Reductive cyclotrimerization of CO and isonitriles with a highly reactive Ca synthon.

Whereas the small molecule activation with β-diketiminate (BDI)Mg complexes of type (BDI)Mg-Mg(BDI) is extensively investigated, lack of similar Ca reagents prevents studies on Ca reactivity. Herein, we report on small molecule activation with dinitrogen complexes of type (BDI)Ca(N)Ca(BDI) which acts as Ca synthon by release of N and two electrons. Reaction of (BDI*)Ca(THP) with CO led to formation of a deltate product with the cyclic CO dianion (1); BDI* = HC[C(Me)N(DIPeP)] (DIPeP = 2,6-(EtCH)-phenyl) and THP = tetrahydropyran. Reaction with the isonitrile CyN[triple bond, length as m-dash]C gave as the major product a complex with the triimino deltate C(NCy) dianion (2) which is unstable in solution. Isolation of the side-product (BDI*)Ca·(CN-Cy) (3) indicates dynamic ligand exchange and Schlenk equilibria. Variation of the isonitrile reagent led to isolation of (BDI*)Ca·(CN-R) (4: R = xylyl, 5: R = Bu). Crystal structures and NMR studies in solution are discussed for complexes 1-5. We also report an extensive DFT study on the reductive trimerization of MeN[triple bond, length as m-dash]C with this Ca synthon. The key intermediate (BDI)Ca(MeNC)Ca(BDI) contains dianionic MeNC. Contrary to expectation, C-C coupling does not proceed by nucleophilic attack at a second MeNC reagent. Electron transfer results in two bridging MeNC˙ radical anions. This rare singlet diradicaloid reacts further by radical coupling to [MeNC-CNMe]. Differences with Mg reactivity are discussed.