Can Related Pyridine−Alkoxide Titanium Complexes Adopt Different

In order to elucidate the reasons why similar complexes adopt different geometries, without having a significant impact on the bond distances, the str...
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Organometallics 2009, 28, 1329–1335

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Can Related Pyridine-Alkoxide Titanium Complexes Adopt Different Geometries? A Combined Experimental and Theoretical Study Rosa Fandos,† Beatriz Gallego,† Marı´a Isabel Lo´pez-Solera,‡ Antonio Otero,*,‡ Ana Rodrı´guez,§ Marı´a Jose´ Ruiz,† Pilar Terreros,| and Tanja van Mourik⊥ Departamento de Quı´mica Inorga´nica, Orga´nica y Bioquı´mica, UniVersidad de Castilla-La Mancha, Facultad de Ciencias del Medio Ambiente, AVda. Carlos III, s/n 45071 Toledo, Spain, Departamento de Quı´mica Inorga´nica, Orga´nica y Bioquı´mica, UniVersidad de Castilla-La Mancha, Facultad de Quı´micas, Campus de Ciudad Real, 13071 Ciudad Real, Spain, Departamento de Quı´mica Inorga´nica, Orga´nica y Bioquı´mica, UniVersidad de Castilla-La Mancha, ETS Ingenieros Industriales, AVda. Camilo Jose´ Cela, 3, 13071 Ciudad Real, Spain, Instituto de Cata´lisis y Petroleoquı´mica, CSIC, Cantoblanco, 28049, Madrid, Spain, and School of Chemistry, UniVersity of St. Andrews, North Haugh, St. Andrews, Fife KY16 9ST, Scotland, U.K. ReceiVed October 24, 2008

A series of new titanium pyridine-alkoxide complexes, [TiCp*Cl{2,6-(OCH2)2py-κ3-O,N,O}] (2), [TiCp*{2,6-(OCH2)py(CH2OH)-κ1-O}{2,6-(OCH2)2py-κ3-O,N,O}] (3), [TiCp*Me2{2-(OCH2)py-κ1-O}] (4), [TiCp*Me{2-(OCH2)py-κ1-O}2] (5), and [TiCp*(O){2-(OCH2)py-κ2-O,N})] (6), have been synthesized. All of these compounds were characterized by NMR spectroscopy. The single-crystal structures of [TiCp*Me{2,6-(OCH2)2py-κ3-O,N,O}] (1), 2, 3, and 4 were determined and revealed the presence of piano and nonpiano stool geometries. The molecular structures of 1, 2, and 3 were also studied by means of density functional theory (DFT) in an attempt to rationalize the possible reasons for the stabilization of one or other geometry. Introduction Oxygen donor ligands such as alkoxides and aryloxides are extremely versatile ligands, and they are among the most widely used in organometallic and coordination chemistry.1 Accordingly, such ligands have been traditionally used as ancillary frameworks for group 42 and 53 metal centers because they usually yield kinetically stable complexes. In recent years, we have been interested in the development of studies based on the preparation of several families of cyclopentadienyl-containing early transition metal complexes with different classes of

* To whom correspondence should be addressed. E-mail: antonio.otero@ uclm.es. † Facultad de Ciencias del Medio Ambiente. ‡ Facultad de Quı´micas. § ETS Ingenieros Industriales. | Instituto de Cata´lisis y Petroleoquı´mica. ⊥ University of St. Andrews. (1) Bradley, D. C.; Mehrotra, R. C.; Singh, A.; Rothwell, I. P. Alkoxo and Aryloxo DeriVatiVes of Metals; Academic Press: London, 2001. (2) Selected references: (a) Hill, J. E.; Balaich, G.; Fanwick, P. E.; Rothwell, I. P. Organometallics 1993, 12, 2911. (b) Thorn, M. G.; Fanwick, P. E.; Rothwell, I. P.; Chesnut, R. W. Chem. Commun. 1999, 2543. (c) Thorn, M. G.; Etheridge, Z. C.; Fanwick, P. E.; Rothwell, I. P.; Chesnut, R. W. J. Organomet. Chem. 1999, 591, 148. (d) Tshuva, E. Y.; Versano, M.; Goldberg, I.; Kol, M.; Weitman, H.; Goldschmidt, Z. Inorg. Chem. Commun. 1999, 2, 371. (e) Thorn, M. G.; Vilardo, J. S.; Lee, J.; Hanna, B.; Fanwick, P. E.; Rothwell, I. P.; Chesnut, R. W. Organometallics 2000, 19, 5636. (f) Mulford, D. R.; Fanwick, P. E.; Rothwell, I. P. Polyhedron 2000, 19, 35. (3) Selected references: (a) Chamberlain, L. R.; Rothwell, I. P.; Huffman, J. C. Inorg. Chem. 1984, 23, 2578. (b) Chamberlain, L. R.; Rothwell, I. P.; Folting, K.; Huffman, J. C. J. Chem. Soc., Dalton Trans. 1987, 155. (c) Fanwick, P. E.; Ogilvy, A. E.; Rothwell, I. P. Organometallics 1987, 6, 73. (d) Vilardo, J. S.; Salberg, M. M.; Parker, J. R.; Fanwick, P. E.; Rothwell, I. P. Inorg. Chim. Acta 2000, 299, 135.

multifunctional ligands.4 We recently described5 the synthesis of a monocyclopentadienyl titanium complex with a chelating pyridine dialkoxide ligand, namely, [TiCp*Me{2,6-(OCH2)2pyκ3-O,N,O}], and we are now interested in the preparation and structural characterization of this and related complexes. Although the monocyclopentadienyl complexes of titanium normally have a three- or four-legged piano stool structure (pseudosquare pyramidal SP),6 we found in this study that two of the synthesized complexes containing a pyridine dialkoxide pincer moiety have an unusual nonpiano stool arrangement (distorted trigonal-bipyramidal TBP) of the ligands around the metal center, whereas other related complexes have the wellestablished piano stool structure. We report here the results of this study together with electronic structure calculations at the density functional theory (DFT) level.

Results and Discussion The complexes [TiCp*X{2,6-(OCH2)2py-κ3-O,N,O}] [X ) Me (1),5 Cl (2)] can be obtained by reaction of 1 equiv of 2,6pyridinedimethanol with [TiCp*X3] (X ) Me, Cl), respectively (Scheme 1). Complex 2 was isolated in moderate yield (67%) as an orange air-sensitive solid that is soluble in toluene and THF but insoluble in diethyl ether and pentane. (4) See as an illustrative example: Conde, A.; Fandos, R.; Otero, A.; Rodriguez, A. Organometallics 2008, 27, 6090. (5) Fandos, R.; Herna´ndez, C.; Otero, A.; Rodrı´guez, A.; Ruiz, M. J.; Terreros, P. Chem. Eur. J. 2003, 9, 671. (6) (a) The Cambridge Structural Database: a quarter of a million structures and rising. Allen, F. H. Acta Crystallogr. 2002, B58, 380. (b) ConQuest 1.10. New Software for searching the Cambridge Structural Database and visualizing crystal structures. Bruno, I. J.; Cole, J. C.; Edington, P. R.; Kessler, M.; Macrae, C. F.; McCabe, P.; Pearson, J.; Taylor, R. Acta Crystallogr. 2002, B58, 389.

10.1021/om8010239 CCC: $40.75  2009 American Chemical Society Publication on Web 01/23/2009

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Figure 1. ORTEP drawings of complexes 1 and 2 with the atomic labeling schemes; hydrogen atoms are omitted for clarity; thermal ellipsoids are at the 30% level of probability. Scheme 1

Complex 2 was characterized spectroscopically. The 1H NMR spectrum of 2 in C6D6 shows a singlet at 2.19 ppm corresponding to the Cp* methyl groups. The methylene protons of the alkoxide moiety appear as two doublets at 5.45 and 5.66 ppm, as expected from their equivalence,7 whereas the aromatic pyridinic protons appear as two multiplets at 6.21 and 6.65 ppm. These values are comparable to those previously found for complex 1.5 Accordingly, we propose a similar structural disposition for 2 (see Scheme 1), where two oxygen atoms of the pyridine alkoxide ligand and the pyridine nitrogen atom are bonded to the Ti center in a symmetrical coordination environment. Single-crystal X-ray diffraction studies on 1 and 2 were carried out. The molecular structures of these complexes are shown in Figure 1, and selected bond lengths and angles are listed in Table 1. As expected, the pyridine alkoxide ligand is bonded to the metal center in both complexes through the oxygen and nitrogen atoms in a pincer fashion. All distances and angles are similar in both complexes (see Table 1) and are within the range found for analogous complexes.8 The studies indicate that complexes exhibit unusual distorted trigonal-bipyramidal (TBP) titanium centers. Thus, the equatorial groups are displaced toward the axial nitrogen pyridine atom, forexample,in1,C(1)-Ti(1)-N(1))88.38(4)°,O(1)-Ti(1)-N(1) ) 73.2(3)°, O(2)-Ti(1)-N(1) ) 73.4(3)° (see Table 1). This structure is significantly different from the well-established three- or four-legged piano stool (also viewed as pseudosquare pyramidal SP) structure found in the majority of the monocyclopentadienyl titanium complexes described previously.6 In fact, although the first example of a mono(cyclopentadienyl)-metal complex with a TBP configuration was described several years ago, they are extremely rare.9a,b It could additionally be considered that the nonpiano stool geometries of 1 and 2 may (7) In CDCl3 the methylenic protons of 2 give rise to only one signal (at 5.77 ppm). (8) (a) Boyle, T. J.; Sewell, R. M.; Ottley, L. A. M.; Pratt, H. D., III; Quintana, C. J.; Bunge, S. D. Inorg. Chem. 2007, 46, 1825. (b) Zaitsev, K. H.; Bermeshev, M. V.; Karlov, S. S.; Oprunenko, Y. F.; Churakov, A. V.; Howard, J. A. K.; Zaitseva, G. S. Inorg. Chim. Acta 2007, 360, 2507.

Fandos et al.

arise from geometric distortions in the well-established fourlegged piano stool geometry.9c However, an exhaustive analysis of the L-Ti-Ct (Ct is the centroid of the Cp* ligand) values allows one to definitively exclude the presence of an “angular trans influence” distortion in our complexes. In order to gain more of an insight into this chemistry, the protonolysis reaction of complex 1 was carried out with 1 equiv of 2,6-pyridinedimethanol, and this led to isolation of 3 (see Scheme 2). Complex 3 was isolated in 75% yield as air-sensitive yellow crystals that are insoluble in toluene, THF, and dichloromethane. This compound is sufficiently soluble in CDCl3 to allow its characterization by 1H NMR spectroscopy, but all attempts to acquire a 13C NMR spectrum were unsuccessful due to its high unstability in solution, giving rise rapidly to a mixture of decomposition products. The 1H NMR spectrum shows a singlet at 1.98 ppm for the Cp* ligand and two doublets at 5.47 and 5.73 ppm due to the two equivalent methylene protons of the dialkoxide ligand coordinated to the titanium atom. The spectrum also shows resonances corresponding to the monoalkoxide ligand (see Experimental Section). These data suggest, in principle, a similar structural disposition to 1, where a methyl ligand was replaced by a monoalkoxide one (see Scheme 2). A single-crystal X-ray diffraction study of 3 was also carried out. An ORTEP plot and the atom-labeling scheme for 3 are shown in Figure 2, and selected bond lengths and angles are listed in Table 1. The molecule consists of discrete units in which a pyridine alkoxide ligand is bonded to the metal in a pincer fashion (as in 1 and 2) and the other one as terminal alkoxide ligand. However, the geometry around the titanium atom corresponds to the commonly found four-legged piano stool (pseudosquare pyramidal SP), which is well-documented for monocyclopentadienyl titanium derivatives.1 The value of O(3)-Ti(1)-N(1) (137°) is noteworthy and, in this case, the four legs of the piano stool are the bonds formed between the titanium and the three oxygen atoms and the pyridinic nitrogen. This molecular conformation determines that the bond distances between the pincer ligand and the titanium atom are slightly different than those found in 1 and 2 complexes. The Ti-O bonds are slightly longer, while the Ti-N bond is shorter, but the differences are very small. On the basis of these data, we can suggest that the piano or nonpiano stool environment (SP versus TBP geometry) around the titanium center in our complexes does not have a significant effect on the bond distances found. In order to elucidate the reasons why similar complexes adopt different geometries, without having a significant impact on the bond distances, the structures of 1, 2, and 3 were studied at the DFT level. The BP86/LANL2DZ-optimized structures of compounds 1, 2, and 3 are in good agreement with their corresponding X-ray structures, as can be seen from the values displayed in Table 2. In agreement with the experimental observations, the optimized geometry of 3 exhibits the expected piano stool structure [∠(N-Ti-O) ) 135°], whereas 2 does not [∠(N-Ti-Cl) ) 85°]. The computed potential energy profiles are shown in Figure 3. The profiles of complexes 1 and 2 show one minimum [at ∠(N-Ti-C) ) 91 ° (complex 1) and at ∠(N-Ti-Cl) ) 85° (complex 2)]. The profile of complex 2 shows a near-minimum visible in the area around 130°. Complex 3 shows three minima, and the one corresponding to the X-ray structure (indicated by a star in Figure 3) is actually the least stable of these. The deepest minimum corresponds to a structure that is more similar to that of 1 and 2, with a similarly small N-Ti-O angle (92°).

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Table 1. Bond Lengths [Å] and Angles [deg] for 1, 2, and 3 1

2

Ti(1)-Cp* Ti(1)-O(1) Ti(1)-O(2) Ti(1)-C(1) Ti(1)-N(1) O(1)-C(8) O(2)-C(9)

2.105(5) 1.911(5) 1.912(5) 2.113(7) 2.145(7) 1.394(8) 1.396(8)

Ti-Cp* Ti(1)-O(1) Ti(1)-O(2) Ti(1)-Cl(1) Ti(1)-N(1) O(1)-C(1) O(2)-C(7)

C(1)-Ti(1)-N(1) O(2)-Ti(1)-O(1) O(2)-Ti(1)-N(1) O(1)-Ti(1)-N(1) C(1)-Ti(1)-O(1) C(1)-Ti(1)-O(2) Ct-Ti(1)-N(1) Ct-Ti(1)-Cl(1) Ct-Ti(1)-O(1)

88.38(4) 142.7(2) 73.4(3) 73.2(3) 98.46(3) 96.65(6) 163.3 110.3 103.5

Cl(1)-Ti(1)-N(1) O(2)-Ti(1)-O(1) O(2)-Ti(1)-N(1) O(1)-Ti(1)-N(1) Cl(1)-Ti(1)-O(1) Cl(1)-Ti(1)-O(2) Ct-Ti(1)-N(1) Ct-Ti(1)-C(1) Ct-Ti(1)-O(1)

3

4

Bond Lengths [Å] 2.092(5) Ti(1)-Cp* 1.907(19) Ti(1)-O(1) 1.905(19) Ti(1)-O(2) 2.3338(9) Ti(1)-O(3) 2.146(2) Ti(1)-N(1) 1.399(4) O(1)-C(7) 1.394(4) O(2)-C(1) O(3)-C(8) O(4)-C(14)

2.088(3) 1.940(6) 1.956(7) 1.854(7) 2.123(9) 1.40(1) 1.39(1) 1.37(1) 1.42(1)

Ti(1)-Cp* Ti(1)-O(1) Ti(1)-N(1) Ti(1)-C(1) Ti(1)-C(2) N(1)-C(8) N(1)-C(4) O(1)-C(3)

2.402(3) 1.881(2) 2.296(2) 2.137(3) 2.152(3) 1.347(3) 1.347(3) 1.391(3)

Angles [deg] 84.35(7) O(3)-Ti(1)-N(1) 140.76(9) O(2)-Ti(1)-O(1) 73.51(9) O(2)-Ti(1)-N(1) 73.29(9) O(1)-Ti(1)-N(1) 98.03(6) O(3)-Ti(1)-O(1) 98.91(7) O(3)-Ti(1)-O(2) 163.5 Ct-Ti(1)-O(3) 108.5 Ct-Ti(1)-O(1) 104.7 Ct-Ti(1)-N(1)

137.0(3) 132.7(3) 73.0(3) 73.1(3) 92.4(3) 90.5(3) 111.4 111.3 111.5

O(1)-Ti(1)-C(1) O(1)-Ti(1)-C(2) C(1)-Ti(1)-C(2) O(1)-Ti(1)-N(1) C(1)-Ti(1)-N(1) C(2)-Ti(1)-N(1) Ct-Ti(1)-C(1) Ct-Ti(1)-O(1) Ct-Ti(1)-N(1)

86.2(1) 126.9(1) 84.4(1) 73.62(8) 140.1(1) 81.1(1) 109.9 120.1 109.9

Scheme 2

Table 2. Comparison of Key Structural Features of the X-ray Structures and BP86/LANL2DZ-Optimized Structures ∠(N-Ti-X)a complex 1 complex 2 complex 3

X-ray

BP86

88 84 137

91 85 135

R(Ti-Cp*)b X-ray 2.11 2.09 2.09

BP86 2.16 2.14 2.15

Angles in degrees. Complex 1: X ) C(methyl). Complex 2: X ) Cl. Complex 3: X ) O. b Distance (in Å) from the Ti atom to the center of the Cp* ring. a

Figure 2. ORTEP drawing of complex 3 with the atomic labeling scheme; hydrogen atoms are omitted for clarity; thermal ellipsoids are at the 30% level of probability.

The optimized structures are shown in Figure 4, and key structural features are displayed in Table 3. The three different structures of complex 3 are labeled 3a, 3b, and 3c. Structure 3a corresponds to the X-ray crystal structure (the minimum in the potential energy curve labeled “propeller” in Figure 3); 3b corresponds to the shallow minimum at ∠(N-Ti-O) ) 131° in the curve labeled “book”, and 3c corresponds to the deepest minimum at ∠(N-Ti-O) ) 92°. The three minima of complex 3 differ in two ways: (i) the relative orientation of the Cp* ring

Figure 3. Relaxed potential energy profiles on varying the N-Ti-X angle (complex 1, X ) C(methyl); complex 2, X ) Cl; complex 3, X ) O). The stars indicate the structures corresponding to the X-ray crystal structures. For clarity, the minimum of the profile for complex 1 was placed at 5 kJ/mol and not at 0 kJ/mol as for complexes 2 and 3.

and the Ti-containing ring system, quantified by the angle between the Cp* plane and the plane of the pyridine moiety of the Ti-containing ring (labeled τ1 in Table 3), and (ii) the orientation of the OH-containing pyridine ring with respect to the Ti-containing ring system, as quantified by the torsion defined by the Ca-Nb-Cc-Cd atom sequence (labeled τ2 in Table 3; see Figure 5 for the labeling of the atoms). The orientation of the rings in 3b and 3c (τ2 ∼ 0°) resembles a book, and we therefore labeled these “books” in Figure 3. In 3a (τ2 ∼ 80°), the orientation of the (hydroxymethyl)pyridine ring differs from the book orientation such as to form a structure reminiscent of a two-blade propeller, and we labeled this orientation “propeller”. Complexes 1, 2, and 3c have an almost perpendicular arrangement of the Cp* ring with respect to the other part of the complex (τ1 ∼ 65-75°), an arrangement enabled by the small N-Ti-X angle in these structures. The larger N-Ti-O angle in 3a and 3b forces the Cp* ring away from perpendicular toward a near-parallel arrangement (τ1 ∼ 26-27°). In 3a, the OH-containing pyridine ring is rotated by almost 80° (τ2 ∼ 79°) as compared to 3b, thereby blocking any possible structural relaxation to a perpendicular arrangement. The corresponding potential energy curve (labeled “propeller” in Figure 3) therefore does not display a minimum around 90°.

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Figure 6. Unit cell of complex 3. Scheme 3

Figure 4. The BP86/LANL2DZ-optimized structures of complexes 1, 2, and 3. Table 3. Comparison of the Torsion Angles Describing the Geometries of the Optimized Structures of 1, 2, and 3 1 2 3a 3b 3c

∠(N-Ti-X)

τ1a

91 85 135 131 92

70.5 74.2 25.7 27.5 65.5

τ2b

Scheme 4 79.3 0.9 -5.2

a Angle (in degrees) between the Cp* plane and the plane of the pyridine ring of the Ti-containing ring system (planes are defined by three atoms in the ring). b Torsion angle (in degrees) defined by the Ca-Nb-Cc-Cd atom sequence (see Figure 5 for the labeling of the atoms).

Figure 5. Atom labeling in compound 3 used for the definition of τ 2.

In conclusion, the potential energy surface of complex 3 displays several minima in the gas phase. The lowest of these minima involves a near-perpendicular arrangement between the Cp* ring and the rest of the complex, and this does not show the expected piano stool arrangement of the atoms coordinated to the Ti atom. The minimum corresponding to the X-ray crystal structure, which does have the usual piano stool arrangement, is about 20 kJ/mol higher in energy. The preference for the piano stool arrangement in the crystal phase is probably the result

crystal packing forces. The crystal structure may be stabilized by π-π interactions between OH-containing pyridine rings on adjacent complexes (see Figure 6). Complexes 1 and 2 exhibit only one minimum, and this has a near-perpendicular structure. The piano stool structure is not stable in either the gas phase (probably due to the absence of blocking of structural relaxation) or the solid phase (possibly due to the absence of stabilizing interactions between the pyridine rings). In order to gain a better understanding of the coordination modes of pyridinic alcohols with titanium centers, in the last part of this study, we considered the use of 2-pyridinemethanol in the protonolysis process. Thus, the reaction [TiCp*Me3] with 1 and 2 equiv of 2-pyridinemethanol, respectively, gave the complexes [TiCp*Me2{2-(OCH2)py-κ1-O}] (4) and [TiCp*Me{2(OCH2)py-κ1-O}2] (5) (Scheme 3), which were isolated in moderate (64%) and low (31%) yields as orange and yellow solids, respectively. These complexes are extremely unstable toward hydrolysis, and even under an inert atmosphere, they decompose slowly due to the presence of adventitious water to yield the same oxo derivative [TiCp*(O){2-(OCH2)py-κ2-O,N}] (6), which can be directly synthesized by reaction of 5 with water (Scheme 4). The 1H NMR spectrum of complex 4 in CDCl3 shows a singlet at 1.83 ppm corresponding to the Cp* ligand. The

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plexes 3 and 4 do have the usual piano stool (pseudosquare pyramidal SP) geometry. Electronic structure calculations at the DFT level on complexes 2 and 3 revealed that the nonpiano stool structure, which has a near-perpendicular arrangement of the Cp* ring with respect to the rest of the complex, is the preferred structure in the gas phase for both compounds. Additionally, it was found that for complex 3 the piano stool structure is a local minimum on the gas-phase potential energy surface but that such a structure does not exist for complex 2. It appears that the nonpiano stool structure results from a favorable near-perpendicular arrangement of the Cp* ring and the rest of the complex, whereas the piano stool structure is probably favored in the solid state due to crystal packing forces. Figure 7. ORTEP drawing of complex 4 with the atomic labeling scheme; hydrogen atoms are omitted for clarity; thermal ellipsoids are at the 30% level of probability.

methylene protons of alkoxide moiety and the aromatic pyridinic protons appear as a singlet at 5.78 ppm and four multiplets at 7.24, 7.53, 7.80, and 8.14 ppm, respectively. Nevertheless, the two methyl groups bonded to the metal center give rise to only one signal (at 0.22 ppm), indicating that in solution the pyridinic moiety can rotate freely. The 1H NMR spectra of 5 and 6 are similar to that of 4, except that the protons of the pyridinic ligands in 5 and 6 give rise to several multiplets, as expected from their inequivalence (see Experimental Section). Additionally, the IR spectrum of 6 exhibits a strong band at 966 cm-1 that can be assigned to a ν(TidO) stretching mode.10On the basis of these data, we propose that the complex 5 presents in solution a structure in which the alkoxide ligand binds in a monodentate fashion and shows dynamic behavior, while the oxo complex 6 contains probably the alkoxide ligand coordinated in a bidentate fashion (see Scheme 4). In order to gain an insight into the proposed structures, a single-crystal X-ray diffraction study on 4 was carried out. An ORTEP plot and the atom-labeling scheme of 4 are shown in Figure 7, and selected bond lengths and angles are listed in Table 1. The geometry around the titanium center corresponds to a piano stool structure (pseudosquare pyramidal SP), in which the four legs are the bonds formed between the titanium atom and the two methyl groups and the oxygen and nitrogen atoms of the pyridinic moiety, which behaves as a bidentate κ2-O,N ligand. The Ti(1)-O(1) distance [1.881(2) Å] is shorter than those in complexes 2 and 3, while the Ti(1)-N(1) distance [2.296(2) Å] is longer. The Ti-C distances [2.137 and 2.152 Å] are only slightly longer than that in complex 1. Nevertheless, the structural data for complex 4 are consistent with those of similar compounds.10

Concluding Remarks New monocyclopentadienyl complexes of titanium that contain pyridine-alkoxide ligands have been synthesized and characterized by NMR spectroscopy and X-ray crystallography. In this study, interesting structural features were identified in that an unusual disposition was found in complexes 1 and 2, which have an unexpected distorted trigonal-bipyramidal titanium centers (TBP). Conversely, the structurally related com(9) (a) Liu, A. H.; Murray, R. C.; Dewan, J. C.; Santarsiero, B. D.; Schrock, R. R. J. Am. Chem. Soc. 1987, 109, 1282. (b) du Plooy, K. E.; Moll, U.; Wocadlo, S.; Massa, W.; Okuda, J. Organometallics 1995, 14, 3129. (c) Lin, Z.; Hall, M. B. Organometallics 1993, 12, 19. (10) Fandos, R.; Herna´ndez, C.; Otero, A.; Rodrı´guez, A.; Ruiz, M. J.; Terreros, P. J. Chem. Soc., Dalton Trans. 2000, 299.

Experimental Section The preparation and handling of the compounds described here was performed with the exclusion of air and moisture under a nitrogen atmosphere using standard vacuum line and Schlenk techniques. All solvents were dried and distilled under a nitrogen atmosphere. Complex 1 [TiCp*Me{2,6-(OCH2)2py-κ3-O,N,O}] was prepared by literature procedures,5 as were [TiCp*Me3]11 and [TiCp*Cl3].12 The commercially available compound (HOCH2)2py was used as received from Aldrich, and (HOCH2)py was distilled before use. 1H and 13C NMR spectra were recorded on a Mercury Varian FT (200 MHz) spectrometer. Trace amounts of protonated solvents were used as references, and chemical shifts are reported in units of parts per million relative to SiMe4. IR spectra were recorded with a Nicolet Magna-IR 550 spectrophotometer in the region 4000-400 cm-1 or with a Jasco-IR4100 spectrophotometer equipped with a Miracle Single Reflection ATR Diamond/ZnSe in the region 4000-560 cm-1. Crystallization of [TiCp*Me{2,6-(OCH2)2py-K3-O,N,O}] (1). Yellow crystals, suitable for an X-ray diffraction study, were obtained by cooling a saturated solution of complex 1 in toluene to -30 °C. Synthesis of [TiCp*Cl{2,6-(OCH2)2py-K3-O,N,O}] (2). Et3N (0.41 mL, 2.92 mmol) was added dropwise at room temperature to a mixture of [TiCp*Cl3] (0.424 g, 1.46 mmol) and (HOCH2)2py (0.200 g, 1.46 mmol) in toluene (8 mL). The mixture was stirred for 2 h at room temperature and filtered to remove Et3NHCl. The orange solution was dried in vacuum to yield an orange solid (0.352 g, 67%), which was characterized as 2. Orange crystals, suitable for an X-ray diffraction study, were obtained by cooling a saturated solution of complex 2 in toluene to -30 °C. IR (Nujol/PET, cm-1): 1607 (m), 1575 (m), 1476 (m), 1451 (m), 1317 (m), 1098 (vs), 1064 (s), 795 (m), 759 (vs), 737 (m), 613 (m), 486 (s), 480 (s). 1H NMR (C6D6, rt): δ (ppm) 2.19 (s, 15 H, Cp*), 5.45 (d, 2J ) 18.69 Hz, 2 H, CH2), 5.66 (d, 2J ) 18.69 Hz, 2 H, CH2), 6.21 (m, 2 H, py), 6.65 (m, 1 H, py). 1H NMR (CDCl3, rt): δ (ppm) 2.06 (s, 15 H, Cp*), 5.77 (s, 4 H, CH2), 7.26 (m, 2 H, py), 7.74 (m, 1 H, py). 13 C{1H} NMR (CDCl3, rt): δ (ppm) 12.6 (Cp*), 80.9 (CH2), 116.7 (Cp*), 128.4 (py), 139.4 (py), 168.1 (pyipso). Anal. Calcd for C27H22NO2ClTi: C, 57.41; H, 6.23; N, 3.94. Found: C, 57.46, H, 6.18; N, 4.05. Synthesis of [TiCp*{2,6-(OCH2)py(CH2OH)-K1-O}{2,6-(OCH2)2pyK3-O,N,O}] (3). To a solution of [TiMeCp*{κ3-O,N,O-(OCH2)2py}] (1) (0.227 g, 0.676 mmol) in toluene at -78 °C was added (HOCH2)2py (0.094 g, 0.676 mmol). The stirred mixture was left to reach room temperature slowly and was then stirred for a further 2 h. The solvent was removed under reduced pressure, and the resulting oil was washed with pentane (5 mL) to yield a yellow solid (0.233 g, 75%), which was characterized as 3. Yellow crystals, (11) Mena, M.; Pellinghelli, M. A.; Royo, P.; Serrano, R.; Tiripicchio, A. J. Chem. Soc., Chem. Commun. 1986, 1118. (12) Hidalgo-Llina´s, G.; Mena, M.; Palacios, F.; Royo, P.; Serrano, R. J. Organomet. Chem. 1988, 340, 37.

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Fandos et al.

Table 4. Crystal Data and Structure Refinement for 1, 2, 3, and 4 empirical formula formula weight temperature (K) wavelength (Å) crystal system space group a (Å) b (Å) c (Å) β (deg) volume (Å3) Z density (calculated) (g/cm3) absorption coefficient (mm-1) F(000) crystal size (mm3) index ranges reflections collected independent reflections data/restraints/parameters goodness-of-fit on F2 final R indices [I > 2σ(I)] largest diff. peak and hole

1

2

3

4

C17H22ClNO2Ti 355.71 180(2) 0.71073 monoclinic P21/c 14.427(1) 8.2811(5) 14.4114(8) 109.024(3) 1627.8(2) 4 1.451 0.696 744 0.26 × 0.16 × 0.03 -20 e h e 20 -11 e k e 11 -20 e l e 15 28825 4961 [R(int) ) 0.0894] 4961/0/204 0.995 R1 ) 0.0555 wR2 ) 0.1427 0.781 and -0.516

C18H25NO2Ti 335.29 250(2) 0.71073 monoclinic P21/c 11.702(5) 10.447(4) 14.589(6) 105.950(7) 1715.0(12) 4 1.299 0.505 712 0.33 × 0.27 × 0.17 -10 e h e 10 -8 e k e 9 -12 e l e 12 4048 1228 [R(int) ) 0.0620] 1228/0/205 1.134 R1 ) 0.0532 wR2 ) 0.1275 0.384 and -0.197

C24H30N2O4Ti 458.40 180(2) 0.71073 monoclinic P21/n 7.834(5) 12.959(7) 21.517(12) 94.42(1) 2178(2) 4 1.398 0.427 968 0.32 × 0.22 × 0.13 -9 e h e 9 -15 e k e 15 -25 e l e 25 7463 3793 [R(int) ) 0.1192] 3793/0/281 0.880 R1 ) 0.1084 wR2 ) 0.2244 1.348 and -0.462

C18H27NOTi 321.31 160(2) 0.71073 monoclinic P21/n 8.661(2) 15.027(3) 13.301(3) 99.634(4) 1706.7(6) 4 1.250 0.501 688 0.24 × 0.17 × 0.11 -11 e h e 11 -19 e k e 18 -17 e l e 17 13631 4170 [R(int) ) 0.0859] 4170/0/197 0.978 R1 ) 0.0595 wR2 ) 0.1420 0.727 and -0.404

suitable for an X-ray diffraction study, were obtained from a saturated solution of complex 3 in THF. 1H NMR (CDCl3, rt): δ (ppm) 1.98 (s, 15 H, Cp*), 3.46 (t, 2J ) 5.13 Hz, 1H, CH2OH), 4.76 (d, 2J ) 5.13 Hz, 2 H, CH2OH), 5.26 (s, 2 H, CH2), 5.47 (d, 2 J ) 18.69 Hz, 2 H, CH2), 5.73 (d, 2J ) 18.69 Hz, 2 H, CH2), 6.88-7.73 (m, 6 H, py). Anal. Calcd for C24H30O4N2Ti: C, 62.92; H, 6.54; N, 6.11. Found: C, 62.96; H, 6.52; N, 5.76. Synthesis of [TiCp*Me2{2-(OCH2)py-K1-O}] (4). To a solution of [TiCp*Me3] (0.237 g, 1.039 mmol) in toluene (5 mL) at -78 °C was added (HOCH2)py (0.100 mL, 1.039 mmol). The solution was stirred for 1 h at room temperature. The solvent was removed under reduced pressure. The resulting oil was washed with pentane (5 mL) to yield an orange solid (0.216 g, 64%), which was characterized as complex 4. Orange crystals, suitable for an X-ray diffraction study, were obtained by cooling a saturated solution of complex 4 in THF to -30 °C. 1H NMR (CDCl3, rt): δ (ppm) 0.22 (s, 6 H, Me), 1.83 (s, 15 H, Cp*), 5.78 (s, 2 H, CH2), 7.24 (m, 1 H, py), 7.53 (m, 1 H, py), 7.80 (m, 1 H, py), 8.14 (m, 1 H, py). 13 C{1H} NMR (CDCl3, rt): δ (ppm) 12.2 (Cp*), 54.2 (Me), 76.8 (CH2), 120.6 (Cp*), 122.3 (py), 122.5 (py), 138.3 (py), 148.6 (py), 166.8 (pyipso). Synthesis of [TiCp*Me{2-(OCH2)py-K1-O}2] (5). To a solution of [Cp*TiMe3] (0.335 g, 1.469 mmol) in toluene (5 mL) at -78 °C was added (HOCH2)py (0.283 mL, 2.938 mmol). The solution was stirred for 30 min at room temperature. The solvent was removed under reduced pressure. The resulting oil was washed with pentane (5 mL) to yield a yellow solid (0.188 g, 31%), which was characterized as complex 5. 1H NMR (CDCl3, rt): δ (ppm) 0.43 (s, 3 H, Me), 1.88 (s, 15 H, Cp*), 5.39 (d, 2J ) 16.50 Hz, 2 H, CH2), 5.51 (d, 2J ) 16.50 Hz, 2 H, CH2), 7.10 (m, 2 H, py), 7.41 (m, 2 H, py), 7.62 (m, 2 H, py), 8.41 (m, 2 H, py). 13C{1H} NMR (CDCl3, rt): δ (ppm) 11.8 (Cp*), 48.6 (Me), 76.8 (CH2), 120.7 (Cp*), 121.9 (py), 122.2 (py), 137.4 (py), 149.0 (py), 164.4 (pyipso). Synthesis of [TiCp*(O){2-(OCH2)py-K2-O,N}] (6). To a solution of [TiCp*Me{2-(OCH2)py-κ1-O}2] (5) (0.167 g, 0.403 mmol) in THF (5 mL) was added H2O (7.24 µL, 0.403 mmol). The solution was stirred overnight at room temperature. The resulting suspension was filtered and the solid dried under vacuum to afford complex 6 as a yellow solid. IR bands (cm-1): 620 (m), 698 (s), 732(s), 897 (m), 940 (m), 966 (s), 1027 (w), 1067 (m), 1123 (m), 1378 (w), 1428 (m), 1487 (w), 1590 (w), 2911 (w), 2955 (w), 3022 (w), 3046 (w), 3069 (w). 1H NMR (CDCl3, rt): δ (ppm) 1.64 (s, 15 H, Cp*),

5.43 (s, 2 H, CH2), 7.25 (m, 1 H, py), 7.28 (m, 1 H, py), 7.76 (m, 1 H, py), 8.59 (m, 1 H, py). 13C{1H} NMR (CDCl3, rt): δ (ppm) 11.7 (Cp*), 73.6 (CH2), 119.5 (Cp*), 121.6 (py), 122.1 (py), 138.7 (py), 149.4 (py), 168.5 (pyipso). Anal. Calcd for C16H21N1O2Ti: C, 62.55; H, 6.89; N, 4.56. Found: C, 62.57; H, 6.84; N, 4.53. Computational Methodology. The structures of complexes 1, 2, and 3 were optimized with the BP8613,14 functional and the LANL2DZ15 basis set using the Gaussian 03 software suite.16 The starting geometries were taken from the X-ray crystal structures of the three compounds. All calculations employed Gaussian’s “ultrafine” integration grid. Relaxed potential energy profiles were created by optimizing the geometry over a range of fixed N-Ti-X angles (complex 1, X ) C(methyl); complex 2, X ) Cl; complex 3, X ) O). Crystal Structure Determination. A summary of crystal data collection and refinement parameters for all compounds is given in Table 4. Single crystals were obtained by crystallization from saturated solutions. The single crystals were mounted on a glass fiber and transferred to a Bruker X8 APEX II CCD-based diffractometer equipped with a graphite monochromated Mo KR radiation source (λ ) 0.71073 Å). Data were integrated using SAINT,17 and an absorption correction was performed with the (13) Becke, A. D. Phys. ReV. A 1988, 38, 3098. (14) Perdew, J. P. Phys. ReV. B 1986, 33, 8822. (15) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (16) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision B.04; Gaussian Inc.: Pittsburgh, PA, 2003. (17) SAINT+ V7.12a. Area-Detector Integration Program. BrukerNonius AXS: Madison, WI, 2004.

Related Pyridine-Alkoxide Titanium Complexes program SADABS.18 The software package SHELXTL version 6.1219 was used for space group determination, structure solution, and refinement by full-matrix least-squares methods based on F2. All non-hydrogen atoms were refined with anisotropic thermal parameters. Hydrogen atoms were placed using a “riding model” and included in the refinement at calculated positions. The crystal of compound 3 diffracts weakly, but the results obtained are sufficient for an adequate description of its geometry.

(18) Sheldrick, G. M. SADABS Version 2004/1. A Program for Empirical Absorption Correction. University of Go¨ttingen: Go¨ttingen, Germany, 2004. (19) SHELXTL-NT Version 6.12. Structure Determination Package. Bruker-Nonius AXS: Madison, WI, 2001.

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Acknowledgment. This work was supported by the Ministerio de Ciencia e Innovacio´n (MCINN), Spain (Grant Nos. CTQ2005-08123-C02-01/BQU, CTQ2006-11845/BQU, and Consolider-Ingenio 2010 ORFEO CSD2007-00006) and the Junta de Comunidades de Castilla-La Mancha, Spain (Grant No. PCI08-0010). Supporting Information Available: Text, tables, figures, and CIF files giving full experimental data for the crystallographic studies of complexes 1, 2, 3, and 4. This material is available free of charge via the Internet at http://pubs.acs.org. OM8010239