Synthesis and Structure of Heterospin Compounds Based on the [Mn₆(O)₂Piv₁₀]-Cluster Unit and Nitroxide

Reaction of [Mn6(O)2Piv10(Thf)4]·Thf with 2,4,4,5,5-pentamethyl-4,5-dihydro-1H-imidazolyl-3-oxide-1-oxyl (NIT-Me) produces different heterospin compounds depending of the solvent used in the synthesis. Among the latter a new molecular magnet [Mn6(O)2Piv10(Thf)2(NIT-Me)Mn6(O)2Piv10(Thf)(CH2Cl2)(NIT-Me)] with Tc = 3.5 K has been found

Problem of a Wide Variety of Products in the Cu(hfac)₂−Nitroxide System

The stereochemically flexible Cu(hfac)2 metal−ligand system when combined with polyfunctional nitroxides leads to a variety of solids with varying structure and composition. While investigating the products of Cu(hfac)2 interaction with spin-labeled pyrazole 4,4,5,5-tetramethyl-2-(1-methyl-1H-pyrazol-4-yl)-imidazoline-3-oxide-1-oxyl, we have isolated a family of (12) heterospin compounds differing in structure and composition in the solid state. In synthetic systems, these compounds often cocrystallize and must be separated mechanically. It is also shown that minor variation of the structure of the solid heterospin complex can substantially change the magnetic properties of compounds

Redox-Induced Change in the Ligand Coordination Mode

The reaction of cobalt­(II) pivalate with a spin-labeled Schiff base (HL1) in organic solvents formed trinuclear complex [Co3(Piv)2L12L22]·Solv (Solv is Me2CO and/or C7H16 and CH3CN) containing both nitroxide L1 and the product of its single-electron reduction, nitrone L2. The formation of [Co3(Piv)2L12L22] was a consequence of an unusual phenomenon, which we called “redox-induced change in the ligand coordination mode”. A reduction of L1 to L2 led to a change in the set of donor atoms and even in the size of the metallocycle. This phenomenon was also found for mononuclear [CrL12L2] and [FeL12L2]·Me2CO

Redox-Induced Change in the Ligand Coordination Mode

The reaction of cobalt­(II) pivalate with a spin-labeled Schiff base (HL1) in organic solvents formed trinuclear complex [Co3(Piv)2L12L22]·Solv (Solv is Me2CO and/or C7H16 and CH3CN) containing both nitroxide L1 and the product of its single-electron reduction, nitrone L2. The formation of [Co3(Piv)2L12L22] was a consequence of an unusual phenomenon, which we called “redox-induced change in the ligand coordination mode”. A reduction of L1 to L2 led to a change in the set of donor atoms and even in the size of the metallocycle. This phenomenon was also found for mononuclear [CrL12L2] and [FeL12L2]·Me2CO

2D and 3D Cu(hfac)₂ Complexes with Nitronyl Nitroxide Biradicals

Reactions between Cu(hfac)2 and nitronyl nitroxide biradicals 1,4-bis[4-(4,4,5,5-tetramethyl-3-oxide-1-oxyl-4,5-dihydro-1H-imidazol-2-yl)pyrazol-1-yl]butane (L4) and 1,8-bis[4-(4,4,5,5-tetramethyl-3-oxide-1-oxyl-4,5-dihydro-1H-imidazol-2-yl)pyrazol-1-yl]octane (L8) gave respectively a framework compound [Cu(hfac)2]2L4 and a layered polymer compound [Cu(hfac)2]2L8. The framework of [Cu(hfac)2]2L4 consists of 66-membered condensed metallocycles. Inside the framework, the structure has macrohelixes (pitch ∼25 Å) extending along the [001] crystallographic direction. All the helixes have the same direction of winding; the crystals, therefore, are optically active, the structure corresponding either to P-isomer (P41212) or to M-isomer (P43212). The long distances between the Cu atoms and the O atoms of the coordinated >N−O groups (Cu−O 2.351−2.467 Å) are responsible for ferromagnetic exchange interactions in Cu2+−O−NN−O−Cu2+−O−N< exchange clusters

Synthesis, Structure, and Magnetic Properties of (6−9)-Nuclear Ni(II) Trimethylacetates and Their Heterospin Complexes with Nitroxides

New polynuclear nickel trimethylacetates [Ni6(OH)4(C5H9O2)8(C5H10O2)4] (6), [Ni7(OH)7(C5H9O2)7(C5H10O2)6(H2O)]·0.5C6H14·0.5H2O (7), [Ni8(OH)4(H2O)2(C5H9O2)12] (8), and [Ni9(OH)6(C5H9O2)12(C5H10O2)4]·C5H10O2·3H2O (9), where C5H9O2 is trimethylacetate and C5H10O2 is trimethylacetic acid, have been found. Their structures were determined by X-ray crystallography. Because of their high solubility in low-polarity organic solvents, compounds 6−9 reacted with stable organic radicals to form the first heterospin compounds based on polynuclear Ni(II) trimethylacetate and nitronyl nitroxides containing pyrazole (L1−L3), methyl (L4), or imidazole (L5) substituent groups, respectively, in side chain [Ni7(OH)5(C5H9O2)9(C5H10O2)2(L1)2(H2O)]·0.5C6H14·H2O (6+1a), [Ni7(OH)5(C5H9O2)9(C5H10O2)2(L2)2(H2O)]·H2O (6+1b), [Ni7(OH)5(C5H9O2)9(C5H10O2)2(L3)2(H2O)]·H2O (6+1c), [Ni6(OH)3(C5H9O2)9(C5H10O2)4(L4)]·1.5C6H14 (6‘ ‘), and [Ni4(OH)3(C5H9O2)5(C5H10O2)4(L5)]·1.5C7H8 (4). Their structures were also determined by X-ray crystallography. Although Ni(II) trimethylacetates may have varying nuclearity and can change their nuclearity during recrystallization or interactions with nitroxides, this family of compounds is easy to study because of its topological relationship. For any of these complexes, the polynuclear framework may be derived from the [Ni6] polynuclear fragment {Ni6(μ4-OH)2(μ3-OH)2(μ2-C5H9O2−O,O‘)6(μ2-C5H9O2−O,O)(μ4-C5H9O2−O,O,O‘,O‘)(C5H10O2)4}, which is shaped like an open book. On the basis of this fragment, the structure of 7-nuclear compounds (7 and 6+1a−c) is conveniently represented as the result of symmetric addition of other mononuclear fragments to the four Ni(II) ions lying at the vertexes of the [Ni6] open book. The 9-nuclear complex is formed by the addition of trinuclear fragments to two Ni(II) ions lying on one of the lateral edges of the [Ni6] open book. This wing of the 9-nuclear complex preserves its structure in another type of 6-nuclear complex (6‘ ‘) with the boat configuration. If, however, two edge-sharing Ni(II) ions are removed from [Ni6] (one of these lies at a vertex of the open book and the other, on the book-cover line), we obtain a 4-nuclear fragment recorded in the molecular structure of 4. Twinning of this 4-nuclear fragment forms highly symmetric molecule 8, which is a new chemical version of cubane

Redox-Induced Change in the Ligand Coordination Mode

The reaction of cobalt­(II) pivalate with a spin-labeled Schiff base (HL¹) in organic solvents formed trinuclear complex [Co₃(Piv)₂L¹₂L²₂]·Solv (Solv is Me₂CO and/or C₇H₁₆ and CH₃CN) containing both nitroxide L¹ and the product of its single-electron reduction, nitrone L². The formation of [Co₃(Piv)₂L¹₂L²₂] was a consequence of an unusual phenomenon, which we called “redox-induced change in the ligand coordination mode”. A reduction of L¹ to L² led to a change in the set of donor atoms and even in the size of the metallocycle. This phenomenon was also found for mononuclear [CrL¹₂L²] and [FeL¹₂L²]·Me₂CO

Thermally Induced Spin Transitions in Nitroxide−Copper(II)−Nitroxide Spin Triads Studied by EPR

Thermally induced spin transitions in a family of heterospin polymer chain complexes of Cu2+ hexafluoroacetylacetonate with two pyrazole-substituted nitronyl nitroxides are studied using electron paramagnetic resonance (EPR) spectroscopy. The structural rearrangements at low temperatures induce spin transitions in exchange-coupled spin triads of nitroxide−copper(II)−nitroxide. The values of exchange interactions in spin triads of studied systems are typically on the order of tens to hundreds of inverse centimeters. The large magnitude of exchange interaction determines the specific and very informative peculiarities in EPR spectra due to the predominant population of the ground state of a spin triad and spin exchange processes. The variety of these manifestations depending on structure and magnetic properties of spin triads are described. EPR is demonstrated as an efficient tool for the characterization of spin transitions and for obtaining information on the temperature-dependent sign and value of the exchange interaction in strongly coupled spin triads

Cu(hfac)₂ Complexes with Acyclic Nitroxide Prone to Single-Crystal to Single-Crystal Transformation and Showing Mechanical Activity

The heterospin solid phases of the chain polymer [Cu­(hfac)2LEt]∞ and bicyclic molecule [Cu­(hfac)2LEt]2-I (LR = pyrazolyl-substituted tert-butylnitroxide; 1-R-5-(tert-butyl-oxylamino)­pyrazole, R = Et, Pr) were found to undergo spontaneous transformation into the bicyclic molecule [Cu­(hfac)2LEt]2-II. The single-crystal to single-crystal (SC–SC) transformation of [Cu­(hfac)2LEt]2-I into [Cu­(hfac)2LEt]2-II was recorded by X-ray diffraction analysis of the crystal as a function of time. At 255–277 K, the [Cu­(hfac)2LEt]2-I → [Cu­(hfac)2LEt]2-II SC–SC transformation proceeded for 12–18 h. The [Cu­(hfac)2LEt]∞ → [Cu­(hfac)2LEt]2-II SC–SC phase transformation was accompanied by a change in the crystal shape, spontaneous mechanical displacements of crystals, and a change in color from orange to dark green. This process started, to a certain extent, already in the crystals lying under the layer of the mother solution. After the crystals were separated from the solution, the SC–SC transformation [Cu­(hfac)2LEt]∞ → [Cu­(hfac)2LEt]2-II occurred completely within 4 h at room temperature. Under normal conditions, [Cu­(hfac)2LPr]2-I also undergoes transformation into [Cu­(hfac)2LPr]2-II. At the macro level, the transformation [Cu­(hfac)2LPr]2-I → [Cu­(hfac)2LPr]2-II is accompanied by spontaneous fragmentation of crystals, visualized as a scatter of small particles of the formed phase in different directions. The reverse transformation [Cu­(hfac)2LPr]2-II → [Cu­(hfac)2LPr]2-I occurs when [Cu­(hfac)2LPr]2-II is cooled below 225 K. When [Cu­(hfac)2LPr]2-II was heated above 300 K, the irreversible SC–SC phase transformation [Cu­(hfac)2LPr]2-II → [Cu­(hfac)2LPr]∞ was observed, which caused a pronounced change in the color of the crystals from dark green to orange. Heat treatment of the [Cu­(hfac)2LPr]∞ single crystal at 303 K on a diffractometer for 1 day or more caused partial melting of the starting crystal, disappearance of X-ray diffraction reflections from the sample under study, and appearance of reflections corresponding to the formation of the new polymer complex [Cu­(hfac)2L*Pr]∞, where L*Pr is the product of transformation of the radical including the oxidation of LPr and migration of the nitroxide O atom to the heterocycle, leading to the formation of 5-(tert-butylimino)-1-propyl-1,5-dihydro-4H-pyrazol-4-one (L*Pr). The results of the X-ray diffraction study of the phase transformations completely agreed with the data of magnetochemical measurements for the complexes. Having replaced the acyclic nitroxides LEt and LPr by their diamagnetic structural analogues LPEt (2,2-dimethyl-1-(1-ethyl-1H-pyrazol-5-yl)­propan-1-one) and LPPr (2,2-dimethyl-1-(1-propyl-1H-pyrazol-5-yl)­propan-1-one), we obtained the complexes [Cu­(hfac)2LPEt]∞, [Cu­(hfac)2(LPPr)2], and [(Cu­(hfac)2)3(LPPr)2], for which the transformations are absolutely not characteristic. It was also found that polymorphic transformations are also uncharacteristic of complexes of other metals with the acyclic nitroxides under study ([Zn­(hfac)2LEt]2, [Zn­(hfac)2LPr]2, [Mn­(hfac)2LEt]2). Thus, it was shown that the presence of both the Cu­(II) ion and coordinated O–N group of acyclic nitroxide in the solid phase are favorable conditions for the emergence of stereochemical nonrigidity and multiple phase transformations in the compounds of Cu­(hfac)2 with acyclic nitroxides

Cu(hfac)₂ Complexes with Acyclic Nitroxide Prone to Single-Crystal to Single-Crystal Transformation and Showing Mechanical Activity

The heterospin solid phases of the chain polymer [Cu­(hfac)2LEt]∞ and bicyclic molecule [Cu­(hfac)2LEt]2-I (LR = pyrazolyl-substituted tert-butylnitroxide; 1-R-5-(tert-butyl-oxylamino)­pyrazole, R = Et, Pr) were found to undergo spontaneous transformation into the bicyclic molecule [Cu­(hfac)2LEt]2-II. The single-crystal to single-crystal (SC–SC) transformation of [Cu­(hfac)2LEt]2-I into [Cu­(hfac)2LEt]2-II was recorded by X-ray diffraction analysis of the crystal as a function of time. At 255–277 K, the [Cu­(hfac)2LEt]2-I → [Cu­(hfac)2LEt]2-II SC–SC transformation proceeded for 12–18 h. The [Cu­(hfac)2LEt]∞ → [Cu­(hfac)2LEt]2-II SC–SC phase transformation was accompanied by a change in the crystal shape, spontaneous mechanical displacements of crystals, and a change in color from orange to dark green. This process started, to a certain extent, already in the crystals lying under the layer of the mother solution. After the crystals were separated from the solution, the SC–SC transformation [Cu­(hfac)2LEt]∞ → [Cu­(hfac)2LEt]2-II occurred completely within 4 h at room temperature. Under normal conditions, [Cu­(hfac)2LPr]2-I also undergoes transformation into [Cu­(hfac)2LPr]2-II. At the macro level, the transformation [Cu­(hfac)2LPr]2-I → [Cu­(hfac)2LPr]2-II is accompanied by spontaneous fragmentation of crystals, visualized as a scatter of small particles of the formed phase in different directions. The reverse transformation [Cu­(hfac)2LPr]2-II → [Cu­(hfac)2LPr]2-I occurs when [Cu­(hfac)2LPr]2-II is cooled below 225 K. When [Cu­(hfac)2LPr]2-II was heated above 300 K, the irreversible SC–SC phase transformation [Cu­(hfac)2LPr]2-II → [Cu­(hfac)2LPr]∞ was observed, which caused a pronounced change in the color of the crystals from dark green to orange. Heat treatment of the [Cu­(hfac)2LPr]∞ single crystal at 303 K on a diffractometer for 1 day or more caused partial melting of the starting crystal, disappearance of X-ray diffraction reflections from the sample under study, and appearance of reflections corresponding to the formation of the new polymer complex [Cu­(hfac)2L*Pr]∞, where L*Pr is the product of transformation of the radical including the oxidation of LPr and migration of the nitroxide O atom to the heterocycle, leading to the formation of 5-(tert-butylimino)-1-propyl-1,5-dihydro-4H-pyrazol-4-one (L*Pr). The results of the X-ray diffraction study of the phase transformations completely agreed with the data of magnetochemical measurements for the complexes. Having replaced the acyclic nitroxides LEt and LPr by their diamagnetic structural analogues LPEt (2,2-dimethyl-1-(1-ethyl-1H-pyrazol-5-yl)­propan-1-one) and LPPr (2,2-dimethyl-1-(1-propyl-1H-pyrazol-5-yl)­propan-1-one), we obtained the complexes [Cu­(hfac)2LPEt]∞, [Cu­(hfac)2(LPPr)2], and [(Cu­(hfac)2)3(LPPr)2], for which the transformations are absolutely not characteristic. It was also found that polymorphic transformations are also uncharacteristic of complexes of other metals with the acyclic nitroxides under study ([Zn­(hfac)2LEt]2, [Zn­(hfac)2LPr]2, [Mn­(hfac)2LEt]2). Thus, it was shown that the presence of both the Cu­(II) ion and coordinated O–N group of acyclic nitroxide in the solid phase are favorable conditions for the emergence of stereochemical nonrigidity and multiple phase transformations in the compounds of Cu­(hfac)2 with acyclic nitroxides