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The range of complexes reported to date is still small, these electropositive metal organometallics already display a range of chemistry as yet unseen in late metal–N-Heterocyclic carbenes

F-Block N-Heterocyclic Carbene Complexes

CHEMICAL REVIEWS, no. 8 (2009): 3599-3611

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摘要

N-Heterocyclic carbenes (NHCs) can bind as two-electron s-donor ligands to lanthanide and actinide metal cations. In this review we summarise how the incorporation of an anionic group (alkoxide or amido), to form heterobidentate NHC ligands, allows the synthesis of a range of f-block NHC adducts. The tethering group also allows the labili...更多

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简介
  • N-Heterocyclic carbene (NHC) ligands are heterocycles that bind as soft, two-electron s-donors through the NCN carbon atom, and are used widely as strongly basic phosphine analogues, to support late transition metal complexes.[1].
重点内容
  • N-Heterocyclic carbene (NHC) ligands are heterocycles that bind as soft, two-electron s-donors through the NCN carbon atom, and are used widely as strongly basic phosphine analogues, to support late transition metal complexes.[1]
  • Very little chemistry of the electropositive metal–NHC fragment has been reported to date
  • The first lanthanide NHC complexes were isolated by Arduengo et al in 1994, who synthesised the first lanthanide(II) and lanthanide(III) complexes.[4]
  • It is clear that NHCs are better than trialkyl or aryl phosphines as donor ligands in f-block coordination chemistry
  • The range of complexes reported to date is still small, these electropositive metal organometallics already display a range of chemistry as yet unseen in late metal–NHC
结果
  • Complex 20 may be quenched with a variety of electrophiles; for example the reaction with Me3SiCl in d8-thf smoothly silylates the NHC backbone to afford [Y(L9)(N0)2] (21) (where L9 = ButNCH2CH2[C{NC(SiMe3)CHNBut}]) in quantitative yield, with concomitant elimination of KCl, Scheme 10.
  • KC8 reduction appears to allow dme-coordination to a divalent samarium complex, which loses K(I) more readily; a crude 1H NMR spectrum on freshly reduced 10 shows two separate N0 resonances consistent with formation of KN0 and [Sm(L)(N0)(dme)]; germane to this, the lanthanide silylamide complexes that reduce dinitrogen eliminate KN0 as a by-product.[25,29] attempts to prepare Sm(II) amido-NHC complexes from divalent [Sm(N0)2]2 give 10 as the only isolable product.[17] Briefly heating a solution of the KC8 reduced Sm species results in the purple solution instantaneously turning red giving [Sm(L)(N0)(m-OCH3)]2 23, the ether cleavage product, after work-up, Scheme 11.26
  • The widespread use of metal NHC complexes in homogeneous catalysis includes many systems in which the carbene is generated by an in situ deprotonation.[1] Occasionally, this can result in abnormally bound carbenes—i.e. ligands bound through a backbone C4 or C5 carbon, as a result of an H migration from the backbone to C2, e.g. I, Fig. 11.36 It has been predicted that the C4-NHC ligand is a stronger electron donor than a C2-NHC,[37] and shown that C4-bound derivatives e.g. J, Fig. 11, can function as better catalysts.1b,38 With the increasing use of NHCs to enhance early metal catalyst systems for reactions such as C–C bond formation and polymerisation, the behaviour and occurrence of abnormallybound carbene adducts is of widespread relevance and importance in homogeneous catalysis and small molecule reactivity.[39] The reduction chemistry described here in addition to the growing number of abnormal carbenes indicates that caution should be employed when generating catalysts in situ.
  • The reduction chemistry of 13 has far proven unsuccessful in terms of small molecule activation, or isolation of low oxidation state complexes, it has provided a convenient entry-point to NHC lanthanide mono-iodide precursors and the authors are currently exploring their metathesis chemistry; for example, 25 reacts readily with NaN3 to afford dimeric [Nd(L9)(N0)(m-k1:k1-N3)]2 (28), Scheme 16.22
结论
  • The range of complexes reported to date is still small, these electropositive metal organometallics already display a range of chemistry as yet unseen in late metal–NHC complexes, and of potential relevance to homogeneous catalysis, and small molecule activation chemistry.
表格
  • Table1: Metal–NHC M–C bond lengths in structurally characterised f-block NHC adducts; Ccene chemical shifts and 1JYC coupling constants included where relevanta
  • Table2: Substrates studied as potential competitor ligands with the metal-coordinated NHC group in 11 (published data are in reference 17 unless otherwise indicated)
Download tables as Excel
基金
  • We thank the EPSRC, the Leverhulme foundation, the Royal Society, and the Nuffield Foundation for funding
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