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«A Dissertation Submitted to the Graduate Faculty of the Louisiana State University and Agricultural and Mechanical College in partial fulfillment of ...»

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1H NMR spectra of [Re(CO)3(MAE)]PF6 (5 mM) in DMSO-d6 at 25 °C before (top) and 20 min after (bottom) addition of OH– (1.7 mM). The multiplet at ~4.2 ppm arises from one of the C7H2 protons.

To confirm that OH– is acting as a catalyst and does not coordinate directly to Re, a 1.7 mM NaOH and a 0.54 mM NaOH solution were prepared from the same stock solution of [Re(CO)3(MAE)]PF6. If hydroxide adds to Re and is not acting as a catalyst, decreasing the hydroxide concentration to 0.54 mM would not only decrease the rate of reaction but would also decrease the percentage of the new product. When the solutions were monitored by NMR spectroscopy, the 1.7 mM NaOH sample reached equilibrium in 20 min as before, whereas the

0.54 mM NaOH sample required more time (1-2 h) to reach equilibrium, as expected. In both solutions, the new product abundance was the same (13%). Thus, hydroxide is not coordinating but is acting as a catalyst.

In the identical 1.7 mM NaOH study, but with [Re(CO)3(EAE)]BF4, the new minor set of signals appeared and reached its maximum intensity in 40 min (12% minor isomer, 88% major isomer). The new endo-NH signal appears more downfield (5.78 ppm) than the exo-NH signal (4.40 ppm) of the original isomer. The new N4H signal appeared upfield (6.77 ppm) of the initial N4H signal (7.03 ppm). The faster rate of isomerization of [Re(CO)3(MAE)]PF6 versus [Re(CO)3(EAE)]BF4 is attributable to the slightly greater N1H acidity of [Re(CO)3(MAE)]PF6, as suggested by its shorter NH to ND exchange half-life than that of [Re(CO)3(EAE)]BF4.

When Cl– was added to a base-isomerized sample of [Re(CO)3(MAE)]PF6, the N1H signal of the minor endo-N1H/exo-N1CH3 isomer shifted only minimally downfield (∆δ ∼ 0.28 ppm at 150 mM Cl–), while the N4H signal was not shifted. The NH signals of the starting major exo-N1H/endo-N1CH3 isomer shifted as described previously. This observation highlights a new application of Cl– addition as an aid in identifying which isomer of a fac-[Re(CO)3L]n complex has an endo-NH/exo-alkyl and which has an exo-NH/endo-alkyl terminal amine. The small ∆δ for the N4H signal further indicates that this proton is not very acidic. The isomerization at N1 should have no effect on the electronic character of N4. This result and the relatively long halflife for H to D exchange indicate a lower acidity for the N4H. This lower acidity in turn means that N4 is relatively electron rich, and thus N4 is poised to accept a proton during the reaction pathway to [Re(CO)3(DAE)]BF4 (Scheme 3.2).

3.4 Conclusions As shown by Natile et al.30 for a stable PtII compound with a closely related sevenmembered ring, we conclude that the ReI seven-membered chelate rings in [Re(CO)3(MAE)]PF6 and [Re(CO)3(EAE)]BF4 products shown in Scheme 3.1 arise from intramolecular attack by a terminal amine on a coordinated acetonitrile. The novel DAE ligand in [Re(CO)3(DAE)]BF4 provides an interesting and highly unusual variation having an sp2 N donor derived from an sp3 primary amine. Complexed DAE has a five-membered chelate ring and a dangling NH2 group.

DAE is formed via a series of steps, and the likely pathway has two proton transfer steps and a Re–N bond disruption step, Scheme 3.2. The stable PtII and ReI complexes with seven-membered chelate rings cannot convert to a complex with the DAE-type ligand (Scheme 3.3), because the endocyclic nitrogen bears an alkyl or aryl group rather than the proton needed for the second proton transfer step forming the dangling NH2 group in DAE, as shown in Scheme 3.2. Thus, the seven-membered chelate ring of the MAE-type ligand serves as a model for one likely intermediate in the formation of the DAE ligand. Two types of evidence suggest that the sp2 NH donor bound to Re in this MAE-type ring is poised to undergo dissociation and protonation.

First, [Re(CO)3(MAE)]PF6 has a large Re–N4–C11 angle, indicating strain and favoring Re–N bond breaking. Second, N4H is relatively non-acidic (revealed by slow H to D exchange and by weak interaction with chloride ion of the N4H of the minor endo-N1H/exo-N1CH3 isomer of [Re(CO)3(MAE)]PF6), indicating an electron-rich N ready to accept a second proton.

Our studies on chloride interaction complement reports of the use in anion receptors36 of transition-metal fragments to serve as scaffolds onto which H-bonding donor groups can be connected.23 Chloride and other anions interact with the pyrazole NH’s of fac-[ReI(CO)3(generic pyrazole)3]+ cations.22 This work is related to our present findings on chloride interactions with six-coordinate ReI tricarbonyl complexes. However, we expand the field of the interaction of anions with NH groups in metal complexes by showing that such interactions provide a useful approach for both interpreting NMR data and for elucidating the structure of isomers; such information is useful for probing properties of Re analogues of 99mTc radiopharmaceuticals.

3.5 References

1. Lipowska, M.; He, H.; Malveaux, E.; Xu, X.; Marzilli, L. G.; Taylor, A. T. J. Nucl. Med.

2006, 47, 1032-1040.

2. Alberto, R.; Schibli, R.; Schubiger, A. P.; Abram, U.; Pietzsch, H. J.; Johannsen, B. J. Am.

Chem. Soc. 1999, 121, 6076-6077.

3. Desbouis, D.; Struthers, H.; Spiwok, V.; Küster, T.; Schibli, R. J. Med. Chem. 2008, 51, 6689-6698.

4. Wei, L.; Babich, J.; Zubieta, J. Inorg. Chim. Acta 2005, 358, 3691-3700.





5. Schibli, R.; Schubiger, A. P. Eur. J. Nucl. Med. Mol. Imaging 2002, 29, 1529-1542.

6. Rattat, D.; Eraets, K.; Cleynhens, B.; Knight, H.; Fonge, H.; Verbruggen, A. Tetrahedron Lett. 2004, 45, 2531-2534.

7. Lipowska, M.; Cini, R.; Tamasi, G.; Xu, X.; Taylor, A. T.; Marzilli, L. G. Inorg. Chem. 2004, 43, 7774-7783.

8. Lipowska, M.; Marzilli, L. G.; Taylor, A. T. J. Nucl. Med. 2009, 50, 454-460.

9. Lipowska, M.; He, H.; Xu, X.; Taylor, A. T.; Marzilli, P. A.; Marzilli, L. G., Manuscript in Preparation.

10. Christoforou, A. M.; Marzilli, P. A.; Fronczek, F. R.; Marzilli, L. G. Inorg. Chem. 2007, 46, 11173-11182.

11. Christoforou, A. M.; Fronczek, F. R.; Marzilli, P. A.; Marzilli, L. G. Inorg. Chem. 2007, 46, 6942-6949.

12. He, H. Y.; Lipowska, M.; Christoforou, A. M.; Marzilli, L. G.; Taylor, A. T. Nucl. Med.

Biol. 2007, 34, 709-716.

13. He, H.; Lipowska, M.; Xu, X.; Taylor, A. T.; Marzilli, L. G. Inorg. Chem. 2007, 46, 3385He, H.; Lipowska, M.; Xu, X.; Taylor, A. T.; Carlone, M.; Marzilli, L. G. Inorg. Chem.

2005, 44, 5437-5446.

15. Bartholomä, M.; Valliant, J.; Maresca, K. P.; Babich, J.; Zubieta, J. Chem. Commun.

(Cambridge, U.K.) 2009, 5, 493-512.

16. Banerjee, S. R.; Babich, J.; Zubieta, J. Inorg. Chem. Commun. 2004, 481-484.

17. Michelin, R. A.; Mozzon, M.; Bertani, R. Coord. Chem. Rev. 1996, 147, 299-338.

18. Kukushkin, V. Y.; Pombeiro, A. J. L. Chem. Rev. 2002, 102, 1771-1802.

19. Natile, G.; Intini, F. P.; Bertani, R.; Michelin, R. A.; Mozzon, M.; Sbovata, S. M.; Venzo, A.; Seraglia, R. J. Organomet. Chem. 2005, 690, 2121-2127.

20. Cornacchia, D.; Pellicani, R.; Intini, F. P.; Pacifico, C.; Natile, G. Inorg. Chem. 2009, DOI:

10.1021/ic9011372.

21. Pérez, J.; Riera, L. Chem. Soc. Rev. 2008, 37, 2658-2667.

22. Nieto, S.; Pérez, J.; Riera, L.; Miguel, D.; Golen, J. A.; Rheingold, A. L. Inorg. Chem. 2007, 46, 3407-3418.

23. Nieto, S.; Pérez, J.; Riera, L.; Riera, V.; Miguel, D. Chem.-Eur.J. 2006, 12, 2244-2251.

24. Schmidt, S. P.; Trogler, W. C.; Basolo, F. Inorg. Synth. 1990, 28, 160-165.

25. Perera, T.; Fronczek, F. R.; Marzilli, P. A.; Marzilli, L. G., Manuscript in Preparation.

26. Edwards, D. A.; Marshalsea, J. J. Organomet. Chem. 1977, 131, 73-91.

27. Otwinowski, Z.; Minor, W. Macromolecular Crystallography, part A; New York Academic Press: New York, 1997; Vol. 276.

28. Sheldrick, G. M. "SHELXL97, Program for Crystal Structure Solution and Refinement," University of Gottingen: Gottingen, Germany, 1997.

29. Maresca, L.; Natile, G.; Intini, F. P.; Gasparrini, F.; Tiripicchio, A.; Camellini, M. T. J. Am.

Chem. Soc. 1986, 108, 1180-1185.

30. Tiripicchio, A.; Camellini, M. T.; Maresca, L.; Natile, G. Acta Crystallogr., Sect. C 1990, C46, 549-551.

31. Kukushkin, Y. N.; Kiseleva, N. P.; Zangrando, E.; Kukushkin, V. Y. Inorg. Chim. Acta 1999, 285, 203-207.

32. Syamala, A.; Chakravarty, A. R. Inorg. Chem. 1991, 30, 4699-4704.

33. Britten, J. F.; Lock, C. J. L.; Pratt, W. M. C. Acta Crystallogr., Sect. B 1982, 38, 2148-2155.

34. Maheshwari, V.; Carlone, M.; Fronczek, F. R.; Marzilli, L. G. Acta Crystallogr., Sect. B 2007, B63, 603-611.

35. Perera, T.; Marzilli, P. A.; Fronczek, F. R.; Marzilli, L. G., Manuscript in Preparation.

36. Beer, P. D. Chem. Commun. (Cambridge, U.K.) 1996, 689-696.

–  –  –

The chemistry of amidine complexes of several metals (including platinum, iridium, cobalt, and manganese) has been described in several reviews.1-3 Because the fac-{MI(CO)3} (M = Tc or Re) core is important in radiopharmaceuticals,4-6 the acetonitrile reaction chemistry of ReI complexes has recently become a subject of scrutiny.7-9 Both of the amidine carbon-to-nitrogen bonds have double-bond character. The bond between the amidine carbon and the superbasic coordinated nitrogen10-12 leads to two configurations dictating the description of amidine stereochemistry as E or Z (Figure 4.1). When the remote nitrogen has two different substituents, the two resulting configurations about the bond to the amidine carbon lead to a total of four conceivable configurations depicted in Figure 4.1 (E, E′, Z, and Z′ labels follow previous usage).13,14 Whereas detection of isomers involving the two configurations about the bond between the amidine carbon and the metal-bound nitrogen is a common occurrence for amidine ligands as well as iminoether ligands,15 there have been few reports of isomers resulting from restricted rotation about the other amidine carbon-to-nitrogen bond, in part because amidine ligands often have a symmetrical remote NH2 or NR2 grouping, thus precluding such isomers. Also, in previously studied ReI carbonyl compounds, ring formation restricted the amidine stereochemistry.7,8 *Reproduced with permission from American Chemical Society: Perera, T.; Fronczek, F. R.;

Marzilli, P. A.; Marzilli, L. G., “Superbasic Amidine Monodentate Ligands in fac-[Re(CO)3(5,5′Me2bipy)(Amidine)]BF4 Complexes: Dependence of Amidine Configuration on the Remote Nitrogen Substituents,” Inorg. Chem. 2010, DOI: 10.1021/ic100714m. Copyright 2010 American Chemical Society.

Most reports on monodentate amidine ligands of interest to the current study on complexes with the fac-{ReI(CO)3} core involve pseudo square planar PtII complexes, about which many studies have been performed in view of the demonstrated cytotoxicity of many iminoether and amidine Pt complexes.13,16-18 Iminoether and amidine ligands are related. Natile and co-workers have contributed substantially to this field because the first PtII compound with a trans configuration shown to have anticancer activity was an iminoether complex;19 these and other investigators later extended such studies to the evaluation of ketimine20 and amidine13,21,22 Pt complexes.

Figure 4.1.

Conceivable fac-[Re(CO)3(L)(HNC(CH3)NHR)]+ isomers, in which L is a bidentate ligand denoted by N–N donor atoms.

In reports on Pt complexes, the dependence of the amidine ligand configuration on the bulk and the presence of NH groups of the remote amidine NR2, NHR, or NH2 group in the ligand have been assessed.14,22,23 Both amidine ligands in trans-[Pt(HNC(CH3)NHCH3)2Cl2] have the Z configuration,22 thought to be stabilized by strong intramolecular H-bonds between chloride and the remote NH group.22 The amidine ligands in cis-[Pt(HNC(CH3)N(CH3)2)2Cl2] lack a remote NH group, however, and adopt the E configuration.23 Belluco et al. reported that, on the basis of 1H NMR data, the addition reactions of primary amines and secondary amines to cis- and trans-[PtCl2(PhCN)2] afforded a complicated mixture of amidine complexes, with the amidine having a mixture of E, E′, Z, and Z′ configurations in CD2Cl2.14 In a more recent study, Marzano et al. conducted additional chemical studies of some of these amidine complexes because they have promising antitumor activity.13 These investigators noted that the reaction forming cis-[PtCl2(HNC(Ph)NHCH3)2] afforded a mixture of E, E′, Z, and Z′ isomers, unlike such reactions with acetonitrile derivatives; heating converted the product to the Z isomer stabilized by intramolecular H-bonding to chloride ligands.13 Both steric effects and H-bonding play a role in dictating stereochemistry in these Pt amidine complexes.22,23 Nucleophilic attack of pyrazole on a coordinated, metal-activated nitrile resulted in the formation of a pyrazolylamidino chelate ring in ReI carbonyl compounds.7 We recently reported some unusual ReI amidine complexes formed by attack of primary or secondary amine terminal groups of polyamines on coordinated acetonitrile, in one case giving a seven-membered chelate ring.8 The starting complex in that study8 had three coordinated acetonitrile ligands, and the attacking amines were complicated.

To assess ReI amidine chemistry, we have now investigated amidine products formed by treating fac-[Re(CO)3(5,5′-Me2bipy)(CH3CN)]BF4 (a complex with only one coordinated acetonitrile) with ammonia and amines (Figure 4.2). In the resulting complexes, such as facRe(CO)3(5,5′-Me2bipy)(HNC(CH3)NHR)]BF4, the monodentate amidine ligand can conceivably have any of the four possible configurations. Unlike in the cases of many other Re and Pt complexes, these configurations will not be confounded by H-bonding interactions or controlled by ring formation. Only steric and solvent effects and intrinsic electronic structures of the amidine ligands will influence geometry and stability. Two-dimensional NMR spectroscopy, in conjunction with structural characterization of several complexes by single-crystal X-ray crystallography, has been utilized to evaluate which monodentate amidine ligand configurations (Figure 4.1) are favored in solution.

Figure 4.2.

[Re(CO)3(L)(HNC(CH3)NHR)]+ isomers observed upon treatment of [Re(CO)3(5,5′Me2bipy)(CH3CN)]+ with RNH2 in acetonitrile at 25 °C.



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