Computational Studies of Reactions of Insertion of Rhodium(I) and Iridium(I) into N–H, N–CH₃, and NCH₂–H Bonds of the Diarylamine-Based PNP Pincer Ligands

This work presents the investigation by DFT methods of the mechanism of N–Me and N–H oxidative addition in reactions of the secondary amine form of the PNP pincer ligand 4-Me-2-(iPr2P)-C6H3)2NH (or PN(H)P), its N-methylated derivative 4-Me-2-(iPr2P)-C6H3)2NMe (or PN(Me)P), and a version of the latter whose aromatic rings are “tied” with a CH2CH2 linker (or TPN(Me)P) with Rh(I) and Ir(I). Reactions were considered by starting from (κ3-PN(H)P)MCl, (κ3-PN(Me)P)MCl, and (κ3-TPN(Me)P)MCl (M = Rh, Ir). Oxidative addition from (κ3-PN(H)P)MCl to give (PNP)M(H)(Cl) is predicted to proceed with essentially no barrier via direct migration of H from N to the metal. The analogous direct migration of Me from N to the metal is predicted to be the dominant mechanism for both Rh systems, with the calculated barrier for (κ3-PN(Me)P)RhCl of 21.8 kcal/mol being in reasonable agreement with the experimental value of 24.0(18) kcal/mol. For Ir, an alternative pathway that involves initial NCH2–H oxidative addition, followed by CH2 extrusion and C–H recombination, is calculated to be competitive with direct Me transfer, especially for the “tied” ligand where it is preferred. This alternative pathway entails prohibitively high barriers for both Rh systems (>35 kcal/mol), which can be traced to the high energy of the intermediate in which a CH2 carbene is bound to a RhIII center. In general, the energies of all barriers and intermediates are lower with the “tied” ligand. DFT calculations also evaluate the energetics of the NCH2–H oxidative addition intermediates. These were observed experimentally for only the “tied” ligand system (for both Rh and Ir), and the DFT energies are consistent with these observations

Computational Studies of Reactions of Insertion of Rhodium(I) and Iridium(I) into N–H, N–CH₃, and NCH₂–H Bonds of the Diarylamine-Based PNP Pincer Ligands

This work presents the investigation by DFT methods of the mechanism of N–Me and N–H oxidative addition in reactions of the secondary amine form of the PNP pincer ligand 4-Me-2-(iPr2P)-C6H3)2NH (or PN(H)P), its N-methylated derivative 4-Me-2-(iPr2P)-C6H3)2NMe (or PN(Me)P), and a version of the latter whose aromatic rings are “tied” with a CH2CH2 linker (or TPN(Me)P) with Rh(I) and Ir(I). Reactions were considered by starting from (κ3-PN(H)P)MCl, (κ3-PN(Me)P)MCl, and (κ3-TPN(Me)P)MCl (M = Rh, Ir). Oxidative addition from (κ3-PN(H)P)MCl to give (PNP)M(H)(Cl) is predicted to proceed with essentially no barrier via direct migration of H from N to the metal. The analogous direct migration of Me from N to the metal is predicted to be the dominant mechanism for both Rh systems, with the calculated barrier for (κ3-PN(Me)P)RhCl of 21.8 kcal/mol being in reasonable agreement with the experimental value of 24.0(18) kcal/mol. For Ir, an alternative pathway that involves initial NCH2–H oxidative addition, followed by CH2 extrusion and C–H recombination, is calculated to be competitive with direct Me transfer, especially for the “tied” ligand where it is preferred. This alternative pathway entails prohibitively high barriers for both Rh systems (>35 kcal/mol), which can be traced to the high energy of the intermediate in which a CH2 carbene is bound to a RhIII center. In general, the energies of all barriers and intermediates are lower with the “tied” ligand. DFT calculations also evaluate the energetics of the NCH2–H oxidative addition intermediates. These were observed experimentally for only the “tied” ligand system (for both Rh and Ir), and the DFT energies are consistent with these observations

Exhaustive Chlorination of [B₁₂H₁₂]²⁻ without Chlorine Gas and the Use of [B₁₂Cl₁₂]²⁻ as a Supporting Anion in Catalytic Hydrodefluorination of Aliphatic C−F Bonds

The fully chlorinated closo-dodecaborate salt Cs2[B12Cl12] was prepared in high yield from Cs2[B12H12] and SO2Cl2 in acetonitrile at refluxing temperature. [Ph3C]2[B12Cl12] was obtained by simple metathesis reactions. Catalytic hydrodefluorination of benzotrifluoride sp3 C−F bonds was accomplished using [Ph3C]2[B12Cl12] as a precatalyst and Et3SiH as a stoichiometric reagent. Full consumption of the sp3 C−F bonds in p-FC6H4CF3 and C6F5CF3 with a turnover number up to 2000 was achieved

Binuclear Palladium Complexes Supported by Bridged Pincer Ligands

A series of binucleating ligands each containing two tridentate pincer sites based on a central, monoanionic aryl (C) donor flanked by neutral phosphinito (P) and imino (N) donors, [(PCN)-(CH2)n-(PCN)] (1), or a central, monoanionic amido (N) flanked by neutral phosphine (P) and imino donors (N), [(PNN)-(CH2)n-(PNN)] (4), are presented. The metalating sites were linked through the condensation of two equivalents of m-hydroxybenzaldehyde or the purposively built asymmetric diarylamine [(H)­N­(2-C­(O)­H-4-Me-C6H3)­(2-P­(iPr)2-4-Me-C6H3)] (3) with primary α,ω-diamines (1,2-diaminoethane, n = 2; 1,4-diaminobutane, n = 4), which simultaneously constructed the imino arms and bridged the pincer cores. Ligands 1 and 4 were then used in the synthesis of neutral, square-planar palladium­(II) complexes 5 and 6, [(PCN-Cn)­Pd2X2, (PNN-Cn)­Pd2X2; n = 2, 4; X = Cl, OAc, OTf]. The difference in the trans influence of the central donor was demonstrated through X-ray crystal structures of 5b-Cl, (PCN-C4)­Pd2Cl2, and 6b-Cl, (PNN-C4)­Pd2Cl2. The (PNN-Cn)­Pd2X2 complexes (6) proved to be redox-active, presumably via oxidation of the ligand, and cyclic voltammetry illustrated the extent to which electronic communication between the two pincer sites is mediated by the length of the bridge between them. (PCN-Cn)­Pd2OTf2 (5-OTf) and (PNN-Cn)­Pd2OTf2 (6-OTf) complexes reacted with hydride donors such as Et3SiH or β-hydride-containing alkoxides such as NaOiPr to generate bridging-hydride monocations [(PCN)­Pd-H-Pd­(PCN)-Cn]­[OTf] (5-H, n = 2, 4) and [(PNN)­Pd-H-Pd­(PNN)-C2]­[OTf] (6a-H), where the supported Pd–Pd separation was also found to be affected by the bridge length. In the case of the (CH2)4-bridged (PNN-C4)­PdOTf (6b-OTf), a bridging-hydride monocation was not observed. Instead, the formation of [(PN­(H)­N)­Pd-Pd­(PNN)-C4]­[OTf] (6b-H), protonated at the amido N of the ligand with a direct Pd­(I)–Pd­(I) bond, was recorded

Binuclear Palladium Complexes Supported by Bridged Pincer Ligands

A series of binucleating ligands each containing two tridentate pincer sites based on a central, monoanionic aryl (C) donor flanked by neutral phosphinito (P) and imino (N) donors, [(PCN)-(CH₂)_<i>n</i>-(PCN)] (<b>1</b>), or a central, monoanionic amido (N) flanked by neutral phosphine (P) and imino donors (N), [(PNN)-(CH₂)_<i>n</i>-(PNN)] (<b>4</b>), are presented. The metalating sites were linked through the condensation of two equivalents of <i>m</i>-hydroxybenzaldehyde or the purposively built asymmetric diarylamine [(H)­N­(2-C­(O)­H-4-Me-C₆H₃)­(2-P­(<i>i</i>Pr)₂-4-Me-C₆H₃)] (<b>3</b>) with primary α,ω-diamines (1,2-diaminoethane, <i>n</i> = 2; 1,4-diaminobutane, <i>n</i> = 4), which simultaneously constructed the imino arms and bridged the pincer cores. Ligands <b>1</b> and <b>4</b> were then used in the synthesis of neutral, square-planar palladium­(II) complexes <b>5</b> and <b>6</b>, [(PCN-C_<i>n</i>)­Pd₂X₂, (PNN-C_<i>n</i>)­Pd₂X₂; <i>n</i> = 2, 4; X = Cl, OAc, OTf]. The difference in the <i>trans</i> influence of the central donor was demonstrated through X-ray crystal structures of <b>5b-Cl</b>, (PCN-C₄)­Pd₂Cl₂, and <b>6b-Cl</b>, (PNN-C₄)­Pd₂Cl₂. The (PNN-C_<i>n</i>)­Pd₂X₂ complexes (<b>6</b>) proved to be redox-active, presumably via oxidation of the ligand, and cyclic voltammetry illustrated the extent to which electronic communication between the two pincer sites is mediated by the length of the bridge between them. (PCN-C_<i>n</i>)­Pd₂OTf₂ (<b>5-OTf</b>) and (PNN-C_<i>n</i>)­Pd₂OTf₂ (<b>6-OTf</b>) complexes reacted with hydride donors such as Et₃SiH or β-hydride-containing alkoxides such as NaO<i>i</i>Pr to generate bridging-hydride monocations [(PCN)­Pd-H-Pd­(PCN)-C_<i>n</i>]­[OTf] (<b>5-H</b>, <i>n</i> = 2, 4) and [(PNN)­Pd-H-Pd­(PNN)-C₂]­[OTf] (<b>6a-H</b>), where the supported Pd–Pd separation was also found to be affected by the bridge length. In the case of the (CH₂)₄-bridged (PNN-C₄)­PdOTf (<b>6b-OTf</b>), a bridging-hydride monocation was not observed. Instead, the formation of [(PN­(H)­N)­Pd-Pd­(PNN)-C₄]­[OTf] (<b>6b-H</b>), protonated at the amido N of the ligand with a direct Pd­(I)–Pd­(I) bond, was recorded

One-Pot Synthesis of 1,3-Bis(phosphinomethyl)arene PCP/PNP Pincer Ligands and Their Nickel Complexes

A one-pot synthesis of arene-based PCP/PNP ligands has been developed. The reaction of 1,3-bis­(bromomethyl)­benzene or 2,6-bis­(bromomethyl)­pyridine with various chlorophosphines in acetonitrile afforded bis-phosphonium salts. These salts can then be reduced by magnesium powder to yield PCP or PNP ligands. In comparison to traditional synthetic methods for making PCP/PNP ligands involving the use of secondary phosphines, this new alternative method allows for the use of chlorophosphines, which are cheaper, safer to handle, and have a broader range of commercially available derivatives. This is especially true for the chlorophosphines with less bulky alkyl groups. Moreover, the one-pot procedure can be extended to allow for the direct synthesis of PCP/PNP nickel complexes. By using nickel powder as the reductant, the resulting nickel halide was found to directly undergo metalation with the PCP or PNP ligand to generate nickel complexes in high yields

A Series of Pincer-Ligated Rhodium Complexes as Catalysts for the Dimerization of Terminal Alkynes

A series of pincer complexes of Rh has been prepared and tested as catalysts for the dimerization of terminal alkynes. The pincers included aryl/bis­(phosphinite) POCOP, aryl/bis­(phosphine) PCP, and diarylamido/bis­(phosphine) PNP ligands. Rh^I complexes of the general form (pincer)­Rh­(S^<i>i</i>Pr₂) or (pincer)­Rh­(H₂) were used as catalysts. In addition, the apparent donating ability of the pincer ligands was gauged through the carbonyl stretching frequencies in (pincer)­Rh­(CO) complexes by IR spectroscopy. All surveyed Rh complexes acted as catalysts for dimerization of 4-ethynyltoluene, 1-hexyne, or trimethysilylacetylene. The products were a mixture of <i>E</i>- and <i>gem</i>-enyne isomers, with small amounts of oligomers in some cases. The <i>Z</i>-enyne isomers were not observed except in two reactions. None of the catalysts showed useful selectivity for either the <i>E</i>- or the <i>gem</i>-enyne product. However, the POCOP-based catalysts bearing P^<i>i</i>Pr₂ donor arms performed faster and possessed apparently greater longevity (up to 20 000 TON) than the previously reported pincer Rh catalysts

Synthesis and Characterization of PBP Pincer Iridium Complexes and Their Application in Alkane Transfer Dehydrogenation

This work reports on the synthesis of several new complexes of Ir supported by a diarylboryl/bis­(phosphine) PBP pincer ligand. The previously reported complexes (PBP)­Ir­(Ph)­(Cl) (<b>1</b>) and (PBP)­Ir­(H)­(Cl) (<b>2</b>) were converted to the new complexes (PBP)­IrH₄ (<b>3</b>) and (PBP)­Ir­(Ph)­(H) (<b>4</b>). Complexes <b>3</b> and <b>4</b> serve similarly as precatalysts for transfer dehydrogenation of cyclooctane. The turnover numbers achieved were relatively modest but were increased (to 220 at 200 °C) when 1-hexene was used as a sacrificial hydrogen acceptor vs <i>tert</i>-butylethylene. The dicarbonyl complex (PBP)­Ir­(CO)₂ (<b>6</b>) was also synthesized, by the reaction of CO with either <b>3</b> or <b>4</b>. Intermediates (PB^PhP)­Ir­(H)­(CO)₂ (<b>5</b>) and (PBP)­IrH₂(CO) (<b>7</b>) were observed in these reactions. Complex 7 could be obtained in pure form by comproportionation of <b>3</b> and <b>6</b>. Solid-state structures of <b>3</b> and <b>6</b> were determined by X-ray crystallography

Halobenzenes and Ir(I): Kinetic C−H Oxidative Addition and Thermodynamic C−Hal Oxidative Addition

A (PNP)Ir fragment undergoes facile, room-temperature oxidative addition of C−H bonds in arenes and haloarenes in preference to aromatic carbon−halogen bonds. This preference, however, is determined to be kinetic in nature. Oxidative addition of C−Cl and C−Br is preferred thermodynamically. The products of the C−Cl or C−Br oxidative addition are separated from the C−H oxidative addition products by a high activation barrier and are only accessible at >100 °C. Of the C−H oxidative addition products of chlorobenzene, the isomer with the o-ClC6H4 ligand has the lowest energy

Synthesis and Characterization of PBP Pincer Iridium Complexes and Their Application in Alkane Transfer Dehydrogenation

This work reports on the synthesis of several new complexes of Ir supported by a diarylboryl/bis­(phosphine) PBP pincer ligand. The previously reported complexes (PBP)­Ir­(Ph)­(Cl) (<b>1</b>) and (PBP)­Ir­(H)­(Cl) (<b>2</b>) were converted to the new complexes (PBP)­IrH₄ (<b>3</b>) and (PBP)­Ir­(Ph)­(H) (<b>4</b>). Complexes <b>3</b> and <b>4</b> serve similarly as precatalysts for transfer dehydrogenation of cyclooctane. The turnover numbers achieved were relatively modest but were increased (to 220 at 200 °C) when 1-hexene was used as a sacrificial hydrogen acceptor vs <i>tert</i>-butylethylene. The dicarbonyl complex (PBP)­Ir­(CO)₂ (<b>6</b>) was also synthesized, by the reaction of CO with either <b>3</b> or <b>4</b>. Intermediates (PB^PhP)­Ir­(H)­(CO)₂ (<b>5</b>) and (PBP)­IrH₂(CO) (<b>7</b>) were observed in these reactions. Complex 7 could be obtained in pure form by comproportionation of <b>3</b> and <b>6</b>. Solid-state structures of <b>3</b> and <b>6</b> were determined by X-ray crystallography