RESUMEN

Rollover cyclometalated complexes constitute a family of derivatives which differ from classical cyclometalated species in certain aspects. Various potential application fields have been developed for both classes of compounds, which have both similarities and differences. In order to uncover the relationships and distinctions between these two families of compounds, four Pt(II) cyclometalated complexes derived from 2-phenylpyridine (ppy) and 2,2'-bipyridine (bpy), assumed as prototypical ligands, were compared. For this study, an electron rich isostructural and isoelectronic pair of compounds, [Pt(N^C)Me(PPh3)], and an electron-poorer compound, [Pt(N^C)Cl(PPh3)] were chosen (N^C = ppy or bpy). DFT calculations, cyclic voltammetry, and UV-Vis spectra also helped to shed light into these species. Due to the presence of the more electronegative nitrogen in place of a C-H group, the rollover bpy-H ligand becomes a slightly weaker donor than the classical ppy-H ligand, and hence, generates (slightly) more stable cyclometalated complexes, lower energy frontier molecular orbitals, and electron-poorer platinum centers. On the whole, it was revealed that classical and rollover complexes have overall structural similarity, which contrasts to their somewhat different chemical behavior.

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RESUMEN

A new bipyridine ligand, 6-(1-phenylbenzyl)-2,2'-bipyridine, has been prepared by a multistep synthesis starting from the corresponding substituted pyridine. The coordinating properties of the new ligand have been tested with two d(8) metal ions, Pt(ii) and Pd(ii), to give the cyclometalated complexes [Pt(N,N,C)Cl] and [Pd(N,N,C)Cl], where N,N,C is a terdentate deprotonated bipyridine containing a new stereogenic carbon atom directly generated by C-H bond activation. The single-crystal of the platinum complex has been solved by X-ray diffraction. DFT calculations confirm the presence of a PtH interaction that stabilizes one of the two possible conformers by 14.7 kJ mol(-1) for Pt and 12.9 kJ mol(-1) for Pd. The energy barrier to pass from one conformer to the other is 25.4 and 23.8 kJ mol(-1) respectively. Under different reaction conditions, regioselective activation of a pyridine C-H bond gave the less common cyclometalated rollover complex [Pt(L-H)Me(DMSO)], which was isolated and characterised.

RESUMEN

The reaction of the cyclometalated rollover complex [Pt(bpy-H)(Me)(DMSO)] (bpy-H = cyclometalated 2,2'-bipyridine) with two diphosphines, dppm (1,1-bis(diphenylphosphino)methane) and dppe (1,2-bis(diphenylphosphino)ethane), was investigated. According to the reaction conditions, dppm behaves as a monodentate, bridging or chelated ligand, whereas dppe gave only chelated species. Some aspects of the reactivity of the isolated species were studied, including protonation with [H3O·18-crown-6][BF4] and coordination reactions of mononuclear complexes, obtaining, inter alia, rare examples of unsymmetrical organometallic species with bridging dppm.

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RESUMEN

Rollover cyclometalation involves bidentate heterocyclic donors, unusually acting as cyclometalated ligands. The resulting products, possessing a free donor atom, react differently from the classical cyclometalated complexes. Taking advantage of a "rollover"/"retro-rollover" reaction sequence, a succession of oxidative addition and reductive elimination in a series of platinum(II) complexes [Pt(N,C)(Me)(PR3)] resulted in a rare C(sp(2))-C(sp(3)) bond formation to give the bidentate nitrogen ligands 3-methyl-2,2'-bipyridine, 3,6-dimethyl-2,2'-bipyridine, and 3-methyl-2-(2'-pyridyl)-quinoline, which were isolated and characterized. The nature of the phosphane PR3 is essential to the outcome of the reaction. This route constitutes a new method for the activation and functionalization of C-H bond in the C(3) position of bidentate heterocyclic compounds, a position usually difficult to functionalize.

RESUMEN

Rollover cyclometalation of 2-(2'-pyridyl)quinoline, L, allowed the synthesis of the family of complexes [Pt(L-H)(X)(L')] and [Pt(L*)(X)(L')][BF4] (X = Me, Cl; L' = neutral ligand), the former being the first examples of Pt(II) rollover complexes derived from the ligand L. The ligand L* is a C,N cyclometalated, N-protonated isomer of L, and can also be described as an abnormal-remote pyridylene. The corresponding [Pt(L-H)(Me)(L')]/[Pt(L*)(Me)(L')](+) complexes constitute an uncommon Brønsted-Lowry acid-base conjugated couple. The species obtained were investigated in depth through NMR and UV-vis spectroscopy, cyclic voltammetry, and density functional theory (DFT) methods to correlate different chemico-physical properties with the nature of the cyclometalated ligand (e.g., L vs bipy or L* vs L) and of the neutral ligand (DMSO, CO, PPh3). The crystal structures of [Pt(L-H)(Me)(PPh3)], [Pt(L-H)(Me)(CO)] and [Pt(L*)(Me)(CO)][BF4] were determined by X-ray powder diffraction methods, the latter being the first structure of a Pt(II)-based, protonated, rollover complex to be unraveled. The isomerization of [Pt(L*)(Me)(PPh3)](+) in solution proceeds through a retro-rollover process to give the corresponding adduct [Pt(L)(Me)(PPh3)](+), where L acts as a classical N,N chelating ligand. Notably, the retro-rollover reaction is the first process, among the plethora of Pt-C bond protonolysis reactions reported in the literature, where a Pt-C(heteroaryl) bond is cleaved rather than a Pt-C(alkyl) one.

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