Download or read book Synthesis of Pincer-type NHC Ligands with Phosphorus Functionalities and Their Ag(I), Cu(I) and Ir(I/III) Organometallic Complexes written by Xianghao Liu and published by . This book was released on 2011 with total page 166 pages. Available in PDF, EPUB and Kindle. Book excerpt: Although the first metal complexes with the cyclometalated PCP pincer ligands (Figure 1a) were reported about 30 years ago, it is still a hot and fast developing topic covering all aspects of chemistry: mechanistic, structural, catalytic, synthetic, theoretical, and coordination studies. Pincer-based metal complexes possess a unique balance of stability and reactivity that can be controlled by systematic ligand modifications and variation of the metal centre.Numerous metal complexes with related pincer-type NHC ligands (Figure 1b) have been reported and well characterized (with e.g. CCC or CNC donor sets). Largely because the NHC ligands have a lesser tendency to dissociate from the metal centre than phosphines, their complexes exhibit advantages over phosphine metal complexes, such as improved air and thermal stability, which are highly desirable in e.g. organometallic chemistry and homogeneous catalysis. The aim of this thesis was to design and synthesize partial/full NHC analogues to the PCP framework, which may result in CCP, CCOP and CCC pincer ligands. Among the transition metals, silver and iridium are at the focus of our work because (i) NHC ligands bound to Ag(I) can generally be transferred to another metal and (ii) Ir complexes are good candidates for the challenging Csp3-H bond activation and hydrogen transfer reactions. Three different types of ligand precursors have been successfully synthesized: 1) bis-imidazolium salts with a methylene bridge connecting the aromatic ring to each of the two imidazole rings; 2) hybrid phosphino-imidazolium salts with a methylene bridge connecting the aromatic ring to the imidazole ring and to the phosphine group; 3) hybrid phosphinito-imidazolium salts where the corresponding oxygen and nitrogen atoms, respectively, are directly connected to the aromatic ring. The corresponding metal carbene complexes were prepared in situ from these imidazolium precursors in the presence of the weak base Cs2CO3 and isolated.The bis-NHC ligandTwo mutually meta positions of the aromatic ring were blocked with methyl groups to avoid possible ortho-C-H activation on the ring so that only pincer-type complexes would form if metalation was to occur on the aromatic ring. The bromides in the imidazolium dibromide salt were replaced with non-coordinating counterions PF6 to suppress their high hygroscopic character.An initial coordination chemistry study was performed by reaction of 1 with Ag2O as a weak base. However, owing to possible H-bonding between the NCHN units and a fluorine atom of PF6, Ag2O was not able to deprotonate the imidazolium to generate the silver NHC complex. However, the corresponding Cu(I) complex was obtained by treating the imidazolium salt with [Cu(NCMe)4]PF6 and Cs2CO3 in MeCN at 60 _C. Surprisingly, a transmetalation reaction between 4 and AgOTF afforded the desired Ag(I) complex 5. This reaction is particularly noteworthy since carbene transmetalation reactions always feature the opposite reaction (from Ag to Cu).The short distances observed by X-ray diffraction for 4 and 5 between the metal and the aromatic rings led us to examine whether these d10 metal centers are better described as T/Y-shaped tri-coordinated or linearly di-coordinated, and the results of detailed DFT calculations suggested the absence of a chemical bond between the Cu(I) or Ag(I) center and parts of the aromatic ring. The proximity observed experimentally between the aromatic carbon and the metals thus results from the combined effects of a chelating ligand structure and the metal preferred coordination geometry.In the cases of iridium complexes, the nature of the metal precursors played an important role. In complex 7, the cod ligand was preferably chelated to the Ir center and the two bridging CNHC^CH^CNHC ligands connect the Ir(I) centres in such a way that each metal is bonded to two mutually cis carbene donors, resulting in a dicationic 20-membered dimetallacycle. While cyclooctene (coe) is more labile than 1,5-cyclooctadiene (cod), it was easily replaced by the solvent molecule MeCN which coordinates to the Ir center through its nitrogen in complex 6. The requirement for shorter reacting times (compared with the reaction using [Ir(cod)(μ-Cl)]2) also showed the higher lability of the complex resulting from a more weakly bound coe ligand compared to cod. Unlike the situation found in the figure-of-eight complex 7, the aromatic ring was metalated, resulting in the formation of an Ir-C and an Ir-H bond. Mixtures of solvents were used to increase the solubility of the ligand (soluble in MeCN) and [Ir(coe)2(μ-Cl)]2 (soluble in THF). It is worth mentioning that MeCN seemed to have stabilized the pincer-type Ir(III) hydride because when bis-imidazolium dibromide, which is soluble in THF, was treated under the same conditions but in the absence of MeCN, only decomposition of the Ir precursor was observed.Hybrid phosphino-NHC ligandIn order to limit possible problems associated with the sensitivity of uncoordinated phosphine groups, we introduced the P-donor moiety in the final step of the preparation. Diethyl ether was used as the solvent in the first step to precipitate the mono-substituted derivative, thus preventing further substitution eventhough 1,3-bis(chloromethyl)benzene was present in large excess. [...].