Synthesis and Structure of Alkaline Earth Bis{hydrido-tris(3,5-diisopropyl-pyrazol-1-yl)borate} Complexes: Ae(Tp)₂...

The synthesis and structural characterization of Ae(Tp^iPr2)₂ (Ae = Mg, Ca, Sr, Ba; Tp^iPr2 = hydrido-tris(3,5-diisopropyl-pyrazol-1-yl)borate) are reported. In the crystalline state, the alkaline earth metal centers are six-coordinate, even the small Mg²⁺ ion, with two κ³-N,N',N''-Tp ligands, disposed in a bent arrangement (B···Ae···B < 180°). However, contrary to the analogous Ln(Tp^iPr2)₂ (Ln = Sm, Eu, Tm, Yb) compounds, which all exhibit a bent-metallocene structure close to C symmetry, the Ae(Tp^iPr2)₂ compounds exhibit a greater structural variation. The smallest Mg(Tp^iPr2)₂ has crystallographically imposed C₂ symmetry, requiring both bending and twisting of the two Tp ligands, while with the similarly sized Ca²⁺ and Sr²⁺, the structures are back toward the bent-metallocene C symmetry. Despite the structural variations, the B···M···B bending angle follows a linear size-dependence for all divalent metal ions going from Mg²⁺ to Sm²⁺, decreasing with increasing metal ion size. The complex of the largest metal ion, Ba²⁺, forms an almost linear structure, B···Ba···B 167.5°. However, the "linearity" is not due to the compound approaching the linear metallocene-like geometry, but is the result of the pyrazolyl groups significantly tipping toward the metal center, approaching "side-on" coordination. An attempt to rationalize the observed structural variations is made.