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

Reactions of BrMg(CH2)(m)CH=CH2 (m = 4, a; 5, b; 6, c) and AsCl3 (0.30 equiv) give the arsines As((CH2)(m)CH=CH2)(3) (58-70%), which when added to iron tricarbonyl sources yield trans-Fe(CO)(3)(As((CH2)(m)CH-CH2)(3))(2) (66-70%). Reactions with Grubbs catalyst (18 mol %, CH2Cl2, reflux) and then hydrogenations (ClRh(PPh3)(3)/60-80 degrees C) afford gyroscope-like complexes trans-tiebar above startFe(CO)(3)(As((CH2)(n))(3)As) (4a-c, n = 2m + 2; 41-59%/two steps) of idealized D3h symmetry. Additions of NO+BF4- give the isoelectronic and isosteric cations [tiebar above startFe(CO)(2)(NO)(As((CH2)(n))(3)As)]+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-[tiebar above startFe(CO)(3)(H)(As((CH2)(n))(3)tiebar above endAs)]+BArf- (6a-c+ BArf-; 98-99%). Crystal structures of 4a-c and 5b(+)BF(4-) are determined. That of 4c suggests enough van der Waals clearance for the Fe(CO)3 moiety to rotate within the As(CH2)(14)As 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(+)BF(4)(-) and 6a(+)BAr(f)(-) 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(+)BAr(f)(-) decoalesce. The data give Delta H-double dagger/Delta S-double dagger 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.