Rhodium aryloxide complexes containing terdentate nitrogen ligands, C-O bond formation, hydrogen bonding with phenol and oxidative addition reactions. Molecular structure of an Rh(III)-acetyl complex

Article abstract of DOI:10.1016/s0020-1693(97)05821-0

The new rhodium(I) phenoxide complexes [Rh(OPh)(2,6-(CH=NR²)₂C₅H₃N)] (R²=i-Pr (3), t-Bu (4)) containing strongly electron-donating N-N′-N ligands, have been prepared by a metathesis reaction of [RhCl(2,6-(CH=NR²)₂C₅H₃N)] (R²=i-Pr (1), t-Bu (2)) with NaOPh. These rhodium(I) phenoxide complexes 3 and 4, which are very sensitive to O₂ but stable towards H₂O, give with phenol the adducts [Rh(OPh) (2,6-(CH=NR₂)₂C₅H₃N)]·HOPh (R²=i-Pr (5), t-Bu (6)), which contain strong O-H?O hydrogen bonds. The hydrogen bonded phenol could not be extracted with diethyl ether, while no exchange of the hydrogen bonded phenol and the phenoxide ligand in 4 is observed on the NMR time scale. However, a small excess of phenol results in exchange of the hydrogen bonded phenol, the coordinated phenoxide ligand and free phenol on the NMR time scale. Reaction of 3 and 4 with p-nitrophenol afforded [Rh(OC₆H₄-(NO₂-4))(2,6-(CH=NR²) ₂C₅H₃N)]·HOPh (R²=i-Pr (7), t-Bu (8)) in which the formed phenol is hydrogen bonded to the Rh(I)-OC₆H₄-(NO₂-4) moiety. The O-H?O bond is less strong than in 5 and 6, as the hydrogen bonded phenol could be removed by diethyl ether. Treatment of 3 with acetyl chloride and benzoyl chloride in benzene at room temperature gave phenylacetate and [RhCl₂(C(O)CH₃) (2,6-(C(H)=N-J-Pr)₂C₅H₃N)] (15), and phenylbenzoate and [RhCl₂(C(O)Ph)(2,6-(C(H)=N-i-Pr)₂C₅H ₃N)] (19), respectively. Complex 15 and the analogous complex [RhCl₂(C(O)CH₃)(2,6-(C(H)=N-t-Bu)₂C ₅H₃N)] (16) could also be prepared directly from acetyl chloride and 1 or 2, respectively. The single crystal X-ray determination of complex 16, monoclinic, space group P2₁/c, a=10.0477(5), b=11.7268(6), c=19.2336(9) A?, β=92.041(4)°, Z=4, R₁=0.0281, shows that the acetyl group occupies an axial position, while the N-N′-N ligand is positioned equatorially. In solution this geometry remains unchanged as was shown by variable temperature ¹H NMR measurements. When the oxidative addition of acetyl chloride to 3 was carried out at -78°C in toluene the intermediate complex [RhCl(OPh)(C(O)Me)(2,6-(C(H)=N-i-Pr)₂C₅H₃N)] (11) could be isolated, which at room temperature reductively eliminates phenyl-acetate with formation of 1. Oxidative addition of acetyl chloride to 4 at room temperature gives [RhCl(OPh)(C(O)Me)(2,6-(C(H)=N-t-Bu)₂C₅H₃N)] (12) which yields phenylacetate and 2 at 70°C in benzene by reductive elimination. Treatment of 3 with two equivalents of benzyl chloride afforded a mixture of [RhCl(OPh)(CH₂Ph)(2,6-(C(H)=N-i-Pr)₂C₅H ₃N)] (13) and [RhCl₂(CH₂Ph)(2,6-(C(H)=N-i-Pr)₂C ₅H₃N)] (17) and some non-characterizable organic products, while 4 only yielded [RhCl(OPh)(CH₂Ph)(2,6-(C(H)=N-t-Bu)₂C₅H ₃N)] (14).