Syntheses and crystal structures of dinuclear complexes containing d-block and f-block luminophores. Sensitization of NIR luminescence from Yb(III), Nd(III), and Er(III) centers by energy transfer from Re(I)and Pt(II)-bipyrimidine metal centers

Mononuclear complexes [Re(bpym)(CO)3Cl] and [Pt(bpym)(CC-C6H4CF3)2] (bpym = 2,2'-bipyrimidine), in which one of the bipyrimidine sites is vacant, have been used as "complex ligands" to prepare heterodinuclear d-f complexes in which a lanthanide tris(1,3-diketonate) unit is attached to the secondary bipyrimidine site to evaluate the ability of d-block chromophores to act as antennae for causing sensitized near-infrared (NIR) luminescence from adjacent lanthanide(III) centers. The two sets of complexes so prepared are [Re(CO)3Cl(?-bpym)Ln(fod)3] (abbreviated as Re-Ln; where Ln = Yb, Nd, Er) and [(F3C-C6H4-CC)2Pt(?-bpym)Ln(hfac)3] (abbreviated as Pt-Ln; where Ln = Nd, Gd). Members of both series have been structurally characterized; the metal-metal separation across the bipyrimidine bridge is ?6.3 Å in each case. In these complexes, the 3MLCT (MLCT = metal to ligand charge-transfer) luminescences of the mononuclear [Re(bpym)(CO)3Cl] and [Pt(bpym)(CC-C6H4CF3)2] complexes are quenched by energy transfer to those lanthanides (Ln = Yb, Nd, Er) that have low-lying f-f states capable of NIR luminescence; as a result, sensitized NIR luminescence is seen from the lanthanide center following excitation of the d-block unit. In the solid state, quenching of the luminescence from the d-block chromophore is complete, indicating efficient d -> f energy transfer, as a result of the short metal-metal separation across the bipyrimidine bridge. In a CH2Cl2 solution, partial dissociation of the dinuclear complexes into the mononuclear units occurs, with the result that some 3MLCT luminescence is observed from mononuclear [Re(bpym)(CO)3Cl] or [Pt(bpym)(CC-C6H4CF3)2] present in the equilibrium mixture. Solution UV-vis and luminescence titrations, carried out by the addition of portions of Ln(fod)3(H2O)2 or Ln(hfac)3(H2O)2 to the d-block complex ligands, indicate that binding of the lanthanide tris(1,3-diketonate) unit at the secondary bipyrimidine site to give the d-f dinuclear complexes occurs with an association constant of ca. 105 M-1.