The 2,2′:6′:2″-terpyridine ligand has literally shaped the coordination chemistry of transition metal complexes in a plethora of fields. Expansion of the ligand bite by amine functionalities between the pyridine units in the tridentate N,N’-dimethyl-N,N’-dipyridine-2-yl-pyridine-2,6-diamine ligand (ddpd) modifies the properties of corresponding transition metal complexes, comprising redox chemistry, molecular dynamics, magnetism and luminescence. The origins of these differences between ddpd and tpy complexes will be elucidated and comprehensively summarized with respect to first row transition metal complexes with d²–d¹⁰ electron configurations. Emerging applications of these ddpd complexes complementary to those of the well-known terpyridine ligand will be highlighted.

Mono-, di- & trinuclear ternary complexes of Dy³⁺& Tb³⁺with pyrazole & oligo-diketonates are both luminescent and single molecule magnets. Quantum yields &U_effvalues decrease with higher nuclearity & reduced intramolecular Ln–Ln distance.

Herein we report the synthesis and characterization of a novel bis-tridentate 1,3,4-thiadiazole ligand (L = 2,5-bis[(2-pyridylmethyl)thio]methyl-1,3,4-thiadiazole). Two new mononuclear complexes of the type [M^II(L)₂](ClO₄)₂ (with M = Fe^II (C1) and Co^II (C2)) have been synthesized, containing the new ligand (L). In both complexes the metal centers are coordinated by an N₄S₂-donorset and each of the two ligands is donating to the metal ion with just one of the tridentate pockets. The iron(II) complex (C1) is in the low spin [LS] state below room temperature and shows an increase in the magnetic moment only above 300 K. In contrast, the cobalt(II) complex (C2) shows a gradual spin crossover (SCO) with T_1/2 = 175 K. To our knowledge, this is the first cobalt(II) SCO complex with an N₄S₂-coordination.

A new family of complexes (^tBu₄N){[Ln^III(O₂CBu^t)₄][12-MC-_{Mn(III)N(shi)}-4]}·5CH₂Cl₂ (Ln = Gd (1) and Tb (2)), (^tBu₄N)₂{[Y^III(O₂CBu^t)₄][12-MC-_{Mn(III)N(shi)}-4]}(ClO₄) (3); where shiH₃ = salicylhydroxamic acid; Bu^tCO₂⁻ = pivalate ions; ^tBu₄N = tetrabutylammonium and ClO₄⁻ = perchlorate ions, has been isolated. The reaction of salicylhydroxamic acid with Mn(O₂CBu^t)₂·2H₂O, Ln(NO₃)₃·xH₂O, ^tBu₄NClO₄ in the presence of morpholine (C₄H₉NO) led to the isolation of compounds 1–3. The complexes belong to the 12-MC-4 family of Metallacrowns (MCs) possessing a central {Mn₄^IIILn^III(µ-NO)₄}¹¹⁺ core with the four Mn^III atoms occupying the periphery positions, while the formed [Mn–N–O] repeating unit, assists in the accommodation of the Ln^III atom in the center of the ring. Peripheral ligation is provided by four η¹:η¹:μ pivalate ions. Direct current magnetic susceptibility (dc) measurements revealed the presence of predominant antiferromagnetic exchange interactions within the metal centers. A 1-J fitting model was used in order to quantify the Mn^III–Mn^III interactions and fitting of the data, for the diamagnetic Y^III analogue, gave J = −3.74 cm⁻¹ and g_Mn(III) = 2.07. Fitting of the {Mn₄Gd} compound using a 2-J model, counting additionally for the Mn^III–Gd^III interactions, revealed values of J₁ = −3.52 cm⁻¹, J₂ = −0.45 cm⁻¹, and g_Mn(III) = 1.99.

A series of new bis(pyrazol-3-yl)pyridines (L^R) N,N′-disubstituted by 4-functionalized 2,6-dibromophenyl groups have been synthesized to study the effect of a distal substituent on the spin-crossover (SCO) behaviour of the iron(II) complexes [Fe(L^R)₂](ClO₄)₂ by variable-temperature magnetometry, NMR spectroscopy, and X-ray diffraction. The SCO-assisting tendency of the substituents with different electronic and steric properties (i.e., the bromine atom and the methyl group) in the para-position of the 2,6-dibromophenyl group is discussed. Together with earlier reported SCO-active iron(II) complexes with N,N′-disubstituted bis(pyrazol-3-yl)pyridines, these new complexes open the way for this family of SCO compounds to emerge as an effective ‘tool’ in revealing structure–function relations, a prerequisite for successful molecular design of switchable materials for future breakthrough applications in sensing, switching, and memory devices.