One‐Dimensional Ferromagnetically Coupled Bimetallic Chains Constructed with trans‐[Ru(acac)2(CN)2]−: Syntheses,Structures, Magnetic Properties,and Density Functional Theoretical Study

Four cyano‐bridged 1D bimetallic polymers have been prepared by using the paramagnetic building block trans‐[Ru(acac)₂(CN)₂]^? (Hacac=acetylacetone): {[{Ni(tren)}{Ru(acac)₂(CN)₂}][ClO₄]?CH₃OH}_n ( 1 ) (tren=tris(2‐aminoethyl)amine), {[{Ni(cyclen)}{Ru(acac)₂(CN)₂}][ClO₄]? CH₃OH}_n ( 2 ) (cyclen=1,4,7,10‐tetraazacyclododecane), {[{Fe(salen)}{Ru(acac)₂(CN)₂}]}_n ( 3 ) (salen^2?=N,N′‐bis(salicylidene)‐o‐ethyldiamine dianion) and [{Mn(5,5′‐Me₂salen)}₂{Ru(acac)₂(CN)₂}][Ru(acac)₂(CN)₂]? 2 CH₃OH ( 4 ) (5,5′‐Me₂salen=N,N′‐bis(5,5′‐dimethylsalicylidene)‐o‐ethylenediimine). Compounds 1 and 2 are 1D, zigzagged NiRu chains that exhibit ferromagnetic coupling between Ni^II and Ru^III ions through cyano bridges with J=+1.92 cm^?1, z J′=?1.37 cm^?1, g=2.20 for 1 and J=+0.85 cm^?1, z J′=?0.16 cm^?1, g=2.24 for 2 . Compound 3 has a 1D linear chain structure that exhibits intrachain ferromagnetic coupling (J=+0.62 cm^?1, z J′=?0.09 cm^?1, g=2.08), but antiferromagnetic coupling occurs between FeRu chains, leading to metamagnetic behavior with T_N=2.6 K. In compound 4 , two Mn^III ions are coordinated to trans‐[Ru(acac)₂(CN)₂]^? to form trinuclear Mn₂Ru units, which are linked together by π–π stacking and weak Mn???O* interactions to form a 1D chain. Compound 4 shows slow magnetic relaxation below 3.0 K with ?=0.25, characteristic of superparamagnetic behavior. The Mn^III???Ru^III coupling constant (through cyano bridges) and the Mn^III???Mn^III coupling constant (between the trimers) are +0.87 and +0.24 cm^?1, respectively. Compound 4 is a novel single‐chain magnet built from Mn₂Ru trimers through noncovalent interactions. Density functional theory (DFT) combined with the broken symmetry state method was used to calculate the molecular magnetic orbitals and the magnetic exchange interactions between Ru^III and M (M=Ni^II, Fe^III, and Mn^III) ions. To explain the somewhat unexpected ferromagnetic coupling between low‐spin Ru^III and high‐spin Fe^III and Mn^III ions in compounds 3 and 4 , respectively, it is proposed that apart from the relative symmetries, the relative energies of the magnetic orbitals may also be important in determining the overall magnetic coupling in these bimetallic assemblies.     

Field‐Induced Slow Magnetic Relaxation in a Mononuclear Manganese(III)–Porphyrin Complex

We report on a novel manganese(III)–porphyrin complex with the formula [Mn^III(TPP)(3,5‐Me₂pyNO)₂]ClO₄?CH₃CN ( 2 ; 3,5‐Me₂pyNO=3,5‐dimethylpyridine N‐oxide, H₂TPP=5,10,15,20‐tetraphenylporphyrin), in which the Mn^III ion is six‐coordinate with two monodentate 3,5‐Me₂pyNO molecules and a tetradentate TPP ligand to build a tetragonally elongated octahedral geometry. The environment in 2 is responsible for the large and negative axial zero‐field splitting (D=?3.8 cm^?1), low rhombicity (E/|D|=0.04) of the high‐spin Mn^III ion, and, ultimately, for the observation of slow magnetic‐relaxation effects (E_a=15.5 cm^?1 at H=1000 G) in this rare example of a manganese‐based single‐ion magnet (SIM). Structural, magnetic, and electronic characterizations were carried out by means of single‐crystal diffraction studies, variable‐temperature direct‐ and alternating‐current measurements and high‐frequency and ‐field EPR spectroscopic analysis followed by quantum‐chemical calculations. Slow magnetic‐relaxation effects were also observed in the already known analogous compound [Mn^III(TPP)Cl] ( 1 ; E_a=10.5 cm^?1 at H=1000 G). The results obtained for 1 and 2 are compared and discussed herein.     

Redox non-innocence of thioether crowns: spectroelectrochemistry and electronic structure of formal nickel(III) complexes of aza-thioether macrocycles

The Ni^II complexes [Ni([9]aneNS₂‐CH₃)₂]²⁺ ([9]aneNS₂‐CH₃=N‐methyl‐1‐aza‐4,7‐dithiacyclononane), [Ni(bis[9]aneNS₂‐C₂H₄)]²⁺ (bis[9]aneNS₂‐C₂H₄=1,2‐bis‐(1‐aza‐4,7‐dithiacyclononylethane) and [Ni([9]aneS₃)₂]²⁺ ([9]aneS₃=1,4,7‐trithiacyclononane) have been prepared and can be electrochemically and chemically oxidized to give the formal Ni^III products, which have been characterized by X‐ray crystallography, UV/Vis and multi‐frequency EPR spectroscopy. The single‐crystal X‐ray structure of [Ni^III([9]aneNS₂‐CH₃)₂](ClO₄)₆?(H₅O₂)₃ reveals an octahedral co‐ordination at the Ni centre, while the crystal structure of [Ni^III(bis[9]aneNS₂‐C₂H₄)](ClO₄)₆?(H₃O)₃? 3H₂O exhibits a more distorted co‐ordination. In the homoleptic analogue, [Ni^III([9]aneS₃)₂](ClO₄)₃, structurally characterized at 30 K, the Ni? S distances [2.249(6), 2.251(5) and 2.437(2) Å] are consistent with a Jahn–Teller distorted octahedral stereochemistry. [Ni([9]aneNS₂‐CH₃)₂](PF₆)₂ shows a one‐electron oxidation process in MeCN (0.2 M NBu₄PF₆, 293 K) at E_1/2=+1.10 V versus Fc⁺/Fc assigned to a formal Ni^III/Ni^II couple. [Ni(bis[9]aneNS₂‐C₂H₄)](PF₆)₂ exhibits a one‐electron oxidation process at E_1/2=+0.98 V and a reduction process at E_1/2=?1.25 V assigned to Ni^II/Ni^III and Ni^II/Ni^I couples, respectively. The multi‐frequency X‐, L‐, S‐, K‐band EPR spectra of the 3+ cations and their 86.2 % ⁶¹Ni‐enriched analogues were simulated. Treatment of the spin Hamiltonian parameters by perturbation theory reveals that the SOMO has 50.6 %, 42.8 % and 37.2 % Ni character in [Ni([9]aneNS₂‐CH₃)₂]³⁺, [Ni(bis[9]aneNS₂‐C₂H₄)]³⁺ and [Ni([9]aneS₃)₂]³⁺, respectively, consistent with DFT calculations, and reflecting delocalisation of charge onto the S‐thioether centres. EPR spectra for [⁶¹Ni([9]aneS₃)₂]³⁺ are consistent with a dynamic Jahn–Teller distortion in this compound.     

Three Distinct Redox States of an Oxo‐Bridged Dinuclear Ruthenium Complex

A series of [{(terpy)(bpy)Ru}(μ‐O){Ru(bpy)(terpy)}]ⁿ⁺ ( [RuORu]ⁿ⁺ , terpy=2,2′;6′,2′′‐terpyridine, bpy=2,2′‐bipyridine) was systematically synthesized and characterized in three distinct redox states (n=3, 4, and 5 for Ru^II,III₂ , Ru^III,III₂ , and Ru^III,IV₂ , respectively). The crystal structures of [RuORu]ⁿ⁺ (n=3, 4, 5) in all three redox states were successfully determined. X‐ray crystallography showed that the Ru? O distances and the Ru‐O‐Ru angles are mainly regulated by the oxidation states of the ruthenium centers. X‐ray crystallography and ESR spectra clearly revealed the detailed electronic structures of two mixed‐valence complexes, [Ru^IIIORu^IV]⁵⁺ and [Ru^IIORu^III]³⁺ , in which each unpaired electron is completely delocalized across the oxo‐bridged dinuclear core. These findings allow us to understand the systematic changes in structure and electronic state that accompany the changes in the redox state.     

Hydroxo‐Bridged Dimers of Oxo‐Centered Ruthenium(III) Triangle: Synthesis and Spectroscopic and Theoretical Investigations

The homometallic hexameric ruthenium cluster of the formula [Ru^III₆(μ₃‐O)₂(μ‐OH)₂((CH₃)₃CCO₂)₁₂(py)₂] ( 1 ) (py=pyridine) is solved by single‐crystal X‐ray diffraction. Magnetic susceptibility measurements performed on 1 suggest that the antiferromagnetic interaction between the Ru^III centers is dominant, and this is supported by theoretical studies. Theoretical calculations based on density functional methods yield eight different exchange interaction values for 1 : J₁=?737.6, J₂=+63.4, J₃=?187.6, J₄=+124.4, J₅=?376.4, J₆=?601.2, J₇=?657.0, and J₈=?800.6 cm^?1. Among all the computed J values, six are found to be antiferromagnetic. Four exchange values (J₁, J₆, J₇ and J₈) are computed to be extremely strong, with J₈, mediated through one μ‐hydroxo and a carboxylate bridge, being by far the largest exchange obtained for any transition‐metal cluster. The origin of these strong interactions is the orientation of the magnetic orbitals in the Ru^III centers, and the computed J values are rationalized by using molecular orbital and natural bond order analysis. Detailed NMR studies (¹H, ¹³C, HSQC, NOESY, and TOCSY) of 1 (in CDCl₃) confirm the existence of the solid‐state structure in solution. The observation of sharp NMR peaks and spin‐lattice time relaxation (T1 relaxation) experiments support the existence of strong intramolecular antiferromagnetic exchange interactions between the metal centers. A broad absorption peak around 600–1000 nm in the visible to near‐IR region is a characteristic signature of an intracluster charge‐transfer transition. Cyclic voltammetry experiments show that there are three reversible one‐electron redox couples at ?0.865, +0.186, and +1.159 V with respect to the Ag/AgCl reference electrode, which corresponds to two metal‐based one‐electron oxidations and one reduction process.     

Robust and efficient amide-based nonheme manganese(III) hydrocarbon oxidation catalysts: substrate and solvent effects on involvement and partition of multiple active oxidants

Two new mononuclear nonheme manganese(III) complexes of tetradentate ligands containing two deprotonated amide moieties, [Mn(bpc)Cl(H₂O)] ( 1 ) and [Mn(Me₂bpb)Cl(H₂O)] ? CH₃OH ( 2 ), were prepared and characterized. Complex 2 has also been characterized by X‐ray crystallography. Magnetic measurements revealed that the complexes are high spin (S=5/2) Mn^III species with typical magnetic moments of 4.76 and 4.95 μ_B, respectively. These nonheme Mn^III complexes efficiently catalyzed olefin epoxidation and alcohol oxidation upon treatment with MCPBA under mild experimental conditions. Olefin epoxidation by these catalysts is proposed to involve the multiple active oxidants Mn^V?O, Mn^IV?O, and Mn^III? OO(O)CR. Evidence for this approach was derived from reactivity and Hammett studies, KIE (k_H/k_D) values, H₂¹⁸O‐exchange experiments, and the use of peroxyphenylacetic acid as a mechanistic probe. In addition, it has been proposed that the participation of Mn^V?O, Mn^IV?O, and Mn^III? OOR could be controlled by changing the substrate concentration, and that partitioning between heterolysis and homolysis of the O? O bond of a Mn‐acylperoxo intermediate (Mn? OOC(O)R) might be significantly affected by the nature of solvent, and that the O? O bond of the Mn? OOC(O)R might proceed predominantly by heterolytic cleavage in protic solvent. Therefore, a discrete Mn^V?O intermediate appeared to be the dominant reactive species in protic solvents. Furthermore, we have observed close similarities between these nonheme Mn^III complex systems and Mn(saloph) catalysts previously reported, suggesting that this simultaneous operation of the three active oxidants might prevail in all the manganese‐catalyzed olefin epoxidations, including Mn(salen), Mn(nonheme), and even Mn(porphyrin) complexes. This mechanism provides the greatest congruity with related oxidation reactions by using certain Mn complexes as catalysts.     

Alcohol oxidation reactions catalyzed by ruthenium–carbonyl complexes of thioarylazoimidazoles

Alcohols are oxidized by N‐methylmorpholine‐N‐oxide (NMO), Bu^tOOH and H₂O₂ to the corresponding aldehydes or ketones in the presence of catalyst, [RuH(CO)(PPh₃)₂(SRaaiNR′)]PF₆ ( 2 ) and [RuCl(CO)(PPh₃)(S_κRaaiNR′)]PF₆ ( 3 ) (SRaaiNR′ ( 1 ) = 1‐alkyl‐2‐{(o‐thioalkyl)phenylazo}imidazole, a bidentate N(imidazolyl) (N), N(azo) (N′) chelator and S_κRaaiNR′ is a tridentate N(imidazolyl) (N), N(azo) (N′), S_κ‐R is tridentate chelator; R and R′ are Me and Et). The single‐crystal X‐ray structures of [RuH(CO)(PPh₃)₂(SMeaaiNMe)]PF₆ ( 2a ) (SMeaaiNMe = 1‐methyl‐2‐{(o‐thioethyl)phenylazo}imidazole) and [RuH(CO)(PPh₃)₂(SEtaaiNEt)]PF₆ ( 2b ) (SEtaaiNEt = 1‐ethyl‐2‐{(o‐thioethyl)phenylazo}imidazole) show bidentate N,N′ chelation, while in [RuCl(CO)(PPh₃)(S_κEtaaiNEt)]PF₆ ( 3b ) the ligand S_κEtaaiNEt serves as tridentate N,N′,S chelator. The cyclic voltammogram shows Ru^III/Ru^II (~1.1 V) and Ru^IV/Ru^III (~1.7 V) couples of the complexes 2 while Ru^III/Ru^II (1.26 V) couple is observed only in 3 along with azo reductions in the potential window +2.0 to ?2.0 V. DFT computation has been used to explain the spectra and redox properties of the complexes. In the oxidation reaction NMO acts as best oxidant and [RuCl(CO)(PPh₃)(S_κRaaiNR′)](PF₆) ( 3 ) is the best catalyst. The formation of high‐valent Ru^IV=O species as a catalytic intermediate is proposed for the oxidation process. Copyright © 2014 John Wiley & Sons, Ltd.     

A 1,2,4‐Triazole‐based Polynuclear Mixed‐valence MnIIMnIII Complex: Synthesis,Characterization, and Magnetic Property

A mixed‐valence Mn complex {[Mn^IIMn^III(HL)₂(4,4′‐bpy)(H₂O)₂] · (ClO₄)(DMF)₃(4,4′‐bpy)_0.5}_n ( 1 ) [H₂L = 3‐(2‐phenol)‐5‐(pyridin‐2‐yl)‐1,2,4‐triazole] was synthesized and characterized by X‐ray single‐crystal structure analysis and magnetic susceptibility. Single‐crystal X‐ray analysis revealed that complex 1 has a dinuclear core, in which adjacent central Mn^III atoms are linked by 4,4′‐bipyridine to form an infinite one‐dimensional (1D) molecular configuration. According to the Mn surrounding bond lengths and bond valence sum (BVS) calculations, we demonstrated that the Mn atom coordinated to the pyridine N atoms is in the +2 oxidation state, while another Mn atom coordinated to the phenolic oxygen atoms is in the +3 oxidation state. Magnetic susceptibility data of the complex 1 indicate that the ferromagnetic interaction dominates in this complex.     

Oxygen Evolution Catalyzed by a Mononuclear Ruthenium Complex Bearing Pendant SO3− Groups

Rational molecular design of catalytic systems capable of smooth O? O bond formation is critical to the development of efficient catalysts for water oxidation. A new ruthenium complex was developed, which bears pendant SO₃^? groups in the secondary coordination sphere: [Ru(terpy)(bpyms)(OH₂)] (terpy=2,2′:6′,2′′‐terpyridine, bpyms=2,2′‐bipyridine‐5,5′‐bis(methanesulfonate)). Water oxidation driven by a Ce⁴⁺ oxidant is distinctly accelerated upon introduction of the pendant SO₃^? groups in comparisons to the parent catalyst, [Ru(terpy)(bpy)(OH₂)]²⁺ (bpy=2,2′‐bipyridine). Spectroscopic, electrochemical, and crystallographic investigations concluded that the pendant SO₃^? groups promote the formation of an O? O bond via the secondary coordination sphere on the catalyst, whereas the influence of the pendant SO₃^? groups on the electronic structure of the [Ru(terpy)(bpy)(OH₂)]²⁺ core is negligible. The results of this work indicate that modification of the secondary coordination sphere is a valuable strategy for the design of water oxidation catalysts.     

Redox Behaviour of Cymantrene Fischer Carbene Complexes in Designing Organometallic Multi‐tags

A series of Group 7 Fischer carbene complexes, [Cp(CO)₂Mn^I=C(OEt)Ar] (Cp=cyclopentadienyl, Ar=Th=thienyl ( 1 a ), Ar=Fu=furyl ( 2 a ), Ar=Fc=ferrocenyl ( 3 a )) and biscarbene complexes, [Cp(CO)₂Mn?C(OEt)?Ar′?(OEt)C?Mn(CO)₂Cp] (Ar′=Th′=2,5‐thienylene ( 1 b ), Ar′=Fu′=2,5‐furylene ( 2 b ), Ar′=Fc′=1,1′‐ferrocendiyl ( 3 b )) was synthesized and characterized. Chemical oxidation of [Cp(CO)₂Mn?C(OEt)Fc] ( 3 a ) and isolation of the oxidised species [3 a][PF₆] possessing a Mn^II centre proved possible below ?30 °C in dichloromethane solution. The ESR spectrum of the transiently stable radical cation, [3 a][PF₆] , confirmed the presence of a low‐spin Mn^II centre characterized by a rhombic g tensor (g_x=1.975, g_y=2.007 and g_z=2.130) in frozen dichloromethane at 77 K with ⁵⁵ Mn hyperfine coupling constants A₁, A₂ and A₃ of 115, 33 and 43 G, respectively. Electrochemical studies demonstrated the influence of the Ar substituent on the oxidation potential. All complexes showed that the redox potentials of carbene double bond reduction and Mn^I oxidation were dependent on the type of Ar group, but only 3 b showed resolved oxidations for the two Mn^I centres. Surprisingly, Mn^I oxidation occurs at lower potentials than ferrocenyl oxidation. Density functional theory (DFT) calculations were carried out to delineate the nature of the species involved in the oxidation and reduction processes and clearly confirm that oxidation of Mn^I is favoured over that of ferrocene.     

Three‐Dimensional Antiferromagnetic Order of Single‐Chain Magnets: A New Approach to Design Molecule‐Based Magnets

Two one‐dimensional compounds composed of a 1:1 ratio of Mn^III salen‐type complex and Ni^II oximato moiety with different counter anions, PF₆^? and BPh₄^?, were synthesized: [Mn(3,5‐Cl₂saltmen)Ni(pao)₂(phen)]PF₆ ( 1 ) and [Mn(5‐Clsaltmen)Ni(pao)₂(phen)]BPh₄ ( 2 ), where 3,5‐Cl₂saltmen^2?=N,N′‐(1,1,2,2‐tetramethylethylene)bis(3,5‐dichlorosalicylideneiminate); 5‐Clsaltmen^2?=N,N′‐(1,1,2,2‐tetramethylethylene)bis(5‐chlorosalicylideneiminate); pao^?=pyridine‐2‐aldoximate; and phen=1,10‐phenanthroline. Single‐crystal X‐ray diffraction study was carried out for both compounds. In 1 and 2 , the chain topology is very similar forming an alternating linear chain with a [‐Mn^III‐ON‐Ni^II‐NO‐] repeating motif (where ‐ON‐ is the oximate bridge). The use of a bulky counteranion, such as BPh₄^?, located between the chains in 2 rather than PF₆^? in 1 , successfully led to the magnetic isolation of the chains in 2 . This minimization of the interchain interactions allows the study of the intrinsic magnetic properties of the chains present in 1 and 2 . While 1 and 2 possess, as expected, very similar paramagnetic properties above 15 K, their ground state is antiferromagnetic below 9.4 K and paramagnetic down to 1.8 K, respectively. Nevertheless, both compounds exhibit a magnet‐type behavior at temperatures below 6 K. While for 2 , the observed magnetism is well explained by a Single‐Chain Magnet (SCM) behavior, the magnet properties for 1 are induced by the presence in the material of SCM building units that order antiferromagnetically. By controlling both intra‐ and interchain magnetic interactions in this new [Mn^IIINi^II] SCM system, a remarkable AF phase with a magnet‐type behavior has been stabilized in relation with the intrinsic SCM properties of the chains present in 1 . This result suggests that the simultaneous enhancement of both intrachain (J) and interchain (J′) magnetic interactions (with keeping J ? J′), independently of the presence of AF phase might be an efficient route to design high temperature SCM‐based magnets.     

Four‐Center Oxidation State Combinations and Near‐Infrared Absorption in [Ru(pap)(Q)2]n (Q=3,5‐Di‐tert‐butyl‐N‐aryl‐1,2‐benzoquinonemonoimine,pap=2‐Phenylazopyridine)

The complex series [Ru(pap)(Q)₂]ⁿ ([ 1 ]ⁿ–[ 4 ]ⁿ; n=+2, +1, 0, ?1, ?2) contains four redox non‐innocent entities: one ruthenium ion, 2‐phenylazopyridine (pap), and two o‐iminoquinone moieties, Q=3,5‐di‐tert‐butyl‐N‐aryl‐1,2‐benzoquinonemonoimine (aryl=C₆H₅ ( 1⁺ ); m‐(Cl)₂C₆H₃ ( 2⁺ ); m‐(OCH₃)₂C₆H₃ ( 3⁺ ); m‐(tBu)₂C₆H₃ ( 4 ⁺)). A crystal structure determination of the representative compound, [ 1 ]ClO₄, established the crystallization of the ctt‐isomeric form, that is, cis and trans with respect to the mutual orientations of O and N donors of two Q ligands, and the coordinating azo N atom trans to the O donor of Q. The sensitive C? O (average: 1.299(3) Å), C? N (average: 1.346(4) Å) and intra‐ring C? C (meta; average: 1.373(4) Å) bond lengths of the coordinated iminoquinone moieties in corroboration with the N?N length (1.292(3) Å) of pap in 1 ⁺ establish [Ru^III(pap⁰)(Q^.?)₂]⁺ as the most appropriate electronic structural form. The coupling of three spins from one low‐spin ruthenium(III) (t_2g⁵) and two Q^.? radicals in 1 ⁺– 4 ⁺ gives a ground state with one unpaired electron on Q^.?, as evident from g=1.995 radical‐type EPR signals for 1 ⁺– 4 ⁺. Accordingly, the DFT‐calculated Mulliken spin densities of 1 ⁺ (1.152 for two Q, Ru: ?0.179, pap: 0.031) confirm Q‐based spin. Complex ions 1 ⁺– 4 ⁺ exhibit two near‐IR absorption bands at about λ=2000 and 920 nm in addition to intense multiple transitions covering the visible to UV regions; compounds [ 1 ]ClO₄–[ 4 ]ClO₄ undergo one oxidation and three separate reduction processes within ±2.0 V versus SCE. The crystal structure of the neutral (one‐electron reduced) state ( 2 ) was determined to show metal‐based reduction and an EPR signal at g=1.996. The electronic transitions of the complexes 1 ⁿ– 4 ⁿ (n=+2, +1, 0, ?1, ?2) in the UV, visible, and NIR regions, as determined by using spectroelectrochemistry, have been analyzed by TD‐DFT calculations and reveal significant low‐energy absorbance (λ_max>1000 nm) for cations, anions, and neutral forms. The experimental studies in combination with DFT calculations suggest the dominant valence configurations of 1 ⁿ– 4 ⁿ in the accessible redox states to be [Ru^III(pap⁰)(Q^.?)(Q⁰)]²⁺ ( 1 ²⁺– 4 ²⁺)→[Ru^III(pap⁰)(Q^.?)₂]⁺ ( 1 ⁺– 4 ⁺)→[Ru^II(pap⁰)(Q^.?)₂] ( 1 – 4 )→[Ru^II(pap^.?)(Q^.?)₂]^? ( 1 ^?– 4 ^?)→[Ru^III(pap^.?)(Q^2?)₂]^2? ( 1 ^2?– 4 ^2?).     

Density Functional Calculations of 55Mn, 14N and 13C Electron Paramagnetic Resonance Parameters Support an Energetically Feasible Model System for the S2 State of the Oxygen‐Evolving Complex of Photosystem II

Metal and ligand hyperfine couplings of a previously suggested, energetically feasible Mn₄Ca model cluster ( SG2009^?1 ) for the S₂ state of the oxygen‐evolving complex (OEC) of photosystem II (PSII) have been studied by broken‐symmetry density functional methods and compared with other suggested structural and spectroscopic models. This was carried out explicitly for different spin‐coupling patterns of the S=1/2 ground state of the Mn^III(Mn^IV)₃ cluster. By applying spin‐projection techniques and a scaling of the manganese hyperfine couplings, computation of the hyperfine and nuclear quadrupole coupling parameters allows a direct evaluation of the proposed models in comparison with data obtained from the simulation of EPR, ENDOR, and ESEEM spectra. The computation of ⁵⁵Mn hyperfine couplings (HFCs) for SG2009^?1 gives excellent agreement with experiment. However, at the current level of spin projection, the ⁵⁵Mn HFCs do not appear sufficiently accurate to distinguish between different structural models. Yet, of all the models studied, SG2009^?1 is the only one with the Mn^III site at the Mn_C center, which is coordinated by histidine (D1‐His332). The computed histidine ¹⁴N HFC anisotropy for SG2009^?1 gives much better agreement with ESEEM data than the other models, in which Mn_C is an Mn^IV site, thus supporting the validity of the model. The ¹³C HFCs of various carboxylates have been compared with ¹³C ENDOR data for PSII preparations with ¹³C‐labelled alanine.     

Electrochemical Properties of the Oxo‐Manganese‐Phenanthroline Complex Immobilized on Ion‐Exchange Polymeric Film and Its Application as Biomimetic Sensor for Sulfite Ions

A biomimetic sensor containing the oxo‐bridged dinuclear manganese‐phenanthroline complex incorporated into a cation‐exchange polymeric film deposited onto glassy carbon electrode for detection of sulfite was studied. Cyclic voltammetry at the modified electrode in universal buffer showed a two electron oxidation/reduction of the couple Mn^IV(μ‐O)₂Mn^IV/Mn^III(μ‐O)₂Mn^III. The sensor exhibited electrocatalytic property toward sulfite oxidation with a decrease of the overpotential of 450 mV compared with the glassy carbon electrode. A plot of the anodic current versus the sulfite concentration for potential fixed (+0.15 V vs. SCE) at the sensor was linear in the 4.99×10^?7 to 2.49×10^?6 mol L^?1 concentration range and the concentration limit was 1.33×10^?7 mol L^?1. The mediated mechanism was derived by Michaelis? Menten kinetics. The calculated kinetics values were Michaelis? Menten rate constant= =1.33 µmol L^?1, catalytic rate constant=6.06×10^?3 s^?1 and heterogeneous electro‐chemical rate constant=3.61×10^?5 cm s^?1.     

An Unusually Delocalized Mixed‐Valence State of a Cyanidometal‐Bridged Compound Induced by Thermal Electron Transfer

The heterometallic complexes trans ‐[Cp(dppe)FeNCRu(o ‐bpy)CNFe(dppe)Cp][PF₆]_n ( 1 [PF₆]_n , n =2, 3, 4; o ‐bpy=1,2‐bis(2,2′‐bipyridyl‐6‐yl)ethane, dppe=1,2‐bis(diphenylphosphino)ethane, Cp=1,3‐cyclopentadiene) in three distinct states have been synthesized and fully characterized. 1 ³⁺[PF₆]₃ and 1 ⁴⁺[PF₆]₄ are the one‐ and two‐electron oxidation products of 1 ²⁺[PF₆]₂, respectively. The investigated results suggest that 1 [PF₆]₃ is a Class II mixed valence compound. 1 [PF₆]₄ after a thermal treatment at 400 K shows an unusually delocalized mixed valence state of [Fe^III‐NC‐Ru^III‐CN‐Fe^II], which is induced by electron transfer from the central Ru^II to the terminal Fe^III in 1 [PF₆]₄, which was confirmed by IR spectroscopy, magnetic data, and EPR and Mössbauer spectroscopy.     

Correspondence of RuIIIRuII and RuIVRuIII Mixed Valent States in a Small Dinuclear Complex

The diruthenium(III) compound [(μ‐oxa){Ru(acac)₂}₂] [ 1 , oxa^2?=oxamidato(2?), acac^?=2,4‐pentanedionato] exhibits an S=1 ground state with antiferromagnetic spin‐spin coupling (J=?40 cm^?1). The molecular structure in the crystal of 1? 2 C₇H₈ revealed an intramolecular metal–metal distance of 5.433 Å and a notable asymmetry within the bridging ligand. Cyclic voltammetry and spectroelectrochemistry (EPR, UV/Vis/NIR) of the two‐step reduction and of the two‐step oxidation (irreversible second step) produced monocation and monoanion intermediates (K_c=10^5.9) with broad NIR absorption bands (ε ca. 2000 M ^?1 cm^?1) and maxima at 1800 ( 1 ^?) and 1500 nm ( 1 ⁺). TD‐DFT calculations support a Ru^IIIRu^II formulation for 1 ^? with a doublet ground state. The 1 ⁺ ion (Ru^IVRu^III) was calculated with an S=3/2 ground state and the doublet state higher in energy (ΔE=694.6 cm^?1). The Mulliken spin density calculations showed little participation of the ligand bridge in the spin accommodation for all paramagnetic species [(μ‐oxa){Ru(acac)₂}₂]ⁿ, n=+1, 0, ?1, and, accordingly, the NIR absorptions were identified as metal‐to‐metal (intervalence) charge transfers. Whereas only one such NIR band was observed for the Ru^IIIRu^II (4d⁵/4d⁶) system 1 ^?, the Ru^IVRu^III (4d⁴/4d⁵) form 1 ⁺ exhibited extended absorbance over the UV/Vis/NIR range.     

Effect of Heme–Heme Interactions and Modulation of Metal Spins by Counter Anions in a Series of Diiron(III)‐μ‐hydroxo Bisporphyrins: Unusual Stabilization of Two Different Spins in a Single Molecular Framework

A new family of five ethene‐bridged diiron(III)‐μ‐hydroxo bisporphyrins with the same core structure but different counter anions, represented by the general formula [Fe₂(bisporphyrin)]OH ? X (X=counter anion), is reported herein. In these complexes, two different spin states of Fe are stabilized in a single molecular framework. Protonation of the oxo‐bridged dimer 1 by strong Brønsted acids such as HI, HBF₄, HPF₆, HSbF₆, and HClO₄ produces the μ‐hydroxo complexes with I₅^? ( 2 ), BF₄^? ( 3 ), PF₆^? ( 4 ), SbF₆^? ( 5 ), and ClO₄^? ( 6 ) as counter anions, respectively. The X‐ray structures of 2 and 6 have been determined, which provide a rare opportunity to investigate structural changes upon protonation. Spectroscopic characterization has revealed that the two iron(III) centers in 2 are nonequivalent with nearly high and admixed‐intermediate spins in both the solid state and solution. Moreover, the two different Fe^III centers of 3 – 5 are best described as having admixed‐high and admixed‐intermediate spins with variable contributions of S=5/2 and 3/2 for each state in the solid, but two different admixed‐intermediate spins in solution. In contrast, the two Fe^III centers in 6 are equivalent and are assigned as having high and intermediate spin states in the solid and solution, respectively. The X‐ray structures reveal that the Fe? O bond length increases on going from the μ‐oxo to the μ‐hydroxo complexes, and the Fe‐O(H)‐Fe unit becomes more bent, with the dihedral angle decreasing from 150.9(2)° in 1 to 142.3(3)° and 143.85(2)° in 2 and 6 , respectively. Variable‐temperature magnetic data have been subjected to a least‐squares fitting using the expressions derived from the spin Hamiltonians H=?2JS₁?S₂?μ?B+D[${S{{2\hfill \atop z\hfill}}}$ ?1/3S(S+1)] (for 2 , 3 , 4 , and 5 ) and H=?2JS₁?S₂ (for 6 ). The results show that strong antiferromagnetic coupling between the two Fe^III centers in 1 is attenuated to nearly zero (?2.4 cm^?1) in 2 , whereas the values are ?46, ?32.6, ?33.5, and ?34 cm^?1 for 3 , 4 , 5 , and 6 , respectively.     

Large Hexadecametallic {MnIII–LnIII} Wheels: Synthesis,Structural, Magnetic,and Theoretical Characterization

The synthesis, gas sorption studies, magnetic properties, and theoretical studies of new molecular wheels of core type {Mn^III₈Ln^III₈} (Ln=Dy, Ho, Er, Y and Yb), using the ligand mdeaH₂, in the presence of ortho‐toluic or benzoic acid are reported. From the seven wheels studied the {Mn₈Dy₈} and {Mn₈Y₈} analogues exhibit SMM behavior as determined from ac susceptibility experiments in a zero static magnetic field. From DFT calculations a S=16 ground state was determined for the {Mn₈Y₈} complex due to weak ferromagnetic Mn^III–Mn^III interactions. Ab initio CASSCF+RASSI‐SO calculations on the {Mn₈Dy₈} wheel estimated the Mn^III–Dy^III exchange interaction as ?0.1 cm^?1. This weak exchange along with unfavorable single‐ion anisotropy of Dy^III/Mn^III ions, however, led to the observation of SMM behavior with fast magnetic relaxation. The orientation of the g‐anisotropy of the Dy^III ions is found to be perpendicular to the plane of the wheel and this suggests the possibility of toroidal magnetic moments in the cluster. The {Mn₈Ln₈} clusters reported here are the largest heterometallic Mn^IIILn^III wheels and the largest {3d–4f} wheels to exhibit SMM behavior reported to date.     

Characterising lone-pair activity of lead(II) by 207Pb solid-state NMR spectroscopy: coordination polymers of [N(CN)2]- and [Au(CN)2]- with terpyridine ancillary ligands

A series of lead(II) coordination polymers containing [N(CN)₂]^? (DCA) or [Au(CN)₂]^? bridging ligands and substituted terpyridine (terpy) ancillary ligands ([Pb(DCA)₂] ( 1 ), [Pb(terpy)(DCA)₂] ( 2 ), [Pb(terpy){Au(CN)₂}₂] ( 3 ), [Pb(4′‐chloro‐terpy){Au(CN)₂}₂] ( 4 ) and [Pb(4′‐bromo‐terpy)(μ‐OH₂)_0.5{Au(CN)₂}₂] ( 5 )) was spectroscopically examined by solid‐state ²⁰⁷Pb MAS NMR spectroscopy in order to characterise the structural and electronic changes associated with lead(II) lone‐pair activity. Two new compounds, 2 and [Pb(4′‐hydroxy‐terpy){Au(CN)₂}₂] ( 6 ), were prepared and structurally characterised. The series displays contrasting coordination environments, bridging ligands with differing basicities and structural and electronic effects that occur with various substitutions on the terpyridine ligand (for the [Au(CN)₂]^? polymers). ²⁰⁷Pb NMR spectra show an increase in both isotropic chemical shift and span (Ω) with increasing ligand basicity (from δ_iso=?3090 ppm and Ω=389 ppm for 1 (the least basic) to δ_iso=?1553 ppm and Ω=2238 ppm for 3 (the most basic)). The trends observed in ²⁰⁷Pb NMR data correlate with the coordination sphere anisotropy through comparison and quantification of the Pb? N bond lengths about the lead centre. Density functional theory calculations confirm that the more basic ligands result in greater p‐orbital character and show a strong correlation to the ²⁰⁷Pb NMR chemical shift parameters. Preliminary trends suggest that ²⁰⁷Pb NMR chemical shift anisotropy relates to the measured birefringence, given the established correlations with structure and lone‐pair activity.     

Proton Order–Disorder Phenomena in a Hydrogen‐Bonded Rhodium–η5‐Semiquinone Complex: A Possible Dielectric Response Mechanism

A newly synthesized one‐dimensional (1D) hydrogen‐bonded (H‐bonded) rhodium(II)–η⁵‐semiquinone complex, [Cp*Rh(η⁵‐p‐HSQ‐Me₄)]PF₆ ([ 1 ]PF₆; Cp*=1,2,3,4,5‐pentamethylcyclopentadienyl; HSQ=semiquinone) exhibits a paraelectric–antiferroelectric second‐order phase transition at 237.1 K. Neutron and X‐ray crystal structure analyses reveal that the H‐bonded proton is disordered over two sites in the room‐temperature (RT) phase. The phase transition would arise from this proton disorder together with rotation or libration of the Cp* ring and PF₆^? ion. The relative permittivity ε_b′ along the H‐bonded chains reaches relatively high values (ca., 130) in the RT phase. The temperature dependence of ¹³C CP/MAS NMR spectra demonstrates that the proton is dynamically disordered in the RT phase and that the proton exchange has already occurred in the low‐temperature (LT) phase. Rate constants for the proton exchange are estimated to be 10^?4–10^?6 s in the temperature range of 240–270 K. DFT calculations predict that the protonation/deprotonation of [ 1 ]⁺ leads to interesting hapticity changes of the semiquinone ligand accompanied by reduction/oxidation by the π‐bonded rhodium fragment, producing the stable η⁶‐hydroquinone complex, [Cp*Rh³⁺(η⁶‐p‐H₂Q‐Me₄)]²⁺ ([ 2 ]²⁺), and η⁴‐benzoquinone complex, [Cp*Rh⁺(η⁴‐p‐BQ‐Me₄)] ([ 3 ]), respectively. Possible mechanisms leading to the dielectric response are discussed on the basis of the migration of the protonic solitons comprising of [ 2 ]²⁺ and [ 3 ], which would be generated in the H‐bonded chain.