Stepwise Synthesis, Structures, and Reactivity of Mono-, Di-, and Trimetallic Metal Complexes with a 6π + 6π Quinonoid Zwitterion^†

The benzoquinonemonoimine N,N‘-dineopentyl-2-amino-5-alcoholate-1,4-benzoquinonemonoiminium [C6H2(NHCH2t-Bu)2(O)2] 6, which is a rare example of an organic zwitterion being more stable than its canonical form, is best described as constituted of two chemically connected but electronically not conjugated 6π electron subunits. The two successive acidities of 6 allow the preparation of mono-, di-, and trimetallic complexes in which the control of the π-system delocalization becomes possible. Reaction of 6 with NaOt-Bu results in monodeprotonation of one N−H function, and the isolated sodium salt 9, which is stable under N2, reacts with chloride-bridged Pd(II) homodimetallic complexes, [AuCl(PPh3)] or trans-[NiCl(Ph)(PPh3)2], to afford the monometallic complexes 10−15 in which the π-system is localized. A second in situ deprotonation of the remaining N−H amino function of 10 with NaH followed by reaction with [Pd(8-mq)(μ-Cl)]2 (8-mq = orthometalated 8-methylquinoline) affords the homodimetallic complex 17 in which the π-system of the quinonoid ligand is delocalized between the two metal centers. Deprotonation of both N−H amino functions of the square-planar complex trans-[Ni(N,O)2] 15 with NaH and reaction with [Pd(8-mq)(μ-Cl)]2 affords the heterotrimetallic (Pd, Ni, Pd) complex 18 in which the π-system of the two quinonoid ligands is delocalized between the three metal centers. The crystal structures of the monometallic complexes 10 and 13 and of the dipalladium complex 17 are reported and consequences of metal coordination discussed. Complex 15 was tested in catalytic ethylene oligomerization with AlEtCl2 as cocatalyst.