Selective synthesis, reactivity and luminescence of unsymmetrical bis-cyclometalated Pt(iv) complexes

Pt(ii) complexes cis-N,N-[PtCl(C^N)(N'^C'H)], where C^N represents a monocyclometalated 2,6-diaryl- or 2-arylpyridine ligand and N'^C'H is an N-coordinated 2-arylpyridine, are selectively obtained from bridge-cleavage reactions of dimers [Pt₂(μ-Cl)₂(C^N)₂] with excess N'^C'H at room temperature; isolation and characterization of derivatives of this kind is reported for the first time. Oxidation with PhICl₂ affords Pt(iv) complexes [PtCl₂(C^N)(C'^N')], bearing two cyclometalated ligands in an unsymmetrical arrangement. The abstraction of the two chlorides using AgOTf at 120 °C in the presence of an additional 2-arylpyridine ligand leads to mer isomers of tris-cyclometalated Pt(iv) complexes if C^N derives from a 2-arylpyridine, whereas it results in a reductive C-C coupling if C^N is a monocyclometalated 2,6-diarylpyridine. Complexes [PtCl₂(C^N)(C'^N')] show phosphorescence in frozen PrCN glasses arising from essentially ³LC excited states localized on the cyclometalated ligand with the lowest π-π* transition energy. The combined photophysical data and computational results substantiate a variable degree of MLCT admixture into the emitting state depending on the atom trans to the metalated carbon of the chromophoric ligand (Cl or N), which has an appreciable effect on the characteristics of the observed luminescence.