A sequential and mild approach toward the synthesis of poly(limonene)dicarbonate (PLDC) has been developed using readily available limonene oxide (LO) and CO 2 as renewable reagents and an air-stable Al(III) complex as catalyst for the alkene-rich poly-(limonene)carbonate (PLC). The developed sequence allows for the stepwise construction of different PLDC polymers, using PLC as a synthetic intermediate with molecular weights of up to 15.3 kg/mol and tunable glass-transition temperature (T g ) values of up to an unprecedented 180 °C using a commercially available mixture of cis/ trans (+)-LO.

Copper enzymes play important roles in the binding and activation of dioxygen in biological systems. Key copper/dioxygen intermediates have been identified and studied in synthetic analogues of the metalloprotein active sites, including the μ-η(2):η(2)-peroxodicopper(II) motif relevant to type III dicopper proteins. Herein, we report the synthesis and characterization of a bioinspired dicopper system that forms a stable μ-η(1):η(1)-peroxo complex whose Cu-O-O-Cu torsion is constrained to around 90° by ligand design. This results in sizeable ferromagnetic coupling between the copper(II) ions, which is detected by magnetic measurements and HF-EPR spectroscopy. The new dicopper peroxo system is the first with a triplet ground state, and it represents a snapshot of the initial stages of O2 binding at type III dicopper sites.

Pyrazolate-based μ-1,2-peroxo dicopper(II) complex 1 undergoes clean 1e oxidation at low potential (-0.59 V vs Fc/Fc) to yield the rather stable μ-1,2-superoxo dicopper(II) complex 3, which was characterized by spectroscopic methods (ν̃(O-O) = 1070 cm, Δ(O-O) = -59 cm) and analyzed by DFT calculations. 3 is also formed via H-atom abstraction from the corresponding μ-1,1-hydroperoxo dicopper(II) complex 2, while 3 itself is able to abstract H-atoms from weaker X-H bonds such as TEMPO-H to re-form 2. Kinetic and thermodynamic analyses evidence a concerted proton-electron transfer pathway for these processes. The thermodynamic square scheme reveals a bond dissociation free energy of 71.7 ± 1.1 kcal mol for the hydroperoxo OO-H bond of 2.

Flavo-diiron nitric oxide reductases (FNORs) are a subclass of nonheme diiron proteins in pathogenic bacteria that reductively transform NO to NO, thereby abrogating the nitrosative stress exerted by macrophages as part of the immune response. Understanding the mechanism and intermediates in the NO detoxification process might be crucial for the development of a more efficient treatment against these bacteria. However, low molecular weight models are still rare, and only in a few cases have their reductive transformations been thoroughly investigated. Here, we report on the development of two complexes, based on a new dinucleating pyrazolate/triazacyclononane hybrid ligand L, which serve as model systems for nonheme diiron active sites. Their ferrous nitrile precursors [L{Fe(R'CN)}(μ-OOCR)](X) (1) can be readily converted into the corresponding nitrosyl adducts ([L{Fe(NO)}(μ-OOCR)](X), 2). Spectroscopic characterization shows close resemblance to nitrosylated nonheme diiron sites in proteins as well as previous low molecular weight analogues. Crystallographic characterization reveals an anti orientation of the two {Fe(NO)} (Enemark-Feltham notation) units. The nitrosyl adducts 2 can be (electro)chemically reduced by one electron, as shown by cyclic voltammetry and UV/vis spectroscopy, but without the formation of NO. Instead, various spectroscopic techniques including stopped-flow IR spectroscopy indicated the rapid formation, within few seconds, of two well-defined products upon reduction of 2a (R = Me, X = ClO). As shown by IR and Mössbauer spectroscopy as well as X-ray crystallographic characterization, the reduction products are a diiron tetranitrosyl complex ([L{Fe(NO)}](ClO), 3a') and a diacetato-bridged ferrous complex [LFe(μ-OAc)](ClO) (3a″). Especially 3a' parallels suggested products in the decay of nitrosylated methane monooxygenase hydroxylase (MMOH), for which NO release is much less efficient than for FNORs.

The μ-1,2-peroxo dicopper(II) complex (2) of a compartmental bis(tetradentate) pyrazolate-based ligand is shown to convert, upon protonation, to the corresponding μ-1,1-hydroperoxo dicopper(II) complex (3). The transformation is cleanly reversed with base, and an apparent pK(a) = 22.2 ± 0.3 for the Cu2OOH unit in MeCN has been determined. The unprecedented stability of 3 (t(1/2) = 9 h in nitrile solvents at room temperature, giving the hydroxo-bridged dicopper complex) has allowed for its structural characterization by X-ray diffraction. While the O-O bond length (1.462(3) Å) barely changes upon protonation from 2 to 3, the O-O stretching frequency is much higher in the hydroperoxo complex 3 (860 cm(-1)). 3 mediates 2e(-) oxo transfer to the nucleophilic substrate PPh3 but is not activated for H-atom abstraction.