Gyroscope-Like Complexes Based on Dibridgehead Diphosphine Cages That Are Accessed by Three-Fold Intramolecular Ring Closing Metatheses and Encase Fe(CO)3, Fe(CO)2(NO)+, and Fe(CO)3(H)+ Rotators

Reactions of trans-Fe­(CO)3(P­((CH2)mCHCH2)3)2 (m = a/4; b/5, c/6, e/8) and Grubbs’ catalyst (12–24 mol %, CH2Cl2, reflux) give the cage-like trienes trans-Fe­(CO)3(P­((CH2)mCHCH­(CH2)m)3P) (3a–c,e, 60–81%). Hydrogenations (ClRh­(PPh3)3, 60–80 °C) yield the title compounds trans-Fe­(CO)3(P­((CH2)n)3P) (4a–c,e, 74–86%; n = 2m + 2), which have idealized D3h symmetry. A crystal structure of 4c suggests enough van der Waals clearance for the Fe­(CO)3 moiety to rotate within the three P­(CH2)14P linkages; structures of E,E,E-3a show rotation to be blocked by the shorter P­(CH2)4CHCH­(CH2)4P linkages. Additions of NO+BF4– give the isoelectronic and isosteric cations [Fe­(CO)2(NO)­(P­((CH2)n)3P)]+BF4– (5a–c+BF4–; 81–98%). Additions of [H­(OEt2)2]+BArf– (BArf = B­(3,5-C6H3(CF3)2)4) afford the metal hydride complexes mer,trans-[Fe­(CO)3(H)­(P­((CH2)n)3P)]+BArf– (6a–c,e+BArf–; 98–99%). The behavior of the rotators in the preceding complexes is probed by VT NMR. At ambient temperature in solution, 5a,b+BF4– and 6a+BArf– show two sets of P­(CH2)n/2 13C NMR signals (2:1), whereas 5c+BF4– and 6b,c+BArf– show only one. At higher temperatures, the signals of 5b+BF4– coalesce; at lower temperatures, those of 5c+BF4– and 6b+BArf– decoalesce. These data give ΔH⧧/ΔS⧧ values (kcal/mol and eu) of 8.3/–28.4 and 9.5/–6.5 for Fe­(CO)2(NO)+ rotation (5b,c+) and 6.1/–23.5 for Fe­(CO)3(H)+ rotation (6b+). 13C CP/MAS NMR spectra show that the Fe­(CO)3 moiety in polycrystalline 4c (but not 4a) undergoes rapid rotation between −60 and 95 °C. Approaches to minimizing these barriers and developing molecular gyroscopes are discussed.