David J. R. Brook

Synthesis of 1,5-diisopropyl substituted 6-oxoverdazyls

1,5-Diisopropyl-6-oxo-verdazyl free radicals were synthesized via the condensation of BOC protected isopropyl hydrazine with phosgene, deprotection with aqueous HCl, condensation with aldehydes to form tetrazanes and finally oxidation to give the free radicals. The introduction of isopropyl groups results in free radicals that show greater solubility in a variety of solvents and are more stable than their methyl substituted counterparts. ESR shows reduced hyperfine coupling to the isopropyl methine hydrogens consistent with this hydrogen being in the plane of the verdazyl ring.

Strong ferromagnetic metal–ligand exchange in a nickel bis(3,5-dipyridylverdazyl) complex

A new 1,5-dipyridyl verdazyl, synthesized from the corresponding dipyridyl hydrazone, coordinates nickel(ii) to form a structurally characterized, pseudooctahedral complex analogous to Ni(terpy)(2)(2+). The unusually short Ni-verdazyl distance results in strong ferromagnetic exchange (J(Ni-rad) = +300, J(rad-rad) = +160 cm(-1)) between all three paramagnetic species along with a metal-ligand charge transfer band in the electronic spectrum.

Structure-Property Relationships of Stable Free Radicals: Verdazyls with Electron-Rich Aryl Substituents

Substitution of the 3 position of 6-oxoverdazyl free radicals with electron-rich arylamines, phenols, and aryl ethers elicits changes in the UV-vis spectra and in the pK(a) of the aryl substituents consistent with the verdazyl being electron withdrawing. The pK(a) of substituents is decreased: in 80% methanol phenols 3a and 3b have pK(a) of 10.4 and 10.9, respectively, while the ammonium ion from protonation of 3j has pK(a) = 2.4. On the basis of these measurements, Hammett parameters for the verdazyl have been estimated: sigma(p)(-) = +0.48 and sigma(m) = +0.27. The longest wavelength band in the visible spectrum is red-shifted with increasingly electron-rich aromatic rings and with increasingly polar solvents, consistent with a transition from the highest fully occupied orbital to the radical SOMO. Exceptions occur when additional interactions occur between verdazyl and substituent; hydrogen bonding in the case of 3c and steric interference for 3f. Measurements such as ESR and electrochemistry that are dependent largely on the SOMO are relatively insensitive to changes in substituent.

Carbonyl and Thiocarbonyl Stabilized 1,4-dihydropyrazines: synthesis and characterization

Three analogues of the 1,4-dihydropyrazine, 3,4,7,8-tetrahydro-4,4,8,8-tetramethyl-2,6-dioxa-4a,8a-diazaanthracene-1,5-dione (DDTTA), have been synthesized. 2,4,4,6,8,8-Hexamethyl-3,4,7,8-tetrahydro-2,4a,6,8a-tetraazaanthracene-1(2H),5(6H)-dione (HTTA) is synthesized by chlorination of the previously reported 5,6-dihydro-1,3,5,5-tetramethylpyrazin-2(1H)-one with tert-butyl hypochlorite and self condensation of the resulting α-chloromethylimine in the presence of diisopropylethylamine in dimethylformamide (DMF). Thioxo derivatives 3,4,7,8-tetrahydro-4,4,8,8-tetramethyl-5-thioxo-2,6-dioxa-4a,8a-diazaanthracen-1-one (DDTTA–S) and 3,4,7,8-tetrahydro-4,4,8,8-tetramethyl-2,6-dioxa-4a,8a-diazaanthracene-1,5-dithione (DDTTA–S2) have been synthesized by direct thionation of DDTTA with phosphorus pentasulfide in pyridine. All three molecules have been characterized spectroscopically. In addition the crystal structure of HTTA has been determined. Radical cations obtained by one electron oxidation of the dihydropyrazines have been characterized by electron spin resonance spectroscopy.

Free radical complexes of copper(I): geometry and ground state

Abstract DFT calculations were performed on the singlet and triplet states of a series of copper(I)–bis(radical) complexes. For a copper(I)–bis(iminonitroxide) complex, the results are consistent with experimental data, predicting a triplet ground state and an excited singlet at 38.7 cm−1. For two related copper(I)–bis(verdazyl) complexes, the calculations predict a triplet ground state and singlet excited states at 58.6 and 34.9 cm−1, respectively; however both the minimized structure and singlet–triplet separation are inconsistent with experimental data. We suggest that the difference between theory and experiment is a result of intermolecular interactions within the crystal lattice.

Coordination Chemistry of Verdazyl Radicals

The past 17 years have seen the growth of the coordination chemistry of verdazyls—stable free radicals first reported in 1963. Though verdazyls are weakly basic ligands, the ability to synthesize derivatives with chelating substituents, along with the steric similarity of verdazyls to aromatic azine ligands, has resulted in a variety of coordination compounds. Coordination compounds have been reported with Mn2+, Co2+, Ni2+, Cu+, Cu2+, Zn2+, Ru2+, Ag+, Cd2+, Tb3+, Dy3+, Gd3+ and Hg2+. These studies have resulted in observations of strong ferromagnetic exchange (up to 400 cm−1), ligand-based redox processes, and non-innocent behavior. On the whole, verdazyls are weakly basic ligands in which the half-filled pi orbital can act as either an electron acceptor or an electron donor. The richness of the metal-verdazyl interaction, along with the structural diversity of verdazyls, provides many further opportunities for novel chemistry, as well as fertile ground for the introduction of undergraduates to research. GRAPHICAL ABSTRACT

Oxidation of Electron Donor-Substituted Verdazyls: Building Blocks for Molecular Switches

Species that can undergo changes in electronic configuration as a result of an external stimulus such as pH or solvent polarity can play an important role in sensors, conducting polymers, and molecular switches. One way to achieve such structures is to couple two redox-active fragments, where the redox activity of one of them is strongly dependent upon environment. We report on two new verdazyls, one subsituted with a di-tert-butyl phenol group and the other with a dimethylaminophenyl group, that have the potential for such behavior upon oxidation. Oxidation of both verdazyls with copper(II) triflate in acetonitrile gives diamagnetic verdazylium ions characterized by NMR and UV-vis spectroscopies. Deprotonation of the phenol-verdazylium results in electron transfer and a switch from a singlet state to a paramagnetic triplet diradical identified by electron spin resonance. The dimethylaminoverdazylium 9 has a diamagnetic ground state, in line with predictions from simple empirical methods and supported by density functional theory calculations. These results indicate that verdazyls may complement nitroxides as spin carriers in the design of organic molecular electronics.

Synthesis and structure of di-μ-bromo-bis[(1,5-dimethyl-6-oxo-3-(2-pyridyl)verdazyl)copper(I)]

Reaction of the verdazyl precursor 1,4,5,6-tetrahydro-2,4-dimethyl-6-(2′-pyridyl)-1,2,4,5-tetrazin-3(2H)-one (pvdH3) with oxygen and copper(I) bromide in acetonitrile results in precipitation of the crystalline diradical di-μ-bromo-bis[(1,5-dimethyl-6-oxo-3-(2-pyridyl)verdazyl)copper(I)]. The solid shows a slightly anisotropic ESR spectrum and magnetic susceptibility measurements indicate weak antiferromagnetic exchange between the free radicals, consistent with the assignment as a copper(I) ion coordinated to a free radical.

Spin distribution in copper(I) phosphine complexes of verdazyl radicals

Copper(I) forms mixed ligand coordination compounds with the stable, paramagnetic bipyridine analogue, 1,5-dimethyl-3-(2-pyridyl)-6-oxoverdazyl (pyvd) and a variety of monodentate and bidentate phosphine ligands. These compounds were characterized in solution by titration, UV-vis spectra, ESR spectra and electrospray mass spectrometry. Coordination of the phosphine gives a metal–ligand charge transfer transition in the UV-vis that is red shifted by more electron donating phosphines. ESR indicates that spin density on copper increases, both with strongly electron donating phosphines and with weakly basic phosphite ligands. This can be explained by the presence of two different orbital interactions: with more donating phosphines the interaction is predominantly through the filled copper(I) d-orbitals, but with weakly donating phosphite ligands the interaction is through an empty copper(I) p-orbital. The differing spin transfer mechanisms may have implications in the design of molecular magnetic systems.

Molecular motion in zinc hydrazone grid complexes

Four new grid forming hydrazone ligands substituted with methoxy and dimethylamino groups were synthesised. Combination of these ligands with zinc triflate in acetonitrile resulted in self-assembly to form grids as indicated by 1H NMR and ES-MS. 1H NMR also showed thermally induced rotation of the intercalated phenyl ring in both the new grids, and in three previously reported grid compounds. Of these seven, five were amenable to study by variable temperature 1H NMR. Though the observed rates varied by more than an order of magnitude depending upon ligand structure and level of deprotonation, activation energies were similar (∼60 kJ/mol) for all complexes studied. This suggests a model in which dissociation of the central pyrimidine ligand precedes phenyl group rotation with an enthalpy of dissociation near zero. The rate of rotation of the phenyl ring increases with an introduction of electron-donating substituents on the phenyl ring, possibly due to an increased repulsion between π systems.