Bond Cleavage - ACS Publications - American Chemical Society

Feb 24, 2009 - Alexander M. Kirillov,† Piotr Smolenski,† Matti Haukka,‡ M. Fátima C. ... 1049-001, Lisbon, Portugal, Department of Chemistry, UniVersi...
0 downloads 0 Views 303KB Size
Download PDF
Organometallics 2009, 28, 1683–1687

1683

Unprecedented Metal-Free C(sp3)-C(sp3) Bond Cleavage: Switching from N-Alkyl- to N-Methyl-1,3,5-triaza-7-phosphaadamantane Alexander M. Kirillov,† Piotr Smolen´ski,† Matti Haukka,‡ M. Fa´tima C. Guedes da Silva,†,§ and Armando J. L. Pombeiro*,† Centro de Quı´mica Estrutural, Complexo I, Instituto Superior Te´cnico, TU Lisbon, AVenida RoVisco Pais, 1049-001, Lisbon, Portugal, Department of Chemistry, UniVersity of Joensuu, P.O. Box 111, FI-80101, Joensuu, Finland, and UniVersidade Luso´fona de Humanidades e Tecnologias, AVenida do Campo Grande, 376, 1749-024, Lisbon, Portugal ReceiVed October 24, 2008

N-Alkyl-1,3,5-triaza-7-phosphadamantane halides [PTA-R]X (X ) I-, Br-) with more than two carbon atoms in the alkyl chain are shown to easily convert, under mild conditions in aqueous medium, into the corresponding N-methyl derivatives, thus representing a novel type of metal-free C(sp3)-C(sp3) bond cleavage that affects inert, unstrained, and unbranched aliphatic groups. This unusual transformation is promoted by Cu ions and is proposed to occur via a postulated ammonium ylide. In the presence of Cu ions, this C-C bond dissociation is also a key point for the formation of a unique 2D Cu(I) coordination polymer with a rare P,N-coordination mode of the PTA cage. The search for mild and selective methods of C-C bond activation in various substrates has been of long-standing interest in view of both fundamental and practical importance in, for example, organic synthesis, organometallic and environmental chemistry, catalysis, and biochemistry. Most of the reported examples of C-C bond activation concern the dissociation of strained, cyclic, and/or multiple C-C bonds in the presence of metal centers.1-3 In contrast, the cleavage of unstrained and inert C(sp3)-C(sp3) bonds remains little explored4 and typically limited to high-temperature hydrocarbon cracking,5 photoinduced electron transfer,6 mass spectrometry ionization,7 or enzymatic systems.8 In particular, efficient and mild metal-free C-C bond dissociation reactions on the secondary and/or primary C(sp3) atoms within unbranched alkyl moieties are virtually unknown. * To whom correspondence should be addressed. Phone: +351 218419237. Fax: +351 218464455. E-mail: [email protected]. † Instituto Superior Te´cnico. ‡ University of Joensuu. § Universidade Luso´fona de Humanidades e Tecnologias. (1) For selected reviews, see: (a) Jun, C.-H. Chem. Soc. ReV. 2004, 33, 610. (b) Rybtchinski, B.; Milstein, D. Angew. Chem., Int. Ed. 1999, 38, 870. (c) Murakami, M.; Ito, Y. In Topics in Organometallic Chemistry; Murai, S., Ed.; Springer: Berlin, 1999; Vol. 3, pp 97-129. (d) Crabtree, R. H. Chem. ReV. 1985, 85, 245. (2) Shilov, A. E.; Shul’pin, G. B. ActiVation and Catalytic Reactions of Saturated Hydrocarbons in the Presence of Metal Complexes; Kluwer Academic Publishers: Dordrecht, 2000. (3) For selected recent examples, see: (a) Gunay, A.; Jones, W. D. J. Am. Chem. Soc. 2007, 129, 8729. (b) Mueller, J.; Wuertele, C.; Walter, O.; Schindler, S. Angew. Chem., Int. Ed. 2007, 46, 7775. (c) Anstey, M. R.; Yung, C. M.; Du, J.; Bergman, R. G. J. Am. Chem. Soc. 2007, 129, 776. (d) Draghici, C.; Brewer, M. J. Am. Chem. Soc. 2008, 130, 3766. (4) Niwa, T.; Yorimitsu, H.; Oshima, K. Angew. Chem., Int. Ed. 2007, 46, 2663, and references therein. (5) Olah, G. A.; Molnar, A. Hydrocarbon Chemistry; John Wiley & Sons: Hoboken, NJ, 2003. (6) (a) Gaillard, E. R.; Whitten, D. G. Acc. Chem. Res. 1996, 29, 292. (b) Lee, L. Y. C.; Ci, X.; Giannotti, C.; Whitten, D. G. J. Am. Chem. Soc. 1986, 108, 175. (7) (a) Hammerum, S.; Norrman, K.; Solling, T. I.; Andersen, P. E.; Bo Jensen, L.; Vulpius, T. J. Am. Chem. Soc. 2005, 127, 6466, and references therein. (b) Yadav, L. D. S. Organic Spectoscopy: Springer: Berlin, 2005. (8) Frey, P. A.; Hegeman, A. D. Enzymatic Reaction Mechanisms; Oxford University Press US: New York, 2007.

Scheme 1

Being interested in the coordination behavior of 1,3,5-triaza7-phosphaadamantane (PTA)9 (one of the most relevant ligands in aqueous organometallic chemistry),10,11 we have investigated the reactivity of N-alkyl-1,3,5-triaza-7-phosphaadamantane halides [PTA-R]X (R ) nBu, nPr, Et; X ) I-, Br-). Herein, we report that such compounds surprisingly undergo an unprecedented R-C-C bond cleavage within the alkyl arm of [PTAR]X, generating the N-methyl-1,3,5-triaza-7-phosphaadamantane [PTA-Me]X and the corresponding aldehyde (Scheme 1). The evidence for such cleavage and formation of the resulting products is based on a combination of 1H, 13C{1H}, and 31P{H} NMR, GC, IR, FAB+-MS, elemental, and single-crystal X-ray structural analyses. That metal-free C-C bond rupture proceeds in aqueous solution, at ambient temperature, even in the absence of air (under dinitrogen) or in the dark. It is accelerated by heating, (9) (a) Jaremko, Ł.; Kirillov, A. M.; Smolen´ski, P.; Lis, T.; Pombeiro, A. J. L. Inorg. Chem. 2008, 47, 2922. (b) Kirillov, A. M.; Smolen´ski, P.; Guedes da Silva, M. F. C.; Pombeiro, A. J. L. Eur. J. Inorg. Chem. 2007, 2686. (c) Smolen´ski, P.; Pombeiro, A. J. L. Dalton Trans. 2008, 87. (10) For a comprehensive review, see: Phillips, A. D.; Gonsalvi, L.; Romerosa, A.; Vizza, F.; Peruzzini, M. Coord. Chem. ReV. 2004, 248, 955, and references therein. (11) For recent examples, see: (a) Dutta, B.; Scolaro, C.; Scopelliti, R.; Dyson, P. J.; Severin, K. Organometallics 2008, 27, 1355. (b) Miranda, S.; Vergara, E.; Mohr, F.; de Vos, D.; Cerrada, E.; Mendia, A.; Laguna, M. Inorg. Chem. 2008, 47, 5641. (c) Marchi, A.; Marchesi, E.; Marvelli, L.; Bergamini, P.; Bertolasi, V.; Ferretti, V. Eur. J. Inorg. Chem. 2008, 2670. (d) Erlandsson, M.; Landaeta, V. R.; Gonsalvi, L.; Peruzzini, M.; Phillips, A. D.; Dyson, P. J.; Laurenczy, G. Eur. J. Inorg. Chem. 2008, 620. (e) Mebi, C. A.; Nair, R. P.; Frost, B. J. Organometallics 2007, 26, 429. (f) Huang, R.; Frost, B. J. Inorg. Chem. 2007, 46, 10962. (g) Gossens, C.; Dorcier, A.; Dyson, P. J.; Rothlisberger, U. Organometallics 2007, 26, 3969. (h) Rossin, A.; Gonsalvi, L.; Phillips, A. D.; Maresca, O.; Lledos, A.; Peruzzini, M. Organometallics 2007, 26, 3289.

10.1021/om801026a CCC: $40.75  2009 American Chemical Society Publication on Web 02/24/2009

1684 Organometallics, Vol. 28, No. 6, 2009

KirilloV et al. Scheme 2

and the quantitative conversion of [PTA-R]+ to [PTA-Me]+ can be achieved within 5 h of reflux (Scheme 1). The PTA core and the halide counterion in [PTA-R]X play a determinant role, since no cleavage is detected for either [NBu4]I or [PTABu][BPh4] (even under refluxing conditions for an extended time). Too prolonged reaction times (20 h of reflux) promote the C-N bond cleavage (elimination of the whole n-alkyl arm), as a side reaction, in accord with the detection of minor amounts of PTA phosphine oxide and butanal in the case of [PTA-Bu]I. The addition of an excess of KI or I2 to, for example, [PTABu]I does not lead to a noticeable acceleration of the C-C bond cleavage, but I2 promotes the oxidation of the PTA core to phosphine oxide. Besides, neither the known10 hydrolysis of PTA at high temperatures affecting its core by multiple C-N bond cleavages nor the recently reported12 metal insertion into the PTA core12 (when performed in the presence of copper) appears to be followed. The observed C-C bond dissociation is promoted by copper ions and was studied in detail using [PTA-Bu]I, featuring the formation of new [PTA-Me]+ and [PTA-Bu]+ copper(I) compounds (Scheme 2). Hence, the mixture of a diluted (ca. 10-3 M) aqueous solution of Cu(NO3)2 and HNO3 with a diluted aqueous solution of [PTA-Bu]I, followed by slow evaporation of the obtained solution in air at room temperature, leads to the neutral 2D Cu(I) polymer [Cu2(µ-I)3(PTA-Me)]n (1) (Scheme 2, a). This reaction involves (i) an R-C-C bond cleavage with concomitant formation of propanal (detected by GC and 1H NMR analyses) and (ii) the reduction of Cu(II) to Cu(I) (with phosphine and I-).9b The X-ray crystal structure of 1 (Figure 1) reveals the rare P,N-coordination mode13 of the PTA cage, not reported previously for any derivative with the alkylated (12) Mena-Cruz, A.; Lorenzo-Luis, P.; Romerosa, A.; Serrano-Ruiz, M. Inorg. Chem. 2008, 47, 2246. (13) (a) Lidrissi, C.; Romerosa, A.; Saoud, M.; Serrano-Ruiz, M.; Gonsalvi, L.; Peruzzini, M. Angew. Chem., Int. Ed. 2005, 44, 2568. (b) Frost, B. J.; Mebi, C. A.; Gingrich, P. W. Eur. J. Inorg. Chem. 2006, 1182. (c) Mohr, F.; Falvello, L. R.; Laguna, M. Eur. J. Inorg. Chem. 2006, 3152.

PTA core, which in 1 behaves as a bridging bidentate ligand. The use of diluted solutions of reagents is essential for the C-C bond cleavage and consequent formation of 1, since more concentrated water solutions (ca. 10-2-10-1 M) of copper(II) nitrate and [PTA-Bu]I cause an immediate precipitation of the Cu(I) complex [CuI2(PTA-Bu)2]I (2) (Scheme 2, b) with P-coordinated [PTA-Bu]+. However, the application of the synthetic procedure of 1 directly to [PTA-Me]I (Scheme 2, c) instead of [PTA-Bu]I leads to the formation of another Cu(I) product, [Cu2I2(µ-I)2(PTA-Me)2] (3), which, as shown by X-ray crystallography (Figure 1), possesses only the conventional

Figure 1. Thermal ellipsoid (50% probability) representation of crystal structures of 1 (left) and 3 (right). H atoms are omitted for clarity. Cu green, P orange, I deep purple, N blue, C gray. Inset shows a schematic comparative representation of the architectures observed in the crystal packing diagrams of compounds 1 and 3; projection of one metal-organic layer along the ac plane (1) or the a axis (3).

Metal-free C(sp3)-C(sp3) Bond CleaVage

P-coordination of the [PTA-Me]+ ligands. Interestingly, the [PTA-Bu]+ ligands in 2 also undergo the above C-C bond rupture, and for example, a suspension of 2 in water, upon refluxing in air, leads to compound 3 (Scheme 2, d). This points out the generality of the observed C-C bond cleavage, which thus also occurs at coordinated [PTA-R]+ species. The molecular structure of 1 is composed of two symmetry nonequivalent Cu centers possessing a distorted tetrahedral geometry filled by three iodine atoms and one [PTA-Me]+ ligand adopting the alternating P- or N-coordination. All iodine atoms and [PTA-Me]+ moieties act in a bridging mode, thus generating infinite two-dimensional polymeric layers (Figure 1, left). They are built from two kinds of zigzag-type 1D chains based on the -Cu2I2-I-Cu2I2-I- and -[PTA-Me]-Cu2I2-[PTA-Me]Cu2I2- motifs, with the repeating periods of 7.2901(2) and 8.9157(2) Å, respectively, being equivalent to the a and c unit cell dimensions. The Cu(µ-I)2Cu core is nonplanar, possessing a Cu · · · Cu distance of 3.012(2) Å, while the shortest Cu · · · Cu separations within the Cu(µ-I)Cu and Cu[PTA-Me]Cu fragments are 5.021(2) and 6.808(2) Å, respectively. The cagelike geometry of the [PTA-Me]+ spacer is practically not affected by its unconventional P,N-coordination mode. In general, most of the bonding parameters in 1 (Table S1) are comparable to those reported for related PTA-9b,13 or [PTA-Me]14,15-containing compounds, as well as for various phosphine/iodide Cu clusters.16 The solid-state structure of 3 (Figure 1, right) reveals some common features to those of 1, namely, the presence of dinuclear Cu(µ-I)2Cu cores, similar bonding parameters (Table S1), and the tetrahedral geometry around the Cu centers. Nevertheless, the structure of 3 is not polymeric and is composed of discrete dimeric [Cu2I2(µ-I)2(PTA-Me)2] units bearing two terminal iodine atoms and two P-coordinated [PTA-Me]+ ligands. In contrast to 1, [PTA-Me]+ does not bridge to another metal. The distinct structural characters of 1 and 3 can also be shown by the inset in Figure 1, representing their architectures within one metal-organic layer in the crystal packing diagrams. In our case, the cleavage does not appear to proceed via a photoinduced electron-transfer reaction (it occurs in the dark), in contrast to documented6 C-C bond rupture reactions of amine derivatives, nor via a free radical process since no inhibiting effect was detected in the presence of a radical trap such as CBrCl3 or 2,6-di-tert-butyl-4-methylphenol (BHT). Hence, the heterolysis of the C-C bond of the N+CH2-R′ moiety attached to the PTA cage possibly occurs preferably to the homolysis, eventually leading, in the presence of halide, to the postulated ammonium ylide moiety (PTA)N+-CH2- and the alkyl halide R′X.17a Protonation of the ylide17 by H2O (which, however, we could not prove by attempted NMR experiments with D2O) could lead to the N+-CH3 group of the [PTA-Me]I product, (14) (a) Pruchnik, F. P.; Smolen´ski, P.; Gałdecka, E.; Gałecki, Z. New J. Chem. 1998, 1395. (b) Smolen´ski, P.; Pruchnik, F. P.; Ciunik, Z.; Lis, T. Inorg. Chem. 2003, 42, 3318. (15) For examples, see: (a) Romerosa, A.; Saoud, M.; CamposMalpartida, T.; Lidrissi, C.; Serrano-Ruiz, M.; Peruzzini, M.; Garrido, J. A.; Garcı´a-Maroto, F. Eur. J. Inorg. Chem. 2007, 2803. (b) Mebi, C. A.; Frost, B. J. Z. Anorg. Allg. Chem. 2007, 633, 368. (c) Romerosa, A.; CamposMalpartida, T.; Lidrissi, C.; Saoud, M.; Serrano-Ruiz, M.; Peruzzini, M.; Garrido-Cardenas, J. A.; Garcia-Maroto, F. Inorg. Chem. 2006, 45, 1289. (d) Dorcier, A.; Dyson, P. J.; Gossens, C.; Rothlisberger, U.; Scopelliti, R.; Tavernelli, I. Organometallics 2005, 24, 2114. (e) Kovacs, J.; Joo´, F.; Benyei, A. C.; Laurenczy, G. Dalton Trans. 2004, 2336. (16) For examples, see the Cambridge Structural Database (CSD); Allen, F. H. Acta Crystallogr. 2002, B58, 380. (17) (a) Nitrogen, Oxygen and Sulfur Ylide Chemistry; Clark, J. S., Ed.; Oxford University Press: Oxford, 2002. (b) Gaebert, C.; Siegner, C.; Mattay, J.; Toubartz, M.; Steenken, S. J. Chem. Soc., Perkin Trans. 2 1998, 2735.

Organometallics, Vol. 28, No. 6, 2009 1685

whereas hydrolysis (without requiring O2 as oxidant) of the liberated R′X would form the aldehyde, a transformation related to the Sommelet reaction (i.e., conversion of organohalides to aldehydes by action of water and hexamethylenetetramine, an amine analogue of PTA).18 Moreover, ammonium ylides are typical intermediates in some related reactions of ammonium salts and tertiary amines (e.g., the Stevens and Sommelet-Hauser rearrangements).19 Hence, although the detailed mechanism of the observed C-C bond cleavage is still unclear, it conceivably involves steps known for the above-mentioned reactions.18,19 Aldehydes are also common products in photoinduced6 or metalpromoted20 C-C bond cleavages in some amine derivatives. C-C bond dissociation reactions are also known to occur under mass spectrometry conditions.7 In view of the yet unknown mass spectroscopic behavior of alkylated PTA derivatives, we investigated [PTA-R]I (R ) nBu, nPr, Et, Me) and 1-3 by FAB+-MS spectrometry (see Supporting Information). All [PTA-R]+ species are rather stable toward fragmentation on account of the detection of the molecular ion peaks [M]+ with 100% relative intensity. Nevertheless, the R-C-C bond cleavage is also quite a favorable process in [PTA-Bu]I, leading to the [(PTA-Me) - H]+ species at m/z ) 171 (35%), conceivably in the iminium form (PTAopen)N+dCH2 involving a partial opening of the PTA cage. Similar peaks are detected for the N-propyl and N-ethyl PTA derivatives but with lower intensities (12% and 5%, respectively). The β-C-C bond cleavage is also shown by [PTA-Bu]I as a minor intensity (6%) signal due to [PTA-Et]+, whereas no evidence for the C-N bond rupture is found (absence of the PTA peak). The latter process is also not detected for [PTA-Me]I, but occurs for [PTAPr]I and [PTA-Et]I [intense (48% and 15%, respectively) PTA peaks at m/z ) 157]. Elimination of the methyl group is not detected in [PTA-Me]I. In 1-3, the N-alkyl PTA ligands are more stable toward fragmentation but show a more intricate behavior than that of the corresponding free species. In general, the observed fragmentation patterns display features established under mass spectrometry conditions for various alkylamines,7 namely, revealing the possibility of (i) R-cleavage of the C-C bond with elimination of alkyl radicals or alkenes and (ii) ring openings with formation of various iminium ions.21 In summary, we have described a novel type of metal-free C(sp3)-C(sp3) bond cleavage within the N-alkylated aminophosphines that proceeds in aqueous medium, even under very mild conditions (room temperature). This unusual reaction broadens the scope of important organic transformations known for tertiary amines and ammonium salts.17a,18,19 The R-cleavage of the n-butyl arm of [PTA-Bu]+ in the presence of Cu(II) opens up the route for a novel 2D coordination polymer, [Cu2(µI)3(PTA-Me)]n 1, with a rare P,N-coordination mode of the PTA core, thus extending the application of PTA derivatives as spacers for building metal-organic frameworks. (18) (a) Angyal, S. J. Org. React. 1954, 8, 197. (b) Li, J. J.; Corey, E. J. Name Reactions of Functional Group Transformations; Wiley: New York, 2007. (19) (a) Vanecko, J. A.; Wan, H.; West, F. G. Tetrahedron 2006, 62, 1043, and references therein. (b) Marko´, I. E. In ComprehensiVe Organic Synthesis, Vol. 3; Trost, B. M., Fleming, I., Eds.; Pergamon: Oxford, 1991; Chapter 3.10. (20) Wang, F.; Sayre, L. M. J. Am. Chem. Soc. 1992, 114, 248. (21) (a) Kalsi, P. S. Spectroscopy of Organic Compounds, 6th ed.; New Age International Publ., 2004. (b) Affolter, C.; Buhlmann, P. Structure Determination of Organic Compounds: Tables and Spectral Data; Springer: Berlin, 2000.

1686 Organometallics, Vol. 28, No. 6, 2009

KirilloV et al.

Experimental Section

J(HB-P) ) 0 Hz, 2H), 4.09 (s, 2H), 4.26 and 4.44 (J(HAHB) ) 15.3 Hz, 2H), 4.40 (s, 2H), 4.86 and 4.91 (J(HAHB) ) 2.1 Hz, 2H), 4.81 and 4.97 (J(HAHB) ) 11.6 Hz, 2H). 31P{1H} NMR (DMSOd6) δ/ppm: -83.3 (s br). No reliable 13C{1H} NMR spectrum could be accumulated due to the gradual decomposition of 1 in DMSOd6 during the long acquisition time. IR (KBr, cm-1): 2990w, 2950w, 1446w, 1408w, 1307m sh, 1242s br, 1186m, 1134m, 1091m, 1052w, 984m, 932m, 894w, 870w, 844w, 805s, 745s, 559m, and 443w. Anal. Calcd for C7H15Cu2I3N3P (Mr ) 680.0 g/mol): C, 12.36; N, 6.18; H, 2.22. Found: C, 12.77; N, 6.60; H, 2.58. Synthesis of [CuI2(PTA-Bu)2]I (2). To an aqueous solution (15 mL) of Cu(NO3)2 · 2.5H2O (0.50 mmol, 116 mg) and HNO3 (0.50 mmol) (the acid was added to avoid the spontaneous hydrolysis of the metal salt) was added an aqueous solution (5 mL) of [PTABu]I (1.00 mmol, 341 mg) with continuous stirring at room temperature. The resulting white suspension was stirred for 2 h. The solid was then filtered off, washed with H2O (3 × 10 mL), and dried in vacuo, giving 2 as a white microcrystalline powder in ca. 55% (240 mg) yield, based on copper(II) nitrate. Compound 2 is soluble in DMSO but insoluble in water and common organic solvents. X-ray quality single crystals of 2 could not be obtained either by the slow evaporation of the mother liquor or upon recrystallization of 2 from DMSO. 1H NMR (DMSO-d6) δ/ppm: 0.93 (t, 3J(H-H) ) 7.4 Hz, 6H, N+CH2CH2CH2CH3), 1.29 (sextet, 3 J(H-H) ) 7.4 Hz, 4H, N+CH2CH2CH2CH3), 1.65 and 2.81 (2m, 8H, N+CH2CH2CH2CH3), 3.84 and 4.94 (J(HAHB) ) 15.0 Hz, 3 J(HA-P) ) 14.0 Hz, 3J(HB-P) ) 7.4 Hz, 8H, PCHAHBN), 4.36 (s, 4H, PCH2N+), 4.33 and 4.52 (J(HAHB) ) 13.7 Hz, 4H, NCHAHBN), 4.80 and 4.98 (J(HAHB) ) 11.4 Hz, 8H, NCHAHBN+). 31 P{1H} NMR (DMSO-d6) δ/ppm: -80.2 (s br). 13C{1H} and HMQC 13C-1H NMR (DMSO-d6) δ/ppm: 18.6 (s, N+CH2CH2CH2CH3), 24.8 (s, N+CH2CH2CH2CH3), 26.3 (s, N+CH2CH2CH2CH3), 66.4 (s, N+CH2CH2CH2CH3), 50.8 (d, 2 J(C-P) ) 18.2 Hz, PCH2N), 57.0 (d, 2J(C-P) ) 23.1 Hz, PCH2N+), 74.5 (s, NCH2N), 83.8 (s, NCH2N+). IR (KBr, cm-1): 2958m br, 2909w, 2874w, 1463m, 1413m, 1384m, 1312s br, 1252m, 1222m, 1123m, 1081s, 1031s, 989m, 962w, 927m, 904w, 866w, 814s, 766m, 744s, 692w, 588w, 563s, 461w, and 441w. Anal. Calcd for C20H42CuI3N6P2 (Mr ) 872.79 g/mol): C, 27.52; N, 9.63; H, 4.85. Found: C, 27.99; N, 10.09; H, 5.20. Synthesis of [Cu2I2(µ-I)2(PTA-Me)2] (3). Method A. The use of a synthetic procedure similar to that applied for 1, but directly employing [PTA-Me]I (0.20 mmol, 60 mg) instead of [PTA-Bu]I, leads to the formation of 3 as colorless X-ray quality crystals in ca. 46% yield based on copper(II) nitrate. Compound 3 can also be obtained in a quicker way by using more concentrated aqueous solutions of reactants (method B). Method B. To an aqueous solution (15 mL) of Cu(NO3)2 · 2.5H2O (0.50 mmol, 116 mg) and HNO3 (0.50 mmol) (the acid was added to avoid the spontaneous hydrolysis of the metal salt) was added an aqueous solution (5 mL) of [PTA-Me]I (1.00 mmol, 299 mg) with continuous stirring at room temperature. The resulting white suspension was stirred for 2 h. The solid was then filtered off, washed with H2O (3 × 10 mL), and dried in vacuo, giving 3 as a white microcrystalline powder in ca. 57% yield, based on copper(II) nitrate. Further crops of product (ca. 10%, i.e. total yield ca. 67%, 165 mg) as well as colorless X-ray quality crystals could be obtained upon evaporation of the filtrate for several days in air and at room temperature. Compound 3 is soluble in DMSO but insoluble in water and common organic solvents. 1H NMR (DMSO-d6) δ/ppm: 2.66 (s, 6H, N+CH3), 3.82 and 4.03 (J(HAHB) ) 14 Hz, 3J(HA-P) ) 7.6 Hz, 3J(HB-P) ) 0 Hz, 8H, PCHAHBN), 4.23 and 4.55 (J(HAHB) ) 12.8 Hz, 4H, NCHAHBN), 4.41 (s, 4H, PCH2N+), 4.79 and 5.01 (J(HAHB) ) 11.9 Hz, 8H, NCHAHBN+). 1H NMR (D2O) δ/ppm: 2.79 (s, 6H, N+CH3), 3.91 and 4.02 (J(HAHB) ) 12 Hz, 3J(HA-P) ) 9.2 Hz, 3J(HB-P) ) 3.4 Hz, 8H, PCHAHBN), 4.44 and 4.60 (J(HAHB) ) 15 Hz, 4H, NCHAHBN), 4.40 (s, 4H, PCH2N+), 4.85

General Procedures. All synthetic work was performed in air. All chemicals were obtained from commercial sources and used as received. 1,3,5-Triaza-7-phosphaadamantane (PTA),22 N-methyl1,3,5-triaza-7-phosphaadamantane iodide [PTA-Me]I,22 N-ethyl1,3,5-triaza-7-phosphaadamantane iodide [PTA-Et]I,23 and N-propyl-1,3,5-triaza-7-phosphaadamantane iodide [PTA-Pr]I23 were prepared by published methods (see Supporting Information for the synthesis of other ligands). C, H, and N elemental analyses were carried out by the Microanalytical Service of the Instituto Superior Te´cnico. The FAB+-MS spectra were obtained on a Trio 2000 instrument by bombarding 3-nitrobenzyl alcohol (m-NBA) matrixes of the samples with 8 keV (ca. 1.18 × 1015 J) Xe atoms. Mass calibration for data system acquisition was achieved using CsI. Infrared spectra (4000-400 cm-1) were recorded on a BIORAD FTS 3000MX instrument in KBr pellets. 1H, 13C{1H}, and 31 P{1H} NMR spectra were measured on a Bruker 300 UltraShield spectrometer at ambient temperature. The 31P NMR chemical shifts are relative to an external 85% H3PO4 aqueous solution. Gas chromatography (GC) analyses were performed on a Perkin-Elmer Clarus 500 gas chromatograph with a BP20 SGE capillary column (P/N 054424). The identification of aldehydes was based on the comparison of the obtained chromatograms with those of authentic samples. C-C Bond Cleavage Investigation. The mixtures composed of (a) the aqueous (25 mL) solution of [PTA-Bu]I (0.50 mmol, 171 mg), (b) the aqueous (25 mL) solution of [PTA-Bu]Br (0.50 mmol, 147 mg), (c) the H2O (10 mL)/acetone (10 mL) solution of [PTA-Bu][BPh4] (0.15 mmol, 80 mg), (d) the aqueous solution (25 mL) of [PTA-Pr]I (0.50 mmol, 164 mg), (e) the aqueous solution (25 mL) of [PTA-Et]I (0.50 mmol, 157 mg), (f) the aqueous solution (25 mL) of [NBu4]I (0.50 mmol, 185 mg), (g) the reaction solution for the synthesis of 1 (see below), (h) the reaction solution for the synthesis of 2 (see below), and (i) the reaction solution for the synthesis of 3 (method C, see below), contained in a Schlenk flask connected with a reflux condenser, were refluxed for 5-20 h (or, in a separate experiment, stored in air at 25 °C for several weeks) with continuous stirring. The reaction referred to in (a) was also performed (i) under dinitrogen atmosphere, (ii) in the dark, (iii) in the presence of an excess of KI or I2, and (iv) in the presence of radical traps such as CBrCl3 (1.50 mmol) or 2,6-di-tert-butyl4-methylphenol (BHT) (1.00 mmol). During the experiments listed in (a-e) the reaction system was periodically flushed with air (optional) and the reaction gases were slowly bubbled through CDCl3 (optional), which was then analyzed by 1H NMR, 13C{1H} NMR, and GC techniques, aiming at the identification of volatile products resulting from the C-C or C-N bond cleavage reactions. The reaction mixture (liquid phase) was cooled (optional) and evaporated in vacuo. The obtained residue was then analyzed by 1 H, 13C{1H}, and 31P{1H} NMR spectroscopies. Synthesis of [Cu2(µ-I)3(PTA-Me)]n (1). To a diluted aqueous solution (100 mL) of Cu(NO3)2 · 2.5H2O (0.10 mmol, 23 mg) and HNO3 (0.10 mmol) (the acid was added to avoid the spontaneous hydrolysis of the metal salt) was added a diluted aqueous solution (50 mL) of [PTA-Bu]I (0.20 mmol, 68 mg) at room temperature. The resulting colorless solution was passed through a filter paper, and the filtrate was left in a beaker to evaporate in air at room temperature, affording colorless X-ray quality crystals in 3 weeks. They were isolated by filtration and dried in air to give 1 in ca. 42% yield (29 mg), based on copper(II) nitrate. Compound 1 is soluble in DMSO but insoluble in water and common organic solvents. 1H NMR (DMSO-d6) δ/ppm: 2.72 (s, 3H, N+CH3), PTA core: 3.79 and 3.97 (J(HAHB) ) 12.9 Hz, 3J(HA-P) ) 13.2 Hz, (22) (a) Daigle, D. J., Jr.; Vail, S. L. J. Heterocycl. Chem. 1974, 11, 407. (b) Daigle, D. J. Inorg. Synth. 1998, 32, 40. (23) Pruchnik, F. P.; Smolen´ski, P. Appl. Organomet. Chem. 1999, 13, 829.

3

Metal-free C(sp3)-C(sp3) Bond CleaVage

Organometallics, Vol. 28, No. 6, 2009 1687

and 4.97 (J(HAHB) ) 12 Hz, 8H, NCHAHBN+). 31P{1H} NMR (DMSO-d6) δ/ppm: -81.9 (s br). 31P{1H} NMR (D2O) δ/ppm: -82.9 (s br), [for comparison, 31P{1H} NMR of free [PTA-Me]I ligand, δ/ppm: -86.0 (s, in DMSO-d6) or -85.0 (s, in D2O)]. 13 C{1H} NMR (DMSO-d6) δ/ppm: 46.2 (s, PCH2N), 48.6 (s, N+CH3), 55.3 (s, PCH2N+), 68.5 (s, NCH2N), 79.4 (s, NCH2N+). IR (KBr, cm-1): 2974w, 2940w, 1449s, 1304s, 1290m, 1243s, 1118s, 1091s, 1024s, 982m, 928s, 894m, 870w, 812s, 769s, 745s, 648w, 561s, 464w, and 443w. Anal. Calcd for C14H30Cu2I4N6P2 (Mr ) 979.1 g/mol): C, 17.17; N, 8.58; H, 3.09. Found: C, 17.57; N, 9.04; H, 3.46. Method C. A suspension of 2 (0.10 mmol, 100 mg) in water (50 mL) was refluxed in air for ca. 5 h. The resulting mixture was then filtered off, washed with H2O, and dried in vacuo, giving 3 as a white powder, which has been found to contain some unidentified impurities. X-ray Crystal Structure of 1. The X-ray diffraction data for 1 were collected with a Nonius Kappa CCD diffractometer using Mo KR radiation. Crystals were mounted in inert oil within the cold gas stream of the diffractometer. The DENZOSCALEPACK program package24 was used for cell refinements and data reductions. The structures were solved by direct methods using the SHELXS-97 program.25 Structural refinements were carried out with the SHELXL-97 program.25 The N2 and P1 atoms were positionally disordered over two alternative sites. Both sites were shared by the N and P atoms with occupancies 0.76 and 0.24. The atoms sharing the same site were refined with equal coordinates and anisotropic displacement parameters. H atoms were positioned geometrically and constrained to ride on their parent atoms, with C-H ) 0.98-0.99 Å, and Uiso ) (1.2-1.5)Ueq (parent atom). The highest peak and the deepest hole are located at 0.83 and 0.76 Å from the Cu2 atom, respectively. Crystal data of 1: C7H15Cu2I3N3P, M ) 679.97, monoclinic, space group P21/c, a ) 7.2901(2) Å, b ) 24.1108(6) Å, c ) 8.9157(2) Å, R ) 90°, β ) 109.664(2)°, γ )

90°, V ) 1475.72(7) Å3, T ) 120(2) K, Z ) 4, Dcalcd ) 3.061 Mg m-3, µ ) 9.245 mm-1, F(000) ) 1240, θ ) 2.96-27.00, no. of reflections ) 28591 reflections, no. of reflections with I > 2σ(I) ) 3223 (Rint ) 0.0499), R1 ) 0.0460, wR2 ) 0.0860. X-ray Crystal Structure of 3. The X-ray diffraction data for 3 were collected using a Bruker AXS-KAPPA APEX II diffractometer with graphite-monochromated Mo KR radiation. Data were collected at 150 K using omega scans of 0.5° per frame, and a full sphere of data was obtained. Cell parameters were retrieved using Bruker SMART software and refined using Bruker SAINT on all the observed reflections. Absorption corrections were applied using SADABS. Structure was solved by direct methods using the SHELXS-97 package25 and refined with SHELXL-9725 with the WinGX graphical user interface.26 All H atoms were inserted in calculated positions. Crystal data of 3: C14H30Cu2I4N6P2, M ) 979.08, orthorhombic, space group P212121, a ) 10.7336(6) Å, b ) 13.5973(9) Å, c ) 18.1057(11) Å, R ) β ) γ ) 90°, V ) 2642.5(3) Å3, T ) 150(2) K, Z ) 4, Dcalcd ) 2.461 Mg m-3, µ ) 6.418 mm-1, F(000) ) 1824, θ ) 2.94-25.03; no. of reflections ) 2509, no. of reflections with I > 2σ(I) ) 2209 (Rint ) 0.0000); R1 ) 0.0327, wR2 ) 0.0614.

(24) Otwinowski, Z.; Minor, W. Methods in Enzymology, Vol. 276, Macromolecular Crystallography; Carter, C. W., Jr.; Sweet, R. M., Eds.; Academic Press: New York; 1997; Part A, pp 307-326. (25) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112.

OM801026A

Acknowledgment. This work was supported by the FCT, through its POCI 2010 (FEDER funded) and “Science 2007” programs. We thank Dr. M. L. Kuznetsov for fruitful discussions. Supporting Information Available: Synthesis of [PTA-Bu]I and derived compounds, mass spectroscopy data, additional structural representations (Figures S1, S2), bonding parameters (Table S1), and CIF files for 1 and 3, and selected supporting NMR spectra (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org.

(26) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837.