J. CHEM. SOC. DALTON TRANS. 1990 357 1 Charge Distribution in Schiff-base Biquinone Complexes. Electrochemical, Spectroelectrochemical and Electron Spin Resonance Studies of Complexes of Co"', Ni", Zn", Cu", and Cd" with the N-(2'-Hydroxy-3',5'-di-t-butylphenyl)-4,6- di-t-butyl-o-benzoquinone Imine Ligand System in CH,Cl,t Bhaskar G. Maiya, Yuanjian Deng, and Karl M. Kadish * Department of Chemistry, University of Houston, Houston, TX 77204-564 I, U.S.A. Five different Schiff - base biquinone complexes were examined by electrochemical, spectroelectrochemical, and e.s.r. methods: [MI'( hbqi),] and [Col'l( hbqi) (hbsqi)] [M = Ni, Zn, Cu, or Cd; hbqi is the anion of N- (2'-hydroxy-3',5'-t-di-butylphenyl) -4,6-di-t-butyl-o-benzoquinone imine and hbsqi is singly reduced hbqi, N- (2'-hydroxy-3',5'-di-t-butylphenyl)-4,6-di-t-butyl-o- benzosemiquinone iminate(2-)] .Each compound was characterized by e.s.r. and u.v.-visible spectroscopy as well as by electrochemistry and spectroelectrochemistry in CH,CI, containing 0.1 mol dms N Bu",CIO,. The first one-electron oxidation and the first one-electron reduction of the complexes both occur at ligand-based orbitals. The voltammetric and e.s.r. data are self consistent and indicate that an oxidized hbqi ligand dissociates from [M"(hbqi)J after a one-electron abstraction. Singly oxidized [Colll(hbqi) (hbsqi)] does not undergo a loss of ligand and [Colll(obsq) (hbsqi)] + is electrogenerated where obsq is singly oxidized hbqi. This complex does not exhibit coupling between the two unpaired electrons. The one-electron reduction of [M1I(hbqi),] (M = Ni, Zn or Cd) produces a stable [M"(hbqi)(hbsqi)]- species in which the unpaired electron resides on the hbsqi ligand.The stable reduction product of [Colll(hbqi) (hbsqi)] is identified as [Coili( hbsqi),] -, a species in which the two unpaired electrons independently interact with the cobalt nucleus and two hydrogens. Finally, e.s.r. spectra suggest that a structural change in [Cu"(hbqi),] occurs upon going from the solid state to methylene chloride solutions at low temperature. Numerous metal-o-quinone complexes have been studied as to their spectroscopic, magnetic, and electrochemical properties.'-' Redox potentials for transition-metal o- quinone c o m p l e ~ e s , ~ - ~ ~ ~ - ~ ~ including those of hbqi the anion of N-(2'-hydroxy-3',5'-di-t-butylphenyl)-4,6-di-t-butyl- o-benzoquinone imine,2 have been published but the products generated in the one-electron oxidation or reduction of these latter derivatives have never been reported.This is dofie in the present study which presents electrochemical and spectroscopic data for five transition-metal complexes which contain either hbqi or hbsqi ligands where hbsqi is singly reduced hbqi, i.e. N- (2'- hydroxy-3',5'-di-t- butylphenyl)-4,6-di-t- but yl-o- benzosemi- quinone iminate(2 -). The investigated compounds are represented as [Co"'(hbqi)- (hbsqi)] and [M"(hbqi),] where M = Ni, Zn, Cu, or Cd. U.v.- visible and e.s.r. techniques were used to characterize the neutral complexes as well as the products of each one-electron oxidation or reduction in methylene chloride containing 0.1 mol dmP3 tetra-n-butylammonium perchlorate.In addition, e.s.r. results are presented which describe the change in electronic structure of [Cu"(hbqi),] upon going from the solid state to CH,Cl, solutions at 298 and 128 K. As will be shown in this paper, the charge distribution in each electro-oxidized or electroreduced biquinone complex will depend upon the structure of the co-ordinated ligand, the net charge on the complex, and the type of co-ordinated metal ion. Experimental Instrumentation and Methods.-U.v.-visible spectra were recorded with an IBM 9430 spectrophotometer, a Perkin-Elmer 330 spectrophotometer or a Tracor Northern 1710 holographic [M"( hbqi)d 1 [Co"'( hbqi) (hbsqi)] optical spectrometer/multichannel analyzer. Electroreduction or electro-oxidation of each complex was carried out under an inert atmosphere using Schlenk techniques, after which the compounds were transferred to an e.s.r cell which had been modified for use on a Schlenk line. E.s.r.spectra were recorded on an IBM model ER-100D system. Quantitative e.s.r. measurements were performed with a Bruker ESP-300 e.s.r. spectrometer. Spin intensities were compared before and after exhaustive electrolysis. Diphenylpicrylhydrazyl (dpph) was used as the g marker (g = 2 0036 rf: 0.0002). t Non-S.I. unit employed: G = T.3572 J. CHEM. SOC. DALTON TRANS. 1990 la A - I > A 1.2 0.8 0.4 0.0 -0.4 -0.8 Potential / V vs. s.c.e. Figure 1. Cyclic voltammograms of [M"(hbqi),] [M = Ni" (a), Zn" (b), Cu" (c), or Cd" (43 in CH,CI,, 0.1 mol dm-3 NBu",CIO, at 298 K (solid line) and 196 K (broken line) The e.s.r.spectra of several complexes were also obtained after in-situ electrolysis using a modified e.s.r./electrochemical ce11.I6 The spectra of the final products were obtained after 1-3 min electrolysis and were identical to those obtained after bulk controlled-potential electrolysis where complete radical generation required 15-20 min. In both cases, an oxidation of the singly reduced species regenerated the original complex in high yield (9&95%), as determined by u.v.-visible spectra. Cyclic voltammetry and bulk controlled-potential coulo- metry were carried out with an IBM 225 voltammetric analyzer or a PAR model 174A/175 polarographic analyzer/potentiostat which was coupled with a PAR model RE 0074 X-Y recorder.Both the working and counter electrodes were platinum. A platinum minigrid electrode was used for the thin-layer spectroelectrochemical cell. Details of the construction of this cell have been rep~rted.'~ Potentials were measured versus a saturated calomel electrode (s.c.e) which was separated from the bulk solution by means of a fritted-glass-disk junction. Potentials for thin-layer spectroelectrochemical measurements were applied with a PAR model 175 potentiostat and spectra were obtained with a Tracor Northern 1710 spectrometer multichannel analyzer which had an operating wavelength region of 30&-930 nm. All experiments were carried out at 298 2 K, unless otherwise specified. Materials-The complexes [M"(hbqi),] where M = Ni, Zn, Cu, or Cd were synthesized according to published procedures;' [Co"'(hbqi)(hbsqi)] was prepared using a method reported by Larsen and Pierpont., The final products were recrystallized from a CH,Cl,-CH30H mixture and their purity confirmed by comparison with u.v.-visible, 'H n.m.r., or e.s.r.spectra reported in the literature. 1,2 The supporting electrolyte was 0.1 mol dmP3 tetra-n-butylammonium perchlorate for both electrochemical and spectroelectrochemical experiments. This salt was pur- chased from Fluka Chemical Co., twice recrystallized from ethyl alcohol, and stored in a vacuum oven at 3 13 K. Methylene chloride was distilled over calcium hydride prior to use. Results Electrode Reactions of [M"(hbqi),] and [Co"'(hbqi)- (hbsqi)].-Each [M"(hbqi),] complex undergoes two or three reductions and two oxidations in CH2C1,, 0.1 mol dm-3 NBu",CIO,.Only the first oxidation and the first reduction were examined as to their reaction products in this study and gave cyclic voltammograms of the type shown in Figure 1. The measured peak and half-wave potentials for the illustrated reactions and those at more positive or more negative potentials are summarized in Table 1. The complexes of Ni", Zn", and Cd" all undergo a reversible one-electron diffusion-controlled reduction at room tempera- ture. These reactions occur at E - -0.64 to -0.72 V and are mV. The complex [Cu"(hbqi),] differs from the other three derivatives in that it undergoes a quasi-reversible reduction located at Eil = -0.46 V (scan rate = 0.1 V s-').However i /v* remains constant over the scan rate range of 0.05-0.5 V The complex [Ni"(hbqi),] undergoes a reversible room tem- perature oxidation at E+ = +0.83 V; [Zn"(hbqi),] undergoes an irreversible room-temperature oxidation at Ep = +0.85 V for a potential scan rate of 0.1 V s-' (solid line, Figure I), but this oxidation becomes reversible when the scan rate is increased to values larger than 0.5 V s-' or when the solution temperature is lowered to 196 K. A voltammogram under this latter condition is sh.own in Figure 1. The complexes [Cd"(hbqi)J and [Cu"(hbqi),] both undergo irreversible oxidations at room temperature. The oxidation of [Cd11(hbqi)2] occurs at E,, = + 1.04 V for a scan rate of 0.1 V s-' and the shape of the oxidation peak is given by IEp - Ep,21 = 210 mV.This process becomes quasi-reversible at 196 K (see broken line in Figure 1) and the overall data suggest the occurrence of a chemical reaction following an electron-transfer step (i.e. an ex. type mechanism).' * The oxidation of [Cu"(hbqi),] also becomes quasi-reversible at 196 K (broken line, Figure 1) and the peak-to- peak separation is 150 mV for a scan rate of 0.1 V s-'. The complex [Co"'(hbqi)(hbsqi)] undergoes a reversible one- electron room-temperature oxidation at Eil = +0.15 V and a reversible one-electron room-temperature reduction at E+ = -0.42 V. These values are both comparable to E+ values given in the literature., A second irreversible reduction is located at Ep = - 1.27 V for a scan rate of 0.1 V s-'. This reaction remains irreversible at all temperatures down to 196 K, and contrasts with data in the literature where the second reduction process is reported to be quasi-reversible at room temperature.," characterized by a constant ip/v 9 - and an IE,.- E,.I = 65 f 5 U. V.-Visible Spectra of Neutral, Oxidized, and Reduced Complexes.-The [M"(hbqi),] complexes all have strong absorption bands centred at 700-900 and 340-450 nm. The measured wavelength maxima and molar absorptivities in CH,Cl,, 0.1 mol dm-3 NBun4C104 are listed in Table 2 and are similar to values in the literature for the same complexes in either neat CH2Cl, or neat toluene.'*2 The [M"(hbqi),] derivatives all have similar u.v.-visible spectra and because of this it has been suggested that the absorptions are due to ligand- based transitions.' Figure 2 shows time-resolved u.v.-visible spectra obtained during the first one-electron oxidation of [Ni"(hbqi),] and the spectral data before and after electro-oxidation of each [M"(hbqi),] species are summarized in Table 2.The oxidations are all irreversible on the thin-layer spectroelectrochemical time- scale ( M 2 min) and the resulting spectra are therefore due to a product of the chemical reaction following electron abstraction. The reduction of [Ni"(hbqi),] is electrochemically andJ. CHEM. SOC. DALTON TRANS. 1990 3573 Table 1. Peak and half-wave potentials (V versus s.c.e.) for the room-temperature oxidation or reduction of hbqi and hbsqi complexes in CH,CI,, 0.1 rnol dm-3 NBu",ClO, (n.r. = no reaction).Oxidation Reduction A A I \ r \ Compound First (E+) Second (E,") First (E+) Second (E,") Third (E,") [Ni"( hbqi),] 0.83 1.21 - 0.72 - 1.07 - 1.70 [Zn"( h bqi),] 0.81 1.17 -0.66 - 1.02 -.167 [Cd"( h bqi),] - 1.04 (E,") n.r. - 0.64 - 0.88 (E+) - 1.65 [Cu"( hbqi),] 0.67 (E,") 1.33' - 0.46 - 0.88 n.r. [Co"'( hbqi)( hbsqi)] 0.15 n.r. - 0.42 - 1.27 n.r. " Peak potential at 0.1 V s-'. E+ at scan rate 20.5 V s-'. ' Ep at 0.4 V s-'. Quasi-reversible reaction, IEp, - Epal = 0.16 V at 0.02 V s-'. Table 2. Maximum absorbance wavelengths (Amax) and corresponding molar absorptivities (&/dm3 mol- cm-') of neutral, reduced and oxidized hbqi and hbsqi complexes in CH,Cl,, 0.1 mol dm-3 NBu",ClO,. Compound Neutral Reduced Oxidized [Ni"( h bqi),] [Zn"(h bqi),] 431 (18.1), 769 (29.9), 844 (27.9) 368 (7.8), 430 (9.8), 527 (3.8), 735 (40.1), 796 (34.9) 413 (21.6), 836 (18.7) 410 (13.9), 783 (15.6) 417 (16.3), 697 (19.7), 773 (16.3), 854 (15.9) 374 (14.6), 745 (16.7) 326 (8.0), 438 (11.4), 766 (30.3), 824 (sh), (25.0) 402 (22.4), 807 (15.0) 462 (12.9), 766 (14.6), 896 (12.4) [Cu"( hbqi),] [Cd"( hbqi),] [Co"'(hbqi)(hbsqi)] 430 (6.31, 532 (2.2), 742 (35.2), 797 (sh) (30.6) 388 (18.9), 436 (12.1), 519 (9.2), 891 (10.1) 399 (16.1), 796 (21.5) 404 (25.4), 556 (6.1) 378 (19.5) 458 (29.1), 464 (sh) (28.8), 930 (13.7) 0.0 I 300 500 700 900 h f nrn Figure 2.U.v.-visible spectral changes obtained for [Ni"(hbqi),] in CH,CI,, 0.1 mol dm-3 NBu",CIO, during the first one-electron (a) controlled-potential oxidation at + 1.00 V and (b) controlled-potential reduction at -0.86 V. The initial and final spectra are given by solid lines and the intermediate spectra by broken lines spectrally reversible and the electrogenerated product has peaks at 393 and 41 3 nm [see Figure 2(b)].Similar reversible spectral changes occur during reduction of [Zn"(hbqi),], [Cu"(hbqi),] or [Cd"(hbqi),] and the spectral data for the neutral and singly reduced complexes are summarized in Table 2. Table 2 also summarizes the spectra changes obtained after one-electron electro-oxidation or electroreduction of [Co"'- (h bqi)( h bsqi)]. Singly oxidized [Co"'( h bq i)( h bsqi)] has peaks at 458,464, and 930 nm while the singly reduced product of the original complex has peaks at 404 and 556 nm (see Figure 3). The neutral cobalt(rI1) complex has a band at 1 030 nm which remains after bulk electrolysis at -0.70 V.E.S.R. Spectroscopy-E.s.r. data for the neutral complexes of Cu" and Co"' as well the singly oxidized or singly reduced hbqi derivatives are summarized in Table 3. The reductions are reversible on both the thin-layer and bulk controlled-potential time-scales. In contrast the oxidations are irreversible and none of the initial species, except for [Co"'(hbqi)(hbsqi)], could be regenerated after reduction of the singly oxidized complex. The e.s.r. behaviour of the investigated complexes varied as a function of the metal ion and the results are represented in three groups for the purpose of discussion. These are [M"(hbqi),] (M = Zn, Cd, or Ni), [Cu"(hbqi),], and [Co"'- (hbqi)( hbsqi)]. [Zn"( hbqi),], [Cd"(hbqi),] and [Ni"( hbqi),] . The room- temperature e.s.r.spectrum of singly reduced [Zn"(hbqi),] in CH,C1, is shown in Figure 4(a) and is similar to the spectrum for singly reduced [Cd"(hbqi),] under the same solution conditions. Both complexes show a hyperfine splitting pattern consistent with a coupling of the unpaired electron to one nitrogen ( I = 1) and four hydrogens ( I = 3) (see Table 3). This pattern is invariant with changes in complex concentration over a range of 10-4-10-3 mol dm-3 as well as with changes in the modulation amplitude from 0.2 to 2.0 G. The spectrum of singly reduced [Ni"(hbqi),] has a broad signal centred at g = 2.018 and a peak-to-peak separation, AHpp, of 27 G [see Figure (4b)I. This spectrum is also invariant with changes in complex concentration or modulation amplitude. The chemical oxidation of hbqi in benzene results in an e.s.r.spectrum which consists of 29 lines with AN = 10.0 G, AH = 2.5 G (2 H), and AH = 1.2 G (2 H).19 This spectrum can be compared to a theoretical spectrum of singly oxidized hbqi [i.e. (6'-oxo-3',5'-di-t- butylcyclohexa-2',4'-dien- 1 -ylidene-3574 J. CHEM. SOC. DALTON TRANS. 1990 Table 3. Room-temperature e.s.r. parameters of hbqi and hbsqi complexes in CH,Cl,, 0.1 mol dm-3 NBu",CIO,." f- Compound [ Nil1( h bqi),] [I Zn"( h bqi),] [Cu"( h bqi) ,] [Cd"(hbqi),] [Co"'( hbqi)(hbsqi)] Neutral h -l 7 g AM AN AH 2.198', 122' 2.095 2.005 9.2 3.4 Reduced h \ g AM AN AH 2.018 2.003 7.0 1.9 2.003 2.003 6.5 2.1 2.003 14.6 14.6 Oxidized AM AN - g 2.019 2.006 2.006 2.007 2.005' 15.9 7.6 " All coupling constants are reported in Gauss.' g , , at 128 K. A,,'". g, at 128 K. ' Value for the central multiplet shown in Figure 7(b). - AH 4.1 (2 H), 2.6 (2 H) 1.51 1.0 ' 0.5 A 0 0.0 1 I 300 500 700 900 Figere 3. U.v.-visiWe spectral changes observed for [Co"'(hbqi)(hbsqi)] in CH,CI,, 0.1 rnol dm-3 NBu",CIO, during (a) one-electron controlled-potential oxidation at +0.40 V and (b) one-electron controlled-potential reduction at -0.70 V. The initial and final spectra are given by solid lines and the intermediate spectra by broken lines htnlll amino)-4,6-di- t- bu t yl-o- benzosemiquininone, o bsq] which would have 27 lines if the unpaired electron interacted with one nitrogen and two sets of two equivalent hydrogens. The spectrum obtained after bulk electro-oxidation of [Cd"(hbqi),] in CH,CI,, 0.1 mol dmP3 NBu",ClO, is shown in Figure 5 and has twenty-six resolved lines, several of which show additional unresolved splittings.Similar spectra are obtained for each oxidized [M"(hbqi),] complex. The spectrum of oxidized [M"(hbqi),] differs from both the theoretical and experiment- ally observed spectrum of oxidized hbqi and was analyzed in terms of an interaction of the unpaired electron with one nitrogen (AN z 11.8 G) and two sets of two hydrogens [AH = 3.5 (2 H) and 1.5 G (2 H)]. Figure 4. Room-temperature e.s.r. spectra obtained after controlled- potential reduction of (a) [Zn"(hbqi),] at -0.82 V and (b) [Ni"(hbqi),] at -0.84 V in CH,Cl,, 0.1 mol dm-3 NBu",ClO, - 4 G g =2.00 Figure 5. Room-temperature e.s.r.spectrum obtained after controlled- potential oxidation of [Cd"(hbqi),] at 1.20 V in CH,CI,, 0.1 mol dm-3 NBu",CIO, [Cu"(hbqi),]. The e.s.r. spectrum of magnetically diluted [Cu"(hbqi),] (5% w/w in 3,5-di-t-butylcatechol) at 298 K is shown in Figure 6(a). The value of gll (2.032) is less than g1 (2.182) and AllC" = 105 G and these results agree with values of 2.028 (gll), 2.226 (gJ, and 105 G ( A given in the literature for the same compound under similar experimental conditions. Solid samples of [Cu"(hbqi),] doped in either 3,5-di-t- butylcatechol or [Zn"(hbqi),] also have the same spectral patterns between 298 and 128 K. Millimolar solutions of [Cu"(hbqi),] in CH2Cl, at 298 K show a broad e.s.r. signal centred at g = 2.140 which is similar to a room-temperature spectrum reported in the literature.' The low-temperature spectrum of [Cu"(hbqi),] in CH,Cl, is shown in Figure 6(b) and has never been reported.Surprisingly, the value of gll (2.198) is greater than g, (2.095), which contrasts with the order of g values observed for the same species in the solid state. The e.s.r. spectrum of singly reduced [Cu"(hbqi),] hasJ. CHEM. SOC. DALTON TRANS. 1990 3575 Figure 6. E.s.r. spectra of [Cu"(hbqi),] (a) in 3,5-di-t-butylcatechol ( z 5% wjw copper complex) at 298 K and (6) in CH,CI, (= 1 mmol dm-3) at 128 K 20 G I CJ = 2.00 - (a ) Figure 7. Room-temperature e.s.r. spectra of [Co"'(hbqi)(hbsqi)] in CH,Cl,, 0.1 mol dm-3 NBu",CIO, (a) before controlled-potential oxidation or reduction, (b) after controlled-potential oxidation at +0.40 V, and (c) after controlled-potential reduction at -0.70 V.The central multiplet and the broad signals at its wings in (b) are obtained at modulation amplitudes of 0.2 (-) and 4.0 G (- - - -), respectively complex multiplets with an intensity ratio that could not be analyzed in terms of any conceivable model of hyperfine coupling. For this reason the spectrum was not analyzed with respect to a possible structure. [Co"'(hbqi)(hbsqi)]. The neutral, singly oxidized, and singly reduced [Co"'(hbqi)(hbsqi)] complexes all show room- temperature e.s.r. spectra in CH2Cl,, 0.1 mol dm-3 NBu",ClO,. These spectra are illustrated in Figure 7 and the spectral data are summarized in Table 3. The measured spin intensity almost doubles after either reduction or oxidation of the complex and this is consistent with the electro-oxidized or electroreduced product containing two unpaired electrons.An analysis of the spectra for these electrogenerated derivatives also suggests that there are two unpaired electrons in the singly oxidized and singly reduced products. The initial [Co"'(hbqi)(hbsqi)] complex has a signal centred at g = 2.005 [Figure 7(a)] and can be analyzed in terms of one unpaired electron interacting with the cobalt nucleus ( I = 5) and a hydrogen. The hyperfine coupling values for these ineteractions are A'" = 9.2 G and AH = 3.4 G, both of which are similar to values reported for the compound in toluene.2 The spectrum of singly oxidized [Co"'(hbqi)(hbsqi)] [Figure 7(b)] exhibits a central complex multiplet pattern for a modulation amplitude of 0.2 G.This results from an interaction of the unpaired electron with one nitrogen and two sets of two equivalent hydrogens. Additional broad signals are also observed at both ends of this spectrum when the modulation amplitude is set to 4.0 G [see broken line in Figure 7(6)]. These signals are equally spaced and number eight, including two 'hidden' lines inside the main intense multiplet. No additional splittings are observed for the eight broad lines, even at modulation amplitudes as low as 0.5 G. The e.s.r. spectrum of singly reduced [Co"'(hbqi)(hbsqi)] [Figure 7(c)] is centred at g = 2.003 and has peaks which are separated by 14.6 G. Additional lines are not observed when the modulation amplitude is varied over a range of 1.0-5.0 G. Thus, the observed nine-line spectrum might suggest that there is an interaction of the unpaired electrons with the cobalt nucleus and a proton, as is the case for the neutral complex.Discussion It has been suggested that the oxidation of hbqi will result in a ligand having a 'benzoquinone-like' structure with double carbon-oxygen and carbon-nitrogen bonds.2b This would lead to a lowering of the complexing ability of the oxidized ligand (i.e. obsq) (compared to that of hbqi) and dissociation from the metal after electro-oxidation of [M"(hbqi),]. The first oxidation of each [M"(hbqi),] complex is irreversible on the bulk electrolysis time-scale and the voltammetric data are consistent with the generation of a radical obsq ligand2' which then dissociates.The rate of this ligand dissociation is fast and, according to the scan-rate and temperature-dependence data from the cyclic voltammograms in Figure 1, follows the order: Cd > Cu > Zn > Ni. E.s.r. data for the oxidation product (Figure 5) suggest that the dissociated ligand is similar to, but not identical with obsq. It was not possible to isolate any of the singly oxidized complexes on the spectroelectrochemical time-scale other than the one generated from [Co"'(hbqi)(hbsqi)]. A dis- proportionation reaction has been reported to occur after electro-oxidation of iron,g vanadium,20 nickel,21 and zinc 22 catecholate or semiquinonate complexes in aprotic media but a similar reaction does not appear to occur for electro-oxidized [M"(hbqi),] in CH,C12, 0.1 mol dm-3 NBu",ClO,.Spectroelectrochemical data provide no information with respect to the metal-ligand bonding properties in [MI1- (hbqi),] and this is not surprising since the spectral bands3576 J. CHEM. SOC. DALTON TRANS. 1990 between 350 and 900 nm are due to ligand-based transitions.' Nevertheless, the decreased band intensity after electro- oxidation or electroreduction is consistent with both electron transfers involving ligand-based orbitals. The e.s.r. data for singly reduced [Zn"(hbqi),] and [Cd"(hbqi),] suggest that the unpaired electron is localized on only one of the two ligands in electrogenerated [M"- (hbqi)(hbsqi)]- 2c (where M = Zn or Cd) and further suggests that this electron interacts with one nitrogen and four hydrogens of the hbsqi radical.lg This is similar to the case of SnPh,(hb~qi),~~" where the unpaired electron interacts with the nitrogen and two sets of equivalent hydrogens.Singly reduced [Ni"(hbqi),] has a broad e.s.r. signal which may indicate the formation of a nickel(I1)-stabilized ligand radical species, as is the case for singly reduced [Ni(cbqo),] (cbqo = 4-chloro-1,2- benzoquinone 2-0ximate).,~ E.s.r. data for [Cu"(hbqi),] in the solid state indicate that the unpaired electron occupies the dz2 orbital of the Cu" and that the complex has an axially compressed tetragonal di~tortion.',~~,~' On the other hand, it should be noted that the majority of other tetragonally distorted, six-co-ordinate copper complexes have glI > g , which is consistent with an elongation or a weaker field along the tetragonal axis.26 The spectrum of [Cu"(hbqi),] in Figure 6(b) exhibits features similar to the above copper(I1) complexes and is consistent with a tetragonally distorted ligand geometry and axial elongation.The fact that gll is greater than g , suggests that the unpaired electron is situated in the dx2 - yz orbital.26 Temperature-dependent structural changes have been reported for several copper(1r) complexes containing o- quinone type ligands. For example, [Cu"(dbsq),] (dbsq = 3,5-di-t-butyl-o-benzosemiquinone) and [Cu"(bipy)(dbcat)] l3 (bipy = 2,2'-bipyridine and dbcat = 3,5-di-t-butylcatecholate) both undergo a dimer-monomer equilibrium upon going from the solid state to frozen liquid solution^.^' A change in structure of the monomeric copper(I1) complex has not been reported nor is there a cross-over of the gll and g , values for metal-quinone complexes.Thus, the observed cross-over upon going from the solid state to CH2C12 solutions of [Cu"(hbqi)J is novel in that it provides the first example in metal-quinone chemistry. The addition of one electron to [Co"'(hbqi)(hbsqi)] may occur at the metal or at the hbqi ligand.,' The observed nine- line splitting pattern in Figure 7(c) and the small coupling constant of 14.6 G both seem to suggest a ligand-based reduction to generate [Co"'(hbsqi),] -. The two hbsqi radicals in the reduced complex do not interact with each other but rather appear to interact independently with the cobalt nucleus and one hydrogen. The first one-electron oxidation of [Co"'(hbqi)(hbsqi)] has been postulated to produce [C~"'(hbqi)~]+ 2c which would be e.s.r.silent. However, as seen in Figure 7(b), the oxidized cobalt complex does have an e.s.r. spectrum and this spectrum seems to result from a sum of signals due to two species. The central part of the spectrum shows coupling of an unpaired spin to one nitrogen and to two sets of two hydrogens while the other unpaired electron shows only a cobalt hyperfine structure with a coupling constant of 15.9 G. One possible explanation for these features is that a disproportionation occurs after oxidation of [Co"'(hbqi)(hbsqi)] to give [Co"'(ob~q)(hbqi)]~ + and [Co"'(hbsqi),]- in solution. [Co"'(hbsqi),]- should exhibit e.s.r spectral features similar to those shown in Figure 7(c) and is different from what is actually obtained for singly oxidized [Co"'(hbqi)(hbsqi)] [Figure 7(b)].Furthermore, a bulk electrolysis of the neutral complex at +0.40 V gives only 1.0 k 0.1 electrons transferred. There is a fairly well defined current-time curve during reduction which indicates the absence of coupled chemical reactions. Thus, both e.s.r. and volt- ammetric data suggest that a disproportionation of the oxidized cobalt complex does not occur. All of the data are self consistent in indicating that the one- electron oxidation of [Co"'(hbqi)(hbsqi)] generates [Co"'- (obsq)(hbsqi)]+. The unpaired electron on the obsq ligand in this oxidized complex interacts with one nitrogen and two sets of two hydrogens. The unpaired electron on the hbsqi ligand which was originally coupled with the cobalt nucleus and one proton then becomes localized close to the cobalt atom in [Co"'(obsq)(hbsqi)] + after electro-oxidation. This results in an increase in the coupling constant, A'', from 9.2 G in the initial complex to 15.9 G in the oxidized species.There is an apparent lack of interaction between the two unpaired spins and this may indicate the presence of mutually orthogonal spin orbitals in the singly oxidized cobalt complex.28 As discussed earlier, this is also true for the singly reduced cobalt complex. Acknowledgements The support of the Robert A. Welch Foundation (E680) is gratefully acknowledged. We thank Professor L. Kevan for use of his e.s.r. spectrometer and Mr. Guan-Dao Lei for help with the spin-intensity measurements.References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 A. Y. Girgis and A. L. Balch, Znorg. Chem., 1975, 14,2724. (a) S. K. Larsen and C. G. Pierpont, J. Am. Chem. Soc., 1988, 110, 1827; (b) L. A. delearie, R. C. Haltiwanger, and C. G. Pierpont, Inorg. Chem., 1989, 28, 644, (c) C. L. Simpson, S. R. Boone, and C. G. Pierpont, ibid., p. 4379. C. G. Pierpont and R. M. Buchanan, Coord. Chem. Rev., 1981,38,45. W. Kaim, Coord. Chem. Rev., 1987,76, 187. M. Haga, E. S. Dodsworth, and A. B. P. Lever, Inorg. Chem., 1986,25, 447 and refs. therein. H. H. Downs, R. M. Buchanan, and C. G. Pierpont, Inorg. Chem., 1979,18,1736. R. M. Buchanan and C. G. Pierpont, J. Am. Chem. Soc., 1980, 102, 495 1. S. E. Jones, D-H. Chin, and D. T. Sawyer, Inorg.Chem., 1981,20,4257. S . E. Jones, L. E. Leon, and D. T. Sawyer, Znorg. Chem., 1982,21,3692. S. R. Cooper, Y. B. Koh, and K. N. Raymond, J. Am. Chem. Suc., 1982,104,5092. D. J. Gorden and R. F. Fenske, Inorg. Chem., 1982,21,2907. L. A. deLearie and C. G. Pierpont, J. Am. Chem. SOC., 1986,108,6393. R. M. Buchanan, C. Wilson-Blumenberg, C. Trapp, S. K. Larson, D. L. Greene, and C. G. Pierpont, Inorg. Chem., 1986,25,3070. M. E. Cass, N. R. Gorden, and C. G. Pierpont, Inorg. Chem., 1986,25, 3962. N. W. Lynch, M. Valentine, and D. N. Hendrickson, J. Am. Chem. SOC., 1982,104,6982. X . H. Mu and K. M. Kadish, Electroanalysis, 1990,2, 15. X. Q. Lin and K. M. Kadish, Anal. Chem., 1985,57,1498. R. S . Nicholson and I. Shah, Anal. Chem., 1964,36, 706. E. P. Ivakhnenko, Zh.Org. Khim., 1983,19,886. P. J. Bosserman and D. T. Sawyer, Inorg. Chem., 1982,21, 1545. C . Benelli, A. Dei, D. Gatteschi, and L. Pardi, Inorg. Chem., 1988,27, 2831. M. E. Bodini, G. Copia, R. Robinson, and D. T. Sawyer, Inorg. Chem., 1983,22, 126. (a) H. B. Stegmann and K. Scheffler, Chem. Ber., 1970,103,1279; (b) C. B. Castellani, A. Buttafava, 0. Carugo, and A. Poggi, J . Chem. SOC., Dalton Trans., 1988, 1497. B. J. Hathaway and A. A. G. Tomlinson, Coord. Chem. Rev., 1970,5,1. C . J. Ballhausen, in 'Introduction to Ligand Field Theory,' McGraw- Hill, New York, 1962, 134. B. A. Goodman and J. B. Rayner, Adu. Inorg. Chem. Radiochem., 1970, 13, 135. J. S. Thompson and J. C. Calabrese, J. Am. Chem. Soc., 1986, 108, 1903. 0. Kahn, R. Prins, J. Reedijk, and J. S. Thompson, Inorg.Chem., 1987,26,3557. Received 21st May 1990; Paper 0/02224AJ. CHEM. SOC. DALTON TRANS. 1990 357 1Charge Distribution in Schiff-base Biquinone Complexes. Electrochemical,Spectroelectrochemical and Electron Spin Resonance Studies of Complexes ofCo"', Ni", Zn", Cu", and Cd" with the N-(2'-Hydroxy-3',5'-di-t-butylphenyl)-4,6-di-t-butyl-o-benzoquinone Imine Ligand System in CH,Cl,tBhaskar G. Maiya, Yuanjian Deng, and Karl M. Kadish *Department of Chemistry, University of Houston, Houston, TX 77204-564 I, U.S.A.Five different Schiff - base biquinone complexes were examined by electrochemical,spectroelectrochemical, and e.s.r. methods: [MI'( hbqi),] and [Col'l( hbqi) (hbsqi)] [M = Ni, Zn, Cu,or Cd; hbqi is the anion of N- (2'-hydroxy-3',5'-t-di-butylphenyl) -4,6-di-t-butyl-o-benzoquinone imineand hbsqi is singly reduced hbqi, N- (2'-hydroxy-3',5'-di-t-butylphenyl)-4,6-di-t-butyl-o-benzosemiquinone iminate(2-)] .Each compound was characterized by e.s.r. and u.v.-visiblespectroscopy as well as by electrochemistry and spectroelectrochemistry in CH,CI, containing 0.1mol dms N Bu",CIO,. The first one-electron oxidation and the first one-electron reduction of thecomplexes both occur at ligand-based orbitals. The voltammetric and e.s.r. data are self consistentand indicate that an oxidized hbqi ligand dissociates from [M"(hbqi)J after a one-electronabstraction. Singly oxidized [Colll(hbqi) (hbsqi)] does not undergo a loss of ligand and[Colll(obsq) (hbsqi)] + is electrogenerated where obsq is singly oxidized hbqi.This complex doesnot exhibit coupling between the two unpaired electrons. The one-electron reduction of[M1I(hbqi),] (M = Ni, Zn or Cd) produces a stable [M"(hbqi)(hbsqi)]- species in which theunpaired electron resides on the hbsqi ligand. The stable reduction product of [Colll(hbqi) (hbsqi)]is identified as [Coili( hbsqi),] -, a species in which the two unpaired electrons independentlyinteract with the cobalt nucleus and two hydrogens. Finally, e.s.r. spectra suggest that a structuralchange in [Cu"(hbqi),] occurs upon going from the solid state to methylene chloride solutions atlow temperature.Numerous metal-o-quinone complexes have been studiedas to their spectroscopic, magnetic, and electrochemicalproperties.'-' Redox potentials for transition-metal o-quinone c o m p l e ~ e s , ~ - ~ ~ ~ - ~ ~ including those of hbqi theanion of N-(2'-hydroxy-3',5'-di-t-butylphenyl)-4,6-di-t-butyl-o-benzoquinone imine,2 have been published but the productsgenerated in the one-electron oxidation or reduction of theselatter derivatives have never been reported.This is dofie in thepresent study which presents electrochemical and spectroscopicdata for five transition-metal complexes which contain eitherhbqi or hbsqi ligands where hbsqi is singly reduced hbqi, i.e. N-(2'- hydroxy-3',5'-di-t- butylphenyl)-4,6-di-t- but yl-o- benzosemi-quinone iminate(2 -).The investigated compounds are represented as [Co"'(hbqi)-(hbsqi)] and [M"(hbqi),] where M = Ni, Zn, Cu, or Cd.U.v.-visible and e.s.r. techniques were used to characterize the neutralcomplexes as well as the products of each one-electronoxidation or reduction in methylene chloride containing 0.1 moldmP3 tetra-n-butylammonium perchlorate. In addition, e.s.r.results are presented which describe the change in electronicstructure of [Cu"(hbqi),] upon going from the solid state toCH,Cl, solutions at 298 and 128 K. As will be shown in thispaper, the charge distribution in each electro-oxidized orelectroreduced biquinone complex will depend upon thestructure of the co-ordinated ligand, the net charge on thecomplex, and the type of co-ordinated metal ion.ExperimentalInstrumentation and Methods.-U.v.-visible spectra wererecorded with an IBM 9430 spectrophotometer, a Perkin-Elmer330 spectrophotometer or a Tracor Northern 1710 holographic[M"( hbqi)d1[Co"'( hbqi) (hbsqi)]optical spectrometer/multichannel analyzer.Electroreductionor electro-oxidation of each complex was carried out under aninert atmosphere using Schlenk techniques, after which thecompounds were transferred to an e.s.r cell which had beenmodified for use on a Schlenk line. E.s.r. spectra were recordedon an IBM model ER-100D system. Quantitative e.s.r.measurements were performed with a Bruker ESP-300 e.s.r.spectrometer. Spin intensities were compared before and afterexhaustive electrolysis. Diphenylpicrylhydrazyl (dpph) wasused as the g marker (g = 2 0036 rf: 0.0002).t Non-S.I. unit employed: G = T3572 J. CHEM. SOC.DALTON TRANS. 1990laA - I >A1.2 0.8 0.4 0.0 -0.4 -0.8Potential / V vs. s.c.e.Figure 1. Cyclic voltammograms of [M"(hbqi),] [M = Ni" (a), Zn" (b),Cu" (c), or Cd" (43 in CH,CI,, 0.1 mol dm-3 NBu",CIO, at 298 K (solidline) and 196 K (broken line)The e.s.r. spectra of several complexes were also obtainedafter in-situ electrolysis using a modified e.s.r./electrochemicalce11.I6 The spectra of the final products were obtained after1-3 min electrolysis and were identical to those obtained afterbulk controlled-potential electrolysis where complete radicalgeneration required 15-20 min. In both cases, an oxidation ofthe singly reduced species regenerated the original complex inhigh yield (9&95%), as determined by u.v.-visible spectra.Cyclic voltammetry and bulk controlled-potential coulo-metry were carried out with an IBM 225 voltammetric analyzeror a PAR model 174A/175 polarographic analyzer/potentiostatwhich was coupled with a PAR model RE 0074 X-Y recorder.Both the working and counter electrodes were platinum.Aplatinum minigrid electrode was used for the thin-layerspectroelectrochemical cell. Details of the construction of thiscell have been rep~rted.'~ Potentials were measured versusa saturated calomel electrode (s.c.e) which was separated fromthe bulk solution by means of a fritted-glass-disk junction.Potentials for thin-layer spectroelectrochemical measurementswere applied with a PAR model 175 potentiostat and spectrawere obtained with a Tracor Northern 1710 spectrometermultichannel analyzer which had an operating wavelengthregion of 30&-930 nm.All experiments were carried out at298 2 K, unless otherwise specified.Materials-The complexes [M"(hbqi),] where M = Ni, Zn,Cu, or Cd were synthesized according to published procedures;'[Co"'(hbqi)(hbsqi)] was prepared using a method reported byLarsen and Pierpont., The final products were recrystallizedfrom a CH,Cl,-CH30H mixture and their purity confirmed bycomparison with u.v.-visible, 'H n.m.r., or e.s.r. spectra reportedin the literature. 1,2 The supporting electrolyte was 0.1 mol dmP3tetra-n-butylammonium perchlorate for both electrochemicaland spectroelectrochemical experiments. This salt was pur-chased from Fluka Chemical Co., twice recrystallized fromethyl alcohol, and stored in a vacuum oven at 3 13 K.Methylenechloride was distilled over calcium hydride prior to use.ResultsElectrode Reactions of [M"(hbqi),] and [Co"'(hbqi)-(hbsqi)].-Each [M"(hbqi),] complex undergoes two or threereductions and two oxidations in CH2C1,, 0.1 mol dm-3NBu",CIO,. Only the first oxidation and the first reductionwere examined as to their reaction products in this study andgave cyclic voltammograms of the type shown in Figure 1. Themeasured peak and half-wave potentials for the illustratedreactions and those at more positive or more negative potentialsare summarized in Table 1.The complexes of Ni", Zn", and Cd" all undergo a reversibleone-electron diffusion-controlled reduction at room tempera-ture.These reactions occur at E - -0.64 to -0.72 V and aremV. The complex [Cu"(hbqi),] differs from the other threederivatives in that it undergoes a quasi-reversible reductionlocated at Eil = -0.46 V (scan rate = 0.1 V s-'). However i /v*remains constant over the scan rate range of 0.05-0.5 VThe complex [Ni"(hbqi),] undergoes a reversible room tem-perature oxidation at E+ = +0.83 V; [Zn"(hbqi),] undergoesan irreversible room-temperature oxidation at Ep = +0.85 Vfor a potential scan rate of 0.1 V s-' (solid line, Figure I), but thisoxidation becomes reversible when the scan rate is increased tovalues larger than 0.5 V s-' or when the solution temperature islowered to 196 K. A voltammogram under this latter conditionis sh.own in Figure 1.The complexes [Cd"(hbqi)J and [Cu"(hbqi),] both undergoirreversible oxidations at room temperature. The oxidation of[Cd11(hbqi)2] occurs at E,, = + 1.04 V for a scan rate of 0.1 V s-'and the shape of the oxidation peak is given by IEp - Ep,21 = 210mV.This process becomes quasi-reversible at 196 K (see brokenline in Figure 1) and the overall data suggest the occurrence of achemical reaction following an electron-transfer step (i.e. an ex.type mechanism).' * The oxidation of [Cu"(hbqi),] also becomesquasi-reversible at 196 K (broken line, Figure 1) and the peak-to-peak separation is 150 mV for a scan rate of 0.1 V s-'.The complex [Co"'(hbqi)(hbsqi)] undergoes a reversible one-electron room-temperature oxidation at Eil = +0.15 V and areversible one-electron room-temperature reduction at E+ =-0.42 V.These values are both comparable to E+ values givenin the literature., A second irreversible reduction is located atEp = - 1.27 V for a scan rate of 0.1 V s-'. This reactionremains irreversible at all temperatures down to 196 K, andcontrasts with data in the literature where the second reductionprocess is reported to be quasi-reversible at room temperature.,"characterized by a constant ip/v 9 - and an IE,. - E,.I = 65 f 5U. V.-Visible Spectra of Neutral, Oxidized, and ReducedComplexes.-The [M"(hbqi),] complexes all have strongabsorption bands centred at 700-900 and 340-450 nm. Themeasured wavelength maxima and molar absorptivities inCH,Cl,, 0.1 mol dm-3 NBun4C104 are listed in Table 2 and aresimilar to values in the literature for the same complexes ineither neat CH2Cl, or neat toluene.'*2 The [M"(hbqi),]derivatives all have similar u.v.-visible spectra and because ofthis it has been suggested that the absorptions are due to ligand-based transitions.'Figure 2 shows time-resolved u.v.-visible spectra obtainedduring the first one-electron oxidation of [Ni"(hbqi),] and thespectral data before and after electro-oxidation of each[M"(hbqi),] species are summarized in Table 2.The oxidationsare all irreversible on the thin-layer spectroelectrochemical time-scale ( M 2 min) and the resulting spectra are therefore due to aproduct of the chemical reaction following electron abstraction.The reduction of [Ni"(hbqi),] is electrochemically anJ.CHEM. SOC. DALTON TRANS. 1990 3573Table 1. Peak and half-wave potentials (V versus s.c.e.) for the room-temperature oxidation or reduction of hbqi and hbsqi complexes in CH,CI,, 0.1rnol dm-3 NBu",ClO, (n.r. = no reaction).Oxidation ReductionA AI \ r \Compound First (E+) Second (E,") First (E+) Second (E,") Third (E,")[Ni"( hbqi),] 0.83 1.21 - 0.72 - 1.07 - 1.70[Zn"( h bqi),] 0.81 1.17 -0.66 - 1.02 -.167[Cd"( h bqi),] - 1.04 (E,") n.r. - 0.64 - 0.88 (E+) - 1.65[Cu"( hbqi),] 0.67 (E,") 1.33' - 0.46 - 0.88 n.r.[Co"'( hbqi)( hbsqi)] 0.15 n.r. - 0.42 - 1.27 n.r." Peak potential at 0.1 V s-'. E+ at scan rate 20.5 V s-'. ' Ep at 0.4 V s-'. Quasi-reversible reaction, IEp, - Epal = 0.16 V at 0.02 V s-'.Table 2. Maximum absorbance wavelengths (Amax) and corresponding molar absorptivities (&/dm3 mol- cm-') of neutral, reduced and oxidized hbqiand hbsqi complexes in CH,Cl,, 0.1 mol dm-3 NBu",ClO,.Compound Neutral Reduced Oxidized[Ni"( h bqi),][Zn"(h bqi),]431 (18.1), 769 (29.9), 844 (27.9)368 (7.8), 430 (9.8), 527 (3.8), 735 (40.1),796 (34.9)413 (21.6), 836 (18.7)410 (13.9), 783 (15.6)417 (16.3), 697 (19.7), 773 (16.3), 854 (15.9)374 (14.6), 745 (16.7)326 (8.0), 438 (11.4), 766 (30.3), 824 (sh),(25.0)402 (22.4), 807 (15.0) 462 (12.9), 766 (14.6), 896 (12.4) [Cu"( hbqi),][Cd"( hbqi),][Co"'(hbqi)(hbsqi)]430 (6.31, 532 (2.2), 742 (35.2), 797 (sh)(30.6)388 (18.9), 436 (12.1), 519 (9.2), 891 (10.1)399 (16.1), 796 (21.5)404 (25.4), 556 (6.1)378 (19.5)458 (29.1), 464 (sh) (28.8), 930 (13.7)0.0 I300 500 700 900h f nrnFigure 2.U.v.-visible spectral changes obtained for [Ni"(hbqi),] inCH,CI,, 0.1 mol dm-3 NBu",CIO, during the first one-electron (a)controlled-potential oxidation at + 1.00 V and (b) controlled-potentialreduction at -0.86 V. The initial and final spectra are given by solidlines and the intermediate spectra by broken linesspectrally reversible and the electrogenerated product has peaksat 393 and 41 3 nm [see Figure 2(b)]. Similar reversible spectralchanges occur during reduction of [Zn"(hbqi),], [Cu"(hbqi),]or [Cd"(hbqi),] and the spectral data for the neutral and singlyreduced complexes are summarized in Table 2.Table 2 also summarizes the spectra changes obtained afterone-electron electro-oxidation or electroreduction of [Co"'-(h bqi)( h bsqi)].Singly oxidized [Co"'( h bq i)( h bsqi)] has peaksat 458,464, and 930 nm while the singly reduced product of theoriginal complex has peaks at 404 and 556 nm (see Figure 3).The neutral cobalt(rI1) complex has a band at 1 030 nm whichremains after bulk electrolysis at -0.70 V.E.S.R. Spectroscopy-E.s.r. data for the neutral complexes ofCu" and Co"' as well the singly oxidized or singly reduced hbqiderivatives are summarized in Table 3. The reductions arereversible on both the thin-layer and bulk controlled-potentialtime-scales. In contrast the oxidations are irreversible and noneof the initial species, except for [Co"'(hbqi)(hbsqi)], could beregenerated after reduction of the singly oxidized complex.The e.s.r.behaviour of the investigated complexes varied asa function of the metal ion and the results are represented inthree groups for the purpose of discussion. These are[M"(hbqi),] (M = Zn, Cd, or Ni), [Cu"(hbqi),], and [Co"'-(hbqi)( hbsqi)].[Zn"( hbqi),], [Cd"(hbqi),] and [Ni"( hbqi),] . The room-temperature e.s.r. spectrum of singly reduced [Zn"(hbqi),] inCH,C1, is shown in Figure 4(a) and is similar to the spectrumfor singly reduced [Cd"(hbqi),] under the same solutionconditions. Both complexes show a hyperfine splitting patternconsistent with a coupling of the unpaired electron to onenitrogen ( I = 1) and four hydrogens ( I = 3) (see Table 3). Thispattern is invariant with changes in complex concentration overa range of 10-4-10-3 mol dm-3 as well as with changes in themodulation amplitude from 0.2 to 2.0 G.The spectrum of singlyreduced [Ni"(hbqi),] has a broad signal centred at g = 2.018and a peak-to-peak separation, AHpp, of 27 G [see Figure (4b)I.This spectrum is also invariant with changes in complexconcentration or modulation amplitude.The chemical oxidation of hbqi in benzene results in ane.s.r. spectrum which consists of 29 lines with AN = 10.0 G,AH = 2.5 G (2 H), and AH = 1.2 G (2 H).19 This spectrumcan be compared to a theoretical spectrum of singly oxidizedhbqi [i.e. (6'-oxo-3',5'-di-t- butylcyclohexa-2',4'-dien- 1 -ylidene3574 J. CHEM. SOC. DALTON TRANS. 1990Table 3. Room-temperature e.s.r. parameters of hbqi and hbsqi complexes in CH,Cl,, 0.1 mol dm-3 NBu",CIO,."f-Compound[ Nil1( h bqi),][I Zn"( h bqi),][Cu"( h bqi) ,][Cd"(hbqi),][Co"'( hbqi)(hbsqi)]Neutralh-l 7g AM AN AH2.198', 122'2.0952.005 9.2 3.4Reducedh\g AM AN AH2.0182.003 7.0 1.92.0032.003 6.5 2.12.003 14.6 14.6OxidizedAM AN - g2.0192.0062.0062.0072.005' 15.9 7.6" All coupling constants are reported in Gauss.' g , , at 128 K. A,,'". g, at 128 K. ' Value for the central multiplet shown in Figure 7(b).- AH4.1 (2 H),2.6 (2 H)1.511.0 '0.5A00.0 1 I300 500 700 900Figere 3. U.v.-visiWe spectral changes observed for [Co"'(hbqi)(hbsqi)]in CH,CI,, 0.1 rnol dm-3 NBu",CIO, during (a) one-electroncontrolled-potential oxidation at +0.40 V and (b) one-electroncontrolled-potential reduction at -0.70 V.The initial and final spectraare given by solid lines and the intermediate spectra by broken lineshtnlllamino)-4,6-di- t- bu t yl-o- benzosemiquininone, o bsq] whichwould have 27 lines if the unpaired electron interacted withone nitrogen and two sets of two equivalent hydrogens. Thespectrum obtained after bulk electro-oxidation of [Cd"(hbqi),]in CH,CI,, 0.1 mol dmP3 NBu",ClO, is shown in Figure 5 andhas twenty-six resolved lines, several of which show additionalunresolved splittings. Similar spectra are obtained for eachoxidized [M"(hbqi),] complex. The spectrum of oxidized[M"(hbqi),] differs from both the theoretical and experiment-ally observed spectrum of oxidized hbqi and was analyzed interms of an interaction of the unpaired electron with onenitrogen (AN z 11.8 G) and two sets of two hydrogens [AH =3.5 (2 H) and 1.5 G (2 H)].Figure 4.Room-temperature e.s.r. spectra obtained after controlled-potential reduction of (a) [Zn"(hbqi),] at -0.82 V and (b) [Ni"(hbqi),]at -0.84 V in CH,Cl,, 0.1 mol dm-3 NBu",ClO,- 4 G g =2.00Figure 5. Room-temperature e.s.r. spectrum obtained after controlled-potential oxidation of [Cd"(hbqi),] at 1.20 V in CH,CI,, 0.1 mol dm-3NBu",CIO,[Cu"(hbqi),]. The e.s.r. spectrum of magnetically diluted[Cu"(hbqi),] (5% w/w in 3,5-di-t-butylcatechol) at 298 K isshown in Figure 6(a). The value of gll (2.032) is less than g1(2.182) and AllC" = 105 G and these results agree with values of2.028 (gll), 2.226 (gJ, and 105 G ( A given in the literature forthe same compound under similar experimental conditions.Solid samples of [Cu"(hbqi),] doped in either 3,5-di-t-butylcatechol or [Zn"(hbqi),] also have the same spectralpatterns between 298 and 128 K.Millimolar solutions of [Cu"(hbqi),] in CH2Cl, at 298 Kshow a broad e.s.r.signal centred at g = 2.140 which is similarto a room-temperature spectrum reported in the literature.' Thelow-temperature spectrum of [Cu"(hbqi),] in CH,Cl, is shownin Figure 6(b) and has never been reported. Surprisingly, thevalue of gll (2.198) is greater than g, (2.095), which contrastswith the order of g values observed for the same species in thesolid state.The e.s.r. spectrum of singly reduced [Cu"(hbqi),] haJ.CHEM. SOC. DALTON TRANS. 1990 3575Figure 6. E.s.r. spectra of [Cu"(hbqi),] (a) in 3,5-di-t-butylcatechol( z 5% wjw copper complex) at 298 K and (6) in CH,CI, (= 1 mmoldm-3) at 128 K20 G I CJ = 2.00 - (a )Figure 7. Room-temperature e.s.r. spectra of [Co"'(hbqi)(hbsqi)] inCH,Cl,, 0.1 mol dm-3 NBu",CIO, (a) before controlled-potentialoxidation or reduction, (b) after controlled-potential oxidation at+0.40 V, and (c) after controlled-potential reduction at -0.70 V. Thecentral multiplet and the broad signals at its wings in (b) are obtained atmodulation amplitudes of 0.2 (-) and 4.0 G (- - - -), respectivelycomplex multiplets with an intensity ratio that could not beanalyzed in terms of any conceivable model of hyperfinecoupling.For this reason the spectrum was not analyzed withrespect to a possible structure.[Co"'(hbqi)(hbsqi)]. The neutral, singly oxidized, andsingly reduced [Co"'(hbqi)(hbsqi)] complexes all show room-temperature e.s.r. spectra in CH2Cl,, 0.1 mol dm-3 NBu",ClO,.These spectra are illustrated in Figure 7 and the spectral dataare summarized in Table 3. The measured spin intensity almostdoubles after either reduction or oxidation of the complex andthis is consistent with the electro-oxidized or electroreducedproduct containing two unpaired electrons. An analysis of thespectra for these electrogenerated derivatives also suggests thatthere are two unpaired electrons in the singly oxidized andsingly reduced products.The initial [Co"'(hbqi)(hbsqi)] complex has a signal centredat g = 2.005 [Figure 7(a)] and can be analyzed in terms of oneunpaired electron interacting with the cobalt nucleus ( I = 5)and a hydrogen.The hyperfine coupling values for theseineteractions are A'" = 9.2 G and AH = 3.4 G, both of whichare similar to values reported for the compound in toluene.2The spectrum of singly oxidized [Co"'(hbqi)(hbsqi)] [Figure7(b)] exhibits a central complex multiplet pattern for amodulation amplitude of 0.2 G. This results from an interactionof the unpaired electron with one nitrogen and two sets of twoequivalent hydrogens. Additional broad signals are alsoobserved at both ends of this spectrum when the modulationamplitude is set to 4.0 G [see broken line in Figure 7(6)].Thesesignals are equally spaced and number eight, including two'hidden' lines inside the main intense multiplet. No additionalsplittings are observed for the eight broad lines, even atmodulation amplitudes as low as 0.5 G.The e.s.r. spectrum of singly reduced [Co"'(hbqi)(hbsqi)][Figure 7(c)] is centred at g = 2.003 and has peaks which areseparated by 14.6 G. Additional lines are not observed when themodulation amplitude is varied over a range of 1.0-5.0 G.Thus, the observed nine-line spectrum might suggest that thereis an interaction of the unpaired electrons with the cobaltnucleus and a proton, as is the case for the neutral complex.DiscussionIt has been suggested that the oxidation of hbqi will result in aligand having a 'benzoquinone-like' structure with doublecarbon-oxygen and carbon-nitrogen bonds.2b This would leadto a lowering of the complexing ability of the oxidized ligand(i.e.obsq) (compared to that of hbqi) and dissociation from themetal after electro-oxidation of [M"(hbqi),]. The first oxidationof each [M"(hbqi),] complex is irreversible on the bulkelectrolysis time-scale and the voltammetric data are consistentwith the generation of a radical obsq ligand2' which thendissociates. The rate of this ligand dissociation is fast and,according to the scan-rate and temperature-dependence datafrom the cyclic voltammograms in Figure 1, follows the order:Cd > Cu > Zn > Ni. E.s.r. data for the oxidation product(Figure 5) suggest that the dissociated ligand is similar to, butnot identical with obsq.It was not possible to isolate any of the singly oxidizedcomplexes on the spectroelectrochemical time-scale otherthan the one generated from [Co"'(hbqi)(hbsqi)].A dis-proportionation reaction has been reported to occur afterelectro-oxidation of iron,g vanadium,20 nickel,21 and zinc 22catecholate or semiquinonate complexes in aprotic media but asimilar reaction does not appear to occur for electro-oxidized[M"(hbqi),] in CH,C12, 0.1 mol dm-3 NBu",ClO,.Spectroelectrochemical data provide no information withrespect to the metal-ligand bonding properties in [MI1-(hbqi),] and this is not surprising since the spectral band3576 J. CHEM. SOC. DALTON TRANS. 1990between 350 and 900 nm are due to ligand-based transitions.'Nevertheless, the decreased band intensity after electro-oxidation or electroreduction is consistent with both electrontransfers involving ligand-based orbitals.The e.s.r.data for singly reduced [Zn"(hbqi),] and[Cd"(hbqi),] suggest that the unpaired electron is localizedon only one of the two ligands in electrogenerated [M"-(hbqi)(hbsqi)]- 2c (where M = Zn or Cd) and further suggeststhat this electron interacts with one nitrogen and fourhydrogens of the hbsqi radical.lg This is similar to the case ofSnPh,(hb~qi),~~" where the unpaired electron interacts with thenitrogen and two sets of equivalent hydrogens. Singly reduced[Ni"(hbqi),] has a broad e.s.r. signal which may indicate theformation of a nickel(I1)-stabilized ligand radical species, as isthe case for singly reduced [Ni(cbqo),] (cbqo = 4-chloro-1,2-benzoquinone 2-0ximate).,~E.s.r.data for [Cu"(hbqi),] in the solid state indicate thatthe unpaired electron occupies the dz2 orbital of the Cu" andthat the complex has an axially compressed tetragonaldi~tortion.',~~,~' On the other hand, it should be noted thatthe majority of other tetragonally distorted, six-co-ordinatecopper complexes have glI > g , which is consistent with anelongation or a weaker field along the tetragonal axis.26 Thespectrum of [Cu"(hbqi),] in Figure 6(b) exhibits features similarto the above copper(I1) complexes and is consistent with atetragonally distorted ligand geometry and axial elongation.The fact that gll is greater than g , suggests that the unpairedelectron is situated in the dx2 - yz orbital.26Temperature-dependent structural changes have beenreported for several copper(1r) complexes containing o-quinone type ligands.For example, [Cu"(dbsq),] (dbsq =3,5-di-t-butyl-o-benzosemiquinone) and [Cu"(bipy)(dbcat)] l3(bipy = 2,2'-bipyridine and dbcat = 3,5-di-t-butylcatecholate)both undergo a dimer-monomer equilibrium upon goingfrom the solid state to frozen liquid solution^.^' A change instructure of the monomeric copper(I1) complex has not beenreported nor is there a cross-over of the gll and g , values formetal-quinone complexes. Thus, the observed cross-over upongoing from the solid state to CH2C12 solutions of [Cu"(hbqi)Jis novel in that it provides the first example in metal-quinonechemistry.The addition of one electron to [Co"'(hbqi)(hbsqi)] mayoccur at the metal or at the hbqi ligand.,' The observed nine-line splitting pattern in Figure 7(c) and the small couplingconstant of 14.6 G both seem to suggest a ligand-basedreduction to generate [Co"'(hbsqi),] -. The two hbsqi radicalsin the reduced complex do not interact with each other butrather appear to interact independently with the cobalt nucleusand one hydrogen.The first one-electron oxidation of [Co"'(hbqi)(hbsqi)] hasbeen postulated to produce [C~"'(hbqi)~]+ 2c which would bee.s.r.silent. However, as seen in Figure 7(b), the oxidized cobaltcomplex does have an e.s.r. spectrum and this spectrum seems toresult from a sum of signals due to two species. The central partof the spectrum shows coupling of an unpaired spin to onenitrogen and to two sets of two hydrogens while the otherunpaired electron shows only a cobalt hyperfine structure with acoupling constant of 15.9 G.One possible explanation for thesefeatures is that a disproportionation occurs after oxidationof [Co"'(hbqi)(hbsqi)] to give [Co"'(ob~q)(hbqi)]~ + and[Co"'(hbsqi),]- in solution. [Co"'(hbsqi),]- should exhibite.s.r spectral features similar to those shown in Figure 7(c) and isdifferent from what is actually obtained for singly oxidized[Co"'(hbqi)(hbsqi)] [Figure 7(b)]. Furthermore, a bulkelectrolysis of the neutral complex at +0.40 V gives only1.0 k 0.1 electrons transferred. There is a fairly well definedcurrent-time curve during reduction which indicates the absenceof coupled chemical reactions. Thus, both e.s.r.and volt-ammetric data suggest that a disproportionation of theoxidized cobalt complex does not occur.All of the data are self consistent in indicating that the one-electron oxidation of [Co"'(hbqi)(hbsqi)] generates [Co"'-(obsq)(hbsqi)]+. The unpaired electron on the obsq ligand inthis oxidized complex interacts with one nitrogen and two setsof two hydrogens. The unpaired electron on the hbsqi ligandwhich was originally coupled with the cobalt nucleus and oneproton then becomes localized close to the cobalt atom in[Co"'(obsq)(hbsqi)] + after electro-oxidation. This results in anincrease in the coupling constant, A'', from 9.2 G in the initialcomplex to 15.9 G in the oxidized species.There is an apparentlack of interaction between the two unpaired spins and this mayindicate the presence of mutually orthogonal spin orbitals in thesingly oxidized cobalt complex.28 As discussed earlier, this isalso true for the singly reduced cobalt complex.AcknowledgementsThe support of the Robert A. Welch Foundation (E680) isgratefully acknowledged. We thank Professor L. Kevan for useof his e.s.r. spectrometer and Mr. Guan-Dao Lei for help withthe spin-intensity measurements.References12345678910111213141516171819202122232425262728A. Y. Girgis and A. L. Balch, Znorg. Chem., 1975, 14,2724.(a) S. K. Larsen and C. G. Pierpont, J. Am. Chem. Soc., 1988, 110,1827; (b) L. A. delearie, R. C. Haltiwanger, and C. G. Pierpont,Inorg. Chem., 1989, 28, 644, (c) C. L. Simpson, S. R. Boone, andC. G. Pierpont, ibid., p. 4379.C. G. Pierpont and R. M. Buchanan, Coord. Chem. 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