Orientation and Stereodynamic Paths of Planar Monodentate Ligands in Square Planar Nickel N₂S Complexes

Abstract

The well-established presence of histidine donors in binding sites of Ni-containing biomolecules prompts the study of orientational preference and stereodynamic nature of flat monodentate ligands (L = imidazoles, pyridine and an N-heterocyclic carbene) bound to planar N 2 SNi moieties. Square planar [N 2 SNiL] n+ complexes are accessed through bridge-splitting reactions of dimeric, thiolate-S bridged [N 2 SNi] 2 complexes. The solid state molecular structures of three mononuclear products, and three monothiolate bridged dinickel complexes, reveal that the plane of the added monodentate ligand orients largely orthogonal to the N 2 SNiL square plane. Variable temperature 1 H NMR characterization of dynamic processes and ground state isomer ratios of imidazole complexes in their stopped exchange limiting spectra, readily correlate with DFT-guided interpretation of Ni-L rotational activation barriers. Full DFT characterization find Ni-L bond lengthening as well as a tetrahedral twist distortion in the transition state, reaching a maximum in the NHC complex, and relating mainly to the steric hindrance derived both from the ligand and the binding pocket. In the case of the imidazole ligands a minor electronic contribution derives from intramolecular electrostatic interactions (imidazole C-2 C-H δ+ --S δ− interaction). Computational studies find this donor-acceptor interaction is magnified in O-analogues, predicting coplanar arrangements in the ground state of N 2 ON imid Ni complexes.

pH Dependent Reversible Formation of a Binuclear Ni2 Metal-Center within a Peptide Scaffold

“…These data are consistent with the structural models depicted in Scheme 1; at pH 7.4, the data is consistent with each peptide containing two mononuclear nickel sites that collapses into a binuclear cysteinate bridged Ni 2 center at high pH, the formation of which is well represented in small molecule Ni-thiolate chemistry [47,48]. two mononuclear nickel sites that collapses into a binuclear cysteinate bridged Ni2 center at high pH, the formation of which is well represented in small molecule Ni-thiolate chemistry [47,48].…”

“…Nickel thiolates are susceptible to oxidative damage by O 2 and H 2 O 2 [52,53], yet the NiSOD active-site is robust against oxidative damage effected by such species. Possible explanations for protection of the NiSOD active-site against Ni-S oxidative damage by ROSs have been proposed, including electronic fine tuning of the Ni-S moiety via the mixed amine/amide coordination environment [54,55] and a fast rate of the O 2 − disproportionation reaction relative to the rate of oxidation of the coordinate cysteinate sulfur atoms [45,48,49]. In this study we have shown that oxidation of the Ni-S-Cys bond by O 2 is slow for {Ni 2 (SOD mds )} at pH = 7.4, with a half-life of nearly 7 h under the reaction conditions investigated (ambient O 2 concentration, 1.0 mM {Ni 2 (SOD mds )}).…”

“…This is further validated by time-dependent DFT calculations simulating the S K-edge X-ray absorption spectra (vide infra). two mononuclear nickel sites that collapses into a binuclear cysteinate bridged Ni2 center at high pH, the formation of which is well represented in small molecule Ni-thiolate chemistry [47,48].…”

Synthesis and Characterization of Novel Template Complexes

“…The weak band at about 600 nm in the visible region of the spectrum, which is so characteristic of tetrahedral and octahedral complexes,28 indicates symmetry distortion effect that causes deviation from planarity due to chiral distortion29–31 that lead to lack of both plane of symmetry and inversion center. It can be assigned to the weakest energy transition of d xy → d.…”

The roles of chalcogenides in O2 protection of H2ase active sites

“…The simple dinickel buttery complex precursor may be split by various nucleophiles including carbenes, imidazoles, thiolates, and selenoates. [49][50][51] With the chalcogenates, entry to NiFe bimetallics is possible making use of the (h5-C 5 H 5 ) Fe(CO)(NCCH 3 ) 2 + receiver species outlined in Fig. 6.…”