Article pubs.acs.org/Organometallics
Pyridonate-Supported Titanium(III). Benzylamine as an Easy-To-Use Reductant Eugene Chong,† Wei Xue,† Tim Storr,‡ Pierre Kennepohl,† and Laurel L. Schafer*,† †
Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia, Canada V6T 1Z1 Department of Chemistry, Simon Fraser University, 8888 University Drive, Burnaby, British Columbia, Canada V5A 1S6
‡
S Supporting Information *
ABSTRACT: The reaction of bis(3-phenyl-2-pyridonate)Ti(NMe2)2 with excess benzylamine leads to an unexpected reduction of the metal center from Ti(IV) to Ti(III). The reduced titanium species was isolated and revealed as tris(3phenyl-2-pyridonate)Ti(NH2Bn)2. Ammonia and N-benzyl-1phenylmethanimine are released as byproducts of the reaction, thereby confirming benzylamine as a mild reductant. This new pyridonate-supported titanium(III) complex has been fully characterized, and experimental data and theoretical calculations confirm a d1 metal center with no ligand-based radical character. This Ti(III) complex does not react with aminoalkenes, suggesting that radical species are not viable intermediates for the hydroaminoalkylation or hydroamination reaction.
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INTRODUCTION Reduced titanium species are versatile reagents and catalysts for mediating various bond-forming reactions.1−14 Many of these previous reports involve the use of titanocene complexes for which titanium(III) complexes, such as Cp2TiCl, can be generated from Cp2TiCl2 by the addition of stoichiometric amounts of reducing metals.15−18 Indeed, strong reducing agents, including alkali-metal and alkaline-earth-metal amalgams, naphthalenides, and metal−alkyl reagents are typically used for such reductions, which could lead to undesirable overreduced metal impurities or salt byproducts.19 Alternative methods have been reported for the generation of titanium(III) species from titanium(IV) precursors under salt-free conditions. Such approaches include the thermal homolysis of select Ti− O20 or Ti−C bonds21 or the use of organosilicon reducing agents.22 Recent reports have also shown that delocalized radicals formed by the addition of alkyl radicals to arenes can reduce Ti(IV) to Ti(III).23,24 To our knowledge, the use of a simple amine has not been reported for the clean reduction of Ti(IV) to Ti(III) species, although the use of tertiary amines, such as NEt3, are known reductants for mid to late transition metals.19,25−29 In this work, we report the reduction of Ti(IV) pyridonate complexes to pyridonate-supported Ti(III) compounds using benzylamine as the reducing agent. Previous studies in our group have focused on bis(N,O)ligated titanium systems30−34 for applications in the catalytic synthesis of amines using hydroamination35,36 and hydroaminoalkylation37,38 reactions. Recent reactivity investigations of group 4 pyridonate complexes have revealed unique trends that distinguish these systems from reported amidate and ureate hydroamination precatalysts.32,34 Specifically, group 4 pyridonate complexes (e.g., 1) can be used to selectively promote hydroaminoalkylation over hydroamination (eq 1).34 © 2015 American Chemical Society
This difference in reactivity results from an unexpected change in mechanism when using amidate vs pyridonate catalysts.32,34 We have proposed pyridonate dinuclear species as key intermediates for promoting hydroaminoalkylation preferentially.32,34 Alternatively, such mechanistic variability could result from accessing reduced metal species that would promote a different reaction path. Benzylamine is a known challenging substrate for hydroamination, which is prone to promoting catalyst decomposition and often gives mixtures of products in titanium-catalyzed hydroamination reactions.39−42 The nature and the possible identity of such decomposed species have not been reported thus far. Herein, we provide insight for such species using our N,O-ligated titanium systems with pyridonates as the supporting ligand in stoichiometric reaction studies with benzylamine. We report the synthesis and characterization of a novel tris(2-pyridonate)titanium(III) complex using benzylamine as an unexpected reductant. This newly obtained titanium(III) species has also been evaluated for its viability in aminoalkene hydroamination/hydroaminoalkylation. Received: June 3, 2015 Published: September 28, 2015 4941
DOI: 10.1021/acs.organomet.5b00469 Organometallics 2015, 34, 4941−4945
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RESULTS AND DISCUSSION The reaction of bis(2-pyridonate)-supported titanium complex 134 (Scheme 1) with benzylamine gave a deeply colored
sign of bond elongation that would be consistent with a ligandcentered radical (Table S1 in the Supporting Information).45 Notably, complex 2 is NMR silent due to its paramagnetic character. Given that the tris(3-phenyl-2-pyridonate)titanium(III) complex 2 was isolated from the reaction using the bis(3phenyl-2-pyridonate)titanium(IV) precursor 1 (Scheme 1), we speculated that ligand redistribution of complex 1 preceded the reduction of the metal center from Ti(IV) to Ti(III). The hemilabile nature of the pyridonate ligands may facilitate such ligand redistribution. Thus, tris(3-phenyl-2-pyridonate)Ti(NMe2) (3) was first prepared from the protonolysis reaction of Ti(NMe2)4 and 3 equiv of 3-phenyl-2-pyridone (Scheme 2).
Scheme 1. Reaction of 1 with Benzylamine To Access a Ti(III) Complex
solution that has a 1H NMR spectrum with broad signals of low intensity, suggesting the formation of paramagnetic species. Fortuitously, small quantities of brown crystals suitable for Xray crystallographic analysis were isolated from recrystallization in a toluene/pentane mixture. The identity of the isolated compound was revealed to be the tris(3-phenyl-2-pyidonate)titanium(III) complex 2 with two neutrally coordinated benzylamines (Figure 1).
Scheme 2. Synthesis of Tris(3-phenyl-2pyridonate)Ti(NMe2)
In the solid state, complex 3 exhibits a Cs-symmetric structure with pseudo-pentagonal-bipyramidal coordination geometry (Figure 2). Similar to the case for 2, the pyridonate ligands
Figure 2. ORTEP representation of the solid-state molecular structure of 3 plotted with 50% probability ellipsoids for non-hydrogen atoms. The THF solvent molecule is omitted for clarity. Selected bond lengths (Å) and angles (deg): Ti1−O1, 2.0889(11); Ti1−O2, 2.0572(10); Ti1−O3, 2.0432(9); Ti1−N1, 2.1864(12); Ti1−N2, 2.2101(12); Ti1−N3, 2.2053(12); Ti1−N4, 1.8799(12); O1−Ti1− N1, 62.11(4); O2−Ti1−N2, 61.58(4); O3−Ti1−N3, 61.87(4); N4− Ti1−O1, 94.54(4); N4−Ti1−N1, 156.43(4).
Figure 1. ORTEP representation of the solid-state molecular structure of 2 plotted with 50% probability ellipsoids for non-hydrogen atoms. Selected bond lengths (Å) and angles (deg): Ti1−O1, 1.9790(15); Ti1−O2, 2.1156(15); Ti1−O3, 2.1091(15); Ti1−N2, 2.1998(18); Ti1−N3, 2.2565(18); Ti1−N4, 2.204(2); Ti1−N5, 2.2128(19); N4− C34, 1.481(3); N5−C41, 1.485(3); O2−Ti1−N2, 60.98(6); O3− Ti1−N3, 60.59(6); N4−Ti1−O1, 94.11(7); N4−Ti1−N5 176.20(8); Ti1−O1−C1, 136.56(14); C34−N4−Ti1, 124.10(14); C41−N5−Ti1, 114.83(14).
in 3 bind to titanium in an asymmetric κ2-N,O-chelating motif, with the oxygen donor (Ti−Oavg 2.0631(10) Å) binding more closely than the nitrogen donor (Ti−Navg 2.2006(12) Å) to the metal center. To determine if 3 is a suitable precursor to the reduced complex 2, it was reacted with 3 equiv of benzylamine (Scheme 3). Using the alternative titanium(IV) precursor 3, the
Complex 2 adopts a C1-symmetric pentagonal-bipyramidal coordination geometry with two pyridonate ligands exhibiting a κ2-N,O-chelating motif with the oxygen donor (Ti−Oavg 2.1124(15) Å) binding closer to the titanium center than the nitrogen donor (Ti−Navg 2.2282(18) Å) and the third pyridonate with a κ1-O binding motif (with a much shorter Ti−O bond, 1.9790(15) Å). This variable binding mode of the pyridonate ligands illustrates the hemilabile nature of these ligands. The coordination of two neutral benzylamine donors in the axial positions is evidenced by a long Ti−Navg bond length (2.208(2) Å). An examination of ligand bond lengths shows no
Scheme 3. Reduction of 3 to 2 with Benzylamine
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DOI: 10.1021/acs.organomet.5b00469 Organometallics 2015, 34, 4941−4945
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Organometallics titanium(III) complex 2 can be isolated in reproducibly high yields (83−93%). Consistent with the initial isolation conditions, complex 2 can be recrystallized from a toluene/ pentane mixture, and X-ray crystallographic analysis confirmed the formation of product 2 (Figure 1). Thus, the reliable synthesis of the titanium(III) complex 2 from the tris(2pyridonate)titanium complex 3 suggests that bis(2-pyridonate) titanium complex 1 is susceptible to ligand redistribution prior to reduction of the metal center with easily handled benzylamine. The paramagnetic character of 2 has been further confirmed by EPR spectroscopy (Figure 3). The spectrum is consistent
Figure 4. Spin density plot of 2.
quenched reaction mixture by GC-MS analysis shows the presence of three compounds: benzylamine, 3-phenyl-2pyridone, and a signal with m/z 195 corresponding to Nbenzyl-1-phenylmethanimine.46 The broad signals observed in the 1H NMR spectrum of the reaction mixture are due to this imine byproduct, while complex 2 is NMR silent. The formation of N-benzyl-1-phenylmethanimine is accompanied by the release of ammonia. This byproduct can be trapped as the NH4Cl salt47 and was identified by 1H NMR spectroscopy in d6-DMSO, which revealed a diagnostic triplet of 14NH4Cl centered at δ 7.12 with a coupling constant of 51 Hz (1J14N−1H).45 Thus, the release of N-benzyl-1-phenylmethanimine and ammonia as the only observed byproducts from the reaction rationalizes simple benzylamine as a stoichiometric reductant in this reaction. With the isolation of 2 in high yield and identification of Nbenzyl-1-phenylmethanimine and ammonia as the only byproducts of the reaction, we propose the following process for benzylamine-promoted reduction in Scheme 4. The pyridonatesupported Ti(IV) complex 3 undergoes an amido exchange with 1 equiv of benzylamine to give Ti(IV) intermediate A with the release of dimethylamine. On the basis of the evidence that titanium complexes are capable of α-C−H activation of amines
Figure 3. EPR spectrum (toluene, 77 K) of 2 overlaid with a simulation.
with an axially elongated octahedral Ti(III) complex with gx = 1.948, gy = 1.958, and gz = 1.990. L/G = 0. Hyperfine splittings due to the naturally occurring 47Ti and 49Ti isotopes, which constitutes 12.83% natural abundance and have none-zero nuclear spins, can also be seen in the spectrum. More importantly, hyperfine interactions that could arise from radical behavior in the 2-pyridonates, should a ligand-based radical be formed, are not present. These observations suggest localization of the unpaired electron on the titanium metal center. Density functional theory (DFT) calculations have been used to further investigate the geometric and electronic structure of complex 2.45 The calculations reproduce the experimental coordination sphere bond lengths to within ±0.06 Å (Table S3 in the Supporting Information). The calculation also supports the experimental assignment of a d1 Ti(III) metal center, with the spin density based at the metal center (Figure 4). Thus, the predicted electronic structure is in good agreement with the EPR analysis, with insignificant 2-pyridonate ligand radical behavior. Careful analysis of the reaction mixture in d6-benzene by 1H NMR spectroscopy and identification of organic byproducts by GC-MS clarifies how benzylamine functions as a reductant (eq 2). Upon reaction completion the 1H NMR spectrum has
Scheme 4. Postulated Process for Reduction of PyridonateSupported Ti(IV) to Ti(III) with Benzylamine
broad signals of weak intensity and signals for 3 and benzylamine are no longer present, although a new singlet appears at δ 4.60 in the reaction mixture.45 Examination of the 4943
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diffractometer at the Department of Chemistry, University of British Columbia, by Jacky C.-H. Yim or Scott Ryken. EPR Data Acquisition and Process. EPR data were collected on an Elexsys E500 series continuous wave EPR spectrometer (Bruker, Billerica, MA, USA). The spectrometer was operated at a frequency of 9.40 GHZ (X-band) at 77 K, 100 kHz field modulation, 1 G modulation amplitude, and 0.64 mW microwave power. Each spectrum recorded was the averaged result over five scans. Frequency calibration53 was independently verified using 2,2-diphenyl-1-picrylhydrazyl (DPPH, g = 2.0036) (Sigma-Aldrich, St Louis, MO, USA) as an external standard. Spectra were simulated using EasySpin. Synthesis of Tris(3-phenyl-2-pyridonate)Ti(NMe2) (3). A Teflon-capped 20 mL vial, equipped with a magnetic stir bar, was charged with 3-phenyl-2-pyridone (0.257 g, 1.50 mmol) and ∼3 mL of benzene. In this vial, Ti(NMe2)4 (0.112 g, 0.500 mmol) dissolved in ∼2 mL of benzene was quantitatively added. The mixture was stirred at room temperature for 4 h, forming a deep red-brown solution, and the solvent was removed in vacuo. The product was dissolved in hot toluene (∼10 mL), and storing at −35 °C overnight afforded a tan brown microcrystalline solid (0.275 g, 91%). Single crystals suitable for X-ray crystallography were obtained from a concentrated solution of the complex in THF left at room temperature over a couple of days. 1 H NMR (400 MHz, C6D6): δ 8.01 (d, 6H, 3JHH = 7.2 Hz, Ph-H), 7.77 (dd, 3H, 3JHH = 5.2, 4JHH = 1.8 Hz, Ar-H), 7.32 (dd, 3H, 3JHH = 7.4, 4 JHH = 1.8 Hz, Ar-H), 7.26 (t, 6H, 3JHH = 7.7 Hz, Ph-H), 7.13 (t, 3H, 3 JHH = 7.5 Hz, Ph-H), 6.06 (dd, 3H, 3JHH = 7.4, 5.2 Hz, Ar-H), 3.61 (s, 6H, − N(CH3)2). 13C NMR (100 MHz, C6D6): δ 171.5, 141.8, 139.4, 136.8, 129.1, 129.0, 127.9, 122.1, 113.2, 49.0. MS (EI): m/z 602 (M+), 558 (M+ − NMe2). Anal. Calcd for C35H30N4O3Ti: C, 69.77; H, 5.02; N, 9.30. Found: C, 70.12; H, 5.14; N, 8.97. Synthesis of Tris(3-phenyl-2-pyridonate)Ti(NH2Bn)2 (2). In a small vial, complex 3 (30.1 mg, 0.0500 mmol) and benzylamine (16.4 μL, 0.150 mmol) were mixed in 1 mL of benzene. The resulting brown solution was transferred to a J. Young NMR tube and heated to 65 °C overnight, which gave a deep brown-red solution. The solution was filtered through Celite and concentrated in vacuo. The crude residue was recrystallized from a solution of the complex in toluene (∼2 mL) at −35 °C, layered with pentane (∼4 mL), overnight to give the product as a brown microcrystalline solid (36 mg, 93%). The reaction was repeated twice using the same procedure at a larger scale over a reaction time of 2 h, using 3 (60.3 mg, 0.100 mmol) and benzylamine (32.8 μL, 0.300 mmol), to afford 64 mg (83%) and 66 mg (85%) of the product. Single crystals suitable for X-ray crystallography were obtained from slow diffusion of pentane into a solution of the complex in toluene overnight. MS (EI): m/z 558 (M+ − 2BnNH2), 388 (M+ − 2BnNH2 − L), where L = 3-phenyl-2-pyridonate. Anal. Calcd for C47H42N5O3Ti: C, 73.05; H, 5.48; N, 9.06. Found: C, 72.70; H, 5.82; N, 9.37.
to generate structurally characterized metallaziridine species,32,48 and the known reactivity of group 4 pyridonates32,34 for hydroaminoalkylation reactions, we propose that the benzylic C−H bond is cleaved by β-hydrogen abstraction by a pyridonate ligand to form the titanaziridine intermediate B. Group 4 metallaziridine species are known to have two resonance structures,49,50 in which the other resonance form of Ti(IV) species B is the formally Ti(II) species C that has the metal bound by a η2-CN imine linkage. Considering the literature precedent that the η2-imine can be displaced from the metal center of a titanaziridine50 and that comproportionation of Ti(II) and Ti(IV) can quantitatively yield Ti(III) in the synthesis of (Cp2TiIIICl)2 from Cp2TiII(η2-C2(SiMe3)2) and Cp2 Ti IV Cl 2 ,51 we postulate that A and C undergo a comproportionation reaction to yield D. This Ti(III) species D is then trapped by 2 equiv of benzylamine to give the isolable complex 2. The 0.5 equiv of the released benzylamine and phenylmethanimine then undergo a condensation reaction to afford N-benzyl-1-phenylmethanimine and ammonia as byproducts of the reaction. With the reduced Ti complex 2 in hand we tested this complex as a potential intramolecular hydroaminoalkylation or hydroamination precatalyst in eq 1. Attempts to convert 2,2diphenylaminoalkene with 20 mol % 2 in toluene solvent to either hydroamination or hydroaminoalkylation cyclized products resulted in no product formation. These results suggest that Ti(III) species supported by pyridonate ligands are not viable intermediates for these catalytic processes. In summary, we have demonstrated that a tris(2-pyridonate)titanium(IV) complex can be cleanly reduced to titanium(III) using benzylamine as an easy-to-use reducing agent. Such reactivity may also rationalize why some amine substrates are challenging substrates in catalytic reactions.39−42 The lack of productive hydroamination or hydroaminoalkylation reactivity using 2 suggests that such Ti(III) species do not promote desirable reactivity but rather deleterious catalyst decomposition pathways. Work exploring the generality of benzylamine as a useful reductant and the reactivity of such readily accessed reduced Ti(III) species is underway.
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EXPERIMENTAL SECTION
General Considerations. All reactions were conducted in ovendried glassware under an atmosphere of dry dinitrogen using standard Schlenk line and glovebox techniques. Benzene, toluene, and pentane were passed over activated alumina columns and stored over activated 4 Å molecular sieves prior to use. d6-Benzene and d8-toluene were dried over 4 Å molecular sieves, degassed by three freeze−pump−thaw cycles, and stored in Teflon-sealed Schlenk flasks prior to use. Ti(NMe2)4 was purchased from Sigma-Aldrich and used as received. Benzylamine was distilled over CaH2 and degassed by three freeze− pump−thaw cycles prior to use. Complex 1 and 3-phenyl-2-pyridone were synthesized according to literature procedures.34 N-Benzyl-1phenylmethanimine was synthesized from benzylamine and benzaldehyde using a literature procedure,52 and 1H NMR spectral data match reported literature values.46 1H and 13C NMR spectra were recorded on a Bruker 400 MHz Avance spectrometer at ambient temperature, and chemical shifts are given relative to the corresponding residual protio solvent. Mass spectra were recorded on a Kratos MS-50 spectrometer using an electron impact (70 eV) source. Elemental analyses were recorded on a Carlo Erba EA 1108 Elemental Analyzer. GC/MS analyses were conducted on an Agilent 7890A GC equipped with a 5975C inert XL CI mass detector using methane as the chemical ionization agent. Single-crystal X-ray structure determinations were performed on a Bruker X8 APEX II or Bruker APEX DUO
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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.5b00469. Crystallographic data for 2 and 3 (CIF) NMR spectra and calculations (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail for L.L.S.:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the NSERC for financial support of this work and a postgraduate scholarship to E.C. We thank Jacky C.-H. Yim and 4944
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Coupling Reactions and More; de Meijere, A., Bräse, S., Oestreich, M., Eds.; Wiley-VCH: Weinheim, Germany, 2014; pp 1135−1258. (37) Roesky, P. W. Angew. Chem., Int. Ed. 2009, 48, 4892. (38) Chong, E.; Garcia, P.; Schafer, L. L. Synthesis 2014, 46, 2884. (39) Doye, S. Synlett 2004, 1653. (40) Haak, E.; Bytschkov, I.; Doye, S. Angew. Chem., Int. Ed. 1999, 38, 3389. (41) Shi, Y.; Ciszewski, J. T.; Odom, A. L. Organometallics 2001, 20, 3967. (42) Heutling, A.; Pohlki, F.; Doye, S. Chem. - Eur. J. 2004, 10, 3059. (43) Zhang, Z.; Schafer, L. L. Org. Lett. 2003, 5, 4733. (44) Yim, J. C.-H.; Bexrud, J. A.; Ayinla, R. O.; Leitch, D. C.; Schafer, L. L. J. Org. Chem. 2014, 79, 2015. (45) See the Supporting Information. (46) Liu, L.; Zhang, S.; Fu, X.; Yan, C.-H. Chem. Commun. 2011, 47, 10148. (47) Anderson, J. S.; Rittle, J.; Peters, J. C. Nature 2013, 501, 84. (48) Manßen, M.; Lauterbach, N.; Dörfler, J.; Schmidtmann, M.; Saak, W.; Doye, S.; Beckhaus, R. Angew. Chem., Int. Ed. 2015, 54, 4383. (49) Cummings, S. A.; Tunge, J. A.; Norton, J. R. Top. Organomet. Chem. 2005, 10, 1. (50) Durfee, L. D.; Hill, J. E.; Fanwick, P. E.; Rothwell, I. P. Organometallics 1990, 9, 75. (51) Arp, H.; Zirngast, M.; Marschner, C.; Baumgartner, J.; Rasmussen, K.; Zark, P.; Müller, T. Organometallics 2012, 31, 4309. (52) Moonen, K.; Stevens, C. V. Synthesis 2005, 2005, 3603. (53) Krzystek, J.; Sienkiewicz, A.; Pardi, L.; Brunel, L. C. J. Magn. Reson. 1997, 125, 207.
Scott Ryken for assistance with X-ray crystallography. The authors thank Westgrid for access to computational resources. This work was undertaken, in part, thanks to funding from the Canada Research Chairs program.
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