Anion Radical Ligands Research Articles

Cis-Bis(2,2′-Azopyridinido)dicarbonylruthenium(II)

An [Ru(apy)2Cl2] precursor (apy = 2,2′-azopyridine) in 2-methoxyethanol was heated under a pressurized CO atmosphere to afford a diradical complex, [Ru(apy·−)2(CO)2], containing one-electron-reduced azo anion radical ligands. The electronic states of the complex were characterized by spectroscopic techniques and computational studies. Magnetic measurements revealed the existence of antiferromagnetic interactions in the diradical complex.

Nickel(II) and Cobalt(III) bis(dioxolene) complexes with di(2-pyridyl)imine ligands: Synthesis and magnetic properties

Here the synthesis, the molecular structure and the magnetic propertiesof four novel six-coordinated bis-dioxolene nickel and cobalt complexes with various di(pyridyl)imines as ancillary N-donor ligands is discussed. Despite the analogous composition the obtained compounds show different charge distribution dependently on the nature of complexing metal. Bis-o-semiquinonato nickel(II) complexes demonstrate the strong ferromagnetic exchange interaction between unpaired electrons of the high-spin nickel ion and anion-radical ligands. Cobalt complex containing naphtyl-substituted ancillary ligand demonstrates the proceeding of solid-state redox-isomeric transformation at temperatures above 220 K.

Magnetic and luminescent properties of Cd(ii)- and Fe(ii)-anion radical frameworks: various networks or structures influenced by metal ion sizes or in situ forming mechanisms of anion radical ligand

The preparation, X-ray crystallography, EPR, and magnetic and luminescent properties of three new metal–anion radical frameworks are described herein. The anion radical ligand (bipo−˙) and co-ligand, 1,4-benzenedicarboxylate (BDC2−), coordinate to Cd(II) centers, leading to an interpenetrated three-dimensional (3D) metal–organic frameworks (MOF), [Cd3(bipo−˙)4(BDC)]n (1). Although [Fe3(bipo−˙)4(BDC)]n (2) and 1 possess the same space group (tetragonal, P42/nbc), and similar rigid spirocycle-like chain and powder X-ray diffraction (PXRD) patterns, 2 exhibits an interesting spirocycle-like one-dimensional (1D) chain network, in which BDC2− behaves not as bridging co-ligand but as a counteranion, according to smaller Fe(II) ion size and long Fe1⋯Fe1′ distance between rigid chains. Thus, metal ion sizes induce different networks of isomorphous 1 and 2. Moreover, the different forming mechanisms of bipo−˙ ligand also result in different structural MOFs: the in situ deprotonation of initial Hbipo−˙ leads to an 1D Fe(II) MOF (2); the in situaldol condensation and then deprotonation of initial hydrochloride salt of imidazo[1,2-a]pyridin-2(3H)-one results in a two-dimensional (2D) Fe(II) MOF, [Fe3(bipo−˙)2(μ2-OH)2(μ2-H2O)(BDC)]n (3). More noteworthy, in 2, BDC2− suffers large distortion, mainly attributed to its strong electrostatic interaction with the Fe(II) cation on the rigid spirocycle chain. Compounds 2 and 3 exhibit broad EPR signals, ascribed to the strong iron–anion radical antiferromagnetic coupling. 1 shows an unusual magnetic phase transition at ∼70 K, and intense fluorescence emission which can even be excited by visible light (460 nm). Both 2 and 3 show strong antiferromagnetic interaction between Fe(II) cation and radical. Compared to 1, two Fe(II) MOFs exhibit obvious blue-shift fluorescence emissions.

Isostructural Metal–Anion Radical Coordination Polymers with Tunable Phosphorescent Colors (Deep Blue, Blue, Yellow, and White) Induced by Terminal Anions and Metal Cations

Five phosphorescent metal-anion radical coordination polymers based on a new anion radical ligand generated by in situ deprotonation of a stable zwitterionic radical are described. The N,O,N-tripodal anion radical ligand links metal cations, which leads to five isostructural coordination polymers, [M(3)(bipo(-.))(4)(L)(2)](n) (M=Cd or Mn, Hbipo(-.)=2,3'-biimidazo[1,2-a]pyridin-2'-one, L=Cl(-), HCOO(-) or SCN(-)). The isostructural coordination polymers exhibit novel one-dimensional spirocycle-like structures. Three isostructural Cd(II) coordination polymers display unusual phosphorescent color changes (blue, yellow, and white) induced by terminal anions. Significantly, the Cd(II) coordination polymer with terminal Cl(-) possesses moderate quantum yield, and shows a bright white-light phosphorescence emission, which is independent of excitation wavelength and can even be excited by visible light. Upon adjusting the metal cation to Mn(II), two isostructural Mn(II) coordination polymers reveal deep-blue-light phosphorescence emissions that are independent of terminal anions. As radical-based coordination polymers, some of them show antiferromagnetic interactions between radical species or radical and metal center.

New metal-anion radical framework materials: CoII compounds showing ferromagnetic to antiferromagnetic phase transition at about 344 K, and ZnII compounds exhibiting terminal anion ligand induced direct white-light-emission

The preparation, X-ray crystallography, EPR, magnetic and luminescent investigation of new metal-anion radical framework materials based on a new anion radical ligand generated by in situ deprotonation of a stable zwitterionic radical are described herein. N,O,N-tripodal anion radical ligand (bipo(-)˙) links metal cations, giving rise to four isostructural one-dimensional metal-organic frameworks, [M(bipo(-)˙)(L)](n) [M = Zn, L = HCOO(-) (1), SCN(-) (1a), N(3)(-) (1b); M = Co, L = Br(-) (3)]. The tripodal bipo(-)˙ ligand and one co-ligand, 1,4-benzenedicarboxylate, coordinate to metals leading to two isostructural two-dimensional metal-organic frameworks, [M(bipo(-)˙)(BDC)(0.5)](n) [M = Zn (2) and Co (4)]. The two Co(II) compounds are the first examples that exhibit unusual ferromagnetic to antiferromagnetic phase transition with transition temperature over room temperature, which can be demonstrated by the cooling and warming measurements of susceptibility. Compound 4 also exhibits long-range magnetic ordering with the critical temperature at about 44 K proved by ac susceptibility measurements. The metal-radical frameworks exhibit distinctly different fluorescence emissions. Especially, the isomorphous one-dimensional Zn(II) compounds show interesting terminal anion ligand-induced photoluminescent color changes, including direct and invariable white-light-emission with terminal SCN(-) ligand.

Singlet Diradical Complexes of Chromium, Molybdenum, and Tungsten with Azo Anion Radical Ligands from M(CO)6Precursors

The homoleptic diamagnetic complexes M(mer-L)(2), M = Cr, Mo,W (1a,b, 2a,b, and 4a,b), were obtained by reacting the hexacarbonyls M(CO)(6) with the tridentate ligands 2-[(2-N-arylamino)phenylazo]pyridine (HL = NH(4)C(5)N=NC(6)H(4)N(H)C(6)H(4)(H) (HL(a)) or NH(4)C(5)N=NC(6)H(4)N(H)C(6)H(4)(CH(3)) (HL(b))) in refluxing n-octane. In the case of M = Mo, the dinuclear compounds [Mo(L)(pap)](2)(mu-O) (3a,b) (pap = 2-(phenylazo)pyridine), were obtained as second products in moist solvent. X-ray diffraction analysis for Cr(L(b))(2) (1b), Mo(L(a))(2) (2a), and W(L(a))(2) (4a) reveals considerably distorted-octahedral structures with trans-positioned azo-N atoms and cis-positioned 2-pyridyl-N and anilido nitrogen atoms. Whereas the N(azo)-M-N(azo) angle is larger than 170 degrees, the other two trans angles are smaller, at about 155 degrees (M = Cr, 1b) or 146 degrees (M = Mo, W; 2a, 4a), due to the overarching bite of the mer-tridentate ligands. The bonds from M to the neutral 2-pyridyl-N atoms are distinctly longer by more than 0.08 A than those to the anilido or azo nitrogen atoms, reflecting negative charge on the latter. The N-N bond distances vary between 1.339(2) A for 1b and 1.373(3) A for 4a, clearly indicating the azo radical anion oxidation state. Considering the additional negative charge on anilido-N, the mononuclear complexes are thus formulated as M(IV)(L*(2-))(2). The diamagnetism of the complexes as shown by magnetic susceptibility and (1)H NMR experiments is believed to result from spin-spin coupling between the trans-positioned azo radical functions, resulting in a singlet diradical situation. The experimental structures are well reproduced by density functional theory calculations, which also support the overall electronic structure indicated. The dinuclear 3a with N-N distances of 1.348(10) A for L(a) and 1.340(9) A for pap is also formulated as an azo anion radical-containing molybdenum(IV) species, i.e., [Mo(IV)(L*(2-))(pap*-)](2)(mu-O). All compounds can be reversibly reduced; the Cr complexes 1a,b are also reversibly oxidized in two steps. Electron paramagnetic resonance spectroscopy indicates metal-centered spin for 1a+ and 1a- and g approximately 2 signals for 2a-, 3a+, 3a-, and 4a-. Spectroelectrochemistry in the UV-vis-NIR region showed small changes for the reduction of 2a, 3a, and 4a but extensive spectral changes for the reduction and oxidation of 1a.

Reduced and excited states of the intermediates (α-diimine)(C5R5)Rh in hydride transfer catalysis schemes: EPR and resonance Raman spectroscopy, and comparative DFT calculations of Co, Rh and Ir analogues

The electronic structures of the highly air-sensitive intermediates (N[caret]N) (C(5)Me(5))Rh, (N[caret]N = 2,2'-bipyridine (bpy), 2,2'-bipyrimidine (bpym), 2,2'-bipyrazine (bpz) and 3,3'-bipyridazine (bpdz)) of hydride transfer catalysis schemes were studied through resonance Raman (rR) spectroscopy and through EPR of the reduced forms [(N[caret]N) (C(5)Me(5))Rh](.-). The rR results are compatible with a predominant MLCT character of the lowest excited states [ (N[caret]N) (C(5)Me(5))Rh]*, and the EPR spectra of the reduced states reveal the presence of anion radical ligands, (N[caret]N) (.-), coordinated by unusually electron rich rhodium(i) centres. The experimental results, including the assignments of electronic transitions, are supported by DFT calculations for the model compounds [(N[caret]N)(C(5)H(5))Rh](o)/(.-), (N[caret]N) = bpy or bpym. The calculations confirm a significant but not complete mixing of metal and ligand orbitals in the lowest unoccupied MO which still retains about 3/4 pi* (N[caret]N) character. DFT calculations on (bpy)(C(5)H(5))M and [(bpy)(C(5)H(5))ClM](+), M = Co, Rh, Ir, agree with the experimental results such as the differences between the homologues, especially the different LUMO characters of the precursor cations in the case of Co-->d(M)) and Rh or Ir (-->pi*(bpy)).

High-frequency (245 GHz) and X-band EPR study of stable dicopper radical complexes

Stable, readily accessible and in part even structurally established paramagnetic complex cations [Cu2(µ-η4-BL)L2]+˙, BL = bridging ligand, L = Ph2P(CH2)nPPh2 or 2 PPh3, were studied by high-frequency (245 GHz) and X-band EPR spectroscopy. The dicopper(I)/anion radical ligand formulation as opposed to a mixed-valent dimetal description is supported by the high-frequency EPR results which show g components between 2.0220 and 1.9968. The largest (rhombic) g anisotropy Δg was found for the system with BL = azodi-tert-butyl formate and L = Ph2P(CH2)6PPh2, and the smallest (axial) Δg splittings for 1,2,4,5-tetrazine-bridged species. There is a clear relation between Δg as obtained from the high-field measurements and the 63,65Cu hyperfine coupling as measured from solution spectra in the X band.

Zweikernige Bisphosphankupfer(I)-Komplexe mit dem 2,2′-Bipyrimidin-Brückenliganden: Strukturelle Analogie zu „Blauen“ Kupferzentren und Reduktion zu paramagnetischen Spezies / Dinuclear Bis(phosphane)copper(I) Complexes with 2,2′-Bipyrimidine as Bridging Ligand: Structural Analogy to “Blue” Copper Centers and Reduction to Paramagnetic Species

Dinuclear complexes [(PR3)2Cu(μ: η4-bpym)Cu(PR3)2](BF4)2 with the bis-chelating bridging ligand 2,2′-bipyrimidine (bpym) and various monodentate and bidentate phosphanes were synthesized via a four component “self assembly” reaction. The reversible electrochemical reduction to monocationic radical complexes was studied by cyclic voltammetry and electron spin resonance (ESR). Highly resolved ESR spectra revealed the presence of the bpym anion radical ligand via resolved hyperfine coupling with 1H, 14N, 31P, 63Cu and 65Cu nuclei. The variation of phosphane ligands was accompanied by significant changes of electrochemical potentials which were rationalized considering electronic and structural effects. The configurational flexibility of Cu(I) in this environment is evident from the X-ray crystal structure analysis of [(PPh3)2Cu(μ: η4-bpym)Cu(PPh3)2](BF4)2 - H2O• CH3OH for which the coordination at the metal can be described as 3 + 1 (NN′P + P′), qualitatively similar to the (NN′S + S′) arrangement around the type 1 Cu centers of “blue” copper proteins. Intramolecular cooperative π donor/π acceptor/π donor “sandwich” interactions provide parts of the driving force for this highly distorted coordination geometry which illustrates the proclivity of Cu(I) towards such an “entatic state” arrangement.

Spectroelectrochemistry and localization of added electrons in ruthenium (II) mixed-ligand complexes

The ruthenium (II) mixed-ligand complexes cis-[Ru(bpy)2(NCS)2] (I), [Ru(bpy)2(pn)]2+ (II) and [Ru(bpy)2(phen)]2+ (III) (bpy, 2,2′-bipyridine; pn, 1,2-diaminopropane; phen, 1,10-phenanthroline) were subjected to two (for I and II), and three (for III) stepwise, one-electron reductions, and one-electron oxidations (all fully reversible); the products were studied in situ by solution UV—vis-near IR spectroscopy. The first two reductions took place in all cases on separate bpy ligands, while the third reduction of III took place on phen. Bands of the reduced species in the near IR—vis region are observed and assigned to anion radical ligands. Novel features of the LMCT bands of the oxidized species are presented and discussed.

Binuclear radical complexes of heavy-metal fragments containing ruthenium, osmium, rhodium and gold

Homobinuclear chelate complexes of the 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine anion radical ligand with the metal fragments [Os(bpy)]2+(d6), [Rh(norbonadiene)]+(d8) and [Au(PPh3)]+(d10) have been studied by ESR spectroscopy. The unique suitability of this radical ligand for coordination of heavy metal fragments and for ESR investigations is discussed.

ChemInform Abstract: Paramagnetic Copper(I) Dimers.

AbstractThe reaction of the binucleating α‐diimine (I) with the Cu complex (II) yields a binuclear diamagnetic complex, which on electrochemical as well as on chemical one‐electron reduction leads to the paramagnetic Cu(I) dimer (IIIa) with an anion radical ligand.

ChemInform Abstract: The Transition Metal Coordination Chemistry of Anion Radicals

A. Scope and introduction B. Formation of anion radicals C. Ligand properties of anion radicals D. Formation of anion radical complexes (i) Redox reactions of complexes (ii) Coordination of “free” anion radicals (iii) Photochemical generation (iv) Thermal electron transfer between closed-shell species E. Survey of anion radical ligands and of their complexes . . , (i) Carbon ligands (ii) Nitrogen ligands (iii) Oxygen ligands (iv) Phosphorus ligands (v) Sulfur ligands F. Spectroscopy and structure (i) Magnetic resonance and magnetism (ii) Electrochemistry (iii) Electronic spectroscopy (UV/VIS/NIR) ......... (iv) Structural studies and vibrational spectroscopy ..... G. Reactivity of anion radical complexes Acknowledgements References

The transition metal coordination chemistry of anion radicals

A. Scope and introduction B. Formation of anion radicals C. Ligand properties of anion radicals D. Formation of anion radical complexes (i) Redox reactions of complexes (ii) Coordination of “free” anion radicals (iii) Photochemical generation (iv) Thermal electron transfer between closed-shell species E. Survey of anion radical ligands and of their complexes . . , (i) Carbon ligands (ii) Nitrogen ligands (iii) Oxygen ligands (iv) Phosphorus ligands (v) Sulfur ligands F. Spectroscopy and structure (i) Magnetic resonance and magnetism (ii) Electrochemistry (iii) Electronic spectroscopy (UV/VIS/NIR) ......... (iv) Structural studies and vibrational spectroscopy ..... G. Reactivity of anion radical complexes Acknowledgements References