[acs symposium series] inorganic chemistry: toward the 21st century volume 211 ||...

2

Oxidation-Reduction Photochemistry of Polynuclear Complexes in Solution

DANIEL G. NOCERA, ANDREW W. MAVERICK, JAY R. WINKLER, CHI-MING CHE, and HARRY B. GRAY California Institute of Technology, Arthur Amos Noyes Laboratory, Pasadena, CA 91125

Three classes of polynuclear complexes containing metal-metal bonds possess emissive excited states that undergo oxidation-reduction reactions in solution: the prototypes are Re2Cl2-8(d4·d4), Pt2(P2O5H2)4-4 (d8·d8), and Mo6Cl2-14 (d4)6. Two-electron oxidations of Re2Cl2-8 and Pt2(P2O5H2)4-4 have been achieved by one-electron acceptor quenching of the excited complexes in the presence of Cl-, followed by one-electron oxidation of the Cl--trapped mixed-valence species. Two-electron photochemical oxidation-reduction reactions also could occur by excited-state atom transfer pathways, and some encouraging preliminary observations along those lines are reported.

Present systems for photochemical energy conversion emphasize one-photon/one-electron-relay/catalyst schemes, as i n the famous Ru(bpy)| +/water s p l i t t i n g experiments of many workers(1-14). A l l such systems are designed to take advantage of the fact that upon excitation the metal complex sensitizer i s a better oxidant and reductant than i t s ground state, and as a result i t can oxidize poor donors and reduce weak acceptors provided i t l i v e s long enough i n solution to do so (Figure 1). Net storage of photochemi c a l energy i s then achieved by employing separate catalysts to convert the photogenerated species into useful products(1).

It has been our goal for some time to run photochemical energy storage reactions without relay molecules or separate catalysts. We have concentrated on the photochemistry of polynuclear metal complexes i n homogeneous solutions, because we believe i t should be possible to f a c i l i t a t e multielectron transfer processes at the available coordination sites of such cluster species.

One problem we have had to overcome i n developing metal-cluster oxidation-reduction photochemistry i s the tendency of excited clusters to dissociate into r a d i c a l fragments (for

0097-615 6/ 83/0211 -0021 $06.00/0 © 1983 American Chemical Society

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In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

2 2 I N O R G A N I C C H E M I S T R Y : T O W A R D T H E 21ST C E N T U R Y

example, Mri2(CO)io)* But after several years of research we have found that (at least) three broad classes of metal-metal bonded systems have attractive excited-state properties:

Electronic Configuration Example

(1) d ^ d 1 * Re 2Cl8~

(2) d 8-d 8 Pt 2(P 20 5H2)Îr

(3) ( d l *) 6 Mo 6 C l f ^

In the f i r s t case the metal-metal bond i s quite strong i n the ground state, and i t i s weakened only s l i g h t l y i n the lowest singlet (16δ*) excited state. In the second case the lowest excited state (3do*po) possesses a r e l a t i v e l y strong metal-metal bond. In the th i r d case the nature of the long-lived excited state i s not well understood at this time. It may have some relationship to a δ+δ* or δ+π* excited state i n a d1* binuclear complex, however.

Binuclear Complexes ι * 2-Evidence suggests that the Αδδ state of Re 2Cls i s not

eclipsed (the δ bond i s gone), and that Franck-Condon factors are responsible for the r e l a t i v e l y long li f e t i m e of this state (Figure 2)(15,16.17). Various electron acceptors (e.g., TCNE) quench the R e ^ l e ^ luminescence i n nonaqueous solutions, thereby producing Re 2 C l e " and the reduced acceptor(17) . A transient signal attributable to TCNE" was observed i n flash k i n e t i c spectroscopic studies of dichloromethane solutions containing TCNE and (BuifN)2Re2Cl8; the decay of the transient was found to follow second-order kinetics (k = 3 χ 10 9 M"1 s - 1 ) .

The luminescence of Re2Cl§~* also i s quenched by secondary and t e r t i a r y aromatic amines i n a c e t o n i t r i l e solution(17). Neither the electronic absorption nor the emission spectrum of Re2Cl§" changes i n the presence of the quenchers, and no evidence for the formation of new chemical species was observed i n flash spectroscopic or steady-state emission experiments. The results of these experiments suggest that the products of the quenching reaction form a strongly associated ion pair, Re2Cl8~'D+.

The two reduction potentials involving Re2Cl8~* (-/2~*; 2-*/3-) have been estimated from the results of spectroscopic and electrochemical experiments (-0.51 and 0.90 V vs. SCE)(17). The δδ* singlet provides a f a c i l e route to an extremely powerful i n organic oxidant, Re2Cl8~ (E° = 1.24 V vs_. SCE), a species that has not been generated cleanly by other means.

In recent experiments D. G. Nocera has extended this work to include two-electron photochemical oxidation of Re2Cls . The strategy involved here i s to generate Re2Clê by acceptor quenching

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N O C E R A E T A L . Polynuclear Complexes in Solution

Figure 1. Modified Latimer diagram illustrating the relative reduction potentials of a metal complex (M) and its excited state (M*).

.§* 1 [SS*] ( D 4 D ) ( 1 . 7 5 6 V )

680 nm absorption

1/ 1/ Re uuRe

/I /I

0.14 ̂ LLS emission

(CH3CN solution)

V ] ( D 4 H )

Figure 2. Selected electronic spectroscopic properties of Re2Cl8

2~-

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24 I N O R G A N I C C H E M I S T R Y : T O W A R D T H E 21ST C E N T U R Y

of 166*, and then by adding C l ~ to trap the mixed-valence species, Re2Clg". Acceptors whose potentials are high enough can then oxidize Re2Clg" to Re2Cl§ (Figure 3) .

Several d 8«d 8 complexes of rhodium(I), iridium(I), and platinum(II) possess r e l a t i v e l y long-lived excited states (many are i n the ys range)(18,19,20). The excited state i s believed to be a do*po t r i p l e t (Figure 4). The beautiful green phosphorescence (517 nm; l i f e t i m e VLO us) exhibited by Pt2 (P2°5H2)i+~ i s a n

interesting case i n point(20). Spectroscopic studies on Ba2Pt2~ (Ρ2θ5Η2)^ at 4 Κ have revealed that the Pt-Pt bond i s much stronger i n the excited state than i n the ground state (Pt-Pt stretching frequency: 160 cm"1 (excited state), 110 cm"1 (ground state); Pt-Pt distance: 2.7 Â (excited state), 2.93 Â (ground state)(21). ^_

The phosphorescence of Pt2(P2°5)i+H8 i n aqueous solution i s quenched by 1,1-bis(2-sulfoethyl)-4,4'-bipyridinium inner s a l t (BSEP). Transient absorption attributable to BSEP" ( X m a x ^ 610 nn) i s observed i n f l a s h k i n e t i c spectroscopic studies of aqueous solutions containing P t 2 ( ? 2 ° 5 ) k s - B ~ and BSEP, thereby establishing an electron transfer quenching mechanism:

Pt 2(P20 5)i +He"* + BSEP —9—>· Pt2(P205)i+H8~ + BSEP~

Stern-Volmer analysis of the quenching yields k q = 5.5 χ 10 9 M"1

s" 1([Pt 2(P 205)i +H8 ] ~ 10"1* M; 0.1 M NaClO^; 25°C). Both the quenching reaction and the bimolecular back electron transfer (k - 1 χ 10 9 M"1 s" 1 for P t 2 ^ O s ) ^ " and BSEP") are near the diffusion l i m i t for such processes i n aqueous solution at 25°C.

The 3A 2 u(do*po) state of Pt2(P205)i*H8 i s an extremely powerful one-electron reductant i n aqueous solution. Preliminary experiments have shown that species such as 0s(NH3)5Cl 2 ( E ] / 2

=

-1.09 V vs. SCE) and nicotinamide (Εχ/2 • -1.44 V vs. Ag/AgCl; CH30H, pH 7.2) are readily reduced by P t 2 ( Ρ 2 0 5 ) ^ Η 8 \ From these and related experiments i t i s apparent^that Pt2(P2°5)4 H8" i s a

stronger reducing agent than Ru(bpy)3 +* i n aqueous solution. C.-M. Che has demonstrated that the back reaction of photo-

generated Pt2(P205H2)^~ with a reduced acceptor can be inhibited by a x i a l ligand binding. Addition of CI" traps the mixed-valence species, Pt2(P205H2)i +Cl 4", which i s rapidly oxidized by the acceptor to Ρί2(Ρ2θ5Η2)ι^ΐ!Γ (Figure 5). Hexanuclear Clusters

The (d i +) 6 clusters (M 6xfi : M = Mo, W; X - C l , Br, I) exhibit red luminescence, and the excited state lifetimes are remarkably long:

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N O C E R A E T A L . Polynuclear Complexes in Solution

Figure 3. Modified Latimer diagram for the Re2Cl8

2'/Re2Cl9 system (E°/V vs. SCE).

Pz pa

per

4 ! _ da»

4 f — *>•

dcr* per

emission

(0.01 to 20/xs)

d 8 (dTT)6 ( d z2 ) 2 . (d z

2) 2 (dTT)6 (dcr*)'

Figure 4. Electronic structure of d 8 · d 8 systems that highlights the emissive da*pa triplet state.

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26 I N O R G A N I C C H E M I S T R Y ! T O W A R D T H E 21ST C E N T U R Y

2.5 eV

2 C I 4 + ) ^ ^ ( R 2 c £ + )

A A"

Figure 5. Modified Latimer diagram for the Pt2(P205)4H8

4-/Pt2(P205)H8Cl2

4-system.

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2. N O C E R A E T A L . Polynuclear Complexes in Solution 27

M X Lifetime (ys)(Bu uN* s a i t , CHqCN soin) Mo C l 180 Mo Br 130 Mo I 50 W C l 2 W Br 15 W I 30

In recent studies the diamagnetism of the clusters has been confirmed(22), which accords with the closed-shell (^ig) ground state obtained i n an SCF-X*-SW calculation(23). The positions of the emission and weak absorption bands at low temperature suggest excited state energies of ̂ 2 eV for a l l three clusteç ions(22,24). As t h | energies of these bands are similar for Mo^Cl^ and MogBrj^, i t appears that both HOMO and LUMO are largely metal-centered. Also, i t may be deduced from the symmetry of the EPR spectrum of MogCly^ that the oxidized cluster i s a x i a l l y d istorted i n some way(22). Although there are several possible explanations for the apparent reorganization, an attractive one consistent with the values of the £ tensor components i s that an eg cluster o r b i t a l i s depopulated on oxidation, and that as a result the parent 0^ cluster undergoes a tetragonal d i s t o r t i o n to give a 2 A i g (D^) or 2Αχ (0^ ν) ground state for MogCl^. Determination of the nature of the Mo-Mo and Mo-Cl interactions i n the half-occupied cluster o r b i t a l w i l l require a f u l l analysis of the £ parameters based on structural information for the oxidized ion that i s not available at present.

Electrochemical studies have shown that the cluster ions undergo simple one-electron oxidation i n aprotic solvents.

Reduction potenitals of (Bu^N^MgX^ complexes i n CH3CN solution at 25°C.

Μ/Χ E 1 / 2(V)/Ag/Ag + E 1 / 2(V)/SCE(est.)

Mo/Cl 1.29 1.60 Mo/Br 1.07 1.38 W/Cl 0.83 1.14

The ΜβΧχι* products are powerful oxidizing agents, rivaled by only a few chemical oxidants. Also, the position of the oxidation wave r e l a t i v e tg those of the free halide ions confirms that the halides i n MgXjIJ are firmly bound.

The luminescent excited state of ΜοβΟΙ^ reacts rapidly with electron acceptors(24). The powerfully oxidizing MogClïi+is produced i n these reactions. Experiments with BSEP as acceptor i n

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28 I N O R G A N I C C H E M I S T R Y : T O W A R D T H E 21 ST C E N T U R Y

particular confirm that the ion's attractive photochemical properties, o r i g i n a l l y inferred from i t s photophysical and electrochemical behavior i n nonaqueous media, are retained i n aqueous solution as well. These properties, namely, the presence of long-lived excited states and the capacity for rapid thermal and photochemical redox reactions withou£ major structural change, are expected to persist i n the MogBr^IJ and WgCl^ ions also.

Atom Transfer Processes

Excited state atom transfer could be very attractive as a means to photochemical energy storage, because the accompanying structural rearrangements could create s i g n i f i c a n t k i n e t i c barriers to the energy-wasting back reactions. What i s more, atom transfers t y p i c a l l y are multielectron events, as are most useful energy storage reactions. Recent work i n this area has involved systems that are well suited for photochemical O-atom and C l + transfers.

Several luminescent (^650 nm) dioxorhenium(V) systems(25) have been investigated as potential 0-atom transfer agents. The emission quantum yields measured with 436 nm excitation are about 0.03 for trans-Re02(pyridine) ί and i t s isotopically-substituted derivatives i n pyridine solution. The excited state lifetimes of these ions vary from 4 to 17 ys.

Though the electronic spectra of the dioxorhenium(V) ions indicate that their excited states possess activated metal-oxygen bonds(25), i t has not been possible to demonstrate any simple oxygen atom transfer photochemistry with these species. The thermodynamic and k i n e t i c barriers to oxygen atom transfer from Re02 +* must be quite large. As a res u l t , attention has been directed toward a mono-oxo rhenium(V) compound, ReOCl3(PPti3)2, i n which the oxygen atom acceptor i s PPI13. J. R. Winkler has found that i r r a d i a t i o n of this species i n noncoordinating solvents leads to a green product whose infrared spectrum exhibits phosphorus-oxygen stretching bands but no rhenium-oxygen stretches, In addition, preliminary laser flash spectra suggest that the formation of this green product i s rapid (y7 ys) and obeys f i r s t -order k i n e t i c s . These data imply a unimolecular photochemical reductive elimination process i n which an oxygen atom i s transferred from rhenium to coordinated PPI13.

Evidence obtained i n recent experiments by D. G. Nocera suggests that energetic species generated by i r r a d i a t i o n of Re2Cl8~ may undergo atom transfer reactions (Figure 6). A key observation i n this 2context i s that a dichloromethane solution of Re2Cl 8" and P t C l 6 " produces Re 2Cl9 upon i r r a d i a t i o n (>560 nm). Solutions i d e n t i c a l with those used for the photochemical experiment do not react i n the dark over a period of f i v e days at 50°C. It i s reasonable to speculate that the reaction involves that fraction of the higher excited states of Re2Cl8*" that are believed to bypass the θδ* singlet channel, thereby producing a high energy

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NOCERA ET AL. Polynuclear Complexes in Solution

Cl Cl 2-* I I jP Re s « Re

Λ Λ

| ç i a

A

ci α 9 ci V ci ? , Re. ,.Re

/ Xa7 \ Cl C l Cl

Figure 6. Proposed atom (CV) transfer pathway for the photochemical oxidation of Re2Cl8

2~ to Re2Cl9~.

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30 I N O R G A N I C C H E M I S T R Y : T O W A R D T H E 21ST C E N T U R Y

(probably Cl-bridged) intermediate(15) that would be i d e a l l y suited for C l + transfer to form Re2Clg. In any case, these preliminary photochemical experiments are encouraging, because they demonstrate that Re2Cl8~ can be converted to Re2Clg by C l +

transfer, thereby effecting with low energy v i s i b l e l i g h t a net two-electron photoredox reaction i n homogeneous solution.

Acknowledgment

D. MacKenzie assisted with the (Bu l tN) 2M 6X l l f emission l i f e time measurements. Research at the C a l i f o r n i a Institute of Technology was supported by National Science Foundation Grants CHE78-10530 and CHE81-20419. This i s Contribution No. 6703 from the Arthur Amos Noyes Laboratory.

Literature Cited1. Gray, H. B.; Maverick, A. W. Science 1981, 214, 1201. 2. Balzani, V.; Bolletta, F.; Gandolfi, M. T.; Maestri, M.

Top. Curr. Chem. 1978, 75, 1. 3. Gafney, H. D.; Adamson, A. W. J. Am. Chem. Soc. 1972, 94,

8238. 4. Brugger, P.-A.; Grätzel, M. J. Am. Chem. Soc. 1980, 102,

2461. 5. Tsutsui, Y.; Takauma, K.; Nishijima, T.; Matsuo, T.

Chem. Lett. 1979, 617. 6. Lehn, J.-M.; Sauvage, J. P.; Ziessel, R. Nouv. J. Chim. 1981,

5, 291. 7. Brugger, P.-Α.; Cuendet, P.; Grätzel, M. J. Am. Chem. Soc.

1981, 102, 2923. 8. Kalyanasundaram, K.; Grätzel, M. Angew. Chem. Int. Ed. Engl.

1980, 19, 646. 9. Borgarello, E.; Kiwi, J.; Pelizzetti, E.; Visca, M.; Grätzel,

M. J. Am. Chem. Soc. 1981, 103, 6324. 10. Kirch, M.; Lehn, J.-M.; Sauvage, J.-P. Helv. Chim. Acta 1979,

62, 1345. 11. Chan, S.-F.; Chou, M.; Creutz, C.; Sutin, N. J. Am. Chem.

Soc. 1981, 103, 369. 12. Shafirovich, V. Ya.; Khannonov, N. K.; Strelets, V. V.

Nouv. J. Chim. 1981, 4, 81. 13. Krishnan, C. V.; Sutin, N. J. Am. Chem. Soc. 1981, 103, 2141. 14. Brown, G. M.; Brunschwig, B. S.; Creutz, C.; Endicott, J. F.;

Sutin, N. J. Am. Chem. Soc. 1979, 101, 1298. 15. Fleming, R. H.; Geoffroy, G. L.; Gray, Η. Β.; Gupta, Α.;

Hammond, G. S. J. Am. Chem. Soc. 1976, 98, 48. 16. Miskowski, V. M.; Goldbeck, R. Α.; Kliger, D. S.; Gray, H. B.

Inorg. Chem. 1979, 18, 86. 17. Nocera, D. G.; Gray, H. B. J. Am. Chem. Soc. 1981, 103, 7349. 18. Milder, S. J.; Goldbeck, R. Α.; Kliger, D. S.; Gray, H. B.

J. Am. Chem. Soc. 1980, 102, 6761.

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2. NOCERA ET AL. Polynuclear Complexes in Solution 31

19. Smith, T. P. Ph.D. Thesis, California Institute of Technology, Pasadena, California, 1982.

20. Che, C.-M.; Butler, L. G.; Gray, H. B. J. Am. Chem. Soc. 1981, 103, 7796.

21. Rice, S. F. Ph.D. Thesis, California Institute of Technology, Pasadena, California, 1982.

22. Maverick, A. W. Ph.D. Thesis, California Institute of Technology, Pasadena, California, 1982.

23. Cotton, F. Α.; Stanley, G. G. Chem. Phys. Lett. 1978, 58, 1978.

24. Maverick, A. W.; Gray, H. B. J. Am. Chem. Soc. 1981, 103, 1298.

25. Winkler, J. R.; Gray, H. B. J. Am. Chem. Soc., submitted 1982. RECEIVED September 24, 1982

Discussion A.W. Adamson, University of Southern California: You spoke

of the bond energy being larger in the excited state, in the case of Pt2(P205H2)^ . It seems to me that what the i.r. data indicate is that the bond strength, that is, the bond force constant, may be larger. The concept of an excited state bond energy probably requires some care of definition. In the case of ground state molecules, one ordinarily would like the sum of all bond energies to add up to the atomization energy. Depending on the definition used, this might not be the case with an excited state species. Could you comment on this matter?

Η. B. Gray: Care of definition is certainly needecT The reference dissociation limit must be specified. Discussion of this point has been published (1).

(1) Rice, S. F.; Gray, H. B. J . Am. Chem. Soc. 1981, 103, 1593. D

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32 I N O R G A N I C C H E M I S T R Y ! T O W A R D T H E 21 ST C E N T U R Y

C.H. L a n g f o r d , Concordia U n i v e r s i t y : Gray r i g h t l y emphas i z e s the v a l u e of the search f o r l o n g - l i v e d e x c i t e d s t a t e s as i n i t i a t o r s of p h o t o r e a c t i o n i n homogeneous s o l u t i o n . T h i s i s because the r e a c t i v e e x c i t e d s t a t e must a c c o m p l i s h an encounter w i t h a r e a c t i o n p a r t n e r . The long l i f e t i m e does impose a d i s a d v a n t a g e . I t u s u a l l y i m p l i e s a f u l l y r e l a x e d e x c i t e d s t a t e which i s prevented from r e l a x i n g t o the ground s t a t e by a s u i t a b l e b a r r i e r . T h i s i s one of the l o s s p r o c e s s e s d i s c u s s e d by B o l t o n which l i m i t s the e f f i c i e n c y of energy s t o r a g e . In the c o n t e x t of energy c o n v e r s i o n , I suspect t h a t n a t u r e uses an a l t e r n a t e s t r a t e g y . Both c h l o r o p h y l l and r h o d o p s i n appear to a c h i e v e an i r r e v e r s i b l e step very f a s t w i t h o u t w a i t i n g f o r f u l l r e l a x a t i o n . Thus, e f f i c i e n c y photochemical s y n t h e s i s of ender-g o n i c p r o d u c t s c o u l d be argued t o r e q u i r e assembly f i r s t t o a v o i d a d i f f u s i o n encounter requirement then the e x a m i n a t i o n of " s e l e c t i o n r u l e s " t o d i s c o v e r u s e f u l i r r e v e r s i b l e pathways of energy d e g r a d a t i o n . H o l l e b o n e (_1) has proposed an approach t o such s e l e c t i o n r u l e s .

(1) H o l l e b o n e , B.R. ; L a n g f o r d , C H . ; Serpone, N. Coord. Chem. Revs. 1981, 39, 101.

H. B. Gray: We must remember th a t we are s t i l l i n the i n i t i a l stages of systematic study of i n o r g a n i c o x i d a t i o n - r e d u c t i o n photochemistry. Nature has indeed some s l i c k ways to optimize photochemical energy conversion. I am c o n f i d e n t t h a t i n o r g a n i c chemists w i l l do as w e l l or b e t t e r , perhaps even before the t u r n of the century!

A.B.P. L e v e r , York U n i v e r s i t y : The next few y e a r s w i l l see growth i n our knowledge of two e l e c t r o n photoredox r e a g e n t s as suggested by the atom t r a n s f e r work you have r e p o r t e d . The p u r p o s e f u l d e s i g n of two e l e c t r o n r e a g e n t s might l e a d t o sequent i a l two e l e c t r o n redox or c o n c e r t e d two e l e c t r o n redox. For purposes such as the o x i d a t i o n of water, how do you see such r e a g e n t s d e v e l o p i n g ?

H. B. Gray: Work on the e x c i t e d s t a t e s of soluble metal oxo species t h a t i n a sense are the homogeneous-s o l u t i o n analogues of T i 0 2 - t y p e m a t e r i a l s i s a promising d i r e c t i o n to take, i n my view.

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2. N O C E R A E T A L . Polynuclear Complexes in Solution 33

D. Cole-Hamilton, L i v e r p o o l U n i v e r s i t y : P r o f e s s o r Gray, some of the platinum complexes t h a t you have prepared a t the same time as producing hydrogen c o n t a i n , f o r m a l l y a t l e a s t , P t ( I I I ) . Can these compounds act as o x i d i s i n g agents, e i t h e r t h e r m a l l y or p h otochemically?

H. B. Gray: Yes, c e r t a i n of the b i n u c l e a r P t ( I I I ) complexes act as m i l d thermal o x i d i z i n g agents, and they are more potent o x i d i z e r s when i r r a d i a t e d .

D.R. M c M i l l i n , Purdue U n i v e r s i t y : You r e f e r r e d t o a r e a c t i v e e x c i t e d s t a t e of R e 2 C l 8

l * which had a l i f e t i m e of the o r d e r of a microsecond i n CH 2C1 2 s o l u t i o n and suggested t h a t i t c o u l d be a s s i g n e d t o the s i n g l e t s t a t e of the δ 1 δ * 1 c o n f i g u r a t i o n . That would r e q u i r e i n t e r s y s t e m c r o s s i n g to the a s s o c i a t e d t r i p l e t s t a t e to occur w i t h a r a t e constant of the order of 10 6

sec 1 which seems q u i t e low f o r a t r a n s i t i o n metal complex, e s p e c i a l l y one i n v o l v i n g a t h i r d row i o n . Would you comment on t h i s p o i n t ?

Η. B. Gray: M u l t i c o n f i g u r a t i o n SCF c a l c u l a t i o n s by P. J . Hay i n d i c a t e t h a t the χδδ*- 3δδ* energy s e p a r a t i o n i s over 1 eV, and there i s no evidence f o r i n t e r v e n i n g s t a t e s t h a t c ould provide a f a c i l e i n t e r system pathway. Thus a r e l a t i v e l y small s i n g l e t - * t r i p l e t i n t e r s y s t e m c r o s s i n g r a t e constant i s not a l l t h at p e c u l i a r .

R.A. Walton, Purdue U n i v e r s i t y : The q u e s t i o n as to r o t a t i o n a l conformation ( e c l i p s e d or staggered) t h a t c h a r a c t e r i z e s the e x c i t e d s_tate geometries of the complex anions [ R e 2 C l 8 ] 2

and [ M o ^ l g ] 1 * has been addressed by P r o f e s s o r Gray (JL). There i s o b v i o u s l y no q u e s t i o n t h a t a s t e r i c a l l y h i n d e r e d complex such as M o 2 C l ^ ( P - n - B u 3 ) ^ (1) w i l l r e t a i n the e c l i p s e d geometry t h a t p e r t a i n s to the ground s t a t e , thereby p r o v i d i n g an i n t e r e s t i n g comparison w i t h [ M o 2 C l e ] ' ' . I should l i k e to mention t h a t we have now prepared the s t e r i c a l l y h i n d e r e d quadruply bonded d i r h e n i u m ( I I I ) complex [ R e 2 C l „ ( P M e 2 P h ) h ] ( P F 6 ) 2 ( 2 ) , a molecule t h a t i s i s o e l e c t r o n i c w i t h the unhindered o c t a c h l o r o d i -r h e n a t e ( I I I ) a n i o n , [ R e 2 C l 8 ] 2 . A study of i t s e m i s s i o n b e h a v i o r would perhaps be of i n t e r e s t .

(1) Miskowski, V.M.; Goldbeck, R.A.; K l i g e r , D.S.; Gray, H.B. I n org. Chem. 1979, 18, 86.

(2) Dunbar, K.; Walton, R.A., unpublished work.

Η. B. Gray: Indeed i t would. Please send me a sample!

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[acs symposium series] inorganic chemistry: toward the 21st century volume 211 ||...

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