Conversion of Azomethine Moiety to Carboxamido Group at Cobalt(III) Center in Model Complexes of Co-Containing Nitrile Hydratase

Abstract

The Co(III) complex of the Schiff base ligand N-2-mercaptophenyl-2'-pyridylmethyl-enimine (PyASH), namely, [Co(PyAS)(2)]Cl (1), has been synthesized via an improved method and its structure has been determined by X-ray crystallography. The two deprotonated ligands are arranged in mer configuration around the Co(III) center and the overall coordination geometry is octahedral. The coordinated azomethine function in 1 is rapidly converted into carboxamido group upon reaction with OH(-). The product is the bis carboxamido complex (Et(4)N)[Co(PyPepS)(2)] (2), reported by us previously. Reaction of H(2)O(2) with 1 in DMF affords [Co(PyASO(2))(PyPepSO(2))] (3), a species with mixed imine and carboxamido-N donor centers as well as S-bound sulfinates. Further reaction with H(2)O(2) in the presence of NaClO(4) converts 3 into the previously reported bis carboxamido/sulfinato complex Na[Co(PyPepSO(2))(2)] (4). The reaction conditions for the various transformation reactions for complexes 1-4 and the structure of 3 are also reported. The mechanism of the -CH=NR + [O] --> -C(=O)NHR transformation has been discussed. The reactions reported here provide convenient alternate routes for the syntheses of Co(III) complexes with coordinated carboxamide, thiolate, and/or sulfinate donors as models for the Co-site in the Co-containing nitrile hydratase(s).

CoIII Complexes with Square‐Planar N2S2‐ and N2(SO2)2‐Type Ligands as An Active Site Structural Model for Nitrile Hydratase – Biological Implications of an Amidate Coordination

“…The redox potential is high enough to maintain the Co center in the 3+ oxidation state and is consistent with those of previously reported N 2 S 2 -type Co complexes that contain two amidate nitrogen atoms and two sulfur atoms as coordinating atoms. [31][32][33] The redox potential in methanol is clearly much higher than that in acetone, which is in agreement with the tendencies for the equilibrium constant Table S6). The redox potential values showed a significantly larger positive shift with an increase in AN, which suggests that the attractive interaction of the carbonyl oxygen atoms with the solvent molecules causes an increase in the Lewis acidity of the Co III center and promotes the axial coordination of a monodentate ligand.…”

Spectroscopic differences between heterocyclic benzothiazoline, -thiazole and imine containing ligands and comparison of the Co and Cu pyridine benzothiazole and imine complexes

“…Mascharak and coworkers previously reported the Co(III) complex of Py(Bt) [11]. This reaction was carried out in DMF using NaH as base and [Co(NH 3 ) 5 Cl]Cl 2 as the starting metal salt and yielded [Co(PyAS) 2 ]Cl with a 2:1 metal to ligand ratio (Note: the un-cyclized ligand N-2-mercaptophenyl-2 0 -pyridylmethyl-enimine is denoted PyASH where PyAS À is the deprotonated form of the ligand).…”

“…The cyclization of the product has an intrinsic advantage because it serves to protect the thiol group from undergoing oxidation to the corresponding disulfide, and the ring easily opens in the presence of base to coordinate the imine nitrogen to a metal ion [9][10][11][12][13][14].…”

Ligand Transformations and Efficient Proton/Water Reduction with Cobalt Catalysts Based on Pentadentate Pyridine‐Rich Environments

“…This step has been documented. [9] A detailed density functional theory (DFT) [10] study was also performed to evaluate details of the hydroxy to amide conversion mechanism ( Figure 2). Calculations indicate that the transformation requires atmospheric 3 O 2 to react with the C À H function of the intermediate hydroxy complex.…”

“…Thus, the metal center helps stabilize the radical intermediate I and makes the CÀH hydrogen removal event nearly isoenergetic. [9] An intersystem crossing (triplet to singlet surface) from species I followed by the removal of the hydroxy hydrogen by the hydroperoxo radical gives rise to the amide complex 4 and the overall process is favored by about 38 kcal mol À1 . Geometry optimization of intermediate I on the singlet surface results in the transfer of the hydroxy hydrogen from the metal complex to the hydroperoxo radical, giving rise to 4.…”