Inorg. Chem. 1987, 26, 4024-4029
4024
Contribution from the Departments of Chemistry, Colgate University, Hamilton, New York 13346, and University of New Haven, West Haven, Connecticut 06516
Photo- and Thermochemistry of cis -[Cr(en),F,]+ and cis -[Cr(en),F(H,O)l2+ in Acidic Aqueous Solution Bruce Herbert, David Reinhard, Michael J. Saliby, and Peter S . Sheridan* Received May 28, 1987 The cis-[Cr(en),F,]+ ion is shown to undergo three consecutive aquations in acidic aqueous solution, leading to cis-[Cr(en),,F(H20)l2+,[Cr(en)(en-H)F(H,O),]-'+, and [Cr(en)F(H20)J2+, respectively. Kinetic studies of the first reaction confirm earlier findings of an acid-catalyzed and an acid-independent path (kobsd= k, + kl[H+], with k, = 6.3 X lo4 s-' and kl = 4.8 X lo-) M-' s-I at 70 O C ; for kobsd,AH* = 90.8 f 1.0 kJ mol-' and AS* = -40.7 f 2.4 J mol-' K-I). Ring opening upon aquation of ~is-[Cr(en),F(H~O)]~' is not acid catalyzed ( k = 1.88 X s-' at 70 "C, AH* = 83.4 f 1.9 kJ mol-', and AS* = 74.0 f 3.8 J mo1-l K-I). Ligand field irradiation of cis-[Cr(en),F2]+ in acidic solution causes efficient (9 = 0.35 f 0.04) amine labilization; the resulting [Cr(en)(en-H)F2(H,0)]z+ loses monodentate ethylenediamine when heated in solution. The ~is-[Cr(en),F(H,O)]~+ ion is also photosensitive, generating [Cr(en)(en-H)F(H,O),]-'+ and [Cr(en-H),F,(H,O),]*+ in successive photoinduced aquations. The photochemical results are discussed in terms of available photolability models.
Introduction
The photochemical study of six-coordinate Cr(II1) complexes is now entering its fourth It has been 20 years since Adamson, first proposed the empirical rules that catalyzed a burst of activity testing which of t h e ligands will be lost upon ligand field excitation of t h e c o m p l e ~ . ~ - Several ~' more theoretically based models quickly followed,2z-25and it has now been a decade since V a n q u i ~ k e n b o r n e presented ~ ~ . ~ ~ a photolability model, based on the angular overlap approximation t o MO theory, which has become the current standard. The more important tests of these photolysis models have involved photochemical studies of Cr(111)-fluoro-amine species,1'~2s-32 as the unique properties of t h e F ligand set t h e m a p a r t from t h e other Cr(II1)-halo-amine c~mplexes.~~~~~ In t h e decade since this work, high-pressure mechanistic studies3sand pulsed laser studies36have explored the photophysics of the photochemical process, and work has proceeded on chemical studies of o t h e r bis(amine) system^,^^-^^ but t h e r e has been a paucity of work on complexes with the cis-[CrN,X,]+ chromophore. The thermal reactivity of most cis-dihalotetrakis(amine)chromium(III) complexes in aqueous solutions hinders their photochemical study, b u t the unusually robust Cr-F moiety suggests t h a t studies of cis-[CrN,F,]+ ions could be important tests of t h e photolability models. The [Cr(tren)F,]+ ion (where tren is P,p,@"-triaminotriethylamine,a tripodal, tetradentate amine that forces a cis configuration on the two fluorides) efficiently releases fluoride upon p h o t o l y ~ i s a, ~result ~ not anticipated by photolability models. The possible constraints of the tren ligand call into question whether the models could be expected t o apply to such a complex, however, and studies on less sterically constrained systems are needed. In a kinetic study of t h e aquation of cis-[Cr(en),F,]+, Garner,' includes a footnote describing an experiment t h a t compared t h e rates of aquation of HCIO, solutions of cis-[Cr(en),F,]+ in a "black-painted reaction flask and ...in an uncoated flask exposed during the daylight to diffuse sunlight and laboratory fluorescent light and during nights to light from a 150-W incandescent bulb 20 cm from t h e pair of flasks". No difference in t h e rates of fluoride release was observed, leading t o the conclusion that "The rate of aquation of cis-[Cr(en),F2lf is unaffected by visible light ...." Given t h e more recent evidence of Cr(II1)-amine photosensitivity and t h e importance of Cr(II1)-F complexes in t h e testing of photochemical models, we have pursued a more thorough study of t h e thermal a n d photochemical behavior of cis- [Cr(en),F,]+ a n d its reaction products.
Experimental Section Materials. cis-[Cr(en),F,]' was prepared as the iodide salt by the method of Vaughn;42 purity was ensured by comparison of electronic spectra [observed maxima at 516 nm (t73) and 378 nm (c37); literature42 *To whom correspondence should be addressed at Colgate University.
0020-166918711326-4024$01.50/0
values at 516 nm (€75.5) and 378 nm (t39.5)] and by chromium and fluorine analyses. Other reagents were of reagent grade and used without
(1) Plane, R. A.; Hunt, J. P. J . A m . Chem. SOC.1957, 79, 3343. (2) Balzani, V.; Carassiti, V. Photochemistry of Coordination Compounds; Academic: New York, 1970. (3) Zinato, E. In Concepts of Inorganic Photochemistry; Adamson, A. W., Fleishauer, P. D., Eds.; Wiley Interscience: New York, 1975. (4) Adamson, A. W. J . Phys. Chem. 1967, 71, 798. (5) Wasgestian, H. F.; Schlafer, H. L. Z . Phys. Chem. (Munich) 1968,57, 282. (6) Wasgestian, H. F.; Schlafer, H. L. Z . Phys. Chem. (Munich) 1968.62, 127. (7) Zinato, E.; Lindholm, R. D., Adamson, A. W. J . Am. Chem. SOC.1969, 91, 1076. ( 8 ) Riccieri, P.; Schlafer, H. F. Inorg. Chem. 1970, 9, 727. (9) Vogler, A. J . Am. Chem. SOC.1971, 93, 5912. (10) Kirk, A. D.; Moss, K. C.; Valentin, J. G. Can. J . Chem. 1971, 48, 1524. (11) Pyke, S. C.; Linck, R. G. J . A m . Chem. SOC.1971, 93, 5281. (12) Kutal, C.; Adamson, A. W. J . A m . Chem. SOC.1971, 93, 5581. (13) Kirk, A. D. J . A m . Chem. SOC.1971, 93, 283. (14) Manfrin, M. F.; Moggi, L.; Balzani, V. Inorg. Chem. 1971, 10, 207. (15) Zinato, E.; Furlani, C.; Lanna, G.;Riccieri, P. Inorg. Chem. 1972, 11, 1746. (1 6) Riccieri, P.; Zinato, E. Proceedings of the XIVInternational Conference of Coordination Chemistry; IUPAC: Toronto, Canada, 1972; p 252. (17) Gandolfi, M. T.; Manfrin, M. F.; Moggi, L.; Balzani, J. J . Am. Chem. SOC.1972, 94, 7152. (18) Zinato, E.; Riccieri, P. Inorg. Chem. 1973, 12, 1451. (19) Kutal, C.; Adamson, A. W. Inorg. Chem. 1973, 12, 1990. (20) Gandolfi, M. T.; Manfrin, M. F. Moggi, L.; Balzani, J. Inorg. Chem. 1974, 13, 1342. (21) Zinato, E.; Riccieri, P.; Adamson, A. W. J. Am. Chem. SOC.1974, 96, 375. (22) Zink, J. I. J . Am. Chem. SOC.1972, 94, 8039. (23) Wrighton, M.; Gray, H. B.; Hammond, G. S. Mol. Photochem. 1973, 5 , 165. (24) Furlani, C. Theor. Chim. Acta 1974, 34, 233. (25) Bifano, C.; Linck, R. G. Inorg. Chem. 1974, 13, 609. (26) Vanquickenborne, L. G.; Ceulemans, A. J . Am. Chem. SOC.1977, 99, 2208. (27) Vanquickenborne, L. G.;Ceulemans, A. J . Am. Chem. SOC.1978, 100, 475. (28) Wright, R. E.; Adamson, A. W. Inorg. Chem. 1977, 16, 3360. (29) Wong, C. F. C.; Kirk, A. D. Inorg. Chem. 1977, 16, 3148. (30) Manfrin, M. F.; Sandrini, D.; Juris, A,; Gandolfi, M. F. Inorg. Chem. 1978, 18, 90. (31) Wirth, G.; Linck, R. G. J . A m . Chem. SOC.1973, 95, 5913. (32) Kane-Maguire, N. A. P.; Wallace, K. C.; Speece, D. G. Inorg. Chem. 1986, 25, 4650. (33) Vanquickenborne, L. G.; Ceulemans, A. Inorg. Chem. 1979, 18, 897. (34) Kirk, A. D. Inorg. Chem. 1979, 18, 2326. (35) (a) Angerman, K.; van Eldik, R.; Kelm, H.; Wasgestian, F. Inorg. Chem. 1981, 20, 955. (b) Angerman, K.; Schmidt, R.; van Eldik, R.; Kelm, H.; Wasgestian, F. Inorg. Chem. 1982, 21, 1175. (36) Waltz, W. L.; Lilie, J.; Lee, S. H. Inorg. Chem. 1984, 23, 1768.
0 1987 American Chemical Society
Inorganic Chemistry, Vol. 26, No. 24, 1987 4025
Photo- and Thermochemistry of cis-[Cr(en),F,]+ further purification. Column chromatography media was Sephadex SP C-25 (40-1 20 mesh) cation-exchange resin; eluants were aqueous solutions of NaBF,, Na2S04,NaNO,, HCIO,, or NaOH. Analyses. Chromium was determined spectrophotometrically as chromate ion at 372 nm after oxidation of Cr(II1) with alkaline peroxide.43 A Cr(V1) standard curve was prepared with solutions of [KCr(SO,),]-l 2 H 2 0 that had undergone identical alkaline oxidations. Fluoride ion was determined with a fluoride ion electrode (Corning) in combination with a double-junction reference electrode (Corning 476067) and an Orion 601A digital pH meter. Fluoride standard series were prepared with N a F dissolved in 0.10 M HOAc/O.lO M NaOAc buffer solutions. Fluoride was also measured by titration with Th4+.,, Electronic spectra were recorded on a Beckman Acta MIV spectrophotometer. Reaction Conditions. Kinetic studies were done in dilute HCIO,; ionic strength was controlled with NaC10,. Reactions were monitored spectrophotometrically. For the photochemical studies, irradiation was done with a 200-W Hg short-arc lamp in an Oriel housing. For preparative purposes, a Corning glass filter that absorbed light below about 400 nm was used; quantum yield studies were done with an interference filter (Oriel) that passed light at 502 nm (fwhh of ca. 20 nm). Quantum yields were based on spectral changes during the first 5-10% of reaction, and incident intensity was determined by using Reinecke a~tinometry.,~ Photolyses were performed at room temperature, and identical samples held in the dark showed no detectable spectral changes during the photolysis periods.
Results Thermal Reactions and Aquation Products. Reaction of cis[Cr(en),F,]+ in 0.40 M HC104 at 40 O C causes a slight blue shift of the maximum, with isosbestics at 506, 395, and 363 nm. Isosbestics are maintained for about 2 h, after which time they are lost and the low energy absorption band undergoes a red shift (vide infra). When a solution of c i ~ - [ C r ( e n ) ~ F is ~ ]allowed + to react just until loss of isosbestics ( 5 h at 25 OC in 0.1 M HC104), ion-exchange chromatography reveals the presence of four species in solution: (a) The first band (eluted with 0.15 M Na2S04solution) is the red-violet cis-[Cr(en),F,]+ starting material (I; A, at 516 and 378 nm). (b) The second band displays A, at 505 nm (e 72.5) and 373 nm (e 32.0), consistent with the published spectrum of cis-[Cr(en),F( H 2 0 ) ]2+ (II).42 (c, d) Under the described conditions, the third and fourth bands are too dilute to be carefully characterized, but more extensive reaction of ~ i s - [ C r ( e n ) ~ F (1 ~ ] h+ at 65 "C) yields more of these products. The more rapidly eluted material (111; removed with values at 518 and 376 nm, an eluant of 0.3 M HC104) has A, while the less rapidly moving cation (IV) has A, values of 530 nm (e 42.1) and 387 nm (e 24.4). These results show that, in acidic aqueous solution, cis-[Cr(en),FZ]+ undergoes three successive aquations; the first releases fluoride ion, leading to the c i ~ - [ C r ( e n ) ~ F ( H ~ Oion ) ] ~(reaction + l ) , and is followed by aquation of the monofluoro ion to give [Cr(en)(en-H)F(H,0)Z]3+ with a protonated, monodentate ethylenediamine ligand (reaction 2). Hydrolysis of the pendant ethylenediamine (reaction 3) leads to the eventual aquation product, IV, [Cr(en)F(HZO)J2+. (Kinetic studies of the first two reactions are presented below.) cis-[Cr(en),F,]+ + H 2 0 c i ~ - [ C r ( e n ) ~ F ( H , O ) ]+ ~ +F (1) c i ~ - [ C r ( e n ) ~ F ( H ~ O+ ) ] H30+ ~+ [Cr(en)(en-H)F( H20)2]3+ (2)
- -
[Cr(en)(en-H)F(HZO),]3+
+ H20
-
[Cr(en)F(H,O),]*+
+ en-H+
(3)
(37) Kirk, A. D.; Namasivayam, C.; Ward, T. Inorg. Chem. 1986, 25, 2225. (38) Gowin, E.; Wasgestian, F. Inorg. Chem. 1985, 24, 3106. (39) Cimonlino, M. C.; Linck, R. G.Inorg. Chem. 1981, 20, 3499. (40) Saliby, M.; Sheridan, P. S.; Madan, S. K. Inorg. Chem. 1980, 19, 1291. (41) Fehrmann, K. R.;Garner, C. S . J. Am. Chem. SOC.1961, 83, 1276. (42) Vaughn, J. W.; Yeoman, A. M. Inorg. Chem. 1976, 15, 2320. (43) Haupt, G. J. Res. Nut/. Bur. Stand. U.S.A. 1952, 48, 414. (44) Horton, C. A. In Treatise on Analytical Chemistry; Kolthoff, I. M., Elving, P. J., Eds.; Interscience: New York, 1961; Part 11, Vol. 7, p 307. (45) Wegner, E. E.; Adamson, A. W. J. A m . Chem. SOC.1966, 88, 394.
300
400
500
600
Figure 1. Spectral changes upon heating a chromatographically separated sample of ~is-[Cr(en)~F(H,O)]~' in 0.40 M HCIO, solution.
1 O''
,
0.3
1'
[H
0.5
Figure 2. Graph of koM vs [H'] (70 "C) for the aquation of cis-[Cr(en),F,]+. The y intercept corresponds to the rate constant for the acid-independent path; the slope is proportional to k l , the rate constant for the acid-dependent path.
Plots o f t vs A for both the difluoro and the chromatographically isolated ~ i s - [ C r ( e n ) ~ F ( H , O ) ]ion ~ + allow prediction of the wavelengths expected for the isosbestic points for reaction 1. Comparison of these values with the observed values 505, 398, 360 nm 506, 395, 362 nm
predicted observed
further supports the supposition that c i ~ - [ C r ( e n ) ~ F ( H ~ Ois) ] ~ + the only product formed upon aquation of ~ i s - [ C r ( e n ) ~ F ~ ] + . Supporting evidence for reaction 2 comes from a comparison of the aquation of a chromatographically separated sample of ~is-[Cr(en),F(H,O)]~+ with the aquation of a separate sample of c i ~ - [ C r ( e n ) ~ F ( H ~Iz+Hz0.46 O)] The spectral changes accompanying aquation of these separate samples are indistinguishable and show an isosbestic at 553 nm; a second isosbestic is observed initially at about 408 nm but is lost near the end of the reaction. These spectral changes are shown in Figure 1 and represent the changes caused by the complete loss of a bidentate ethylenediamine (reactions 2 and 3). After completion of the reaction between I1 and IV, product solutions were loaded onto a cation-exchange column and the resin was washed with water. Analysis of the washings for F ion (with Th4+)showed that
