[MII(H2dapsc)]-[Cr(CN)6] (M = Mn, Co) Chain and Trimer Complexes: Synthesis, Crystal Structure, Non-Covalent Interactions and Magnetic Properties

Abstract

Four new heterometallic complexes combining [M^II(H₂dapsc)]²⁺ cations with the chelating H₂dapsc {2,6-diacetylpyridine-bis(semicarbazone)} Schiff base ligand and [Cr(CN)₆]^3- anion were synthesized: {[M^II(H₂dapsc)]Cr^III(CN)₆K(H₂O)_2.5(EtOH)_0.5}_n·1.2n(H₂O), M = Mn (1) and Co (2), {[Mn(H₂dapsc)]₂Cr(CN)₆(H₂O)₂}Cl·H₂O (3) and {[Co(H₂dapsc)]₂Cr(CN)₆(H₂O)₂}Cl·2EtOH·3H₂O (4). In all the compounds, M(II) centers are seven-coordinated by N₃O₂ atoms of H₂dapsc in the equatorial plane and N or O atoms of two apical -CN/water ligands. Crystals 1 and 2 are isostructural and contain infinite negatively charged chains of alternating [M^II(H₂dapsc)]²⁺ and [Cr^III(CN)₆]^3- units linked by CN-bridges. Compounds 3 and 4 consist of centrosymmetric positively charged trimers in which two [M^II(H₂dapsc)]²⁺ cations are bound through one [Cr^III(CN)₆]^3- anion. All structures are regulated by π-stacking of coplanar H₂dapsc moieties as well as by an extensive net of hydrogen bonding. Adjacent chains in 1 and 2 interact also by coordination bonds via a pair of K⁺ ions. The compounds containing Mn^II (1, 3) and Co^II (2, 4) show a significant difference in magnetic properties. The ac magnetic measurements revealed that complexes 1 and 3 behave as a spin glass and a field-induced single-molecule magnet, respectively, while 2 and 4 do not exhibit slow magnetic relaxation in zero and non-zero dc fields. The relationship between magnetic properties and non-covalent interactions in the structures 1-4 was traced.

Keywords: crystal structure; dc and ac magnetic properties; field-induced single-molecule magnetism; metamagnetic transition; non-covalent interactions; seven-coordinate PBP complexes; spin-glass.

Scheme 1

Some diacetylpyridine-based acyclic pentadentate (N₃O₂) ligands: R = NH₂ (H₂dapsc), 2-OHC₆H₄ (H₄daps); 4-OCH₃C₆H₄ (H₂dapmbh), C₆H₅ (H₂dapbh), C₆H₅-C₆H₄ (Biph), CH₃CHOH (l-daplh).

Figure 1

Asymmetric unit in 1 with atom numbering scheme (ORTEP drawing with 50% probability ellipsoids, solvate water molecules are omitted for clarity). Colored atoms: Cr (green), K (grey), Mn (magenta), N (blue), O (red). Symmetry codes: (a) (−x, y − 0.5, −z), (b) (1 − x, y − 0.5, 1 − z), (c) (−x, y + 0.5, −z) and (d) (1 − x, y + 0.5, 1 − z).

Figure 2

Infinite CN-bridged {[M^II(H₂dapsc)]²⁺[Cr^III(CN)₆]³⁻}_n chains in 1 interacting by π-π stacking of H₂dapsc ligands and coordination bonds via K⁺/O in the (1 0 –1) plane. Interchain coordination bonds via pairs of K⁺ cations are shown by black bonds. Hydrogen bonds are shown by red dashed lines (see Tables S1 and S2 for hydrogen bond geometry in 1 and 2, respectively).

Figure 3

Part of the structure 1 displaying zigzag infinite chain of π-stacked coplanar [Mn(H₂dapsc)] units; the [Cr(CN)₆] anions, H atoms and most of water molecules are omitted for clarity. The Mn-Mn distances are 8.2341(6) and 8.2357(6) Å for 1 and 2 interactions, respectively (magenta dotted lines). Shortened C…C contacts (<3.6 Å) between H₂dapsc fragments are shown by black dashed lines.

Figure 4

Asymmetric unit in 3 with atom numbering scheme (ORTEP drawing with 50% probability ellipsoids). Colored atoms: Cl (light green), Cr (green), Mn (magenta), N (blue), O (red). Symmetry code: (e) (1 − x, −y, −z).

Figure 5

The ac layer in the structure 3. Intermetallic distances (in Å) are shown (magenta color for Mn…Mn, green color for Mn…Cr). Hydrogen bonds are shown by red dashed lines (see Table S3 for hydrogen bond geometry).

Figure 6

Asymmetric unit in 4 with atom numbering scheme (ORTEP drawing with 50% probability ellipsoids). Colored atoms: Cl (light green), Co (cyan), Cr (green), N (blue), O (red). Symmetry code: (e) (1 − x, −y, −z).

Figure 7

The ab layer in the structure 4 (solvate water and ethanol are omitted for clarity). Intermetallic distances (in Å) are shown (cyan color for Co…Co, green color for Co…Cr). Hydrogen O-H…O/N-H…N bonds are shown by red/blue dashed lines (see Table S4 for hydrogen bond geometry).

Figure 8

Temperature dependencies of magnetic susceptibility χ_mol and effective magnetic moment μ_eff (insets) for 1 (a), 2 (b), 3 (c) and 4 (d). The blue and green colors of the symbols distinguish the data for Mn- and Co-based complexes, respectively. The dashed lines on the insets show the estimations of effective magnetic moment for a unit cell with the Co, Cr and Mn spins, indicated on the legends.

Figure 9

M(H) loop (a) and its first derivative (b) at 2 K for complex 2.

Figure 10

Temperature dependencies of χ’ (a) and χ″ (b) at different frequencies for complex 1 in a zero dc field.

Figure 11

Frequency dependencies of χ’ (a) and χ″ (b) at different temperatures for complex 3 in 1500 Oe dc field.

Figure 12

The dependence of lnτ vs. 1/T for 3 in 1500 Oe dc field and its approximation using the Arrhenius formula (solid line). The points were obtained from the frequency dependencies χ″(ν) at different temperatures (Figure 11, main text). Approximation parameters: activation energy U_eff = 4.1 K, a pre-exponential factor τ₀ = 4.2·10⁻⁶ s.