Tetranuclear Fe Clusters with a Varied Interstitial Ligand: Effects on the Structure, Redox Properties, and Nitric Oxide Activation

Abstract

A new series of tetranuclear iron clusters displaying an interstitial μ4-F ligand was prepared for comparison to μ4-O analogs. With a single NO coordinated as a reporter of small molecule activation, the μ4-F clusters were characterized in five redox states, from FeII3{FeNO}8 to FeIII3{FeNO}7, with N–O stretching frequencies ranging from 1680 cm−1 to 1855 cm−1, respectively. Despite accessing more reduced states with an F− bridge, a two electron reduction of the distal Fe centers is necessary for the μ4-F clusters to activate NO to the same degree as the μ4-O system; consequently, NO reactivity is observed at more positive potentials with μ4-O than μ4-F. Moreover, the μ4-O ligand better translates redox changes of remote metal centers to diatomic ligand activation. The implication for biological active sites is that the higher charge bridging ligand is more effective in tuning cluster properties, including the involvement of remote metal centers, for small molecule activation

A Thermodynamic Model for Redox-Dependent Binding of Carbon Monoxide at Site-Differentiated, High Spin Iron Clusters

“…From this perspective, 3 contains a masked apical Fe II site whose affinity for CO is modulated relative to 5 by two terms, one accounting for the energetic cost of redox reorganization and the other for the effect of changes in redox states of the remote metals (Figure 8). Although our data for these and related clusters 39–41,69–70 is most consistent with a valence-localized assignment, an analogous scheme can be constructed for a valence-delocalized system, where the internal electron transfer ( i -ET) term is replaced by a term accounting for the energetic penalty of trapping an electron at Fe4, assuming the CO bound product is valence-localized.…”

“…A single crystal X-ray diffraction study confirms the formation of a tetranuclear iron cluster (Figure 3a), where the bond metrics within the Fe 4 (μ 4 -O) motif are diagnostic of metal oxidation states. 39–41 For the structurally homologous pyrazolate bridged clusters [LFe 3 O(PhPz) 3 Fe][OTf] n (n = 1–3), the distances between the distal, six-coordinate iron centers (Fe1, Fe2, Fe3, respectively) and the interstitial oxygen atom (O1) elongate upon reduction (average Fe1/2/3-O1 distances: 1.96 Å for Fe III and 2.07 Å for Fe II ). 41 The observation of two long (2.1480(19) and 2.093(2) Å) and one short (1.983(2) Å) bond distance between the interstitial oxygen (O1) and the iron centers Fe1, Fe2, and Fe3 suggests a valence localized [Fe II 2 Fe III ] assignment for the basal triiron core of 3 .…”

“…However, with few exceptions, 46–49 Fe III centers generally display no affinity for CO. 50 Alternatively, we envisioned that an internal electron transfer ( i -ET) from a distal Fe II site might accommodate coordination of CO (Figure 7a). Based on the diagnostic features associated with the basal core Fe II centers in these clusters 39–41 , Mössbauer spectroscopy serves as a convenient tool to determine whether redox reorganization accompanies CO binding. 51 The zero field Mössbauer spectrum (80 K) obtained by freezing a CO-saturated solution of 3 in 2,6-lutidine ( f.p.…”

Nitric oxide activation facilitated by cooperative multimetallic electron transfer within an iron-functionalized polyoxovanadate–alkoxide cluster

“…Likewise, electronic storage across a metal-oxide base has been observed previously, in the case of [LFe 3 (PhPz) 3 EFeNO] n + (L = 1,3,5-triarylbenzene ligand motif, E = O, F). 56 , 57 Agapie and coworkers demonstrate through spectroscopic and crystallographic investigations that changes in oxidation state of the tetra-iron cluster, [LFe 3 (PhPz) 3 OFeNO] 1+ , occur exclusively across the distal iron centres. Mössbauer parameters of complexes [LFe 3 (PhPz) 3 OFeNO] n + ( n = 1, 2, 3) reveal little change in the isomer shift of the apical iron centre upon oxidation of the cluster core, indicating the retention of the {FeNO} 7 state for the iron–nitrosyl adduct across the redox series.…”

Remote Ligand Modifications Tune Electronic Distribution and Reactivity in Site-Differentiated, High-Spin Iron Clusters: Flipping Scaling Relationships

“…Two reasonable simulations were obtained, both of which afford Mössbauer parameters for one subsite (50% total iron) which are consistent with the presence of two high spin, six-coordinate Fe II centers (δ ~ 1.1 mm/s, |ΔEQ| ~ 3.2 mm/s) within the triiron core. 5,52,[64][65][66][67] The relative intensity of the sharp resonance near 1 mm/s indicates the presence of one ferric ion whose isomer shift and quadrupole splitting depend on how the Lorentzian feature near -0.5 mm/s is modelled, with δ bounded between 0.34-0.47 mm/s. Isomer shifts in this range are common for high spin, six-coordinate ferric centers in O/N rich ligand environments, 35,[54][55][56]68 suggesting a [Fe II 2Fe III ] redox level for the triiron core identical to that inferred from the solid state structure.…”