Activation of M–Cl Bonds with Phosphine–Alanes: Preparation and Characterization of Zwitterionic Gold and Copper Complexes

The triphosphine–alane [<i>i</i>Pr₂P­(<i>o</i>-C₆H₄)]₃Al (<b>1</b>) was prepared by coupling ortho-lithiated diisopropylphenylphosphine with AlCl₃. Reactions of <b>1</b> with gold and copper chlorides afforded the zwitterionic cage complexes <b>2</b> and <b>3</b>. The three phosphine arms coordinate symmetrically to the coinage metal, while the aluminum center abstracts the chloride. Coordination of the related diphosphine–alane [<i>i</i>Pr₂P­(<i>o</i>-C₆H₄)]₂AlCl (<b>4</b>) to CuCl is also accompanied by a shift of the chloride atom from copper to aluminum. However, the ensuing highly electrophilic Cu⁺ center engages in weak intra- and intermolecular Cl→Cu interactions, resulting in the original polymeric complex <b>5</b>. The structures of all complexes have been ascertained spectroscopically and crystallographically, and their bonding situations have been analyzed by DFT calculations

Amino and Alkyl B‑Substituted P‑Stabilized Borenium Salts

The ability of the phosphino-naphthyl moiety to stabilize amino- and alkylborenium cations has been studied. Surprisingly, the phosphine–aminochloroborane precursor <b>2</b> was found to exist in neutral open form (without P→B interaction) in benzene solution and in the solid state but to ionize spontaneously in chloroform to generate the P-stabilized borenium salt <b>3</b>. Addition of gallium trichloride shifts the process forward and affords the corresponding tetrachlorogallate borenium salt <b>3′</b>. The phosphine group of <b>2</b> remains available for external reactivity, as shown by the ready formation of the corresponding phosphine gold­(I) chloride complex <b>4</b>. The P-stabilized cyclohexylborenium cation <b>6</b> has also been prepared by reacting the corresponding bromoborane <b>5</b> with gallium tribromide. Compound <b>6</b> is a very rare example of an alkyl-substituted borenium salt. The structures of <b>2</b>, <b>3′</b>, and <b>4</b>–<b>6</b> have been unambiguously ascertained by multinuclear NMR spectroscopy and X-ray crystallography

Combined Experimental/Computational Study of Iridium and Palladium Hydride PP(O)P Pincer Complexes

The diphosphine–phosphine oxide {[<i>o</i>-<i>i</i>Pr₂P­(C₆H₄)]₂P­(O)­H} (<b>1</b>) has been prepared, and its coordination to Ir and Pd has been explored. Using [IrCl­(cyclooctene)₂]₂, the pincer hydride complex {(<i>o</i>-<i>i</i>Pr₂PC₆H₄)₂P­(O)]­IrHCl} (<b>2</b>) is readily obtained by phosphine-assisted P­(O)–H bond activation. Coordination of CO to Ir affords the corresponding octahedral complex {(<i>o</i>-<i>i</i>Pr₂PC₆H₄)₂P­(O)]­IrHCl­(CO)} (<b>3</b>) as a single stereoisomer. The electronic properties of the PP­(O)­P ligand have been compared with those of related PEP frameworks on the basis of ν_CO stretching frequencies. Treatment of <b>1</b> with [Pd­(P<i>t</i>Bu₃)₂] gives the palladium hydride complex {(<i>o</i>-<i>i</i>Pr₂PC₆H₄)₂P­(O)]­PdH} (<b>4</b>). The mechanism of P­(O)–H bond activation at Pd has been investigated computationally. Complex <b>4</b> reacts with methyl acrylate at room temperature, giving {(<i>o</i>-<i>i</i>Pr₂PC₆H₄)₂P­(O)]­PdCH­(Me)­CO₂Me} (<b>7</b>) as the result of regioselective insertion into the Pd–H bond

Phosphino-Boryl-Naphthalenes: Geometrically Enforced, Yet Lewis Acid Responsive P → B Interactions

Three naphthyl-bridged phosphine-borane derivatives <b>2</b>-BCy₂, <b>2</b>-BMes₂, and <b>2</b>-BFlu, differing in the steric and electronic properties of the boryl moiety, have been prepared and characterized by spectroscopic and crystallographic means. The presence and magnitude of the P → B interactions have been assessed experimentally and theoretically. The naphthyl linker was found to enforce the P → B interaction despite steric shielding, while retaining enough flexibility to respond to the Lewis acidity of boron

Phosphino-Boryl-Naphthalenes: Geometrically Enforced, Yet Lewis Acid Responsive P → B Interactions

Three naphthyl-bridged phosphine-borane derivatives <b>2</b>-BCy₂, <b>2</b>-BMes₂, and <b>2</b>-BFlu, differing in the steric and electronic properties of the boryl moiety, have been prepared and characterized by spectroscopic and crystallographic means. The presence and magnitude of the P → B interactions have been assessed experimentally and theoretically. The naphthyl linker was found to enforce the P → B interaction despite steric shielding, while retaining enough flexibility to respond to the Lewis acidity of boron

Chelating Assistance of P–C and P–H Bond Activation at Palladium and Nickel: Straightforward Access to Diverse Pincer Complexes from a Diphosphine–Phosphine Oxide

The diphosphine–phosphine oxide (DPPO) {[<i>o</i>-<i>i</i>-Pr₂P-(C₆H₄)]₂P­(O)­Ph} (<b>1</b>) reacts with [Ni­(cod)₂] (cod = 1,4-cyclooctadiene) to give the diphosphine–phosphide oxide κ^P,P(O),P pincer complex <b>3</b>. According to DFT calculations, the Ph–P­(O) bond activation involves a three-center P,C_ipso,Ni transition state. Reaction of the DPPO ligand <b>1</b> with [(nbd)­Pd­(ma)] (nbd = 2,5-norbornadiene and ma = maleic anhydride) affords the [(DPPO)­Pd­(ma)] complex <b>4</b>. Upon heating, the ma coligand is displaced and the κ^P,P(O),P palladium pincer complex <b>2</b> is obtained. The dinuclear complex {(DPPO)­[Pd­(ma)]₂} (<b>6</b>) has also been authenticated. X-ray diffraction analysis showed an original situation in which the oxygen atom of the central phosphine oxide moiety bridges the two palladium centers. Addition of trifluoromethanesulfonic acid to DPPO <b>1</b> affords the trifunctional phosphine–phosphine oxide–phosphonium derivative <b>7</b>. Upon reaction with [Pd₂(dba)₃], the palladium hydride κ^P,O(P),P pincer complex <b>8</b> is cleanly formed as the result of P⁺–H bond activation. Complex <b>8</b> is readily deprotonated by DBU (DBU = 1,8-diazabicycloundec-7-ene), and spontaneous oxidative addition of the Ph–P­(O) bond gives the diphosphine–phosphide oxide κ^P,P(O),P pincer complex <b>2</b>. Conversely, addition of trifluoromethanesulfonic acid on <b>2</b> does not give back the palladium hydride <b>8</b> but leads to the diphosphine–hydroxy phosphine κ^P,P(OH),P pincer complex <b>9</b>

Chelating Assistance of P–C and P–H Bond Activation at Palladium and Nickel: Straightforward Access to Diverse Pincer Complexes from a Diphosphine–Phosphine Oxide

The diphosphine–phosphine oxide (DPPO) {[<i>o</i>-<i>i</i>-Pr₂P-(C₆H₄)]₂P­(O)­Ph} (<b>1</b>) reacts with [Ni­(cod)₂] (cod = 1,4-cyclooctadiene) to give the diphosphine–phosphide oxide κ^P,P(O),P pincer complex <b>3</b>. According to DFT calculations, the Ph–P­(O) bond activation involves a three-center P,C_ipso,Ni transition state. Reaction of the DPPO ligand <b>1</b> with [(nbd)­Pd­(ma)] (nbd = 2,5-norbornadiene and ma = maleic anhydride) affords the [(DPPO)­Pd­(ma)] complex <b>4</b>. Upon heating, the ma coligand is displaced and the κ^P,P(O),P palladium pincer complex <b>2</b> is obtained. The dinuclear complex {(DPPO)­[Pd­(ma)]₂} (<b>6</b>) has also been authenticated. X-ray diffraction analysis showed an original situation in which the oxygen atom of the central phosphine oxide moiety bridges the two palladium centers. Addition of trifluoromethanesulfonic acid to DPPO <b>1</b> affords the trifunctional phosphine–phosphine oxide–phosphonium derivative <b>7</b>. Upon reaction with [Pd₂(dba)₃], the palladium hydride κ^P,O(P),P pincer complex <b>8</b> is cleanly formed as the result of P⁺–H bond activation. Complex <b>8</b> is readily deprotonated by DBU (DBU = 1,8-diazabicycloundec-7-ene), and spontaneous oxidative addition of the Ph–P­(O) bond gives the diphosphine–phosphide oxide κ^P,P(O),P pincer complex <b>2</b>. Conversely, addition of trifluoromethanesulfonic acid on <b>2</b> does not give back the palladium hydride <b>8</b> but leads to the diphosphine–hydroxy phosphine κ^P,P(OH),P pincer complex <b>9</b>

B‑Centered Reactivity of Persistent P‑Stabilized Boryl Radicals

A new P-stabilized boryl radical [<i>i</i>Pr₂P­(naph)­BMes]^• <b>2a</b> was obtained by reduction of the corresponding phosphino-bromoborane <b>1a</b> with Na­(Hg). The persistent radical <b>2a</b> has been characterized by EPR, and its structure has been thoroughly studied by DFT. The corresponding Gomberg-type dimer has been analyzed by NMR and XRD, and the Gibbs free energy associated with the dimerization process has been evaluated by VT EPR. The replacement of the Ph substituents at phosphorus for <i>i</i>Pr groups has a slight but noticeable impact: it increases the spin density at boron and favors the radical over its Gomberg-type dimer. An original cross-coupling product between <b>2a</b> and the trityl radical Ph₃C^• has also been authenticated crystallographically. The P-stabilized boryl radicals <b>2a,b</b> are readily trapped by TEMPO to give the corresponding B–O adducts <b>3a,b</b> (naphthyl-bridged phosphine-boranes without P → B interaction). The reaction of <b>2a,b</b> with Ph₃CCl substantiates their ability to participate in halogen transfer reactions

B‑Centered Reactivity of Persistent P‑Stabilized Boryl Radicals

A new P-stabilized boryl radical [<i>i</i>Pr₂P­(naph)­BMes]^• <b>2a</b> was obtained by reduction of the corresponding phosphino-bromoborane <b>1a</b> with Na­(Hg). The persistent radical <b>2a</b> has been characterized by EPR, and its structure has been thoroughly studied by DFT. The corresponding Gomberg-type dimer has been analyzed by NMR and XRD, and the Gibbs free energy associated with the dimerization process has been evaluated by VT EPR. The replacement of the Ph substituents at phosphorus for <i>i</i>Pr groups has a slight but noticeable impact: it increases the spin density at boron and favors the radical over its Gomberg-type dimer. An original cross-coupling product between <b>2a</b> and the trityl radical Ph₃C^• has also been authenticated crystallographically. The P-stabilized boryl radicals <b>2a,b</b> are readily trapped by TEMPO to give the corresponding B–O adducts <b>3a,b</b> (naphthyl-bridged phosphine-boranes without P → B interaction). The reaction of <b>2a,b</b> with Ph₃CCl substantiates their ability to participate in halogen transfer reactions

Catalytic Dehydrogenation of (Di)Amine-Boranes with a Geometrically Constrained Phosphine-Borane Lewis Pair

The <i>o</i>-phenylene bridged phosphine-borane <i>i</i>Pr₂P­(<i>o</i>-C₆H₄)­B­(Fxyl)₂ <b>2</b> was prepared. Despite ring strain, it adopts a closed form, as substantiated by NMR, XRD, and DFT analyses. However, the corresponding open form is only slightly higher in energy. The dormant Lewis pair <b>2</b> proved to efficiently catalyze the dehydrogenation of a variety of amine- and diamine-boranes under mild conditions. The corresponding phosphonium-borate <i>i</i>Pr₂PH­(<i>o</i>-C₆H₄)­BH­(Fxyl)₂ <b>3</b> was authenticated as a key intermediate of these dehydrogenation reactions. The propensity of <b>3</b> to release H₂ plays a major role in the catalytic turnover