Kamaluddin Abdur-Rashid

Catalytic Solvolysis of Ammonia Borane

An energy source with a low environmental impact remains a crucial goal for our society. While energy consumption is a broader concern, transportation is an area of keen interest. Hydrogen is an attractive alternative to petrochemical resources because its combustion produces only water as a by-product. Unfortunately, the physical properties of hydrogen, which complicate its safe, efficient, and economical storage, remain a significant barrier toward establishing hydrogen as a viable source of energy. Of the known hydrogen storage technologies (i.e. compression and liquefaction, metal hydrides, chemical hydrides , and carbon nanotube adsorption) chemical hydrides have the highest gravimetric storage capacity. Despite recent determinations by the Department of Energy (DOE) on the status of sodium borohydride, ammonia borane remains one of the most compelling candidates for hydrogen storage because of its higher hydrogen content (19.6 wt %) and stability. 6] Indeed, the aforementioned DOE report goes so far as to suggest that the decision to not use sodium borohydride should not impact continued research on ammonia borane (AB). Moreover, applications outside of transportation remain equally worthy of consideration, not only as a means to further the refinement of developing technologies, but also to encourage the development of critical aspects connected with the establishing of new energy sources, such as the supply and distribution channels. Several homogeneous catalysts have been shown to catalyze the release of one equivalent of hydrogen from ammonia borane at ambient temperature. For example, a very efficient homogeneous iridium catalyst for the dehydrogenation of ammonia borane was reported by Goldberg and coworkers, who demonstrated the fast release of hydrogen within 20 minutes at room temperature. Relevant pincertype catalysts have shown similar efficacies as demonstrated by the research groups of Fagnou and Schneider. Manners and co-workers have demonstrated that pincerbased catalysts can catalyze the linear polymerization of ammonia borane to form poly(aminoborane). Baker and coworkers described the acid-initiated dehydrogenation of ammonia borane as well as a homogeneous nickel-containing catalyst capable of effecting the dehydrogenation of ammonia borane wherein a 94% yield of hydrogen was observed in three hours at 60 8C. Despite these advances, dehydrogenation of ammonia borane remains limited both in terms of hydrogen yield and reaction rate. In contrast, the hydrolysis of ammonia borane in the presence of a heterogeneous catalyst can provide up to three equivalents of hydrogen per mole of ammonia borane at room temperature at satisfactory rates. Several reports have appeared (see for example Xu and Chandra, Manners and co-workers, Ramachandran and Gagare, and Jagirdar and co-workers), which detailed heterogeneous catalysts containing noble or basic metals and used for the hydrolysis of ammonia borane. Unfortunately, these systems require relatively high catalyst loadings and the catalysts have proven difficult to recover with no option for reuse. Recently, reusable monodisperse nickel nanoparticles have emerged as useful catalysts that display five cycles of catalytic activity. Nonetheless, the most practical issue—the systemic wt % of hydrogen—is rarely addressed for hydrolysis-based systems. For example, the system wt % of hydrogen for the hydrolysis of ammonia triborane (where the system weight is defined as NH3B3H7 + water + catalyst) is 6.1% when a base metal heterogeneous catalyst is used. The comparison of this value with the modified DOE target of 7.5% systemic gravimetric capacity for the year 2015 shows that the systemic wt % of hydrogen is among the most significant hurdles for the development of an efficient system for the generation of hydrogen by means of hydrolytic methods. That is, the requirement for the reaction media (i.e. organic solvent or water in the case of solvolytic or hydrolytic processes), which contributes greatly to the total weight of the system, significantly diminishes the hydrogen wt % of the system. Herein, we describe a system for the solvolysis of ammonia borane that constitutes significant progress toward addressing the issues described above. The simple and robust system displays rapid and quantitative evolution of hydrogen from ammonia borane and employs a homogeneous iridium catalyst with exceptionally low loadings and minimal use of solvent. [*] Dr. T. W. Graham, Dr. C.-W. Tsang, X. Chen, Dr. R. Guo, Dr. W. Jia, Dr. S.-M. Lu, Dr. C. Sui-Seng, C. B. Ewart, Dr. D. Amoroso, Dr. K. Abdur-Rashid Kanata Chemical Technologies Inc. 101 College Street, Office 230, MaRS Centre, South Tower, Toronto, ON, M5G 1L7 (Canada) Fax: (+ 1)416-981-7814 E-mail: damoroso@kctchem.com kamal@kctchem.com Homepage: http://www.kctchem.com

Electro-optical properties of the first rhenium-hydrazone complex, fac-Re(CO)3(dpknph)*Cl

Reaction between Re(CO)5Cl and dpknph in PhMe under reflux gave fac-Re(CO)3(dpknph)Cl in good yield. Both dpknph and fac-Re(CO)3(dpknph)Cl exhibit rich electro-optical properties that are sensitive to their surroundings and point to the potential use of these compounds in nonlinear optics and molecular sensing. Spectroscopic and electrochemical measurements on solutions of dpknph and fac-Re(CO)3(dpknph)Cl show that the metal complex undergoes faster electron/charge-transfer than the free ligand. Solvent variations show that the rate increases in the following order: DMSO>DMF>MeCN.

Optosensing properties of fac-Re(CO)(3)(dpknph)Cl (dpknph=di-2-pyridyl ketone p-nitrophenyl hydrazone).

Optical and thermodynamic measurements on fac-Re(CO)(3)(dpknph)Cl in polar non aqueous solvents revealed the existence of two interlocked conformational forms for fac-Re(CO)(3)(dpknph)Cl. The equilibrium distribution of the low (alpha-) and high (beta-) energy conformations is solvent dependent, controlled by the dipole moment of the solvent molecules and their orientation around the total dipole of fac-Re(CO)(3)(dpknph)Cl. The interplay between the alpha- and beta-conformations of fac-Re(CO)(3)(dpknph)Cl, allowed calculations of their extinction coefficients, by forcing the equilibrium to shift to one conformation, using chemical stimuli. In DMSO and DMF extinction coefficients of 87 000+/-2000 and 35 000+/-2000 M(-1) cm(-1) were calculated for the beta- and alpha-conformations of fac-Re(CO)(3)(dpknph)Cl at lambda(max.), respectively. Thermo-optical measurements on fac-Re(CO)(3)(dpknph)Cl, allowed calculations of the activation parameters for the interconversion between the alpha- and beta-conformations of fac-Re(CO)(3)(dpknph)Cl. In DMSO and DMF changes in enthalpy (DeltaH(ø)) of -11.2+/-1.3 and 10.9+/-0.5 kJmol(-1), entropy (DeltaS(ø)) of -12.7+/-4.3 and 29.4+/-1.7 JK(-1) mol(-1), and free energy (DeltaG(ø)) of -7.5+/-0.2 and+2.2+/-0.2 kJmol(-1) and hence equilibrium constants of 20.9+/-1.7 and 0.4+/-0.1 were calculated for fac-Re(CO)(3)(dpknph) at 295 K. The high values for the extinction coefficients and low values for the activation parameters for the interconversion between the alpha- and beta-conformations of fac-Re(CO)(3)(dpknph)Cl, in polar non aqueous solvents allowed the use of these systems as molecular sensors to probe their structural relaxation and interactions with their surroundings. These systems (fac-Re(CO)(3)(dpknph)Cl and surrounding solvent molecules) optically sense chemical and physical stimuli and their sensing power depends on the intensity and nature of these stimuli, i.e. the systems exhibit a high degree of sensitivity and selectivity.

Synthesis of chiral aminophosphines from chiral aminoalcohols via cyclic sulfamidates.

Protic aminophosphines with multiple chiral centers were synthesized in good yields and high purity by the nucleophilic ring-opening of N-protected cyclic sulfamidates with metal phosphides, followed by hydrolysis and deprotection. This synthetic approach is clean, scalable, and high yielding. The method provides an efficient alternative route for the synthesis of chiral aminophosphines.

Highly active iridium catalysts for the hydrogenation of ketones and aldehydes

The pressure hydrogenation capabilities of the iridium pincer complexes IrH2Cl[((i)Pr2PC2H4)2NH] (1) and IrH3[((i)Pr2PC2H4)2NH] (2) are described and compared to related results obtained previously in transfer hydrogenation. Complex 1 was shown to act as a convenient air-stable entry point to the active catalyst 2, in the presence of base and hydrogen gas. The catalysts are active in a range of solvents, including CH2Cl2 and CHCl3, in contrast to related ruthenium systems. This class of iridium complexes is very effective for the direct hydrogenation of a wide range of carbonyl compounds including ketones, diketones, alpha,beta-unsaturated ketones and aldehydes. A catalytic cycle is proposed for this system which involves an ionic heterolytic bifunctional hydrogenation mechanism.

Kinetics of oxidation of ascorbate by tetranuclear cobalt(III) complexes (‘hexols’) in aqueous solution

The kinetics of oxidation of L-ascorbic acid (H2A) by cobalt(III) hexols, [Co{CoL4(µ-OH)2}3]6+[L4=(NH3)4, (en)2, or tren; en = ethane-1,2-diamine, tren = tris(2-aminoethyl)amine], was studied as a function of pH, L-ascorbic acid concentration, temperature and ionic strength, using stopped-flow and conventional spectrophotometric techniques. The rate of the reaction is first order with respect to the concentration of each reactant and increases as [H+] decreases. The kinetic data indicate involvement of the monoprotonated and deprotonated ascorbate species (HA– and A2-) in the redox process. For L4=(NH3)4 the rate constants k2 and k3 are 0.22 ± 0.02 and (5.51 ± 0.09)× 105 dm3 mol–1 s–1 respectively at 25 °C, and the corresponding activation parameters are ΔH2‡= 103 ± 7 kJ mol–1, ΔS2‡= 89 ± 22 J K–1 mol–1 and ΔH3‡= 46 ± 3 kJ mol–1 and ΔS3‡= 19 ± 11 J K–1 mol–1. The variations in rate constants and activation parameters for the series of complexes mentioned above are discussed. The Fuoss theory was applied to the redox process to estimate the ion-pair formation constant and the rate constant for the electron transfer.

[(R)-2,2-Bis(diphenyl­phosphan­yl)-1,1′-binaphthyl-κ2P,P′]{2-[(2R)-1,2-diamino-1-(4-meth­oxy­phen­yl)-3-methyl­but­yl]-5-meth­oxy­phenyl-κC1}hydrido­ruthenium(II) benzene monosolvate

In the title complex, [Ru(C19H25N2O2)H(C44H32P2)]·C6H6, the RuII ion is in a distorted octahedral coordination environment with the hydride H atom trans to the tertiary carbinamine N atom, giving an H—Ru—N angle of 160.8 (12)°. The equatorial sites are occupied by two P atoms, the secondary carbinamine N atom and a coordinated C atom.

KINETICS AND MECHANISM OF THE OXIDATION OF L-ASCORBIC ACID BY CIS-DIAQUA COBALT(III) AMMINE COMPLEXES

The kinetics of oxidation of L-ascorbic acid by cis-diaquacobalt(III) complexes, [CoL4(H2O)2]3+(L4=(NH3)4, (en)2 or tren; en = ethane-1,2-diamine, tren = tris(2-aminoethyl)amine] was studied as a fuction of pH, L-ascorbic acid concentration, temperature, ionic strength and methanol content of the solvent using stopped-flow and conventional spectrophotometry. The results indicated that only the ascorbate monoanion, HA–, is involved in the redox process with the cobalt(III) species. The rate constants for the [Co(tren)(H2O)2]3+ and [Co(tren)(H2O)(OH)]2+ species (k2 and k5) are 0.26 ± 0.09 and 1.25 ± 0.03 dm3 mol–1 s–1 respectively at 30 °C, and the corresponding activation parameters are ΔH2‡= 124 ± 9 kJ mol–1, ΔS2‡= 137 ± 30 J K–1 mol–1 and ΔH5‡= 82 ± 2 kJ mol–1, ΔS5‡= 26 ± 6 J K–1 mol–1. The variations in the rate constants and thermodynamic parameters for the series of complexes is discussed. The Marcus cross-relationship for electron transfer has been applied to the redox process to confirm the outer-sphere mechanism and to estimate the self-exchange rate constant for the [CoL4(H2O)(OH)]2+/+ couple.

Reactivity of the tri-µ-hydroxo-bis[triamminecobalt(III)] ion in halide media

The kinetics of acid hydrolysis of trans-[(H2O)(NH3)3Co(µ-OH)2Co(NH3)3(H2O)]4+ in acidic chloride and bromide solutions was studied using stopped-flow and conventional spectrophotometric techniques, at an ionic strength of 1.0 mol dm–3 and over the ranges 0.10 ⩽[H+]⩽ 0.50 mol dm–3, 0.10 ⩽[X–]⩽ 0.50 mol dm–3, 25 ⩽T⩽ 35 °C and in binary aqueous cosolvent mixtures containing up to 40% methanol by volume. The rate equation observed was of the form kobs=(kc[H+]+k′c)[X–]. At 25 °C the rate constants kc were 1.10 × 10–2 and 1.22 × 10–3 dm6 mol–2 s–1 in chloride and bromide solutions respectively for the [H+]-dependent term. The corresponding values for the [H+]-independent pathway, k′c, were 4.5 × 10–4 and 0.7 × 10–4 dm3 mol–1 s–1. The activation parameters in chloride solution were ΔH‡(kc)= 51 ± 14 kJ mol–1, ΔS‡(kc)=–110 ± 48 J K–1 mol–1; ΔH‡(k′c)= 132 ± 32 kJ mol–1 and ΔS‡(k′c)= 136 ± 105 J K–1 mol–1. At constant [H+](0.5 mol dm–3) the reaction was first order with respect to [Cl–] and [Br–] and the rate constants, kd, were 4.5 × 10–3 and 3.3 × 10–3 dm3 mol–1 s–1 respectively.