Protonation and deprotonation of TpOs(NHPh)Cl2:: An unusually inert amido ligand

Protonation of the Os(IV) amido complex TpOs(NHPh)Cl-2 (1) to give the aniline complex [TpOs(NH2Ph)Cl-2]OTf (2) requires excess triflic acid (HOTf). Complex 1 is unreactive with HCl and other moderately strong acids. Consistent with the low basicity of 1, the aniline complex 2 is extremely acidic and is deprotonated by stoichiometric addition of weak bases such as Cl- or H2O. No reaction is observed between 1 and methyl triflate (CH3OTf) at ambient temperatures. Upon heating, CH3OTf removes the chloride ligands from 1 to give CH3Cl and the amidobis(triflate) complex TpOs(NHPh)(OTf)(2) (3). Attack at the amido nitrogen is not observed. Complex 1 is thus very inert to protonation and electrophilic attack at nitrogen. A deprotonated form of 1, TpOs[NPh(MgBr)]Cl-2 (4), is generated on reaction of PhMgBr with TpOs(N)Cl-2. Complex 4 is extremely basic and will protonate to 1 with weak acids such as CH3CN, DMSO, and acetic anhydride. Thus, I has a low acidity as well as a low basicity; it is both less acidic and less basic than aniline. The inertness of 1 is ascribed to partial Os-N pi bonding and to the oxidizing nature of the Os(IV) center.