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)2Pz)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)2Pz)2]Cu(L) (where L = C2H4, cis-CH3HC═CHCH3, and CO) have been prepared from [(Ph3B)CH(3,5-(CH3)2Pz)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 κ2N 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)2Pz)2)]Li(THF) and [(Ph3B)CH(3,5-(CH3)2Pz)2]Cu(CO) or simply hovering above the metal site as in [(Ph3B)CH(3,5-(CH3)2Pz)2)]Cu(C2H4) and [(Ph3B)CH(3,5-(CH3)2Pz)2)]Cu(cis-CH3HC═CHCH3). The 13C and 1H resonances of the ethylene carbon and protons of [(Ph3B)CH(3,5-(CH3)2Pz)2)]Cu(C2H4) appear at δ 81.0 and 3.71 ppm in CD2Cl2, respectively. The characteristic CO frequency for [(Ph3B)CH(3,5-(CH3)2Pz)2]Cu(CO) has been observed at υ̅ 2092 cm-1 by infrared spectroscopy and is lower than that of free CO suggesting moderate M → CO π-back-donation. A detailed analysis of these complexes has been presented herein.