Partially Shielded Fe(CO)(3) Rotors: Syntheses, Structures, and Dynamic Properties of Complexes with Doubly trans Spanning Diphosphines, trans-($)over-barFe(CO)(3)(PhP((CH2)(n))(2)PPh)
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2017 American Chemical Society. Reactions of Fe(CO)3(n4-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. Variableerature 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; G273 K 12.8 kcal/mol), but that in 4a is not (RT to 105 C; G378 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).
