PHOTOCHEMICAL CO2 REDUCTION BY RHENUIM AND RUTHENIUM COMPLEXES. Page: 4 of 4
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1-2 mM Re complex with or without 1 M TEA were pressurized by
-2 atmospheric pressure of Ar or CO2. The sample was excited with
the third harmonic (355 nm, 6 ns) of a Continuum Surelite-1
Nd:YAG laser. The excitation energy was typically 20 mJ cm-2 per
pulse. Photochemical reduction of rhenium and ruthenium species
was conducted in either CH3CN/Et3N (4:1 v/v) under vacuum or N2-
bubbled DMF/TEOA (4:1 v/v). The solution was irradiated either
with monochromated light or light through a cut-off filter using a 75
W lamp at 25 0C.
Electrochemical measurements were conducted with BAS 100b
electrochemical analyser from Bioanalytical Systems using solutions
(1 mM) in acetonitrile. A glassy carbon electrode, SCE, and Pt wire
were used with a standard H-shape cell.
Gas and solution phase DFT calculations were carried out using
the hybrid B3LYP method and the Hay-Wadt VDZ (n+l) ECP
(LANL2DZ ECP) 5d basis set. Geometry optimizations and
frequency analyses were performed using the Gaussian 03 package of
Results and Discussion
Thermodynamic hydricity, the ability to donate a hydride ion
(H-), for metal hydrides and dihydrobenzonicotinamides in
acetonitrile has been determined through systematic studies of
electrochemistry and acidity by DuBois and his coworkers.9
Hydricity is defined as the free-energy change for dissociation of H-,
in contrast with acidity and homolytic cleavage. Experimental
estimates of hydricity of substrate DH are obtained from cycles based
on combinations of the processes from the free-energy changes as
AG0H - AGH+(D-H) - 2 E0(D+/D-) + AG0H+(H2) + E0(NHE), where
AG0H-, AG0H+(D-H), E(D+/D-), AG0H+(H2), and E(NHE) are
hydricity of DH, acidity of DH, reduction potential of DH, H2 acidity,
and reduction potential of H+, respectively. Hydricities obtained in
our preliminary calculations in acetonitrile solution are compared to
experimental values in Table 1. The computed electronic energy
hydricity differs somewhat from the hydricity free energy
(comparable to that derived from experimental data) in that it does
not include the contribution of zero-point and thermal energy or
entropic effects, but these effects are generally small and trends can
be compared. The strongest hydride donor, CHO-, will transfer a
hydride to any acceptor listed below it. Several metal complexes
seem capable of hydride transfer to CO2 in acetonitrile:
Cp(bpy)Mo(CO)(H) from the metal and Re(bpy)(CO)3(CHO) and
Re(CO)3(dcb)(CHO) from the C-H of the formyl ligand.
Our preliminary calculations predict that the reduction of
acetone to 2-propanol can take place in acetonitrile by the sequential
transfer of a proton from solution followed by a hydride from
[Ru(bpy)2(pbnHH)]2+ in two slightly exoergic steps in disagreement
with the previously published proposed mechanism.7 The
calculations also predict that neither [Ru(bpy)2(pbnHH)]2+ nor
[Ru(bpy)2(pbn)]2+ can reduce CO2. We are therefore exploring the
development of a stronger hydride donor than [Ru(bpy)2(pbnHH)]2+
with the guidance of calculations of hydricities to screen the
In order to test if this type of scenario will work, we have
prepared several new complexes including [Ru(bpy)2(pbnHH)]2+,
Re(pbn)(CO)3C1 and [Re(pbn)(CO)3(PCy3)]+. We examined the acid-
base and electrochemical properties of the ground and excited states
of Re(pbn)(CO)3C1 and [Re(pbn)(CO)3(PCy3)]+. The titration of a
solution containing Re(pbn)(CO)3C1 or [Re(pbn)(CO)3(PCy3)]+ by
acid shows the disappearance of the lowest MLCT absorption band
and the appearance of the red-shifted band indicating the formation
of the protonated species as found in [Ru(bpy)2(pbn)]2+ to form
Table 1. Computed Electronic Energy Hydricities in Acetonitrile
(B3LYP/LANL2DZ level of theory)
Hydride Donor Product Hydricity
CHO- CO -61
CH3CHN- CH3CN 0
isopropoxide acetone 24
Cp(bpy)(C)MoH (+-CH3CN) Cp(bpy)(C)Mo(NCCH3)+ 39a
[Re'(bpy)(C)3(CH)] [Re'(bpy)(C)4]+ 45
[Re'(CQ)3(dcb)(CHQ)] [Re'(CQ)4(dcb)]+ 49b
HC00- C02 51 (43)
N-Me-nicH N-Me-nic+ 60
N-Bz-nicH N-Bz-nic+ 61 (59)
Cp(bpy)(CC)MoH Cp(bpy)(CC)Mo+ 65a
para-monohydroquinone para-benzoquinone 69
[CpRe(NC)(CC)(CHC)] [CpRe(NC)(C)2]+ 70 (55)
H2 (+ CH3CN) CH3CNH+ 80 (76)
[RuI(bpy)2(pbnHH)]2+ [Ru"(bpy)2(pbnH+)]3+ 92
isopropanol CH3C(OH)CH3+ 94
[IrII(bpy)2(pbnHH)]3+ [Ir"'(bpy)2(pbnH+)]4+ 104
para-hydroquinone para-monohydroquinone+ 128
a LANL2DZ/6-31 G(d,p) 6D basis; b dcb=(4,4')-dichloro-(2,2')-bipyridine;
Experimental value corresponds to Cp*, not Cp as calculated;
N-Me-nicH =N-methylnicotinamide, N-Bz-nicH=N-benzylnicotinamide;
calculations compared to DuBois AG values (in parentheses) are in blue.
[Ru(bpy)2(pbnH)]3+. The excited-state properties under presence and
absence of acid, and the reactivity of the ground and excited-state
species toward CO2 activation are currently investigated using
[Ru(bpy)2(pbnHH)]2+, Re(pbn)(CO)3C1 and [Re(pbn)(CO)3(PCy3)]+.
The results will be presented at the ACS National Meeting in New
Acknowledgement. Work performed at Brookhaven National
Laboratory was funded under contract DE-AC02-98CH10886 with
the U.S. Department of Energy and supported by its Division of
Chemical Sciences, Geosciences, & Biosciences, Office of Basic
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FUJITA,E.; MUCKERMAN, J.T. & TANAKA, K. PHOTOCHEMICAL CO2 REDUCTION BY RHENUIM AND RUTHENIUM COMPLEXES., article, November 30, 2007; United States. (digital.library.unt.edu/ark:/67531/metadc894585/m1/4/: accessed September 23, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.