B.K Bennett, S. Pitteri, L. Pilobello, S. Lovell, W. Kaminsky, J. M.
Mayer. Osmium(IV) complexes TpOs(X)Cl2 and their Os(III) counterparts: Oxidizing
compounds with an unusual resistance to ligand substitution. J Chem Soc DALTON
23 (2001) 3489-3497.
TpOs(IV)(X)Cl-2 complexes (Tp = hydrotris(1-pyrazolyl)borate) are formed
upon treatment of TpOs(NPPh3)Cl-2 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)Cl-2 (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 TpOsCl(3) 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)Cl-2
(L = PPh3, py). A more general route to Os(III) complexes involves reduction
of 1 by decamethylferrocene (Cp*Fe-2) to give [Cp*Fe-2][TpOs(OTf)Cl-2], which
undergoes substitution at 65 degreesC forming the Os(III) complexes TpOs(L)Cl-2
(L = MeCN, C6H5CN, PPh3, pyrimidazole, 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)Cl-2]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)Cl-2]BF4, appear to
be in part a consequence of the electrophilic character of the Os(IV)
center.