4.2. Structure of phosphane ligands
The molecular masses of the prepared phosphanes were verified with measurements of accurate mass peaks. The structural characterization was based mainly on 1H, 13C{1H}, and 31P{1H} NMR techniques. In addition, to assign the 1H and 13C{1H} NMR shifts two-dimensional H,H-correlated COSY-90, H,C-correlated HSQC or C,H-correlated HETCOR, or long-range C,H-correlated COLOC NMR spectra were needed, especially for the alkyl-substituted arylphosphanes and mixed arylalkylphosphanes. The molecular structures of successfully crystallized ligands 9–11, 13, 23, and 27 were further determined by X-ray crystallography at the University of Joensuu. Qualitative pictures of the phosphane structures obtained from the quantum mechanical calculations carried out at the University of Joensuu were useful in the studies of activity and selectivity trends in hydroformylation reactions.
4.2.1. 31P{1H} NMR spectra and cone angles
The 31P NMR shifts were characteristic for each of the phosphane ligand types (aromatic phosphanes, pyridylphosphanes, mixed arylalkylphosphanes). In general, as the substituents near the phosphorus atom became more sterically demanding, the 31P nuclei experienced an increasing shielding effect and the phosphanes showed a shift from –2.6 to –27.7 ppm, while the cone angles increased from 149 to 223. The 31P NMR shifts, yields, and calculated cone angles of phosphane ligands 1–30 are presented in Table 1. The 31P NMR resonances were singlets if not stated otherwise.
As expected, the 31P nuclei of meta- or para-substituted phenylphosphanes (18–20 and 3–4) were deshielded (δP = –2.6 to –4.3 ppm) relative to the ortho-substituted ones. The shielding effect caused by the o-alkyl substituents were also detected for the o-alkyl-substituted arylalkylphosphanes, whose 31P NMR shifts in ppm were less than those of unsubstituted arylalkylphosphanes [87], [88]. The shielding of the 31P nuclei in polyaromatic phosphanes 6–7 and 17 was increased (low δP-values), whereas in the polyaromatic o-phenyl-substituted phenylphosphane 14 the shielding was lesser. The polyaromatic ligands have different electronic properties than the o-alkyl-substituted phenylphosphanes, including stronger ring current effect, which along with the steric bulk affected the 31P NMR shift.
For monodentate phosphorus ligands the cone angle is defined as the apex angle of a cylindrical cone, centered at 2.28 Å from the center of the phosphorus atom, which touches the outermost atoms of the model. For phosphanes containing different substituents an average for the three substituents is taken [41]. In general, the structures of free ligands are relatively flexible. Optimization of the o-substituted phosphanes mostly led to conformations where the substituents were located outside the cone. The o-methyl-substituted arylphosphanes (8 and 17) and o-alkyl-substituted mixed arylisopropylphosphanes (24, 25 and 29) exhibited much lower steric repulsion than the others and the o-alkyl substituent of the ligands 8, 17, 24, 25, and 29 was located inside the cone.
Table 1. Yields, 31P NMR shifts (δP), and calculated cone angles (θ ) of ligands (L).
| L | Name | Yield [%] | δP [ppm] | θ [°] | |
|---|---|---|---|---|---|
| 1 | (o-trifluoromethylphenyl)diphenylphosphane | 97 | −9.4 (q, 4JPF 53 Hz) | 174 | |
| 2 | tris(o-trifluoromethylphenyl)phosphane | 92 | −15.6 (q, 4JPF 55 Hz) | 221 | |
| 3 | (p-trifluoromethylphenyl)diphenylphosphane | 98 | −4.1 | 149 | |
| 4 | tris(p-trifluoromethylphenyl)phosphane | 68 | −4.3 | 149 | |
| 5 | (o-selenomethylphenyl)diphenylphosphane | 30 | −10.4 | − | |
| 6 | (9-anthryl)diphenylphosphane | 72 | −22.9 | 176 | |
| 7 | bis(9-anthryl)phenylphosphane | 61 | −27.7 | 207 | |
| 8 | (o-methylphenyl)diphenylphosphane | 53 | −10.7 | 151 | |
| 9 | bis(o-methylphenyl)phenylphosphane | 87 | −19.0 | 158 | |
| 10 | (o-ethylphenyl)diphenylphosphane | 50 | −14.0 | 169 | |
| 11 | bis(o-ethylphenyl)phenylphosphane | 71 | −23.5 | 194 | |
| 12 | (o-isopropylphenyl)diphenylphosphane | 23 | −13.8 | 187 | |
| 13 | (o-cyclohexylphenyl)diphenylphosphane | 72 | −13.6 | 184 | |
| 14 | (o-phenylphenyl)diphenylphosphane | 69 | −11.9 | 191 | |
| 15 | (2,4,5-trimethylphenyl)diphenylphosphane | 64 | −11.8 | 159 | |
| 16 | (2,5-dimethylphenyl)diphenylphosphane | 92 | −9.5 | 159 | |
| 17 | (2-methylnaphthyl)diphenylphosphane | 24 | −16.5 | 223 | |
| 18 | (m-isopropylphenyl)diphenylphosphane | 33 | −2.6 | 161 | |
| 19 | bis(m-isopropylphenyl)phenylphosphane | 30 | −2.8 | 161 | |
| 20 | tris(m-isopropylphenyl)phosphane | 58 | −3.0 | 184 | |
| 21 | (3-methyl-2-pyridyl)diphenylphosphane | 47 | −6.3 | 151 | |
| 22 | (2,5-dimethylphenyl)bis(3-pyridyl)phosphane | 30 | −25.0 | 181 | |
| 23 | (2,5-dimethylphenyl)bis(4-pyridyl)phosphane | 12 | −15.7 | − | |
| 24 | (o-methylphenyl)diisopropylphosphane | 35 | −4.8 | 165 | |
| 25 | (o-cyclohexylphenyl)diisopropylphosphane | 84 | −4.4 | 172 | |
| 26 | (o-methylphenyl)dicyclohexylphosphane | 82 | −11.6 | 181 | |
| 27 | (o-cyclohexylphenyl)dicyclohexylphosphane | 76 | -14.6 | 211 | |
| 28 | bis(o-methylphenyl)isopropylphosphane | 81 | -22.3 | 198 | |
| 29 | (2-methylnaphthyl)diisopropylphosphane | 41 | 6.6 | 199 | |
| 30 | (2-methylnaphthyl)dicyclohexylphosphane | 66 | -8.1 | 185 | |
Mingos’ statistical analysis has provided some interesting insights into the Tolman cone angle. Specifically, it has demonstrated that the cone angle in real complexes vary much more than previously believed and that there are systematic periodic differences in the average cone angles. The cone angles may also be affected by the steric requirements of the co-ligands and the coordination number of the complex. More surprisingly, the analysis suggests that even within a single complex containing two or more phosphane ligands the cone angle may vary considerably. [3]
The cone angles were larger in the free ortho-alkyl-substituted ligands than in their Rh(acac)CO(L) complexes, mainly because the phosphane ligands in Rh(acac)CO(L) complexes were surrounded by other ligands that caused steric pressure on the phosphane side chains in the direction of the phosphane atom. For ligands 26 and 27, the difference between the free and Rh(acac)CO(L)-coordinated ligand was about 10° [V]. In both the free and coordinated forms the o-alkyl-substituents of 26 and 27 were oriented outside the cone. In the case of ligands 8 and 17 the o-methyl substituent, which was oriented inside the cone in the free ligands, was oriented outside the cone in the Rh(acac)CO(L) coordinated state [IV]. The dissimilar orientation gave a 5° difference between the cone angles of the free and coordinated ligand 8 and as much as 64° for 17.
Although the above-mentioned statistical analysis and the results of the cone angle calculations based on optimized free ligand structures are not alone sufficient for the estimation of steric requirements of the coordinated ligands, they should give a qualitative idea of the steric size of the ligand. Moreover, in most cases the ab initio Hartree-Fock (HF) calculations and the determined X-ray crystal structures of the free ligands have indicated a similar orientation for the o-alkyl substituent. These results confirm the reliability of the HF calculations and make them valuable, particularly for the non-crystallizable phosphane ligands.
4.2.2. 1H and 13C{1H} NMR spectra
The 1H and 13C{1H} NMR spectra were characteristic for each type of phosphane. In general, the 1H NMR shifts of aromatic and cyclohexyl spin systems were not of first order. This complicated the interpretation and even with the assistance of two-dimensional spectra the 1H spectra could not be fully assigned. The 13C{1H} NMR spectra contained only a few overlapping and broad resonance peaks whose precise interpretation was not possible. Some differences in the 1H and 13C NMR shifts due to electronic and steric dissimilarities of the phosphanes are described in this section. The main emphasis is on the characterization of o-alkyl-substituted phenylphosphanes. NMR shifts of the m-isopropyl-substituted phenylphosphanes 18–20 and chromium carbonyl derivatives, which have not been published previously, are presented in detail.
4.2.2.1. o-Substituted arylphosphanes
The differences in the 1H and 13C NMR shifts of phosphane ligands modified with electron-withdrawing and electron-releasing functionality become clear in a comparison of the shifts of o-trifluoromethyl-substituted phenylphosphanes with those of o-selenomethyl-substituted and o-alkyl-substituted phenylphosphanes. The aromatic protons of electron-withdrawing o-trifluoromethyl-substituted ligands were mainly deshielded in relation to the protons of electron-releasing o-selenomethyl-substituted and o-alkyl-substituted ligands; for example, the shift of H6 was 7.2 ppm for ligand 1 and 6.8 ppm for ligands 5 and 8 (see Scheme 11 for numbering of hydrogen and carbon atoms).
The differences in the 13C NMR shifts of the o-substituted phenyl ring are summarized in Table 2. The size of the o-alkyl substituent affected the 13C NMR shifts of the substituted phenyl ring. The carbon C2 in all the electron-releasing o-substituted arylphosphanes was least shielded, as expected, since generally the replacement of hydrogen by more electronegative carbon causes an increased δC-value of that deshielded carbon. The carbon C1 was most shielded (lowest δC-value) in the o-(i-Pr)-substituted phenylphosphane (12) and least shielded (highest δC-value) in the o-SeMe-substituted phenylphosphane (5), whereas in o-CF3-substituted phenylphosphane (1) and o-alkyl-substituted phenylphosphanes (8, 10, 13) the shielding of carbon C1 was between them. Relative to the shift of carbon C3 of the o-CF3-substituted phenyl ring, the shifts of the o-Me- and o-Et-substituted phenyl rings were increased like the shifts of the o-SeMe-substituted phenyl ring, whereas the shifts of the o-(i-Pr)- and o-Cy-substituted phenyl rings were decreased. The shifts of carbons C2 and C4-C6 of all comparable o-alkyl-substituted phenylphosphanes were similar to those of o-selenomethyl-substituted phenylphosphane 5. For these electron-releasing ligands (5, 8, 10, 12, 13) the carbon C2 was deshielded (higher δC-value) and the carbons C4-C6 were shielded (lower δC-value), in contrast to the carbons of electron-withdrawing ligand 1.
Table 2. 13C NMR shifts (ppm) for the o-substituted phenyl rings of phosphane ligands 1, 5, 8, 13, and 14. R indicates the o-substituent of the phenyl ring. See Scheme 11 for numbering of carbon atoms.
| δC | 1 | 5 | 8 | 13 | 14 |
|---|---|---|---|---|---|
| (R = CF3) | (R = SeMe) | (R = Me) | (R = Cy) | (R = Ph) | |
| C1 | 136.6 | 139.2 | 136.0 | 134.8 | 135.9 |
| C2 | 134.9 | 139.3 | 142.2 | 152.2 | 148.4 |
| C3 | 126.4 | 130.0 | 130.0 | 126.1 | 130.1 |
| C4 | 131.6 | 129.4 | 128.6 | 129.0 | 127.3 |
| C5 | 128.9 | 126.2 | 126.0 | 125.9 | 127.2 |
| C6 | 136.1 | 133.4 | 132.7 | 133.4 | 134.1 |
In general, as the o-alkyl-substituent became more bulky with both types of ligands (phenylphosphanes 8, 10, 12, 13 and phenylalkylphosphanes 24–27) the carbons C1 and C3 showed a tendency to shielded shifts and carbons C2 and C7 to deshielded shifts. The shifts of C1, C2 and C3 of the o-alkyl-substituted phenylphospanes calculated from the empirical equations express this as well [89]. The tendency of the carbon C7 to deshield was also seen for the electronically different o-phenyl-substituted phenylphosphane 14. The trends are illustrated in Scheme 12, in which the cone angles of the free ligands are used to characterize the steric size of the ligands created by the o-substituents. It should be noted, however, that although there is a correlation between the cone angle and the 13C NMR shifts of C1-C3 there is no obvious dependence. Most likely, the shifts reflect the electronic effects caused by the particular o-alkyl substituent. This correlation is relevant, however, to the interpretation of the catalytic results below.
4.2.2.2. m-Isopropyl-substituted phenylphosphanes
The 1H and 13C{1H} NMR data for m-isopropyl-substituted phenylphosphanes 18–20, which are reported here for the first time, are presented in Table 3, and the numbering of the hydrogen and carbon atoms is given in Scheme 13. The 1H NMR resonances of protons Hm−1 were septets and protons Hm−2 were doublets, whereas the 1H NMR resonances of aromatic spin systems were not of first order. In the 13C{1H} NMR spectra, the shifts were doublets if the coupling constant are reported and singlets if not.
Table 3. 1H and 13C NMR shifts (ppm) and coupling constants (Hz) for the m-isopropyl-substituted phenylphosphane ligands 18–20. See Scheme 13 for numbering of hydrogen and carbon atoms.
| δH | δC | 18 | 19 | 20 | |||
|---|---|---|---|---|---|---|---|
| JHH | JCP | ||||||
| Hm−1 | Cm−1 | 2.8 | 34.0 | 2.8 | 33.0 | 2.8 | 34.0 |
| 3JHH | − | 7 | − | 7 | − | 7 | − |
| Hm−2 | Cm−2 | 1.2 | 23.9 | 1.2 | 23.9 | 1.2 | 23.9 |
| 3JHH | − | 7 | − | 7 | − | 7 | − |
| − | C1 | − | 136.7 | − | 137.0 | − | 137.2 |
| − | 1JCP | − | 10 | − | 10 | − | 10 |
| H2 | C2 | 7.2–7.3 | 132.3 | 7.2–7.3 | 132.1 | 7.2–7.3 | 132.0 |
| − | 2JCP | − | 24 | − | 23 | − | 22 |
| − | C3 | − | 148.9 | − | 148.8 | − | 148.7 |
| − | 3JCP | − | 8 | − | 8 | − | 7 |
| H4 | C4 | 7.2 | 126.8 | 7.2 | 126.8 | 7.2 | 126.7 |
| H5 | C5 | 7.2–7.3 | 128.4 | 7.2–7.3 | 128.4 | 7.2–7.3 | 128.3 |
| − | 3JCP | − | 6 | − | 6 | − | 6 |
| H6 | C6 | 7.1 | 131.0 | 7.1 | 131.0 | 7.1 | 131.0 |
| − | 2JCP | − | 15 | − | 16 | − | 16 |
| − | C7 | − | 137.4 | − | 137.7 | − | − |
| − | 1JCP | − | 11 | − | 11 | − | − |
| H8 | C8 | 7.3 | 133.7 | 7.3 | 133.6 | − | − |
| − | 2JCP | − | 19 | − | 19 | − | − |
| H9 | C9 | 7.2–7.3 | 128.4 | 7.2–7.3 | 128.3 | − | − |
| − | 3JCP | − | 7 | − | 7 | − | − |
| H10 | C10 | 7.3 | 128.6 | 7.3 | 128.5 | − | − |
The 1H NMR resonances appeared in the same region for all three m-isopropylphenylphosphanes (18–20). The 1H shifts of H2, H5, and H9 overlapped and appeared in the range 7.2 to 7.3 ppm. In the assigment of H6 and H2, recording of the COSY spectrum was necessary where correlation peaks were observed between H6 and H5 and between H5 and H4. This additional information made it possible to assign the shifts of C2 and C6 on the basis of HSQC spectra.
The shifts of aromatic carbons varied from 126.7 to 148.9 ppm. In general, the carbons C2-C5 of the m-isopropyl-substituted phenyl ring and C9-C10 of the unsubstituted phenyl ring were shielded a little and carbons C1 and C7 deshielded as the number of m-isopropyl-substituted phenyl rings increased and the phosphanes became more bulky. The carbon C3 gave a chemical shift in the lowest field at around 148.7 to 148.9 ppm.
4.2.2.3. Isopropyl and cyclohexyl groups directly bonded to phosphorus
The 1H NMR data of the isopropyl groups directly bonded to phosphorus (ligands 24, 25, 29) are presented in Table 4. The 1H NMR shifts of the cyclohexyl groups appeared at 0.8 to 2.1 ppm for arylcyclohexylphosphanes 26, 27, and 30; the exact peaks for the cyclohexyl groups could not be determined owing to the overlapping of signals. The 13C{1H} NMR data for phosphane ligands containing isopropyl or cyclohexyl groups directly bonded to phosphorus are presented in Table 5. In the 13C{1H} NMR spectra, the the shifts were doublets if the coupling constants are reported and mainly singlets if not. As can be seen, for ligand 26 the shift of carbon C7 was broad, while for ligand 27 it was partly overlapping with the shift of carbon Co-2. Scheme 14 shows the numbering of the H and C atoms.
Figure Scheme 14. Numbering of hydrogen and carbon atoms in NMR measurements. R means o-methyl- or o-cyclohexyl-substituted aryl ring.
Table 4. 1H NMR shifts (ppm), multiplicities and coupling constants (Hz) for the isopropyl group directly bonded to phosphorus. See Scheme 14 for numbering of hydrogen and carbon atoms.
| δH | 24 | 25 | 29 | ||||
|---|---|---|---|---|---|---|---|
| JHH, JHP | |||||||
| H7 | 2.1 (dsep) | 2.1 (dsep) | 2.7 (broad) | ||||
| 3JHH | 7 | 7 | − | ||||
| 2JHP | 2 | 2 | − | ||||
| H8 | 0.9 (dd) | 0.9 (dd) | 0.7 (dd) | ||||
| 3JHH | 7 | 7 | 7 | ||||
| 3JHP | 12 | 12 | 14 | ||||
| H8’ | 1.1 (dd) | 1.1 (dd) | 1.3 (dd) | ||||
| 3JHH | 7 | 7 | 7 | ||||
| 3JHP | 15 | 15 | 17 | ||||
Table 5. 13C NMR shifts (ppm) and coupling constants of doublets (Hz) for isopropyl and cyclohexyl groups directly bonded to phosphorus. See Scheme 14 for numbering of hydrogen and carbon atoms.
| δC | 24 | 25 | 26 | 27 | 29 | 30 | |
|---|---|---|---|---|---|---|---|
| JCP | |||||||
| C7 | 24.1 | 24.2 | ∼33.7 | ∼34.5 | 25.5 | 35.8 | |
| 1JCP | 13 | 13 | − | − | 14 | 13 | |
| C8 | 19.3 | 19.3 | 29.0 | 29.3 | 21.2 | 30.5 | |
| 2JCP | 11 | 11 | 7 | 8 | 14 | 11 | |
| C8’ | 20.2 | 20.3 | 30.3 | 30.8 | 23.0 | 33.5 | |
| 2JCP | 19 | 19 | 14 | 17 | 29 | 25 | |
| C9 | − | − | 27.1 | 27.2 | − | 27.0 | |
| 3JCP | − | − | 9 | 7 | − | 8 | |
| C9’ | − | − | 27.2 | 27.3 | − | 26.9 | |
| 3JCP | − | − | 13 | 12 | − | 13 | |
| C10 | − | − | 26.4 | 26.5 | − | 26.4 |
The COLOC NMR spectra, which were measured only for the o-alkyl-substituted aryldialkylphosphanes 24, 27, and 29, showed long-range 13C–1H (3JCH) couplings and the nonequivalent carbons C8 and C8’ and correspondingly C9 and C9’ were possible to assign on the basis of these couplings. Such long-range coupling is possible only if the two nuclei belong to the same alkyl group. Thus, in the isopropylphosphane ligands 24, 25, and 29, and in the cyclohexylphosphane ligands 26, 27, and 30, the two alkyl groups directly bonded to phosphorus are equivalent to each other. Earlier unsubstituted phenyldiisopropylphosphane [90] and phenyldicyclohexylphosphane [88], [91], [92] have showed similar magnetic nonequivalency of the carbons and protons as well. Assignment of 13C NMR shifts of the isopropyl and cyclohexyl carbons in the ligands 24, 25, 26, 27, and 29 agreed with the earlier published shielding order (C7)>(C8’), (C8)>(C9’), (C9)>(C10) [91]. The 2JCP and 3JCP values for C8’ and C9’ were considerably larger than those for C8 and C9 as has been noted for unsubstituted phenyldicyclohexylphosphane [90], [91], [92]. Apparently, the observed nonequivalency of the protons and carbons in the NMR spectra of the o-alkyl-substituted aryldialkylphosphanes was a consequence of steric crowding. Therefore, the rotation around the P–C bonds was rather restricted in the NMR timescale.
4.2.3. X-ray crystal structures
Ligands 9, 10, 11, 13, 18, 23, and 27 formed single crystals, whose X-ray crystal structures were determined at the University of Joensuu. The bond distances and angles of the structures were in the predictable range; in general, there were only minor differences in the bond angles and distances. The X-ray crystal structures of ligands 9, 18, 23, and 27 are presented in Figure 1.
In all structures with a single alkyl substituent in ortho position of the phenyl ring, the substituent was located outside the cone owing to the steric demands (see ligands 9 and 27 in Fig. 1). The m-isopropyl substituent (ligand 18) was oriented analogously. In the crystal structure of (2,5-dimethylphenyl)bis(4-pyridyl)phosphane 23 the m-methyl substituent was coordinated inside the cone and the o-methyl substituent outside the cone.
The COLOC NMR spectra of ligands 24, 27, and 29 indicated that carbons C8 and C8’ were magnetically nonequivalent (see Scheme 14). In the crystal stucture of ligand 27, the nonequivalency of carbons C8 and C8’ was observed in one of the cyclohexyl rings directly bonded to phosphorus, where the C8–C7–P and C8’–C7–P angles were dissimilar (109.42° and 117.93°). In the other cyclohexyl ring the angles were almost equal (108.49° and 109.97°). The dissimilarity may have an effect on the NMR spectra if the rotation around the P–C bonds has become limited in the NMR timescale.
In the free ligand state and in both Rh-coordinated complexes, Rh(CO)(Cl)(L)2 and Rh(acac)CO(L), the o-alkyl substituent was similarly oriented outside the cone close to the Rh-center [II], [V]. Apparently, in most cases the crystal structure of the free ligand gives a qualitatively picture of the orientation of the o-alkyl-substituent in the Rh-coordinated complexes and so can assist in the prediction of catalytical behavior. However, sterically smaller o-alkyl substituents (like methyl) can more easily orientate inside the cone than can bulkier substituents and it is riskier to use their crystal structures to estimate the orientation of the smaller size o-substituent in metal complexes.
