Organometallics 1996, 14, 3418-3422
3418
Synthesis and Characterization of Cationic Square-Planar Iridium(1) Complexes Containing Chiral Diphosphines Franco Morandini" and Giuseppe Pilloni Dipartimento di Chimica Inorganica, Metallorganica ed Analitica and Centro CNR, Via F. Marzolo 1, I-35131 Padova, Italy
Giambattista Consiglio Technisch-Chemisches Laboratorium, E. T. H. Zentrum, Universitatstrasse 6, CH-8092 Zurich, Switzerland
Antonio Mezzetti Dipartimento di Scienze e Tecnologie Chimiche, Via del Cotonificio 108, I-33100 Udine, Italy Received February 1, 1 9 9 P Cationic square-planar complexes of the type [Ir(diphosphine)21Cl (diphosphine = (R$)and rac-cypenphos, (S,S)-chiraphos, (R)and rac-prophos, (R)-phenphos)have been synthesized starting from [Ir(COE)2C112 or [Ir(C2H4)2C1]2. The complexes were fully characterized through lH- and 31P{1H}-NMRspectroscopy. With the C1 diphosphines both cis and trans isomers are formed with low selectivity. Using racemic cypenphos no diastereoselectivity in the formation of the complexes was observed. For the racemic prophos the diastereoselectivity is different for the two geometrical isomers formed. A study of the electrochemical behavior of these complexes in acetonitrile with some attention to the characteristics of the electron transfer process has been carried out. The results are compared with those previously obtained in the reduction of the corresponding dppe derivative and provide a tool for the determination of the relative basicity of the ligands. The reduction was found to proceed by a single two-electron step to a d10 anion, which is quenched by the protic solvent to give the final metal hydride derivative which in the case of complexes 2 and 3 were observed by NMR spectroscopy.
Introduction Electron-richtransition metal complexes are believed to be efficient agents for stereospecific reactions of organic substrates both from a stoichiometric and a catalytic point of view.1-5 In particular complexes of the type [Ir(diphosphine)zlCl(in which the diphosphine is chiral) can be of interest as catalyst precursors for enantioselective hydrogenation.6 During attempts t o prepare compounds of the type (q5-CgH7)Ir(diphosphine) to be compared with the previously reported analogous rhodium complexes7 via displacement of the diolefin from (v5-CgH7)Ir(COD)(COD = 1,5-cyclooctadiene)or (q5-CgH7)Ir(COE)2(COE = cyclooctene) with chiral diphosphines (Chart l), we observed formation of cationic complexes of the type [Ir(diphosphine)zl+as the ~~
*Abstract published in Advance ACS Abstracts, June 1, 1995. (1)Werner, H.Angew. Chem., Int. Ed. Engl. 1963,22,927. ( 2 )For example, compare: (a) Davies, S. G.; McNally, J. P.; Smallridge, A. J. Adv. Organomet. Chem. 1990,30, 1. (b) Davies, S. G . Pure Appl. Chem. 1988,60,13.( c ) Dalton, D. M.; Fernandez, J. M.; Emerson, K, Larsen, R. D.; Arif, A. M.; Gladysz, J. A. J . Am. Chem. SOC.1990,112,9198. (3)Consiglio, G.;Morandini, F. Chem. Rev. 1987,87,761. (4)Morandini, F.;Consiglio, G.; Sironi, A,; Moret, M. J . Organomet. Chem. 1989,370,305. (5)Morandini, F.; Consiglio, G. J . Organomet. Chem. 1990,390,C64. (6)(a)Chang, Y.N. C.; Meyer, D.; Osborn, J. A. J.Chem. Soc., Chem. Commun. 1990, 869. (b) Mashima, K.; Akutagawa, T.; Zhang, X.; Takaya, H.; Taketomi, T.; Kumobayashi, H.;Akutagawa, S. J. Organomet. Chem. 1992,428,213. (b) Zhang, X.;Taketomi, T.; Yoshizumi, T.; Kumobayashi, H.; Akutagawa, T.; Mashima, K.; Takaya, H.J.Am. Chem. SOC.1993,115, 3318. (7)Morandini, F.;Pilloni, G . ; Consiglio, G . ; Sironi, A,; Moret, M. Organometallics 1993,12,3495.
major component. These complexes are also of interest because of diastereoselectivity phenomena during complexation and of their relationship t o possible nonlinear effects in catalysis.8 In the present work we report on the synthesis, the multinuclear NMR characterization, and the electrochemical behavior of iridium(1)complexes of formula [Ir(diphosphine)2]Clobtained by reacting the dimers [Ir(COE)2C112 or [Ir(C2H&C112 with various chiral diphosphines.
Results and Discussion Preparation and Spectroscopic Properties. In contrast to the similar reaction with triphenylphosphine, by reacting the complex (v5-CgH,)Ir(COD)with the diphosphines of Chart 1,mixtures of compounds are (8)Guillaneux, D.; Zhao, S.-H.; Samuel, 0.; Rainford, D.; Kagan,
H.B.J . Am. Chem. SOC.1994,116,9430.
0276-7333/95/2314-341~~09.00/0 0 1995 American Chemical Society
Cationic Square-Planar Iridium(I) Complexes
Organometallics, Vol. 14, No. 7,1995 3419
Chart 2
Isomer A
Isomer B
obtained in which it is possible to recognize by NMR analysis complexes of the type [Iridiphosphine)zl+as the major component. The reaction seems to occur with displacement of the CgH7- leading to formation of stable planar cationic complexes of the type [Ir(diphosphine)~]+.In fact, we were unable to ascertain the previously observed formation of compounds corresponding to the migration of the cyclopentadienyl ligand to the cyclooctadiene ligand.1° The complexes [Ir(diphosphine)zlCl can be prepared in a pure form by reacting dimeric [Ir(COE)zClIz in toluene at room temperature with an excess of the appropriate diphosphine. Better results are obtained when the cyclooctene ligand is first substituted with ethylene a t 0 "C in diethyl ether solution, followed by the addition of the appropriate diphosphine. In the latter case the yields of the reactions are quite high, normally close to 90%. The orange microcrystalline compounds behave as 1:l electrolytes of the type [Ir(diphosphine)zlCl in methanol solution.'l The 31P{lH}-NMR spectra of the complexes containing the optically pure CZ chiral diphosphines (S,S)-chiraphos, a, (Chart 1) and (R,R)-cypenphos, b, contain a single signal for the four equivalent P atoms, as expected for a square-planar iridium(1) complex. The chemical shift value (6 50.03) found for the singlet of [Ir{(S,S)-chiraphos}~lCl,1, is close to that of [Ir(dppe)zlC1, 2,12(6 50.28). Analogously, [Ir{(R,R)-cypenphos}~1C1, 3, exhibits only a singlet centered at 6 22.79. The 31P{1H}-NMR spectrum of the compound prepared starting with racemic cypenphos shows two singlets with the same intensity at 6 22.79 and 24.71. The two species formed correspond to the two possible diastereomers [Ir{(R*,R*)-cypenphos}zlCland [Ir{(R,S)cypenphos}~lCl,respectively, in which two homo- or heterochiral diphosphine molecules are coordinated to the metal. The equal intensity of the two lines shows that there is no diastereoselectivity in the formation of the bis(diph0sphine) complex. The complexes containing C1 chiral phosphines give rise to more complicated 31P{lH}-NMR spectra since the two P atoms of the diphosphine are inequivalent, and the two geometrical isomers A and B are formed (Chart 2). Thus, the 31P{lH}-NMR spectrum of the prophos c derivative, [Ir{(R)-prophos}~lC1,4, shows two sets of A A W patterns that are characterized by the different magnitude of the J(P,P) coupling constants. The experimental spectrum together with the simulated spectra for both isomers A and B are reported in Figure 1. The 31P{1H}-NMR spectrum of the trans isomer A shows only cis-J(P,P') coupling constants in the range (9)Marder, T. B.; Williams, I. D. J . Chem. Soc., Chem. Commun. 1987, 1478. (10) Merola, J. S.; Kacmarcik, R. T. Organometallics 1989, 8 , 778. (11)Geary, W. J. Coord. Chem. Rev. 1971, 7,81. (12) Vaska, L.; Catone, D. L. J. Am. Chem. Soc. 1966, 88, 5324.
I
'
PPm
"
'
1
"
50
'
'
l
"
'
45
'
I
"
'
'
40
1
'
~
'
35
Figure 1. 31P{ lH}-NMR spectrum of [Ir((R)-prophos}2]Cl, 4: (a) simulated spectrum of isomer A,(b) experimental spectrum of the mixture of isomers A and B (CD2C12,room temperature); (c) simulated spectrum of isomer B. 20-25 Hz, in agreement with the presence of two pairs of chemically equivalent trans P atoms. The resonances at lower field are due to the phosphorus atoms closer to the methyl group of the backbone.13J4 In isomer B the two chemically inequivalent P atoms occupy trans positions, as indicated by the large J(P,P') coupling constant of 271.6 Hz. The two geometrical isomers are present in a A B molar ratio of 59:41. This ratio was also confirmed by integration of the signals due to the methyl protons of the prophos ligands centered at 6 0.86 and 0.79 in the 'H-NMR spectrum. When racemic prophos is used, four different isomers form, which correspond to the diastereomers (containing homo- or heterochiral ligands) of each geometrical isomer. The diastereomers containing the homochiral ligands are obviously formed as pairs of enantiomers ((R,R)and (S,S),respectively). The cis:trans molar ratio is 54:46 for the homochiral diastereomer (i.e., for the sum of the (RP)and (8,s) compounds) and is 4555 for the heterochiral diastereomer. On the other hand, the diastereomeric ratio { (R,S):[(S,S) (R,R)I} is different for the cis and the trans isomers, being larger for the cis isomer (62:38)than for the trans one (54:46). In both cases the diastereomer containing two heterochiral ligand molecules prevails.
+
(13) Morandini, F.; Consiglio, G . ;Piccolo, 0.Znorg. Chin. Acta 1982, 57, 15. (14) Brunner, H;Stumpf, A. J . Organomet. Chem. 1993,459, 139.
3420 Organometallics, Vol. 14, No. 7, 1995
Morandini et al.
The complex containing the phenphos d analogue, [Ir{ (R)-phenphos}zICl,5, displays a similar 31P{'H}-NMR
pattern and isomeric composition as observed for the (R)-prophos derivative 4. The spectrum consists also systems for the in this case of two superimposed A.4" two isomers A and B with an A:B ratio of 56:44 as determined by integration of the 31P{lH)-NMR spectrum. The subspectrum of the trans isomer appears as two pseudotriplets since the four cis-2Jpp coupling constants have fortuitously the same value. We attempted to change the isomeric distribution obtained for the complexes [Ir{(R)-prophos}~lC1and [Ir{(R)-phenphos}~lC1by means of electrocatalytic cyclovoltammetry and/or controlled-potential coulometry experiments in acetonitrile-tetraethylammonium perchlorate (0.2 M) solution. These tests stemmed from the observation that the electrogenerated tetrahedral anion [Ir(prophos)zl- turned out long-lived enough to be potentially capable of triggering isomerization of the depolarizer (t0.5 = 3 s, vide infra). However, if the catholyte is dried under vacuum and the residue dissolved in deuterated dichloromethane,the 31P{ 'H}-NMR spectrum shows no change in the isomeric composition both for the prophos and the phenphos derivatives. Electrochemistry. The electrochemical reduction of square-planar bis(diphosphine) complexes of rhodium and iridium has been thoroughly studied by a number of research groups during the past 20 years.15-18 One of us first proposed and subsequently confirmedla that the electrochemical reduction of these complexes in acetonitrile proceeds via a single two-electron step t o the anionic intermediate [M(dppe)zl- quenched by proton abstraction from the medium to give the final hydride derivative HM(dppe)z. Other g r o ~ p s envis~~J~ aged the reduction t o proceed via a one-electron transfer, generating the zero-valent compound followed by H atom abstraction from the solvent t o give the metal hydride and subsequent reduction of the solvent radical. More recent work of Eisenberg et al.,19 employing a purely chemical approach to the reduction of [Rh(dppe)zl+, concluded that the two-electron-reduction product does undergo reaction with protic solvents by acid-base chemistry, while H atom sources are not at all involved in the metal hydride formation. As expected, the electrochemical behavior of the complexes [Ir{(S,S)-chiraphos}~l+,1,[Ir{(R)-prophos}zl+, 4, and [Ir{(R)-phenphos}~]+, 5, is quite similar to that observed for the dppe analogue 2 under identical experimental c~nditions.'~J~ Thus, the cyclic voltammograms always exhibit a single reduction peak with the directly associated oxidation response in the reverse scan, which is coupled to an irreversible chemical reaction (Figures 2-4). The occurrence of two-electron tranfers has been verified chronoamperometrically by comparison of the diffusion currents with 2 and confirmed by controlled potential coulometries. That a concerted two-electron reaction, i.e., an EE mechanism (15)Pilloni, G.; Vecchi, E.; Martelli, M. J . Electroanal. Chem. 1973, 45, 483.
(16)Teo, B. K.; Ginsberg, A. P.; Calabrese, J. C. J. Am. Chem. SOC. 1976,98,3027. (17) Sofranko, J.A.;Eisenberg, R.; Kampmaier, J. A. J . Am. Chem. SOC.1979,101, 1042. (18)Pilloni, G.; Zotti, G.; Martelli, M. Inorg. Chem. 1982,21, 1283. (19) Kunin, A. J.; Nanni, E. J.; Eisenberg, R. Inorg. Chem. 1985, 24, 1852.
Figure 2. Cyclic voltammogram of 1.92 mM [Ir{(S,S)chiraphos}&l, 1,in CH&N, 0.2 M TEAP,at 25 "C and at a scan rate of 100 mV s-l.
A
W
Figure 3. Cyclic voltammogram of 1.92 mM [Ir(dppe)& C1,2, in CHsCN, 0.2 M TEAP,at 25 "C and at a scan rate of 100 mV s-l. with Ez" very anodic of does occur in complex 2 as well as in its rhodium analogue has been already proved unambiguously.la (20) Myers, R. L.;Shain, I. Anal. Chem. 1969, 4 1 , 980.
Cationic Square-Planar Iridium(I) Complexes
V
Figure 4. Cyclic voltammogram of 2.70 mM [Ir{(R)phenphos}21Cl, 5, in CH&N, 0.2 M TEAP,at 25 "C and at a scan rate of 100 mV s-l. However, it is of interest to see how the modification in the backbone of the diphosphine affects the IrWlr(-I) redox behavior. The redox couples are centered at E112= -2.110,-2.135,-2.155,-2.170,and -2.225 V (as mean values of the potentials for cathodic and anodic peak currents) for 2, 5, 4, 3, and 1, respectively. The shifting of El12 values to more negative potentials on going from R = H (dppe) to Ph (phenphos),Me (prophos), cypenphos, and finally to chiraphos (which has two methyl groups on the backbone) is in accordance with the relative electron-donating capacities of these substituents. Thus, the chiraphos ligand is expected to be the most electron-donating of the three ligands and [Ir{(S,S)-chiraphos)zl+, 1, is the most difficult of the five complexes to reduce. Moreover, an inspection of the shape of the voltammetric responses (see Figures 2-4) clearly indicates that the electron transfer is essentially reversible for 2 (peak width, E,,z - E, = 30 mV, and peak to peak separation, AE, = 38 mV, at the scan rate used), while the wave is quasireversible for 1,4, and 3 (Ep/2- E, = 40 mV; AE, = 60 mV) and approaches the irreversible behavior in the case of 5 (Ep/2- E, = 65 mV; AE, = 130 mV).21In line with these observations, the shape and the position of the waves displayed by complexes 1, 3,4, and 5 are markedly dependent upon the cyclic scan rate over the experimental range used (20-1000 mV s-l), while they are virtually independent for 2. The slowed down rate of heterogeneous electron transfer in depolarizers 1, 3, 4, and 5, compared with that of 2, apparently reflects the relative greater conformational rigidity arising from the steric demand of the substituents in the backbone. Competing with and/or alternative to the multielectron tranfer mechanism could be the disproportionation (21)Rayan, M. D. J. Electrochem. SOC.1978, 125, 547.
Organometallics, Vol. 14, No. 7, 1995 3421 mechanism.21 A detailed report by Eisenberg et al.19 describing the chemical reduction of [Rh(dppehl+through variation of solvent and temperature conditions established that the intermediate one-electron reduction product, Rh(dppelzo,is in equilibrium with the disproportionated [Rh(dppehl-/Rh(dppe)zI+ion pair. The ionic species appear to predominate by far in polar solutions and/or at low temperatures, while the radical species is favored in nonpolar solvents at room temperature. Since the shape and position of the voltammograms were found to be independent of the depolarizer concentration, the irreversible disproportionation reaction is not an important pathway under the conditions used. Consistently, this result could reveal that the first electron transfer is the slow step in the electrochemical reduction of complex 5, so that the zero-valent complex is rapidly reduced to the corresponding anion a t the electrode surface. Unfortunately, the peak current were found to be always equal to unity at ratios ipa/iPc scan rates large enough (20.5V s-l) to make the wave irreversible and assure that the coupled chemical reaction is not occurring a t any appreciable extent. Thus, the identity of the slow step remains still undetermined. So far as the chemical reaction following charge transfer, it can once again be ascribed to quenching of the primary two-electron reduction product by proton sources to give the hydride derivative. The final hydride products were recognized for the complexes 2 and 3 by lH-NMR spectroscopy. For both complexes the resonance of the hydride proton appears as a quintet centered a t 6 -12.47 ppm (JPH= 8.5 Hz) and 6 -13.18 (JPH= 7.3 Hz) in toluene-& solution, respectively.22 Indeed, in close analogy with our previous results for the [M(dppe)zl+ s y s t e m ~ , ~deliberate ~J~ addition of increasing amounts of water leads t o the stepwise disappearance of the coupled anodic peak, the cathodic one being unchanged, in the cyclic voltammograms of 1, 3, 4, and 5. Owing to incomplete reversibility of the electron transfer processes, the rate constant value for the chemical step in the reduction of 1,3,4 and 6 has been determined by double-potential step chronoamperome t ~ y . ~The 3 pseudo-first-order rate constants at 25 "C in nominally anhydrous acetonitrile were found to be 0.17,0.35,0.24,and 0.13 s-l for complexes 1,3,4, and 5, respectively. These values are compatible with the value of 0.40 s-l that we previously obtained by cyclic voltammetryla and here confirmed by chronoamperometric method for complex 2 under identical conditions.
Conclusion We have reported here the preparation of new chiral biddiphosphine) iridium complexes. The formation of these complexes is accompanied in the cases examined by a very low extent of diastereoselectivity. Of interest is the response of the electrochemical measurements which give information about the different basicity of the ligands investigated. These methods can be useful in finding a possible rationale for the difference in (22) Schnabel, R. C.; Roddick, D. M. Organometallics 1993,12,704 and references therein. (23)Schwarz, W. M.; Shain, I. J. Phys. Chem. 1966,69, 30.
Morandini et al.
3422 Organometallics, Vol. 14, No. 7, 1995
[Ir{(RJZ)-cypenphos))2]Cl,3. lH-NMR 7.76-7.14 (m, 40H, C6H6); 2.30-0.89 (m, 16H, CH CH2). 31P-NMR: 22.77 (s). Anal. Found: C, 62.78; H,5.08. Calcd for C58H56P4ClIr: C, 63.06; H, 5.10. Experimental Section [Ir{(R*JZ*)-cypenphos)dCl+ [Ir{(R,S)-cypenphosl~lC& Materials. All reactions and manipulations were carried 3'. lH-NMR: 7.77-6.79 (m, 40H, C6H5); 2.27-0.93 (m, 16H, out under nitrogen. The solvents were dried and degassed CH CH2). 31P-NMR: 24.71 ( 8 ) ; 22.77 (s). Anal. Found: C, before use. lH- and 31P{1H}-NMRspectra were recorded on a 62.90; H, 5.16. Calcd for C58H56P4C1Ir: C, 63.06; H, 5.10. JEOL FX 90 Q or on a AM 400 Bruker spectrometer. Positive [Ir{(R)-prophos)2lCl,4. 'H-NMR 7.80-6.86 (m, 40H, 6 values in ppm are downfield from internal Me4Si ('HI or c&6); 2.45-2.09 (m, 6H, CH CHd; 0.86,0.79 (m, 6H, CH3). external 85%H3P04 (31P).Anhydrous CH&N was purchased 31P-NMR: isomer A, truns-(R$),AAXX' pattern, 35.2 (PI,P3), from Aldrich and used as received. The electrolyte tetraethyl50.2 (Pz, P4), (51,~ = 5 3 . 4 = 24.2,51,3 = 271.1,51,4 = 5 2 , 3 = 20.7, ammonium perchlorate (TEAP,C. Erba) was dried in vacuo J 2 , 4 = 271.3 Hz); isomer B, cis-(R&),AAXX' pattern, 34.8 (PI, at 75 "C prior t o use. High-purity argon, further purified from = 25.1, 51,s = J z ,=~271.6, 5 1 , 4 = J 2 , 3 = Pz), 51.9 (P3, P4) (51,~ oxygen by passage over reduced copper at 450 "C, was used 20.7,53,4 = 23.2 Hz). Anal. Found: C, 61.71; H, 5.08. Calcd in electrochemical experiments. Cyclooctene and 1,5-cycloocfor C~H52P4C11r:C, 61.62; H, 4.98. tadiene were obtained from Jansen, 1,2-bis(diphenylphosphi[Ir{(R*JZRI)-prophos}2lC1 + [Ir{(R,S)-prophos}2lCl,4 . no)ethane (dppe) and triphenylphosphine from Fluka and were 'H-NMR: 7.70-6.80 (m, 40H, C6H5); 2.36-1.60 (m, 6H, CH used without purification. 1,2-Bis(diphenylphosphino~~ycloCH2); 0.82 (m, 6H, CH3l3lP-NMR: isomer A, trum-(R,S), pentane ( c y p e n p h o ~ ) (2S,3S)-2,3-bis(diphenylphosphino),~~ AAXX' pattern, 40.2 (PI, P3), 54.6 (Pz, P4), (51,~ = 5 3 , 4 = 17.4, butane (chiraphos),26a(R)-1,2-bis(diphenylphosphino)propane (prophos),26b(R)-l-phenyl-l,2-bis(diphenylphosphino)ethane 5 1 . 3 = 265.3, 5 1 , 4 = 52,s = 21.8, 5 2 . 4 = 265.2 Hz); isomer B, cis(R,S),AAXX' pattern, 30.8 (PI,Pz),55.7 (P3, P4) (J1,2= 25.4, ( p h e n p h o ~ ) [Ir(COE)zC1]2,28 ,~~ [Ir(COD)C112,29 (v5-CgH,)Ir5 1 . 3 = 5 2 , 4 = 270.9, 5 1 , 4 = 5 2 . 3 = 23.4, J 3 , 4 = 22.0 Hz). Anal. (COD),l0and (q5-C9H7)Ir(COE)230were prepared according to Found: C, 61.52; H, 4.92. Calcd for C54H52P4ClIr: C, 61.62; published procedures. Spectral simulations were performed H, 4.98. with PANIC (Bruker Spectrospin AG) using positive truns[Ir{(R)-phenphos}&l, 5. 'H-NMR: 7.51-6.35 (m, 40H, V p p and negative cis-2Jpp. The truns-2Jppvalues obtained for CeH5); 3.35-2.48 (m, 6H, CH CH2). 31P-NMR: isomer A, the cis isomers were used as starting values in the data set of truns-(RJU,pseudoA& pattern, 31.4 (t),54.6 (t),(JPP = 23.2 the corresponding trans species. Hz); isomer B, cis-(R,R),AAXX' pattern, 30.8 (PI, Pz), 55.7 The conductivity experiments were carried out at 25 "C with (P3, P4) (51,~ = 25.4, 5 1 , 3 = 5 2 . 4 = 270.9, 5 1 , 4 = J z = , ~23.4, 5 3 . 4 a Radiometer CDM3 instrument on a cu. methanol = 22.0 Hz). Anal. Found: C, 65.58; H, 4.86. Calcd for solution of the complexes. C&&4ClIr: c , 65.33; H, 4.80. General Procedure for the Preparation of the [Ir(diphos)e]Cl Complexes. [Ir(CzH4)2C1]2 was generated in Apparatus and Procedure for the Electrochemical situ by bubbling ethylene into a suspension of [Ir(COE)&ll2 Measurements. All experiments were performed on anhy(0.3 g, 0.33 mmol) in ethyl ether at 0 "C for 30 min as drous deoxygenated acetonitrile solutions with 0.2 M TEAP previously reported.31 The solution was then reacted with a as the supporting electrolyte, using a conventional three5-fold excess of the appropriate diphosphine in 20 mL of electrode liquid-jacketed cell. Cyclic voltammetry measuretoluene or dichloromethane at room temperature. After 8 h, ments were performed with an Amel 551 potentiostat moduthe solvent was removed under reduced pressure and 20 mL lated by an Amel 566 function generator, and the recording of n-hexane was added to the residue. The orange microcrysdevice was either an Amel model 863 X-Y recorder or a talline compounds were filtered off,washed with n-hexane, and Hewlett-Packard 7090 A measurement plotting system, dedried in vacuo. Recrystallization was from dichloromethand pending on the scan rate employed. The working electrode n-hexane. Yields are up t o 90%. Elemental analyses and was a planar platinum microelectrode (ca. 0.3 mm2) surNMR parameters (6, CD2C12) for the complexes are as follows. rounded by a platinum spiral counter electrode. [Ir{(S,S)chiraphos}dCl,1. 'H-NMR 7.49-7.17 (m, 40H, Controlled potential electrolyses were performed with an C6H5); 1.83 (m, 4H, CHI; 0.67(m, 12H, CH3). 31P-NMR 50.03 Amel 552 potentiostat linked to an Amel 731 digital integrator. (e). Anal. Found: C, 61.73; H, 5.07. Calcd for C5sHssP4ClIr: The working electrode was a mercury pool, and the counter C, 62.24; 5.22. was external, the connection being made through an appropriate salt bridge. In all cases silved0.l M silver perchlorate in (24) Visentin, G.; Piccolo, 0.;Consiglio, G. J. Mol. Catal. ISSO, 61, CH&N, separated from the test solution by 0.2 M TEAP in L1. CH3CN solution sandwiched between two fritted disks, was (25)Allen, D. L.; Gibson, V. C.; Green, M. L. H.; Skinner, J. F.; Bashkin, J.; Grebenik, P. D. J. Chem. SOC.,Chem. Commun. 1985, used as the reference electrode. Compensation for iR drop was 895. achieved by positive feedback. Ferrocene was added at the (26)(a) Fryzuk,M. D.; Bosnich, B. J.Am. Chem. Soc. 1977,99,6262. end of each experiment as the internal reference. All poten(b) Fryzuk, M. D.; Bosnich, B. J. Am. Chem. SOC.1978, 100, 5491. tials are referred to the ferroceniudferrocene redox couple (E" (27) (a) King, R. B.; Bakos, J.; Hoff,C. D.; Marko', L. J. Org. Chem. 1979,44,1729.(b) Brown, J. M.; Murrer, B. A. Tetrahedron Lett. 1979, = +0.010V relative to the actual Ag/AgC104reference electrode 4859. and +0.420 V relative to SCE). (28) Van der Ent, A.; Onderdelinden, A. L. Inorg. Synth. 1990,28,
reactivity shown by these ligands in stoichiometricand catalytic reacti0ns.3,~~
+
+
+
+
+
90. (29) Herde, J. L.; Lambert, J. C.; Senoff, C. V. Inorg. Synth. 1974, 15, 18. (30) Merola, J. S.; Kacmarcik, R. T.; Van Engen, D. J. Am. Chem. Soc. 1986, 108, 329. (31)Abad, J. A. Inorg. Chim. Acta 1986, 121, 213.
Acknowledgment. We thank Mr. A. Ravazzolo
(CNR,Padua, Italy) for skillful technical assistance. OM9500836
