Post‐Metalation Modification of a Macrocyclic Dicobalt(III) Metallohost by Site‐Selective Ligand Exchange for Guest Recognition Control

Abstract

We propose post‐metalation modification as a useful strategy to control the guest recognition behavior of a metal‐containing macrocyclic host. This is based on the ligand exchange of the axial ligands of a cobalt(III) dinuclear macrocyclic host, [LCo2X4]2+ (X=axial amine ligand). Four piperidine ligands in [LCo2(pip)4]2+ (pip=piperidine) were site‐selectively replaced with primary amines. The competitive experiments revealed that the order of the affinity toward the cobalt centers in [LCo2X4]2+ is primary amine > secondary amine > tertiary amine and that the piperidine‐coordinating complex, [LCo2(pip)4]2+, was reasonably reactive among the isolable complexes. Indeed, two piperidine ligands at the diagonal positions in [LCo2(pip)4]2+ were site‐selectively replaced with pyridine or acetate ion. The replacement of piperidine with acetate ion significantly enhanced the recognition ability towards Na+.

Guest Recognition Control Accompanied by Stepwise Gate Closing and Opening of a Macrocyclic Metallohost

“…One acetato ligand coordinates to one of the cobalt(III) ions and Na + ion, which is located in the O 6 cavity, in a bridging fashion. Similar Co–Na bridging was observed in the crystal structure of [LCo 2 (pip) 2 (OAc) 2 ⋅Na](OTf), which has two acetato ligands at the diagonal positions [5] …”

“…Thus, we attempted to re‐open the apertures of a host structure after once closing them (Scheme 1 b). Indeed, we achieved a two‐step conversion, open→close→open, by using a macrocyclic cobalt(III) metallohost platform, [LCo 2 (pip) 4 ](OTf) 2 (pip=piperidine) (Scheme 1 c); [5, 6, 10] we closed this open metallohost [LCo 2 (pip) 4 ](OTf) 2 by the introduction of bridging ligands, 1,6‐hexanediamine (hda), on both apertures of the macrocyclic cavity, then re‐opened the closed metallohost [LCo 2 (hda) 2 ](OTf) 2 by the introduction of acetato ligand in the second step. By taking advantage of this successive structural conversion, we successfully controlled the guest uptake behavior of the metallohost in an on‐demand fashion.…”

“…We chose [LCo 2 (pip) 4 ](OTf) 2 as the starting complex for the post‐metalation modification method to close the metallohost structure, because this complex was reasonably reactive among the isolable complexes [5] . When hda was added to [LCo 2 (pip) 4 ](OTf) 2 in CD 3 CN, a new set of signals gradually appeared in the 1 H NMR spectrum of the mixture.…”

“…We have recently developed the post-metalation modification method, [5,6] by which we can introduce av ariety of substituents/groupsi nto ah ost scaffold in ar eversible fashion underm ild conditions by using ligand exchange. We used hexacoordinate metal ions, which are placed in the tetradentate chelate coordination sites in organic host molecules, so that we can introduce variousk inds of organic ligands at the two axial positions (Scheme 1a).…”

“…We chose [LCo 2 (pip) 4 ](OTf) 2 as the startingc omplex for the post-metalation modification methodt oc lose the metallohost structure, because this complex was reasonably reactive amongt he isolable complexes. [5] When hda was added to [LCo 2 (pip) 4 ](OTf) 2 in CD 3 CN, an ew set of signals gradually appearedi nt he 1 HNMR spectrum of the mixture. The spectrum after 1dshowed ah ighly symmetrical spectral pattern with only one set of signals for the salophu nit (H 2 saloph = N,N'-disalicylidene-o-phenylenediamine) and three signals fort he À (CH 2 ) 6 À moieties in the hda ligand ( Figure 1b,S upporting Information).…”