Organometallics in Superacidic Media: Generation of Highly Electrophilic (Fluoroalkyl)phosphine Pt(II) Cationic Complexes

Organometallics in Superacidic Media:  Generation of Highly Electrophilic (Fluoroalkyl)phosphine Pt(II) Cationic Complexe

Synthesis, Structure, and Reactivity of [(dfepe)Pt(μ-H)]₂. An Unusual Example of Conformational Polymorphism

Treatment of the methyl complex (dfepe)Pt(Me)(O2CCF3) (dfepe = (C2F5)2PCH2CH2P(C2F5)2) with 1 atm of H2 in acetone at 20 °C cleanly affords the hydride-bridged dimer [(dfepe)Pt(μ-H)]2 (1), which crystallizes in both green (1a, monoclinic) and purple (1b, orthorhombic) forms. Diffraction data indicate that 1a and 1b are conformational polymorphs which primarily differ in the degree of rotation about the Pt−Pt bond (interchelate angle = 7° (1a), 49° (1b)) and intermolecular crystal packing. DSC experiments reveal a mildly exothermic (0.51 kJ mol-1) irreversible phase transition from 1a to 1b at 91 °C (mp of 1b = 112 °C). In CH2Cl2, 1 exhibits a distinctive dπ → dσ* transition at 428 nm (ε ∼ 4000 mol-1 cm-1) and a reversible redox couple at +0.63 V (CH2Cl2, vs SCE). [(dfepe)Pt(μ-H)]2 serves as a versatile precursor to (dfepe)Pt0 compounds:  exposure of acetone solutions of 1 to 1 atm of C2H4, C2F4, or CO results in clean conversions to (dfepe)Pt(η2-C2H4) (2), (dfepe)Pt(η2-C2F4) (3), and (dfepe)Pt(CO) (4) (ν(CO) = 2044 cm-1), respectively. With the exception of 3, these Pt(0) derivatives readily revert to the hydride dimer under 1 atm of H2 at ambient temperature. Under 2 atm of CO, the reversible formation of (dfepe)Pt(CO)2 (5) from 4 is observed. X-ray data for 1a:  a = 23.527(5) Å, b = 10.451(2) Å, c = 16.623(3) Å, β = 110.20(3)°, monoclinic, C2/c, Z = 4, R = 0.0670. Data for 1b:  a = 13.354(3) Å, b = 16.232(3) Å, c = 18.525(4) Å, orthorhombic, Pccn, Z = 4, R = 0.0423

New Electrophilic Iridium(I) Complexes: H−H and C−H Bond Heterolysis by [(dfepe)Ir(μ-X)]₂ (X = O₂CCF₃, OTf)

New electrophilic dimeric iridium(I) complexes [(dfepe)Ir(μ-X)]2 (dfepe = (C2F5)2PCH2CH2P(C2F5)2; X = O2CCF3, OTf) have been prepared and their reactions with H2 and cyclopentane examined. Treatment of [(cod)Ir(O2CCF3)]2 with dfepe produced an ionic product [(dfepe)Ir(cod)]+[(dfepe)Ir(O2CCF3)2]- (1), which in refluxing benzene rearranged with loss of cyclooctadiene to form [(dfepe)Ir(μ-O2CCF3)]2 (2). The corresponding reaction of [(cod)Rh(O2CCF3)]2 with dfepe yielded [(dfepe)Rh(μ-O2CCF3)]2 (3) directly. X-ray diffraction analysis of 2 revealed a hinged dimeric geometry with an unusually large interplanar angle of 82.7° defined by the two 4-coordinate metal centers (Ir(1)−Ir(2) = 4.307 Å). The triflate-bridged analogue of 2 was prepared via an indirect route:  addition of 1 equiv of triflic acid to (dfepe)Ir(η3-C3H5) yielded the allyl hydride complex (dfepe)Ir(η3-C3H5)(H)(OTf) (4), which eliminated propylene in refluxing heptane to quantitatively afford [(dfepe)Ir(μ-O3SCF3)]2 (5). The structure of 4 was confirmed by X-ray diffraction. In contrast to [(dfepe)Ir(μ-Cl)]2, the acetate- and triflate-bridged analogues 2 and 5 are reactive toward both H2 and alkane C−H bonds. Treatment of 2 with H2 (20 °C) or cyclopentane (150 °C) cleanly afforded (dfepe)2Ir2(μ-H)2(H)(μ-O2CCF3) (6) and CpIr(dfepe), respectively. Surprisingly, the corresponding reactions of 5 are significantly slower, suggesting that the concomitant release of the stronger acid CF3SO3H may inhibit these heterolysis reactions

Synthesis of [(dfepe)Pt(Me)(NC₅F₅)]⁺B(C₆F₅)₄⁻, a Highly Active Ethylene Dimerization Catalyst

The synthesis of cationic adducts (dfepe)Pt(Me)(L)+ (dfepe = (C2F5)2PCH2CH2P(C2F5)2; L = MeCN, CO, C2H4, C5F5N, μ-Cl) are reported. Treatment of (cod)Pt(Me)Cl with AgSbF6 in acetonitrile followed by the addition of dfepe afforded (dfepe)Pt(Me)(CH3CN)+SbF6−. Addition of B(C6F5)3 to (dfepe)Pt(Me)(O2CCF3) in methylene chloride afforded the structurally characterized borane association product (dfepe)Pt(Me)[(O2CCF3)B(C6F5)3] in high yield. Attempts to displace the [(O2CCF3)B(C6F5)3]− anion with donor ligands resulted in loss of borane and regeneration of (dfepe)Pt(Me)(O2CCF3). Addition of the mesitylenium acid (1,3,5-C6H4Me3)+B(C6F5)4− to (dfepe)PtMe2 in methylene chloride at ambient temperatures resulted in chloride abstraction and the precipitation of the chloride-bridged dimeric complex [{(dfepe)Pt(Me)}2(μ-Cl)]+B(C6F5)4−, which has been structurally characterized. In contrast, treatment of (dfepe)PtMe2 with (1,3,5-C6H4Me3)+B(C6F5)4− in pentafluoropyridine at ambient temperature resulted in the precipitation of the structurally characterized pentafluoropyridine adduct [(dfepe)Pt(Me)(NC5F5)]+B(C6F5)4− in good yield. Exposure of [(dfepe)Pt(Me)(NC5F5)]+B(C6F5)4− to 1 atm of CO in o-difluorobenzene gave the carbonyl complex [(dfepe)Pt(Me)(CO)]+B(C6F5)4−. In marked contrast to previously reported platinum systems, [(dfepe)Pt(Me)(NC5F5)]+B(C6F5)4− is a very active ethylene dimerization catalyst at ambient temperature (600 psi ethylene, 22 °C in ortho-difluorobenzene, 150 turnovers h−1). The ethylene adduct [(dfepe)Pt(Me)(η2-C2H4)]+B(C6F5)4− has been spectroscopically characterized at −20 °C

Synthesis of [(dfepe)Pt(Me)(NC₅F₅)]⁺B(C₆F₅)₄⁻, a Highly Active Ethylene Dimerization Catalyst

The synthesis of cationic adducts (dfepe)Pt(Me)(L)+ (dfepe = (C2F5)2PCH2CH2P(C2F5)2; L = MeCN, CO, C2H4, C5F5N, μ-Cl) are reported. Treatment of (cod)Pt(Me)Cl with AgSbF6 in acetonitrile followed by the addition of dfepe afforded (dfepe)Pt(Me)(CH3CN)+SbF6−. Addition of B(C6F5)3 to (dfepe)Pt(Me)(O2CCF3) in methylene chloride afforded the structurally characterized borane association product (dfepe)Pt(Me)[(O2CCF3)B(C6F5)3] in high yield. Attempts to displace the [(O2CCF3)B(C6F5)3]− anion with donor ligands resulted in loss of borane and regeneration of (dfepe)Pt(Me)(O2CCF3). Addition of the mesitylenium acid (1,3,5-C6H4Me3)+B(C6F5)4− to (dfepe)PtMe2 in methylene chloride at ambient temperatures resulted in chloride abstraction and the precipitation of the chloride-bridged dimeric complex [{(dfepe)Pt(Me)}2(μ-Cl)]+B(C6F5)4−, which has been structurally characterized. In contrast, treatment of (dfepe)PtMe2 with (1,3,5-C6H4Me3)+B(C6F5)4− in pentafluoropyridine at ambient temperature resulted in the precipitation of the structurally characterized pentafluoropyridine adduct [(dfepe)Pt(Me)(NC5F5)]+B(C6F5)4− in good yield. Exposure of [(dfepe)Pt(Me)(NC5F5)]+B(C6F5)4− to 1 atm of CO in o-difluorobenzene gave the carbonyl complex [(dfepe)Pt(Me)(CO)]+B(C6F5)4−. In marked contrast to previously reported platinum systems, [(dfepe)Pt(Me)(NC5F5)]+B(C6F5)4− is a very active ethylene dimerization catalyst at ambient temperature (600 psi ethylene, 22 °C in ortho-difluorobenzene, 150 turnovers h−1). The ethylene adduct [(dfepe)Pt(Me)(η2-C2H4)]+B(C6F5)4− has been spectroscopically characterized at −20 °C

Investigation of Iridium ^CF₃PCP Pincer Catalytic Dehydrogenation and Decarbonylation Chemistry

The iridium fluorinated pincer complex (^CF₃PCP)­Ir­(cod) (^CF₃PCP = 2,6-C₆H₃(CH₂P­(CF₃)₂)₂) catalyzes hydrogen transfer from cyclooctane (coa) to <i>tert</i>-butylethylene (tbe) in 1/1 coa/tbe at 200 °C to give cyclooctene (coe) and neohexane (tba) at an initial rate of 40 TO h^–1. In 5/1 coa/tbe, higher initial activity (155 TO h^–1) and higher turnovers (2580 TON’s after 1450 min) are found. Samples of 95% tbe contain significant amounts of isoprene (2-methyl-1,3-butadiene), which reacts with (^CF₃PCP)­Ir­(cod) to initially form (^CF₃PCP)­Ir­(isoprene). Alkene inhibition studies show that (^CF₃PCP)Ir is only modestly inhibited (67% reduced initial activity) in the presence of 800 equiv of added coe. Unlike donor pincer systems, no decrease in activity is noted under 1 atm of N₂ or in the presence of excess water. Hydrogenation of (^CF₃PCP)­Ir­(L) (L = cod, isoprene) did not produce (^CF₃PCP)­Ir­(H)_<i>x</i> but instead afforded the first example of the unusual aryl-bridged bimetallic complex [(μ-1κ²(<i>P</i>,<i>C</i>),2κ²(<i>P</i>′,<i>C</i>)-^CF₃PCP)­Ir­(H)₂]₂(μ-^CF₃PCPH)­(μ-H), which has been isolated and crystallographically characterized. Ir­(I) pincer complexes (^CF₃PCP)­Ir­(L) (L = MeP­(C₂F₅)₂, CO, dfepe (dfepe = (C₂F₅)₂PCH₂CH₂P­(C₂F₅)₂)) also serve as moderately active aldehyde decarbonylation catalyst precursors for 2-naphthaldehyde with similar activities in diglyme (1.7 TO h^–1, 152 °C) and in 1,4-dioxane (0.052 TO h^–1, 94 °C). The catalyst resting states are the corresponding five-coordinate carbonyl complexes (^CF₃PCP)­Ir­(MeP­(C₂F₅)₂)­(CO), (^CF₃PCP)­Ir­(CO)₂, and [(^CF₃PCP)­Ir­(CO)]₂(μ-dfepe). DFT studies indicate that the preferred catalyst resting state for alkane dehydrogenation, (^CF₃PCP)­Ir­(cod), can be ascribed to the lower steric requirements of the CF₃-substituted pincer ligand

Acceptor PCP Pincer Iridium(I) Chemistry: Stabilization of Nonmeridional PCP Coordination Geometries

The preparation of a series of four-coordinate complexes (CF3PCP)Ir(L) (L = CO, DBU, nbe, coe, MeP(C2F5)2 (dfmp)) and five-coordinate complexes (CF3PCP)Ir(L)(L′) (L = L′ = CO, dfmp, nbd, cod, (C2F5)2PCH2CH2P(C2F5)2 (dfepe); L = PhCN, L′ = C2H4) from dehydrohalogenation of (CF3PCP)Ir(C2H4)(H)Cl with Et3N in the presence of trapping ligands is reported. (CF3PCP)Ir(L) and (CF3PCP)Ir(L)2 for L = CO, dfmp have been structurally characterized and establish a distorted-trigonal -bipyramidal coordination geometry for (CF3PCP)Ir(L)2 with a bent PCP unit and inequivalent axial and equatorial L coordination sites. (CF3PCP)Ir(L)(L′) systems (L = L′ = CO, C2H4; L = PhCN, L′ = C2H4) are highly fluxional, with ligand site interconversion free energy barriers determined by VT NMR of 9.7 kcal mol−1 (L = L′ = CO), 12.2 kcal mol−1 (L = L′ = C2H4), and 16.1 kcal mol−1 (L = C2H4, L′ = PhCN). A dissociative site exchange mechanism is proposed. (CF3PCP)Ir(L) complexes readily undergo oxidative addition reactions. Addition of H2 to (CF3PCP)Ir(CO) reversibly forms trans-(CF3PCP)Ir(CO)(H)2 at ambient temperatures. In contrast, addition of H2 to (CF3PCP)Ir(dfmp) affords fac,cis-(CF3PCP)Ir(dfmp)(H)2 as the major product, with an unusual facially coordinated pincer group. VT NMR monitoring of the reaction of (CF3PCP)Ir(CO) with H2 established the initial formation of fac,cis-(CF3PCP)Ir(CO)(H)2 followed by conversion to mer,cis-(CF3PCP)Ir(CO)(H)2 prior to isomerization to mer,trans-(CF3PCP)Ir(CO)(H)2. The unusual stability of (CF3PCP)Ir(L)2 and fac,cis-(CF3PCP)Ir(L)(H)2 complexes is attributable to the increased stability of nonplanar (PCP)M moieties possessing strongly π-accepting phosphorus groups

Acceptor Pincer Coordination Chemistry of Platinum: Reactivity Properties of (^CF₃PCP)Pt(L)⁺ (L = NC₅F₅, C₂H₄)

Synthetic strategies toward the synthesis of electron-poor pincer complexes (CF3PCP)PtH and (CF3PCP)Pt(η2-H2)+ are described. Metathesis of (CF3PCP)PtCl with hydride reagents does not lead to (CF3PCP)PtH; (CF3PCP)PtCl with KH in tetrahydrofuran (THF) afforded an unusual metallated bimetallic pincer product (CF3PCP)Pt[κ1-C,κ3−P,C,P-2,6-(CHP(CF3)2)(CH2P(CF3)2)-C6H3]PtCl, which has been structurally characterized. Chloride abstraction from (CF3PCP)PtCl or protonolysis of (CF3PCP)PtMe in the presence of H2 gives the structurally characterized hydride-bridged dimer {(CF3PCP)Pt}2(μ-H)+. In the presence of trapping ligands H2O, C2H4, or pentafluoropyridine, the corresponding complexes (CF3PCP)Pt(L)+ (L = H2O, C2H4, NC5F5) are cleanly produced and have been structurally characterized. The C2H4 and NC5F5 adducts may be alternatively prepared by methide abstraction from (CF3PCP)PtMe with Ph3C+B(C6F5)4− in the presence of trapping ligand. Evidence for the transient formation of (CF3PCP)PtH from treatment of (CF3PCP)PtCl or (CF3PCP)Pt(NC5F5)+ with Et3Si+B(C6F5)4− is presented. (CF3PCP)Pt(C2H4)+ serves as a catalyst for ethylene hydrogenation (0.30 turnovers h−1, 70 °C) and hydrosilation with Et3SiH (460 turnovers h−1, RT) and Cl3SiH (5 turnovers h−1, RT). At elevated temperatures, (CF3PCP)Pt(C2H4)+ also exhibits limited ethylene dimerization activity (0.07 turnovers h−1, 155 °C) and 1-butene isomerization (0.9 turnovers h−1, 80 °C)

Acceptor PCP Pincer Iridium(I) Chemistry: Stabilization of Nonmeridional PCP Coordination Geometries

The preparation of a series of four-coordinate complexes (CF3PCP)Ir(L) (L = CO, DBU, nbe, coe, MeP(C2F5)2 (dfmp)) and five-coordinate complexes (CF3PCP)Ir(L)(L′) (L = L′ = CO, dfmp, nbd, cod, (C2F5)2PCH2CH2P(C2F5)2 (dfepe); L = PhCN, L′ = C2H4) from dehydrohalogenation of (CF3PCP)Ir(C2H4)(H)Cl with Et3N in the presence of trapping ligands is reported. (CF3PCP)Ir(L) and (CF3PCP)Ir(L)2 for L = CO, dfmp have been structurally characterized and establish a distorted-trigonal -bipyramidal coordination geometry for (CF3PCP)Ir(L)2 with a bent PCP unit and inequivalent axial and equatorial L coordination sites. (CF3PCP)Ir(L)(L′) systems (L = L′ = CO, C2H4; L = PhCN, L′ = C2H4) are highly fluxional, with ligand site interconversion free energy barriers determined by VT NMR of 9.7 kcal mol−1 (L = L′ = CO), 12.2 kcal mol−1 (L = L′ = C2H4), and 16.1 kcal mol−1 (L = C2H4, L′ = PhCN). A dissociative site exchange mechanism is proposed. (CF3PCP)Ir(L) complexes readily undergo oxidative addition reactions. Addition of H2 to (CF3PCP)Ir(CO) reversibly forms trans-(CF3PCP)Ir(CO)(H)2 at ambient temperatures. In contrast, addition of H2 to (CF3PCP)Ir(dfmp) affords fac,cis-(CF3PCP)Ir(dfmp)(H)2 as the major product, with an unusual facially coordinated pincer group. VT NMR monitoring of the reaction of (CF3PCP)Ir(CO) with H2 established the initial formation of fac,cis-(CF3PCP)Ir(CO)(H)2 followed by conversion to mer,cis-(CF3PCP)Ir(CO)(H)2 prior to isomerization to mer,trans-(CF3PCP)Ir(CO)(H)2. The unusual stability of (CF3PCP)Ir(L)2 and fac,cis-(CF3PCP)Ir(L)(H)2 complexes is attributable to the increased stability of nonplanar (PCP)M moieties possessing strongly π-accepting phosphorus groups

Polyhydride (Fluoroalkyl)phosphine Complexes of Iridium. Synthesis, Dynamics, and Reactivity Properties of (dfepe)₂Ir₂(μ-H)₃(H)

The synthesis and reactivity properties of new dimeric iridium polyhydrides incorporating the acceptor ligand (C2F5)2PCH2CH2(C2F5)2 (dfepe) are reported. Hydrogenolysis of (dfepe)Ir(η3-C3H5) (prepared by metathesis of [(dfepe)Ir(μ-Cl)]2 with allylmagnesium chloride) afforded (dfepe)2Ir2(μ-H)3(H) (3) in high yield as an air-stable red crystalline solid. A triply bridged ground-state geometry for 3 was deduced from low-temperature NMR data and was confirmed by X-ray crystallography. Hydride site exchange mechanisms are proposed which are consistent with VT 1H and 31P NMR data. Although 3 is formally coordinatively saturated, hydride bridge dissociation readily occurs and leads to ligand addition reactions. Thus, treatment of tetrahydride 3 with 1 atm of H2 at 20 °C quantitatively affords the hexahydride dimer [(dfepe)Ir(μ-H)2(H)2]2 (5). In the absence of H2, 5 rapidly loses H2 in solution at 20 °C to re-form 3. The structure of 5 has been determined by X-ray crystallography. 3 also reacts with CO to give (dfepe)Ir(CO)2H (6), which loses CO under 1 atm of H2 to reversibly afford (dfepe)Ir(CO)H3 (7). The trihydride undergoes thermal H/D exchange with both D2 (20 °C) and benzene-d6 (120 °C), presumably via the intermediacy of (dfepe)Ir(CO)H. The tetrahydride 3 also undergoes H/D exchange with D2 and benzene-d6 under similar conditions. In the presence of tert-butylethylene, dehydrogenation of cyclopentane by 3 at 120 °C quantitatively affords CpIr(dfepe); a likely intermediate in this process is the dihydride dimer [(dfepe)Ir(μ-H)]2. (dfepe)Ir(η3-C3H5) also reacts directly with cyclopentane at 120 °C to give CpIr(dfepe)