Ruthenium(II) Complexes Bearing Carboxylato and 2-Oxocarboxylato Ligands

Synthesis, Structure and Reactivity of a Mononuclear N,N,O-Bound Fe(II) α-Keto-Acid Complex

A bulky, tridentate phenolate ligand (ImPh2 NNOtBu ) was used to synthesise the first example of a mononuclear, facial, N,N,O-bound iron(II) benzoylformate complex, [Fe(ImPh2 NNOtBu )(BF)] (2). The X-ray crystal structure of 2 reveals that the iron centre is pentacoordinate (τ=0.5), with a vacant site located cis to the bidentate BF ligand. The Mössbauer parameters of 2 are consistent with high-spin iron(II), and are very close to those reported for α-ketoglutarate-bound non-heme iron enzyme active sites. According to NMR and UV-vis spectroscopies, the structural integrity of 2 is retained in both coordinating and non-coordinating solvents. Cyclic voltammetry studies show that the iron centre has a very low oxidation potential and is more prone to electrochemical oxidation than the redox-active phenolate ligand. Complex 2 reacts with NO to form a S=3 /2 {FeNO}7 adduct in which NO binds directly to the iron centre, according to EPR, UV-vis, IR spectroscopies and DFT analysis. Upon O2 exposure, 2 undergoes oxidative decarboxylation to form a diiron(III) benzoate complex, [Fe2 (ImPh2 NNOtBu )2 (μ2 -OBz)(μ2 -OH)2 ]+ (3). A small amount of hydroxylated ligand was also observed by ESI-MS, hinting at the formation of a high-valent iron(IV)-oxo intermediate. Initial reactivity studies show that 2 is capable of oxygen atom transfer reactivity with O2 , converting methyl(p-tolyl)sulfide to sulfoxide.

Phosphine-functionalised tris(pyrazolyl)methane ligands and their mono- and heterobimetallic complexes

The synthesis, characterisation and reactivity of new phosphine-functionalised tris(pyrazolyl)methane ligands (TpmPR₂, 2a-c, with R = Ph, nBu, iPr) are presented. The reaction of 2a-c with [Rh(CO)₂Cl]₂ furnished N,P-heterochelate carbonyl complexes (3a-c), which were used to quantify the donor abilities of the ligands via IR spectroscopy. The coordination flexibility was demonstrated by treating representative members of this new ligand class with [CpRu(acn)₃][PF₆] (acn = acetonitrile), [(tht)AuCl] (tht = tetrahydrothiophene) and [Pd(allyl)Cl]₂ providing either a N,N,P-heteroscorpionate complex (4) or the P-coordinated complexes (5,6) without any involvement of the pyrazolyl entities. With 2a and [Pd(cod)Cl₂], another P,N-heterochelate complex (7) was obtained, which served as a precursor for a heterobimetallic complex containing palladium and copper (8). Detailed NMR spectroscopic and X-ray crystallographic investigations have been performed on all new complexes.

Substitution behaviour of amine-bridged dinuclear Pt(II) complexes with bio-relevant nucleophiles

Complexes of the type [Pt2(N,N,N',N'-tetrakis(2-pyridylmethyl)diamine(HO)2]4+ and [Pt2(N,N,N',N'-tetrakis(2-pyridylmethyl)diamine(Cl)2]2+ were used to study their reactions with a series of bio-relevant nucleophiles, viz. thiourea, L-methionine and guanosine-5'-monophosphate (5'-GMP2-) as a function of nucleophile concentration and temperature. The reactions with the sulfur containing nucleophiles (thiourea and L-methionine) were followed under pseudo-first-order conditions by stopped-flow and UV-Vis spectrophotometry. The reaction with 5'-GMP2- was carried out under second order conditions and studied by NMR spectroscopy. The results indicate that the bridged dinuclear complexes remain intact after coordination of the studied nucleophiles for an extended period of time, which differs significantly from that reported for other multinuclear platinum complexes in the literature.

Iron(II) complexes of sterically bulky alpha-ketocarboxylates. structural models for alpha-ketoacid-dependent nonheme iron halogenases

Reaction of the sterically hindered alpha-ketocarboxylate 2,6-di(mesityl)benzoylformate (MesBF) with the iron(II) complexes LFeCl 2 [L = N, N, N', N'-tetramethylpropylenediamine (Me 4pda) or 6,6'-dimethyl-2,2'-bipyridine (dmby)] yielded LFe(Cl)(MesBF) ( 1 or 2). X-ray crystal structures of these complexes showed that they closely model the active site structure of the nonheme iron halogenase enzyme SyrB2. A similar synthetic procedure using benzoylformate with L = dmby yielded (dmby)Fe[(O 2CC(O)Ph)] 2 ( 3) instead, demonstrating the need for the sterically hindered alpha-ketocarboxylate to assemble the halogenase model compounds. In order to make reactivity comparisons among the structurally related iron(II) complexes of benzoylformates of varying steric properties, the complexes [LFe(O 2CC(O)Ar)] n ( 4- 6) were prepared, where L' = tris(pyridylmethyl)amine (tpa) and Ar = 2,6-dimesitylphenyl, 2,6-di p-tolylphenyl, or 2,4,6-trimethylphenyl, respectively. X-ray structures for the latter two cases ( 5 and 6) revealed dinuclear topologies ( n = 2), but UV-vis and (1)H NMR spectroscopy indicated that all three complexes dissociated in varying degrees to monomers in CH 2Cl 2 solution. Although compounds 1- 6 were oxidized by O 2, oxidative decarboxylation of the alpha-ketocarboxylate ligand(s) only occurred for 3. These results indicate that the steric hindrance useful for structural modeling of the halogenase active site prohibits functional mimicry of the enzyme.

Influence of the bridging ligand on the substitution behaviour of dinuclear Pt(ii) complexes. An experimental and theoretical approach

A series of dinuclear Pt(II) complexes of the type [Pt2(N,N,N',N'-tetrakis(2-pyridylmethyl)diamine(H2O)2]4+ were synthesized. Acid-base titrations, and concentration and temperature dependent stopped-flow measurements of the reaction with chloride were performed to study the thermodynamic and kinetic behaviour of the dinuclear bridged complexes. The results indicate that there is a clear interaction between the two Pt(II) centres, which becomes weaker as the aliphatic chain increases in length. From a certain chain length onwards, the Pt(II) centres become independent of each other and exhibit identical thermodynamic and kinetic properties. The experimental results are discussed in reference to structures obtained by DFT (BP86/LACVP*) calculations.