A Copper–Nitroxide Adduct Exhibiting Separate Single Crystal-to-Single Crystal Polymerization–Depolymerization and Spin Crossover Transitions

Abstract

A complex cascade of solid-state processes initiated by variation of temperature was found for the heterospin complex [Cu(hfac)2L(Me/Et)] formed in the reaction of copper(II) hexafluoroacetylacetonate [Cu(hfac)2] with stable nitronyl nitroxide 2-(1-methyl-3-ethyl-1H-pyrazol-4-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (L(Me/Et)). The cooling of the compound below 260 K initiated a solid-state chemical reaction, which led to a depolymerization of chains and formation of a pair heterospin complex [Cu(hfac)2L(Me/Et)2][[Cu(hfac)2]3L(Me/Et)2]. Further decrease in temperature below 144 K led to a spin transition accompanied by a drastic decrease in the effective magnetic moment from 2.52 to 2.24 μB. When the compound was heated, the order of effects was reversed: at first, the magnetic moment abruptly increased, and then the molecular fragments of the pair cluster united into polymer chains. Two hysteresis loops correspond to this cascade of temperature-induced structural transformations on the experimental dependence μeff(T): one at high (T↑ = 283 K and T↓ = 260 K) and the other at low (T↑ = 161 K, T↓ = 144 K) temperature. The spin transitions were also recorded for the [[Cu(hfac)2]3L(Bu/Et)2] and [[Cu(hfac)2]5L(Bu/Et)4] molecular complexes, which are models of the trinuclear fragment of the {[Cu(hfac)2]3L(Me/Et)2} pair cluster.

Ligand Structure Effects on Molecular Assembly and Magnetic Properties of Copper(II) Complexes with 3-Pyridyl-Substituted Nitronyl Nitroxide Derivatives

“…χ M T gradually increases with decreasing temperature and reaches 1.68 cm 3 ·K·mol –1 at 2 K, indicating the domination of ferromagnetic exchange interactions, which are typical for axial coordination of NO groups to central Cu 2+ ions. 1,2,10,14,24 The experimental dependence of χ M T ( T ) is well described by the model including the contribution of the three-spin {RA- J CuR -Cu 2+ A- J CuR -RB} exchange cluster ( H = −2 J CuR ( S CuA S RA + S RB S CuA )) and isolated Cu 2+ ion spins from coordination units {CuO 4 N 2 } according to the Curie law, with the best fit parameters as follows: g R = 2.00 (fixed), g Cu 2+ = 2.10, J CuR = +16 cm –1 , and zJ ′ = +0.07 cm –1 .…”

“…Chains with a head-to-head motif of [Cu(hfac) 2 L 6-Br ] n are similar to the previously described coordination polymers for complexes based on pyrazolyl-substituted derivatives of nitroxides. 9,10,23,24 Alternating octahedral {CuO 6 } and {CuO 4 N 2 } coordination units contain chelate hfac ligands in a coplanar arrangement. This arrangement should be less spatially crowded compared with the cis position spacing in {CuO 5 N} units for the [Cu(hfac) 2 L 6-Me ] 2 dimer complex.…”

“…1,2,7,9−14,23,24 At the same time, a variety of structural motifs generates the diversity of magnetic properties. 1,2,10,11,23,24 The short Cu–N Py bonds (2.003(9) and 2.021(8) Å) were observed only for the mononuclear [Cu(hfac) 2 L 2-Me ] complex due to the reduced spatial bulkiness of pentacoordinated {CuO 4 N} units. For the [{Cu(hfac) 2 } 2 (H 2 O)L 2-Me ] complex with two Cu(hfac) 2 fragments, which coordinate the same L 2-Me ligand, the Cu–N Py bond in the {CuO 4 N} units becomes longer (2.298(3) Å), whereas the coordination sphere of the other Cu atom tends to a distorted octahedral environment {CuO 6 } by including an O NO atom and H 2 O molecule in axial positions (Cu–O NO = 2.461(3) Å and Cu–O H 2 O = 2.344(3) Å).…”

A Hidden Coordination-Bond Torsional Deformation as a Sign of Possible Spin Transition in Nickel(II)-Bis(nitroxide) Compounds

“…Namely, the Cu-O-N-C sp 2 angular deformation causes the exchange coupling switch. On the other hand, Ovcharenko et al reported the spin-transition Cu 2+ -nitroxide compounds [ 52 , 53 ], and the mechanism involves a switch of the role of axial and equatorial coordinations [ 54 ]. Rey et al reported another type of the spin-transition Cu 2+ -nitroxide materials [ 55 ], where the coordination environment is changed between a square pyramid and a trigonal bipyramid.…”

An indication of spin-transition accompanied by an order-disorder structural transformation in [Ni(phpyNO)2(NCS)2] (phpyNO = tert-butyl 5-phenyl-2-pyridyl nitroxide)

“…What "spin crossover" covers may be extended to multi-centred systems showing an essentially similar spinmultiplicity switch at their ground state. [11][12][13] The spin state is regulated by ferro-/antiferromagnetic balance continuum in multi-centred systems, 11 in place of the Hund/auau principles, or potential/kinetic exchange interactions, in one-centred systems.…”