Mercury Complexes of N-Heterocyclic Carbenes Derived from

Organometallics 2009, 28, 3793–3803 DOI: 10.1021/om8011745

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Mercury Complexes of N-Heterocyclic Carbenes Derived from Imidazolium-Linked Cyclophanes: Synthesis, Structure, and Reactivity Murray V. Baker,*,† David H. Brown,†,‡ Rosenani A. Haque,† Peter V. Simpson,† Brian W. Skelton,† Allan H. White,† and Charlotte C. Williams† †

Chemistry M313, School of Biomedical, Biomolecular and Chemical Sciences, The University of Western Australia, Crawley, WA 6009, Australia, and ‡Nanochemistry Research Institute, Department of Applied Chemistry, Curtin University of Technology, GPO Box U1987, Perth WA 6845, Australia Received December 11, 2008

A range of mercury-imidazolylidene complexes have been prepared by reaction of imidazoliumlinked cyclophanes with mercury(II) acetate. For cyclophanes based on meta-xylyl groups the mercury complexes are mononuclear and exhibit unprecedented structures in which the mercury atom is bound within the cyclophane ring, the two NHC donors being mutually trans. For cyclophanes based on ortho-xylyl or 2,6-lutidinediyl groups the mercury complexes are dinuclear with a [Hg(μ-L)2Hg] core, in which each mercury atom is coordinated by one NHC from each cyclophane unit. The mononuclear and dinuclear complexes have been structurally characterized. Studies of the NHC-pyridinophane mercury complex indicated that the NHC-Hg bonding is labile, suggesting that the dinuclear complex based on the [Hg(μ-L)2Hg] core existed in equilibrium with a mononuclear complex [HgL]. The labile nature of the NHC-Hg bonding was further established by a redox transmetalation reaction of a dinuclear NHC-Hg(II) complex with a Pd(0) source, which afforded an NHC-Pd(II) complex.

Introduction N-Heterocyclic carbenes (NHCs) are of much interest and their complexes well-known. Complexes of NHCs with almost every transition metal have been reported together with their applications, particularly in the area of catalysis.1-4 We have been interested in a particular class of NHCs where imidazolium-based carbenes constitute part of a cyclophane structure.5,6 We and others have reported the synthesis and characterization of complexes of a variety of NHC-cyclophane ligands with a range of different metals *To whom correspondence should be addressed. Tel: +61 8 6488 2576. E-mail: [email protected]. (1) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290–1309. (2) Hahn, F. E.; Jahnke, M. C. Angew. Chem., Int. Ed. 2008, 47, 3122–3172. (3) Normand, A. T.; Cavell, K. J. Eur. J. Inorg. Chem. 2008, 2781–2800. (4) Kantchev, E. A. B.; O’Brien, C. J.; Organ, M. G. Angew. Chem., Int. Ed. 2007, 46, 2768–2813. (5) Baker, M. V.; Bosnich, M. J.; Brown, D. H.; Byrne, L. T.; Hesler, V. J.; Skelton, B. W.; White, A. H.; Williams, C. C. J. Org. Chem. 2004, 69, 7640–7652. (6) Baker, M. V.; Brown, D. H. Mini Rev. Org. Chem. 2006, 3, 333–354. (7) Shi, Z.; Thummel, R. P. Tetrahedron Lett. 1995, 36, 2741–2744. (8) Baker, M. V.; Skelton, B. W.; White, A. H.; Williams, C. C. J. Chem. Soc., Dalton Trans. 2001, 111–120. (9) Magill, A. M.; McGuinness, D. S.; Cavell, K. J.; Britovsek, G. J. P.; Gibson, V. C.; White, A. J. P.; Williams, D. J.; White, A. H.; Skelton, B. W. J. Organomet. Chem. 2001, 617-618, 546–560. r 2009 American Chemical Society

[Pd(II), Ni(II), Pt(II), Rh(I), Ir(I), Au(I), and Ag(I)] (e.g., 1, 2, and 3).7-23 Metal complexes of NHC-cyclophanes have exhibited exceptional catalytic activity for Heck reactions (10) Garrison, J. C.; Simons, R. S.; Kofron, W. G.; Tessier, C. A.; Youngs, W. J. Chem. Commun. 2001, 1780–1781. (11) Garrison, J. C.; Simons, R. S.; Talley, J. M.; Wesdemiotis, C.; Tessier, C. A.; Youngs, W. J. Organometallics 2001, 20, 1276–1278. (12) Baker, M. V.; Skelton, B. W.; White, A. H.; Williams, C. C. Organometallics 2002, 21, 2674–2678. (13) Baker, M. V.; Brown, D. H.; Haque, R. A.; Skelton, B. W.; White, A. H. Dalton Trans. 2004, 3756–3764. (14) Barnard, P. J.; Baker, M. V.; Berners-Price, S. J.; Skelton, B. W.; White, A. H. Dalton Trans. 2004, 1038–1047. (15) Melaiye, A.; Sun, Z.; Hindi, K.; Milsted, A.; Ely, D.; Renekerm, D. H.; Tessier, C. A.; Youngs, W. J. J. Am. Chem. Soc. 2005, 127, 2285–2291. (16) Baker, M. V.; Brayshaw, S. K.; Skelton, B. W.; White, A. H.; Williams, C. C. J. Organomet. Chem. 2005, 690, 2312–2322. (17) Baker, M. V.; Brown, D. H.; Simpson, P. V.; Skelton, B. W.; White, A. H.; Williams, C. C. J. Organomet. Chem. 2006, 691, 5845–5855. (18) Baker, M. V.; Brown, D. H.; Hesler, V. J.; Skelton, B. W.; White, A. H. Organometallics 2007, 26, 250–252. (19) Willans, C. E.; Anderson, K. M.; Junk, P. C.; Barbour, L. J.; Steed, J. W. Chem. Commun. 2007, 3634–3636. :: (20) Hahn, F. E.; Langenhahn, V.; Lugger, T.; Pape, T.; Le Van, D. Angew. Chem., Int. Ed. 2005, 44, 3759–3763. (21) Hahn, F. E.; Radloff, C.; Pape, T.; Hepp, A. Chem. Eur. J. 2008, 14, 10900–10904. (22) Kaufhold, O.; Stasch, A.; Pape, T.; Hepp, A.; Edwards, P. G.; Newman, P. D.; Hahn, F. E. J. Am. Chem. Soc. 2009, 131, 306–317. (23) Kaufhold, O.; Stasch, A.; Edwards, P. G.; Hahn, F. E. Chem. Commun. 2007, 1822–1824.

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(Pd),8,9 as well as interesting antimicrobial (Ag)15,24 and antimitochondrial (Au) biological activity.14,25

Of the vast array of NHC-metal complexes that have been reported, mercury complexes are among the least studied despite being one of the first NHC-metal complexes isolated. In 1968 Wanzlick and Scho¨nherr reported the first NHCmercury complex, made by reaction of an imidazolium salt with a basic mercury source, mercury(II) acetate.26 Since that first report, all NHC-mercury complexes reported in the literature have been prepared by this one-pot mercuration method.27-38 The majority of the NHC-mercury(II) complexes are mononuclear, with linear Hg(NHC)2 units (e.g., 433 and 531), although in some cases association with counterions distorts the Hg environment toward tetrahedral (e.g., 627). Lin and co-workers have reported helical dinuclear complexes of the form [(NHC)2Hg]2 (e.g., 7).30,31 Linear and tetrahedral mono-NHC Hg complexes are also known (e.g., 828 and 930). (24) Kascatan-Nebioglu, A.; Panzner, M. J.; Tessier, C. A.; Cannon, C. L.; Youngs, W. J. Coord. Chem. Rev. 2007, 251, 884–895. (25) Barnard, P. J.; Wedlock, L. E.; Baker, M. V.; Berners-Price, S. J.; Joyce, D. A.; Skelton, B. W.; Steer, J. H. Angew. Chem., Int. Ed. 2006, 5966–5970. (26) Wanzlick, H.-W.; Scho¨nherr, H. J. Angew. Chem., Int. Ed. Engl. 1968, 7, 141–142. (27) Arduengo, A. J.III; Harlow, R. L.; Marshall, W. J.; Prakasha, T. K. Heteroat. Chem. 1996, 7, 421–426. (28) Bildstein, B.; Malaun, M.; Kopacka, H.; Ongania, K.-H.; Wurst, K. J. Organomet. Chem. 1998, 552, 45–61. (29) Bildstein, B.; Malaun, M.; Kopacka, H.; Ongania, K.-H.; Wurst, K. J. Organomet. Chem. 1999, 572, 177–187. (30) Chen, J. C. C.; Lin, I. J. B. J. Chem. Soc., Dalton Trans. 2000, 839–840. (31) Lee, K.-M.; Chen, J. C. C.; Lin, I. J. B. J. Organomet. Chem. 2001, 617-618, 364–375. (32) Buron, C.; Stelzig, L.; Guerret, O.; Gornitzka, H.; Romanenko, V.; Bertrand, G. J. Organomet. Chem. 2002, 664, 70–76. (33) Catalano, V. J.; Malwitz, M. A.; Etogo, A. O. Inorg. Chem. 2004, 43, 5714–5724. (34) Arduengo, A. J.III; Tapu, D.; Marshall, W. J. Angew. Chem., Int. Ed. 2005, 44, 7240–7244. (35) Wan, X.-J.; Xu, F.-B.; Li, Q.-S.; Song, H.-B.; Zhang, Z.-Z. Organometallics 2005, 24, 6066–6068. (36) Scheele, U. J.; Dechert, S.; Meyer, F. Inorg. Chim. Acta 2006, 359, 4891–4900. (37) Liu, Q.-X.; Yin, L.-N.; Feng, J.-C. J. Organomet. Chem. 2007, 692, 3655–3663. (38) Liu, Q.-X.; Zhao, X.-J.; Wu, X.-M.; Guo, J.-H.; Wang, X.-G. J. Organomet. Chem. 2007, 692, 5671–5679.

Ligand transfer from mercury to other transition metals or main group elements is an established synthetic method.39 Recently a series of lanthanoid(II) and -(III) complexes and various Sn(IV) complexes have been prepared by redox transmetalation using metal(0) sources and mercury(II) complexes.40-47 Transmetalation using NHC-mercury(II) complexes has not been reported, although transmetalation involving silver-NHC complexes is now a routine method for the synthesis of other metal-NHC complexes.48,49 In this paper we report the first syntheses of mercury-NHC complexes derived from imidazolium-linked cyclophanes. The cyclophane structures include ortho- (I) and meta-xylyl (II and III) based cyclophanes, as well as the pyridinophane IV. We also report examples of structurally characterized mononuclear metal complexes involving an NHC-linked meta-xylyl cyclophane, only one example of which has been reported19 previously. NMR studies suggest that the NHC ligands on the mercury complexes are labile. Lability is also indicated by the reaction of a mercury(II)-NHC cyclophane complex with a Pd(0) source to give a Pd(II)-NHC cyclophane complex, the first reported example of a redox transmetalation reaction of a mercury-NHC complex. (39) Wardell, J. L., In Comprehensive Organometallic Chemistry, 1st ed.; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon Press: Oxford, 1982; Vol. 2. (40) Deacon, G. B.; Delbridge, E. E.; Skelton, B. W.; White, A. H. Eur. J. Inorg. Chem. 1998, 543–545. (41) Wu, Y. J.; Ding, L.; Zhou, Z. X.; Du, C. X.; Wang, W. L. J. Organomet. Chem. 1998, 564, 233–239. (42) Deacon, G. B.; Feng, T.; Forsyth, C. M.; Gitlits, A.; Hockless, D. C. R.; Shen, Q.; Skelton, B. W.; White, A. H. J. Chem. Soc., Dalton Trans. 2000, 961–966. (43) Deacon, G. B.; Forsyth, C. M. Organometallics 2003, 22, 1349–1352. (44) Deacon, G. B.; Forsyth, C. M.; Nickel, S. J. Organomet. Chem. 2002, 647, 50–60. (45) Deacon, G. B.; Fallon, G. D.; Forsyth, C. M.; Harris, S. C.; Junk, P. C.; Skelton, B. W.; White, A. H. Dalton Trans. 2006, 802–812. (46) Hitzbleck, J.; Deacon, G. B.; Ruhlandt-Senge, K. Eur. J. Inorg. Chem. 2007, 592–601. (47) Cole, M. L.; Deacon, G. B.; Forsyth, C. M.; Junk, P. C.; Konstas, K.; Wang, J. Chem. Eur. J. 2007, 13, 8092–8110. (48) Garrison, J. C.; Youngs, W. J. Chem. Rev. 2005, 105, 3978–4008. (49) Lin, I. J. B.; Vasam, C. S. Coord. Chem. Rev. 2007, 251, 642–670.

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Results and Discussion Synthesis of Cyclophanes. The cyclophane salts I 3 2PF6, II 3 2Br, III 3 2Br, and IV 3 2Br were prepared following published procedures.5,8,13 The bromide salts II 3 2Br and III 3 2Br were converted quantitatively into the corresponding PF6 salts by salt metathesis with KPF6 in water. The PF6 salts of the cyclophanes are considerably more soluble than the corresponding bromide salts in common organic solvents such as acetonitrile and acetone.

Mercury Complexes Derived from I, II, and III. The salts of the mercury complexes 10-12 were prepared by reaction of Hg(OAc)2 with the corresponding cyclophane salts I 3 2PF6III 3 2PF6. Except for the reaction of I 3 2PF6 with Hg(OAc)2 (which used excess Hg(OAc)2), the reactions were conducted with a cyclophane:Hg(OAc)2 ratio of 1:1. The reactions and workups were conducted without the exclusion of air or moisture. The reaction of I 3 2PF6 with 1 equiv of Hg(OAc)2 in DMSO at 90 °C was very slow and invariably afforded an impure product. When excess Hg(OAc)2 was used, however, the tetra-DMSO solvate of 10 3 4PF6 was obtained after 4 days and was isolated as a white powder in 69% yield. The solvation pattern was confirmed by 1H NMR, microanalytical data, and X-ray studies (see Structural Studies). Reaction of the cyclophane salts II 3 2PF6 and III 3 2PF6 with 1 equiv of Hg(OAc)2 in refluxing acetonitrile for 1 day afforded the mononuclear complexes 11 and 12, which were obtained as colorless, crystalline PF6 salts in 50-60% yield. Complexes 11 and 12 represent structurally characterized examples of mononuclear metal complexes of meta-xylyllinked NHC cyclophanes. In the above reactions it was convenient to work with the PF6 salts of the imidazolium cyclophanes. The PF6 salts of 10-12 were not soluble in water, so removal of excess or residual Hg(OAc)2 was easily performed by washing the crude reaction product with water. The Hg-NHC complexes 10-12, as their PF6 salts, were readily soluble in DMSO, dmf, acetone, and acetonitrile, and insoluble in water, dichloromethane, and diethyl ether. The complexes are tolerant of air and moisture. Exposure of d6-DMSO solutions of these complexes to air and moisture for seven days resulted in no visible change in their 1H NMR spectra. The mononuclear complexes 11 3 2PF6 and 12 3 2PF6 are, however, unstable to prolonged heating. Upon heating

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at 100 °C for two days in d6-DMSO, the solution of 10 3 4PF6 showed no decomposition, while a solution of 12 3 2PF6 displayed some minor decomposition and a solution of 11 3 2PF6 indicated significant decomposition (based on the 1 H NMR spectra of the solutions).

The NMR spectral features of 10-12 are consistent with their solid-state structures (see Structural Studies). The complexes 10-12 exhibit rigid conformations in solution with no evidence of dynamic behavior. In the 1H NMR spectrum the benzylic protons appear as sharp AX patterns for complexes 10 and 11 and a sharp AB pattern for complex 12. The imidazolyl ring protons for 10 (δ 6.69) are upfield compared to the corresponding protons of 11 (δ 7.90) and 12 (δ 8.07), consistent with the imidazolyl protons in 10 being positioned in a region of magnetic shielding (between the aromatic rings of the cyclophane). The rigid behavior of 11 and 12 presumably reflects the inability of the metasubstituted benzene rings to “flip” from one possible orientation to the other, a process that would require either a C-H group (in 11) or a C-CH3 group (in 12) to swing through a 10-membered metallocyclic ring, past the Hg center. The 13C NMR spectrum for each complex exhibits a signal corresponding to the carbene carbon (C-Hg: 10, δ = 175.4 ppm; 11, δ = 170.6 ppm; 12, δ = 170.0 ppm) within the range reported in the literature for other mercury-carbene carbon signals (δ 170 -185 ppm).27,32,34,36 No 1H-199Hg or 13C-199Hg coupling was observed. Like the NMR studies discussed above, mass spectrometric studies of 11 and 12 were consistent with these complexes being mononuclear. The mass spectrum of 11 3 2PF6 shows an ion cluster consistent with the isotope pattern expected for M - PF6, including 685 202 (C22H20N200 4 HgPF6) and 687 (C22H20N4 HgPF6). Similarly, the mass spectrum of complex 12 3 2PF6 shows an ion cluster consistent with the isotope pattern expected for M - PF6, including 769 (C28H32N200 4 HgPF6) and 771 (C28H32N202 4 HgPF6). Mercury Complexes Derived from Pyridinophane IV. The reactions of IV 3 2Br with Hg(OAc)2 were more complicated than the examples above. The reaction of equimolar amounts of IV 3 2Br and Hg(OAc)2 in DMSO at 70 °C yielded a mixture of mercury-NHC complexes. The 1H and 13C NMR spectra suggested that two products were formed: both spectra contained two sets of resonances that could be attributed to different pyridinophane complexes. In addition, the chemical shifts of signals in the 1H NMR spectra

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Figure 1. 1H NMR spectra (500.13 MHz) for a sample prepared by dissolving crystals of 13 in d6-DMSO: (a) recorded at 25 °C immediately; (b and c) recorded 1 week later, at 25 and 53 °C respectively; (d) the same sample after removal of solvent and redissolution in fresh d6-DMSO, recorded immediately at 25 °C.

showed significant variability from sample to sample. For these reasons, interpretation of the NMR spectra was not straightforward. Fortunately, a complex identified as the dinuclear complex 13 could be crystallized from solutions of the crude product in either DMSO or DMSO-acetone, the crystals being amenable to X-ray studies (see Structural Studies). Interestingly, the reaction of the bromide-free salt IV 3 2PF6 with Hg(OAc)2 did not afford a complex.

When crystals of 13 were dissolved in d6-DMSO and examined immediately by 1H NMR spectroscopy, the 1H NMR spectrum (Figure 1a) showed the expected resonances: a pair of doublets for the benzylic protons, a single set of signals for the pyridyl H3/4/5 protons, and a single resonance for the imidazolyl H4/5 protons. The simplicity of the spectrum indicates that in the complex the two pyridinophane units are equivalent and that within each pyridinophane the pyridine units are equivalent, the imidazolyl H4 and H5 protons are equivalent, and the sets of benzylic protons are equivalent. Presumably the pyridine units are “flapping” back and forth in the complex rapidly on the NMR time scale so that the NMR spectrum reflects an “average” conformation of the pyridinophane units rather than specific syn and anti conformations that are evident in the solid-state structures (see Structural Studies). This process does not exchange the benzylic protons, and thus they remain nonequivalent. This averaging is consistent with observations of other dinuclear metal complexes of the pyridinophane IV.14 When the above solution of 13 was re-examined by 1H NMR spectroscopy one week later (Figure 1b), resonances attributed to a new species were present: a new set of pyridyl

H3/4/5 protons, a new singlet for imidazolyl H4/5 protons, and a broad signal (Wh/2 ca. 80 Hz) attributed to benzylic protons. The broad signal became sharper, but was nevertherless still a broad singlet (Wh/2 10 Hz), when the sample was heated to 53 °C (Figure 1c), and at this temperature the resonances due to 13 were much less intense than in the spectrum recorded at room temperature (Figure 1). After the sample was stripped of solvent and redissolved in d6-DMSO, the 1H NMR spectrum (Figure 1d) contained only those resonances that were present in the spectrum of the original fresh solution (Figure 1a). The new signals in Figures 1b and c are tentatively attributed to the monomeric complex 14, analogous to the structurally characterized meta-cyclophane complex 11. Rapid interconversion of the syn- and anti-conformations of 14 (Scheme 1) would account for the apparent equivalence of all the benzylic protons (which appear as a slightly broadened singlet in the 1H NMR spectrum at 53 °C, Figure 1c). Here, the pyridyl groups are able to “flip” through the 10-membered metallocyclic ring (unlike the case for the benzene rings in 11 and 12; see above). It may be that transient coordination of the pyridine N atoms to the Hg center facilitates the interconversion of the conformers of 14 in a way that is not possible for the meta-xylyl complex 11. In the 1H NMR spectra of samples containing 13, there was noticeable sample-to-sample variability in the chemical shifts of the signals due to 13, in particular the triplet due to pyridyl-H4 and the downfield benzylic doublet. This variability may be a consequence of solvolysis reactions, where some (or all) of the Br- ions dissociate from the Hg centers of 13 on dissolution in d6-DMSO. If this is the case, the NMR signals attributed to 13 should actually be the concentrationweighted average of signals due to rapidly equilibrating [Hg (μ-L)2Hg]2+ units having zero, one, two, three, or four associated Br- ions. Consistent with this suggestion, addition of small amounts of bromide (as NaBr) to d6-DMSO solutions containing both 13 and 14 resulted in changes in the chemical shifts of the pyridyl-H4 and the downfield benzylic doublet (Figure 2), with very minor changes to the chemical shifts of other signals due to 13. Interestingly, in this experiment there were no changes to the signals due to 14, consistent with the formulation of this complex as a

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Figure 2. 1H NMR chemical shifts (500.13 MHz, d6-DMSO) of the signal corresponding to the pyridyl-H4 (O) protons and one set of the benzylic protons (0) of a solution of 13 upon addition of bromide (as NaBr). Scheme 1

monomeric [HgL]2+ species with no associated bromide ions. However, consistent with 13 and 14 being in equilibrium, addition of the NaBr resulted in the slow increase in the amount of 13 and an associated decrease in the amount of 14. Similarly, dilution or heating (see Figure 1c) of a solution of 13 and 14 results in an increase in 14 and a decrease in 13, which is consistent with 14 being a product of the dissociation of 13 into six species (1 equiv of 13 dissociated to 2 equiv of 14 and 4 equiv of Br-). 13 C NMR spectra of solutions containing only 13 or only 14 could not be obtained because of the relatively rapid equilibration between 13 and 14. 13C NMR spectra of solutions containing both 13 and 14 (d6-DMSO) showed signals expected for this mixture of complexes. There were twelve 13C resonances in all, six for each complex. Assignment of particular resonances to either 13 or 14 was made by comparison of spectra for two samples containing different relative amounts of the two compounds. Signals at δ 176.4 and 176.8 ppm were attributed to carbene carbons in 13 and 14, respectively. Mass spectroscopic studies (FAB-HRMS) were undertaken with d6-DMSO solutions containing both 13 and 14. In the mass spectra an ion cluster centered near m/z 623 was

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assigned to the cation 14 with an associated bromide ion (e.g., 623.0467, [L202Hg79Br]+; 625.0468, [L202Hg81Br]+). Peaks corresponding to 13, [Br2Hg(μ-L)2HgBr2]+, or 13 less a bromide ion, [Br2Hg(μ-L)2HgBr]+, were not detected. However, a solid sample of 13, when suspended in a m-nitrobenzyl alcohol matrix and examined by FAB lowresolution mass spectrometry, produced an ion cluster centered near m/z 1326, consistent with 13 less a bromide 79 81 + or ion, [Br2Hg(μ-L)2HgBr]+ (e.g., 1326, [L202 2 Hg2 Br2 Br] 200 202 79 81 + [L2 Hg Hg Br Br2] ). Not surprisingly, this sample also gave an ion cluster centered near m/z 623, corresponding to [LHgBr]+. Unfortunately, ions near m/z 1326 could not be detected in a high-resolution experiment. Structural Studies. The structurally characterized arrays are of two types: mononuclear and binuclear, variously (un) solvated. In the former, a single mercury atom, linearly bound by the pair of carbene carbon atoms of the macrocycle, lies within the latter; such is the case with 12 3 2PF6 and 11 3 2PF6, the cations of which are depicted in Figure 3(a,b), with cation geometries presented in Table 1. In 12 3 2PF6 3 CH3CN, one formula unit, devoid of crystallographic symmetry, comprises the asymmetric unit of the structure. The two carbene rings on either side of the mercury atom are essentially coplanar and coplanar with the mercury center (Table 1); the pair of C6 aromatic rings lie cis/“syn” to one side of the carbenoid, quasi-parallel, with the acetonitrile molecule included between the C6 rings; on the other side of the plane, an FPF5 anion approaches. The Hg 3 3 3 F interaction is long, with little perturbation of C-Hg-C from linearity (Figure 3(a); Table 1); fluorine-methyl-hydrogen atom distances lie about the van der Waals’ limit, suggesting inhibition of a closer anion approach (F 3 3 3 H(C(121), C (321)) 2.64, 2.36 A˚). In 11 3 2PF6, the mercury atom relates similarly to the carbene rings; the mercury atom on this occasion is disposed on a crystallographic inversion center, one-half of the formula unit comprising the asymmetric unit of the structure. Consistent with the symmetry, the aromatic C6 rings now lie mutually anti and obligate parallel to each other, with the mercury atom approached from either side by the pair of FPF5 anions (Figure 3(b)). Hg 3 3 3 F are shorter here than in 12 3 2PF6 3 MeCN, perhaps because of the greater accessibility of the metal atom (Table 1). Despite this pair of stronger interactions, Hg-C may be shorter in this complex (2.054(8) A˚) than in 12 3 2PF6 3 MeCN (2.080(5), 2.082(5) A˚). Mercury-carbon (carbene) distances in both complexes are similar to those reported elsewhere for other carbene complexes. In both complexes there are close contacts to the neighboring carbon atoms of the benzene rings (Table 1). Also in both complexes, evidence of strain is found in the large deviations of the bridging atoms from their adjoining ring planes, not found in the other complexes (Table 1, cf. Tables 2 and 3); these deviations are greater about the sixmembered rings. In some cases, deviations about the imidazole rings are in opposite senses, indicative of some slight twist within the array. The binuclear arrays are of the form [LNHCNHC NHC ] (or [LAn )2HgLAn (HgLAn 2 )2L 2 Hg(μ-L 2 ]) (anions/ solvent), a pair of bidentate cyclophane units (LNHC) coordinating a pair of mercury atoms, each of which also carries a pair of ancillary ligands (LAn), either unidentate anions (Br (13)) or solvent (DMSO (10)) molecules. Such arrays are found in 10 3 4PF6 3 2DMSO 3 2EtOH (LAn being DMSO) and in 13 ( 3 nS) (two examples: 3 6DMSO and 3 2DMSO 3 2H2O;

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Figure 3. (a, b) Projections of cations 12 and 11 (in 12 3 2PF6 3 CH3CN and 11 3 2PF6), showing their associations with one and two anions, respectively (the array of 11 3 2PF6 is centrosymmetric). The deviation of the bridging methylene groups from the planes of their associated C6/C3N2 rings is evident. Table 1. Selected Geometries for 12 3 2PF6 3 2CH3CN and 11 3 2PF6 12 3 2PF6 3 2CH3CN

11 3 2PF6

Distances (A˚) Hg-C(22) Hg-C(42) Hg 3 3 3 F Hg 3 3 3 C(12) Hg 3 3 3 C(32)

2.080(5) 2.082(5) 2.989(4) 2.834(6) 2.840(5)

C(22)-Hg-C(42) Hg 3 3 3 F-P F 3 3 3 Hg-C(22) F 3 3 3 Hg-C(42)

178.7(2) 153.5(3) 108.1(2) 73.1(2)

2.054(8) 2.924(7) 2.750(6)

Angles (deg) 180 96.3(3)

Interplanar Dihedral angles (θ, deg) and out-of-Plane Atom Deviations (δ, A˚) (θ) C3N2/C3N2 C3N2(2)/C6(1) (3) C3N2(4)/C6(1) (3) C6/C6 (δ) Hg/C3N2(2) (4) C(1)/C6(1) C(4) C(2)/C6(3) C(3) C(1)/C3N2(2) C(2) C(3)/C3N2(4) C(4)

6.2(3) 84.0(2) 89.1(2) 85.7(3) 87.3(2) 7.0(2)

0 84.4(4)

0.19(1) 0.02(1) 0.35(1) 0.25(1)

0.08(2)

0.32(1) 0.06(1) 0.07(1) 0.09(2) 0.01(1)

0.19(2) 0.21(2) (C(2)) 0.08(2) 0.26(3)

LAn being bromide). All of these arrays conform to some crystallographic symmetry: in 10 3 4PF6 3 2DMSO 3 2EtOH, one-half of the formula unit, devoid of crystallographic symmetry, comprises the asymmetric unit of the structure, whereas in 13( 3 nS), three such half-units comprise the asymmetric unit. Complex 10 is cationic, whereas the two forms of 13 are molecular. Projections of the cations/ molecules are given in Figure 4, with geometries presented comparatively in Table 2. In complex 10, the pairs of aromatic rings are directed away from the cation core, at similar inclinations to the central (carbene-)C4 plane. In three of the four independent molecules of 13, on each ligand one aromatic C5N plane is directed inward,

the other outward, anti as in 11 3 2PF6 above, obligate parallel to their centrosymmetric counterparts on the other macrocycle. The C5N ring inclinations to the central C4 plane are quite diverse throughout the four examples (Table 3). Because of the difference in ortho versus meta incorporation of the six-membered aromatic rings, the Hg 3 3 3 Hg distance (3.4618(3) A˚) is much shorter in 10 than in 13 (Table 2), cf. also the van der Waals sum (3.10 A˚50). Other differences in the cation core geometries may be contingent on Hg 3 3 3 Hg (e.g., C-Hg-C are appreciably smaller in the examples of 13 cf. 10) or on the difference in the “LAn” substituents (all of the latter may be considered more or less strongly bound, albeit less so than the carbene carbon atoms, which impose a predominant quasi-linearity on the mercury atom coordination environments), so that the mercury atoms have some quasi-four-coordinate component in their bonding. In 10, Hg-C are somewhat shorter, with C-Hg-C more nearly linear, than in the examples of 13, as might be expected with the “softer” bromide anion interactions of the latter, despite the much longer Hg-LAn distance. LAn-HgLAn angles in 10 and 13 are generally less than 90°, consistent with the latter. The Hg(μ-LNHC)2Hg array has a counterpart in the recently reported [Ag(μ-LNHC)2Ag] 3 2PF6, essentially devoid of close cation/anion interaction, in which Ag-C are 2.096(9), 2.084(9) A˚, C-Ag-C 171.3(3)°.11 The angular descriptors LAn-Hg-C are unsymmetrical in varying degrees throughout the complexes 10 and 13, which may be related to the proximity of bridging methylene hydrogen atoms, or aromatic rings, depending on the disposition of the latter vis- a-vis the molecular core. With respect to hydrogen atom contacts, in 10, O(2) has contacts to either side from H(2B), H(4B) (x, 1-y, 1-z) at ca. 2.6 A˚, those to O(1) being rather more distant to H(1A, 1B) at 2.9, 2.8 A˚. In 13( 3 nS), all bromine atoms have at least one intramolecular hydrogen contact at 2.8 A˚ or less (Table 2). With respect to (six-membered) aromatic ring dispositions, it is of interest to note the diversity found in the various cation/ molecule components of 10 and 13. In 10 both aromatic rings are directed away from the molecular core, mutually syn, anti to the mercury atoms, as in 11. In 13 3 6DMSO (centrosymmetric) one ring of the independent ligand lies over the core, one away mutually anti, as in 12, with Br 3 3 3 C distances to the carbon atoms of the inwardly directed ring being found (50) Bondi, A. J. Phys. Chem. 1964, 68, 441–451.

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NHC n+ Table 2. Selected Geometries for [LNHC(HgLAn ] Systems 2 )2 L

13 3 2DMSO 3 2H2O 13 3 6DMSO

a

mol.2a

mol.1

mol.3a

10 3 4PF6a,b

Distances (A˚) Hg-C(22) Hg-C(42) Hg-LAn(1) Hg-LAn(2) Hg 3 3 3 Hg C(22) 3 3 3 C(42)

2.109(6) 2.088(6) 2.8589(7) 2.9880(7) 5.7945(7) 4.978(7)

2.127(10) 2.063(10) 2.8772(11) 2.9013(12) 5.9499(8) 4.892(9), 5.117(9)

2.055(12) 2.062(10) 2.9664(11) 2.9597(12) 5.6510(9) 5.022(11)

2.070(12) 2.130(11) 2.9120(13) 2.9787(14) 5.6721(10) 4.938(11)

2.081(2) 2.078(2) 2.499(2) 2.520(2) 3.4618(3) 3.171(4)

Angles (deg) LAn(1)-Hg-LAn(2) LAn(1)-Hg-C(22) LAn(2)-Hg-C(22) LAn(1)-Hg-C(42) LAn(2)-Hg-C(42) C(22)-Hg-C(42)

87.81(2) 100.3(1) 95.4(1) 98.4(2) 98.4(2) 157.2(2)

91.38(3) 95.5(2) 109.3(3) 104.9(2) 86.9(2) 153.7(3)

85.76(3) 98.7(2) 94.8(3) 95.5(3) 96.9(3) 162.3(3)

85.59(4) 98.1(2) 98.7(2) 97.0(3) 95.8(3) 159.8(3)

82.21(7) 85.57(8) 91.44(8) 102.33(8) 91.97(8) 171.76(9)

Interplanar Dihedral Angles (θ, deg)c and out-of-Plane Atom Deviations (δ, A˚) (θ) C3N2/C3N2 C3N2(2)/C6 (1) (3) C3N2(4)/C6 (1) (3) C6/C6 C4/C3N2 (2) (4) /C6 (1) (3) (δ) Hg/C3N2 (2) (4) LAn(1)/C4 (2) Hg/C4 C(1)/C6(1) C(4) C(2)/C6(3) C(3) C(1)/C3N2(2) C(2) C(3)/C3N2(4) C(4)

16.1(3) 87.5(3) 81.1(2) 81.1(3) 83.4(3) 29.9(2) 89.4(2) 88.8(2) 43.5(2) 73.2(2)

3.1(5) 51.8(4) 67.5(4) 54.9(4) 64.5(4) 61.7(3) 3.0(3) 5.8(4) 32.0(3) 70.3(3)

11.6(5) 85.5(4) 82.2(5) 87.5(4) 11.6(5) 23.7(4) 88.9(3) 89.9(3) 43.0(3) 66.6(4)

12.3(5) 87.5(5) 89.7(5) 83.7(4) 78.0(4) 26.5(4) 85.6(3) 85.9(4) 45.0(3) 71.5(3)

5.5(1) 88.3(1) 83.0(1) 88.3(1) 87.2(1) 89.6(1) 80.7(1) 81.2(1) 45.5(1) 44.7(1)

0.12(1) 0.11(1) 2.107(5) -1.942(6) 0.075(3) 0.11(1) 0.05(1) 0.00(1) 0.06(1) 0.01(1) 0.04(1) -0.01(1) 0.06(1)

0.52(2) 0.59(2) 2.10(1) -1.98(1) 0.053(7) 0.15(6) -0.05(2)e -0.06(2)e 0.11(2) 0.11(2) -0.06(2)d 0.02(2) -0.12(2)d

0.16(2) 0.08(2) 2.04(1) -1.99(1) 0.023(6) 0.08(2) 0.01(2) 0.32(2) 0.05(2) 0.04(2) 0.03(2) 0.03(2) -0.06(2)

0.06(2) 0.22(2) 2.02(1) -1.98(1) 0.013(7) 0.08(2) 0.11(2) 0.02(2) 0.12(2) 0.12(2) -0.05(2) 0.03(2) 0.06(2)

0.347(5) 0.139(6) 1.269(4) -1.957(4) 0.034(2) 0.08(2) 0.11(2) 0.022(7) 0.025(6) 0.12(2) -0.05(2) 0.071(6) 0.053(6)

a Molecules 2,3 of 13 and the cation of 10 are centrosymmetric, so that C(22 or 42) may be appropriately transformed. b Hg-O(0)-S(n) (n = 1, 2) are 133.3(1), 120.8(1)°; Hg 3 3 3 Hg0 -C(22,42),LAn(1,2) are 85.60(6), 86.34(7), 149.98(5), 126.65(5)°. The O(1)-Hg 3 3 3 Hg0 -O(20 ) torsion angle is (cis) -18.2 (1)°; C(22)-Hg 3 3 3 Hg0 -C(42) -1.7(1)°. O(1)-Hg 3 3 3 Hg0 -C(42,220 ) are 71.5(2), -106.7(1)°; O(2)-Hg 3 3 3 Hg0 -C(42,220 ) are -90.3(1), 91.5(1)°. c For C6 read C5N in entries for 13 3 nS. d In some cases here, 2-fold rotation images may apply (italicized). e Read C(2),C(4) respectively (in vertical sequence). In 13 3 6DMSO, Br(1) 3 3 3 H(1B,3B0 ) are 2.7, 2.8 A˚; Br(2) 3 3 3 H(2A,4A0 ) 2.9, 2.8 A˚ (est.); Br(1) 3 3 3 C(11,16) are 4.165(7), 3.955(7) A˚. In 13 3 2DMSO 3 2H2O (molecule 1): Br(11) 3 3 3 H(11A,41A0 ) are 2.7, 2.7 A˚; Br(12) 3 3 3 H(22B) 2.7 A˚ (est.). (molecule 2) Br(21) 3 3 3 H(21B,23B0 ) are 2.7, 2.8 A˚; Br(22) 3 3 3 H(24A0 ) 2.6 A˚ (est.). (molecule 3) Br(31) 3 3 3 H(31B) is 2.7 A˚; Br(32) 3 3 3 H(32A,34A0 ) are 2.7, 2.6 A˚ (est.). The C3N2/C3N2 interplanar dihedral angle across the mercury atom is 34.0(4)°.

around 4 A˚ or less for Br(1), but not notably shorter than those to the inner carbon atoms of the other ring (both bromine atoms having at least one contact Br 3 3 3 H at 2.8 A˚ or less). In 13 3 2DMSO 3 2H2O, the conformations of the ligands of the centrosymmetric molecules 2 and 3 are similar to each other, the C6 aromatic rings within each ligand being mutually anti, parallel across the inversion center to their inversion-related counterparts in the other ligand with similar Br 3 3 3 H,C contact distributions found. In molecule 1, however, with 2-symmetry (rather than 1), although the C6 aromatic rings within each ligand are mutually syn, one ligand has both aromatic rings directed over the molecular core, the other both away (anti to the mercury atoms); contacts for this and the other forms are summarized in the footnote to Table 2. There is a very considerable interplanar dihedral angle between the pair of C3N2 planes to either side of the mercury atom (34.0(4)°).

Transmetalation Chemistry. The possibility of using Hg-NHC complexes as NHC transfer reagents was investigated, using the mercury ortho-cyclophane complex 10 as a model compound. Reaction of 10 3 4PF6 with the Pd(0) source Pd(PPh3)4 in CH3CN afforded the Pd(II) complex 15 (Scheme 2). During the experiment, a silver-colored liquid (presumably elemental Hg) pooled at the bottom of the flask. The reaction was conveniently followed by NMR spectroscopy. The 31P NMR spectrum displayed resonances at δ 18.0 (for coordinated PPh3 of 15)18 and δ -3.6 (for the liberated PPh3). The 1H NMR spectrum showed resonances consistent with an ortho-cyclophane moiety coordinated to a PdL2 fragment (a set of two sharp benzylic doublets at δ 4.22 and 5.89, a signal for the NHC ring protons at δ 6.91, and arene protons at δ 7.3-7.7). The 13C NMR spectrum exhibited a doublet of doublets for the carbene

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Table 3. Selected Geometries for 16, 17, 19, and 20

16 (X, Y = PPh3,CH3CN)

17a (X, Y = PPh3,Br)

19b (X, Y = Br2)

20 (X, Y = I2)

Distances (A˚) Pd-C(22) Pd-C(42) Pd-X Pd-Y

1.972(1) 2.036(1) 2.3462(4) 2.050(2)

1.996(2) 2.022(2) 2.3387(6) 2.4614(3)

1.962(6) 1.970(6) 2.4728(9) 2.4789(9)

1.984(4) 1.984(4) 2.6378(4) 2.6510(4)

Angles (deg) C(22)-Pd-C(42) C(22)-Pd-X C(22)-Pd-Y C(42)-Pd-X C(42)-Pd-Y X-Pd-Y

83.02(5) 92.61(4) 174.20(6) 175.39(4) 92.30(6) 92.15(4)

84.33(8) 93.29(6) 172.85(6) 175.59(7) 89.99(5) 92.65(2)

85.5(2) 89.5(2) 174.1(2) 174.0(2) 89.1(2) 95.71(3)

82.2(1) 91.9(1) 171.5(1) 172.91(9) 89.3(1) 96.53(1)

Interplanar Dihedral Angles (θ, deg) and Out-of-Plane Palladium Atom Deviations (δ, A˚) (θ) C2X2/C6

(1) (3) C3N2 (2) (4) C6/C6 C3N2/C3N2 C6(1)/C3N2 (2) (4) C6(3)/C3N2 (2) (4) (δ) Pd/C2X2 Pd/C3N2 (2) (4)

31.82(5) 35.79(6) 89.69(5) 89.50(6) 67.21(7) 79.63(7) 66.52(7) 73.65(7) 69.82(7) 66.97(7)

30.57(7) 32.10(7) 82.18(8) 83.32(7) 62.28(9) 83.28(11) 64.16(9) 75.16(9) 72.79(9) 67.41(9)

0.008(1) 0.039(3) 0.064(3)

0.004(1) 0.042(4) 0.058(4)

25.7(2) 33.1(2) 87.4(2) 87.4(2) 58.8(3) 87.3(3) 69.8(2) 69.7(2) 71.8(2) 69.5(3) 0.058(3) 0.04(1) 0.03(1)

44.7(1) 36.8(1) 87.9(1) 89.3(1) 80.6(1) 70.1(2) 65.6(2) 69.1(2) 74.8(2) 63.3(2) 0.047(2) 0.332(7) 0.077(6)

a Ref 18; note however, that the present data represent an improvement in precision over the earlier, consequent upon subsequent rerefinement on F 2 (rather than |F|). b Ref 8. The accuracy of the results for this species (impure compound?) is questionable.

carbons, centered at δ 160.9 (2Jtrans C-P = 144 Hz, 2Jcis C-P = 14 Hz). During isolation of the Pd-NHC complex from the reaction mixture, 15 underwent solvolysis with acetonitrile, affording complex 16, which was isolated as its PF6 salt. We have previously isolated the monotriphenylphosphine complexes 17 3 BPh4 and 18 3 PF6 from reaction mixtures containing excess triphenylphosphine.16,18 The instability of 15 toward solvolysis is thus not surprising and presumably is a result of unfavorable steric interactions of the two triphenylphosphine ligands and the bis-NHC ligand. Although the overall isolated yield of 16 3 2PF6 from 10 3 4PF6 was low (30%), this sequence represents the first example of a redox transmetalation involving a NHC-mercury complex.

In the solvated palladium complex 16, formulated as [LPd (PPh3)(NCMe] 3 2PF6 3 1/2C6H6, one formula unit, devoid of crystallographic symmetry, comprises the asymmetric unit

of the structure (Figure 5, Table 3). The two o-xylyl groups are oriented syn, both being anti to the palladium atom. Geometries listed in Table 3 are compared with those of [LPdX2]( 3 nS) (X = Br, nS = 2 CH3CN (19) (caveat!); X = I, nS = 0.5 MeOH (20))8 and with the adduct X1 = PPh3, X2 = Br (17),18 which all have similar conformations of the cyclophane ligand. Pd-C distances are similar throughout except Pd-C(42) in the PPh3 adducts, which are appreciably lengthened, presumably due to the trans effect of the coordinated triphenylphosphine ligand. In 17, 19, and 20 the angle at Pd subtended by the donor atoms of the nonchelate ligands is larger than the P-Pd-N angle in 16, perhaps due to the increased bulk of the halide substituents (Table 3). For 16, deviations of the bridging C(1,4) from plane 1 are 0.055 (3), 0.042(3), C(2,3)/plane 3 0.062(3), 0.026(3); C(1,2)/plane 2 0.013(3), 0.063(3), C(3,4)/plane 4 0.024(3), 0.044(3) A˚, indicative of diminished strain (cf. the mercury systems 11 and 12). One of the benzene solvent components is disposed about a crystallographic inversion center; the other may be considered as included by the ligand (Figure 5).

Experimental Section General Considerations. Nuclear magnetic resonance spectra were recorded using Bruker Avance 500 (500.13 MHz for

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Figure 4. (a) Projections of the centrosymmetric cation 10 in 10 3 4PF6. (b) Projection of the centrosymmetric molecule of 13, in 13 3 6DMSO. (c) Projection of molecule 1 (with 2 symmetry) in 13 3 2DMSO 3 2H2O showing its unique conformation. Scheme 2

1

H, 125.77 MHz for 13C, and 202.46 MHz for 31P) and Bruker ARX300 (300.14 MHz for 1H, 75.48 MHz for 13C, and 121.50 MHz for 31P) spectrometers at ambient temperature, except where otherwise stated. 1H and 13C chemical shifts were referenced to solvent resonances, and 31P chemical shifts were referenced to an external 85% H3PO4 solution. Assignments of 13C NMR spectra were made with the aid of DEPT, 1 H-13C COSY, and HSQC spectra. Microanalyses were performed by the Microanalytical Laboratory at the Research School of Chemistry, Australian National University, Canberra. Mass spectra were obtained by Dr. A. Reeder using a VG Autospec mass spectrometer via fast atom bombardment

(FAB) with a cesium ion source and a m-nitrobenzyl alcohol matrix. Synthesis of Cyclophane Salts. The cyclophane salts I 3 2PF6, II 3 2Br, III 3 2Br, and IV 3 2Br were prepared by procedures based on the methods of Baker et al.8,12 and Magill et al.9 Cyclophane Salt II 3 2PF6. A solution of II 3 2Br (100 mg, 0.200 mmol) in 5 mL of water was added to a solution of KPF6 (73.3 mg, 0.400 mmol) in 5 mL of water. The resulting white precipitate was collected and dried under vacuum to leave the desired cyclophane salt as a white powder (77 mg, 63%). An analytically pure sample was obtained by recrystallization from acetonitrile/water. 1H NMR (500.13 MHz, CD3CN): δ

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Baker et al.

5.30 (s, 8H, 8 benzylic CH2), 6.74 (s, 2H, 2 Ar H2), 7.42 (d, 4H, 2 Ar H4/H6), 7.52 (m, 6H, 2 imidazolium H4/H5 and 2 Ar H5), 8.33 (s, 2H, s, 2 NCHN) ppm. 13C NMR (125.77 MHz, CD3CN): δ 53.6 (benzylic CH2), 118.3 (Ar CH), 124.4 (imidazolium C4/C5), 126.9 (Ar CH), 130.9 (Ar CH), 136.5 (NCHN), 136.6 (Ar C) ppm. Anal. Calcd for C22H22F12N4P2: C, 41.79; H, 3.51; N, 8.86. Found: C, 41.88; H, 3.44; N, 9.04. Cyclophane Salt III 3 2PF6. This compound was prepared in the same way as II 3 2PF6. Yield: 56%. 1H NMR (500.13 MHz, CD3CN): δ 1.65 (s, 6H, 2 C2-CH3), 2.31 (s, 12H, 2 C4-CH3/ 2 C6-CH3), 5.26 (A part of AB multiplet, 2JH,H = 15 Hz, 4H, 4 benzylic CHH), 5.36 (B part of AB multiplet, 2JH,H = 15 Hz, 4H, 4 benzylic CHH), 6.80 (s, 2H, 2 Ar H5) 7.11 (s, 2H, 2 NCHN), 7.68 (s, 4H, 2 imidazolium H4/H5) ppm. 13C NMR (125.77 MHz, CD3CN): δ 15.5 (C2-CH3), 20.2 (C4-CH3/C6-

Figure 5. Projection of the cation 16 (in 16 3 2PF6 3 11/2C6H6), showing the included benzene molecule.

CH3), 49.4 (benzylic CH2), 125.2 (imidazolium C4/C5), 128.5 (C), 132.9 (NCHN), 139.0 (Ar C), 142.0 (Ar C) ppm. Anal. Calcd for C28H34F12N4P2: C, 46.94; H, 4.78; N, 7.82. Found: C, 46.69; H, 4.62; N, 7.80. Synthesis of the Mercury Complexes. Mercury Complex 10 3 4PF6. Hg(OAc)2 (75 mg, 0.235 mmol) was added to a solution of I 3 2PF6 (100 mg, 158 mmol) in DMSO (6 mL). The mixture was heated at 90 °C for 4 days. A clear solution was obtained. The solvent was removed under reduced pressure to give a white powder, which was collected and dissolved in acetonitrile (4 mL) and filtered through a small glass pipet plugged with about 1 cm of silica. The filtrate was collected and removal of the solvent from the filtrate gave the complex as a white powder (129 mg, 69%). A single crystal of this complex suitable for X-ray diffraction studies was obtained from slow diffusion of ethanol into a solution of the salt in DMSO. 1H NMR (500.13 MHz, d6-DMSO): δ 2.53 (s, 24H, 4 (CH3)2SO), 5.50 (A part of an AX pattern, 2JH,H = 15 Hz, 8H, 8 benzylic CHH), 5.90 (X part of an AX pattern, 2JH,H = 15 Hz, 8H, 8 benzylic CHH), 6.69 (s, 8H, 8 imidazolyl H4/H5), 7.72-7.74 (AA0 part of AA0 XX0 pattern, 8H, 8 Ar H), 7.98-8.00 (8H, XX0 part of AA0 XX0 pattern, 8H, 8 Ar H) ppm. 13C NMR (125.77 MHz, d6-DMSO): δ 40.4 (CH3), 54.0 (benzylic CH2), 123.9 (imidazolyl C4/C5), 130.9 (Ar CH), 134.6 (Ar CH), 133.3 (Ar C), 175.4 (C-Hg) ppm. Anal. Calcd for C44H40Hg2F24N8P4 3 4(CH3)2SO: C, 31.63; H, 3.27; N, 5.68. Found: C, 31.30; H, 2.94; N, 5.99. Mercury Complex 11 3 2PF6. Hg(OAc)2 (28 mg, 0.088 mmol) was added to a solution of II 3 2PF6 (55 mg, 0.087 mmol) in acetonitrile (40 mL), and the mixture was heated at reflux for 24 h. A clear solution was obtained. Removal of solvent under reduced pressure gave a white powder, which was collected, washed with water (2 3 mL), and dried. The white powder was recrystallized from acetonitrile to afford colorless crystals (39 mg, 53%), mp (dec) 253-292 °C. Crystals suitable for X-ray diffraction studies were obtained from acetonitrile/water. 1 H NMR (500.13 MHz, d6-DMSO): δ 5.07 (A part of AX

Table 4. Crystal/Refinement Data [10](PF6)4 3 2DMSO 3 2EtOHa formula Mr cryst syst space group a/A˚ b/A˚ c/A˚ R/deg β/deg γ/deg V/A˚3 Dc (g cm-3) (Z) μMo/mm-1 specimen/mm Tmin/max 2θmax/deg Nt N (Rint) No R1 wR2 (a, (b)) S

C60H88F24Hg2N8 O8P4S6 2222.8 triclinic P1 (#2) 13.108(1) 14.518(1) 14.772(1) 106.265(2) 105.549(2) 108.820(2) 2347 1.573 3.6 0.18,0.15,0.10 0.80 70 42039 20 467 (0.029) 16 238 0.036 0.089 (0.050) 1.03

[11](PF6)2 C22H20F12Hg N4P2 831.0 triclinic P1 (#2) 8.451(2) 9.533(2) 9.622(2) 62.078(4) 64.551(4) 72.364(4) 613.4 2.249 6.5 0.16,0.11,0.04 0.28 65 11997 4375 (0.094) 4064 0.069 0.20 (0.126) 0.97

[12](PF6)2 3 MeCN

[13] 3 6DMSOb

[13] 3 2DMSO 3 2H2Oc

[16](PF6)6 3 3/2C6H6d

C30H35F12Hg N5P2 956.2 monoclinic P21/n (#14) 11.735(1) 14.561(2) 20.476(2)

C52H72Br4Hg2 N12O6S6 1874.4 monoclinic C2/c (#15) 26.397(3) 16.474(2) 16.159(2)

C44H52Br4Hg2 N12O4S4 1597.9 monoclinic C2/c (#15) 37.779(6) 27.298(4) 22.682(4)

101.521(3)

101.015(2)

116.164(3)

3428 1.852 4.7 0.20,0.12,0.09 0.65 65 49805 12 362 (0.064) 8804 0.051 0.14 (0.070, 9.2) 1.04

6898 1.805 7.0 0.17,0.14,0.12 0.79 59 33770 8769 (0.054) 5690 0.058 0.17 (0.097) 1.04

20995 1.517 6.8 0.70,0.12,0.08 0.40 50 97326 18 421 (0.089) 8304 0.067 0.18 (0.090) 0.94

C51H47F12N5 P3Pd 1157.3 triclinic P1 (#2) 13.921(1) 13.968(1) 14.856(1) 72.928(2) 72.060(2) 64.827(2) 2442 1.574 5.7 0.45,0.44,0.43 0.90 75 48166 24 991 (0.021) 19 579 0.041 0.127 (0.085, 1.89) 0.86

a Variata. The solvent component in some regions was subject to serious disorder and insusceptible of meaningful modeling and suppressed using “SQUEEZE”.53 b Crystals desolvated very rapidly in ambience! C, O of the DMSO solvent molecules were modeled with isotropic displacement parameters. Components S, O of DMSO(3) were modeled as disordered over pairs of sites, occupancies 0.5, with associated idealized geometries. c Difference map residues were modeled in terms of DMSO molecules (U(C(301,302)) with isotropic forms in refinement), with less well-defined components as water molecule oxygen atoms (O(02-05) having site occupancies 0.5 and isotropic displacement parameter forms; associated hydrogen atoms were not located). d One of the anions was modeled with its fluorine atoms disordered over pairs of sites, occupancies refining to 0.649(5) and complement; the latter component with idealized geometries.

Article pattern, 2JH,H = 14 Hz, 4H, 4 benzylic CHH), 5.74 (X part of AX pattern, 2JH,H = 14 Hz, 4H, 4 benzylic CHH), 7.67-7.73 (m, 6H, 6 Ar H4/H5/H6) 7.90 (s, 4H, 4 imidazolyl H4/H5), 7.93 (s, 2H, 2 Ar H2) ppm. 13C NMR (125.77 MHz, d6-DMSO): δ 53.8 (benzylic CH2), 125.2 (imidazolyl C4/C5), 132.0 (Ar CH), 133.5 (Ar CH), 133.7 (Ar CH), 134.3 (Ar C), 170.6 (C-Hg) ppm. MS (FAB): m/z 685 (M - PF6, 202 C22H20N200 4 HgPF6), 687 (M - PF6, C22H20N4 HgPF6). Anal. Calcd for C22H20N4HgP2F12: C, 31.80; H, 2.43; N, 6.74. Found: C, 31.91; H, 2.42; N, 6.88. Mercury Complex 12 3 2PF6. This compound was prepared in the same way as mercury complex 11 3 2PF6, starting from III 3 2 PF6. Yield: 59%, mp (dec) 251-284 °C. Crystals suitable for Xray diffraction studies were obtained from acetonitrile/water. 1H NMR (500.13 MHz, d6-DMSO): δ 1.83 (s, 12H, 2 C4-CH3/2 C6-CH3), 2.53 (s, 6H, 2 C2-CH3), 5.58 (A part of AB pattern, 2 JH,H = 15 Hz, 4H, 4 benzylic CHH), 5.64 (B part of AB pattern, 2JH,H = 15 Hz, 4H, 4 benzylic CHH), 7.10 (s, 2H, 2 Ar-H5), 8.07 (s, 4H, 2 imidazolyl H4/H5) ppm. 13C NMR (125.77 MHz, d6-DMSO): δ 16.3 (C4-CH3/C6-CH3), 18.2 (C2CH3), 48.6 (benzylic CH2), 125.8 (imidazolyl C4/C5), 132.9 (Ar CH), 130.6 (Ar C), 137.8 (Ar C), 139.7 (Ar C), 170.0 (C-Hg) ppm. MS (FAB): m/z 769 (M - PF6, C28H32N200 4 HgPF6), 771 (M PF6, C28H32N202 4 HgPF6). Anal. Calcd for C28H32N4HgP2F12: C, 36.75; H, 3.52; N, 6.12. Found: C, 36.70; H, 3.69; N, 6.09. Mercury Complexes 13 and 14 3 2Br. A solution of the pyridinophane IV 3 2Br (20 mg, 0.040 mmol) and Hg(OAc)2 (12.6 mg, 0.040 mmol) in DMSO (ca. 1 mL) was heated to 70 °C under nitrogen in a sealed flask fitted with a Young’s tap, for 17 h. After removal of the volatiles under reduced pressure the residue was triturated with dry acetone under a nitrogen atmosphere to afford a white solid (19.5 mg). Crystals suitable for X-ray diffraction studies were grown from a concentrated solution of the complex in d6-DMSO, as well as by diffusion of vapors between neat acetone and a solution of the complex in DMSO. An analytically pure sample of 13 was obtained by diffusion of acetone vapor into a solution of the product in DMSO. Mp (dec): 247-269 °C. Anal. Calcd for C40H36N12Hg2Br4: C, 34.18; H, 2.58; N, 11.96. Found: C, 33.85; H, 2.89; N, 11.96. Spectroscopic data for 13 (obtained from freshly prepared solutions): 1 H NMR (500.13 MHz, d6-DMSO): δ 5.30 (d, 2JH,H = 16.1 Hz, 8H, 8 benzylic CHH), 5.69 (d, 2JH,H = 16.1 Hz, 8H, 8 benzylic CHH), 7.20 (s, 8H, 4 imidazolyl-H4/H5), 7.31 (d, 3JH,H = 7.7 Hz, 8H, 4 py-H3 and py-H5), 7.61 (t, 3JH,H = 7.7 Hz, 4H, 4 py-H4) ppm. 13C NMR (125.77 MHz, d6-DMSO): δ 53.9 (CH2), 121.3 (imidazolyl C4/C5), 124.0 (py C3/C5), 137.5 (py C4), 154.3 (py C2/C6), 176.4 (Hg-C) ppm. LRMS (FAB): m/z (solid suspension in m-nitrobenzyl alcohol) 1326 (M - Br) [(C20H18N6)2(202Hg)2(79Br)81 2 Br or 202 (C20H18N6)200 Hg79Br(81Br)2 requires 1326]. Spectro2 Hg scopic data for 14 3 2Br [obtained from aged solutions]: 1H NMR (500.13 MHz, d6-DMSO, 53 °C): δ 5.47 (br s, Wh/2 10 Hz, 8H, 4 benzylic CH2), 7.65 (d, 3JH,H = 7.7 Hz, 4H, 2 py-H3 and py-H5), 7.83 (s, 4H, 2 imidazolyl-H4 and H5), 7.91 (t, 3JH,H = 7.7 Hz, 2H, 2 py-H4) ppm. 13C NMR (125.77 MHz, d6-DMSO): δ 54.7 (CH2), 123.9 (imidazolyl C4/C5), 124.8 (py C3/C5), 140.8 (py C4), 153.5 (py-C2/C6), 176.8 (Hg-C) ppm. HRMS (FAB): m/z (solution in d6-DMSO): 623.0467 (M - Br) 79 (C20H18N202 6 Hg Br requires 623.0483). Palladium Complex 16 3 2PF6. In a nitrogen-filled drybox, Pd (PPh3)4 (18 mg, 0.016 mmol) was added to a stirred solution of 10 3 4PF6 (16 mg, 0.008 mmol) in acetonitrile (2 mL). The stirring was continued for 24 h, resulting in a red solution along with a small quantity of a silver-colored liquid. The mixture was filtered through Celite, and the solvent was removed from the filtrate under reduced pressure. The residue was dissolved in a mixture of dichloromethane/ethyl acetate (4:1, 2 mL) and


3803

filtered through a glass pipet plugged with silica, eluting with the same solvent mixture. The filtrate was stripped of solvent, and the residue was dissolved in acetonitrile. Addition of diethyl ether to the acetonitrile solution afforded a pale yellow precipitate. The precipitate was collected, washed with diethyl ether (3 2 mL), and dried in vacuo. Yield: 5 mg, 30%. An analytically pure sample of this complex and crystals suitable for X-ray analysis were obtained by layering a solution of the complex in acetonitrile with benzene. 1H NMR (500.13 MHz, d6-DMSO): δ 1.95 (s, 3H, 1 CH3CN), 4.42 (A part of AX pattern, 2JH,H = 14 Hz, 2H, 2 benzylic CHH), 5.21 (A part of AX pattern, 2JH,H = 14 Hz, 2H, 2 benzylic CH0 H0 ), 5.80 (X part of AX pattern, 2 JH,H = 14 Hz, 2H, 2 benzylic CHH), 6.20 (X part of AX pattern, 2JH,H = 14 Hz, 2H, 2 benzylic CH0 H0 ), 6.94 (s, 2H, 2 imidazolyl H4/H5), 7.27 (d, 5JH,P = 1 Hz, 2H, 2 imidazolyl H40 /H50 trans to PPh3), 7.39-7.50 (m, 8H, 8 Ar H), 7.54-7.61 (m, 15H, Ar-H) ppm. 13C NMR (125.77 MHz, d6-DMSO): δ 1.76 (CH3), 52.6 (benzylic-CH2), 53.1 (benzylic CH2) 118.3 (CtN), 124.1 (d, 4JC,P = 4 Hz, imidazolyl C40 /C50 trans to PPh3), 124.4 (imidazolyl C4/C5), 128.0 (d, J = 49 Hz, Ar C, PPh3), 130.8 (d, J = 11 Hz, Ar CH, PPh3), 131.4 (xylyl Ar CH), 131.5 (xylyl Ar CH), 133.2 (xylyl Ar CH), 133.3 (xylyl Ar CH), 133.4 (d, J = 2 Hz, Ar CH, PPh3), 134.6 (d, J = 12 Hz, Ar CH, PPh3), 135.0 (xylyl Ar C), 135.7 (xylyl Ar C), 152.4 (d, 2JC,P = 7 Hz, C-Pd cis to PPh3), 160.4 (d, 2JC,P = 148 Hz, C-Pd trans to PPh3) ppm. Anal. Calcd for C42H38PdF12N5P3 3 1.5C6H6: C, 52.93; H, 4.09; N, 6.05. Found: C, 53.22; H, 4.11; N, 6.11. Structure Determinations. Full spheres of CCD area-detector diffractometer data were measured (Bruker AXS instrument, ω-scans, T ca. 153 K; monochromatic Mo KR radiation, λ = 0.71073 A˚), yielding Nt(otal) reflections, these merging to N independent (Rint cited) after “empirical”/multiscan absorption correction (proprietary software), which were used in the full matrix least-squares refinements on F2 (non-hydrogen atom anisotropic displacement parameter forms; hydrogen atom treatment following a riding model; reflection weights: (σ2(F 2) + (aP)2 (+ bP))-1 (P = (Fo2 + 2Fc2)/3)); No reflections with F > 4σ(F) were considered “observed”. Computation used the SHELXL-97 program.51,52 Pertinent results are given below and in the tables and figures (the latter showing the nonhydrogen atoms with 50% probability amplitude displacement envelopes, hydrogen atoms (where shown) having arbitrary radii of 0.1 A˚); individual divergences in procedure are recorded as “variata”. Full cif depositions reside with the Cambridge Crystallographic Data Center, CCDC #705682-705687.

Acknowledgment. We thank the Australian Research Council for a Discovery Grant (to M.V.B. and A.H.W.) and an Australian Postgraduate Award (to C.C.W.), the Universiti Sains Malaysia for a postgraduate research scholarship (to R.A.H.), the Gleddon Trust for a Robert and Maude Gleddon Postgraduate Scholarship (to P.V.S.), and Curtin University of Technology for a Research and Teaching Fellowship (to D.H.B.) Supporting Information Available: Cif files for 10 3 4PF6 3 2 DMSO 3 2EtOH, 11 3 2PF6, 12 3 2PF6 3 CH3CN, 13 3 6DMSO, 13 3 2DMSO 3 2H2O, and 16 3 2PF6 3 11/2C6H6. This material is available free of charge via the Internet at http://pubs.acs.org. (51) Sheldrick, G. M. SHELXL-97. A Program for Crystal Structure Refinement; University of Go¨ttingen: Go¨ttingen, Germany, 1997. (52) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112–122. (53) van der Sluis, P.; Spek, A. L. Acta Crystallogr., Sect. A 1990, 46, 194–201.

Mercury Complexes of N-Heterocyclic Carbenes Derived from

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