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Some aspects of organometallic electrochemistry
Materials
Chemistry
and Physics,
29 (1991)
117
117-131
Review
Some aspects of organometallic electrochemistry D. Osella Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica Universith di Torino, Via P. Giuria 7, 10125 Turin (Italy)
dei Materiali,
The electrochemical investigations in the field of organometallic chemistry may be useful to: a) understand the stabilizing effect of the organic chains on the metallic frame, with the aim of obtaining stable ions (electron reservoirs); b) estimate (in a direct way) the electronic delocalization on the complex, predicted by theoretical calculations; c) evaluate the geometrical changes following or accompanying the electron transfers; d) test (in a direct way) the electronic unsaturation of the metallic core, predicted by the electron count; e) plan efficient and selective electrosyntheses. In particular, the storage of electrical energy into chemical form by means of bond breaking and reforming in a reversible manner (point a) might be of some interest in the wider context of developing molecular batteries [ 1 ]. The aim of this review article is to show some examples of these features, mainly taken from the author’s own work. The main technique employed for such investigations is by far cyclic voltammetry (CV) which allows a wide choice of potential scan rate (v) and then a wide time-scale range of electrochemical events. Further, the electrochemical armory generally consists of coulometry (to assess the number of electrons transferred, n), DC polarography, rotating disk electrode (RDE) voltammetry (having high sensitivity because of the forced hydrodynamic convection) and all the spectroscopic techniques to be employed in situ (IR, UV/VIS, ESR and sometimes NMR) in order to follow the chemical transformation caused by the imposition of the electrical charge. The reader is certainly familiar with the CV experiment, which measures the current at a given potential when the potential itself is swept in a forward and then in a reverse scan (triangular potential ramp). A cathodic response of a fully reversible reduction process is reported below as an example, (Fig. 1) E, being the inversion potential [2]. Three important parameters can be extracted from such a voltammogram: the average value between the cathodic and anodic peak potentials, which
0254-0584/91/$3.50
0 1991
-
Elsevier Sequoia, Lausanne
118
POTENTIAL,
V -SCE
Fig. 1. A cathodic response of a fully reversible reduction
(in the case of reversible processes) has a thermodynamic meaning: E”’ = $(E,“+ E,‘); the difference between the anodic and cathodic peak potentials, which is modulated by the scan rate and (within the Marcus theory [3]) may indicate the rearrangements undergone by the species as a consequence of the charge imposition: AE, = Epa- E,=; the ratio between the anodic and cathodic peak current intensities, which is also modulated by the scan rate and gives (making use of the Nicholson-Sham approach [4]) the life-time of the electrogenerated ions: ipa/ipc. In binary metal carbonyl complexes the HOMO and LUMO are generally metal-metal bonding and antibonding in character, respectively. Thus, the anodic oxidation and the cathodic reduction cause quick and irreversible decomposition. The seminal work from the Dessy group [ 5 ] clearly indicated that several dinuclear metal carbonyl and metal cyclopentadienyl complexes, L,M-ML,, (L= CO, Cp), undergo a two-electron reduction process, followed by the complete separation of the metallic units: L,M-ML,
+ 2e- -
2MI,-
As an example, the CV response of COAX in CH&l, at a Pt electrode is shown below (Pig. 2). In principle, the coordination of large organic chains, interacting with the bimetallic frame in a multicentered U/T fashion, should be able to stabilize the electrogenerated ions. In order to test this idea, we synthesized a number of butatriene hexacarbonyl-diiron complexes from the dehydroxylation reactions of alkyne dials by iron carbonyls (Pig. 3) [S]:
119
0 I!
~OChCO -
COAX
CLXCO), =
t 2 e-
-
7c0(co)4-
~Oc)4C~CqcO)r
‘c’ 0.800 -
1.600 1.200 .-
-1.200 1.200
b A‘1”
”
I
I
I
0.400
0.800
0.000
I
I
-0.800
-1.200
1
-0.400
-1.600
E “1. 5cE Fig.
2. The CV response of Co,(CO),
in CH,Cl, at a F’t electrode.
\ R “’
Fig. 3. Structure of butatriene hexacarbonyl-diiron complex.
3Fe(CO& + OHR&CmCC&OH
Fe2(CO)G(R&=C=C=CR2)
+ Fe(OH)2 + 9CO
From the CV responses of a number of butatriene complexes, one can observe that complex 1 undergoes a single-step two-electron process, complexes 2-4 two distinct one-electron reductions (Fig. 4). The chemical reversibility increases on passing from 1 to 4. The difference between the formal electrode potentials of the two subsequent reductions, AE” =E”(O/l -) -E”(l/2 -), increases from 1 (thetwo steps are overlapped in CV) to 2-4. A possible explanation of these features is that the reduction of butatriene complexes results in a lengthening of the Fe -Fe bond [6 I.
120
i (PA) Fe2(CO)b(H2C=C=C=CH2)
50
(1)
0
50
50
Fe2(C0)6(HMeC=C=C=CHe)
(2)
Fe2(C0)6(Ne2C=C=C=CMe2)
(3)
Fe2(CO)6(Ph2C=C=CzCPh2)
(4)
(c)
+ 50
Fig.4. CV responsesof butatriene complexes.
Indeed, theoretical calculations indicate a non-degenerate LUMO having a metal-metal antibonding character with negligible contributions from the butatriene frontier MOs. However the organic chain acts as a bridge between the metals, altering its conformation in order to maintain effective metal-ligand orbital overlap and at the same time, clasping the metal atoms together. Two inner MOs guarantee the Fe-butatriene interactions and hence the chemical reversibility of the reduction process. As the steric bulk of the butatriene substituents increases (on passing from 1 to 4) the flexibility of the organic chain becomes less, the chemical
121
reversibility increases, the second reduction step becomes more difficult and AE”, is larger. The coordination of large organic chains interacting with a polymetallic frame should also increase the electronic delocalization over the entire cluster. For example, Thorn and Hoffmann (71 suggested that polynuclear metallacyclopentadienyl systems might exhibit a six-r-electron armaticity. In order to test this hypothesis, we synthesized a number of Fe3(CO)8 (alkyne)z derivatives [ 8 1. The cathodic part of the cyclic voltammogram recorded using a Hg working electrode in an acetonitrile solution of Fe3(CO)s(PhC2Ph)z is shown below (Fig. 5). The analysis of the CV response with scan rate is indicative of two subsequent one-electron fast transfers (electrochemically reversible processes) uncomplicated by following fast chemical events (chemically reversible processes). This full reversibility allows an evaluation of the formal electrode potentials E”, having thermodynamic meaning. The E”’ values, as expected, are sensitive to the electronic effects of the substituents R of the alkynes. Good free energy correlations can be obtained by plotting the E”’ values against the sum of the Taft 8 values of all the substituents, regardless of their position in the cycle. This confirms a high electron delocalization over the entire cluster [8]. In striking contrast, ‘flyover-bridge’ derivatives, where localized Fe-C (T bonds have been suggested [ 91, show a linear correlation between E”’ values and the optimized abscissa function a,, where the values of the substituents adjacent to the metallic frame are weighted up to 80%. The difference between the formal electrode potentials of the two subsequent reductions, AE ” = E”(O/l -) - E”‘( 1 - /2 -) is almost constant for all the Fe,(CO)8(RC2R)2 derivatives (ca. 600 mV) regardless of the substituents
(a>
Fig. 5. (a) Structure solution.
(b) of Fe3(CO)8(PhCzPh)z;
’ (b)
Cyclic
voltammogram
of (a) in acetonitrile
122
R in the metallacycle, the solvent and the electrode material. This suggests that both electrons are added to a nondegenerate LUMO (the dianion is ESR silent). The AE” can be assumed to represent (to a first approximation) the energy required to overcome electron repulsion within the same MO, i.e. cu. 58 KJ/mol. It is noteworthy that a similar value has been found for aromatic hydrocarbons [ lo]. Electron Trcznqfm Chain (ETC) catalysis is a general method to take advantage of electron transfer processes in order to accelerate otherwise too slow transformations [ 111. Generally, in metal cluster chemistry a reduction process is employed to initiate the cycle. The key feature is the preparation of @-lived anions, which can rapidly undergo substitution reactions and then (by virtue of their more negative potentials) transfer the extra electron to the unreduced substrate (ECE mechanism). M,(CO),
+ 1e- -
[M,(CO),]-+L-
[ M,(CO), j -
PUCO),-,Ll-+cO
MACO),- ,Ll- + M&W, [M,(CO),_,L]-
initiation
-
M,(CO),_,L+
M&O&, - I L + W&W, Ile-
termination
The reasonable stability of the Fe3(C0)8(PhCzPh)2 monoanion suggests that an ETC reaction can take place on such a substrate [S]. Indeed, the electrolysis of Fe3(C0)8(PhC2Ph), at the first reduction potential in the presence of excess P(OMe)a rapidly transforms the parent cluster in both apical and equatorial isomers Fe,(CO),[P(OMe),](PhC,Ph), (yields ca. 50 and 25, respectively) Fig. 6. The ETC process is not regioselective as expected by the closeness of the E”(O/l -) values of the two isomers. Indeed, the 2:l molecular ratio obtained corresponds to the statistical Fe&,ical/Fe~quatorial ratio
181.
The Fes(CO),(alkyne) derivatives are formally electronically unsaturated (46e). Shilling and Hoffmann [ 121 predicted that the alkyne orientation over the Fe, triangle in the model compound Fe,(CO)9(HC2H) is a consequence of the different energies of the frontier MOs. Thus, the perpendicular orientation is preferred for the neutral 46e species, but a parallel orientation is forecast for the corresponding 48e species, namely the [Fe,(CO),(HC,H)]2dianion (Fig. 7). A similar behaviour can be predicted using the Polyhedral Skeletal Electron Pair (PSEP) approach [ 131. All the Fes(CO)&alkyne) derivatives exhibit two fully reversible oneelectron reduction processes. The exhaustive electrolysis of Fe,(CO),(MeC,Me) at the potential of the second reduction process produces a stable, diamagnetic dianion so that it can be studied by ‘H and 13C NMR spectroscopy [ 141. While the parent compound shows two clear distinct resonances for the acetylenic carbons as well for the methyl substituents, the dianion shows a single resonance for each these groups indicating an increased symmetry after the reduction process, i.e. the coordination of the 2-butyne chain in a parallel fashion.
123
I-
E"= -0.98 V E"= - 1.03 V
-co 1+P(oMe)3
[Fe3(CO),P(OMe)3(PhC2Ph)2]
A-
decomposition
products
1
IFe3(CO)7P(OMe)3(PhCzPh)zl two isomers
Fig. 6. The electrolysis
of Fe,(CO),(PhC,Ph),
with excess
P(OMe),.
Interestingly, this reorientation facilitates the second reduction step, making EO’(l - /2 -) less negative than that expected on the basis of electrostatic effects. Indeed, AE”, for all the series is cu. 200 mV (Fig. 8). The back-donation from the two equivalent metal atoms to the alkyne plays an important role in the preference for the perpendicular coordination of the organic chain over the metallic triangle [ 151. Indeed, the analogous M3(CO)9(PhC2Ph) [M =Ru, OS] derivatives result to be highly unstable due to the higher electronegativities of such metals [ 151. It is however possible to obtain the stable 0s3(C0)7(PhzPCH2PPhz) (I PhC,Ph) derivative where the presence of good a-donor and poor r-acceptor phosphine groups provides the two equivalent osmium atoms with sufficient electron density for backelectrochemical scenario of donation The (Fig. 9) 1161. similar to that of the isoelectronic Os,(dppm)(CO),(PhC,Ph) is Fe3(CO)9(PhCzPh) derivative, but a single step two-electron reduction is observed instead of two subsequent, one-electron reductions [ 171. The dianion is ESR silent and the large AE, value indicates that the I c-, II reorientation is concomitant with the reduction process. Interestingly, the stable dianion [Os,(dppm)(CO),(PhC,Ph)]2can be saturated dihydrido species protonated to electronically the H,Os,(dppm)(CO),(PhC,Ph), where the parallel coordination of the alkyne over the metallic triangle is maintained [ 171. This reaction represents a redox mediated hydrogenation (Fig. 10). It is worth noting that in the case of OS, the coordination change from I (46e-) to II (48e-) can be achieved by chemical means through the coordination of a further CO ligand. Electrochemistry is able to discriminate
124
S 9 (tzp* 9 CO groups (Fe3(C0MHC~H)
46e
1
[F~s(CO)~(HC~H)I~48e
Fig. 7. Energy band scheme for 46e neutral species and 48e dianion 1121.
125
close-trigonal-bipyramid PSEP EAH Fig.
: S=6 : 46 8.
nido-octahedron
; n=5
PSEP
e-
EAN
Structural
change
Fig. 9. Triosmium
complex
consequence
structure
: 5=J
;
n=mj
: 40 e-
on redox
reaction.
[lS].
\A! :/ ;,i. /\ ..i.
...c ..’
yes’
I \ I’ Ph2pb/PPh2 Hz
Ph,P \
C/PPh2 Hz 4
+ Fig.
10. Redox
I
mediated
hydrogenation.
this change in the electronic structure. As a matter of fact, the bubbling of CO into the electrochemical cell quickly transforms 0s3(dppm)(CO)7 (I PhC,Ph) into Os,(dppm)(CO), (II PhC,Ph); in the meantime the cathodic response changes from a chemically reversible to a chemically irreversible two-electron reduction, as expected for a electronically saturated compound (LUMO having metal-metal antibonding character) (Pig. 11). Interestingly, the unstable dianion [Os,(CO),(dppm)(PhC2Ph)]zpartially converts into
126
VISCE
V
[Os3(dppm)(w+.
PhQPh)]
v/SCE
Fig. 11. CV curves of triosmium complexes.
[Os,(CO),(dppm)(PhCzPh)]2the 0s~(CO)~(dppm)(PhC2Ph)
by CO loss, as testified by the reformation of (O/-- 2) peak system during multiple CV scans
II?]. Other well documented examples of reversible geometrical modifications in organometalIic derivatives are: redox ~ter~onve~ion between Ru~II)(~~-C~H~)~ and Ru(O)(q*CGIL&+XIG) I 18 I; redox interconversion between Ose(CO) 18,capped-trigonal bipyramid (hypercloso structure), to [Os6(CO)i8]“-, close-octahedron [19]; redox ~ter~o~ve~ion from [ Ru~(c~ene)~S2 I2+, &so-trigonal bipyramid, to Ru,(cymene),S,, nido-octahedron [ 201. The capability to coordinate and then activate small organic moieties is an essential requirement for the use of metal clusters in catalysis [21]_ Addition of organic molecules to electronically and coordinatively unsaturated clusters is usually easy since ligand dissociation is not required. So far few examples of unsaturated clusters have been reported [ 221, the prototype being Os&-W&O),O. The cluster OS~(~-H)~(CO)~~ was shown to contain a triangular array of three osmium atoms with ten terminally bonded carbonyls and two hydrides bridging the shortest OS-OS edge. The cluster possesses 46e, two fewer that the 48e required by the RAN rule and usually found in triangular clusters of the iron triad. Several approaches have been utilized to rationalize the
127
unusual bonding scheme, in particular Churchill et al. [23] described the Os-H,-0s core in terms of a doubly-protonated double-bond and Broach and Williams [24] in terms of a four-center, four-electron bond. Finally, Sherwood and Hall [25] confirmed that the major part of the bonding in the Os-H,-OS system is composed of two three-center, two-electron bonds, as in diborane with an additional ‘&-- tzg’ bonding interaction. Indeed, Os,(H),(CO),, does behave as an electron deficient compound: its typical reactivity consists of nucleophilic addition of Lewis bases L (phosphines, phosphites, arsines, stibines, CO, nitriles, isonitriles, etc.) to form the 1:l adducts OS&-H)(H)(CO),,,L having the 48e closed-shell configuration (Fig. 12). The CV technique is able to reveal the passage from the unsaturated to the saturated electron structure. For instance, bubbling of CO into the solution causes the transformation of OS~(H)~(CO)~~ into Os,(p-H)(H)(CO), 1 and produces a different electrochemical response. A two-electron, chemically irreversible reduction process is observed at a more negative potential (Fig. 13(b), peak D) for the latter species instead of an one-electron, chemically reversible process (peak system A/C), plagued by absorption phenomena (peak B), found for the latter (Fig. 13(a)), where the ‘Os-H,-OS’ core represents the reduction center [26]. Also the electrochemical behavior of
Fig. 12. Ligand breaking of Os=Os
double bond.
Fig. 13. CV curves indicating transform
from unsaturated to saturated electron
structure
128 Ph
I+ Ph
co
1
f’hz
(c;)s?/P,CH KO),Os
/\
I
2
/PPhz
pc”o,,
Fig. 14. Transformation
of unsaturated 0s3(H)(CO),[Ph2PCH,P(Ph)C,H,j
with CO addition.
the unsaturated (46e) Osa(H)(CO)a [ Ph2PCH2P(Ph)CGH,] cluster was studied by means of electrochemical and spectroscopic techniques. The transforthis very reactive compound into the saturated mations of Os~(~)(CO~~[Ph~PCH~P(Ph~(~~H~~and Os~(CO)~*(Ph~PCH~PPh~)clusters by subsequent CO addition (Fig. 14) was followed step-by-step by cyclic voltammetry [27]. The Os,(H)(CO)s[Ph,PCH,P(Ph)C,H,] compound exhibits an one-electron, chemically reversible reduction process (peak system A/B (Fig. 15a)). When CO is bubbled at room temperature through its solution in the electrochemical cell, after 2 h, [Os3(H)(CO)9{Ph2PCH2P(Ph)C,H,}] is formed quantitatively. The CV response changes during this transformation: a new peak, D, grows at more negative potential followed in the reverse scan by small peaks likely due to the reoxidation processes of electrogenerated fragments. Comparison of current intensities of the peaks A and D indicates that Os,(H)(CO),[ Ph2PCH2P(Ph)C& I undergoes a chemically irreversible one-electron reduction. F’inally, the stirring of an acetonitriIe solution of 0s3(H)(CO),[Ph,PCH,P(Ph)C,H,] under CO atmosphere for a week at room temperature gradually, but completely, turns the 0s,(H)(CO)9[ Ph,PC&P(Ph)CSH,I into the Os~(CO~~~(dppm)cluster. The CV response shows a new peak E followed in the reverse scan by small reoxidation peaks of the
129
2.000 -
-o.oDo gr_
-
----/
----_=7--
--_ d
I
A
\
I '\
-1.000 -
"
:
B I
I-23%00 8.000
I
I
-1.200
-1.600
I
-0.400
-o.wo
I
I
-0.600
-O.mo
I
I
I
-1.200
-1.400
--L.._ -2.000
_-I -2.a
I
1
I
-l.bOO
-l.EmO
-2.000
6.ODD
4.000
‘p Q
x 2.000
-O.OJ>
B
I -2.03 .40D 1
-1.000
11-l-
-
?oo
E vs.SCE
Fig. 15. The CV response changes during the transformation of 0s,(H)(CO),[Ph2PCH,P(Ph)C,H,].
fragmentation products (Fig. 15b). Comparison of the peak current intensities recorded on the same solution before and after the transformation (peaks A and E respectively) indicates that peak E corresponds to a chemically irreversible two-electron reduction process, likely a complex ECE one as elegantly demonstrated by Downard et al. for Osa(CO),a [28]. The negative shift of potential on passing from Osa(CO),a to Os,(CO),,(dppm) is consistent with the higher cr donor/r acceptor ratio of diphosphines with respect to
130
CO ligand. The CV technique is able to follow these transformations, and in this context, the term e~ctroc~~cal spectroscopy coined by Heinze [29] seems quite appropriate.
Acknowledgement
I am grateful to my colleagues and talented research students; their names are to be found in the references. Finally, I thank the Council of National Research (CNR, Rome) for the financial support within the Progetto Finalizzato ‘Chimica Fine-Nuove Sintesi’ and the Johnson Matthey for a loan of precious metals.
References 1 A. Sgamellotti, Proceedings CISCI 90, San Benedetto de1 Pronto, Italy, l-5 October 1990, lL11, p. 461. 2 P. T. Kissinger and W. R. Heineman, J. Chem. Educ., 60 (1983) 702. 3 R. S. Nicholson and I. Sbain, Anal. Chem., 34 (1964) 706. 4 W. E. Geiger, Prog. Iwg. Ch,ern.,33 (1985) 275. 5 R. E. Dessy, P. M. Weissman and R. L. Pohl, J. Am Chem. Sot., 88 (1966) 5117. 6 D. Osella, 0. Gambino, R. Gobetto, P. Zanello, G. Las&i, C. E. Housecroft and S. M. Owen, Organinne t., 9 (1990) 1792. 7 D. L. Thorn and R. Hoffmann, Nouv. J. Chim., 3 (1979) 39. 8 D. Osella, G. Arman, M. Botta, R. Gobetto, F. Las&i and P. Zanello, ~ga~~~., 8 (1989) 620. 9 D. Osella, M. Botta, R. Gobetto, F. Lascbi and P. Zanello, Orguvwmet., 7 (1988) 283. 10 T. M. McKinney, Electroanal. Ch.em., 10 (1980) 97. 11 (a) J. M. Saveant,Act. Ch.em. Res., 13 (1980) 323; (b) J. K. Kochi, J. Organomet. Chem., 300 (1986) 139; (c) D. Astruc, Angew. Chem. Int. Ed. En&, 27 (1988) 643. 12 B. E. R. Shilling and R. Hoffmann, J. Am. Chem. Sot., 101 (1979) 3456. 13 For a recent review see: D. M. P. Mingos, Ace. Chem. Res., I7 (1984) 311. 14 D. Osella, R. Gobetto, P. Montangero, P. Zanello and A. Cinquantini,Orgunomet., 5 (1986)
1247. 15 (a) V. Busetti, G. Granozzi, S. Aime, R. Gobetto, D. Osella, Organome t., 3 (1984) 1510; (b) S. Aime, R. Bertoncello, V. Busetti, R. Gobetto, G. Granozzi and D. Osella, Iwg. Cha., 25 (1986) 4004. 16 J. A. Clucas, A. Dolby, M. M. Harding and A. K. Smith, J. Chem. Sot. Chcnn Commun., (1987) 1829. 17 D. O&la, A. K. Smith and P. Zanello, unpublished work. 18 E. 0. Fischer and C. H. Elschenbroich, Churn. Be?-., IO3 (1970) 162. 19 B. Tulyathan and W. E. Geiger, J. Am. Chem. Sot., IO7 (1985) 5960. 20 J. R. Lockemeyer, T. B. Rauchfuss and A. L. Rheingold, J. Am. Chem Sot., 111 (1989) 5733. 21 G. Lavigne, H. D. Kaesz, in H. Knozinger, B. C. Gates, L. Guczi, teds.), Metal CleLsters in CaMysis, Elsevier, Amsterdam, 1986. 22 B. F. G. Johnson, J. Lewis and P. Kilty, J, Chem. Sot. A (1968) 2859. 23 M. R. Churchill, F. J. Hollander and J. P. Hutchinson, Zwg. Cha., 16 (1977) 2697. 24 R. W. Broach and M. J. Williams, Inorg. Chem., 18 (1979) 314. 25 D. E. Sherwood and M. B. Hall, Inorg. C&m., 21 (1982) 3458.
131 26 D. Osella,
E. Stein, C. Nervi, P. Zanello, F. Laschi, A. Cinquantini, E. Rosenberg and J. Fiedler, Organomet., 10 (1991) 1929. 27 D. Osella, A. K. Smith and P. Zanello, unpublished work. 28 A. J. Downard, B. H. Robinson, J. Simpson and A. M. Bond, J. @-ganomet. Chem., 320 (1987) 363. 29 J. Heinze, Angew. Chem. ht. Ed. Engl., 23 (1984) 831.
Chemistry
and Physics,
29 (1991)
117
117-131
Review
Some aspects of organometallic electrochemistry D. Osella Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica Universith di Torino, Via P. Giuria 7, 10125 Turin (Italy)
dei Materiali,
The electrochemical investigations in the field of organometallic chemistry may be useful to: a) understand the stabilizing effect of the organic chains on the metallic frame, with the aim of obtaining stable ions (electron reservoirs); b) estimate (in a direct way) the electronic delocalization on the complex, predicted by theoretical calculations; c) evaluate the geometrical changes following or accompanying the electron transfers; d) test (in a direct way) the electronic unsaturation of the metallic core, predicted by the electron count; e) plan efficient and selective electrosyntheses. In particular, the storage of electrical energy into chemical form by means of bond breaking and reforming in a reversible manner (point a) might be of some interest in the wider context of developing molecular batteries [ 1 ]. The aim of this review article is to show some examples of these features, mainly taken from the author’s own work. The main technique employed for such investigations is by far cyclic voltammetry (CV) which allows a wide choice of potential scan rate (v) and then a wide time-scale range of electrochemical events. Further, the electrochemical armory generally consists of coulometry (to assess the number of electrons transferred, n), DC polarography, rotating disk electrode (RDE) voltammetry (having high sensitivity because of the forced hydrodynamic convection) and all the spectroscopic techniques to be employed in situ (IR, UV/VIS, ESR and sometimes NMR) in order to follow the chemical transformation caused by the imposition of the electrical charge. The reader is certainly familiar with the CV experiment, which measures the current at a given potential when the potential itself is swept in a forward and then in a reverse scan (triangular potential ramp). A cathodic response of a fully reversible reduction process is reported below as an example, (Fig. 1) E, being the inversion potential [2]. Three important parameters can be extracted from such a voltammogram: the average value between the cathodic and anodic peak potentials, which
0254-0584/91/$3.50
0 1991
-
Elsevier Sequoia, Lausanne
118
POTENTIAL,
V -SCE
Fig. 1. A cathodic response of a fully reversible reduction
(in the case of reversible processes) has a thermodynamic meaning: E”’ = $(E,“+ E,‘); the difference between the anodic and cathodic peak potentials, which is modulated by the scan rate and (within the Marcus theory [3]) may indicate the rearrangements undergone by the species as a consequence of the charge imposition: AE, = Epa- E,=; the ratio between the anodic and cathodic peak current intensities, which is also modulated by the scan rate and gives (making use of the Nicholson-Sham approach [4]) the life-time of the electrogenerated ions: ipa/ipc. In binary metal carbonyl complexes the HOMO and LUMO are generally metal-metal bonding and antibonding in character, respectively. Thus, the anodic oxidation and the cathodic reduction cause quick and irreversible decomposition. The seminal work from the Dessy group [ 5 ] clearly indicated that several dinuclear metal carbonyl and metal cyclopentadienyl complexes, L,M-ML,, (L= CO, Cp), undergo a two-electron reduction process, followed by the complete separation of the metallic units: L,M-ML,
+ 2e- -
2MI,-
As an example, the CV response of COAX in CH&l, at a Pt electrode is shown below (Pig. 2). In principle, the coordination of large organic chains, interacting with the bimetallic frame in a multicentered U/T fashion, should be able to stabilize the electrogenerated ions. In order to test this idea, we synthesized a number of butatriene hexacarbonyl-diiron complexes from the dehydroxylation reactions of alkyne dials by iron carbonyls (Pig. 3) [S]:
119
0 I!
~OChCO -
COAX
CLXCO), =
t 2 e-
-
7c0(co)4-
~Oc)4C~CqcO)r
‘c’ 0.800 -
1.600 1.200 .-
-1.200 1.200
b A‘1”
”
I
I
I
0.400
0.800
0.000
I
I
-0.800
-1.200
1
-0.400
-1.600
E “1. 5cE Fig.
2. The CV response of Co,(CO),
in CH,Cl, at a F’t electrode.
\ R “’
Fig. 3. Structure of butatriene hexacarbonyl-diiron complex.
3Fe(CO& + OHR&CmCC&OH
Fe2(CO)G(R&=C=C=CR2)
+ Fe(OH)2 + 9CO
From the CV responses of a number of butatriene complexes, one can observe that complex 1 undergoes a single-step two-electron process, complexes 2-4 two distinct one-electron reductions (Fig. 4). The chemical reversibility increases on passing from 1 to 4. The difference between the formal electrode potentials of the two subsequent reductions, AE” =E”(O/l -) -E”(l/2 -), increases from 1 (thetwo steps are overlapped in CV) to 2-4. A possible explanation of these features is that the reduction of butatriene complexes results in a lengthening of the Fe -Fe bond [6 I.
120
i (PA) Fe2(CO)b(H2C=C=C=CH2)
50
(1)
0
50
50
Fe2(C0)6(HMeC=C=C=CHe)
(2)
Fe2(C0)6(Ne2C=C=C=CMe2)
(3)
Fe2(CO)6(Ph2C=C=CzCPh2)
(4)
(c)
+ 50
Fig.4. CV responsesof butatriene complexes.
Indeed, theoretical calculations indicate a non-degenerate LUMO having a metal-metal antibonding character with negligible contributions from the butatriene frontier MOs. However the organic chain acts as a bridge between the metals, altering its conformation in order to maintain effective metal-ligand orbital overlap and at the same time, clasping the metal atoms together. Two inner MOs guarantee the Fe-butatriene interactions and hence the chemical reversibility of the reduction process. As the steric bulk of the butatriene substituents increases (on passing from 1 to 4) the flexibility of the organic chain becomes less, the chemical
121
reversibility increases, the second reduction step becomes more difficult and AE”, is larger. The coordination of large organic chains interacting with a polymetallic frame should also increase the electronic delocalization over the entire cluster. For example, Thorn and Hoffmann (71 suggested that polynuclear metallacyclopentadienyl systems might exhibit a six-r-electron armaticity. In order to test this hypothesis, we synthesized a number of Fe3(CO)8 (alkyne)z derivatives [ 8 1. The cathodic part of the cyclic voltammogram recorded using a Hg working electrode in an acetonitrile solution of Fe3(CO)s(PhC2Ph)z is shown below (Fig. 5). The analysis of the CV response with scan rate is indicative of two subsequent one-electron fast transfers (electrochemically reversible processes) uncomplicated by following fast chemical events (chemically reversible processes). This full reversibility allows an evaluation of the formal electrode potentials E”, having thermodynamic meaning. The E”’ values, as expected, are sensitive to the electronic effects of the substituents R of the alkynes. Good free energy correlations can be obtained by plotting the E”’ values against the sum of the Taft 8 values of all the substituents, regardless of their position in the cycle. This confirms a high electron delocalization over the entire cluster [8]. In striking contrast, ‘flyover-bridge’ derivatives, where localized Fe-C (T bonds have been suggested [ 91, show a linear correlation between E”’ values and the optimized abscissa function a,, where the values of the substituents adjacent to the metallic frame are weighted up to 80%. The difference between the formal electrode potentials of the two subsequent reductions, AE ” = E”(O/l -) - E”‘( 1 - /2 -) is almost constant for all the Fe,(CO)8(RC2R)2 derivatives (ca. 600 mV) regardless of the substituents
(a>
Fig. 5. (a) Structure solution.
(b) of Fe3(CO)8(PhCzPh)z;
’ (b)
Cyclic
voltammogram
of (a) in acetonitrile
122
R in the metallacycle, the solvent and the electrode material. This suggests that both electrons are added to a nondegenerate LUMO (the dianion is ESR silent). The AE” can be assumed to represent (to a first approximation) the energy required to overcome electron repulsion within the same MO, i.e. cu. 58 KJ/mol. It is noteworthy that a similar value has been found for aromatic hydrocarbons [ lo]. Electron Trcznqfm Chain (ETC) catalysis is a general method to take advantage of electron transfer processes in order to accelerate otherwise too slow transformations [ 111. Generally, in metal cluster chemistry a reduction process is employed to initiate the cycle. The key feature is the preparation of @-lived anions, which can rapidly undergo substitution reactions and then (by virtue of their more negative potentials) transfer the extra electron to the unreduced substrate (ECE mechanism). M,(CO),
+ 1e- -
[M,(CO),]-+L-
[ M,(CO), j -
PUCO),-,Ll-+cO
MACO),- ,Ll- + M&W, [M,(CO),_,L]-
initiation
-
M,(CO),_,L+
M&O&, - I L + W&W, Ile-
termination
The reasonable stability of the Fe3(C0)8(PhCzPh)2 monoanion suggests that an ETC reaction can take place on such a substrate [S]. Indeed, the electrolysis of Fe3(C0)8(PhC2Ph), at the first reduction potential in the presence of excess P(OMe)a rapidly transforms the parent cluster in both apical and equatorial isomers Fe,(CO),[P(OMe),](PhC,Ph), (yields ca. 50 and 25, respectively) Fig. 6. The ETC process is not regioselective as expected by the closeness of the E”(O/l -) values of the two isomers. Indeed, the 2:l molecular ratio obtained corresponds to the statistical Fe&,ical/Fe~quatorial ratio
181.
The Fes(CO),(alkyne) derivatives are formally electronically unsaturated (46e). Shilling and Hoffmann [ 121 predicted that the alkyne orientation over the Fe, triangle in the model compound Fe,(CO)9(HC2H) is a consequence of the different energies of the frontier MOs. Thus, the perpendicular orientation is preferred for the neutral 46e species, but a parallel orientation is forecast for the corresponding 48e species, namely the [Fe,(CO),(HC,H)]2dianion (Fig. 7). A similar behaviour can be predicted using the Polyhedral Skeletal Electron Pair (PSEP) approach [ 131. All the Fes(CO)&alkyne) derivatives exhibit two fully reversible oneelectron reduction processes. The exhaustive electrolysis of Fe,(CO),(MeC,Me) at the potential of the second reduction process produces a stable, diamagnetic dianion so that it can be studied by ‘H and 13C NMR spectroscopy [ 141. While the parent compound shows two clear distinct resonances for the acetylenic carbons as well for the methyl substituents, the dianion shows a single resonance for each these groups indicating an increased symmetry after the reduction process, i.e. the coordination of the 2-butyne chain in a parallel fashion.
123
I-
E"= -0.98 V E"= - 1.03 V
-co 1+P(oMe)3
[Fe3(CO),P(OMe)3(PhC2Ph)2]
A-
decomposition
products
1
IFe3(CO)7P(OMe)3(PhCzPh)zl two isomers
Fig. 6. The electrolysis
of Fe,(CO),(PhC,Ph),
with excess
P(OMe),.
Interestingly, this reorientation facilitates the second reduction step, making EO’(l - /2 -) less negative than that expected on the basis of electrostatic effects. Indeed, AE”, for all the series is cu. 200 mV (Fig. 8). The back-donation from the two equivalent metal atoms to the alkyne plays an important role in the preference for the perpendicular coordination of the organic chain over the metallic triangle [ 151. Indeed, the analogous M3(CO)9(PhC2Ph) [M =Ru, OS] derivatives result to be highly unstable due to the higher electronegativities of such metals [ 151. It is however possible to obtain the stable 0s3(C0)7(PhzPCH2PPhz) (I PhC,Ph) derivative where the presence of good a-donor and poor r-acceptor phosphine groups provides the two equivalent osmium atoms with sufficient electron density for backelectrochemical scenario of donation The (Fig. 9) 1161. similar to that of the isoelectronic Os,(dppm)(CO),(PhC,Ph) is Fe3(CO)9(PhCzPh) derivative, but a single step two-electron reduction is observed instead of two subsequent, one-electron reductions [ 171. The dianion is ESR silent and the large AE, value indicates that the I c-, II reorientation is concomitant with the reduction process. Interestingly, the stable dianion [Os,(dppm)(CO),(PhC,Ph)]2can be saturated dihydrido species protonated to electronically the H,Os,(dppm)(CO),(PhC,Ph), where the parallel coordination of the alkyne over the metallic triangle is maintained [ 171. This reaction represents a redox mediated hydrogenation (Fig. 10). It is worth noting that in the case of OS, the coordination change from I (46e-) to II (48e-) can be achieved by chemical means through the coordination of a further CO ligand. Electrochemistry is able to discriminate
124
S 9 (tzp* 9 CO groups (Fe3(C0MHC~H)
46e
1
[F~s(CO)~(HC~H)I~48e
Fig. 7. Energy band scheme for 46e neutral species and 48e dianion 1121.
125
close-trigonal-bipyramid PSEP EAH Fig.
: S=6 : 46 8.
nido-octahedron
; n=5
PSEP
e-
EAN
Structural
change
Fig. 9. Triosmium
complex
consequence
structure
: 5=J
;
n=mj
: 40 e-
on redox
reaction.
[lS].
\A! :/ ;,i. /\ ..i.
...c ..’
yes’
I \ I’ Ph2pb/PPh2 Hz
Ph,P \
C/PPh2 Hz 4
+ Fig.
10. Redox
I
mediated
hydrogenation.
this change in the electronic structure. As a matter of fact, the bubbling of CO into the electrochemical cell quickly transforms 0s3(dppm)(CO)7 (I PhC,Ph) into Os,(dppm)(CO), (II PhC,Ph); in the meantime the cathodic response changes from a chemically reversible to a chemically irreversible two-electron reduction, as expected for a electronically saturated compound (LUMO having metal-metal antibonding character) (Pig. 11). Interestingly, the unstable dianion [Os,(CO),(dppm)(PhC2Ph)]zpartially converts into
126
VISCE
V
[Os3(dppm)(w+.
PhQPh)]
v/SCE
Fig. 11. CV curves of triosmium complexes.
[Os,(CO),(dppm)(PhCzPh)]2the 0s~(CO)~(dppm)(PhC2Ph)
by CO loss, as testified by the reformation of (O/-- 2) peak system during multiple CV scans
II?]. Other well documented examples of reversible geometrical modifications in organometalIic derivatives are: redox ~ter~onve~ion between Ru~II)(~~-C~H~)~ and Ru(O)(q*CGIL&+XIG) I 18 I; redox interconversion between Ose(CO) 18,capped-trigonal bipyramid (hypercloso structure), to [Os6(CO)i8]“-, close-octahedron [19]; redox ~ter~o~ve~ion from [ Ru~(c~ene)~S2 I2+, &so-trigonal bipyramid, to Ru,(cymene),S,, nido-octahedron [ 201. The capability to coordinate and then activate small organic moieties is an essential requirement for the use of metal clusters in catalysis [21]_ Addition of organic molecules to electronically and coordinatively unsaturated clusters is usually easy since ligand dissociation is not required. So far few examples of unsaturated clusters have been reported [ 221, the prototype being Os&-W&O),O. The cluster OS~(~-H)~(CO)~~ was shown to contain a triangular array of three osmium atoms with ten terminally bonded carbonyls and two hydrides bridging the shortest OS-OS edge. The cluster possesses 46e, two fewer that the 48e required by the RAN rule and usually found in triangular clusters of the iron triad. Several approaches have been utilized to rationalize the
127
unusual bonding scheme, in particular Churchill et al. [23] described the Os-H,-0s core in terms of a doubly-protonated double-bond and Broach and Williams [24] in terms of a four-center, four-electron bond. Finally, Sherwood and Hall [25] confirmed that the major part of the bonding in the Os-H,-OS system is composed of two three-center, two-electron bonds, as in diborane with an additional ‘&-- tzg’ bonding interaction. Indeed, Os,(H),(CO),, does behave as an electron deficient compound: its typical reactivity consists of nucleophilic addition of Lewis bases L (phosphines, phosphites, arsines, stibines, CO, nitriles, isonitriles, etc.) to form the 1:l adducts OS&-H)(H)(CO),,,L having the 48e closed-shell configuration (Fig. 12). The CV technique is able to reveal the passage from the unsaturated to the saturated electron structure. For instance, bubbling of CO into the solution causes the transformation of OS~(H)~(CO)~~ into Os,(p-H)(H)(CO), 1 and produces a different electrochemical response. A two-electron, chemically irreversible reduction process is observed at a more negative potential (Fig. 13(b), peak D) for the latter species instead of an one-electron, chemically reversible process (peak system A/C), plagued by absorption phenomena (peak B), found for the latter (Fig. 13(a)), where the ‘Os-H,-OS’ core represents the reduction center [26]. Also the electrochemical behavior of
Fig. 12. Ligand breaking of Os=Os
double bond.
Fig. 13. CV curves indicating transform
from unsaturated to saturated electron
structure
128 Ph
I+ Ph
co
1
f’hz
(c;)s?/P,CH KO),Os
/\
I
2
/PPhz
pc”o,,
Fig. 14. Transformation
of unsaturated 0s3(H)(CO),[Ph2PCH,P(Ph)C,H,j
with CO addition.
the unsaturated (46e) Osa(H)(CO)a [ Ph2PCH2P(Ph)CGH,] cluster was studied by means of electrochemical and spectroscopic techniques. The transforthis very reactive compound into the saturated mations of Os~(~)(CO~~[Ph~PCH~P(Ph~(~~H~~and Os~(CO)~*(Ph~PCH~PPh~)clusters by subsequent CO addition (Fig. 14) was followed step-by-step by cyclic voltammetry [27]. The Os,(H)(CO)s[Ph,PCH,P(Ph)C,H,] compound exhibits an one-electron, chemically reversible reduction process (peak system A/B (Fig. 15a)). When CO is bubbled at room temperature through its solution in the electrochemical cell, after 2 h, [Os3(H)(CO)9{Ph2PCH2P(Ph)C,H,}] is formed quantitatively. The CV response changes during this transformation: a new peak, D, grows at more negative potential followed in the reverse scan by small peaks likely due to the reoxidation processes of electrogenerated fragments. Comparison of current intensities of the peaks A and D indicates that Os,(H)(CO),[ Ph2PCH2P(Ph)C& I undergoes a chemically irreversible one-electron reduction. F’inally, the stirring of an acetonitriIe solution of 0s3(H)(CO),[Ph,PCH,P(Ph)C,H,] under CO atmosphere for a week at room temperature gradually, but completely, turns the 0s,(H)(CO)9[ Ph,PC&P(Ph)CSH,I into the Os~(CO~~~(dppm)cluster. The CV response shows a new peak E followed in the reverse scan by small reoxidation peaks of the
129
2.000 -
-o.oDo gr_
-
----/
----_=7--
--_ d
I
A
\
I '\
-1.000 -
"
:
B I
I-23%00 8.000
I
I
-1.200
-1.600
I
-0.400
-o.wo
I
I
-0.600
-O.mo
I
I
I
-1.200
-1.400
--L.._ -2.000
_-I -2.a
I
1
I
-l.bOO
-l.EmO
-2.000
6.ODD
4.000
‘p Q
x 2.000
-O.OJ>
B
I -2.03 .40D 1
-1.000
11-l-
-
?oo
E vs.SCE
Fig. 15. The CV response changes during the transformation of 0s,(H)(CO),[Ph2PCH,P(Ph)C,H,].
fragmentation products (Fig. 15b). Comparison of the peak current intensities recorded on the same solution before and after the transformation (peaks A and E respectively) indicates that peak E corresponds to a chemically irreversible two-electron reduction process, likely a complex ECE one as elegantly demonstrated by Downard et al. for Osa(CO),a [28]. The negative shift of potential on passing from Osa(CO),a to Os,(CO),,(dppm) is consistent with the higher cr donor/r acceptor ratio of diphosphines with respect to
130
CO ligand. The CV technique is able to follow these transformations, and in this context, the term e~ctroc~~cal spectroscopy coined by Heinze [29] seems quite appropriate.
Acknowledgement
I am grateful to my colleagues and talented research students; their names are to be found in the references. Finally, I thank the Council of National Research (CNR, Rome) for the financial support within the Progetto Finalizzato ‘Chimica Fine-Nuove Sintesi’ and the Johnson Matthey for a loan of precious metals.
References 1 A. Sgamellotti, Proceedings CISCI 90, San Benedetto de1 Pronto, Italy, l-5 October 1990, lL11, p. 461. 2 P. T. Kissinger and W. R. Heineman, J. Chem. Educ., 60 (1983) 702. 3 R. S. Nicholson and I. Sbain, Anal. Chem., 34 (1964) 706. 4 W. E. Geiger, Prog. Iwg. Ch,ern.,33 (1985) 275. 5 R. E. Dessy, P. M. Weissman and R. L. Pohl, J. Am Chem. Sot., 88 (1966) 5117. 6 D. Osella, 0. Gambino, R. Gobetto, P. Zanello, G. Las&i, C. E. Housecroft and S. M. Owen, Organinne t., 9 (1990) 1792. 7 D. L. Thorn and R. Hoffmann, Nouv. J. Chim., 3 (1979) 39. 8 D. Osella, G. Arman, M. Botta, R. Gobetto, F. Las&i and P. Zanello, ~ga~~~., 8 (1989) 620. 9 D. Osella, M. Botta, R. Gobetto, F. Lascbi and P. Zanello, Orguvwmet., 7 (1988) 283. 10 T. M. McKinney, Electroanal. Ch.em., 10 (1980) 97. 11 (a) J. M. Saveant,Act. Ch.em. Res., 13 (1980) 323; (b) J. K. Kochi, J. Organomet. Chem., 300 (1986) 139; (c) D. Astruc, Angew. Chem. Int. Ed. En&, 27 (1988) 643. 12 B. E. R. Shilling and R. Hoffmann, J. Am. Chem. Sot., 101 (1979) 3456. 13 For a recent review see: D. M. P. Mingos, Ace. Chem. Res., I7 (1984) 311. 14 D. Osella, R. Gobetto, P. Montangero, P. Zanello and A. Cinquantini,Orgunomet., 5 (1986)
1247. 15 (a) V. Busetti, G. Granozzi, S. Aime, R. Gobetto, D. Osella, Organome t., 3 (1984) 1510; (b) S. Aime, R. Bertoncello, V. Busetti, R. Gobetto, G. Granozzi and D. Osella, Iwg. Cha., 25 (1986) 4004. 16 J. A. Clucas, A. Dolby, M. M. Harding and A. K. Smith, J. Chem. Sot. Chcnn Commun., (1987) 1829. 17 D. O&la, A. K. Smith and P. Zanello, unpublished work. 18 E. 0. Fischer and C. H. Elschenbroich, Churn. Be?-., IO3 (1970) 162. 19 B. Tulyathan and W. E. Geiger, J. Am. Chem. Sot., IO7 (1985) 5960. 20 J. R. Lockemeyer, T. B. Rauchfuss and A. L. Rheingold, J. Am. Chem Sot., 111 (1989) 5733. 21 G. Lavigne, H. D. Kaesz, in H. Knozinger, B. C. Gates, L. Guczi, teds.), Metal CleLsters in CaMysis, Elsevier, Amsterdam, 1986. 22 B. F. G. Johnson, J. Lewis and P. Kilty, J, Chem. Sot. A (1968) 2859. 23 M. R. Churchill, F. J. Hollander and J. P. Hutchinson, Zwg. Cha., 16 (1977) 2697. 24 R. W. Broach and M. J. Williams, Inorg. Chem., 18 (1979) 314. 25 D. E. Sherwood and M. B. Hall, Inorg. C&m., 21 (1982) 3458.
131 26 D. Osella,
E. Stein, C. Nervi, P. Zanello, F. Laschi, A. Cinquantini, E. Rosenberg and J. Fiedler, Organomet., 10 (1991) 1929. 27 D. Osella, A. K. Smith and P. Zanello, unpublished work. 28 A. J. Downard, B. H. Robinson, J. Simpson and A. M. Bond, J. @-ganomet. Chem., 320 (1987) 363. 29 J. Heinze, Angew. Chem. ht. Ed. Engl., 23 (1984) 831.