Communication pubs.acs.org/Organometallics
SET Oxidation of Li/X Phosphinidenoid Complexes by TEMPO Vitaly Nesterov,† Sebastian Schwieger,† Gregor Schnakenburg,† Stefan Grimme,‡ and Rainer Streubel*,† †
Institut für Anorganische Chemie der Rheinischen Friedrich-Wilhelms-Universität Bonn, Gerhard-Domagk-Straße 1, 53121 Bonn, Germany ‡ Mulliken Center for Theoretical Chemistry, Institut für Physikalische und Theoretische Chemie, Rheinische Friedrich-Wilhelms-Universität Bonn, Beringstraße 4, 53115 Bonn, Germany S Supporting Information *
ABSTRACT: Oxidation of in situ generated Li/X phosphinidenoid complexes [(OC)5W{(Me3Si)2CHP(X)Li/12-crown4}] (X = F (2), Cl (6)) with TEMPO at −78 °C led to the lithium halogenophosphinite complexes [(OC) 5 W{(Me3Si)2CHP(X)OLi/12-crown-4}] (4) via an unknown pathway possibly involving transient P-nitroxyl complex derivatives (3). Whereas 4a (X = F) could be isolated directly, 4b (X = Cl) underwent facile hydrolysis to give the lithium phosphonite complex [(OC)5W{(Me3Si)2CHP(OH)OLi/12crown-4}] (7) as the final product; the structures of complexes 4a and 7 were firmly established by single-crystal X-ray analysis. DFT calculations on the assumed intermediate P-nitroxyl complexes 3a,b reveal a preference for homolytic N−O over P−O bond cleavage, as underlined by the small bond dissociation energies (in kcal/mol: 3a, P−O 53.2 and N−O 26.0; 3b, P−O 35.4 and N−O 20.8). lithium fluorophosphinite complex 4a was observed after slow warming (ca. 12 h) (Scheme 1).
Single-electron-transfer (SET) processes play an important role in chemistry and biology,1 and most often, one or more reactive species with unpaired electrons are involved. Among the most investigated spin labels is the stable nitroxyl radical 2,2,6,6tetramethylpiperidin-1-oxyl (TEMPO),2 which is known to react with carbanions, generating the corresponding radicals via oxidation and which can be readily trapped, for example, by another 1 equiv of the reagent to produce alkoxyamines.3 While the application of TEMPO as oxygen transfer reagent for transition-metal complexes has been demonstrated previously,4 to the best of our knowledge, no such oxidation reactions were reported for organophosphorus compounds.5 It was recently demonstrated that Li/X phosphinidenoid tungsten complexes (X = F, Cl) can be oxidized by triarylcarbenium salts and the two radicals, the P−X phosphanyl complexes and triarylmethyls, couple in situ to form P−C bonds followed by elimination (X = Cl)6 or rearrangement (X = F).7 The combination of the high oxidation potential of TEMPO8 with the perspective of trapping the P-functional phosphanyl complex without facing such follow-up reactions prompted us to carry out this study. Here, we report on reactions of TEMPO with Li/X phosphinidenoid complexes (X = F, Cl) that lead finally to oxidized, oxygen-insertion-like products having a P−OLi substructure. DFT calculations provide the first insights into the bonding of the assumed intermediates, the P-nitroxyl substituted phosphane complexes as well as open-shell phosphinoyl-type complexes, being the decomposition products of the latter, formed via homolytic N−O bond cleavage. When Li/F phosphinidenoid complex 2,9 obtained in situ via lithiation of phosphane complex 1,10 was treated with TEMPO (2 equiv)11 at low temperature, a selective formation of the © 2012 American Chemical Society
Scheme 1. Proposed Reaction of Li/F Phosphinidenoid Complex 2 with TEMPO
Pure 4a (28% isolated yield) was obtained after crystallization. In the solid state 4a is a stable compound, but in THF solution it slowly decomposes at ambient temperatures to give a mixture of unidentified products. The 31P{1H} NMR spectrum of 4a displays a doublet with 183W satellites at 154.0 ppm (1JP,F = 956.2 and 1JP,W = 307.7 Hz), which are astoundingly similar to those of 19 (154.2 ppm, 1JP,F = 806.2 and 1JP,W = 286.1 Hz). Complex 4a was also subjected to singlecrystal X-ray analysis (Figure 1), and the unique combination of a P−F and a P−OLi unit12 was unequivocally confirmed. Received: February 7, 2012 Published: April 17, 2012 3457
dx.doi.org/10.1021/om300099g | Organometallics 2012, 31, 3457−3459
Organometallics
Communication
Scheme 2. Reaction of Li/Cl Phosphinidenoid Complex 6 with TEMPO
Figure 1. Molecular structure of lithium fluorophosphinite complex 4a in the crystal form (50% probability level, hydrogen atoms are omitted for clarity). Selected bond lengths (Å) and angles (deg): W−P = 2.532(3), P−C(1) = 1.831(11), P−F = 1.621(7), P−O(1) = 1.501(9), O(1)−Li = 1.80(2); C(1)−P−W = 116.9(4), O(1)−P−C(1) = 107.2(5), F−P−C(1) = 100.2(5), O(1)−P−F = 102.8(5), P−O(1)− Li = 165.3(9).
The 31P{1H} NMR reaction monitoring revealed that the reaction starts around 0 °C, giving an intermediate in small amounts, which, on the basis of its parameters (217.5 ppm; 1JP,F = 998.2 and 1JP,W = 339.5 Hz), was assigned to P-nitroxyl complex 3a. At about 4 °C the ratio between 2, 3a, and 4a was about 19:1:4 and upon warming the amounts of 2 and 3a started to decrease in favor of 4a. Unfortunately, further spectroscopic measurements using reaction solutions did not provide insights. The tentative assignment of 3a is further supported through a comparison with the NMR data reported for the related complex [W(CO)5{(Me3Si)2HCP(F)Cl}] (213.0 ppm; 1JP,F = 1015.8 and 1JP,W = 347.0 Hz).6 Taking different substituents R into account, it is also in quite good agreement with the data of [W(CO)5{MeP(F)OMe}] (194.5 ppm; 1JP,F = 1010.0, 1JP,W = 342.0 Hz).13 A preliminary study of the reaction of complex 2 with TEMPO and triphenylcarbenium tetrafluoroborate as cooxidant showed an accelerated formation of 3a at lower temperatures (below −20 °C) if 5 equiv of TEMPO (1 equiv of 2 and [Ph3C]BF4) was used. Although a more selective conversion of 2 into 3a occurred, byproducts known from the reaction of 2 with [Ph3C]BF4 (in the absence of TEMPO)7 were formed as well and, hence, precluded characterization of 3a. Nevertheless, the formation of 3a points to a trapping reaction of a transiently formed P−F phosphanyl complex radical7 with TEMPO. We then became interested to probe if this SET oxidation reaction can be exploited using other systems, and therefore, the reaction of the Li/Cl phosphinidenoid complex 6,14 obtained via chlorine−lithium exchange from dichlorophosphane complex 5,15 with TEMPO (2 equiv) was carried out. At −20 °C a slow but selective conversion of 6 (in comparison to 2) took place and a product assigned to lithium chlorophosphonite complex 4b (151.1 ppm; 1JP,W = 318.0 Hz) was formed.16 Here, no evidence for further intermediates was obtained (Scheme 2). All attempts to isolate complex 4b failed, as it appeared to be thermally labile and moisture sensitive; therefore, the nature of 4b could not be further ascertained. Finally, treatment of complex 4b with a slight excess of water was successful and yielded selectively monolithium phosphonite complex 7 (119.7 ppm; 1JP,W = 283.5 Hz), which was obtained in pure form after crystallization (73% yield). The molecular structure of 7 was established by single-crystal X-ray
Figure 2. Molecular structure of monolithium phosphonite complex 7 in the crystal form (50% probability level, hydrogen atoms except H1 are omitted for clarity). Selected bond lengths (Å) and angles (deg): W(1)−P(1) = 2.532(9), P(1)−C(1) = 1.820(3), P(1)−O(1) = 1.610(3), P(1)−O(2) = 1.539(3); C(1)−P(1)−W(1) = 116.3(12), O(2)−P−O(1) = 107.2(5), O(2)−P−C(1) = 105.6(16), O(1)−P− C(1) = 102.5(16), P(1)−O(2)−Li(1) = 147.4(3).
analysis (Figure 2), confirming that during the hydrolysis (of the P−Cl function) one trimethylsilyl group was finally substituted by a hydrogen atom. One of the most prominent features of complex 7 is the dimer, which is bonded via two O(1)−H(1)···O(2) hydrogen bonds (O(1)···O(2) distance, 2.667(3) Å; O−H···O bond angle, 169.1(2)°). In order to get the first insight into the bonding and decomposition of the assumed intermediates, the P-nitroxyl phosphane complexes 3, DFT calculations at the PW6B95-D3/ def2-TZVP//TPSS-D3/def2-TZVP level of theory were performed.17 It was found that for P-halogeno-substituted phosphane complexes 3a,b homolytic cleavage of the O−N bonds is more favorable in comparison to P−O bonds, which is especially pronounced for the case of 3a (Table 1). The computed dissociation energies using the two density functionals showed differences of only 2 kcal/mol, which is within the typical accuracy of this level of theory for such thermochemical problems.18 Interestingly, the phosphinoyl-type complexes 8a,b, generated via homolytic O−N bond cleavage, display (calculated) spin densities (Figure 3) that are largely concentrated on the W(CO)5 fragment. According to the Mulliken population analysis, the oxygen atom of the O−P moiety carries only about 30% of the spin density, while the value for phosphorus is even lower (about 10%); only slight differences were found for X = F or X = Cl. 3458
dx.doi.org/10.1021/om300099g | Organometallics 2012, 31, 3457−3459
Organometallics
Communication
Table 1. Computed Dissociation Energies De, Bond Dissociation Energies (BDE), and Free Enthalpies of Dissociation (Gdiss(298)) in kcal/mol
(2) For recent reviews, see: (a) Vogler, T; Studer, A. Synthesis 2008, 13, 1979. (b) Ciriminna, R; Pagliaro, M. Org. Process Res. Dev. 2010, 14, 245. (c) Tebben, L.; Studer, A. Angew. Chem., Int. Ed. 2011, 50, 5034. (3) (a) Sholle, V. D.; Golubev, V. A.; Rozantsev, E. G. Dokl. Akad. Nauk SSSR 1971, 200, 137. (b) Whitesides, G. M.; Newirth, T. L. J. Org. Chem. 1975, 40, 3448. (c) Nagashima, T.; Curran, D. P. Synlett 1996, 330. (d) Dalko, P. I. Tetrahedron Lett. 1999, 40, 4035. (4) For recent examples, see: (a) Lippert, C. A.; Soper, J. D. Inorg. Chem. 2010, 49, 3682. (b) Fortier, S.; Kaltsoyannis, N.; Wu, G.; Hayton, T. W. J. Am. Chem. Soc. 2011, 133, 14224. (c) Fortier, S.; Brown, J. L.; Kaltsoyannis, N.; Wu, G.; Hayton, T. W. Inorg. Chem. 2012, 51, 1625. (5) Application of TEMPO for the trapping of photochemically generated phosphinoyl radicals is known to produce the corresponding P(V) nitroxyl derivatives: (a) Kolczak, U.; Rist, G.; Dietliker, K.; Wirz, J. J. Am. Chem. Soc. 1996, 118, 6477. (b) Baxter, J. E.; Davidson, R. S. Makromol. Chem., Rapid Commun. 1987, 8, 311. (6) Ö zbolat-Schön, A.; Bode, M.; Schnakenburg, G.; Anoop, A.; van Gastel, M.; Neese, F.; Streubel, R. Angew. Chem., Int. Ed. 2010, 49, 6894. (7) Nesterov, V.; Ö zbolat-Schön, A.; Schnakenburg, G.; Shi, L.; Cangönül, A.; van Gastel, M.; Neese, F.; Streubel, R., Chem. Asian. J. DOI: 10.1002/asia.201200161. (8) (a) Rychnovsky, S. D.; Vaidyanathan, R.; Beauchamp, T.; Lin, R.; Farmer, P. J. J. Org. Chem. 1999, 64, 6745. (b) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877. (9) Ö zbolat, A.; von Frantzius, G.; Hoffbauer, W.; Streubel, R. Dalton Trans. 2008, 2674. (10) Phosphane complex 1 is unreactive toward TEMPO at ambient temperature. (11) While 1 equiv of TEMPO is sufficient to form lithium halogenophosphinite complexes 4a,b, 2 equiv was used to achieve a rapid and cleaner conversion of thermally labile Li/X phosphinidenoid complexes. (12) Recently, an anionic phosphane complex bearing a P−OLi structural unit (lithium hydrogenophosphonite tungsten(0) complex) was reported using a different reaction sequence; see: Duan, L.; Schnakenburg, G.; Streubel, R. Organometallics 2011, 30, 3246. (13) Bienewald, F.; Tran Huy, N. H.; Mathey, F. C. R. Acad. Sci. Paris 1999, 2 (II), 701. (14) 16. Ö zbolat, A.; von Frantzius, G.; Pérez, J. M.; Nieger, M.; Streubel, R. Angew. Chem., Int. Ed. 2007, 46, 9327. (15) Khan, A. A.; Wismach, C.; Jones, P. G.; Streubel, R. Dalton Trans. 2003, 2483. (16) A preliminary study showed that the P-trityl-substituted Li/Cl phosphinidenoid pentacarbonyltungsten complex reacts with TEMPO (same conditions) to give a product analogous to the proposed 4b, having very similar 31P NMR data (160.5 ppm, 1JP,W = 319.2 Hz) and being structurally confirmed. (17) The quantum chemical DFT calculations have been performed with slightly modified versions of the TURBOMOLE suite of programs. (a) TURBOMOLE; Ahlrichs, R., et al.; Universität Karlsruhe: 2009; see http://www.turbomole.com. (b) Ahlrichs, R.; Bär, M.; Häser, M.; Horn, H.; Kölmel, C. Chem. Phys. Lett. 1989, 162, 165−169. (c) Tao, J.; Perdew, J. P.; Staroverov, V. N.; Scuseria, G. E. Phys. Rev. Lett. 2003, 91, 146401. (d) Zhao, Y.; Truhlar, D. G. J. Phys. Chem. A 2005, 109, 5656−5667. (e) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. J. Chem. Phys. 2010, 132, 154104. (f) Grimme, S.; Ehrlich, S.; Goerigk, L. J. Comput. Chem. 2011, 32, 1456−1465. (g) Schäfer, A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1994, 100, 5829−5835. (h) Weigend, F.; Ahlrichs, R. Phys. Chem. Chem. Phys. 2005, 7, 3297− 3305. (18) Goerigk, L.; Grimme, S. Phys. Chem. Chem. Phys. 2011, 13, 6670.
PW6B95-D3 complex
X
cleaved bond
TPSS-D3 De
De
BDEa
Gdiss(298)
3a
F
3b
Cl
P−O O−N P−O O−N
52.5 26.6 37.2 22.6
54.8 28.6 37.5 24.0
53.2 26.0 35.4 20.8
35.2 8.1 18.8 4.2
Dissociation energy plus zero-point vibrational and H(0) − H(298) thermal corrections.
a
Figure 3. Plots of spin-density isosurfaces (contour value of ±0.005) for radicals 8a,b (PW6B95/def2-TZVP level). The Mulliken spindensity populations for 8a are 0.105 for P, 0.287 for O, and 0.411 for W. The corresponding values for 8b are 0.099 for P, 0.297 for O, and 0.396 for W.
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ASSOCIATED CONTENT
S Supporting Information *
Text giving experimental details and results and details of the quantum chemical calculations, tables giving optimized Cartesian coordinates, and CIF files giving crystal structure data for 4a and 7. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Fax: +49 228 739616. Tel: +49 228 735345. E-mail: r.
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the Deutsche Forschungsgemeinschaft (SFB 813 “Chemistry at spin centers” and HBFG 4010) for financial support. G.S. and S.S. thank Prof. A. C. Filippou for support.
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REFERENCES
(1) Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH: Weinheim, Germany, 2001; Vols. 1−4. 3459
dx.doi.org/10.1021/om300099g | Organometallics 2012, 31, 3457−3459
