Cationic Lewis base-free β-diketiminate (BDI) complexes of Mg and Ca have been isolated as their B(CF) salts. The cation (BDI)Mg shows an extraordinarily strong Lewis acidity that can compete with strong Lewis acids like B(CF) and (BDI)AlMe. Its highly electrophilic nature is exemplified by isolation of an 3-hexyne adduct.

Attempted synthesis of the donor-acceptor complex (BDI)Ca ←Al (BDI) complex by reaction of (BDI)Ca in the form of its B(C F ) salt with (BDI)Al in benzene led to dearomatization of the solvent and formation of (BDI)Ca (C H )Al (BDI) (BDI=CH[C(CH )N-Dipp] , Dipp=2,6-diisopropylphenyl). The C H anion is strongly puckered and its boat form features four long (ca. 1.50 Å) and two short (ca. 1.34 Å) C-C bond distances. The flagpole positions of the C H anion chelate an Al cation giving a norbornadiene-like fragment with Al in the 7-position. The C=C double bonds of this alumina-norbornadiene strongly coordinate to the Ca metal ion. The complex is stable in solution up to 80 °C. Several mechanisms for its formation are discussed including a highly likely frustrated Lewis pair type mechanism in which benzene is activated by the Lewis acid (BDI)Ca followed by nucleophilic attack by the Lewis base (BDI)Al .

The low‐valent ß‐diketiminate complex (DIPPBDI)Al is stable in benzene but addition of catalytic quantities of [(DIPPBDI)CaH]2 at 20 °C led to (DIPPBDI)Al(Ph)H (DIPPBDI=CH[C(CH3)N‐DIPP]2, DIPP=2,6‐diisopropylphenyl). Similar Ca‐catalyzed C−H bond activation is demonstrated for toluene or p‐xylene. For toluene a remarkable selectivity for meta‐functionalization has been observed. Reaction of (DIPPBDI)Al(m‐tolyl)H with I2 gave m‐tolyl iodide, H2 and (DIPPBDI)AlI2 which was recycled to (DIPPBDI)Al. Attempts to catalyze this reaction with Mg or Zn hydride catalysts failed. Instead, the highly stable complexes (DIPPBDI)Al(H)M(DIPPBDI) (M=Mg, Zn) were formed. DFT calculations on the Ca hydride catalyzed arene alumination suggest that a similar but more loosely bound complex is formed: (DIPPBDI)Al(H)Ca(DIPPBDI). This is in equilibrium with the hydride bridged complex (DIPPBDI)Al(μ‐H)Ca(DIPPBDI) which shows strongly increased electron density at Al. The combination of Ca‐arene bonding and a highly nucleophilic Al center are key to facile C−H bond activation.

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