8.3.2 [Fe2S2] CLUSTERS 62
8.3.3 [Fe3S4] CLUSTERS 64
8.3.3.1 Linear Clusters 64
8.3.3.2 CuboidalClusters 65
8.3.4 [Fe4S4] CLUSTERS 67
8.3.4.1 Thiolate, Halide, and Related Clusters 67
8.3.4.2 The [Fe4S4]3þOxidation State 69
8.3.4.3 The [Fe4S4]1þOxidation State 69
8.3.4.4 StructuralTrends of Fe4S4Clusters 70
8.3.4.5 Reactions withp-acceptor Ligands 71
8.3.4.5.1 Isonitrile clusters 71
8.3.4.5.2 Phosphine clusters 72
8.3.4.6 Fe4S4Peptide Clusters 73
8.3.4.7 Site-differentiated Fe4S4Clusters 74
8.3.4.8 Fe4S6Clusters 76
8.3.5 HEXANUCLEAR (Fe6S6, Fe6S8, Fe6S9) CLUSTERS 77
8.3.5.1 Prismane (Fe6S6) Clusters 77
8.3.5.2 Basket (Fe6S6) Clusters 78
8.3.5.3 Fe6S8Clusters 79
8.3.5.4 Fe6S9Clusters 80
8.3.6 HEPTANUCLEAR (Fe7S6) AND OCTANUCLEAR (Fe8S6, Fe8S8, Fe8S9, Fe8S12) CLUSTERS 81
8.3.6.1 Fe7S6Cluster 81
8.3.6.2 Fe8S6Clusters 81
8.3.6.3 Fe8S8Clusters 82
8.3.6.4 Fe8S9Clusters 82
8.3.6.5 Fe8S12Cluster 84
8.3.7 HIGHER NUCLEARITY CYCLIC CLUSTERS 84
8.3.7.1 Fe16S16Clusters 85
8.3.7.2 Fe18S30Clusters 85
8.3.8 SUMMARY 86
8.3.9 REFERENCES 87
8.3.1 SCOPE AND FUNDAMENTAL LIGAND TYPES
The impetus for the development of iron–sulfur cluster chemistry over the last three decades derives largely from the occurrence of iron–sulfur clusters in an extensive variety of proteins and enzymes.1–5Five cluster types (1)–(5) have been demonstrated by protein crystallography and are shown inFigure 1. Clusters (1)–(3) represent the fundamentalset found in proteins (ferredoxins) from a variety of prokaryotic and eucaryotic sources. Rhomb-like cluster (1) is especially preva- lent in green plants. Cuboidal cluster (2) and cubane-type cluster (3) are found in bacterialsources
61
and differ by a single iron atom and its terminal ligand. Cluster (4) is a variation of (3) in which a terminal cysteinate is replaced by an exogenous ligand or a carboxylate sidechain, and (5), the most complex protein-bound cluster, is thePcluster of nitrogenase in the as-isolated form of the iron–molybdenum protein.6,7In addition, there are variations on these themes in which terminal cysteinate is replaced by alkoxide from serinate, the imidazole ring of histidine, and carboxylate from glutamate or aspartate. However, the great majority of protein-bound clusters feature tetrahedralfour-coordinate iron sites with terminalcysteinate ligation. Except for oxidized (1), the clusters are mixed-valence FeIII,II, often with delocalized electronic structures and coupled magnetic interactions.8,9
Provided here is an account of synthetic iron–sulfur cluster chemistry intended to summarize leading results since the subject was briefly treated in Comprehensive Coordination Chemistry (CCC, 1987), Volume 4, but also contains certain earlier findings. Structural types and synthetic reactions affording them are emphasized. Synthetic methods are selective, not exhaustive. Reactions are presented in minimalform; indicated stoichiometries are not necessarily those used in practice.
Methods for and structures of sulfide clusters generally extend to selenide clusters, which are not included. Space does not allow detailed consideration of electronic properties. Not included are iron thiolates, organometallic, carbonyl, nitrosyl, or certain other types of abiological clusters which have been treated recently,10 and iron–sulfur proteins themselves. As will become evident, the scope of synthetic clusters far exceeds the biological set (1)–(5). Presented in Scheme 1 are synthetic reactions for four clusters of varying nuclearity by reaction of elemental sulfur with a common mononuclear precursor, [Fe(SEt)4]2.11 These reactions have the common theme of sulfur as a source of sulfide by reduction with thiolate and/or FeII, a frequent reaction in synthesis. Clusters are generally isolated as dark red or black salts of quaternary ammonium or phosphonium cations, soluble in polar solvents such as acetonitrile, Me2SO, and DMF, and unstable to air, particularly in solution. Throughout, cluster refers to the entire molecule [FemSpLl]z with generalized terminal ligands L and core to the FemSp portion thereof. Redox potentials are referenced to the standard calomel electrode. As will become evident, the Fe2S2
rhomb is the fundamental building block in iron–sulfur cluster chemistry.
8.3.2 [Fe2S2]CLUSTERS
As prepared, these clusters are in the oxidized form, contain the planar rhombic [Fe2(2-S)2]2þ core, and are of three general types. Thiolate-ligated cluster (6a) is an analogue of protein site (1) and may be prepared by reactions (1)11 and (2)12 from mononuclear precursors and by ligand
Fe SãCys Fe
CysãS Fe SR
Fe RS
Fe SãCys Fe
CysãS
S S S
S Fe
RS S Fe
S S Fe
SãCys CysãS
CysãS SãCys
S
S S
Fe SãCys Fe
CysãS
Fe
OH-/ 2/-O2CR Fe
CysãS
S S S
S
(5)
CysãS Fe Fe
S S S
Fe S Fe
S
Fe S
Fe S
S Fe Fe S
SãCys SãCys CysãS
ã
ã
(1) (2) (3)
(4)
Cys Cys
H O
Figure 1 Structurally defined native clusters.
substitution reaction (3)13 (R=alkyl, R0=aryl), which may be shifted toward product by removalof RSH. Related methods starting from FeCl3, sulfur, and NaSR are also available.14 2ẵFeðSRị42ỵ2S! ẵFe2S2ðSRị42ỵRSSRỵ2RS ð1ị
2ẵFeðSRị41ỵ2S! ẵFe2S2ðSRị42ỵ2RSSR ð2ị
ẵFe2S2ðSRị42ỵ4R0SHé ẵFe2S2ðSR0ị42ỵ4RSH ð3ị Clusters (6b) with L=halide have been obtained from (6a) by reaction (4)19, and are useful starting materials for the preparation of clusters (6b) with L=ArOand C4H4N, as in ligand substitution reaction (5)20–22. This reaction has also been used with NaSR to prepare thiolate clusters (6a), including those containing short cysteinyl peptides as closer models of protein site (1).15–17 Peptide clusters are also accessible by reaction (3). Bis(pentasulfide) cluster (6c) is obtained by the reaction of [Fe(SPh)4]2with five equivalents of (PhCH2S)2S.18
ẵFe2S2ðSRị42ỵ4R0COX! ẵFe2S2X42ỵ4R0COSR ð4ị
ẵFe2S2X42ỵ4NaL! ẵFe2S2L42ỵ4NaX ð5ị
Fe S
Fe S RS SR
RS SR
(6a)
2-
Fe S
Fe S L L
L L
(6b)
2-
Fe S
Fe S S S
S S
(6c)
2- S S
S S
S S
As a class, [Fe2S2]2þ clusters are air sensitive, exhibit a diamagnetic ground state with thermally accessible excited states arising from antiferromagnetic coupling of two S=5/2 spins, and characteristic absorption and isotropically shifted 1H NMR spectra that allow their
[FeCl4]2-
4NaSEt, MeCN
[Fe(SEt)4]2-
[Fe3S4(SEt)4]3- [Fe6S9(SEt)2]4- [Fe2S2(SEt)4]2-
[Fe4S4(SEt)4]2- [Fe3S4(SR)4]3- [Fe6S9(SR)2]4- 80o C
MeCN
1S, MeCN 1.5S, 85o
C, MeCN 1.4S, Me2CO
1FeCl2
, 1NaSEt MeCN
60oC, MeCN 4RSH
MeCN
2RSH MeCN
Scheme 1
identification.11,14 Cores are essentially isometric with the usual bond distances Fe–S2.30 A˚
and Fe–Fe2.70 A˚; iron sites exhibit distorted tetrahedralstereochemistry. Clusters (6a) can be reduced chemically or electrochemically to [Fe2S2(SR)4]3.13,23–25 These species can be examinedin situ but none has ever been isolated because of their extreme reactivity, one aspect of which is spontaneous reaction (6)26 where both reactant and product have the same mean oxidation state (Fe2.5þ). As for reduced protein site (1), [Fe2S2(SR)4]3 species are characterized by an antiferromagnetically coupled S=1/2 ground state, an axialor rhombic EPR spectrum observa- ble at or below ca. 100 K23–25 and a trapped-valence FeIIIFeII ground state. Consequently, this state is an intrinsic property and not a consequence of protein structure and environment:
2ẵFe2S2ðSRị43! ẵFe4S4ðSRị42ỵ4RS ð6ị
8.3.3 [Fe3S4]CLUSTERS
Two types of trinuclear clusters, linear [Fe3S4(SR)4]3(7) and cuboidal[Fe3S4(SR)3]3(8), have been prepared and differ in core structure, oxidation state, and stability. Both clusters have been observed in proteins; cluster (8) is an analogue of protein site (2):
RS Fe RS
S S
Fe Fe
SR SR S
S
3-
(7)
Fe SR Fe
RS Fe RS
S S
S S Fe
S Fe
S Fe S S SR
SR RS
3-
(8)
8.3.3.1 Linear Clusters
The first trinuclear cluster (7) prepared was [Fe3S4(SEt)4]3in 1983,11obtained by carefulcontrolof stoichiometry and scale in reaction (7)11,27. The initialcomplex may be used as such or generatedin situ.27Isolated clusters with R=Ph11and mes28have been prepared by ligand substitution analogous to reaction (3). The structure of [Fe3S4(SPh)4]3 contains the all-ferric core [Fe3S4]1þ buil t of two nearly planar Fe2S2rhombs sharing a common iron atom and disposed at a dihedralangle of 89.4. The rhombs are nearly isometric with those in (6a), and the three FeIII sites have distorted tetrahedralgeometry.11 These sites (S=5/2) are antiferromagnetically coupled to generate an S=1/2 ground state; [Fe3S4(SEt)4]3behaves as a Curie paramagnet at 5300 K.29Linear cluster (7) has been found in a partially unfolded form of the enzyme aconitase where it was identified by comparison of spectroscopic properties with those of [Fe3S4(SEt)4]3.30 Subsequent comparative MCD spectra further support the presence of (7).31 Recently, reconstitution of recombinant human cytosolic iron regulatory protein 1 (cytosolic aconitase) with FeII and Na2S afforded a product containing (7), as shown by EPR and EXAFS analysis.32It is probable that the cluster is fully ligated by cysteinate in both proteins. An Fe4S4cluster is present in the functional protein;
no examples of the linear cluster in vivohave been found. A minor amount of the linear cluster has also been detected in pyruvate formate-lyase activating enzyme:33
3ẵFeðSEtị42ỵ4S! ẵFe3S4ðSEtị43ỵ5=2 EtSSEtỵ3EtS ð7ị The fully oxidized nature of cluster (7) generates significant reactivity properties. The cluster cannot be reversibly reduced, but it does react with FeCl2 to form an Fe4S4 cluster and with reduced metalcomplexes to afford heterometalcubane-type clusters.1,34 Reaction (8)11 proceeds in high yield. Reaction (9)27 produces a cluster containing a loosely bound diamagnetic Mo(CO)3unit and a paramagnetic (S=2) [Fe3S4]0core fragment which serves as an electronic analogue of protein site (2) in the same oxidation state. Reaction (10)28,35,36
(R=Et, mes) conveys one of the two routes to NiFe3S4clusters. Reactions (8)–(10) and those in Scheme 2 are ones of fragment condensation in which rearranged Fe3S4 is a fragment of the product cluster. EPR and Mo¨ssbauer spectra support the core formulations [CoFe3S4]2þ (S=1/2){CoII(S=3/2)þ[Fe3S4]0(S=2)} and [NiFe3S4]1þ(S=3/2){NiII(S=1)þ[Fe3S4]1 (S=5/2)},28,35 indicating that (9) is formed by a one-electron reduction and (10)/(11) by two- electron reduction of [Fe3S4]1þ. Cluster (10) is isolated with four thiolate ligands because of the lability of ligated AsPh3. While PPh3is bound in (11), it is readily displaced in the absence of free phospine by a variety of ligands.35These reductive rearrangement reactions are characterized by core reduction, structural alteration from a linear to a cuboidal geometry, and binding of the incoming metal. The reactions in Scheme 2 can be likened to inner sphere electron transfer in which there is a persistent intermediate that resembles the final product:
ẵFe3S4ðSEtị43ỵFeCl2ỵEtS! ẵFe4S4ðSEtị42ỵ1=2 EtSSEtỵ2Cl ð8ị
ẵFe3S4ðSEtị43ỵ ẵMoðCOị3ðMeCNị3 ! ẵðCOị3MoFe3S4ðSEtị33ỵ1=2 EtSSEtỵ3MeCN ð9ị
ẵFe3S4ðSRị43ỵ ẵNiðPPh3ị4 ! ẵðPh3PịNiFe3S4ðSRị32ỵEtSỵ3PPh3 ð10ị
Fe S S
Co Fe
S
Fe S
S S
S
S
S Fe S
S S
Fe Fe
S S S S
Fe S S
Ni Fe
S
Fe S
S S
S
S
Fe S S
Ni Fe
S
Fe S
S S
PPh3
S 2-
(9)
3-
3-
(10)
2-
(11) (7)
THF/Me2CO xs Co[P(OMe)3]3(S ) Ni(AsPh3)4 MeCN Ni(PPh3)4 MeCN
Scheme 2
8.3.3.2 Cuboidal Clusters
Protein site (2) was discovered after sites (1) and (3) were well established. An analogue of (2) proved more difficult to synthesize than those of the other two sites. Despite numerous earlier attempts, an analogue cluster (8) was not prepared until1995.37,38The key to obtaining this cluster in substance is the use of the trifunctionalsemirigid cavitand ligand LS3to form the 3:1 site-differentiated cluster (12) (Section 8.3.4.7) in Scheme 3. This cluster undergoes the regiospecific substitution reactions (12)!(13)!(14). Further reaction with N-methylmidodiacetate removes the unique iron atom, yielding [Fe3S4(LS3)]3(15), the only isolated example of cuboidal (8). The structure of (15) resembl es closely the cubane cluster [Fe4S4(LS3)Cl]2, showing that removalof an iron atom does not cause any major changes in the remainder of the core structure. Cluster (15) exhibits the three-member electron transfer series (11);38only the centralmember has been isolated. Allattempts to prepare examples of (8) with monofunctional thiolate ligands have thus far been unsuccessful.
ẵFe3S4ðLS3ị4$ ẵFe3S4ðLS3ị3$ ẵFe3S4ðLSị32 ð11ị
2 -
3 - 3 -
(13)
(14) (15) [Fe(Meida)2]2-
2 -
2 4
5 6
4'3' 2'
(12)
S Me
Me
Me Fe
S SEt
S
S
S Fe
S S
Fe Fe Fe
S
S Fe
S S
Fe Fe Fe
S
S
S S
Fe Fe Fe
RS SR
OSO2CF3
SR (Et3NH)(TfO)
EtSH
O N
O
O O
Me
RS SR
SR
RS SR SR
(Et4N)2(Meida) TfO-
(Et4N)2(Meida)
Scheme 3
Cluster (2) is usually found in proteins that lack a fourth cysteinyl residue in a normal position for binding to an Fe4S4cluster. Cluster (15) is a structural and electronic analogue of site (2) in the mixed-valence [Fe3S4]0 oxidation state.38 Double exchange between the two sites of a delocalized FeIIIFeII pair produces a spin S=9/2 which is antiferromagnetically coupled to the S=S/2 spin at the FeIIIsite to generate anS=2 ground state. The existence of this state in (15), established by Mo¨ssbauer spectroscopy and comparison with proteins, is thus an intrinsic prop- erty of the cluster and is independent of protein structure. Protein clusters are generally isolated in the fully oxidized [Fe3S4]1þ state, which is characterized by an EPR signalnearg2.01 corres- ponding to an S=1/2 ground state. When this cluster is treated with FeII under reducing conditions in reaction (12), an [Fe4S4]2þcluster is formed in which one iron atom is coordinated by an exogenous ligand (water, hydroxide) or a noncysteinate residue. Reaction (13)38 is closely related to this process. The reverse of reaction (12), probably passing through [Fe4S4]3þ which releases the iron atom not bound by cysteinate, is the means of formation of protein cluster (2) in the oxidized state. Chemicaloxidation of [Fe4S4(Stibt)4]2generates an EPR signalattributed to an [Fe3S4]1þcluster39. The initialcluster has been oxidized and isolated in the [Fe4S4]3þstate.40In proteins, the reduced nucleophilicity of the [Fe3S4]1þ portion of [Fe4S4]3þ and the lack of a terminalcysteinate ligand contribute to the ease of removalof FeII:
ẵFe3S41ỵỵFe2ỵỵe! ẵFe4S42ỵ ð12ị
ẵFe3S4ðLS3ị3ỵ ẵFeCl42! ẵFe4S4ðLS3ịCl2ỵ3Cl ð13ị The structures of (8) and (15) suggest that the clusters might bind a metal at the voided site, thereby forming cubane-type cores. This expectation has been realized for both the native and synthetic clusters. Incubation of proteins having Fe3S4 sites with excess metalion under reducing
conditions leads to the formation of MFe3S4clusters with divalent ions of the first transition series and CuI, CdII, PbII, and T1I.34,41–43Reaction (14)44occurs with certain reduced metalcompounds and thiophilic metals to afford a series of heterometal cubanes with L-M=Ph3P-CoII/NiII/CuI/AgI, (OC)3Mo0/W0, and NC-CuI/AgI, and M=T1I. This reaction affords the [CoFe3S4]1þoxidation level, one electron more reduced than (9). Both reaction (14) and those of Scheme 2 produce isoelectronic [NiFe3S4]1þclusters. Reaction (14) includes nonredox binding of thiophilic metals and inner-sphere redox reactions with CoI and NiI. Products of the first reaction type are formulated as {M1þþ[Fe3S4]0} and retain the S=2 ground state of (15). Those of the second are {M2þþ[Fe3S4]1} with S=1 (Co) and 3/2 (Ni). A significant feature of these clusters is that the heterometalis incorporated into cores with the reduced fragments [Fe3S4]0,1, which function as cluster ligands. The fully oxidized core [Fe3S4]1þis evidently insufficiently nucleophilic to act as a general ligand. Only the soft thiophiles TlI45 (weakly) and CuI41,46 have been shown to coordi- nate to this oxidation state of protein-bound clusters. Electrochemical potentials for metal ion release are consistent with a low binding affinity of [Fe3S4]1þ.43 Lastly, ground spin states of heterometal protein clusters are interpretable in terms of antiparallel spin coupling between tetrahedralMII and [Fe3S4]0,1 fragments.34 The large majority of synthetic [MFe3S4]z clusters also follow this regularity:28,35
ẵFe3S4ðLS3ị3ỵ ẵMLn0z! ẵLMFe3S4ðLS3ị2;3ỵ ðn1ịL0 ð14ị
8.3.4 [Fe4S4]CLUSTERS
Cubane-type clusters (16) constitute the largest family of iron–sulfur clusters. The first examples were synthesized in 1972–73.47,48 Hundreds of clusters with variant ligands and different oxida- tion states have been prepared subsequently, those with the [Fe4S4]2þcore being by far the most common. These clusters are considered in three basic groups followed by other types containing specialized ligation.
Fe S
S Fe
Fe S RS Fe
S SR RS
SR
3-, 2-, 1-
(16)
8.3.4.1 Thiolate, Halide, and Related Clusters
As shown in Figure 2, homoleptic thiolate-ligated clusters constitute a four-member electron transfer series whose core oxidation state ranges from all-ferrous [Fe4S4]0to [Fe4S4]3þwith mean oxidation state Fe2.75þ. The all-ferric state [Fe4S4]4þremains hypothetical. Isoelectronic protein sites and synthetic clusters, illustrated by thiolate-ligated species, are arranged vertically. Thus the clusters [Fe4S4(SR)4]2 (16) are analogues of protein site (3) in the same oxidation state in ferredoxin and high-potentialproteins. The [Fe4S4(SR)4]3,2,1oxidation states have been isol- ated. Severalexamples of [Fe4S4(SR)4]4, containing the all-ferrous core [Fe4S4]0, have been detected electrochemically, but at potentials indicative of extreme sensitivity to oxidation (e.g., [Fe4S4(SAr)4]3/4at 1.5 V to1.7 V.26,49). Because other oxidation states are usually obtained by oxidation or reduction of [Fe4S4(SR)4]2, these clusters are first considered.
Clusters in the [Fe4S4]2þstate were originally prepared by the self-assembly reaction (15)47,48. Subsequently, convenient reactions (16)52,53 and (17)52,53 were introduced in which sulfur is the source of sulfide. These reactions are among the very few in which cluster assembly has been resolved into detectable steps.50 Thus, under the stoichiometry of reaction (17), reaction (18a)
forms the adamantaine-like complex [Fe4(SR)10]2,51 which reacts quantitatively with sulfur to give the product cluster in reaction (18b).50 The sum (18a)þ(18b)=(17). If the reactant mole ratio is increased to RS:FeIII: S 5:1:1, a different pathway described by sequentialreacations (19a), (1), and (19b)26,50 is followed. The product is generated in the last step by spontaneous dimerization in methanolsolution. The sum 4(19a)þ2(1)þ(19b)=(17). Reactions (16) and (17) have been conducted in methanoland water. Many of the reactions leading to the formation of [Fe4S4(SR)4]2 have been summarized.50 Reaction (20)54,55 is particularly useful for, but not restricted to, the formation of arythiolate clusters. The reaction may be shifted completely to the right by a suitable amount of R0SH or removalof product thiol. In this and related systems, mixed clusters of monofunctional ligands are detectable by NMR but are not separable owing to the facile ligand redistribution reaction (21)54(n=1–3).
4FeCl3ỵ6RSỵ4HSỵ4OMe! ẵFe4S4ðSRị42ỵRSSRỵ 12Clỵ4MeOH ð15ị
4FeCl2ỵ10RSỵ4S! ẵFe4S4ðSRị42ỵ3RSSRỵ 8Cl ð16ị
4FeCl3ỵ14RSỵ4S! ẵFe4S4ðSRị42ỵ5RSSR ỵ12Cl ð17ị
4FeCl3ỵ14RS! ẵFe4ðSRị102ỵ2RSSRỵ 12Cl ð18aị
ẵFe4ðSRị102ỵ4S! ẵFe4S4ðSRị42ỵ3RSSR ð18bị
FeCl3ỵ5RS! ẵFeðSRị42ỵ1=2 RSSRỵ3Cl ð19aị
2ẵFe2S2ðSRị42! ẵFe4S4ðSRị42ỵRSSRỵ2RS ð19bị
ẵFe4S4ðSRị42ỵnR0SHé ẵFe4S4ðSRị4nðSR0ịn2ỵnRSH ð20ị
ð4nịẵFe4S4ðSRị42ỵnẵFe4S4ðSR0ị42é4ẵFe4S4ðSRị4nðSR0ịn2 ð21ị The nucleophilic nature of coordinated thiolate sustains reaction with electrophiles such as weak protonic acids,56acylhalides,19and sulfonium cations.57Reaction (22)19is illustrative; intermedi- ate species with n=1–3 are detectable by NMR. The reaction is irreversible; the clusters [Fe4S4X4]2 (X=Cl, Br) are available in high yield by this method. These clusters can be directly assembled in the reaction systems FeX3/H2S/Ph4PX58 and FeCl3/Na2S/R4NBr.59 The iodo cluster was first made by the reaction of [Fe4S4Cl4]2 with NaI in acetonitrile,19 and
core oxidation state:
[Fe4S4]0 [Fe4S4]1+ [Fe4S4]2+ [Fe4S4]3+
3Fe(II) + Fe(III)
4Fe(II) 2Fe(II) +2Fe(III) Fe(II) + 3Fe(III) Fe protein† Fdred Fdox/HPred HPox
proteins:
analogues: [Fe4S4(SR)4]4- *[Fe4S4(SR)4]3- *[Fe4S4(SR)4]2- *[Fe4S4(SR)4]1- Fd = ferredoxin. HP = "high-potential" protein. *Isolated. †Nitrogenase.
Figure 2 Electron transfer series of native and synthetic [Fe4S4] clusters, showing core oxidation states and formalmetaloxidation states. The [Fe4S4]0state in a protein is known only in the Fe protein of nitrogenase.
subsequently by the assembly reaction (23)60,61 Halide clusters have been isolated only in the [Fe4S4]2þ state. Their principal feature is ligand lability, leading to extensive use in generalized reaction (24) as precursors to differently substituted clusters. By substitution reactions (20), (24) and others, clusters with a diverse array of ligands have been prepared or generated in solution:
ẵFe4S4ðSRị42ỵnR0COCl! ẵFe4S4ðSRị4nCln2ỵnR0COSR ð22ị
4FeðCOị5ỵ4SỵI2ỵ2I! ẵFe4S4I42ỵ20CO ð23ị
ẵFe4S4X42ỵ4Lz! ẵFe4S4L4z0ỵ4X ð24ị The family of thiolate cluster [Fe4S4(SR)4]2 is exceptionally large, including among others those with the simplest ligand (R=H),62–64exceptionally bulky ligands (R = 2,4,6-triisopropyl- benzenethiolate,39,40 2,4,6-triisopropylbenzylthiolate,65 adamantane-1-thiolate66), water-solubilizing ligands (R=CH2CH2OH,67,68(CH2)2CO269), crown ether ligands,70,71macrocyclic tetrathiolates,72–74 and dendrimeric thiolates (R=dendron).75,76Clusters with other ligand types such as phenolate77 and dithiocarbamate78 have been prepared. Based on kinetics investigations, mechanisms have been proposed for the substitution of bound thiolate with another thiolate in the presence of weak acid,79,80 and for reaction (24) with X=Clor Brand L=RS.81–83
8.3.4.2 The [Fe4S4]3þOxidation State
This state is an analogue of the oxidized form of cluster (3) of high-potentialiron–sulfur proteins, a subset of ferredoxins demarcated by access to this state. It was first detected by electrochemical oxidation of [Fe4S4(SBut)4]2 and related clusters with bulky ligands.84–86 Thereafter, the redox couple [Fe4S4(Stibt)4]2/1was found to be chemically reversible, and [Fe4S4-(Stibt)4]1was isolated from the oxidation of the cluster dianion in dichloromethane with [Cp2Fe](BF)4].40Other oxidants of moderate strength also effect this oxidation.39 In some instances, [Fe4S4(SR)4]1 clusters have been generated by -irradiation in host single crystals of [Fe4S4(SR)4]2 salts for spectroscopic study.87,88While certain [Fe4S4-(SR)4]1clusters can be generated by chemical or electrochemical oxidation of [Fe4S4(SR)4]2at moderate potentials (near 0 V), they are clearly much less stable than the precursor dianions. The great majority of oxidation reactions observed electrochemically are irreversible. Conditions have been found with aqueous polymer dispersions that permit observation of the redox couples [Fe4S4-(SAd)4]3/2/1and demonstration of proton-coupled electron transfer reactions.66,89Overall, [Fe4S4(SR)4]1 clusters are best stabilized in dry weakly basic solvents such as dichloromethane and with large hydrophobic ligands that protect the electrophilic [Fe4S4]3þcore from nucleophilic attack.90,91 Only (Bu4N)[Fe4S4(Stibt)4] has been isolated in substance. Its EPR and Mo¨ssbauer spectroscopic properties establish an S=1/2 ground state and sustain the description of the cluster as an analogue of the HPoxsite.
8.3.4.3 The [Fe4S4]1þOxidation State
This state is an analogue of the reduced form of protein cluster (3), and in isolable form is found mainly as the thiolate clusters [Fe4S4(SR)4]3. These species are strongly reducing, and in aprotic solvents show reversible [Fe4S4(SR)4]3/2couples at ca.1.0 V to1.5 V. The clusters are usually prepared by reduction of [Fe4S4]2þ clusters, as in reaction (25)26,95,96 which utilizes the reductant sodium acenaphthylenide in solvents such as acetonitrile/THF. The clusters can also be conveniently obtained in a single-step synthesis using the reaction system FeCl2/2NaSR/4NaSH in DMF.92 Excess thiolate is added to neutralize the protons released from hydrosulfide. The oxidant, which removes one electron per four equivalents of FeII, has not been identified. Reduction of [Fe4S4- (SCH2CH2OH)4]2 with dithionite68 and [Fe4S4(SCH2CH2CO2)4]6with [Cr(EDTA)]493 in aqu- eous solution to the [Fe4S4]1þstate have been reported. Redox potentials of [Fe4S4]2þ/1þcouples of thiolate clusters are markedly increased upon passing from a non-aqueous solvent to water. For example, the [Fe4S4(SCH2CH2CO2)4]6/7potentialin methanol94and the [Fe4S4(SCH2CH2OH)4]2/3potential