Imino Nitroxide Research Articles

Electron Spin Resonance Spin Probe Technique for Investigating Non‐TEMPO Radicals Dispersed in Nanospaces of a Crystalline Zn Complex

An ESR spin probe technique with non‐TEMPO radicals, such as nitronyl nitroxide (NN), benzonitronyl nitroxide (BzNN), and iminonitroxide (IN) radicals, was used for a porous metal‐organic framework (MOF), [(ZnI2)3(TPT)2] (ZnTPT; TPT = tris(4‐pyridyl)‐1,3,5‐triazine), at room temperature. The principal values of g and hyperfine coupling (A) tensors estimated from spectral reproduction were different from those for organic matrices for some of these radicals. These results indicate that host‐guest interactions occur between the ZnTPT matrix and guest radicals. Thus, when using NN, BzNN, and IN radicals as spin probes for a porous MOF, the interaction between the metal atoms or organic ligands in host materials and guest radicals should be considered. The experimental ESR spectra for the derivatives of NN or BzNN radicals were reproduced only by the rigid‐limit component in the ESR time scale. However, those for the derivatives of IN radicals were approximately reproduced only by rotational diffusion around the z‐axis perpendicular to the plane in the IN group. Interestingly, this reproduction was not around the y‐axis of the principal axes of the g tensors, parallel to the molecular long axis, as previously observed in a few organic matrices. The IN radicals dispersed in the ZnTPT matrix are expected to be accommodated in cylindrical or pseudocylindrical nanospaces sandwiched by the pyridyl or triazine rings of TPT in ZnTPT. These findings show that the ESR spin probe technique using non‐TEMPO radicals can be used to investigate the chemical and biological structures of nanosized materials.

Influence of the Coordinated Transition Metal Ion on Magnetic Relaxation of Lanthanide Based Complexes with Imino Nitroxide Biradical Ligands.

In spite of achievement of a lot of Ln-radical SMMs, how to improve magnetic behavior of Ln-radical system remains challenging. Here, two series of Ln-radical complexes have successfully been built using an imino nitroxide biradical, namely, [Ln2 (hfac)6 (ImPhPyobis)2 ] (LnIII =Gd 1, Tb 2, Dy 3) and [Ln2 Cu2 (hfac)10 (ImPhPyobis)2 ] (LnIII =Gd 4, Dy 5; hfac=hexafluoroacetylacetonate and ImPhPyobis=5-(4-oxypyridinium-1-yl)-1,3-bis(1'-oxyl-4',4',5',5'-tetramethyl-4,5-hydro-1H-imidazol-2-yl)benzene). For these biradical-metal complexes, two imino nitroxide biradicals bind two Ln(III) ions via their oxygen atoms coming from 4-oxypyridinium units to produce a binuclear {Ln2 O2 } unit. Those imino nitroxide groups are free for complexes 1-3, however one of imino nitroxide groups of the biradical is ligated to the copper(II) ion for complexes 4 and 5. The distinct magnetic relaxation behaviors are observed for two Dy derivatives, as revealed by ac magnetic studies: complex 3 presents one magnetic process with the effective energy barrier(Ueff ) of 74.0 K while complex 5 exhibits dual relaxation processes with Ueff values for the fast- and slow-relaxation being 20.2 K and 30.9 K, respectively, which implies that the second coordination sphere of Dy ion plays a critical role for magnetic relaxation.

Magnetic and Structural Properties of Organic Radicals Based on Thienyl- and Furyl-Substituted Nitronyl Nitroxide

Magnetic properties of organic radicals based on thienyl- and furyl-substituted nitronyl nitroxide (NN) and iminonitroxide (IN) were investigated by measuring the temperature dependence of the magnetization. The magnetic behavior of 2-benzo[b]thienyl NN (2-BTHNN) is interpreted in terms of the two-magnetic-dimer model, in which one dimer exhibits ferromagnetic (FM) intermolecular interaction and the other dimer shows antiferromagnetic (AFM) interaction. The existence of two dimers in 2-BTHNN is supported by crystal structure analysis. The magnetic behaviors of 2-bithienyl NN, 4-(2′-thienyl)phenyl NN (2-THPNN), 2- and 3-furyl NN, 2-benzo[b]furyl NN, and 3-benzo[b]thienyl IN are also reported. The one-dimensional alternating AFM nature observed in 2-THPNN is consistent with its crystal structure.

Sodium salts of 2-hydroxy-3,5-dinitrophenyl-substituted nitronyl and imino nitroxides

Sodium salts of 2-hydroxy-3,5-dinitrophenyl-substituted nitronyl and imino nitroxides

Synthesis and Crystal Structures of Two Metal-Radical Complexes

Two new M(II)-radical complexes [M(hfac)2(IM-o-QN)] (M = Ni(1), Zn(2); IM-o-QN = 4'-quinoxalinyl-substituted imino nitroxide; hfac = hexafluoroa-cetylacetonate) have been synthesized and characterized by X-ray diffraction analysis, element analyses, IR and UV-Visible spectroscopy. X-ray analysis reveal that the structures of both complexes are similar configuration and differently spatial symmetries. The complex 1 crystallizes in the triclinic Pī space group with the respective cell parameters: a = 9.1189(18) &#197, b = 9.836(2) &#197, c = 18.537(4) &#197, α = 75.92(3)°, β= 81.95(3)°, γ = 69.32(3)°, V = 1506.1(5) &#1973, Z = 2, whereas the complex 2 is in monoclinic C2/c space group with the respective cell parameters: a = 26.996(5) &#197, b = 9.5223(19) &#197, c = 23.961(5) &#197, α = 90.00°, β= 91.07(3)°, γ= 90.00°, V = 6158(2) &#1973, Z = 8. In two new M(II)-radical complexes, the central M(II)(Ni(1) and Zn(2)) ions are coordinated by four oxygen atoms from two hfac and two nitrogen atoms from imino nitroxide radicals to form a distorted octahedron. Additionally, the optical properties and thermal analysis of the two complexes are reported.

New method for the synthesis of heterospin metal complexes with nitroxides

A new approach was developed to synthesize transition metal complexes with nitroxides based on the simultaneous involvement of nitronyl nitroxide and imino nitroxide in the reaction with metal. It was shown that the reaction of a metal with nitronyl nitroxide can afford a metal complex containing two different radicals in the metal coordination sphere in the case when imino nitroxide is generated in the reaction medium via a redox process. The reaction of a metal with imino nitroxide also can give mixed-ligand complexes, in which the metal coordinates both the starting imino nitroxide and its reduction product — the corresponding amidine oxide. This compound can be prepared by an alternative synthesis using the reaction of metal with sterically hindered amidine oxide, resulting in the formation of mixed-ligand coordination compounds through the coordination of both the starting amidine oxide and its oxidation product — imino nitroxide. In the latter case, the following conditions have to be met: the reaction should be performed in the presence of oxygen and transition metal, which can easily change its oxidation state under ambient conditions (CoII, MnII, FeII). To synthesize mixed-ligand complexes with transition metals that are not prone to change the oxidation state under ambient conditions (NiII, ZnII), a specially prepared mixture of nitronyl nitroxide and imino nitroxide should be added to the reaction mixture. It is worth noting that the reaction can be performed using nitronyl nitroxide and imino nitroxide belonging to different series, which significantly extends the scope of the method.The bibliography includes 156 references.

Intermolecular Spin-Crossover-like Phenomenon Sensitive to Applied External Pressure in Heterospin Crystals

The interaction of Cu(acF6)2 with the stable imino nitroxide 2-(pyridin-3-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-1-oxyl (ImN-mPy) at a ratio of 2:1 led to the formation of a new complex [...

A diarylethene annulated isomer as a highly-conductive molecular wire evaluated by the exchange interaction between two nitroxides.

Bis(imino nitroxide) bridged by a diarylethene annulated isomer was synthesized and the exchange interaction between the two radicals was determined by the simulation of the ESR spectrum. The exchange interaction was found to be as large as that of the closed-ring isomer, suggesting that the annulated isomer can be used as a highly-conductive molecular wire in molecular electronics.

Through the challenging synthesis of tetraphenylporphyrin derivatives bearing nitroxide moieties

Tetraphenylporphyrin derivatives a synthetic heterocycles with convenient preparation and a richness of properties which make them attractive in broad fields such as energy, life and materials sciences. Thus, in the quest for new radical architectures, tetraphenylporphyrins are prime candidates. To this end, we designed free-base tetraphenylporphyrins bearing nitronyl and imino nitroxide moieties covalently bonded to the para-position of the meso-phenyl substituent. Their detailed synthesis and characterization are reported here.

Mononuclear manganese(iii) complexes with reduced imino nitroxide radicals by single-electron transfer and intermolecular hydrogen bonds as an intramolecular structural driving force.

Manganese(iii) complexes were synthesized by one-electron transfer from a Mn(ii) ion to the imino nitroxide radical 2-(2-imidazolyl)-4,4,5,5-tetramethylimidazoline-1-oxyl (IMImH) in methanol. After the manganese ions attained the +III oxidation state, the imino nitroxide radicals were found to be irreversibly reduced in the complexes. Depending on the synthesis conditions, two complexes differing by their counter-anions were isolated as single crystals. These are [Mn(IMHIm)2(MeOH)2]ClO4·H2O (1) and [Mn(IMHIm)2(MeOH)2]PF6 (2), which crystallize in the monoclinic P21/n and triclinic P1[combining macron] space groups, respectively. The two complexes show Jahn-Teller distortions typical of Mn(iii) centres and only reduced radicals are coordinated, as indicated by the N-O bond lengths and electroneutrality. In addition, the crystal structure analyses reveal two intermolecular hydrogen bonding networks. One involves counter-anions, water molecules and reduced radicals, and the other involves coordinated methanol molecules and imidazole moieties. These intermolecular interactions are driving forces that stabilize the two complexes. They also suggest that the tautomer is in the amino imine-oxide form after reduction of the radical and reveal the deprotonation of the imidazole ring, which is required for electroneutrality. This assessment is supported by single-crystal X-ray diffraction, EPR and Raman spectroscopy as well as magnetic and electrochemical studies.

Planar Benzo[1,2- b:4,5- b']dithiophene Derivatives Decorated with Nitronyl and Imino Nitroxides.

Four weakly antiferromagnetic interacting biradicals of benzo[1,2- b:4,5- b']dithiophene (BDT) and BDT extended with two thiophenes (BDTTh2) linked with nitronyl and imino nitroxides (NN and IN) as BDT-NN, BDT-IN, BDTTh2-NN, and BDTTh2-IN were designed, synthesized, and characterized. Short intermolecular π-π distances were found (3.42 Å) for BDT-NN, whereas larger ones were found for BDT-IN (3.54 Å) and BDTTh2-NN (3.67 Å), respectively. Intramolecular magnetic interaction ( Jintra,exp/ kB) of BDT-NN (-26 K) is much larger than for BDT-IN (-5.3 K), while it is reduced for the dithiophene-extended molecule BDTTh2-NN (-2.3 K). Intermolecular interactions ( zJinter,exp/ kB) of BDT-NN (-6.5 K) and BDT-IN (-6.0 K) are stronger than for BDTTh2-NN (-4.6 K). Such large intermolecular couplings resulting from good π-stacking mark BDT-IN and BDTTh2-NN as promising crystalline materials with similar sized Jintra and Jinter. In addition, we also extracted a coupling within the chain of Jchain/ kB = -2.2 K and a coupling between the chains of zJinterchain = -1.5 K for BDTTh2-NN by a Heisenberg chain model. Intra- and intermolecular interactions and spin densities were examined by DFT studies.

Revisiting the Ullman's Radical Chemistry for Phthalocyanine Derivatives.

Phthalocyanine derivatives do not cease to gain attention due to their numerous properties and applications (e.g., sensor, PDT). This makes them a unique scaffold for the design of new material. In this context, we were interested to develop the synthesis of an imino nitroxide-substituted phthalocyanine by Ullman's procedure; a challenge due to the intrinsic low solubility of most phthalocyanine derivative in much solvents. To overcome this solubility problem, we designed a phthalocyanine with bulky neopentyl substituents in peripheral positions as counterpart to the imino nitroxide moieties. The imino nitroxide-substituted phthalocyanine was obtained by condensation of a monoformyl-substituted phthalocyanine with 2,3-bis(hydroxylamino)-2,3-dimethylbutane in refluxing THF-MeOH (2:1) mixture in the presence of p-toluenesulfonic acid monohydrate, follow by oxidation with PbO2 . Characterization was performed by electrochemistry, UV/Vis and EPR spectroscopy in solution as well as SQUID in solid state.

Simultaneous Introduction of Two Nitroxides in the Reaction: A New Approach to the Synthesis of Heterospin Complexes.

A new approach to the synthesis of multispin compounds has been developed, namely, the simultaneous introduction of two different stable nitroxides (nitronyl nitroxide and imino nitroxide) in a reaction with a metal ion. An important characteristic of the new method is that nitronyl nitroxide and imino nitroxide introduced in the reaction with the metal are the products of different series; i.e., the nitronyl nitroxide molecule differs from the imino nitroxide molecule not only in one additional oxygen molecule per molecule but also in another substituent in the side chain of the organic paramagnet. This possibility was demonstrated on the synthesis of multispin compounds [Ni2(A1)(L2)2(Piv)(MeOH)], [Ni2(L1)(A2)2(Piv)(H2O)], [Co2(A1)(L2)2(Piv)(MeOH)], and [Co3(L1)2(A2)2(Piv)2], in which Ln and An differ in the substituent in the phenyl ring. The number of multispin compounds that can be synthesized by the proposed method is almost unlimited. The heterospin complexes of transition metals with coordinated nitronyl nitroxide and imino nitroxide in one molecule contain energy-different exchange interaction channels that differ in both magnitude and sign, as confirmed by the quantum-chemical analysis of exchange channels in [Ni(B1)(B2)2](NO3)2. The series of mixed-radical complexes may include compounds with nontrivial magnetic properties such as [Co2(A1)(L2)2(Piv)(MeOH)], which experiences bulk magnetic ordering below 3.5 K.

Core-Modified Porphyrin Diradicals with a C═C Unit: Redox-Driven Magnetic Switching

We explore the intramolecular spin interactions of the core-modified porphyrin diradicals with a C═C unit (R-(C═C) and R-(C═C)2+) featuring (C═C)porphyrin and (C═C)porphyrin2+ as the couplers and verdazyl, nitronyl nitroxide, and imino nitroxide as spin sources (R) at the B3LYP/6-31G(d) level and the C═C effect through comparison with the porphine-coupled diradicals (R-(Null)). Structurally, modifications of porphine through introducing radical groups to the edge sites and a C═C unit to its core lead to a nonplanar diradical structure featuring a curved (C═C)porphyrin coupler and twist linkages of radical groups. Although such nonplanar structures seem unfavorable to the spin coupling between spin sources, our results suggest that the core modification with a C═C unit noticeably enhances the spin couplings in R-(C═C) and R-(C═C)2+ compared with R-(Null) with a planar porphine coupler, and R-(C═C) possess mild ferromagnetic couplings but R-(C═C)2+ present strong antiferromagnetic ones, indicating that two-...

Ferrocene-based diradicals of imino nitroxide, nitronyl nitroxide and verdazyl, and their cations are possible SMM: A quantum chemical study

Six diradicals designed from imino nitroxide, verdazyl and nitronyl nitroxide monoradicals coupled via the ferrocene moiety and six corresponding triradical cations are quantum chemically investigated. The transoid conformation is employed for considerations of general stability. All biradicals are found as very weakly and antiferromagnetically coupled. This agrees with experiment. The cations have strong antiferromagnetic spin-coupling. The charge and spin population distributions, spin alternation pattern, and the disjoint nature of SOMOs can be used to explain the nature and extent of magnetic interaction. Calculated EPR characteristics identify the neutral species as well as their cations as possible single molecule magnets.

Spin-dimer networks: engineering tools to adjust the magnetic interactions in biradicals

Magneto-structural correlations in stable organic biradicals have been studied on example of weakly exchange coupled models with nitronyl nitroxide and imino nitroxide spin-carrying entities.

Syntheses, structures and magnetic properties of four-spin Mn-Imino nitroxide radical complexes

Based on the nitroxide radicals, four-spin complexes [Mn(hfac)2(IMpPhCl)]2·NITpPhCl (1) and [Mn(hfac)2(IMmPhCl)]2·NITmPhCl (2) (IMpPhCl = 2-(4'-chlorophenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl, IMmPhCl = 2-(3'-chlorophenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl, hfac = hexafluoroacetylacetonate) have been synthesized and characterized by single-crystal X-ray diffraction. The X-ray crystal structure analyses show that the structures of the two compounds are similar and the imino nitroxide radical ligand acts as a bridge ligand linking two Mn(II) ions through the oxygen atom of the N–O group to form a four-spin system. Two kinds of nitroxide radicals: nitronyl nitroxide and imino nitroxide radicals coexist in the cyclic Mn(II) complexes. The magnetic studies show that there exists an antiferromagnetic interaction between Mn(II) ions and the imino nitroxide radical ligands, which is explained by spin polarization mechanism. The antiferromagnetic interaction of Mn−Rad in complex 2 (J1 = −9.36 cm−1) is stronger than that in complex 1 (J1 = −9.19 cm−1), which is consistent with crystal structure of complexes (The bond length of the shortest Mn-O in complex 2 (2.1625 Å) is smaller than complex 1 (2.1898 Å)).

Synthesis and Straightforward Quantification Methods of Imino Nitroxide-Based Hexaradical Architecture on a Cyclotriphosphazene Scaffold.

The synthesis of a homogeneous neutral hexaradical architecture consisting of six imino nitroxide radical moieties covalently bonded on a cyclotriphosphazene scaffold was reported. The synthesis of hexaradical imino nitroxide compounds follows the Ullman procedure involving the condensation of 2,3-bis(hydroxylamino)-2,3-dimethylbutane with hexa-(4-formylphenoxy)cyclotriphosphazene (3) followed by oxidation of the condensation product hexa-[4-(1-hydroxy-4,4,5,5-tetramethyl-2-imidazoline-2-yl)phenoxy]cyclotriphosphazene (2) by NaIO4. Characterization of hexaradical was performed by X-ray and SQUID in solid state and by EPR, absorption spectroscopy, and electrochemistry in solution. CV of 1 shows an oxidation peak at 1.184 V (vs SCE) and a reduction peak at -0.883 V, both characteristics of the presence of phenyl imino nitroxide (7) moieties, suggesting that the contribution of the cyclotriphosphazene core is negligible. Attention was particularly focused on developing methods, UV-vis spectroscopy and square-wave voltammetry, to quantify the number of radicals in a way to confirm easily and rapidly the polyradicals' structure.

Copper(II) coordination compounds with sterically constraining pyrenyl nitronyl nitroxide and imino nitroxide

2-(1′-Pyrenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (PyrNN) and 2-(1′-pyrenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-1-oxyl (PyrIN) form pentacoordinate metal–radical complexes with Cu(hfac)2: [Cu(hfac)2PyrNN] and [Cu(hfac)2PyrIN], 1 and 2, respectively. In 1, one aminoxyl unit of PyrNN coordinates to Cu(II) in a distorted square pyramidal environment. In 2, an unusual example of monodentate Cu–N coordination of the imino nitroxide, PyrIN, gives an environment intermediate between square pyramidal and trigonal bipyramid. Both complexes show novel close contacts of the pyrene π-electron cloud with the Cu(II) ion. Compound 1 has a particularly short Cu–O(N) bond of 1.948Å, giving strong intramolecular antiferromagnetic (AFM) spin-pairing and an absence of significant paramagnetic susceptibility response. The magnetic behavior of compound 2 indicates copper–radical intramolecular ferromagnetic exchange of 2J=151cm−1, and intermolecular AFM spin pairing of 2J=−220cm−1 due to close antiparallel contacts between non-coordinated N–O units. EPR spectra for both complexes are silent at 293 and 77K. UB97D/6-31G(d) computations find strong antiferromagnetic (2J=−213cm−1) and ferromagnetic (2J=610cm−1) intramolecular metal–radical exchange in 1 and 2, respectively in agreement with experimental results.

Heterospin MnIII complex as a reaction product of the redox-induced change in the ligand coordination mode

The reaction of MnII chloride with imino nitroxide radical, 2-(2-hydroxy-5-nitrophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-1-oxyl (HL2), affords the MnIII complex [MnL22L3]·Me2CO, a distinctive feature of which is the simultaneous presence in the ligand shell of both the initial imino nitroxide and the product of its reduction 2-(2-hydroxy-5-nitrophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide (HL3). The reaction involves the oxidation of MnII to MnIII and the reduction of the imino nitroxide radical to the corresponding amidine oxide along with a change in the coordination mode of the heterocyclic ligand on going from L2 to L3. The MnIII ion forms with L2 six-membered metallocycles typical of Schiff bases, whereas with L3 MnIII forms a seven-membered metallocycle due to the coordination of L3 by oxygen atoms of the phenol and N-oxide It was found in a similar reaction of NiII chloride with imino nitroxide HL2 that no oxidation of the metal occurred and bis(chelate) [NiL22(H2O)2]·2Me2CO was formed in the solid phase. The molecular and crystal structures of the compounds were determined, and their magnetic properties were studied.