1027
Inorg. Chem. 1982, 21, 1027-1030
Contribution from the Department of Chemistry, Washington State University, Pullman, Washington 991 64
Electrochemistry of N-Heterocyclic Complexes of Rhodium(1) and Iridium(1) W. A. FORDYCE, K. H. POOL, and G. A. CROSBY*
Received June 15, 1981 Cyclic voltammograms and EPR spectra of stable reduced species are reported for some complexes of the general formula [M(chel)(diene)]+: M = Rh(I), Ir(1); chel = 2,2'-bipyridine, 1,lO-phenanthroline, 4,7-diphenyI-l,lO-phenanthroline, 2,2'-bipyrazine; diene = 1,5-cyclooctadiene, 2,5-norbomadiene. The complexes showed two reversible one-electron reductions in acetonitrile at a Pt wire electrode. EPR spectra of the one-electron-reduced species were typically singlets (AH,, 8 G) at a g value of 2.001. The data indicate that the redox orbital is ligand T * ~ ~ ~ ~ .
Introduction
Table I. Cyclic Voltammetry Dataa
In the preceding article' we discussed the electronic spectroscopy of series of complexes of the general formula [M(~hel)(diene)]'.~ We report here on the cyclic voltammetry of these complexes in acetonitrile and the EPR spectroscopy of stable reduced species. Numerous investigators have examined the electrochemistry of complexes containing chelating N-heterocyclic ligands in nonaqueous solvents. Two recent r e v i e w ~ 4discuss ~ many of these reports. The majority of the studies have been concerned with complexes of first-row transition metals, Ru(II), and Os(I1); DeArmond and co-workers5-' have conducted an extensive study of Rh(II1) and Ir(II1) complexes. Electrochemical investigations of Rh(1) and Ir(1) complexes are less numerous. Dessy et al.12J3 have published electrochemical data for several Rh(1) and Ir(1) species with Abonded ligands. DeArmond and co-workers examined the electrochemistry of [Rh(chel),]+ (chel = bpy? phen6) and the EPR spectra of electrochemically generated [Rh(chel),lo7 during the course of their study of Rh(II1) complexes. Other complexes examined have contained primarily pho~phine'~'' and phosphite'* ligands. Recently Sofranko et al. reexamined the electrochemistry of [Rh(diphos),]+ l 9 and described the
'
(1) Fordyce, W. A.; Crosby, G. A. Inorg. Chem., preceding paper in this
(2)
(3) (4) (5) (6) (7)
(8) (9)
(IO) (11) (12)
issue. Key to abbreviations used in this paper: M = Rh(I), Ir(1); chel = en (ethylenediamine), bpy (2,2'-bipyridine), phen (1,lO-phenanthroline), 4,7-Ph2-phen (4,7-diphenyl- 1,lO-phenanthroline),bpz (2,2'-bipyrazine); diene = cod (1,5-cyclooctadiene), nbd (2,5-norhrnadiene), 1 $hex (1,Shexadiene). Budnikov, G. K.; Troepal'skaya, T. V. Usp. Khim. 1979.48, 829. Kapoor, R. C.; Kishan, J. J . Sci. Ind. Res. 1979, 38, 674. Kew, G.; DeArmond, K.; Hanck, K. J . Phys. Chem. 1974, 78, 727. Kew, G.; Hanck, K.; DeArmond, K. J: Phys. Chem. 1975, 79, 1828. Caldararu, H.; DeArmond, M. K.; Hanck, K. W.; Sahini, V. E. J . Am. Chem. Soc. 1976, 98,4455. Hanck, K.; DeArmond, K.; Kew, G.; Kahl, J.; Caldararu, H. In "Characterizationof Solutes in non-Aqueous Solvents";Mamantov, G., Ed.;Plenum Press: New York, 1977. Kahl, J. L.; Hanck, K. W.; DeArmond, K. J . Phys. Chem. 1978,82, 540. Kahl, J. L.; Hanck, K. W.; DeArmond, K. J . Phys. Chem. 1979,83, 2606. Kahl, J. L.; Hanck, K. W.; DeArmond, K. J . Phys. Chem. 1979,83, 2611. Dessy, R. E.; Stary,F. E.; King, R. B.; Waldrop, M. J. Am. Chem. Soc. 1966,88,471.
(13) Decisy, R. E.; King, R. B.; Waldrop, M. J . Am. Chem. SOC.1966, 88, 5112. (14) Olson, D. C.; Keim, W. Inorg. Chem. 1969, 8, 2028. (15) Pilloni, G.; Valcher, S.;Martelli, M. J . Electroanal. Chem. Interfacial Electrochem. 1973, 40, 63. (16) Pilloni, G.; Vecchi, E.; Martelli, M. J . Electroanal. Chem. Interfacial Electrochem. 1973, 45, 483. (17) Pilloni, G.; Martelli, M. J . Electroanal. Chem. Interfacial Electrochem. 1977 - - .-, 47 .. , 1-19- . (18) Pilloni, G.; Zotti, G.; Martelli, M. J . Electroanal. Chem. Interfacial Electrochem. 1975, 63, 424. (19) Sofranko, J. A.; Eisenberg, R.; Kampmeier, J. A. J . Am. Chem. SOC. 1979, 101, 1042.
0020-1669/82/ 1321-1027$01.25/0
complex
(peak EPCI)b
(peak EPCII)b
[Rh@py)(cod)I C10, -1.26 -1.73 [ Rh(phen)(cod)] C10, -1.27 -1.74 [Rh(4,7-Ph2 phen)(cod)]ClO, -1.23 -1.65 [Rh(bpz)(cod)]PF, + bpzC -0.75 -1.29 [Rh(bpy)(nbd)I C10, -1.28 -1.76 [ Rh(phen)(nbd)]ClO, -1.29 -1.76 [Ir(bpy)(cod) 1C10, -l.Ogd -1.55 [Ir(bpy)(nbd) 1PF, -1.10 -1.57 a In 0.1 M TEAP in acetonitrile at a Pt wire, scan rate 50-500 mV/s. Potential of cathodic peak in volts. 1:10 molar ratio of complex to free ligand. Weak adsorption peak at -0.96 V in anodic scan
electrocatalytic capabilities of [ Rh(diphos),lo 19q20 [diphos = 1,2-bis(diphenylphosphino)ethane]. Our goal was to characterize the redox orbital2' (Le., the orbital involved in the oxidation or reduction) of the complexes [M(chel)(diene)]+ and compare the results with the spectroscopic characterization of the acceptor orbital (i.e., the lowlying orbital involved in electronic excitation).' A previous report22of the electrochemistry of some of these complexes has been reinterpreted.
Experimental Section Materials. Ligand abbreviations are defined in ref 2. All complexes were prepared as described previously.' Electrolyte solutions for electrochemical measurements were prepared from acetonitrile distilled over CaH2. Reagent grade [(C2H5)4N]C104(TEAP) was twice recrystallized from water and dried in vacuo over P205. Cyclic Voltammograma Cyclic voltammograms were obtained with use of a PAR Model 174A polarographic analyzer and a PAR Model RE0074 X-Y recorder. Scan rates ranged from 50 to 500 mV/s. The PAR Model 303 three-electrode cell consisted of a Ag/AgCl in 0.1 M tetraethylammonium chloride in acetonitrile reference electrode, a platinum wire auxiliary electrode, and a platinum wire working electrode. The Pt wire working electrode was preconditioned in degassed 0.1 M TEAP in acetonitrile electrolyte solution by scanning the accessible potential range several times before addition of sample. Voltammograms of all samples (- lo-' M) were recorded in 0.1 M TEAP in acetonitrile degassed with acetonitrile-saturated nitrogen. Rather than compensate for iR drop, we compared AE, values with the AE, value of the ferrocene/ferrocenium couple under the same experimental condition^,^' namely 70 mV. All potentials reported were corrected to the aqueous SCE reference with use of the reported potential of the ferrocene/ferrocenium couple (0.307 V vs. aqueous SCE).24 (20) Sofranko, J. A.; Eisenberg, R.; Kampmeier, J. A. J . Am. Chem. Soc. 1980, 102, 1163. (21) Vlcek, A. A . Reu. Chim.Miner. 1968, 5 , 299. (22) Makrlik, E.; Hanzlik, J.; Camus, A,; Mestroni, G.; Zassinovich, G. J . Organomet. Chem. 1977, 142,95. (23) Gagne, R. R.; Koval, C. A.; Linsensky, G. C. Inorg. Chem. 1980, 19, 2855.
0 1982 American Chemical Society
1028 Inorganic Chemistry, Vola21, No. 3, 1982
0 -
Fordyce, Pool, and Crosby
--s--c\
0 -
-0.8
-1.2
v
vs.
-1.6 SCE
-2.C 0 -
Figure 1. Cyclic voltammograms of (a) [Rh(bpy)(cod)]C104and (b) [Ir(bpy)(nbd)]PF, in 0.1 M TEAP in acetonitrile; scan rate 200 mV/s.
I 1
I
I
-0.5
-2.0
V
vs.
SCE
Figure 3. Cyclic voltammograms of (a-c) 1.17
X low3M [Rh(bpz)(cod)]PF, and (d) 1.17 X lo-’ M [Rh(bpz)(cod)]PF6and 1.17 X lo-* M bpz in 0.1 M TEAP in acetonitrile; scan rate 200 mV/s.
Table 11. EPR Spectral Data
complex a --i 0 -
