Muhammad U. Anwar

Lanthanide complexes of tritopic bis(hydrazone) ligands: single-molecule magnet behavior in a linear Dy(III)3 complex.

Tritopic pyridinebis(hydrazone)-based ligands typically produce square M(9) [3 × 3] grid complexes with first-row transition-metal ions (e.g., M = Mn, Fe, Co, Cu, Zn), but with larger lanthanide ions, such coordination motifs are not produced, and instead linear trinuclear complexes appear to be a preferred option. The reaction of 2pomp [derived from pyridine-2,6-bis(hydrazone) and 2-acetylpyridine] with La(III), Gd(III), and Dy(III) salts produces helical linear trinuclear [Ln(3)(2pomp)(2)]-based complexes, where each metal ion occupies one of the three tridentate ligand pockets. Two ligands encompass the three metal ions, and internal connections between metal ions occur through μ-O(hydrazone) bridges. Coligands include benzoate, nitrate, and N,N-dimethylformamide. The linear Dy(III)(3) complex exhibits single-molecule magnet behavior, demonstrated through alternating-current susceptibility measurements. Slow thermal magnetic relaxation was detected in an external field of 1800 Oe, where quantum-tunneling effects were suppressed (U(eff) = 14 K).

Self-assembled Ln(III)4 (Ln = Eu, Gd, Dy, Ho, Yb) [2 × 2] square grids: a new class of lanthanide cluster.

Self-assembly of the Ln(III) ions (Ln = Eu, Gd, Dy, Ho, Yb) into square [2 × 2] grid-like arrays has been readily effected using simple, symmetric ditopic ligands based on a carbohydrazone core. The metal ions are connected via single atom bridges (e.g., μ2-O(hydrazone), μ2-OH, μ2-OMe, μ2-1,1-N3(-), μ4-O), depending on reaction conditions. The Gd(III)4 examples exhibit intramolecular antiferromagnetic exchange (-J < 0.11 cm(-1)), and in one Dy(III)4 example, with a combination of μ2-1,1-N3(-), and μ4-O bridges linking adjacent metal ions, SMM behavior is observed. One thermally driven relaxation process is observed in the temperature range 10-25 K (τ0 = 6.5(1) × 10(-7) s, U(eff) = 110(1) K) in the presence of an 1800 Oe external field, employed to suppress a second quantum based relaxation process. The extended group of Ln(III) ions which submit to this controlled self-assembly, typical of the transition metal ions, indicates the general applicability of this approach to the lanthanides. This occurs despite the anticipated limitations based on larger ionic radii and coordination numbers, and is an encouraging sign for extension to larger grids with appropriately chosen polytopic ligands.

Polynuclear lanthanide (Ln) complexes of a tri-functional hydrazone ligand – mononuclear (Dy), dinuclear (Yb, Tm), tetranuclear (Gd), and hexanuclear (Gd, Dy, Tb) examples

The lanthanide coordination chemistry of a tri-functional vanillin-hydrazone-oxime ligand reveals a variety of different products, depending on reaction conditions, with mono-nuclear (Dy), dinuclear (Yb, Tm), tetranuclear (Gd) and hexanuclear (Gd, Tb, Dy) examples. The Ln6 (Ln = Gd, Dy, Tb) complexes form in the presence of both triethylamine and acetic acid, and have unique, flat hexanuclear structures built on a μ3-O bridged triangular core, with the six lanthanide ions bridged further through μ-acetate and μ-Ohydrazone connections in an expanded fused triangular array. Similar reaction conditions with Yb(III) and Tm(III) lead preferentially to dinuclear systems, while in the presence of a competitive benzoate ligand a rectangular Gd4 complex results. Variable temperature DC magnetic data for the Gd(III) complexes reveal weak antiferromagnetic exchange. AC magnetic data on the other polynuclear complexes down to 2 K, both in the absence and presence of external bias fields, revealed no significant out of phase signals normally indicative of SMM behavior. However, the mononuclear Dy(III) complex shows frequency dependent AC signals and maxima in the temperature range 2-20 K in the presence of an external bias field, indicative of SMM behaviour, with Ueff = 36(1) K, and τ0 = 4.4(2) × 10(-6) s.

Polynuclear Fen Complexes (n = 1, 2, 4, 5) of Polytopic Hydrazone Ligands with Fe(II), Fe(III) and Mixed Oxidation State Combinations

The iron coordination chemistry of some polytopic hydrazone based ligands is examined. The complexes derive from a general self-assembly strategy, where ligand design can be used to devise specific polymetallic [n × n] grid architectures. However, as part of any complex equilibrium process, oligomeric entities can also occur, particularly when ligand tautomeric flexibility is considered, and examples of mononuclear, dinuclear, tetranuclear, and pentanuclear complexes have been observed within a related class of ligands. In addition, ligand site donor composition can lead to coordination spheres that stabilize both high spin Fe(II) and Fe(III) sites, with evidence for Fe(II) spin crossover. Structural and magnetic properties are examined, which reveal the presence of antiferromagnetic exchange in the polynuclear systems.

μ-O Bridged Mn10 Assemblies with Open O6 Sites for Binding Extra Guests: Structural, Magnetic, and Surface Studies

High nuclearity [Mn(10)M(2)] clusters have been achieved through a self-assembly approach where multiple coordinating functional groups are incorporated into one ligand. When the hydrazone group appended with an oxime function as a reactive intermediate is used, the attachment of a vanillin subunit creates a ligand (L4) with three coordinating groups, which in their own right lead to cluster assemblies. The trifunctional ligand L4 produces a series of self-assembled, mixed oxidation state (Mn(II)/Mn(III)) Mn(10)M(2) based clusters with an overall linear structure comprising two connected pentanuclear Mn(5) halves, which bind alkali metal cations (M = Li, Na, K, Rb, Cs) and H(3)O(+) in the vanillin (O(6)) end pockets, created by the assembly of three ligands around each Mn(5) subunit. Antiferromagnetic exchange dominates the spin coupling in the Mn(10) complexes, and surface studies on highly oriented pyrolytic graphite (HOPG) clearly show the arrangement of metal ions (Mn, Cs) in the Mn(10)Cs(2) linear cluster assembly.

Approaching polymetallic ‘Assemblies of Assemblies’ using ligands with multiple functionality – novel Mn10M2 (M = alkali metal) chains with ‘ionophoric’ end cavities

A tri-functional ligand with hydrazone, oxime and o-vanillin subunits uses separate encoded coordination instructions to create a novel extended spin-coupled cluster where the hydazone oxygen and oxime groups of three ligands combine to create bridges linking ten Mn ions together, leaving organized O(6) end cavities capable of coordinating oxo-philic cations.

Oligonuclear Fe Complexes (Fe, Fe4, Fe6, Fe9) Derived from Tritopic Pyridine Bis-Hydrazone Ligands—Structural, Magnetic, and Mössbauer Studies

Tri-topic pyridine bis-hydrazone ligands produce polynuclear complexes with Fe(II) and Fe(III) salts with varying nuclearity and metal ion oxidation states. Mononuclear, tetranuclear, hexanuclear, and nonanuclear examples are discussed using structural, magnetic and Mössbauer data. In one case, although X-ray data suggest a [3 × 3] Fe9 grid (space group P42/n), careful examination of the structure, in conjunction with magnetic and Mössbauer data, indicates an unusual situation where the corner and center sites are present at unit occupancy, whereas side site occupancy is ∼0.6.