Nitrosylation of Nitric‐Oxide‐Sensing Regulatory Proteins Containing [4Fe‐4S] Clusters Gives Rise to Multiple Iron–Nitrosyl Complexes

The reaction of protein‐bound iron–sulfur (Fe‐S) clusters with nitric oxide (NO) plays key roles in NO‐mediated toxicity and signaling. Elucidation of the mechanism of the reaction of NO with DNA regulatory proteins that contain Fe‐S clusters has been hampered by a lack of information about the nature of the iron‐nitrosyl products formed. Herein, we report nuclear resonance vibrational spectroscopy (NRVS) and density functional theory (DFT) calculations that identify NO reaction products in WhiD and NsrR, regulatory proteins that use a [4Fe‐4S] cluster to sense NO. This work reveals that nitrosylation yields multiple products structurally related to Roussin's Red Ester (RRE, [Fe₂(NO)₄(Cys)₂]) and Roussin's Black Salt (RBS, [Fe₄(NO)₇S₃]. In the latter case, the absence of ³²S/³⁴S shifts in the Fe?S region of the NRVS spectra suggest that a new species, Roussin's Black Ester (RBE), may be formed, in which one or more of the sulfide ligands is replaced by Cys thiolates.     

Heterometallic Clusters [CuSn3S9]5− and [Cu6Sn6S20]10−: Solvothermal Synthesis and Characterization of 4f–3d Thiostannates

Two types of 4f–3d thiostannates with general formula [Hen]₂[Ln(en)₄(CuSn₃S₉)] ? 0.5 en ( Ln1 ; Ln=La, 1 ; Ce, 2 ) and [Hen]₄[Ln(en)₄]₂[Cu₆Sn₆S₂₀] ? 3 en ( Ln2 ; Ln=Nd, 3 ; Gd, 4 ; Er, 5 ) were prepared by reactions of Ln₂O₃, Cu, Sn, and S in ethylenediamine (en) under solvothermal conditions between 160 and 190 °C. However, reactions performed in the range from 120 to 140 °C resulted in crystallization of [Sn₂S₆]^4? compounds and CuS powder. In 1 and 2 , three SnS₄ tetrahedra and one CuS₃ triangle are joined by sharing sulfur atoms to form a novel [CuSn₃S₉]^5? cluster that coordinates to the Ln³⁺ ion of [Ln(en)₄]³⁺ (Ln=La, Ce) as a monodentate ligand. The [CuSn₃S₉]^5? unit is the first thio‐based heterometallic adamantane‐like cluster coordinating to a lanthanide center. In 3 – 5 , six SnS₄ tetrahedra and six CuS₃ triangles are connected by sharing common sulfur atoms to form the ternary [Cu₆Sn₆S₂₀]^10? cluster, in which a Cu₆ core is enclosed by two Sn₃S₁₀ fragments. The topological structure of the novel Cu₆ core can be regarded as two Cu₄ tetrahedra joined by a common edge. The Ln³⁺ ions in Ln1 and Ln2 are in nine‐ and eightfold coordination, respectively, which leads to the formation of the [CuSn₃S₉]^5? and [Cu₆Sn₆S₂₀]^10? clusters under identical synthetic conditions. The syntheses of Ln1 and Ln2 show the influence of the lanthanide contraction on the quaternary Ln/Cu/Sn/S system in ethylenediamine. Compounds 1 – 5 exhibit bandgaps in the range of 2.09–2.48 eV depending on the two different types of clusters in the compounds. Compounds 1 , 3 , and 4 lost their organic components in the temperature range of 110–350 °C by multistep processes.     

Synthesis of Well‐Defined Bicapped Octahedral Iron Clusters [(trenL)2Fe8(PMe2Ph)2]n (n=0, −1)

The synthesis of polynuclear clusters with control over size and cluster geometry remains an unsolved challenge. Herein, we report the synthesis and characterization of open‐shell octairon clusters supported by two heptaamine ligands [o‐H₂NC₆H₄NH(CH₂)₂]₃N (^trenLH₉). The crystal structure of the all‐ferrous species ([^trenL)₂Fe₈(PMe₂Ph)₂] ( 1 ) displays a bicapped octahedral geometry with Fe? Fe distances ranging from 2.4071(6) to 2.8236(5) Å, where the ligand amine units are formally in amine, amide, and imide oxidation states. Several redox states of the octairon cluster are accessible, as ascertained using cyclic voltammetry. The one‐electron‐reduced clusters [M]⁺[(^trenL)₂Fe₈(PMe₂Ph)₂]^? (M=Bu₄N ( 2 a ); (15‐crown‐5)Na(thf) ( 2 b )) were isolated and characterized. Variable‐temperature magnetic susceptibility data indicates that the exchange coupling within the [Fe₈] core is antiferromagnetic which is attenuated upon reduction to the mixed valent anion.     

Controlled Expansion of a Strong‐Field Iron Nitride Cluster: Multi‐Site Ligand Substitution as a Strategy for Activating Interstitial Nitride Nucleophilicity

Multimetallic clusters have long been investigated as molecular surrogates for reactive sites on metal surfaces. In the case of the μ₄‐nitrido cluster [Fe₄(μ₄‐N)(CO)₁₂]^?, this analogy is limited owing to the electron‐withdrawing effect of carbonyl ligands on the iron nitride core. Described here is the synthesis and reactivity of [Fe₄(μ₄‐N)(CO)₈(CNAr^Mes2)₄]^?, an electron‐rich analogue of [Fe₄(μ₄‐N)(CO)₁₂]^?, where the interstitial nitride displays significant nucleophilicity. This characteristic enables rational expansion with main‐group and transition‐metal centers to yield unsaturated sites. The resulting clusters display surface‐like reactivity through coordination‐sphere‐dependent atom rearrangement and metal–metal cooperativity.     

Control of the Transition between Ni‐C and Ni‐SIa States by the Redox State of the Proximal FeS Cluster in the Catalytic Cycle of [NiFe] Hydrogenase

[NiFe] hydrogenase catalyzes the reversible cleavage of H₂. The electrons produced by the H₂ cleavage pass through three Fe–S clusters in [NiFe] hydrogenase to its redox partner. It has been reported that the Ni‐SI_a, Ni‐C, and Ni‐R states of [NiFe] hydrogenase are involved in the catalytic cycle, although the mechanism and regulation of the transition between the Ni‐C and Ni‐SI_a states remain unrevealed. In this study, the FT‐IR spectra under light irradiation at 138–198 K show that the Ni‐L state of [NiFe] hydrogenase is an intermediate between the transition of the Ni‐C and Ni‐SI_a states. The transition of the Ni‐C state to the Ni‐SI_a state occurred when the proximal [Fe₄S₄]_p^2+/+ cluster was oxidized, but not when it was reduced. These results show that the catalytic cycle of [NiFe] hydrogenase is controlled by the redox state of its [Fe₄S₄]_p^2+/+ cluster, which may function as a gate for the electron flow from the NiFe active site to the redox partner.     

Controlled Incorporation of Nitrides into W‐Fe‐S Clusters

Incorporation of monatomic 2p ligands into the core of iron–sulfur clusters has been researched since the discovery of interstitial carbide in the FeMo cofactor of Mo‐dependent nitrogenase, but has proven to be a synthetic challenge. Herein, two distinct synthetic pathways are rationalized to install nitride ligands into targeted positions of W‐Fe‐S clusters, generating unprecedented nitride‐ligated iron–sulfur clusters, namely [(Tp*)₂W₂Fe₆(μ₄‐N)₂S₆L₄]^2? (Tp*=tris(3,5‐dimethyl‐1‐pyrazolyl)hydroborate(1?), L=Cl^? or Br^?). ⁵⁷Fe Mössbauer study discloses metal oxidation states of W^IV₂Fe^II₄Fe^III₂ with localized electron distribution, which is analogous to the mid‐valent iron centres of FeMo cofactor at resting state. Good agreement of Mössbauer data with the empirical linear relationship for Fe–S clusters indicates similar ligand behaviour of nitride and sulfide in such clusters, providing useful reference for reduced nitrogen in a nitrogenase‐like environment.     

On the Critical Effect of the Metal (Mo vs. W) on the [3+2] Cycloaddition Reaction of M3S4 Clusters with Alkynes: Insights from Experiment and Theory

Whereas the cluster [Mo₃S₄(acac)₃(py)₃]⁺ ([ 1 ]⁺, acac=acetylacetonate, py=pyridine) reacts with a variety of alkynes, the cluster [W₃S₄(acac)₃(py)₃]⁺ ([ 2 ]⁺) remains unaffected under the same conditions. The reactions of cluster [ 1 ]⁺ show polyphasic kinetics, and in all cases clusters bearing a bridging dithiolene moiety are formed in the first step through the concerted [3+2] cycloaddition between the C?C atoms of the alkyne and a Mo(μ‐S)₂ moiety of the cluster. A computational study has been conducted to analyze the effect of the metal on these concerted [3+2] cycloaddition reactions. The calculations suggest that the reactions of cluster [ 2 ]⁺ with alkynes feature ΔG^≠ values only slightly larger than its molybdenum analogue, however, the differences in the reaction free energies between both metal clusters and the same alkyne reach up to approximately 10 kcal mol^?1, therefore indicating that the differences in the reactivity are essentially thermodynamic. The activation strain model (ASM) has been used to get more insights into the critical effect of the metal center in these cycloadditions, and the results reveal that the change in reactivity is entirely explained on the basis of the differences in the interaction energies E_int between the cluster and the alkyne. Further decomposition of the E_int values through the localized molecular orbital‐energy decomposition analysis (LMO‐EDA) indicates that substitution of the Mo atoms in cluster [ 1 ]⁺ by W induces changes in the electronic structure of the cluster that result in weaker intra‐ and inter‐fragment orbital interactions.     

Adamantyl‐ and Furanyl‐Protected Nanoscale Silver Sulfide Clusters

The silver salts of 1‐adamantanethiol (AdSH) and furan‐2‐ylmethanethiol (FurCH₂SH) were successfully applied as building blocks for ligand‐protected Ag₂S nanoclusters. The reaction of the silver thiolates [AgSAd]_x and [AgSCH₂Fur]_x with S(SiMe₃)₂ and 1,5‐bis(diphenylphosphino)pentane (dpppt) afforded three different clusters with 58, 94 and, 190 silver atoms. The intensely colored compounds [Ag₅₈S₁₃(SAd)₃₂] ( 1 ), [Ag₉₄S₃₄(SAd)₂₆(dpppt)₆] ( 2 ), and [Ag₁₉₀S₅₈(SCH₂Fur)₇₄(dpppt)₈] ( 3 ) were structurally characterized by single‐crystal X‐ray diffraction and exhibit different cluster core geometries and ligand shells. The diameters of the well‐defined sphere‐shaped nanoclusters range from 2.2 nm to 3.5 nm.     

Solvent‐Free Ball‐Milling Subcomponent Synthesis of Metallosupramolecular Complexes

Subcomponent self‐assembly from components A , B , C , D , and Fe²⁺ under solvent‐free conditions by self‐sorting leads to the construction of three structurally different metallosupramolecular iron(II) complexes. Under carefully selected ball‐milling conditions, tetranuclear [Fe₄( AD ₂)₆]^4? 22‐component cage 1 , dinuclear [Fe₂( BD ₂)₃]^2? 11‐component helicate 2 , and 5‐component mononuclear [Fe( CD ₃)]²⁺ complex 3 were prepared simultaneously in a one‐pot reaction from 38 components. Through subcomponent substitution reaction by adding subcomponent B , the [Fe₄( AD ₂)₆]^4? cage converts quantitatively to the [Fe₂( BD ₂)₃]^2? helicate, which, in turn, upon addition of subcomponent C , transforms to [Fe( CD ₃)]²⁺, following the hierarchical preference based on the thermodynamic stability of the complexes.     

Synthesen und Strukturen von Übergangsmetall‐Komplexen mit Dithiophosphinato‐ und Trithiophosphonato‐Liganden

Syntheses and Structures of Transition Metal Complexes with Dithiophosphinato and Trithiophosphinato Ligands The reactions of MnCl₂ with Ph₂P(S)(SSiMe₃) produced [Mn(S₂PPh₂)₂(thf)₂] ( 1 ) and [Mn(S₂PPh₂)₂(dme)] ( 2 ) (DME = 1,2‐Dimethoxyethane). The compounds [Co₆(S₃PPh)₂(μ⁴‐S)₂(μ³‐S)₂(PPh₃)₄] ( 3 ), [Co₂(S₃PPh)₂(PPh₃)₂] ( 4 ), [Ni(S₂PPh)(PPhEt₂)₂] ( 5 ), [Ni(S₃PPh)(PPhEt₂)₂] ( 6 ) and [Cu₄(S₃PPh)₂(dppp)₂] ( 8 ) [dppp = 1,3‐Bis(diphenylphosphanyl)propane] were obtained from reactions of first‐row transition metal halides with PhP(S)(SSiMe₃)₂ in the presence of tertiary phosphines. In a reaction of PhP(S)(SSiMe₃)₂ with PhPEt₂ PhPEt₂PS₂Ph ( 7 ) was isolated. All compounds were characterized by X‐ray crystallography.     

A Mixed‐Valence {Fe13} Cluster Exhibiting Metal‐to‐Metal Charge‐Transfer‐Switched Spin Crossover

It is promising and challenging to manipulate the electronic structures and functions of materials utilizing both metal‐to‐metal charge transfer (MMCT) and spin‐crossover (SCO) to tune the valence and spin states of metal ions. Herein, a metallocyanate building block is used to link with a Fe^II‐triazole moiety and generates a mixed‐valence complex {[(Tp^4‐Me)Fe^III(CN)₃]₉[Fe^II₄(trz‐ph)₆]}?[Ph₃PMe]₂?[(Tp^4‐Me)Fe^III(CN)₃] ( 1 ; trz‐ph=4‐phenyl‐4H‐1,2,4‐triazole). Moreover, MMCT occurs between Fe^III and one of the Fe^II sites after heat treatment, resulting in the generation of a new phase, {[(Tp^4‐Me)Fe^II(CN)₃][(Tp^4‐Me)Fe^III(CN)₃]₈ [Fe^IIIFe^II₃(trz‐ph)₆]}? [Ph₃PMe]₂?[(Tp^4‐Me)Fe^III(CN)₃] ( 1 a ). Structural and magnetic studies reveal that MMCT can tune the two‐step SCO behavior of 1 into one‐step SCO behavior of 1 a . Our work demonstrates that the integration of MMCT and SCO can provide a new alternative for manipulating functional spin‐transition materials with accessible multi‐electronic states.     

Systematic Investigation of High‐Sensitivity Luminescent Sensing for Polyoxometalates and Iron(III) by MOFs Assembled with a New Resorcin[4]arene‐Functionalized Tetracarboxylate

A new family of resorcin[4]arene‐based metal–organic frameworks (MOFs), namely, [Eu(HL)(DMF)(H₂O)₂] ? 3 H₂O ( 1 ), [Tb(HL)(DMF)(H₂O)₂] 3 H₂O ( 2 ), [Cd₄(L)₂(DMF)₄(H₂O)₂] 3 H₂O ( 3 ) and [Zn₃(HL)₂(H₂O)₂] 2 DMF ? 7 H₂O ( 4 ), have been constructed from a new resorcin[4]arene‐functionalized tetracarboxylic acid (H₄L=2,8,14,20‐tetra‐ethyl‐6,12,18,24‐tetra‐methoxy‐4,10,16,22‐tetra‐carboxy‐methoxy‐calix[4]arene). Isostructural 1 and 2 exhibit charming 1D motifs built with the cup‐like HL^3? anions and rare earth cations. Compounds 3 and 4 show a unique sandwich‐based 2D layer and a fascinating 3D framework, respectively. Remarkably, compounds 1 and 2 display intensive red and green emissions triggered by the efficient antenna effect of organic ligands under UV light. More importantly, systematic luminescence studies demonstrate that Ln‐MOFs 1 and 2 , as efficient multifunctional fluorescent materials, show highly selective and sensitive sensing of Fe³⁺, polyoxometalates (POMs), and acetone, which represents a rare example of a sensor for quantitatively detecting three different types of analytes. This is also an exceedingly rare example of Fe³⁺ and POMs detection in aqueous solutions employing resorcin[4]arene‐based luminescent Ln‐MOFs. Furthermore, the possible mechanism of the sensing properties is deduced.     

Spectroscopic Characterization of an Eight‐Iron Nitrogenase Cofactor Precursor that Lacks the “9th Sulfur”

Nitrogenases catalyze the reduction of N₂ to NH₄⁺ at its cofactor site. Designated the M‐cluster, this [MoFe₇S₉C(R‐homocitrate)] cofactor is synthesized via the transformation of a [Fe₄S₄] cluster pair into an [Fe₈S₉C] precursor (designated the L‐cluster) prior to insertion of Mo and homocitrate. We report the characterization of an eight‐iron cofactor precursor (designated the L*‐cluster), which is proposed to have the composition [Fe₈S₈C] and lack the “9^th sulfur” in the belt region of the L‐cluster. Our X‐ray absorption and electron spin echo envelope modulation (ESEEM) analyses strongly suggest that the L*‐cluster represents a structural homologue to the l ‐cluster except for the missing belt sulfur. The absence of a belt sulfur from the L*‐cluster may prove beneficial for labeling the catalytically important belt region, which could in turn facilitate investigations into the reaction mechanism of nitrogenases.     

Solvothermal Synthesis and Characterization of the New Iron Thioantimonates(III) [Fe(C6H18N4)]FeSbS4 and [Fe(C4H13N3)2]Fe2Sb4S10 Containing FeII and FeIII and Protein‐Analogous [2FeIII‐2S]2+ Clusters

The two new compounds [Fe(tren)]FeSbS₄ ( 1 ) (tren = tris(2‐aminoethyl)amine) and [Fe(dien)₂]Fe₂Sb₄S₁₀ ( 2 ) (dien = diethylendiamine) were prepared under solvothermal conditions and represent the first thioantimonates(III) with iron cations integrated into the anionic network. In both compounds Fe³⁺ is part of a [2Fe^III‐2S] cluster which is often found in ferredoxines. In addition, Fe²⁺ ions are present which are surrounded by the organic ligands. In ( 1 ) the Fe²⁺ ion is also part of the thioantimonate(III) network whereas in ( 2 ) the Fe²⁺ ion is isolated. In both compounds the primary SbS₃ units are interconnected into one‐dimensional chains. The mixed‐valent character of [Fe(tren)]FeSbS₄ was unambiguously determined with Mössbauer spectroscopy. Both compounds exhibit paramagnetic behaviour and for ( 1 ) a deviation from linearity is observed due to a strong zero‐field splitting. Both compounds decompose in one single step.     

Magnetic Properties of Two New Fe4 Single‐Molecule Magnets in the Solid State and in Frozen Solution

Two novel tetranuclear, star‐shaped iron(III) clusters, [Fe₄(acac)₆(Br‐mp)₂] and [Fe^III₄(acac)₆(tmp)₂], are described. Both have S=5 ground states resulting from antiferromagnetic nearest‐neighbour superexchange interactions, with J=?8.2 cm^?1 and J=?8.5 cm^?1 for 1 and 2 , respectively. Energy barriers for the relaxation of the magnetisation of approximately 12 cm^?1 were derived from AC susceptibility measurements. Magnetic resonance measurements revealed a zero‐field splitting parameter D=?0.34 cm^?1 for both complexes. AC susceptibility measurements in solution demonstrated that the complexes are reasonably stable in solution. Interestingly, the magnetisation relaxation slows down significantly in frozen solution, in contrast to what is generally observed for single‐molecule magnets. This was shown to result from a large increase in τ₀, the prefactor in the Arrhenius equation, with the energy barrier remaining unchanged.     

Salts of Anionic Metal Carbonyl Clusters with Cryptand[2.2.2](Na+), DB‐18‐crown‐6(Na+), and Paramagnetic Cp*2Cr+ Cations Obtained by Reduction

Reduction of neutral metal clusters (Co₄(CO)₁₂, Ru₃(CO)₁₂, Fe₃(CO)₁₂, Ir₄(CO)₁₂, Rh₆(CO)₁₆, {CpMo(CO)₃}₂, {Mn(CO)₅}₂) by decamethylchromocene (Cp*₂Cr) or sodium fluorenone ketyl in the presence of cryptand[2.2.2] and DB‐18‐crown‐6 was studied. Nine new salts with paramagnetic Cp*₂Cr⁺, cryptand[2.2.2](Na⁺), and DB‐18‐crown‐6(Na⁺) cations and [Co₆(CO)₁₅]^2– ( 1 , 2 ), [Ru₆(CO)₁₈]^2– ( 3 – 4 ) dianions, [Rh₁₁(CO)₂₃]^3– ( 6 ) trianions, and new [Ir₈(CO)₁₈]^2– ( 5 ) dianions were obtained and structurally characterized. The increase of nuclearity of clusters under reduction was shown. Fe₃(CO)₁₂ preserves the Fe₃ core under reduction forming the [Fe₃(CO)₁₁]^2– dianions in 7 . The [CpMo(CO)₃]₂ and [Mn(CO)₅]₂ dimers dissociate under reduction forming mononuclear [CpMo(CO)₃]^– ( 8 ) and [Mn(CO)₅]^– ( 9 ) anions. In all anions the increase of negative charge on metal atoms shifts the bands attributed to carbonyl C–O stretching vibrations to smaller wavenumbers in agreement with the elongation of the C–O bonds in 1 – 9 . In contrast, the M–C(CO) bonds are noticeably shortened at the reduction. Magnetic susceptibility of the salts with Cp*₂Cr⁺ is defined by high spin Cp*₂Cr⁺ (S = 3/2) species, whereas all obtained anionic metal clusters and mononuclear anions are diamagnetic. Rather weak magnetic coupling between S = 3/2 spins is observed with Weiss temperature from –1 to –11 K. That is explained by rather long distances between Cp*₂Cr⁺ and the absence of effective π–π interaction between them except compound 7 showing the largest Weiss temperature of –11 K. The {DB‐18‐crown‐6(Na⁺)}₂[Co₆(CO)₁₅]^2– units in 2 are organized in infinite 1D chains through the coordination of carbonyl groups of the Co₆ clusters to the Na⁺ ions and π–π stacking between benzo groups of the DB‐18‐crown‐6(Na⁺) cations.     

Tridentate Thiolate Ligands: Application to the Synthesis of the Site-Differentiated [4Fe-4S] Cluster having a Hydrosulfide Ligand at the Unique Iron Center

We have designed new trithiols Temp(SH)₃ and Tefp(SH)₃ that can be synthesized conveniently in short steps and are useful for preparation of crystalline [3:1] site-differentiated [4Fe-4S] clusters suitable for X-ray structural analysis. The ethanethiolate clusters (PPh₄)₂[Fe₄S₄(SEt)(TempS₃)] ( 4a ) and (PPh₄)₂[Fe₄S₄(SEt)(TefpS₃)] ( 4b ) were prepared as precursors, and the unique iron sites were then selectively substituted. Upon reaction with H₂S, (PPh₄)₂[Fe₄S₄(SH)(TempS₃)] ( 6a ) and (PPh₄)₂[Fe₄S₄(SH)(TefpS₃)] ( 6b ), which model the [4Fe-4S] cluster in the β subunit of (R)-2-hydroxyisocaproyl-CoA dehydratase, were synthesized. Clusters 6a and 6b were further converted to the sulfido-bridged double cubanes (PPh₄)₄[{Fe₄S₄(TempS₃)}₂(μ²-S)] ( 6b ) and (PPh₄)₄[{Fe₄S₄(TefpS₃)}₂(μ²-S)] ( 7b ), respectively, via intermolecular condensation with the release of H₂S. Conversely, addition of H₂S to 7a , 7b afforded the hydrosulfide clusters 6a , 6b . The molecular structures of the clusters reported herein were elucidated by X-ray crystallographic analysis. Their redox properties were investigated by cyclic voltammetry.     

Electron‐Rich Diferrous–Phosphane–Thiolates Relevant to Fe‐only Hydrogenase: Is Cyanide “Nature's Trimethylphosphane”?

The two‐step one‐pot oxidative decarbonylation of [Fe₂(S₂C₂H₄)(CO)₄(PMe₃)₂] ( 1 ) with [FeCp₂]PF₆, followed by addition of phosphane ligands, led to a series of diferrous dithiolato carbonyls 2 – 6 , containing three or four phosphane ligands. In situ measurements indicate efficient formation of 1 ²⁺ as the initial intermediate of the oxidation of 1 , even when a deficiency of the oxidant was employed. Subsequent addition of PR₃ gave rise to [Fe₂(S₂C₂H₄)(μ‐CO)(CO)₃(PMe₃)₃]²⁺ ( 2 ) and [Fe₂(S₂C₂H₄)(μ‐CO)(CO)₂(PMe₃)₂(PR₃)₂]²⁺ (R=Me 3 , OMe 4 ) as principal products. One terminal CO ligand in these complexes was readily substituted by MeCN, and [Fe₂(S₂C₂H₄)(μ‐CO)(CO)₂(PMe₃)₃(MeCN)]²⁺ ( 5 ) and [Fe₂(S₂C₂H₄)(μ‐CO)(CO)(PMe₃)₄(MeCN)]²⁺ ( 6 ) were fully characterized. Relevant to the H_red state of the active site of Fe‐only hydrogenases, the unsymmetrical derivatives 5 and 6 feature a semibridging CO ligand trans to a labile coordination site.     

Metal‐Templated,Tight Loop Conformation of a Cys‐X‐Cys Biomimetic Assembles a Dimanganese Complex

With the goal of generating anionic analogues to MN₂S₂ ? Mn(CO)₃Br we introduced metallodithiolate ligands, MN₂S₂^2? prepared from the Cys‐X‐Cys biomimetic, ema^4? ligand (ema=N,N′‐ethylenebis(mercaptoacetamide); M=Ni^II, [V^IV≡O]²⁺ and Fe^III) to Mn(CO)₅Br. An unexpected, remarkably stable dimanganese product, (H₂N₂(CH₂C=O(μ‐S))₂)[Mn(CO)₃]₂ resulted from loss of M originally residing in the N₂S₂^4? pocket, replaced by protonation at the amido nitrogens, generating H₂ema^2?. Accordingly, the ema ligand has switched its coordination mode from an N₂S₂^4? cavity holding a single metal, to a binucleating H₂ema^2? with bridging sulfurs and carboxamide oxygens within Mn‐μ‐S‐CH₂‐C‐O, 5‐membered rings. In situ metal‐templating by zinc ions gives quantitative yields of the Mn₂ product. By computational studies we compared the conformations of “linear” ema^4? to ema^4? frozen in the “tight‐loop” around single metals, and to the “looser” fold possible for H₂ema^2? that is the optimal arrangement for binucleation. XRD molecular structures show extensive H‐bonding at the amido‐nitrogen protons in the solid state.     

A Brønsted‐Ligand‐Based Iron Complex as a Molecular Switch with Five Accessible States

A mononuclear Fe^II complex, prepared with a Brønsted diacid ligand, H₂L (H₂L=2‐[5‐phenyl‐1H‐pyrazole‐3‐yl] 6‐benzimidazole pyridine), shows switchable physical properties and was isolated in five different electronic states. The spin crossover (SCO) complex, [Fe^II(H₂L)₂](BF₄)₂ ( 1_A ), exhibits abrupt spin transition at T_1/2=258 K, and treatment with base yields a deprotonated analogue [Fe^II(HL)₂] ( 1_B ), which shows gradual SCO above 350 K. A range of Fe^III analogues were also characterized. [Fe^III(HL)(H₂L)](BF₄)Cl ( 1_C ) has an S=5/2 spin state, while the deprotonated complexes [Fe^III(L)(HL)], ( 1_D ), and (TEA)[Fe^III(L)₂], ( 1_E ) exist in the low‐spin S=1/2 state. The electronic properties of the five complexes were fully characterized and we demonstrate in situ switching between multiple states in both solution and the solid‐state. The versatility of this simple mononuclear system illustrates how proton donor/acceptor ligands can vastly increase the range of accessible states in switchable molecular devices.