Single crystal structural analysis of [FeII(tame)2]Cl2⋅MeOH (tame=1,1,1‐tris(aminomethyl)ethane) as a function of temperature reveals a smooth crossover between a high temperature high‐spin octahedral d 6 state and a low temperature low‐spin ground state without change of the symmetry of the crystal structure. The temperature at which the high and low spin states are present in equal proportions is T 1/2=140 K. Single crystal, variable‐temperature optical spectroscopy of [FeII(tame)2]Cl2⋅MeOH is consistent with this change in electronic ground state. These experimental results confirm the spin activity predicted for [FeII(tame)2]2+ during its de novo artificial evolution design as a spin‐crossover complex [Chem. Inf. Model. 2015, 55, 1844], offering the first experimental validation of a functional transition‐metal complex predicted by such in silico molecular design methods. Additional quantum chemical calculations offer, together with the crystal structure analysis, insight into the role of spin‐passive structural components. A thermodynamic analysis based on an Ising‐like mean field model (Slichter–Drickammer approximation) provides estimates of the enthalpy, entropy and cooperativity of the crossover between the high and low spin states.

The complexation of nickel(ii) with acetylacetonate bis(thiosemicarbazone) NS ligands with varying substituents has revealed that two isomers can exist independently in solution. These isomers differ according to the formation of either a 5,6,5-membered (symmetric) or a 4,7,5-membered (asymmetric) chelate ring arrangement. These two isomers have distinctly different properties. The symmetric complex (sym-[Ni(acacR)]) is unstable in the presence of air and slowly converts to the oxidised analogue sym-[Ni(acacRO)] with a carbonyl group installed at the apical C-atom. The mechanism of this O-atom transfer reaction is still unclear but kinetic and spectroelectrochemical experiments in addition to Density Functional Theory calculations have identified a single electron oxidised Ni-ligand radical complex as a key intermediate. By contrast the asymmetric complex, asym-[Ni(acacR)] is inert to ligand oxidation.

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The complexation reaction mechanism of acetylacetone bis-thiosemicarbazone ligands (H 2 acacR) with Cu II is explored using a variety of physical methods. The complexes form via a complicated multistep mechanism that is initiated by ring opening of the pyrazoline form of the ligand and leads, ultimately in air, to an oxidised ketone form of the ligand. Tetradentate N 2 S 2 coordinated forms of the intermediate [Cu (acacR)] [a]

The tetradentate N2S2 Schiff base ligands derived from condensing S-methyl or S-benzyl dithiocarbazate with acetylacetone have been found to be versatile chelators for nickel and undergo a variety of ligand centred oxidations.

The copper coordination chemistry of bis(dithiocarbazate) Schiff base ligands derived from acetylacetone yields a diversity of products depending on the reaction conditions and presence of oxygen. Both di- and trivalent copper complexes have been isolated and characterised.

CpsB is a metal ion-dependent hydrolase involved in the biosynthesis of capsular polysaccharides in bacterial organisms. The enzyme has been proposed as a promising target for novel chemotherapeutics to combat antibiotic resistance. The crystal structure of CpsB indicated the presence of as many as three closely spaced metal ions, modelled as Mn, in the active site. While the preferred metal ion composition in vivo is obscure Mn and Co have been demonstrated to be most effective in reconstituting activity. Using isothermal titration calorimetry (ITC) we have demonstrated that, in contrast to the crystal structure, only two Mn or Co ions bind to a monomer of CpsB. This observation is in agreement with magnetic circular dichroism (MCD) and electron paramagnetic resonance (EPR) data that indicate the presence of two weakly ferromagnetically coupled Co ions in the active site of catalytically active CpsB. While CpsB is known to be a phosphoesterase we have also been able to demonstrate that this enzyme is efficient in hydrolyzing the β-lactam substrate nitrocefin. Steady-state and stopped-flow kinetics measurements further indicated that phosphoesters and nitrocefin undergo catalysis in a conserved manner with a metal ion-bridging hydroxide acting as a nucleophile. Thus, the combined physicochemical studies demonstrate that CpsB is a novel member of the dinuclear metallohydrolase family.

Bis-thiosemicarbazones derived from the b-diketone benzoylacetone (H 3 banR, R ¼ Me, Et, Ph) are potentially tetradentate N 2 S 2 ligands whose coordination chemistry with copper is reported. In the absence of oxygen and in the presence of base they form anionic Cu II complexes of the fully deprotonated ligands [Cu II (banR)]-. Upon exposure to atmospheric oxygen they undergo a complex series of reactions leading to two types of products; one a ligand oxidised ketone complex [Cu II (banRO)] and the other an unprecedented dimeric di-Cu III complex [(Cu III (banR)) 2 ] depending on the R substituent. Time-resolved UV-vis spectroscopy, cyclic voltammetry, spectroelectrochemistry, and electron paramagnetic resonance (EPR) spectroscopy have been used to identify intermediates on the way to stable products formed under both anaerobic and aerobic conditions. It is found that both ligand-centred and Cu-centred oxidation reactions are occurring in parallel leading to this unusually complicated mixture of products.