Thermally induced order–disorder transition in crystals of lithium complexes of 1,5-Bis[2-(diphenylphosphorylmethyl)phenoxy]-3-oxapenthane [LiL](ClO4) and [LiL](NCS): Effect of the anion on the temperature of transition

Compounds of 1,5-bis[2-(diphenylphosphorylmethyl)phenoxy]-3-oxapenthane (L) with lithium salts LiXL, where Х– = ClO4, NCS, or I, and LiBrL ⋅ 2H2O, are synthesized. The IR spectra and the thermal behavior of complexes are studied. X-ray diffraction studies of isostructural [LiL](ClO4) (I) and [LiL](NCS) (II) crystals are performed. In the [LiL]+ complex, the ligand coordinates the Li atom by five O atoms, forming an environment in the shape of a trigonal bipyramid [in II at 174 K, the Li–O bonds are1.829(3)–2.341(3) Å]. It is found that crystals I and II undergo a reversible phase transition of the order–disorder type with the change of symmetry (P21/n ↔ P$$\bar 1$$). The temperatures of transition of crystals I and II differ by ~70°. This distinction is explained by different types of disordering of the phenyl rings of the L ligand in the monoclinic phase, namely, dynamic in I and static in II. The analysis of intermolecular contacts allows one to relate the state of disordering of ligand L in the crystal to the strength of its interactions with the anion: in II the NCS– anion is firmly fixed by the С–Н···S and С–Н···N hydrogen bonds (at 174 K, H···S is 2.77 and H···N is 2.36 Å); in I the С–Н···О interactions of the ClO$$\bar 4$$ anion with the ligand are weaker (at 215 K, Н···О ≥ 2.37 Å) and the anion is disordered.