Kinetic Aspects for the Reduction of CO₂and CS₂with Mixed-Ligand Ruthenium(II) Hydride Complexes Containing Phosphine and Bipyridine

Abstract

A new water-soluble ruthenium hydride complex [Ru(H)(bpy)2(PTA)]PF6 (bpy = 2,2'-bipyridine, PTA = 1,3,5-triaza-7-phosphaadamantane) (1a) was prepared. 1a reacted with CO2 and CS2 to give the corresponding formate and dithioformate complexes, respectively. Both the insertions of CO2 and CS2 into the Ru-H bond of 1a followed second-order kinetics. The second-order rate constant (k2) of CO2 insertion reaction varied from (9.40 ± 0.41) × 10(-4) M(-1) s(-1) in acetone to (1.13 ± 0.08) × 10(-1) M(-1) s(-1) in methanol; moreover, the ln(k2) is in good linear relationship with the acceptor number (AN) of the solvent used. Although, the k2 of CS2 insertion reaction ranged from (3.43 ± 0.10) M(-1) s(-1) in methanol to (24.0 ± 0.5) M(-1) s(-1) in N,N-dimethylformamide, which is 1000 times faster than CO2 insertion. Generally, the k2 of CS2 insertion increased with the static dielectric constant (D(s)) of the reaction medium investigated. For comparison purposes, we further investigated the reactivity of [Ru(H)(bpy)2(PPh3)]PF6 (PPh3 = triphenylphosphine) (1b) with CO2 and CS2. 1b reacted with CO2 slowly in the methanol with a k2 of (1.46 ± 0.09) × 10(-3) M(-1) s(-1), yielding a formate complex [Ru(η(1)-OC(H)═O)(bpy)2(PPh3)]PF6 (2b). The reaction of 1b with CS2 is 1000 times faster than that of CO2. The structures of 1a, 1b, and 2b were determined by X-ray crystallographic analysis.

Carbon Dioxide Insertion into Bridging Iron Hydrides: Kinetic and Mechanistic Studies

“…Executing the reaction in toluene at room temperature leads to greater product speciation as compared to THF as evidenced by the number of C=O stretches in IR spectra of the reaction mixture. Previously, hydridicity of metal hydrides has been correlated to solvent acceptor number, consistent with the observed solvent effect for reaction of 1 with CO 2 , , , . Taken together with the reactivity of Fe 3 H 3 L Et/Me with CO which results in H 2 reductive elimination, we proposed that the hydrides in these clusters are coordinatively fluxional and capable of adopting semi‐bridging or terminal modes.…”

“…The solvent dependence of the hydride transfer reaction in prior systems correlates with the solvent acceptor number (AN), and rate constants for a few of these systems have been reported in a number of solvents , , , , . Given that reported reactivity studies have not been conducted in toluene, we first must estimate the value of k in benzene for these reported compounds by determining the linear function correlating AN to rate constant (Entries 1–4 in Table , Figure S12).…”

“…There are few reports detailing the kinetic parameters for hydride transfer from a transition metal center to CO 2 . Independent of this dearth of detailed kinetic studies, two mechanistic pathways are proposed: normal insertion wherein an H – attacks the π* orbital of CO 2 , and abnormal insertion in which a metallocarboxylate is generated , .…”

“…Coordinatively unsaturated complexes typically proceed through an inner sphere mechanism: CO 2 binds to the metal centers at a vacant coordination site, followed by a concerted M–H bond breaking and C–H bond forming event to generate the (formato)metal complex . Contrastingly, the outer sphere mechanism results from hydride insertion followed by reorientation and coordination of the resultant formate, and is expected for coordinatively saturated complexes . Here, the highly polarized transition state can be stabilized by exogenous Lewis acids or secondary coordination sphere interactions , …”

Activation of heteroallenes by coordinatively unsaturated nickel(ii) alkyl complexes supported by the hydrotris(3-phenyl-5-methyl)pyrazolyl borate (TpPh,Me) ligand

“…[1][2][3][4][5] Apart from CO 2 , other unsaturated small molecules, such as sulfur containing heteroallenes (CS 2 , RNCS, and OCS), can serve as potential building blocks for C 1 chemistry. [8][9][10][11][12][13][14][15][16][17][18] Further transformations of these substrates can be achieved via cleavage of the carbon-sulfur bond, similar to sulfur abstraction observed in low valent transition metal complexes. The activation of these compounds by transition metal complexes with M-C and M-H bonds frequently results in the formation of insertion products.…”