Some phosphinite complexes of Rh and Ir, their intramolecular reactivity and DFT calculations about their application in biphenyl metathesis

Biphen(OPR2) (with R: Ph, iPr, Cy) is reacted with [Rh(COE)2Cl]2. The corresponding μ-chloro-bridged dimers are received. An X-ray analysis of [Biphen(OPCy2)RhCl]2 is included. This compound shows a dynamic behaviour in solution, ascribed to a monomer/dimer equilibrium. The difference of the Biphen ligands to Milsteins PCP pincer-type ligand is shown. A catalytic cycle for biphenyl metathesis containing the coupling of oxidative addition and reductive elimination of the bridging C–C single bond in the biphenyl fragment using RhI/III complexes and the concept of chelating assistance was calculated using DFT (B3PW91/LANL2DZ). According to the calculations the activation energy of the oxidative addition is about 30 kcal/mol and for the reductive elimination about 19 kcal/mol. The fac–RhIII complex is by far the most stable compound, but the formation of it is kinetically strongly disfavoured. Pre-catalysts (COD)M(Ph-O-PR2) (M: Rh, Ir) were synthesized by pre-coordinating the phosphinite to the metal (X-ray structures of four such compounds included) followed by treatment with 2 equiv. of sec. BuLi (X-ray structures of two such compounds included). In case of Ir this synthesis is complicated by C–H activation (X-ray structure of (COD)Ir(H)(Cl)(2-Br-phenyl-O-(diisopropylphosphinite)) included) and fast oxidative addition of the Ph-C–Halide bond. For (COD)Ir(H)(Cl)(2-phenyl-O-(diisopropylphosphinite)) the C–H activation is reversible and thermodynamic parameters for the ring closure reaction were determined by VT-NMR measurement (ΔH = −21.1 ± 0.5 kJ/mol, ΔS = −62.8 ± 1.7 J/(mol K)). The pre-catalysts were reacted with Biphen(OPR2) to enter the calculated catalytic cycle. With Rh as center metal this reaction works out cleanly to give new complexes with the three P-atoms coordinated to one Rh center. No hemi-labile character was found for these P-donors even at 105 °C in toluene. If (COD)Rh(2-phenyl-O-(diisopropylphosphinite)) is reacted with 2 equiv. of 2-iodo-phenyl-O-(diisopropylphosphinite) oxidative addition of one C–Iodo bond is observed and the corresponding mer–RhIII complex is received. Upon treatment with 2 equiv. of sec. BuLi the resulting product is(Biphen(OPiPr2))RhI(2-phenyl-O-(diisopropylphosphinite)) rather than mer–RhIII(2-phenyl-O-(diisopropylphosphinite))3. Reaction of [Rh(COD)Cl]2 with 3 equiv. of 2-bromo-phenyl-O-(diphenylphosphinite) shows a fast scrambling of the chlorine into all possible ortho positions of the phenolate rings in the final RhIII reaction product.

The possibility of the re-organization of biphenyl units according to a metathesis reaction, a yet unprecedented transformation, is examined theoretically (DFT: B3PW91/LANL2DZ) and with first experimental approaches using mainly Rh as center metal and phosphinite ligands of the type Pho-X-O-(PR2) (X = H, Br, I or Pho-X-O-(PR2), R: Ph, Cy, iPr), which function as ligand and substrate both in one. The intramolecular reactivity between the center metal (Rh, Ir) and the ortho substituents of the phosphinite ligands is examined.Figure optionsDownload as PowerPoint slide