Article pubs.acs.org/IC
A Copper−Nitroxide Adduct Exhibiting Separate Single Crystal-toSingle Crystal Polymerization−Depolymerization and Spin Crossover Transitions Victor Ovcharenko,*,† Sergey Fokin,† Elvina Chubakova,† Galina Romanenko,† Artem Bogomyakov,† Zhanna Dobrokhotova,⊥ Nikita Lukzen,† Vitaly Morozov,† Marina Petrova,† Maria Petrova,∥ Ekaterina Zueva,∥ Igor Rozentsveig,‡ Elena Rudyakova,‡ Galina Levkovskaya,‡ and Renad Sagdeev§ †
International Tomography Center, SB RAS, Institutskaya Str., 3a Novosibirsk 630090, Russian Federation A. E. Favorsky Irkutsk Institute of Chemistry, SB RAS, 1 Favorsky Str., Irkutsk 664033, Russian Federation ⊥ N. S. Kurnakov Institute of General and Inorganic Chemistry, RAS, 31 Leninsky Ave., Moscow 119991, Russian Federation ∥ Kazan National Research Technological University, 68 K. Marx Str., Kazan 420015, Russian Federation § Kazan (Volga Region) Federal University, 18 Kremlevskaya Str., Kazan 420008, Russian Federation ‡
S Supporting Information *
ABSTRACT: A complex cascade of solid-state processes initiated by variation of temperature was found for the heterospin complex [Cu(hfac)2LMe/Et] formed in the reaction of copper(II) hexafluoroacetylacetonate [Cu(hfac)2] with stable nitronyl nitroxide 2-(1-methyl-3-ethyl-1H-pyrazol-4yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1oxyl (LMe/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)2LMe/Et2][[Cu(hfac)2]3LMe/Et2]. 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]3LBu/Et2] and [[Cu(hfac)2]5LBu/Et4] molecular complexes, which are models of the trinuclear fragment of the {[Cu(hfac)2]3LMe/Et2} pair cluster.
■
INTRODUCTION The development of methods for the synthesis of Cu(II) complexes with nitroxides underlies the creation of unusual heterospin compounds, which exhibit thermo- and photoinduced spin transitions.1−3 Since the classic spin crossover4 is impossible in Cu(II) complexes with diamagnetic ligands, including the diamagnetic structural analogs of nitroxides,5,6 an essential condition for this effect is coordination of an additional paramagnetic center, i.e., formation of at least a two-center exchange cluster.7 A spin-crossover-like phenomenon is observed when the external effect changes the mutual orientation of the paramagnetic centers and consequently the character of interaction of odd electrons.7−11 The thermally induced change in the distances between the paramagnetic centers in the {>N−•O−Cu(II)} or {>N−•O−Cu(II)−O•− NN−•O−Cu(II)} and {>N−•O−Cu(II)−O•−NN−•O−Cu(II)−O•−NN−•O−Cu(II)} exchange clusters and the central {>N−•O−Cu(II)−O•−NN−•O−Cu2+−O•−NN−•O···O•−NN− •O−Cu2+−O•−NN−•O−Cu(II)−O•− NN−•O−Cu(II)−O•−NN−•O} groups changes from 3.625 Å at 85 K to 4.232 Å at 290 K). At the same time, the calculations did not confirm any significant exchange ( 256 K.
calculations agree with experiment better than the results of molecular calculations. Note that when the interradical interchain exchange is neglected, that is, in the approximation with all exchange clusters in crystal regarded as magnetically isolated, the agreement between the calculated and experimental μeff(T) curves is appreciably worse (green line, Figure 3). Below 140 K three-spin exchange clusters containing the Cu(3) atoms with strong internal antiferromagnetism are the paramagnetic centers with effective spin S = 1/2. On the basis of mathematical considerations, one can show that these centers are linked into a magnetic chain with neighboring threespin exchange clusters containing Cu(1) atoms via the effective exchange parameter J0(eff) = (2/3)J0 < 0. Thus, at T < 140 K the magnetic chain consists of alternating three-spin exchange clusters and one-spin fragments. At lowered temperatures, the effective spins 1/2 are aligned oppositely to the spins of the three-spin clusters. This is associated with the initial decrease in the total effective magnetic moment below 140 K. In the three-spin exchange clusters with the positive exchange integral, spin alignment into the state with the resulting spin S = 3/2 occurs concurrently. This leads to an increase in the effective magnetic moment of the compound at lowered temperatures after reaching a certain minimum at 20 K (Figures 2 and 3). The mutually complementary data of Figures 1 and 2 for the range 150−160 K show that in complexes with spin-labeled pyrazoles, a spin transition can occur in molecular solid phases. Can we regard the {[Cu(hfac)2]3LMe/Et2} fragments responsible for the spin transition as individual trinuclear molecules, for which the shortest distance to the adjacent {Cu(hfac)2LMe/Et2} mononuclear fragments is ∼2.7 Å (Figure 1)? The Cu−N E
DOI: 10.1021/acs.inorgchem.6b00140 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
the trinuclear fragments of these complexes at 295 K are very similar: 2.353(2) and 2.351(4) Å (Figures 1 and 4). The experimental μ e f f (T) dependence for [[Cu(hfac)2]3LBu/Et2] is shown in Figure 5. At room temperature
distance, which is 2.568(3) Å at 290 K, can be regarded (based on XRD data and structural criteria developed for Jahn−Teller ions29) as a quite admissible axial distance. It follows from the CCDC statistical database (November 2014; version 5.36; updates May 2015) that in the compound containing a Cu··· O−N fragment, two types of Cu−O distances are most characteristic: 1.9−2.0 and 2.3−2.6 Å, which correspond to the position of the O atoms in the equatorial plane of the Cu bipyramid and in the axial positions, respectively. For the Cu− N bond, the most characteristic distances are 1.9−2.3 Å. Fewer distances lie in the range 2.3−2.6 Å (Figure 1S). For [Cu(hfac)2LMe/Et] at 240 K (i.e., after the phase transformation), however, the Cu−N distance (2.744(3) Å) is certainly too long, and we can suppose that the Cu−N chemical bond is absent in the fragments in question. Consequently, for solids with a molecular structure, spin transitions are possible in Cu(II) complexes with spin-labeled pyrazoles; previously, the transitions were recorded only for polymer chain compounds or cyclic dimers.2,9 This is an important result, which affects further development of the theoretical description of spin transitions in heterospin systems.30,31 Note that previous attempts to synthesize these compounds failed.9,13 Selective synthesis of heterospin molecular compounds capable of exhibiting a spin transition is problematic because it is very difficult to predict the required modification of the paramagnetic organic molecule. The situation is further complicated by the fact that temperature variation initiates many coherent intra- and intermolecular motions of atoms, which provide the mechanical stability of the single crystal as an entity, especially during the phase transformation.8 In addition, copper heterospin complexes with nitroxides are stereochemically nonrigid compounds, and hence one can isolate many heterospin complexes using the same paramagnetic ligand depending on the synthesis conditions and reagent ratio.32 Therefore, we had to perform a few hundred experiments before we managed to isolate molecular Cu(hfac)2 complexes with pyrazolyl-substituted nitronyl nitroxides that exhibit spin crossover by varying the set of R and R′ substituents in the pyrazole ring, the reagent ratio, solvent, and synthesis conditions. One of these complexes was [[Cu(hfac)2]3LBu/Et2], whose molecular structure is shown in Figure 4. Details of the structural study and selected bond lengths and angles for this complex are listed in Table 2S. According to their topology, the trinuclear [[Cu(hfac)2]3LBu/Et2] molecules are similar to the trinuclear fragments of [Cu(hfac)2LMe/Et2][[Cu(hfac)2]3LMe/Et2] formed at T < 260 K (Figures 1 and 2). The Cu−ONO bond lengths in
Figure 5. Dependence μeff(T) for [[Cu(hfac)2]3LBu/Et2].
μeff is 4.14 μB, which corresponds to five noninteracting paramagnetic centers with spin S = 1/2, two of which have g = 2.0, and the other three have g = 2.22. When the complex is cooled from 170 to 150 K, μeff decreases to 3.97 μB. When the complex is cooled from 140 to 105 K, μeff smoothly decreases to ∼3.60 μB, and then further to 3.13 μB with a plateau at 60 K. This value corresponds to three noninteracting paramagnetic centers with spin S = 1/2, two of which have g ≈ 2.15, and the third one has g = 2.0. The form of the μeff(T) dependence for [[Cu(hfac)2]3LBu/Et2], which is very similar to the dependence described previously for the [Cu(hfac)2LMe/H] polymer chain complex,32 points to a spin transition in the heterospin complex. Regretfully, the single crystals of the compound decomposed during the X-ray diffraction experiment, which precluded the obtaining of data on the temperature dynamics of the structure of [[Cu(hfac)2]3LBu/Et2] below 170 K. When the products of the reaction of Cu(hfac)2 and LBu/Et were studied, one more molecular complex was isolated with an unusual Cu(hfac)2/paramagnetic ligand ratio (5/4). [[Cu(hfac)2]5LBu/Et4] is a kinetic product that can be obtained as described in the Experimental Section. The crystal structure of the compound is formed by the centrosymmetric [[Cu(hfac)2]5LBu/Et4] molecules (Figure 6). The vertices of the
Figure 6. [[Cu(hfac)2]5LBu/Et4] molecule at 295 K.
square bipyramid of the central Cu1 atom are occupied by the O atoms of NO groups (Cu(1)−ONO = 2.303(3) Å, Table 2). The donor N atoms of the ligand occupy one of the vertices of the square bipyramids at the Cu(2) atom of the {Cu(2)O5N} coordination units at a distance of 2.388(3) Å. The environment is completed by the O atom of the NO group of the
Figure 4. Structure of the [[Cu(hfac)2]3LBu/Et2] molecule at 295 K. F
DOI: 10.1021/acs.inorgchem.6b00140 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Table 2. Selected Bond Lengths (Å), Calculated Exchange Parameters J (cm−1) for [[Cu(hfac)2]5LBu/Et4], and Fraction of the Low-Temperature Phase (w) at Different Temperatures T (K) 295 V (Å3) Cu−O
175
JCu(1)−O•−N JN−O•···O•−N JCu(3)−O•−N
32 −2 −370
JCu(1)−O•−N JN−O•···O•−N JCu(3)−O•−N1 w
23 −1041
3805.6(2) 3707.0(1) 2.287(2) 2.175(2) 2.673(2) 2.681(2) 1.957(2) 1.959(2) 2.380(2) 2.379(2) Gaussian09 UB3LYP/TZVP 35 −101 −4 16 −750 −764 ORCA 3.0 UB3LYP/def2-TZVP 16 −84 9 6 −373 −386 Quantum Espresso 5.0 GGA+U 24 −62 −734 −736
0
0.07
Cu−N
3895(1) 2.303(3) 2.683(3) 1.960(2) 2.388(3)
240
JCu(1)−O•−N JN−O•···O•−N JCu(3)−O•−N
0.46
neighboring LBu/Et molecule with a distance of 2.683(3) Å. The terminal Cu(3) atoms have a square pyramidal environment and coordinate one ONO atom of the paramagnetic ligand. The Cu(3)−ONO distance is short, 1.960(2) Å. Note that two of the four LBu/Et molecules are coordinated by the copper ions via the N atoms of the pyrazole ring and one of the O atoms of the NO groups, while the other two LBu/Et molecules perform the bridging function only via the O atoms of the paramagnetic fragment of the ligand. When the crystal was cooled to 240 K, the X-ray diffraction experiment revealed only a slight shortening of all interatomic distances and insignificant changes in the angles (Table 2). The cooling of [[Cu(hfac)2]5LBu/Et4] from 295 to 240 K actually led to an insignificant compression of the crystal. Further cooling of the compound leads to a structural transition and the ensuing spin transition (Figure 7). Despite the considerable rearrangements of coordination units, the phase transition occurs without crystal decay during the repeated cooling−heating cycles, which allowed us to trace the structural dynamics of the complex. The μeff value at 300 K tends to 4.01 μB corresponding to a system of five weakly interacting centers with spin 1/2 and g = 2.07. The contribution of two terminal copper ions and two LBu/Et coordinated by these ions is absent because of the dominance of strong antiferromagnetic interactions (Table 2), which lead to compensation of spins already at room temperature. When the high-temperature phase is cooled from 240 to 80 K, μeff decreases to 3.21 μB. The reverse course of the μeff(T) dependence for [[Cu(hfac)2]5LBu/Et4] repeats the curve over the whole temperature range. In the temperature range 80−5 K, μeff corresponds to three independent spins with S = 1/2 and an average g-factor of ∼2.13, which agrees with the data on the structural rearrangement of solid [[Cu(hfac)2]5LBu/Et4] and the appearance of strong antiferromagnetic exchange interactions in the {>N−•O−Cu2+−•O−NN−•O−Cu2+−•O−NN−•O−Cu(II)−O•− NN−•O− Cu(II)−O • −NN−•O−Cu(II)−O•−N
