Phenylalanine-Based Co2+ and Cd2+ 1D Coordination Polymers: Structural Properties and Catalytic Application for Solvent-Free Aerobic Oxidation of Cycloalkene

Two 1D coordination polymers (CPs) with general formula [M(L)(H2O)(AcO)]n, (M = Co (1) or Cd (2), AcO = acetate anion and L denotes l-phenylalanine based ligand), were synthesized and fully characterized by various spectroscopies (UV-vis, FTIR, and NMR), thermal techniques, magnetic measurements (for 1), and single-crystal and powder X-ray diffraction studies. They can be described as "ribbon-like" 1D polymers constructed through a zigzag arrangement. The polymeric structure is developed due to the coordination mode adopted by the amino acid ligand, classified as μ3-N1O1:O1:O2, which simultaneously links three metal centers. This moiety also plays an important role as a magnetic coupler between metal centers in the cobalt system, which shows a weak antiferromagnetic interaction. Both CPs have also been used in the catalytic oxidation of cyclohexene with molecular oxygen (O2) as an oxidant. Under mild conditions, both compounds demonstrated remarkable catalytic activity, with the cobalt system being more efficient than the cadmium analogue (conversion: 73 and 58% and selectivity for the major product, 2-cyclohexanone: 63 and 55%, for 1 and 2, respectively). Leaching experiments and the results obtained using a radical quencher are consistent with a radical-mediated mechanism for the Co compound. The presence of the superoxide radical was also confirmed using EPR spectroscopy and DMPO as a spin trap, which was further validated by DFT calculations. The activity observed for the Cd analogue is attributed to the organic scaffold assisted by the templating effect of the metal ion.

Multiple rotor modes and how to trigger them: complex cation ordering in the family of relaxing hybrid formates

A combination of structural, dielectric and calorimetric studies is used to describe a highly atypical behaviour of novel hybrid formate [NH₃(CH₂)₃NH₂(CH₂)₃NH₃][Mn(HCOO)₃]₃, incorporating large triprotonated molecular cations. Two successive phase transitions, switching between fast multiple rotor modes, and the surprising probable coexistence of static and dynamic disorder are discussed for this compound.

Effect of Alkali and Trivalent Metal Ions on the High-Pressure Phase Transition of [C2H5NH3]MI 0.5MIII 0.5(HCOO)3 (MI = Na, K and MIII = Cr, Al) Heterometallic Perovskites

We report the high-pressure behavior of two perovskite-like metal formate frameworks with the ethylammonium cation (EtAKCr and EtANaAl) and compare them to previously reported data for EtANaCr. High-pressure single-crystal X-ray diffraction and Raman data for EtAKCr show the occurrence of two high-pressure phase transitions observed at 0.75(16) and 2.4(2) GPa. The first phase transition involves strong compression and distortion of the KO₆ subnetwork followed by rearrangement of the -CH₂CH₃ groups from the ethylammonium cations, while the second involves octahedral tilting to further reduce pore volume, accompanied by further configurational changes of the alkyl chains. Both transitions retain the ambient P2₁/n symmetry. We also correlate and discuss the influence of structural properties (distortion parameters, bulk modulus, tolerance factors, and compressibility) and parameters calculated by using density functional theory (vibrational entropy, site-projected phonon density of states, and hydrogen bonding energy) on the occurrence and properties of structural phase transitions observed in this class of metal formates.

A novel co-crystallization molecular ferroelectric induced by the ordering of sulphate anions and hydrogen atoms

The co-crystallization complex [EG]·[Cu(phen)₂·SO₄] undergoes a paraelectric–ferroelectric phase transition around 272 K which was confirmed by the observation of a large dielectric anomaly.

Magnetic layered perovskites of [CH3C(NH2)2]2[M(HCOO)4] (M = Co2+ and Ni2+): synthesis, structures and properties

Two layered perovskites of the formula [CH₃C(NH₂)₂]₂[M(HCOO)₄] (M = Co and Ni) exhibit anisotropic thermal expansion behavior and weak ferromagnetism.

A Series of Bimetallic Ammonium AlNa Formates

A series of AlNa bimetallic ammonium metal formate frameworks (AlNa AMFFs) have been prepared by employing various ammoniums from NH₄⁺ to large linear polyammoniums. The series consists of six perovskites of (4¹² ⋅6³ ) topology for mono-ammoniums, two chiral (4⁹ ⋅6⁶ ) frameworks incorporating polyethylene ammoniums, two niccolites with (4¹² ⋅6³ )(4⁹ ⋅6⁶ ) topology containing diammoniums, and two layered compounds made of 2D (4,4) AlNa formate sheets intercalated by small diammoniums. The first ten compounds present the structural hierarchy of (4¹² ⋅6³ )_m (4⁹ ⋅6⁶ )_n framework topologies for (m, n)=(1, 0), (0, 1), and (1, 1), respectively, in parallel to the homometallic AMFFs for divalent metals. The symmetry lowering, asymmetric formate bridges, and different hydrogen-bonding strengths appeared in the bimetallic structures owing to the different charge and size of Al³⁺ and Na⁺ seemingly inhibits the occurrence of phase transitions for more than half the AlNa AMFFs within the series, and the bimetallic members undergoing phase transitions show different transition behaviors and dielectric properties compared with the homometallic analogs. Anisotropic/negative/zero thermal expansions of the materials could be rationally attributed to the librational motion, or flip movement between different sites, of the ammonium cations, and the coupled change of AlNa formate frameworks. The thermal and IR spectroscopic properties have also been investigated.

Study of the relationship between magnetic field and dielectric properties in two ferromagnetic complexes

Two heterometallic niccolite structure frameworks [NH₂(CH₃)₂][Cr^IIIM^II(HCOO)₆] (M = Fe, Ni) were reported and characterized by single crystal X-ray diffraction, dielectric and magnetic susceptibility measurement.

Syntheses, structures, and magnetic properties of two unique Cu(ii)-based coordination polymers involving a crystal-to-crystal structural transformation from a 1D chain to a 3D network

Promoted by the DMF solvent, the 1D chain structure of 1 can be irreversibly transformed into the 3D sod network structure of 2 in a crystal-to-crystal fashion, which is accompanied by a drastic magnetic change.

Ferro- or antiferromagnetic interactions controlled by ditopic or chelating auxiliary ligands in 3D metal-organic frameworks

Three 3D Co(ii) metal-organic frameworks of the general formula [Co(DNNDC)(NN)]_n were reported, where DNNDC = 1,5-dinitronaphthalene-3,7-dicarboxylate and NN = bridging or chelating dipyridyl ligands. The MOFs exhibit similar 3D [Co(DNNDC)]_n frameworks in which double-carboxylate-bridged [Co(OCO)₂]_n chains are cross-linked by naphthalene backbones. The bridging dipyridyl ligands [4,4'-bipyridine (bpy) and 1,2-bis(4-pyridyl)ethane (bpe)] serve as additional linkers between the chains to generate a reinforced framework with new net topology, while the chelating ligand 1,10-phenanthroline (phen) reinforces the frameworks through π-π stacking and C-HO interactions. Remarkably, varying the auxiliary ligand leads to a change in the sign of the magnetic exchange through the double carboxylate bridge: the MOFs with the ditopic bpy and bpe ligands show ferromagnetic interactions, while the one with the chelating phen ligand shows antiferromagnetic exchange. The difference in magnetic exchange is attributable to the subtle change in the coordination conformation of the carboxylate bridge.

A Variety of Phase-Transition Behaviors in a Niccolite Series of [NH3 (CH2 )4 NH3 ][M(HCOO)3 ]2

A niccolite series of [bnH₂²⁺][M(HCOO)₃]₂ (bnH₂²⁺=1,4‐butyldiammonium) shows four kinds of metal‐dependent phase transitions, from high temperature para‐electric phases to low‐temperature ferro‐, antiferro‐, glass‐like, and para‐electric phases. The conformational flexibility of bnH₂²⁺ and the different size, mass, and bonding character of the metal ion lead to various disorder‐order transitions of bnH₂²⁺ in the lattice and relevant framework modulations, thus different phase transitions and dielectric responses. The magnetic members display a coexistence or combination of electric and magnetic orderings in the low‐temperature region.

Phase transitions and dielectric properties of a hexagonal ABX3 perovskite-type organic–inorganic hybrid compound: [C3H4NS][CdBr3]

The perovskite-type hybrid thiazolium tribromocadmate(ii) exhibits two phase transitions at 180 and 146 K, accompanied by remarkable dielectric responses.

Synthesis and characterization of novel niccolites [(CH3)2NH2][FeIIIMII(HCOO)6] (MII = Zn, Ni, Cu)

We report the synthesis, structural, magnetic, Raman and IR studies of novel niccolite-type heterometallic [(CH₃)₂NH₂][Fe^IIIM^II(HCOO)₆] compounds (M^II = Ni, Zn and Cu). The Ni and Cu compounds order ferromagnetically at 42 and 28.5 K, respectively. The monoclinic Cu member exhibits different structural changes on cooling than the trigonal Zn and Ni compounds.

Temperature-dependent studies of [(CH3)2NH2][FeIIIMII(HCOO)6] frameworks (MII = Fe and Mg): structural, magnetic, dielectric and phonon properties

We report temperature-dependent studies of [(CH₃)₂NH₂]Fe^IIIMg^II(HCOO)₆] and [(CH₃)₂NH₂]Fe^IIIFe^II(HCOO)₆] and discuss the origin of different behaviors of both compounds.

A heterometallic strategy to achieve a large magnetocaloric effect in polymeric 3d complexes

A large magnetocaloric effect was realized in polymeric 3d complexes for the first time by complementarity of magnetic orbital interactions in a heterometallic system.

Phase transitions, prominent dielectric anomalies, and negative thermal expansion in three high thermally stable ammonium magnesium-formate frameworks

We present three Mg-formate frameworks, incorporating three different ammoniums: [NH4][Mg(HCOO)3] (1), [CH3CH2NH3][Mg(HCOO)3] (2) and [NH3(CH2)4NH3][Mg2(HCOO)6] (3). They display structural phase transitions accompanied by prominent dielectric anomalies and anisotropic and negative thermal expansion. The temperature-dependent structures, covering the whole temperature region in which the phase transitions occur, reveal detailed structural changes, and structure-property relationships are established. Compound 1 is a chiral Mg-formate framework with the NH4(+) cations located in the channels. Above 255 K, the NH4(+) cation vibrates quickly between two positions of shallow energy minima. Below 255 K, the cations undergo two steps of freezing of their vibrations, caused by the different inner profiles of the channels, producing non-compensated antipolarization. These lead to significant negative thermal expansion and a relaxor-like dielectric response. In perovskite 2, the orthorhombic phase below 374 K possesses ordered CH3CH2NH3(+) cations in the cubic cavities of the Mg-formate framework. Above 374 K, the structure becomes trigonal, with trigonally disordered cations, and above 426 K, another phase transition occurs and the cation changes to a two-fold disordered state. The two transitions are accompanied by prominent dielectric anomalies and negative and positive thermal expansion, contributing to the large regulation of the framework coupled the order-disorder transition of CH3CH2NH3(+). For niccolite 3, the gradually enhanced flipping movement of the middle ethylene of [NH3(CH2)4NH3](2+) in the elongated framework cavity finally leads to the phase transition with a critical temperature of 412 K, and the trigonally disordered cations and relevant framework change, providing the basis for the very strong dielectric dispersion, high dielectric constant (comparable to inorganic oxides), and large negative thermal expansion. The spontaneous polarizations for the low-temperature polar phases are 1.15, 3.43 and 1.51 μC cm(-2) for 1, 2 and 3, respectively, as estimated by the shifts of the cations related to the anionic frameworks. Thermal and variable-temperature powder X-ray diffraction studies confirm the phase transitions, and the materials are all found to be thermally stable up to 470 K.