Spin-Transition and Ferromagnetic Interactions in Copper(II) Complexes of a 3-Pyridyl-Substituted Imino Nitroxide. Dependence of the Magnetic Properties upon Crystal Packing

Abstract

2-(3-Pyridyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-1-oxy, IM-3Py (1), reacts with copper(II) bis(hexafluoroacetylacetonate), Cu(hfac)2, to give three complexes. One of them is a centrosymmetric three-spin species, Cu(hfac)2(IM-3Py)2 (4), where two nitroxide ligands are pyridyl bound to the metal and the spins independent from 300 to 5 K. The two other complexes, [Cu(hfac)2]4(IM-3Py)2 (α-phases, 5, and β-phases, 6) are six-spin clusters comprising four copper(II) ions linked by two tridentate IM-3Py units. Their molecular structures are almost identical, but they crystallize in different space groups. For both complexes, two octahedral endo metal ions are involved in a cyclic structure and the two others are exocyclic in a trigonal-bipyramidal environment. On the one hand, the magnetic properties of these two clusters are similar since, for both, the high-temperature behavior can only be accounted for by large ferromagnetic interactions; on the other hand they are dramatically different because the α-phase has a S = 3 ground spin state, while the β-phase exhibits two transitions resulting in two independent spins below 70 K. The transition temperatures are T C1 ↑ = 211 K, T C1 ↓ = 152 K, T C2 ↑ = 80 K, and T C2 ↓ = 70 K. With the help of ab initio calculations on model fragments, the presence of a ferromagnetic interaction through the pyridyl ring has been tentatively rationalized invoking McConnell's electron transfer mechanism. The high-temperature transition has been assigned to a change of the N-imino coordination from equatorial in a trigonal-pyramidal coordination to axial in a square-planar pyramidal environment at the exocyclic copper center. The low-temperature transition corresponds to a switch from axial to equatorial of the nitroxyl coordination at the endocyclic octahedral metal ion. Relevant crystallographic parameters are as follows: 4, triclinic, P1̄, a (Å) = 6.955(2), b (Å) = 11.485(2), c (Å) = 14.012(3), α (deg) = 112.55(1), β (deg) = 98.79(1), γ (deg) = 90.28(1), Z = 1; 5, monoclinic, C2/c, a (Å) = 25.319(4), b (Å) = 17.653(3), c (Å) = 19.973(4), β (deg) =96.60(2), Z = 4; 6, triclinic, P1̄, a (Å) = 12.200(2), b (Å) = 13.389(3), c (Å) = 15.457(3), α (deg) = 102.54(2), β (deg) = 110.16(2), γ (deg) = 92.57(2), Z = 1.

Breathing Crystals from Copper Nitroxyl Complexes

“…When the sample is cooled to ∼225 K, μ eff gradually decreases to ∼2.3 β. The short axial Cu-O L distance (2.237Å) in the octahedral CuO 6 units is the reason for the predominance of the antiferromagnetic contribution in >N-• O-Cu 2+ -O • -N< exchange clusters in the temperature range 225-300 K. 37,46 The results of the structural studies of [Cu(hfac) 2 L Et ] after the phase transition show that at 188 K the Cu-O L distance in CuO 6 units increases by only ∼0.02Å. This bond lengthening, however, leads to a drastic decrease in the antiferromagnetic contribution and the dominance of ferromagnetic interactions in >N-• O-Cu 2+ -O • -N< exchange clusters.…”

“…36,38 The reason is quite evident: structural rearrangements as profound as these generally lead to crystal decay. 36,37 For this compound, χ T decreased slowly when the temperature decreased from 100 to 30 K and then started to increase. This unusual effect certainly deserved a special study, but the crystal structure of the compound was not studied below the transition point because the single crystals exploded below 30 K. 40 Later it was found, however, that all the magnetic effects described in, 36,[38][39][40] had the same nature and were associated with a structural phase transition, which was induced by the external treatment and changed the distances and angles between the paramagnetic centers in heterospin exchange clusters and hence the exchange-coupling energy.…”

“…The Cu-O L distance, which is long enough at room temperature (2.484Å), is the reason for the predominance of ferromagnetic exchange interaction in Cu 2+ -O • -N< exchange clusters. 20,37,[46][47][48] The shift of the coordinated nitroxyl O L atoms from axial to equatorial position below 150 K leads to a change of ferromagnetic exchange to a strong antiferromagnetic interaction in half of all Cu 2+ -O • -N< exchange clusters. As a result, spins are compensated in these units, which leads to the √ 2-fold decrease in μ eff .…”

“…Later, a modification of a tetranuclear [Cu(hfac) 2 ] complex with 3-pyridyl-substituted imino nitroxide was described, with a similar structure, and a 2-step transition (at 140-152 and 70-80 K) was recorded for it. 37 The high-temperature transition was assumed to be a pseudospin transition, and the magnetic effect was correlated to the ordinary change of the equatorial coordination of the imine N atom at the exocyclic metal atom to the axial coordination in a square pyramid. Quantum-chemical calculations confirmed the main ideas used to explain these spin-crossover-like effects in Cu (II) complexes with nitronyl nitroxides.…”

Spin transitions in non-classical systems

“…13, 15 The reason is evident, that is, significant structural rearrangements in the complex usu ally lead to breakdown of the crystal. 13, 14 However, struc ture determination below the transition temperature is necessary for detailed analysis of the structural changes responsible for the spin transitions. Noteworthy are also changes in the χT parameter of a chain complex of Cu(hfac) 2 with nitronylnitroxyl biradical.…”