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

Abstract

The Group 1 complexes, [M(Me 6 [18]aneN 6)][BAr F ] (M = Li-Cs; Me 6 [18]aneN 6 = 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 CH 2 Cl 2 solution, and characterised spectroscopically ( 1 H, 13 C{ 1 H}, 7 Li, 23 Na, and 133 Cs NMR), by microanalysis and, for M = Li, K, and Rb, by single crystal X-ray analysis. The structures show N 6 -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 N 6 plane, with the [BAr F ] - anions above and below, leading to two K + species in the asymmetric unit (a hexagonal planar [K(Me 6 [18]aneN 6)] + cation and a '[K(Me 6 [18]aneN 6)(κ 1 -BAr F) 2 ] - anion', with long axial K⋯F interactions). The Rb + ion sits above the N 6 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(Me 3 tacn) 2 ] + (M = Na, K; distorted octahedral, N 6 donor set) and half-sandwich cations [Li(Me 3 tacn)(thf)] + (distorted tetrahedron, N 3 O donor set), [Li(Me 4 cyclen)(OH 2)] +, and [Na(Me 4 cyclen)(thf)] + (both distorted square pyramids with N 4 O donor sets) were also prepared (Me 3 tacn = 1,4,7-trimethyl-1,4,7-triazacyclononane, Me 4 cyclen = 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(Me 3 tacn) 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.