Dinitrogen Activation with Low-Valent Strontium

Abstract

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.