Monitoring the electronic features of rigid C1-bridged Group 4 ansa-metallocenes by using the dynamic and structural properties of their s-cis-η4-butadiene complexes

Abstract The six-membered ring annulated C 1 -bridged ansa-metallocene [(4-cyclopentadienylidene)-4,7,7-trimethyl-4,5,6,7-tetrahydroindenyl]zirconium dichloride ( 6a ) was treated with “butadiene-magnesium” to yield the corresponding (s- cis - η 4 -butadiene)ansa-zirconocene complex as a mixture of two diastereoisomers ( 8a/8a′ ). Their mutual interconversion was monitored by dynamic 1 H NMR spectroscopy at high temperatures. The Gibbs activation energy of the thermally induced 8a ⇌8a′ (butadiene)metalocene ring topomerization is ΔG inv ≠ (368 K) = 20.2 ± 0.5 kcal mol −1 . This is about 8 kcal mol −1 higher than observed for the analogous ( σ 2 , π -butadiene)metallocene ring inversion process in the parent bis(cyclopentadienyl)zirconium system. The corresponding (s- cis - η 4 -butadiene)ansa-hafnocene complex ( 9/9′ ) was prepared analogously. Its ( η 4 -butadiene)Hf inversion barrier ( ΔG inv ≠ (268 K) = 14.9 ± 0.5 kcal mol −1 ) is about 7 kcal mol −1 higher than that of the (s- cis - η 4 -butadiene)HfCp 2 reference. The (s- cis - η 4 -butadiene)[7- n -butyl-(4-cyclopentadienylidene)4,7-dimethyl-4,5,6,7-tetrahydroindenyl]zirconium complex 8b was characterized by X-ray diffraction. The η-butadiene ligand exhibits only a small CC bond alteration (1.38(2), 1.36(2) and 1.40(2) A) and a relatively small difference of the butadiene C terminal -Zr (2.324(12) and 2.301(13) A) vs. C internal -Zr (2.445(13) and 2.422(13) A) bond lengths. This points to an increased π complex character of the rigid ansa-metallocene complex compared with the parent (s- cis - η 4 -butadiene)zirconocene complex 2a whose structure was determined as a reference ( 2a : butadiene C terminal Zr, 2.332(5) and 2.302(5) A, C internal Zr, 2.491(6) and 2.480(5) A). A theoretical extended Huckel theory analysis has revealed that the pronounced increase in the butadiene metallocene topomerization barrier on going from the “normal” bent metallocene (D(1)ZrD(2), 126.2°, where D(1) and D(2) denote the centroids of the Cp rings) to the strained C 1 -bridged ansa-metallocene complex 8 containing a markedly opened bent metallocene wedge ( 8b : D(1)ZrD(2), 114.7°) is electronic in origin, probably caused by closing the energy gap between the 2a 1 and b 1 frontier orbitals. We conclude that the stereoelectronic features of the bent metallocenes can be controlled by the application of specific steric constraints.