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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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Synthesis of [Re(CO)3(N(SO2R)dpa)]+ complexes Synthesis of the Porphyrin T(N(SO2C6H4)dpa)P and its 1:4 Re Adduct, [{Re(CO)3}4T(N(SO2C6H4)dpa)P](BF4)4. We have utilized the synthetic approach reported by members of our group29 to prepare T(R1R2NSO2C6H4)P (R1 = N-py-n-CH2 (n = 2 or 4) and R2 = alkyl) in order to synthesize a porphyrin (T(N(SO2C6H4)dpa)P) bearing four dpa moieties linked via tertiary sulfonamide groups (Scheme 6.1). Because the porphyrin is not water-soluble, we utilized the [Re(CO)3(CH3CN)3]+ precursor to form a 1:4 adduct between T(N(SO2C6H4)dpa)P and Re (see below). Alessio and co-workers utilized the [Re(CO)3(DMSO)3]+ precursor to

synthesize [Re(CO)3(bipyridine)(DMSO)]+,37 which was then used to prepare 1:1 and 1:4

adducts between meso-tetra(4-pyridyl)porphyrin (TpyP(4)) and ReI(CO)3(bipyridine) units.38 In these porphyrin-Re conjugates a peripheral meso pyridyl group was directly bound to Re. In [{Re(CO)3}4T(N(SO2C6H4)dpa)P](BF4)4, three of the Re-bound atoms (a peripheral tertiary sulfonamide and two pyridyl groups) are to the porphyrin but linked via an aryl group.

Structural Results. The Re complexes reported here exhibit a pseudo octahedral structure, with the three carbonyl ligands occupying one face. The remaining three coordination

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[Re(CO)3(N(SO2Me)dpa)]PF6 (1) (Figure 6.2), [Re(CO)3(N(SO2tmb)dpa)]PF6 (2) (Figure 6.3), and [Re(CO)3(N(dansyl)dpa)]BF4 (3) (Figure 6.4). The unusual and most interesting structural feature is that the central N, N2, is bound to Re. Crystal data and details of the structural refinement for these complexes are summarized in Table 6.1. The atom numbering systems in the ORTEP figures are used to describe the solid-state data. The asymmetric unit of 1 contains one [Re(CO)3(N(SO2Me)dpa)]+ cation and half of two crystallographically independent PF6– anions;

one anion lies on an inversion center, and the other anion lies on a twofold axis and is disordered.

In the molecular structure of [Re(CO)3(N(SO2Me)dpa)]PF6 (1) (Figure 6.2), the Re–N2 bond distance (2.2826(16) Å) involving the central nitrogen is significantly longer than the Re– N1 (2.1736(17) Å) and the Re–N3 (2.1948(18) Å) bond distances (Table 6.1). The complex of the parent amine, [Re(CO)3(N(H)dpa)]Br, has a central N–Re bond distance of 2.187(4) Å,18 but when the NH proton is replaced by a CH2CO2H substituent (in [Re(CO)3(N(CH2CO2H)dpa)]Br) the N–Re bond distance is longer (2.230(5) Å).18 Because such bond lengthening can be attributed to steric rather than to electronic effects, the similar length of the Re–N bond for this sp3 tertiary N and for the sulfonamide N (N2) indicates that the sulfonamide N is a relatively strong donor. In [Re(CO)3(N(SO2Me)dpa)]PF6 (1) (Figure 6.2), the angles of the sulfonamide nitrogen are close to 109° (Table 6.2), a result clearly illustrating that the hybridization of the sulfonamide nitrogen has changed from sp2 to sp3 upon binding to Re. Re–N(sp3) bond distances in [Re(CO)3L]+ complexes with prototypical aliphatic NNN donor ligands are ∼2.23-2.29 Å,39,40 and Re–N(sp2) bond distances are ∼2.17-2.19 Å.41 The Re–N(pyridyl) bond distances of 1 (2.1736(17) and 2.1948(18) Å) and [Re(CO)3(dpa)]Br (2.177(5) and 2.183(5) Å)18 are statistically very similar, and thus any effects of having a tertiary sulfonamide N anchoring the two chelate rings versus having a traditional sp3 nitrogen anchoring the rings are minimal.

Figure 6.2.

ORTEP plots of the cations in [Re(CO)3(N(SO2Me)dpa)]PF6 (1). Thermal ellipsoids are drawn with 50% probability.

Figure 6.3.

ORTEP plots of the cations in [Re(CO)3(N(SO2tmb)dpa)]PF6 (2). Thermal ellipsoids are drawn with 50% probability.

Figure 6.4.

ORTEP plots of the cations in [Re(CO)3(N(dansyl)dpa)]BF4 (3). Thermal ellipsoids are drawn with 50% probability.

Table 6.1.

Crystal Data and Structure Refinement for [Re(CO)3(N(SO2Me)dpa)]PF6 (1), [Re(CO)3(N(SO2tmb)dpa)]PF6 (2), and [Re(CO)3(N(dansyl)dpa)]BF4 (3)

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R = (∑||Fο| - |Fc||)/∑|Fο|; bwR2 = [∑[w(Fο2 - Fc2)2]/∑[w(Fο2)2]]1/2, in which w = 1/[σ2(Fο2) + a (dP)2 + (eP)] and P = (Fο2 + 2Fc2)/3, d = 0.0368, 0.0463, and 0.0399 and e = 5, 0, and 2.129 for [Re(CO)3(N(SO2Me)dpa)]PF6 (1), [Re(CO)3(N(SO2tmb)dpa)]PF6 (2), and [Re(CO)3(N(dansyl)dpa)]BF4 (3), respectively.

Table 6.2.

Selected Bond Distances (Å) and Angles (deg) for [Re(CO)3(N(SO2Me)dpa)]PF6 (1) and [Re(CO)3(N(SO2tmb)dpa)]PF6 (2) and [Re(CO)3(N(dansyl)dpa)]BF4 (3)

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The Re–N1, Re–N2, and Re–N3 bond distances of [Re(CO)3(N(SO2tmb)dpa)]PF6 (2) (Figure 6.3) and [Re(CO)3(N(dansyl)dpa)]BF4 (3) (Figure 6.4) are not statistically different from the relevant bond distances of 1 (Table 1). In both 2 and 3, the Re–N2 bond distance is significantly longer than the Re–N1 and Re–N2 bond distances, as found for 1.

To the best of our knowledge, there are four literature examples in which tertiary sulfonamides have been shown to bind to metals.24,26-28 In all these previous cases, the N is positioned advantageously by ligand rings. In three of these examples, the likelihood of the tertiary sulfonamides coordinating to the metal is improved via good positioning of the group within a macrocyclic cavity created by chelating N binding sites.24,26,27 The fourth example is a cyclic 7-azanorbornadiene derivative of Fe(CO)3.28 In each of these examples, the metal-bound tertiary sulfonamide nitrogen and the donors of the adjacent chelate rings occupy the face of an octahedron24 or have a similar arrangement in complexes with other geometries.26,27 In no case do these three donor atoms and the metal atom define a common plane.

In Pt(N(SO2Me)dpa)Cl2, a Pt complex containing the (N(SO2Me)dpa) ligand (Supporting Information), the central nitrogen is not bound to Pt; the bidentate ligand binding mode is confirmed by X-ray crystallography. Bond angles pertaining to the relevant angles of the sulfonamide nitrogen are close to 120° (not shown) and indicate sp2 hybridization. In contrast to the clear evidence that binding of the tertiary sulfonamide nitrogen in 1 changes hybridization at the central nitrogen from sp2 to sp3, a Cu complex of a pyridine containing macrocycle having tertiary sulfonamide nitrogens that are bound and unbound to Cu,26 has bond angles of 110.58°,

114.91°, 100.89°, and 98.84° at the bound sulfonamide nitrogen and 114.00°, 114.17°, and

112.71° at the unbound sulfonamide nitrogen. This observation suggests constrained bond angles. The Cu-N bond length of 2.346(7) Å is within the range of a typical Cu-N bond length.26 However, there is no clear evidence to indicate either sp2 or sp3 hybridization at the sulfonamide nitrogen.

NMR Spectroscopy. All complexes reported were characterized by NMR spectroscopy in DMSO-d6. In the free ligands (N(SO2Me)dpa, N(SO2tmb)dpa, and N(dansyl)dpa), the methylene groups appear as a singlet (Table 6.3). Upon coordination to metal, regardless of the bi or tri dentate binding mode, the methylene groups appear as two doublets because the methylene protons are no longer magnetically equivalent.

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As mentioned above, N(SO2Me)dpa acts as a bidentate ligand in Pt(N(SO2Me)dpa)Cl2 (unpublished results) and as a tridentate ligand in [Re(CO)3(N(SO2Me)dpa)]PF6 (1). For 1 the signals of the methylene protons appear as two doublets and have a coupling constant (J = 16.6 Hz) indicating geminal coupling. For Pt(N(SO2Me)dpa)Cl2, the coupling constant, although still indicative of geminal coupling, is smaller (J = 15.0 Hz) than that in the tridentate complex 1.

However, the coupling constants of [Re(CO)3(N(SO2tmb)dpa)]PF6 (2), (J = 16.0 Hz) and Pt(N(SO2tmb)dpa)Cl2 (unpublished results, J = 15.6 Hz) are not much different. Thus, the value of J is not indicative of bidentate versus tridentate binding mode. There is no trend in shift to suggest that the NMR signals (Table 6.3) of the methylene protons can be used to distinguish between bidentate and tridentate ligand binding modes.

The methyl group 13C NMR signal in the free N(SO2Me)dpa ligand appears at 39.24 ppm.

The signal does not shift much in Pt(N(SO2Me)dpa)Cl2 (37.17 ppm) but it has a considerably different shift in 1 (32.88 ppm); the signal is thus upfield compared to free ligand. Ordinarily, one expects that metal coordination will produce a downfield shift. These results indicate that 13C NMR shifts of the methyl group may be used to distinguish between bidentate and tridentate binding modes of the ligand in [Re(CO)3(N(SO2Me)dap)]PF6 (1) and Pt(N(SO2Me)dpa)Cl2. The upfield shift in 1 can be attributed to the rehybridization of the N.

However, the most convenient NMR signal to distinguish between the bidentate and tridentate ligand binding modes of the N(SO2Me)dpa ligand is the methyl 1H signal. In 1, the 1H methyl signal moves downfield by 0.76 ppm upon binding of the N(SO2Me)dpa ligand to Re;

however, upon binding of the N(SO2Me)dpa ligand to Pt, the methyl signal shifts downfield by only 0.10 ppm. This observation can best be explained by the inductive effect resulting from a direct N–Re bond in 1 (Figure 6.2) and the absence of a N–Pt bond in Pt(N(SO2Me)dpa)Cl2 (Supporting Information).

COSY spectra of the free N(SO2Me)dpa ligand and [Re(CO)3(N(SO2Me)dpa)]PF6 (1, Figure 6.2) in DMSO-d6 aided in the assignment of the aromatic signals. The most downfield doublet (8.50 ppm) of the N(SO2Me)dpa ligand belongs to the pyridyl H6/6′ protons (explained by the close proximity to nitrogen). In 1 (Figure 6.2), the pyridyl H6/6′ signal appears at 8.89 ppm. All the aromatic signals appear more downfield for 1 than in the free ligand. Selected 1H

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Pt(N(SO2Me)dpa)Cl2 (unpublished results) are given in Table 6.3. The pyridyl H6/6′ protons of Pt(N(SO2Me)dpa)Cl2 appear more downfield than in 1. The same trend is observed for N(SO2tmb)dpa, [Re(CO)3(N(SO2tmb)dpa)]PF6 (2), and Pt(N(SO2tmb)dpa)Cl2.

2D NMR spectroscopy aided in the assignment of signals for T(N(SO2C6H4)dpa)P and [{Re(CO)3}4 T(N(SO2C6H4)dpa)P](BF4)4 (4). A 1H NMR spectrum of 4 in DMSO-d6 (Figure 6.5) shows that the beta hydrogen signal (Hβ, Scheme 6.1) is downfield by 0.30 ppm when compared to the corresponding signal of the free porphyrin (Table 6.4). The pyridyl H6/6′ signal of 4 appears at 9.01 ppm versus 8.55 ppm of T(N(SO2C6H4)dpa)P, indicating the presence of a Re–N(pyridyl) bond. An interesting spectral change found upon coordination of the four Re moieties to T(N(SO2C6H4)dpa)P is that the phenylene signals give rise to a singlet for 4 (Figure 6.5). Upon binding of the porphyrin to Re, the signals of the phenylene ortho and meta protons of the aryl group have a similar chemical shift (8.84 ppm) because the signal of the phenylene meta protons (the protons closest to Re) shifts downfield (by 0.60 ppm) for 1 versus the smaller shift of the ortho proton signal (0.37 ppm). Then the resulting similar shifts lead to non-first-order spectra in this region, a result confirming the structure proposed.

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In the 1H NMR spectrum for [{Re(CO)3}4T(N(SO2C6H4)dpa)P](BF4)4 (4) in DMSO-d6 (Figure 6.5), the methylene group signals appear as two doublets (J = 16.6 Hz), a value identical to that of 1 (J = 16.6 Hz). The methylene protons of 4 projecting toward and away from the carbonyl ligands are designated as endo- and exo-CH protons, respectively. An NOE cross-peak seen in the ROESY spectrum of 4 in DMSO-d6, between phenylene signal (8.84 ppm) and the methylene signal (5.97 ppm) aided in assigning this CH signal to the endo-CH proton. An NOE cross-peak between the pyridyl H3/3′ the methylene signal (4.97 ppm) signal allowed assignment of the latter signal to exo-CH. For the free T(N(SO2C6H4)dpa)P ligand in DMSO-d6, the methylene CH2 singlet at 4.90 ppm has NOE cross-peaks with the pyridyl H3/3′ signal as well as phenylene mH. This observation further illustrates that the equivalence and free rotation of the

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[{Re(CO)3}4T(N(SO2C6H4)dpa)P](BF4)4. Also the assignment of the methylene signals of [{Re(CO)3}4T(N(SO2C6H4)dpa)P](BF4)4 described above can be extended to assign the relatively downfield methylene 1H NMR signal to the endo-CH signal and the upfield signal to the exo-CH signal in complexes 1 and 2.

Figure 6.5.

Comparison of the 1H NMR spectra of T(N(SO2C6H4)dpa)P (bottom) and [{Re(CO)3}4T(N(SO2C6H4)dpa)P](BF4)4 (top) in DMSO-d6 at 25 °C.

6. 4 Summary and Conclusions Three novel ligands and their [Re(CO)3L]PF6 complexes have been synthesized and characterized as a prelude to radiopharmaceutical studies. In two of these complexes, structural evidence establishes the existence of a bond between a tertiary sulfonamide N and Re. Because there are distinctive NMR spectral features associated with such binding in the N(SO2R)dpa ligands, it is clear that in all cases the tertiary sulfonamide N binds.

Tertiary sulfonamides when coordinated appear to be relatively good donors, as judged by the Re–N bond length. However, monodentate tertiary sulfonamides generally do not bind metals. Thus, tertiary sulfonamides have been observed to bind metals only when geometrical restraints are enforced.24,26-28 The tertiary sulfonamide nitrogen, when bound as an internal donor atom anchoring two chelate rings, adopts a pyramidal stereochemical preference with the result that the N enforces a facial tridentate geometry. This binding is favored in octahedral complexes such as the [Re(CO)3L]+ complexes presented in this study. In square planar complexes, however, binding at tertiary sulfonamide N does not take place because coordination of the other donors of the adjacent chelate rings so created would not force the sulfonamide N into the coordination plane.

The conjugation approach described here has potentially wide applications. This prospect is supported by our results showing that a sulfonamide can be used to conjugate the {Re(CO)3}+ unit to a porphyrin. We do not see any limitation in conjugating other molecules to such a core or to extending this chemistry to 99mTc analogues.

6.5 References

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

2. Liu, S. Chem. Soc. Rev. 2004, 33, 445-461.

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

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