Dinitrogen Activation with Low‐Valent Strontium

DFT calculations on β-diketiminate (BDI) complexes with the full series of alkaline-earth (Ae) metals show that (BDI)AeAe(BDI) complexes of the heavier Ae metals (Ca, Sr, Ba) have long weak Ae─Ae bonds that are prone to homolytic bond cleavage. However, isolation of (BDI)Sr(μ-N₂)Sr(BDI) with a side-on bridging N₂ ^2- dianion should thermodynamically be feasible. Attempts to stabilize such a complex with the super bulky BDI* ligand failed (BDI* = HC[(Me)C = N-DIPeP]₂, DIPeP = 2,6-Et₂CH-phenyl). First, N₂ fixation with a Sr complex was enabled by a heterobimetallic approach. Reduction of (^DIPePNN)Sr with potassium gave (^DIPePNN)₂Sr₂K₂(N₂) (6-Sr); ^DIPePNN = ^DIPePN-Si(Me)₂CH₂CH₂Si(Me)₂-N^DIPeP. A similar Ca product was also isolated (6-Ca). Crystal structures reveal a N₂ ^2- anion with side-on bonding to Ae²⁺ and end-on coordination to K⁺. DFT calculations and Atoms-In-Molecules analyses show mainly ionic bonding. Both 6-Ae complexes are synthons for hitherto unknown (BDI*)AeAe(BDI*) (Ae = Ca, Sr) and react by releasing N₂ and two electrons. Although surprisingly stable in benzene, the reduction of I₂ and H₂ is facile. Fast reaction with Teflon led to formation of crystalline [(^DIPePNN)SrKF]₂ (7), which is labile and decomposed to KF and (^DIPePNN)Sr. Latter reactivity underscores potential use of 6-Ae complexes as very strong, hydrocarbon-soluble reducing agents.