Syntheses and Structures of Digold Complexes of Macrobicyclic

Nov 28, 2011 - Dibridgehead Diphosphines That Can Turn Themselves Inside Out. Michael Stollenz, Nattamai Bhuvanesh, Joseph H. Reibenspies, and John ...
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Syntheses and Structures of Digold Complexes of Macrobicyclic Dibridgehead Diphosphines That Can Turn Themselves Inside Out Michael Stollenz, Nattamai Bhuvanesh, Joseph H. Reibenspies, and John A. Gladysz* Department of Chemistry, Texas A&M University, P.O. Box 30012, College Station, Texas 77842-3012, United States S Supporting Information *

ABSTRACT: When isomers of the dibridgehead diphosphine P((CH2)14)3P (2) are treated with Me2SAuCl, 2·(AuCl)2 is obtained in high yields. With in,in-/out,out-2 (97:3 equilibrium mixture), only a single isomer can be detected by lowtemperature NMR, which is assigned as out,out-2·(AuCl)2 on the basis of the crystal structure. With in,out-2, in,out2·(AuCl)2 is obtained, as confirmed by a crystal structure. Both lattices show hydrogen bonding between the AuCl moieties of one molecule and the PCH2 hydrogen atoms of a neighboring molecule. This requires access to the backside of the P−Au−Cl linkages, distorting the diphosphine cages. When out,out-2·(AuCl)2 is treated with CH3Li, out,out2·(AuCH3)2 can be isolated (91%). Analogous hydrogen bonding is impossible, and this molecule crystallizes with a much more symmetrical cage, the center of which is occupied by a methylcyclopentane solvate.

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lthough innumerable types of diphosphine ligands have been employed in coordination or organometallic chemistry, bicyclic dibridgehead diphosphines remain essentially unexplored.1 We recently developed the first synthesis of aliphatic macrobicyclic dibridgehead diphosphines. 2,3 This involved the demetalation of the monoplatinum chelate complex 1 (Scheme 1, bottom right), which was in turn prepared via a 3-fold intramolecular ring-closing metathesis/ hydrogenation sequence.4 As shown in Scheme 1 (top), the resulting diphosphine 2 is capable of turning itself inside out,2b a topological process that has been termed homeomorphic isomerization5 but has been rigorously characterized only for a few molecules.6,7 This involves pulling one methylene chain through the macrocycle defined by the other two. This process inverts, in a pairwise manner, the in/out sense of the bridgehead atoms,7 a transformation that would otherwise require pyramidal inversion. The in,in and out,out isomers rapidly equilibrate in toluene at −80 °C (Keq = 97:3; ΔG⧧193 K = 11.5 (in,in → out,out) or 10.4 kcal/mol (out,out → in,in)), as assayed by 31P NMR.2b The in,out isomer, available by thermal epimerization (150 °C; ΔG⧧423 K = 33.8 kcal/mol), undergoes a degenerate homeomorphic isomerization. Low-temperature 31P NMR spectra illustrated previously2b exhibit two equal signals that coalesce upon warming, allowing the barrier to be determined (ΔG⧧200 K = 8.5 kcal/mol). Phenomena that exchange externally and internally directed functionality are of potential use in transport, sequesterization, and delivery processes. Hence, we have been interested in (1) exploring the coordination chemistry of 2 with metal fragments for which the steric and electronic properties can easily be varied, (2) establishing the availability of bimetallic complexes, © 2011 American Chemical Society

and (3) probing the structures and dynamic properties of the resulting adducts. In this communication, we report the syntheses and crystal structures of a series of digold(I) derivatives, including fascinating hydrogen-bonding motifs that control diphosphine cage conformations in the solid state. Thus, in,in-/out,out-2 and the commercial gold(I) chloride source Me2SAuCl (2 equiv) were combined in THF under subdued or red light (Scheme 1, path A). Workup afforded a white powder that was moderately light sensitive, turning light purple. This behavior proved common to all of the digold complexes below. Microanalysis and mass spectrometry data supplied in the Supporting Information agreed with the formulation 2·(AuCl) 2 (83%). A UV/visible spectrum (CH2Cl2) showed the lowest energy absorption to be at 235 nm, the weak tail of which was no longer detectable at 290 nm. 8 The 31P NMR spectrum showed a signal downfield of that of 1 (δ 20.9 vs −30.1 ppm). When spectra were recorded at −90 °C in CD2Cl2, no decoalesence or line-broadening phenomena were observed. Although this result remains consistent with a rapidly equilibrating mixture of isomers, it was considered very unlikely that both AuCl moieties could be accommodated in an in,in conformation. Hence, this was taken as evidence for the exclusive formation of out,out-2·(AuCl)2. The isomeric species derived from in,out-2 was sought. For this purpose, it proved expedient to repeat the preceding reaction with a 54:46 mixture of in,in-/out,out-2 and in,out-2, prepared by (incomplete) thermal epimerization (Scheme 1, Received: October 3, 2011 Published: November 28, 2011 6510

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Scheme 1. Digold Complexes of the Macrobicyclic Dibridgehead Phosphine 2

path B). A chromatographic workup gave out,out-2·(AuCl)2 (48%) and in,out-2·(AuCl)2 (42%). The 1H and 13C NMR spectra of in,out-1·(AuCl)2 were similar to those of out,out2·(AuCl)2. Only one 31P signal was observed at room temperature (22.6 ppm), although the phosphorus nuclei are not symmetry equivalent. When 31P NMR spectra were recorded between 0 and −90 °C in CD2Cl2, the signal broadened and additional features appeared.9 This behavior was not first order, and given the possible complication of additional equilibria noted below, further investigation has been deferred. The crystal structures of out,out-2·(AuCl)2 and in,out2·(AuCl)2 were determined as described in the Supporting Information. The molecular structures, depicted in Figures 1 and 2, were in accord with the assigned stereochemistries. However, several additional features of interest were apparent. The structure of out,out-2·(AuCl)2 exhibited a distorted diphosphine cage with a phosphorus−phosphorus distance of 7.02 Å and distinctly nonlinear Au−P−P−Au vector, as reflected by an average Au−P−P angle of 143°. The closest

Figure 1. Crystal structure of out,out-2·(AuCl)2: (top) molecular structure with anisotropic thermal ellipsoids (50% probability level); (bottom) intermolecular hydrogen bonds present in the lattice.

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forms one such bond (Cl···C = 3.70 Å; Cl···H = 2.79 Å; Cl···H−C = 154°). These interactions require a distorted bicyclic cage that allows each chlorine atom to approach the backside of a phosphorus atom. The analogous hydrogen atoms in phosphonium salts have appreciable acidities,11 as reflected by the generation of ylides for Wittig reactions. Accordingly, related hydrogen bonds,12 including other P−C−H···Cl−Au systems involving aliphatic phosphine ligands, have been documented crystallographically.13 Interestingly, in,out-2·(AuCl)2 also exhibited a distorted cage and a similar intermolecular hydrogen bonding motif (Figure 2). The phosphorus−phosphorus distance is comparable (7.16 Å), and the Au−P−P angle involving the out gold atom is 133°. Each chlorine atom now participates in a single hydrogen bond (Cl···C = 3.61−3.65 Å, Cl···H = 2.68−2.76 Å, Cl···H−C = 144−167°). Again, there is no sign of aurophilic interactions (closest gold−gold distance 4.49 Å). If the distortions in the cages in the preceding structures are induced by hydrogen bonding, they should be reduced when the chloride ligand is replaced with an alkyl or aryl group. Accordingly, out,out-2·(AuCl)2 and CH3Li (2.3 equiv) were combined at −78 °C. As shown in Scheme 1 and in accord with much literature precedent,14 workup gave the bis(methylgold) complex out,out-2·(AuCH3)2 as an analytically pure white solid in 91% yield. The 1H and 13C NMR spectra showed new phosphorus-coupled methyl signals but were otherwise very similar to those of out,out-2·(AuCl)2. The 31P NMR signal was considerably downfield of those of 2·(AuCl)2 (36.9 ppm). Crystallization from methylcyclopentane gave the monosolvate out,out-2·(AuCH3)2·(C5H9CH3). Two independent molecules were found in the unit cell, differing in the gauche/anti conformational patterns of the methylene chains. One is depicted in Figure 3, and the other is given in the

Figure 2. Crystal structure of in,out-2·(AuCl)2: (top) molecular structure with anisotropic thermal ellipsoids (50% probability level); (bottom) intermolecular hydrogen bonds present in the lattice.

Figure 3. Molecular structure of out,out-2·(AuCH3)2·(C5H9CH3): (left) thermal ellipsoid plot for one of the two independent molecules in the unit cell (50% probability level); (right) space filling representation.

gold−gold distance in the lattice (4.98 Å) was far greater than those associated with aurophilic interactions (2.50−3.50 Å). 10 The origin of this distortion becomes apparent in the packing diagram excerpt in Figure 1 (bottom), which reveals a stacking motif. The chlorine atom Cl2 (see central molecule) forms hydrogen bonds with two PCH2 hydrogen atoms of a neighboring molecule (Cl···C = 3.56−3.64 Å; Cl···H = 2.67− 2.68 Å; Cl···H−C = 150−163°). The chlorine atom Cl1 in turn

Supporting Information. The methylcyclopentane molecule occupied the middle of the diphosphine cage in each, filling most of the space (Figure 3, right). As expected, there were no hydrogen-bonding interactions and the cages were more symmetrical, with longer phosphorus−phosphorus distances (12.00−12.12 Å) and average Au−P−P angles closer to 180° 6512

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(169°). To the best of our knowledge, only three other methylgold(I) complexes of phosphines have been structurally characterized.14 Interestingly, in each case the structure of the corresponding chlorogold complex has also been determined.14c,15 We observe, as in the other pairs of compounds, that the P−AuCl bonds, which are associated with the stronger gold Lewis acid, are shorter than the P−AuCH3 bonds (2.196(6)− 2.225(4) vs 2.288(2)−2.304(2) Å). The data for the digold complexes can be compared to those reported for the bis(borane) adducts in,in-/out,out-2·(BH3)2 and in,out-2·(BH3)2 described earlier.2b The 31P NMR chemical shifts (15.8−14.7 ppm) are similar to those of the chlorogold complexes, and no decoalesence phenomena are observed at −90 °C in CD2Cl2. Interestingly, the former also crystallized as an out/out methylcyclopentane solvate, out,out2·(BH3)2·(C5H9CH3). It exhibited an approximately linear B−P−P−B vector, with a phosphorus−phosphorus distance of 13.21 Å and an average P−P−B angle of 166°. Finally, for all of the digold complexes, the most intense ion in the MALDI+ and ESI+ mass spectra was a monogold species of the formula [2·Au]+, consistent with the cage structure shown in Scheme 2.

One other potential conformational equilibrium merits comment at this time. In the absence of any distortion in the diphosphine cage of in,out-2·(AuCl)2, it would be more difficult to accommodate an in AuCl group. If the steric bulk of the substituent on gold is sufficiently increased, an in group should become impossible. However, an alternative conformation may be accessible. When only half of one of the methylene chains is pulled through the macrocycle defined by the other two, the in/ out sense of only one phosphorus atom is inverted. As shown in Scheme 2, this would afford a “crossed chain” out,out isomer (3). Can such a species become the ground state, or at minimum a crystallographically observable isomer? Such issues will be probed in future reports, together with further development of the coordination chemistry of 2, the rich nature of which has been established by this initial study.

ASSOCIATED CONTENT

S Supporting Information *

Text, a figure, tables, and CIF files giving experimental procedures and crystallographic data. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

(1) (a) Hinton, R. C.; Mann, F. G. J. Chem. Soc. 1959, 2835−2843. (b) Davis, M.; Mann, F. G. Chem. Ind. (London) 1962, 1539−1540. (2) (a) Nawara, A. J.; Shima, T.; Hampel, F.; Gladysz, J. A. J. Am. Chem. Soc. 2006, 128, 4962−4963. (b) Stollenz, M.; Barbasiewicz, M.; Nawara-Hultzsch, A. J.; Fiedler, T.; Laddusaw, R. M.; Bhuvanesh, N.; Gladysz, J. A. Angew. Chem., Int. Ed. 2011, 50, 6647−6651; Angew. Chem. 2011, 123, 6777−6781. (3) For macrobicyclic dibridgehead diphosphorus compounds based upon triarylphosphines and phosphites, see: (a) Däbritz, F.; Jäger, A.; Bauer, I. Eur. J. Org. Chem. 2008, 5571−5576. (b) Däbritz, F.; Theumer, G.; Gruner, M.; Bauer, I. Tetrahedron 2009, 65, 2995−3002. (c) Review of earlier papers in this series: Bauer, I.; Habicher, W. D. Collect. Czech. Chem. Commun. 2004, 69, 1195−1230. (4) This template strategy has enabled the synthesis of many monometallic chelate complexes of dibridgehead diphosphines, but it does not deliver the free diphosphines per se. For other types of complexes, see: (a) Shima, T.; Hampel, F.; Gladysz, J. A. Angew. Chem., Int. Ed. 2004, 43, 5537−5540; Angew. Chem. 2004, 116, 5653− 5656. (b) Wang, L.; Hampel, F.; Gladysz, J. A. Angew. Chem., Int. Ed. 2006, 45, 4372−4375; Angew. Chem. 2006, 118, 4479−4482. (c) Wang, L.; Shima, T.; Hampel, F.; Gladysz, J. A. Chem. Commun. 2006, 4075−4077. (d) Hess, G. D.; Hampel, F.; Gladysz, J. A. Organometallics 2007, 26, 5129−5131. (e) Skopek, K.; Barbasiewicz, M.; Hampel, F.; Gladysz, J. A. Inorg. Chem. 2008, 47, 3474−3476. (5) Park, C. H.; Simmons, H. E. J. Am. Chem. Soc. 1968, 90, 2429− 2431. (6) (a) Haines, A. H.; Karntiang, P. J. Chem. Soc., Perkin Trans. 1 1979, 2577−2587. (b) Wareham, R. S.; Kilburn, J. D.; Turner, D. L.; Rees, N. H.; Holmes, D. S. Angew. Chem., Int. Ed. Engl. 1995, 34, 2660−2662; Angew. Chem. 1995, 107, 2902−2904. (c) Saunders, M.; Krause, N. J. Am. Chem. Soc. 1990, 112, 1791−1795. (d) Alder, R. W.; Heilbronner, E.; Honegger, E.; McEwan, A. B.; Moss, R. E.; Olefirowicz, E.; Petillo, P. A.; Sessions, R. B.; Weisman, G. R.; White, J. M.; Yang, Z.-Z. J. Am. Chem. Soc. 1993, 115, 6580−6591. (7) Review of in/out isomerism: Alder, R. W.; East, S. P. Chem. Rev. 1996, 96, 2097−2111. (8) UV/visible data for Et3PAuCl (nm, CH3CN): 235, 229, 215, 204, 196. Savas, M. M.; Mason, W. R. Inorg. Chem. 1987, 26, 301−307. (9) In some runs two very closely spaced peaks appeared (ca. 50:50), and in others shoulders became evident. All samples remained homogeneous. Species of the type 3 (Scheme 2) may participate in any equilibria, and the rational generation of such “crossed chain” systems is under study. (10) Schmidbaur, H.; Schier, A. Chem. Soc. Rev. 2012, in press (DOI: 10.1039/c1cs15182g). (11) Representative pK a (DMSO) values include 22 for [Ph3PCH3]+Cl− and 17.4−17.6 for [Ph3PCH2C6H5]+X−: (a) Zhang, X.-M.; Bordwell, F. G. J. Am. Chem. Soc. 1994, 116, 968−972. (b) Cheng, J.-P.; Liu, B.; Zhao, Y.; Sun, Y.; Zhang, X.-M.; Lu, Y. J. Org. Chem. 1999, 64, 604−610. (12) Jones, P. G.; Ahrens, B. Chem. Commun. 1998, 2307−2308, and references therein. (13) For related systems with C−H···Cl−Au linkages, see: (a) Freytag, M.; Jones, P. G. Chem. Commun. 2000, 277−278. (b) Bardaji, M.; Jones, P. G.; Laguna, A. J. Chem. Soc., Dalton Trans. 2002, 3624−3629. (14) (a) Gavens, P. D.; Guy, J. J.; Mays, M. J.; Sheldrick, G. M. Acta Crystallogr. 1977, B33, 137−139. (b) Hong, X.; Cheung, K.-K.; Guo, C.-X.; Che, C.-M. J. Chem. Soc., Dalton Trans. 1994, 1867−1871. (c) Liu, L.-K.; Luh, L.-S.; Wen, Y.-S.; Eke, U. B.; Mesubi, M. A. Organometallics 1995, 14, 4474−4482. (15) (a) Baenziger, N. C.; Bennett, W. E.; Soboroff, D. M. Acta Crystallogr. 1976, B32, 962−963. (b) Schmidbaur, H.; Wohlleben, A.; Wagner, F.; Orama, O.; Huttner, G. Chem. Ber. 1977, 110, 1748− 1754.

Scheme 2. Additional Structures and Equilibria

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AUTHOR INFORMATION

Corresponding Author *Tel: 979-845-1399. E-mail: [email protected].

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ACKNOWLEDGMENTS We thank the National Science Foundation (No. CHE0719267) for support. 6513

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