Understanding the Reactivity of Strained Sandwich Compounds with Aluminum or Gallium in Bridging Positions: Experiments and DFT Calculations

The aluminum and gallium dichlorides (Mamx)­ECl2 1a (E = Al; 82%) and 1b (E =
Ga; 79%) (Mamx = 2,4-di-tert-butyl-6-[(dimethylamino)­methyl]­phenyl)
reacted with dilithioferrocene or dilithioruthenocene to give [1]­ferrocenophanes
(2a, 2b) and [1]­ruthenocenophanes (3a, 3b), respectively. The galla[1]­ruthenocenophane 3b could be isolated from the reaction mixture through precipitation
into hexane (50%), while 2a, 2b, and 3a underwent ring-opening polymerization under the reaction
conditions of their formation reactions to give metallopolymers (Mw (DLS) between 8.07 and 106 kDa). Monomer 3b was polymerized using Karstedt’s catalyst resulting
in an Mw of 28.6(±6.3) kDa. In order
to get an indication of the structure of polymers, bis­(ferrocenyl)
compounds (Mamx)­EFc2 (E = Al (4a), 51%; E
= Ga (4b), 49%) were prepared and characterized by single
crystal X-ray analysis. DFT calculations shed some light on the unexpected
high reactivity of these new strained sandwich species. Optimized
geometries of known aluminum and gallium-bridged [1]­ferrocenophanes
(Al­(Pytsi) (6a), Ga­(Pytsi) (6b); Pytsi =
[dimethyl­(2-pyridyl)­silyl]­bis­(trimethylsilyl)­methyl) and [1]­ruthenocenophanes
(Al­(Me2Ntsi) (7a), Ga­(Me2Ntsi)
(7b); Me2Ntsi = [(dimethylamino)­dimethylsilyl]­bis­(trimethylsilyl)­methyl)
matched very well with experimental molecular structures. Geometries
of species 2a, 2b, 3a, and 3b were optimized (BP86/TZ2P) and the structural influence
of the tBu group of the Mamx ligand in ortho position
was evaluated by optimizing molecular structures of the four unknown
species where the ortho-tBu group was replaced by
an H atom (2aH, 2bH, 3aH, and 3bH). The most pronounced
structural effect was seen as a change of the orientation of the bridging
moiety with respect to the sandwich unit. As the tBu group was removed, the aromatic ligand moved toward the freed-up
space. The energetics (ΔE, ΔH298K, and ΔG298K) accompanied
by the structural changes were evaluated by a hydrogenolysis reaction
of strained species resulting in Cp2M (M = Fe, Ru) and respective aluminum and gallium dihydrides. This nonisodesmic
reaction showed that [1]­metallocenophanes equipped with the ortho-tBu group were on average 5.5 kcal/mol higher strained
(ΔH298K) than species where the tBu group was lacking. The investigation of the isodesmic
reaction between strained species and Cp2M yielding bis­(metallocenyl)
compounds revealed that the ortho-tBu group sterically
interacts with one of the metallocenyl units. The bis­(metallocenyl)
compounds are model compounds for the respective metallopolymers and
one can conclude that even though the ortho-tBu group
imposes additional strain on the starting metallocenophanes, this
effect cancels out in ROPs because the ortho-tBu
group imposes a similar strain on the resulting polymers. The uncovered
steric repulsion between the ortho-tBu group and
the sandwich moieties probably causes the ortho-tBu to act as an unusually sensitive NMR probe of the tacticity of
the polymers.