Synthesis, structure, spectral properties and theoretical studies of two half-sandwich titanium-complexes with adamantoxy ligands

Synthesis, structure, spectral properties and theoretical studies of two half-sandwich titanium-complexes with adamantoxy ligands

Journal of Molecular Structure 1142 (2017) 248e254 Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: http:/...

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Journal of Molecular Structure 1142 (2017) 248e254

Contents lists available at ScienceDirect

Journal of Molecular Structure journal homepage: http://www.elsevier.com/locate/molstruc

Synthesis, structure, spectral properties and theoretical studies of two half-sandwich titanium-complexes with adamantoxy ligands Vojtech Varga a, Karel Mach a, Jirí Pinkas a, Jirí Kubista a, Katarína Szarka b,  bert Gyepes b, c, * Ro J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, v.v.i., Dolejskova 3, 182 23 Prague 8, Czech Republic  cesta 3322, 945 01 Koma rno, Slovak Republic Department of Chemistry, Faculty of Education, J. Selye University, Bratislavska c Department of Inorganic Chemistry, Charles University, Hlavova 2030, 128 40 Prague 2, Czech Republic a

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 December 2016 Received in revised form 17 April 2017 Accepted 17 April 2017 Available online 20 April 2017

Two novel half-sandwich Ti complexes, both incorporating two adamantoxy (OAd) ligands coordinated to their central atoms were synthesized. The complexes were characterized by 1H, 13C, 19F NMR, EI-MS and IR spectroscopy and by single-crystal X-ray diffraction. In both complexes, the coordination environment is pseudo-tetrahedral and is assembled of two O-coordinated adamantoxy and one h5-coordinated permethylcyclopentadienyl ligand. The fourth ligand in complex 1 is a methyl group coordinated through a regular s-bond, whereas in complex 2 the same coordination site is occupied by the methyl group of the balancing (C6F5)3BMeˉ anion. DFT computations complemented with NBO analyses of 2 have unveiled, that in addition to the electrostatic interactions occurring between the half-sandwich titanocene moiety and the anionic ligand, delocalization of one methyl CeH bond into the available acceptor orbital on the central atom takes place. AIM analyses of 2 have revealed the presence of a Bond Critical Point between the metal atom and the anionic methyl group. These results prove foundation for the description of 2 as a zwitterionic complex coupled with a concurrent Ti/CeH agostic interaction in its molecule. The comparison of 2 with its analogous complex equipped with two Ot-Bu ligands instead of both OAd has suggested only a minor change in the extent of the agostic interaction, despite significant geometric differences between the two complexes. © 2017 Elsevier B.V. All rights reserved.

Keywords: Half-sandwich complex Ionic complex Zwitterionic complex Solid-state structure Density functional theory

1. Introduction Half-sandwich titanium complexes in combination with methylaluminoxane are known to form effective catalysts for polymerization of styrene, which are capable to produce syndiotactic polystyrene [1e3]. The nature of the active catalytic species has been studied in single-site cationic systems, which were generated by mixing [(h5-C5Me5)TiMe3] with [B(C6F5)3]. The resulting ionic complex [(h5-C5Me5)TiMe2]þ[MeB(C6F5)3]- was, however, observable by 1H NMR spectroscopy only at low temperatures, since at ambient temperatures it decomposed rapidly [4]. The modification of the titanium component by introducing an alkoxy or aryloxy group instead of one methyl led only to a slight increase of the thermal stability of products [(h5-C5Me5)Ti(OR)Me][(m-Me)

* Corresponding author. Department of Chemistry, Faculty of Education, J. Selye University, Bratislavsk a cesta 3322, 945 01 Kom arno, Slovak Republic. E-mail address: [email protected] (R. Gyepes). http://dx.doi.org/10.1016/j.molstruc.2017.04.072 0022-2860/© 2017 Elsevier B.V. All rights reserved.

B(C6F5)3] [5,6]. The first thermally stable complexes of this type [(h5-C5Me5)Ti(Ot-Bu)2][(m-Me)B(C6F5)3] (3) [ref. [7]] and [(h5C5Me5)Ti(Oi-Pr)2][(m-Me)B(C6F5)3] (4) [ref. [8]] were obtained by mixing pentamethylcyclopentadienyltitanium(methyl)dialkoxides with tris(pentafluorophenyl)borane (cf. Chart 1), and their structure in solution and in solid state were investigated. In this article we report on the synthesis of the half-sandwich titanocene(methyl)diadamantoxide [(h5-C5Me5)TiMe(OAd)2] (Ad ¼ 1-adamantyl) (1) and the zwitterionic complex [(h5-C5Me5) Ti(OAd)2][MeB(C6F5)3] (2) made thereof by reacting 1 with [B(C6F5)3]. Even if complexes 2 and 3 exhibit no catalytic activity towards styrene, the analysis of their solid-state structures complemented with computational studies are important for interpreting the role of tert-alkoxy ligands in influencing the electronic and steric relationships around the central atoms. The nature of the titanocene-oxo bond was studied previously by computational methods and a comparison of crystal structures of a number of titanocene alkoxides including the trivalent [(h5-C5Me5)2Ti(OAd)]

V. Varga et al. / Journal of Molecular Structure 1142 (2017) 248e254

Chart 1.

was also done [9]. The current work extends the previously reported systems to zwitterionic complexes incorporating agostic interaction. 2. Materials and methods 2.1. General comments and methods Pentamethylcyclopentadienyltitanium(methyl)di(1adamantoxide) 1 was prepared under either nitrogen or argon atmosphere. Its reaction with B(C6F5)3 and isolation of [(h5-C5Me5) Ti(OAd)2][MeB(C6F5)3] (Ad ¼ 1-adamantyl) (2) was performed in all-sealed vacuum systems using magnetically breakable seals. Crystals of 1 and 2 for EI-MS measurements and melting point determinations were placed into glass capillaries in a glovebox Labmaster 130 (mBraun) under purified nitrogen (concentrations of oxygen and water were lower than 2.0 ppm). 1H (299.98 MHz), 13 C (75.44 MHz) and 19F (282.22 MHz) NMR spectra were recorded on a Varian Mercury 300 spectrometer at 298 K. Chemical shifts (d/ ppm) are given relative to the solvent signal (C6D6: dH 7.15, dC 128.0). EI-MS spectra were obtained on a VG-7070E mass spectrometer at 70 eV. Samples in sealed capillaries were opened and inserted into the direct inlet under argon. The mass spectra are represented by the peaks of relative abundance higher than 7% and by important peaks of lower intensity. The EI-MS samples of 1 and 2 were completely evaporated without changing the fragmentation pattern; this proves the uniformity of the compounds. Samples of 1 and 2 in KBr pellets were prepared in a glovebox Labmaster 130 (mBraun) and their IR spectra were recorded in an air-protecting cuvette on a Nicolet Avatar FT IR spectrometer in the range of 400e4000 cm1. Melting points were measured on a Koffler block in sealed glass capillaries and are uncorrected.

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evolution of methane. The resulting slightly yellow solution was warmed to 60  C for 1 h and then, the formed methane and hexane were slowly evaporated under vacuum until the solution became saturated. After cooling to 23  C overnight the mother liquor was decanted, crystalline product was washed out with condensing hexane vapour and dried in vacuum. Yield of nearly colourless crystalline solid 0.44 g (88%). M.p. 143  C. EI-MS (150  C): m/z (relative abundance) 500 (Mþ; 5), 487 (20), 486 (51), 485 ([M  Me]þ; 100), 484 (19), 483 (15), 350 ([M  Me  C10H15]þ; 6), 334 ([M  Me  OC10H15]þ; 4), 200 (14), 199 ([M  Me  OC10H15  C10H15]þ; 18), 198 (10), 197 (7), 195 (11), 181 (8), 136 (17), 135 ([C10H15]þ; 97), 105 (22), 95 (9), 94 (7), 93 (66), 91 (11), 81 (27), 79 (68), 77 (12), 69 (11), 67 (38), 64 (12), 55 (26).1H NMR (C6D6): 0.55 (s, 3H, TiMe); 1.46e1.60 (m, 12H, CH2); 1.91 (d, 3 JHH ¼ 2.7 Hz, 12H, CH2); 2.00 (s, 15H, C5Me5); 2.00e2.08 (partially overlapped m, 6H, CH). 13C {1H} NMR (C6D6): 12.1 (C5Me5); 31.5 (CH); 36.7 (CH2); 37.7 (TiMe); 47.5 (CH2); 80.2 (OC); 119.7 (C5Me5). IR (KBr, cm1): 2906 (vs), 2849 (s), 2678 (vw), 2654 (vw), 1451 (m), 1407 (vw), 1375 (w), 1363 (vw), 1347 (s), 1300 (m), 1142 (vs), 1115 (vs), 1088 (vs), 1005 (s), 958 (m), 931 (w), 812 (w), 791 (m), 754 (s), 682 (m), 625 (vw), 505 (m), 473 (m), 415 (m).

2.2.2. Synthesis of [(h5-C5Me5)Ti(OAd)2][MeB(C6F5)3] (2) Compound 1 (0.29 g, 0.58 mmol) was dissolved in 20 mL of hexane and the solution was poured to [B(C6F5)3] 0.30 g, 0.59 mmol under stirring. The reaction mixture was warmed to 60  C for 2 h and then concentrated to saturation (about 7 mL) by slow evaporation to vacuum. Cooling to 5  C overnight afforded yelloworange crystals. Yield 0.82 g (81%). M.p. 148e150  C, melts with decomposition and gas evolution. 1 H NMR (C6D6): 0.53 (br s, w½ ¼ 24 Hz, 3H, MeB); 1.50e1.78 partly overlapped (m, 24H, CH2); 1.74 (s, 15H, C5Me5); 1.88e1.96 (m, 6H, CH). 13C {1H} NMR (C6D6): 12.5 (C5Me5); 31.4 (CH); 36.9 (CH2); 46.5 (CH2); 90.7 (OC); 130.9 (C5Me5); 137.6 (d of multiplets, 1 JCF ¼ 256 Hz, m-CF, C6F5); 149.0 (d of multiplets, 1JCF ¼ 237 Hz, o-CF, C6F5); MeB, Cipso, p-CF signals were not detected. 19F (C6D6): 132.5 (d, 3JFF ¼ 20.7 Hz, 6F, o-CF); 160.5 (m, 3F, p-CF); 165.4 (br s, 6F, mCF). IR (KBr, cm1): 2935 (s,sh), 2917 (s), 2856 (m), 1642 (m), 1512 (s), 1460 (vs), 1381 (w), 1349 (w), 1298 (m), 1268 (m), 1182 (w), 1111 (s), 1080 (vs), 1057 (s), 1000 (s), 973 (s), 960 (m), 934 (w), 905 (vw), 879 (vw), 841 (vw), 798 (m), 758 (m), 685 (w), 661 (vw), 571 (w), 476 (w), 425 (m).

2.3. X-ray single-crystal studies 2.2. Chemicals Solvents hexane and toluene were dried by refluxing over LiAlH4 and stored as solutions of green dimeric titanocene [(m-h5:h5C10H8)(m-H)2{Ti(h5-C5H5)}2] [ref. [10]] on a vacuum line. Benzened6 (C6D6) (Sigma Aldrich) was degassed, distilled under vacuum on singly tucked-in permethyltitanocene [Ti(C5Me5)(C5Me4CH2)] [ref. [11]], and stored as its solution on a vacuum line. 1-Adamantol (C10H16O) (Sigma Aldrich) was weighed on air and degassed. The pentamethylcyclopentadienyltitanium trimethyl [(h5- C5Me5) TiMe3] was purchased from Sigma Aldrich, opened under nitrogen in a glovebox, and dissolved in degassed hexane to give 0.1 M solution. B(C6F5)3 (Strem Chemicals) was analyzed by IR spectroscopy (ATR method) for purity, and only pure samples were used for synthesis of 2. 2.2.1. Preparation of [(h5- C5Me5)TiMe(OAd)2] (1) A solution of [(h5- C5Me5)TiMe3] (0.1 M, 10 mL, 1.0 mmol) in hexane was added to 1-adamantanol (0.31 g, 2.04 mmol) under

Suitable single crystals of 1 and 2 were mounted into Lindemann glass capillaries in a Labmaster 130 (mBraun) glovebox under purified nitrogen. Diffraction data were collected on an EnrafNonius Kappa CCD diffractometer using graphite-monochromated MoKa (0.71073 Å) radiation at 150 K using an Oxford Cryostream cooling unit. Data collection, data reduction and cell refinement were performed using COLLECT [12] and DENZO [13]. The phase problem was solved by direct methods (SIR-2004) [14] and refined by full-matrix least-squares on F2 (SHELXL-2014) [15]. All nonhydrogen atoms were refined with no constrains and using anisotropic thermal parameters, with the exception of the disordered adamantoxy moiety in 1, where the carbon atoms in the part with lower occupancy were refined isotropically. Hydrogen atoms in 1 and 2 were put into idealized positions and refined isotropically, with the exception of H(31A), H(31B), and H(31C) of the bridging methyl group in 2, which were refined with no constrains. Relevant crystallographic data are given in Table 1. Molecular graphics was carried out with PLATON [16] and Raster3D [17].

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Table 1 Crystallographic data and refinement details for 1 and 2. Identification code Empirical formula Formula weight Temperature Wavelength Crystal system, space group Unit cell dimensions

Volume Z, Calculated density Absorption coefficient F(000) Crystal size Theta range for data collection Limiting indices Reflections collected/unique Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2s(I)] R indices (all data) Largest diff. peak and hole

1 C31H48O2Ti 500.59 150(2) K 0.71073 Å Monoclinic, P21/n a ¼ 12.1551(4) Å b ¼ 17.9882(5) Å c ¼ 13.3054(3) Å b ¼ 108.3162(16) 2761.81(14) Å3 4, 1.204 Mg/m3 0.335 mm1 1088 0.43  0.51  0.23 mm3 1.970 to 27.448 15h  15, 23k  23, 17l  17 12365/6313 [Rint ¼ 0.0535] Full-matrix least-squares on F2 6313/0/341 1.042 R1 ¼ 0.0511, wR2 ¼ 0.1109 R1 ¼ 0.0935, wR2 ¼ 0.1289 0.312 and 0.526 e.Å3

2.4. Computational studies DFT calculations of 2 have been carried out using Gaussian 09, Revision D.01 [18]. Geometry optimizations and NBO analyses were done using Grimme's functional including dispersion (B97D) [19] and the 6-31þG(d,p) basis set used for all atoms. NBO analyses were performed with a standalone version of NBO 5.G [20]. Molar volumes were computed with precision higher than default with Gaussian. For Counterpoise calculations of 2 and 3, the basis set 631þG(d,p) was used on the optimized geometries and the complexes were divided into two fragments, with the MeB(C6F5)3 moieties separated out as anions. Topology analysis was done using Multiwfn 3.3.9 [21].

3. Results 3.1. Synthesis and spectral studies Compound [(h5-C5Me5)Ti(OAd)2Me] (Ad is 1-adamantyl) (1) was obtained by reacting [(h5-C5Me5)TiMe3] with at least two-fold molar excess of 1-adamantol with evolution of methane. Attempts to use a higher excess of 1-adamantanol for reaction with (h5C5Me5)TiMe(OAd)2 did not lead further to the formation of [(h5C5Me5)Ti(OAd)3]. 1H and 13C NMR spectra of [(h5-C5Me5)Ti(1OAd)2Me] (1) corresponded to the expected molecular structure, where the signal for methyl group bounded to titanium central atom appeared at dH 0.55 ppm and dC 37.7 ppm. The reaction of the complex 1 with B(C6F5)3 led to vanishing of the TiMe signal in both 1 H and 13C NMR spectra and emerging a new broad singlet signal in 1 H NMR at 0.53 ppm attributable to MeB(C6F5)3 group. The generation of the cationic titanium center in [(h5-C5Me5)Ti(OAd)2] [MeB(C6F5)3] (2) led to downfield shift of proximal quaternary carbons in 13C NMR spectra (dC (OC) 80.2 ppm for 1 and 90.7 ppm for 2; dC (C5Me5) 119.7 ppm for 1 and 130.9 ppm for 2). Contrary to that, the signal of methyl groups C5Me5 was highfield shifted in 1H NMR spectrum (dH 2.00 ppm for 1 and 1.74 ppm for 2), which is probably a result of shielding by proximal anionic borate group [MeB(C6F5)3]. The cation-anion proximity was further corroborated by the difference between the chemical shifts of meta- and parafluorine (D ¼ jdm-Fj  jdp-F j) in 19F NMR spectrum, a probe

2 C52H55BF15O2Ti 1055.67 150(2) K 0.71073 Å Monoclinic, P21/c a ¼ 21.1017(7) Å b ¼ 10.3157(3) Å c ¼ 22.2343(7) Å b ¼ 95.5370(10) 4817.4(3) Å3 4, 1.456 Mg/m3 0.277 mm1 2180 0.570  0.417  0.284 mm3 1.840 to 27.000 26h  26, 13k  13, 28l  28 67995/10468 [Rint ¼ 0.0331] Full-matrix least-squares on F2 10468/0/658 1.039 R1 ¼ 0.0409, wR2 ¼ 0.0999 R1 ¼ 0.0593, wR2 ¼ 0.1141 0.745 and 0.507 e.Å3

introduced by Horton [22]. The obtained value D ¼ 4.9 ppm is very similar to values obtained for analogous complexes [(h5-C5Me5) Ti(Ot-Bu)2][MeB(C6F5)3] (D ¼ 5.0 ppm) [7] and [(h5-C5Me5)Ti(OiPr)2][MeB(C6F5)3] (D ¼ 4.9 ppm) [8] and support strong cationanion interaction fulfilling an inner sphere ion pair (ISIP) model [23].

3.2. Solid-state structures Complex 1 crystallized in a monoclinic lattice with space group P21/n. In the solid-state structure, one of the adamantoxy ligands was struck by disorder, since it acquired two distinct positions rotated by approximately 60 around the axis interconnecting the oxygen and its tertiary carbon neighbour. The occupancy factors of both positions have been refined to be 0.85:0.15 and as a consequence of this inequality, the carbon atoms of the adamantoxy moiety acquiring the lower site occupation factor were refined only isotropically and with thermal parameters fixed to 0.04. The molecular structure of 1 is drawn in Fig. 1. Compared with the related and sterically more crowded (h5C5Me5)2TiOAd,9 molecule 1 has both its Ti-O-Cpivot angles lower than the analogous angle in the bent sandwich complex, where its value was 174.96(13). The deviation of the TieOeC angle from the ideal tetrahedral was found to be enforced by the increased steric demands of more voluminous ligands. Additionally, the decreased crowding around the metal in 1 led to both TieO interatomic distances becoming shorter, which was 1.8546(14) Å in (h5C5Me5)2TiOAd; however, this TieO shortening led to the simultaneous O-Cpivot prolongation, which reached 1.308(3) Å. Complex 2 crystallized in a monoclinic lattice with space group P21/c. Its solid-state structure is stabilized by the presence of a solvating hexane, of which one half is located in the asymmetric part of the unit cell, while its symmetrically dependant part is generated through the crystallographic center of symmetry. The structure of 2 is thus effectively a hemisolvate. The molecular structure is depicted in Fig. 2. The solid-state structure of molecule 2 suggested its bridging methyl group is s-bonded to boron, whereas all its CeH bonds fulfilled the criteria for possible agostic bonding interaction with the titanium. Since the methyl C(31) atom shows an sp3

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3.3. Computational studies of 2

Fig. 1. The solid-state structure of 1 with thermal ellipsoids drawn on 30% probability level. The disordered adamantyl moiety on O(2) is omitted for clarity. Selected bond distances and angles: Ti(1)eO(1) ¼ 1.7941(16) Å; Ti(1)eO(2) ¼ 1.7802(16) Å; Ti(1)CE(1) ¼ 2.0734(11) Å; Ti(1)eC(31) ¼ 2.119(2) Å; O(1)-Cpivot ¼ 1.414(3) Å; O(2)Cpivot ¼ 1.420(3) Å; CE(1)-Ti(1)-O(1) ¼ 117.20(7); CE(1)-Ti(1)-O(2) ¼ 120.60(6); CE(1)Ti(1)-C(31) ¼ 109.94(8); Ti(1)-O(1)-C(11) ¼ 162.86(16); Ti(1)-O(2)-C(21) ¼ 170.57(15).

Fig. 2. The solid-state structure of 2 with thermal ellipsoids drawn on 30% probability level. Hydrogen atoms excepting on the bridging methyl group are omitted for clarity. The solvating hexane molecule is not drawn. Selected bond distances and angles: Ti(1) eO(1) ¼ 1.7542(14) Å; Ti(1)eO(2) ¼ 1.7619(14) Å; Ti(1)-CE(1) ¼ 2.0492(9) Å; Ti(1) eC(31) ¼ 2.4444(19) Å; B(1)eC(31) ¼ 1.672(3) Å; CE(1)-Ti(1)-O(1) ¼ 118.57(6); O(1) eC(11) ¼ 1.433(2) Å; O(2)eC(21) ¼ 1.431(2) Å; CE(1)-Ti(1)-O(2) ¼ 117.46(6); CE(1)Ti(1)-C(31) ¼ 117.46(5) Å; Ti(1)-C(31)-B(1) ¼ 165.43(14); Ti(1)-O(1)C(11) ¼ 169.28(14); Ti(1)-O(2)-C(21) ¼ 167.90(14).

hybridization, its bonding to titanium has to be accomplished via agostic bonding of its CeH bonds. All conditions required for its occurrence are fulfilled: the d0 titanium atom has empty orbitals to interact with the s-CeH bonding orbital, the TieC and TieH distances are comparable, and TieCeH angles are below 100 [24]. The actual presence of agostic interaction was confirmed by computational studies.

Computational studies of 2 provided a detailed insight into the anticipated agostic interaction of the anionic methyl group with the central atom. Initial geometry optimizations of the molecule, which has been placed in vacuum, have retained its molecular arrangement. The optimization procedure, carried out as the first step of computational studies has reached a geometry virtually identical with the solid-state structure and yielded the TieC(Me) interatomic separation 2.438 Å. The bending at the bridging methyl group was also retained, resulting in the TieCeB angle becoming 162.87 ; the methyl hydrogen atoms were oriented such that one was bisected by this plane, whereas the other two hydrogen atoms were oriented outwards from this plane symmetrically to both its sides. For the in-plane hydrogen, the respective TieH interatomic separation became 2.148 Å and the TieCeH angle 61.76 , permitting a notable bending of the hydrogen towards the metal. Due to the aforementioned TieCeB bending, the TieH separation for the other two hydrogens was elongated to approximately 2.5 Å, thus the three methyl hydrogen atoms became significantly unequal in interacting with the central atom. The TieH Mayer bond orders confirmed this inequality, which yielded 0.1 for the in-plane hydrogen, whereas roughly three times lower values were obtained for the out-ofplane atoms (Table 2). Since the TieCMe Mayer bond order reached simultaneously a value 0.33, this was suggesting the importance of one single CeH interaction with the metal. This particular in-plane interaction was examined in detail by Natural Bond Orbital (NBO) analysis by determining the 2nd order perturbation of the Fock matrix in the NBO basis and by observing the natural charges of the atoms involved. The dominant interaction between the metal and carbon was of dipole origin with natural charges Ti: 1.72 and C: 1.03. Since the natural charge of the inplane hydrogen was 0.23, its interaction with the metal had to be preferably of covalent character. The Lewis representation of the molecule by NBO yielded a methyl group attached by a regular sbond to the boron of the anionic part, whereas the interaction of this methyl with the metal was treated as delocalization to the proper metallic acceptor orbitals. In total, there were three distinct CeH delocalizations found from the in-plane CeH bond to the metal, which resulted in the decrease of the respective CeH bond occupancy to 1.92. For comparison, the CeH bond occupancies for both out-of-plane hydrogens became 1.97, which supported the assumption of their marginal participation in the agostic interaction. NBO Perturbation Theory has determined also the energies of three significant in-plane CeH delocalizations to the metal. The weakest of them with energy 10.75 kJ/mol occurred to the acceptor orbital mixed mostly from the 3dxy and 3dyz orbitals, while the delocalization energy to the 3dz [2] became 18.37 kJ/mol and even 19.46 kJ/mol to the 4s. In addition to NBO theory, the presence of agostic interaction was also confirmed by performing a topology analysis through the Atoms in Molecules [25] (AIM) approach against the charge density.

Table 2 Mayer Bond Orders and NBO Occupancy Numbers related to the agostic interaction in 2. Atom

H(31A) H(31B) H(31C)

Mayer Bond Orders to neighbouring atoms C(31)

Ti(1)

0.742 0.833 0.878

0.097 0.034 0.027

NBO Occupancies of CeH bonds to C(31)

1.923 1.966 1.972

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Fig. 3. Contour plot of electron density Laplacian of critical points (CP's) and bond paths for 2 in the plane formed by Ti(1) and the C-H bond (C(31)-H(31A)) of the anion. Positive values of the Laplacian are represented by full lines, negative values by dashed ones; its contour values represent 0.001, ±2$10n, ±4$10n and ±8$10n, where n is a whole number in the range <-3; 3>. Brown dots represent atomic CP's (3,-3), blue squares bond CP's (3,-1), red lines the bond paths. The plane of the C5Me5 ligand skeleton is approximately perpendicular to the upper left corner of the Figure. CP's (3,þ1) and (3,þ3) have been omitted for clarity. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

The existence of agostic interaction was identified through locating a critical point (CP) and the paths interconnecting the CP with its neighbouring atoms. AIM analysis of 2 has uncovered the existence of a Bond Critical Point (BCP) located about halfway between the metallic atom and the methyl group involved in this interaction (Fig. 3). The most important topological parameters of this CP are given in Table 3. A low electron density coupled with a higher value of the Laplacian in this BCP, together with AIM charges obtained by integration in the atomic basins indicate an ionic interaction [26]. The BCP ellipticity value, computed as (l1/l2)-1, where l1 and l2 denote the eigenvalues of the charge density Hessian after its diagonalization, displays an unusually large value, which is an indicator of the structural instability of agostic interactions [27]. Another graphic representation of both the covalent and noncovalent interactions is the Laplacian of electron density (V2 r), which allows for a quick depiction of the CeH agostic interaction (Fig. 3) that occurs between regions of charge concentration and charge depletion. Further NBO results have confirmed the anticipated zwitterionic character of 2, permitting to separate the molecule into a cationic and an anionic part. This property was devised from the sum of natural charges on the (C6F5)3BMe moiety, which yielded 0.80 and thus suggested its anionic character, while the balancing positive charge with the same magnitude was obtained on the metal carrying the (h5-C5Me5) and the two alkoxy ligands. For a detailed elucidation of comparing the effect(s) of the O-

donating ligands on the geometry of the final complexes and also on the extent of the agostic interaction of the bridging anionic methyl group, computational studies of 2 have been extended to the analogic [(h5-C5Me5)Ti(t-BuO)2][MeB(C6F5)3] (3) [7]. The optimized geometries of 2 and 3 differed markedly in the rotation of agostic methyl hydrogen atoms with respect to the metal. This rotation was quantified by the CE-TieCeH dihedral angle (CE denotes the centroid of the h5-C5Me5 ligand, C the methyl in agostic interaction and H its hydrogen atoms oriented towards the h5C5Me5 ring), which was 5.31 in 2, whereas for 3 the same value became 22.24 . Since excepting their alkoxy ligands both complexes are identical, the disparate orientation of their bridging CeH bonds had to be prompted by the different orientation and the steric demands of the alkoxy ligands and not possible differences in their electronic structure. The unequal steric requirement of these substituents have expressed themselves in differing Ti-O-Cpivot angles, which were 136.56 and 166.47 for 2 in contrast with 155.63 and 164.36 for 3. The lower Ti-O-Cpivot angle in the otherwise more voluminous 2 (molar volume 574.086 cm3/mol) compared with 3 (molar volume 478.594 cm3/mol) results from the respective AdO ligand attempting to decrease its repulsion with the other O-coordinated ligand being rotated slightly towards the anionic part of the molecule. This steric arrangement directs the alkoxy oxygen bridges in 2 to a position, which enables two of the three bridging methyl hydrogens to participate in weak intramolecular hydrogen bonds (Fig. 4).

Table 3 Important topological parameters related to the agostic interaction in 2. Individual columns provide data for Bond Critical Point and they include (form left to right) the interatomic separation of the BCP to the participating atoms, its electron density, Laplacian, ellipticity and the AIM charges of the atoms involved. r(BCPeatom) (Å) Ti(1)

C(31)

H(31A)

1.189

1.310

1.119

r(rBCP) (e/Å3)

V2r(rBCP) (e/Å5)

εBCP

qAIM Ti(1)

C(31)

H(31A)

0.035

0.123

1.041

1.868

0.515

0.061

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Fig. 4. View of 2 along the B-Me-Ti interconnection (left) and rotated by the vertical axis (right). The interatomic distances between the bridging oxygen atoms and their neighbouring agostic hydrogens are 2.49 Å and 2.65 Å. Atom legend: C: black sticks, H: grey sticks, O: red sticks, B: violet sticks, Ti: green balls. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Even if the orientation of the bridging methyl groups is different in 2 and 3, the total energy of their agostic interaction shows no significant differences. Since both central atoms are formally of d0 configuration, there are appropriate metallic acceptor orbitals available for both methyl groups rotated differently. In addition to the agostic interactions having a similar extent, the total interaction energies determined by a Counterpoise computation taken between the anionic and cationic parts of 2 and 3 became also remarkably similar: 377.39 kJ/mol for 2 and -378.00 kJ/mol for 3. The interaction energies put forward the noteworthy similarity of complexes, despite having different alkoxy ligands in their molecules.

Acknowledgements This work was supported by Czech Science Foundation (V.V. by project 14-08531S, K.M. and J.P. by project P207/12/2368) and by the OP VVV “Excellent Research Teams” (Project No. CZ.02.1.01/0.0/ 0.0/15_003/0000417 – CUCAM). The authors are grateful for computational resources provided on the bose cluster at the J. Heyrovsky Institute of Physical Chemistry, Academy of Sciences of Czech Republic, v.v.i. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.molstruc.2017.04.072.

4. Conclusions Two new complexes [(h5-C5Me5)Ti(OAd)2Me] and [(h5-C5Me5) Ti(OAd)2][MeB(C6F5)3] have been synthesized and structurally characterized. The first complex was prepared by reacting its parent [(h5-C5Me5)TiMe3] with 1-adamantol. This complex reacted further with B(C6F5)3 yielded the zwitterionic [(h5-C5Me5)Ti(OAd)2] [MeB(C6F5)3]. Both complexes were characterized by NMR, EI-MS and IR spectroscopy. X-ray diffraction studies of single crystals confirmed the pseudo-tetrahedral coordination environment around the central atoms, including both alkoxy ligands O-coordinated to the metals. Density Functional Theory studies of [(h5C5Me5)Ti(OAd)2][MeB(C6F5)3] have confirmed the zwitterionic character of the complex. The [MeB(C6F5)3] moiety of the molecule is attached to the cationic part not only through electrostatic forces, but also through an agostic interaction occurring between the metal and the anionic methyl group, which acts as a bridge linking the borate and the titanium. A comparison of this complex with its closely related analogue [(h5-C5Me5)Ti(Ot-Bu)2][(m-Me)B(C6F5)3] has shown an insignificant energy difference (0.61 kJ/mol) in the attraction between the cationic and anionic part of the complex, despite the different arrangement of their alkoxy ligands and also their agostic methyl groups. The larger steric demand of the adamantoxy ligands results in the increased repulsion between the alkoxy ligand, which is however counterbalanced by their facilitated electrostatic attraction to the borate anion. The two complexes with agostic interaction their molecules are remarkably similar in stability, despite carrying ligands with different volumes in their molecules.

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