Chapter 25
Atom-Transfer Reactivity of Binuclear d Complexes
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Photochemical and Electrocatalytic Reactions David C. Smith and Harry B. Gray Arthur Amos Noyes Laboratory, California Institute of Technology, Pasadena, CA 91125 3
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The long-lived (dσ*pσ) state of binuclear d complexes undergoes a variety of reactions. One prominent reac tion, photooxidative addition of halocarbons, apparently proceeds by halogen atom transfer rather than outersphere electron transfer. Excited-state hydrogen atom transfer occurs in reactions between several binuclear d complexes and a number of organic and organometallic substrates. Specific results for P t ( P O H ) 4 4 - and I r ( T M B ) 2 (TMB = 2,5-diisocyano-2,5-dimethylhexane) are discussed. Production of a hole in the dσ* orbital is believed to be an important factor in these photochemical atom-transfer reactions. Electrochemical generation of such a hole produces a highly reactive intermediate that can undergo atom abstraction, thereby yielding net oxidation of an organic substrate. The net reaction is electrocatalytic in metal complex. 8
2
2
5
2
+
2
4
A n electronic excited state of a metal complex is both a stronger reductant and a stronger oxidant than the ground state. Therefore, complexes with relatively long-lived excited states can participate in intermolecular elec tron-transfer reactions that are uphill for the corresponding ground-state species. Such excited-state electron transfer reactions often play key roles in multistep schemes for the conversion of light to chemical energy (1). While electron-transfer processes are common in inorganic photochemistry, excited-state atom transfer is limited to a small class of inorganic complexes. For UC>2 , the diradical excited state (-U-0) is active in alcohol oxidation (2). The primary photoprocess is hydrogen atom abstraction by the oxygen-centered radical. Photoaddition to a metal center via atom transfer has been observed for binuclear metal complexes such as Re (CO)io (3-5). The primary photoprocess is metal-metal bond homolysis. The photogenerated metal radical undergoes thermal atomabstraction reactions. Until recently, atom transfer to a metal-localized excited state had not been observed. Atom transfer to a metal complex is facilitated if localized electron or hole generation occurs at one or more open coordination sites. Binuclear d8 complexes have been found to undergo photochemical atom transfer to one 2+
2
0097-6156/89/0394-0356$06.00/0 ο 1989 American Chemical Society
In The Challenge of d and f Electrons; Salahub, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
25.
SMITH & GRAY
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Atom-TransferReactivity ofBinuclear d [Complexes
of the metal centers (Roundhill, D. M.; Gray, H . B.; Che, C.-M. Accts. Chem. Res., in press, 6-8). These complexes possess open coordination sites in addition to an electronic structure that localizes the electron (hole) necessary for atom transfer to the metal center. Much effort has been directed toward elucidating the electronic structure of binuclear d& metal complexes (9-14). Several years ago, a simple molecular-orbital model was presented to explain the electronic spectroscopic features (15). Starting from a monomer orbital scheme, two square-planar units areTïrought together in a face-to-face orientation (Figure 1). The orbitals perpendicular to the molecular plane, d 2 and p , interact strongly, yielding redicted to be a powerful reductant, with E°(M2 /3M2*) estimated to range rom -0.8 to -2.0 V vs SSCE in CH3CN. That this state is a powerful reductant has been confirmed by investigation of the electron-transfer quenching of 3M2* by a series of pyridinium acceptors with varying reduction potentials (13). For several binuclear complexes, the excitedstate reduction potential cannot be calculated accurately due to the irreversibility of the ground-state electrochemistry; but it can be estimated from bimolecular electron-transfer quenching experiments. For systems that are powerful excited-state reductants, photoreduction of alkyl halides is observed (6,16). This reaction was initially interpreted to be an outer-sphere electron transfer to form the radical anion, which rapidly decomposes to yield R« and X " . Subsequent thermal reactions yield the observed products, an S R N I mechanism (Figure 3a). While such a mechanism, S R N I , appears plausible for a metal complex with E ( M / 3 M * ) < -1.5 V (SSCE), it seems unlikely for complexes with E ° ( M / 3 M * ) > -1.0 V (SSCE). Reduction potentials for alkyl halides of interest are generally more negative than -1.5 V (SSCE) (17). Alkyl halide photoreduction is observed for binuclear d& complexes whose excited-state reduction potentials are more positive than -1.0 V (SSCE) in CH3CN. An alternative pathway to outer-sphere electron transfer, which yields similar photoredox products with alkyl halides, is excited-state atom transfer (Figure 3b). Data obtained for Pt2(P20sH2)4 ~ indicate that alkyl Z
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+
Î
0
+
2
2
+
2
2
4
In The Challenge of d and f Electrons; Salahub, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
z
THE CHALLENGE OF d AND f ELECTRONS
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Figure 2. P i c t o r i a l representation of the M2-localized hole in a 3(da*pa) state.
In The Challenge of d and f Electrons; Salahub, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
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25.
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SMITH & GRAY
Atom-Transfer Reactivity ofBinuclear d Complexes
b.
RX + M-M
°M-M
•
+ RX
•M-M-X + R.
RX + . M - M . ·
•M-M-X + R. R. =
/
X-M-M-X + C H C H 2
2
\
R. = .CR'X (R* = alkyl, aryl) R-M-M-X
Figure 3. a. S R ^ I mechanistic scheme for halocarbon photooxida tive addition to binuclear d^ complexes, b. Atom-transfer mechanism for halocarbon photooxidative addition.
In The Challenge of d and f Electrons; Salahub, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.
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THE CHALLENGE OF d AND f ELECTRONS
and aryl halides react with the 3(
