Strain-controlled, photochemically, or thermally promoted haptotropic shifts of cyclopentadienyl ligands in group 8 metallocenophanes

Cyclopentadienyl (Cp) ligands in moderately strained 1]- and 2]ferrocenophanes Fe{(eta5-C5H4)2(ERx)y}: Fe{(eta5-C5H4)2SiMe2} (1), Fe{(eta5-C5H4)CH2}2 (10)] and highly strained 2]ruthenocenophanes Ru{(eta5-C5H4)CR2}2 {R = H (15), Me (16)}] are susceptible to partial substitution by P donors and form mixed-hapticity metallocycles-M(L2){(eta5-C5H4)(ERx)y(eta1-C5H4)}]: Fe(dppe){(eta5-C5H4)SiMe2(eta1-C5H4)}] (5), Fe(dmpe){(eta5-C5H4)SiMe2(eta1-C5H4)}] (6), Fe(dmpe){(eta5-C5H4)(CH2)2(eta1-C5H4)}] (11), Ru(dmpe){(eta5-C5H4)(CH2)2(eta1-C5H4)}] (17), Ru(dmpe){(eta5-C5H4)(CMe2)2(eta1-C5H4)}] (18), and Ru(PMe3)2{(eta5-C5H4)(CH2)2(eta1-C5H4)}] (19)-through haptotropic reduction of one eta5-, pi-bound Cp to eta1, sigma-coordination. These reactions are strain-controlled, as highly ring-tilted 2]ruthenocenophanes 15 and 16 tilt angles (alpha) approximately 29-31 degrees ] react without irradiation to form thermodynamically stable products, while moderately strained n]ferrocenophanes 1 and 10 (alpha approximately 19-22 degrees ) require photoactivation. The iron-containing photoproducts 5 and 11 are metastable and thermally retroconvert to their strained precursors and free phosphines at 70 degrees C. In contrast, the unprecedented ring-opening polymerization (ROP) of the essentially ring-strain-free adduct 6 to afford poly(ferrocenyldimethylsilane) Fe(eta5-C5H4)2SiMe2]n (Mw approximately 5000 Da) was initiated by the thermal liberation of small amounts of P donor. Unlike reactions with bidentate analogues, monodentate phosphines promoted photolytic ROP of ferrocenophanes 1 and 10. MALDI-TOF analysis suggested a cyclic structure for the soluble poly(ferrocenyldimethylsilane), 8-cyclic, produced from 1 in this manner. While the polymer likewise produced from 10 was insoluble, the initiation step in the ROP process was modeled by isolation of a tris(phosphine)-substituted ring-opened ferrocenophane Fe(PMe3)3{(eta5-C5H4)(CH2)2(C5H5)}]OCH2CH3] (13OCH2CH3]) generated by irradiation of 10 and PMe3 in a protic solvent (EtOH). Studies of the cation 13 revealed that the Fe center reacts with a Cp- anion with loss of the phosphines to form Fe(eta5-C5H5){(eta5-C5H4)(CH2)2(C5H5)}] (14) under conditions identical to those of the ROP experiments, confirming the likelihood of "back-biting" reactions to yield cyclic structures or macrocondensation to produce longer chains.