NH NMR Shifts of New Structurally Characterized fac-[Re(CO)₃(polyamine)]ⁿ⁺ Complexes Probed via Outer-Sphere Hydrogen-Bonding Interactions to Anions, Including the Paramagnetic [Re^IVBr₆]²⁻ Anion

Abstract

fac-[Re I (CO) 3 L] n complexes serve as models for short-lived fac-[ 99m Tc I (CO) 3 L] imaging tracers (L = tridentate ligands forming two five-membered chelate rings defining the L face). Dangling groups on L, needed to achieve desirable biodistribution, complicate the NMR spectra, which are not readily understood. Using less complicated L, we found that NH groups (exo-NH) projecting toward the L face sometimes showed an upfield shift attributable to steric shielding of the exo-NH group from the solvent by the chelate rings. Our goal is to advance our ability to relate these spectral features to structure and to solution properties. To investigate whether exo-NH groups in six-membered rings exhibit the same effect and whether the presence of dangling groups alters the effect, we prepared new fac-[Re(CO) 3 L] n complexes that allow direct comparisons of exo-NH shifts for six-membered vs. five-membered chelate rings. New complexes were structurally characterized with the following L: dipn (N-3-(aminopropyl)-1,3-propanediamine); N'-Medipn (3,3′-diamino-Nmethyldipropylamine); N,N-Me 2 dipn (N,N-dimethyldipropylenetriamine); aepn (N-2-(aminoethyl)-1,3-propanediamine); trpn (tris-(3-aminopropyl)amine); and tren (tris-(2-aminoethyl)amine). In DMSO-d 6 , the upfield exo-NH signals were exhibited by all complexes, indicating that the rings sterically shield the exo-NH groups from bulky solvent molecules. This interpretation was supported by exo-NH signal shift changes caused by added halide and [ReBr 6 ] 2− anions, consistent with outer-sphere H-bond interactions between these anions and the exo-NH groups. For fac- [Re(CO) 3 (dipn)]PF 6 in acetonitrile-d 3 , the exo-NH signal shifted further downfield in the series, Cl − > Br − > I − , and the plateau in shift change required lower concentration for smaller anions. These results are consistent with steric shielding of the exo-NH groups by the chelate rings. Nevertheless, despite its size, the shape and the charge of [ReBr 6 ] 2− allowed the dianion to induce large upfield paramagnetic shifts of the exo-NH signal of fac-[Re(CO) 3 (dipn)]PF 6 . This dianion shows promise as an outer-sphere H-bonding paramagnetic shift reagent. 21 For both isomers in the solid state, the coordinated carboxyl group is deprotonated, whereas the dangling carboxyl group is protonated. Two types of terminal NH's were defined and unambiguously identified through the crystallography of the two isomers. In one isomer, the terminal amine has an endo-NH proton (defined as the proton projecting toward the carbonyl ligands, Figure 1) with a normal relatively downfield shift (5.84 ppm, DMSO-d 6 ) for a terminal secondary amine NH signal. In the other isomer, this amine has an exo-NH proton (defined as the proton projecting away from the carbonyl ligands, Figure 1). 21 The signal of this terminal secondary amine exo-NH proton was observed at a rather upfield position (5.36 ppm, DMSO-d 6 ). NIH Public AccessIn later studies, 24 We hypothesized that the shift differences might be attributable in ...

Iminoether Complexes of the Type, fac‐[Re(CO)3L{HNC(CH3)OCH3}]BF4 (L = Bipyridine or a Substituted Bipyridine): Synthesis and Properties

“…In order to assess this explanation, we conducted an experiment that we have used previously. [6,40,41] When Cl -was added to solutions of the fac-[Re(CO) 3 …”

“…0.21 ppm), a finding that is consistent with the exposure of this proton to the solvent. [6,40,41] The half-life for the exchange reaction of the N3 H to N3 D on addition of D 2 O (pH 6.5, 100 μL) to 4 (5 mm, 550 μL [D 6 ]dmso) was ca. 40 min.…”

Complexes Possessing Rare “Tertiary” Sulfonamide Nitrogen-to-Metal Bonds of Normal Length: fac-[Re(CO)3(N(SO2R)dien)]PF6 Complexes with Hydrophilic Sulfonamide Ligands

“…All Re–N bond distances in 6 are generally similar to the Re–N(sp 3 ) distances observed in [Re(CO) 3 L] + complexes with prototypical NNN donor ligands, which range from ∼2.21 to 2.29 Å as the bulk of substituents on the N atoms increases. 46,50 However, the distances from Re to the central N2 (2.2763(14) Å in 4 and 2.2686(19) Å in 6 ) are significantly longer than the Re-N1 distances (2.2377(14) Å for 4 and 2.221(2) Å for 6 ) and Re–N3 distances (2.2072(15) Å for 4 and 2.216(2) Å for 6 ) (Table 2). These Re–N2 distances in 4 and 6 are significantly longer than the Re–N2 distance in [Re(CO) 3 ( N (H)dien)]PF 6 46 (2.201(3) Å), but they are comparable to the Re–N2 distance of [Re(CO) 3 ( N (Me)dien)]PF 6 .…”

“…38,46,50 The cation of [Re(CO) 3 ( N (SO 2 Me)dien)]PF 6 ( 4 ) and one of the cations of [Re(CO) 3 ( N (SO 2 tol)dien)]PF 6 ( 6 ) (the one shown in Figure 2) have five-membered chelate rings of different chirality (λ or δ). However, the other independent cation in the asymmetric unit of 6 (not shown) has chelate rings of the same chirality.…”

“…Both the shifts and the characteristic shift separation between these signals (Δ = ∼1.5 ppm in DMSO- d 6 ) are useful for assigning such NH NMR signals. 46,50 …”