Michael J. Rose

Ruthenium Nitrosyls Derived from Tetradentate Ligands Containing Carboxamido-N and Phenolato-O Donors: Syntheses, Structures, Photolability, and Time Dependent Density Functional Theory Studies

In order to examine the role(s) of designed ligands on the NO photolability of {Ru-NO}(6) nitrosyls, a set of three nitrosyls with ligands containing two carboxamide groups along with a varying number of phenolates have been synthesized. The nitrosyls namely, (NEt(4))(2)[(hybeb)Ru(NO)(OEt)] (1), (PPh(4))[(hypyb)Ru(NO)(OEt)] (2), and [(bpb)Ru(NO)(OEt)] (3) have been characterized by X-ray crystallography. Complexes 1-3 are diamagnetic, exhibit nu(NO) in the range 1780-1840 cm(-1) and rapidly release NO in solution upon exposure to low power UV light (7 mW/cm(2)). Density Functional Theory (DFT) and Time Dependent DFT (TDDFT) calculations on 1-3 indicate considerable contribution of ligand orbitals in the MOs involved in transitions leading to NO photolability. The results of the theoretical studies match well with the experimental absorption spectra as well as the parameters for NO photorelease and provide insight into the transition(s) associated with loss of NO.

Heck Coupling of Olefins to Mixed Methyl/Thienyl Monolayers on Si(111) Surfaces

The Heck reaction has been used to couple olefins to a Si(111) surface that was functionalized with a mixed monolayer comprised of methyl and thienyl groups. The coupling method maintained a conjugated linkage between the surface and the olefinic surface functionality, to allow for facile charge transfer from the silicon surface. While a Si(111) surface terminated only with thienyl groups displayed a surface recombination velocity, S, of 670 ± 190 cm s(-1), the mixed CH3/SC4H3-Si(111) surfaces with a coverage of θSC4H3 = 0.15 ± 0.02 displayed a substantially lower value of S = 27 ± 9 cm s(-1). Accordingly, CH3/SC4H3-Si(111) surfaces were brominated with N-bromosuccinimide, to produce mixed CH3/SC4H2Br-Si(111) surfaces with coverages of θBr-Si < 0.05. The resulting aryl halide surfaces were activated using [Pd(PPh3)4] as a catalyst. After activation, Pd(II) was selectively coordinated by oxidative addition to the surface-bound aryl halide. The olefinic substrates 4-fluorostyrene, vinylferrocene, and protoporphyrin IX dimethyl ester were then coupled (in dimethylformamide at 100 °C) to the Pd-containing functionalized Si surfaces. The porphyrin-modified surface was then metalated with Co, Cu, or Zn. The vinylferrocene-modified Si(111) surface showed a linear dependence of the peak current on scan rate in cyclic voltammetry, indicating that facile electron transfer had been maintained and providing evidence of a robust linkage between the Si surface and the tethered ferrocene. The final Heck-coupled surface exhibited S = 70 cm s(-1), indicating that high-quality surfaces could be produced by this multistep synthetic approach for tethering small molecules to silicon photoelectrodes.

Thiolate S-oxygenation controls nitric oxide (NO) photolability of a synthetic iron nitrile hydratase (Fe-NHase) model derived from mixed carboxamide/thiolate ligand.

In order to determine the origin of the NO photolability of the active site of Fe-containing nitrile hydratase (Fe-NHase), a model complex of the NO-bound active site (dark form) has been isolated and structurally characterized. The model, NEt(4)[(Cl(2)PhPepS)Fe(NO)(DMAP)] (2), is derived from a tetradentate ligand comprising carboxamido N and thiolato S donor centers much like the donors present in the active site of Fe-NHase. This {Fe-NO}(6) nitrosyl effectively mimics the NO-bound active site in terms of structural and spectroscopic parameters. However, this model lacks the key property of NO photolability. Interestingly, S-oxygenation of the model complex results in formation of Na[(Cl(2)PhPep{SO(2)}(2))Fe(NO)(DMAP)] (3), in which the -S donors are oxygenated to -SO(2) moieties, and this species exhibits NO photolability. These results indicate that S-oxygenation could be the key reason for the observed NO photolability of the active site of the dark form of Fe-NHase.

Dual Coordination Modes of Ethylene-Linked NP2 Ligands in Cobalt(II) and Nickel(II) Iodides

Here we report the syntheses and crystal structures of a series of cobalt(II) and nickel(II) complexes derived from (R)NP2 ligands (where R = OMe(Bz), H(Bz), Br(Bz), Ph) bearing ethylene linkers between a single N and two P donors. The Co(II) complexes generally adopt a tetrahedral configuration of general formula [(NP2)Co(I)(2)], wherein the two phosphorus donors are bound to the metal center but the central N-donor remains unbound. We have found one case of structural isomerism within a single crystal structure. The Co(II) complex derived from (Bz)NP2 displays dual coordination modes: one in the tetrahedral complex [((Bz)NP2)Co(I)(2)]; and the other in a square pyramidal variant, [((Bz)NP2)Co(I)(2)]. In contrast, the Ni(II) complexes adopt a square planar geometry in which the P(Et)N(Et)P donors in the ligand backbone are coordinated to the metal center, resulting in cationic species of formula [((R)NP2)Ni(I)](+) with iodide as counterion. All Ni(II) complexes exhibit sharp (1)H and (31)P spectra in the diamagnetic region. The Co(II) complexes are high-spin (S = 3/2) in the solid state as determined by SQUID measurements from 4 to 300 K. Solution electron paramagnetic resonance (EPR) experiments reveal a high-spin/low-spin Co(II) equilibrium that is dependent on solvent and ligand substituent.

Hybrid organic/inorganic band-edge modulation of p-Si(111) photoelectrodes: effects of R, metal oxide, and Pt on H2 generation.

The efficient generation of dihydrogen on molecularly modified p-Si(111) has remained a challenge due to the low barrier heights observed on such surfaces. The band-edge and barrier height challenge is a primary obstruction to progress in the area of integration of molecular H2 electrocatalysts with silicon photoelectrodes. In this work, we demonstrate that an optimal combination of organic passivating agent and inorganic metal oxide leads to H2 evolution at photovoltages positive of RHE. Modulation of the passivating R group [CH3 → Ph → Naph → Anth → Ph(OMe)2] improves both the band-edge position and ΔV (Vonset - VJmax). Subsequent atomic layer deposition (ALD) of Al2O3 or TiO2 along with ALD-Pt deposition results in to our knowledge the first example of a positive H2 operating potential on molecularly modified Si(111). Mott-Schottky analyses reveal that the flat-band potential of the stable Ph(OMe)2 surface approaches that of the native (but unstable) hydride-terminated surface. The series resistance is diminished by the methoxy functional groups on the phenyl unit, due to its chemical and electronic connectivity with the TiO2 layer. Overall, judicious choice of the R group in conjunction with TiO2|Pt effects H2 generation on p-Si(111) photoelectrodes (Voc = 207 ± 5.2 mV; Jsc = -21.7 mA/cm(2); ff = 0.22; ηH2 = 0.99%). These results provide a viable hybrid strategy toward the operation of catalysts on molecularly modified p-Si(111).

Criss-crossed dinucleating behavior of an N4 Schiff base ligand: formation of a μ-OH,μ-O2 dicobalt(III) core via O2 activation.

We report the synthesis and structural characterization of a dicobalt(III) complex with a μ-OH,μ-O2 core, namely μ-OH,μ-O2-[Co(enN4)]2(X)3 [1(ClO4)3 and 1(BF4)3]. The dinuclear core is cross-linked by two N4 Schiff base ligands that span each cobalt center. The formally Co(III)-Co(III) dimer is formed spontaneously upon exposure of the mononuclear Co(II) complex to air and exhibits a ν(O-O) value at 882 cm(-1) that shifts to 833 cm(-1) upon substitution with (18)O2. The CV of 1(BF4)3 exhibits a reversible {Co(III)-Co(III)}↔{Co(III)-Co(IV)} redox process, and we have investigated the oxidized {Co(III)-Co(IV)} species by EPR spectroscopy (g = 2.02, 2.06; S = 1/2 signal) and DFT calculations.

Iron Hydride Detection and Intramolecular Hydride Transfer in a Synthetic Model of Mono-Iron Hydrogenase with a CNS Chelate

We report the identification and reactivity of an iron hydride species in a synthetic model complex of monoiron hydrogenase. The hydride complex is derived from a phosphine-free CNS chelate that includes a Fe-C(NH)(═O) bond (carbamoyl) as a mimic of the active site iron acyl. The reaction of [((O═)C(HN)N(py)S(Me))Fe(CO)2(Br)] (1) with NaHBEt3 generates the iron hydride intermediate [((O═)C(HN)N(py)S(Me))Fe(H)(CO)2] (2; δFe-H = -5.08 ppm). Above -40 °C, the hydride species extrudes CH3S(-) via intramolecular hydride transfer, which is stoichiometrically trapped in the structurally characterized dimer μ2-(CH3S)2-[((O═)C(HN)N(Ph))Fe(CO)2]2 (3). Alternately, when activated by base ((t)BuOK), 1 undergoes desulfurization to form a cyclometalated species, [((O═)C(NH)NC(Ph))Fe(CO)2] (5); derivatization of 5 with PPh3 affords the structurally characterized species [((O═)C(NH)NC)Fe(CO)(PPh3)2] (6), indicating complex 6 as the common intermediate along each pathway of desulfurization.

Platinum-Enhanced Electron Transfer and Surface Passivation through Ultrathin Film Aluminum Oxide (Al2O3) on Si(111)–CH3 Photoelectrodes

We report the preparation, stability, and utility of Si(111)-CH3 photoelectrodes protected with thin films of aluminum oxide (Al2O3) prepared by atomic layer deposition (ALD). The photoelectrodes have been characterized by X-ray photoelectron spectroscopy (XPS), photoelectrochemistry (Fc in MeCN, Fc-OH in H2O), electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV) simulation. XPS analysis of the growing Al2O3 layer affords both the thickness, and information regarding two-dimensional versus three-dimensional mode of growth. Impedance measurements on Si(111)|CH3|Al2O3 devices reveal that the nascent films (5-30 Å) exhibit significant capacitance, which is attenuated upon surpassing the bulk threshold (∼30 Å). The Al2O3 layer provides enhanced photoelectrochemical (PEC) stability evidenced by an increase in the anodic window of operation in MeCN (up to +0.5 V vs Ag) and enhanced stability in aqueous electrolyte (up to +0.2 V vs Ag). XPS analysis before and after PEC confirms the Al2O3 layer is persistent and prevents surface corrosion (SiOx). Sweep-rate dependent CVs in MeCN at varying thicknesses exhibit a trend of increasingly broad features, characteristic of slow electron transport kinetics. Simulations were modeled as slow electron transfer through a partially resistive and electroactive Al2O3 layer. Lastly, we find that the Al2O3 ultrathin film serves as a support for the ALD deposition of Pt nanoparticles (d ≈ 8 nm) that enhance electron transfer through the Al2O3 layer. Surface recombination velocity (SRV) measurements on the assembled Si(111)|CH3|Al2O3-15 device affords an S value of 4170 cm s(-1) (τ = 4.2 μs) comparable to the bare Si(111)-CH3 surface (3950 cm s(-1); τ = 4.4 μs). Overall, the results indicate that high electronic quality and low surface defect densities can be retained throughout a multistep assembly of an integrated and passivated semiconductor|thin-film|metal device.

Steric spacing of molecular linkers on passivated Si(111) photoelectrodes.

Surfaces with high photoelectrochemical and electronic quality can be prepared by tethering small molecules to single-crystalline Si(111) surfaces using a two-step halogenation/alkylation method (by Lewis and co-workers).1-7 We report here that the surface coverage of custom-synthesized, phenyl-based molecular linkers can be controlled by varying the steric size of R-groups (R=CH3, C6H11, 2-ethylhexyl) at the periphery of the linker. Additionally, the linkers possess a para triflate group (-O2SCF3) that serves as a convenient analytical marker and as a point of covalent attachment for a redox active label. Quantitative X-ray photoelectron spectroscopy (XPS) measurements revealed that the surface coverage systematically varies according to the steric size of the linker: CH3 (6.7±0.8%), CyHex (2.9±1.2%), EtHex (2.1±0.9%). The stability of the photoelectrochemical cyclic voltammetry (PEC-CV) behavior was dependent on an additional methylation step (with CH3MgCl) to passivate residual Si(111)-Cl bonds. Subsequently, the triflate functional group was utilized to perform Pd-catalyzed Heck coupling of vinylferrocene to the surface-attached linkers. Ferrocene surface coverages measured from cyclic voltammetry on the ferrocene-functionalized surfaces Si(111)-8a/CH3-Fc (R=CH3) and Si(111)-8c/CH3-Fc (R=2-EtHex) are consistent with the corresponding Fe 2p XPS coverages and suggest a ∼1:1 conversion of surface triflate groups to vinyl-Fc sites. The surface defect densities of the linker/CH3 modified surfaces are dependent on the coverage and composition of the organic layer. Surface recombination velocity (SRV) measurements indicated that n-Si(111)-8a/CH3 and the ferrocene coupled n-Si(111)-8a/CH3-Fc exhibited relatively high surface carrier lifetimes (4.51 and 3.88 μs, respectively) and correspondingly low S values (3880 and 4510 cm s(-1)). Thus, the multistep, linker/Fc functionalized surfaces exhibit analogously low trap state densities as compared to the fully passivated n-Si(111)-CH3 surface.

H2 Photogeneration Using a Phosphonate-Anchored Ni-PNP Catalyst on a Band-Edge-Modified p-Si(111)|AZO Construct

We report the fabrication of a {semiconductor}|{metal oxide}|{molecular catalyst} construct for the photogeneration of dihydrogen (H2) under illumination, including band-edge modulation of the semiconductor electrode depending on the identity of Si(111)-R and the metal oxide. Briefly, a synergistic band-edge modulation is observed upon (i) the introduction of a p-Si|n-AZO heterojunction and (ii) introduction of an organic dimethoxyphenyl (diMeOPh) group at the heterojunction interface; the AZO also serves as a transparent and conductive conduit, which was capped with an ultrathin layer (20 Å) of amorphous TiO2 for stability. A phosphonate-appended PNP ligand and its Ni complex were then adsorbed to the p/n heterojunction for photoelectrochemical H2 generation (figures of merit: Vonset ≈ + 0.03 V vs NHE, Jmax ≈ 8 mA cm(-2) at 60 mM TsOH).