Thermal spin-crossover in the [M3Zn6Cl6L12] (M = Zn, FeII; L = 5,6-dimethoxy-1,2,3-benzotriazolate) system: structural, electrochemical, Mössbauer, and UV-Vis spectroscopic studies

Abstract

Fusion of pentanuclear Kuratowski-type coordination units leads to homo- and heterononanuclear coordination compounds, two of which are presented, having structural formulae [Zn(9)Cl(6)(OMe(2)bta)(12)]·DMF (1), and [Fe(II)(3)Zn(6)Cl(6)(OMe(2)bta)(12)]·DMF (2), respectively; (OMe(2)btaH = 5,6-dimethoxy-1,2,3-benzotriazole; DMF = N,N'-dimethylformamide). Single crystal X-ray structure analyses reveal the presence of {M(3)Zn(6)L(12)}(6+) cores (M = Zn or Fe(II); L = 5,6-dimethoxy-1,2,3-benzotriazolate) in which the M(II) ions are bridged by μ(3)-OMe(2)bta ligands. In both compounds, the six peripheral Zn ions are tetracoordinated, whereas the remaining three metal ions M are hexacoordinated. The charge of each {M(3)Zn(6)L(12)}(6+) moiety is balanced by six chloride anions that are monodentately bound to the peripheral Zn ions. Based on differences in experimental Fe-N-donor bond lengths (deduced from single crystal data of 2 recorded at 223 K), two out of three Fe(II) ions are found in a high-spin (HS) state, whereas one Fe(II) ion shows a low-spin (LS) state. The assignment of different energetic ground states of Fe(II) ions is corroborated by spectroscopic studies: Both solid-state and solution UV-Vis spectra of 2 (at ambient temperature) display absorption bands owing to the presence of both HS and LS Fe(II) ions. Removal of occluded DMF molecules from the crystal lattices of 1 and 2 in high vacuum leads to fully desolvated powders, termed hereafter 1a and 2a, respectively. Mössbauer studies on 2a show that all three Fe(II) ions are in HS state at 160 K, and upon cooling to 7 K, the central Fe(II) ion undergoes a HS→LS transition while the HS states of the other Fe(II) ions remains unchanged. The cyclic voltammogram of 2 (chloroform solution) exhibits a single reversible oxidation regardless of different Fe(II) spin states in the nonanuclear core of 2.

Charge Delocalization and Bulk Electronic Conductivity in the Mixed-Valence Metal–Organic Framework Fe(1,2,3-triazolate)2(BF4)x

“…The Fe1–N1 and Fe2–N2 distances, where Fe1 and Fe2 correspond to the tetrahedral corner and center iron­(II) sites, respectively, are 1.961(2) and 1.977(3) Å, consistent with Fe–N distances for compounds with low-spin Fe II ions and strong-field ligands . Metal–organic frameworks exhibiting low-spin Fe II centers are quite rare in the literature, although there have been several reports suggesting that triazolate-based ligands can stabilize low-spin octahedral Fe II centers in zero-, one-, and three-dimensional coordination compounds.…”

Photophysical properties of Kuratowski-type coordination compounds [MIIZn4Cl4(Me2bta)6] (MII = Zn or Ru) featuring long-lived excited electronic states

“…20 The series of isostructural compounds featuring open-shell 3d-metal centres in the central octahedral site of the pentanuclear units was recently extended via synthesis and characterization of their Fe(II), Ni(II), and Cu(II) counterparts. 19 It should be noted here, that structurally similar homopentanuclear complexes were previously reported; 23,24,25 their systematic development with respect to catalytic applications, and their use as rigid building blocks in supramolecular coordination compounds, 29, 30 or infinite 3D coordination networks, 26,27,31 however, constitutes a recent finding, the potential of which shall be fully exploited in the future.…”

CuN6 Jahn–Teller centers in coordination frameworks comprising fully condensed Kuratowski-type secondary building units: phase transitions and magneto-structural correlations

“…It should be noted in this context that the compound [Fe II 3 Zn 6 Cl 6 L 12 ] (L = 5,6dimethoxy-1,2,3-benzotriazolate), which is a heterononanuclear dimer of condensed Kuratowski units, shows thermally induced spin-crossover. 28 Embedding Fe(II) centers in [Cu(ta) 2 ]-type frameworks might thus lead to robust compounds exhibiting stimulus responsive spin-crossover hysteresis, which could be employed in sensing or information processing applications. On the other hand, employing less symmetric 1,2,3-triazolate ligands might lead to a symmetry reduction in [Cu(ta) 2 ] networks, 29 and the resulting compounds might display ferroelectric properties, that is spontaneous electric polarization.…”