The molecular dynamics and reactivity of transition metal and main group eta-1-indenyl complexes

Abstract

<p>The molecular dynamics of a landmark compound in fluxional organometallic chemistry, (η5 -C5 H5 )Fe(CO) 2 (η1 -C9 H7 ), 1.9 , were reexamined by use of 2D-EXSY and single selective inversion NMR techniques. Contrary to an important early report, the present work establishes unambiguously that this compound is quasi-fluxional. The barrier (ΔG [double dagger] ) to [1,5] iron shifts in 1.9 is 20 ± 2 kcal mol-1 and the intermediate isoindene, 1.10 , is sufficiently long-lived so as to yield a [4+2] cycloadduct, 2.2 , with tetracyanoethylene. The tris(σ-indenyl)silanes, (1-C9 H7 ) 3 SiX where X = H, Me or allyl (3.18, 3.19 and 3.20 , respectively) undergo a series of [1,5] silatropic shifts which interconvert the RRR, RRS, RSS , and SSS isomers, where the R and S labels refer to the absolute configuration of C(1) in each indenyl ring. The molecular dynamics of these tris(σ-indenyl)silanes have been elucidated by means of selective inversion experiments together with 2D-EXSY data; the barrier (ΔG [double dagger] ) to [1,5] silicon shifts in these compounds is ∼24 kcal mol-1 , and the exchange pathways between indenyl sites can be conveniently mapped onto a hypercube. Moreover, each of the three tris(σ-indenyl)silanes examined yielded the corresponding triple Diels-Alder adduct, 3.21, 3.22 and 3.23 , respectively, upon reaction with tetracyanoethylene. Despite the steric demands of the ligands in these tris(σ-indenyl)silanes and tris(benzonorbornyl)silanes, it has been demonstrated that chemical reactions can still be carried out at the silicon centers in these molecules. Whilst examining the factors which affect silicon migrations in indenylsilanes, the silicon-substituted dibenzo[a,d ]fulvalene, 4.24 , was unexpectedly obtained in low yield from the reaction of indene and chlorotrimethylsilane in the presence of excess base. This compound has been characterized by use of both NMR spectroscopy and X-ray crystallography, and can be rationally prepared in acceptable yield starting from the dimer of trimethylsilylindene, 4.31 . In continuation of these studies, a series of indenylsilanes bearing methyl substituents (4.7 ) and a π-bonded chromium fragment (4.38 ) on indene have been prepared, as has a compound in which a dicobalt cluster is situated adjacent to the migrating silicon center (4.52 ). Data obtained from 2D-EXSY and single selective inversion NMR studies involving these complexes demonstrated that such structural perturbations have little impact on the barrier to [1,5] silicon shifts (ΔG[double dagger] ∼24 kcal mol-1 ) in this class of compounds. Finally, in an attempt to generate a compound capable of participating in intramolecular Diels-Alder reactions, the indenylsilanes, 5.15-5.17 , were prepared; the quasi-fluxional nature of this molecular system was revealed by 2D-EXSY NMR. Preliminary reactivity studies demonstrated that the unactivated dienophiles tethered to the indenyl framework in these molecules do not undergo the desired intramolecular [4+2] cycloaddition process.</p>