To selectively capture molecules possessing an electron-acceptor character like nitric oxide, Hitoshi Miyasaka and colleagues, RIKEN Spring-8 Center, Hyogo, Japan, have synthesized one-dimensional chain compounds possessing a high donor character. They used 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).

CH₂Cl₂ occupies the spaces between chains of [−{M₂}−phz−] repeating units, until NO guest molecules trigger stepwise transformations with hysteresis absorption/desorption behavior: in a first gate-opening absorption step, NO is physisorpt by the gas-recognition ability of the pores. In a second gate-opening absorption step host/guest chemical interactions occur.
A charge transfer between the host [−{M₂}−phz−] framework and the NO molecules plays a key role for the specific adsorption behavior. The phz unit, which is activated by a π back-donation mechanism from the [M₂] units, and its near-field region, trap the NO molecules involving charge transfer.

The team thinks that the fine-tuning of the electrondonating ability of framework components is a promising strategy to achieve the advanced selective absorption properties.

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• Selective NO Trapping in the Pores of Chain-Type Complex Assemblies Based on Electronically Activated Paddlewheel-Type [Ru₂^II,II]/[Rh₂^II,II] Dimers,
  Wataru Kosaka, Kayo Yamagishi, Akihiro Hori, Hiroshi Sato, Ryotaro Matsuda, Susumu Kitagawa, Masaki Takata, Hitoshi Miyasaka,
  J. Am. Chem. Soc. 2013.
  DOI: 10.1021/ja4076056