Group 13 and 14 coordination complexes and reagents for the electrodeposition of tin and lead from supercritical fluids

Abstract

Supercritical fluid electrodeposition is a new technique being developed that aims to deposit technologically important p-block materials at the nanoscale. This requires significant synthetic effort in precursor development, and investigations into fundamental p-block coordination chemistries, primarily through the use of neutral phosphine and arsine ligands, have been undertaken to aid the design process. The compounds synthesised have been characterised by microanalysis, IR spectroscopy and multinuclear NMR spectroscopy (1H, 11B, 19F, 31P{1H}, 27Al and 119Sn, where appropriate), while single crystal X-ray structure determinations have been performed for representative examples.

AlX3 (X = Cl, Br, I) form both [AlX3(PMe3)] and [AlX3(PMe3)2] depending on the reagent stoichiometry employed. While all reported arsine complexes are four-coordinate, neutral diphosphine ligands show a preference for six-coordinate Al; the crystal structures of [AlCl2{o-C6H4(PR2)2}2][AlCl4] (R = Me, Ph) and [AlCl2{Me2P(CH2)2PMe2}2][AlCl4] are reported, along with [(AlCl3)2{?-R2P(CH2)2PR2}] (R = Me, Cy). The importance of solvent choice and hydrolysis in these systems is discussed. Steric and electronic effects are considered in comparing the behaviour of MX3 (M = Al, Ga, In) towards phosphines.

A number of new phosphine complexes of BX3 (X = F, Cl, Br, I) have been synthesised, and [BF3(PMe3)] is crystallographically authenticated. Flexible diphosphine ligands yield bridging [(BX3)2{?-R2P(CH2)2PR2}] (R = Me, Et), while rigid chelating ligands produce the unusual ionic [BX2{o-C6H4(PMe2)2}]+ and [BCl2{o-C6H4(AsMe2)2}]+. The d(B–P) are consistent with the order of Lewis acidity being BI3 > BBr3 > BCl3 > BF3. The rare [B2F7]- ion is thought to drive the unexpected formation of [BF2{Ph2P(O)CH2P(O)Ph2}][B2F7].

The effect of the weakly coordinating fluoroanions [BF4]- and [SiF6]2- on the coordination environments around Pb(II) in di- and tri-imine complexes is investigated, with several different coordination modes observed. A method is devised to synthesise some Pb(II) diphosphine complexes; the crystal structures of [Pb{o-C6H4(PMe2)2}(H2O)(SiF6)]·H2O and [Pb(L–L)(NO3)2] (L–L = Me2P(CH2)2PMe2, o-C6H4(PMe2)2) are reported, which reveal chelating diphosphines and intermolecular anion interactions, giving extended structures. Adventitious O2 forms the bridging ligand complex [Pb{Et2(O)P(CH2)2P(O)Et2}2(NO3)2].

The well-defined and stable [NnBu4][SnX3] and [PPh4][PbX3] (X = Cl, Br, I) precursors are shown by cyclic voltammetry to give reproducible metal deposition and stripping peak features in CH2Cl2, with the reduction potential becoming more accessible Cl ? Br ? I. Good quality thin films of Sn and Pb can be electrodeposited from CH2Cl2 using these precursors, which are analysed by SEM, EDX and XRD. Subsequent work in the group has shown that [NnBu4][SnCl3] can be used to deposit 13 nm Sn nanowires from supercritical CH2F2, highlighting the potential of this versatile electrolyte system for future applications in p-block materials deposition at the extreme nanoscale, and device fabrication.

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Identifiers

ORCID for Gill Reid: orcid.org/0000-0001-5349-3468

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