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Anodic oxidation of the complexes NEt4[Cr(X)(CO)5](X = CN,I). Reactivity of the electrogenerated neutral species
EIeetrochimica
Pergamon PII: Soo13463q%po3167
Arm, Vol. 42, No. 10, pp. 1549-1559, 1997 Copyright 0 1997 Elsevier Science Lrd. Printed in Great Britain. All rights reserved 00134686/97 $17.00 + 0.00
Anodic oxidation of the complexes NEtJCr(X)(CO)s](X
= CNJ).
Reactivity of the electrogenerated neutral species W. P. Fehlhammer,“?
M. Fritz,” D. Flonerb and C. Moinetb*
“Institut fur Anorganische
und Analytische Chemie der Freien Universitat Berlin, FabeckstraBe 34-36, D-14195 Berlin, Deutschland bLaboratoire d’Electrochimie, URA CNRS n”439, Universite de Rennes I, Campus de Beaulieu, 35042 Rennes Cedex, France
(Received 25 March 1996; in revised form 5 July 1996) Abstract-The cyclic voltammograms in DMF or acetonitrile of [Cr(CN)(C0)5]- (Ia) and [Cr(I)(CO)S]- (Ha) show two anodic waves. Under an inert atmosphere, only the first one corresponding to a one-electron-transfer appears reversible for scan rates of 0.1 V s-i. In DMF, in the presence of oxygen, the reversibility totally disappears. Macroscale electrolysis in a batch cell at a potential corresponding to the plateau of the first wave involves two moles of electrons per mole of starting material; during electrolysis, decomposition with CO evolution is observed for both complexes while in the case of IIa, additional formation of iodine takes place. In the presence of oxygen, CO2 is produced during electrolysis. The chemical transformations in MeCN of the two neutral species Cr(CN)(CO)s (Ib) and Cr(I)(CO), (IIb) were studied by cyclic voltammetry immediately after their generation by one-electron oxidation in a flow cell of the parent complexes Ia and IIa. A comparative study of solutions in MeCN of Ia, IIa and Cr(COk, which have been subject to uv irradiation for x 15 min shows that in both cases the first step of the chemical transformation consists in a ligand exchange with formation of [Cr(CN)(NCMe)(CO)a] or [Cr(NCMe)(CO)r], respectively. Ib (Ia) and IIb (Ha) also proved to be efficient redox catalysts for the eiecttooxidation of the cyanide ion and of triphenylphosphine. 0 1997 Elsevier Science Ltd. All rights reserved. Key words: Chromium
complexes, oxidation, electrocatalysis.
INTRODUCTION The anodic oxidation in organic solvents of the anionic complexes [Cr(X)(CO)s]- (X = Cl, Br, I, CN) has been studied by a number of research groups. From cyclic voltammetry at a platinum electrode in dichloromethane solution, Lloyd ef al. [l] inferred a primary oxidation process for the chloro, iodo and cyano complexes consisting of a reversible one-electron-transfer reaction; attempts to prepare the oxidized species by chemical oxidation had failed due to their instability, however. Bond et al. [2] reported on the voltammetry at platinum in acetone of the three halogen0 complexes; at room temperature, the first oxidation step again was found to be a
reversible one-electron process whereas the second step leading to a cationic species appeared non-reversible. In the case of pentacarbonyl(iodo)chromate, however, reversibility of this second step was observed at -75°C. In a subsequent study by Bond and Colton [3], the thermal decomposition of the neutral complex Cr(I)(CO)s which had been prepared by chemical oxidation of the corresponding anion in acetone at low temperature, was followed electrochemically and found to proceed by disproportionation according to Scheme 1. 2 Cr(IKOh
+ [Cr(I)W%l-
+ [Cr(I)(CO)5]+ 1 decomposition
Scheme 1. *Author to whom correspondence should be addressed. t Present Address: Deutsches Museum, Museumsinsel 1, D-80538 Miinchen, Deutschland.
In the sixties Behrens et al. had reported on the reaction of Na[Cr(I)(COh] with iodine at low temperature which resulted in a blue solid identified
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W. P. Fehlhammer et al.
1550
as Cr(I)(CO)s [4]. From there a synthetic route was opened to other brightly coloured neutral species formulated as and Cr(CN)(CO)s (green) Cr(NCS)(CO)s (red) [5]. Note, however, that parts of this work have later been questioned [l, 61. Oxidation in appropriate solvents of [Cr(CN)(C0)5]- by arene diazonium salts in an inert atmosphere has been developed by our group to an efficient synthetic method for novel functional isocyanides. Carried out in, say, chloroform, this “radical alkylation of cyano complexes” gives [Cr(CO)rCNCClr] in high yield. In the presence of oxygen, however, a totally different product, [(OC)$ZrCNC(=O)NCCr(Co)s], was obtained from the same solvent [7, 81. In a preliminary note [9], we have already reported on the different course the electrochemical oxidation of [Cr(CN)(CO)r]- takes under oxygen as compared to that under an inert atmosphere. In order to complete the previous work, this paper presents studies of the anodic oxidation of [Cr(CN)(CO)j]and [Cr(I)(CO)s]- under various conditions. Particular emphasis is put on the question of stabilities and relative reactivities towards selected reagents of the oxidized species.
EXPERIMENTAL Electrochemistry All experiments were carried out at room temperature. The solutions were deaerated and saturated with dinitrogen except in cases where the investigations were performed in the presence of oxygen. Conventional electrochemical equipment was used for cyclic voltammetry (EGG PAR model 362 scanning potentiostat with an XY recorder). For both, cyclic voltammetry and voltammetry at a rotating electrode, the working electrode was a disc of vitreous carbon (diameter 3 mm) of platinum (diameter 2 mm). All potentials refer to the saturated calomel electrode (see) and have not been corrected for the Ohmic drop. Controlled potential electrolyses were carried out in a batch cell [lo] at a platinum or vitreous carbon electrode (diameter of disc 4cm). For the preparation of the solutions, 10m3to 2 low3 mole of the respective substrate (NEt&r(CN)(CO)s], NEt4[Cr(I)(C0)5]) was dissolved in 150 ml of DMF or acetonitrile; the concentration of the base electrolyte (NBu;[BF.+] or LiC104) was 0.1 M. Gas analyses were performed either by mass spectrometry (CO and COz) or by reaction with an aqueous solution of calcium hydroxide or sodium hydroxide followed by an acidometric titration (COZ) or by reaction with an aqueous solution of ammoniacal silver nitrate (CO). Continuous electrolyses were carried out in a flow cell [l 11 fitted with a graphite felt anode (diameter
4 cm, thickness 1.2 cm) and two electrical circuits. The solutions studied contained 0.25 g of NEt[Cr(CN)(CO)s] or NEt_JCr(I)(C0)5], respectively, dissolved in 250 ml of acetonitrile, and were 0.1 M in LiC104 (as base electrolyte). They were deaerated and passed through the porous anode at a rate of 4 ml per minute. Current intensities for one-electron processes were imposed (ie, 18.5 mA with il = 12.5 mA for the upstream circuit and iz = 6 mA for the downstream circuit for NE&[Cr(CN)(CO)s and 14.5 mA with il = 10 mA and iz = 4.5 mA for NEtJCr(I)(CO)s]). The irradiation experiments were carried out with a Hohensonne loo-type 1081-mercury lamp (uu + ir, 300W) which was placed at a distance of approximately 20 cm off the voltametric cell. The complexes Cr(C0)6, NEtJCr(CN)(C0)5] and NEtJCr(I)(C0)5] were 4-6 x 10m3M, and the base electrolyte, NBu;BF+ was 0.1 M in acetonitrile solution. Cyclic voltammograms were recorded before and after 15 min of uv irradiation. Starting materials NEt[Cr(CN)(CO)s] and NEt[Cr(I)(CO)s] were prepared according to published procedures [12,13]. Analytical grade dimethyl formamide (DMF), acetonitrile and acetone were purchased from SDS, and lithium perchlorate purum from Fluka, and used without further purification. Tetra(nbutyl)ammonium tetrafluoroborate purum from Fluka was recrystallized from ethanol-water prior to use.
RESULTS 1. Electrochemical studiesin the absence of oxygen 1.1. Cyclic voltammetry. At vitreous carbon or platinum electrodes, at scan rates of 0.1 V s-l, the first anodic process appears reversible for both [Cr(CN)(C0)5](Ia) and [Cr(I)(CO)S]- (IIa) in DMF, acetonitrile and acetone (Table 1). At lower
Table 1. Voltammetric data for NEt.&r(CN)(CO)s] (Ia) and NEt.+[Cr(I)(CO)s] (Ha) at a vitreous carbon electrode; in (a) DMF, (b) acetonitrile, (c) acetone; scan rate 0.1 V s-l Solvent
[Cr(CWCW- (W (4 (b) [Cr(I)(CO)s]- (IIa)
[Zi (b) (c)
Ep.1”
EPA”
Epal”
0.82 0.71 0.74 0.61 0.47 0.55 (0.73)”
0.72 0.61 0.66 0.52 0.39 0.42 (0.66)*
1.15 1.38 I .25 0.95 0.99
“VW,. *Electrolyte NEhClOd; VA~/A~SCI [2].
1.05 (1.24)b
Anodic oxidation of chromium complexes
Fig. 1. Voltammograms of NEti[Cr(CN)(C0)5] (Ia) in acetonitrile [substrate concentration = 4 x IO-’ M, c (electrolyte NBu;BFd) = 0.1 M; vitreous carbon electrode]. (a) scan rate 0.2 V SK’; ~ first scan, - - - second scan; (b) scan rate 0.02 V s-l; (c) scan rate 0.2 V s-l; ~ first scan, - - ~ second scan. -.-.-.- third scan.
scan rates (0.01 V s-i), reversibility is only observed for the iodo complex Ha. For both complexes, a second, irreversible step occurs at more anodic potentials. In acetonitrile, a new reversible process can be detected at approximately 1 V,, (Fig. 1, peaks Z-2’) after oxidation of
1551
the cyano complex Ia either (i) up to a potential (1.35 V,,,) corresponding to the second oxidation step for a sweep rate of 0.2 V s-i (Fig. l(a)), (ii) up to the potential of the first step (1.08 V,,,) for a 0.02 V s-’ sweep rate (Fig. l(b)) or (iii) up to the potential of the first step (1.05 V,,.,) on the second scan for 0.2 V s-l sweep rate (Fig. l(c)). The voltammograms at a scan rate of 0.1 V s-’ of the solution salt of Ia in slightly acidic (pH 4.8) and neutral aqueous solutions again exhibit a first reversible system (Ep, = 0.56 V,,,. Ep, = 0.47 V,,,) followed by an irreversible one at a more anodic potential (Ep, = 1.1 V,,,). Reversibility of the first step is still observed at the much lower sweep rate of 0.01 V SK’. In slightly basic aqueous media (pH 9.2) however, reversibility is already lost at a scan rate of 0.1 vs ‘. 1.2. Batch cell electrolysis. Electrooxidation at vitreous carbon or platinum anodes in DMF or acetonitrile of the cyanochromium complex Ia was performed at a potential corresponding to the plateau of the first wave (DMF: E = 0.85 V,,,; MeCN: E = 0.9 V,,,). In both media, CO is evolved during electrolysis while no cyanogen (Nd---C=N) has been detected. After consumption of two moles of electrons per mole of starting material, voltammograms showed the initial first and second waves to have disappeared. Electrooxidation (acetonitrile, vitreous carbon anode) of the iodo species IIa was carried out at the potential of the first anodic process (E = 0.9 V,,,.) involving two moles of electrons per mole of starting material. Again, CO evolution was established. A cyclic voltammogram recorded at the end of the electrolysis showed the presence of a reducible species (Fig. 2) which was identified as iodine. 1.3. Flow cell electrol.vsis. In order to study the chemical transformations of the primary products of the anodic oxidation, the electrolyses of the cyano (Ia) and iodo complexes (IIa) in acetonitrile were performed in a flow cell fitted with a graphite felt porous anode. Due to the high efficiency of the graphite felt electrode, the complexes are oxidized in high yields in spite of very short contact times between solution and anode; thus most of the chemical transformation of the electrogenerated neutral species Ib and IIb takes place off the electrode. In terms of the theoretical amount of electricity required for this process-one mole of electrons per mole of complex-more than 85% of the starting material has reacted. In the case of complex Ia, the resulting solution is yellow at the cell outlet, yet turns red quickly. By cyclic voltammetry, a new reversible system is observed at a potential more anodic than that of the initial first anodic wave (Fig. 3). The voltammogram is unchanged after two h. (Note that the time between the oxidation at the porous anode and the recording of the first voltammograms was about 10 min as a rule).
W. P. Fehlhammer et al.
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The iodo complex Ha gives rise to a green solution at the outlet of the cell which then turns yellow. The green colour probably results from a mixture of yellow, the final colour, and blue due to unstable Cr(I)(CO)S [4]. A voltammogram recorded at a rotating disc electrode, immediately after collecting the solution at the cell outlet showed several anodic and cathodic waves (Fig. 4(b)): two waves (1 and 4) for the non-oxidized starting material; two new anodic waves (2 and 3); a first cathodic wave (1’) for the reduction of non-decomposed neutral [Cr(I)(C0)5] (IIb); and a second cathodic wave (5’). After one hour (Fig. 4(c)), waves 1, 1’ and 4 have decreased while waves 2 and 3 have increased. Wave 3 is indicative of a new reversible system (Fig. 5(b), waves 2 and 5 are attributed to a mixture of iodine and iodide (Fig. 5(c)). 2. Electrochemical studies in the presence of oxygen In two preliminary notes [9, 141 we have demonstrated the effect of dioxygen on the course of the anodic oxidation of organometallic compounds and the specific role the solvent plays in this connection. Here, we present the full experimental data on [Cr(CN)(C0)5]together with new results on [CrU)(CO)51-. 2.1. Cyclic voltammetry. Admission of dioxygen to
a solution of Ia in DMF causes dramatic changes in the morphology of the cyclic voltammogram (Fig. 6): the first anodic process is no longer reversible, and the I
second anodic step has disappeared. In acetonitrile, in contrast, no striking modifications are observed of the first oxidation, apart from a slight decrease in intensity of the cathodic peak [9]. A similar phenomenon had been observed with ferrocene under the same conditions [14]. Here, in the presence of dioxygen, electrogenerated ferricinium was shown by cyclic voltammetry to be much more stable in acetonitrile than in DMF. In the case of the iodo complex IIa, the overall influence of molecular oxygen on its anodic oxidation is less pronounced at the time scale of cyclic voltammetry. In DMF, only a partial decrease in reversibility of the first anodic step is shown in the voltammogram. In acetonitrile, voltammograms recorded at a scan rate of 0.1 V s-r are the same, no matter whether dioxygen was present or absent. 2.2. Batch cell electrolysis. Macroscale electrolyses in the presence of dioxygen of the cyano complex Ia were performed in DMF and acetonitrile at a working potential corresponding to the first anodic wave. At platinum or vitreous carbon, both CO and CO2 evolution characterized by mass spectrometry [9] occurred, the amount of COZ depending on the solvent used. In DMF, one mole of CO* has been observed per mole of starting compound, in acetonitrile only 0.5. Coulometric data in DMF differ from those in acetonitrile in a similar way: While in DMF two moles of electrons are transferred per mole of
JA
50
Fig. 2. Voltammograms of NEt.+[Cr(I)(CO)s](IIa) in acetonitrile [substrate concentration = 1.4 x lo-* M, c (electrolyte LiC104) = 0.1 M; vitreous carbon electrode; 0.2 V s-l]. Before electrolysis (E = 0.8 V,,,) at a vitreous carbon anode, --- after electrolysis.
Anodic oxidation of chromium complexes
1553
Fig. 3. Voltammograms of NEt&r(CN)(C0)5] (Ia) in acetonitrile [substrate concentration = 2.9 x IO-’ M, c (electrolyte LiClO4) = 0.1 M; vitreous carbon electrode; 0.2 V S-I]. Before electrolysis at a graphite felt porous anode, - - - after electrolysis.
chromium complex, it is only approximately 1.6 moles of electrons in acetonitrile. It is worth mentioning that in acetonitrile the current intensity decreases to about 10% of the initial value after consumption of only 0.3 moles of electrons, ie, shortly after the electrolysis has started. Interestingly, a very similar solvent dependence has been observed for macroscale electrolyses of ferrocene. In DMF, anodic oxidation occurred with formation of 0.5 moles of CO2 also involving two moles of electrons per mole of ferrocene. Here, CO* is probably formed by electrochemical oxidation of some kind of ferricinium-Oz-DMF complex as no CO2 was evolved during the decomposition of chemically prepared ferricinium in DMF in the presence of 02, In acetonitrile, only approximately 0.3 moles of electrons per mole of ferrocene were involved in the anodic oxidation, and no CO2 evolution was observed. On the other hand a mixture of ferrocene and ferricinium in acetonitrile totally decomposes to a brown precipitate within a few minutes in the presence of oxygen. 3. Ekctrocatalytic properties of chromium complexes Ia and IIa. The catalytic oxidation of triphenylphosphine in acetonitrile by the electrogenerated radical cations of Cr(NCMe)(COh and Cr(CO)5(py) (py = pyridine) has previously been described by Hershberger et al. [15]. The pentacarbonyl(cyano) (Ia) and -(iodo)
chromates (IIa) have now been found to exhibit similar electrocatalytic properties. Thus, cyanide (CN-) whose anodic oxidation is reacts rapidly with electrogenerated slow, Cr(CN)(CO)S (Ib) in DMF as shown by the loss of reversibility in cyclic voltammetry and the increase of the anodic current. A macroscale electrolysis in the same medium of a mixture of [Cr(CN)(CO)$ and CN- at vitreous carbon (E = 0.85 V,) involved two moles of electrons per mole of complex Ia, yet one mole of electrons per mole of cyanide anion. CO evolution characterized by reaction with an aqueous solution of ammoniacal silver nitrate occurred during the second part of the electrolysis. No cyanogen has been detected by mass spectrometry, however. This is in accord with a mechanism previously proposed by Inoue and Tsutsumi for the anodic oxidation of cyanide which circumvents the formation of cyanogen [ 161. Catalytic electrooxidation in DMF, acetonitrile or acetone of triphenylphosphine (Fig. 7) or p-toluenesulfinic acid by Cr(I)(COb, is again indicated by an increase in the cyclic voltammogram of the anodic peak and a decrease of the cathodic peak. 4. Electrochemical studies of uu-irradiated carbonylcbromium complexes It is known that w irradiation of Cr(CO)h in acetonitrile causes ligand substitution with formation of Cr(NCMe)(CO)S and [Cr(NCMe)2(C0)4] [17].
W. P. Fehlhammer et al.
1554
r-
a
c! Jb
5cI-
> ./
1
E vsc?
i /IN
Fig. 4. Voltammograms of NEt4[Cr(I)(C0)5] (Ha) in acetonitrile [substrate concentration = 2.2 x lo-) M, c (electrolyte LiCIO4)= 0.1 M; rotating vitreous carbon electrode (2000rpm)]. Before electrolysis at a graphite felt porous anode, - - - approx. 10 min after electrolysis, -.-.-.- approx. 1 h after electrolysis. More recently, Compton et al. [18] have shown that three new oxidation waves are observed at 0.9, 0.4 and -0.1 V,, after illumination at 320 mn of Cr(CO)e in acetonitrile. The waves were attributed to [Cr(NCMe)(C0)5], [Cr(NCMe)2(CO)4] and [Cr(NCMe)j(CO)3] respectively. In order to understand the electrochemical transformations of the electrogenerated neutral species Ib and IIb, solutions in acetonitrile of Cr(CO)e, [Cr(CN)(C0)5]and [Cr(I)(CO),]- have been irradiated with a mercury lamp and subsequently studied by cyclic voltammetry. the initially colourless After uv irradiation, solution of Cr(C0)6 is yellow-orange. New waves are observed at less anodic potentials which correspond to the two reversible systems Ep,l = 0.33 V,, and Ep,2 = Epa~ = 0.41 V,,,, 0.92 V,,,, Epa = 0.84 V,, (Fig. 8(a))..
For [Cr(CN)(CO)j]-, the initially colourless solution also turns yellow during irradiation. The voltammogram (Fig. 8(b)) now shows two new anodic peaks, Ep, = 0.14 and 1.01 V,,,, at the first anodic scan; in the case of the non-irradiated cyano complex Ia, the peak at Ep, = 1.01 V,, appears only at the second anodic scan (cf. 1.1). For [Cr(I)(CO)S]- (Da), two new reversible systems with Ep,, = 0.42 V,,, Ep,,O.34 V,, and Ep,2 = 0.94 V,,, , Epc2= 0.85 V,, were observed (Fig. 8(c)). DISCUSSION As described previously [l, 21, the electrochemical oxidation in organic solvents of [Cr(CN)(CO)& and [Cr(I)(CO)$ involves two one-electron steps (cf. Scheme 2).
Anodic oxidation of chromium complexes
1555 4
b
+.
i
/
’ ’
EVsce
i
Fig. 5. Voltammograms of NEti[Cr(I)(CO)s] (Ha) in acetonitrile [substrate concentration = 2.2 x 10S3 M, c (electrolyte LiClOd) = 0.1 M; vitreous carbon electrode; 0.2 V s-l]. _ Before electrolysis at a graphite felt porous anode, --~ approx. I h after electrolysis, -‘-_ voltammograms of 12 (5.5 x 10m4 M) + NBGI (I.1 x 10m3 M) in acetonitrile (for comparison)
[CrWKOW
* -
Cr(W(COh
Ir
Cr(W(CO)5
IP
e [Cr(X)(CW+
For the iodo complex IIb, a ligand exchange with the solvent occurs (Scheme 3). Cr(I)(COb + MeCN + [Cr(NCMe)(COb]
+ l/2 12
Scheme 2 (X = CNJ).
Scheme 3.
Our experiments show that of the neutral complexes, Cr(I)(COh (IIb) has a much higher stability than Cr(CN)(CO)S (Ib), and that both cations [Cr(X)(CO)s]+ (X = CN (Ic), I (11~))resulting from one-electron oxidations of Ib and IIb, respectively, are extremely unstable.
Experimental results confirming this hypothesis are: (i) the amount of iodine present in the bulk at the end of the macroscale electrolysis (Fig. 2). (ii) the mixture of iodine and iodide ions that is found in the medium after electrolysis in a flow cell and complete transformation of IIb (Fig. 4, curve (c)) (Notice, that unreacted [Cr(I)(CO)S]- is oxidized by iodine (Fig. 4, curve (b), waves 1 and 4).), and (iii) a new compound (Fig. 4, curve (c), wave 3) which is identified as Cr(NCMe)(CO)J by comparison with the product from uv irradiation in acetonitrile of Cr(CO), (Fig. 8(a), peak 2) or [Cr(I)(CO)S]- (Fig. 8(c), peak 2) respectively; the potential of peak 2 in Fig. 8(a) is in accord with the values published by Pickett et al. [19], Hershberger et al. [15] and Compton et al. [18]. By analogy, peaks 1 of Fig. 8(a) and 8(c) are attributed to Cr(NCMe)2(C0)4] [15, 18, 191.
1. Chemical transformations of the neutral species Ib and IIb in an inert atmosphere
First of all it should be pointed out that the chemical transformation of the neutral species Cr(X)(CO)s (X = CN (Ib),I (IIb)) very much depends on the nature of X. Though studied in detail only in acetonitrile, similar phenomena are most likely to be found in DMF solutions.
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W. P. Fehlhammer et al.
Identical observations had earlier been made by Behrens and Herrmann in a study of reactions with mono- and bidentate N- and P-ligands of their supposed Cr(I)(COb; just as in our experiments, both penta- and tetracarbonyl derivatives of Cr(CO)e had formed along with elemental iodine by obviously homolytic cleavage of the Cr-I bond [20]. To explain the chemical transformation of IIb in acetonitrile, it thus appears unnecessary to consider a disproportionation reaction of this species as suggested by Bond and Colton (Scheme 1) [3]. In our case, the ligand exchange with the solvent is probably faster than disproportionation. Further, it is interesting to note that, at the voltammetry scale, the stability of [Cr(NCMe) (CO)J]+ seems higher than that of [Cr(I)(C0)5]+ (11~). Obviously, the chemical transformation of IIc also proceeds by ligand exchange with the solvent (Scheme 4). [Cr(I)(C0)5]+ + MeCN + [Cr(NCMe)(C0)5]+
i
/
’
:
1
I
lo-
+ l/2 I2
Scheme 4. This follows from the fact that cathodic peaks for the reduction of both [Cr(NCMe)(CO)S]+ and iodine have been detected after oxidation of the complex anion IIa into the complex cation IIc (Fig. 5(a)). In contrast to IIb, a carbonyl ligand instead of CN is readily replaced by a solvent molecule in the cyanochromium radical Ib (Scheme 5). This common type of substitution is particularly favoured by a low electron density at the metal which precludes any strong x back-bonding to CO. A pathway involving organometallic radicals has also been suggested for
b
Ev see
Fig. 7. Voltammograms of NEt[Cr(I)(CO)s] (Ha) in acetone [substrate concentration = 2.7 x 1O-3 M, c (electrolyte NBuIBFd) = 0.1 M; vitreous carbon electrode; 0.1 V S-I]. Before addition of PPh3, ---after addition of PPh, (c = 4.1 x 1O-3 M).
various carbonyl substitution reactions which are catalyzed by di- and polynuclear transition metal complexes [21, 221. Cr(CN)(C0)5 + MeCN + [Cr(CN)(NCMe)(C0)4]
+ CO
Scheme 5. In accordance with this mechanism, the new reversible system (Ep. = 1.01 V,,) observed in the cyclic voltammogram of Ia (Fig. 1, peaks 2-2’) or
Fig. 6. Voltammograms of NEt4[Cr(CN)(CO)s] (Ia) in dimethyl formamide [substrate concentration = 2.3 x IO-’ M, c (electrolyte NBuZBF4) = 0.1 M; vitreous carbon electrode 0.2 V s-i]. Under Nz, ---under OZ.
after electrolysis of Ia in the flow cell (Fig. 3(b), waves 2-2’) is also present after uv irradiation of Ia in the same medium (Fig. 8(b), wave 2). The oxidation potential of this system clearly differs from the value for Cr(NCMe)(COh (Ep, = 0.92 V,,,). Cyclic voltammetry (Fig. 3(b)) also shows that [Cr(CN)(NCMe)(C0)4]+ is more stable than Ic. The latter undergoes a fast carbonyl ligand exchange with acetonitrile in S~U~Unascendi at the electrode (Scheme 6).
Anodic oxidation of chromium complexes
1557
a
hA
Fig. 8. Voltammograms of (a) Cr(CO)h (c = 5.9 x 10-l M), (b) NEt4[Cr(CN)(CO)s] (la) (c = 4.1 NEt@(I)(C0)5] (Ha) (c = 4.7 x IO-) M) in acetonitrile [c (electrolyte NBu:BFd) = 0.1 M; vitreous 0.2 V SK’]. __ Before ut’ irradiation, - -- after 15 min of irradiation.
[Cr(CN)(C0)5]+ + MeCN -+ [Cr(CN)(NCMe)(CO)J+
+ CO
Scheme 6. A cathodic
peak assigned to the reduction
of
[Cr(CN)(C0)5]-
10-j M) and (c) carbon electrode:
[Cr(CN)(NCMe)(C0)4]+ is observed after oxidation of the cyano complex anion Ia into the cation Ic (Fig. l(a), Fig. 3(a)). The coulometric data as obtained from an exhaustive electrolysis in a batch cell can be explained by an ECE mechanism for the oxidation of [Cr(CN)(CO)s]- (Scheme 7).
--) Cr(CN)(CO)s
Cr(CN)(CO)5 + MeCN + [Cr(CN)(NCMe)(CW [Cr(CN)(NCMe)(C0)4
x
-+ [Cr(CN)(NCMe)(C0)411+ Scheme 7.
+ CO
-+ decomposition
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W. P. Fehlhammer et al.
In the case of the iodo system II, the mechanism of the electrochemical oxidation is far more complicated due to the additional step of a chemical oxidation of the starting material by iodine (Scheme 8).
CONCLUSIONS From this study it becomes obvious, that the primary products resulting from one- or two-electron oxidations of [Cr(CN)(CO)5]- and [Cr(I)(CO)5]- are
- i< [Cr(I)(CO)sl- + Cr(I)(C0)5 Cr(I)(CO)5 + MeCN + Cr(NCMe)(C0)5 [Cr(I)(CD)s]- + l/2
12 +
cr(1)(co)s
+ l/2 1~ + 1-
- IP
Cr(NCMe)(C0)5
-+ [Cr(NCMe)(C0)5]+
+ decomposition
I- 4 l/2 12 Scheme 8. 2. Chemical transformations of [Cr(CN)(CO)$
in
the presence of dioxygen
The particular effect of dioxygen on the course of the electrochemical oxidation of [Cr(CN)(CO)5]- can probably be explained by a radical coupling between the two paramagnetic species [Cr(CN)(C0)5] and 02. To account for the accumulated experimental evidence on the oxidation of the cyanochromium complex system, on the one hand, and similar findings in the system ferroceneiferricinium, on the other, we propose an adduct with an 02-bridge between two chromium moieties. In fact, a p-peroxodichromium structure analogous to that of related p-peroxodicobalt systems [23-261 appears more likely than a peroxide link between two ligands (here CO or CN) as suggested for 19-electron complexes [27,28]. In the latter case, formation of one mole of CO2 per mole of starting material would have to be expected in DMF as well as in acetonitrile in analogy to the formation via an ON-O-O-NO bridged intermediate of [Ni(Cl)(NOz)dppe] from [Ni(Cl)(NO)dppe] and dioxygen [29]. The p-peroxodichromium species is probably able to oxidize DMF on the surface of the electrode. Attempts to identify the product of oxidation of DMF failed. As in the case of the ferrocene/ferricinium system [14] we assume that the oxidized form of the solvent and the reduced form of the peroxo complex react to give some kind of adduct which is oxidized at the anode with evolution of CO2. This hypothesis explains (i) the increase of the anodic current in cyclic voltammetry (Fig. 6) and (ii) the formation of one mole of CO2 per mole of starting material [Cr(CN)(CO5]- (Ia). Note, however, that, as in the ferrocene case, 0.5 mole of CO2 actually derive from DMF, ie, the p-peroxodichromium species decomposes by giving rise to only one mole of CO2 along with other oxidation products. In acetonitrile, coupling between the primary oxidation product Cr(CN)(CO)5 and molecular oxygen occurs more slowly.
not stable in organic solvents. As paths of decay, ligand exchange reactions of the neutral chromium(I) complexes Cr(CN)(C0)5 (Ib) or Cr(I)(C0)5 (IIb) have been established in acetonitrile as solvent. While Ib undergoes a simple CO substitution for acetonitrile, a solvent-induced inner redox reaction of the type LMCT occurs in the case of IIb leading to a chromium(O) species and elementary iodine. However, the life-times in solution of the neutral complexes Ib and IIb turned out to be long enough render them efficient redox catalysts. :Er(CN)(CO)5]- has thus recently been used to avoid poisoning of the vitreous carbon electrode during the anodic oxidation of cyclopentadienyl($-cyclohexadienyl)iron in acetonitrile [30]. Decomposition in solution of the doubly oxidized cationic complexes F%CNWW + and [Cr(I)(CO)5]+, on the other hand, appears to be extremely fast. A second point of emphasis in this work was concerned with the action of molecular oxygen on paramagnetic organometallic species in nonaqueous media. Oxidative decomposition (with CO2 evolution) of the carbonylchromium systems was particularly fast and efficient in DMF, a solvent which can obviously be oxidized by the product resulting from the reaction between dioxygen and Cr(CN)(CO)5. In MeCN which is much less prone to oxidation, these side reactions proceed much slower. Nevertheless, our results unequivocally demonstrate that oxygen has to be eliminated by flushing with an inert gas for all kinds of electrochemical studies including anodic oxidations. REFERENCES 1. M. K. Lloyd, J. A. McCleverty,
D. G. Orchard, J. A. Connor, M. B. Hall, I. H. Hillier, E. M. Jones and G. K. McEwen, J. Chem. Sot., Dalton Trans. 1743 (1973). 2. A. M. Bond, J. A. Bowden and R. Colton, inorg. Chem.
13, 602 (1974). 3. A. M. Bond and R. Colton, Inorg. Chem. 15, 446 (1976). 4. H. Behrens and H. Zizlsperger, Z. Narurforsch. 16b, 349 (1961).
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oxidation
of chromium
complexes
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5. H. Behrens and D. Herrmann,
Z. Naturforsch. Zlb, 1236 (1966). 6. M. C. R. Symons, S. W. Bratt and J. L. Wyatt, J. Chem.
18. R. G. Compton,
Sot., Dalton Trans. 991 (1982). 7. W. P. Fehlhammer and F. Deael. Annew. Chem. 91. 80 (1979); Angew. Chem. Inr., Ed. Engl.-18, 75 (1979);‘W.
3641 (1993). 19. C. J. Pickett and D. Pletcher, J. Organomer. Chem. 102, 327 (1975). 20. H. Behrens and D. Herrmann, Z. anorg. allg. Chem.
P. Fehlhammer, F. Degel and G. Beck, Chem. Ber. 120, 461 (1987). 8. W. P. Fehlhammer and M. Fritz, Chem. Rev. 93, 1243 (1993).
9. W. P. Fehlhammer and C. Moinet, J. Organomet. Chem. 260, C55 (1984). IO. G. Jacob and C. Moinet, Bull. Sot. Chim. Fr. I-291 (1983). Il. C. Moinet and E. Raoult.. Bull. Sot. Chim. Fr. 128,214 (1991). 12. W.P. Fehlhammer, W. A. Herrmann and K. ofele in Handbuch
der
Priiparativen
Anorganischen
Chemie,
(Edited by G. Brauer) vol. III, p. 1997-1999. Enke Verlag, Stuttgart (198 I). 13. E. W. Abel, I. S. Butler and J. G. Reid, J. Chem. Sot. 2068 (1963). 14. W. P. Fehlhammer and C. Moinet, J. Electroanal. Chem.
158, 187 (1983).
15. J. W. Hershberger, R. J. Klinger and J. K. Kochi, J. Am. Chem. Sot. 104, 3034 (1982). 16. T. Inoue and S. Tsutsumi, J. Am. Chem. Sot. 87, 3525 (1965).
17. G. R. Dobson, M. F. Amr El Sayed, I. W. Stoltz and R. K. Sheline, Inorg. Chem. 1, 526 (1962).
R. Barghout, I. C. Eklund, A. C. Fisher, S. G. Davies, M. R. Metzler, A. M. Bond, R. Colton and J. N. Walter, J. Chem. Sot., Dalton Trans.
351, 225 (1967). 21. M. 0. Albers, N. J. Coville and E. Singleton, J. Chem. Sot., Dalton Trans. 1069 (1982). 22. M. 0. Albers, N. J. Coville and E. Singleton, J. Organomei. Chem. 326, 229 (1987). 23. F. R. Frmzek, W. P. Schaefer and R. E. Marsh. Inorg. Chem. 14, 611 (1975). 24. M. Crawford, S. A. Bedel, R. I. Patel, L. W. Young and R. Nakon, Inorg. Chem. 18, 2075
(1979). 25. S. R. Pickens and A. E. Martell, Inorg. Chem. 19, 15
(1980). and M. Seymour, Polyhedron 1, 21 (1982). 27. H. Kojima, S. Takahashi and N. Hagihara. J. Chem. 26. J. D. Ortego
Sot., Chem. Commun. 230 (1973). 28. N. A. Vol’kenau and V. A. Petrakova, J. Organomet. Chem. 233, C7 (1982). 29. R. Ugo, S. Bhaduri, B. F. G. Johnson, A. Khair, A. Pickard and Y. Benn-Taarit. J. Chem. Sot.. Chem. Commun. 694 (1976). 30. N. Guennec and C. Moinet, J. Organomet. Chem. 503, 171 (1995).
Pergamon PII: Soo13463q%po3167
Arm, Vol. 42, No. 10, pp. 1549-1559, 1997 Copyright 0 1997 Elsevier Science Lrd. Printed in Great Britain. All rights reserved 00134686/97 $17.00 + 0.00
Anodic oxidation of the complexes NEtJCr(X)(CO)s](X
= CNJ).
Reactivity of the electrogenerated neutral species W. P. Fehlhammer,“?
M. Fritz,” D. Flonerb and C. Moinetb*
“Institut fur Anorganische
und Analytische Chemie der Freien Universitat Berlin, FabeckstraBe 34-36, D-14195 Berlin, Deutschland bLaboratoire d’Electrochimie, URA CNRS n”439, Universite de Rennes I, Campus de Beaulieu, 35042 Rennes Cedex, France
(Received 25 March 1996; in revised form 5 July 1996) Abstract-The cyclic voltammograms in DMF or acetonitrile of [Cr(CN)(C0)5]- (Ia) and [Cr(I)(CO)S]- (Ha) show two anodic waves. Under an inert atmosphere, only the first one corresponding to a one-electron-transfer appears reversible for scan rates of 0.1 V s-i. In DMF, in the presence of oxygen, the reversibility totally disappears. Macroscale electrolysis in a batch cell at a potential corresponding to the plateau of the first wave involves two moles of electrons per mole of starting material; during electrolysis, decomposition with CO evolution is observed for both complexes while in the case of IIa, additional formation of iodine takes place. In the presence of oxygen, CO2 is produced during electrolysis. The chemical transformations in MeCN of the two neutral species Cr(CN)(CO)s (Ib) and Cr(I)(CO), (IIb) were studied by cyclic voltammetry immediately after their generation by one-electron oxidation in a flow cell of the parent complexes Ia and IIa. A comparative study of solutions in MeCN of Ia, IIa and Cr(COk, which have been subject to uv irradiation for x 15 min shows that in both cases the first step of the chemical transformation consists in a ligand exchange with formation of [Cr(CN)(NCMe)(CO)a] or [Cr(NCMe)(CO)r], respectively. Ib (Ia) and IIb (Ha) also proved to be efficient redox catalysts for the eiecttooxidation of the cyanide ion and of triphenylphosphine. 0 1997 Elsevier Science Ltd. All rights reserved. Key words: Chromium
complexes, oxidation, electrocatalysis.
INTRODUCTION The anodic oxidation in organic solvents of the anionic complexes [Cr(X)(CO)s]- (X = Cl, Br, I, CN) has been studied by a number of research groups. From cyclic voltammetry at a platinum electrode in dichloromethane solution, Lloyd ef al. [l] inferred a primary oxidation process for the chloro, iodo and cyano complexes consisting of a reversible one-electron-transfer reaction; attempts to prepare the oxidized species by chemical oxidation had failed due to their instability, however. Bond et al. [2] reported on the voltammetry at platinum in acetone of the three halogen0 complexes; at room temperature, the first oxidation step again was found to be a
reversible one-electron process whereas the second step leading to a cationic species appeared non-reversible. In the case of pentacarbonyl(iodo)chromate, however, reversibility of this second step was observed at -75°C. In a subsequent study by Bond and Colton [3], the thermal decomposition of the neutral complex Cr(I)(CO)s which had been prepared by chemical oxidation of the corresponding anion in acetone at low temperature, was followed electrochemically and found to proceed by disproportionation according to Scheme 1. 2 Cr(IKOh
+ [Cr(I)W%l-
+ [Cr(I)(CO)5]+ 1 decomposition
Scheme 1. *Author to whom correspondence should be addressed. t Present Address: Deutsches Museum, Museumsinsel 1, D-80538 Miinchen, Deutschland.
In the sixties Behrens et al. had reported on the reaction of Na[Cr(I)(COh] with iodine at low temperature which resulted in a blue solid identified
1549
W. P. Fehlhammer et al.
1550
as Cr(I)(CO)s [4]. From there a synthetic route was opened to other brightly coloured neutral species formulated as and Cr(CN)(CO)s (green) Cr(NCS)(CO)s (red) [5]. Note, however, that parts of this work have later been questioned [l, 61. Oxidation in appropriate solvents of [Cr(CN)(C0)5]- by arene diazonium salts in an inert atmosphere has been developed by our group to an efficient synthetic method for novel functional isocyanides. Carried out in, say, chloroform, this “radical alkylation of cyano complexes” gives [Cr(CO)rCNCClr] in high yield. In the presence of oxygen, however, a totally different product, [(OC)$ZrCNC(=O)NCCr(Co)s], was obtained from the same solvent [7, 81. In a preliminary note [9], we have already reported on the different course the electrochemical oxidation of [Cr(CN)(CO)r]- takes under oxygen as compared to that under an inert atmosphere. In order to complete the previous work, this paper presents studies of the anodic oxidation of [Cr(CN)(CO)j]and [Cr(I)(CO)s]- under various conditions. Particular emphasis is put on the question of stabilities and relative reactivities towards selected reagents of the oxidized species.
EXPERIMENTAL Electrochemistry All experiments were carried out at room temperature. The solutions were deaerated and saturated with dinitrogen except in cases where the investigations were performed in the presence of oxygen. Conventional electrochemical equipment was used for cyclic voltammetry (EGG PAR model 362 scanning potentiostat with an XY recorder). For both, cyclic voltammetry and voltammetry at a rotating electrode, the working electrode was a disc of vitreous carbon (diameter 3 mm) of platinum (diameter 2 mm). All potentials refer to the saturated calomel electrode (see) and have not been corrected for the Ohmic drop. Controlled potential electrolyses were carried out in a batch cell [lo] at a platinum or vitreous carbon electrode (diameter of disc 4cm). For the preparation of the solutions, 10m3to 2 low3 mole of the respective substrate (NEt&r(CN)(CO)s], NEt4[Cr(I)(C0)5]) was dissolved in 150 ml of DMF or acetonitrile; the concentration of the base electrolyte (NBu;[BF.+] or LiC104) was 0.1 M. Gas analyses were performed either by mass spectrometry (CO and COz) or by reaction with an aqueous solution of calcium hydroxide or sodium hydroxide followed by an acidometric titration (COZ) or by reaction with an aqueous solution of ammoniacal silver nitrate (CO). Continuous electrolyses were carried out in a flow cell [l 11 fitted with a graphite felt anode (diameter
4 cm, thickness 1.2 cm) and two electrical circuits. The solutions studied contained 0.25 g of NEt[Cr(CN)(CO)s] or NEt_JCr(I)(C0)5], respectively, dissolved in 250 ml of acetonitrile, and were 0.1 M in LiC104 (as base electrolyte). They were deaerated and passed through the porous anode at a rate of 4 ml per minute. Current intensities for one-electron processes were imposed (ie, 18.5 mA with il = 12.5 mA for the upstream circuit and iz = 6 mA for the downstream circuit for NE&[Cr(CN)(CO)s and 14.5 mA with il = 10 mA and iz = 4.5 mA for NEtJCr(I)(CO)s]). The irradiation experiments were carried out with a Hohensonne loo-type 1081-mercury lamp (uu + ir, 300W) which was placed at a distance of approximately 20 cm off the voltametric cell. The complexes Cr(C0)6, NEtJCr(CN)(C0)5] and NEtJCr(I)(C0)5] were 4-6 x 10m3M, and the base electrolyte, NBu;BF+ was 0.1 M in acetonitrile solution. Cyclic voltammograms were recorded before and after 15 min of uv irradiation. Starting materials NEt[Cr(CN)(CO)s] and NEt[Cr(I)(CO)s] were prepared according to published procedures [12,13]. Analytical grade dimethyl formamide (DMF), acetonitrile and acetone were purchased from SDS, and lithium perchlorate purum from Fluka, and used without further purification. Tetra(nbutyl)ammonium tetrafluoroborate purum from Fluka was recrystallized from ethanol-water prior to use.
RESULTS 1. Electrochemical studiesin the absence of oxygen 1.1. Cyclic voltammetry. At vitreous carbon or platinum electrodes, at scan rates of 0.1 V s-l, the first anodic process appears reversible for both [Cr(CN)(C0)5](Ia) and [Cr(I)(CO)S]- (IIa) in DMF, acetonitrile and acetone (Table 1). At lower
Table 1. Voltammetric data for NEt.&r(CN)(CO)s] (Ia) and NEt.+[Cr(I)(CO)s] (Ha) at a vitreous carbon electrode; in (a) DMF, (b) acetonitrile, (c) acetone; scan rate 0.1 V s-l Solvent
[Cr(CWCW- (W (4 (b) [Cr(I)(CO)s]- (IIa)
[Zi (b) (c)
Ep.1”
EPA”
Epal”
0.82 0.71 0.74 0.61 0.47 0.55 (0.73)”
0.72 0.61 0.66 0.52 0.39 0.42 (0.66)*
1.15 1.38 I .25 0.95 0.99
“VW,. *Electrolyte NEhClOd; VA~/A~SCI [2].
1.05 (1.24)b
Anodic oxidation of chromium complexes
Fig. 1. Voltammograms of NEti[Cr(CN)(C0)5] (Ia) in acetonitrile [substrate concentration = 4 x IO-’ M, c (electrolyte NBu;BFd) = 0.1 M; vitreous carbon electrode]. (a) scan rate 0.2 V SK’; ~ first scan, - - - second scan; (b) scan rate 0.02 V s-l; (c) scan rate 0.2 V s-l; ~ first scan, - - ~ second scan. -.-.-.- third scan.
scan rates (0.01 V s-i), reversibility is only observed for the iodo complex Ha. For both complexes, a second, irreversible step occurs at more anodic potentials. In acetonitrile, a new reversible process can be detected at approximately 1 V,, (Fig. 1, peaks Z-2’) after oxidation of
1551
the cyano complex Ia either (i) up to a potential (1.35 V,,,) corresponding to the second oxidation step for a sweep rate of 0.2 V s-i (Fig. l(a)), (ii) up to the potential of the first step (1.08 V,,,) for a 0.02 V s-’ sweep rate (Fig. l(b)) or (iii) up to the potential of the first step (1.05 V,,.,) on the second scan for 0.2 V s-l sweep rate (Fig. l(c)). The voltammograms at a scan rate of 0.1 V s-’ of the solution salt of Ia in slightly acidic (pH 4.8) and neutral aqueous solutions again exhibit a first reversible system (Ep, = 0.56 V,,,. Ep, = 0.47 V,,,) followed by an irreversible one at a more anodic potential (Ep, = 1.1 V,,,). Reversibility of the first step is still observed at the much lower sweep rate of 0.01 V SK’. In slightly basic aqueous media (pH 9.2) however, reversibility is already lost at a scan rate of 0.1 vs ‘. 1.2. Batch cell electrolysis. Electrooxidation at vitreous carbon or platinum anodes in DMF or acetonitrile of the cyanochromium complex Ia was performed at a potential corresponding to the plateau of the first wave (DMF: E = 0.85 V,,,; MeCN: E = 0.9 V,,,). In both media, CO is evolved during electrolysis while no cyanogen (Nd---C=N) has been detected. After consumption of two moles of electrons per mole of starting material, voltammograms showed the initial first and second waves to have disappeared. Electrooxidation (acetonitrile, vitreous carbon anode) of the iodo species IIa was carried out at the potential of the first anodic process (E = 0.9 V,,,.) involving two moles of electrons per mole of starting material. Again, CO evolution was established. A cyclic voltammogram recorded at the end of the electrolysis showed the presence of a reducible species (Fig. 2) which was identified as iodine. 1.3. Flow cell electrol.vsis. In order to study the chemical transformations of the primary products of the anodic oxidation, the electrolyses of the cyano (Ia) and iodo complexes (IIa) in acetonitrile were performed in a flow cell fitted with a graphite felt porous anode. Due to the high efficiency of the graphite felt electrode, the complexes are oxidized in high yields in spite of very short contact times between solution and anode; thus most of the chemical transformation of the electrogenerated neutral species Ib and IIb takes place off the electrode. In terms of the theoretical amount of electricity required for this process-one mole of electrons per mole of complex-more than 85% of the starting material has reacted. In the case of complex Ia, the resulting solution is yellow at the cell outlet, yet turns red quickly. By cyclic voltammetry, a new reversible system is observed at a potential more anodic than that of the initial first anodic wave (Fig. 3). The voltammogram is unchanged after two h. (Note that the time between the oxidation at the porous anode and the recording of the first voltammograms was about 10 min as a rule).
W. P. Fehlhammer et al.
1552
The iodo complex Ha gives rise to a green solution at the outlet of the cell which then turns yellow. The green colour probably results from a mixture of yellow, the final colour, and blue due to unstable Cr(I)(CO)S [4]. A voltammogram recorded at a rotating disc electrode, immediately after collecting the solution at the cell outlet showed several anodic and cathodic waves (Fig. 4(b)): two waves (1 and 4) for the non-oxidized starting material; two new anodic waves (2 and 3); a first cathodic wave (1’) for the reduction of non-decomposed neutral [Cr(I)(C0)5] (IIb); and a second cathodic wave (5’). After one hour (Fig. 4(c)), waves 1, 1’ and 4 have decreased while waves 2 and 3 have increased. Wave 3 is indicative of a new reversible system (Fig. 5(b), waves 2 and 5 are attributed to a mixture of iodine and iodide (Fig. 5(c)). 2. Electrochemical studies in the presence of oxygen In two preliminary notes [9, 141 we have demonstrated the effect of dioxygen on the course of the anodic oxidation of organometallic compounds and the specific role the solvent plays in this connection. Here, we present the full experimental data on [Cr(CN)(C0)5]together with new results on [CrU)(CO)51-. 2.1. Cyclic voltammetry. Admission of dioxygen to
a solution of Ia in DMF causes dramatic changes in the morphology of the cyclic voltammogram (Fig. 6): the first anodic process is no longer reversible, and the I
second anodic step has disappeared. In acetonitrile, in contrast, no striking modifications are observed of the first oxidation, apart from a slight decrease in intensity of the cathodic peak [9]. A similar phenomenon had been observed with ferrocene under the same conditions [14]. Here, in the presence of dioxygen, electrogenerated ferricinium was shown by cyclic voltammetry to be much more stable in acetonitrile than in DMF. In the case of the iodo complex IIa, the overall influence of molecular oxygen on its anodic oxidation is less pronounced at the time scale of cyclic voltammetry. In DMF, only a partial decrease in reversibility of the first anodic step is shown in the voltammogram. In acetonitrile, voltammograms recorded at a scan rate of 0.1 V s-r are the same, no matter whether dioxygen was present or absent. 2.2. Batch cell electrolysis. Macroscale electrolyses in the presence of dioxygen of the cyano complex Ia were performed in DMF and acetonitrile at a working potential corresponding to the first anodic wave. At platinum or vitreous carbon, both CO and CO2 evolution characterized by mass spectrometry [9] occurred, the amount of COZ depending on the solvent used. In DMF, one mole of CO* has been observed per mole of starting compound, in acetonitrile only 0.5. Coulometric data in DMF differ from those in acetonitrile in a similar way: While in DMF two moles of electrons are transferred per mole of
JA
50
Fig. 2. Voltammograms of NEt.+[Cr(I)(CO)s](IIa) in acetonitrile [substrate concentration = 1.4 x lo-* M, c (electrolyte LiC104) = 0.1 M; vitreous carbon electrode; 0.2 V s-l]. Before electrolysis (E = 0.8 V,,,) at a vitreous carbon anode, --- after electrolysis.
Anodic oxidation of chromium complexes
1553
Fig. 3. Voltammograms of NEt&r(CN)(C0)5] (Ia) in acetonitrile [substrate concentration = 2.9 x IO-’ M, c (electrolyte LiClO4) = 0.1 M; vitreous carbon electrode; 0.2 V S-I]. Before electrolysis at a graphite felt porous anode, - - - after electrolysis.
chromium complex, it is only approximately 1.6 moles of electrons in acetonitrile. It is worth mentioning that in acetonitrile the current intensity decreases to about 10% of the initial value after consumption of only 0.3 moles of electrons, ie, shortly after the electrolysis has started. Interestingly, a very similar solvent dependence has been observed for macroscale electrolyses of ferrocene. In DMF, anodic oxidation occurred with formation of 0.5 moles of CO2 also involving two moles of electrons per mole of ferrocene. Here, CO* is probably formed by electrochemical oxidation of some kind of ferricinium-Oz-DMF complex as no CO2 was evolved during the decomposition of chemically prepared ferricinium in DMF in the presence of 02, In acetonitrile, only approximately 0.3 moles of electrons per mole of ferrocene were involved in the anodic oxidation, and no CO2 evolution was observed. On the other hand a mixture of ferrocene and ferricinium in acetonitrile totally decomposes to a brown precipitate within a few minutes in the presence of oxygen. 3. Ekctrocatalytic properties of chromium complexes Ia and IIa. The catalytic oxidation of triphenylphosphine in acetonitrile by the electrogenerated radical cations of Cr(NCMe)(COh and Cr(CO)5(py) (py = pyridine) has previously been described by Hershberger et al. [15]. The pentacarbonyl(cyano) (Ia) and -(iodo)
chromates (IIa) have now been found to exhibit similar electrocatalytic properties. Thus, cyanide (CN-) whose anodic oxidation is reacts rapidly with electrogenerated slow, Cr(CN)(CO)S (Ib) in DMF as shown by the loss of reversibility in cyclic voltammetry and the increase of the anodic current. A macroscale electrolysis in the same medium of a mixture of [Cr(CN)(CO)$ and CN- at vitreous carbon (E = 0.85 V,) involved two moles of electrons per mole of complex Ia, yet one mole of electrons per mole of cyanide anion. CO evolution characterized by reaction with an aqueous solution of ammoniacal silver nitrate occurred during the second part of the electrolysis. No cyanogen has been detected by mass spectrometry, however. This is in accord with a mechanism previously proposed by Inoue and Tsutsumi for the anodic oxidation of cyanide which circumvents the formation of cyanogen [ 161. Catalytic electrooxidation in DMF, acetonitrile or acetone of triphenylphosphine (Fig. 7) or p-toluenesulfinic acid by Cr(I)(COb, is again indicated by an increase in the cyclic voltammogram of the anodic peak and a decrease of the cathodic peak. 4. Electrochemical studies of uu-irradiated carbonylcbromium complexes It is known that w irradiation of Cr(CO)h in acetonitrile causes ligand substitution with formation of Cr(NCMe)(CO)S and [Cr(NCMe)2(C0)4] [17].
W. P. Fehlhammer et al.
1554
r-
a
c! Jb
5cI-
> ./
1
E vsc?
i /IN
Fig. 4. Voltammograms of NEt4[Cr(I)(C0)5] (Ha) in acetonitrile [substrate concentration = 2.2 x lo-) M, c (electrolyte LiCIO4)= 0.1 M; rotating vitreous carbon electrode (2000rpm)]. Before electrolysis at a graphite felt porous anode, - - - approx. 10 min after electrolysis, -.-.-.- approx. 1 h after electrolysis. More recently, Compton et al. [18] have shown that three new oxidation waves are observed at 0.9, 0.4 and -0.1 V,, after illumination at 320 mn of Cr(CO)e in acetonitrile. The waves were attributed to [Cr(NCMe)(C0)5], [Cr(NCMe)2(CO)4] and [Cr(NCMe)j(CO)3] respectively. In order to understand the electrochemical transformations of the electrogenerated neutral species Ib and IIb, solutions in acetonitrile of Cr(CO)e, [Cr(CN)(C0)5]and [Cr(I)(CO),]- have been irradiated with a mercury lamp and subsequently studied by cyclic voltammetry. the initially colourless After uv irradiation, solution of Cr(C0)6 is yellow-orange. New waves are observed at less anodic potentials which correspond to the two reversible systems Ep,l = 0.33 V,, and Ep,2 = Epa~ = 0.41 V,,,, 0.92 V,,,, Epa = 0.84 V,, (Fig. 8(a))..
For [Cr(CN)(CO)j]-, the initially colourless solution also turns yellow during irradiation. The voltammogram (Fig. 8(b)) now shows two new anodic peaks, Ep, = 0.14 and 1.01 V,,,, at the first anodic scan; in the case of the non-irradiated cyano complex Ia, the peak at Ep, = 1.01 V,, appears only at the second anodic scan (cf. 1.1). For [Cr(I)(CO)S]- (Da), two new reversible systems with Ep,, = 0.42 V,,, Ep,,O.34 V,, and Ep,2 = 0.94 V,,, , Epc2= 0.85 V,, were observed (Fig. 8(c)). DISCUSSION As described previously [l, 21, the electrochemical oxidation in organic solvents of [Cr(CN)(CO)& and [Cr(I)(CO)$ involves two one-electron steps (cf. Scheme 2).
Anodic oxidation of chromium complexes
1555 4
b
+.
i
/
’ ’
EVsce
i
Fig. 5. Voltammograms of NEti[Cr(I)(CO)s] (Ha) in acetonitrile [substrate concentration = 2.2 x 10S3 M, c (electrolyte LiClOd) = 0.1 M; vitreous carbon electrode; 0.2 V s-l]. _ Before electrolysis at a graphite felt porous anode, --~ approx. I h after electrolysis, -‘-_ voltammograms of 12 (5.5 x 10m4 M) + NBGI (I.1 x 10m3 M) in acetonitrile (for comparison)
[CrWKOW
* -
Cr(W(COh
Ir
Cr(W(CO)5
IP
e [Cr(X)(CW+
For the iodo complex IIb, a ligand exchange with the solvent occurs (Scheme 3). Cr(I)(COb + MeCN + [Cr(NCMe)(COb]
+ l/2 12
Scheme 2 (X = CNJ).
Scheme 3.
Our experiments show that of the neutral complexes, Cr(I)(COh (IIb) has a much higher stability than Cr(CN)(CO)S (Ib), and that both cations [Cr(X)(CO)s]+ (X = CN (Ic), I (11~))resulting from one-electron oxidations of Ib and IIb, respectively, are extremely unstable.
Experimental results confirming this hypothesis are: (i) the amount of iodine present in the bulk at the end of the macroscale electrolysis (Fig. 2). (ii) the mixture of iodine and iodide ions that is found in the medium after electrolysis in a flow cell and complete transformation of IIb (Fig. 4, curve (c)) (Notice, that unreacted [Cr(I)(CO)S]- is oxidized by iodine (Fig. 4, curve (b), waves 1 and 4).), and (iii) a new compound (Fig. 4, curve (c), wave 3) which is identified as Cr(NCMe)(CO)J by comparison with the product from uv irradiation in acetonitrile of Cr(CO), (Fig. 8(a), peak 2) or [Cr(I)(CO)S]- (Fig. 8(c), peak 2) respectively; the potential of peak 2 in Fig. 8(a) is in accord with the values published by Pickett et al. [19], Hershberger et al. [15] and Compton et al. [18]. By analogy, peaks 1 of Fig. 8(a) and 8(c) are attributed to Cr(NCMe)2(C0)4] [15, 18, 191.
1. Chemical transformations of the neutral species Ib and IIb in an inert atmosphere
First of all it should be pointed out that the chemical transformation of the neutral species Cr(X)(CO)s (X = CN (Ib),I (IIb)) very much depends on the nature of X. Though studied in detail only in acetonitrile, similar phenomena are most likely to be found in DMF solutions.
1556
W. P. Fehlhammer et al.
Identical observations had earlier been made by Behrens and Herrmann in a study of reactions with mono- and bidentate N- and P-ligands of their supposed Cr(I)(COb; just as in our experiments, both penta- and tetracarbonyl derivatives of Cr(CO)e had formed along with elemental iodine by obviously homolytic cleavage of the Cr-I bond [20]. To explain the chemical transformation of IIb in acetonitrile, it thus appears unnecessary to consider a disproportionation reaction of this species as suggested by Bond and Colton (Scheme 1) [3]. In our case, the ligand exchange with the solvent is probably faster than disproportionation. Further, it is interesting to note that, at the voltammetry scale, the stability of [Cr(NCMe) (CO)J]+ seems higher than that of [Cr(I)(C0)5]+ (11~). Obviously, the chemical transformation of IIc also proceeds by ligand exchange with the solvent (Scheme 4). [Cr(I)(C0)5]+ + MeCN + [Cr(NCMe)(C0)5]+
i
/
’
:
1
I
lo-
+ l/2 I2
Scheme 4. This follows from the fact that cathodic peaks for the reduction of both [Cr(NCMe)(CO)S]+ and iodine have been detected after oxidation of the complex anion IIa into the complex cation IIc (Fig. 5(a)). In contrast to IIb, a carbonyl ligand instead of CN is readily replaced by a solvent molecule in the cyanochromium radical Ib (Scheme 5). This common type of substitution is particularly favoured by a low electron density at the metal which precludes any strong x back-bonding to CO. A pathway involving organometallic radicals has also been suggested for
b
Ev see
Fig. 7. Voltammograms of NEt[Cr(I)(CO)s] (Ha) in acetone [substrate concentration = 2.7 x 1O-3 M, c (electrolyte NBuIBFd) = 0.1 M; vitreous carbon electrode; 0.1 V S-I]. Before addition of PPh3, ---after addition of PPh, (c = 4.1 x 1O-3 M).
various carbonyl substitution reactions which are catalyzed by di- and polynuclear transition metal complexes [21, 221. Cr(CN)(C0)5 + MeCN + [Cr(CN)(NCMe)(C0)4]
+ CO
Scheme 5. In accordance with this mechanism, the new reversible system (Ep. = 1.01 V,,) observed in the cyclic voltammogram of Ia (Fig. 1, peaks 2-2’) or
Fig. 6. Voltammograms of NEt4[Cr(CN)(CO)s] (Ia) in dimethyl formamide [substrate concentration = 2.3 x IO-’ M, c (electrolyte NBuZBF4) = 0.1 M; vitreous carbon electrode 0.2 V s-i]. Under Nz, ---under OZ.
after electrolysis of Ia in the flow cell (Fig. 3(b), waves 2-2’) is also present after uv irradiation of Ia in the same medium (Fig. 8(b), wave 2). The oxidation potential of this system clearly differs from the value for Cr(NCMe)(COh (Ep, = 0.92 V,,,). Cyclic voltammetry (Fig. 3(b)) also shows that [Cr(CN)(NCMe)(C0)4]+ is more stable than Ic. The latter undergoes a fast carbonyl ligand exchange with acetonitrile in S~U~Unascendi at the electrode (Scheme 6).
Anodic oxidation of chromium complexes
1557
a
hA
Fig. 8. Voltammograms of (a) Cr(CO)h (c = 5.9 x 10-l M), (b) NEt4[Cr(CN)(CO)s] (la) (c = 4.1 NEt@(I)(C0)5] (Ha) (c = 4.7 x IO-) M) in acetonitrile [c (electrolyte NBu:BFd) = 0.1 M; vitreous 0.2 V SK’]. __ Before ut’ irradiation, - -- after 15 min of irradiation.
[Cr(CN)(C0)5]+ + MeCN -+ [Cr(CN)(NCMe)(CO)J+
+ CO
Scheme 6. A cathodic
peak assigned to the reduction
of
[Cr(CN)(C0)5]-
10-j M) and (c) carbon electrode:
[Cr(CN)(NCMe)(C0)4]+ is observed after oxidation of the cyano complex anion Ia into the cation Ic (Fig. l(a), Fig. 3(a)). The coulometric data as obtained from an exhaustive electrolysis in a batch cell can be explained by an ECE mechanism for the oxidation of [Cr(CN)(CO)s]- (Scheme 7).
--) Cr(CN)(CO)s
Cr(CN)(CO)5 + MeCN + [Cr(CN)(NCMe)(CW [Cr(CN)(NCMe)(C0)4
x
-+ [Cr(CN)(NCMe)(C0)411+ Scheme 7.
+ CO
-+ decomposition
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W. P. Fehlhammer et al.
In the case of the iodo system II, the mechanism of the electrochemical oxidation is far more complicated due to the additional step of a chemical oxidation of the starting material by iodine (Scheme 8).
CONCLUSIONS From this study it becomes obvious, that the primary products resulting from one- or two-electron oxidations of [Cr(CN)(CO)5]- and [Cr(I)(CO)5]- are
- i< [Cr(I)(CO)sl- + Cr(I)(C0)5 Cr(I)(CO)5 + MeCN + Cr(NCMe)(C0)5 [Cr(I)(CD)s]- + l/2
12 +
cr(1)(co)s
+ l/2 1~ + 1-
- IP
Cr(NCMe)(C0)5
-+ [Cr(NCMe)(C0)5]+
+ decomposition
I- 4 l/2 12 Scheme 8. 2. Chemical transformations of [Cr(CN)(CO)$
in
the presence of dioxygen
The particular effect of dioxygen on the course of the electrochemical oxidation of [Cr(CN)(CO)5]- can probably be explained by a radical coupling between the two paramagnetic species [Cr(CN)(C0)5] and 02. To account for the accumulated experimental evidence on the oxidation of the cyanochromium complex system, on the one hand, and similar findings in the system ferroceneiferricinium, on the other, we propose an adduct with an 02-bridge between two chromium moieties. In fact, a p-peroxodichromium structure analogous to that of related p-peroxodicobalt systems [23-261 appears more likely than a peroxide link between two ligands (here CO or CN) as suggested for 19-electron complexes [27,28]. In the latter case, formation of one mole of CO2 per mole of starting material would have to be expected in DMF as well as in acetonitrile in analogy to the formation via an ON-O-O-NO bridged intermediate of [Ni(Cl)(NOz)dppe] from [Ni(Cl)(NO)dppe] and dioxygen [29]. The p-peroxodichromium species is probably able to oxidize DMF on the surface of the electrode. Attempts to identify the product of oxidation of DMF failed. As in the case of the ferrocene/ferricinium system [14] we assume that the oxidized form of the solvent and the reduced form of the peroxo complex react to give some kind of adduct which is oxidized at the anode with evolution of CO2. This hypothesis explains (i) the increase of the anodic current in cyclic voltammetry (Fig. 6) and (ii) the formation of one mole of CO2 per mole of starting material [Cr(CN)(CO5]- (Ia). Note, however, that, as in the ferrocene case, 0.5 mole of CO2 actually derive from DMF, ie, the p-peroxodichromium species decomposes by giving rise to only one mole of CO2 along with other oxidation products. In acetonitrile, coupling between the primary oxidation product Cr(CN)(CO)5 and molecular oxygen occurs more slowly.
not stable in organic solvents. As paths of decay, ligand exchange reactions of the neutral chromium(I) complexes Cr(CN)(C0)5 (Ib) or Cr(I)(C0)5 (IIb) have been established in acetonitrile as solvent. While Ib undergoes a simple CO substitution for acetonitrile, a solvent-induced inner redox reaction of the type LMCT occurs in the case of IIb leading to a chromium(O) species and elementary iodine. However, the life-times in solution of the neutral complexes Ib and IIb turned out to be long enough render them efficient redox catalysts. :Er(CN)(CO)5]- has thus recently been used to avoid poisoning of the vitreous carbon electrode during the anodic oxidation of cyclopentadienyl($-cyclohexadienyl)iron in acetonitrile [30]. Decomposition in solution of the doubly oxidized cationic complexes F%CNWW + and [Cr(I)(CO)5]+, on the other hand, appears to be extremely fast. A second point of emphasis in this work was concerned with the action of molecular oxygen on paramagnetic organometallic species in nonaqueous media. Oxidative decomposition (with CO2 evolution) of the carbonylchromium systems was particularly fast and efficient in DMF, a solvent which can obviously be oxidized by the product resulting from the reaction between dioxygen and Cr(CN)(CO)5. In MeCN which is much less prone to oxidation, these side reactions proceed much slower. Nevertheless, our results unequivocally demonstrate that oxygen has to be eliminated by flushing with an inert gas for all kinds of electrochemical studies including anodic oxidations. REFERENCES 1. M. K. Lloyd, J. A. McCleverty,
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