Synthesis, reactivity, structures, and dynamic properties of gyroscope like iron complexes with dibridgehead diphosphine cages: pre- vs. post-metathesis substitutions as routes to adducts with neutral dipolar Fe(CO)(NO)(X) rotors

Abstract

Three routes are explored to the title halide/cyanide complexes trans-Fe(CO)(NO)(X)(P((CH₂)₁₄)₃P) (9c-X; X = Cl/Br/I/CN), the Fe(CO)(NO)(X) moieties of which can rotate within the diphosphine cages (ΔH^‡/ΔS^‡ (kcal mol⁻¹/eu⁻¹) 5.9/−20.4 and 7.4/−23.9 for 9c-Cl and 9c-I from variable temperature ¹³C NMR spectra). First, reactions of the known cationic complex trans-[Fe(CO)₂(NO)(P((CH₂)₁₄)₃P)]⁺ BF₄⁻ and Bu₄N⁺ X⁻ give 9c-Cl/-Br/-I/-CN (75–83%). Second, reactions of the acyclic complexes trans-Fe(CO)(NO)(X)(P((CH₂)_mCHCH₂)₃)₂ and Grubbs’ catalyst afford the tris(cycloalkenes) trans-Fe(CO)(NO)(X)(P((CH₂)_mCHCH(CH₂)_m)₃P) (m/X = 6/Cl,Br,I,CN, 7/Cl,Br, 8/Cl,Br) as mixtures of Z/E isomers (24–41%). Third, similar reactions of trans-[Fe(CO)₂(NO)(P((CH₂)_mCHCH₂)₃)₂]⁺ BF₄⁻ and Grubbs’ catalyst afford crude trans-[Fe(CO)₂(NO)P((CH₂)_mCHCH(CH₂)_m)₃P)]⁺ BF₄⁻ (m = 6, 8). However, the CC hydrogenations required to consummate routes 2 and 3 are problematic. Crystal structures of 9c-Cl/-Br/-CN are determined. Although the CO/NO/X ligands are disordered, the void space within the diphosphine cages is analyzed in terms of horizontal and vertical constraints upon Fe(CO)(NO)(X) rotation and the NMR data. The molecules pack in identical motifs with parallel P–Fe–P axes, and without intermolecular impediments to rotation in the solid state.