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

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journal contribution
posted on 03.05.2010, 00:00 by Jesús R. Berenguer, Julio Fernández, Belén Gil, Elena Lalinde, Sergio Sánchez
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−Cα(acetylide) bond (Pt−Cd = 2.7, Cd−Cα ∼ 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−Cα bonds (Pt−Cd ∼ 2.85; Cd−Cα 2.470(10)−2.551(5) Å) of different PtII 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 25, 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.