Coordination Complexes of a Neutral 1,2,4‐Benzotriazinyl Radical Ligand: Synthesis,Molecular and Electronic Structures,and Magnetic Properties

A series of d‐block metal complexes of the recently reported coordinating neutral radical ligand 1‐phenyl‐3‐(pyrid‐2‐yl)‐1,4‐dihydro‐1,2,4‐benzotriazin‐4‐yl ( 1 ) was synthesized. The investigated systems contain the benzotriazinyl radical 1 coordinated to a divalent metal cation, Mn^II, Fe^II, Co^II, or Ni^II, with 1,1,1,5,5,5‐hexafluoroacetylacetonato (hfac) as the auxiliary ligand of choice. The synthesized complexes were fully characterized by single‐crystal X‐ray diffraction, magnetic susceptibility measurements, and electronic structure calculations. The complexes [Mn( 1 )(hfac)₂] and [Fe( 1 )(hfac)₂] displayed antiferromagnetic coupling between the unpaired electrons of the ligand and the metal cation, whereas the interaction was found to be ferromagnetic in the analogous Ni^II complex [Ni( 1 )(hfac)₂]. The magnetic properties of the complex [Co( 1 )(hfac)₂] were difficult to interpret owing to significant spin–orbit coupling inherent to octahedral high‐spin Co^II metal ion. As a whole, the reported data clearly demonstrated the favorable coordinating properties of the radical 1 , which, together with its stability and structural tunability, make it an excellent new building block for establishing more complex metal–radical architectures with interesting magnetic properties.     

Effect of Organic Dicarboxylates with Different Rigidity on the Construction of Cobalt(II) Complexes with a Semi‐rigid Tripyridyl‐bis‐amide Ligand

Three coordination compounds with dimensions from 0D to 2D, namely, [Co(bppdca)₂(HL1)₂] ( 1 ) [Co(bppdca)(L2)(H₂O)] · 2H₂O ( 2 ) and [Co(bppdca)(L3)] · 3H₂O ( 3 ) [bppdca = N,N′‐bis(pyridine‐3‐yl)pyridine‐2,6‐dicarboxamide, H₂L1 = 2,5‐pyridinedicarboxylic acid, H₂L2 = 4,4′‐oxybisbenzoic acid, H₂L3 = 2‐carboxymethylsulfanyl nicotinic acid] were hydrothermally synthesized and structurally characterized. Single crystal X‐ray diffraction analysis reveals that complex 1 is a discrete 0D complex, in which the bppdca ligand and the H₂L1 act as the terminal groups to coordinate with the Co^II ions. In coordination polymer 2 , two bppdca ligands coordinate in anti configuration with two Co^II ions to generate a 28‐membered Co₂(bppdca)₂ loop, which is further extended into 1D ladder‐like double chain by pairs of L2 ligands. In 3 , the Co^II ions are linked by bppdca ligands to generate 1D wave‐like chain, which is further connected by the L3 to form a 2D network. Finally, the coordination compounds 1 – 3 are extended into 3D supramolecular frameworks through the hydrogen bonding interactions. The Co^II ions and the bppdca ligands in the title coordination compounds exhibit different coordination characters and conformations. The effect of organic dicarboxylates with different rigidity and length on the structures of Co^II coordination compounds was investigated. In addition, the fluorescence and electrochemical behaviors of coordination compounds 1 – 3 were reported.     

Redox‐Active‐Ligand‐Mediated Formation of an Acyclic Trinuclear Ruthenium Complex with Bridging Nitrido Ligands

Coordination of a redox‐active pyridine aminophenol ligand to Ru^II followed by aerobic oxidation generates two diamagnetic Ru^III species [ 1 a (cis) and 1 b (trans)] with ligand‐centered radicals. The reaction of 1 a / 1 b with excess NaN₃ under inert atmosphere resulted in the formation of a rare bis(nitrido)‐bridged trinuclear ruthenium complex with two nonlinear asymmetrical Ru‐N‐Ru fragments. The spontaneous reduction of the ligand centered radical in the parent 1 a / 1 b supports the oxidation of a nitride (N^3?) to half an equivalent of N₂. The trinuclear omplex is reactive toward TEMPO‐H, tin hydrides, thiols, and dihydrogen.     

Site‐Selective Ligand Exchange on a Heteroleptic TiIV Complex Towards Stepwise Multicomponent Self‐Assembly

A series of heteroleptic [Ti 1 ₂X]^? complexes have been selectively constructed from a mixture of Ti^IV ions, a pyridyl catechol ligand (H₂ 1 ; H₂ 1 =4‐(3‐pyridyl)catechol), and various bidentate ligands (HX) in the presence of a weak base, in addition to a previously reported [Ti 1 ₂(acac)]^? (acac=acetylacetonate) complex. Comparative studies of these Ti^IV complexes revealed that [Ti 1 ₂(trop)]^? (trop=tropolonate) is much more stable than the [Ti 1 ₂(acac)]^? complex, which allows the replacement of acac with trop on the [Ti 1 ₂(acac)]^? complex. This Ti^IV‐centered site‐selective ligand exchange reaction also takes place on a heteronuclear Pd^II? Ti^IV ring complex with the preservation of the Pd^II‐centered coordination structures. Intra‐ and intermolecular linking between two Ti^IV centers with a flexible or a rigid bis‐tropolone bridging ligand provided a tetranuclear and an octanuclear Pd^II? Ti^IV complex, respectively. These higher‐order structures could be efficiently constructed only through a stepwise synthetic route.     

Crystal structure and magnetic properties of a linear tetranuclear CoII cluster

Polynuclear complexes are an important class of inorganic functional materials and are of interest particularly for their applications in molecular magnets. Multidentate chelating ligands play an important role in the design and syntheses of polynuclear metal clusters. A novel linear tetranuclear Co^II cluster, namely bis{μ₃‐(E)‐2‐[(2‐oxidobenzylidene)amino]phenolato}bis{μ₂‐(E)‐2‐[(2‐oxidobenzylidene)amino]phenolato}bis(1,10‐phenanthroline)tetracobalt(II), [Co₄(C₁₄H₁₁NO₂)₄(C₁₂H₈N₂)₂], was prepared under solvothermal conditions through a mixed‐ligand synthetic strategy. The structure was determined by X‐ray single‐crystal diffraction and bulk purity was confirmed by powder X‐ray diffraction. The complex molecule has a centrosymmetric tetranuclear chain‐like structure and the four Co^II ions are located in two different coordination environments. The Co^II ions at the ends of the chain are in a slightly distorted octahedral geometry, while the two inner Co^II ions are in five‐coordinate distorted trigonal bipyramidal environments. A magnetic study reveals ferromagnetic Co^II…Co^II exchange interactions for the complex.     

Metal(II) Ion Complexes with 5‐(Pyrazin‐2‐yl)‐2,4‐dihydro‐3H‐1,2,4‐triazole‐3‐thione; Synthesis,Structural Characterization,Acid‐base,and Complexing Properties in Solution

The study reports the synthesis of complexes Co(HL)Cl₂ ( 1 ), Ni(HL)Cl₂ ( 2 ), Cu(HL)Cl₂ ( 3 ), and Zn(HL)₃Cl₂ ( 4 ) with the title ligand, 5‐(pyrazin‐2‐yl)‐1,2,4‐triazole‐5‐thione (HL), and their characterization by elemental analyses, ESI‐MS (m/z), FT‐IR and UV/Vis spectroscopy, as well as EPR in the case of the Cu^II complex. The comparative analysis of IR spectra of the metal ion complexes with HL and HL alone indicated that the metal ions in 1 , 2 , and 3 are chelated by two nitrogen atoms, N(4) of pyrazine and N(5) of triazole in the thiol tautomeric form, whereas the Zn^II ion in 4 is coordinated by the non‐protonated N(2) nitrogen atom of triazole in the thione form. pH potentiometry and UV/Vis spectroscopy were used to examine Co^II, Ni^II, and Zn^II complexes in 10/90 (v/v) DMSO/water solution, whereas the Cu^II complex was examined in 40/60 (v/v) DMSO/water solution. Monodeprotonation of the thione triazole in solution enables the formation of the L:M = 1:1 species with Co^II, Ni^II and Zn^II, the 2:1 species with Co^II and Zn^II, and the 3:1 species with Zn^II. A distorted tetrahedral arrangement of the Cu^II complex was suggested on the basis of EPR and Vis/NIR spectra.     

Magnetic investigations of a two‐dimensional coordination polymer with a three‐dimensional supramolecular framework: poly[[bis[μ2‐1,4‐bis(1,2,4‐triazol‐1‐yl)butane]bis(thiocyanato‐κN)cobalt(II)] dihydrate]

In the title mixed‐ligand metal–organic polymeric complex, {[Co(NCS)₂(C₈H₁₂N₆)₂]·2H₂O}_n, the asymmetric unit contains a divalent Co^II cation, which sits on an inversion centre, two halves of two crystallographically distinct and centrosymmetric 1,4‐bis(1,2,4‐triazol‐1‐yl)butane (BTB) ligands, one N‐bound thiocyanate ligand and one solvent water molecule. The Co^II atom possesses a distorted {CoN₆} octahedral geometry, with the equatorial positions taken up by triazole N atoms from four different BTB ligands. The axial positions are filled by thiocyanate N atoms. In the crystal, each Co^II atom is linked covalently to four others through the distal donors of the tethering BTB ligands, forming a neutral (4,4)‐topology two‐dimensional rhomboid grid layer motif, which is coincident with the (11) crystal planes. Magnetic investigations show that weak antiferromagnetic coupling exists between Co^II atoms in the complex.     

A Paramagnetic Copper(III) Complex Containing an Octahedral CuIIIS6 Coordination Polyhedron

Only the second octahedral, paramagnetic copper(III ) complex (S=1) has now been synthesized and characterized. Six thiolato bridging ligands in the heterotrinuclear species [LCo^IIICu^IIICo^IIIL](ClO₄)₃⋅2 Me₂CO (L=1,4,7‐tris(4‐tert‐butyl‐2‐sulfidobenzyl)‐1,4,7‐triazacyclononane) stabilize this rare electron configuration. A section of the structure of the reduced form (Cu^II, S=½) is shown. XAS, EXAFS, and EPR spectroscopy prove unambiguously that the one‐electron oxidation to the copper(III ) is metal‐ rather than ligand‐centered.     

Construction of Mononuclear Copper(II) and Trinuclear Cobalt(II) Complexes Based on Asymmetric Salamo‐Type Ligands

Mononuclear copper(II) and trinuclear cobalt(II) complexes, namely [Cu(L¹)]₂ · CH₂Cl₂ and [{Co(L²)(EtOH)}₂Co(H₂O)] · EtOH {H₂L¹ = 4,6‐dichloro‐6′‐methyoxy‐2,2′‐[1,1′‐(ethylenedioxydinitrilo)dimethylidyne]diphenol and H₃L² = 6‐ethyoxy‐6′‐hydroxy‐2,2′‐[1,1′‐(ethylenedioxydinitrilo)dimethylidyne]diphenol}, were synthesized and characterized by elemental analyses, IR and UV/Vis spectroscopy, and single‐crystal X‐ray diffraction. In the Cu^II complex, the Cu^II atom is four‐coordinate, with a N₂O₂ coordination sphere, and has a slightly distorted square‐planar arrangement. Interestingly, the obtained trinuclear Co^II complex is different from the common reported 2:3 (L:Co^II) salamo‐type Co^II complexes. Infinite 2D layer supramolecular structures are formed via abundant intermolecular hydrogen bonding and π ··· π stacking interactions in the Cu^II and Co^II complexes.     

Not Limited to Iron: A Cobalt Heme–NO Model Facilitates N–N Coupling with External NO in the Presence of a Lewis Acid to Generate N2O

Some bacterial heme proteins catalyze the coupling of two NO molecules to generate N₂O. We previously reported that a heme Fe–NO model engages in this N?N bond‐forming reaction with NO. We now demonstrate that (OEP)Co^II(NO) similarly reacts with 1 equiv of NO in the presence of the Lewis acids BX₃ (X=F, C₆F₅) to generate N₂O. DFT calculations support retention of the Co^II oxidation state for the experimentally observed adduct (OEP)Co^II(NO?BF₃), the presumed hyponitrite intermediate (P^.+)Co^II(ONNO?BF₃), and the porphyrin π‐radical cation by‐product of this reaction, and that the π‐radical cation formation likely occurs at the hyponitrite stage. In contrast, the Fe analogue undergoes a ferrous‐to‐ferric oxidation state conversion during this reaction. Our work shows that cobalt hemes are chemically competent to engage in the NO‐to‐N₂O conversion reaction.     

A new coordination mode of 6‐methylnicotinic acid in trans‐tetraaquabis(6‐methylpyridine‐3‐carboxylato‐κO)cobalt(II) tetrahydrate

The title compound, [Co(C₇H₆NO₂)₂(H₂O)₄]·4H₂O, contains a Co^II ion lying on a crystallographic inversion centre. The Co^II ion is octahedrally coordinated by two 6‐methylpyridine‐3‐carboxylate ligands in axial positions [Co—O = 2.0621 (9) Å] and by four water molecules in the equatorial plane [Co—O = 2.1169 (9) and 2.1223 (11) Å]. There are also four uncoordinated water molecules. The 6‐methylpyridine‐3‐carboxylate ligands are bound to the Co^II ion in a monodentate manner through a carboxylate O atom. There is one strong intramolecular O—H...O hydrogen bond, and six strong intermolecular hydrogen bonds of type O—H...O and one of type O—H...N in the packing, resulting in a complex three‐dimensional supramolecular structure.     

A Bio‐Inspired Switch Based on Cobalt(II) Disulfide/Cobalt(III) Thiolate Interconversion

Disulfide/thiolate interconversion supported by transition‐metal ions is proposed to be implicated in fundamental biological processes, such as the transport of metal ions or the regulation of the production of reactive oxygen species. We report herein a mononuclear dithiolate Co^III complex, [Co^IIILS(Cl)] ( 1 ; LS=sulfur containing ligand), that undergoes a clean, fast, quantitative and reversible Co^II disulfide/Co^III thiolate interconversion mediated by a chloride anion. The removal of Cl^? from the Co^III complex leads to the formation of a bis(μ‐thiolato) μ‐disulfido dicobalt(II) complex, [Co₂^II,IILSSL]²⁺ ( 2 ²⁺). The structures of both complexes have been resolved by single‐crystal X‐ray diffraction; their magnetic, spectroscopic, and redox properties investigated together with DFT calculations. This system is a unique example of metal‐based switchable Mⁿ₂‐RSSR/2 M⁽ⁿ⁺¹⁾‐SR (M=metal ion, n=oxidation state) system that does not contain copper, acts under aerobic conditions, and involves systems with different nuclearities.     

A Pentanuclear Cobalt Complex with two [CoII(CH3O)3]– Units Wrapping a Triangular [CoIII3O]7+ Core: Synthesis,Structure, and Magnetic Properties

The reaction of Hppko (Hppko = phenyl 2‐pyridyl ketone oxime) and CoCl₂ · 6H₂O in the CH₃OH solvent with the presence of triethylamine (NEt₃) at room temperature and the exposure to air resulted in the formation of a new pentanuclear, mixed‐valence cobalt complex with the molecular formula [{Co^II(CH₃O)₃}₂{Co^III₃(μ₃‐O)(ppko)₃}Cl₂]. X‐ray single crystal analysis displays a trigonal bipyramid configuration with the terminal two Co^II ions wrapping an triangle [Co^III₃O]⁷⁺ core. The intermolecular C–H ··· O and C–H ··· Cl interactions form a 2D network framework. The analysis of magnetic susceptibility revealed the dominant antiferromagnetic interactions and strong orbital contribution of Co^II ions.     

Crystal structure and spectroscopic characterization of a cobalt(II) tetraazamacrocycle: completing a series of first‐row transition‐metal complexes

The tetraazamacrocyclic ligand 1,4,8,11‐tetramethyl‐1,4,8,11‐tetraazacyclotetradecane (TMC) has been used to bind a variety of first‐row transition metals but to date the crystal structure of the cobalt(II) complex has been missing from this series. The missing cobalt complex chlorido(1,4,8,11‐tetramethyl‐1,4,8,11‐tetraazacyclotetradecane‐κ⁴N )cobalt(II) chloride dihydrate, [CoCl(C₁₄H₃₂N₄)]Cl·2H₂O or [Co^IICl(TMC)]Cl·2H₂O, crystallizes as a purple crystal. This species adopts a distorted square‐pyramidal geometry in which the TMC ligand assumes the trans‐I configuration and the chloride ion binds in the syn‐methyl pocket of the ligand. The Co^II ion adopts an S = spin state, as measured by the Evans NMR method, and UV–visible spectroscopic studies indicate that the title hydrated salt is stable in solution. Density functional theory (DFT) studies reveal that the geometric parameters of [Co^IICl(TMC)]Cl·2H₂O are sensitive to the cobalt spin state and correctly predict a change in spin state upon a minor perturbation to the ligand environment.     

Lability‐Controlled Syntheses of Heterometallic Clusters

A bulky bidentate ligand was used to stabilize a macrocyclic [Fe^III₈Co^II₆] cluster. Tuning the basicity of the ligand by derivatization with one or two methoxy groups led to the isolation of a homologous [Fe^III₈Co^II₆] species and a [Fe^III₆Fe^II₂Co^III₂Co^II₂] complex, respectively. Lowering the reaction temperatures allowed isolation of [Fe^III₆Fe^II₂Co^III₂Co^II₂] clusters with all three ligands. Temperature‐dependent absorption data and corresponding experiments with iron/nickel systems indicated that the iron/cobalt self‐assembly process was directed by the occurrence of solution‐state electron‐transfer‐coupled spin transition (ETCST) and its influence on reaction intermediate lability.     

mer‐Bis(1,4‐dibenzoylthiosemicarbazidato‐κ3O,N,O′)cobalt(II)

The title complex, [Co(C₁₅H₁₂N₃O₂S)₂], consists of an octahedrally coordinated Co^II ion, with two crystallographically independent 1,4‐dibenzoylthiosemicarbazidate ligands in a tridentate mer coordination [Co—O = 2.064 (3)–2.132 (3) Å and Co—N = 2.037 (3)–2.043 (3) Å]. There are intermolecular N—H...S hydrogen bonds involving one ligand and strong π–π stacking interactions involving the other ligand, resulting in a three‐dimensional supramolecular framework. The hydrogen bonds and π–π interactions, as well as different intramolecular aryl–benzamide H—C...H(—N) distances, give rise to a difference in conformation between the two ligands.     

Hydrothermal Synthesis of Two New Transition Metal Coordination Polymers with Mixed Ligands

Hydrothermal reactions of 1, 2, 4‐benzenetricarboxylic acid, 1, 10‐phenanthroline and transition metal cations including Zn^II or Co^II, in basified aqueous solution gave rise to two complexes, [Zn₃(btrc)₂(1, 10‐phen)₂(H₂O)₂]_n ( 1 ), and [Co₃(btrc)₂(1, 10‐phen)₂(H₂O)₂]_n ( 2 ) (btrc = 1, 2, 4‐benzenetricarboxylate, and 1, 10‐phen = 1, 10‐phenanthroline). 1 2 crystalize isotypically in the triclinic space group P1¯. The btrc ligand acts as multi‐dentate bridging ligand in both compound 1 and 2 to link up transition metal atoms into lamella networks, which are further attached into three‐dimensional frameworks through complex hydrogen bonding and π‐π interactions. The photoluminescence spectrum for compound 1 has also been studied. The corresponding reaction with Cu²⁺ follows another pathway.     

Synthesis and characterization of a novel series of metallothiocarbohydrazone polymers and their adducts

A novel linear polymeric pentadentate (O₂N₂S‐sites) ligand (H₃L) bearing both soft and hard donors was prepared by the reaction of a bifunctional carbonyl compound, 4,6‐diacetylresorcinol, with a bifunctional hydrazide compound, thiocarbohydrazide. Mono‐ and binuclear Cu^II and Ni^II complexes/each monomeric unit of the polymeric ligand were obtained depending on the pH of the reaction medium and the metal ion. Adducts with 1,10‐phenanthroline (Phen) and 2,2′‐bipyridyl (Bpy) were obtained. Anomalous dimeric Co^II/Co^III complexes of the polymeric ligand were obtained in which two molecules of the linear polymeric ligand trapped two cobalt ions (Co^II and Co^III) in each monomeric unit. These structures are very interesting in that they contain Co^II/Co^III, side by side, as high‐spin square planar coordinated Co^II ions and low‐spin (diamagnetic) octahedral coordinated Co^III ions. The suggested structures of the complexes have been elucidated on the basis of elemental and thermal analyses, conductance, and magnetic susceptibility measurements as well as spectral studies (electronic, IR, and ESR spectra). © 2007 Wiley Periodicals, Inc. Heteroatom Chem 18:100–107, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20239     

Transition metal complexes with tridentate salicylaldimine derived from 3,5-di-t-butylsalicylaldehyde

Several new binuclear Cu^II, Ni^II, OV^IV and Mn^II complexes of tridentate salicylaldimine (H₂L), obtained from 3,5-di-t-butylsalicylaldehyde and o-aminophenol, have been prepared and characterized by analytical, spectroscopic (i.r., u.v.–vis., e.s.r.) techniques, magnetic and thermal measurements. The adduct formation or dissociation of these complexes in the presence of strongly coordinating solvents like pyridine and DMSO did not take place. The complexation of Co^II with H₂L is accompanied by intramolecular electron transfer from the metal to the coordinated ligand yielding the radical ligand Co^III complex (g = 2.003, A ^Co = 10 G). The e.s.r. spectra of the Cu^II, OV^IV and Mn^II complexes in the solid state and in solution are very broad due to intramolecular dipolar antiferromagnetic interactions.     

The Charge Transfer Approach to Heavier Main‐Group Element Radicals in Transition‐Metal Complexes

The Co^II and Fe^II complexes 1Co and 1Fe with a coordinated phosphorus radical were easily obtained through a charge‐transfer approach from the M^I precursors LM^I(tol) (M=Co, Fe; L=CH(MeC=NDipp)₂, Dipp=2,6‐i Pr₂C₆H₃) to the diazafluorenylidene‐substituted phosphaalkene 1 . Structural, magnetic, and computational studies on 1Co and 1Fe indicate a weak antiferromagnetic interaction between the high‐spin M^II ion and the phosphorus radical, resulting in a triplet and quartet ground state, respectively. Complexes 1Co and 1Fe are the first examples of phosphorus‐radical‐coordinated transition‐metal complexes synthesized by charge transfer, providing a new approach to access radicals of heavier main‐group elements.