1D hetero-bimetallic regularly alternated 4f–3d coordination polymers based onN-oxide-4,4′-bipyridine (bipyMO) as a linker: photoluminescence and magnetic properties

Abstract

Heterotopic divergent ligand N-oxide-4,4'-bipyridine (bipyMO) has been herein exploited for the preparation of hetero-bimetallic coordination polymers where Ln(hfac)3 and M(hfac)2 nodes regularly alternate (Hhfac = 1,1,1,5,5,5-hexafluoro-2,4-pentanedione), bipyMO being able to selectively use its two potential coordination sites to discriminate the metal ions. The synthesis of three coordination polymers, [Ln(hfac)3M(hfac)2(bipyMO)2]n (Ln = Eu, M = Zn, 1; Ln = Eu, M = Cu, 2, Ln = Dy, M = Co, 3), was carried out by reacting the appropriate [M(hfac)2(bipyMO)]n and [Ln(hfac)3] precursors in toluene in the presence of a given stoichiometric amount of bipyMO. The products were characterized by elemental analysis, X-ray powder diffraction, and FTIR spectroscopy. Single crystal X-ray diffraction studies carried out on 2 showed that it was formed by chains containing the hexa-coordinated 3d metal (Cu(hfac)2[N]2) and the octa-coordinated lanthanide (Eu(hfac)3[O]2) nodes, where [N] and [O] stand for the donor atom of the bridging divergent ligand. The X-ray powder diffraction patterns of the three compounds and the comparison of their cell constant values allowed establishing that the derivatives were isotypic. Photoluminescence (PL) studies on microcrystalline sample powders evidenced a bright red emission for 1 with an absolute PL quantum yield of 0.24. The sensitized emission of Eu3+ can be excited in a wide wavelength range, from UV to visible, up to ≈450 nm. Conversely, europium emissions are not detectable in 2 due to the presence of Cu(hfac)2(bipyMO) moieties whose strong absorption overlaps Eu3+ transitions. Magnetic measurements conducted on 3 revealed the presence of a weak ferromagnetic interaction below 2.1 K. An ac susceptibility study highlighted a slow relaxation of the magnetization of 3 with an applied static magnetic field of 0.1 T, which could be equally fitted with a Orbach-direct or a Raman-direct mechanism. No relaxation dynamics was detected without the application of a static magnetic field.

Composition–Thermometric Properties Correlations in Homodinuclear Eu3+ Luminescent Complexes

“…As in bipyMO analogues, − the complexes show different europium coordination numbers (CNs) depending on the number of pyrzMO ligands bonded to the two metal centers: CN = 8 in 1 – 3 ; CN = 9 in 4 . The differences in the crystal field components of the 5 D 0 → 7 F J multiplets observed in the PL spectra of the complexes are due to small variations in the Eu 3+ coordination polyhedra related to CNs as well as to R and R′ substituents at the β-diketonato moieties.…”

“…Although intensity-based ratiometric luminescent thermometers have been the most greatly studied, the development of single-emitter (single-transition) thermometers is still very important for fluidodynamic and aerodynamic applications, where luminescent complexes are used in the so-called temperature-sensitive paints (TSPs) to map the surface temperature distributions. With TSPs the drawbacks deriving from the use of single emitters are bypassed by using the ratio of an image taken under test conditions to an image taken at a known reference condition. − Recently, we focused on lanthanide coordination chemistry with the heterotopic divergent N -oxide ligand 4,4′-bipyridine N -oxide (bipyMO). − The different affinities of 4f metal ions toward O- and N-donor ligands afford the synthesis of lanthanide dinuclear complexes with the composition [Ln 2 (β-diketonate) 6 (bipyMO) x ] ( x = 2, 3 depending on the β-diketonate) where the oxygen atom of bipyMO bridges two lanthanide ions and the nitrogen donor atom is not coordinated. − In this context, we developed a family (16 members) of homodinuclear Eu 3+ and Gd 3+ luminescent compounds to study the effect of the β-diketonato and N -oxide ligands on the thermometric properties of the complexes. We used four β-diketonato ligands with different steric and electronic properties commonly used for the synthesis of europium luminescent derivatives: thenoyltrifluoroacetone (Htta), dibenzoylmethane (Hdbm), benzoyltrifluoroacetone (Hbta), and hexafluoroacetylacetone (Hhfac).…”

Stoichiometrically Controlled Assembly of Lanthanide Molecular Complexes of the Heteroditopic Divergent Ligand 4′-(4-Pyridyl)-2,2′:6′,2″-terpyridine N-Oxide in Hypodentate or Bridging Coordination Modes. Structural, Magnetic, and Photoluminescence Studies

“…One of the most relevant contributions that ditopic organic ligands introduced into the field of molecular magnetism is the possibility of preparing the so-called single-chain magnets (SCMs), one-dimensional coordination polymers featuring significant intrachain magnetic interactions among anisotropic spin bearers but negligible interactions among the different chains, that can present an opening of the magnetic hysteresis loop at cryogenic temperatures . Moreover, the use of divergent organic ligands has been envisaged as promising for the preparation of ordered structures of SMMs with modulable magnetic properties, , and heterotopic ligands have been used to couple SMM units in one-dimensional − or three-dimensional coordination polymeric networks. Moving to discrete molecular entities, we found a coordination chemistry bottom-up approach has been used to induce a weak magnetic bias between different paramagnetic centers in SMMs, slowing their dynamics, to prepare polymetallic grids, to increase the number of addressable states in molecular candidates as qbits, or in a molecular switch, allowing movement from two to three available states within a single molecular entity …”

Homodinuclear Lanthanide Complexes with the Divergent Heterotopic 4,4′‐Bipyridine N‐Oxide (bipyMO) Ligand

“…These polymers are characterized by the presence of two different regularly alternated coordination sites, M*[N] 2 and M*[O] 2 , (M* is the complementary metal coordination sphere and [N] and [O] specify the donor atom of the connector). Heterometallic CPs have been prepared for 3d [M II ‐M′ II = Mn‐Cu] or for 3d–4f metal centres [Ln III = Eu, M II = Cu, Zn; Ln III = Dy, M II = Co] . In these heterometallic polymers the perfect alternation of the coordination sites (Cu*[N] 2 and Mn*[O] 2 or M*[N] 2 and Ln*[O] 2 ) showed the efficient discrimination operated by bipyMO.…”

“…With the intent to extend our synthetic studies, on the coordination chemistry of the bipyMO ligand, its reactivity of the bipyMO ligand towards oxophylic open metal sites, as in [Ln(β‐dik) 3 ], appeared quite attractive. In this paper we report the synthesis of Eu 2 (dbm) 6 (bipyMO) 2 1 , Tb 2 (dbm) 6 (bipyMO) 2 2 , Eu 2 (tta) 6 (bipyMO) 2 3 Eu 2 (hfac) 6 (bipyMO) 3 4 , Tb 2 (hfac) 6 (bipy>MO) 3 ( 5 ) (Hdbm = dibenzoylmethane, Htta = thenoyltrifluoroacetone, Hhfac = hexafluoroacetylacetone).…”