Kinetically stabilized 1,1'-bis[(E)-dipnosphenyl]ferrocenes: syntheses, structures, properties, and reactivity

Kinetically stabilized 1,1'-bis[(E)-diphosphenyl]ferrocenes were synthesized by taking advantage of extremely bulky substituents, 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (denoted as Tbt) and 2,6-bis[bis(trimethylsilyl)methyl]-4-[tris(trimethylsilyl)methyl]phenyl (denoted as Bbt) groups, and characterized by the spectroscopic and X-ray crystallo-graphic analyses. The electronic structures of the 1,1'-bis[(E)-diphosphenyl]ferrocenes were determined by analyzing electronic spectra, the transitions of which were reasonably assigned based on theoretical calculations. In the cyclic vol-tammograms, there were two well-defined reversible one-electron reduction couples corresponding to the intramolecular two diphosphene units. Furthermore, the 1,1'-bis[(E)-diphosphenyl]ferrocene was found to undergo ligand-exchange reactions with group 6 metal carbonyl complexes along with the E-to-Z isomerization of the diphosphene moieties, leading to the formation of the corresponding 1,1'-bis[(Z)-diphosphenyl]ferrocene group 6 metal tetracarbonyl complexes, [M(CO) ₄{(Z,Z)-(BbtP=PC ₅H₄)₂Fe}] (M = Cr, Mo, and W). The molecular structures of these complexes were determined by spectroscopic analyses ( ¹H, ¹³C, and ³¹PNMR spectra, and UV-vis spectra), and that of the tungsten complex was determined by X-ray crystallographic analysis. Several types of d→ π*_p=p electron transitions due to the iron and group 6 metals were detected by using UV-vis spectroscopy, and these results were supported by theoretical calculations.