Cu(ii)-mediated tautomerization for the pyrazole-nitrile coupling reaction

Abstract

This study investigates the mechanism of prototropic tautomerization in metal-bound asymmetric pyrazole (R-PzH) ligands during Cu(II)-mediated PzH-MeCN coupling reactions. Intrinsic prototropic tautomerization of metal-bound ligands has not been previously documented. Various new bis-pyrazolylamidino Cu(II) complexes, [Cu(R-Pz(HN[double bond, length as m-dash]C(Me)))2(ClO4)2], from the coupling reaction, and tetrakis pyrazole Cu(II) complexes, [Cu(R-PzH)4(ClO4)2], with symmetric and asymmetric C-monosubstituted R-PzH ligands were synthesized and characterized. Kinetic UV-vis studies revealed slower coupling rates with asymmetric R-PzH ligands, further reduced with increasing substituent size due to steric hindrance impeding tautomerization. Structural analysis showed preferential binding of asymmetric 3(5)-R-PzH to Cu(II) ions at the C5 position of PzH, requiring a tautomeric shift to the C3 position for the subsequent coupling reaction. DFT calculations confirmed the greater stability of the C5-tautomeric [Cu(R-PzH)4(ClO4)2] complexes over their C3 counterparts, explaining the rate discrepancies. A mechanism involving counteranion-mediated proton transfer during tautomerization is proposed to account for coupling product formation.

Graphical abstract: Cu(ii)-mediated tautomerization for the pyrazole-nitrile coupling reaction

Supplementary files

Article information

Article type
Communication
Submitted
17 Oct 2024
Accepted
14 Dec 2024
First published
17 Dec 2024

Dalton Trans., 2025, Advance Article

Cu(II)-mediated tautomerization for the pyrazole-nitrile coupling reaction

C. Hsieh, C. Chen, M. Chiang and Y. Horng, Dalton Trans., 2025, Advance Article , DOI: 10.1039/D4DT02904F

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