One of the most surprising aspects of the chemistry of centered hexazirconium clusters is their redox behavior. Because the lower oxidation states of zirconium are generally inaccessible via solution-phase synthesis, it had been generally expected that the clusters would behave as very strong reducing agents when excised from the solid state.32–35 As it turns out, the reactivity of these clusters is not so simply described. If care is taken to confine their reactivity to simple electron transfer, most of these clusters are not particularly strong reducing agents and will not reduce protons in acidic aqueous solution. Cyclic voltammetry (see Chapter 2.15) was important in clarifying the nature of hexazirconium cluster reactivity and, as an added benefit, reveals much about the systematics of cluster electronic structure.36–40
2.65.3.2 Cyclic Voltammetric Results and Electronic Structure
Redox potentials of [(Zr6Z)X12]L6 clusters involve cluster frontier orbitals (HOMOs and LUMOs). The bonding schemes shown in Figures 3 and 6 predict that the frontier orbitals of interstitially stabilized clusters may have no interstitial atom character, i.e., they are of the wrong symmetry to include contributions from the interstitial atom valence orbitals. The reason for this general property is simple: when cage orbitals strongly interact with an interstitial atom, Zr6—Z bonding orbitals are stabilized and their antibonding counterparts are destabilized. Cage orbitals that are of the wrong symmetry to interact remain in the intervening energy range and form the frontier orbitals. The bonding schemes also predict a HOMO–LUMO gap that is sensible in view of the usual observed numbers of cluster bonding electrons (CBEs) in both main-group- and transition-metal-centered clusters (14 and 18 CBEs, respectively).
The comparative redox behavior of [(Zr6Z)Cl18]n clusters provides excellent support for the qualitative scheme just described. Figure 4shows cyclic voltammograms for these species in the presence of excess chloride in MeCN. Consider first the Be-centered species, (Zr6BeCl18)3,4,5 (11, 12, and 13 CBEs). All exhibit good stability in MeCN on the time scale of CV scans; three consecutive redox waves are clearly observed at1.45 V,1.04 V, and 0.56 V, which respect- ively correspond to (Zr6BeCl18)5/6, (Zr6BeCl18)4/5, and (Zr6BeCl18)3/4redox couples. The presence of three waves and their nearly equal spacing is indicative of the degeneracy of the cluster HOMO. The spacing (E1=2), are characteristically 0.4–0.55 V when they correspond to successive redox couples involving the same or degenerate orbitals. Shifts of this magnitude were also observed in electrochemical studies in basic AlCl3/ImCl ionic liquids36 and aqueous
Centred Zirconium Clusters 777
solution.38In cases where both cluster oxidation and reduction can be observed, this characteristic allows an estimation of the HOMO–LUMO gap, as illustrated at the top of Figure 3 for the N-centered cluster. The oxidation wave (the (Zr6NCl18)2/3, 13/14 CBE, couple) and the reduct- ion wave (the (Zr6NCl18)3/4, 14/15 CBE, couple) are separated by 1.73 V. However, a second reduction wave (the (Zr6NCl18)4/5, 15/16 CBE, couple) lies some 0.53 V further negative than the first reduction wave. Since this second reduction presumably corresponds to placing a second electron in the cluster LUMO, the 0.53 V gap is just the result of electron–electron repulsion.
Since E1=2 values of this magnitude are characteristic, we can estimate the cluster HOMO–
LUMO gap to be about 1.2 eV (ẳ1.730.53 eV). A similar gap is deduced for the C-centered cluster, as is evident from the close similarities in the spacing of waves in the voltammograms for Figure 3 Cyclic voltammograms for [(Zr6Z)Cl18]4, ZẳBe, B, C, N: (a) waves for oxidation of the 14 e
species, (b) waves for the [(Zr6Z)Cl18]3/4couples.
these two clusters. The same chain of reasoning was applied to the analysis of data for (Zr6FeCl18)4obtained in Cl-rich ionic liquids and a gap of1.45 eV or greater was estimated.36 These HOMO–LUMO gap estimates agree well with thresholds for absorption in optical spectra;
Be-, B-, and C-centered chloride clusters all exhibit HOMO–LUMO transitions at the appropriate wavelengths in the near-IR region, while the lowest energy transition for Fe- and Mn-centered clusters are in the visible region.28
Not surprisingly, cluster charge is the most important determinant of reduction potential in MeCN. Reduction potentials (E1=2’s) of species with the same charge change modestly on moving through the series Be!B!C, despite the different CBE counts and interstitial atoms. E1=2for the (Zr6ZCl18)3/4redox couples (marked ‘‘b’’ inFigure 4) increases from0.56 V, to 0.41 V, and to0.29 V as Z changes from Be to B to C. These shifts of 0.15 V can be attributed to the increase in interstitial (Z) electronegativity on moving through this series. As the Zr—Z bonds become more polarized towards the Z atom, the positive charge on the Zr6cage increases and the oxidation moves to more positive potential. Of course, in the N-centered case, the (Zr6NCl18)3/4 redox couple does not lie at ca.0.14 V, but it is instead observed at a much morenegativevalue (E1=2ẳ 1.35 V), since the oxidation of the (Zr6NCl18)4 ion involves electron loss from the a2u
orbital that lies1.2 eV above a filledt2g set.
To summarize, the redox behavior of [(Zr6Z)X12]-based clusters can be systematically under- stood because electron transfers involve cluster frontier orbitals that are by nature nonbonding with respect to Zr6—Z interactions. Zr6—Z bonding electrons lie in more deeply occupied orbitals and their (unoccupied) antibonding counterparts lie well above the LUMO. As a consequence of these features, the oxidation and reduction potentials of clusters bearing identical terminal ligands (e.g., H2O or X), are determined mostly by the cluster’s charge with the electronegativity of the interstitial atom (Z) and the supporting halides (X) having secondary roles.36,37,40
[Zr6 Cl12]Cl6 [(Zr6 Z)Cl12]Cl6 n–
∆E(t1u–t1u )
t1u(p)
*
*
Ζ
1t1u
1t2g a2u
t2u 2t2g
2t1u t2u
t1u t2g
t2g
a2u
a1g
a1g
a1g(s) eu
eu
Figure 4 Molecular orbitals for [(Zr6Z)Cl18]4, derived from an empty cluster, [(Zr6&)Cl18], and an main- group atom, Z. The energy gap between the bondingt1uand the antibondingt1u*
orbitals is indicated.
Centred Zirconium Clusters 779