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(μ-bpym)Ln(fod)3] (abbreviated as
Re−Ln; where Ln = Yb, Nd, Er) and [(F3C−C6H4−CC)2Pt(μ-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.