Monoanionic, Bis(pyrazolyl)methylborate [(Ph3B)CH(3,5-(CH3)2Pz)2)]- as a Supporting Ligand for Copper(I)-ethylene, cis-2-Butene, and Carbonyl Complexes

The monoanionic bidentate ligand [(Ph3B)CH(3,5-(CH3)(2)Pz)(2))](-) has been prepared from lithium bis(pyrazolyl)methanide and triphenylborane. This useful new ligand is closely related to the well-established bis(pyrazolyl)borate and bis(pyrazolyl)methane ligands but has key differences to both analogues as well. The ethylene, cis-2-butene, and carbon monoxide adducts [(Ph3B)CH(3,5-(CH3)(2)Pz)(2))]Cu(L) (where L = C2H4, cis-CH3HC-CHCH3, and CO) have been prepared from [(Ph3B)CH(3,5-(CH3)(2)Pz)(2))]Li(THF), copper(I) triflate, and the corresponding coligand. These complexes have been characterized by NMR spectroscopy and X-ray crystallography. In all cases the bis(pyrazolyl) moiety is bound in kappa N-2 fashion with the BPh3 group rotated to sit over the metal center, sometimes coordinating to the metal via phenyl carbons as in [(Ph3B)CH(3,5-(CH3)(2)Pz)(2))]Li(THF) and [(Ph3B)CH(3,5-(CH3)(2)Pz)(2))]Cu(CO) or simply hovering above the metal site as in [(Ph3B)CH(3,5-(CH3)(2)Pz)(2))]Cu(C2H4) and [(Ph3B)CH(3,5-(CH3)(2)Pz)(2))]Cu(cis-CH3HC=CHCH3). The C-13 and H-1 resonances of the ethylene carbon and protons of [(Ph3B)CH(3,5-(CH3)(2)Pz)(2))]Cu(C2H4) appear at delta 81.0 and 3.71 ppm in CD2Cl2, respectively. The characteristic CO frequency for [(Ph3B)CH(3,5-(CH3)(2)Pz)(2))]Cu(CO) has been observed at (upsilon) over bar 2092 cm(-1) by infrared spectroscopy and is lower than that of free CO suggesting moderate M -> CO pi-back-donation. A detailed analysis of these complexes has been presented herein.