Tris(pyrazolyl)borate Copper Hydroxide Complexes Featuring Tunable Intramolecular H-Bonding.

A modular synthesis provides access to a series of new tris(pyrazolyl)borate ligands XpyMeTpK that possess a single functionalized pendant pyridyl (py) or pyrimidyl (pyd) arm designed to engage in tunable intramolecular H-bonding to metal-bound functionalities. To illustrate such H-bonding interactions, a series of [XpyMeTpCu]2(μ-OH)2 (6a-6e) complexes were synthesized from the corresponding XpyMeTpCu-OAc (5a-5e) complexes. Single crystal X-ray structures of three new dinuclear [XpyMeTpCu]2(μ-OH)2 complexes reveal H-bonding between the pendant heterocycle and bridging hydroxide ligands while the donor arm engages the copper center in an unusual monomeric DMAPMeTpCu-OH complex. Vibrational studies (IR) of each bridging hydroxide complex reveal reduced νOH frequencies that tracks with the H-bond accepting ability of the pendant arm. Reversible protonation studies that interconvert [XpyMeTpCu]2(μ-OH)2 and [XpyMeTpCu(OH2)]OTf species indicate that the acidity of the corresponding aquo ligand decreases with increasing H-bond accepting ability of the pendant arm.