Syntheses, Reactivity, Structures, and Dynamic Properties of Gyroscope-like Iron Carbonyl Complexes Based on Dibridgehead Diarsine Cages
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2016 American Chemical Society. 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/213C 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 H/S 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.
