Mononuclear, Five-Coordinate Lanthanide Amido and Aryloxide Complexes Bearing Tetradentate (N₂O₂) Schiff Bases

Abstract

Two monomeric, five-coordinate lanthanide complexes, [bis-5,5'-(1,3-propanediyldiimino)-2,2-dimethyl-4-hexene-3-onato]samarium[2,6-bis(tert-butyl)-4-methylphenoxide] and [bis-5,5'-(1,3-propanediyldiimino)-2,2-dimethyl-4-hexene-3-onato]erbium[2,6-bis(tert-butyl)-4-methylphenoxide], were isolated from the reactions of 2,6-bis(tert-butyl)-4-methylphenol with [bis-5,5'-(1,3-propanediyldiimino)-2,2-dimethyl-4-hexene-3-onato]lanthanide[bis(trimethylsilyl)amido] (lanthanide = Er(3+) and Sm(3+)). The purified phenoxides were recovered in excellent yields and analytical purity, and the reactions proceeded cleanly without Schiff-base degradation or cluster formation. Analogously, [bis-3,3'-(1,3-propanediyldiimino)-1-phenyl-2-butene-1-onato]erbium[bis(trimethylsilyl)amido] was also directly converted to [bis-3,3'-(1,3-propanediyldiimino)-1-phenyl-2-butene-1-onato]erbium[2,6-bis(tert-butyl)-4-methylphenoxide]; however, a less sterically demanding alcohol (i.e., ethanol) yielded a neutral trinuclear oxo alkoxide species with each dianionic Schiff base asymmetrically bridging through micro-oxo interactions. In this polynuclear cluster, each symmetry-related, seven-coordinate erbium(III) ion exhibits monocapped trigonal prismatic geometry, which assembles by sharing triangular capped faces. Single-crystal X-ray diffraction revealed square-pyramidal metal coordination in each five-coordinate lanthanide ion with varied S(4) ruffling of the "square base" donor atoms and the six-membered propylene diamine chelate ring adopting the boat conformation. To contrast the effect of subtle ligand changes, we also report the synthesis and characterization of [bis-5,5'-(2,2-dimethyl-1,3-propanediyldiimino)-2,2-dimethyl-4-hexene-3-onato]samarium[bis(trimethylsilyl)amido], having gem-dimethyl substituents appended to the propylene bridge central carbon. The six-membered diamine chelate ring in this compound adopts the chair conformation without metal-hydrocarbon interaction. Also presented are qualitative activity observations and polymerization data for the polymerization of rac-lactide and epsilon-caprolactone using the five-coordinate lanthanide amidos and phenoxides.

Synthesis and structural characterization of two five coordinate aluminum alkyl and bis(trimethylsilyl)amino complexes bearing acyclic tetradentate Schiff bases

“…The regions where the two Schiff-bases in 2 and 3 do not superimpose are near the linker groups and the second t Bu group which is rotated about the C(1)-C(2) bond in 2 relative to 3. Although the ligand set in 2 is virtually identical (with the exception of one less methylene group in the linker) to that for the earlier trivalent lanthanide complexes [9], the ionic radius of 5-coordinate Al 3+ is smaller. The net result of replacing a trivalent lanthanide ion with the smaller Al 3+ ion is therefore to "pull" all of the ligands radially toward the central metal ion.…”

“…1,2-diaminoethane (Acros), absolute ethanol (EtOH, Pharmco), Et 3 Al solution (Aldrich) and benzene (C 6 H 6 ) were used as received whereas alkali earth bis(trimethy lsilyl)amides (Gelest) and rac-lactide were first purified by double sublimation at ~10 −5 Torr. 2,2-Dimethyl-3,5-hexanedione [10] and bis-5,5′-(1,3-propanediyldiimino)-2,2-dimethyl-4-hexene-3-one [9] were synthesized using literature methods. Toluene (C 7 H 8 ) and deuterated benzene (C 6 D 6 ) with sodium metal and pentane (C 5 H 12 , CaH 2 ) were dried and distilled immediately before use.…”

“…In our hands we, for the first time, wanted to evaluate "saturated" SB Al 3+ complexes for the polymerization of racemic lactide. The advantages of these complexes would be the lack of aromaticity and tunable peripheral steric bulk, both precluding the possibility of oligomerization in the solution and solid-state species [9].…”

“…Our secondary interest in these Group 13 complexes stems from their utility as benchmarks to isoelectronic five-coordinate rare earth Schiff-base complexes [9]. The present study was also designed to compare Al 3+ structural features and activities with those of these heavier congeners.…”

Crystal structure of 2-bromo-4-chloro-6-[(4-chlorophenylimino) methyl]phenol, C13H8BrCl2NO

“…Schiff base ligands have played an important role in the development of coordination chemistry related to catalysis and enzymatic reactions [1,2], magnetism and molecular architectures [3,4] and biological properties [5,6]. 3-Bromo-5-chlorosalicylaldehyde is an important intermediate in synthesizing pesticides and drugs, andanactive aldehydeeasilytoreact with many amines, but the Schiff base with 4-chloroanilinehas notbeenreportedsofar.…”