Template for Communications for the Journal of the ...



Reactions of Iridium Hydride Pincer Complexes with Dioxygen:

New Peroxo Complexes and Reversible O2 Binding

D. Bridget Williams, Werner Kaminsky, James M. Mayer,* and Karen I. Goldberg*

Department of Chemistry, University of Washington, Box 351700, Seattle, Washington, 98195-1700

RECEIVED DATE (automatically inserted by publisher); mayer@chem.washington.edu; goldberg@chem.washington.edu

Organometallic reactivity is utilized in a range of organic transformations, but organometallic oxidation chemistry remains relatively under-developed.[i] Molecular oxygen is the most attractive terminal oxidant for chemical processes, in terms of cost, atom economy, and the potential for few byproducts.1 Recent studies have described elegant palladium catalysts for the aerobic oxidation of alcohols and other substrates, and have implicated mechanisms where O2 serves only to re-oxidize the palladium and does not interact with the substrates.1a In contrast, insertion of O2 into a palladium-hydride bond has been suggested in an alternative mechanism and recently this reactivity has been directly observed in model systems.[ii] Understanding the various modes of reactivity of oxygen with organometallic complexes will be key to developing effective homogeneous catalysis with this oxidant.

Reported here are reactions of O2 with the iridium pincer hydride complexes (tBuPCP)Ir(H)(X) [X = Cl (1)[iii]a, Ph (2)3b, H (3)3c, OH (4)3d and CCPh (5)3e] [tBuPCP = k3-C6H3(CH2PtBu2)2]. Our long-term goal is to couple O2-reactivity with the ability of such iridium-pincer complexes to catalytically activate C–H bonds in saturated hydrocarbons.[iv] The chemistry described here involves loss of H–X and reversible binding of O2 to form mono- and bis-peroxo complexes. Reactions of iridium compounds with O2 to form Ir(O2) species are well known, starting with Vaska’s complex.[v] The products reported here, however, include the first unsaturated iridium peroxide and the first late-transition metal bis-peroxo complex.

The hydride-chloride complex, (tBuPCP)Ir(H)(Cl) (1), is unreactive with 1 atm of O2 (research grade, < 0.5 ppm H2O) in benzene-d6 over 12 h at 343 K. This was surprising given the unsaturated nature of complex 1. In contrast, the phenyl-hydride complex 2 reacts rapidly with 1 atm O2 in benzene solution at 298 K to give two species, 6 and 7, in addition to small amounts of unidentified products (1H NMR: 10.7 (s), 15.8 (s)). Decomposition is observed after ca. 40 min; however, isolated 7 can be stored in the solid state for several months. Both 6 and 7 have 1H and 31P{1H} NMR spectra that indicate C2v symmetry, with equivalent phosphorus nuclei, methylene protons and tBu groups.[vi]

The ratio of 6 to 7 depends on the concentration of oxygen in solution. Under 5 atm O2 at 25°C, 1 is converted completely to 6, and X-ray quality crystals of 6 were grown from pentane under these conditions. Structure solution was accomplished (to R = 2.7%, Rw = 6.6%) using a disorder model for the twinned crystals in which molecules stack in one direction along the axis in some domains of the I crystals but in the opposite direction in other domains. The structure of the (pincer)iridium-bis(peroxo), (tBuPCP)Ir(η2-O2)2 (6) is shown in Figure 1. Due to the disorder, the bond lengths are not very well defined, but they appear to fall in normal ranges. For instance, the O–O distance of 1.54 (3) Å is similar to that in other iridium peroxo complexes,5g,[vii] implying that 6 is a bis-(O22–) complex with a formal 5+ Ir oxidation state.

[pic]

Figure 1. Molecular structure of (tBuPCP)Ir(η2-O2)2 (7), with thermal ellipsoids shown at 50% probability. Selected bond lengths (Å) and angles (deg): Ir(1)-O(1) 2.008(10); Ir(1)-O(2) 2.040(9); O(1)-O(2) 1.54(3); Ir(1)-C(1) 2.023(12); Ir(1)-P(1) 2.380(4); O(1)-Ir(1)-C(1) 87.3(8); O(1)-Ir(1)-O(2) 44.7(9); O(2)-O(1)-Ir(1) 68.7(7); O(1)-O(2)-Ir(1) 66.6(7).

Under vacuum, 6 cleanly forms 7 which is assigned as the 5-

coordinate mono-peroxo IrIII complex (tBuPCP)Ir(η2-O2) (eq 1). Treatment of 7 with O2 gives 6 by 1H and 31P{1H} NMR, establishing the reversible equilibrium in eq 1. The IR spectrum of 7 in a KBr pellet shows a Ir(O2) stretch at 893.5 cm-1 which is absent in the 18O-labeled complex, apparently shifting to ca. 811.1 cm-1.[viii] Characteristic of late-metal peroxo complexes,[ix] complex 7 reacts with SO2 to give the iridium sulfato complex, (tBuPCP)Ir(k2-SO4) (8), based on IR, 1H, and 31P{1H} NMR spectroscopies. [Mass spec?]

[pic] (1)

The dihydride complex (tBuPCP)Ir(H)2 (3) reacts with 5 atm O2 to give a mixture of 6, a new species 9 (ratio ~2:1), and trace amounts of 7 and (tBuPCP)Ir(H)(OH) (4) (eq 2). Over time the amount of 9, 7, and 4 decreases and 6 is the sole product. Complex 4 was identified by comparison with NMR spectra of an authentic sample, particularly the hydride signal at -31.0 ppm.3d Independently synthesized 4 is converted by 5 atm of O2 to cleanly give 6 in ca. 30 minutes. Monitoring the reaction of 3 with 1 atm O2 by 1H NMR shows the appearance and growth of a peak for water,which gradually shifts under the tert-butyl signals. This is supported by the observation of D2O via 2H NMR in the analogous reaction between the dideuteride complex, (tBuPCP)Ir(D)2, and 5 atm O2.

[pic] (2)

Complex 9 is tentatively assigned as the trans-dihydride peroxo complex trans-(tBuPCP)Ir(H)2(O2) based on a hydride signal at -6.8 ppm in the 1H NMR which integrates to two protons relative to the methylene protons of the ligand. This complex appears to be analogous to trans-dihydride species (iPrPCP)Ir(CO)(H)2,[x]a [(tBuPNP)Ir(CO)(H)2]PF6,10b and (tBuPNP)Ir(Ph)(H)2,10c all of which have hydride signals between -6 and -10 ppm.

The acetylide (tBuPCP)Ir(H)(CCPh) (5) also reacts with 5 atm O2 to very rapidly form 6. To our surprise, removal of the O2 by freeze-pump-thaw degassing yields not only 7, but also regenerates 5 (22%) (eq 3). This reaction indicates that not only can 6 convert to 7 under vacuum, but that the dioxygen ligand of 7 can also apparently be displaced.

[pic] (3)

The most likely pathway for these transformations is O2-induced reductive elimination from (tBuPCP)Ir(H)(X) to generate 7 directly (eq 4). O2 promoted reductive elimination of C-O and C-N bonds from NiII has been reported.[xi] The dihydride and acetylide complexes, 3 and 5, do not readily reductively eliminate at 298 K in the absence of O2,[xii] ruling out pre-equilibrium loss of HX followed by O2 binding to IrI. The tentative identification of 9 as a dihydride-peroxo complex, in essence an O2-adduct of 3, supports this pathway, although a cis isomer would appear more likely to lead to reductive elimination. In an attempt to observe a similar adduct, (tBuPCP)Ir(H)(tolyl) (2a) was reacted with 5 atm of O2 at 253 K in toluene, but the sole product was 6, similar to the 298 K reaction between 2 and O2. The hydrido-chloride complex 1 is inert presumably because loss of HCl from the metal center is unfavorable. The displacement of O2 from 7 by HCCPh (vide infra) may proceed by a similar associative mechanism.

[pic] (4)

The peroxo complexes are not especially reactive. For instance, they are inert to ethylene and cyclohexene. Complex 6 is a slow catalyst for the oxidation of PPh3 to OPPh3 by O2, giving 3.4 turnovers over three days under 5 atm of O2 in C6D6 (by 1H NMR and GC/MS). Surprisingly, the mono-peroxo complex 7 does not oxidize PPh3, perhaps because of the instability of the intermediate oxo complex that would likely be formed. Treatment of isolated 7 with 5 atm H2 gives water and the tetrahydride 10 (eq 5), which is readily formed by addition of H2 to 3. Similarly, 7 + D2 yields (tBuPCP)Ir(D)4 (10-d4) and D2O by 2H NMR.

[pic] (5)

The only oxygenated products formed in any of these reactions are water and trace amounts of the hydroxy-hydride, 4, from 3 + O2 (eq 2), do we really know that there is no water in the oxygen?, and water from 7 + H2 (eq 5). In none of the other (tBuPCP)Ir(H)(X) reactions is there any evidence for O2 insertion into an Ir–H or Ir–C bond. This contrasts recent reports of O2 insertion into metal-hydride bonds.2,[xiii] In the reaction of 2 with O2, neither hydroxy-phenyl nor hydrido-phenoxy complexes were observed by 1H NMR spectroscopy, and no phenol was seen by GC [GC experiment@]. The reaction of 5 with O2 forms HCCPh, not an oxygenated product [Must check this!]. Further work is needed to better define the basic pathways for reactions of O2 with organometallic compounds.

In summary, a number of iridium pincer hydride compounds (tBuPCP)Ir(H)(X) react with O2 to give IrIII and IrV peroxo complexes (tBuPCP)Ir(O2)n, n = 1 (7), or 2 (6). Dioxygen in most cases appears to promote reductive elimination of HX rather than inserting into an Ir–H or Ir–C bond. Yet the peroxo complex 7 reacts with hydrogen to form water and the tetrahydride complex 10. This type of reaction could be a potential alternative to more traditional hydrogen scavengers such as t-butylethylene. Studies are continuing in order to more fully develop the organometallic chemistry of O2 and incorporate it into new oxidation catalysis.

Acknowledgements. Support from the NSF Center for the Activation and Transformation of Chemical Bonds (CATSB) is gratefully acknowledged. We thank A. S. Goldman for helpful discussions, and Mira Kanzelberger.

Supporting Information Available: Full experimental details and crystallographic data for 6.

Table of Contents Graphic:

[pic]

Abstract:

Iridium pincer hydride complexes (tBuPCP)Ir(H)(X) [X = Ph (2), H (3), OH (4) and CCPh (5)] react with O2 in benzene solution to give the peroxo complexes (tBuPCP)Ir(O2)n [tBuPCP = k3-C6H3(CH2PtBu2)2; n = 1 (6), 2 (7)]. The bis(peroxo) complex 7 has been characterized by an X-ray crystal structure. Despite significant disorder, the structure reveals a pentagonal bipyramidal molecule with both peroxo ligands and the aryl carbon of the pincer ligand in the pentagonal plane. Under vacuum, complex 7 is converted to the mono-peroxo complex 6, which is reversed under O2. The reaction of the hydride-acetylide 5 with O2 is also reversible. The O2 reactions of (tBuPCP)Ir(H)(X) typically appear to proceed by dioxygen-induced reductive elimination of HX, rather than by insertion of O2 into an Ir–H or Ir–C bond. (tBuPCP)Ir(H)(Cl) is unreactive with dioxygen, possibly because elimination of HCl does not occur. Complex 6 reacts with H2 (5 atm) to give (tBuPCP)Ir(H)4 and water, and 7 (but not 6) oxidizes PPh3.

-----------------------

([i]) (a) Stahl, S. S. Angew. Chem. Int. Ed. 2004, 43, 3400-3420. (b) Sheldon, R. A.; Kochi, J. K. Metal Catalysed Oxidations of Organic Compounds; Academic Press: NewYork, 1981. (c) Dioxygen Activation and Homogeneous Catalytic Oxidation; Simándi, L. I., Ed.; Elsevier Science Publishing Company Inc.: New York, 1991. (d) Dible, B. R.; Sigman, M. S. J. Am. Chem. Soc. 2003, 125, 872-873. (e) Stahl, S. S. Science 2005, 309, 1824-1826.

([ii]) (a) Denney, M. C.; Smythe, N. A.; Cetto, K. L.; Kemp, R. A.; Goldberg, K. I. J. Am. Chem. Soc. 2006, 128, 2508-2509. (b) Konnick, M. M.; Gandhi, B. A.; Guzei, I. A.; Stahl, S. S. Angew. Chem. Int. Ed. 2006, 45, 2904-2907.

([iii]) (a) Moulton, C. J.; Shaw, B. L. J. Chem. Soc., Dalton, 1976, 1020-1024. (b) Göttker-Schnetmann, I.; White, P. S.; Brookhart, M. Organometallics 2004, 23, 1766-1776. (c) Gupta, M.; Hagen, C.; Kaska, W. C.; Cramer, R. E.; Jensen, C. M. J. Am. Chem. Soc. 1997, 119, 840-841. (d) Morales-Morales, D.; Lee, D. W.; Wang, Z.; Jensen, C. M. Organometallics 2001, 20, 1144-1147. (e) Personal communication with A. S. Goldman.

(4) (a) Gupta, M.; Hagen, C.; Flesher, R. J.; Kaska, W. C.; Jensen, C. M. Chem. Commun. 1996, 2083-2084. (b) Liu, F.; Pak, E. B.; Singh, B.; Jensen, C. M.; Goldman, A. S. J. Am. Chem. Soc. 1999, 121, 4086-4087. (c)

([iv]) (a) Vaska, L. Science 1963, 140, 809-810. (b) Roundhill, D. M.; Allen, G. H.; Bechtold, R. A.; Beaulieu, W. B. Inorg. Chim. Acta. 1981, 54, L99-L100. (c) Gopinathan, S.; Joseph, K.; Gopinathan, C. Polyhedron 1985, 4, 955-958. (d) Lawson, H. J.; Atwood, J. D. J. Am. Chem. Soc. 1989, 111, 6223-6227. (e) Bianchini, C.; Meli, A.; Peruzzini, M.; Vizza, F. J. Am. Chem. Soc. 1990, 112, 6726-6728. (f) Barbaro, P.; Bianchini, C.; Laschi, F.; Midollini, S.; Moneti, S.; Scapacci, G.; Zanello, P. Inorg. Chem. 1994, 33, 1622-1630.

([v]) See Supporting Information for full experimental and structural details. Caution should be taken when adding pressures of O2 to organic solvents.

([vi]) (a) Nolte, M. J.; Singleton, E.; Laing, M. J. Am. Chem. Soc. 1975, 97, 6396-6400. (b) Mezzetti, A.; Novelli, V.; Del Zotto, A.; Rigo, P. Acta Cryst. 1987, C43, 425-428. (c) Wang, J.-C.; Chou, L.-Y.; Hsien, W.-Y. Acta Cryst. 1994, C50, 879-882.

(8) Comment on overlap with ligand stretches?Using the simple harmonic oscillator equation the calculated shift is 842 cm-1. Ligand stretches are also located in this region.

([vii]) Valentine, J. S. Chem. Rev. 1973, 73, 235-245.

([viii]) (a) Rybtchinski, B.; Ben-David, Y.; Milstein, D. Organometallics 1997, 16, 3786-3793. (b) Kloek, S. M.; Heinekey, D. M.; Goldberg, K. I. Organometallics 2006, 25, 3007-3011. (c) Personal communication with S. M. Kloek.

([ix]) (a) Han, R.; Hillhouse, G. L. J. Am. Chem. Soc. 1997, 119, 8135-8136. (b) Lin, B. L.; Clough, C. R.; Hillhouse, G. L. J. Am. Chem. Soc. 2002, 124, 2890-2891.

([x]) (a) Krogh-Jespersen, K.; Czerw, M.; Summa, N.; Renkema, K. B.; Achord, P. D.; Goldman, A. S. J. Am. Chem. Soc. 2002, 124, 11404-11416. (b) Personal communication with A. S. Goldman.

([xi]) Other M-H plus O2 reactions

................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download