Reaction of Secondary Phosphine Oxides with Rhodium (I)

Jun 22, 2010 - †Evonik Oxeno GmbH, Paul-Baumann-Strasse 1, 45772 Marl, Germany, and #Institut f¨ur Chemie der. Universit¨at Rostock, A.-Einstein-S...
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Organometallics 2010, 29, 3139–3145 DOI: 10.1021/om100171p

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Reaction of Secondary Phosphine Oxides with Rhodium(I) Andrea Christiansen,§,† Detlef Selent,§ Anke Spannenberg,§ Wolfgang Baumann,§ Robert Franke,† and Armin B€ orner*,§,# §

Leibniz-Institut f€ ur Katalyse an der Universit€ at Rostock, A.-Einstein-Strasse 29a, 18059 Rostock, Germany, † Evonik Oxeno GmbH, Paul-Baumann-Strasse 1, 45772 Marl, Germany, and #Institut f€ ur Chemie der Universit€ at Rostock, A.-Einstein-Strasse 3a, 18059 Rostock, Germany Received March 3, 2010

Five secondary phosphine oxides (SPOs) differing in the steric and electronic properties of the P-substituents were reacted with [Rh(acac)(cod)]. Due to the tautomeric equilibrium, SPOs are able to coordinate to metal atoms via their trivalent tautomers. Their complexation behavior is strongly dependent on the substituents at the phosphorus atom. Electron-rich tertiary alkyl groups do not favor the reaction, whereas electron-poor phosphinous acids immediately coordinate, resulting in “quasi-chelate” complexes with a hydrogen bond in the backbone. The latter remains intact even in alcohols. Additionally, the SPOs were reacted with [Rh(cod)2]BF4, resulting in the corresponding BF2-capped complexes.

Introduction Secondary phosphine oxides A (Scheme 1), also abbreviated as SPOs,1 are weak acids.2 Therefore they are subject to tautomerism in solution with phosphinous acids B.3 Due to the tautomeric equilibrium, SPOs are able to coordinate to metal atoms4 via their trivalent tautomers B; therefore for the former we recently suggested the term preligands.5 One of the characteristic properties of SPOs compared to phosphines is their high stability toward oxygen. This has been explained by the dominance of pentavalent form A in solution.3,6 Metal complexes of phosphinous acids B have been found to form efficient catalysts in homogeneous catalysis.7 The first application dated back to the 1980s, when the groups of van Leeuwen and Matsumoto used them in the Pt- and

Rh-catalyzed hydroformylation of linear olefins.8,9 Only recently have SPOs been successfully applied as preligands in various other transition metal catalyzed reactions, e.g., in cross-coupling reactions of electronically deactivated chlorinated aromatics,7a,10 in the hydrolysis of nitriles,11 in Pd- and Pt-catalyzed cycloaddition reactions,12 and in the Ir- and Rh-catalyzed asymmetric hydrogenation of imines and olefins.13,14 The reaction of selected SPOs with several metals has been broadly investigated.4 Generally, SPOs add oxidatively to platinum(0) under the formation of a Pt-H bond.4 In contrast, palladium(II), nickel(II), ruthenium(II), and

*Corresponding author. Tel: þþ49-381-1281-202. Fax: þþ49-3811281-51202. E-mail: [email protected]. (1) In the literature also the term secondary phosphine chalcogenides is used: Walther, B.; Hartung, H.; Maschmeier, M.; Baumeister, U.; Messbauer, B. Z. Anorg. Allg. Chem. 1988, 566, 121–130. (2) (a) Grayson, M.; Farley, C. E.; Streuli, C. A. Tetrahedron 1967, 23, 1065–1078. (b) Quin, L. D. A Guide to Organophosphorus Chemistry; Wiley-Interscience: New York, 2000. (3) Chatt, J.; Heaton, B. T. J. Chem. Soc. A 1968, 2745–2757. (4) (a) Roundhill, D. M.; Sperline, P. R.; Beaulieu, W. B. Coord. Chem. Rev. 1978, 26, 263–279. (b) Walther, B. Coord. Chem. Rev. 1984, 60, 67–105. (5) Dubrovina, N. V.; B€ orner, A. Angew. Chem., Int. Ed. 2004, 43, 5883–5886. (6) Corbridge, D. E. C. Phosphorus: An Outline of its Chemistry, Biochemistry and Uses, 5th ed.; Elsevier: Amsterdam, 1995; p 336. (7) (a) Ackermann, L. Synthesis 2006, 10, 1557–1571. (b) Ackermann, L.; Born, R.; Spatz, J. H.; Althammer, A.; Gschrei, C. J. Pure Appl. Chem. 2006, 78, 209–214. (c) Ackermann, L. Synlett 2007, 4, 507–526. (d) Ackermann, L. Chiral Secondary Phosphine Oxides and HeteratomSubstituted Secondary Phosphine Oxides as Preligands. In Phosphorus Ligands in Asymmetric Catalysis; B€orner, A., Ed.; Wiley-VCH: Weinheim, 2008; Vol. I-III, pp 831-847. (8) van Leeuwen, P. W. N. M.; Roobeek, C. F.; Wife, R. L; Frijns, J. H. G. J. Chem. Soc., Chem. Commun. 1986, 31–33.

(9) (a) van Leeuwen, P. W. N. M.; Roobeek, C. F. EP 82.576 1983. (b) Matsumoto, M.; Tamura, M. J. Mol. Catal. 1983, 19, 365–376. (10) (a) Li, Y. Angew. Chem., Int. Ed. 2001, 40, 1513–1516. (b) Li, G. Y.; Zheng, G.; Noonan, A. F. J. Org. Chem. 2001, 66, 8677–8681. (c) Li, G. Y.; Fagan, P. J.; Watson, P. L. Angew. Chem., Int. Ed. 2001, 40, 1106– 1109. (d) Li, G. Y.; Marshall, W. J. Organometallics 2002, 21, 590–591. (e) Li, G. Y. J. Org. Chem. 2002, 67, 3643–3650. (f) Wolf, C.; Lerebours, R. J. Org. Chem. 2003, 68, 7077–7084. (g) Wolf, C.; Lerebours, R.; Tanzani, E. H. Synthesis 2003, 68, 2069–2073. (h) Wolf, C.; Lerebours, R. Org. Lett. 2004, 6, 1147–1150. (i) Yang, W.; Wang, Y.; Korte, J. R. Org. Lett. 2003, 5, 31131–3134. (j) Ackermann, L. Org. Lett. 2005, 7, 3123–3125. (k) Ackermann, L.; Born, R.; Spatz, J. H.; Meyer, D. Angew. Chem., Int. Ed. 2005, 44, 7216–7219. (11) (a) Ghaffar, T.; Parkins, A. W. Tetrahedron Lett. 1995, 36, 8657– 8660. (b) Jiang, X.-b.; Minnaard, A. J.; Feringa, B. L.; de Vries, J. G. J. Org. Chem. 2004, 69, 2327–2331. (c) For the application in the amidation of nitriles, see: Cobley, C. J.; van der Heuvel, M.; Abbadi, A.; de Vries, J. G. Tetrahedron Lett. 2000, 41, 2467–2470. (12) (a) Bigeault, J.; Giordano, L.; Buono, G. Angew. Chem., Int. Ed. 2005, 44, 4753–4757. (b) Bigeault, J.; Giordano, L.; de Riggi, I.; Gimbert, Y.; Buono, G. Org. Lett. 2007, 9, 3567–3570. (c) Bigeault, J.; de Riggi, I.; Gimbert, Y.; Giordano, L.; Buono, G. Synlett 2008, 1071–1075. (13) (a) Jiang, X.-b.; Minnaard, A. J.; Hessen, B.; Feringa, B. L.; Duchateau, A. L. L.; Andrien, J. G. O.; Boogers, J. A. F.; de Vries, J. G. Org. Lett. 2003, 5, 1503–1506. (b) Reetz, M. T.; Sell, T.; Goddard, R. Chimia 2003, 57, 290–292. (c) Jiang, X.-b.; Van den Berg, M.; Minnaard, A. J.; Feringa, B. L.; de Vries, J. G. Tetrahedron: Asymmetry 2004, 15, 2223–2229. (d) Dubrovina, N. V.; Jiao, H.; Tararov, V. I.; Spannenberg, A.; Kadyrov, R.; Monsees, A.; Christansen, A.; B€orner, A. Eur. J. Org. Chem. 2006, 15, 3412–3420.

r 2010 American Chemical Society

Published on Web 06/22/2010

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Scheme 1. Equilibrium of Pentavalent and Trivalent Phosphorus Species and Formation of Metal Complexes

rhodium(I) mostly react retaining their oxidation states.15 Of particular interest for catalytic applications is the complexation behavior of SPOs and its dependence on their electronic properties. Strongly electron-withdrawing substituents on the phosphorus can shift the equilibrium depicted in Scheme 1 toward the phosphinous acid B;3,16 hence electron-poor SPOs should be predestined to form metal complexes. However, up to now, systematic investigations on these relations cannot be found in the literature. However, they are crucial for the assessment of catalytic properties of SPOs. For the study presented herein SPOs 1-5 were chosen, differing mainly in their electronic properties. Recently, we reported a detailed study based on NMR and IR investigations on the tautomeric equilibrium of these SPOs and its dependence on solvent and temperature.17 Only with SPO 5 in the tautomeric equilibrium was the relevant phosphinous acid found. With the other, more electron-rich SPOs, the pentavalent PdO structure was dominant.

The preligands 1-5 were reacted with [Rh(acac)(cod)] (acac = acetyl acetonate, cod = 1,5-cylooctadiene) and [Rh(cod)2]BF4, respectively, representing Rh complexes frequently employed for the generation of precatalysts in homogeneous (14) For more recent applications in homogeneous catalysis, compare: (a) Ackermann, L. Synlett 2007, 507–526. (b) Pfaltz, A.; Ribourdouille, Y.; Feng, X.; Ramalingam, B.; Pugin, B.; Spindler, F. WO 2007135179 A1 20071129. (c) Pugin, B.; Landert, H.; Gschwend, B.; Pfaltz, A., Spindler, F. WO 2009065783 A1 20090528. (d) Ackermann, L.; Barfuesser, S. Synlett 2009, 808–812. (e) Ackermann, L.; Potukuchi, H. K.; Kapdi, A. R.; Schulzke, C. Chem.;Eur. J. 2010, 16, 3300–3303. (f) Ackermann, L.; Potukuchi, H. K. Synlett 2009, 17, 2852–2856. (15) So far, the oxidative addition to rhodium was described only with secondary phosphites but not with secondary phosphine oxides, e.g.: (a) Bennett, M. A.; Mitchell, T. R. B. J. Organomet. Chem. 1974, 70, C30–C32. (b) Trzeciak, A. M.; Ziolkowski, J. J. Pol. J. Chem. 2003, 77, 749– 756. (16) (a) Griffiths, J. E.; Burg, A. B. J. Am. Chem. Soc. 1960, 82, 1507– 1508. (b) Magnelli, D. D.; Tesi, G.; Lowe, I. U.; McQuistion, W. E. Inorg. Chem. 1966, 5, 457–461. (c) Hoge, B.; Th€osen, C.; Herrmann, T.; Panne, P.; Pantenburg, I. J. Fluorine Chem. 2004, 125, 831–851. (d) Hoge, B.; Neufeind, S.; Hettel, S.; Wiebe, W.; Thoesen, C. J. Organomet. Chem. 2005, 690, 2382–2387. (e) Hoge, B.; Garcia, P.; Willner, H.; Oberhammer, H. Chem.;Eur. J. 2006, 12, 3567–3574. (17) Christiansen, A.; Garland, M.; Li, C.; Selent, D.; Ludwig, R.; Spannenberg, A.; Baumann, W.; B€ orner, A. Eur. J. Org. Chem. 2010, 2733–2741. (18) In the reaction with (Rh(cod)Cl)2, which is also a frequently used catalyst precursor, several products were obtained. Thus, application of SPO 3 gave rise to a mixture of compounds, the major product being [HRh(P(OH)Ph2)3] (δ 96.0 ppm, JRh-P = 142.4 Hz) with ca. 31% signal intensity in the 31P NMR spectrum, which is in accordance with results reported in ref 15b. (19) For the reaction of heteroatom-substituted secondary phosphine oxides (HASPOs) with [Rh(cod)2]BF4 see: Christiansen, A.; Selent, D.; Spannenberg, A.; K€ ockerling, M.; Reinke, H.; Baumann, W.; Jiao, H.; A.; B€ orner, A. Manuscript in preparation.

Christiansen et al. Scheme 2. Formation of Quasi-Chelate Complexes Starting from [Rh(acac)(cod)]

catalysis.18,19 Metal complexes were investigated by NMR spectroscopy and X-ray structural analysis.

Results and Discussion Reaction with [Rh(acac)(cod)]. By reaction of two equivalents of SPOs with [Rh(acac)(cod)] in THF the acetyl acetonate anion was replaced by two monodentate phosphinous acids to give the zwitterionic, quasi-bidentate Rh(I) complexes 6-9 (Scheme 2).20 In the course of complexation acetyl acetone was liberated, whereas no competing reaction between the SPO and the latter has been observed.21 After workup complexes 6-9 were obtained as orange solids, which are air-stable. While the perfluorinated ligand 5 reacted at -78 °C immediately to Rh complex 9 (monitored by NMR spectroscopy), electron-rich di-tert-butylphosphine oxide (1) gave only traces of the expected complex even at elevated temperatures (þ80 °C).22 Complexes 6-9 were characterized by NMR spectroscopy and elemental analysis. From 31P and 1H NMR spectroscopic data it became evident that in CD2Cl2 solution an intramolecular hydrogen bond bridge was formed in the backbone, which leads to a “quasi-chelate” complex structure. The 31P NMR spectrum displays only a single doublet, characterizing two equivalent phosphorus atoms. In the 1H NMR spectrum the resonance of the bridging hydrogen atom was broad and barely visible at ambient temperature. At 195 K a very sharp signal, e.g., for complex 7 at δ 16.6, was found.23 However, even at that temperature only a single doublet could be observed in the 31P NMR spectrum (δ 90.6, J 157 Hz). Apparently, the equivalence of the two phosphorus atoms remains due to a rapid exchange of the bridging proton. Hence, in the liquid phase a time-averaged symmetric position of the bridging H atoms between the coordinated phosphinous acids can be assumed. In addition, we determined the chemical shift of the 103Rh center of 7 (-238 ppm, THF, ambient temperature), which is found in the region characteristic for Rh(cod)(bisphosphine) cations.24 (20) In contrast, with Pt(0) diphenylphosphine oxide can either coordinate as a donor ligand or add oxidatively to give hydroplatinum(II) complexes. See e.g.: Appleby, T.; Woollins, J. D. Coord. Chem. Rev. 2002, 235, 121–140. (21) Only during heating of Ph2P(O)H in pure acetyl acetone this feature was reported. See ref 1. (22) Interestingly, there are no literature examples for the structural motif Rh(tert-Bu2PO)2, where two di-tert-butylphosphino units are coordinated to rhodium. Moreover, there is only one record reporting on a single Rh-PtBu2O unit coordinated to Rh: Merceron-Saffon, N.; Gornitzka, H.; Baceiredo, A.; Bertrand, G. J. Organomet. Chem. 2004, 689, 1431–1435. (23) Werner et al. reported a resonance at δ 17 ppm (at -60 °C, in toluene-d6) for a bridging hydrogen atom in cyclopentadienylnickel phosphonate complexes: Werner, H.; Khac, T. N. Z. Anorg. Allg. Chem. 1981, 475, 241-250, and a resonance at δ 14.3 ppm (C6D6) in cyclopentadienylrhodium phosphonate complexes: Werner, H.; Freser, R. Z. Anorg. Allg. Chem. 1979, 458, 301-308.

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Table 1. Coupling Constants and Chemical Shifts of Rh Complexes 6-9 in 31P NMR Spectroscopy compound

δ 31P NMR (CD2Cl2) (ppm)

JRh-P (Hz)

6 7 8 9

92.3 93.0 91.8 74.0

159.7 160.2 161.5 180.0

The P-Rh coupling constants of complexes 6-9 in the 31P NMR spectra indicate a slight correlation with the substitution pattern of the applied SPOs: electron-poor SPOs cause an enhancement of the P-Rh coupling constants compared to electron-rich SPOs (Table 1). While quasi-chelate complex 6 displays a coupling constant of 159.7 Hz (CD2Cl2), complex 9, deriving from the fluorinated pyridyl phosphine oxide, is characterized by a coupling constant of 180.0 Hz. However, there is only a small increase of ca. 2 Hz comparing complex 6 and complex 8. Similar observations hold for the chemical shifts in the 31P NMR spectra. While there is hardly a difference between complexes 6, 7, and 8, a significant shift to higher field could be observed for complex 9 (ca. 15 ppm). X-ray crystal structure analysis of compound 6, bearing the most electron-rich phosphinous acid, and 9, with the most withdrawing substituents at phosphorus, additionally confirmed the suggested structural motifs (Figure 1). In 6 the P-O as well as the Rh-P bond distances are not significantly different, whereas they are equally long in 9. The P-O bond distances are in between the typical range of a single and a double bond. However, it becomes evident from the molecular structures of 6 and 9 that the bridging H atoms between the coordinated phosphinous acids forming the six-membered rings are not symmetrically situated between the two oxygen atoms in the solid state (e.g., for 6: O1-H 1.03(4), H 3 3 3 O2 1.42(4), O1 3 3 3 O2 2.437(2) A˚, O1-H 3 3 3 O2 171(3)°). This observation is in contrast to our NMR spectroscopic observations in solution. There are also several sometimes controversial discussions in the literature, where a symmetric position of the proton was assumed.25 One would expect a shortening of the P-Rh bond length following an increase of the electron-withdrawing character of the substituents. Indeed this tendency becomes apparent comparing complexes 6 and 9. The P-Rh-P bond angle in complex 9 is enlarged by 3° compared to that in 6. In the coordinated phosphinous acid of complex 6 the P-O bond as well as the P-C bond length are lengthened compared to those in the preligand bis(p-tolyl)phosphine oxide (2).17 Reaction with [Rh(cod)2]BF4. It is known that hydrogenbonded quasi-chelate complexes consisting of a phosphinous (24) Ernsting, J. M.; Elsevier, C. J.; de Lange, W. G. J.; Timmer, K. Magn. Reson. Chem. 1991, 29, S118–S124. (25) (a) Sperline, R. P.; Roundhill, D. M. Inorg. Chem. 1977, 16, 2612–2617. (b) Gray, G. M.; Kraihanzel, C. S. J. Organomet. Chem. 1978, 146, 23–37. See also references quoted in ref 23. (26) Sperline, R. P.; Beaulieu, W. B.; Roundhill, D. M. Inorg. Chem. 1978, 7, 2032–2035. (27) (a) Kong, P. C.; Roundhill, D. M. Inorg, Chem. 1972, 11, 749– 759. (b) Sperline, R. P.; Dickson, M. K.; Roundhill, D. M. J. Chem. Soc., Chem. Commun. 1977, 62–63. (c) Dixon, K. R.; Rattray, A. D. Inorg. Chem. 1977, 16, 209–211. (d) Tooke, D. M.; Mills, A. M.; Spek, A. L.; Vlugt, J. I.; Vogt, D. Acta Crystallogr., E: Struct. Rep. Online 2004, E60 (7), m943– m945. (28) (a) Berry, D. E.; Bushnell, G. W.; Dixon, K. R. Inorg. Chem. 1982, 21, 957–960. (b) Duncan, J. A. S.; Stephenson, T. A.; Walkinshaw, M. D.; Hedden, D.; Roundhill, D. M. J. Chem. Soc., Dalton Trans. 1984, 801–807. (c) Varshney, A.; Gray, G. M. J. Organomet. Chem. 1990, 391, 415–429.

Figure 1. Molecular structure of Rh complexes 6 and 9 with selected bond lengths and angles. Thermal ellipsoids are drawn at the 30% probability level. Hydrogen atoms (except H attached to O1) are omitted for clarity. Selected bond lengths (A˚) and angles (deg) for complex 6: P1-Rh1 2.3121(5); P2-Rh1 2.3092(5); P1-O1 1.5620(15); P2-O2 1.5543(14); P1-Rh1-P2 85.07(2). Selected bond lengths (A˚) and angles (deg) for complex 9: P1-Rh1 2.2565(6); P2-Rh1 2.2559(6); P1-O1 1.544(2); P2-O2 1.547(2); P1-Rh1-P2 88.13(2).

acid and a phosphinito ligand can be transformed into chelate complexes by a capping reaction with a BF2 unit.26 As reagents, BF3,1,27 BF3-Et2O,28 and HBF429 have been suggested. For example, nickel, iridium, and platinum complexes could be synthesized in good yields. Remarkably only a single Rh complex of the type [(Z2)Rh(PR2O)2BF2] (Z 6¼ PR2O) has been prepared, but it was obtained only in low yield.30 In our preliminary trials the reaction of [Rh(PPh2O)2H(cod)] (7) with BF3-Et2O gave only a poor yield of the desired BF2-capped complex. Moreover, the formation of several side products was observed. In contrast the direct reaction of SPOs 2-5 with [Rh(cod)2]BF4 in a ratio of 2:1 (29) Werner, H.; Khac, T. N. Angew. Chem., Int. Ed. Engl. 1977, 16, 324–325. (30) Arena, C. G.; Nicol o, F.; Drommi, D.; Bruno, G.; Faraone, F. J. Chem. Soc., Dalton Trans. 1996, 4357–4363.

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Scheme 3. Formation of BF2-Capped Complexes Starting from [Rh(cod)2]BF4 and SPOs 2-5

directly yielded BF2-bridged Rh complexes 10-13 in moderate to excellent yield (57-95%) (Scheme 3).31 As expected, the electron-rich (t-Bu)2P-OH (1) did not react, obviously due to its poor coordination properties at rhodium, which is the precondition for the subsequent esterification with BF4(vide supra). Complexes 10-13 were characterized by NMR spectroscopy and elemental analysis. In comparison to the corresponding H-bridged quasi-chelate complexes 6-9 we found a downfield shift in the 31P NMR spectrum as well as an increase in the Rh-P coupling constants. The structures of complexes 11 and 13 were additionally confirmed by X-ray structural analysis (Figure 2). The stronger the electron-withdrawing properties of the P-aryl substituents, the shorter the P-Rh bonds due to a stronger Rh-P back-bonding (similar to quasi-chelate complexes 6 and 9 in Figure 1). Simultaneously the bite angle P-Rh-P is widened. In comparison with the quasi-chelate complexes (Figure 1) the BF2 substitution results in a decrease of the bite angle and the P-Rh distances. Influence of the Solvent on the Complexation Behavior of SPOs. A common property of hydrogen bond bridges is their strong dependency on the solvent used. In general, intramolecular hydrogen bonds are not stabilized in protic and polar solvents. Surprisingly, we also obtained the quasi-chelate complex [Rh(PPh2O)2D(cod)] (7-d) in an NMR experiment in deuterated methanol (Scheme 4). A downfield shift up to 6 ppm as well as a larger P-Rh coupling constant (2-3 Hz) could be observed compared to the measurements in deuterated dichloromethane and THF (further details are listed in the Experimental Part). In spite of the strong hydrogen-bonding acceptor property32 of methanol, the quasi-chelate structure was maintained. As a side product red crystals of rhodium(I) complex 14 were formed (Scheme 4 and Figure 3). In this

Figure 2. Molecular structure of BF2-capped complexes 11 and 13 with selected bond lengths and angles. Thermal ellipsoids are drawn at the 30% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (A˚) and angles (deg) for complex 11: P1-Rh1 2.2520(7); P2-Rh1 2.2793(8); P2-O2 1.579(2); P1-O1 1.580(2); P1-Rh1-P2 83.84(3). Selected bond lengths (A˚) and angles (deg) for complex 13: P2-Rh1 2.2265(7); P1-Rh1 2.2369(6); P1-O1 1.555(2); P2-O2 1.559(2); P1Rh1-P2 86.24(2). Scheme 4. Reaction of SPO 3 with [Rh(acac)(cod)] in Deuterated Methanol

(31) The reaction proceeds probably via A, which was indicated by P and 13F NMR spectroscopy. For a comparable ruthenium species see also: den Reijer, J.; R€ uegger, H.; Pregosin, P. S. Organometallics 1998, 17, 5213–5215. 31

(32) (a) Dubrovina, N. V.; Shuklov, I. A.; Birkholz, M.-N.; Michalik, D.; Paciello, R.; B€ orner, A. Adv. Synth. Catal. 2007, 349, 2183–2187. (b) For a review, see: Shuklov, I. A.; Dubrovina, N. V.; B€orner, A. Synlett 2007, 2925–2943.

complex 1,5-cyclooctadiene is replaced by two molecules of Ph2P-OCD3 formed by partial esterification of two phosphorus ligands of 3 with the deuterated solvent. Interestingly, esterification of diphenylphosphinous acid did not take place in the absence of [Rh(acac)(cod)].17 Similarly to that found in 7 also in 14 the pseudochelate structure via the intramolecular hydrogen bond remained intact. The intramolecular hydrogen bond was confirmed by X-ray analysis (Figure 3; O2-D 1.16(4), D 3 3 3 O3 1.26(4), O2 3 3 3 O3 2.416(3) A˚, O2-D 3 3 3 O3 176(3)°).

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Table 2. Coupling Constants and Chemical Shifts of Rh Complex 7 in 31P NMR Spectroscopy (121 MHz) in Different Solvents, Listed with Increasing Polarity32 solvent

δ 31P NMR (ppm) a

THF-d8 CD2Cl2 CD3OD CF3CD2OD a

90.8 92.3 96.9 101.2

JRh-P (Hz) 159.0a 159.7 162.1 164.5

Measured at 161 MHz.

polarity of solvents and the chemical shifts as well as the Rh-P coupling constants in their NMR spectra. Reacting SPOs 2-5 with [Rh(cod)2]BF4 resulted in the corresponding BF2-capped complexes in good yields. Work is in progress to elucidate the potential of the Rh complexes as precursors in the hydroformylation of olefins as well as in other catalytic reactions.33

Experimental Part Figure 3. Molecular structure of complex 14 with selected bond length and angles. Thermal ellipsoids are drawn at the 30% probability level. With the exception of the O2-D 3 3 3 O3 bond, hydrogen atoms are excluded for clarity. Selected bond lengths (A˚) and angles (deg): P3-Rh1 2.3164(9); P2-Rh1 2.3236(11); P1-Rh1 2.2895(9); P4-Rh1 2.2784(11); P3-O3 1.564(2); P(2)-O(2) 1.567(2); P1-O1 1.6308(16); P4-O4 1.6265(16); P3-Rh1-P2 87.41(4); P3-Rh1-P4 91.93(4); P1-Rh1-P2 97.99(4).

When a sample of Rh[(P(p-tolyl)O]2H](cod) (6) was dissolved in deuterated methanol, H/D exchange of the bridging H atom in the ligand backbone was observed by 1H NMR spectroscopy. While the sharp signal for the bridging H atom in CD2Cl2 at 210 K was visible (δ 16.9), it could not be detected in CD3OD due to the H/D exchange. Since complex 6 is only slightly soluble in methanol, the experiment was repeated in a mixture of CD3OD/CD2Cl2. Again, H/D exchange took place; hence no signal for the bridging H atom could be detected. When Ph2P(O)D (3) was reacted with [Rh(acac)(cod)] in deuterated trifluoroethanol, also complex 7-d was formed as the main product, displaying a deuterium bond bridge. However, the formation of the rhodium complex was slow due to the poor solubility of the rhodium precursor in this solvent. Similar to the reaction in methanol, a downfield shift in the 31P NMR spectrum as well as an increased Rh-P coupling constant resulted (121 MHz; δ 101.2, J 164.5 Hz) compared to the less polar solvents THF and CD2Cl2. There is a clear correlation between the polarity of solvents and the chemical shifts as well as the Rh-P coupling constants in the 31 P NMR spectra: Polar solvents cause a downfield shift and a larger coupling constant (Table 2)!

Conclusion and Summary The ability of SPOs to react after tautomerization with [Rh(acac)(cod)] is strongly dependent on the ligand structure. Thus, with the bulky, strongly electron-rich SPO 1 the desired Rh complex was not obtained, whereas preligands 2-5 led to Rh complexes bearing a phosphinous acid and a phosphinito ligand. The zwitterionic complexes are stabilized by an intramolecular hydrogen bond bridge, leading to a quasi-chelate complex structure. Unexpectedly, this structural principle is retained even in alcohols as solvents. Also, it became evident that there is a correlation between the

General Procedures. If not stated otherwise, all reactions were carried out under an inert atmosphere (argon 5.0) and with standard Schlenk techniques. Commercially available compounds were used as purchased. The solvents were dried by conventional procedures and distilled under an argon atmosphere. If possible, reactions were monitored via NMR spectroscopy. The yields listed below are isolated yields; melting points are not corrected. NMR spectra were recorded on a Bruker AV 300 and a Bruker AV 400 MHz spectrometer. Chemical shifts are reported in ppm and relative to deuterated solvents. TMS was used as standard for 1H and 13C NMR spectra, and H3PO4 for 31P NMR spectroscopy; 103Rh is referenced to Ξ = 3.16 MHz. Unless stated otherwise, NMR spectra were taken at ambient temperature. Mass spectra were recorded on an AMD 402 spectrometer. The synthesis and commercial sources, respectively, of secondary phosphine oxides 1-5 are listed in a previous paper.17 X-ray Crystal Structure Analysis of Compounds 6, 9, 11, 13, and 14. Data were collected on a STOE IPDS diffractometer using graphite-monochromated Mo KR radiation. The structures were solved by direct methods (SHELXS-97: Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112-122, and SIR2004: Burla, M. C.; Caliandro, R.; Camalli, M.; Carrozzini, B.; Cascarano, G. L.; De Caro, L; Giacovazzo, C.; Polidori, G.; Spagna, R. J. Appl. Crystallogr. 2005, 38, 381-388, respectively) and refined by full-matrix least-squares techniques on F2 (SHELXL-97: Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112-122). XP (Bruker AXS) was used for graphical representations. Compound 6: space group P1, triclinic, a = 6.8633(2) A˚, b = 11.6332(4) A˚, c = 20.9725(7) A˚, R = 104.909(3)°, β = 98.667(3)°, γ = 90.515(3)°, V = 1597.65(9) A˚3, Z = 2, Fcalcd = 1.394 g 3 cm-3, 23 217 reflections measured, 6296 independent reflections, of which 5119 were observed (I > 2σ(I)), R1 (I > 2σ(I)) = 0.0232, wR2 (all data) = 0.0529, 390 refined parameters. Compound 9: space group P21/n, monoclinic, a = 13.8014(5) A˚, b = 15.8180(7) A˚, c = 15.8529(6) A˚, β = 90.310(3)°, V = 3460.8(2) A˚3, Z = 4, Fcalcd = 1.828 g 3 cm-3, 43 042 reflections measured, 6096 independent reflections, of which 4976 were observed (I > 2σ(I)), R1 (I > 2σ(I)) = 0.026, wR2 (all data) = 0.0626, 515 refined parameters. Compound 11: space group P21/n, monoclinic, a = 12.546(3) A˚, b = 17.317(4) A˚, c = 13.706(3) A˚, β = 100.84(3)°, V = 2924.6(10) A˚3, Z = 4, Fcalcd = 1.504 g 3 cm-3, 10 470 reflections measured, 5630 independent reflections, of which 4330 were observed (I > 2σ(I)), R1 (I > 2σ(I)) = 0.0265, wR2 (all data) = 0.0532, 377 refined parameters. (33) Christiansen, A.; Li, Ch.; Garland, M.; Selent, D.; Ludwig, R.; Franke, R.; B€ orner, A. ChemCatChem 2010, in press.

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Compound 13: space group P21/n, monoclinic, a = 11.8043(3) A˚, b = 17.4829(4) A˚, c = 16.1498(4) A˚, β = 106.718(2)°, V = 3192.01(13) A˚3, Z = 4, Fcalcd = 1.985 g 3 cm-3, 52 198 reflections measured, 7331 independent reflections, of which 5423 were observed (I > 2σ(I)), R1 (I > 2σ(I)) = 0.0316, wR2 (all data) = 0.0614, 519 refined parameters. Compound 14: space group P1, triclinic, a = 10.924(2) A˚, b = 11.766(2) A˚, c = 19.580(4) A˚, R = 106.88(3)°, β = 91.35(3)°, γ = 111.89(3)°, V = 2209.8(8) A˚3, Z = 2, Fcalcd = 1.411 g 3 cm-3, 8069 reflections measured, 8069 independent reflections, of which 6156 were observed (I > 2σ(I)), R1 (I > 2σ(I)) = 0.0272, wR2 (all data) = 0.0551, 536 refined parameters. [(C8H12)Rh{(p-tolyl)2PO}2H] (6). To a solution of bis(p-tolyl)phosphine oxide (2) (1.1 mmol, 0.255 g) in THF (4 mL) was added [Rh(acac)(cod)] (0.55 mmol, 0.172 g) under stirring. After a few minutes the formed orange precipitate was mixed with Et2O (10 mL), and the resulting solution covered with Et2O (5 mL) and stored at -26 °C for 15 h. After filtration of the product, washing with Et2O (2  5 mL), and drying in vacuo 0.328 g (87%) of complex 6 was obtained. 1H NMR (300 MHz, CD2Cl2): δ 16.9 (br, 1 H, O 3 3 H 3 3 O), 7.36 (m, 8 H, C6H4), 7.05 (m, 8 H, C6H4), 4.46 (br, 4 H, CH, C8H12), 2.30 (s, 12 H, CH3), 2.22 (m, 8 H, CH2, C8H12). 13C NMR (76 MHz, CD2Cl2): δ 139.7 (s, CCH3), 137.8 (m, PC6H4), 131.0 (“t”, J = 6.3 Hz, C6H4), 128.7 (“t”, J = 5.2 Hz, C6H4), 96.6 (dd, J = 5.3, 11.9 Hz, CH, C8H12), 30.5 (s, CH2, C8H12), 21.3 (s, CH3). 31P NMR (121 MHz, CD2Cl2): δ 92.3 (d, JRh-P = 159.7 Hz). Anal. Calcd for C36H41O2P2Rh: C 64.48, H 6.16, P 9.24, Rh 15.35. Found: C 64.6, H 6.05, P 9.57, Rh 15.06. Mp: 173-175 °C (THF/Et2O). [(C8H12)Rh{(C6H5)2PO}2H] (7). Diphenylphosphine oxide (3) (6.45 mL, 0.2 M in THF, 1.29 mmol) was added dropwise via a syringe to a stirred solution of [Rh(acac)(cod)] (0.2 g, 0.645 mmol) in THF (4 mL) at -45 °C. After 10 min stirring at -45 °C the reaction solution was slowly warmed to room temperature. The color turned from light yellow to orange, and the product complex began to precipitate as an orange solid.34 The solution of the catalyst was reduced to 5 mL in vacuo and then mixed with diethyl ether (6 mL). The precipitate was filtered, washed with diethyl ether, and dried in vacuo. Yield: 0.321 g (81%). Alternatively, the synthesis could be performed in toluene. NMR data in CD2Cl2: 1H NMR (300 MHz): δ 16.6 (at 195 K, s, 1H, O 3 3 H 3 3 O), 7.53 (m, 8 H, C6H5), 7.26 (m, 12 H, C6H5), 4.52 (br, 4 H, CH, C8H12), 2.23 (m, 8 H, CH2, C8H12). 13C NMR (76 MHz): δ 140.9 (m, CP), 131.0 (t, J = 6.1 Hz, C6H5), 129.7 (s, C6H5), 128.0 (t, J = 4.9 Hz, C6H5), 97.2 (m, CH, C8H12), 30.5 (s, CH2, C8H12). 31P NMR (121 MHz): δ 93.0 (d, JRh-P = 160.2 Hz). NMR data in THF-d8: 1H NMR (400 MHz): δ 16.2 (br, 1 H, O 3 3 H 3 3 O), 7.52 (m, 8 H, C6H5), 7.20 (m, 12 H, C6H5), 4.50 (br, 4 H, CH, C8H12), 2.21 (m, 8 H, CH2, C8H12). 31P NMR (162 MHz): δ 90.8 (d, JRh-P = 159 Hz);. 103 Rh NMR (12.6 MHz): δ -238. 1H NMR (193 K, 400 MHz): δ 16.63 (s, 1 H, O 3 3 H 3 3 O), 7.49 (m, 8 H, C6H5), 7.24 (m, 12 H, C6H5), 4.45 (br, 4 H, CH, C8H12), 2.20 (m, 8 H, CH2, C8H12). 31P NMR (193 K, 162 MHz): δ 91.0 (d, JRh-P = 157 Hz). NMR data in CD3OD: 1H NMR (300 MHz): δ 7.51 (m, 8 H, C6H5), 7.30 (m, 12 H, C6H5), 4.54 (br, 4 H, CH, C8H12), 2.27 (m, 8 H, CH2, C8H12). 31P NMR (121 MHz): δ 96.9 (d JRh-P = 162.1 Hz). Anal. Calcd for C32H33O2P2Rh: C 62.55, H 5.41, P 10.08, Rh 16.75. Found: C 62.62, H 5.41, P 10.06, Rh 16.53. Mp: 168-169 °C (THF/Et2O), 170-172 °C (MeOH). When Ph2P(O)H/D was reacted with [Rh(acac)(cod)] in deuterated trifluoroethanol, also complex 7 was formed as main product. 31P NMR (121 MHz, CF3CD2OD): δ 101.2 (d, JRh-P = 164.5 Hz). The formation of the rhodium complex was slow (34) 31P NMR studies (121 MHz, CD2Cl2) showed that the conversion was complete after 8 min stirring at room temperature. With increasing reaction time a second doublet occurred at δ 84.7 (JRh-P = 139.9 Hz), which gained in proportion with proceeding reaction time.

Christiansen et al. due to the poor solubility of the rhodium precursor in CF3CD2OD. [(C8H12)Rh{(p-F-C6H4)2PO}2H] (8). To a solution of bis( p-fluorophenyl)phosphine oxide (4) (1.01 mmol, 0.241 g) in THF (4 mL) was added [Rh(acac)(cod)] (0.51 mmol, 0.157 g). After 8 min stirring at room temperature the volatiles were removed in vacuo. The residue was dissolved in CH2Cl2 (1 mL) and carefully covered with Et2O (10 mL). After 12 h at -26 °C the obtained precipitate was filtered, washed with Et2O, and dried in vacuo to afford 0.134 g (19%) of complex 8. 1H NMR (400 MHz, CD2Cl2): δ 7.46 (m, 8 H, C6H4), 6.97 (m, 8 H, C6H4), 4.48 (br, 4 H, CH, C8H12), 2.24 (m, 8 H, CH2, C8H12); O 3 3 H 3 3 O was not detectable at room temperature. 13C NMR (75 MHz, CD2Cl2): δ 163.9 (d, JC-F = 249.5 Hz, CF), 136.5 (m, CP), 133.0, 115.3 (2 m, C6H4), 97.8 (m, CH, C8H12), 30.5 (s, CH2, C8H12). 31P NMR (162 MHz, CD2Cl2): δ 91.8 (d, JPRh = 161.5 Hz). Anal. Calcd for C32H29F4O2P2Rh: C 55.99, H 4.26, P 9.02, Rh 14.99. Found: C 55.4, H 4.06, P 8.97, Rh 14.94. Mp: 129131 °C (CH2Cl2/hexane). [(C8H12)Rh{(C5NF4)2PO}2H] (9). To a solution of [Rh(acac)(cod)] (0.37 mmol, 0.115 g) in THF (2 mL) at -78 °C was slowly added under stirring a THF solution of 5 (0.2 M, 0.37 mmol, 0.371 g). After a few minutes the volatiles were removed and the residue evaporated to dryness in vacuo. The obtained foam was dissolved in a little CH2Cl2 and carefully covered with a 15-fold amount of diethyl ether. After five days 0.268 mg (80%) of complex 9 as red crystals suitable for X-ray structural analysis was obtained; the crystals were washed with diethyl ether and dried in vacuo. 1H NMR (300 MHz, CD2Cl2): δ 14.63 (br, 1 H, O 3 3 H 3 3 O), 4.85 (br, 4 H, CH, C8H12), 2.38 (br, 8 H, CH2, C8H12). 13C NMR (76 MHz, CD2Cl2): δ 145.9, 142.6 (2 C), 139.1, 131.0 (m, 4 H, C5NF4), 106.0 (m, CH, C8H12), 30.4 (s, CH2, C8H12). 19F NMR (282 MHz, CD2Cl2): δ -88.4, -132.4 (2 m). 31P NMR (121 MHz, CD2Cl2): δ 74.0 (d, JRh-P = 180.0 Hz). Anal. Calcd for C28H13F16N4O2P2Rh: C 37.11, H 1.45, P 6.84, Rh 11.36, N 6.18. Found: C 37.09, H 1.43, P 6.75, Rh 11.21, N 5.82. Mp: 110-112 °C. [(C8H12)Rh{(p-CH3-C6H4)2PO}2BF2] (10). A suspension of [Rh(cod)2]BF4 (0.2 mmol, 0.088 g) in THF (2 mL) was cooled to -40 °C, and 2 mL of ligand solution (0.4 mmol 2, 0.1 g) was added dropwise under stirring via a syringe. After 10 min stirring at -70 °C the reaction solution was slowly warmed to room temperature overnight. The clear, orange solution was reduced to 2 mL in vacuo, and then a layer of diethyl ether (10 mL) was added. After 3 days at 8 °C the crystalline precipitate was filtered off and dried in vacuo. Yield: 0.089 g (57%). 1H NMR (300 MHz, CD2Cl2): δ 7.35 (m, 8 H, C6H4), 7.07 (m, 8 H, C6H4), 4.58 (br, 4 H, CH, C8H12), 2.31 (s, 12 H, CH3), 2.28 (m, 8 H, CH2, C8H12). 13C NMR (76 MHz, CD2Cl2): δ 140.6 (s, CCH3), 134.6 (m, PC6H4), 131.0 (t, J = 6.5 Hz, C6H4), 129.0 (t, J = 5.4 Hz, C6H4), 100.2 (m, CH, C8H12), 30.3 (s, CH2, C8H12), 21.4 (s, CH3). 19F NMR (282 MHz, CD2Cl2): δ -137.3. 31P NMR (121 MHz, CD2Cl2): δ 101.2 (“d”,35 JRh-P ≈ 163.7 Hz). Anal. Calcd for C36H40BF2O2P2Rh: C 60.19, H 5.61, P 8.62. Found: C 58.3, H 5.29, P 8.63. Mp: 154-156 °C (THF/Et2O). [(C8H12)Rh{(C6H5)2PO}2BF2] (11). To a mixture of [Rh(cod)2]BF4 (0.42 mmol, 0.170 g) and diphenylphosphine oxide (3) (0.169 g, 0.84 mmol) was slowly added THF (8 mL) at room temperature under stirring. After 3 h reaction time the orange solution was filtered, and the resulting filtrate was reduced in vacuo to one-fourth of its volume and covered with a layer of diethyl ether (16 mL). After 3 days at 8 °C the orange crystals were filtered off, washed with diethyl ether, and dried in vacuo. Yield: 0.264 g (95%). Crystals suitable for X-ray structural analysis were obtained from CD2Cl2. 1H NMR (300 MHz, CD2Cl2): δ 7.49 (m, 8 H, C6H5), 7.29 (m, 12 H, C6H5), 4.61 (br, 4 H, CH, C8H12), 2.30 (br, 8 H, CH, C8H12). 13C NMR (76 MHz, (35) Additional coupling with the fluorine atoms of the BF2 unit leads to a spectrum of higher order.

Article CD2Cl2): δ 137.8, 131.1 (2 m, C6H5), 130.5 (s, C6H5), 128.4 (m, C6H5), 100.8 (m, CH, C8H12), 30.3 (s, CH2, C8H12). 19F NMR (282 MHz, CD2Cl2): δ -137.5. 31P NMR (121 MHz, CD2Cl2): δ 101.7 (“d”,35 J ≈ 163.8 Hz). Anal. Calcd for C32H32BF2O2P2Rh: C 58.04, H 4.87, P 9.35, Rh 15.54. Found: C 58.1, H 4.88, P 9.37, Rh 15.35. Mp: 177-178 °C. [(C8H12)Rh{(p-F-C6H4)2PO}2BF2] (12). A suspension of [Rh(cod)2]BF4 (0.4 mmol, 0.164 g) in toluene (3 mL) was cooled to -70 °C, and 4 mL of ligand solution (0.8 mmol 4 in toluene, 0.192 g) was added dropwise under stirring via a syringe. After 10 min stirring at -70 °C the reaction solution was slowly warmed to room temperature overnight. The orange precipitate was filtered, washed with diethyl ether, and dried in vacuo. Yield: 0.258 g (87%). 1H NMR (300 MHz, CD2Cl2): δ 7.47 (m, 8 H, C6H4), 7.00 (m, 8 H, C6H4), 4.60 (br, 4 H, CH, C8H12), 2.33 (br, 8 H, CH2, C8H12). 13C NMR (101 MHz, CD2Cl2): δ 164.3 (d, JC-F = 251.0 Hz, CF), 134.0 (m, CP), 133.3 (dd, J = 7.6 Hz, J = 15.33 Hz, C6H4), 115.7 (dt, JCF = 5.8 Hz, C6H4), 101.4 (m, CH, C8H12), 30.3 (s, CH2, C8H12). 19F NMR (282 MHz, CD2Cl2): δ -109.6 (m, CF), -137.6, (m, BF2). 31P NMR (121 MHz, CD2Cl2): δ 100.5 (“d”,35 JRh-P ≈ 165.3 Hz). Anal. Calcd for C32H28BF6O2P2Rh: C 52.35, H 3.84, P 8.44. Found: C 50.78, H 4.08, P 8.36. Mp: 162-164 °C (toluene). [(C8H12)Rh{(C5NF4)2PO}2BF2] (13). A suspension of [Rh(cod)2]BF4 (0.37 mmol, 0.150 g) in THF (3 mL) was cooled to -78 °C, and 3.69 mL of ligand solution (0.2 M in THF, 0.74 mmol of 5) was added slowly under stirring via a syringe. The reaction solution was allowed to warm to room temperature, and after 3.5 h the volatiles were removed in vacuo. The obtained foam was dissolved in a small amount of CH2Cl2 and carefully covered with a 7-fold amount of diethyl ether. After several hours the yellow-orange precipitate was filtered off, washed with diethyl ether, and dried in vacuo. The filtrate was reduced in vacuo and retreated with diethyl ether, and the precipitate handled as described above. Crystals suitable for X-ray structural analysis were obtained from CD2Cl2. The overall yield of complex 13 amounts to 0.24 g (68%). When the reaction was monitored via NMR spectroscopy, the intermediate formation of quasi-chelate complex 9 was observed. 1H NMR (300 MHz,

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CD2Cl2): δ 5.00 (br, 4 H, CH, C8H12), 2.56-2.36 (m, 8 H, CH2, C8H12). 13C NMR (76 MHz, CD2Cl2): δ 145.9 (m, C5NF4), 142.7 (m, C5NF4), 139.2 (m, C5NF4), 108.5 (m, CH, C8H12), 30.4 (s, CH2, C8H12). 19F NMR (282 MHz, CD2Cl2): δ -87.2, -131.4 (2 m, C5NF4), -138.3 (BF2). 31P NMR (121 MHz, CD2Cl2): δ 79.6 (“d”,35 JRh-P ≈ 188.1 Hz). Anal. Calcd for C28H12BF18N4O2P2Rh: C 35.25, H 1.27, P 6.49, Rh 10.79, N 5.87. Found: C 35.3, H 1.39, P 6.53, Rh 10.73, N 5.73. Mp: 230 °C (dec). [(C6H5)2P(OCD3)}2Rh{(C6H5)2PO]2D (14). While in the absence of [Rh(acac)(cod)] no esterification of Ph2P(O)D (3) to Ph2P-OCD3 and Ph2P(O)-OCD3, respectively, took place in CD3OD, an immediate reaction was observed in the presence of the metal complex. Complex 14 was crystallized in CD3OD and analyzed by means of X-ray structural analysis. The CD3 groups can neither be detected by 1H NMR spectroscopy nor be unambiguously assigned in the 13C NMR spectrum due to their low signal intensity. 1H NMR (400 MHz, CD2Cl2): δ 7.34 (m, 4 H), 7.27 (m, 8 H), 7.18 (m, 12 H), 7.08 (m, 8 H), 7.04 (m, 8 H). 13C NMR (75 MHz, CD2Cl2): δ 143.8, 135.7 (2 m, CP), 132.8, 131.6 (2 “t”, C6H5), 129.9, 128.6 (2 s, C6H5), 127.9, 127.3 (2 “t”, C6H5). 31 P NMR (121 MHz, CD2Cl2): δ 131.7 and 97.2 (2 AA0 BB0 X systems). Deuterated methyl and hydroxyl groups were not observed.

Acknowledgment. Financial support for the experimental research was provided by Evonik Oxeno GmbH. One of us (A.Ch.) is grateful for a research scholarship from the Degussa Stiftung. We thank PD Dr. B. Hoge (University of Cologne) for a generous gift of SPO 5. We acknowledge skilled technical assistance by Dr. O. Zayas, Mrs. H. Borgwald, Mrs. M. Geisendorf, and Ms. K. Romeike. Supporting Information Available: Tables of crystallographic data in cif file format of compounds 6, 9, 11, 13, and 14. This material is available free of charge via the Internet at http:// pubs.acs.org.