Syntheses, Reactivity, Structures, and Dynamic Properties of Gyroscope-like Iron Carbonyl Complexes Based on Dibridgehead Diarsine Cages

Reactions of BrMg­(CH2)mCHCH2 (m = 4, a; 5, b; 6, c) and AsCl3 (0.30 equiv) give the arsines As­((CH2)mCHCH2)3 (58–70%), which when added to iron tricarbonyl sources yield trans-Fe­(CO)3(As­((CH2)mCHCH2)3)2 (66–70%). Reactions with Grubbs’ catalyst (18 mol %, CH2Cl2, reflux) and then hydrogenations (ClRh­(PPh3)3/60–80 °C) afford gyroscope-like complexes trans-Fe­(CO)3(As­((CH2)n)3As) (4a–c, n = 2m + 2; 41–59%/two steps) of idealized D3h symmetry. Additions of NO+BF4– give the isoelectronic and isosteric cations [Fe­(CO)2(NO)­(As­((CH2)n)3As)]+BF4– (5a–c+ BF4–; 81–98%), and [H­(OEt2)2]+BArf– (BArf = B­(3,5-C6H3(CF3)2)4) gives the hydride complexes mer,trans-[Fe­(CO)3(H)­(As­((CH2)n)3As)]+BArf– (6a–c+ BArf–; 98–99%). Crystal structures of 4a–c and 5b+BF4– are determined. That of 4c suggests enough van der Waals clearance for the Fe­(CO)3 moiety to rotate within the As­(CH2)14As linkages; that of 4a shows rotation to be blocked by the shorter As­(CH2)10As linkages. The rotator dynamics in these complexes are probed by VT NMR. At ambient temperature in solution, 5a+BF4– and 6a+BArf– give two sets of P­(CH2)n/2 13C NMR signals (2:1), while 5b,c+BF4– and 6b,c+BArf– give only one. At lower temperatures the signals of 5b+BF4– and 6b+BArf– decoalesce. The data give ΔH⧧/ΔS⧧ values (kcal/mol and eu) of 7.7/–22.1 and 5.4/–22.7 for Fe­(CO)2(NO)+ and Fe­(CO)3(H)+ rotation. These barriers are distinctly lower than in diphosphine analogues, consistent with the longer iron–arsenic vs −phosphorus bonds increasing the interior dimensions of the diarsine cage.