Inorg. Chem. 1986, 25, 3037-3043 excited ST' for C O ( H ~ O ) , ~should + therefore be close to the 'A, ground state in energy. The analysis of Winkler et al.31 as well as much earlier considerations by Friedman et al.32suggest that the energy difference is only about 4 kcal mol-' with a rather much larger equilibrium distance for the ST,state. The latter result is in agreement with spectroscopic results19 as well as our calculations for Co(NH3):+. Thus the ST,state has an equilibrium bond length that is much closer to the one for the ground state of the Co(I1) complex. This suggests a possible electron-transfer / ~case + . of ~~ path via the ST,or 3Tlstate for C O ( H ~ ~ ) ~ ~In+the Co(NH3):+ the ST' is too high in energy,lg however, to be of any importance as a preequilibrium state for electron transfer. VI. Conclusions Electron exchange between C O ( N H ~ ) ! ~and + C O ( N H ~ ) , ~in+ their respective ground states is spin forbidden in the absence of spin-orbit coupling. We suggest that the 'E state for CO(NH3),'+ is closer in energy to the ground state (4Tl) than believed previously. This suggestion is supported by INDO calcuations. In fact for small metal-ligand distances there is a crossing between the 2E and 4T, energy surfaces. Thus for Co-N distances appropriate for electron transfer, the 2E state is below the 4T, state.
3037
Due to spin-orbit coupling the 4T, states pass adiabatically to the 2E state. Electron exchange between the 'E state of C O ( N H ~ ) , ~ + and the 'Al state of C O ( N H ~ ) , ~is+spin-allowed. The attempt was made to estimate whether the electron transfer between the two complexes, held close to each other in various conformations, was also spatially adiabatic. As for Fe(H20)62+/3+, we found that CO(NH~),'+/~+ was close to the border to nonadiabatic electron transfer. The 2E intermediate state, according to our calculations with all Co-N distances equal, lowers the thermal barrier for electron transfer somewhat. Since Jahn-Teller distortion of the 2E state takes place, the barrier is again increased. Our calculations suggest, however, that the 2E barrier is still lower than or is about the same as the 4T1barrier. This is in agreement with experimental result^.^^^ Acknowledgment. We are grateful for continued support from NFR, the Swedish Natural Science Research Council (S.L., K.S.), and from the United States Army through CRDC Chemical Systems Technology Center Award DAAA 15-85-C-0034 (M.C.Z.). Registry No. CO(NH&~+,15365-75-0; Co(NH3)2+,14695-95-5.
Contribution from the Department of Chemistry, Boston University, Boston, Massachusetts 02215 , and National Bureau of Standards, Gaithersburg, Maryland 20899
Mechanisms of Ligand-to-Metal Intramolecular Electron Transfer in Cobalt(111)-Amine Complexes Containing a Coordinated Radical' Kevin D. Whitburn,? Morton Z. Hoffman,*$Nina V. Brezniak,* and Michael G. Simicg Received January 15. 1986 The one-electron reduction of Co(II1) complexes containing nitrophenyl ligands possessing differing lead-in and bridging groups by radiolytically generated 'COT and 'C(CH3)z0H radicals in neutral and acidic aqueous solution results in the formation of coordinated nitrophenyl ligand radicals. The UV-visible absorption spectra, the acid-base properties, and the decay kinetics of the transient intermediates were examined by pulse radiolysis. In neutral solution, the coordinated ligand radicals decay via intramolecular electron transfer from the coordinated nitrophenyl radical donor to the Co(II1) acceptor. The values of the intramolecular electron-transfer rate constants depend on the isomeric position of the nitro group on the phenyl moiety, the structure of the bridging molecule between the redox sites, and the nature of the lead-in group to the metal center. Bridging structures between the initial radical site and the metal center of varying length, flexibility,and r-conjugation are incorporated into the 18 complexes studied. Correlation of the values of AH* and AS* of electron transfer with the structural relationship of the donor and acceptor sites leads to the proposition that four different mechanisms of intramolecular electron transfer operate in these complexes: through chain, direct and indirect ligand bypass, and nonadiabatic transfer. Protonation of the coordinated nitro radical greatly diminishes the rate of intramolecularelectron transfer in the nitrophenyl carboxylato complexes; in most cases, protonation affects only the driving force for electron transfer while leaving the mechanism unchanged.
Introduction The study of the rates and mechanisms of electron-transfer reactions is fundamental to the understanding of many important biological redox sequences, including the respiratory chain and photosynthesis.2 At the basis of these biological processes is the controlled sequential transfer of electrons between protein molecules that contain specific redox-active sites. Beyond its biological relevance, the understanding of the mechanisms of electron transfer between separated donor and acceptor sites has application to redox processes on electrode surfaces and to the reactivity of intermediates in homogeneous and heterogeneous catalysis. When the donor and acceptor sites are on a single molecule, electron transfer can occur intramolecularly without the kinetic influences of reactant diffusion and precursor substitution. These intramolecular systems, based conceptually on early developments by T a ~ b e model ,~ the "precursor complex" that precedes electron transfer in bimolecular redox reaction^.^
Present address: Department of Chemistry, Framingham State College, Framingham, MA 01701. *Boston University. *National Bureau of Standards.
0020-1669/86/1325-3037$01.50/0
The relationship between the rate of intramolecular electron transfer (IET) and the distance between the donor and acceptor sites has been examined by a number of investigators with a ' ~ relationship particular focus on long-range r e a c t i ~ i t y . ~ - The ~
______~
(1) This research was supported by a grant from the Office of Basic Energy Sciences, Division of Chemical Sciences, US. Department of Energy, to Boston University. (2) Isied, S. S. Prog. Inorg. Chem. 1984, 32, 443. (3) Isied, S . S.; Taube, H. J . Am. Chem. SOC.1973, 95, 8198. (4) Sutin, N. Acc. Chem. Res. 1968, 1, 225. (5) Winkler, J. R.; Nocera, D. G.; Yocom, K. M.; Bordignon, E.; Gray, H. B. J . Am. Chem. SOC.1982. 104. 5798. (6) Kostic, N. M.; Margalit, R.: Che, C.-M.; Gray, H. B. J . Am. Chem. SOC.1983, 105, 7765. (7) Nocera, D. G.; Winkler, J. R.; Yocom, K. M.; Bordignon, E.; Gray, H. B. J . Am. Chem. SOC.1984, 106, 5145. (8) Isied, S. S.; Worosila, G.;Atherton, S . J. J . Am. Chem. SOC.1982, 104, 7659. (9) Isied, S . S.; Kuehn, C.; Worosila, G. J . Am. Chem. SOC.1984, 106, 1722. (10) Peterson-Kennedy, S . E.; McGourty, J. L.; Hoffman, B. M. J . Am. Chem. SOC.1984, 106, 5010. (11) Ho,P. S.; Sutoris, C.; Liang, N.; Margiolash, E.; Hoffman, B. M. J . Am. Chem. SOC.1985, 107, 1070. (12) Calcaterra, L. T.; Closs, G. L.; Miller, J. R. J . Am. Chem. SOC.1983, 105, 670.
(13) Miller, J. R.; Calcaterra, L. T.; Closs, G. L. J . Am. Chem. SOC.1984, 106, 3047.
0 1986 American Chemical Society
Whitburn et al.
3038 Inorganic Chemistry, Vol. 25, No. 17, 1986 between the mechanism of electron transfer and the structure of the molecular medium between the redox-active sites has also been studied systematically. The role of the bridging chain has been investigated for IET across polypeptides in which flexible and rigid amino acid residues have been positioned between the donor and acceptor metal ions in a series of binuclear comple~es.’~Binuclear complexes in which the donor and acceptor metal ions are separated by organic bridging molecules having variable flexibility and extent of a-conjugation have been systematically s t ~ d i e d . ’ ~ Depending on the precise nature of the bridging ligand, either through-the- bridge or bridge-bypass mechanisms are operative. Another experimental approach to the study of IET in metal complexes involves the generation of a nitrophenyl donor radical at a remote ligand site of a pentaamminecobalt(II1) complex.16 The rate of IET, investigated as a function of the isomeric position of the nitro group in a series of mono- and dinitrobenzoato complexes, depends on the electron spin density a t or adjacent to the carboxyl group leading into the Co(II1) acceptor enter.^'*'^ The generally slow rate of IET in these Co(II1) complexes relative to molecular vibration has been attributed to the poor overlap of the a-orbitals of the ligand with the u-orbitals of the Co(II1) enter."^'^ The insulating effect of the carboxyl lead-in group may reinforce inherent structural tendencies for transferring an electron from the ligand radical directly into the metal center in an “intramolecular outer-sphere” mechanism that bypasses the lead-in gro~p.’~,~~ In this paper we report the rates of IET studied as a function of solution medium for 18 Co(II1) complexes containing nitrophenyl ligands that possess organic bridging structures of variable length, flexibility, a-conjugation, and isomeric disposition of the nitro group on the aromatic portion of the remote ligand and variable lead-in groups to the acceptor center. From the observed activation parameters, four mechanisms of IET are proposed t o be operative in these complexes, A preliminary account of some of these results has been published previously.” Experimental Section Preparation of Complexes. General literature methods were used to prepare (nitrophenyl carboxylato)pentaamminecobalt(III) complexes as the perchlorate salt^.^^*^' The parent nitrophenyl acids used in the syntheses were purchased from Aldrich and I C N Biochemicals. Any parent acids received that were off-white were recrystallized from boiling methanol. Analyses of melting points before and after recrystallization were taken as general indicators of the purity of these starting materials. Crude samples of the complexes were, in general, not purified of unreacted acid by the literature method of repeated washings with methanol and ether, because several of the complexes were soluble in these organic solvents. Instead, the unreacted acid was removed by dissolving the crude sample in a minimum amount of water a t 70 OC to which a saturated N a H C 0 3 solution was added dropwise until the pH was -9. The solution was then quickly chilled to 0 OC with stirring on a salt-ice bath to minimize any base hydrolysis of the precipitating complex; any reacted acid remained in solution as its sodium salt. The filtered complex was then recrystallized two to three times from water at 70 OC and dried in a desiccator. Elemental analyses for Co, N, C, and H were satisfactory, indicating the presence of, at most, negligible quantities of the free ligand parent acids. UV-visible spectra were taken of the nitrophenyl carboxylato complexes; characteristic d-d bands having absorbance maxima at 500-502 nm with e IO2 M-’ cm-’ were observed. Other complexes were kindly provided by other researchers: (0nitrophenyl)cyano)pentaamminecobalt(III) perchlorate and (3-(@nitrophenyl)carbonyl)-2,4-dimethyl-1,5,8,12-tetraazacyclotetradeca1,3-
-
(a) Isied, S. S.; Vassilian, A. J . Am. Chem. SOC.1984,106, 1726;(b) Isied. S . S . : Vassilian, A. J . Am. Chem. SOC.1984,106, 1732. (15) Haim, A. Pure Appl. Chem. 1983,55, 89. (16) Hoffman, M. Z.; Simic, M. J . Am. Chem. SOC.1972,94, 1757. (17) Simic, M. G.; Hoffman, kf.Z.; Brezniak, N. V. J . Am. Chem. SOC. 1977, 99, 2166. (18) Neta, P.;Simic, M. G.; Hoffman, M. Z . J . Phys. Chem. 1976,80,2018. (19) Wieghardt, K.; Cohen, H.; Meyerstein, D. Ber. Bunsen-Ges. Phys. Chem. 1978,82, 388. (20) Hoffman, M. Z.; Simic, M. J . Am. Chem. SOC.1970,92, 5533. (21) Whitburn, K.D.; Hoffman, M. Z.; Simic, M. G.; Brezniak, N. V. Inorg. Chem. 1980,29, 3180. (22) Gould, E.S.; Taube, H. J . Am. Chem. SOC.1964,86, 1319. (23) Dockal, E. R.;Everhart, E. T.; Gould, E. S . J . Am. Chem. SOC.1971, 93, 5561. (14)
Table I. Co(II1) Complexes Studied’ ligand X o-nitrobenzoate m-nitrobenzoate p-nitrobenzoate (o-nitropheny1)acetate CH2 (m-nitropheny1)acetate CH2 @-nitropheny1)acetate CH2 (2,4-dinitrophenyl)acetate CH2 o-nitrocinnamate CH=CH m-nitrocinnamate CH=CH p-nitrocinnamate CH=CH @-nitropheny1)butyrate (CH,), @-nitroglycyl)benzoate CONHCH2 (p-nitropheny1)diglycinate (CONHCHJ2 @-nitropheny1)cyanide (@-nitropheny1)amino)sulfonyl @-nitropheny1)sulfon yl p-nitrophenoxide nitrotetraazamacrocycle* “(NH3)SCo-Y-X-PhN02: Macrocycle structure:
CN>&$+W2 N”\
Y
abbrev
02C 02C 02C 02C 02C 02C 02C 02C 02C 02C 02C 02C 02C NC NHSO2 OS02 0
ONBZ MNBZ PNBZ ONPA MNPA PNPA DNPA ONCM MNCM PNCM PNPB PNGB PNDG PNNC PNNS PNOS PNPO NCYC
X = bridging group: Y = lead-in group.
1 CHD
diene)cobalt(III) hexafluorophosphate (Dr. R. J. Balahura, University of Guelph); ((@-nitrophenyl)amino)sulfonyl)pentaamminecobalt(III~ perchlorate and (p-nitrophenoxo)pentaamminecobalt(III) perchloratez4 (Dr. R. B. Jordan, University of Alberta); (0-nitropheny1)sulfonyl)pentaamminecobalt(II1) perchlorate (Dr. W. G. Jackson, Australian National University). The reduced Co(I1) form of the macrocyclic complex was characterized spectrally by reduction with V(I1) ~olution.~’ The complexes used in this study are of the form Co”’-Y-X-PhN02 (where X is the bridging group and Y is the lead-in group) and are shown in Table I. Radiation Techniques. Pulse radiolyses were conducted with the Febetron 705 apparatus at the US.Army Natick Research and Development Center or the van de Graaff accelerator at the Center for Fast Kinetics Research (CFKR) at the University of Texas, Austin. Transient absorption spectra were obtained by optical spectrophotometry with time resolutions of < O S c(s. The radiation dose per pulse was determined from SCN- dosimetry.26 Experiments were normally run at 25 OC; activation parameters were obtained with temperature control to zk0.2 OC. Continuous radiolyses were performed with the 6oCo y-source at Boston University. Fricke dosimetry2’ was used to evaluate the dose rate (D,) of the y-source, for which a value of 1.1 X lo2Gy min-’ was determined. Solutions to be irradiated were prepared in water purified through Millipore systems. Reagent grade sodium formate (Fisher) and 2propanol (Eastman) were used, and solutions were buffered with sodium phosphate and adjusted to the desired acidic pH with perchloric acid. Solutions were purged of air by bubbling for -30 min with high-purity N 2 0 . Analyses for Co 2+ after y-irradiation were performed by using the method of Kitson2;gith a Cary 118 spectrophotometer. Yields of Coaq2+were linear with radiation dose. The extent of radical-induced reaction was kept to 95% of (24) Solutions of the PNPO complex undergo efficient photoredox upon exposure to rwm light; freshly prepared solutions were kept in darkness. (25) Guenther, P. R.;Linck, R. G. J . Am. Chem. Soc. 1969,91, 3769. (26) Baxendale, J. H.; Bevan, P. L. T.; Stott, D. A. Trans. Faraday SOC. 1968,6432398. (27) Matheson, M. S.; Dorfman, L. M. Pulse Radiolysis; MIT Press: Cambridge, MA, 1969. (28) Kitson, R. E.Anal. Chem. 1950,22, 664.
Inorganic Chemistry, Vol. 25, No. 17, 1986 3039
Intramolecular Electron Transfer
Table 11. Spectral Maxima and Rate Constants of Formation of One-Electron-Reduced Transient SDecies PH 7
~~
h (nm> Figure 1. Transient absorption spectra measured 60 ps after the pulse irradiation of N20-saturatedsolutions containing 52 pM PNPA and 0.1 M 2-propanol at pH 7.2 (0)and pH 0.7 (0). The spectra are corrected for loss of the substrate.
complex
A,,:
ONBZ MNBZ PNBZ ONPA MNPA PNPA DNPA OMCM MNCM PNCM PNPB PNGB PNDG PNNC PNNS PNOS PNPO NCYC
290 330 290 290 300 295 300, 380 300 310, 370 300, 400 330 325 320 325 300
