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.

Graphical abstractThe 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 full-size imageDownload as PowerPoint slide