A structural study of new tetrakis(1H-pyrazol-1-yl)methanes

Tetrakis(1H-pyrazol-1-yl)methanes are very rare compounds of which only two are known: the unsubstituted 1 obtained classically by Hückel in 1937 from carbon tetrachloride and prepared again several times and the 3,5-dimethyl substituted 2 obtained serendipitously by Pombeiro in 2009. We have now extended this group to include four new derivatives 8, 9, 11 and 12 bearing methyl groups. The X-ray crystal structure of the four compounds has been determined. They have been studied by NMR both in solution (H, C, N) and in the solid state (C and N). DFT calculations of the six compounds (geometries, energies and absolute shieldings) have been used to discuss the experimental observations.
We thank Dr. Duane Choquesillo-Lazarte (LEC, IACT-CSIC) for compound 12 X-ray data collection. This work was carried out with financial support from the Ministerio de Ciencia, Innovación y Universidades (Project PGC2018-094644-B-C22) and Direccion General de Investigacion en Innovacion de la Comunidad de Madrid (PS2018/EMT-4329 AIRTEC-CM). Computer, storage and other resources from the CTI (CSIC) are gratefully acknowledged. Thanks are also due to University of Aveiro and FCT/MEC for the financial support of the QOPNA research unit (FCTUID/QUI/00062/2019) through national founds and, where applicable, co-financed by the FEDER, within the PT2020 Partnership, and also to the Portuguese NMR network. Support from the UNED is also greatly acknowledged