Aromatic Imines in the Titanocene Coordination Sphere

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Aromatic Imines in the Titanocene Coordination SphereTitanaaziridine vs 1‑Aza-2-titanacyclopent-4-ene Structures Florian Loose, Inka Plettenberg, Detlev Haase, Wolfgang Saak, Marc Schmidtmann, André Schaf̈ er, Thomas Müller,* and Rüdiger Beckhaus* Institute of Chemistry, Carl von Ossietzky University Oldenburg, D-26111 Oldenburg, Federal Republic of Germany S Supporting Information *

ABSTRACT: Two independent synthetic routes to η2-imine titanocene complexes were developed. On one hand side, ligand exchange reactions of bis(trimethylsilyl)acetylene by (pTolyl)HCNPh (3) employing the Rosenthal reagent Cp2Ti{η2-C2(SiMe3)2} (1) lead to Cp2Ti{η2-(p-Tolyl)CHNPh} (5), exhibiting a titanaaziridine structure. On the other hand, the direct reductive complexation of 3 by using Cp2TiCl2 (2) and Mg as reducing agent leads also to 5, one of the rare known titanoceneaziridines without additional ligands. By using the ketimine (p-Tolyl)2CNPh (4) instead of the aldimine 3, an unexpected coordination mode was found by X-ray diffraction, exhibiting an azatitanacyclopent-4-ene structure involving one tolyl fragment. In such a way, via the reductive complexation of 4, employing 2 or Cp*TiCl3 (12), the 1-aza-2titanacyclopent-4-ene complexes 6 and 13 are formed. Density functional calculations at the M06-2X level identify these new complexes 6 and 13 as 1-aza-2-titanacyclopent-4-enes, in agreement with an analysis based on the experimental structural parameters. A theoretical study of the bonding between the titanocene fragment and the imine ligand reveals that steric factors are more pronounced for titanaaziridines and disfavor their formation compared to azatitanacyclopentenes. This provides a rationalization for the preferred formation of titanoceneaziridines in the case of aldimine ligands and azatitanacyclopentenes when ketimines are applied. Whereas titanoceneaziridine 5 undergoes insertion reactions into the Ti−C carbon σ-bond with aldehydes, ketones, or carbodiimides to the five-membered titanacycles 20 and 21, complex 6 appears to be inert in comparable reactions.

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amidines12 are prepared via zirconocene imine complexes. Employing Brintzinger-type ansa-zirconocene derivatives, the synthesis of chiral amino acids13 and esters14 or enantiomeric pure allylic amines15 becomes available. The in situ generation of η2-imine titanium complexes, derived from transient (η2olefine)Ti(OPri)2 derivativesand comparable intermediatesis proven by a broad range of coupling reactions.16−22 Particularly, η2-imine complexes are essential intermediates in catalytic transformations of olefins and N-alkyl arylamines leading to hydroaminoalkylation products, as shown for tantal amides,23 bis(pyridonate)zirconiumamides24 and tantalamidates, particularly in enantioselective amine synthesis.25 Bis(πindenyl)titanocenedimethyl complexes are useful precatalysts in styrene hydroaminomethylations.26 The role of metallaaziridines in catalytic synthesis of amines by employing C−C bond forming reactions5,27−30 or in stoichiometric reactions31−33 has been summarized during the last years. In many cases the imine complexes are discussed as intermediates in catalytic as well as stoichiometric transformations. As one of the first titanoceneaziridines, Teuben et al. described the reaction of titanium(III)alkyls with benzalani-

INTRODUCTION The use of imines (Schiff bases) as ligands is characterized by a long tradition in organometallic and coordination chemistry. The incorporation of imine functionalities into polydentate organic molecules is an important aspect for the development of supramolecular complexes1 as well as new types of homogeneous catalysts.2 The coordination of imines to late transition metals are often characterized by a η1-coordination mode involving the nitrogen lone pair (I).3,4 However, in the case of early transition metals, particularly in a low valent state, η2-imine complexes (II) are found, exhibiting the formation of a metallaaziridine three-membered ring (III).5,6

Nevertheless, imine transition metal complexes are often involved in organic synthesis. In such a way, the formation of 2amino alcohols via the coupling of imines with aldehydes or ketones promoted by niob(III) complexes is described as one of the early examples.7 Later on, pyrroles,8 α-trisubstituted amines,9 allylic amines,10 tetrahydrochinolines,11 and α-amino © XXXX American Chemical Society

Received: July 23, 2014

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line.34 Generally, the formation of imine complexes of early transition metals became available by thermolysis of metal amides35 (procedure A; Scheme 1), rearrangements of iminoacyl derivatives6,36−43 (procedure B), ligand exchange reactions44 (procedure C), as well as thermolysis of metallocene alkyl-8−10,45,46 or arylamides47 (procedure D).

The three-membered titanacycle 5 shows a remarkable thermal stability up to 176 °C in the solid state. In the 1H NMR spectrum of the titanoceneaziridine 5 in C6D6 as solvent, two Cp resonances at 6.01 and 5.99 ppm are found, which are highfield shifted compared to the signals of starting compound 1 (6.45 ppm).60 Of high diagnostic value is the shift of the aldimine proton RNCR−H from 8.43 ppm in 3 to 5.64 ppm in complex 5. For the known phosphane imine complex 744 (1H NMR data summarized in Table 1), a stronger shift up to

Scheme 1. General Synthetic Procedures Leading to η2Imine Complexesa)

Table 1. Comparison of Selected 1H NMR Data of the Imine 3 and the Complexes 5 and 7 δ [ppm] Cp CH

Imine 3

Titanoceneaziridine 5

Imine complex 744

8.43

5.99; 6.01 5.64

5.11; 5.20 2.87

2.87 ppm is observed. Whereas 7 decomposed in solution without excess of PMe3, the ligand free titanoceneaziridine 5 appears to be stable also in solution for weeks at room temperature. Suitable single crystals of compound 5 for X-ray diffraction could be obtained from n-hexane as solvent. To the best of our knowledge, 5 is the first structurally characterized titanoceneaziridine without additional ligands besides the Cp ligands. For comparison, selected structurally characterized titanaaziridines are summarized in Scheme 3.

a

A: thermolysis of transition metal amides; B: rearrangement of iminoacyl units; C: ligand exchange; D: alkyl-H or aryl-H elimination from organylmetalamides.

Additionally further procedures are known, employing nonmetallocene derivatives.48−58 Here we wish to report the formation and reactivity of titanoceneaziridine complexes without additional ligands, formed in a multigram scale, employing aldimines. By using ketimines, an azabutadiene coordination mode is observed.

Scheme 3. Structurally Characterized Titanaaziridines (744, 8,24 9,61 10,6 1143)

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RESULTS AND DISCUSSION Imine Complexation. When reacting the Rosenthal reagent 159 with N-(4-methylbenzylidene)aniline (3) for 17 h at 60 °C in n-hexane as solvent, a color change from orange to dark green is observed (Scheme 2). Alternatively, instead of 1, the titanocenedichloride (2) can be used for the direct complexation of 3 employing magnesium as reducing agent in THF as solvent. In both cases the extremely air and moisture sensitive η2-iminetitanocene complex 5 can be isolated in up to 77% yield in the form of a intensively green colored solid. Scheme 2. Synthetic Routes Leading to the Imine Complexes 5 and 6 The X-ray diffraction analysis of 5 shows the expected C1− N1 bond elongation (1.403(2) Å) compared to free imines (av 1.27 Å),62 which appears also larger as for the titanaaziridine 7 (1.383(2) Å),44 indicating a stronger back-donation in 5 compared to 7. In agreement with this, the hybridization of N1 and C1 changes from sp2 to sp3. The bond lengths Ti1−N1 (1.987(2) Å) and Ti1−C1 (2.185(2) Å) are found in the expected range of comparable titanaaziridines 7−11 (Table 2). In particular also the Ti1−N1 bond lengths of 5 and 7 are nearly identical, whereas the Ti1−C1 distance of 5 (2.185(2) Å) is clearly shorter compared to 7 (2.302(2) Å).44 This indicates that the bonding situation of the imine in complex 5 is closer to a η2-bonding mode. B

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indicating the formation of a three-membered titanaaziridine structure, comparable to 5, in solution. On the other hand, in the case of 5 no dynamic processes are observed in a similar temperature range.72

Figure 1. Molecular structure of titanoceneaziridine 5 (Thermal ellipsoids are drawn at 50% probability, hydrogen atoms are omitted for clarity). Selected bond lengths [Å] and angles [deg]: Ti1−N1 1.987(2), Ti1−C1 2.185(2), C1−N1 1.403(2), N1−Ti1−C1 38.93(6), N1−C1−Ti1 62.86(9), C1−N1−Ti1 78.21(10).

Table 2. Selected Structural Data of the Imine 4 Compared to the Titanium Imine Complexes 5−11, 13 Compound

Ti−N [Å]

Ti−C [Å]

C−N [Å]

∑∠C [deg]

∑∠N [deg]

1.987(2) 2.02(1) 1.941(1) 1.992(2) 2.046(2) 1.883(5) 1.855(2) 1.846(4) 1.883(2)

2.185(2) 2.67(12) 2.399(2) 2.302(2) 2.219(3) 2.163(6) 2.150(2) 2.158(5) 2.132(3)

1.283(1) 1.403(2) 1.32(2) 1.393(2) 1.382(2) 1.368(4) 1.446(8) 1.410(3) 1.421(7) 1.417(4)

360 309 360 360 310 349 345 353 353 323

339 360 360 360 333 355 360 360 359

a

4 5 6b 13b 744 824 961 10a6 10b6 1143 a

Figure 2. Molecular structure of complex 6. Hydrogen atoms are omitted for clarity.

Because of the low quality of the X-ray structure solution, we abandon a detailed discussion of further bond length and angles. However, we also synthesized the comparable complex 13 with a slightly modified ligand system starting from Cp*TiCl3 (12) instead of Cp2TiCl2 (2). Reacting 12 with ketimine 4 in the presence of Mg as reducing agent, a color change from orange red to dark violet occurred. After workup, 13 is obtained in the form of large violet crystals in moderate yields. The imine complex 13 appears to be stable under MS conditions, leading to an M+ signal in LIFDI measurements. Scheme 4. Synthesis of the Azabutadiene Complex 13

Imine. b1-Aza-2-titanacyclopent-4-ene.

Employing the sterically more demanding imine 4 instead of 3, the formation of compound 6 occurs via ligand exchange reaction under the same conditions as shown in the case of 5. In such a way, 6 is obtained in the form of a dark violet powder. By using toluene instead of n-hexane, better yields could be achieved. In addition to the possibility to build up the titanium imine complexes 5 and 6 via the Rosenthal reagent 159 (Scheme 2), we also developed a synthetic route to titanoceneaziridines directly starting with bis(π-cyclopentadienyl)titanium(IV)dichloride 2 (Scheme 2). For this reductive complexation of an imine to the titanium center, we used magnesium as reducing agent to generate in situ a titanocene fragment, which is trapped by the imines 3 and 4, forming 5 and 6 in good yields (up to 83%). The 1H NMR spectrum of 6 shows just one singlet corresponding to the cyclopentadienyl ligands at 5.12 ppm and one CH3-signal at 2.12 ppm at room temperature, apart from aromatic related signals. Contrary to our expectations, we discovered by X-ray diffraction that 6 shows a new type of coordination of the imine 4 via the nitrogen atom and one of the bulky p-tolyl groups. In agreement with this azabutadiene coordination mode, an envelope rearrangement is found for 6 in solution (ΔG#223 = 42.2 kJ mol−1).72 There are no hinds

The molecular structure in the solid state of 13 is shown in Figure 3. It is obvious that in complex 13 the imine 4 is within the same coordination mode as in the biscylopentadienyl substituted complex 6. The N1−C11 bond length in 13 (1.393(2) Å) is elongated in a characteristic manner compared to the N1−C7 distance in 4 (1.283(1) Å). On the other hand, the C11−C12 distance (1.425(2) Å) is shortened in comparison to C7−C8 in 4 (1.497(1) Å). The sum of angles around C11 with 360° indicates a sp2-hybridization. Furthermore, the titanium imine carbon distance (Ti1−C11 2.399(2) Å) is found to be significant longer as in 5 (2.185(2) Å), indicating a weak contact. On the other hand, the Ti1−C13 (2.247(2) Å) distance is in agreement with a typical azatitanacyclopent-4-ene structure (compare 14−16, Table 3). Additionally, in the former aromatic ring C12−C17, localized CC double bonds are found between C14−C15 (1.352(3) Å) as well as C16−C17 (1.366(2) Å), compared to the well-balanced C−C distances in the C19−C24 ring. C

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may be caused by the ligand 4 that does not represent a classical 1-azabuta-1,3-diene due to the fact that the C−C double bond is within an aromatic system. The contribution of the η4−π bonding mode could be analyzed64 by the difference Δ = [(Ti1−C11 + Ti−C12)/2 − (Ti1−N1 + Ti1−C13)/2] = 0.326 Å. In comparison to mainly η4-bonded 1-aza-1,3-butadienes, this value is quite large,65−68 but compared with 14 (0.324 Å),63 15 (0.328 Å),64 and 16 (0.541 Å),64 it is in good agreement. It is evident that the ratio of the η4-π bonding is larger than in 1664 and in the same order as in 1463 and 15.64 Table 3 shows also that the internal N−C and C−C bond lengths are close to those of 14 and 15. Furthermore, N1, C11, C12, and C13 of 13 are, likewise, inplane. The fold angle of the central five-membered ring system (illustrated in Figure 4) of 67.6° is almost the same, as in compound 14 (67.0°).63 Of a high diagnostic value for the coordination mode of the ketimine 4 in complex 13 is the chemical shift of the proton signal localized at the C13 atom (213 K, toluene-d8, 1H, 5.83 ppm). Compared with the corresponding proton in 4 (7.96 ppm), a high field shift is observed. Generally, for Cp*(Cl)Ti(η4-diene)69,70 and Cp(Cl)Ti(η4-azabutadiene),63,64 prone and supine isomers are known. Also in the case of 13, the minor supine isomer is detectable in solution in a ratio of 1:10 (supine:prone, 213 K) (Figure 5). Further discussion is focused on the main prone isomer. The high-field shift of the 1H NMR signal of the Ti−CH proton corresponds to the change of the hybridization of the coordinated carbon atom and is in good agreement with the 1H NMR data reported by Scholz63,64 for classical titanium 1azabutadiene complexes. The chemical shift of the signal related to the corresponding carbon atom of 102.2 ppm is identical with the values observed by Scholz (101.4−104.3 ppm)63,64 for 14 and 15 and similar complexes. The rehybridization of the carbon atom is also proven by the 1JC,H coupling at this position. The coupling constant is lowered from 160 Hz in the free imine 4 to 148 Hz in complex 13. In relation to the coupling constants reported by Scholz in the range of 136−141 Hz,63,64 there seems to be a slightly higher s-ratio at the carbon atom in 13 compared to titanium complexes of 1azabutadienes. Quantum mechanical calculations71 at the density functional M06-2X level of theory72 were applied to investigate the bonding situation in titanoceneaziridine complexes such as 5 and in azatitanacyclopentenes 6 and 13. The molecular structures obtained by the calculations for imine 4 and for the titanium complexes 5 and 13 are, in the case of all relevant structural parameters, very close to the results from the X-ray diffraction (XRD) measurements (see Table S7, Supporting Information). This suggests that the calculated molecular structures at this level of theory are well suited for (i) a theoretical analysis of the bonding situation and (ii) for the analysis of the factors which favor the formation of fivemembered azatitanacyclopentene isomers over their threemembered titanoceneaziridine isomers. In agreement with the conclusions drawn from the XRD data, also the calculated structural parameters identify complex 5 as titanoceneaziridine. The computed Ti−C and Ti−N distances for complex 5 (Ti−N 196.6 pm; Ti−C 213.2 pm) are typical for single bonds between titanium and the main group element in titanocene complexes (Ti−N 195.1 pm for Cp2Ti(NH2)2; Ti−C 213.2 pm for Cp2TiMe2, calculated at

Figure 3. Molecular structure of complex 13. Hydrogen atoms are omitted for clarity. Thermal ellipsoids are shown at the 50% probability level. Selected bond lengths (Å) and angles (deg): Ti1− N1 1.941(1), Ti1−C11 2.399(2), Ti1−C12 2.441(2), Ti1−C13 2.247(2), N1−C11 1.393(2), C11−C12 1.425(2), C12−C13 1.438(2), C13−C14 1.430(2), C14−C15 1.352(3), C15−C16 1.426(3), C16−C17 (1.366(2), C12−C17 1.445(2), C19−C20 1.400(2), C20−C21 1.386(2), C21−C22 1.3902(2), C22−C23 1.395(2), C23−C24 1.389(2), C19−C24 1.394(2), C11−Ti1−N1 35.48(5), Ti1−N1−C11 90.50(9), N1−C11−Ti1 54.02(7)).

Table 3. Comparison of the Structural Data of 13 with the Known Complexes 14−16 (Bond Lengths [Å]) Ti1−N1 Ti1−C13 Ti1−C11 Ti1−C12 N1−C11 C11−C12 C12−C13

13

1463

1564

1664

1.941(1) 2.247(2) 2.399(2) 2.441(2) 1.393(2) 1.425(2) 1.438(2)

1.902(3) 2.147(3) 2.323(3) 2.374(3) 1.376(4) 1.394(4) 1.449(4)

1.920(1) 2.136(1) 2.332(2) 2.381(1) 1.385(2) 1.386(2) 1.442(2)

2.016(3) 2.278(4) 2.660(7) 2.715(9) 1.354(5) 1.375(6) 1.446(6)

We interpret this new coordination mode of imine 4 as an extension of the 1-azabutadiene complexes 14−16 synthesized by Scholz et al.63,64 For comparison, selected structurally characterized 1-azabutadiene complexes are summarized in Scheme 5. Scheme 5. Structurally Characterized 1-Azabutadiene Complexes (14,63 15,64 1664)

The bond lengths within 13 are similar to that reported by Scholz for compounds 14−16. Notably interesting is the similarity of 13 to the likewise monocyclopentadienyl substituted derivatives 1463 and 15.64 As shown in Table 3 the Ti1−N1 and Ti1−C13 distances of 13 are 1.941(1) Å and 2.247(2) Å, just slightly elongated in comparison to 14 and 15. Also the distances Ti−C11 (2.399(2) Å) and Ti−C12 (2.441(2) Å) are elongated. This indicates a somewhat weaker interaction between the titanium center and the ligand. This D

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Figure 4. (a) Plane 1 (N1, C11, C12, C13) of 13 (H atoms and further ligands omitted for clarity). (b) Fold angle plane 1/plane 2 (Ti1, C13, N1) 67.6° (H atoms omitted for clarity).

molecules (QTAIM)75,76 predicts a three-membered cyclic structure involving the titanium atom and the CN group of the imine (see Figure 6a). A QTAIM analysis of complex 6 indicates the existence of bond paths between the nitrogen atom and titanium atom and between the titanium atom and the ortho-carbon atom, Co, of one of the tolyl substituents and suggests the formation of a five-membered azatitanacycle in a strongly folded envelope conformation (see Figure 6b,c). Judging from the relatively large separation of the ortho-carbon atom and the titanium atom (Ti−C = 228.3 pm) and from the calculated WBI of 0.54, the Ti−Co bond (see Figure 7) in complex 6 is somewhat

Figure 5. Prone and supine isomer of complex 13.

the same level of theory). A natural bond orbital (NBO) analysis72,73 predicts for the Ti−C and for the Ti−N linkage in titanoceneaziridine 5 Wiberg bond indices (WBI)74 which are about three-quarters of that calculated for typical Ti−C and Ti−N single bonds in titanocene complexes (WBI(TiC(5)) = 0.74, WBI(TiN(5)) = 0.73 vs WBI(TiC(Cp2TiMe2) = 0.91, WBI(TiN(Cp2Ti(NH2)2) = 1.07). The innercyclic C−N bond in titanoceneaziridine 5 is elongated compared to the CN double bond in imine 3 (127.0 pm (3) vs 139.3 pm (5), see also Table S7), which indicates the expected decrease in CN bond order. This is further supported by the calculated WBI for this C−N bond in titanoceneaziridine 5 (WBI(CN(5)) = 1.06), which is significantly smaller than the WBI for the CN bond in imine 3 (WBI(CN(3) = 1.80). Similar structural parameters are predicted for the closely related but experimentally not available titanoceneaziridines 18 and 19 (see Tables S7 and S8). In addition, a topological analysis of the calculated electron density of complex 5 based on the quantum theory of atoms in

Figure 7. Pertinent calculated structural parameters and Wiberg bond indices (WBI, in parentheses) of imine 4, its radical dianion [4]2−, and titanocene complex 6 (bond lengths in pm, at M06-2X/6-311+G(d,p)(C,H,N), SDD (Ti); R = p-tolyl).

weaker than in titanoceneaziridine 5 (Ti−C = 213.2 pm, WBI(TiC(5)) = 0.74, see Figure 7). In contrast, the bond parameter of the Ti−N linkages in complex 6 are similar to those computed for aziridine 5 (6: Ti−N 202.4 pm, WBI(TiN)

Figure 6. Contour plots of the calculated Laplacian of the electron density, ∇2ρ(r), (a) in the Ti−C−N plane of titanoceneaziridine 5, (b) in the Ti−Co−N plane of 1-aza-2-titanacyclopent-4-ene 6, and (c) in the Co−bcp(CiC)−N plane of 6 (Relevant parts of the molecular graphs of the cations are projected onto the respective contour plot). The bond paths which follow the line of maximum electron density between bonded atoms are shown by solid black lines, and the corresponding bond critical points (bcps) are shown as green spheres. Red spheres mark ring critical points. Red contours indicate regions of local charge accumulation (∇2ρ(r) < 0), blue contours indicate regions of local charge depletion ((∇2ρ(r) > 0)). Calculated at M06-2X/def2tzvp (Ti), 6-311+G (C,H,N). E

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Figure 8. Relative energies of isomeric titanaaziridine/azatitanacyclopentene pairs calculated at M062X/6-311+G(d,p) (C,H,N), SDD (Ti).

= 0.70; 5: Ti−N 196.6 pm, WBI(TiN) = 0.73).77 A more detailed comparison between the structural parameter calculated for the free imine 4 and for the ligated imine in titanocene complex 6 reveals some interesting details. In the ligated imine the C−N bond is elongated compared to the free imine (by 8.0 pm, see Figure 7), while the neighboring C−Ci bond is significantly shortened (by 8.9 pm). Finally, the Ci−Co bond is again elongated to some extent (by 4.6 pm). A very similar sequence in bond lengths changes is also predicted by the calculation for the dianion of imine 4 [4]2− (see Figure 7). This suggests at least partial reduction of the imine ligand by the titanocene fragment upon complexation and supports the formulation of complex 6 (and likewise 13) as 1-aza-2titanacyclopent-4-ene. In agreement with the experiment, the calculations found titanoceneaziridine 5 to be more stable than its cyclopentene isomer 17, and for the pair of isomers cyclopentene 13 and aziridine 19, the reversed stability order is predicted (see Figure 8). In this case, it is interesting to note that the energy difference between the prone and supine conformers of complex 13 is 7.4 kJ mol−1 in favor of the prone isomer. Inclusion of solvent effects and entropy contributions at T = 298 K results in a free Gibbs energy difference between these two conformers of ΔG298 (prone/supine) = −5 kJ mol−1. This value is in reasonable agreement with an estimate made on the basis of the relative concentrations of both isomers at different temperatures, as measured by NMR spectroscopy (ΔG213(prone/ supine) = −4 kJ mol−1). This suggests that also in toluene solution the prone isomer is the dominant conformer. Interestingly, the titanocene aziridine complex with ketimine 4, complex 18, has essentially the same energy as the experimentally obtained cyclopentene complex 6. This suggests that effects of entropy, solvation, and/or lattice effects in the solid state, which are not accounted for in the present gas-phase calculation, favor the formation of compound 6 under the experimental conditions. The analysis of the thermodynamic cycle shown in Figure 9 provides a more detailed understanding of the factors that favor the formation of cyclopentene complexes such as 6 and 13 over that of three-membered

Figure 9. Thermodynamic cycle for the formation of either titanaaziridines or azatitanacyclopentenes from titanocenes and imines (see also Table 4).

aziridenes, i.e. 5.78,79 For this analysis the exothermic formation of the respective titanocene complex from imine and titanocene intermediate, CpTiX, is subdivided into several steps. The endothermic preparation steps, quantified by the preparation energies for imine and titanocenes, E prep (imine) and Eprep(CpTiX), describe the structural and electronic changes which occur in the imine ligand and in the titanocene complex when they are transformed from their free optimized closedshell ground state structures to the molecular structures they adopt in the respective complex. The complexation energy, F

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Table 4. Calculated Preparation Energies, Eprep, Complexation Energies, Ecomp, and Ligand Dissociation Energy ELDE for Compounds 5, 6, 13, and 17−19 (M06-2X/6-311+G(d,p) (C,H,N), SDD (Ti)) Cpd.

R1

Cp

X

ELDE [kJ mol−1]

Eprep(imine) [kJ mol−1]

Eprep(CpTiX) [kJ mol−1]

Ecomp [kJ mol−1]

5 18 19 17 6 13

H p-Tolyl p-Tolyl H p-Tolyl p-Tolyl

Cp Cp Cp* Cp Cp Cp*

Cp Cp Cl Cp Cp Cl

−172 −139 −229 −127 −139 −277

162 204 224 76 76 91

61 72 60 78 78 64

−395 −415 −513 −281 −293 −432

Scheme 6. Reactions of titanoceneaziridine 5 with carbonyl compounds and carbodiimides

Ecom, is the energy released when the titanocene complex, either titanoceneaziridine or azatitanacyclopentene, is formed from the prepared ligand and the titanocene fragment. The ligand dissociation energy ELDE is calculated as the difference between the endothermic preparation energies and the strongly exothermic complexation energy (see Figure 9). In general, the complexation energies calculated for the formation of the titanoceneaziridines 5, 18, and 19 are by 81−122 kJ mol−1 larger than those predicted for the formation of the isomeric azatitanacyclopentenes 17, 6, and 13 (Table 4). This is in qualitative agreement with the conclusions drawn from the analysis of the bonding parameter which suggested that the Ti− N linkages are of similar strength in titanoceneaziridines and in azatitanacyclopentenes, but that the interaction between the titanium and the carbon atom is somewhat weaker in azatitanacyclopentenes. Interestingly, the preparation energies for the titanocene fragments are very similar for all six investigated complexes (Eprep(CpTiX) = 60−78 kJ mol−1, see Table 4). This brings the preparation energy for the imine ligand into focus, which is calculated to be significantly higher for the formation of aziridine complexes (by 86−133 kJ mol−1, see Table 4). In addition, the imine preparation energy Eprep(imine) increases from the aldimine 3 to the ketimine 4 and increases further when the Cp2Ti fragment is changed to the Cp*TiCl fragment. Obviously, the increase of the ligand preparation energy parallels the expected increase of the steric requirements during the formation of the aziridine complexes. Therefore, the higher steric demand on the imine ligand is one of the decisive factors which disfavor titanoceneaziridine complexes compared to the isomeric azatitanacyclopentenes. In the case of the isomer pair 5/17, the formation of the titanoceneaziridine 5 is driven by its more favorable complexation energy Ecomp and for the pair 6/18 the increase of Eprep(imine) for the aziridine complex 18 neutralizes already the more favorable Ecomp of aziridine 18. Finally, in the case of the isomeric Cp* titanocene complexes 13/19, the higher

complexation energy Ecomp calculated for 19 does not compensate the significantly increased preparation energy of ketimine 4 in complex 19. Reactivity of Titanoceneaziridine 5. In order to get more detailed reactivity knowledge of titanoceneaziridine 5, reactions with polar multiple bonds were investigated (Scheme 6). In all cases insertion reactions are found with aldehydes, ketones, and carbodiimides. The titanaoxazolidine 20a, formed by the insertion of acetone, generates in 1H NMR spectrometry two signals at 1.25 and 1.41 ppm for both methyl groups at the central fivemembered ring. This indicates significantly different chemical surroundings above and below the ring. The difference in the chemical shift of the signals resulting from the cyclopentadienyl ligands at 5.93 and 6.32 ppm supports this thesis. The singlet at 5.44 ppm of the methine proton is slightly shifted upfield compared to the titanoceneaziridine 5 and within the range of organic oxazolidines.80,81 Of high diagnostic value for the characterization of imines, imine complexes, and subsequent products are results of 15N,1H HMBC NMR experiments. In such an experiment for the five-membered rings 20a−20d, 15N shifts of 173.2 up to 175.7 ppm are found (Table 5). In the case Table 5. Comparison of selected 15N-NMR data of the imine 3, 4 and the complexes 5, 20, as determined by 15N,1H HMBC NMR-experiments (50.66 MHz, 500 MHz, 305 K, δ15N relative to δ15N(NH3) = 0)

G

Compound

δ [ppm]

Solvent

3 4 5 20a 20b 20c 20d

327.8 329.3 226.2 173.2 177.7 173.4 175.7

C6D6 C6D6 C6D6 toluene-d8 C6D6 C6D6 C6D6

dx.doi.org/10.1021/om500750y | Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

more pronounced for titanaaziridines and disfavor their formation. To get a further understanding of the hydroaminoalkylation reaction, that is thought to proceed via metallaaziridines as key intermediates in the catalytic cycle, we investigated the reactivity of complex 5 as model compound. We noticed that substrates with a reactive double bond, such as aldehydes, ketones, and carbodiimides, undergo insertion reaction within the titanium carbon bond of the titanoceneaziridine 5.

of the three-membered titanoceneaziridine 5, a shift of 226.2 ppm is observed. This upfield shift indicates the abolition of the double bond character between the nitrogen and the nearby carbon atom in the staring imines 3 and 4 (327.8, 329.3 ppm). Between the ring protons of compound 20d a coupling constant of 9.5 Hz is observed in the 1H NMR study. This coupling constant indicates that the protons are in an antiarrangement. Therefore, the phenyl and the p-tolyl groups have to be in a gauche-configuration.72 Complex 20a crystallizes monoclinic in the space group C2/ c. The molecular structure of 20a is shown in Figure 10.

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EXPERIMENTAL SECTION

General Methods. All reactions except the synthesis of imine 3 were carried out under rigorous exclusion of air and moisture using standard Schlenk and glovebox techniques. All solvents and reagents were obtained from commercial suppliers and used as received unless otherwise noted. N-(4-Methylbenzylidene)aniline (3) was prepared according to the procedure of Ramsden and Nongkunsarn,83 and Cp2Ti(η2-BTMSA) (1)59 was synthesized according to a literature procedure as well as Cp*TiCl3 (12).84 The used solvents, except within the synthesis of 3, were dried with common drying agents and freshly distilled under nitrogen atmosphere prior to use. Given chemical shifts of 15N result out of 15N,1H HMBC NMR experiments with nitromethane as external standard (δ = 378.9 ppm vs NH3). All discussed yields are isolated yields. Preparation of N-[Di(4-methylphenyl)methylen]aniline 4. To 5.0 g (130 mmol) of potassium in 200 mL of toluene was added 12 mL (130 mmol) of dry aniline. After stirring the reaction mixture at room temperature overnight, 26.9 g (130 mmol) of 4,4′dimethylbenzophenone in 200 mL of toluene was added. The deep blue reaction mixture was stirred overnight at room temperature, quenched with 500 mL of water, and extracted two times with 400 mL of chloroform. The combined organic layers were dried with MgCl2, filtered, and completely evaporated in vacuum. Recrystallization of the crude product from 75 mL of ethanol leads to 15.5 g (54 mmol, 42%) of the desired product as orange crystals. Mp: 80 °C. 1H NMR (499.87 MHz, 300 K, CDCl3) [ppm]: δ = 2.30 (s, 3H, 4-Me), 2.40 (s, 3H, 4′Me), 6.72 (d, 2H, 3J = 7.7 Hz, 7-H), 6.92 (t, 1H, 3J = 7.3 Hz, 9-H), 6.99 (d, 2H, 3J = 8.1 Hz, 2-H), 7.04 (d, 2H, 3J = 8.1 Hz, 3-H), 7.14 (t, 2H, 3J = 7.8 Hz, 8-H), 7.20 (d, 2H, 3J = 8.0 Hz 3′-H), 7.63 (d, 2H, 3J = 8.0 Hz, 2′-H). 13C{1H}-NMR (125.71 MHz, 300 K, CDCl3) [ppm]: δ = 21.6 (4-Me), 21.7 (4′-Me), 121.3 (C-7), 123.1 (C-9), 128.6 (C-3), 128.7 (C-8), 129.1 (C-3′), 129.6 (C-2′), 129.8 (C-2), 133.6 (C-1), 137.5 (C-1′), 138.7 (C-4), 141.2 (C-4′), 143.7 (C-6), 168.5 (C-5). 15 N{1H}-NMR (50.66 MHz, 305 K, C6D6) [ppm]: δ = 329.3. IR (ATR, cm−1): 3022 (w), 2921 (w), 1620 (m), 1605 (m), 1590 (m), 1567 (w), 1506 (w), 1482 (m), 1446 (w), 1405 (w), 1312 (m), 1293 (m), 1262 (m), 1222 (m), 1154 (m), 1138 (m), 1111 (m), 1070(m), 984 (m), 944 (m), 902 (m), 828 (s), 811 (m), 764 (s), 733 (m), 698 (s), 674 (m), 659 (m), 631 (m), 616 (m), 567 (m). Anal. Calcd for C21H19N: C, 88.38; H, 6.71; N, 4.91. Found: C, 88.05; H, 6.97; N, 4.90. Preparation of Cp2Ti(η2-PhN=CH-p-Tolyl) 5. Method A: Cp2Ti(η2-BTMSA) 1 (1.045 g, 3 mmol) and 586 mg (3 mmol) of N-(4-methylbenzyliden)aniline 3 were dissolved in 30 mL of n-hexane. The reaction mixture was stirred for 17 h at 60 °C. After cooling down to room temperature, the green precipitate of 5 was collected, washed with 8 mL of n-hexane, and dried in vacuum. Yield: 868 mg (2.3 mmol), 77%. Method B: Bis(cyclopentadienyl)titanium(IV)-dichloride 2 (9.76 g, 39 mmol), N-(4-methylbenzylidene)aniline 3 (7.65 g, 39 mmol), and magnesium (0.95 g, 39 mmol) were dissolved in 250 mL of tetrahydrofuran. The resulting mixture was stirred for 16 h at room temperature. After evaporation of the solvent in vacuum, the residue was washed with toluene (4 × 50 mL). The combined filtrates were concentrated in vacuum, and the resulting green solid was washed three times with 50 mL of n-hexane and dried under vacuum. Yield: 5.61 g (15 mmol), 38%. Mp: 167 °C (dec.). 1H NMR (499.87 MHz, 305 K, C6D6) [ppm]: δ = 2.32 (s, 3H, 5-Me), 5.63 (s, 1H, 1-H), 6.00 (s, 5H, Cp), 6.02 (s, 5H, Cp), 6.82 (t, 1H, 3J = 7.1 Hz, 9-H), 7.09−

Figure 10. Molecular structure of titanaoxazolidine 20a. Hydrogen atoms are omitted for clarity. Thermal ellipsoids are shown at the 50% probability level. Selected bond lengths (Å) and angles (deg): Ti1−N1 2.051(2), Ti1−O1 1.841(2), N1−C1 1.436(4), O1−C2 1.424(3), C1−C2 1.424(3), O1−Ti1−N1 80.52(9), C2−O1−Ti1 123.30(17), N1−C1−C2 106.9(2), O1−C2−C1 105.0(2).

The Ti1−N1 bond length of titanaoxazolidine 20a is 2.051(2) Å, in good agreement with the sum of the covalence radii of 2.02 Å.82 In contrast, the Ti1−O1 bond length is 1.841(2) Å, slightly shortened in comparison to the sum of the corresponding covalence radii but within the range of the titanium oxygen bonds of Rothwells alkoxy substituted titanaaziridines 10a and 10b.6 The bond between C1 and C2 is somewhat shortened, probably by the geometry of the central five-membered ring. The central titanacyclopentane ring is not planar due to the tetracoordination of the carbon atoms C1 and C2. The sp3 configuration is indicated by the sums of angles around these atoms of 331° and 328°.

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CONCLUSIONS In summary, we have prepared the new cyclopentadienyl substituted titanium imine complexes 5 and 6. At first we found that these complexes can be synthesized via an exchange of the bis(trimethylsilyl)acetylene ligand in the Rosenthal reagent 1. Alternatively, the reductive complexation of imines, employing Cp2TiCl2 (2) and magnesium as reducing agent, leads selectively to the imine complexes 5 and 6 in a multigram scale. While studying the titanium complexes 5 and 6 we noticed the so far unknown coordination mode of the ketimine 4 in the complexes 6 and 13, in which the imine is bonded via the nitrogen atom and one of the ortho-carbon atoms of the aryl group. The results of density functional calculations at the M06-2X level support the formulation of these complexes as azatitanacyclopentenes. In addition, an analysis of the bonding between the titanocene fragment and the imine ligand reveals the importance of steric factors for the competition between both coordination modes of the aryl imine. Steric effects are H

dx.doi.org/10.1021/om500750y | Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

7.12 (m, 4H, 4-H, 8-H), 7.16 (m, 4H, 3-H, 7-H). 13C{1H}-NMR (125.71 MHz, 300 K, C6D6) [ppm]: δ = 21.5 (5-Me), 116.3 (Cp), 118.1 (Cp), 119.0 (C-1), 123.6 (C-3), [128.7, 128.5, 128.3] (C-7, C-8, C-9), 129.3 (C-4), 134.0 (C-5), 150.8 (C-2), 160.7 (C-6). 15N{1H}NMR (50.66 MHz, 305 K, C6D6) [ppm]: δ = 226.2. IR (ATR, cm−1): 3016 (w), 2917 (w), 2875 (w), 1624 (w), 1587 (m), 1505 (w), 1484 (m), 1439 (w), 1357 (w), 1312 (m), 1279 (w), 1169 (w), 1015 (m), 801 (s), 749 (s), 692 (s), 538 (s), 506 (m), 446 (m), 404 (m). MS (CI, i-butane): m/z (%) = 373 (100) [M]+, 196 (34) [C14H14N]+. Anal. Calcd for C24H23NTi: C, 77.22; H, 6.21; N, 3.75. Found: C, 76.84; H, 5.87; N, 3.69. Preparation of Cp2Ti(PhN=C(p-Tolyl)2) 6. Method A: Cp2Ti(η2BTMSA) 1 (174 mg, 0.5 mmol) and 142 mg (0.5 mmol) of N-[di(4methylphenyl)methylen]aniline (4) were dissolved in 5 mL of toluene. The reaction mixture was stirred for 17 h at 60 °C, and the solvent was completely evaporated from the deep red solution in vacuum. The residue was recrystallized from 15 mL of n-hexane at 4 °C. Yield: 56 mg (0.12 mmol), 23%. Method B: Bis(cyclopentadienyl)titanium(IV)dichloride 2 (7.469 g, 30 mmol), N-[di(4-methylphenyl)methylen]aniline 4 (8.561 g, 30 mmol), and magnesium (0.729 g, 30 mmol) were dissolved in 150 mL of tetrahydrofuran. After stirring the reaction mixture for 16 h at room temperature, the solvent was evaporated in vacuum. The residue was dissolved in 75 mL of toluene, and the precipitate of MgCl2 was washed with toluene (2 × 75 mL). The combined filtrates were concentrated in vacuum, and the resulting dark violet solid was washed four times with 20 mL of n-hexane to yield 11.518 g (25 mmol, 83%) of the desired product 6. Mp: 160 °C (dec.). 1 H NMR (499.87 MHz, 305 K, C6D6) [ppm]: δ = 2.12 (s, 6H, Me), 3.71 (s(br), 2H, o,o′-p-Tolyl), 5.12 (s, 10H, Cp), 6.47 (m, 1H, NPh(p)), 6.74−7.06 (m, 8H, N-Ph, p-Tolyl, not resolved), 7.65 (s(br), 2H, o,o′-p-Tolyl). 13C{1H}-NMR (125.71 MHz, 305 K, C6D6) [ppm]: δ = 20.7 (Me), 105.1 (Cp), 114.1 (Ti−C), [121.4, 123.1, 127.4, 128.8, 128.9, 129.1, 129.3, 129.4, 129.6, 129.8, 130.0] (N-Ph, p-Tolyl(o,o′, m,m′)), 125.6 (N-Ph(p)), 130.3 (p-Tolyl(i)), 131.1 (p-Tolyl(p)), 138.0 (N−C(p-Tolyl)2), 154.6 (N-Ph(i)). IR (ATR, cm−1): 3011 (w), 2913 (w), 2855 (w), 1587 (m), 1505 (m),1477 (m), 1446 (w), 1418 (w), 1336 (m), 1317 (w), 1280 (m), 1261(w), 1224 (m), 1107 (w), 1071 (w), 1017 (m), 796 (s), 769 (m), 724(m), 701 (m). MS (EI, 70 eV): m/z (%) = 462 (19) [M-H]+, 370 (25) [C24H20NTi]+, 285 (45) [C21H19N]+, 270 (23), 195 (53) [C14H13N]+, 178.0 (100) [Cp2Ti]+. Anal.Calcd for C31H29NTi: C, 80.34; H, 6.31; N, 3.02. Found: C, 75.16; H, 5.51; N, 2.78. (C,H failed due to extrem high reactivity) Preparation of Cp*ClTi(PhNC(p-Tolyl)2) 13. Pentamethylcyclopentadienyltitaniumtrichloride 12 (1.000 g, 3.45 mmol), N-[di(4methylphenyl)methylen]aniline 4 (0.985 g, 3.45 mmol), and magnesium (83 mg, 3.45 mmol) were dissolved in 40 mL of tetrahydrofuran. After stirring the reaction mixture for 16 h at room temperature, the solvent was evaporated in vacuum. The residue was dissolved in 20 mL of toluene, and the precipitate of MgCl2 was washed with toluene (2 × 10 mL). The combined filtrates were concentrated in vacuum, and the resulting solid was recrystallized from 12 mL of n-hexane to yield 953 mg (1.89 mmol, 55%) of 13 as deep violet plates. Mp: 121 °C (dec.). 1H NMR (499.87 MHz, 213 K, Told8, prone-isomer given) [ppm]: δ = 1.64 (s, 15H, Cp*), 1.99 (s, 3H, 9Me), 2.03 (s, 3H, 13-Me), 5.83 (d, 1H, 3J = 6.2 Hz, 11-H), 6.45 (d, 1H, 3J = 9.3 Hz, 14-H), 6.49 (d, 1H, 3J = 7.6 Hz, 8-H), 6.55 (d, 1H, 3J = 7.8 Hz, 7-H), 6.59 (d, 1H, 3J = 6.3 Hz, 12-H), 6.75 (t, 1H, 3J = 6.8 Hz, 1-H), 6.93−7.13 (m, 5H, 2-H, 3-H, 8′-H), 7.23 (d, 1H, 3J = 9.2 Hz, 7′-H), 7.93 (d, 1H, 3J = 7.7 Hz, 15-H). 13C{1H}-NMR (125.71 MHz, 213 K, Tol-d8) [ppm]: δ = 11.7 (Cp*), 20.1 (13-Me), 21.0 (9Me), 102.2 (C-11, 1JCH = 147.8 Hz), 105.4 (C-10), 107.1 (C-13), 121.4 (C-2/C-3/C-8′), 123.1 (C-1), 123.5 (C-7′), 123.6 (Cp*), 128.2 (C-2/C-3/C-8′), 128.4 (C-6), 129.4 (C-2/C-3/C-8′), 129.7 (C-8), 129.8 (C-7), 129.9 (C-9), 131.4 (C-12/C-15), 132.8 (C-12/C-15), 133.2 (C-14), 137.1 (C-5), 148.5 (C-4). IR (ATR, cm−1): 3052 (w), 3027 (w), 3012 (w), 2955 (w), 2854 (m), 2725 (w), 1586 (m), 1510 (m), 1480 (s), 1448 (m), 1375 (m), 1333 (s), 1304 (s), 1283 (s), 1222 (m), 1176 (s), 1100 (m), 1075 (m), 1025 (s), 995 (m), 937 (s), 892 (m), 859 (m), 822 (s), 804 (s), 796 (s), 757 (s), 733 (s), 701 (m), 689 (s). MS (LIFDI): m/z (%) = 503 (100) [M]+. HRMS (LIFDI):

calcd 503.1854, found 503.1867. Anal. Calcd for C31H34ClNTi: C, 73.89; H, 6.80; N, 2.78. Found: C, 73.27; H, 6.46; N, 2.74. Preparation of Titanaoxazolidine 20a. Titanoceneaziridine 5 (508 mg, 1.36 mmol) was dissolved in 4 mL of benzene. While adding 100 μL (1.36 mmol) of acetone, the reaction mixture colored immediately dark orange. After stirring at room temperature for 16 h, the solvent was removed completely in vacuum. The residue was washed with 5 mL of n-hexane. Drying the dark orange solid in vacuum yielded 277 mg (0.64 mmol, 47%) of the desired product. Mp: 123 °C (dec.). 1H NMR (500.13 MHz, 298 K, C6D6) [ppm]: δ = 1.25 (s, 3H, 10-Me), 1.41 (s, 3H, 10-Me), 2.07 (s, 3H, 9-Me), 5.44 (s, 1H, 5-H), 5.93 (s, 5H, Cp), 6.32 (s, 5H, Cp), 6.37 (d, 2H, 3J = 8.3 Hz, 3H), 6.59 (t, 1H, 3J = 7.1 Hz, 1-H), 6.94 (d, 2H, 3J = 7.7 Hz, 7-H), 7.03 (t, 2H, 3J = 7.7 Hz, 2-H), 7.22 (d, 2H, 3J = 7.7 Hz, 8-H). 13C{1H}NMR (125.77 MHz, 298 K, C6D6) [ppm]: δ = 21.1 (9-Me), 27.6 (10Me), 30.1 (10-Me), 90.9 (C-5), 93.6 (C-10), 116.9 (C-1), 117.3 (Cp), 117.6 (Cp), 118.8 (C-3), 128.2 (C-8), 128.3 (C-2), 128.8 (C-7), 135.8 (C-9), 142.4 (C-6), 157.4 (C-4). 15N{1H}-NMR (50.66 MHz, 298 K, Tol-d8) [ppm]: δ = 173.2.IR (ATR, cm−1): 3067 (w), 2965 (w), 2921 (w), 2801 (w), 2361 (w), 2342 (w), 1587 (w), 1480 (w), 1440 (w), 1354 (w), 1236 (m), 1139 (m), 1044 (w), 1016 (m), 971 (m), 945 (w), 880 (m), 801 (s), 761 (m), 694 (m).MS (CI, i-butane): m/z (%) = 432 (28) [M + H]+, 373 (100) [M-C3H6O]+, 301 (28) [M-2Cp]+, 196 (42) [C14H14N]+. HRMS (CI, i-butane): calcd 432.1801, found 432.1802. Anal.Calcd for C27H29NOTi: C, 75.17; H, 6.78; N, 3.25. Found: C, 73.20; H, 6.33; N, 2.85. Preparation of Titanaoxazolidine 20b. Titanoceneaziridine 5 (249 mg, 0.67 mmol) was dissolved in 4 mL of benzene. While adding 50 μL (0.67 mmol) of cyclobutanone, the reaction mixture colored immediately orange. After stirring at room temperature for 16 h, the solvent was removed completely in vacuum. The residue was washed with n-hexane (1 × 1 mL and 2 × 2 mL). Drying the orange solid in vacuum yielded 129 mg (0.29 mmol, 43%) of the desired product. Mp: 134 °C (dec.). 1H NMR (500.87 MHz, 305 K, C6D6) [ppm]: δ = 0.86−0.92 (m, 1H, 4-H), 1.40−1.49 (m, 1H, 4-H), 2.09 (s, 3H, 9-Me), 2.29−2.49 (m, 4H, 3-H, 5-H), 5.50 (s, 1H, 1-H), 6.01 (s, 5H, Cp), 6.15 (d, 2H, 3J = 8.1 Hz, 11-H), 6.33 (s, 5H, Cp), 6.60 (t, 1H, 3J = 7.2 Hz, 13-H), 6.98 (d, 2H, 3J = 7.6 Hz, 7-H), 7.04 (t, 2H, 3J = 7.5 Hz, 12H), 7.38 (d, 2H, 3J = 7.4 Hz, 8-H). 13C{1H}-NMR (125.71 MHz, 305 K, C6D6) [ppm]: δ = 12.7 (C-4), 21.1 (9-Me), 36.4 (C-3/C-5), 40.5 (C-3/C-5), 91.3 (C-1), 95.1 (C-2), 116.5 (C-13), 117.1 (Cp, C-11), 117.3 (Cp), 128.1 (C-8), 128.3 (C-12), 129.0 (C-7), 136.1 (C-9), 141.9 (C-6), 157.0 (C-10). 15N{1H}-NMR (50.66 MHz, 305 K, C6D6) [ppm]: δ = 177.7. IR (ATR, cm−1): 3068 (w), 2968 (w), 2942 (w), 2916 (w), 2764 (w), 2360 (m), 2342 (w), 1588 (m), 1488 (m), 1449 (w), 1248 (m), 1113 (m), 1088 (m), 1050 (m), 1015 (s), 884 (m), 796 (s), 749 (s), 713 (m), 691 (s). MS (CI, i-butane): m/z (%) = 444 (73) [M + H]+, 373 (100) [M-C4H6O]+, 313 (54) [M-2Cp]+. Anal.Calcd for C28H29NOTi: C, 75.85; H, 6.59; N, 3.16. Found: C, 75.13; H, 6.71; N, 2.77. Preparation of Titanaoxazolidine 20c. Titanoceneaziridine 5 (500 mg, 1.34 mmol) and 1-indanone (180 mg, 1.34 mmol) were suspended in 10 mL of n-hexane. While stirring the suspension for 16 h at room temperature, the color turned from green to orange. The orange solid was filtered out and washed with n-hexane (2 × 10 mL, 4 × 3 mL). Yield: 419 mg (0.83 mmol), 62%. Mp: 132 °C (dec.). 1H NMR (500.13 MHz, 298 K, C6D6) [ppm]: δ = 1.20−1.24 (m, 1H, 4H), 1.97 (s, 3H, 14-Me), 2.28−2.40 (m, 2H, 3-H, 4-H), 2.97−3.01 (m, 1H, 3-H), 5.87 (s, 1H, 1-H), 6.05 (s, 5H, Cp), 6.33 (d, 2H, 3J = 7.5 Hz, 16-H), 6.38 (s, 5H, Cp), 6.58 (t, 1H, 3J = 6.3 Hz, 18-H), 6.72 (d, 2H, 3J = 7.1 Hz, 12-H/13-H), 6.94−7.04 (m, 4H, 12-H/13-H, 17-H), 7.19−7.23 (m, 2H, 7-H/10-H, 8-H/9-H), 7.35 (t, 1H, 3J = 6.7 Hz, 8H/9-H), 7.87 (d, 1H, 3J = 7.0 Hz, 7-H/10-H).13C{1H}-NMR (125.77 MHz, 298 K, C6D6) [ppm]: δ = 21.1 (14-Me), 30.4 (C-4), 38.5 (C-3), 91.7 (C-1), 105.0 (C-2), 117.0 (C-18), 117.6 (Cp), 118.3 (Cp, C-16), 121.4 (C-7/C10), 124.6 (C-7/C-10, C-12/C13), 126.8 (C-8/C-9), 128.2 (C-8/C-9), 128.3 (C-12/C-13), 128.4 (C-17), 135.7 (C-14), 140.1 (C-11), 144.0 (C-5), 149.6 (C-6), 157.5 (C-15).15N{1H}-NMR (50.66 MHz, 305 K, C6D6) [ppm]: δ = 173.4. IR (ATR, cm−1): 3022 (w), 2921 (w), 2847 (w), 2360 (m), 2342 (w), 1684 (w), 1602 (w), I

dx.doi.org/10.1021/om500750y | Organometallics XXXX, XXX, XXX−XXX

Organometallics

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1506 (w), 1488 (w), 1457 (w), 1285 (w), 1248 (w), 1175 (m), 1102 (m), 1059 (m), 1012 (m), 800 (s), 752 (s), 725 (s), 668 (s).MS (CI, ibutane): m/z (%) = 441 (2) [M-Cp+H]+, 373 (8) [M-C9H8O]+, 310 (14) [M-C14 H13 N] + , 196 (100) [C14H14N] +.Anal. Calcd for C33H31NOTi: C, 78.41; H, 6.59; N, 3.16. Found: C, 78.47; H, 6.44; N, 2.50. Preparation of Titanaoxazolidine 20d. Titanoceneaziridine 5 (367 mg, 0.98 mmol) was dissolved in 3 mL of benzene. While adding 100 μL (0.98 mmol) of benzaldehyde, the color of the mixture immediately changed from green to orange. After stirring the mixture for 16 h at room temperature, the solvent was removed in vacuum. The residue was washed with 15 mL of n-hexane and dried in vacuum. Yield: 251 mg (0.52 mmol), 53%. Mp: 140 °C (dec.). 1H NMR (499.87 MHz, 305 K, C6D6) [ppm]: δ = 2.03 (s, 3H, 9-Me), 5.50 (d, 2H, 3J = 9.5 Hz, 5-H), 6.03 (d, 2H, 3J = 8.1 Hz, 3-H), 6.08 (d, 1H, 3J = 9.8 Hz, 10-H), 6.12 (s, 5H, Cp), 6.32 (s, 5H, Cp), 6.59 (t, 1H, 3J = 7.2 Hz, 1-H), 6.88 (d, 2H, 3J = 7.8 Hz, 7-H), 6.99−7.02 (m, 4H, 8-H, 13H), 7.18−7.22 (m, 5H, 2-H, 12-H, 14-H).13C{1H}-NMR (125.77 MHz, 298 K, C6D6) [ppm]: δ = 21.1 (9-Me), 92.3 (C-5), 93.5 (C-10), 116.3 (Cp), 116.4 (C-3), 116.6 (C-2), 118.0 (Cp, C-1), 127.5 (C-13), 128.1 (C-7), 128.2 (C-14), 128.3 (C-12), 129.0 (C-8), 136.1 (C-9), 141.2 (C-6), 143.9 (C-11), 157.2 (C-4).15N{1H}-NMR (50.66 MHz, 305 K, C6D6) [ppm]: δ = 175.7. IR (ATR, cm−1): 3083 (w), 3026 (w), 2361 (w), 2343 (w), 1588 (m), 1487 (m), 1450 (w), 1270 (m), 1202 (w), 1052 (m), 1016 (s), 884 (m), 803 (s), 750 (w), 700 (m).MS (CI, i-butane): m/z (%) = 480 (1) [M + H]+, 373 (2) [M-C7H6O]+, 196 (100) [C14H14N]+. HRMS (CI, i-butane): calcd 479.1729, found 479.1739. Anal.Calcd for C31H29NOTi: C, 77.66; H, 6.10; N, 2.92. Found: C, 77.92; H, 6.18; N, 2.71. Preparation of Titanaimidazolidine 21. Titanoceneaziridine 5 (200 mg, 0.54 mmol) and 1,3-di-p-tolylcabodiimide (120 mg, 0.54 mmol) were dissolved in 2 mL of benzene. The reaction mixture was stirred for 2 days at room temperature, and the solvent was removed in vacuum. Washing the residue with n-hexane (2 × 10 mL) and drying the solid in vacuum yielded 136 mg (0.17 mmol, 31%) of the dark green titanaimidazolidine 13. Mp: 113 °C (dec.). 1H NMR (300.13 MHz, 295 K, THF-d8) [ppm]: δ = 1.96 (s, 3H, Me), 2.17 (s, 3H, Me), 2.38 (s, 3H, Me), 4.27 (s, 1H, N−CH), 5.60 (s, 5H, Cp), 6.32 (m, 2H, Ar), 6.41 (s, 5H, Cp), 6.44 (m, 2H, Ar), 6.68−7.30 (m, 12H, Ar), 7.66 (m, 1H, Ar). 13C{1H}-NMR (125.69 MHz, 305 K, THF-d8) [ppm]: δ = 20.7 (Me), 20.9 (Me), 21.1 (Me), 87.8 (N−CH), 114.4 (Cp), 117.6 (Cp), 121.6 (CHar), 122.0 (CHar), 122.9 (CHar), 124.6 (CHar), 125.6 (CHar), 126.3 (CHar), 127.4 (Car-Me), 128.4 (CHar), 128.5 (CHar), 129.6 (CHar), 129.7 (CHar), 130.0 (CHar), 130.8 (CHar), 133.6 (CarMe), 135.3 (Car), 142.3 (Car-Me), 147.7 (Car), 149.2 (Car), 152.8 (Car), 153.4 (N−C−N). IR (ATR, cm−1): 3067 (w), 3017 (w), 2962 (w), 2915 (w), 2856 (w), 1594 (w), 1520 (s), 1503 (s), 1450 (m), 1379 (m), 1288 (m), 1252 (m), 1215 (m), 1102 (m), 1015 (m), 975 (m), 911(m), 811 (s), 753 (m), 694 (m), 682 (s).MS (CI, i-butane): m/z (%) =594 [M-H]+ (15), 420 [C29H30N3] + (100), 401 [C25H25N2Ti]+ (55), 330 (36), 316 (29), 279 [C19H23N2]+ (16), 223 [C15H15N2]+ (86), 196 [C14H14N]+ (47), 108 (35). Anal.Calcd for C39H37N3Ti: C, 78.65; H, 6.26; N, 7.06. Found: C, 78.81; H, 6.63; N, 6.84.

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ACKNOWLEDGMENTS This work was supported by the Deutsche Forschungsgemeinschaft (DFG) (BE 1400/7-1). The High End Computing Resource Oldenburg (HERO) at the CvO University is thanked for computer time.

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REFERENCES

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S Supporting Information *

Detailed spectra, crystallographic data, and computational details. This material is available free of charge via the Internet at http://pubs.acs.org.

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