THEORETICAL AND EXPERIMENTAL STUDIES OF THE PROTONATED TERPYRIDINE CATION - AB-INITIO QUANTUM-MECHANICS CALCULATIONS, AND CRYSTAL-STRUCTURES OF 2 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 liga nd is when the N-C-C-N torsion angles are trans but that in the proton ated forms, the cis arrangement is prevalent being stabilised by intra molecular N-H ... N hydrogen bonds. Results are consistent with the ex perimental crystal structure data found in the literature and also wit h two crystal structures reported here in which two different ion pair s formed between protonated terpyridine cations and lanthanate(III) ni trate 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. W ater molecules are present as hydrogen bond accepters in the diprotona ted 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. Comp ound 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 sa marium atom is 1 l-co-ordinate, the Sm-O bond lengths vary between 2.4 94(5) and 2.742(5) Angstrom while the Sm-N bond lengths vary between 2 .626(4) and 2.650(5) Angstrom.