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.