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Indenyl–titanium–trichloride complexes: correlations between electrochemical and UV–vis-spectroscopic data
Journal of Electroanalytical Chemistry 533 (2002) 127 /133 www.elsevier.com/locate/jelechem
Indenyl titanium trichloride complexes: correlations between electrochemical and UV vis-spectroscopic data /
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Thomas Weiß a, Karuppannan Natarajan b, Heinrich Lang a,*, Rudolf Holze a,* a
Institut fu¨r Chemie, Technische Universita¨t Chemnitz, Strasse der Nationen 62, D-09107 Chemnitz, Germany b Department of Chemistry, Bharathiar University, Coimbatore 641 046, India Received 28 February 2002; received in revised form 13 May 2002; accepted 3 August 2002
Dedicated to Professor Gottfried Huttner on the occasion of his 65th birthday in grateful recognition of his numerous contributions to organometallic chemistry.
Abstract Substituent effects in several titanium(IV) /trichloride half-sandwich complexes with h5-coordinated cyclopentadienyl or indenyl ligands including (h5-C5H5)TiCl3 (1), (h5-C9H7)TiCl3 (2), (h5-1-SiMe3 /C9H6)TiCl3 (3), (h5-1-Me /C9H6)]TiCl3 (4), (h5-4-Me / C9H6)]TiCl3 (5), [h5-5,6-(CH2)3 /C9H5]TiCl3 (6), (h5-2-SiMe3 /C9H6)TiCl3 (7), (h5-1,3-Me2-C9H5)TiCl3 (8), (h5-4,7-Me2 / C9H6)TiCl3 (9), [h5-3,4-(CH2)3 /C9H5]TiCl3 (10), (h5-1-SiMe3 /4,7 /Me2 /C9H4)TiCl3 (11) and (h5-1,3,4,7-Me4 /C9H3)TiCl3 (12) were studied with cyclic voltammetry (CV) and UV /vis spectroscopy. Cyclic voltammograms of 1 /12 showed current waves caused by the reversible redox process of the Ti(IV)/(III) redox couple at electrode potentials typical for d0-titanium centers. In UV /vis spectra, two major absorption bands between 400 /430 and 530 /590 nm were observed. The latter is assigned to a ligand to metal charge transfer (LMCT). A comparison shows that the alkylation or Me3Si-substitution (complexes 3 /12) induces a shift of the corresponding absorption to longer wavelengths indicating a smaller energy gap between the frontier orbitals. Based on the experimental data an additive increment system for Me3Si and alkyl-substituted h5-indenyl derivatives of titanium(IV) /trichloride species is proposed. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Cyclopentadienyl; Titanocene; Indenyl /titanium /trichloride; Spectroelectrochemistry
1. Introduction Indenyl ligands can successfully be applied as pbound annulenes in the synthesis of half-sandwich and sandwich complexes with group 4-transition metals. These complexes are promising novel homogenous catalysts for Ziegler/Natta polymerization and copolymerization of olefins [1 /15]. The introduction of substituents at different positions of the h5-coordinated indenyl ligand can influence both sterical as well as electronic features of the resulting h5indenyl transition metal complexes [16 /18]. This in turn affects reactivity and hence the reaction chemistry by fine-tuning the energy of the frontier orbitals and, thus,
* Corresponding authors. Tel.: /49-5311-509; fax: /49-5311-832 E-mail addresses: [email protected] (H. Lang), [email protected] (R. Holze).
the electronic situation at the transition metal center [19]. For simple titanocenes of the general type (h5C5H5)2TiCl3 [20 /25] and (h5-C5H5)TiCl2(OR) [26] results pertaining to these effects have been reported whereas less is known about species like (h5-C5H5)TiCl3 or (h5-C9H7)TiCl3. The situation becomes more complicated for indenyl half-sandwich complexes because of the increasing number of possible substituent isomers. For example, only one isomer is expected in monosubstituted (h5-C5H4R)TiCl3, whereas four stereo isomers in h5-coordinated indenyl complexes of type (h5C9H6R)TiCl3 are possible, since the substituent R can be attached in position 1(3), 2, 4(7) or even 5(6), respectively (Fig. 1). This makes predictions of the effect of the substituents R at the p-perimeter on the transition metal center more difficult. These effects of type and position of the substituent can be studied with UV/vis-spectroscopy and cyclic voltammetry (CV). We have prepared and studied the Me3Si- and alkyl-substituted (h5-inde-
0022-0728/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 7 2 8 ( 0 2 ) 0 1 0 8 0 - X
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Fig. 1. Numbering scheme of h5-coordinated indenyl transition metal complexes; the positions 1(3), 4(7), 5(6) and 8(9) are equivalent.
nyl)titaniumtrichlorides 3/12, for comparison (h5C5H5)TiCl3 (1) and (h5-C9H7)TiCl3 (2) were included (Fig. 2) [27 /29]. Based on the experimental data an additive increment system is proposed.
2. Experimental The preparation and full characterization by elemental analysis and spectroscopy (IR, 1H, 13C{1H}-NMR) of complexes 1 /12 have been described elsewhere in detail (for 1 [27], for 2, 4, 5, 6 [16], for 3, 7, 8, 9, 10 [27,29]). UV /vis spectroscopy was carried out with a Lambda 40 spectrometer (Perkin/Elmer) using 10 mm cuvets at a resolution of 5 nm. The concentration of the complexes was 1 /103 mol l 1 in methylene chloride. The validity of the Lambert /Beer law was confirmed for (h5-C9H7)TiCl3 (2) for concentrations ranging from 103 to 104 mol 1 l1 at 25 8C. In addition spectra of (h6-Men C6H6n )TiCl4 (13: n/0; 14: n/1; 15: n/2) were obtained by using a solution of TiCl4 (0.4556 mol l 1) in methylene chloride (1.0 ml TiCl4 in 20 ml methylene chloride) and adding three drops of the
corresponding benzene compound to 2.0 ml of the TiCl4 solution. For CV a platinum disc electrode (0.125 cm2, EDI 101 T, Radiometer), a platinum wire counter electrode and a saturated calomel electrode filled with methylene chloride as solvent of the electrolyte solution were used with the standard three electrode glass cell of the digital electrochemical analyzer (DEA) 101 and the electrochemical interface IMT 102 (Radiometer). Experiments were run with VOLTALAB-3.1 software (V. 2.0) at a scan rate dE /dt/200 mV s 1. The measured electrode potentials were converted taking the redox potential EFeC of the ferricenium/ferrocene couple as a reference [30]. Electrolyte solutions were prepared from tetrabutylammonium hexafluorophosphate (Fluka, dried in an oil pump vacuum at 120 8C) 0.1 M in methylene chloride (distilled over CaH2) with 5.0 mg of the complex in 5.0 ml of methylene chloride. This composition was equal in all experiments. Thus data are comparable although no IR -compensation was attempted. All experiments were carried out with solutions saturated with argon at 25 8C.
3. Results and discussion The UV /vis spectrum of the half sandwich complex (h5-1-SiMe3 /C9H7)TiCl3 (3) is shown in Fig. 3, it is representative for all other compounds 2/12. The common feature for all complexes 2/12 (Fig. 2) are two absorption maxima between 400/430 and 530/590 nm with the latter being of lower intensity. Further weak absorption features, especially their position, in the long
Fig. 2. Selected TiCl3 half-sandwich complexes. (Complexes 6 and 10 can formally be considered as dimethyl substituted derivatives thereof.)
T. Weiß et al. / Journal of Electroanalytical Chemistry 533 (2002) 127 /133
Fig. 3. UV /vis spectrum of (h5-1-SiMe3 /C9H7)TiCl3 (3) in methylenechloride (10 3 M); the weak additional absorption features around 400 nm are most likely caused by electronic absorptions due to p 0/p*transitions of the indenyl ligand.
wavelength range correlate strongly with the polarity of the solvent [29]. With increasing polarity of the solvent the band is red shifted; this indicates a strong polarity of the excited state of the molecule (presumably an exciplex state). This state is stabilized more effectively by a polar solvent, consequently the corresponding highest occupied molecular orbital /lowest unoccupied molecular orbital-difference (HOMO /LUMO) is diminished [31]. The latter solvatochromic effect is typical for complexes with a change of the electron distribution during the electronic transition (charge transfer, CT) and hence, ligand-to-metal charge transfer (LMCT) transitions can be identified with these results. The lowest energy absorption band in the UV /vis spectrum of a d0-halfsandwich complex arises from a CT between the HOMO and the LUMO [32]. This assignment was checked by correlating the values of lmax of complexes 2, 4 and 8 with complexes of type (h6-Men C6H6n )TiCl4 (13: n /0; 14: n /1; 15: n/2) in
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methylene chloride as solvent (Fig. 4). The (h6-benzene)TiCl4 adducts show only one very characteristic absorption band in the UV/vis spectrum at 403 nm for (h6-C6H6)TiCl4 (13), 415 nm for (h6-C6H5Me)TiCl4 (14) and 430 nm for (h6-C6H4-1,3-Me2)TiCl4 (15). This demonstrates, that an increasing number of methyl substituents at the p-coordinated aromatic ring induces a significant shift of the lmax from 403 (13) to 430 nm (15). A similar phenomenon was observed with ferrocene complexes [33]. This observation implies a LMCT transition mechanism [34]. A complete list of results is provided in Table 1. A comparison of lmax of the unsubstituted cyclopentadienyl- and indenyl /titanium /trichloride complexes 1 and 2 reveals a significant bathochromic shift of lmax from 395 nm for 1 to 528 nm for 2. This shift is caused by the additional 6-membered p-system in 2 causing typical conjugation effects. This is also typical when Table 1 UV /vis data (lmax) of complexes 1 /12 Compound
lmax/nm Emax/eV lmax(calcd.)/nm
o /l cm 1 mol 1
1 2 3 4 5 6 7 8 9 10 11 12
395 528 538 550 548 548 541 572 577 573 592 621
2136 936 1051 672 n.a. 896 630 1085 n.a. 701 n.a. n.a.
3.139 2.348 2.305 2.255 2.263 2.263 2.292 2.168 2.149 2.164 2.095 1.997
528 541 550 552 548 541 572 576 574 589 620
a
n.a., Not analyzed. Readings taken at a single concentration only.
a
Fig. 4. Relationship between the CT-absorption of 13 /15 and the absorption of the indenyl /titanium /trichloride complexes 2, 4 and 8.
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going from small to large chromophores and indicates a smaller energy difference between the HOMO and the LUMO in 2. With additional substituents at the h5coordinated indenyl ligands, e.g. Me3Si or alkyl groups (complexes 3/12) a further red-shift to even higher lmax values is found (see Table 1). In order to quantify the observed shifts the differences in lmax were calculated by comparing the respective values with lmax of 2. In case of the symmetrically disubstituted ligands the shifts were divided by 2. Results are listed in Table 2. The effect of a methyl group in position 1(3) is nearly twice the effect of the Me3Si-group in the same position. This can be explained at least in part by the fact that a methyl group shows only a /I effect, whereas the Me3Si-group shows a combined /I (I, inductive) and / M-effect (M, mesomeric). The latter results in a reduced electron density at the aromatic p-system. A generally large bathochromic shift is observed upon substitution in positions c1(3) and 4(7). The shift of lmax amounts to about 1/5% of lmax of 2. The understanding of this changes will be used to identify correlations between e.g. the catalytic activity of complexes 1 /12 in a-olefin polymerization and the observed shifts [35] as was done elsewhere for nickel a-diimine systems [36]. A representative CV of (h5-1-SiMe3 /C9H7)TiCl3 (3) starting at EFeC //0.2 V is shown in Fig. 5. The reduction current wave is caused by the reduction process Ti(IV) 0/Ti(III), the corresponding oxidation process is observed also. A conceivable oxidation process beyond the reoxidation of Ti(III) for complexes 1 /12 was not observed in the range of electrode potentials investigated because the titanium(IV) complexes studied have a fairly stable d0-electron configuration of the transition metal center. Table 3 lists reduction potentials Ered, DE -values (DE /Ered/Eox) and E0 (E0 /(Eox/Ered)/2) of complexes 1/12. Similar values of E0, Ered and DE have been reported previously [37]. Extended Hu¨ckel calculations of complexes 2, 3, 4 and 7 have identified the HOMO of the complex as a ligand-attributed MO, whereas the LUMO is a MO with a contribution from the d2z -orbital of the titanium metal Table 2 Dlmax values for Me3Si and Me substituents in position 1(3), 2, 4(7) and 5(6) (equivalent positions as indicated in brackets, see Fig. 1) Group
Dlmax/nm in position 1(3)
Me SiMe3 a b c d
22 13
2
4(7)
a,b a
24 10
c
a
Calculated as lmax(substituted indene)lmax(2). Calculated as [lmax(8)lmax(2)]/2. Calculated as [lmax(9)lmax(2)]/2. Calculated as [lmax(6)lmax(2)]/2.
5(6) 10
d
Fig. 5. Cyclic voltammogram of (h5-1-SiMe3C9H7)TiCl3 (3) in methylene chloride.
Table 3 Ered, E0 and DE values of 1 /12 Compound
E0/V
DE /V
Ered/V
Ered,calcd./V
1 2 3 4 5 6 7 8 9 10 11 12
0.82 0.82 1.24 0.89 0.85 0.95 0.92 1.04 0.86 0.93 0.90 0.88
0.236 0.234 0.170 0.230 0.627 0.650 0.180 0.276 0.160 0.120 0.690 0.305
0.94 0.94 1.33 1.00 0.95 1.27 1.01 1.18 0.94 0.99 1.25 1.19
0.94 1.32 1.06 0.94 1.20 1.00 1.18 0.94 1.06 1.32 1.18
with an energy close to that of the metal orbital [29]. Accordingly the reduction potential provides information about the electronic properties of the metal center. Mediated by the Ti/D distance (D is centroid of the h5-coordinated 5-membered indenyl ring), the substituents T of the p-bound aromatic system in 1 /12 show effects on the electronic properties of the metal center. An electron donating substituent R at the p-perimeter will cause a higher electron density at the titanium(IV) center and make it more difficult to reduce, whereas electron withdrawing groups at the corresponding h5coordinated cyclopentadienyl or indenyl ligands decrease the reduction overpotential when compared with the parent complexes (h5-C5H5)TiCl3 (1) and (h5C9H7)TiCl3 (2), respectively, indicating higher Lewis acidity of the metal center. In contrast to the UV /vis data obtained for complexes 1 and 2 (vide supra) the reduction potentials Ered of these complexes do not differ (Table 3). This implies that the annulated 6-membered ring of the indenyl building block in 2 has no significant effect on the metal center. By comparing the data summarized in Table 3 one recognizes that the reduction potentials for
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3 /12 are shifted to more negative values by more electron donating substituents such as Me3Si and Me. In addition, this effect strongly depends on the position of the respective substituent R. As outlined before with the UV /vis data an attempt is made to identify electrode potential increments ascribed to the substituents in certain positions. The values of DEred obtained are listed in Table 4. The reduction potentials of complexes 3, 4, 6, 8 and 9 depend significantly on the substituents R in position 1(3) (complexes 3, 4, 8) 4 (7) (complex 9) and 5(6) (complex 6) of the indenyl ligand (Table 4). The effect of the SiMe3-group is about three times the effect of the Me-group in position(s) 1(3). The reduction potentials of complexes 3, 4, 6, 8 and 9 as compared with that of 2 are shifted to more negative values (2, /0.94; 3, /1.33; 4, /1.00; 6, /1.27; 9, /0.94 V (Table 3)). To prove and generalize the procedure for the definition of increments (Dlmax and DEred values) data for the indenyl titanium trichloride complexes 2 (nonsubstituted), 4 (position 1) and 8 (positions 1 and 3) and 2, 5 (position 4) and 9 (positions 4 and7) were plotted versus the number of Me-substituents (Figs. 6 and 7). A
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Table 4 DEred values for Me3Si- and Me-substituents in position 1(3), 2, 4(7) and 5(6) Group
DEred/V in position 1(3)
Me SiMe3 a b c d
0.12 0.38
2
4(7)
a d
0.0 0.06
b
5(6) 0.13
c
d
Calculated as [Ered(8)Ered(2)]/2. Calculated as [Ered(9)Ered(2)]/2. Calculated as [Ered(6)Ered(2)]/2. Calculated as EredEred(2) (substituted indene).
change of the transition energy EUV vis showing a linear decrease with the number of methyl groups is observed. In both cases */UV /vis-spectroscopy and CV */a linear relationship between the number of Me groups at the p-bound annulene and EUV vis and Ered can be formulated within the error limits (Figs. 6 and 7). This indicates that the substituent effects at least in position(s) 1(3) and 4(7) are additive. Such additive substituent effects are well known for alkyl-substituted
Fig. 6. Correlation between the number of methyl substituents and the Emax as well as Ered of the complexes 2, 4 and 8. EUV vis: Y//0.09x/2.35. Ered: Y //0.12x/0.92.
Fig. 7. Correlation between the number of methyl substituents and the Emax as well as Ered of the complexes 2, 5 and 9. EUV vis: Y//0.10x/2.35. Ered: Y //0.94.
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benzenes [32], where a bathochromic shift of lmax was also observed. As an example, the increment for a Me-group in position(s) 1(3) in the h5-coordinated indenyl building block can be determined from the slope of the linear function (see Fig. 6). The increments obtained were Dlmax //0.0909/0.002 eV and DEred //0.129/0.03 V for a Me group in position(s) 1(3). For a Me group in position(s) 4(7) Dlmax //0.109/0.008 eV and DEred / 0.0 V were obtained. Values for Dlmax for a Me-group in position(s) 1(3) and 4(7) are in the same range indicating similar substituent effects. However, values of DEred show a stronger substituent effect for a Me group in position(s) 1(3) when compared with position(s) 4(7). This difference is a consequence of the fact that electrochemistry probes MO energies, whereas UV / vis-spectroscopy probes differences between MOs. Thus a substituent might cause a shift of the energy of an MO without affecting the energy difference and vice versa. This is discussed in detail below (see Fig. 8). The calculated increment value of Dlmax /0.0909/ 0.002 eV (:/22 nm) for a Me substituent in position 1(3) (complexes 2, 4 and 8) is similar to the result of Dlmax /0.077 eV (:/19 nm) reported by Kim et al. [16] without any significant influence of the changed solvent benzene versus methylene chloride on the absolute band positions. Incremental changes of electrochemical data were reported for disubstituted ferrocene systems of general type (h5-C5H4R)2Fe (increment for R /Me: 0.07 V) as well as for titanocene dichlorides of type (h5C5H5n Men )2TiCl2 (n /1, 2, . . ., 5; increment for Me: 0.05 V) [23,24]. In both cases, the reduction is hindered by the donicity of the substituent to the metal center. Our value for the Me substituent in 1(3) position of the indenyl ligand DEred /0.09 eV is typical for a Me group in organometallic compounds. In order to test our set of increments complexes 10, 11 and 12 (c.f. Fig. 1) have been synthesized. The general equation for the substituent induced shift is:
value(x)value[(h5 -C9 H7 )TiCl3 ]
n X
Me(i)
0
n X
SiMe3 (i)
0
where Me(i) is the increment for Me; and SiMe3(i) is the increment for SiMe3; n is the number of substituents in position i; i is the position i in the h5-coordinated indenyl unit: positions 1 and 3, 4 and 7, 5 and 6 are equivalent. The expected lmax of the complexes 10 /12 were calculated by adding the appropriate Dlmax values (Table 2) to the lmax of complex 2. A remarkable agreement of calculated and experimental data was found as listed in Tables 1 and 3. The changes of the energy levels of the frontier orbitals (HOMO and LUMO) in the metal complexes represented in Fig. 2 effected by substitution in the pbound aromatic system can be interpreted in a molecular-orbital model. The reduction of the titanium(IV) to titanium(III) corresponds to an electron transfer into the LUMO of the corresponding species 1 /12. The HOMO of a d0-configurated half-sandwich complex is mainly Cp’-ligand based (Cp’ /any h5-cyclopentadienyle-type ligand, e.g. indenyl). Calculated MO energies (B3LYP) reported elsewhere [38] for 2 present the LUMO as a weakly antibonding MO with mostly metal character. Increasing the energy of LUMO by means of substitution at the indenyl ligand results in a correspondingly more negative reduction potential. Hence Ered indicates */in a first approximation */the energy of the LUMO levels [39]. In this model, the absorption EUV vis at long wavelength energy edge, should correspond to the HOMO /LUMO energy gap. Based on the experimental data (Table 3) the frontier orbital interactions of electron donating substituents SiMe3 and Me are analyzed with respect to complexes (h5-1-SiMe3 / C9H7)TiCl3 (3), (h5-1-Me /C9H7)TiCl3 (4), (h5-4-Me/ C9H7)TiCl3 (5) and (h5-2-SiMe3 /C9H7)TiCl3 (7). Fig. 8 illustrates qualitatively the relationship between mo-
Fig. 8. Relationship between substituent location, type of substituent, frontier orbital energies and CT absorption energies. The influence of (a) Me in Pos. 1(3), (b) Me in Pos. 4(7), (c) SiMe3 in Pos. 1(3) and (d) SiMe3 in Pos. 2 of the indenyl moiety.
T. Weiß et al. / Journal of Electroanalytical Chemistry 533 (2002) 127 /133
lecular structure, frontier orbital energies and CT absorption energies as well as reduction potentials. The indenyl /titanium /trichloride complexes 3 and 7 have very similar CT absorptions at 2.292 and 2.305 eV. Moving the 2-SiMe3 group to position 1 causes only a minute decrease of the absorption energy. In contrast, the same change of a siloxy group from position 2 to 1 in yellow colored bis(indenyl)-based zirconocene dichloride complexes caused a decrease in the LMCT absorption energy from 2.90 to 2.67 eV [37]. Compared with the monosilyl substituted indenyl complexes 3 and 7 the absorption energies of the methyl substituted titaniumtrichloride complexes 4 (2.255 eV) and 5 (2.263 eV) are also very similar to each other, but smaller than those of 3 and 7. Both Me and SiMe3 of the appropriate p-bound annulene decrease the HOMO /LUMO energy gap. In particular, electron donating substituents in position 1(3) (Fig. 8a and c) affect the energy separation of the HOMO and LUMO level by destabilization (increasing energy). In contrast, movement of the donating group in position 4(7) (Fig. 8b) or 2 (Fig. 8d) destabilizes only the HOMO level compared with the parent compound 2. In each case, the HOMO p-orbital level is raised as was found in electrochemical and spectroscopic studies of ferrocene derivatives with electron donating substituents [34]. Only substituents with donating capacity such as SiMe3 (3: Ered //1.33 eV) and Me (4: Ered //1.00 eV) in position 1 destabilize the LUMO energy level in comparison to mother complex 2 (Ered //0.94 eV).
Acknowledgements Financial support of the Fonds der Chemischen Industrie and Bayer AG is gratefully acknowledged. Data and helpful suggestions by E. Meichel and R. Blaudeck as well as stimulating discussions of the UV / vis spectroscopic data with S. Spange are appreciated. U. Bo¨hme (Technische Universita¨t Bergakademie Freiberg) provided valuable assistance during the MOcalculations.
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Indenyl titanium trichloride complexes: correlations between electrochemical and UV vis-spectroscopic data /
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Thomas Weiß a, Karuppannan Natarajan b, Heinrich Lang a,*, Rudolf Holze a,* a
Institut fu¨r Chemie, Technische Universita¨t Chemnitz, Strasse der Nationen 62, D-09107 Chemnitz, Germany b Department of Chemistry, Bharathiar University, Coimbatore 641 046, India Received 28 February 2002; received in revised form 13 May 2002; accepted 3 August 2002
Dedicated to Professor Gottfried Huttner on the occasion of his 65th birthday in grateful recognition of his numerous contributions to organometallic chemistry.
Abstract Substituent effects in several titanium(IV) /trichloride half-sandwich complexes with h5-coordinated cyclopentadienyl or indenyl ligands including (h5-C5H5)TiCl3 (1), (h5-C9H7)TiCl3 (2), (h5-1-SiMe3 /C9H6)TiCl3 (3), (h5-1-Me /C9H6)]TiCl3 (4), (h5-4-Me / C9H6)]TiCl3 (5), [h5-5,6-(CH2)3 /C9H5]TiCl3 (6), (h5-2-SiMe3 /C9H6)TiCl3 (7), (h5-1,3-Me2-C9H5)TiCl3 (8), (h5-4,7-Me2 / C9H6)TiCl3 (9), [h5-3,4-(CH2)3 /C9H5]TiCl3 (10), (h5-1-SiMe3 /4,7 /Me2 /C9H4)TiCl3 (11) and (h5-1,3,4,7-Me4 /C9H3)TiCl3 (12) were studied with cyclic voltammetry (CV) and UV /vis spectroscopy. Cyclic voltammograms of 1 /12 showed current waves caused by the reversible redox process of the Ti(IV)/(III) redox couple at electrode potentials typical for d0-titanium centers. In UV /vis spectra, two major absorption bands between 400 /430 and 530 /590 nm were observed. The latter is assigned to a ligand to metal charge transfer (LMCT). A comparison shows that the alkylation or Me3Si-substitution (complexes 3 /12) induces a shift of the corresponding absorption to longer wavelengths indicating a smaller energy gap between the frontier orbitals. Based on the experimental data an additive increment system for Me3Si and alkyl-substituted h5-indenyl derivatives of titanium(IV) /trichloride species is proposed. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Cyclopentadienyl; Titanocene; Indenyl /titanium /trichloride; Spectroelectrochemistry
1. Introduction Indenyl ligands can successfully be applied as pbound annulenes in the synthesis of half-sandwich and sandwich complexes with group 4-transition metals. These complexes are promising novel homogenous catalysts for Ziegler/Natta polymerization and copolymerization of olefins [1 /15]. The introduction of substituents at different positions of the h5-coordinated indenyl ligand can influence both sterical as well as electronic features of the resulting h5indenyl transition metal complexes [16 /18]. This in turn affects reactivity and hence the reaction chemistry by fine-tuning the energy of the frontier orbitals and, thus,
* Corresponding authors. Tel.: /49-5311-509; fax: /49-5311-832 E-mail addresses: [email protected] (H. Lang), [email protected] (R. Holze).
the electronic situation at the transition metal center [19]. For simple titanocenes of the general type (h5C5H5)2TiCl3 [20 /25] and (h5-C5H5)TiCl2(OR) [26] results pertaining to these effects have been reported whereas less is known about species like (h5-C5H5)TiCl3 or (h5-C9H7)TiCl3. The situation becomes more complicated for indenyl half-sandwich complexes because of the increasing number of possible substituent isomers. For example, only one isomer is expected in monosubstituted (h5-C5H4R)TiCl3, whereas four stereo isomers in h5-coordinated indenyl complexes of type (h5C9H6R)TiCl3 are possible, since the substituent R can be attached in position 1(3), 2, 4(7) or even 5(6), respectively (Fig. 1). This makes predictions of the effect of the substituents R at the p-perimeter on the transition metal center more difficult. These effects of type and position of the substituent can be studied with UV/vis-spectroscopy and cyclic voltammetry (CV). We have prepared and studied the Me3Si- and alkyl-substituted (h5-inde-
0022-0728/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 7 2 8 ( 0 2 ) 0 1 0 8 0 - X
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Fig. 1. Numbering scheme of h5-coordinated indenyl transition metal complexes; the positions 1(3), 4(7), 5(6) and 8(9) are equivalent.
nyl)titaniumtrichlorides 3/12, for comparison (h5C5H5)TiCl3 (1) and (h5-C9H7)TiCl3 (2) were included (Fig. 2) [27 /29]. Based on the experimental data an additive increment system is proposed.
2. Experimental The preparation and full characterization by elemental analysis and spectroscopy (IR, 1H, 13C{1H}-NMR) of complexes 1 /12 have been described elsewhere in detail (for 1 [27], for 2, 4, 5, 6 [16], for 3, 7, 8, 9, 10 [27,29]). UV /vis spectroscopy was carried out with a Lambda 40 spectrometer (Perkin/Elmer) using 10 mm cuvets at a resolution of 5 nm. The concentration of the complexes was 1 /103 mol l 1 in methylene chloride. The validity of the Lambert /Beer law was confirmed for (h5-C9H7)TiCl3 (2) for concentrations ranging from 103 to 104 mol 1 l1 at 25 8C. In addition spectra of (h6-Men C6H6n )TiCl4 (13: n/0; 14: n/1; 15: n/2) were obtained by using a solution of TiCl4 (0.4556 mol l 1) in methylene chloride (1.0 ml TiCl4 in 20 ml methylene chloride) and adding three drops of the
corresponding benzene compound to 2.0 ml of the TiCl4 solution. For CV a platinum disc electrode (0.125 cm2, EDI 101 T, Radiometer), a platinum wire counter electrode and a saturated calomel electrode filled with methylene chloride as solvent of the electrolyte solution were used with the standard three electrode glass cell of the digital electrochemical analyzer (DEA) 101 and the electrochemical interface IMT 102 (Radiometer). Experiments were run with VOLTALAB-3.1 software (V. 2.0) at a scan rate dE /dt/200 mV s 1. The measured electrode potentials were converted taking the redox potential EFeC of the ferricenium/ferrocene couple as a reference [30]. Electrolyte solutions were prepared from tetrabutylammonium hexafluorophosphate (Fluka, dried in an oil pump vacuum at 120 8C) 0.1 M in methylene chloride (distilled over CaH2) with 5.0 mg of the complex in 5.0 ml of methylene chloride. This composition was equal in all experiments. Thus data are comparable although no IR -compensation was attempted. All experiments were carried out with solutions saturated with argon at 25 8C.
3. Results and discussion The UV /vis spectrum of the half sandwich complex (h5-1-SiMe3 /C9H7)TiCl3 (3) is shown in Fig. 3, it is representative for all other compounds 2/12. The common feature for all complexes 2/12 (Fig. 2) are two absorption maxima between 400/430 and 530/590 nm with the latter being of lower intensity. Further weak absorption features, especially their position, in the long
Fig. 2. Selected TiCl3 half-sandwich complexes. (Complexes 6 and 10 can formally be considered as dimethyl substituted derivatives thereof.)
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Fig. 3. UV /vis spectrum of (h5-1-SiMe3 /C9H7)TiCl3 (3) in methylenechloride (10 3 M); the weak additional absorption features around 400 nm are most likely caused by electronic absorptions due to p 0/p*transitions of the indenyl ligand.
wavelength range correlate strongly with the polarity of the solvent [29]. With increasing polarity of the solvent the band is red shifted; this indicates a strong polarity of the excited state of the molecule (presumably an exciplex state). This state is stabilized more effectively by a polar solvent, consequently the corresponding highest occupied molecular orbital /lowest unoccupied molecular orbital-difference (HOMO /LUMO) is diminished [31]. The latter solvatochromic effect is typical for complexes with a change of the electron distribution during the electronic transition (charge transfer, CT) and hence, ligand-to-metal charge transfer (LMCT) transitions can be identified with these results. The lowest energy absorption band in the UV /vis spectrum of a d0-halfsandwich complex arises from a CT between the HOMO and the LUMO [32]. This assignment was checked by correlating the values of lmax of complexes 2, 4 and 8 with complexes of type (h6-Men C6H6n )TiCl4 (13: n /0; 14: n /1; 15: n/2) in
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methylene chloride as solvent (Fig. 4). The (h6-benzene)TiCl4 adducts show only one very characteristic absorption band in the UV/vis spectrum at 403 nm for (h6-C6H6)TiCl4 (13), 415 nm for (h6-C6H5Me)TiCl4 (14) and 430 nm for (h6-C6H4-1,3-Me2)TiCl4 (15). This demonstrates, that an increasing number of methyl substituents at the p-coordinated aromatic ring induces a significant shift of the lmax from 403 (13) to 430 nm (15). A similar phenomenon was observed with ferrocene complexes [33]. This observation implies a LMCT transition mechanism [34]. A complete list of results is provided in Table 1. A comparison of lmax of the unsubstituted cyclopentadienyl- and indenyl /titanium /trichloride complexes 1 and 2 reveals a significant bathochromic shift of lmax from 395 nm for 1 to 528 nm for 2. This shift is caused by the additional 6-membered p-system in 2 causing typical conjugation effects. This is also typical when Table 1 UV /vis data (lmax) of complexes 1 /12 Compound
lmax/nm Emax/eV lmax(calcd.)/nm
o /l cm 1 mol 1
1 2 3 4 5 6 7 8 9 10 11 12
395 528 538 550 548 548 541 572 577 573 592 621
2136 936 1051 672 n.a. 896 630 1085 n.a. 701 n.a. n.a.
3.139 2.348 2.305 2.255 2.263 2.263 2.292 2.168 2.149 2.164 2.095 1.997
528 541 550 552 548 541 572 576 574 589 620
a
n.a., Not analyzed. Readings taken at a single concentration only.
a
Fig. 4. Relationship between the CT-absorption of 13 /15 and the absorption of the indenyl /titanium /trichloride complexes 2, 4 and 8.
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going from small to large chromophores and indicates a smaller energy difference between the HOMO and the LUMO in 2. With additional substituents at the h5coordinated indenyl ligands, e.g. Me3Si or alkyl groups (complexes 3/12) a further red-shift to even higher lmax values is found (see Table 1). In order to quantify the observed shifts the differences in lmax were calculated by comparing the respective values with lmax of 2. In case of the symmetrically disubstituted ligands the shifts were divided by 2. Results are listed in Table 2. The effect of a methyl group in position 1(3) is nearly twice the effect of the Me3Si-group in the same position. This can be explained at least in part by the fact that a methyl group shows only a /I effect, whereas the Me3Si-group shows a combined /I (I, inductive) and / M-effect (M, mesomeric). The latter results in a reduced electron density at the aromatic p-system. A generally large bathochromic shift is observed upon substitution in positions c1(3) and 4(7). The shift of lmax amounts to about 1/5% of lmax of 2. The understanding of this changes will be used to identify correlations between e.g. the catalytic activity of complexes 1 /12 in a-olefin polymerization and the observed shifts [35] as was done elsewhere for nickel a-diimine systems [36]. A representative CV of (h5-1-SiMe3 /C9H7)TiCl3 (3) starting at EFeC //0.2 V is shown in Fig. 5. The reduction current wave is caused by the reduction process Ti(IV) 0/Ti(III), the corresponding oxidation process is observed also. A conceivable oxidation process beyond the reoxidation of Ti(III) for complexes 1 /12 was not observed in the range of electrode potentials investigated because the titanium(IV) complexes studied have a fairly stable d0-electron configuration of the transition metal center. Table 3 lists reduction potentials Ered, DE -values (DE /Ered/Eox) and E0 (E0 /(Eox/Ered)/2) of complexes 1/12. Similar values of E0, Ered and DE have been reported previously [37]. Extended Hu¨ckel calculations of complexes 2, 3, 4 and 7 have identified the HOMO of the complex as a ligand-attributed MO, whereas the LUMO is a MO with a contribution from the d2z -orbital of the titanium metal Table 2 Dlmax values for Me3Si and Me substituents in position 1(3), 2, 4(7) and 5(6) (equivalent positions as indicated in brackets, see Fig. 1) Group
Dlmax/nm in position 1(3)
Me SiMe3 a b c d
22 13
2
4(7)
a,b a
24 10
c
a
Calculated as lmax(substituted indene)lmax(2). Calculated as [lmax(8)lmax(2)]/2. Calculated as [lmax(9)lmax(2)]/2. Calculated as [lmax(6)lmax(2)]/2.
5(6) 10
d
Fig. 5. Cyclic voltammogram of (h5-1-SiMe3C9H7)TiCl3 (3) in methylene chloride.
Table 3 Ered, E0 and DE values of 1 /12 Compound
E0/V
DE /V
Ered/V
Ered,calcd./V
1 2 3 4 5 6 7 8 9 10 11 12
0.82 0.82 1.24 0.89 0.85 0.95 0.92 1.04 0.86 0.93 0.90 0.88
0.236 0.234 0.170 0.230 0.627 0.650 0.180 0.276 0.160 0.120 0.690 0.305
0.94 0.94 1.33 1.00 0.95 1.27 1.01 1.18 0.94 0.99 1.25 1.19
0.94 1.32 1.06 0.94 1.20 1.00 1.18 0.94 1.06 1.32 1.18
with an energy close to that of the metal orbital [29]. Accordingly the reduction potential provides information about the electronic properties of the metal center. Mediated by the Ti/D distance (D is centroid of the h5-coordinated 5-membered indenyl ring), the substituents T of the p-bound aromatic system in 1 /12 show effects on the electronic properties of the metal center. An electron donating substituent R at the p-perimeter will cause a higher electron density at the titanium(IV) center and make it more difficult to reduce, whereas electron withdrawing groups at the corresponding h5coordinated cyclopentadienyl or indenyl ligands decrease the reduction overpotential when compared with the parent complexes (h5-C5H5)TiCl3 (1) and (h5C9H7)TiCl3 (2), respectively, indicating higher Lewis acidity of the metal center. In contrast to the UV /vis data obtained for complexes 1 and 2 (vide supra) the reduction potentials Ered of these complexes do not differ (Table 3). This implies that the annulated 6-membered ring of the indenyl building block in 2 has no significant effect on the metal center. By comparing the data summarized in Table 3 one recognizes that the reduction potentials for
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3 /12 are shifted to more negative values by more electron donating substituents such as Me3Si and Me. In addition, this effect strongly depends on the position of the respective substituent R. As outlined before with the UV /vis data an attempt is made to identify electrode potential increments ascribed to the substituents in certain positions. The values of DEred obtained are listed in Table 4. The reduction potentials of complexes 3, 4, 6, 8 and 9 depend significantly on the substituents R in position 1(3) (complexes 3, 4, 8) 4 (7) (complex 9) and 5(6) (complex 6) of the indenyl ligand (Table 4). The effect of the SiMe3-group is about three times the effect of the Me-group in position(s) 1(3). The reduction potentials of complexes 3, 4, 6, 8 and 9 as compared with that of 2 are shifted to more negative values (2, /0.94; 3, /1.33; 4, /1.00; 6, /1.27; 9, /0.94 V (Table 3)). To prove and generalize the procedure for the definition of increments (Dlmax and DEred values) data for the indenyl titanium trichloride complexes 2 (nonsubstituted), 4 (position 1) and 8 (positions 1 and 3) and 2, 5 (position 4) and 9 (positions 4 and7) were plotted versus the number of Me-substituents (Figs. 6 and 7). A
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Table 4 DEred values for Me3Si- and Me-substituents in position 1(3), 2, 4(7) and 5(6) Group
DEred/V in position 1(3)
Me SiMe3 a b c d
0.12 0.38
2
4(7)
a d
0.0 0.06
b
5(6) 0.13
c
d
Calculated as [Ered(8)Ered(2)]/2. Calculated as [Ered(9)Ered(2)]/2. Calculated as [Ered(6)Ered(2)]/2. Calculated as EredEred(2) (substituted indene).
change of the transition energy EUV vis showing a linear decrease with the number of methyl groups is observed. In both cases */UV /vis-spectroscopy and CV */a linear relationship between the number of Me groups at the p-bound annulene and EUV vis and Ered can be formulated within the error limits (Figs. 6 and 7). This indicates that the substituent effects at least in position(s) 1(3) and 4(7) are additive. Such additive substituent effects are well known for alkyl-substituted
Fig. 6. Correlation between the number of methyl substituents and the Emax as well as Ered of the complexes 2, 4 and 8. EUV vis: Y//0.09x/2.35. Ered: Y //0.12x/0.92.
Fig. 7. Correlation between the number of methyl substituents and the Emax as well as Ered of the complexes 2, 5 and 9. EUV vis: Y//0.10x/2.35. Ered: Y //0.94.
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benzenes [32], where a bathochromic shift of lmax was also observed. As an example, the increment for a Me-group in position(s) 1(3) in the h5-coordinated indenyl building block can be determined from the slope of the linear function (see Fig. 6). The increments obtained were Dlmax //0.0909/0.002 eV and DEred //0.129/0.03 V for a Me group in position(s) 1(3). For a Me group in position(s) 4(7) Dlmax //0.109/0.008 eV and DEred / 0.0 V were obtained. Values for Dlmax for a Me-group in position(s) 1(3) and 4(7) are in the same range indicating similar substituent effects. However, values of DEred show a stronger substituent effect for a Me group in position(s) 1(3) when compared with position(s) 4(7). This difference is a consequence of the fact that electrochemistry probes MO energies, whereas UV / vis-spectroscopy probes differences between MOs. Thus a substituent might cause a shift of the energy of an MO without affecting the energy difference and vice versa. This is discussed in detail below (see Fig. 8). The calculated increment value of Dlmax /0.0909/ 0.002 eV (:/22 nm) for a Me substituent in position 1(3) (complexes 2, 4 and 8) is similar to the result of Dlmax /0.077 eV (:/19 nm) reported by Kim et al. [16] without any significant influence of the changed solvent benzene versus methylene chloride on the absolute band positions. Incremental changes of electrochemical data were reported for disubstituted ferrocene systems of general type (h5-C5H4R)2Fe (increment for R /Me: 0.07 V) as well as for titanocene dichlorides of type (h5C5H5n Men )2TiCl2 (n /1, 2, . . ., 5; increment for Me: 0.05 V) [23,24]. In both cases, the reduction is hindered by the donicity of the substituent to the metal center. Our value for the Me substituent in 1(3) position of the indenyl ligand DEred /0.09 eV is typical for a Me group in organometallic compounds. In order to test our set of increments complexes 10, 11 and 12 (c.f. Fig. 1) have been synthesized. The general equation for the substituent induced shift is:
value(x)value[(h5 -C9 H7 )TiCl3 ]
n X
Me(i)
0
n X
SiMe3 (i)
0
where Me(i) is the increment for Me; and SiMe3(i) is the increment for SiMe3; n is the number of substituents in position i; i is the position i in the h5-coordinated indenyl unit: positions 1 and 3, 4 and 7, 5 and 6 are equivalent. The expected lmax of the complexes 10 /12 were calculated by adding the appropriate Dlmax values (Table 2) to the lmax of complex 2. A remarkable agreement of calculated and experimental data was found as listed in Tables 1 and 3. The changes of the energy levels of the frontier orbitals (HOMO and LUMO) in the metal complexes represented in Fig. 2 effected by substitution in the pbound aromatic system can be interpreted in a molecular-orbital model. The reduction of the titanium(IV) to titanium(III) corresponds to an electron transfer into the LUMO of the corresponding species 1 /12. The HOMO of a d0-configurated half-sandwich complex is mainly Cp’-ligand based (Cp’ /any h5-cyclopentadienyle-type ligand, e.g. indenyl). Calculated MO energies (B3LYP) reported elsewhere [38] for 2 present the LUMO as a weakly antibonding MO with mostly metal character. Increasing the energy of LUMO by means of substitution at the indenyl ligand results in a correspondingly more negative reduction potential. Hence Ered indicates */in a first approximation */the energy of the LUMO levels [39]. In this model, the absorption EUV vis at long wavelength energy edge, should correspond to the HOMO /LUMO energy gap. Based on the experimental data (Table 3) the frontier orbital interactions of electron donating substituents SiMe3 and Me are analyzed with respect to complexes (h5-1-SiMe3 / C9H7)TiCl3 (3), (h5-1-Me /C9H7)TiCl3 (4), (h5-4-Me/ C9H7)TiCl3 (5) and (h5-2-SiMe3 /C9H7)TiCl3 (7). Fig. 8 illustrates qualitatively the relationship between mo-
Fig. 8. Relationship between substituent location, type of substituent, frontier orbital energies and CT absorption energies. The influence of (a) Me in Pos. 1(3), (b) Me in Pos. 4(7), (c) SiMe3 in Pos. 1(3) and (d) SiMe3 in Pos. 2 of the indenyl moiety.
T. Weiß et al. / Journal of Electroanalytical Chemistry 533 (2002) 127 /133
lecular structure, frontier orbital energies and CT absorption energies as well as reduction potentials. The indenyl /titanium /trichloride complexes 3 and 7 have very similar CT absorptions at 2.292 and 2.305 eV. Moving the 2-SiMe3 group to position 1 causes only a minute decrease of the absorption energy. In contrast, the same change of a siloxy group from position 2 to 1 in yellow colored bis(indenyl)-based zirconocene dichloride complexes caused a decrease in the LMCT absorption energy from 2.90 to 2.67 eV [37]. Compared with the monosilyl substituted indenyl complexes 3 and 7 the absorption energies of the methyl substituted titaniumtrichloride complexes 4 (2.255 eV) and 5 (2.263 eV) are also very similar to each other, but smaller than those of 3 and 7. Both Me and SiMe3 of the appropriate p-bound annulene decrease the HOMO /LUMO energy gap. In particular, electron donating substituents in position 1(3) (Fig. 8a and c) affect the energy separation of the HOMO and LUMO level by destabilization (increasing energy). In contrast, movement of the donating group in position 4(7) (Fig. 8b) or 2 (Fig. 8d) destabilizes only the HOMO level compared with the parent compound 2. In each case, the HOMO p-orbital level is raised as was found in electrochemical and spectroscopic studies of ferrocene derivatives with electron donating substituents [34]. Only substituents with donating capacity such as SiMe3 (3: Ered //1.33 eV) and Me (4: Ered //1.00 eV) in position 1 destabilize the LUMO energy level in comparison to mother complex 2 (Ered //0.94 eV).
Acknowledgements Financial support of the Fonds der Chemischen Industrie and Bayer AG is gratefully acknowledged. Data and helpful suggestions by E. Meichel and R. Blaudeck as well as stimulating discussions of the UV / vis spectroscopic data with S. Spange are appreciated. U. Bo¨hme (Technische Universita¨t Bergakademie Freiberg) provided valuable assistance during the MOcalculations.
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