Synthesis and Reactivity Studies of Benzo-Substituted Bis(indenyl) Iron and Zirconium Complexes: The Difference a Methyl Group Can Make

The synthesis of the sterically hindered 1,3,4,7-tetrasubstituted
indenyl ligand 1,3-(CHMe2)2-4,7-Me2-C9H3 is accomplished via initial preparation
of 4,7-dimethylindene and subsequent installation of isopropyl groups
on the five-membered ring. Synthesis of the corresponding bis­(indenyl)
iron complex (η5-C9H3-1,3-(CHMe2)2-4,7-Me2)2Fe (3) and comparison to a bis­(indenyl) iron analogue devoid of benzo
substituents, (η5-C9H5-1,3-(CHMe2)2)2Fe (4), through variable-temperature
NMR studies and electrochemistry, establishes the new ligand as both
more sterically demanding and slightly more electron rich. Alkali-metal
reduction of (η5-C9H3-1,3-(CHMe2)2-4,7-Me2)2ZrCl2 (5) yields an equilibrium mixture of the η5,η9 sandwich complex 7 and the
cyclometalated hydride 8, indicating both that benzo
binding is still possible when the six-membered ring is substituted
and that ligand activation can be modulated by the choice of substituents,
as the η5,η9 zirconium sandwich 9, which lacks methyl groups on the benzo ring, does not cyclometalate
under ambient conditions. The reactivity of 8 was explored,
demonstrating that the cyclometalated species can act as a source
of both Zr­(II), via ligand-induced reductive elimination, and Zr­(IV),
through insertion or σ bond metathesis, depending on the added
reagent. Addition of H2 to 8 gives (η5-C9H3-1,3-(CHMe2)2-4,7-Me2)2ZrH2 (17),
which upon prolonged thermolysis results in benzo CC bond
insertion into the Zr hydride. The reaction rate in comparison to
that of the bis­(indenyl) zirconium dihydride analogue 19, without benzo substituents, suggests that the methyl groups on
the six-membered ring significantly reduce the rate of intramolecular
insertion. These studies show that benzo substitution accomplishes
both major intended goals: destabilizing the interaction of the benzo
ring with low-valent metals while reducing the rate of insertion of
a benzo CC bond into a metal hydride in high-oxidation-state
complexes.