Engineering topics

Bullock, Ronald Morris

Publications and source records attributed to Bullock, Ronald Morris.

3 records

DOE OSTI2024

Synthesis and Structural Investigation of Rigid Naphthyridine-Bis(carbene) for Trigonal Planar Coordination of Coinage Metals

Coinage metal complexes, particularly Cu(I) and Au(I), supported by N-heterocyclic carbenes are of broad interest in organometallic synthesis, catalysis, and luminescent materials. The d 10 coinage metals can adopt varied linear, trigonal planar, and tetrahedral geometries. However, two-coordinate, linear Cu(I) and Au(I) complexes supported by sterically demanding monodentate or chelating carbenes are generally observed. In most cases, chelating ligands generate multinuclear species with linear geometries at the corresponding Cu(I) centers rather than mononuclear complexes. In this report, we synthesized two bis(carbene) ligands anchored by a flexible bipyridine and a rigid naphthyridine backbone with tunable proximal and distal steric properties at the wingtips to examine the influence of backbone rigidity and directionality of carbene donors on the formation of trigonal planar coinage metal species. Here, the bipyridine-bis(carbene) (ImPy) 2 ligand exclusively stabilizes dinuclear chloride complexes of Cu(I) and Ag(I), whereas the naphthyridine-bis(carbene) (NBC) stabilizes mononuclear, trigonal planar chloride complexes of Cu(I) and Ag(I) and a dinuclear chloride Au(I) complex.

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DOE OSTI2022

Isolation of a Cu–H Monomer Enabled by Remote Steric Substitution of a N-Heterocyclic Carbene Ligand: Stoichiometric Insertion and Catalytic Hydroboration of Internal Alkenes

Transient Cu–H monomers have long been invoked in the mechanisms of substrate insertion in Cu–H catalysis. Their role from Cu–H aggregates has been mostly inferred since ligands to stabilize these monomeric intermediates for systematic studies remain limited. Within the last decade, new sterically demanding N-heterocyclic carbene (NHC) ligands have led to isolable Cu–H dimers and, in some cases, spectroscopic characterization of Cu–H monomers in solution. In this work, we report an NHC ligand, IPr*R, containing para R groups of CHPh 2 and CPh 3 on the ligand periphery for the isolation of a Cu–H monomer for insertion of internal alkenes. This reactivity has not been reported for (NHC)CuH complexes despite their common application in Cu–H-catalyzed hydrofunctionalization. Changing from CHPh 2 to CPh 3 impacts the relative concentration of Cu–H monomers, rate of alkene insertion, and reaction of a trisubstituted internal alkene. Specifically, for R = CPh 3 , monomeric (IPr*CPh 3 )CuH was isolated and provided >95% monomer (10 mM in C 6 D 6 ). In contrast, for R = CHPh 2 , solutions of [(IPr*CHPh 2 )CuH] 2 are 80% dimer and 20% (IPr*CHPh 2 )CuH monomer at 25 °C based on 1 H, 13 C, and 1 H– 13 C HMBC NMR spectroscopy. Quantitative 1 H NMR kinetic studies on cyclopentene insertion into Cu–H complexes to form the corresponding Cu–cyclopentyl complexes demonstrate a strong dependence on the rate of insertion and concentration of the Cu–H monomer. Only (IPr*CPh 3 )CuH, which has a high monomer concentration, underwent regioselective insertion of a trisubstituted internal alkene, 1-methylcyclopentene, to give (IPr*CPh 3 )Cu(2-methylcyclopentyl), which has been crystallographically characterized. We also demonstrated that (IPr*CPh 3 )CuH catalyzes the hydroboration of cyclopentene and methylcyclopentene with pinacolborane.

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DOE OSTI2020

Splitting of multiple hydrogen molecules by bioinspired diniobium metal complexes: a DFT study

Splitting of molecular hydrogen (H 2 ) into bridging and terminal hydrides is a common step in transition metal chemistry. In this paper, we propose a novel organometallic platform for cleavage of multiple H 2 molecules, which combines metal centers capable of stabilizing multiple oxidation states, and ligands bearing positioned pendant basic groups. Using quantum chemical modeling, we show that low-valent, early transition metal diniobium(II) complexes with diphosphine ligands featuring pendant amines can favorably uptake up to 8 hydrogen atoms, and that the energetics are favored by the formation of intramolecular dihydrogen bonds. This result suggests new possible strategies for the development of hydrogen scavenger molecules that are able to perform reversible splitting of multiple H 2 molecules.

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