Stabilised silicate and borate solutions for foliar agricultural sprays

This thesis describes the synthesis and characterisation of seven organic tetraalkylammonium hydroxide salts potentially capable of inhibiting the polycondensation of bioactive silicic acid. The preparation of the hydroxide salts [Me3NCH2CH(OH)CH2OH][OH] (TAA(1)), [Et2MeNCH2CH(OH)CH2OH][OH] (TAA(2)), [Et3NCH2CH(OH)CH2OH][OH] (TAA(3)), [Me2(CH2CH2OH)NCH2CH(OH)CH2OH][OH] (TAA(4)), [Et2(CH2CH2OH)NCH2CH(OH)CH2OH][OH] (TAA(5)), [Me3N(CH2CH(OH)(OCH2CH2)nOCH2CH(OH)CH2NMe3][OH]2 (TAA(6)), Me2(CH2CH(OH)CH2OH)N(CH2)2N(CH2CH(OH)CH2OH)Me2][OH]2 (TAA(7)), is reported, together with stability trials of some acidified formulations containing these salts and silicic acid. TAA(1) was used in formulations for controlled glasshouse pot trials of crops including potato, wheat, cabbage and tomato to test the efficacy of silicic acid formulations on plant growth. The cabbage and wheat trial results report small improvements in foliar yield (up to a 30% improvement in dry weight of stem and leaf produce in cabbage; up to a 47% improvement in wheat), while the potato and tomato results report negligible or no improvements. Assays were carried out on leaf digests of the wheat and cabbage produce to determine silicon uptake by AAS, and a small increase in Si concentration (10-25% for cabbage; 23-45% for wheat) was reported on the leaves sprayed with silicic acid prototypes. The synthesis and characterisation of a number of novel polyoxidoborate compounds containing organic amines was also reported, due to the similarity of the cations to those used in the formulations. All of these compounds contain 6-membered B3O3 boroxole rings within their structures. A total of twenty-eight new non-metal cation polyborate salts are reported; of which 18 of these contain the pentaborate anion, [B5O6(OH)4]- , and one contains the tetraborate dianion, [B4O5(OH)4]2-. The crystal structures of seventeen salts containing these polyborate anions are reported: [Me3N(CH2)2NMe3][B4O5(OH)4]·2H2O·2B(OH)3 (1a), [Me3NCH2CH2NMe3][B5O6(OH)4]2 (1b), [Me3N(CH2)3NMe3][B5O6(OH)4]2.0.5H2O (2a), [Me3NCH2CH=CH2][B5O6(OH)4] (2b), [CH3(C3H3N2)(CH2)6(C3H3N2)CH3][B5O6(OH)4]2 (3a), [C2H5(C3H3N2)(CH2)6(C3H3N2)C2H5][B5O6(OH)4]2 (4a), [CH3(C3H3N2)CH2(C6H4)CH2(C3H3N2)CH3][B5O6(OH)4]2 (5a), [CH3(C4H8N)(CH2)6(C4H8N)CH3] [B5O6(OH)4]2 (6a and 6b) (two polymorphs), [C2H5(C4H8N)(CH2)6(C4H8N)C2H5][B5O6(OH)4]2 (7a), [C4H9(C4H8N)(CH2)6(C4H8N)C4H9] [B5O6(OH)4]2·4B(OH)3 (8a), [C3H5(C4H8N)(CH2)6(C4H8N)C3H5][B5O6(OH)4]2 (9a), [MeNHC(NH2)NH2] [B5O6(OH)4]·H2O (18a), [Me2NC(NH2)NH2][B5O6(OH)4] (19a), [Me2NC(NHMe)N(Me)2] [B5O6(OH)4]·B(OH)3 (20a), [NH2NHC(NH2)NH2][B5O6(OH)4] (21a), and [C7H14N3][B5O6(OH)4] (22a). All of these compounds were characterized using spectroscopic (FTIR, multi-element NMR) and analytical (elemental analysis and thermal analysis) techniques. In the solid state, all of the reported polyborate salts form giant H-bonded anionic lattices. In some cases, cation-anion H-bonding interactions are displayed in the structures. The tetraborate (1a) requires two 'spacer' molecules of boric acid and water to fully incorporate the dication into the vacancies within the lattice. Of the pentaborate products, the "herringbone" and "brickwall" structures were particular prevalent in accommodating bulkier dications. Of the reported pentaborate salts, the anionic lattice in [C4H9(C4H8N)(CH2)6(C4H8N)C4H9][B5O6(OH)4]2 (8a) was particularly interesting, containing the unique "α,α,β,β" configuration, featuring two C(8) β-chains, with each anionic unit forming hydrogen-bonding interactions with two adjacent molecules of boric acid.