Lanthanides and actinides: Annual survey of their organometallic chemistry covering the year 2019
Résumé
This review summarizes the progress in organo-f-element chemistry during the year 2019. Organo-felement
chemistry, including Sc, Y, the lanthanides and the actinides, has been a flourishing research area
for many years. The mainly ionic and Lewis acid character of the lanthanide metals provides a vast array
of intriguing structural features supported by numerous organic ligands. In this year’s edition several new
types of complexes are presented, including the first scandacyclopropene complex [Cp*(BuC(NiPr)2)Sc
(g2-PhCCPh)][K(crypt)] displaying an aromatic metallacycle, the first lanthanide-aluminabenzene complexes
[(1-Me-3,5-tBu2-C5H3Al)(l-Me)Ln(2,4-di-tbutylpentadienyl)] (Ln = Y, Lu) and the first scandium
phosphonioketene complex [LSc(g2-COCHPPh3)I] (L = [MeC(NDIPP)CHC(NDIPP)Me], which all showed
interesting reactivities. Furthermore, a wide range of lanthanide alkyl complexes were synthesized and
structurally characterized, including the first isolated ScMe3 derivatives [Sc(AlMe4)3(Al2Me6)0.5] and
[(Me3TACN)ScMe3]. A very important finding in divalent lanthanide chemistry was the synthesis of the
first neutral divalent Dy and Tb sandwich complexes, Ln(C5iPr5)2, which were investigated for their
magnetic properties. The reactivity of divalent metallocenes towards transition metals precursors or
As0 provided unprecedented multimetallic complexes, for example [(Cp*2Sm)4As8], [{(Cp*)2Sm}3{(l-
O4C4)(l-g2-CO)2(l-g1-CO)(CO)5Re2}SmCp*2(thf)] and [Cp*2Yb(taphen)MMe2YbCp*2] (M = Ni, Pt;
taphen = 4,5,9,10-tetraazaphenanthrene). New reactivity of lanthanide complexes was unveiled, such
as the direct dinitrogen to hydrazine conversion using a low-valent Sc complex or the reduction of CS2
using different divalent Yb complexes affording for the first time a CS2 2 bridging unit as shown in the
complex [Yb2(DippForm)4(CS2)] or an intriguing acetylendithiolate bridged Yb(III) complex Yb2L4(C2S2)
(L = (OtBu)3SiO). Numerous new lanthanide catalyzed homo- and co-polymerization processes involving
polar or non-polar monomers were reported, including efficient and stereoselective polymerization of omethoxystyrene,
vinylpyridine or isoprene. The regio-, diastereoselective and stereoregular cyclopolymerization
of different ether and thioether substituted 1,6-heptadienes was reported. A wide range of
hydrofunctionalization reactions were developed, among them an efficient hydrophosphinylation process
of styrenes and alkynes. It was further shown that alkyllanthanide halides could undergo efficient
halogen/lanthanide exchange with arylhalides and vinylhalides providing useful organolanthanide transfer
reagents, for example in the stereoselective Zweifel olefination. Organolanthanide complexes have
also found new applications in material sciences, for example, Ce(C5H4iPr)3 was employed for the
formation of an ultrathin CeO2 overlayer on a Pt electrode via atomic layer deposition to improve lowtemperature
solid oxide fuel cells. An increasingly studied field is the area of endohedral metallafullerenes
(EMF) which gave rise to a large number of unprecedented lanthanide compounds with unusual
cages, as well as dimetalfullerenes with interesting single molecular magnet (SMM) properties and
new insights on direct Ln-Ln bonds.
Hydrocarbyl complexes of the actinides continued to flourish, in spite of the challenges presented by
synthesis and characterization. The first examples of structurally-characterized uranium(IV) homoleptic
aryl complexes and transuranic hydrocarbyl Np(III) complex have been reported. An experimental and
computational study has demonstrated that f-orbitals have a structure-directing role in overlap-driven
covalency in carbene-stabilised metalla-allene complexes and 13C NMR shift has be shown to be a simple
and direct probe of the actinide-carbon bond covalency in the acetylides. Small molecule activation
chemistry has provided some unusual and important results, including a uranium(V) carbene complex
coordinated end-on to dinitrogen and a stable dinuclear U(IV) dihydride complex which reacted with CO2 and CO/H2 to form methoxide and ultimately methanol. New ligands and binding modes resulted
from actinide main group chemistry, with reports of the first examples of terminal g1-cyaarside ligands
(C„As), bridging diarsaallene (As = C = As)2 and trapped radical dianion of the phosphoethynolate
(OCP2) ligand. The bis-CptBu2 metallocene stablised thorium phosphinidene continued to demonstrate
a wealth small molecule reactivity, including reductive coupling, heterocycle formation and E–H
(E = P, N, C) bond activation. Two examples of U(II) complexes are reported, [K(crypt)][(C5Me4H)3U]
and [K(crypt)][U(NR2)3] (R = SiMe3). The first direct assembly of a uranium tri-rhenium triple inverse
sandwich complex was reported, both experimental and computation data are consistent with atypical
Cp-bonding, with electron density redistributed from Re(I) to U(III). Isopropyl substituted cyclopentadienyl
ligands have enabled the synthesis, reactivity and magnetic properties of U(III) metallocenes, including
base-free cationic species. The first example of a monomeric thorium terminal dihyrido compound
(CpAr5)(Cp*)ThH2(THF) (Ar = 3,5-tBu2-C6H3) has been synthesized. The full characterisation of the
organoamericium(III) compound (C5Me4H)3Am provided a unique insight into Am-C bonding. The Th
(IV)/Th(III) redox couple has been experimentally determined for a range of Th(IV) and Th(III) organometallics.
The first uranium phosphaazaallene has been synthesized by reaction of a bis-phosphide complex
with tert-butyl cyanide. Actinide EMFs continued to be an active area of research; molecular structures,
synthetic and purification methodologies are reported. How best to computationally model the distinct
properties of actinide EMFs was the subject of some debate. Thorium complexes have found application
in catalysis, in the selective dihydroboration of nitriles, the hydroboration of imines and polymerization
of isoprene.
Domaines
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