Synthesis and Reactivity of a Nucleophilic Palladium(II) Carbene

Two formal palladium carbene complexes, [PC­(sp²)­P]­Pd­(PR₃) (<b>3</b>: R = Me; <b>4</b>: R = Ph) were isolated and characterized from [PC­(sp³)­H₂P] ([PC­(sp³)­H₂P] = bis­[2-(di-isopropylphosphino)­phenyl]­methane, ⁱPr₂P-C₆H₄-CH₂-C₆H₄-PⁱPr₂). Structural studies and DFT calculations indicate that the interaction between palladium and carbon is best described as a single bond, associated with nucleophilic character at that carbon atom. The characteristics of <b>3</b> were probed by reactions with electrophiles (MeI), acids (MeOH and HCl), and <i>para</i>-toluidine

C–H Activation Reactions of a Nucleophilic Palladium Carbene

The reactivity of a nucleophilic palladium carbene, [PC­(sp²)­P]­Pd­(PMe₃) (<b>1</b>; [PC­(sp²)­P] = bis­[2-(diisopropylphosphino)­phenyl]­methylene), toward the C–H bonds of CH₃COCH₃, CH₃CN, Ph–CCH, fluorene, and 9,10-dihydroanthracene was investigated. All surveyed substrates reacted with <b>1</b>. However, there was no detectable reaction of <b>1</b> with Ph₂CH₂. It is proposed that the p<i>K</i>_a values of the studied C–H bonds govern their reactivity toward <b>1</b>: our results show that substrates with a p<i>K</i>_a higher than 29, such as Ph₂CH₂ (p<i>K</i>_a = 32.2), do not react even with prolonged heating

Ag(I) and Tl(I) Precursors as Transfer Agents of a Pyrrole-Based Pincer Ligand to Late Transition Metals

A PNP ligand, PN^pyrP ((PN^pyrP)H = 2,5-bis­((di-<i>iso</i>-propylphosphino)­methyl)­pyrrole), which employs a pyrrole unit as a central anionic nitrogen donor, was designed. The corresponding group 10 metal chlorides as well as iridium and ruthenium compounds were isolated. In order to conduct this work, [(PN^pyrP)­Tl] and [(PN^pyrP)­Ag]₂ were synthesized and characterized. The thallium and silver species were paramount in the formation of the iridium and ruthenium complexes, which could not be isolated using (PN^pyrP)H or the corresponding lithium pyrrolide salt. Interestingly, the solid state molecular structure of [(PN^pyrP)­Tl] indicates that the metal center engages in an η² intermolecular interaction with the backbone of a neighboring pyrrole molecule instead of the expected bonding to the phosphine arms

An Adaptable Chelating Diphosphine Ligand for the Stabilization of Palladium and Platinum Carbenes

Group 10 metal carbenes are proposed in catalytic transformations; however, their isolation remains difficult without the presence of a heteroatom donor. The adaptable cis and trans coordinating ligand P^terP (1,2-bis­(2-(diisopropylphosphino)­phenyl)­benzene) is key in stabilizing two-coordinate palladium and platinum(0) precursors. Reacting these precursors with di-<i>p</i>-tolyldiazomethane ((<i>p</i>-tol)₂CN₂) leads to the formation of the unprecedented trigonal-planar diarylcarbenes [(P^terP)­MC­(<i>p</i>-tol)₂] (M = Pd, Pt), upon transformation of the trans coordinating ligand into a wide-bite, cis-coordinating ligand. Both palladium and platinum diarylcarbenes were characterized by multinuclear NMR spectroscopy. The unusual stability of the platinum analogue allowed its characterization via X-ray crystallography. Furthermore, the reactivity of the palladium and platinum diarylcarbenes with Ph₂SiH₂ and CH₃I was investigated

Aryl and Benzyl C–H Activation of N‑Substituted PNP Ligands

The synthesis of the <i>N</i>-aryl-substituted PNP pro-ligands H­(PN^naphP) (<i>N</i>-di­(2-diisopropyl­phosphine-4-methylphenyl)­naphthylamine) and H­(PN^tolP) (<i>N</i>-di­(2-diisopropylphosphine-4-methylphenyl)-<i>o</i>-tolylamine) is reported. The corresponding iridium­(III) complexes, [(PN^naphP)­Ir­(H)­Cl], [(<i>o</i>-methyl-PN^tolP)­Ir­(H)­Cl], [(<i>o</i>-aryl-PN^tolP)­Ir­(H)­Cl], [(PN^naphP)­Ir­(H)₂], [(<i>o</i>-methyl-PN^tolP)­Ir­(H)₂], and [(<i>o</i>-aryl-PN^tolP)­Ir­(H)₂], were also synthesized and structurally characterized, along with reaction intermediates, demonstrating various ligand coordination modes

C–H Activation Reactions of a Nucleophilic Palladium Carbene

The reactivity of a nucleophilic palladium carbene, [PC­(sp²)­P]­Pd­(PMe₃) (<b>1</b>; [PC­(sp²)­P] = bis­[2-(diisopropylphosphino)­phenyl]­methylene), toward the C–H bonds of CH₃COCH₃, CH₃CN, Ph–CCH, fluorene, and 9,10-dihydroanthracene was investigated. All surveyed substrates reacted with <b>1</b>. However, there was no detectable reaction of <b>1</b> with Ph₂CH₂. It is proposed that the p<i>K</i>_a values of the studied C–H bonds govern their reactivity toward <b>1</b>: our results show that substrates with a p<i>K</i>_a higher than 29, such as Ph₂CH₂ (p<i>K</i>_a = 32.2), do not react even with prolonged heating

Flexible Coordination of Diphosphine Ligands Leading to cis and trans Pd(0), Pd(II), and Rh(I) Complexes

A series of diphosphine ligands ^<i>i</i>Pr₂P–C₆H₄–X–C₆H₄–P^<i>i</i>Pr₂ (for ligand L¹, X = CH₂; for ligand L², X = CH₂CH₂) was investigated to determine the preference for cis/trans coordination to palladium(0), palladium­(II), and rhodium­(I). Increasing the length of the bridging alkyl backbone from one to two carbons changes the geometry of the resulting palladium­(II) complexes, with L¹ coordinating preferentially cis, while L² coordinates in a trans fashion. Coordination to Pd(0) leads to L¹Pd­(dba) and L²Pd­(dba), in which both ligands accommodate a P–M–P angle close to 120°. L² was found to coordinate cis in a rhodium­(I) complex ([L²Rh­(nbd)]­[BF₄], where nbd = norbornadiene)

Synthesis and Reactivity of a Nucleophilic Palladium(II) Carbene

Two formal palladium carbene complexes, [PC­(sp²)­P]­Pd­(PR₃) (<b>3</b>: R = Me; <b>4</b>: R = Ph) were isolated and characterized from [PC­(sp³)­H₂P] ([PC­(sp³)­H₂P] = bis­[2-(di-isopropylphosphino)­phenyl]­methane, ⁱPr₂P-C₆H₄-CH₂-C₆H₄-PⁱPr₂). Structural studies and DFT calculations indicate that the interaction between palladium and carbon is best described as a single bond, associated with nucleophilic character at that carbon atom. The characteristics of <b>3</b> were probed by reactions with electrophiles (MeI), acids (MeOH and HCl), and <i>para</i>-toluidine

Three-Coordinate Nickel Carbene Complexes and Their One-Electron Oxidation Products

The synthesis and characterization of two new carbene complexes, (dtbpe)­NiCH­(dmp) (<b>1</b>; dtbpe = 1,2-bis­(di-<i>tert</i>-butylphosphino)­ethane; dmp = 2,6-dimesitylphenyl) and (dippn)­NiCH­(dmp) (<b>2</b>; dippn = 1,8-bis­(di-<i>iso</i>-propylphosphino)­naphthalene), are described. Complexes <b>1</b> and <b>2</b> were isolated by photolysis of the corresponding side-bound diazoalkane complexes, exemplified by (dtbpe)­Ni­{η²-N₂CH­(dmp)} (<b>3</b>). The carbene complexes feature Ni–C distances that are short and Ni–C–C angles at the carbene carbon that are intermediate between 120° and 180° (155.7(3)° and 152.3(3)°, respectively). The difference between the two carbenes became obvious when their reactivity toward 1-electron oxidizing agents was studied: the oxidation of <b>1</b> led to an internal rearrangement and the formation of a nickel­(I) alkyl [{κ²-P,C-di-<i>tert</i>-butylphosphino-di-<i>tert</i>-butyl-PCH­(dmp)­ethane}­Ni]­[BAr^F₄] (<b>4</b>), while the oxidation of <b>2</b> allowed the isolation of an unrearranged product, formulated as the cationic nickel­(III) carbene complex­[(dippn)­NiCH­(dmp)]­[BAr^F₄] (<b>6</b>). Both oxidations are chemically reversible and the respective reductions lead to the neutral carbene complexes, <b>1</b> and <b>2</b>

Group 10 Metal Complexes Supported by Pincer Ligands with an Olefinic Backbone

The coordination chemistry of 2,2′-bis­(di-<i>iso</i>-propyl­phosphino)-<i>trans</i>-stilbene (<i>t</i>PCHCHP) with group 10 metal centers in a variety of oxidation states is reported; different coordination modes were observed depending on the oxidation state of the metal. With metal centers in the 0 or +1 oxidation state ((<i>t</i>PCHCHP)­Ni, [(<i>t</i>PCHCHP)­Pd]₂, (<i>t</i>PCHCHP)­NiCl, (<i>t</i>PCHCHP)­NiI), η² coordination of the olefin occurs, whereas, with metals in the +2 oxidation state, C–H activation of the backbone, followed by rapid H–X reductive elimination, was observed, leading to an η¹ coordination of the backbone in (<i>t</i>PCCHP)­MCl (M = Ni, Pd, Pt). Employing the methyl-substituted analogue, 2,2′-bis­(di-<i>iso</i>-propylphosphino)-<i>trans</i>-diphenyl-1,2-dimethylethene (<i>t</i>PCMeCMeP), forced an η² coordination of the olefin in [(<i>t</i>PCMeCMeP)­NiCl]₂­[NiCl₄]. The synthesis of the hydride complex (<i>t</i>PCCHP)­NiH was attempted, but, instead, led to the formation of (<i>t</i>PCHCHP)­Ni, indicating that the vinyl form of the backbone can function as a hydrogen acceptor. All metal complexes were characterized by multinuclei NMR spectroscopy, X-ray crystallography, and elemental analysis