Acceptor PCP Pincer Iridium Chemistry: (CF3PCP)IrIII Coordination Properties

The underappreciated influence of ancillary halide on metal–ligand proton tautomerism

Syntheses of Vaska-type complexes [IrP₂X(CO)] (P = phosphine, X = halide) with all four common halides (fluoride, chloride, bromide, and iodide) was attempted using a protic and hemilabile imidazolyl di-tert-butyl phosphine ligand. In the solid-state, all four complexes were found to be ionic with the halides in the outer-sphere, and the fourth coordination site of the square plane occupied by the imidazole arm of the ligand. In solution, however, the chloride complex was found to be in equilibrium with an octahedral Ir^III–H species at room temperature. For the bromide and iodide analogs, the corresponding Ir^III–H species were also observed but only after heating the solutions. The neutral Ir^I Vaska's analogs for X = Cl, Br, and I were obtained upon addition of excess halide salt, albeit heating was required for X = Br and I. The Ir^III–H species are proposed to originate from tautomerization of minor amounts of the electron rich neutral Vaska analog (halide inner-sphere and phosphines monodentate) that are in equilibrium with the ionic species. Heating is required for the larger anions of bromide and iodide to overcome a kinetic barrier associated with their movement to an inner-sphere position prior to tautormerization. For the fluoride analog, the Ir^III–H was not observed, attributable to strong hydrogen bonding interactions of the imidazolyl proton with the fluoride anion.

Ligand protonated Ir^I bisphosphine carbonyl complexes isolated as halide salts equilibrate with their neutral Ir^III–H congeners in solution. The equilibrium constant and energy barrier to interconversion are dependent on the identity of the halide.

Group 9 and 10 Metal Complexes of an Ylide-Substituted Phosphine: Coordination versus Cyclometalation and Oxidative Addition

Ylide-substituted phosphines (YPhos) have been shown to be excellent ligands for several transition metal catalyzed reactions. Investigations of the coordination behavior of the YPhos ligand YSPPh2 (1) [with YS = (Ph3P)(SO2Tol)C] toward group 9 and 10 metals revealed a surprisingly diverse coordination chemistry of the ligand. With Ni(CO)4, the formation of a di- as well as tricarbonyl complex is observed depending on the reaction conditions. In [( κP,η 2 -benzene-1)Ni(CO)2] the phosphine ligand also coordinates via a phosphonium bound phenyl group to the metal leading to a unique nickel η 2 -arene interaction, which can be viewed as an intermediate state toward P-C bond activation. Full cleavage of the P-C bond takes place with [Rh(COD)Cl]2 leading to a complex salt with [( κP,κO-1)Rh(COD)]+ as cation and a dirhodium trichloride complex anion. Here, YSPPh2 underwent P-C bond cleavage to thus act as an anionic diphosphine ligand. In contrast, in [( κP,κO-1)Rh(COD)]+ as well as [( κP,κO-1)Rh(CO)Cl], formed from the reaction of 1 with [Rh(CO)2Cl]2, the YPhos ligand acts as bidentate ligand complexing the metal via the phosphine and sulfonyl moiety with an intact PPh3 unit. A further type of coordination is observed with [Ir(COD)Cl]2. Here, phosphine coordination is accompanied by C-H activation at one of the phosphonium bound phenyl groups leading to a cyclometalated complex.

H2 addition to (Me4PCP)Ir(CO): studies of the isomerization mechanism

Reduced steric demand of the Me4PCP pincer ligand (PCP = κ3-C6H4-1,3-[CH2PR2]2, R = Me), allows for a more open metal center. This is evident through structure and reactivity comparisons between (Me4PCP)Ir derivatives and other (R4PCP)Ir complexes (R = tBu, iPr, CF3). In particular, isomerization from cis-(R4PCP)Ir(H)2(CO) to trans-(R4PCP)Ir(H)2(CO) is more facile when R = Me than when R = iPr. Deuterium incorporation in the hydride ligands from solvent C6D6 was observed during this isomerization when R = Me. This deuterium exchange has not been observed for other analogous R4PCP iridium complexes. A kinetic study of the cis/trans isomerization combined with computational studies suggests that the cis/trans isomerization proceeds through a migratory-insertion pathway involving a formyl intermediate.

Crystal structure of an iridium(III) complex of the [C(dppm)2] PCP pincer ligand system and its conjugate CH acid form

The crystal structures of [Ir^III(CO)(C(dppm)₂-κ³P,C,P′)ClH]Cl and [Ir^III(CO)(CH(dppm)₂-κ³P,C,P′)ClH]Cl₂ have been determined. Both complexes show a slightly distorted octa­hedral coordinated Ir^III centre. The PCP pincer ligand system is arranged in a meridional manner.

After the successful creation of the newly designed PCP carbodi­phospho­rane (CDP) ligand [Reitsamer et al. (2012 ▸). Dalton Trans.41, 3503–3514; Stallinger et al. (2007 ▸). Chem. Commun. pp. 510–512], the treatment of this PCP pincer system with the transition metal iridium and further the analysis of the structures by single-crystal diffraction and by NMR spectroscopy were of major inter­est. Two different iridium complexes, namely (bis­{[(di­phenyl­phosphan­yl)meth­yl]di­phenyl­phosphanyl­idene}methane-κ³P,C,P′)carbonyl­chlorido­hydridoiridium(III) chloride di­chloro­methane tris­olvate, [Ir^III(CO){C(dppm)₂-κ³P,C,P′}ClH]Cl·3CH₂Cl₂ (1) and the closely related (bis­{[(di­phenyl­phosphan­yl)meth­yl]di­phenyl­phosphanyl­idene}methanide(1+)-κ³P,C,P′)carbonyl­chlorido­hy­dridoirid­ium(III) dichloride–hydro­chloric acid–water (1/2/5.5), [Ir^III(CO){CH(dppm)₂-κ³P,C,P′)ClH]Cl}₂ (2), have been designed and both complexes show a slightly distorted octa­hedral coordinated Ir^III centre. The PCP pincer ligand system is arranged in a meridional manner, the CO ligand is located trans to the central PCP carbon and a hydride and chloride are located perpendicular above and below the P₂C₂ plane. With an Ir—C_CDP distance of 2.157 (5) Å, an Ir—CO distance of 1.891 (6) Å and a quite short C—O distance of 1.117 (7) Å, complex 1 presents a strong carbonyl bond. Complex 2, the corresponding CH acid of 1, shows an additionally attached proton at the carbodi­phospho­rane carbon atom located anti­periplanar to the hydride of the metal centre. In comparison with complex 1, the Ir—C_CDP distance of 2.207 (3) Å is lengthened and the Ir—C—O values indicate a weaker trans influence of the central carbodi­phospho­rane carbon atom.

Iridium complexes of perimidine-based N-heterocyclic carbene pincer ligandsviaaminal C–H activation

The reactions ofN,N′-bis(phosphinomethyl)dihydroperimidine pro-ligands H₂C(NCH₂PR₂)₂C₁₀H₆-1,8 (R = Ph1a, R = Cy1b) with iridium(i) substrates have been investigated and shown to readily result in chelate-assisted C–H activation processes.

Dehydrogenation of n-Alkanes by Solid-Phase Molecular Pincer-Iridium Catalysts. High Yields of α-Olefin Product

We report the transfer-dehydrogenation of gas-phase alkanes catalyzed by solid-phase, molecular, pincer-ligated iridium catalysts, using ethylene or propene as hydrogen acceptor. Iridium complexes of sterically unhindered pincer ligands such as (iPr4)PCP, in the solid phase, are found to give extremely high rates and turnover numbers for n-alkane dehydrogenation, and yields of terminal dehydrogenation product (α-olefin) that are much higher than those previously reported for solution-phase experiments. These results are explained by mechanistic studies and DFT calculations which jointly lead to the conclusion that olefin isomerization, which limits yields of α-olefin from pincer-Ir catalyzed alkane dehydrogenation, proceeds via two mechanistically distinct pathways in the case of ((iPr4)PCP)Ir. The more conventional pathway involves 2,1-insertion of the α-olefin into an Ir-H bond of ((iPr4)PCP)IrH2, followed by 3,2-β-H elimination. The use of ethylene as hydrogen acceptor, or high pressures of propene, precludes this pathway by rapid hydrogenation of these small olefins by the dihydride. The second isomerization pathway proceeds via α-olefin C-H addition to (pincer)Ir to give an allyl intermediate as was previously reported for ((tBu4)PCP)Ir. The improved understanding of the factors controlling rates and selectivity has led to solution-phase systems that afford improved yields of α-olefin, and provides a framework required for the future development of more active and selective catalytic systems.

Dehydrogenation of formic acid by Ir-bisMETAMORPhos complexes: experimental and computational insight into the role of a cooperative ligand

The synthesis of Ir-complexes with three bisMETAMORPhos ligands is reported. The activity of these systems towards HCOOH dehydrogenation and the dual role of the ligand during catalysis is discussed, using spectroscopic and computational methods.

The synthesis and tautomeric nature of three xanthene-based bisMETAMORPhos ligands (La–Lc) is reported. Coordination of these bis(sulfonamidophosphines) to Ir(acac)(cod) initially leads to the formation of Ir^I(L^H) species (1a), which convert via formal oxidative addition of the ligand to Ir^III(L) monohydride complexes 2a–c. The rate for this step strongly depends on the ligand employed. Ir^III complexes 2a–c were applied in the base-free dehydrogenation of formic acid, reaching turnover frequencies of 3090, 877 and 1791 h^–1, respectively. The dual role of the ligand in the mechanism of the dehydrogenation reaction was studied by ¹H and ³¹P NMR spectroscopy and DFT calculations. Besides functioning as an internal base, bisMETAMORPhos also assists in the pre-assembly of formic acid within the Ir-coordination sphere and aids in stabilizing the rate-determining transition state through hydrogen-bonding.

Pyrrole-based new diphosphines: Pd and Ni complexes bearing the PNP pincer ligand

A new class of diphosphine PNP pincer ligand, 2,5-bis(diphenylphosphinomethyl)pyrrole 2, was synthesized by the reaction between Ph2PH and 2,5-bis(dimethylaminomethyl)pyrrole in 90% yield. The analogous reaction of Ph2PH with 1,9-bis(dimethylaminomethyl)diphenyldipyrrolylmethane readily afforded a PNNP type diphosphine ligand, 1,9-bis(diphenylphosphinomethyl)diphenyldipyrrolylmethane 5 in 92% yield. These phosphine compounds were oxidized with H2O2 and S8 to give the corresponding phosphoryl and thiophosphoryl compounds 6-9 in very good yields. The reaction of the PNP pincer ligand 2 with [PdCl2(PhCN)2] in the presence of Et3N afforded the mononuclear Pd(II) complex, [PdCl{C4H2N-2,5-(CH2PPh2)2-κ(3)PNP}] 10 in 87% yield. Conversely, treatment of 2 with [PdCl2(PhCN)2] in the absence of Et3N gave the dinuclear Pd(II) complex [Pd2Cl4{μ-C4H3N-2,5-(CH2PPh2)2-κ(2)PP}2], the structure which is proposed based on the spectroscopic data. When 2 was treated with Pd(0) precursor [Pd2(dba)3]·CHCl3 the dinuclear Pd(I) complex [Pd2{μ-C4H2N-2,5-(CH2PPh2)2-κ(2)PN,κ(1)P}2], 12, was obtained in 23% yield. The formation of complex 12 is solvent dependent, which transforms into complex 10 in CDCl3 as studied by variable temperature (1)H and (31)P NMR methods. Treatment of 2 with [Ni(OAc)2]·4H2O gave the mononuclear Ni(II) pincer complex [Ni(OAc){C4H2N-2,5-(CH2PPh2)2-κ(3)PNP}], 13, which upon treatment with an excess of LiCl or LiBr or KI afforded the respective halide ion substituted Ni(II) complexes, [NiX{C4H2N-2,5-(CH2PPh2)2-κ(3)PNP}] (X = Cl, Br, and I), 14-16, in very good yields. The structures of 5, 2,5-bis(diphenylphosphorylmethyl)pyrrole 6, 10, 12, and 14-16 were determined by the single crystal X-ray diffraction method. In the structure of 12, two short contacts between the diagonally positioned Pd and P atoms are observed. To understand these weak interactions, density functional theory (DFT) calculations were done and an interaction MO diagram is presented.

Acceptor pincer Ru(II) chemistry

A series of new acceptor pincer Ru(II) complexes are reported. The carbonyl complex ((CF(3))PCP)Ru(CO)Cl(2)(-)Et(3)NH(+) is obtained from the reaction of (CF(3))PCPH with [(cod)Ru(μ-Cl)(2)](n). Chloride displacement with (CF(3))PCPH, CO, PPh(3), or C(2)H(4) gave complexes of the type ((CF(3))PCP)Ru(CO)(L)Cl, or in the case of (CF(3))PCPH, the bridged dimeric complex [((CF(3))PCP)Ru(CO)Cl](2)(μ-(CF(3))PCPH). Chloride abstraction from ((CF(3))PCP)Ru(CO)(L)Cl, L = CO or PPh(3) by (Et(3)Si)(2)(μ-H)(+)B(C(6)F(5))(4)(-) followed by Et(3)N addition produced ((CF(3))PCP)Ru(CO)(L)(H) products. Reaction of cis-((CF(3))PCP)Ru(CO)(2)Cl with (Et(3)Si)(2)(μ-H)(+)B(C(6)F(5))(4)(-) in the presence of excess CO afforded ((CF(3))PCP)Ru(CO)(3)(+). The reaction of (CF(3))PCPH with (PPh(3))(3)Ru(H)(O(2)CR) (R = Me or Ph) produced the corresponding carboxylate complexes ((CF(3))PCP)Ru(PPh(3))(O(2)CR).

Diastereoselective synthesis and coordination chemistry of enantiopure keto-bis-(2-phosphametallocene)s

Enantiopure 2-(chlorocarbonyl)phosphametallocenes [MCp*(2-{COCl}-3,4-Me(2)-5-Ph-PC(4))] (M = Fe, Ru) react with phospholide anions to give 2-phosphametallocene-2'-acylphospholides K[MCp*(2-CO-2'-{3',4'-Me(2)-5'-PhPC(4)}-3,4-Me(2)-5-Ph-PC(4))] (M = Fe, Ru) and these have been converted into keto-bis-(2-phosphametallocene)s through reaction with [FeClCp*(tmeda)]; templation of this process with CuBr gives rise to the C(2)- (or pseudo-C(2)- when M = Ru) symmetric form of [{MCp*(3,4-Me(2)-5-Ph-PC(4))}(2)-2,2'-(CO)] (M = Fe, Ru; Fe, Fe) with high (>95%) diastereoselectivity. X-Ray structures of these ligands coordinated to [RuCp*Cl] and [PtCl(2)] centres show that the spatial orientation of the very flexible keto-bis-(2-phosphametallocene) structure is highly responsive to the coordination sphere of the chelated platinum or ruthenium centre.

Triethylsilyl Perfluoro-Tetraphenylborate, [Et(3)Si][F(20)-BPh(4)], a widely used Non-Existent Compound

The commonly used triethylsilyl per-fluoro-tetraphenylborate salt, [Et(3)Si(+)][F(20)-BPh(4) (-)], has been misidentified. As prepared, the cation is a hydride-bridged silane adduct [R(3)Si-H-SiR(3) (+)]. Under favorable circumstances it can be an effective source of the triethylsilylium ion Et(3)Si(+) but in the absence of a stabilizing base the potent electrophilicity of Et(3)Si(+) decomposes the "inert" F(20)-BPh(4) (-) counterion.