Organic Chemistry and Theory

190 Pages·1978·3.037 MB·English

by  Vincenzo Balzani, Fabrizio Bolletta, Maria Teresa Gandolfi (auth.)

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Fortschritte der Chemischen Forschung
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Springer-Verlag
Berlin Heidelberg New York 1978
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Contents
Bimolecular Electron Transfer Reactions of the Excited
States of Transition Metal Complexes
Vincenzo Baizani, Fabrizio Bolletta, Maria Teresa Gandolfi,
and Mauro Maestri
Photochemical Reactivity of Keto Imino Ethers.
Type I Rearrangement and (2 + 2)-Photocycloaddition
to the Carbon-Nitrogen Double Bond
Tad H. Koch, David R. Anderson, John M. Bums, Geoffrey C. Crockett,
Kent A. Howard, Joseph .S Keute, Ronald M. Rodehorst,
and Richard J. Sluski 65
Computational Methods of Correlation Energy
Petr (2hrsky and Ivan Huba6 97
A Quantitative Measure of Chemical Chirality and Its
Application to Asymmetric Synthesis
James Dugundji, Rosemarie Kopp, Dieter Marquarding, and Ivar Ugi 165
Author Index 26-75 181
Editorial Board:
Prof. Dr. Michael J. S. Dewar Department of Chemistry, The University of Texas
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Sendal, Japan 980
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Blaise Pascal, B. P. 296/R8, F-67008 Strasbourg-Cedex
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Department of Chemistry, Lexington, KY 40506, USA
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and Technology, South Kensington,
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Bimolecular Electron Transfer Reactions of the Excited
States of Transition Metal Complexes
Vincenzo Balzani, Fabrizio BoUetta, Maria Teresa Gandolfi,
and Mauro Maestri
Istituto Chimico "G. Ciamician" dell'Universitfi, Bologna, Italy
Table of Contents
List of Symbols . . . . . . . . . . . . . . . . . . . . . 3
1. Introduction . . . . . . . . . . . . . . . . . . . . . 4
2. Excited States of Transition Metal Complexes . . . . . . . . . . 4
3. Quenching of Excited States . . . . . . . . . . . . . . . 6
4. Quenching by Chemical Reaction . . . . . . . . . . . . . . 8
5. Thermodynamic Aspects of Excited State Electron Transfer
Reactions . . . . . . . . . . . . . . . . . . . . . . 10
6. Kinetic and Theoretical Aspects of Outer-Sphere Electron Transfer
Reactions . . . . . . . . . . . . . . . . . . . . . . 14
6.1. Thermal Reactions . . . . . . . . . . . . . . . . . 14
6.2. Excited State Reactions . . . . . . . . . . . . . . . 21
7. Electron Transfer Reactions Induced by Excited State Electron Transfer
Quenching . . . . . . . . . . . . . . . . . . . . . 25
8. Conversion of Light Energy into Chemical Energy . . . . . . . . 27
9. Complexes Containing 2,2'-Bipyridine, 1,10-Phenanthroline or Their
Derivatives as Ligands . . . . . . . . . . . . . . . . . . 28
9.1. Introduction . . . . . . . . . . . . . . . . . . . 28
9.2. Spectroscopic Properties . . . . . . . . . . . . . . . 29
9.3. Redox Properties of the Excited States . . . . . . . . . . 36
9.4. Quenching Processes . . . . . . . . . . . . . . . . 37
9.5. Conversion of Light Energy into Chemical Energy . . . . . . 41
9.6. Kinetic Aspects . . . . . . . . . . . . . . . . . . 45
10. Uranyl Ion . . . . . . . . . . . . . . . . . . . . . 48
10.1. Excited State Properties . . . . . . . . . . . . . . . 48
10.2. Quenching by Organic Molecules . . . . . . . . . . . . 49
10.3. Quenching by Metal Cations . . . . . . . . . . . . . . 50
10.4. Quenching by Halide Ions . . . . . . . . . . . . . . 51
V. Balzani et al.
11. Formation and Reaction of Solvated Electrons . . . . . . . . . 52
11.1. Introduction . . . . . . . . . . . . . . . . . . . 52
11.2. Photoelectron Production . . . . . . . . . . . . . . 52
11.3. Production of Excited States in Reactions of Solvated Electrons. 54
12. Other Systems . . . . . . . . . . . . . . . . . . . 55
12.1. Metal Ions . . . . . . . . . . . . . . . . . . . 55
12.2. Miscellaneous . . . . . . . . . . . . . . . . . . 57
13. References . . . . . . . . . . . . . . . . . . . . . 57
Excited Statr Rodox Reactions
List of Symbols
bpy = 2,2'-bipyridine
DMA = N,N-dimethylaniline
DMF = Dimethylfotmamide
DMSO = Dimethylsulfoxide
EDTA = Ethylendiaminetetraacctate ion
Et = Ethyl
Me = Methyl
NHE = Normal Hydrogen Electrode
p2+ = 1,1'-dimethyl-4,4'-bipyridine 2+ (paraquat)
phen = 1,10-phenantbxoline
SCE = Saturated Calomel Electrode
terpy = 2,2',2"-terpytidine
TPTZ = 2,4,6-tripyfidil-s-triazine
0 = phenyl
/CH3
Rl = O=C-O-CH
~cn a
/CH2CH2~
R2 = O=C-O-CH CH2
~CH2CH2 "/
R3 =- O=C-O-CH2-C6Hs
o=y_o t
R4 =
R5 =
R6 =
.V inazlaB et al.
1. Introduction
It is well known that for each molecule there is the so-called ground state and there
are many electronically excited states that can be obtained by visible rO ultraviolet
irradiation. In general, the ground state is the one that is responsible for the ordi-
nary chemistry, while the electronically excited states are those responsible for the
photochemical reactions. Each electronically excited state is virtually a new mole-
cule with respect to the corresponding ground state and thus it can exhibit differ-
ent chemical properties. This means that photochemistry offers a new dimension
to chemistry l), the excited state dimension, which, although mostly unexplored,
is very promising for the progress of chemical research.
As will be shown later, one of the most important consequences of electronic
excitation is that of increasing the electronic affinity and decreasing the ionization
potential of a molecule. As a consequence, those electronically excited states which
live long enough to encounter other species can easily be involved in intermolecular
electron transfer reactions. Until a few years ago, however, the attention of inor-
ganic photochemists was mainly focused on intramolecular photoredox reactions
and ligand photosubstitution reactions 2' 3). In the field of organic photochemistry,
on the other hand, the electron transfer reactions of electronically excited states
have been extensively investigated in the past decade in connection with fluores-
cence quenching and exciplex formation. As the luminescence emission is the most
useful "handle" to establish whether an excited state is involved in intermolecular
processes, the study of the electron-transfer reactions in transition metal photo-
chemistry only began when luminescent complexes were made available for sensiti-
zation and quenching experiments 4). There are at least three different reasons for
the tremendous surge of interest and activity in the study of the electron-transfer
reactions of electronically excited transition metal complexes. Firstly, these reac-
tions are very promising for the conversion of light energy (including solar energy)
into chemical energy. Secondly, they can lead to complexes having unusual oxida-
tion states and thus unusual chemical properties. Finally, they allow us to check
the theories of outer-sphere electron-transfer reactions over a broad range of free
energy change.
In this review article, we discuss the fundamental basis of the bimolecular
electron-transfer reactions of electronically excited transition metal complexes and
then collect and examine the data so far obtained in this field. Although a wide
range of systems are discussed, we focus primarily on quantitative studies, the
majority of which involves Werner-type complexes in fluid solution.
2. Excited States of Transition Metal Complexes
The assignment of the various bands which appear in the absorption spectra of
transition metal complexes is a very difficult problem because the absorption
spectra reflect, of course, the complexity of the electronic structure of these mole-
cules. From a photochemical point of view 2), it is convenient to make reference to
Excited State Redox Reactions
schematic molecular orbital (MO) diagrams such as that shown in Fig. .1 In this very
simplified diagram, each MO is labelled as metal (M) or ligand (L) according to its
P n l
predominant localization. Thus, for example, the alg, eg and tt, bonding MO's,
which result from the combination of metal and ligand orbitals of appropriate
tlu,all
tl~ 't2~l'tl"'t2" ~ ttg,t2w0tlu,t2u ~)*~
n S alg
0
(n-1)d t2g'eg t2g
I tlg't20'tlu't2" / tlg't2g,tlu't2" "11
w ll't.
als, %, tlu ~,
alg, %,tl. J
metal molecular orbitals ligand
orbitals orbitals
Fig. 1. Schematic orbital energy diagram representing various types of electronic transitions in
octahedral complexes. A line connects an atomic orbital to that molecular orbital in which it
has the greatest participation. 1: metal centered (MC)transitions; 2: ligand centered (LC)
transitions; 3a: ligand-to-metal charge transfer (LMCT) transitions; 3b: metal-to-ligand charge
transfer (MLCT) transitions
symmetry, are labelled with L because they receive the greatest contribution from
the ligand orbitals. In the ground electronic configuration of transition metal com-
plexes in their usual oxidation states, the o L and rr L orbitals are completely filled,
the Mn orbitals are either partially or completely filled and the higher orbitals are
usually empty. Using the diagram of Fig. 1, it is possible to make a classification of
the various electronic transitions according to the localization of the MO's involved.
Specifically, we may identify three fundamental types of electronic transitions:
1. Transitions between MO's predominantly localized on the central metal; these
are usually called metal centered (MC), ligand field or d-d transitions.
2. Transitions between MO's predominantly localized on the ligands. These are
usually called ligand centered (LC) or intra-ligand transitions.
3. Transitions between MO's of different localization which cause the displace-
ment of the electronic charge from the ligands to the metal or viceversa. These
transitions are generally called charge transfer (CT) transitions and more specifically
can be distinguished into ligand-to-metal charge transfer (LMCT) and metal-to-ligand
charge transfer (MLCT) transitions.
Another important type of electronic transition is the charge transfer to solvent
(CTTS) transition. Recently, it has also been found s) that electronic transitions may
occur between MO's which are localized on different ligands of the same complex.
v. Balzani et al.
These transitions have been called inter-ligand transmetallic charge transfer transi-
tions. Other types of transitions are present in polynuclear transition metal com-
plexes ,6 )7 but we will not deal with such compounds.
It must be pointed out that the classification into MC, LC and CT transitions
(or excited states) is somewhat arbitrary and loses its meaning whenever the states
involved cannot be described with localized MO configurations. The chemical and
physical properties of these orbitally different excited states have been examined
in detail by several authors ,2 ,3 ,6 a-~o) and will not be further discussed here.
It should be pointed out that the energy ordering of the various orbitals may
be different from that shown in Fig. 1. For the Ru(bpy) +2 complex, for example,
the 7r~.(tlu) orbital is thought to be lower than the o~(eg) orbital 11). More generally,
the excited state ordering is extremely sensitive to the type of the ligands and the
nature and oxidation state of the metal. For example, the lowest excited states of
Ir(phen)Cl~ 12), Ir(phen)2Cl~ )21 and Ir(phen) + l )a are MC, MLCT and LC respec-
tively; the lowest excited state of Rh(bpy)2Cl ~ is MC, whereas that of Ir(bpy)2Cl~
is MLCT14); and the lowest excited state of lrC13- )51 is MC, whereas that of
lrCl 2- t6) is LMCT. Further complications arise from the fact that the energy
splitting between the spin states (e.g., singlet and triplet) belonging to the same
orbital configuration is very large for the CM excited states because the two inter-
acting electrons reside on the same atom, whereas it is smaller for the CT excited
states. It follows that the excited state ordering may be different in the spin-allowed
and spin-forbidden manifolds. Finally, it should be recalled that the presence of
heavy metals brings about a considerable degree of spin-orbit coupling and that
this effect, being related to the central metal atom, is different for the different
kinds of excited states. The LC excited states are scarcely affected, whereas for the
CM and CT excited states it may become meaningless to talk about discrete spin
states17, 18). In the current literature, however, the spin label is generally used even
when its meaning is doubtful.
3. Quenching of Excited States
In a fluid solution, an excited state which has a lifetime long enough to encounter
other species can be deactivated ("quenched") in a bimolecular process. The inti-
mate mechanism of the quenching process is often difficult to elucidate 4' 19-21),
but the final result (Fig. 2) is either (i) the electronic energy transfer from the ex-
cited state to the quencher, (ii) a chemical reaction between the excited state and
the quencher or (iii) the deactivation of the excited state by some catalytic action
of the quencher. Different quenching processes may also occur simultaneously,
which causes noticeable complications in the study of these systems. The kinetic
aspects of the interaction between an excited state and a quencher in a fluid
solution have been extensively discussed 4' ,02 22-24). The quenching processes are
usually studied by measuring at different quencher concentrations one or more of
the following quantities: (a) the emission or reaction efficiency of the excited state
under continuous illumination or (b) the decay of the excited state emission or