Lanthanides and actinides: Annual survey of their organometallic chemistry covering the year 2018

Edelmann, F. T., Farnaby, J. H. , Jaroschik, F. and Wilson, B. (2019) Lanthanides and actinides: Annual survey of their organometallic chemistry covering the year 2018. Coordination Chemistry Reviews, 398, 113005. (doi: 10.1016/j.ccr.2019.07.002)

Abstract

This review summarizes the progress in organo-f-element chemistry during the year 2018. A continuing trend for many years, which remained important in 2018, was the synthesis and investigation of reactive trivalent lanthanide mono- and bis(alkyl) or (benzyl) complexes supported by a variety of non-cyclopentadienyl ligands such as amidinates, β-diketiminates or NHC ligands. An important contribution was the synthesis of the homoleptic, solvent-free dibenzyl complexes [Ln(CH2Ph)2]n (Ln = Eu, Sm, Yb) which served as precursors for the synthesis of the first divalent lanthanide imides [(THF)Ln(μ3-NDipp)]4. Lanthanide carbene chemistry has also been of growing interest. Functionalized NHC ligands were employed to unveil new reactivity as demonstrated in the synthesis of homoleptic lanthanide complexes with aryloxide-tethered NHC ligands, Ln(LR)3 (LR = 2-O-3,5-tBu2C6H2(1-C{N(CH)2N(R)}), R = iPr, tBu, Mes), which reacted with CO2 by selective insertion into Ln–C(NHC) bonds. A diverse reactivity towards small unsaturated molecules was observed for phosphino and thiophosphinoyl alkylidene lanthanide complexes as well as for phosphinidene lanthanide complexes. Furthermore, the trinuclear mixed oxo/alkyl complexes L13Ln3(μ2-CH3)3(μ3-CH3)(μ3-O) (with L1 = PhC(NC6H3iPr2-2,6)2; Ln = Sc, Y, Lu, Dy) undergo non-redox oxygen transfer with PhNCS or CS2 despite the presence of reactive Ln-alkyl bonds. The synthesis of pseudo-Grignard reagents PhLnI (Ln = Eu, Yb) was investigated and their synthetic potential in organometallic chemistry demonstrated. The first lanthanide-cyclobutadienyl complexes were obtained as anionic “tuck-in” complexes [M{η4-C4(SiMe3)4}{η4-C4(SiMe3)3-κ-(CH2SiMe2)}]2− (Ln = Y, Dy), showing square-shaped cyclobutadienyl ligands. Further progress has been made in the understanding of “new” divalent lanthanide chemistry, especially the influence of the ligand size. The small CpMe ligand formed highly reactive complexes, e.g. [K(crypt)][Y(CpMe)3], with the larger lanthanides, which reacted with the solvent to give unprecedented reductive THF-ring opening. However, with smaller lanthanides the complexes [(18-crown-6)K(μ-CpMe)K(18-crown-6)][CpMe3Ln] (Ln = Tb, Ho), displaying an inverse sandwich as counter-cation, could be isolated. The reactivity of the highly bulky complex SmCpAr-Et2 (CpAr-Et = C5(4-EtC6H4)5) towards a large range of small molecules was investigated, revealing only reaction with cuminil to afford the first trivalent lanthanide decaaryllanthanidocene complex SmCpAr-Et2(Ar′C(O)C(O)Ar′) (Ar′ = 4-iPrC6H4). Following the important discovery of recent years on SMM (single molecule magnet) behavior of Dy metallocenes, the quest for even better SMMs continued in 2018. Several new record-holding complexes were synthesized based on polyisopropyl-cyclopentadienyl complexes, with the best complex to date being [(C5iPr5)(C5Me5)Dy] [BAr4], showing magnetic hysteresis up to 80 K and an effective energy barrier to reversal of magnetization Ueff = 1541 cm−1. Several remarkable lanthanide arene complexes have been prepared and structurally characterized. For example, the bimetallic inverse sandwich La2+ complex salt [K(18-crown-6)(THF)2][(Cp″2La)2(μ-η6:η6-C6H6)]·THF (Cp″ = C5H3(SiMe3)2-1,3) reduces hydrocarbons such as naphthalene, anthracene, or cyclooctatetraene to give La3+ complexes of the hydrocarbon anions. Samarium-arene bonding has also been observed in the rare samarium(II) aryloxide Sm(OAriPr6)2 [AriPr6 = −C6H3-2,6-(C6H2-2,4,6-iPr3)2] and in the remarkable tetranuclear samarium(II) inverse sandwich complex (µ-η6:η6-C7H8)[KSmL3]2 (L = OSi(OtBu)3). Several new triple-decker complexes of the type Ln2(COT″)3 (COT″ = bis(trimethylsilyl)cyclooctatetraenyl dianion) have been isolated and structurally characterized. The synthesis and structural characterization of four unsolvated divalent lanthanide cyclononatetraenyl sandwich complexes, Ln(Cnt)2 (Ln = Sm, Eu, Tm, Yb; Cnt = η9-cyclononatetraenyl) have also been achieved. Single-crystal X-ray diffraction studies revealed that these neutral sandwich complexes are rigorously linear. A rare heterobimetallic [1] ferrocenophane terbium(III) complex has been found to exhibit single-ion magnet behavior. Reduction of the scandium precursor Sc(nacnac)(OAr)(OCP) (nacnac– = [ArNC(CH3)]2CH, Ar = 2,6-iPr2C6H3) with KC8 afforded a binuclear scandium complex comprising a unique [OCPPCO]4− central motif formed through P–P radical coupling. Heterobimetallic Sm/Co polyarsenides [(CptttCo)2As4Sm(C5Me4R)2] (Cpttt = 1,2,4-C5H2tBu3, R = Me, nPr) were synthesized from the reaction of divalent Sm complexes Cp*2Sm, Cp*2Sm(THF)2 or (C5Me4nPr)2Sm with [(CptttCo)2(μ,η2:2-As2)2], while the Sm/Sb multimetallic complex [(Cp*2Sm)4(μ4,η2:2:2:2-Sb8)] was synthesized from the oxidation of Cp*2Sm with activated antimony. The chemistry of endohedral lanthanide metallofullerenes continued to be an active field of research in 2018. Significant achievements have also been made in the area of organolanthanide catalysis. For example, a variety of highly active lanthanide catalysts for the polymerization of polar substituted styrene monomers as well as the polymerization of 2-vinylpyridine have been developed. Half-sandwich complexes of scandium have been successfully employed in the copolymerization of myrcene with ethylene and propylene. New organolanthanide-catalyzed reactions include the diastereo- and enantioselective C(sp)–H addition of terminal alkynes to 3,3-substituted cyclopropenes and the catalytic hydrothiomethylation of olefins and dienes with a series of methyl-alkyl sulfides. Moreover, the first-time scandium-catalyzed C(sp3)–H alkylation of N,N-dimethyl anilines with olefins has been investigated. Significantly less results over previous years have been published in 2018 on the use of organolanthanide precursors in materials science. The synthesis and reactivity of complexes with actinide-element multiple bonds, e.g. Acdouble bondC, Acdouble bondN, Acdouble bondP, is a highly active research area. The synthesis of uranium(IV) silyl-phosphino-carbene complexes was reported, among which the bis(carbene) complex [U{C(SiMe3)(PPh2)}(BIMPTMS)(µ-Cl)Li(TMEDA)(µ-TMEDA)0.5]2 (36%) showing a 3-center U–C–P character and a short Udouble bondCcarbene bond. A rare U(IV) imido species [K(THF)3][(PN)U(NH)(iPr2P(C6H3Me)N(C6H2Me2CH2)] was isolated, in which one of the PN ligands has been cyclometallated. This complex was proposed to have been formed by the addition of the ligand C–H of a nearby methyl group to a uranium nitride intermediate. The synthesis and the diverse reactivity of the first base-free terminal phosphinidene thorium(IV) complex, Th(CptBu3)2(PR), towards heterounsaturated small molecules was reported. The functionalization of CO and tert-butyl nitrile have been reported for a Th(IV) bis(phosphido) complex, (C5Me5)2Th{P(Mes)(H)}2, involving intramolecular proton transfer reactions. The synthesis and characterization of the first Np(II) organometallic complex [K(crypt)][Np(Cp″)3] (crypt = 2,2,2-cryptand, Cp″ = C5H3(SiMe3)2) was achieved by reduction of a trivalent precursor. The synthesis and characterization of a neutral U(II) complex, U(NHAriPr6)2, and the very reactive uranium(III) cation [U(NHAriPr6)2][BArF24] supported by an amido bis(arene) ligand was reported. Good progress has been made in metallofullerene chemistry of the actinides, as evidenced by the structural characterization of the dimetallic actinide endohedral metallofullerene (EMF) U2@C80, revealing short U–U bonding inside the cage. Actinide complexes have also found new applications in catalysis, as shown by the first example of selective hydroboration of aldehydes and ketones using actinide catalysis.

• Item Type: Articles
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Farnaby, Dr Joy and Wilson, Mr Bradley
• Authors: Edelmann, F. T., Farnaby, J. H., Jaroschik, F., and Wilson, B.
• College/School: College of Science and Engineering > School of Chemistry
• Journal Name: Coordination Chemistry Reviews
• Publisher: Elsevier
• ISSN: 0010-8545
• ISSN (Online): 1873-3840
• Published Online: 10 August 2019