Elizabeth T. Papish

Hydrotris(triazolyl)borate complexes as functional models for Cu nitrite reductase: the electronic influence of distal nitrogens.

Hydrotris(triazolyl)borate (Ttz) ligands form CuNO(x) (x = 2, 3) complexes for structural and functional models of copper nitrite reductase. These complexes have distinct properties relative to complexes of hydrotris(pyrazolyl)borate (Tp) and neutral tridentate N-donor ligands. The electron paramagnetic resonance spectra of five-coordinate copper complexes show rare nitrogen superhyperfine couplings with the Ttz ligand, indicating strong σ donation. The copper(I) nitrite complex [PPN](+)[(Ttz(tBu,Me))Cu(I)NO(2)](-) has been synthesized and characterized and allows for the stoichiometric reduction of NO(2)(-) to NO with H(+) addition. Anionic Cu(I) nitrite complexes are unusual and are stabilized here for the first time because Ttz is a good π acceptor.

Iridium and Ruthenium Complexes of N-Heterocyclic Carbene- and Pyridinol-Derived Chelates as Catalysts for Aqueous Carbon Dioxide Hydrogenation and Formic Acid Dehydrogenation: The Role of the Alkali Metal

Hydrogenation reactions can be used to store energy in chemical bonds, and if these reactions are reversible, that energy can be released on demand. Some of the most effective transition metal catalysts for CO2 hydrogenation have featured pyridin-2-ol-based ligands (e.g., 6,6′-dihydroxybipyridine (6,6′-dhbp)) for both their proton-responsive features and for metal–ligand bifunctional catalysis. We aimed to compare bidentate pyridin-2-ol based ligands with a new scaffold featuring an N-heterocyclic carbene (NHC) bound to pyridin-2-ol. Toward this aim, we have synthesized a series of [Cp*Ir(NHC-pyOR)Cl]OTf complexes where R = tBu (1), H (2), or Me (3). For comparison, we tested analogous bipy-derived iridium complexes as catalysts, specifically [Cp*Ir(6,6′-dxbp)Cl]OTf, where x = hydroxy (4Ir) or methoxy (5Ir); 4Ir was reported previously, but 5Ir is new. The analogous ruthenium complexes were also tested using [(η6-cymene)Ru(6,6′-dxbp)Cl]OTf, where x = hydroxy (4Ru) or methoxy (5Ru); 4Ru and 5Ru were both reported previously. All new complexes were fully characterized by spectroscopic and analytical methods and by single-crystal X-ray diffraction for 1, 2, 3, 5Ir, and for two [Ag(NHC-pyOR)2]OTf complexes 6 (R = tBu) and 7 (R = Me). The aqueous catalytic studies of both CO2 hydrogenation and formic acid dehydrogenation were performed with catalysts 1–5. In general, NHC-pyOR complexes 1–3 were modest precatalysts for both reactions. NHC complexes 1–3 all underwent transformations under basic CO2 hydrogenation conditions, and for 3, we trapped a product of its transformation, 3SP, which we characterized crystallographically. For CO2 hydrogenation with base and dxbp-based catalysts, we observed that x = hydroxy (4Ir) is 5–8 times more active than x = methoxy (5Ir). Notably, ruthenium complex 4Ru showed 95% of the activity of 4Ir. For formic acid dehydrogenation, the trends were quite different with catalytic activity showing 4Ir ≫ 4Ru and 4Ir ≈ 5Ir. Secondary coordination sphere effects are important under basic hydrogenation conditions where the OH groups of 6,6′-dhbp are deprotonated and alkali metals can bind and help to activate CO2. Computational DFT studies have confirmed these trends and have been used to study the mechanisms of both CO2 hydrogenation and formic acid dehydrogenation.

Dramatic tuning of ligand donor properties in (Ttz)CuCO through remote binding of H+ (Ttz = hydrotris(triazolyl)borate)

Complexes with bulky hydrotris(triazolyl)borate (Ttz) ligands, TtzCuCO, were used to probe how acids change the donor properties of Ttz ligands. (Ttz(tBu,Me))CuCO shows four distinct protonation states and a gradual increase in the CO stretch. The increased electrophilic nature of the Cu center upon protonation leads to enhanced C-H activation catalysis.

Tris(triazolyl)borate ligands of intermediate steric bulk for the synthesis of biomimetic structures with hydrogen bonding and solubility in hydrophilic solvents

Tris(triazolyl)borate ligands (Ttz) of intermediate steric bulk were synthesized to investigate their potential for hydrogen bonding and improved solubility in hydrophilic solvents as applied to biomimetic chemistry. The crystal structure of 3-phenyl-5-methyl-1,2,4-triazole (Htz(Ph,Me)) revealed hydrogen bonding and pi stacking interactions. The new ligand salt, potassium tris(3-phenyl-5-methyl-1,2,4-triazolyl)borate (KTtz(Ph,Me)) was synthesized as the first example of a Ttz ligand of intermediate steric bulk. Metathesis between KTtz(Ph,Me) and NaCl followed by recrystallization produced [NaTtz(Ph,Me)].6CH3OH in which the geometry around the sodium is octahedral with an unusual N(3)O(3) donor set; this structure also shows that a hydrogen bonding network is formed by methanol molecules and triazole nitrogens. (Ttz(Ph,Me))ZnCl was synthesized and characterized crystallographically as [(Ttz(Ph,Me))ZnCl].0.5CH3OH in which the zinc is tetrahedral and the triazole rings are within hydrogen bonding distance of CH(3)OH. All of these new compounds are methanol soluble to varying degrees and Htz(Ph,Me) and KTtz(Ph,Me) are soluble in methanol/water mixtures.

Biomimetic Chemistry with Tris(triazolyl)borate Ligands: Unique Structures and Reactivity via Interactions with the Remote Nitrogens

A review of the structures and reactivity of tris(triazolyl)borate (Ttz) ligands and closely related triazole-based ligands is presented, with an emphasis on complexes of the transition metals. Ttz ligands can form mononuclear complexes or coordination polymers of various dimensionalities. The extra nitrogens in Ttz ligands (relative to trispyrazolylborate, Tp, ligands) can bind to additional metals or acidic hydrogens, and through the latter interactions water solubility is improved. Ttz ligands are also weaker donors than Tp ligands, and this often leads to an increased coordination number in metal complexes. An in-depth discussion of recently developed bulky Ttz ligands and their use in forming biomimetic structures and catalyzing reactions is included.

Protonation of Transition Metal Hydrides to Give Dihydrogen Complexes: Mechanistic Implications and Catalytic Applications

Publisher Summary This chapter discusses the protonation of transition-metal hydride complexes, which plays an important role in many catalytic processes. These processes include ionic hydrogenation and the reduction of H+ to H2. Ionic hydrogenation involves the sequential transfer of H+ and H- to olefins, ketones, or imines. If H2 addition to a transition-metal complex generates an acidic dihydrogen complex capable of transferring H+ and H- to a substrate, then the possibility of catalytic ionic hydrogenation exists. The mechanism of ionic hydrogenation is intimately related to that of transfer hydrogenation. Some of the most effective systems for transfer hydrogenation use 2-propanol as a hydrogen source and transfer H+ to the metal and H- to a tethered amino group.

Crystal structures of bis- and hexakis[(6,6'-di-hydroxy-bipyridine)copper(II)] nitrate coordination complexes.

Two copper(II) complexes, a dinuclear and a hexanuclear complex with bridging hydroxyl and nitrate ligands, were obtained from reaction of copper nitrate with dihydroxybipyridine at neutral and slightly acidic pH. Formation of multi-nuclear complexes contrasts with the equivalent sulfate compounds which formed discrete mononuclear complexes. The complexes feature intramolecular and intermolecular hydrogen bonding.

Ethyl[tris(3-tert-butyl-5-methylpyrazol-1-yl)hydridoborato]zinc(II).

The X-ray crystal structure of the title compound, [Zn(C(2)H(5))(C(24)H(40)BN(6))], or Tp(tBu,Me)ZnEt [Tp(tBu,Me) is tris(3-tert-butyl-5-methylpyrazolyl)hydridoborate], reveals a distorted tetrahedral geometry around the Zn atom. The Zn center is coordinated by three N atoms of the borate ligand and by one C atom of the ethyl group. The present structure and other tetrahedral Tp zinc alkyl complexes are compared with similar Ttz ligands (Ttz is 1,2,4-triazolylborate), but no major differences in the structures are noted, and it can be assumed that variation of the substitution pattern of Tp or Ttz ligands has little or no influence on the geometry of alkylzinc complexes. Refinement of the structure is complicated by a combination of metric pseudosymmetry and twinning. The metrics of the structure could also be represented in a double-volume C-centered orthorhombic unit cell, and the structure is twinned by one of the orthorhombic symmetry operators not present in the actual structure. The twinning lies on the borderline between pseudomerohedral and nonmerohedral. The data were refined as being nonmerohedrally twinned, pseudomerohedrally twinned and untwinned. None of the approaches yielded results that were unambiguously better than any of the others: the best fit between structural model and data was observed using the nonmerohedral approach which also yielded the best structure quality indicators, but the data set is less than 80% complete due to rejected data. The pseudomerohedral and the untwinned structures are complete, but relatively large residual electron densities that are not close to the metal center are found with values up to three times higher than in the nonmerohedral approach.

Crystal structure of 4′-bromo-2′,5′-dimeth­oxy-2,5-dioxo-[1,1′-biphen­yl]-3,4-dicarbo­nitrile [BrHBQ(CN)2] benzene hemisolvate

The geometry of the title hemibiquinone is different from previous examples and may be correlated with the weak interactions in the crystal.

Crystal structure of (perchlorato-κO)(1,4,7,10-tetra­aza­cyclo­dodecane-κ4N)copper(II) perchlorate

The crystal structure of (perchlorato-κO)(1,4,7,10-tetraazacyclododecane-κ4 N)copper(II) perchlorate is reported. The crystal was grown from a solution of methanol at ambient temperature which resulted in no co-crystallization of solvent.