Structural Evaluation and Solution Integrity of Alkali Metal Salt Complexes of the Manganese 12-Metallacrown-4 (12-MC-4) Structural Type

Self-Assembled Heterometallic Complexes by Incorporation of Calcium or Strontium Ion into a Manganese(II) 12-Metallacrown-3 Framework Supported by a Tripodal Ligand with Pyridine-Carboxylate Motifs: Stability in Their Manganese(III) Oxidized Form

We report on the isolation of a new family of μ-carboxylato-bridged metallocrown (MC) compounds by self-assembly of the recently isolated hexadentate tris(2-pyridylmethyl)amine ligand tpada^2- incorporating two carboxylate units with metal cations. Twelve-membered MCs of manganese of the type 12-MC-3, namely, [{Mn^II(tpada)}₃(M)(H₂O)_n]²⁺ (Mn₃M) (M = Mn²⁺ (n = 0), Ca²⁺ (n = 1), or Sr²⁺ (n = 2)), were structurally characterized. The metallamacrocycles connectivity consisting in three -[Mn-O-C-O]- repeating units is provided by one carboxylate unit of the three tpada^2- ligands, while the second carboxylate coordinated a fourth cation in the central cavity of the MC, Mn²⁺ or an alkaline earth metal, Ca²⁺ or Sr²⁺. Mn₃Ca and {Mn₃Sr}₂ join the small family of heterometallic manganese-calcium complexes and even rarer manganese-strontium complexes as models of the OEC of photosystem II (PSII). A 8-MC-4 of strontium of the molecular wheel type with four -[Sr-O]- repeating unit was also isolated by self-assembly of the tpada^2- ligand with Sr²⁺. This complex, namely, [Sr(tpada)(OH₂)]₄ (Sr₄), does not incorporate any cation in the central cavity but instead four water molecules coordinated to each Sr²⁺. Electrochemical investigations coupled to UV-visible absorption and EPR spectroscopies as well as electrospray mass spectrometry reveal the stability of the 12-MC-3 tetranuclear structures in solution, both in the initial oxidation state, Mn^II₃M, as well as in the three-electrons oxidized state, Mn^III₃M. Indeed, the cyclic voltammogram of all these complexes exhibits three-successive reversible oxidation waves between +0.5 and +0.9 V corresponding to the successive one-electron oxidation of the Mn(II) ion into Mn(III) of the three {Mn(tpada)} units constituting the ring, which are fully maintained after bulk electrolysis.

Synthesis and crystal structure of two manganese-based 12-metallacrown-4 complexes: Na2(3-chloro-benzoate)2[12-MCMn(III)N(shi)-4](DMF)6 and MnNa(3-chloro-benzoate)3[12-MCMn(III)N(shi)-4](DMF)(H2O)4·4DMF·0.72H2O

Similar synthetic schemes yield two different metallacrown (MC) complexes: bis-(μ-3-chloro-benzoato)hexa-kis-(di-methyl-formamide)-tetra-kis-(μ4-N,2-dioxido-benzene-1-carboximidato)tetra-manganese(III)disodium(I), [Mn4Na2(C7H4ClO2)2(C7H4NO3)4(C3H7NO)6] or Na2(3-chloro-benzoate)2[12-MCMn(III)N(shi)-4](DMF)6, 1, and tetra-μ-aqua-tris-(μ-3-chloro-benzoato)(di-methyl-formamide)-tetra-kis-(μ4-N,2-dioxido-benzene-1-carboximidato)penta-manganese(III)sodi-um(I) di-methyl-formamide tetra-solvate 0.72-hydrate, [Mn5Na(C7H4ClO2)3(C7H4NO3)4(C3H7NO)(H2O)4]·4C3H7NO·0.718H2O or MnNa(3-chloro-benzo-ate)3[12-MCMn(III)N(shi)-4](DMF)(H2O)4·4DMF·0.72H2O, 2, where shi3- is salicyl-hydrox-imate and DMF is N,N-di-methyl-formamide. Both complexes have the same framework consisting of four MnIII ions in the MC ring and four shi3- ligands, resulting in an overall square-shaped mol-ecule. The MnIII ions are either five- or six-coordinate with elongated bond lengths in the apical or axial direction, respectively. The structure of 1 is nearly planar, and the MC binds two Na+ ions on opposite faces of the MC central cavity. The 3-chloro-benzoate anions also bind on opposite faces of the MC and form bridges between the central Na+ ions and the ring MnIII ions. For 1 the metallacrown mol-ecule, except for the central Na+ ion, exhibits whole mol-ecule disorder over two sets of sites. Both moieties are centrosymmetric and are related to each other by a pseudo-mirror operation with opposite sense of rotation around the Na⋯Na axis. The occupancy ratio of the main disorder of the metallacrown mol-ecules and 3-chloro-benzoate anions refined to 0.9276 (9):0.0724 (9). The structure of 2 is slightly domed, and the MC binds both an MnII ion and an Na+ ion in the MC central cavity. The MnII ion is located on the convex side of the MC, while the Na+ ion binds to the concave side. Complex 2 represents the first instance of a [12-MCMn(III)N(shi)-4] mol-ecule binding both 3d transition metal and alkali metal ions in the central cavity. In addition, three 3-chloro-benzoate anions bind on the convex side of the MC and connect the MnII ion to three of the ring MnIII ions.

Elucidation of 1H NMR Paramagnetic Features of Heterotrimetallic Lanthanide(III)/Manganese(III) 12-MC-4 Complexes

The paramagnetic one-dimensional ¹H NMR spectra of twelve Ln^IIINa^I(OAc)₄[12-MC_Mn^III(N)shi-4] complexes, where Ln^III is Pr^III-Yb^III (except Pm^III) and Y^III, are reported. Their solid-state isostructural nature is confirmed in methanol-d₄ solution, as a similar pattern in the ¹H NMR spectra is observed along the series. Notably, a relatively well-resolved spectrum is reported for the Gd^III complex. The chemical shift data are analyzed using the "all lanthanides" method, and the Fermi contact and pseudo-contact contributions are calculated for the lanthanide-induced shift (LIS). For the Tb^III-Yb^III complexes, the pseudo-contact contributions are typically 1 order of magnitude higher than the Fermi contact contributions; however, for the Gd^III complex, the Fermi contact is the main contribution to the paramagnetic chemical shift. For the methyl protons of the axial acetate (^-OAc) ligands, the LIS is opposite in sign, with respect to that of the aromatic salicylhydroximate (shi^3-) protons, because of structural rearrangements that occur upon dissociation of the Na^I cation in solution. The calculated crystal field parameters (B_Ln) for the Tb^III (360 cm^-1), Dy^III (250 cm^-1), Ho^III (380 cm^-1), Er^III (410 cm^-1), Tm^III (620 cm^-1), and Yb^III (380 cm^-1) complexes are not constant, likely as a consequence of the inaccuracy of the Bleaney's constants and, to a smaller extent, the small structural changes that occur in solution. Overall, the metallacrown scaffold retains structural integrity and similarity in solution for the entire series; however, small structural features, which do not affect the overall similarity, do likely occur.

Sm(iii)[12-MCGa(III)shi-4] as a luminescent probe for G-quadruplex structures

The luminescent anionic metallacrown Sm12-MC-4 exhibits similarity in shape and size to the guanine tetrad and is able to form complexes with G-quadruplex assembly.

A Heterobimetallic Anionic 3,6-Connected 2D Coordination Polymer Based on Nitranilate as Ligand

In order to synthesize new coordination polymers with original architectures and interesting magnetic properties, we used the nitranilate ligand (C₆O₄(NO₂)₂^2- = C₆N₂O₈^2-), derived from the dianionic ligand dhbq^2- (2,5-dihydroxy-1,4-benzoquinone = H₂C₆O₄^2-). The use of this bis-bidentate bridging ligand led to [(DAMS)₂{FeNa(C₆N₂O₈)₃}·CH₃CN]_n (1) (DAMS⁺ = C₁₆H₁₇N₂⁺ = 4-[4-(dimethylamino)-α-styryl]-1-methylpyridinium), a 2D heterometallic coordination polymer presenting an unprecedented structure for any anilato-based compound. This structural type is a 3,6-connected 2D coordination polymer derived from the well-known honeycomb hexagonal structure, where Fe(III) ions alternate with Na⁺ dimers (as Na₂O₁₂ units) in the vertices of the hexagons and with an additional [Fe(C₆N₂O₈)₃]^3- anion located in the center of the hexagons connecting the three Na⁺ dimers. The magnetic properties of compound 1 show the presence of paramagnetic isolated high spin Fe(III) complexes with a zero field splitting, |D| = 8.5 cm^-1.

A novel octanuclear nickel(II)-molybdenum(VI) heterometallic cluster based on the salicylhydroxamate ligand

Salicylhydroxamic acid (H3shi) is known for its strong coordination ability and multiple coordination modes, and can easily coordinate to metal cations to form compounds with five- or six-membered rings, as well as mono-, di- and multinuclear compounds with interesting structures having potential applications in organic chemistry, coordination chemistry, and the materials and biological sciences. A novel octanuclear nickel(II)-molybdenum(VI) heterometallic cluster based on the salicylhydroxamate ligand, namely di-μ3-acetato-di-μ2-acetato-di-μ3-hydroxido-di-μ3-oxido-tetraoxidooctakis(pyridine-κN)bis(μ5-salicylhydroxamato)hexanickel(II)dimolybdenum(VI) monohydrate, [Mo2Ni6(C7H4NO3)2(C2H3O2)4O5(OH)2(C5H5N)8]·H2O, (I), was synthesized by the reaction of sodium molybdate, nickel acetate and salicylhydroxamic acid in a dimethylformamide/pyridine/methanol solution at room temperature. The salicylhydroxamate(3-) (shi(3-)), acetate and oxide ligands adopt complicated coordination modes and link six Ni(II) and two Mo(VI) cations into the octanuclear heterometallic cluster. All of the metal cations exhibit octahedral coordination geometries and are connected to each other through the sharing of corners, edges or planes. The heterometallic clusters are further connected to form two-dimensional supramolecular layers through weak C-H...O hydrogen bonds. Studies of the magnetic properties of the title compound reveal antiferromagnetic interactions between the Ni(II) cations.

Complexation of some alkali and alkaline earth metal cations by macrocyclic compounds containing four pyridine subunits – a DFT study

Tetradentate pyridine-based macrocyclic compounds offer useful ligands capable of efficient and selective complexation of M = Li⁺, Na⁺, K⁺, Be²⁺, Mg²⁺and Ca²⁺.

Crystal structure of di-μ-chloro-acetato-hexa-kis-(di-methyl-formamide)-tetra-kis-(μ-N,2-dioxido-benzene-1-carboximidato)tetra-manganese(III)disodium dimethyl-formamide disolvate

The title compound consists of a macrocyclic ring with an Mn^III—N—O repeat unit that occurs four times, producing a mol­ecule with an overall square structure. Two Na⁺ ions are captured above and below the central cavity of the mol­ecule.

The synthesis, crystal structure, and FT–IR data for the title compound, [Na₂Mn₄(C₂H₂ClO₂)₂(C₇H₄NO₃)₄(C₃H₇NO)₆]·2C₃H₇NO or Na₂(O₂CCH₂Cl)₂[12-MC_Mn^IIIN(shi)-4](DMF)₆·2DMF, where MC is metallacrown, shi³⁻ is salicyl­hydroximate, and DMF is N,N-di­methyl­formamide, is reported. The macrocyclic metallacrown consists of an –[Mn^III—N—O]₄– ring repeat unit and the metallacrown captures two Na⁺ ions in the central cavity above and below the plane of the metallacrown. Each Na⁺ ion is seven-coordinate and is bridged to two ring Mn^III ions, through either a coordinating DMF mol­ecule or a chloro­acetate anion. The ring Mn^III ions have either a tetra­gonally distorted octa­hedral geometry or a distorted square-pyramidal geometry. Weak C—H⋯O inter­actions, in addition to pure van der Waals forces, contribute to the overall packing of the mol­ecules. The complete molecule has inversion symmetry and is disordered over two sets of sites with an occupancy ratio of 0.8783 (7):0.1217 (7). The solvent molecule is also disordered over two sets of sites, with an occupancy ratio of 0.615 (5):0.385 (5).

Enforcement of a high-spin ground state for the first 3d heterometallic 12-metallacrown-4 complex

Applying a novel magnetic director approach, we have realized Cu(ii)(DMF)₂Cl₂[12-MC_{Fe(III)N(Shi)}-4](DMF)₄·2DMF as the first heterometallic transition metal 12-MC-4 complex in order to provide this type of compound with a high-spin ground state.

Sodium and potassium ion directed self-assembled multinuclear assembly of divalent nickel or copper and L-leucine derived ligand

L-leucine derived ligand (H(2)L(L-leu)), KOH, and Ni(II) salt in 2:2:1 ratio self-assembled into a rather large (approximately 13 A) supramolecular assembly with the formula [K{Ni(HL(L-leu))(2)}(3)](+) (1). Structural characterization showed three [Ni(HL(L-leu))(2)] units encapsulated K(+) similar to organic crown ethers/cryptand. Substituting Ni(II) with Cu(II) and K(+) with Na(+) in the above reaction resulted in a set of structurally identical assemblies with the general formula [M'{M(HL(L-leu))(2)}(3)](+), where M' is either K(+) or Na(+) and M is either Cu(II) or Ni(II); [Na{Ni(HL(L-leu))(2)}(3)]ClO(4) (2), Na{Ni(HL(L-leu))(2)}(3)]OTf (3), [K{Cu(HL(L-leu))(2)}(3)]ClO(4) (4), [Na{Cu(HL(L-leu))(2)}(3)]ClO(4) (5), [K{Cu(HL(L-leu))(2)}(3)]NO(3) (6). Electrospray Ionization (ESI)-mass spectra of the assemblies in MeOH showed the retention of assemblies in solution. Visible spectroscopic studies showed retention of assembly 1 in N,N-dimethylformamide (DMF) which is stable even after the addition of 5 equiv of [18]-crown-6. The assemblies in 2-6 show various degrees of dissociation to [M(HL(L-leu))(2)] and M', in stronger H-bonding methanol. The dissociation can be reversed upon addition of excess KNO(3)/NaNO(3) salt. Structural characterization of [Cu(HL(L-leu))(2)(MeCN)] (7) along with its transformation to [K{Cu(HL(L-leu))(2)}(3)](+) in the presence of K(+) salt demonstrated that the assembly formation proceeds through an alkali metal ion induced ligand reorientation within the [Cu(HL(L-leu))(2)] units which is further stabilized by six strong H-bonds holding the assembly. Interestingly, visible spectra of 1 and 2 shows that minor structural changes caused by replacing K(+) with Na(+) is sufficient to shift the d-d transition of Ni(II) by approximately 70 nm, thereby providing an indirect way of distinguishing K(+) and Na(+), none of which have spectroscopic signature in the visible range.

Metallacrown-based compartments: selective encapsulation of three isonicotinate anions in non-centrosymmetric solids

Upon crystallization in high yields, metallacrowns form compartments that are capable of asymmetrically encapsulating three isonicotinate guests.

[NaCuII4] Cluster from Alkali Template Assembly of Two Asymmetric End-On Azido-Bridged [CuII2] Units

The cluster [NaCu4L2(N3)2](ClO4) [1; H3L is 2-(2'-hydroxyphenyl)-1,3-bis[3'-aza-4'-(2''-hydroxyphenyl)prop-4'-en-1'-yl]-1,3-imidazolidine] has been synthesized and structurally characterized. Complex 1 is formed by the template assembly of two [Cu2L(N3)] neutral fragments through their weak oxophilic interactions with a central Na+ cation as observed in metallacrowns. The cluster exhibits a combination of ferro- and antiferromagnetic interactions. End-on N3- bridging of copper ions within the [Cu(II)2] units facilitates stabilization of S = 1 magnetic subunits that mutually cancel via antiferromagnetic coupling as mediated by the O...Na+...O bridges.