Martin T. Lemaire

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.

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.

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.

Progress and design challenges for high‑spin molecules

In this short critical review, selected examples of current (within the past two years) synthetic efforts toward the construction of high-spin molecules are explored, including the use of metal complexes containing stable free radical ligands, lanthanide or actinide complexes, and other coordination clusters, or a completely different approach, taking advantage of non-Heisenberg exchange in fully delocalized mixed-valence complexes (spin-dependent delocalization, SDD, or double exchange). A description of reported work in this regard is followed by a brief general discussion that highlights what the future may hold for high-spin molecule design.

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).

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.

Structure and magnetic properties of a nickel(II) complex of a tridentate verdazyl radical: strong ferromagnetic metal-radical exchange coupling

Magnetic susceptibility measurements on a structurally characterized nickel(II) complex of a tridentate verdazyl radical indicate very strong ferromagnetic Ni–verdazyl intramolecular exchange interactions (J > +200 cm−1).

Structure and magnetism of a verdazyl radical clathrate hydrate. Strong intermolecular magnetic interactions derived from π-stacking within ice-like channels

Deprotonation of 1,5-dimethyl-6-oxoverdazyl-3-carboxylic acid (2H) in acetonitrile/water affords crystals of the sodium salt of the carboxylate radial anion as a hydrated material (3). The solid state structure of 3 consists of a 2-D array of antiparallel stacked verdazyl carboxylate radical anions (2-) pillared between chains of sodium cations. These layers are separated by structurally ordered water layers. These layers consist of fused polygons based on OH⋯O hydrogen bonds that assemble into a complex 2-D hydrogen bonded network. The alternating layers are also linked via an orthogonal hydrogen bonded network. The overall structure resembles a 2-D ionic clathrate hydrate with radical stacks contained within channels. Analysis of temperature dependent magnetic susceptibility measurements reveal very strong antiferromagnetic exchange interactions (J = −290 cm−1) which can be ascribed to the 1-D stacks of radicals within the hydrate structure.

Iron(II) complexes containing thiophene- substituted ''bispicen'' ligands — Spin-crossover, ligand rearrangements, and ferromagnetic interactions

The synthesis and characterization of three new tetradentate “bispicen-type” ligands containing a substituted thiophene heterocycle are described [2,5-thienyl substituents = H (7), Ph (8), or 2-thi...

Spin-crossover in a homoleptic cobalt(II) complex containing a redox-active NNO ligand

We report the preparation of a divalent cobalt complex containing the ligand 1-(2-pyridylazo)-2-phenanthrol (papl). The complex is neutral and homoleptic containg two equivalents of the anion of papl. Variable temperature magnetic susceptibility measurements indicate a gradual spin-crossover between the S = 1/2 and S = 3/2 states of the complex. This is corroborated by single crystal X-ray diffraction experiments at 147 and 325 K, which reveal differences in the coordinate bond distances that are consistent with spin-crossover observed in other cobalt(II) complexes. The complex is redox-active and features intense, ligand-centered visible absorptions bands, which are completely changed in alcohol solvents (methanol, ethanol) and indicate oxidation of the complex to cobalt(III) in these polar, protic solvents.