Synthesis and reactivity of heterometallic RhO–M (M=Si, Ti) complexes

Offsets between hyperconjugations, p→d donations and Pauli repulsions impact the bonding of E-O-E systems. Case study on elements of Group 14

The nature of the E-O chemical bond (E = C, Si, Ge, Sn) is investigated in a wide range of model derivatives, such as oxonium cations, hydrogenated/methylated/fluorinated/chlorinated ethers and acyclic oligomers incorporating the E-O-E moiety. By means of density functional theory (DFT) calculations and natural bond orbital (NBO) techniques, we propose a bonding mechanism that explains the structural contrast between the organic and the inorganic counterparts of all these derivatives: the interplay between stabilizing interactions like LP(O)→σ*(E-X) hyperconjugations and LP(O)→d(E) donations with LP(O)⋯σ(E-X) vicinal Pauli repulsions (X = H, C, O, F, Cl) dictates the equilibrium structures in terms of E-O-E angles and E-O bond lengths. In addition, the present work represents the first study of oxonium ions that describes the structural discrepancies among organic derivatives and their heavier analogues. Another novel outcome for ethers and oligomers is that the two non-equivalent lone pair electrons (LPs) at the oxygen atoms impact in different manners the geometries of such derivatives, i.e. the s/p LP is correlated with the bending behaviour of the E-O-E units, while the pure p LP mainly dictates the short E-O bond distances of inorganic derivatives. Lastly, we evaluate the impact of the number of electronegative substituents, e.g. F, Cl or OEH₃ groups, on the bond patterns developed for hydrogenated or methylated ethers.

Multinuclear rhodium complexes supported by tetra-tert-butoxy disiloxide ligand: Synthesis, structure, and reactivity

Rhodium siloxide complexes have been used as homogeneous catalysts and model compounds for silica-supported metal catalysts. However, rhodium complexes with multidentate siloxide ligands have rarely been prepared. In this study,...

Titanium thiosalicylate complexes: functional metalloligands for the construction of redox-active heterometallic architectures

The titanium complex [TiCp*(thiosal)(thiosalH)] (1) has been synthesised by reaction of [TiCp*Me₃], Cp* = η⁵-C₅Me₅, with thiosalicylic acid (H₂thiosal). Complex 1 reacts with [M(μ-OH)(COD)]₂ (M = Rh, Ir) to yield the corresponding early-late heterobimetallic complexes [TiCp*(thiosal)₂M(COD)] (M = Rh (2); Ir (3)). Carbon monoxide replaces the COD ligand in 2 and 3 leading to the respective dicarbonyl complexes [TiCp*(thiosal)₂M(CO)₂] (M = Rh (4); Ir (5)). Compound 4 reacts with PPh₃ to yield the monocarbonyl derivative [TiCp*(thiosal)₂Rh(CO)(PPh₃)] (6). The reaction of compound 1 with LiⁿBu yields the tetrametallic complex [{TiCp*(thiosal)₂Li}₂(THF)₃(H₂O)] (7). Compound 7 reacts with [RuCp*Cl(COD)] yielding the heterometallic complex [TiCp*(thiosal)₂RuCp*] (8). The molecular structures of compounds 4, 5 and 7 have been studied by X-ray diffraction. From cyclic voltammetric (CV) and square wave voltammetric (SWV) experiments, we observed that attachment of the titanium moiety of precursor 1 to a late transition metal moiety through the sulfur atoms has a significant influence on the reduction behaviour of the Ti(iv) metal centre. Thus, monometallic 1 exhibits an irreversible reduction process at -1.15 V vs. SCE, whereas the CVs of heterobimetallic compounds 2-6 are characterized by the reversible or quasi-reversible one-electron reduction of the Ti(iv)/Ti(iii) system, suggesting a significant stabilization of the Ti(iii) reduced species. Likewise, substitution of the M(COD) diolefin fragment in 2 and 3 by the M(CO)₂ carbonyl-containing moiety (in compounds 4 and 5) leads to a significant anodic shift in the titanium E_1/2 reduction redox potentials.