Aza-macrocyclic complexes of the Group 1 cations – synthesis, structures and density functional theory study

Abstract

The Group 1 complexes, [M(Me₆[18]aneN₆)][BAr^F] (M = Li–Cs; Me₆[18]aneN₆ = 1,4,7,10,13,16-hexamethyl-1,4,7,10,13,16-hexaazacyclooctadecane; BAr^F = tetrakis{3,5-bis(trifluoromethyl)-phenyl}borate), are obtained in high yield by reaction of the macrocycle with M[BAr^F] in anhydrous CH₂Cl₂ solution, and characterised spectroscopically (¹H, ¹³C{¹H}, ⁷Li, ²³Na, and ¹³³Cs NMR), by microanalysis and, for M = Li, K, and Rb, by single crystal X-ray analysis. The structures show N₆-coordination to the metal ion; the small ionic radius for Li⁺ leads to a puckered conformation. In contrast, the K⁺ ion fits well into the N₆ plane, with the [BAr^F]⁻ anions above and below, leading to two K⁺ species in the asymmetric unit (a hexagonal planar [K(Me₆[18]aneN₆)]⁺ cation and a ‘[K(Me₆[18]aneN₆)(κ¹-BAr^F)₂]⁻ anion’, with long axial K⋯F interactions). The Rb⁺ ion sits above the N₆ plane, with two long axial Rb⋯F interactions in one cation and two long, mutually cis Rb⋯F interactions in the other. The unusual sandwich cations, [M(Me₃tacn)₂]⁺ (M = Na, K; distorted octahedral, N₆ donor set) and half-sandwich cations [Li(Me₃tacn)(thf)]⁺ (distorted tetrahedron, N₃O donor set), [Li(Me₄cyclen)(OH₂)]⁺, and [Na(Me₄cyclen)(thf)]⁺ (both distorted square pyramids with N₄O donor sets) were also prepared (Me₃tacn = 1,4,7-trimethyl-1,4,7-triazacyclononane, Me₄cyclen = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane). Density functional theory (DFT) calculations, using the BP86 and B3LYP functionals, show that the accessibility of the [M(Me₃tacn)₂]⁺ sandwich cations depends strongly on the M⁺ ionic radius, such that it is sufficiently large to avoid steric clashing between the Me groups of the two rings, and small enough to avoid very acute N–M–N chelate angles. The calculations also show that coordination to the Group 1 cation involves significant donation of electron density from the p-orbitals on the N atoms of the macrocycle, rather than purely electrostatic interactions.