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.
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abstract
Reactions of trans-Fe(CO)3(P((CH2)mCHCH2)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)mCHCH(CH2)m)3 P) (3a-c,e, 60-81%). Hydrogenations (ClRh(PPh3)3, 60-80 C) yield the title compounds trans- Fe(CO)3(P((CH2)n)3 P) (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)4CHCH(CH2)4P linkages. Additions of NO(+)BF4(-) give the isoelectronic and isosteric cations [ Fe(CO)2(NO)(P((CH2)n)3 P)](+)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)3 P)](+)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 (13)C 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 H()/S() 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(+)). (13)C 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.
