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)

The new pro-ligand 4-methyl-4'- (carbonylamino(2-(tert-butoxycarbonylamino) ethyl)) -2,2'-bipyridyl (L-1) has been prepared and used to synthesise the complex fac-(ReCl)-Cl-I(CO)(3)(L-1) 1 and the complex salts [M-II (bipy)(2)(L-1)] (PF6)(2) (M = Ru-II 8 or Os-II 15). Deprotection with trifluoroacetic acid affords the amine-functionalised derivatives fac-(ReCl)-Cl-I(CO)(3)(L-2) 2, [M-II (bipy)(2)(L-2)] (PF6)(2) (M = Ru-II 9 or Os-II 16) which react with the dianhydride of diethylenetriamine pentaacetic acid (DTPA) to give the binuclear complex {fac - (ReCl)-Cl-I(CO)(3)}(2)(L-3) 3 and the complex salts [{M-II (bipy)(2)}(2)(L-3)] (PF6)(4) (M = Ru-II 10 or Os-II 17). 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-I Cl(CO)(3)}(2)(L-3) Ln(III) (Ln = Nd 4, Er 5, Yb 6 or Y 7) and [{M-II (bipy)(2)}(2)(L-3) 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 L-1 and [Ru-II (bipy)(2)((LH)-H-2)] (H2PO4)(3) center dot (CH3)(2)CO center dot 0.8H(2)O were obtained, the latter revealing the presence of H2PO4- counter anions, the source of which is presumed to be hydrolysis of PF6- ions.