Syntheses, crystal structure and theoretical modelling of tetrahedral mono-β-diketonato titanocenyl complexes

Polyhedron 28 (2009) 966–974

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Syntheses, crystal structure and theoretical modelling of tetrahedral mono-b-diketonato titanocenyl complexes Annemarie Kuhn, Alfred Muller, Jeanet Conradie * Department of Chemistry, University of the Orange Free State, Bloemfontein 9300, South Africa

a r t i c l e

i n f o

Article history: Received 22 October 2008 Accepted 26 December 2008 Available online 23 February 2009 Keywords: Titanocene b-Diketone DFT Crystal structure

a b s t r a c t A comparative investigation on three novel bis(cyclopentadienyl) mono(b-diketonato) titanium(IV) complexes, [Cp2TiIV(R1COCHCOR2)]+ClO4 (Cp = g5-C5H5), i.e. [Cp2Ti(tfba)]+, [Cp2Ti(tfth)]+ and [Cp2Ti(tfba)]+ where tfba = CF3COCHCOC6H5, tfth = CF3COCHCOC4H3S and tffu = CF3COCHCOC4H3O, has been performed based on structural data and DFT calculations. The preparation of [Cp2TiIV(b-diketonato)]+ClO4 involves the reaction of Cp2TiCl2 with AgClO4 and the respective b-diketones. The crystal structures show that the structures are isomorphous. All the complexes exhibit p-stacking between one Cp ring and the aromatic R-group ring, i.e. the C6H5, C4H3S and C4H3O fragments, respectively. The DFT calculations show O p bonding. that the formal 16-electron count of these d0 titanium(IV) complexes is increased via Ti The bonding mode in the [Cp2Ti(b-diketonato)]+ complexes is different from that in Cp2Ti(OR)2 and Cp2Ti(dioxolene) complexes. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Titanocene dichloride [Cp2TiIVCl2] (Cp = g5-C5H5) is a convenient starting material for the preparation of many other organometallic compounds of titanium [1]. It has found application as a catalyst or catalyst component for a wide variety of hydrometallation and carbometallation reactions, as well as for other transformations involving Grignard reagents. In addition, titanocene dichloride [2] and budotitane, [TiIV(ba)2(OEt)2] [3] (ba = the b-diketonato ligand CH3COCHCOC6H5) exhibit variable anti-tumor activity for a variety of human tumors with reduced toxicity. Cationic [Cp2TiIV(b-diketonato)]+ complexes were first prepared by Doyle and Tobias [4]. Although only one crystal structure of a titanium(IV) compound containing the [Cp2Ti(b-diketonato)]+ cation, [Cp2Ti(acac)][Ir3(l3-S)2(CO)6] [5], is known, structures of the analogous d1 titanium(III) compounds [Cp2TiIII(R1COCHCOR2)], with R1, R2 = CH3 or C6H5 [6,7], have been reported. The coordination sphere of these complexes does not change when switching the redox state of the central coordinating metal from titanium(III) to titanium(IV), but the change in electron density on the central coordinating titanium should be reflected by changes in the bond lengths of the bonds to titanium. The electronic structure of the bent Cp2Ti2+ fragment had been rationalized by Lauher and Hoffmann years ago [8]. Structure determinations and extended Hückel calculations on [Cp2TiIV(OR)2] complexes containing monodentate OR ligands

* Corresponding author. Tel.: +27 51 4012194; fax: +27 51 4446384. E-mail address: [email protected] (J. Conradie). 0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2008.12.060

showed that short Ti–O bonds and large Ti–O–R angles resulted from in-plane Ti O p bonding involving an orbital of al symmetry on the Ti atom [9]. Since the chemistry of the bent metallocene fragment is mainly determined by the three low lying frontier orbitals, 1a1, b2 and 2a1 of Cp2Ti2+, we found it worthwhile to investigate both by experimental structure and computational chemical calculations the way that the bidentate oxygen donor b-diketonato ligand binds to the bent Cp2Ti2+ fragment. In this study we report the synthesis and characterization by Xray crystallography of three new [Cp2TiIV(R1COCHCOR2)]+ complexes with coordinated oxygen donor bidentate b-diketonato ligands. The need to further understand the titanium–oxygen bond led to theoretical DFT studies on the structure of the present and related bis(g5-cyclopentadienyl)titanium(IV) complexes. 2. Results and discussion 2.1. Synthesis of complexes The synthesis of the novel mono-b-diketonato titanium(IV) salts [Cp2Ti(CF3COCHCOR)]+ClO4 with CF3COCHCOR = CF3COCHCOC6H5 (tfba) (1), CF3COCHCOC4H3S (tfth) (2) and CF3COCHCOC4H3O (tffu) (3) is based on an anion metathesis reaction (which is driven by precipitation of one of the products), followed by a b-diketone substitution reaction according to Scheme 1. An important step is the solvation of the titanocene dichloride (Cp2TiCl2) which hydrolyses in pure water with the displacement of the two Cl ligands, forming a wide range of cationic species such as [Cp2Ti(H2O)(OH)]+, [Cp2Ti(H2O)(H2O)]2+ and [Cp2Ti(OH)2]

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2 Cl +

OH2

Cl H2O / THF

Ti

+

Cl

Ti

O

OH2 2 AgClO4

R

OH

+

O

OH

R

Ti

OH

ClO4

2 ClO4

CF3

Ti

H + HClO4 O

-2AgCl

CF3 (and other aquated species)

(and other aquated species)

R = C6H5 (1), C4H3S (2), C4H3O (3)

Scheme 1. Synthesis of [Cp2Ti(CF3COCHCOR)]+ClO4 complexes.

[10]. Addition of silver perchlorate (AgClO4) removes the Cl ions by precipitating silver chloride (AgCl) while the hydrolysed perchlorate titanium salt, e.g. [Cp2Ti(H2O)(OH)]+ClO4 remains in solution. Finally, the b-diketone replaces the hydrolysed leaving groups, forming an ionic species, [Cp2Ti(b-diketonato)]+ClO4, with ClO4 as the counter-ion. The problem of the insolubility of the bdiketones in water was overcome by means of a mixed solvent system, using a 1:2 ratio of H2O and THF. It was found that these salts could also be synthesized in anhydrous THF, in which case, instead of hydrolysis in water, a similar solvated species could be formed by displacement of the two Cl ligands in Cp2TiCl2 with OC4H4 instead of OH2. All the synthesized salts are insoluble in water, hexane and ether, slightly soluble in organic solvents such as chloroform, dichloromethane and are soluble in acetone, THF and ethanol. However, dissolving the product in ethanol caused it to decompose. The [Cp2Ti(b-diketonato)]+ cation is very sensitive to moisture in solution, but has been found to be stable in the solid state. 2.2. X-ray structure analysis The molecular diagrams showing the atomic labelling, accompanied by diagrams showing the molecular packing of 1, 2 and 3, respectively, are presented in Fig. 1. Crystal data and details of data collection and refinement are summarized in Table 1. Selected bond lengths and angles for the molecules are listed in Table 2. Structural data for similar complexes are also included in Table 2. Geometrical aspects: The crystal structures of complexes 1–3 revealed a pseudo-tetrahedral structure with the centroids of the cyclopentadienyl rings and the two oxygen atoms of the b-diketonato ring in the ancillary positions of the central titanium(IV) atom. It is evident, from examination of the bond angles (see Table 2), that the coordination tetrahedron is distorted. The (centroid Cp1)–Ti–(centroid Cp2) angle is on average ca. 134° for complexes 1–3, which is a significant deviation from the ideal tetrahedral bond angle of 109.5°. This large angle is attributed to steric hindrance between the Cp ligands. In contrast, the O1–Ti–O2 angle is on average ca. 86°, which is considerably smaller than the standard 109.5° angle. This small angle is attributed to constraints in accommodating the chelated b-diketonato ligand. The other angles around the Ti atom, ranging from 103.4° to 108.4°, approach those for an ideal tetrahedral geometry. Typical C–C and C–O single and double bond lengths are: C–C (1.54 Å), C@C (1.34 Å), C–O (1.43 Å), C@O (1.23 Å) [11]. The large double bond character of the C–C and C–O bonds in the chelate ring of the b-diketonato ligand (see Table 2) emphasize effective electron delocalization in the pseudo-aromatic core of the b-diketonato ligand. This electron delocalization enhances electronic communication between the R-groups of the b-diketonato ligand and the Ti centre. The b-diketonato ligand in [Cp2Ti(b-diketonato)]+ binds, within experimental error (taken as three times the bond e.s.d.), symmetrically to the titanium core in 1 and 2 and asymmetrically in 3, with a difference between the two Ti–O bond lengths of

0.014, 0.000 and 0.023 Å, respectively. This implies that the electron donating properties of the substituted non-CF3 groups Ph, C4H3S, C4H3O (vR = 2.21 [12], 2.10 [13], unknown) of the b-diketonato ligand do not show an observed effect on the Ti–O bond length in the solid state compared to the stronger electron withdrawing property of the CF3 group (vCF3 ¼ 3:01 [12]). The Ti–O bond lengths are shorter in the TiIV complexes (average 1.996 Å) compared to the TiIII analogues (average 2.076 Å), see Table 2, and this indicates that the more electron deficient TiIV centre binds oxygen more strongly than TiIII. The same trend is observed in the Ti–Cp bonds: the Ti–(centroid Cp) bonds (average 2.036 Å) for the TiIV complexes are ca. 0.03 Å shorter than those for the TiIII complexes (average 2.062 Å). The Cp rings of all the complexes tabulated in Table 2 are in a staggered conformation. From an examination of the Cambridge Structural Database (CSD) [14] it can be seen that the staggered sandwich conformation in bent metallocene Cp2TiIV(O-ligand)2 and Cp2TiIV(O,O0 -ligand) complexes is highly favored, although in many cases it was found that the energy barrier to cyclopentadienyl rotation is very small [15]. It has been shown that the choice of conformation is attributed largely to crystal packing effects [16] highlighted in the crystals of Cp2NbCl2 and Cp2MoCl2, where both the staggered and eclipsed conformations are found in the same crystal. Crystallographic aspects: The structures 1–3 are isomorphous, i.e. they crystallize in the same space group, P21/n, Z = 4, with unit cell parameters varying slightly to accommodate the differences in the substituted R-groups and are situated on fairly similar coordinates within the unit cell. The refinements of [Cp2Ti(tfba)]+ (1) and [Cp2Ti(tfth)]+ (2) showed large thermal vibrations on parts of the periphery of the molecules, such as the Cp ring and the CF3 and ClO4 groups. These large thermal vibrations may be attributed to the fact that the collections were done at room temperature and possibly also to loose packing environments in the periphery of the molecules. The thermal vibrations were treated by disordered refinement techniques, involving geometric and anisotropic restraints to obtain more satisfactory refinement results. In the data collection of 3, where sub-zero temperatures were utilized, these disorders became less pronounced and were considered negligible in the final refinements. Site occupancies for both disordered parts of 1 and 2 (indicated as A and B in Fig. 1) were refined freely but restricted to add up to one. These refined to ratios which are shown in Table 3. Hydrogen-interactions, although relatively weak, also play a part in site occupation ratios. There are H-interactions between the atoms in the higher occupied site, e.g. the interactions C2– H2F1A have higher occupation rates of 56.9% and 83.7% (position A) for the disordered F atoms in [Cp2Ti(tfba)]+ (1) and [Cp2Ti(tfth)]+ (2), respectively. Similarly, H-interactions with perchlorate exist, which lead to the higher occupation rates of 80.0% and 58.9% (position A) for the disordered O atoms in the perchlorate ion. The fact that the Cp ring, marked C21 to C26 in the molecular diagram of Fig. 1, is not disordered in all three cases, while the other

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Fig. 1. Molecular structure indicating the numbering scheme (with 30% probability displacement ellipsoids, left) and packing viewed along the a-axis (right) of (a) [Cp2Ti(tfba)]+ClO4 and (b) [Cp2Ti(tfth)]+ClO4 at T = 20° and (c) [Cp2Ti(tffu)]+ClO4 at T = 123°. Monomeric cationic entities in the crystal are linked by p–p stacking interactions (indicated by double-headed arrows) forming dimeric units (perchlorate ions have been removed for the sake of clarity).

Cp ring (marked C11 to C16) is disordered in [Cp2Ti(tfba)]+ (1) and [Cp2Ti(tfth)]+ (2) may be attributed to the fact that the non-disordered Cp rings are involved in the observed p-stacking as described below, forcing this ring (C21 to C26) into a more restricted packing environment. Packing features. The isomorphous complexes 1–3 have very similar packing arrangements, with p–p stacking between one Cp ring and the R-group ring of the b-diketonato ligand, i.e. the C6H5, C4H3S and C4H3O fragments of tfba, tfth and tffu, respectively. The p–p stacking interactions are between pairs of Ti complexes, forming dimeric units, see Fig. 1 (right). The planes of the p–p stacked rings are nearly parallel in [Cp2Ti(tfth)]+ (2), with only a small deviation of 0.9° between the

planes, compared to 7.4° and 11.0° in the corresponding complexes 3 and 1, respectively. Geometrical parameters describing the p–p stacking [17] are given in Fig. 2. In all three cases (tfba, tfth and tffu), the rings are displaced from each other, in a slipped or offset alignment. The centroid–centroid separation, Rcen, of 3.724, 3.590 and 3.667 Å and the displacement (centre-normal) angle, h, of 14.4°, 10.4° and 12.6° corresponds to a horizontal displacement of the ring centroids, d, of 0.926, 0.648 and 0.800 Å, respectively. The shortest C–H separations between the rings are 3.264, 3.486 and 3.217 Å. These parameters fall well within those required for effective p–p stacking [18]. The packing pattern observed here appears to be unique to the TiIV bis(cyclopentadienyl) mono(b-dikeanalogues, i.e. tonato) complexes because the TiIII

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A. Kuhn et al. / Polyhedron 28 (2009) 966–974 Table 1 Crystal data and structure refinement for [Cp2Ti(CF3COCHCOR)]+ClO4 with R = C6H5, C4H3S and C4H3O.

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 (mm3) Theta range for data collection (°) Index ranges 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)] R indices (all data) Largest difference in peak and hole (e Å3)

[Cp2Ti(tfba)]+ClO4

[Cp2Ti(tfth)]+ClO4

[Cp2Ti(tffu)]+ClO4

TiO6C20H16ClF3 492.68 red, cubic monoclinic P21/n

TiO6C18H14ClF3S 498.70 red, plate monoclinic P21/n

TiO7C18H14ClF3 482.64 red, plate monoclinic P21/n

10.088(2) 16.613(3) 12.443(3) 90 96.86(3) 90 2060.4(7) 4 1.588 20 0.71073 0.607 1000 0.12  0.12  0.12 2.06–25.00 11 6 h 6 11, 11 6 k 6 19, 14 6 l 6 14 11 213 3622 [0.1037] 100.0 semi-empirical from equivalents 0.9307 and 0.9307 full-matrix least-squares on F2 3622/367/391 1.010 R1 = 0.0601, wR2 = 0.1294 R1 = 0.1852, wR2 = 0.1812 0.355 and 0.385

9.872(2) 15.346(3) 13.134(3) 90 97.46(3) 90 1973.1(7) 4 1.679 20 0.71073 0.737 1008 0.20  0.14  0.06 2.05–28.30 13 6 h 6 9, 20 6 k 6 18, 17 6 l 6 13 13 654 4871 [0.0727] 99.3 semi-empirical from equivalents 0.9571 and 0.8666 full-matrix least-squares on F2 4871/319/383 0.969 R1 = 0.0517, wR2 = 0.1166 R1 = 0.1628, wR2 = 0.1555 0.300 and 0.427

10.0303(5) 16.0338(9) 11.6124(6) 90 97.046(2) 90 1853.45(17) 4 1.730 123 0.71073 0.677 976 0.27  0.11  0.07 2.41–28.32 13 6 h 6 13, 21 6 k 6 20, 15 6 l 6 15 17 806 4606 [0.0316] 99.4 semi-empirical from equivalents 0.9542 and 0.8384 full-matrix least-squares on F2 4606/0/271 1.046 R1 = 0.0339, wR2 = 0.0787 R1 = 0.0465, wR2 = 0.0858 0.586 and 0.364

Table 2 Selected bond lengths (Å) and angles (°) for [Cp2TiIV(b-diketonato)]+ and Cp2TiIII(b-diketonato) complexes. b-Diketonato ligand

Ti–O1 Ti–O1 Ti–O2 O1–C1 O2–C3 C1–C2 C2–C3 Ct1–Tib,c Ct1–Tib,c Ct2–Tib O1O2 O1–Ti–O2 Ct1–Ti–Ct2b O1–C1–C2 C1–C2–C3 O2–C3–C2 ud

xe a b c d e

[Cp2TiIVb]+

Cp2TiIIIb 5a

tfba

tfth

tffu

acac

acac6a

ba7a

dbm7a

1.979(4) 1.979(4) 1.993(5) 1.281(7) 1.283(8) 1.389((9) 1.350(9) 2.007(13) 2.059(15) 2.023(3) 2.705 85.91(19) 136.3 120.7(6) 124.7(6) 127.1(6) 46.6 7.9

1.996(3) 1.996(3) 1.996(3) 1.277(4) 1.283(5) 1.408(6) 1.359(6) 2.000(2) 2.033(2) 2.037(2) 2.720 85.98(12) 133.4 121.8(4) 123.0(4) 127.8(4) 48.4 19.4

1.9914(12) 1.9914(12) 2.0144(13) 1.283(2) 1.281(2) 1.411(3) 1.369(3) 2.0333(9)

1.969 1.969 1.981 1.295 1.279 1.344 1.404 2.033

2.068 2.068 2.068 1.287 1.260 1.360 1.390 2.066

2.085 2.085 2.078 1.261 1.268 1.388 1.385 2.067

2.067 2.067 2.087 1.261 1.286 1.401 1.388 2.060

2.0333(8) 2.729 85.90(5) 133.3 123.06(16) 121.79(16) 128.15(17) 47.4 19.9

2.036 2.710 86.6 133.8 123.4 126.1 123.4 47.5 16.8

2.052 2.775 84.3 134.4 126.8 123.4 125.7 45.9 4.6

2.070 2.758 83.0 135.2 123.8 124.9 125 46.6 1.0

2.064 2.740 82.5 134.3 124.8 122.8 125.4 44.9 13.3

acac, CH3COCHCOCH3; ba, CH3COCHCOC6H5; dbm, C6H5COCHCOC6H5. Ct1, centroid of plane defined by C11–C15; Ct2, centroid of plane defined by C21–C25. Cp1 disordered, two positions. u, angle between the two Cp planes. x, dihedral angle between the b-diketonato ligand plane (through atoms O1–C1–C2–C3–O2) and the O1–Ti–O2 plane.

[Cp2TiIII(CH3COCHCOC6H5)] [6] and [Cp2TiIII(C6H5COCHCOC6H5)] [7], display no p–p stacking. These Cp2TiIII(b) complexes do not have a counter-ion such as ClO4 in the case of TiIV which might influence the observed packing.

2.3. Theoretical studies Density functional theory (DFT) calculations were carried out on complexes 1–3 and [Cp2TiIV(CH3COCHCOCH3)]+ (4) to study

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Table 3 Site occupancies (%) for disordered parts (marked A and B in Fig. 1) of the Cp1-ring and the CF3 and ClO4 groups in [Cp2Ti(tfba)]+ClO4 (1) and [Cp2Ti(tfth)]+ClO4 (2). Crystal data collected at T = 20°. Complex

Disordered group

[Cp2Ti(tfba)]+ (1) [Cp2Ti(tfth)]+ (2)

CF3 FA:FB

ClO4 OA:OB

Cp1 CA:CB

56.9:43.1 83.7:16.3

80.0:20.0 58.9:41.1

42.0:58.0 80.0:20.0

Surface normal R cen Ring centroid θ

Rcd

ϕ

d d = displacement of ring centroids Rcen = centroid-centroid separation Rcd = closest contact distance between rings θ = centre-normal angle ϕ =angle between π-stacked planes Fig. 2. Definition of the p–p stacking geometrical parameters, Rcen (centroid– centroid separation), Rcd (closest contact distance) and h (centre-normal angle) between the two aromatic rings involved in slipped or offset non-parallel alignment of p–p stacking. Note: The two benzene rings displayed in the figure can be any aromatic ring, e.g. C6H5, C4H3O or C4H3O.

the ground state geometrical and electronic properties of the bdiketonato interactions with the titanium centre in monomeric complexes of the type [Cp2TiIV(R1COCHCOR2)]+. Since these density functional methods are applied to b-diketonato titanocenyl complexes for the first time and reported here, some measure on the reliability of the approach was obtained by comparing the calculated data with the known single crystal X-ray diffraction structural data of 1–4. The root-mean-square distances (RMSD)

calculated for non-hydrogen atoms for the best three-dimensional superposition of calculated structures on experimental structures give a qualitative measurement of the accuracy of the ground state geometry of the calculated structures. Excellent agreement between experimental and theoretical structures is obtained, as reflected by the RMSD values of 0.20, 0.08, 0.07 and 0.04 Å for 1–4, respectively. For 1 and 2, containing disordered parts, the parts with the highest site occupancy were used in the RMSD calculation. The largest deviation from the experimental structure was found for 1, containing a large % of disordered parts. Fig. 3 displays the optimized structures of 1–4 with selected calculated geometrical parameters. All the bonds in the Ti-b-diketonato ring structure of 1–4 were reproduced by DFT calculations to within 0.01–0.03 Å of the experimental values. Since comparisons of experimental metal–ligand bond lengths with calculated bond lengths below a threshold of 0.02 Å are considered as meaningless [19], the computational method used thus gives a good account of the experimental bond lengths. The O1–Ti–O2 angles were calculated accurately to within 1°. The DFT optimized structures containing titanocene with the Cp rings in the staggered or in the eclipsed conformation are approximately equi-energetic, with no preference for either conformation. This result is in agreement with the experimental observation that the choice of conformation is attributed to crystal packing effects and is counter-ion dependant [16]. An interesting observation for experimental structure 2 is that the sulfur on the aromatic thienyl group is orientated towards the methine hydrogen of the b-diketonato ligand; this is in contrast to the orientation in the uncoordinated Htfth ligand [20]. The same result was found for the orientation of the O atom of the furyl group in the crystal structure of 3 [21]. Theoretical calculations, however, did not give any preference for the orientation of the thienyl or furyl group in the optimized ground state structure of 2 or 3, respectively. The energy needed for rotation between the orientations is 11.3 kcal/mol for 2, showing that this rotation may easily occur at room temperature [22], see Fig. 4. Electronic structure: A bent Cp2Ti2+ fragment has C2v symmetry if the Cp ligands have an eclipsed geometry, and Cs symmetry if the

Fig. 3. The PW91/TZP calculated minimum energy geometries of 1–4. Angles (°) and bond lengths (Å) are as indicated.

A. Kuhn et al. / Polyhedron 28 (2009) 966–974

971

relative energy /kcal/mol

-5642 -5644 -5646 -5648 -5650 -5652 -5654 -5656 0

50

100

150

200

dihedral angle Fig. 4. Rotation of 2 from the isomer with the S orientated towards the methine H to the isomer with S orientated in the same direction as the O, as a function of the O1–C1–C31–S dihedral angle where all other internal coordinates have been optimized at each point.

rings are staggered. The Cs symmetry is preserved when a symmetrical b-diketonato ligand is complexed to Cp2Ti2+. The electronic structure of [Cp2Ti(acac)]+ with Cs symmetry is thus presented here as a representative example of the [Cp2Ti(b-diketonato)]+ complexes 1–4. To understand the way in which the acac ligand binds, the interaction between the frontier orbitals of a bent Cp2Ti2+ fragment [8] and the HOMOs of the acac ligand is schematically illustrated in Fig. 5. The usual in-plane r-type Ti–O interaction is observed where the unoccupied b2 LUMO+1 of Cp2Ti2+ interacts with the occupied b2 HOMO of acac (forming HOMO5 of [Cp2Ti(acac)]+) and the unoccupied 2a1 LUMO+2 of Cp2Ti2+ interacts with the occupied a1 HOMO2 of acac (forming HOMO6 of [Cp2Ti(acac)]+). The [Cp2Ti(acac)]+ complex is formally a 16-electron d0 complex if only Ti–O r-bonding is considered. The electron count is increased by Ti O p donation. A moderate to weak ptype interaction is observed in the HOMO2 and HOMO7 of the [Cp2Ti(acac)]+ complex, see Fig. 5 for a schematic presentation of the interaction and Fig. 6 for presentation of the orbitals involved in Ti–O bonding. The above described Ti–O interactions result in Ti–O bonds of an average 1.99 Å for 1–4, slightly longer than that for determined for Cp2TiIV(OR)2 complexes containing two monodentate OR ligands, i.e. 1.96 Å for Cp2Ti(cyanoacetato)2 [23] and 1.93 Å for Cp2Ti(benzoato)2 [9] and comparable to the Ti–O bond length of 1.97 Å in [g2-OC(Ph)@C(Ph)O]TiCp2, containing a dioxolate bidentate ligand [24]. One interesting feature of the [Cp2Ti(b-diketonato)]+ structures is the degree of out-of-plane folding of the b-diketonato ligand, as schematically shown in Fig. 7. The folding of [Cp2Ti(b-diketonato)]+ along the OO axis is x = 7.9°, 19.4°, 19.9° and 16.8° for 1–4, respectively. Similar folding is observed in the series of d0 penta metallacyclic structures, i.e. Cp2Ti(dioxolene) [24], Cp2Ti(dithiolene) [25] and Cp2Ti(diselenolene) [26] complexes, with an increasing degree of folding along the OO, SS and SeSe axes of 38°, 42–50° and 48–52°, respectively. It was rationalized by Lauher and Hoffmann [8] using extended Hückel (EH) calculations that the folding in the Cp2Ti(dithiolene) complexes was due to a stabilizing interaction between the empty acceptor LUMO 1a1 orbital of the Cp2Ti2+ fragment and the HOMO b1 orbital of the dithiolene ligand, which was only possible by a symmetry lowering of the Cp2Ti(dithiolene) complex to the C2 conformation and associated folding of the TiS2C2 metallacycle. In this study we confirmed this Ti L (L = O, S, Se) p interaction (between the empty acceptor 1a1 LUMO of the Cp2Ti2+ and the b1 HOMO of the ligand) with DFT calculations for the above pentacyclic series, Cp2(TiO2C2R2),

Fig. 5. A schematic molecular orbital (MO) diagram for the interaction between Cp2Ti2+ and the acac unit in 4. The y-axis denotes the relative energy in eV for the molecular orbitals of the [Cp2Ti(acac)]+ complex 4. The MO energy levels indicated in blue do not have any Ti–O interaction.

Fig. 6. Selected occupied MO’s of 4, illustrating the Ti–O r and Ti

O p interaction.

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R1 O Ti

L

ω

Ti

L

θ

Ti O

Cp2TiIV(L,L−BID)

θ

Ti L

R2 [Cp2TiIV(β)]+

ϕ

R

ϕO

R

R

L = O, S, Se

e.g. Cp2TiIV(dioxolene)

O

θ

Cp 2TiIV(OR)2

Fig. 7. Folding round the LL axis in the Cp2Ti(LL-BID) complexes (LL-BID = bidentate ligand with donor atoms L, L = O, S or Se) is described by the dihedral angle, x, between the cyclic ligand plane and the L–Ti–L plane. In the Cp2Ti(OR)2 complexes containing monodentate ligands OR, the large h and u angles describe the degree of bending along Ti–O–R.

Fig. 8. The 1a1 type MO of the [Cp2Ti(b-diketonato)]+, Cp2Ti(dioxolene), Cp2Ti(dithiolene), Cp2Ti(diselenolene) and Cp2Ti(OR)2 complexes illustrating the 1a1–p interaction.

Cp2(TiS2C2R2) and Cp2Ti(Se2C2R2) (R = CH3), see Fig. 8. However, no such Ti O p interaction was observed for the hexacyclic [Cp2Ti(b-diketonato)]+ complexes 1–4 where the 1a1 LUMO of the Cp2Ti2+ fragment remains non-bonding in [Cp2Ti(acac)]+, see Fig. 7. Thus we conclude that the folding in the [Cp2Ti(b-diketonato)]+ complexes is not due to the stabilization of the same electronic interaction as in the Cp2Ti(dioxolene) [24], Cp2Ti(dithiolene) [25] and Cp2Ti(diselenolene) [26] series. Bending along Ti–O–R, stabilized by Ti O p interactions, is observed in Cp2TiIV(OR)2 complexes. Extended Hückel (EH) calculations [9] revealed that the in-plane Ti O p interaction involving the 1a1 LUMO of Cp2Ti2+ and the b1 HOMO of the ligand was made possible by the large O–Ti–O (h = 89–91°) and Ti–O–R (u = 143–148°) angles [9,23, 27]. The in-plane Ti O p interaction was reproduced with DFT in the present study for Cp2TiIV(OCH3)2 and is illustrated in Fig. 8. The strain of the b-diketonato ring structure in [Cp2Ti(b-diketonato)]+ complexes with a O–Ti–O angle of ca. 86°, however, makes large O–Ti–O and Ti–O–C angles impossible. Thus stabilization through in-plane Ti O p interaction is thus geometrically not possible for [Cp2TiIV(b-diketonato)]+ complexes. The potential energy as a function of the folding along the OO, SS and SeSe axes, with all other internal coordinates being optimized at each point, was calculated for the model complexes [Cp2Ti(b-diketonato)]+, Cp2Ti(dioxolene), Cp2Ti(dithiolene) and Cp2Ti(diselenolene). A plot of potential energy as a function of folding indicates that it takes less than 1 kcal/mol to fold the [Cp2Ti(acac)]+ complex from the minimum energy conformation, with a folding angle of 17°, to planar. This energy is substantially increased in the pentacyclic series, where the energy needed is 7.4, 16.7 and 16.0 kcal/mol for the Cp2Ti(O2C2(CH3)2), Cp2Ti(S2C2(CH3)2) and Cp2Ti(Se2C2(CH3)2) complexes, respectively. 3. Conclusions New titanocenyl complexes of the type [Cp2Ti(CF3COCHCOR)]+ClO4 with R = C6H5, C4H3S and C4H3O were synthesized and characterized. Crystallographic studies showed that all com-

plexes exhibit p–p stacking between one Cp ring and the aromatic R-group ring, i.e. the C6H5, C4H3S and C4H3O fragments, respectively. DFT calculations show that [Cp2TiIV(b-diketonato)]+ complexes behave differently to Cp2TiIV(OR)2 and Cp2TiIV(dioxolene) complexes in that no significant bonding interaction between the 1a1 LUMO of the Cp2Ti2+ fragment and the coordinating ligand is observed. 4. Experimental 4.1. Materials and apparatus Solid reagents (Merck, Aldrich and Sigma) were used without further purification. Liquid reactants and solvents were distilled prior to use using standard purification techniques [28] and water was double distilled. Melting points were determined with a Reichert Thermopan microscope equipped with a Kofler hot-stage and are uncorrected. NMR measurements were recorded on a Bruker Advance DPX 300 NMR spectrometer. Chemical shifts are reported as d values relative to SiMe4 (0 ppm). IR spectra were recorded from neat samples on a Digilab FTS 2000 Fourier transform spectrometer utilizing a He–Ne laser at 632.6 nm. 4.2. Synthesis Safety note: Perchlorate complexes are potentially explosive, should be treated with caution and handled in small quantities. 4.2.1. Preparation of [Cp2Ti(tfba)]+ClO4– (1) Titanocene dichloride, Cp2TiCl2 (249.0 mg, 1.0 mmol) was dissolved in H2O/THF, 1:2 mixture (9 ml) and stirred under nitrogen for ½ h. The solution was cooled down on an ice-bath before silver perchlorate, AgClO4 (406 mg, 1.96 mmol), dissolved in water (2 ml) was added. The cooled mixture was stirred for a further ½ h. AgClO4 is light sensitive and the reaction must be shielded from light. Silver chloride, a white precipitate, was filtered off and washed with H2O/THF (2 ml). Trifluorobenzoylacetone, Htfba (432 mg,

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2.0 mmol), dissolved in cold THF (2 ml, ca. 4 °C) was added dropwise to the orange filtrate while stirring, causing a slight colour change. After ½ h of stirring the solution was removed and left for 7 days to precipitate (turned dark brown/black). The dark sticky precipitate was collected and washed with water (2  100 ml) and diethyl ether (4  50 ml). Recrystallisation from DCM/hexane afforded the pure product. Spectroscopically pure crystals suitable for single X-ray analysis were obtained by recrystallisation from DCM/hexane. Yield 51% (246 mg). M.p. = 200 °C. Colour: metallic brown. mCO = 1554 cm1. NMR: dH (300 MHz, acetone–d6)/ppm: 7.16 (s, 10H, 2  C5H5), 7.55 (s, 1H, CH), 7.68 (t, 2H, C6H5), 7.86 (t, 1H, C6H5), 8.35 (d, 2H, C6H5). Anal. Calc. for TiC20H16O6ClF3: C, 48.75; H, 3.27. Found: C, 48.13; H, 3.22%. 4.2.2. Preparation of [Cp2Ti(tfth)]+ClO4– (2) Preparation as for 1, but replacing the trifluorobenzoylacetone with trifluorothenoylacetone, Htfth (461 mg, 2.0 mmol). Yield 48% (235 mg). M.p. = 191 °C. Colour: dark brown. mCO = 1566 cm1. NMR: dH (300 MHz, acetone–d6)/ppm 7.13 (s, 10H, 2  C5H5), 7.38 (s, 1H, CH), 7.50 (t, 1H, C4H3S), 8.42 (d, 1H, C4H3S), 8.56 (d, 1H, C4H3S). Anal. Calc. for TiC18H14SO6ClF3: C, 43.35; H, 2.83. Found: C, 43.77; H, 2.87%. 4.2.3. Preparation of [Cp2Ti(tffu)]+ClO4– (3) Preparation as for 1, but replacing the trifluorobenzoylacetone with trifluorofuroylacetone, Htffu (412 mg/2.0 mmol). Yield 43% (203 mg). M.p. = 195 °C. Colour: brown. mCO = 1568 cm1. NMR: dH (300 MHz, acetone–d6)/ppm: 7.00 (t, 1H, C4H3O), 7.13 (s, 10H, 2  C5H5), 7.18 (s, 1H, CH), 8.07 (d, 1H, C4H3O), 8.28 (d, 1H, C4H3O). Anal. Calc. for TiC18H14O7ClF3: C, 44.79; H, 2.92. Found: C, 44.71; H, 2.87%. 4.3. X-ray crystal structure determination The X-ray intensity data for 1 and 2 were measured on a Bruker SMART 1K CCD area detector and that for 3 on a Bruker X8 Apex II 4K CCD area detector. Both instruments were equipped with a graphite monochromator and a 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 4.00 cm from the crystal. Data collection temperature for 1 and 2 was room temperature (293 K), whereas the temperature during the collection of 3 was 150(2) K, using an Oxford 700 series cryostream cooler. The initial unit cell and data collection of 3 were achieved by the APEX2 software [29] utilizing COSMO [30] 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 initial unit cell and data collection for 1 and 2 were achieved by the SMART-NT software [31], set up for data collection of more that a hemisphere of reciprocal space. A total of 1350 frames were collected with a scan width of 0.3° in u and exposure times of 10 s per frame. The frames were integrated using a narrow frame integration algorithm and reduced with the Bruker SAINTPLUS and XPREP software [32] packages, respectively. Analysis of all of the data collections showed no significant decay during the data collection. Data were corrected for absorption effects using the multi-scan technique SADABS [33]. All the structures were solved by the direct methods package SIR97 [34] and refined using the WINGX software package [35] incorporating SHELXL [36]. The largest peaks on the final difference electron densities and the deepest holes were all within 1 Å from nonhydrogen atoms and presented no physical meaning in the final refinements. Aromatic protons were placed in geometrically idealized positions (C–H = 0.93–0.98 Å) and constrained to ride on their parent atoms with Uiso(H) = 1.2Ueq(C). Disorders on the perchlorate

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ions, CF3 moieties and those on the Cp rings were treated with various geometrical and vibrational restraints. Atomic scattering factors were taken from the International Tables for Crystallography Volume C [37]. The molecular plot was drawn using the DIAMOND program [38] 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. 4.4. Quantum computational methods Pure Density Functional Theory (DFT) calculations were carried out using the Amsterdam Density Functional 2007 (ADF) program system [39] with the PW91 (Perdew–Wang, 1991) exchange and correlation functional [40]. The TZP (Triple f polarized) basis set, a fine mesh for numerical integration (5.2 for geometry optimizations), a spin-restricted formalism and full geometry optimization (gas-phase) with tight convergence criteria, as implemented in the ADF 2007 program, were used. The accuracy of the computational method was evaluated by comparing the root-mean-square deviations (RMSD’s) between the optimized molecular structure and the crystal structure, using the non-hydrogen atoms in the molecule. RMSD values were calculated using the ‘‘RMS Compare Structures” utility in CHEMCRAFT Version 1.5 [41]. Whether artificially generated atomic coordinates or coordinates obtained from X-ray crystal data were used in the input files, optimizations for each compound resulted in the same optimized geometry. The optimized structures were verified as a minimum through frequency calculations. Unless indicated, no symmetry limitations were imposed in the calculations. Acknowledgements Financial assistance from the South African National Research Foundation under Grant No. 61093 and the Central Research Fund of the University of the Free State is gratefully acknowledged. Appendix A. Supplementary data CCDC 693411, 693410 and 6934 contains the supplementary crystallographic data for [(C5H5)2Ti(CF3COCHCOR)]+ClO4 with R = C6H5 (1), C4H3S (2) and C4H3O (3), respectively. 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) 1223336-033; or e-mail: [email protected]. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.poly.2008.12.060. References [1] N.J. Long, Metallocenes: An Introduction to Sandwich Complexes, Blackwell Science, London, 1998. p. 23. [2] P. Köpf-Maier, Eur. J. Clin. Pharmacol. 47 (1994) 1. [3] B.K. Keppler, C. Friesen, H.G. Moritz, H. Vongerichten, E. Vogel, Struct. Bonding 78 (1991) 97. [4] (a) G. Doyle, R.S. Tobias, Inorg. Chem. 6 (1967) 1111; (b) G. Doyle, R.S. Tobias, Inorg. Chem. 7 (1968) 2484. [5] M. Casado, J.J. Perez-Torrente, M.A. Ciriano, A.J. Edwards, F.J. Lahoz, L.A. Oro, Organometallics 18 (1999) 5299 (CSD Ref.: LIHDUB). } gel, F. Marken, Inorg. Chim. Acta 235 [6] A.M. Bond, R. Colton, U. Englert, H. Hu (1995) 117 (CSD Ref.: HIDPOZ). [7] S.S. Yun, I. Suh, E.H. Kim, S. Lee, J. Coord. Chem. 51 (2000) 219 (CSD Ref.: MOYJEP, MOYJIT). [8] J.W. Lauher, R. Hoffmann, J. Am. Chem. Soc. 98 (1976) 1729. [9] D.M. Hoffman, N.D. Chester, R.C. Fay, Organometallics 2 (1983) 48. [10] J.H. Toney, T.J. Marks, J. Am. Chem. Soc. 107 (1985) 947. [11] R.C. Weast, Handbook of Chemistry and Physics, 63rd Ed., The Chemical Rubber Co., Ohio, p. F180. [12] W.C. Du Plessis, J.C. Erasmus, G.J. Lamprecht, J. Conradie, T.S. Cameron, M.A.S. Aquino, J.C. Swarts, Can. J. Chem. 77 (1999) 378.

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