A Readily Accessible Chiral NNN Pincer Ligand with a Pyrrole Backbone and Its Ni(II) Chemistry: Syntheses, Structural Chemistry, and Bond Activations

A new class of chiral C2-symmetric N-donor pincer ligands, 2,5-bis­(2-oxazolinyldimethylmethyl)­pyrroles
(PdmBox)­H, was synthesized starting from the readily available ethyl
2,2-dimethyl-3-oxobutanoate (1). The synthesis of the
ligand backbone was achieved by oxidative enole coupling with CuC12 followed by Paal–Knorr-type pyrrole synthesis. The
corresponding protioligands (RPdmBox)H (R = iPr: 5a; Ph: 5b) were obtained by condensation
with amino alcohols and subsequent zinc-catalyzed cyclization. Reaction
of the lithiated ligands with [NiCl2(dme)] yielded the
corresponding square-planar nickel­(II) complexes [(RPdmBox)­NiCl]
(6a/b). Salt metathesis of 6a with the corresponding
alkali or cesium salts in acetone led to the formation of air- and
moisture-stable [(iPrPdmBox)­NiX] (X =
F (7), X = Br (8), X = I (9), X = N3 (10), X = OAc (11).
Furthermore, the conversion of [(iPrPdmBox)­NiF]
(7) with hydride transfer reagents such as PhSiH3 led to the stable hydrido species [(iPrPdmBox)­NiH] (27), the stoichiometric transformations
of which were studied. Treatment of 6a with organometallic
reagents such as ZnEt2, PhLi, PhCCLi, NsLi, or
(4FBn)2Mg­(THF)2 gave the corresponding
alkyl, alkynyl, or aryl complexes. The availability of the new nonisomerizable
PdmBox pincer ligands allowed the comparative study of their ligation
to square-planar complexes as helically twisted spectator ligands
as opposed to the enforced planar rigidity of their iso-PmBox analogues
and the way this influences the reactivity of the Ni complexes.