Synthesis, Structures, and Dynamic Behavior of Intramolecularly Base

Dec 11, 2009 - Keith Izod,* John Stewart, William Clegg, and Ross W. Harrington. Main Group Chemistry Laboratories, School of Chemistry, Bedson Buildi...
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Organometallics 2010, 29, 108–116 DOI: 10.1021/om900854m

Synthesis, Structures, and Dynamic Behavior of Intramolecularly Base-Stabilized Diphosphatetrylenes Containing a Five-Membered Chelate Ring Keith Izod,* John Stewart, William Clegg, and Ross W. Harrington Main Group Chemistry Laboratories, School of Chemistry, Bedson Building, Newcastle University, Newcastle upon Tyne, NE1 7RU, U.K. Received October 2, 2009

The reactions between GeCl2(1,4-dioxane) and 2 equiv of [{(Me3Si)2CH}(C6H4-2-NMe2)P]K and between SnCl2 and 2 equiv of [{(Me3Si)2CH}(C6H4-2-NMe2)P]Li give the corresponding diphosphatetrylenes [{(Me3Si)2CH}(C6H4-2-NMe2)P]2E [E = Ge (5), Sn (6)] as orange crystalline solids in good yield. X-ray crystallography reveals that 5 and 6 adopt similar, but not identical structures in the solid state; each crystallizes as a discrete monomeric diphosphatetrylene, with one chelating phosphide ligand and one ligand bound solely through its P atom. The two compounds differ in the orientation of the noncoordinated nitrogen atom: in 5 the Ge 3 3 3 N distance (>5 A˚) is clearly too great for bonding, whereas in 6 the Sn 3 3 3 N distance is just 3.306(3) A˚, implying a weak Sn-N bonding interaction. In solution both 5 and 6 exhibit dynamic behavior. However, whereas 5 undergoes both chelating-terminal ligand exchange and epimerization via inversion at phosphorus, compound 6 adopts a pseudo-trigonal-bipyramidal structure in solution at low temperatures. DFT calculations suggest that such a species, the putative intermediate in the associative mechanism for chelating-terminal ligand exchange, is a low-energy minimum on the potential energy surface.

Introduction The synthesis of stable low-oxidation-state group 14 compounds (diorganotetrylenes), formal analogues of carbenes, remains a challenging and fascinating area of main group chemistry. By far the most progress in this area has been in the synthesis of diaminotetrylenes (R2N)2E [E = Si, Ge, Sn, Pb] and their cyclic counterparts, analogues of the Arduengo-type N-heterocyclic carbenes.1,2 Somewhat surprisingly, far less is known about the corresponding diphosphatetrylenes (R2P)2E, and few such compounds have been isolated and structurally characterized.3-8 This may, at least in part, be attributed to the large barrier toward planarization of trisubstituted phosphorus centers, which significantly hampers the stabilization of diphosphatetrylenes through P-E pπ-pπ interactions.9 Among the known diphosphatetrylenes both monomeric species such as [{(Tripp)2FSi}(iPr3Si)P]2E [E = Ge (1), Sn (2), Pb; Tripp = 2,4,6-iPr3C6H2]3 and dimeric species such as *Corresponding author. E-mail: [email protected]. (1) For recent reviews of heavier tetrylene chemistry see: (a) Barrau, J.; Rima, G. Coord. Chem. Rev. 1998, 178-180, 593. (b) Tokitoh, N.; Okazaki, R. Coord. Chem. Rev. 2000, 210, 251. (c) Kira, M. J. Organomet. Chem. 2004, 689, 4475. (d) Weidenbruch, M. Eur. J. Inorg. Chem. 1999, 373. (e) Klinkhammer, K. W. In Chemistry of Organic Germanium, Tin and Lead Compounds; Rappoport, Z., Ed.; Wiley: New York, 2002; Vol. 2, pp 283-357. (f) Veith, M. Angew. Chem., Int. Ed. Engl. 1987, 26, 1. (g) Hill, N. J.; West, R. J. Organomet. Chem. 2004, 689, 4165. (h) Gehrhus, B.; Lappert, M. F. J. Organomet. Chem. 2001, 617, 209. (i) K€uhl, O. Coord. Chem. Rev. 2004, 248, 411. (j) Zemlyanskii, N. N.; Borisova, I. V.; Nechaev, M. S.; Khrustalev, V. N.; Lunin, V. V.; Antipin, M. Yu.; Ustynyuk, Yu. A. Russ. Chem. Bull. Int. Ed. 2004, 53, 980. (k) Zabula, A. V.; Hahn, F. E. Eur. J. Inorg. Chem. 2008, 5165. pubs.acs.org/Organometallics

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{(tBu2P)2Pb}2,4 {((Me3Si)2P)2Pb}2,5 and {(iPr2P)2Ge}26 have been isolated. In addition, several ate complexes and (2) For selected references to diamidotetrylenes see: (a) Driess, M.; Yao, S.; Brym, M.; van Wullen, C.; Lentz, D. J. Am. Chem. Soc. 2006, 128, 9628. (b) Driess, M.; Yao, S.; Brym, M.; van Wullen, C. Angew. Chem., Int. Ed. 2006, 45, 4349. (c) Hill, N. J.; Moser, D. F.; Guzei, I. A.; West, R. Organometallics 2005, 24, 3346. (d) Fjeldberg, T.; Hope, H.; Lappert, M. F.; Power, P. P.; Thorne, A. J. J. Chem. Soc., Chem. Commun. 1983, 639. (e) Olmstead, M. M.; Power, P. P. Inorg. Chem. 1984, 23, 413. (f) Tang, Y.; Felix, A. M.; Zakharov, L. N.; Rheingold, A. L.; Kemp, R. A. Inorg. Chem. 2004, 43, 7239. (g) Avent, A. G.; Drost, C.; Gehrus, B.; Hitchcock, P. B.; Lappert, M. F. Z. Anorg. Allg. Chem. 2004, 630, 2090. (h) Gans-Eichler, T.; Gudat, D.; Nieger, M. Angew. Chem., Int. Ed. 2002, 41, 1888. (i) Chorley, R. W.; Hitchcock, P. B.; Lappert, M. F.; Leung, W.-P.; Power, P. P.; Olmstead, M. M. Inorg. Chim. Acta 1992, 198, 203. (j) Braunschweig, H.; Hitchcock, P. B.; Lappert, M. F.; Pierssens, L. J.-M. Angew. Chem., Int. Ed. Engl. 1994, 33, 1156. (k) Lappert, M. F.; Slade, M. J.; Atwood, J. L.; Zaworotko, M. J. J. Chem. Soc., Chem. Commun. 1980, 621. (l) Bazinet, P.; Yap, G. P. A.; Richeson, D. S. J. Am. Chem. Soc. 2001, 123, 11162. (m) Mansell, S. M.; Russell, C. A.; Wass, D. F. Inorg. Chem. 2008, 47, 11367. (n) Zabula, A. V.; Hahn, F. E.; Pape, T.; Hepp, A. Organometallics 2007, 26, 1972. (o) Hahn, F. E.; Wittenbecher, L.; LeVan, D.; Zabula, A. V. Inorg. Chem. 2007, 46, 7662. (p) Zabula, A. V.; Pape, T.; Hepp, A.; Hahn, F. E. Organometallics 2008, 27, 2756. (q) Hahn, F. E.; Zabula, A. V.; Pape, T.; Hepp, A.; Tonner, R.; Haunschild, R.; Frenking, G. Chem.;Eur. J. 2008, 14, 10716. (r) Hahn, F. E.; Heitmann, D.; Pape, T. Eur. J. Inorg. Chem. 2008, 1039. (s) Charmant, J. H.; Haddow, M. F.; Hahn, F. E.; Heitmann, D.; Fr€olich, R.; Mansell, S. M.; Russel, C. A.; Wass, D. F. Dalton Trans. 2008, 6055. (3) Driess, M.; Janoschek, R.; Pritzkow, H.; Rell, S.; Winkler, U. Angew. Chem., Int. Ed. 1995, 34, 1614. (4) Cowley, A. H.; Giolando, D. M.; Jones, R. A.; Nunn, C. M.; Power, J. M. Polyhedron 1988, 7, 1909. (5) Goel, S. C.; Chiang, M. Y.; Rauscher, D. J.; Buhro, W. E. J. Am. Chem. Soc. 1993, 115, 160. (6) Druckenbrodt, C.; du Mont, W.-W.; Ruthe, F.; Jones, P. G. Z. Anorg. Allg. Chem. 1998, 624, 590. (7) Yao, S.; Brym, M.; Merz, K.; Driess, M. Organometallics 2008, 27, 3601. r 2009 American Chemical Society

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mixed group 2/group 14 phosphide and phosphinidene complexes have been reported.10,11 In related carbene chemistry Bertrand and co-workers have isolated a range of “push-pull” phosphinosilyl- and phosphinophosphonio-carbenes [(R2P)(R3Si)C: and [(R2P)(R3P)C:]þ, respectively] and moderately stable aryl- and alkyl-substituted phosphinocarbenes [(R2P)ArC: (Ar = e.g., 2,4,6-Me3C6H2)].12 With particular relevance to this report, Bertrand and co-workers have also recently shown that, by careful selection of the substituents, heterocycle composition, and synthetic route, a stable P-heterocyclic carbene is accessible, in which the phosphorus atoms adjacent to the carbene center approach planarity.13 We recently reported the synthesis, solid-state structures, and dynamic behavior of the intramolecularly base-stabilized diphosphatetrylenes [{(Me3Si)2CH}(C6H4-2-CH2NMe2)P]2E [E = Ge (3),14 Sn (4)15]. These compounds crystallize as discrete monomers, with one phosphide ligand binding the group 14 element center through its phosphorus and nitrogen atoms, to give a six-membered chelate ring, and one ligand binding through the phosphorus atom only; the second amino group in each case has no close contacts with the group 14 element center. Compounds 3 and 4 are highly dynamic in solution, although their dynamic behavior differs due to the increased polarity of the Sn-P bond in 4 compared to the more covalent Ge-P bond in 3. For compound 3 variable-temperature 1H and 31P{1H} NMR spectroscopy indicates that, in addition to dynamic exchange between the chelating and terminal phosphide ligands, at higher temperatures there is a dynamic equilibrium between diastereomers, most likely due to inversion at the phosphorus center(s). For compound 4 variable-temperature 1H and 31 P{1H} NMR studies indicate that inversion at phosphorus is more facile than in 3, resulting in rapid exchange between diastereomers, such that only exchange between the chelating and terminal phosphide ligands may be observed. The negative entropy of activation for this latter process is (8) Rivard, E.; Sutton, A. D.; Fettinger, J. C.; Power, P. P. Inorg. Chim. Acta 2007, 360, 1278. (9) Kapp, J.; Schade, C.; El-Nahasa, A. M.; Schleyer, P. von R. Angew. Chem., Int. Ed. Engl. 1996, 35, 2236. (10) Arif, A. M.; Cowley, A. H.; Jones, R. A.; Power, J. M. J. Chem. Soc., Chem. Commun. 1986, 1446. (11) For examples see: (a) Westerhausen, M.; Hausen, H.-D.; Schwarz, W. Z. Anorg. Allg. Chem. 1996, 622, 903. (b) Westerhausen, M.; Krofta, M.; Wiberg, N.; N€oth, H.; Pfitzner, A. Z. Naturforsch. B 1998, 53, 1489. (c) Westerhausen, M.; Hausen, H.-D.; Schwarz, W. Z. Anorg. Allg. Chem. 1995, 621, 877. (d) Westerhausen, M.; Krofta, M.; Schneiderbauer, S.; Piotrowski, H. Z. Anorg. Allg. Chem. 2005, 631, 1391. (e) Allan, R. E.; Beswick, M. A.; Cromhout, N. L.; Paver, M. A.; Raithby, P. R.; Trevithick, M.; Wright, D. S. Chem. Commun. 1996, 1501. (f) Garcia, F.; Hahn, J. P.; McPartlin, M.; Pask, C. M.; Rothenberger, A.; Stead, M. L.; Wright, D. S. Organometallics 2006, 25, 3275. (12) (a) Buron, C.; Gornitzka, H.; Romanenko, V.; Bertrand, G. Science 2000, 288, 834. (b) Despagnet, E.; Gornitzka, H.; Rozhenko, A. B.; Schoeller, W. W.; Bourissou, D.; Bertrand, G. Angew. Chem., Int. Ed. 2002, 41, 2835. (c) Despagnet-Ayoub, E.; Sole, S.; Gornitzka, H.; Rozhenko, A. B.; Schoeller, W. W.; Bourissou, D.; Bertrand, G. J. Am. Chem. Soc. 2003, 125, 124. (d) Despagnet, E.; Miqueu, K.; Gornitzka, H.; Dyer, P. W.; Bourissou, D.; Bertrand, G. J. Am. Chem. Soc. 2002, 124, 11834. (e) Lavallo, V.; Mafhouz, J.; Canac, Y.; Donnadieu, B.; Schoeller, W. W.; Bertrand, G. J. Am. Chem. Soc. 2004, 126, 8670. (13) (a) Martin, D.; Baceiredo, A.; Gornitzka, H.; Schoeller, W. W.; Bertrand, G. Angew. Chem., Int. Ed. 2005, 44, 1700. (b) Masuda, J. D.; Martin, D.; Lyon-Saunier, C.; Baceiredo, A.; Gornitzka, H.; Donnadieu, B.; Bertrand, G. Chem. Asian J. 2007, 2, 178. (14) Izod, K.; McFarlane, W.; Allen, B.; Clegg, W.; Harrington, R. W. Organometallics 2005, 24, 2157. (15) Izod, K.; Stewart, J.; Clark, E. R.; McFarlane, W.; Allen, B.; Clegg, W.; Harrington, R. W. Organometallics 2009, 28, 3327.

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consistent with exchange proceeding through an associative pathway, via a pseudo-trigonal-bipyramidal intermediate, and this proposition is supported by DFT calculations.

Scheme 1

We now report the synthesis of the related diphosphatetrylenes [{(Me3Si)2CH}(C6H4-2-NMe2)P]2E [E = Ge (5), Sn (6)], in which the phosphide ligands have the potential to form five-membered chelate rings, and show that the contraction in chelate ring size has a significant effect on both the solid state structures and solution behavior of these species.

Results and Discussion Synthesis and Structural Characterization. The reaction between GeCl2(1,4-dioxane) and 2 equiv of the potassium phosphide [{(Me3Si)2CH}(C6H4-2-NMe2)P]K16 in THF cleanly gives the diphosphagermylene [{(Me3Si)2CH}(C6H4-2-NMe2)P]2Ge (5), after a straightforward workup, as orange crystals in good yield (Scheme 1). The tin homologue [{(Me3Si)2CH}(C6H4-2-NMe2)P]2Sn (6) may be obtained in a similar fashion as orange crystals from the reaction between SnCl2 and 2 equiv of the lithium phosphide [{(Me3Si)2CH}(C6H4-2-NMe2)P]Li.16 Compounds 5 and 6 are soluble in common organic solvents, including light petroleum, and may be obtained as single crystals suitable for X-ray crystallography by recrystallization from either cold hexamethyldisiloxane/diethyl ether or cold n-hexane, respectively. Compound 5 is stable toward both heat and light; however, compound 6 decomposes on heating above ca. 50 °C or on exposure to ambient light for several weeks. Compounds 5 and 6 crystallize as discrete monomers with similar, but not identical, structures; the molecular structures of 5 and 6 are shown in Figures 1 and 2, respectively, along with selected bond lengths and angles. In both compounds the group 14 atom is coordinated by the phosphorus and nitrogen atoms of one phosphide ligand, generating a slightly puckered five-membered chelate ring; the P(1)-E-N(1) bite angles for 5 and 6 are 86.01(10)° and 80.73(8)°, respectively, consistent with the greater size of Sn(II) compared to Ge(II). (16) Izod, K.; Stewart, J. C.; Clegg, W.; Harrington, R. W. Dalton Trans. 2007, 257.

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Figure 1. Molecular structure of 5 with 40% probability ellipsoids and with H atoms omitted for clarity. Selected bond lengths (A˚) and angles (deg): Ge-P(1) 2.3894(13), Ge-P(2) 2.3938(13), Ge-N(1) 2.165(4), P(1)-C(1) 1.887(4), P(1)-C(8) 1.849(4), C(1)-Si(1) 1.893(5), C(1)-Si(2) 1.899(5), P(2)-C(16) 1.897(4), P(2)-C(23) 1.854(5), C(16)-Si(3) 1.884(4), C(16)Si(4) 1.896(4), P(1)-Ge-P(2) 85.82(4), P(1)-Ge-N(1) 86.01(10), P(2)-Ge-N(1) 92.40(10).

Figure 2. Molecular structure of 6 with 40% probability ellipsoids and with H atoms omitted for clarity. Selected bond lengths (A˚) and angles (deg): Sn-P(1) 2.6110(11), Sn-P(2) 2.5995(10), Sn-N(1) 2.408(3), Sn...N(2) 3.306(3), P(1)-C(1) 1.906(4), P(1)-C(8) 1.861(4), C(1)-Si(1) 1.904(4), C(1)-Si(2) 1.899(4), P(2)-C(16) 1.889(3), P(2)-C(23) 1.842(4), C(16)Si(3) 1.895(4), C(16)-Si(4) 1.901(4), P(1)-Sn-P(2) 94.84(4), P(1)-Sn-N(1) 80.73(8), P(2)-Sn-N(1) 88.32(8).

The second phosphide ligand in both compounds is primarily bound to the group 14 center through its phosphorus atom. The principal difference between the structures of 5 and 6 lies in the orientation of the amino group in this second, terminal, phosphide ligand. In 5 the amino group of the terminal phosphide ligand is oriented away from the germanium center, and there is no short Ge 3 3 3 N(2) contact, thus conferring a trigonal-pyramidal geometry on the germanium center. In contrast, in 6 the amino group of the terminal phosphide ligand is directed toward the tin center, such that the Sn 3 3 3 N(2) distance is just 3.306(3) A˚; this is significantly longer than the Sn-N(1) distance [2.408(3) A˚], but is substantially less than the sum of the van der Waals radii of tin and nitrogen (3.72 A˚). This suggests that there is a weak

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bonding interaction between these two atoms and results in a geometry at tin that may best be described as lying somewhere between three-coordinate trigonal pyramidal and four-coordinate pseudo-trigonal bipyramidal. The Ge-P(1) and Ge-P(2) distances in 5 [2.3894(13) and 2.3938(13) A˚, respectively] are similar to the Ge-P distances in 3 [2.4023(4) and 2.4114(4) A˚]14 and related compounds; for example, the Ge-P distances in {(iPr2P)2Ge}2 range from 2.3981(11) to 2.4261(11) A˚,6 whereas the Ge-P distance in [CH{(CMe)(2,6-iPr2C6H3N)}2]Ge{P(SiMe3)2} is 2.3912(8) A˚.7 Similarly, the Sn-P(1) and Sn-P(2) distances in 6 [2.5995(10) and 2.6110(11) A˚, respectively] are comparable to the corresponding distances in 4 [2.6407(8) and 2.5906(9) A˚]15 and lie in the range of Sn-P distances in the few previously reported tin(II) phosphides; for comparison, the Sn-P distance in monomeric 2 is 2.567(1) A˚,3 whereas the Sn-P distances in the ate complex (tBu2P)Sn(μ-tBu2P)2Li(THF) are 2.684(4), 2.671(4), and 2.702(3) A˚.10 In both 5 and 6 the phosphorus atoms in the chelating and terminal ligands are distinctly pyramidal [for 5, sum of angles at P(1) = 304.07°, P(2) = 309.90°; for 6, sum of angles at P(1) = 302.82°, P(2) = 316.67°]. Compounds 5 and 6 are chiral at each phosphorus atom and at the group 14 element centers; both compounds crystallize as the ESPchSPtR diastereomer (where Pch and Pt refer to the phosphorus atoms in the chelating and terminal ligands, respectively). Solution Behavior. Compounds 5 and 6 are highly dynamic in solution. Below 60 °C the variable-temperature 1H NMR spectra of 5 in d8-toluene are rather complex, containing many broad and overlapping resonances, which are somewhat difficult to interpret. However, at 60 °C the 1H NMR spectrum of 5 contains a single set of ligand resonances, consistent with both rapid interconversion between diastereomers and rapid exchange between the chelating and terminal ligands. At lower temperatures the 1H NMR spectra suggest the presence of multiple species, one of which significantly predominates. The variable-temperature 31P{1H} NMR spectra of 5 in d8-toluene are more informative: at 60 °C the 31P{1H} spectrum of 5 consists of a reasonably sharp singlet at -40.8 ppm (Figure 3). As the temperature is reduced, this signal broadens and decoalesces, until, at -41 °C, the spectrum consists of two equal intensity, broad signals at -57.0 (A) and -37.3 ppm (B). As the temperature is reduced further, peak A sharpens and resolves into a doublet (JPP = 75.7 Hz), while peak B begins to broaden and decoalesce further until, at -60 °C, the spectrum consists of the doublet A, a broad peak of equal intensity at -38.7 ppm (C), and two minor peaks of equal intensity at -54.3 (D) and -33.3 ppm (E). The low-field peaks decoalesce further as the temperature is reduced until, at -90 °C, the spectrum consists of a doublet at -57.7 ppm (A), a broad singlet at -44.4 ppm (C), and three low-intensity singlets at -55.1 (D), -37.4 (F), and -32.3 ppm (G); at -100 °C peaks A and C are resolved as a pair of equal intensity doublets (JPP = 94.9 Hz), although the minor signals D, F, and G are of only very low intensity at this temperature. A low-temperature 31P EXSY spectrum of 5 in d8-toluene reveals that, at -60 °C, exchange between A and C is rapid, whereas exchange between A and D, between C and D, and between A and E is slower on the NMR time-scale; exchange between D and E is not observed at this temperature. This behavior is consistent with rapid exchange both between diastereomers and between the terminal and chelating

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Figure 4. 1H NMR spectrum of 6 at -80 °C in d8-toluene [*free phosphine {(Me3Si)2CH}(C6H4-2-NMe2)PH, T = solvent].

Figure 3. Variable-temperature d8-toluene.

31

P{1H} NMR spectra of 5 in

ligands within each diastereomer at elevated temperatures. At low temperatures these dynamic processes are frozen out and signals are observed corresponding to the two distinct phosphorus environments in three of the four possible diastereomers of 5, one of which predominates. The dynamic behavior observed for 5 closely resembles that observed for the related diphosphagermylene 3; however, whereas for 3 epimerization appears to be favored over chelating-terminal ligand exchange, for 5 exchange of the terminal and chelating ligands in the major diastereomer appears to be rapid in comparison to diastereomer interconversion. This may be attributed to the formation of a favorable five-membered chelate ring in 5 compared to the six-membered chelate ring in 3. The tin analogue 6 exhibits rather different behavior in solution. Above ambient temperature the 1H NMR spectra of 6 in d8-toluene consist of a single set of ligand signals, consistent with rapid exchange on the NMR time-scale both between diastereomers and between the chelating and terminal phosphide ligands. As the temperature is reduced below ambient, these signals broaden and decoalesce until, at -80 °C, the spectrum contains two equal intensity singlets at 0.29 and 0.44 ppm due to the SiMe3 protons, a complex multiplet at 0.86 ppm due to the methine protons, singlets at 2.07 and 2.70 ppm due to the NMe2 protons, and four multiplets centered at 6.48, 6.86, 7.04, and 7.72 ppm due to the aromatic protons (Figure 4). These signals clearly arise from a compound containing a single ligand environment in which the CH(SiMe3)2 and NMe2 groups are diastereotopic, implying that both nitrogen atoms are coordinated to the tin center at this temperature. The 31P{1H} NMR spectrum of 6 in d8-toluene at 50 °C consists of a slightly broadened singlet at -44.0 ppm,

Figure 5. Variable-temperature 31P{1H} NMR spectra of 6 in d8-toluene [the sharp signal observed at ca. 71 ppm is due to a small amount of the free phosphine {(Me3Si)2CH}PH(C6H42-NMe2)].

exhibiting poorly resolved satellites due to coupling to 117/119Sn (JPSn = 1070 Hz) (Figure 5), again consistent with rapid exchange both between diastereomers and between the chelating and terminal ligands. As the temperature is reduced, this signal broadens and shifts to higher field until, at -20 °C, only a very broad singlet at approximately -51 ppm is observed. As the temperature is reduced further, this signal continues to move upfield but now sharpens until, at -80 °C, the spectrum consists of a lone singlet at -59.1 ppm exhibiting satellites due to coupling to 117/119Sn (JPSn = 1034 Hz). We were unable to observe a signal in the room-temperature 119Sn{1H} NMR spectrum of 6, possibly due to excessive signal broadening due to dynamic exchange at this temperature. However, at 40 °C the spectrum consists of a very broad signal at approximately 712 ppm, on which coupling to 31P is not resolved (Figure 6). At -78 °C this signal has shifted significantly upfield and a broad peak at

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Figure 6.

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Sn{1H} NMR spectra of 6 in d8-toluene at (a) 40 °C and (b) -90 °C. Scheme 2. [R = CH(SiMe3)2]

474 ppm is observed; at -90 °C this signal resolves into a sharp binomial triplet at 455 ppm (JPSn = 1050 Hz). The variable-temperature 1H, 31P{1H}, and 119Sn{1H} spectra of 6 are consistent with the formation of a highly symmetrical species at low temperature in which the two ligands are equivalent. The equivalence of the two NMe2 groups and the diastereotopic nature of the N-methyl groups indicate that the nitrogen atoms are not subject to inversion, and so both amino groups are coordinated to tin in this species. The large upfield shift of the 119Sn signal at low temperatures is also consistent with an increase in coordination number at the tin center. These spectra are therefore consistent with the formation at low temperature of a pseudo-trigonal-bipyramidal species 60 analogous to the intermediate proposed for chelating-terminal ligand exchange via an associative pathway (Scheme 2). DFT Calculations. In order to gain greater insight into the dynamic behavior of 5 and 6, we have carried out DFT calculations on the model complexes [Me(C6H4-2NMe2)P]2E [E = Ge (5a), Sn (6a)]. While replacement of the bulky CH(SiMe3)2 groups in 5 and 6 with significantly smaller Me groups necessarily underestimates the steric compression at the Ge and Sn centers, the central cores of the model compounds are identical with those found in 5 and 6, and so we believe that meaningful conclusions may be drawn from these calculations. For both 5a and 6a minima were located for all four possible diastereomers (Figure 7); minimum 5a3 corresponds to the stereoisomer of 5 observed crystallographically [it should be noted that replacement of the (Me3Si)2CH group in the former with a Me group in the latter results formally in opposite chirality designations for the P stereocenters in 5 and 5a and in 6 and 6a]. The major structural features of 5 are replicated well in 5a3, which contains a three-coordinate, trigonal-pyramidal germanium atom coordinated by the N and P atoms of one phosphide ligand and the P atom of a second phosphide ligand [sum of angles at Ge: 299.43°]. The calculated bond lengths within the GeNP2 core of 5a3 are typically overestimated by approximately 0.01-0.1 A˚. For example, the Ge-Pch, Ge-Pt, and Ge-N bond lengths are 2.397, 2.391, and 2.288 A˚, respectively [cf. 2.3894(13),

Figure 7. Optimized geometries of the four ground-state diastereomers of 5a (5a1-5a4) and of the intermediate geometry 5a5, with relative energies (kJ mol-1) in square brackets. H atoms are omitted for clarity. Relative energies for the corresponding minimum energy geometries for 6a are given in italics.

2.3938(13), and 2.165(4) A˚, respectively, for 5]; however, the P-Ge-N bite angle in 5a3 [86.33°] is very similar to the corresponding angle in 5 [86.01(10)°]. For both 5a and 6a the lowest energy diastereomer is calculated to be the ERPchRPtR isomer 5a1/6a1, although the ESPchRPtR diastereomers 5a2 and 6a2 lie just 8.4 and 1.7 kJ mol-1 higher in energy, respectively. The ERPchSPtR and ESPchSPtR diastereomers 5a3 and 5a4 lie 18.0 and 37.2 kJ mol-1 higher in energy than 5a1, whereas the corresponding diastereomers of the tin analogue 6a3 and 6a4 lie 4.2 and 22.2 kJ mol-1 higher in energy than the lowest energy diastereomer 6a1.

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Scheme 3

In addition to 5a1-5a4 and 6a1-6a4 we located minimum energy geometries for both 5a and 6a in which there is one strong E-N contact and one secondary, longer E 3 3 3 N contact (5a5 and 6a5, respectively). These geometries correspond to the structure determined crystallographically for the diphosphastannylene 6. For 6a5 the Sn-N and Sn 3 3 3 N distances are 2.450 and 3.660 A˚, respectively; these distances compare with Sn-N and Sn 3 3 3 N distances of 2.408(3) and 3.306(3) A˚ in the crystal structure of 6. The angles at the Sn center in 6a5 are very similar to the corresponding angles in 6; for example, the P-Sn-N bite angles in 6a5 and 6 are 80.38° and 80.73(8)°, respectively. Geometries 5a5 and 6a5, which represent intermediates in the formation of pseudo-trigonalbipyramidal species (see below), are -27.6 and -22.6 kJ mol-1, respectively, more stable than the lowest energy diastereomers (5a1 and 6a1) that do not exhibit secondary E 3 3 3 N contacts. It has been shown previously that trigonal-pyramidal pnictine centers may undergo inversion via one of two distinct mechanisms: vertex-inversion, via a trigonal-planar transition state, or edge-inversion, via a T-shaped transition state (Scheme 3). Calculations by Arduengo and Dixon, and others, reveal that electronegative substituents favor edgeinversion, whereas electropositive substituents favor vertexinversion.17 Thus, PF3 and PF2H undergo edge-inversion via a T-shaped transition state, whereas PFH2 and PH3 undergo vertex-inversion via a trigonal-planar transition state. For the model compounds 5a and 6a transition states were located for vertex-inversion of the phosphorus atoms in both the chelating (transition states 5a6 and 6a6, respectively) and terminal phosphide ligands (5a7 and 6a7); the barriers to inversion in each case are calculated to be 94.6 (5a6), 77.4 (6a6), 46.4 (5a7), and 30.1 (6a7) kJ mol-1, with respect to the ground-state geometries 5a1 and 6a1 (Figure 8). We were unable to locate a transition state corresponding to edgeinversion at phosphorus in any of these cases, consistent with the presence of electropositive Ge and Sn atoms adjacent to the phosphorus centers. As expected, inversion at the phosphorus atom in the chelating ligand is a substantially higher energy process than inversion at the phosphorus atom of the terminal ligand in both 5a and 6a. We also note that inversion at either phosphorus center is significantly more favorable in 6a than in 5a. This is consistent with the presence of the more electropositive tin atom in the former compound. It has been shown previously that electropositive atoms significantly decrease the barrier to vertex-inversion in (17) (a) Dixon, D. A.; Arduengo, A. J.III; Fukunaga, T. J. Am. Chem. Soc. 1986, 108, 2461. (b) Dixon, D. A.; Arduengo, A. J.III. J. Chem. Soc., Chem. Commun. 1987, 498. (c) Dixon, D. A.; Arduengo, A. J.III. J. Am. Chem. Soc. 1987, 109, 338. (d) Schwerdtfeger, P.; Laakkonen, L. J.; Pyykk€o, P. J. Chem. Phys. 1992, 96, 6807. (e) Schwerdtfeger, P.; Hunt, P. Adv. Mol. Struct. Res. 1999, 5, 223. (f) Schwerdtfeger, P.; Boyd, P. D. W.; Fischer, T.; Hunt, P.; Liddell, M. J. Am. Chem. Soc. 1994, 116, 9620. (g) G€oller, A.; Clark, T. Chem. Commun. 1997, 1033.

Figure 8. Optimized geometries for transition states involving inversion at phosphorus in 5a and 6a with H atoms omitted for clarity; energies relative to 5a1 and 6a1 (kJ mol-1) are given in square brackets.

tertiary phosphines and arsines; for example, NMR studies yield free energies of activation for inversion at phosphorus in the compounds iPrPhP(EMe3) of 136.8, 89.5, and 80.8 kJ mol-1 for E = C, Ge, and Sn, respectively.18 Thus, inversion at either phosphorus center in 6a should be more energetically favorable than in 5a. Our previous DFT calculations on the model complexes [Me(C6H4-2-CH2NMe2)P]2E [E = Ge (3a), Sn (4a)], which contain six-membered chelate rings, revealed that, in addition to inversion at either of the phosphorus centers, these molecules may epimerize through inversion at the germanium or tin center, either via a vertex-inversion mechanism or via an unusual hybrid edge/vertex-inversion mechanism, in which the geometry at the group 14 element center is midway between trigonal planar and T-shaped. All attempts to obtain a transition state for inversion at the germanium center of 5a converged to transition state 5a8 (Figure 9), in which the germanium atom adopts a geometry midway between trigonal planar and T-shaped (P-Ge-P 154.51°). Attempts to locate a transition state corresponding to either pure edge- or pure vertex-inversion at the germanium center for this compound were unsuccessful. A similar hybrid edge/ vertex-inversion mechanism was found for the tin center in the diphosphastannylene 4a (P-Sn-P 157.73°), but not for the germanium center in the diphosphagermylene 3a, which undergoes inversion via a transition state that is close to trigonal planar in nature (i.e., via a vertex-inversion mechanism). In contrast, inversion at the tin center in 6a proceeds via an essentially edge-inversion mechanism; the calculated transition state for inversion at tin (6a8) has a T-shaped geometry in which the P-Sn-P angle is 169.17°. Transition states 5a8 and 6a8 are calculated to be 241.0 and 289.2 kJ mol-1 higher in energy than the corresponding ground states 5a1 and 6a1, clearly suggesting that inversion at the group 14 center via this mechanism is highly disfavored. Compounds 5a and 6a are valence isoelectronic with tertiary pnictines PnR3 (Pn = As, Sb). Previous calculations (18) (a) Beachler, R. D.; Casey, J. P.; Cook, R. J.; Senkler, G. H., Jr.; Mislow, K. J. Am. Chem. Soc. 1987, 109, 338. (b) Beachler, R. D.; Mislow, K. J. Am. Chem. Soc. 1971, 93, 773. (c) Beachler, R. D.; Andose, J. D.; Stackhouse, J.; Mislow, K. J. Am. Chem. Soc. 1972, 94, 8060. (d) Driess, M.; Merz, K.; Monse, C. Z. Anorg. Allg. Chem. 2000, 626, 2264.

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Minima 5a9 and 6a9 are highly symmetrical, possessing a C2 axis that bisects the P-Sn-P angle, making the two phosphide ligands equivalent; minima 5a10 and 6a10 have no such symmetry, and so the two ligands are inequivalent. The low-temperature 1H NMR spectrum of 6 in d8-toluene contains a single set of ligand resonances and is consistent with coordination of both amino groups to the tin center, while the 31P{1H} and 119Sn{1H} NMR spectra of 6 also indicate a symmetrical complex in which both amino groups are bound to tin. These spectra are clearly consistent with compound 6 adopting a structure similar to minimum 6a9 at low temperatures.

Conclusions

Figure 9. Calculated transition states for inversion at germanium (5a8) and tin (6a8) and optimized geometries for the pseudo-trigonal-bipyramidal intermediates 5a9 and 5a10 with H atoms omitted for clarity. Energies (kJ mol-1) relative to the ground states 5a1 and 6a1 are given in square brackets [relative energies for 6a9 and 6a10 in italics].

reveal that barriers to vertex-inversion of trigonal-pyramidal tertiary pnictines increase, whereas barriers to edge-inversion decrease, with increasing atomic number of the pnictogen atom.17 Unfortunately, in the case of 5a8 and 6a8 we are unable to directly compare the barriers to inversion at the group 14 element centers since these compounds are subject to somewhat different inversion mechanisms. It has been shown previously that electronegative substituents favor edge- over vertex-inversion in the isoelectronic tertiary pnictine compounds PnR3 (see above). In the present case the electronegativities of the substituents are preserved, while the electronegativity of the inversion center decreases on going from 5a to 6a. That the germanium and tin centers in 5a and 6a undergo inversion via different mechanisms suggests that the important factor in determining the mechanism is the electronegativity difference between the central atom and the substituents. Thus, the greater electronegativity difference between Sn and P in 6a favors an edgeinversion mechanism, whereas the lower electronegativity difference between Ge and P in 5a favors a hybrid edge/ vertex-inversion mechanism. Attempts to locate a transition state involving a twocoordinate germanium or tin center, corresponding to exchange of the terminal and chelating phosphide ligands via a dissociative mechanism, were unsuccessful, with optimizations consistently converging to give geometries containing a four-coordinate, pseudo-trigonal-bipyramidal group 14 element center with PRPR stereochemistry (5a9 and 6a9). This corresponds to the transition state for exchange between the chelating and terminal phosphide ligands via an associative pathway. However, both 5a9 and 6a9 are calculated to be minima, rather than transition states; in fact minima 5a9 and 6a9 are calculated to be -50.6 and -28.5 kJ mol-1, respectively, more stable than the corresponding “ground states” 5a1 and 6a1. Indeed, the lowest energy minima that we located in this study correspond to the PSPR diastereomers of these pseudo-trigonal-bipyramidal intermediates (5a10 and 6a10, respectively).

The contraction in chelate ring size on going from the sixmembered chelate ring in the diphosphagermylene 3 to the five-membered chelate ring in 5 has only minimal impact on the nature of the solid-state structures and dynamic behavior of these compounds; both compounds crystallize as discrete monomers containing a trigonal-pyramidal germanium center bound by one chelating and one terminal phosphide ligand. In contrast, the difference in chelate ring size between the diphosphastannylenes 4 and 6 significantly affects both the solid-state structures and dynamic behavior of these compounds. Compounds 4 and 6 both crystallize as discrete monomers in which one phosphide ligand chelates the tin center, while the second phosphide ligand acts principally as a terminal P-donor ligand. However, in 6 the amino group of the terminal ligand is directed toward the tin center, and there is evidence for a weak Sn 3 3 3 N bonding interaction between these two atoms; in 4 the amino group of the terminal ligand is directed away from the tin center and no such interaction is apparent. Compounds 5 and 6 exhibit rather different dynamic behavior in solution. Compound 5 behaves similarly to 3, undergoing rapid exchange at elevated temperatures between the terminal and chelating phosphide ligands and between diastereomers; at low temperatures three of the possible four diastereomers of 5 are observed, one of which significantly predominates. In contrast, while at elevated temperatures compound 6 is subject to rapid exchange between the terminal and chelating phosphide ligands and between individual diastereomers, at low temperatures this compound adopts a pseudo-trigonal-bipyramidal structure in which both amino groups are bound to the tin center. DFT calculations on the model compounds 5a and 6a suggest that such pseudo-trigonal-bipyramidal species are low-energy minima on the potential energy surface. These calculations also reveal that epimerization at phosphorus occurs via a vertex-inversion process and that this process is more favorable for 6a than 5a. Epimerization through inversion at the group 14 element center is highly disfavored and proceeds via an edge-inversion mechanism for 6a, but via an unusual hybrid edge/vertex-inversion mechanism for 5a.

Experimental Section All manipulations were carried out using standard Schlenk techniques under an atmosphere of dry nitrogen. Diethyl ether, THF, n-hexane, and light petroleum (bp 40-60 °C) were dried prior to use by distillation under nitrogen from sodium, potassium, or sodium/potassium alloy; hexamethyldisiloxane was

Article

(19) Benet, S.; Cardin, C. J.; Cardin, D. J.; Constantine, C. P.; Heath, P.; Rashid, H.; Teixeira, S.; Thorpe, J. H.; Todd, A. K. Organometallics 1999, 18, 389. (20) Flack, H. D. Acta Crystallogr., Sect. A 1983, 39, 876.

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Table 1. Crystallographic Data for 5 and 6

formula fw cryst size (mm)

5

6

C30H58GeN2P2Si4 693.7 0.60  0.53  0.14 triclinic P1 10.0421(8) 13.8295(11) 15.5082(12) 95.525(2) 108.821(2) 105.651(2) 1922.9(3) 2 1.026 0.578-0.870 11 226 5014 0.032 4442 368 0.055 0.100 1.333

C30H58N2P2Si4Sn 739.8 0.32  0.30  0.22 orthorhombic Pca21 18.283(5) 13.124(2) 16.4317(14)

)

cryst syst space group a (A˚) b (A˚) c (A˚) R (deg) β (deg) γ (deg) 3942.6(13) V (A˚3) Z 4 0.872 μ (mm-1) transmn coeff range 0.768-0.831 reflns measd 22 347 unique reflns 7632 0.060 Rint 6269 reflns with F2 > 2σ params 368 2 a 0.035 R (on F, F > 2σ) 0.070 Rw (on F2, all data)a 1.036 goodness of fita 0.025(15) abs struct param20 -3 0.46, -0.51 0.62, -0.63 max., min. electron density (e A˚ ) P P P a R= Fo| - |Fc / |Fo|; Rw = [ w(Fo2 - Fc2)2/ P P Conventional w(Fo2)2]1/2; S = [ w(Fo2 - Fc2)2/(no. data - no. params)]1/2 for all data. )

dried by distillation under nitrogen from calcium hydride. THF and hexamethyldisiloxane were stored over activated 4 A˚ molecular sieves; diethyl ether, n-hexane, and light petroleum were stored over a potassium film. Deuterated toluene was distilled from potassium, deoxygenated by three freeze-pump-thaw cycles, and was stored over activated 4 A˚ molecular sieves. Germanium(II) chloride was prepared as its 1,4-dioxane adduct by a previously published procedure;19 tin(II) chloride was dried with chlorotrimethylsilane prior to use. The compounds [{(Me3Si)2CH}(C6H42-NMe2)P]Li and [{(Me3Si)2CH}(C6H4-2-NMe2)P]K were prepared by previously published procedures.16 All other compounds were used as supplied by the manufacturer. 1 H and 13C{1H} NMR spectra were recorded on a JEOL Eclipse500 spectrometer operating at 500.16 and 125.65 MHz, respectively, or a Bruker Avance300 spectrometer operating at 300.15 and 75.47 MHz, respectively; chemical shifts are quoted in ppm relative to tetramethylsilane. 31P{1H} and 119Sn{1H} NMR spectra were recorded on a JEOL Eclipse500 spectrometer operating at 202.35 and 186.50 MHz, respectively; chemical shifts are quoted in ppm relative to external 85% H3PO4 and external Me4Sn, respectively. Elemental analyses were obtained by the Elemental Analysis Service of London Metropolitan University. [{(Me3Si)2CH}(C6H4-2-NMe2)P]2Ge (5). To a stirred solution of GeCl2(1,4-dioxane) (0.29 g, 1.26 mmol) in THF (20 mL) was added, dropwise, a solution of [{(Me3Si)2CH}(C6H42-NMe2)P]K (0.88 g, 2.51 mmol) in THF (20 mL). The reaction mixture was stirred at room temperature for 16 h. Solvent was removed in vacuo, and the sticky brown solid was extracted into light petroleum (20 mL) and filtered. Solvent was removed in vacuo from the filtrate, and the sticky solid was crystallized from cold (-30 °C) hexamethyldisiloxane containing a few drops of diethyl ether as orange blocks of 5. Yield: 0.56 g, 64%. Anal. Calcd for C30H58N2P2Si4Ge: C, 51.98; H, 8.37; N, 4.04. Found: C, 51.90; H, 8.28; N, 4.11. 1H NMR (d8-toluene, 333 K): δ 0.13 (36H, s, SiMe3), 0.92 (2H, s, CHP), 2.59 (12H, s, NMe2), 6.67-7.87 (8H, m, aryl). 13C{1H} NMR (C6D6, 297 K): δ 3.46 (SiMe3), 3.99 (SiMe3), 6.69 [d, JPC = 41.3 Hz, CHP], 45.56 (NMe2), 120.32, 124.48, 129.37, 134.29 (aryl), 135.99 [d, JPC = 22.7 Hz, aryl], 157.28 [d, JPC = 10.3 Hz, aryl]. 31P{1H} NMR (d8-toluene, 297 K): δ -43.2 (s, br). [{(Me3Si)2CH}(C6H4-2-NMe2)P]2Sn (6). To a stirred solution of SnCl2 (0.21 g, 1.76 mmol) in cold (-78 °C) THF (20 mL) was added, dropwise, a solution of [{(Me3Si)2CH}(C6H4-2NMe2)P]Li (1.12 g, 3.52 mmol) in THF (20 mL), excluding light as much as possible. The reaction mixture was allowed to attain room temperature and was stirred for 16 h. Solvent was removed in vacuo, and the sticky brown solid was extracted into light petroleum (20 mL) and filtered. Solvent was removed in vacuo from the filtrate, and the sticky solid was crystallized from cold (-30 °C) n-hexane as orange blocks of 6. Yield: 0.72 g, 56%. Anal. Calcd for C30H58N2P2Si4Sn: C, 48.71; H, 7.90; N, 3.79. Found: C, 48.72; H, 7.89; N, 3.86. 1H NMR (d8-toluene, 294 K): δ 0.28 (36H, s, SiMe3), 0.78 (2H, br s, CHP), 2.63 (12H, s, NMe2), 6.73-7.66 (8H, m, aryl). 13C{1H} NMR (d8-toluene, 296 K): δ 4.01 (SiMe3), 9.03 [d, JPC = 52.7 Hz, CHP], 48.68 (NMe2), 119.47, 125.67, 126.23, 135.57 (aryl), 144.63 [d, JPC = 41.3 Hz, aryl], 153.90 (aryl). 31P{1H} NMR (d8-toluene, 295 K): δ -45.4 [br s, JSnP = 1060 Hz]. 119Sn{1H} NMR (d8-toluene, 183 K): δ 455 [t, JSnP = 1050 Hz]. Crystal Structure Determinations of 5 and 6. Measurements were made at 150 K on Bruker AXS SMART CCD and Nonius KappaCCD diffractometers using graphite-monochromated Mo KR radiation (λ = 0.71073 A˚). Cell parameters were refined from the observed positions of all strong reflections. Intensities

Organometallics, Vol. 29, No. 1, 2010

were corrected semiempirically for absorption, based on symmetry-equivalent and repeated reflections. The structures were solved by direct methods and refined on F2 values for all unique data. Table 1 gives further details. All non-hydrogen atoms were refined anisotropically, and H atoms were constrained with a riding model; U(H) was set at 1.2 (1.5 for methyl groups) times Ueq for the parent atom. Programs were Bruker AXS SMART and SAINT, Nonius COLLECT and EvalCCD, and SHELXTL for structure solution, refinement, and molecular graphics.21 DFT Calculations. Geometry optimizations and single-point energy calculations were performed on the gas phase molecules with the Gaussian03 suite of programs (revision D.01)22 on a 224-core Silicon Graphics Altix 4700, with 1.6 GHz Montecito Itanium2 processors, via the EPSRC National Service for Computational Chemistry Software (http://www.nsccs.ac.uk). Ground-state optimizations for the model compounds 5a and 6a were carried out using the B3LYP hybrid functional23 with an Lanl2dz effective core potential basis set24 for Sn and a 6-31G(d,p) all-electron basis set on all other atoms25 [B3LYP/6-31G(d,p), (21) (a) SMART and SAINT software for CCD diffractometers; Bruker AXS Inc.: Madison, WI, 1997. (b) COLLECT software; Nonius BV: Delft, The Netherlands, 2000. (c) Duisenberg, A. J. M.; Kroon-Batenburg, L. M. J.; Schreurs, A. M. M. J. Appl. Crystallogr. 2003, 36, 220. (d) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112. (22) Frisch, M. J.; et al. Gaussian 03, Revision D.01; Gaussian, Inc.: Wallingford, CT, 2004. (23) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Stephens, P. J.; Devlin, F. J.; Chablowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623. (c) Hertwig, R. H.; Koch, W. Chem. Phys. Lett. 1997, 268, 345. (24) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (b) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (c) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (25) (a) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213. (b) Francl, M. M.; Petro, W. J.; Hehre, W. J.; Binkley, J. S.; Gordon, M. S.; DeFrees, D. J.; Pople, J. A. J. Chem. Phys. 1982, 77, 3654. (26) (a) Binkley, J. S.; Pople, J. A.; Hehre, W. J. J. Am. Chem. Soc. 1980, 102, 939. (b) Gordon, M. S.; Binkley, J. S.; Pople, J. A.; Pietro, W. J.; Hehre, W. J. J. Am. Chem. Soc. 1982, 104, 2797. (c) Pietro, W. J.; Francl, M. M.; Hehre, W. J.; Defrees, D. J.; Pople, J. A.; Binkley, J. S. J. Am. Chem. Soc. 1982, 104, 5039. (d) Dobbs, K. D.; Hehre, W. J. J. Comput. Chem. 1986, 7, 359.

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Lanl2dz; default parameters were used throughout]. Transition states were initially located using the QST3 method at the HF/ 3-21G* level of theory26 prior to a full transition-state geometry optimization at the B3LYP/6-31G(d,p),Lanl2dz level of theory. Final single-point energies for the optimized geometries were obtained at the MP2/6-31G(d,p),Lanl2dz level of theory.27 Minima were confirmed by the absence of imaginary vibrational frequencies and transition states by the presence of a single imaginary vibrational frequency. The accuracy of transition states (27) (a) Møller, C.; Plesset, M. S. Phys. Rev. 1934, 46, 618. (b) Head-Gordon, M.; Pople, J. A.; Frisch, M. J. Chem. Phys. Lett. 1988, 153, 503. (c) Saebø, S.; Alml€of, J. Chem. Phys. Lett. 1989, 154, 83. (d) Frisch, M. J.; Head-Gordon, M.; Pople, J. A. Chem. Phys. Lett. 1990, 166, 275. (e) Frisch, M. J.; Head-Gordon, M.; Pople, J. A. Chem. Phys. Lett. 1990, 166, 281. (f) Head-Gordon, M.; Head-Gordon, T. Chem. Phys. Lett. 1994, 220, 122.

Izod et al. was judged by consideration of the principal displacement vectors of the imaginary vibrational mode; in each case this was consistent with the appropriate inversion process.

Acknowledgment. The authors are grateful to the EPSRC for support. Supporting Information Available: For 5 and 6 details of structure determination, atomic coordinates, bond lengths and angles, and displacement parameters in CIF format. For 5a and 6a details of DFT calculations, final atomic coordinates, and energies. Full details of ref 22. This material is available free of charge via the Internet at http://pubs.acs.org. Observed and calculated structure factor details are available from the authors upon request.