Issue 10, 2025

Borane-tethered heteroscorpionate zinc catalysts for the hydroboration of carbon dioxide, isocyanates, esters and nitriles

Abstract

This work describes the synthesis, characterization and catalytic hydroboration activity of a family of zinc bis(κ2-borohydride) complexes containing borane-functionalized heteroscorpionate ligands. The new zinc bis(κ2-borohydride) complexes with R-substituted bis(3,5-dimethylpyrazolyl)methane ligands with or without borane functionalities [(L1–4)Zn(BH4)2]1–4 (1, R = H; 2, R = CH2CH[double bond, length as m-dash]CH2; 3, R = CH2CH2CH2(9-borabicyclo[3.3.1]nonane) or CH2CH2CH2(9-BBN); 4, R = CH2CH2CH2BCy2) were synthesized by reacting the corresponding dichloride complexes [(L1–4)ZnCl2] with two equivalents of NaBH4 in THF. The new complexes were characterized by NMR spectroscopy, FTIR spectroscopy and elemental analysis, while complex 3 was characterized by single-crystal X-ray diffraction. The borane-functionalized complexes 3 and 4 catalyzed the hydroboration of CO2 at 1 bar pressure, an array of isocyanates and esters and the dihydroboration of a set of nitriles with pinacolborane (HBPin) at catalyst loadings of 1 mol% and temperature of 60 °C in yields of 32–99%. In the cases of CO2 and isocyanates, the methylated products (along with PinBOBPin) were preferred after 16 h. The catalytic reactions utilizing 1 mol% of the unfunctionalized complexes 1 and 2 achieved yields in the range of 7–24%, an average 4-fold decrease in catalytic activities, confirming that the catalytic intermediates benefit from the intra- or intermolecular stabilization of incorporated boranes.

Graphical abstract: Borane-tethered heteroscorpionate zinc catalysts for the hydroboration of carbon dioxide, isocyanates, esters and nitriles

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Article information

Article type
Paper
Submitted
03 Dec 2024
Accepted
10 Jan 2025
First published
22 Jan 2025
This article is Open Access
Creative Commons BY license

Dalton Trans., 2025,54, 4244-4254

Borane-tethered heteroscorpionate zinc catalysts for the hydroboration of carbon dioxide, isocyanates, esters and nitriles

T. F. C. Cruz and L. F. Veiros, Dalton Trans., 2025, 54, 4244 DOI: 10.1039/D4DT03363A

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