Selective NO Trapping in the Pores of Chain-Type Complex Assemblies Based on Electronically Activated Paddlewheel-Type [Ru₂^II,II]/[Rh₂^II,II] Dimers

The design of porous materials that undergo selective adsorption of a specific molecule is a critical issue in research on porous coordination polymers or metal–organic frameworks. For the purpose of the selective capture of molecules possessing an electron-acceptor character such as nitric oxide (NO), one-dimensional chain compounds possessing a high donor character have been synthesized using 4-chloroanisate-bridged paddlewheel-type dimetal­(II, II) complexes with M = Ru and Rh and phenazine (phz) as the chain linker: [M₂(4-Cl-2-OMePhCO₂)₄(phz)]·n(CH₂Cl₂) (M = Ru, 1; Rh, 2). These compounds are isostructural and are composed of chains with a [−{M₂}–phz−] repeating unit and CH₂Cl₂ occupying the void space between the chains. Compounds 1 and 2 change to a new phase (1-dry and 2-dry) upon evacuating the crystallization solvent (CH₂Cl₂) and almost lose their pores in the drying process: no void space in 1-dry and 31.8 ų, corresponding to 2.9% of the cell volume, in 2-dry. Nevertheless, the compounds show a unique gas accommodation ability. Accompanied by a structural transformation (i.e., the first gate-opening) at low pressures of <10 kPa, both compounds show a typical physisorption isotherm for O₂ (90 K) and CO₂ (195 K), with the adsorption amount of ca. 2–4 gas molecules per [M₂] unit. In addition, the adsorption isotherm for NO (121 K) involves the first gate-opening followed by a second gate-opening anomaly at NO pressures of ≈52 kPa for 1-dry and ≈21 kPa for 2-dry. At the first gate-opening, the absorbed amount of NO is ca. 4 molecules per [M₂] unit, and then it reaches 8.4 and 6.3 for 1-dry and 2-dry, respectively, at 95 kPa. Only the isotherm for NO exhibits hysteresis in the desorption process, and some of the NO molecules are trapped in pores even after evacuating at 121 K, although it recovers to the original dried sample on heating to room temperature. The adsorbed NO molecules accrue a significant electron donation from the host framework even in the [Rh₂] derivative, indicating that such simple porous compounds with electron-donor characteristics are useful for the selective adsorption of NO.