Organometallics 2009, 28, 1613–1615
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Reversible Carbonylation of an [NCN]PtMe Pincer Complex and Direct Evidence of Alkyl Migration Margaret L. Scheuermann,† Arnold L. Rheingold,‡ and B. Scott Williams*,† W. M. Keck Science Center, Claremont Colleges, 925 N. Mills AVenue, Claremont, California 91711, and Department of Chemistry and Biochemistry, UniVersity of California, San Diego, UH 5128, 9500 Gilman DriVe, MC 0332, La Jolla, California 92093-0332 ReceiVed January 21, 2009 Summary: [NCN]PtMe (1) reacts with CO initially to form an adduct (2) and then again to form a carbonyl acyl complex (3). Reaction of 3 with Me3NO generates 2 by alkyl migration rather than carbonyl deinsertion. Formal carbonyl insertion reactions have been extensively studied both as fundamental mechanisms and as key steps in important catalytic processes.1 Ever since the seminal study of a d6 octahedral manganese complex by Noack and Calderazzo,2 it has generally been accepted that the mechanism by which metal alkyl carbonyl complexes M(CO)Me convert to acyl complexes M(C(O)Me) is alkyl migration rather than carbonyl insertion. As a consequence of the principle of microscopic reversibility, the reverse reaction is thought to proceed by alkyl migration rather than carbonyl deinsertion. However, in most of the cases studied, the data are ambiguous,3 and in at least one iron case, carbonyl insertion has been clearly implicated.4 For square-planar d8 complexes, the preponderance of evidence suggests that migration is the dominant (if not exclusive) mechanism,5 but the only system in which an example of migration has been clearly demonstrated is that of Van Leeuwen.6 In these latter examples, a series of [PP′]M(CO)R ([PP′] ) a nonsymmetric bidentate phosphine, R ) alkyl, M ) Pd, Pt) complexes were prepared. Metal acyl formation was shown to occur with a stereochemistry consistent with migration rather than insertion. Insertion followed by isomerization could be ruled out because the product of migration was shown to be a kinetic rather than a thermodynamic product. We report herein the synthesis, characterization, and reactivity of a new platinum carbonyl complex and its insertion product, as well as unambiguous evidence of a migratory deinsertion pathway in the presence of Me3NO. Despite [NamineCarylNamine]PtX complexes having been known since the early 1980s,7 the first example in which X ) alkyl * To whom correspondence should be addressed. E-mail: swilliams@ jsd.claremont.edu. † Claremont Colleges. ‡ University of California, San Diego. (1) (a) Parshall, G. W.; Ittel, S. D. In Homogeneous Catalysis. The Applications and Chemistry of Catalysis by Soluble Transition Metal Complexes, 2nd ed.; Wiley: New York, 1992. (b) Collmann, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. In Principles and Applications of Transition Metal Chemistry: University Science Books: Mill Valley, CA, 1987. (2) Noack, K.; Calderazzo, F. J. Organomet. Chem. 1967, 10, 101. (3) Cavell, K. J. Coord. Chem. ReV. 1996, 155, 209. (4) Flood, T. C.; Campbell, K. D. J. Am. Chem. Soc. 1984, 106, 2853. (5) Anderson, G. K.; Cross, R. J. Acc. Chem. Res. 1984, 17, 67. (6) van Leeuwen, P. W. N. M.; Roobeek, C. F.; van der Heijden, H. J. Am. Chem. Soc. 1994, 116, 12117. (7) Grove, D. M.; van Koten, G.; Louwen, J. N.; Noltes, J. G.; Spek, A. L.; Ubbels, H. J. C. J. Am. Chem. Soc. 1982, 104, 6609.
Scheme 1. Reactivity of [NCN]PtCH3 with CO
(1; Scheme 1) was recently reported.8 When 1 is placed under 1 atm of CO in THF, one CO molecule rapidly displaces a chelate arm to form 2. Such hemilabile behavior of nitrogen pincer arms is well precedented.9 Over the course of hours to days (depending upon the CO pressure), 2 incorporates a second equivalent of CO to form the carbonyl acyl complex 3. All three complexes have been characterized by combustion analysis and spectroscopy (see the Supporting Information). In addition, crystals suitable for X-ray crystallography were obtained of both 1 and 3 (Figures 1 and 2; see the Supporting Information for details).10,11 The terminal νCO frequencies in the IR spectra of 2 and 3 (2055 and 2063 cm-1, respectively) are consistent with the presumption that the aryl carbon of the ligand (trans to CO in 3) has a stronger trans influence than does the amine ligand (trans to CO in 2). However, in this case the trans influence is partially offset by the fact that 2 has a more π-rich metal center (the cis ligand is CH3) than does 3 (where the cis ligand is a π-withdrawing Ac group). The 1JPtCO coupling constants (1148 Hz for 3 and 1930 Hz for 2) show a much stronger dependence on trans influence, presumably because 1JPtC is primarily determined by σ rather than π interactions. In contrast, comparisons of 1, in which the aryl group is trans to methyl, and 3, in which it is trans to a carbonyl, seem contradictory. 1JPtCaryl couplings for 1 (636 Hz) and 3 (1101 Hz) are consistent with methyl having a stronger trans influence than carbonyl,12 but the Pt-aryl bond lengths in 1 (1.970(8), 1.957(9) Å) and 3 (2.050(5), 2.058(5) Å) suggest the opposite. The unexpectedly long Pt-Caryl bond in 3, however, may be the result of dechelation of one arm of the NCN ligand. Van Koten and co-workers have noted that a tridentate [NCN]PtBr complex has an anomalously short Pt-Caryl bond due to the constraints imposed by binding both amine arms.14 Upon thermolysis at 68 °C in THF-d8, 3 undergoes conversion to a mixture of 2 and its geometric isomer, 2′ (Scheme 2). Clean product ratios could not be obtained because 2 and 2′ thermally interconvert under the reaction conditions (see below) and because 2′ undergoes decomposition to uncharacterized organic products and platinum black.15 (8) Madison, B. L.; Thyme, S. B.; Keene, S.; Williams, B. S. J. Am. Chem. Soc. 2007, 129, 9538. (9) For example, see Vuzman, D.; Poverenov, E.; Shimon, L. J. W.; Diskin-Posner, Y.; Milstein, D. Organometallics 2008, 27, 2627.
10.1021/om900050p CCC: $40.75 2009 American Chemical Society Publication on Web 02/26/2009
1614 Organometallics, Vol. 28, No. 6, 2009
Communications Scheme 3. Generation of Labeled Carbonyl Species
Figure 1. ORTEP drawing of 1, with ellipsoids at the 30% probability level. One of the two symmetry-independent, but chemically identical, molecules is shown.
Figure 2. ORTEP drawing of 3, with ellipsoids at the 30% probability level. One of the two symmetry-independent, but chemically identical, molecules is shown.
conversion of 2 to 2′ in 22 h) or much more rapidly in the presence of a trace of CO (ca. 40% conversion of 2 to 2′ in 22 h). The interconversion between 2 and 2′ presumably occurs via the coordination of a fifth ligand, but whether this ligand is the free arm of the NCN moiety, a solvent molecule, or a molecule of CO depends upon the reaction conditions. In keeping with this hypothesis, exposure of 2 to 13CO results in immediate formation of the 13CO-labeled analogue 2* (Scheme 3). Two other labeled species were also prepared using 13CO in this fashion. Over time, incorporation of a second equivalent of 13CO affords the doubly labeled 3**. When 3** is placed under unlabeled CO, it undergoes conversion to singly labeled 3*, in which the label is at the acyl carbon. We had hoped that thermolysis of the singly labeled 3* would allow us to track the fate of the labeled carbonyl in the conversion to 2 and 2′, but complete scrambling of labeled and unlabeled CO was observed in the products. Even when the reaction was carried out with a stream of argon bubbling through the solution, complete scrambling was observed. One possible explanation for this exchange is that either methyl migration or carbonyl deinsertion from 3* occurs to form the five-coordinate species [NCN]Pt(12CO)(13CO)Me.17 Because of the inherent fluxionality of five-coordinate species, the
Scheme 2. Thermolysis of 3
By heating 3 to 100 °C, 2′:2 ratios as high as 2 could be obtained, suggesting that 2′ is accessible from 2 and may in fact be more stable.16 This would be unsurprising, since 2 contains two strong mutually trans donors (aryl and methyl), whereas in 2′, these ligands are mutually cis. The relief of this electronic “trans repulsion” can be seen in the dramatic increase of the one-bond Pt-C couplings to the methyl group (42 Hz for 2, 88 Hz for 2′). Direct interconversion between 2 and 2′ occurs at 68 °C in THF-d8 either in the absence of CO (ca. 5%
(10) Crystal and structure refinement parameters for 1: at 130(2) K, C17H30N2Pt, fw ) 457.52, orthorhombic, space group Pbca, a ) 24.3432(5) Å, b ) 10.3495(5) Å, c ) 27.6545(6) Å, V ) 6966.8(4) Å3, Z ) 16, dcalcd ) 1.745 g cm-3, µ(Mo KR) ) 0.705 mm-1, Rint ) 0.0727, R1 ) 0.0164, wR2 ) 0.1275 (I > 2σ(I)), CCDC 662710. Data were collected on Bruker SMART/Enraf-Nonius KappaCCD automatic diffractometers using graphitemonochromated Mo KR (λ ) 0.710 73 Å) radiation. Structures were solved by direct methods and refined by full-matrix least-squares refinement using the SHELXL9711 programs. Crystal and structure refinement parameters for 3: at 100(2) K, C19H30N2O2Pt, fw ) 513.54, monoclinic, space group P21/n, a ) 16.7499(4) Å, b ) 8.8572(2) Å, c ) 27.0704(7) Å, β ) 95.2760(10)°, V ) 3999.07(17) Å3, Z ) 8, dcalcd ) 1.706 g cm-3, µ(Cu KR) ) 0.705 mm-1, Rint ) 0.0237, R1 ) 0.0361, wR2 ) 0.0884 (I > 2σ(I)), CCDC 662710. Data were collected on a Bruker APEX II diffractometer using graphite-monochromated Cu KR (λ ) 1.541 78 Å) radiation. Structures were solved by direct methods and refined by full-matrix leastsquares refinement using the SHELXL9711programs. (11) Sheldrick, G. M. SHELXL-97, Program for Refinement of Crystal Structures; University of Go¨ttingen, Go¨ttingen, Germany, 1997. (12) Interestingly, previous reports7,8 of [NCN]PtX complexes have exhibited 1JPtC couplings from the aryl carbon to platinum of 584-1014 Hz, making this the largest such coupling known. We speculate that this may imply that the carbonyl binds almost exclusively through π-backbonding and thus does not compete with the aryl ring for the σ orbital which carries the Pt-C coupling.13 (13) Appelton, T. G.; Clark, H. C.; Manzer, L. E. Coord. Chem. ReV. 1973, 10, 335. (14) Terheijden, J.; van Koten, G.; Muller, F.; Grove, D. M.; Vrieze, K.; Nielsen, E.; Stam, C. H. J. Organomet. Chem. 1986, 315, 401.
Communications Scheme 4. Reaction of 3 with Trimethylamine N-Oxide
carbonyls could thereby become equivalent. A second possibility is that, even under a constant argon purge, exchange of liberated CO with 2 and 2′ is sufficiently rapid that mechanistic information could not be gained from a labeling study. In order to obtain an unambiguous mechanistic result for any decarbonylation reaction of this complex, it would be necessary to eliminate both the possibility of a five-coordinate intermediate and the liberation of free CO which could exchange with the product. In sharp contrast to the thermal reactivity of 3*, when monolabeled 3* was allowed to react with Me3NO at room temperature,18 clean formation of 2* was observed, with no formation of either the unlabeled analogue 2 or the isomer 2′* (15) While the formation of platinum black and the back reaction with liberated CO have prevented us from obtaining either clean kinetics or an equilibrium constant for the interconversion of 2 and 2′, the rate of decomposition increases with the increase in the concentration of 2′, suggesting that it is 2′ rather than 2 which undergoes degradation. (16) This reaction is under kinetic rather than thermodynamic control; the buildup of 2′ relative to 2 suggests but certainly does not prove that it is the more stable isomer. (17) For an example of such a reaction pathway, see: Schultz, C. S.; DeSimone, J. M.; Brookhart, M. J. Am. Chem. Soc. 2001, 123, 9172.
Organometallics, Vol. 28, No. 6, 2009 1615
(Scheme 4). Since Me3NO acts by abstraction of a molecular CO unit,19 this presumably generates the intermediate A. Carbonyl deinsertion should lead to 2′*, while methyl migration should lead to 2*. Since it was shown in a separate experiment that 2′ does not quickly and quantitatively convert to 2 under the reaction conditions (interconversion being slow and actually favoring 2′), initial formation of 2′, which then rapidly and irreversibly isomerizes to 2, can be ruled out.20 This represents, therefore, a rare example of a reaction in which the mechanism of acyl ligand decarbonylation can be clearly shown to be alkyl migration.21 Attempts to elucidate the mechanism of the thermal interconversion of 2, 2′, and 3 are ongoing in our laboratory.
Acknowledgment. We thank Prof. D. O’Leary for assistance with NMR spectroscopy, K. I. Goldberg for helpful scientific discussions, and Dr. Werner Kaminsky for the solution of the crystal structure of 1. B.S.W. thanks the Dreyfus Foundation for a Faculty Startup Award (No. SU03-061). Supporting Information Available: Text, figures, tables, and CIF files giving experimental details for the syntheses and reactions described herein, along with the characterization data for 1-3 and full structural data for 1 and 3. This material is available free of charge via the Internet at http://pubs.acs.org. OM900050P (18) Identical results were obtained when a solution of 3 was added via syringe to a solution of Me3NO at 68 °C. (19) Reference 1b, p 264. (20) While isomerization of A to place the acyl trans to aryl followed by CO migration could be postulated, this seems highly unlikely, as it would require these very donating groups to arrange themselves mutually trans, leaving the weakly donating amine trans to the open site. (21) It should be noted that the thermal decarbonylation of 3 and decarbonylation of 3 in the presence of Me3NO may well proceed via different mechanisms and intermediates. Similarly, the carbonylation of 2 in the presence of CO is not the formal microscopic reverse of either of these decarbonylation reactions because of the presence of CO (and the absence of Me3NO).
