An Easy Conversion from Platinum(II) Reagents to Platinum(IV) Products: κ³ to κ² Coordination Mode Interconversion, Phenyl Migration, and Ortho C–H Activation Cascade in a Hemilabile “Click”-Triazole Scorpionate Platinum System

A series of platinum phenyl olefin complexes has been prepared that bear hemilabile “Click”-triazole based scorpionate ligands (Tt^R). Mild heating of the olefin adducts initiates a reaction sequence to form stable, cationic Pt­(IV) hydride metallacycles. An attractive mechanism involves κ³/κ² conversion, phenyl migration, and ortho C–H activation. Activation parameters for the overall C–C bond-forming and C–H bond-breaking process were obtained. Insertion products from mono- and disubstituted olefins reveal a kinetic preference for phenyl migration to the less sterically hindered olefin position but a thermodynamic preference for the β-substituted metallacycle isomer, in which steric bulk is further away from the metal center and the Tt^R ligand. Thermolysis converts the kinetically favored products to their thermodynamically stable isomers via a reversible C–C bond cleavage and formation reaction. EXSY NMR and deuterium-labeling studies reveal facile scrambling processes in the metallacycles due to the hemilabile ligand

Regioselectivity of Addition to the Azavinylidene Ligand in Tp′W(CO)(η²‑HCCH)(NCHMe): Electrophilic Addition versus Oxidation and Radical Coupling

The Tp′W­(CO)­(HCCH)­(NCHMe) (Tp′ = hydridotris­(3,5-dimethylpyrazolyl)­borate) molecule contains an electron-rich 1-azavinylidene ligand where the preferred site for electrophile addition is the α-nitrogen. As is observed in other cases for electrophile addition to a 1-azavinylidene with a <i>cis</i>-alkyne ligand, this fragment will add small electrophiles such as H⁺, Me⁺, and Et⁺ to the α-nitrogen lone pair, maintaining a W^II metal center and forming the coordinated imine products [Tp′W­(CO)­(HCCH)­(NHCHMe)]­[BF₄], [Tp′W­(CO)­(HCCH)­(N­(Me)CHMe)]­[OTf], and [Tp′W­(CO)­(HCCH)­(N­(Et)CHMe)]­[OTf]. In contrast, when a one-electron oxidant is employed, net electrophile addition at the β-carbon can be achieved. Changing the nature of the electrophile to [CPh₃]­[BF₄] leads to a rapid one-electron oxidation and selective radical combination to form the imido product [Tp′W­(CO)­(HCCH)­(NCH­(CH₃)­CPh₃)]­[BF₄]. This net addition of an electrophile at the 1-azavinylidene β-carbon concomitantly oxidizes the metal center to W^IV and raises the CO stretching frequency to 2109 cm^–1 versus 1946 cm^–1 for the W^II imine complex. This reactivity is also seen with [CPh₂(C₆H₄OMe)]­[BF₄]. This variable reactivity demonstrates the flexibility of the 1-azavinylidene ligand when it is paired with an adjacent alkyne ligand

C–H and C–C Bond Formation Promoted by Facile κ³/κ² Interconversions in a Hemilabile “Click”-Triazole Scorpionate Platinum System

A series of platinum complexes bearing 3-fold symmetrical “Click”-triazole-based scorpionate ligands (Tt^R) has been prepared. Metalation of these weakly donating ligands results in neutral Pt­(II) complexes that display a κ² coordination mode. Such complexes are susceptible to oxidative addition using a variety of electrophilic alkyl and allyl reagents to generate isolable cationic κ³ Pt­(IV) complexes. Protonation of the κ² precursors results in Pt­(IV) dimethyl hydride cations. Thermolysis of the dimethyl hydride species at 35 °C in the presence of a trapping π-acid ligand initiates reductive elimination of methane and formation of a Pt­(II) species of the type [Tt^PhPtMe­(L)]­[BF₄] (L  CO, ethylene, propylene, <i>cis</i>-2-butene, <i>trans</i>-2-butene, isobutylene) in good yield. Furthermore, the κ³ σ-allyl complexes [Tt^RPt­(Ph)₂(CH₂CHCH₂)]­[I] cleanly undergo C_sp2–C_sp2 reductive coupling to form biphenyl at ambient temperatures. The Tt^R ligand serves as a homofunctional hemilabile ligand and exhibits a lower barrier κ³/κ² interconversion to generate reactive unsaturated five-coordinate complexes than the well-studied Tp′PtMe₂H complex, which requires thermolysis at temperatures above 100 °C

Regioselectivity of Addition to the Azavinylidene Ligand in Tp′W(CO)(η²‑HCCH)(NCHMe): Electrophilic Addition versus Oxidation and Radical Coupling

The Tp′W­(CO)­(HCCH)­(NCHMe) (Tp′ = hydridotris­(3,5-dimethylpyrazolyl)­borate) molecule contains an electron-rich 1-azavinylidene ligand where the preferred site for electrophile addition is the α-nitrogen. As is observed in other cases for electrophile addition to a 1-azavinylidene with a <i>cis</i>-alkyne ligand, this fragment will add small electrophiles such as H⁺, Me⁺, and Et⁺ to the α-nitrogen lone pair, maintaining a W^II metal center and forming the coordinated imine products [Tp′W­(CO)­(HCCH)­(NHCHMe)]­[BF₄], [Tp′W­(CO)­(HCCH)­(N­(Me)CHMe)]­[OTf], and [Tp′W­(CO)­(HCCH)­(N­(Et)CHMe)]­[OTf]. In contrast, when a one-electron oxidant is employed, net electrophile addition at the β-carbon can be achieved. Changing the nature of the electrophile to [CPh₃]­[BF₄] leads to a rapid one-electron oxidation and selective radical combination to form the imido product [Tp′W­(CO)­(HCCH)­(NCH­(CH₃)­CPh₃)]­[BF₄]. This net addition of an electrophile at the 1-azavinylidene β-carbon concomitantly oxidizes the metal center to W^IV and raises the CO stretching frequency to 2109 cm^–1 versus 1946 cm^–1 for the W^II imine complex. This reactivity is also seen with [CPh₂(C₆H₄OMe)]­[BF₄]. This variable reactivity demonstrates the flexibility of the 1-azavinylidene ligand when it is paired with an adjacent alkyne ligand

Regioselectivity of Addition to the Azavinylidene Ligand in Tp′W(CO)(η²‑HCCH)(NCHMe): Electrophilic Addition versus Oxidation and Radical Coupling

The Tp′W­(CO)­(HCCH)­(NCHMe) (Tp′ = hydridotris­(3,5-dimethylpyrazolyl)­borate) molecule contains an electron-rich 1-azavinylidene ligand where the preferred site for electrophile addition is the α-nitrogen. As is observed in other cases for electrophile addition to a 1-azavinylidene with a <i>cis</i>-alkyne ligand, this fragment will add small electrophiles such as H⁺, Me⁺, and Et⁺ to the α-nitrogen lone pair, maintaining a W^II metal center and forming the coordinated imine products [Tp′W­(CO)­(HCCH)­(NHCHMe)]­[BF₄], [Tp′W­(CO)­(HCCH)­(N­(Me)CHMe)]­[OTf], and [Tp′W­(CO)­(HCCH)­(N­(Et)CHMe)]­[OTf]. In contrast, when a one-electron oxidant is employed, net electrophile addition at the β-carbon can be achieved. Changing the nature of the electrophile to [CPh₃]­[BF₄] leads to a rapid one-electron oxidation and selective radical combination to form the imido product [Tp′W­(CO)­(HCCH)­(NCH­(CH₃)­CPh₃)]­[BF₄]. This net addition of an electrophile at the 1-azavinylidene β-carbon concomitantly oxidizes the metal center to W^IV and raises the CO stretching frequency to 2109 cm^–1 versus 1946 cm^–1 for the W^II imine complex. This reactivity is also seen with [CPh₂(C₆H₄OMe)]­[BF₄]. This variable reactivity demonstrates the flexibility of the 1-azavinylidene ligand when it is paired with an adjacent alkyne ligand

Sequential Nitrene Transfers to an Organometallic Half-Sandwich Iridium Complex

Nitrene transfer to an iridium­(Cp*) complex with a pyridyl-amide bidentate ligand drives an unexpected outer-sphere C–H activation and amide functionalization reaction mediated by this metal center. This is a significant departure from C–N bond-forming processes in mononuclear rhodium and iridium catalysts, which require C–H activation at the metal center prior to nitrene transfer and insertion (an inner-sphere C–H insertion pathway). The mechanism likely involves a high oxidation state iridium-imido reactive species capable of hydrogen atom abstraction and a radical-based rearrangement during the formation of the ultimate iridium product

Oxygen Atom Transfer to a Half-Sandwich Iridium Complex: Clean Oxidation Yielding a Molecular Product

The oxidation of [Ir­(Cp*)­(phpy)­(NCAr^F)]­[B­(Ar^F)₄] (<b>1</b>; Cp* = η⁵-pentamethylcyclopentadienyl, phpy = 2-phenylene-κ<i>C</i>^1′-pyridine-κ<i>N</i>, NCAr^F = 3,5-bis­(trifluoromethyl)­benzonitrile, B­(Ar^F)₄ = tetrakis­[3,5-bis­(trifluoromethyl)­phenyl]­borate) with the oxygen atom transfer (OAT) reagent 2-<i>tert-</i>butylsulfonyliodosobenzene (sPhIO) yielded a single, molecular product at −40 °C. New Ir­(Cp*) complexes with bidentate ligands derived by oxidation of phpy were synthesized to model possible products resulting from oxygen atom insertion into the iridium–carbon and/or iridium–nitrogen bonds of phpy. These new ligands were either cleaved from iridium by water or formed unreactive, phenoxide-bridged iridium dimers. The reactivity of these molecules suggested possible decomposition pathways of Ir­(Cp*)-based water oxidation catalysts with bidentate ligands that are susceptible to oxidation. Monitoring the [Ir­(Cp*)­(phpy)­(NCAr^F)]⁺ oxidation reaction by low-temperature NMR techniques revealed that the reaction involved two separate OAT events. An intermediate was detected, synthesized independently with trapping ligands, and characterized. The first oxidation step involves direct attack of the sPhIO oxidant on the carbon of the coordinated nitrile ligand. Oxygen atom transfer to carbon, followed by insertion into the iridium–carbon bond of phpy, formed a coordinated organic amide. A second oxygen atom transfer generated an unidentified iridium species (the “oxidized complex”). In the presence of triphenylphosphine, the “oxidized complex” proved capable of transferring one oxygen atom to phosphine, generating phosphine oxide and forming an Ir–PPh₃ adduct in 92% yield. The final Ir–PPh₃ product was fully characterized

Bis(acetylacetonate) Tungsten(IV) Complexes Containing a π-Basic Diazoalkane or Oxo Ligand

Tungsten­(IV) bis­(acetylacetonate) (acetylacetonate = acac) complexes were synthesized by incorporating either a diazoalkane or an oxo ligand into the coordination sphere of a tungsten­(II) reagent. The reaction of free diazoalkane (N₂CRR′) with W­(CO)₃(acac)₂ leads to loss of two carbon monoxide ligands and coordination of the diazoalkane reagent through the terminal nitrogen to produce W­(CO)­(acac)₂(N₂CRR′). This monomer is best formulated as a tungsten­(IV) complex. A second example of converting a d⁴ tungsten carbonyl complex to a d² product involves oxidation of W­(CO)­(acac)₂(PhCN) with <i>m</i>-chloroperoxybenzoic acid (MCPBA) to replace the CO ligand with an oxygen atom. This increase in metal oxidation state causes rotation of the nitrile ligand by 90° relative to the two bidentate acac ligands. Electrophilic addition at the nitrogen of the π-bound nitrile ligand using methyl triflate (MeOTf) and subsequent nucleophilic addition at carbon with sodium trimethoxyborohydride, Na­[HB­(OMe)₃], reduces the CN bond stereoselectively and produces the neutral imine complex W­(O)­(acac)₂(PhHCNMe) with a diastereomeric ratio of 11:1

Seeking a Mechanistic Analogue of the Water–Gas Shift Reaction: Carboxamido Ligand Formation and Isocyanate Elimination from Complexes Containing the Tp′PtMe Fragment

A series of stable, isolable Tp′Pt­(IV) carboxamido complexes of the type Tp′PtMe₂(C­(O)­NHR) (R = Et, ⁿPr, ⁱPr, ^tBu, Bn, Ph) has been synthesized by addition of amide nucleophiles to the carbonyl ligand in Tp′Pt­(Me)­(CO) followed by trapping of the Pt­(II) intermediate with methyl iodide as the methylating reagent. These compounds mimic elusive intermediates resulting from hydroxide addition to platinum-bound CO in the Water–Gas Shift Reaction (WGSR). Seeking parallels to WGSR chemistry, we find that deprotonation of the carboxamido NH initiates elimination and the isocyanate-derived products form; the resulting platinum fragment can be protonated to reoxidize the metal center and generate Tp′PtMe₂H, the synthetic precursor to Tp′Pt­(Me)­(CO). Mechanistic studies on the formation of and elimination from Tp′PtMe₂(C­(O)­NHR) suggest a stepwise process with deprotonation from a Pt­(IV) species as the key step prompting elimination

Probing the Oxidation Chemistry of Half-Sandwich Iridium Complexes with Oxygen Atom Transfer Reagents

The new complexes [Ir­(Cp*)­(phpy)­3,5-bis­(trifluoromethyl)­benzonitrile]⁺ (<b>1-NCAr</b>^<b>+</b>) and [Ir­(Cp*)­(phpy)­(styrene)]⁺ (<b>1-Sty</b>^<b>+</b>, Cp* = η⁵-pentamethylcyclopentadienyl, phpy = 2-phenylene-<i>κC</i>^<i>1</i>′-pyridine-<i>κN</i>) were prepared as analogues of reported iridium water oxidation catalysts, to study their reactions with oxygen atom transfer (OAT) reagents at low temperatures. In no case was the desired product, an Ir­(V)­oxo complex, observed by spectroscopy. Instead, ligand oxidation was implicated. Oxidation of <b>1-NCAr</b>^<b>+</b> with the OAT reagent dimethyldioxirane (DMDO) yielded dioxygen when analyzed by GC, but formation of a heterogeneous or paramagnetic species was simultaneously observed. This amplifies uncertainty over the actual identity of iridium catalysts in the harsh oxidizing conditions required for water oxidation. Catalyst stability was then assessed for a reported styrene epoxidation mediated by [Ir­(Cp*)­(phpy)­(OH₂)]⁺ (<b>1-OH</b>_<b>2</b>^<b>+</b>). It was found that the OAT reagent iodosobenzene (PhIO) extensively oxidized the organic ligands of <b>1-OH</b>_<b>2</b>^<b>+</b>. Acetic acid was detected as a decomposition product. In addition, both the molecular structure and the aqueous electrochemistry of <b>1-OH</b>_<b>2</b>^<b>+</b> are described for the first time. Oxidative scans revealed rapid decomposition of the complex. All of the above experiments indicate that degradation of the organic ligands in catalysts built with the Ir­(Cp*)­(phpy) framework are facile under oxidizing conditions. In separate experiments designed to promote ligand substitution, an unexpected silver-bridged, dinuclear Ir­(III) species with terminal hydrides, [{Ir­(Cp*)­(phpy)­H}₂Ag]⁺ (<b>2</b>), was discovered. The source of Ag⁺ for complex <b>2</b> was identified as AgCl