Chiral S-1,1′-bi-2-naphthol (S-BINOL) as a synthon for supramolecular hydrogen-bonded {(S-BINOLATn−)(S-BINOL)n}-strands with naphthyl-paneled cavities or channels for a Cd(NH3)4-fragment (n = 2) or [Ag(NH3)2]+ (n = 1). Part 2

Abstract

The molecule 1,1′-bi-2-naphthol (BINOL) shows a propensity for supramolecular, hydrogen-bonded strand formation when crystallized with its deprotonated form BINOLAT²⁻ or BINOLAT⁻ (in conc. ammonia). The strand adapts from racemic rac-BINOL to enantiomeric S-BINOL and from di- to monocationic metal ions but keeps the overall strand feature. The structure of [Cd²⁺(S-BINOLAT²⁻-κ²O,O′)(NH₃)₄](S-BINOL)₂(H₂O)(MeOH)₂ (1) can be correlated to the known structures of [M(NH₃)₆]²⁺(rac-BINOLAT²⁻)(rac-BINOL)₂ (M = Ni, Cd) despite the direct Cd-O,O′-BINOLAT chelate coordination in the former. With Ag⁺ as a monocationic metal structures of the formula [Ag(NH₃)₂]⁺(S-BINOLAT⁻)(S-BINOL)(EtOH) (2) and [Ag(NH₃)₂]⁺(S-BINOLAT⁻)(S-BINOL)(H₂O)₂(MeOH) (3) are formed from ethanol or methanol, respectively. Crystallization of Ag⁺/NH₃ with rac-BINOL from EtOH also yields 2 with a spontaneous resolution to an enantiomeric excess of ca. 40%. Simultaneous differential thermoanalysis, thermogravimetry and mass spectrometry (DTA-TG-MS) show that the EtOH, MeOH and H₂O solvent of crystallization together with the NH₃ ligands can be removed before the BINOL moieties. X-Ray powder diffraction (XRPD) still shows the sample of 1 to be crystalline after the loss of solvent of crystallization. Upon heating to 130 °C the needle-shaped crystals of 2 keep their shape, yet darken (formation of Ag by XRPD) and turn amorphous, with small crystal “prickles” (BINOL by XRPD) forming on the surface. The difference in metal coordination between 1 and [Cd(NH₃)₆]²⁺(rac-BINOLAT²⁻)(rac-BINOL)₂ is evidenced by a strong maximum at 591 nm in the emission spectrum of 1 which is absent in the emission spectra of both BINOL and [Cd(NH₃)₆]²⁺(rac-BINOLAT²⁻)(rac-BINOL)₂. This maximum is assigned to a metal–ligand transition due to the direct Cd-O,O′-BINOLAT chelate coordination in 1.