Julie L. Boyer

Scott R. Wilson

Thomas B. Rauchfuss

Comptes Rendus Chimie

Available online at www.sciencedirect.comC. R. Chimie 11 (2008) 922e925http://france.elsevier.com/direct/CRAS2C/Preliminary communication / CommunicationNanoscale ensembles using building blocks inspired bythe [FeFe]-hydrogenase active siteJulie L. Boyer, Thomas B. Rauchfuss*, Scott R. WilsonDepartment of Chemistry, University of Illinois at Urbana-Champaign, 600 South Mathews Avenue, Urbana, IL 61801, USAReceived 23 August 2007; accepted after revision 10 January 2008Available online 15 May 2008AbstractThe hydrogenase model [Fe2(S2C3H6)(CN)2(CO)4]2 was employed as a molecular tecton for the construction of supramolec-ular aggregates. IR spectroscopy indicated that cyanide bridged aggregates are formed when [Fe2(S2C3H6)(CN)2(CO)4]2 wastreated with Lewis acids such as Zn(tetraphenylporphyrinate), [Cu(NCMe)(2,20-bipyridine)]PF6 and [Cu(NCMe)4]PF6. Condensa-tion of [Fe2(S2C3H6)(CN)2(CO)4]2 with the tritopic Lewis acid [Cp*Rh]2þ afforded the novel expanded tetrahedron cage,{[Fe2(S2C3H6)(CN)2(CO)4]6[Cp*Rh]4}4. The tetrahedron cage was characterized crystallographically as the PPN salt. To citethis article: Julie L. Boyer et al., C. R. Chimie 11 (2008). 2008 Académie des sciences. Published by Elsevier Masson SAS. All rights reserved.Keywords: Cyanide; Supramolecular chemistry; Cage compounds; Lewis acids; Carbonyl ligands1. IntroductionCyanometallates are well known to serve as ligandsfor Lewis acidic metal centers [1e4]; this area datesback to the synthesis of Prussian Blue and the Hofmannclathrates [5e9]. Inspired by these precedents,researchers have recently learned how to generate mo-lecular cubiodal cages via condensations of cyanometal-lates and the Lewis acids wherein both bear blockingcoligands [10,11]. With certain coligands, one canobtain molecular cages that display distinctive hosteguest or magnetic properties [12e15]. The inventionof new cyanometallate cages depends on the develop-ment of new cyanometallate building blocks [16e19].* Corresponding author.E-mail address: rauchfuz@uiuc.edu (T.B. Rauchfuss).1631-0748/$ - see front matter  2008 Académie des sciences. Publisheddoi:10.1016/j.crci.2008.01.017In evaluating new cyanometallate building blocks,we focused on the dianions [Fe2(SR)2(CN)2(CO)4]2,which have been of intense interest as models for theactive site of the [FeFe]-hydrogenases [20e22]. Suchspecies exhibit Bronsted basicity, redox activity, andan ambidentate cyanide ligand. In addition to the pos-sibility of incorporated functionality into multimetallicensembles, such species are attractive building blocksfor several reasons: (i) they are readily prepared witha range of thiolate bridges and counter cations [23],(ii) the Fe(CN)(CO)2 vertices are fluxional via a turn-stile rotation (Scheme 1) [24] which allows the cyanideligands to readily reorient to accommodate the geomet-ric preference of templating Lewis acids, and possiblyto correct non-optimally folded cages of the annealingstructure and (iii) the Bronsted basicity of the FeeFebond in these dianionic species has been demonstrated.We were particularly intrigued by the prospect ofby Elsevier Masson SAS. All rights reserved.FeS(CH2)nCCCC CCSFeOONNOOFeS(CH2)nCCCC CCSFeOOOONNFeS(CH2)nCCCC CCSFeOOONONScheme 1.923J.L. Boyer et al. / C. R. Chimie 11 (2008) 922e925elucidating pathways to potentially multifunctional en-sembles that could open the door to the realm of cata-lytically active coordination cages.2. Results and discussionWe initially surveyed the ability of [Fe2(S2C3H6)(CN)2(CO)4]2 to serve as a ligand by screening its in-teraction with several metal ions, using FT-IR spectros-copy as an assay.1 Solutions of [Fe2(S2C3H6)(CN)2(CO)4]2 were found to react with MeCN solutionsof Lewis acids such as [Cu(NCMe)(bipy)]PF6 and[Cu(NCMe)4]PF6 in a 1:1 ratio to give soluble adducts.In each case, adduct formation was signaled by shiftsin the nCN band, 37 cm1, to higher energy. A shiftin the nCO bands to higher energy was also observed,indicative of a Lewis acid adduct formation [25]. Sim-ilarly, the highly lipophilic salt (PPN)2[Fe2(S2C3H6)(CN)2(CO)4] (PPNþ ¼ Ph3PNPPh3þ) was found to form1:2 adducts with ZnTPP (TPP¼meso-tetraphenylpor-phyrin dianion). IR spectra for [Fe2(S2C3H6)(CN)2(CO)4][ZnTPP]22 exhibited nCN band at 2100 cm1,a shift of 25 cm1 vs. (PPN)2[Fe2(S2C3H6)(CN)2(CO)4], consistent with the formation of FeCNeZnlinkages.The preceding condensations clearly indicated theability of this dimetallic dicyanide as a supramoleculartecton. We therefore directed our attention to the morechallenging possibility of generating three-dimensionalensembles. In this case, we turned to the use ofCp*Rh2þ, which is a versatile reagent in stabilizinghigh molecular weight coordination cages [26]. In thediaxial orientation, the two cyanide vectors in [Fe21 IR spectra of the titration of [Cu(NCMe)4]PF6 and [bipyCu(NCMe)]PF6 to [Fe2(S2C3H6)(CN)2(CO)4]2 are given in the supple-mentary material.(S2C3H6)(CN)2(CO)4]2 define an angle of w115,which should favor the formation of adamantoid or ex-tended tetrahedral cages when reacted with the half-sandwich [Cp*Rh(NCMe)3](PF6)2 [27].IR spectral analysis demonstrated that 1.5 equiv[Fe2(S2C3H6)(CN)2(CO)4]2 condensed readily with[Cp*Rh(NCMe)3](PF6)2 in acetonitrile solution (Eq.(1)). The IR spectrum of the product solution featureda band assigned to nm-CN at 2108 cm1, shifted tohigher energy vs. starting [Fe2(S2C3H6)(CN)2(CO)4]2(2075 cm1). The three nCO bands (1979, 1935,1913 cm1) were also shifted to higher energy vs.starting [Fe2(S2C3H6)(CN)2(CO)4]2 (1963, 1922,1884 cm1). Titrations indicated that cage formationwas strongly favored since the treatment of (NEt4)2[Fe2(S2C3H6)(CN)2(CO)4] with 0.5 equiv of [Cp*Rh(NCMe)3](PF6)2 gave aggregates and unreacted(NEt4)2[Fe2(S2C3H6)(CN)2(CO)4]. At 1/0.67 ratio of(NEt4)2[Fe2(S2C3H6)(CN)2(CO)4] to [Cp*Rh(NCMe)3](PF6)2, the IR spectrum indicated the presence of a sin-gle condensed aggregate (Fig. 1). ESI-MS analysis ofthe reaction (PPN)2[Fe2(S2C3H6)(CN)2(CO)4]þ 0.67[Cp*Rh(NCMe)3](PF6)2 showed a strong current atm/z¼ 811 and 1261, corresponding to {[Fe2(S2C3H6)(CN)2(CO)4]6[Cp*Rh]4}4 and {PPN{[Fe2(S2C3H6)(CN)2(CO)4]6[Cp*Rh]4}}3, respectively.6½Fe2ðS2C3H6ÞðCNÞ2ðCOÞ42 þ 4½CpRhðNCMeÞ32þ/ f½Fe2ðS2C3H6ÞðCNÞ2ðCOÞ46½CpRh4g4 ð1ÞFragile crystals of the Fe12Rh4(CN)12 cage were ob-tained using a variety of different counter ions Q(Q¼NEt4, NBu4, PPN). The tetra-PPNþ salt affordedthe highest quality crystals. Crystallographic analysisindicates that cage consists of an expanded tetrahedronas predicted by the Stang analysis (Fig. 2) [27]. FourCp*Rh vertices are connected by six [Fe2(S2C3H6)2200 2150 2100 2050 2000 1950 1900 1850 1800Fig. 1. IR spectrum of (NEt4)2[Fe2(S2C3H6)(CN)2(CO)4] (top),(NEt4)2[Fe2(S2C3H6)(CN)2(CO)4] þ 0.5[Cp*Rh(NCMe)3](PF6)2(middle), (NEt4)2[Fe2(S2C3H6)(CN)2(CO)4]þ 0.67[Cp*Rh(NCMe)3](PF6)2 (bottom). All spectra were obtained in MeCN solution.924 J.L. Boyer et al. / C. R. Chimie 11 (2008) 922e925(CN)2(CO)4]2 linkers. The average Rh/Rh distancein the expanded tetrahedron is 11.50 Å, dwarfing thew5 Å edge distance reported in previous simple cyano-metallate tetrahedra [28,29]. The dicyanide linkers areindeed diaxially oriented (see Scheme 1). In this way,the propanedithiolate project toward the cage interior.Preliminary crystallographic analysis of the NEt4þ saltrevealed the same tetrahedral framework indicatingthat the counter ions do not influence the topology ofthe ensemble.The 1H NMR spectrum of MeCN solution of (PPN)4{[Fe2(S2C3H6)(CN)2(CO)4]6[Cp*Rh]4} displayed alarge upfield shift (0.5e0.75 ppm) for the methyleneprotons, consistent with the encapsulation of theFig. 2. Molecular structure of the anion {[Fe2(S2C3H6)(CN)2(CO)4]6[Cp*Rh[Fe2(S2C3H6)(CN)2(CO)4] shown on only one face for clarity. (Right) hyS¼ yellow, Fe¼ red, and Rh¼ green. (For interpretation of the referencesof this article.)dithiolate straps within the tetrahedral cage. The spec-trum also indicates the presence of multiple rotatamers,which reinforces the rotational flexibility and complex-ity of the [Fe2(S2C3H6)(CN)2(CO)4]2 building block.The condensation of dimetallic dicyanides to givesupertetrahedra enjoys some generality as IR spectros-copy indicated that both (NEt4)2[Ru2(S2C3H6)(CN)2(CO)4] and (NEt4)2[Fe2(S2C2H4)(CN)2(CO)4] form3:2 adducts with [Cp*Rh(NCMe)3](PF6)2. While [M2(SR)2(CN)2(CO)4]2 has shown to be a verstile build-ing block, the condensed aggregrates were metastable,depositing insoluble materials upon either standing orconcentration. The fragility of the aggregrates pre-cluded protonation and catalysis studies.3. ConclusionsAlthough cages containing metalemetal bonds arevery rare, it is interesting to note that PdePd bondsare found in the very first reported molecular cyanome-tallate cage, [(C5H4Me)Mn(CO)2(CN)Pd(CO)]4 [30].Thus, both the present result and the historical recordsuggest that dimetallic cyanides are promising precur-sors to new classes of molecular cyanometallate cages.In principle, such cages, provided that they are suffi-ciently robust, would combine both catalytic andhost-guest properties.4. Experimental sectionReactions and manipulations were performed undera nitrogen atmosphere, using standard Schlenk-linetechniques. Electrospray ionization-mass spectra(ESI-MS) were acquired using a Micromass QuattroQHQ quadrupoleehexapoleequadrupole instrument.FeFeS SOCOCCOCOCp*Rh RhCp*Cp*RhRhCp*FeFeSSOC COOCOCFeFeSSCOCOCOCO]4}4 (left) and schematic representation of the tetrahedron cage withdrogen atoms are omitted for clarity. N¼ blue, C¼ gray, O¼ red,to color in this figure legend, the reader is referred to the web version925J.L. Boyer et al. / C. R. Chimie 11 (2008) 922e925Infrared spectra were obtained on a Mattson GalaxySeries FT-IR 3000.4.1. (PPN)2[Fe2(S2C3H6)(CN)2(CO)4]A solution of 1.44 g (0.0023 mol) (NEt4)2[Fe2(S2C3H6)(CN)2(CO)4] in 7 mL MeOH was added drop-wise to a stirred solution of 2.65 g (0.0046 mol) PPNClin 5 mL MeOH. Addition of 20 mL of degassed H2Oprecipitated a red oily solid which was recrystallizedfrom MeCN by the addition of Et2O. Yield: 3.026 g(92%). IR (MeCN, cm1): nCN¼ 2075; nCO¼ 1963,1921, 1883. 1H NMR (CD3CN): d 1.648 (m, 2H,S2C2H4CH2), 1.839 (m, 4H, S2C2H4CH2), 7.468e7.652 (m, 60H, Ph). Anal. Calcd for C81H66Fe2N4O4P4S2$CH3CN (found): C, 66.44 (66.35); H, 4.64(4.60); N, 4.67 (4.86).4.2. (PPN)4{[Fe2(S2C3H6)(CN)2(CO)4]6[Cp*Rh]4}A solution of 0.0305 g (0.047 mmol) [Cp*Rh(NCMe)3](PF6)2 in 10 mL MeCN was added dropwiseto a stirred solution of 0.102 g (0.07 mmol) (PPN)2[Fe2(S2C3H6)(CN)2(CO)4] in 15 mL MeCN. Dropwiseaddition of 50 mL of degassed H2O precipitated a redsolid. The deep red product was extracted into 20 mLof MeCN. IR (MeCN, cm1): nCN¼ 2107; nCO¼1978, 1934, 1912. ESI-MS (m/z): 811 {[Fe2(S2C3H6)(CN)2(CO)4]6[Cp*Rh]4}4, 1261 {PPN{[Fe2(S2C3H6)(CN)2(CO)4]6[Cp*Rh]4}}3 Crystals were grown inthe course of one week at 11 C from layering a solu-tion of MeCN with hexane and Et2O. Solutions of(PPN)4{[Fe2(S2C3H6)(CN)2(CO)4]6[Cp*Rh]4} provedmetastable with respect to depositing insoluble mate-rial upon either concentration or standing. The instabil-ity of the tetrahedron cage prohibited the quantitativeremoval of the PPNPF6 contaminant.AcknowledgementsThis research was supported by the US Depart-ment of Energy (DEFG02-90ER 14146). 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Nanoscale ensembles using building blocks inspired by the [FeFe]-hydrogenase active site

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Nanoscale ensembles using building blocks inspired by the [FeFe]-hydrogenase active site

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