A tetracobalt(II) cluster with a two vertex truncated dicubane topology endogenously supported by carboxylate-based (2-pyridyl)methylamine ligands: magneto-structural and DFT studies

A reaction between CoCl2 and L3-(CO2-)2 (2 : 1 stoichiometry) in CH3OH affords a discrete complex [CoII4-{L3-(CO2-)2}2(μ3-OCH3)2(CH3OH)2(H2O)2Cl2] (1) [L3-(CO2-)2 = 3-[N-{2-(pyridin-2-yl)methyl}amino]-bis(propionate)]. The structure of 1 reveals two terminal mononuclear CoII{L3-(CO2-)2}Cl units connected by a dimeric CoII2(μ3-OCH3)2(CH3OH)2(H2O) unit present in the centre through two methoxo (μ3-OCH3)- and two carboxylate (μ-1,1-OCO-) bridges affording a tetranuclear coordination cluster of Co(II) with a defective dicubane topology. In 1, Co1 (terminal) has distorted octahedral CoIIN2O3Cl and the central Co2 has CoIIO6 coordination. Such coordination arrangements afford the observed topology. Variable-temperature magnetic studies reveal anti-ferromagnetic coupling in 1. Three magnetic exchange interactions (one anti-ferromagnetic and two ferromagnetic: J1 = +3.3 cm-1 (Co⋯Co 3.176 Å; μ-1,1-OCO- and μ3-OCH3 bridges), J2 = -2.5 cm-1 (Co⋯Co 3.228 Å; μ-1-OCO- and μ3-OCH3 bridges) and J3 = +10.6 cm-1 (Co⋯Co 3.084 Å; two μ3-OCH3 bridges)) have been identified, with the inclusion of the orbital reduction parameter (α = Aκ = 1.38), spin-orbit coupling (λ = -158 cm-1) and axial distortion (energy gap Δ = -975 cm-1 between singlet and doublet levels), rationalized by density functional theory (DFT) calculations.

Crystal structure of poly[bis(μ2-1,4-di(1H-imidazol-1-yl)benzene-κ2 N:N′)-(μ2-tetraoxidomolybdato(VI)-κ2 O:O′)cobalt(II)], C24H20N8O4MoCo

C24H20N8O4MoCo, monoclinic, C2/c (no. 15), a = 19.3535(7) Å, b = 10.0870(4) Å, c = 12.4174(5) Å, β = 97.3700(10)°, V = 2404.08(16) Å3, Z = 4, R gt(F) = 0.0218, wR ref(F 2) = 0.0577, T = 298(2) K.

Crowding out: ligand modifications and their structure directing effects on brucite-like {Mx(μ3-OH)y} (M = Co(ii), Ni(ii)) core growth within polymetallic cages

Previous employment of the ligands 2-methoxy-6-[(methylimino)methyl]phenol (L₁H) and 2-methoxy-6-[(phenylimino)methyl]phenol (L₂H) has resulted in the self-assembly of pseudo metallocalix[6]arene complexes of general formulae: [M₇(μ₃-OH)₆(L_x)₆](NO₃)_y (M = Ni(ii), x = 1, y = 2 (1) and Co(ii/iii), x = 2, y = 3 (2)). Extrapolating upon this work, we report the coordination chemistry of ligands 2-methoxy-6-{[(2-methoxyphenyl)imino]methyl}phenol (L₃H), 2-[(benzylimino)methyl]-6-methoxyphenol (L₄H), 2-[(benzylamino)methyl]-6-methoxyphenol (L₅H) and 2-[(benzylamino)methyl]-4-bromo-6-methoxyphenol (L₆H), whose structures are modifications of ligands L_1-2H. These ligands are employed in the synthesis and characterisation of the dimetallic complex [Ni(ii)₂(L₃)₃(H₂O)](NO₃)·2H₂O·3MeOH (3); the monometallic complexes [Ni(ii)(L₄)₂] (4) and [Co(iii)(L₄)₃]·H₂O·MeOH (5a); and the tetranuclear pseudo metallocalix[4]arene complexes: [(NO₃)⊂Co(ii)₄(μ₃-OH)₂(L₅)₄(H₂O)₂](NO₃)·H₂O (6), [(NO₃)⊂Ni(ii)₄(μ₃-OH)₂(L₅)₄(H₂O)₂](NO₃)·H₂O (7) and [Ni(ii)₄(μ₃-OH)₂(L₆)₄(NO₃)₂]·MeCN (8). The tetrametallic 'butterfly' core topologies in 6-8 are discussed with respect to their structural and topological relationship with their heptanuclear [M₇] (M = Co(ii), Ni(ii)) pseudo metallocalix[6]arene ancestors (1 and 2).

New polyaminocarboxylate macrocycles containing phenolate binding units: synthesis, luminescent and relaxometric properties of their lanthanide complexes

The synthesis of two new polyaminocarboxylate macrocycles incorporating one or two intracyclic phenol units (H₄L¹ and H₈L², respectively) is described. The 12-membered H₄L¹ macrocycle leads to soluble and stable mononuclear Ln^III complexes of [(L¹)Ln(H₂O)₂]^- composition (Ln = Eu, Tb and Gd) in aqueous solutions. In Tris buffer (pH 7.4), the [(L¹)Tb(H₂O)₂]^- complex displays a suitable efficiency for sensitized emission (η_sens = 48%) and a high luminescence quantum yield (Φ = 22%), which is worthy of note for a bis-hydrated terbium complex. Besides, luminescence experiments show that bidentate endogenous anions (citrate, carbonate, and phosphate) do not displace the two inner-sphere water molecules of this complex. In contrast, the possible presence of LMCT states causes the europium complex to be weakly luminescent. The [(L¹)Gd(H₂O)₂]^- complex is characterized by high relaxivity (r = 7.2 s^-1 mM^-1 at 20 MHz) and a very short water residence time of the coordinated water molecules (τ = 9 ns), promising values for the realisation of macromolecular systems with high relaxivities. Thus, the Tb and Gd complexes of the H₄L¹ macrocycle exhibit several improvements in terms of luminescent (lower excitation energy, higher brightness) and relaxometric (shorter τ_M) properties compared to the corresponding LnPCTA complexes, where a phenol moiety substitutes a pyridine ring. On the other hand, the 24-membered H₈L² macrocycle including two phenol units in its structure leads to dinuclear complexes of [(L²)Ln₂]^2- composition. Its terbium complex shows a long luminescence lifetime (2 ms) and a high quantum yield (43%) in aqueous solutions, making this compound a new promising candidate for time-resolved applications.

Tetranuclear Ni(ii) and Co(ii) Schiff-base complexes with an M4O6 defective dicubane-like core: zero-field SMM behavior in the cobalt analogue

Two tetranuclear Ni₄ and Co₄ complexes were prepared and characterized. Their magnetic properties were thoroughly studied and it was revealed that the Co₄ compound behaves as a zero-field single-molecule magnet.

Syntheses, crystal structures and steady state and time-resolved fluorescence properties of a PET based macrocycle and its dinuclear ZnII/CdII/HgII complexes

The present report deals with solution spectroscopic (absorption and emission) and colorimetric studies of a Robson type tetraaminodiphenol macrocyclic ligand and its ZnII2, CdII2 and HgII2 compounds.

Synthesis, structure and magnetic properties of phenylhydroxamate-based coordination clusters

The strategic recombination of preformed coordination clusters in the presence of polymodal bridging ligands has successfully led to the characterisation of five new compounds of structural and magnetic interest. Indeed using the dinuclear complex [M2(H2O)(piv)4(Hpiv)4] (M = Co, Ni; Hpiv = pivalic acid) as starting material and reacting it with phenylhydroxamic acid (H2pha) has yielded the four tetrametallic coordination clusters [Co4(Hpha)2(piv)6(Hpiv)4] (1), [Ni4(Hpha)2(piv)6(Hpiv)2(DMF)2] (2), [Co4(Hpha)2(piv)6(EtOH)2(H2O)2] (3), [Ni4(Hpha)2(piv)6(EtOH)2(H2O)2] (4) and the hexanuclear complex [Co6(Hpha)4(piv)8(EtOH)2]·EtOH (5). All the compounds have been structurally characterised revealing a particular binding mode for the hydroxamate ligand. The study of their magnetic properties has been performed and the modelling of these properties has been done using the appropriate hamiltonians for each compound. The experimental data and their modelling show non-zero spin ground states for compounds 4 and 5.