Steric Control in the Synthesis and Stereochemistry of the Asymmetric “Head-to-Head” Isomer of Rhodium Binuclear [rh2(μ-htp5)2(μ-dppm)2](pf6)2 (htp5 = l, 5-diisocyano-l, l, 5-Triphenylpentane; Dppm = bis(Diphenylphosphino)Methane)

Charles A. Daws, Michael G. Hill, John P. Bullock, Kent R Mann

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Abstract

The reaction of the asymmetrical diisocyanide ligand l, 5-diisocyano-l, l, 5-triphenylpentane (HTP5) with [Rh(dppm)(CO)Cl]2 (dppm = bis(diphenylphosphino)methane) results in the exclusive formation of the “head-to-head” RR and 55 isomers of the face-to-face binuclear complex [Rh2(μ-HTP5)2(μ-dppm)2](PF6)2. Herein we report the synthesis and characterization of this complex by 31P NMR, homonuclear 2D δ/j resolved 31P NMR, infrared, and electronic spectra, cyclic voltammetry, and X-ray crystallography. The complex C100H88F12N4P6Rh2 crystallizes in the monoclinic space group C2/c with a = 28.74 (1) Å, b = 19.126 (9) Å, c = 21.91 (1) Å, β = 119.74 (4)°, V = 10455 (1) Å3, and Z = 4. The crystal structure and 31PNMR spectra indicate that only the head-to-head RR and 55 isomers exist in both the solid state and solution. Steric interactions among the pendant phenyl rings direct the dication formation reaction to produce exclusively the head-to-head RR and 55 isomers. These steric interactions also result in different rotameric conformations of the Rh-P bonds at the two Rh centers. The differences in the phenyl ring orientations produce different Rh-P bond lengths, 31P chemical shifts, and v(CN) stretching frequencies for the “head” and “tail” ends of the molecule. These results contrast with the less hindered compound [Rh2(μ-dimen)2(μ-dppm)2](PF6)2 (dimen = 1, 8-diisocyano-p-menthane) in which both the “head-to-head” and the “head-to-tail” isomers exist in a 1:1 ratio, and the spectroscopically observable differences in the two ends are less. Further, the radical formed by one-electron oxidation of [Rh2-(HTP5)2(dppm)2](PF6)2 is studied by EPR and UV-vis-IR spectroelectrochemistry.

Original languageEnglish (US)
Pages (from-to)2948-2955
Number of pages8
JournalInorganic Chemistry
Volume31
Issue number13
DOIs
StatePublished - Jan 1 1992

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