Tuning the properties of the osmium nitrido group in TpOs(N)X2 by changing the ancillary ligand.

TpOs(N)(OAc)2 (2) is formed upon reaction of TpOs(N)Cl2 (1) with excess silver acetate (Tp = hydrotrispyrazolylborate). Treatment of 2 with protic acids HX gives the osmium(VI) complexes TpOs(N)X2, where X = trifluoroacetate (TFA, 3), trichloroacetate (TCA, 4), tribromoacetate (TBA, 5), bromide (6), oxalate (X2 = O2C2O2, 7), or nitrate (ONO2, 8). Cyclic voltammetry studies of 1-8 show irreversible reductions of OsVI to OsV, varying over a range of 0.63 V. Much smaller relative variations are observed in 15N NMR chemical shifts, v(Os identical to N) stretching frequencies, and optical absorbances. Compounds 1-8 all react with PPh3 by nucleophilic attack at the nitride ligand, yielding TpOs(NPPh3)X2. The reactions are accelerated by more electron withdrawing ligands X. The relative rates correlate with the peak reduction potentials although the effect is small: the rates vary by only 10(2) while the 0.63 V change in Ep,c corresponds to a change in equilibrium constant for electron transfer of approximately 10(11). Compounds 1-8 also react with triphenylboron, BPh3, with formation of borylanilido complexes TpOs[N(Ph)BPh2]X2. However, rate constants for reactions with BPh3 to yield OsIV boryl-amido complexes do not in general correlate with one-electron-reduction potentials. This is likely due to the mechanism of the BPh3 reactions being a two-step process and not simply nucleophilic attack.