Syntheses, properties, and structural characteristics of several new [tetrakis(1-pyrazolyl)borato]samarium(III) complexes: comparative study of the B(pyrazolyl)4 and BH(pyrazolyl)3 ligation

Although reactions of samarium(III) chloride, SmCl3 · 6H2O, with potassium hydrotris(1-pyrazolyl)borate K[BH(pz)3] (pz = 1-pyrazolyl) in a molar ratio of (1/1) in THF afford [SmCl{BH(pz)3}2(Hpz)], similar reactions with K[B(pz)4] gave rise to separation of anhydrous H[B(pz)4]. The homoleptic eight-coordinate complex [Sm{B(pz)4}3] obtained from SmCl3 · 6H2O and threefold moles of K[B(pz)4] was allowed to react with twofold moles of K[BH(pz)3] to give a mixture of three major species [Sm{B(pz)4}n{BH(pz)3}(3 − n)] (n = 2, 1, 0), whereas similar reactions of [Sm{BH(pz)3}3] with K[B(pz)4] did not proceed at all. The acetylacetonato (acac) complex [Sm{B(pz)4}2(acac)], derived from the triflate “Sm{B(pz)4}2(OTf)”, was treated with twofold moles of K[BH(pz)3] and showed its quantitative conversion to [Sm{BH(pz)3}2(acac)]. However, analogous reaction of [Sm{BH(pz)3}2(acac)] with K[B(pz)4] did not proceed. Accordingly, samarium(III) ion was determined to prefer coordination of BH(pz)3 ligand to that of B(pz)4, indicating less σ-donating electronic character of the latter. The complexes [Sm{B(pz)4}2(L–L)] (L–L = β-ketoenolato) in toluene-d8 exhibited 1H NMR spectroscopic equivalence of all four pyrazolyl groups at high temperatures, and are regarded as a new class of B(pz)4 complexes, showing fast intramolecular exchange of their coordinated and uncoordinated pyrazolyl groups. Four compounds were crystallographically characterized.