Neutral and Anionic Monomeric Zirconium Imides Prepared via Selective C=N Bond Cleavage of a Multidentate and Sterically Demanding β-Diketiminato Ligand.

Abstract

A sterically encumbering multidentate β-diketiminato ligand, tBu L2 (tBu L2=[ArNC(tBu)CHC(tBu)NCH2 CH2 N(Me)CH2 CH2 NMe2 ]- , Ar=2,6-iPr2 C6 H3 ), is reported in this study along with its coordination chemistry to zirconium(IV). Using the lithio salt of this ligand, Li(tBu L2) (4), the zirconium(IV) precursor (tBu L2)ZrCl3 (6) could be readily prepared in 85 % yield and structurally characterized. Reduction of 6 with 2 equiv of KC8 resulted in formation of the terminal and mononuclear zirconium imide-chloride [C(tBu)CHC(tBu)NCH2 CH2 N(Me)CH2 CH2 NMe2 ]Zr(=NAr)(Cl) (7) as the result of reductive C=N cleavage of the imino fragment in the multidentate ligand tBu L2 by an elusive ZrII species (tBu L2)ZrCl (A). The azabutadienyl ligand in 7 can be further reduced by 2 e- with KC8 to afford the anionic imide [K(THF)2 ]{[CH(tBu)CHC(tBu)NCH2 CH2 N(Me)CH2 CH2 N(Me)CH2 ]Zr=NAr} (8-2THF) in 42 % isolated yield. Complex 8-2THF results from the oxidative addition of an amine C-H bond followed by migration to the vinylic group of the formal [C(tBu)CHC(tBu)NCH2 CH2 N(Me)CH2 CH2 NMe2 ]- ligand in 7. All halides in 6 can be replaced with azides to afford (tBu L2)Zr(N3 )3 (9) which was structurally characterized, and reduction with two equiv of KC8 also results in C=N bond cleavage of tBu L2 to form [C(tBu)CHC(tBu)NCH2 CH2 N(Me)CH2 CH2 NMe2 ]Zr(=NAr)(N3 ) (10), instead of the expected azide disproportionation to N3- and N2 . Solid-state single crystal structural studies confirm the formation of mononuclear and terminal zirconium imido groups in 7, 8-Et2 O, and 10 with Zr=NAr distances being 1.8776(10), 1.9505(15), and 1.881(3) Å, respectively.