14904
J. Phys. Chem. 1996, 100, 14904-14907
In Situ FTIR Spectroscopy Studies on Electrochemical Redox Processes of High Nuclearity Osmium Carbonyl Clusters Wen-Feng Lin,†,‡ Wing-Tak Wong,*,† and Shi-Gang Sun‡ Department of Chemistry, The UniVersity of Hong Kong, Pokfulam Road, Hong Kong, and State Key Laboratory for Physical Chemistry of the Solid Surface, Department of Chemistry, Xiamen UniVersity, Xiamen 361005, China ReceiVed: May 7, 1996; In Final Form: June 25, 1996X
The electrochemical redox processes of two high nuclearity osmium carbonyl clusters [(Ph3P)2N]2[Os10C(CO)24]‚PPN2 (1) and Os6(CO)18 (6) have been studied by electrochemical in situ FTIR. The five oxidation states of 1, i.e., [Os10C(CO)24]0/1-/2-/3-/4-, have been characterized. There are no significant structural changes for these species. Hence, the ability of this decanuclear cluster to act as an electron reservoir has been demonstrated. The structural rearrangement associated with the two-electron reduction of bicapped tetrahedral 6 to octahedral dianion [Os6(CO)18]2- and [Os6(CO)18]4- tetraanion has also been investigated.
1. Introduction High nuclearity osmium carbonyl clusters have been shown to behave as electron “sinks” and “sources”, stabilizing the reversible one-electron transfer reaction, and hence, they exist in a range of oxidation states.1-4 The multiple oxidation states of the clusters have been extensively studied by electrochemical methods such as cyclic voltammetry. The relationship between the structure and electrochemical reactivity of osmium and other transition metal carbonyl clusters is interesting. A variety of clusters undergo multiple one-electron redox steps. Generally, these compounds have “closed" structures or contain ligands or interstitial heteroatoms able to stabilize the different oxidation states of clusters generated electrochemically. Several examples of multielectron transfer reactions involving structural changes have been also reported.5-7 It has been shown that the HOMO of transition metal carbonyl clusters is usually a σ-bonding MO of metal-metal bonding character while the LUMO is usually metal-metal antibonding in character.8 As a result of this, it has been found that clusters are subject to fragmentation by electrochemical oxidation or reduction, which leads to metal carbonyl fragments of low nuclearity. However, the metal core cohesion may be reinforced against fragmentation by chelation, capping by donor ligands, or the presence of interstitial heteroatoms (C, H, N, P, S, etc.) bonded to the metal framework.9 In this paper, we report an investigation of the redox chemistry of the two high nuclearity osmium carbonyl clusters [Os10C(CO)24]2- anion (1) and Os6(CO)18 (6) using electrochemical in situ FTIR spectroscopy.10,11 This technique allows the characterization of unstable cluster species that are normally difficult to study at ambient conditions.
74-5041 clean air package. The details of spectroelectrochemical cell and in situ FTIRS experiments were described in refs 10 and 11. Potential controlling and cyclic voltammetry were carried out with a XHD-II potentiostat. The working electrode used in both cyclic voltammetry and IR experiments was a platinum disk of ca. 6.0 mm diameter or glassy carbon electrode of 3 mm diameter. It was polished with alumina powder of 1.0, 0.3, and 0.05 µm diameter and cleaned in an ultrasonic bath followed by a washing with distilled water and drying with a soft tissue. The reference electrode (SCE) was separated from the cell by a solution bridge. Normalized differential reflectance spectra were obtained as ∆R/R ) [R(E2) - R(E1)]/R(E1), where R(E2) and R(E1) correspond to the single-beam spectrum obtained at potentials E2 and E1, respectively. In the resulting spectra the positivegoing bands indicate that more IR has been absorbed at E1 than at E2, and the negative-going bands correspond to a greater IR absorption at E2 than at E1 (e.g., upward and downward bands correspond respectively to loss and gain of compounds in the thin layer between the electrode and the CaF2 IR window at E2 compared to that at E1). For each single-beam spectrum 100 interferograms were collected at 8 cm-1 resolution. All experiments were performed at a temperature of 20 ( 2 °C. The samples of [(Ph3P)2N]2[Os10C(CO)24] and Os6(CO)18 were prepared and purified using the methods in the literature.12-14 All solvents (CH2Cl2 and THF) were distilled prior to use. The supporting electrolyte for all solvents was 0.1 M tetra-nbutylammonium tetrafluoroborate (NBu4BF4). The solutions were deaerated with nitrogen gas of 99.99% purity prior to use and maintained under a nitrogen atmosphere throughout the measurements. 3. Results and Discussion
2. Experimental Section Details of in Situ FTIR Experimental Setup. In situ FTIR experiments were carried out with a Nicolet 730 FTIR apparatus equipped with a globar IR source and a liquid-nitrogen-cooled MCT-B detector. The instrument was purged throughout the experiment with clean air freed of CO2 and H2O by a Balston * Corresponding author. † The University of Hong Kong. ‡ Xiamen University. X Abstract published in AdVance ACS Abstracts, August 15, 1996.
S0022-3654(96)01300-7 CCC: $12.00
3.1. In Situ FTIR Results. 3.1.1. Redox Processes of [Os10C(CO)24]2-. Cyclic Voltametric (CV) studies showed that in CH2Cl2 or THF solution at a platinum or glassy carbon electrode, salts of the dianionic cluster [Os10C(CO)24]2- (1) undergo two oxidation and two reduction steps as illustrated in Scheme 1. These CV data are essentially identical with the results from previous reports.2,9 The first oxidation is a simple one-electron reversible (∆Ep ) 61 mV, E1/2 ) +0.78 V) process. The second one is irreversible (Ep ) +1.21 V), and associated with this step is a © 1996 American Chemical Society
Osmium Carbonyl Clusters
J. Phys. Chem., Vol. 100, No. 36, 1996 14905
SCHEME 1: Electrochemical Redox Processes of the Dianionic Cluster [Os10C(CO)24]2- (1)
subsequent reduction at +0.35 V. The reduction occurs by two electrons in steps. The addition of the first electron is fast and quasi-reversible (∆Ep ) 98 mV, E1/2 ) -1.19 V), while the addition of the second one is kinetically slow and irreversible (Ep ) -1.28 V). Associated with this second reduction step is a reoxidation at a more positive potential of -0.58 V. Figure 1 shows the in situ subtractively normalized interfacial FTIR (SNIFTIR) spectra for the oxidation processes of [Os10C(CO)24]2- (1) in CH2Cl2 solution and at a glassy carbon electrode. Two bipolar bands were observed. The two positivegoing peaks in Figure 1a at 2035 and 1989 cm-1 are attributed to the absorption of carbonyl νCO of [Os10C(CO)24]2- (1)9 (which has only two different environments for 24 carbonyls (12 of each type) and thus only shows two IR bands) at a potential E1 of 0.5 V, and the two negative-going peaks at 2058 and 2010 cm-1 are assigned to the absorption of the oxidation product νCO of [Os10C(CO)24]•- (2)9 at a potential E2 of 0.8 V. As mentioned above, the oxidation of 1 to 2 at 0.8 V is reversible. As indicated in Figure 1a′, the bipolar bands reversed when the potential was stepped from 0.8 to 0.5 V, i.e., 2 was reduced to 1 at 0.5 V. Figure 1b shows that when the potential shifted
further from 0.8 V to a more positive value of 1.25 V, the [Os10C(CO)24]•- (2) undergoes a further oxidation to the product [Os10C(CO)24]0 (3), demonstrating two downward bands at 2081 and 2042 cm-1. The above result fits well with the previous data.9 It is worth noting that when the potential was stepped negatively from 1.25 to 0.8 V, 3 could not be reduced to 2 (see Figure 1c). To reduced 3, the potential must be stepped to a more negative value below 0.35 V. This reduction is a oneelectron step, as shown in Figure 1d. When the potential shifted from 1.25 to 0 V, 3 was reduced only to 2 but not to 1. Figure 2 shows the in situ SNIFTIR spectra for the reduction processes of [Os10C(CO)24]2- (1). It can be seen from Figure 2a that when the potential was stepped from -0.5 to -1.2 V, 1 was reduced (two upward bands at 2035 and 1989 cm-1 correspond to the loss of 1 in the thin layer) to [Os10C(CO)24]•3(4), yielding two downward bands at 1998 and 1963 cm-1. A weak negative peak at 1934 cm-1 could also be detected. This
Figure 1. In situ SNIFTIR spectra of 5.0 × 10-5 M [N(PPh3)2]2[Os10C(CO)24] in 0.1 M NBu4BF4-CH2Cl2 at a glass carbon electrode: (a) E1 ) 0.5 V/SCE, E2 ) 0.8 V; (a′) E1 ) 0.8 V, E2 ) 0.5 V; (b) E1 ) 0.8 V, E2 ) 1.25 V; (c) E1 ) 1.25 V, E2 ) 0.8 V; (d) E1 ) 1.25 V, E2 ) 0.0 V.
Figure 2. In situ SNIFTIR spectra of 5.0 × 10-5 M [N(PPh3)2]2[Os10C(CO)24] in 0.1 M NBu4BF4-CH2Cl2 at a glass carbon electrode: (a) E1 ) -0.5 V/SCE, E2 ) -1.2 V; (a′) E1 ) -1.2 V, E2 ) -0.5 V; (b) E1 ) -1.1 V, E2 ) -1.3 V; (b′) E1 ) -1.3 V, E2 ) -1.1 V; (c) E1 ) -1.3 V, E2 ) 0.8 V.
14906 J. Phys. Chem., Vol. 100, No. 36, 1996
Lin et al.
Figure 4. In situ SNIFTIR spectra of 5.0 × 10-4 M Os6(CO)18 in 0.1 M NBu4BF4-THF at a glass carbon electrode, where E1 ) 0.3 V/SCE and E2 ) -0.2 V.
Figure 3. In situ SNIFTIR spectra of 5.0 × 10-5 M [N(PPh3)2]2[Os10C(CO)24] in 0.1 M NBu4BF4-CH2Cl2 at a platinum electrode: (a) E1 ) 0.5 V/SCE, E2 ) 0.8 V; (b) E1 ) 0.8 V, E2 ) 1.25 V; (c) E1 ) -0.5 V, E2 ) -1.2 V; (d) E1 ) -1.3 V, E2 ) -1.1 V; (e) E1 ) -1.3 V, E2 ) 0.8 V.
band may be primarily assigned to the IR absorption of [Os10C(CO)24]4- (5), the species produced by further slow reduction of 4 (as illustrated below). The reduction of 1 at -1.2 V is a one-electron reversible process. As indicated by parts b, b′, and a′ of Figure 2, when the potential was changed from -1.1 to -1.3V, 1 was reduced to 2, and when the potential was shifted back from -1.3 to -1.1 V, 2 was oxidized back to 1. By keeping the potential at -1.3 V for a long time, [Os10C(CO)24]•3- (4) was reduced slowly to [Os10C(CO)24]4(5), generating two peaks at 1976 and 1934 cm-1. As shown in Figure 2c, the potential was kept at -1.3 V for 8 min and the potential difference spectrum was then measured between -1.3 and 0.8 V. The four upward bands correspond to the mixture of 4 (1998 and 1963 cm-1 bands) and 5 (1976 and 1934 cm-1 bands) at -1.3 V, and the two downward peaks at 2034 and 1989 cm-1 correspond to 1 produced at 0.8 V. A weak downward peak at 2058 cm-1 corresponding to 2 (produced by further oxidation of 1) was also measured at 0.8 V. These data reveal that both the reduction of 1 and the oxidation of 4 are processes of the one-electron-step. The above results at a glassy carbon (GC) electrode can also be obtained at a Pt electrode (Figure 3). However, there is still a small difference in the detailed comparison between Figure 3e and Figure 2c. At -1.3 V a larger amount of 5 (originating 1976 and 1934 cm-1 bands) was detected at a Pt electrode (Figure 3e) than that at a GC electrode (Figure 2c). This may
Figure 5. In situ time-resolved SNIFTIR spectra of 5.0 × 10-4 M Os6(CO)18 in 0.1 M NBu4BF4-THF at a glass carbon electrode, where E1 ) 0.3 V/SCE and E2 ) -0.2 V: (a) t ) 2 s; (b) t ) 4 s; (c) t ) 6 s; (d) t ) 8 s; (e) t ) 10 s.
indicate that 5 is produced more rapidly by further reduction of 4 and is more stable at a Pt electrode than at a GC electrode. In summary, the one-electron step-by-step redox processes of [Os10C(CO)24]2-, which involved the five oxidation states, [Os10C(CO)24]0/1-/2-/3-/4-, have been characterized by electrochemical in situ FTIR for the first time at the molecular level. 3.1.2. Redox Processes of Os6(CO)18. The cyclic voltammetry studies demonstrated that Os6(CO)18 (6) undergoes two reduction steps in CH2Cl2 or THF at both GC and Pt electrodes.5 The first step is close to 0.0 V, and the second one is at a negative potential of -2.10 V. It was also established that each wave involved a transfer of two electrons as given in eqs 1 and 2:
Os6(CO)18 + 2e- S [Os6(CO)18]2- E° ) +0.04 V
(1)
[Os6(CO)18 ] 2- + 2e- S [Os6(CO)18]4- E° ) -2.10 V (2) The first process was firmly established as a reversible process,
Osmium Carbonyl Clusters
J. Phys. Chem., Vol. 100, No. 36, 1996 14907 TABLE 1: Summary of the in Situ IR Data for the Reduction Process of Os6(CO)18 species Os6(CO)18
structure bicapped tetrahedron
[Os6(CO)18]2- octahedron [Os6(CO)18 ]4- pentagonal pyramid
Figure 6. In situ SNIFTIR spectra of 5.0 × 10-4 M Os6(CO)18 in 0.1 M NBu4BF4-THF at a glass carbon electrode, where E1 ) -1.6 V/SCE and E2 ) -2.4 V.
and the second one was only a tentatively assigned, since the tetraanion could not be isolated.5 The in situ SNIFTIR spectroscopy for the first reduction process of Os6(CO)18 is shown in Figure 4. The three upward bands at 2075, 2062, and 2037 cm-1 correspond to the absorption of carbonyls of Os6(CO)18 at a potential E1 of 0.3 V. The neutral Os6(CO)18 has a bicapped tetrahedral structure (C2V symmetry) with three sets of Os(CO)3 fragment and thus yields three IR absorption bands as indicated above. The downward band at 1994 cm-1 corresponds to the absorption of the reduction product [Os6(CO)18]2- at potential E2 of -0.2 V. The dianion [Os6(CO)18]2- has an octahedral structure and thus only one IR absorption band for the carbonyls. Figure 5 shows the time-dependent SNIFTIR spectra for the reduction process of Os6(CO)18. It is shown that both the upward bands (corresponding to the loss of Os6(CO)18) and the downward band (corresponding to the gain of [Os6(CO)18]2-) increase with respect to time (each spectrum takes 2 s). These results illustrated that the reduction is a slow process owing to the structural change involved in the charge-transfer step, which has a large activation barrier. Figure 6 shows the in situ FTIR spectrum for the second reduction process represented in eq 2. The loss feature of an upward band at 1996 cm-1 corresponded to [Os6(CO)18]2- at a potential E1 of -1.6 V. The gain feature of a main downward band at 1872 cm-1 and a weak downward band at 1967 cm-1 at a potential E2 of -2.4 V corresponded to the absorption of the reduction product, tetraanion [Os6(CO)18]4-, which was proposed to have a pentagonal pyramid structure5 and was therefore a nido-cluster. The above data are summarized in the Table 1 for clarity. 4. Conclusions The present work demonstrated a successful study of in situ FTIR on electrochemical redox processes of two high nuclearity osmium carbonyl clusters [Os10C(CO)24]2- (1) and Os6(CO)18 (6). The main points can be summarized as follows. (a) The
existing potential (V/SCE) 0.3 -0.2 to -1.6 -2.4
IR data (cm-1) 2075, 2062, and 2037 1996 1967 w, 1872
electrochemical one-electron step-by-step redox processes of [Os10C(CO)24]2-, which involved five oxidation states, e.g., [Os10C(CO)24]0/1-/2-/3-/4-, showing no significant changes in structure, have been characterized in situ and at the molecular level. The high stability of the tetracapped octahedral “[Os10C(CO)24]” core has been demonstrated. This high stability allows the decanuclear cluster to act as an electron reservoir. The detailed IR data for these five species are shown in Scheme 1. (b) The structural changes in the two-electron reduction processes of 6 have been characterized, e.g., from the bicapped tetrahedral structure of neutral Os6(CO)18 (which yields three IR bands at 2075, 2062, and 2037 cm-1) to the octahedral dianion [Os6(CO)18]2- (which generates an IR band at 1996 cm-1). The latter can be further reduced to the tetraanion [Os6(CO)18]4-, giving a main band at 1872 cm-1 and a weak band at 1967 cm-1. This tetraanion [Os6(CO)18]4- may be a nido-cluster having a pentagonal pyramid structure. Acknowledgment. This work was supported by the Hong Kong Research Grants Council, The University of Hong Kong, the State Key Laboratory for Physical Chemistry of the Solid Surface in Xiamen University, and the Chinese National Science Foundation. References and Notes (1) Lemoine, P. Coord. Chem. 1988, 83, 169. (2) Drake, S. R. Polyhedron 1990, 4, 455. (3) Drake, S. R.; Barley, M. H.; Johnson, B. F. G.; Lewis, J. J. Chem. Soc., Chem. Commun. 1987, 1657. (4) Geiger, W. E. In Progress in Inorganic Chemistry; Lippard, S. J., Ed.; J. Wiley: New York, 1985; Vol. 33, p 275. (5) Tulyathan, B.; Geiger, W. E. J. Am. Chem. Soc. 1985, 107, 5960. (6) Linden, J. G. M.; Paulissen, M. L. H.; Schmitz, J. E. J. J. Am. Chem. Soc. 1983, 105, 1903. (7) Drake, S. R.; Barley, M. H.; Johnson, B. F. G.; Lewis, J. Organometallics 1988, 7, 806. (8) Lauher, J. W. J. Am. Chem. Soc. 1979, 101, 2604. (9) Drake, S. R.; Johnson, B. F. G.; Lewis, J.; McQueen, R. C. S. J. Chem. Soc., Dalton Trans. 1987, 1051. (10) Sun, S. G.; Yang D. F.; Tian Z. W. J. Electroanal. Chem. 1990, 289, 177. (11) . Lin, W. F.; Sun, S. G. Electrochim. Acta 1996, 41, 803. (12) Jackson, P. F.; Johnson, B. F. G.; Lewis, J.; Nelson, W. J. H.; McPartlin, M. J. Chem. Soc., Dalton Trans. 1982, 2099. (13) Eady, C. R.; Johnson, B. F. G.; Lewis, J. J. Organomet. Chem. 1972, 37, C39. (14) Eady, C. R.; Johnson, B. F. G.; Lewis, J. J. Chem. Soc., Dalton Trans. 1975, 2606.
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