Cobalt complexes appended with para - And meta -arylcarboxylic acids: Influence of cation, solvent, and symmetry on hydrogen-bonded assemblies

Article

pubs.acs.org/crystal

Cobalt Complexes Appended with para- and meta-Arylcarboxylic

Acids: Inﬂuence of Cation, Solvent, and Symmetry on

Hydrogen-Bonded Assemblies

Girijesh Kumar, Himanshu Aggarwal, and Rajeev Gupta*

Department of Chemistry, University of Delhi, Delhi -110 007 India

S

* Supporting Information

ABSTRACT: The present work shows the syntheses, structures,

and hydrogen-bonding-based self-assembly of Co³⁺coordination

complexes containing appended arylcarboxylic acid groups. The

carboxylic acid groups are present at either the para or the meta

position of the appended arene ring. Every Co³⁺coordination

complex oﬀers both hydrogen-bond donors (in O−H groups of

arylcarboxylic acid) and acceptors (in CO_amidegroups as well as

CO_acidof arylcarboxylic acid) in a single molecule. As a con-

sequence, all molecules self-assemble in a highly complementary

and unique structural fashion via an array of inter- and intramolecular hydrogen bonding. The position of carboxylic acid, presence of cation,

type of cation, and solvent of crystallization has signiﬁcantly aﬀected/altered the self-assembly process of complementary functional groups.

come under the category of homosynthons.¹⁶However, when a

suﬃciently polar group, which can potentially form one or more

INTRODUCTION

■

The design and self-assembly of hydrogen bond (H-bond)-containing

synthons¹is an important research activity in supramolecular

H-bonds, is associated with a carboxylic acid based synthon, it

comes under the class of heterosynthons.¹⁷The main advantage

chemistry.²The motivation for preparing intricate synthetic systems

of a heterosynthon-based approach is that several geometrically

diﬀerent building blocks can be easily programmed into

supramolecular structures, which are otherwise extremely

diﬃcult to achieve with homosynthons. Few intriguing examples

with speciﬁc one- (1D), two- (2D), and three-dimensional (3D)

architectures³⁻¹⁰is often inspired from nature where biological

molecules self-assemble in unique, yet energetically limited,

architectural forms.^1,2In addition, research activities are also driven

show that the heterosynthons can be used to change the

by the requirement of newer materials with structure-speciﬁc

properties and tailor-made applications.¹¹One of the major challenges

dimensionality of carboxylic acids and rationally modulate the

periodicity of the pattern.^17aFurther, in the case of several

in this ﬁeld is to understand the parameters that control the molecular

self-assembly and thus help the scientiﬁc community to take a small,

competing synthons, the self-assembly pattern tends to follow a

typical hierarchical principle,¹⁸that is, the best H-bond donor

yet vital, step toward predicting the ﬁnal outcome of the self-

assembled architectures.^3,12Recent progress in the vast ﬁeld of supra-

pairs with the best H-bond acceptor, the second best donor with

the second-best acceptor, and so on. The situation becomes more

molecular chemistry is targeted to understand the molecular self-

complex, although interesting, when a certain number of water

assembly of synthetically designed organic molecules that act as the

molecules co-crystallize with carboxylic acid based motifs and

molecular building blocks and contain H-bond-sensitive functional

groups.¹²In addition, signiﬁcant progress has also been made to look

actively participate and/or compete in the self-assembly. In such

instances, a water molecule may act as “glue” and connect two

into the coordination-driven self-assemblies where metal-to-ligand

bonding generates diverse topological architectures.¹³

carboxylic acids together, or a water molecule may also function as a

“spacer” and elongate the framework.¹⁹Of course, such a situation

The molecular synthons based on carboxylic acid groups have

been extremely important and abundant in the self-assembled

again qualiﬁes within the broad domain of heterosynthons.

structures.^7,12a,b,14For H-bond-based structures; carboxylic acid

Our research has been focusing on developing coordination

groups are quite interesting for several reasons.^7,12a,14One of the

complexes as the building blocks for the construction of ordered

structures.²⁰A coordination complex as the building block oﬀers

most important reasons is that such a synthon exhibits dual

character as the oxygen atom of the −COOH group can act as

an acceptor for H-bonds, whereas the hydroxyl group can act

structural rigidity that has the ability to place the auxiliary

functional groups in a preorganized conformation. Such auxiliary

as a donor.^12aTherefore, carboxylic acid groups are self-

functional groups could then be utilized to either coordinate a

secondary metal ion or be involved in the self-assembly. The ﬁrst

complementary in nature and tend to create intermolecularly

linked dimers.^14cCommon binding modes for the carboxylic acids

based synthons are cyclic dimers, trimers, and higher polygons,

and chains, tapes, and other catemeric motifs.¹⁵Notably,

examples where two carboxylic acid based groups are involved

Received: August 12, 2012

Revised: November 9, 2012

Published: November 9, 2012

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8.21 (t, 1H, H₁₀). ¹³C NMR (CDCl₃, 400 MHz): δ = 51.99 (C₁),

166.46(C₂), 126.42 (C₃), 131.06 (C₄), 119.31 (C₅), 140.97 (C₆), 160.98

(C₇), 148.60 (C₈), 126.17 (C₉), 139.90 (C₁₀).

synthetic strategy has allowed us to prepare the {Mⁿ⁺−Co³⁺−

Mⁿ⁺} heterobimetallic complexes (Mⁿ⁺= Zn²⁺, Cd²⁺, Hg²⁺, and

Cu⁺)^20a−cand {Co³⁺−Zn²⁺}^20eor {Co³⁺−Cd²⁺}^20fnetworks of a

highly ordered nature. The second strategy has permitted us to

introduce H-bond-based functional groups that have the

propensity to self-assemble. In a recent work,²⁰ⁱwe showed the

self-assembly of Co³⁺-based building blocks appended with

phenol and catechol groups. An important aspect of this work

was that these complexes oﬀered both H-bond donors (in O−H

group) as well as acceptors (in OC_amidegroup) in a single

molecule and thus operated in a highly complementary manner.

We anticipated that the replacement of a phenol group by

carboxylic acid may lead to a more complex situation as a

−COOH group exhibits dual character and oﬀers both H-bond

acceptor and donor.^14a,bHerein, we report several Co³⁺

complexes appended with p- and m-carboxylic acids as the

building blocks to understand their self-assembly through

hydrogen bonding (H-bonding). We also attempt to understand

the eﬀect of cations, solvent of crystallization, and symmetry on

the self-assembly process.

2,6-Bis(3-ethylbenzoate-carbomyl)pyridine (H₂L^m‑COOEt). This li-

gand was synthesized similarly using ethyl-3-amino benzoate in place of

methyl-4-amino benzoate. Yield: 2.46 g, 89%. Anal. Calcd for

C₂₅H₂₃N₃O₆·H₂O: C, 62.62%; H, 5.26%; N, 8.76%. Found: C,

62.70%; H, 5.38%; N, 8.65%. FTIR (KBr, selected peaks): 3334

1

(NH), 1702 (COO), 1686 (CO) cm⁻¹. H NMR (CDCl₃, 400

MHz): δ =10.60 (s, 2H, CONH), 1.10 (t, 6H, H₁/H_1′), 4.06 (q, 4H, H₂/

H_2′), 7.70 (d, 2H, H₅/H_5′, J = 8 Hz), 7.40 (m, 2H, H₆/H_6′), 8.20 (d, 2H,

H₇/H_7′), 8.48 (s, 2H, H₉/H_9′), 8.52 (d, 2H, H₁₂/H_12′), 8.54 (t, 1H, H₁₃).

¹³C NMR (CDCl₃, 400 MHz): δ = 14.17 (C₁), 60.80 (C₂), 166.26 (C₃),

130.30 (C₄), 125.42 (C₅), 128.99 (C₆), 124.10 (C₇), 137.78 (C₈),

122.11 (C₉), 161.29 (C₁₀), 148.78 (C₁₁), 125.28 (C₁₂), 139.63 (C₁₃).

Synthesis of Cobalt(III) Complexes with Protected Carboxylic

Acid Groups. [Co(BL^p‑COOEt)₃] (1^P). The ligand HBL^p‑COOEt(0.50 g, 1.85

mmol) was dissolved in dinitrogen-ﬂushed DMF and treated with solid

NaH (44.4 mg, 1.85 mmol). The resulting mixture was then stirred for

30 min at room temperature, and solid [Co(H₂O)₆](ClO₄)₂] (223.2

mg, 0.61 mmol) was added to the aforementioned mixture. After 30 min

of stirring, dry O₂was purged to the solution for 2 min. The solution was

ﬁnally stirred for 1 h. The reaction mixture was ﬁltered, followed by the

removal of the solvent under reduced pressure. The crude product was

isolated after washing with diethyl ether. The crude product thus

obtained was dissolved in DMF and subjected to vapor diﬀusion of

diethyl ether, which aﬀorded a crystalline product within 2 days. Yield:

1.2 g, 75%. Anal. Calcd for C₄₅H₃₉N₆O₉Co·6H₂O: C, 55.44%, H, 5.27%,

N, 8.52%. Found: C, 55.81%; H, 5.41%; N, 8.06%. FTIR (KBr, selected

peaks): 3426 (OH), 1717, 1664 (COO), 1636, 1593 (CO) cm⁻¹.

Absorption spectrum [λ_max, nm, CH₃OH (ε, M⁻¹cm⁻¹)]: 640 (40), 530

(sh, 270), 500 (sh, 310). Conductivity (∼1 mM, CH₃OH, 298 K): Λ_M=

8 Ω⁻¹cm²mol⁻¹(the range for 1:1 electrolyte in CH₃OH is 80−115).

¹H NMR (CDCl₃, 400 MHz): δ = 1.31−1.35 (m, 9H, H₁), 4.30−4.23

EXPERIMENTAL SECTION

■

Materials and Methods. Reagents of analytical grade were

procured from Sigma-Aldrich, Alfa-Aesar, and Spectrochem and were

used without further puriﬁcation. The solvents were puriﬁed using

standard literature methods.²¹

Synthesis of Ligands. 2-(4-Ethylbenzoate-carbomyl)pyridine

(HBL^p‑COOEt). Pyridine-2-carboxylic acid (1.0 g, 0.08 mmol) was added

asa solid in one portion to a suspension ofethyl-4-amino benzoate(1.35 g,

0.08 mmol) in pyridine (10 mL), and the mixture was stirred at 40 °C

for 40 min. Triphenylphosphite (5.89 g, 0.019 mmol) was added

dropwise over 10 min to the aforementioned mixture, and the

temperature was increased to 90 °C. The reaction mixture was stirred

for 4 h at 90 °C. On cooling to room temperature, a colorless precipitate

was obtained that was ﬁltered, washed with water, and dried. The crude

product was puriﬁed by recrystallization with aqueous CH₃OH. Yield:

2.02 g, 92%. Anal. Calcd for C₁₅H₁₄N₂O₃: C, 66.66%; H, 5.22%; N,

10.36%. Found: C, 66.62%; H, 5.38%; N, 10.54%. FTIR (KBr, selected

(m, 6H, H₂), 7.23−7.25 (m, 3H, H₅protons for three asymmetric

ligands), 7.02, 6.52, 6.22 (br, 3H, H₆), 7.93−7.96 (t, 3H, H₁₀protons for

three asymmetric ligands), 7.70, 7.85 (m/m, 3H, H₁₁protons for three

asymmetric ligands), 7.64, 7.41 (d, 3H, H₁₂protons for three

asymmetric ligands), 9.07, 8.60, 8.38 (d, 3H, H₁₃protons for three

asymmetric ligands).

[Co(BL^m‑COOEt)₃] (2^P). This complex was synthesized using an identical

scale in a similar manner as that of complex 1^Pusing ligand HBL^m‑COOOEt

in place of HBL^p‑COOOEt. Yield: 0.98 g, 62%. Anal. Calcd for

C₄₅H₃₉N₆O₉Co·2H₂O: C, 59.87%, H, 4.80%, N, 9.31%. Found: C,

59.47%; H, 4.46%; N, 9.11%. FTIR (KBr, selected peaks): 3433 (OH),

1718, (COO), 1600 (CO) cm⁻¹. Absorption spectrum [λ_max, nm,

CH₃OH (ε, M⁻¹cm⁻¹)]: 655 (30), 525 (sh, 190), 500 (sh, 225).

Conductivity (∼1 mM, CH₃OH, 298 K): Λ_M= 5 Ω⁻¹cm²mol⁻¹(the

range for 1:1 electrolyte in CH₃OH is 80−115). ¹H NMR (CDCl₃, 400

MHz): δ =1.17−1.26 (m, 9H, H₁), 4.24−4.26 (m, 6H, H₂), 7.86, (m,

3H, H₅), 6.80, 6.85 (m, 3H, H₆), 7.70 (m, 3H, H₇protons for three

asymmetric ligands), 8.19, 7.99 (s, 3H, H₉), 7.49, 7.52 (d, 3H, H₁₂

protons for three asymmetric ligands), 7.77 (m/t, 3H, H₁₃protons for

three asymmetric ligands), 7.18, 7.28 (m, 3H, H₁₄protons for three

asymmetric ligands). 9.10, 8.71, 8.38 (d, 3H, H₁₅protons for three

asymmetric ligands).

1

peaks): 3336 (N-H), 1712 (COO), 1543 (CO) cm⁻¹. H NMR

(CDCl₃, 400 MHz): δ = 10.23 (s, 2H, CONH), 1.39 (s, 3H, H₁), 4.37

(q, 2H, H₂), 8.09 (d, 1H, H₅), 7.87 (d, 1H, H₆), 8.31 (d, 1H, H₁₀), 7.93−

8.07 (m, 1H, H₁₁), 7.53 (m, 1H, H₁₂), 8.63 (d, 1H, H₁₃). ¹³C NMR

(CDCl₃, 400 MHz): δ = 14.33 (C₁), 60.82 (C₂), 166.14 (C₃), 148.00

(C₄), 122.50 (C₅), 118.80 (C₆), 137.78 (C₇), 162.16 (C₈), 149.29 (C₉),

126.74 (C₁₀), 130.82 (C₁₁), 125.96 (C₁₂), 141.70 (C₁₃).

2-(3-Ethylbenzoate-carbomyl)pyridine (HBL^m‑COOEt). This ligand

was synthesized in a similar manner as that of HBL^p‑COOEtusing ethyl-3-

amino benzoate in place of ethyl-4-amino benzoate. Yield: 1.90 g, 87%.

Anal. Calcd for C₁₅H₁₄N₂O₃: C, 66.66%; H, 5.22%; N, 10.36%. Found

C; 66.38%; H, 5.47%; N, 10.31%. FTIR (KBr, selected peaks): 3336

(N-H), 1713 (COO), 1696, 1612 (CO) cm⁻¹. ¹H NMR (CDCl₃, 400

MHz): δ = 10.14 (s, 2H, CONH), 1.40 (t, 3H, H₁), 4.39 (q, 2H, H₂),

7.25 (m, 1H, H₅), 7.48 (m, 1H, H₆), 6.82 (d, 1H, H₇), 8.62 (s, 1H, H₉),

7.81 (d, 1H, H₁₂), 8.27 (m, 1H, H₁₃), 7.91 (m, 1H, H₁₄), 8.22 (d, 1H,

H₁₅). ¹³C NMR (CDCl₃, 400 MHz): δ = 14.33 (C₁), 61.16 (C₂), 166.18

(C₃), 148.02 (C₄), 122.46 (C₅), 115.22 (C₆), 120.55 (C₇), 137.86 (C₈),

126.64 (C₉), 162.20 (C₁₀), 149.62 (C₁₁), 123.96 (C₁₂), 129.21 (C₁₃),

125.37 (C₁₄), 131.35 (C₁₅).

(Et₄N)[Co(L^p‑COOMe)₂] (3^P). The ligand H₂L^p‑COOMe(1.0 g, 2.30 mmol)

was dissolved in CH₂Cl₂(50 mL), and to it was added Co(OAc)₂·4H₂O

(0.287 g, 1.15 mmol) and K₂CO₃(7.99 g, 57.88 mmol). The resulting

mixture was reﬂuxed for 4 h. To the resulting yellow-red solution was

added solid Et₄NCl·xH₂O (0.764 g, 4.62 mmol), and stirring was

continued for an additional 2 h at room temperature in open air.

Exposure to air resulted in a dark coloration of the solution.

Subsequently, solvent was removed under reduced pressure, followed

by the addition of CH₃CN. The solution was ﬁltered, and the ﬁltrate was

2,6-Bis(4-methylbenzoate-carbomyl)pyridine (H₂L^p‑COOMe). This

ligand was also synthesized in a similar manner using pyridine-2,

6-dicarboxylic acid and methyl-4-amino benzoate in a 1:2 ratio. The

crude product was recrystallized from aqueous CH₃OH. Yield: 2.54 g,

98%. Anal. Calcd for C₂₃H₁₉N₃O₆·2H₂O: C, 58.85%; H, 4.94%; N, 8.95%.

Found: C, 58.74%; H, 5.18%; N, 9.03%. FTIR (KBr, selected peaks):

3337 (NH), 1714, 1686 (COO), 1665 (CO) cm⁻¹. ¹H NMR (CDCl₃,

400 MHz): δ = 9.62 (s, 2H, CONH), 3.94 (s, 6H, H₁/H_1′),

7.88 (d, 4H, H₄/H_4′), 8.14 (d, 4H, H₅/H_5′), 8.56 (d, 2H, H₉/H_9′, J = 8 Hz),

1

concentrated to /₄th of its original volume. Addition of diethyl ether

resulted in the isolation of a dark green precipitate, which was ﬁltered

and dried in vacuum. The compound was recrystallized by diﬀusing the

vapors of Et₂O to a CH₃CN solution of the crude product. Yield: 1.4 g,

58%. Anal. Calcd for C₅₄H₅₄CoN₇O₁₂·2H₂O: C, 59.61; H, 5.37; N, 9.01;

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Found: C, 59.14%; H, 4.97%; N, 8.77%. FTIR (KBr, selected peaks):

3448 (OH), 1718 (COO), 1611 (CO) cm⁻¹. Conductivity (CH₃CN,

∼1 mM solution at 298 K): Λ_M= 125 Ω⁻¹cm²mol⁻¹(the range for 1:1

electrolyte in CH₃CN is 120−160). Absorption spectrum (λ_max, nm,

CH₃OH (ε, M⁻¹cm⁻¹): 651 (160), 568 (sh, 60). ¹H NMR (CDCl₃, 400

MHz): δ = 3.80 (s, 12H, H₁/H_1′), 6.92 (d, 8H, H₄/H_4′, J = 8 Hz), 7.58

(d, 8H, H₅/H_5′, J = 8 Hz), 7.70 (d, 4H, H₉/H_9′, J = 8 Hz), 7.83 (t, 2H,

H₁₀), 3.02 (q, 8H, −CH₂−, Et₄N⁺), 1.15 (t, 12H, −CH₃, Et₄N⁺). ¹³C

NMR (CDCl₃, 400 MHz): δ = 7.50 (−CH₃, Et₄N⁺), 51.99 (−CH₂−,

Et₄N⁺), 52.48 (C₁), 167.44 (C₂), 139.21(C₃), 124.36 (C₄), 125.70 (C₅),

150.65 (C₆), 167.12 (C₇), 156.33 (C₈), 126.41 (C₉), 129.90 (C₁₀).

(Et₄N)[Co(L^m‑COOEt)₂] (4^P). The complex 4^Pwas synthesized using an

identical scale and method as that of 3^P, however, using ligand

H₂L^m‑COOEtin place of H₂L^p‑COOMe. Yield: 1.36 g, 57%. Anal. Calcd for

C₅₈H₆₂CoN₇O₁₂·6H₂O: C, 57.28; H, 6.13; N, 8.06. Found: C, 57.01%;

H, 6.56%; N, 7.71%. FTIR (KBr, selected peaks): 3422 (OH), 1718

(COO), 1618 (CO) cm⁻¹. Conductivity (CH₃CN, ∼1 mM solution

at 298 K): Λ_M= 145 Ω⁻¹cm²mol⁻¹(the range for 1:1 electrolyte in

CH₃CN is 120−160). Absorption spectrum (λ_max, nm, CH₃OH (ε, M⁻¹

cm⁻¹): 631 (60), 562 (sh, 30). ¹H NMR (CDCl₃, 400 MHz): δ = 1.28 (t,

12H, H₁/H_1′), 4.18 (q, 8H, H₂/H_2′), 7.15 (d, 4H, H₅/H_5′), 6.98 (m, 4H,

H₆/H_6′), 7.48 (d, 4H, H₇/H_7′), 7.34 (s, 4H, H₉/H_9′), 7.62 (d, 4H, H₁₂/

∼1 mM, 298 K): Λ_M= 10 Ω⁻¹cm²mol⁻¹(the range for 1:1 electrolyte in

CH₃OH is 80−115). Absorption spectrum (λ_max, nm, CH₃OH (ε, M⁻¹

cm⁻¹): 645 (30), 525 (sh, 250), 485 (sh, 290). ¹H NMR (DMSO-d₆, 400

MHz): δ = 7.46−7.52 (m, 6H, H₃protons for three asymmetric ligands),

7.23, 7.02, 6.24 (d, 6H, H₄protons for three asymmetric ligands), 7.90−

7.94 (m, 3H, H₈protons for three asymmetric ligands), 8.33, 8.02

(m, 3H, H₉protons for three asymmetric ligands), 7.67 (d, 3H, H₁₀protons

for three asymmetric ligands), 8.92, 8.73, 8.02 (d, 3H, H₁₁protons for

three asymmetric ligands).

[Co(BL^m‑COOH)₃] (2). This compound was synthesized in a similar

manner as discussed for complex 1. Yield: 0.096 g, 53%. Anal. Calcd for

C₃₉H₂₇CoN₆O₉·CH₂Cl₂: C, 55.38; H, 3.37; N, 9.69. Found: C, 55.12%;

H, 3.41%; N, 10.09%. FTIR (KBr, selected peaks): 3422 (OH), 1702

(COO), 1605 (CO) cm⁻¹. Conductivity (CH₃OH, ∼1 mM, 298 K):

Λ_M= 15 Ω⁻¹cm²mol⁻¹(the range for 1:1 electrolyte in CH₃OH is

80−115). Absorption spectrum (λ_max, nm, CH₃OH (ε, M⁻¹cm⁻¹): 645

(25), 540 (sh, 190), 505 (230). ¹H NMR (CDCl₃, 400 MHz): δ = 5.30

(s, 2H, CH₂Cl₂), 7.04, 6.79 (m/br, 3H, H₃protons for three asymmetric

ligands), 7.70 (t, 3H, H₄protons for three asymmetric ligands), 6.45 (br,

3H, H₅protons for three asymmetric ligands), 7.46 (s, 3H, H₇protons

for three asymmetric ligands), 7.32−7.38 (d/d, 6H, H₁₀/H₃protons for

three asymmetric ligands), 8.32 (t, 3H, H₁₁protons for three asymmetric

ligands), 7.88−7.95 (m, 3H, H₁₂protons for three asymmetric ligands),

8.95, 8.72, 8.16 (d, 3H, H₁₃protons for three asymmetric ligands).

(Et₄N)[Co(L^p‑COOH)₂] (3). The complex 3 was synthesized in a similar

manner as that of complex 1. Yield: 0.48 g, 51%. Anal. Calcd for

C₅₀H₄₆CoN₇O₁₂·8H₂O: C, 52.68; H, 5.48; N, 8.60. Found: C, 52.42%;

H, 5.65%; N, 8.58%. FTIR (KBr, selected peaks): 3404 (OH), 1702

(COO), 1570 (CO) cm⁻¹. Conductivity (DMF, ∼1 mM solution at

298 K): Λ_M= 70 Ω⁻¹cm²mol⁻¹(the range for 1:1 electrolyte in DMF is

65−160). Absorption spectrum (λ_max, nm, CH₃OH (ε, M⁻¹cm⁻¹): 660

(120), 594 (sh, 80). ¹H NMR (DMSO-d₆, 400 MHz): δ = 12.7 (br, 4H,

−COOH), 6.73 (d, 8H, H₃/H_3′, J = 8 Hz), 7.45 (d, 8H, H₄/H_4′, J = 9

Hz), 7.64 (d, 4H, H₈/H_8′, J = 8 Hz), 7.99 (t, 2H, H₉), 3.16 (q, 8H,

H

_12′), 7.71 (t, 2H, H₁₃), 3.22 (q, 8H, −CH₂−, Et₄N⁺), 1.18 (t, 12H,

−CH₃, Et₄N⁺). ¹³C NMR (CDCl₃, 400 MHz): δ = 7.52 (−CH₃, Et₄N⁺),

52.33 (−CH₂−, Et₄N⁺), 14.23 (C₁), 60.59 (C₂), 167.30 (C₃), 138.34

(C₄), 125.02 (C₅), 123.94 (C₆), 128.29 (C₇), 145.57 (C₈), 126.87 (C₉),

167.16 (C₁₀), 156.23 (C₁₁), 130.34 (C₁₂), 131.35 (C₁₃).

Na[Co(L^p‑COOMe)₂] (5^P). This compound was synthesized in a similar

manner as discussed for 1^Pusing the following reagents: H₂L^p‑COOMe

(0.50 g, 1.153 mmol), NaH (0.055 g, 2.307 mmol), and [Co(H₂O)₆]-

(ClO₄)₂] (0.211 g, 0.58 mmol). Yield: 0.57 g, 53%. Anal. Calcd for

C₄₆H₃₄N₆O₁₂CoNa·H₂O: C, 57.39%; H, 3.77%; N, 8.73%. Found: C,

57.13%; H, 3.97%; N, 8.98%. FTIR (KBr, selected peaks): 3405 (OH),

1709 (COO), 1608, 1587 (CO) cm⁻¹. Absorption spectra (λ_max, nm,

DMF (ε, M⁻¹cm⁻¹): 630 (190), 565 (sh, 140), 545 (sh, 170).

Conductivity (∼1 mM, CH₃OH, 298 K): Λ_M= 90 Ω⁻¹cm²mol⁻¹(the

range for 1:1 electrolyte in CH₃OH is 80−115). ¹H NMR (DMSO-d₆,

400 MHz): 3.75 (s, 12H, H₁/H_1′), 7.48 (m, 8H, H₄/H_4′, J = 8 Hz), 6.76

(d, 8H, H₅/H_5′, J = 8 Hz), 7.67 (d, 4H, H₉/H_9′), 8.02 (t, 2H, H₁₀/H_10′).

¹³C NMR (DMSO-d₆, 400 MHz): δ = 51.91 (C₁), 166.45 (C₂), 151.05

1

−CH₂−, Et₄N⁺), 1.12 (t, 12H, −CH₃, Et₄N⁺). H NMR (DMSO-d₆/

D₂O, 400 MHz): δ = 6.73 (d, 8H, H₃/H_3′, J = 8 Hz), 7.45 (d, 8H, H₄/

H_4′, J = 9 Hz), 7.65 (d, 4H, H₈/H_8′, J = 8 Hz), 8.02 (t, 2H, H₉), 3.16 (q,

8H, −CH₂−, Et₄N⁺), 1.12 (t, 12H, −CH₃, Et₄N⁺). ¹³C NMR (DMSO-

d₆, 400 MHz): δ = 17.05 (−CH₃, Et₄N⁺), 51.35 (−CH₂−, Et₄N⁺),

167.14 (C₁), 139.99 (C₂), 123.85 (C₃), 125.71 (C₄), 150.63 (C₅),

166.39 (C₆), 155.77 (C₇), 126.15 (C₈), 129.09 (C₉).

(C₃), 124.58 (C₄), 123.88 (C₅), 140.11 (C₆), 166.04 (C₇), 155.66 (C₈),

126.39 (C₉), 128.93 (C₁₀).

(Et₄N)[Co(L^m‑COOH)₂] (4). This complex was again synthesized in a

similar manner as that noted for complex 1. Yield: 0.54 g, 60%. Anal.

Calcd for C₅₀H₄₆CoN₇O₁₂: C, 60.30; H, 4.66; N, 9.85. Found: C,

59.95%; H, 4.96%; N, 9.62%. FTIR (KBr, selected peaks): 3427 (OH),

1718 (COO), 1570 (CO) cm⁻¹. Conductivity (DMF, ∼1 mM

solution at 298 K): Λ_M= 75 Ω⁻¹cm²mol⁻¹(the range for 1:1 electrolyte

in DMF is 65−160). Absorption spectrum (λ_max, nm, CH₃OH (ε, M⁻¹

Na[Co(L^m‑COOEt)₂] (6^P). This compound was synthesized in a similar

manner as discussed for 1^Pusing the following reagents: H₂L^m‑COOEt

(0.50 g, 1.083 mmol), NaH (0.057 g, 2.38 mmol), and [Co(H₂O)₆]-

(ClO₄)₂] (0.198 g, 0.54 mmol). Yield: 0.54 g, 50%. Anal. Calcd for

C₅₀H₄₂N₆O₁₂CoNa·H₂O: C, 58.94%; H, 4.35%; N, 8.25%. Found: C,

58.62%; H, 4.59%; N, 8.41%. FTIR (KBr, selected peaks): 3404 (OH),

1709 (COO), 1609, 1593 (CO) cm⁻¹. Absorption spectra (λ_max, nm,

CH₃OH (ε, M⁻¹cm⁻¹): 630 (120), 560 (sh, 80), 550 (sh, 90).

Conductivity (∼1 mM, CH₃OH, 298 K): Λ_M= 105 Ω⁻¹cm²mol⁻¹(the

range for 1:1 electrolyte in CH₃OH is 80−115). ¹H NMR (DMSO-d₆,

400 MHz): δ = 1.28 (t, 12H, H₁/H_1′), 4.19 (q, 8H, H₂/H_2′), 6.96 (d, 4H,

H₅/H_5′), 7.02 (m, 4H, H₆/H_6′), 6.90 (d, 4H, H₇/H_7′, J = 8 Hz), 7.43 (s,

4H, H₉/H_9′), 7.60 (d, 4H, H₁₂/H_12′), 7.94 (t, 2H, H₁₃/H_13′). ¹³C NMR

((DMSO-d₆, 400 MHz): 14.05 (C₁), 60.52 (C₂), 166.40 (C₃), 139.46

(C₄), 127.96 (C₅), 122.89 (C₆), 123.42 (C₇), 145.96 (C₈), 126.29 (C₉),

165.36 (C₁₀), 155.39 (C₁₁), 129.72 (C₁₂), 131.10 (C₁₃).

1

cm⁻¹): 646 (130), 572 (sh, 60). H NMR (DMSO-d₆, 400 MHz):

δ = 12.57 (br, 4H, −COOH), 6.90 (d, 4H, H₃/H_3′), 6.99 (m, 4H, H₄/H_4′),

7.41 (d, 4H, H₅/H_5′, J = 8 Htz), 7.24 (s, 4H, H₇/H_7′), 7.53 (d, 4H, H₁₀/

H

_10′, J = 8 Hz), 7.91 (t, 2H, H₁₁), 3.18 (q, 8H, −CH₂−, Et₄N⁺), 1.13 (t,

12H, −CH₃, Et₄N⁺). ¹H NMR (DMSO-d₆/D₂O, 400 MHz): δ = 6.89

(d, 4H, H₃/H_3′), 7.00 (m, 4H, H₄/H_4′), 7.41 (d, 4H, H₅/H_5′, J = 8 Hz),

7.27 (s, 4H, H₇/H_7′), 7.53 (d, 4H, H₁₀/H_10′, J = 8 Hz), 7.91 (t, 2H, H₁₁),

3.15 (q, 8H, −CH₂−, Et₄N⁺), 1.11 (t, 12H, −CH₃, Et₄N⁺). ¹³C NMR

(DMSO-d₆, 400 MHz): δ = 7.05 (−CH₃, Et₄N⁺), 51.32 (−CH₂−,

Et₄N⁺), 166.85 (C₁), 124.39 (C₂), 127.68 (C₃), 130.46 (C₄), 127.12

(C₅), 139.29 (C₆), 123.45 (C7), 166.54 (C₈), 155.74 (C₉), 130.66 (C₁₀),

145.99 (C₁₁).

Synthesis of Cobalt(III) Complexes with Deprotected

Carboxylic Acid Groups. [Co(BL^p‑COOH)₃] (1). The complex 1 was

obtained after the base-catalyzed deprotection of complex 1^P. The

compound 1^Pwas dissolved in a mixture of THF/H₂O (3:1, v/v) and

treated with 3 equiv of NaOH. The reaction mixture was stirred for 12 h

at room temperature. The resulting solution was neutralized by 3 N

HCl. Removal of THF under vacuum resulted in the precipitation of the

product. The crude product was recrystallized from water, which

produced a highly crystalline product within 2−3 days. Yield: 0.12 g,

66%. Anal. Calcd for C₃₉H₂₇N₆O₉Co·3H₂O: C, 56.99; H, 3.98; N, 10.04.

Found: C, 56.66%; H, 4.16%; N, 10.08%. FTIR (KBr, selected peaks):

3402 (OH), 1702 (COO), 1581 (CO) cm⁻¹. Conductivity (CH₃OH,

H₃O⁺[Co(L^p‑COOH)₂] (5). This complex was obtained after the base-

catalyzed deprotection of complex 5^P(Na⁺salt) as follows: Compound

5^Pwas dissolved in a mixture of THF/H₂O (3:1, v/v) and treated with 4

equiv of NaOH. The reaction mixture was stirred for 12 h at room

temperature. The resulting solution was neutralized by 4 N HCl.

Removal of THF under vacuum resulted in the precipitation of the

product. The crude product was recrystallized from water, which

produced a highly crystalline product within 4−5 days. Yield: 0.16 g,

55%. Anal. Calcd for C₄₂H₂₉N₆O₁₃Co·11H₂O: C, 46.59%; H, 4.75%; N,

7.76%. Found: C, 46.90; H, 4.59; N, 7.65. FTIR (KBr, selected peaks):

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3402 (OH), 1702 (COO), 1604 (CO) cm⁻¹. Conductivity (CH₃OH,

∼1 mM solution at 298 K): Λ_M= 105 Ω⁻¹cm²mol⁻¹(the range for 1:1

electrolyte in CH₃OH is 80−115). Absorption spectrum (λ_max, nm, CH₃OH

(ε, M⁻¹cm⁻¹): 630 (120), 560 (sh, 65), 540 (sh, 95). ¹H NMR (DMSO-d₆,

400 MHz): δ = 6.72 (d, 8H, H₃/H_3″, J = 8 Hz), 7.45 (d, 8H, H₄/H_4′, J = 8

Hz), 7.64 (d, 4H, H₈/H_8′, J = 8 Hz), 7.98 (t, 2H, H₉/H_9′). ¹³C NMR

(DMSO-d₆, 400 MHz): δ = 167.23 (C₁), 150.56 (C₂), 125.44 (C₃), 123.77

(C₄), 140.00 (C₅), 166.48 (C₆), 155.80 (C₇), 125.99 (C₈), 129.05 (C₉).

H₃O⁺[Co(L^m‑COOH)₂] (6). This complex was again synthesized in a

similar manner as that noted for complex 5. Yield: 0.16 g, 60%. Anal.

Calcd for C₄₂H₂₉N₆O₁₃Co·2H₂O: C, 54.79%; H, 3.61%; N, 9.13%.

Found: C, 54.57%; H, 3.35%; N, 9.32%. FTIR (KBr, selected peaks):

(d, 4H, H₇/H_7′), 7.54 (d, 4H, H₁₀/H_10′, J = 8 Hz), 7.92 (t, 2H, H₁₁/H_11′).

¹³C NMR (DMSO-d₆, 400 MHz): δ = 166.84 (C₁), 145.96 (C₂), 127.60

(C₃), 123.39 (C₄), 127.13 (C₅), 139.41 (C₆), 130.38 (C₇/C₁₁), 166.56

(C₈), 155.79 (C₉), 124.56 (C₁₀).

H₃O⁺[Co(L^p‑COOH)₂] (7). This complex was again synthesized in a

similar manner as that noted for complex 5. However, recrystallization

was achieved by the vapor diﬀusion of diethyl ether to a DMF solution of

crude complex. Yield: 0.14 g, 50%. Anal. Calcd for C₄₂H₂₉CoN₆O₁₃·

2H₂O·DMF: C, 54.39%; H, 4.06%; N, 9.87%. Found: C, 54.68%; H,

4.33%; N, 9.70%. FTIR (KBr, selected peaks): 3404 (OH), 1702 (COO),

1604 (CO) cm⁻¹. Conductivity (CH₃OH, ∼1 mM solution at 298 K):

Λ_M= 110 Ω⁻¹cm²mol⁻¹(the range for 1:1 electrolyte in CH₃OH is 80−

115). Absorption spectra (λ_max, nm, CH₃OH (ε, M⁻¹cm⁻¹): 625 (95), 560

(sh, 50), 545 (sh,70). ¹H NMR (DMSO-d₆, 400 MHz): δ = 7.94 (s, 1H,

−CHO, DMF), 7.45 (d, 8H, H₃/H_3′, J = 8 Hz), 6.73 (d, 4H, H₄/H_4′, J = 8

3410 (OH), 1712, 1690 (COO), 1604, 1582 (CO) cm⁻¹

.

Conductivity (CH₃OH, ∼1 mM solution at 298 K): Λ_M= 85 Ω⁻¹

cm²mol⁻¹(the range for 1:1 electrolyte in CH₃OH is 80−120).

Absorption spectrum (λ_max, nm, CH₃OH (ε, M⁻¹cm⁻¹): 625 (100), 560

(sh, 60), 545 (sh, 75). ¹H NMR (DMSO-d₆, 400 MHz): δ = 6.90 (d, 4H,

H₃/H_3′), 6.99 (d, 4H, H₄/H_4′), 7.41 (d, 4H, H₅/H_5′, J = 8 Hz), 7.24

Hz), 7.65 (d, 4H, H₈/H_8′, J = 8 Hz), 8.00 (t, 2H, H₉/H_9′, J = 8 Hz). ¹³

C

NMR (DMSO-d₆, 400 MHz): δ = 162.30 (−CHO, DMF), 167.11 (C₁),

150.58 (C₂), 123.86 (C₃), 125.68 (C₄), 139.96 (C₅), 166.45 (C₆), 155.74

(C₇), 126.11 (C₈), 129.10 (C₉), 30.69 (CH₃, DMF), 34.35 (CH_3′DMF).

H₃O⁺[Co(L^m‑COOH)₂] (8). This complex was again synthesized in a

similar manner as that mentioned for complex 7. Yield: 0.15 g, 57%.

Anal. Calcd for C₄₂H₂₉CoN₆O₁₃·2H₂O·DMF: C, 54.39%; H, 4.06%; N,

9.87%. Found: C, 54.19%; H, 4.27%; N, 9.73%. FTIR (KBr, selected

peaks): 3425 (OH), 1702 (COO), 1584 (CO), cm⁻¹. Conductivity

(CH₃OH, ∼1 mM solution at 298 K): Λ_M= 95 Ω⁻¹cm²mol⁻¹(the

range for 1:1 electrolyte in CH₃OH is 80−115). Absorption spectra

(λ_max, nm, CH₃OH (ε, M⁻¹cm⁻¹): 640 (90), 585 (sh, 75), 570 (85). ¹H

NMR (DMSO-d₆, 400 MHz): δ = 7.92 (s, 1H, −CHO, DMF), 6.92 (d,

4H, H₃/H_3′), 6.98 (t, 4H, H₄/H_4′), 7.42 (d, 4H, H₅/H_5′, J = 8 Hz), 7.24

Scheme 1. Preparative Routes for the Protected Complexes 1^P

a

and 2^Pand Their Deprotected Complexes 1 and 2

(s, 4H, H₇/H_7′), 7.54 (d, 4H, H₁₀/H_10′, J = 8 Hz), 7.94 (t, 2H, H₁₁/H_11′

,

J = 8 Hz). ¹³C NMR (DMSO-d₆, 400 MHz): δ = 162.29 (−CHO, DMF),

166.69 (C₁), 149.94 (C₂), 127.69 (C₃), 123.50 (C₄), 127.04 (C₅), 129.25

(C₆), 130.48 (C₇/C₁₁), 166.49 (C₈), 155.73 (C₉), 124.39 (C₁₀), 30.72 (CH₃,

DMF), 34.38 (CH_3′, DMF).

Physical Methods. The FTIR spectra (KBr disk, 4000−400 cm⁻¹)

were recorded with a Perkin-Elmer FTIR 2000 spectrometer. The

conductivity measurements were carried out in organic solvents with a

digital conductivity bridge from the Popular Traders, India (model no.:

PT 825). The elemental analysis data were obtained with an Elementar

Analysen Systeme GmbH Vario EL-III instrument. The NMR spectral

measurements were carried out with a GEOL spectrometer (400 MHz)

with TMS as the internal standard. The solution absorption spectra

were recorded with a Perkin-Elmer Lambda-25 spectrophotometer.

a

Conditions: (a) NaH, DMF, N₂; (b) [Co(H₂O)₆](ClO₄)₂; (c) excess

O₂; (d) NaOH.

a

Scheme 2. Preparative Routes for the Protected Complexes 3^P−6^Pand Their Deprotected Complexes 3−8

a

Conditions: (a) K₂CO₃, CH₂Cl₂; (b) [Co(H₂O)₆](ClO₄)₂; (c) Et₄NCl·xH₂O in O₂atmosphere; (d) NaH, DMF, N₂; (e) [Co(H₂O)₆](ClO₄)₂;

(f) excess O₂; (g) NaOH.

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Figure 1. (a) Thermal ellipsoidal representation (30% probability level with partial numbering scheme) of complex 1; hydrogen atoms and solvent

molecules are omitted for clarity. (b) H-bonding interactions between diﬀerent synthons. (c) A view of packing diagram along the c axis.

The diﬀused reﬂectance spectra were recorded with a Perkin-Elmer

Lambda-35 spectrophotometer. Thermal gravimetric analysis (TGA)

and diﬀerential scanning calorimetry (DSC) were performed with

DTG-60 Shimadzu and TA-DSC Q200 instruments, respectively, at a

5 °C min⁻¹heating rate under nitrogen.

(O1W) and C12, C13, C14, N3, and O4 atoms of the DMF molecule

(for complex 8) were reﬁned isotropically. Complex 2 showed a

positionally disordered water molecule O2W, and C40, Cl1, and Cl2

atoms of the dichloromethane molecule. These atoms were solved by

using the PART command, and their positions were reﬁned isotropically

with the site occupancy factor (SOF) of O2WA (0.428) and O2WB

(0.572), C40A (0.22) and C40B (0.78), Cl1A (0.22) and Cl1B (0.78),

and Cl2A (0.22) and Cl2B (0.78). The oxygen atoms O3W (for

complex 5), O2W (for complex 6), O1W (for complex 7), and O1W

(for complex 8) were modeled as the hydronium ions. The hydronium

ion was modeled based on its pyramidal geometry and its ability to

accept one and donate three hydrogen bonds. The hydrogen atoms of

the hydronium ion were located for complex 5, but not for the remaining

complexes; however, the molecular formulas for all complexes include

these hydrogen atoms. The poor crystal quality (R_int: ca. 11%) and the

disorders associated with the lattice water molecules and hydronium ion

were reﬂected in the high R values of complex 6. The solvent accessible

voids (SAVs) were calculated using PLATON.^23dDetails of the

crystallographic data collection and structural solution parameters are

provided in Table 1.

Crystallography. The intensity data for complexes 1, 2, 3, 4, and 6

were collected at 298 K, whereas for complexes 5, 7, and 8, the data were

collected at 150 K on an Oxford XCalibur CCD diﬀractometer equipped

with graphite monochromatic Mo Kα radiation (λ = 0.71073 Å).²²Data

were processed with XCalibur S SAINT while the empirical absorption

correction was applied using spherical harmonics implemented in the

SCALE3 ABSPACK scaling algorithm.²²The structures were solved by

the direct methods using SIR-97^23aand reﬁned by the full-matrix least-

squares reﬁnement techniques on F²using the program SHELXL-97^23b

incorporated in the WINGX 1.8.05 crystallographic collective pack-

age.^23cThe hydrogen atoms were ﬁxed at the calculated position with

isotropic thermal parameters; however, the hydrogen atoms of the

lattice water molecules (except O14W) and a hydronium ion for

complex 5 were located from the diﬀerence Fourier map. The non-

hydrogen atoms were reﬁned anisotropically. However, the positionally

disordered water molecule O14W (for complex 5) and hydronium ion

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Figure 2. (a) Thermal ellipsoidal representation (30% probability level with partial numbering scheme) of complex 2; hydrogen atoms and solvent

molecules are omitted for clarity. (b) H-bonding interactions between diﬀerent synthons. (c) A view of packing diagram along the c axis.

Table 2. Selected Bond Lengths [Å] for Complexes 1−8 (Geometry around the Co³⁺Ion)

a

bond

1

2

3

4

5

6

7

8

Co1−N1^i,iv,v(8)

Co1−N2^iv(8)

Co1−N3

Co1−N4

Co1−N5

1.954(3)

1.960(3)

1.944(3)

1.925(3)

1.951(3)

1.960(3)

1.971(2)

1.923(3)

1.937(2)

1.940(3)

1.944(3)

1.958(2)

1.861(2)

1.948(2)

1.979(2)

1.858(2)

1.969(2)

1.953(2)

1.852(2)

1.960(2)

1.958(2)

1.847(2)

1.956(2)

1.946(3)

1.852(3)

1.954(3)

1.951(3)

1.851(3)

1.951(3)

1.989(8)

1.860(8)

1.957(8)

1.971(7)

1.868(8)

1.991(7)

1.954(3)

1.850(3)

1.950(3)

1.955(3)

1.847(3)

1.956(3)

1.949(3)

1.854(4)

Co1−N6

a

Symmetry transformations used to generate equivalent atoms for complex 8: (i) −x, −y, z; (iv) −y, x, −z; (v) y, −x, −z.

and HBL^m‑COOEt, are the coupling products of pyridine-2-carboxylic

acid with ethyl-4-amino benzoate or ethyl-3-amino benzoate,

respectively. Similarly, the tridentate ligands, H₂L^p‑COOMeand

H₂L^m‑COOEt, were synthesized by coupling pyridine-2,6-dicarboxylic

acid either with methyl-4-amino benzoate or with ethyl-3-amino

benzoate, respectively. The complexes 1^P−6^Pwere synthesized by

treating the deprotonated form of either bidentate or tridentate

ligands with the Co(II) salt. The ﬁnal Co(III) state was achieved

RESULTS AND DISCUSSION

Synthesis and Characterization. Two types of Co³⁺-based

■

coordination complexes have been synthesized and studied in the

present work. The ﬁrst category has compounds 1 and 2, where

bidentate ligands have been used and the general formula is

[Co³⁺(Ligand)₃]. The second class has complexes 3−8, where

tridentate ligands are present with the general formula

(cation)[Co³⁺(Ligand)₂]. The bidentate ligands, HBL^p‑COOEt

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Table 3. Selected Bond Angles [deg] for Complexes 1−8 (Geometry around the Co³⁺Ion)

a

bond

1

2

3

4

5

6

7

8

^(8)v,iv,iN1−Co1−N2^iv

N1−Co1−N3

N1−Co1−N4

N1−Co1−N5

N1−Co1−N6

N2−Co1−N3

N2−Co1−N4

N2−Co1−N5

N2−Co1−N6

N3−Co1−N4

N3−Co1−N5

N3−Co1−N6

N4−Co1−N5

N4−Co1−N6

N5−Co1−N6

82.31(12)

94.56(12)

89.47(12)

176.42(12)

95.76(12)

176.15(12)

94.75(12)

94.72(12)

85.90(12)

82.95(12)

88.49(12)

96.67(12)

92.78(12)

174.77(12)

81.99(11)

82.34(11)

87.87(11)

94.57(11)

96.21(11)

176.27(11)

95.01(11)

176.54(11)

86.20(11)

93.98(10)

83.29(11)

175.87(11)

93.06(11)

95.72(11)

89.13(11)

82.91(11)

81.88(10)

162.90(10)

90.39(10)

99.69(10)

90.28(10)

81.03(10)

100.44(10)

177.51(10)

96.71(10)

92.30(10)

97.41(10)

92.10(10)

81.53(10)

162.76(10)

81.38(10)

81.84(9)

163.77(10)

90.36(9)

98.63(9)

91.57(9)

81.93(9)

98.77(9)

179.13(9)

97.47(9)

92.11(9)

97.60(9)

90.52(9)

81.97(9)

163.76(10)

81.79(9)

81.93(12)

163.58(11)

90.30(12)

98.52(12)

92.28(12)

81.68(11)

99.68(12)

178.17(12)

96.89(12)

91.61(12)

97.89(12)

90.53(12)

82.11(12)

163.43(11)

81.33(12)

80.1(3)

161.8(3)

92.2(3)

95.1(3)

90.8(3)

81.7(3)

103.4(3)

173.5(3)

95.0(3)

90.6(3)

103.1(3)

92.2(3)

81.1(3)

161.6(3)

80.6(3)

81.73(14)

163.74(13)

90.28(14)

96.67(14)

91.46(14)

82.01(14)

97.08(13)

177.89(14)

99.55(13)

91.59(14)

99.58(14)

91.35(14)

81.54(13)

163.36(13)

81.83(13)

81.94(9)

a

Symmetry transformations used to generate equivalent atoms for complex 8: (i) −x, −y, z; (iv) −y, x, −z; (v) y, −x, −z.

Table 4. Selected H-Bond Parameters for Complexes 1 and 2

a

complex

D−H···A

d(H···A) (Å)

d(D···A) (Å)

∠(DHA) (deg)

1

O5−H5A···O2ⁱ

O7−H7A···O3ⁱⁱ

O9−H9A···O2ⁱⁱⁱ

O5−H5A···O1ⁱ

O7−H7A···O6ⁱⁱ

O9−H9A···O3ⁱⁱⁱ

1.88

1.80

1.88

1.91

1.83

1.85

2.700(2)

2.589(3)

2.701(4)

2.681(5)

2.648(4)

2.649(5)

174

161

174

156

178

164

2

a

Symmetry transformations used to generate equivalent atoms for complex 1: (i) 1/2 − x, 1/2 + y, 1/2 − z; (ii) 3/2 − x, 1/2 + y, 1/2 − z; (iii) 1/2

− x, −1/2 + y, 1/2 − z; for complex 2: (i) 1 − x, 1 − y, 1 − z; (ii) 2 − x, −y, −z; (iii) 1 − x, −y, −z.

through dioxygen-mediated oxidation. The complexes 1^P−6^P

contain protected carboxylic acids groups at the periphery. The

subsequent deprotection using base-assisted hydrolysis aﬀorded

the ﬁnal complexes 1−8 with free carboxylic acid groups at the

periphery (Schemes 1 and 2). The complexes 1, 2, and 3−8 were

isolated as brick-red and deep green crystalline solids, respectively,

after recrystallization. Complexes 1 and 2 are neutral in nature as the

3+ charge on the Co(III) ion is balanced by three monoanionic

bidentate ligands. On the other hand, the remaining complexes

with tridentate ligands are monoanionic in nature. In the cases of

complexes 3 and 4, Et₄N⁺balances the charge, whereas in complexes

5−8, the charge is balanced by the protonated form of the water

molecule (i.e., hydronium ion, H₃O⁺). The evidence for the

presence of H₃O⁺has come from the conductivity measurements²⁴

that show a 1:1 electrolytic nature for complexes 5−8 as well

as an acidic nature of their aqueous and/or methanolic solution.

The protonation has most probably occurred during the

deprotection step where excess NaOH was neutralized by the

addition of HCl.

The deprotected complexes 1−8 show broad signals between

3402 and 3427 cm⁻¹, which have been assigned to ν_O−Hdue to

both free carboxylic acid (−COOH) and water molecules that

are present in the lattice (Table S1, Supporting Information).²⁵

Importantly, such −COOH stretches were absent in their

precursor complexes (1^P−6^P) with the protected carboxylic acid

groups. The signals between 1702 and 1718 cm⁻¹correspond

to the ν_COOof free carboxylate groups, whereas protected

complexes, 1^P−6^P, show these stretches at slightly higher

values. In addition, the proton NMR spectra display a broad

feature between 11 and 12 ppm for the presence of free

−COOH groups, which disappeared on the addition of D₂O.

The D₂O-exchange experiments strongly support the presence

of free carboxylic acid groups in the deprotected complexes.

All complexes show well-resolved signals in their proton as well as

carbon NMR spectra, and these techniques are the simplest method

to characterize the present complexes (Figures S1−S18, Supporting

Information). The absorption spectra for all cobalt complexes were

studied in CH₃OH, and the λ_maxwere observed within the range of

625−650 nm (Figures S19−S24, Supporting Information). The

observed feature is due to the d−d transition of the Co(III) ion.

The high-energy features were observed below 350 nm and could

tentatively be assigned to intraligand transitions.

Thermal Studies. Thermogravimetric analysis (TGA) and

diﬀerential scanning calorimetric (DSC) analysis were performed

on all complexes to study the thermal stability of the hydrogen-

bonded networks and the fate of lattice solvent molecules (Figures

S25−S32, Supporting Information). Typically, the loss of lattice water

molecules was observed in the temperature range of 20−100 °C,

whereas complexes 7 and 8 show the loss of DMF at relatively higher

temperature (between 140 and 180 °C). For complex 1, the observed

weight loss (11.37%) ﬁts nicely with the calculated value of 11.04%

for the loss of ﬁve water molecules. For complex 2, the weight loss

(obs, 8.76%; calcd, 9.47%) corresponds to the loss of one CH₂Cl₂

molecule. For complex 3, the weight loss (obs, 9.65%; calcd, 9.60%) is

related to the loss of six water molecules. A weight loss between 28

and 170 °C (obs, 14.69%; calcd, 14.86%) corresponds to the loss of

one water and one Et₄N⁺cation for complex 4. The TG curve of 5

shows the weight loss (obs, 18.60%; calcd, 18.50%) for the loss of 10

water molecules and 1 hydronium ion. For 6, the weight loss (obs,

10.74%; calcd, 9.94%) corresponded to the loss of four water

molecules and one hydronium ion. In a similar manner, complex 7

shows the weight loss (obs, 11.03%; calcd, 10.97%) for the loss of one

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Figure 3. (a) Thermal ellipsoidal representation (30% probability level with partial numbering scheme) of complex 3; hydrogen atoms, cation, and

solvent molecules are omitted for clarity. (b) Figure showing H-bonding interactions involving water molecules (shown in red color). (c) A view of

packing diagram along the c axis. (d) A portion of the network showing the presence of Et₄N⁺cations (space-ﬁlling mode) within the void.

water, one hydronium ion, and one DMF molecule. Similarly,

complex 8 shows the weight loss (obs, 11.07%; calcd, 11.43%) for the

loss of two water molecules, one hydronium ion, and one DMF. All

complexes show the second weight loss above 270 °C, which

corresponds to the decomposition of the framework. In DSC

analysis, complexes 1−6 show exothermic features between 20 and

140 °C for the loss of lattice water molecules. On the other hand,

complexes 7 and 8 display a broad feature for the loss of a DMF

molecule in the exothermic region of 140−180 °C. In most cases, a

nice match was observed between the thermal weight loss to that of

crystallographically observed lattice solvent molecules as well as

microanalysis results (cf. Table S2, Supporting Information).

Crystallography. All deprotected cobalt(III) compounds

have been characterized by the single-crystal X-ray diﬀraction

analysis to understand their molecular structures, orientation of

the appended arylcarboxylic acid groups, and H-bond-based

self-assembly. The relevant crystallographic data and structure

solution parameters are summarized in Table 1, whereas the

selected bond lengths and bond angles are given in Tables 2 and 3.

The H-bonding geometries are listed in Tables 4−7.

Crystal Structures of Complexes 1 and 2. Complexes 1 and 2

crystallized in monoclinic (P2₁/n space group) and triclinic cell

systems (P1 space group), respectively. The molecular structures

̅

and self-assemblies of complexes 1 and 2 are shown in Figures 1

and 2, respectively. Three bidentate pyridine-amide ligands

appended with either p- or m-arylcarboxylic acid groups are

arranged octahedrally around the Co³⁺ion (Figures 1a and 2a).

The ligands coordinate the Co³⁺ion via a ﬁve-membered chelate

ring between amide-N and pyridyl-N atoms. The Co−N_amide

bond distances (avg 1.949 Å) are comparable to Co−N_pyridine

bond distances (avg. 1.939 Å), suggesting a symmetrical bonding

from all donors (Table 2).^20aAll diagonal angles are more or less

linear in nature, ranging from 174.8° to 176.4°, again suggesting a

symmetrical arrangement of donors, resulting in a nearly

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Figure 4. (a) Thermal ellipsoidal representation (30% probability level with partial numbering scheme) of complex 4; cation and hydrogen atoms are

omitted for clarity. (b) A portion of the network showing intermolecular H-bonding between diﬀerent molecules. (c) Space-ﬁlling model of the packing

diagram. (d) A portion of the network showing the presence of Et₄N⁺cations (space-ﬁlling mode) within the void.

Table 5. Selected H-Bond Parameters for Complexes 3 and 4

a

complex

D−H···A

d(H···A) (Å)

d(D···A) (Å)

∠(DHA) (deg)

3

O5−H5A···O2Wⁱ

O7−H7A···O3Wⁱⁱ

O9−H9A···O4Wⁱⁱⁱ

O11−H11A···O7W^iv

O5−H5A···O2ⁱ

1.85

2.656(5)

2.595(5)

2.724(5)

2.592(5)

2.668(4)

2.631(3)

2.629(3)

2.584(3)

169

168(4)

175(7)

163(7)

159(4)

160(3)

172(4)

1.67(6)

1.83(7)

1.61(8)

1.80(5)

1.71(3)

1.76(4)

1.75(5)

4

O7−H7A···O1ⁱⁱ

O9−H9A···O4ⁱⁱⁱ

O11−H11A···O3^iv

a

Symmetry transformations used to generate equivalent atoms for complex 3: (i) 2 − x, 1 − y, 1 − z; (ii) 1 + x, −1 + y, z; (iii) −x, 1 − y, 1 − z; (iv)

−x, 1 − y, −z; for complex 4: (i) −1/2 + x, 1/2 − y, −1/2 + z; (ii) 1/2 + x, 1/2 − y, 1/2 + z; (iii) −1/2 + x, 1/2 − y, 1/2 + z; (iv) 1/2 + x, 1/2 − y,

−1/2 + z.

octahedral geometry (Table 3). Notably, signiﬁcant reduction of

the bond angles from 90° involving the ﬁve-membered chelate

rings was observed. Such an observation indicates a tight

chelation; in fact, the average angle is only ∼83°. In both 1 and 2,

the crystal structures clearly illustrate the meridional (mer)

geometric form at the metal, as the dihedral angle between the

coordinating planes comprising either three amide-N atoms or

three pyridyl-N atoms is close to 180°. The occurrence of the mer

form has made all three associated ligands asymmetrical to each

other, and this ligand asymmetry was observed in IR and ¹H NMR

spectra with 3-fold signals.^20a,26aIn both 1 and 2, the presence of the

mer form is quite unique and interesting, which places the three

appended arylcarboxylic acid groups to diﬀerent directions. Such a

situation is in complete contrast to the cases where three bidentate

ligands are arranged in facial (fac) form and the appended groups

are projected in a nearly C₃symmetric fashion.^27,28

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Figure 5. (a) Thermal ellipsoidal representation (30% probability level with partial numbering scheme) of complex 5; hydrogen atoms, cation, and

solvent molecules are omitted for clarity. (b) A portion of the network showing H-bonding between molecular components. (c) A view of packing

diagram along the a axis. (d) A space-ﬁlling representation of the network in a view along the a axis; water molecules are shown in red color.

Notably, both complexes 1 and 2 are appended with the

arylcarboxylic acid groups. A −COOH group is unique as it is capable

of functioning both as H-bond donor (through H atom) as well as

acceptor (through O atom). In addition, both complexes 1 and 2 also

have O_amidegroups that can act as the H-bond acceptors. Thus, a

combination of H-bond donors (H_acid) and H-bond acceptors

(O_amideand O_acid) is being oﬀered. As a consequence, both complexes

self-assemble into complementary supramolecular networks via

intermolecular H-bonding between −COOH and O_amidegroups

(Table 4). In addition, complex 2 also shows conventional

intermolecular H-bonding between H_acidand O_acidgroups, resulting

in the generation of the [COOH]₂dimer.^14bFor 1, the carboxylic

O−H groups O5−H5A, O7−H7A and O9−H9A interact with the

complementary O_amidegroups O2, O3, and O2^#, forming O5−

H5A···O2ⁱ, O7−H7A···O3ⁱⁱ, and O9−H9A···O2ⁱⁱⁱmotifs (Figures 1b

and 1c). The heteroatom separations between the interacting

groups were found to be between 2.589(3) and 2.701(4) Å,

while the O−H···O_amideangles were between 161 and 174°.

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Figure 6. (a) Thermal ellipsoidal representation (30% probability level with partial numbering scheme) of complex 6; hydrogen atoms, cation, and

solvent molecules are omitted for clarity. (b) A view showing the H-bonding between carboxylic acids, lattice water molecules, and hydronium ions. (c)

A view of packing diagram along the a axis. (d) A space-ﬁlling representation of the network in a view along the a axis; water molecules and hydronium

ions are shown in red and blue color, respectively.

These O−H_acid···O_amideinteractions led to the generation of 1D

chains. These chains are further connected to each other via

O−H_acid···O_amideH-bonding interactions to generate the 2D net-

work. There are additional H-bonding involving O_acidatoms O4 and

O6 to that of lattice water molecules O1W and O2W (O4···O2W:

3.032 Å and O6···O1W: 2.789 Å), which strengthens the network.

Similarly, in complex 2, the carboxylic O−H groups (O5−

H5A and O9−H9A) interact with the complementary O_amideatoms

O1 and O3, forming the O5−H5A···O1ⁱand O9−H9A···O3ⁱⁱⁱ

motifs (Figure 2b,c). The heteroatom separations and O−H···O_amide

angles were found to be 2.681(5) and 2.649(5) Å, and 156 and 164°,

respectively. The carboxylic acid O−H group (O7−H7A) further

interacts with the O_acid(O7−H7A···O6ⁱⁱ: 2.648(4) Å) to generate a

[COOH]₂cyclic dimer^14bthat results in the formation of a 2D

network. The D−H···A bond angle for the [COOH]₂cyclic dimer

was found to be 178°, which is very close to the ideal angle of 180°

for an isolated dimer.^14bThe crystal structure of complex 1 shows

solvent accessible voids (SAVs) with a volume of 1130 Å³, which is

ca. 25% of the unit cell volume (4462 Å³). Interestingly, complex 2

does not oﬀer any considerable void, and we believe that a highly

complementary interaction between the interacting groups has

prevented the generation of void space.

Crystal Structures of Complexes 3 and 4. Complexes 3 and 4

were found to crystallize in triclinic (P1 space group) and mono-

̅

clinic cell systems (P2₁/n space group), respectively. Both of

these complexes are anionic in nature, and the charge is being

balanced by the Et₄N⁺cation. The molecular structures of 3 and

4 are quite similar and show that the central Co³⁺ion is meridionally

coordinated by two tridentate pyridine-amide based ligands (Figures

3a and 4a).^20a,26The octahedral Co³⁺ion is coordinated to four

anionic N_amideatoms in the basal plane and two N_pyridineatoms in the

axial position.^20aThe Co−N_amideand Co−N_pyridinedistances were

found to be in the ranges of 1.948(2)−1.979(2) and 1.847(2)−

1.861(2) Å, respectively (Table 2). In both cases, axial pyridine rings

are trans to each other and make an angle in excess of 177°. The

diagonal N_amidegroups make angles of nearly 163° with the central

Co³⁺ion in both cases (Table 3). The bonding parameters for

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Table 6. Selected H-Bond Parameters for Complexes 5 and 6

a

complex

D−H···A

d(H···A) (Å)

d(D···A) (Å)

∠(DHA) (deg)

5

O1W−H1A···O3ⁱ

O2W−H2A··· O1ⁱⁱ

O3W−H3A···O1

O3W−H3B···O12

O4W−H4A···O4ⁱⁱ

O5−H5A···O6Wⁱⁱⁱ

O6W−H6A···O2^iv

O6W−H6B···O8^v

O7−H7A···O5W^vi

O8W−H8A···O6^vii

O8W−H8B···O3

O9−H9A···O10W^viii

O10W−H10A···O10^ix

O10W−H10B···O12

O11−H11A···O4W^x

O5W−H55B···O6

O7W−H77A···O2

O9W−H99B···O4

O11W−H111···O6

O1W−H1B···O1ⁱⁱ

O5−H5A···O3ⁱⁱⁱ

1.82

1.79

2.05

2.08

1.91

1.78

2.10

1.85

1.74

2.15

1.90

1.82

1.79

2.08

1.76

2.04

1.94

1.89

1.98

1.86(15)

1.90

1.81

1.85

1.86

2.645(4)

2.705(4)

2.871(4)

3.001(5)

2.789(4)

2.607(4)

2.993(4)

2.787(5)

2.561(5)

2.982(5)

2.814(6)

2.616(6)

2.600(6)

2.828(5)

2.578(5)

2.902(5)

2.755(6)

2.819(5)

2.808(6)

2.537(14)

2.658(14)

2.62(2)

163

157

158

157

165

166

156

158

165

142

156

158

149

131

165

159

167

156

147

6

153(17)

153

O7−H7A···O2W

O9−H9A···O10^iv

O11−H11A···O2^v

171

2.669(17)

2.630(11)

173

157

a

Symmetry transformations used to generate equivalent atoms for complex 5: (i) 1 − x, −y, 1 − z; (ii) 1 − x, 1 − y, 1 −z; (iii) 1 + x, y, z; (iv) x, y,

1 + z; (v) −1 + x, y, 1 + z; (vi) 2 − x, 1 − y, −z; (vii) −1 + x, y, z; (viii) −x, −y, −z; (ix) x,1 + y, z; (x) −x, 1 − y, 1 − z; for complex 6: (ii) −1 + x,

−1 + y, z; (iii) 2 − x, 1 − y, −z; (iv) 2 − x, 1 − y, 1 − z; (v) 1 − x, 2 − y, 1 − z.

complexes 3 and 4 are in line with our earlier systems as well as to

that of literature examples with similar ligands.^20a,26

acid O−H groups were found to interact symmetrically with the

complementary O_amideatoms, forming the O5−H5A···O2ⁱ, O7−

H7A···O1ⁱⁱ, O9−H9A···O4ⁱⁱⁱ, and O11−H11A···O3^ivmotifs,

respectively. For these motifs, the heteroatom separations

between 2.584(3) and 2.668(4) Å and O−H···O_amideangles

between 159 and 172° were observed. A highly symmetrical array

of H-bonds between H_acidand O_amidegroups has prevented these

functional groups from further interacting with other atoms/groups.

As an example, even highly stable intermolecular H-bonded

[COOH]₂dimers were not formed.¹⁴Complex 4 showed very

small SAVs of 214 Å³(only ca. 4% of the unit cell volume), which

were almost fully occupied by the Et₄N⁺cations (Figure 4d).

Crystal Structures of Complexes 5 and 6. Both complexes 5

These molecules are appended with four peripheral

H-bonding donor −COOH groups and have the ability to self-

assemble, considering the fact that there are H-bonding

acceptors in O_acidand O_amidegroups (Table 5). For complex 3,

the −COOH groups act as H-bond donors, while the lattice

water molecules function as acceptors, although CO_amide

groups are also present. In this case, for every −COOH group,

two water molecules were present in the crystal lattice and found

to be signiﬁcantly involved in H-bonding with the −COOH

groups. The carboxylic acid O−H groups interact with the lattice

water molecules, forming the O5−H5A···O2W, O7−

H7A···O3W, O9−H9A···O4W, and O11−H11A···O7W motifs,

respectively (Figure 3b,c). The heteroatom separations and the

O−H_acid···O_waterangles were found to be between 2.592(5) and

2.724(5) Å and in excess of 163°, respectively. Interestingly,

because of the presence of lattice water molecules, the carboxylic

acid O−H groups were not interacting at all with the CO_amide

groups, a feature that has been typically observed in the

remaining systems. The lattice water molecules are arranged in

the form of an octagon within the H-bonded network. Complex

3 does not oﬀer adequate SAVs as the void space is ﬁlled with the

lattice water molecules and Et₄N⁺cations, as shown in Figure 3d.

In the case of complex 4, the −COOH groups that are

appended at the meta position have resulted in a highly

symmetrical and interesting self-assembly. Each building block

shows H-bonding interaction with four neighboring building

blocks, and the extension of such interactions generates a 2D

sheet (Figure 4b,c). Importantly, no water molecule was found in

the crystal lattice of complex 4, thus minimizing the competition

from the water molecules, as noted in complex 3. The carboxylic

and 6 crystallized in a triclinic cell system with the P1 space

̅

group. In both compounds, the negative charge on the complex

anion was balanced by the presence of a hydronium ion (H₃O⁺).

The Co³⁺ion has octahedral geometry where the basal plane is

composed of N_amidegroups while the axial positions are occupied

by the N_pyridineatoms (Figures 5a and 6a).^20a,26The Co−N_amide

and Co−N_pyridinedistances were found to be in the ranges of

1.946(3)−1.991(7) and 1.851(3)−1.868(8) Å, respectively

(Tables 2 and 3).

Notably, there were 13 lattice water molecules and 1 H₃O⁺in

complex 5, whereas only 2 water molecules and a hydronium ion

were present in complex 6. In both 5 and 6, the hydronium ion

plays a very important role. It not only balances the charge but

also shows interesting H-bonding interactions with the

carboxylic acid O−H groups, O_amideatoms, and lattice water

molecules (Table 6). In 5, the H_acidgroups O5−H5A, O7−H7A,

O9−H9A, and O11−H11A were interacting with the hydronium

ion O3W, and water molecules O6W, O5W, O10W, and

O4W, respectively (Figure 5b). The resultant O5−H5A···O6Wⁱⁱⁱ,

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Figure 7. (a) Thermal ellipsoidal representation (30% probability level with partial numbering scheme) of complex 7; hydrogen atoms, cation, and

solvent molecules are omitted for clarity. (b) A view showing the H-bonding between carboxylic acids, lattice water molecules, and hydronium ions. (c)

Packing diagram in a view along the a axis. (d) Packing diagram (space-ﬁlling mode) in a view along the c axis; water molecules and hydronium ions are

shown in red and blue color, respectively.

O7−H7A···O5W^vi, O9−H9A···O10Wⁱⁱⁱ, and O11−H11A···O4W^x

motifs have the heteroatom separations between 2.561 and 2.616 Å,

while the O−H···O_waterangle was within 158−165° (Table 6).

These interactions resulted in the generation of a chain running

along the a axis (shown in magenta color; Figure 5c). One such

chain further connects to other parallel running chains in the same

direction (shown in pink color; Figure 5c) through the hydronium

ion as well as lattice water molecules (shown as red color ball;

Figure 5c). These interactions led to the generation of a closely

packed 2D network that does not show any SAVs (Figure 5d).

In the case of complex 6, the carboxylic O−H groups O5−

H5A, O7−H7A, O9−H9A, and O11−H11A interact with the

O_amide, O_carboxylate, and hydronium ion (O3, O2W, O10, and O2,

respectively) (Figure 6b,c). The O5−H5A···O3ⁱⁱⁱ, O7−

H7A···O2W, O9−H9A···O10^iv, and O11−H11A···O2^vmotifs

have the heteroatom separations between 2.620(2) and

2.669(17) Å (Table 6). The average O−H···O_amideand

O−H···O_waterbond angles were found to be ca. 161° and ca.

171°, respectively. These interactions resulted in the generation of a

chain running along the a axis (shown in yellow color; Figure 6c).

A yellow chain further connects with the parallel running chains

in the same direction (shown in pink color; Figure 6c) through

the formation of the [COOH]₂dimer between H_acidand

O_carboxylategroups as well as via the hydronium ion. For the

[COOH]₂dimer, the O−H···O bond angle of 173° and

separation of 2.669(17) Å were noted.^14bThe lattice water

molecules (shown as red color ball; Figure 6c) do not play any

signiﬁcant role in the connection of two chains and only interact

with the O_amideatoms (Figure 6c). The crystal structure of

complex 6 showed a very small SAV of 61 Å³(2.5% of the unit

cell volume), which is partially occupied by the hydronium ion

and lattice water molecules (Figure 6d).

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Figure 8. (a) Thermal ellipsoidal representation (30% probability level with partial numbering scheme) of complex 8; hydrogen atoms, cation, and

solvent molecules are omitted for clarity. (b, c) A portion of the network showing intermolecular H-bonding between diﬀerent molecules. (d) A view of

packing diagram (space-ﬁlling mode) along the c axis.

Table 7. Selected H-Bond Parameters for Complexes 7 and 8

a

complex

D−H···A

d(H···A) (Å)

d(D···A) (Å)

∠(DHA) (deg)

7

O5−H5A···O13ⁱ

O7−H7A···O1ⁱⁱ

O9−H9A···O3ⁱⁱⁱ

O11−H11A···O9^iv

O3−H3A···O1ⁱ

1.79

1.78

1.75

1.91

1.81

2.629(6)

2.609(6)

2.505(6)

2.726(5)

2.599(5)

174

170

148

163

157

8

a

Symmetry transformations used to generate equivalent atoms for complex 7: (i) 2 − x, 2 − y, 1 − z; (ii) x, −1 + y, z; (iii) −x, 2 − y, 1 − z; (iv) −x,

2 − y, −z; for complex 8: (i) 1/2 + y, −1/2 + x, 1/2 + z.

Crystal Structures of Complexes 7 and 8. Complex 7

the ranges of 1.950(3)−1.956(3) and 1.847(3)−1.861(5) Å,

respectively (Tables 2 and 3).^20a,26

Complex 7 was recrystallized from DMF, and as a result, one

DMF was found to co-crystallize in the lattice that was actively

participating in the H-bonding. In 7, the H_acidgroups O5−H5A,

crystallized in a triclinic cell system with the P1 space group,

̅

whereas complex 8 crystallized in a tetragonal cell system with

the P42₁c space group. As noted for complexes 5 and 6, in both 7

and 8, the negative charge on the complex anion is balanced by

the hydronium ion (H₃O⁺). In each case, the octahedral Co³⁺ion

is surrounded in a N₆donor environment (Figures 7a and 8a).

The Co−N_amideand Co−N_pyridinedistances were found to be in

O7−H7A, O9−H9A, and O11−H11A interact with O_DMF

,

O

_amide, and O_carboxylatesatoms (Figure 7b). The O5−H5A···O13ⁱ,

O7−H7A···O1ⁱⁱ, O9−H9A···O3ⁱⁱⁱ, and O11−H11A···O9^ivmotifs

88

dx.doi.org/10.1021/cg3011629 | Cryst. Growth Des. 2013, 13, 74−90

Crystal Growth & Design

Article

have the heteroatom separations between 2.505(6) and 2.726(5)

Å while the O−H···O_donorangles were within 163−174° (Table

7). These interactions resulted in the generation of a double

chain running along the a axis (shown in pink color; Figure 7c).

One such double chain further connects with the neighboring

chains through the lattice water molecules (shown in red color),

hydronium ions (shown in blue color), and DMF molecules

(shown in gray color, Figure 7c). All such interactions resulted in

a 2D sheetlike architecture. The structure of complex 7 showed

SAVs of 453 Å³in volume (18% of the unit cell volume of 2548

Å³) and cavities of 14 × 12 Å², which are occupied by DMF,

hydronium ions, and lattice water molecules (Figure 7d).

Complex 8 shows a highly symmetrical and complementary

H-bonding where every building block was found to interact with

four neighboring molecules (Figure 8b,c). Here, the H_acidgroup

O3−H3A interacts with the complementary O_amideatom O1,

forming the O3−H3A···O1ⁱmotif with the heteroatom

separation of 2.597(7) Å and O−H···O_amideangle of 155°

(Table 7). Such interactions resulted in the creation of a chain

running along the c axis (shown in violet color; Figure 8d).

Further, one such chain was found to be connected with the

other parallel running chain (shown in pink color) via

O−H···O_amidesynthons. Because of a symmetrical and highly

complementary H-bonding interaction, complex 8 does not oﬀer

any void space (Figure 8d). Furthermore, the symmetrical nature

of H-bonding deprives the hydronium ion of interacting with

other functional groups, and it remains isolated within the lattice.

isolated entity within the H-bonded network and occupied the

voids (see Figures 3d and 4d, for example).

Finally, the type(s) of lattice solvent molecules (water vs

DMF) also aﬀected the network generation. For example, the

presence of DMF in complex 7 showed that it not only actively

participated in the H-bonding but also complemented the lattice

water molecules in the network creation. For complex 8,

analytical data and diﬀraction studies established the presence of

DMF in the lattice. However, the highly disordered nature of this

DMF molecule has restricted us in evaluating its role in the H-

bonding network.

It is opined that the coordination complexes appended with H-

bond-sensitive functional groups provide a far more challenging

situation to study the H-bonded structures as a coordination

complex signiﬁcantly inﬂuences the placement of the functional

group and generates unconventional geometrical motifs.^14,27,28

Our ongoing work is exploring placing the appended carboxylic

acid groups at multiple position(s) on the arene ring to further

increase the number of carboxylic acid groups per building block

to evaluate their role in the H-bonding-based network creation.

ASSOCIATED CONTENT

* Supporting Information

■

S

X-ray crystallographic data for complexes 1−8 in CIF format,

NMR spectra (Figures S1−S18), absorption spectra (Figures

S19−S24), TGA-DSC plots (Figures S25−S32), and tables. This

material is available free of charge via the Internet at http://pubs.

acs.org.

SALIENT FEATURES AND CONCLUSIONS

■

AUTHOR INFORMATION

Corresponding Author

*Fax: + 91 - 11 - 2766 6605. Tel: + 91 - 11 - 2766 6646, ext. 172.

E-mail: rgupta@chemistry.du.ac.in.

Notes

■

There are a number of structural features that categorically

distinguish two sets of complexes, that is, complexes with p-

arylcarboxylic acid appended groups to that of m-arylcarboxylic

acid appended groups. For example, the numbers of co-

crystallized water molecules were found to be much higher in

the case of complexes with p-arylcarboxylic acid appended

groups than that of meta analogues. Therefore, in complexes with

p-arylcarboxylic appended groups, the H-bonding always took

place through the lattice water molecules. Thus, water molecules

functioned as the “spacers” and elongated the H-bonded

network.¹⁹In complete contrast, complexes with m-arylcarbox-

ylic acid appended groups self-assembled through complemen-

tary H_acid···O_amidesynthons. As a consequence of highly

symmetrical and complementary H-bonding, the complexes

with m-arylcarboxylic appended groups either oﬀered small SAVs

or did not oﬀer at all. In essence, because of the closely packed

network in meta cases, the solvent molecules did not co-

crystallize in large numbers. However, in cases of para analogues,

a loosely packed network (due to not so highly symmetrical

H-bonding) oﬀered voids that could be occupied by the solvent

molecules. Thus, the symmetrical versus not highly symmetrical

nature of self-assembly in para versus meta systems dictates the

fate of co-crystallized solvent molecules and packing behavior.

Notably, the typically stable and frequently observed [COOH]₂

dimer has only been noted in two cases: complexes 2 and 6; both

of them have m-arylcarboxylic acid appended groups.

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

R.G. gratefully acknowledges the ﬁnancial support from the

Department of Science and Technology (DST), New Delhi.

Crystallographic data and analytical facilities were provided by

the CIF-USIC facility of this university. G.K. thanks CSIR for the

award of SRF. We dedicate this paper to Professor Gautam R.

Desiraju on the occasion of his 60th birthday.

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Article Doi

Cobalt complexes appended with para - And meta -arylcarboxylic acids: Influence of cation, solvent, and symmetry on hydrogen-bonded assemblies

DOI: 10.1021/cg3011629

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