Syntheses and structures of novel cyclic and dinuclear organorhodoximes: a homologous series of di- to penta-methylene-bridged complexes‡

Abstract

The compound [Rh(Hdmg)₂(PPh₃)]^– ([Rh]^–), synthesized by reduction of [Rh]–Cl with NaBH₄ in methanolic KOH, reacted with 1,2-disubstituted ethanes XCH₂CH₂X′ (X/X′ = Cl/OMe or Br/Br) forming [Rh]–CH₂CH₂OMe 1a as well as [Rh]–Br and ethylene as heterolytic fragmentation products. Heterolytic fragmentation of 1a enforced by protonation with acids (CF₃SO₃H, CD₃CO₂D) generated MeOH, H₂CCH₂ and [Rh]–O₃SCF₃ and [Rh]–O₂CCD₃, respectively. Reaction of [Rh]^– with XCH₂CH₂X′ (X/X′ = Cl/Cl, Cl/Br or Cl/OPh) afforded the dinuclear complex [Rh]CH₂CH₂[Rh] 2a. The anion [Rh]^– reacted with Cl(CH₂)₃Cl to give [Rh]–CH₂CH₂CH₂Cl 1b, whereas Br(CH₂)₃Br was reacted with excess and equimolar amounts of [Rh]^–, yielding [Rh]CH₂CH₂CH₂[Rh] 2b and [Rh{(CH₂)₃ON  C(Me)C(Me)NO}(Hdmg)(PPh₃)] 3b, respectively. Similar reactions carried out with Br(CH₂)_nBr (n = 4 or 5) yielded [Rh]–(CH₂)₅Br 1d, [Rh](CH₂)_n[Rh] (n = 4 2c or 5 2d) and [Rh{(CH₂)_nON  C(Me)C(Me)NO}(Hdmg)(PPh₃)] (n = 4 3c or 5 3d), respectively. All complexes were fully characterized by NMR spectroscopy (¹H, ¹³C, ³¹P). The ³¹P-{¹H} NMR spectra of dinuclear complexes 2a and 2b exhibit typical AA′ patterns of AA′XX′ systems (A = ³¹P, X = ¹⁰³Rh) due to considerable ⁵J(³¹P–³¹P) and ⁶J(³¹P–³¹P) couplings (36.7, 11.2 Hz), respectively. The crystal structures of the dinuclear rhodoximes 2a–2c and of the cyclic organorhodoxime 3b have been determined. The two (Hdmg)₂ planes in the di- and tetra-methylene-bridged complexes 2a and 2c are parallel with distances of 4.5 (2a) and 7.1 Å (2c), respectively, and exhibit an ecliptic conformation. In the trimethylene-bridged complex 2b, the two (Hdmg)₂ planes include an angle of 45.0(1)° and exhibit a staggered conformation, which minimizes electrostatic repulsion between the O–H–O moieties and the steric interference between two methyl groups. In all three complexes the oligo-methylene bridges are fully staggered. In 3b the six-membered ring (1-oxa-2-aza-3-rhodacyclohexane) exhibits a distorted chair conformation. The distance between the two O atoms in the O–H–O bridge [O(2)· · ·O(3) 2.58(1) Å] is distinctly shorter than those that are not connected via a hydrogen bridge [O(1)· · ·O(4) 3.30(1) Å].