Hydrogenation of the (PONOP)Ir(I)CH(3) complex [PONOP = 2,6-bis(di-tert-butylphosphinito)pyridine] yields the unexpected trans-dihydride species (PONOP)IrCH(3)(H)(2). Mechanistic investigations have revealed that this reaction proceeds via proton-catalyzed H(2) cleavage, a pathway that circumvents the intermediacy of the typically invoked cis-dihydride isomer. Protonation yields the cationic (PONOP)Ir(CH(3))(H)(+) complex, which is then trapped by H(2) to yield an eta(2)-H(2) complex. Deprotonation of this species yields the trans-dihydride. Intermediates in the proposed pathway have been confirmed by independent low-temperature syntheses and spectroscopic observations

Bernskoetter, Wesley H.

Brookhart, Maurice

Findlater, Michael

PubMed

Carolina Digital Repository

Proton-catalyzed Hydrogenation of a d8 Ir(I) Complex Yields atrans Ir(III) DihydrideMichael Findlater, Wesley H. Bernskoetter, and Maurice Brookhart*Department of Chemistry, The University of North Carolina at Chapel Hill, Chapel Hill, NorthCarolina 27599-3290Square planar d8 iridium(I) complexes, including the iconic Vaska’s complex, (trans-IrCl(CO)[P(C6H5)3]2),1 have been used to explore a wide range of oxidative additionreactions of d8 transition metal complexes. These studies established mechanisms rangingfrom concerted cleavage of nonpolar substrates, stepwise addition of polar substrates, andradical chain reactions of certain RX substrates.2 In light of the importance of transitionmetal hydride complexes in numerous catalytic transformations, including olefinhydrogenation3 and hydroformylation,4 the concerted addition of dihydrogen to Ir(I) centershas received particular scrutiny.5 Oxidative addition of dihydrogen to afford a kineticallypreferred cis-dihyride complex is the prevailing pathway. Factors shown to influence thekinetics and thermodynamics of H2 addition include the mode of substrate approach, metalbasicity, and the steric and electronic nature of the ancillary ligands. In cases where thetrans-dihydride isomer is observed, prior formation of a cis-dihyride intermediate istypically invoked.6Milstein has recently reported that hydrogenation of Ir(I) phenyl complex 1 yields the transdihydride complex 2 as the kinetic and thermodynamic product.7 The mechanism proposedinvolved intermediacy of the dearomatized complex 3 formed by water-assisted protontransfer, followed by α2-binding of H2 cis to the phenyl group and transfer of hydrogen fromα2-H2 to the methine carbon of the bridge. DFT calculations and deuterium labeling resultssupported this proposal.8(1)brookhart@email.unc.edu.Supporting Information Available: Experimental details and pertinent NMR spectra. This material is available free of charge athttp://pubs.acs.org.NIH Public AccessAuthor ManuscriptJ Am Chem Soc. Author manuscript; available in PMC 2011 April 7.Published in final edited form as:J Am Chem Soc. 2010 April 7; 132(13): 4534–4535. doi:10.1021/ja100168w.NIH-PA Author ManuscriptNIH-PA Author ManuscriptNIH-PA Author ManuscriptWe report here the hydrogenation of a related Ir(I) methyl complex which yields a trans-dihydride species but via a quite different mechanism involving proton-catalyzed H2cleavage, a pathway which circumvents the intermediacy of the cis-dihydride isomer.We recently described the synthesis of an Ir(I) methyl complex supported by the neutralpincer ligand 2,6-bis(di-tert-butylphosphinito)pyridine, (PONOP)Ir(CH3) (4-Me), and itsprotonation with a non-coordinating acid to yield a remarkably stable five-coordinate,sixteen-electron complex, (PONOP)Ir(H)(Me)+ (4-MeH+).9 This complex was found toequilibrate rapidly with an unobserved Ir(I) s-methane complex, (PONOP)Ir(CH4)+, prior tomethane loss. To investigate the stability of the related Ir(III) methyl dihydride complex, afrozen benzene-d6 solution of 4-Me was treated with 1 atm of dihydrogen at −196°C.Warming the solution to ambient temperature and shaking overnight afforded completeconversion to the unexpected trans-dihydride complex (PONOP)Ir(CH3)H2 (4-MeH2).10The 31P{1H} NMR spectrum of 4-MeH2 displays a singlet at 182.6 ppm, shifted marginallyupfield relative to that for 4-Me. The corresponding 1H NMR spectrum exhibits a 2H tripletof quartets at −9.06 ppm (2JP-H = 17 Hz, 3JH-H = 2.4 Hz) corresponding to the Ir-Hfragments and a 3H triplet of triplets at 1.05 ppm (3JP-H = 5 Hz, 3JH-H = 2.8 Hz) assigned tothe Ir-CH3 moiety.10(2)Evacuation of the dihydrogen atmosphere from 4-MeH2 resulted in reversion to 4-Me overca. 1 day at 23°C under a static vacuum. 4-MeH2 is stable under dihydrogen in benzenesolution at 23°C, but eliminates CH4 at temperatures above 60°C. Monitoring of theconcentrations of [H2] and both iridium methyl species by NMR spectroscopy in samplescontaining less than 1 atm of dihydrogen afforded a Keq of 748(34) M−1 (23 °C) for thehydrogenation of 4-Me.10Initial kinetic experiments revealed the rates of hydrogenation were non-uniform. Theseobservations led to speculation that trace amounts of water played a role in the reaction.Indeed, parallel NMR tube experiments in which samples of 4-Me were spiked with >10equiv. of water or methanol (added via syringe) revealed complete hydrogenation to 4-MeH2 in a matter of minutes for the methanol or water treated samples compared to hoursfor the control hydrogenation reaction. Two possible mechanisms for the methanol- orwater-assisted cleavage of dihydrogen to produce the trans-dihydride, 4-MeH2, are shownin Figure 1.11 An alternative mechanism in which α-elimination from 4-Me forms aFindlater et al. Page 2J Am Chem Soc. Author manuscript; available in PMC 2011 April 7.NIH-PA Author ManuscriptNIH-PA Author ManuscriptNIH-PA Author Manuscripttransient carbene intermediate, followed by 1,2-addition of H2, was ruled out on the basis ofkinetic isotope effect experiments.10The first mechanism (Figure 1a) proceeds by the classic cis-addition of H2 to the metalcenter followed by base-assisted isomerization of the unobserved cis-dihydride complex tothe trans-dihydride species. Water (or alcohol) acts as the base, permitting transientformation of an iridium(I) methyl hydride anion, which could isomerize to reestablish themethyl group trans to the pyridyl nitrogen, followed by protonation with the conjugate acidto afford 4-MeH2. To assay the ability of base to catalyze the formation of 4-MeH2, parallelhydrogenation reactions with 4-Me were conducted. One sample of the hydrogenationmixture was treated with approximately 5 equiv of triethylamine at −196 °C prior to thewarming of benzene-d6 solutions. Monitoring by NMR spectroscopy revealed no detectablerate enhancement for conversion of 4-Me to 4-MeH2 for the amine-containing sample.Since no rate enhancement was observed in the presence of a superior base, it is unlikelythat water/alcohol is acting as a base to accelerate the formation of the trans-dihydridespecies.The second mechanism (Figure 1b), utilizes water/alcohol as a weak acid to protonate 4-Me,generating small quantities of the iridum(III) methyl hydride cation, 4-MeH+. Subsequentcoordination of dihydrogen trans to the iridium-hydride ligand and deprotonation by theconjugate base would yield the observed trans-dihydride complex. Previous isolation of 4-MeH+ (vida supra), offers strong evidence for the viability of this species as an intermediateand permits direct investigation of the subsequent reactions along the proposedhydrogenation pathway.A frozen solution of 4-MeH+ in methylene chloride-d2 was treated with 1 atm of dihydrogenat −196 °C and the tube transferred to a pre-cooled (−100 °C) NMR probe. Uponthawing, 1H and 31P NMR spectroscopy indicated near complete conversion (>90%) to thedihydrogen-hydride species, 4-MeH(H2)+. The presence of the hydride was confirmed by a1H triplet at −13.37 ppm (2JP-H = 17 Hz) and the coordinated dihydrogen was observed as a2H broad singlet at −1.98 ppm (JHD = 34 Hz in the α2-HD complex). Additionally the Ir-CH3 resonance appears at 0.39 ppm and the 31P{1H} NMR spectrum exhibits a singlet at174.1 ppm.Further evidence in support of the proposed mechanism of hydrogenation (Figure 1b) wasgarnered via in situ deprotonation of 4-MeH(H2)+. Deprotonation of the bound dihydrogenmolecule by a conjugate base is a key step in the proposed mechanism for formation of thetrans-dihydride species without the intermediacy of the cis-dihydride isomer. Significantly,addition of 10 equiv. of triethylamine to a methylene chloride-d2 solution of 4-MeH(H2)+ at−90 °C (eq 2) resulted in complete conversion to 4-MeH2 with concomitant formation ofthe (H)NEt3B(ArF)4 salt (ArF = 3,5-(CF3)C6H3).(3)Findlater et al. Page 3J Am Chem Soc. Author manuscript; available in PMC 2011 April 7.NIH-PA Author ManuscriptNIH-PA Author ManuscriptNIH-PA Author ManuscriptExperiments employing either D2O or CH3OD revealed rapid incorporation of deuteriuminto the methyl group of 4-Me. This exchange clearly occurs via deuteration at iridium togive 4-MeD+ followed by reversible reductive coupling to yield 4-(CH3D)+. Thisobservation is consistent with the equilibrium indicated in the proposed mechanism (Figure1b).9In summary, we report that proton-catalyzed hydrogenation of an Ir(I) complex yields atrans-dihydride iridium(III) complex without the intermediacy of the cis-dihydride isomer.The proposed mechanism, shown in Figure 1b, is supported by independent verification ofthe elementary reaction steps along the proposed pathway.11 Since the bridge atoms areoxygen, the “Milstein mechanism” cannot apply here.7,8 It is remarkable that two quitedifferent mechanisms, both water-mediated, can apply to very similar systems. The unusualproton-catalyzed net oxidative addition of dihydrogen seen here serves as an alternativepathway for dihydrogen cleavage by metal complexes sufficiently basic to be protonated byweak acids such as water.Supplementary MaterialRefer to Web version on PubMed Central for supplementary material.AcknowledgmentsWe acknowledge funding by the NSF (Grant No. CHE-0650456) as part of the Center for Enabling NewTechnologies Through Catalysis and the NIH (Grant No. GM 28938).References1. Vaska L, DiLuzio JW. J Am Chem Soc 1962;84:679–80.2. See for example, (a) Deutsch PP, Eisenberg R. Chem Rev 1988;88:1147–1161. (b) Abu-HasanaynF, Goldman AS, Krogh-Jespersen K. Inorg Chem 1994;33:5122–5130. (c) Labinger JA, Bercaw JE.Nature 2002;417:507–514. [PubMed: 12037558]3. Boerner, A.; Holz, J. Transition Metals of Organic Synthesis. Wiley-VCH; Weinheim: 2004.4. Ojima, I.; Tsai, C-Y.; Tzamarioudaki, M.; Bonafoux, D. The hydroformylation reaction. Wiley;Hoboken: 2000. Organic Reactions.5. (a) Kubas GJ. Acc Chem Res 1988;21:120–128. (b) Jessop PG, Morris RH. Coord Chem Rev1992;121:155–284. (c) Heinekey DM, Oldham WJ Jr. Chem Rev 1993;93:913–926. (d) EsteruelasMA, Oro LA. Chem Rev 1998;98:577–588. [PubMed: 11848909] (e) Kubas, GJ. Metal Dihydrogenand σ-Bond complexes: Structure, Theory and Reactivity. Kluwer; New York: 2001. (f) JohnsonCE, Eisenberg R. J Am Chem Soc 1985;107:3148–3160.6. See for example: (a) Salem H, Shimon LJW, Diskin-Posner Y, Leitus G, Ben-David Y, Milstein D.Organometallics 2009;28:4791–4806. (b) Rybtchinski B, Ben-David Y, Milstein D.Organometallics 1997;16:3786–3793. (c) Yoshida T, Otsuka S. J Am Chem Soc 1977;99:2134. (d)Paonessa RS, Trogler WC. J Am Chem Soc 1982;104:1138.7. Ben-Ari E, Leitus G, Shimon LJM, Milstein D. J Am Chem Soc 2006;128:15390. [PubMed:17132002]8. Iron MA, Ben-Ari E, Cohen R, Milstein D. Dalton Trans 2009:9433–9439. [PubMed: 19859598]9. Bernskoetter WH, Hanson SK, Buzak SK, Davis Z, White PS, Swartz R, Goldberg KI, BrookhartM. J Am Chem Soc 2009;131:8603–8613. [PubMed: 19489584]10. See Supporting Information for these details and an alternative mechanism suggested by areviewer.11. A reviewer notes that the counteranions differ for the water and methanol catalysed reactions (−OHor −OCH3) versus the low temperature protonation studies (B(ArF)4−), thus the latter speicesshould be viewed as model compounds.Findlater et al. Page 4J Am Chem Soc. Author manuscript; available in PMC 2011 April 7.NIH-PA Author ManuscriptNIH-PA Author ManuscriptNIH-PA Author ManuscriptFigure 1.Proposed mechanisms for water-catalyzed dihydrogen cleavage: (a) water as base, and (b)water as acidFindlater et al. Page 5J Am Chem Soc. Author manuscript; available in PMC 2011 April 7.NIH-PA Author ManuscriptNIH-PA Author ManuscriptNIH-PA Author Manuscript

Proton-Catalyzed Hydrogenation of a d 8 Ir(I) Complex Yields a trans Ir(III) Dihydride

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Proton-Catalyzed Hydrogenation of a d 8 Ir(I) Complex Yields a trans Ir(III) Dihydride

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