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Published on: March 19, 2020
Magnetic circular dichroism study of a dicobalt(II) complex with mixed 5- and 6-coordination: a spectroscopic model
James A Larrabee1, W Rainey Johnson, Adam S Volwiler
1Department of Chemistry and Biochemistry, Middlebury College, Middlebury, Vermont 05753, USA. larrabee@middlebury.edu
Abstract:
The magnetic circular dichroism (MCD) study of [Co(2)(mu-OH)(mu-Ph(4)DBA)(TMEDA)(2)(OTf)], in which Ph(4)DBA is the dinucleating bis(carboxylate) ligand dibenzofuran-4,6-bis(diphenylacetate) and TMEDA is N,N,N',N'-tetramethylethylenediamine, is presented. This complex serves as an excellent spectroscopic model for a number of dicobalt(II) enzymes and proteins that have both the mu-hydroxo, mu-carboxylato bridging and asymmetric 6- and 5-coordination. The low-temperature MCD spectrum of the model complex shows bands at 490, 504, and 934 nm arising from d-d transitions on the 6-coordinate Co(II) and bands at 471, 522, 572, 594, and 638 nm arising from d-d transitions on the 5-coordinate Co(II). The most intense MCD bands are at 504 and 572 nm for 6- and 5-coordinate Co(II), respectively, and these two bands are found in the MCD spectra of dicobalt(II)-substituted methionine aminopeptidase from Escherichia coli (CoCoMetAP), glycerophosphodiesterase from Enterobacter aerogenes (CoCoGpdQ), aminopeptidase from Aeromonas proteolytica (CoCoAAP), and myohemerythrin from Themiste zostericola (CoCoMyoHry). These dicobalt(II)-substituted proteins are known to have one 5- and one 6-coordinate Co(II) bridged by one or two carboxylates and either a water or a hydroxide. The uncertainty of the bridging water's state of protonation is problematic, as this is a likely candidate for the attacking nucleophile in the dimetallohydrolases. Analysis of the variable-temperature variable-field (VTVH) MCD data determined that the Co(II) ions in the model complex are ferromagnetically coupled with a J of 3.0 cm(-1). A comparison of all dicobalt(II) complexes and dicobalt(II)-substituted protein active sites with the mu-hydroxo/aqua, mu-carboxylato bridging motif reveals that J is either zero or negative (antiferromagnetic) in the mu-aqua systems and positive (ferromagnetic) in the mu-hydroxo systems. It was also determined that the Co(II) ions in CoCoAAP and CoCoMyoHry are ferromagnetically coupled, each with a J of 3.4 cm(-1), which suggests that these ions have a mu-hydroxo bridging ligand.
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