Erschienen in: European Journal of Inorganic Chemistry ; 2018 (2018), 6. - S. 713-721 https://dx.doi.org/10.1002/ejic.201701459 The [Fe{(SePPh2)2N}2] Complex Revisited: X-ray Crystallography, Magnetometry, High-Frequency EPR, and Mössbauer Studies II Reveal Its Tetrahedral Fe Se4 Coordination Sphere Eleftherios Ferentinos,[a] Spyros Chatziefthimiou,[b][‡] Athanassios K. Boudalis,[b][‡‡] Michael Pissas,[b] Guinevere Mathies,[c][‡‡‡] Peter Gast,[c] Edgar J. J. Groenen,*[c] Yiannis Sanakis,*[b] and Panayotis Kyritsis*[a] Abstract: The synthesis and characterization of complex electronic structure of 1 was elucidated by magnetometry, [Fe{(SePPh2)2N}2](1) is described. X-ray crystallography shows high-frequency EPR, and Mössbauer spectroscopic studies, that this system contains a tetrahedral FeSe4 coordination which reveal zero-field splitting (ZFS) parameters typical of high sphere. The structural features of 1 are compared with those of spin S = 2 tetrahedral FeII sites. Accurate ZFS parameters (D = similar first row transition element complexes, including a re- +8.22 cm–1, E/D = 0.104) were obtained by analysis of the EPR cently reported preparation of [Fe{(SePPh2)2N}2](1SP) exhibiting spectra and compared with those of the analogous a highly unusual square planar FeSe4 coordination sphere. The [Fe{(SPPh2)2N}2] complex. Introduction its active site have been investigated.[6] The effects of replacing Cys by Se-Cys have been investigated in various metallo- Selenium exhibits a rich and intensively investigated biological proteins, such as metallothionein,[7] Cyt P ,[8] azurin,[9] and chemistry.[1] In that respect, the bioinorganic chemistry of sel- 450 sulfite oxidase.[10] Moreover, sulfide S2– has been replaced by enium, mainly expressed via coordination of either seleno- selenide Se2– in the iron-sulfur active site of numerous cystein (Se-Cys) or selenide (Se2–) to transition metals in metal- proteins,[11] like the [4Fe-4S] cluster of [FeFe]-hydrogenase[12a] loproteins or synthetic analogues, has been extensively ex- (Se/S substitution has also been carried out in the [2Fe] cluster plored. More specifically, X-ray crystallography has revealed the of the latter[12b]), the iron protein of nitrogenase,[13] the photo- coordination of Se-Cys to nickel in the Ni-Fe-Se hydrogenase,[2] system I polypeptide PsaC,[14] aconitase,[15] high potential iron- as well as to molybdenum in the active site of formate dehydro- sulfur protein,[16] as well as [2Fe-2S][17] and 2[4Fe-4S] ferredox- genase[3] and nicotinate dehydrogenase.[4] The Ni-Fe-Se hydro- ins.[18] genase has been employed for the photocatalytic production [5] As far as the synthetic bioinorganic chemistry of selenium of H2 under increased O2 levels, while chemical analogues of is concerned, a range of multinuclear clusters containing iron 2– [19] [a] Inorganic Chemistry Laboratory, Department of Chemistry, coordinated to Se and various thiols, as well as tungsten- National and Kapodistrian University of Athens, iron-sulfur/selenium clusters,[20] have been investigated by Panepistimiopolis 15771, Athens, Greece Holm and co-workers. The coordination of selenium to transi- E-mail: [email protected] tion metals has also been explored by employing the so called http://users.uoa.gr/~kyritsis/home.html ′ ′ [b] Institute of Nanoscience and Nanotechnology, N.C.S.R. “Demokritos”, dichalcogenidoimidodiphosphinato ligands, R2P(E)NHP(E )R 2 ′ [21] Aghia Paraskevi 15310, Attiki, Greece (E, E′ =O,S,Se,Te;R,R = alkyl or aryl groups). Among this – [22] E-mail: [email protected] family of chelating ligands, L = [Ph2P(Se)NP(Se)Ph2] , contain- https://inn.demokritos.gr/en/prosopiko/i.sanakis ing E = E′ = Se as donor atoms, and R = R′ = Ph as peripheral [c] Huygens-Kamerlingh Onnes Laboratory, Department of Physics, groups bonded to the P atoms of the P–N–P backbone, has Leiden University, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands afforded the following structurally characterized homoleptic E-mail: [email protected] complexes with first series transition metal ions (structures de- https://www.universiteitleiden.nl/en/staffmembers/edgar-groenen#tab-1 posited in the Cambridge Structural Database, CSD[23]): octahe- [‡] Present address: EMBL-Hamburg c/o Deutsches Elektronen-Synchrotron, III [24] [24] II [25] [26] dral M L3,M=V, Cr, tetrahedral M L2,M=Mn, Co, Hamburg, Germany [27] II [28] I [29] [‡‡] Present address: Institut de Chimie de Strasbourg Zn, square planar Ni L2 and tetranuclear [Cu 4L3]BF4. – (UMR 7177, CNRS-Unistra), Université de Strasbourg, With respect to the coordination of [Ph2P(Se)NP(Se)Ph2] to 4 rue Blaise Pascal, CS 90032, 67081 Strasbourg, France FeII, it should first be noted that the following literature com- [‡‡‡] Present address: Department of Chemistry, University of Konstanz, plexes have been shown, by X-ray crystallography, to unequivo- Universitätsstraße 10, 78464 Konstanz, Germany Supporting information and ORCID(s) from the author(s) for this article are cally contain a tetrahedral FeSe4 coordination sphere: [30] [31] available on the WWW under https://doi.org/10.1002/ejic.201701459. [N(CH3CH2)4]2[Fe(SePh)4], [Fe(1,10-phen)3][Fe(SePh)4], 713 Konstanzer Online-Publikations-System (KOPS) URL: http://nbn-resolving.de/urn:nbn:de:bsz:352-2-e7hbu1honj565 [32] [33] [PPh4]2[Fe(Se4)2], [N(CH3)4]2[Fe{SeC(CH3)3}4], [bis(1,3-di- X-ray Crystallography and IR Spectroscopy tert-butyl-1H-imidazol-3-ium)] [Fe(SePh) ],[34] [Fe{(SePiPr ) - 2 4 2 2 The molecular structure of 1 is shown in Figure 1. The crystal N} ][35] and [Fe(dmise) ][BF ] , dmise = N,N′-dimethylimidazole- 2 4 4 2 data, as well as the refinement statistics, of the structure of 1 selone.[36] In addition, on the basis of spectroscopic and room are listed in Table S1 (Supporting Information). Selected bond temperature magnetometry studies, the [Fe{(SePPh ) CH} ] 2 2 2 lengths and angles of 1, are listed in Tables S2 and S3, respec- complex, bearing a ligand which is similar to L, has been pro- [37] tively. posed to also contain a tetrahedral FeSe4 core. Based on the above, the recently reported crystal structure of a preparation of FeL2 containing a square planar FeSe4 coordination sphere [38] (1SP, CSD code FOMWIP) was rather unexpected and prompted us to revisit the FeL2 system. X-ray crystallography revealed that our preparation of FeL2 (1), presented herein, exhibits clearly a tetrahedral FeSe4 coordi- nation sphere. The magnetic and spectroscopic properties of 1 were investigated by magnetometry, as well as by IR, high- frequency EPR and Mössbauer spectroscopy. Comparisons are [38] [35] being made with the properties of 1SP, [Fe{(SePiPr2)2N}2] [39] and [Fe{(SPPh2)2N}2] complexes. Recent high-frequency EPR II studies of the latter have revealed distinct S =2Fe sites, the Figure 1. Molecular structure of complex 1. The H atoms are omitted for spin-Hamiltonian parameters of which have been accurately de- clarity. Color coding: Iron (brown), selenium (magenta), phosphorus (light termined.[39b] This complex is a synthetic analogue of the re- brown), nitrogen (blue), carbon (gray). duced form of the iron-sulfur protein rubredoxin, which con- The FeSe core of complex 1 exhibits a distorted tetrahedral tains a [Fe(S-Cys) ] active site.[40] To our best knowledge, re- 4 4 geometry, as reflected by the magnitude of the Se–Fe–Se placement of the iron-coordinating Cys residues of rubredoxin angles which lie between 101.7(1)° and 115.7(1)° (Table S2). The by Se-Cys has not yet been reported. two endocyclic Se–Fe–Se bite angles amount to 115.7(1)° and In the following, the interplay between literature tetrahedral are similar to those of the [Fe{(SePiPr ) N} ][35] and and square planar FeII complexes, including their spin states 2 2 2 [M{(SePPh ) N} ],M=Mn,[25] Co,[26] Zn,[27] complexes listed in and Mössbauer spectroscopic signatures, is analyzed. Moreover, 2 2 2 Table 1. The dihedral angle between the planes defined by the the effects on the D (axial) and E (rhombic) zero-field splitting endocyclic Se–Fe–Se angles of complex 1 is 86.8°, comparable (ZFS) components of FeII coordination to [Ph P(Se)NP(Se)Ph ]–, 2 2 to those of the [M{(SePPh ) N} ], M = Mn, Co, Zn, complexes as opposed to [Ph P(S)NP(S)Ph ]–,[39b] in a similar tetrahedral 2 2 2 2 2 (Table 1). The significantly smaller values, 81.3° and 82.4°, re- S =2FeE site,E=S,Se,arealso discussed. 4 spectively, for the sites 1 and 2 of the recently reported [Fe{(SePiPr2)2N}2] complex reveal a more profound deviation from the value (90.0°) of the ideal tetrahedral structure, appar- Results and Discussion ently imposed by the iPr peripheral groups of the ligand. In addition, the latter structure exhibits a different space group Chemical Synthesis compared with the rest of those listed in Table 1. The FeSePNPSe rings in the structure of 1 adopt a pseudo-boat con- Complex 1 was synthesized by mixing FeCl2 and the formation, with the P and Se atoms at the apices, in agreement – [Ph2P(Se)NP(Se)Ph2] ligand, under anaerobic conditions, in or- with the structure of the analogous Mn, Co, Zn complexes der to minimize the formation of byproduct FeIII species. A dif- (Table 1). ferent preparation of the same complex (1SP) has been recently The averaged P–Se and P–N bond lengths of [38] reported. The proposed crystal structure of 1SP, containing a Ph2P(Se)NHP(Se)Ph2 and 1 are listed in Table 2. Close inspection square planar FeSe4 coordination sphere, will be compared, in reveals that the average P–Se bond length of 1 is increased the following, with that of the present complex 1.