title: Synthesis and Reactivity of Bis-Unsubstituted Trispyrazolylborate Lanthanide Complexes
Creators: name: Chowdhury, Tajrian
Publisher: name: University of Glasgow
Year: 2024

description: 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.