Iridium(I) carbonyl complex [Ir(2,6-(PtBu2CH2)2C6H3)(CO)] undergoes reversible C–H bond activation of benzene and a series of fluorobenzenes on UV irradiation. Exclusive ortho-selectivity is observed in reactions of fluorobenzene and 1,2-difluorobenzene

Chaplin, Adrian B.

Emerson-King, Jack

Habershon, Scott

Hauser, Simone A.

UV irradiation of [Ir(2,6-(PtBu2CH2)2C6H3)(CO)] generates the transient and reactive 14 VE Ir(i) fragment {Ir(2,6-(PtBu2CH2)2C6H3)}, which subsequently undergoes C–H bond activation of benzene and fluorobenzenes.</p

Simone A. Hauser

Jack Emerson-King

Scott Habershon

Adrian B. Chaplin

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Chemical Communications

UV-light promoted C–H bond activation of benzene and fluorobenzenes by an iridium(<scp>i</scp>) pincer complex

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     warwick.ac.uk/lib-publications      Original citation: Hauser, Simone A., Emerson-King, Jack, Habershon, Scott and Chaplin, Adrian B.. (2017) UV-light promoted C–H bond activation of benzene and fluorobenzenes by an iridium(i) pincer complex. Chemical Communications, 53 (26). pp. 3634-3636. Permanent WRAP URL: http://wrap.warwick.ac.uk/88074                                 Copyright and reuse: The Warwick Research Archive Portal (WRAP) makes this work of researchers of the University of Warwick available open access under the following conditions.  This article is made available under the Creative Commons Attribution 3.0 (CC BY 3.0) license and may be reused according to the conditions of the license.  For more details see: http://creativecommons.org/licenses/by/3.0/    A note on versions: The version presented in WRAP is the published version, or, version of record, and may be cited as it appears here.  For more information, please contact the WRAP Team at: wrap@warwick.ac.uk  3634 | Chem. Commun., 2017, 53, 3634--3636 This journal is©The Royal Society of Chemistry 2017Cite this:Chem. Commun., 2017,53, 3634UV-light promoted C–H bond activation ofbenzene and fluorobenzenes by an iridium(I)pincer complex†Simone A. Hauser, Jack Emerson-King, Scott Habershon and Adrian B. Chaplin*Iridium(I) carbonyl complex [Ir(2,6-(PtBu2CH2)2C6H3)(CO)] under-goes reversible C–H bond activation of benzene and a series offluorobenzenes on UV irradiation. Exclusive ortho-selectivity isobserved in reactions of fluorobenzene and 1,2-difluorobenzene.Epitomised by applications in the catalytic dehydrogenation ofalkanes, iridium complexes of phosphine-based pincer ligandsare widely recognised for their capacity to activate C–H bonds.1With fluoroaryls representing valuable synthons in organicchemistry,2,3 we have targeted use of these iridium compoundsfor carrying out selective C–H bond activation reactions ofpartially fluorinated benzenes (C6H6nFn, nr 3).3 The presenceof fluorine substituents results in significantly stronger C–Hbonds than benzene and, correspondingly, fluorobenzenesrepresent challenging substrates.3,4 Previous work by Milsteinand Ozerov employing neutral (PNP) and anionic (PNP*) pincerligands has highlighted the potential of iridium pincers,although under moderate temperature regimes (o100 1C) thesesystems showed poor regioselectivity in the activation of fluoro-benzene (Scheme 1).5 As C–H bond activation is thermodynami-cally favoured ortho to the fluorine substituents,4 indiscriminateand irreversible oxidative addition reactions of transient 14 VEIr(I) intermediates {Ir(PNP)}+/{Ir(PNP*)} are implicated.We postulated that use of anionic pincer ligands bearingcentral aryl donors could promote selective activation of fluoro-benzene substrates through increased reaction reversibilityimparted by the high trans-influence aryl donor.6,7 With a viewto testing this hypothesis we selected [Ir(PCP)(CO)] 1 (PCP =2,6-(PtBu2CH2)2C6H3)8 as a well-defined precursor for the lowcoordinate and formally 14 VE Ir(I) fragment {Ir(PCP)}, throughphotochemically promoted dissociation of the carbonyl ligand.In this way, subsequent products of C–H bond activation wouldbe trapped on re-coordination of the carbonyl ligand (in aclosed system). Initial experiments using benzene as the sub-strate supported this reasoning, with [Ir(PCP)(C6D5)D(CO)]2-d69 generated on irradiation of a 20 mM C6D6 solution of 1at RT using a 100 W Hg arc lamp (quartz J. Young’s NMR tube,Scheme 2). Following this reaction by periodic analysis using31P NMR spectroscopy, however, indicated that conversion of 1(d31P 82.0) to 2-d6 (d31P 52.3) plateaued at 62% after ca. 2 h totalirradiation. On the same timeframe, irradiation of indepen-dently synthesised 2 resulted in an equivalent reaction compo-sition. In contrast, both 1 and 2 are thermally stable onextended heating at 80 1C in C6D6 solution (8 h) and no isotopeexchange was observed for 2 (to 2-d6).10 Together these resultsindicate establishment of a photostationary mixture of 1 and2-d6, mediated through light induced carbonyl dissociationfrom both species.11To gain deeper understanding of the photolysis experiments, aseries of DFT and TD-DFT calculations were performed (see ESI† forfull details). In line with expectation, the computed free energy forcarbonyl dissociation to form {Ir(PCP)} is a significantly endergonicprocess (DG298K = +194.4 kJ mol1). While the subsequent C–Hbond activation of benzene is exothermic (DH = 15.3 kJ mol1),Scheme 1 C–H bond activation of fluorobenzene using Ir-pincers. PNP =2,6-(PtBu2CH2)2C5H3N, PNP* = (4-Me-2-(iPr2P)C6H3)2N, COE = cyclooctene,NBE = norbornene, NBA = norbornane.Department of Chemistry, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL,UK. E-mail: a.b.chaplin@warwick.ac.uk; Web: http://go.warwick.ac.uk/abchaplin,www.twitter.com/chaplinlab† Electronic supplementary information (ESI) available: Full experimental andcomputational details, NMR, IR and UV-vis spectra, TD-DFT analysis of 1 and 2,and optimised geometries in xyz format. CCDC 1520522–1520525. For ESI andcrystallographic data in CIF or other electronic format see DOI: 10.1039/c6cc09807jReceived 9th December 2016,Accepted 26th February 2017DOI: 10.1039/c6cc09807jrsc.li/chemcommChemCommCOMMUNICATIONOpen Access Article. Published on 17 March 2017. Downloaded on 03/05/2017 10:23:39.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineView Journal  | View IssueThis journal is©The Royal Society of Chemistry 2017 Chem. Commun., 2017, 53, 3634--3636 | 3635the Ir(III) product lies thermodynamically further uphill from 1(DG298K = +238.0 kJ mol1). Such energetics are characteristicof an unfavourable equilibrium reaction, although one thatwould be offset by the use of the substrate as the solvent.12Re-coordination of the carbonyl ligand counteracts the unfavour-able thermodynamics (DG298K = 95.0 kJ mol1), however, 2 isstill calculated to be +141.3 kJ mol1 higher in free energy than1 + C6H6. Together these results are consistent with the lack of anythermal reaction observed for either 1 or 2 in C6D6 and highlightthe important promoting role of UV-irradiation in the formationof 2 (and reformation of 1). In this context, analysis of 1 byTD-DFT identified a number of singlet–singlet electronic transi-tions between 195 and 235 nm (i.e. UV) that can be attributed tocarbonyl dissociation. A representative example is shown inFig. 1 (full details provided in ESI†). Similar excitations are alsocalculated for 2 between 190 and 260 nm, suggesting that itwould be difficult to selectively enact the UV-promoted dissocia-tion of carbonyl from either 1 or 2.13To further explore this C–H bond activation chemistry,20 mM solutions of 1 in fluorobenzene, 1,2-difluorobenzene,and 1,3,5-trifluorobenzene were irradiated for a total of 8 h at RT(Scheme 3). Similar to that observed in benzene, analysis by 1Hand 31P NMR spectroscopy indicated formation of photostationarymixtures composed of 1 and Ir(III) products of C–H bond activation3; as the minor and major components, respectively. Supportingour hypothesis, and contrasting with reactions of analogous PNPand PNP* systems (vide supra), C–H bond activation of fluoro-benzene (3a) and 1,2-difluorobenzene (3b) proceeded with exclu-sive ortho-selectivity. Both possible rotamers of each isomer areformed, although in disparate proportions (Scheme 3).Consistent with these observations and manifestation ofthe ‘‘ortho fluorine eﬀect’’,4 the alternative regioisomers of 3a(410 kJ mol1) and 3b (418 kJ mol1), and their respective5-coordinate precursors [Ir(PCP)(2-FC6H4)H] (424 kJ mol1)and [Ir(PCP)(2,3-F2C6H3)H] (426 kJ mol1), are calculated tobe significantly higher in free energy. As for the formation of 2from 1, computed reaction free energies suggest the formationof 3 occur outside of thermally accessible regimes and insteadare presumably driven through nuanced diﬀerences in the photo-physical properties of 1 and 3. Moreover, increasingly favourableoverall reactions correlate with number of fluorine substituents:DG298K/kJ mol1 = +141.3 (2), +128.1 (3a), +121.0 (3b), +119.1 (3c).With the above in mind and to independently verify theirstructures, analytically pure samples of 3 were isolated thoughdehydrohalogenation of [Ir(PCP)HCl]14 using K[OtBu] in therespective fluorobenzene at 75 1C, followed by reaction withcarbon monoxide (yield = 32–54%). The structures of thesenew compounds, and for comparison 2,9 were fully verified insolution by 1H, 13C, 19F and 31P NMR, IR and UV-vis spectroscopy,and in the solid-state by X-ray crystallography (3a, 3b – Fig. 2;2, 3c – ESI†). Notable spectroscopic markers include lowfrequency hydride resonances at d 9.07 (2), 8.86/9.68 (3a),8.88/9.66 (3b), 9.46 (3c) that show 3JPH coupling ofca. 17 Hz, and 31P resonances at d 51.1 (2), 54.5/51.7 (3a),54.7/52.2 (3b), 54.0 (3c) in C6D12 solution (data for majorrotamers underlined). The structures of major rotamers of 3aand 3b, bearing the fluorine atoms proximal to the carbonylligand, were definitively established in solution through NOESYexperiments and corroborated in the solid-state (Fig. 2). Like-wise, the hydride resonances of the minor rotamers showcharacteristic 1hJFH coupling of ca. 10 Hz. The observed relativeconformational preferences are also reproduced in silico (see ESI†).Complexes 3 are thermally stable on extended heating at 80 1Cin C6D6 solution: after 8 h,o5% 1 is formed and for 3a and 3bthe ratio of rotamers was unchanged, as determined in situ by31P NMR spectroscopy.In conclusion, we have demonstrated that selectiveC–H bond activation reactions of fluorobenzenes can beachieved under mild conditions using photolysis of [Ir(2,6-(PtBu2CH2)2C6H3)(CO)] 1 as a means to generate the reactiveScheme 2 UV-promoted interconversion between 1 and 2 ([Ir] = 20mM, RT).Fig. 1 Representative electronic excitation related to carbonyl dissocia-tion from 1; wavelength, oscillator strength (f) and % contribution of therepresented natural transition orbitals (rendered with an orbital isosurfacevalue of 0.02).Scheme 3 UV-promoted C–H bond activation of fluorobenzenes (top;[Ir] = 20 mM, 8 h, RT). Major rotamers depicted in red, minor rotamersdepicted in blue (bottom).Communication ChemCommOpen Access Article. Published on 17 March 2017. Downloaded on 03/05/2017 10:23:39.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Online3636 | Chem. Commun., 2017, 53, 3634--3636 This journal is©The Royal Society of Chemistry 201714 VE Ir(I) fragment {Ir(2,6-(PtBu2CH2)2C6H3)} in solution. Thiswork not only showcases the ability of iridium pincer complexesto mediate challenging C–H bond activations, but more generallyhighlights a potentially useful catalyst design principle that wehope will stimulate the development of new organic transforma-tions employing partially fluorinated benzenes as building blocks.We gratefully acknowledge financial support from the SwissNational Science Foundation (S. A. H.), EPSRC (J. E.-K.), RoyalSociety (A. B. C.) and University of Warwick. Crystallographicdata was collected using a diﬀractometer purchased throughsupport from Advantage West Midlands and the EuropeanRegional Development Fund. We also thank the ScientificComputing Research Technology Platform at the University ofWarwick for providing high-performance computing facilities.Notes and references1 (a) K. Krogh-Jespersen, M. Czerw, K. Zhu, B. Singh, M. Kanzelberger,N. Darji, P. D. Achord, K. B. Renkema and A. S. Goldman, J. Am.Chem. Soc., 2002, 124, 10797–10809; (b) R. Ghosh, X. Zhang,P. Achord, T. J. Emge, K. Krogh-Jespersen and A. S. Goldman,J. Am. Chem. Soc., 2007, 129, 853–866; (c) W. H. Bernskoetter, P. S.White, K. I. Goldberg and M. Brookhart, J. Am. Chem. Soc., 2009, 131,8603–8613; (d) J. Choi, A. H. R. MacArthur, M. Brookhart andA. S. Goldman, Chem. Rev., 2011, 111, 1761–1779; (e) J. Choi,D. Y. Wang, S. Kundu, Y. Choliy, T. J. Emge, K. Krogh-Jespersenand A. S. Goldman, Science, 2011, 332, 1545–1548.2 For representative examples see: (a) M. Lafrance, C. N. Rowley,T. K. Woo and K. Fagnou, J. Am. Chem. Soc., 2006, 128, 8754–8756;(b) H.-Q. Do and O. Daugulis, J. Am. Chem. Soc., 2008, 130,1128–1129; (c) Y. Nakao, N. Kashihara, K. S. Kanyiva andT. Hiyama, J. Am. Chem. Soc., 2008, 130, 16170–16171; (d) X. C.Cambeiro, T. C. Boorman, P. Lu and I. Larrosa, Angew. Chem., Int.Ed., 2012, 52, 1781–1784; (e) P. Ricci, K. Kra¨mer, X. C. Cambeiro andI. Larrosa, J. Am. Chem. Soc., 2013, 135, 13258–13261; ( f ) X. C.Cambeiro, N. Ahlsten and I. Larrosa, J. Am. Chem. Soc., 2015, 137,15636–15639; (g) J. Takaya, S. Ito, H. Nomoto, N. Saito, N. Kirai andN. Iwasawa, Chem. Commun., 2015, 51, 17662–17665; (h) M. Simonetti,G. J. P. Perry, X. C. Cambeiro, F. Julia´-Herna´ndez, J. N. Arokianathar andI. Larrosa, J. Am. Chem. Soc., 2016, 138, 3596–3606.3 (a) E. Clot, O. Eisenstein, N. Jasim, S. A. Macgregor, J. E. McGradyand R. N. Perutz, Acc. Chem. Res., 2011, 44, 333–348; (b) S. D. Pike,M. R. Crimmin and A. B. Chaplin, Chem. Commun., 2017, 53, DOI:10.1039/c6cc09575e.4 (a) M. E. Evans, C. L. Burke, S. Yaibuathes, E. Clot, O. Eisenstein andW. D. Jones, J. Am. Chem. Soc., 2009, 131, 13464–13473; (b) E. Clot,C. Me´gret, O. Eisenstein and R. N. Perutz, J. Am. Chem. Soc., 2009,131, 7817–7827; (c) E. Clot, M. Besora, F. Maseras, C. Me´gret,O. Eisenstein, B. Oelckers and R. N. Perutz, Chem. Commun., 2003,490–491.5 (a) L. Fan, S. Parkin and O. V. Ozerov, J. Am. Chem. Soc., 2005, 127,16772–16773; (b) E. Ben-Ari, M. Gandelman, H. Rozenberg, L. J. W.Shimon and D. Milstein, J. Am. Chem. Soc., 2003, 125, 4714–4715;(c) E. Ben-Ari, R. Cohen, M. Gandelman, L. J. W. Shimon, J. M. L.Martin and D. Milstein, Organometallics, 2006, 25, 3190–3210.6 D. Y. Wang, Y. Choliy, M. C. Haibach, J. F. Hartwig, K. Krogh-Jespersen and A. S. Goldman, J. Am. Chem. Soc., 2016, 138, 149–163.7 During the preparation of this manuscript the C–H bond activationof fluorobenzene using related Ir-POCOP pincer complexes has beendescribed: L. P. Press, A. J. Kosanovich, B. J. McCulloch andO. V. Ozerov, J. Am. Chem. Soc., 2016, 138, 9487–9497.8 (a) K. Krogh-Jespersen, M. Czerw, K. Zhu, B. Singh, M. Kanzelberger,N. Darji, P. D. Achord, K. B. Renkema and A. S. Goldman, J. Am.Chem. Soc., 2002, 124, 10797–10809; (b) D. Morales-Morales,R. Redo´n, Z. Wang, Do W. Lee, C. Yung, K. Magnuson and C. M.Jensen, Can. J. Chem., 2001, 79, 823–829.9 M. Kanzelberger, B. Singh, M. Czerw, K. Krogh-Jespersen andA. S. Goldman, J. Am. Chem. Soc., 2000, 122, 11017–11018.10 Acid catalysed oxidative addition of C(sp)–H bonds to 1 have,however, been reported: J. D. Hackenberg, S. Kundu, T. J. Emge,K. Krogh-Jespersen and A. S. Goldman, J. Am. Chem. Soc., 2014, 136,8891–8894.11 Consistent with this suggestion, photolysis of 20 mM solutionsof 1 and 2 in C6D6 under 1 atm of CO at RT resulted in prolonged(46 h) establishment of similar photostationary mixtures of 1 : 2-d6(1: 1.4, corresponding to 58% 2-d6) to those obtained under anargon atmosphere (after ca. 2 h). For discussion of mechanisticpathways in related photochemical reactions of [Rh(PMe3)2(CO)Cl]see: G. P. Rosini, W. T. Boese and A. S. Goldman, J. Am. Chem. Soc.,1994, 116, 9498–9505.12 J. E. Wheatley, C. A. Ohlin and A. B. Chaplin, Chem. Commun., 2014,50, 685–687.13 Notably using a long pass (glass) filter (o330 nm): no reaction wasobserved when a 20 mM solution of 1 was irradiated in C6D6 for 2 hat RT, while o5% formation of 1 was observed when a 20 mMsolution of 2 was irradiated in C6D6 for 2 h at RT.14 C. J. Moulton and B. L. Shaw, J. Chem. Soc., Dalton Trans., 1976,1020.Fig. 2 Solid-state structures of 3a (occupancy of disordered fluorinesubstituents in parenthesis) and 3b. Thermal ellipsoids at 50% probabilitylevel; hydrogen atoms omitted for clarity. Selected bond lengths [Å] andangles [1]: 3a, Ir1–C2, 1.931(6); Ir1–C4, 2.159(5); Ir1–C12, 2.115(5); Ir1–P10,2.3394(15); Ir1–P11, 2.3474(15); C2–Ir1–C12, 83.8(2); C4–Ir1–C12,178.2(2); P10–Ir1–P11, 155.84(5). 3b, Ir1–C2, 1.928(2); Ir1–C4, 2.150(2);Ir1–C12, 2.111(2); Ir1–P10, 2.3511(6); Ir1–P11, 2.3397(6); C2–Ir1–C12,89.07(9); C4–Ir1–C12, 173.63(8); P10–Ir1–P11, 157.41(2).ChemComm CommunicationOpen Access Article. Published on 17 March 2017. Downloaded on 03/05/2017 10:23:39.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Online

UV-light promoted C–H bond activation of benzene and fluorobenzenes by an iridium(i) pincer complex

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UV-light promoted C–H bond activation of benzene and fluorobenzenes by an iridium(i) pincer complex

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