Hideki Furutachi

Bimetallic dioxygen complexes derived from ‘end-off’ compartmental ligands

Abstract This article deals with recent progress in bimetallic dioxygen complexes derived from ‘end-off’ compartmental ligands having a phenolic or alcoholic oxygen as the endogenous bridge. Focus is placed on oxygenation of μ-phenoxo and μ-alkoxo bridged dimetal(II) complexes (metal(II)=Co(II), Fe(II), and Cu(II)) in view of the electronic and stereochemical nature of the compartmental ligands and bimetallic core structures.

Aliphatic C−H Bond Activation Initiated by a (μ-η2:η2-Peroxo)dicopper(II) Complex in Comparison with Cumylperoxyl Radical

A (mu-eta(2):eta(2)-peroxo)dicopper(II) complex, [Cu(2)(H-L)(O(2))](2+) (1-O(2)), supported by the dinucleating ligand 1,3-bis[bis(6-methyl-2-pyridylmethyl)aminomethyl]benzene (H-L) is capable of initiating C-H bond activation of a variety of external aliphatic substrates (SH(n)): 10-methyl-9,10-dihydroacridine (AcrH(2)), 1,4-cyclohexadiene (1,4-CHD), 9,10-dihydroanthracene (9,10-DHA), fluorene, tetralin, toluene, and tetrahydrofuran (THF), which have C-H bond dissociation energies (BDEs) ranging from approximately 75 kcal mol(-1) for 1,4-CHD to approximately 92 kcal mol(-1) for THF. Oxidation of SH(n) afforded a variety of oxidation products, such as dehydrogenation products (SH((n-2))), hydroxylated and further-oxidized products (SH((n-1))OH and SH((n-2))=O), dimers formed by coupling between substrates (H((n-1))S-SH((n-1))) and between substrate and H-L (H-L-SH((n-1))). Kinetic studies of the oxidation of the substrates initiated by 1-O(2) in acetone at -70 degrees C revealed that there is a linear correlation between the logarithms of the rate constants for oxidation of the C-H bonds of the substrates and their BDEs, except for THF. The combination of this correlation and the relatively large deuterium kinetic isotope effects (KIEs), k(2)(H)/k(2)(D) (13 for 9,10-DHA, approximately > 29 for toluene, and approximately 34 for THF at -70 degrees C and approximately 9 for AcrH(2) at -94 degrees C) indicates that H-atom transfer (HAT) from SH(n) (SD(n)) is the rate-determining step. Kinetic studies of the oxidation of SH(n) by cumylperoxyl radical showed a correlation similar to that observed for 1-O(2), indicating that the reactivity of 1-O(2) is similar to that of cumylperoxyl radical. Thus, 1-O(2) is capable of initiating a wide range of oxidation reactions, including oxidation of aliphatic C-H bonds having BDEs from approximately 75 to approximately 92 kcal mol(-1), hydroxylation of the m-xylyl linker of H-L, and epoxidation of styrene (Matsumoto, T.; et al. J. Am. Chem. Soc. 2006, 128, 3874).

Polynuclear zinc(II) complexes of phenol–imine and –amine macrocycles

The template condensation of 2,6-diformyl-4-methylphenol with 1,11-diamino-3,6,9-trioxaundecane using zinc(II) acetate as the metal source, followed by addition of an excess of NaClO4, gave a tetranuclear zinc(II) complex [Zn4L1(µ-O2CMe)4][ClO4]21, where H2L1 is a [2 + 2] tetra-Schiff-base macrocycle. A single-crystal X-ray analysis of the dichloromethane adduct, 1·2CH2Cl2, revealed that this complex cation is centrosymmetric and contains two pairs of zinc ions bridged by a phenolic oxygen and two acetate ligands. Recrystallisation of this complex from a hydrous methanolic solution gave another tetranuclear complex, [Zn4L1(µ-OH)2(µ-O2CMe)2][ClO4]2·2H2O 2, which was also structurally characterised. While the four zinc ions are encapsulated in the macrocycle as observed for 1, in complex 2 the two dimeric units formed by phenoxo- and acetato-bridges are further connected by two hydroxyo bridges. Proton NMR spectroscopy has shown that this structural conversion is reversible and depends on the presence of water or acetic acid in solution. Treatment of complex 1(or 2) with NaBH4 in methanol gave a dinuclear zinc complex, [Zn2L2][ClO4]2·H2O3, together with a metal-free phenol–amine macrocycle, H2L2, which is a reduced form of H2L1. Both complex 3 and H2L2, have been characterised on the basis of spectroscopic data.

Oxidation Reactivity of Bis(μ‐oxo) Dinickel(III) Complexes: Arene Hydroxylation of the Supporting Ligand

In the nick(el) of time: Bis(mu-oxo) dinickel(III) complexes 2 (see scheme), generated in the reaction of 1 with H(2)O(2), are capable of hydroxylating the xylyl linker of the supporting ligand to give 3. Kinetic studies reveal that hydroxylation proceeds by electrophilic aromatic substitution. The lower reactivity than the corresponding mu-eta(2):eta(2)-peroxo dicopper(II) complexes can be attributed to unfavorable entropy effects.

Nuclear Resonance Vibrational Spectroscopy and DFT study of Peroxo-Bridged Biferric Complexes: Structural Insight into Peroxo Intermediates of Binuclear Non-heme Iron Enzymes†

Binuclear non-heme iron enzymes utilize O2 to catalyze a variety of reactions, including hydrogen atom abstraction, desaturation, electrophilic aromatic substitution, and so on. In most cases, their catalytic cycles begin with the reductive binding of O2 by biferrous centers to form high-spin antiferromagnetically coupled (AFC) peroxo-bridged biferric intermediates. These peroxo intermediates can either react with substrate or convert to more reactive high-valent species. Because of their transient nature, structural information must be deduced from spectroscopic data, which are rich for some peroxo intermediates, while for others too limited for geometric and electronic structural insight. The peroxo intermediate of W48F/D84E ribonucleotide reductase (RR), referred to as P, does exhibit distinct spectral features. These include electronic absorption (Abs) and resonance Raman (rR) spectra that are equivalent to those of cis m-1,2 end-on peroxo-bridged Fe2 model complexes, thus providing a basis for the computational model of P as a cis m-1,2 peroxo-bridged Fe2 species (with the (Glu)4(His)2 ligand set of this protein active site). However, P is not reactive and must convert to a second-peroxo-level intermediate P’ that does not have Abs spectral features for rR-based structural elucidation. For systems that do not have chromophores or are photoactive, nuclear resonance vibrational spectroscopy (NRVS) is an alternative to rR spectroscopy. NRVS is a synchrotron-based technique that probes vibrational side bands of Fe nuclear transitions. Its spectral intensity is determined by the amount of Fe displacement in each normal mode, thus allowing the specific investigation of the Fe active site with high sensitivity and without the limitation of the selection rules of rR spectroscopy. In this study, we establish the basis for the NRVS analysis of peroxo-bridged Fe2 intermediates, based on structurally well-characterized synthetic model complexes. We have measured the NRVS spectra of [Fe2(mOH)(mO2)(6Me2-BPP)2] + (1) and [Fe2(mO)(mO2)(6Me2-BPP)2] (2 ; Figure 1; 6Me2-BPP = N,N-bis(6methyl-2-pyridylmethyl)-3-aminopropionate). These complexes are cis m-1,2 peroxo-bridged species, the former with an additional hydroxo bridge and the latter with an oxo

Structure and dioxygen-reactivity of copper(I) complexes supported by bis(6-methylpyridin-2-ylmethyl)amine tridentate ligands

The structure and dioxygen-reactivity of copper(I) complexes R supported by N,N-bis(6-methylpyridin-2-ylmethyl)amine tridentate ligands L2R[R (N-alkyl substituent)=-CH2Ph (Bn), -CH2CH2Ph (Phe) and -CH2CHPh2(PhePh)] have been examined and compared with those of copper(I) complex (Phe) of N,N-bis[2-(pyridin-2-yl)ethyl]amine tridentate ligand L1(Phe) and copper(I) complex (Phe) of N,N-bis(pyridin-2-ylmethyl)amine tridentate ligand L3(Phe). Copper(I) complexes (Phe) and (PhePh) exhibited a distorted trigonal pyramidal structure involving a d-pi interaction with an eta1-binding mode between the metal ion and one of the ortho-carbon atoms of the phenyl group of the N-alkyl substituent [-CH2CH2Ph (Phe) and -CH2CHPh2(PhePh)]. The strength of the d-pi interaction in (Phe) and (PhePh) was weaker than that of the d-pi interaction with an eta2-binding mode in (Phe) but stronger than that of the eta1 d-pi interaction in (Phe). Existence of a weak d-pi interaction in (Bn) in solution was also explored, but its binding mode was not clear. Redox potentials of the copper(I) complexes (E1/2) were also affected by the supporting ligand; the order of E1/2 was Phe>R>Phe. Thus, the order of electron-donor ability of the ligand is L1Phe<L2R<L3Phe. This was reflected in the copper(I)-dioxygen reactivity, where the reaction rate of copper(I) complex toward O2 dramatically increased in the order of R<R<R. The structure of the resulting Cu2/O2 intermediate was also altered by the supporting ligand. Namely, oxygenation of copper(I) complex R at a low temperature gave a (micro-eta2:eta2-peroxo)dicopper(II) complex as in the case of Phe, but its O-O bond was relatively weakened as compared to the peroxo complex derived from Phe, and a small amount of a bis(micro-oxo)dicopper(III) complex co-existed. These results can be attributed to the higher electron-donor ability of L2R as compared to that of L1Phe. On the other hand, the fact that Phe mainly afforded a bis(micro-oxo)dicopper(III) complex suggests that the electron-donor ability of L2R is not high enough to support the higher oxidation state of copper(III) of the bis(micro-oxo) complex.

Diphenoxo-bridged NiCo and CuCo complexes of macrocyclic ligands:synthesis, structure and electrochemical behaviour

Heterodinuclear di-µ-phenoxo-M II Co II (M = Ni or Cu) complexes have been derived from phenol-based dinucleating ligands (L m,n ) 2- , comprising of two 2,6-di(iminomethyl)-4-methylphenolate entities linked by two lateral chains (CH 2 ) m (m = 2 or 3) and (CH 2 ) n (n = 3 or 4) at the imino nitrogens. The crystal structure of [CuCo(L 2,3 )(dmf) 2 (H 2 O)][ClO 4 ] 2 (dmf = dimethylformamide) has been determined. The copper ion resides at the N 2 O 2 site formed by the ethylene lateral chain and assumes a square-pyramidal geometry together with a dmf oxygen at the apex. The Co at the site of the trimethylene lateral chain assumes a pseudo-octahedral geometry together with a dmf and a water molecule at the axial positions. The Cu · · · Co separation doubly bridged by the phenolic oxygens is 2.998(2) A. The NiCo complexes are paramagnetic (S Ni = 0), whereas the CuCo complexes show a strong antiferromagnetic interaction between the two metal ions. Cyclic voltammograms of the NiCo complexes show two quasi-reversible couples attributable to the stepwise reductions: Ni II Co II → Ni I Co II → Ni I Co I . The Ni I Co II and Ni I Co I complexes were prepared in solution by coulometry and characterized by visible spectroscopy. Similarly, the CuCo complex of (L 3,3 ) 2- is reduced stepwise to Cu I Co II and then to Cu I Co I . On the other hand, the CuCo complexes of (L 2,3 ) 2- and (L 2,4 ) 2- showed unusual electrochemical behaviour at the electrode suggesting a scrambling or site exchange of the metal ions.