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

Abstract

TpOsIV(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 E1/2 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.