«A Dissertation Submitted to the Graduate Faculty of the Louisiana State University and Agricultural and Mechanical College in partial fulfillment of ...»
Spectra are shown in Figure 2.6 and in Supporting Information. An important goal of the current study is to understand factors influencing NH shifts for 6-membered rings. Our interest focuses on the dependence of shifts on through-space and solvent effects, and thus we must factor out the through-bond inductive effect on shift of the extra methylene group in the 6membered rings vs. 5-membered rings of fac-[Re(CO)3L]n complexes. The through-bond inductive effect is best assessed by considering the shift of the signal of the central NH group.
As illustrated in Figure 2.6, the central NH signal of [Re(CO)3(dipn)]+ in DMSO-d6, is more upfield (6.01 ppm, Table 2.4) than that of [Re(CO)3(dien)]+ (6.98 ppm).24 The more upfield shift of the NH signal of a central N joining 6-membered rings than for an N joining 5-membered rings24 can also be observed in acetonitrile-d3 and acetone-d6 (Table 2.4). Furthermore, the shift of the central NH of [Re(CO)3(aepn)]+ is 6.47 ppm; thus, the shift for the compound with one 5and one 6-membered ring is almost exactly between the shifts for the 5,5- and 6,6- compounds.
This relationship is also found for the central NH signal of [Re(CO)3(N,N-Me2dipn)]+ (6.34 ppm) vs. that of its corresponding dien analogue, [Re(CO)3(N,N-Me2dien)]+ (7.02 ppm).24 Thus, there is an inductive through-bond, upfield-shifting effect of ~0.3 to 0.5 ppm for every 5- to 6membered ring change. This finding is also true for acetone-d6 and acetonitrile-d3, even though the specific values quoted above are for DMSO-d6.
If this inductive effect alone were influencing the shifts of the exo-NH and endo-NH signals, then these also would be shifted upfield by ~0.3 to 0.5 ppm for every 5- to 6-membered ring change (Figure 2.6). For [Re(CO)3(dipn)]+, in DMSO-d6, the upfield exo-NH signal (3.78 ppm) is more upfield than the exo-NH signal for [Re(CO)3(dien)]+ (4.14 ppm),24 and the endoNH signal (5.22 ppm) is also more upfield than for [Re(CO)3(dien)]+ (5.43 ppm).24 A comparison of NH shifts in DMSO-d6 for this pair of complexes (Figure 2.6) and several more pairs of complexes [trpnH vs. trenH (Supporting Information); N′-Medipn vs. N′-Medien;24 and N,NMe2dipn vs. N,N-Me2dien24] indicates that the exo-NH signal is upfield by an average of ~0.4 ppm and the endo-NH signal is upfield by an average of ~0.2 ppm for complexes with two 6membered rings vs. those with two 5-membered rings. We attribute these differences to the inductive effect and suggest that the effect on signals of terminal amine proton signals is smaller than on central secondary amine NH signals. At present not enough information exists to interpret the causes of these small differences, but it is clear that the shifts of the NH2 signals of one chelate ring depend slightly on the features of the other chelate ring (cf. Figure 2.6).
The NH protons of [Re(CO)3(tacn)]+ (7) may be considered to resemble closely the central NH protons of Re tricarbonyl complexes containing 5-membered rings such as [Re(CO)3(dien)]+. These NH protons are directed toward solvent, away from the hydrophobic pocket. The 1H NMR shifts of the NH signals of [Re(CO)3(tacn)]+ in DMSO-d6 (7.06 ppm), acetonitrile-d3 (5.57 ppm), and acetone-d6 (6.62 ppm) are very similar to those of [Re(CO)3(dien)]+ (6.98, 5.57, and 6.57 ppm in DMSO-d6, acetonitrile-d3, and acetone-d6, respectively).24 Because [Re(CO)3(tacn)]+ lacks competing exo-NH and endo-NH protons, we use [Re(CO)3(tacn)]+ as a control to help interpret the effect of Cl– upon the central NH signals of the new complexes in the studies to be described next.
Interaction of exo-NH Groups with the Cl– Anion. The effects of Cl– addition on NH shifts for 5 mM solutions of several complexes in DMSO-d6 were assessed. Upon the addition of Et4NCl, the observed shift changes, ∆δ, of the exo-NH signals of 1, 2, 4, 5 and 6 (Figures 2.7-2.9 and A.5 and A.6, Supporting Information) were downfield (+ values). (The smaller ∆δ’s for the endo-NH and the central NH signals are discussed below.) For [Re(CO)3(dipn)]+ (1), the shift changes of the exo-NH signal (∆δ = ∼1.4 ppm, plateau at [Cl–] of ∼75 mM, Figure 2.7) were comparable to those for [Re(CO)3(dien)]+ (∆δ = ∼1.2 ppm, plateau at [Cl–] of ∼100 mM).24 Following a reported treatment of the ∆δ data,24 we calculated the equilibrium constant for ion-pairing ([Re(CO)3L]n+ + Xm- ↔ [Re(CO)3L]n+,Xm-) in DMSO-d6 at 25 °C. A value of 168 ± 26 M–1 was calculated for the equilibrium constant for [Re(CO)3(dipn)]+ + Cl- ↔ [Re(CO)3(dipn)]+,Cl-. This value is comparable to that reported for [Re(CO)3(dien)]+ (93 ± 11 M–1).24 The non-bonded distance between the two exo-NH’s in [Re(CO)3(N′-Medipn)]PF6 (2), a complex with two representative 6-membered conformations, is
2.475 Å, a value similar to the 2.509 Å distance in [Re(CO)3(dien)]PF6,24 a representative compound with two 5-membered chelate rings.
The standard method for calculating ion-pairing equilibrium constants in DMSO-d6 gave values for the [Re(CO)3(aepn)]+,Cl– equilibrium of 210 ± 12 M–1 (exo-N1H ∆δ plateau = 1.33 ppm) and 188 ± 13 M–1 (exo-N3H ∆δ plateau = 1.49 ppm) for the NH’s. In the molecular structure of 6 (Figure 2.2), the distance between the two exo-NH’s is 2.314 Å. For [Re(CO)3(trenH)]2+ (4), this distance is 2.571 Å, and Cl– ion-pairing caused a large ∆δ for the exo-NH (1.2 ppm). For [Re(CO)3(trpnH)]2+ (5), the distance between the exo-NH’s is 2.336 Å, and the exo-NH ∆δ plateau = 1.5 ppm. However, the ion-pairing equilibrium constant could not be determined well, possibly because of the extra charge and the dangling charged group (see below).
Effect of Cl– on ∆δ of the NH signals of [Re(CO)3(dipn)]PF6 (1) and [Re(CO)3(tacn)]PF6 (7) (designated as tacn-NH) in DMSO-d6 at 25 °C.
Effect of Cl– on ∆δ of the NH signals of [Re(CO)3(N′-Medipn)]PF6 (2) in DMSO-d6 at 25 °C.
Effect of Cl– on ∆δ of the NH signals of [Re(CO)3(aepn)]PF6 (6) in DMSO-d6 at 25 °C. A COSY spectrum was used to assign the NH signals.
The new results on exo-NH signals reported in this section are consistent with our previous interpretations as follows: the chloride ion interacts with the two exo-NH groups; this interaction involves the formation of H-bonds to chloride; and the two chelate rings sterically impede access of the solvent to the exo-NH’s.
Effect on endo-NH and Central NH Signals of Cl– Anion Interaction with the exoNH and Central NH Groups. DMSO-d6 As Solvent. The plots of NH signal shift vs. Cl– concentration (Figures 2.7-2.9 and in Figures A.5 and A.6, Supporting Information) are revealing. As the Cl– concentration is increased, particularly beyond the concentration at which the large ∆δ of the exo-NH signal plateaus, the endo-NH signals (and sometimes the central NH signal) shift upfield (-∆δ). At higher Cl– concentration the shift changes reverse and the signal may shift downfield slightly (+∆δ) from the upfield-shifted position. These ∆δ are not large (0.2 ppm and usually 0.1 ppm), but similar trends were found both in acetonitrile, see below, and in earlier studies.24 Previously no attempt was made to explain the small shifts, but it is now clear that these small ∆δ's are real and are interpretable.
The ∆δ for endo-NH and central NH signals can be explained by invoking two counteracting factors, one upfield-shifting and the other downfield-shifting. One or the other factor prevails in some cases, and the two nearly cancel each other in the other cases.
The downfield shifting factor arises from Cl– H bonding with the NH group as discussed above. However, both the endo-NH and central NH groups are H-bonded to solvent and are downfield; thus the the ∆δ’s are small in comparison to the ∆δ observed for the less solvated exoNH groups, which have upfield signals in the absence of Cl– and exhibit large downfield ∆δ’s in the presence of Cl–.
The upfield-shifting factor arises from the fact that Cl– H-bonding with an NH group will result in the release of electron density from the N-H bond into the N-Re and N-C bonds. In the present study, the exo-NH bonds of the terminal amines are affected by this H-bonding. In turn, the electron density in the endo-NH bonds (and less so in the central NH bond) will increase, causing an upfield shift change (-∆δ). This explanation, which we believe is compelling, adds additional evidence that the ion-pairing at the exo-NH site involves H-bonding. Because the interaction of Cl– at the exo-NH site is favorable, this upfield-shifting factor is most likely to prevail over the downfield-shifting factor at low Cl– concentration. This reasoning explains the shift changes shown in Figures 2.7 to 2.9 and A.5 to A.9.
For [Re(CO)3(dipn)]+ (1) and [Re(CO)3(aepn)]+ (6) (Figure 2.2), the shift patterns of the two endo-NH signals DMSO-d6 (Figures 2.7 and 2.9) are informative. At low Cl– concentration the two endo-NH signals shift upfield. We believe this behavior is clear evidence for the preferred ion-pairing of the Cl– to the exo-NH protons, which causes the electron density to increase near the endo-NH protons and the consequent upfield shift.
For [Re(CO)3(dipn)]+ (1), as the Cl– concentration is increased and the ion-pairing at the exo-NH site is saturated, the Cl– added to the solution then builds to a sufficient concentration to interact detectably with the central NH, as can be deduced from the reversal of the direction of shift changes of the central NH signal (Figure 2.7). This H-bonding of the central NH begins to reverse the direction of the shift change around the plateau Cl– concentration.
For [Re(CO)3(dipn)]+ the endo-NH ∆δ is ∼ -0.1 ppm. However, for complexes in which the central N has an alkyl group, the endo-NH signal upfield shift change is smaller, such as for [Re(CO)3(N′-Medipn)]+ (2) (∆δ ∼ -0.03 ppm, Figure 2.8), [Re(CO)3(trenH)]2+ (4) (∆δ ∼ -0.04 ppm, Figure A.5, Supporting Information), and [Re(CO)3(trpnH)]2+ (5) (∆δ ∼ -0.07 ppm, Figure A.6, Supporting Information). This finding is readily explained by the fact that the central NH Hbonding site is absent. Thus, after the first anion to interact binds at the preferred exo-NH binding site, the next anion to interact with this ion pair necessarily forms an H-bond to the endo-NH proton. As a result, the changes in shift from the two factors (upfield shift from electron density changes from exo-NH interaction and downfield shift from endo-NH H-bonding) nearly cancel, and only very small endo-NH ∆δ values are observed (Figure 2.8 and Figures A.5 and A.6, Supporting Information).
[Re(CO)3(aepn)]+ (6) in DMSO-d6 exhibits interesting behavior as the Cl– concentration increases (Figure 2.9). The secondary Cl– anion interaction with the central NH shifts this signal downfield. This is the same shift behavior we observed previously with [Re(CO)3(dien)]+.24 As we suggested above, the extra methylene group of the 6-membered rings leads to electron donation to the NH groups. Thus, the central NH is more electron-rich when attached to a 6membered ring. The lower electron density of a 5-membered ring of [Re(CO)3(aepn)]+ (6) is also indicated by the slight downfield shift of the upfield-shifted 5-membered ring endo-NH signal (Figure 2.9). No such ∆δ is exhibited by the 6-membered ring endo-NH signal for [Re(CO)3(aepn)]+ (6) (Figure 2.9).
When Cl– was added to a 5 mM solution of [Re(CO)3(tacn)]+ (7) in DMSO-d6, only downfield shifting of the NH signal was observed (maximum ∆δ = 0.17 ppm, light blue full circles, Figure 2.7). As mentioned above, the NH protons of 7 are directed toward solvent (Figure 2.4). Also, our analysis above comparing central NH shifts to the [Re(CO)3(tacn)]+ NH shift indicates clearly that the NH groups in [Re(CO)3(tacn)]+ are like the central NH of linear triamines. Thus, the downfield shift observed supports our interpretation that at high Cl– concentration, the central NH groups form H bonds to added Cl– anion.
For [Re(CO)3(trenH)]2+ (4), the Cl– ion-pairing caused a large ∆δ for the exo-NH (1.2 ppm) and a small negative ∆δ for the endo-NH (-0.04 ppm) signal (Figure A.5, Supporting Information). This behavior is similar to that of other complexes without the dangling groups, such as [Re(CO)3(N′-Medipn)]+ (2). This similarity suggests no synergism involving the dangling NH3+ (∆δ = 0.8 ppm). Indeed, the protons of the dangling NH3+ group of [Re(CO)3(trenH)]2+ cannot come close enough to interact with the Cl– hydrogen bonded to the exo-NH groups in the 1:1 ion pair. After rotation of torsion angles using Chem3D software, the closest distance between endo-NH and NH3+ protons is ∼3.65 Å. Both protons could interact with a Cl– anion in an ion pair. However, this type of synergistic ion-pair interaction appears to be unfavorable because neither plateauing of ∆δ at low added Cl– anion concentration nor significant endo-NH ∆δ values were observed. Similar results were obtained for [Re(CO)3(trpnH)]2+ (5) (Figure A.6, Supporting Information).
Interaction of Cl–, Br– and I– Anions with [Re(CO)3(dipn)]+. Acetonitrile-d3 As In order to compare the effect of halide size on ion-pairing interactions with Solvent.
[Re(CO)3(dipn)]+, the halide titration experiments were performed in acetonitrile-d3, because we were concerned that the larger halide anions might bind too weakly in DMSO-d6.
As expected from past studies with Et4NCl,24,32 the weakness of the interactions of acetonitrile with the NH as compared to DMSO facilitate Cl– interaction with the exo-NH groups, leading to larger ∆δ and lower Cl– ion concentration for leveling off of ∆δ (∆δ = 3 ppm, plateau at ∼10 mM Cl–, Figure A.7 Supporting Information). As found previously,24 the sharpness of the shift changes makes the NMR method for determining ion pairing equilibrium constants inaccurate.
The above titration was repeated with a “buffering” amount of 50 mM Et4NPF6. Aliquots of a stock solution of 150 mM Et4NCl and 5 mM [Re(CO)3(dipn)]+ were added into a 5 mM [Re(CO)3(dipn)]+ / 50 mM Et4NPF6 acetonitrile-d3 solution. The ∆δ values obtained (Figure A.7 (right), Supporting Information) were almost identical to those obtained without added PF6– (Figure A.7 (left), Supporting Information).
When the Et4NBr salt was used, the final ∆δ (2.5 ppm) was slightly less than for Et4NCl;