Partially Shielded Fe(CO)₃ Rotors: Syntheses, Structures, and Dynamic Properties of Complexes with Doubly <i>trans</i> Spanning Diphosphines, <i>trans</i>-Fe(CO)₃(PhP((CH₂)_<i>n</i>)₂PPh)

Reactions of Fe­(CO)₃(η⁴-benzylideneacetone) and PhP­((CH₂)_<i>m</i>CHCH₂)₂ (<i>m</i> = <b>a</b>, 4; <b>b</b>, 5; <b>c</b>, 6) give <i>trans</i>-Fe­(CO)₃(PhP­((CH₂)_<i>m</i>CHCH₂)₂)₂ (<b>2a</b>–<b>c</b>, 28–70%), which are treated with Grubbs’ catalyst (15 mol %; refluxing CH₂Cl₂). NMR analyses of the crude <i>inter</i>ligand metathesis products <i>trans</i>-Fe­(CO)₃(PhP­((CH₂)_<i>m</i>CHCH­(CH₂)_<i>m</i>)₂PPh) (<b>3a</b>–<b>c</b>, 30–31%) suggest <i>Z</i>/<i>E</i> CC mixtures and/or byproducts from <i>intra</i>ligand metathesis or oligomers. Subsequent hydrogenations (5 bar/cat. Rh­(Cl)­(PPh₃)₃ or PtO₂) afford <i>trans</i>-Fe­(CO)₃(PhP­((CH₂)_<i>n</i>)₂PPh) (<b>4a</b>–<b>c</b>, 69–77%; <i>n</i> = 2<i>m</i> + 2, <i>synperiplanar</i> phenyl groups), which density functional theory calculations show to be more stable than isomers derived from other metathesis modes. Crystallizations give (<i>E</i>,<i>E</i>)-<b>3a</b> and <b>4b</b>, the X-ray structures of which are determined and analyzed. Variable-temperature ¹³C­{¹H} NMR experiments show that rotation of the Fe­(CO)₃ moiety in <b>4b</b> is rapid on the NMR time scale (RT to 0 °C; Δ<i>G</i>^⧧_{273 K} ≤ 12.8 kcal/mol), but that in <b>4a</b> is not (RT to 105 °C; Δ<i>G</i>^⧧_{378 K} ≥ 17.9 kcal/mol). These data indicate rotational barriers lower than those in analogues in which three methylene chains connect the phosphorus atoms, <i>trans</i>-Fe­(CO)₃(P­((CH₂)_<i>n</i>)₃P)