Marcel
Eilers
,
Saskia
Bültena
,
Kevin
Schwitalla
,
Marc
Schmidtmann
and
Rüdiger
Beckhaus
*
Institut für Chemie, Carl von Ossietzky Universität Oldenburg, D-26111 Oldenburg, Germany. E-mail: ruediger.beckhaus@uol.de
First published on 10th December 2024
A series of titanium amidinato complexes were synthesized by stoichiometric insertion reactions of carbodiimides into bis(π–η5:σ–η1-pentafulvene)titanium complexes. NMR studies and single-crystal X-ray diffraction showed κ1N coordination of the former carbodiimides to the metal center. DFT calculations were performed, confirming the clear preference for a single nitrogen atom coordinating to the metal center with a high energy transition state for the formation of a chelating heteroallyl ligand. Depending on the pentafulvene ligand, additional insertion reactions of carbodiimides into the remaining Ti–Cexo bond were observed. This allows for a stepwise insertion of the corresponding carbodiimides and offers the possibility to further functionalize the complexes. The reactivity of the remaining pentafulvene ligand is further demonstrated in reactions with H-acidic and multiple bond substrates.
Due to the broad variability of the substitution patterns, carbodiimides (I) are important building blocks with many different organic and catalytic processes relying on this reactive class. They are widely used in organic synthesis and probably best known for their catalytic use in the synthesis of peptides, amides, and esters,1 for example in the total synthesis of penicillin discovered by Sheehan and co-workers.2 They may also be used in hydroboration reactions3 or in guanylation reactions leading to the formation of guanidine-based ligands.4 Only a few carbodiimide-based metal complexes have been synthesized, such as carbodiimido titanium complexes by salt metathesis reactions (Fig. 2a).5 The central carbon atom of the carbodiimides is positively polarized and can therefore be attacked by nucleophiles. This led to the insertion of several carbodiimides into various metal–element bonds, for example M–C, M–O, M–B and M–N bonds, resulting in the formation of chelating heteroallyl complexes (Fig. 2b).6 Some of these titanium complexes have been shown to be useful in catalytic living polymerisation reactions by exploiting the insertion reaction.7 Remarkable results were achieved by the groups of Rosenthal and Tonks with unusual four-membered heterometallacarbene complexes by replacing the alkyne in the Cp2Ti(btmsa) complex with various carbodiimides (Fig. 2c).8
On the other hand, examples for 1,2-insertion reactions of the carbodiimide into M–C bonds (Fig. 2d),9 and (reversible) cycloaddition reactions10 with MN complexes were reported. For many years, our group has been pursuing the fundamental chemistry and synthetic applications of bis(π–η5:σ–η1-pentafulvene)complexes of group 4 and 5 transition metals. These have been shown to be excellent precursors for the functionalization of transition metal complexes by either E–H bond activation11 or multiple bond substrate insertion11k,l,12 into the frustrated M–Cexo bond in almost always salt free reactions under mild conditions and in high yields. Within our studies, we recently reported on the insertion reaction of carbodiimides into a vanadium pentafulvene complex, forming a vanadium κ1N amidinato complex.11l This led to further investigations into reactions of pentafulvene complexes with cumulated substrates and the recently published reaction of the bis(π–η5:σ–η1-pentafulvene)titanium complex 1 with propa-1,2-diene and the formation of chained titanium–allyl complexes (Fig. 2e).13 Continuing our recent investigations, we herein report on the reaction of bis(π–η5:σ–η1-pentafulvene)titanium complexes with either one or two equivalents of carbodiimides. We further demonstrate the reactivity of the obtained titanium κ1N amidinato complexes (Fig. 2f) towards E–H activation and multiple bond substrate insertion.
With less sterically demanding substituents, the reaction of 1 with iso-propyl and cyclohexyl carbodiimides at room temperature resulted in an immediate colour change of the solutions from blue (1a) and green (1b) to red (2 and 3). The colour change can be explained by the prior coordination of one nitrogen atom of the carbodiimide to the metal center. By following the reaction with 1H NMR spectroscopy, an additional set of signals for the adduct formation was observed (see ESI, Fig. S3†). The characteristic iPr–H signals for the carbodiimide adduct are found at δ1H = 3.65 and 3.94 ppm and are low field shifted compared to the free iPrNC
NiPr (δ1H = 3.33 ppm) and to the product signals of 2a (δ1H = 3.10 and 3.25 ppm).
Coordination of strong donors to bis(π–η5:σ–η1-pentafulvene)titanium complexes has been previously observed by our group in reactions with NHCs,14 and the formation of a Lewis acid–base complex between CyNC
NCy and PhBCl2 as an intermediate in the insertion reaction of carbodiimides into a boron–aryl bond was found by Cowley and co-workers.15 The prior coordination of carbodiimides to titanium centers has only been found in theoretical studies.16
After stirring the red solutions for several hours, the formation of green precipitates was observed. The supernatant was decanted and the solids washed with a small amount of n-hexane and dried in high vacuum. The obtained green solids were isolated in moderate (3b: 52%) to very good yields (2a: 93%, 2b: 80%, 3a: 80%) at the gram scale. They are air and moisture sensitive and were therefore stored in a glovebox (Scheme 1).
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Scheme 1 Insertion reactions of 1 with symmetrical carbodiimides to form titanium κ1N amidinato complexes 2 and 3. |
The products were characterized by NMR spectroscopy and the coordination mode was determined by single-crystal X-ray diffraction (SC-XRD). In accordance with the asymmetric chemical environment at the titanium center, in the 1H NMR spectra the eight characteristic signals for the eight independent cyclopentadienyl (Cp) protons are found in the range of δ1H = 3.96–6.79 ppm (2) and δ1H = 3.41–6.17 ppm (3). The signals for the protons of the respective carbodiimides are observed at higher fields, next to the signals for the adamantyl (CH/CH2) and para-tolyl (CH3) protons. For 2, one adamantyl signal gets significantly shifted to lower fields (δ1H = 4.17–4.26 ppm), probably due to hydrogen bridging to the imine nitrogen atom of the former carbodiimide. The 13C{1H} chemical shifts of the former quaternary carbon atoms of the carbodiimides are found at δ13C{1H} = 173.1 (2a), 172.5 (2b), 175.2 (3a) and 175.1 (3b) ppm, which are significantly shifted to lower fields compared to the free carbodiimides (cf. R1 = Cy: 139.8 ppm).17 For 3a, the 15N signals, obtained from a 1H,15N-HMBC spectrum, show values of δ15N = 276.7 and 278.2 ppm for the nitrogen atoms by coupling with the respective iso-propyl protons. A second set of signals can be observed in the NMR spectra of 2b. A characteristic shift of δ13C{1H} = 160.1 ppm for the central carbon atom of the former carbodiimide indicates the migration of a cyclohexyl group from one nitrogen to the other and the formation of an η1-imino complex 2b′ as a side-product (see ESI, Fig. S6†). The chemical shift is between the free carbodiimide (R = Cy: 139.8 ppm)17 and the insertion product 2bδ13C{1H} = 172.5 ppm and would match with literature (δ13C{1H} = 157.9 ppm).9 However, a more precise characterization was difficult due to the absence of single-crystals, poor solubility in common solvents such as benzene, toluene, THF, DCM or chloroform and signal overlap.
The formation of the titanium κ1N amidinato complexes 2 and 3 was also determined by SC-XRD. Suitable crystals were obtained either directly out of the reaction mixtures or by slow evaporation of concentrated benzene-d6 or n-hexane solutions. The solid-state structures of 2a and 3a are shown in Fig. 3, the structures of 2b and 3b can be found in the ESI.†
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Fig. 3 Molecular structures of complex 2a and 3a. Thermal ellipsoids drawn at the 50% probability level. Hydrogen atoms and solvent molecules are omitted for clarity. |
The molecular structures of 2 and 3 confirm the insertion of the carbodiimides into one of the two Ti–Cexo bonds. With a slightly shortened, when compared to the sum of covalent radii (∑rcov(Ti–N) = 2.07 Å),18 Ti–N single bond (2a: 2.0292(15) Å, 2b: 2.0466(18) Å, 3a: 2.0499(5) Å, 3b: 2.0595(17) Å), the complexes are in accordance with a complex from Beweries and co-workers ((Ti–N) = 2.0562(9) Å).9 The former Cp–Cexo bonds (cf.1b: 1.453(3) Å),19 are elongated to approximately 1.52 Å (2a: 1.517(2) Å, 2b: 1.523(3) Å, 3a: 1.5187(8) Å, 3b: 1.521(3) Å) due to the change in hybridization of the carbon atom. The newly formed Cexo–CN bonds (2a: 1.556(3) Å, 2b: 1.538(3) Å, 3a: 1.5643(7) Å, 3b: 1.567(3) Å) are elongated C(sp3)–C(sp2) single bonds (1.51)20 with the former central carbon atom of the carbodiimides being sp2 hybridized. The remaining Ti–Cexo bond lengths (2a: 2.446(2) Å, 2b: 2.469(2) Å, 3a: 2.4634(6) Å, 3b: 2.469(2) Å) are only slightly elongated compared to the starting material (1b 2.408(2) Å/2.392(2) Å). Overall, complexes 2 and 3 differ only slightly in bond lengths.
We calculated the free Gibbs energies of 2a and the possible chelating coordination mode of the inserted carbodiimide and found a high transition state of 141.6 kJ mol−1 for the exchange reaction between the two nitrogen atoms. The mechanism seems to be associative due to the formation of a hetero-allyl-type transition state, but at the same time, the Ti–Cexo bond gets significantly elongated and the Cp–Cexo bond retained to the plane of the Cp ring, indicating an ambiguous mechanism with a transition of the σ–η5:π–η1 coordination mode to η4 (see Fig. 4). In contrast to previously reported insertion products and our previously synthesized titanium allyl complex,13 the coordination of only one nitrogen to the titanium center is the thermodynamic minimum, while the heteroallyl coordination was found to be the transition state of the exchange reaction.
Of the starting complexes 1a and 1b, only 1b reacted with an additional equivalent of carbodiimide, despite the absence of significant differences in Ti–Cexo bond lengths. After 16 hours of stirring, the reaction mixtures turned from green to yellow to purple. After workup, the obtained complexes 4a (78%) and 4b (58%) were isolated as purple solids (Scheme 2).
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Scheme 2 Reaction of 1b with two equivalents of carbodiimides to form bis(κ1N-amidinato) products 4a and 4b. |
The formation of the titanium bis(κ1N-amidinato) complexes 4a and 4b was observed in solution by NMR spectroscopy. The 1H NMR spectra revealed the symmetrical structure by giving only four characteristic signals in sets of two for the Cp protons. Additionally, the complete set of signals is reduced by a factor of two, which demonstrates the high symmetry of the complexes. In the 13C{1H} NMR spectra, only one signal can be observed for the carbon atoms of the former carbodiimides (4a: δ13C{1H} = 173.8 ppm, 4b: δ13C{1H} = 175.3 ppm), and for 4a, the 15N values were found to be δ15N = 260.9 and 267.5 ppm. SC-XRD of crystals obtained from saturated benzene-d6 solutions by slow evaporation confirms these findings. The molecular structures of 4a and 4b are shown in Fig. 5, the structural parameters can be found in the ESI.†
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Fig. 5 Molecular structures of complexes 4a (left), 4b (middle) and 4c (right). Thermal ellipsoids drawn at the 50% probability level. Hydrogen atoms and solvent molecules are omitted for clarity. |
The molecular structures of 4a and 4b confirm the double insertion of the carbodiimides into the two Ti–Cexo bonds with two Ti–N single bonds (4a: 2.055(3) Å, 2.058(3) Å; 4b: 2.0659(11) Å, 2.0742(12) Å). Now both former Cp–Cexo bonds are elongated due to the insertion and change in hybridization of the carbon atom (4a: 1.524(5) Å, 1.523(4) Å; 4b: 1.5170(19) Å, 1.5131(19) Å). The newly formed Cexo–CN bonds (4a: 1.557(4) Å, 1.564(4) Å; 4b: 1.5642(18) Å, 1.5655(18) Å) are also elongated C(sp3)–C(sp2) bonds.
We were also able to carry out a stepwise double insertion reaction by using n-hexane as the solvent in the first step and reacting the isolated complexes 3 with another carbodiimide to obtain the asymmetric complex 4c, which was carried out on the NMR scale. The formation of the mixed complex 4c was observed in both cases (Scheme 3).
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Scheme 3 Reaction of 3 with different carbodiimides to form the asymmetric bis(κ1N-amidinato) complex 4c. |
The 1H NMR spectrum of 4c shows the asymmetry of the structure by giving eight characteristic signals for the eight Cp protons. Both sets of signals for the cyclohexyl and the iso-propyl protons can be observed, and the 13C{1H} NMR spectrum shows two signals for the central carbon atom of the former carbodiimides (δ13C{1H} = 174.3 ppm (NCN–Cy), δ13C{1H} = 174.5 ppm (NCN–iPr)). Notably, these double insertion reactions were not observed in reactions of 1 with propa-1,2-diene.13 The solid-state structure of 4c is also shown in Fig. 5.
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Scheme 4 Reaction of 2a with primary aromatic amines to give 5 and with an ammonium borate to give 6. |
Characteristic singlets for the Cp–CexoH protons are found at δ1H = 2.80 ppm (5a) and δ1H = 2.75 ppm (5b) with the corresponding N–H signals at δ1H = 8.38 ppm (5a) and δ1H = 7.87 ppm (5b). The quaternary carbon atoms of the former carbodiimide are found at δ13C{1H} = 163.8 ppm (5a) and at δ13C{1H} = 165.2 ppm (5a), and in the IR spectra, the corresponding N–H frequencies are found at 3363 cm−1 (5a) and 3325 cm−1 (5b). In addition, single crystals of 5a were obtained from a saturated benzene-d6 solution in the NMR-tube, that were suitable for SC-XRD and confirmed the proposed protonation of the Cexo carbon atom. The molecular structure of 5a is shown in Fig. 6, and the structure of 5b is given in the ESI.†
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Fig. 6 Molecular structure of complex 5a. Thermal ellipsoids drawn at the 50% probability level. Hydrogen atoms (except for H3 and H6) are omitted for clarity. |
The obtained molecular structure of 5a confirms the protonation of the Cexo with an elongated C1–C6 bond to 1.5168(19) Å and a newly formed Ti1–N3 single bond (1.9872(13) Å), which is relatively short in comparison to the sum of the covalent radii, while the Ti1–N1 single bond has been elongated from 2.031(3) Å to 2.0741(12) Å in the process.
Interestingly, the reaction of 2a with the ammonium borate [Et2NHiPr][BPh4], did not lead to the protonation of the Cexo carbon atom of the pentafulvene ligand, but instead to the protonation of the CN nitrogen atom of the former carbodiimide (Scheme 4, bottom). This protonation could be an indication of the non-selective reactions of 2a with alcohols and much stronger acids such as etheric HCl or HOTf, because both the fulvene function and the former carbodiimide can be involved in the reaction. Unfortunately, no single crystals of 6 could be obtained due to the low stability of the cationic complex in solution, as it decomposes in solvents such as THF, DCM and CDCl3 within a few hours. In the solid state, the complex is much more stable and can be stored in a glovebox without decomposing. The protonation of the nitrogen atom was detected in the solution NMR spectra. In the 1H NMR spectrum, a broad doublet is found at δ1H = 6.73 ppm with a coupling constant of 3J = 10.0 Hz. This signal is coupling to one of the iPr-CH protons, resulting in a doublet (3J = 10.0 Hz) of a septet (3J = 6.3 Hz). In the 13C{1H} NMR, the quaternary carbon of the former carbodiimide is shifted to δ13C{1H} = 172.5 ppm and in the 11B{1H} NMR, only one signal can be observed at δ11B{1H} = −6.6 ppm. The characteristic v(N–H) band can also be observed in the IR spectrum at 3391 cm−1.
In the 1H NMR spectra, two singlets for the methyl groups of the inserted acetone are observed at δ1H = 1.21 and 1.41 ppm. In the 13C{1H} NMR spectra, the chemical shift for the quaternary carbon atom of the former acetone is shifted to δ13C{1H} = 110.0 ppm. The 15N chemical shifts of the two nitrogen atoms were found at δ15N = 240.6 and 260.9 ppm. Single crystals were obtained from a saturated solution of benzene-d6 by slow evaporation and SC-XRD analysis provided the molecular structure confirming the formation of 7 (Fig. 7).
The insertion of acetone leads to the formation of a highly elongated C28–C38 bond of 1.6284(12) Å and the elongation of the former Cp–Cexo bond to 1.5211(12) Å. The resulting Ti1–O1 bond length of 1.8689(7) Å is in the range of a shortened single bond when compared to the sum of the covalent radii (∑rcov(Ti–O) = 1.99 Å).18 The Ti1–N1 single bond has been slightly elongated from 2.031(3) Å to 2.0528(8) Å in the process.
We were also able to react 2a with acetonitrile and 4-chlorobenzonitrile, which also inserted into the remaining Ti–Cexo bond, while the product of the reaction with acetonitrile additionally underwent a 1,3-H-shift to form the amido complex 8 (Scheme 5, middle). This shift cannot be observed on the NMR timescale, leading to the isolation of the amido complex 8 in 77% yield. In the 1H NMR spectrum, the characteristic singlets for the two terminal alkene protons are found at δ1H = 3.77 ppm and 4.30 ppm. This is in accordance with the absence of the former methyl group of acetonitrile. The proton of the formed amido ligand is observed as a broad singlet at δ1H = 7.35 ppm. The corresponding 15N cores were observed at δ15N = 232.8 (NCN), 258.5 (NCN) and 262.5 (NH) ppm. In the IR spectrum, the characteristic v(N–H) band is found at 3351 cm−1.
We were also able to obtain the molecular structure through SC-XRD from single crystals of a saturated benzene-d6 solution. The molecular structure of 8 is shown in Fig. 7. The insertion of acetonitrile and the following 1,3-H-shift resulted in the formation of a new elongated C28–C38 single bond (1.5429(13) Å). The 1,3-H-shift is seen in the C38–C39 double bond (1.3505(13) Å), which is now a slightly elongated C(sp2)C(sp2) bond (1.32 Å).20 The resulting new Ti1–N3 bond with 1.9768(8) Å is in the range of a shortened single bond, and the Ti1–N1 single bond has been slightly elongated from 2.031(3) Å to 2.0562(8) Å in the process.
To negotiate this 1,3-H-shift, we used 4-chlorobenzonitrile and observed a colour change from green to pink and a pink precipitate formed, which was isolated by decantation of the supernatant and washing of the residue with a small amount of n-hexane to result in an 80% yield (Scheme 5, bottom). The aromatic protons are found in the 1H NMR spectrum at δ1H = 6.61–7.11 ppm. Additionally, crystals suitable for SC-XRD analysis were obtained from a saturated benzene-d6 solution by slow evaporation of the solvent. The molecular structure is shown in Fig. 7. The insertion of 4-chlorobenzonitrile resulted in the formation of a new elongated C6–C16 single bond (1.5695(14) Å). With 1.2709(13) Å, the former triple bond of the nitrile now lies in the range of a shortened C(sp2)N double bond (1.28 Å).20 The resulting new Ti1–N1 bond is 1.9861(9) Å, which is in the range of a shortened single bond. The Ti1–N2 single bond has been slightly elongated from 2.031(3) Å to 2.0461(9) Å in the process.
Footnote |
† Electronic supplementary information (ESI) available. CCDC 2402660–2402670 and 2404156. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4dt03261f |
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