Synthetic ability of dinuclear mesocates containing 1,3-bis(diazinecarboxamide)benzene bridging ligands to form complexes of increased nuclearity. Crystal structures, magnetic properties and theoretical studies

Abstract

Triple stranded Ni^II₂-metallacyclic complexes Na_2.5[Ni₂(bpcb)₃]·0.5OH·18.5H₂O (1) and Na₂[Ni₂(bpzcb)₃]·16H₂O (2), and double stranded Cu^II₂-metallacyclic complexes [Cu₂(bpcb)₂(H₂O)₂]·8H₂O (3) and [Cu₂(bpzcb)₂(H₂O)₂]·4H₂O (4) have been assembled from the tailored bisbidentate bridging ligands, 1,3-bis(pyrimidine-2-carboxamide)benzene (H₂bpcb) and 1,3-bis(pyrazine-2-carboxamide)benzene (H₂bpzcb), and the corresponding nitrate salts of the metal ions. Following the “complex as ligand” strategy, 1 can be assembled with either Ni²⁺, Co²⁺ ions or the [Mn(acen)Cl] complex to afford unique, neutral, bent trinuclear molecules [M^IINi^II₂(bpcb)₃]·xH₂O (5 and 6) and the 2D honeycomb-like complex (PPh₄){[Ni₂(bpcb)₃]₂[Mn(acen)]₃} (7), respectively. In these cases, the Ni₂ units are linked to the corresponding metal ions through amidate oxygen atoms and the outward nitrogen atom of one of the pyrimidine rings of the bcpb ligand. The assembly of 2 with Ln³⁺ ions (Ln³⁺ = Tb, Gd) leads to one dimensional complexes of formula [{[Ni₂(bpzcb)₃]Tb(H₂O)₅}(CF₃SO₃)·THF·5H₂O]_n (8) and [{[Ni₂(bpzcb)₃]Ln(H₂O)₄(NO₃)}·2THF·nH₂O]_n (9 and 10) (Ln³⁺ = Gd and Tb), where the dinuclear Ni₂ units are joined to two Ln³⁺ ions exclusively through amidate oxygen atoms of two different ligands. The analyses of the magnetic data indicate that 1–4 exhibit intradinuclear ferromagnetic interactions between the metal ions through a spin polarisation mechanism, as supported by DFT calculations. Trinuclear complexes 5 and 6 show predominant antiferromagnetic coupling, which is a result of an antiferromagnetic interaction between one of the Ni²⁺ ions of the Ni₂ unit and the M²⁺ ion through the pyrimidine bridging fragment that is stronger than the polarised ferromagnetic interaction between the Ni²⁺ ions through the bpcb ligand in the dinuclear [Ni₂(bpcb)₃]²⁻ moiety. Complex 7 shows a dominant antiferromagnetic interaction between the Ni²⁺ and Mn²⁺, whereas the Ni₂Ln (Ln³⁺ = Gd, Tb) chain complexes present ferromagnetic interactions inside the Ni₂ mesocate unit as well as between the Ni²⁺ ions of the Ni₂ unit and the Ln³⁺ ions. The magnetic exchange interactions in these new materials have been experimentally analysed and supported by theoretical DFT studies.