Rhomboidal Heterometallic Alkynyl Based Pt₂Cd₂ Clusters: Structural, Photophysical, and Theoretical Studies

Reactions between [Pt(CCR)₄]²⁻ (R = Tol a, C₆H₄OMe-4 b, C₆H₄OMe-3 c) and Cd²⁺ depend on the media and the alkynyl substituent, leading to the formation of yellow tetranuclear solvate complexes [Pt(CCR)₄Cd(acetone)]₂ 1a,b(acetone)₂ and [Pt(CCC₆H₄OMe-3)₄Cd(dmso)]₂ 1c(dmso)₂ or white polymeric solvate-free species [Pt(CCR)₄Cd]_x 1′a−c. Treatment of 1a,b(acetone)₂ or 1′a−c with N-donor ligands affords a series of tetranuclear clusters [Pt(CCR)₄CdL]₂ (L = py; 2a−c. R = Tol; L = NC₅H₄CH₃-4 3, NC₅H₄CF₃-4 4, pzH 5). X-ray crystallographic studies reveal that, in the tolyl complexes (2a, 4, and 5), the Cd-L²⁺ unit is closely bonded to one Pt−C_α(acetylide) bond (Pt−Cd = 2.7, Cd−C_α ∼ 2.48 Å), and the resulting “Pt(CCTol)₄CdL” unit dimerizes by two additional η²-Cd-acetylide and a weaker Pt···Cd bonding interaction leading to a planar unsymmetrical rhomboidal metal core. By contrast, the m-methoxyphenyl derivatives (2c, 1c(dmso)₂) form symmetrical Pt₂Cd₂ cores, with each Cd bonded (coordination number, C.N. = 5) to the incoming ligand (pyridine 2c, dmso 1c(dmso)₂) and four Pt−C_α bonds (Pt−Cd ∼ 2.85; Cd−C_α 2.470(10)−2.551(5) Å) of different Pt^II fragments. Evidence of ligand dissociation was found for the solvate (1a,b(acetone)₂, 1c(dmso)₂) and NC₅H₄CF₃-4 (4) derivatives by NMR and UV−vis absorption spectra. All tetranuclear aggregates exhibit bright blue to green luminescence in the solid state. Time-dependent density functional theory (TD-DFT) calculations were performed to shed light on the nature of the electronic transitions. In the solvate 1a,b(acetone)₂ and 1c(dmso)₂, emissions have been assigned to a platinum-alkynyl to cadmium charge transfer (³MLM′CT), mixed with some intraligand ³IL(CCR) character. In the imine derivatives 2−5, they are suggested to come from an excited state of large Pt(d)/πCCR→π*(imine) MLL′CT character, mixed with some Pt(d)/π(CCR)→Pt₂Cd₂/π*CCR (ML′M′CT) contribution.