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or hydrazido ligands
Polyhedron 25 (2006) 259–265 www.elsevier.com/locate/poly
Titanium(IV) complexes with amidinate and/or hydrazido ligands Jeffrey M. Pietryga, Jamie N. Jones, Charles L.B. Macdonald, Jennifer A. Moore, Alan H. Cowley * Department of Chemistry and Biochemistry, The University of Texas at Austin, 1 University Station A5300, Austin, TX 78712-0165, USA Received 8 April 2005; accepted 24 June 2005 Available online 11 August 2005 Dedicated to Professor Malcolm H. Chisholm on the occasion of his 60th birthday.
Abstract The complex [Ti2Cl4(NNMe2)2(py)3] (1) has been prepared via the reaction of TiCl4 with LiN(H)NMe2, followed by treatment with pyridine. Preparation of the complex [TiCl(PhC{N(SiMe3)2}2)2(MeNNMe2)] (2) was carried out by treatment of TiCl4 with Li[PhC{N(SiMe3)2}]tmeda, followed by reaction with LiN(Me)NMe2. The structures of 1 and 2 were determined by single crystal X-ray diffraction. The single crystal X-ray structure of Li[PhC{N(SiMe3)2}]tmeda (3) has also been determined. 2005 Elsevier Ltd. All rights reserved. Keywords: Titanium; Amidinate; Hydrazido; Complexes; X-ray structures
1. Introduction Several single-source precursors to titanium nitride have been reported over the past 15 years, including homoleptic titanium amides and mixed titanium amide/alkyl compounds [1], amides such as [CpTiCl2{N(SiMe3)2}] [2] and [TiCl2(NH-t-Bu)2(NH2-t-Bu)2]n [3], several imides [4], adducts of the type [TiCl4(NH3)2] [5] and azides such as [Cp2Ti(N3)2] [6] and [Ti(NMe2)2(N3)2(py)2] [7]. Ideally, the TiN single source precursor should possess reasonable volatility in order to facilitate vapor transport. In turn, this implies the avoidance of oligomerization. A second desirable precursor feature is an all-nitrogen coordination environment around the titanium atom since the presence of, e.g., titanium– chlorine or titanium–carbon bonds can result in the presence of chlorine or carbon impurities in the TiN films or particles. *
Corresponding author. Tel.: +1 512 471 7484; fax: +1 512 471 6822. E-mail address: [email protected] (A.H. Cowley). 0277-5387/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2005.06.035
In the present work, two strategies were employed in an effort to prepare monomeric titanium nitride precursors with all-nitrogen coordination spheres. The first approach featured the use of alkylhydrazido ligands; the second involved the use of alkylhydrazido ligands in conjunction with amidinate ligands. A major potential advantage of hydrazido ligands stems from the expectation that N–N bond cleavage will be more facile than N–C bond cleavage owing to the order of bond enthalpies N–C > N–N. In principle, this should result in lower deposition temperatures and less carbon incorporation for titanium hydrazides than for precursors with Ti–N–C linkages. Such a view is supported by the observation [8] that the decomposition of methylhydrazine on a titanium surface is initiated by N–N bond cleavage at modest temperatures (200 C). Moreover, Winter et al. [9,10] have demonstrated that alkylhydrazide ligands can be employed for the deposition of high quality TiN films; however, the inclusion of chloride ligands in these precursors led to chloride incorporation in the films that were grown at lower temperatures. A second potential advantage of the use of hydrazido ligands is
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that they are capable of binding to titanium in an g2-fashion, thus facilitating the achievement of an allnitrogen coordination environment. A similar comment applies to the use of amidinate ligands which also are capable of exhibiting an g2 -binding mode. In the present contribution, we describe the reaction of TiCl4 with LiN(H)NMe2, followed by treatment with pyridine and the reaction of TiCl4 with Li[PhC{N(SiMe3)2}]tmeda, followed by treatment with LiN(Me)NMe2. Both products were characterized by single-crystal X-ray diffraction. We also report the Xray crystal structure of Li[PhC{N(SiMe3)2}]tmeda. Although the latter is a known compound [11], it has not been structurally characterized previously.
immediately with degassed perfluorinated polyether oil. The data for 1 were collected at 293(2) K on a Siemens P 4 diffractometer and the data for 2 and 3 were collected at 153(2) K on a Nonius Kappa CCD diffractometer. Each data set was corrected for Lorentz and polarization effects. All three structures were solved by direct methods, and refined by full-matrix least-squares on F2 using the SHELXTL PLUS software package [13]. All non-hydrogen atoms were allowed anisotropic thermal motion and hydrogen atoms, which were included at calculated positions, were refined using a riding model and a general isotropic thermal parameter. The total number of reflections, collection ranges, and final R values are listed in Table 1. 2.4. Synthesis of [Ti2Cl4(NNMe2)2(py)3] (1)
2. Experimental 2.1. General procedures All manipulations and reactions were performed under catalyst-scrubbed Ar, using a combination of glove box and standard Schlenk techniques. Dichloromethane was purified by distillation under N2 from calcium hydride, and all other solvents were distilled under N2 from sodium/benzophenone ketyl. Additional degassing of solvents was performed using either vigorous Ar bubbling, or freeze/pump/thaw cycling. A 1.6 M hexane solution of n-BuLi was purchased from a commercial source, and used without further purification. Titanium(IV) chloride was purchased from a commercial source, and purified as necessary by vacuum distillation. 1,1,-Dimethylhydrazine was purchased from a commercial source, and purified by distillation. Trimethylhydrazine [12] and Li[PhC{N(SiMe3)2}]tmeda [11] were prepared and purified using literature methods. 2.2. Spectroscopic measurements All CI mass spectra were recorded on a Finnigan MAT TSQ-700 instrument; the samples were sealed in glass capillaries under Ar in a glove box. NMR spectra were recorded at 295 K on a GE QE-300 spectrometer. The CD2Cl2 used in these measurements was purchased in sealed vials, either pure or with 1% (v/v) tetramethylsilane. The THF-d8 was purchased from a commercial source and vacuum distilled under Ar prior to use. The 1H NMR chemical shifts are reported relative to tetramethylsilane and are referenced either directly or to solvent. 2.3. Single crystal X-ray structure determinations Crystals of [Ti2Cl4(NNMe2)2(py)3] (1), [TiCl(PhC{N(SiMe3)2}2)2(MeNNMe2)] (2) and Li[PhC{N(SiMe3)2}]tmeda (3) of suitable quality were collected directly from Schlenk-type flasks under Ar pressure, and covered
A solution of NH2NMe2 (3.8 g, 63 mmol) in 40 mL of tetrahydrofuran was chilled to 78 C and 25 mL of an n-BuLi solution (1.6 M in hexane) was added over a period of 10 min. The reaction mixture was allowed to warm to room temperature over several hours, then stirred overnight. Titanium tetrachloride (5.2 mL, 16 mmol) was added via syringe to 80 mL of tetrahydrofuran at 10 C, to which was added the solution of LiN(H)NMe2 over a period of 10 min. The reaction mixture was allowed to warm to room temperature overnight, following which the solvent was removed under reduced pressure and the resulting solids extracted with CH2Cl2 (3 · 75 mL). Filtration through a glass frit covered with diatomaceous earth resulted in a red colored filtrate. The volume of the latter solution was reduced to 100 mL, and pyridine (0.5 mL, 6 mmol) was added to a 30 mL aliquot. Cooling of this mixture to 20 C for a period of days produced large red crystals of 1 in 40% yield. 1 H NMR (300 MHz, 298 K, CD2Cl2): d 8.83 (m, 6H, m-py), 7.79 (t, 3H, p-py), 7.35 (t, 6H, o-py), 3.47 (s, 12H, Me2N). MS (CI+, CH4): m/e 556 [Ti2Cl3(NNMe2)2(py)3]+ (5%), 475 [Ti2Cl3(NNMe2)2(py)2]+ (24%), 434 [Ti2Cl3N3C2H7(py)2]+ (27%), 396 [Ti2Cl3(NNMe2)2(py)]+ (49%), 355 [Ti2Cl3N3C2H7(py)]+ (100%), 317 [Ti2Cl3(NNMe2)2]+ (23%). 2.5. Synthesis of [TiCl(PhC{N(SiMe3)2}2)2(MeNNMe2)] (2) Titanium tetrachloride (2.0 mL, 18 mmol) was added via syringe to a solution of Li[PhC{N(TMS)}2]tmeda (13.9 g, 36 mmol) in 100 mL of tetrahydrofuran. The reaction mixture was stirred overnight, after which the volatiles were removed under reduced pressure and the resulting orange-red solid was extracted with diethyl ether (2 · 70 mL). After filtration through a glass frit covered with diatomaceous earth, the red filtrate was concentrated and maintained at 20 C overnight. The intermediate, TiCl2(PhC{N(SiMe3)2}2)2, was isolated as
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Table 1 Selected crystal data and data collection refinement parameters for 1–3
Empirical formula Formula weight Crystal size (mm) Crystal class Space group ˚) a (A ˚) b (A ˚) c (A b () ˚ 3) U (A Z l (Mo-Ka, mm 1) T (K) Diffractometer Radiation ˚) k (A Measured reflections Independent reflections Rint Range in 2h () Range in h Range in k Range in l Data/restraints/parameters R1 [I > 2r(I)] wR2
1
2
3
C21H31Cl8N7Ti2 760.93 0.15 · 0.09 · 0.07 monoclinic P21/n 16.9961(7) 11.1852(5) 18.3199(7) 106.343(1) 3342.0(2) 4 1.142 293(2) Siemens P4
C33H65ClN6OSi4Ti 757.62 0.13 · 0.11 · 0.04 monoclinic P21/c 11.664(2) 21.175(4) 17.730(4) 100.42(3) 4307.0(1) 4 0.404 153(2) Nonius Kappa CCD Graphite-monochromated Mo-Ka 0.71073 14 627 7560 0.0708 5.98 6 2h 6 50.00 13 6 h 6 13 25 6 k 6 25 21 6 l 6 21 7560/0/430 0.0539 0.0979
C19H39LiN4Si2 386.66 0.40 · 0.30 · 0.21 monoclinic P21/n 10.584(5) 13.234(5) 18.147(5) 91.535(5) 2540.9(2) 4 0.149 153(2) Nonius Kappa CCD
17 961 5891 0.0478 2.86 6 2h 6 50.00 20 6 h 6 18 13 6 k 6 11 11 6 l 6 21 5891/0/347 0.0567 0.1477
red crystals in 45% yield. This is a slight variation of the literature methods [11,14,15]. [1H NMR (300 MHz, 298 K, CD2Cl2): d 7.41 (m, 3H, o,p-Ph), 7.28 (m, 2H, m-Ph), 0.05 (s, 18H, Me3Si)]. Trimethylhydrazine (0.23 g, 3 mmol) in 30 mL of diethyl ether was lithiated by treatment with 1.9 mL of a solution of n-BuLi (1.6 M in hexane) at 78 C. The reaction mixture was allowed to assume room temperature and stirring was continued overnight. A solution of TiCl2(PhC{N(SiMe 3)2}2) 2 (1.0 g, 1.5 mmol) in 50 mL of diethyl ether was then added, and the reaction mixture was stirred overnight. Following filtration through a glass frit covered with diatomaceous earth, the resulting orange filtrate was concentrated and chilled to 78 C, affording an orange oil. A crop of orange-red crystals of 2 (0.22 g (0.33 mmol, 22% yield)) formed upon storage of the oil at ambient temperature. 1H NMR (300 MHz, 298 K, THF-d8): d 7.44 (m, 6H, Ph), 7.31 (m, 4H, Ph), 2.74 (s, 3H, MeN), 2.36 (s, 6H, Me2N), 0.07 (s, 18H, Me3Si); MS (CI+, CH4) m/e 683 [M + 1, 14%], 647 [M-Cl , 15%], 609 [TiCl2(PhC{N(SiMe3)2}2)2, 100%].
reagent was allowed to react with TiCl4 in a 4:1 mole ratio in THF solution. The synthesis of 1 was completed by the addition of pyridine in an effort to prevent oligomerization (Scheme 1). 1H NMR spectroscopic assay provided two clues to the structure of 1, namely (i) no N–H signal was evident thus suggesting that the hydrazido ligand is attached to titanium in an g1 or g2 fashion, and (ii) the hydrazido and pyridine ligands are present in a 2:3 mole ratio. However, it was necessary to appeal to X-ray crystallography to provide more definitive structural information. LiN(H)NM e2 + TiCl4
"TiCl2(NNM e2)"
pyridine Me
Me
Py
N
Cl
N
Py
3. Results and discussion
Cl
Ti Cl
Ti N
Cl
N
3.1. Synthesis and preliminary identification of [Ti2Cl4(NNMe2)2(py)3] (1) The initial phase of the synthesis was monolithiation of H2NNMe2 to form LiN(H)NMe2. This lithium
33 850 5816 0.0713 6.56 6 2h 6 54.98 13 6 h 6 13 16 6 k 6 17 23 6 l 6 19 5816/150/245 0.0710 0.1699
Me
Py Me 1 Scheme 1.
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3.2. Single crystal X-ray structure of [Ti2Cl4(NNMe2)2(py)3] (1) The X-ray analysis reveals a dititanium structure in which the two coordination environments are quite distinct (Fig. 1 and Table 2). One titanium atom [Ti(1)] is seven coordinate and features a pyridine molecule, two chlorides, and two g2-attached [Me2NN]2 ligands.
Fig. 1. Molecular structure of [Ti2Cl4(NNMe2)2(py)3] (1) showing the atom numbering scheme. Thermal ellipsoids are set to 30% probability. All hydrogen atoms and two molecules of CH2Cl2 of crystallization have been omitted for clarity.
The overall geometry at Ti(1) can be described as pentagonal bipyramidal. Thus, the sum of bond angles in the pentagonal plane described by the five nitrogen atoms is 360.0(1). Note, however, that the angles in this plane are very distorted and range from 38.1(1) to 101.0(1). The departures from the ideal bond angle of 72 are caused by the very small bite angles of the g2-hydrazide ligands. If, however, a pseudo-trigonal bipyramidal geometry is considered by defining the midpoints of the N–N moieties as ‘‘ligands’’ the ‘‘bond angles’’ in the equatorial plane would be 120.3, 118.1, and 121.6. The axial ligands at N(1) are distorted from linearity as reflected by the Cl–Ti–Cl bond angle of 160.99(6). Note that both of these chloride ligands are inclined away from the more ‘‘crowded’’ side of the molecule. The geometry at Ti(2) is octahedral. Two pyridine molecules are arranged in a trans fashion while the two chlorides and the two imido nitrogens [N(3) and N(4)] each possess a mutually cis arrangement. The departures from the ideal angle of 90 are modest and of the order of 5–6 (see Table 2). The view down the Ti(1)–Ti(2) vector shows that the C(13)–C(14)–N(3)– N(4)–N(1)–N(2) and N(6)–N(7)–C(11)–C(12) planes are close to orthogonal (Fig. 2). The two hydrazides feature an imido nitrogen [N(3) and N(4)], each of which is formally r-bonded to Ti(1)
Table 2 ˚ ) and angles () for 1–3 Selected bond lengths (A 1 Bond lengths Ti(1)–N(1) Ti(1)–N(2) Ti(1)–N(3) Ti(1)–N(4) Ti(1)–N(5) Ti(1)–Cl(1) Ti(1)–Cl(2) Ti(2)–N(3) Ti(2)–N(4) Ti(2)–N(6) Ti(2)–N(7) Ti(2)–Cl(3) Ti(2)–Cl(4) Bond angles N(2)–Ti(1)–N(4) Ti(1)–N(2)–N(4) Ti(1)–N(4)–N(2) N(1)–Ti(1)–N(3) Ti(1)–N(1)–N(3) N(1)–N(3)–Ti(1) Cl(1)–Ti(1)–Cl(2) Ti(1)–N(3)–Ti(2) Ti(1)–N(4)–Ti(2) N(3)–Ti(2)–N(4) Cl(3)–Ti(2)–Cl(4)
2 2.224(2) 2.208(4) 1.941(4) 1.956(4) 2.272(4) 2.366(1) 2.375(1) 1.853(4) 1.859(4) 2.223(4) 2.230(4) 2.461(1) 2.465(1) 38.6(1) 60.9(2) 80.5(2) 38.1(1) 59.8(2) 82.1(2) 170.0(2) 94.4(2) 93.7(2) 88.5(2) 87.8(1)
Ti–N(1) Ti–N(2) Ti–N(3) Ti–N(4) Ti–N(5) Ti–N(6) Ti–Cl
Ti–N(1)–C(1) N(1)–C(1)–N(2) C(1)–N(2)–Ti(1) N(1)–Ti(1)–N(2) Ti(1)–N(3)–C(14) N(3)–C(14)–N(4) C(14)–N(4)–Ti N(3)–Ti(1)–N(4) Ti(1)–N(5)–N(6) N(5)–N(6)–Ti(1) N(5)–Ti(1)–N(6) N(2)–Ti(1)–Cl
3 2.221(3) 2.124(3) 2.211(3) 2.126(3) 1.897(3) 2.196(3) 2.375(1)
88.5(2) 116.1(3) 92.3(2) 62.7(1) 89.3(2) 115.4(3) 92.5(2) 62.5(1) 81.8(2) 58.8(2) 39.4(1) 158.08(8)
Li–N(1) Li–N(2) N(1)–C(1) N(2)–C(1)
N(1)–Li–N(2) Li–N(1)–C(1) N(1)–C(1)–N(2) C(1)–N(2)–Li
2.021(6) 2.035(6) 1.330(3) 1.329(3)
69.1(2) 85.5(2) 119.8(2) 85.0(2)
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263
TMS
TMS
N 2Ph
N Li(tmeda)+ TiCl4
Ph
TMS Cl
N
Ti
N
N
TMS
TMS
Cl
Ph N TMS
LiN(Me)NMe2
TMS = Si(CH3)3
TMS TMS
Ph N
N
Fig. 2. View of the structure of [Ti2Cl4(NNMe2)2(py)3] (1) along the Ti–Ti vector. All hydrogen atoms and two molecules of CH2Cl2 of crystallization have been omitted for clarity.
Ph
N
Ti N Cl TMS Me
N
TMS
N Me
Me
2
and coordinated to Ti(2). The geometries at N(3) and N(4) are close to trigonal planar as reflected by the sums of bond angles, 352.9(3) and 348.3(3), respectively. The remaining nitrogens [N(1) and N(2)] are of the amido type and form donor–acceptor bonds to Ti(1). As expected, the average Ti–N bond distance for the imido ˚ ) is considerably shorter than that nitrogens (1.948(4) A ˚ ). The Ti(1)–N(3)– for the amido nitrogens (2.216(4) A Ti(2)–N(4) core is close to planar (sum of bond angles = 346.8(2)); however, the average Ti–N bond distance to ˚ ) is longer than that to Ti(2) Ti(1) (1.948(4) A ˚ ). As anticipated on the basis of the higher (1.856(4) A coordination number of Ti(1), the N(3)–Ti(1)–N(4) bond angle (83.3(1)) is more acute than the N(3)– Ti(2)–N(4) bond angle (88.5(2)). Overall, the close-toplanar Ti2N2 core is similar to those reported for other dinuclear imido-bridged titanium complexes [16].
Scheme 2.
3.4. Single crystal X-ray structure of [TiCl(PhC{N(SiMe3)2}2)2(MeNNMe2)] (2) The X-ray crystal structure confirms the presence of one chloride ligand in the product. Thus, even though there is sufficient space around the titanium atom (Fig. 3 and Table 2), the second equivalent of LiN (Me)NMe2 fails to react. The coordination sphere comprises two g2-bonded amidinate ligands, one chloride, and one g2-bonded trimethylhydrazide. The titanium(IV) center is therefore heptacoordinate. The
3.3. Synthesis and preliminary identification of [TiCl(PhC{N(SiMe3)2}2)2(MeNNMe2)] (2) The initial phase of the synthesis involved the lithiation of trimethylhydrazine to form LiN(Me)NMe2. The latter was allowed to react in Et2O solution with 0.5 equiv. of TiCl2(PhC{N(SiMe3)2}2)2 which was prepared by a slight variation of the literature methods [11,16]. The overall process is summarized in Scheme 2. The reason for selecting the trimethylhydrazide ligand was to minimize the chance of dimerization by imido bridge formation (as happens in the case of 1). 1H NMR spectroscopic examination of the product revealed the presence of amidinate and hydrazide resonances in a 2:1 mole ratio. The CI+ mass spectrum of 2 evidenced low intensity peaks attributable to [Ti(PhC{N(SiMe3)2}2)2(MeNNMe2)]+ and [TiCl(PhC{N(SiMe3)2}2)2(MeNNMe2)]+ suggesting that the identity of the product is [TiCl(PhC{N(SiMe3)2}2)2(MeNNMe2)]. This suggestion was confirmed by X-ray crystallography.
Fig. 3. Molecular structure of [TiCl(PhC{N(SiMe3)2}2)2(MeNNMe2)] (2) showing the atom numbering scheme. Thermal ellipsoids are set to 30% probability. All hydrogen atoms and the methyl groups on the silicon atoms have been omitted for clarity.
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geometry of 2 does not fall in an easily described category. Nitrogen N(2) and the chloride ligand possess a mutually trans relationship with a Cl–Ti–N(2) bond angle of 158.08(8) and, together with N(1) these atoms roughly describe a trigonal plane. Likewise, nitrogen atoms N(3) and N(6) are almost directly opposite each other (\N(6)–Ti–N(3) = 171.6(1)) and, in turn, nitrogen atoms N(3), N(4), N(5) and N(6) are close to planar. The ˚ for N(5). maximum deviation from said plane is 0.185 A The N(1)–N(2)–Cl and N(3)–N(4)–N(5)–N(6) planes are roughly orthogonal, e.g., N(2) forms angles of 88.9(1) and 91.4(1) with N(3) and N(6), respectively. As in the case of 1, the N–Ti bond distance for the r-bonded nitrogen of the hydrazide ligand, N(5) ˚ ) is shorter than that for the donor nitrogen, (1.897(3) A ˚ ). Although the two amidinate ligands N(6) (2.196(3) A are, as expected, bonded in an g2-fashion, the two Ti–N bond distances are unequal. For example, the ˚ longer than that N(3)–Ti bond distance is 0.085(3) A for the N(4)–Ti bond and the N(2)–Ti bond distance ˚ . The exceeds the N(1)–Ti bond distance by 0.097(3) A average bite angle of the amidinate ligands in 2 is 62.6(1) and thus considerably larger than that of the trimethylhydrazide ligand (39.4(1)).
4. Concluding remarks Although LiN(H)NMe2 is sufficiently reactive toward TiCl4 to cause both LiCl and HCl elimination reactions, only two of the four chloride ligands are displaced. Repetition of this reaction with higher lithium hydrazide/ TiCl4 mole ratios did not change the outcome and 1 was the sole isolated product. Furthermore, the pronounced tendency of the hydrazide (2-) ligand to adopt a bridging position resulted in a dinuclear structure. The use of pyridine ligands to saturate the titanium centers, and thus preclude dimerization, was unsuccessful. In the case of 2, despite the increase of size in going from hydrazide to amidinate ligands, again it was not possible to eliminate all the chloride ligands of TiCl4.
Acknowledgments The authors are grateful to the National Science Foundation (CHE-024008) and the Robert A. Welch Foundation (F-135) for financial support.
Appendix A. Supplementary data 3.5. Single crystal X-ray structure of Li[PhC{N(SiMe3)2}]tmeda (3) The structure of 3 (Fig. 4 and Table 2) is similar to those of other lithium amidinates [17]. In contrast to the structure of the titanium compound 2, the C–N bond distances in the lithio derivative 3 are equal within experimental error and shorter than the average C–N bond distance in 2.
Full crystallographic data for the structural analyses have been deposited with the Cambridge Crystallographic Data Centre, CCDC Nos. 268289 (1), 268290 (2) and 268291 (3). Copies of this information may be obtained free of charge from the Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44 1223 336 033; e-mail:[email protected] or http://www. ccdc.cam.ac.uk). Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.poly.2005.06.035.
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Fig. 4. Molecular structure of Li[PhC{N(SiMe3)2}]tmeda (3) showing the atom numbering scheme. Thermal ellipsoids are set to 30% probability.
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Titanium(IV) complexes with amidinate and/or hydrazido ligands Jeffrey M. Pietryga, Jamie N. Jones, Charles L.B. Macdonald, Jennifer A. Moore, Alan H. Cowley * Department of Chemistry and Biochemistry, The University of Texas at Austin, 1 University Station A5300, Austin, TX 78712-0165, USA Received 8 April 2005; accepted 24 June 2005 Available online 11 August 2005 Dedicated to Professor Malcolm H. Chisholm on the occasion of his 60th birthday.
Abstract The complex [Ti2Cl4(NNMe2)2(py)3] (1) has been prepared via the reaction of TiCl4 with LiN(H)NMe2, followed by treatment with pyridine. Preparation of the complex [TiCl(PhC{N(SiMe3)2}2)2(MeNNMe2)] (2) was carried out by treatment of TiCl4 with Li[PhC{N(SiMe3)2}]tmeda, followed by reaction with LiN(Me)NMe2. The structures of 1 and 2 were determined by single crystal X-ray diffraction. The single crystal X-ray structure of Li[PhC{N(SiMe3)2}]tmeda (3) has also been determined. 2005 Elsevier Ltd. All rights reserved. Keywords: Titanium; Amidinate; Hydrazido; Complexes; X-ray structures
1. Introduction Several single-source precursors to titanium nitride have been reported over the past 15 years, including homoleptic titanium amides and mixed titanium amide/alkyl compounds [1], amides such as [CpTiCl2{N(SiMe3)2}] [2] and [TiCl2(NH-t-Bu)2(NH2-t-Bu)2]n [3], several imides [4], adducts of the type [TiCl4(NH3)2] [5] and azides such as [Cp2Ti(N3)2] [6] and [Ti(NMe2)2(N3)2(py)2] [7]. Ideally, the TiN single source precursor should possess reasonable volatility in order to facilitate vapor transport. In turn, this implies the avoidance of oligomerization. A second desirable precursor feature is an all-nitrogen coordination environment around the titanium atom since the presence of, e.g., titanium– chlorine or titanium–carbon bonds can result in the presence of chlorine or carbon impurities in the TiN films or particles. *
Corresponding author. Tel.: +1 512 471 7484; fax: +1 512 471 6822. E-mail address: [email protected] (A.H. Cowley). 0277-5387/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2005.06.035
In the present work, two strategies were employed in an effort to prepare monomeric titanium nitride precursors with all-nitrogen coordination spheres. The first approach featured the use of alkylhydrazido ligands; the second involved the use of alkylhydrazido ligands in conjunction with amidinate ligands. A major potential advantage of hydrazido ligands stems from the expectation that N–N bond cleavage will be more facile than N–C bond cleavage owing to the order of bond enthalpies N–C > N–N. In principle, this should result in lower deposition temperatures and less carbon incorporation for titanium hydrazides than for precursors with Ti–N–C linkages. Such a view is supported by the observation [8] that the decomposition of methylhydrazine on a titanium surface is initiated by N–N bond cleavage at modest temperatures (200 C). Moreover, Winter et al. [9,10] have demonstrated that alkylhydrazide ligands can be employed for the deposition of high quality TiN films; however, the inclusion of chloride ligands in these precursors led to chloride incorporation in the films that were grown at lower temperatures. A second potential advantage of the use of hydrazido ligands is
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that they are capable of binding to titanium in an g2-fashion, thus facilitating the achievement of an allnitrogen coordination environment. A similar comment applies to the use of amidinate ligands which also are capable of exhibiting an g2 -binding mode. In the present contribution, we describe the reaction of TiCl4 with LiN(H)NMe2, followed by treatment with pyridine and the reaction of TiCl4 with Li[PhC{N(SiMe3)2}]tmeda, followed by treatment with LiN(Me)NMe2. Both products were characterized by single-crystal X-ray diffraction. We also report the Xray crystal structure of Li[PhC{N(SiMe3)2}]tmeda. Although the latter is a known compound [11], it has not been structurally characterized previously.
immediately with degassed perfluorinated polyether oil. The data for 1 were collected at 293(2) K on a Siemens P 4 diffractometer and the data for 2 and 3 were collected at 153(2) K on a Nonius Kappa CCD diffractometer. Each data set was corrected for Lorentz and polarization effects. All three structures were solved by direct methods, and refined by full-matrix least-squares on F2 using the SHELXTL PLUS software package [13]. All non-hydrogen atoms were allowed anisotropic thermal motion and hydrogen atoms, which were included at calculated positions, were refined using a riding model and a general isotropic thermal parameter. The total number of reflections, collection ranges, and final R values are listed in Table 1. 2.4. Synthesis of [Ti2Cl4(NNMe2)2(py)3] (1)
2. Experimental 2.1. General procedures All manipulations and reactions were performed under catalyst-scrubbed Ar, using a combination of glove box and standard Schlenk techniques. Dichloromethane was purified by distillation under N2 from calcium hydride, and all other solvents were distilled under N2 from sodium/benzophenone ketyl. Additional degassing of solvents was performed using either vigorous Ar bubbling, or freeze/pump/thaw cycling. A 1.6 M hexane solution of n-BuLi was purchased from a commercial source, and used without further purification. Titanium(IV) chloride was purchased from a commercial source, and purified as necessary by vacuum distillation. 1,1,-Dimethylhydrazine was purchased from a commercial source, and purified by distillation. Trimethylhydrazine [12] and Li[PhC{N(SiMe3)2}]tmeda [11] were prepared and purified using literature methods. 2.2. Spectroscopic measurements All CI mass spectra were recorded on a Finnigan MAT TSQ-700 instrument; the samples were sealed in glass capillaries under Ar in a glove box. NMR spectra were recorded at 295 K on a GE QE-300 spectrometer. The CD2Cl2 used in these measurements was purchased in sealed vials, either pure or with 1% (v/v) tetramethylsilane. The THF-d8 was purchased from a commercial source and vacuum distilled under Ar prior to use. The 1H NMR chemical shifts are reported relative to tetramethylsilane and are referenced either directly or to solvent. 2.3. Single crystal X-ray structure determinations Crystals of [Ti2Cl4(NNMe2)2(py)3] (1), [TiCl(PhC{N(SiMe3)2}2)2(MeNNMe2)] (2) and Li[PhC{N(SiMe3)2}]tmeda (3) of suitable quality were collected directly from Schlenk-type flasks under Ar pressure, and covered
A solution of NH2NMe2 (3.8 g, 63 mmol) in 40 mL of tetrahydrofuran was chilled to 78 C and 25 mL of an n-BuLi solution (1.6 M in hexane) was added over a period of 10 min. The reaction mixture was allowed to warm to room temperature over several hours, then stirred overnight. Titanium tetrachloride (5.2 mL, 16 mmol) was added via syringe to 80 mL of tetrahydrofuran at 10 C, to which was added the solution of LiN(H)NMe2 over a period of 10 min. The reaction mixture was allowed to warm to room temperature overnight, following which the solvent was removed under reduced pressure and the resulting solids extracted with CH2Cl2 (3 · 75 mL). Filtration through a glass frit covered with diatomaceous earth resulted in a red colored filtrate. The volume of the latter solution was reduced to 100 mL, and pyridine (0.5 mL, 6 mmol) was added to a 30 mL aliquot. Cooling of this mixture to 20 C for a period of days produced large red crystals of 1 in 40% yield. 1 H NMR (300 MHz, 298 K, CD2Cl2): d 8.83 (m, 6H, m-py), 7.79 (t, 3H, p-py), 7.35 (t, 6H, o-py), 3.47 (s, 12H, Me2N). MS (CI+, CH4): m/e 556 [Ti2Cl3(NNMe2)2(py)3]+ (5%), 475 [Ti2Cl3(NNMe2)2(py)2]+ (24%), 434 [Ti2Cl3N3C2H7(py)2]+ (27%), 396 [Ti2Cl3(NNMe2)2(py)]+ (49%), 355 [Ti2Cl3N3C2H7(py)]+ (100%), 317 [Ti2Cl3(NNMe2)2]+ (23%). 2.5. Synthesis of [TiCl(PhC{N(SiMe3)2}2)2(MeNNMe2)] (2) Titanium tetrachloride (2.0 mL, 18 mmol) was added via syringe to a solution of Li[PhC{N(TMS)}2]tmeda (13.9 g, 36 mmol) in 100 mL of tetrahydrofuran. The reaction mixture was stirred overnight, after which the volatiles were removed under reduced pressure and the resulting orange-red solid was extracted with diethyl ether (2 · 70 mL). After filtration through a glass frit covered with diatomaceous earth, the red filtrate was concentrated and maintained at 20 C overnight. The intermediate, TiCl2(PhC{N(SiMe3)2}2)2, was isolated as
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Table 1 Selected crystal data and data collection refinement parameters for 1–3
Empirical formula Formula weight Crystal size (mm) Crystal class Space group ˚) a (A ˚) b (A ˚) c (A b () ˚ 3) U (A Z l (Mo-Ka, mm 1) T (K) Diffractometer Radiation ˚) k (A Measured reflections Independent reflections Rint Range in 2h () Range in h Range in k Range in l Data/restraints/parameters R1 [I > 2r(I)] wR2
1
2
3
C21H31Cl8N7Ti2 760.93 0.15 · 0.09 · 0.07 monoclinic P21/n 16.9961(7) 11.1852(5) 18.3199(7) 106.343(1) 3342.0(2) 4 1.142 293(2) Siemens P4
C33H65ClN6OSi4Ti 757.62 0.13 · 0.11 · 0.04 monoclinic P21/c 11.664(2) 21.175(4) 17.730(4) 100.42(3) 4307.0(1) 4 0.404 153(2) Nonius Kappa CCD Graphite-monochromated Mo-Ka 0.71073 14 627 7560 0.0708 5.98 6 2h 6 50.00 13 6 h 6 13 25 6 k 6 25 21 6 l 6 21 7560/0/430 0.0539 0.0979
C19H39LiN4Si2 386.66 0.40 · 0.30 · 0.21 monoclinic P21/n 10.584(5) 13.234(5) 18.147(5) 91.535(5) 2540.9(2) 4 0.149 153(2) Nonius Kappa CCD
17 961 5891 0.0478 2.86 6 2h 6 50.00 20 6 h 6 18 13 6 k 6 11 11 6 l 6 21 5891/0/347 0.0567 0.1477
red crystals in 45% yield. This is a slight variation of the literature methods [11,14,15]. [1H NMR (300 MHz, 298 K, CD2Cl2): d 7.41 (m, 3H, o,p-Ph), 7.28 (m, 2H, m-Ph), 0.05 (s, 18H, Me3Si)]. Trimethylhydrazine (0.23 g, 3 mmol) in 30 mL of diethyl ether was lithiated by treatment with 1.9 mL of a solution of n-BuLi (1.6 M in hexane) at 78 C. The reaction mixture was allowed to assume room temperature and stirring was continued overnight. A solution of TiCl2(PhC{N(SiMe 3)2}2) 2 (1.0 g, 1.5 mmol) in 50 mL of diethyl ether was then added, and the reaction mixture was stirred overnight. Following filtration through a glass frit covered with diatomaceous earth, the resulting orange filtrate was concentrated and chilled to 78 C, affording an orange oil. A crop of orange-red crystals of 2 (0.22 g (0.33 mmol, 22% yield)) formed upon storage of the oil at ambient temperature. 1H NMR (300 MHz, 298 K, THF-d8): d 7.44 (m, 6H, Ph), 7.31 (m, 4H, Ph), 2.74 (s, 3H, MeN), 2.36 (s, 6H, Me2N), 0.07 (s, 18H, Me3Si); MS (CI+, CH4) m/e 683 [M + 1, 14%], 647 [M-Cl , 15%], 609 [TiCl2(PhC{N(SiMe3)2}2)2, 100%].
reagent was allowed to react with TiCl4 in a 4:1 mole ratio in THF solution. The synthesis of 1 was completed by the addition of pyridine in an effort to prevent oligomerization (Scheme 1). 1H NMR spectroscopic assay provided two clues to the structure of 1, namely (i) no N–H signal was evident thus suggesting that the hydrazido ligand is attached to titanium in an g1 or g2 fashion, and (ii) the hydrazido and pyridine ligands are present in a 2:3 mole ratio. However, it was necessary to appeal to X-ray crystallography to provide more definitive structural information. LiN(H)NM e2 + TiCl4
"TiCl2(NNM e2)"
pyridine Me
Me
Py
N
Cl
N
Py
3. Results and discussion
Cl
Ti Cl
Ti N
Cl
N
3.1. Synthesis and preliminary identification of [Ti2Cl4(NNMe2)2(py)3] (1) The initial phase of the synthesis was monolithiation of H2NNMe2 to form LiN(H)NMe2. This lithium
33 850 5816 0.0713 6.56 6 2h 6 54.98 13 6 h 6 13 16 6 k 6 17 23 6 l 6 19 5816/150/245 0.0710 0.1699
Me
Py Me 1 Scheme 1.
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3.2. Single crystal X-ray structure of [Ti2Cl4(NNMe2)2(py)3] (1) The X-ray analysis reveals a dititanium structure in which the two coordination environments are quite distinct (Fig. 1 and Table 2). One titanium atom [Ti(1)] is seven coordinate and features a pyridine molecule, two chlorides, and two g2-attached [Me2NN]2 ligands.
Fig. 1. Molecular structure of [Ti2Cl4(NNMe2)2(py)3] (1) showing the atom numbering scheme. Thermal ellipsoids are set to 30% probability. All hydrogen atoms and two molecules of CH2Cl2 of crystallization have been omitted for clarity.
The overall geometry at Ti(1) can be described as pentagonal bipyramidal. Thus, the sum of bond angles in the pentagonal plane described by the five nitrogen atoms is 360.0(1). Note, however, that the angles in this plane are very distorted and range from 38.1(1) to 101.0(1). The departures from the ideal bond angle of 72 are caused by the very small bite angles of the g2-hydrazide ligands. If, however, a pseudo-trigonal bipyramidal geometry is considered by defining the midpoints of the N–N moieties as ‘‘ligands’’ the ‘‘bond angles’’ in the equatorial plane would be 120.3, 118.1, and 121.6. The axial ligands at N(1) are distorted from linearity as reflected by the Cl–Ti–Cl bond angle of 160.99(6). Note that both of these chloride ligands are inclined away from the more ‘‘crowded’’ side of the molecule. The geometry at Ti(2) is octahedral. Two pyridine molecules are arranged in a trans fashion while the two chlorides and the two imido nitrogens [N(3) and N(4)] each possess a mutually cis arrangement. The departures from the ideal angle of 90 are modest and of the order of 5–6 (see Table 2). The view down the Ti(1)–Ti(2) vector shows that the C(13)–C(14)–N(3)– N(4)–N(1)–N(2) and N(6)–N(7)–C(11)–C(12) planes are close to orthogonal (Fig. 2). The two hydrazides feature an imido nitrogen [N(3) and N(4)], each of which is formally r-bonded to Ti(1)
Table 2 ˚ ) and angles () for 1–3 Selected bond lengths (A 1 Bond lengths Ti(1)–N(1) Ti(1)–N(2) Ti(1)–N(3) Ti(1)–N(4) Ti(1)–N(5) Ti(1)–Cl(1) Ti(1)–Cl(2) Ti(2)–N(3) Ti(2)–N(4) Ti(2)–N(6) Ti(2)–N(7) Ti(2)–Cl(3) Ti(2)–Cl(4) Bond angles N(2)–Ti(1)–N(4) Ti(1)–N(2)–N(4) Ti(1)–N(4)–N(2) N(1)–Ti(1)–N(3) Ti(1)–N(1)–N(3) N(1)–N(3)–Ti(1) Cl(1)–Ti(1)–Cl(2) Ti(1)–N(3)–Ti(2) Ti(1)–N(4)–Ti(2) N(3)–Ti(2)–N(4) Cl(3)–Ti(2)–Cl(4)
2 2.224(2) 2.208(4) 1.941(4) 1.956(4) 2.272(4) 2.366(1) 2.375(1) 1.853(4) 1.859(4) 2.223(4) 2.230(4) 2.461(1) 2.465(1) 38.6(1) 60.9(2) 80.5(2) 38.1(1) 59.8(2) 82.1(2) 170.0(2) 94.4(2) 93.7(2) 88.5(2) 87.8(1)
Ti–N(1) Ti–N(2) Ti–N(3) Ti–N(4) Ti–N(5) Ti–N(6) Ti–Cl
Ti–N(1)–C(1) N(1)–C(1)–N(2) C(1)–N(2)–Ti(1) N(1)–Ti(1)–N(2) Ti(1)–N(3)–C(14) N(3)–C(14)–N(4) C(14)–N(4)–Ti N(3)–Ti(1)–N(4) Ti(1)–N(5)–N(6) N(5)–N(6)–Ti(1) N(5)–Ti(1)–N(6) N(2)–Ti(1)–Cl
3 2.221(3) 2.124(3) 2.211(3) 2.126(3) 1.897(3) 2.196(3) 2.375(1)
88.5(2) 116.1(3) 92.3(2) 62.7(1) 89.3(2) 115.4(3) 92.5(2) 62.5(1) 81.8(2) 58.8(2) 39.4(1) 158.08(8)
Li–N(1) Li–N(2) N(1)–C(1) N(2)–C(1)
N(1)–Li–N(2) Li–N(1)–C(1) N(1)–C(1)–N(2) C(1)–N(2)–Li
2.021(6) 2.035(6) 1.330(3) 1.329(3)
69.1(2) 85.5(2) 119.8(2) 85.0(2)
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TMS
TMS
N 2Ph
N Li(tmeda)+ TiCl4
Ph
TMS Cl
N
Ti
N
N
TMS
TMS
Cl
Ph N TMS
LiN(Me)NMe2
TMS = Si(CH3)3
TMS TMS
Ph N
N
Fig. 2. View of the structure of [Ti2Cl4(NNMe2)2(py)3] (1) along the Ti–Ti vector. All hydrogen atoms and two molecules of CH2Cl2 of crystallization have been omitted for clarity.
Ph
N
Ti N Cl TMS Me
N
TMS
N Me
Me
2
and coordinated to Ti(2). The geometries at N(3) and N(4) are close to trigonal planar as reflected by the sums of bond angles, 352.9(3) and 348.3(3), respectively. The remaining nitrogens [N(1) and N(2)] are of the amido type and form donor–acceptor bonds to Ti(1). As expected, the average Ti–N bond distance for the imido ˚ ) is considerably shorter than that nitrogens (1.948(4) A ˚ ). The Ti(1)–N(3)– for the amido nitrogens (2.216(4) A Ti(2)–N(4) core is close to planar (sum of bond angles = 346.8(2)); however, the average Ti–N bond distance to ˚ ) is longer than that to Ti(2) Ti(1) (1.948(4) A ˚ ). As anticipated on the basis of the higher (1.856(4) A coordination number of Ti(1), the N(3)–Ti(1)–N(4) bond angle (83.3(1)) is more acute than the N(3)– Ti(2)–N(4) bond angle (88.5(2)). Overall, the close-toplanar Ti2N2 core is similar to those reported for other dinuclear imido-bridged titanium complexes [16].
Scheme 2.
3.4. Single crystal X-ray structure of [TiCl(PhC{N(SiMe3)2}2)2(MeNNMe2)] (2) The X-ray crystal structure confirms the presence of one chloride ligand in the product. Thus, even though there is sufficient space around the titanium atom (Fig. 3 and Table 2), the second equivalent of LiN (Me)NMe2 fails to react. The coordination sphere comprises two g2-bonded amidinate ligands, one chloride, and one g2-bonded trimethylhydrazide. The titanium(IV) center is therefore heptacoordinate. The
3.3. Synthesis and preliminary identification of [TiCl(PhC{N(SiMe3)2}2)2(MeNNMe2)] (2) The initial phase of the synthesis involved the lithiation of trimethylhydrazine to form LiN(Me)NMe2. The latter was allowed to react in Et2O solution with 0.5 equiv. of TiCl2(PhC{N(SiMe3)2}2)2 which was prepared by a slight variation of the literature methods [11,16]. The overall process is summarized in Scheme 2. The reason for selecting the trimethylhydrazide ligand was to minimize the chance of dimerization by imido bridge formation (as happens in the case of 1). 1H NMR spectroscopic examination of the product revealed the presence of amidinate and hydrazide resonances in a 2:1 mole ratio. The CI+ mass spectrum of 2 evidenced low intensity peaks attributable to [Ti(PhC{N(SiMe3)2}2)2(MeNNMe2)]+ and [TiCl(PhC{N(SiMe3)2}2)2(MeNNMe2)]+ suggesting that the identity of the product is [TiCl(PhC{N(SiMe3)2}2)2(MeNNMe2)]. This suggestion was confirmed by X-ray crystallography.
Fig. 3. Molecular structure of [TiCl(PhC{N(SiMe3)2}2)2(MeNNMe2)] (2) showing the atom numbering scheme. Thermal ellipsoids are set to 30% probability. All hydrogen atoms and the methyl groups on the silicon atoms have been omitted for clarity.
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geometry of 2 does not fall in an easily described category. Nitrogen N(2) and the chloride ligand possess a mutually trans relationship with a Cl–Ti–N(2) bond angle of 158.08(8) and, together with N(1) these atoms roughly describe a trigonal plane. Likewise, nitrogen atoms N(3) and N(6) are almost directly opposite each other (\N(6)–Ti–N(3) = 171.6(1)) and, in turn, nitrogen atoms N(3), N(4), N(5) and N(6) are close to planar. The ˚ for N(5). maximum deviation from said plane is 0.185 A The N(1)–N(2)–Cl and N(3)–N(4)–N(5)–N(6) planes are roughly orthogonal, e.g., N(2) forms angles of 88.9(1) and 91.4(1) with N(3) and N(6), respectively. As in the case of 1, the N–Ti bond distance for the r-bonded nitrogen of the hydrazide ligand, N(5) ˚ ) is shorter than that for the donor nitrogen, (1.897(3) A ˚ ). Although the two amidinate ligands N(6) (2.196(3) A are, as expected, bonded in an g2-fashion, the two Ti–N bond distances are unequal. For example, the ˚ longer than that N(3)–Ti bond distance is 0.085(3) A for the N(4)–Ti bond and the N(2)–Ti bond distance ˚ . The exceeds the N(1)–Ti bond distance by 0.097(3) A average bite angle of the amidinate ligands in 2 is 62.6(1) and thus considerably larger than that of the trimethylhydrazide ligand (39.4(1)).
4. Concluding remarks Although LiN(H)NMe2 is sufficiently reactive toward TiCl4 to cause both LiCl and HCl elimination reactions, only two of the four chloride ligands are displaced. Repetition of this reaction with higher lithium hydrazide/ TiCl4 mole ratios did not change the outcome and 1 was the sole isolated product. Furthermore, the pronounced tendency of the hydrazide (2-) ligand to adopt a bridging position resulted in a dinuclear structure. The use of pyridine ligands to saturate the titanium centers, and thus preclude dimerization, was unsuccessful. In the case of 2, despite the increase of size in going from hydrazide to amidinate ligands, again it was not possible to eliminate all the chloride ligands of TiCl4.
Acknowledgments The authors are grateful to the National Science Foundation (CHE-024008) and the Robert A. Welch Foundation (F-135) for financial support.
Appendix A. Supplementary data 3.5. Single crystal X-ray structure of Li[PhC{N(SiMe3)2}]tmeda (3) The structure of 3 (Fig. 4 and Table 2) is similar to those of other lithium amidinates [17]. In contrast to the structure of the titanium compound 2, the C–N bond distances in the lithio derivative 3 are equal within experimental error and shorter than the average C–N bond distance in 2.
Full crystallographic data for the structural analyses have been deposited with the Cambridge Crystallographic Data Centre, CCDC Nos. 268289 (1), 268290 (2) and 268291 (3). Copies of this information may be obtained free of charge from the Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44 1223 336 033; e-mail:[email protected] or http://www. ccdc.cam.ac.uk). Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.poly.2005.06.035.
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Fig. 4. Molecular structure of Li[PhC{N(SiMe3)2}]tmeda (3) showing the atom numbering scheme. Thermal ellipsoids are set to 30% probability.
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