Electrochemical oxidation of tetra-(S)-mandelatodirhodium(II) and tetraformatodirhodium(II)

Po~yMron Vol. 9, No. 9, pp. 1135-1140, printed in Great Britain

1990 0

ELECTROCHEMICAL OXIDATION TETRA-(s)-MANDELATODIRHODIUM(II) TETRAFORMATODIRHODIUM(II) A. SZYMASZEK

0277-5387/90 $3.00+.00 1990 pergamon Pms plc

OF AND

and F. P. PRUCI-INIK*

Institute of Chemistry, University of Wroclaw, 50-383 Wroclaw, Poland (Received 9 October 1989 ; accepted 30 November 1989)

Abstract-Rhodium complexes [Rh2{(S)-C6H5CHOHCOO}J and [Rh,(O&H),] were investigated by electrochemical and spectral techniques in organic solvents and results were compared with those for [Rh2(02CCH3)4]. The complexes were reversibly oxidized to yield Rh”-Rh”’ dimers. The half-wave potentials for [Rh2(02CR)4LJ (R = H, CH3, CH(OH) C,H,) were found to bear a linear relationship with the RCOOH acid dissociation constant. The oxidation process becomes more favourable as the donor number of axial ligand increases. Electronic and ESR spectra of Rh”-Rh”’ complexes were investigated.

The binuclear rhodium(I1) carboxylate complexes are widely studied, because of the interest in the theory of their electronic structure, reactivity and catalytic properties. ‘3’They also exhibit cytostatic and anticancer activity. l-3 Electrochemical investigations of the [Rh2 (OORC),L,] complexes allowed better recognition of their electronic structure and reactivity. Rhodium(I1) carboxylates oxidate reversibly to Rh”-Rh”’ complexes, while their reduction is irreversible.4-6 Oxidation potentials of [Rh,(OOCR), LJ were discovered to decrease with an increase in the electron-donor properties of the R groups, and to decrease with an increase of the donor number of ligands, L, bound along the Rh-Rh axis (Fig. 1). The rhodium(I1) complexes with the chiral carboxylates are interesting from the point of view of their application in the asymmetric synthesis of the organic compounds. The interest in rhodium(I1) chiral carboxylates has been maintained in recent years by their potential applications in chiral organic syntheses. Tetra-(S)-mandelatodirhodium catalyses cycloaddition reactions of dimethyl diazomalonate to 1-alkenes to give chiral cyclopropane compounds. 7 The [Rh,(PhCHOHCOO),] and [Rh,(PhCHOH C00)2(N-N)2C12] (Ph = C6HS, N-N = 2,2’-bipyridine, 1 , 10-phenanthroline) complexes are precursors of the effective catalysts for reduction of olefins * Author to whom correspondence should be addressed.

Fig. 1. Structure of tetracarboxylatodirhodium(I1).

and ketones.’ The [Rh2(02CR)4] complexes also catalyse the oxidation of olefins and alkyl aromatic hydrocarbons. It seemed interesting to study the electrochemical oxidation of tetramandelatodirhodium(I1) and compare its electrochemical properties with that of [Rh2(02CR)J dirhodium(I1) carboxylates. Prior to this, we have examined the electrochemical properties of rhodium(I1) mandelate in methanol.9 To our knowledge, electrochemical properties of rhodium(I1) formate were not investigated. EXPERIMENTAL [R~{(~)-C~H&HOHCOO).I(H~O)~],‘~ tRb(Oz CCH,),(H,O)&” and [Rh,(02CH),],‘* were synthesized by the literature methods. All complexes were stored over molecular sieves (3 .&), at room temperature. Tetra-n-butylammonium tetrafluoroborate, n-Bu4N.BF4 (Fluka), used as supporting

1135

1136

A. SZYMASZEK

and F. P. PRUCHNIK

electrolyte, was recrystallized from an acetoneether (1 : 1) mixture and dried in DCICUO.The solvents were dried over molecular sieves. Tetrahydrofuran (THF) and dimethylformamide (DMF) were distilled in txm.m over CaH, and dichloromethane and acetonitrile over P40 ,,,. For both electrochemical and spectroscopic experiments all solvents contained 0.1 M n-Bu,NBF,. Measurements were performed on a Polarographic Analyser PA-4 (Laboratorni Pristroje) using a three-electrode system. The working electrode was a planar platinum electrode (s = 0.98 cm*). A coiled platinum wire served as the counter-electrode. Potentials were measured relative to a saturated calomel electrode (SCE), which was separated from the examined solution by a bridge filled with electrolyte. The solution in the bridge was periodically exchanged. Ferrocene (Fecp:/Fecp,)‘3 was used as an internal standard. Coulometric measurements were made with a Radelkis OH-404 apparatus. The anode was platinum foil (s = 6.5 cm’), the cathode-mercury (s = 8 cm*). The coulometric electrolyses were carried out in an electrolyser with cells separated by a glass sinter (G-4). The reference electrode (SCE) was separated from the oxidation solution by means of an electrolytic bridge. Solutions in both cells were stirred magnetically. All measurements were performed at room temperature (20f 2°C) in solutions deaerated by a stream of argon (p.a. grade). Electronic absorption spectra were recorded on a Cary 14 spectrophotometer. The ESR spectra were

measured at liquid nitrogen temperature, on a Jeol 3JSMX spectrophotometer. RESULTS

AND DISCUSSION

The cyclic voltammogram of the [Rh,((S)C6HSCHOHCOO)4(H20)2] solution in DMF on the platinum electrode vs SCE is shown in Fig. 2. In the potential range O-l.5 V it shows a single wave, corresponding to one-electron oxidation (Fig. 2). Analogous voltammograms were obtained for [Rhz(C6HsCHOHC00)4] in CH$12, CH3CN and THF solutions and for [Rh2(02CH)4] and [Rh2(02CCH3)4] in DMF solution. During the electro-oxidation of all dirhodium(I1) carboxylates in each solution the potential separations (& - Ep+) were in the range 60-70 mV, at the cyclic sweep rates u = 0.02-0.5 V s- ’ (Table 1). The ratios of the

E(V)

1.L

I

I

1.3

1.2

/

1.1

1.0

0.9

Fig. 2. Cyclic voltammogram of 6 x lop4 M [Rh,((S)CgH5CHOHCOO)4(H20)2] in DMF/O.l M n-Bu,NBF, on the platinum electrode vs SCE, scan rate 200 mV s- ‘.

Table 1. Electrochemical data for oxidation of [Rh,(O,CR),(H,O),] in DMF (0.1 M n-Bu, NBF,) on the platinum electrode (vs SCE) ; u = 0.2 V s- ’ Coulomettic determination of the number of electrons transformed/ Mol of [Rh2(02CR)4L2]

Ep.aU (V)

Ep,c- -%a (mV)

E,, 2 (V)

DWO~CCH,Llh

1.015

70

0.990

0.97 f 0.05

(0.5 mM) [Rh2((S)-C,HSCHOHC02)4L21 (0.5 mM)

1.200

70

1.170

0.97+0.05

WdWW&~1

1.240

70

1.215

0.95 +0.05

Compound

0.8

(0.1 mM) “Present potentials were determined at the presence of Fecp+/Fecp in the same electrolyte (E,,,,,Fecp+/Fecp = 0.540 V vs SCE). ‘Oxidation potential of [Rh2(02CCH3)4L2] in DMF (0.1 M n-Bu,NBF,) is 40 mV lower than the literature value for oxidation of [Rh,(02CCH3)4L2] in H20rJ and 20 mV lower than its oxidation in 0.1 M H,SO, given by Wilson and Taube.4 [Rh2{(S)-CsH,CHOHC0,}4L2] is insoluble in water. ‘[Rh,(O,CH),L,] is slightly soluble in DMF. L = H20.

Tetra-(S)-mandelatodirhodium(I1)

1137

and tetrafonnatodirhodium(I1)

E1/2(V)

1.25 t

0.95

I

1

2

3

4

5

6

7

0

9

KA / KCH~OH

Fig. 3. Plot of the half-wave potentials (V) measured for electro-oxidation of [Rh,(OICR),(H,O),] in DMF/O.l M n-Bu4NBF4 vs the KA/KCH,COOH: (1) R = CH, ; (2) R = PhC(OH)H ; (3) R = H.

(Fig. 3). The oxidation potentials are higher for cathodic to the anodic peak currents, i,,,/i,,,, were complexes with stronger acids, which, therefore, close to unity at all scan rates, indicating the absence stabilize the lower oxidation states of the metal. of coupled chemical reactions.14 Moreover, the The effect of axial ligand complexation on the potentials of the anode peak (E,,& and formal It was found potentials of all the complexes were independent of electrode reactions was investigated. that the half-wave potentials decrease with increasthe scanning rate : the ~+Jz,“’ ratio was constant ing donor number of the solvent (Fig. 4).” The over the range of scan rates. Such properties are donor number can be taken as a measure of the indicative of a one-electron diffusion-controlled interaction of the axial ligand with the central Nernstian oxidation. Thus, the electrochemical oxiatoms. The oxidation process becomes more dation of rhodium(I1) mandelate and rhodium(I1) favourable as the interaction of the axial ligand formate proceed as for the other [Rh,(O,CR),] becomes stronger. The shift of the oxidation potencomplexes. tial with strength of the axial ligand-rhodium bond The stability of rhodium(I1) carboxylato comindicates that the HOMO levels are destabilized plexes, [Rh2(02CR)4L2], depends on dissociation with increasing binding ability of ligands. The constants of RCOOH acids. This is proved by the linear relationship of half-wave potentials with disdifferences in the peak potentials, E = Ep,c- Ep,a, sociation constants, KHA, or with ratio KHA/K-H,COOHfor each solvent are near the theoretical value of

Fig. 4. Dependence of the formal electrode potential of [Rh2{(S)-C6H5CHOHCOO}4(H20)2] donor number of the solvent.

on the

1138

A. SZYMASZEK

58 mV, corresponding one electron.

to the reversible

transfer

and F. P. PRUCHNIK of

and absorption spectra were recorded. The resulting absorption spectra are given in Figs 5 and 6 and [Rh2(02CR),] (R = H, Me, PhCHOH) were in Table 2. oxidized coulometrically at potentials 100 mV more The absorption electronic spectra of the anodic than Elj2 (Table 1). The complete coulo[Rh2(02CR)J type complexes consist of two weak metric oxidation of complexes [Rh2(0&CH& bands in the visible region, band 1(620-500 nm) and band II (45&425 nm). Whereas band II remains (HdYd and [Rh~{(S)-C6H~CHOHCOO}4(H20)21 in DMF was followed by the change of colour almost constant, band I is strongly influenced by of the solutions from blue to light brown. For changes in the axial ligands and shifts to lower wavelengths depending on the donor atom in the [Rh2(02CCH3)2(H20)2] the complex regained the blue colour after 24 h. Somewhat different following sequence: I > Br > Cl > 0 > S > behaviour occurred with [Rh2{(S)-CgHsCHOH N > As > P.‘~2~8~‘oc~1~1g The electronic absorption COO)4(H20)2]. The colour of this solution also spectra of the [Rh2(02CR),L2]+ complexes, in turned light brown after complete oxidation, but addition to the band in the range 400-500 nm, exhiabout 20 min after electrolysis was complete, the bit a new band in the 60@-800 nm range. The energy colour changed to yellow-celadon, under argon of the latter also depends on the type of ligands atmosphere. After 24 h the reduction process was coordinated along the Rh-Rh axis. For [Rh2(02C almost completed and the solution changed to the CH,MH,W +5lc2* the band occurred at 770 nm and was assigned to the G(RhRh) + 6*(RhRh)” original blue colour. Reversibility of the oxidation Whereas, in the [Rh2(02CCH& process of rhodium(I1) formate, acetate and mandel- transition. (NH,),]+ complex this band is located at 570 nm.22 ate was confirmed by the complete reduction of Since in the spectra of [Rh2(02CR)4(DMF)2]+ [Rh2(02CR)Jf complexes at a potential of +0.6 V. (R = CH,, PhCHOH) complexes the band is Similar to electro-oxidation, the consumption of 1.0 electromdimer was observed, and the cyclic observed at 770 nm, the DMF molecules should be regarded as coordinated via the oxygen and not voltammetry of the obtained species gave identical current-voltage curves to those obtained from the the nitrogen atom. In the latter case the strong shift of this band to lower wavelengths should original neutral complex. be observed. During the coulometric oxidation of [Rh2(02C The Rh”Rh”’ complexes in DMF solutions CH,),(H,O),] and [Rh2{(S)-C6H,CHOHCOO)4 (H,O),] in DMF (0.1 M n-Bu4NBF4) the ESR undergo reduction to [Rh2(02CR)& both in a

*t’ 1.8 1.6-

l.L1.21.00.80.6-

I

350

I

450

550

I

650

750

850

X(nm)

L

Fig. 5. Electronic absorption spectra obtained during the oxidation of 1 x lo3 M [Rh2(02C CH,),(H,O),] in DMF/O.l M n-Bu4NBF4, I&, = + 1.1 V/SCE : (1) before electrolysis; (2) after electrolysis ; (3) 24 h after electrolysis.

Tetra-(S)-mandelatodirhodium(I1)

1139

and tetraformatodirhodium(I1)

*t 1.8 t

Fig. 6. Electronic absorption spectra obtained during the oxidation of 1 x lo3 M [Rh,{(S)CgH5CHOHCOO}4(H20)] in DMF/O.l M n-Bu,NBF,, Eapp= + 1.270 V/SCE: (1) before electrolysis ; (2) after electrolysis ; (3) 40 min after electrolysis ; (4) 24 h after electrolysis.

Table 2. UV and visible absorption maxima of the [Rh2(02CR),L2] complexes in DMF (0.1 M n-Bu,NBF,) platinum electrode vs SCE

Maxima, nm (E, molar extinction coefficient) After oxidation Before oxidation

Compound

[Rh,@zCCHd&~l

on the

Oxidation potential ECV)

580(285) ; 460( 125)

770(450) ; 500sh( 150) ; 410sh(275)

1.1 v

580(195) ; 450(75)

770( 150) ; 500sh(205) and 20 min after current was cut off 590(210) and no band at 450 nm

1.25 V

(1 mM)

[Rh~{(S)-CsH~CHOH0~)4L~l (1 mM)

L = H,O.

neutral atmosphere and in air. The rate of this process for the mandelate complex exceeds that for the acetate complex. The faster reduction of the mandelate complex is expected because the oxidation potential of the [Rh2(02CCHOHPh)4

(DMF),]+ is higher and this complex undergoes partial reduction after 20-30 min, while [Rhz(02 CCH,),(DMF),]+ is more stable, and even after 24 h its reduction was not complete (Figs 5 and 6).

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