Differentiating Chemically Similar Lewis Acid Sites in Heterobimetallic Complexes: The Rare-Earth Bridged Hydride (C5Me5)2Ln(μ-H)2Ln′(C5Me5)2 and Tuckover Hydride (C5Me5)2Ln(μ-H)(μ‑η1:η5‑CH2C5Me4)Ln′(C5Me5) Systems

The challenge of differentiating the chemistry of two closely related Lewis acidic metals in heterobimetallic complexes was addressed by studying hydrogenolysis and C–H bond activation reactions of bimetallic rare-earth hydride complexes. Hydrogenolysis of equimolar amounts of Cp*2Lu­(η3-C3H5) (1-Lu) and Cp*2Y­(η3-C3H5) (1-Y) (Cp* = C5Me5) forms a mixture of hydride complexes, the heterobimetallic compound Cp*2Lu­(H)2YCp*2 (2-Lu/Y) and the homobimetallic compounds (Cp*2LuH)2 (2-Lu/Lu) and (Cp*2YH)2 (2-Y/Y). This mixture can be analyzed and differentiated by 1H NMR spectroscopy due to the I = 1/2 89Y nucleus to reveal these three products in a ratio of approximately 86:10:4, respectively. Heating this mixture leads to C–H bond activation and formation of tuckover hydride complexes, the heterobimetallic compounds Cp*2Y­(μ-H)­(μ-η1:η5-CH2C5Me4)­LuCp* (3-Y/Lu) and Cp*2Lu­(μ-H)­(μ-η1:η5-CH2C5Me4)­YCp* (3-Lu/Y) and the homobimetallic compounds Cp*2Lu­(μ-H)­(μ-η1:η5-CH2C5Me4)­LuCp* (3-Lu/Lu) and Cp*2Y­(μ-H)­(μ-η1:η5-CH2C5Me4)­YCp* (3-Y/Y). This mixture could also be analyzed in detail by 1H NMR spectroscopy to reveal a ratio of products of approximately 17:37:19:27, respectively. In contrast to these results with similarly sized Y and Lu metals, hydrogenolysis of a mixture of pairs of allyl complexes with metal centers that differ greatly in size forms just the heterobimetallic product: hydrogenolysis of equimolar amounts of Cp*2La­(η3-C3H5) (1-La) and 1-Lu forms just Cp*2Lu­(H)2LaCp*2 (2-Lu/La), and hydrogenolysis of 1-La and 1-Y forms only Cp*2Y­(H)2LaCp*2 (2-Y/La). Heating the heterobimetallic hydride complex 2-Lu/La generates a single C–H bond activation product, the heterobimetallic tuckover hydride product Cp*2Lu­(μ-H)­(μ-η1:η5-CH2C5Me4)­LaCp* (3-Lu/La). Similarly, heating 2-Y/La forms only Cp*2Y­(μ-H)­(μ-η1:η5-CH2C5Me4)­LaCp* (3-Y/La). In both 3-Lu/La and 3-Y/La, the larger metal, La, is located in the seven-coordinate, not the eight-coordinate, metal site. The experimentally determined locations of these metals were not predictable by standard density functional theory (DFT) calculations, but addition of a dispersion correction led to qualitative agreement with experiment. More elaborate random phase approximation (RPA) calculations of ground-state energies produced nearly quantitative agreement, emphasizing the importance of nonbonding interactions in the analysis of these compounds.