Rh(I)COD complexes

The unavailability o f crystallographic data and carbonyl stretching frequencies for analogous 5-, 6- and 7-membered carbene complexes made impossible a fair comparison between carbenes with different ring sizes. Consequently, [Rh(NHC)(COD)Cl] and [Rh(NHC)(CO)2Cl] complexes with different ring-size carbenes and the same substituents on the nitrogens were synthesised.

The treatment o f [Rh(COD)Cl] 2 with 2 equiv. o f the corresponding free carbene in THF gave complexes [Rh(5-Mes)(COD)Cl], [Rh(6-Mes)(COD)Cl], [Rh(7-Mes)(COD)Cl], [Rh(7-XyI)(COD)Cl] and [Rh(7-ToO(COD)Cl] as yellow air-stable solids in good yields (Scheme 4.6).

21 Burling, S.; Douglas, S.; Mahon, M. F.; Nama, D.; Pregosin, P. S.; Whittlesey, M. K.

Organometallics 2006,25, 2642.

22 Complexes [Rh(5-MesXCOD)Cl] and [RhfS-MesXCOXCl] were first reported by Herrmann et al.: Denk, K.; Sirsch, P.; Herrmann, W. A. J. Organomet. Chem. 2002, 649, 219. The crystal of structure of [Rh(5-MesXCOD)Cl] has been recently reported by Grubbs and co-workers: Blum, Angela P.; Ritter, Tobias; Grubbs, Robert H. Organometallics 2007, 26, 2122. Complexes [Rh(6- MesXCOD)Cl] and[Rh(6-Me«XCO)2Cl] were published by Buchmeiser et al. but without a crystal structure, see ref 11.

[Rh(COD)CI]2 Risk ^NR THF RN NR Cl—Rh—7 n = 0; R = Mes n = 1; R = Mes

n = 2; R = Mes, Xyl, o-tol

Scheme 4.6. Synthesis o f [Rh(NHCXCOD)Cl] (NHC = 5-Mes, 6-Mes, 7-Mes, 7-Xyl, Tol*).

Free carbenes, 5-Mes, 6-Mes, 7-Mes and 7-Xyl, were obtained according to the procedure described in Chapter 2. 7-Tol° was generated in situ by deprotonation o f the corresponding salt with K[HMDS] and reacted with [Rh(COD)Cl]2. Assuming there is not free rotation about the M - Cn h c bond, four different conformations can be envisaged for [Rh(7-

Tol°)(COD)Cl] depending on the orientation o f the methyl groups o f the o-tolyl rings, two

syn and two anti. The and 13C NMR show only one resonance for the two methyl

groups, which confirms formation o f one o f the syn isomers. However, it is not clear whether the methyl groups are positioned on the face o f the chloride or the COD ligand. A more thorough examination o f the !H and 13C NMR spectra shows that the second syn and the anti isomers are also formed in traces.

Crystals o f [Rh(5-, 6-, 7-Mes)(COD)Cl] suitable for X-ray diffraction were obtained by layering a dichloromethane solution o f the corresponding complex with hexane. The crystal structures are shown in Figure 4.9 and selected bond lengths and angles can be found in Table 4.6.

It is clear from the crystallographic data that expansion o f the NHC-ring leads to wide N-

C n h c - N angles. A significant increase o f this angle is observed between the five-

membered ring [Rh(5-Mes)(COD)Cl] (106.8(3)°) and the expanded ring carbenes [Rh(6- Mes)(COD)Cl] and [Rh(7-Mes)(COD)Cl] (117.5(4)° and 118.0°, respectively). This leads to dramatic changes in the Cmcs-N-Cnhc angles that are again compressed from an average o f 126.7° in [Rh(5-Mes)(COD)Cl] to 120.3° in [Rh(6-Mes)(COD)Cl] and 118.1° [Rh(7- MesXCOD)Cl]. As a result o f this, the mesityl substituents on the nitrogens come closer to the metal centre in the expanded carbenes, virtually blocking the two faces o f the metal coordination sphere. Therefore, the Rh-Ccarbene distances for expanded ring carbenes [Rh(7-Mes)(COD)Cl] and [Rh(6-Mes)(COD)Cl] (2.085

A

A

Synthesis o f complexes with expanded N-heterocyclic carbenes as ligands

are significantly longer than the one found for five-membered ring [Rh(5-Mes)(COD)Cl] (2.069(3) A) (Table 4.6).

(a)

(b)

(c)

Figure 4.9. Solid state molecular structures o f [Rh(7-Mes)(COD)Cl] (a), [Rh(6- Mes)(COD)Cl] (b), [Rh(5-Mes)(COD)Cl] (c).

As previously described for silver complexes in Chapters 2 and 3 for 6- and 7-membered carbene ligands, the extra tension originated by the expansion of the ring does not result in a significant increase of the NCN angle, but in an enlargement of the torsion angle p (Cnng- N-N-Cnng), 4.9° for [Rh(6Mes)(COD)Cl] and 29.5° for [Rh(7-Mes)(COD)Cl].

For all three complexes a pyramidal distortion of the nitrogen atoms is observed, with the N-[Cring-CNHC- Cmcs] plane distance being 0.02 and 0.12 A for 7-Mes, 0.05 and 0.07 A for

6-Mes and 0.15 and 0.02 A 5-Mes.

The great steric demands imposed by 7-membered carbenes upon coordination prevent NHC’s with bulkier substituents ffom coordinating to the metal. For example, no reaction was observed between 7-DIPP and [Rh(COD)Cl]2- The metal precursor and the hydrolysis product of the free carbene were recovered instead. Figure 4.10 shows the steric interactions between the aromatic substituents (mesityls) on the carbene with the COD and chloride ligands, which presumably leaves no room for more encumbered N-substituents.

(a) (b)

Figure 4.10. Mercury spacefill depiction of [Rh(7-Mes)(COD)Cl]. (a) Side-view, COD ligand in front; (b) Side-view, chloride ligand in front.

Additionally, the CjsjHC-Rh-Cl angle decreases as a consequence of the steric repulsion between the COD ligand and the mesityl subtituents on the nitrogens, 85.3, 86.7(3) and 93.72(8)° for the 7-, 6- and 5-membered carbene complexes, respectively (Figure 4.10).

Synthesis o f complexes with expanded N-heterocyclic carbenes as ligands

(a) (b) (c)

115

Figure 4.10. Mercury plots of the molecular structures o f (a) [Rh(7-Mes)(COD)Cl], (b) [Rh(6-Mes)(COD)Cl] and (c) [Rh(5-Mes)(COD)Cl]. Hydrogen atoms are omitted for clarity. The blue and red dots represent the centroids defined by the C = C c o d atoms and are used for the measurement of the C=CcoD-Rh-CNHc, C = C co D -R h - C ’= C ’c o d and C = C Cod -

Rh-Cl angles.

Interestingly, in expanded carbenes the carbenic carbon is not in the N-Rh-N plane, i.e., there is a pyramidal distortion of the carbene carbon, presumably as a consequence of the above described. The orthogonal distance between the carbenic carbon and the N-Rh-N plane is 0.11 A for the 6- and 7-membered carbene complexes and 0.05 A in the saturated 5-membered NHC complex.

In complexes [Rh(7-Mes)(COD)Cl] and [Rh(6-Mes)(COD)Cl] the carbene ligand adopts an almost perpendicular arrangement with respect to the coordination plane (defined by the Rh-Cl atoms); the tilt angle 0 (defined by the coordination and N-C-N planes) is 83.6° for [Rh(6-Mes)(COD)Cl] and 80.4° for [Rh(7-Mes)(COD)Cl] complexes. However, the tilt angle 0 is only 63.0° for [Rh(5-Mes)(COD)Cl].

T able 4.6. Bond lengths (A) and angles (°) for [R h(5,6, 7-Mes)(COD)Cl] [Rh(5- Mes)(COD)Cll [Rh(6- Mes)(COD)Cll [Rh(7- Mes)(COD)Cl] Cmm-N-Cnhc (<Xi/a2) (°) 127.4/126.0 120.6/119.9 119.0/117.2 N -CNhc-N (°) 106.8(3) 117.5(4) 118.0 C NHc-Rh-Cl (°) 93.72(8) 86.7(3) 85.3 Rh-C\Hc (A) 2.068(3) 2.075(10) 2.085 N-Cnhc (A) ( a i- s ite /a 2-site) 1.354/1.354 1.365/1.466 1.360/1.352 P (Cna^-N-N-Crioe) 7.4 4.9 29.5

The *H NMR and 13C NMR show three different resonances for the methyl groups at the mesityls for [Rh(5-Mes)(COD)Cl], [Rh(6-Mes)(COD)Cl] and [Rh(7-Mes)(COD)Cl], indicating that in solution the carbene keeps the perpendicular orientation to the coordination plane observed in the solid state. The same behaviour is observed for [Rh(6- Mes)(CO)2Cl] and [Rh(7-Mes)(CO)2Cl] despite the fact that the steric hindrance o f the carbonyl ligands is significantly lower than that o f cyclooctadiene. Interestingly, as previously explained in section 4.2.1.5 the carbene ligand in complex [Rh(7-Cy)(CO)2Cl] rotates at ambient temperature, whereas 7-Mes does not, indicating that the aromatic N- substitution affords a more encumbered ligand. However, in the case o f the biscarbonyl complex [Rh(5-Mes)(CO)2Cl] the !H NMR and 13C NM R show only two resonances, i.e., the two ortho methyls are magnetically equivalent. This implies free rotation o f the carbene ligand about the R Ii- Cn h c bond as a consequence o f the reduced steric demand o f 5-Mes in

comparison with 6-Mes and 7-Mes.