N-silylated benzamidines: versatile building blocks in main group and coordination chemistry

Many kinds of radicals are stable enough to isolate, handle, and store without any special precautions. The diversity in molecular architectures of these stable radicals is sufficiently large that the common factors governing radical stability/persistence, geometric and electronic structure, association/dimerization preferences, and reactivity have generally not been well articulated or appreciated. This review provides a survey of the major classes of stable or persistent organic/organomain group radicals with a view to presenting a unified description of the interdependencies between radical molecular structure and properties.

What's new in stable radical chemistry?

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Packing Problems: High Z′ Crystal Structures and Their Relationship to Cocrystals, Inclusion Compounds, and Polymorphism

This review describes recent advances in the development of thiazyl radicals, particularly dithiadiazolyl (RCNSSN) and dithiazolyl (RCSNSCR) as molecule-based materials. Particular emphasis is placed on their potential applications as molecular conductors, magnets and switchable materials and incorporates an overview of the structure–property relationships required in these molecular systems.

Thiazyl radicals: old materials for new molecular devices

New synthetic methods for heterocyclic 1,3,2-dithiazolyl (DTA) radicals have been developed, and trends in the molecular spin distributions and electrochemical properties of a series of DTA radicals are reported. The crystal structures of [1,2,5]thiadiazolo[3,4-f][1,3,2]benzodithiazol-2-yl (TBDTA) and [1,3,2]pyrazinodithiazol-2-yl (PDTA) have been determined. The structure of TBDTA (at 293 and 95 K) contains two molecules in the asymmetric unit, each of which generates π-stacked arrays, one consisting of antiparallel chains of centrosymmetrically associated dimers, the other comprising parallel chains of unassociated radicals. The structure of PDTA (at 293 and 95 K) is simpler, consisting of slipped stacks of π-dimers. Variable-temperature magnetic susceptibility (χP) measurements on TBDTA indicate essentially paramagnetic behavior for the unassociated radical π-stacks over the range 5−400 K. By contrast PDTA is diamagnetic at all temperatures below 300 K, but between 300 and 350 K the value of χP follows...

Bistabilities in 1,3,2-dithiazolyl radicals.

The reactions of N,N,N[prime]-tris(trimethylsilyl)formamidine with ECl[sub 2] (E = S, Se) afford 1,2,3,5-dithia- and 1,2,3,5-diselenadiazolium chloride, which can be reduced with triphenylantimony to the corresponding dithia- and diselenadiazolyl radicals [HCN[sub 2]E[sub 2]]. The solid state structure of the cofacial dimer [HCN[sub 2]S[sub 2]][sub 2] has been determined by X-ray diffraction; crystals of [HCN[sub 2]S[sub 2]][sub 2] belong to the monoclinic space group P2[sub 1]/n, with a = 6.833(6), b = 16.463(4), c = 19.161(4) A, [beta] = 93.57(4)[degrees], FW = 210.30, Z = 12. The crystal structure consists of stacked dimers, with three dimers in the asymmetric unit. Along the stacking axis the mean intradimer S---S contact is 3.11 A; the mean interdimer contact is 3.76 A. Ab initio calculations with split-valence and larger basis sets have been employed to evaluate the structures and energies of both the gas phase radicals and their dimers. Minor changes in geometry are predicted upon association of the monomers; this relative structural invariance is consistent with low binding energies, the best predictions (including zero point vibrational energy corrections) being ca. 4 kcal/mol (for E = S) and ca. 10 kcal/mol (for E = Se). 30 refs., 6 figs., 9 tabs.

Prototypal 1,2,3,5-dithia- and 1,2,3,5-diselenadiazolyl [HCN2E2]. (E = S, Se) : molecular and electronic structures of the radicals and their dimers, by theory and experiment

Sublimation of 1,2,3,5-dithiadiazolyl in vacuo affords a triclinic phase of the dimer [HCN2S2]2. The crystals belong to the space group P¯1, a = 6.816(3), b = 13.940(2), c = 14.403(3) A, α = 116.830(14), β = 98.64(3), γ = 99.18(3)°, FW = 212.4 (for [HCN2S2]2·[N2]0.08) Z = 6. The crystal structure consists of stacked dimers, with three dimers per asymmetric unit. Pairs of asymmetric units, related by an inversion center, generate a pinwheel motif consisting of six dimers. The columnar structure associated with these pinwheels forms close-packed sets of “molecular tubes”. Cosublimation of the radical in the presence of iodine in the mole ratio (HCN2S2:I = 5:1) yields an iodine-doped hexagonal phase of composition [HCN2S2]6[I]1.1. Crystals of this material belong to the space group P61, a = b = 14.132(16), c = 3.352(5) A, FW = 128.20, Z = 6. The crystal structure consists of sixfold pinwheels in which the now evenly spaced HCN2S2 rings form a spiral about the 61 axis. The iodine atoms lie within the columnar cavity of the pinwheels in a disordered array wrapped tightly about the sixfold screw axis. The single-crystal conductivity of the doped material is 15 S cm-1 at room temperature. Raman spectroscopic and magnetic susceptibility measurements on the doped material are reported.

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Molecular Conductors from Neutral-Radical Charge-Transfer Salts: Preparation and Characterization of an Iodine-Doped Hexagonal Phase of 1,2,3,5-Dithiadiazolyl ([HCN2S2]∙)

C. D. Bryan

Papers

Prototypal 1,2,3,5-dithia- and 1,2,3,5-diselenadiazolyl [HCN2E2]. (E = S, Se) : molecular and electronic structures of the radicals and their dimers, by theory and experiment

Molecular Conductors from Neutral-Radical Charge-Transfer Salts: Preparation and Characterization of an Iodine-Doped Hexagonal Phase of 1,2,3,5-Dithiadiazolyl ([HCN2S2]∙)