- Synthetic inorganic chemistry
- Design and synthesis of new macrocyclic and multidentate ligands involving donor atoms from Groups 15 (P, As, Sb, Bi) and 16 (S, Se, Te)
- Coordination chemistry with s-, p-, d- and f-block metal ions
- Applications in semiconductor deposition and devices
- Metal fluoride complexes for new PET imaging agents
Group 1 and group 2 complexes with soft donor ligands
The neutral group 16 donor chalcogenoethers, ER2 (E = S, Se, Te) are generally regarded as soft, modest σ-donor ligands that form complexes readily with middle and late transition metals in low oxidation states. However, by judicious choice of the reaction conditions and metal ion source, new types of complexes containing soft chalcogenoether ligands with hard, early transition metal ions (including, for example, Ti, Zr, Hf(IV), V(IV), V(III), Nb(V), Ta(V), W(VI), etc.) can be prepared. This opens up the opportunity to explore how these very unusual metal-ligand combinations affect the chemistry and reactivity within these unusual complexes, generating new single source reagents for materials deposition. Similarly, neutral phosphine ligands, PR3, are ubiquitous in transition metal chemistry, owing to their capacity to tune the electronic and steric properties, and hence the reactivity, of the complexes, and to the strong σ-donor properties of the soft phosphine donor functions. This has led to wide utilisation of phosphine co-ligands in many transition metal reagents and catalysts. We have now extended this work to explore the chemistry of soft donor ligands towards alkali metal and alkaline earth cations.
Notable recent contributions include developing a strategy to enable access to very elusive metal-ligand combinations by taking advantage of large, diffuse fluorinated tetra-aryl borate anions.This has led to several new classes of complexes, including unprecedented octathioether macrocyclic coordination to Na+ (Inorg. Chem. 2015, 54, 2497; doi:10.1021/acs.inorgchem.5b00156) and the first alkali metal cation complexes with homoleptic neutral phosphine coordination (Chem. Commun. 2015, 51, 9555; doi:10.1039/C5CC03184B). This work is being extended to alkaline earth and f-block dications with S/Se-donor macrocyclic ligands (Dalton Trans. 2015, 44, 2953; doi:10.1039/C4DT03462G; Dalton Trans. 2012, 42, 89; doi:10.1039/C2DT31692G).
Metal-fluoride coordination chemistry
Until relatively recently metal fluoride coordination complexes, in contrast to the heavier halide counterparts, were rather neglected and little was known about their properties. However, it has become clear that the study of coordination complexes of inorganic fluorides is important and often the properties conferred on the acceptor centre by fluorine co-ligands are very significantly different – this is mainly a reflection of the strong M-F bonding imposing a different chemistry upon the metal centre.
a) High Oxidation State Metal Fluoride Chemistry
b) Fluoride Coordination Complexes as Scaffolds for Next Generation Medical Imaging Agents
The radioisotope 18F is widely used in medical imaging agents for positron emission tomography (PET) since it is easily produced via a cyclotron, has a short half-life (t1/2 = 109 min) and no radioproducts (18O is the decay product). Originally, the 18F was incorporated into organofluorine species, but more recent work has developed a range of non-C-F containing carrier molecules, including species with both metal and non-metal to fluorine bonds. Recent work in our group and in collaboration with GE Healthcare has developed complexes of both AlF3 and GaF3 based upon neutral and anionic macrocyclic N-donor ligands as carrier molecules (Chemical Science, 2014, 5, 381; doi:10.1039/C3SC52104D; Dalton Trans., 2015, 44, 9569; doi:10.1039/C5DT01120E).
Our recent work has shown that [GaF3(BnMe2-tacn)] can be radiofluorinated under very mild conditions in water at sub-30 nM concentration via 18F/19F isotopic exchange (Angew. Chem. Int. Ed. 2018, 57, 6658; 10.1002/anie.201802446).
Electrodeposition of Semiconducting Chalcogenides from Unusual Media
Semiconducting metal chalcogenides are of increasing importance for emerging micro- and nano-electronic applications in renewable energy, solid state memory and 3D metamaterials. However, the demand for smaller, higher performance, more energy-efficient electronics, is pushing conventional materials deposition and device fabrication methods to their intrinsic limits. ‘Top-down’ vapour deposition methods are not suitable for filling very high aspect-ratio vias in structures that may be the basis of future devices. Electrodeposition offers several potentially significant advantages: it is a faster and much lower cost alternative to vapour deposition, and as a ‘bottom-up’ growth method, it can fill 2D and 3D nanostructures selectively. Electrodeposition is therefore an extremely attractive prospect for the production of nano-structured materials and devices.
(a) Electrodeposition of Device-Quality Phase Change Memory (PCM) Materials
PCM is a non-volatile memory type that works on the principle of probing the resistivity of the memory material, which is either amorphous or crystalline, by means of an electrical pulse. Rewriting of the memory bits is achieved by either re-crystallising an amorphous cell with a medium-amplitude voltage SET pulse or re-amorphising a crystalline cell with a high-amplitude voltage RESET pulse (cycling). Materials required for PCM applications are characterised by their high contrast between the resistances of the crystalline and amorphous phases, very fast phase transition speeds (tens of nanoseconds) and high stabilities. Chalcogenide-based PCM materials are among the most promising candidates to scale non-volatile memory beyond the Flash memory architecture, with the ternary Ge2Sb2Te5 (GST-225) the most widely used. Electrodeposition offers a highly effective alternative approach for the deposition of such materials, offering excellent spatial selectivity. In collaboration with Profs. Bartlett and Hector (Chemistry) and de Groot (ECS) and with funding from EPSRC, we have developed a highly tuneable electrolyte system for the electrodeposition of phase change memory materials, including the challenging ternary GST-225, from CH2Cl2. The system allows fine control of the ternary composition across the whole phase diagram, and rapid, highly efficient deposition into arrays of sub-100 nm nano-cells. Using this approach we have produced functional nano-devices with highly promising phase cycling and endurance lifetimes (Materials Horizons, 2015, 2, 420; doi:10.1039/C5MH00030K; RSC Advances, 3, 15645; doi:10.1039/c3ra40739j).
(b) Electrodeposition of Extreme Nanostructured Materials from Supercritical Fluids
Supercritical fluid electrodeposition (SCFED) is a materials deposition technique that exploits the unique properties of supercritical fluids (SCFs), most notably, their lack of surface tension and their high mass transport rates, to enable electrodeposition of materials into high aspect ratio, extremely narrow diameter pores. As part of the EPSRC Programme Grant, ‘Complex Nanostructures by Supercritical Fluid Electrodeposition’ (www.scfed.net) led by Prof. Phil Bartlett we have shown that it is possible to deposit a range of metal and reactive materials from SCFs, including Ge, Ag and Cu, of which 3 nm nanowires have been grown for the latter (Proc. Natl. Acad. Sci., USA 2009, 106, 14768; doi:10.1073/pnas.0901986106; ChemElectroChem, 2014, 1, 187; doi:10.1002/celc.201300131; Phys. Chem. Chem. Phys. 2014, 16, 9202; doi:10.1039/c3cp54955k; Chemistry - A European Journal, 2014, 20, 5019; doi:10.1002/chem.201400179).The development of SCFED to enable the formation of a wide range of elements from the main group (Ga, In, Ge, Sn, Pb, Sb, Bi, Se, Te) has been achieved, and sub-13 nm nanowires of several of these demonstrated, including binary and ternary semiconductors for electronic and optical applications. This work has required development of tailored reagents to provide the source of the elements being deposited, and therefore we have developed several series of unusual complexes based upon dicationic Ge aza-macrocyclic species such as [Ge(Me3-tacn)]2+ and [Ge(Me4-cyclam)]2+ (Angew. Chem. Int. Ed. 2009, 48, 5152; doi:10.1002/anie.200901247) and rare soft phosphine complexes of Ge(II) and Si(IV) (Inorg. Chem. 2010, 49, 752; doi:10.1021/ic902068z; Inorg. Chem. 2013, 52, 5185; doi:10.1021/ic400077z).
(c) Electrodeposition of few-layer transition metal dichalcogenides from weakly coordinating solvents
Under a current EPSRC project we are developing electrolytes that allow the electrodeposition of transition metal dichalcogenide semiconductors, including MoS2 and WS2 and have recently demonstrated electrodeposition of few-Layer MoS2 on graphene for 2D material heterostructures (ACS Adv. Mater. and Interfaces, 2020, 12, 44, 49786; https://dx.doi.org/10.1021/acsami.0c14777).
Single source precursors for chemical vapour deposition of chalcogenide semiconductor thin films
In addition to their intrinsic interest, some of the new families of coordination and organometallic compounds developed in our group are highly effective reagents for the chemical vapour deposition of thin film metal chalcogenide semiconductors. These constitute an important class of materials for many electronics and thermoelectric applications.
In a collaborative project with Prof. Kees de Groot (Electronics and Computer Science) and Prof. Andrew Hector (Chemistry), we have exploited the specific advantages offered by our new compounds, which include their suitability for highly selective deposition onto micro- and nano-patterned regions on lithographically patterned substrates (Chemistry of Materials, 2013, 22, 4442-4449; doi:10.1021/cm302864x; J. Materials Chem. A, 2014, 2, 4865; doi:10.1039/c4ta00341a) and the ability to produce dense arrays of individual metal chalcogenide nanocrystals. The morphology, structure and crystallite orientation are identified using a combination of scanning electron microscopy, atomic force microscopy and thin film X-ray diffraction techniques. These materials exhibit very promising electrical and thermoelectric properties (Journal of Materials Chemistry C, 2105, 3, 423; doi:10.1039/C4TC02327G; ACS Appl. Energy Mater. 2020, 3, 5840; doi.org/10.1021/acsaem.0c00766). Substrate selectivity is a feature that is not typical of CVD, and the origin of this behaviour is currently being investigated, as well as the prospects for utilising it in the fabrication of micro- and nano-devices for phase change memory and thermoelectric applications.
Similar approaches are being adopted for the growth of early transition metal selenide and telluride thin films such as NbSe2, MoSe2, etc.
Organoantimony and organobismuth chemistry
Stibines (SbR3) and bismuthines (BiR3) are the heavier Group 15 analogues of the very widely studied phosphine ligands. Although they are generally regarded as weaker donor ligands, stibines and bismuthines confer a number of interesting features that are not seen in the lighter analogues. Developing the chemistry of these relatively little studied compounds is one of our research themes.
Our recent work on developing new organo-stibine and organo-bismuthine ligand chemistry has revealed that these compounds, unlike their very widely studied lighter phosphine counterparts, can function simultaneously as both donors (towards metal cations) and acceptors (from adjacent Lewis bases) (Coordination Chemistry Reviews 2015, 297-298, 168; doi:10.1016/j.ccr.2015.02.003; Organometallics, 2012, 31, 1025; doi:10.1021/om2010996; Organometallics, 2011, 30, 895; doi:10.1021/om1010148).
This ‘hypercoordination’ leads to distinctly different chemical behaviours and reactivities; current work in the group is exploring the properties, reaction chemistry and the electronic structures and bonding present in these very unusual systems.
More recent work produced the very unexpected dark purple complex [Pd4(µ3-SbMe3)4(SbMe3)4], obtained from [Pd2Cl4(SbMe2Cl)4] and 8 equivalents of MeLi in thf, and containing a Pd4 tetrahedron with each palladium carrying a terminal SbMe3 and with one SbMe3 ligand capping each triangular face – a bonding mode unprecedented for SbMe3.