Trifluoromethyl-Containing β-Diketiminato-Magnesium Complexes: Synthesis, Structures, and Catalysis for the Hydroboration of Carbodiimides

Abstract

Trifluoromethyl-containing β-diketiminate ligands (CF₃)BDI^Et-H = (2,6-di-Et₂C₆H₃)N=C (CF₃)CH=C (CF₃)NH(2,6-di-Et₂C₆H₃), (CF₃)BDI^iPr-H = (2,6-di-ⁱPr₂C₆H₃)N=C (CF₃)CH=C (CF₃)NH(2,6-di-ⁱPr₂C₆H₃) react with MgⁿBu₂ readily to provide THF-ligated four-coordinate monomeric magnesium butyl complexes (CF₃)BDI^EtMgⁿBu (THF) (1) and (CF₃)BDI^iPrMgⁿBu (THF) (2) in THF, and three-coordinate monomeric complexes (CF₃)BDI^EtMgⁿBu (3) and (CF₃)BDI^iPrMgⁿBu (4) in toluene, respectively. The alkane elimination between 4 and NH (SiHMe₂)₂ gives silylamido complex 5 in toluene. Treatments of 4 with heterocumulene tert-butyl isothiocyanate, tert-butyl isocyanate, and N,N′-diisopropylcarbodiimide produce single insertion derivatives (CF₃)BDI^iPrMgN (^tBu)C (ⁿBu)S (6), (CF₃)BDI^iPrMgN (^tBu)C (ⁿBu)O (7), and (CF₃)BDI^iPrMgN (ⁱPr)C (ⁿBu)N (ⁱPr) (8), respectively. All these new compounds were characterized by NMR spectroscopy, and the molecular structures of complexes 1, 2, 4–7 were further confirmed by single-crystal X-ray diffraction analyses. Moreover, representative complexes 2–5 were employed as pre-catalysts for the hydroboration of commercially available carbodiimides bearing aliphatic and aromatic substituents with pinacol borane HBpin, including diisopropylcarbodiimide (DIC), dicyclohexylcarbodiimide (DCC), di-tert-butylcarbodiimide, 2,6-diisopropylphenylcarbodiimide, and unsymmetric 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDCI), in which the three-coordinate monomeric complexes 3 and 4 showed better efficiency giving corresponding N-boryl formamidine products in high yields. A possible mechanistic pathway of the hydroboration reaction has been proposed.