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

Abstract

The aluminum and gallium dichlorides (Mamx)ECl(2)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 (M(w) (DLS) between 8.07 and 106 kDa). Monomer 3b was polymerized using Karstedt's catalyst resulting in an M(w) of 28.6(±6.3) kDa. In order to get an indication of the structure of polymers, bis(ferrocenyl) compounds (Mamx)EFc(2) (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(Me(2)Ntsi) (7a), Ga(Me(2)Ntsi) (7b); Me(2)Ntsi = [(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 (2a(H), 2b(H), 3a(H), and 3b(H)). 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, ΔH(298K), and ΔG(298K)) accompanied by the structural changes were evaluated by a hydrogenolysis reaction of strained species resulting in Cp(2)M (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 (ΔH(298K)) than species where the tBu group was lacking. The investigation of the isodesmic reaction between strained species and Cp(2)M 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.

A Flexible Approach to Strained Sandwich Compounds: Chiral [1]Ferrocenophanes with Boron, Gallium, Silicon, and Tin in Bridging Positions

“…The GaC Cp bond lengths for the two independent molecules in the asymmetric unit were found in the narrow range of 2.007(3) to 2.020(3) Å, whereas the bond to the phenyl group is with 1.971(3) and 1.974(3) Å, respectively, slightly shorter. The GaN bond is with 2.083(2) and 2.102(2) Å expectedly longer than the GaC bonds, but compared to gallium species with similar coordination the GaN bond length is on the short side of the spectrum [GaN bond lengths: 2.112 [(Mamx)Ga(flu)],14 2.133(2) (Ar′GaMe 2 ),15 2.153(3) [(Mamx)GaFc 2 ],8b 2.178(3) [(Ar′Gafc) 2 ],6b 2.192(2) ([( p ‐ t BuAr′)Ga{1,1′‐(η 6 ‐C 6 H 5 ) 2 Cr}] 2 ),6c 2.207(3) Å ([(3‐ t Bu‐6‐Me 2 NCH 2 ‐C 6 H 3 )Ga{1,1′‐(η 6 ‐C 6 H 5 ) 2 Mo}] 2 )6c].…”

“…This second generation of Al‐ and Ga‐bridged sandwich compounds was surprisingly reactive and polymerized under the conditions of their formation reactions. Using DFT calculations, we discovered that the ortho ‐ t Bu group significantly increases the strain of the [1]FCPs and [1]ruthenocenophanes (5.5 kcal mol −1 on average) 8b…”

(N),C,N‐Coordinated Heavier Group 13–15 Compounds: Synthesis, Structure and Applications

“…The Ar CN *MCl 2 (M=Al, Ga) compounds were also utilized as suitable linkers in metallocenyl compounds (Figure A). These compounds were shown to be suitable monomers for the preparation of high molecular‐weight (M w ) polymers by ring‐opening polymerization (Figure B) . The presence of the Ar CN * ligand accelerates ring‐opening polymerization due to a significant increase of the ring strain in the monomers.…”

“…The presence of the Ar CN * ligand accelerates ring‐opening polymerization due to a significant increase of the ring strain in the monomers. Obtained aluminum‐ and gallium containing polymers were used as precursors for functional ceramic materials with tuneable magnetic properties …”

Strained Ferrocenophanes

“…55,56 Steric interactions between the o-tBu group of the Mamx ligand and the ferrocene moiety have been shown to induce additional strain in these aluminum-and gallium-bridged [1]ferrocenophanes. 56 Recent work in this field has focused on the design of ligands with sufficient steric protection to facilitate the formation of [1]ferrocenophanes, yet small enough to allow polymerization to proceed. 55,57 To this end, [1]ferrocenophanes have since been reported containing the bulky Pytsi, Me 2 Ntsi, and Mamx ligands, where the bridging element is part of a fouror five-membered ring ( Figure 6).…”