Heteronuclear bipyrimidine-bridged Ru-Ln and Os-Ln dyads:: low-energy 3MLCT states as energy-donors to Yb(III) and Nd(III)

The complexes [Ru((t)Bu(2)bipy)(bpym)X-2] (X = Cl, NCS) and [M((t)Bu(2)bipy)(2)(bpym)][PF6](2) (M = Ru, Os) all have a low-energy LUMO arising from the presence of a 2,2'-bipyrimidine ligand, and consequently have lower-energy (MLCT)-M-1 and (MLCT)-M-3 states than analogous complexes of bipyridine. The vacant site of the bpym ligand provides a site at which {Ln(diketonate)(3)} units can bind to afford bipyrimidine-bridged dinuclear Ru-Ln and Os-Ln dyads; four such complexes have been structurally characterised. UV/Vis and luminescence spectroscopic studies show that binding of the Ln(III) fragment at the second site of the bpym ligand reduces the (MLCT)-M-3 energy of the Ru or Os fragment still further. The result is that in the dyads [Ru((t)Bu(2)bipy)X-2(mu-bpym)Ln(diketonate)(3)] (X = Cl, NCS) and [Os((t)Bu(2)bipy)(2)(mu-bpym)Ln(diketonate)(3)][PF6](2) the (MLCT)-M-3 is too low to sensitise the luminescent f-f states of Nd(III) and Yb(III), but in [Ru((t)Bu(2)bipy)(2)(mu-bpym)Ln(diketonate)(3)][PF6](2) the (MLCT)-M-3 energy of 13 500 cm(-1) permits energy transfer to Yb(III) and Nd(III) resulting in sensitised near-infrared luminescence on the microsecond timescale.