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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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Implications for the Reaction Pathway Forming [Re(CO)3(DAE)]BF4 from the Structures of [Re(CO)3(MAE)]PF6 and [Re(CO)3(MAEH)F]PF6. We believe that a complex of the [Re(CO)3(MAE)]+ type initially formed as an intermediate, which then rearranged to give [Re(CO)3(DAE)]+ (Scheme 3.2). We propose that the early phase of the reaction between N,NMe2dien and [Re(CO)3(CH3CN)3]BF4 includes an attack of a terminal amine on the sp carbon of a coordinated acetonitrile. This attack leads to formation of a seven-membered ring (as found in [Re(CO)3(MAE)]PF6); this ring then rearranges to give the [Re(CO)3(DAE)]BF4 product. The key reason that the rearrangement is possible relates to the fact that the putative [Re(CO)3(MAE)]+ type intermediate on the pathway to the [Re(CO)3(DAE)]BF4 product would have an endocyclic NH (N3H) rather than an N3Me group in the seven-membered ring of [Re(CO)3(MAE)]PF6 (Scheme 3.3).

Scheme 3.2.

The likely pathway for DAE ligand formation from N,N-Me2dien and acetonitrile (green). Stars indicate the location of the lone pair on the nitrogens known to be used or likely to be used in Re coordination at each stage of the process. The fac-{Re(CO)3}+ fragment is not shown.

Scheme 3.3.

The likely pathway for MAE ligand formation from N,N′,N′′-Me3dien and acetonitrile (green). Stars indicate the location of the lone pair on the nitrogens known to be used or likely to be used in Re coordination at each stage of the process. The fac-{Re(CO)3}+ fragment is not shown. Only one proton can be transferred (in the second step). The hypothetical step which would be needed to form the DAE analogue does not occur because the methyl group can not transfer.

The stable [Re(CO)3(MAE)]+ and [Re(CO)3(EAE)]+ compounds share with the stable Pt complex, cis-[Pt(NH=CPhNButCH2CH2NHBut)Cl2],30 the feature that the endocyclic noncoordinated nitrogen does not have a bound proton. All bonds to N3 in [Re(CO)3(MAE)]PF6 are to carbon. The N3 atom in this ring has no lone pair available to coordinate to Re. However, the similarly situated N3 of the postulated seven-membered ring of the intermediate on the pathway to [Re(CO)3(DAE)]BF4 has one bound proton; this N3H could release its proton as it generates the lone pair needed to form the Re–N3 bond. This proton can serve as the proton needed to convert the dissociating N4H to a NH2 group (Scheme 3.2); this process could be stepwise or concerted. In the Supporting Information, we illustrate some of these points using a scheme based on the X-ray structures.

The large angle within the chelate ring involving N4 (Re–N4–C11 ~ 139°) of [Re(CO)3(MAE)]PF6 provides evidence that strain may predispose N4 to dissociate. The molecular structure of [Re(CO)3(MAEH)F]PF6 (Figure 3.2b) may provide some evidence as to the tendency of N4H to dissociate and be protonated to form an NH2 group. Although the process involves a proton from the HF formed in situ from the PF6– anion or present as an impurity in the AgPF6 reagent, [Re(CO)3(MAEH)F]PF6 may be considered to resemble a species that could be present in the rearrangement pathway discussed in the preceding paragraph if the process is not concerted.

NMR Spectroscopy. All complexes reported were characterized by NMR spectroscopy in DMSO-d6 and acetonitrile-d3. On the basis of the molecular structure of [Re(CO)3(MAE)]PF6, two different types of NH groups are present, as illustrated in Figure 3.3. For fac-[Re(CO)3L]n complexes,7,10 N1H is referred to as an exo-NH proton, as it points away from the carbonyl groups (versus an endo-NH proton pointing toward the carbonyl groups). The N4H is not classified in this way because N4 is sp2 hybridized. Two 1H NMR peaks of [Re(CO)3(MAE)]PF6 in DMSO-d6 decreased in size when D2O was added, indicating that these are NH signals. The broad NH signal at 4.50 ppm (Table 3.3), assigned to the exo-N1H from the COSY cross-peak to the N1CH3 doublet at 2.91 ppm (Figure B.2, Supporting Information), falls within the range observed for exo-NH’s.10 Of the four methyl peaks for [Re(CO)3(MAE)]PF6, only N1CH3 could be a doublet. The sharp NH peak at 7.17 ppm, which must be from N4H, has a COSY cross-peak to a singlet at 2.14 ppm; this must be the C12H3 signal.

Figure 3.3.

Designation of the exo-NH proton (pointing away from the carbonyl groups and toward the hydrophobic pocket of the ligand) of [Re(CO)3(MAE)]PF6 and [Re(CO)3(EAE)]BF4.

N4H is not classified as either endo or exo.

Table 3.3.

1H NMR Chemical Shifts (ppm) of [Re(CO)3(DAE)]BF4 (1), [Re(CO)3(MAE)]PF6 (2), and [Re(CO)3(EAE)]BF4 (4) in DMSO-d6 at 25 °C and Selected NH Shifts in Acetonitriled3a <

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Additional confirmation for the C12H3 signal assignment comes from the presence of a similar methyl singlet at 2.18 ppm in the spectrum of [Re(CO)3(EAE)]BF4, which can have only the C12H3 signal as a singlet because the other three methyl groups are in the ethyl groups. Many signals such as a multiplet at ∼4.2 ppm (assigned by COSY to one of the protons of the C7H2 methylene group bonded to the delocalized N3–C11–N4 amidine group) and the NH signals have similar shifts for both [Re(CO)3(MAE)]PF6 and [Re(CO)3(EAE)]BF4 (Table 3.3). For both, the NH signals are more upfield in acetonitrile versus DMSO, a finding attributable to the weaker interaction of acetonitrile with the NH groups.

For [Re(CO)3(DAE)]BF4 (Figure 3.1), 2D NMR experiments were used to assign the signals in DMSO-d6 (Figures B.3 and B.4, Supporting Information), and NH signals were identified by addition of D2O (Table 3.3). The N1CH3 signals were assigned from an NOE crosspeak at 3.03 and 2.44 ppm. The peak of the central NH (N2H, 7.10 ppm) was identified by COSY and NOE correlations with two multiplets, at 3.19 and 2.97 ppm, from the N2-CH2 groups. The COSY cross-peak between the relatively sharp NH peak at 7.28 ppm (Figure 3.4) and the CCH3 signal at 2.31 ppm (Figure B.4, Supporting Information) assigns these to N4Ha and the methyl group derived from acetonitrile. (See Figure 3.5 for the designations of the N4Ha and N4Hb protons.) This assignment was confirmed by the relatively stronger NOE cross-peak from the CCH3 peak to the N4Hb signal (7.99 ppm) than to the N4Ha signal.

In general, the NH signals of the new compounds were sharp at 25 °C. In contrast, for [Re(CO)3(DAE)]BF4 the NH2 peaks were sharp in DMSO-d6 but broad in acetonitrile-d3 (Figure 3.4), a solvent that interacts weakly with NH groups. We reasoned that the NH2 group may rotate at room temperature, but that DMSO may restrict this rotation by H-bonding to the NH2 protons.

Indeed, all three NH signals were sharp in acetonitrile-d3 at -5 °C. Increasing the temperature resulted in considerable broadening of the NH2 signals (Figure B.5, Supporting Information). The peak width of the central NH (N2H) was hardly affected. The behavior of the NH2 signals above 25 °C was complicated, suggesting that a dynamic process in addition to rotation about the C–N bond occurs at elevated temperatures. Therefore, we performed a ROESY experiment at –5 °C in acetonitrile-d3. The spectrum contained a negative cross-peak between NH signals at 6.58 and

6.02 ppm (Figure B.6, Supporting Information). The magnitude was larger than a negative N4HaN4Hb exchange cross-peak in a ROESY spectrum recorded in DMSO-d6 at room temperature (Figure B.3, Supporting Information). Thus, the rate of rotation of the N4H2 group is faster in acetonitrile-d3 at –5 °C than in DMSO-d6 at 25 °C.

Figure 3.4.

1H NMR spectrum showing NH signals of [Re(CO)3(DAE)]BF4 in DMSO-d6 at 25 °C (top), in acetonitrile-d3 at 25 °C (middle), and in acetonitrile-d3 at -5 °C (bottom).

Figure 3.5.

Designation of the N4Ha and N4Hb protons of [Re(CO)3(DAE)]BF4, indicating the proximity of N4Hb to the methyl group and the rotation of the NH2 group.

NH to ND Exchange. After addition of D2O (100 µL) to DMSO-d6 solutions (5 mM, 600 µL), the two NH2 protons of [Re(CO)3(DAE)]BF4 underwent immediate H to D exchange, whereas the central N2H had an exchange half-life of ∼5 min. A similar experiment for [Re(CO)3(MAE)]PF6 showed that the exo-NH in the terminal NH(CH3) group had a half-life of ∼30 min, and N4H had a half-life of 24 h. The N4H exchange half-life of [Re(CO)3(EAE)]BF4 (24 h) was the same as that of [Re(CO)3(MAE)]PF6. However, the half-life of the exchange of the exo-N1H of [Re(CO)3(EAE)]BF4 was longer (∼1 h) than that of [Re(CO)3(MAE)]PF6, undoubtedly because of the better electron donation of Et vs. Me. These results indicate that the Re-bound sp2 N of the seven-membered ring is relatively electron rich and therefore could protonate and dissociate.

Under similar experimental conditions, the exchange half-lives of the [Re(CO)3(dien)]PF6 complex were found to be ∼1 h, 2 h, and 12 h for the central, exo-NH and endo-NH protons, respectively.

Interaction of NH Protons with the Cl– Anion. When Cl– was added to 5 mM solutions of fac-[Re(CO)3L]+ complexes, downfield shift changes, ∆δ, were observed.10 The ∆δ of the relatively upfield [Re(CO)3(MAE)]PF6 exo-N1H signal reached a plateau of ∼1.3 ppm at 100 mM Cl– in DMSO-d6 (Figure 3.6). This behavior is in agreement with the finding that the exoNH group typically shifts downfield by ~1 ppm in DMSO-d6 on addition of Cl– to facRe(CO)3L]+ complexes having L = dien or simple dien-related derivatives.10 Because H-bonding to solvent causes downfield shifts, the relatively upfield shift of exo-NH signals was attributed to steric hindrance to solvation of the exo-NH groups of [Re(CO)3(dien)]PF6 by virtue of their being located in the pocket (as illustrated for [Re(CO)3(MAE)]PF6 in Figure 3.3 above). This ∆δ behavior was explained by suggesting that the Cl– anion, owing to its small size, can enter the sterically hindered pocket and form H-bonds to the two exo-NH’s. These exo-NH’s are close enough (~2.5 Å) to interact simultaneously with the chloride ion; this two-proton interaction was more favorable than when only one exo-NH was present in the complex.10 A similar explanation accounts for the ∆δ of the exo-NH signal of [Re(CO)3(MAE)]PF6. The plot in Figure 3.6 is very close to that for the exo-NH of [Re(CO)3(dien)]PF6.10 The ∆δ of the N4H signal of [Re(CO)3(MAE)]PF6 exactly paralleled that of the exo-NH, but ∆δ was only ∼0.6 ppm. These results are consistent with synergistic interaction with chloride of both NH protons, which are separated by ~2.5 Å (Figure 3.3).

Figure 3.6.

Change in chemical shift (∆δ, ppm) of the NH signals of [Re(CO)3(MAE)]PF6 (5 mM) caused by added Cl– in DMSO-d6 at 25 °C.

In acetonitrile-d3, on addition of Cl– the relatively upfield [Re(CO)3(MAE)]PF6 exo-N1H signal (at 3.39 ppm, Table 3.3) shifted downfield (∆δ ∼ 3.3 ppm). The N4H signal (at 6.41 ppm) shifted downfield only ∼ 1.8 ppm. The maximum shift change was observed at a much lower concentration of Cl– in acetonitrile-d3 (20 mM, Figure 3.7) than in DMSO-d6 (~100 mM), consistent with the weaker H-bonding of the acetonitrile solvent. The plots for ∆δ for both signals are parallel for the reason given in the preceding paragraph, namely both protons interact with chloride ion.

Figure 3.7.

Change in chemical shift (∆δ, ppm) of the NH signals of [Re(CO)3(MAE)]PF6 (5 mM) caused by added Cl– in acetonitrile-d3 at 25 °C.

When Cl– was added to a solution of [Re(CO)3(DAE)]BF4 in acetonitrile-d3, the three NH signals shifted downfield in parallel (∆δ plateau values at ∼110 mM Cl– = ∼1.5, ~0.8, and ∼2 ppm for N4Ha, N4Hb, and the central NH, respectively, Figure 3.8). A ∆δ of 2 ppm is large for a central NH signal, as the maximum change in shift for the central NH of related N donor L’s in fac-[Re(CO)3L]n complexes is ≤ 1 ppm and occurs at higher Cl– concentration (unpublished results).10 The NH2 group of [Re(CO)3(DAE)]BF4 is not close enough to the central NH to interact synergistically with Cl–. In other cases in which the central NH group was present, the complex had exo-NH groups that could cooperatively bind the Cl– anion.10 Thus, Cl– interacted preferentially at this site. The resulting H-bonded ion pair would have zero overall charge, decreasing the interaction of a second Cl– with the central NH of the complex, which is now a neutral ion pair. However, the competing NH2 site of [Re(CO)3(DAE)]BF4 is far from the positive metal center (Figure 3.5), and this group interacts relatively weakly with Cl–. Thus, the interaction of Cl– with the central NH may be more favorable than in previous cases.10 Nevertheless, this interaction is relatively weak because a fairly high Cl– concentration is needed for ∆δ of the central NH signal of [Re(CO)3(DAE)]BF4 to plateau (Figure 3.8).

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radiopharmaceuticals is the stereochemistry of coordinated secondary amines, which have two configurations that can interconvert via base catalysis.7,10 We assessed isomerization at N1 of [Re(CO)3(MAE)]PF6, resulting from addition of aqueous NaOH to a DMSO-d6 solution. An initial NMR spectrum (Figure 3.9, top) recorded before addition of OH– confirmed that the complex was isomerically pure. After addition of OH– (1.7 mM), a new set of signals observable in the first recorded spectrum (5 min) grew within 20 min to its final intensity (13% new signals vs. 87% initial signals). The new product has a sharp and a broad NH signal (Figure 3.9, bottom) and a methyl doublet (not shown). The broad signal and an associated methyl doublet allow us to establish that the new signals come from the isomer of [Re(CO)3(MAE)]PF6 having the inverted N1 configuration (endo NH, the H points toward the carbonyls). Upon addition of D2O to this base-isomerized sample, the N1CH3 doublet for both isomers immediately became singlets. The new endo-NH signal is more downfield (6.19 ppm) than the exo-NH signal (4.50 ppm), and the new exo-N1CH3 signal (2.56 ppm) is more upfield than the endo-N1CH3 signal (2.91 ppm). The more downfield NH signal correlates with the more upfield CH3 signal. These shift values and correlations agree well with data for chiral [Re(CO)3(N,N′,N′′-Me3dien)]PF6 (ppm: endo-N1H, 6.39, exo-N1CH3 doublet 2.66; and exo-N1H 5.15, endo-N1CH3 doublet 2.94).10 The sharp signal (6.90 ppm, Figure 3.9) upfield of the initial N4H signal (7.18 ppm) was assigned to the N4H of this new endo-N1H/exo-N1CH3 isomer.

Figure 3.9.

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