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Controlling Proton Delivery Through Catalyst Structural Dynamics

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Controlling Proton Delivery Through Catalyst Structural Dynamics Angewandte Communications Chemie International Edition:DOI:10.1002/anie.201607460 Homogeneous Catalysis German Edition:DOI:10.1002/ange.201607460 Controlling Proton Delivery through CatalystStructural Dynamics Allan JayP.Cardenas,Bojana Ginovska, Neeraj Kumar,Jianbo Hou, Simone Raugei, Monte L. Helm, Aaron M. Appel, R. Morris Bullock, and Molly OHagan* Abstract: The fastest synthetic molecular catalysts for H2 structural dynamics in catalysis is demonstrated through the production and oxidation emulate components of the active modulation of catalytic rates over three orders of magnitude site of hydrogenases.The critical role of controlled structural for molecular Ni electrocatalysts for H2 production. This rate dynamics is recognized for many enzymes,including hydro- enhancement is achieved by controlling the structural dynam- genases,but is largely neglected in designing synthetic catalysts. ics involved in key proton-transfer steps. Our results demonstrate the impact of controlling structural Highly active electrocatalysts for the production and Ph C6H4R 2+ R R’ 2+ dynamics on H2 production rates for [Ni(P 2N 2)2] oxidation of H2,such as the [Ni(P 2N 2)2] catalysts R R’ catalysts (R = n-hexyl, n-decyl, n-tetradecyl, n-octadecyl, (P 2N 2 = 1,5-diaza-3,7-diphosphacyclooctane with alkyl or phenyl, or cyclohexyl). The turnover frequencies correlate aryl groups on the Pand Natoms), have been developed by inversely with the rates of chair–boat ring inversion of the incorporating abase in the second coordination sphere of the ligand, since this dynamic process governs protonation at metal to act as aproton relay (Scheme 1), asystem similar to either catalytically productive or non-productive sites.These the active site of the [FeFe]-hydrogenase.[6] Thereactivity of results demonstrate that the dynamic processes involved in proton delivery can be controlled through modification of the outer coordination sphere,inamanner similar to the role of the protein architecture in many enzymes.Asadesign parameter, controlling structural dynamics can increase H2 production rates by three orders of magnitude with aminimal increase in overpotential. The high catalytic efficiency of metalloenzymes is attributed to the active-site structure and control of the active-site Ph C6H4R 2+ Scheme 1. Structure of the nickel(II) complexes: [Ni(P 2N 2)2] . environment, including the precise regulation of substrate Forcomplex characterization, including X-ray structures, see the the delivery and product removal. This regulation is achieved SupportingInformation. through the extended architecture of the protein, interaction with the medium, and controlled structural dynamics,which can dictate active-site access as well as precise positioning of these modular catalysts can be controlled through systematic active components throughout the catalytic reaction.[1] The changes in the steric and electronic characteristics of the structural dynamics of molecular catalysts can also have ligands and the basicity of the pendant amine.[7] Changes to aprofound effect on catalytic performance.Dynamics has the medium composition have substantial effects on the been used as adesign feature to modulate chemoselectivity reactivity of these complexes,with significant rate enhance- [2] [3] and enantioselectivity, to enhance electron transfer, to ments observed in protic ionic liquid/H2Omedia compared to determine enantiomeric excess,[4] and even to turn the activity traditional organic solvents.[8] Proton mobility in these media of the catalyst on or off.[5] However,predicting and control- is critical for fast catalysis,but proton mobility alone does not ling the effects that ligand dynamic processes can have on account for the observed rate enhancement, especially for 1- reactivity is challenging when designing new catalytic systems, C6.[8b] We have now identified the source of the dramatic rate whether synthetic molecular systems or within aprotein enhancement as modulation of the ligand dynamics that framework. In this study,the importance of controlling control the positioning of the pendant amine,and therefore the protonation site.Weobserved aremarkable correlation [*] Dr.A.J.P.Cardenas, B. Ginovska, Dr.N.Kumar,Dr. J. Hou, between ligand structural dynamics and catalytic perfor- Dr.S.Raugei, Dr.M.L.Helm, Dr.A.M.Appel, Dr.R.M.Bullock, mance,which is controlled through different substituents on Dr.M.O’Hagan the periphery of the ligand, seven bonds away from the metal Center for Molecular Electrocatalysis (Scheme 1). Controlling the ligand dynamics led to an Pacific Northwest NationalLaboratory increase in the catalytic activity by more than three orders P.O. Box 999, K2-57, Richland,WA99352 (USA) of magnitude in acetonitrile/water solution, giving H2 pro- E-mail:[email protected] 6 1 duction turnover frequencies of 1.5 10 sÀ .Bycombining Dr.A.J.P.Cardenas Current address:221 Science Center catalyst modification and the use of protic ionic liquids,we 7 1 State University of New York at Fredonia achieved catalytic rates up to 4.5 10 sÀ ,which, to our Fredonia, NY 14063 (USA) knowledge,isthe fastest catalysis for this transformation to Supportinginformation for this article can be found under: date. http://dx.doi.org/10.1002/anie.201607460. Angew.Chem. Int.Ed. 2016, 55,13509 –13513 2016 Wiley-VCH VerlagGmbH &Co. KGaA,Weinheim 13509 Angewandte Communications Chemie To rapidly reduce protons,the six-membered ring con- In many natural catalysts,the outer coordination sphere taining the pendant amine and the metal center of [Ni- modulates dynamic processes essential to catalysis through R R’ 2+ [1] (P 2N 2)2] systems must adopt aboat conformation complex intramolecular and solvent interactions. To emu- (Scheme 2), with the lone pair of the amine oriented endo late this control in amolecular system that is more syntheti- cally accessible,substituents of varying size and structure were incorporated into the outer coordination sphere of the catalyst (Scheme 1) to modulate interconversion of the boat and chair conformers.The boat–chair isomerization rates for Ph C6H4R 2+ the [Ni(P 2N 2)2] series of catalysts were measured by lineshape analysis of the variable-temperature 31P{1H} NMR II spectra for the Ni complexes in CD2Cl2/CD3CN as the solvent to provide the required low-temperature range for Eyring analysis.[11] TheNiII complexes have atrigonal bipyr- amidal structure in which acetonitrile is the fifth ligand. The presence of this fifth ligand results in two different phospho- rus environments in the trigonal bipyramidal structure,which interconvert through dissociation of the acetonitrile,chair– boat isomerization, and re-coordination of acetonitrile.[11] Details of the exchange process,representative spectra, and Eyring analyses are shown in the Supporting Information. As the length of the n-alkyl chain of the substituent in the outer coordination sphere is increased, the rate of chair–boat isomerization decreases,and the turnover frequency (TOF) Scheme 2. Branching of the catalytic cycle occurs due to protonation increases up to three orders of magnitude,asshown in at the inactive exo site or active endo site. Only endo protonation Ph C6H4R 2+ Figure 1. The[Ni(P 2N 2)2] complexes catalyze H2 results in H2 formation (top pathway). The exo protonation is stabilized by boat-to-chairisomerization of the ligand to form ahydro- gen bond (bottom pathway), thereby resulting in aconformer that is thermodynamically and kinetically stabilized to the deprotonation required to resume catalytic activity. with respect to the metal center. Protonation of the chair conformer, which is often the kinetically preferred protona- tion site,positions protons exo to the metal center,where they cannot interact with the metal center.Toproduce H2,these exo-protonated species require isomerization through depro- tonation of the exo position followed by protonation in the endo position. This isomerization process is slow relative to electrocatalysis,and leads to accumulation of the exo- Figure 1. Correlation between catalytic rates of H production mea- protonated species in the electrochemical diffusion layer, 2 sured in protonated dibutyl formamidebis(trifluoromethanesulfonyl)a- effectively attenuating the concentration of the active catalyst mide/water ([(DBF)H]NTf /H O) and boat–chairisomerization rates [7a,9] 2 2 and thereby decreasing the observed rates of catalysis. Ph C6H4R 2+ measured in 4:1CD2Cl2/CD3CN for the [Ni(P 2N 2)2] series. Thestability of the exo-protonated isomers is increased by intramolecular N H···N hydrogen bonding,that is,the “exo À pinch”, which stabilizes the protonation site by more than production, and the plateau of the scan-rate-independent [10] 5pKa units relative to the non-pinched isomer. Thekinetic electrocatalytic current was used to determine the catalytic stability of the protonation site is also increased by the rate,orTOF,inthe media indicated in Table 1byusing presence of the hydrogen-bonding interaction, where access established methods.[12] Foreach catalyst, the catalytic current to the exo-pinched proton by the external base is hindered by (icat)exhibited first-order dependence on catalyst concentra- the second amine.Avoiding the formation of the N H···N tion and plateaued to the steady-state value as the scan rate, À hydrogen bond would greatly accelerate the exo–endo acid concentration, and water content increased (Figures S4– isomerization process and therefore the rate of catalysis. S9 in the Supporting Information). Astrong linear correlation Theformation
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