Structure and magnetic behaviour of mono- and bimetallic chromium(iii) complexes of amine-bis(phenolate) ligands

Abstract

Two lithium amine-bis(phenolate) and four chromium(III) amine-bis(phenolate) complexes have been prepared. The diprotonated tripodal tetradentate ligand precursors 2-tetrahydrofurfuryl-N,N-bis(2-methylene-4-methyl-6-tert-butylphenol), H₂[O₂NO]^BuMe; 2-tetrahydrofurfuryl-N,N-bis(2-methylene-4,6-tert-butylphenol), H₂[O₂NO]^BuBu; 2-pyridylamino-N,N-bis(2-methylene-4,6-methylphenol), H₂[O₂NN′]^MeMe; and 2-pyridylamino-N,N-bis(2-methylene-4,6-tert-butylphenol), H₂[O₂NN′]^BuBu; can be lithiated using n-butyllithium. Isolation of the Li₂[O₂NO]^RR′ compounds generates dimeric {Li₂[O₂NO]^RR′}₂ (R = t-Bu, R′ = Me in 1 and R = R′ = t-Bu in 2) in the solid state as shown by single-crystal X-ray diffraction. The lithiated ligands were used to prepare a series of Cr(III) complexes. Monometallic complexes are obtained when prepared and purified under strictly anhydrous conditions, giving CrCl(THF)[O₂NO]^RR′ (R = t-Bu, R′ = Me in 3 and R = R′ = t-Bu in 4). However, bimetallic Cr complexes are obtained upon recrystallization in air, where adventitious water reacts with the complex resulting in protonation of one of the phenolate groups of the ligand and generating hydroxide, which bridges two Cr(III) centres. Solid-state single-crystal X-ray diffraction studies of {CrCl[O₂NN′]^MeMe}(μ-HO){CrCl[HO₂NN′]^MeMe}, 5, and {CrCl[O₂NN′]^BuBu}(μ-HO){CrCl[HO₂NN′]^BuBu}, 6, were performed. The paramagnetic Cr(III) complexes were also characterized by UV-vis spectroscopy, mass spectrometry and magnetic measurements.