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

The synthesis and structural characterization of Ae(Tp) (Ae = Mg, Ca, Sr, Ba; Tp = 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 Mg ion, with two κ-,',''-Tp ligands, disposed in a bent arrangement (B···Ae···B < 180°). However, contrary to the analogous Ln(Tp) (Ln = Sm, Eu, Tm, Yb) compounds, which all exhibit a bent-metallocene structure close to symmetry, the Ae(Tp) compounds exhibit a greater structural variation. The smallest Mg(Tp) has crystallographically imposed symmetry, requiring both bending and twisting of the two Tp ligands, while with the similarly sized Ca and Sr, the structures are back toward the bent-metallocene symmetry. Despite the structural variations, the B···M···B bending angle follows a linear size-dependence for all divalent metal ions going from Mg to Sm, decreasing with increasing metal ion size. The complex of the largest metal ion, Ba, 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.