Robin G. Hicks

What's new in stable radical 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.

High-temperature metal–organic magnets

For over two decades there have been intense efforts aimed at the development of alternatives to conventional magnets, particularly materials comprised in part or wholly of molecular components. Such alternatives offer the prospect of realizing magnets fabricated through controlled, low-temperature, solution-based chemistry, as opposed to high-temperature metallurgical routes, and also the possibility of tuning magnetic properties through synthesis. However, examples of magnetically ordered molecular materials at or near room temperature are extremely rare, and the properties of these materials are often capricious and difficult to reproduce. Here we present a versatile solution-based route to a new class of metal–organic materials exhibiting magnetic order well above room temperature. Reactions of the metal (M) precursor complex bis(1,5-cyclooctadiene)nickel with three different organics A—TCNE (tetracyanoethylene), TCNQ (7,7,8,8-tetracyanoquinodimethane) or DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone)—proceed via electron transfer from nickel to A and lead to materials containing Ni(II) ions and reduced forms of A in a 2:1 Ni:A ratio—that is, opposite to that of conventional (low Curie temperature) MA2-type magnets. These materials also contain oxygen-based species within their architectures. Magnetic characterization of the three compounds reveals spontaneous field-dependent magnetization and hysteresis at room temperature, with ordering temperatures well above ambient. The unusual stoichiometry and striking magnetic properties highlight these three compounds as members of a class of stable magnets that are at the interface between conventional inorganic magnets and genuine molecule-based magnets.

Switchable materials: A new spin on bistability

Many different kinds of switchable molecules and materials are based on transition metal ions, but similar properties are also possible in organic materials. Now, two separate studies reveal new insights into the ability of organic radicals to associate reversibly and cooperatively in the solid state, and in so doing create bistable, hysteretic materials.

Formazans as β-diketiminate analogues. Structural characterization of boratatetrazines and their reduction to borataverdazyl radical anions

Formazans react with boron triacetate to produce boratatetrazines, which can be reduced to yield borataverdazyl radical anions--the first boron containing verdazyl radicals.

Redox-active, near-infrared dyes based on ‘Nindigo’ (indigo-N,N′-diarylimine) boron chelate complexes

Reactions of indigo-N,N′-diarylimine (‘Nindigo’) derivatives with BF3·Et2O give mono- or bis-BF2 chelate complexes 2 or 3 respectively. The product distribution between 2 and 3 is sensitive to the auxiliary base and solvent. Although the bis-BF2 complexes 3 are isolable, they gradually decompose in solution to the corresponding mono-BF2 species 2; this process is accelerated by water. The instability of 3 is believed to be due to ring stain effects based on structural analyses of 2. Electrochemical studies of 2 reveal one quasi-reversible oxidation process and two irreversible reductions, whereas derivatives of 3 possess a reversible oxidation and two sequential reversible reductions. The electronic spectra of 2 and 3 contain intense (e ∼ 3 × 104 M−1 cm−1) long-wavelength absorptions near 650 nm and 750 nm respectively. Both series of compounds are weakly emissive in the near-infrared. Time-dependent DFT calculations reveal the electronic transitions to be π–π* in nature.

Magnetostructural studies of copper(II)–verdazyl radical complexes

The synthesis, structures, and magnetic properties of several Cu(II) complexes of verdazyl radicals are presented. Reactions of chelating verdazyl radicals with either CuCl2·2H2O or Cu(hfac)2·2H2O produced 1 ∶ 1 Cu ∶ verdazyl complexes with either chloride or hfac ancillary ligands. Structural characterization reveals that the CuCl2 complexes of N,N′-dimethyl-3-(2-pyridyl)-6-oxoverdazyl or N,N′-bis(isopropyl)-3-(2-pyridyl)-6-oxoverdazyl have pseudo-square pyramidal copper ions with verdazyl rings bound in equatorial positions, while the Cu(hfac)2 complex of N,N′-dimethyl-3-(N-methyl-2-imidazolyl)-6-oxoverdazyl is Jahn–Teller distorted pseudo-octahedral and has the verdazyl nitrogen axially bound. Variable temperature magnetic susceptibility studies reveal that equatorially bound verdazyls are strongly antiferromagnetically coupled, while the axially bound radicals are weakly ferromagnetically coupled. Intermolecular magnetic interactions are also an important component of the overall magnetism in these systems.

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.

Transition Metal Complexes of 3-Cyano-and 3-Nitroformazans

The synthesis and characterization of six transition metal complexes of 3-cyano- and 3-nitroformazans are described. Three different formazans were reacted with nickel(II) to produce complexes with bidentate formazan ligands. Mononuclear NiL2 (L = deprotonated formazan) or binuclear hydroxo-bridged (LNi)2(mu-OH) 2 species were produced depending on the steric bulk on the formazan N-aromatic substituents. 1,5-Bis(2-methoxyphenyl)-3-cyanoformazan acts as a tetradentate monoanionic ligand in a copper(II) complex, whereas the analogous 1,5-bis(2-hydroxyphenyl)-3-cyanoformazan binds as a trianion in a tetradentate manner to Fe(III) and Co(III). Crystal structures-the first examples of metal complexes of cyano- or nitroformazans-as well as the electronic spectra of the complexes are discussed in relation to each other as well as that of the uncoordinated formazans.

Electronic structure investigations of neutral and charged ruthenium bis(β-diketonate) complexes of redox-active verdazyl radicals.

The electronic structures of (Vd)Ru(LX)(2) complexes (Vd = 1,5-diisopropyl-3-(2-pyridyl)-6-oxoverdazyl radical; LX = acac or hfac) as neutral, cationic, and anionic species have been investigated experimentally and computationally to probe the interplay between the ruthenium ion and the redox-active verdazyl ligand. The cationic complexes were prepared by oxidation of the corresponding neutral species with silver(I) salts. The hfac-based anionic complex was synthesized by reduction of the neutral species with cobaltocene, but the anionic acac analogue could not be prepared. Experimental (X-ray structures, electronic spectra) and computational (TD-DFT (PCM)) studies reveal that the expression of redox activity of the ligand and metal moieties is highly sensitive to the nature of the ancillary ligands on ruthenium. In the hfac series, the cationic, neutral, and anionic complexes can, respectively, be adequately described as Ru(II) complexes of a coordinated verdazyl cation, neutral radical, and anion. However, the more electron-donating acac coligands facilitate a stronger interaction between ruthenium and verdazyl via Ru(d) to Vd(π*) backbonding which is dependent on the overall charge of the complex and has the net effect of creating a high degree of metal-ligand covalency. Studies on the two cationic complexes reveal further distinctions between the acac- and hfac-containing systems: whereas the former has a significant open-shell singlet contribution to the complex ground state, this open-shell formulation is a minor component of the latter.

Synthesis and Characterization of 3-Cyano- and 3-Nitroformazans, Nitrogen-Rich Analogues of β-Diketimine Ligands

The synthesis and characterization of several formazans containing strong electron-withdrawing substituents (cyano and nitro) in the 3 position of the ligand backbone are described. Reactions of aryldiazonium cations with the conjugated bases of either cyanoacetic acid or nitromethane lead to 1,5-diaryl-3-cyano- or 3-nitroformazans, respectively. When these reactions are carried out in aqueous conditions, the range of aromatic groups is limited by the stability of the diazonium salt. However, 3-nitroformazans containing bulky substituents on the nitrogen atoms (2,6-dimethylphenyl, 2,4,6-trimetyhlphenyl, 2,6-diisopropylphenyl, and 3,5-ditert-butylphenyl) could be made by performing the reactions under nonaqueous and anhydrous conditions. NMR and electronic spectroscopic studies indicate that the 3-nitroformazans exist exclusively as closed ( trans-syn, s-cis) isomers whereas the 3-cyanoformazans exist as mixtures of isomers which are substrate-dependent. The crystal structures of five of the formazans are presented: two 3-nitroformazans, both of which are closed, and three 3-cyanoformazans, two of which are closed and one of which adopts an open ( trans-syn, s-trans) structure. Solid state (diffuse reflectance) spectroscopy has been employed to ascertain the isomeric preferences of the other formazans which could not be crystallographically characterized.