J . A m . Chem. SOC.1989, 1 1 1 , 2535-2541 liquid mercury (excess) for 30 min. Another CH2CIz(10 mL) solution of Et3SiH (5.0 mmol) was then added from the addition funnel. The rate of reaction, determined by following hydrogen evolution, was compared with the rate obtained in the absence of an inhibitor (Table 11). In the Hg test, the rate of reaction was not affected by the presence of excess liquid mercury, and the surface of the mercury bead remained shiny at the end of the reaction. In the DCT test, a complete inhibition of catalysis was observed using 1 mol equiv of DCT. Observation of IntermediatesRelated to the Catalytic Cycle. A typical experiment for the observation of 8 is described as follows. [IrH2S?(PPh3),]SbF6 (S = THF; la; 40 mg, 0.036 mmol) was dissolved in rigorously dry CD2CI2(0.5 mL) in an NMR tube. The sample was cooled to -80 O C (dry ice/acetone). Et3SiH (10.8 fiL, 0.072 mmol) was added via a microsyringe under Ar. The sample was shaken and then
2535
introduced into an NMR probe precooled to -80 O C . The solution gave the variable-temperature 'H NMR spectra as described in the text. T , Experiments. The variable-temperature T , measurements for the hydride resonances of 8 were carried out at 250 MHz in CD,CI, at 193-273 K by the method of ref 37. The data obtained are reported as the following: temperature (K), T I (ms, of the IrH resonance), T , (ms. of the Ir(qZ-HSiEt3)).Only one average TI can be measured above the coalescence temperature: 193, 653, 519; 203, 352, 315; 213, 267, 251; 223, 215, 212; 253, 212; 263, 326; 273, 454.
Acknowledgment. W e thank the National Science Foundation for funding, X.-L.L. thanks the State Education Commission of the People's Republic of China for a fellowship, and we thank Prof. Dennis Lichtenberger for a preprint of his work.
Intramolecular Electron Transfer from Pentacyanoferrate(I1) to Pentaamminecobalt( 111) with 3,3'-Dimethyl-4,4'-bipyridine, 4,4'-Bipyridylacetylene, 1,4-Bis(4-pyridyl)butadiyne, 2,7-Diazapyrene, and 3,tbPhenanthroline as Bridging Ligands: Adiabaticity and the Role of Distance Gyu-Hwan Lee, Leopoldo Della Ciana, and Albert Haim* Contribution f r o m the Department of Chemistry, State University of New York, Stony Brook, New York 11 794-3400. Receioed August 1, 1988 Abstract: Rate constants for intramolecular electron transfer from iron to cobalt in (NH3)5Co"1LFe"(CN)5 (L = 3,3'-dimethy]-4,4'-bipyridine, 4,4'-bipyridylacetylene, 1,4-bis(4-pyridyl)butadiyne, 2,7-diazapyrene, and 3,8-phenanthroline) have been measured at 25 OC and ionic strength 0.10 M. The values of the rate constants (in the same order as the ligands above) are 2.3 X 1.7 X 0.69 X 4.2 X and 9.3 X s-'. The activation free energies for electron transfer display an inverse dependence with respect to the Fe-Co distance in the binuclear complexes. When these free energies are corrected for the solvent reorganization energies, the resulting corrected values are independent of distance and cover the narrow range 13.5 f 0.5 kcal/mol. The metal to ligand charge-transfer bands of Fe"(CN),L3- complexes are shifted to higher energies . shifts in energy are (in the same order as the ligands above) when the remote N of L is coordinated to C O ( N H ~ ) , ~ +The 4.0, 4.3, 1.7, 8.9, and 6.9 kcal/mol. On the basis of the observed trends. it is concluded that the intramolecular electron-transfer reactions proceed in the limiting adiabatic regime.
The measurement of intramolecular rather than intermolecular electron-transfer rates offers distinct advantages for investigating the details of the mechanism of electron transfer.' Complications involved in assembling the reactants are absent, and the transition state has a relatively well-defined geometry. Unfortunately, the search for systems feasible for intramolecular electron-transfer studies is beset by many difficulties.' It is not surprising, therefore, that there is only one type of system for which a wide range of precursor complexes that undergo intramolecular electron transfer can be prepared in high concentration by simply mixing two redox reagents.* In this type of system the lability of water in Fe(CN)SOH,3- and the affinity of the Fe(CN)53-moiety for aromatic
-
nitrogen heterocycles ( N N))s4 are exploited to generate precursor Y
complexes (NC)SFeTINNCo111(NH3), in near quantitative yields U
from reaction 1.
Once formed ( k , pathway), the precursor I
Fe(CN),0H23-
Chart I
T!L7 Py = pyridine
Pz = pyrazine
BP = 4,4'-blpyridine DMBP = 3,3'- dimel hyl-4.4'- bipyrldlne N*
PHEN = 3,8-phenanthroline N a C H 2 C H 2 C N
DAP=2,7-dlazapyrene
BPEA = l , Z - b 1 ~ ( 4 - ~ y r i d y l ) e t h a n e
I
+ Co(NH3),NuN3+
BPP = 1.3- bis(4-pyridyl)propane
kd
n (NC)5Fe11NuNCo11'(NH3)5
+ H 2 0 (1)
complexes generally disappear via intramolecular electron transfer ( I ) Haim, A. f r o g . Inorg. Chem. 1983, 30, 273. ( 2 ) Haim, A. Pure Appl. Chem. 1983, 5.5, 89. (3) Toma, H. E.; Malin, J . M . Inorg. Chem. 1973, 12, 1039. (4) Toma, H. E.; Malin, J. M. Inorg. Chem. 1973, 12, 2080.
0002-7863/89/1511-2535$01.50/0
-
BPM = blsl4-pyr1dyl)methane
N >
N > c = c e N
BPE= trans- 1 . 2 - bis(4-pyrldyl)ethylene
BPA =bis(4-pyrldyl)acetylene
N >
cH
C EC
-C
EC
BPBD = 1 , 4 - b i s l 4 - p y r i d y l ) b u l a d 1 y n e
from Fe(I1) to Co(III), eq 2 (ketpathway), as well as dissociation into the reactants ( k d pathway).2
0 1989 American Chemical Society
2536 J . A m . Chem. Soc., Vol. I 1 1, No. 7, 1989 n
4
(NC),Fe"N N C O " ' ( N H ~ ) ~ (NC),Fe"'N U
Lee et nl.
n NCo"(NH,), U
(2)
In this work, t h e coupling between donor (Fe") and acceptor (Co"') sites via a series of aromatic nitrogen heterocycles was systematically investigated. Among the issues addressed, particular attention was given to the factors influencing the Occurrence and degree of nonadiabaticity, and to t h e role of distance6 in determining intramolecular electron-transfer rates. The ligands utilized in the present work are 3,3'-dimethyl-4,4'-bipyridine (DMBP), 4,4'-bipyridylacetylene (BPA), 1,4-bis(4-pyridyl)butadiyne (BPBD), 2,7-diazapyrene (DAP), and 3,8-phenanthroline ( P H E N ) . The structures of these and other bridging ligands of interest a r e depicted in Chart I.
Experimental Section Materials. BPA was prepared as described in the l i t e r a t ~ r e . ~Caution! The compound was found to cause blistering of the skin several days after contact. Protective gloves must be used in all manipulations involving this compound. DMBP was prepared according to a classical procedure.8 The previously devised synthesis9 was utilized to prepare BPBD. DAP and PHEN were prepared following established proced u r e ~ N-methylpyrazinium . ~ ~ ~ ~ perchlorate (PzCH3+C104-)was prepared by methathesis of PzCH3+I-I5 with sodium perchlorate.6 The water used in all experiments was house-distilled water passed through a Barnstead ion-exchange demineralizer and distilled in a modified, all-glass Corning Model AG-lb apparatus. The argon used to maintain anaerobic conditions was purified by passing it through a column of BTS catalyst kept at 110 O C . Lithium perchlorate (G. F. Smith) and sodium p-toluenesulfonate (NaPTS, Eastman Kodak) were recrystallized from redistilled water. All other reagents were used as received. Preparation of Complexes. (Dimethyl su1foxide)pentaamminecobalt(II1) perchlorate dihydrate was synthesized by the literature procedure.I6 Sodium amminepentacyanoferrate(I1) trihydrateI7 was purified as described previously.I8 Solutions of pentacyanoaquoferrate(I1) [(l-2) X MI were prepared by aquation of Na,[Fe(CN),NH,]3 H z 0 in deaerated, pH 8.0, 0.010 M Tris buffer. The following five pentaamminecobalt(II1) complexes with some of the nitrogen heterocycles listed in Chart I were prepared by a procedure employed previouslyI9 to synthesize related complexes. (Dimethyl sulfoxide)pentaamminecobalt(III) perchlorate dihydrate (0.5 g for BPA, DMBP, BPBD; 0.2 g for DAP, PHEN) and the desired ligand (1 g for BPA, DMBP; 0.5 g for BPBD; 0.2 g for DAP, PHEN) were added to dimethyl sulfoxide (1 mL for BPA, DMBP; 3 mL for BPBD; 2 mL for DAP, PHEN) and heated to 90-95 OC (20 min for BPA, DMBP 12 min for BPBD; 3 h for DAP; 30 min for PHEN). The resulting red-orange solutions were cooled to 0 OC and filtered if necessary (BPBD, DAP, PHEN; unreacted ligand is recovered in this step). A total of 20-30 mL of 0.10 M HCI was added, and the resulting solution was loaded on an ion-exchange column (Dowex 50W-X2, 200-400 mesh, Ht form for all complexes except BPBD; for the latter, Sephadex-SP-C25-120). The unreacted cobalt complex was eluted (2.0 M HCI for Dowex columns; 0.20 M HCI for Sephadex columns). Then, the desired product was eluted (4.0 M HC1 for Dowex columns; 0.50 M HC1 for Sephadex columns). In the case of DAP, the column was heated to -50 OC with a flexible band heater because the complex salt crystallized inside the column. The yellow product solution was evaporated to dryness at -45 "C in a rotary evaporator. The resulting solid was dissolved in the minimum amount of water, and then 72% perchloric acid was added dropwise until orange crystals formed. The mixture was kept at 0 OC for several hours and then filtered. The resulting solid was recrystallized (5) Zawacky, S. K. S.; Taube, H . J . Am. Chem. SOC.1981, 103, 3379. (6) Szecsy, A. P.; Haim, A. J . Am. Chem. SOC.1981, 103, 1679. (7) Tanner, M.;Ludi, A. Chimia 1980, 34, 23. (8) Stoehr, C.; Wagner, M. J . P r a k f . Chem. 1893, 48, 1. (9) Dellaciana, L.; Haim, A. J . Heterocycl. Chem. 1984, 21, 607. (10) Hunig, S.; Gross, J.; Lier, E. F.; Quast, H. Jusfus Liebigs Ann. Chem. 1973, 339. (11) Lier, E. F.; Hunig, S.; Quast, H. Angew. Chem. 1978, 80,799. (12) Ruggli, P.; Schetty, 0. Helv. Chim. Acta 1940, 23, 725. (13) Gill, E. W.; Bracher, A. W. J . Hererocycl. Chem. 1983, 20, 1107. (14) Schwann, T. J.; Miles, N. J. J . Heferocycl. Chem. 1982, 19, 1351. (15) Bahner, C. T.;Norton, L. L. J . Am. Chem. SOC.1950, 72, 2881. (16) Piriz MacColl, C. R.; Beyer, L. Inorg. Chem. 1973, 12, 7. (17) Brauer, G. Handbook of Preparative Inorganic Chemistry, 2nd ed.; Academic Press: New York, 1965; Vol. 2, p 1511. (18) Jwo, J. J.; Haim, A. J . A m . Chem. SOC.1976, 98, 1172. (19) Jwo, J. J.; Gaus, P. L.; Haim, A. J . Am. Chem. SOC.1979,101,6189.
Table I. MLCT Bands of Mononuclear and Binuclear Complexes of Pentacyanoferrate(l1) with Nitrogen Heterocyclesa A,* nm (IO3 X A, M-I crn-')' Fe(CN)5L3(NC)5FeJJLCo"'(NH,)5 BPA 460 (7.6) 495 (7.6) DMBP 383 (4.9) 405 (4.1) BPBD 483 (9.0) 498 (6.7) DAP 441 (5.3) 510 (4.0) PHEN 456 (6.0) 512 (4.2) "Measured in aqueous solutions with [Fe(CN)5L3-]= (1-2) X M, [L] 2 2 X M, [Co(NH,),L3+] 1X M, ionic strength 0.10 M, pH 8. bWavelength for maximum absorbance. 'Molar absorbance at maximum.
-
2-3 times from aqueous perchloric acid: yields 40%, 50%, 15%, 30%, and 20% for the complexes with BPA, DMBP, BPBD, DAP, and PHEN, respectively. Anal. Calcd for [Co(NH3),NCSH4CCC5H4NH](C104)4.2Hz0: C, 18.99; H, 3.72; N, 12.92. Found: C, 18.87; H, 3.67; N, 12.86. Anal. Calcd for ( C O ( ~ H ~ ) ~ N C ~ H ~ C ~ H ~ ~ H ] ( ~ C, 18.89; H, 4.23; N, 12.85. Found: C, 18.97; H, 4.15; N, 12.75. Anal. C, 21.47; Calcd for [CO(NH~)~NC~H~CCCCC~H~NH](CIO~)~.~H~O: H, 3.60; N, 12.52. Found: C, 21.64; H, 3.57; N, 12.48. Anal. Calcd C, 19.45; H, 3.54; for [CO(NH,)~NC,H~CHCHC~H~NH](CIO~)~~H~O: N, 13.22. Found: C, 19.46; H, 3.79; N, 13.13. Anal. Calcd for [Co(NHp)5NCSHzC4H4CSHzNH](ClO,)cH,O: C, 21.98; H, 3.42; N, 12.81. Found: C, 22.02; H, 3.37; N, 12.99. Physical Measurements. Absorption spectra were measured in a Cary 118 spectrophotometer. Kinetic measurements of slow and fast reactions were carried out on a Cary 118 or a Durrum D-110 instrument, respectively. Data handling and processing was described previously in detail.z0~21In all instances, first-order or pseudo-first-order conditions obtained and rate constants were calculated by nonlinear least-squares fitting of A , to t according to A , - A , = ( A , - A , ) exp(-kout). pH and cyclic voltammetric measurements were carried out as described earlier.22
Results Absorption Spectra. Solutions of Fe(CN),L3- complexes were prepared via reaction 3. For L = DMBP and BPA, the solubilities of the ligands in water a r e sufficiently high that reaction 3 can be driven to substantial completion. In contrast, the solubilities k
Fe(CN)SOH23-
+ L .&k 4
Fe(CN),L3-
+ H20
(3)
of DAP, PHEN, and BPBD in water are so low t h a t it is not possible to drive reaction 3 to completion. Therefore, the following procedure was adopted. T h e ligands were dissolved in 40% methanol-water, and Na3[Fe(CN),NH3].3H20 was added to the solution. After aquation of Fe(CN),NH33 to Fe(CN),0H23- and its subsequent reaction with t h e ligand, the solution was diluted with water (and filtered in the case of BPBD) and the absorption spectrum of the resulting solution (now containing
