Orbital control over the metal vs. ligand reduction in a series of neutral and cationic bis(cyclopentadienyl) Ti(IV) complexes

Abstract

Quantum chemistry calculations showed that the first reduction, as experimentally observed by cyclic voltammetry for a series of neutral and cationic functionalised bis(cyclopentadienyl) titanium(IV) derivatives, is metal based, involving the chemically and electrochemically reversible Ti^IV/Ti^III redox couple. The second reduction peak observed is chemically and electrochemically irreversible. For the neutral [Cp₂Ti^IV(ligand)_n] (n = 1 or 2) complexes (Cp = cyclopentadienyl), the second irreversible reduction peak is metal-based, while for the cationic [Cp₂Ti^IV(ligand)]⁺ complexes, the second irreversible reduction peak involves ligand reduction, leading to a Ti^III species coupled to a ligand radical. The electronic properties of the substituents directly influence electron density at the reduction centre when π-interactions between the electro-active centre and the substituent groups exist in the lowest unoccupied molecular orbital (LUMO) of the complexes. In this case, a number of relationships between the electronic properties of the substituents and the reduction potentials have been affirmed, yielding linear relationship between experimentally measured reduction potentials and DFT calculated energies (energy of the LUMO, electron affinity, Mulliken electronegativity and global electrophilicity index). However, the substituent effect is ill defined when the LUMO is highly localised without π-interaction between the electro-active centre and the substituents.

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1. Introduction

A large number of ‘bent-sandwich’ titanocene(IV) derivatives have been developed due to their role in industrial processes, including catalysis^1–6 and as reagents for organic synthesis.^7–9 For the development of homogeneous catalysts it is necessary to fine-tune the electronic properties of the active metal, using functionalised ligands, such that there is electronic interaction between the substituents and the metal centre via the ligand. The redox chemistry (electron transfer, oxidative addition, reductive elimination) of transition metal complexes is one of their defining features. Normally these redox processes take place on the metal, however, certain ligands can actively participate in electron transfer. Although the electrochemical behaviour of [Cp₂TiCl₂] (Cp = cyclopentadienyl) and substituted derivatives has been rigorously studied,^10–17 attempts to directly relate the reduction potentials of substituted complexes to the electronic effects of substituents are often unsuccessful. Therefore understanding the influence of a substituent on the redox-potentials, for example, knowing when and how the electronic properties of the substituents are expected to have an influence on the reduction potentials, will help in the design of new compounds with controlled activity.

In this contribution we present a comparative electrochemical study of two families, neutral and cationic, functionalised bis(cyclopentadienyl) titanium(IV) derivatives, using cyclic voltammetric data and DFT (Density Functional Theory) calculations determining the nature of the reductions, i.e., metal-centred vs. ligand-based reduction and predict the substituent effect on reduction potentials. Substitution of the Cp₂Ti^IVCl₂ derivatives in this study, involves the cyclopentadienyl ligands as well as replacing the labile dichloro ligands with redox active bidentate ligands. Neutral complexes are [(C₅H_5−nMe_n)₂TiCl₂] (n = 0–5) and ansa-compounds [Me₂Si(C₅H_4−nMe_n)₂TiCl₂] (n = 0 or 4) (A1–A7), and [(C₅H₅)₂Ti(biphen)] (A8). Cationic complexes are [(C₅H₅)₂Ti(RCOCHCOR′)]⁺ with combinations of R, R′ = CF₃, C₄H₃S, C₄H₃O, Ph(C₆H₅) and CH₃ (B1–B7), and [(C₅H₅)₂Ti(indolato)]⁺ (B8) see Fig. 1.

Fig. 1 Substituted bis(cyclopentadienyl) Ti(IV) complexes investigated in this study. For the neutral [Cp₂Ti^IV(O,O′-ligand)] complex the bidentate O,O′-ligand is dianionic while for the cationic [Cp₂Ti^IV(O,O′-ligand)]⁺ complexes, it is monoanionic. Abbreviations used for the bidentate ligands: β-diketonato (RCOCHCOR′)⁻ = acac (R, R′ = CH₃, CH₃), ba = (Ph, CH₃), dbm = (Ph, Ph), tfaa = (CF₃, CH₃), tfth = (CF₃, Th), tffu = (CF₃, Fu) and tfba = (CF₃, Ph) and indolato = 3-acetyl-1-benzyl-2-hydroxy-5-methoxyindolate and biphen = 2,2′-biphenol.

2. Experimental

2.1 Theoretical approach

Density functional theory (DFT) calculations were performed using the PW91/TZP (triple ζ polarized) functional¹⁸ and basis sets as implemented in the Amsterdam Density Functional (ADF2014).¹⁹ Geometry relaxed (adiabatic) energies of the molecule (N electron system), the corresponding N − 1 (reduced) and N + 1 (oxidized) electron systems were calculated to determine the electron affinity (EA) and ionization potential (IP) values of complexes.

2.2 Electrochemistry

Electrochemical data of neutral and cationic bis(cyclopentadienyl) Ti^IV complexes form the basis of a density functional theory study presented in this contribution. The cyclic voltammetry measurements of the complexes used in this study, obtained from literature, were obtained under varying experimental conditions; series A1–A7 (THF),¹¹ series B2–B7 (CH₃CN)¹³ and B8 (C₃H₇CN).¹⁴ Cyclic voltammetry measurements done in this study for A1 (DCM), A8 and B1 (CH₃CN), were performed on 0.002 mole dm⁻³ solutions of the complexes in dry solvent with (ⁿBu₄N)(PF₆) as supporting electrolyte and using a glassy carbon working electrode, Pt auxiliary electrode, Ag/Ag⁺ reference electrode. All cited potentials were referenced against the Fc/Fc⁺ couple as suggested by IUPAC,²⁰ with ferrocene used as internal standard.

2.3 Synthesis of compounds

Titanocene dichloride, A1, (Fluka) was purified by recrystallization from hot toluene and complex B1 was prepared as described in ref. 13. In the preparation of [Cp₂Ti(biphen)], A8, titanocene dichloride, [Cp₂TiCl₂] (0.249 g/1.0 mmole) was dissolved in H₂O/THF, 2 : 1 mixture (9 ml) and stirred under nitrogen for ½ h. The solution was cooled down on an ice-bath before silver perchlorate, AgClO₄, (0.406 g/1.96 mmole) dissolved in water (2 ml) was added. The cooled mixture was stirred for a further ½ h. AgClO₄ is light sensitive and the reaction must be light protected. A white precipitate, AgCl, was filtered off and washed with H₂O/THF (2 ml). 2,2′-biphenol (0.372 g, 2 mmole) dissolved in cold THF (2 ml, ca. 4 °C) was added dropwise to the clear orange filtrate while stirring with an immediate colour change (orange to dark red). After ½ h of stirring the solution was removed and left for 4 days to precipitate at room temperature. The precipitate was washed with water, filtered and dissolved in diethyl ether. The solvent was removed with reduced pressure. The solid was washed with MeOH to remove unreacted 2,2′-biphenol yielding pure [Cp₂Ti(biphen)].

Yield 25% (0.0820 g). Colour: red. ¹H NMR (600 MHz, δ/ppm, acetone-d₆): 6.34 (s, 10H, 2× CpH), 6.71 (d, 2H; biphenH), 6.86 (t, 2H, biphenH), 7.18 (t, 2H, biphenH), 7.23 (t, 2H, biphenH). ¹H NMR (300 MHz, δ/ppm, CDCl₃): 6.26 (s, 10H, 2× CpH), 6.67 (d, 2H; biphenH), 6.93 (t, 2H, biphenH), 7.26 (m, 4H, biphenH). Elemental anal. calc. for TiC₂₂H₁₈O₂: C, 72.9; H, 5.0. Found: C, 73.1; H, 4.93%.

3. Results and discussion

3.1 Cyclic voltammetry

In the electrochemical reduction of neutral substituted Cp₂Ti^IV dichloro complexes, A1–A7 (THF¹¹ or DCM), the first two electron transfer reactions are; a reversible redox couple followed by an irreversible reduction peak with a large potential separation of ca. 1.2 V between the electron transfer reactions. See Fig. 2(a) for a representative CV of the series, A1. The first electron transfer was found to be completely reversible at scan rates of 0.05 V s⁻¹, followed by an irreversible reduction in THF.¹¹ The nature of the reduced species [Cp₂TiCl₂]⁻ and [Cp₂TiCl₂]²⁻ was not described,¹¹ though it was suggested that the second reduction could lead to halogen cleavage. In this study, on ground of the DFT study presented below, the first electron transfer, which is completely reversible at scan rates of 0.1 V s⁻¹ (Fig. 2(a)), is ascribed to Ti^IV/Ti^III reduction and the following irreversible reduction is assigned to a second metal Ti^III/Ti^II reduction.

Fig. 2 (a) Cyclic voltammograms (vs. Fc/Fc⁺) of A1, A8 and B1 in 0.1 mol dm⁻³ (ⁿBu₄N)(PF₆) at a scan rate of 0.100 V s⁻¹. A1 is conducted in DCM due to solvent coordination in CH₃CN, giving rise to two oxidation peaks as a result of the solvent coordinated complex.¹⁰ (b) Cyclic voltammograms (vs. Fc/Fc⁺) of [Cp₂Ti(biphen)]A8 in 0.1 mol dm⁻³ (ⁿBu₄N)(PF₆)/(CH₃CN), at scan rates 0.100–0.500 V s⁻¹ showing the dependence of reversibility on scan rate.

For [Cp₂Ti(biphen)] (A8), a one-electron irreversible Ti^IV/Ti^III reduction was observed in CH₃CN at a scan rate of 0.1 V s⁻¹ but as the scan rate increased the corresponding oxidation wave appeared at −1.63 V, with the current ratio, i_pa/i_pc dependent on scan rate (see Table 1 and Fig. 2(b)). However, in THF the reduction of [Cp₂Ti(biphen)] is completely reversible and the corresponding anion, stable enough to be detected by e.s.r. techniques.¹² The reported reduction potential, E_pc (vs. Fc/Fc⁺) = −1.68 V is in good agreement with the value reported in this study, −1.67 V (Table 2). The more negative reduction potential of [Cp₂Ti(biphen)] compared to [Cp₂TiCl₂], i.e., −1.7 vs. −1.3 V respectively, is expected because the Cl ligand is more electron withdrawing than oxygen (bonded to Ti in the O,O-bidentate biphen ligand) and hence the dichloro derivative A1 is easier to reduce than A8.

Table 1 Cyclic voltammetric data (vs. Fc/Fc⁺) in CH₃CN for 0.002 mol dm⁻³[Cp₂Ti^IV(biphen)] (A8) at indicated scan rates

ν (V s^−1)	E pc (V)	ΔEp (V)	E ^0′ (V)	i pc (10^6 A)	i pa/ipc
0.10	−1.701	—	—	—	—
0.20	−1.709	0.079	−1.670	36	0.40
0.30	−1.711	0.081	−1.671	56	0.51
0.40	−1.714	0.084	−1.672	76	0.61
0.50	−1.718	0.091	−1.673	93	0.69
1.00	−1.727	0.110	−1.673	160	0.86

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Table 2 Cyclic voltammetric data (vs. Fc/Fc⁺)^a in the indicated solvent. See Fig. 1 for complex numbering. CV data conducted by this study are reported at 0.10 V s⁻¹ while other studies report data at scan rates between 0.05–0.50 V s⁻¹

Complex	Reduction 1	Reduction 2	Free ligand
	E ^0′/V	E pc/V	E pc/V
A: neutral complexes	Ti^IV/Ti^III	Ti^III/Ti^II		Solvent	Ref.
a SCE relative NHE = 0.244 V and Fc/Fc^+ relative NHE = 0.400 V,^22 therefore Epcvs. Fc/Fc^+ relative SCE = −0.156 V (0.400–0.244 V).
A1 [Cp2TiCl2]	−1.19	−2.58	—	DCM	This study
A1	−1.16	—	—	DCM	10
A1	−0.90	—	—	CH3CN	10
A1	−1.28	—	—	THF	10
A1	−1.31	−2.53	—	THF	11
A2	−1.40	−2.58	—	THF	11
A3	−1.48	−2.64	—	THF	11
A4	−1.59	−2.70	—	THF	11
A5	−1.62	−2.73	—	THF	11
A6	−1.28	−2.52	—	THF	11
A7	−1.62	−2.60	—	THF	11
A8 [Cp2Ti(biphen)]	−1.67	—	−1.93	CH3CN	This study
	−1.68	—	—	THF	12

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Complex	Reduction 1	Reduction 2	Free ligand
	E^o′/V	E pc/V	E pc/V
B: cationic complexes	Ti^IV/Ti^III	ligand		Solvent	Ref.
B1 [Cp2Ti(acac)]^+	−0.85	−2.80	−2.56	CH3CN	This study
B1	−0.86	−2.85	—	C3H7CN	14
B2	−0.84	−2.31	−2.14	CH3CN	13 and 21
B2	−0.85	−2.33	—	C3H7CN	14
B3	−0.82	−2.11	−1.93	CH3CN	13 and 21
B4	−0.63	−2.16	−1.85	CH3CN	13 and 21
B5	−0.62	−1.89	−1.54	CH3CN	13 and 21
B6	−0.62	−1.92	−1.54	CH3CN	13 and 21
B7	−0.62	−1.92	−1.56	CH3CN	13 and 21
B8 [Cp2Ti(indolato)]^+	−0.85	−2.93	−2.63	C3H7CN	14

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A second reduction peak for A8 was not observed in this study, see Fig. 2(a) where the CV scan was reversed at −2.6 V. The second metal Ti^III/Ti^II reduction would only be expected around −3.0 V if the separation between the first and second reduction, was similar to the separation observed in A1. A ligand reduction could be possible for A8, since the free 2,2′-biphenol ligand is redox active, with an irreversible reduction, E_pc (vs. Fc/Fc⁺) = −1.93 V at 0.1 V s⁻¹ (see ESI†) similar to the irreversible reduction of free β-diketones²¹ and functionalized indole ligand.¹⁴

The electrochemical reduction of cationic [Cp₂Ti^IV(β-diketonato)]⁺ complexes B1–B7 in CH₃CN, yielded two separate electron transfers; a reversible redox couple followed by an irreversible reduction peak with a large potential separation between the electron transfer reactions, see Fig. 2(a) for a representative CV of the series, B1. The first reversible electron transfer, is assigned to Ti^IV/Ti^III reduction, while the second irreversible reduction is ascribed to a ligand-based β-diketonato reduction.¹³ The electrochemical reduction of [Cp₂Ti(indolato)]⁺ (B8), reported by Bond,¹⁴ is very similar to that of [Cp₂Ti(acac)]⁺ (B1), with a reversible Ti^IV/Ti^III reduction (E^0′ = −0.85 V and −0.85 V respectively) followed by the second ligand-based irreversible reduction (E_pc = −2.93 V and −2.80 V respectively). In both cases, the ligands themselves are redox active, E_pc = −2.56 and −2.63 V¹⁴ respectively for the free ligands of B1 and B8. In all the examples of this study, the chelation of the bidentate ligand to the metal centre, forming a delocalised π-system, results in the coordinated ligand possessing higher electron density (in the electroactive centre) relative to the uncoordinated ligand and hence is more difficult to reduce at a more negative potential. It is significantly easier to reduce the cationic complexes B1–B8 compared to the neutral complexes A1–A8 because there is a residual positive charge on the complex making it easier to accept an electron.

In summary the observed electrochemical behaviour in the potential range 0 to −2.70 V vs. Fc/Fc⁺, of the neutral and cationic bis(cyclopentadienyl) Ti(IV) derivatives can be simplified (not indicating solvent substitution) and described by the following reactions equations depending on the solvent used.

3.2 Theoretical approach

From a quantum chemistry view, the nature of the reduction centre of a complex can be identified by evaluating the character of the lowest unoccupied molecular orbital (LUMO) of the optimized complex, since reduction involves the addition of an electron into this orbital. Similarly the nature of the second reduction centre can be determined from the LUMO of the reduced complex if the first reduction is chemically reversible. DFT calculations were performed on representative samples of the neutral [Cp₂TiCl₂] (A1) and [Cp₂Ti(biphen)] (A8) and cationic [Cp₂Ti(acac)]⁺ (B1) and [Cp₂Ti(indolato)]⁺ (B8) derivatives. In all four cases, the DFT calculations showed that the first reductions are metal based (Ti^IV/Ti^III) as the LUMO is mainly of d_z² character localized on titanium, see Fig. 4 and 5. The distribution of the electron gained in the first reduction, as visualized by the Mulliken spin density plot of the reduced complex, is shown in Fig. 3.

Fig. 3 The Mulliken spin density plots of the first reduced species showing the distribution of the single unpaired electron located mainly on titanium.

Fig. 4 Molecular energy levels of selected frontier orbitals of complexes, left: [Cp₂TiCl₂] (A1) and right: [Cp₂Ti(biphen)] (A8), and their first and second reduced species. The blue arrows show the electron occupation. Unoccupied orbitals are shown in red. A contour of 6 × 10⁻⁵ e nm⁻³ has been used for the orbital plots. Y-Axis gives the relative energy in eV (1 eV = 96.48 kJ mol⁻¹). Colour code of atoms (online version): Ti (light green), C (black), O (red), H (white).

Fig. 5 Molecular energy levels of selected frontier orbitals of complexes, left: [Cp₂Ti(acac)]⁺: (B1) and right: [Cp₂Ti(indolato)]⁺ (B8), and their first and second reduced species. The blue arrows show the electron occupation. Unoccupied orbitals are shown in red. Y-Axis gives the relative energy in eV (1 eV = 96.48 kJ mol⁻¹). Colour code of atoms (online version): Ti (light green), C (black), O (red), H (white).

The nature of the second reduction centre, determined in the same manner as above but using the optimized reduced species, is more complicated and reveals that the character of the second reduction is not identical in the four examples. For A1, the lowest energy second reduced complex [Cp₂TiCl₂]²⁻ is diamagnetic (q = −2, S = 0) with the second electron added to the β LUMO which is mainly of d_z² character located on titanium (Fig. 4), consistent with, Ti^III/Ti^II second reduction. For A8, the lowest energy second reduced species [Cp₂Ti(biphen)]²⁻ is paramagnetic (q = −2, S = 1), 0.15 eV lower in energy than the diamagnetic option. The second electron is added to the α LUMO, mainly of d_x²−y² character and consistent with the second reduction also being the metal, Ti^III/Ti^II reduction (Fig. 4). The spin plot of the DFT calculated double reduced complex [Cp₂Ti(biphen)]²⁻ in Fig. 6 shows the distribution of the two added unpaired electron, with a configuration.

Fig. 6 The Mulliken density plots of the doubly reduced species showing the distribution of the two unpaired electrons; located mainly on titanium for [Cp₂Ti(biphen)]²⁻ while for [Cp₂Ti(acac)]⁻˙ and [Cp₂Ti(indolato)]⁻˙ the two electrons are located on both titanium and the ligand. [Cp₂TiCl₂]²⁻ is diamagnetic and therefore no unpaired spin density is observed.

For both the cationic complexes [Cp₂Ti(acac)]⁺ (B1) and [Cp₂Ti(indolato)]⁺ (B8), the lowest energy doubly reduced species are paramagnetic (q = −1, S = 1), with the second electron added to the α LUMO. However in this case, the α LUMO is located on the ligand, i.e., the β-diketonato ligand for [Cp₂Ti^III(acac)] and the indolato ligand for [Cp₂Ti^III(indolato)] (Fig. 5), consistent with the second reduction being ligand based. The spin density plots of [Cp₂Ti(acac)]⁻ and [Cp₂Ti^III(indolato)]⁻ (Fig. 6) show the distribution of the two added unpaired electrons, located on titanium as well as on the ligand. These results support the experimental cyclic voltammetry observations in which the cationic complexes B1 and B8 have a reversible metal reduction followed by an irreversible ligand reduction forming an unstable ligand radical. One can also conclude that although the second reduction of [Cp₂Ti(biphen)] (A8) is not observed experimentally by cyclic voltammetry due to the solvent window, one would predict a Ti^III/II reduction instead of the possible ligand reduction as is the case for B1 and B8. According these DFT results, [Cp₂Ti(biphen)] should behave is a similar manner to [Cp₂TiCl₂], with two consecutive metal reductions, even though it has a redox active ligand.

While the character of the LUMOs discussed above gave information on the reduction centres, the energy of the LUMOs is related to the reduction potentials; the lower the energy of the LUMO, the easier an electron can be accepted and the easier the reduction (less negative experimental E_pc). It has been shown that linear relationships exist between experimentally measured reduction potentials E_pc of a series of related compounds and calculated energies; i.e., LUMO energy (E_LUMO), electron affinity (EA), Mulliken electronegativity (χ), global electrophilicity index (ω) and to a variety of empirical quantities, used to describe the electron-withdrawing power of R substituents on the ligand, i.e., Gordy scale group electronegativities, Hammett substituent constants and Lever electronic parameters.^23–28

The following DFT calculated energies, reflecting the electronic properties of substituent groups on the electron density of the titanium, were used:

Energy of the LUMO	E LUMO
Electron affinity:	EA(complex) = E(reduced complex) − E(complex)
Mulliken electronegativity:^29–31	χ = (IP(complex) + EA(complex))/2
	where IP(complex) = E(oxidized complex) − E(complex)
Global electrophilicity index:^32	ω = (μ^2/2η)
	where μ = −(IP + EA)/2 and η = IP − EA

(DFT calculated energy values are recorded in Table S1 of the ESI†).

Strong correlations between LUMO energies, E_LUMO, and reduction potentials, E_pc, have been reported for a related series of compounds, substituted with electron-withdrawing or electron-donating groups. For example, the electro-reduction of functionalised β-diketones²¹ and nitrobenzenes³³ forming radical anions, yielded reduction potentials which gave excellent correlations in the linear relationship between experimental E_pcvs. calculated E_LUMO (R² = 0.99). In these investigations involving ligand reductions, the LUMOs presented effective π-conjugation between the electroactive centre and the substituent groups and hence the electronic properties of the substituents significantly influence the redox properties of the compounds. In the case of metal-centred reductions, the LUMOs can range from being highly metal localised with strong d character to having metal–ligand π-interactions. The investigation of octahedral [Ti(R¹COCHCOR²)₂biphen], containing electron withdrawing or electron donating substituents (R¹,R²), showed that the LUMOs had efficient β-diketonato-metal π-conjugation,³⁴ hence the observed reduction potentials were excellently correlated to LUMO energies (R² = 0.98).

In the present investigation the substituent groups are located at different sites; series A the substituents (R¹, R²…R⁵) are located on the Cp rings while in series B they are located on the β-diketonato ligand (RCOCHCOR′)⁻, see Fig. 1. The LUMO of A1 shows a small but significant π-interaction between titanium and the Cp rings, with 74% metal, and 9.0% Cp rings contributions (Fig. 7). The reduction potentials, E^0′, of A1–A7 correlate linearly with DFT calculated energies, E_LUMO, EA, χ and ω, (R² = 0.80–0,98), see Fig. 8, showing a moderately strong influence of the substituents on the electron density at titanium.

Fig. 7 LUMOs of [Cp₂Ti^IVCl₂] (A1), showing the π-interaction between the metal centre and the Cp rings, with a contour of 3 × 10⁻⁵ e nm⁻³. Colour code of atoms (online version): Ti (light green), C (black), O (red), H (white).

Fig. 8 The relationship between the formal reduction potential, E^0′, of titanium and the indicated DFT calculated energies for complexes A1–A7.

However, the LUMO of complex B1 is metal centred and highly localised with 82% titanium d character. No π-interaction between the metal centre and the acetylacetonato ligand are indicated, thus the electronic properties of the substituents (R,R′) of the β-diketonato ligand in B1–B7 are not expected to have a clearly defined influence on the reduction potential of the Ti^IV/Ti^III couple. This trend is observed in the poor correlation of the linear relationships between the formal reduction potential (E^0′) of the Ti^IV/Ti^III couple and the DFT calculated energies, E_LUMO, EA, χ and ω (see Fig. 9 (top)). The reduction potentials appear in two groups depending on the number of CF₃ groups present on the β-diketonato ligand. The strong electron-withdrawing CF₃ groups clearly affect the electron density at the metal center with a notable shift of 0.2 V in the reduction potentials. Since electron density at the redox centre is regulated by both inductive (through the σ-system) and resonance effects (through the π-system), if direct π resonance (as a result of p-orbital overlap) is absent, CF₃ substituent groups can withdraw electron density from the Ti^IV centre through strong electron-withdrawing induction. In a study of alkyloxy- and aryloxy-functionalized titanocenes, where the characteristic Ti^IV/III reduction was correlated to Hammett substituent constants, a relatively strong influence of the substituents on the electron density at titanium was observed.³⁵ However the LUMOs were not calculated, and thus the substituent effect could not be related to the nature of the LUMO.

Fig. 9 Top: The relationship between the formal reduction potential, E^0′, of Ti(IV) and the indicated DFT calculated energies for B1–B7. Bottom: The relationship between the reduction potential, E_pc, of the β-diketonato ligand and the indicated DFT calculated energies for reduced B1–B7 complexes.

In contrast, when considering the second reduction, the LUMO of the reduced B1 species [Cp₂Ti^III(acac)] is distributed over the acetylacetonato ligand with π-character enhancing the communication between the electro-active ligand centre and the substituent groups (R,R′). In this case the extensive communication between the β-diketonato ligand reduction centre and the substituents, suggest that the electronic properties of the substituent groups of the β-diketonato ligand should have a clearly determinable influence on the second reduction. This is indeed observed when relating the experimental second reduction potentials E_pc (that is ascribed to the reduction of the β-diketonato ligand) to the DFT calculated energies, E_LUMO, EA, χ and ω (see Fig. 9 (bottom)). There is significantly larger substituent effect on the second redox-potentials, than on the first reduction potentials. The energy values are more consistently distributed over the energy range with a linear fit of higher accuracy (R² = 0.96–0.99) compared to the relationships of the first reduction potentials, E^0′versus E_LUMO, EA, χ and ω in Fig. 7 (top), (R² = 0.49–0.95).

4. Conclusions

In summary we demonstrated that the nature of the reduction, i.e., metal-centred vs. ligand-based, of functionalised bis(cyclopentadienyl) titanium(IV) derivatives can be predicted using Quantum Chemistry Techniques. DFT calculations in agreement with reported CV data showed that the first experimentally observed reduction is metal based involving the reversible Ti^IV/III couple, and the second experimentally observed reduction is either metal or ligand-based depending on the electronic structure of the LUMO of the species involved. For the neutral titanium(IV) complexes, the second reduction is metal based, Ti^III/Ti^II, while for the cationic titanium(IV) complexes, it is a ligand reduction. The free bidentate O,O′-ligands, coordinated in both the neutral and cationic complexes, are also redox active but the reduction of the coordinated O,O′-ligands are only observed in the cationic complexes, in agreement with DFT calculations.

We have also shown the conditions needed in the LUMO for the electronic properties of the substituents on the ligands to influence the reduction potentials. The electron-withdrawing or electron-donating properties of the substituents directly influence electron density at the redox centre when π-interactions between the electro-active centre and the substituent groups exist in the LUMO. In this case, a number of relationships between the electronic properties of the substituents and the redox behaviour have been affirmed, showing excellent correlation in the linear relationship between experimentally measured reduction potentials vs. DFT calculated energies, E_LUMO, EA, χ and ω. This was demonstrated for the first reduction in Cp functionalized [Cp₂Ti^IVCl₂] derivatives, series A and the second reduction of β-diketonato functionalized [Cp₂Ti^IV(β-diketonato)]⁺ derivatives, series B. However, the substituent effect was erratic and nonlinear (poor correlation of linear relationships) for the first reduction of series B, confirmed by the LUMO being highly localised with no π-interaction between the metal centre and the substituents on the β-diketonato ligand. Hence using DFT we have been able to determine when it is possible to fine-tune the electronic properties of the electro-active metal, using functionalised ligands for the design of metallocene catalysts.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work has received support from the South African National Research Foundation and the Central Research Fund of the University of the Free State, Bloemfontein, South Africa. A grant of computer time from the Norwegian supercomputing program NOTUR (Grant No. NN4654K) is gratefully acknowledged.

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Footnote

† Electronic supplementary information (ESI) available: Additional electrochemical data, DFT calculated energies and optimized coordinates of the DFT calculations. See DOI: 10.1039/c7nj03746e

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