New bis-, tris- and tetrakis(pyrazolyl)borate ligands with 3-pyridyl and 4-pyridyl substituents: synthesis and coordination chemistry

Abstract

The new ligands dihydrobis[3-(4-pyridyl)pyrazol-1-yl]borate [Bp^4py]⁻, hydrotris[3-(4-pyridyl)pyrazol-1-yl]borate [Tp^4py]⁻, tetrakis[3-(4-pyridyl)pyrazol-1-yl]borate [Tkp^4py]⁻, dihydrobis[3-(3-pyridyl)pyrazol-1-yl]borate [Bp^3py]⁻, hydrotris[3-(3-pyridyl)pyrazol-1-yl]borate [Tp^3py]⁻ and tetrakis[3-(3-pyridyl)pyrazol-1-yl]borate [Tkp^4py]⁻ are derivatives of the well known bis-, tris- and tetrakis-(pyrazolyl)borate cores, bearing 4-pyridyl or 3-pyridyl substituents attached to the pyrazolyl C³ positions. These pyridyl groups cannot chelate to the metal ions in the poly(pyrazolyl) cavity but are externally directed. Structural studies on a range of metal complexes show how, in many cases, coordination of these pendant pyridyl groups to the M(pyrazolyl)_n core of an adjacent metal complex fragment results in formation of coordination oligomers or polymeric networks. [Tl(Bp^3py)], [Tl(Bp^4py)] and [Tl(Tp^4py)] form one-dimensional polymeric chains via coordination of one of their pendant pyridyl units to the Tl(I) centre of an adjacent complex fragment; in contrast, in [Tl(Tp^3py)] coordination of all three pendant pyridyl units to separate Tl(I) neighbours results in formation of a two-dimensional polymeric sheet. In [Tl(Tkp^3py)] and [Tl(Tkp^4py)] the Tl(I) is coordinated by two or three of the four pyrazolyl arms, respectively; bridging interactions of pendant 4-pyridyl groups with adjacent Tl(I) centres result in a two-dimensional sheet forming in each case. In Ag(Tkp^4py) each Ag(I) ion is coordinated by two pyrazolyl rings, and two bridging pyridyl ligands from other complex units, resulting in a one-dimensional chain consisting of pairs of cross-linked zigzag chains. In contrast to these polymeric coordination networks, the structures of [Cu(Tp^4py)] and [(Tp^3py)Cd(CH₃CO₂)] are dimers, with a pendant pyridyl residue from the first metal centre attaching to a vacant coordination site on the second, and vice versa; these dimers are stabilised by π-stacking interactions between sections of the two ligands. [Ni(Tp^3py)₂] is monomeric, with an octahedral coordination geometry arising from two tris(pyrazolyl)borate chelates; the array of pendant 3-pyridyl groups is involved only in intramolecular hydrogen-bonding. [(Tp^4py)Re(CO)₃] is also monomeric, with a facial arrangement of three pyrazolyl ligands and three carbonyls, with the pendant 4-pyridyl groups not further coordinated. [(Tp^2py)Re(CO)₃], based on the related ligand hydrotris[3-(2-pyridyl)pyrazol-1-yl]borate, has a similar fac-(CO)₃(pyrazolyl)₃ coordination geometry.