Dinitrogen Activation with Low‐Valent Strontium

DFT calculations on b‐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. They also show that isolation of (BDI)Sr(m‐N2)Sr(BDI) with a side‐on bridging N22ˉ dianion should be feasible. Attempts to stabilize such a complex with the superbulky BDI* ligand failed (BDI* = HC[(Me)C=N‐DIPeP]2, DIPeP = 2,6‐Et2CH‐phenyl). A heterobimetallic approach enabled first N2 fixation with a Sr complex stabilized with a bulky bis‐amide ligand DIPePNN: DIPePN‐Si(Me)2CH2CH2Si(Me)2‐NDIPeP. Reduction of (DIPePNN)Sr with K/KI gave (DIPePNN)2Sr2K2(N2) (6‐Sr); a similar Ca product was also isolated (6‐Ca). Crystal structures reveal a N22ˉ 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 isolation 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.