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(ER_x)_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)(ER_x)_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 (M_w ≈ 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.