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{(η5-C5H4)2(ERx)y}:  Fe{(η5-C5H4)2SiMe2} (1), Fe{(η5-C5H4)CH2}2 (10)] and highly strained [2]ruthenocenophanes [Ru{(η5-C5H4)CR2}2 {R = H (15), Me (16)}] are susceptible to partial substitution by P donors and form mixed-hapticity metallocycles[M(L2){(η5-C5H4)(ERx)y(η1-C5H4)}]:  [Fe(dppe){(η5-C5H4)SiMe2(η1-C5H4)}] (5),
[Fe(dmpe){(η5-C5H4)SiMe2(η1-C5H4)}] (6), [Fe(dmpe){(η5-C5H4)(CH2)2(η1-C5H4)}] (11), [Ru(dmpe){(η5-C5H4)(CH2)2(η1-C5H4)}] (17), [Ru(dmpe){(η5-C5H4)(CMe2)2(η1-C5H4)}] (18), and [Ru(PMe3)2{(η5-C5H4)(CH2)2(η1-C5H4)}] (19)through haptotropic reduction of one η5-, π-bound Cp to η1, σ-coordination. These reactions
are strain-controlled, as highly ring-tilted [2]ruthenocenophanes 15 and 16 [tilt angles (α) ≈ 29−31°] react
without irradiation to form thermodynamically stable products, while moderately strained [n]ferrocenophanes
1 and 10 (α ≈ 19−22°) require photoactivation. The iron-containing photoproducts 5 and 11 are metastable
and thermally retroconvert to their strained precursors and free phosphines at 70 °C. In contrast, the
unprecedented ring-opening polymerization (ROP) of the essentially ring-strain-free adduct 6 to afford poly(ferrocenyldimethylsilane) [Fe(η5-C5H4)2SiMe2]n (Mw ≈ 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{(η5-C5H4)(CH2)2(C5H5)}][OCH2CH3]
(13[OCH2CH3]) 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(η5-C5H5){(η5-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.