The Interplay of Pincer Ligands and Metal-Carbon Multiple Bonds

Abstract

This thesis investigates the economic, expediently accessible, and unusual N-heterocyclic carbene (NHC) pincer pro-ligands, 1,8-bis(di-R-phosphinomethyl)-2,3-dihydroperimidine (RH2Pm, R = Ph, Cy). The six-membered, unsaturated NHC donors of the RPm ligands present some unique properties, however installation on metals is currently limited by the need to induce double C-H activation (RH2Pm - RPm). In studying the construction and properties of complexes of RH2Pm in a variety of bonding modes, the field may be opened for a variety of applications. Chapter 2 explores installation methods for the RH2Pm ligands which obviate the need for double C-H activation, such as hydride abstraction ([CPh3]BF4) and deprotonation (nBuLi). The former is limited by coordination of the triylium electrophile to the phosphine donors, while the latter results in controllable, sequential deprotonation of each of the two methylene linker arms, albeit without aiding metal installation. In Chapter 3, the impact of carbonyl and thiocarbonyl co-ligands on the installation of RH2Pm is assessed. With [RhCl(CE)(PPh3)2], the insoluble bimetallic products, [Rh2(RH2Pm)2Cl2(CE)2] (E = O, S; R = Ph, Cy) are generated. Upon heating, these afford the pincer compounds [RhCl(RPm)] (R = Ph, Cy) with inexplicable loss of H2 and CE constituents from the precursors. Reduction of [Rh2(PhH2Pm)2Cl2(CO)2] with excess KC8 results in [Rh2(CO)2(PhH2Pm)2], with a Rh-Rh bond preserving the diamagnetism of the precursor. Oxidation of the highly electron rich [Rh2(CO)2(PhH2Pm)2] induces double C-H activation, resulting in the pincer species [Rh(CO)(PhPm)]+. Chapter 4 evaluates the impact of the metal atom itself on RH2Pm pincer installation. The platinum complexes, cis-[PtCl2(CyH2Pm)] and [Pt(nbe)(RH2Pm)] (R = Ph, Cy, nbe = norbornadiene) decompose in refluxing toluene. Reaction of CyH2Pm and [PdCl2(PPh3)2] creates an equilibrium between cis-[PdCl2(CyH2Pm)], and trans-[Pd2Cl4(CyH2Pm)2]. Heating of these species induces H2 loss, affording [PdCl(CyPm)]Cl, while [NiBr(CyPm)]Br forms rapidly at room temperature. Chapter 5 explores the installation reactions of RH2Pm and other pincers onto complexes containing pre-formed metal-carbon multiple bonds. The PhH2Pm ligand binds to [WBr(CMes)(CO)2(pic)2] (pic = picoline) as a cis-bidentate ligand, from which hydride abstraction of the central aminal group generates [WBr(CTol)(PhHPm)(CO)2]BF4. Deprotonation of resulting perimidinium proton (10.1 ppm) may occur, but the resulting purple product could not be isolated. Reaction of a carbido species, [Rh2Cl2(C)(PPh3)4], with PhH2Pm affords a new carbido complex [Rh2(C)(PhH2Pm)2Cl2] (421.8 ppm), which upon heating generates [RhCl(PhPm)]. Like the bimetallic carbonyl complexes of Chapter 3, the fate of the lost H2 and C components remains obscure. Finally, Chapter 6 utilises salts of the triphenylcyclopropenium ion ([C3Ph3]+) to generate rhodium metallacycles. Reaction with [RhCl(CS)(PPh3)2] affords a novel bicyclic complex, [Rh(C5S2Ph3)X2(PPh3)2], resulting from CS transmetallation from a second rhodium equivalent. This species has no isolobal organic analogue, and several indications of aromatic character. The pincer complex [RhCl(PNPtBu)] undergoes electrophilic aromatic substitution (SEAr) with [C3Ph3]PF6 at the pyridine backbone, being too sterically encumbered to permit direct approach to the metal centre. The new thiocarbonyl salts [Rh(CS)(PNPtBu)]OTf fail to undergo clean cycloaddition reactions with a range of alkynes. The decomposition products, fac and mer-[RhCl3(PhPm] demonstrate the first example of such coordinative flexibility by the RPm ligands. New metallacyclobutadienes, [RhCl(C3Ph3)(PhPm)]PF6, are generated, however treatment with alkynes affords cyclopropenylvinyl complexes [RhCl{CR2=C(CO2Me)C3Ph3}(RPm)]PF6 (R = Ph: R2 = H, CO2Me; R = Cy: R2 = H), suggesting an equilibrium of the metallacyclobutadiene and a reconstituted sigma-cyclopropenyl complex. Show more