Search

Homogeneous solar fuels photocatalytic systems often require several additives in solution with the catalyst to operate, such as a photosensitizer (PS), Brønsted acid/base, and a sacrificial electron donor (SED). Tertiary amines, in particular triethylamine (TEA) and triethanolamine (TEOA), are ubiquitously deployed in photocatalysis applications as SEDs and are capable of reductively quenching the PS's excited state. Upon oxidation, TEA and TEOA form TEA•+ and TEOA•+ radical cations, respectively, which decay by proton transfer to generate redox non-innocent transient radicals, TEA• and TEOA•, respectively, with redox potentials that allow them to participate in an additional electron transfer step, thus resulting in net one-photon/two-electron donation. However, the properties of the TEA• and TEOA• radicals are not well understood, including their reducing powers and kinetics of electron transfer to catalysts. Herein, we have used both pulse radiolysis and laser flash photolysis to generate TEA• and TEOA• radicals in CH3CN, and combined with UV/Vis transient absorption and time-resolved mid-infrared spectroscopies, we have probed the kinetics of reduction of the well-established CO2 reduction photocatalyst, fac-ReCl(bpy)(CO)3 (bpy = 2,2′-bipyridine), by these radicals [kTEA• = (4.4 ± 0.3) × 109 M−1 s−1 and kTEOA• = (9.3 ± 0.6) × 107 M−1 s−1]. The ∼50× smaller rate constant for TEOA• indicates, that in contrast to a previous assumption, TEA• is a more potent reductant than TEOA• (by ∼0.2 V, as estimated using the Marcus cross relation). This knowledge will aid in the design of photocatalytic systems involving SEDs. We also show that TEA can be a useful radiolytic solvent radical scavenger for pulse radiolysis experiments in CH3CN, effectively converting unwanted oxidizing radicals into useful reducing equivalents in the form of TEA• radicals. [ABSTRACT FROM AUTHOR]

A series of twelve second coordination sphere (SCS) functionalized manganese tricarbonyl bipyridyl complexes are investigated for their electrocatalytic CO2 reduction properties in acetonitrile. A qualitative and quantitative assessment of the SCS functional groups is discussed with respect to the catalysts' thermodynamic and kinetic efficiencies, and their product selectivities. In probing a broad scope of functional groups, it is clear that only the aprotic ortho‐arylester SCS is capable of promoting the highly desired low‐overpotential proton‐transfer electron‐transfer (PT‐ET) pathway for selective CO production. The ortho‐phenolic analogues cause an increase in overpotential with a product selectivity favoring H2 evolution, consistent with a high‐overpotential pathway via the anionic [Mn−H]− intermediate. Alternative aprotic Lewis base functional groups such as trifluoromethyl, morpholine and acetamide are shown to also be capable of intermediate manganese hydride generation. The tertiary amine substituent, 2‐morpholinophenyl, exhibits a desirable product distribution characteristic of syn‐gas (CO : H2=30 : 48) with an impressive turnover frequency, while the secondary amine group, 2‐acetamidophenyl, induces a notable shift in selectivity with a faradaic yield of 55 % for the formate (HCO2−) product. In addition to their catalytic properties, cyclic voltammetry and infrared spectroelectrochemistry (IR‐SEC) studies are presented to probe pre‐catalyst electronic properties and the two‐electron reduction activation pathway. [ABSTRACT FROM AUTHOR]

We have used fast time-resolved infrared spectroscopy to characterize a series of organometallic methane and ethane complexes in solution at room temperature: W[(CO).sub.5](C[H.sub.4]) and M([[eta].sup.5]--[C.sub.5][R.sub.5])[(CO).sub.2](L) [where M = Mn or Re, R = H or C[H.sub.3] (Re only); and L = C[H.sub.4] or [C.sub.2][H.sub.6]]. In all cases, the methane complexes are found to be short-lived and significantly more reactive than the analogous n-heptane complexes. Re(Cp)[(CO).sub.2](C[H.sub.4]) and Re([Cp.sup.*])[(CO).sub.2](L) [[Cp.sup.*] = [[eta].sup.5] [(C[H.sub.3]).sub.5] and L = C[H.sub.4], [C.sub.2][H.sub.6]] were found to be in rapid equilibrium with the alkyl hydride complexes. In the presence of CO, both alkane and alkyl hydride complexes decay at the same rate. We have used picosecond time-resolved infrared spectroscopy to directly monitor the photolysis of Re([Cp.sup.*])[(CO).sub.3] in scC[H.sub.4] and demonstrated that the initially generated Re([Cp.sup.*])[(CO).sub.2](C[H.sub.4]) forms an equilibrium mixture of Re([Cp.sup.*])[(CO).sub.2](C[H.sub.4])/ Re([Cp.sup.*])[(CO).sub.2](C[H.sub.3])H within the first few nanoseconds ([tau] = 2 ns). The ratio of alkane to alkyl hydride complexes varies in the order Re(Cp)[(CO).sub.2]([C.sub.2][H.sub.6]):Re(Cp)[(CO).sub.2]([C.sub.2][H.sub.5])H > Re([Cp.sup.*])[(CO).sub.2]([C.sub.2][H.sub.6]):Re([Cp.sup.*])[(CO).sub.2] ([C.sub.2][H.sub.5])H [approximately equal to] Re(Cp)[(CO).sub.2] (C[H.sub.4]):Re(Cp) [(CO).sub.2](C[H.sub.3])H > Re([Cp.sup.*])[(CO).sub.2](C[H.sub.4]): Re([Cp.sup.*])[(CO).sub.2](C[H.sub.3])H. Activation parameters for the reactions of the organometallic methane and ethane complexes with CO have been measured, and the [DELTA][H.sup.[double dagger]] values represent lower limits for the C[H.sub.4] binding enthalpies to the metal center of W--C[H.sub.4] (30 kJ x [mol.sup.-1]), Mn--C[H.sub.4] (39 kJ x [mol.sup.-1]), and Re--C[H.sub.4] (51 kJ x [mol.sup.-1]) bonds in W[(CO).sub.5](C[H.sub.4]), Mn(Cp)[(CO) .sub.2](C[H.sub.4]), and Re(Cp)[(CO).sub.2](C[H.sub.4]), respectively. alkane | spectroscopy | supercritical fluid

The photochemistry of Fe(CO)5 is studied in heptane, supercritical (sc) Ar, scXe and scCH4 using time-resolved infrared spectroscopy (TRIR). Direct evidence is provided for the conversion of photogenerated triplet Fe(CO)4 to singlet Fe(CO)4 in conventional and supercritical solution.

The generation of a series of substituted benzoyl radicals by laser flash photolysis is described and the benzoyl radicals are studied by fast time-resolved infrared spectroscopy. These are used as photoinitiators in free radical polymerizations.

Research has been conducted on [Cr(CO) (sub)4 (2,2'-bipyridine)]. The early excited-state dynamics of this compound in CH (sub)2 Cl (sub)2 solution have been investigated via the use of picosecond time-resolved infrared spectroscopy in examining the evolution of the infrared bands caused by the bleached ground-state absorption and the details are reported.

A series of group 7 organometallic noble gas complexes were characterized in supercritical noble gas solution at room temperature using fast time-resolved infrared spectroscopy. Kinetics and activation parameters are presented.

CpRe(Co)sub2(n-heptane), CpRe(Co)sub2(Xe), and CpRe(CO)sub2(Kr) (Cp = etasuper5-Csub5Hsub5) were characterized in n-heptane, supercritical Xe, and Kr solution, using fast time-resolved infrared spectroscopy. CpRE(Co)sub2(XE) and CpRe(CO)sub2(Kr) showed significant stability, and CpRe(CO)sub2(Xe) was found to be less reactive toward CO.