Javier Borau-Garcia

Activation of Water, Ammonia, and Other Small Molecules by PCcarbeneP Nickel Pincer Complexes

Nickel complexes of a PC(carbene)P pincer ligand framework are described. Dehydrobromination of the precursor (PC(sp)(3)P)NiBr in the presence of a donor (PPh3 or NC(t)Bu) leads to the title complexes, which feature a rare nickel-carbene linkage as the pincer ligand anchor point. This strongly donating, nucleophilic carbene center engages in a variety of E-H bond activations (E = H, C, N, O), some of which are reversible. This represents a new mode of bond activation by ligand cooperativity in nickel pincer complexes.

Ligand Cooperation in the Formal Hydrogenation of N2O Using a PCsp2P Iridium Pincer Complex

A new PC(carbene)P pincer ligand with 2,3-benzo[b]thiophene linkers connecting the flanking dialkyl phosphine donors to the central carbene can be attached to Ir(I). The chloro derivative reacts with N2O with loss of N2 to form an iridaepoxide species by addition of an oxygen atom to the Ir═C linkage. This compound reacts with H2 to afford the oxidative addition product, in which the hydride ligands are trans to the Ir-O bond. Heating this dihydride results in slow release of H2O; kinetic and spectroscopic studies show that conversion of the dihydride to its isomer, in which the hydrides are cis to the Ir-O bond, is required for H2O elimination to take place. Together, these reactions constitute the stoichiometric conversion of N2O and H2 to N2 and H2O; further mechanistic studies suggest ways to make the system catalytic.

The influence of electron delocalization upon the stability and structure of potential N-heterocyclic carbene precursors with 1,3-diaryl-imidazolidine-4,5-dione skeletons

Targeting N-heterocyclic carbenes (NHCs) with increased π-acceptor character featuring N-fluorophenyl substituents, the molecular 2-chloro-1,3-bis(fluorophenyl)imidazolidine-4,5-diones (1a–c) were isolated from the condensation of the corresponding formamidine with oxalyl chloride. These formal adducts of NHCs with hydrogen chloride demonstrated reactivity akin to that of alkyl halides: 1,3,1′,3′-tetrakis(2,6-dimethylphenyl)-[2,2′]diimidazolidinyl-4,5,4′,5′-tetraone (2b) was formed via the reductive coupling of 1b, while 1,3-bis(2,6-diisopropylphenyl)-4,5-dioxoimidazolidin-2-yl acetate (3c) was formed as the result of a metathesis reaction with mercury(II) acetate. Chloride abstraction resulted in the formation of imidazolium-4,5-dione salts (4a–c) that decomposed rapidly, except in the case of the kinetically-stabilized 1,3-bis(2,6-diisopropylphenyl)imidazolium-4,5-dione hexafluorophosphate 4c. All imidazolium-4,5-dione hexafluorophosphate salts decomposed to neutral 2-fluoro-1,3-bis(aryl)imidazolidine-4,5-diones (5a–c) via fluoride abstraction. 2-Methoxy-1,3-di(aryl)imidazolidine-4,5-diones (6a–c) were also prepared and they failed to undergo thermolysis and yield the free NHCs. Computational analyses revealed that the instability of NHCs with an oxalamide skeleton, as well as that of imidazolium-4,5-diones, results from a π-framework which extends over both carbonyl moieties and gives rise to a very low energy LUMO, rendering the compounds in question highly electrophilic.

Synthesis and Tunability of Highly Electron-Accepting, N-Benzylated “Phosphaviologens”

We report a structure-property study on phosphoryl-bridged viologen analogues with a remarkably low reduction threshold. Utilizing different benzyl groups for N-quaternization, we were able to confirm the p-benzyl substituent effect on the electronic tunability of the system while maintaining the characteristic chromic response of viologens with two fully reversible one-electron reductions. Due to the considerably increased electron-acceptor properties of the phosphoryl-bridged bipyridine precursor, N-benzylation was found to be very challenging and required the development of new synthetic strategies toward the target viologen species. This study also introduces a new and convenient way for the anion exchange of viologen systems by utilizing methyl triflate. Finally, the practical utility of the new species was verified in simplified proof-of-concept electrochromic devices.