Strained Metallocenophanes: Potential Monomers for Metallopolymers

Abstract

In addition to the known sila[1]ferrocenophane Sp,Sp-119Me, four new planar-chiral sila[1]ferrocenophanes were prepared. Each new sila[1]ferrocenophanes was equipped with a benzyl group in the α-position, either on one or both Cp rings, and substituted on the bridging silicon atom, either by methyl or phenyl groups. While NMR, UV-Vis, and DSC measurements did not show anything uncommon, the structural characterization of this set of four related sila[1]ferrocenophanes by single crystal X-ray analysis revealed unexpectedly large variations of the dihedral angles between both Cp rings (α tilt angle). While DFT calculations predicted α angles in the range of 19.6 to 20.8°, values in the solid state were found between 16.6(2) and 21.45(14)°. However, experimentally determined conformers differ significantly from those predicted for the gas phase. For the sila[1]ferrocenophane with the largest mismatch between the experimental and predicted α angle, it was shown that the orientation of benzyl groups have a significant influence on the ring-tilted structure. It was concluded that packing of molecules in the crystal lattice forces benzyl groups into unusual orientations, resulting in a reduced α angle through steric repulsions. Thermal ring-opening polymerization of the sila[1]ferrocenophanes Sp,Sp-118Me and Sp,Sp-118Ph resulted in soluble, high-molecular-weight polymers. In the next project, a flexible and cost-effective approach was developed to prepare the planar-chiral rac-2,2'-bis(2-methylpropane)-1,1'-dibromoferrocene (rac-137) as a precursor for [1]ferrocenophanes. The key step in this sequence was the reduction of 1,1′-Bis(β-methylpropanoyl)ferrocene using LiAlH4 as a reducing agent. Following the common salt-metathesis approach, the reaction of the dilithio derivative of rac-137 and Me2SiCl2 afforded a new sila[1]ferrocenophane. The obtained strained species was fully characterized by common techniques such as NMR spectroscopy and mass spectrometry. The two new bora[1]ferrocenophanes Sp,Sp-108Mes and Sp,Sp-108Tip equipped with two CHEt2 groups in α-position to bridging element were prepared. The obtained data indicated that by increasing the bulkiness on the sandwich moiety, the conversion towards the targeted compound could improve. Attempts to isolate Sp,Sp-108Mes and Sp,Sp-108Tip were unsuccessful and both monomers polymerized at 100-130 °C. A series of [2]ferrocenophanes bridged by carbon and either silicon (CSi-145), phosphorous (CP-146), and boron (CB-112) were synthesized and characterized. The molecular structures of all three [2]ferrocenophanes showed the α angles of 12.6 (CSi-145), 16.6 (CP-146), and 19.7° (CB-112). Attempts to polymerize CB-112 using thermal and anionic ring-opening polymerization were unsuccessful. A new sila[1]ruthenocenophane (Sp,Sp-115), equipped with 3-pentyl groups on the sandwich unit was synthesized and characterized using common techniques. Attempts to purify the obtained sila[1]ruthenocenophane Sp,Sp-115 through recrystallization and flask-to-flask condensation were unsuccessful. In the next step, the preparation of the phospha[1]ruthenocenophane Sp,Sp-166 was attempted. Following a similar procedure that was applied for the synthesis of the sila[1]ruthenocenophane Sp,Sp-115, the enantiopure (Sp,Sp)-1,1′-diiodo-2,2′-bis(3-pentyl)ruthenocene was lithiated and reacted with PPhCl2 to give the targeted compound. A 1H NMR spectrum taken from the crude mixture showed the formation of Sp,Sp-166 as the main species. Attempts to isolate Sp,Sp-166 through flask-to-flask condensation were unsuccessful. In the next trial, phospha[1]ruthenocenophane Sp,Sp-168 was attempted and isolated through flask-to-flask condensation.