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

Epitomised by applications in the catalytic dehydrogenation of alkanes, iridium complexes of phosphine-based pincer ligands are widely recognised for their capacity to activate C–H bonds. With fluoroaryls representing valuable synthons in organic chemistry, we have targeted use of these iridium compounds for carrying out selective C–H bond activation reactions of partially fluorinated benzenes (C6H6 nFn, n r 3). The presence of fluorine substituents results in significantly stronger C–H bonds than benzene and, correspondingly, fluorobenzenes represent challenging substrates. Previous work by Milstein and Ozerov employing neutral (PNP) and anionic (PNP*) pincer ligands has highlighted the potential of iridium pincers, although under moderate temperature regimes (o100 1C) these systems showed poor regioselectivity in the activation of fluorobenzene (Scheme 1). As C–H bond activation is thermodynamically favoured ortho to the fluorine substituents, indiscriminate and irreversible oxidative addition reactions of transient 14 VE Ir(I) intermediates {Ir(PNP)}/{Ir(PNP*)} are implicated. We postulated that use of anionic pincer ligands bearing central aryl donors could promote selective activation of fluorobenzene substrates through increased reaction reversibility imparted by the high trans-influence aryl donor. With a view to testing this hypothesis we selected [Ir(PCP)(CO)] 1 (PCP = 2,6-(PBu2CH2)2C6H3 ) as a well-defined precursor for the low coordinate and formally 14 VE Ir(I) fragment {Ir(PCP)}, through photochemically promoted dissociation of the carbonyl ligand. In this way, subsequent products of C–H bond activation would be trapped on re-coordination of the carbonyl ligand (in a closed system). Initial experiments using benzene as the substrate supported this reasoning, with [Ir(PCP)(C6D5)D(CO)] 2-d6 9 generated on irradiation of a 20 mM C6D6 solution of 1 at RT using a 100 W Hg arc lamp (quartz J. Young’s NMR tube, Scheme 2). Following this reaction by periodic analysis using P NMR spectroscopy, however, indicated that conversion of 1 (d31P 82.0) to 2-d6 (d31P 52.3) plateaued at 62% after ca. 2 h total irradiation. On the same timeframe, irradiation of independently synthesised 2 resulted in an equivalent reaction composition. In contrast, both 1 and 2 are thermally stable on extended heating at 80 1C in C6D6 solution (8 h) and no isotope exchange was observed for 2 (to 2-d6). 10 Together these results indicate establishment of a photostationary mixture of 1 and 2-d6, mediated through light induced carbonyl dissociation from both species. To gain deeper understanding of the photolysis experiments, a series of DFT and TD-DFT calculations were performed (see ESI† for full details). In line with expectation, the computed free energy for carbonyl dissociation to form {Ir(PCP)} is a significantly endergonic process (DG298K = +194.4 kJ mol ). While the subsequent C–H bond activation of benzene is exothermic (DH = 15.3 kJ mol ),

Epitomised by applications in the catalytic dehydrogenation of alkanes, iridium complexes of phosphine-based pincer ligands are widely recognised for their capacity to activate C-H bonds. 1 With fluoroaryls representing valuable synthons in organic chemistry, 2,3 we have targeted use of these iridium compounds for carrying out selective C-H bond activation reactions of partially fluorinated benzenes (C 6 H 6Àn F n , n r 3). 3 The presence of fluorine substituents results in significantly stronger C-H bonds than benzene and, correspondingly, fluorobenzenes represent challenging substrates. 3,4Previous work by Milstein and Ozerov employing neutral (PNP) and anionic (PNP*) pincer ligands has highlighted the potential of iridium pincers, although under moderate temperature regimes (o100 1C) these systems showed poor regioselectivity in the activation of fluorobenzene (Scheme 1). 5 As C-H bond activation is thermodynamically favoured ortho to the fluorine substituents, 4 indiscriminate and irreversible oxidative addition reactions of transient 14 VE Ir(I) intermediates {Ir(PNP)} + /{Ir(PNP*)} are implicated.
We postulated that use of anionic pincer ligands bearing central aryl donors could promote selective activation of fluorobenzene substrates through increased reaction reversibility imparted by the high trans-influence aryl donor. 6,7With a view to testing this hypothesis we selected [Ir(PCP)(CO)] 1 (PCP = 2,6-(P t Bu 2 CH 2 ) 2 C 6 H 3 À ) 8 as a well-defined precursor for the low coordinate and formally 14 VE Ir(I) fragment {Ir(PCP)}, through photochemically promoted dissociation of the carbonyl ligand.
In this way, subsequent products of C-H bond activation would be trapped on re-coordination of the carbonyl ligand (in a closed system).Initial experiments using benzene as the substrate supported this reasoning, with [Ir(PCP)(C 6 D 5 )D(CO)] 2-d 6 9 generated on irradiation of a 20 mM C 6 D 6 solution of 1 at RT using a 100 W Hg arc lamp (quartz J. Young's NMR tube, Scheme 2).Following this reaction by periodic analysis using 31 P NMR spectroscopy, however, indicated that conversion of 1 3) plateaued at 62% after ca. 2 h total irradiation.On the same timeframe, irradiation of independently synthesised 2 resulted in an equivalent reaction composition.In contrast, both 1 and 2 are thermally stable on extended heating at 80 1C in C 6 D 6 solution (8 h) and no isotope exchange was observed for 2 (to 2-d 6 ). 10 Together these results indicate establishment of a photostationary mixture of 1 and 2-d 6 , mediated through light induced carbonyl dissociation from both species. 11o gain deeper understanding of the photolysis experiments, a series of DFT and TD-DFT calculations were performed (see ESI † for full details).In line with expectation, the computed free energy for carbonyl dissociation to form {Ir(PCP)} is a significantly endergonic process (DG 298K = +194.4kJ mol À1 ).While the subsequent C-H bond activation of benzene is exothermic (DH = À15.3kJ mol À1 ),

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the Ir(III) product lies thermodynamically further uphill from 1 (DG 298K = +238.0kJ mol À1 ).Such energetics are characteristic of an unfavourable equilibrium reaction, although one that would be offset by the use of the substrate as the solvent. 12e-coordination of the carbonyl ligand counteracts the unfavourable thermodynamics (DG 298K = À95.0kJ mol À1 ), however, 2 is still calculated to be +141.3kJ mol À1 higher in free energy than 1 + C 6 H 6 .Together these results are consistent with the lack of any thermal reaction observed for either 1 or 2 in C 6 D 6 and highlight the important promoting role of UV-irradiation in the formation of 2 (and reformation of 1).In this context, analysis of 1 by TD-DFT identified a number of singlet-singlet electronic transitions between 195 and 235 nm (i.e.UV) that can be attributed to carbonyl dissociation.A representative example is shown in Fig. 1 (full details provided in ESI †).Similar excitations are also calculated for 2 between 190 and 260 nm, suggesting that it would be difficult to selectively enact the UV-promoted dissociation of carbonyl from either 1 or 2. 13 To 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 1 H and 31 P NMR spectroscopy indicated formation of photostationary mixtures composed of 1 and Ir(III) products of C-H bond activation 3; as the minor and major components, respectively.Supporting our hypothesis, and contrasting with reactions of analogous PNP and PNP* systems (vide supra), C-H bond activation of fluorobenzene (3a) and 1,2-difluorobenzene (3b) proceeded with exclusive ortho-selectivity.Both possible rotamers of each isomer are formed, although in disparate proportions (Scheme 3).
In conclusion, we have demonstrated that selective C-H bond activation reactions of fluorobenzenes can be achieved under mild conditions using photolysis of [Ir(2,6-(P t Bu 2 CH 2 ) 2 C 6 H 3 )(CO)] 1 as a means to generate the reactive

Fig. 1
Fig.1Representative electronic excitation related to carbonyl dissociation from 1; wavelength, oscillator strength (f) and % contribution of the represented natural transition orbitals (rendered with an orbital isosurface value of 0.02).