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 °C) afford gyroscope-like complexes trans- Fe­(CO)3(As­((CH2) n )3 As) (4a–c, n = 2m + 2; 41–59%/two steps) of idealized D 3h symmetry. Additions of NO+BF4 – give the isoelectronic and isosteric cations [Fe­(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-[Fe­(CO)3(H)­(As­((CH2) n )3 As)]+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.

Reactions of trans-Fe(CO)3(P((CH2)mCH═CH2)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)mCH═CH(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)4CH═CH(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 ΔH(⧧)/ΔS(⧧) 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.

Three routes are explored to the title halide/cyanide complexes trans-Fe(CO)(NO)(X)(P((CH))P) (9c-X; X = Cl/Br/I/CN), the Fe(CO)(NO)(X) moieties of which can rotate within the diphosphine cages (ΔH/ΔS (kcal mol/eu) 5.9/-20.4 and 7.4/-23.9 for 9c-Cl and 9c-I from variable temperature C NMR spectra). First, reactions of the known cationic complex trans-[Fe(CO)(NO)(P((CH))P)] BF and BuN X give 9c-Cl/-Br/-I/-CN (75-83%). Second, reactions of the acyclic complexes trans-Fe(CO)(NO)(X)(P((CH)CH[double bond, length as m-dash]CH)) and Grubbs' catalyst afford the tris(cycloalkenes) trans-Fe(CO)(NO)(X)(P((CH)CH[double bond, length as m-dash]CH(CH))P) (m/X = 6/Cl,Br,I,CN, 7/Cl,Br, 8/Cl,Br) as mixtures of Z/E isomers (24-41%). Third, similar reactions of trans-[Fe(CO)(NO)(P((CH)CH[double bond, length as m-dash]CH))] BF and Grubbs' catalyst afford crude trans-[Fe(CO)(NO)P((CH)CH[double bond, length as m-dash]CH(CH))P)] BF (m = 6, 8). However, the C[double bond, length as m-dash]C hydrogenations required to consummate routes 2 and 3 are problematic. Crystal structures of 9c-Cl/-Br/-CN are determined. Although the CO/NO/X ligands are disordered, the void space within the diphosphine cages is analyzed in terms of horizontal and vertical constraints upon Fe(CO)(NO)(X) rotation and the NMR data. The molecules pack in identical motifs with parallel P-Fe-P axes, and without intermolecular impediments to rotation in the solid state.