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)mCHCH2)2 (m = a, 4; b, 5; c, 6) give trans-Fe­(CO)3(PhP­((CH2)mCHCH2)2)2 (2a–c, 28–70%), which are treated with Grubbs’ catalyst (15 mol %; refluxing CH2Cl2). NMR analyses of the crude interligand metathesis products trans-Fe­(CO)3(PhP­((CH2)mCHCH­(CH2)m)2PPh) (3a–c, 30–31%) suggest Z/E CC mixtures and/or byproducts from intraligand metathesis or oligomers. Subsequent hydrogenations (5 bar/cat. Rh­(Cl)­(PPh3)3 or PtO2) afford trans-Fe­(CO)3(PhP­((CH2)n)2PPh) (4a–c, 69–77%; n = 2m + 2, synperiplanar phenyl groups), which density functional theory calculations show to be more stable than isomers derived from other metathesis modes. Crystallizations give (E,E)-3a and 4b, 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 4b is rapid on the NMR time scale (RT to 0 °C; ΔG⧧273 K ≤ 12.8 kcal/mol), but that in 4a is not (RT to 105 °C; ΔG⧧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, trans-Fe­(CO)3(P­((CH2)n)3P).