h/d exchange between h2–d2o and d2–h2o catalyzed by water soluble tertiary phosphine complexes...

C. R. Acad. Sci. Paris, Serie IIc, Chimie / Chemistry 3 (2000) 601–605© 2000 Academie des sciences / Editions scientifiques et medicales Elsevier SAS. All rights reservedS1387-1609(00)01142-7/FLA

Surface chemistry and catalysis / Chimie des surfaces et catalyse

H/D exchange between H2–D2O and D2–H2Ocatalyzed by water soluble tertiary phosphinecomplexes of ruthenium(II) and rhodium(I)Gabor Kovacsa, Levente Nadasdib,c, Ferenc Jooa,b, Gabor Laurenczyc,*

aInstitute of Physical Chemistry, University of Debrecen, Debrecen, PO Box 7, H-4010 HungarybResearch Group of Homogeneous Catalysis, Hungarian Academy of Sciences, Debrecen, PO Box 7, H-4010 HungarycInstitut de chimie minerale et analytique, universite de Lausanne, BCH, CH-1015 Lausanne-Dorigny, Switzerland

Received 27 January 2000, accepted 30 May 2000

Communicated by Francois Mathey

Abstract – In aqueous solutions, [RuCl2(TPPMS)2]2 (TPPMS=3-sulfonatophenyldiphenylphosphine) catalyzes the exchangebetween H2 and D2O or between D2 and H2O with high efficiency (up to 1 252 turnovers per hour) under mild conditions(298 K, 2 MPa H2). The exchange rate strongly depends on the solution pH. Similar H/D exchange is observed with the watersoluble tertiary phosphine complexes [RhCl(TPPMS)3], [RhCl(TPPTS)3] and [RuCl2(PTA)4] as catalysts (TPPTS= tris(3-sulfona-tophenyl)phosphine, PTA=1,3,5-triaza-7-phosphadamantane) which, however, show lower activity. © 2000 Academie dessciences / Editions scientifiques et medicales Elsevier SAS © 2000 Academie des sciences / Editions scientifiques et medicalesElsevier SAS

H/D exchange / catalysis / ruthenium / rhodium / aqueous solutions

Resume – En solutions aqueuses, [RuCl2(TPPMS)2]2 (TPPMS=3-sulfonatophenyldiphenylphosphine) catalyse l’echange entreH2 and D2O ou entre D2 et H2O avec un haut rendement (superieur a 1 252 turnovers par heure) en conditions douces(298 K, 2 MPa H2). La vitesse d’echange depend fortement du pH de la solution. Un echange H/D similaire est observelorsqu’on utilise comme catalyseurs les complexes de phosphines tertiaires solubles dans l’eau [RhCl(TPPMS)3],[RhCl(TPPTS)3] et [RuCl2(PTA)4], (TPPTS= tris(3-sulfonatophenyl)phosphine, PTA=1,3,5-triaza-7-phosphadamantane), qui,pourtant, manifestent une activite plus faible. © 2000 Academie des sciences / Editions scientifiques et medicales Elsevier SAS

echange H/D / catalyse / ruthenium / rhodium / solutions aqueuses

1. Introduction

Isotope exchange methods are useful tools forlabeling important compounds such as drugs [1, 2]and for mechanistic investigations in reaction kinet-ics. In catalytic hydrogenation by transition metalcomplexes, there is ample possibility for such use ofisotope exchange, since both D2 and D2O, as well as

T2 and T2O, are readily available as source of hydro-gen isotopes. In addition to this, catalytic hydrogena-tion is a much studied and well established processfor practically all the common reducible functionali-ties (C�C, C�C, C�O, C�N, NO2, C�X, etc.) [3, 4].

In the last two decades, much attention has beendevoted to aqueous organometallic catalysis both in

* Correspondence and reprintsE-mail address: [email protected] (G. Laurenczy).

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G. Kovacs et al. / C. R. Acad. Sci. Paris, Serie IIc, Chimie / Chemistry 3 (2000) 601–605

homogeneous solutions and in aqueous/organicbiphasic systems [5–7]. Biphasic procedures havebeen devised for easy catalyst recovery [8] and, inmany cases, these are characterized by mild reactionconditions, less by-products and — in general — byless stress put on the environment [9]. These discov-eries led to use of aqueous–organic biphasic reac-tions in large scale industrial processes [10].

In several cases, it was observed that in aqueousorganometallic catalysis, water played the role of notonly the solvent but also a reactant. Laghmari andSinou [11] were the first to note that in hydrogena-tion of methyl esters of dehydro amino acids (e.g.methyl a-acetamidocinnamate) in D2O/ethyl acetate,biphasic system with Rh-based catalysts prepared insitu from [Rh(COD)Cl]2+sulfonated mono- andbidentate phosphines (COD=1,5-cyclooctadiene),large quantities of deuterium were incorporated intothe product, specifically at the carbon atom alpha tothe carboxylate and the acetamido group (29–75%depending on the substrate and the phosphineused). Based on the results of a detailed study it waslater suggested [12] that deuterium was incorporatedthrough an [Rh]–H+D2O X [Rh]–D+HDO ex-change in the intermediate s-alkylrhodium (abbrevi-ated as [Rh]) hydride and subsequent transfer of Dfrom the rhodium to the alkyl moiety. It should bementioned that (i ) the aqueous phase in these ex-periments was unbuffered, and that (ii ) the extent ofdeuteration was always less than 100 %. Similar sub-stoichiometric deuterium incorporations were ob-served by us in hydrogenation of olefinic acids inD2O with [RhCl(TPPMS)3] [13] and [RhCl(PTA)3] [14]catalysts. Based on the kinetics of these reactionsand on the formation of monohydridorhodium(I)complexes, such as [HRh(TPPMS)3,4], we suggested adirect deuterolysis of the Rh–C bond in the interme-diate s-alkylrhodium derivative. In contrast to theabove findings, methyl itaconate (2-methylenesucci-nate) could hardly be deuterated (less than 5 %),using D2O as deuterium source in catalytic hydro-genation with [RhCl(TPPTS)3] catalyst [15], and therewas less than 3 % deuterium incorporation into a-ac-etamidoacrylic acid when hydrogenated in bufferedD2O solutions (2.4BpHB9.2), with a [Rh(DPPBTS)-(NBD)][O3SCF3] catalyst (NBD=norbornadiene,DPPTS=tetrasulfonated 1,4-bis(diphenylphosphino)-butane) [16]. However, up to 2.4, D could be incor-porated mostly to the CH3 and the adjacent CH2

groups of the methyl 2-methylsuccinate productwhen the hydrogenation of methyl itaconate wasconducted in D2 atmosphere [1]; this result requiresreplacement of several of the original C–H bonds byC–D. Similar observations were made in deuterationof itaconic acid both in H2–D2O and D2–H2O sys-

tems catalyzed by [Pd(QS)2] [17] (QS=1,2-dihy-droxy-9,10-anthraquinone-3-monosulfonate) and inhydrogenation of phospholipid dispersions in D2Ocatalyzed by [Pd(QS)2] [18] and [RhCl(PTA)3] [19]. Itbecame clear from the studies referred to above thatmultiple deuteration requires several reversible hy-dride addition-b-hydride elimination sequences be-fore the release of the final saturated (anddeuterated) product and this is facilitated by thebidentate coordination of itaconate or by the cageeffect operative in the lipid bilayers. However, therole of an M–H l M–D exchange still remainedunexplored. Taking all these findings, it is rathersurprising that up till now the catalytic activity ofwater soluble transition metal phosphine complexesin the isotope exchange between hydrogen andwater has not been studied. Such investigations maybring us closer to the understanding of deuterationand H/D exchange phenomena in reactions such ashydrogenation and hydroformylation, two importantprocesses in aqueous organometallic catalysis.

2. Experimental

All manipulations were done under oxygen-freeconditions using standard Schlenk-techniques.TPPMS [20], [RuCl2(TPPMS)2]2 [20], [RhCl(TPPMS)3][20], PTA [21] and [RuCl2(PTA)4] [22] were preparedas described in the literature. [RhCl(TPPTS)3] wasprepared in situ from [Rh(COD)Cl)]2 (from Johnson-Matthey) and TPPTS (a gift from Dr. I.T. Horvath).D2O and D2 (99.9 %) were purchased from Cam-bridge Isotope Laboratories, H2 was supplied byCarbagas CH.

In a typical experiment 2.6 mM solution of thecatalyst was prepared in 0.1 M KCl in D2O or H2O.The pH was set with diluted HCl or NaOH solutions,and it was measured after the reactions. 2 mL of thissolution together with the excess ligand (if any) wascharged into a medium pressure sapphire NMR tubeof a total volume of 7 mL [23] and pressurized withH2 or D2 up to 2 MPa. The tube was secured on topof a laboratory shaker inside a thermostated chamberand was shaken at 300 rpm. In a separate experi-ment, the same sapphire NMR tube was chargedwith 232 mg of maleic acid and with the catalystsolution. The reactions were followed by 1H or 2HNMR measurements. Spectra were collected on aBruker AC 200 or an AM 360 instrument, the inte-grals were calculated using the WINNMR programon PC, and the 1H chemical shifts were referenced to4,4-dimethyl 4-silapentane sodium sulfonate(TSPSA). The latter and d3-methanol served as inter-nal standards for obtaining the water’s (or D2O’s) H-

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(or D-) concentration by integration. The initial reac-tion rates (number of moles of HDO – reactionproduct – per hour) were calculated from the inte-grals of the HDO peak as a function of time using ca.first fifty minutes, fitting a simple first order polyno-mial (straight line) to the points. The rates weregiven in turnover frequency, TOF (TOF=number of

moles HDO formed in the reaction divided by thenumber of moles of the catalyst and by the durationof the reaction). Each point reported in the figures isan average value of at least three measurements. Therelative error, the reproducibility of the measureddata were estimated to be 15–20 % (error in integra-tion, linear approximation, pH error, etc).

3. Results and discussion

[RuCl2(TPPMS)2]2, [RhCl(TPPMS)3], [RhCl(TPPTS)3]and [RuCl2(PTA)4] were found being active catalystsfor the H/D exchange in both directions (equations(1) and (2)):

H2+D2O X HD+HDO (1)

D2+H2O X HD+HDO (2)

Figure 1 shows the gradual increase of the watersignal intensity in time according to equation (1)with [RuCl2(TPPMS)2]2 as catalyst; from this series ofthe spectra, the time course of the reaction could becalculated and is shown in figure 2. It is noteworthythat the reaction attains the equilibrium correspond-ing to the total amounts of H2 and D2O in the tubein about two hours at 298 K, the initial turnoverfrequency (TOF) of the catalyst being 1 252 h–1. Nosignificant difference in the initial rate of the reac-tions (1) and (2) was observed with the samecatalyst.

[RhCl(TPPMS)3] and [RhCl(TPPTS)3] showed com-parable, albeit somewhat lower catalytic activity atthe same temperature. However, the H–D exchangecatalyzed by [RuCl2(PTA)4] was much slower at 298 Kand was characterized by an inconveniently longinduction period (several hours), so that initial rates,comparable to those of the catalysts with sulfonatedphosphine ligands, could be determined only at343 K. Even then a 2 h induction period precededthe steady phase of the reaction from which timeinterval the catalyst’s activity was obtained. Repre-sentative rate data together with the reaction condi-tions are shown in the table.

Since it is known [24, 25] that the compositionof the hydrido-complexes formed from [Ru-Cl2(TPPMS)2]2 and TPPMS under hydrogen inaqueous solutions is largely influenced by the pH wehave determined the catalytic activity of this complexfor the H/D exchange (equation (1)) in the 15pH512 range. It is seen from figure 3 that in acidicsolutions, below pH 5 the activity of this catalyst isonly slightly changed; however, above this pH there

Figure 1. The intensity change of the HDO signal in the 1H NMRspectrum as a function of time during the reaction of H/Dexchange catalysed by [RuCl2(TPPMS)2]2. The time difference be-tween two following spectra is 10 min, c(Ru)=2.61 mM (in D2O),p(H2)=10 bar, pH=2.5, T=298 K.

Figure 2. A typical reaction curve of H/D exchange in D2Ocatalysed by [RuCl2(TPPMS)2]2. (c(Ru)=2.61 mM (in D2O),p(H2)=20 bar, pH=2.4, t=198 K).

Table. Specific rate of H/D exchange of various catalysts inD2O/H2 system (c(Ru or Rh)=2.61 mM, p[H2]=20 bar).

Catalyst T (°C)pH TOF (h–1)

2.0–5.0[RuCl2(TPPMS)2]2 25 1 2522.0–7.0 25 806[RhCl(TPPMS)3]

6.50[RhCl(TPPTS)3] 25 989[RuCl2(PTA)4] 5.5 25 8.7

338[RuCl2(PTA)4] 5.5 70908[RhCl(PTA)3] 5.2 70

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Figure 3. The rate of H/D exchange as a function of pH catalysedby [RuCl2(TPPMS)2]2 (c(Ru)=2.61 mM (in D2O), p(H2)=20 bar,t=25 °C).

in part — in the known formation of a dihydrideunder basic conditions:

1/2 [RuCl2(TPPMS)2]2+2 TPPMS+2 H2

X [H2Ru(TPPMS)4]+2H++2 Cl– (4)

It is to be mentioned here that at 0.1 MPa totalpressure and 333 K the [HRuCl(TPPMS)3] to[H2Ru(TPPMS)4] transition starts at pH 6 and is com-plete at pH 10, as shown by combined pH–potentio-metric/NMR spectroscopic measurements [24, 25].Although there may be some dependence of thisequilibrium on the hydrogen pressure (presently notknown) therefore a direct comparison may not befully justified, this pH range overlaps to a very largeextent with the pH range of the activity decline incatalysis of H/D exchange.

Concerning the mechanism of the H/D exchange,both reactions (3) and (4) can increase the concen-tration of H+ (HDO) in the aqueous phase and thereverse reactions with D+ would yield HD in the gasphase. It is also likely, that the coordinatively unsatu-rated [HRuCl(TPPMS)3] is more prone to protonationthan [H2Ru(TPPMS)4] with a saturated coordinationsphere, although equilibrium (4) could be rapidly(‘instantaneously’) reversed by titration with acid.However, assuming first order kinetics in H+ (D+)for the reverse reactions (3) and (4), perhaps themost decisive factor in determining the exchangerate is the availability of H+ in (3), in contrast to itslack in (4). Therefore in the forward direction, reac-tion (4) produces the stoichiometric amount of H+

(HDO); however, the catalytic exchange between H2

and D2O can proceed only at a barely detectablelevel.

We also tried to follow the H/D exchange in thepresence of olefins, as possible substrates of hydro-genation, by adding maleic acid to a reaction mix-ture which, in the absence of the olefin, would giverise to a rapidly progressing H/D exchange([RuCl2(TPPMS)2]2+x TPPMS in acidic solution). Im-portantly, this resulted in a complete lack of theisotope exchange reaction between H2 and D2O.Instead, fumaric acid was immediately precipitatedfrom the solution. Clearly, more work should bedone in order to clarify the details of this isomeriza-tion and its relation to the lack of H/D exchange inwater. However, we wish to stress that althoughboth hydrogenation and H/D exchange should in-volve the splitting of H2, one has to be careful ininvolving the findings on H/D exchange systems intodiscussions on the mechanism of hydrogenations.

is a fast decrease of the rate and in strongly alkalinesolutions, negligible or almost no exchange isobserved.

Under the conditions used here for the exchangereaction [RuCl2(TPPMS)2]2 yields [HRuCl(TPPMS)3] inacidic aqueous solutions [24–26]:

1/2 [RuCl2(TPPMS)2]2+TPPMS+H2

X [HRuCl(TPPMS)3]+H++Cl– (3)

The high rates of reaction (1) catalyzed by thiscomplex (TOFmax=1 252 h–1) seem reasonable re-calling the rates of hydrogenation of various olefinicand 2-oxo-acids catalyzed by [HRuX(TPPMS)3] (X=Cl– or CH3COO–) falling in the range of TOF=100–700 h–1 at 333 K and 0.1 MPa total pressure [27].Under similar conditions, [RhCl(TPPMS)3] generallyhydrogenated water soluble olefinic substrates withTOF B 200 h–1; however, in case of fumaric acid, aninitial activity of TOF=1 270 h–1 was determined[28]. Although the mechanisms of the processes ofH/D exchange and olefin hydrogenation may differ,it is reasonable to assume that both require theformation of the appropriate hydrides from theprecatalysts.

The inferior performance of [RuCl2(PTA)4] can berelated to the observed slow and incomplete forma-tion of [HRuCl(PTA)4], [HRu(PTA)5]

+ and[H2Ru(PTA)4] [29, 30]; unfortunately, no quantitativedata are as yet available for the rate of hydrogenactivation by [RuCl2(PTA)4] and for the equilibria ofthe formation of the above hydrides, hindering anyfurther speculation on their role in the catalysis ofH/D exchange.

An explanation for the effect of pH on the rate ofH/D exchange catalyzed by [RuCl2(TPPMS)2]2 in thepresence of excess TPPMS can be found — at least

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Further studies on the mechanism of the catalyticH/D exchange with [RuCl2(TPPMS)2] and other watersoluble transition metal phosphine complexes ascatalysts, on the role of such processes in catalytichydrogenation, as well as on possible utilization [31],are in progress in our laboratories.

Acknowledgements

This work was supported by the Office federal del‘education et de la science, Switzerland (OFESC98.0011) and by the Hungarian National ResearchFoundation (OTKA T29934 and F023159) and the re-search is part of the collaboration within the COSTD10/0001 Working Group. L. Nadasdi is grateful for anOFES fellowship, and G. Kovacs is grateful for a fellow-ship of the Universite de Lausanne. F. Joo thanks John-son-Matthey Pub. Ltd. for a loan of RuCl3.aq andAlbright & Wilson for a gift of tetrakis(hydrox-ymethyl)phosphonium chloride.

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h/d exchange between h2–d2o and d2–h2o catalyzed by water soluble tertiary phosphine complexes...

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