Binding of α,ω-alkyldiammonium ions by cucurbit[n]urils in the gas phase

Abstract

We investigated gas-phase complexes of α,ω-n-alkyldiammonium ions with cucurbit[5]uril (CB[5]), decamethylcucurbit[5]uril (mc5), penta(cyclohexyl)cucurbit[5]uril (CB*[5]), hexa(cyclohexyl)cucurbit[6]uril (CB*[6]), cucurbit[7]uril (CB[7]) and cucurbit[8]uril (CB[8]) using electrospray Fourier transform ion cyclotron resonance mass spectrometry and collision-induced dissociation techniques. The five-membered cucurbit[n]urils (CB[n]s) form singly charged 1:1 and doubly charged 2:1 diamine:CB[n] complexes. All dissociate via loss of neutral α,ω-n-alkyldiamine with only weak dependence of dissociation thresholds on chain length. For a given diamine, threshold energies are in the order CB[5] < mc5 < CB*[5]. This is consistent with guest hydrogen bonding on the portals of the CB[5]s with no threading into the host's interior. The n ≥ 6 CB[n]s form 1:1 complexes with doubly protonated α,ω-n-alkyldiamines. These collisionally dissociate via four channels: loss of singly protonated α,ω-n-alkyldiammonium; fragmentation of the CB[n] cage; loss of neutral α,ω-n-alkyldiamine and fragmentation of the α,ω-n-alkyldiamine. The dissociation threshold energies and branching ratios exhibit strong dependence on the length of the α,ω-n-alkyldiamine and the size of the CB[n]. The data suggest that the optimum α,ω-n-alkyldiamine length for binding CB*[6] is three to four methylene groups; for CB[7], four to five methylene groups and for CB[8], five to six methylene groups, indicating an increasing tendency for the guest to span the host cavity diagonally as the size of the CB[n] increases.