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 = CH₂CHCH₂; 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.