Coordinatively unsaturated titanium catecholate complexes and processes associated therewith
Coordinatively saturated titanium (IV) coordination compounds containing catecholate ligands can be desirable active materials for flow batteries and other electrochemical energy storage systems. Such coordination compounds can be formed advantageously via an intermediate composition containing a coordinatively unsaturated titanium (IV) coordination compound. More specifically, such compositions can include a coordinatively unsaturated titanium (IV) coordination compound having a coordination number of 5 or less and containing two catecholate ligands, wherein the composition is substantially free of non-ligated catechol compound. Methods for forming the coordinatively unsaturated titanium (IV) coordination compounds can include: combining titanium tetrachloride and a catechol compound in an aprotic solvent to form a reaction mixture, and reacting the titanium tetrachloride with the catechol compound in an absence of base to form a coordinatively unsaturated titanium (IV) coordination compound having a coordination number of 5 or less and containing two catecholate ligands and that is insoluble in the reaction mixture.
1. A method comprising:
reacting a titanium tetrachloride with a catechol compound in an aprotic solvent and an absence of base to form a coordinatively unsaturated titanium (IV) coordination compound having a coordination number of 5 or less and comprising two catecholate ligands;
wherein the coordinatively unsaturated titanium (IV) coordination compound is insoluble in the aprotic solvent; and
reacting the coordinatively unsaturated titanium (IV) coordination compound with a ligatable compound in the presence of a base and a second solvent to form a coordinatively saturated titanium (IV) coordination compound having four coordination sites occupied by the two catecholate ligands and two coordination sites occupied by one or more additional ligands.
2. The method of claim 1 , wherein the coordinatively unsaturated titanium (IV) coordination compound has a coordination number of 4.
3. The method of claim 2 , wherein the coordinatively unsaturated titanium (IV) coordination compound is electrically neutral and bears no counterions.
4. The method of claim 1 , comprising no more than about 2 stoichiometric equivalents of the catechol compound relative to the titanium tetrachloride.
5. The method of claim 1 , further comprising:
isolating the insoluble coordinatively unsaturated titanium (IV) coordination compound as a solid and reacting the isolated solid with the ligatable compound in the presence of the base and the second solvent to form the coordinatively saturated titanium (IV) coordination compound having four coordination sites occupied by the two catecholate ligands and two coordination sites occupied by the one or more additional ligands.
6. The method of claim 5 , wherein the insoluble coordinatively unsaturated titanium (IV) coordination compound is substantially free of non-ligated catechol compound.
7. The method of claim 5 , further comprising:
rinsing the insoluble coordinatively unsaturated titanium (IV) coordination compound with a washing solvent after isolating.
8. The method of claim 1 , wherein the second solvent comprises an aqueous solvent.
9. The method of claim 8 , wherein the coordinatively saturated titanium (IV) coordination compound is soluble in the aqueous solvent.
10. The method of claim 1 , wherein the one or more additional ligands comprise an additional catecholate ligand that is the same as or different than the two catecholate ligands.
11. The method of claim 1 , wherein the base comprises one or more alkali metal base, an ammonium base, or a combination thereof.
12. The method of claim 1 , wherein the coordinatively insoluble unsaturated titanium (IV) coordination compound is substantially free of non-ligated catechol compound.
13. The method of claim 5 , wherein the solvent comprises an aqueous solvent.
14. The method of claim 13 , wherein the coordinatively saturated titanium (IV) coordination compound is soluble in the aqueous solvent.
15. The method of claim 5 , wherein the one or more additional ligands comprise an additional catecholate ligand that is the same as or different than the two catecholate ligands.
16. The method of claim 5 , wherein the base comprises one or more alkali metal base, an ammonium base, or a combination thereof.
17. A method comprising:
reacting a titanium tetrachloride with a catechol compound in an aprotic solvent and an absence of base to form a coordinatively unsaturated titanium (IV) coordination compound having a coordination number of 5 or less and comprising two catecholate ligands, wherein the coordinatively unsaturated titanium (IV) coordination compound is insoluble;
isolating the coordinatively unsaturated titanium (IV) coordination compound as a solid;
reacting the coordinatively unsaturated titanium (IV) coordination compound with a ligatable compound in the presence of an alkali metal base and aqueous solvent to form a coordinatively saturated titanium (IV) coordination compound having four coordination sites occupied by the two catecholate ligands and two coordination sites occupied by one or more additional ligands, wherein the coordinatively saturated titanium (IV) coordination compound is soluble in the aqueous solvent; and
incorporating an electrolyte solution comprising the coordinatively saturated titanium (IV) coordination compound in at least one half-cell of a flow battery.
18. The method of claim 17 , wherein the one or more additional ligands comprise an additional catecholate ligand that is the same as or different than the two catecholate ligands.