Mono- and Binuclear Titanates Bearing Podand Diamidoamine

Article pubs.acs.org/Organometallics

Mono- and Binuclear Titanates Bearing Podand Diamidoamine Ligands and Their Use as Catalysts in Siloxane Cross-Linking Martha Höhne,† Andrea Gutacker,‡ Johann Klein,‡ and Esteban Mejía*,† †

Leibniz-Institut für Katalyse, Albert-Einstein-Straße 29a, 18059 Rostock, Germany Adhesive Technologies, Henkel AG & Co KGaA, Henkelstraße 67, 40589 Düsseldorf, Germany

‡

S Supporting Information *

ABSTRACT: The synthesis of titanium(IV) alkoxides bearing diamidoamide ligands was undertaken. We synthesized complexes bearing alkoxide ligands of varying size to assess the influence of the steric demand of the alkoxide ligands on the coordination/dissociation equilibria. Moreover, to evaluate the influence of the electronic properties of these ligands, we prepared analogues including electron-donating trimethylsilyl and electron-withdrawing tosyl N-substituents. Herein, we describe their synthesis, characterization, and coordination behavior as well as some of their decomposition products and catalytic performance in the condensation cross-linking of silicone prepolymers.

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INTRODUCTION Silicone polymers, especially polydimethylsiloxanes, are of paramount importance as adhesives, sealants, insulators, etc. Among these, those that vulcanize at low temperatures under ambient conditions (RTV) represent a considerable fraction of the market.1−4 Traditionally, organometallic tin complexes have been used as curing catalysts with excellent results in terms of shelf life, curing time, and selectivity. Nevertheless, due to its inherent toxicity and the subsequent environmental concerns,5 the use of tin-containing formulations has been restricted and soon will be completely banned.6 Hence, the search for alternative catalysts with comparable (or at best, enhanced) performance in the siloxane cross-linking reaction (Scheme 1) is of high interest.7 The leading alternatives are titanium(IV) alkoxides, which despite their relatively good activities, show important drawbacks like premature deactivation, short shelf life, and incompatibility with common additives like aminosiloxanes.8−11 These shortcomings are due to the high Lewis acidity of the metal center and the inherent hydrolytic instability of the

titanium−alkoxide bond. A “second generation” of catalysts with improved stability has been obtained by addition of acetylacetonates, which stabilize to a certain extent the reactive centers by chelate formation.12 Early transition metal complexes of high valence and pronounced Lewis acidity can be “tamed” by introduction of polydentate, multifunctional ligands having both anionic and neutral donors.13 Prominent representatives of this family are tridentate amidoamine (1)14,15 and amidopyridine ligands (2)16 (Chart 1), in which a neutral tertiary amino donor is capable of

Scheme 1. Typical Cross-Linking (Vulcanization) Reaction for an α,ω-Di-dialkoxyvinylsilyl-Polydimethylsiloxane Silicone

engaging reversible coordination, protecting or “masking” the metal center, whereas the anionic, anchoring amido moieties provide robustness to the complexes. The decoordination− coordination equilibria of the central amino/pyridine moiety in these kinds of complexes (Scheme 1) have been observed in solution by NMR spectroscopy and shown to be dependent on the bulkiness of the additional ligands in axial and equatorial positions (on the penta-coordinated complex).13,14,17

Chart 1. Titanium(IV) Complexes Containing Amidoamine (1) and Amidopyridine (2) Ligands

Received: May 5, 2017 Published: June 27, 2017 © 2017 American Chemical Society

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Organometallics Many of these chelates have been successfully used as nonmetallocene catalysts for the polymerization of olefins, particularly the Zr counterparts of the derivatives presented in Chart 1.18 It has been demonstrated that the neutral donor atom placed between the two anionic amido groups plays a fundamental role in the catalytic activity by stabilizing the active, cationic intermediates during the catalytic cycle. These complexes, especially with amide and alkoxide ligands, have found successful application as initiators in the ring-opening polymerization of cyclic esters such as lactide and εcaprolactone. They undergo controlled polymerization processes and yield polyesters with narrow polydispersity indexes and well-defined molecular weight distributions.17 We have reasoned that due to the hemilabile and protective nature of the central amino functionality in the podand ligand, their titanium complexes may be able to successfully promote the siloxane cross-linking reaction of silicone prepolymers, in which, as discussed above, the presence of nucleophilic additives jeopardizes the activity and shelf-stability of the currently used catalysts. Therefore, the synthesis of titanium(IV) alkoxides bearing diamidoamine ligands was undertaken. Knowing that the steric demand of the alkoxide ligands directly affects the coordination/dissociation equilibrium (shown in Scheme 2), we synthesized complexes bearing alkoxide ligands

Scheme 3. Synthesis of Different Substituted Diethylenetriamine Ligands According to Clark et al.,14 Cloke et al.,15 and Koyama et al.19

In a ligand exchange reaction, the amido ligands should act as an internal base and facilitate the substitution with the tridentate ligands 3a,b. The gain in entropy and stability, as a result of the chelate effect, could be regarded as an additional driving force.22 As electron-donating substituents lower the acidity of the amino protons, 3b had to be deprotonated with nBuLi before coordinating to the metal centers. Due to the LiCl formation, the dichlorotitanium alkoxides were used as metal precursors (Scheme 4).

Scheme 2. Fluxional Behavior of Diamidoamine-Supported Titanium Complexes

Scheme 4. Synthesis of Five-Coordinate Titanium Complexes Bearing Tridentate Diamidoamine Ligands

of varying size. Moreover, to assess the influence of the electronic properties of the tridentate ligand, we prepared analogues including electron-donating trimethylsilyl and electron-withdrawing tosyl N-substituents. Herein, we describe their synthesis, characterization, and coordination behavior as well as some of their decomposition products and catalytic performance in the condensation cross-linking of silicone prepolymers.

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RESULTS AND DISCUSSION Synthesis and Characterization of the Complexes. The first step in the preparation of the series of titanium(IV) complexes bearing tripodand diamidoamino ligands was the synthesis of the corresponding diaminoamine ligand precursors (see Scheme 3). Diethylenetriamine (3) was triply substituted with both electron-donating SiMe3 groups giving 3b and electron-withdrawing tosyl substituents leading to 3a. Unfortunately, the one-pot synthesis of the 3-fold silylated diaminoamine ligand 3b as described by Clark et al.14 led to a mixture of products that could not be separated. Alternatively, deprotonation with n-BuLi and following addition of SiMe3Cl as reported by Cloke et al.15 led to 3b in better yields up to 59%. The insertion of the electron-withdrawing tosyl groups to produce 3a was achieved in good yields (88%) following the procedure of Koyama and co-workers.19 The heteroleptic titanate precursors (4a−e) were prepared by ligand exchange reactions starting from homoleptic TiL4 compounds (L = Cl, NEt2, OR).20,21 First coordination attempts were undertaken by adding the tridentate diamidoamine ligands (3ab) to the diamido(dialkoxo) titanates (4a−c).

Adding a solution of diamido(di-n-butoxy)titanate (4a) in toluene to a stirred solution of 3a in toluene led to a color change from yellow to orange. 5a was precipitated from toluene/n-hexane as an orange solid in a yield of 65%. Its corresponding 1H NMR spectrum shows high complexity. The combination of DEPT-135, HSQC, COSY, and NOESY experiments points out the presence of different isomers of 5a as three types of n-butoxide ligands at room temperature 2453

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Figure 1. COSY spectra of complex 5a (benzene-d8, 500 MHz, 100 °C). The observed magnetic couplings are illustrated by curved arrows in both isomers: the “closed” penta-coordinated (A) and the “open” tetra-coordinated (B).

the square planar transition state, and the facial isomer can be distinguished. Variable-temperature NMR experiments in the range of −80 to 110 °C showed high-temperature decoalescence of the signals so that a H−H correlation experiment (COSY) at 100 °C was recorded (Figure 1). Thus, the H−H correlations among the butyl chains of the three different types of nbutoxide ligands (h, i, and j in Figure 1) were analyzed. From 4.6 to 5.0 ppm (correlation j), three resonances for the O−CH fragments were observed. The two sharp triplets could be assigned to the trigonal bipyramidal isomer A, whereas both O−CH fragments of the tetrahedral isomer B are equivalent and seem to appear as one broad singlet. Attempts to synthesize the silylated analogue using ligand 3b did not lead to the desired five-coordinate complex 5b shown in Scheme 4. Instead, varying the reaction conditions (refluxing in n-heptane for 6 h) led to a brown viscous liquid. Storage of the latter for some days at room temperature led to the formation of a yellow solid that was separated and recrystallized from n-heptane. Some yellow crystals could be isolated and were examined by NMR and X-ray diffraction. The molecular structure of this new complex (6) is depicted in Figure 2. Complex 6 is a centrosymmetric binuclear dimer with amido bridging groups. It may arise from the dimerization of the expected (albeit not isolated) complex 5b, in which the Nbonded silyl groups undergo cyclometalation followed by the loss of one TMS group and one n-butoxide from each titanium center. It contains a perfectly planar Ti2N2 core. The Ti−N bonds of the Ti2N2 four-membered ring display an interesting bond length pattern, shown in Figure 2. With respect to the different Ti−N distances within the four-membered ring, complex 6 can be regarded as an asymmetrically bridging imido type, as already described by Gade et al.,25 including

could be identified (see Figure 1 and Supporting Information). These presumably result from equilibrium between two isomers of 5a as previously observed for similar compounds,13,14,18,23 due to the ability of the amino nitrogen atom to dissociate and recoordinate (Scheme 2). Consequently, the geometry at the metal center changes between tetrahedral and trigonal bipyramidal. Whereas both alkoxide ligands are equivalent when bound to the tetrahedral-coordinated metal center, the trigonal bipyramidal five-coordination results in two different types of alkoxide ligands because one of them is located in an axial position while the other is equatorial.13,14,18,23 The simultaneous presence of the three sets of signals in the NMR spectra indicates the presence of equilibrium between both isomers at the measurement’s temperature (Figure 1, vide infra). Another reason for the complexity of the signals in the 1H NMR could be the Berry pseudorotation, which provokes a fluctuating equilibrium between two penta-coordinated isomers (Scheme 5).14,23,24 Normally, this exchange is as rapid as it could only be detected as an averaged signal in 1H NMR. In case of chelating ligands, these dynamics could be inhibited or slowed down so the NMR signals for the meridional isomer, Scheme 5. Berry Pseudorotation: Equilibrium between Meridional and Facial Isomer with Square Planar Transition State

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the existence of four different signals in the 29Si NMR spectrumdiffering in their chemical shifts from the ones of the free ligand (δ/ppm: 3b 3.33 and 5.59; 5d 0.33, 1.93, 5.20 and 15.51)a coordination of the ligand yielding more than one species can be assumed. According to the mentioned dynamic behavior of complex 5a, this equilibrium between tetrahedral and trigonal bipyramidal geometry at the titanium center is postulated, as well. The corresponding 1H NMR spectrum of 5d offers the expected complexity of signals: the N−CH2 protons of the ethylene fragments resonate between 4.00 and 2.00 ppm. A closer look at this interval offers two intense triplets (Figure 3, signals I and II, correlation m) that are assigned to the flexible eight-membered ring of the tetrahedral-coordinated isomer. At room temperature, complex B, containing the “open chain” ligand with dissociated amino center (Figure 3), seems to be the dominating species. The remaining signals should originate from isomer A. Correlation n (Figure 3) points out the existence of three different ethylene species. They might arise from two different isomers of A which can either be meridional or facial-coordinated. Similar considerations can be applied for the O−CH protons (signal III, Figure 3). Correlation k offers one dominating signal, which presumably corresponds to isomer B with the flexible eight-membered cyclic ligand. Besides, three additional signals can be assigned to the trigonal bipyramid form A. Variable-temperature NMR experiments of 5d over the range of −68 to 90 °C indicated that the sharp triplets I and II at high temperatures, corresponding to the N−CH2 protons at δ 2.48 and 3.39 ppm, gradually coalesced at −20 to −30 °C (Figure 4). With further decrease of the temperature, each of them splits up into two separate signals. High temperatures provide

Figure 2. Capped stick plot of the X-ray molecular structure of complex 6. For clarity, disordered groups of the chelating ligand are depicted in only one orientation. Ti−N bond length pattern of the Ti2N2 four-membered rings shows an asymmetrically bridging imido type.

another ligand system. The same structural motive could be observed for germanium by Karlov et al.26 To stabilize the complex against dimerization and loss of ligands, the steric demand of the alkoxide ligands was increased. Besides, ligand 3b was deprotonated with n-BuLi before adding it to a solution of dichloro(di-isopropoxy)titanate (4b) in toluene to facilitate the coordination. Thus, the formation of LiCl acted as another driving force for the reaction. Complex 5d could be isolated as a deep red liquid (yield 60%). Due to

Figure 3. COSY spectra of complex 5d (benzene-d8, 300 MHz, 25 °C). The observed magnetic couplings are illustrated by curved arrows in both isomers: the “closed” penta-coordinated (A) and the “open” tetra-coordinated (B). 2455

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Figure 5. X-ray molecular structure of complex 7. Hydrogen atoms are omitted for clarity. The thermal ellipsoids correspond to 50% probability. For bond distances and angles, see Supporting Information. Figure 4. Variable-temperature 1H NMR spectra of complex 5d. Enlarged section of N−CH2 protons in the range of −68 to 90 °C (toluene-d8, 500 MHz).

Interestingly, the short Ti−O bond length (measured: Ti−O = 1.796(12) Å for 6; 1.8063(14) Å for 7, calculated single bonding: Ti−O = 1.99 Å,27 calculated double bonding: TiO = 1.74 Å27) and Ti−O−C linearity (Ti−O−C = 176.26(16)° for 6; Ti−O−C = 167.13(16)° for 7) suggests some multiple bond character due to donation of electron density from filled oxygen pπ-orbitals to unfilled d-orbitals of the electron-deficient metal.29−32 Surprisingly, complexes 5c and 5e showed a completely different behavior. Due to the sharp signals of the corresponding 1H NMR spectra (see Supporting Information), a lack of the mentioned dynamics (as observed in complex 5a and 5d) can be assumed. Similar complexes including the sterically demanding threefold tosyl-substituted diethylene triamine and diethylamido ligand made by Mountford et al.17 showed the dynamical behavior in solution. In contrast to them, the combination of the bulky tosyl-substituted diamidoamine ligands with isopropoxides or 2-ethylhexoxides seems to force the tetrahedral geometry at the metal center due to steric repulsion. Catalytic Behavior in Silicone Cross-Linking. Compounds 5a, 5c, 5d, and 5e were tested as catalysts for the crosslinking reaction of α,ω-di-dialkoxyvinylsilyl-polydimethylsiloxane to give three-dimensional networks (Scheme 1). Even though each of them was able to catalyze the cross-linking reaction between the silicone polymers, they showed differences on the skin-over time (SOT) and the depth of cure (DOC) (Table 1). The mechanism of titanium-catalyzed cross-linking reaction is not yet completely understood, although there are some generally accepted assumptions.7,34 van der Weij studied the action of organotin compounds as curing catalysts in condensation-type RTV silicone rubber.33 According to this proposal, a hydrolysis product of the organotin compound is the actual catalyst. In this respect, a trend could be observed: by increasing the bulkiness of the alkoxide ligands, the SOT increased (5a < 5c, 5d < 5e; for further information, see Supporting Information). The sterically demanding ligands cause a kinetic stabilization against hydrolysis, so it requires a

enough energy to cause the dissociation of the amino nitrogen atom, and the geometry at the titanium center is hence tetrahedral. Thus, the resulting eight-membered ring is as flexible as it allows detecting one average signal for the N−CH2 groups. Decreasing the temperature causes coordination of the amino nitrogen atom and thus the formation of the chemically and magnetically different ethylene fragments, which then would resonate at different chemical shifts in the corresponding 1 H NMR spectra. After several months at 4 °C, complex 5d dimerizes analogously to 5b, yielding monoclinic yellow crystals of complex 7. These were recrystallized from n-pentane and characterized by X-ray diffraction. The molecular structure of complex 7 is shown in Figure 5. Similar to complex 6, its core is the planar Ti2N2 ring, which is arranged around a crystallographic center of inversion relating both halves of the molecule to each other. The coordination geometry at the metal centers is strongly distorted but can be regarded to be trigonal bipyramidal. The huge interatomic distance [d(Ti−Ti) = 3.288(6) Å for 6; 3.3488(6)Å for 7] does not indicate any direct interaction between the titanium centers. Instead, the apical nitrogen atoms are bridging both halves of the molecule, and a nonsymmetrical bonding situation can be observed in the Ti2N2 ring, as well.25 The apical nitrogen of 6 and 7 is able to donate an electron lone pair in an empty d-orbital of its titanium metal center. The resulting Ti−N distance (2.2195(17) Å for 6; 2.2325(14) Å for 7) is significantly longer than a covalent Ti−N bond (2.07 Å).27 In contrast, the bridging Ti−N distance (2.047(17) Å for 6; 2.0528(15) Å for 7) is noticeably shorter and could be considered to have covalent bonding character. The Ti−N distances are in a range of 1.9289(17) to 1.9496(14) Å. They are comparable to those of other titanates bearing ligand 4b [N{TMS}{CH2CH2N(TMS)}2].28 2456

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argon prior to utilization. Titanium tetrachloride (≥98%) was purchased from Fluka and used as received. Ti(NEt2)4,21,34 Ti(NEt2)2(n-BuO)2 (4a), Ti(NEt2)2(i-PrO)2 (4b), Ti(NEt2)2((2ethylhexyl)oxy)2 (4c), TiCl2(i-PrO)2 (4d), TiCl2((2-ethylhexyl)oxy)2 (4e),20 and the ligands 3a and 3b14,15 were synthesized according to published procedures. Mass spectra were recorded in a MAT 95-XP spectrometer. NMR spectra were made either in a Bruker AVANCE 500, AVANCE 300 III, or AVANCE 250 II. Chemical shifts (1H, 13C) are given in parts per million relative to SiMe4 and are referenced to signals of the used solvent: benzene-d6 (δH 7.16, δC 128.06), toluened8 (δH 2.09, δC 20.04), THF-d8 (δH 3.58, δC 67.57), or CDCl3 (δH 7.26, δC 77.16). There was no calibration for the 29Si spectra. The peak assignment was made with the help of DEPT, HSQC, COSY, and NOESY experiments. Elemental analyses were performed in a TruSpec CHNS microanalyzer (Leco). IR spectra were recorded in a FT-IR spectrometer (Nicolet 6700 Thermo Electron). Diffraction data were collected on a Kappa APEX II Duo diffractometer (Bruker AXS) using Mo Kα radiation. The structures were solved by direct methods (SHELXS-97) and refined by full-matrix least-squares techniques on F2 (SHELXS-97, SHELXS2014).35 Preparation of Complex 5a. To a stirred suspension of 3a (2.01 g, 3.54 mmol) in 30 mL of toluene, which was cooled to 0 °C, was slowly added a yellow toluene solution of 4a (1.12 mL, 3.54 mmol) through a dropping funnel. The reaction mixture was stirred for 2 h at 0 °C before it was warmed to room temperature. After being stirred for another 5 h at room temperature, the mixture turned orange and cleared up. The solvent was evaporated under reduced pressure. Recrystallization from toluene/n-hexane gave complex 5a as an orange solid. Yield: 1.6 g (2.11 mmol, 65%). Anal. Calcd for C33H47N3O8S3Ti (757.80 g·mol−1): C 52.30, H 6.25, N 5.54, S 12.69. Found: C 52.18, H 6.32, N 5.98, S 12.36. 1H NMR (500 MHz, toluene-d8, 298 K): δ 0.43−0.95 (m, 6H, CH3), 0.96−1.53 (m, 8H, CH2), 1.58−1.64 (m, 9H, Ts−CH3), 2.96−3.64 (m, 8H, N−CH2), 3.85−3.98 (m, 2H, O− CH2), 6.30−6.79 (m, m-Ar-H), 7.05−7.17 (m, o-Ar-H), 7.26−7.45 (m, o-Ar-H), 7.00−8.00 (m, o-Ar-H). 13C NMR (126 MHz, toluene-d8, 298 K): δ 14.5 (CH3), 14.5 (CH3), 14.7 (CH3), 19.9 (CH2), 20.0 (CH2), 21.3 (CH3), 21.5 (CH3), 21.6 (CH3), 32.4 (CH2), 35.8 (CH2), 48.2 (N−CH2), 51.7 (N−CH2), 53.5 (N−CH2), 54.2 (N−CH2), 75.6 (O−CH2), 76.6 (O−CH2), 79.9 (O−CH2), 125.8 (m-CH), 127.9 (mCH), 130.0 (o-CH), 130.1 (o-CH), 135.7 (p-C), 135.9 (p-C), 143.2 (iC), 143.4 (i-C), 143.7 (i-C). CI-MS m/z: 758 [M]+, 684 [M − n BuO]+. IR (neat, cm−1, 297 K): ν = 2956 (m), 2930 (m), 2868 (m). Preparation of Complex 5c. To a stirred suspension of 3a (3.44g, 6.08 mmol) in THF (10 mL) cooled to −60 °C were slowly added 12.12 mmol of n-BuLi (4.85 mL of a 2.5 M solution in nhexane) and an additional 10 mL of toluene. After being stirred for 12 h at room temperature, the solvent was removed in vacuum and an orange solid was isolated. It was suspended in toluene (10 mL) and cooled to −60 °C. A solution of 4d (1.45 g, 6.01 mmol) in toluene (10 mL) was added dropwise. The reaction mixture was warmed to room temperature and stirred for 4 h. Removing the formed LiCl by filtration and evaporating the solvent gave complex 5c as a slightly purple solid. Yield: 3.89 g (5.33 mmol, 89%). Anal. Calcd for C31H43N6O8S3Ti (729.75 g·mol−1): C 51.02, H 5.94, N 5.76, S 13.18. Found: C 51.85, H 6.23, N 5.51, S 12.02. 1H NMR (250 MHz, benzene-d6, 298 K): δ 1.47 (d, 3JHH 6.0 Hz, 12H, CH3), 1.93 (s, 9H, CH3), 2.89 (br s, 4H, NH2), 3.72 (br s, 4H, N−CH2), 5.30 (br s, 2H, O−CH), 6.78 (d, 3JHH 8.0 Hz, 2H, m-Ar-H), 6.97 (d, 3JHH 8.1 Hz, 4H, m-Ar-H), 7.48 (d, 3JHH 8.2 Hz, 2H, o-Ar-H), 8.35 (d, 3JHH 8.3 Hz, 4H, o-Ar-H). 13C NMR (63 MHz, benzene-d6, 298 K): δ 21.2 (CH3), 21.4 (CH3), 25,7 (CH3), 47.5 (CH2), 51.1 (CH2), 82.7 (CH), 125.7 (mCH), 129.3 (m-CH), 129.6 (o-CH), 129.6 (o-CH), 135.5 (p-C), 138.9 (p-C), 142.7 (i-C), 143.3 (i-C). MS m/z (CI): 730 [M]+, 670 [M − i PrO + 1]+, 628 [M − 2iPrO + 1]+, 574 [M − Ts]+. IR (neat, cm−1, 297 K): ν = 2928 (m), 2925 (m), 2892 (m). Preparation of Complex 5d. The three-fold silylated amine 3b (1.32 g, 4.11 mmol) was dissolved in toluene (10 mL) and cooled to −55 °C with stirring. To the chilled solution were slowly added 8.22 mmol of n-BuLi (3.3 mL of a 2.5 M solution in n-hexane) and an additional 7 mL of toluene, giving a white precipitate. The suspension

Table 1. Catalytic Performance of Different Complexes in the Cross-Linking (Curing) of Silicone Rubber complex

SOTa (min)

DOCb (mm)

5a 5c 5d 5e Ti(n-BuO)4 DOTLc

5−7 6 4−5 11−14 8 10

3.0 4.0 4.1 4.0 2.7 3.6

a Skin-over time (SOT) is defined as the time required for the material to form a nontacky surface film. bDepth of cure (DOC) measured as the thickness of the cured material in a 1 cm high probe. Detailed description of the tests can be found in the Supporting Information. c DOTL: dioctyltin laureate (added for reference).

longer time to form the actual catalyst and a first cured skin layer on the silicone. Moreover, the thickness of the cured silicone layer (DOC) obtained with our complexes was shown to be higher than that obtained with the benchmark catalysts Ti(n-BuO)4 and DOTL. This is attributed to the “masking” effect of the podand ligands which stabilizes the reactive center while in the closed state, resulting in a delayed hydrolytic decomposition of the titanium catalyst and protecting it from undesired reactions with nucleophilic additives in the rubber formulation, allowing it to cross-link the polymers over a longer time, leading to thicker cured silicone layers.

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CONCLUSIONS We have prepared and characterized a series of titanium complexes bearing different substituted tridentate diamidoamine ligands in combination with alkoxide ligands. The steric demand of these alkoxide ligands was increased successively. By a purposeful combination of the ligands, we were able to influence the geometry at the metal center; we could observe a temperature-dependent dynamic equilibrium between the “open” tetrahedral and “closed” trigonal bipyramidal coordination geometries in complexes 5a and 5d. It was observed that complexes bearing the three-fold silylated diamidoamine ligand 3b were less stable than their tosylated analogue 3a. When the accompanying ligands were either n-butoxides or isopropoxides, the compounds dimerized at room temperature with loss of an alkoxide ligand and a trimethylsilyl group of the amino nitrogen, yielding the new binuclear complexes 6 and 7, both displaying a planar Ti2N2 ring, as determined by X-ray. Compounds 5a, 5c, 5d, and 5e were tested as catalysts for the curing of RTV silicone rubber in α,ω-di-dialkoxyvinylsilylpolydimethylsiloxane formulations (Scheme 1). All of them displayed good to excellent activities, in most of the cases better than that of the benchmark catalysts Ti(n-BuO)4 and DOTL.

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EXPERIMENTAL SECTION

All operations were carried out under argon using standard Schlenk techniques or in a glovebox. THF, diethyl ether, hexane, and toluene were dried over sodium/benzophenone and distilled under argon. Pentane and heptane were purified with a Grubbs-type column system Pure Solv MD-5. Diethylenetriamine (≥99%), DBU (≥99%), titanium(IV) 2-ethylhexyloxide (95%), p-toluenesulfonyl chloride (≥99%), n-butyllithium (2.5 M in n-hexane), titanium isopropoxide (97%), hexamethyldisilazane (≥99%), and diethylamine (99.5%) were purchased from Sigma-Aldrich and used as received. Chlorotrimethylsilane was purchased from Sigma-Aldrich and was distilled under 2457

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Article

Organometallics

mol−1): C 46.01, H 9.65, N 11.50. Found: C 41.67, H 10.42, N 11.16. 1 H NMR (300 MHz, THF-d8, 298 K): δ 0.13 (s, 36H, 4·TMS), 0.95 (t, 3JHH 7.4 Hz, 6H, CH3), 1.27−1.41 (m, 4H, CH2), 1.77−1.88 (m, 4H, CH2), 3.03−3.13 (m, 2H, CH2), 3.40 (ddd, 3JHH 11.9, 3JHH 6.0, 2 JHH 4.5 Hz, 2H, CH2), 3.46−3.55 (m, 2H, CH2), 3.81 (ddd, 3JHH 11.9, 3 JHH 6.1, 2JHH 4.4 Hz, 2H, CH2), 4.40−4.32 (m, 4H, O−CH2). 13C NMR (75 MHz, THF-d8, 298 K): δ 2.2 (4·TMS), 14.4 (CH3), 19.8 (CH2), 37.8 (CH2), 53.6 (CH2), 60.3 (CH2), 75.2 (O−CH2). 29Si NMR (59.6 MHz, THF-d8, 298 K): δ 3.02. MS m/z (CI): 730 [M]+, 715 [M − CH3]+, 657 [M − nBuO]+, 367 [M/2 + 2H]+. IR (neat, cm−1, 297 K): ν = 2953 (m), 2896 (m), 2844 (m). Preparation of Complex 7. Thermal decomposition at room temperature of compound 5d led to the formation of the binuclear complex 7, which was isolated as yellow crystals and recrystallized from heptane. Yield: 0.12 g (0.016 mmol, 5%). Anal. Calcd for C26H66N6O2Si4Ti2 (703.00 g·mol−1): H 9.46, N 11.96. Found: H 9.20, N 11.87. 1H NMR (300 MHz, benzene-d6, 298 K): δ 0.31 (s, 36H, 4· TMS), 1.35 (d, 2JHH 6.1 Hz, 12H, CH3), 2.79−2.88 (m, 4H, CH2), 3.33−3.43 (m, 8H, CH2), 3.70−3.77 (m, 4H, CH2), 4.64 (qi, 2H, CH). 13C NMR (75 MHz, benzene-d6, 298 K): δ 2.03 (4·TMS), 27.3 (CH3), 52.3 (N−CH2), 60.3 (N−CH2), 75.6 (O−CH2). 29Si NMR (59.6 MHz, benzene-d6, 298 K): δ 2.02. MS m/z (CI): 703 [M]+, 353 [M/2+H]+. IR (neat, cm−1, 297 K): ν = 2950 (m), 2884 (m), 2860 (m), 2834 (m). General Procedure for the Silicone Cross-Linking Experiments. The synthesized compounds were tested in the cross-linking reaction of a silicone rubber formulation consisting of α,ω-didialkoxyvinylsilyl-polydimethylsiloxane with a viscosity of ca. 80000 cST (synthesized following reported procedures, 71%),36 polydimethylsiloxane with a viscosity of about 100 cST (17%), and fumed silica (Aerosil R 104, from Evonik, 11%). Hence, 35 g of the uncured silicone mass and 1.385 mmol of the catalyst were mixed in a dual asymmetric centrifugal mixer (SpeedMixer DAC 150.1 FVZ-K) at 3000 rpm for 150 s. Test strips of the silicone mixture were made and allowed to cure under standard atmosphere. The SOT and the DOC after 24 h were measured following standardized procedures (see Supporting Information).

was stirred for 24 h and then cooled to −40 °C. A colorless solution of 4d (0.97 g, 4.11 mmol) in toluene (3 mL) was added. After being stirred at room temperature for 72 h, the toluene was removed in vacuum. The residue was dissolved in n-heptane. Removing the precipitating LiCl by filtration and evaporating the solvent gave complex 5d as a deep red liquid. Yield: 1.20 g (2.48 mmol, 60%). Anal. Calcd for C19H49N3O2Si3Ti (483.73 g·mol−1): C 47.18, H 10.21, N 8.69. Found: C 40.88, H 9.34, N 9.29. 1H NMR (300 MHz, benzened6, 298 K): δ 0.10−0.44 (m, 27H, 3·TMS), 1.20−1.39 (m, 12H, CH3), 2.43−3.98 (m, 8H, N−CH2) 2.48 (t, 2H, N−CH2), 3.39 (t, 2H, N− CH2), 4.50−4.69 (m, 1H, O−CH), 4.72−4.90 (m, 1H, O−CH). 13C NMR (75 MHz, benzene-d6, 298 K): δ 0.8 (TMS), 1.6 (2·TMS), 27.2 (CH3), 52.7 (CH2), 56.1 (CH2), 76.7 (CH).). 29Si NMR (59.6 MHz, benzene-d6, 298 K): δ 0.33, 1.93, 5.20, 15.51. MS m/z (CI): 484 [M]+, 468 [M − CH3]+, 424 [M − iPrO]+, 382 [M − OiPr + CH3]+, 320 [4b]+. IR (neat, cm−1, 297 K): ν = 2955 (m), 2896 (m), 2863 (m). Preparation of Complex 5e. To a stirred suspension of the threefold tosylated ligand 3a (3.00 g, 5.3 mmol) in toluene (25 mL) cooled to 0 °C was slowly added a solution of 2.00 g (4.44 mmol) of 4c in toluene (10 mL). The reaction mixture was warmed to room temperature, stirred for 2 h, afterward heated to 70 °C for 4 h, and stirred for another 60 h at room temperature. The solvent was evaporated in vacuum, and the residue was washed with n-heptane to remove the unreacted ligand. Complex 5f could be isolated as an orange resinous product. Yield: 1.20 g (1.38 mmol, 78%). Anal. Calcd for C41H63N3O8S3Ti (483.73 g·mol−1): C 56.6, H 7.30, N 4.83, S 11.05. Found: C 56.79, H 7.26, N 4.69, S 10.99. 1H NMR (300 MHz, benzene-d6, 298 K): δ 0.81−0.95 (m, 16H, CH3, CH2), 1.23−1.69 (m, 14H, CH2, CH), 2.38 (s, 3H, CH3), 2.42 (s, 6H, CH3), 2.95−3.26 (m, 4H, CH2), 3.53 (br s, 4H, CH2), 4.64 (br s, 4H, O−CH2), 7.30−7.34 (m, 2H, m-Ar-H), 7.34 (d, 3JHH 8.3 Hz, 4H, m-Ar-H), 7.66 (d, 3JHH 8.2 Hz, 2H, o-Ar-H), 8.02 (d, 3JHH 8.2 Hz, 4H, o-Ar-H). 13C NMR (75 MHz, benzene-d6, 298 K): δ 11.7 (CH3), 14.7 (CH3), 21.5 (CH3), 21.7 (CH2), 24.2 (CH2), 24.5 (CH2), 30.2 (CH2), 31.3 (CH2), 43.6 (CH), 47.8 (CH2), 52.2 (CH2), 126.2 (CH2), 128.6 (CH2), 129.1 (mCH2), 129.3 (m-CH2), 129.8 (o-CH2), 130.5 (o-CH2), 136.2 (p-C), 139.0 (p-C), 144.2 (i-C), 144.4 (i-C). MS m/z (CI): 870 [M]+, 565 [4a]+, 113 [2-ethyl hexane]+, 71 [pentane]+. IR (neat, cm−1, 297 K): ν = 3282 (w), 2957 (m), 2926 (m), 2858 (m). Preparation of Complex 5f. The synthesis was carried out analogously to 5d: 2.33 g (7.29 mmol) of the three-fold silylated amine 3b was deprotonated with 7.29 mmol of n-BuLi (2.92 mL of a 2.5 M solution in n-hexane). Afterward, a solution of 4.54 g (7.29 mmol) of 4e in toluene was added slowly. The reaction mixture was stirred for 6 h. After evaporation of the solvent under reduced pressure, complex 5f was obtained as a dark brown sticky solid. Yield: 1.20 g (1.29 mmol, 26%). Due to its difficult purification and inherent instability, a satisfactory elemental analysis for this complex could not be obtained. 1 H NMR (300 MHz, benzene-d6, 298 K): δ 0.32−0.33 (m, 27H, TMS), 0.92−1.00 (m, 6H, CH3), 1.00−1.08 (m, 6H, CH3), 1.32−1.45 (m, 8H, CH2), 1.45−1.57 (m, 4H, CH2), 1.57−1.72 (m, 4H, CH2), 1.77−1.85 (m, 2H, CH), 3.02−3.06 (m, 2H, N−CH2), 3.45−3.52 (m, 2H, N−CH2), 3.54−3.59 (m, 2H, N−CH2), 3.79−3.88 (m, 2H, N− CH2), 4.32−3.58 (m, 4H, O−CH2). 13C NMR (63 MHz, benzene-d6, 298 K): δ 2.3 (TMS), 11.3 (CH3), 11.4 (CH3), 14.4 (CH3), 14.5 (CH3), 23.7 (CH2), 23.9 (CH2), 29.7 (CH2), 29.8 (CH2), 30.5 (CH2), 30.7 (CH2), 42.3 (CH), 44.0 (CH2), 52.9 (CH2) 59.9 (CH2), 64.9 (CH2), 78,6 (CH2). MS m/z (EI): 624 [M]+, 536 [M − TMS − CH3 + 1]+, 521 [M − TMS − 2CH3 + 1]+, 492 [M − O-2-ethylhexyl]+. IR (neat, cm−1, 297 K): ν = 2858 (m), 2955 (m), 2927 (m). Preparation of Complex 6. In an attempt to prepare 5b, the precursor 4a (1.22 g, 3.60 mmol) was dissolved in toluene (5 mL) and cooled to −30 °C. The ligand 3b was added dropwise (1.15 g, 3.60 mmol), and the reaction mixture was stirred for 18 h at room temperature. The solvent and volatiles were removed in vacuum, and the resulting residue was dissolved in n-pentane (5 mL) and refluxed for 12 h. The solvent was removed to give a brown viscous residue from which yellow crystals were isolated. Recrystallization from nheptane gave complex 6 as yellow prismatic crystals. Yield: 0.14 g (0.19 mmol, 5%). Anal. Calcd for C28H70N6O2Si4Ti2 (730.90 g·

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00335. Further experimental details, NMR and MS spectra of all substances, and crystallographic data of complexes 6 and 7 (PDF) Accession Codes

CCDC 1548179−1548180 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Esteban Mejía: 0000-0002-4774-6884 Notes

The authors declare the following competing financial interest(s): European Patent Application 15200907.2; Inventors: E. Mejia; M. Höhne; U. Kragl; A. Gutacker; J. Klein. Pending (filed on the 17th of December, 2015). 2458

DOI: 10.1021/acs.organomet.7b00335 Organometallics 2017, 36, 2452−2459

Article

Organometallics

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ACKNOWLEDGMENTS Thanks to Dr. A. Spannenberg for the crystallographic measurements and assistance. Financial support by Henkel AG & Co KGaA is gratefully acknowledged.

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DOI: 10.1021/acs.organomet.7b00335 Organometallics 2017, 36, 2452−2459