Allylferrocenylselenide and the synthesis of the first seleno-substituted allenylidene complex: synthesis, spectroscopy, electrochemistry and the effect of electron transfer from the ferrocenylselenyl subunit

Allylferrocenylselenide (2) is prepared from diferrocenyldiselenide (1Se) w hich was characterized along with its sulfur analog 1S by X-ray structure a nalysis. In the crystal lattice the packing is determined by 'point-to-face ' CH . . . pi interactions with close contacts between the CH pi donors and cyclopentadienyl rings as the pi acceptors. Compound 2 is then used in the trapping of the primary butatrienylidene intermediate trans-[ClRu(dppm)(2) =C=C=C=CH2](+). The isolated product, trans-[Cl(dppm)(2)Ru=C=C=C(SeFc)(C4H7 )](+) (3) (Fc = ferrocenyl), represents the first seleno-substituted alleny lidene complex to be reported to date. Compound 3 is formed in a sequence i nvolving regioselective addition of the selenium nucleophile to C-gamma fol lowed by hetero-Cope-rearrangement of the allyl vinyl substituted SeR3+ cat ion. Its spectroscopic properties place 3 at an intermediate position betwe en sulfur and arene substituted all-carbon allenylidene complexes of the sa me metal fragment. The selenoallenylidene complex 3 contains a redox active ferrocenyl substituent attached to the heteroatom giving rise to reversibl e electrochemistry. ESR spectroscopy proves that electron transfer occurs f rom this site and its effect on the spectroscopic properties of 3 is probed by combining electrochemistry and IR or UV-vis/NIR spectroscopy by in situ techniques. In contrast, the reversible reduction primarily involves the a llenylidene ligand as ascertained by ESR spectroscopy. In situ spectro-elec trochemical techniques reveal how the reduction affects the bonding within the unsaturated ligand. (C) 2001 Elsevier Science BN. All rights reserved.