Valence tautomerism in a [2 × 2] Co4 grid complex containing a ditopic arylazo ligand.

We describe the structural and magnetic properties of a tetranuclear [2 × 2] Co4 grid complex containing a ditopic arylazo ligand. At low temperatures and in solution the complex is comprised of Co3+ and singly reduced trianion-radical ligands. In the solid state we demonstrate the presence of valence tautomerization via variable temperature magnetic susceptibility experiments and powder-pattern EPR spectroscopy. Valence tautomerism in polynuclear complexes is very rare and to our knowledge is unprecedented in [2 × 2] grid complexes.

Ligand mixed-valence and electrical conductivity in coordination complexes containing a redox-active phenalenol-substituted ligand.

A new redox-active hydrazone ligand bearing a phenalenol group is described (phpl), which produces neutral six-coordinate Fe and Co complexes (1 & 2) with the ligands identified in different oxidation states; an open-shell anion radical and closed-shell dianion. An intense and very low-energy intervalence charge transfer (IVCT) band is identified in solid-state and in solution in the complexes. Single crystals of 1 are semiconducting (at 300 K, σ = 3.05 × 10-4 S cm-1 with Ea = 245 meV).

Reversible Solution π-Dimerization and Long Multicenter Bonding in a Stable Phenoxyl Radical.

Reversible solution π-dimerization is observed in the stable neutral phenoxyl radical 2,6-bis-(8-quinolylamino)-4-(tert-butyl)phenoxyl baqp and is spectroscopically characterized. This behavior, not previously observed for π-extended phenoxyl radicals, is relevant to the formation of long multicenter bonding in the π-dimer at low temperature akin to previously reported phenalenyl radicals. Our experimental data are supported in a quantitative manner by results from density functional theory (DFT) and ab initio molecular orbital theory calculations. Our theoretical results indicate that the solution dimer features strong bonding interactions between the two phenoxyl rings but that the stability of the dimer is also related to dispersion interactions between the flanking nearly parallel quinolyl rings.

Polynuclear Cu4L4 Copper(II) Aminyl Radical Coordination Complexes.

We describe the structural features and magnetic properties of two polynuclear copper(II) complexes containing a redox-active ligand. These neutral complexes each bear the formula RL4Cu4 (R = tBu, Me) with the ligand in a dianion-aminyl radical oxidation state. X-ray data and density functional theory calculations support an aminyl-type radical character in these complexes, making these the first polynuclear metal aminyl radical complexes.

Molecular and electronic structures of complexes containing 1-(2-pyridylazo)-2-phenanthrol (PAPL): revisiting a redox-active ligand.

Herein we report the molecular structures and electronic properties of neutral, homoleptic, six-coordinate complexes of the general formula M(PAPL)2, where PAPL is the monoanion of 1-(2-pyridylazo)-2-phenanthrol (M = Mn, Ni, Zn). Although, the coordination chemistry of PAPL has been investigated in a few previous reports in the 1970s and 1980s, there are, to our knowledge, no reported single crystal X-ray diffraction studies of any complexes, or any other electronic property or computational studies of complexes containing the PAPL anion until now. The electronic structures of the complexes are probed with a combination of cyclic voltammetry, UV-vis spectroscopy, and spectroelectrochemical measurements. Density functional theory calculations support the redox-active nature of the PAPL ligand. In all complexes we observe two reversible ligand-centered reduction processes, suggesting it may be possible to access the open-shell radical-anionic state of the ligand.

π-Extended and six-coordinate iron(II) complexes: structures, magnetic properties, and the electrochemical synthesis of a conducting iron(II) metallopolymer.

Herein, we describe the preparation of three new bidentate π-extended derivatives of the ligand N-phenyl-2-pyridinalimine (ppi) containing a 3-thienyl (4) substituent at position 4 of the aniline ring or 2-thienyl (6) or phenyl (2) substituents at each of the 2,5 positions of the aniline rings. Three iron(2+) complexes (7-9) containing these ligands were prepared by combining two equivalents each of 2, 4, or 6 with Fe(NCS)(2), and the resulting neutral, six-coordinate complexes were fully characterized, including with single crystal X-ray diffraction experiments in the case of complexes 7 and 9. Variable temperature magnetic susceptibility and Mössbauer experiments confirm the presence of spin-crossover in complexes 7 and 8, and the unusual solid state variable temperature magnetic properties of complex 9 likely result from crystal packing forces. Electropolymerization of the 2,5-dithienyl-substituted complex (9) produces a conducting and electrochromic metallopolymer film (poly-9).

Persistent metal bis(hexafluoroacetylacetonato) complexes featuring a 2,2'-bipyridine substituted triarylamminium radical cation.

Herein, we describe the preparation of a 2,2'-bipyridine derivative containing a redox-active N,N'-(4,4'-dimethoxydiphenylamino) substituent (1), which readily coordinates M(hfac)(2) salts [M = Mn (2), Ni (3), Cu (4)] to generate stable, neutral, and pseudo-octahedral coordination complexes, which have been fully characterized. Cyclic voltammetry and spectroelectrochemical measurements on complexes 2-4 indicate stable one-electron oxidation processes, and the formation of persistent radical cation complexes. The neutral complexes (M = Mn or Ni) were subject to one-electron oxidation with NOPF(6) in acetonitrile, and magnetic moments of the resulting solutions were obtained using the Evans method at different temperatures. Our experimental results suggest that the first reported ferromagnetically coupled metal-triarylamminum radical cation complex is obtained when M = Mn(2+), and antiferromagnetic coupling results when M = Ni(2+). These results are supported by results from density functional theory calculations, which indicate that a π spin polarization mechanism for magnetic exchange coupling is operative in singly oxidized complexes, 2-4.

Hybrid spin-crossover conductor exhibiting unusual variable-temperature electrical conductivity.

We describe the multistep synthesis of a new terthienyl-substituted QsalH ligand and an iron(3+) spin-crossover complex (1) containing this ligand, which electropolymerizes to produce a hybrid-conducting metallopolymer film (poly1). Variable-temperature magnetic susceptibility measurements demonstrate that spin-crossover is operative in the polymer film, and resistivity measurements on indium-tin oxide coated glass slides containing the polymer film exhibit intriguing temperature-dependent profiles.

Bimetallic iron(3+) spin-crossover complexes containing a 2,2'-bithienyl bridging bis-QsalH ligand.

We describe the synthesis of a new 3,3'-diethynyl-2,2'-bithienyl bridging bis-QsalH ligand (5), and the preparation of four bimetallic iron(3+) complexes containing 5 with Cl(-) (6), SCN(-) (7), PF(6)(-) (8), and ClO(4)(-) (9) counteranions. We show with variable temperature magnetic susceptibility, Mossbauer, and electron paramagnetic resonance (EPR) spectroscopy that each complex undergoes a spin-crossover in the solid state. In all four complexes, we observe very gradual and incomplete S = 5/2, 5/2 to S = 1/2, 1/2 spin-crossover processes, with three of the four complexes exhibiting nearly identical magnetic properties. We investigated the electronic properties of the complexes by cyclic and differential pulse voltammetry, and attempted electropolymerization reactions with acetonitrile solutions of the complexes, which were not successful. Each complex features a single iron(3+) reduction wave at approximately -0.7 V (versus ferrocene), and the oxidation of the 2,2'-bithienyl substituent occurs at +1.1 V. These materials represent a new structural paradigm for the study of rare bimetallic iron(3+) spin-crossover complexes.

A conducting metallopolymer featuring valence tautomerism.

A new cobalt bis(semiquinone) valence tautomer is reported featuring a terthienyl substituent, which undergoes electropolymerization to produce a valence tautomeric conducting metallopolymer.

Preparation and magnetic properties of iron(3+) spin-crossover complexes bearing a thiophene substituent: toward multifunctional metallopolymers.

The synthesis of a new 3-ethynylthienyl-substituted QsalH ligand (QsalH is the short form for N-(8-quinolyl)salicylaldimine) (ThEQsalH 3), and the preparation, electronic, and magnetic properties of three homoleptic and cationic iron(3+) complexes containing this ligand with PF(6)(-) 4, SCN(-) 5, and ClO(4)(-) 6 counteranions are reported. In all three complexes a spin-crossover is observed in the solid state by variable temperature magnetic susceptibility measurements and Mossbauer spectroscopy, indicating that the synthetic modification of the QsalH ligand has not significantly altered the electronics at the metal center. This includes the observation of a very rare S = 5/2 to 3/2 spin-crossover in a non-porphyrin iron(3+) complex 5. The molecular structure and magnetic properties of an unusual iron(2+) complex 7 generated by reduction of complex 6 serendipitously during a recrystallization attempt in aerobic acetone solution is also reported. Complexes 4-6 feature iron(3+) reduction and oxidation of the thiophene ring at potentials of approximately -0.7 and +1.2 V (vs Fc), respectively.

5,7-Di-2-pyridyl-2,3-dihydro-thieno[3,4-b][1,4]dioxine.

The title compound, C(16)H(12)N(2)O(2)S, was prepared by a Neigishi cross-coupling reaction to investigate the coordination chemistry of thio-phene-containing ligands. In the mol-ecule, the pyridine rings are twisted from the thio-phene ring by 20.6 (1) and 4.1 (2)°. The six-membered dihydro-dioxine ring is in a half-chair conformation.

N'-(3-Thienylmethyl-ene)pyridine-2-carbohydrazide.

The title compound, C(11)H(9)N(3)OS, was prepared to investigate the coordination chemistry of thio-phene-containing ligands as precursors to inter-esting metallopolymers. The mol-ecule is nearly planar. The angle between the thio-phene and pyridine rings is 8.63 (4)° and features the expected trans configuration about the imine bond. The structure is stabilized by a weak inter-molecular N-H⋯O hydrogen bond. The distance between centroids of adjacent thio-phene rings [3.67 (8) Å] suggests the presence of π-π inter-actions.

Intramolecular pi-dimerization in a 1,1'-bis(verdazyl)ferrocene diradical.

The solid state structure of 1,1'-bis(verdazyl)ferrocene reveals the two radical moieties associated intramolecularly to give the first example of a pi-dimer of a stable verdazyl radical. The pi-dimer structure is not maintained in solution. Magnetic characterization indicates that the radicals are sufficiently strongly antiferromagnetically coupled so as to render the molecule diamagnetic at room temperature.

Preparation and solid-state characterization of the novel mixed biradical *NSNSC-CNSSN*.

Reduction of the radical-cation [*NSSNC-CNSNS][AsF(6)] with ferrocene affords the novel biradical *NSNSC-CNSSN* containing both 1,2,3,5- and 1,3,2,4-isomeric dithiadiazolyl rings. Biradicals form centrosymmetric dimers with pi(*)-pi(*) interactions between different isomeric rings. Biradical *NSNSC-CNSSN* is diamagnetic in the solid state (C = 0.00035, TIP = 6.5 x 10(-)(5) emu/Oe.mol); however, an increase in paramagnetism was observed upon grinding (C = 0.003, TIP = 4.2 x 10(-)(4) emu/Oe.mol).

Mixed valence Mn(II)/Mn(III) [3 x 3] grid complexes: structural, electrochemical, spectroscopic, and magnetic properties.

Mn(II)9 grid complexes with a [Mn9(mu-O)12] core, obtained by self-assembly of a series of tritopic picolinic dihydrazone ligands with Mn(II) salts, have been oxidized by both chemical and electrochemical methods to produce mixed oxidation state systems. Examples involving [Mn(III)3Mn(II)6] and [Mn(III)4Mn(II)5] combinations have been produced. Structures are reported for [Mn9(2poap-2H)6](NO3)6.14H2O (1), [Mn9(2poap-2H)6](ClO4)10.10H2O (3), and [Mn9(Cl2poap-2H)6](ClO4)9.14H2O.3CH3CN (10). Structural studies show distinct contraction of the corner grid sites on oxidation, with overall magnetic properties consistent with the resulting changes in electron distribution. Antiferromagnetic exchange in the outer ring of eight metal centers creates a ferrimagnetic subunit, which undergoes antiferromagnetic coupling to the central metal, leading to S=1/2 (3) and S2/2 (10) ground states. Two moderately intense absorptions are observed on oxidation of the Mn(II) grids in the visible and near-infrared (1000 nm, 700 nm), associated with charge transfer transitions (LMCT, IVCT respectively). Compound 1 crystallized in the monoclinic system, space group P2 1/n, with a=21.308(2) A, b=23.611(2) A, c=32.178(3) A, beta=93.820(2) degrees . Compound 3 crystallized in the tetragonal system, space group I, with a=b=18.44410(10) A, c = 24.9935(3) A. Compound 10 crystallized in the triclinic system, space group P, with a=19.1150(10) A, b=19.7221(10) A, c=26.8334(14) A, alpha=74.7190(10) degrees, beta=77.6970(10) degrees, gamma=64.7770(10) degrees. The facile oxidation of the Mn(II)9 grids is highlighted in terms of their potential use as molecular based platforms for switching and data storage.

Synthesis of multitopic verdazyl radical ligands. Paramagnetic supramolecular synthons.

[structure: see text] The syntheses of several verdazyl radical and diradicals containing pyridine-based multitopic coordination sites are described. These compounds were designed to be paramagnetic analogues of oligopyridine metallosupramolecular building blocks.

Verdazyl radicals as oligopyridine mimics: structures and magnetic properties of M(II) complexes of 1,5-dimethyl-3-(2,2'-bipyridin-6-yl)-6-oxoverdazyl (M = Mn, Ni, Cu, Zn).

The verdazyl radical 1,5-dimethyl-3-(2,2'-bipyridin-6-yl)-6-oxoverdazyl (3) was prepared, and its homoleptic metal complexes M(3)(2)(2+).2X(-) (5, M = Mn(II); 6, M = Ni(II); 7, M = Cu(II); 8, M = Zn(II); X = ClO(4), PF(6)) were characterized by single-crystal X-ray diffraction and variable-temperature magnetic susceptibility measurements. Relevant crystallographic parameters are as follows: 5, monoclinic space group Pna2(1), a = 18.755(4) A, b = 11.154(3) A, c = 16.594(4) A, alpha = 90.00 degrees, beta = 90.00 degrees, gamma = 90.00 degrees, V = 3471.4(13) A(3), and Z = 4; 7, triclinic space group Ponedblac;, a = 9.4638(18) A, b = 9.8442(19) A, c = 18.769(4) A, alpha = 103.746(3) degrees, beta = 92.925(3) degrees, gamma = 94.869(3) degrees, V = 1687.8(6) A(3), and Z = 2; 8, triclinic space group Ponedblac;, a = 9.4858(14) A, b = 9.7919(14) A, c = 18.889(3) A, alpha = 104.196(3) degrees, beta = 92.855(3) degrees, gamma = 94.216(3) degrees, V = 1692.1(4) A(3), and Z = 2. In all cases, the two verdazyl-based ligands bind almost perpendicular to each other in meridional positions, yielding pseudooctahedral metal complexes whose general structural features are strongly reminiscent of metal bis(terpyridine) complexes. The intramolecular metal-verdazyl magnetic exchange coupling is strongly ferromagnetic in 6 (J(Ni-vd) = +240 cm(-1)), and strongly antiferromagnetic in 5 (J(Mn-vd) = -93 cm(-1)). Complex 7 exhibits weak ferromagnetic coupling (J(Cu-vd) = -4.5 cm(-1)). Intramolecular radical-radical coupling in the zinc complex 8 was found to be weakly antiferromagnetic (J(vd-vd) = -8 cm(-1)). Intramolecular radical-radical exchange was generally weak in the four metal complexes, ranging from -10 cm(-1) (for 5) to +2 cm(-1) (for 7). The low-temperature magnetic behavior of 7 and 8 is complex, possibly arising from a combination of intra- and intermolecular interactions.

Synthesis and coordination chemistry of a water-soluble verdazyl radical. structures and magnetic properties of M(H2O)2(vdCO2)(2).2H2O (M = Co, Ni; vdCO2 = 1,5-dimethyl-6-oxo-verdazyl-3-carboxylate).

The synthesis of a verdazyl radical with a carboxylate substituent renders the radical highly soluble in water, thereby permitting the aqueous synthesis of Ni(II) and Co(II) verdazyl complexes which have been structurally and magnetically characterized.

Synthesis and Characterization of Verdazyl Radicals Bearing Pyridine or Pyrimidine Substituents: A New Family of Chelating Spin-Bearing Ligands.

The syntheses and characterization of two new 1,5-dimethyl-6-oxoverdazyl radicals bearing 2-pyridine and 4,6-dimethyl-2-pyrimidine rings as substituents are described. The radical precursors, the corresponding 1,2,4,5-tetrazanes, were prepared by condensation of the bis(1-methylhydrazide) of carbonic acid with the appropriate aromatic aldehyde. Oxidation of 3-(4,6-dimethyl-2-pyrimidyl)-1,5-dimethyl-1,2,4,5-tetrazane 6-oxide (7) with sodium periodate afforded 1,5-dimethyl-3-(4,6-dimethyl-2-pyrimidyl)-6-oxoverdazyl (4), which could be isolated and stored without decomposition. In contrast, attempts to oxidize the analogous 3-(2-pyridyl)-1,5-dimethyl-1,2,4,5-tetrazane 6-oxide (6) with periodate produced the 1,5-dimethyl-3-(2-pyridyl)-6-oxoverdazyl (3) which could not be isolated. However, oxidation of this tetrazane with benzoquinone produced the pyridylverdazyl 3 as a 1:1 complex with hydroquinone. This complex is indefinitely stable in the solid state and provides a means of long-term storage of the pyridylverdazyl. The electronic properties of both radicals have been characterized by EPR spectroscopy, cyclic voltammetry, and MNDO calculations. The radicals have a wide electrochemical window of stability (>1.8 V), and the EPR and computational studies indicate a large spin density residing on N2 and N4.