A Molecular Low‐Coordinate [Fe‐S‐Fe] Unit in Three Oxidation States

Abstract A [Fe‐S‐Fe] subunit with a single sulfide bridging two low‐coordinate iron ions is the supposed active site of the iron‐molybdenum co‐factor (FeMoco) of nitrogenase. Here we report a dinuclear monosulfido bridged diiron(II) complex with a similar complex geometry that can be oxidized stepwise to diiron(II/III) and diiron(III/III) complexes while retaining the [Fe‐S‐Fe] core. The series of complexes has been characterized crystallographically, and electronic structures have been studied using, inter alia, 57Fe Mössbauer spectroscopy and SQUID magnetometry. Further, cleavage of the [Fe‐S‐Fe] unit by CS2 is presented.


Introduction
Nitrogenase (N 2 ase) is an important enzyme that catalysesp rimarily the reduction of dinitrogen to ammonia.T he reaction takes place at the iron-molybdenum co-factor (FeMoco),w hich constitutes aligated[MoFe 7 S 9 C] unit. [1] Despite tremendous advancesi nt he structural elucidation of the co-factor and some intermediate clusters under substrate conversion,aswell as detailed studies of the co-factor's electronic structures, the exact iron site for substrate binding and involved reaction mechanisms are still not fully understood. [2,3] Increasing evidence points to the importance of the Fe-S-Fe belt unit (Figure 1, top). [4,5] This unit is assumed to open up during substrate turnover, as experimentally shown by displacemento ft he sulfideu nit by CO or selenide. [2,6] As such there is inherent interest in molecular models that feature an unsupported [Fe-S-Fe] unit with iron ions in al ow-coordinate environment. However,t he only known examples bearingt hree-coordinate metali ons are a bdiketiminato (nacnac) based iron(II) complex as well as its two-fold reduced iron(I) derivative (Figure1,b ottom). [7,8] The iron(II) complex is susceptible to coordinationb ya mmonia and hydrazines,a nd is even able to cleave the NÀNb ond of the latter. [7,9] We now report on the synthesis of ad inuclear [2Fe-1S] 2 + complex with an unsupportedm onosulfide bridgev ia the reaction of at wo-coordinate iron(I) silylamide with elemental sulfur. Subsequento xidationl eads to the first example of am ixed valent[ 2Fe-1S] 3 + and an "all ferric" [2Fe-1S] 4 + form.T he series of complexes was examined with respectt ot heir spectroscopic and physical properties. The initial [2Fe-1S] 2 + complex was further subjected to av ariety of smallm olecule substrates that are transformed by the N 2 -ase, however showingo nly al imited reactivity or stability.M ost notably,i ts reaction with CS 2 led to rupture of the Fe-S-Fem otif and formation of am ononuclear iron(II) thiocarbonate complex, revealing the structurall ability of the [Fe-S-Fe] unit.
With this unprecedenteds eries of low-coordinate [2Fe-1S] complexes in three differento xidation states in hand, their spectroscopic ande lectronic features were studied in detail.
UV/Vis spectroscopic examination of 2 showedn oa bsorption beyond4 00 nm ( Figure 5A), which is common for low-coordinate iron(II) compounds, [16] but unusualf or m-sulfido diiron(II) complexes. [7,15] In contrast, the mixed valent species 3 exhibited three rather intense maximaa t3 80 nm (e = 11 800 Lmol À1 cm), 470 nm (e = 9540 Lmol À1 cm) and 700 nm (e = 2270 Lmol À1 cm),a nd it is tempting to assign the lowenergy absorption to an intervalence charge transfer (IVCT) transition. For 4 just one pronounced band at 430 nm (e = 5710 Lmol À1 cm) was observed. Zero field 57 Fe Mçssbauer spectroscopy( Figure 5B)r evealed for the iron(II/II) complex 2 a doublet with d = 0.59 mm s À1 and j DE Q j = 0.22 mm s À1 ,i n agreement with data for ar elated three-coordinatei ron(II) complex. [17] These values are slightly smallert han those of the only other known monosulfido-bridged iron(II/II)c omplex showni nF igure 1( bottoml eft; d = 0.86 mm s À1 and j DE Q j = 0.58 mm s À1 ), which can be explained by the weaker donor strength of the silylamide ligands as well as al ess distorted trigonal planar ligand arrangement. [7] The all ferric complex 4 is represented by ad oublet with d = 0.29 mm s À1 and j DE Q j = 3.70 mm s À1 indicatingt he presence of high-spini ron(III) ions. The spectrum of the mixed valent complex 3,r ecorded at 7K, showedt wo doubletsw ith d = 0.36 mm s À1 (j DE Q j = 3.70 mm s À1 )a nd d = 0.57 mm s À1 (j DE Q j = 0.71 mm s À1 ). This evidences distinguishable iron(II/III) positions in solid 3 on the 57 Fe Mçssbauer timescale at 7K,w hereas the smaller separa-  tion in the isomer shifts (Dd(3) = 0.21 mm s À1 vs. Dd(2/4) = 0.30 mm s À1 )i si ndicative of some degree of valence delocalisation. Given the lack of literature precedence of the three-coordinate m-sulfido complexes 3 and 4 their Mçssbauer spectroscopic features are compared best to low coordinate iron complexes bearing a[ 2Fe-2S]m otif in the same oxidation states. [14,15,[18][19][20] Most importantly,s uch mixed valent [2Fe-2S] compounds are shown to exhibit moderate [15,19,20] or strong [14] antiferromagnetic coupling, and give Mçssbauer spectra (at < 10 K) that correspond to either valence localized or delocalized states, respectively.
Additional insights into the electronic situation of 2-4 in solid state were obtained by SQUID measurements (Figure 5D-F). 2 exhibited at 300 KacTv alue of 1.8cm 3 mol À1 K which linearly dropped to ca. 0.1 cm 3 mol À1 Ka t3 0K.T his indicated am oderate antiferromagnetic interaction between the two iron(II) (S = 2) ions with a S = 0g round state. The coupling constant was determined to be J = À53 cm À1 using Ĥ = À2JS A ·S B with g 1 = g 2 = 2.01.
The mixed valent compound 3 showeda t2 95 Kasignificantly lower cTv alue of 0.96 cm 3 mol À1 Kw hich decreased to 0.44 cm 3 mol À1 Ka t8 0Kwith af urtherd rop to 0.38 cm 3 mol À1 K below 20 K, which implies ag round state of S = 1/2. The antiferromagnetic coupling is stronger with J = À115cm À1 (g 1 = 2.08, g 2 = 2.02). For the all-ferric compound 4 as imilar value J = À104 cm À1 (g 1 = g 2 = 2.10, S 1 = S 2 = 5/2) was observed with cT = 1.3 cm 3 ·mol À1 ·K at 300 Kt hat decreased linearly to ca. 0.2 cm 3 mol À1 Kbelow 50 Kdue to a S = 0ground state. The differences in exchange coupling can be explained using asimplified orbital scheme under assumption of an idealized C 2V symmetric ligand environment for each iron atom (Figure5C, zaxis along the Fe-S-Fe unit). Upon oxidation, electrons are removed from the lowest-lying, co-parallel d xy /d x2-y2 orbitals, which have no impact onto the exchange mechanism. As such the variation in J values for 2-4 can be mainly attributed to differences in Fe···Fe distances, with different superexchange contributions due to changes in Fe-(m-S) covalency likely playing af urtherr ole. As ignificant stabilization of the antiferromagnetically coupled ground state upon oxidation from the diiron(II)t ot he mixed-valent iron(II)/iron(III) and diiron(III) states was observed for the series of complexes [(LFe) 2 (m-S) 2 ] 4À/3À/2À (L 2À = bis(benzimidazolato)). [15,19] Having evaluated the redox and electronic properties of 2 we continued with investigations concerning its reactivity towards nitrogenase related small molecules. [21] No reaction with N 2 ,H 2 ,orC Owas observed whereas treatment of 2 with hydrazine derivatives and protono rm ethyl group sources only led to decomposition. Exposure of 2 to CO 2 causedavisible colour change but did not yield any identifiable product, probably due to parallel insertion of CO 2 into the iron silylamide bonds. [22] As such we examined the behavior of 2 towardst he heavierc ongener CS 2 ,w hich is an inhibitor of nitrogenasemediated protono ra cetylene reduction but can also serve as as ubstrate that is mainly converted to H 2 S. [23][24][25] This led to the isolation of the monomeric iron thiocarbonate complex (K{18c6}) 2 [Fe(h 2 -CS 3 )L 2 ], 5 (Scheme 3, Figure 6).
In 5 the iron ion is coordinated by two silylamides and one bidentate thiocarbonate ligand in ad istorted tetrahedral fashion. The two K + {18c6} cations are connected further to the thiocarbonate ligand, each via two sulfur atoms. We explain the formation of 5 by initial insertion of CS 2 into one of the ironsulfur bonds of 2,g iving at hiocarbonate bridgedd imer.T he insertiono fC S 2 into an unsupported M-S-M unit was so far only reported for diuranium complexes. [26] One neutral iron(II) bisamide is then replaced by the potassium crown-ether moieties, which themselves act as aL ewis acid. The mixed valent complex 3 reacted with CS 2 also under rupture of the [Fe-S-Fe] motif. This yieldedt he iron(III) thiocarbonate complex 7,w hich could alternatively be obtained via the oxidation of 6 by silver triflate. For the neutral complex 4 the reactionw ith CS 2 remained inconclusive. The observation of facile Fe-S bond cleavages suggests ar ather weakF e II -S interaction. The displacement of an iron(II) ion by other Lewis acids has possible implications for the situation found in the FeMo cofactor where cleavage of the belt Fe-S-Fe unit is discussed during substrate turnover using the local Lewis acid/basep roperties of the surroundings of the enzyme pocket. [5] The facile insertion of CS 2 into a[ Fe-S-Fe] function also reveals how CS 2 might act as an inhibitor of nitrogenase FeMoco (and other iron-sulfur clusters) which is thought to proceed by blocking of coordination sites [24,25,27] or by insertion into other metal-ligand bonds. [28] Scheme3.Reactionof2 and 3 with CS 2 givingt he thiocarbonatecomplexes 5 and 6.

Conclusions
We have synthesized au nique series of low-coordinate [Fe-S-Fe] complexes in three oxidation states which resembles aF e-S-Fe belt unit in the iron/sulfur/molybdenum co-factor of the nitrogenase enzyme. These complexes were characterized for their magnetic and spectroscopic properties. 57 Fe Mçssbauer spectroscopy showedf or the mixed valent[ 2Fe-1S] 3 + complex localizedv alences tates in solid state at low temperatures. Magnetic measurements revealed for the diferrous [2Fe-1S] 2 + a moderate antiferromagnetic coupling which becomes significantly enhanced for the [2Fe-1S] 3 + and [2Fe-1S] 4 + compounds. Reactivitys tudies on these complexes towards different nitrogenaser elevants ubstrates revealed for CS 2 the facile cleavage of the Fe-S-Fe unit. This led to the formation of an iron thiocarbonatew hich mays uggest ap ossible inhibitory mechanism of CS 2 with respectt ot he reactivity of FeMoco and related Fe/S clusters.

Experimental Section
General considerations:All manipulations were carried out in aglovebox, or using Schlenk-type techniques under ad ry argon atmosphere. Used solvents were dried by continuous distillation over sodium metal for several days, degassed via three freeze-pump cycles and stored over molecular sieves 4 .K {18c6}[FeL 2 ]w as synthesized according to the literature procedure. For details concernining data acquisition of solution and solid-state analyses ( 1 H-NMR spectra, X-ray diffraction analysis, cyclovoltametry,m agnetic measurements and Mçssbauer spectra), see the Supporting Information. After stirring for 2hours, the mixture was filtered, the residue washed with 2 3mLE t 2 Oa nd the combined filtrates were layered with 5mLo fp entane. Storing the solution at À35 8Cf or several days yielded to ad ark red crystalline solid, suitable for X-ray diffraction analysis. Decanting off the supernatant, washing the residue with 2 5mLo fp entane and drying under reduced pressure afforded [K{18c6}][(FeL 2 ) 2 (m-S)], 3,a sadark red crystalline solid (40 mg, 0.023 mmol, 33 %). The aforementioned procedure to synthesize 3l eads to ap ure product according to elemental analysis (vide infra). To obtain am agnetically pure sample several recrystallization steps in Et 2 O/pentane were required, which led to ad ecrease of the yield to less than 10 %. 1   )], 5,( 87 mg, 0.69 mmol, 1equiv) was dissolved in 2mLo fT HF.U pon the addition of AgOTf (17.6 mg, 069 mmol, 1equiv) the solution turned dark red and the precipitation of ag rey solid was observable. After stirring for 2hours, the mixture was filtered and the filtrate was layered with 2mLo fp entane. Storing the solution at À35 8Cf or several days led to the precipitation of dark red crystals, suitable for X-ray diffraction. Decanting off the supernatant, washing of the residue with 2 5mLo fp entane and drying in vacuo afforded 6 as ad ark red crystalline solid. K(18c6)OTf is the major side product of the reaction. As it has almost the same solubility as 6 in Et 2 Oa nd THF,i tw as impossible to obtain an analytically pure sample of 6 upon recrystallization. Therefore 6 could only be characterized by