Organometallics 2010, 29, 1603–1606 DOI: 10.1021/om901037j
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Electrochemistry and MO Computations of Saturated and Unsaturated N-Heterocyclic Silylenes Patrick Zark,† Thomas M€ uller,*,† Robert West‡ Kendrekar Pravinkumar,§ and James Y. Becker*,§ †
Institut f€ ur Reine und Angewandte Chemie, Carl von Ossietzky University Oldenburg, Carl von Ossietzky Strasse 9-11, 26129 Oldenburg, Federal Republic of Germany, ‡Organosilicon Research Center, Department of Chemistry, University of Wisconsin, 1101 University Avenue, Madison, Wisconsin 53706, and § Department of Chemistry, Ben-Gurion University of the Negev, P.O. Box 653, Beer Sheva 84105, Israel Received December 2, 2009
The results of electrochemical investigations by cyclic voltammetry and density functional computations of new saturated (2, 3) and unsaturated N-heterocyclic silylenes (5, 6) are described and compared with the previously known N-heterocyclic silylenes (1, 4). Good correlations have been found between experimental oxidation potentials of saturated 1-3 and unsaturated 4-6 with those of density functional calculations of the electronic properties of these divalent silicon derivatives.
Introduction Silylenes, the divalent silicon analogues of carbenes, are highly reactive and important intermediates in many thermal and photochemical transformations in organosilicon chemistry.1,2 The synthesis of the stable cyclic singlet N-heterocyclic silylenes (NHSi’s) 1 and 4 (Scheme 1) opened the way for an extensive investigation of their physical properties.3-5 Previously, we have investigated the electrochemical properties of 1 and 4 and have studied computationally their structures, frontier orbitals, ionization energies and electron affinities.6 In this paper, we report the results of electrochemical investigations and density functional computations of the new saturated NHSi’s 59 and 69,10 (Scheme 1) and unsaturated
Scheme 1. Chemical Structures of N-Heterocyclic Silylenes 1-6 (Xyl = 2,6-Dimethylphenyl, Dipp = 2,6-Diisopropylphenyl)
NHSi’s 5 and 69,10 (Scheme 1). In the series of saturated NHSi’s 1-3, the substitution pattern at the ring carbon atoms was varied, while in the series of unsaturated NHSi’s the alkyl substituent in compound 4 was substituted with different aryl groups in NHSi’s 5 and 6. The cyclic voltammetric (CV) data of these derivatives have been determined and compared with those of the previously known 1 and 4. Attempts were made to correlate the experimental results with those of density functional calculations of the electronic properties of compounds 1-6.
(1) Gaspar, P. P.; West, R. In The Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chichester, U.K., 1997; Vol. 2, p 2436. (2) Fuchigami, T. Electrochemistry of organosilicon compounds. In Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chichester, U.K., 1998; Vol. 2, Part 2, pp 1187-1232. (3) (a) West, R.; Denk, M. Pure Appl. Chem. 1996, 68, 785. (b) Schmedake, T. A.; Haaf, M.; Apeloig, Y.; M€uller, T.; Bukalov, S.; West, R. J. Am. Chem. Soc. 1999, 121, 9479. (4) Denk, M.; Lennon, R.; Hayashi, R.; West, R.; Haaland, A.; Belyakov, H.; Verne, P.; Wagner, M.; Metzler, N. J. Am. Chem. Soc. 1994, 116, 2691. (5) Reviews: (a) Haaf, M.; Schmedake, T. A.; West, R. Acc. Chem. Res. 2000, 33, 704. (b) Haaf, M.; Schmedake, T. A.; Paradise, B. J.; West, R. Can. J. Chem. 2000, 78, 1526. (c) Hill, N. J.; West, R. J. Organomet. Chem. 2004, 689, 4165. (d) M€uller, T. In: (Ed.), Comprehensive Heterocyclic Chemistry III; Katritzky, A. R., Ed.; Elsevier: Amsterdam, Heidelberg, London, New York, 2008; Vol. 6, pp 690-733 (6) Dhiman, A.; M€ uller, T.; West, R.; Becker, J. Y. Organometallics 2004, 23, 5689–5693. (7) Li, W.; Hill, N. J.; Tomasik, A. C.; Bikzhanova, G.; West, R. Organometallics 2006, 25, 3802. (8) Tomasik, A. C.; Mitra, A.; West, R. Organometallics 2009, 28, 378. (9) Zark, P.; Sch€ afer, A.; Mitra, A.; Haase, D.; Saak, W.; West, R.; M€ uller, T. J. Organomet. Chem. 2010, 695, 398. (10) Kong, L.; Zhang, J.; Song, H.; Cui, C. Dalton Trans. 2009, 5444.
Electrochemistry. Cyclic voltammetric (CV) measurements were carried out in a drybox ([H2O] < 1 ppm; [O2] < 1 ppm), under an inert atmosphere of nitrogen, employing a PAR Model 173 potentiostat/galvanostat combined with a PAR Model 175 universal programmer and Yokogawa Type 3086 XY recorder. They were performed in THF-0.05 M n-Bu4NBPh4 using a conventional three-electrode cell. The working electrode was a glassy-carbon (GC) disk (ca. 3 mm diameter) or Pt disk (ca. 1.5 mm diameter), the reference electrode was Ag/AgCl (with KCl 3 N), and a Pt spiral was the counter electrode. The concentration of substrate was about 2 mM. The solvents THF and 1,2dichlorobenzene (o-DCB) were distilled over Na-ketyl and CaH2, respectively, under an argon atmosphere. The n-Bu4NBPh4 electrolyte (Aldrich) was dried under vacuum (∼30 mmHg) at 85 °C for 24 h before use. The salt n-Bu4NB(C6F5)4 was prepared at the University of Oldenburg under a nitrogen
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Experimental Section
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Table 1. Orbital Energies of Frontier Orbitals and Vertical and Adiabatic Ionization Potentials (IP(vert), IP(ad))a and Electron Affinities (EA(vert), EA(ad))b Computed at B3LYP/6-311þG(d,p)//B3LYP/6-311þG(d,p) entry
compd
E(HOMO) (eV)
E(LUMO) (eV)
IP(vert) (eV)
EA(vert) (eV)
IP(ad) (eV)
EA(ad) (eV)
1 2 3 4 5 6 7 8 9
1 2 3 4 5 6 1c 7c 8c
-5.64 -5.57 -5.52 -5.17 -5.51 -5.56 -5.37 -4.62 -4.15
-0.78 -0.75 -0.79 -0.69 -1.08 -1.11 -0.41 -1.32 -1.33
7.29 7.14 7.04 6.99 7.19 7.11 7.09 6.10 5.34
0.65 0.69 0.58 0.74 0.25 0.16 1.35 0.19 -0.15
7.13 7.01 6.87 6.70 6.88 6.81 6.93 5.77 4.89
0.48 0.50 0.26 0.63 0.05 0.02 0.96 -0.15 -0.63
c
a IP = E(radical cation) - E(neutral). b EA = E(radical anion) - E(neutral) (a more positive EA value actually indicates a smaller electron affinity). Computed at B3LYP/6-31G(d) //B3LYP/6-31G(d).
atmosphere as described before.11 The NHSi’s were prepared according to published procedures7-9 at the Universities of Oldenburg and Wisconsin and kept in sealed tubes under a nitrogen atmosphere, until opened in the glovebox. Computations. The computations12 were done using Gaussian 03. The hybrid density functional B3LYP along with the 6-311þG(d,p) basis set was used for the structure optimization and energy calculations. Further technical details are provided in the Supporting Information.
Results and Discussion Computations. The electrochemistry of some N-heterocyclic silylenes (NHSi’s) was already reported previously, along with computations concerning their oxidation and reduction states.6 With reference to those data, the substituent effects found for the series of saturated NHSi’s 1-3 and unsaturated NHSi’s 4-6 will be rationalized. The frontier orbitals for all NHSi’s are purely π-orbitals with respect to the plane spanned by the NSiN group. The lone pair at silicon is in all cases the HOMO-1 orbital and is orthogonal to the π-system. The extension of the heteroallyl anion type conjugation in the saturated NHSi 1 across the five-membered ring in the unsaturated NHSi 4 results in an increase of the HOMO and LUMO energies (E(HOMO) by 0.47 eV, E(LUMO) by 0.09 eV; see Table 1 and Figure 1). In general, the computed ionization potentials IP and electron affinities EA also reflect these changes in the energy level of the frontier orbitals.12 That is, the unsaturated NHSi 4 is easier to ionize but electron attachment is less favorable than that predicted for 1 (Table 1). Therefore, we will refer in the discussion to the energy levels of the frontier orbitals. In the series of saturated NHSi’s the effects of substituents at the ring carbon atoms on the energy of the frontier orbitals are predicted to be relatively small. In particular, the maximum change in the HOMO energies, ΔE(HOMO), along the series of NHSi’s 1-3 is only 0.12 eV and the effect on the LUMO energy level is even less pronounced (ΔE(LUMO) = 0.04 eV). Nevertheless, the inductive effects of the two methyl groups in NHSi 2 and of the tert-butyl substituent in compound 3 are clearly discernible by the small increase of their HOMO energy level compared to that of NHSi 1 (see Table 1). The computations for the series of unsaturated NHSi’s 4-6 with different substituents at the ring nitrogen atoms (11) (a) Becker, J. Y.; Lee, V. Ya.; Nakamoto, M.; Sekiguchi, A.; Chrostowska, A.; Dargelos, A. Chem. Eur. J. 2009, 15, 8480. (b) LeSuer, R. J.; Buttolph, C.; Geiger, W. E. Anal. Chem. 2004, 76, 6395. (12) All computations were done using: Frisch, M. J.; et al. Gaussian 03, Revision D.02; Gaussian Inc., Pittsburgh, PA, 2003. For details see the Supporting Information.
Figure 1. Qualitative π-MO scheme for the heteroallyl anion like -NSiN- group in the saturated NHSi’s 1-3 and the interaction of the π-MO’s with an additional CdC π-bond to give the five π-MO’s of the unsaturated NHSi’s 4-6 (idealized C2v symmetry was assumed).
point to larger effects on the energy levels of the frontier orbitals. The replacement of the tert-butyl group in 4 by aryl substituents in NHSi’s 5 and 6 lowers the HOMO (by 0.34 eV (5) and by 0.39 eV (6)) and the LUMO energy level (by 0.39 eV (5) and by 0.42 eV (6)) significantly (Table 1). This indicates the electron-withdrawing effect of the aryl substituents compared to the tert-butyl group in 4. The alkyl groups in the ortho position of the aryl substituents enforce a nearly perfect orthogonality of the aryl rings and the five-membered heterocycle in NHSi’s 5 and 6 (dihedral angle between both mean planes 90°).9 As a result, resonance effects become ineffective and only the greater electron-withdrawing effect of the sp2 carbon atom compared to that of an sp3 carbon atom is operative. Electrochemistry. Anodic peak potentials have been measured by cyclic voltammetry (CV) for all divalent derivatives 1-6 under the same experimental conditions in THF using an electrolyte (n-Bu4NBPh4) much more inert than those used previously (n-Bu4NClO4 and n-Bu4NBF4).6 (It is noteworthy that THF was chosen, because this electrolyte is insoluble in a less nucleophilic solvent such as o-dichlorobenzene.) The CV’s were recorded at both glassy-carbon (GC) and Pt working electrodes, and the results are summarized in Table 2. It appeared that the oxidation peaks are
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Table 2. Cyclic Voltammetric Data of Silylenes 1-6a redn (EP)c
oxidn (EP) compd
GCb
Pt
GC or Pt
1 2 3 4 5 6
0.55 (0.05d) 0.50 0.45 0.50 0.60 0.68
ill-defined ill-defined ill-defined 0.42, 0.72 0.50 0.70
none none none none none none
a Peak potentials (EP) are in V and quoted vs Ag/AgCl reference electrode: scan rate, 100 mV/s; [substrate] = ∼2 mM in THF-0.05 M n-Bu4NBPh4. b GC = glassy-carbon working electrode. c No reduction peak was observed upon scanning to -2.5 V. d This is the oxidation potential of the red original sample of disilene 8, the tetramer of NHSi 1 (vide infra, Scheme 2). Within ∼1 min the red color of 8 faded to yellow and then became transparent. Finally this oxidation peak disappeared and a new one emerged at a higher potential (0.55 V), which is attributed to the monomeric form 1.
Scheme 2. Monomer-Dimer-Tetramer Equilibria of 1
Figure 2. Cyclic voltammograms of 1 in THF-0.05 M n-Bu4NBPh4 on a glassy-carbon working electrode at 100 mV/s: (a) ∼10 s after adding the solid red substrate 8; (b) about 10-15 s later; (c) after ∼1 min.
better defined on GC than on Pt. Each silylene afforded one irreversible oxidation wave on GC and Pt, except for 4, which showed two distinct anodic peaks on Pt. The lack of electrochemical reversibility (no reversible couple) indicates that none of the derivatives afford a stable cation-radical intermediate in this medium, probably due to the anticipated high reactivity of the electrogenerated species and the nucleophilicity of the solvent THF. Surprisingly, no reduction waves could be detected under these conditions, upon using either a glassy-carbon or platinum working electrode. It is noteworthy that the saturated silylene 1 is known to be inherently unstable in the solid state; it turns from colorless to red at 25 °C, dimerizes to give the bicyclic silylene dimer 7, and finally forms a tetramer, the disilene 8 (Scheme 2), as was evidenced by its X-ray diffraction data.3b,13 When the red crystals of disilene 8 are dissolved (e.g., in hexane or THF), a dynamic equilibrium exists with the colorless NHSi 1 through the intermediacy of the silylene 7 (Scheme 2). This transformation pattern is also observed now by CV (Figure 2) when the initial oxidation peak at 0.05 V (attributed to the red tetramer 8) disappears gradually to afford a final oxidation peak at 0.55 V (attributed to monomer 1). The color of the THF solution changes accordingly from red to yellow to transparent within 1 min. The reason for a lower than expected current amplitude for 1 stems from its limited solubility in this THF-electrolyte system. This interpretation of the experimental data is in agreement with the fact that the computed HOMO energy level of the disilene 8 is predicted to be significantly higher than those of silylenes 7 and 1 (see Table 1, entries 7-9). Therefore, the disilene 8 is expected to be oxidized at a lower potential than silylene 1, as is observed. Previously, two reduction steps of 1 have been confirmed by chemical reduction with potassium metal that were also confirmed by CV (under experimental conditions different
from those described in this work).6,14 Silylene 4 exhibited only one irreversible reduction wave under those conditions.6 Employing the same experimental conditions, namely running CVs at a GC electrode in THF-n-Bu4NClO4, for all other silylenes resulted in a lack of almost any reduction peak (except for a broad and ill-defined peak observed for 5). Therefore, a more inert electrolyte, n-Bu4NBPh4, was used. Nevertheless, it is rather surprising that no reduction peaks could be detected also with this electrolyte (Table 2) for any of the silylenes studied, either at GC or at Pt. Furthermore, replacing THF with o-DCB in the presence of an even more inert electrolyte (n-Bu4NB(C6F5)4) did not shed light on this issue, because again no reduction peaks were observed in this solvent-electrolyte system by scanning up to -2.25 V. At present, we have no explanation for this phenomenon, although steric and/or preferential adsorption (of the electrolyte used with respect to substrate) effects could play important roles. Also, it should be pointed out that no oxidation peaks (or only ill-defined ones) were observed in this latter solvent-electrolyte system for the various silylenes studied. Comparison between Computation and CV Data. At a first approximation, ignoring solvation, surface and kinetic effects, the first oxidation potential represents the energy level of the highest occupied molecular orbital (HOMO), whereas the reduction potential could be attributed to the lowest unoccupied molecular orbital (LUMO). Therefore, for many classes of compounds cyclic voltammetry (CV) has been used to study the influence of substituents or small variations in the structure and topology on the energy level of the important frontier orbitals. Upon comparison between computational results and CV measurements, a good correlation has been found in the case of the saturated NHSi’s 1-3, and the ease of oxidation is in the order 3 > 2 > 1 (Figure 3). It is noteworthy that the differences in the oxidation potentials among 1-3 are rather small (within 50 mV in experimental values and within 0.12 eV in the HOMO energies). The silylenes substituted with alkyl groups (2 and 3) are easier to oxidize than the NHSi 1, with no substituent at the ethylene bridge, due to the inductive effect of the alkyl groups.
(13) Denk, M.; Green, J. C.; Metzler, N.; Wagner, M. J. Chem. Soc., Dalton Trans. 1994, 2405.
(14) West, R.; Schmedake, T. A.; Haaf, M.; Becker, J.; M€ uller, T. Chem. Lett. 2001, 68.
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or different solvation or adsorption effects during the oxidation of saturated and unsaturated NHSi’s. In addition, since all measured peak potentials are irreversible, they are somewhat unreliable because they could be dependent on solvation, surface, and kinetic effects, Apparently, this is the case here, unlike another known case15 where a good correlation was found even when all measured peak potentials were irreversible.
Conclusion
Figure 3. Plot of the experimental oxidation potentials EP (vs Ag/AgCl) and the HOMO energy levels for NHSi’s 1-6.
On examination of the unsaturated NHSi’s 4-6, the ease of oxidation by CV (on both GC and Pt) is in the order 4 > 5 > 6 and the trend according to calculations of the HOMO energy level is the same, although the linear correlation is just fair, probably due to steric reasons. The aryl groups in 5 and 6 lie in a plane perpendicular to that of the five-membered ring, and they exert a negative inductive effect namely, they serve as electronwithdrawing groups. As such, they lower the HOMO energy levels of 5 and 6 and therefore, make them more difficult to oxidize than 4. This trend exists in both CV data and calculated HOMO energies (Figure 3). The comparison of the data obtained for NHSi’s 1 and 4 is not straightforward. In agreement with the theoretical prediction (Figure 1, Table 1), the experiment showed that the unsaturated NHSi 4 is easier to oxidize than the saturated NHSi 1. Although the data summarized in Figure 3 are rather limited, they suggest, however, that the qualitative correlations found separately for the saturated NHSis 1-3 and the unsaturated compounds 4-6 cannot be extended to accommodate the data for all six silylenes investigated. Possible reasons for that might be different kinetic processes (15) Dhiman, A.; Becker, J. Y.; Minge, O.; Schmidbaur, H; M€ uller, T. Organometallics 2004, 23, 1636 and references therein.
Six cyclic silylene derivatives have been studied by cyclic voltammetry, and each showed one irreversible oxidation peak potential in THF-n-Bu4NBPh4 medium, but no reduction peaks could be detected under these conditions. The quality of the oxidation waves was better on a GC than on a Pt electrode. The oxidation of the saturated cyclic silylenes 1-3 was slightly easier than the corresponding oxidation for the unsaturated derivatives 4-6. The ease of oxidation in the former series was found to be in the order 3 > 2 > 1, as was also confirmed by computational results. As expected, the silylenes substituted with alkyl groups at the ethylene bridge (2 and 3) are easier to oxidize than “naked” 1, due to the inductive effect of the alkyl groups. A good correlation was also found by examining the unsaturated cyclic silylenes 4-6. The ease of oxidation by both CV and computations was in the order 4 > 5 > 6. Silylenes 5 and 6 are more difficult to oxidize than 4 because the N-aryl groups lie in a plane perpendicular to that of the five-membered ring and, as a consequence, they exert only a negative inductive effect.
Acknowledgment. This research was supported by the Israel Science Foundation (Grant No. 317/07) and by the Fonds der Chemischen Industrie (scholarship to P.Z.). J. Y.B. and K.P. are thankful to Mrs. E. Solomon for technical assistance. Supporting Information Available: Text giving a full description of the computational procedures, tables giving absolute energies of computed molecules (Tables S1 and S2) and Cartesian coordinates of all calculated compounds, and figures giving the CV’s of silylenes 1-6. This material is available free of charge via the Internet at http://pubs.acs.org.
