Mechanisms of Photochemical Reactions of Transition-Metal

14

Downloaded by CORNELL UNIV on July 31, 2012 | http://pubs.acs.org Publication Date: April 17, 1998 | doi: 10.1021/ba-1998-0254.ch014

Mechanisms of Photochemical Reactions of Transition-Metal Complexes Elucidated by Pulse Radiolysis Experiments Carol Creutz, Harold A. Schwarz, and Norman Sutin Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973-5000

This chapter illustrates the complementarity of photochemical and ra diation chemical techniques to elucidate elementary pathways in mecha nisticallyrichsystems. Some of the mechanistic conclusions that have resulted from these studies in aqueous media are presented. Extreme (both high and low) oxidation states of transition-metal complexes are included. Reactivity with respect to electron transfer reactions and small-molecule activation are addressed.

Since the early 1970s, the work of our group at Brookhaven National Labora tory has focused on basic research directed toward the conversion of the energy from sunlight into other forms, useful on a practical human time scale. Our approach (illustrated in Figure 1) has involved the use of transition-metal com plexes with good light absorption properties and relatively long excited state lifetimes as sensitizers. Typically, these excited state species possess ~2 eV of excitation energy, and accordingly (depending on ground-state properties, of course), they are powerful reducing and/or oxidizing agents. The energy, captured originally in the light-absorption step, is largely retained through charge-separating, electron transfer reactions of the excited state, which produce ground-state, highly reac tive electron transfer products. In principle, the energy stored (albeit fleetingly) in the charge-separated species can be captured and stored in several ways, such as through electron transfer to appropriate electrodes installed in a suitably designed circuit to either yield a photogalvanic cell or charge a battery. The direction we have taken is mainly a more chemical one, in which the energy ©1998 American Chemical Society

In Photochemistry and Radiation Chemistry; Wishart, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1998.

231


232

PHOTOCHEMISTRY AND RADIATION CHEMISTRY

excitation

exceed state electron transfer

energy storage regeneration

Figure 1. Cartoon outlining the steps involved in converting solar energy (hv) to stored chemical energy or electricity. Electron transfer reactions of the excited state (denoted by an asterisk) with donors (D) or acceptors (A) to yield chargeseparated species (D+orA-) take place in competition with nonproductive emis sion (hv')> nonradiative excited-state decay, and back electron transfer, which produce heat.

from the light is used to drive uphill chemical reactions, such as dissociation of water into its elements or reduction of carbon dioxide. The chemicals pro duced in these reactions could then be used as fuels (energy sources) or as raw materials for other processes. Critical to an efficient overall photoconversion process are high efficiency at each juncture and, in particular, for high yields in the chemical-forming reactions, efficient catalysis by species that can be generated by one-electron transfer reactions. One- and two-electron reduction potentials for hydrogen ion and for carbon dioxide vs. normal hydrogen elec trode ( N H E ) are compared in Figure 2. One role of the catalyst is to stabilize the one-electron reduction (or oxida tion) products, thereby lowering the kinetic barrier for the net two (or more) electron transfer process. Transition-metal catalysts serve to stabilize the oneelectron reduction products and to promote rapid formation of the desired products. M M

1

1

+ H —* Μ +

11

Η· — Μ

Π Ι

H"

+ C0 — M C0 - — M :C0 2

n

2

m

(1)

2

2

-


(2)

14.

CREUTZ ET AL. Mechanisms of Photochemical Reactions

Reduction Potentials vs. NHE, pH 7,25 Two Electron

-2.7 V H

+

"C

One Electron

-0.41 V H

233

H

2

+0.03 V H

+

H"

-0.53 V


C0

CO

2

.1.9 V COo

-0.31V C0

2

-1.2 V *C0 '

:C0 *

2

2

2

HC0 " 2

Standard States: for gases

C 0 , CO 1 atm.; others 1 M . 2

Figure 2. One- and two-electron reduction potentials for hydrogen ion and carbon dioxide vs. NHE (pH 7, 25 °C). Standard states: 1 atm for gases H , C0 , and CO; 1 M for others. 2

2

This chapter focuses on our mechanistic studies of some of these chemical systems.

Flash Photolysis versus Pulse Radiolysis Techniques Clearly, excited state reactions, the true photochemical reactions, are best stud ied direcdy through photochemical methods (although there are systems in which an excited state of interest may not be attainable through direct light absorption but might be attained through a radiation chemical route). What then is the role of radiation chemistry in the mechanistic study of complex photochemical systems? It is a versatile, powerful tool, particularly the pulse radiolysis technique, for the study of the thermal reactions that may be induced by photochemical events. In our work, these follow-up reactions have involved organic radicals and metal ions in unusually high or low oxidation states. We have used U V - v i s spectroscopy to identify these species and probe their fates. First we outline the strengths of pulse radiolysis in these studies; then we return to them in photochemical case studies that illustrate their value. Note that all of these comments pertain to aqueous solutions near room temperature; without a large and reliable database on the kinetics and products of such solutions, the planning and interpretation of these complex experiments would not be pos sible.


234


Sensitivity. Pulse radiolysis has high sensitivity for weakly absorbing species, which is especially useful for monitoring ligand-field absorptions of first-transition-series metal ions. There is no inner filter (or at least less of one): functional photochemical systems of necessity contain species that strongly absorb light in some region of the spectrum. The sensitizer absorption makes it much more difficult to detect signal changes arising from chemical reactions not involving the sensi tizer, especially when the desired signals are small. Synthetic Control. The ionization of water produces hydrated elec tron, e~ , and hydroxyl radical, O H , species with a remarkable range of oxidizing and reducing ability, and these are stable for a remarkably long time in water. By manipulation of scavenger and substrate concentrations, the primary reagents can be readily changed. To give a simple example, in an Ar-saturated aqueous solution, the primary radicals are the hydrated electron, eâ , and hydro xyl radical, - O H , but saturation of the solution with nitrous oxide converts eâq to - O H , thereby eliminating the reducing species and giving a high yield of the oxidizing * O H . By contrast, with use of a solution containing C O 2 and formate, a very high yield of the strongly reducing C 0 ~ radical is produced.


q

q

2

Flexibility. There is ready control of reagent concentrations through dose variations. Furthermore, in the experiments conducted at Brookhaven National Laboratory (BNL), great flexibility is provided by the cell design. The cell consists of a large reservoir of solution which is used to fill and refill (2 m L per change) the working cell area in the side arm. The working area is irradiated with 2-MeV electrons from one side through 0.5-mm-thiek quartz. The solution is monitored at 90° to the electron pulse through a 2-cm path length (which can be increased to 6 cm, effectively through use of mirrors in the sample holder). The solution p H is easily monitored and adjusted through the top port, so that a single stock solution can be used for a p H dependence study. The "purge" gas is easily changed, so that a single solution can be used for a range of, for example, C 0 concentrations. The current B N L setup is also readily adapted for work on time scales ranging from microseconds to minutes, with the long time scale capability being particularly useful. 2

Applications We turn now to concrete examples of the interplay between photochemical and radiation chemical techniques in the form of case histories taken mainly from our work at Brookhaven National Laboratory.

Excited State Characterization. Charge-transfer excited states of transition-metal complexes fulfill many important photochemical roles, and an


14.

235


understanding of their properties has been the focus of many kinds of studies. Metal-to-ligand charge transfer ( M L C T ) excited states have long been of keen interest, and pulse radiolysis techniques have enriched our understanding of these states and aided in their assignments. Typically, M L C T excitation involves transfer of a metal electron to an aromatic ligand bound to the metal, i.e., M-L

^> M - L "

(3)

+

where h is Planck's constant and V T is frequency. The transfer produces an oxidized metal center, M , and a reduced ligand, L~. Since many aromatic anion radicals absorb at relatively low energy, the M L C T excited state might be expected to exhibit low-energy absorptions characteristic of the L " moiety. Thus, the M L C T state of R u ( b p y ) , * Ru(bpy) , or R u ( b p y ) ( b p y - ) where bpy is 2, 2'-bipyridine exhibits bands at 390 and 520 nm, reminiscent of the spectrum of bpy~, produced by reduction with sodium metal in tetrahydrofuran (THF) (1 ). When finally determined by picosecond absorption spec troscopic methods, the spectra of the M L C T states of a series of R u ( N H ) L complexes (2 ) also proved to exhibit characteristic L " chromophores by compar ison with radical spectra determined earlier in pulse radiolysis measurements. In photochemical systems, highly reactive photoproducts such as Ru(bpm) , which is produced by reduction of the M L C T state * R u ( b p m ) are common (bpm = 2,2'-bipyrimidine). In the photochemical system, a sacrifi cial donor, D , such as ethylenediaminetetraacetic acid (EDTA), T E O A (triethanolamine), or oxalate, is used to reduce the excited state ("reductive quench ing"). Such sacrificial reagents owe their high efficiencies to the irreversible character of their two-electron couples. Their one-electron oxidized forms, D , generally rearrange and ultimately provide a second reducing equivalent to the system. However, the complexity of their chemistry generally makes it more difficult to characterize the authentic chemistry of the reactive transition-metal complexes when the two are produced together. Thus, the définitive characteri zation of Ru(bpm) was carried out by pulse radiolysis (3). Protonation of Ru(bpm) to yield R u ( b p m ) ( b p m H ) (pK^ = 6.3) was established, and the chemistries of these two Ru(II)-bound ligand radicals were fully explored. Indeed, Hoffman, Mulazzani, and colleagues, through comparable photochemi cal and radiation chemical studies, mapped out the chemistry of the R u ( L ) ( L " ) series for Ru(bpy) (4) and Ru(bpz) (5). M L C T excited states can also be produced via radiation chemical tech niques. The reduction of Ru(bpy) with e^ yields the M L C T excited state of Ru(bpy) in high yield (6, 7): C


+

n

3

2+

3

2+

in

2+

2

3

3

+

2 +

5

3

2+

+

3

3

n

+

+

n

+

2

2+

2

3

+

3

3

3

3+

+

q

2+

e~ + Rh(bpy) q

3

3+

> * Ru(bpy)

3

2+

(4)

Characterization of "Invisible" Photoproducts. In the late 1970s, our attention was drawn to the work of Lehn and Sauvage, who reported


236


photoproduetioii of H from a system containing Ru(bpy) , R h i b p y ) ^ , triethanolamine, and tetrachloroplatinate (8). We conducted in-depth mechanis tic studies of this system to learn how the H was formed (9). From continuous photolyses, we learned that omission of the platinum salt resulted in completely different products: no H was produced, but a purple-to-brown colored prod uct, ultimately identified as the low-spin d rhodium(I) complex Rh(bpy) , formed instead. Prepared independendy, the latter does not reduce water to dihydrogen in the presence of the platinum salts or colloidal platinum, consis tent with the R h ^ R h and H / H potentials (-0.25 and -0.47 V versus N H E , respectively, at p H 8). 2

3

2+

2

2

8


1

+

2

+

2

[Pt°]

Rh(bpy)

2

+ 2H 0

+

2

H

+ Rh(bpy) (OH) , Κ ~ H T atm at p H 8

2

2

2

+

(5)

4

(The line through the arrow indicates that the reaction does not take place.) Emission quenching and flash photolysis studies revealed that the primary photoreaction involved oxidation of the M L C T excited state * Ru(bpy) by Rh(bpy) (Jfe = 6.2 Χ 10 M " s" ): 3

3

3+

8

* Ru(bpy)

3

2+

1

2+

1

+ Rhibpy)^ -

R u i b p y ) ^ + Rh(bpy)

3

(6)

2+

In the transient absorption studies, bleaching of the distinctive Ru(bpy) ground-state and excited absorption spectra were readily observed, consistent with formation of Ru(bpy) and Rh(bpy) . However, no intense new absorp tions were observed. When triethanolamine was added to the Ru(bpy) + RMbpyta * solution, the re-formation of Ru(bpy) via reduction of Ru(bpy) by T E O A could be observed in the microsecond-to-millisecond regime, de pending upon conditions. 3

3

3+

3

2+

3

3

2+

3

R u i b p y ) ^ + T E O A — Ru(bpy)

2+

2+

3

2+

3

+ TEOA+

3+

(7)

Typically, however, no other color changes occurred in these solutions for times up to a second. O n this very long time scale, the growth of the Rh(I) absorption could be detected, but reliable kinetic data could not be obtained because of diffusion; only a small volume element of the solution was irradiated, and on the time scale of seconds, that volume began to mix with unirradiated solution, giving the appearance that the Rh(I) was being consumed. We also tried to use electrochemical methods to characterize Rh(bpy) , but the latter is reduced more readily than the Rh(bpy) complex, so that only the Rh(I) complex can be prepared by the electrochemical technique. 3

3

3+


2+

14.

237


Finally, to characterize the elusive Rh(bpy) species, we turned to pulse radi olysis methods, using the electron to reduce Rh(III) and T E O A as OH-scavenger (although the chemistry of T E O A differs in photo- and radiation chemistry systems (10)). 3

Rh(bpy)3^ + «s, -

2+

Rhftpy^

(8)

N(CH CH OH) + OH — N(CH CHOH)(CH CH OH) + H 0 2

2

3

2

2

2

2

2


(9) The Rh(bpy) species exhibited only weak absorption features in the visible-near-UV region of the spectrum (ε ο4000, ε 1 0 0 0 M " c m ) . These low intensities suggest that the species is actually an authentic 19-electron, Rh(II) complex. The alternative, Rh (bpy) (bpy~), should exhibit much more intense absorption. The disproportionation of Rh(bpy) occurs by a highly unusual mechanism, rate-determining bpy-loss. 3

2+

35

1

490

m

2

3

Rh(bpy)

3

2+

-1

^± R h ( b p y ) ( H 0 ) 2

2

2

2+

+ bpy,

2+

k = 2 s" {

(10)

1

where f is forward. Rh(bpy)

3

+ Rh(bpy) (H 0)

2+

2

2

2

2 +

- Rh(bpy)

3

+ Rh(bpy)

3+

2

+

(11)

The most surprising outcome of this study was the conclusion that the 19electron species is so stable with respect to loss of ligand. This conclusion is derived from the kinetic data for bpy loss and addition (11). Rh(bpy)

3

2+

?± R h ( b p y ) ( H 0 ) 2

2

2

2+

+ bpy

Jfcf = 2 s" ; k = 0.2 Χ 10 M " s" ; Κ = k /k 1

r

9

1

1

{

r

(12)

= ΙΟ" M 8

where r is reverse. At the time of this discovery, 19-electron species beyond the first transition series were not regarded as electronically viable. Subsequent work by David Tyler and his group demonstrated the existence of many other 19-electron species and the important role such species can play in organometallic systems.

Characterization of an Unstable Photoproduet. Pulse radiolysis provides a unique experimental tool for the characterization of an unstable photoproduet. The highly oxidizing, ligand-field excited state of * Os^TMCX^2*, where T M C is tetramethyl-cyclam, has now been studied In Photochemistry and Radiation Chemistry; Wishart, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1998.


238


Figure 3. Energetics of the ground- and excited-state Os (TMC)0 couples. v

Os (TMC)0 VI

+

2

+

2

fairly extensively (12). As shown in Figure 3, it is a powerful outer-sphere oxidant, capable of being reduced by such poor reductants as the halide ions and hydroxide ion (13). However, relatively little attention has been paid to its Os(V) reduction product. In photochemical experiments, Os(V) can be pro duced by excited state quenching: * Os^TMC)*^ * + Q — Os (TMC)0 2

v

2

+ Q

+

AG° ~ 0 eV

+

(13)

However, photochemical systems are not suitable for characterizing the groundstate Os /Os couple, because the "back" reaction is extremely exergonic. VI

v

Os (TMC)0 v

2

+

+ Q — O s ^ i T M O O ^ + Q, +

AG° — 2 eV

(14)

In general, a self-exchange rate is best estimated from very low driving force reac tions involving similar reaction partners. Furthermore, the Os(V) is unstable to ward Os(III), which renders application of more conventional techniques, such as stopped flow, difficult. Accordingly, we used pulse radiolysis to produce Os(V), O s ^ T M O O ^ + e~ — O s ( T M C ) 0 v

2

+


(15)

14.


239

and studied its reaction, Os (TMC)0 v

2

+

+ B Q ^± O s ^ T M Q O a ^ + B Q " ,

Κ = 0.315

(16)

with the characterized reaction partner benzoquinone (BQ), B Q + e~ τ± B Q " q

= 0.078 V vs. N H E

E

M

B Q + B Q " ^± B Q " + B Q

k

ex

= 6.2 Χ 10 M " 7

(17) s"

1

in order to estimate the self-exchange rate, kn = 1.1 Χ 10 M ~ - s " , of the ground state couple (14). 6


(18)

1

Os (TMC)0 v

2

+

1

1

+ O s ^ T M O O / * — Os™(TMC)0 *2

+ Os (TMC)0 v

2

+

(19)

In both the above rhodium and osmium systems, the oxidation state studied by pulse radiolysis is at a thermodynamic maximum for the system, so that it is very difficult to utilize more conventional techniques such as electrochemical methods or chemical reductants to produce them for study. The difficulties of studying these unstable oxidation states are further exacerbated by their rela tively low molar absorptivities and the overlap of their spectra with those of the higher oxidation states from which they are produced.

Synthetic Control as a Probe of Mechanism. In certain systems, synthetic control of very subde power is possible. The oxidation of water to dioxygen by Ru(bpy) has long (15-17) been of interest because this process is one putative step in the use of Ru(bpy) to mediate the photodecomposition of water into its elements. In the reaction of Ru(bpy) " with water/hydroxide ion, electron transfer to produce Ru(bpy) + O H can be postulated as an elementary step. The O H would be expected to add rapidly to the aromatic ring to yield Ru(bpy) (bpyOH) . Studies of the Ru(bpy) + O H " reaction by stopped-flow techniques reveal an intermediate absorbing at long wave lengths. To test whether this species might be the OH-radical adduct, pulse radiolysis was used to study the reaction of O H with Ru(bpy) . 3

3+

3

3+

3

3

2+

2

3H

2+

3

3+

3

Ruibpyfe * + O H " — Ru(bpy) (bpyOH) 3

Ru(bpy)

3

2

2+

+ O H — Ru(bpy) (bpyOH) 2

2+

2+

2+

(20) (21)

Evidence that common adducts are formed in the two reactions provided sub stantiation for Ru(bpy) (bpyOH) species. Recendy, these have been observed 2

2+


240


in the direct (uncatalyzed) oxidation of water i n zeolites into which the R u i b p y ^ * has been incorporated (18). 3

In contrast to some of the unstable

Carbon Dioxide Complexes.

systems described above, macroeyclic cobalt(I) complexes can be generated by electrochemical or chemical methods in organic solvents, but when mixed with water react with it rapidly. We have conducted extensive studies of tetraazamaeroeyelic cobalt complexes. The cobalt(I) complex was first described by Vaselevskis and Olson (19); its protonation in water was reported by Tait et al. (20); and its binding to C 0 was discovered by Fisher and Eisenberg (21 ). Fujita carried out elegant work in acetonitrile to characterize the binding of C 0 to the macrocycle and its reduction in protic media (22, 23). We wished to carry out parallel studies in aqueous media, but recognized that protonation of the low oxidation state would complicate the work. Pulse radiolysis ultimately proved extremely useful (24, 25). One theme in the C0 -binding systems is an "electron isomerism" reminis cent of the Ru(bpy) /OH and Ru(bpy) /OH* systems described above. In principle, there are two routes to the cobalt(I)-C0 adduct: 2


2

2

3

3+

_

3

2+

2

CoL C0

2 +

+ e~ — C o L q

+ e" — C 0 "

2

q

CoL

+

CoL

2

+

+ C 0 -+ C o L ( C O ) 2

£

+ C 0 " — CoL(C0 )

2 +

2

2

(22)

+

(23)

+

Depending upon the complex, both may be observed. For the case of rac C o L , the two routes yield different isomers; addition of C 0 to Co(I) produces primary rac C o L ( C 0 ) , whereas reaction of C 0 " with C o L results in sec ondary rac C o L ( C 0 ) . The isomers are depicted in Figure 4. Primary rac C o L ( C 0 ) also exhibits coordination isomerism (22); at low temperatures it is six-coordinate, with a solvent molecule occupying the position trans to C 0 , but at high temperatures a purple five-coordinate form predomi nates. 2

+

2

2

2 +

2

+

2

+

2

primary rac C o L C 0

2

+

+ S ^ primary rac C o L ( C 0 ) ( S ) 2

purple

(24)

+

yellow

For S = C H C N (22), the enthalpy change, ΔΗ° = -6.2 kcal m o l " , and the entropy change, aS° = - 2 6 cal K " m o l " , and for S = H 0 (25), ΔΗ° = -6.1 kcal m o l " , and AS° = - 1 9 cal K " m o l " . Ultimately, we learned to adequately control the starting isomer of the cobalt(II) macrocycle and the kinetics of formation of the H , C O , and C 0 adducts so that a full comparison of the thermodynamics and kinetics of these adducts could be made for both stereoisomers of the macrocycle (25). Ironi cally, the binding constants for both C O and C 0 to the rac isomer in water are so great that it would be very difficult to determine them in the absence 1

3

1

1

1

1

2

1

+

2


2

14.


241

Isomers of CoL

XT Ν

χτ«-"^

Ν"

meso L

rac L


Isomers of CoL(X) X

,CH

CH

3

χ

CH

3

secondary, rac

primary,rac

X

3

meso

Figure 4. The chirality of the amine nitrogen in the macrocyclic complex (rac L and meso L) leads to three potential isomers of the axially substituted complexes.

of the protonation processes originally of great concern. Thus, data for the equilibrium C o L C O / + H ^ primary rac C o L ( H ) +

2 +

+ C0

(25)

2

were determined as a function of partial C 0 (or CO) pressure and p H for most accurate data. Combining our results with those of earlier work (20) to the comparisons shown in Table I. As first reported by Tait et al. (20), C o L reacts with Brônsted acids, H A , with a rate constant that depends on acidity of H A . 2

+

primary rac C o L

+

+ H A ^± primary rac C o L ( H )

2+

the led rac the

+ A " , fc A> ^eq (26) H

Indeed, a plot of log &HA versus log Ke for the proton transfer (log( ^coL(H) ) = 11.3) is linear, with a slope of ca. 0.5. Most remarkably, the rate constants for both C O and C 0 addition to the macrocycle fall on the same plot. Protonation and formation of the metal-carbon bond appear to lend them selves to a description in terms of an associative reaction (S 2), in which the low-spin d cobalt(I) metal center serves as the nucleophile. q

2+

2

N

8


242


Table I.

Comparison of CoL(X) Isomers k (298 K), water Primary rac

Reaction CoL + C 0 ^ CoL(C0 ) +

2

2

C o L + CO ^ CoL(CO +

+

if

meso

1.7 Χ ΙΟ 0.38 4.5 Χ ΙΟ 5.0 Χ ΙΟ 3.1 1.6 Χ ΙΟ 3.1 Χ 10 1.2 Χ ΙΟ" 2.5 Χ 10

1.5 Χ ΙΟ 2.5 6.0 Χ ΙΟ 8.0 Χ ΙΟ 10 0.8 Χ ΙΟ 2.4 Χ ΙΟ 8 Χ ΙΟ

k /k f

r

+

Κ


+

3

+

h Κ kf/k

+

1

1

1

8

1

9

9

r

1

8

8

C o L + H 0 ^ CoL(H)

1

6

8

8

4

2

11

M " s" s" M" M" s s" M M " s" s" M" 1

7

8

1

13

1

Unique Thermodynamic Data. Exploitation of combinations of photo-, radiation, and electrochemical techniques has led to the determination of rather remarkable sets of data for aqueous media exemplified for those for bpy. The redox properties in aqueous media are important in many systems because bpy is such a useful ligand for ruthenium, rhodium, and cobalt metal centers. The full reduction scheme shown in Scheme I was determined through a combination of methods. The left-hand branch was characterized through studies of quenching the emission of polypyridyl-ruthenium(II) complexes as a function of p H . The bipyridine radical protonation equilibria and equilibrations (right-hand branch) were studied by pulse radiolysis methods (26). Although the bpy", produced through reduction of bpy by eâ , protonates very rapidly q

-2.13 V bpy"

bpy + e* pK 4.4

+ H

a

+ H

+

+

pK -24 a

-0.97 V bpyH + e*

bpyH

+

pK 0.05 a

.

2

U

bpyH

2

+

+ H

+ β

e

+ H+ p K 8 . 0

+

a

-0.50 V ^

bpyH

2

+

Scheme I.


14.


243

(27) to yield bpyH, equilibration of bpyH with phosphate buffers could be studied on the millisecond time scale. Furthermore, equilibration of bpyH - b p y H with C o ( b p y ) - C o ( b p y ) was utilized to define the reduction potentials for the protonated species. With the reduction potentials established, the quenching rate constants could be interpreted in terms of intrinsic and thermodynamic components to the rates. +

3

+

3

2+


Mechanistic Surprises Both our photochemical and radiation studies have focused on the chemistry of very reactive species in aqueous solution. Indeed, it is because the photo chemical work involved aqueous media that radiation chemistry techniques could be so useful to us. Our pulse radiolysis work has led to a number of highly unusual mechanistic conclusions. In the area of low-oxidation-state chemistry, several of the systems violate standard organometallic dogma. We investigated the rate of hydride formation in another cobalt(I) system, that derived from the high-spin d polypyridyl-cobalt(I) complexes (28). Remarkably, electron transfer was found to be the rate-determining step for formation of the hydride complex, and contributions from Brônsted acid pathways contribute negligibly to the rate. Rather, the hydride formation appears to involve Η-atom transfer from the protonated bpy radical. The "H-atom receptor" may be either Co(bpy) or Co(bpy) as shown in Scheme II. 8

2

2+

2+

Scheme II.



244


Conversion of a metalloearboxylie acid to a hydride complex is supposed to occur via a hydrogen transfer to the metal as C 0 is eliminated: 2

Instead, we found a strikingly different route for the macrocyclie cobalt systems. As shown in Scheme III, the metalloearboxylie acid (1) is essentially unreactive, but its conjugate base, the metallocarboxylate (2), can eliminate C 0 at - 1 s" to form the Co(I) complex (3). The Co(I) complex can then undergo protonation by available proton sources to form the cobalt(III) hydride (4) (24). 2

1

Concluding Remarks We and others have used pulse radiolysis methods to clarify a number of com plex photochemical mechanisms. In the course of these studies we have also been able to learn a great deal of new chemistry, including the electronic ab sorption spectra, thermodynamics, and reaction mechanisms of highly reactive transition-metal centers in both unusually high and low oxidation states. As these data pertain to aqueous media, they contribute in an important way to future work on solar photoconversion in water (the ideal medium from both economic and environmental points of view) and to catalysis in aqueous media in general.

Acknowledgments This research was carried out at Brookhaven National Laboratory under con tract D E - A C 0 2 - 7 6 C H 0 0 0 1 6 with the U.S. Department of Energy and was supported by its Division of Chemical Sciences, Office of Basic Energy Sci-


14.


245

ences. We thank our many collaborators for their invaluable contributions to the work presented here.

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