Accessing Intermediate and High Oxidation States with Tungsten and Iridium Pincer Complexes

The silazane PNP ligand [HN(SiMe2CH2PPh2) 2] has been coordinated to tungsten(0) in three different conformations and these three complexes can be interconverted. The PHNP ligand can be bound as a bidentate ligand through the two phosphine arms as in W(PHNP)(CO)4, as a tridentate facial ligand in W(PHNP)(CO)3, or as a tridentate meridional ligand in W(PHNP)(CO)3. Formal oxidation by protonation of the metal center occurred to form a cationic tungsten hydride species. A related series of cationic tungsten(II) halide complexes was synthesized, [W(PHNP)(CO)3X]+ (X= I, Br, Cl, F) by various routes. All of the tungsten(II) complexes underwent deprotonation at the amine site of the PHNP ligand when triethylamine was added, resulting in neutral seven-coordinate complexes.;A non-heterocyclic bis(imino)aryl ligand with blocking methyl substituents, 4,6-dimethyl-1,3-benzenediphenylimine (NCHN), has been synthesized. Reaction with [Ir(CH2=CH2)2(Cl)]2 under mild conditions led to (NCN)Ir(CH2CH3)(Cl) via C-H activation at the central aryl position of the NCN ligand. This five-coordinate complex proved to be a versatile starting material for modification of the three remaining coordination sites. Neutral nucleophiles including water and triphenylphosphine could be readily added to the vacant sixth coordination site. Protonation of the ethyl group resulted in loss of ethane and formation of a dicationic chloride-bridged (NCN)Ir dimer. Alternatively, the chloride ligand could be abstracted from (NCN)Ir(CH2CH3)(Cl) to access various neutral and cationic species.;Oxidation of (NCN)Ir(CH2CH3)(Cl) to iridium(V) intermediates occurs by electrophilic addition and oxidative addition pathways. Addition of iodine appears to follow an electrophilic addition pathway, resulting in a cationic Ir(V) intermediate, [(NCN)Ir(CH2CH3)(Cl)(I)][I]. The nucleophilic iodide counterion attacks the ethyl group, resulting in ethyl iodide and a mixed halide species. The mixed halide complex undergoes halide exchange to produce a diiodide complex and a dichloride complex. Oxidative addition of a Si-H bond to (NCN)Ir(CH2CH3)(Cl) occurs with triethylsilane, resulting in a seven-coordinate Ir(V) intermediate observed by 1H NMR. Reductive elimination of ethane is fast from the Ir(V) complex and produces an Ir-silyl complex.;Oxygen atom transfer reagents have been employed to try to access the elusive Ir(V) oxo intermediate proposed in the mechanism of catalytic water oxidation by iridium. Reaction of pyridine-N-oxide, sPhIO, and MCPBA with (NCN)Ir(CH2CH3)(Cl) resulted in adduct formation. However, reaction of MCPBA with bis-aqua complex [(NCN)Ir(CH 2CH3)(OH2)2][BF4] resulted in formation of ethylene from the Ir-ethyl group, through a possible Ir(V) oxo species. [(NCN)Ir(CH2CH3)(OH2) 2][BF4] also reacted rapidly with NaIO4 to produce a short-lived intermediate accompanied by formation of ethylene gas, possible through a similar mechanism as the MCPBA reaction. Electrochemistry of the water-soluble [(NCN)Ir(CH2CH3)(OH2) 2][BF4] showed that it was capable of catalytic water oxidation. Electrochemical studies on heterogeneous water oxidation by an iridium pincer complex bound to a metal oxide surface through phosphonate linkages were also conducted, demonstrating catalytic water oxidation.