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Electronic Properties of Monocapped Prismane and Basket Iron-Sulphur

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Electronic Properties of Monocapped Prismane and Basket Iron-Sulphur ELECTRONIC PROPERTIES OF MONOCAPPEDMONOCAPPED PRISMANE AND BASKET IRON-SULPHURIRON-8ULPHUR CLUSTERS B. S. SNYDER, M. S. REYNOLDS and R. H. HOLM* Department of Chemistry, Harvard University, Cambridge, MA 02138, U.S.A. and G. C. PAPAEFTHYMIOUPAPAEFfHYMIOU and R. B. FRANKELTFRANKELt Francis Bitter National Magnet Laboratory, Massachusetts Institute of Technology,Technology, Cambridge, MA 02139, U.S.A.U.S.A. Abstract-The ground state electronic properties of the mixed-valence clusters Fe7S6Fe7S6 (PEq4Cl,(PEt&Cl? (1),(l), Fe6S6(PEt,)4L2Fe,&(PEt&L2 [L = Cl- (2).(2), Br- (4)(4), I-I~ (5) and PhSPhS- (6)], Fe&Se,Fe6Se6 (PEt3)4CI2(PEt3)&12 (3) and [Fe[Fe,S,(PEt),]‘-6S 6(PEt) 6] 1- (9), have been investigated by magnetic susceptibility, magnetization and MossbauerMiissbauer spectroscopic measurements. Cluster 1 has a (idealized C,,)C3v) monocapped prismane structure and clusters 2-6 and 9 adopt the CtiC 2v “basket”"basket" 1 configuration based on the [Fe[Fe6(~2-S)@3-S)4@4-S)]2+,1+6(J.lrS)(j.trS) 4(j.t4-S)] 2+, + core unit. From magnetic prop-prop­ erties, the ground states S = 1/2l/2 (1, 9) and S = 1 (2-5) were established. Unlike thethe other clusters, 6 did not show a Curie region of susceptibility; its ground state was not directly determined but is probably S = 1. MossbauerMiissbauer spectra were successfully analysed in terms of a 1:: 1:: 1 ironiron site population,population. Isomer shifts and quadrupole splittings were assigned to each site. Magnetically perturbed spectra obtained in applied fields of 60-80 kOe were analysed toto give the magnetic hyperfine parameters and magnetic hyperfine fields in clusters 1-51-5 and 9. The magnetic spectra demonstrate antiantiferromagneticferromagnetic spin coupling which affords thethe indicatedindicated ground states. NearlyNearly allall iron-sulphuriron-sulphur clusters are characterized ments. For example, the ground spin states and byby corecore structuresstructures thatthat are built up entirely by ver­ver- associated matters of the electronic structures of tex-sharingtex-sharing and/orand/or edge fusionfusion of Fe2S2 rhombs. II*2,2 cubane-type [Fe4S4(SR)4P-,2-,I-[Fe4S4(SR)4]3-.2-“- clusters have TheThe prototype singlesingle planar rhomb isis found in the been eluciated in some detail by these methods.6-8methods.6-8 binuclearbinuclear speciesspecies [Fe2S2L4]2-.[Fe2S2L412-. Clusters proceed in All of these clusters have the common property of complexitycomplexity fromfrom thisthis typetype toto thosethose with nuc1earitiesnuclearities molecular antiferromagnetism. 3,3, 4,4, 6,6, 7,7, 88 andand 18.18.22*3,3 Excluding organometallics As part of our work on higher-nuclearity Fe-S andand with thethe exceptionexception Of[of [Fe&(PEt,),]“~”Fe6S8(PEt3)6F+,I+ 4,54V5 in clusters, we have recently prepared and described thethe preceding set,set, thesethese clusters have as terminal the structures and certain reactivity aspects of a ligands,ligands, halide, RS-RS or RO-. The electronic prop­prop- new series of hexa- and heptanuclear clusters. 3,9-12 ertieserties ofof manymany ofof thesethese materials have been probed These differ from the above in having a mixed byby applicationapplication ofof MossbauerMiissbauer spectroscopyspectroscopy and by ligandligand set that includes tertiary phosphines and core magnetizationmagnetization andand magneticmagnetic susceptibilitysusceptibility measure- structures not yet found in the absence of phosphine. With reference to Fig. 1, the core geometry of Fe7S6(PEt3)4CI/Fe7S6(PEt3)4C139 (1) approaches that ...*AuthorAuthor toto whomwhom correspondencecorrespondence shouldshould be addressed. ofa monocapped hexagonal prism, while that of Fe,Fe6 3 tt PresentPresent address:address : DepartmentDepartment of Physics, California S6(PR3)4L2,Sb(PR&L2, L = halide I‘O*” 0,1 1 (2-5, 7) and thiolate3thiolate PolytechnicPolytechnic StateState University,University, SanSan Luis Obispo, CA (6, 8), and [Fe6S6(PEt3)6]1+[Fe,S,(PEt1)h]‘+12 12 (9) resembles a 94307,94307, U.S.A.U.S.A. basket, with the lower Fe-S-Fe fragment, which 1 2-5,7 6396,8 9 110 0 Fig. 1.1. Schematic structures of clusters 1-10l-10 illustrating the monocapped prismane (1) and basket (2-10)(2-10) topologies.topologies. Iron atoms are labelled so as to correspond to the site assignments in Table 2. isis not part ofthethe aforementioned rhomb, serving as Their unique core structure suggests that thethe thethe handle. We designate these as "basket"“basket” clusters, basket clusters may have unusual electronic features. asas a means of distinguishing their [Fe6S6][Fe&] core top­top- Ground state properties, previously uninvestigated, ology fromfrom thethe idealizedidealized hexagonal prisms present have been examined by magnetic and MiissbauerMossbauer inin thethe "prismane"“prismane” clusters, [[Fe6S6L6]2-~3-.‘3-‘6Fe6S6L6F-,3-. 13-16 The techniques, and the leading results are reported here. structuresStlllCtUlW ofOf Fe6S6(PBu3)4CI2,FC&j(Ph~)~C12,‘1 II Fe6S6(PEt3)4(S-P­Fe6S6(PEt3)&F CC6H4Br)236H 4Br)z3 and [Fe[Fe6S6(PEt)6]‘+6S 6(PEt) 6] 1+ 12l2 define the basket EXPERIMENTAL topology,topology, which isis manifested in virtually isometric [Fe(1l2-S)(llrS)4(1l4-SW+,I+[Fe(~2-S)(~3-S),(~~-S)]2+~‘+ core units. This unique Preparation ofcompound9compounds topologytopology isis supported,supported, inin part, by the unusual coor­coor- dination at thethe FeS3PFeS,P sites. Whereas FeS3LFeS,L sites in The compounds Fe7S6(PEt 3)4C13 (1),9 Fe6S6 thesethese and numerous other clusters have a distorted (pEt3) 4Cl2 (2), II Fe6Se6(PEt3)4CI2 (3), II Fe6S6 tetrahedraltetrahedral configuration, the phosphine-ligated (PEt3)4Br2 (4), II Fe6S6(pEt3)412 (5),11 Fe6S6 sitessites exhibit a roughly trigonal pyramidal geometry, (PEt3MSPh)2 (6),3 Fe6S6(PEt3MS-p-C6H4Br)z with thethe Fe atom closely approaching the S3 plane. (8)3 and [Fe6S6(PEt3)6](BF4) (9)12 were prepared as These speciesspecies are also among the most reduced previously described. Fe-S clusters, with Fe mean oxidation states of ++2.142.14 inin Fe7S6(PEt3)4Cl3,Fe&(PEt3)&13, ++2.172.17 in [Fe6S6[Fe& Physical measurements (PEt(PEt&]‘+3) 6] I+ and ++2.332.33 in Fe6S6(PEt3)4L2.Fe6S6(PEt&L2. As a resultresult of theirtheir high nuclearities, unusual ligand sets All measurements were performed under strict and previously unencountered core structures, these anaerobic conditions. Magnetic susceptibility and molecules are of current interest as the newest magnetization measurements at applied fields of 10 additionsadditions toto thethe extensive family of Fe-S clusters, kOe and up to 50 kOe, respectively, were carried which now encompasses seven nuclearities and 10 out on a SHE 905 SQUID magnetometer operating corecore structuralstructural types.types. 1-3I-3 Most of these clusters exist between 1.8 and 300 K. Solid state measurements inin twotwo ormore oxidation levels, some ofwhich have were made on finely ground polycrystalline samples been isolated.isolated. (15-35(1535 mg) loaded into precalibrated containers and sealed with epoxy resin under a di~~og~-he~~dinitrogen-helium Fe”’Felli sites, ifif present, cannot bebe recognizedrecognized unam-unam­ atmosphere. Diamagnetic susceptibility correctionsicorrections 17 biguously fromfrom crystallographic data.data. AsAs willwill bebe werewere applied. MlissbauerMossbauer spectra were determined seen, *HIH NMR spectra areare consistentconsistentwithwithretentionretention with a constant-accelerationconsent-~lemtion spectrometer equipped of thethe basket structure of halidehalideclustersclustersininsolution.solution. with a 57COs7Co source in a Rh matrix. Zero-field measure-measure­ Fe&(PEt&Cl,Fe7S6(PEt3)4Ch (1);(1); Fe6S6(PEt3).&12Fe6S6(PEt3)4Clz (2);(2); Fe6Se6Fe6Se6 ments were made between 4.2 and 300 K, with thethe fPEt&&(PEt3)40Z (3)(3); ; Fe~S~~Et~~~r~Fe6S6(PEt3)J)rZ (4)(4); ; Fe~S~~Et~~~I~Fe6S6(PEh)4I2 spectrometer operating in the time mode and the (5); Fe~S~(PEt~)~(SPh)~Fe6S6(pEt3MSPhh (6);(6); k&(PBu~MAFe6S6(PBu3)4CIz (7)(7); ; source maintained at room temperature. Magnetically Fe6S6(PEt3)XS-p-CgH4Br)2Fe6S6(PEt3)4(S-p-C6H4Br)2 tS>(8);; V%S6(PEt3)611+[Fe6S6(PEt3)6] 1+ perturbed spectra were obtained in lon~tudinallylongitudinally (9.(9). applied fields up to 80 kOe, with the source and K. absorber at 4.2 K. PolycrystallinePolycrystaIline samples were dis-dis­ Electronic properties persed in boron nitride powder and sealed with epoxy resin in plastic sample holders. ‘isomerIsomer shifts are Ground spin states, other magnetic aspects andand reported relative to Fe metal at 4.2 K. EPR electronic distributions inin clusters l-71-7 havehave beenbeen spectra were recorded at X-band frequencies at ca examined using thethe methods of magnetic sus-sus­ 19 K on a Varian E-I09E-109 spectrometer. Solution ceptibility, magneti~tionmagnetization and 57Fe MijssbauerMossbauer susceptibilities were determined by a NMR spectroscopy. Related results for cluster 9 have been method. 18’ * Solvent susceptibilities were taken reported”reported 12 and certain data are includedincluded here forfor from the literatureliterature” 19 and corrections for solution comparison purposes. Before each measurement, density changes with temperature were applied. the purity of a given compound was established from its distinctive, isotropi~llyisotropically shifted ‘HIH NMR spectrum (vide infra) previously recorded on an RESULTS AND DISCUSSION spectrum (vi& infia) previously recorded on an analytical sample. 3*11,123.11.12 Magnetic and MiissbauerMossbauer The following clusters, 1-9,l-9, are of primary inter-inter­ results are contained in Tables 1 and 2 respectively,respectively, est inin this investigation. The monocapped prismane and selected data are displayed inin Figs 2-7. The structure of 1 has been crystallographically estab­estab- numerical designation of Fe sites inin Table 2 and lished.lished.’9 The basket configurations are assigned to Fig. 1correspond. The Curie constant in the Curie-Curie­ 2--62-6 on thethe basis of the crystal structures of 7 and Weiss law, xMXM = C/(T-Cf(T- @),8), has the values 8, and thethe correspondence of spectroscopic prop­prop- C = 0.375, 1.0001.OOO and 1.875 emu K mol- ’I for pure erties toto thosethose of the structurally defined clusters.
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