Stoichiometric Reactions

Already in their pioneering work in 2003 Arnold et al. started to probe the reactivity of the metal–NHC bonds in yttrium complexes by monitoring ¹JYC coupling in ¹³C NMR and observing that only strong donors such TMEDA and triphenylphosphane oxide leads to dissociation of the carbene.^[62] Later the same group tested the liability of Y–NHC bond in bis(carbene)

complexes Y-58 in a series of competition experiments with triphenyl- und trimethylphosphine oxides. Although the reaction of bis(trimethyl)silylamido analogue results only in intractable products, the dichloro ligated analogue of Y-58 yields simple triphenyl- und trimethylphosphine oxide adducts with one remaining carbene attached.^[64]

Arnold also reported the treatment of samarium analogue Sm-58 with potassium-intercalated graphite (KC8) in DME yielding dimeric compound 96 containing μ-OMe fragments as a product of DME cleavage. Moreover, interesting bimetallic dimers [(NHC)M(Nʺ)(μ-Nʺ)K(DME)]2 94 with K-bound anionic dicarbene (NHDC) binding simultaneously to yttrium or samarium centres in

“normal fashion” are observed by the reduction of compounds Y- or Sm-58 through the addition of an equimolar amount of potassium naphtalenide (Scheme 1.3.11).^[7] Quenching this yttrium dimer with electrophilic Me3SiCl in THF affords a monomeric complex 95 obtained by backbone silylation and KCl elimination. The same kind of reactivity of alkali metal bonded anionic dicarbenes have been already described in Section 1.2.1.

Scheme 1.3.11. Reactions of rare earth complexes bearing amido functionalised NHCs with KC8. Another example of regioselective C5 silylation was observed with Nd-58. Addition of Me3SiI to this complex results in the formation of the backbone-silylated Nd NHC dimer 97 (Scheme 1.3.12).^[90b] The authors speculated about the mechanism by proposing a nucleophilic substitution of a silyl amide ligand in the first step, followed by the deprotonation of the backbone by a liberated silyl amide. Subsequently, similar to other anionic abnormal carbenes (Section 1.2.1), these compounds would be immediately intercepted by electrophiles such as [SiMe3]⁺. Further reduction of 97 with KC8 proved unsuccessful and yielded only niobium analogue Nd-95 in low yield. Interestingly, by comparing the crystal structures of Nd-58 to its

silylated analogue Nd-95, the influence of SiMe3 is evident in elongation of Nd–CC bond for silylated compound.^[90b]

Furthermore, Nd-97 reacted in a salt elimination reaction showing the exchange of iodine by azide as well as by an aryl amide.^{[90b, 112]} Moreover, starting from Nd-58 the synthesis of unique heterobimetallic rare earth complexes with direct metal–metal bonds have been achieved by applying [Ga(N(Ar)CH)2][K(TMEDA)] (Ar = 2,6-diisopropylphenyl, Dipp) or K[CpFe(CO)2] as reagents for displacement of iodine.^[113]

Scheme 1.3.12. Reactivity of neodymium amido-functionalised NHC complex.

Although the reductive chemistry of Nd-97 was unsuccessful, later the same group was able to achieve an entry in redox chemistry of NHC complexes with lanthanoids by oxidizing the Ce(III) tris(alkoxy-NHC) complex 68a.^{[93a, 106]} Ce(IV) compound 98a is obtained by a treatment of 68a with benzoquinone followed by subsequent ligand redistribution (Scheme 1.3.13).

Further improvement concerning the yield of the reaction can be achieved by the addition of another equivalent of corresponding potassium ligand adduct 42a. Also the usage of XeF2 and [Fe(Cp)2][OTf] as oxidants is possible. The ¹H NMR spectrum of 98a indicates fast fluxional behaviour of the pendant carbenes at RT. The decrease of the temperature results in decoalescence of ¹H NMR signals at –43.15 ºC and finally in the appearance of three sets of proton resonances at –75.15 ºC. Furthermore, the treatment of 98a with two equivalents of 9-borabicyclo[3.3.1]nonane (9-BBN) confirms its bulk structure by producing the borane adduct 99a via interception of pendant NHCs with borane.^[93a] More recently, the oxidation of another previously mentioned cerium alkoxy-tethered NHC complex Ce-72c was also conveniently conducted at RT with Ph3CCl yielding Ce(IV)-72cCl, which is also accessible by oxidation of CeNʺ3 with Ph3CCl and subsequent addition of 71c.^[106]

Scheme 1.3.13. Oxidation of Ce-68a followed by ligand redistribution.

Highly interesting C–Si and C–C bond formations resembling the reactivity of frustrated Lewis pairs by addition-elimination reactions across the M–CC bond have been observed for Ce/Y amido (72) and Sc/Y alkyl complexes (74c and 75c).^{[5a, 95a]} The treatment of these compounds with polar organic E–X substrates (E = silyl, phosphinyl, stannyl, boranyl; X = halide, azide) produces compounds displaying E–NHC and M–X bonds (Scheme 1.3.14, a). The driving force of this reaction is a formation of more stable M–X bonds. The back-conversion to starting complexes can be pushed by thermolysis resulting in liberation of heteroatom-linked hydrocarbon, followed by conversion of remaining halide complex to the starting material via salt metathesis.^[5a]

Scheme 1.3.14. a). C–Si bond formation by addition-elimination reactions on the example of 75c. b).

C–C bond formation by addition-elimination reaction at heterobimetallic Sc-Li compound 102c.

Unfortunately, the complexes mentioned above cannot cleave even the weakest carbon–

halogen bonds by addition across M–CC bond. Only the deployment of lithium carbene “ate”

complex 102c yields the desired C–C bond formation by using Ph3CCl. Hereby, Ph3CCH2SiMe3 and bimetallic Sc complexes 74c are formed (Scheme 1.3.14, b).^[5a]

Using the same frustrated Lewis base/acid type of reactivity the activation of acidic N–H and C–H bonds was also achieved.^[5b] The cerium complex 69a activates various alkynes RC≡CH (R = Me3Si, Ph) at RT yielding a zwitterionic tris(cyclopentadienyl) cerium complexes with incorporated protonated imidazolium group (106a, Scheme 1.3.15, a). The authors proposed that 106a is obtained due to ligand redistribution within an initially formed intermediate product 104a, which is not preferable due its lesser degree of steric protection. This explanation would imply that the second redistribution product 105a should be also formed but since it was not obtained, it probably oligomerises or polymerizes due to its steric unsaturation. Noteworthy, 106a can also be also prepared by a direct reaction of 69a with cyclopentadiene. Further investigations of the same reaction with Y or Sc analogues did not result in activation of alkynes or CpH, although it appears that a dynamic equilibrium of reversible C–H addition exists. These observations again confirmed the very often observed strong dependence of the reactivity of REE complexes on the size and Lewis acidity of the applied metal cation.

Scheme 1.3.15. Cleavage of acidic C–H bonds by rare earth NHC complexes.

Additionally, Y complex 70a reacts with cyclopentadiene yielding polymer [YCp2(LH)2(Cp)]∞

107a (Scheme 1.3.15, b), which contains cyclopentadienyl moieties trapped between two imidazolium units though unusual C–H∙∙∙(π-Cp) interactions. Interestingly, neither indene nor

fluorene react in this manner with 70a despite their higher C–H acidity. Moreover, although pyrrole, indole and diphenylacetophenone form readily addition products with Y-69a and Y-70a due to formation of thermodynamically stable Y–O and Y–N bonds, the same reactions are not observed for cerium analogues. Since the experiments were conducted in THF, a high coordinating ability of this solvent to cerium or lesser Lewis acidity of the respective cation may be the cause for the inertness of the cerium complexes.

Another interesting entry in the reactivity of REE NHC complexes was made by Arnold by reporting an insertion of CO2 into M-72c (M = Y, Ce) and Ce-76b. Unfortunately, the obtained insoluble products could not be properly characterised. An equally insoluble complex is formed by treating Ce-76b with COS. In contrast to Ce-76b, Ce-72c does not react with COS supporting the assumption that a labialization of carbene is necessary for binding such reagents.^[114] However, the Sc NHC complex 68a readily activates three equivalents of CO2 in a “frustrated Lewis pair” reactivity type forming an insoluble polymeric product as well, which was characterised by elemental analysis, FT-IR spectroscopy and solid state NMR. The isolation of soluble insertion products is only possible in case of the reaction with CS2 yielding 108a and 109a containing dithiocarboxylated imidazolium groups (Scheme 1.3.16).^[93b]

Scheme 1.3.16. Activation of carbon disulfide by scandium NHC complex 68a.