Complexes with Poly(dentate) NHC Ligands

The REE adducts with mono(dentate) neutral NHCs mentioned above are not particularly stable and their use is limited as they quickly degrade due to ligand dissociation. An elegant approach for stabilizing NHC complexes of rare earth metals is the application of poly(dentate) ligands, especially with additional anionic donors.

The synthesis of REE complexes supported by donor-functionalised NHC ligands can be achieved in two ways. The most widely applied protocol is a generation of alkali metal NHC adducts (in situ or isolated), which are then used as reagents for transferring the ligand to the REE centre. A second possibility include a direct deprotonation of the ligand precursor with a REE compound, of which mostly amides and alkyls are used as internal bases.

Complexes with Nitrogen-Anchors

One of the first teams to make use of anionic tethers was the group of Arnold. To achieve that, Arnold and co-workers designed an amido-functionalised imidazolium salt 36 (Section 1.2.1, Scheme 1.2.4), which can be conveniently deprotonated with n-BuLi or LiNʺ.^[62] A corresponding lithium amide-bromide adduct 38, also already mentioned in Section 1.2.1, can transfer the NHC ligand to a series of rare-earth amides 58-59 (Scheme 1.3.2, a).^{[62, 90]}

Furthermore, Arnold et al. introduced a related amido-bis(NHC) ligand and reported the synthesis of corresponding yttrium complexes 62a-b by a transamination reaction of YNʺ3 and one equivalent of the lithium chloride adducts 61a or 61b (Scheme 1.3.2, b).^[64]

Scheme 1.3.2. Synthesis of a series of rare earth complexes with anionic N-anchored NHCs.

Gu et al. also examined the formation of rare earth complexes supported by CNC pincer diarylimido linked bis(NHC) ligands and noted a strong dependence of obtained products on the reaction conditions (Scheme 1.3.3).^[91] For example, a reaction of pro-ligand 63b with [MNʺ3(μ-Cl)Li(THF)3] at RT in THF yields the unwanted heterocyclic organic product 64b formed by carbene C–C and C–N coupling. However, at lower temperatures, using the same metal precursor and additional equivalents of n-BuLi bis(amido) complexes 65b are isolated.

Also the treatment of the ligand precursor with 5.0 eq. of NaNʺ and MCl3 at –78 °C yields this product. Interestingly, in contrast to the one pot reaction with NaNʺ, a stepwise addition of NaNʺ to imidazolium precursor 63b at –78 °C, followed by addition of YbCl3 results in formation of cationic [YbL2]⁺ 66b with an inverse crown pentagonal counter anion [{Na(μ-Nʺ}5(μ-Cl)]^–. Finally, a stepwise treatment of 63b with n-BuLi at –30 °C leads after subsequent addition to [MNʺ3(μ-Cl)Li(THF)] (M = Y, Er, Yb) precursors to isolation of zwitterionic complexes 67b.^[91]

More recently the same group confirmed the utility of their synthetic approach for obtaining CNC-pincer alkyl rare earth compounds (65a,c) by applying differently substituted ligands.^[92]

Scheme 1.3.3. Formation of rare earth complexes bearing CNC-pincer NHC ligands.

Complexes with Oxygen-Anchors

Concomitantly to the development of amido-functionalised NHC ligands Arnold introduced alkoxy-functionalised NHC precursors 41a-c (Paragraph 1.2.1, Scheme 1.2.5).^[4] Using the potassium based transmetallation reagent 42a the group was able to access a number of REE NHC complexes such as pseudo octahedral complexes 68a (M = Sc, Y, Ce, Scheme 1.3.4).^[93]

Furthermore, a treatment of REE metallocenes with 42a yields mono- or bis(carbene) cyclopentadienyl complexes (69a and 70a, Scheme 1.3.4).^[5b]

Interestingly, in contrast to unsaturated alkoxy-functionalised NHCs a deprotonation of corresponding precursors of closely related saturated analogues with n-BuLi or KBn in hexanes/toluene yields bicyclic products 71a-c instead of alkali metal NHC adducts (Scheme 1.3.5).^[94] Using these compounds in a protonolysis reaction with rare earth amides and alkyls a series of structurally versatile complexes can be obtained.[5a, 94-95] The remaining proton of the ligand is hereby removed by alkyl- or amide-substituents on the metal precursor acting as internal base and thereby vacating a coordination site.

Scheme 1.3.4. Synthesis of rare earth complexes bearing alkoxy-functionalised imidazol-2-ylidenes.

Scheme 1.3.5. Synthesis of REE complexes supported by alkoxy-functionalised imidazolin-2-ylidenes.

Reaction conditions: i) Y-72a-b: (C6D6), RT; Y-72c: (THF), RT; Ce-72b: (hexanes), RT: Ce-72c:

(toluene), RT ii) (C6D6), 85 °C iii) (hexanes), 0 °C iv) Sc-75c: (hexanes), 0 °C; Y-75c: (toluene), 0 °C v) Y-76a: (C6D6), 85 °C; Ce-76b: (toluene), RT; Ce-76c: (hexanes), RT.

Additionally, an interesting approach for using anionic oxygen-tethered NHCs was made by Shen. She introduced enol-functionalised NHC complexes 78 obtained by in situ reaction of NHC precursor 77 with NaNʺ and REE chlorides in THF at RT (Scheme 1.3.6).^[96]

Scheme 1.3.6. Enol-functionalised NHC complexes of REE.

REE complexes supported by N-(3,5-di-tert-butyl-2-hydroxybenzyl)-functionalised NHC ligands can be also obtained in a one pot reaction with REE amides and alkali metal bases in order to fulfil stoichiometric requirements. In this manner the tris(NHC) complex Y-80b is obtained by a reaction of the N-(3,5-di-tert-butyl-2-hydroxybenzyl)-modified imidazolium pro-ligand 79b with [LiY{N(i-Pr)2})4] and n-BuLi at –78 °C (Scheme 1.3.7).^[97]

Scheme 1.3.7. Synthesis of Y and Yb N-(3,5-di-tert-butyl-2-hydroxybenzyl)-functionalised NHC complexes.

By conducting the same reaction at RT a mono(NHC) yttrium complex 81b bearing a bis(phenolate) ligand and two N-bonded imidazoles is formed. The authors speculated that a cleavage of benzylic C–N bond by hydrogen transfer from phenol to the carbene centre followed by an attack of a N(i-Pr)2-group on the unsaturated ligand fragment would result in the observed release of free amine and formation a methylene-bridged bis(phenolate).^[97]

Further experiments with [Li{Yb(N(i-Pr)2}4], n-BuLi and the pro-ligand in the stoichiometric ratio of 1:1:2 at –78 °C resulted in isolation of bis(NHC) ytterbium compounds 82a-b. Interestingly, all attempts to prepare mono(NHC) ligated complexes were futile.^[98]

Similar to N-(3,5-di-tert-butyl-2-hydroxybenzyl)-functionalised mono(NHC) ligands described above the same group also applied the 1,3-bis(3,5-di-tert-butyl-2-hydroxybenzyl)-functionalised analogue 83. For the preparation of the respective complexes similar procedure, a one pot deprotonation reaction with [LiM{N(i-Pr)2}4(THF)] and n-BuLi in a 2:1:2 molar ratio in THF at –50 °C, is carried out (Scheme 1.3.8).^[99]

Scheme 1.3.8. Lanthanoid complexes bearing tridentate 1,3-bis(3,5-di-tert-butyl-2-hydroxybenzyl)-functionalised NHC ligands.

Remarkably, Shen noted that the solvents used for the crystallization had a significant effect on the solid state structure of the obtained complexes. Anionic [ML2][Li(DME3)] (M = Sm, Er,

THF/toluene mixture the neutral species Sm-85 incorporating alkali metal cation is favoured.

Also the reaction of corresponding ligand precursor with SmNʺ3 and NaNʺ in a 2:1.3 molar ratio affords the Et2O-Na-analogue of 85.^[99] More recently, Ni and co-workers also introduced similar complexes bearing 1,3-bis(3,5-di-tert-butyl-2-hydroxybenzyl)-functionalised NHC ligands derived from 2,4-dihydro-imidazolium and pyrimidium analogues of tridentate NHC ligands 83.^[100]

Indenyl- and Fluorenyl-functionalised NHC complexes

Despite the evident dominance of cyclopentadienyl ligands in the coordination chemistry of lanthanides the obvious choice for additional stabilization of rare earth NHC complexes, indenyl and fluorenyl-substituted NHCs, were only introduced in 2006.^[67] For the preparation of binuclear complex 86 Downing et al. used a twostep procedure (Figure 1.3.1). The group first generated corresponding potassium NHC adduct of their ligand and subsequently, reacted it with yttrium alkyl [Y{(CH2SiMe3)}3(THF)2], which acts as internal base for the removal of the remaining acidic cyclopentadienyl proton.^[101] Concomitantly, Cui and co-workers reported the closely related REE NHC complexes 87 supported by indenyl-substituted NHC ligand via double deprotonation with [LiCH2SiMe3] and [{Ln(CH2SiMe3)3}(THF)2] (Figure 1.3.1).^[102] They later expanded this compound class by introducing more rare earth cations as well as fluorenyl-substituted NHC ligands.^[103]

Figure 1.3.1. Reported indenyl- and fluorenyl functionalised NHC complexes of rare earth metals.

Cyclometallated NHCs

Several reports describe C–H activation of ligand side chains by metal-alkyl groups in rare earth NHC complexes.^{[89, 104]} For example, Okuda presented cyclometallated rare earth compounds 88-89 formed by ortho-metalation of a methyl group of N-mesityl substituent by rare-earth alkyls [MR3(THF)2] (R = CH2SiMe3) in THF at RT (Scheme 1.3.9, a).^[104] Furthermore, an unusual synthesis of formally aryl-metallated rare earth CCC-pincer bis(NHC) complexes 91 was reported by Cui.^[105] The corresponding ligand precursor 90 is hereby deprotonated with n-BuLi in situ in the presence of MCl3 (Scheme 1.3.9, b). Unfortunately, the mechanism of the exchange of chlorides to bromides is unknown.

Scheme 1.3.9. a). C‒H activation by rare earth alkyls, b). CCC pincer NHC complexes of REE.