Metal-to-ligand charge-transfer sensitisation of near-infrared emitting lanthanides in trimetallic arrays M2Ln (M = Ru, Re or Os; Ln = Nd, Er or Yb)

Abstract

The new pro-ligand 4-methyl-4′-(carbonylamino(2-(tert-butoxycarbonylamino)ethyl))-2,2′-bipyridyl (L¹) has been prepared and used to synthesise the complex fac-Re^ICl(CO)₃(L¹) 1 and the complex salts [M^II(bipy)₂(L¹)](PF₆)₂ (M = Ru^II8 or Os^II15). Deprotection with trifluoroacetic acid affords the amine-functionalised derivatives fac-Re^ICl(CO)₃(L²) 2, [M^II(bipy)₂(L²)](PF₆)₂ (M = Ru^II9 or Os^II16) which react with the dianhydride of diethylenetriamine pentaacetic acid (DTPA) to give the binuclear complex {fac-Re^ICl(CO)₃}₂(L³) 3 and the complex salts [{M^II(bipy)₂}₂(L³)](PF₆)₄ (M = Ru^II10 or Os^II17). The latter react with salts Ln(OTf)₃ to afford a series of 12 heterotrimetallic compounds that contain a lanthanide (Ln) ion in the DTPA binding site; {fac-Re^ICl(CO)₃}₂(L³)Ln^III (Ln = Nd 4, Er 5, Yb 6 or Y 7) and [{M^II(bipy)₂}₂(L³)Ln^III](PF₆)(OTf)₃ (M = Ru^II, Ln = Nd 11, Er 12, Yb 13 or Y 14; M = Os^II, Ln = Nd 18, Er 19, Yb 20 or Y 21). All of these trimetallic species display absorption bands ascribed to metal-to-ligand charge-transfer (MLCT) excitations, and luminescence measurements show that these excited states can be used to sensitise near-infrared emission from Ln^III (Ln = Nd, Er or Yb) ions. Single crystal X-ray structures of L¹ and [Ru^II(bipy)₂(L²H)](H₂PO₄)₃·(CH₃)₂CO·0.8H₂O were obtained, the latter revealing the presence of H₂PO₄⁻ counter anions, the source of which is presumed to be hydrolysis of PF₆⁻ ions.