Synthesis and Reactivity of Bis-Unsubstituted Trispyrazolylborate Lanthanide Complexes

This thesis demonstrates the synthetic utility and versatility of the unsubstituted N-donor tridentate scorpionate ligand hydrotris(1-pyrazolyl)borate (Tp) in lanthanide (Ln) coordination chemistry specifically of [Ln(Tp)2(X)] complexes to support a diverse range of reactive ‘X’-anions in the [Ln(Tp)2] + ancillary ligand environment. Lanthanides have remarkable physical (optical, magnetic) and chemical (small-molecule activation, catalytic activity) properties. In the [Ln(Tp)2] + ancillary ligand environment, through the modulation of the sterics, electronics and fundamental nature of ‘X’ anions, the advantageous physical and chemical properties of the Ln(III) ions can be tuned. The current research in the field is introduced in Ch 1, to highlight that most synthetic chemistry of [Ln(Tp)2(X)] complexes were performed in aqueous media and that these [Ln(Tp)2(X)] complexes were studied predominantly for probing the physical properties (optical, magnetic) of the Ln(III) ions. Owing to the scarcity of air- and moisture-sensitive synthetic chemistry to access [Ln(Tp)2(X)] complexes, a gap is seen in the knowledge of the reactive chemistry between the [Ln(Tp)2] + and the comparator [Ln(TpR )2] + (TpR = bulky analogues of Tp) ancillary ligand environments. In literature, it was also observed that the synthesis of reactive [Ln(TpR )2(X)] complexes, supporting reactive ‘X’ anions have often proved difficult or led to complicated reactions and ligand degradation. Utilising the small unsubstituted Tp ligand, this thesis presents rational synthetic routes to access reactive [Ln(Tp)2(X)] complexes for chemical applications (small-molecule activation, redox-chemistry, catalysis) in addition to investigating physical properties (photoluminescence). Hence rediscovering the Tp ligand as a robust ancillary ligand environment to stabilise reactive Ln ions (Ch 2 – Ch 7). The heteroleptic precursor Ln(III) triflate complexes [Ln(Tp)2(OTf)] (Ln = Y, Ch 2; Sm, Ch 5; Eu, Ch 2; Gd, Ch 2; Dy, Ch 4; Yb, Ch 2; OTf = triflate) have been synthesised and fully characterised, to provide an entry point into the chemistry of [Ln(Tp)2] + . Salt metathesis of [Ln(Tp)2(OTf)] with K(Nʹʹ) (Nʹʹ = N(SiMe3)2) in toluene yielded the [bis(silyl)]amides [Ln(Tp)2(Nʹʹ)] (Ln = Y, Ch 2; Sm, Ch 5; Dy, Ch 4; Yb, Ch 2). The complexes [Ln(Tp)2(Nʹʹ)] underwent protonolysis with the bulky alcohol 2,6- tBu2-4-Me-phenol (HOAr) to yield aryloxides [Ln(Tp)2(OAr)] (Ln = Y, Yb; Ch 2). Complexes [Ln(Tp)2(X)] (X = OTf, Nʹʹ) were used to access primary lanthanide amides [Ln(Tp)2(NHArCF3)] (Ln = Y, Dy; ArCF3 = C6H3(CF3)2-3,5; Ch 4), by either metathesis of [Ln(Tp)2(OTf)] with K(NHArCF3) or protonolysis of [Ln(Tp)2(Nʹʹ)] with H2NArCF3 in toluene. The amides [Ln(Tp)2(Nʹʹ)] are reactive towards small-molecule activation, such as activation and functionalisation of carbon dioxide (CO2) to yield isolable monomeric silyloxides [Ln(Tp)2(OSiMe3)] (Ln = Y, Sm; Ch 6) and trimethylsilyl isocyanate (O=C=NSiMe3). In pursuit of hydrides, [Ln(Tp)2(Nʹʹ)] demonstrate insertion of alanes [LB•AlH3] (LB = neutral Lewis base) into Ln–N(Nʹʹ) o-bonds to yield lanthanide-aluminium heterobimetallic trihydride complexes [Ln(Tp)2(u-H)2Al(H)(Nʹʹ)] (Ln = Y, Sm, Dy, Yb; Ch 5). The complexes [Ln(Tp)2(u - H)2Al(H)(Nʹʹ)] exhibit reduction of unsaturated substrates such as carbodiimides and benzophenone, and catalytically dehydrocouples dimethylamine-borane under ambient conditions, elucidating the nature of the complexes as bimetallic hydrides. Reduction of Ln(III) triflates [Ln(Tp)2(OTf)] (Ln = Sm, Eu, Yb) with KC8 in toluene (Ln = Eu, Yb) or THF (Ln = Sm) yielded the Ln(II) complexes: monomeric [Sm(Tp)2(DME)] (Ch 6) and [Yb(Tp)2] (Ch 3) and dimeric [{Eu(Tp)(u-k 1 :n 5 -Tp)}2] (Ch 3). All Ln(II) complexes are intensely coloured and the electronic absorption data show the 4f-5d electronic transitions in Ln(II) (Ln = Eu, Yb). The complex [{Eu(Tp)(u-k 1 :n 5 -Tp)}2] is photoluminescent and single-crystal X-ray diffraction data revealed the first u-k 1 :n 5 -coordination mode of the unsubstituted Tp ligand to Ln(II). The non-classical Ln(II) [Ln(Tp)2] (Ln = Y, Dy) cannot be isolated, where it leads to the formation of the homoleptic Ln(III) [Ln(Tp)3] (Ln = Y, Dy) complexes and fragmentation of the Tp ligand to yield [Y(Tp)2(k 2 -pz)] (pz = pyrazolyl, Ch 6). The formation of [Ln(Tp)3] was observed to be a major limitation in the chemistry of [Ln(Tp)2] + when attempting to stabilise highly-reducing and/or non-traditional ‘X’-anions, for example as also observed in the synthesis and challenging isolation of the parent amides such as [{Y(Tp)2(u-NH2)}2] (Ch 4). The Ln(II) complexes are single-electron reductants for example, [Sm(Tp)2(DME)] reduces CO2 in isolation of the oxalate-bridged dimeric complex [{Sm(Tp)2}2(u-n 2 :n 2 -O2CCO2)] (Ch 6) and [Yb(Tp)2] reduces the redox-active bridging ligand 1,10-phenanthroline-5,6-dione (pd) yielding the O,O′-bound Yb(III) complex [Yb(Tp)2(O,O′-pd)] (Ch 7). To make the synthetic route towards [Ln(Tp)2(O,O′-pd)] accessible to a range of mid to late Ln(III) ions, the [Ln(Tp)2(O,O′-pd)] (Ln = Dy, Yb; Ch 7) complexes were also synthesised by a route from Ln(III) [Ln(Tp)2(OTf)]. Subsequent complexation of Ln(III) B-diketonate precursors [Ln(hfac)3(THF)2] (Ln = Eu, Yb; Ch 7) to the N,N′- binding site of [Ln(Tp)2(O,O′-pd)] yielded the pd•- radical-bridged lanthanide heterobimetallic complexes [(Tp)2Ln(O,O′-N,N′-pd)Ln′(hfac)3] (Ln = Yb, Ln′ = Eu; Ln = Dy, Ln′ = Yb; Ch 7). The light emissive properties of [Ln(Tp)2(O,O′-pd)] and [(Tp)2Ln(O,O′-N,N′-pd)Ln′(hfac)3] were investigated. In conclusion (Ch 8), the thesis summarises the contributions and advances made in the field of Ln(III) [Ln(Tp)2] + and Ln(II) [Ln(Tp)2] chemistry, and looks at potential future directions to access further reactive Ln(III) [Ln(Tp)2(X)] synthetic targets

Synthesis and reactivity of bis-tris(pyrazolyl)borate lanthanide-aluminium heterobimetallic trihydride complexes

Molecular heterobimetallic hydride complexes of lanthanides (Ln) and main group (MG) metals exhibit chemical properties unique from their monometallic counterparts and are highly reactive species, making their synthesis and isolation challenging. Herein, molecular Ln/Al heterobimetallic trihydrides [Ln(Tp)2(μ-H)2Al(H)(Nʹʹ)] 2-Ln (Ln = Y, Sm, Dy, Yb; Tp = hydrotris(1-pyrazolyl)borate; Nʹʹ = N(SiMe3)2) have been synthesised by facile insertion of aminoalane [Me3N•AlH3] into the Ln–N amide bonds of [Ln(Tp)2(Nʹʹ)] 1-Ln. Thus, providing a simple synthetic strategy to access a range of Ln/Al hydrides. Reactivity studies demonstrate that 2-Ln is a heterobimetallic hydride, with evidence for the cooperative nature of 2-Ln shown by the catalytic amine-borane dehydrocoupling under ambient conditions in contrast to its monomeric counterparts

Reduction chemistry yields stable and soluble divalent lanthanide tris(pyrazolyl)borate complexes †

Reduction of the heteroleptic Ln(iii) precursors [Ln(Tp)2(OTf)] (Tp = hydrotris(1-pyrazolyl)borate; OTf = triflate) with either an aluminyl(i) anion or KC8 yielded the adduct-free homoleptic Ln(ii) complexes dimeric 1-Eu [{Eu(Tp)(μ-κ1:η5-Tp)}2] and monomeric 1-Yb [Yb(Tp)2]. Complexes 1-Ln have good solubility and stability in both non-coordinating and coordinating solvents. Reaction of 1-Ln with 2 Ph3PO yielded 1-Ln(OPPh3)2. All complexes are intensely coloured and 1-Eu is photoluminescent. The electronic absorption data show the 4f–5d electronic transitions in Ln(ii). Single-crystal X-ray diffraction data reveal first μ-κ1:η5-coordination mode of the unsubstituted Tp ligand to lanthanides in 1-Eu

Heteroleptic lanthanide(III) complexes: synthetic utility and versatility of the unsubstituted bis-scorpionate ligand framework

The unsubstituted bis-hydrotris(1-pyrazolyl)borate) (Tp) ligand framework has been used to synthesise a range of heteroleptic Ln(III) coordination complexes [Ln(Tp)2(X)]. The precursor complexes [Ln(Tp)2(OTf)] 1-Ln (Ln = Y, Eu, Gd, Yb; OTf = triflate) were synthesised by reaction of Ln(OTf)3 with two equivalents of K(Tp). The 8-coordinate β-diketonate complexes [Ln(Tp)2(hfac)] 2-Ln (Ln = Y, Eu, Yb; hfac = hexafluoroacetylacetonate) were synthesised from Ln(OTf)3 by reacting 1-Ln generated in situ with an equivalent of K(hfac). The 7-coordinate amide complexes [Ln(Tp)2(N″)] 3-Ln (Ln = Y, Yb; N″ = bis(trimethylsilyl)amide) were synthesised from 1-Ln by reaction with K(N″). Reactivity of 3-Ln towards protonolysis was demonstrated by the isolation of the hydroxide dimer [{Y(Tp)2(μ-OH)}2] 4-Y from adventitious reaction with water and the aryloxide complex [Ln(Tp)2(OAr)] 5-Ln (Ln = Y, Yb; OAr = 2,6-tBu2-4-Me-phenoxide) from reaction with H(OAr). Full characterisation data are presented for all complexes, including solid-state molecular structure determination by single-crystal X-ray diffraction

Buta- and penta-dienyl complexes of the actinides

This chapter covers the synthesis, structure and bonding, and reactivity of actinide complexes of buta-, and penta-dienyl ligands. First, the small number of homoleptic, and non-cyclopentadienyl heteroleptic hydrocarbyl actinide complexes are presented in brief. This is followed by an overview of the synthesis and reactivity of heteroleptic hydrocarbyl actinide complexes in a tris-cyclopentadienyl and bis-permethylcyclopentadienyl ancillary ligand environment. The bulk of this chapter then provides comprehensive coverage of actinide complexes with composite four-carbon ligands. First, actinide complexes containing acyclic 2-butene-1,4-diyl and 1,3-butadiene-1,4-diyl ligands. Second, the planar five-membered metallacyclic complexes, actinacyclopentadienes, actinacyclopentatrienes, and an actinacyclopentyne. The actinacyclic complexes display significant differences in bonding and reactivity depending on the level of ligand unsaturation. Third, recent developments in complexes of the cyclobutadienyl dianion are reported. Finally, a conclusion provides a summary and an outlook on future work

Lanthanides and actinides: Annual survey of their organometallic chemistry covering the year 2019

This review summarizes the progress in organo-f-element chemistry during the year 2019. Organo-f-element 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(η2-PhCCPh)][K(crypt)] displaying an aromatic metallacycle, the first lanthanide-aluminabenzene complexes [(1-Me-3,5-tBu2-C5H3Al)(μ-Me)Ln(2,4-di-tbutylpentadienyl)] (Ln = Y, Lu) and the first scandium phosphonioketene complex [LSc(η2-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{(μ-O4C4)(μ-η2-CO)2(μ-η1-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 CS22− 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 o-methoxystyrene, 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 low-temperature 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 η1-cyaarside ligands (Ctriple bondAs−), bridging diarsaallene (As = C = As)2− and trapped radical dianion of the phosphoethynolate (OCP2−radical dot) 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

A student led computational screening of peptide inhibitors against main protease of SARS-CoV-2

The main protease of SARS-CoV-2 is a promising drug target due to its functional role as a catalytic dyad in mediating proteolysis during the viral life cycle. In this study, experimentally proven 14 HIV protease peptides were screened against the main protease of SARS-CoV-2. Fourteen middle and high school “student researchers” were trained on relevant computational tools, provided with necessary biological and chemical background and scientific article writing. They performed the primary screening via molecular docking and the best performing complexes were subjected to molecular dynamics simulations. Molecular docking revealed that HIP82 and HIP1079 can bind with the catalytic residues, however after molecular dynamics simulation only HIP1079 retained its interaction with the catalytic sites. The student researchers were also trained to write scientific article and were involved with drafting of the manuscript. This project provided the student researchers an insight into multi-disciplinary research in biology and chemistry, inspired them about practical approaches of computational chemistry in solving a real-world problem like a global pandemic. This project also serves as an example to introduce scientific inquiry, research methodology, critical thinking, scientific writing, and communication for high school students

Lanthanide amide complexes supported by the bis‐tris(pyrazolyl)borate ligand environment

Synthesis of primary lanthanide amides in the bis-hydrotris(1-pyrazolyl)borate ligand environment has been achieved. Salt metathesis of [Dy(Tp)2(OTf)] 1-Dy (OTf = CF3SO3) with K(Nʹʹ) (Nʹʹ = N(SiMe3)2) in toluene yielded the [bis(silyl)]amide [Dy(Tp)2(Nʹʹ)] 2-Dy. Complexes 1-Ln and 2-Ln were both used to access primary lanthanide amides, where either metathesis of 1-Ln with K(NHArCF3) (ArCF3 = C6H3(CF3)2-3,5) or protonolysis of 2-Ln with H2NArCF3 in toluene yielded the primary amides [Ln(Tp)2(NHArCF3)] 3-Ln (Ln = Y, Dy). The synthesis of parent amides was also attempted, but the metathesis of 1-Y with NaNH2 yielded complicated reaction mixtures, but from which the dimeric parent amide [{Y(Tp)2(μ-NH2)}2] 4-Y and an ʻateʼ-salt [{Y(Tp)2(μ2-OTf)(μ3-OTf)Na(THF)2}2] 5-Y were isolated. Full characterisation data are presented for all complexes, including solid-state molecular structure determination by single-crystal X-ray diffraction

Synthesis and reactivity of bis-tris(pyrazolyl)borate lanthanide-aluminium heterobimetallic trihydride complexes

Molecular heterobimetallic hydride complexes of lanthanides (Ln) and main group (MG) metals exhibit chemical properties unique from their monometallic counterparts and are highly reactive species, making their synthesis and isolation challenging. Herein, molecular Ln/Al heterobimetallic trihydrides [Ln(Tp)2(-H)2Al(H)(Nʹʹ)] 2-Ln (Ln = Y, Sm, Dy, Yb; Tp = hydrotris(1-pyrazolyl)borate; Nʹʹ = N(SiMe3)2) have been synthesised by facile insertion of aminoalane [Me3N•AlH3] into the Ln–N amide bonds of [Ln(Tp)2(Nʹʹ)] 1-Ln. Thus, providing a simple synthetic strategy to access a range of Ln/Al hydrides. Reactivity studies demonstrate that 2-Ln is a heterobimetallic hydride, for example it is able to reduce unsaturated substrates. Further evidence for the cooperative nature of 2-Ln is shown, as 2-Ln can catalytically dehydrocouple amine-borane under ambient conditions in contrast to its monomeric counterparts

A second glass transition observed in single-component homogeneous liquids due to intramolecular vitrification

Data used for figures and tables in manuscrip