«A Dissertation Submitted to the Graduate Faculty of the Louisiana State University and Agricultural and Mechanical College in partial fulfillment of ...»
Preparation of [Re(CO)3(dipn-d5)]PF6. The NH NMR signals of a 5 mM solution of [Re(CO)3(dipn)]PF6 in acetonitrile-d3 (600 µL) disappeared within ∼40 min after addition of D2O (5 µL) and K2CO3 (2 mg). Therefore, the NH groups of [Re(CO)3(dipn)]PF6 (8 mg) in CH3CN (3 mL) were exchanged by using D2O (70 µL) and K2CO3 (8 mg). The reaction mixture was taken to dryness, the residue dissolved in CH3CN, and the solution filtered to remove K2CO3. The filtrate was taken to dryness, and the residue was re-dissolved in CH3CN (3 mL) to give a 5 mM [Re(CO)3(dipn-d5)]PF6 stock solution for use in H NMR-monitored titrations. Later, [Re(CO)3(dipn-d5)]PF6 was prepared more conveniently by using the volatile Et3N instead of K2CO3.
Addition of [Bu4N]2[ReBr6] to [Re(CO)3(dipn-d5)]PF6. A 5 mM solution of [Re(CO)3(dipn-d5)]PF6 in CH3CN (600 µL) was treated with increasing amounts of [Bu4N]2[ReBr6] (1 to 12 mM), and the solution was monitored by 2H NMR spectroscopy after each [ReBr6]2– aliquot was added. An analogous 1H NMR experiment was performed with [Re(CO)3(dipn)]PF6 in acetonitrile-d3. The [Re(CO)3(dipn-d5)]PF6 concentration was kept constant throughout the titrations as described above.
2.3 Results and Discussion Synthesis. A [Re(CO)3(H2O)3]OTf aqueous solution5 (pH ~6) was used to prepare all the complexes crystallized and structurally characterized in this study (Scheme 2.1). All complexes are new except for the tacn complex, which was not previously characterized as a PF6– salt.28,29 This salt was needed for our NMR studies.
Synthesis of complexes X-ray Crystallography. All complexes possess a distorted octahedral structure, with the three carbonyl ligands occupying one face. The three remaining coordination sites are occupied by amine nitrogen atoms (Figures 2.2-2.4). Crystal data and details of the structural refinement for these complexes are summarized in Table 2.1. Ligands and their abbreviations are depicted in Chart 2.1.
The complexes have chelate rings of different sizes: two six-membered chelate rings ([Re(CO)3(dipn)]BF4 (1), [Re(CO)3(N′-Medipn)]PF6 (2), [Re(CO)3(N,N-Me2dipn)]BF4 (3), and [Re(CO)3(trpnH)](PF6)2 (5)); six- and five-membered chelate rings ([Re(CO)3(aepn)]PF6 (6));
two five-membered rings ([Re(CO)3(trenH)](PF6)2 (4)); or three five-membered rings ([Re(CO)3(tacn)]PF6 (7)). For all complexes except [Re(CO)3(tacn)]PF6 (7), N1 and N3 refer to bound terminal nitrogen atoms of the ligand, and N2 denotes the central nitrogen; for [Re(CO)3(trenH)](PF6)2 (4) and [Re(CO)3(trpnH)](PF6)2 (5), the nitrogen atom of the dangling NH3+ group is designated as N4 (Chart 2.1).
Selected Re–N bond lengths and the N–Re–N bond angles are summarized in Table 2.2.
The Re–N bond lengths and N–Re–N bond angles are consistent with those found in similar facRe(CO)3L]+ complexes.23,24 It is useful to compare the N–Re–N angles for two terminal amine groups of [Re(CO)3(dien)]PF6 (87.14(12)°), which has two 5-membered chelate rings,24 with those of [Re(CO)3(aepn)]PF6 (86.74(15)°), which has 5- and 6-membered chelate rings (Figure 2.2), and [Re(CO)3(dipn)]BF4 (84.67(10)°), which has two 6-membered rings (Figure 2.2). The N–Re–N angle relating two terminal N’s decreases significantly as the ring size increases (Table 2.2). The non-bonded distances between N1 and N3 are mostly similar for complexes 1-6, ranging from 3.01-3.14 Å (Table 2.3) regardless of the size of the chelate ring.
The presence of a methyl group on N2 of [Re(CO)3(N′-Medipn)]PF6 (2) is reflected in a longer Re–N2 bond distance for 2 (2.299(2) Å) than for [Re(CO)3(dipn)]BF4 (1) (2.244(3) Å) (Table 2.2). A similar difference in the Re–N2 bond distance is found for the corresponding complexes with two 5-membered rings (L = dien and N′-Medien, Chart 2.1).24 Thus, in [Re(CO)3L]n complexes containing either 6- or 5-membered chelate rings, having a methyl substituent on N2 increases the Re–N2 bond distance by a similar small extent. This same
[Re(CO)3(trpnH)](PF6)2 (5) are considered. The central N in 4 and 5 bears a CH2CH2NH3+; the greater bulkiness of this dangling group does not appear to cause any greater lengthening of the Re–N2 bond than does the methyl group.
ORTEP plots of the cations of [Re(CO)3(dipn)]BF4 (1), [Re(CO)3(N′-Medipn)]PF6 (2), [Re(CO)3(N,N-Me2dipn)]BF4 (3), and [Re(CO)3(aepn)]PF6 (6). Thermal ellipsoids are drawn with 50% probability. Only one of the independent molecules is shown for 1 (Z′ = 3) and for 3 (Z′ = 2).
ORTEP plots of the cations of [Re(CO)3(trenH)](PF6)2 (4) and [Re(CO)3(trpnH)](PF6)2 (5). Thermal ellipsoids are drawn with 50% probability.
ORTEP plot of the cation of [Re(CO)3(tacn)]PF6 (7). Thermal ellipsoids are drawn with 50% probability.
Crystal Data and Structure Refinement for [Re(CO)3(dipn)]BF4 (1), [Re(CO)3(N′-Medipn)]PF6 (2), [Re(CO)3(N,NMe2dipn)]BF4 (3), [Re(CO)3(trenH)](PF6)2 (4), [Re(CO)3(trpnH)](PF6)2 (5), [Re(CO)3(aepn)]PF6 (6), and [Re(CO)3(tacn)]PF6 (7)
R = (∑||Fο| - |Fc||)/∑|Fο|; bwR2 = [∑[w(Fο2 - Fc2)2]/∑[w(Fο2)2]]1/2, in which w = 1/[σ2(Fο2) + (dP)2 + (eP)] and P = (Fο2 + 2Fc2)/3, d a = 0.0356, 0.0245, 0.0412, 0.0241, 0.0294,0.0225, and 0.0131, and e = 7.9046, 0.3934, 0.4422, 3.894, 2.1227, 2.1782, and 1.8054 for complexes 1-7, respectively.
Selected Bond Distances (Å) and Angles (deg) for [Re(CO)3(dipn)]BF4 (1), [Re(CO)3(N′-Medipn)]PF6 (2), [Re(CO)3(N,N-Me2dipn)]BF4 (3), [Re(CO)3(trenH)](PF6)2 (4), [Re(CO)3(trpnH)](PF6)2 (5), [Re(CO)3(aepn)]PF6 (6), and [Re(CO)3(tacn)]PF6 (7) bond distances
N1–Re–N2 84.72(10) 91.47(8) 91.82(14) 78.55(6) 83.62(13) 77.42(14) 77.10(11) N2–Re–N3 85.57(10) 84.63(8) 81.80(15) 77.35(6) 86.11(12) 83.90(14) 77.14(8) N1–Re–N3 84.67(10) 84.32(9) 87.88(14) 88.04(7) 88.80(18) 86.74(15) 77.34(11)
Chelate Ring Conformation. In the next subsection on NMR signal assignments, we discuss the use of COSY NMR spectra and chelate ring torsion angles, which depend on chelate ring conformation. Thus, it is useful to consider chelate ring conformations in the new structures and to compare these to conformations found in previous studies.23,24 From Scheme 2.1, it can be seen that all of the new complexes except 3 and 6 will have a time-averaged plane of symmetry.
However, none have such a plane in the solid state (Figures 2.2 and 2.3).
The N2,N3 6-membered rings in compounds 1, 2, 3, and 6 (Figure 2.2) all have a very similar chair conformation (Chart 2.2). However, the conformation of the N1,N2 ring differs; this ring has the twist boat conformation in 1 (Chart 2.2), the sofa conformation in 2 and 3 (Chart 2.2), and five members in 6 (see below).30,31 When the two chelate rings are not equivalent (as in 3 and 6), L has a 'head' and a 'tail' (htL), and thus the complex is chiral. Also, one of the htL rings may dictate the conformation of the other ring. For 3, in which both rings are 6-membered and flexible, such conformational control of one ring by the other ring is not evident.
Conformations of 6-membered rings In contrast, when one htL ring is 5-membered, the conformation of the ring may be influenced by the other ring. The conformation of 5-membered rings is described by ring pucker (λ or δ) (Figure 2.5). One ring pucker (λ or δ) may be favored. In cases such as [Re(CO)3(tmbSO2-dien)], in which both rings of the htL are 5-membered, this chirality will determine the favored ring pucker of each ring (λ or δ).23 The common pucker designation (λ or δ) is not useful for interpreting NMR data. Instead, we designate the 5-membered ring conformations as endo-C and exo-C, where the ring carbon bound to the terminal N projects toward and away, respectively, from the carbonyl ligands (Figure 2.5). This designation can also be used for 6-membered rings (Figure 2.5). Please note: the ring carbon has both an endo-CH and an exo-CH, regardless of whether or not the carbon is an endo-C or an exo-C. For example, in Figure 2.1, the 6-membered ring shown has an endo-C with its two hydrogens labeled as endoCH and exo-CH.
Designations of the exo-C and endo-C conformations for 5- and 6-membered rings of fac-[Re(CO)3L]n complexes. Re is directed away from the viewer and the nitrogens are blue. The terminal N of the ring of interest is on the right, the central N is on the left, and the terminal N (but not the chain methylene groups) of the other ring is in the axial position. Note that for 5membered rings, the chirality of the ring pucker is also designated.
NMR Spectroscopy. Complexes 1–7 were characterized by NMR spectroscopy in DMSO-d6 (Figures 2.6 and A.1, Supporting Information), acetonitrile-d3, and acetone-d6 at 25 °C (Table 2.4). COSY experiments performed for most of the complexes at 25 °C in DMSO-d6, together with torsion angles obtained from the respective molecular structures, were useful in assigning NMR signals.
1H NMR spectra of [Re(CO)3(dipn)]PF6 (1), [Re(CO)3(aepn)]PF6 (6), and [Re(CO)3(dien)]PF6 (top) in DMSO-d6 at 25 °C (* water peak). Full spectra for 1 and 6 are presented in Figure A.1 in Supporting Information.
In our previous work, the assignment by COSY or other means of an NH signal to an exoNH or endo-NH proton was possible either because the complex contained only one of these types of protons in a secondary amine or because there was only one NH2 group in a unique 5membered chelate ring (the ligand was an htL).23 For example, the ring with the terminal amine in [Re(CO)3(tmbSO2-dien)] has an exo-C conformation in the solid, and the endo-NH exhibited a strong COSY cross-peak to the exo-CH. In [Re(CO)3(aepn)]PF6 (6), the 5-membered chelate ring has an endo-C conformation, and the exo-NH signal exhibited a strong COSY cross-peak to the endo-CH signal of the endo-C.
For a symmetric (non-htL) complex with two identical ethylene chains such as [Re(CO)3(dien)]+, the 5-membered chelate rings undergo rapid change in pucker, with both rings λ (λ,λ) or both δ (δ,δ).24 Over time, any given CH2 group in these rings is alternately endo or exo with respect to the carbonyl ligands. This rapid conformational interchange process averages the torsion angles such that endo-NH and exo-NH to CH couplings average. This averaging is found for each of the NH signals to both CH signals (unpublished data). COSY data cannot be used to assign the signals. However, the position in conformational space of the NH groups moves just slightly as the slight rotation about the Re–N bond occurs during the dynamic process. Thus, the exo-NH signal remains upfield to the endo-NH signal.24 The conformations of 6-membered rings are more diverse than those of 5-membered rings (Chart 2.2), as discussed above. However, the chair conformation is most commonly found for the 6-membered rings in the new structures, and we assume that the solution structures will be dominated by this conformation, even in those symmetrical compounds in which the rings undergo conformational interchange. As will be seen, this assumption is justified by its utility in interpreting the NMR data. In the chair conformation, the exo-NH is related to the endo-CH by the largest H–N–C–H torsion angle (Table A.1, Figure A.2, Supporting Information); thus, for the 6-membered ring(s) in 1, 2, 3, and 6 (which exhibit similar COSY NH-CH cross-peaks), the NH-CH cross-peak having the highest intensity will be the exo-NH-endo-CH cross-peak (see below and Supporting Information).
We begin our discussion with [Re(CO)3(N,N-Me2dipn)]BF4 (3, Figure 2.2), a chiral complex with an unsymmetrical coordinated htL in which dynamic motion cannot interchange the rings. The ring with the terminal NH2 group has the chair conformation. The expected three NH signals (central NH and terminal NH2) were observed for 3 in DMSO-d6 (Table 2.4, Figure A.3, Supporting Information). COSY studies discussed in Supporting Information establish that, although the chelate ring has six members, the exo-NH shift is upfield, as found for 5-membered chelate rings. However, the exo-NH-endo-CH COSY cross-peak is larger than the endo-NH-exoCH COSY cross-peak, unlike the case of the 5-membered chelate ring of [Re(CO)3(tmbSO2dien)] in a previous study in which the largest H-N-C-H torsion angle was ∼157° for a ring in the exo-C conformation (Figure 2.5). For this compound, the endo-NH-exo-CH cross-peak was the strongest HN-CH cross-peak.23 As found for 3, COSY data for [Re(CO)3(dipn)]BF4 (1) in DMSO-d6 (Supporting Information) allowed us to establish that the exo-NH signal is upfield. Furthermore, unlike the case for symmetrical complexes with two 5-membered rings, the NH signals of the complexes with two 6-membered rings can be assigned unambiguously by COSY to either exo-NH or endoNH. A COSY experiment on [Re(CO)3(aepn)]PF6 (6) in DMSO-d6 showed NH-CH correlations (Supporting Information) which leave no doubt about the assignment of the NH signals (Figure 2.6) of the 6-membered ring. NMR results indicate that 6-membered rings in 1, 2, 3, and 6 have an endo-C conformation (Figure 2.5).
The COSY spectrum of 6 establishes that pucker of the 5-membered ring is endo-C in solution, the same conformation as in the solid. Thus the 6-membered ring induces a preferred endo-C conformation in the 5-membered ring in both the solution and solid states. Note that the ring bearing the tmbSO2 group in the case of [Re(CO)3(tmbSO2-dien)] also induces a preferred ring conformation in both states, but this conformation is exo-C.23 For [Re(CO)3(trenH)](PF6)2 (4), both 5-membered chelate rings will time average between endo-C and exo-C conformations in solution. Thus, as expected for a symmetrical complex with two 5-membered rings, the NH-CH COSY cross-peaks in DMSO-d6 have similar intensity and do not allow assignment of the signals to a specific proton (Figure A.4, Supporting Information). However, we can use the clear pattern that the upfield NH signal arises from the exo-NH to assign the NH signals of 4 (Table 2.4).
Factors Influencing Shifts of NH Signals for Six- vs. Five-Membered Rings.
Assignments and shifts of NH signals of new complexes in several solvents are summarized in Table 2.4.