Photocatalysis

DOI: 10.1002/anie.200600543

A Supramolecular Photocatalyst for theProduction of Hydrogen and the SelectiveHydrogenation of Tolane**

Sven Rau,* Bernhard Sch�fer, Dieter Gleich,Ernst Anders, Manfred Rudolph, Manfred Friedrich,Helmar G�rls, William Henry, and Johannes G. Vos

The central processes in natural photosynthesis are light-driven electron transfer from the special-pair chromophore tothe primary acceptor, and the subsequent charge separationto enable the reduction of substrates. The transfer of thesedesign principles to artificial systems has led to the develop-ment of catalytic multicomponent systems for the photo-catalytic reduction of CO2

[1] and for the production ofhydrogen.[2] In these systems, photoredox-active metal com-plexes and separate redox catalysts are used to facilitatedirected photoinduced electron transfer. For example, Gr$t-zel et al. showed that heterogeneous photocatalyst systemscan be used for the generation of hydrogen,[3] and Curraoet al. reported the photochemical[4] splitting of water with asystem consisting of a photoactive silver/silver chloride anodeand a silicon solar cell acting as a cathode. However, in theseheterogeneous systems, the electron-transfer processesdepend on many interfacial parameters that are difficult toinfluence. Homogeneous systems, consisting of a photoactiveRu or Ir complex and an electron relay of Co or Pdcomplexes,[5] which generate H2 in intermolecular reactions,have also been described. Their effectiveness is limited by theinstability of the reduced photocatalysts.

In an intramolecular photocatalyst it should be easier tocontrol a vectorial photoelectron transfer by precise tuning ofthe physical properties and orientation of the molecularcomponents. If it were also possible to slow down charge-recombination processes, efficient photocatalytic systems

may become feasible. With complex 1, we report the firstsupramolecular photocatalyst meeting these requirements.[6]

Under mild conditions (25 8C, irradiation at 470 nm) and inhomogeneous solution, this compound catalyzes the gener-ation of hydrogen and the selective hydrogenation fromtolane to cis-stilbene without added H2.

The heterodinuclear Ru–Pd complex used as photocata-lyst consists of the following three components (see Scheme 1,compound 1):

– a photoactive ruthenium(II) fragment acting as a lightabsorber,[7]

– a PdCl2 unit which, when coordinated at the other end ofthe assembly, acts as a catalytic center,

– a bridging unit (tetrapyridophenazine, tpphz) connectingthe two metal centers through a conjugated reducible p-electron system.[8]

Compounds 1–4 (Scheme 1) were isolated in high yieldsand have been fully characterized.

In the first step of the photocatalytic process theruthenium moiety was excited using 470-nm light. To facilitateefficient electron transfer to the Pd center, the electron donortriethylamine (TEA) was utilized to re-reduce the photo-chemically formed RuIII center.[9] The results obtained in thisstudy are summarized in Table 1. We observed no catalyticdevelopment of hydrogen in the dark (run 6) and found thatthe heterodinuclear complex 1 is the only effective photo-catalyst (runs 1–5, Table 1). The Pd-free complex 2 and thebinuclear Ru complex 3 are poor photocatalysts and formonly very small amounts of H2 (TON= 0.56, runs 9 and 10,Table 1). The replacement of the tpphz bridging ligand bybipyrimidine in complex 4 also resulted in a loss of catalyticactivity (run 11, Table 1). These results show that the presenceall three components in 1 is essential for the catalytic process.With 1, turnover numbers of 56 mol hydrogen per molcatalyst can be achieved.

Table 1 also demonstrates that the amount of photo-catalytically formed hydrogen depends strongly on the TEAconcentration and the exposure time (runs 1–5, Table 1), and

Scheme 1. Structures of complexes 1–4 ; L=4,4’-di-tert-butyl-2,2’-bipyr-idine.

[*] Dr. S. Rau, Dr. B. Sch,fer, Dr. M. Rudolph, Dr. M. Friedrich,Dr. H. G1rlsIAAC FSU JenaLessingstrasse 8, 07743 Jena (Germany)Fax: (+49)364-194-8102E-mail: sven.rau@uni-jena.de

Dr. D. Gleich, Prof. Dr. E. AndersIOMC FSU JenaHumboldtstrasse 10, 07743 Jena (Germany)

W. Henry, Prof. Dr. J. G. VosNational Centre for Sensor ResearchSchool of Chemical Sciences, Dublin City UniversityDublin 9 (Ireland)

[**] We thank Prof. D. Walther (Jena) for support and inspiration, andalso the referees for their helpful comments. This work wassupported financially by the DFG (SFB 436) and Enterprise IrelandSC20030074.

Supporting information for this article is available on the WWWunder http://www.angewandte.org or from the author.

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that chloride ions inhibit the reaction (run 8, Table 1). Adetailed analysis of hydrogen formation as a function ofreaction time shows that the amount of H2 produced increasessteadily and levels off after 1200 min. After about 1800 minno more hydrogen is produced.[10] Initially the rate ofhydrogen formation increases with increasing TEA concen-tration (runs 1–4, Table 1), but at a TEA concentration of0.86 molL�1, the rate becomes relatively independent (ca.1600 nmolmin�1 in runs 4 and 5, Table 1).

Under the same conditions, complex 1 can also catalyzethe selective reduction of tolane to cis-stilbene. Again,turnover numbers of more than 50 are achieved withoutadditional formation of H2 (run 12, Table 1). The exclusivegeneration of the cis isomer as well as the inactivity of thepalladium-free compound 2 (run 13, Table 1) indicate that thealkyne reduction occurs at the palladium center. Since 1 isinert without irradiation (run 14, Table 1), the tolane reduc-tion observed (“without hydrogen”) is also photocatalytic innature. Also without irradiation and without TEA, 1 canreduce tolane to cis-stilbene if hydrogen is added (run 15,Table 1). This confirms that the palladium moiety acts as thecatalytic center.

To obtain further information on the role of the palladiumand the bridge in the photocatalytic reduction process,photophysical, electrochemical, (Table 2), EPR, and NMRstudies as well as theoretical investigations were carried out.It is generally accepted that for tpphz complexes theelectronic coupling between the bipyridine M-coordinationsphere and the central phenazine ring is weak and that the3MLCT state, which is located on the phenazine component,to the ligand may reduce an adjacent metal center.[8] Ourphoto-EPR examinations show that upon irradiation of 1–3 at436 nm in CH3CN/TEA mixtures, an EPR signal is detectedthat can be assigned to a pyrazine-based radical anion. Thisbecomes evident by a comparison with electrochemicallygenerated data reported by Kaim et al.[11] Cyclovoltammetrymeasurements for 1 indicate that the first reduction is

phenazine based (�0.59 V), while an irreversible reductionof the PdII is observed at �0.78 V. The bipyridine ligands arereduced at more negative potentials (between �1.05 and�1.39 V).

In addition, 1H NMR measurements indicate that thesignals of protons adjacent to the Pd center are almostunchanged upon irradiation, moving slightly from 9.20 to9.26 ppm. This indicates that palladium remains coordinatedduring irradiation. Further evidence for this conclusion can bedrawn from the attempted photoreaction between 1/triethyl-amine and cyclohexene. Cyclohexene is not hydrogenatedunder these conditions. The presence of colloidal palladiumwould be expected to result in the catalytic formation ofcyclohexane.[12]

For additional information on the location of the excitedstate and the radical anion formed in the photoreducedcomplex 1, we examined simplified models (M1 and M2),without tert-butyl groups at the bipyridine ligands, by meansof DFT methods using the X-ray data obtained for 2 and 3(Figure 1).[12] A comparison of the structural parameters ofthe DFT-optimized structure ofM2 with those obtained in thesolid state for 2 show very good agreement.[10] The coordina-tion of the {PdCl2} fragment (M1) does not lead to large

Table 1: Examination of the photocatalytic development of hydrogen and hydrogenation of tolane in presence of TEA at 25 8C.[a]

Run Cat. ctolane[molL�1]

cTEA[molL�1]

cTBACl[molL�1]

tirrad. (470 nm)[min]

TONfor H2

[b]TONfor stilbene[c]

TOF

1 1 – 0.00086 0 120 0.15 – 652 1 – 0.0086 0 120 1.45 – 6283 1 – 0.086 0 120 2.25 – 9754 1 – 0.86 0 120 3.6 – 15605 1 – 2.08 0 1750 56.4 – 16766[d] 1 – 2.04 0 0 0 – 07 1 – 0 0 360 0 – 08 1 – 0.16 0.16 150 0.04 – 1.49 2 – 2.08 0 1550 0.56 – 1.910 3 – 2.08 0 1550 0.56 – 1.911 4 – 2.08 0 1550 0 – 012 1 0.026 3.6 0 6960 0 63 47113 2 0.026 3.6 0 6960 0 0 014[e] 1 0.026 3.6 0 0 0 0 015[e] 1 0.026 0 0 0 0 485 4203[e]

[a] Reaction conditions: ccomplex=5.2G10�5 molL�1, solvent: acetonitrile; TBACl= tetrabutylammonium chloride, TON [mol productmol�1 catalyst],TOF [nmol product min�1] . [b] Turnover number determined by GC analysis of the products in the N2 carrier gas. [c] Determined by GC or GC–MSanalysis of the products. [d] Sample without irradiation after 360 min. [e] Sample without irradiation after 6000 min.

Table 2: Photophysical and electrochemical data for compounds 1–3.

Cmpd lmax,abs. [nm][a] lmax,em. [nm][a] taer [ns][b] Ered [V]

[c]

1 445 650[d] 27 �0.59, �0.78,[e] �1.05,�1.24, �1.39

2 445 638 154 �0.61, �1.08, �1.30,�1.48, �1.76

3 445 652[d] 27/130 �0.33, �1.05, �1.24,�1.48

[a] 293 K, CH3CN. [b] taer is the lifetime of the excited state in air-saturated solvent. [c] In CH3CN versus Ag/AgCl (Fc/Fc+=0.83 V).[d] Very weak. [e] Peak potential of irreversible Pd reduction; Eox(Ru)-[1,2,3]=1.6 V.

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changes in the structural parameters. As expected, in bothcompounds the HOMO is centered on the ruthenium center,while the LUMO is located on the pyrazine ring. The reducedstate of M1 and M2 was simulated by a decrease of the totalcharge by one. In both model systems the SOMO is situatedon the pyrazine ring and corresponds to the LUMO in theinitial complexes; this is in excellent agreement with the EPRand emission spectra (Figure 2a,b).

Since chloride ions inhibit hydrogen generation with 1, achloride ion was removed from the palladium center in thereduced model complex M1, and the structure was optimizedonce more (M1D). This optimized structure indicates not onlythe relocation of the SOMO to the Pd center (Figure 2c[13]),but also the regeneration of the photoredox-active {RuII–tpphz} fragment, which can once again enter into a photo-catalytic cycle with TEA.

The experimental and theoretical results suggest that inboth catalytic processes 1 transfers two electrons to thesubstrates during the catalytic cycle. At this stage no detailedmechanistic data are available, but a likely reaction pathwaymay include the following steps: a twofold photoreduction of1 by Et3N according to Equation (1) (see reference [9]), andthe subsequent reduction of the protons at the Pd centeraccording to Equation (2). In the presence of tolane thephotoreduced catalyst can reduce the alkyne as described byEquation (3).

2Et3Nþ 1 ðhnÞ��! ð1þ 2 e�Þ þ 2Hþ þ oxidation products of Et3N ð1Þ

2Hþ þ ð1þ 2 e�Þ ! H2 þ 1 ð2Þ

2Hþ þ ð1þ 2 e�Þ þ Ph-C�C-Ph ! cis-Ph-CH¼CH-Phþ 1 ð3Þ

At present, we assume that after the first photoelectron-transfer step and the generation of the RuIII–phenazine(�)–PdII radical, RuIII is reduced to RuII by TEA. After thechloride loss from the PdII center, electron transfer fromphenazine(�) to the PdII center takes place yielding a PdI

moiety. When this sequence of steps is repeated, two electronsare stored in the molecule. These electrons are then utilized toreduce two protons[9] originating from oxidized TEA. Alter-natively, the deprotonated, photooxidixed triethylamine canalso transfer another electron to the singly reduced catalyst.[14]

At present we are not able to differentiate between these twopossibilities. In the presence of tolane, a fast photo-inducedreduction of the substrate is observed. In both catalyticsystems, the Pd center is therefore acting both as an electronstorage site and as a catalytic center.

The supramolecular assembly presented here opens upvarious possibilities for increasing the catalytic efficiency. Forexample, the HOMO/LUMO difference in the bridging-ligand/metal fragment can be adjusted by derivatization of thebridge, by introduction of other metals at the periphery, or byvariation of the coordinated anions. Moreover, attachment ofthese oligonuclear metal complexes to TiO2 surfaces couldinfluence not only the direction but also the kinetics of thephotoinduced electron transfer. These investigations will alsofacilitate a more detailed insight into the reaction cycle andmay as well lead to new photocatalytic reactions with othersubstrates.

Received: February 9, 2006Revised: April 26, 2006Published online: August 17, 2006

Figure 1. Solid-state structures of 2 (top) and 3 (bottom).

Figure 2. SOMOs of a)M1, b)M2, and c) the model after chloridedissociation (M1D).

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.Keywords: homogeneous catalysis · hydrogen · palladium ·ruthenium · supramolecular chemistry

[1] J. Hawecker, J.-M. Lehn, R. Ziessel, J. Chem. Soc. Chem.Commun. 1983, 536.

[2] J.-M. Lehn, J.-P. Sauvage, Nouv. J. Chim 1977, 1, 449.[3] K. Kalyanasundaram, J. Kiwi, M. Gr$tzel, Helv. Chim. Acta

1978, 61, 2720.[4] A. Currao, V. R. Reddy, M. K. van Veen, R. E. I. Schropp, G.

Calzaferri, Photochem. Photobiol. Sci. 2004, 3, 1017, andreferences therein.

[5] J. I. Goldsmith, W. R. Hudson, M. S. Lowry, T. H. Anderson, S.Bernhard, J. Am. Chem. Soc. 2005, 127, 7502 – 7510; J. R. Fisher,D. J. Cole-Hamilton, J. Chem. Soc. Dalton Trans. 1984, 809 – 813.

[6] After submission of this manuscript a publication by Sakai et al.appeared (online publishing date: March 28, 2006) describings aheterodinuclear ruthenium–platinum photocatalyst capable ofproducingH2 with a TON of 2.4: H. Ozawa,M. Haga, K. Sakai, J.Am. Chem. Soc. 2006, 128, 4926 – 4927.

[7] B. Dietzek, W. Kiefer, J. Blumhoff, L. BMttcher, S. Rau, D.Walther, U. Uhlemann, M. Schmitt, J. Popp, Chem. Eur. J. 2006,12, 5105.

[8] C. Chiorboli, M. A. J. Rodgers, F. Scandola, J. Am. Chem. Soc.2003, 125, 483 – 491.

[9] R. Konduri, H. Ye, F. M. MacDonnell, S. Serroni, S. Campagna,K. Rajeshwar,Angew. Chem. 2002, 114, 3317;Angew. Chem. Int.Ed. 2002, 41, 3185 – 3187.

[10] See the Supporting Information.[11] J. Fees, W. Kaim, M. Moscherosch, W. Matheis, J. KlOma, M.

KrejcOk, S. ZQlis, Inorg. Chem. 1993, 32, 166 – 174.[12] M. W. van Laren, C. J. Elsevier Angew. Chem. 1999, 111, 3926 –

3928; Angew. Chem. Int. Ed. 1999, 38, 3715 – 3717.[13] The following references can be found in the background

information. All calculations (Gaussian03)[Za] were carried outwithout symmetry restrictions with the hybrid functionalB3LYP.[Zb] Ru and Pd were described by means of quasi-relativistic effective nuclear potentials (“small core”).[Zc] Level I:standard basis set 6-311G(d,p) (implemented in Gaussian03) forH, C, N and Cl;[Zd] level II: 6-311G(d,p) basis set, solventacetonitrile (CPCM model[Ze]). References [Za–Ze] are given inthe Supporting Information.

[14] We thank one of the referees for pointing out this importantmechanistic alternative.

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