Dinitrogen Activation with Low‐Valent Strontium

Abstract

Abstract 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‐N 2 )Sr(BDI) with a side‐on bridging N 2 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‐Et 2 CH‐phenyl). First, N 2 fixation with a Sr complex was enabled by a heterobimetallic approach. Reduction of ( DIPeP NN)Sr with potassium gave ( DIPeP NN) 2 Sr 2 K 2 (N 2 ) ( 6 ‐Sr); DIPeP NN =  DIPeP N‐Si(Me) 2 CH 2 CH 2 Si(Me) 2 ‐N DIPeP . A similar Ca product was also isolated ( 6 ‐Ca). Crystal structures reveal a N 2 2− anion with side‐on bonding to Ae 2+ 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 N 2 and two electrons. Although surprisingly stable in benzene, the reduction of I 2 and H 2 is facile. Fast reaction with Teflon led to formation of crystalline [( DIPeP NN)SrKF] 2 ( 7 ), which is labile and decomposed to KF and ( DIPeP NN)Sr. Latter reactivity underscores potential use of 6 ‐Ae complexes as very strong, hydrocarbon‐soluble reducing agents.