Carbon-fluorine bonds are the strongest known single bonds to carbon and as a consequence can prove very hard to cleave. Alhough vinyl and aryl C-F bonds can undergo oxidative addition to transition metal complexes, this reaction has appeared inoperable with aliphatic substrates. We report the addition of C(sp(3))-F bonds (including alkyl-F) to an iridium center via the initial, reversible cleavage of a C-H bond. These results suggest a distinct strategy for the development of catalysts and promoters to make and break C-F bonds, which are of strong interest in the context of both pharmaceutical and environmental chemistry.

The isomerization of olefins by complexes of the pincer-ligated iridium species ((tBu)PCP)Ir ((tBu)PCP = κ(3)-C(6)H(3)-2,6-(CH(2)P(t)Bu(2))(2)) and ((tBu)POCOP)Ir ((tBu)POCOP = κ(3)-C(6)H(3)-2,6-(OP(t)Bu(2))(2)) has been investigated by computational and experimental methods. The corresponding dihydrides, (pincer)IrH(2), are known to hydrogenate olefins via initial Ir-H addition across the double bond. Such an addition is also the initial step in the mechanism most widely proposed for olefin isomerization (the "hydride addition pathway"); however, the results of kinetics experiments and DFT calculations (using both M06 and PBE functionals) indicate that this is not the operative pathway for isomerization in this case. Instead, (pincer)Ir(η(2)-olefin) species undergo isomerization via the formation of (pincer)Ir(η(3)-allyl)(H) intermediates; one example of such a species, ((tBu)POCOP)Ir(η(3)-propenyl)(H), was independently generated, spectroscopically characterized, and observed to convert to ((tBu)POCOP)Ir(η(2)-propene). Surprisingly, the DFT calculations indicate that the conversion of the η(2)-olefin complex to the η(3)-allyl hydride takes place via initial dissociation of the Ir-olefin π-bond to give a σ-complex of the allylic C-H bond; this intermediate then undergoes C-H bond oxidative cleavage to give an iridium η(1)-allyl hydride which "closes" to give the η(3)-allyl hydride. Subsequently, the η(3)-allyl group "opens" in the opposite sense to give a new η(1)-allyl (thus completing what is formally a 1,3 shift of Ir), which undergoes C-H elimination and π-coordination to give a coordinated olefin that has undergone double-bond migration.

PCP"-pincer-ligated iridium complexes have been found to be highly effective catalysts for the dehydrogenation of alkanes. We report a computational and experimental study of the effect on catalytic activity resulting from systematically varying steric crowding by the substitution of methyl groups for the phosphino tert-butyl groups of ( R4 PCP)Ir ( R4 PCP = κ 3 -C 6 H 3 -2,6-(CH 2 PR 2 ) 2 ; R = t Bu or Me). DFT calculations for ( R4 PCP)Ir species (R 4 = t Bu 4 or t Bu 3 Me) indicate that the ratedetermining step in the n-alkane/1-alkene transfer dehydrogenation cycle is β-H elimination by ( R4 PCP)Ir(n-alkyl)(H). It is calculated that the transition state for this step is ca. 10 kcal/mol lower for ( tBu3Me PCP)Ir than for ( tBu4 PCP)Ir (relative to the corresponding free ( R4 PCP)Ir). However, this catalytically favorable effect is calculated to be partially offset by the strong binding of 1-alkene to ( tBu3Me PCP)Ir in the resting state, so the overall barrier is thus lower by only ca. 4 kcal/mol. Further Me-for-t Bu substitutions have a smaller effect on the transition states, and the calculated energy of the olefin-bound resting states is lowered by comparable amounts; therefore these additional substitutions are predicted to have little overall favorable effect on catalytic rates. ( tBu3Me PCP)IrH 4 has been synthesized and isolated, and its catalytic activity has been investigated. It is indeed found to be a more active catalyst precursor than ( tBu4 PCP)IrH 4 for alkane transfer dehydrogenation. ( tBu2Me2 PCP)IrH 4 was also synthesized and as a catalyst precursor is found to afford somewhat lower activity than ( tBu3Me PCP)IrH 4 . However, synthetic precursors of ( tBu2Me2 PCP)IrH 4 tended to yield dinuclear clusters, while complex mixtures were observed during catalysis that were not amenable to characterization. It is therefore not clear if the lesser catalytic activity of ( tBu2Me2 PCP)Ir vs ( tBu3Me PCP)Ir derivatives is due to the energetics of the actual catalytic cycle or due to deactivation of this catalyst via the facile formation of clusters.

A pincer-ligated iridium complex, (PCP)Ir (PCP = κ(3)-C6H3-2,6-[CH2P(t-Bu)2]2), is found to undergo oxidative addition of C(sp(3))-O bonds of methyl esters (CH3-O2CR'), methyl tosylate (CH3-OTs), and certain electron-poor methyl aryl ethers (CH3-OAr). DFT calculations and mechanistic studies indicate that the reactions proceed via oxidative addition of C-H bonds followed by oxygenate migration, rather than by direct C-O addition. Thus, methyl aryl ethers react via addition of the methoxy C-H bond, followed by α-aryloxide migration to give cis-(PCP)Ir(H)(CH2)(OAr), followed by iridium-to-methylidene hydride migration to give (PCP)Ir(CH3)(OAr). Methyl acetate undergoes C-H bond addition at the carbomethoxy group to give (PCP)Ir(H)[κ(2)-CH2OC(O)Me] which then affords (PCP-CH2)Ir(H)(κ(2)-O2CMe) (6-Me) in which the methoxy C-O bond has been cleaved, and the methylene derived from the methoxy group has migrated into the PCP Cipso-Ir bond. Thermolysis of 6-Me ultimately gives (PCP)Ir(CH3)(κ(2)-O2CR), the net product of methoxy group C-O oxidative addition. Reaction of (PCP)Ir with species of the type ROAr, RO2CMe or ROTs, where R possesses β-C-H bonds (e.g., R = ethyl or isopropyl), results in formation of (PCP)Ir(H)(OAr), (PCP)Ir(H)(O2CMe), or (PCP)Ir(H)(OTs), respectively, along with the corresponding olefin or (PCP)Ir(olefin) complex. Like the C-O bond oxidative additions, these reactions also proceed via initial activation of a C-H bond; in this case, C-H addition at the β-position is followed by β-migration of the aryloxide, carboxylate, or tosylate group. Calculations indicate that the β-migration of the carboxylate group proceeds via an unusual six-membered cyclic transition state in which the alkoxy C-O bond is cleaved with no direct participation by the iridium center.

(PCP)Ir (PCP = kappa(3)-C(6)H(3)-2,6-[CH(2)P(t-Bu)(2)](2)) is found to undergo oxidative addition of the methyl-oxygen bond of electron-poor methyl aryl ethers, including methoxy-3,5-bis(trifluoromethyl)benzene and methoxypentafluorobenzene, to give the corresponding aryloxide complexes (PCP)Ir(CH(3))(OAr). Although the net reaction is insertion of the Ir center into the C-O bond, density functional theory (DFT) calculations and a significant kinetic isotope effect [k(CH(3))(OAr)/k(CD(3))(OAr) = 4.3(3)] strongly argue against a simple insertion mechanism and in favor of a pathway involving C-H addition and alpha-migration of the OAr group to give a methylene complex followed by hydride-to-methylene migration to give the observed product. Ethoxy aryl ethers, including ethoxybenzene, also undergo C-O bond cleavage by (PCP)Ir, but the net reaction in this case is 1,2-elimination of ArO-H to give (PCP)Ir(H)(OAr) and ethylene. DFT calculations point to a low-barrier pathway for this reaction that proceeds through C-H addition of the ethoxy methyl group followed by beta-aryl oxide elimination and loss of ethylene. Thus, both of these distinct C-O cleavage reactions proceed via initial addition of a C(sp(3))-H bond, despite the fact that such bonds are typically considered inert and are much stronger than C-O bonds.