Thermodynamic Study on Dioxygen Binding of Diiron(II) and Dicobalt(II) Complexes Containing Various Dinucleating Ligands

Abstract

A new dinucleating ligand containing a sterically bulky imidazolyl group, Ph-Htidp (N,N,N′,N′-tetrakis[(1-methyl-4,5-diphenyl-2-imidazolyl)methyl]-1,3-diamino-2-propanol), and its μ-alkoxo-diiron(II) complexes [Fe2(Ph-tidp)(RCO2)](ClO4)2, (RCO2 = C6H5CO2 (1), C6F5CO2 (2), CF3CO2 (3), and C2H5CO2 (4)), were synthesized. The structure of complex 1 was determined by X-ray crystallography. Complex 1 crystallizes in the monoclinic space group P21/c with a = 13.464(2), b = 19.223(4), c = 31.358(4) Å, β = 92.84(2)°, and Z = 4. The complex has a doubly-bridged structure with μ-alkoxo of Ph-tidp and μ-benzoate; the two iron centers have a distorted five-coordinate structure with N3O2 donor set. All the complexes showed fairly good reversible oxygenation below −30 °C in CH2Cl2, which was monitored by UV-vis and NMR spectroscopies, and dioxygen up-take measurements. Introduction of 4,5-diphenyl substituents into 2-imidazolyl group stabilized the μ-peroxo diiron species against irreversible oxidation, just as introduction of 6-methyl substituent into 2-pyridyl group did. Phenyl substituents appear to weaken the electron donor ability of a dinucleating ligand to stabilize divalent oxidation state of iron and to form a hydrophobic cavity for a O2 binding site, which would suppress the irreversible oxidation and facilitate the reversible oxygenation. Dioxygen affinities of the Ph-tidp and Me4-tpdp diiron(II), and the tpdp and bpmp dicobalt(II) complexes were measured, [Fe2(Me4-tpdp)(RCO2)]2+ (RCO2 = C6H5CO2 and RCO2 = CF3CO2) and [Co2(L)(RCO2)]2+ (L = tpdp, RCO2 = CH3CO2, and L = bpmp, RCO2 = C6F5CO2, and CF3CO2), where Me4-tpdp, tpdp, and bpmp are N,N,N′,N′-tetrakis[(6-methyl-2-pyridyl)methyl]-1,3-diamino-2-propanolate, N,N,N′,N′-tetrakis(2-pyridylmethyl)-1,3-diamino-2-propanolate, and 2,6-bis[bis(2-pyridylmethyl)aminomethyl]-4-methylphenolate, respectively. Within a series of the Ph-tidp diiron(II) complexes, dioxygen affinity is well correlated with electron donor ability of bridging carboxylates (1 (C6H5CO2) > 2 (C6F5CO2) > 3 (CF3CO2)). In contrast to the above trend, dioxygen affinities of the Ph-tidp complexes are lower than those of the Me4-tpdp complexes, although electron donor abilities of the Me4-tpdp complexes are weaker than those of the Ph-tidp complexes. Significant enhancement of dioxygen affinity was observed for both iron and cobalt complexes with 2,6-bis(aminomethyl)phenolate bridging skeleton compared to the complexes with a 1,3-diamino-2-propanolate bridging one. Thermodynamic study suggested that the observed enhancement is mainly attributable to a favorable entropy effect along with a steric effect.

Thermodynamics of Complex Formation of Silver(I), Cadmium(II) and Cobalt(II) with Open‐Chain Polyamines in Dimethyl Sulfoxide and Molecular Dioxygen Binding to Cobalt(II) Complexes

“…As far as cobalt() complexes are concerned, different N-alkylation of polyamines also has important effects on the dioxygen affinity of these complexes: [6] electron transfer between the metal centre and di-oxygen should depend on the available electron density at the central metal ion, which is in turn greatly affected by the electron-donor properties of the ligands in the coordination sphere. [26,27] As an extension of previous works we report here on results concerning the complex formation of Ag I , Cd II and Co II with the differently N-methylated polyamines 1,4,7,10-tetraazadecane (trien) and 1,1,4,7,10,10-hexamethyltriethylenetetraamine (hmtrien) in the aprotic solvent dimethyl sulfoxide (DMSO). For silver(), data relating to trien complexation in DMSO had been published previously.…”

Affinity of Polypyridines Towards CdII and CoII Ions: a Thermodynamic and DFT Study

“…Examples of the attractive properties and applications of these ligands are their potential use as building blocks of sequestering agents for metals of environmental importance, such as Cd or Pb, [4] fluorescent chemosensors, [3e] catalysts, [3f] or models for small molecule carriers, as in Cu I or Co II dioxygen complexes. [13][14][15][16][17][18] In this context, some of our published papers concerning the complexation of transition-metal ions like Cd II and Co II [11] have shown that steric effects and N-methylation strongly influence not only the stoichiometry and nature of all metal ion complexes formed, but also the selectivity of these ligands toward metal ions. Systematic studies confirm that different N-alkylation of polyamines, or merely the substitution of only one amine with one alcohol group, causes important differences in the dioxygen affinity of Co II complexes: [11b,11e] this affinity greatly depends on the elec-complexes are formed, whereas entropy changes counteract complex formation.…”

Solvent Role on Cobalt(II) Dioxygen Carriers Based on Simple Polyamine Ligands

“…Bubbling N 2 after 24 h does not produce any change in the electronic spectrum, suggesting that an irreversible process has already occurred. When the [Co(dien) 2 ] 2 complex is involved, no change in the 2 , the oxidation reaction (Eqn. 2) is very slow, and k 2 cannot be determined.…”

“…O 2 Uptake in DMSO. The formation of the superoxo adduct [Co(dien) 2 , and the electron density on the metal center is slightly lower in the former case; ii) on the other hand, the value of logK O 2 has been related [4] to the sum of logK a of the ligands (which is a measure of the simple s-donor ability of a ligand) that are reported here in H 2 O for dmdien (SlogK a 22.59) and available for dien (SlogK a 23.22) [24]. As this trend is not expected to be affected very differently in DMSO, the large difference in the logK O 2 values for dien and dmdien does not seem to be accounted for by these arguments alone, as DSlogK a is only ca.…”

“…1 are bearing the O 2 ligand opposite to an amino group. The dien ligand in the [Co(dien) 2 3.2. Complexation Equilibria in Water.…”