We report the extensive investigation of Li and H dynamics in Li 6 C 60 and Li 6 C 60 H y , by combining 7 Li and 1 H solid state NMR measurements with DC/AC conductivity, in order to evaluate the potential application of these systems for energy-storage purposes. 7 Li NMR results show a local motion of Li ions above 200 K in both pristine and hydrogenated compounds, with activation energies of 90-150 meV and correlation times of about 30 ps. Evidences of Li interdiffusive dynamics are given by conductivity measurements in Li 6 C 60 already above 120 K, with activation energies of 240 meV, suggesting that ionic conductivity is of the order of 10 −5 S•cm −1 at room temperature, with correlation times of about 150 ps. On the other hand, the Li 6 C 60 H y behaves like a semiconductor with a high energy gap (ca. 2.5 eV), suggesting that diffusion of intercalated Li ions is prevented. 1 H NMR measurements indicate the absence of H motions for the whole temperature range investigated (up to 360 K), neither on macroscopic or local scale. Li 6 C 60 good properties for H 2-storage are confirmed in terms of absorption capacity (5 wt% H 2), moreover we found that around 35% of lithium segregates in LiH form, leaving Li 4 C 60 H 40 as the final hydrogenation product.
Rollover" cyclometalation is a particular case of metal-mediated C−H bond activation, and the resulting complexes constitute an emerging class of cyclometalated compounds. In the case of 2,2′-bipyridine "rollover cyclometalation" has been used to synthesize the complexes [Pt(bipy-H)(Me)(L)] (L = PPh 3 , PCy 3 , P(OPh) 3 , P(p-tolyl) 3 ), whose protonation produces a series of stable corresponding pyridylenes [Pt(bipy*)(Me)(L)] + . The unusual bipy* ligand may be described as an abnormalremote heterocyclic chelated carbene or simply as a mesoionic cyclometalated ligand. These cationic species spontaneously convert in solution, through a retro-rollover reaction, to the corresponding isomers [Pt(bipy)(Me)(L)] + , where the 2,2′-bipyridine is coordinated in the classical N,N bidentate mode. Isomerization is achieved at different rates (ranging over three orders of magnitude), depending on the nature of the phosphane ligand, the most basic (PCy 3 ) providing the fastest reaction. The mesoionic species [Pt(bipy*)(Me)(L)] + contain two Pt−C bonds: the balance between the Pt−C(sp 2 ) and Pt−C(sp 3 ) bond rupture is subtle, and competition is observed according to the reaction conditions. In the presence of an external neutral ligand L′ methane is released to give the cationic derivatives [Pt(bipy-H)(L)(L′)] + , whereas reaction of the neutral [Pt(bipy-H)(Me)(L)] with HCl may follow different routes depending on the nature of the neutral ligand L. Assuming all reactions take place through the formation of a hydride intermediate, quantum chemical calculations show that computed energy barriers are qualitatively consistent with observed reaction rates.
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.
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.
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