Synthesis and Structure of Alkaline Earth Bis{hydrido-tris(3,5-diisopropyl-pyrazol-1-yl)borate} Complexes: Ae(TpiPr2)2 (Ae = Mg, Ca, Sr, Ba)

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 kappa(3)-N,N',N ''-Tp(iPr2) ligands, disposed in a bent arrangement (B center dot center dot center dot Ae center dot center dot center dot B < 180 degrees). 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-s symmetry, the Ae(Tp(iPr2))(2) compounds exhibit a greater structural variation. The smallest Mg(Tp(iPr2))(2) has crystallographically imposed C-2 symmetry, requiring both bending and twisting of the two Tp(iPr2) ligands, while with the similarly sized Ca2+ and Sr2+, the structures are back toward the bent-metallocene C-s symmetry. Despite the structural variations, the B center dot center dot center dot M center dot center dot center dot 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 center dot center dot center dot Ba center dot center dot center dot B 167.5 degrees. 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.