Iridium Pincer Complexes with an Olefin Backbone - Organometallics

Organometallics , 2015, 34 (17), pp 4262–4271. DOI: 10.1021/acs.organomet.5b00495. Publication Date (Web): August 14, 2015. Copyright © 2015 Americ...
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Iridium Pincer Complexes with an Olefin Backbone Alexey V. Polukeev and Ola F. Wendt* Centre for Analysis and Synthesis, Department of Chemistry, Lund University, P.O. Box 124, S-22100 Lund, Sweden S Supporting Information *

ABSTRACT: Among the large variety of pincer complexes, those with carbon− carbon double bonds in the backbone have received little attention. Here, we report the reactions of complex (PCCP)IrPh (3) and its derivatives with small molecules. Compound 3 readily adds CO to give the 18e adduct (PCCP)IrPhCO (5a), which upon heating undergoes isomerization into the thermodynamically more stable isomer (PCCP)IrCO(Ph) (5b), via reversible loss of CO. Reaction of 5 with hydrogen leads to the formation of saturated carbonyl compounds (PC−CP)IrCO (9) and (PC−CP)IrH(CO)H (10). In contrast to the hydrides (PCCP)IrH3 (6) and (PC−CP)IrH4 (7), which are in tautomeric equilibrium via insertion of one of the hydrides into the olefin moiety, the former compounds do not isomerize into the olefin form. Protonation of 5 with CF3COOH gives a complex with an agostic methylene group, 11, which undergoes a rare transformation for Ir pincers, which is insertion of CO into the Ir−Ph bond with subsequent formation of (PCCP)IrOCOCF3 (12) and Ph−CHO. The trihydride 6 reacts with CO to give 9, which can add a second molecule of CO to reversibly form the dicarbonyl 13. Exposure of 6 to CO2 leads to the formate (PC−CP)Ir(H)OC(O)H (14). Complex 3 can take up a molecule of dioxygen to give peroxide (PCCP)IrPhO2 (8); a similar reaction is observed for the saturated complex 9, with formation of (PC−CP)Ir(CO)O2 (15). XRD structures as well as reactivity point to a higher degree of O−O bond activation in 15.

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source of an olefin functionality obtained through βelimination. Thus, in an iridium pincer complex with a cyclohexyl backbone we have recently reported an intramolecular coupling reaction involving three or four unactivated Csp3−H bonds to give coordinated carbon−carbon double bonds under the extrusion of dihydrogen.9 Here, we focus on the chemistry of the resulting pincer complexes bearing an olefin moiety as a central donating group.

INTRODUCTION Pincer complexes are complexes of transition metals with terdentate, typically meridional ligands. The original PCP pincer architecture, as designed by Moulton and Shaw1 in the 1970s, consisted of a central anionic carbon ligand and two neutral, donating groups such as phosphines in the cis positions. Since then, a lot of modifications of pincer ligands have been done, introducing neutral or charged heteroatoms to different binding sites and giving rise to PNP, PSiP, NCN, PCN, SCS, OCO, CCC, and many other families of ligands.2,3 These compounds have been intensively studied in recent decades because of their high potential in catalysis2,3 and ability to perform fine-tuning of electronic and steric properties of the molecules. In particular, the success of the so-called metal−ligand cooperation approach4 has stimulated a search for new pincer ligands capable of changing their coordination mode depending on the other ligands bound to the metal. In this respect, pincer ligands with a C−C double bond as a central donating group are interesting since an olefin moiety can interact with the metal atom in several ways, by for example coordination in an η2-fashion, oxidative addition of one of the vinylic C−H bonds, or acting as a hydrogen acceptor. A few examples of compounds falling into the formal definition of a pincer complex but with a central alkene group are reported in the literature,5 but little is known about their properties. A limited number of studies include Bennett’s work on some complexes of 2,2′-bis(diphenylphosphino)-trans-stilbene6 and a recent publication from Iluc7 about complexation with related 2,2′bis(di-isopropylphosphino)-trans-stilbene. We have a long-standing interest in aliphatic pincer ligands based on a cyclohexyl framework,8 and these are a potential © XXXX American Chemical Society

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EXPERIMENTAL SECTION

General Considerations. All manipulations were conducted under an inert gas atmosphere using standard Schlenk, high-vacuumline, and glovebox techniques unless otherwise stated. All solvents were distilled under vacuum from Na/benzophenone. Trifluoroacetic acid was distilled from phosphorus oxide. Hydrocarbon deuterated solvents were distilled under vacuum from Na/benzophenone; CD2Cl2 was distilled under vacuum from calcium hydride. NMR spectra were recorded on a Varian Unity INOVA 500 MHz instrument. 1H and 13C NMR chemical shifts are reported in parts per million and referenced to the signals of deuterated solvents. 31P{1H} NMR chemical shifts are reported relative to an external 85% solution of phosphoric acid. When multiplets cannot be resolved in complex 1H spectra due to overlap with other resonances, the center of the signal according to the 2D spectrum is reported. IR spectra were recorded on a Bruker Alpha spectrometer. Elemental analyses were performed by H. Kolbe Microanalytisches Laboratorium, Mülheim an der Ruhr, Germany. Satisfactory elemental analysis for 12 was not obtained, because this compound is an oily substance and even after 1 week at high vacuum contains some amount of trifluoroacetic acid. Complex 10 was not subjected to elemental analysis because all samples always contain Received: June 9, 2015

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DOI: 10.1021/acs.organomet.5b00495 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

at P2). 13C{1H} NMR (126 MHz, C6D6): δ 186.46 (dd, 2JPC = 9.3 Hz, 2 JPC = 7.7 Hz, Ir-CO), 148.34 (br, 16-C and 12-C), 136.91 (dd, 2JPC = 7.9 Hz, 2JPC = 5.6 Hz, 11-C), ca. 128.5 (overlapping with solvent, 13-C + 15-C), 121.15 (d, 2JPC = 1.1 Hz, 14-C), 68.80 (dd, 2JP2C = 9.5 Hz, 2 JP1C = 2.8 Hz, 1-C), 54.99 (dd, 1JP2C = 30.2 Hz, 3JP1C = 1.4 Hz, 10-C), 46.35 (dd, 2JP1C = 8.0 Hz, 3JP2C = 3.6 Hz, 2-C), 38.24 (dd, 1JP1C = 25.7 Hz, 3JP2C = 3.0 Hz, C(CH3)3 at P1), 37.31 (d, 3JP2C = 7.6 Hz, 5-C), 37.22 (dd, 1JP1C = 13.1 Hz, 3JP2C = 4.7 Hz, C(CH3)3 at P1), 36.44 (d, 2 JP2C = 1.4 Hz, 6-C), 36.33 (s, 3-C), 34.24 (dd, 2JP2C = 17.8 Hz, 2JP1C = 3.3 Hz, 9-C), 33.05 (d, 1JP1C = 19.2 Hz, 7-C), 32.06 (dd, 2JP2C = 8.3 Hz, 4JP1C = 7.0 Hz, C(CH3)3 at P2), 32.03 (d, 2JP2C = 2.0 Hz, CH3), 31.65 (d, 1JP2C = 22.0 Hz, 8-C), 30.72 (dd, 2JP1C = 4.4 Hz, 4JP2C = 1.0 Hz, C(CH3)3 at P1), 30.3 (br, C(CH3)3 at P1), 27.60 (d, 4JP2C = 1.9 Hz, 4-C), 27.07 (dd, 2JP2C = 2.4 Hz, 4JP1C = 1.4 Hz, C(CH3)3 at P2), 25.52 (dd, 2JP2C = 3.4 Hz, 4JP1C = 1.8 Hz, CH3). 31P{1H} NMR (202 MHz, C6D6): δ 68.59 (d, 2JP1P2 = 301.0 Hz, P1), 12.11 (d, 2JP1P2 = 301.0 Hz, P2). IR (hexane), cm−1: 1943 (CO). Anal. Calcd for C31H52IrOP2: C, 53.58; H, 7.54. Found: C, 53.66; H, 7.41 (mixture with 5a). Synthesis of Oxygen Complex 8. Complex 3 (0.025 g, 0.038 mmol) was dissolved in 5 mL of benzene, and the stirred solution was exposed to air. The red-orange solution turned nearly colorless and was evaporated to give 8 as a pale yellow powder in almost quantitative yield. X-ray quality crystals were grown by slow evaporation of a benzene solution of 8.

small amounts of 9. Compounds 1,10 2,9 3,9 and 49 were synthesized according to the previosly published procedures Synthesis of CO Adduct 5a. Complex 3 (0.030 g, 0.045 mmol) was dissolved in 5 mL of benzene in a nitrogen-atmosphere glovebox, and the solution was freeze−pump−thaw degassed and refilled with 1 atm of CO. The red-orange solution rapidly turned colorless, indicating formation of 5a. Evaporation of the solvent under vacuum gave 5a in quantitative yield.

1 H NMR (500 MHz, C6D6): δ 8.73 (br s, 1H, 16-H), 8.53 (br s, 1H, 12-H), 7.27 (br m, 1H, 15-H), 7.18 (br m, 1H, 13-H), 7.13 (apparent t, 3JH13H14 = 3JH15H14 = 7.1 Hz, 1H, 14-H), 2.24−2.10 (m, 2H, 6-H + 8HA), 2.05−1.94 (m, 1H, 2-H), 1.99 (d, 3H, 3JP2H = 13.8 Hz, CH3), 1.85−1.71 (m, 3H, 3-HA + 4-HA + 5HA), 1.61 (dd, 1H, 3JP2H = 22.5 Hz, 2JP1H = 5.4 Hz, 9-H), 1.60−1.52 (m, 1H, 5-HB), 1.51−1.45 (m, 1H, 7-HA), 1.44−1.36 (m, 1H, 4-HB), 1.33−1.24 (m, 3H, 3-HA + 7-HB + 8-HB), 1.21 (d, 9H, 3JP1H = 13.1 Hz, tBu at P1), 1.11 (br d, 3JP1H = 12.1 Hz, tBu at P1), 0.94 (d, 3H, 3JP2H = 12.9 Hz, CH3), 0.67 (d, 3JP2H = 14.5 Hz, tBu at P2). 13C{1H} NMR (126 MHz, C6D6): δ 184.30 (apparent t, 2JP1C = 2JP2C = 4.9 Hz, CO), 151.56 (br s, 16-C), 146.45 (br s, 12-C), 135.45 (apparent t, 2JP1C = 2JP2C = 10.6 Hz, 11-C), 128.3 (overlapping with solvent signal, 15-C), 125.93 (br s, 13-C), 121.65 (s, 14-C), 62.37 (dd, 2JP2C = 12.3 Hz, 2JP1C = 5.5 Hz, 1-C), 54.21 (dd, 1 JP2C = 30.4, 3JP1C = 2.0 Hz, 10-C), 43.76 (dd, 2JP1C = 5.2 Hz, 3JP2C = 2.7 Hz, 2-C), 38.91 (d, 3JP2C = 5.7 Hz, 5-C), 38.13 (dd, 1JP1C = 22.0 Hz, 3JP2C = 5.6 Hz, C(CH3)3 at P1), 37.51 (d, 2JP2C = 1.4 Hz, 6-C), 36.37 (s, 3-C), 36.13 (dd, 1JP1C = 14.4 Hz, 3JP2C = 2.8 Hz, C(CH3)3 at P1), 34.28 (d, 1JP2C = 22.4 Hz, 8-C), 34.24 (dd, 2JP2C = 23.0 Hz, 2JP1C = 3.1 Hz, 9-C), 32.99 (d, 1JP1C = 19.6 Hz, 7-C), 31.66 (apparent t, 2 JP2C = 4JP1C = 8.2 Hz, C(CH3)3 at P2), 31.16 (dd, 2JP1C = 4.8 Hz, 4JP2C = 0.7 Hz, C(CH3)3 at P1), 30.26 (very br s, C(CH3)3 at P1), 29.29 (dd, 2JP2C = 4.0 Hz, 4JP1C = 0.9 Hz, CH3), 29.26 (d, 2JP2C = 1.8 Hz, CH3), 27.25 (d, 4JP2C = 1.7 Hz, 4-C), 26.85 (dd, 2JP2C = 2.6 Hz, 4JP1C = 1.1 Hz, C(CH3)3 at P2). 31P{1H} NMR (202 MHz, C6D6): δ 62.75 (d, 2 JP1P2= 312.7 Hz, P1), −12.90 (d, 2JP1P2 = 312.7 Hz, P2). IR (hexane), cm−1: 1936 (CO). Anal. Calcd for C31H52IrOP2: C, 53.58; H, 7.54. Found: C, 53.66; H, 7.41 (mixture with 5b). Isomerization of 5a into 5b. Complex 5b (0.015 g, 0.022 mmol) was dissolved in 0.6 mL of deuterobenzene in the glovebox, the solution was transferred to a J. Young NMR tube, and the reaction was monitored by NMR spectroscopy at specified temperatures (60−80 °C). In the end an equilibrium mixture consisting of ca. 85% 5b and ca. 15% 5a was formed.

H NMR (500 MHz, C6D6): δ 9.05 (br, 1H, 16-H), 7.84 (br, 1H, 12-H), 7.25 (m, 2H, 13-H + 15-H), 7.09 (apparent tt, 3JHH = 7.2 Hz, 4 JHH = 1.3 Hz, 1H, 14-H), 3.83 (d, 3JP2H = 18.9 Hz, 1H, 9-H), 2.75 (apparent td, 2JHH = 3JHH = 14.0 Hz, 2JP1H = 7.7 Hz, 1H, 7-HA), 2.74− 2.69 (m, overlapping, 1H, 6-H), 2.52 (apparent tt, J1 = 13.1 Hz, J2 = 9.0 Hz, 1H, 5-HA), 2.31 (dddd, 1JHH = 13.0 Hz, 3JHH = 5.2 Hz, 2JP2H = 10.4 Hz, 4JP1H = 1.9 Hz, 8-HA), 2.02 (apparent td, 2JHH = 14.0 Hz, 3JHH = 4.7 Hz, 2JP2H = 7.7 Hz, 1H, 7-HA), 2.05−1.99 (m, 1-H, 7-HB), 1.99− 1.92 (m, overlapping, 1H, 2-H), 1.92−1.83 (m, overlapping, 2H, 3-HA + 4-HA), 1.73−1.60 (m, overlapping, 3H, 3-HB + 4-HB + 5-HB), 1.56 (d, 3JP1H = 12.2 Hz, 9H, C(CH3)3 at P1), 1.52 (d, 3JP2H = 13.4 Hz, 3H, CH3), 1.42−1.33 (m, 1H, 8-HB), 0.87 (d, 3JP2H = 11.6 Hz, 3H, CH3), 0.84 (d, 3JP2H = 16.3 Hz, 9H, C(CH3)3 at P2), 0.79 (br d, 9H, C(CH3)3 at P1). 13C{1H} NMR (126 MHz, C6D6): δ 144.4 (br s, 16C), 141.4 (br s, 12-C), 126.6 (br s, 15-C), 125.8 (br s, 13-C), 124.06 (dd, 2JPC = 9.7 Hz, 2JPC = 5.4 Hz, 11-C), 122.60 (d, 5JPC = 1.3 Hz, 14C), 81.51 (dd, 2JP2C = 13.1 Hz, 2JP1C = 3.6 Hz, 1-C), 56.39 (d, 2JP2C = 16.6 Hz, 9-C), 48.93 (d, 1JP2C = 26.1 Hz, 10-C), 41.99 (dd, 2JP1C = 5.6 Hz, 3JP2C = 0.6 Hz, 2-C), 39.56 (dd, 1JP1C = 10.3 Hz, 3JP2C = 5.3 Hz, C(CH3)3 at P1), 39.02 (d, 1JP1C = 20.6 Hz, 7-C), 37.97 (s, 6-C), 37.02 (dd, 1JP1C = 12.7 Hz, 3JP2C = 5.6 Hz, C(CH3)3 at P1), 32.37 (dd, 1JP2C = 10.2 Hz, 3JP1C = 2.6 Hz, C(CH3)3 at P2), 31.74 (apparent t, 2JP2C = 4 JP1C = 1.6 Hz, C(CH3)3 at P1), ca. 30.6 (very br, C(CH3)3 at P1), 29.02 (d, 3JP2C = 6.3 Hz, 5-C), 28.26 (s, CH3), 27.34 (apparent t, 2JP2C = 4JP1C = 2.2 Hz, C(CH3)3 at P2), 27.27 (apparent t, 2JP2C = 4JP1C = 3.8 Hz, CH3), 25.08 (d, 3JP1C = 13.2 Hz, 3-C), 25.02 (dd, 1JP2C = 24.5 Hz, 3 JP1C = 1.0 Hz, 8-C), 17.33 (d, 4JP2C = 2.2 Hz, 4-C). 31P NMR (202 MHz, C6D6): δ 5.51 (d, 2JP1P2 = 421.2 Hz, P1), −5.56 (d, 2JP1P2 = 421.2 Hz, P2). Anal. Calcd for C30H51IrO2P2: C, 51.63; H, 7.37. Found: C, 51.82; H, 7.42. Synthesis of Carbonyl Complex 9. A Straus flask was charged with complex 3 (0.030 g, 0.045 mmol) and 10 mL of benzene inside the nitrogen-atmosphere glovebox, and the solution was freeze−pump− thaw degassed and refilled with 1 atm of H2. After stirring for 15 min, accompanied by color change from red-orange to nearly colorless, the solution was once more degassed, refilled with 1 atm of CO, and 1

1 H NMR (500 MHz, C6D6): δ 8.56−8.53 (br m, 2H, 12-H and 16H), 7.17 (br t, 3JHH = 7.3 Hz, 2H, 13-H and 15-H), 7.06 (br t, 3JHH = 7.3 Hz, 1H, 14-H), 2.50−2.43 (m, 1H, 6-H), 2.21−2.08 (m, 1H, 2-H), 2.06−1.94 (m, overlapping, 4H, 3-HA + 5-HA + 5-HB + 8-HA), 1.93− 1.86 (m, overlapping, 2H, 4-HA + 8-HB), 1.83 (d, 3JP2H = 13.9 Hz, 3H, CH3), 1.56−1.50 (m, 2H, 7-HA + 7-HB), 1.49−1.40 (m, overlapping, 1H, 4-HB), 1.36 (dd, overlapping, 3JP2H = 26.8 Hz, 3JP1H = 2.1 Hz, 1H, 9-H), 1.37−1.30 (m, overlapping, 1H, 3-HB), 1.26 (d, 3JP1H = 11.9 Hz, 9H, C(CH3)3 at P1), 1.10 (3JP1H = 13.4 Hz, 9H, C(CH3)3 at P1), 0.97 (d, 3JP2H = 11.9 Hz, 3H, CH3), 0.75 (d, 3JP2H = 14.2 Hz, 9H, C(CH3)3

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DOI: 10.1021/acs.organomet.5b00495 Organometallics XXXX, XXX, XXX−XXX

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28.3 Hz, 10-C), 42.16 (d, 1JP1C = 23.9 Hz, 7-C), 38.85 (d, 1JP2C = 32.9 Hz, 8-C), 34.97 (dd, 1JP1C = 20.0 Hz, 3JP2C = 3.3 Hz, C(CH3)3 at P1), 33.62 (dd, 1JP1C = 21.1 Hz, 3JP2C = 3.2 Hz, C(CH3)3 at P1), 31.26 (dd, 1 JP2C = 13.0 Hz, 3JP1C = 4.9 Hz, C(CH3)3 at P2), 30.72 (d, 2JP1C = 3.5 Hz, C(CH3)3 at P1), 30.34 (dd, 3JP2C = 4.4 Hz, 4JP1C = 1.7 Hz, 5-C), 29.55 (d, 2JP1C = 4.1 Hz, C(CH3)3 at P1), 28.27 (dd, 2JP2C = 4.4 Hz, 4 JP1C = 1.4 Hz, C(CH3)3 at P2), 23.32 (apparent t, 2JP2C = 4JP1C = 2.3 Hz, CH3), 20.01 (d, 3JP1C = 17.0 Hz, 3-C), 18.90 (d, 4JP1C = 2.0 Hz, 4C). 31P{1H} NMR (202 MHz, C6D6): δ 72.74 (d, 2JP1P2 = 271.3 Hz), 66.61 (d, 2JP1P2 = 271.3 Hz). IR (hexane), cm−1: 1972 (CO), 1802 (H−Ir−H asymm.). Protonation of 5a and 5b. Formation of 11 and 12. A J. Young NMR tube was charged with a mixture of 5a and 5b (0.020 g, 0.032 mmol) as well as 0.7 mL of deuterobenzene inside the nitrogenatmosphere glovebox, and one drop of trifluoroacetic acid was added. NMR spectra indicate simultaneous formation of 11 and some amount of 12. After several days the reaction is complete, and only 12 can be observed by NMR in the solution.

stirred overnight. Evaporation of the solvent under vacuum gave 9 as a yellow powder (0.026 g, 94%). A note: this reaction is somewhat unstable and may produce some amount of 10. If so, the compound should be redissolved in benzene and, after addition of several drops of tert-butylethylene, heated overnight at 120 °C.

H NMR (500 MHz, C6D6): δ 2.43 (ddd, 2JHH = 14.7 Hz, 3JHH = 6.6 Hz, 2JP1H = 7.9 Hz, 1H, 7-HA), 2.24−2.14 (m, overlapping, 2H, 7HB + 8-HA), 1.80−1.63 (m, overlapping, 5H, 2-H + 4-HA + 4-HB + 6H + 8-HB), 1.62−1.39 (m, overlapping, 3-HA + 3-HB + 5-HA + 5-HB + 9-HA + 9-HB), 1.48 (dd, 3JP2H = 12.2 Hz, 5JP1H = 2.1 Hz, 3H, CH3), 1.34 (dd, 3JP1H = 10.7 Hz, 5JP2H = 2.0 Hz, 9H, C(CH3)3 at P1), 1.30 (dd, overlapping, 3JP2H = 8.1 Hz, 5JP1H = 1.8 Hz, CH3), 1.29 (dd, 3JP1H = 10.8 Hz, 5JP2H = 2.0 Hz, 9H, C(CH3)3 at P1), 1.21 (dd, 3JP2H = 11.4 Hz, 5JP1H = 2.2 Hz, 9H, C(CH3)3 at P2). 13C{1H} NMR (126 MHz, C6D6): δ 196.30 (apparent t, 2JP1C = 2JP2C = 5.9 Hz, Ir-CO), 69.69 (d, 2 JP2C = 6.6 Hz, 1-C), 64.69 (dd, 2JP2C = 7.9 Hz, 3JP1C = 1.9 Hz, 9-C), 47.52 (dd, 2JP1C = 11.1 Hz, 3JP2C = 7.6 Hz, 2-C), 45.63 (dd, 2JP1C = 3.9 Hz, 3JP1C = 0.9 Hz, 6-C), 42.57 (dd, 1JP2C = 21.0 Hz, 3JP1C = 3.3 Hz, 10-C), 41.50 (dd, 1JP1C = 19.6 Hz, 3JP2C = 3.0 Hz, 7-C), 36.19 (dd, 1 JP1C = 4.5 Hz, 3JP2C = 2.0 Hz, C(CH3)3 at P1), 36.06 (d, 1JP1C = 4.6 Hz, C(CH3)3 at P1), 32.89 (dd, 2JP2C = 6.4 Hz, 4JP1C = 1.6 Hz, CH3), 31.23 (dd, 1JP2C = 12.2 Hz, 3JP1C = 6.0 Hz, C(CH3)3 at P2), 30.34 (dd, 2 JP1C = 4.5 Hz, 4JP2C = 1.7 Hz, C(CH3)3 at P1), 30.25 (m, overlapping, 5-C), 30.04 (dd, overlapping, 1JP2C ≈ 26 Hz, 8-C), 29.68 (dd, 2JP1C = 4.0 Hz, 4JP2C = 1.5 Hz, C(CH3)3 at P1), 28.31 (dd, 2JP2C = 4.1 Hz, 4JP1C = 1.9 Hz, C(CH3)3 at P2), 24.30 (dd, 2JP2C = 2.6 Hz, 4JP1C = 1.0 Hz, CH3), 20.43 (dd, 3JP1C = 13.5 Hz, 4JP2C = 2.1 Hz, 3-C), 18.29 (d, 4JP1C = 1.6 Hz, 4-C). 31P{1H} NMR (202 MHz, C6D6): δ 86.24, 84.92 (AB system, 2JP1P2 = 270.0 Hz). IR (hexane), cm−1: 1924 (CO). Anal. Calcd for C25H47IrOP2: C, 48.60; H, 7.67. Found: C, 48.79; H, 7.47. Synthesis of Complex 10. A Straus flask was charged with complex 5b (0.030 g, 0.043 mmol) and 5 mL of benzene inside the nitrogenatmosphere glovebox, the solution was freeze−pump−thaw degassed, and the flask was refilled with 4 atm of H2. After heating on an oil bath for 12 h at 80 °C, the volatiles were removed and the residue was dissolved in pentane and filtered through a thin layer of Celite. Evaporation of the solvent gave 10 as a pale yellow powder (0.022 g, 82%). 1

1 H NMR (500 MHz, C6D6): δ selected signals 7.41 (br, 2H, o-H), 6.72 (apparent t, J = 7.1 Hz, 1H, p-H), 6.64 (apparent t, J = 7.6 Hz, 2H, m-H), 1.34 (d, 3JP2H = 13.9 Hz, 3H, CH3), 1.10 (d, 3JP1H = 13.1 Hz, 9H, C(CH3)3 at P1), 1.01 (d, 3JP2H = 11.4 Hz, 3H, CH3), 0.72 (d, 3 JP1H = 14.9 Hz, 9H, C(CH3)3 at P1), 0.54 (d, 3JP2H = 15.9 Hz, 9H, C(CH3)3, at P2), −0.34 (dd, 3JP2H = 27.2 Hz, 2JHH = 20.6 Hz, 1H, 9HB), −4.06 (dd, 3JP1H = 3.2 Hz, 2JHH = 20.6 Hz, 1H, 9-HA). 31P{1H} NMR (202 MHz, C6D6): δ 72.29 (d, 2JP1P2 = 252.4 Hz), 38.68 (d, 2 JP1P2 = 252.4 Hz). 13C{1H} NMR (126 MHz, CD2Cl2): δ 177.92 (apparent t, 2JP1C = 2JP2C = 5.3 Hz), 160.2 (br, 16-C + 12-C), 116.96 (m, 11-C), 116.9 (br, 13-C or 15-C), 114.6 (13-C or 15-C), 124.94 (s, 14-C), 52.12 (dd, 2JP1C = 7.5 Hz, 3JP2C = 3.8 Hz, 2-C), 51.78 (d, 1JP2C = 27.9 Hz, 10-C), 38.83 (dd, 1JP1C = 22.1 Hz, 3JP2C = 1.9 Hz, C(CH3)3 at P1), 38.25 (dd, 1JP1C = 14.7 Hz, 3JP2C = 3.0 Hz, C(CH3)3, at P1), 37.85 (s, 6-C), 35.79 (d, 3JPC = 7.8 Hz, 3 or 5-C), 34.93 (s, 3 or 5-C), 34.29 (dd, 1JPC = 19.0 Hz, 3JPC = 2.6 Hz, 7 or 8-C), 33.90 (dd, 1JP2C = 14.1 Hz, 3JP1C = 5.1 Hz, C(CH3)3, at P2), 31.14 (dd, 1JPC = 27.7 Hz, 3JPC = 2.1 Hz, 7 or 8-C), 30.74 (dd, 2JP1C = 3.2 Hz, 4JP2C = 0.8 Hz, C(CH3)3 at P1), 30.7 (br, C(CH3)3 at P1), 30.47 (d, 2JP2C = 2.8 Hz, CH3), 29.88 (dd, 2JP2C = 3.4 Hz, 4JP1C = 0.5 Hz, CH3), 27.9 (br, 9-C), 27.48 (dd, 2 JP2C = 3.5 Hz, 4JP1C = 1.1 Hz, CH3), 27.43 (apparent t, 2JP2C = 4JP1C = 1.6 Hz, C(CH3)3 at P2), 26.12 (d, 4JPC = 2.2 Hz, 4-C). 31P{1H} NMR (202 MHz, CD2Cl2, 13CO sample): δ 72.92 (dd, 2JP1P2 = 253.7 Hz, 2 JP1C = 5.6 Hz, P1), 38.68 (d, 2JP1P2 = 253.7 Hz, 2JP2C = 4.9 Hz, P2).

H NMR (500 MHz, C6D6): δ 2.57−2.49 (m, 1H, 8-HA), 2.34−2.27 (m, 1H, 7-HA), 2.06−1.90 (m, overlapping, 3H, 7-HB + 9-HA + 9-HB), 1.85−1.80 (m, 1H, 2-H), 1.74−1.56 (m, overlapping, 3-HA + 4-HA + 4-HB + 5-HA + 8-HB), 1.57 (d, 3JP2H = 8.3 Hz, 3H, CH3), 1.40−1.35 (m, overlapping, 5-HB), 1.37−1.33 (m, overlapping, 3-HB), 1.33 (d, 3 JP1H = 12.7 Hz, 9H, C(CH3)3 at P1), 1.30 (d, 3JP1H = 12.4 Hz, 9H, C(CH3)3 at P1), 1.24 (d, 3JP2H = 10.1 Hz, 3H, CH3), 1.20 (d, 3JP2H = 13.4 Hz, 9H, C(CH3)3 at P2), 0.88−0.81 (m, 1H, 6-H), −10.57 (apparent tdd, 2JP1H = 2JP2H = 14.3 Hz, 2JHH = 7.5 Hz, 4JHH = 2.2 Hz, 1H, Ir−H), −10.79 (apparent tdd, 2JP1H = 2JP2H = 15.2 Hz, 2JHH = 7.5 Hz, 4JHH = 3.1 Hz, 1H, Ir−H). 13C{1H} NMR (126 MHz, C6D6): δ 179.92 (dd, 2JPC = 5.5 Hz, 2JPC = 4.4 Hz, Ir−CO), 72.32 (d, 2JP2C = 8.2 Hz, 9-C), 53.60 (d, 2JP2C = 8.3 Hz, 1-C), 47.43 (dd, 2JP1C = 9.9 Hz, 3 JP2C = 6.4 Hz, 2-C), 44.84 (d, 2JP2C = 3.3 Hz, 6-C), 42.94 (d, 1JP2C = 1

H NMR (500 MHz, C6D6): δ 2.87−2.79 (m, overlapping, 2H, 8HA + 9-H), 2.64−2.52 (m, overlapping, 2H, 2-H + 6-H), 2.06−2.03 (m, 1H, 5-HA), 2.00−1.88 (m, 2H, 7-HA + 7-HB), 1.85 (d, 3JP2H = 14.8 Hz, 3H, CH3), 1.82−1.77 (m, 1H, 4-HA), 1.66−1.60 (m, 1H, 8-HB), 1.58−1.51 (m, 2H, 3-HA + 3-HB), 1.43 (m, overlapping, 4-HB), 1.40 (d, 3JP1H = 13.8 Hz, 9H, C(CH3)3 at P1), 1.39 (d, 3JP1H = 15.0 Hz, 9H, 1

C

DOI: 10.1021/acs.organomet.5b00495 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

Synthesis of Oxygen Complex 15. A J. Young NMR tube was charged with complex 9 (0.020 g, 0.032 mmol) and 0.7 mL of deuterobenzene inside the nitrogen-atmosphere glovebox. Oxygen gas was bubbled through the solution using a rubber septum and a needle for 15 min. Evaporation of the volatiles under vacuum gave 15 as a pale yellow powder in almost quantitative yield.

C(CH3)3 at P1), 1.28 (d, 3JP2H = 17.0 Hz, 9H, C(CH3)3 at P2), 1.23 (d, 3JP2H = 14.7 Hz, 3H, CH3), 1.22 (m, overlapping, 3-HB). 13C{1H} NMR (126 MHz, CD2Cl2): δ 84.17 (dd, 2JP2C = 13.7 Hz, 2JP1C = 5.7 Hz, 1-C), 53.07 (dd, 1JP2C = 31.3 Hz, 3JP1C = 1.8 Hz, 10-C), 44.23 (dd, 2 JP2C = 18.2 Hz, 2JP1C = 1.3 Hz, 9-C), 43.31 (apparent t, 2JP1C = 3JP2C = 3.2 Hz, 2-C), 39.10 (dd, 1JP1C = 25.1 Hz, 3JP2C = 2.7 Hz, C(CH3)3 at P1), 37.97 (d, 3JP2C = 5.5 Hz, 5-C), 36.52 (apparent t, 2JP1C = 3JP2C = 1.3 Hz, 6-C), 36.06 (dd, 1JP1C = 18.0 Hz, 3JP2C = 1.8 Hz, C(CH3)3 at P1), 35.33 (s, 3-C), 33.51 (dd, 1JP2C = 12.2 Hz, 3JP1C = 5.0 Hz, C(CH3)3 at P2), 32.54 (d, 1JP1C = 24.0 Hz, 8-C), 30.77 (dd, 2JP1C = 4.2, 4JP2C = 0.8 Hz, C(CH3)3 at P1), 30.57 (d, 1JP1C = 20.1 Hz, 7-C), 29.95 (br s, C(CH3)3 at P1), 29.49 (d, 2JP2C = 1.2 Hz, CH3), 27.06 (dd, 2JP2C = 2.6 Hz, 4JP1C = 0.8 Hz, C(CH3)3 at P2), 25.76 (d, 4JPC = 1.8 Hz, 4-C), 25.08 (dd, 2JP2C = 3.7 Hz, 4JP1C = 0.8 Hz, CH3). 31P{1H} NMR (202 MHz, CD2Cl2): δ 76.73 (d, 2JP1P2 = 214.5 Hz, P1), 5.25 (d, 2 JP1P2 = 214.5 Hz, P2). 19F{1H} NMR (470 MHz, CD2Cl2): δ −75.9. Synthesis of Dicarbonyl Complex 13. A J. Young NMR tube was charged with complex 9 (0.017 g, 0.028 mmol) and 0.6 mL of deuterobenzene inside the nitrogen-atmosphere glovebox. The solution was degassed and refilled with 1 atm of CO; complex 13 was simultaneously formed in quantitative yield according to NMR. Evaporation of the solvent recovered complex 9.

H NMR (500 MHz, C6D6): δ 2.78−2.72 (m, 1H, 8-HA), 2.28−2.21 (m, 1H, 7-HA), 2.01−1.94 (m, 1H, 7-HB), 1.91−1.85 (m, 2H, 9-HA + 9-HB), 1.85−1.77 (m, overlapping, 2-H + 8-HB), 1.75−1.55 (overlapping, 4H, 3-HA + 4-HA + 4-HB + 5-HA), 1.51 (d, 3JP1H = 12.4 Hz, 9H, C(CH3)3 at P1), 1.43−1.38 (m, overlapping, 1H, 5-HB), 1.34 (d, 3 JP2H = 13.5 Hz, 3H, CH3), 1.37−1.32 (m, overlapping, 1H, 3-HB), 1.26 (d, 3JP2H = 14.0 Hz, 9H, C(CH3)3 at P2), 1.22 (d, 3JP1H = 12.2 Hz, 9H, C(CH3)3 at P1), 1.11 (d, 3JP2H = 10.1 Hz, 3H, CH3), 1.01−0.94 (m, 1H, 6-H). 13C{1H} NMR (126 MHz, C6D6): δ 178.31 (apparent t, 2 JP1C = 2JP2C = 8.2 Hz, Ir−CO), 69.58 (d, 2JP2C = 6.1 Hz, 9-C), 48.56 (dd, 2JP2C = 12.0 Hz, 2JP1C = 2.4 Hz, 1-C), 47.84 (dd, 2JP2C = 6.1 Hz, 3 JP2C = 4.4 Hz, 6-C), 45.02 (d, 2JP1C = 1.9 Hz, 2-C), 42.50 (d, 1JP2C = 27.0 Hz, 10-C), 39.77 (dd, 1JP1C = 10.5 Hz, 3JP2C = 6.2 Hz, C(CH3)3 at P1), 37.26 (d, 1JP1C = 22.0 Hz, 7-C), 36.18 (dd, 1JP1C = 15.8 Hz, 3JP2C = 2.9 Hz, C(CH3)3 at P1), 33.78 (dd, 1JP2C = 9.6 Hz, 3JP1C = 7.7 Hz, C(CH3)3 at P2), 30.89 (br m, overlapping, 2 C(CH3)3 at P1), 29.02 (d, 2JP2C = 3.1 Hz, CH3), 28.26 (apparent t, 2JP2C = 4JP1C = 2.4 Hz, C(CH3)3 at P2), 28.03 (d, 3JP2C = 6.3 Hz, 5-C), 26.13 (d, 1JP2C = 29.5 Hz, 8-C), 25.33 (apparent t, 2JP2C = 4JP1C = 2.9 Hz, CH3), 20.38 (d, 3 JP1C = 14.0 Hz, 3-C), 18.43 (d, 4JP1C = 2.1 Hz, 4-C). 31P{1H} NMR (202 MHz, C6D6): δ 54.22 (d, 2JP1P2 = 350.8 Hz), 48.98 (d, 2JP1P2 = 350.8 Hz). IR (hexane), cm−1: 1951 (CO). Anal. Calcd for C25H47IrO3P2: C, 46.21; H, 7.29. Found: C, 46.22; H 7.26. Crystallography. XRD-quality crystals of compounds 8 and 15 were obtained through slow crystallization from benzene or hexane. Intensity data were collected with an Oxford Diffraction Excalibur 3 system, using ω-scans and Mo Kα (λ = 0.710 73 Å) radiation.11 The data were extracted and integrated using Crysalis RED.12 The structures were solved by direct methods and refined by full-matrix least-squares calculations on F2 using SHELXL13 and SIR-92.14 Molecular graphics were generated using CrystalMaker 8.3.5.15 CCDC deposition numbers 1405157 and 1405158. 1

H NMR (500 MHz, C6D6): δ 2.70−2.62 (m, 1H, 8-HA), 2.01 (d, JHH = 11.2 Hz, 1H, 9-HA), 1.95 (ddd, 2JHH = 15.4 Hz, 3JHH = 7.6 Hz, 2 JP1H = 7.7 Hz, 1H, 7-HA), 1.90−1.80 (m, overlapping, 2H, 3-HA + 7HB), 1.71−1.65 (m, 1H, 4-HA), 1.58−1.50 (m, 1H, 5-HA), 1.32 (d, 3 JP1H = 13.2 Hz, 9H, C(CH3)3 at P1), 1.28 (m, overlapping, 8-HB), 1.27 (m, overlapping, CH3), 1.24 (m, overlapping, 9-HB) 1.20 (d, 3JP1H = 12.7 Hz, 9H, C(CH3)3 at P1),), 1.19 (d, 3JP2H = 13.7 Hz, 9H, C(CH3)3 at P2), 1.19 (m, overlapping, 4-HB), 1.14 (m, overlapping, 2H), 1.06 (d, 3JP2H = 12.9 Hz, 3H, CH3), 1.04−0.94 (m, overlapping, 3HB + 6-H). 13C{1H} NMR (126 MHz, C6D6): δ 69.77 (d, 2JP2C = 15.1 Hz, 9-C), 64.06 (br d, 2JP2C = 8.3 Hz, 1-C), 52.28 (dd, 2JP1C = 12.0 Hz, 3 JP2C = 4.0 Hz, 2-C), 48.45 (dd, 2JP2C = 2.3 Hz, 3JP1C = 0.9 Hz, 6-C), 42.29 (dd, 1JP2C = 22.8 Hz, 3JP1C = 1.3 Hz, 10-C), 38.32 (d, 1JP2C = 32.1 Hz, 8-C), 38.03 (d, 1JP1C = 16.0 Hz, 7-C), 37.52 (dd, 1JP1C = 17.2 Hz, 3JP2C = 1.0 Hz, C(CH3)3 at P1), 36.43 (dd, 1JP1C = 16.7 Hz, 3JP2C = 5.4 Hz, C(CH3)3 at P1), 36.16 (d, 3JP2C = 5.0 Hz, 5-C), 34.2 (br, 3-C), 32.83 (dd, 1JP2C = 11.0 Hz, 3JP1C = 3.9 Hz, C(CH3)3 at P2), 30.66 (d, 2 JP1C = 4.7 Hz, C(CH3)3 at P1), 30.58 (d, 2JP2C = 8.2 Hz, CH3), 30.47 (dd, 2JP1C = 4.9 Hz, 4JP2C = 0.9 Hz, C(CH3)3 at P1), 28.20 (dd, 2JP2C = 4.2 Hz, 4JP1C = 0.7 Hz, C(CH3)3 at P2), 27.02 (s, 4-C), 22.94 (d, 2JP2C = 2.4 Hz, CH3). 31P{1H} NMR (202 MHz, C6D6): δ 79.73 (br d, 2 JP1P2 = 176 Hz), 59.81 (br d, 2JP1P2 = 176 Hz). Reaction of Complex 6 with CO2. Observation of Complex 14. A J. Young NMR tube was charged with complex 3 (0.015 g, 0.023 mmol) and 0.7 mL of C6D6 inside the nitrogen-atmosphere glovebox. The solution was freeze−pump−thaw degassed and refilled with 1 atm of H2. After shaking for 5 min, accompanied by color change from redorange to nearly colorless, the solution was once more degassed and refilled with an amount of CO2 corresponding to 4 atm pressure using a calibrated gas bulb. After shaking for 10 min, the NMR spectra indicated formation of a ca. 61:39 6/14 mixture. Upon heating, slow formation of complex 9 along with several unidentified compounds was observed. Characterization of 14: 1H NMR (500 MHz, C6D6): δ 9.27 (s, 1H, Ir-OC(O)H), −32.96 (dd, 1H, 2JPH = 12.7 Hz, 2JPH = 16.0 Hz, Ir−H). 13C{1H} NMR (126 MHz, C6D6): δ 172.1 (CO). 31P{1H} NMR (202 MHz, C6D6): δ 61.00 (d, 2JPP = 331.8 Hz), 51.40 (d, 2JPP = 331.8 Hz). 1

2

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RESULTS AND DISCUSSION As reported previously,9 olefin complex 3 can be readily synthesized upon heating a benzene solution of hydridochloride 1 in the presence of base and tert-butylethylene as a hydrogen acceptor (Scheme 1). Complex 3 is an orange-red air-sensitive compound with good solubility in nonpolar organic solvents. Some reactivity of 3 with small molecules is presented in Scheme 2. Thus, 3 reacts with chloroform, giving mainly chloro complex 4.9 Another way of cleaving the Ir−Ph bond is reaction with hydrogen, which Scheme 1. Synthesis of Olefin Complex 3

D

DOI: 10.1021/acs.organomet.5b00495 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics 3

JP2H = 26.8 Hz, 3JP1H = 2.1 Hz) ppm; three signals are observed in the aromatic region, indicating fast rotation of the phenyl group, likely because it became more remote from the bulky tert-butyl groups. In the 13C{1H} NMR spectrum, the Ir−CO resonance is observed at 186.5 (dd, 2JPC = 9.3 Hz, 2JPC = 7.7 Hz) and the double-bond signals appear at 68.8 (dd, 2JP2C = 9.5 Hz, 2JP1C = 2.8 Hz) and 34.2 (dd, 2JP2C = 17.8 Hz, 2JP1C = 3.3 Hz) ppm. The ν(C−O) for 5b comes at 1943 cm−1 in hexane and is very similar to that of 5a. When 13CO was used to prepare 5b, only the signal at 68.8 ppm, corresponding to the quaternary olefin carbon, revealed an additional 13C−13C splitting of ca. 18.2 Hz. These data are consistent with the characterization of 5b as a geometrical isomer of 5a, where CO and Ph switched their positions. As will be shown below, and in agreement with the previous report by Bennett,6b such compounds have trigonal-bipyramidal (TBP) geometry with the P atoms occupying axial positions. 2D NOESY spectra indicate that in 5a CO occupies an equatorial position that is close to the cyclohexyl ring, while in 5b the phenyl group reveals an NOE interaction with the cyclohexyl protons. No intermediate during this transformation could be detected by NMR, but at temperatures around 100 °C the colorless solution becomes slightly reddish, which may indicate the presence of a 16e compound, likely the parent complex 3 resulting from dissociation of CO or the product of insertion of CO into the Ir−Ph bond (Scheme 4). When a mixture of 5a

Scheme 2. Reactivity of Olefin Complex 3 toward Small Molecules

rapidly leads to a mixture of tautomers 6 and 7, as reported previously.9 Complex 3 is coordinatively unsaturated and readily adds CO or O2 molecules upon exposure to the corresponding atmospheres. At the same time, 3 remains inert to CO2 or N2. The 18e adduct with CO (5a) is characterized by an AX system (62.75, −12.90 ppm, 2JP1P2 = 312.7 Hz) in the 31P{1H} NMR spectrum. The olefin proton appears at 1.61 ppm (dd, 1H, 3JP2H = 22.5 Hz, 2JP1H = 5.4 Hz), rotation of the phenyl group is hindered, and five signals from it are observed. In the 13 C{1H} NMR spectrum, the CO signal resonates at 184.3 ppm (apparent t, 2JP1C = 2JP2C = 4.9 Hz), while the olefinic carbons appear at 62.4 (2JP2C = 12.3 Hz, 2JP2C = 5.5 Hz, 1-C) and 34.2 (dd, 2JP2C = 23.0 Hz, 2JP1C = 3.1 Hz) ppm. The IR spectrum of a solution of 5a in hexane reveals an intense ν(C−O) band at 1936 cm−1. Heating a solution of 5a in C6D6 results in the appearance of NMR signals of a new complex, 5b, which is in equilibrium with 5a (Scheme 3). Complex 5b has 31P NMR signals at 68.59 (d, 2 JP1P2= 301.0 Hz, P1) and 12.11 (d, 2JP1P2 = 301.0 Hz, P2). In the 1H NMR spectrum the olefin proton appears at 1.36 (dd,

Scheme 4. Possible Pathways for Isomerization of 5a to 5b; Experiments Favor the Path via 3

Scheme 3. Isomerization of 5a and Selected NOE Interactions (Red Dashed Lines) in 5a and 5b and 5b was heated at 80 °C under a 13CO atmosphere, incorporation of 13CO into both 5a and 5b was observed. This is more consistent with a dissociative pathway that includes the loss of CO from Ir. A kinetic study of the conversion of 5a to 5b (see the SI for details) shows the standard behavior for a reversible first-order reaction with only steady-state concentrations of any intermediate. Performing the experiment at different temperatures resulted in an Eyring plot that gave ΔH⧧ = 28.5 ± 0.8 kcal/mol and ΔS⧧ = 5.8 ± 2.5 cal/(mol K) for the forward reaction as well as ΔH⧧ = 27.8 ± 0.8 kcal/mol and ΔS⧧ = −0.3 ± 2.3 cal/(mol K) for the reverse. This gives an equilibrium constant at 61 °C of 7.5, in fairly good agreement with the value of 6.1 obtained from the NMR distribution. The entropies have fairly large errors, but the enthalpies of activation clearly point to a dissociative mechanism with substantial bond breaking in the transition state, and based on this and the results above we favor the mechanism via complex E

DOI: 10.1021/acs.organomet.5b00495 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics 3. It should be noted, however, that at certain conditions insertion of CO into Ir−Ph bonds is possible (see below); besides, there is some evidence that CO may also undergo insertion into Cipso−Ir bond (see the SI). Therefore, the relatively large errors in ΔH⧧ and ΔS⧧ may be a result of a competition of two mechanisms. In contrast to 3, 5 does not react with H2 at ambient temperature, but upon heating, two new compounds are formed, namely, carbonyl complex 9 and trans-dihydride 10 (Scheme 5), characterized by an AB system (86.24 and 84.92

Scheme 6. Reaction of 5 with Trifluoroacteic Acid

Scheme 5. Reaction of 5 with Hydrogen within several hours it is converted to compound 12. The 31 1 P{ H} spectrum of 11 reveals two doublets (72.29 and 38.68 ppm, 2JP1P2 = 252.4 Hz), and two high-field signals at −0.34 (dd, 3JP2H = 27.2 Hz, 2JHH = 20.6 Hz) and −4.06 (dd, 3JP1H = 3.2 Hz, 2JHH = 20.6 Hz) are observed in the 1H NMR spectrum. The chemical shifts of 11 are slightly dependent on the amount of CF3COOH added, indicating the presence of a tight ion pair. Limited solubility did not allow us to acquire a carbon spectrum in C6D6. In CD2Cl2 the reaction is less clean, but some important observations were made. Surprisingly, both high-field signals reveal a weak −CH2− type correlation with a broad carbon resonance at 27.9 ppm in the 1H−13C HSQC spectrum and are therefore assigned to the methylene group. A one-bond C−H coupling of 86 Hz for the signal at −4.06 ppm indicates an agostic interaction with iridium for one of the C−H bonds. The other signal at −0.34 ppm reveals a 1JC−H of 144 Hz, which is significantly higher than typical 1JC−H couplings for Csp3−H bonds (ca. 125 Hz) and is close to the value for Csp2−H bonds (ca. 155 Hz), indicating noticeable rehybridization of the nonagostic part of the −CH2− moiety.18 Using CF3COOD shows that it is the hydrogen resonating at −0.34 ppm that comes from the acid. Remarkably, despite the absence of an agostic interaction (because of geometry constraints, both C− H bonds cannot be oriented toward Ir at the same time) and some Csp2−H character, this hydrogen appears at negative chemical shifts, indicating a remote influence of the iridium atom on the shielding. The formation of 11 presumably takes place through outer-sphere protonation of the double bond since the incoming hydrogen occupies the position remote from the iridium atom (as shown by deuteration). An alternative mechanism, namely, β-insertion of the double bond into an iridium hydride formed via protonation, would likely place the deuterium label into the agostic position. A reduced electron density on Ir in 11 compared to 5 presumably leads to a weakening of the back-donation to CO, which, in turn, reduces the strength of the Ir−CO bond and enables the insertion of CO into the Ir−Ph bond. Subsequent elimination of benzaldehyde leads to trifluoroacetate complex 12. It should be noted that, because of the high binding energy of CO to iridium pincer complexes, a more common reaction is the deinsertion of CO, as exemplified by decarbonylative dehydrogenation or catalytic decarbonylation reactions;19,20 the opposite reaction is very rare.21 Previously, Bennett performed protonation of Rh and Ir complexes with a 2,2′bis(diphenylphosphino)-trans-stilbene ligand with HCl.6b The proton was reported to add to the double bond, while the chlorine, to Ir. No agostic bonding of the resulting −CH2− group was observed, likely because of geometry differences between 5 and stilbene-based complexes. Reactivity of Trihydride 6 with CO and CO2. Complex 6 reacts with CO to give a number of unidentified products,

ppm, 2JP1P2 = 270.0 Hz, 9) and two distorted doublets with a “roof” effect (72.74, 66.61, 2JP1P2 = 271.3 Hz, 10) in the 31 1 P{ H} NMR spectrum. The NMR spectral features of 9 and 10, including the vanishing of the olefin >CH− group and the appearance of a new −CH2− group near 60−70 ppm, are similar to those described previously for 7.9 The carbonyl group resonates at 196.3 (apparent t, 2JP1C = 2JP2C = 5.9 Hz) ppm for 9 and at 179.9 (dd, 2JPC = 5.5 Hz, 2JPC = 4.4 Hz) for 10. The hydride signals appear at −10.57 (tdd, 2JP1H = 2JP2H = 14.3 Hz, 2 JHH = 7.5 Hz, 4JHH = 2.2 Hz) and −10.79 ppm (tdd, 2JP1H = 2 JP2H = 15.2 Hz, 2JHH = 7.5 Hz, 4JHH = 3.1 Hz). In the IR spectrum of 9, a band at 1924 cm−1 corresponding to ν(C−O) is observed; this value is very close to that for the symmetrical (PCyP)IrCO16 (1920 cm−1), indicating that a ligand with a coupled α-carbon and tert-butyl group has essentially identical electronic properties with the symmetrical PCyP ligand. The IR spectrum of 10 reveals a strong band at 1972 cm−1 from CO and a broad medium-intense band at 1802 cm−1, which is characteristic for ν(Ir−H) in the trans dihydride complexes.17 It seems reasonable that under heating 5 transforms into coordinatively unsaturated 3, which reacts with hydrogen to give a mixture of 6 and 7. This mixture recaptures a CO molecule to give 9 and 10. It is remarkable that CO acts as an anchor, which shifts the equilibrium between hydrido-olefin and saturated compounds to the side of the latter: no unsaturated hydride complex can be detected. Thus, one can conclude that the olefin backbone weakens the Ir−CO interaction compared to the saturated backbone, likely due to competition between the CO and double bond for Ir dπorbitals. An interesting transformation happens when trifluoroacetic acid is added to a solution of complex 5 in C6D6 (Scheme 6). First, formation of complex 11 is observed by NMR spectroscopy, but this complex is relatively unstable, and F

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compound (1.95 ppm), in agreement with a decreased backbonding to the double bond. The olefinic carbons appear at 81.5 ppm (dd, 2JP2C = 13.1 Hz, 2JP1C = 3.6 Hz) and 56.4 ppm (d, 2JP2C = 16.6 Hz). A phenyl group reveals two apparent triplets integrating as 1H and 2H and two broad singlets from ortho protons. Interestingly, a steric interaction of these protons with one of the tert-butyls of the P atoms is observed, which results in a broadening of the signals of tBu due to hindered rotation of the CH3 groups. Complex 8 has a trigonalbipyramidal geometry around iridium (Figure 1). The O−O

which overnight are converted to carbonyl complex 9 (Scheme 7). If an excess of CO is used, dicarbonyl 13 is formed, which is Scheme 7. Reaction of 6 with CO

characterized by an AX system in the 31P{1H} NMR spectrum (79.73 and 59.81, br d, 2JP1P2 = 176 Hz). Broad signals from CO groups are observed at 191.9 and 182.0 ppm; they seem to be in exchange via a dissociative mechanism, which is confirmed by loss of one of the CO’s to give 9 upon evacuation. In the IR spectrum complex 13 is characterized by two intense bands at 1948 and 1909 cm−1. Neutral dicarbonyl compounds are rare for Ir pincers and were previously observed for systems with reduced steric bulk around Ir.22 Therefore, it may be concluded that in 9 the Ir atom is less screened than in the symmetrical (PCyP)IrCO, where no dicarbonyl is formed. Complex 6 undergoes a reversible reaction with CO2 with formation of the insertion product 14 (Scheme 8). The 31P

Figure 1. Molecular structure of complex 8 with thermal ellipsoids at the 30% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ir−C1 2.151(5), Ir−C32 2.188(5), Ir−C11 2.108(6), Ir−O1 2.030(3), Ir−O2 2.068(4), C1− C32 1.415(9), O1−O2 1.451(5), P−Ir−P 160.79(5), C1−Ir−C11 130.0(2), C1−Ir1−O1 137.1(2).

Scheme 8. Reaction of 6 with CO2

distance of 1.45 Å is significantly longer than in free dioxygen (1.21 Å) and falls in the range observed for Ir(η2-O2) compounds (1.43−1.53 Å24). These data, together with the observation of an O−O band at 860 cm−1 as well as an Ir−O band at 464 cm−1 in the IR spectrum (KBr), argue for the formulation of 8 as an Ir(III) peroxo complex rather than an Ir(I) dioxygen complex. At the same time, the reactivity of 8 is relatively low. The pincer ligand in 8 appears to be fairly resistant to oxidation, and heating of a solution of 8 results only in reversible decoordination of the O2 molecule. For instance, at 100 °C in a closed NMR tube under air, formation of 3 is observed, with the 8:3 ratio being ca. 6:1. Prolonged heating of a solution of 8 in an open vial results in the recovery of 3 in a high yield. Even heating of the benzene solution of 8 under O2 at 80 °C overnight does not result in significant changes, but at higher temperatures decomposition to a number of unidentified product is observed. External cyclooctene does not react with 8. Complex 8 is also almost unactive in a typical test reaction for peroxo compounds, namely, oxidation of triphenylphosphine. Thus, under an air or O2 atmosphere 8 is inert toward PPh3, and only under 5 atm of O2 does very slow oxidation take place, resulting in formation of less than 0.1 equiv of OPPh3 overnight. Carbonyl complex 9 can also form an adduct with oxygen, compound 15 (Scheme 10). This compound is characterized by a distorted AX system (54.22 and 48.98 ppm, d, 2JP1P2 = 350.8 Hz) in the 31P{1H} NMR spectrum. The carbonyl group resonates at 178.31 (apparent t, 2JP1C = 2JP2C = 8.2 Hz), and the CO stretching frequency of 1951 cm−1 is higher than that in 9 (1924 cm−1), which is consistent with the coordination of an electron-withdrawing ligand. Complex 15 also has a TBP geometry (Figure 2), with an O−O bond length of 1.51 Å, which is significantly longer than that in 8 and unambiguously suggests the formulation of 15 as an Ir(III) peroxo complex.

NMR signals at 61.00 and 51.40 (d, 2JPP = 331.8 Hz) point to a saturated structure. The formate group is characterized by a resonance at 9.27 ppm in the 1H NMR spectrum as well as a signal at 172.1 ppm in the 13C{1H} spectrum. The hydride appears at −32.96 ppm (dd, 2JPH = 12.7 Hz, 2JPH = 16.0 Hz). These spectral data are similar to those previously reported for a related formate complex, [(tBu2PCH2)2CH]Ir(H)O−CHO.23 Upon heating complex 14, it is transformed into 9 and some unidentified decomposition products. Complexation of 3 and 9 with Oxygen. Exposure of the orange-red solution or powder of 3 to air results in the formation of the colorless dioxygen complex 8 (Scheme 9). The 31P{1H} NMR spectrum of 8 reveals an AB system (5.51, −5.56 ppm, 2JP1P2 = 421.2 Hz) with a high value of 31P−31P coupling, indicating mutually trans arrangement of the phosphines. The CH− proton resonates at 3.83 ppm (d, 3 JPH = 18.9 Hz, 1JCH = 164 Hz, for comparison, 156 Hz for 3) and is significantly low-field shifted compared to the parent Scheme 9. Reaction of Complex 3 with Dioxygen

G

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conclusion since such couplings are typical for sp2-type carbons, but this criterion should be treated with some caution; should the hybridization of carbons in a metallacyclopropane be similar to cyclopropane itself, such compounds might also reveal C−H couplings of the same magnitude. Table 1 summarizes NMR and XRD data for iridium olefin compounds recently synthesized in our group. Among the four compounds bearing a formal Ir(III) fragment bound to an olefin (6, 8, 17, 18), 8 and 17 are the least electron-rich and reveal NMR signals in the normal region for an olefin moiety. The C−C bond distance in 8 of 1.415(9) Å can be compared to typical CC (∼1.34 Å) and C−C (∼1.54 Å) bond lengths, as well as the bond length in cyclopropane (∼1.51 Å). For the two other compounds (6, 18) the NMR signals are considerably high-field shifted, indicating increased contribution of metallacyclopropane (and thus formally Ir(V)) structures. Remarkably, this contribution seems to be roughly similar to the one in compound 3 with a formal Ir(I) metal center. The latter compound has the highest “olefin” character among formally Ir(I) compounds, based on the 1H NMR chemical shift. Complexes 4 and 16 are slightly shifted toward the cyclopropane limit compared to 3, which tentatively can be ascribed to the lower trans influence of halogens compared to phenyl and, in turn, stronger interaction with Ir. It is interesting that coordination of CO to 3 also slightly increases the contribution of metallacyclopropane, again indicating that the donation from the olefin is more important than the π-back-donation. Thus, the contribution of the metallacyclopropane structure increases as follows: 8, 17 < 6, 18 < 3 < 4, 5a, 5b, 16. At the same time it is not straightforward to say whether this contribution is big enough to warrant a description of the latter group as metallacyclopropanes with iridium in a higher oxidation state. While the chemical shifts of the “olefin” group for these complexes are observed in the aliphatic region, it is important to note that the 13C resonance of cyclopropane is observed at −2.8 ppm, i.e., at much higher field than those of any Ir compound discussed above. The C−C distances in 4 and 16 of ca. 1.44 Å fall well between those of single and double C− C bonds and do not exceed the typical values for Ir−olefin moieties, which can be found in the CCDC (ca. 1.42−1.44 Å).

Scheme 10. Reaction of Complex 9 with Dioxygen

Figure 2. Molecular structure of complex 15 with thermal ellipsoids at the 30% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ir−C1 2.195(6), Ir−C9 1.832(8), Ir−O1 2.049(6), Ir−O2 2.117(5), O1−O2 1.514(8), C9− O3 1.16(1), P−Ir−P 156.40(7), C1−Ir−C9 98.5(3), C1−Ir1−O1 105.2(2).

The reactivity of 15 is in agreement with an increased degree of activation of dioxygen. Thus, instead of releasing O2 upon heating, as was observed for 8, 15 decomposes. In contrast to 8, complex 15 rapidly oxidizes one equivalent of PPh3 at room temperature, but the reaction is not catalytic presumably due to decomposition. Remarks on the Relative Contribution of π-Olefin/ Metallacyclopropane Structures. Some of the olefin compounds reported here reveal 1H and 13C NMR signals that lie in the aliphatic region rather than in the typical olefin one and therefore deserve some comments. Previously,9 we assigned complexes 3 and 4 as Ir(I) with olefin moieties, primarily based on C−C bond lengths, despite relatively highfield NMR shifts of the olefin groups. A one-bond C−H coupling of 156 Hz measured for 39 at first glance supports this

Table 1. NMR (C6D6) and XRD Data on Some Ir Olefin Complexes

39 d(C−C) XRD, Å δ 9-H, ppm δ 9-C, ppm δ 1-C, ppm

1.425(7) 1.421(8) 1.95 49.5 76.6

69

49

5a

5b

1.441(5) 2.51 46.1 81.3

1.52 35.3 63.6

1.61 34.2 62.4 H

1.36 34.2 68.8

8

1625

1.415(9)

1.438(15)

3.83 56.4 81.5

1.68 16.8 68.9

1725

1825

3.50 44.3 103.4

∼ 1.8 23.0 79.3

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(c) van der Boom, M. E.; Milstein, D. Chem. Rev. 2003, 103, 1759− 1792. (d) Morales-Morales, D. Rev. Soc. Quim. Mex. 2004, 48, 338− 346. (e) Leis, W.; Mayer, H. A.; Kaska, W. C. Coord. Chem. Rev. 2008, 252, 1787−1797. (f) Choi, J.; MacArthur, A. H. R.; Brookhart, M.; Goldman, A. S. Chem. Rev. 2011, 111, 1761−1779. (g) O’Reilly, M. E.; Veige, A. S. Chem. Soc. Rev. 2014, 43, 6325−6369. (4) Peters, R., Ed. Cooperative Catalysis: Designing Efficient Catalysts for Synthesis; Wiley-VCH: Weinheim, 2015. (a) Gunanathan, C.; Milstein, D. Acc. Chem. Res. 2011, 44, 588−602. (b) Kohl, S. W.; Weiner, L.; Schwartsburd, L.; Konstantinovski, L.; Shimon, L. J. W.; Ben-David, Y.; Iron, M. A.; Milstein, D. Science 2009, 324, 74−77. (c) Askevold, B.; Nieto, J. T.; Tussupbayev, S.; Diefenbach, M.; Herdtweck, E.; Holthausen, M. C.; Schneider, S. Nat. Chem. 2011, 3, 532−537. (d) Grutzmacher, H. Angew. Chem., Int. Ed. 2008, 47, 1814− 1818. (5) Crocker, C.; Errington, R. J.; Markham, R.; Moulton, C. J.; Odell, K. J.; Shaw, B. L. J. Am. Chem. Soc. 1980, 102, 4373−4379. Crocker, C.; Errington, R. J.; Mcdonald, W. S.; Odell, K. J.; Shaw, B. L.; Goodfellow, R. J. J. Chem. Soc., Chem. Commun. 1979, 498−499. (a) Campos, J.; Lopez-Serrano, J.; Alvarez, E.; Carmona, E. J. Am. Chem. Soc. 2012, 134, 7165−7175. (b) Baratta, W.; Ballico, M.; Del Zotto, A.; Zangrando, E.; Rigo, P. Chem. - Eur. J. 2007, 13, 6701− 6709. (c) Baratta, W.; Herdtweck, E.; Martinuzzi, P.; Rigo, P. Organometallics 2001, 20, 305−308. (d) Bennett, M. A.; Longstaff, P. A. J. Am. Chem. Soc. 1969, 91, 6266−6280. (e) Vigalok, A.; Rybtchinski, B.; Shimon, L. J. W.; Ben-David, Y.; Milstein, D. Organometallics 1999, 18, 895−905. (a) Gusev, D. G.; Lough, A. J. Organometallics 2002, 21, 2601−2603. (b) Kuznetsov, V. F.; AbdurRashid, K.; Lough, A. J.; Gusev, D. G. J. Am. Chem. Soc. 2006, 128, 14388−14396. (6) (a) Bennett, M. A.; Clark, P. W. J. Organomet. Chem. 1976, 110, 367−381. (b) Bennett, M. A.; Johnson, R. N.; Tomkins, I. B. J. Organomet. Chem. 1976, 118, 205−232. (7) (a) Barrett, B. J.; Iluc, V. M. Organometallics 2014, 33, 2565− 2574. (b) Barrett, B. J.; Iluc, V. M. Inorg. Chem. 2014, 53, 7248−7259. (8) (a) Sjovall, S.; Wendt, O. F.; Andersson, C. J. Chem. Soc. Dalton 2002, 1396−1400. (b) Olsson, D.; Arunachalampillai, A.; Wendt, O. F. Dalton Trans. 2007, 5427−5433. (c) Arunachalampillai, A.; Olsson, D.; Wendt, O. F. Dalton Trans. 2009, 8626−8630. (d) Jonasson, K. J.; Wendt, O. F. J. Organomet. Chem. 2014, 759, 15−18. (9) Polukeev, A. V.; Marcos, R.; Ahlquist, M. S. G.; Wendt, O. F. Chem. Sci. 2015, 6, 2060−2067. (10) Polukeev, A. V.; Gritcenko, R.; Jonasson, K. J.; Wendt, O. F. Polyhedron 2014, 84, 63−66. (11) Crysalis CCD; Oxford Diffraction Ltd.: Abingdon, Oxfordshire, UK, 2005. (12) Crysalis RED; Oxford Diffraction Ltd.: Abingdon, Oxfordshire, UK, 2005. (13) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, A64, 112−122. (14) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Burla, M. C.; Polidori, M. J. Appl. Crystallogr. 1994, 27, 435−436. (15) CrystalMaker Software; Begbroke Science Park: Yarnton, Oxfordshire, OX5 1PF, U.K., 2010. (16) Jonasson, K. J.; Polukeev, A. V.; Wendt, O. F. RSC Adv. 2015, 5, 15534−15538. (17) See, for example: Rybtchinski, B.; Ben-David, Y.; Milstein, D. Organometallics 1997, 16, 3786−3793. (18) For some examples where the agostic C−H bond reveals decreased C−H coupling, while other C−H bonds at the same carbon demonstrate increased C−H couplings, see: (a) Tempel, D. J.; Brookhart, M. Organometallics 1998, 17, 2290−2296. (b) DunlopBriere, A. F.; Budzelaar, P. H. M.; Baird, M. C. Organometallics 2012, 31, 1591−1594. (19) (a) Morales-Morales, D.; Redón, R.; Wang, Z.; Lee, D. W.; Yung, C.; Magnuson, K.; Jensen, C. Can. J. Chem. 2001, 79, 823−829. (b) Kloek, S. M.; Heinekey, D. M.; Goldberg, K. I. Organometallics 2006, 25, 3007−3011. (c) Melnick, J. G.; Radosevich, A. T.; Villagrán, D.; Nocera, D. G. Chem. Commun. 2010, 46, 79−81. (d) Polukeev, A.

The C−O stretching frequencies in 5a and 5b (1936 and 1943 cm−1, correspondingly), which may serve as an indirect probe of the oxidation state of Ir, are also observed between clearly Ir(I) (9, 1924 cm−1) and Ir(III) ((PCyP)IrH(Ph)CO, 1951 cm−1 for syn and 1966 cm−1 for anti, and 10, 1972 cm−1). All in all, we conclude that both limiting structures have considerable contribution to the group of complexes with the most high-field NMR chemical shifts. However, as noted, the C−C distances agree well with Ir−olefin complexes in the literature, and it can be noted that there is also literature precedence for substantial upfield shifts of 1H and 13C NMR signals (2.06 and 36.5, respectively, in an Ir(I)−cyclooctene complex).26 We therefore suggest that the complexes described here also be primarily classified as olefin complexes with Ir in the lower oxidation state.

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CONCLUSIONS In summary, we have presented some chemistry of iridium pincer complexes with an olefin backbone and compounds derived from them. A reactivity pattern that seems to go through most of such compounds is a tautomeric equilibrium between an iridium olefin hydride and the corresponding insertion product. Depending on the ligands on Ir, one or another form can be preferred. The chemistry of the saturated form more or less resembles other iridium pincers with an aliphatic backbone. A noticeable feature of the olefin form is a relatively weak π-basicity, which leads to weaker binding of CO and O2 compared to other Ir pincer complexes. This leads to a lower reactivity of the olefin-based peroxo complex.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00495. Details of the kinetic measurements, NMR spectra, and a characterization of the product of CO insertion into the Cα−Ir bond (PDF) Crystallographic data (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Swedish Research Council, the Knut and Alice Wallenberg Foundation, and the Crafoord Foundation is gratefully acknowledged.

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REFERENCES

(1) Moulton, C. J.; Shaw, B. L. J. Chem. Soc., Dalton Trans. 1976, 1020−1024. (2) (a) Morales-Morales, D.; Jensen, C. M., Eds. The Chemistry of Pincer Compounds; Elsevier Science: Amsterdam, 2007. (b) van Koten, G.; Milstein, D., Eds. Organometallic Pincer Chemistry; Springer: Heidelberg, Topics in Organometallic Chemistry, 2013; Vol. 40, pp 1− 356. (c) Szabó, K. J.; Wendt, O. F., Eds. Pincer and Pincer-Type Complexes; Wiley-VCH: Weinheim, 2014. (3) (a) Albrecht, M.; van Koten, G. Angew. Chem., Int. Ed. 2001, 40, 3750−3781. (b) Singleton, J. T. Tetrahedron 2003, 59, 1837−1857. I

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Organometallics V.; Petrovskii, P. V.; Peregudov, A. S.; Ezernitskaya, M. G.; Koridze, A. A. Organometallics 2013, 32, 1000−1015. (20) Adams, J. J.; Arulsamy, N.; Roddick, D. M. Organometallics 2012, 31, 1439−1447. (21) Musa, S.; Filippov, O. A.; Belkova, N. V.; Shubina, E. S.; Silantyev, G. A.; Ackermann, L.; Gelman, D. Chem. - Eur. J. 2013, 19, 16906−16909. (22) (a) Adams, J. J.; Arulsamy, N.; Roddick, D. M. Organometallics 2011, 30, 697−711. (b) Bezier, D.; Brookhart, M. ACS Catal. 2014, 4, 3411−3420. (23) McLoughlin, M. A.; Keder, N. L.; Harrison, W. T. A.; Flesher, R. J.; Mayer, H. A.; Kaska, W. C. Inorg. Chem. 1999, 38, 3223−3227. (24) (a) Laing, M.; Nolte, M. J.; Singleton, E. J. Chem. Soc., Chem. Commun. 1975, 660−661. (b) Lebel, H.; Ladjel, C.; Belanger-Gariepy, F.; Schaper, F. J. Organomet. Chem. 2008, 693, 2645−2648. (c) Verat, A. Y.; Fan, H.; Pink, M.; Chen, Y.-S.; Caulton, K. G. Chem. - Eur. J. 2008, 14, 7680−7686. (d) Kinauer, M.; Scheibel, M. G.; Abbenseth, J.; Heinemann, F. W.; Stollberg, P.; Würtele, C.; Schneider, S. Dalton Trans. 2014, 43, 4506−4513. (25) Jonasson, K. J.; Polukeev, A. V.; Marcos, R.; Ahlquist, M. S. G.; Wendt, O. F. Angew. Chem., Int. Ed. 2015, 54, 9372−9375. (26) For a recent Ir(I) olefin structure see: Phillips, N.; Tang, C. Y.; Tirfoin, R.; Kelly, M. J.; Thompson, A. L.; Gutmann, M. J.; Aldridge, S. Dalton Trans. 2014, 43, 12288−12298.

J

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