Luminescent Platinum(II) Dimers with a Cyclometallating Aryldiamine Ligand

Triflate salts of three (Pt(pip2NCN))2(μ-L)2+ (pip2NCNH = 1,3-bis(piperidylmethyl)benzene) dimers bridged by a
series of nitrogen-donor ligands (L = pyrazine (pyz), 1,2-bis(4-pyridyl)ethane (bpa), trans-1,2-bis(4-pyridyl)ethylene
(bpe)) are reported. These complexes have been fully characterized by 1H NMR spectroscopy and elemental
analysis. The X-ray crystal structures of [(Pt(pip2NCN))2(μ-pyz)](CF3SO3)2 and [(Pt(pip2NCN))2(μ-bpe)](CF3SO3)2·2CH2Cl2 are reported. [(Pt(pip2NCN))2(μ-pyz)](CF3SO3)2:  triclinic, P1̄, a = 12.5240(5) Å, b = 14.1570(6) Å, c = 14.2928(6) Å, α = 106.458(1)°, β = 92.527(1)°, γ = 106.880(1)°, V = 2303.46(17) Å3, Z = 2. [(Pt(pip2NCN))2(μ-bpe)](CF3SO3)2·2CH2Cl2:  monoclinic, P21/c, a = 10.1288(6) Å, b = 16.3346(9) Å, c = 17.4764(10) Å, β =
90.882(2)°, V = 2891.1(3) Å3, Z = 2. These structures and solution measurements provide evidence for the
strong trans-directing properties of the pip2NCN- ligand. The electronic structures of these complexes and those
of the 4,4‘-bipyridine (bpy) dimer, (Pt(pip2NCN))2(μ-bpy)2+, also have been investigated by UV−visible absorption
and emission spectroscopies, as well as cyclic voltammetry. The accumulated data indicate that variations in the
bridging ligands provide remarkable control over the electronic structures and photophysics of these complexes.
Notably, the bpa dimer exhibits a broad, low-energy emission from a metal-centered 3LF excited state, whereas
the bpe and bpy dimers exhibit structured emission from a lowest pyridyl-centered 3(π−π*) excited state. In contrast,
the pyz dimer exhibits remarkably intense yellow emission tentatively assigned to a triplet metal-to-ligand charge-transfer excited state.