Reductive Cyclotrimerization of CO and Isonitriles with a Highly Reactive CaI Synthon

Abstract

Whereas the small molecule activation with b-diketiminate (BDI) Mg^I complexes of type (BDI)Mg-Mg(BDI) is extensively investigated, lack of similar Ca^I reagents prevents studies on Ca^I reactivity. Herein, we report on small molecule activation with dinitrogen complexes of type (BDI)Ca(N₂)Ca(BDI) which acts as Ca^I synthon by release of N₂ and two electrons. Reaction of [(BDI*)Ca(THP)]₂(N₂) with CO led to formation of a deltate product with the cyclic C₃O₃^2- dianion (1); BDI* = HC[C(Me)N(DIPeP)]₂ (DIPeP = 2,6-(Et₂CH)-phenyl) and THP = tetrahydropyran. Reaction with the isonitrile CyN≡C gave as the major product a complex with the triimino deltate C₃(NCy)₃^2– dianion (2) which is unstable in solution. Isolation of the side-product (BDI*)₂Ca∙(CN–Cy) (3) indicates dynamic ligand exchange and Schlenk equilibria. Variation of the isonitrile reagent led to isolation of (BDI*)₂Ca∙(CN–R) (4: R = xylyl, 5: R = tBu). 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≡C with this Ca^I synthon. The key intermediate (BDI)Ca(MeNC)Ca(BDI) contains dianionic MeNC^2-. 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]^2-. Differences with Mg^I reactivity are discussed.