«A Dissertation Submitted to the Graduate Faculty of the Louisiana State University and Agricultural and Mechanical College in partial fulfillment of ...»
ORTEP plot of [Re(CO)3(CH3CN)3]PF6. Thermal ellipsoids are drawn with 50% probability.
The [Re(CO)3L(HNC(CH3)OCH3)]BF4 complexes (L = bipy (2), 4,4′-Me2bipy (3), 5,5′Me2bipy (4), and 6,6′-Me2bipy (5)) have an acetimidate ligand in the Z configuration in the solid
Me2bipy)(HNC(CH3)OCH2CH3)]BF4 (Figure D1, Supporting Information).
In all of the [Re(CO)3L(HNC(CH3)OCH3)]BF4 structures (2-5, Figure 5.2), the distance of the Re−CO bond trans to the acetimidate group does not differ significantly from those of the other Re−CO bonds (not shown); the acetimidate ligand has no trans influence. However, a negative trans influence is present for [Re(CO)3(4,4′-Me2bipy)BF4] (unpublished results), in which the distance of the Re−CO bond trans to Re−F is significantly shorter (1.902(2) Å) than the other Re−CO bond distances (1.920(2) Å and 1.928(2) Å.
ORTEP plots of the cations of (a) [Re(CO)3(bipy)(HNC(CH3)OCH3)]BF4 (2), (b) [Re(CO)3(4,4′-Me2bipy)(HNC(CH3)OCH3)]BF4 (3), (c) [Re(CO)3(5,5′Me2bipy)(HNC(CH3)OCH3)]BF4 (4) and (d) [Re(CO)3(6,6′-Me2bipy)(HNC(CH3)OCH3)]BF4 (5).
The uncomplexed 6,6′-Me2bipy ligand in 5 is omitted for clarity. Thermal ellipsoids are drawn with 50% probability.
Crystal Data and Structure Refinement for [Re(CO)3(CH3CN)3]PF6 and [Re(CO)3(L) (HNC(CH3)OCH3)]BF4 (L = bipy, 4,4′-Me2bipy, 5,5′-Me2bipy, and 6,6′-Me2bipy) [Re(CO)3CH3CN)3]PF6 bipy 4,4′-Me2bipy 5,5′-Me2bipy 6,6′-Me2bipy
empirical C9H9N3O3Re·PF6 C16H15N3O4Re·BF4 C18H19N3O4Re·BF4 C18H19N3O4Re·BF4 C18H19N3O4Re·BF4·0.5(C12H12N2) formula
and d = 0.0388, 0.0104, 0.0167, 0.0203, and 0.0274 and e = 4.4179, 2.0041, 2.412, 1.2932, and 4.5142 for [Re(CO)3(CH3CN)3]PF6, and [Re(CO)3(L) (HNC(CH3)OCH3)]BF4, in which L = bipy, 4,4′-Me2bipy, 5,5′-Me2bipy, and 6,6′-Me2bipy, respectively.
The Re−N (Figure 5.2) bond lengths of complexes 2-4 are comparable with typical Re sp2 nitrogen bond lengths, which range from 2.14 to 2.18 Å.6 However, the Re−N1 (2.211(3) Å) and Re−N2 (2.213(3) Å) bond lengths for 5, (L = 6,6′-Me2bipy), are significantly longer than the next longest Re−N bond length of the other complexes reported in this study (Re−N3 of [Re(CO)3(5,5′-Me2bipy)(HNC(CH3)OCH3)]BF4 (4) = (2.1860(18) Å). The Re−N1 and Re−N2 bond distances are thus longer for [Re(CO)3(6,6′-Me2bipy)(HNC(CH3)OCH3)]BF4 (5), in which the highest distortion in planar aromatic ligands is expected. Also, it should be noted that the Re−N3 bond lengths of [Re(CO)3(Me2bipy)(HNC(CH3)OCH3)]BF4 do not depend on the position of the methyl substituents, but are longer than the 2.084(5) Å Re–N bond length20 of [Re(HNC(CH3)OCH3)2Cl4], the only reported molecular structure to contain a Re−acetimidate bond.
Figure 5.3 shows that the plane of the iminoether ligand is projected along the length of the bipyridine plane except [Re(CO)3(4,4′-Me2bipy)(HNC(CH3)OCH3)]BF4 (3), in which the acetimidate ligand is almost perpendicular.
Hydrogen bonding between the N3H and F1 of the BF4– anion in 3 may contribute toward this difference. Thus, the orientation of the iminoether ligand does not depend on bipy bulk. Even with the bulkiness of the methyl groups at the 6,6′ position of the 6,6′-Me2bipy ligand, the orientation of the acetimidate ligand is similar to that of the unsubstitued bipy. Figure 5.4 compares the molecular structures of 2 and 5 through overlaying Re atoms and N and O atoms of the acetimidate ligand, rms = 0.033.
For [Re(CO)3L(HNC(CH3)OCH3)]BF4 complexes (see Figure 5.2 for atom numbering), the bond angles range between 74.44(7) and 75.21(5)° for N1−Re−N2, 81.49(6) to 86.72(5)° for N1−Re−N3, and 80.33(5) to 81.66(7)° for N2−Re−N3. Although most of the differences are significant, no distinct trend could be identified.
Relative orientations of [Re(CO)3L(HNC(CH3)OCH3)]BF4 when the structures are viewed with the aromatic rings in the plane and the acetimidate ligand projected toward the viewer. L = bipy (a), 4,4′-Me2bipy (b), 5,5′-Me2bipy (c), and 6,6′-Me2bipy (d).
Overlay of Re, N3, and O4 atoms of the acetimidate ligands of [Re(CO)3(bipy)(HNC(CH3)OCH3)]BF4 (gold) and [Re(CO)3(6,6′-Me2bipy) (HNC(CH3)OCH3)]BF4 (purple) when the structure is viewed with the aromatic ring of the least distorted structure on the plane and the iminoether ligand projected along the y axis (r.m.s. = 0.033).
For complexes 2-5 (Figure 5.2), the iminoether N-to-C (N3–C16) bond shows typical double-bond character, while the iminoether N-to-O (C16–O4) distance is longer than the typical sp2 C=O bond length (1.21 Å) and closer to, though somewhat less than that of the average sp2 C–O bond length (1.34 Å).28 As an example, in [Re(CO)3(4,4′-Me2bipy)(HNC(CH3)OCH3)]BF4 (3), these bond distances are 1.279(3) Å (N3–C16) and 1.327(3) Å (C16–O4). In complexes 2-5, even though extensive delocalization is not reflected in the bond lengths, the C16–O4–C18 angle is close to 120° (Table 5.2), as observed by Natile and co-workers in iminoether PtII complexes,10 and may support the resonance argument (a lone pair of electrons on oxygen contributes to the formation of a delocalized system having double-bond character along the O–C–N bonds). This argument has also been proposed for IrIII complexes bearing an iminoether ligand.16 Resonance may also explain the quasi-planarity of the acetimidate ligand, as evidenced by N–C–O–C torsion angles ranging from 171.67(18)° to 177.55(18)° (Table 5.2).
In a previous study,29 we noted that the electron delocalization along the N-C-N bonds was supported by relevant bond lengths and angles for monodentate amidine ligands in [Re(CO)3(5,5′-Me2bipy)(HNC(CH3)NHR)]BF4 (R = isopropyl, isobutyl, tert-butyl, and benzyl).
In those complexes the N-to-C bond distance of the Re-bound N was only slightly shorter than the other amidine C–N bond. These observations were reflected in the ease of rotation about the N–C bonds giving rise to different configurations about N–C bonds in amidine complexes.
However, only the E′ isomers formed crystals in the above complexes.29 When R = H as in [Re(CO)3(5,5′-Me2bipy)(HNC(CH3)NH2)]BF4, the Z isomer crystallized.29 All of the structures having an iminoether ligand reported in this study have the Z configuration as noted above. The Z configuration is sterically favored as the oxygen of the iminoether ligand is less bulky than the NHR group of amidine ligands.
Selected Bond Distances (Å) and Angles (deg) for [Re(CO)3L(HNC(CH3)OCH3)]BF4 (L = Bidentate Ligand)
Out-of-Plane Distortions. Bipyridines in metal complexes having M–N distances within the typical range (∼2.0-2.2 Å) are not planar;30 out-of-plane distortions of these complexes can be described by several parameters: bowing (θB), twisting (θT) and S-shaped deformation (ds).31
Me2bipy)(HNC(CH3)OCH3)]BF4 is the most distorted, with an exceptionally large twist angle (θT = 12.4°) and a large bow angle (θB = 11.0°). This result may arise from steric repulsion between the methyl groups at the 6,6′ positions and the carbonyl ligands. [Re(CO)3(6,6′Me2bipy)(HNC(CH3)OCH3)]BF4 also has the largest dihedral angle (θdi = 16.7°) of the acetimidate complexes reported here.
Earlier studies from our laboratory have focused on the effects on structure of ligand bulk on [PtLCl2] complexes.32 Bearing in mind that Re–N bond distances are longer than Pt–N bond distances, we sought to analyze the effect of Re–N bond distances on distortions of ReI bipyridine complexes studied here. Typical Pt–N bond distances of [PtLCl2] complexes, where L = bipy, Me2bipy, ranged from 2.017(3) - 2.032(3) Å.32,33 For acetimidate complexes the Re–N bond distances (pyridyl N) varied from 2.1706(17) - 2.1786(18) Å, with the notable exception of [Re(CO)3(6,6′-Me2bipy)(HNCC(CH3)OCH3)]BF4, in which the Re–N bond distances are ∼2.21 Å ( as discussed above). This complex is also distinguished by its exceptionally large twist angle (θT = 12.4°) compared to [Pt(6,6′-Me2bipy)Cl2] (θT = 6.1°).
Ligand Deformation in Octahedral Rhenium Complexes a
is within the range (θP = 2.9 - 12.5°) reported by Hazell, who also noted that θP decreases with increasing M–N distance.31 This finding is valid for the comparison of [Re(CO)3(6,6′Me2bipy)(HNC(CH3)OCH3)]BF4 with other ReI complexes in this study (the other complexes reported here have very similar M-N distances and a generalization cannot be made regarding them). For [Pt(6,6′-Me2bipy)Cl2] the θP value (θP = 10.9°)32 is comparatively larger than for isomers having methyl groups at the 4,4′ and 5,5′ positions, but [Re(CO)3(6,6′Me2bipy)(HNC(CH3)OCH3)]BF4 has the smallest θP value (θP = 4.0°) in comparison with others in the series. In PtLCl2, distortion is characterized more by twisting than bowing, except for [Pt(6,6′-Me2bipy)Cl2].32 But for the Re complexes reported here, distortion is characterized more by bowing than by twisting, except when L = 6,6′-Me2bipy (Table 5.3). Among the complexes discussed in Table 5.3, [Re(CO)3(6,6′-Me2bipy)(HNC(CH3)OCH3)]BF4 (5) has the highest dihedral angle (θdi = 16.7°). A dihedral angle of 20.2° in [Pt(6,6′-Me2bipy)Cl2] reveals that the 6,6′ isomer had the highest dihedral angle in PtLCl2 complexes.32 Thus, the highest dihedral angle is for the 6,6′ isomer, irrespective of whether the complex is square planar or octahedral.
NMR Spectroscopy. All complexes reported were characterized by NMR spectroscopy in several solvents (CDCl3, DMSO-d6 and acetonitrile-d3). The iminoether NH signals appear more downfield in DMSO-d6 (8.57-8.72 ppm) than in CDCl3 (7.41-7.62 ppm) or in acetonitriled3 (7.18-7.31 ppm).The more downfield chemical shifts of the N3H signals in DMSO-d6 may be explained by looking at the molecular structures of these complexes (Figure 5.2), in which N3H points toward the solvent; thus, more downfield signals are observed in hydrogen bonding
[Re(CO)3L(HNC(CH3)OCH3)]BF4 (5 mM, 550 µL DMSO-d6) showed that the half-life for the exchange reaction of N3H with D2O is ∼40 min. In a similar study of ReI complexes bearing unusual amidine ligands (having seven-membered chelate rings), the NH of the sp2N took longer for NH-ND exchange in DMSO-d6 (half life = 1 day).9 In a study of complexes of the type [Re(CO)3(5,5′-Me2bipy)(HNC(CH3)NHR)]BF4 out of four conceivable isomers, E′ and Z isomers were observed in acetonitrile-d3 and E, E′ and Z isomers were observed in CDCl3 and CD2Cl2; the exchange reaction between the E and E′ isomers was fast enough to be observed on the NMR time scale, even though the exchange reaction between E′ and Z isomers was slow.29 The bond distance of the Re-bound N and C is shorter for iminoether complexes (∼1.28 Å) than for amidine complexes (∼1.31 Å). Thus, the double bond character of the bond between the Re-bound N and C is higher in iminoether complexes (Table 5.2) versus amidine complexes, indicating that E to Z interchange is slow, leading to possible absence of the E isomer.
Interaction of NH Protons with the Cl– Anion. Downfield shift changes, ∆δ, for NH groups directed away from solvent were observed when Cl– was added to 5 mM solutions of fac
Me2bipy)(HNC(CH3)OCH3)]BF4 in DMSO-d6, the already downfield NH signal of the acetimidate ligand shifted only slightly downfield (∆δ ∼ 0.21 ppm, plot not included).
5.4 Conclusions Because the fac-[Re(CO)3L(L′)]n compounds containing acetonitrile as the labile ligand isolated here in good yield and purity can be used as intermediates, we propose that facRe(CO)3(CH3CN)3]+ is a versatile precursor. Iminoether complexes of ReI bearing dimethyl-2,2'bipyridine ligands favor the Z configuration in the solid state; there is no evidence from NMR spectra that the E or E′ isomers exist in solution. This finding is in contrast to that of amidine complexes, where, E, E′ and Z isomers were observed in CDCl3 in [Re(CO)3(5,5′Me2bipy)(amidine)]BF4 complexes obtained by the addition of aliphatic amines at the rheniumcoordinated nitrile.29 For fac-[Re(CO)3L(HNC(CH3)OR')]BF4 bearing the bipy moiety and iminoether ligands reported here, distortion is characterized more by bowing, than by twisting, except when L = 6,6′-Me2bipy; in PtLCl2, distortion is characterized more by twisting than bowing, except for [Pt(6,6′-Me2bipy)Cl2].32
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