Substitution kinetics of biphenol at dichlorobis(acetylacetonato-O,O′)titanium(IV): Isolation, characterization, crystal structure and enhanced hydrolytic stability of the product bis(acetylacetonato-O,O′)(biphenyldiolato-O,O′)titanium(IV)

Polyhedron 28 (2009) 209–214

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Substitution kinetics of biphenol at dichlorobis(acetylacetonato-O,O0 )titanium(IV): Isolation, characterization, crystal structure and enhanced hydrolytic stability of the product bis(acetylacetonato-O,O0 )(biphenyldiolato-O,O0 )titanium(IV) Tsietsi A. Tsotetsi, Annemarie Kuhn, Alfred Muller, Jeanet Conradie * Department of Chemistry, University of the Free State, Nelson Mandela Drive, Bloemfontein 9301, South Africa

a r t i c l e

i n f o

Article history: Received 13 October 2008 Accepted 21 October 2008 Available online 4 December 2008 Keywords: Acetylacetone Titanium Substitution Kinetics

a b s t r a c t Novel bis(acetylacetonato-O,O0 )(biphenyldiolato-O,O0 )titanium(IV) is synthesized and characterized by ‘X-ray crystallography and other physical methods. The kinetics of substitution of bidentate 2,20 -biphenyldiolato for the two monodentate Cl ligands in Ti(CH3COCHCOCH3)2Cl2 proceeds via a 7-coordinated transition state according to an associative mechanism. The Ti(CH3COCHCOCH3)2biphen complex exhibits high hydrolytic stability. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction Complexes of titanium(IV) are widely studied for a variety of purposes, mainly serving as a catalyst in different organic reactions as well as for antitumor activity [1,2]. Titanium alkoxides are excellent precursors for the deposition of metal oxides used in optoelectronics, high-Tc superconductors and ceramic materials [3,4]. Titanium alkoxy systems are, for example, effective catalysts in a variety of processes such as the Diels–Alder reaction [5], C–C bond forming reactions [6], esterification reactions including ones involved in the production of dialkyl phthalate plasticisers [7], polymerization of alkenes and alkynes [8,9], asymmetric and enantioselective reactions [10–12] and many more [12–14]. However, since titanium alkoxides are very sensitive to hydrolysis, a problem frequently encountered when used as catalysts, is that as the catalytic reaction commences, there is some cleavage of the Ti–OR bonds due to the reaction with water that is produced as by-product in the reaction, for example in esterification reactions [15]. Studies on sol–gel systems involving [Ti(OR)4] have shown that the rate of hydrolysis of the metal alkoxide can be significantly reduced by the presence of bulky [16], or chelating ligands as acetylacetonate and glycols [17]. Thus, producing titanium(IV) complexes exhibiting enhanced water resistance, is extremely valuable. Electron-rich oxygenbased ligands may lead to resistance towards hydrolysis. The O,O0 -chelate bidentate 1,1-bi-2-naphthol ligand in the (BINO* Corresponding author. Tel.: +27 51 4012194; fax: +27 51 4446384. E-mail address: [email protected] (J. Conradie). 0277-5387/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2008.10.025

Late)TiX2 catalysts have resulted in development of an impressive number of highly enantioselective processes [18] such as the cyanosylation of aldehydes [19], Diels–Alder cycloadditions [20] hetero Diels–Alder cycloaddition [21], carbonyl-ene reaction [22], Mukaiyama aldol condensation [23] and asymmetric addition of alkyl groups to aldehydes, to name a few [24]. In this paper we describe the synthesis of a remarkably hydrolytically stable catalytic complex containing three electron donating O,O0 -chelate bidentate ligands: bis(acetylacetonato-O,O0 )(biphenyldiolato-O,O0 )titanium(IV). 2. Results and discussion 2.1. Reactivity of Ti(acac)2Cl2 with 2,20 -biphenyldiol The complex Ti(acac)2Cl2 1 (Hacac = acetylacetone = CH3COCHCOCH3) was prepared by treating TiCl4 with two equivalents of acetylacetone and isolated and purified by recrystallization before use [25,26]. Reaction of complex 1 with 2,20 -biphenyldiol in 1:1 ratio in CH3CN yielded the mixed ligand complex, Ti(acac)2(biphen) 2 (H2biphen = 2,20 -biphenyldiol = HOC6H4C6H4OH). Even with the use of higher ligand ratios, 1:2 or 1:3, the reaction yielded 2 rather than a product possessing more than one biphenolato ligand in the coordination sphere of Ti. This is similar to what was found for the reaction between Ti(acac)2Cl2 and 1,10 -methylenedi-2-naphtol [27]. Stoichiometric amounts of Ti(acac)2Cl2 and H2biphen (representing second-order reaction conditions), stirred at room temperature for 25 h under an inert atmosphere, generated the product Ti(acac)2(biphen) 2 in low yields (<10%).

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2.2. Spectral study of Ti(acac)2(biphen) 2

Table 1 Crystal and structure refinement data for Ti(acac)2biphen (2).

The proton NMR spectrum of 2 showed methyl protons of acac as a singlet at 2.05 ppm and the methine proton of acac also as a singlet at 5.81 ppm. The biphenolato ring protons were arranged in a 2:2:2:2 ratio (d; t; t; d pattern) at 6.88; 7.00; 7.21; 7.39 ppm, compared to the 4:4, m; m pattern at 7.07; 7.33 ppm in the uncoordinated ligand. The disappearance of the phenolic– OH protons in the spectrum of 2, compared to the uncoordinated ligand, suggests that the bidentate ligand is chelated through the two oxygens to the Ti(IV) centre. The methine proton of the chelated ring in both Ti(acac)2biphen (5.81 ppm) and parent complex, Ti(acac)2Cl2 (6.00 ppm) is slightly downfield shifted relative to the uncoordinated Hacac (5.50 ppm). This is due to the formation of the six-membered, planar pseudo-aromatic system when acetylacetone coordinates to the Ti(IV) centre.

Formula Formula weight Crystal colour/habit Crystal system Space group Unit cell dimension a (Å) b (Å) c (Å) a (°) b (°) c (°) Volume (Å3) Z Dcalc (Mg m3) Temperature (°C) Wavelength (Å) Absorption coefficient (mm1) F(0 0 0) Crystal size (mm) h Range for data collection (°) Index ranges

2.3. X-ray structure of (2) A molecular diagram showing the numbering scheme of the title compound Ti(acac)2biphen, 2, is presented in Fig. 1a. Crystal data and details for data collections and refinements are summarized in Table 1. Selected bond lengths and angles for 2 and for comparison, also of 1, are displayed in Fig. 1b and c, respectively. The Ti(acac)2biphen, 2, structure consists of an octahedrally coordinated titanium atom with all three ligands acting as bidentate. The octahedral coordination around titanium is distorted to accommodate the three chelated ligands, i.e., the two six-membered acetonato and seven-membered biphenolato rings. The bond angles vary from 83° to 103° compared to 90° in a perfect octahedron. Due to the strain in the ring, the ligand O–Ti–O angle increases from 82.8° for the six-membered acetonato chelated ligand to 89.9° for the seven-membered biphenolato ring, which is in the same trend as the angle of 94.0° found for an eight-membered 1,10 -methylenedi-2-naphtol (mbinaph) ring in the structure of Ti(acac)2mbinaph [27]. The Ti–O bond lengths in 2 are in the order Ti–Obiphen (1.860 Å) < Ti–Oacac axial (1.985 Å) < Ti–Oacac equatorial (2.013 Å) as illustrated in Fig. 1b. The formal Ti–O single bond is 1.94–1.99 Å (Ti–O distances found in rutile, TiO2 [28]) and Ti@O double bond is 1.61–1.68 Å [29]. Therefore the shortest Ti–O distance is clearly shorter than one expected for a single bond, suggesting that this Ti–O bond possesses partial double bond character arising from donation of the electrons from the py and pz filled oxygen orbitals to the empty titanium d orbitals [30]. The Ti–Oacac bonds trans to another acac oxygen, are shorter (ca. 1.99 Å) than those trans to the oxygen of the chelated dianion biphenalato ligand (ca. 2.01 Å). The Ti–ligand bond lengths in 1 are in the order Ti–Oacac axial (1.924–1.936 Å) < Ti–Oacac equatorial (1.967–

a

b H3C

R indices (all data) Largest difference in peak and hole (e Å3)

1.978 Å) < Ti–Cl (2.277 Å) as illustrated in Fig. 1c. Both Ti–Oacac axial and Ti–Oacac equatorial are slightly longer in 2 than in 1, but the Ti–Obiphen bond in 2 is considerably shorter than any Ti–Oacac bond in either 1 or 2. Therefore, a strong electron donation to the titanium centre is expected due to the short Ti–Obiphen bond. The compound Ti(acac)2biphen, 2, crystallizes in the monoclinic centrosymmetric space group C2/c with Z = 4, resulting in only half the molecule necessary for the asymmetric unit as it is situated on a twofold rotational axis. Intermolecular p–p stacking of the chelated b-diketonato acac moieties is clearly observed in Ti(acac)2biphen (Fig. S1a Supplementary material). The planes of the p–p stacked rings are parallel with the rings displaced from each other in a slipped or offset alignment resulting in a centroid–centroid

c

O

dax=1.9850(14)

O

dL=1.8601(14) deq=2.0132(15) Ti 89.9°

O H3C

15.8796(13) 9.7062(6) 13.2038(11) 90 93.097(2) 90 2032.1(3) 4 1.406 173 0.71073 0.457 896 0.46  0.34  0.21 2.46–28.00 20  h  20, 7  k  12, 17  l  16 6844 2358 (0.0307) 96.1 semi-empirical from equivalents 0.9101 and 0.8173 full-matrix least-squares on F2 2358/0/134 1.188 R1 = 0.0395, wR2 = 0.1213 R1 = 0.0538, wR2 = 0.1544 0.567 and 0.752

Reflections collected Independent reflections (Rint) Completeness to h = 28.35 (%) Absorption correction Max. and min. transmission Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2r(I)]

CH3

O

C22H22O6Ti 430.30 orange, cuboid monoclinic C2/c

82.8°

CH3

H3C

Cl

dCl=2.277 91.7-93.5°

O 97.2 -86.5° H3C

CH3

dax=1.924-1.936

deq=1.967-1.978 Ti

O

O

O

O

Cl

O

CH3

Fig. 1. (a) The molecular structure (30% probability displacement ellipsoids) of Ti(acac)2biphen (2) showing the numbering scheme (primed labels indicate atoms generated by symmetry operations). Diagram indicating coordination, distances (Å) and angles (°) around the titanium centre of (b) Ti(acac)2biphen (2) and (c) Ti(acac)2Cl2 (1).

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separation of the acetylacetono backbones of 3.619 Å and a centrenormal angle h of 8.6°. The geometrical parameters describing p–p stacking [31,32] are given in Supplementary material Fig. S1b. 2.4. Hydrolytic stability of (2) Complex 1 is extremely moisture sensitive and hydrolyzes easily [33]. The final stage of hydrolysis is the formation of TiO2. Keppler and Heim [33] evaluated the rate of hydrolysis, showing that complex 1 precipitates within 5 s when dissolved in dry acetonitrile treated with 0.01% water. Complex 2, however, exhibits a high hydrolytic stability and is ‘‘air stable” for more than 3 years. The solution stability was tested under the same conditions as Keppler and Heim, i.e., 0.01% water/CH3CN, and it was found that 2 does not precipitate within 6 weeks. The strong electron donation to the titanium centre observed in the form of a short Ti–Obiphen bond may contribute to the enhanced hydrolytic stability of 2 relative to 1. 2.5. Substitution kinetics The kinetics of substitution of bidentate 2,2-biphenyldiolato for the two monodentate Cl ligands in Ti(CH3COCHCOCH3)2Cl2 takes place in two kinetically distinct steps, including a solvent pathway.

Table 2 Kinetic data and activation parameters for the substitution of 2,20 -biphenyldiolato for the two monodentate Cl ligands in Ti(acac)2Cl2. T (°C)

k1 (dm3 mol1 s1)

103 kS (s1)

D H# (kJ mol1)

DS# (J mol1 K1)

DG # (kJ mol1)

15.0 25.0a 36.2

0.062(2) 0.153(4) 0.365(5)

0.12(3) 4.61(7) 13.3(1)

59.6

60.8

77.7

a

k2 = 0.0015(2) s1 for the second step at 25°.

are given in Table 2 and the results plotted as a function of the incoming H2biphen concentration in Fig. 2b. The general rate law applicable to the first [H2biphen] dependent step is given by Rate = {kS + k1[H2biphen]}[Ti(acac)2Cl2] = kobs[Ti(acac)2Cl2] [35,36] with the pseudo-first-order rate constant kobs = kS + k1[H2biphen] and k1 the second-order rate constant for the substitution process and kS the first-order rate constant for a solvent pathway. A noticeable solvent path is observed, e.g. kS = 0.00461(7) s1 at 25 °C. That Ti(CH3COCHCOCH3)2Cl2 1 indeed forms a solvent coordinated species is also confirmed on 1H NMR. 1 H NMR in CDCl3 (a non-coordinating solvent) showed one set of proton signals corresponding to the signals of the protons of the methyl groups and the methine protons of the acac ligand coordi-

ð1Þ

Both reaction steps were observed when following the reaction on a UV–Vis spectrophotometer, see Fig. 2a. The rate constant for the second step (0.0015(2) s1 at 25 °C) is ca. 10 times slower than that of the first step (0.153(4) dm3 mol1 s1 at 25 °C). The first step was followed at 340 cm1 where the second reaction did not have a change in absorbance. The two stages and the rate constants were determined independently. The H2biphen was in excess, so first-order kinetics in [H2biphen] was observed for the first step. The second step was [H2biphen] independent, consistent with the second step being rate-limiting ring closure [34]. Rate constants for the first and second stages of substitution at Ti(acac)2Cl2

nated to Ti(CH3COCHCOCH3)2Cl2, but in CD3CN solution (a coordinating solvent), two sets of signals were observed, see Supporting material Fig. S2. Since the 1H NMR spectrum in CDCl3 confirmed the purity of 1, the second species in CD3CN must correspond to the seven-coordinate [Ti(CH3COCHCOCH3)2(Cl2)(CD3CN)] solvent coordinated species. Each signal of the solvent coordinated species split since the methyl and methine groups are not chemical equivalent in [Ti(CH3COCHCOCH3)2(Cl2)(CD3CN)]. The activation parameters, the entropy of activation, DS#, and activation enthalpy, DH#, were determined from a temperature dependence study. The large negative activation entropy of

Fig. 2. (a) Absorbance vs. time data of the UV monitoring of the reaction between 0.0002 mol dm3 Ti(CH3COCHCOCH3)2Cl2, 1, and 0.040 mol dm3 H2biphen at 336 nm illustrating the two reaction steps observed. (b) Temperature and H2biphen concentration dependence of the first reaction step of the substitution of 2,2-biphenyldiolato for the two monodentate Cl ligands in Ti(CH3COCHCOCH3)2Cl2. Inset: linear dependence between ln(k1/T) and 1/T, as predicted by the Eyring equation (k1 is the second-order rate constant for the first reaction step in Eq. (1)).

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Scheme 1. Schematic presentation of the proposed associative mechanism of the substitution reaction between Ti(acac)Cl2 (1) and H2biphen to afford Ti(acac)2(biphen) (2).

60.8 J mol1 K1, suggests that the substitution process proceeds via an associative mechanism. This is expected, since octahedral complexes of TiIV (d0) are coordinatively unsaturated, i.e., Ti(acac)2Cl2 is a 12-electron species, and therefore would prefer an associatively activated pathway. The incoming H2biphen ligand is thus expected to compete with the solvent to form a 7-coordinated transition state. When considering the nature of the transition state of the first reaction, an activated complex in which the biphenolato binds to the Ti nucleus through only one of the O-atoms, is expected to form first. This transition state may then convert to another transition state in which the biphenol ligand becomes bidentate. Stable Ti(IV) complexes with coordination numbers greater than six, for example, 7- and 8-coordination, are known [37]. The common geometry for 7-coordinated Ti(IV) complexes is pentagonal bipyramidal, as in the case of Ti(dithioacetato)3Cl [38] and Ti(N,N-diethyldithiocarbamato)4 [39]. In a dissociative mechanism, the 5-coordinated cationic species, [Ti(acac)2Cl]+, formed by breaking the first Ti–Cl bond, is considerably less likely. The proposed reaction pathway for the substitution the reaction between Ti(acac)2Cl2 and H2biphen may thus be presented as in Scheme 1 where the first [H2biphen] dependent step is the formation of a 7-coordinated [Ti(CH3COCHCOCH3)2(Cl2)(H2biphen)] complex, and the second [H2biphen] independent step being rate-limiting ring closure. 3. Conclusions The substitution of bidentate 2,2-biphenyldiolato for the two monodentate Cl ligands in Ti(CH3COCHCOCH3)2Cl2 proceeded via a 7-coordinated transition state according to an associative mechanism. The product of the substitution reaction, Ti(CH3COCHCOCH3)2biphen, is ‘‘air stable” and also exhibits high resistance towards hydrolysis. 4. Experimental 4.1. Materials and apparatus Solid reagents used in preparations (Merck, Aldrich and Fluka) were used without further purification. Liquid reactants and solvents were distilled prior to use; water was doubly distilled.

Organic solvents were dried according to published methods [40]. 4.2. Synthesis of 2 Complex 1 was synthesized according to published methods [25,26]. To 15 ml CH3CN, 2,20 -biphenyldiol (0.186 g, 1 mmol) was added and stirred at RT. Complex 1 (0.317 g, 1 mmol) in 15 ml CH3CN was added drop by drop to give an orange red solution which was refluxed for 4–6 h. The reaction mixture was then cooled to RT and filtered. The precipitate was washed in MeOH to remove unreacted biphenol, filtered, dried and stored under N2 atmosphere. Yield: 0.333 g (77.5%). M.p. >250 °C. 1H NMR (d/ppm, CDCl3): 2.05 (s, 12H, 4  CH3); 5.81 (s, 2H, acacH); 6.88 (d, 2H, biphenH); 7.00 (t, 2H, biphenH); 7.21 (t, 2H, biphenH); 7.39 (d, 2H, biphenH). kmax(CH3CN)/nm 334 (e/dm+3 mol1 cm1 4890(30)). Elemental Anal. Calc. for C22H22O6Ti: C, 61.4; H, 5.2. Found: C, 61.2; H, 5.1%. 4.3. Spectroscopy and spectrophotometry NMR measurements at 25 °C were recorded on a Bruker Advance II 600 NMR spectrometer [1H(600.130 MHz)]. The chemical shifts were reported relative to SiMe4 (0.00 ppm). Positive values indicate downfield shift. IR spectra were recorded from neat samples on a Digilab FTS 2000 infrared spectrophotometer. UV–Vis spectra were recorded on a Cary 50 Probe UV–Vis spectrophotometer. 4.4. Kinetic measurements The substitution reaction was monitored on the UV–Vis (by monitoring the change in absorbance at the indicated wavelength) spectrophotometer. All kinetic measurements were monitored under pseudo-first-order conditions with [H2biphen] 10–200 times the concentration of the Ti(acac)2Cl2 complex in CH3CN solution. The concentration Ti(acac)2Cl2 ffi 0.0002 mol dm3. Kinetic measurements, under pseudo-first-order conditions for different concentrations of Ti(acac)2Cl2 at a constant [H2biphen], confirmed that the concentration of Ti(acac)2Cl2 did not influence the value of the observed kinetic rate constant. A linear relationship between

T.A. Tsotetsi et al. / Polyhedron 28 (2009) 209–214

UV absorbance, A, and concentration, C, confirmed the validity of the Beer–Lambert law (A = eCl with l = path length = 1 cm) for the complexes 1 and 2 at experimental wave lengths kmax = 340 nm (e/dm3 mol1 cm1 is 1830(20) for 1 and 4890(30) for 2). The observed first-order rate constants were obtained from least-square fits of absorbance versus time data [41]. 4.5. Calculations Pseudo-first-order rate constants, kobs, were calculated by fitting kinetic data [41] to the first-order equation [42] [A]t = [A]0 e(kobs t) with [A]t and [A]0 the concentration of the indicated species at time t and at t = 0 (UV–Vis). The experimentally determined pseudo-first-order rate constants were converted to second-order rate constants, k1, by determining the slope of the linear plots of kobs against the concentration of the incoming biphenolato ligand. Non-zero intercepts implied that kobs = k1[biphen] + k1 and that the first-order rate constant for a solvent pathway, k1, in the proposed reaction mechanism exists. All kinetic mathematical fits were done utilizing the fitting program MINSQ [41]. The error of all the data are presented according to crystallographic conventions, for example kobs = 0.0236(1) s1 implies kobs = (0.0236 ± 0.0001) s1. The activation parameters were determined from the Eyring relationship [42] and the activation free energy DG# = DH#  TDS#. 4.6. X-ray crystal structure determination Crystals of Ti(acac)2biphen (2) were obtained from recrystallization in chloroform. X-ray intensity data for 2 was measured on a Bruker X8 Apex II 4K CCD area detector, equipped with a graphite monochromator and Mo Ka fine-focus sealed tube (k = 0.71073 Å) operated at 1.5 kW power (50 kV, 30 mA). The detector was placed at a distance of 3.75 cm from the crystal. Sample temperature was kept constant at 100(2) K using an Oxford 700 series cryostream cooler. The initial unit cell and data collection of 2 were achieved by the Apex2 software [43] utilizing COSMO [44] for optimum collection of more than a hemisphere of reciprocal space. A total of 948 frames were collected with a scan width of 0.5° in u and x with an exposure time of 20 s per frame. The frames were integrated using a narrow frame integration algorithm and reduced with the Bruker SAINTPLUS and XPREP software [45] packages respectively. Analysis of the data collections showed no significant decay during the data collection. Data were corrected for absorption effects using the multi-scan technique SADABS [46]. The structure was solved by the direct methods package SIR97 [47] and refined using the WINGX software package [48] incorporating SHELXL [49]. The largest peaks on the final difference electron densities and the deepest holes were all within 1 Å from nonhydrogen atoms which presented no physical meaning in the final refinements. Aromatic protons were placed in geometrically idealized positions (C–H = 0.95 Å) and constrained to ride on their parent atoms with Uiso(H) = 1.2 Ueq(C). Initial positions of the methyl protons were obtained from a Fourier difference map and refined as fixed rotors with Uiso(H) = 1.5 Ueq(C) and C–H = 0.98 Å. Atomic scattering factors were taken from the International Tables for Crystallography Volume C [50]. The molecular plot was drawn using the DIAMOND program [51] with a 30% thermal envelope probability for non-hydrogen atoms. Hydrogen atoms were drawn as arbitrary sized spheres with a radius of 0.135 Å. Acknowledgements Financial assistance by the South African National Research Foundation under Grant No. 2067416 and the Central Research Fund of the University of the Free State is gratefully acknowledged.

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Appendix A. Supplementary data CCDC 705149 contains the supplementary crystallographic data for Ti(acac)2biphen. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected]. Text and figures giving further explanations is provided in the supplementary data. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.poly.2008.10.025.

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