Osmium(iv) complexes TpOs(X)Cl2 and their Os(iii) counterparts: oxidizing compounds with an unusual resistance to ligand substitution

Abstract

TpOs^IV(X)Cl₂ complexes (Tp = hydrotris(1-pyrazolyl)borate) are formed upon treatment of TpOs(NPPh₃)Cl₂ with the protic acids HX: triflic acid (HOTf), HCl, HBr, CF₃COOH, and CH₃COOH. The reaction with acetic acid is slow but is catalyzed by HOTf. The triflate ligand in TpOs(OTf)Cl₂ (1) is remarkably inert and does not undergo simple substitution reactions. For instance, no reaction is observed between 1 and anhydrous Cl⁻ salts, but conversion to TpOsCl₃ occurs upon addition of small amounts of H₂O or HCl. Substitution appears to be catalyzed by protic reagents. Treatment of 1 with PPh₃ or pyridine (py) yields the substituted osmium(III) complexes TpOs(L)Cl₂ (L = PPh₃, py). A more general route to Os(III) complexes involves reduction of 1 by decamethylferrocene (Cp*₂Fe) to give [Cp*₂Fe][TpOs(OTf)Cl₂], which undergoes substitution at 65 °C forming the Os(III) complexes TpOs(L)Cl₂ (L = MeCN, C₆H₅CN, PPh₃, py, imidazole, and NH₃) in 70–90% yields. Oxidation of the neutral Os(III) complexes with [NO]BF₄ in CH₂Cl₂ affords Os(IV) salts of the formula [TpOs(L)Cl₂]BF₄ in near quantitative yields. This indirect synthetic approach yields osmium(IV) complexes that are not accessible by direct substitution. The Os(IV) complexes are strong oxidants, with E_1/2 values from +0.00 to +0.70 V in MeCN vs. Cp₂Fe^+/0. The inertness of the triflate ligand in 1, and the acetonitrile ligand in [TpOs(NCMe)Cl₂]BF₄, appear to be in part a consequence of the electrophilic character of the Os(IV) center.