• cluster
• DFT
• electron transfer
• TDDFT

Phosphido (PR2-) ligands bind terminally to an electron rich transition element only in a very few special cases, and quite commonly prefer to bridge two such metals. Therefore, the structural unit M2(mu-PR2) (M= late transition metal) is often employed to stabilize polymetallic and/or cluster structures toward fragmentation reactions. For example, an entire family of 44 e- triangular clusters of platinum sharing the general formulas Pt3(mu -PBu2t)3(CO)2X or [Pt3(mu -PR2)3(L)3]+X- has been described in the past decades. The remarkable stability of the Pt3(mu -PR2)3 core and the structural flexibility of the M2(mu -PR2) bridges allowed also the uncommon observation of the skeletal bond-stretch isomers, i.e. isomers differing only for the length of one metal-metal bond. In our research group some clusters of this family are being used as building blocks to prepare large molecular assemblies in which two or more cluster units are connected by covalently linked organic spacers.
This PhD thesis is a part of this program and its main targets were to prepare suitable cluster precursors and to start to investigate, both experimentally and theoretically, electron-transfer processes occurring at the cluster-spacer covalent junction (generally a sigma Pt-C≡C- bond).
The neutral or cationic derivatives Pt3(mu-PBu2t)3(CO)2X [1, X = H; 4, X = Cl; 5, X = I; 6, X = CCPh] and [Pt3(mu-PBu2t)3(CO)3]CF3SO3 (2+, CF3SO3-) have been prepared and characterized spectroscopically and, often, by single-crystal X-ray diffraction (1, 2+CF3SO-3 , 4 and 6).
In order to gain useful information about the effect of terminal ligands on the bond-stretch isomerism, observed also in these complexes, an exhaustive theoretical analysis was carried out, based on DFT method as implemented in the package Gaussian03. For each of the species full geometry optimizations were attempted in vacuum and in CH2Cl2 (using the CPCM model) and starting from the X-ray structure, when available. The calculations were performed both on simplified models, using hydrogen atoms instead of tert-butyls, and on the real systems. At the best of our knowledge this is the first time that DFT calculation have been performed on triangular platinum derivatives without any approximation.
The electrochemistry of all compounds has been investigated by cyclic voltammetry, revealing the presence of a rich and interesting redox behaviour, with two one-electron oxidation processes centred on the cluster unit. All the oxidized forms have been also investigated by DFT methods in order to elucidate which part of the molecule is involved in the redox process.
The Sonogashira-type reaction between 4 and ethynylferrocene, in the presence of catalytic amounts of Cu(I) salts, afforded Pt3(mu-PBu2t)3(CO)2CCFc, 7, which was investigated by cyclic voltammetry and spectroelectrochemistry; the latter showed the presence of broad bands in the NIR region growing after electrochemical oxidation. The origin of the NIR bands was investigated by Time Dependent DFT techniques on 7 and 7+ both as isolated molecules in vapour phase and in dichloromethane solution, to have a direct comparison with the experimental measurements. The effect on the calculated spectrum of the substituents on phosphorus atoms was also investigated.
Encouraged by the good results obtained by the experimental and theoretical analysis of compounds containing a single cluster unit, we synthesized two new derivatives: {Pt3}CCFcCC{Pt3}, 8 and {Pt3}CCFcFcCC{Pt3}, 9, containing two cluster moieties connected by a bis(ethynyl)ferrocene and a bis(ethynyl)biferrocene spacer, respectively. The X-ray structure of compound 9 showed a cisoidal displacement of the platinum clusters with respect to the biferrocene spacer. NMR DOSY spectroscopy were suggested that the solid state configuration is maintained also in solution.
The electrochemical study of 8 and 9 showed a redox behaviour richer than in 7. The spectroelectrochemistry of 9 revealed the growth of a NIR band during the first and the second oxidation. The removal of a further electron caused a former red shift followed by the disappearance of the band. TDDFT calculations were performed for the neutral and the monoxidized compound to enlighten the nature of NIR band.
Finally, the triiodo- 42 e- system Pt3(mu-PBu2t)3I3, 10, was prepared by the oxidation with iodine of the 44 e-{Pt3}I, 5, with iodine. At the best of our knowledge 10 is the first trinuclear platinum cluster derivative containing bridging phosphides stable with only with 42 valence electrons. Due to the decreased of valence electrons in 10 compared to 5, the cyclic voltammetry reveals a different redox behaviour with two successive one-electron reductions. Also for compound 10 the X-ray structure was available and starting from it we have performed DFT calculations on the 10 and on the reduced forms, 10- and 102-.