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Photoelectron spectroscopy of titanium(IV) tert-butoxide
Chemical Physics Letters 427 (2006) 47–50 www.elsevier.com/locate/cplett
Photoelectron spectroscopy of titanium(IV) tert-butoxide Igor Novak b
a,*
, Branka Kovacˇ
b
a Charles Sturt University, P.O. Box 883, Orange, NSW 2800, Australia Physical Chemistry Division, ‘R. Bosˇkovic´’ Institute, HR-10002 Zagreb, Croatia
Received 26 April 2006; in final form 1 June 2006 Available online 21 June 2006
Abstract The electronic structure of titanium(IV) tert-butoxide Ti(OC(CH3)3)4 has been investigated by UV photoelectron spectroscopy (UPS) and DFT/OVGF calculations. This is the first reported UPS spectrum of a titanium alkoxide. We discuss the nature of metal–ligand bonding on the basis of empirical arguments. Ó 2006 Elsevier B.V. All rights reserved.
1. Introduction Titanium(IV) alkoxides, Ti(OR)4 are important precursors in the production of TiO2. Ti(OR)4 undergo hydrolysis (giving TiO2 and alkanol) and condensation reactions producing macromolecular networks which are important for sol–gel processing [1]. Transition metal alkoxides readily hydrolyze and polymerize which makes it impossible to study their electronic structure experimentally. Titanium(IV) tertiary butoxide Ti(OC(CH3)3)4 is an exception due to the presence of bulky t-butyl groups which quench reactivity; it exists as a monomer in solution and is quite volatile [2] thus making it suitable for the electronic structure analysis via UV photoelectron spectroscopy (UPS). In view of these properties it is not surprising that very few studies of Ti–O bonding have been reported. In one theoretical study (based on MP2 and DFT calculations) the Ti–O bond in X3TiOCH3 (X = H,F,Cl,Br) was described as having triple bond character [3]. This character was reflected in the nearly linear geometry of the Ti–O–C bond. The analysis of the nature of Ti–O bond on the basis of experimental data rather than theoretically derived bond analysis is thus of interest. In this work we present the *
Corresponding author. Fax: +61 263657659. E-mail address: [email protected] (I. Novak).
0009-2614/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.06.048
experimental investigation of Ti–O bonding by UV photoelectron spectroscopy (UPS) of titanium(IV)-tertbutoxide Ti(t-BuO)4. 2. Experimental and computational methods The sample compound Ti(OC(CH3)3)4 was purchased from Aldrich and used without further purification after checking it’s identity and purity by NMR spectroscopy. The HeI/HeII photoelectron spectra were recorded on the Vacuum Generators UV-G3 spectrometer and calibrated with small amounts of Xe or Ar gas which was added to the sample flow. The spectral resolution in HeI and HeII spectra was 25 meV and 70 meV, respectively when measured as FWHM of the 3p1 2P3/2 Ar+ Ar(1S0) line. The sample was sufficiently volatile for its spectrum to be recorded at 30 °C. The reproducible spectra were obtained and showed no signs of decomposition. The quantum chemical calculations were performed with GAUSSIAN 03 program [4] including full geometry optimization of the neutral molecule at B3LYP/6-311G* level as the first step. Subsequently, the optimized geometry was used as the input into the single point calculation using OVGF method at 6-31 G level [5]. This method obviates the need for using Koopmans approximation and provides vertical ionization energies with typical deviation of 0.2– 0.4 eV from experimental values. The calculations suggest
48
I. Novak, B. Kovacˇ / Chemical Physics Letters 427 (2006) 47–50
that the molecule has S4 symmetry with Ti–O bond lengths ˚ , O–Ti–O bond angle of 109.2° and Ti–O–C of 1.797 A angle of 149.8° (see below).
There are no experimental reports regarding molecular structure of titanium alkoxides in the gas phase. However, the molecular structure of solid titanium tetramethoxide Ti(OCH3)4 had been determined by single crystal X-ray diffraction [6]. The molecules form tetrameric units. Ti–O ˚ , O–Ti–O bond lengths were in the range 1.786–2.078 A angle was 99.6° and Ti–O–C angle 140.0°. Our calculated geometry thus appears to be consistent with the available experimental data. We performed the same type of calculations for mono, di and tri titanium(IV) tertbutoxides. The Ti–O–C bond angles were found to be 179.8°, 161.2° and 163.7°,in TiH3OC(CH3)3, TiH2(OC(CH3)3)2 and TiH(OC(CH3)3)3, respectively. The magnitude of Ti–O–C angle appears to depend on the extent of steric crowding between bulky t-Bu groups. 3. Results and discussion The photoelectron spectra of the title molecule and tertiary butanol are shown in Fig. 1. The assignment is summarized in Table 1. Due to the high density of ionic states above 11 eV we confine our analysis to the bands whose ionization energies are <11 eV. The bands below 11 eV comprise two manifolds with maxima at 8.85 eV and 9.7– 10.2 eV. The intensity ratio of the two manifolds is 3:5, respectively (Table 1). This ratio is consistent with the assignment obtained by OVGF calculations. The bands can be attributed to ionizations from eight oxygen lone pairs which span representations 2e + 2a + 2b in the S4 point group. The detailed assignment of lone pair ionizations to individual bands is given in Table 1 and was derived from Greens functions (OVGF) calculations. The comparison of the UPS spectrum of Ti(OC(CH3)3)4 with the spectra of reference molecules: uranium hexamethoxide U(OCH3)6 [7] and t-butanol (Fig. 1), supports the proposed assignment. In the UPS of uranium hexamethoxide the 12 oxygen lone pairs appear in the region 8.80–12.79 eV, while in t-butanol the oxygen lone pairs appear at 10.25 ev and 11.5 eV.
Fig. 1. HeI and HeII photoelectron spectra of Ti(OC(CH3)3)4 and (CH3)3COH.
Table 1 Experimental (Ei/eV) and calculated (OVGF/eV) vertical ionization energies, assignments and relative band intensities in UPS of titanium(IV) tetrabutoxidea Ti(OC(CH3)3)4
Ei
OVGF
Assignment
Relative intensity
X–A
8.85
8.65, 8.65, 8.69
e+a
1 (HeI)
B–C
9.70
9.54, 9.54, 9.57
e+b
1 (HeII) 1.6 (HeI)
D–E
10.2 (10.5)
9.93, 10.02
a+b
F–etc.
11–14.5 9.70, 9.70
e
H3TiOC(CH3)3 a
1.62 (HeII) 1.6 (HeI) 1.62 (HeII) 10.4 (HeI) 4.3 (HeII)
The Ei in brackets corresponds to high energy shoulder.
The variation of relative band intensities on going from HeI to HeII excitation provides insight into the character of ionized orbitals. On going from HeI to HeII radiation, an increase in the relative intensities of bands below 11 eV compared to those above 11 eV is noted (Table 1). However, within the 8–11 eV region itself there is no
I. Novak, B. Kovacˇ / Chemical Physics Letters 427 (2006) 47–50
apparent change in the relative intensities (Table 1). In the spectra of U(OCH3)6 the change in relative intensities of bands within the 8–13 eV region had been noted [7]. It was attributed to ninefold increase in U5f orbital photoionization cross-section on going from HeI to HeII radiation [7,8]. Why is there no intensity variation amongst bands <11 eV in UPS of Ti(OC(CH3)3)4? We recall that HeII/ HeI photoionization cross-sections ratios for C2p, O2p, Ti3d and Ti4s orbitals are 0.31, 0.64, 0.79 and 0.74, respectively [8]. The bands corresponding to orbitals localized on t-butyl groups (and thus having prominent C2p character) would be expected to show a decrease in intensity (compared to O or Ti bands) as was indeed observed for manifolds above 11 eV (Fig. 1). The HeII/HeI cross-section ratio for O2p is smaller than for Ti3d or Ti4s, hence the bands corresponding to orbitals with metal character can be expected to show enhancement of relative intensity. However, since no enhancement was observed, we conclude that orbital ionizations within 8.85–10.2 eV range correspond to orbitals with little metal and predominantly O2p lone pair character. This conclusion is tentative, because the difference between cross-section ratios for the O2p and Ti3d orbitals is not very pronounced. However, we note two arguments which indirectly support our assignment of ionizations as being predominantly O2p: (a) Ionizations from orbitals with Ti3d character appear above 11 eV as seen in the UPS spectra of TiCl4 and CH3TiCl3 [9,10]. (b) Bands corresponding to Ti3d and to C2p orbital ionizations in the spectrum of CH3TiCl3 show distinctly different intensity behaviour on going from HeI to HeII radiation. Ti3d band is reduced in intensity by half while C2p bands decrease in intensity five times [9]. Unfortunately, due to the properties of titanium alkoxides mentioned in the Section 1, no comparison with UPS spectrum of another titanium alkoxide can be made to corroborate the point about O2p character of bands <11 eV. The absence of metallic character has relevance for the nature of Ti–O bonding and will be discussed next. Theoretical approaches which analyze metal–ligand (M– L) bonding in the early transition metal d0 complexes have used natural bond order (NBO) or atoms-in-molecules (AIM) analyses. Ti–O bond was described as a triple bond comprising back-donation type p-bonding (O2p ! Ti3d) in addition to the conventional r-bond [3]. While p-bonding is favored by the linearity of Ti–O–C bond, this is not necessarily the case as Kaupp has demonstrated [11]. He has shown that ligand’s electronegativity, bond angles, pbonding and r-bonding are interdependent factors which can not be rationalized through simple principles of metal–ligand bonding like Bent’s rule. (According to this rule, bonds to electronegative ligands have less metal d character and larger L–M–L angles than do bonds to the
49
less electronegative ligands). In Ti(OC(CH3)3)4 bulky t-Bu substituents preclude the formation of linear Ti–O–C bonds. UPS data help us to unravel p- and r-bonding through the analysis of orbital ionization energies and relative band intensity variations (see above). Comparison of oxygen lone pair energies in Ti(OC(CH3)3)4 and t-butanol reveals that oxygen lone pairs are destabilized (i.e. have lower ionization energies) by up to 1.35 eV in the former. The net electron density transfer from ligand to metal would lead to destabilization of the orbitals of the latter and stabilization of the orbitals of the former which is the opposite of the observed effects. The connection between electronic energy levels and geometry does exists. The multiple, linear Ti–O–C bond leads to stabilization of oxygen lone pairs as was indicated by OVGF results for linearly bonded H3TiOC(CH3)3 vs. nonlinearly bonded Ti(OC(CH3)3)4 (Table 1). Since O2p destabilization was observed in the UPS spectrum of Ti(OC(CH3)3)4 we propose that Ti–O bond has some ionic character in complexes where Ti–O– C subunits are bent. The presence of oxygen lone pair bands below 11 eV is a further indication of this net electron transfer. The comparison of the vibrational spectra of titanium alkoxides [12,13] and t-BuOH shows little change in C–O stretching frequencies. Such a change (an increase) would be expected to be indicative of multiple ligand–metal bonding. 4. Conclusion Ti alkoxides are highly susceptible not only to nucleophilic substitution reactions (e.g. hydrolysis), but also to condensations/additions (e.g. alkoxolation/bridging). The detailed microscopic information about such processes is very difficult to obtain yet these reactions govern the emerging structures and morphologies of macromolecular networks. On the other hand, controlling the growth of such networks is a must for designing better sol–gel technologies. One usually collects macroscopic information (equilibrium constants for hydrolysis/condensation, calorimetric measurements) and subsequently builds a model to gauge the chemical composition of the alkoxide network [14]. The theoretical models incorporate the analysis of partial charges and basicities of oxygen centres, the stability of Ti–O bond and coordination/charge on electrophilic Ti atoms [15]. Our results provide evidence about the electronic structure/density of two principal nodes in the network: oxygen and titanium and can thus be useful for designing better theoretical models. On the basis of our results we propose some guidelines for model building and understanding of Ti–alkoxide gel networks. Multiple Ti–O bonds and Ti O electron transfer are not important factors in determining structure and reactivity of such networks. Oxygen centres in such networks are quite basic (electron rich) even if they are in terminal positions. Coordination expansion (CE), i.e. the tendency of some metals (Ti) to increase their coordination number beyond their
50
I. Novak, B. Kovacˇ / Chemical Physics Letters 427 (2006) 47–50
normal valency, is due to Lewis basicity of Ti centres. CE is the main driving force behind the formation/growth of titanium alkoxide networks [14]. Titanium is of course a Lewis acid, but its electron deficiency (Lewis acidity) is probably enhanced by coordinating oxygen atoms which have high electron affinity. This in turn enhances CE tendency; the two factors thus appear to act synergistically. References [1] U. Schubert, J. Mater. Chem. 15 (2005) 3701. [2] D.C. Bradley, Chem. Rev. 89 (1989) 1317. [3] J.A. Dobado, J.M. Molina, R. Uggla, M.R. Sundberg, Inorg. Chem. 39 (2000) 2831. [4] M.J. Frisch et al., GAUSSIAN 03, Revision C2, GAUSSIAN Inc., Pittsburgh PA, 2003. [5] W. Von Niessen, J. Schirmer, L.S. Cederbaum, Comp. Phys. Rep. 1 (1984) 57.
[6] D.A. Wright, D.A. Williams, Acta Cryst. B24 (1968) 1107. [7] B.E. Bursten, M. Casarin, D.E. Ellis, I. Fragala, T.J. Marks, Inorg. Chem. 25 (1986) 1257. [8] J.J. Yeh, Atomic Calculation of Photoionization Cross-sections and Asymmetry Parameters, Gordon and Breach, Langhorne, 1993. [9] C.N. Field, J.C. Green, N. Kaltsoyannis, G.S. McGrady, A.N. Moody, M. Siggel, M. De Simone, J. Chem. Soc., Dalton Trans. (1997) 213. [10] B.E. Bursten, J.C. Green, N. Kaltsoyannis, M.A. MacDonald, K.H. Sze, J.S. Tse, Inorg. Chem. 33 (1994) 5086. [11] M. Kaupp, Chem. Eur. J. 5 (1999) 3631. [12] (a) H.A. Ory, Anal. Chem. 32 (1960) 509; (b) C.T. Lynch, K.S. Mazdiyasni, J.S. Smith, W.J. Crawford, Anal. Chem. 36 (1964) 2332. [13] K.S. Finnie, V. Luca, P.D. Moran, J.R. Bartlett, J.L. Woolfrey, J. Mater. Chem. 10 (2000) 409. [14] J. Blanchard, M. In, B. Schaudel, C. Sanchez, Eur. J. Inorg. Chem. (1998) 1115. [15] F. Biechel, J. Dubuc, M. Henry, New J.Chem. 28 (2004) 764.
Photoelectron spectroscopy of titanium(IV) tert-butoxide Igor Novak b
a,*
, Branka Kovacˇ
b
a Charles Sturt University, P.O. Box 883, Orange, NSW 2800, Australia Physical Chemistry Division, ‘R. Bosˇkovic´’ Institute, HR-10002 Zagreb, Croatia
Received 26 April 2006; in final form 1 June 2006 Available online 21 June 2006
Abstract The electronic structure of titanium(IV) tert-butoxide Ti(OC(CH3)3)4 has been investigated by UV photoelectron spectroscopy (UPS) and DFT/OVGF calculations. This is the first reported UPS spectrum of a titanium alkoxide. We discuss the nature of metal–ligand bonding on the basis of empirical arguments. Ó 2006 Elsevier B.V. All rights reserved.
1. Introduction Titanium(IV) alkoxides, Ti(OR)4 are important precursors in the production of TiO2. Ti(OR)4 undergo hydrolysis (giving TiO2 and alkanol) and condensation reactions producing macromolecular networks which are important for sol–gel processing [1]. Transition metal alkoxides readily hydrolyze and polymerize which makes it impossible to study their electronic structure experimentally. Titanium(IV) tertiary butoxide Ti(OC(CH3)3)4 is an exception due to the presence of bulky t-butyl groups which quench reactivity; it exists as a monomer in solution and is quite volatile [2] thus making it suitable for the electronic structure analysis via UV photoelectron spectroscopy (UPS). In view of these properties it is not surprising that very few studies of Ti–O bonding have been reported. In one theoretical study (based on MP2 and DFT calculations) the Ti–O bond in X3TiOCH3 (X = H,F,Cl,Br) was described as having triple bond character [3]. This character was reflected in the nearly linear geometry of the Ti–O–C bond. The analysis of the nature of Ti–O bond on the basis of experimental data rather than theoretically derived bond analysis is thus of interest. In this work we present the *
Corresponding author. Fax: +61 263657659. E-mail address: [email protected] (I. Novak).
0009-2614/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.06.048
experimental investigation of Ti–O bonding by UV photoelectron spectroscopy (UPS) of titanium(IV)-tertbutoxide Ti(t-BuO)4. 2. Experimental and computational methods The sample compound Ti(OC(CH3)3)4 was purchased from Aldrich and used without further purification after checking it’s identity and purity by NMR spectroscopy. The HeI/HeII photoelectron spectra were recorded on the Vacuum Generators UV-G3 spectrometer and calibrated with small amounts of Xe or Ar gas which was added to the sample flow. The spectral resolution in HeI and HeII spectra was 25 meV and 70 meV, respectively when measured as FWHM of the 3p1 2P3/2 Ar+ Ar(1S0) line. The sample was sufficiently volatile for its spectrum to be recorded at 30 °C. The reproducible spectra were obtained and showed no signs of decomposition. The quantum chemical calculations were performed with GAUSSIAN 03 program [4] including full geometry optimization of the neutral molecule at B3LYP/6-311G* level as the first step. Subsequently, the optimized geometry was used as the input into the single point calculation using OVGF method at 6-31 G level [5]. This method obviates the need for using Koopmans approximation and provides vertical ionization energies with typical deviation of 0.2– 0.4 eV from experimental values. The calculations suggest
48
I. Novak, B. Kovacˇ / Chemical Physics Letters 427 (2006) 47–50
that the molecule has S4 symmetry with Ti–O bond lengths ˚ , O–Ti–O bond angle of 109.2° and Ti–O–C of 1.797 A angle of 149.8° (see below).
There are no experimental reports regarding molecular structure of titanium alkoxides in the gas phase. However, the molecular structure of solid titanium tetramethoxide Ti(OCH3)4 had been determined by single crystal X-ray diffraction [6]. The molecules form tetrameric units. Ti–O ˚ , O–Ti–O bond lengths were in the range 1.786–2.078 A angle was 99.6° and Ti–O–C angle 140.0°. Our calculated geometry thus appears to be consistent with the available experimental data. We performed the same type of calculations for mono, di and tri titanium(IV) tertbutoxides. The Ti–O–C bond angles were found to be 179.8°, 161.2° and 163.7°,in TiH3OC(CH3)3, TiH2(OC(CH3)3)2 and TiH(OC(CH3)3)3, respectively. The magnitude of Ti–O–C angle appears to depend on the extent of steric crowding between bulky t-Bu groups. 3. Results and discussion The photoelectron spectra of the title molecule and tertiary butanol are shown in Fig. 1. The assignment is summarized in Table 1. Due to the high density of ionic states above 11 eV we confine our analysis to the bands whose ionization energies are <11 eV. The bands below 11 eV comprise two manifolds with maxima at 8.85 eV and 9.7– 10.2 eV. The intensity ratio of the two manifolds is 3:5, respectively (Table 1). This ratio is consistent with the assignment obtained by OVGF calculations. The bands can be attributed to ionizations from eight oxygen lone pairs which span representations 2e + 2a + 2b in the S4 point group. The detailed assignment of lone pair ionizations to individual bands is given in Table 1 and was derived from Greens functions (OVGF) calculations. The comparison of the UPS spectrum of Ti(OC(CH3)3)4 with the spectra of reference molecules: uranium hexamethoxide U(OCH3)6 [7] and t-butanol (Fig. 1), supports the proposed assignment. In the UPS of uranium hexamethoxide the 12 oxygen lone pairs appear in the region 8.80–12.79 eV, while in t-butanol the oxygen lone pairs appear at 10.25 ev and 11.5 eV.
Fig. 1. HeI and HeII photoelectron spectra of Ti(OC(CH3)3)4 and (CH3)3COH.
Table 1 Experimental (Ei/eV) and calculated (OVGF/eV) vertical ionization energies, assignments and relative band intensities in UPS of titanium(IV) tetrabutoxidea Ti(OC(CH3)3)4
Ei
OVGF
Assignment
Relative intensity
X–A
8.85
8.65, 8.65, 8.69
e+a
1 (HeI)
B–C
9.70
9.54, 9.54, 9.57
e+b
1 (HeII) 1.6 (HeI)
D–E
10.2 (10.5)
9.93, 10.02
a+b
F–etc.
11–14.5 9.70, 9.70
e
H3TiOC(CH3)3 a
1.62 (HeII) 1.6 (HeI) 1.62 (HeII) 10.4 (HeI) 4.3 (HeII)
The Ei in brackets corresponds to high energy shoulder.
The variation of relative band intensities on going from HeI to HeII excitation provides insight into the character of ionized orbitals. On going from HeI to HeII radiation, an increase in the relative intensities of bands below 11 eV compared to those above 11 eV is noted (Table 1). However, within the 8–11 eV region itself there is no
I. Novak, B. Kovacˇ / Chemical Physics Letters 427 (2006) 47–50
apparent change in the relative intensities (Table 1). In the spectra of U(OCH3)6 the change in relative intensities of bands within the 8–13 eV region had been noted [7]. It was attributed to ninefold increase in U5f orbital photoionization cross-section on going from HeI to HeII radiation [7,8]. Why is there no intensity variation amongst bands <11 eV in UPS of Ti(OC(CH3)3)4? We recall that HeII/ HeI photoionization cross-sections ratios for C2p, O2p, Ti3d and Ti4s orbitals are 0.31, 0.64, 0.79 and 0.74, respectively [8]. The bands corresponding to orbitals localized on t-butyl groups (and thus having prominent C2p character) would be expected to show a decrease in intensity (compared to O or Ti bands) as was indeed observed for manifolds above 11 eV (Fig. 1). The HeII/HeI cross-section ratio for O2p is smaller than for Ti3d or Ti4s, hence the bands corresponding to orbitals with metal character can be expected to show enhancement of relative intensity. However, since no enhancement was observed, we conclude that orbital ionizations within 8.85–10.2 eV range correspond to orbitals with little metal and predominantly O2p lone pair character. This conclusion is tentative, because the difference between cross-section ratios for the O2p and Ti3d orbitals is not very pronounced. However, we note two arguments which indirectly support our assignment of ionizations as being predominantly O2p: (a) Ionizations from orbitals with Ti3d character appear above 11 eV as seen in the UPS spectra of TiCl4 and CH3TiCl3 [9,10]. (b) Bands corresponding to Ti3d and to C2p orbital ionizations in the spectrum of CH3TiCl3 show distinctly different intensity behaviour on going from HeI to HeII radiation. Ti3d band is reduced in intensity by half while C2p bands decrease in intensity five times [9]. Unfortunately, due to the properties of titanium alkoxides mentioned in the Section 1, no comparison with UPS spectrum of another titanium alkoxide can be made to corroborate the point about O2p character of bands <11 eV. The absence of metallic character has relevance for the nature of Ti–O bonding and will be discussed next. Theoretical approaches which analyze metal–ligand (M– L) bonding in the early transition metal d0 complexes have used natural bond order (NBO) or atoms-in-molecules (AIM) analyses. Ti–O bond was described as a triple bond comprising back-donation type p-bonding (O2p ! Ti3d) in addition to the conventional r-bond [3]. While p-bonding is favored by the linearity of Ti–O–C bond, this is not necessarily the case as Kaupp has demonstrated [11]. He has shown that ligand’s electronegativity, bond angles, pbonding and r-bonding are interdependent factors which can not be rationalized through simple principles of metal–ligand bonding like Bent’s rule. (According to this rule, bonds to electronegative ligands have less metal d character and larger L–M–L angles than do bonds to the
49
less electronegative ligands). In Ti(OC(CH3)3)4 bulky t-Bu substituents preclude the formation of linear Ti–O–C bonds. UPS data help us to unravel p- and r-bonding through the analysis of orbital ionization energies and relative band intensity variations (see above). Comparison of oxygen lone pair energies in Ti(OC(CH3)3)4 and t-butanol reveals that oxygen lone pairs are destabilized (i.e. have lower ionization energies) by up to 1.35 eV in the former. The net electron density transfer from ligand to metal would lead to destabilization of the orbitals of the latter and stabilization of the orbitals of the former which is the opposite of the observed effects. The connection between electronic energy levels and geometry does exists. The multiple, linear Ti–O–C bond leads to stabilization of oxygen lone pairs as was indicated by OVGF results for linearly bonded H3TiOC(CH3)3 vs. nonlinearly bonded Ti(OC(CH3)3)4 (Table 1). Since O2p destabilization was observed in the UPS spectrum of Ti(OC(CH3)3)4 we propose that Ti–O bond has some ionic character in complexes where Ti–O– C subunits are bent. The presence of oxygen lone pair bands below 11 eV is a further indication of this net electron transfer. The comparison of the vibrational spectra of titanium alkoxides [12,13] and t-BuOH shows little change in C–O stretching frequencies. Such a change (an increase) would be expected to be indicative of multiple ligand–metal bonding. 4. Conclusion Ti alkoxides are highly susceptible not only to nucleophilic substitution reactions (e.g. hydrolysis), but also to condensations/additions (e.g. alkoxolation/bridging). The detailed microscopic information about such processes is very difficult to obtain yet these reactions govern the emerging structures and morphologies of macromolecular networks. On the other hand, controlling the growth of such networks is a must for designing better sol–gel technologies. One usually collects macroscopic information (equilibrium constants for hydrolysis/condensation, calorimetric measurements) and subsequently builds a model to gauge the chemical composition of the alkoxide network [14]. The theoretical models incorporate the analysis of partial charges and basicities of oxygen centres, the stability of Ti–O bond and coordination/charge on electrophilic Ti atoms [15]. Our results provide evidence about the electronic structure/density of two principal nodes in the network: oxygen and titanium and can thus be useful for designing better theoretical models. On the basis of our results we propose some guidelines for model building and understanding of Ti–alkoxide gel networks. Multiple Ti–O bonds and Ti O electron transfer are not important factors in determining structure and reactivity of such networks. Oxygen centres in such networks are quite basic (electron rich) even if they are in terminal positions. Coordination expansion (CE), i.e. the tendency of some metals (Ti) to increase their coordination number beyond their
50
I. Novak, B. Kovacˇ / Chemical Physics Letters 427 (2006) 47–50
normal valency, is due to Lewis basicity of Ti centres. CE is the main driving force behind the formation/growth of titanium alkoxide networks [14]. Titanium is of course a Lewis acid, but its electron deficiency (Lewis acidity) is probably enhanced by coordinating oxygen atoms which have high electron affinity. This in turn enhances CE tendency; the two factors thus appear to act synergistically. References [1] U. Schubert, J. Mater. Chem. 15 (2005) 3701. [2] D.C. Bradley, Chem. Rev. 89 (1989) 1317. [3] J.A. Dobado, J.M. Molina, R. Uggla, M.R. Sundberg, Inorg. Chem. 39 (2000) 2831. [4] M.J. Frisch et al., GAUSSIAN 03, Revision C2, GAUSSIAN Inc., Pittsburgh PA, 2003. [5] W. Von Niessen, J. Schirmer, L.S. Cederbaum, Comp. Phys. Rep. 1 (1984) 57.
[6] D.A. Wright, D.A. Williams, Acta Cryst. B24 (1968) 1107. [7] B.E. Bursten, M. Casarin, D.E. Ellis, I. Fragala, T.J. Marks, Inorg. Chem. 25 (1986) 1257. [8] J.J. Yeh, Atomic Calculation of Photoionization Cross-sections and Asymmetry Parameters, Gordon and Breach, Langhorne, 1993. [9] C.N. Field, J.C. Green, N. Kaltsoyannis, G.S. McGrady, A.N. Moody, M. Siggel, M. De Simone, J. Chem. Soc., Dalton Trans. (1997) 213. [10] B.E. Bursten, J.C. Green, N. Kaltsoyannis, M.A. MacDonald, K.H. Sze, J.S. Tse, Inorg. Chem. 33 (1994) 5086. [11] M. Kaupp, Chem. Eur. J. 5 (1999) 3631. [12] (a) H.A. Ory, Anal. Chem. 32 (1960) 509; (b) C.T. Lynch, K.S. Mazdiyasni, J.S. Smith, W.J. Crawford, Anal. Chem. 36 (1964) 2332. [13] K.S. Finnie, V. Luca, P.D. Moran, J.R. Bartlett, J.L. Woolfrey, J. Mater. Chem. 10 (2000) 409. [14] J. Blanchard, M. In, B. Schaudel, C. Sanchez, Eur. J. Inorg. Chem. (1998) 1115. [15] F. Biechel, J. Dubuc, M. Henry, New J.Chem. 28 (2004) 764.