Carbene complexes. Part 18. Synthetic routes to electron-rich olefin-derived monocarbenerhodium(I) neutral and cationic complexes and their chemical and physical properties

Abstract

Electron-rich olefins of general type [[graphic omitted]R]₂(L^R ₂; R = Me, Et, Ph, 4-MeC₆H₄, 4-MeOC₆H₄, or 2-MeOC₆H₄) undergo reaction with a variety of rhodium(I) precursors via ligand displacement or chloride-bridge cleavage to afford monocarbenerhodium(I) complexes, such as [RhCl(L^R)(PPh₃)₂], [Rh(cod)Cl(L^R)], or [Rh(CO)Cl(L^R)(PPh₃)][L^R=[graphic omitted]R, cod = cyclo-octa-1,5-diene]; complexes [RhCl(L′^Me)(PPh₂)X][L′^Me=[graphic omitted]Me, X = CO or PPh₃] have similarly been obtained from the olefin L′^Me ₂. From these, further complexes may be obtained by ligand (neutral or anionic) exchange processes: trans-[RhBr(L^R)(PPh₃)₂], trans-[Rh(CO)(L^R)(PPh₃)₂]X (X = Br, Cl, ClO₄, or I), [Rh(CO)X(L^R)(PPh₃)](X = BH₄ or ClO₄), cis-[Rh(CO)₂X(L^R)](X = Cl or NO₃), [Rh(cod)X(L^R)](X = CH₂SiMe₃, ClO₄, or NO₃), cis-[Rh(cod)-(L^R)(PPh₃)][ClO₄], and [Rh(CO)₃(L^R)][ClO₄]. In many of the reactions some of these ligand displacements at Rh^I proceed without retention of stereochemistry and it is likely that the observed product is the thermodynamically preferred isomer. Other chemical properties of the monocarbene-rhodium(I) complexes relate to (i) rare examples of the displacement of L^R from Rh by PPh₃ or Ph₂PCH₂CH₂PPh₂ under rather forcing conditions, and (ii) oxidative addition (not particularly facile) of HCl, [NMe₂CHCl]Cl, or C₂(CN)₄. The 45 new complexes have been characterised by analysis and spectroscopy (i.r. and ¹H and ³¹P n.m.r.) and, where appropriate, relative molecular mass determination, and electrical conductivity. From J(³¹P–¹⁰³Rh) coupling constants it is concluded that L^R has a greater trans influence than PPh₃[also indicated by ν(Rh–Cl)] but a lower cis influence.