Nitroxide Biradicals Research Articles

Effect of solvent nature on spin exchange in rigid nitroxide biradicals

Intramolecular electron spin exchange as a function of temperature and solvent viscosity and polarity has been studied by X-band electron paramagnetic resonance (EPR) spectroscopy in two rigid nitroxide biradicals existing in one spatial conformation only. Temperature variations of the isotropic hyperfine splitting constanta and exchange integral value |J/a| were measured from EPR spectra and subsequently analyzed. The interaction of polar solvent molecules with >N-O fragments of nitroxide groups led to a slight decrease of the |J/a| value with the increase of temperatureT. In contrast, the interaction of polar solvent molecules with functional groups inside the bridge resulted in a noticeable increase of |J/a| vs.T. In the last case, a coverse relationship between the values of |J/a| and the hyperfine splitting constanta has been observed for solvents with different polarity.

Pyridine-substituted nitronyl nitroxide biradicals: a triplet (S = 1) ground state lasting out N-methylation

A 3,5-pyridine-substituted biradical of nitronyl nitroxide (1) has been designed, synthesized and the magnetic properties fully characterized. The ground-state spin multiplicity of 1 has been found to be triplet (S = 1) with a singlet–triplet energy gap of 2J/kB = 40 K from magnetic susceptibility measurements on a magnetically diluted system dispersed in organic polymer films. The 3,5-substituted pyridine 1 has a hydrogen-accepting site which is more accessible to hydrogen donors than previously known biradicals with sterically hindered 2,6-pyridine frameworks. N-Methylation of 1 has yielded a stable cationic species in a trifluoromethanesulfonate salt (2+·TfO−). The ground state of the cation 2+ has been found to be triplet as well with 2J/kB = 32 K from magnetic susceptibility measurements for magnetically diluted films. The magnetic susceptibility of neat crystalline solids of 1 and 2+·TfO− has been explained by Heisenberg exchange coupling models based on their X-ray crystal structures. It is well known that the energy preference of a high-spin ground state for m-phenylene, or m-xylylene, coupling units is disturbed in such cases as the π-conjugation is affected by heteroatomic substitution, an ionic charge, or molecular conformation. The present experimental results show that the high-spin preference in 1 and 2+ is little influenced by the heterocycle or the ionic charge. Intermolecular noncovalent bonds such as hydrogen bonding and electrostatic interactions are a driving force for crystallization of open-shell molecules in a controllable manner. The ground-state triplet biradicals serve as building blocks for molecule-based magnets of S > 1/2 based on intermolecular noncovalent bonding architecture.

Regularities of the Spin Exchange Coupling Through a Bridge in Nitroxide Biradicals

AbstractFor Abstract see ChemInform Abstract in Full Text.

Synthesis and crystal structure of a new copper(II) binuclear complex bridged by the reduced derivative of a nitronyl nitroxide biradical

A new copper(II) binuclear complex, [Cu(hfac)2]2(IMH)2Ph(OMe)2 ((IMH)2Ph(OMe)2 = 2,5-dimethoxyl-1,4-bis(4′,4′,5′,5′-tetramethylimidazoline-3′-oxide)benzene, hfac = hexafluoroacetonate), bridged by a reduced derivative of a nitronyl nitroxide biradical has been synthesized and characterized by X-ray crystallography. The complex crystallizes in the monoclinic space group P21/n, with a = 10.002(14), b = 19.950(3), c = 14.504(2) Å, β = 108.842(3)° and Z = 2. The structure refined to a final R value of 0.0644. The complex contains two copper(II) ions bridged by a reduced derivative of a nitronyl nitroxide biradical with a Cu ··· Cu separation of 8.430(2) Å.

Regularities of the spin exchange coupling through a bridge in nitroxide biradicals

The current state, achievements, problems and prospects of the intramolecular electron spin exchange interaction as a function of the nitroxide biradical composition and structure, type of the radical ring, temperature and the solvent nature are considered on the basis of the literature data and the results of our own experiments.

Intramolecular spin exchange in flexible short-chain biradicals in solutions with various viscosity

Intramolecular electron spin exchange as a function of temperature and solvent viscosity has been studied by X-band electron paramagnetic resonance (EPR) spectroscopy in two short-chain flexible nitroxide biradicals. Certain thermodynamic parameters of the conformational rearrangements were calculated from the EPR spectra. The process of spin exchange in these biradicals dissolved in six different alcohols is compared with that in a nonpolar solvent (toluene) and with the thermodynamic characteristics of the solvents. No correlation is found on a macroscopic level (solvent viscosity) but on a microscopic level (solvent longitudinal relaxation times) a distinct correlation is seen. The original results are compared with literature data.

Study on the heteroatom influence in pyridine-based nitronyl nitroxide biradicals with phenylethynyl spacers on the molecular ground state.

Novel pyridine-based nitronyl nitroxide (NIT) biradicals, 3,5-bis[4-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)phenylethynyl)]pyridine (1) and 2,6-bis[4-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)phenylethynyl)]pyridine (2), and monoradicals, 4-(5-bromopyridine-3-ylethynyl)-1-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)benzene (3), 4-trimethylsilylethynyl-1-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)benzene (4), and 4-trimethylsilylethynyl-1-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)pyridine (5), were synthesized and investigated by ESR and UV-vis spectroscopy. The solution EPR measurements of the biradicals gave well-resolved, nine-line spectra with exact half line spacing as compared to monoradicals (giso = 2.0067) with isotropic line spacing /aN/= 7.36 G. This indicates strong, intramolecular exchange coupling (J >> 7 x 10(-4) cm(-1); J/aN >> 1) of the biradicals with in the limit of EPR. The temperature dependence on the Deltams = +/-2 signal intensity of biradicals follow Curie behavior down to 4 K ascertaining the triplet ground state or its near-degeneracy with the singlet state. UV-vis studies of 1-5 show characteristic differences in the extinctions of n-pi transitions around 600 nm. Both biradicals 1 and 2 were crystallized in monoclinic space groups C2/c and P2(1)/a with the intraradical distances 1.54 and 1.47 nm, respectively. Computational studies of the biradicals 1, 2, and 1,3-bis[4-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)phenylethynyl)]benzene (T) were performed by the AM1/CAS(8,8) method to calculate the singlet-triplet (DeltaEST) energy difference and the spin density distribution. Results show that the position of the pyridyl nitrogen in 1 and 2 in comparison with T does not alter the triplet ground-state spin multiplicities supporting the obtained experimental results.

Meta-Phenylene-bridged bis(imino nitroxide) biradicals as potential high-spin ligands

Frozen-solution ESR spectra of 4-hydroxy-1,3-phenylenebis(imino nitroxide) (1) and 2-hydroxy-5-methyl-1,3-phenylenebis(imino nitroxide) (2) in toluene showed fine structures due to the triplet states (imino nitroxide stands for 2-substituted 4,4,5,5-tetramethylimidazolin-3-yloxyl). The temperature dependence of the Δms=±2 signal intensity for 1 followed the Curie law, suggesting that 1 has a ground triplet state. On the other hand, the triplet signal intensity of 2 did not obey the Curie law. The intramolecular magnetic coupling seems to depend on the twist angle between the imino nitroxide group and benzene ring. Complexation with Cu(CH3CO2)2·H2O in ethanol gave a mononuclear complex, Cu(L1)2, where L1 is the anionic form derived from 1. The X-ray diffraction study of Cu(L1)2 revealed that the central copper(II) ion had a twisted square plane; the dihedral angle between two chelating OCuN planes was 45°. The magnetic measurements of Cu(L1)2 indicated the presence of intramolecular ferromagnetic interaction, which is ascribed mainly to a ground quartet formation at the directly bonded radicalcopperradical moiety with Jradicalcopper/kB=+132±5 K, where the Heisenberg spin Hamiltonian is defined as H=−2JSiSj.

Use of Sodium Triacetoxyborohydride in the Synthesis of Nitroxide Biradicals.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Synthesis and properties of Mn(hfac) 2 complexes of isoindoline nitroxide radicals

The Mn(hfac) 2 (hfac=hexafluoroacetylacetonate) complexes of one isoindoline nitroxide monoradical, 1, TMIO (1,1,3,3-tetramethylisoindolin-2-yloxyl) and two isoindoline nitroxide biradicals, 2 (bis(1,1,3,3-tetramethylisoindolin-2-yloxyl-5-yl)methane) and 3 (1,1-bis(1,1,3,3-tetramethylisoindolin-2-yloxyl-5-yl)ethane) have been prepared and characterized. X-ray crystallography was performed on all complexes revealing that the [Mn(hfac) 2 2] n complex exists as linear chains while the [Mn(hfac) 2 3] 2 complex exists as cyclic units comprised of two biradical units and two Mn(hfac) 2 units. Magnetic measurements revealed reasonably strong Mn–nitroxide antiferromagnetic exchange interactions ( J) in the range −162 to −214 K which suggests that isoindoline nitroxides are useful candidates for the preparation of ferrimagnetic materials based on Mn(hfac) 2. The magnetic data also revealed that the biradicals exhibit weak antiferromagnetic intramolecular exchange interactions.

Use of sodium triacetoxyborohydride in the synthesis of nitroxide biradicals

A family of nitroxide biradicals was synthesized for the purpose of creating terminal groups with high relaxivity for linkage to dendrimers for use as targeted MR contrast agents for articular cartilage. The structure of the nitroxide biradicals strongly influenced their MR relaxivities. The addition of a carbon atom between the nitroxyl rings and the bridging nitrogen increased the relaxivity of the nitroxide biradical by ∼14% over what was expected by doubling the relaxivity of the monomeric nitroxide. Conversely in nitroxide biradicals without the additional carbon atoms, steric hindrance restricted the relaxivity to ∼35% below what was expected by doubling the relaxivity of the monomeric nitroxide. Nitroxide biradicals that have a carbon atom incorporated between the nitroxyl rings and the bridging nitrogen exhibit MR relaxivity values ∼223% greater than that of the corresponding monomeric nitroxide. By linking these nitroxide biradicals to a dendrimer, it should be possible to synthesize targeted magnetic resonance contrast agents with higher relaxivities than dendrimer-linked nitroxides with single nitroxyl rings as terminal groups. These dendrimer-linked dinitroxides should be capable of differentiating normal and abnormal articular cartilage on the basis of the glycosaminoglycan concentration.

Synthesis, crystal structure and magnetic properties of a novel nitroxide biradical. Theoretical investigation of the exchange mechanisms

The novel nitroxide biradical 1,8-bis(3-ethinyl-2,2,5,5-tetramethyl-3-pyrroline-1-oxyl)-naphthalene was synthesized and its structure and magnetic properties were investigated. Using SQUID measurements an antiferromagnetic exchange coupling of 2 J=−3.5 K was evaluated. Temperature-dependent measurements of the half-field EPR signal intensity and broken symmetry DFT computations confirm this result. To unravel the mechanisms of the intramolecular exchange interaction calculations on model systems were performed. These revealed a strong ferromagnetic through-bond interaction via the 1,8-substituted naphthalene bridge and a competing strong antiferromagnetic through-space interaction via the acetylene groups. Both interactions are of the same order of magnitude leading to the weak overall coupling observed.

Electrospray mass spectrometry of stable iminyl nitroxide and nitronyl nitroxide free radicals.

Electrospray ionization (ESI) mass spectra have been recorded for a range of substituted nitronyl nitroxide and iminyl nitroxide monoradicals and biradicals. Secondary species formed in the ESI source were observed as the dominant ions in both the iminyl nitroxide and nitronyl nitroxide spectra. Daughter ion spectrometry was used to establish fragmentation mechanisms for the nitronyl nitroxide and iminyl nitroxide moieties as well as the secondary species under ESI conditions.

Electron Dipole–Dipole ESEEM in Field-Step ELDOR of Nitroxide Biradicals

The use of a rapid stepping of the magnetic field for investigation of electron dipole–dipole ESEEM in pulsed X-band ELDOR is described. The magnetic field jump, synchronized with a microwave pumping pulse, is positioned between the second and the third pulses of the stimulated echo pulse sequence. This echo is measured as a function of the delay between the first and the second pulses. The data are analyzed for a Fourier transform resulting in a Pake resonance pattern. To remove the electron-nuclear contributions to ESEEM, time traces with pumping were divided by those without. This resulted in complete elimination of electron-nuclear contributions, which is seen from the absence of peaks at nuclear frequencies and the similarity of results for protonated and deuterated solvents. For increasing the electron–electron modulation depth, a scanning of the magnetic field during the microwave pumping is proposed. The interspin distances and their distribution are determined for two long-chained (ca. 2 nm) nitroxide biradicals in glassy toluene and in frozen nematic liquid crystal 4-cyano-4′-pentyl-biphenyl. For the latter solvent, the alignment of the axis connecting two nitroxides in biradicals is quantitatively analyzed.

Unique Behavior of Nitroxide Biradicals in the Controlled Radical Polymerization of Styrene

The use of various binitroxides, with both radical sites of similar reactivity, as mediator in the controlled radical polymerization of styrene was examined and the rate constants of the reactions involved were determined at 130 °C. Typical features of a controlled radical polymerization were observed in the early stage of the polymerization, leading to the formation of two-arm macromolecules containing the binitroxide at the core. At higher conversion, however, continuous decomposition reaction resulted in a break of the two-arm macromolecules into a dead chain having an unsaturation end group and a living chain capped by a modified binitroxide. The latter had the free nitroxide site inactivated by conversion into hydroxylamine. The extent of this side reaction was much larger than in classical nitroxide-mediated controlled radical polymerization of styrene. This feature was assigned to the structure of the growing chains. As they contain the binitroxide at the core, the activation reaction produces a pr...

Structure and Magnetic Property of a Novel Nitronyl Nitroxide Biradical

AbstractA new biradical of nitronyl nitroxide, 1,3‐[di‐2‐(4,4,5,5‐tetramethyl‐4,5‐dihydro‐1H‐imidazolyl‐1‐oxyl‐3‐oxide) phenoxy]propane [BNITPhOPr (1)], was prepared and characterized. The temperature dependence of the magnetic susceptibility values were observed for 1 in the range of 2–300 K. The observed data was successfully simulated giving the exchange integral J = −0.47 cm−1, θ = −0.94 K. This result indicates a weak antiferromagnetic spin exchange interaction between the biradicals.

Electron dipole–dipole interaction in ESEEM of nitroxide biradicals

Fourier transform of the standard primary electron spin echo envelope modulation (ESEEM) of nitroxide biradicals in organic glass shows a peak ascribed to electron–electron dipole–dipole interaction. The dipolar couplings obtained in imidazoline biradical and double spin-labeled peptide Trichogin GA IV are in good agreement with pulsed electron–electron double resonance (PELDOR) data. The intensity of this peak is increased substantially in a new three-pulse relaxation-induced dipolar modulation enhancement (RIDME) experiment. This experiment involves the non-resonant partner spin in the formation of ESEEM via a relaxation-induced change of the local dipolar field experienced by the resonant spin.

Magnetic properties of phenol-substituted nitronyl nitroxide biradicals as building blocks of organic salt ferrimagnets

A practical difficulty in constructing organic molecule-based ferrimagnetics is co-crystallization of two distinct molecules with different spin quantum numbers. Coulombic energy between cationic and anionic molecules in organic salts is a promising driving force for co-crystallization. We investigated magnetic properties of four kinds of phenoxide-substituted nitronyl nitroxide biradical salts; a potassium and a sodium salt of 2,4-bis(1-oxyl-3-oxido-4,4,5,5-tetramethyl-2-imidazolin-2-yl)-phenol ( 1a and 1b), and a potassium and a sodium salt of 4-methyl-2,6-bis(1-oxyl-3-oxido-4,4,5,5-tetramethyl-2-imidazolin-2-yl)-phenol ( 2a and 2b). For 1a and 2a, it was found from the analysis of magnetic susceptibility in the solid state and ESR spectra in diluted frozen solutions that the ground-state spin multiplicities changed from triplet ( S=1) into singlet ( S=0) on ionization. For 1b and 2b, magnetic susceptibility in the solid state suggested the weakening of their intramolecular ferromagnetic interaction or the conversion of the ground-state spin multiplicities into singlet.

Phenol-substituted nitronyl nitroxide biradicals with a triplet (S = 1) ground state

The crystal structures of two kinds of nitronyl nitroxide biradical with a phenolic substituent, 2,4-bis(1-oxyl-3-oxido-4,4,5,5-tetramethyl-2-imidazolin-2-yl)phenol (1a) and 4-methyl-2,6-bis(1-oxyl-3-oxido-4,4,5,5-tetramethyl-2-imidazolin-2-yl)phenol (2a) have been solved. The biradical 1a belongs to the monoclinic system with space group P21/n, a = 11.806(4) Å, b = 25.330(5) Å, c = 7.337(7) Å, β = 104.65(4)°, and Z = 4, while the biradical 2a to the orthorhombic system with Pbca, a = 20.206(4) Å, b = 40.405(4) Å, c = 11.888(3) Å, and Z = 8. From the magnetic susceptibility in the crystalline solid state, both 1a and 2a are found to have a triplet (S = 1) ground state with the intramolecular ferromagnetic interaction of 2J/kB = 26.0 ± 0.5 K for 1a and 12 ± 1 K for 2a. The triplet ground states are confirmed by the EPR measurements on the isolated molecules in diluted glassy solutions. The singlet–triplet energy gaps of the biradicals with a phenolic hydroxy substituent are found to be reduced as compared with a parent non-substituted biradical.

CIDEP of fullerene C 60 biradical bisadducts by intramolecular triplet–triplet quenching: a novel spin polarization mechanism for biradicals

[60]fullerene bisadducts containing the C 2 ν -symmetrical 2,2,5,5-tetramethylpyrrolidine-1-oxyl moiety have been synthesized. Their EPR spectrum is typical of nitroxide biradicals with electron exchange interaction J of the same order of the nitrogen hyperfine coupling. From the EPR spectrum the absolute value of J was obtained, while the sign was measured by ENDOR. When the derivatives are photoexcited strong spin polarization is observed with A/E/A/E or E/A/E/A patterns, according to a positive or a negative sign of J. This behavior is accounted for by an intramolecular triplet–triplet interaction. The analogy with the polarization of transient spin correlated radical pairs is pointed out.