Details

Autor(en) / Beteiligte

• Rahimi, Naser
• Zargarian, Davit

Titel

Cationic tetra- and pentacoordinate complexes of nickel based on POCN- and POCOP-type pincer ligands: synthesis, characterization, and ligand exchange studies

Ist Teil von

• New journal of chemistry, 2021-09, Vol.45 (33), p.1563-1573

Ort / Verlag

Cambridge: Royal Society of Chemistry

Erscheinungsjahr

2021

Quelle

Alma/SFX Local Collection

Beschreibungen/Notizen

• Stirring acetonitrile solutions of the charge-neutral pincer complexes (POCN)NiBr (1, POCN = κ P ,κ C ,κ N -{2-(i-Pr) 2 PO,6-CH 2 {c-N(CH 2 ) 5 }-C 6 H 3 ) and (POCOP)NiBr (2, POCOP = κ P ,κ C ,κ P′ -2,6-(i-Pr 2 OP) 2 C 6 H 3 ) with AgSbF 6 facilitates Br − abstraction to give the corresponding cationic acetonitrile adducts [(POCN)Ni(NCMe)] + , 1a , and [(POCOP)Ni(NCMe)] + , 2a . Treating 1a and 2a with pyridine (py), 2,2′-bipyridine (bipy), phenanthroline (phen), or 4,4′-bipyridine (bipy*) gave the corresponding monocationic adducts [(POCN or POCOP)Ni(ligand)] + (ligand = py: 1b and 2b ; κ N ,κ N′ -bipy: 1c and 2c ; κ N ,κ N′ -phen: 1d and 2d ) and the dicationic dinuclear adducts [(POCN or POCOP) 2 Ni 2 (μ-bipy*)] 2+ ( 1e and 2e ). The new adducts 1a-1d and 2b-2e have been characterized by NMR spectroscopy; complex 1e proved to be insoluble and could not be analyzed by NMR. Single crystal X-ray diffraction studies were used to establish the solid-state structures of 1a-1e , and 2b-2d . UV-vis spectra have also been recorded for the pentacoordinated complexes 1c , 1d , 2c , and 2d . Studying the equilibria that govern the displacement of halides in (pincer)NiX (X = Cl, Br) by the in-coming nucleophiles py, bipy, and phen has allowed us to determine the following order for the relative nucleophilicities of the ligands involved in the halide substitution equilibria: Cl − > Br − > phen > bipy > py > MeCN. Similar K eq measurements showed that cationic species are better stabilized with the POCN platform compared to POCOP. This implies that POCN is a better net donor of electron density compared to POCOP, such that the relative Lewis acidity (electrophilicity) of the cationic fragments should follow the order [(POCOP)Ni] + > [(POCN)Ni] + . Cyclic voltammetry measurements on the bipy adducts showed reversible, one-electron oxidation events occurring at a lower value for [(POCN)Ni(bipy)] + , implying a more electron-rich Ni( ii ) centre with POCN vs. POCOP. Consistent with these assertions, mixing (POCN)NiBr and [(POCOP)Ni(phen)] + in acetonitrile gave [(POCN)Ni(phen)] + and (POCOP)NiBr. Similarly, the K eq value of 0.19 for the equilibrium exchange (POCN)NiCl + (POCOP)NiBr (POCN)NiBr + (POCOP)NiCl indicates that the [(POCOP)Ni] + fragment is better stabilized by Cl − vs. Br − . These exchange reactions do not occur in THF or CH 2 Cl 2 , implying that they are driven by the nucleophilic character of the solvent. The closely related pincer ligands POCN and POCOP display different electron donating properties and different degrees of resistance to ligand exchange reactions proceeding via cationic reaction intermediates.