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Iridium Models of Rhodium Intermediates in Hydroformylation Catalysis: Isolation and Molecular Structures of Fluxional ae and ee Isomers Golnar Abkai,† Sebastian Schmidt,† Tobias Rosendahl,†,‡ Frank Rominger,† and Peter Hofmann*,†,‡ †
Organisch-Chemisches Institut, Universität Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany Catalysis Research Laboratory (CaRLa), Universität Heidelberg, Im Neuenheimer Feld 584, 69120 Heidelberg, Germany
‡
S Supporting Information *
ABSTRACT: We report the synthesis and single-crystal molecular structures of two stereoisomers of trigonalbipyramidal hydrido dicarbonyl Ir complexes [(L2)Ir(H)(CO)2] with ae (axial−equatorial) and ee (equatorial− equatorial) ligand P coordination and fluxional behavior in solution. L2 is a new chelating bisphosphite with unprecedented high selectivity in Rh-catalyzed bis-hydroformylation of butadiene to adipic aldehyde. These Ir analogues are ideal stabilized structural models for nonseparable ae and ee Rhhydroformylation resting state isomers [(L2)Rh(H)(CO)2]. With Ir, both stereoisomers with the same ligand could be characterized independently for the first time.
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It is well-known that reactions involving iridium complexes are slower than reactions of their rhodium congeners.10 Considering gross molecular geometries and structural details, both metals are perfectly comparable, however.11−13 Thus, the option to utilize Ir model compounds and Ir model chemistry to characterize the structures of species of only fleeting existence in rhodium-catalyzed hydroformylation reactions can be highly instrumental for ligand design and mechanistic understanding. This is of particular interest for the Rh-catalyzed n-selective bis-hydroformylation of 1,3-butadiene to adipic aldehyde, one of the so-called “dream reactions” of industrial chemistry (eq 1)
arious studies of different types of chelating phosphorus ligand systems employed in Rh hydroformylation catalysis have gathered an appreciable amount of information about the catalyst resting state.1−8 Independent of the chelating ligand used, five-coordinate hydrido dicarbonyl complexes with trigonal-bipyramidal structures have been identified for both rhodium and iridium as the metal center. Chelate ligands can either occupy the axial−equatorial (ae) or equatorial− equatorial (ee) positions in combination with two CO ligands and the hydride ligand in an axial position (Figure 1). In most cases for Rh a rapid equilibrium between the ae- and ee-coordinated forms was observed, and both configurational isomers are active precatalyst species initiating the catalytic cycle in hydroformylation reactions.1,9
tackled multiply but without success since decades. For this long-sought transformation we have recently started a synthetic, structural, and theoretical investigation of mechanistically relevant Ir compounds considered to mimic intermediates of the highly complicated catalytic cycles with Rh. In previous studies, using new types of chelating bisphosphite ligands for rhodium-catalyzed hydroformylation of 1alkenes and of butadiene, where the adipic aldehyde selectivity reached 50%,14 we have observed fast equilibria between the two ae and ee isomers for both rhodium and iridium complexes
Figure 1. Fast equilibrium between ae- and ee-coordinated resting state isomers of the hydrido dicarbonyl complexes (M = Rh, Ir) at room temperature (left) and bis-phosphite ligand used in this study (right). © 2014 American Chemical Society
Received: December 27, 2013 Published: June 5, 2014 3212
dx.doi.org/10.1021/om401139u | Organometallics 2014, 33, 3212−3214
Organometallics
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of the type [(bis-phosphite ligand)M(H)(CO)2] (M = Rh, Ir).15 Even at temperatures as low as −80 °C none of these equilibria could be frozen out using NMR spectroscopic techniques. For Ir and the bis-phosphite ligand shown in Figure 1 we observed that the two isomers form a mixture both in the (amorphous) solid state and in solution. For all such trigonalbipyramidal complexes of Rh and Ir, to the best of our knowledge so far only one of the two interconverting isomers could be isolated by crystallization. In earlier work15 we have already reported the single-crystal X-ray diffraction structure of the ee-coordinated isomer of the iridium model compound. The expected close similarity of iridium and rhodium structures was shown by comparing DFT-optimized rhodium and iridium geometries with the experimentally characterized ee iridium species. Using our published synthesis but changing the solvent for crystallization from dichloromethane to toluene, we were now able to also crystallize the second isomer with an aecoordinated phosphite ligand (Figure 2). This represents the
Figure 4. Superposition of two analogous single-crystal ae molecular structures for Ir and Rh differing only with regard to the central metal and the ligand backbone.
In summary, the structural characterization of the two isomeric Ir complexes nicely shows the usefulness and practicability of investigating iridium model complexes in order to uncover ligand-determined structural details of Rh species within the catalytic cycle of the hydroformylation reaction. Not only have we confirmed the existence of both postulated and computed isomeric forms of the trigonalbipyramidal resting state but this strategy of studying Ir model complexes can also be transferred to other structures, in particular to those which are too short-lived to be detected in Rh catalysis, as e.g. 16-VE species or olefin-coordinated intermediates.16 We consider Ir model chemistry as particularly promising in the area of ligand design and ligand optimization for novel types of hydroformylation reactions, in particular for the unsolved problem of butadiene hydroformylation to adipic aldehyde.14 Ir model compounds relevant in this context will be reported separately.
Figure 2. Isolation of the ae and ee isomers of [(tMeTriptyphosphite)Ir(H)(CO)2], depending on the solvent used for crystallization.
first case where both isomers could be obtained as single crystals and confirms once more the similar stability of the two isomeric forms, as has already been shown in our earlier study.15 In the molecular structures of both isomers (Figure 3), a larger P1−Ir−P2 bite angle (105.5°) is found in the ee isomer
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EXPERIMENTAL SECTION
General Experimental Details. All manipulations were performed under an inert atmosphere of purified argon (standard vacuum line, Braun glovebox) using standard Schlenk techniques. The solvents used were purified and dried according to literature procedures, degassed by freeze−pump−thaw cycles, and stored under an atmosphere of argon. 1H and 13C{1H} NMR data were reported in units of δ relative to TMS referenced to the residual solvent resonance as internal reference. 31P{1H} NMR spectra were externally referenced to 85% H3PO4; low-temperature measurements were calibrated at room temperature. HP-NMR experiments were carried out in HPNMR tubes by Wilmad LabGlass. Materials. IrCl3·xH2O and Rh(acac)(CO)2 were supplied by BASF SE. The tMe-Triptyphosphite ligand (TTP) and the tMe-Rucaphosphite ligand were prepared according to a published report.14 [Ir(COE)2Cl]2 was prepared according to known procedures.17 [(tMe-Triptyphosphite)Ir(H)(CO)2]. This compound has been prepared from the corresponding (TTP)Ir(CO)Cl complex via hydride transfer using Na[HB(OMe)3] under a CO atmosphere.15 Crystals with the ee-coordinated phosphite ligand suitable for X-ray diffraction analysis were obtained by isothermic distillation of a saturated solution in CH2Cl2 in an apparatus consisting of two Schlenk tubes connected by a frit. The ae-coordinated isomer was obtained by isothermic distillation of a saturated solution in toluene using the same Schlenk technique (CCDC 943739). These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www. ccdc.cam.ac.uk/data_request/cif. [(tMe-Rucaphosphite)Rh(H)(CO)2]. For the preparation of this compound, Rh(acac)(CO)2 was treated with the tMe-Rucaphosphite ligand in C6D6 under an argon atmosphere.15 The resulting solution was then transferred to an HP-NMR tube, and the solution was pressurized with 12 bar of CO and H2 (1/1). Crystals suitable for Xray analysis precipitated from this sample after the NMR measurement.
Figure 3. ORTEP representations of the ae-coordinated complex [(tMe-Triptyphosphite)Ir(H)(CO)2] (left) and the ee-coordinated form of the same compound (right). Ellipsoids are drawn at the 50% probability level. Ligand hydrogens are omitted for clarity.
as expected, while this angle in the ae-coordinated form is 99.0°. The Ir−CO distances vary slightly between 1.91 Å in the ae isomer and 1.90 Å (COeq) and 1.95 Å (COax) in the ee isomer, respectively. The close similarity of the iridium and rhodium structures can now be shown directly by comparing the single-crystal molecular structure of the Ir ae isomer with our earlier ae rhodium complex15 that bears a slightly different bis-phosphite ligand (Figure 4). The comparison of the DFT-optimized structures each with rhodium and iridium as the central metal also shows high similarity (see the Supporting Information). 3213
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
Note
S Supporting Information *
Tables, figures, and a CIF file giving crystal data, atomic coordinates, bond distances, bond angles, and anisotropic displacement parameters and computational details and Cartesian coordinates of the computed structures. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author
*E-mail for P.H.:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the Fonds der Chemischen Industrie (fellowship to S.S.) and the Deutsche Forschungsgemeinschaft (SFB 623, TP D4) for financial support of this work. The Catalysis Research Laboratory (CaRLa) is jointly funded by Heidelberg University, BASF SE, and the state of Baden-Württemberg. We gratefully acknowledge support from these institutions. We thank Jun.Prof. Dr. Katrin Schuhen for careful proofreading of the manuscript.
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REFERENCES
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