Metal complexes of substituted catecholates and redox flow batteries containing the same

1. A composition comprising:

a coordination compound comprising a substituted catecholate ligand, the substituted catecholate ligand having a structure of

in a neutral form or a salt form;

wherein n is an integer ranging between 1 and 4, such that one or more Z are bound to the substituted catecholate ligand at an open aromatic ring position, each Z being the same or different when more than one Z is present; and

wherein Z is a heteroatom functional group selected from the group consisting of A 1 R A1 , A 2 R A2 , A 3 R A3 , and CHO;

wherein A 1 is —(CH 2 ) a — or —(CHOR)(CH 2 ) a —, R A1 is —OR 1 or —(OCH 2 CH 2 O) b R 1 , a is an integer ranging between 0 and about 6, with the proviso that R 1 is not H when a is 0 and R A1 is —OR 1 , and b is an integer ranging between 1 and about 10;

wherein R is H, C 1 -C 6 alkyl, heteroatom-substituted C 1 -C 6 alkyl, or C 1 -C 6 carboxyalkyl; and

wherein R 1 is H, methyl, ethyl, a C 2 -C 6 polyol bound through an ether linkage or an ester linkage, or C 1 -C 6 carboxyalkyl;

wherein A 2 is —(CH 2 ) c — or —CH(OR 2 )(CH 2 ) d —, R A2 is —NR 3 R 4 , a carbon-linked amino acid, or —C(═O)XR 5 , X is —O— or —NR 6 —, c is an integer ranging between 0 and about 6, and d is an integer ranging between 0 and about 4;

wherein R 2 , R 3 , R 4 and R 6 are independently selected from the group consisting of H, C 1 -C 6 alkyl, or heteroatom-substituted C 1 -C 6 alkyl; and

wherein R 5 is H, C 1 -C 6 alkyl, heteroatom-substituted C 1 -C 6 alkyl, a C 2 -C 6 polyol bound through an ester linkage, a hydroxyacid bound through an ester linkage, a polyglycol acid bound through an ester linkage, an amino alcohol bound through an ester linkage or an amide linkage, an amino acid bound through an ester linkage or an amide linkage, or —(CH 2 CH 2 O) b R 1 ; and

wherein A 3 is —O— or —NR 2 —, R A3 is —(CHR 7 ) e OR 1 , —(CHR 7 ) e NR 3 R 4 , —(CHR 7 ) e C(═O)XR 5 , or —C(═O)(CHR 7 ) f R 8 , e is an integer ranging between 1 and about 6, with the proviso that e is not 1 when A 3 is —O—, and f is an integer ranging between 0 and about 6;

wherein R 7 is H or OH;

wherein R 8 is H, C 1 -C 6 alkyl, heteroatom-substituted C 1 -C 6 alkyl, a C 2 -C 6 polyol bound through an ether linkage or an ester linkage, a hydroxyacid bound through an ether linkage or an ester linkage, a polyglycol acid bound through an ether linkage or an ester linkage, an amino alcohol bound through an ether linkage, an ester linkage, or an amide linkage, an amino acid bound through an ether linkage, an ester linkage, or an amide linkage, a carbon-linked amino acid, or —(OCH 2 CH 2 O) b R 1 ;

wherein the substituted catecholate ligand has a structure selected from the group consisting of

wherein the coordination compound has a formula of

D g M(L 1 )(L 2 )(L 3 );

wherein M is a transition metal, D is NH 4 + , Li + , Na + , or K + , g is an integer ranging between 0 and 6, and L 1 , L 2 and L 3 are ligands, at least one of L 1 , L 2 and L 3 being the substituted catecholate ligand.

2. The composition of claim 1 , wherein at least two of L 1 , L 2 and L 3 are substituted catecholate ligands.

3. The composition of claim 2 , wherein L 1 and L 2 are substituted catecholate ligands and L 3 is an unsubstituted catecholate ligand.

4. The composition of claim 1 , wherein L 1 is the substituted catecholate ligand and L 2 and L 3 are unsubstituted catecholate ligands.

5. The composition of claim 1 , wherein each of L 1 , L 2 and L 3 are substituted catecholate ligands.

6. The composition of claim 1 , wherein the transition metal is Ti.

7. The composition of claim 1 , wherein any of L 1 to L 3 that are not the unsubstituted catecholate ligand comprises one or more ligands that are independently selected from the group consisting of an unsubstituted catecholate, ascorbate, citrate, glycolate, a polyol, gluconate, a hydroxyalkanoate, acetate, formate, benzoate, malate, maleate, phthalate, sarcosinate, salicylate, oxalate, a urea, a polyamine, aminophenolate, acetylacetonate, and lactate.

8. An electrolyte solution comprising the composition of claim 1 .

9. The electrolyte solution of claim 8 , wherein the electrolyte solution is an aqueous solution.

10. The electrolyte solution of claim 9 , further comprising:

a buffer, a supporting electrolyte, a viscosity modifier, a wetting agent, or any combination thereof.

11. The electrolyte solution of claim 9 , wherein the aqueous solution has a pH ranging between about 1 and about 13.

12. The electrolyte solution of claim 9 , wherein the transition metal is Ti.

13. The electrolyte solution of claim 9 , wherein a concentration of the coordination compound in the aqueous solution ranges between about 0.5 M and about 3 M.

14. A flow battery comprising an electrolyte solution comprising the composition of claim 1 .

15. The flow battery of claim 14 , wherein the transition metal is Ti.