Practical Applications of Cage Metal Complexes

57Fe Mössbauer and DFT study of the electronic and spatial structures of the iron(ii) (pseudo)clathrochelates: the effect of ligand field strength

“…The nature of a given caging ligand (first of all, that of its regular donor or H-donor groups, the geometrical parameters of its molecule and the degree of its protonation as well) is known 1 to strongly affect the ability of this ligand, as a host, to encapsulate the appropriate organic or inorganic ions, or neutral molecules as guests. In the case of transition metal cage complexes (clathrochelates 2 ), as metal-centered compounds, the field strength of the corresponding macropolycyclic ligand also plays a very important role in their thermodynamic and kinetic stability, and the physicochemical (first of all, redox) and spectral characteristics as well. Indeed, cage complexes of this type with the encapsulated metal ion(s) are known to be chemically robust coordination compounds possessing unusual chemical, optical and redox properties.…”

“…Most of them are formed by the iron( ii )-centered clathrochelate ligands and monomers. Indeed, the Fe 2+ cation is known 2 to be the most efficient templating metal ion for coordination-driven self-assembly of the macropolycyclic tris-α-dioximates formed by capping with various Lewis acidic agents, such as the inorganic and elementoorganic derivatives of p-block elements {boron( iii ), tin, germanium, zirconium and hafnium( iv ), antimony( v ) etc. }.…”

“…As a result, a wide range of (poly)cage iron( ii ) complexes and their hybrid, oligo- and polymeric derivatives of the targeted structure, geometry and functionality have been prepared and structurally characterized to date. 2…”

“…Earlier, 57 Fe Mössbauer experimental and theoretical DFT studies have been widely used for the evaluation of the electronic structures of a number of bioactive iron-containing coordination compounds and cluster systems (first of all, the natural iron-based azaheterocyclic enzimes, 12 their synthetic 13 and biorelevant oxygen- and sulfur-containing clusters and metalloorganic analogs 14 ), including the iron clathrochelates. 2 The low-spin states of an overwhelming majority of the earlier-described mono- and binuclear (pseudo)cage iron( ii ) tris-α-diiminates, the derivatives of α-dioximes, α-oximehydrazones and azineoximes, both in solution and in the solid state can be clearly seen: (i) from their solid-state 57 Fe Mössbauer spectra; 2 a (ii) from their solution NMR spectra (the absence of any paramagnetic shift or paramagnetic broadening), and (iii) from the smaller values of Fe–N distances in their molecules (approximately 1.90–1.95 Å), 2 as compared with those characteristics of the high-spin iron( ii ) complexes with N -donor ligands (typically 2.1–2.2 Å). 15 Indeed, three bonding t 2g → e 1 + a 1 orbitals of the low-spin iron( ii ) (pseudo)clathrochelates are fully occupied due to very high ligand field strength, characteristic of a given type of their (pseudo)encapsulating ligands.…”

“…The average bite (chelate N–Fe–N) angle α in the aforementioned (pseudo)macrobicyclic tris-α-diiminate molecules is close to 80°. 2…”