Preparation, X-ray Characterization, and Reactivity of the Rod-like and Angular Germanium- and Titanium(IV)-Capped Iron(II) Bis-Clathrochelates and Their Mono- and Bis-Capped (Semi)clathrochelate Precursors

Transmetalation of the bis{triethylantimony(V)}-capped iron(II) tris-α-dioximate with n-butylboronic acid afforded the mixed antimony, boron cross-linked clathrochelate with single reactive antimony(V)-based apical fragment. This macrobicyclic precursor easily underwent the transmetalation reactions with germanium and titanium(IV) alkoxides to give the rod-like and angular Fe^II₂M^IV-trinuclear bis-clathrochelates. Those of the aforementioned diantimony(V)-capped complex with 3- and 4-carboxyphenylboronic acids afforded the monoboron-capped iron(II) semiclathrochelates, undergoing a double-cyclization (macrobicyclization) with germanium- and titanium(IV)-based capping agents. The reactions in the low-temperature range unexpectedly gave the stable 2:1 associates, formed by the bridging of two carboxyl-terminated macrobicyclic molecules of the mixed carboxylboron, triethylantimony-capped iron(II) clathrochelate with a triethylantimony(V)-based linker fragment. The obtained complexes were characterized using elemental analysis, MALDI-TOF, ¹H and ¹³C{¹H} NMR and UV-vis spectra, and single-crystal XRD experiments. The encapsulated iron(II) ion in their 3D-molecules is situated almost in the center of its FeN₆-coordination polyhedron possessing a truncated trigonal-pyramidal geometry. Fe-N distances fall in the range 1.887(7)-1.945(4) Å characteristic of the low-spin iron(II) complexes. The cross-linking titanium and germanium(IV) ions in the corresponding bis-clathrochelate molecules form the octahedral M^IVO₆-coordination polyhedra, the M^IV-O distances of which vary from 1.946(2) to 1.964(2) Å and from 1.879(7) to 1.907(6) Å, respectively.