Laga In Partial Fulfillment of the Requirements for the degree of Bachelor of Science CALIFORNIA INSTITUTE OF TECHNOLOGY Pasadena, California 2014 (Defended June Stephanie M

It has been my pleasure to work with the extraordinary members of the Gray group and the Caltech community. Single crystals of the compound suitable for X-ray diffraction studies were obtained and confirmed the dimer structure. Using electrochemical methods, the redox properties of the dimer were evaluated and it was found to be an electrocatalyst for proton reduction in acetonitrile.

Because hydrogen gas is difficult to handle and store, hydrogenation of CO2 and later dehydrogenation of the liquid product, formic acid, has been proposed as a hydrogen storage system. The studies here demonstrate the efficiency of the complex as a precatalyst for the desired reaction, with good conversion of the starting formic acid to CO2 and H2. To better understand the properties of the triphosphine cobalt complex, a synthetic procedure for substituting electron-donating groups (eg, methoxy groups) in the ligand was investigated.

To provide the electrons needed for NADPH production, H2O is oxidized at the manganese group of the oxygen-evolving complex (OEC). The potential at which hydrogen is evolved was found to be surprisingly correlated with the electronic properties of the ligand. Electrochemical studies of these dicobalt compounds revealed two nicely separated Co(III/II) pairs and also two solvated Co(II/I) pairs, indicating the stability of the mixed valence intermediates.

The complex was prepared by treating the doubly proton-bridged mononuclear glyoxime, first obtained by Schrauzer and Holland,52 with trimethoxyboron under anhydrous conditions to give the previously unknown compound.

Figure 1.1. A model device architecture for water splitting. The process of interest is electrochemical proton reduction (4e - + 4H + → 2H 2 ) as shown on the lower, photocathode component

Ray Crystallography of the BO 4 -

In support of this idea, IVCT bands were not observed in the expected 700-1000 nm region during spectroelectrochemical studies of the complex. If the counterion is omitted from consideration, C2 symmetry is observed in the complex due to the orientation of the two large macrocycles. Finally, a signal was found at 18.4 ppm, attributable to the two bridging protons of the complex.

This assignment is consistent with the bimetallic Peters system described above, which showed two one-electron events for each of the two paired CoIII/II and CoII/I couples. Two closely spaced but clearly separated signals were observed for a 5 mM sample of the dimer in NBu4PF6-acetonitrile solution, implying that two unique processes occurred during the oxidative movement. The UV-visible spectrum of the dimer, at a concentration of 2.23 mM in acetonitrile, revealed two bands at 542 and 623 nm with extinction coefficients of 129 and 77 M-1 cm-1, respectively.

To determine the ability of dimer 1 to catalyze the reduction of protons to hydrogen, cyclic voltammograms of the free dimer were compared with solutions of the dimer with increasing concentrations of an organic proton source. Cyclic voltammetry of 1 in the presence of increasing acid concentrations indicates the initiation of a catalytic reaction near Co(II/I) potentials of the metal complex. The complex was synthesized by treatment of proton-bridged mononuclear dimethylglyoxymate with trimethoxy boron under anhydrous conditions.

First, heating formic acid (10 equivalents) and a solution of [Co(trifos)(MeCN)]+ (2), in CD3CN at 70 °C for four hours, led to the appearance of a singlet in the 1H liquid of the mixture. NMR at  4.58 ppm, indicating the presence of hydrogen gas. The analysis of the kinetics of the reaction showed that the rate of gas evolution increased after a slow initiation period or lag phase (Figure 2.5). Kinetics of dehydrogenation of formic acid in the presence (blue) and absence (red) of 1 mol% [nBu4N][HCO2]HCO2H.

Cyclic voltammograms of 2 on a glassy carbon electrode in 1.3 mM formic acid showed enhanced current at -1.8 V versus Fc+/0. Cyclic voltammograms of 1 (0.6 mM) in acetonitrile solution containing 0.1 M [nBu4N][PF6] and ferrocene in the presence and absence of formic acid. A cobalt formate complex was isolated via the following procedure: the addition of 1 equivalent of base, [nBu4N][HCO2]HCO2H, to a 1:1 acetonitrile:tetrahydrofuran solution of 2 resulted in an immediate color change of the mixture from blue to yellow.

A small KIE (1-3) would indicate a rate-determining step for breaking the The kinetic isotope effects for the reaction were measured by replacing formic acid with HCOD2D, DCO2H and DCO2D (Figure 1.10). Numerical simulations of the reaction kinetics are consistent with a mechanism involving an initiation period followed by a rapid zero-order decay.

Figure 1.9. 11 B NMR of the BO 4 - -bridged dimeric cobaloxime.

We studied the development and synthesis of triphos ligand with methoxy electron donor (Figure 2.11). These syntheses can be divided into two parts: the formation of the diaryl phosphine 6 or 7 and the formation of the tripodal ligand 8. The 1H NMR spectrum also shows resonances in the 7-8 ppm range, indicating aryl protons, and a singlet at δ 4.05 ppm, assigned as methoxy protons, OCH3.The.

The 31P{1H} NMR spectrum shows only a singlet at 19.2 ppm, confirming the purity of the phosphine oxide (see Figures B3 and B4). Two methods, reported in the literature, have been explored for the formation of the triphos-OMe ligand 8 from the diaryl phosphine 7. In Method II (Scheme 1.4), the reaction conditions include Ni(PPh3)2Cl2 and Et3N in DMF.

Purification of the ligand was not particularly effective, so the products generated via Methods I and II were analyzed by NMR to determine which route afforded 8 in the highest purity. The 1H NMR spectrum of the mixtures from Method I showed peaks in the aromatic region, 7-8 ppm, and peaks in the 3-4 ppm region, tentatively assigned to methoxy protons, OCH3. The reaction mixture shows a signal in the 31P NMR spectrum at -29.5 ppm suggesting the formation of the desired ligand.

GCMS of methoxy ligands obtained from Method I have not been consistent with the expected mass of the ligand structure. However, these results were only from ligands purified by silica gel chromatography and are therefore assumed to be a result of degradation of the product on silica gel. Vapor diffusion of the complex in DCM with diethyl ether led to the formation of the best crystals.

More likely, the reaction of the trichloride and the phosphine in Method I did not go to completion, leaving an unbound phosphine. Also, it is suspected that the use of VKM in the recrystallization of the cobalt complex may. The proposed mechanism involves the formation of a cobalt(III) hydride species via protonation of cobalt(I), followed by reduction of the nascent CoIII-H to generate a highly reactive CoII-H.

Figure 2.11. Proposed substituted cobalt species.

SUPPORTING INFORMATION Experimental Details

The brown solid was washed in DCM and filtered again to yield a dark brown solid (937 mg, mmol, 70%). After recrystallization in MeCN/diethyl ether, a small broad peak at 20 ppm disappears in the 11B NMR.

SUPPORTING INFORMATION Experimental Details

Electrochemical analyzes were performed in a three-electrode cell consisting of a glassy carbon working electrode (surface area = 0.07 cm2), a platinum wire counter electrode, and a silver wire reference electrode. Low-temperature diffraction (and scanning) data were collected on a Bruker Kappa four-circle diffractometer coupled to a Bruker APEX II CCD detector with graphite monochromated radiation Mo K (=0.71073 Å), and the structure of compound 4 was solved by direct methods using SHELXS and improved with respect to F2 on all data by full matrix least squares with SHELXL-2013 using established improvement techniques. All hydrogen atoms were included in the model at geometrically calculated positions and refined using the riding model.

The isotropic displacement parameters of all hydrogen atoms were fixed to 1.2 times the U value of the atoms to which they are bonded (1.5 times for methyl groups). All disordered atoms were refined using similarity constraints as well as rigid bond constraints on anisotropic displacement parameters. Cobalt formate, complex 4, in the orthorhombic space group Pna21 with one molecule in the asymmetric unit.

Note that these distances have been chosen to provide the best fit to the X-ray data and thus avoid the introduction of systematic errors.