Theoretical and experimental studies of the protonated terpyridine cation. Ab initio quantum mechanics calculations, and crystal structures of two different ion pairs formed between protonated terpyridine cations and nitratolanthanate(III) anions

Ab initio quantum mechanics calculations have been carried out on all possible conformations of the terpyridine ligand and its mono- and di-protonated forms. Results show that the lowest energy form of the ligand is when the N-C-C-N torsion angles are trans but that in the protonated forms, the cis arrangement is prevalent being stabilised by intramolecular N-H ... N hydrogen bonds. Results are consistent with the experimental crystal structure data found in the literature and also with two crystal structures reported here in which two different ion pairs formed between protonated terpyridine cations and lanthanate(III) nitrate anions have been prepared and analysed structurally. Compound 1 consists of discrete diprotonated terpyridine cations and hexanitrato-lanthanate anions, namely 3[H(2)terpy](2+)2[La(NO3)(6)](3-). 3H(2)O. Water molecules are present as hydrogen bond accepters in the diprotonated terpyridyl cavities. Each lanthanum atom is 12-co-ordinate and the La-O bond lengths vary between 2.609(11) and 2.765(10) Angstrom. Compound 2 consists of a diprotonated [H(2)terpy](2+) cation together with a [Sm(terpy)(NO3)(4)](-) anion, and a NO3- anion which is present as a hydrogen bond acceptor in the diprotonated terpyridyl cavity. The samarium atom is 1 l-co-ordinate, the Sm-O bond lengths vary between 2.494(5) and 2.742(5) Angstrom while the Sm-N bond lengths vary between 2.626(4) and 2.650(5) Angstrom.