An Alkyne as Central Noninnocent Ligating Unit in Pincer Coordination: A Rigid Bis(indenopyridine) [NCCN]-Ligand for Early Transition Metals

Stille coupling of two indenopyridine derivatives with
a distannylated
acetylene has provided convenient access to a novel bis(indenopyridine)-substituted
acetylene ligand (1) featuring a rigid backbone. Upon
coordination to early transition metals such as titanium, activation
of the central alkyne unit was expected to induce a significant geometric
distortion. This in turn would drive the reorientation of the indenopyridine
wingtip units and combined with the central acetylene bonding give
rise to an overall meridional arrangement. The latter is an essential
feature for achieving defined [NCCN]-type coordination. In a first
study of the coordination behavior of such a ligand, a highly symmetrical
titanium(IV) dichloride complex (2) has been synthesized
in which the metal center is stabilized by X2-type interaction
with the π-coordinated alkyne. Subsequent alkylation of [NCCN]TiCl2 (2) with various organolithium and Grignard
reagents provided access to a series of bis(alkyl)titanium complexes,
demonstrating the synthetic accessibility of well-defined organometallic
derivatives based on this ligand platform.