Enhancing electron accepting ability of triarylboron via π-conjugation with 2,2′-bipy and metal chelation: 5,5′-bis(BMes ₂)-2,2′-bipy and its metal complexes

The synthesis of a new diboron compound, 5,5′-bis(BMes₂)-2,2′-bipy (B2bipy, 1), has been accomplished. The conjugation of the 2,2′-bipy unit with the two BMes₂ groups in 1 has been found to greatly enhance the electron-accepting ability of the boron centers, compared to the biphenyl analogue. 1 is an effective chelate ligand for metal ions and three metal complexes; namely, [Cu^I(B2bipy)(PPh₃)₂][BF₄], 2, Pt^II(B2bipy)Ph2, 3a, and Pt^II(B2bipy)Me₂, 3b, have been synthesized. The binding of metal ions to B2bipy was found to further significantly increase the electron-accepting ability/Lewis acidity of the boron centers. Cyclic voltammetry diagrams revealed that the free ligand and all three complexes display two reversible reduction peaks that are separated by 0.31-0.40 V. The first reduction potential (E_1/2) for 1, 2, 3a, and 3b was found to be -1.69, -1.36, -1.34, and -1.38 V, respectively, versus FeCp₂^+/0 in DMF, which supports that the electron-accepting ability of the metal complexes is much greater than that of BMes(C₆F₅)₂. The strong electron-accepting ability/Lewis acidity of the metal complexes and the free ligand was manifested by their ability to sequentially bind to 2 equiv of F- ions in non-alcoholic solvents with K₁ ≥ 10⁸ and 109 M^-1 for 1 and 3a, respectively, and to 1 equiv of F- ions in the presence of methanol/ethanol or water. The crystal structures of 1, its 2:1 fluoride adduct [NBu₄]2[1F₂], and 3a were established by X-ray diffraction analyses. The 1:1 fluoride adduct of 2, [Cu^I(B2bipyF)(PPh₃)₂], 2F, isolated directly from the reaction of 2 with NBu4F in a 4:1 mixture of CH₃OH and CH₂Cl₂ was characterized. All metal complexes are intensely colored and display a characteristic metal-to-ligand (B2bipy) charge transfer absorption band in the visible region, which was found to be highly sensitive toward binding by anions such as fluorides.