Synthesis, electrochemical and DFT study of octahedral bis(β-diketonato)-titanium(IV) complexes

Article abstract of DOI:10.1016/j.ica.2016.08.010

The synthesis and electrochemistry of seven new and four known bis(β-diketonato)-titanium(IV) complexes are reported. Integrated electrochemical and quantum chemistry techniques have been used to show that the reduction potential of this series of eleven bis(β-diketonato)-titanium(IV) complexes, [Ti(R¹COCHCOR²)₂biphen], are significantly influence by the electronic properties of the 1,3-β-diketonato-substituents (R¹, R²), due to the efficient β-diketonato-metal π-conjugated character of the lowest unoccupied molecular orbital (LUMO) of the complexes. Electron density in the redox center is controlled by both inductive and resonance effects as seen by the spin density plots. Electron-withdrawing substituents make the reduction potentials more positive by stabilizing the reduced species. Both metal and ligand based redox processes are observed; the reversible metal Ti^IV/Ti^IIIreduction is followed by a considerably more negative, irreversible ligand reduction. The density functional theory calculated electronic structure of the first reduction is a Ti^IIIspecies while the second reduced species, is Ti^IIIcoupled to a β-diketonato radical. A number of important relationships between the electronic properties of the substituents and the redox behaviour have been developed. The experimentally measured reduction potentials, E_pc, gave excellent correlation in the linear relationship between experimental E_pcvs. calculated LUMO energies, E_LUMO(R²?=?0.98).

Full text of DOI:10.1016/j.ica.2016.08.010

Accepted Manuscript
Synthesis, Electrochemical and DFT study of octahedral bis(β-diketonato)-ti-
tanium(IV) complexes
Annemarie Kuhn, Jeanet Conradie
PII:
S0020-1693(16)30462-5
http://dx.doi.org/10.1016/j.ica.2016.08.010
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Inorganica Chimica Acta
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Please cite this article as: A. Kuhn, J. Conradie, Synthesis, Electrochemical and DFT study of octahedral bis(β-
diketonato)-titanium(IV) complexes, Inorganica Chimica Acta (2016), doi: http://dx.doi.org/10.1016/j.ica.
2016.08.010
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Synthesis, Electrochemical and DFT study of octahedral bis(β-
diketonato)-titanium(IV) complexes
Annemarie Kuhn and Jeanet Conradie*
Department of Chemistry, PO Box 339, University of the Free State, Bloemfontein, 9300, South Africa,
Tel: 27-51-4012194, Fax: 27-51-4446384
conradj@ufs.ac.za
Abstract
The synthesis and electrochemistry of seven new and four known bis(β-diketonato)-titanium(IV)
complexes are reported. Integrated electrochemical and quantum chemistry techniques have been used
to show that the reduction potential of this series of eleven bis(β-diketonato)-titanium(IV) complexes,
[Ti(R1COCHCOR2)2biphen], are significantly influence by the electronic properties of the 1,3-β-
diketonato-substituents (R1, R2), due to the efficient β-diketonato-metal π-conjugated character of the
lowest unoccupied molecular orbital (LUMO) of the complexes. Electron density in the redox center is
controlled by both inductive and resonance effects as seen by the spin density plots. Electron-
withdrawing substituents make the reduction potentials more positive by stabilizing the reduced species.
Both metal and ligand based redox processes are observed; the reversible metal TiIV/TiIII reduction is
followed by a considerably more negative, irreversible ligand reduction. The density functional theory
calculated electronic structure of the first reduction is a TiIII species while the second reduced species, is
TiIII coupled to a β-diketonato radical. A number of important relationships between the electronic
properties of the substituents and the redox behavior have been developed. The experimentally
measured reduction potentials gave excellent correlation in the linear relationship between experimental
Epc vs calculated ELUMO (R2 = 0.98).
Keywords
Electrochemistry; DFT; β-diketone; titanium(IV); electronegativity
1

TOC
Research highlights
• Cyclic voltammetry of 11 bis(1,3-disubstituted-β-diketonato)-titanium(IV) complexes
• Formal reduction potential relates to electronic parameters of substituents.
• Formal reduction potential relates to DFT calculated energies of the LUMO.
• Formal reduction potential relates to DFT calculated electron affinities.
2

1. Introduction
The β-diketonates are a flexible class of chelating ligands which have been used in transition metal
chemistry for more than 100 years [1]. The well documented class of compounds, octahedral bis(β-
diketonato)titanium(IV) complexes arose, partly due to their application in the field onocology, for
example see Figure 1 for tumour-inhibiting budotitane [2,3,4,5,6]. In the field of catalysis, research into
post-metallocene catalysts for the polymerization of ethylene resulted in the development of a number
of highly active structurally similar bidentate chelated titanium(IV) catalysts, see Figure 1 [7,8,9,10,11].
Bis(β-diketonato)titanium-(IV) complexes may assist in the development of optically active
enantioselective catalysts. It has been shown that the bis(β-diketonato)titanium-(IV) fragment due to its
own dissymmetric (chiral) electronic structure, can discriminate between enantiomers of chiral π-donor
ligands, for example, binding O,O'- bidentate 1,1-bi-2-naphtholate ligand (BINOL) in bis(β-
diketonato)Ti(BINOL) resulted in an impressive, highly enantioselective process [12]. Applying this
electronic stereochemical control to catalytic reactions has opened new research efforts in catalysis.
Figure 1. (a) Antitumour agent, Budotitane beloning to the bis(diketonato)titanium-(IV) complexes and
structurally similar (b) highly active single-site ethylene polymerization titanium catalysts.
However, for these application the hydrolysis of titanium(IV) is a real and urgent problem [13], and
therefore water resistance titanium (IV) complexes, are extremely valuable. The rate of hydrolysis can
be significantly lowered by introducing electron-rich oxygen-based ligands. Here we have prepared 7
new bis(β-diketonato)-(2,2’-biphenyldiolato) titanium(IV) complexes containing three O,O′-chelated
bidentate ligands with enhanced hydrolytic stability [14,15]. Knowledge of the chemical and
electrochemical activity of these complexes towards reduction and oxidation, facilitates the study and
understanding of their mechanistic action during different catalytic processes. Hence we are interested
in how the electronic properties of the substituents (R1, R2 groups attached to the β-diketonate system)
influences the reduction potential of the TiIV/TiIII couple and in predicting redox potentials with the aim
3

of designing organometallic molecules with specific redox potentials. We hereby present a combined
electrochemical and computational chemistry (using DFT) study of a series of eleven
[TiIV(R1COCHCOR2)2biphen] complexes (seven are reported here for the first time) with substituents
ranging from electron-withdrawing (e.g. CF3) to electron-donating properties (e.g. CH3), with R1, R2 =
CF3, CF3 (1), CF3, Ph (2) [15], CF3, Fu (3), CF3, Th (4), CF3, CH3 (5), CF3, CH2(CH3) (6), CF3,
CH(CH3)2 (7), CF3, C(CH3)3 (8), Ph, Ph (9) [15] and Ph, CH3 (10) [15] and CH3, CH3 (11) [14] where
Ph = phenyl (C6H5), Fu = furyl (C4H3O) and Th = thienyl (C4H3S) and biphen = 2,2’-biphenyldiolato
(Figure 2). Linear relationships were obtained by comparing experimentally measured reduction
potentials Epc of the [Ti(β-diketonato)2biphen] series to calculated energies such as LUMO energy
(ELUMO), electron affinity (EA) and to a variety of electronic parameters describing the electron–
withdrawing power of the R1 and R2 substituents on the β-diketonato ligand (R1COCHCOR2)−.
Figure 2. [Ti(R1COCHCOR2)2biphen] complexes, with two strongly electron-withdrawing CF3
substituents (red), one CF3 substituent (green) and two non-CF3 electron-donating substituents, Ph, CH3
(blue). Abbreviations used for the β-diketonato ligand: (R1COCHCOR2)− = hfaa (R1, R2 = CF3, CF3),
tfba = (CF3, Ph), tffu = (CF3, Fu), tfth = (CF3, Th), tfaa = (CF3, CH3), tfma = (CF3, CH2CH3), tfdma =
(CF3, CH(CH3)2), tftma = (CF3, C(CH3)3), dbm = (Ph, Ph), ba = (Ph, CH3) and acac = (CH3, CH3).
2. Experimental and Computational Methods
2.1
General
NMR measurements were recorded on a Bruker Avance DPX 300 NMR spectrometer at 298 K.
Chemical shifts are reported as values relative to SiMe4 (0 ppm). Cyclic voltammetry was performed
utilizing a BAS 100 B/W Electrochemical Analyzer linked to a personal computer utilizing the
BAS100W Version 2.3 software.
4

2.2
Synthesis of [Ti(β-diketonato)2biphen] complexes (1) – (11)
The [Ti(β-diketonato)2Cl2] complexes used as starting material for the [Ti(β-diketonato)2biphen]
synthesis, were synthesised according to published methods [16,17] and the [Ti(β-diketonato)2biphen]
complexes were synthesized with slight changes as reported previously [14, 15]. To stirred solution of
2,2’-biphenyldiol (0.186g, 1 mmol in 15 ml CH3CN) at RT, a solution of [Ti(β-diketonato)2Cl2] (1
mmol in 15 ml CH3CN) was added dropwise at room temperature with an immediate colour change
(clear/grey to orange/red). After refluxing for 4.6 h, the reaction mixture was cooled and solvent
evaporated to dryness. The residue was washed in MeOH (non-fluorinated [Ti(β-diketonato)2biphen]
complexes) or CH3CN (fluorinated [Ti(β)2biphen] complexes) to dissolve unreacted biphenol. The pure
product, obtained after recrystallization from dichloromethane/n-hexane, was stored under N2
atmosphere. The characterization data of the complexes is provided in the Supporting Information.
2.3
Cyclic Voltammetry
Cyclic voltammetry measurements [18,19] were performed on 0.002 mol dm-3 compound solutions
in dry acetonitrile (Aldrich, Biotech grade 99.93+% purity, anhydrous, kept under purified argon)
containing 0.1 mol dm-3 tetra-n-butylammonium hexafluorophosphate, (nBu4N)(PF6) (Fluka
electrochemical grade), as supporting electrolyte and under a blanket of purified argon at 25 °C. A
three-electrode cell, with a glassy carbon (surface area 7.07 x 10-6 m2) working electrode, Pt auxiliary
electrode and a Ag/Ag+ (0.010 mol dm-3 AgNO3 in CH3CN) reference electrode [20] mounted on a
Luggin capillary, was used [21,22]. All temperatures were kept constant to within 0.5 °C. Successive
experiments under the same experimental conditions showed that all reduction and formal reduction
potentials were reproducible within 0.005 V. All cited potentials were referenced against the Fc/Fc+
couple as suggested by IUPAC [23]. Ferrocene exhibited a formal reduction potential E°' = 0.072 V vs.
Ag/Ag+, a peak separation ∆Ep = Epa – Epc = 0.070 V and ipc/ipa = 0.99 under our experimental
conditions. Epa (Epc) = anodic (cathodic) peak potential and ipa (ipc) = anodic (cathodic) peak current. E°'
(Fc/Fc+) = 0.400 V vs. NHE [24] and SCE = 0.244 V vs. NHE.
2.4
Quantum Computational Methods and empirical parameters
Density functional theory (DFT) calculations were performed using the PW91/TZP (Triple ζ
polarized) functional [25] and basis sets as implemented in the Amsterdam Density Functional
(ADF2013) [26]. The functional, PW91, previously gave excellent results for geometry (good
agreement between calculated and experimental) and for energies (good agreement between calculated
and experimental isomer distribution) [15]. Geometry relaxed (adiabatic) energies of the neutral
5

molecule (N electron system) and the N+1 (anion) and N+2 (dianion) electron systems, were calculated.
These energies were used to obtain the electron affinity (EA) values of complexes, as calculated by:
EA(complex) = E(complex)− − E(complex).
To determine the correct spin state of the dianion, namely S = 0 or S = 1, calculations with a
selection of additional functionals were done, namely the GGA functionals OLYP (Handy-Cohen and
Lee-Yang-Parr) [27,28,29,30], S12g [31], OPBE [32], the meta-GGA functionals M06-L [33], TPSS
[34,35] as well as the hybrid functionals B3LYP (Becke 1993 and Lee-Yang-Parr) [36], B3LYP* [37]
and S12h [31].
Solvent effects were taken into account for all optimized structures reported here. The COSMO
(Conductor like Screening Model) model of solvation [38,39,40] was used as implemented [41] in ADF.
The cavity type was Esurf [42] and the solvent used was CH3CN.
Different empirical parameters can be used to quantify the electronic effect of the various
substituents, R1 and R2, of the β-diketonato ligand, (R1COCHCOR2)−, i.e., Gordy scale group
electronegativities, χR1 + χR2 [43,44], Hammett meta substituent constant, σR1 + σR2 [45,46], and the
Lever electronic parameter, EL [47,48,49,50]. The pKa of the free β-diketone [51,52,53], describing the
acidic strength of the free β-diketone, can also be used to describe the influence of the β-diketonato
ligand on the metal to which it is coordinated.
3. Results and Discussion
3.1 Preparation and solution behaviour of bis(β-diketonato)-(2,2’-biphenyldiolato) titanium(IV)
The reaction of 2,2'-biphenol in a 1:1 ratio with [Ti(β-diketonato)2Cl2] in CH3CN yielded, [Ti(β-
diketonato)2biphen]. Using even higher ligand ratios of 2:1 or 3:1, yielded the same product instead of
titanium products with more than one biphenolato ligand in the coordination sphere. This is similar to
what was found for the reaction between Ti(acac)2Cl2 and 1,1'-methylene-di-2-naphthol.[54]. The
biphenolato complexes exhibit high hydrolytic stability compared to the parent compounds; tested under
the same conditions (0.01 % water/CH3CN ) as used by Keppler and Heim [55], it was found that the
biphenolato complexes precipitate only slightly within 4 weeks compared to the dichloro complexes
which precipitate within seconds. The solubility and hydrolytic stability of the CF3-substituted bis(β-
diketonato) biphen titanium(IV) complexes (soluble in DCM, chloroform, acetonitrile, acetone) differs
from those of the non-CF3 analogues; for example, where the former are soluble in acetonitrile but
hydrolyze more easily, the latter are poorly soluble but more stable to hydrolysis.
The solution behavior of the parent compounds, bis(β-diketonato) dichloro titanium(IV) is well
documented [16,17,56]. They exist as an equilibrium mixture of monomeric isomers which are
6

fluxional and mutually transform each other on the time scale of nuclear magnetic resonance (NMR).
Further, the CF3 substituted analogues undergo a partial hydrolysis reaction, in which the hydrolyzed
dinuclear {Ti(β-diketonato)2Cl}2(µ-O) is in equilibrium with the mononuclear [Ti(β-diketonato)2Cl2]
[57,58].
1
Results obtained from variable temperature H and 19F NMR spectroscopy showed that bis(β-
diketonato)biphenolato complexes (with asymmetric β-diketonate ligands) also exist as an equilibrium
mixture of 3 cis-geometric isomers. However, there was no evidence for the formation of a partially
hydrolyzed complex or the spontaneous equilibrium (hydrolysis) dimerization reaction in solution as is
the case for the parent dichloro compound. The solid state, partially hydrolyzed, oxo- and biphenolato-
bridged dinuclear, complex [59] (Figure 3) has been reported but its formation is through the chelation
of H2biphen with the parent dinuclear {Ti(β-diketonato)2Cl}2(µ-O) complex (in equilibrium with the
mononuclear [Ti(β-diketonato)2Cl2] as indicated above) and not from the hydrolysis of [Ti(β-
diketonato)2biphen].
Figure 3. Structure of hydrolysed dimeric, {Ti(hfaa)2}2(µ-O)(µ-biphen) complex (right), with a
bridging biphenolato ligand [59] that formed from {Ti(hfaa)2Cl}2(µ-O) (left).
3.2 Computational Chemistry Investigation
The neutral and reduced [TiIV(R1COCHCOR2)2biphen] complexes (1) – (11) were optimized using
DFT. The optimized coordinates are provided in the Electronic Supporting Information, the calculated
energies of the LUMO (lowest unoccupied molecular orbitals) (ELUMO) and calculated electron affinity
(EA) are given in Table 1. The relative energies (eV), calculated by a selection of functionals for the
dianion of complex (11), [Ti(acac)2biphen]-2, are given in Table 2. The results show that the dianion is
paramagnetic with S = 1.
Insight into the electronic structure of the [Ti(β-diketonato)2biphen] complexes allows one to
understand and predict the electrochemical behavior. Frontier molecular orbitals, HOMOs (highest
occupied molecular orbitals) and LUMOs, play an important role in redox reactions, for example,
7

during reduction, an electron is transferred into the LUMO of the neutral species and this gained
electron will be observed in the HOMO of the reduced species. Evaluation of the frontier molecular
orbitals of the [Ti(β-diketonato)2biphen] complexes, showed that the top three HOMOs are mainly
biphenol-ligand based, the LUMO is mainly metal based, and the LUMO+1 is mainly β-diketonato-
ligand based, with none of the first five LUMOs being biphenol-ligand based.
In most of cases reduction occur by adding an electron to the lowest unoccupied molecular orbital
(LUMO) of a complex. However, sometimes molecular orbital reorganization occurs after the addition
of an electron, leading to the use of a higher unoccupied orbital [60]. Therefore the frontier orbitals of
the first reduced anion and the HOMO of the second reduced dianion of [TiIV(R1COCHCOR2)2biphen]
complexes (1) – (11) have been examined. Figure 4 show the DFT calculated frontier orbitals of (a)
neutral [TiIV(ba)2biphen], HOMO which is ligand-biphenol based and LUMO which is mainly metal
based; (b) HOMO of the first reduced anion, [Ti(ba)2biphen]− which is mainly metal based (very similar
to the LUMO of the neutral species) and (c) the HOMO of the second reduced dianion,
[Ti(ba)2biphen]−2, which is mainly ligand-β-diketonato based. From these molecular orbitals, it can be
deduced that a first reduction should be (Ti(IV)→Ti(III)) and a second reduction, β-diketonato→β-
diketonato−. The character of the top five LUMOs of the neutral complex supports that a
biphenyldiolato ligand reduction would be unlikely.
Figure 4. DFT presentation of frontier molecular orbitals of (a) neutral [Ti(ba)2biphen], (b) first
reduced anion (q = -1, S = ½) and (c) second reduced dianion (q = -2, S = 1)
8

The Mulliken spin density plots in Figure 5 (a) – (c) portray the electron density of the added
unpaired electron in the HOMO of the reduced complexes (9) – (11) and Figure 5 (d), of the two added
unpaired electrons of the dianionic species of (10). These spin density plots illustrates the efficient β-
diketonato-metal π-conjugated character of the orbitals and show the effects of electron-withdrawing
induction (as a result of substituent group electronegativities) and resonance (as a result of π-orbital
overlap). Resonance stabilizes the complex by delocalizing charge from the β-diketonato backbone into
an aromatic side group(s). As the number of phenyl groups attached to the β-diketonato ligand
increases; the relative electron spin density on TiIII decreases, i.e., the Mulliken spin populations are
0.635, 0.418 and 0.331 respectively for complexes with β-diketonato ligands, acac (CH3, CH3) (11), ba
(CH3, Ph) (10) and dbm (Ph, Ph) (9), with the remaining electron density distributed over the β-
diketonato backbone and phenyl rings. Similarly, as the inductive electron–withdrawing CF3
substituents increase, the relative electron density on TiIII decreases i.e., 0.635, 0.492 and 0.381
respectively for complexes with β-diketonato ligands, acac (CH3, CH3) (11), tfaa (CF3, CH3) (5) and
hfaa (CF3, CF3) (1) (See Figure S1 of the Supporting information).
Electron spin density on Ti for the dianionic species of (11), (10) and (9) are 0.857, 0.485 and 0.353
respectively, with the rest of the unpaired spin (ca. 1.1, 1.5 and 1.6 e- respectively) distributed mainly
over the β-diketonato ligands. Thus the dianionic species of [Ti(β-diketonato)2(biphen)] can be
described as titanium(III)-β-diketonato radical complexes, see the spin density plot of
[Ti(ba)2(biphen)]2-, Figure 5 (d).
Figure 5. Mulliken spin density plots of the reduced complexes (a) [Ti(acac)2biphen]− (11) (b)
[Ti(ba)2biphen]− (10), (c) [Ti(dbm)2biphen]− (9) and dianion (d) [Ti(ba)2biphen]2- (10) . The electron
9

density on TiIII for the anions in (a) – (c) is 0.635, 0.418 and 0.331 respectively. The spin plots use an
electron contour of 0.002 e/Å3. Colour code of atoms (online version): Ti (purple), C (black), O (red), H
(white).
The electrochemical study presented below focuses on the first redox process. A linear relationship
between the calculated energy of the LUMO (ELUMO) of the neutral species and the experimental
reduction potential (Epc) can theoretically be derived [61]. The energy needed to reduce a complex
(measured in volts as the reduction potential Epc), is also closely related to electron affinity of a complex
(EA, the amount of energy released when an electron is added to a molecule in the gaseous state)
[62,63]. The DFT calculated energies, ELUMO and EA are given in Table 1 for complexes (1) – (11) and
discussed further in section on Relationships.
3.3
Cyclic Voltammetry
The electrochemical data for the TiIV/TiIII couple at 0.100 V s-1 is summarised in Table 1 and the
comparative cyclic voltammograms, shown in Figure 7. Data and CV graphs for scan rates 0.050 -
0.500 V s-1 are provided in the supporting information. Complexes (2) - (11) exhibit a distinctive one
electron, electrochemically reversible TiIV/TiIII couple according to Scheme 1 with 0.062 ≤ ∆Ep ≤ 0.091
V which are similar to that of the ferrocene under the same conditions (∆Ep = 0.070 V). The redox
process is completely irreversible for (1). The redox process for (2) - (11) is chemically partially
reversible, with 0.56 ≤ ipa/ipc ≤ 0.94, at ν = 0.100 V s-1, with relatively constant ipa/ipc ratios over the
scanning rate of 0.100 – 0.500 V s-1. Figure 6 gives the CVs of [Ti(tfth)2biphen], (4), at different scan
rates. The linear plot of the difference in current intensity of the anodic and cathodic peaks (ipa - ipc)
against the square root of the scan rate, also given in Figure 6, supports that the first reduction wave is
reversible.
[TiIII(β-diketonato)2biphen]
[TiIII(β-diketonato)2biphen]
[TiIV(β-diketonato)2biphen] + e-
Scheme 1. The reduction of [Ti(β-diketonato)2biphen].
10

Figure 6. Cyclic voltammograms (vs. Fc/Fc+) of [Ti(tfth)2biphen] (0.002 mol dm-3) at scan rates 0.050,
0.100, 0.200, 0.300, 0.400 and 0.500 V s-1.
A comparison of the cyclic voltammograms of the [Ti(β-diketonato)2biphen] series, listed in order of
increasing reduction potentials, see Figure 7, show that the ease of reduction depends on the number of
strongly electron-withdrawing CF3 groups in the complex. Complex (1) containing β-diketonato ligands
with two strongly electron-withdrawing CF3 groups, is reduced most easily (has the most anodic
(positive) potential, Epc = -0.730 V) followed by complexes (2) – (8) containing β-diketonato ligands
with one CF3 groups (average Epc = -1.050 V) and reduced least easily, are complexes (9) – (11)
containing non-fluorinated β-diketonato ligands with weakly electron-donating CH3 and Ph groups
(average Epc = -1.360 V). The difference in reduction potentials between the three groups is ca. 0.320 V
and 0.310 V respectively. The significant influence of the electronic properties of substituent groups, R1
and R2, on reduction potentials, suggests that efficient electronic communication via conjugation
between the electro-active metal centre and the substituent R groups exists (supported by DFT
calculated electronic structures, see Figure 4 and Figure 5). The electronic effect of the substituents
other than CF3, i.e., R = Ph, Fu, Th, CH3, CH2CH3, CH(CH3)2 and C(CH3)3 is slighter, for example, the
[Ti(CF3COCHCOR)2biphen] group has a difference in reduction potentials of ca. 0.100 V (see the CVs
of complexes (2) – (8) in Figure 7). Electron density at the metal center is lowered by an interplay of
inductive effects through the σ-system (if the substituent is electron-withdrawing) and resonance effects
through the π-system (if direct resonance between the substituent group and the electro-active center in
11

the reduced species exists) see Figure 5. For this reason, complexes containing ring-structures i.e., Ph =
C6H5, Fu = C4H3O and Th = C4H3S (e.g. complexes (2) – (4)) tend to be reduced more easily than
complexes containing chain or branched-structures, i.e., CH3, CH2CH3, CH(CH3)2 and C(CH3)3 (e.g.
complexes (5) – (8)). The ring-structures, Ph, Fu, Th, have lower group electronegativities (see Table
1), i.e., weaker σ-electron-withdrawing power but stronger π-electron-withdrawing power compared to
the chain structures. This trend is observed clearly for the reduction of the free β-diketone [64].
The branched structures, complexes (5) – (8) show a small but meaningful shift towards more
negative reduction potentials when increasing the branched size of the R2 substituent from Me, Et, iPr to
tBu, the potentials are -1.030 V (5), -1.075 V (6), -1.076 V (7) and -1.108 V (8) (see Table 1). This
trend follows the σ-electron-donating order of Me < Et < iPr < tBu.
12

Figure 7. Comparative cyclic voltammograms (vs. Fc/Fc+) at a scan rate of 0.100 V s-1 for
[Ti(R1COCHCOR2)2biphen] series (1) – (11) (0.002 mol dm-3), showing the TiIV/TiIII couple. The CVs
shown in red represent the complex containing β-diketonato ligands with two strongly electron-
withdrawing CF3 substituents, those in green with one CF3 substituent and those in blue contain, non-
fluorinated electron-donating substituents, Ph, CH3.
A wide potential scan of [Ti(ba)2biphen] see Figure 8, shows two redox processes; the TiIV/III redox
couple (Epc = -1.387 V vs Fc/Fc+) and a more cathodic irreversible reduction at Epc = -2.206 V vs
Fc/Fc+. Uncoordinated β-diketones are electrochemically active with reported reduction potentials
ranging from -1.200 to -2.100 V for a similar series of β-diketones (Epc = -1.243 V for Hhfaa and Epc = -
2.138 V for Hba) [64]. The reduction potential of a metal-chelated β-diketonato ligand, generally
occurs at a more negative cathodic potential than for uncoordinated β-diketones [65], because the
coordination of the β-diketone to a metal centre, forming a conjugated ring system, results in the
coordinated β-diketonato ligand possessing higher electron density compared to the uncoordinated β-
diketone. Therefore it can be inferred that the second redox process is a ligand reduction. This is
further supported by the DFT investigation in which the frontier molecular orbitals of the optimized
anion [Ti(ba)2biphen]¯ indicates that the first reduction is metal based while the optimized dianion
[Ti(ba)2biphen]2 indicates that the second reduction is ligand based (see Figure 4). The CVs of
¯
uncoordinated β-diketone, Hba, and coordinated ligand ba in [Ti(ba)2biphen] (10), are compared in
Figure 8, showing a negative potential shift of 0.08 V. This is in the same order as the shift of 0.17 V
for the coordinated ba ligand in tetrahedral [Cp2TiIV(ba)]+ClO4¯ [65] (Cp = cyclopentadienyl).
Figure 8. Wide potential scan (vs. Fc/Fc+) of [Ti(ba)2biphen] (0.002 mol dm-3) at 200 mV s-1 with
ferrocene used as an internal standard (Fc/Fc+, E0' = 0 V). CV of uncoordinated ligand Hba reduction,
(blue) is inserted for comparison.
13

Comparative cyclic voltammograms of [TiIV(β-diketonato)2biphen] and the related parent compound,
[TiIV(β-diketonato)2Cl2] [66] show an anodic shift of 0.600 V (see Figure 9). The chloro-ligands
withdraw more electron density from metal centre compared to the electron donating O,O′-
biphenyldiolato ligands, making titanium in [Ti(β-diketonato)2Cl2] easier to reduce (at less negative
potentials) than the corresponding [TiIV(β-diketonato)2biphen] complexes. This effect can also be seen
from the DFT calculated Mulliken charge on titanium, i.e., 1.225 in [TiIV(acac)2Cl2], is lower than 1.794
in [TiIV(acac)2biphen]. The substituents (R1, R2) of the β-diketonato ligand appear to have similar
electronic influences on the titanium(IV) reduction for both [TiIV(β-diketonato)2biphen] and [Ti(β-
diketonato)2Cl2]; for example, a shift of 0.34 V for CF3-β-diketonato (tfba and tfaa) complexes,
compared to non-CF3-β-diketonato (dbm and ba) complexes, is observed (see Figure 9) suggesting that
the complexes might have similar conjugated LUMO systems. The spin density plots (see Figure 10) of
these two (reduced) bis(β-diketonato)-titanium(IV) complexes, [Ti(acac)2Cl2]− and [Ti(acac)2biphen]−,
showing the distribution of the added unpaired electron, illustrate the close resemblance of the orbitals
involved in the reduction, as well as the effective β-diketonato-metal π-conjugated nature of the orbitals.
The related behaviour is important because one can anticipate the reduction potentials of the dichloro
complexes (based on the biphen complexes), which are otherwise difficult to observe due to complex
instability (hydrolysis) [57,58], adsorption of the complex at the electrode surface and electrode
passivation [66].
14

Figure 9. Comparative cyclic voltammograms (vs. Fc/Fc+) at a scan rate of 0.100 V s-1 for
[Ti(R1COCHCOR2)2biphen] and [Ti(R1COCHCOR2)2Cl2] series (0.002 mol dm-3), showing the
TiIV/TiIII couple. The CVs shown in green represent the complexes containing β-diketonato ligands
with one strongly electron-withdrawing CF3 substituents and those in blue contain, non-fluorinated
electron-donating substituents, Ph, CH3. Vertical dotted lines indicate average Epc values.
Figure 10. Mulliken spin density plots of the reduced complexes (a) [Ti(acac)2biphen]− (11) and (b)
[Ti(acac)2Cl2]−.
15

Table 1. Empirical parameters, Gordy scale group electronegativities χR1 + χR2, Hammett meta substituent constant, σR1 + σR2, and the Lever
electronic parameter, EL and pKa of the free β-diketone, Calculated LUMO energies (ELUMO), electron affinities (EA), and experimental reduction
potential (Epc vs. Fc/Fc+) of [TiIV(R1COCHCOR2)2biphen] complexes (1) – (11).
σR + σR'
EA
(eV)
ELUMO
(eV)
Epc vs
Fc/Fc+ (V)
E°' vs
Complex
no
χR + χR'
a
R1
R2
pKaa
EL
β-diketonato
a,c
a,b
Fc/Fc+ (V)
hfaa
tfba
tffu
1
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CF3
Ph
CF3
Ph
4.71
6.30
6.02
5.22
5.24
0.86
0.49
0.49
0.52
0.361
0.36
-
0.17
0.05
0.05
0.05
0.03
-
3.055 -4.524
2.559 -3.834
2.491 -3.776
2.525 -3.790
2.374 -3.832
2.354 -3.763
2.371 -3.773
2.370 -3.735
2.209 -3.338
1.824 -3.086
1.677 -2.571
-0.730
-0.991
-1.012
-1.028
-1.030
-1.075
-1.076
-1.108
-1.358
-1.387
-1.388
2
-0.960
-0.971
-0.990
-0.985
-1.030
-1.031
-1.063
-1.277
-1.344
-1.348
3
Fu
tfth
4
Th
6.497 5.11
tfaa
5
CH3
CH2CH3
6.30
6.64
5.35
5.32
5.30
5.28
4.42
4.55
4.68
tfma
tfdma
tftma
dbm
ba
6
7
CH(CH3)2 6.80
-
8
C(CH3)3
Ph
7.13
9.35
8.70
8.95
0.33
0.12
-0.009
-0.138
-
9
-0.04
-0.06
-0.08
10
11
CH3
CH3
Ph
acac
CH3
a
For a comprehensive list of pKa values, Gordy scale group electronegativities, Hammett meta substituent constants and the Lever electronic
parameter related to β-diketones, see reference [67] (Appendix).
b The group electronegativity (χR + χR') on the Gordy scale: χCF3 = 3.01, χPh = 2.21, χFu = 2.23, χTh = 2.10, χCH3 = 2.34, and χCH2CH3 = 2.27, χCH(CH3)2
= 2.27 and χC(CH3)3 = 2.27.
c The Hammett meta substituent σ constants: σCF3 = 0.43, σPh = 0.06, σFu = 0.06, σTh = 0.09, σCH3 = -0.069, σCH2CH3 = -0.07, σC(CH3)3 = -0.10,.
16

Table 2. Relative energies (eV) calculated by a selection of functionals for [Ti(acac)2biphen]-2.
S
meta-GGA
hybrid
GGA
M06-L TPSS B3LYP B3LYP*
S12h OLYP OPBE BP86 S12g
0
1
0.06
0.00
0.06
0.00
0.20
0.00
0.14
0.00
0.25
0.00
0.08
0.00
0.10
0.00
0.03
0.00
0.01
0.00
3.4
Relationships
The reduction potential Epc for the reversible TiIV/III redox process of [TiIV(R1COCHCOR2)2biphen]
complexes (1) – (11) are compare to calculated energies of the LUMO (ELUMO) and electron affinity
(EA), as well as to empirical parameters that are related to the electron density on the β-diketone, i.e.,
pKa of the β-diketone (R1COCH2COR2), the sum of the group electronegativities χR1 + χR2 of the
substituents, R1 and R2, the sum of the Hammett constants σR1 + σR2 and Lever electronic parameter, EL.
(tabulated in Table 1). The linear relationships between reduction potentials Epc and these parameters
are illustrated in Figure 11 and Figure 12.
An excellent agreement between Epc and the calculated ELUMO (R2 = 0.98 for gas phase calculations
and 0.94 for calculations including CH3CN as solvent) exits (see Figure 11), hence the nature of the
LUMO dominates the redox chemistry. The LUMO, although located mainly on the metal, has a
delocalized π-electron system, distributed over the conjugated β-diketone backbone and aryl substitutes
(Figure 4), allowing for communication between the metal and substituents, R1 and R2, of the β-
diketonato ligand. The Mulliken spin plots (Figure 5) clearly show that the unpaired electron density is
distributed over the β-diketone backbone and aryl substitutes. The excellent correlation Epc vs ELUMO for
gas phase calculations makes it possible to predict redox potentials based on the LUMO energies, with a
high degree of accuracy. The correlation between Epc vs. EA (R2 = 0.92 for gas phase calculations and
0.96 for calculations using CH3CN as solvent) can also be used. For Epc vs. EA a better accuracy for
calculations using the continuum model for solvent simulation than for gas phase calculations is
obtained, while for Epc vs ELUMO, the gas phase calculations gave a better accuracy.
17

Figure 11. The linear relationships between reduction potential Epc of [TiIV(R1COCHCOR2)2biphen]
and calculated energies of the LUMO (ELUMO) and electron affinity (EA). The values shown in red
represent the complex containing β-diketonato ligands with two strongly electron-withdrawing CF3
substituents, those in green with one CF3 substituent and those in blue contain, non-fluorinated electron-
donating substituents, Ph, CH3.
The correlation of Epc of metal-β-diketonato complexes vs the selected empirical parameters (which
quantify the electronic effect of the various substituents, R1 and R2, of the β-diketonato ligand,
(R1COCHCOR2)−, generally show good linear fits [65,66,67,68,69,70].
Results for the
[TiIV(R1COCHCOR2)2biphen] complexes are presented in Figure 12. In general as the electron
withdrawing ability of the substituent groups increase, the electron density at the titanium metal center
decreased and the complex is reduced more readily at a more positive (less negative) potential. This
once again suggests that efficient electronic communication exists between the substituent R groups and
the electro-active titanium metal center (supported by DFT calculate electronic structures). However,
the (Gordy scale) group electronegativities, χR1 + χR2 performed the worst as an indicator for reduction
18

potentials (see Figure 12, R2 = 0.89) because this parameter relies solely on inductive electron
withdrawing effects of the substituents while the electron density at the metal center is reduced by an
interplay of inductive effects through the σ-system (if the substituent is electron-withdrawing) and
resonance effects through the π-system (if the complex contains aromatic groups R1 and/or R2). This
results in the opposite trend for the following [Ti(β-diketonato)2biphen] complexes which contain
aromatic substituents; (dbm, χR1 + χR2 = 4.42, Epc = -1.317 V) compared to (ba, χR1 + χR2 = 4.55, Epc = -
1.387 V) and (tfba, χR1 + χR2 = 5.22, Epc = -0.960 V) compared to (tfaa, , χR1 + χR2 = 5.35, Epc = -1.030
V), i.e. it appears as if the electron withdrawing ability of the substituent groups increases but the Ti
reduces less readily. However this contradiction occurs because the phenyl group withdraws electron
density from the metal center through resonance effects, (see this effect in the spin density plots, Figure
5).
Figure 12. The linear relationships between reduction potential Epc of [TiIV(R1COCHCOR2)2biphen]
and (a) pKa of the β-diketone, (b) the group electronegativities χR1 + χR2 of the substituents, R1 and R2,
(c) the sum of Hammett constants σR1 + σR2 of R1 and R2 and (d) Lever electronic parameter, EL. The
values shown in red represent the complex containing β-diketonato ligands with two strongly electron-
withdrawing CF3 substituents, those in green with one CF3 substituent and those in blue contain, non-
fluorinated electron-donating substituents, Ph, CH3.
19

4 Conclusion
[TiIV(β-diketonato)2biphen] complexes undergo two redox processes in acetonitrile; a reversible
metal TiIV/TiIII reduction followed by a considerably more negative, irreversible ligand reduction. The
electronic properties of the 1,3-β-diketonate (R1COCHCOR2)- substituents significantly influenced the
value of the TiIV/III reduction potential indicating excellent electronic communication between the
substituents R1, R2 and the electro-active titanium(IV) center. Electron withdrawing groups make
reduction potentials more positive by stabilizing the reduced species, while electron-donating
substituents have the opposite effect. Several important relationships between the substituents and
redox behaviour were developed, i.e., linear trends between the reduction potentials Epc of the TiIV/III
couple and DFT calculated parameters (LUMO energies, ELUMO and electron affinity, EA), and between
several electronic parameters used to describe the electron–withdrawing power of the R1 and R2
substituents. We have shown that redox-potentials give an excellent correlation in the linear
relationship with calculated LUMO energies (R2 = 0.98), and it is therefore possible to predict redox
potentials with a high degree of accuracy. The poor correlation of Epc vs group electonegativities of the
R1 and R2 substituents on the β-diketonato ligand, is due aromatic substituents being more electron-
withdrawing than predicted by the exclusively inductive group electonegativity parameter. The spin
density plots of the reduced species clearly show the resonance electron–withdrawing effect of aromatic
substituents.
Acknowledgment.
The National Research Foundation of South Africa and the Central Research Fund of the University of
the Free State, Bloemfontein.
Supporting Information.
Tables, Figures, Characterization data, optimized coordinates. This information is available free of
charge via the Internet.
20

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Graphical abstract
Table of content briefs
CV and DFT of bis(β-diketonato)-titanium(IV) complexes