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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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We initiated our study with a freshly prepared CD2Cl2 solution because equilibrium was not reached for ∼3 h after crystals of 2 were dissolved in CD2Cl2. The E isomer was initially abundant (E′:E:Z = 37:37:28; Figure 4.8, bottom). Addition of 1% and then 2% of acetonitrile-d3 into the CD2Cl2 solution of 2 (these first two additions were recorded prior to equilibrium in order to have abundant E′ and E isomers) showed an increase in the relative abundance of the E′ isomer, as reflected in the intensities of the H6/6′ signals (Figure 4.8). At 21% acetonitrile-d3, the E′ isomer H6/6′ signal was observed but not the E isomer signal (Figure 4.8). Thus, the one set of signals observed in 100% acetonitrile-d3 is that of the E′ isomer, with a negligible (if any) contribution of an E isomer.

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and CDCl3. 1H NMR data for complexes 2, 3, and 4 in CD2Cl2 and CDCl3 (Table C.

1, Supporting Information) indicate that complexes 3 and 4 have E′, E (in most cases), and Z isomers, as found above for 2. In CDCl3 and CD2Cl2 (as in acetonitrile-d3), the signals of the methyl group derived from acetonitrile have a more downfield shift (∼2.2 ppm) characteristic for the E and E′ isomers and a more upfield shift (∼2.0 ppm) characteristic for the Z isomer. An N3H signal having a shift upfield of 4.60 ppm in acetonitrile-d3, CD2Cl2, or CDCl3 was another indication allowing the assignment of signals to the E′ isomer. As found in the case of every solvent for compounds with non-anisotropic R groups, the H6/6′ signal of the E′ isomer in CD2Cl2 or CDCl3 is more downfield than that of the E isomer, which in turn is more downfield than this signal for the Z isomer (Table C.1). This relationship holds true for all solvents, including DMSO-d6 (see below).

These shift analogies were supported by the spectral changes upon dissolution of crystals, which also aided in the assignments. Dissolution of crystals of 3 (R = isobutyl) in CDCl3 gave NMR features (Table C.2) similar to those observed for 2. A 1H NMR spectrum recorded in 5 min contained mostly one large set of peaks of the E isomer, identified by NMR shifts that closely resemble those of the E isomer of 2. Dissolution of crystals of 3 in CD2Cl2 also gave NMR features similar to those observed for 2 (Tables C.1 and C.2), for which three distinct sets of signals were observed, with features expected for the three isomers.

A 1H NMR spectrum of 4 (R = tert-butyl) initially recorded within 5 min of dissolution in CDCl3 showed signals for the three isomers with the E and E′ isomers in abundance. With time, peaks assignable to the Z isomer grew. However, the spectrum obtained upon dissolution of 4 in CD2Cl2 has only two sets of signals. The peaks clearly assignable to the Z isomer were sharp, and these grew with time. Although the signals attributable to the E′ isomer were broad at 25 °C, all signals in the regions characteristic of the E and E′ isomers, including the informative H6/6′ signal, were sharp at 0 °C; these results are consistent with the absence of the E isomer in this solvent (Tables C.1 and C.2).

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Some E′:E:Z equilibrium ratios have been mentioned above for different solvents. In this section we discuss the data further. Because the situation is complicated, we begin with a brief summary.

The E′ isomer is favored by high bulk and polar solvents. The E isomer is insensitive to bulk but favored by low solvent polarity. The Z isomer is favored by low bulk and low solvent polarity.

For 2, E′:E:Z equilibrium ratios in CDCl3 (5:32:63) versus CD2Cl2 (19:19:62) show that while the percentage of the E′ isomer has increased, the abundance of the Z isomer has remained more or less constant. In other words, E′ has increased and E has decreased as the dielectric constant of the solvent increased from 4.8 (CDCl3) to 9.1 (CD2Cl2). This same relationship is valid for 3, and as mentioned above, signals for the E isomer of 4 are no longer observed even in

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Me2bipy)(HNC(CH3)NHR)]BF4 compounds, the E′ isomer is least abundant in CDCl3 (5-15%).

As shown in Table C.2, the abundance of the E′ isomer (64-82%) was greater in the higher dielectric constant (37.5) solvent, acetonitrile-d3, and even higher in DMSO-d6 (dielectric constant = 47.2). Although we do not discuss the NMR studies in DMSO-d6 in detail, the isomer and signal assignments followed the methods discussed for other solvents. For example, an NMR spectrum of 2 recorded in DMSO-d6 showed mostly one set of signals (E′) initially, and a second set of peaks grew with time, reaching equilibrium within 30 min (E′:Z = 78:22). The N4H signals of both the E′ and Z isomers appear more downfield in DMSO-d6 than in acetonitrile, consistent with the good H-bonding and solvation properties of DMSO.

Compared to all solvents used for 2, 3, and 4, the abundance of the Z isomer was lowest (13-22%) in DMSO-d6 (Table C.2). The Z isomer was favored most in CDCl3 (57-64% abundance). Thus, a low dielectric solvent favors the Z isomer for all [Re(CO)3(5,5′Me2bipy)(HNC(CH3)NHR)]BF4 compounds studied.

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isomers for complexes 2 and 3 in CDCl3 and CD2Cl2, the Z′ isomer appears not to be present in several solvents ranging from CDCl3 to DMSO-d6. The Z′ isomer is probably destabilized by interligand steric clashes involving the basal plane defined by the 5,5′-Me2bipy N atoms and the trans carbonyl C atoms (Figure C.7, Supporting Information). Such clashes are likely to be severe in octahedral complexes but are likely to be less important in square-planar complexes, and the Z′ isomer has been reported for PtII.14 Complexes with the fac-{ReI(CO)3} core are generally sterically undemanding compared to other octahedral complexes because ReI–N bonds are longer than typical M–N bonds for metal ions in an octahedral environment and the CO ligands are relatively non-bulky. Nevertheless, even in this favorable case, steric clashes appear to preclude formation of significant amounts of the Z′ isomer.

The unstable nature of the Z′ isomer suggests that the pathway for the interconversion of the E′ to Z isomers passes through the E isomer and not through the Z′ isomer. Furthermore, the findings for 2 in CDCl3, namely that the E′/E ratio remained constant while the amount of Z increased with time and that only E′-to-E EXSY cross-peaks were present in the ROESY spectrum, suggest that the E′-to-E interconversion is facile. Thus, the slow steps in the E′-to-Z interconversion are the E-to-Z interconversion. A likely reaction pathway scheme is shown in Figure C.8, Supporting Information.

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in acetonitrile-d3 at 25 °C recorded at 15 min and 1 day were very similar, containing two very broad NH signals (6.30 and 5.93 ppm) and one fairly sharp NH signal at 5.45 ppm (each integrating to one proton) for the major Z isomer. Sharp H6/6′ signals were observed, with the smaller signal downfield, suggesting that ~12% E′ isomer was also present. The signal at 5.45 ppm did not shift with temperature and can be assigned to N3H of the Z isomer. At 5 °C, the two broad NH signals of equal intensity became sharp (6.60 and 5.81 ppm). At 35 °C, the broad signals merged to give one peak (6.09 ppm), the total intensity remaining the same. These results

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(HNC(CH3)NH2)]BF4 and that elevated temperature increases the rate of rotation around the C– NH2 bond. The NH peaks of the minor E′ isomer could not be identified. The H6/6′ and amidine CH3 signals were used to obtain the ratio of the E′ and Z isomers (Table 4.4).

Structural analysis of fac-[Re(CO)3(5,5′-Me2bipy)(HNC(CH3)OCH3)]BF4, an iminoether analogue of the amidine complexes in this study, has revealed that the iminoether ligand has the Z configuration in the solid state.26 The oxygen of the iminoether ligand is sterically less bulky than the NHR group of [Re(CO)3(5,5′-Me2bipy)(HNC(CH3)NHR)]BF4 complexes. Combining these findings for [Re(CO)3(5,5′-Me2bipy)(ligand)]BF4 complexes in which the ligand is an amidine or an iminoether, we suggest that the Z configuration is favored electronically, but the E′ configuration is favored by steric effects.

Robustness of the Isopropylamidine Ligation. When [Re(CO)3(5,5′-Me2bipy) (HNC(CH3)NHCH(CH3)2)]BF4 (2) in acetonitrile-d3 or CDCl3 was treated with a fivefold excess of 4-dimethylaminopyridine, no major changes in spectral features of the amidine complex were observed, even up to 2 months, indicating that the isopropylamidine ligand is not readily replaced. [Re(CO)3(5,5′-Me2bipy)(4-dimethylaminopyridine)]BF4 was synthesized, and the NMR shifts were recorded in acetonitrile-d3 and CDCl3 as a control.

4.4 Conclusions The [Re(CO)3(5,5′-Me2bipy)(CH3CN)]BF4 complex (1) readily forms [Re(CO)3(5,5′Me2bipy)(amidine)]BF4 complexes. The facility of this reaction for a low-oxidation-state, thirdrow transition metal complex very likely reflects the fact that the CO ligands reduce the electron density on the metal via π back-bonding. In these complexes, the amidine ligand is attached robustly, but it does not exhibit a trans influence.

Because the amidine grouping is not in a ring and the ligand is monodentate, the configuration is not restricted. Furthermore, there is no interligand hydrogen bonding possible to influence the stereochemistry. Thus, the configuration is influenced primarily by electronic and steric effects. The E′ isomer crystallized for [Re(CO)3(5,5′-Me2bipy)(HNC(CH3)NHR)]BF4 (R = methyl, isopropyl, isobutyl, tert-butyl, and benzyl), whereas the Z isomer crystallized for [Re(CO)3(5,5′-Me2bipy)(HNC(CH3)NH2)]BF4. Increased bulk of the R group favors the E′ configuration of the amidine ligand because the alkyl group projects away from the basal plane.

The Z configuration is favored electronically, as evidenced by the structure of [Re(CO)3(5,5′Me2bipy)(HNC(CH3)NH2)]BF4 and also by the structure of several [Re(CO)3(L)(iminoether)]BF4 complexes analyzed in unpublished work.

The exchange reaction between the E′ and Z isomers of [Re(CO)3(5,5′-Me2bipy) (HNC(CH3)NHR)]BF4 (R = isopropyl, isobutyl and tert-butyl) complexes is too slow to be observed on the NMR time scale. The isomerization rate is slow because there is double-bond character in the bond between the amidine C and the N bound to Re. However, the isomerization rate between the E′ and E isomers is fast because there is less double-bond character in the bond between the amidine C and the remote N. Exchange between the E′ and Z isomers is likely to follow a pathway through the E isomer because the Z′ isomer (not observed) is likely to be unstable as a result of strong steric clashes between the R group and the basal ligands in this isomer.

4.5 References

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11. Decouzon, M.; Gal, J.-F.; Maria, P.-C. Rapid Commun. Mass. Spectrom. 1993, 7, 599-602.

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Leito, I. J. Phys. Chem. A 2007, 111, 1245-1250.

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Bertani, R. J. Biol. Inorg. Chem. 2007, 12, 477-493.

14. Belluco, U.; Benetollo, F.; Bertani, R.; Bombieri, G.; Michelin, R. A.; Mozzon, M.; Tonon, O.; Pombeiro, A. J. L.; da Silva, F. C. G. Inorg. Chim. Acta 2002, 334, 437-447.

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A.; Gandin, V.; Marzano, C. J. Med. Chem. 2007, 50, 4775-4784.

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P.; Caputo, P.; Natile, G. J. Med. Chem. 1993, 36, 510-512.

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

27. Otwinowski, Z.; Minor, W. Macromolecular Crystallography, Part A, Methods in Enzymology; New York Academic Press: New York, 1997; Vol. 276, pp. 307-326.

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