A series of Rh complexes derived from a PBP-type pincer ligand have been synthesized and characterized. It was previously reported that reaction of [(COD)RhCl]2 with 2,2′-bis(diisopropylphino)triphenylborane (1) resulted in a mixture of complexes containing a Z-type borane interaction (2-Cl), a boryl pincer (3a-Cl), and a η2 binding of the B–Ph bond to Rh (4-Cl). In this work, we demonstrate that analogous complexes are accessible by replacement of chloride with potentially bidentate acetylacetonate, carboxylate, and trifluoromethanesulfonate ligands. In addition, a new type of isomer was observed in complexes with acetate and pivalate, where the carboxylate bridges between Rh and B (3b-OAc, 3b-OPiv). All of these types of complexes are isomeric, and the preference for particular isomers for different anionic ligands varies. These isomers differ and are related by a change in the coordination mode of the oxygenous ligands and the migration of the Ph group between B and Rh.

Reactions of a trivalent diboryl Ir complex Ir-(Bpin)2 supported with a POCOP-type pincer ligand with a series of small molecules have been explored (pin = pinacolate). Ir-(Bpin)2 deoxygenated CO2 to make Ir-CO and O(Bpin)2. Attempts to deoxygenate other carbonyl compounds were not successful. The reaction with CO led to loss of B2pin2 by reductive elimination and its replacement on Ir with CO. Ir-(Bpin)2 did not react with p-trifluoromethylbenzaldehyde, acetone, ethyl acetate or benzonitrile. The reaction with ethyl formate resulted in formal dehydroethoxylation of this substrate, with formation of Ir-CO and HBpin and EtOBpin. Finally, treatment of Ir-(Bpin)2 with ethylene resulted in diboration of ethylene to produce pinB-CH2CH2-Bpin and Ir-(C2H4).Download high-res image (147KB)Download full-size image

The manuscript reports the synthesis, characterization, and analysis of electronic structure in a series of complexes of small perfluorocarbon ligands with the (PNP)Rh fragment (where PNP is a diarylamido/bis(phosphine) pincer ligand). Reactions of (PNP)Rh(TBE) as the source of (PNP)Rh with CHF3 and C2HF5 produced perfluoroalkylidene complexes (PNP)RhCF2 and (PNP)RhC(F)(CF3). (PNP)RhCF2 could also be obtained via the reaction of (PNP)Rh(TBE) with Me3SiCF3/CsF, with an admixture of (PNP)Rh(C2F4), where TBE = tert-butylethylene. Abstraction of fluoride from these neutral (PNP)RhCxFy complexes was successful, although only abstraction from (PNP)RhCF2 allowed unambiguous identification of the Rh product, [(PNP)RhCF]+. DFT computational studies allowed comparison of relative energies of (PNP)Rh(C2F4) and [(PNP)Rh(C2F3)]+ isomers as well as comparisons between the electronic structure of the CF2, C2F4, and CF+ complexes and their hydrocarbon analogues.

While investigating rhodium-catalyzed Negishi coupling, it was observed that the (PNP)Rh fragment readily inserted into zinc–carbon bonds to form isolable molecules with covalent rhodium–zinc bonds.

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) (1) and (PBP)Ir(H)(Cl) (2) were converted to the new complexes (PBP)IrH4 (3) and (PBP)Ir(Ph)(H) (4). Complexes 3 and 4 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 tert-butylethylene. The dicarbonyl complex (PBP)Ir(CO)2 (6) was also synthesized, by the reaction of CO with either 3 or 4. Intermediates (PBPhP)Ir(H)(CO)2 (5) and (PBP)IrH2(CO) (7) were observed in these reactions. Complex 7 could be obtained in pure form by comproportionation of 3 and 6. Solid-state structures of 3 and 6 were determined by X-ray crystallography.

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 tert-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.

The unexpectedly facile insertion of Rh or Ir into a B–Ph bond (reversible for Rh) converts a borane/bis(phosphine) precursor into a boryl/bis(phosphine) PBP pincer ligand. Interconversions between the boryl/borane/borate central functionality are demonstrated in reactions with dihydrogen.

Correction for ‘Convenient C-alkylation of the [HCB11Cl11]− carborane anion’ by Rodrigo Ramírez-Contreras and Oleg V. Ozerov, Dalton Trans., 2012, 41, 7842–7844.

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 (2), 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 (3) and (PCP)Re(O)I2 (4). Treatment of 2 with LiAlH4 resulted in the isolation of (PCP)ReH6 (5) upon workup. NMR data suggest a classical hexahydride nature for 5. 5 reacted with PMe3 and 4-dimethylaminopyridine (DMAP) by losing H2 and forming adducts (PCPiPr)ReH4(PMe3) (6) and (PCP)ReH4(DMAP) (7). On the other hand, reaction of 5 with CO resulted in loss of all the hydrides and formation of the Re(I) compound (PCPiPr)Re(CO)3 (8). Finally, thermolysis of the hexahydride 5 led to loss of half the hydrides as H2 and formation of the dimeric species formulated as (PCPiPr)2Re2H6 (9).

Following the report on the successful use of SiNN pincer complexes of iridium as catalysts for dehydrogenative borylation of terminal alkynes (DHBTA) to alkynylboronates, this work examined a wide variety of related pincer ligands in the supporting role in DHBTA. The ligand selection included both new and previously reported ligands and was developed to explore systematic changes to the SiNN framework (the 8-(2-diisopropylsilylphenyl)aminoquinoline). Surprisingly, only the diarylamido/bis(phosphine) PNP system showed any DHBTA reactivity. The specific PNP ligand (bearing two diisopropylphosphino side donors) used in the screen showed DHBTA activity inferior to SiNN. However, taking advantage of the ligand optimization opportunities presented by the PNP system via the changes in the substitution at phosphorus led to the discovery of a catalyst whose activity, longevity, and scope far exceeded that of the original SiNN archetype. Several Ir complexes were prepared in a model PNP system and evaluated as potential intermediates in the catalytic cycle. Among them, the (PNP)Ir diboryl complex and the borylvinylidene complex were shown to be less competent in catalysis and thus likely not part of the catalytic cycle.

Correction for ‘Ligand survey results in identification of PNP pincer complexes of iridium as long-lived and chemoselective catalysts for dehydrogenative borylation of terminal alkynes’ by Chun-I Lee et al., Chem. Sci., 2015, DOI: 10.1039/c5sc02161h.

New carborane anions carrying one or three triflyloxy substituents are described. The mono-triflyloxy substituted carborane can be halogenated to give pentabromo and decachloro derivatives with preservation of the B–OTf linkage. The use of [HCB11Cl10OTf]− as a weakly coordinating anion is demonstrated.

This paper presents the synthesis and characterization of a series of pincer ligands and their Ni, Pd, Pt, and Rh complexes. The ligands under examination are based on a diarylamine which is modified either by two phosphino (-PR2) substituents in the ortho-positions (PNP ligands) or by a combination of a phosphino and an iminyl (−CH═NX) substituent (PNN ligands). The ligands can be broken down into three groups: (a) C2v-symmetric PNP ligands with identical side −PR2 donors, (b) Cs-symmetric PNP′ ligands with different −PR2 side donors, and (c) PNN ligands containing a −PiPr2 side donor. All of the ligands under study readily formed square-planar complexes of the types (PNZ)PdCl, (PNZ)Pd(OAc), and (PNZ)RhCO, where PNZ is the corresponding anionic tridentate pincer ligand. For select PNP ligands, (PNP)NiCl and (PNP)PtCl were also studied. The (PNZ)MCl complexes (M = Ni, Pd, Pt) underwent quasireversible oxidation in cyclic voltammetry experiments. Based on the close similarity of formal potentials for Ni, Pd, and Pt analogs, and based on the previous literature evidence, these oxidation events are ascribed primarily to the PNZ ligand, and the E1/2 values can be used to compare the ease of oxidation of different ligands. A (PNP)PdCl complex containing methoxy substituents para- to the central nitrogen underwent two quasireversible oxidations. Two mono-oxidized complexes were isolated and structurally characterized in comparison to their neutral analog, revealing minimal changes in the bond distances and angles. Several other neutral complexes were also structurally characterized. The carbonyl stretching frequency in (PNZ)RhCO complexes was used to gauge the donating ability of the various pincer ligands toward the metal. Comparison of E1/2 values for (PNZ)PdCl and νCO values for (PNZ)RhCO revealed that the two are not consistently correlated across all the studied ligands and can be tuned to different degrees through judicious ligand alteration.

A comparison of Rh and Ir complexes of the SiNN ligand (combining Si–H, amido, and quinoline donors) reveals its great degree of adaptability. The amido donor can function as a boryl group acceptor, and the Si–H/metal interaction is highly variable. In contrast to Ir analogues, complexes of Rh do not catalyze dehydrogenative borylation of terminal alkynes but do act as modest benzene borylation catalysts.

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.

The alkylidyne complex (C5Me5)Ta(≡CPh)(PMe3)2Cl (1) was first reported by Schrock in 1978, but little if any follow-up work on 1 or other group 5 metal alkylidynes has been reported. This work discloses two avenues of reactivity of 1. Treatment of 1 with 3-hexyne resulted in the formation of a tantalacyclobutadiene. Abstraction of chloride from 1 led to a mixture of products that included isomers of [(C5Me5)Ta(═CHPh)(CH2PMe2)(PMe3)]+ (4), in which a C–H bond of a PMe3 ligand was added across the Ta≡C bond. The C–H activation was found to be reversible, and the equilibrium mixture functioned as an equivalent of a cationic Ta alkylidyne in reaction with 3-hexyne, producing a cationic tantalacyclobutadiene (6). Compounds 4 and 6 were structurally characterized in the solid state by XRD methods, with 6 being the first structurally characterized metallacyclobutadiene in group 5.

Herein we report the synthesis and spectroscopic observation of a Pt complex featuring a P2Si═ ligand containing a central silylene donor connected to phosphines via two o-arylene linkers. Species of the type (P2Si═)PtR undergo a net 1,2-migration of R from Pt to Si, which leads to highly unsaturated Pt cations displaying only a very distant interaction with the weakly coordinating carborane anion.

This article describes a well-defined pincer-Rh catalyst for C–S cross-coupling reactions. (POCOP)Rh(H)(Cl) serves as an active precatalyst for the coupling of aryl chlorides and bromides with aryl and alkyl thiols under reasonable conditions (3% mol cat., 110 °C, 2–24 h, >90% yield). For select substrates, >90% yields were obtained with catalyst loading as low as 0.1%. Key mechanistic intermediates have been isolated and fully characterized, including (POCOP)Rh(Ph)(SPh) (6a) and (POCOP)Rh(SPh2) (6b). The aryl/bis(phosphinite) (POCOP)Rh system has been shown to favor aryl thiolate reductive elimination at elevated temperatures and in some cases at room temperature, compared with the analogous diarylamido/bis(phosphine) (PNP)Rh pincer system. Concerted reductive elimination has been studied with 6a directly and in the presence of aryl bromide and aryl chloride traps. This investigation demonstrates a clear rate dependence on aryl chloride concentration during catalysis, a dependence that is absent when using aryl bromides. The rate of catalysis is dramatically reduced or brought to zero for ortho-tolyl halides, which can be traced to slower C–S coupling and slower carbon–halogen oxidative addition for ortho-substituted aryls. The influence of the sterics in the thiol component is less straightforward. The S–H oxidative addition product (POCOP)Rh(H)(SPh) (16) has been fully characterized and its reactivity has been examined, resulting in the isolation of the sodium-thiolate adduct (POCOP)Rh(NaSPh) (19). The solid-state structure of 19 shows Na interactions not only with sulfur, but also with a neighboring Rh and the chelating aryl carbon of the pincer framework. The reactivity of 16 and 19 indicates that these potential side products should not hinder catalysis.

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. RhI complexes of the general form (pincer)Rh(SiPr2) 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 E- and gem-enyne isomers, with small amounts of oligomers in some cases. The Z-enyne isomers were not observed except in two reactions. None of the catalysts showed useful selectivity for either the E- or the gem-enyne product. However, the POCOP-based catalysts bearing PiPr2 donor arms performed faster and possessed apparently greater longevity (up to 20 000 TON) than the previously reported pincer Rh catalysts.Keywords: alkyne dimerization; catalysis; enyne; phosphine; pincer; rhodium

Reaction of CO2 with a Pd(I)–Pd(I) dimer supported by amido/bis(phosphine) pincer PNP ligands produces free CO in the presence of Me3SiCl and Me3SiOTf.

Rhodium complexes supported by the aryl/bis(phosphinite) POCOP pincer ligand undergo reactions that constitute a RhI/RhIII synthetic cycle for C–N coupling analogous to the classical Pd0/PdII Buchwald–Hartwig chemistry. (POCOP)Rh(Ar)(X) complexes (X = Cl, Br) can be readily obtained by oxidative addition of ArX to the (POCOP)Rh fragment generated in situ from (POCOP)Rh(H)(Cl) (1) and NaOtBu. (POCOP)Rh(Ar)(X) complexes react with anilines and diphenylamine in the presence of an equimolar amount of NaOtBu to give RhIII aryl/amido complexes (POCOP)Rh(Ar)(NHAr′) and (POCOP)Rh(Ar)(NPh2). The intermediate (POCOP)Rh(p-F3CC6H4)(OtBu) (7) was isolated and shown to react irreversibly with p-MeC6H4NH2 to give (POCOP)Rh(p-F3CC6H4)(NHC6H4Me-p) (5). The latter undergoes reductive elimination of the diarylamine product p-F3CC6H4NHC6H4Me-p upon heating. The kinetics of this reaction point to a first-order process, and DFT calculations located a transition state for concerted C–N reductive elimination. Complex 1 effected catalytic formation of diarylamines from anilines and aryl chlorides and bromides at 115 °C in the presence of NaOtBu with modest turnover numbers of <15. In a separate reaction, 5 was degraded by NaOtBu under catalytic conditions; it is possible that it is one of the reasons for limited catalytic turnover. Reactions of 7 with pyrrolidine and N-methylaniline resulted in the formation of C6H5CF3, HOtBu, and imine complexes of (POCOP)Rh. This ostensibly proceeds via β-hydrogen elimination from the unobserved aryl/amido intermediate, followed by loss of C6H5CF3 by C–H reductive elimination. DFT calculations were consistent with this pathway and indicated that it possesses a significantly lower barrier than the concerted C–N reductive elimination.

This paper explores the potential for C–F reductive elimination from Rh(III) pincer complexes. A DFT computational study indicated that concerted C–F reductive elimination from (POCOP)Rh(CHCH2)(F) (3) (where POCOP is κ3P,C,P-2,6(iPr2PO)2C6H3, and aryl/bis(phosphinite) pincer ligand) possesses an experimentally plausible activation barrier of ΔG⧧ = 28.3 kcal/mol. This barrier is considerably lower than that calculated (35.7 kcal/mol) for the analogous C–F reductive elimination from (POCOP)Rh(Ph)(F) (1). The difference is ascribed to the need for a partial rotation of a phenyl or vinyl group in the transition state, with the phenyl being more encumbered by the steric bulk of the supporting pincer ligand. DFT calculations did not analyze the full range of possible side reactions, which have proven to be dominant. The attempted synthesis of 1 was unsuccessful because of competing C–C reductive elimination at the stage of the preparation of the (POCOP)Rh(CHCH2)(I) precursor. DFT calculations predicted C–C reductive elimination to be facile but markedly unfavorable in the monomeric unit because of inherent strain in the product. That strain is apparently relieved in dimerization that takes place with opening up of the pincer to become bridging between two Rh centers. The opening up of the pincer, and thus dimerization and C–C reductive elimination, was prevented by the use of the tBuPOCOP ligand (κ3P,C,P-2,6(iPr2PO)2-3,5-But2C6H3), which allowed isolation of (tBuPOCOP)Rh(CHCH2)(I) (11). Compound 11 was converted to (tBuPOCOP)Rh(CHCH2)(OTf) (12) via reaction with AgOTf. However, treatment of 11 with AgF or of 12 with CsF failed to produce (tBuPOCOP)Rh(CHCH2)(F), resulting instead in multiple products containing P–F bonds. On the other hand, 12 was cleanly converted to (tBuPOCOP)Rh(CHCH2)(OBut) (13) via reaction with NaOBut and then to (tBuPOCOP)Rh(CHCH2)(OC6H4F-p) (14) by treatment of 13 with p-fluorophenol. Neither 13 nor 14 gave any evidence of C–O reductive elimination. Instead, thermolysis of either 13 or 14 resulted in dehydroalkoxylation to the bimetallic divinylacetylene complex (tBuPOCOP)Rh(CH2═CHC≡CCH═CH2)Rh(tBuPOCOP) (15) as the major product.

Compounds with carbon–boron bonds are versatile intermediates for building more complex molecules via the elaboration of the carbon–boron bonds into other carbon–element bonds. The synthesis of carbon–boron bonds by catalytic dehydrogenative borylation of carbon–hydrogen bonds with dialkoxyboranes (RO)2BH is particularly attractive. It has been demonstrated for a variety of carbon–hydrogen bond types but not for the C(sp)–H bonds of terminal alkynes, for which hydroboration of the triple bond is a competing process. We report a new iridium catalyst that is strictly chemoselective for C–H borylation of terminal alkynes. The key to the success of this catalyst appears to be the new ancillary SiNN pincer ligand that combines amido, quinoline, and silyl donors and gives rise to structurally unusual Ir complexes. A variety of terminal alkynes (RC≡C—H) can be converted to their alkynylboronates (RC≡C—Bpin, where pin = pinacolate) in high yield and purity within minutes at ambient temperature.

This manuscript explores new chemistry that can be related to the unobserved 14-electron [(FPNP)Pt]+ transient (FPNP = (4-F-2-(iPr2P)C6H3)2N). Its reactivity can be accessed via abstraction of triflate from (FPNP)PtOTf (1) with K[B(C6F5)4] in various solvents serving as substrates. With benzene, toluene and fluorobenzene, net heterolytic splitting of an aromatic C–H bond across the N–Pt bond is observed, without any detectable intermediates, leading to the [(FPN(H)P)Pt–Ar]+ products (Ar = Ph, 3a; Ar = C6H4Me, 3b (ortho), 3c (meta), 3d (para); Ar = o-C6H4F, 3e). The latter can be alternatively prepared by protonation of the neutral (FPNP)Pt–Ar compounds. Compounds 3a–3e do not release free arene under thermolysis at 80 °C, and compounds 3b/c/d do not interconvert under ambient temperature. With chlorobenzene and bromobenzene, the kinetic product is the κ1-Cl or κ1-Br adduct [(FPNP)Pt–Cl–Ph]+ (4) or [(FPNP)Pt–Br–Ph]+ (10). Compound 4 rearranges into a C–H splitting product [(FPN(H)P)Pt–C6H4Cl]+ (3f), while 10 slowly reacts by formal transfer of Br atom to Pt. An analogous Cl atom transfer to Pt is observed upon the reaction of 1 with K[B(C6F5)4] in dichloromethane, producing [(FPNP)PtCl][B(C6F5)4] (9a) which features an oxidized FPNP ligand framework. X-Ray diffractometry established structures of [(FPN(H)P)Pt–C6H4F-o][B(C6F5)4] (3e, disordered rotamers), [(FPN(H)P)Pt–C6H4Me][B(C6F5)4] (disordered meta- and para-isomers 3c/d), and [(FPNP)PtCl][HCB11Cl11] (9b). DFT calculations at the PBE0 and M06-L levels on the free [(FPNP)Pt]+ cation predict a relatively small (10–12 kcal mol−1) separation between the singlet and the triplet states. The relatively low triplet energy is probably related to the viability of the unexpected halogen atom abstraction reactions.

Analysis of the structures of three (PNP)Pd–Pd(PNP) dimers [where PNP stands for anionic diarylamido/bis(phosphine) pincer ligands] has been carried out with the help of single-crystal X-ray diffractometry and density functional theory (DFT) calculations on isolated molecules. The three dimers under study possess analogous ancillary ligands; two of them differ only by an F versus Me substituent in a remote (five bonds away from Pd) position of the pincer ligand. Despite these close similarities, X-ray structural determinations revealed two distinct structural motifs: a highly symmetric molecule with a long Pd–Pd bond or a highly distorted molecule with Pd–Pd bonds ca. 0.14 Å shorter. DFT calculations on a series of (PNP)Pd–Pd(PNP) dimers (as molecules in the gas phase) confirmed the existence of these distinct minima for dimers carrying large isopropyl substituents on the P-donor atoms (as in the experimental structure). These minima are nearly isoergic conformers. Evidently, the electronically preferred symmetric structure for the dimer (with a square-planar environment about Pd and a linear N–Pd–Pd–N vector) is not sterically possible with the preferred Pd–Pd distance. Thus, the minima correspond to either a symmetric structure with a long Pd–Pd bond distance or a structure with a short Pd–Pd distance but with substantial distortions in the Pd coordination environment to alleviate steric conflict. This notion is supported by finding only a single minimum (symmetric and with short Pd–Pd bonds) for each of the dimers carrying smaller substituents (H or Me) on the P atoms, regardless of the remote substitution.

A rhodium(II) complex of a diarylamido/bis(phosphine) PNP pincer ligand, (PNP)Rh(OTf) (2, where OTf = O3SCF3 and PNP = [κ3-P,N,P-(4-Me-2-(iPr2P)-C6H3)2N]), has been prepared by oxidation of the rhodium(I) precursor (PNP)Rh(H2C═CHBut) (1) with AgOTf. A series of related rhodium(II) complexes of the general formula (PNP)Rh(X) (where X = OAc (3), OSiPh3 (4), OC6H4F (5), Cl (6)) was synthesized via simple anion metathesis reactions starting from 2. In addition, complexes 3 and 6 could be prepared by hydrogen atom abstraction from (PNP)Rh(H)(OAc) (7) or (PNP)Rh(H)(Cl) (8) with TEMPO ((2,2,6,6-tetramethylpiperidin-1-yl)oxyl). Solid-state X-ray structures of compounds 2–6 revealed an approximately square-planar environment about Rh. Analysis of the structural features of 2–6, EPR spectroscopic data, and DFT computational studies are most consistent with a +2 oxidation state for rhodium. Reactions of 2, 3, 5, and 6 with H2 were explored. The reaction of 2 with H2 gave the new complex (PN(H)P)Rh(H)2(OTf) (9), and the reaction of 3 with H2 produced (PNP)Rh(H)(OAc) (7), whereas the reaction of 5 with H2 gave the known (PNP)Rh(H2) (10), all with complete consumption of the starting rhodium(II) complexes. In contrast, the reaction of 6 with H2 produced a mixture of (PNP)Rh(H)(Cl) (8) and (PN(H)P)Rh(H)2(Cl) (11) in apparent equilibrium with 6 and H2. (PNP)Rh(H2) (10) was identified as an elongated dihydrogen complex.

Reactions of a series of phenyl esters with a (PNP)Rh fragment have been studied. PhO2CPh only underwent C–H oxidative addition (OA). PhO2CCF3 chiefly underwent acyl–oxygen OA. PhO2CBut and PhO2CNEt2 initially underwent OA of an ortho-C–H bond of the phenyl group but continued thermolysis led to the phenyl–oxygen OA products.

A method for C-alkylation of [HCB11Cl11]− anions using potassium tert-butoxide and alkyl iodides in tert-butanol is presented. Trimethylammonium salts of the corresponding C-alkylated derivatives are easily isolated and obtained in good yields.

The Pd(I)−Pd(I) dimer [(FPNP)Pd−]2 reacts with O2 upon exposure to light to produce either the superoxide (FPNP)PdO2 or the peroxide [(FPNP)PdO−]2, which exist in equilibrium with free O2. Both complexes contain square-planar Pd(II) centers. The unpaired electron density in (FPNP)PdO2 is localized on the superoxide ligand.

Addition of TiCl4 or ZrCl4 to (PNP)Rh(CH2CHtBu) (1) rapidly gives complexes (PNP)Rh(MCl3)(Cl) (M = Ti, 2; Zr, 3) in 75–77% yield (PNP = (4-Me-2-(iPr2P)–C6H3)2N). Compound 2 can also be synthesized via a reaction of (PNP)RhCl with TiCl3 or of (PNP)TiCl3 with 1/2 [(cod)RhCl]2.

A serendipitously discovered construction of a carbazole nucleus by lithiation of N-methylated bis(4-methyl-2,6-dibromophenyl)amine is described. It was used to synthesize an NNN pincer-type ligand that combines a central carbazole site (N-methylated in the precursor ligand) with two flanking aldimine donors bearing mesityl substituents. The installation of this ligand on Pd was accomplished via an N−Me cleaving reaction with (COD)PdCl2 producing MeCl and (NNN)PdCl (where NNN is an anionic carbazolyl/bis(imine) pincer ligand). Several (NNN)PdX complexes were characterized spectroscopically. (NNN)PdOTf (−OTf = triflate or −O3SCF3) readily reacted with stoichiometric amounts of water in benzene or dichloromethane to give a cationic water adduct [(NNN)Pd(OH2)]OTf. An X-ray diffraction study on a single crystal of (NNN)PdCl revealed an almost perfectly square planar environment about Pd and an almost perfectly planar carbazole/bis(imine) conjugated system. Cyclic voltammetry of (NNN)PdCl showed quasi-reversible oxidation at E1/2 = 0.72 V vs Fc/Fc+ which is most likely ligand-based.

The use of weakly coordinating anions BArF4 (where ArF = 3,5-(CF3)2C6H3) and CB11H12 allows one to access clean reactions of the [(PNP)Pd]+ fragment (PNP = bis(2-iPr2P4̅-Me-phenyl)amido) with the B–H bond in catecholborane (CatBH) and catecholdiboron (CatBBCat). In both cases, a net heterolytic cleavage of B–H or B–B takes place, with the nitrogen atom of PNP being a recipient of a boryl fragment. The resultant products [(PN(BCat)P)PdH]+ (2) and [(PN(BCat)P)PdBCat]+ (3) were isolated as either BArF4 or CB11H12 salts and fully characterized. They are susceptible to hydrolysis, with the B–N bond hydrolyzing selectively and rapidly at RT to give [(PN(H)P)PdH]+ (1) and [(PN(H)P)PdBCat]+ (4). Notably, 4 and 2 are isomers, but they do not interconvert even under thermolysis at 90 °C. The Pd–B bond in 4 can be further hydrolyzed more slowly, to give 1. On the other hand, a Pd–B bond was formed from the Pd–H bond in 2 by reaction with excess CatBH (and evolution of H2), producing 3.

The diarylamido/bis(phosphine) PNP pincer ligand (2-iPr2P-4-MeC6H3)2N has been evaluated as a scaffold for supporting a BF2 fragment. Compound (PNP)BF2 (6) was prepared by simple metathesis of (PNP)Li (5) with Me2SBF3. NMR spectra of 6 in solution are of apparent C2 symmetry, suggestive of a symmetric environment about boron. However, a combination of X-ray structural studies, low-temperature NMR investigations, and DFT calculations consistently establish that the ground state of this molecule contains a classical four-coordinate boron with a PNBF2 coordination environment, with one phosphine donor in PNP remaining “free”. Fortuitous formation of a single crystal of (PNP)BF2·HBF4 (7), in which the “free” phosphine is protonated, furnished another structure containing the same PNBF2 environment about boron for comparison and the two PNBF2 environments in 6 and 7 are virtually identical. DFT studies on several other diarylamido/bis(phosphine) pincer (PNP)BF2 systems were carried out and all displayed a similar four coordinate PNBF2 environment in the ground state structures. The symmetric appearance of the room-temperature NMR spectra is explained by the rapid interconversion between two degenerate four-coordinate, C1-symmetric ground-state forms. Lineshape analysis of the 1H and 19F NMR spectra over a temperature range of 180–243 K yielded the activation parameters ΔH‡ = 8.1(3) kcal mol−1 and ΔS‡ = −6.0(15) eu, which are broadly consistent with the calculated values. Calculations indicate that the exchange of phosphine donors at the boron center proceeds by an intrinsically dissociative mechanism.

A pincer-based (POCOP)Rh catalyst is demonstrated to be an active and well-defined catalyst for the coupling of select aryl and alkyl Grignards with aryl iodides. The proposed intermediacy of oxidative addition of aryl halides to (POCOP)RhI is supported by the isolation of the oxidative addition product.

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.

The (PNP)Rh fragment (2) can be conveniently accessed by dissociation of L from the four-coordinate complexes (PNP)Rh(L) (L = SPri2, 3; L = H2C═CHBut, 10), which contain the tridentate PNP pincer ligand (2-iPr2P-4-Me-C6H4)2N−. A new and a more straightforward synthesis of 10 is reported, yielding 65% of 10 based on RhCl3(H2O)x. A number of new aryl halide oxidation addition products (AHOAP) of the general formula (PNP)Rh(Ar)(Hal) (Hal = Cl, Br, I) have been synthesized through oxidative addition (OA) reactions of meta- and para-substituted aryl halides with 3 or 10. The rotation about the Rh−Caryl bond is restricted, resulting in rotamers for the meta-substituted aryls that are distinct on the NMR spectroscopy time scale. Reactions of some aryl halides containing a p-NO2 or p-CO2Me with 3 or 10 led to the observation of products of C−H OA that are ostensibly stabilized by coordination to the NO2 or CO2Me group. Analogous C−H OA products were observed for the halide-free nitrobenzene and ethyl benzoate, as well. However, the C−H OA products are thermodynamically unstable with respect to the isomeric AHOAP, to which they convert upon thermolysis. A Hammett-style analysis of the relative electronic effects of the para substituents X in the OA reactions of p-HalC6H4X with 10 was carried out. The positive values of ρ obtained (ρ = 1.51(15) for Ar−Cl, ρ = 0.70(9) for Ar−Br, and ρ = 0.92(9) for Ar−I) illustrate the increase in the OA rate with increasing electron-withdrawing effect of X. Issues relating to the usefulness of the Hammett parameters in comparing various OA reactions are discussed. Comparison with analogous studies on the OA of aryl halides to Pd(0) complexes leads to the notion that the electronic effects have an impact on the rate similar to, but less pronounced than, that of the (PNP)Rh system, possibly indicative of an earlier transition state for the OA of aryl halides with (PNP)Rh.

Expedient and economical methods for the synthesis of undecahalogenated carborane anions have been developed.

Syntheses of several Mn complexes supported by a monoanionic amido/bis(phosphino) PNP ligand (PNP = [2-P(CHMe2)2-4-MeC6H3]2N) from anhydrous MnCl2 are reported. Treatment of (PNP)Li (2) with MnCl2 in tetrahydrofuran (THF) led to isolation of either (PNP)Mn(μ-Cl)2Li(THF)2 (5) or (PNP)MnCl (6), depending on the workup. Reaction of 6 with 2 equiv of MeLi resulted in isolation of (PNP)Mn(μ-Me)2Li(THF)2 (7) that is structurally similar to 5. Reduction of 6 in the presence of pyridine produced material analytically consistent with (PNP)Mn(py)3 (8), whereas reduction in the presence of 2,2′-bipyridine gave fully characterized (PNP)Mn(bipy) (9). Compounds 5 and 7 display magnetic moments indicative of high-spin Mn(II) (S = 5/2). The magnetic moment of 9 (S = 2) was interpreted as an antiferromagnetic combination of a high-spin Mn(II) center and a singly reduced bipyridine ligand. Addition of a single CO ligand to 9 generated diamagnetic, low-spin (PNP)Mn(bipy)(CO) (10). Solid-state structures of 5, 7, 9, and 10 were determined by X-ray diffraction methods and used in conjunction with density functional theory studies to analyze the electronic nature of the (PNP)Mn complexes under study.

A terminal palladium (II) fluoride complex (FPNP)PdF (where FPNP is a an anionic fluoro-substituted diarylamido/bis(phosphine) pincer ligand) has been prepared and characterized spectroscopically and structurally. An X-ray diffraction study revealed an approximately square–planar environment about Pd and a short Pd–F bond distance. (FPNP)PdF reacted with silanes containing electron-withdrawing groups on Si by exchange of fluoride with one of the substituents on Si. An analysis of the 19F chemical shifts of both the Pd-bound fluoride and of the fluorines on the backbone of the FPNP ligand is provided.A terminal palladium fluoride supported by an FPNP ligand has been synthesized and characterized. This complex possesses an approximately square–planar environment about Pd and a short Pd–F bond. It reacts with silane reagents containing electron-withdrawing groups by exchange of fluorine with a substituent on silicon.

The diarylamido/bis(phosphine) PNP pincer ligand (2-iPr2P-4-MeC6H3)2N has been evaluated as a scaffold for supporting a BF2 fragment. Compound (PNP)BF2 (6) was prepared by simple metathesis of (PNP)Li (5) with Me2SBF3. NMR spectra of 6 in solution are of apparent C2 symmetry, suggestive of a symmetric environment about boron. However, a combination of X-ray structural studies, low-temperature NMR investigations, and DFT calculations consistently establish that the ground state of this molecule contains a classical four-coordinate boron with a PNBF2 coordination environment, with one phosphine donor in PNP remaining “free”. Fortuitous formation of a single crystal of (PNP)BF2·HBF4 (7), in which the “free” phosphine is protonated, furnished another structure containing the same PNBF2 environment about boron for comparison and the two PNBF2 environments in 6 and 7 are virtually identical. DFT studies on several other diarylamido/bis(phosphine) pincer (PNP)BF2 systems were carried out and all displayed a similar four coordinate PNBF2 environment in the ground state structures. The symmetric appearance of the room-temperature NMR spectra is explained by the rapid interconversion between two degenerate four-coordinate, C1-symmetric ground-state forms. Lineshape analysis of the 1H and 19F NMR spectra over a temperature range of 180–243 K yielded the activation parameters ΔH‡ = 8.1(3) kcal mol−1 and ΔS‡ = −6.0(15) eu, which are broadly consistent with the calculated values. Calculations indicate that the exchange of phosphine donors at the boron center proceeds by an intrinsically dissociative mechanism.

Correction for ‘Convenient C-alkylation of the [HCB11Cl11]− carborane anion’ by Rodrigo Ramírez-Contreras and Oleg V. Ozerov, Dalton Trans., 2012, 41, 7842–7844.

This manuscript explores new chemistry that can be related to the unobserved 14-electron [(FPNP)Pt]+ transient (FPNP = (4-F-2-(iPr2P)C6H3)2N). Its reactivity can be accessed via abstraction of triflate from (FPNP)PtOTf (1) with K[B(C6F5)4] in various solvents serving as substrates. With benzene, toluene and fluorobenzene, net heterolytic splitting of an aromatic C–H bond across the N–Pt bond is observed, without any detectable intermediates, leading to the [(FPN(H)P)Pt–Ar]+ products (Ar = Ph, 3a; Ar = C6H4Me, 3b (ortho), 3c (meta), 3d (para); Ar = o-C6H4F, 3e). The latter can be alternatively prepared by protonation of the neutral (FPNP)Pt–Ar compounds. Compounds 3a–3e do not release free arene under thermolysis at 80 °C, and compounds 3b/c/d do not interconvert under ambient temperature. With chlorobenzene and bromobenzene, the kinetic product is the κ1-Cl or κ1-Br adduct [(FPNP)Pt–Cl–Ph]+ (4) or [(FPNP)Pt–Br–Ph]+ (10). Compound 4 rearranges into a C–H splitting product [(FPN(H)P)Pt–C6H4Cl]+ (3f), while 10 slowly reacts by formal transfer of Br atom to Pt. An analogous Cl atom transfer to Pt is observed upon the reaction of 1 with K[B(C6F5)4] in dichloromethane, producing [(FPNP)PtCl][B(C6F5)4] (9a) which features an oxidized FPNP ligand framework. X-Ray diffractometry established structures of [(FPN(H)P)Pt–C6H4F-o][B(C6F5)4] (3e, disordered rotamers), [(FPN(H)P)Pt–C6H4Me][B(C6F5)4] (disordered meta- and para-isomers 3c/d), and [(FPNP)PtCl][HCB11Cl11] (9b). DFT calculations at the PBE0 and M06-L levels on the free [(FPNP)Pt]+ cation predict a relatively small (10–12 kcal mol−1) separation between the singlet and the triplet states. The relatively low triplet energy is probably related to the viability of the unexpected halogen atom abstraction reactions.

Expedient and economical methods for the synthesis of undecahalogenated carborane anions have been developed.

New carborane anions carrying one or three triflyloxy substituents are described. The mono-triflyloxy substituted carborane can be halogenated to give pentabromo and decachloro derivatives with preservation of the B–OTf linkage. The use of [HCB11Cl10OTf]− as a weakly coordinating anion is demonstrated.

While investigating rhodium-catalyzed Negishi coupling, it was observed that the (PNP)Rh fragment readily inserted into zinc–carbon bonds to form isolable molecules with covalent rhodium–zinc bonds.

The manuscript reports the synthesis, characterization, and analysis of electronic structure in a series of complexes of small perfluorocarbon ligands with the (PNP)Rh fragment (where PNP is a diarylamido/bis(phosphine) pincer ligand). Reactions of (PNP)Rh(TBE) as the source of (PNP)Rh with CHF3 and C2HF5 produced perfluoroalkylidene complexes (PNP)RhCF2 and (PNP)RhC(F)(CF3). (PNP)RhCF2 could also be obtained via the reaction of (PNP)Rh(TBE) with Me3SiCF3/CsF, with an admixture of (PNP)Rh(C2F4), where TBE = tert-butylethylene. Abstraction of fluoride from these neutral (PNP)RhCxFy complexes was successful, although only abstraction from (PNP)RhCF2 allowed unambiguous identification of the Rh product, [(PNP)RhCF]+. DFT computational studies allowed comparison of relative energies of (PNP)Rh(C2F4) and [(PNP)Rh(C2F3)]+ isomers as well as comparisons between the electronic structure of the CF2, C2F4, and CF+ complexes and their hydrocarbon analogues.

A novel PNN-type pincer ligand has been accessed via imine reduction with LiAlH4 to provide the phosphine diamino proligand, PNHNH (1). The ligand, 1, as well as PNHP can be metallated directly, via N–H cleavage, with L2ReOX2(OEt) precursors to access six-coordinate (PNP)ReOCl2 (2) and (PNNH)ReOX2 (3-Cl, X = Cl; 3-Br, X = Br) in good yield. 3-Cl and 3-Br undergo dehydrohalogenation upon treatment with NEt3, furnishing the five-coordinate phosphine/diamido PNN-type compounds (PNN)ReOX (4-Cl, X = Cl; 4-Br, X = Br) in excellent yield, presenting as a mixture of rotameric diasteromers. The reversibility of this deprotonation, and the coordinative unsaturation of 4-Cl is shown in reactions with HCl(aq) and PMe3 providing 3-Cl and 4-PMe3, respectively. Treatment of 4-Cl with AgOAc, AgOTf, or NaHBEt3 lead to formation of (PNN)ReO(OAc) (4-OAc), (PNN)ReO(OTf) (4-OTf), and (PNN)ReO(H) (4-H), all isolated in excellent yields in varying diasteromeric ratios. The nature of the isomerism was analyzed based on solid-state structural studies and solution NMR data.

Platinum complexes of the diimino/carbazolyl NNN pincer ligand have been prepared and characterized. Cyclometallation of the mesityl substituent was observed to occur unexpectedly easily in (NNN)PtOTf, which also catalyzed cyclometallation in (NNN)PtMe.

Reaction of CO2 with a Pd(I)–Pd(I) dimer supported by amido/bis(phosphine) pincer PNP ligands produces free CO in the presence of Me3SiCl and Me3SiOTf.

Reactions of a series of phenyl esters with a (PNP)Rh fragment have been studied. PhO2CPh only underwent C–H oxidative addition (OA). PhO2CCF3 chiefly underwent acyl–oxygen OA. PhO2CBut and PhO2CNEt2 initially underwent OA of an ortho-C–H bond of the phenyl group but continued thermolysis led to the phenyl–oxygen OA products.

Addition of TiCl4 or ZrCl4 to (PNP)Rh(CH2CHtBu) (1) rapidly gives complexes (PNP)Rh(MCl3)(Cl) (M = Ti, 2; Zr, 3) in 75–77% yield (PNP = (4-Me-2-(iPr2P)–C6H3)2N). Compound 2 can also be synthesized via a reaction of (PNP)RhCl with TiCl3 or of (PNP)TiCl3 with 1/2 [(cod)RhCl]2.

Following the report on the successful use of SiNN pincer complexes of iridium as catalysts for dehydrogenative borylation of terminal alkynes (DHBTA) to alkynylboronates, this work examined a wide variety of related pincer ligands in the supporting role in DHBTA. The ligand selection included both new and previously reported ligands and was developed to explore systematic changes to the SiNN framework (the 8-(2-diisopropylsilylphenyl)aminoquinoline). Surprisingly, only the diarylamido/bis(phosphine) PNP system showed any DHBTA reactivity. The specific PNP ligand (bearing two diisopropylphosphino side donors) used in the screen showed DHBTA activity inferior to SiNN. However, taking advantage of the ligand optimization opportunities presented by the PNP system via the changes in the substitution at phosphorus led to the discovery of a catalyst whose activity, longevity, and scope far exceeded that of the original SiNN archetype. Several Ir complexes were prepared in a model PNP system and evaluated as potential intermediates in the catalytic cycle. Among them, the (PNP)Ir diboryl complex and the borylvinylidene complex were shown to be less competent in catalysis and thus likely not part of the catalytic cycle.

Correction for ‘Ligand survey results in identification of PNP pincer complexes of iridium as long-lived and chemoselective catalysts for dehydrogenative borylation of terminal alkynes’ by Chun-I Lee et al., Chem. Sci., 2015, DOI: 10.1039/c5sc02161h.

A method for C-alkylation of [HCB11Cl11]− anions using potassium tert-butoxide and alkyl iodides in tert-butanol is presented. Trimethylammonium salts of the corresponding C-alkylated derivatives are easily isolated and obtained in good yields.

A pincer-based (POCOP)Rh catalyst is demonstrated to be an active and well-defined catalyst for the coupling of select aryl and alkyl Grignards with aryl iodides. The proposed intermediacy of oxidative addition of aryl halides to (POCOP)RhI is supported by the isolation of the oxidative addition product.