540
The Journal of Physical Chemistry, Vol. 82, No. 5, 1978
J. L.
Kahl, K. W. Hanck, and K. DeArmond
Electrochemistry of Iridium-Bipyridine Complexes J. L. Kahl, Kenneth W. Hanck,” and Keith DeArmond” Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27607 (Received October 1 7 , 1977)
Cyclic voltammetry data have been obtained for the bis-, bisesquis-, and tris(2,2’-bipyridine)(bpy) complexes of iridium(II1) in acetonitrile. A reaction mechanism is proposed based on analysis of electrochemical and luminescence data. For the bisesquis and tris complexes the mechanism involves successive one electron transfers producing stable species on an electrochemical time scale. For [Ir(bpy),Clz]+the electron transfer sequence is interrupted at several steps by the elimination of a chloride. These elimination reactions are time dependent and can be resolved by the variation of the scan rate. The classification of the redox orbitals as localized or delocalized is also discussed.
Introduction The electro~hemical~-~ and 1uminescence&l0properties of a number of d6 metal ions complexed to a-type conjugated ligands have been reported. Of particular interest are the bis, bisesquis, and tris complexes of Fe(II),11-13 O S ( I I ) , ~ ~Co(III),lg J~J~ Rh(III),20~21 and Ir(III)21-25with 2,2’-bipyridine (bpy). The determination of the electron transfer mechanism of the bis, bisesquis, and tris bpy complexes of Ir(II1) in acetonitrile is the subject of this paper. The reaction of IrCl, with bpy26-29leads primarily to the bis [Ir(bpy),C12]+species and an “impurity” which can only be removed by successive recrystallizations or careful work with chromatographic columns. Extensive work with this reaction2, shows an entire sequence of products: [Ir( ~ P Y ) & ~ ( H12t ~ O )[ W ~ P Y2(H2O) ) 21 [ W P Y )2(H20)(bpy)J3+,and the corresponding hydroxides. These compounds can be cycled betwed the acid form (Ir-OH2) and the base form (Ir-OH) by addition of acid or base. The compound [Ir(bpy)2(OH)(bpy)]2+is also a product of the synthesis24of the difficult to synthesize [Ir(bpy),I3+ under controlled conditions. This monodentate bpy compound is of interest since it has been proposed as a stable reaction intermediate in ligand displacement reactions.= The synthesis of [Ir(b~y),]~+ is carried out under controlled halide free conditions in an iridium sulfatebisulfate fused salt medium. Column chromatography is employed to separate the tris complex from the approximately ten other products.24 The bis, bisesquis, and tris complexes are easily characterized by their emission spectra at 77 K in a ethanol-methanol glass. Two distinct electron transfer mechanisms are known for d6 diimine complexes: (1) a sequence involving only successive electron transfer steps-hrough at least the first two steps as for Fe(II), Ru(II), and Os(I1) complexes; (2) a sequence involving electron transfer steps coupled to rapid chemical reactions as for the Rh(II1) complexes. One of the purposes of this research was to determine into which class the complexes of Ir(II1) fall. The first electron transferred to these d6 species presumably enters the same orbital from which luminescent emission occurs. Both electrochemistry and spectroscopy can therefore be used to study the orbitals involved. Our data will be utilized to characterize the “redox” orbitals of each of the Ir(II1) bpy complexes studied as localized or delocalized. Experimental Section Reagents. The acetonitrile (AN) was Aldrich Spectrophotometric Grade further purified by a method discussed elsewhere.20 Karl Fisher titrations indicate 4 X lo-, M H20 in the AN. Voltammetric scans done with 6.0 X 0022-365417812082-0540$0 1.OO/O
M HzO added to the complexes showed no significant changes however controlled potential coulometric results were ambiguous probably due to unusual hydroxide and aquo products.23 The tetraethylammonium perchlorate (TEAP) used as the electrolyte was Eastman White Label further purified by titration with silver nitrate and four recrystallizations from deionized H,O. The complexes were synthesized using IrC13.nH20 from Engelhard Industries and bpy from Fisher Scientific Co. Synthesis. The preparation of [Ir(bpy)2C12]N03has been described in the l i t e r a t ~ r e . ~To ~ - ~the ~ reaction mixture 100 mL of H 2 0 was added, checked for neutrality, and chromatographed on a Cellex P (H+ form) ion exchange column with 0.01 M and then 0.05 M “0,. The 0.05 M HNOBfraction was flash evaporated to dryness on a Rotovac and repeatedly chromatographed in methanol on Sephadex LH-20. The large green luminescent band eluted from the column gave yellow crystals of [Ir(bpy)2C1,]N03on addition of 2-propanol and evaporation. ‘H NMR in MezSO-d6showed doublets a t 9.90 and 8.07 ppm, triplets at 9.09, 8.73, and 7.75 ppm, and a quartet at 8.38 ppm vs. TMS. 13C NMR in Me2SO-d6showed chemical shifts of 157.6, 151.0,141.4, 140.8, 128.7, and 125.1 ppm vs. TMS. Anal. Calcd for [Ir(bpy),Cl2]N0,: C, 37.68; H, 2.53; N, 10.99; C1, 11.12. Found: C, 37.53; H, 2.33; N, 10.73; C1, 10.99, [Ir(bpy),](NO,), was prepared by the method of Flynn and DemasZ4except that no attempt was made to convert to the perchlorate salt. lH NMR in Me2SO-dGshowed doublets at 9.28 and 8.14 and triplets at 8.74 and 8.08 ppm vs. TMS. 13CNMR in Me2SO-d6showed chemical shifts of 155.7,151.1,142.9,130.1,and 126.7 ppm vs. TMS. Anal. Calcd for [Ir(bpy)B](N03)3-2H20:C, 40.82; H, 3.20; N, 14.28. Found: C, 41.08; H, 3.32; N, 13.74. [Ir(bpy)z(OH)(bpy)](N03)z was the major product of the [Ir(bpy),] (NO,), synthesis when the reaction time was shortened to 3 h. Further purification was carried out as for [Ir(bpy),](NO,), except that the 0.05 M HNO, fraction from the Cellex P column contained the bisesquis complex. This fraction was repeatedly chromatographed in methanol on Sephadex LH-20. The green luminescent band eluted from the column gave light yellow crystals of [Ir(bpy),(OH)(bpy)](NO,), on addition of 2-propanol and evaporation. IH NMR in Me2SO-d6showed doublets at 9.18, 8.28, 8.17, 7.36, 7.29, and 6.83 ppm and triplets at 8.64,8.44, 7.98, and 7.72 ppm vs. TMS. 13C NMR in Me2SO-d6 showed chemical shifts of 156.9, 156.0, 154.3, 152.1, 150.2, 141.5, 139.6, 129.7, 125.8, and 122.9 ppm vs. TMS. Anal. Calcd for [Ir(bpy)2(OH)(bpy)](N03)2:C, 44.94, H, 3.14; N, 13.98. Found: C, 44.54; H, 3.14; N, 13.25. All three compounds showed their characteristic emission at 77 K in methanol-ethanol glass and in a 0.1 0 1978 American Chemical Society
The Journal of Physical Chemistry, Vol. 82, No. 5, 1978 541
Electrochemistry of Iridium-Bipyridine Complexes
T
200"R
1
0-
k-
Flgure 2. Cyclic voltammogram of [Jr(bpy),CI]+, 6.1 X 74 mVIs.
-0 6
-1 0
-1 4
-1 8
-2 2
V vs SCE Flgure 1. Cyclic voltammograms of [Ir(bpy),CI,]+, 6.1 X v = 3.00 V/s, (b) v = 0.37 VIS, (c) v = 74 mVls.
M: (a)
M TEAP-AN snow. Each compound was stored under dry N2 or in a vacuum desiccator in the dark at all times. Appuratus. Emission spectra were obtained on an Aminco-Bowman spectrophotofluorometer. Electrochemical measurements were made with a Princeton Applied Research (PAR) Model 173 potentiostat with a PAR 176 current to voltage converter with IR compensation. A triangle wave generator based on a circuit by Bull and or a PAR 175 universal programmer was interfaced to the PAR 173. Electrochemical data were recorded on a Nicolet 1090A explorer digital oscilloscope which was interfaced to a Houston Instrument 2000 omnigraphic X-Y recorder. A conventional H-type electrochemical cell was used. A series of 22-mm2geometric area platinum wire electrodes were used as working electrodes. They were cleaned in hot 15 M nitric acid, rinsed twice in deionized water, and twice in AN. After drying they were prescanned 20 times from -0.5 to -2.5 V vs. SCE in 0.1 M TEAP-AN under normal experimental conditions. Each electrode was used for one scan only and then recleaned. Controlled potential coulometric (CPC) experiments were done with platinum gauze electrodes measuring 6.6 cm2 in surface area which were cleaned as the wire electrodes above. Further experimental conditions have been described previously20except that a 3.5 M NaC1-agar salt bridge was used. All electrode potentials are in reference to the SCE. All compounds were dissolved in 0.1 M TEAP-AN. IR compensation was used whenever necessary.
Results Table I contains peak potentials as a function of scan rate for each compound and the free ligand. Reduction peaks are designated by increasing positive Roman numerals, I through VI, proceeding toward more negative potentials. No peaks were observed between +2.00 and -0.60 V. The bis, bisesquis, and tris complexes all show
M: v =
a totally irreversible oxidation (designated peak -I) at a potential more positive than +2.00 V (Table I). [ I r ( b p ~ ) ~ C / ~ ]Cyclic +. voltammograms of the bis complex a t decreasing scan rates starting a t 3.0 VJs are shown in Figure 1. CPC at -1.20 V which corresponds to a one electron reduction (n = 0.98) of the starting material yields a [Ir(bpy)&l]+species with reduction peaks at -1.39 and -1.60 V which can be seen in Figure 2. This same solution had a voltammetric peak a t approximately +1.2 V due to the oxidation of free chloride in s01ution.l~The magnitude of the peak indicates the presence of 1mol of free chloride per mole of complex. When a stepwise CPC study was done first at -1.20 V (n = 0.98) and then at -1.70 (n = 1-87),there was a large cathodic current at -2.30 V and a peak at +1.2 V indicating two free chlorides in solution. A loss of two chlorides at this stage would correspond to an [Ir(bpy)2]0species in solution. When a CPC study was done first at -1.80 V ( n = 2.10) and then at -2.20 V ( n = 0.90), a large cathodic current a t -2.30 V and a peak at +1,2 V indicating two free chlorides in solution were observed. Emission spectra a t 77 K for all CPC reduction products produced no evidence for free bpy in solution.20 Figure 3 shows the effects of variation of reversal potential for the bis complex a t 9.0 V/s. These data are characteristic of that measured for scan rates from 3.0 to 100 v/s. [Ir(bpy)2(OH)(bpy)12+. Figure 4 shows the effects of variation of the potential at which reversal occurs for the bisesquis complex at 9.0 V/s. There was no variation in the electron transfer sequence for scan rates from 74 mV/s to 100 V/s. A stepwise CPC study was done first at -1.50 V (n = 2.02), then at -2.00 V (n = l.lO), and finally at -2.30 V ( n = 0.92). No evidence for free bpy was found in the solutions at any point during the electrolysis. [ I r ( b ~ y ) ~ ]Figure ~ + . 5 shows the effects of variation of the potential at which reversal occurs for the tris complex at 9.0 V/s. There was no variation in the electron transfer sequence for scan rates from 74 mV/s to 100 V Js. When a stepwise CPC study was done first at -1.14 V ( n = 2.10), then at -1.50 V (n = 0.99), and finally at -2.05 V ( n = 1.11) no evidence for free bpy was observed in solution a t any point. The heterogeneous standard rate constant k, was calculated for each of the electron transfers studied. The method used has been outlined by N i c h ~ l s o n .The ~ ~ values obtained are given in Table 11. The kinetic data for couples I, 11, and V of the bis complex are based on fast scan rates to avoid errors due to the elimination of chlorides from the bis complex at slow scan rates.
542
The Journal of Physical Chemistry, Vol. 82, No. 5, 1978
J. L. Kahl, K. W. Hanck, and K. DeArmond
1
-.
T 1
1SOilA
0-
~
0-
I
-0.6
I
-1.0
I
-1.4
I
-1.8
I
1
I
I
-16
-2.2
V vs SCE Flgure 3. Cyclic voltammograms of [Ir(bpy),CI,]+, 9.0 Vis: (a) 1.0 X M, (b) 1.0 X M, (c) 6.1 X M.
-1.0
I
-1.4
I
1
I
-2,2
-1.8
V v s SCE Figure 5. Cyclic voltammograms of [Ir(bpy),13+, 9.0 VIS: (a) 3.0 X M, (b) 2.6 X M, (c) 4.0 X M.
CIrL2C 121"
-I
-e
T
***
200uA
1
[ I - L * C I*?*
4
dp
4
0-
c
1; 200"P
II 200uR
i
r -0.6
-1.0
-1.4
-1.8
-2.2
V vs SCE Figure 4. Cyclic voltammograms of [Ir(bpy),(OH)(bpy)12+,9.0 V/s: (a) 3.5 X M, (b) 3.5 X M, (c) 2.6 X M.
Discussion Electrochemistry. [Ir(bpy),Cl,]+. An electron transfer mechanism is proposed in Figure 6 which is consistent with the coulometric, voltammetric, and spectroscopic (no free bpy) results obtained. Wave I represents a one electron process followed by a relatively slow chemical reaction. Reversal of a fast scan
Figure 6. Proposed reaction mechanism for [Ir(bpy),C12]+, L = bpy. Roman numerals indicate peak number.
soon after ( E ~ Cshows ) ~ a corresponding oxidation peak where ipa/ip, = 1-00while at 74 mV/s the same scan yields ipa/ip, = 0.82. A scan from -1.20 to -0.60 V after CPC at -1.20 V shows no oxidation peak. Wave I1 (Figure lc)_isa composite of two one electron processes: [Ir(bpy),C1,]o
+e-
[Ir(bpy),Cl,]-
Epc = -1.34 V
[Ir(bpy),Cl]+ %[ I r ( b ~ y ) , C l ] ~ Epo=-1.39 V
(1) (2)
The Journal of Physical Chemistry, Vol. 82, No. 5, 1978 543
Electrochemistry of Iridium-Bipyridine Complexes
TABLE I: Variation of Anodic (EP) and Cathodic 0.1M TEAP-AN at 22-mmz pt wire2 v
Epc
100 20 10 5 1 0.1
100 20 10 5 1 0.1
100 20 10 5 1 0.1
Epa
b
-I t2.124 t2.097 t2.084 t2.064 +2.043 t2.006
b b b b b b
-I t2.190 t2.148 t2.090 t2.068 t2.036 t2.008
b b b b b b
-I t2.124 t2.106 t2.086 t2.064 t2.050 +2.048
b b b b b b
-I t2.574 t2.438 t2.406 t2.375 t2.354 t2.284
b b b b
b
Epc
Epa
Epc
M
Epa
I1 -1.249 -1.264 -1.272 -1.261 -1.257 -1.238
Epc
I11
-0.966 -0.972 -0.972 -0.980 -0.996 -1.002
-0.888 -0.770 -1.058 -0.856 -0.772 -1.030 -0.856 -0.772 -1.016 -0.860 -0.770 -1.012 -0.862 -0.790 -1.012 -0.861 -0.790 -1.012
-1.384 -1.390 a -1.391 a -1.461 -1.399 -1.484 -1.412 -1.631
C. [Ir(bpy),] ,+,4.0 X I1 I11 -0.938 -1.294 -1.154 -0.942 -1.212 -1.112 -0.936 -1.200 -1.102 -0.934 -1.196 -1.102 -0.943 -1.196 -1.116 -0.948 -1.197 -1.128
D. bpy, 1.3 X 100 20 10 5 1 0.1 a
I1
I -2.374 -2.252 -2.228 -2.205 -2.186 -2.164
-1.980 -2.052 -2.070 -2.082 -2.092 -2.098
Not present at this scan rate,
-2.954 -2.800 -2.784 -2.726 -2.680 -2.628
Epa
IV b b b b b b
B. [Ir(bpy),(OH)(bpy)’JZ+, 4.1 X lo-+ M I1 I11 IV -1.310 -1.146 -1.950 -1.794 -2.232 -2.048 -1.274 -1.158 -1.898 -1.810 -2.174 -2.064 -1.264 -1.158 -1.898 -1,818 -2.170 -2.062 -1.260 -1.170 -1.894 -1.816 -2.158 -2.064 -1.261 -1.168 -1.892 -1.821 -2.157 -2.050 C c -1,251 -1.169 -1.894 -1.821
C
Epc
I1 ads
a a a a
-1.418 -1.362 -1.355 -1.346 -1.350 -1.339
I
a a a
Epa
-1.047 -1.060 -1.066 -1.069 -1.070 -1.056
I -1.106 -1.084 -1.070 -1.068 -1.060 -1.059
Epa
A. [Ir(bpy)zClz]t,8.0 X
Epc
I -1.197 -1.158 -1.151 -1.149 -1.143 -1.143
(Epc) Peak Potentials as a Function of v for Cyclic Voltammograms in
V -1.740 -1.762 -1.760 -1.764 -1.770 -1.777
Epa
V -2.502 -2.305 -2.420 -2.307 -2.407 -2.307 -2.403 -2.304 C a a
a
V -2.490 -2.286 -2.424 C -2.414 c -2.408 c c a a a
M
IV -1.946 -1.862 -1.846 -1.844 -1.842 -1.838
b b b b b b
-2.067 -1.963 -1.936 -1.918 -1.894 a
Epc
-2.202 -2.126 -2.110 -2.110 -2.112 -2.112
VI -1.992 -2.488 -2.254 -2.014 -2.382 C -2.020 -2.366 C -2.024 -2.348 c -2.035 -2.348 a -2.034 -2.324 a
lo-’ M
b b b b b b
Not present at any scan rate.
Not a well-defined peak.
Ep in V vs. SCE,u in V/S,
Ep average of at least three scans, all values are f 5 mV. TABLE 11: Standard Rate Constants (ks, cm/s) for Bis, Bisesquis, and Tris Bpy-Iridium Complexes
I I1 I11 IV V VI a
[Ir(bP5%C1Zl 0.21 i 0.02 0.24 f 0.03 a b 0.15 f 0.01
-(OH)--
(bPY)lZt [WPY)313+ bPY 0.15 i 0.01 0.30 f 0.05 0.05 f 0.01 0.14 f 0.01 0.29 f 0.01 b 0.25 f 0.06 0.18 f 0.03 0.13 f 0.02 0.21 i 0.01 b 0.17 f 0.02 b Peaks poorly defined, Irreversible process.
At fast scan rates wave I1 (Figure 3) is due to reaction 1 while at slow scan rates wave I1 (Figure 2) is due to reaction 2. Wave I11 occurs only at slow scan rates (Figure 1) or after CPC at -1.20 V (Figure 2) indicating it is a one electron reduction of the monochloro [Ir(bpy)zC1]O.The adsorption spike at -1.63 V is present only at slow scan rates and is likely associated with a strongly adsorbed reactant of wave 11,32 Le., [Ir(bpy)2C1]+species. Verifying this interpretation is the fact that CPC at -1.20 V produces a black deposit on the electrode while the remaining solution (Figure 2) exhibits no adsorption spike. Wave IV is an irreversible one electron reduction of [Ir(bpy),C12]-which is present at the electrode surface only a t fast scan rates (Figure 3). The fast following chemical reaction involving the loss of one chloride forms the [Ir( b p ~ ) ~ C lspecies ]whose oxidation wave is observed in Figure 3 a t -1.39 V. If enough time is allowed for the second chloride to be lost (CPC time scale) the product would be [Ir(bpy)2]owhich may be responsible for the large cathodic current a t -2.30 V. A scan from -2.20 to -0.60
V after CPC at -2.20 V shows none of the oxidation peaks in Figure 3. Wave V involves the one electron reduction of [Ir(bpy)2C1]-(Figure 3) and cannot be due to free bpy in solution since emission spectra of CPC product does not indicate the presence of free bpy. Roffia and C i a n have ~ ~ ~done the cyclic voltammetry of [Ir(bpy),Cl,] C104in N,N-dimethylformamide at a hanging mercury drop electrode. Their work at 2.5 V/s shows a distinctly different electron transfer mechanism than the data presented here. This is perhaps due to the different solvent used in their work. [Ir(bpy),(OH)(bpy)12+.An electron transfer mechanism is proposed in Figure 7 which is consistent with the voltammetric (Figure 4), coulometric, and spectroscopic (no free bpy) results obtained. Figure 4 shows four sequential one electron reductions and corresponding oxidations of the starting material. No variation in the voltammetry is observed from 74 mV/s up to 100 V/s indicating no preceding or following chemical reactions associated with these four electron transfers. Wave V is a one electron irreversible process which only at 100 V/s has a small oxidation peak upon scan reversal. [ I r ( b ~ y ) ~ ] An ~ ’ . electron transfer mechanism is proposed in Figure 8 which is consistent with the voltammetric (Figure 5), coulometric, and spectroscopic (no free bpy) results obtained. There is no change in the voltammetry for waves I thru V from 74 mV/s up to 100 V/s. Wave VI is irreversible up to a scan rate of 100 V/s where it begins to show a small oxidation wave on potential scan reversal as for wave V of the bisesquis complex. Redox Orbital Character. V 1 ~ e has k ~discussed ~ ~ ~ the concept of a “redox orbital” in the process of correlating
544
The Journal of Physical Chemistry, Vol. 82, No. 5, 1978
C Ir i p (OH 1 M I 3 + -I A
A
/-e-
CIrLp(OH)M12+
1 I [I r i p(OH 1 M I ” I‘ I1
A A
+E!-/
+e-/
CIrL2 (OH 1 MI0
r
+e-j
IIi
E Ir L 2(Ob 1 M I ’ -
[
+e-j
IV
C I r Lp ( O H 1 M I 2 V
+e-/
C I r L 2(OH 1 M I 3 -
Flgure 7. Proposed reaction mechanism for [Ir(bpy),(OH)(bpy)] ’+, L = bipy, M = monodentate bpy. Roman numerals Indicate peak number.
***
-I
\
+e-
I
Ii
[II;.L?lI+
I‘ III c II-LJ +e-, I‘ IV CIrLsl’+e-, I‘ v +e-
C Ir L3? +e- i
VI
Flgure 0. Proposed reaction mechanism for [Ir(bpy),13+, L = bpy. Roman numerals indicate peak number.
the spectral and redox properties of coordination complexes. Generally, the redox orbital should determine the rate and the mechanism (i.e., simple transfer, chemical reaction, etc.). For example, compounds in which the redox orbitals are delocalized over a number of atoms are generally expected to exchange electrons more rapidly than those in which the redox orbital is localized on a single atom. However, heterogenous rates17 measured for a number of diimine complexes indicate that the situation may not always be so simple when comparing predominantly metal T type with predominant ligand a type orbitals. Moreover, systems in which the redox orbital is substantially delocalized (i.e., 7 electron systems) are expected to give redox reactions which proceed with retention of structure while those in which the redox orbital is localized are more likely to undergo chemical reaction.
J. L. Kahl,
K. W.
Hanck, and
K. DeArmond
Such ideas have been utilized to understand and correlate electrochemistry, and spectroscopy observed for d6 diimjnes20i22of Rh(II1) and Ru(II).14 The d6 complexes of the second and third row transition metal ions all give a photoluminescence. From these data, the lowest empty orbital2I of the emitting species can be designated as a “localized orbital” (metal d type) or “delocalized orbital” (a type having ligand character and id some cases metal d a character). In general, this lowest empty orbital of spectroscopy should be identical with the redox orbital utilized in the first reduction step of the d6 complex. The spectroscopic data20i23,24 indicate that for all three Ir complexes, the redox orbital should be delocalized. Consistent with these conclusions, the reduction of all three d6 species produces a species stable on the voltammetric scale. Moreover the character of the successive reductions of the tris (five additional steps) and the bisesquis complexes (four additional steps) imply37that these processes also involve delocalized redox orbitals. The magnitude of the rate constants for these couples (Table 11) are all comparable and in agreement with those reported for the analogous couples of the diimine complexes of Rh(1II)y Ru(II),17and Os(II).l’ Thus the kinetic data corroborates the characterization of these redox orbitals as delocalized. The chemical reaction for the bis complex involving loss of C1- may according to the discussion above imply that a redox orbital having localized character (metal d a type) lies close in energy to the lowest redox orbital. Such a result is consistent with the photochemical loss of Cl- found by Balzani and c o - ~ o r k e r sfor ~ ~ the analogous [Ir(phen)&l2]+ compounds. For this material, the luminescence and the photochemistry are found to originate from different states suggesting that distinct but energetically close orbitals may be associated with these two processes. Close examination of the spacing of the redox potentials for the compounds provides further insight into the character of the redox orbitals. For example, the pattern (Table I) of the redox potentials for the tris indicates two triads, that of the bisesquis, a pair and then a triad, while the bis complex (fast sweep) shows a pair and then a single potential within the experimental potential range. In all three cases, the spacing between the two sets of potentials is -0.6 V while the spacing within the first grouping is smaller (-0.18) than that within the second grouping (-0.24). The similarity of the spacings within the first grouping and within the second grouping may indicate that the redox orbitals of the three complexes involve orbitals having a comparable degree of delocalization. Likewise, the similarity of the spacing between the groupings implies that the fourth (tris), third (bisesquis), and third (bis) electrons each involve an analogous interaction. The larger magnitude of this spacing (-0.6 eV) may indicate that the interaction corresponds to an electron pairing. Such similarities likely could not be sustained for the distinct orbital splitting patterns associated with C2 bis and D3 tris compounds. Therefore, the suggestion that the redox orbitals are confined to a single ring delocalized orbital rather than a multi-ring delocalized orbital has some appeal. Such a scheme has been utilized to r a t i o n a l i ~ e ~ ~ ~ ~ ~ the electrochemistry and the luminescence of [Rh(bpy)3l3+, [Rh (phen)3]3+, [Rh(phen) 2( bpy)]3+, and [Rh(phen)( b p ~ ) ~ ] ~Multiple ’. emission lifetimes and time resolved spectra have been measured for mixed bis diimine complexes of Ir(III), i.e., [IrLL’C12]+.39J”Such data, by analogy to that for the mixed ligand Rh(III), imply that the redox
Dielectric Relaxation in Dimethyl Carbonate
The Journal of Physical Chemistry, Vol. 82, No. 5, 1978 545
orbitals for the bis Ir(II1) complexes are likely single ring delocalized orbitals, hence a similar situation for the other (tris, bisesquis) Ir(II1) complexes would not be surprising.
T. Sali and S. Aoyagui, J . flectroanal. Chem., 58, 401 (1975). N. Tanaka and Y. Sato, flectrochim. Acta, 13, 335 (1968). N. Tokel-Takvotyan, R. E. Hemingway, and A. J. Bard, J. Am. Chem. Soc., 95, 6582 (1973). S. Roffia and M. Ciano, J. flectroanal. Chem., 77, 349 (1977). J. Van Houten and R. J. Watts, J. Am. Chem. Soc., 98,4853(1976). T. Saji and S. Aoyagul, J . flectroanal. Chem., 63, 31 (1975). F. Zuloaga and M. Kasha, Photochem. Photobiol., 7,549 (1966). N. Tanaka and Y. Sato, Bull. Chem. Soc., Jpn., 41, 2059 (1968). G. Kew, K. DeArmond, and K. Hanck, J. phys. Chem.,78,727(1974). K. DeArmond and J. Hillis, J . Chem. Phys., 54, 2247 (1971). K. Hanck, K. DeArmond, G. Kew, J. Kahl, and H. Caldararu in "Characterization of Solutes in Non-Aqueous Solvents", Plenum Press, New York, N.Y., 1977. R. Watts, J. Harrington, and J. Van Houten, J . Am.'Chem. Soc.,
Summary The redox sequence for the Ir(II1) complexes are similar to those sequences found for the analogous Fe(II), Ru(II), and Os(II1) complexes rather than those found for the tris and bis Rh(II1) complexes. Such a sequence is consistent with the delocalized redox orbital character postulated for these d6 starting materials. Ultimately, ESR s t ~ d y ~ofl v ~ ~ the stable reduced species may be useful in determining the relative amount of metal and ligand character for the reduced species and in describing the interaction between the ligands within a complex.
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Acknowledgment. This research was supported by the National Science Foundation (CF 40894 and CHE7605716).
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Microwave Dielectric Relaxation of Some Lithium Salts in Dimethyl Carbonate D. Saar,+ J. Brauner, H. Farber, and S. Petruccl" Department of Nectrlcal Engineering and Chemistry, Polytechnic Institute of New York, Brooklyn and Farmingdale Campuses, Brooklyn, New York 11201 (Received August 30, 1977)
Complex permittivities (real and imaginary parts) of the solvent dimethyl carbonate (DMC) and of solutions of LiBr, LiSCN, and LiC104 in DMC at 25 "C, in the frequency range 0.45-67 GHz (wavelength 67-0.45 cm) are reported. The solvent shows a relaxation process which appears to be of the Debye type, within experimental error, with a relaxation frequency of 22 GHz. The electrolyte solutions show an additional relaxation process also of the Debye type, within experimental error, with relaxation frequencies in the range 1-1.5 GHz. Infrared spectra for LiSCN solutions show this salt to be completely associated to contact ion pairs or higher aggregates previously interpreted as ion-pair dimers. The correlation between the quantity (to - c m ) / p 2(using literature values for the dipole moments of the ion pairs) and the calculated concentration of the pairs, C,, is linear. It is concluded that, at the concentration investigated, the additional relaxation process observed in the electrolyte solutions is mainly due to rotational relaxation of contact ion-pair dipoles.
Introduction Lithium salts show remarkable solubility in relatively polar media of quite low permittivity. These systems are relevant to the development of energy sources such as batteries using nonaqueous solvents, and alkali metals as electrodes. Unfortunately, our knowledge of the structures This work is part of the thesis of D. Saar, in partial fulfillment for the requirements of Doctor in Philosophy (Chemistry),Polytechnic Institute of New York. 0022-3654/78/2082-0545$0 1 .OO/O
and dynamics of electrolytes in these media is quite limited, the most common misconception being that free ions are the preponderant species of these media, or that (at the other extreme) these systems may be treated as solvated fused ionic salts. Recently, Chabanel et a1.l have reported infrared spectra of LiSCN in the solvent dimethyl carbonate. Later2 dielectric constant measurements of the same system, together with other Li+ salts in DMC have been published. These data have been interpreted as indicating that the salts exist as contact ion pairs and 0 1978 American Chemical Society
