Laurence K. Thompson

Polynuclear coordination complexes—from dinuclear to nonanuclear and beyond

Abstract Polytopic ligands based on aromatic or open chain diazine and alkoxo-fragments form an ideal focus for the simultaneous coordination of metal centers in contiguous groupings, juxtaposed closely enough that magnetic spin–spin exchange occurs between adjacent pairs of metal ions. Examples of polynuclear complexes M n ( n =2–9) will be discussed in terms of their structures, and principally their magnetic properties, with a focus on self-assembly reactions. Ligand design elements that lead to predictable cluster and grid structures will be emphasized. Magneto-structural studies and correlations will be highlighted with antiferromagnetic and ferromagnetic examples, in addition to recent studies with high nuclearity grids and clusters. Ferromagnetic grids with high spin ground states present exciting possibilities for applications of these nano-scale molecular systems in, e.g. information storage technology.

Magnetic [n x n] (n = 2-5) grids by directed self-assembly.

Polytopic hydrazone-based ligands are discussed in the context of the design attributes of the ligand and the power of self-assembly as a methodology for the synthesis of polymetallic systems with specific and predetermined organization of the metal centers in a closely spaced bridged arrangement. Magnetic exchange coupling occurs as a result of the close proximity of the metal ions. Homometallic, heterometallic, and mixed-spin-state [n x n] (n = 2-5) square grids are highlighted and discussed in terms of their structural and magnetic properties. Antiferromagnetic, ferromagnetic, and ferrimagnetic examples are described.

Synthesis, structure, and magnetism of a novel alkoxide bridged nonacopper(II) (Cu9O12) [3 3] square grid generated by a strict self-assembly process

The nonanuclear copper complex [Cu9(2paop-H)6](NO3)12�9H2O (2paop = I) was prepd. and its crystal structure detd. The complex forms a [3 � 3] square grid of copper atoms with the ligand bonding in a heptadentate manner by five nitrogen atoms and two bridging oxygen atoms. Variable temp. magnetic susceptibility and magnetization data show that the complex displays predominantly ferromagnetic behavior at low temp., but a small intramol. antiferromagnetic exchange component may be present. [on SciFinder(R)]

Azide Bridged Dicopper and Dinickel Complexes: Structure and Magnetism

Abstract The quest for coordination complexes with novel magnetic properties, which may have potentially useful applications in materials science, has focussed attention on superexchange bridges with the ability to propagate ferromagnetic coupling between paramagnetic transition metal centers. The flexi-dentate azide ligand can bridge two metal centers in two ways, in the μ2-1,1- or μ2-1,3- fashion. In the 1,3-mode superexchange is exclusively antiferomagnetic in nature, whereas in the 1,1-mode the magnetic role of the azide bridge depends in large measure on the angle it subtends at the two metal centers. For dicopper(II) systems with small angles (<108°) ferromagnetic coupling dominates, while at larger angles the azide has been shown to propagate antiferromagnetic exchange. Control of the molecular dimensions, which in turn can influence the geometry at the azide, can lead to control of the magnetic properties. Examples of dicopper(II) and dinickel(II) complexes illustrating the ferromagnetic and antif...

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-assembly of mixed-valence Co(II/III) and Ni(II) clusters: azide-bridged 1D single chain coordination polymers comprised of tetranuclear units, tetranuclear Co(II/III) complexes, ferromagnetically coupled azide-bridged tetranuclear, and hexanuclear Ni(II) complexes: synthesis, structural, and magnetic properties

One-pot reactions between 2,6-diformyl-4-methylphenol (DFMP) and 2-aminoethanol (AE) in the presence of cobalt(II) salts [Co(ClO4)2, CoCl2, Co(CH3CO2)2, Co(NO3)2] and sodium azide result in the self-assembly of novel one-dimensional single chain mixed-valence cobalt coordination polymers {[Co2(II)Co2(III) (HL)2(OCH3)2(N3)3]ClO(4).5H2O.CH3OH}n (1), {[Co2(II)Co2(III) (HL)2(OCH3)2(N3)3]Cl.H2O}n (2) in which tetra-cobalt cationic units are bridged by symmetrical 1,3-azides, forming single chains; mixed valence neutral tetranuclear clusters [Co2(II)Co2(III) (HL)2(OCH3)2(N3)4]CH3OH.2H2O (3), [Co2(II)Co2(III)(HL)2(OCH3)2(N3)2(CH3CO2)2].2CH3OH.2H2O (4), and the cationic cluster [Co2(II) Co2(III) (HL)2(OCH3)2(CH3OH)2(N3)2](NO3)2 (5). In all these reactions, H3L, the potentially pentadentate (N2O3), trianionic double Schiff base ligand 2,6-bis[(2-hydroxy-ethylimino)-methyl]-4-methylphenol is formed. The reaction between DFMP and AE in the presence of nickel(ii) salts and sodium azide in methanol-water mixture results in the self-assembly of ferromagnetically coupled hexanuclear complexes [Ni6(H2L)2(HL-1)2(H2O)2(N3)6](ClO4)(2).2CH3OH (6), and [Ni6(H2L)2(HL-1)2(CH3OH)2(N3)6](BF4)2 (7), involving double (H3L) and single (H2L-1) Schiff base ligands, and a neutral tetranuclear complex [Ni4(H2L)2(OCH3)2(CH3CO2)2(N3)2] (8) with only double Schiff-base (H3L). In these complexes, the nature of the anion and the reaction conditions seem to play an important role in directing the formation of tetranuclear, hexanuclear or polymeric clusters. All complexes involve divacant double cubane-type cores containing three to four different types of bridging ligands (phenoxy, azido, methoxy/alkoxy, and acetate). Variable temperature magnetic properties of these spin coupled clusters have been investigated and magneto-structural correlations have been established.

Self-assembled polymetallic square grids ([2 × 2] M4, [3 × 3] M9) and trigonal bipyramidal clusters (M5)—structural and magnetic properties

New self-assembled grids and clusters are reported, with square [2 × 2] M4 (M = Mn(II)4, Cu(II)4), trigonal-bipyramidal Mn(II)5, and square [3 × 3] M9 (M = Mn(II), Cu(II)) examples. These are based on a series of ditopic and tritopic hydrazone ligands involving pyridine, pyrimidine and imidazole end groups. In all cases the metal centres are bridged by hydrazone oxygen atoms with large (>125°) bridge angles, leading to antiferromagnetic exchange for all the Mn systems (J = −2 to −5 cm−1), which results in S = 0 (Mn4), and S = 5/2 (Mn5, Mn9) ground states. The copper systems have a 90° alternation of the Jahn–Teller axes within the Cu4 and Cu8 grid rings (Cu9), which leads to magnetic orbital orthogonality, and dominant ferromagnetic coupling. For the Cu9 grid antiferromagnetic exchange between the ring and the central copper leads to a S = 7/2 ground state, while for the Cu4 grids S = 4/2 ground states are observed. The magnetic data have been treated using isotropic exchange models in the cases of the Cu4 and Cu9 grids, and the Mn5 clusters. However due to the enormity of a fully isotropic calculation a simplified model is used for the Mn9 grid, in which the outer Mn8 ring is treated as the equivalent of an isolated magnetic chain, with no coupling to the central metal ion.

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.

Ligating properties of tridentate Schiff base ligands, 2-[[(2-pyridinylmethyl)imino]methyl] phenol (HSALIMP) and 2-[[[2-(2-pyridinyl)ethyl]imino]methyl]phenol (HSALIEP) with zinc(II), cadmium(II), nickel(II) and manganese(III) ions. X-ray crystal structures of the [Zn(SALIEP)(NO3)]2 dimer, [Mn(SALIEP)2](ClO4), and [Zn(AMP)2(NO3)2]

Abstract A series of zinc(II), cadmium (II), nickel(II) and manganese (III) complexes with two potentially tridentate (NNO) Schiff base ligands, 2-[[(2-pyridinylmethyl)imino]methyl]phenol (HSALIMP) and 2-[[[2-(2-pyridinyl)ethyl]imino]methyl]phenol (HSALIEP) have been synthesized and structurally characterized through IR, 1 H, 13 C, and 2-D NMR spectroscopy and in two cases by X-ray crystallography. The Schiff base ligands HSALIMP and HSALIEP are generated from 1+1 condensation of salicylaldehyde with 2-aminomethylpyridine and 2(2-aminoethyl)pyridine, respectively. HSALIMP behaves as a tridentate (NNO) or a bidentate (NN) ligand, depending upon the nature of the anion. On reaction with zinc(II) salts HSALIMP gives two types of complexes, (a) [Zn(SALIMP)X] 2 · y H 2 O (X=NO 3 , y =0 ( I ); X=I, y =3 ( II ) and (b) [Zn(HSALIMP)Cl 2 ]·H 2 O ( III ). HSALIMP reacts with nickel(II) bromide to form a mononuclear complex [Ni(HSALIMP) 2 Br 2 ] ( VIII ), in which one metal ion combines with two neutral HSALIMP ligands. However HSALIEP, a similar ligand with one additional carbon atom in the chain, acts as a tridentate ligand (NNO), with the phenolate oxygen deprotonated, irrespective of the nature of the anion, forming dimeric compounds of 1:1 metal:ligand stoichiometry with zinc(II) and cadmium(II) ions, [Zn(SALIEP)X] 2 · y H 2 O (X=NO 3 , y =0 ( IV ); X=Cl, y =2 ( V ); X=I, y =1 ( VI ), [Cd(SALIEP) (NO 3 )] 2 ·C 2 H 5 OH ( VII ) and a monomeric complex of 1:2 metal:ligand stoichiometry with manganese(III) ion, [Mn(SALIEP) 2 ](ClO 4 ) ( IX ). The single crystal X-ray structures of the [Zn(SALIEP)(NO 3 )] 2 ( IV ) dimer, [Mn(SALIEP) 2 ](ClO 4 ) ( IX ), and [Zn(AMP) 2 (NO 3 ) 2 ] ( X ), a minor product obtained from the filtrate of I , have been determined. IV consists of centrosymmetric dimer in which deprotonated phenolates bridge the two five-coordinate metal atoms and link the two halves of the dimer. The structure of IX consists of a centrosymmetric molecule in which the manganese(III) ion is bound to two tridentate SALIEP ligands in a nearly regular octahedral arrangement. The structure of X consists of a centrosymmetric monomer in which the six coordinate octahedral zinc(II) ion is bonded equtorially to two bidentate 2-aminomethylpyridine ligands and axially to two monodentate nitrate groups.

Field-dependent anisotropy change in a supramolecular Mn(II)- [3 x 3] grid.

The magnetic anisotropy of a novel Mn(II)- [3x3] grid complex was investigated by means of high-field torque magnetometry. Torque vs field curves at low temperatures demonstrates a ground state with S>0 and exhibits a torque step due to a field-induced level crossing at B(*) approximately 7.5 T, accompanied by an abrupt change of magnetic anisotropy from easy-axis to hard-axis types. These observations are discussed in terms of a spin Hamiltonian formalism.