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Electrochemical properties of two new binuclear ethyne-bridged cobalt complexes: bis-[(η5-cyclopentadienyl)cobaltη4-1,3-cyclopentadien-5-exo-yl)]-ethyne (1) and di-cobaltocenylium-ethyne-di-hexafluorophosphate (2)
Ekctrmhimica Acta, Vol. 38, No. Il. pp. 1527-1533,t993 Printed in Great Britain
00134686/93 $6.00+ 0.00 Q 1993.Perpmon Prrslt&i
ELECTROCHEMICAL PROPERTIES OF TWO NEW BINUCLEAR ETHYNE-BRIDGED COBALT COMPLEXES: BIS-[(q5-CYCLOPENTADIENYL)COBALT(q4-1,3CYCLOPENTADIEN-5-EXO-YL)]-ETHYNE (1) and DI-COBALTOCENYLIUM-ETHYNE-DIHEXAFLUOROPHOSPHATE (2) HERWIG SCHOTTENBBRGER, CHRISTOPHERRIEKERand DAGMAROBENDORF* Institut fiir Anorganische und Analytische Chemie der Universitiit Innsbruck, Innrain 52 a, A-6020, Austria (Received 2 November 1992; in revisedjiirm
12 January 1993)
Ahstract-The results of an electrochemical study of two new z-bridged cobalt complexes [($C,H,)Co{(r14-C,H,~C~C-(r14-C~Hs)} ‘Wrl’-C,H,)l (1) aad C(I’-C,H,)C~((~~‘-C,H~-C~~‘CSH3}Co(qs-C,H,)]‘+ 2 PF; (2) are discussed. Reduction of (2) proceeds in two reversible one-electron steps from the dication to the neutral compound. A AE” separation of 18OmV indicates interaction between the two ethyne-bridged cobalt centers. Further reduction of (2) leads to formation of a monoanion and subsequent decomposition. Oxidation of (1) occurs by an (E)ECE (E = electrochemical step, C = chemical step) process. The character of the chemical reaction has been probed by cyclic voltammetry in solvents of varying donor ability.
Kev words: ethyne-bridged binuclear cobalt complexes, cyclic voltammetry, chronoamperometry, redox mechanism, sol&t dep&dence.
INTRODUCTION Organometallic compounds, in particular those forming mixed-valence complexes, are of increasing interest as they serve as models of synthetic conducting polymers and provide additional insight regarding intramolecular electron and charge transfer or delocalization. Furthermore they have also been recognized as playing a major role in the new emerging opto-electronic technologies[l-31. n-bridged bi- and polynuclear complexes have been subject to extensive investigations in order to get information about intervalence transfer properties as a function of distance between two or more interacting groups, as a function of the nature of the bridge between them and as a function of their relative orientation and chemical environment[l-81. Though mixed-valence sandwich compounds with various metal centers have been prepared and studied, the most common examples are still members of the ferrocene series. As cobalt cyclopentadiene complexes can serve as electron-donor as well as electron-acceptor groups, either in the form of a neutral cobaltocene or cobaltocenium substituent, x-bridged cobaltocenium complexes might also be of interest in the design of tunable organometallic materials for nonlinear optics[l]. We want to report here on the electrochemical properties of two new x-bridged cobalt complexes: the cobalt analogue of diferrocenylacetylene, [(~5-C,H,)Co{(q5-C5H&C==C-(~5-C5H,)~Co(q5C,H,)J*+ 2 PF, (9, and the corresponding endohydride, [(q’ - C5H5)Co{(q4 - C,H,) - C-C -(q4-
C5H5))Co(q5-C,H,)] (1). Our interest in the electrochemical properties of these compounds was based on the attempts to find a more convenient way to prepare bi- or polynuclear bridged metallocene complexes. In general the synthesis of substituted q’-cyclopentadienyl cobalt compounds, [(q5-C5H4R)Co(~5-C5H5)], where R represents an organic group, can be achieved either by oxidative elimination of a ring substituent from [(q4C5H5R)Co(q5-C,H,)]-compounds by chemical oxidizing agents like tritylium salts (PhJ+X-; X= PF;, BFJ or electrochemical oxidation[P-141. However, in some cases our attempts to prepare binuclear bridged metallocene complexes by chemical oxidation of the (~4-cyclopentadiene)(~5-cyclopentadienyl)-cobalt moiety of the complex failed and we hoped that electrochemical oxidation might be more successful. Though hydride compounds like (1) have been reported as intermediates in the synthesis of heteronuclear bi- and termetallocenes[lS] nothing is known about their redox properties. In order to get more insight into the redox mechanism we investigated (1) and (2) by cyclic voltammetry and related techniques. EXPERIMENTAL Physical measurements
Cyclic voltammetry experiments were carried out with a HEKA potentiostat/galvanostat PG 28 system. Measurements with scan rates up to 1 Vs- ’ were recorded on a Philips PM 8133 X-Y recorder.
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H.SCHOITENBERGER et al.
1
For faster scan rates data acquisition was achieved via an IBM compatible computer using the HEKA IBM-Elektrochemie system and a Hewlett-Packard 7475A plotter. For all measurements a three electrode configuration and two different electrode combinations were used: either a platinum disk working electrode (diameter 1 mm) sealed in soft glass, a platinum wire counter electrode coiled around the glass mantle of the working electrode and a silver wire pseudoreference electrode or a platinum ring working electrode, a glassy carbon counter electrode and a silver/silver chloride reference electrode. All potentials were calibrated against the formal potential of the Fc/Fc+ couple (0.42OV vs. Ag/AgCl). Solutions of the complexes were ca. 0.4mM in acetonitrile, unless otherwise stated, and contained 0.1 M ntetrabutylammonium hexafluorophosphate (TBAH) as supporting electrolyte. Acetonitrile, dichloromethane, pyridine and TBAH were obtained from . Fluka Chemicals (high purity). All solvents were dried according to literature procedures and freshly distilled before use. All manipulations and electrochemical measurements were performed under argon. E” values were taken where possible as the average of the anodic and cathodic peak potentials. Electrochemical reversibility was judged on the basis of peak-to-peak separations (A&), relative to the predicted value of 59mV for a reversible one electron transfer[l6,17]. Infrared spectra of KBr moldings were recorded on a Biorad Model FTS-40 or a Pye-Unicam SP3-300 spectrometer. ‘H NMR spectra were recorded on either a Varian EM 360L (60MHz) or on a Bruker AM-300 (13C spectra) with tetramethylstiane as internal standard. Mass spectra were obtained with a MAT CH7 mass spectrometer. Ultrasonication involved the use of a Bandelin Sonorex RK 255 S cleaning bath.
dissolved in 20ml of dimethoxyethane and 0.23ml (0.17 g ; 1.5 mmol) tetramethylethylenediamine and 0.351 g (1.05 mmol) of cobaltocenium hexafluorophosphate were added. The reaction mixture was ultrasonicated for 45min at 0°C. After removal of the solvent, the precipitate was dissolved in diethylether and subjected to chromatography (Al,O,: Merck; activity 3). The product was eluted with diethylether as an orange band. After removal of the solvent 0.177g (29% yield) of a spectroscopically pure product were obtained. ‘H NMR (C,D,): 6 2.60 (pseudo-q, 4H, J = 2.1 Hz), 3.39 (pseudo-t, 2H, J = 2.1 Hz), 4.37 (s, lOH), 4.95 (pseudo-t, 4H, J = 2.1 Hz). “C NMR (C,D,): 6 40.92 (d); 43.00 (d); 75.03 (d); 77.92 (s) 79.29 (d). ir (KBr): 3090; 3050; 2910; 2850; 1405; 1283; 1185; 1108; 1062; 1008; 952; 858; 807cm-‘. Mass spectrum: m/z 402 (100%). Di-cobaltocenylium-ethyne-di-hexaj?uorophosphate (2). To a solution of 0.402g (l.Ommol) of (la) in
CH,Cl, 0.814g (2.1 mmol) of tritylium hexafluorophosphate were added at ambient temperature and subsequently stirred for 90min. The solvent was removed in uacuo. The precipitate was dissolved in acetonitrile and chromatographed over A&O, (Merck, activity 3). A solvent gradient of diethylether/acetonitrile (l/l v/v) to pure acetonitrile was applied. The product was eluted as the final yellow band with pure acetonitrile. Depending on the condition of the alumina used, it may occur that the product is completely retarded. In this case the column may be flushed with saturated aqueous NH,Cl solution. This procedure leads to a remobilisation of the product, so that subsequent elution with methanol/acetonitrile yields the desired product as a yellow band. After removal of the solvent the product may be crystallized by slowly diffusing diethylether into a acetonitrile solution of (2). The same product may also be obtained by a straight forward reaction sequence without isolation of (qs-cyclopentadieny1)C(q4-1,3-cyclopentadiene)-5ethynyll-cobalt; ie by in situ deprotonation to (la) and reaction with an additional equivalent of Compound preparation cobaltocinium hexafluorophosphate. For this “one pot” reaction sequence DME is preferred as the Bis[($ - cyclopentadienyl)(t14 - 1,3 - cyclopentadien 5-exo-yl)cobalt]-ethyne (1). To a solution of 0.214g ’ solvent. ‘H NMR (CD,CN): 6 5.77 (s, lOH), 5.78 (pseudoof (~5-cyclopentadienyl)[(~4-l,3-cyclo(1 mmol) t, 4 H, J = 2.1 Hz), 5.98 (pseudo-t, 4H, J = 2.1 Hz). pentadiene-5-ethynyl]-cobalt[18] in 8Oml of nhexane 0.625ml of a methyllithium solution (1.6 13C (CD,(X): 6 86.61 (d), 87.63 (d), 87.73 (d) (only H-bearing carbons observed). ir (KBr) 3120; 1505; molar in diethylether) were added slowly at 0°C. An 1420; 1400; 835; 563; 500; 445cm-‘. Mass specimmediate reaction was observed with evolution of trum: mole peak not observed, no characteristic methane and the formation of a yellow precipitate fragments. (la). The mixture was stirred for an additional 30min at the ambient temperature and subsequently W - cyclopentadienyl)(q4 - 1,3 - cyclopentadiene) the solvent was removed in ULICUO. The residue was cobalt. The compound was prepared by a simplified
Properties of two new cobalt complexes
way to a published procedure[19]. Cobaltocenium hexafluorophosphate was suspended in a diluted toluene solution of sodium bis(Zmethoxyethoxy) aluminium hydride (70% in toluene; commercially available from Lancaster) at ambient temperature. The work-up procedure was similar to the one given in[19]. CAS registry numbers: (n5cyclopentadienyl)[(~41,3 - cyclopentadiene) - 5 - ethyne- 1,2 - diyl] - cobalt [131276-83-O]; [q4-1,3-cyclopentadiene]($-cyclopentadienyl)cobalt [33032-03-0][18]; Sodium bis(Z methoxyethoxy) aluminium hydride [22722-98-l]; Cobaltocenium [12241-42-81; N,N,N’,N’-tetramethylethylenediamine [ 110-18-91.
RESULTS
AND DISCUSSION
The ethyne-bridged cobalt complexes (1) and (2) have been synthesized in a similar way to a recently published procedure[l8] (Scheme 1). More experimental details as well as the results of physical measurements such as NMR, MS and ir are given in the Experimental Section. The results of the electrochemical investigation shall be discussed in detail. According to literature reports[!&1 l] complex (2) was supposed to be the product of an electrochemical oxidation of 1. Therefore the cyclic voltammogram of this compound is discussed first. Figure 1 shows the cyclic voltammogram of (2) as a function of scan range. In the range from 0 to - 1.5 V only two waves at -0.66 and -0.84 V vs. AgjAgCl are detected. Electrochemical reversibility of these waves
1529
is indicated by invariance of the current function iJv ‘12C and peak potential with scan rate, as well as peak-to-peak separatiqns of 6OmV, which is almost the theoretical value for a reversible one electron transferC16, 173. The waves lie very close to the redox potential reported for the Cp2Co/Cp2Co+ couple (- 0.89 V vs. Ag/AgCl) and can thus be easily assigned to the successive one electron reductions of the two cobaltocene units. The shift of the redox potential to more positive values relative to cobaltocene is possibly due to the electron withdrawing effect of the ethyne bridge.[6]. From the potential , difference Ago between two charge transfer steps the extent of the delocalization of the valence electrons and the degree of interaction between the two redox centers can be estimated[20]. In this case a Ago value of 180mV, which is slightly greater than that reported for biferrocenylacetylene[6] indicates a weak interaction between the two metal centers via the conjugated ethyne bridge. While reduction of bisfulvalene dimetal complexes and termetallocenesC21, 221 in acetonitrile leads to stable anions and d&ions, reduction of (2) does not lead to stable reduction products but, apparently, fast homogenous reactions occur after the charge transfer (Fig. la). At - 1.8 V an irreversible reduction peak appears with a peak height similar to that of the first two peaks which is most probably due to the one electron reduction of one of the cobaltocene centers. Chemical complications are indicated by the decreasing peak current ratios of the peaks at -0.66 and - 0.84 V and the appearance of a new oxidation peak at about - 1.03 V. The nature of the chemical reaction and reaction products could not be deter-
co
Scheme 1. Synthesis of the ethyne-bridged cobalt complexes (1)and (2).
H. SCHOTIENBEXGF% et al.
1530 a
b
,
1
J
0
-I
-2
I
0
-I
V (vs. Ag/AgCl)
Fig. 1. Cyclic voltammogram of (2) in acetonitrile/O.l M TBAH at a scan rate of lOOmVs_'; (a) scan range from 0 to - 2.OV;(h) scan range from 0 to - 1.5 V. mined unambiguously. However, studies of other ethyne-bridged heteronuclear bimetallocenes suggest that the instability of the reduction products might be due to transformations occuring at the ethyne bridge. Figure 2a shows the cyclic voltammogram of (1) in different solvents. Dichloromethane had to be added in all cases because of the low solubility of (1) in pure
IA
acetonitrile. In acetonitrile/dichloromethane three peaks are observed: an oxidation
(15 : 1), peak at
0.38 V and two reduction peaks at 0.28 and -0.84 V. Peak 3 can only be observed after having scanned past 0.4V in positive potential direction. This means that this peak must be due to a product formed after the oxidation process of (1). Chemical complications for the charge transfers at 0.38 and - 0.84 V are also indicated by the variation of the current functions with scan rate. Figure 3 shows plots of current functions iJv ‘/‘C vs. scan rate for the redox process in acetonitrile/dichloromethane (15 : 1). The i&1’2C value for peak 1 decreases with increased scan rate and increases for peak 3 reaching limiting values at higher scan rates whereas the current function of peak 2 remains almost constant. At low scan rates (20mV s-r) for peak 1 a current function of about four times larger than the theoretically expected value for a reversible one electron transfer is observed which reaches a limiting value between two and three at higher sweep rates (5 Vs-‘). The current functions of peaks 2 and 3 are equal to n-values of about 2 and 1, respectively. This is consistent with chronoamperometric measurements which yield at 1.2V a value of about n = 4. Stepping to - 1.2V
(b)
P
c
lOxkdatbnpeakI(Fyl2a)at038V llWuctkmpepk2(Fig2a)at0.28V
30
p
l
Redudm
P
peak 3 CFii 2a) at -O.&IV
5 IQ 20 tc.1
2: d
c-L__
-_
l---
---_
-Y
V (vs. Ag/AqCU I -I
I
0
I I
Fig. 2. Cyclic voltammogram of (1) in different solvents/ 0.1 M TBAH: (a) acetoaitrile/dichloromethane (15 : l), scan acetonitrile/ C = 0.87mM; rate = 1OOmVs-l, (h) (C = 17.6mM), (15 : l)/pyridine dichloromethane C = 0.44mM, scan rate = 1OOmVs- ’ ; (c) dichloromethane, C = 0.31 mM, scan rate = 1 Vs-‘.
Fig. 3. Plot of current function iJv’l”C vs. scan rates for redox processes of (1) in acetonitrile/dichloromethane (15 : 1).
Properties of two new cobalt complexes after electrolysis at 1.2V (for 3s) leads to a rapid decrease of the n-value from about 4 to 1 within 3s. This behavior suggests than an (E)ECE process is occurring. In order to assign the waves 1-3 (Figure 2a) to particular redox reactions of (1) we reinvestigated the redox behavior of the most simple (t)4-cyclocomplex, pentadiene)($-cyclopentadienyl)cobalt [(r$-C,HS)Co(q4-CsH,)] (3), in acetonitrile. The cyclic voltammogram shows three waves (Fig. 4c and d) though they are somewhat distorted by adsorption effects due to the low solubility of (3) in CH,CN. The first wave (peaks l/l’) at 0.3V (AE, 90 mV) can be attributed to the quasireversible oxidation of (3) to the monocation. It seems that acetonitrile is stabilizing the oxidation product at least within the time scale of cyclic voltammetry. Adding dichloromethane, pyridine or 1,4-lutidine to an acetonitrile solution of (3) leads to the disappearance of this wave with a consequent increase of the current function of wave 2. Waves 2 and 3 at -0.87 V (AE, 60mV) and - 1.8 V (AE, 1lOmV) can be attributed to the subsequent reduction of cobaltocenium which was formed from (3) by oxidative deprotonation. Comparing these results with the redox potentials found for (1) the following assignment can be made: peak 1 in Fig. 2a is attributed to a formal “two electron” process[23] due to the statistical oxidation of
V (vs
Ag/AgCl) k
fi’$
V lvs Ag/AgCll
voltammograms of [($ - C5H@JoC4Fig. 4. Cyclic C.HAl (3) in acetonitrile/O.l M TBAH at different scan ., rates and scan ranges, C’= 0.4mM: (a) scan range from -0.15 to 1 V, scan rate = lOOmVs_‘; (b) scan range from 0.8 to - 1.5V, scan rate = lOOmVs_‘; (c) scan range from -2.2 to 0.8V (start at -1.4V). scan rate = SOOmVs-‘; (d) scan range from -2.2 to 0.8 V (start at - 1.4V), scan rate = lOOmVs_‘. “l_,I
EA U):ll-0
1531
the two noninteracting metal centers at very similar potentials leading to the dication of (1). Peak 2 might be the corresponding reduction peak. The increased current function of peak 1 as well as the observation of only one oxidation peak indicates that the oxidation of (1) is followed by a chemical reaction leading to an electroactive species that is more easily oxidized than (1). However, the chemical reaction is not an oxidative abstraction of the endohydrogen atom in (l), leading to the formation of (2), since in this case two reversible peaks at -0.66 and -0.84V are expected to appear during the scan to negative potential. Instead the irreversible reduction wave of peak 3 is observed, which remains irreversible within the range of scan rates studied (20mVs-‘-5VsK’). In order to gain more information about the nature of the chemical reaction additional measurements with different concentrations and in different solvents were performed. Within the concentration range of 0.3-1.2mM the appearance of the cyclic voltammogram is not concentration dependent. This excludes the possibility of dimerization. Changing the solvents has more effect on the appearance of the cyclic voltammogram. Adding a 40-fold (relative to the concentration of (1)) excess of pyridine to the solution (Fig. 2b) leads to the formation of peaks 1, 2 and 3 at 0.38, 0.12 and -l.O4V, respectively. At 0.22 V peak 2’ corresponding to peak 2 is observed during the second scan. The most drastic change is observed in pure dichloromethane (Fig. 2c), though the measurements are complicated by adsorption effects that could not be avoided. The oxidation process at 0.46V (peak 1) is completely irreversible within the range of scan rates studied (20mVs-‘-2VsK’) and as a consequence of oxidation up to three more or less irreversible peaks appear in the negative potential range at -0.70, -0.92 and - 1.07V. Partial removal of dichloromethane and addition of acetonitrile (final solvent composition: CH,Cl,/CH,CN = 4 : 12) leads to exactly the same voltammogram as shown in Fig. 2a. These results suggest that the chemical step in the (E)ECE process is the coordination of solvent molecules (eg acetonitrile, pyridine) to the dication of(l), leading to a free j$s-(cyclopentadien)-ethyne] (4) and the monocation of the solvent adduct of a cyclopentadienyl cobalt complex [$-CpCo(L), J +. The latter is readily oxidized at 0.38V to the dication giving rise to the increasing current function of peak 1. The irreversible reduction peak at -0.84V (peak 3) is most possibly due to further reduction of the monocation[24,25]. This is in good agreement with the redox potentials reported[24-271 for the oxidation and reduction of various [$-CpCo(L)J+ compounds. While in acetonitrile/dichloromethane (15 : 1) the oxidation/reduction of the [usCpCo(CH,CN),]+/[~5-CpCo(CH,CN),]2+ couple occurs at almost the same potential as the oxidation/ reduction of (1), addition of pyridine shifts the redox potential to more negative values (peak 2/2’ in Fig. 2b at 0.17V). Scheme 2 shows the possible redox mechanism for the oxidation of (1). A quantitative treatment of the redox mechanism is probably more complex, since compounds like [$-CpCo(L),]*’ (L = solvent molecule) are very reactive[24_27J and
H. SCHO~
1532
0F.a et al.
TI
+ L ( L=Solvent moteade)
CO
2
L/I“L L
+’
2
lx. 0.33v
( for L =CH,CN )
do
+
L’I’L L
+ f Scheme 2. Suggested mechanism for the oxidation of (1) and consequent solvolysis of the oxidation product in solvents of bigher donor ability.
recombination of the monocation [$-CpCo(L),]+ with the diene (4) leading to the dication of (l), as comproportionation/disproportionation well as equilibria according to equation (1) have to be taken into account,
Cl1+ 2ccPwLM*+
s
[112’ +
2ccpc4L),I+ (1)
Unfortunately, electrochemical oxidation of [(q4C,H,R)Co(t$-C,H,)] complexes does not provide a more convenient way to prepare binuclear metallocenes, because after the oxidation of (1) solvolysis seems to occur more readily than oxidative abstraction of a proton. Whether this is due to the endo position of the hydrogen atom in (1) or a general feature of [(q4-C5H,R)Co(qS-C,HJJ complexes, containing a second metallocene group R, remains to be studied. Acknowledgement-Financial support by the Fonds zuc Fiirderung der Wissenschaft und Forschung, Vienna, Austria (project no. P 7592-CHE) is gratefully acknowledged.
REFERENCES 1. H. S. Nalwa, Appl. Organomet. Chem. 5,349 (1991). 2. W. E. Geiger Jr and N. G. Connelly, Adu. Organomet. them. zq 87 (1985). 3. T.-Y. Donn. C.-C. Schei. M.-Y. Hwana T.-Y. L&e.S.-K. Yeh and YT:S.Wen, Orgmunnetallics 11,573 (1992). 4. W. H. Morrison, S. Krogsrud and D. N. Hendrickson, Inorg. Chem. l&l998 (1973).
5. M. J. Powers and T. J. Meyer, J. Am. them. Sot. 100, 4393 (1978). 6. C. LeVanda, K. Bechgaard and D. 0. Cowan, J. Org. Chem. 41,270O (1976). 7. N. Dowling and P. M. Henry, lnorg. Chem. 21, 4088 (1982). 8. N. Dowlin& P. M. Henry, N. A. Lewis and H. Taube, Inorg. Chem. 20.2345 (1981). 9. N. El Murr and E. Laviron, Can. J. Chem. 54, 3357 (1976). 10. N. El Murr and E. Laviron, Tetrahedron I.&t. 875 (1975). 11. k. Ei Murr, Y. Dusausoy, J. E. Shea@ and M. Agnew, J. them. Sot., Dalton Trans. 901(1979). 12. J. M. O’Connor and J. A. Johnson, Synlett. 57 (1989). 13. H. Schottenberger, G. Ingram and D. Obeidorf, J. Oraanomet. Chem. 4% 109 (1992). 14. M.- Andre, H. Schottmbetgtr, &. Ingram, P. Jaitner and K. E. Schwarzhans, Chromatographia 30, 543 (1990). 15. G. Ingram, P. Jaitner and K. E. Schwarzhans, Z. Naturforsch. 45b, 781(1990). 16. R. S. Nicholson and I. Shain, Anal. Chem. 36, 706
(1964). 17. J. A. Bard and L. R. Faulkner, Electrochemical Methods, Fundamentals and Applications, Wiley, New York (1980). 18. M. Wildschek, C. Rieker, P. Jaitner, H. Schottenberger and K. E. Schwarzhans, J. Organomet. Gem. 396, 355 (1990). 19. M. L. H. Green, L. Pratt and G. Wilkinson, J. them. sot. 3753 (1959). 20. D. E. Richardson and H. Taube, Coord. Chem Reu. 60, 107 (1984). 21. D. Obendorf, H. Schottenberger and C. Riekv Organometalli& 10, 1293 (1991). 22. D. Obendorf, C. Rieker and G. Ingram, J. e&roar& them. 326,81(1992). --3
Properties of two new cobalt complexes 23. J. B. Flanagan, S. Margel, A. J. Bard and F. Anson, J. Am. &em. Sot. 100.4248 (1978). 24. K.-D. Plitzko and’ V. Bdkelheide, Organometallics 7, 1573 (1988). 25. U. Kiille, J. Organomet. Gem. 184,379 (1980).
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26. N. Kuhn, H. Briiggemann, M. Winter and V. M. De Bellis, J. Orucmnomet.Chem. 320.391 (1987). 27. K. Broadlec N. G. Connelly add W.‘E. Gkiger, J. them. Sot., Dalton Trans 121 (1983).
00134686/93 $6.00+ 0.00 Q 1993.Perpmon Prrslt&i
ELECTROCHEMICAL PROPERTIES OF TWO NEW BINUCLEAR ETHYNE-BRIDGED COBALT COMPLEXES: BIS-[(q5-CYCLOPENTADIENYL)COBALT(q4-1,3CYCLOPENTADIEN-5-EXO-YL)]-ETHYNE (1) and DI-COBALTOCENYLIUM-ETHYNE-DIHEXAFLUOROPHOSPHATE (2) HERWIG SCHOTTENBBRGER, CHRISTOPHERRIEKERand DAGMAROBENDORF* Institut fiir Anorganische und Analytische Chemie der Universitiit Innsbruck, Innrain 52 a, A-6020, Austria (Received 2 November 1992; in revisedjiirm
12 January 1993)
Ahstract-The results of an electrochemical study of two new z-bridged cobalt complexes [($C,H,)Co{(r14-C,H,~C~C-(r14-C~Hs)} ‘Wrl’-C,H,)l (1) aad C(I’-C,H,)C~((~~‘-C,H~-C~~‘CSH3}Co(qs-C,H,)]‘+ 2 PF; (2) are discussed. Reduction of (2) proceeds in two reversible one-electron steps from the dication to the neutral compound. A AE” separation of 18OmV indicates interaction between the two ethyne-bridged cobalt centers. Further reduction of (2) leads to formation of a monoanion and subsequent decomposition. Oxidation of (1) occurs by an (E)ECE (E = electrochemical step, C = chemical step) process. The character of the chemical reaction has been probed by cyclic voltammetry in solvents of varying donor ability.
Kev words: ethyne-bridged binuclear cobalt complexes, cyclic voltammetry, chronoamperometry, redox mechanism, sol&t dep&dence.
INTRODUCTION Organometallic compounds, in particular those forming mixed-valence complexes, are of increasing interest as they serve as models of synthetic conducting polymers and provide additional insight regarding intramolecular electron and charge transfer or delocalization. Furthermore they have also been recognized as playing a major role in the new emerging opto-electronic technologies[l-31. n-bridged bi- and polynuclear complexes have been subject to extensive investigations in order to get information about intervalence transfer properties as a function of distance between two or more interacting groups, as a function of the nature of the bridge between them and as a function of their relative orientation and chemical environment[l-81. Though mixed-valence sandwich compounds with various metal centers have been prepared and studied, the most common examples are still members of the ferrocene series. As cobalt cyclopentadiene complexes can serve as electron-donor as well as electron-acceptor groups, either in the form of a neutral cobaltocene or cobaltocenium substituent, x-bridged cobaltocenium complexes might also be of interest in the design of tunable organometallic materials for nonlinear optics[l]. We want to report here on the electrochemical properties of two new x-bridged cobalt complexes: the cobalt analogue of diferrocenylacetylene, [(~5-C,H,)Co{(q5-C5H&C==C-(~5-C5H,)~Co(q5C,H,)J*+ 2 PF, (9, and the corresponding endohydride, [(q’ - C5H5)Co{(q4 - C,H,) - C-C -(q4-
C5H5))Co(q5-C,H,)] (1). Our interest in the electrochemical properties of these compounds was based on the attempts to find a more convenient way to prepare bi- or polynuclear bridged metallocene complexes. In general the synthesis of substituted q’-cyclopentadienyl cobalt compounds, [(q5-C5H4R)Co(~5-C5H5)], where R represents an organic group, can be achieved either by oxidative elimination of a ring substituent from [(q4C5H5R)Co(q5-C,H,)]-compounds by chemical oxidizing agents like tritylium salts (PhJ+X-; X= PF;, BFJ or electrochemical oxidation[P-141. However, in some cases our attempts to prepare binuclear bridged metallocene complexes by chemical oxidation of the (~4-cyclopentadiene)(~5-cyclopentadienyl)-cobalt moiety of the complex failed and we hoped that electrochemical oxidation might be more successful. Though hydride compounds like (1) have been reported as intermediates in the synthesis of heteronuclear bi- and termetallocenes[lS] nothing is known about their redox properties. In order to get more insight into the redox mechanism we investigated (1) and (2) by cyclic voltammetry and related techniques. EXPERIMENTAL Physical measurements
Cyclic voltammetry experiments were carried out with a HEKA potentiostat/galvanostat PG 28 system. Measurements with scan rates up to 1 Vs- ’ were recorded on a Philips PM 8133 X-Y recorder.
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H.SCHOITENBERGER et al.
1
For faster scan rates data acquisition was achieved via an IBM compatible computer using the HEKA IBM-Elektrochemie system and a Hewlett-Packard 7475A plotter. For all measurements a three electrode configuration and two different electrode combinations were used: either a platinum disk working electrode (diameter 1 mm) sealed in soft glass, a platinum wire counter electrode coiled around the glass mantle of the working electrode and a silver wire pseudoreference electrode or a platinum ring working electrode, a glassy carbon counter electrode and a silver/silver chloride reference electrode. All potentials were calibrated against the formal potential of the Fc/Fc+ couple (0.42OV vs. Ag/AgCl). Solutions of the complexes were ca. 0.4mM in acetonitrile, unless otherwise stated, and contained 0.1 M ntetrabutylammonium hexafluorophosphate (TBAH) as supporting electrolyte. Acetonitrile, dichloromethane, pyridine and TBAH were obtained from . Fluka Chemicals (high purity). All solvents were dried according to literature procedures and freshly distilled before use. All manipulations and electrochemical measurements were performed under argon. E” values were taken where possible as the average of the anodic and cathodic peak potentials. Electrochemical reversibility was judged on the basis of peak-to-peak separations (A&), relative to the predicted value of 59mV for a reversible one electron transfer[l6,17]. Infrared spectra of KBr moldings were recorded on a Biorad Model FTS-40 or a Pye-Unicam SP3-300 spectrometer. ‘H NMR spectra were recorded on either a Varian EM 360L (60MHz) or on a Bruker AM-300 (13C spectra) with tetramethylstiane as internal standard. Mass spectra were obtained with a MAT CH7 mass spectrometer. Ultrasonication involved the use of a Bandelin Sonorex RK 255 S cleaning bath.
dissolved in 20ml of dimethoxyethane and 0.23ml (0.17 g ; 1.5 mmol) tetramethylethylenediamine and 0.351 g (1.05 mmol) of cobaltocenium hexafluorophosphate were added. The reaction mixture was ultrasonicated for 45min at 0°C. After removal of the solvent, the precipitate was dissolved in diethylether and subjected to chromatography (Al,O,: Merck; activity 3). The product was eluted with diethylether as an orange band. After removal of the solvent 0.177g (29% yield) of a spectroscopically pure product were obtained. ‘H NMR (C,D,): 6 2.60 (pseudo-q, 4H, J = 2.1 Hz), 3.39 (pseudo-t, 2H, J = 2.1 Hz), 4.37 (s, lOH), 4.95 (pseudo-t, 4H, J = 2.1 Hz). “C NMR (C,D,): 6 40.92 (d); 43.00 (d); 75.03 (d); 77.92 (s) 79.29 (d). ir (KBr): 3090; 3050; 2910; 2850; 1405; 1283; 1185; 1108; 1062; 1008; 952; 858; 807cm-‘. Mass spectrum: m/z 402 (100%). Di-cobaltocenylium-ethyne-di-hexaj?uorophosphate (2). To a solution of 0.402g (l.Ommol) of (la) in
CH,Cl, 0.814g (2.1 mmol) of tritylium hexafluorophosphate were added at ambient temperature and subsequently stirred for 90min. The solvent was removed in uacuo. The precipitate was dissolved in acetonitrile and chromatographed over A&O, (Merck, activity 3). A solvent gradient of diethylether/acetonitrile (l/l v/v) to pure acetonitrile was applied. The product was eluted as the final yellow band with pure acetonitrile. Depending on the condition of the alumina used, it may occur that the product is completely retarded. In this case the column may be flushed with saturated aqueous NH,Cl solution. This procedure leads to a remobilisation of the product, so that subsequent elution with methanol/acetonitrile yields the desired product as a yellow band. After removal of the solvent the product may be crystallized by slowly diffusing diethylether into a acetonitrile solution of (2). The same product may also be obtained by a straight forward reaction sequence without isolation of (qs-cyclopentadieny1)C(q4-1,3-cyclopentadiene)-5ethynyll-cobalt; ie by in situ deprotonation to (la) and reaction with an additional equivalent of Compound preparation cobaltocinium hexafluorophosphate. For this “one pot” reaction sequence DME is preferred as the Bis[($ - cyclopentadienyl)(t14 - 1,3 - cyclopentadien 5-exo-yl)cobalt]-ethyne (1). To a solution of 0.214g ’ solvent. ‘H NMR (CD,CN): 6 5.77 (s, lOH), 5.78 (pseudoof (~5-cyclopentadienyl)[(~4-l,3-cyclo(1 mmol) t, 4 H, J = 2.1 Hz), 5.98 (pseudo-t, 4H, J = 2.1 Hz). pentadiene-5-ethynyl]-cobalt[18] in 8Oml of nhexane 0.625ml of a methyllithium solution (1.6 13C (CD,(X): 6 86.61 (d), 87.63 (d), 87.73 (d) (only H-bearing carbons observed). ir (KBr) 3120; 1505; molar in diethylether) were added slowly at 0°C. An 1420; 1400; 835; 563; 500; 445cm-‘. Mass specimmediate reaction was observed with evolution of trum: mole peak not observed, no characteristic methane and the formation of a yellow precipitate fragments. (la). The mixture was stirred for an additional 30min at the ambient temperature and subsequently W - cyclopentadienyl)(q4 - 1,3 - cyclopentadiene) the solvent was removed in ULICUO. The residue was cobalt. The compound was prepared by a simplified
Properties of two new cobalt complexes
way to a published procedure[19]. Cobaltocenium hexafluorophosphate was suspended in a diluted toluene solution of sodium bis(Zmethoxyethoxy) aluminium hydride (70% in toluene; commercially available from Lancaster) at ambient temperature. The work-up procedure was similar to the one given in[19]. CAS registry numbers: (n5cyclopentadienyl)[(~41,3 - cyclopentadiene) - 5 - ethyne- 1,2 - diyl] - cobalt [131276-83-O]; [q4-1,3-cyclopentadiene]($-cyclopentadienyl)cobalt [33032-03-0][18]; Sodium bis(Z methoxyethoxy) aluminium hydride [22722-98-l]; Cobaltocenium [12241-42-81; N,N,N’,N’-tetramethylethylenediamine [ 110-18-91.
RESULTS
AND DISCUSSION
The ethyne-bridged cobalt complexes (1) and (2) have been synthesized in a similar way to a recently published procedure[l8] (Scheme 1). More experimental details as well as the results of physical measurements such as NMR, MS and ir are given in the Experimental Section. The results of the electrochemical investigation shall be discussed in detail. According to literature reports[!&1 l] complex (2) was supposed to be the product of an electrochemical oxidation of 1. Therefore the cyclic voltammogram of this compound is discussed first. Figure 1 shows the cyclic voltammogram of (2) as a function of scan range. In the range from 0 to - 1.5 V only two waves at -0.66 and -0.84 V vs. AgjAgCl are detected. Electrochemical reversibility of these waves
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is indicated by invariance of the current function iJv ‘12C and peak potential with scan rate, as well as peak-to-peak separatiqns of 6OmV, which is almost the theoretical value for a reversible one electron transferC16, 173. The waves lie very close to the redox potential reported for the Cp2Co/Cp2Co+ couple (- 0.89 V vs. Ag/AgCl) and can thus be easily assigned to the successive one electron reductions of the two cobaltocene units. The shift of the redox potential to more positive values relative to cobaltocene is possibly due to the electron withdrawing effect of the ethyne bridge.[6]. From the potential , difference Ago between two charge transfer steps the extent of the delocalization of the valence electrons and the degree of interaction between the two redox centers can be estimated[20]. In this case a Ago value of 180mV, which is slightly greater than that reported for biferrocenylacetylene[6] indicates a weak interaction between the two metal centers via the conjugated ethyne bridge. While reduction of bisfulvalene dimetal complexes and termetallocenesC21, 221 in acetonitrile leads to stable anions and d&ions, reduction of (2) does not lead to stable reduction products but, apparently, fast homogenous reactions occur after the charge transfer (Fig. la). At - 1.8 V an irreversible reduction peak appears with a peak height similar to that of the first two peaks which is most probably due to the one electron reduction of one of the cobaltocene centers. Chemical complications are indicated by the decreasing peak current ratios of the peaks at -0.66 and - 0.84 V and the appearance of a new oxidation peak at about - 1.03 V. The nature of the chemical reaction and reaction products could not be deter-
co
Scheme 1. Synthesis of the ethyne-bridged cobalt complexes (1)and (2).
H. SCHOTIENBEXGF% et al.
1530 a
b
,
1
J
0
-I
-2
I
0
-I
V (vs. Ag/AgCl)
Fig. 1. Cyclic voltammogram of (2) in acetonitrile/O.l M TBAH at a scan rate of lOOmVs_'; (a) scan range from 0 to - 2.OV;(h) scan range from 0 to - 1.5 V. mined unambiguously. However, studies of other ethyne-bridged heteronuclear bimetallocenes suggest that the instability of the reduction products might be due to transformations occuring at the ethyne bridge. Figure 2a shows the cyclic voltammogram of (1) in different solvents. Dichloromethane had to be added in all cases because of the low solubility of (1) in pure
IA
acetonitrile. In acetonitrile/dichloromethane three peaks are observed: an oxidation
(15 : 1), peak at
0.38 V and two reduction peaks at 0.28 and -0.84 V. Peak 3 can only be observed after having scanned past 0.4V in positive potential direction. This means that this peak must be due to a product formed after the oxidation process of (1). Chemical complications for the charge transfers at 0.38 and - 0.84 V are also indicated by the variation of the current functions with scan rate. Figure 3 shows plots of current functions iJv ‘/‘C vs. scan rate for the redox process in acetonitrile/dichloromethane (15 : 1). The i&1’2C value for peak 1 decreases with increased scan rate and increases for peak 3 reaching limiting values at higher scan rates whereas the current function of peak 2 remains almost constant. At low scan rates (20mV s-r) for peak 1 a current function of about four times larger than the theoretically expected value for a reversible one electron transfer is observed which reaches a limiting value between two and three at higher sweep rates (5 Vs-‘). The current functions of peaks 2 and 3 are equal to n-values of about 2 and 1, respectively. This is consistent with chronoamperometric measurements which yield at 1.2V a value of about n = 4. Stepping to - 1.2V
(b)
P
c
lOxkdatbnpeakI(Fyl2a)at038V llWuctkmpepk2(Fig2a)at0.28V
30
p
l
Redudm
P
peak 3 CFii 2a) at -O.&IV
5 IQ 20 tc.1
2: d
c-L__
-_
l---
---_
-Y
V (vs. Ag/AqCU I -I
I
0
I I
Fig. 2. Cyclic voltammogram of (1) in different solvents/ 0.1 M TBAH: (a) acetoaitrile/dichloromethane (15 : l), scan acetonitrile/ C = 0.87mM; rate = 1OOmVs-l, (h) (C = 17.6mM), (15 : l)/pyridine dichloromethane C = 0.44mM, scan rate = 1OOmVs- ’ ; (c) dichloromethane, C = 0.31 mM, scan rate = 1 Vs-‘.
Fig. 3. Plot of current function iJv’l”C vs. scan rates for redox processes of (1) in acetonitrile/dichloromethane (15 : 1).
Properties of two new cobalt complexes after electrolysis at 1.2V (for 3s) leads to a rapid decrease of the n-value from about 4 to 1 within 3s. This behavior suggests than an (E)ECE process is occurring. In order to assign the waves 1-3 (Figure 2a) to particular redox reactions of (1) we reinvestigated the redox behavior of the most simple (t)4-cyclocomplex, pentadiene)($-cyclopentadienyl)cobalt [(r$-C,HS)Co(q4-CsH,)] (3), in acetonitrile. The cyclic voltammogram shows three waves (Fig. 4c and d) though they are somewhat distorted by adsorption effects due to the low solubility of (3) in CH,CN. The first wave (peaks l/l’) at 0.3V (AE, 90 mV) can be attributed to the quasireversible oxidation of (3) to the monocation. It seems that acetonitrile is stabilizing the oxidation product at least within the time scale of cyclic voltammetry. Adding dichloromethane, pyridine or 1,4-lutidine to an acetonitrile solution of (3) leads to the disappearance of this wave with a consequent increase of the current function of wave 2. Waves 2 and 3 at -0.87 V (AE, 60mV) and - 1.8 V (AE, 1lOmV) can be attributed to the subsequent reduction of cobaltocenium which was formed from (3) by oxidative deprotonation. Comparing these results with the redox potentials found for (1) the following assignment can be made: peak 1 in Fig. 2a is attributed to a formal “two electron” process[23] due to the statistical oxidation of
V (vs
Ag/AgCl) k
fi’$
V lvs Ag/AgCll
voltammograms of [($ - C5H@JoC4Fig. 4. Cyclic C.HAl (3) in acetonitrile/O.l M TBAH at different scan ., rates and scan ranges, C’= 0.4mM: (a) scan range from -0.15 to 1 V, scan rate = lOOmVs_‘; (b) scan range from 0.8 to - 1.5V, scan rate = lOOmVs_‘; (c) scan range from -2.2 to 0.8V (start at -1.4V). scan rate = SOOmVs-‘; (d) scan range from -2.2 to 0.8 V (start at - 1.4V), scan rate = lOOmVs_‘. “l_,I
EA U):ll-0
1531
the two noninteracting metal centers at very similar potentials leading to the dication of (1). Peak 2 might be the corresponding reduction peak. The increased current function of peak 1 as well as the observation of only one oxidation peak indicates that the oxidation of (1) is followed by a chemical reaction leading to an electroactive species that is more easily oxidized than (1). However, the chemical reaction is not an oxidative abstraction of the endohydrogen atom in (l), leading to the formation of (2), since in this case two reversible peaks at -0.66 and -0.84V are expected to appear during the scan to negative potential. Instead the irreversible reduction wave of peak 3 is observed, which remains irreversible within the range of scan rates studied (20mVs-‘-5VsK’). In order to gain more information about the nature of the chemical reaction additional measurements with different concentrations and in different solvents were performed. Within the concentration range of 0.3-1.2mM the appearance of the cyclic voltammogram is not concentration dependent. This excludes the possibility of dimerization. Changing the solvents has more effect on the appearance of the cyclic voltammogram. Adding a 40-fold (relative to the concentration of (1)) excess of pyridine to the solution (Fig. 2b) leads to the formation of peaks 1, 2 and 3 at 0.38, 0.12 and -l.O4V, respectively. At 0.22 V peak 2’ corresponding to peak 2 is observed during the second scan. The most drastic change is observed in pure dichloromethane (Fig. 2c), though the measurements are complicated by adsorption effects that could not be avoided. The oxidation process at 0.46V (peak 1) is completely irreversible within the range of scan rates studied (20mVs-‘-2VsK’) and as a consequence of oxidation up to three more or less irreversible peaks appear in the negative potential range at -0.70, -0.92 and - 1.07V. Partial removal of dichloromethane and addition of acetonitrile (final solvent composition: CH,Cl,/CH,CN = 4 : 12) leads to exactly the same voltammogram as shown in Fig. 2a. These results suggest that the chemical step in the (E)ECE process is the coordination of solvent molecules (eg acetonitrile, pyridine) to the dication of(l), leading to a free j$s-(cyclopentadien)-ethyne] (4) and the monocation of the solvent adduct of a cyclopentadienyl cobalt complex [$-CpCo(L), J +. The latter is readily oxidized at 0.38V to the dication giving rise to the increasing current function of peak 1. The irreversible reduction peak at -0.84V (peak 3) is most possibly due to further reduction of the monocation[24,25]. This is in good agreement with the redox potentials reported[24-271 for the oxidation and reduction of various [$-CpCo(L)J+ compounds. While in acetonitrile/dichloromethane (15 : 1) the oxidation/reduction of the [usCpCo(CH,CN),]+/[~5-CpCo(CH,CN),]2+ couple occurs at almost the same potential as the oxidation/ reduction of (1), addition of pyridine shifts the redox potential to more negative values (peak 2/2’ in Fig. 2b at 0.17V). Scheme 2 shows the possible redox mechanism for the oxidation of (1). A quantitative treatment of the redox mechanism is probably more complex, since compounds like [$-CpCo(L),]*’ (L = solvent molecule) are very reactive[24_27J and
H. SCHO~
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0F.a et al.
TI
+ L ( L=Solvent moteade)
CO
2
L/I“L L
+’
2
lx. 0.33v
( for L =CH,CN )
do
+
L’I’L L
+ f Scheme 2. Suggested mechanism for the oxidation of (1) and consequent solvolysis of the oxidation product in solvents of bigher donor ability.
recombination of the monocation [$-CpCo(L),]+ with the diene (4) leading to the dication of (l), as comproportionation/disproportionation well as equilibria according to equation (1) have to be taken into account,
Cl1+ 2ccPwLM*+
s
[112’ +
2ccpc4L),I+ (1)
Unfortunately, electrochemical oxidation of [(q4C,H,R)Co(t$-C,H,)] complexes does not provide a more convenient way to prepare binuclear metallocenes, because after the oxidation of (1) solvolysis seems to occur more readily than oxidative abstraction of a proton. Whether this is due to the endo position of the hydrogen atom in (1) or a general feature of [(q4-C5H,R)Co(qS-C,HJJ complexes, containing a second metallocene group R, remains to be studied. Acknowledgement-Financial support by the Fonds zuc Fiirderung der Wissenschaft und Forschung, Vienna, Austria (project no. P 7592-CHE) is gratefully acknowledged.
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Properties of two new cobalt complexes 23. J. B. Flanagan, S. Margel, A. J. Bard and F. Anson, J. Am. &em. Sot. 100.4248 (1978). 24. K.-D. Plitzko and’ V. Bdkelheide, Organometallics 7, 1573 (1988). 25. U. Kiille, J. Organomet. Gem. 184,379 (1980).
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26. N. Kuhn, H. Briiggemann, M. Winter and V. M. De Bellis, J. Orucmnomet.Chem. 320.391 (1987). 27. K. Broadlec N. G. Connelly add W.‘E. Gkiger, J. them. Sot., Dalton Trans 121 (1983).