J. Am. Chem. SOC. 1981, 103, 3379-3387
3319
Intramolecular Electron-Transfer Reactions in Bridged Binuclear Ru( 11)-Co( 111) Molecules Susan K. S. Zawacky and Henry Taube* Contribution from the Department of Chemistry, Stanford University, Stanford, California 94305. Received September 15, 1980
Abstract: Intramolecular electron transfer in Ru(I1)-Co(II1) binuclear complexes containing pyrazine, 4,4’-bipyridine, and selected pyridinecarboxylate anions as bridging ligands has been studied. For the series (H20)Ru11(NH3)4(R)Co111(NH3)$+, where R equals
with 3-pyridylacetate, 4-pyridylacetate, nicotinate, isonicotinate, 3-cinchomeronate, 4-cinchomeronate, and pyrazinecarboxylate as bridging ligands, specific rates at 25 O C are 0.566 X 7.83 X 1.80 X lo-,, 12.4 X lo-,, 1.1 X 42 X and 10.13 X s-l, respectively. Related complexes with 4-cinchomeronate as bridging ligand but with iodide or sulfate substituting for water at the trans position of the ruthenium undergo electron transfer with specific rates at 25 OC of 0.5 and 5 s-l, respectively. The pyrazinecarboxylate-bridgedspecies undergoes facile photoinduced electron transfer, and its spectrum exhibits a shoulder at 390 nm (t = 400 i 50) which is absent from the spectra of both constituent mononuclear complexes. For the complexes (03S)Ru11(NH3)4(L)Co111(NH3)2+ where L = pyrazine and 4,4’-bipyridine, specific rates for electron transfer at 25 “C are 0.128 and 4 X lo4 s-l, respectively. These results indicate that electron transfer in the pyridinecarboxylato complexes approaches the adiabatic regime. The surprisingly low rate for the 4,4’-bipyridine-bridged complex, especially in comparison to the rate for the pyrazine complex, raises the possibility that electron transfer in this species is strongly nonadiabatic.
T h e work to be reported is a refinement and extension of the earlier preliminary results of Isied and Taubela on the rates of intramolecular electron transfer based on the Co(II1)-Ru(I1) redox system. Activation parameters have now been determined, and a n important correction on the rate constant for the isonicotinate-bridged species is reported. The work has been extended to related bridging groups, and the effect on the rate of electron transfer attending substitution on Ru(I1) in the position trans to the bridging group has been examined. A glossary of abbreviations used in the paper follows. 3-cinH represents the form in which the residue C O ( N H ~replaces )~ a proton on the meta-carboxyl of cinchomeronic acid
4-cinH, we followed the procedure to be outlined. Forty millimoles (6.8 g ) of cinchomeronic acid was dissolved in 23 mL of 1.75 M NaOH at was added, and the solution 75 OC, 2.0 g of [CO(NH~),(H~O)](CIO~)~ was maintained at 75 ‘C with stirring for 2 h. The solution was allowed to cool slowly to room temperature and to stand for 2-3 h, whereupon the relatively insoluble [CO(NH~)~(~-~~~H)](CIO~)~ was removed by filtration. Solid NaCIO, was then added to the filtrate to precipitate [CO(NH,)~(~-C~~H)](CIO~)~, which was collected by filtration, washed with saturated NaCIO, solution, ethanol, and ether, and dried under vacuum. Yield: ca. 0.8 g of the 4-cin complex. The complexes [ C O ( N H , ) ~ ( ~ Z ) ] C ~and ~ . ~ [Co(NH,)’(bpyH)]H~O C14.Hz0 were prepared as described by Gould, Morland, and Johnson‘ and isolated as chloride salts following purification by gel filtration chromatography with use of Bio-Gel P-2 (Bio-Rad Laboratories) with 0.1 M HC1 or 0.5 M LiCl and 0.01 M HCl as eluant. N P C 0 . H Ruthenium Compounds. Samples of [ R u ( N H ~ ) ~ ] used C I ~ to prepare solutions of Ru(NH&~+were recrystallized by the method of Armor.5 The compound tranr-[Ru(NHJ4(SOZ)C1]C1was prepared by the method COzH of Vogt, Katz, and Wiberley6 according to Armor’s procedure.’ Preparation of Ru(III)Co(III) Binuclear Complexes. Binuclear and 4-cinH implies that Co(NHJ5 is in the trans position; pz complexes of the form [(S04)Ru111(NH3)4(L)Co(NH3)5]C13~2HZ0, represents pyrazine, bpy, 4,4’-bipyridine, MES, morpholinewhere L = nicotinate, 3- and 4-pyridylacetate, isonicotinate, 3-cinH, and ethanesulfonic acid (used here as a buffer because of its very low 4-cinH, were prepared according to the method of Isied and Taube’ and affinity for metal ions),2 HTFMS, trifluoromethanesulfonic acid, then purified by gel chromatography with use of Bio-Gel P-2 with H T F A , trifluoroacetic acid; HTos, p-toluenesulfonic acid, and 0.05-0.10 M HC1 as eluant. The complex [(S04)Ru111(NH$4(pyraziNaTos, sodium p-toluenesulfonate. necarboxylate)Cor1’(NH3)5]C13~2Hz0 was prepared by a modification of this method. A 150-mg sample of tran~-[Ru(NH~)~(S0~)Cl]Cl was Experimental Section combined with ca. 100 mg of NaHCO, and 190 mg of [ C O ( N H , ) ~ ( ~ ~ Preparation of Cobalt(II1) Complexes. Complexes of the type [Coraziniumcarboxylate)]C13 in 8-10 mL of water under argon, and the (NH,)s(L)] (C104)2,where L = nicotinate, 3-pyridylacetate, 4-pyridylsolution was allowed to react for 1-2 min. One milliliter of 5 M HCI acetate, isonicotinate, and pyrazinecarboxylate, were prepared by the was then added with stirring, followed quickly by 6-8 drops of 30% H20> method of Taube and Gould,’ as modified by Isied.lb The complexes The resulting orange solution was mixed with ca. 75 mL of 1 M HC1, [CO(NH,)~(~-C~~H)](CI~~)~ and [CO(NH~)~(~-~~~H)](CIO~)~ were and acetone was added with stirring until the solution was just turbid prepared by a method modified slightly from that used for the other (100-150 mL), when it was set to chill in an ice bath or refrigerator to carboxylate complexes. To obtain a maximum yield of the 3-cinH cominduce precipitation of the product. Yield: ca. 50%. Attempts to purify plex, we dissolved 20 mmol (3.4 g) of cinchomeronic acid (Aldrich) in the complex either by ion exchange or gel filtration were unsuccessful, was 10 mL of 3.5 M NaOH at 50 OC, 1.0 g of [CO(NH~)~(H~O)](C~O~), owing to disproportionation of the complex on the column. It was puadded, and the solution was maintained at 5 0 4 0 OC with stirring for 2 rified by redissolving it in a minimum volume of 1 M HCI and either h. The solution was cooled slowly to room temperature, and the [Corepeating the original precipitation procedure or adding solid NH4PF6 (NH3)5(3-cinH)](C104)zwas collected by filtration, washed with dilute to precipitate the hexafluorophosphate salt. aqueous NaC10, solution, ethanol, and ether, and dried under vacuum. It was desired to replace coordinated sulfate by another ligand in Yield: 0.6-0.8 g of the 3-cinH complex. To enhance the yield of the certain of the Ru(II1)-Co(II1) ions. The sulfato form of the iso(1) (a) Isied, S. S.; Taube H. J. Am. Chem. Soc. 1973,95,8198. (b) Isied, S.S.Ph.D. Thesis, Stanford University, 1974. (2) Good, N. F.; Winget, G . D.; Winter, W.; Connolly, T. N.; Izawa, S.; Singh, R. M. M. Biochemistry 1966, 5, 467. (3) Taube, H.; Gould, E. S. J . Am. Chem. SOC.1964, 86, 1318.
0002-786318 111503-3379$01.25/0
(4) Gould, E. S.;Morland, R.B.; Johnson, N. A. Inorg. Chem. 1976,15, 1929. ( 5 ) Armor, J. N. Ph.D. Thesis, Stanford University, 1970. (6) Vogt, L. H.; Katz, J. L.; Wiberley, S. E. Inorg. Chem. 1965,4, 1157.
0 198 1 American Chemical Society
3380 J. Am. Chem. SOC.,Vol. 103, No. 12, 1981
Zawacky and Taube
Table I. Microanalysis for Ru(III)-Co(III) Binuclear Complexes compd
L
r
C
H
N
Ru
co
calcd obsd
7.2 7.45
3.5 3.52
14.0 14.18
calcd obsd
10.7 10.30
5.2 4.90
20.8 20.71
calcd obsd
12.2 12.39
5.4 4.96
20.3 20.20
calcd obsd
12.2 11.4
5.4 5.24
20.4 19.35
calcd obsd
11.7 12.01
4.92 4.92
19.5 19.79
14.1 14.3
8.2 8.18
calcd obsd
11.1 11.09
5.20 4.85
18.68 18.42
calcd obsd
8.66 8.76
5.38 4.97
22.21 22.36
14.57 14.48
8.49 8.26
calcd obsd
11.1 11.15
5.47 5.35
21.65 21.82
15.6 15.65
9.08 9.05
CalCd obsd
16.2 17.01
5.3 5.23
20.7 20.39
calcd obsd
11.0 11.46
4.35 4.91
18.3 18.41
13.2 13.3
7.7 7.50
Gild obsd
17.3 17.30
3.9 3.90
14.5 14.44
calcd obsd
12.4 11.59
3.65 3.55
15.9 14.41
10.5 10.0
6.1 5.5
c1
13.9 13.19
'CO2Co(NH3l5J
-
27.4 28.81
nicotinate-bridged binuclear complex was converted to [CIRU(NH~)~- (111)-bridged complex, can be followed by monitoring the disappearance (isonicotinate)Co(NHJ5]C14-2H20 by dissolving the original compound of the Ru(I1) absorption during the reaction, but in practice the reaction in a minimum of 1 M HCI at 10 OC, adding excess Ru(NH3):+ or between the Ru(II)-Co(III) binuclear and the Ru(II1) product (reaction ascorbic acid, and then reoxidizing the binuclear after ca. 30 s by adding 1) interferes with that of interest, resulting in curvature in first-order rate aqueous bromine solution until the red color of the Ru(I1) species was Ru"LCo"' + Ru"'L = Ru"'LCO"' + Ru"L (1) discharged; the chloro species was then precipitated by using 8-10 volumes of acetone and purified by gel filtration. The sulfato form of the plots. This complication can be avoided by exploiting small but signif4-cinH-bridged binuclear was converted to [ I R U ( N H ~ ) ~ ( ~ - C ~ ~ H ) Cicant O - differences in the T T * absorptions of the Ru"LCO"' species and (NH,)5]Cl4.2H20 by a slightly different procedure. A 200-mg sample the corresponding Ru"L complex and using an excess of the external of [(S04)Ru(NH3)4(4-cinH)Co(NH3),]C13~2H,0 was dissolved in a reducing agent so that the ruthenium remains in the reduced state folminimum volume of 0.05 M HTFMS, and chloride ion was removed lowing the intramolecular electron transfer. Under these conditions, the from solution by gel filtration with 0.02 M HTFMS as eluant. The net changes observed is given by eq 2. Gould and Fan' have shown that eluate (ca. 20 mL) was cooled to 10 OC, and 10 mL of a solution 0.3 M Ru"LCO'~' + reductant Ru"L + Co2+ + oxidized product (2) in NaI and 0.1 M in I2 was added with stirring. After ca. 10 min, the solution was extracted with ether to remove I2 and loaded onto a column Ru(NH&~+ reduces most (pentaamminecarboxylate)cobalt(III) comof AG 50W-X2resin, from which IRU(NH~)~(~C~~H)CO(NH~)~~+ was plexes with a specific rate of 0.1 M-' SKI or less, and ascorbate reduces eluted with 5 M HCI and precipitated as the chloride salt by adding Co(II1) complexes even more slowly*so that with use of low (
