Communication Cite This: Organometallics XXXX, XXX, XXX−XXX
1,2-CH Bond Activation of Pyridine across a Transient Titanium Alkylidene Radical and Re-Formation of the TiCHtBu Moiety Takashi Kurogi,† Matthias E. Miehlich,‡ Dominik Halter,‡ and Daniel J. Mindiola*,† †
Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States Inorganic Chemistry, Department of Chemistry and Pharmacy, Friedrich-Alexander University Erlangen-Nürnberg, 91058 Erlangen, Germany
‡
S Supporting Information *
ABSTRACT: Reduction of the titanium alkylidene [(PNP)Ti CHtBu(OTf)] (PNP− = N[2-PiPr2-4-methylphenyl]2−) with KC8 in the presence of pyridine results in formation of the transient titanium(III) alkylidene radical [(PNP)TiCHtBu)] (A) or the adduct [(PNP)TiCHtBu)(NC5H5)] (B), which activates the C−H bond of pyridine to form the titanium(III) pyridyl alkyl complex [(PNP)Ti(CH2tBu)(η2-NC5H4)] (1) in 64% yield as brown microcrystals. Lowtemperature X-band EPR spectroscopy and single-crystal X-ray diffraction studies confirm the identity of 1 as a d1 metal-centric radical with superhyperfine coupling to one nitrogen atom and having a side-on pyridyl moiety, which results in formation of the two isomeric forms 1a,b. Oxidation of 1 with [FeCp*2][OTf] cleanly promotes α-hydrogen abstraction to re-form [(PNP)TiCHtBu(OTf)] with concurrent elimination of pyridine and FeCp*2. Re-formation of the alkylidene moiety most likely stems from an intermediate such as [(PNP)Ti(CH2tBu)(η2-NC5H4)(OTf)] (C).
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d1 alkylidene radical [(PNP)TiCHtBu] (A) or its pyridine adduct [(PNP)TiCHtBu)(NC5H5)] (B). A cyclic voltammogram of [(PNP)TiCHtBu(OTf)] in [NnBu4][OTf]/1,2-C6H4F2 showed two irreversible cathodic waves at Epc = −2.62 and −2.92 V (versus the FeCp2+/0 couple referenced at 0.0 V).11 Accordingly, chemical reduction of the titanium alkylidene triflate complex [(PNP)TiCHtBu(OTf)] with 1 equiv of KC8 in a mixture of benzene and pyridine resulted in an immediate color change from red to dark brown. After workup, and crystallization from pentane at −35 °C, complex [(PNP)Ti(CH2tBu)(η2-NC5H4)] (1) was isolated as a brown crystalline material in 64% yield (Scheme 1). The 1H NMR spectrum of a crystalline sample of 1 reveals a C1-symmetric system, given the presence of approximately 20 broad resonances in the −2.69 to 13.60 ppm range,11 in accord with a paramagnetic complex. A solid-state structure of a single crystal of 1 gave some surprises (Figure 1). Two isomers in a 78:22 occupancy ratio are observed, with the pyridyl nitrogen being oriented in (isomer 1a) or out (isomer 1b) of the groove defined by the Cneopentyl−Ti−Cpyridyl angle (Figure 1).11 Despite this, the Ti1−C32 distance of 2.1556(12) Å for the neopentyl group is in the expected range for Ti(III)−alkyl bonds,2b,9,12 whereas the Ti1−N2A and Ti1−C27A (isomer 1a) as well as Ti1−N2B and Ti1−C27B (isomer 1b) distances are in the range for the few examples of titanium(IV) pyridyl
ntramolecular C−H bond activation by a metal−carbon multiple bond was first documented by Hessen and coworkers with the transient titanium complex [Cp2TiCHtBu] adding benzene to form the unstable alkyl phenyl species [Cp2Ti(CH2tBu)(C6H5)].1 Since then, other seminal examples encompassing Ti,2 V,3 Cr,4 Mo,5 and W5 possessing metal− carbon multiple bonds have been shown to intermolecularly activate C−H bonds in arenes and alkanes among many other heterosubstituted hydrocarbons. In fact, some of these systems can even engage in CH4 activation,2g,6 complementing the original work by Wolczanski and Cummins with the transient imido intermediate [(tBu3SiNH)2TiNSitBu3].7 We have been exploring metal−carbon multiple bonds containing radicals in systems such as [(Menacnac)VCHtBu(THF)][BPh 4 ] ( M e − nacnac = [ArNC(CH 3 )] 2 CH − , Ar = 2,6-iPr2C6H3)8 or the transient [(PNP)VCHtBu]3a (PNP− = N[2-PiPr2-4-methylphenyl]2−) and their propensity to engage in C−H activation and small-molecule activation, including redox reactions. We inquired if a d1 radical species such as [(PNP)TiCHtBu] could be isolated or generated and whether this system displayed redox reactivity at the metal and/or alkylidene moiety. Prior results using the sterically crowded β-diketiminate ligand on titanium, as in the case of [( tBu nacnac)TiCH t Bu(OTf)] ( tBu nacnac − = [ArNC(tBu)]2CH−),9 resulted in C−H activation of the iPr methyl group of the tBu nacnac ligand. 10 Herein, we report intermolecular C−H bond activation reactivity of the transient © XXXX American Chemical Society
Received: October 16, 2017
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DOI: 10.1021/acs.organomet.7b00770 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics Scheme 1. Synthetic Route to the Mononuclear Titanium(III) Pyridyl Complex Isomers 1a,b via Reduction of [(PNP)TiCHtBu(OTf)] by KC8 with Pyridine and Subsequent Oxidation To Release Pyridine and Re-Form the Alkylidene Ligand
Figure 2. CW X-band EPR spectrum of 1 recorded in frozen toluene solution (ca. 1 mM) at 8 K (black line) along with a simulated spectrum (red line). Conditions: frequency, 8.957 GHz; MW, 1.0 mT; MF, 100 kHz; MW power, 1.0 mW.
C−X (X = H, F) activation of pyridine derivatives by [(PNP)TiCtBu], we have suggested that there is an entropic penalty at the transition state since pyridine must dissociate in order to perfectly place the C−H bond across the TiCHtBu ligand.14 The presence of the crystallographic isomers 1a,b likely arises from demetalation of the pyridyl nitrogen and rotation about the Ti1−C27A or Ti1−C27B bond (Figure 1). Surprisingly, preforming the reduction of [(PNP)TiCHtBu(OTf)] with KC8 in benzene resulted in no reaction, thus suggesting that pyridine most likely forms the radical K+(pyridine•−),15 which then engages in reduction of the titanium(IV) alkylidene precursor. To probe the reactivity of the pyridyl moiety in 1, we oxidized this species with [FeCp*2][OTf] (Cp* = C5Me5) in C6D6 over 15 min. Examination of the reaction mixture revealed clean formation of FeCp*2 (1 equiv), pyridine (1 equiv), and [(PNP)TiCHtBu(OTf)] (Scheme 1),11 therefore suggesting that oxidation promotes α-hydrogen abstraction to extrude the pyridine and re-form the [TiCHtBu] linkage, presumably via the coordinatively saturated pyridyl species [(PNP)Ti(CH2tBu)(η2-NC5H4)(OTf)] (C) (Scheme 1). In conclusion, we have shown how a transient titanium(III) alkylidene species A activates, intermolecularly, the C−H of pyridine to form the pyridyl species 1, which exists as a mixture of isomers based on X-ray diffraction analysis. Whereas X-band EPR spectroscopy at low temperature shows rhombic symmetry and a metal centered radical, the use of an outersphere one-electron oxidant ([FeCp*2][OTf]), reforms the alkylidene moiety in [(PNP)TiCHtBu(OTf)].
Figure 1. Solid-state structures of the pyridyl isomers 1a,b displaying thermal ellipsoids at the 50% probability level. H atoms and cocrystallized solvents have been omitted for clarity. Selected distances (Å) and angles (deg) for both isomers: Ti1−C32, 2.1556(12); Ti1−N1, 2.0961(10); Ti1−P1, 2.6114(3); Ti1−P2, 2.6016(4); P1−Ti1−P2, 148.856(12). Selected distances (Å) and angles (deg) for 1a: Ti1−C27A, 2.1278(12); Ti1−N2A, 2.1449(11); N1−Ti1−N2A, 145.36(4); C32−Ti1−N2A, 102.19(5); C32−Ti1− C27A, 138.56(5); C27A−Ti1−N2A, 36.93(5). Selected distances (Å) and angles (deg) for 1b: Ti1−C27B, 2.1449(11); Ti1−N2B, 2.1278(12); N1−Ti1−N2B, 108.63(4); C32−Ti1−N2B, 138.56(5); C32−Ti1−C27B, 102.19(5); C27B−Ti1−N2B, 36.93(5).
complexes13 and are akin to those in the V(III)-substituted pyridyl alkyl derivatives [(PNP)V(CH2tBu)(η2-NC5RH3)] (R = CH3, Ph),3a for which no rotational isomers were crystallographically observed. Given the S = 1/2 nature of 1 by the Evans method (μeff = 1.77 μB, 300 K), we collected X-band EPR spectra in toluene at 8 K (Figure 2). The EPR spectrum clearly shows rhombic symmetry with g1 = 1.987 (Wx = 1.5 mT), g2 = 1.964 (Wy = 1.1 mT), and g3 = 1.932 (Wz = 2.0 mT) along with a superhyperfine coupling to one 14N nucleus (I = 1, 99.6%) stemming from either the pyridyl or PNP ligand (A1 = 58.8 MHz, A2 = 64.5 MHz, A3 = 0 MHz). The g values are consistent with the unpaired electron in 1 being metal centric. We propose the formation of 1 to occur via one-electron reduction of [(PNP)TiCHtBu(OTf)] to form the d1 radical [(PNP)TiCHtBu] (A), which then binds pyridine to form the adduct [(PNP)TiCHtBu(py)] (B). We argue against [(PNP)TiCHtBu(OTf)] binding pyridine prior to reduction since we do not observe any evidence for coordination of the Lewis base in the absence of reductant. Once reduced, 1,2-CH bond addition of pyridine across TiCHtBu would result in formation of 1. On the basis of prior theoretical work involving
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00770. Crystallographic data and tables, experimental details, and spectroscopic data (PDF) Accession Codes
CCDC 1578990 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge B
DOI: 10.1021/acs.organomet.7b00770 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics
(9) Basuli, D.; Bailey, B. C.; Watson, L. A.; Tomaszewski, J.; Huffman, J. C.; Mindiola, D. J. Organometallics 2005, 24, 1886−1906. (10) Basuli, D.; Bailey, B. C.; Huffman, J. C.; Mindiola, D. J. Organometallics 2005, 24, 3321−3334. (11) See the Supporting Information. (12) Selected examples of Ti(III) alkyl complexes: (a) Luinstra, G. A.; Cate, L. C. T.; Heeres, H. J.; Pattiasina, J. W.; Meetsma, A.; Teuben, J. H. Organometallics 1991, 10, 3227−3237. (b) Hagadorn, J. R.; Arnold, J. J. Am. Chem. Soc. 1996, 118, 893−894. (c) Johnson, A. R.; Davis, W. M.; Cummins, C. C. Organometallics 1996, 15, 3825− 3835. (d) Love, J. B.; Clark, H. C. S.; Cloke, F. G. N.; Green, J. C.; Hitchcock, P. B. J. Am. Chem. Soc. 1999, 121, 6843−6849. (e) Bailey, B. C.; Bauli, F.; Huffman, J. C.; Mindiola, D. J. Organometallics 2006, 25, 3963−3968. (f) Basuli, F.; Adhikari, D.; Huffman, J. C.; Mindiola, D. J. J. Organomet. Chem. 2007, 692, 3115−3120. (g) Trunkely, E. F.; Epshteyn, A.; Zavalij, P. Y.; Sita, L. R. Organometallics 2010, 29, 6587−6593. (h) Grant, L. N.; Ahn, S.; Manor, B. C.; Baik, M.-H.; Mindiola, D. J. Chem. Commun. 2017, 53, 3415−3417. (13) (a) Hao, L.; Harrod, J. F.; Lebuis, A.-M.; Mu, Y.; Shu, R.; Samuel, E.; Woo, H.-G. Angew. Chem., Int. Ed. 1998, 37, 3126−3129. (b) Piglosiewicz, I. M.; Kraft, S.; Beckhaus, R.; Haase, D.; Saak, W. Eur. J. Inorg. Chem. 2005, 2005, 938−945. (14) Fan, H.; Fout, A. R.; Bailey, B. C.; Pink, M.; Baik, M.-H.; Mindiola, D. J. Dalton Trans. 2013, 42, 4163−4174. (15) Kalyanaraman, V.; Rao, C. N. R.; George, M. V. J. Chem. Soc. B 1971, 2406−2409.
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
*E-mail for D.J.M.:
[email protected]. ORCID
Takashi Kurogi: 0000-0002-8804-757X Dominik Halter: 0000-0003-0733-8955 Daniel J. Mindiola: 0000-0001-8205-7868 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the U.S. National Science Foundation (CHE0848248 and CHE-1152123) and the University of Pennsylvania for financial support of this research. T.K. acknowledges financial support from the JSPS (Japan Society for the Promotion of Science) for postdoctoral fellowships. The authors also thank Dr. Patrick J. Carroll for discussion of crystallographic data at the University of Pennsylvania. Prof. Karsten Meyer and Prof. Michael Zdilla are also thanked for insightful discussions and use of their EPR instrument for data collection.
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DOI: 10.1021/acs.organomet.7b00770 Organometallics XXXX, XXX, XXX−XXX
