PRIMERAS PÁGINAS

cell, as well as a disordered toluene molecule. The molecular structure (Figure 5.5) reveals a rhodium center coordinated in a square planar geometry by the carbene ligand, the chloride, and the cyclooctadiene ligand, as is expected for a d8 _{metal center. The C1–Rh1 bond}

(2.01305(3) Å) is within the range of carbene-rhodium bonds in structurally related imidazolidin-2-ylidene rhodium(I) complexes.58 _{The angles around the rhodium center are}

between 85 and 94°, with the sum of angles amounting to 350°. This confirms the slightly distorted square planar coordination. The N–C1 bond lengths (1.33880(1) and 1.35018(1) Å) and the N1–C1–N2 bond angle (108.091(1)°) are in the typical range of imidazolidin-2-ylidene ligands.

The 1_{H NMR spectrum of 11 in CDCl}

3 shows the expected signals of the SINpMe ligand and

the cyclooctadiene ligand. Broadened doublets at 1.89 and 2.26 ppm correspond to the CH2

groups of the cod, with the CH=CH protons resonating at 3.67 and 5.11 ppm. The resonance of the NHC backbone protons is split into a complex multiplet at 3.14-3.39 ppm. The signal corresponding to the CH2 groups of the N-substituents is found at 5.92 ppm and the signals

between 7.49 and 8.45 ppm were assigned to the naphthyl groups. Recording the 1_{H NMR}

spectrum in CD2Cl2 rather than CDCl3 leads to a notable change: The singlet at 5.92 is split into

two doublets at 5.67 and 6.13 ppm with a coupling constant of 15.0 Hz, while none of the other signals are affected. This may indicate that the rotation of the naphthyl groups around the H2C–

In the 13_C{1_{H} NMR spectrum, the signals at 28.6 and 32.9 ppm were assigned to the CH} 2

groups of the cod ligand. The CH=CH carbon atoms are found at 68.6 and 99.7 ppm as doublets with 1_{JC–Rh coupling constants of 14.6 and 6.2 Hz, respectively. The NHC backbone carbons}

resonate at 48.6 ppm, the CH2 groups substituents at 52.6 ppm, and the naphthyl CH carbons

between 123.8 and 133.8 ppm. While no signal corresponding to the carbene carbon atom was observed in the 13_C{1_{H} NMR spectrum, a} 1_H/13_{C HMBC experiment revealed an additional}

resonance at 214.1 ppm due to coupling of the carbene carbon to the CH2 groups of the carbene

ligand. The chemical shift is consistent with related imidazolidin-2-ylidene rhodium(I) complexes.58 _{A list of all} 1_{H and} 13_C{1_{H} NMR signals of 11 is given in Table 5.2.}

5.3.2 Synthesis of (1,3-Bis(1-(1-naphthyl)ethyl)imidazolidin-2-ylidene)(1,5-cycloocta- diene)rhodium(I) Chloride, (SINpEt)(cod)RhCl (12)

Scheme 5.20. Synthesis of [(SINpEt)RhCl(cod)] (12).

While the reaction of silver oxide with the chiral imidazolinium salt 4 produced a mixture of compounds, the synthesis of the corresponding rhodium complex [(SINpEt)RhCl(cod)] (12) using the silver carbene transfer route was successful (Scheme 5.20). 12 was prepared in the same manner as 11. After purification by column chromatography, 12 was obtained as a light yellow powder in 61% yield. Several attempts to obtain X-ray quality crystals remained unsuccessful. However, the NMR spectra indicate that the structure of 12 is analogous to that of 11.

Due to the lower symmetry of the SINpEt ligand in 12 as compared to SINpMe in 11, the

1_{H NMR spectrum of 12 recorded in CDCl}

3 is more complex since all hydrogen atoms are now

magnetically inequivalent. The methylene protons of the cod ligand give rise to broad multiplets in the range of 0.82 to 2.33 ppm, while the olefinic protons of cod resonate at 2.91, 3.51, 5.00, and 5.09 ppm. Multiplets corresponding to the NHC backbone are observed at 3.40, 3.69, and 3.89 ppm. The CH3 groups bound to the chiral centers give rise to doublets at 1.89 and 1.69

with 3_{JHH coupling constants of 7.0 and 7.1 Hz, respectively. The protons bound to the}

neighboring carbon atoms, which are stereocenters, are observed at 7.04 and 7.85 ppm. While the signal at 7.04 ppm is split into a quartet with 3_{JHH = 7.1 Hz, the signal at 7.85 overlaps with}

N N Rh Cl 0.5 Ag2O 0.5 [(cod)RhCl]2 N N+ Cl- 4 12

a signal assigned to the naphthyl substituents. The naphthyl groups resonate in the range of 7.33 to 9.24 ppm.

In accord with the 1_{H NMR spectrum of 12, the reduced symmetry with respect to 11 leads to}

a fairly complex 13_C{1_{H} NMR spectrum. Whereas in 11, the two “sides” of the NHC ligand}

as well as the cod ligand were magnetically equivalent, every carbon in 12 gives rise to a signal of its own. The chemical shifts of the signals corresponding to the same positions, however, are almost identical for 11 and 12. The cod ligand gives rise to signals between 27.3 and 32.6 ppm (CH2 carbons) and between 66.7 and 99.4 ppm (CH=CH carbons). The NHC backbone

resonates at 44.4 and 46.4 ppm. The CH3 groups are observed at 20.5 and 20.0 ppm, and the

chiral carbons at 53.8 and 56.2 ppm. Fourteen signals between 121.4 and 128.9 ppm correspond to the CH carbons of the naphthyl groups while the quaternary naphthyl carbons give rise to six signals between 130.3 and 142.0 ppm. The resonance of the carbene carbon was observed at 213.0 ppm in a 1_H/13_{C HMBC spectrum. This chemical shift is nearly identical to the}

corresponding signal of complex 11. The full assignment is given in Table 5.2.

5.3.3 Synthesis of (1,3-Bis(1-naphthylmethyl)benzimidazolin-2-ylidene)(1,5-

cycloocta-diene)rhodium(I) Chloride, [(BNpMe)(cod)RhCl] (13)

The preparation of 13 followed the same procedure as 11 and 12. Stirring benzimidazolium salt

XL with a slight excess of Ag2O in the presence of [(cod)RhCl]2 (Scheme 5.21) and subsequent

purification of the crude product by column chromatography gave 13 as a bright yellow powder in a yield of 67%. Rather than by column chromatography, pure 13 can also be obtained by recrystallizing the crude product from toluene.

Scheme 5.21. Synthesis of [(BNpMe)RhCl(cod)] (13).

X-ray quality crystals were obtained by layering a toluene solution of 13 with n-hexane. 13 crystallizes in the triclinic space group P-1 with two molecules in the unit cell. The molecular structure of 13 (Figure 5.6) is analogous to that of 11, with a square planar coordination geometry at the rhodium center. The L1–Rh1–L2 angles (L = ligand) are in the range of 87.6 to 93.8° with a sum of angles of 360°. The Rh1–C1 bond, at 2.017(2) Å, is comparable to related complexes [(NHC)RhCl(cod)] containing benzimidazolin-2-ylidene ligands.59 _{The N–C1 bond}

N N Rh Cl 0.5 Ag2O 0.5 [(cod)RhCl]2 N N+ Cl- XL 13

lengths (1.357(3) and 1.361(3) Å) and N1–C1–N2 angle (105.9(2)°) are almost identical to those in the related complex [(BMe)RhCl(cod)] (BMe = 1,3-dimethylbenzimidazolin-2- ylidene).59a

Figure 5.6. Solid-state X-ray structure of 13 (thermal ellipsoids at 50% probability; H atoms omitted for clarity).

The 1_{H NMR spectrum of 13 recorded in CDCl3 is very similar to that of 11 with the addition}

of resonances of the backbone protons. The largest differences are observed for one of the olefinic cod signals, which is observed at 3.31 ppm (11: 3.67 ppm) and the CH2 groups of the

N-substituents, which give rise to two doublets at 6.32 and 7.28 ppm with a coupling constant

of 16.7 Hz. The remaining resonances are only marginally shifted with respect to the spectrum of 11. The splitting of the methylene signals into two doublets was observed for 11 in CD2Cl2

solution and indicates that the rotation of the naphthyl groups around the C7–C8 axis is hindered.

In the 13_C{1_{H} NMR spectrum of 13, most signals are identical to those of 11. The only major}

difference is the chemical shift of the resonance of the carbene carbon. For 13, a doublet at 199.1 ppm with a 1_J

C–Rh coupling constant of 51.2 Hz was assigned to the carbene (11: 214.0

ppm). Due to coupling with rhodium, the carbene signal is split into a doublet with a coupling constant of 51.2 Hz. See Table 5.2 for a full list of 1_{H and} 13_C{1_{H} NMR signals.}

Table 5.2. H and C NMR chemical shifts of complexes 11, 12, and 13 (assignment: see Figure 5.7).

Figure 5.7. Assignment scheme for rhodium complexes 11, 12, and 13.

5.3.4 Reaction of NHC-Rhodium Complexes with Chloride Abstracting Agents

In analogy to the work of Dorta and co-workers on iridium complexes of naphthyl-substituted NHC ligands,36 _{we attempted to prepare a chelating NHC-rhodium complex in which a}

naphthyl substituent coordinates to the metal center by removing the chloride ligand. Sodium tetrakis(3,5-bis(trifluoromethyl)phenyl)borate (NaBArF_{4) and silver tetrafluoroborate were}

employed as chloride abstracting agents. The reactions were performed in CDCl3 to allow

monitoring by NMR spectroscopy.

1_{H /} 13_{C [(SINpMe)RhCl(cod)] (11) [(SINpEt)RhCl(cod)] (12) [(BNpMe)RhCl(cod)] (13)}

C1 – 214.1 – 213.0 – 199.1 H2 / C2 5.92 52.6 7.04/7.85 53.8/56.2 6.32/7.28 50.2 H3 / C3 – – 1.89/1.69 20.5/20.0 – – C4 – 132.1 – 135.9/142.0 – 131.9 H5 / C5 7.54 126.1 7.54/7.33 124.0/121.4 6.97-7.03 123.6 H6 / C6 7.48 125.4 7.49/7.45 125.3/124.6 7.36 125.4 H7 / C7 7.84 128.6 7.84/7.80 128.7/127.5 7.85 128.2 H8 / C8 7.90 128.7 7.90/7.84 128.9/128.3 7.98 129.0 H9 / C9 7.56 126.1 7.56/7.54 126.1/126.2 7.61-7.67 126.3 H10 / C10 7.64 126.7 7.68/7.73 126.4/127.0 7.74 126.9 H11 / C11 8.45 123.9 8.53/9.24 124.0/126.0 8.42 122.8 C12 – 131.7 – 131.5/130.4 – 130.6 C13 – 133.8 – 134.0/134.1 – 133.7 H14 / C14 3.19/3.35 48.6 3.40/3.69/ 3.89 44.4/46.4 – 135.4 H15 / C15 – – – – 6.97-7.03 111.0 H16 / C16 – – – – 7.04-7.09 122.8 CH (cod) 3.66/5.11 68.6/99.7 2.91/3.51/ 5.00/5.09 70.0/66.7/ 99.4/98.2 3.31/5.07 69.3/100.5 CH2 (cod) 1.89/2.26 28.6/32.9 0.82/1.17/ 1.43/1.66/ 1.66/2.25/ 1.83/2.33 32.6/ 27.3/ 32.1/ 29.4 1.47-2.06 28.3/32.6 2 4 5 6 7 8 9 10 11 12 13 14 1 15 16 N N Rh Cl 3

Reacting complex 11 with one equivalent of NaBArF