Curtis M. Zaleski

Controllable Formation of Heterotrimetallic Coordination Compounds: Systematically Incorporating Lanthanide and Alkali Metal Ions into the Manganese 12-Metallacrown-4 Framework

The inclusion of Ln(III) ions into the 12-MC-4 framework generates the first heterotrimetallic complexes of this molecular class. The controllable and deliberate preparations of these compounds are demonstrated through 12 crystal structures of the Ln(III)M(I)(OAc)4[12-MCMn(III)(N)shi-4](H2O)4·6DMF complex, where OAc(-) is acetate, shi(3-) is salicylhydroximate, and DMF is N,N-dimethylformamide. Compounds 1-12 have M(I) as Na(I), and Ln(III) can be Pr(III) (1), Nd(III) (2), Sm(III) (3), Eu(III) (4), Gd(III) (5), Tb(III) (6), Dy(III) (7), Ho(III) (8), Er(III) (9), Tm(III) (10), Yb(III) (11), and Y(III) (12). An example with M(I) = K(I) and Ln(III) = Dy(III) is also reported (Dy(III)K(OAc)4[12-MCMn(III)(N)shi-4](DMF)4·DMF (14)). When La(III), Ce(III), or Lu(III) is used as the Ln(III) ions to prepare the Ln(III)Na(I)(OAc)4[12-MCMn(III)(N)shi-4] complex, the compound Na2(OAc)2[12-MCMn(III)(N)shi-4](DMF)6·2DMF·1.60H2O (13) results. For compounds 1-12, the identity of the Ln(III) ion affects the 12-MCMn(III)(N)shi-4 framework as the largest Ln(III), Pr(III), causes an expansion of the 12-MCMn(III)(N)shi-4 framework as demonstrated by the largest metallacrown cavity radius (0.58 Å for 1 to 0.54 Å for 11), and the Pr(III) causes the 12-MCMn(III)(N)shi-4 framework to be the most domed structure as evident in the largest average angle about the axial coordination of the ring Mn(III) ions (103.95° for 1 to 101.69° for 11). For 14, the substitution of K(I) for Na(I) does not significantly affect the 12-MCMn(III)(N)shi-4 framework as many of the structural parameters such as the metallacrown cavity radius (0.56 Å) fall within the range of compounds 1-12. However, the use of the larger K(I) ion does cause the 12-MCMn(III)(N)shi-4 framework to become more planar as evident in a smaller average angle about the axial coordination of the ring Mn(III) ions (101.35°) compared to the analogous Dy(III)/Na(I) (7) complex (102.40°). In addition to broadening the range of structures available through the metallacrown analogy, these complexes allow for the mixing and matching of a diverse range of metals that might permit the fine-tuning of molecular properties where one day they may be exploited as magnetic materials or luminescent agents.

Effects of the Central Lanthanide Ion Crystal Radius on the 15-MCCuII(N)pheHA-5 Structure

Twenty crystal structures of the Ln(III)[15-MC(CuII(N)pheHA)-5](3+) complex, where pheHA = phenylalanine hydroxamic acid and where Ln(III) = Y(III) and La(III)-Tm(III), except Pm(III), with the nitrate and/or hydroxide anion are used to assess the effect of the central metal ion on the metallacrown structure. Each Ln(III)[15-MC(CuII(N)pheHA)-5](3+) complex is amphiphilic with a hydrophobic side consisting of the phenyl groups of the pheHA ligand and a side without the aromatic residues. Three general structures are observed for the Ln(III)[15-MC(CuII(N)pheHA)-5](3+) complexes. In the Type 1 structures, the central metal ion does not bind a nitrate anion on the metallacrowns hydrophobic face, and two adjacent metallacrowns dimerize through their phenyl groups producing a hydrophobic compartment. In the Type 2 structures, the central metal ion binds a nitrate in a bidentate fashion on the hydrophobic face. There are two distinct types of Type 2 metallacrowns, designated A and B. Type 2A metallacrowns have a water molecule bound to the central metal ion on the hydrophilic face, while Type 2B metallacrowns have a monodentate nitrate ion bound on the hydrophilic face to the central metal ion. The Type 2 metallacrowns also dimerize via the phenyl groups to form a hydrophobic compartment. In Type 3 structures, the central metal ion binds a nitrate in a bidentate fashion on the hydrophobic side, but instead of forming dimers, the metallacrowns pack in a helical arrangement to give either P or M one-dimensional helices. Regardless of the type of metallacrown, the overall trend observed is that as the Ln(III) ion crystal radius increases, the metallacrown cavity radius also increases while the metallacrown becomes more planar. This conclusion is demonstrated by a decrease in the oxime oxygen distances to the oxime oxygen mean plane and a decrease in the ring Cu(II) distances to the Cu(II) mean plane as the metallacrown cavity radius increases and the lanthanide crystal radius increases. In addition, a decrease in the O(oxime)-Cu(II)-N(oxime)-O(oxime) torsion (dihedral) angles is also observed as the metallacrown cavity radius and the lanthanide crystal radius both increase. These observations help explain the thermodynamic preferences for Ln(III) ions within this class of metallacrowns and may be used to design compartments capable of binding guests in different orientations within chiral, soft solids.

A cationic 24-MC-8 manganese cluster with ring metals possessing three oxidation states [MnII4MnIII6MnIV2(μ4-O)2(μ3-O)4(μ3-OH)4(μ3-OCH3)2(pko)12](OH)(ClO4)3

Reaction of Mn(ClO4)2 with di-pyridyl ketone oxime, (2-py)2CNOH, gives the novel cluster [MnII4MnIII6MnIV2(μ4-O)2(μ3-O)4(μ3-OH)4(μ3-OCH3)2(pko)12](OH)(ClO4)3 1. It is the only example of a 24-MC-8, and the first metallacrown with ring metal ions in three different oxidation states. Magnetic measurements show antiferromagnetic behavior.

Using small molecule complexes to elucidate features of photosynthetic water oxidation

The molecular oxygen produced in photosynthesis is generated via water oxidation at a manganese–calcium cluster called the oxygen-evolving complex (OEC). While studies in biophysics, biochemistry, and structural and molecular biology are well known to provide deeper insight into the structure and workings of this system, it is often less appreciated that biomimetic modelling provides the foundation for interpreting photosynthetic reactions. The synthesis and characterization of small model complexes, which either mimic structural features of the OEC or are capable of providing insight into the mechanism of O2 evolution, have become a vital contributor to this scientific field. Our group has contributed to these findings in recent years through synthesis of model complexes, spectroscopic characterization of these systems and probing the reactivity in the context of water oxidation. In this article we describe how models have made significant contributions ranging from understanding the structure of the water-oxidation centre (e.g. contributions to defining a tetrameric Mn3Ca-cluster with a dangler Mn) to the ability to discriminate between different mechanistic proposals (e.g. showing that the Babcock scheme for water oxidation is unlikely).

Structural and physical characterization of tetranuclear [Mn(II)3Mn(IV)] and [Mn(II)2Mn(III)2] valence-isomer manganese complexes.

Two tetranuclear Mn complexes with an average Mn oxidation state of +2.5 have been prepared. These valence isomers have been characterized by a combination of X-ray crystallography, X-ray absorption spectroscopy, and magnetic susceptibility. The Mn(II)3Mn(IV) tetramer has the Mn ions arranged in a distorted tetrahedron, with an S = 6 ground spin state, dominated by ferromagnetic exchange among the manganese ions. The Mn(II)2Mn(III)2 tetramer also has a distorted tetrahedral arrangement of Mn ions but shows magnetic behavior, suggesting that it is a single-molecule magnet. The X-ray absorption near-edge structure (XANES) spectra for the two complexes are similar, suggesting that, while Mn XANES has sufficient sensitivity to distinguish between trinuclear valence isomers (Alexiou et al. Inorg. Chem. 2003, 42, 2185), similar distinctions are difficult for tetranuclear complexes such as that found in the photosynthetic oxygen-evolving complex.

Heterometallic Mixed 3d-4f Metallacrowns: Structural Versatility, Luminescence, and Molecular Magnetism

The synthesis of metal-containing coordination complexes has relevance in a number of applications, including catalysis, gas separations and storage, data storage, contrast agents, and drug delivery. As such, methods towards the a priori design of these materials are of great importance. Unlike organic synthesis, which is governed by well-known and understood reaction mechanisms, inorganic synthesis is dominated by labile metal-ligand dative bonds, whose reversibility in solution can lead to unanticipated molecules. However, the metallamacrocycles known as metallacrowns (MCs) provide the opportunity to investigate controllable inorganic syntheses, which can lead to systematic alterations of these molecules. Thus, entire classes of inorganic molecules may be designed with specific properties. In this review, 3d-4f metallacrowns will be examined, with emphasis on (1) synthetic and design aspects; (2) luminescence properties; and (3) magnetic behavior. GRAPHICAL ABSTRACT

The Nature of the Bridging Anion Controls the Single-Molecule Magnetic Properties of DyX4M 12-Metallacrown-4 Complexes

A family of DyX4M(12-MCMnIII(N)shi-4) compounds were synthesized and magnetically characterized (X = salicylate, acetate, benzoate, trimethylacetate, M = NaI or KI). The bridging ligands were systematically varied while keeping the remainder of the MC-geometry constant. The type of monovalent cation, necessary for charge balance, was also altered. The dc magnetization and susceptibility of all compounds were similar across the series. Regardless of the identity of the countercation, the Dy(Hsal)4M 12-MC-4 compounds were the only compounds to show frequency-dependent ac magnetic susceptibility, a hallmark of single-molecule magnetism. This indicates that the nature of the bridging ligand in the 12-MCMnIII(N)shi-4 compounds strongly affects the out-of-phase magnetic properties. The SMM behavior appears to correlate with the pKa of the bridging ligand.

One-Dimensional Coordination Polymers of 12-Metallacrown-4 Complexes: {Na2(L)2[12-{\rm{M}}{{\rm{C}}_{{\rm{M}}{{\rm{n}}^{{\rm{III}}}}{\rm{(N)shi}}}}-4]} n , where L is Either −O2CCH2CH3 or −O2CCH2CH2CH3

The metallacrown one-dimensional coordination polymers {Na2(O2CCH2CH3)2[12-MCMnIII(N)shi\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}

Crystal structure of tetra­aqua­(di­methyl­formamide)­tetra­kis­(μ-N,2-dioxido­benzene-1-carboximidato)tetra­kis­(μ-tri­methyl­acetato)­tetra­manganese(III)sodiumyttrium–di­methyl­formamide–water (1/8.04/0.62)

{\rm{M}}{{\rm{C}}_{{\rm{M}}{{\rm{n}}^{{\rm{III}}}}{\rm{(N)shi}}}}