From tetranuclear to pentanuclear [Co-Ln] (Ln = Gd, Tb, Dy, Ho) complexes across the lanthanide series: effect of varying sequence of ligand addition.

Two new families of cobalt(ii/iii)-lanthanide(iii) coordination aggregates have been reported: tetranuclear [LnCoL2(N-BuDEA)2(O2CCMe3)4(H2O)2]·(MeOH)n·(H2O)m (Ln = Gd, 1; Tb, 2; Dy, 3; n = 2, m = 10 for 1 and 2; n = 6, m = 2 for 3) and pentanuclear LnCoIICoL2(N-BuDEA)2(O2CCMe3)6(MeOH)2 (Ln = Dy, 4; Ho, 5) formed from the reaction of two aggregation assisting ligands H2L (o-vanillin oxime) and N-BuDEAH2 (N-butyldiethanolamine). A change in preference from a lower to higher nuclearity structure was observed on going across the lanthanide series brought about by the variation in the size of the LnIII ions. An interesting observation was made for the varying sequence of addition of the ligands into the reaction medium paving the way to access both structural types for Ln = Dy. HRMS (+ve) of solutions gave further insight into the formation of the aggregates via different pathways. The tetranuclear complexes adopt a modified butterfly structure with a more complex bridging network while trapping of an extra CoII ion in the pentanuclear complexes destroys this arrangement putting the Co-Co-Co axis above the Ln-Ln axis. Direct current (dc) magnetic susceptibility measurements reveal weak antiferromagnetic coupling in 1. Complexes 2 and 5 display no slow magnetic relaxation, whereas complexes 3 and 4 display out-of-phase signals at low temperature in ac susceptibility measurements. All compounds were analyzed with DFT and CASSCF calculations and informations about the single-ion anisotropies and mutual 4f-4f/4f-3d magnetic interactions were derived.

Solvent-induced structural transformation from heptanuclear to decanuclear [Co-Ln] coordination clusters: trapping of unique counteranion and understanding of aggregation pathways.

Five new cobalt(ii/iii)-lanthanide(iii)-based coordination aggregates, [LnIII3CoII2CoIII2(L1)2(O2CCMe3)8(OH)4(OMe)2(H2O)4]·Ln(η1-O2CCMe3)2(η2-O2CCMe3)2(MeOH)2·2MeOH·2H2O (where Ln = Tb (1), Ho (3), and H2L1 = N-(2-hydroxyethyl)-salicylaldimine), TbIII3CoII3CoIII4(L1)4(O2CCMe3)9(OH)10(H2O) (4) and LnIII3CoII2CoIII5(L1)4(O2CCMe3)10(OH)10 (Ln = Dy (5), Ho (6)) have been synthesized and characterized, including structural analysis via single-crystal X-ray diffraction. The dysprosium analogue (2) of 1 and 3 was previously reported by us. The heptanuclear monocationic clusters in 1 and 3 were formed by placement of seven metal ions (4 Co and 3 Ln) in a vertex shared dicubane structure from the control of two Schiff base anions and crystallized in the presence of in situ generated and literature unknown counter anions Tb(η1-O2CCMe3)2(η2-O2CCMe3)2(MeOH)2- and Ho(η1-O2CCMe3)2(η2-O2CCMe3)2(MeOH)2-. Interesting solvent-induced cluster structure transformation was observed on dissolving the heptanuclear aggregates in MeCN for the formation of decanuclear clusters 4-6. These high nuclearity clusters consist of a vertex shared heptanuclear dicubane part and a curved trinuclear chain linking the two cubic halves. The dicubane unit differs from that of the heptanuclear precursors in the presence of CoII/III at the shared vertex as opposed to LnIII and the absence of OMe- bridges. HRMS (+ve) analysis shed light on the pathway of formation of these heptanuclear molecules, while at the same time revealing a different aggregation process for the decanuclear clusters.

Synthetic diversity and change in nuclearity in [Co-Dy] coordination aggregates: bridge removal, solvent induced structural reorganization and AC susceptibility measurements.

Three new cobalt(ii/iii)-dysprosium(iii) complexes, [DyIII3CoII2CoIII2(L1)2(O2CCMe3)8(OH)4(OMe)2(H2O)4]·Dy(η1-O2CCMe3)2(η2-O2CCMe3)2(MeOH)2·4H2O (1), [DyIII3CoII2CoIII2(L2)2(O2CCMe3)8(OH)4(OMe)2(MeOH)2(H2O)2]·Dy(η1-O2CCMe3)2(η2-O2CCMe3)2(MeOH)2·4MeOH (2) and DyIII2CoII2CoIII2(L2)2(O2CCMe3)10(OH)2 (3) have been reported. In the heptanuclear 3d-4f monocationic aggregates in 1 and 2 the three dysprosium and four cobalt sites are arranged into a vertex shared dicubane structure, induced by two structure-directing ligands. Interestingly, a unique and previously unknown dysprosium(iii)-pivalate based counter anion, Dy(η1-O2CCMe3)2(η2-O2CCMe3)2(MeOH)2-, was trapped by the monocationic cores during crystallization. MeCN induced structural rearrangement of 2 through loss of OMe- bridges and dysprosium(iii) ions at the shared vertex resulted in the hexanuclear 3d-4f neutral aggregate 3, in which two dysprosium and four cobalt sites exhibit a near planar disposition. HRMS(+ve) of solutions of 1 and 2 revealed the pathway for aggregation processes and solvent induced structural transformation along with the importance of bridging OMe- in directing the formation of these compounds. Magnetic studies show a non-zero out-of-phase component in the AC susceptibility measurements of 1 but not in 2 and 3, although 1 and 2 have a very similar {CoIII2CoII2DyIII3} core and another DyIII center. Ab initio single-ion calculations point to the different single-ion anisotropic behavior for DyIII centers (energy in cm-1 and g-tensors) as well as negative and positive D values for CoII sites in 1 and 2 respectively reaffirming the experimental result. However, calculations envision that, zero-field out-of-phase signal and no out-of-phase signal in 1 and 2 respectively do not solely generate from the single-ion Dy/Co anisotropies and the overall relaxation mechanism can be understood by considering the exchange interactions between DyIII-DyIII and DyIII-CoII centres.

Unusually Distorted Pseudo-Octahedral Coordination Environment Around CoII from Thioether Schiff Base Ligands in Dinuclear [CoLn] (Ln = La, Gd, Tb, Dy, Ho) Complexes: Synthesis, Structure, and Understanding of Magnetic Behavior.

The synthesis, structural characterization, and magnetic behavior of a new family of binuclear CoII-LnIII complexes of formula [LnIIICoIIL2(NO3)3]·H2O (Ln = La, 1; Gd, 2; Tb, 3; Dy, 4; Ho, 5; HL = 3-methoxy-N-(2-(methylsulfanyl)phenyl)salicylaldimine) have been reported. The chosen ligand system HL has adjacent soft ONS and hard OO binding pockets ideal for selective coordination of CoII and 4f metal ions. All the complexes 1-5 exhibit a CoII center in a highly distorted octahedral geometry with the LnIII centers in bicapped square-antiprism geometry. The unique distortion about the CoII center is introduced by the coordination of 4f metal ions in the hard OO coordination site. The distortion is further supported by the presence of -SMe groups giving an S donor atom which owing to its larger size can support large bond distances and angles. The geometry around the CoII centers is intermediate between meridional and facial geometric isomers. The magnetic properties of these complexes have been investigated by a "full model" approach using CONDON with the experimental magnetochemical analysis revealing ferromagnetic Co-Ln coupling in compounds 2-5. Ab initio calculations on the X-ray crystal structures of 1-5 paint a semiquatitative picture about the contribution of the individual anisotropic centers toward the overall magnetic properties of the compounds. Theoretical analysis predicts 1 and 2 as weak single-ion magnet (SIM) and single-molecule magnet (SMM) respectively with CoII being solely responsible for the complex anisotropy. In 2, JCoGd plays a crucial role in preserving the anisotropy contribution of Co by channelizing relaxation via a higher excited exchange doublet. Because of the inefficiency of JCoTb, JCoDy, and JCoHo in quenching single-ion Ln fragment transverse anisotropy and preserving CoII high axial anisotropy (favoring rhombicity), 3-5 lack SMM behavior.

Entrapment of a Pseudo-Tetrahedral CoII Center by Thioether Sulfur Bound {Co2 (µ-L)} Fragments: Synthesis, Field-Induced Single-Ion Magnetism and Catechol Oxidase Mimicking Activity.

Simultaneous incorporation of both CoII and CoIII ions within a new thioether S-bearing phenol-based ligand system, H3 L (2,6-bis-[{2-(2-hydroxyethylthio)ethylimino}methyl]-4-methylphenol) formed [Co5 ] aggregates [CoII CoIII 4 L2 (µ-OH)2 (µ1,3 -O2 CCH3 )2 ](ClO4 )4 ⋅H2 O (1) and [CoII CoIII 4 L2 (µ-OH)2 (µ1,3 -O2 CC2 H5 )2 ](ClO4 )4 ⋅H2 O (2). The magnetic studies revealed axial zero-field splitting (ZFS) parameter, D/hc=-23.6 and -24.3 cm-1 , and E/D=0.03 and 0.00, respectively for 1 and 2. Dynamic magnetic data confirmed the complexes as SIMs with Ueff /kB =30 K (1) and 33 K (2), and τ0 =9.1×10-8  s (1), and 4.3×10-8  s (2). The larger atomic radius of S compared to N gave rise to less variation in the distortion of tetrahedral geometry around central CoII centers, thus affecting the D and Ueff /kB values. Theoretical studies also support the experimental findings and reveal the origin of the anisotropy parameters. In solutions, both 1 and 2 which produce {CoIII 2 (µ-L)} units, display solvent-dependent catechol oxidation behavior toward 3,5-di-tert-butylcatechol in air. The presence of an adjacent CoIII ion tends to assist the electron transfer from the substrate to the metal ion center, enhancing the catalytic oxidation rate.

Thioether sulfur-bound [Cu2] complexes showing catechol oxidase activity and DNA cleaving behaviour.

Rational ligand design approaches allowed {Cu2(µ-OH/OMe)} cores to be accommodated within µ-phenoxido bis(tetradentate) and µ-phenoxido bis(tridentate) ligands having thioether donors. The complexes [Cu2(µ-H2L1)(µ-OH)](ClO4)2·2H2O (1), [Cu2(µ-L2)(µ-OH)(OH2)](ClO4)2 (2a) and [Cu2(µ-L2)(µ-OCH3)(OH2)](ClO4)2 (2b) were obtained from an N2O3S2 donor set bearing the H3L1 ligand (2,6-bis-[{2-(2-hydroxyethylthio)ethylimino}methyl]-4-methylphenol) and N2OS2 donor set containing the HL2 ligand (4-methyl-2,6-bis-[{2-(methylthio)phenylimino}methyl]phenol) without showing double phenoxido bridging or any type of preformed inter-fragment aggregation. Previously, we showed that H3L (2,6-bis[((2-(2-hydroxyethoxy)ethyl)imino)methyl]-4-methylphenol), the ether analogue of H3L1, in the presence of carboxylate anions, was responsible for the self-aggregation of preformed {Cu2} fragments and gave two types of [Cu4] complexes comprising [Cu4O] and [Cu4(OH)2] cores (T. S. Mahapatra, A. Bauzá, D. Dutta, S. Mishra, A. Frontera and D. Ray, ChemistrySelect, 2016, 1, 64-74). The molecular structures of 1, 2a and 2b were determined via single crystal X-ray diffraction and solution studies, which indicated the presence of [Cu2] species. This was further confirmed via UV-vis spectroscopy and HRMS analysis. The synthesized complexes were screened for their potential as catalysts for the catalytic oxidation of 3,5-di-tert-butylcatechol (3,5-DTBCH2). A change in the mechanism of catalytic oxidation was observed with a change in the ligand backbone. All three complexes also showed DNA binding properties, which were further substantiated via molecular docking studies. Their DNA binding affinities were quantitatively ascertained using their intrinsic binding constant, Kb, values which were found to be 4.2 × 104, 5.6 × 104 and 4.8 × 104 M-1, respectively. Furthermore, the complexes displayed efficient DNA cleavage behaviour with pBR322 and the oxidative path was established in presence of ROS, singlet oxygen, 1O2, and the superoxide anion, O2·-.

Competitive coordination aggregation for V-shaped [Co3] and disc-like [Co7] complexes: synthesis, magnetic properties and catechol oxidase activity.

Unique dependence on the nature of metal salt and reaction conditions for coordination assembly reactions of varying architecture and nuclearity have been identified in V-shaped [Co3L4] and planar disc-like [Co7L6] compounds: [CoL2(µ-L)2(µ-OH2)2(CF3CO2)2] (1) and [Co(µ-L)6(µ-OMe)6]Cl2 (2) (HL = 2-{(3-ethoxypropylimino)methyl}-6-methoxyphenol). At room temperature varying reaction conditions, cobalt-ligand ratios and use of different bases allowed unique types of coordination self-assembly. The synthetic marvel lies in the nature of aggregation with respect to the two unrelated cores in 1 and 2. Complex 1 assumes a V-shaped arrangement bound to L(-), water and a trifluoroacetate anion, while 2 grows around a central Co(II) ion surrounded by a {Co} hexagon bound to methoxide and L(-). Magnetic measurements revealed that the intermetallic interactions are antiferromagnetic in nature in the case of complex 1 and ferromagnetic in the case of 2 involving high spin cobalt(ii) ions with stabilization of the high-spin ground state in the latter case. In MeCN solutions complexes 1 and 2 showed catalytic oxidation of 3,5-di-tert-butylcatechol (3,5-DTBCH2) to 3,5-di-tert-butylbenzoquinone (3,5-DTBQ) in air. The kinetic study in MeCN revealed that with respect to the catalytic turnover number (kcat) 2 is more effective than 1 for oxidation of 3,5-DTBCH2.