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Photolyses of trans-Fe(CO)3(As((CH2)n)3As) (n =a, 10; b, 12; c, 14) in the presence of PMe3, or reactions of trans-[Fe(CO)2(NO)(As((CH2)n)3As)]+ BF4− and n-Bu4N+ Cl−, affordthe air stable title complexes As((CH2)n)3As (8a−c) in 79−34%yields. With 8a, the in, in and out, out isomers are separable andeach is crystallographically characterized. With 8b,c, the isomersrapidly interconvert by homeomorphic isomerization, but eachcrystallizes (contrathermodynamically) as an out, out isomer.Reactions of 8c with H2O2 or 8b with BH3 give 8c·2O or 8b·2BH3,respectively (85−94%). Reactions of 8c with MCl2 (M = Pt, Pd,Ni), Rh(CO)(Cl), and Fe(CO)3 sources afford the correspondingcage-like complexes trans-MLn(As((CH2)14)3As) (86−51%). Thecrystal structures of 8c·2O and the PtCl2 and PdCl2 adducts are determined and compared to those of 8a−c and diphosphorusanalogs. The corresponding distibine Sb((CH2)14)3Sb is analogously prepared, but precursors necessary for the bismuth analogcould not be accessed due to the diminished BiR3 Lewis basicity 

UV irradiation of yellow CH2Cl2 solutions of trans-Fe(CO)3(P-((CH2)10)3P) (2a) and PMe3 (10 equiv) gives, in addition to the previously reported dibridgehead diphosphine P((CH2)10)3P (46%), a green paramagnetic complex that crystallography shows to be the trigonal-bipyramidal iron(I) radical trans-[Fe(CO)2(Cl)(P((CH2)10)3P)]• (1a•; 31% after workup). This is a rare example of an isolable species of the formula [Fe(CO)4−n(L)n(X)]• (n = 0−3, L =two-electron-donor ligand; X = one-electron-donor ligand). Analogous precursors with longer P(CH2)nP segments (n = 12, 14, 16, 18) give only the demetalated diphosphines, and a rationale is proposed. The magnetic susceptibility of 1a•,assayed by Evans’ method and SQUID measurements, indicates a spin (S) of 1/2. Cyclic voltammetry shows that 1a• undergoes a partially reversible one-electron oxidation, but no facile reduction. The UV−visible, EPR, and 57Fe Mössbauer spectra are analyzed in detail. Complex 2a is similarly studied, and, despite the extra valence electron, exhibits a comparable oxidation potential (ΔE1/2 ≤0.04 V). The crystal structure shows a cage conformation, solvation level, disorder motif, and unit cell parameters essentially identical to those of 1a•. DFT calculations provide much insight regarding the structural, redox, and spectroscopic properties. 

Photolyses of trans- Fe(CO)3(P((CH2)n)3P (n = 10 (a), 12 (b), 14 (c),16 (d), 18 (e)) in the presence of PMe3 provide the first economical and scalable route to macrobicyclic dibridgehead diphosphines P((CH2)n)3P (1). These are isolated as mixtures of in,in/out,out isomers that equilibrate with degenerate in,out/out,in isomers at 150 °C via pyramidal inversion at phosphorus. For the entire series, VT 31P NMR data establish or bound Keq rates, and activation parameters for a variety of phenomena, many of which involve homeomorphic isomerizations, topological processes by which certain molecules can turn themselve sinside out. This provides the first detailed mapping of such trends in homologous series of aliphatic bicyclic compounds XE((CH2)n)3EX with any type of bridgehead. Isomeric diborane adducts 1 a,d·2BH3 are also characterized. Crystal structures of out,out-1 a and in,in-1 a·2BH3 aid isomer assignments and reveal unusual cage conformations. 

Reactions of trans-[[upper bond 1 start]Fe(CO)2(NO)(As((CH2)n)3As[upper bond 1 end])]+ BF4− (n = 10, 12, 14) and Bu4N+ Cl− afford the title compounds As((CH2)n)3As, which upon reaction (n = 14) with MCl2 (M = Pt, Ni), Rh(CO)(Cl), and Fe(CO)3 sources reconstitute cage like complexes trans-[upper bond 1 start]MLn(As((CH2)14)3A[upper bond 1 end]s). Reactions with H2O2 and BH3 give the corresponding arsine oxides and boranes. Crystal structures of metal-free species reveal out,out isomers, but cage complex formation is proposed to entail homeomorphic isomerization to in,in isomers with endo directed lone pairs. 

The diphosphine complexes cis - or trans -PtCl 2 (P((CH 2 ) n ) 3 P) ( n = b/12, c/14, d/16, e/18) are demetalated by MCX nucleophiles to give the title compounds (P((CH 2 ) n ) 3 )P (3b–e, 91–71%). These “empty cages” react with PdCl 2 or PtCl 2 sources to afford trans -MCl 2 (P((CH 2 ) n ) 3 P). Low temperature 31 P NMR spectra of 3b and c show two rapidly equilibrating species (3b, 86 : 14; 3c, 97 : 3), assigned based upon computational data to in , in (major) and out , out isomers. These interconvert by homeomorphic isomerizations, akin to turning articles of clothing inside out (3b/c: Δ H ‡ 7.3/8.2 kcal mol −1 , Δ S ‡ −19.4/−11.8 eu, minor to major). At 150 °C, 3b, c, e epimerize to (60–51) : (40–49) mixtures of ( in , in / out , out ) :  in , out isomers, which are separated via the bis(borane) adducts 3b, c, e·2BH 3 . The configurational stabilities of in , out -3b, c, e preclude phosphorus inversion in the interconversion of in , in and out , out isomers. Low temperature 31 P NMR spectra of in , out -3b, c reveal degenerate in , out / out , in homeomorphic isomerizations (Δ G ‡Tc 12.1, 8.5 kcal mol −1 ). When ( in , in / out , out )-3b, c, e are crystallized, out , out isomers are obtained, despite the preference for in , in isomers in solution. The lattice structures are analyzed, and the D 3 symmetry of out , out -3c enables a particularly favorable packing motif. Similarly, ( in , in / out , out )-3c, e·2BH 3 crystallize in out , out conformations, the former with a cycloalkane solvent guest inside.