Self-Assembly of Chromium(II) Metal-Ion and Pyrazine Ligand into One-Dimensional Coordination Polymers

Abstract

The first examples of a one-dimensional (1D) coordination polymer featuring a chromium/pyrazine (pyz) repeating motif have been synthesized from Cr(OTf)₂-based precursors (OTf: triflate). In nitrile-based solvents (RCN), a fine control of the synthesis conditions allows the selective formation of both chain and ladder-like compounds: {Cr(pyz)(RCN)₂(OTf)₂}_n (1, R: Ph) and {Cr₂(pyz)₃(RCN)₂(OTf)₄}_n•X_n (2, R: Ph; 3, R: Me, X: CH₂Cl₂; 4, R: Me), respectively. Single-crystal X-ray diffraction experiments reveal the systematic presence of octahedral Cr(II) ions bridged by pyrazine ligands, forming the 1D coordination polymers. In 2–4, these Cr-pyrazine chains are paired by additional bridging pyrazines resulting in ladder-like arrangements. In these structures, two triflate ions are coordinated via oxygen atoms to the Cr ions in a trans- or cis- configuration for 1–3 and 4, respectively. In the trans- configuration, the coordinating oxygen atoms of the triflate ions align with the direction of the Jahn–Teller elongation observed at each metal node. In contrast, the chromium Jahn–Teller axis in 4 involves a pyrazine ligand, forming the rungs of the ladder structure and one triflate anion. The +II oxidation state of the Cr ions was further confirmed by X-ray absorption spectroscopy, which demonstrates the lack of intramolecular electron-transfer between the chromium center and the pyrazine and the stability of the Cr(II) ions under the given reaction conditions and coordination environments. Magnetic susceptibility measurements reveal the presence of weak antiferromagnetic interactions, which dominate between S = 2 Cr(II) spins in all compounds. Using a Heisenberg chain model for classical S = 2 spins, as developed by Fisher (H = −2JΣ_i=1ⁿS⃗_i·S⃗_i+1), the exchange interaction (J) through the neutral pyrazine is estimated at −3.1, −3.3, −3.4, and −2.5 K for 1–4, respectively. Combining magnetic and calorimetric measurements at low temperatures, a long-range antiferromagnetic ordering is detected below 20 K in all four materials.