New HATNA(CN)6 ligand in the design of dianion magnetic assemblies with lanthanides {Cryptand(K+)}2{HATNA(CN)6·3LnIII(TMHD)3}2− (Ln = Gd, Tb and Dy)

Abstract

Hexaazatrinaphthylenehexacarbonitrile ligand, HATNA(CN)6, when reduced to dianion state, forms crystalline {Cryptand[2.2.2](K+)}2{HATNA(CN)6}2−·2C6H4Cl2 (1) salt whose structure, optical and magnetic properties are studied. The HATNA(CN)6}2− dianions absorb light in the whole visible and a wide NIR range from 900 up to 2200 nm and have singlet EPR silent ground state. The dianions form trinuclear {Cryptand[2.2.2](K+)}2{HATNA(CN)6·3LnIII(TMHD)3}2−·3C6H5CH3 complexes where Ln is Gd (2), Tb (3) and Dy (4), and TMHD is 2,2,6,6-tetramethyl-3,5-heptanedionate. The {HATNA(CN)6}2− dianions are strongly distorted in coordination assemblies and have the length of the La-N{HATNA(CN)6} bonds of 2.58–2.66 Å. Dianions preserve low-energy absorption at the coordination of three metal complexes with maxima of the bands at 1440–1460 nm. DFT calculations support strong acceptor properties of HATNA(CN)6 and the non-degenerated LUMO for this ligand. As a result, dianions have singlet ground state whereas triplet state is positioned higher by 139 meV only (1620 K) and is thermally unattainable at 300 K. Coordination of the lanthanide complexes increases the singlet–triplet gap up to 210 meV (2440 K). Complexes 2–4 show weak antiferromagnetic coupling between high-spin lanthanide ions. J value estimated by PHI program for the gadolinium-complex is close to zero. Among 2–4 only gadolinium-containing complex 2 manifests an EPR signal with a complicated structure which can be approximated with D = 0.19 and E = 0.02 cm−1.