NMR spectroscopy, X-ray crystallography, and electrochemical studies, the ligand binds in cis fashion. In the bis(acetonitrile) complexes of rhodium and palladium [((iPr)DPDBFphos)M(CH(3)CN)(2)](BF(4))(n) (M = Rh, Pd; n = 1, 2), the ligand adopts a trans coordination geometry. Density functional theory (DFT, M06-L) calculations predict that the trans conformer is energetically more favorable than the cis by 3.5 kcal/mol. Cyclic voltammograms of the bis(acetonitrile) Pd(II) and Rh(I) complexes contain reversible and quasi-reversible reduction events, respectively, which are preliminarily assigned as metal-based redox reactions.

High-Performance Pressure-Sensitive Adhesives from Renewable Triblock Copolymers

A part of broader efforts to develop sustainable alternatives to polymers derived from nonrenewable feedstocks, biorenewable and biodegradable ABA triblock copolymers are being actively explored for thermoplastic elastomers and other applications. A key technology for such copolymers is in pressure sensitive adhesives (PSAs), where, for example, the “stickies” problem in paper recycling could be mitigated by facilitating degradation of contaminating residues after paper pulping. We recently developed renewable and hydrolytically degradable poly(lactide)-b-poly(menthide)-b-poly(lactide) (PLA-PM-PLA) triblock copolymers that were microphase separated, elastomeric, and effective as poly(L-lactide) crystallization nucleation agents. High molar mass (∼100 kg/mol) PLA-PM-PLA samples containing small PLA (5−10 kg/mol) segments formulated with 40 wt % of a rosin ester tackifier miscible with the central PM component gave PSAs with respectable peel adhesion, probe tack and sheer strength values (3.2 N cm−1, 1.1 N, and ∼2500 min at room temperature, respectively). With a key goal being to improve performance at high temperatures for broader applicability, we developed variants with poly(α-methylene-γ-butyrolactone) (PMBL) end blocks that exhibit very high Tg values (170−190 °C). 5 While these PMBL-PM-PMBL triblocks were found to exhibit excellent mechanical properties (e.g., high elongation at break values and low hysteresis), difficulties were encountered in efforts to explore PSA formulations due to their poor solubility in common organic solvents. We report herein the preparation of poly(γ-methyl-α-methylene-γ-butyrolactone)-b -poly(menthide)-b-poly(γ-methyl-α-methylene-γ-butyrolactone) (PMeMBL-PM-PMeMBL), the end blocks of which can be accessed from biomass via levulinic acid. Characterization of the new triblock copolymer revealed the requisite elastomeric properties and enhanced solubility in organic solvents for the preparation of tackifier modified blends that exhibited impressive PSA properties significantly improved relative to previously reported sustainable polymer-containing formulations. Samples of the triblock copolymer PMeMBL-PM-PMeMBL were synthesized using procedures similar to that used for the preparation of PMBL-PM-PMBL, substituting γ-methyl-αmethylene-γ-butyrolactone (MeMBL; Figure 1) for α-methylene-γ-butyrolactone (see Supporting Information (SI) for details). H NMR spectroscopy and SEC data (Figure S1− S3) support the successful formation of two low dispersity triblocks containing 9.6 and 16.8 wt % PMeMBL (Table 1). The triblocks were characterized by differential scanning calorimetry (DSC), atomic force microscopy (AFM), and small-angle X-ray scattering (SAXS) (Figures S4−S6). DSC traces of the PMeMBL-PM-PMeMBL samples revealed two glass transition temperatures around −26 and 210 °C, consistent with microphase-separated PM midblocks (Tg ≈ −26 °C) and PMeMBL (Tg ≈ 225 °C) 7 end blocks. Further support for microphase separation came from room temperature AFM images of thermally annealed (150 °C for 1 d) thin films that featured circular bright spots (smaller, hard PMeMBL domains) in a darker background (soft PM; Figure S6) consistent with spherical inclusions of PMeMBL in a matrix of PM. SAXS profiles of similarly annealed samples contained a principal reflection consistent with domain spacings of 21 and 22 nm for the 5-100-5 and 10-100-10 triblocks, respectively (Figure S5). Although well-defined higher-order reflections were not observed, broad features at higher q values are consistent with form factor scattering from an array of spherical particles. Based on these data and considering the low hard segment (PMeMBL) contents, we posit that the copolymers adopt a microphase separated but disorganized spherical morphology. Analysis of tensile stress−strain curves (Figure S7 and Table S1) for the triblock copolymers revealed tensile strengths (2−4 MPa) lower than those reported for PMBL-PM-PMBL (3−12 MPa) or commercial poly(styrene)-b-poly(butadiene)-b-poly(styrene) (SBS) TPEs (20−40 MPa), but higher than PLAPM-PLA (<1.7 MPa) and PMBL-PBA-PMBL (0.7 MPa). Notably, the PMeMBL-PM-PMeMBL samples demonstrated strain values in excess of 1600% similar to what was reported for PMBL-PM-PMBL; mechanical failure of these triblocks was not observed due to instrumental limitations. Based on our earlier PLA-PM-PLA triblock studies, we formulated blends of the PMeMBL-PM-PMeMBL copolymers with the rosin ester (RE) tackifiers Sylvalite RE-85 and Sylvalite RE-10L (Arizona Chemicals). A representative PSA formulation was prepared by combining PMeMBL(10)-PM(100)PMeMBL(10) (100 parts, 61 wt %) with Sylvalite RE-85 (40

Nickel-Catalyzed Decarbonylation of Aromatic Aldehydes

We report here the first systematic study of nickel-catalyzed decarbonylation of aromatic aldehydes under relatively mild conditions. Aldehydes with electron donating groups at para and ortho positions are generally successful with our method. For aldehydes with electron-withdrawing groups, significantly higher yields were achieved for ortho-substituted substrates than para ones, probably due to the effects of steric hindrance or electron donors at the ortho position to suppress the Tishchenko reaction, an undesirable side reaction toward homocoupled esters.

Tripodal N,P Mixed-Donor Ligands and Their Cobalt Complexes: Efficient Catalysts for Acceptorless Dehydrogenation of Secondary Alcohols

A new tetradentate tripodal ligand, iPrPPPNHPyMe, and the cobalt complexes were synthesized and characterized. The well-defined cobalt complexes efficiently catalyzed acceptorless dehydrogenation of secondary alcohols into ketones.

Cobalt-Catalyzed Acceptorless Dehydrogenative Coupling of Primary Alcohols to Esters

A novel catalytic system with a tripodal cobalt complex is developed for efficiently converting primary alcohols to esters. KO tBu is found essential to the transformation. A preliminary mechanistic study suggests a plausible reaction route that involves an initial Co-catalyzed dehydrogenation of alcohol to aldehyde, followed by a Tishchenko-type pathway to ester mediated by KO tBu.