Half-sandwich complexes of osmium containing guanidine-derived ligands

Abstract

Pyridinyl- and phosphano-guanidino complexes of formula [(η⁶-p-cymene)OsCl(H₂L)][SbF₆] (cymene = MeC₆H₄iPr; H₂L = N,N′-bis(p-Tolyl)-N′′-(2-pyridinylmethyl)guanidine, H₂L1 (1) and N,N′-bis(p-Tolyl)-N′′-(2-diphenylphosphanoethyl)guanidine, H₂L2 (2)) have been prepared from the dimer [{(η⁶-p-cymene)OsCl}₂(μ-Cl)₂] and H₂L in the presence of NaSbF₆. Treatment of complex 2 with HCl renders the phosphano–guanidinium complex [(η⁶-p-cymene)OsCl₂(H₃L₂)][SbF₆] (3). Compounds 1 and 2 react with AgSbF₆ rendering the cationic aqua complexes [(η⁶-p-cymene)Os(H₂L)(OH₂)][SbF₆]₂ (H₂L = H₂L1 (4), H₂L2 (5)). Addition of monodentate ligands L to compound 4 affords complexes of formula [(η⁶-p-cymene)Os(H₂L1)L][SbF₆]₂ (L = py (6), 4-(NHMe)py (7), CO (8), P(OMe)₃ (9)). Treatment of complexes 4 and 5 with NaHCO₃ renders the monocationic complexes [(η⁶-p-cymene)Os(κ³N,N′,N′′-HL1)][SbF₆] (10) and [(η⁶-p-cymene)Os(κ³N,N′,P-HL2)][SbF₆] (11), respectively, in which the HL ligand adopts a fac-κ³ coordination mode. The new complexes have been characterised by analytical and spectroscopic means, including the determination of the crystal structures of the compounds 1–4, 6, 8, and 11, by X-ray diffractometric methods. The phosphano–guanidino complexes 2 and 5 exhibit a temperature dependent fluxional process in solution. The new 18 electron complexes 1, 2, 6, and 8–10 are active catalysts for the Friedel–Crafts reaction between trans-β-nitrostyrene and N-methyl-2-methylindole. Conversions greater than 90% were obtained. Proton NMR studies support a mechanism involving the Brønsted-acid activation of trans-β-nitrostyrene through the NH functionalities of the coordinated guanidine ligands.