Ligands In Coordination Sphere Research Articles

Hydrogen Evolving Mono‐Nuclear Manganese Complexes with Carefully Positioned Pyridine Base Ligands

AbstractEnormous efforts have been made by the scientific community in the development of bio‐mimetic/bio‐inspired hydrogen‐evolving complexes as catalysts for an alternative hydrogen energy carrier and a renewable energy sources. In this regard, new bio‐inspired mono‐nuclear Mn(I) carbonyl model complexes fac‐[(Mn(CO)3(κ2‐SN2C7H5)(κ1‐PPh2Py))] 1 and fac‐[(Mn(CO)3(κ2‐S2NC7H4)(κ1‐PPh2Py))] 2 with 2‐mercaptobenzimidazole (N,N) and 2‐mercaptobenzothiazole (S,N) ligands and a N‐based phosphine ligand (PPh2Py) have been synthesized, spectroscopically characterized and evaluated as electro‐catalysts for hydrogen generation. In contrast to related mono‐nuclear (N,N/S,N)−Mn−PPh3 complexes,, the newly introduced pyridine nitrogen atom of the PPh2Py ligand placed in an axial position in the second coordination sphere of the Mn(I) complexes, provides a first site for protonation and enables a fast intra‐molecular proton transfer to the central metal atom with a weaker acid. Theoretical DFT calculations enable a detailed picture of catalytic pathways, assign redox transitions and structural changes. Both the molecular complexes were tested as electro‐catalysts for hydrogen generation in CH3CN. They effectively catalyzed the electrochemical proton reduction with the weaker acetic acid as the proton source and displayed a turnover frequency (TOF/s−1; O.P., η/V) of 610; 1.02 and 615; 1.12, respectively. The computer‐aided design of dedicated proton transfer pathways from second coordination sphere ligands is thus able to allow a hydrogen evolution reactivity with a weaker acid (acetic acid).

Ynone Co-Coordination at a Nickel Borane Complex: An Assessment of Secondary Coordination Sphere Effects.

Secondary coordination sphere ligand effects can be used to direct or organize small molecule substrates at a metal center. Herein, we assess the bifunctional ambiphilic diphosphine, tri-tert-butylboranyldiphosphinoethane (ttbbpe) and its ability to influence stereoselective substrate coordination, while appended to nickel. This report takes a synthetic/computational approach to test the impacts and limitations associated with ligand-directed substrate coordination using [Ni(ttbbpe)(η2:η2-COD)] (COD = 1,5-cyclooctadiene) and ynones (alkynes having an α-carbonyl group at the propargylic position) as model substrates.

Site-specific electronic structure of covalently linked bimetallic dyads from nitrogen K-edge x-ray absorption spectroscopy.

A nitrogen K-edge x-ray absorption near-edge structure (XANES) survey is presented for tetrapyrido[3,2-a:2',3'-c:3″,2″-h:2‴,3‴-j]phenazine (tpphz)-bridged bimetallic assemblies that couple chromophore and catalyst transition metal complexes for light driven catalysis, as well as their individual molecular constituents. We demonstrate the high N site sensitivity of the N pre-edge XANES features, which are energetically well-separated for the phenazine bridge N atoms and for the individual metal-bound N atoms of the inner coordination sphere ligands. By comparison with the time-dependent density functional theory calculated spectra, we determine the origins of these distinguishable spectral features. We find that metal coordination generates large shifts toward higher energy for the metal-bound N atoms, with increasing shift for 3d < 4d < 5d metal bonding. This is attributed to increasing ligand-to-metal σ donation that increases the effective charge of the bound N atoms and stabilizes the N 1s core electrons. In contrast, the phenazine bridge N pre-edge peak is found at a lower energy due to stabilization of the low energy electron accepting orbital localized on the phenazine motif. While no sensitivity to ground state electronic coupling between the individual molecular subunits was observed, the spectra are sensitive to structural distortions of the tpphz bridge. These results demonstrate N K-edge XANES as a local probe of electronic structure in large bridging ligand motifs, able to distinctly investigate the ligand-centered orbitals involved in metal-to-ligand and ligand-to-ligand electron transfer following light absorption.

Encapsulated Neutral Ruthenium Catalyst for Substrate-Selective Oxidation of Alcohols.

The neutral complex dichloro-{diethyl[(5-phenyl-1,3,4-oxadiazol-2-ylamino)-(4-trifluoro-methylphenyl)methyl]phosphonate} (p-cymene)-ruthenium(II) was encapsulated inside a self-assembled hexameric host obtained upon reaction of 2,8,14,20-tetra-undecyl-resorcin[4]arene and water. The formation of an inclusion complex was inferred from a combination of spectral measurements (MS, UV/Vis spectroscopy, 1 H and DOSY NMR). The 31 P and 19 F NMR spectra are consistent with motions of the ruthenium complex inside the self-assembled capsule. Molecular dynamics simulations carried out on the inclusion complex confirmed these intra-cavity movements and highlighted possible supramolecular interactions between the ruthenium first coordination sphere ligands and the inner part (aromatic rings) of the capsule. The embedded ruthenium complex was assessed in the catalytic oxidation (using NaIO4 as oxidant) of mixtures of three arylmethyl alcohols into the corresponding aldehydes. The reaction kinetics were shown to vary as a function of the substrates' size, with the oxidation rate varying in the order benzylalcohol >4-phenyl-benzylalcohol >9-anthracenemethanol. Control experiments realized in the absence of hexameric capsule did not allow any discrimination between the substrates.

Crystal structure of hexa-kis-(N,N-di-methyl-form-amide-κO)iron(III) μ-chlorido-bis-(tri-chlorido-cadmium).

The title compound, [Fe(C3H7NO)6][Cd2Cl7], crystallizes in the trigonal space group R and is assembled from discrete [Fe(DMF)6]3+ cations (DMF = N,N-di-methyl-formamide) and [Cd2Cl7]3- anions. In the cation, the iron(III) atom, located on a special position of site symmetry, is coordinated by six oxygen atoms from DMF ligands with all Fe-O distances being equal [2.0072 (16) Å]. A slight distortion of the octa-hedral environment of the metal comes from the cis O-Fe-O angles deviating from the ideal value of 90° [86.85 (7) and 93.16 (7)°] whilst all the trans angles are strictly 180°. The central Cl atom of the [Cd2Cl7]3- anion is also located on a special position of site symmetry and bridges two corner sharing, tetra-hedrally coordinated CdII atoms. The two Cd atoms and the central Cl atom are colinear. The two sets of terminal chloride ligands on either side of the dumbbell-like anion are rotated relative to each other by 30°. In the crystal, the cations and anions, stacked one above the other along the c-axis direction, are held in place principally by electrostatic inter-actions. There are also C-H⋯Cl and C-H⋯O inter-actions, but these are rather weak. Of the six crystal structures reported to date for ionic salts of [Fe(DMF)6] n+ cations (n = 2, 3), five contain FeII ions. The title compound is the second example of a stable compound containing the [Fe(DMF)6]3+ cation. The existence of both [Fe(DMF)6]2+ and [Fe(DMF)6]3+ cations shows that the DMF ligand coordination sphere can accommodate changes in the charge and spin states of the metal centre.

Ligand coordination sphere effect of Schiff base cis‐dioxomolybdenum(VI) complexes in selective catalytic oxidation of alcohols

AbstractSeveral cis‐dioxomolybdenum(VI) complexes with Schiff bases‐derived ligands were synthesized and fully characterized. The catalytic performances of these complexes were tested in the alcohol oxidation under solvent‐free condition using H2O2 as oxidant giving high results. The influence of the oxygen, sulfur, and nitrogen atom within the coordination sphere around the molybdenum center was studied (S &gt; N &gt; O). From this study, we suggest that there exists a relationship between the electronegativity of the atom and the catalytic performance in alcohol oxidation.

Synthesis, characterization, and biological properties of rhenium(I) tricarbonyl complexes bearing nitrogen-donor ligands

Rhenium(I) tricarbonyl complexes have properties that make them valuable for various biomedical applications, such as imaging, cancer treatment, and bactericidal uses. The ability to modify the ligand coordination sphere of these complexes enables researchers to fine-tune and optimize their properties for biological use. In this study, we explored the role of axial nitrogen-donor ligands. Specifically, the compounds fac-[Re(CO)3(phen)(L)]+, where phen = 1,10-phenanthroline and L = pyridine (Re-py), piperidine (Re-pip), morpholine (Re-morph), and thiomorpholine (Re-thio), were synthesized and characterized. X-ray crystal structures of these complexes show that they obtain an expected pseudo-octahedral geometry with the three CO ligands arranged in a facial manner. Additionally, the X-ray crystal structure of a byproduct from these reactions, the hydroxo-bridged dinuclear Re compound [(CO)3(phen)Re(μ-OH)Re(phen)(CO)3]+, is described. The photophysical properties of these complexes were investigated in detail, revealing that they are photoluminescent in air-equilibrated pH 7.4 phosphate-buffered saline with quantum yields ranging from 1.7 to 3.1%. Both the quantum yields and emission energies were found to correlate with the basicity of the axial nitrogen donor, whereby more basic ligands give rise to smaller quantum yields and lower-energy emissions. These four compounds were further evaluated for their potential as fluorescence microscopy imaging agents. Of the four compounds, only Re-py showed detectable intracellular luminescence in HeLa cells. Lastly, the cytotoxicities of these compounds in HeLa cells were determined. None of the four compounds is significantly cytotoxic as reflected by their 50% growth inhibitory concentrations that exceed 30 μM.

Asymmetry in the Ligand Coordination Sphere of the [FeFe] Hydrogenase Active Site Is Reflected in the Magnetic Spin Interactions of the Aza-propanedithiolate Ligand.

[FeFe] hydrogenases are very active enzymes that catalyze the reversible conversion of molecular hydrogen into protons and electrons. Their active site, the H-cluster, contains a unique binuclear iron complex, [2Fe]H, with CN– and CO ligands as well as an aza-propane-dithiolate (ADT) moiety featuring a central amine functionality that mediates proton transfer during catalysis. We present a pulsed 13C-ENDOR investigation of the H-cluster in which the two methylene carbons of ADT are isotope labeled with 13C. We observed that the corresponding two 13C hyperfine interactions are of opposite sign and corroborated this finding using density functional theory calculations. The spin polarization in the ADT ligand is shown to be linked to the asymmetric coordination of the distal iron site with its terminal CN– and CO ligands. We propose that this asymmetry is relevant for the enzyme reactivity and is related to the (optimal) stabilization of the iron-hydride intermediate in the catalytic cycle.

Coordination polymers and supramolecular solid-state architectures constructed from an organometallic tecton, bis(4-pyridyl)mercury

Four new 1-D coordination polymers have been assembled using bis(4-pyridyl)mercury (py2Hg) as spacer: 1∞[Mn(hfac)2(py2Hg)] 1, 1∞[Zn(CH3COO)2(py2Hg)]∙2H2O 2; 1∞[Ni(dipdtp)2(py2Hg)] 3 (dipdtp = O,O′-diisopropyldithiophosphato), 1∞[Zn(H2O)4(py2Hg)]∙(18C6)(ClO4)24 (18C6 = crown ether 1,4,7,10,13,16-hexaoxacyclooctadecane). The crystal structures of 1, 3, and 4 consist of linear chains, while 2 shows a zigzag topology. The zigzag chains in 2 are assembled into supramolecular layers through weak Hg⋯O interactions, and are further linked into a 3-D framework by tetrameric water clusters. The structure of 3 is expanded into 2-D layers through weak Hg⋯S interactions. In crystal 4, the 18C6 molecules act as second coordination sphere ligands. The supramolecular 3-D network is sustained by hydrogen bonds and Hg⋯O interactions. The system [Cu2(valpa)2]∙(py2Hg) 5 was obtained from a co-crystallization process of py2Hg and the binuclear complex [Cu2(valpa)2] (H2valpa is the Schiff base resulted from the condensation reaction between o-vanillin and propanolamine). The py2Hg molecules in 5 interact each other through N⋯Hg bonds generating layers with square meshes. The organometallic tecton, through the mercury atom, is involved in various weak interactions (Hg⋯O, Hg⋯N, Hg⋯S), which play an important role in sustaining the solid state architectures.

The mixed ligand complexes of Co(II), Ni(II), Cu(II) and Zn(II) with coumarilic acid/1,10-phenanthroline

The coumarilate (coum−) and 1,10-phenanthroline (phen) mixed ligand complexes of Co(II) (1), Ni(II) (2) Cu(II) (3) and Zn(II) (4) were synthesized and structural characterizations were performed by using elemental analysis, magnetic susceptibility, solid-state UV–Vis, FTIR spectra, thermoanalytical TG-DTG/DTA and single-crystal X-ray diffraction methods. The Co(II) and Ni(II) complexes are salt-type compounds, and they have two moles phen ligands bound as bidentate, two moles aqua ligands in coordination sphere and two moles anionic coum− ligand outside of the coordination unit as the counter-ion of the molecular structure. At the same time, the Co(II) and Ni(II) complexes have five moles of aqua ligands as hydrated water outside of the molecules. It was obtained that Cu(II) and Zn(II) complex structures contain one mole of phen ligand, two moles coordinated (coum−) ligand and a one-mole aqua ligand, and the molecules (Cu(II) and Zn(II)) have fivefold structure and obey square pyramidal geometry. Thermal decomposition of each complex started with dehydration (the first removal is the dehydration of complex Co(II) and Ni(II) as removal of hydrate–aqua molecules), and then, the decomposition of organic parts was observed. The thermal dehydration of the complexes takes place in one (Cu(II) and Zn(II)) or two (Co(II) and Co(II)) steps. The decomposition mechanism, thermal decomposition steps and thermal stability of the investigated complexes provide useful data for the interpretation of their structures. The final decomposition products were found to be metal oxides. Some biological applications (antifungal/antibacterial) were performed using structurally characterized compounds.

Novel mixed ligand complexes of acesulfame / nicotinamide with some transition metals. Synthesis, crystal structural characterization, and biological properties

The salt type mixed ligand metal complexes of CoII (I), NiII (II), CuII (III), ZnII (IV) and MnII (V) were synthesized using neutral ligand nicotinamide and anionic ligand acesulfame. The structural characterizations of complexes were performed by using elemental analysis, magnetic susceptibility, solid-state UV–Vis, FTIR spectra, thermoanalytic TG-DTG/DTA, and single crystal X-ray diffraction methods. The complexes of I, III, IV, and V were obtained suitable crystal form for single crystal analysis by SC-XRD diffraction. The complexes of II was believed that the others structures so, the spectroscopic results of all the complexes are suitable for each other. The complexes are salt type compounds, and they have 2 mol nicotinamide ligands as monodentate coordination to metal center by pyridine N atoms and 4 mol aqua ligands in coordination sphere and 2 mol anionic acesulfamate ligands located outside of coordination unit as a counter ion. The metal(II) cationic atoms in complexes have octahedral geometry. Thermal decomposition steps of compounds are started with dehydration.

A combined experimental and computational study on the reaction of fluoroarenes with Mg-Mg, Mg-Zn, Mg-Al and Al-Zn bonds.

Through a combined experimental and computational (DFT) approach, the reaction mechanism of the addition of fluoroarenes to Mg-Mg bonds has been determined as a concerted SNAr-like pathway in which one Mg centre acts as a nucleophile and the other an electrophile. The experimentally determined Gibbs activation energy for the addition of C6F6 to a Mg-Mg bond of a molecular complex, ΔG‡298 K(experiment) = 21.3 kcal mol-1 is modelled by DFT with the ωB97X functional, ΔG‡298 K(DFT) = 25.7 kcal mol-1. The transition state for C-F activation involves a polarisation of the Mg-Mg bond and significant negative charge localisation on the fluoroarene moiety. This transition state is augmented by stabilising closed-shell Mg···F ortho interactions that, in combination with the known trends in C-F and C-M bond strengths in fluoroarenes, provide an explanation for the experimentally determined preference for C-F bond activation to occur at sites flanked by ortho-fluorine atoms. The effect of modification of both the ligand coordination sphere and the nature and polarity of the M-M bond (M = Mg, Zn, Al) on C-F activation has been investigated. A series of highly novel β-diketiminate stabilised complexes containing Zn-Mg, Zn-Zn-Zn, Zn-Al and Mg-Al bonds has been prepared, including the first crystallographic characterisation of a Mg-Al bond. Reactions of these new M-M containing complexes with perfluoroarenes were conducted and modelled by DFT. C-F bond activation is dictated by the steric accessibility, and not the polarity, of the M-M bond. The more open coordination complexes lead to enhanced Mg···F ortho interactions which in turn lower the energy of the transition states for C-F bond activation.

Tridentate phosphine ligands bearing aza-crown ether lariats

Crown ethers are useful macrocycles that act as size-selective binding sites for alkali metals. These frameworks have been incorporated into a number of macromolecular assemblies that use simple cations as reporters and/or activity triggers. Incorporating crown ethers into secondary coordination sphere ligand frameworks for transition metal chemistry will lead to new potential methods for controlling bond formation steps, and routes that couple traditional ligand frameworks with these moieties are highly desirable. Herein we report the syntheses of a family of tridentate phosphine complexes bearing tethered aza-crown ethers (lariats) designed to modularize the variation of aza-crown size, lariat length, and distal phosphine substituents, followed by the synthesis and solid-state structures of Mo(III) complexes bearing cations in the pendent crown ethers.

DFT modeling of structures and redox potentials of wild-type, Nickel-substituted and mutated (N47S/M121L, HPAz) Azurin

Azurin (Az) from Pseudomonas aeruginosa is a redox active protein belonging to the family of cupredoxins. Cupredoxins span a wide range of reduction potentials (E0) going from stellacyanin having the lowest potential of ca. 184mV to rusticyanin showing the higher potential of ca. 680mV. Several works have been devoted to the understanding of the factors influencing E0 by changing primary coordination sphere ligands or exploring secondary coordination sphere mutations. To this goal, a series of Az mutants have been designed and showed that E0 could be tuned over a very broad range (between 90mV and 640mV) without significantly perturbing the metal binding site. Among these mutants, the HPAz variant showed the highest E0 value (970mV) ever reported for Az while a significant lowering of E° value (−590mV) has been reached in a Ni-substituted Az. In this paper, we computed the B3LYP energies and Gibbs free energies of oxidized and reduced models of wild-type Az, Ni-substituted Az and two mutants (N47S/M121L and HPAz) of the protein Az to estimate their E0. The results show that the employed strategy is able to reproduce the experimental lowering or increasing of E0 among the studied Az proteins.

Thiolate Spin Population of Type I Copper in Azurin Derived from 33S Hyperfine Coupling.

The electron transfer mediating properties of type I copper proteins stem from the intricate ligand coordination sphere of the Cu ion in their active site. These redox properties are in part due to unusual cysteine thiol coordination, which forms a highly covalent copper-sulfur (Cu-S) bond. The structure and electronic properties of type I copper have been the subject of many experimental and theoretical studies. The measurement of spin delocalization of the Cu(II) unpaired electron to neighboring ligands provides an elegant experimental way to probe the fine details of the electronic structure of type I copper. To date, the crucial parameter of electron delocalization to the sulfur atom of the cysteine ligand has not been directly determined experimentally. We have prepared 33S-enriched azurin and carried out W-band (95 GHz) electron paramagnetic resonance (EPR) and electron-electron double resonance detected NMR (EDNMR) measurements and, for the first time, recorded the 33S nuclear frequencies, from which the hyperfine coupling and the spin population on the sulfur of the thiolate ligand were derived. The overlapping 33S and 14N EDNMR signals were resolved using a recently introduced two-dimensional correlation technique, 2D-EDNMR. The 33S hyperfine tensor was determined by simulations of the EDNMR spectra using 33S hyperfine and quadrupolar tensors predicted by QM/MM DFT calculations as starting points for a manual spectral fit procedure. To reach a reasonable agreement with the experimental spectra, the 33S hyperfine principal value, Az, and one of the corresponding Euler angles had to be modified. The final values obtained gave an experimentally determined sulfur spin population of 29.8 ± 0.7%, significantly improving the wide range of 29-62% reported in the literature. Our direct, experimentally derived value now provides an important constraint for further theoretical work aimed at unravelling the unique electronic properties of this site.

Amidinate Ligands in Zinc coordination sphere: Synthesis and structural diversity

A one-pot reaction involving neosilyllithium and three different carbodiimides (RN=C=NR, R=cyclohexyl, isopropyl and tert-butyl) in diethyl ether, followed by the addition of anhydrous ZnCl2, afforded, in high yield, corresponding homoleptic zinc amidinate complexes having the molecular formulae [Zn{CyN =C(CH2SiMe3)NCy}2] (1), [Zn{ iPrN =C(CH2SiMe3)N iPr}2] (2) and [Zn{ tBuN =C(CH2SiMe3) NtBu}2] (3), respectively, and amidinato moieties in the zinc coordination sphere. Solid state structures of complexes 1-3 are reported thereafter - all the three complexes are isostructural, and each of them consists of two four-membered metallacycles. We report the syntheses and structural characterization of three homoleptic zinc amidinate complexes derived from one pot reaction of carbodiimides (RN=C=NR, R = cyclohexyl, isopropyl, tert-butyl), neosilyllithium and anhydrous zinc chloride in diethyl ether at ambient temperature 25 °C.

Divergent assembly mechanisms of the manganese/iron cofactors in R2lox and R2c proteins

A manganese/iron cofactor which performs multi-electron oxidative chemistry is found in two classes of ferritin-like proteins, the small subunit (R2) of class Ic ribonucleotide reductase (R2c) and the R2-like ligand-binding oxidase (R2lox). It is unclear how a heterodimeric Mn/Fe metallocofactor is assembled in these two related proteins as opposed to a homodimeric Fe/Fe cofactor, especially considering the structural similarity and proximity of the two metal-binding sites in both protein scaffolds and the similar first coordination sphere ligand preferences of MnII and FeII. Using EPR and Mössbauer spectroscopies as well as X-ray anomalous dispersion, we examined metal loading and cofactor activation of both proteins in vitro (in solution). We find divergent cofactor assembly mechanisms for the two systems. In both cases, excess MnII promotes heterobimetallic cofactor assembly. In the absence of FeII, R2c cooperatively binds MnII at both metal sites, whereas R2lox does not readily bind MnII at either site. Heterometallic cofactor assembly is favored at substoichiometric FeII concentrations in R2lox. FeII and MnII likely bind to the protein in a stepwise fashion, with FeII binding to site 2 initiating cofactor assembly. In R2c, however, heterometallic assembly is presumably achieved by the displacement of MnII by FeII at site 2. The divergent metal loading mechanisms are correlated with the putative in vivo functions of R2c and R2lox, and most likely with the intracellular MnII/FeII concentrations in the host organisms from which they were isolated.

Coordination modification and sorption properties of mesoporous chromium(III) terephthalate

Mesoporous chromium(III) terephthalate MIL-101 was modified with pyrazine, 1,4-diaza[2.2.2]bicyclooctane, and 4,4′-bipyridyl by the replacement of aqua ligands in chromium coordination sphere by coordinated organic ligands. Effect of ligand size on the sorption characteristics of the modified frameworks toward nitrogen and carbon dioxide was studied. It was shown that bulky 4,4′-bipyridyl can block the mesocavities of metal-organic framework. It was shown that basic centers in the modified frameworks increase adsorption heat of carbon dioxide by about 1.5 times.

Modulation of active site electronic structure by the protein matrix to control [NiFe] hydrogenase reactivity.

Control of the reactivity of the nickel center of the [NiFe] hydrogenase and other metalloproteins commonly involves outer coordination sphere ligands that act to modify the geometry and physical properties of the active site metal centers. We carried out a combined set of classical molecular dynamics and quantum/classical mechanics calculations to provide quantitative estimates of how dynamic fluctuations of the active site within the protein matrix modulate the electronic structure at the catalytic center. Specifically we focused on the dynamics of the inner and outer coordination spheres of the cysteinate-bound Ni-Fe cluster in the catalytically active Ni-C state. There are correlated movements of the cysteinate ligands and the surrounding hydrogen-bonding network, which modulate the electron affinity at the active site and the proton affinity of a terminal cysteinate. On the basis of these findings, we hypothesize a coupling between protein dynamics and electron and proton transfer reactions critical to dihydrogen production.

Structure, ligands and substrate coordination of the oxygen-evolving complex of photosystem II in the S2 state: a combined EPR and DFT study

The S2 state of the oxygen-evolving complex of photosystem II, which consists of a Mn4O5Ca cofactor, is EPR-active, typically displaying a multiline signal, which arises from a ground spin state of total spin ST = 1/2. The precise appearance of the signal varies amongst different photosynthetic species, preparation and solvent conditions/compositions. Over the past five years, using the model species Thermosynechococcus elongatus, we have examined modifications that induce changes in the multiline signal, i.e. Ca(2+)/Sr(2+)-substitution and the binding of ammonia, to ascertain how structural perturbations of the cluster are reflected in its magnetic/electronic properties. This refined analysis, which now includes high-field (W-band) data, demonstrates that the electronic structure of the S2 state is essentially invariant to these modifications. This assessment is based on spectroscopies that examine the metal centres themselves (EPR, (55)Mn-ENDOR) and their first coordination sphere ligands ((14)N/(15)N- and (17)O-ESEEM, -HYSCORE and -EDNMR). In addition, extended quantum mechanical models from broken-symmetry DFT now reproduce all EPR, (55)Mn and (14)N experimental magnetic observables, with the inclusion of second coordination sphere ligands being crucial for accurately describing the interaction of NH3 with the Mn tetramer. These results support a mechanism of multiline heterogeneity reported for species differences and the effect of methanol [Biochim. Biophys. Acta, Bioenerg., 2011, 1807, 829], involving small changes in the magnetic connectivity of the solvent accessible outer MnA4 to the cuboidal unit Mn3O3Ca, resulting in predictable changes of the measured effective (55)Mn hyperfine tensors. Sr(2+) and NH3 replacement both affect the observed (17)O-EDNMR signal envelope supporting the assignment of O5 as the exchangeable μ-oxo bridge and it acting as the first site of substrate inclusion.