Condensation of 1,4-dimethoxybenzene (DMB) with paraformaldehyde in the presence of BF3.O(C2H5)2 gave novel para-bridged pentacyclic pillar DMB (DMpillar[5]arene). Moreover, para-bridged pentacyclic hydroquinone (pillar[5]arene) was prepared. Pillar[5]arene formed 1:1 host-guest complexes with dialkyl viologen and alkyl pyridinium derivatives. However, pillar[5]arene did not form complexes with the diadamantyl viologen derivative since a bulky adamantyl group was unable to thread the cavity of pillar[5]arene.

A series of pillar[5]arene derivatives with alkyl groups of different length were synthesized. The new alkyl-substituted pillar[5]arene derivatives 1,4-bis(ethoxy)pillar[5]arene (C2), 1,4-bis(propoxy)pillar[5]arene (C3), 1,4-bis(butoxy)pillar[5]arene (C4), 1,4-bis(pentyloxy)pillar[5]arene (C5), 1,4-bis(hexyloxy)pillar[5]arene (C6), and 1,4-bis(dodecanoxy)pillar[5]arene (C12) were obtained by Lewis acid-catalyzed condensation of dialkoxybenzene monomers with paraformaldehyde. The conformational characteristics of the pillar[5]arene derivatives were investigated by dynamic (1)H NMR measurements. When the alkyl substituents were bulkier than methyl groups, the rotation of phenolic units in the pillar[5]arenes was suppressed and their conformation was immobilized. As their length increased, the alkyl substituents packed at the upper and lower rims and thus lowered the conformational freedom of the pillar[5]arenes.

A six-coordinate bis(μ-oxo)nickel(III) complex, [Ni2(μ-O)2(Me3-tpa)2]2+ (1), was synthesized by the reaction of [Ni2(μ-OH)2(Me3-tpa)2]2+ (2) with 1 equiv of hydrogen peroxide in methanol at −90 °C, where Me3-tpa = tris(6-methyl-2-pyridylmethyl)amine. The 6-methyl groups of Me3-tpa have a significant influence on the formation and stabilization of the high-valent bis(μ-oxo)dinickel(III) species. The reaction of 2 with a large excess of hydrogen peroxide (>10 equiv) afforded a novel bis(μ-superoxo)dinickel(II) complex, [Ni2(μ-O2)2(Me3-tpa)2]2+ (3), thus, the reaction demonstrates a unique conversion of a NiIII(μ-O)2NiIII core into a NiII(μ-OO)2NiII core upon exposure to hydrogen peroxide. Complexes 1, 2, and 3 have been characterized by X-ray crystallography and various physicochemical techniques. Complex 1 has a Ni(μ-O)2Ni core and the average Ni−O and Ni−N bond distances (1.871 and 2.143 Å, respectively) are significantly shorter than those of 2 (2.018 and 2.185 Å, respectively), suggesting that 1 is a bis(μ-oxo)dinickel(III) complex. Complex 3 consists of a Ni(μ-OO)2Ni core with two μ-1,2-O−O bridges to form a six-membered ring with chair conformation and the O−O bond distance is 1.345(6) Å. The resonance Raman spectrum of a powdered sample of 3 measured at ∼110 K showed an isotope-sensitive band at 1096 cm-1 (1044 cm-1 for an 18O-labeled sample), indicating that 3 is a bis(μ-superoxo)dinickel(II) complex. Thermal decomposition of both 1 and 3 in acetone at −20 °C under N2 atmosphere resulted in partial hydroxylation of a methyl group of Me3-tpa in yields of 21−27% for both complexes. For complex 3, a carboxylate complex, [Ni(Me2-tpaCOO)(OH2)]+ (4), where one of the three methyl groups of Me3-tpa is oxidized to carboxylate, was also isolated as a decomposed product under N2 atmosphere. During the decomposition process of 3, dioxygen evolution was simultaneously observed. The electrospray ionization mass spectrometry (ESI-MS) of 3 revealed the formation of 1 during the decomposition process. These results suggest that one possible decomposition pathway of 3 is a disproportionation of two coordinated superoxides to dioxygen and peroxide followed by the O−O bond scission of peroxide to regenerate 1, which is responsible for the hydroxylation and the oxidation of the 6-methyl group of Me3-tpa.