Richard E. P. Winpenny

The structures and magnetic properties of complexes containing 3d- and 4f-metals

Heterometallic complexes containing 3d-/4f-metals have been made with a variety of ligands including Schiff-bases, pyridonates and amino-alcohols. The majority of species with Schiff-base ligands are trinuclear with Cu2Ln cores, and with other ligands larger oligomers are found, ranging as large as Cu12La8 and Cu12Ln6 clusters (Ln = Y, Nd, Sm or Gd). The magnetic properties displayed by these polynuclear species are discussed, and the magnetic coupling between CuII and GdIII is always found to be ferromagnetic.

The coordination chemistry of 2-pyridone and its derivatives

Abstract This review covers the coordination chemistry of 2-pyridone (2-hydroxypyridine) and its derivatives from 1968 to 1993. The derivatives studied have chiefly been 2-pyridone itself and those substituted in the 6-position of the ring, in particular 6-methyl-2-pyridone and 6-chloro-2-pyridone. These ligands have found their major usage as 1,3-bridging ligands akin to carboxylates. A large number of dimeric complexes have been synthesized with elements including Cr, Mo, W, Tc, Re, Ru, Os, Rh, Ir, Pd, Pt and Cu, and in which the metal-metal bond order varies from four to zero. The structural chemistry of these dimeric complexes is discussed, as are physical studies designed to increase the understanding of metal-metal bonding. Larger polymetallic arrays can be made with first-row metals such as chromium, cobalt and copper. Work on platinum complexes of 2-pyridone has been important in modelling the interaction of cis -[Pt(NH 3 ) 2 CI 2 ] with uracil nucleobases; this work is also reviewed. Dimeric rhodium(I) and iridium(I) complexes display interesting photochemistry, and this work is described. A recent development is the use of 2-pyridones as bridging ligands in heterometallic assemblies, especially of copper and lanthanoids and Group 2 metals.

Inter-ligand reactions: in situ formation of new polydentate ligands

Two ligands have been synthesized by derivatisation of cyanuric chloride: 6-(diethylamino)-2,4-disulfanyl-1,3,5-triazine (H2SSta) 1 and 6-(diethylamino)-2-hydroxo-4-sulfanyl-1,3,5-triazine (H2OSta) 2 have been characterised by X-ray crystallography, which shows intermolecular hydrogen bonding in the solid state, leading to dimers of 1 and ribbons of 2. On reaction with metal salts both ligands undergo oligomerisation reactions. Compound 1 reacts with nickel chloride to form a mononuclear complex, [Ni{(Sta)S(S2ta)}] 3. In 3 two triazine ligands have reacted, to form a tetradentate ligand in which two triazine rings are bridged by a sulfur group, with a co-ordinated disulfide group present on one ring and a co-ordinated thiolate on the second. Compound 2 reacts with cobalt(II) chloride to form a cage complex, [Co6NaO(OStaH)7{S(Ota)2}2(O2CPh)2(H2O)2] 4. This complicated structure contains two polydentate ligands formed by linking triazine groups through a bridging sulfur. The cage contains four cobalt(II) and two cobalt(III) sites which are assigned by bond length considerations. The compound [Co(OSta)3] 5 co-crystallises with 4, and its structure has also been determined.

A Family of Polynuclear Cobalt and Nickel Complexes Stabilised by 2-Pyridonate and Carboxylate Ligands

The synthesis and structural characterisation of a series of cobalt and nickel cages are reported. Eight of these structures contain a [M10(mu3-OH)6(eta2, mu3-xhp),(eta2, mu2-O2CR)6]2+ core (where M = Co or Ni; xhp = 6-chloro- or 6-methyl-2-pyridonate: R = Me, Ph, CHMe2, CH2Cl, CHPh2 or CMe3), where the ten metal atoms describe a centred-tricapped-trigonal prism (ttp). The cage contains six hydroxide ligands around the central metal, and the exterior is coated with pyridonate and carboxylate ligands. For four of the cages additional metal centres are found attached to the upper and/or lower triangular faces of the trigonal prism, generating dodeca- and undecanuclear cages. Three further cages are reported that contain a metal core based on an incomplete centred-tetraicosahedron. These cages involve trimethylacetate as a ligand in company with either 6-methyl-2-pyridonate or 6-chloro-2-pyridonate. Comparison of these latter structures with the trigonal prisms reveal that they can be described as a pentacapped-trigonal prism missing one edge. Magnetic studies of three of the nickel cages with trigonal prismatic cores show spin ground states of S = 8, 4 and 2 for Ni12, Ni11 and Ni10 cages, respectively.

Elucidating the mode of action of a corrosion inhibitor for iron

Two polymetallic iron(III) complexes 1 and 2 have been synthesised from the known corrosion inhibitor 3-(4-methylbenzoyl)-propionic acid HL1 and their crystal structures determined. Coordination geometries extracted from these structures have been used as the basis for molecular modelling onto idealised iron(III) oxide surfaces as an aid to understanding the efficacy of inhibitors of the 4-keto acid type. The proposed mode of action involves 1,3-bridging didentate coordination of the carboxylate function of L1 to two FeIII ions, hydrogen-bond formation between the 4-keto group of L1 and a bridging surface hydroxy group, as well as close packing of the aromatic end groups, which should generate a hydrophobic barrier on the surface. Adsorption isotherm experiments have been used to compare the strengths of binding of related carboxylic acids onto iron(III) oxide surfaces and indicate that the presence of the 4-keto function leads to the formation of significantly more stable surface complexes.

Families of High Nuclearity Cages

Abstract The structural diversity displayed by polymetallic cages is discussed, concentrating on the chemistry of cobalt and nickel stabilised by carboxylate and pyridonate ligands. The structures can be broken down into “families of cages”, five of which are identified. Adamantine-cages have been made with hepta- and nona-nuclear cores. Tricapped trigonal prisms with deca-, undeca- and dodeca-nuclear metal arrays have been made for Co and Ni, and related cages for Cr and Fe have also been synthesised. Perhaps the most aesthetically pleasing family are the metal wheels, which vary in nuclearity from eight to eighteen, and which have been found for the metals from Ti to Ni - excluding Mn. Cages based on oligomers of cubanes form the fourth family, and the fifth group is related to simple mineral archetypes such as the rock-salt or cadmium iodide structure.

Modeling Surface Engineering: Use of Polymetallic Iron Cages and Computer Graphics To Understand the Mode of Action of a Corrosion Inhibitor

Simple organic molecules can have many functions. The active ingredient in the corrosion inhibitor 3-(4-methylbenzoyl)propionate works because it addresses the metal sites of a surface through carboxylate groups, forms hydrogen bonds with surface hydroxide groups (see picture), and provides excellent surface coverage through efficient packing of substituted aromatic groups.

The preparation and crystal structures of Cu(Hdpa)(chp)2 and a new phase of Cu(dpa)2 [Hdpa = 2-(pyridyl-2-amino)-pyridine, Hchp = 6-chloro-2-hydroxypyridine]

The synthesis and crystal structure of Cu(Hdpa)(chp)2 (I) are reported. The copper environment is six coordinate with Hdpa and one of the chp ligands bonding in a chelating fashion. The second pyridine is bound through N, the O atom takes up the sixth coordination site but is also hydrogen bonded to the Hdpa ligand of a second Cu(Hdpa)(chp)2 unit, forming a centrosymmetric dimer. A new (orthorhombic) phase of Cu(dpa)2 (II) has also been found, in which distorted tetrahedral copper(II) complexes are linked through a network of H bonds between the deprotonated dpa ligand and CH bonds meta- to the pyridyl nitrogen donor.

NEW HIGH-SPIN CLUSTERS FEATURING TRANSITION METALS

Three possible routes to polynuclear transition metal complexes are discussed. The first route, oligomerization induced by desolvation of small cages, is exemplified by synthesis of a dodecanuclear cobalt cage, and by reactions which give octa– and dodecanuclear chromium cages. The second route involves linking cages through organic spacers, and is illustrated by use of phthalate to link together cobalt and nickel cages. For the nickel case a complex consisting of four cubanes and a sodium octahedron is found. The third route involves the use of water to introduce hydroxide bridges into cages. One method of introducing water is to use hydrated metal salts; the transformation of a Cu6Na cage into a Cu12La8 illustrates this approach. Alternatively, adventitious water within solvents can be used as a source, and this approach has led to a Co24 cage. The structures and magnetic properties of these various cages are discussed.

Synthesis and studies of a trinuclear Mn(II) carboxylate complex

We report a carboxylate triangle consisting of three manganese(II) centres which is made from manganese(II) carbonate and pivalic acid. The magnetic exchange within the triangle is extremely weak, and antiferromagnetic. Several models have been used to fit the magnetic data, and the best fit uses two weak antiferromagnetic coupling constants of J(1)=-0.588 cm(-1) and J(2)=-0.855 cm(-1). Exchange interactions between the metal centres has been calculated using DFT adopting all the three possible Heisenberg models for a trinuclear system and the results are compared with experimental values. Spin density distribution is used to analyse the nature of the coupling between the metal centres. EPR spectroscopy has been used to explore the nature of the ground state. Recrystallisation of the trinuclear compound from MeCN gives a polymer, while oxidation in air leads to a known compound--an edge-sharing bitetrahedral (MnIII2MnII4) cage.