«A Dissertation Submitted to the Graduate Faculty of the Louisiana State University and Agricultural and Mechanical College in partial fulfillment of ...»
The relevant angles of the amidine moiety are all ~120° (Table 4.2), and these facts all indicate electron delocalization along the N–C–N bonds. Furthermore, in all cases as exemplified by 2, the N4–C16 bond is slightly but significantly longer than the N3–C16 bond (Table 4.2), suggesting slightly less double-bond character. These slight differences are reflected in the ease of rotation about the N–C bonds.
The orientation of the amidine ligand of 4 is different from that of compounds 2, 3, and 7.
(Figure C.2, Supporting Information shows different orientations of the amidine ligands when the Me2bipy plane is oriented in a similar manner). These differences in orientation are present in the solid state; there are no indications to imply their presence in solution.
Me2bipy)(HNC(CH3)NHCH2C6H5)]BF4 (5, Figure 4.3), shows that the phenyl ring of the amidine moiety is stacked above the bipyridine ligand. The closest C-to-C non-bonded distances are 3.584, 3.586, and 3.953 Å. Stacking is depicted in side and top-down views of the stacked rings in Supporting Information (Figure C.3). Effects of this stacking on NMR shifts of the bipyridine signals are discussed later.
NMR Spectroscopy. General Considerations. All complexes reported in this study were characterized by NMR spectroscopy in acetonitrile-d3, and selected complexes were studied in other solvents. COSY and ROESY experiments aided in the assignment of the signals of complexes 2, 3, and 4 in acetonitrile-d3 and of 2 in CDCl3 and CD2Cl2. (For the NMR discussion, the atom numbering in Figure 4.2 is used.) NMR spectra recorded immediately after dissolving crystals of [Re(CO)3(5,5′-Me2bipy)(HNC(CH3)NHR)]BF4 (R = isopropyl (2), isobutyl (3), tertbutyl (4), benzyl (5) and methyl (6)) complexes in several solvents showed two to three sets of signals, which changed in intensity until equilibrium was reached. In general, one set of signals grew slowly, and all other sets (one or two sets, depending on solvent) decreased with time.
Because rotation about the bond between the amidine carbon and the Re-bound N is expected to be slow compared to the rotation about the bond between the amidine carbon and the remote N, this behavior indicates that the first signals observed are from the E′ or E isomer (the solid has the E′ isomer). This reasoning allowed us to assign unambiguously the initial set(s) of peaks to an isomer with the amidine ligand in the E′ or E configuration and the second set to the isomer with the amidine ligand in the Z′ or Z configuration. However, 2D NMR data in combination with studies in several solvents and in mixtures of solvents allowed us to conclude that in every case the E′ and Z isomers of [Re(CO)3(5,5′-Me2bipy)(HNC(CH3)NHR)]BF4 complexes coexisted in polar solvents such as acetonitrile-d3. Only in solvents of low polarity do we observe signals for the E isomer. The Z′ isomer was not found in any solvent. As an illustration of our solution studies and 2D NMR analysis of amidine complexes in polar solvents, we first discuss 2 (Figure 4.3) in acetonitrile-d3; we then extend the discussion to other compounds in this solvent and eventually to other solvents.
Acetonitrile-d3. Upon dissolution of 2 (Figure 4.3) in acetonitrile-d3, the NMR spectrum showed two sets of signals (Figure 4.5). The less intense set of signals grew in intensity until equilibrium was reached (~15 min, Figure 4.5). Integration as well as the slow isomerization of the complex on dissolution allowed us to identify all signals attributable to each of the two sets. Two singlets for the CCH3 groups derived from acetonitrile occur at 1.89 (Z′ or Z) and 2.07 (E′ or E) ppm. The four broad peaks between 4.5 and 6.1 ppm (Figure 4.6) were identified as NH signals by addition of K2CO3/D2O. A CH multiplet at 3.15 ppm (integrating to one proton) for the E′ or E isomer has a COSY cross-peak with the CH3 doublet (0.74 ppm) from the isopropyl group (Figure 4.7). This CH multiplet correlated in turn with an NH signal at 6.10 ppm, assigning this signal to N4H (Table 4.3). (See Figure 4.2 for designations of the N3H and N4H protons.) Likewise, a similar correlation was observed for peaks of the Z′ or Z isomer of 2; the CH3 doublet (ppm) correlated with the CH multiplet (3.69 ppm), which correlated with the N4H signal at 5.57 ppm (Figure 4.7).
1H NMR spectra as a function of time after crystals of the E′ isomer of [Re(CO)3(5,5′-Me2bipy)(HNC(CH3)NHCH(CH3)2)]BF4 (2) were dissolved in acetonitrile-d3 at 25 °C.
1H NMR spectrum of [Re(CO)3(5,5′-Me2bipy)(HNC(CH3)NHCH(CH3)2)]BF4 (2) in acetonitrile-d3 at 25 °C.
A strong NOE cross-peak seen in the ROESY spectrum (Figure C.4, Supporting Information) between the N3H signal (5.33 ppm) and the CCH3 singlet (1.89 ppm) for the Z′ or Z isomer, together with a strong NOE peak between this CCH3 singlet and the CH multiplet (3.69 ppm), establishes that this is the Z isomer. The absence of a N4H-CCH3 NOE cross-peak excludes the presence of the Z′ isomer. There is no NOE peak between the N3H signal (4.51 ppm) and the CCH3 signal (2.07 ppm) attributable to the E′ or E isomer, a result consistent with the assignment of these signals to the E′ isomer. The presence of a very strong NOE peak between the N3H signal (4.51 ppm) and the CH multiplet (3.15 ppm) confirms beyond doubt that this isomer indeed has the E′ configuration because the distance between the N3H and CH protons is short for the E′ isomer (2.18 Å) but is too long (3.51 to 4.39 Å, Supporting Information) to give a strong NOE peak for all of the other isomers. Thus, we conclude that the two sets of signals observed for 2 in acetonitrile-d3 arise from the E′ and Z isomers. The alternative conclusion that the signals arise from a mixture of rapidly interconverting (E′/E and Z′/Z) isomers is not supported by the solvent and temperature dependence studies described below.
The 5,5′-Me2bipy signals of 2 were completely assigned by using NOE and COSY crosspeaks between the 5/5′ CH3 signals and the H6/6′ and H4/4′ signals for 2 (Figure C.5, Supporting Information). The occurrence of only one set of 5,5′-Me2bipy signals for each isomer indicates rapid rotation about the Re–N3 bond.
Acetonitrile-d3. The same methods described above revealed that two isomers formed upon dissolution of crystals for compounds 3 to 6. For all except 5 (see below), the clear chemical shift patterns for the H6/6′ signals in acetonitrile-d3 (Table 4.3) indicate that these are E′ and Z isomers. For 3 and 4, these conclusions were supported by 2D NMR spectra. For both isomers of [Re(CO)3(5,5′-Me2bipy)(HNC(CH3)NHR)]BF4 (R = isopropyl (2), isobutyl (3), tert-butyl (4), and methyl (6)) complexes, the N3H signals (N bound to Re) are more upfield (4.30-5.34 ppm) than the N4H signals (5.57-6.80 ppm) in acetonitrile-d3. The signals of the methyl group derived from acetonitrile range from 2.01-2.08 ppm for the E′ isomers and from 1.86-1.95 ppm for the Z isomers. Thus, for the typical R group, the isomers are easily identified.
(HNC(CH3)NHCH2C6H5)]BF4 (5) Arising from Stacking. For complexes 2, 3, 4 and 6, with non-anisotropic R groups, the average shifts in acetonitrile-d3 are 8.84 (H6/6′), 8.04 (H4/4′), and 8.27 (H3/3′) ppm for E′ isomer signals and 8.73 (H6/6′), 8.04 (H4/4′), and 8.29 (H3/3′) ppm for Z isomer signals. (The H4/4′ and H3/3′ signals of the E′ isomer overlap with the H4/4′ and H3/3′
(HNC(CH3)NHCH2C6H5)]BF4 (5, Figure 4.3), the Z isomer H6/6′, H4/4′ and H3/3′ signals (8.76, 8.05, and 8.27 ppm, respectively) have the typical values; however, the signals for the E′ isomer all appear upfield (8.64, 7.96 and 8.05 ppm, respectively) to typical values of the E′ isomer of compounds 2, 3, 4, and 6. (For 5, the upfield-shifted H3/3′ doublet of the E′ isomer overlaps with the H4/4′ doublet of the Z isomer.) The upfield shifts of the 5,5′-Me2bipy signals of the E′ isomer of 5 can be reasonably explained only by the stacking of the phenyl moiety over the 5,5′-Me2bipy ligand in solution. This explanation was apparent when the complex was first prepared, and it derived support when the solid-state structure showed a stacking interaction (Figure C.3, Supporting Information). As we noted above and for reasons expanded upon below, the major form that can be present upon dissolution has to be either the E′ or E isomer. The E isomer (Figure 4.1) cannot stack and can be excluded because the upfield shifts cannot be explained without phenyl/5,5′-Me2bipy stacking.
Free rotation around the Re–N3 bond, which must occur in solution, explains both the presence of one signal for each type of 5,5′-Me2bipy proton and the upfield shifts of the H6/6′ and H4/4′ signals.
Me2bipy)(HNC(CH3)NHCH2CH(CH3)2)]BF4 (3). 1H NMR spectra of 3 (5 mM) recorded from
-15 to 55 °C in acetonitrile-d3, showed no change in the equilibrium ratios of the H6/6′ and the NH NMR signals and no major shift in these signals. Thus, the E′-to-Z interchange is too slow on the NMR time scale to influence spectra. The presence of only small EXSY cross-peaks at 65 °C in a ROESY spectrum in acetonitrile-d3 (not shown) further confirms that the interchange between the E′ and Z isomers of 3 is slow on the NMR time scale. In addition, at -15 °C no additional signals, which would indicate the presence of the E or Z′ isomers, were observed.
data for a series of amidine complexes in acetonitrile-d3 at room temperature allowed us to determine the E′:Z equilibrium ratio (Table 4.4). The similarity of these E′:Z ratios for 2 (R = isopropyl), 3 (R = isobutyl), and 6 (R = methyl) indicates that a ratio of ∼65:35 may be the ‘normal’ ratio of E′ to Z isomers in acetonitrile. Put differently, the ratio is dictated by the relative bulk of the N4H and the CCH3, which project toward the equatorial plane, and by the natural relative energy of the molecular orbitals. The higher equilibrium ratio (E′:Z = 82:18) for 4 (R = tert-butyl) than the baseline value establishes that as the bulk of the amidine ligand R substituent rises above a threshold size, such as for R = tert-butyl, the high bulk favors the E′ isomer. Likewise, although the benzyl group in 7 has only moderate bulk, phenyl/5,5′-Me2bipy stacking favors the E′ isomer in acetonitrile (Table 4.4).
Isomers of 2 Present in Less Polar Solvents, CD2Cl2 and CDCl3. When crystals of [Re(CO)3(5,5′-Me2bipy)(HNC(CH3)NHCH(CH3)2)]BF4 (2) were dissolved in CD2Cl2 and CDCl3, signals for three isomers were observed (Figures 4.8 and C.6, Supporting Information).
Two sets of signals maintained the same ratio and decreased with time. The third set grew in intensity and has all the characteristics expected for the Z isomer.
In CDCl3, of the two sets of signals that decreased with time, one set had medium intensity (designated as m), and the other had weak intensity (designated as w). In a ROESY spectrum of 2 (Figure C.6, Supporting Information) in CDCl3 at 25 °C, EXSY peaks between the respective signals of the m and w sets are present as follows (ppm): CH multiplets, 3.50 (m) and 3.13 (w); N3H, 5.22 (m) and 4.30 (w); N4H, 6.07(m) and 5.77(w); and CH3 doublets, 1.07(m) and 0.80 (w). The m signals were assigned to the E isomer by an N4H-N3H NOE peak. Also, an NOE cross-peak between the CCH3 (2.23 ppm) signal and the CH multiplet (3.50 ppm) establishes without doubt that the m set belongs to the E isomer. The w signals belong to the E′ isomer, as confirmed by EXSY peaks above. The most intense set of signals was assigned to the Z isomer by NOE cross-peaks relating the CCH3 signal (2.03 ppm) to the N3H signal (5.75 ppm) and to the CH multiplet of the isopropyl group (3.67 ppm). A CCH3-N3H NOE peak together with a CCH3-CH NOE peak is uniquely expected for the Z isomer (Figure 4.1). Thus, the three sets of 1H NMR signals observed for 2 in CDCl3 belong to the E′, E, and Z isomers present in the equilibrium ratio of 5:32:63, respectively.
1H NMR spectra illustrating the distribution of E′, E, and Z isomers of [Re(CO)3(5,5′-Me2bipy)(HNC(CH3)NHCH(CH3)2)]BF4 (2) in CD2Cl2 (with percent acetonitriled3 noted on trace) and acetonitrile-d3 at 25 °C. Note: the bottom two spectra were recorded before equilibrium was reached.
In CD2Cl2, as found in the less polar CDCl3 solvent, 2 exhibits three sets of 1H NMR signals.
Two sets of these signals have essentially equal intensity and are connected by E-E′ EXSY peaks (ROESY spectrum at 0 °C, not shown) as follows (ppm): CH multiplets, 3.51 and 2.95; N3H,
5.11 and 4.06; N4H, 5.96 and 5.85; and CH3 doublets, 1.03 and 0.80. The third set of signals is established as belonging to the Z isomer by NOE peaks relating the CCH3 signal (1.97 ppm) to the N3H signal (5.16 ppm) and to the CH multiplet of the isopropyl group (3.67 ppm).
Equilibrium for 2 in CD2Cl2 (E′:E:Z = 19:19:62) is reached in 3 h.
the polar solvent acetonitrile-d3, the signals for the E′ isomer could alternatively represent a rapidly interconverting mixture of E and E′ isomers. In solvents such as CD2Cl2, the separate signals for these isomers can be observed. Thus, in solvents of low polarity, it is clear that both isomers are present and that interchange is slow on the NMR time scale. We designed an experiment to assess whether in acetonitrile-d3 a significant amount of an E isomer in rapid exchange with the major E′ isomer might be present but undetectable in acetonitrile-d3.