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Unusual Structures and Physical Properties in Organometallic Chemistry
John Wiley & Sons
Copyright © 2002 John Wiley & Sons Ltd
All right reserved. ISBN: 0-471-49635-9
Chapter One
Structure and Electrochemistry of Transition Metal Carbonyl Clusters with Interstitial or Semi-Interstitial Atoms: Contrast between Nitrides or Phosphides and Carbides PIERO ZANELLO Dipartimento di Chimica dell'Università di Siena, Via Aldo Moro, 53100 Siena, Italy
Transition-metal carbonyl clusters containing interstitial or semi-interstitial atoms have been the subject of many studies, particularly in view of the fact that the insertion of interstitial atoms inside the metal cage of the clusters often increases the number of valence electrons (hence affecting to some extent the reactivity), leaving essentially unaltered the molecular geometry with respect to the original species. Their preparative, structural, spectroscopic (NMR) and theoretical aspects have been elucidated and their possible use as catalysts has been proposed. In addition, their electrochemical behaviour has been mostly reviewed in a series of articles devoted to a systematic examination of the electrochemical behaviour of homo- and hetero-metal carbonyl clusters.
In this present paper, we should like to focus more specifically on the different, and in some cases contrasting, electrochemical behaviour of homoleptic transition-metal carbonyl clusterscontaining interstitial or exposed N, P atoms with respect to the ITLITL-containing analogues. Since these nitride or phosphide carbonyl clusters can be considered as a link between organometallic and coordination compounds, it is hoped that a detailed comparison of their redox aptitude can help theoreticians in understanding more and more the extent to which the nature of such interstitial heteroatoms might affect the electron mobility inside such compounds.
We will examine here only those complexes for which the X-ray structures have been solved-discussions of the structural details are given in the relevant literature references, or mostly in References. Even if in many cases there are not sufficient electrochemical data to allow comparisons to be made between nitride/phosphide-containing metal clusters and their carbide analogues, we think it is useful to give literature references to the X-ray structures of all of the complexes known so far.
The molecular structures and electrochemical responses presented here are adapted from the original figures quoted in the text. Unless otherwise specified, all the electrode potentials are referred to the saturated-calomel electrode.
1 INTRODUCTION
Although carbide-containing transition-metal carbonyl clusters have been known for a long time, delays were experienced before electrochemists began to deal with them, so that their redox chemistry was adequately, although roughly explored by chemical routes. In fact, the use of chemical reagents does not allow the redox properties of a molecule to be finely tuned. For instance, Chini's group in Milan pioneeringly investigated not only the synthetic and structural aspects of metal clusters, but also their redox chemistry. Thus, one can find in the literature the structural characterization of a few redox couples of metal carbonyl clusters obtained 'blindly' by using chemical reagents. In this connection, Figures 1-3 show the molecular structures of one member of each of the couples, [[[Co.sub.3][Ni.sub.9](C)[(CO).sub.20]].sup.3-/2-], [[[Rh.sub.12][(C).sub.2][(CO).sub.23]].sup.4-/3], and [[[Co.sub.13][(C).sub.2][(CO).sub.24]].sup.4-/3].
In all of these cases, the redox congeners are isostructural with each other and only minor variations in the metal-metal, metal-[carbon.sub.(carbonyl)], and metal-[carbon.sub.(carbide)] bonding distances occur upon addition/removal of one electron. As previously mentioned, the use of chemical reagents does not allow the multiple redox states of a molecule to be adequately determined. For instance, [[[Co.sub.3][Ni.sub.9](C)[(CO).sub.20]].sup.3-] not only undergoes the chemically reversible one-electron removal process, [[[Co.sub.3][Ni.sub.9](C)[(CO).sub.20]].sup.3-/2-] ([E.sup.0'] = -0.30 V), but it is also able to add two electrons in a single step ([E.sub.0'] = -1.71 V), affording the pentaanion [[[Co.sub.3][Ni.sub.9](C)[(CO).sub.20]].sup.5-], which, however, is a transient species ([t.sub.1/2] [approximately equal to] 1 s) (Figure 4).
Analogously, the chemical picture concerned with the redox change [[[Rh.sub.12][(C).sub.2][(CO).sub.23]].sup.4-/3-] appears to be correct as far as the full stability within the family is concerned. Indeed, as Figure 5 illustrates, the redox ability is more extended in that [[[Rh.sub.12][(C).sub.2][(CO).sub.23]].sup.4-] not only undergoes reversibly the cited one-electron oxidation ([E.sup.0'] = -0.46 V), but also exhibits a second irreversible one-electron removal ([E.sub.p] = -0.16 V), as well as a single two-electron reduction to the corresponding hexaanion [[[Rh.sub.12][(C).sub.2][(CO).sub.23]].sup.6-] ([E.sup.0'] = -1.62 V), which, however, is relatively short-lived ([t.sub.1/2] [approximately equal to] 1 s).
Even more instructive is the case of the redox family of [[[Co.sub.13][(C).sub.2] [(CO).sub.24]].sup.4-]. As Figure 6 proves, the tetraanion not only undergoes reversible the cited one-electron oxidation to the trianion [[[Co.sub.13][(C).sub.2][(CO).sub.24]].sup.3-] ([E.sup.0'] = -0.54 V), but it also exhibits the chemically reversible sequential access to the corresponding penta- ([E.sup.0'] = -1.06 V) and hexa- ([E.sup.0'] = -1.68 V) anions, [[[Co.sub.13][(C).sub.2][(CO).sub.24]].sup.5-/6-], respectively.
Furthermore, the usefulness of electrochemical studies in the present field is not limited to the discovery of multiple, stable or unstable, redox states of clusters, but also to the eventual conversion of a molecule to a somewhat related species by simple redox processes.
For instance, the dianion [[[Ru.sub.6](C)[(CO).sub.16]].sup.2-], the octahedral geometry of which is shown in Figure 7(a) undergoes an irreversible two-electron oxidation ([E.sub.p] = +0.48 V, vs Ag/AgCl) to the neutral more carbonylated congener [Ru.sub.6](C)[(CO).sub.17], which in turn undergoes an irreversible reduction ([E.sub.p] = -0.47 V) (Figure 7(b)).
As confirmation, [Ru.sub.6](C)[(CO).sub.17], the octahedral geometry of which is shown in Figure 8(a), exhibits a quite complementary voltammetric response (Figure 8(b)), thus pointing out that, upon two-electron addition, it converts again to the decarbonylated dianion [[[Ru.sub.6](C)[(CO).sub.16]].sup.2-].
By way of comparison, the isostructural and isoelectronic non-carbide dianion [[[Ru.sub.6][(CO).sub.18]].sup.2-] (Figure 9) also exhibits in dichloromethane solution a two-electron oxidation coupled to fast chemical complicated behaviour, although this it occurs at a notably lower potential value ([E.sub.p] = -0.36 V, vs Ag/AgCl).
Finally, as an alternative to the thermally induced phosphine substitution, which affords a series of not easily separable products, the anodic oxidation of [[[Ru.sub.6](C)[(CO).sub.16]].sup.2-] in the presence of phosphines selectively leads to the monosubstituted neutral species [Ru.sub.6](C)[(CO).sub.16](P[R.sub.3]). In this connection, Figure 10 shows the molecular structure of [Ru.sub.6](C)[(CO).sub.16](P[Ph.sub.2]Et).
2 HOMONUCLEAR CLUSTERS
2.1 HOMONUCLEAR IRON CLUSTERS
2.1.1 [Fe.sub.4](C)[(CO).sub.13] and [[[Fe.sub.4](C)[(CO).sub.12]].sup.2-] versus [[[Fe.sub.4](N)[(CO).sub.12]].sup.-]
As Figure 11 illustrates, the three 62-cluster-valence-electron (CVE) complexes [[[Fe.sub.4](C)[(CO).sub.12]].sup.2-], [Fe.sub.4](C)[(CO).sub.13] and [[[Fe.sub.4](N) [(CO).sub.12]].sup.-] possess a butterfly geometry.
It has been briefly reported that the dianion [[[Fe.sub.4](C)[(CO).sub.12]].sup.2-] undergoes, in nonaqueous solvents, four oxidation steps, with only the first two of these having features of transient chemical reversibility. This means that the corresponding 61/60-CVE congeners [[[Fe.sub.4](C)[(CO).sub.12]].sup.-,0] are only partially stable and tend to decompose. As a matter of fact, oxidation under CO atmosphere affords [Fe.sub.4](C)[(CO).sub.13].
Quite opposite is the redox ability of the monoanion [[[Fe.sub.4](N)[(CO).sub.12]].sup.-]. As Figure 12 shows, this undergoes in acetonitrile solution two sequential one-electron reductions at [E.sup.0'] = -1.23 V and -1.58 V, respectively, with both having features of chemical reversibility.
Indeed, over the long time-scales of macroelectrolysis only the 63-CVE dianion [[[Fe.sub.4](N)[(CO).sub.12]].sup.2-] remains quite stable. Furthermore, in the presence of triphenylphosphine, the electrochemical reduction triggers the electrocatalytic substitution of one carbonyl ligand, affording [[[Fe.sub.4](N)[(CO).sub.11](P[Ph.sub.3])].sup.-]. The electrochemical pathway quite parallels the thermal one, which also allowed the obtainment and consequent structural characterization of [[Fe.sub.4](N)[(CO).sub.11](P[Me.sub.2]Ph)].sup.-]. The molecular structures of these substituted complexes are shown in Figure 13. In both cases, the phosphine ligand replaces one carbonyl on the wingtip, i.e. the less coordinated iron vertex of the [Fe.sub.4] butterfly.
On the other hand, it seems useful to take into account that the electro-catalytic substitution does not proceed in the presence of diferrocenyl-phosphine, whereas the thermal activation remains operative, affording [[[[Fe.sub.4](N)[(CO).sub.11](PPh([C.sub.5][H.sub.4]Fe[C.sub.5] [H.sub.5]).sub.2])].sup.-], the molecular structure of which is shown in Figure 14.
2.1.2 Fe.sub.5](C)[(CO).sub.15] and [[[Fe.sub.5](C)[(CO).sub.14].sup.2-] versus [[[Fe.sub.5](N)[(CO).sub.14].sup.-]
[[[Fe.sub.5](C)[(CO).sub.14]].sup.2-], [Fe.sub.5](C)[(CO).sub.15] and [[[Fe.sub.5](N) [(CO).sub.14]].sup.-] constitute the object of the first comparative electrochemical study of isoelectronic and isostructural carbide and nitride clusters. The square pyramidal geometries of the 74-CVE clusters [Fe.sub.5](C)[(CO).sub.15], [[[Fe.sub.5](C)[(CO).sub.14]].sup.2-] and [[[Fe.sub.5](N)[(CO).sub.14]].sup.-] are illustrated in Figure 15.
As Figure 16 shows, [Fe.sub.5](C)[(CO).sub.15] in dichloromethane solution, at low temperature (-15 °C), undergoes a single, uncomplicated, two-electron reduction ([E.sup.0'] = -0.11 V) to the 76-CVE dianion [[[Fe.sub.5](C)[(CO).sub.15]].sup.2-]. Increasing the temperature, the current ratio [i.sub.pa]/[i.sub.pc] decreases and a new re-oxidation wave appears at higher potential values ([E.sub.p] = +0.13 V), which proved to be due to the oxidation of the decarbonylated dianion [[[Fe.sub.5](C)[(CO).sub.14]].sup.2-], formed as a consequence of the chemical complication following the two-electron addition.
As a matter of fact, [[[Fe.sub.5](C)[(CO).sub.14]].sup.2-] exhibits in dichloromethane solution either a single two-electron oxidation ([E.sup.0'] = +0.07 V) or a single two-electron reduction ([E.sup.0'] = -1.58 V).
The easy reduction of [Fe.sub.5](C)[(CO).sub.15] and the instability of its dianion are likely to be responsible for the fact that simple reaction of [Fe.sub.5](C)[(CO).sub.15] with P[Me.sub.2]Ph affords the trisubstitued species [Fe.sub.5](C)[(CO).sub.12][(P[Me.sub.2]Ph).sub.3]. The molecular structure of this latter species shows that replacement of carbonyl ligands takes place at the basal iron atoms (Figure 17).
In its turn, [[[Fe.sub.5](N)[(CO).sub.14]].sup.-] undergoes two separate one-electron reductions, with only the first one being chemically reversible (Figure 18).
The 74/75/76-CVE [[[Fe.sub.5](N)[(CO).sub.14]].sup.-/2-/3-] redox sequence occurs in aceto-nitrile solution at [E.sup.0'] = -0.90 V and [E.sub.p] - -1.40 V, respectively, whereas in dichloromethane solution it occurs at [E.sup.0'] = -1.04V and [E.sub.p] = -1.50 V, respectively.
As in the case of [[[Fe.sub.4](N)[(CO).sub.12]].sup.-], evidence has been gained that in the presence of phosphines the first cathodic reduction of [[[Fe.sub.5](N)[(CO).sub.14]].sup.-] affords the monosubstituted species [[[Fe.sub.5](N)[(CO).sub.13](P[R.sub.3])].sup.-] via an electron-transfer chain catalytic process.
2.1.3 [[[Fe.sub.6](C)[(CO).sub.16]].sup.2-] versus [[[Fe.sub.6](N)[(CO).sub.15]].sup.3-]
The octahedral geometries of the 86-CVE cluster dianions [[[Fe.sub.6](C)[(CO).sub.16]].sup.2-] and [[[Fe.sub.6](N)[(CO).sub.15]].sup.3-] are illustrated in Figure 19. In these cases, the carbide or nitride atoms are fully encapsulated inside the [Fe.sub.6] cage.
As deducible from Figure 20, [[[Fe.sub.6](C)[(CO).sub.16]].sup.2-] undergoes in dichloroethane solution essentially irreversible redox changes. Only the first one-electron oxidation ([E.sub.p] = +0.20 V) has some features of chemical reversibility, indicating that the 85-CVE monoanion [[[Fe.sub.6](C)[(CO).sub.16]].sup.-], even if short-lived, is able to exist. In contrast, both of the 84-CVE neutral species [Fe.sub.6](C)[(CO).sub.16], or the tetranion [[[Fe.sub.6](C)[(CO).sub.16]].sup.4-], immediately decompose.
As illustrated in Figure 21, the redox ability of the nitride [[[Fe.sub.6](N)[(CO).sub.15]].sup.3-] is also characterized by three, substantially irreversible, one-electron anodic steps.
In addition, in this case the corresponding 85-CVE dianion [[[Fe.sub.6](N)[(CO).sub.15]].sup.2-] is able to exist for short periods of time ([t.sub.1/2] [approximately equal to] 15 s). It is, however, interesting to note that it has been proved that, upon exhaustive three-electron oxidation, the original octahedral trianion releases one Fe(CO) group and converts to the previously examined square-pyramidal pentairon species [[[Fe.sub.5](N)[(CO).sub.14]].sup.-].
2.2 HOMONUCLEAR RUTHENIUM CLUSTERS
2.2.1 [Ru.sub.5](C)[(CO).sub.15] versus [[[Ru.sub.5](N)[(CO).sub.14].sup.-]
The only possible structural comparison between carbide and nitride complexes of ruthenium can be found in the 74-CVE clusters [Ru.sub.5](C)[(CO).sub.15] and [[[Ru.sub.5](N)[(CO).sub.14].sup.-], the square pyramidal structures of which are shown in Figure 22.
[Ru.sub.5](C)[(CO).sub.15] undergoes in dichloromethane solution either an irreversible two-electron oxidation ([E.sub.p] = +0.49 V, vs Ag/[Ag.sup.+]), or two sequential irreversible one-electron reductions ([E.sub.p] = -0.58 and -1.04 V, respectively), which induce reorganization to the isoelectronic (and likely isostructural) dianion [[[Ru.sub.5](C)[(CO).sub.14]].sup.2-]. This latter, in turn, undergoes an irreversible oxidation ([E.sub.p] = +0.2 V) to the neutral carbonylated congener [Ru.sub.5](C)[(CO).sub.15].
As happens for [Fe.sub.5](C)[(CO).sub.15], [Ru.s
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