Synthesis of a Rhodium Carbonyl Phosphaalkenyl–Phosphido

Communication pubs.acs.org/Organometallics

Synthesis of a Rhodium Carbonyl Phosphaalkenyl−Phosphido Complex: A Phosphorus Congener of Schiff Base Type N,N′-Chelating Monoanionic Ligands Teruyuki Matsumoto, Takahiro Sasamori,* Hideaki Miyake, and Norihiro Tokitoh* Institute for Chemical Research, Kyoto University, Gokasho Uji, Kyoto 611-0011, Japan S Supporting Information *

ABSTRACT: A monoanionic, bidentate phosphaalkenyl−phosphido ligand was designed, synthesized, and used for the complexation of a rhodium carbonyl fragment. The characterization of the latter revealed delocalized π electrons on the phosphaalkenyl−phosphide moiety. The catalytic activity of this rhodium carbonyl complex has been demonstrated in a hydrosilylation reaction.

chiff base type N,N′-chelating monoanionic ligands, i.e. βdiketiminate ligands 1 and anilido-imine ligands 2 (Scheme 1), have been successfully employed for the stabilization of

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ligands with a low-coordinated phosphorus atom.7 We have also developed the phosphorus analogues of anilido−imine ligands and reported the synthesis of their rhodium carbonyl complexes, which exhibited a strong trans influence due to the sp2 hybridization of the phosphorus atom.8 Despite their prospective rewards, P,P′-chelating monoanionic ligands have not yet been intensively explored. Only one example of an attempted synthesis of a P,P′-chelating monoanionic ligand was reported by Ionkin et al.,9 who postulated the generation of a phosphorus analogue of a β-diketiminato ligand, which allegedly underwent ready intramolecular cyclization after its formation. In this paper, we report the synthesis, complexation, and some catalytic activity of the monoanionic, P,P′-chelating, phosphaalkenyl−phosphido ligand 3. The design is based on the steric congestion of the low-coordinated phosphorus atoms in order to avoid potential intramolecular cyclization. As a suitable protecting group for the reactive PC double bond, the 2,4,6-tri-tert-butylphenyl (Mes*) group was chosen,10 since such bulky substituents on the phosphorus atoms are expected to protect not only the PC double bond but also the coordinated metal center. As a suitable precursor for ligand 3, the corresponding protonated phosphaalkenyl−phosphine 4 was prepared (Scheme 2). A phospha-Peterson reaction between 511 and Mes*P(Li)SiMe3 was used for the transformation of a carbonyl group into a phosphaethenyl moiety.12 Despite the reactive P C double bond present in 4, it was obtained as pale yellow crystals, which are stable under ambient atmospheric conditions. Phosphaalkenyl−phosphine 4 was fully characterized by solution spectroscopy and in the solid state by X-ray crystallographic analyses.13 The 31P NMR spectrum of 4 showed two sets of doublets at −62.7 ppm (sp3-hybridized P) and at 265.2 ppm (sp2-hybridized P) with a 4JPP coupling

Scheme 1

many transition metals and main-group elements in unusual oxidation and coordination states.1 Monoanionic N,N′chelating ligands have accordingly attracted much interest as they offer not only offer fundamental insights into coordination chemistry but also desirable practical applications, e.g., in catalytic olefin polymerization reactions, where they can act as supporting ligands.2 On the other hand, the coordination chemistry of low-coordinated organophosphorus compounds has recently drawn a great deal of attention in its own right, mainly due to their unique electronic features.3 These lowcoordinated organophosphorus compounds exhibit low-lying π*-orbital levels with both strong electron-accepting character and trans influence,4 and they have been identified as exciting research targets and promising ligands for transition-metalmediated catalysis. However, the coordination chemistry of these low-coordinated sp2-hybridized organophosphorus compounds is still less developed in comparison to their sp3hybridized phosphine analogues, mostly as a result of the extremely high reactivity of the former with respect to the latter. On the basis of these considerations, phosphorus analogues of N,N′-chelating monoanionic ligands bearing sp2-hybridized phosphorus atoms have become attractive research targets in coordination chemistry. Mathey5 and Mindiola6 have independently reported the synthesis of monoanionic P,N-chelating © 2014 American Chemical Society

Received: January 21, 2014 Published: March 7, 2014 1341

dx.doi.org/10.1021/om500065n | Organometallics 2014, 33, 1341−1344

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spectrum, however, 6 showed only one ddd signal, due to the carbonyl groups at δC 192.1 ppm (2JCP = 32.7 and 38.2 Hz, 1 JCRh = 66.0 Hz). Even an NMR measurement of 6 at −100 °C in toluene-d8 was unable to resolve this signal and resulted only in broadening. The integration of this signal relative to an external standard indicated that the signal should correspond to two overlapping CO moieties. Furthermore, we found that a C6D6 solution of 6 was inert toward ethylene gas bubbling, suggesting that no dissociation equilibrium of CO from the rhodium center is present. Thus, we concluded that in solution both CO moieties should easily exchange with each other even at low temperature.17 The variable-temperature (room temperature to −100 °C) UV/vis spectra of 6 in hexane did not show any temperature dependence (λmax 539 (ε 3400), 450 (3400), 368 (11000) nm). In the IR spectrum, the carbonyl stretching frequencies of 6 were observed at νCO 2030 and 1973 cm−1 (KBr), which are slightly shifted in comparison to those of the related compounds bearing imino-phosphido ligands (νCO 1989 and 2046 cm−1).8 The solid-state structure of crystalline rhodium phosphaalkenyl−phosphide 6 is shown in Figure 1. Monoanionic phosphaalkenyl−phosphido ligand 3 is bound to the rhodium center in a P,P′-chelating fashion. The central six-membered Rh−P−C−C−C−P ring shows a quasi-planar alignment, and the internal angle sum is approximately 718°, indicative of a delocalization of the π electrons over the ring skeleton. The angle sum around Rh is 366°, showing a slightly distorted square planar structure. A bond length for P1−C1 of 1.774(3) Å represents an intermediate value between typical P−C single bonds (1.79−1.83 Å)18 and PC double bonds (1.60−1.70 Å).15 While the bond angle sum around P2 (ca. 360°) reflects a planar geometry consistent with sp2 hybridization, the bond angle sum around P1 (351.2°) is indicative of slight pyramidalization. The theoretically optimized structure of 6OPT exhibits structural parameters which are in agreement with the experimentally observed values (Figure 2). However, optimized geometric parameters of a less hindered model compound with Me substituents (6Me) are also in good agreement with the experimentally observed values (Figure 2), suggesting that the structural features of 6 should not be perturbed by the steric hindrance of the bulky Mes* groups. The observed pyramidalized geometries should be characteristic for the presence of phosphorus, because the observed structures of P,N-analogues of 7 and 88 and the theoretically optimized structure of the N,N′ analogue of 6NNOPT exhibit planar geometries around the N atoms (Figure 2). In spite of the slightly pyramidalized geometry of the P1 atom, the P1−C1 bond length of 1.774(3) Å is shorter than the corresponding P−C bond length of 1.845(2) Å in protonated species 4, indicating a substantial contribution of canonical structure 3B (Figure 1). In addition, the C1−C2 and C2−C3 bond lengths of 6 are 1.437(3) and 1.428(3) Å, respectively, suggesting that 3 exhibits an electronic resonance structure between 3A and 3B (Figure 1) caused by the coordination toward the Rh moiety. The calculated HOMO of 6 shows π-conjugated character of the P−C6H4−CP moiety, and especially the π orbitals on the P1−C1 and C2−C3 bonds indicate a conjugative contribution (Figure 3). Nevertheless, the P2−C3 bond length of 1.677(3) Å is comparable to the corresponding PC bond length of 1.678(2) Å in 4, indicating a decreased electronic effect toward the P2−C3 moiety as a result of the complexation with the rhodium atom. In the case of the N,P analogues 7 and 8 (Figure 2), the trans influence of the P moiety should be,

Scheme 2

constant of 37 Hz. The 1H NMR spectrum of 4 exhibited a characteristic doublet for the P−H moiety at 6.51 ppm (1JPH = 236 Hz), which is slightly shifted to low field in comparison to the signals for diarylphosphines (ca. 5 ppm).14 The doublet arising from the PCH moiety was observed at 9.08 ppm (2JPH = 25 Hz), which is slightly shifted to low field relative to (E)-Mes*PCHPh at 8.12 ppm (2JPH = 25.3 Hz).15 Attempted deprotonation reactions of 4 with n-BuLi, LDA, NaH, and KH resulted in the formation of complicated mixtures, from which it was difficult to isolate or use the lithiated species 3 as an appropriate precursor for complexation reactions. Therefore, we decided to use protonated ligand 4 itself for the reaction with a transition-metal halide complex under basic conditions. When 4 was treated with 1/2 equiv of [RhCl(CO)2]2 in the presence of triethylamine at room temperature for 3 h, rhodium phosphaalkenyl−phosphide 6 was obtained quantitatively in the form of air- and moisturestable reddish purple crystals. The 31P NMR spectrum of 6 showed two sets of doublets of doublets at 117.0 ppm (2JPP = 49 Hz, 1JPRh = 127 Hz) for P1 and at 172.0 ppm (2JPP = 49 Hz,1JPRh = 160 Hz) for P2 (Figure 1). The signal for P1 is shifted to low field in comparison to that of [(dippe)Rh(PPh2)(PHPh2)] (dippe = 1,2-bis(diisopropylphosphino)ethane; δP(PPh2) −47.2 to −46.3).16 In the 13C NMR

Figure 1. Molecular structure of rhodium phosphaalkenyl−phosphide 6. Displacement ellipsoids were drawn at the 50% probability level. Hydrogen atoms, benzene, and hexane are omitted for clarity. Selected bond lengths (Å) and angles (deg): P1−Rh, 2.2661(7); P2−Rh, 2.2473(7); Rh−C4, 1.921(3); Rh−C5, 1.892(3); C4−O1, 1.101(3); C5−O2, 1.133(3); P1−C1, 1.774(3); P2−C3, 1.677(3); C1−C2, 1.437(3); C2−C3, 1.428(3); P1−Rh−P2, 87.44(3); C4−Rh−C5, 93.08(13); Rh−P1−C1, 126.40(9); Rh−P2−C3, 125.89(10); P1− C1−C2, 123.8(2); C1−C2−C3, 124.8(2); C2−C3−P2, 129.9(2). 1342

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Organometallics

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Scheme 3

phosphorus congener of N,N′-chelating monoanionic ligands, and its rhodium carbonyl complex 6. The unique properties derived from the low-coordinated sp2-hybridized phosphorus atom have been revealed on the basis of spectroscopic and Xray crystallographic analyses. The phosphaalkenyl−phosphido ligand 3 should be a very promising candidate for a supporting capacity in catalytic applications. Further investigations into the chemical reactivity and catalytic reactivity of metal complexes are currently in progress and will be reported in due course.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

Text, figures, and CIF files giving experimental details and chemical data of the newly obtained compounds, crystallographic data for 4 and 6, and details of the theoretical calculations. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 2. Experimentally observed and theoretically optimized structural parameters for 6 and related compounds.

Corresponding Author

*E-mail for T.S.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This paper is dedicated to Professor Renji Okazaki on the occasion of his 77th birthday. This work was partially supported by Grants-in-Aid for Scientific Research (B) (No. 22350017), Young Scientist (A) (No. 23685010), Scientific Research on Innovative Areas, “New Polymeric Materials Based on ElementBlocks” [#2401] (No. 25102519), Scientific Research on Innovative Areas, “Stimuli-responsive Chemical Species for the Creation of Functional Molecules” [#2408] (No. 24109013), and MEXT Project of Integrated Research on Chemical Synthesis from Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan.

Figure 3. Graphic representation of the HOMO of 6 from different angles (this figure is shown in color in the Supporting Information).

irrespective of sp3 or sp2 hybridization, stronger in comparison to that of the N moiety. Indeed, longer Rh−CO bond lengths for the CO ligands in positions trans to the P atom were observed. In the case of the P,P′ ligand 6, the trans influences of both P moieties are almost identical. As previously concluded, the lack of a distinct sp3-/sp2-hybridized pattern in 6 should result from comparable contributions of the canonical forms 3A and 3B, which is reflected in similar Rh−CO bond lengths. These bond lengths are moreover comparable to those in trans positions of the P atoms in 7 and 8. The strong trans influence of both P sites in 6 should also be responsible for the dynamic CO exchange in solution, as observed in the NMR spectra. As a preliminary investigation into the catalytic activity of 6, we examined a 1,4-hydrosilylation reaction. Even though 6 bears CO substituents on the rhodium atom,19 we expected 6 to work as a catalyst for the reaction of 2-cyclohexen-1-one with triethylsilane. The treatment of 2-cyclohexen-1-one with 3 equiv of triethylsilane in the presence of 0.5 mol % of 6 in benzene under reflux conditions for 4 h afforded the corresponding silylenolate 9 in an almost quantitative yield (Scheme 3), thus demonstrating the ability of 6 to work as a hydrosilylation catalyst.20 In summary, we succeeded in the synthesis and characterization of phosphaalkenyl−phosphido ligand 3, which is a

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REFERENCES

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(c) Nagahora, N.; Sasamori, T.; Watanabe, Y.; Furukawa, Y.; Tokitoh, N. Bull. Chem. Soc. Jpn. 2007, 80, 1884−1900. (4) Ozawa, F.; Kawagishi, S.; Ishiyama, T.; Yoshifuji, M. Organometallics 2004, 23, 1325−1332. (5) Grundy, J.; Donnadieu, B.; Mathey, F. J. Am. Chem. Soc. 2006, 128, 7716−7717. (6) Basuli, F.; Tomaszewski, J.; Huffman, J. C.; Mindiola, D. J. J. Am. Chem. Soc. 2003, 125, 10170−10171. (7) Neutral N,P- and P,P′-chelating ligands with a double-bonded phosphorus atom have been reported. For examples, see: (a) Brauer, D. J.; Liek, C.; Stelzer, O. J. Organomet. Chem. 2001, 626, 106−112. (b) Dugal-Tessier, J.; Dake, G. R.; Gates, D. P. Org. Lett. 2010, 12, 4667−4669. (8) Sasamori, T.; Matsumoto, T.; Tokitoh, N. Polyhedron 2010, 29, 425−433. (9) Ionkin, A. S.; Marshall, W. J.; Fish, B. M.; Schiffhauer, M. F.; Davidson, F.; McEwen, C. N.; Keys, D. E. Organometalllics 2007, 26, 5050−5058. (10) The Mes* group has been used as an effective protective group especially for low-coordinated phosphorus compounds. For examples, see: (a) Yoshifuji, M. Pure Appl. Chem. 2005, 77, 2011−2020. (b) Yoshifuji, M.; Shima, I.; Inamoto, N.; Hirotsu, K.; Higuchi, T. J. Am. Chem. Soc. 1981, 103, 4587−4589. (11) Tokitoh, N.; Matsumoto, T.; Sasamori, T. Heterocycles 2008, 76, 981−987. (12) For the recent developments on the phospha-Peterson reactions, see: Yam, M.; Chong, J. H.; Tsang, C. W.; Patrick, B. O.; Lam, A. E.; Gates, D. P. Inorg. Chem. 2006, 45, 5225−5234 and references cited therein. (13) Experimental details and crystallographic data for compounds 4 and 6 are shown in the Supporting Information. (14) For examples, see: (a) Yokoyama, Y.; Takahashi, K. Bull. Chem. Soc. Jpn. 1987, 60, 3485−3489. (b) Rivard, E.; Sutton, A. D.; Fettinger, J. C.; Power, P. P. Inorg. Chim. Acta 2007, 360, 1278−1286. (15) Yoshifuji, M.; Toyota, K.; Inamoto, N. Tetrahedron 1988, 44, 1363−1367. (16) Han, L. B.; Tilley, T. D. J. Am. Chem. Soc. 2006, 128, 13698− 13699. (17) An energy barrier of 13.7 kcal/mol was calculated at the B3PW91/6-31G(d) (6-311G(3d) for P, Lanl2DZ for Rh) level for the CO exchange in the model compound 6Me with methyl instead of the Mes* groups. (18) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen, A. G.; Taylor, R. J. Chem. Soc., Perkin Trans. 2 1987, S1−S19. (19) As CO ligands generally coordinate strongly to transition-metal centers, many transition-metal complexes with CO ligands do not exhibit catalytic activity without photoinduced elimination of at least one CO ligand. (20) Complex 6 was found to be inert toward 2-cyclohexen-1-one or triethylsilane in C6D6 at room temperature. Although heating of the solution of 6 and 2-cyclohexen-1-one at 60 °C for 15 h resulted in no change, that of the solution of 6 and triethylsilane under the same conditions afforded a complicated mixture, indicating the 4-hydrosilylation catalyzed by 6 could be initiated by the reaction of 6 with triethylsilane. However, the detailed reaction mechanism is unclear at present.

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dx.doi.org/10.1021/om500065n | Organometallics 2014, 33, 1341−1344