Proton
-
Coupled Electron Transfer between 4
-
Cyanophenol
and Photoexcited Rhenium(I) Complexes with Different
Protonatable Sites
Journal:
Inorganic Chemistry
Manuscript ID:
ic-2012-00834c.R2
Manuscript Type:
Article
Date Submitted by the Author:
29-Jun-2012
Complete List of Authors:
Bronner, Catherine; Georg-August-University Göttingen, Institute of
Inorganic Chemistry
Wenger, Oliver; University of Goettingen, Institute of Inorganic Chemistry
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Submitted to Inorganic Chemistry
1
Proton-Coupled Electron Transfer between 4-
Cyanophenol and Photoexcited Rhenium(I) Complexes
with Different Protonatable Sites
Catherine Bronner and Oliver S. Wenger
*
Georg-August-Universität, Institut für Anorganische Chemie, Tammannstrasse 4, D-37077 Göttingen,
Germany
oliver.wenger@chemie.uni-goettingen.de
RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required
according to the journal that you are submitting your paper to)
ABSTRACT
Two rhenium(I) tricarbonyl diimine complexes, one of them with a 2,2´-bipyrazine (bpz) and a pyridine
(py) ligand in addition to the carbonyls ([Re(bpz)(CO)
3
(py)]
+
), and one tricarbonyl complex with a 2,2´-
bipyridine (bpy) and a 1,4-pyrazine (pz) ligand ([Re(bpy)(CO)
3
(pz)]
+
) were synthesized and their
photochemistry with 4-cyanophenol in acetonitrile solution was explored. Metal-to-ligand charge
transfer (MLCT) excitation occurs towards the protonatable bpz ligand in the [Re(bpz)(CO)
3
(py)]
+
complex while in the [Re(bpy)(CO)
3
(pz)]
+
complex the same type of excitation promotes an electron
away from the protonatable pz ligand. This study aimed to explore how this difference in electronic
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excited-state structure affects the rates and the reaction mechanism for photoinduced proton-coupled
electron transfer (PCET) between 4-cyanophenol and the two rhenium(I) complexes. Transient
absorption spectroscopy provides clear evidence for PCET reaction products, and significant H/D
kinetic isotope effects are observed in some of the luminescence quenching experiments. Concerted
proton-electron transfer is likely to play an important role in both cases, but a reaction sequence of
proton transfer and electron transfer steps cannot be fully excluded for the 4-cyanophenol /
[Re(bpz)(CO)
3
(py)]
+
reaction couple. Interestingly, the rate constants for bimolecular excited-state
quenching are on the same order of magnitude for both rhenium(I) complexes.
INTRODUCTION
In view of the importance of proton-coupled electron transfer (PCET) in photosynthesis,
1-3
respiration,
4
nitrogen or carbon dioxide fixation
5
there have been numerous investigations exploring the
fundamentals of PCET in recent years.
6-10
Phenols have played a prominent role in such studies,
11-16
partly because phenolic functions occur in biologically relevant PCET systems but also because they are
simple enough for mechanistic investigations in purely artificial systems. A question of central interest
in such studies is often whether the electron and the proton are transferred in a concerted manner or
whether there are individual (consecutive) steps of electron transfer and proton transfer.
17-18
A variety of
different experimental techniques have been employed including electrochemical,
10,15
EPR,
19
and optical
spectroscopic methods.
20-21
Many experimental investigations performed until now focus on PCET between molecules in their
electronic ground states, but recently there has been increasing interest in PCET reactivity of
photoexcited molecules or metal complexes.
22-33
Such investigations appear interesting in the context of
direct light-to-chemical energy conversion, but much is yet to be learned about PCET involving
electronically excited states.
28
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Against this background we have conducted a comparative study of the excited-state PCET chemistry
of the two rhenium(I) tricarbonyl diimine complexes shown in Scheme 1 with 4-cyanophenol as a
common reaction partner. Rhenium complexes of this type have long been known as luminophors and
photooxidants,
34-37
and they were frequently employed as sensitizers for photoinduced electron
transfer.
38-44
Both complexes from Scheme 1 have protonatable nitrogen atoms at the ligand periphery,
either at a 2,2’-bipyrazine (bpz) chelating agent (left) or at a monodentate 1,4-pyrazine (pz) ligand
(right). Based on prior studies of photoexcited ruthenium(II) 2,2’-bipyrazine complexes with phenols as
reaction partners we anticipated that photoexcitation of the two rhenium complexes from Scheme 1
would induce PCET chemistry when 4-cyanophenol is present at sufficiently high concentration.
25,28-29,33
Based on these prior investigations we pictured that in aprotic solvent 4-cyanophenol might form
hydrogen bonds to the bpz ligand of [Re(bpz)(CO)
3
(py)]
+
(py = pyridine) and to the pz ligand of
[Re(bpy)(CO)
3
(pz)]
+
(bpy = 2,2’-bipyridine), and such encounter adducts would appear to be reasonable
precursors for PCET events.
Scheme 1. MLCT excitation and PCET chemistry in two distinct 4-cyanophenol / rhenium reaction
couples. ET = electron transfer, PT = proton transfer.
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The long-lived
3
MLCT (metal-to-ligand charge transfer) state which is populated after photoexcitation
is localized on the bidentate bpz and bpy ligands (upper half of Scheme 1). Consequently, MLCT
excitation of [Re(bpz)(CO)
3
(py)]
+
is expected to increase the basicity of the nitrogen atoms at the
periphery of the bpz ligand relative to the ground state (which is beneficial for proton transfer), but at
the same time the MLCT-excited electron is in the middle of the electron transfer pathway between 4-
cyanophenol and the metal center (lower left corner of Scheme 1). By contrast, MLCT excitation of
[Re(bpy)(CO)
3
(pz)]
+
opens a direct electron transfer pathway from the phenol to the metal center (lower
right corner of Scheme 1), but MLCT excitation in this case is expected to decrease the basicity of the
uncoordinated pz nitrogen atom relative to the ground state. We deemed it interesting to explore to what
extent, if at all, these two fundamentally different scenarios affect the photoinduced chemistry of 4-
cyanophenol / rhenium reaction couples.
RESULTS AND DISCUSSION
X-ray crystal structures. Yellow monocrystals of [Re(bpz)(CO)
3
(py)](PF
6
) were grown by slow
diffusion of pentane into an acetone solution. This compound crystallizes in the monoclinic C2/c space
group with two molecules of the complex and two hexafluorophosphate counter ions in the asymmetric
unit. The [Re(bpz)(CO)
3
(py)]
+
cation is depicted in Figure 1a (note that only one crystallographically
independent cation is represented). A solvate of this structure has already been described by Rillema et
al.
45
The [Re(bpy)(CO)
3
(pz)]
+
complex (Figure 1b) was crystallized as the hexafluorophosphate salt by
slow evaporation of acetone in an acetone/water mixture at 0°C, affording yellow plates of
[Re(bpy)(CO)
3
(pz)]
2
(PF
6
)
2
·(H
2
O)·((CH
3
)
2
CO). The complex crystallizes in the monoclinic C2/c space
group with one rhenium(I) complex and one PF
6
-
ion in the asymmetric unit. In addition, a disordered
acetone solvent molecule and a water molecule are present, the latter being located on a C2 axis.
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