Small Structural Variations Have Large Effects on the Assembly Properties and Spin State of Room Temperature High Spin Fe(II) Iminopyridine Cages

Abstract

Small changes in steric bulk at the terminus of bis-iminopyridine ligands can effect large changes in the spin state of self-assembled Fe­(II)-iminopyridine cage complexes. If the added bulk is properly matched with ligands that are either sufficiently flexible to allow twisted octahedral geometries at the Fe centers or can assemble with unusual mer configurations at the metals, room temperature high spin Fe­(II) cages can be synthesized. These complexes maintain their high spin state in solution at low temperatures and have been characterized by X-ray crystallographic and computational methods. The high spin M2L3 meso-helicate and M4L6 cage complexes display longer N–Fe bond distances and larger interligand N–Fe–N bond angles than their diamagnetic counterparts, and these structural changes invert the ligand selectivity in narcissistic self-sorting and accelerate subcomponent exchange rates. The paramagnetic cages can be easily converted to diamagnetic cages by subcomponent exchange under mild conditions, and the intermediates of the exchange process can be visualized in situ by NMR analysis.

No, Not That Way, the Other Way: Creating Active Sites in Self-Assembled Host Molecules

“…52,54 The packing around the Fe centers is much more challenging, and any complexes formed are not diamagnetic, but are paramagnetic, with high-spin Fe centers (Scheme 5). 55 The increased bulk around the pyridyl centers forces two changes to the coordination environment, allowing the assembly of three methylpyridines around each Fe center. The Fe-N bond lengths are increased by ca.…”

“…The mer 3 fac stereochemistry at the metals is also very useful for creating paramagnetic assemblies. 55 Even though the diaminofluorenol ligand is very rigid (and therefore should not be amenable to assembly with MePyCHO), the highly twisted coordination angles at the mer centers in 6 are already perfectly positioned to incorporate more steric bulk (Scheme 6, b). Paramagnetic cage 6•Me forms very easily when 6•H is heated with MePyCHO and Fe(ClO 4 ) 2 in CH 3 CN and shows NMR peaks in the characteristic  = -100 ppm to +200 ppm region.…”

“…all the assemblies were too small to act as hosts for anything bigger than a perchlorate anion. The next step was to take the diaminofluorene scaffold, which successfully formed M 4 L 6 assemblies with internalized functional groups 55,56,62 and simply make the cage bigger by making the ligands longer. This was not necessarily a good idea: by lengthening the ligand, the assembly will be larger, but the 'gaps' between the ligands are also larger.…”

“…) Assembly of diamagnetic or paramagnetic helicates upon aldehyde terminus variation; b) structures of diamagnetic helicate 3•H and paramagnetic helicate 3•Me; c) 'switch-off' paramagnetism via subcomponent exchange 55. …”

Tunable Fullerene Affinity of Cages, Bowls and Rings Assembled by PdII Coordination Sphere Engineering

“…[13] When considering other sterically more demanding donor functionalities, we recently started to substitute pyridines with quinolines, which carry an hydrogen atom on the annelated benzene ring that causes steric congestion aroundt he metal binding site. [14] We tested this methodo nb is-monodentate banana-shapedl igandsw ith ad ibenzo-2.2.2-bicyclo-octane backbonea nd showedt hat the pyridine derivative L 1 leads to the formation of atypical[Pd 2 L 1 4 ] 4 + cage while 6-quinolinyl derivative L 2 cleanly forms bowl [Pd 2 L 2 3 (MeCN) 2 ] 4 + when palladium precursor[ Pd(MeCN) 4 ](BF 4 ) 2 was mixed with L 2 in a2 :3 ratio in acetonitrile. [15] As explanation for the latter finding, we indeed identifieds teric congestion aroundt he coordination site, as observed in the single crystal X-ray structure of the bowl-shaped compound.…”