Discovery of New Reactivity at Complexes of Rhenium Supported by PCP, PNN, and PNP Pincer Ligands and the Study of Pincers Containing Ancillary Antimony Donors

The chemistry of pincer ligands is such that a wide variety of transition metals and main group elements are capable of being supported, stabilized, and isolated in often, highly reactive forms. The pincer ligands which have found the most utility are those based on the 1,3- bis(dialkylphosphinomethyl)benzene and 2,2’-bis(dialkylphosphine)diphenylamine ligand backbones. Herein, we have taken steps and report on the development of complexes of rhenium supported by PCP and PNP-related pincer ligands. Described is the formation and characterization of the first complexes of rhenium supported by the parent PCP pincer ligand and the subsequent formation of multiple polyhydrides. The substitution of dihydrogen allowed access to mixed ligand species supported by the PCP pincer. The general salt metathesis reactivity as well as the generation of supposed five-coordinate (PCP)Re+ cations and their insertion reactivity was explored. Attention was then turned to the unusual and counter-intuitive reactivity of a novel rhenium-oxo-acetate supported by the PCP pincer. Against expected pKa values, the rhenium-acetate undergoes completely irreversible hydrolysis to form a highly fluxional rhenium dioxo with an acetic acid adduct, featuring a significant hydrogen-bonding interaction. Aside from the substitution chemistry at (PCP)Re, the parent ligand was also found to, for the first time, be capable of undergoing a base-assisted dearomatization forming a “pseudo-carbene” which performs a metal-ligand cooperative activation of small molecules such as carbon dioxide. The nature of the rhenium-carbon bonding was studied spectroscopically, crystallographically, and computationally to determine significant Re-C π-bonding interactions. PNP and proton-responsive PNN-type pincers were introduced into the coordination sphere of high-valent rhenium as well. They exhibited pronounced rotameric isomerism which was able to be studied on the NMR timescale. The reduction of high-valent (PNP)Re fragments also allowed for the isolation of Re(I)/Mn(I) dicarbonyls which exhibit metal-ligand cooperativity in the activation of acidic substrates such as formic acid. Finally, pincer ligands containing antimony donors in varying oxidation states were examined and their direct comparisons to phosphorous were made as well as their possible application in the binding of small molecules and formation of novel aromatics

Discovery of New Reactivity at Complexes of Rhenium Supported by PCP, PNN, and PNP Pincer Ligands and the Study of Pincers Containing Ancillary Antimony Donors

The chemistry of pincer ligands is such that a wide variety of transition metals and main group elements are capable of being supported, stabilized, and isolated in often, highly reactive forms. The pincer ligands which have found the most utility are those based on the 1,3- bis(dialkylphosphinomethyl)benzene and 2,2’-bis(dialkylphosphine)diphenylamine ligand backbones. Herein, we have taken steps and report on the development of complexes of rhenium supported by PCP and PNP-related pincer ligands. Described is the formation and characterization of the first complexes of rhenium supported by the parent PCP pincer ligand and the subsequent formation of multiple polyhydrides. The substitution of dihydrogen allowed access to mixed ligand species supported by the PCP pincer. The general salt metathesis reactivity as well as the generation of supposed five-coordinate (PCP)Re+ cations and their insertion reactivity was explored. Attention was then turned to the unusual and counter-intuitive reactivity of a novel rhenium-oxo-acetate supported by the PCP pincer. Against expected pKa values, the rhenium-acetate undergoes completely irreversible hydrolysis to form a highly fluxional rhenium dioxo with an acetic acid adduct, featuring a significant hydrogen-bonding interaction. Aside from the substitution chemistry at (PCP)Re, the parent ligand was also found to, for the first time, be capable of undergoing a base-assisted dearomatization forming a “pseudo-carbene” which performs a metal-ligand cooperative activation of small molecules such as carbon dioxide. The nature of the rhenium-carbon bonding was studied spectroscopically, crystallographically, and computationally to determine significant Re-C π-bonding interactions. PNP and proton-responsive PNN-type pincers were introduced into the coordination sphere of high-valent rhenium as well. They exhibited pronounced rotameric isomerism which was able to be studied on the NMR timescale. The reduction of high-valent (PNP)Re fragments also allowed for the isolation of Re(I)/Mn(I) dicarbonyls which exhibit metal-ligand cooperativity in the activation of acidic substrates such as formic acid. Finally, pincer ligands containing antimony donors in varying oxidation states were examined and their direct comparisons to phosphorous were made as well as their possible application in the binding of small molecules and formation of novel aromatics

Synthesis and Reactivity of Three-Coordinate (dtbpe)Rh Silylamides: CO₂ Bond Cleavage by a Rhodium(I) Disilylamide

Rhodium­(I) silylamide complexes supported by the 1,2-bis­(di-<i>tert</i>-butylphosphino)­ethane (dtbpe) ligand have been prepared and their structures and reactivity studied. Although the complexes degrade over time to release the corresponding silylamines, they react cleanly with silver­(I) salts to transfer the amido group at ambient temperature. The bis­(trimethylsilyl)­amide complex (dtbpe)­Rh–N­(TMS)₂ reacts with CO₂ to form a carbamate complex that decomposes via loss of hexamethyldisiloxane to form a bis­(μ-isocyanate) dimer, suggesting that silylamides may be useful nitrene-group and nitrogen-atom sources through selective N–Si bond cleavage

Complexes of High-Valent Rhenium Supported by the PCP Pincer

The PCP ligand can be introduced into the coordination sphere of Re by metalation in a reaction with L₂Re­(O)­Cl₃ precursors, leading to an octahedral (PCP)­Re­(O)­Cl₂ (<b>2</b>), in which the aryl donor of PCP is cis to the oxo ligand. Halide exchange with Me₃SiBr or Me₃SiI furnished analogous (PCP)­Re­(O)­Br₂ (<b>3</b>) and (PCP)­Re­(O)­I₂ (<b>4</b>). Treatment of <b>2</b> with LiAlH₄ resulted in the isolation of (PCP)­ReH₆ (<b>5</b>) upon workup. NMR data suggest a classical hexahydride nature for <b>5</b>. <b>5</b> reacted with PMe₃ and 4-dimethylaminopyridine (DMAP) by losing H₂ and forming adducts (PCP^iPr)­ReH₄(PMe₃) (<b>6</b>) and (PCP)­ReH₄(DMAP) (<b>7</b>). On the other hand, reaction of <b>5</b> with CO resulted in loss of all the hydrides and formation of the Re­(I) compound (PCP^iPr)­Re­(CO)₃ (<b>8</b>). Finally, thermolysis of the hexahydride <b>5</b> led to loss of half the hydrides as H₂ and formation of the dimeric species formulated as (PCP^iPr)₂Re₂H₆ (<b>9</b>)

Irreversible Hydrolysis of PCP-Supported Rhenium(V) Acetates

Complexes (PCP^R)­Re­(O)­(OAc)₂ [R = ⁱPr (<b>4a</b>) and ^tBu (<b>4b</b>); PCP = κ³-<i>P</i>,<i>C</i>,<i>P</i>-2,6-(R₂PCH₂)₂C₆H₃] undergo unexpected irreversible hydrolysis to yield (PCP^R)­Re­(O)­(OAc)­(OH) (<b>3a</b>/<b>3b</b>) and free AcOH. <b>3a</b> and <b>3b</b> are highly fluxional in solution, possibly via AcOH loss and the intermediacy of (PCP^R)­Re­(O)₂, which was isolated for R = ^tBu (<b>5b</b>)

Complexes of High-Valent Rhenium Supported by the PCP Pincer

The PCP ligand can be introduced into the coordination sphere of Re by metalation in a reaction with L₂Re­(O)­Cl₃ precursors, leading to an octahedral (PCP)­Re­(O)­Cl₂ (<b>2</b>), in which the aryl donor of PCP is cis to the oxo ligand. Halide exchange with Me₃SiBr or Me₃SiI furnished analogous (PCP)­Re­(O)­Br₂ (<b>3</b>) and (PCP)­Re­(O)­I₂ (<b>4</b>). Treatment of <b>2</b> with LiAlH₄ resulted in the isolation of (PCP)­ReH₆ (<b>5</b>) upon workup. NMR data suggest a classical hexahydride nature for <b>5</b>. <b>5</b> reacted with PMe₃ and 4-dimethylaminopyridine (DMAP) by losing H₂ and forming adducts (PCP^iPr)­ReH₄(PMe₃) (<b>6</b>) and (PCP)­ReH₄(DMAP) (<b>7</b>). On the other hand, reaction of <b>5</b> with CO resulted in loss of all the hydrides and formation of the Re­(I) compound (PCP^iPr)­Re­(CO)₃ (<b>8</b>). Finally, thermolysis of the hexahydride <b>5</b> led to loss of half the hydrides as H₂ and formation of the dimeric species formulated as (PCP^iPr)₂Re₂H₆ (<b>9</b>)

High-Turnover Aromatic C–H Borylation Catalyzed by POCOP-Type Pincer Complexes of Iridium

The catalytic C–H borylation of arenes with HBpin (pin = pinacolate) using POCOP-type pincer complexes of Ir has been demonstrated, with turnover numbers exceeding 10 000 in some cases. The selectivity of C–H activation was based on steric preferences and largely mirrored that found in other Ir borylation catalysts. Catalysis in the (POCOP)Ir system depends on the presence of stoichiometric quantities of sacrificial olefin, which is hydrogenated to consume the H₂ equivalents generated in the borylation of C–H bonds with HBpin. Smaller olefins such as ethylene or 1-hexene were more advantageous to catalysis than sterically encumbered <i>tert</i>-butylethylene (TBE). Olefin hydroboration is a competing side reaction. The synthesis and isolation of multiple complexes potentially relevant to catalysis permitted examination of several key elementary reactions. These experiments indicate that the C–H activation step in catalysis ostensibly involves oxidative addition of an aromatic C–H bond to the three-coordinate (POCOP)Ir species. The olefin is mechanistically critical to gain access to this 14-electron, monovalent Ir intermediate. C–H activation at Ir­(I) here is in contrast to the olefin-free catalysis with state-of-the-art Ir complexes supported by neutral bidentate ligands, where the C–H activating step is understood to involve trivalent Ir-boryl intermediates

High-Turnover Aromatic C–H Borylation Catalyzed by POCOP-Type Pincer Complexes of Iridium

The catalytic C–H borylation of arenes with HBpin (pin = pinacolate) using POCOP-type pincer complexes of Ir has been demonstrated, with turnover numbers exceeding 10 000 in some cases. The selectivity of C–H activation was based on steric preferences and largely mirrored that found in other Ir borylation catalysts. Catalysis in the (POCOP)Ir system depends on the presence of stoichiometric quantities of sacrificial olefin, which is hydrogenated to consume the H₂ equivalents generated in the borylation of C–H bonds with HBpin. Smaller olefins such as ethylene or 1-hexene were more advantageous to catalysis than sterically encumbered <i>tert</i>-butylethylene (TBE). Olefin hydroboration is a competing side reaction. The synthesis and isolation of multiple complexes potentially relevant to catalysis permitted examination of several key elementary reactions. These experiments indicate that the C–H activation step in catalysis ostensibly involves oxidative addition of an aromatic C–H bond to the three-coordinate (POCOP)Ir species. The olefin is mechanistically critical to gain access to this 14-electron, monovalent Ir intermediate. C–H activation at Ir­(I) here is in contrast to the olefin-free catalysis with state-of-the-art Ir complexes supported by neutral bidentate ligands, where the C–H activating step is understood to involve trivalent Ir-boryl intermediates

Chalcogen Extrusion from Heteroallenes and Carbon Monoxide by a Three-Coordinate Rh(I) Disilylamide

We report the reactions of several heteroallenes (carbon disulfide, carbonyl sulfide, and phenyl isocyanate) and carbon monoxide with a three-coordinate, bis­(phosphine)-supported Rh­(I) disilylamide (<b>1</b>). Carbon disulfide reacts with <b>1</b> to afford a silyltrithiocarbonate complex similar to an intermediate previously invoked in the deoxygenation of CO₂ by <b>1</b>, and prolonged heating affords a structurally unusual μ-κ²(<i>S</i>,<i>S</i>′):κ²(<i>S</i>,<i>S</i>′)-trithiocarbonate dimer. Carbonyl sulfide reacts with <b>1</b> to afford a structurally unique Rh­(SCNCS) metallacycle derived from two insertions of OCS and N-to-O silyl-group migrations. Phenyl isocyanate reacts with <b>1</b> to afford a dimeric bis­(phenylcyanamido)-bridged complex resulting from multiple silyl-group migrations and nitrogen-for-oxygen metathesis, akin to reactivity previously observed with carbon dioxide. The ability of <b>1</b> to activate carbon–chalcogen multiple bonds via silyl-group migration is further supported by its reactivity with carbon monoxide, where a nitrogen-for-oxygen metathesis is also observed with expulsion of hexamethyldisiloxane. For all reported reactions, intermediates are observable under appropriate conditions, allowing the formulation of mechanisms where insertion of the unsaturated substrate is followed by one or more silyl-group migrations to afford the observed products. This rich variety of reactivity confirms the ability of metal silylamides to activate exceptionally strong carbon–element multiple bonds and suggests that silylamides may be useful intermediates in nitrogen-atom and nitrene-group-transfer schemes

Chalcogen Extrusion from Heteroallenes and Carbon Monoxide by a Three-Coordinate Rh(I) Disilylamide

We report the reactions of several heteroallenes (carbon disulfide, carbonyl sulfide, and phenyl isocyanate) and carbon monoxide with a three-coordinate, bis­(phosphine)-supported Rh­(I) disilylamide (<b>1</b>). Carbon disulfide reacts with <b>1</b> to afford a silyltrithiocarbonate complex similar to an intermediate previously invoked in the deoxygenation of CO₂ by <b>1</b>, and prolonged heating affords a structurally unusual μ-κ²(<i>S</i>,<i>S</i>′):κ²(<i>S</i>,<i>S</i>′)-trithiocarbonate dimer. Carbonyl sulfide reacts with <b>1</b> to afford a structurally unique Rh­(SCNCS) metallacycle derived from two insertions of OCS and N-to-O silyl-group migrations. Phenyl isocyanate reacts with <b>1</b> to afford a dimeric bis­(phenylcyanamido)-bridged complex resulting from multiple silyl-group migrations and nitrogen-for-oxygen metathesis, akin to reactivity previously observed with carbon dioxide. The ability of <b>1</b> to activate carbon–chalcogen multiple bonds via silyl-group migration is further supported by its reactivity with carbon monoxide, where a nitrogen-for-oxygen metathesis is also observed with expulsion of hexamethyldisiloxane. For all reported reactions, intermediates are observable under appropriate conditions, allowing the formulation of mechanisms where insertion of the unsaturated substrate is followed by one or more silyl-group migrations to afford the observed products. This rich variety of reactivity confirms the ability of metal silylamides to activate exceptionally strong carbon–element multiple bonds and suggests that silylamides may be useful intermediates in nitrogen-atom and nitrene-group-transfer schemes