Chiral Titanium(IV) Complexes Containing Polydentate Ligands

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

Chiral Titanium(IV) Complexes Containing Polydentate Ligands Based on α‑Pinene. Catalytic Activity in Sulfoxidation with Hydrogen Peroxide Irene Reviejo,† Vanessa Tabernero,† Marta E. G. Mosquera,† Javier Ramos,‡ Tomás Cuenca,† and Gerardo Jiménez*,†

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†

Departamento de Química Orgánica y Química Inorgánica, Instituto de Investigación en Química “Andrés M. del Río” (IQAR) Universidad de Alcalá, Campus Universitario, 28805 Alcalá de Henares, Spain ‡ Macromolecular Physics Department, Instituto de Estructura de la Materia, Consejo Superior de Investigaciones Científicas, Serrano 113bis, 28006, Madrid, Spain S Supporting Information *

ABSTRACT: The reaction of TiCl4−n(OiPr)n (n = 0, 2, 4) with various terpenoid preligands based on α-pinene, C7H6Me3(OH)(NCH2CH2G) (G = NH2, I; NHMe, II; OH, III), stereoselectively affords a series of new chiral titanium(IV) complexes. Such complexes are either octahedral, [TiCl2X(OC7H6Me3NCH2CH2G-κ3N,N,O)] (X = Cl, G = NH2, 1; X = OiPr, G = NH2, 2; G = NHMe, 3), or trigonal bipyramidal, [Ti(OiPr)3(OC7H6Me3NCH2 CH2G-κ 2N,O)] (G = NH2 , 4; NHMe, 5) and [TiX(O iPr)(OC7H6Me3NCH2CH2O-κ3N,O,O*)] (X = Cl, 6; OiPr, 8), depending on the acidity of titanium, the reaction conditions and the nature of the pendant ending group of the terpenoid ligand. Density functional theory (DFT) calculations have been carried out to assess the stability of the multiple possible diastereoisomers allowing us to propose structural suggestions. The new titanium complexes efficiently catalyze the selective oxidation of various types of sulfides to either sulfoxides or sulfones using aqueous hydrogen peroxide, under mild conditions. All compounds have been characterized by multinuclear NMR spectroscopy and the molecular structure for some of them has been determined by X-ray diffraction.

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INTRODUCTION Sulfide oxidation is currently of much interest, both in industry and the academic world, because the final products, sulfoxides and sulfones, are appealing intermediates for the preparation of a wide variety of commodities as well as compounds with biological and pharmaceutical activity.1 In addition, this process is also important in the desulfurization of fuels.2 One of the challenges in the oxidation field is to avoid polluting agents that can cause an environmental impact. In this regard, the best strategy is the design of efficient catalysts that can perform using clean oxidants.3 Among the oxidants considered environmentally benign, hydrogen peroxide occupies a predominant position,3b as it is inexpensive and easy to use, provides high atom efficiency (47% active oxygen), and has water as its only byproduct.4 Among the different catalytic systems available, complexes bearing Schiff base ligands have shown remarkable activity as catalysts for the oxidation of a variety of different substrates;5 features such as their easy synthesis, high thermal and hydrolytic stability, and high oxygen affinity make them ideal for oxidation processes.5,6 Complexes of group 4 metals in high oxidation states, particularly those of Ti(IV), have proved excellent and versatile catalysts in a wide range of processes.4h,7 Furthermore, titanium one of the most abundant metals on Earth, is © XXXX American Chemical Society

cheap, nontoxic, and biocompatible, which makes it an attractive option for organic synthesis.8 There are an extensive number of catalytic systems based on Ti(IV) complexes bearing polydentate Schiff bases, mono- or dianionic, such as [ONO], [ONN], or [ONS] systems.9 Other highly active systems bear tetradentate ligands such as trialkanolamine10 and triphenolamine11 ligands, or the extensively used salen ligands.12 In recent years, the development of new chiral Schiff base ligands is expanding, due to their utility in asymmetric synthesis. Another great challenge in today’s society is the development of more sustainable chemical methods that minimize environmental impacts. For this reason, green chemistry is becoming more relevant in all fields.13 One of the principles included in this philosophy is the use of renewable feedstocks. Biomass is becoming a relevant feedstock for the manufacture of high added value products.14 The great advantage of biomass is its renewable and biodegradable character and the fact that it contains functional groups that allow a straightforward derivatization, rendering new and Special Issue: In Honor of the Career of Ernesto Carmona Received: March 20, 2018

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DOI: 10.1021/acs.organomet.8b00163 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Scheme 1. Synthesis of Preligands I−III

valuable products.15 Among components present in biomass, monoterpenes have been drawing increasing attention, in particular, α-pinene, limonene, and Δ3-carene, due to their accessibility, availability as pure enantiomers in Nature and easy and stereoselective functionalization.13 As part of one of our ongoing research lines, focused on the use of Schiff bases based on monoterpenes to prepare new chiral complexes, we describe herein the stereoselective synthesis of a new family of Ti(IV) complexes bearing a chiral polydentate ligand based on α-pinene. These complexes are characterized by multinuclear NMR spectroscopy and additionally, DFT calculations are performed to give support to some structural features proposed for these compounds. Finally, their catalytic activity on sulfoxidation of various types of sulfides, to either sulfoxides or sulfones, using aqueous hydrogen peroxide is studied.

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RESULTS AND DISCUSSION The Schiff base type preligands used in this work have been synthesized from α-pinene following known procedures.16 However, slight modifications have been introduced in these protocols to improve both the efficiency and the yield (Scheme 1). Spectroscopic characterization has also been completed with 15N NMR chemical shifts (see the Experimental Section). These reactions are stereoselective, rendering the enantiomerically pure preligands C7H6Me3(OH)(NCH2CH2G) (G = NH2 (I); NHMe (II); OH (III)). Preligand II, which was synthesized by addition of methylethylenediamine to a solution of ketol in toluene at 90 °C and in the presence of BF3·OEt2, is reported here for the first time. The structure of preligand C7H6Me3(OH)(NCH2CH2OH) (III) has been confirmed by means of an X-ray diffraction study on a single crystal of the corresponding hydrochloride derivative. The most relevant aspect of this structure is the anti-planar disposition of the hydroxyl group of the pinene moiety and the CMe2 fragment (Figure 1). The protonation of the nitrogen atom was confirmed by the value of the C3−N1−C11, slightly larger (126.4(4)°) than the one expected for an imine nitrogen (sp2) with a lone pair of electrons, indicating that this is involved in the formation of a N−H dative bond. The presence of the chloride anions generates a hydrogen bonding network that arranges the molecules in parallel chains, of two molecules wide, along the a axis. The chloride anion interacts with two OH groups of one molecule and the NH group of the neighboring one, generating and unsual synthon as displayed in Figure 2. The strongest interaction is stablished with O(1). Synthesis and Characterization of Titanium Complexes. The treatment of preligand I with TiCl4 in toluene at −78 °C and in absence of an external base affords the new derivative [TiCl3(OC7H6Me3NCH2CH2NH2-κ3N,N*,O)] (1) as a pale-yellow solid. The formation of this complex is the

Figure 1. ORTEP drawing of III·HCl (hydrogen atoms are omitted for clarity, and thermal ellipsoids are set at 30% probability). Selected bond distances (Å) and angles (deg): N1−C7 1.281(4), N1−C13 1.465(4), N1−C7−C11 121.8(3), N1−C7−C8 118.1(3).

Figure 2. H-bond interactions N(1)···Cl 3.142(4) Å, N(1)−H···Cl 2.219(4) Å, N(1)−H···Cl 152.7(2)°; O(1)···Cl 3.069(3) Å, O(1)− H···Cl 2.213(4) Å, O(1)−H···Cl 2.288(3)°; O(2)···Cl 3.075(3) Å, O(2)−H···Cl 2.288(3) Å, O(2)−H···Cl 167.9(3)°.

result of the protonolysis of a Ti−Cl bond, through the selective deprotonation of the hydroxy group in the preligand and the elimination of HCl (Scheme 2). The new monoanionic terpenoid ligand is coordinated in a tridentate fashion, bound by the alkoxo group and both nitrogen atoms (as deduced from the spectroscopic analysis), affording a mononuclear complex that exhibits a distorted octahedral structure. For such a coordination environment Scheme 2. Synthesis of Complex 1

B


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Organometallics

one chloride completing the equatorial plane, whereas the other two occupy the axial positions and are noticeably deviated from linearity (Cl2−Ti1−Cl1, 162°). Bond lengths and angles values are in the usual ranges observed in the literature.17 Ti−N(2) length (2.222(7)) lies in the typical interval for simple bonds, which further proves the nondeprotonation of the amino moiety. In the asymmetric unit there are two independent molecules that are connected by two N−H···Cl H-bonds between Cl(6) and N(40) of one molecule and Cl(5) and N(30), respectively, of the other (Figure 5).

around titanium and depending on the ligand’s coordination mode, two stereoisomers can be contemplated for complex 1, the fac and the mer diastereoisomers (Figure 3). However, only one of them is generated, according to the spectroscopic analysis, which indicates that the reaction proceeds in a stereoselective manner.

Figure 3. Possible diastereoisomers of 1.

The spectroscopic behavior of 1 indicates a C1 symmetry, in agreement with the proposed structure and the chirality of the terpenoid ligand. The 1H NMR spectrum shows four multiplets for the diastereotopic protons of the methylene groups of the chain linking both nitrogen atoms, as a result of the ligand’s coordination to the metal center. Resonances corresponding to the amino protons are correctly assigned by TOCSY-1D and HSQC 1H−15N experiments, confirming the nondeprotonation of this group. The chemical shifts for the amine nitrogen, in the 15N NMR spectrum, and for the imine carbon, in the 13C NMR spectrum, provide a powerful tool to establish the coordination of the amine and imine groups, respectively. Thus, the resonance of the amino nitrogen at 51 ppm and the resonance of the imino carbon at 197.0 ppm, both shifted downfield from those observed for the free ligand, confirming the binding of both nitrogen atoms to the metal center. Although, the stereochemistry of complex 1 (fac or mer) cannot be stablished by NMR spectroscopy, it is unambiguously assigned by X-ray diffraction analysis. The solid-state structure of 1 is illustrated in Figure 4. This structure confirms the tridentate coordination of the ligand in a mer disposition, rendering two condensed five-membered metallacycles. The titanium atom shows a pseudo-octahedral environment with

Figure 5. H-bond interactions N(30)···Cl(6) 3.374(6) Å, H(30B)··· Cl(6) 2.543(7) Å, N(30)−H···Cl(6) 151.9(3)°; N(40)···Cl(5) 3.334(7) Å, H(40A)···Cl(5) 2.529(7) Å, N(40)−H···Cl(5) 140.2(3)°.

In addition, DFT geometrical optimizations performed on both possible stereoisomers of complex 1, showed that the mer is more stable than the fac by 18.7 kcal/mol (see Figure S1 and Table S1 for a comparison of geometrical parameters), in good agreement with the experimental results discussed above. The reaction of preligands I and II with TiCl2(OiPr)2 in dichloromethane and in the absence of an external base (in order to drive the process via isopropanol elimination) gives t wo new mono al koxo complexes, [TiCl 2 (O i Pr)(OC7H6Me3NCH2CH2NH2-κ3N,N*,O)] (2) and [TiCl2(OiPr)(OC7H6Me3NCH2CH2NHMe-κ3N,N*,O)] (3), respectively (Scheme 3). These complexes are formed as a Scheme 3. Synthesis of Complexes 2 and 3

consequence of the selective protonolysis of one Ti−OiPr bond with elimination of an equivalent of isopropanol and the coordination of both nitrogen atoms to the titanium center, as inferred from its spectroscopic behavior. Again, the process selectively proceeds via hydroxyl deprotonation, while the amine group remains unchanged rendering a monoanionic tridentate ligand. Both compounds are isolated as pale-yellow solids in high yields and were fully characterized by elemental analysis and 1 H, 13C, and 15N NMR spectroscopy. Complexes 2 and 3 show a spectroscopic behavior similar to that observed for complex 1, confirming their C1 symmetry and the coordination of both nitrogen atoms. The main difference in the 1H NMR spectra of 2 and 3 is the set of resonances assigned to the remaining isopropoxide group, two doublets centered at 1.35 ppm, integrating for 3 protons each, and a septuplet at 4.85 ppm, integrating for one proton. The 13C and 15N NMR spectra features are in line with the coordination of the amine (δN ∼

Figure 4. ORTEP drawing of complex 1 (hydrogen atoms are omitted for clarity, and thermal ellipsoids are set at 30% probability). Selected bond distances (Å) and angles (deg): Ti(1)−O(1) 1.772(7), Ti(1)− N(10) 2.158(8), Ti(1)−N(30) 2.213(9), Cl(3)−Ti(1)−Cl(1) 161.91(16), O(1)−Ti(1)−N(10) 75.7(3), O(1)−Ti(1)−N(30) 149.4(3), Ti2−O2, 1.783(7), Ti2−N20 2.138(7), Ti2−N40 2.224(8), Cl2−Ti2−Cl4, 163.14(11), O2−Ti2−N20 76.1(3), O2− Ti2−N40 150.3(3). C


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Organometallics 45) and imine (δC ∼ 194, δN ∼ 290) groups. In addition, as observed for 1, only one of the multiply plausible stereoisomers is rendered, as deduced from the NMR analysis; therefore, DFT calculations were carried out to assess the relative stability of the possible diastereoisomers. Again, DFT calculations for complex 2 corroborate that the mer stereoisomer is more favorable than the fac by 24.2 kcal/ mol (Figure S2). However, even accepting a mer disposition of the tridentate ligand, three different diastereoisomers can be proposed depending on the relative location of the isopropoxide group: Diastereoisomer TiOiPrCl2−I shows the OiPr group in the equatorial plane, whereas diastereoisomers TiOiPrCl2−II and TiOiPrCl2−III feature the OiPr ligand in each of the two nonequivalent axial positions (Figure 6).

solvent effects, 4.6 and 3.8 in CH2Cl2 (synthesis solvent) and 5.4 and 4.6 kcal/mol in CHCl3 (NMR experiments solvent). NMR calculations were performed by reoptimization in CHCl3 of all conformers in Table 1. The different conformers for each diastereoisomer were taken into account because they are very close in energy (within 1 kcal/mol); thus, they could not be negligible for the NMR chemical shifts calculation, as explained in the “Computational Methods” section. Comparisons of computed and experimental 1H and 13C chemical shifts for each diastereoisomer are illustrated in Figure 7. The better linear fit and the lower root-mean squared deviation (RMSD) for both 1H and 13C chemical shifts are found for diastereoisomer TiOiPrCl2−I. In the former case, the protons of the amine group were not taken into account, due to the experimental uncertainty in its determination. Taking into consideration both energy calculations and calculated NMR chemical shifts, it can be concluded that diastereoisomer TiOiPrCl2−I is the most stable and therefore most likely to be formed in the reaction. In view of these results, we decided to explore the reactivity of preligands I and II with Ti(OiPr)4, trying to force the deprotonation of the amine group. However, such reactions do not go beyond the hydroxyl deprotonation either, specifically forming the corresponding pentacoordinated trialkoxo complexes [Ti(OiPr)3(OC7H6Me3NCH2CH2NH2-κ2N,O)] (4) and [Ti(OiPr)3(OC7H6Me3NCH2CH2NHMe-κ2N,O)] (5). Such complexes are formed by protonolysis of a single Ti− OiPr bond and the coordination of the imino nitrogen, while the amine group remains uncoordinated, as inferred from 15N NMR analysis (Scheme 4). Complexes 4 and 5 show a distorted trigonal bipyramidal geometry around the metal center where the terpenoid fragment acts as a monoanionic bidentate ligand. Both compounds are isolated as pale-yellow oils in high yields and characterized by spectroscopic methods. The spectroscopic behavior of these complexes is apparently in disagreement with the expected C1 symmetry. The 1H NMR spectra of both complexes show a single set of resonances for the three remaining isopropoxide groups, two doublets, and a septuplet centered at 1.17, 1.18, and 4.65 ppm, respectively. To justify this finding, requires the assumption that these pentacoordinated complexes show a fluxional behavior in solution, either Berry pseudorotation or via an imine discoordination/ coordination process, that allows the isopropoxide groups to interchange their positions, rendering them equivalent on the NMR scale time. The 13C and 15N NMR data confirm the coordination of the imine group (δC ∼ 190, δN ∼ 294) and the noncoordination of the amine end (δN ∼ 19). Additionally, the nondeprotonation of the amine group is verified by the resonance attributed to the methyl in the NHMe group in complex 5, which appears as a doublet in the 1H NMR spectrum due to its coupling to the proton. Additionally, DFT optimizations on complexes 2 and 4 have been performed for both the amine group coordinated (octahedral geometry) and uncoordinated (trigonal bipyramidal geometry). The octahedral geometry (tridentate mode) is enthalpically more stable that the trigonal bipyramidal (bidentate mode) in both cases: For 2, it is more stable by 19.2 kcal/mol, and for 4, it is only more stable by 1.6 kcal/mol (see Figure S5).18 In light of these results, we decided to calculate the entropic term to determine its repercussion on the free energy difference on each case. The entropic

Figure 6. Diastereoisomers TiOi PrCl2−I, TiOiPrCl2−II, and TiOiPrCl2−III of 2 and 3.

In order to stablish the relative stability of these diastereoisomers, DFT calculations were carried out at the M06-2X/6-311++G** level, using the crystal structure of 1 as a model to build the structures of 2 and 3, through the replacement of one Cl atom by an −OiPr group (see the “Computational Methods” section). A relaxed potential energy surface (PES) scan was performed for each diastereoisomer in order to identify the possible local minimum structures, by rotating the OiPr group (the scan coordinate was defined as the Ti−O−C(iPr)−H dihedral angle). The results are shown in Figure S3. The different minima located for each diastereoisomer were fully optimized (Figure S4), and their relative energies were collected in Tables 1 and S4. In all cases, the metal center Table 1. Relative Energies (kcal/mol) and Dihedral Angles (deg) of the Fully Optimized Minima for Each Diastereoisomer in Dichloromethane (CH2Cl2)a diastereoisomer TiOiPrCl2−I TiOiPrCl2−II

TiOiPrCl2−III

conformer

φCH2Cl2

ΔECH2Cl2

RMSD

a b a b c a b c

−159.0 49.6 −73.6 −5.5 21.0 10.0 172.0 −23.5

0.3 0.0 5.0 4.6 4.8 3.8 4.2 4.7

0.308 0.090 0.058 0.069 0.058 0.155 0.056 0.078

a

The root mean squared deviation (RMSD, Å) represents the geometrical differences resulting from the all-heavy-atom superimposition of the gas-phase and solvated structures.

retained an octahedral-like coordination. The distance Ti−O1 is slightly shorter for diastereoisomer TiOiPrCl2−I than those found for diastereoisomers TiOiPrCl2−II and TiOiPrCl2−III, by 0.05 and 0.08 Å, respectively. Diastereoisomer TiOiPrCl2−I shows lower energy than those of diastereoisomers Ti− OiPrCl2−II and TiOiPrCl2−III both in the gas phase, by 8.1 and 7.0 kcal/mol, respectively, and by taking into account D


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Organometallics

Figure 7. Correlation of calculated versus experimental 1H (a) and 13C (b) chemical shifts for compound 2.

ligand is capable of adopting different coordination modes, depending on the acidity of the titanium atom. This last point justifies the coordination of the amino group in complexes 1−3 with no coordination in complexes 4 and 5. In the former, the presence of chloro ligands, good electron-withdrawing groups, yields a highly electron deficient titanium center and, therefore, the coordination of the NHR group. However, in the latter the presence of three isopropoxide groups, with a significant πdonating ability, makes the titanium atom electronically less deficient and sterically more hindered, preventing the coordination of the amine group. Table 2 shows a comparison of selected spectroscopic data that serve as reference arguments to support these conclusions.

Scheme 4. Synthesis of Complexes 4 and 5

differences between the octahedral and the trigonal bipyramidal dispositions for theses complexes are 5.6 and 11.0 kcal/ mol (TΔS terms equal to 1.7 and 3.3 kcal/mol at 298.15 K) for 2 and 4, respectively, which is not high enough to invert the stability order of each disposition for 2 but is high enough for 4. Thus, whereas the free energy difference (ΔGocta→tbp) for 2 is 17.6 kcal/mol, confirming that the octahedral arrangement is more stable, for 4, the ΔGocta→tbp is −1.7 kcal/mol, indicating that the more stable disposition for it is the trigonal bipyramidal geometry. Therefore, DFT calculations indicate the unbound state is more likely for complex 4, in good agreement with the experimental results discussed above. Bringing together all of the experimental evidence, several conclusions can be drawn: (i) In no case does the amine group react to give the corresponding amide derivative. (ii) The terpenoid ligand has proved to have the ability to bind to the metal center in a stereoselective fashion. (iii) The mer coordination is the most favorable one. (iv) The terpenoid

Table 2. Relevant Chemical Shifts (ppm) to Establish the Coordination Mode of the Terpenoid Ligand in Complexes 1−5

I 1 2 4 II 3 5 E

CN (13C)

CN (15N)

NHR (15N)

CMeO (13C)

177 197 192 190 177 194 190

317

18 51 43 18 22 50 20

76 107 100 89 76 99 89

292 294 318 290 294


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Organometallics

κ3N,O,O*})}3(μ3-O)]Cl (7), with an outer sphere Cl− anion. This species is the result of the hydrolysis of complex 6. As can be seen in Figure 9, compound 7 is a trinuclear complex comprised of three “Ti(OiPr)(C12H19NO2)” units

Facing the impossibility of deprotonating the amine group in compounds I and II, we decided to examine similar reactions using terpenoid compound III with a terminal OH group, noticeably more acidic than those with the amine functionalities, NH2 or NHMe. To conduct the comparative study between the OH and the NHR reactivity, we have analyzed the reactivity of III with two starting titanium compounds, TiCl2(OiPr)2 and Ti(OiPr)4, under different reaction conditions. Reaction of TiCl2(OiPr)2 with an equivalent of III in the absence of an external base leads to a mixture of products that could not be elucidated due to the complexity exhibited by their NMR spectra. However, when the reaction is performed in the presence of an equivalent of NEt3, it specifically proceeds with the protonolysis of one Ti−Cl and one Ti−OiPr bond with concomitant formation of the new pentacoordinated derivative [TiCl(O i Pr)(OC 7 H 6 Me 3 NCH 2 CH 2 Oκ3N,O,O*)] (6), as a result of the deprotonation of both hydroxyl functionalities (Scheme 5). Scheme 5. Synthesis of Complex 6

Figure 9. ORTEP drawing of complex 7 (hydrogen atoms are omitted for clarity, and thermal ellipsoids are set at 30% probability). Selected bond distances (Å) and angles (deg): O3−Ti3 1.781(4), O7−Ti3 1.849(4), O8−Ti3 2.026(4), O6−Ti3 2.036(4), O10−Ti3 1.954(3), N6−Ti3 2.188(5), O3−Ti3−O10 152.18(2), O10−Ti3−O6 73.32(1), Ti1−O10−Ti3 109.41(2).

Complex 6 features a distorted trigonal bipyramidal structure where the terpenoid fragment acts as a dianionic tridentate ligand. On the basis of such a molecular geometry, two different diastereoisomers can be considered depending on the relative orientation of the chloride and isopropoxide groups in relation to the terpenoid ligand (Figure 8, isomers A and B).

related by a C3 axis. These fragments are assembled by the ending oxygen atom of the ethylenic chain and by a μ3-oxo bridging ligand. Each terpenoid ligand is doubly deprotonated and coordinated in a dianionic tridentate fashion, in a mer disposition. The central core, Ti3O4, consists of three titanium atoms with three alternating terpenic oxygens, forming a sixmembered cycle in a chair conformation where the three titanium atoms are capped by a μ3-oxo ligand (Figure 10, left). Each metal center exhibits a distorted octahedral geometry, where the ligand is coordinated in a tridentate manner, adopting a mer disposition (Figure 10, right). Again H-bond interactions involving the chloride anion are observed, which in this case are weak C−H···Cl contacts. As in III·HCl, the chloride anion displays three H-bond interactions, in this case with three different molecules, which lead to an arrangement in layers along the crystal packing (Figure 11). When the reaction of TiCl2(OiPr)2 with III is performed by doubling the molar amount of III and of NEt3, in a 1(Ti)/ 2(III)/2(NEt3) molar ratio, it proceeds with the protonolysis of all Ti−Cl and Ti−OiPr bonds present in the starting material, giving a new mononuclear complex stabilized by the coordination of two dianionic tridentate terpenoid ligands [Ti(OC7H6Me3NCH2CH2O-κ3N,O,O*)2] (8) (Scheme 6). The reaction is stereoselective with only one of the possible diastereoisomers formed. On the basis of the solid structure of 7, we also suggest a mer coordination for the terpenoid ligands in compound 8, which reduces the number of possible diastereoisomers to two that are different in the relative orientation of both ligands (Figure 12). However, these cannot be distinguished from the available structural data. The 1H NMR spectrum of 8 shows a single set of resonances for both terpenoid ligands in agreement with a C2 symmetry

Figure 8. Possible isomers of complex 6.

Nevertheless, once again, the formation of 6 proceeds in a stereoselective manner, rendering only one diastereoisomer, as inferred from the NMR analysis. Complex 6 is isolated as paleyellow solid and was fully characterized by elemental analysis and 1H, 13C, and 15N NMR spectroscopy. The NMR spectroscopic data for 6 accords with a C1 symmetry, consistent with the presence of a chiral terpenoid ligand. The 1H NMR spectrum of 6 shows two sets of resonances for the terpenoid and the alkoxo ligands respectively, in a 1:1 integration ratio. Chemical shifts for both carbon atoms bonded to oxygen atoms, in the 13C NMR spectrum, corroborate the deprotonation of both hydroxyl groups, while the 13C and 15N NMR resonances for the imino moiety (δC = 197.2 y δN = 296) prove its coordination to the titanium atom. When a toluene solution of 6 was maintained under not strictly anhydrous conditions at −20 °C for several days, colorless crystals suitable for X-ray diffraction precipitated out. Crystallographic analysis shows these belong to a new cationic trinuclear complex [{Ti(OiPr)(μ2-O{OC7H6Me3NCH2CH2OF


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Organometallics

Figure 10. Core (left) and arrangement around titanium centers (right) in complex 7.

of its carbon (196.1 ppm) and its nitrogen (295 ppm), in the C and 15N NMR spectra, respectively. We were also interested in exploring the reaction of 1 equiv of III with Ti(OiPr)4 in dichloromethane. Under these conditions, the reaction leads to the formation of a new mononuclear complex [Ti(OiPr)2(OC7H6Me3NCH2CH2Oκ3N,O,O*)] (9), via deprotonation of both OH groups and elimination of 2 equiv of isopropanol and concomitant formation of a dianionic tridentate terpenoid ligand (Scheme 7). 13

Scheme 7. Synthesis of Complex 9

Figure 11. H-bond interactions C(28)···Cl 3.506(2) Å, C(28)−H··· Cl 2.658(2) Å, C(28)−H···Cl 143.8(1)°; C(11)···Cl 3.857(2) Å, C(11)−H···Cl 2.939(3) Å, C(11)−H···Cl 154.60(3)°; C(35)···Cl 3.770(3) Å, C(35)−H···Cl 2.816(3) Å, C(35)−H···Cl 162.03(4)°

Spectroscopic analysis of 9 confirms a C1 symmetry, in agreement with the chiral nature of the terpenoide ligand. 13C and 15N NMR spectra confirm the double deprotonation of the ligand, and the coordination of the imino group to the titanium atom (δC = 193.6, δN = 296). Unlike complexes 4 and 5, in 9 both isopropoxide ligands are inequivalent, which can be attributed to the different coordination mode of the terpenoid ligand in these complexes. In compound 9, the terpenoid ligand shows a dianionic tridentate coordination, hence it exhibits an additional covalent bond with the metal center, preventing any fluxional behavior in solution. Catalytic Activity. Recently, we disclosed and demonstrated the applicability of H2O2 as oxidant in different oxidation reactions using titanium cyclopentadienyl-silsesquioxane derivatives.19 Encouraged by these exciting results, we decided to investigate the catalytic activity of complexes 1− 6, 8, and 9 in sulfoxidation reactions using H2O2 as oxidant (Scheme 8).

Scheme 6. Synthesis of Complex 8

Scheme 8. General Sulfoxidation Reaction

Figure 12. Possible mer diastereisomers of complex 8.

for this complex. The coordination of the ending terpenoid oxygen atom downfield shifted C−O carbon resonance (95.0 ppm), compared to that found for the free ligand (76.4 ppm). Like in the previous cases, the coordination of the imino nitrogen is clearly established on the basis of the chemical shift G


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Organometallics We first explored the sulfoxidation of thioanisole (SMePh) as a model substrate to assess the catalytic potential of these complexes. Such oxidations were carried out either in NMR tubes, in deuterated solvent, or in sealed vials, regularly taking aliquots. The evolution of the process was monitored by 1H NMR spectroscopy, using mesitylene as internal standard. Catalytic studies were performed in methanol, at room temperature, using a 0.5 mol % loading of catalyst and 1 equiv of H2O2 (30% in water). The results are collected in Table 3.

Table 4. Selective Sulfoxidation of Thioanisole to Methylphenylsulfone Catalyzed by Complexes 1−6, 8, and 9a

Table 3. Selective Sulfoxidation of Thioanisole to Methylphenylsulfoxide Catalyzed by Complexes 1−6, 8, and 9a

entry

catalyst

% conv.b

% SO2 yieldb

1 2 3 4 5 6 7 8

1 2 3 4 5 6 8 9

>99 >99 >99 >99 >99 >99 >99 >99

90 91 69 85 98 78 75 81

a

entry

catalyst

t (min)

% conv.b

% SO yieldb

1 2 3 4 5 6 7 8

1 2 3 4 5 6 8 9

5 5 5 40 60 30 60 45

>99 >99 >99 86 >99 97 91 93

99 95 96 78 75 93 85 88

Reaction conditions: 0.52 mmol of substrate, 3 equiv of H2O2 (30% aqueous solution), and 0.5 mol % catalyst in MeOH (1 mL) 24 h at 60 °C. bYield determined by1H NMR spectroscopy analysis, using mesitylene as internal standard.

Table 5. Selective Oxidation of Sulfides to Sulfoxides Catalyzed by 2a

a

Reaction conditions: 0.52 mmol of substrate, 1 equiv of H2O2 (30% aqueous solution), and 0.5 mol % catalyst in MeOH (1 mL) at room temperature. bYield determined by1H NMR spectroscopy analysis, using mesitylene as internal standard.

The most effective catalysts are octahedral complexes 1−3, containing a monoanionic tridentate terpenoid ligand. These oxidize thioanisole to the sulfoxide in a quantitative and chemoselective manner in only 5 min (Table 3, entries 1−3). Among the pentacoordinated complexes, the best results are obtained with 6 (entry 6). However, it requires a longer reaction time and the selectivity toward the sulfoxide is slightly lower. On the basis of these results, we decided to pursue the complete oxidation of thioanisole to the sulfone by increasing the substrate:oxidant molar ratio. Thus, when the oxidation process was performed at 60 °C in methanol using 0.5 mol % catalyst loading, 3 equiv of H2O2 (30% in water), quantitative conversions were achieved in all cases (Table 4). Regardless of the complex used, the oxidation of thioanisole to methylphenylsulfone is virtually quantitative within a period of 24 h. On the basis of the above-mentioned results, we selected 2 to further explore the scope of these complexes as sulfoxidation catalysts, investigating the oxidation of a wide variety of sulfides with aqueous H2O2 under the optimized reaction conditions found for thioanisole (in methanol, 0.5 mol % 2 and 1 equiv of H2O2, 5 min at room temperature). Results are summarized in Table 5. We were pleased to find that all sulfides were quantitatively and chemoselectively oxidized to the corresponding sulfoxide in a very short period of time (5 min). In view of these results, we decided to pursue the complete oxidation of these sulfides to sulfones by increasing the molar amount of the oxidant. Thus, when the oxidation of these substrates was performed in

a

Reaction conditions: 0.52 mmol substrate, 1 equiv of H2O2 (30% aqueous solution) and 0.5 mol % of catalyst in MeOH (1 mL) 5 min at room temperature. bYield determined by 1H NMR spectroscopy analysis, using mesitylene as internal standard

methanol using 0.5 mol % 2 loading and 3 equiv of H2O2 (30% in water) at 60 °C, selective oxidation to sulfone was obtained (Table 6). Complex 2 has proved to be an active and selective catalyst for the sulfoxidation of an extensive variety of sulfides both toward the sulfoxide and the sulfone. Conversions are quantitative and selectivity considerably high for both products. Tolerance to several functional groups, such as OH, OMe, CC, or COOEt, was observed, even when the moieties are susceptible to oxidation as well. Although a little excess of oxidant was required to achieve high sulfone yields, this did not affect the substituents in the substrates tested.

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CONCLUSIONS In summary, a series of new titanium(IV) complexes, bearing a chiral polydentate ligand based on α-pinene, have been H


Article

Organometallics

pressure to yield I as an off-white solid (2.03 g, mmol, 81%). 1H NMR (400 MHz, CDCl3): δ 0.81, 1.28, 1.43 (s, 3 × 3H, 3Me), 1.51, 2.30 (m, 2 × 1H, CH2),1.88 (bs, 2H, NH2), 2.00 (m, 1H, CH), 2.03 (m, 1H, CH), 2.47 (m, 2H, CH2), 2.96 (m, 2H, CH2NH2), 3.27 (m, 2H, CH2N). 13C{1H} NMR (100.6 MHz, CDCl3): δ 22.9, 27.3, 28.3 (3 Me), 28.2 (CH2), 33.7 (CH2), 38.3 (CH), 38.5 (CMe2), 42.6 (CH2N), 50.4 (CH), 53.3 (CH2N), 75.7 (CMeO), 176.9 (CN). 15 N NMR (40.5 MHz, CDCl3): δ 18(NH2CH2), 317 (CN). Synthesis of C7H6Me3(OH)(NCH2CH2NHMe) (II). Following a procedure similar to that described for I (90 °C, 24 h) using Nmethylethylenediamine (in 1(ketol)/2(amine) molar ratio), II was obtained as a pale-yellow oil, in 95% yield. 1H NMR (400 MHz, CDCl3): δ 0.81, 1.28, 1.42 (s, 3 × 3H, 3Me), 1.49, 2.29 (m, 2 × 1H, CH2), 1.99 (m, 1H, CH), 2.01 (m, 1H, CH), 2.45 (s, 3H, NHMe), 2.47 (bs, 1H, NHMe), 2.47 (m, 2H, CH2), 2.83 (m, 2H, CH2NH2), 3.32 (m, 2H, CH2N). 13C{1H} NMR (100.6 MHz, CDCl3): δ 22.8, 27.4, 28.2 (3 Me), 28.4 (CH2), 33.6 (CH2), 36.5 (NHMe), 36.5 (CH), 38.5 (CMe2), 50.3 (CH2N), 50.3 (CH), 52.3 (CH2N), 76.4 (CMeO), 176.7 (CN). 15N NMR (40.5 MHz, CDCl3): δ 22(NH2CH2), 318 (CN). Anal. Calcd. for C13H24N2O (224.35 g/mol): C, 69.60; H, 10.78; N, 12.49. Found: C, 69.89; H, 11.10; N, 12.22. Synthesis of C7H6Me3(OH)(NCH2CH2OH) (III). Following a procedure similar to that described for I (100 °C, 24 h) using ethanolamine (in a 1(ketol)/10(amine) molar ratio), III was obtained as a dark brown oil, in 89%. 1H NMR (400 MHz, CDCl3): δ 0.82, 1.30, 1.43 (s, 3 × 3H, 3Me), 1.53, 2.32 (m, 2 × 1H, CH2), 2.02 (m, 1H, CH), 2.04 (m, 1H, CH), 2.49 (m, 2H, CH2), 3.34 (m, 2H, CH2N = ), 3.84 (m, 2H, CH2OH). 13C{1H} NMR (100.6 MHz, CDCl3): δ 22.9, 27.3, 28.1 (3 Me), 28.3 (CH2), 34.2 (CH2), 38.3 (CH), 38.5 (CMe2), 50.4 (CH), 52.6 (CH2N), 62.2 (CH2OH), 76.4 (CMeO), 178.3 (CN). 15N NMR (40.5 MHz, CDCl3): δ 310 (CN). Synthesis of [TiCl3{(OC7H6Me3NCH2CH2NH2)-κ3N,N*,O}] (1). A solution of TiCl4 (0.10 mL, 0.95 mmol) in toluene (15 mL) was added to a solution of C7H6Me3(OH)(NCH2CH2NH2) (I) (0.20 g, 0.95 mmol) in toluene (20 mL) at −78 °C. The reaction mixture was allowed to reach room temperature and then stirred for 1 h; a paleyellow suspension developed. The solvent was then removed under vacuum to yield 1 as a pale-yellow solid (0.28 g, 81%). 1H NMR (400 MHz, CDCl3): δ 1.06, 1.35, 2.00 (s, 3 × 3H, 3Me), 2.09 (m, 1H, CH), 2.10, 2.42 (m, 2 × 1H, CH2), 2.21 (m, 1H, CH), 2.79 (m, 2H, CH2), 3.59, 3.78 (m, 2 × 1H, CH2NH2), 3.65, 4.05 (m, 2 × 1H, CH2N), 4.05, 4.39 (m, 2 × 1H, NH2). 13C{1H} NMR (100.6 MHz, CDCl3): δ 22.3, 23.5, 27.3 (3 Me), 27.8 (CH2), 33.0 (CH2), 41.3 (CH), 42.1 (CMe2), 45.2 (CH2N), 51.4 (CH2N), 54.0 (CH), 107.5 (CMeO), 197.0 (CN). 15N NMR (40.5 MHz, CDCl3): δ 51 (NH2CH2). Anal. Calcd. for C12H21Cl3N2OTi (363.53 g/mol): C, 39.65; H, 5.82; N, 7.71. Found: C, 39.67; H, 5.85; N, 7.78. Synthesis of [TiCl2(OiPr)(OC7H6Me3NCH2CH2NH2-κ3N,N*,O)] (2). A solution of C7H6Me3(OH)(NCH2CH2NH2) (I) (0.57 g, 2.72 mmol) in dichloromethane (20 mL) was added at room temperature to a solution of TiCl2(OiPr)2 (0.64 g, 2.72 mmol) in dichloromethane (30 mL). The solution was stirred for 2 h, and the solvent was then removed under vacuum giving 2 as a pale-yellow solid (1.01 g, 96%). 1 H NMR (400 MHz, CDCl3): δ 1.03, 1.31, 1.80 (s, 3 × 3H, 3Me), 1.34, 1.36 (d, 3JHH = 6.3, 2 × 3H, OCHMe2), 1.96 (m, 1H, CH), 2.00, 2.31 (m, 2 × 1H, CH2), 2.10 (m, 1H, CH), 2.70 (m, 2H, CH2), 3.25, 3.78 (m, 2 × 1H, NH2), 3.41, 3.68 (m, 2 × 1H, CH2NH2), 3.51, 4.03 (m, 2 × 1H, CH2N), 4.83 (sp, 3JHH = 6.3, 1H, OCHMe2). 13C{1H} NMR (100.6 MHz, CDCl3): δ 23.5, 24.0, 27.5 (3 Me), 24.6, 24.7 (OCHMe2), 27.8 (CH2), 32.6 (CH2), 40.8 (CH), 41.5 (CMe2), 44.2 (CH2N), 49.1 (CH2N), 54.1 (CH), 82.9 (OCHMe2), 99.0 (CMeO), 193.6 (CN). 15N NMR (40.5 MHz, CDCl3): δ 44 (NH2CH2), 292 (CN). Anal. Calcd. for C15H28Cl2N2O2Ti (387.17 g/mol): C, 46.53; H, 7.29; N, 7.24. Found: C, 46.53; H, 7.35; N, 7.28 Synthesis of [TiCl 2 (O i Pr)(OC 7 H 6 Me 3 NCH 2 CH 2 NHMeκ3N,N*,O)] (3). A solution of C7H6Me3(OH)(NCH2CH2NHMe) (II) (0.30 g, 1.34 mmol) in dichloromethane (20 mL) was added at room temperature to a solution of TiCl2(OiPr)2 (0.32 g, 1.34 mmol) in dichloromethane (30 mL). The solution was stirred for 2 h, and the

Table 6. Selective Oxidation of Sulfides to Sulfones Catalyzed by 2a

a

Reaction conditions: 0.52 mmol of substrate, 3 equiv of H2O2 (30% aqueous solution), and 0.5% mol catalyst in MeOH (1 mL) 24 h at 60 °C. bYield determined by 1H NMR spectroscopy analysis, using mesitylene as internal standard

synthesized and fully characterized. All reported synthetic approaches are stereoselective and only one of the final possible diastereoisomers has been obtained. The coordination mode of the terpenoid ligand is rationalized on the basis of the titanium atom electronic deficiency and the nature of the pendant ending group (NHR or OH) of the ligand precursor. DFT calculations have been carried out to assess the stability of the different possible diastereoisomers for some of the complexes. The synthesized complexes act as highly efficient catalysts for the chemoselective oxidation of a wide variety of sulfides, either to the corresponding sulfoxide or the sulfone, using near-stoichiometric amount of H2O2 (30% in water) and under mild conditions.

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EXPERIMENTAL SECTION

General Methods. All manipulations were performed under an argon atmosphere (argon type U−N45, O2 and H2O < 3 ppm) using standard Schlenk techniques or in an MBraun Unilab-MB-20-G glovebox (O2 and H2O < 0.1 ppm). Solvents were dried by means of a MBraun solvent purification system. Deuterated solvents were degassed by several freeze−pump−thaw cycles and stored over activated 4 Å molecular sieves. NMR spectra were recorded on a Bruker Avance II 400 spectrometer at room temperature (1H, 400 MHz; 13C, 100.6 MHz; 15N, 40.5 MHz). Chemical shifts (δ) are expressed in ppm referenced against tetramethylsilane (TMS), using as internal reference residual protons of the deuterated solvent for 1H or carbons of the deuterated solvent for 13C{1H}. 15N chemical shifts were measured by 1H−15N HMBC, or 1H−15N HSQC experiments, using the instrument’s internal reference. X-ray crystallographic data were collected using a Nonius Kappa CCD instrument operating at 200 K. Elemental analyses were performed on a Perkin−Elmer 2400 instrument. Analytical values for 5 and 8 are far from the expected ones since their high instability hinders their purification, causing their hydrolysis. Synthesis of C7H6Me3(OH)(NCH2CH2NH2) (I). A mixture of ethylenediamine (8 mL, 119 mmol) and BF3·OEt2 (0.88 mL, 7.1 mmol) was syringed into an ampule containing a solution of ketol (2 g, 11.9 mmol) in toluene (40 mL) over activated molecular sieves (4 Å). The reaction mixture was heated at 90 °C for 4 h and then the molecular sieves were filtered off via a fritted funnel and washed with diethyl ether (2 × 10 mL). The volatiles were removed under reduced I


Article

Organometallics

(m, 6H, OCHMe2), 1.52 (m, 1H, CH), 2.00 (m, 1H, CH), 2.51, 2.70 (m, 2 × 1H, CH2), 3.48, 4.47 (m, 2 × 1H, CH2N), 4.60, 4.71 (m, 2 × 1H, CH2O), 4.65 (m, 1H, OCHMe2). 13C{1H} NMR (100.6 MHz, CDCl3): δ 23.3, 25.3, 27.7 (3 Me), 25.6, 25.7 (OCHMe2), 28.0 (CH2), 32.2 (CH2), 40.4 (CMe2), 41.0 (CH), 53.1 (CH), 53.3 (CH2N), 71.8 (CH2O), 78.3 (OCHMe2), 97.0 (CMeO), 196.5 (CN). Anal. Calcd. for C36H57ClN3O10Ti3 (871.02 g/mol): C, 49.63; H, 6.61; N, 4.82. Found: C, 49.38; H, 6.79; N, 4.65. Synthesis of [Ti(OC7H6Me3NCH2CH2O-κ3N,O,O*)2] (8). A solution of C7H6Me3(OH)(NCH2CH2OH) (III) (87 mg, 0.37 mmol) and NEt3 (100 μL, 0.73 mmol) in toluene (20 mL) was added at room temperature to a solution of Ti(OiPr)2Cl2 (0.15 g, 0.73 mmol) in toluene (30 mL). The solution was stirred for 2 h and the ammonium salt formed was filtered off via cannula filter. The solvent was evaporated to yield 8 as a pale-yellow solid (0.13 g, 76%). 1H NMR (400 MHz, CDCl3): δ 1.02, 1.30, 1.63 (s, 3 × 3H, 3Me), 1.94 (m, 1H, CH), 2.06 (m, 1H, CH), 2.32 (m, 2H, CH2), 2.66, 2.82 (m, 2 × 1H, CH2), 3.80, 4.03 (m, 2 × 1H, CH2N), 4.48, 4.74 (m, 2 × 1H, CH2O). 13C{1H} NMR (100.6 MHz, CDCl3): δ 23.5, 26.1, 27.8 (3 Me), 28.3 (CH2), 31.9 (CH2), 40.3 (CH), 40.8 (CMe2), 53.6 (CH), 56.5 (CH2N), 72.0 (CH2O), 95.0 (CMeO), 196.1 (CN). 15 N NMR (40.5 MHz, CDCl3): δ 295 (H2CN). Synthesis of [Ti(OiPr)2(OC7H6Me3(NCH2CH2O-κ3N,O,O*)] (9). Ti(OiPr)4 (78 μL, 0.26 mmol) was added at room temperature to a solution of C7H6Me3(OH)(NCH2CH2OH) (III) (56 mg, 0.26 mmol) in dichloromethane (5 mL). The solution was stirred for 2 h, and the solvent was then removed under vacuum giving 9 as a darkyellow oil (83 mg, 83%). 1H NMR (400 MHz, C6D6): δ 0.70, 1.04, 1.71 (s, 3 × 3H, 3Me), 1.37−1.47 (bm, 12H, 2 OCHMe2), 1.16, 2.07 (m, 2 × 1H, CH2), 1.56 (m, 1H, CH), 1.89, 2.11 (m, 2 × 1H, CH2), 1.90 (m, 1H, CH), 3.22, 3.40 (m, 2 × 1H, CH2N), 4.20, 4.55 (m, 2 × 1H, CH2O), 4.86, 4.97 (bm, 2 × 1H, OCHMe2). 13C{1H} NMR (100.6 MHz, C6D6): δ 23.1, 26.6, 27.5 (3 Me), 26.1, 26.6 (2 OCHMe2), 28.3 (CH2), 30.7 (CH2), 39.9 (CH), 40.1 (CMe2), 52.9 (CH), 54.1 (CH2N), 71.2 (CH2O), 76.2, 76.5 (2 OCHMe2), 91.7 (CMeO), 193.6 (CN). 15N NMR (40.5 MHz, CDCl3): δ 296 (H2CN). Anal. Calcd. for C18H33NO4Ti (375.33 g/mol): C, 57.60; H, 8.86; N, 3.73. Found: C, 55.65; H, 8.25; N, 4.34. Sulfoxidation Procedure. Sulfoxidation reactions were carried out in a J. Young NMR tube or in a sealed vial equipped with a magnetic stirrer and immersed in an oil bath at the appropriate temperature. A mixture of substrate (0.52 mmol) with mesitylene (37 μL) in methanol (1 mL) and 1 equiv of H2O2 (30% in water, 53 μL) was added to a stirred mixture of catalyst (0.5 mol %), The course of the reaction was monitored by 1H NMR spectroscopy. When the reactions were conducted in vials, aliquots of 0.05 mL were taken at regular intervals, treated with MnO2 and MgSO4, and filtered through a syringe filter before being analyzed. A procedure similar to that described for sulfoxides was adopted for sulfones by using 3 equiv of H2O2 (30% in water, 122 μL) and heating to 60 °C. Single-Crystal X-ray Structure Determination for III·HCl, 1· 0.5C6H6, and 7·C7H8. Data collection was performed at 200(2) K, with the crystals covered with perfluorinated ether oil. Single crystals were mounted on a Bruker-Nonius Kappa CCD single crystal diffractometer equipped with a graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). Multiscan20 absorption correction procedures were applied to the data. The structure was solved using the WINGX package,21 by direct methods (SHELXS-97) and refined using full-matrix least-squares against F2 (SHELXL-97).22 All nonhydrogen atoms were anisotropically refined except for C2, C4, C5, C6, C45, and C46 in compound 7·C7H8 that are disordered, the disorder has been treated and those atoms left isotropic. For 1, there are two independent molecules of the compound and one solvent molecule C6H6 per unit cell; in one of the molecules, one carbon atom in the NCCNTi metallacycle, C3, is disordered in two positions. Also, in the same molecule the pinenne ring shows some positional disorder, which in this case was not treated. Hydrogen atoms were geometrically placed and left riding on their parent atoms except for the hydrogen atoms involved in the H-bonding network in III·HCl

solvent was then removed under vacuum giving 3 as a pale-yellow solid (0.45 g, 84%). 1H NMR (400 MHz, CDCl3): δ 1.03, 1.30, 1.78 (s, 3 × 3H, 3Me), 1.36, 1.37 (d, 3JHH = 6.25, 2 × 3H, OCHMe2), 1.98 (m, 1H, CH), 2.00, 2.30 (m, 2 × 1H, CH2), 2.10 (m, 1H, CH), 2.68 (m, 2H, CH2), 2.73 (d, 3JHH = 6.03, 3H, NHMe), 3.06, 3.44 (m, 2 × 1H, CH2NHMe), 3.51, 3.98 (m, 2 × 1H, CH2N), 4.42 (m, 1H, NHMe), 4.90 (sp, 3JHH = 6.25, 1H, OCHMe2). 13C{1H} NMR (100.6 MHz, CDCl3): δ 23.5, 23.6, 27.6 (3 Me), 24.5, 24.6 (OCHMe2), 27.8 (CH2), 32.4 (CH2), 38.4 (NHMe), 40.8 (CH), 41.6 (CMe2), 48.3 (CH2N), 53.7 (CH2N), 54.2 (CH), 83.1 (OCHMe2), 99.5 (CMeO), 193.7 (CN). 15N NMR (40.5 MHz, CDCl3): δ 50 (NHMeCH2), 290 (H2CN). Anal. Calcd. for C16H30Cl2N2O2Ti (401.20 g/mol): C, 47.90; H, 7.54; N, 6.98. Found: C, 47.53; H, 7.26; N, 6.73. Synthesis of [Ti(OiPr)3(OC7H6Me3NCH2CH2NH2-κ2N,O)] (4). Ti(OiPr)4 (67 μL, 0.23 mmol) was added at room temperature to a solution of C7H6Me3(OH)(NCH2CH2NH2) (I) (48 mg, 0.23 mmol) in dichloromethane (5 mL). The solution was stirred for 2 h, and the solvent was then removed under vacuum giving 4 as a yellow oil (89 mg, 89%). 1H NMR (400 MHz, CDCl3): δ 0.96, 1.27, 1.55 (s, 3 × 3H, 3Me), 1.16, 1.17 (d, 2 × 9H, OCHMe2, 3JHH = 6.15), 1.18, 2.25 (m, 2 × 1H, CH2), 1.82 (m, 1H, CH), 2.01 (m, 1H, CH), 2.55, 2.69 (m, 2 × 1H, CH2), 1.36 (m, 2H, NH2), 2.82, 3.35 (m, 2 × 1H, CH2NH2), 3.35, 3.70 (m, 2 × 1H, CH2N), 4.66 (sp, 3JHH = 6.15, 3H, OCHMe2). 13C{1H} NMR (100.6 MHz, CDCl3): δ 23.5, 27.8, 29.1 (3 Me), 26.4, 26.4 (OCHMe2), 28.6 (CH2), 31.9 (CH2), 39.8 (CH), 40.1 (CMe2), 41.2 (CH2N), 52.8 (CH), 24.1 (CH2N), 73.2 (3 OCHMe2), 89.1 (CMeO), 189.9 (CN). 15N NMR (40.5 MHz, CDCl 3 ): δ 18 (NH 2 CH 2 ), 294 (CN). Anal. Calcd. for C21H42N2O4Ti (434.44 g/mol): C, 58.06; H, 9.74; N, 6.45. Found: C, 58.13; H, 9.91; N, 6.31. Synthesis of [Ti(OiPr)3(OC7H6Me3NCH2CH2NHMe-κ2N,O)] (5). Ti(OiPr)4 (67 μL, 0.23 mmol) was added at room temperature to a solution of C7H6Me3(OH)(NCH2CH2NHMe) (II) (50 mg, 0.22 mmol) in dichloromethane (5 mL). The solution was stirred for 2 h, and the solvent was then removed under vacuum giving 5 as a yellow oil (82 mg, 82%). 1H NMR (400 MHz, CDCl3): δ 0.95, 1.26, 1.53 (s, 3 × 3H, 3Me), 1.15, 2.24 (m, 2 × 1H, CH2), 1.17, 1.18 (d, 3JHH = 6.08, 2 × 3H, OCHMe2), 1.81 (m, 1H, CH), 1.86 (m, 1H, NHMe), 2.00 (m, 1H, CH), 2.41 (d, 3JHH = 6.24, 3H, NHMe), 2.57, 2.70 (m, 2 × 1H, CH2), 2.62, 3.28 (m, 2 × 1H, CH2NHMe), 3.49, 3.70 (m, 2 × 1H, CH2N), 4.65 (sp, 3JHH = 6.08, 1H, OCHMe2). 13C{1H} NMR (100.6 MHz, CDCl3): δ 23.4, 27.8, 29.0 (3 Me), 26.4, 26.4 (OCHMe2), 28.6 (CH2), 32.8 (CH2), 36.3 (NHMe), 39.8 (CH), 40.1 (CMe2), 50.7 (CH2N), 50.7 (CH2N), 52.7 (CH), 76.2 (3 OCHMe2), 89.0 (CMeO), 189.7 (CN). 15N NMR (40.5 MHz, CDCl3): δ 20 (NH2CH2), 294 (CH2NC). Synthesis of [TiCl(OiPr)(OC7H6Me3NCH2CH2O-κ3N,O,O*)] (6). A Schlenk flask was charged with a solution of C7H6Me3(OH)(NCH2CH2OH) (III) (0.13 g, 0.60 mmol) and NEt3 (83 μL, 0.60 mmol) in toluene (30 mL). Ti(OiPr)2Cl2 (0.14 g, 0.60 mmol) was added in solid portions with vigorous stirring. The solution was stirred for 2 h and the ammonium salt formed was filtered off via cannula filter. Solvent was removed from the resulting solution to yield 6 as a pale-yellow solid (0.19 g, 91%). 1H NMR (400 MHz, C6D6): δ 0.65, 0.98, 1.74 (s, 3 × 3H, 3Me), 1.52 (d, 3JHH = 6.14, 2 × 3H, OCHMe2), 1.65 (m, 1H, CH), 1.89 (m, 1H, CH), 1.89, 2.71 (m, 2 × 1H, CH2), 2.20, 2.32 (m, 2 × 1H, CH2), 3.02, 4.89 (m, 2 × 1H, CH2N), 5.13 (m, 2 × 1H, CH2O), 5.39 (sp, 3JHH = 6.14, 1H, OCHMe2). 13C{1H} NMR (100.6 MHz, C6D6): δ 23.1, 26.7, 27.4 (3 Me), 25.2, 25.3 (OCHMe2), 28.2 (CH2), 31.5 (CH2), 40.3 (CH), 40.6 (CMe2), 53.0 (CH), 54.2 (CH2N), 73.7 (CH2O), 81.8 (OCHMe2), 97.4 (CMeO), 197.2 (CN). 15N NMR (40.5 MHz, C6D6): δ 296 (CH2NC). Anal. Calcd. for C15H26ClNO3Ti (351.69 g/mol): C, 51.23; H, 7.45; N, 3.98. Found: C, 51.65; H, 8.02; N, 3.76 Synthesis of [{Ti(OiPr)(μ 2 -O{OC 7 H 6 Me 3 NCH 2 CH 2 Oκ3N,O,O*})}3(μ3-O)]Cl (7). When a toluene solution of 6, in the presence of trace water, was kept at −20 °C for several days, 7 was precipitated out as colorless crystals. 1H NMR (400 MHz, CDCl3): δ 0.83, 1.96 (m, 2 × 1H, CH2), 0.99 1.25, 1.64, (s, 3 × 3H, 3Me), 1.16 J


Organometallics

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(H2, H1, and H11) that were found in the Fourier map and refined freely. For 7·C7H8, a disordered molecule of toluene per molecule of 7 was present in the asymmetric unit. No chemical sense could be done of the disordered solvent molecule so squeeze procedure23 was applied to remove its contribution from the structure factors. Fullmatrix least-squares refinements were carried out by minimizing ∑w(Fo2 − Fc2)2 with the SHELXL-97 weighting scheme and stopped at shift/err < 0.001. In the case of 7·C7H8, although crystallized in a noncentrosymmetric group, both enantiomers are present in the structure and were refined as a 2-component inversion twin. Crystallographic data for the structure reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as CCDC 1830516 [III·HCl], 1830518 [1·0.5C6H6], and 1830517 [7· C7H8]. Computational Methods. All calculations were performed with Gaussian16.18 Three different functionals (B3LYP, M06-L and M062X) using several basis sets (6-31G**, 6-311G**, 6-311+G**, and 6311++G**) have been selected to perform geometry optimization calculations of complex 1 ([TiCl3(OC7H6Me3NCH2CH2NH2κ3N,N*,O)]). These simulations are intended to serve as a small benchmark set to choose the most suitable DFT functional for these kinds of complex. All tested methods perform well to reproduce the experimental X-ray structures with errors in distances and angles below 0.02 Å and 2.2°, respectively (see Table S3). Therefore, M062X/6-311+G** is selected to perform all geometry, energy, and NMR calculations. Frequency calculations at 298.15K have been performed both to confirm the absence of imaginary frequencies which indicates that the structure is a local minimum and to calculate free energies. Solvation effects were taken into account the Polarizable Continuum Model (PCM) using the integral equation formalism variant (IEFPCM).24 Two solvents have been used, dichloromethane (ε = 8.93) and chloroform (ε = 4.7113). All other parameters for PCM method were kept in their default values in Gaussian16. In all cases, reoptimizations of the gas phase structures were done. 1 H and 13C chemical shifts for all diastereoisomers of complex 2 [TiCl2(OiPr){(OC7H6Me3NCH2CH2NH2)-κ3N,N*,O}] in chloroform were calculated as follows (see Table S4). Hydrogen and carbon shielding constants were computed using the perturbation theory and the Gauge Invariant Atomic Orbitals (GIAO) [ref]. Chemical shifts, δ(1H) and δ(13C) are referred to the usual TMS standard through the following equation:

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Gerardo Jiménez: 0000-0002-7057-4750 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge Spanish MINECO (project CTQ2014-58270-R) and Universidad de Alcalá (project UAH-AE2017-2). I.R. acknowledges Spanish MECD (fellowship FPU13/03374). J.R. acknowledges Spanish MINECO (contract RYC-2011-09585).

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DEDICATION Dedicated to Prof. Ernesto Carmona on the occasion of his 70th birthday. REFERENCES

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δ(13Ci) = δ(13C)TMS − δ(13Ci)complex δ(1Hi) = δ(1H)TMS − δ(1Hi)complex The chemical shifts of the different conformers for a certain diastereoisomer (see Table S4) were taken into account using Boltzmann weighting factors. Summation of the weighted tensors across all conformers gives the Boltzmann-weighted average NMR shielding tensors for the candidate structure.

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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.8b00163. Tables of crystal data, NMR spectra of representative complexes, and tables and figures of DFT calculations (PDF) Accession Codes

CCDC 1830516−1830518 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. K


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