Postfunctionalized Metalloligand-Based Catenated Coordination

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Post-functionalized Metalloligands Based Catenated Coordination Polymers: Syntheses, Structures and Effect of Labile Sites on Catalysis Saurabh Pandey, Girijesh Kumar, and Rajeev Gupta Cryst. Growth Des., Just Accepted Manuscript • Publication Date (Web): 26 Mar 2019 Downloaded from http://pubs.acs.org on March 27, 2019

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Crystal Growth & Design

Post-functionalized Metalloligands Based Catenated Coordination Polymers: Syntheses, Structures and Effect of Labile Sites on Catalysis Saurabh Pandey,† Girijesh Kumar‡ and Rajeev Gupta†* †Department ‡Department

of Chemistry, University of Delhi, Delhi 110007, India of Chemistry, Panjab University, Chandigarh, India

ABSTRACT: In this work, pyridyl-appended Co3+ complexes (1 and 2) have been post-functionalized by using 4(bromomethyl)benzoic acid thus changing the functionalities from pyridyl-N donors to carboxylate-O donors. Using two such postfunctionalized metalloligands (3 and 4), several homo- and heterometallic coordination polymers (HCPs) have been synthesized. Single crystal structural analyses revealed that all HCPs presented intriguing one-dimensional catenated architectures. Postsynthetic modification induced flexibility was found to be responsible for the nearly identical architectures for two sets of HCPs starting from two different post-functionalized metalloligands 3 and 4. Two sets of HCPs differed by the presence (3a – 3d) or absence (4a – 4b) of labile coordinated water molecules that demonstrated profound effect on the heterogeneous catalysis of Knoevenagel condensation reactions and cyanation reactions.

INTRODUCTION Design and synthesis of coordination polymers (CPs) are important due to their well-defined architectures1–10 and potential applications in catalysis,11–17 gas sorption,18–22 optics,23–25 magnetism,26,27 and devices.28–30 In recent time, extensive efforts have been devoted to design next-generation CPs with fascinating structural features and topologies.31–36 Constructing an organized and well-designed framework material has always been a challenging task.37 One of the landmark achievements in this field is the utilization of a metalloligand as a well-defined building block for the construction of a desirable and ordered architecture.2,8,38–55 Today, the focus is to design improved metalloligands that can generate next-generation CPs with tunable architectures for various custom-made applications.36,38,39 Notably, majority of the applications in CPs are related to the porosity such materials offer and therefore continuous efforts have been placed to enhance their porosity.56,57 Such a feat can be achieved primarily by introducing lengthened or extended organic linkers those can coordinate to the secondary metals to afford extended architectures of increased porosity.56–62 Similarly, one could utilize metalloligands having extended appended groups to potentially coordinate secondary metal ions to generate desirable CPs with enhanced porosity. In this context, a very few examples are available in literature wherein organic ligands containing extended functional groups have been used.61,62 However, one could also think about introducing extended functional groups by the mean of post-synthetic modification, an approach recently developed in CP research.63,64 In this work, efforts have been made to lengthen the appended functional groups by post– synthetic modification of already known metalloligands.46 In particular, we have used two pyridyl-appended cobaltbased metalloligands46 for the post-functionalized modification using 4-bromomethylbenzoic acid, thus changing the functionalities from pyridyl-N donors to carboxylate-O donors. The resultant post–functionalized metalloligands after reaction with secondary metal ions produced one-dimensional CPs with intriguing catenated architectures, 3a – 3d and 4a – 4b. Post-

synthetic modification approach induced flexibility has resulted in nearly identical structures of two sets of CPs starting from two different post-functionalized metalloligands. Two sets of CPs differ by the presence (3a – 3d) or absence (4a – 4b) of labile coordinated water molecules that illustrate profound effect on the heterogeneous catalysis of Knoevenagel condensation reactions and cyanation reactions.

EXPERIMENTAL SECTION Materials Commercially available reagents of analytical grade were used as received without further purification. Solvents were purified using the standard literature procedures. The ligands H2L3py and H2L4py and their Co3+ complexes 1 and 2 were synthesized according to our previous report.46 Physical Measurements FTIR spectra were recorded with Perkin Elmer Spectrum-two spectrometer having Zn-Se ATR. NMR spectral measurements were carried out with a Jeol 400 MHz spectrometer. Absorption spectra were recorded either with PerkinElmer Lambda25 or Lambda35 spectrophotometers. Elemental analysis data were obtained with an Elementar Analysen Systeme GmbH Vario ELIII instrument. Thermal gravimetric analysis (TGA) was carried out with DTG 60 Shimadzu at 5 °C min−1 heating rate under the nitrogen atmosphere. Inductively coupled plasma mass spectroscopy (ICP-MS) measurements were carried out using Thermo Scientific iCAP Q. Powder X-ray diffraction (PXRD) studies were performed either with an X’Pert Pro from Panalytical or Bruker AXS D8 Discover instrument (Cu Kα radiation, λ = 1.54184 Å) with a slow scan rate at room temperature. Brunauer– Emmett–Teller (BET) surface area measurements were carried out by using nitrogen adsorption-desorption technique in an automated surface area and porosity analyzer (Quantachrome).

Synthesis of Post-Functionalized Metalloligands (PFMLs) PFML 3. Complex 1 was treated (100 mg, 0.121 mmol) with 4bromomethylbenzoic acid (114 mg, 0.534 mmol) in dimethyl

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sulfoxide (DMSO, 6 mL) at room temperature for 2 h under the magnetic stirring. After completion of the reaction, solvent was removed under reduced pressure to afford a sticky product which upon triturating with acetone afforded a yellow product. This product was separated, dried and was dissolved in methanol. Vapor diffusion of diethyl ether at room temperature produced crystalline product within 5-6 d. Yield: 0.148 g (80%). Anal. Calcd. for C66H50Br3CoN10O122H2O: 52.50%; H, 3.60%; N, 9.28%; Found: C, 52.42%; H, 3.55%; N, 9.80%. FTIR spectrum (Zn-Se ATR, selected peaks): 1708, 1641 and 1623 (C=O) and 3089, 2927 (C-H) cm-1. Absorption spectrum: (λ, nm, DMSO (ε, M-1 cm-1): 665 (70). 1H NMR spectrum (DMSO-d6, 400 MHz) δ= 8.97 (s, 4H), 8.90 (d, J = 5.8 Hz, 4H), 8.07 (t, J = 7.8 Hz, 4H), 7.95 (d, J = 8.1 Hz, 8H), 7.76 – 7.71 (m, 2H), 7.65 (d, J = 7.8 Hz, 2H), 7.50 (d, J = 8.4 Hz, 4H), 7.43 (d, J = 8.0 Hz, 8H), 5.85 (s, 8H). 13C NMR spectrum (100 MHz, DMSO-d6) δ 167.28 (s,), 167.16 (s, J = 12.0 Hz), 153.80 (s), 145.02 (s), 143.93 (s), 143.99 (s), 143.23(s), 142.83 (s), 141.49 (s), 139.10 (s), 132.07 (s), 130.56 (s), 129.08 (s), 128.88 (s), 127.32 (s), 62.85 (s). PFML 4. PFML 4 was synthesized using a similar method as followed for PFML 3 however using complex 2. Yield: 155 mg (92%). Anal. Calcd. for C66H48CoN10O12Br4H2O: C, 57.27%; H, 4.08%; N, 10.12%; Found: C, 56.87%; H, 3.95%; N, 10.05%. FTIR spectrum (Zn-Se ATR, selected peaks): 3100 (C–H; asym.), 2929 (C–H; sym.) 1705, and 1621 (C=O). Absorption spectrum (λ, nm, DMSO (ε, M-1 cm-1): 697 (230). 1H NMR spectrum(400 MHz, DMSO-d6) δ 8.59 (d, J = 6.6 Hz, 8H), 8.29 (t, J = 7.8 Hz, 2H), 8.05 (d, J = 8.0 Hz, 8H), 7.88 (d, J = 7.7 Hz, 4H), 7.34 (d, J = 8.1 Hz, 8H), 7.19 (d, J = 6.5 Hz, 8H), 5.64 (s, 8H). 13C NMR spectrum (100 MHz, DMSO-d6) δ 167.28 (s), 166.98 (s), 161.95 (s), 153.82 (s), 145.12 (s), 139.72 (s), 132.05 (s), 130.65 (s), 128.88 (s), 128.70 (s), 128.64 (s), 125.70 (s), 61.94 (s). Synthesis of Coordination Polymers [(3)Mn(H2O)2]Br.C2H6SO.25H2O (3a).65 HCP 3a was synthesized by layering a solution of PFML 3 (100 mg, 0.067 mmol) in DMSO (4 mL) over an aqueous solution of Mn(OAc)2.4H2O (35.14 mg, 0.147 mmol) dissolved in water (4 mL). The crystals were obtained after 8–10 days. Yield: 0.94 g (79%). Anal. Calcd. for C66H46CoN10O12MnBr(H2O)2.20H2O: C, 45.01%; H, 5.15%; N, 7.95%; Found: C, 44.74%; H, 4.81%; N, 7.78%. FTIR spectrum (Zn-Se ATR, selected peaks): 3390 (OH), 3079 (C–H; asymm), 2936 (CH2; symm), 1625 (COO), 1597 (C=O) cm-1. Diffused reflectance absorption spectra (λmax, nm): 662. [(3)Co(H2O)2]Br.24H2O (3b).65 PFML 3 (100 mg, 0.067 mmol) was treated with Co(OAc)2.4H2O (36.71 mg, 0.147 mmol) in DMF (6 mL) at room temperature for 2 h under the magnetic stirring. After 2 h, solvent was removed under the reduced pressure and the crude product was washed with methanol and dried under vacuum. This product was dissolved in water and vapors of acetone were diffused. After a period of one week, dark red crystals were obtained. Yield 72 mg, (65%). Anal. Calcd. for C66H46Co2N10O12Br(H2O)2. 14H2O: C, 47.84%; H, 4.74%; N, 8.45; Found: C, 47.53%; H, 4.59%; N, 8.42%. FTIR spectrum (Zn-Se ATR, selected peaks): 3392 (OH), 3081 (C–H; asymm), 2933 (C–H; symm), 1631 (COO), 1595 (C=O) cm-1. Diffused reflectance absorption spectra (λmax, nm): 672. [(3)Ni(H2O)2]Br.4H2O (3c).65 HCP 3c was synthesized using an identical method as discussed for 3b, however, using Ni(OAc)2.4H2O (36.68 mg, 0.1474 mmol). The crude compound was dissolved in water and vapor diffusion with acetone for a period of 6–8 d afforded red colored crystalline product. Yield: 64 mg (65%). Anal. Calcd. for C66H46CoN10O12NiBr(H2O)2.5H2O: C, 53.03%; H, 4.05%; N, 9.37%; Found: C, 53.42%; H, 4.55%; N, 9.80%. FTIR spectrum (Zn-Se ATR, selected peaks): 3384 (OH), 3076 (CH2; asymm.), 2992 (CH2; symm.), 1629 (COO), 1596

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(C=O) cm-1. Diffused reflectance absorption spectra (λmax, nm): 663. [(3)Zn]Br.6H2O (3d).65 HCP 3d was synthesized by layering a MeOH solution of PFML 3 (100 mg, 0.067 mmol, 4 mL) over an aqueous solution of Zn(OAc)2.2H2O (32.35 mg, 0.1474 mmol, 4 mL) with an intermediate layer of 1-butanol (4 mL). After a period of 8–10 d, red crystalline product of 3d was obtained. Yield: 74 mg (75%). Anal. Calcd. for C66H46CoN10O12ZnBr.6H2O: C, 53.44%; H, 3.94%; N, 9.44%; Found: C, 53.12%; H, 3.93%; N, 9.29%. FTIR spectrum (Zn-Se ATR, selected peaks): 3070 (C–H; asymm.), 2928 (C–H; symm.), 1630 (COO), 1619 (C=O) cm-1. Diffused reflectance absorption spectra (λmax, nm): 668. [(4)Mn].8H2O (4a). HCP 4a was synthesized by the reaction of PFML 4 (100 mg, 0.0666 mmol) with Mn(OAc)2.4H2O (35.94 mg, 0.1467 mmol) in DMF (5 mL) at room temperature for 2 h. After completion of the reaction, solvent was removed under the reduced pressure and the crude product was isolated after washing with diethyl ether. This product was dissolved in water and vapor diffusion of acetone afforded red crystalline material within 8–10 d. Yield: 68 mg (63%). Anal. Calcd. for C66H46CoN10O12Mn.6H2O: C, 56.90%; H, 4.20%; N, 10.05%; Found: C, 56.92%; H, 3.87%; N, 9.90%. FTIR spectrum (Zn-Se ATR, selected peaks): 3062 (CH2; asymm), 2953 (CH2; symm), 1644 (COO), 1594 (C=O) cm-1. Diffused reflectance absorption spectra (λmax, nm): 699. [(4)Cd].8H2O (4b). HCP 4b was synthesized by layering a solution of PFML 4 (100 mg, 0.0666 mmol) in DMSO (4 mL) over an aqueous solution of Cd(OAc)2.2H2O (38.91 mg, 0.146 mmol) in water (4 mL). Yield: 71 mg (72%). Anal. Calcd. for C66H46CoN10O12Cd.8H2O: C, 53.32%; H, 4.20%; N, 9.42%; Found: C, 53.42%; H, 4.55%; N, 9.80%. FTIR spectrum (Zn-Se ATR, selected peaks): 3045 (CH2; asymm), 2928 (CH2; symm), 1641 (COO), 1620 (C=O) cm-1. Diffused reflectance absorption spectra (λmax, nm): 679. Crystallography The intensity data were collected at 293 K with an Oxford XCalibur CCD diffractometer equipped with graphite monochromatic MoKα radiation (λ = 0.71073 Å).66 Data were processed with XCalibur S SAINT while the empirical absorption correction was applied using spherical harmonics implemented in the SCALE3 ABSPACK scaling algorithm.67 The structures were solved by the direct methods using SIR-9768 and refined by the full-matrix least squares refinement techniques on F2 using program SHELXL-9769 incorporated in WINGX version 2014.1 crystallographic collective package.70 The solvent accessible voids (SAVs) were calculated using PLATON.71 Additional refinement details have been included in SI. Details of the crystallographic data collection and structure solution parameters are provided in Table 1 whereas Tables S1 and S2 (SI) contain the selected bonding parameters.

RESULTS AND DISCUSSION Design Aspects and Synthesis. Post-functionalized metalloligands (PFMLs) 3 and 4 were synthesized by treating pyridine-appended metalloligands 1 or 2 (as the case may be)46 with 4bromomethylbenzoic acid in DMSO (Scheme 1). FTIR spectra of PFMLs 3 and 4 showed characteristic νC=O stretches for arylcarboxylic acid at 1708 cm-1 and 1707 cm-1, respectively,55 whereas, bands at 3089 and 2927 cm-1 and 3100 and 2929 cm-1 are assigned to the C–H stretches of methylene group for 3 and 4, respectively (Figures S1 and S2, Supporting Information).72 1H NMR spectra of PFMLs 3 and 4 displayed the characteristic singlet for the methylene group (–CH2–) at 5.85 and 5.64 ppm, respectively (Figures S3 and S5, SI).73,74 This feature confirmed


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that all four pyridyl groups have been functionalized. 13C NMR and 4, signals for –CH2- group were noted at 62.8 and 61.9 for PFML 3 Table 1. Crystallographic data collection and structure refinement parameters for PFMLs 3 and 4 and HCPs 3a, 3d, 4a and 4b. 3

4

3a

3d

4a

4b

Formula

C66H54Br3CoN10O14

C66H64BrCoN10O23

C68H50BrCoMnN10O40S

C132H92Br2Co2N20Zn2O36

C66H66CoMnN10O22

C66H46CdCoN10O20

Fw

1509.85

1508.1365

1873.02

2942.68

1465.16

1470.46

T (K)

298(2)

298(2)

298(2)

298(2)

298(2)

298(2)

Crystal System

Monoclinic

Monoclinic

Monoclinic

Orthorhombic

Tetragonal

Tetragonal

Space Group

C2/c

P2/c

P21/n

Pna21

I 41/a

I 41/a

a (Å)

26.6350(10)

7.6066(9)

15.6220(8)

19.3548(6)

17.683(4)

17.5602(8)

b (Å)

14.9763(4)

15.5949(14)

28.6251(13)

19.8101(6)

17.683(4)

17.5602(8)

c (Å)

16.4404(5)

28.832(2)

20.8157(9)

17.6823(7)

21.469(5)

21.6199(14)

α (o)

90°

90°

90°

90

90

90

β()

102.412(3)°

94.647(9)

107.3150(10)

90

90

90

γ()

90°

90°

90°

90

90

90

V (Å3)

6404.7(4)

3409.0(6)

8886.6(7)

6779.8(4)

6713(3)

6666.7(6)

Z

4

2

4

2

4

4

d (Mg/m3)

1.566

1.531

1.400

1.441

1.271

1.465

F (000)

3056

1552

3804

2992

2640

2988

1.051

1.061

1.059

1.059

1.139

1.034

0.0744, 0.1597

0.0823, 0.1120

0.0987, 0.2775

0.0823, 0.2028

0.0859, 0.2382

0.1168, 0.3009

0.1499, 0.1848

0.2028, 0.2231

0.1699, 0.3122

0.1120.2231

0.0987, 0.2461

0.2353, 0.3732

1.835 and -0.791

1.118 and -1.157

1.473 and -2.215

1.800 and -1.412

0.599 and -0.415

1.584 and -0.429

o

o

Goodness-offit on F2 R1,wR2 [I>2(I)] R1,wR2[all data] Largest diff. peak and hole ( e.Å-3)

aR

1

= ∑ ∥Fo| − |Fc∥ / ∑ | Fo|; wR2 = {∑ [w(/|Fo|2 − |Fc|2)2] / ∑[w Fo 4]}1/2.

respectively (Figures S4 and S6, SI).73,74 UV-Visible spectra of PFMLs 3 and 4 exhibited bathochromically shifted absorption maxima (ca. 15 nm and 40 nm) when compared to their parent metalloligands 1 and 2, respectively (Figures S7 and S8, SI).46 This is most probably due to the incorporation of additional πelectron fragments.75 A close similarity between the XRPD patterns of as-synthesized PFMLs to that of simulated ones from their single crystal diffraction data revealed the phase purity of the bulk samples (Figures S9 and S10, SI). Thermal gravimetric analysis (TGA) for PFMLs 3 and 4 were performed in the temperature ranges of 30 – 800 °C. TGA exhibited the loss of lattice water molecules between 30 – 100 °C whereas both PFMLs were stable up to 300 °C and decayed beyond this temperature sequentially. For PFML 3, an observed weight loss (1.94%) fits with the calculated weight loss of 2.38% for the release of two water molecules (Figure S11, SI). For PFML 4, a weight change (obsd./cacld. (%); 5.14/5.21) corresponded to the loss of four lattice water molecules (Figure S12, SI). The two sets of HCPs were synthesized by treating PFMLs 3 and 4 with the appropriate M(OAc)2 salt (Scheme 2). The crystalline HCPs, once formed, were largely insoluble in most of the common organic solvents. As the present HCPs contain two different metals (except 3b) where differentiating between such metals would be difficult solely based on the difference of their electron densities from X-ray diffraction studies, ICP-MS analyses were carried out. Indeed, ICP-MS results revealed the

presence of two different metals (%) for the following HCPs as follows: 3a, Co/Mn (3.94/3.91); 3c, Co/Ni (3.82/3.85); 3d, Co/Zn (3.61/3.75); and 4a, Co/Mn (3.81/3.75). Importantly, these analyses establish the presence of two different metals in approximately 1:1 ratio in all these HCPs.

O R R O

N N N

Co N

O R'

O DMSO

N R N R

HO

O

O

Br

R' O

N N N

Co N

N Br

R= N

N

(1)

(2)

R' =

(3)

O N R' N R' O

COOH

N Br

(4)

COOH

Scheme 1. Synthetic route for the preparation of post-functionalized metalloligands 3 and 4.

All six HCPs exhibited strong stretches (1625 – 1641 cm-1) due to the νCOO groups, in their FTIR spectra.50–53 In addition,



broad features at 3382 – 3416 cm-1 are indica tive of the presence of coordinated and/or lattice water molecules (Figures S13–S18, SI).53 HCPs 3a–3d, synthesized from PFML 3, displayed λmax in the range of 662–674 nm in their diffused reflectance absorption spectra (Figure S19, SI). On the other hand, HCPs 4a and 4b, originated from PFML 4, showed λmax at 699 and 679 nm, respectively (Figure S20, SI). In both cases, such features are tentatively assigned to the d–d transitions based on both primary (Co3+ ion) and secondary metals (except for Zn2+ and Cd2+ ions).50,52,53 XRPD patterns of the freshly prepared samples of HCPs 3a3d as well as 4a-4b were nearly similar to those simulated from the single crystal diffraction data indicative of the phase purity of the bulk samples (Figures S21–S26, SI). To probe thermal profiles of all six HCPs, TGA studies were performed in the temperature ranges of 30–800 °C (Figures S27 and S28, SI). For all six HCPs, a good match was observed for the weight losses between 30 – 120 C corresponding to the lattice and/or coordinated water molecules, as shown in Table 2. Subsequent to the weight loss for such water molecules, all HCPs were thermally stable up to 300 – 325 C. Beyond this temperature sequential decomposition of PFMLs followed by their parent metalloligands took place (ca. 40 – 60 %) which continues up to ca. 600 C and is finally leading to metal oxides (35 – 50 %). Two representative HCPs, 3a and 4a, were also investigated for their porosity measurements using Brunauer–Emmett–Teller (BET) studies (Figures S29 and S30, SI). HCP 3a and 4a respectively showed surface area of 18.7 m2/g (average pore size: 134.5 Å) and 22.9 m2/g (average pore size: 339.4 Å).62 These results indicate low surface area as a result of compact packing of 1D chains in the crystal lattice (cf. crystal structures).

PFML 3

OH2 O O M OO O O OH2 O

M2+

N

N

O

O

O

NN N

O N

Co N NN

O N

O OH2 O O O M O O O OH2 O

where M2+ = Mn (3a), Co (3b), Ni (3c), Zn (3d) N

N

PFML 4

M2+

O

O O M

O O

O

O

NN N

O O

O N 2+

where M

O

NN

O O

Co N O

O M

O O

O O

O

N

= Mn (4a), Cd (4b)

Scheme 2. Synthetic route for the preparation of homo- and heterometallic coordination polymers 3a – 3d and 4a – 4b.

Table 2. Thermal gravimetric analysis data for HCPs 3a – 3d and 4a – 4b.

HCP

Weight loss (Obsd. %)

Weight loss (Calcd. %)

3a 3b 3c 3d 4a 4b

22.03 17.16 8.01 7.35 9.65 9.97

22.50 17.39 8.43 7.28 10.07 9.68

Loss of water molecules 22 16 7 6 6 8

Crystal Structures Crystal structures of PFMLs 3 and 4. Both PFMLs 3 and 4 were crystallographically characterized and were found to crystallize in monoclinic cells with C2/c and P2/c

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space groups, respectively (for asymmetric unit, see Figure S31, SI). X-ray diffraction analyses revealed that in both cases all four pyridyl groups were functionalized with 4-(bromomethyl)benzoic acid and thus are present in the pyridinium form (Figures 1a and 1d). The positive charge on the resultant PFMLs is balanced by the combinations of bromide ion(s) and anionic arylcarboxylate group(s). In case of PFML 3, all four pyridyl-N functionalized arylcarboxylic groups are in their protonated form (i.e., COOH) whereas the parent metalloligand has 1 charge. Therefore, overall 3+ charge is balanced by three bromide anions. Notably, one of the bromide ions lies on bisects of the plane and thereby contributes two negative charges. In contrast, in PFML 4, out of four pyridyl-N functionalized arylcarboxylic groups; two each are present in neutral protonated (i.e., COOH) and anionic deprotonated (i.e., COO) forms. Therefore an overall monocationic charge is balanced by a bromide ion. However, crystal structure exhibits two Br- ions with half occupancy and thus both of them together contribute 1– charge. In PFMLs, geometry of the central Co(III) ion remained largely unaffected after the post– functionalization of the pyridyl groups.46 In both cases, avg. Co– Npyridyl and Co–Namide bond distances were found to be in range of 1.870 – 1.873 Å and 1.970 – 2.010 Å, respectively (Table S1, SI). These values are quite similar to that of our earlier cobalt-based metalloligands.46,50-53 PFML 3 forms H-bonding based assembly via bromide ion, Br2, which connects two molecules through O6-H group of the carboxylic acid (O6-H6…Br2: 2.442 Å) (Figure 1b). Such an interaction generates a 1D chain. Further, such 1D chains interact with the adjacent ones through other weak interactions and finally generates a two-dimensional (2D) packing (Figure 1c). In case of PFML 4, individual molecules are connected to each other via COOH…OC(OH) H-bonding interactions (O3–H2…O5: 1.723 Å). Such a bonding produces an overall 2D architecture manifested by H-bonding interactions and generating square-grid architecture (Figures 1e and 1f). The topology of the resultant H-bonded network is sql with {44·62} point symbol (Figure 1g). Crystal Structures of HCPs 3a–3d.65 Single crystal X-ray diffraction analyses were carried out for all four HCPs, 3a – 3d (for asymmetric unit, see Figure S32, SI). Although, all four HCPs presented a very similar 1D polymeric chain structure; 3d was different by the absence of metalcoordinated water molecules when compared to the remaining three HCPs, 3a – 3c. Therefore, crystal structures of 3a (as a representative case) and 3d are discussed in details.77 HCP 3a crystallized in monoclinic cell with P21/n space group. The asymmetric unit contains one PFML 3, one Mn(II)ion, two coordinated water, one bromide ion, one DMSO and twenty five lattice water molecules. A 1D polymeric chain is generated via the coordination of secondary Mn(II) ions to the arylcarboxylate fragments originating from PFML 3 (Figures 2a–2c). Each Mn(II) ion exhibits six-coordinated octahedral geometry. Out of six coordination sites, four are satisfied with Ocaroxylate atoms (O5, O8, O10 and O12) coming from four different arms of two PFMLs. The remaining two axial sites are occupied by the water molecules (O1W and O2W). 1D chains are further connected to the parallelrunning neighboring ones through various H-bonding interactions involving Ocarboxylate and lattice water molecules as well as C-H…π interactions to generate a 2D sheet–like architecture (Figure 2d). PLATON analysis exhibited that only 2.7% solvent accessible voids (SAVs) per unit cell volume existed due to the compact packing of 1D chains. From the topological point of view, each Mn atom is connected to two PFMLs having four arms and thus can be simplified as a four-connected node while each PFML can be viewed as a linear linker connecting two Mn centers (Figure 2e). Therefore, the network is constituted of 2,2,2,4 connected 4nodal net having Schlafli symbol of {122}{12}10-{22}{2}10.


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Reaction of Co(II) and Ni(II) ions with PFML 3 afforded HCPs 3b77,78 and 3c77,79 and both crystallized in triclinic cell with P-1 space group. The crystal structures and packing as well as

topologies of HCPs 3b and 3c were nearly identical to that of 3a (Figures S33 and S34, SI).77-79

Figure 1. (a) Stick model diagram with partial numbering scheme for PFML 3; hydrogen atoms, anions, and solvent molecules are omitted for clarity. (b) Figure showing H-bonding interactions between Br- ion (shown as golden yellow color sphere) and arylcarboxylic acid fragments. (c) A view of the packing diagram in space fill mode along c axis. (d) Stick model diagram with partial numbering scheme for PFML 4; hydrogen atoms, anions, and solvent molecules are omitted for clarity. (e) Figure showing H-bonding interactions between different molecules (shown in assorted colors). (f) A view of packing diagram in space fill mode along c axis. (g) Topological representation of PFML 4.



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Figure 2. (a) Stick model diagram with partial numbering scheme for HCP 3a; hydrogen atoms, bromide anion, and solvent molecules are omitted for clarity. (b and c) Figures showing H-bonding interactions involving various water molecules (shown as red color spheres). (d) A view of packing diagram in the space fill mode along c axis. (e) Topological representation of HCP 3a. HCP 3d, prepared by treating PFML 3 with Zn(II) salt, crystallized in orthorhombic cell with Pna21 space group. The asymmetric unit is comprised of one PFML, one Zn(II) ion, one bromide ion and six lattice water molecules. The Zn(II) ion acquired tetrahedral geometry where all four coordination sites are satisfied by Ocarboxyalate groups stemming from two different PFMLs. In this case as well, 1D polymeric chain was generated by the coordination of secondary Zn(II) ions to the PFML 3 (Figures 3a-3d). Each 1D chain is further connected to the

adjacent chains via H-bonding between lattice water molecules and Oamide atoms as well as via C–H…π interactions (O1…O4W: 2.764 Å; O4W…H-C22: 3.055 Å) (Figure 3b). As a result, 1D chains are stacked to generate a layered structure. The crystal structure of 3d exhibits 8.4% SAVs per unit cell volume which is filled with the lattice water molecules. Topological analysis revealed a similar network topology for 3d as observed in case of 3a – 3c.


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Figure 3. (a) Stick model diagram with partial numbering scheme for HCP 3d; hydrogen atoms, bromide anion, and solvent molecules are omitted for clarity. (b and d) Figures showing H-bonding interactions involving water molecules (shown as red color sphere). (c) A view of packing diagram in the space fill mode along c axis. (e) Topological representation of HCP 3d.



Page 8 of 15

Figure 4. (a) Stick model diagram with partial numbering scheme for HCP 4a; hydrogen atoms and solvent molecules are omitted for clarity. (b) Topological representation of HCP 4a. (c) Stick model diagram with partial numbering scheme for HCP 4b; hydrogen atoms and solvent molecules are omitted for clarity. (d and f) Figures showing H-bonding interactions involving water molecules (shown as red color sphere). (e) A view of packing diagram in the space fill mode along c axis. (g) Topological representation of HCP 4b. Crystal Structures of HCPs 4a and 4b. To understand both effect and impact of the position of appended groups, metalloligand 2 offering para-pyridyl groups was functionalized. The resultant para-pyridyl-N functionalized PFML 4, on reaction with Mn(II) and Cd(II) salts, produced HCPs 4a and 4b, respectively. Both HCP 4a and 4b crystallized in tetragonal cell with I41/a space group. In both cases, asymmetric unit contained one fourth of PFML 4, one secondary M(II) ion and two lattice water molecules (for asymmetric unit, see Figure S35, SI). In both cases, geometry of the secondary metal ion was nearly square-antiprism wherein all eight coordination sites were satisfied by the bidentate Ocarboxylate groups (Figures 4a and 4c). The Mn–O bond distances were found to be in the range of 2.284 – 2.367 Å whereas Cd–O bond lengths were noted between 2.411 – 2.416 Å (Table S1, SI). These bonds are quite longer than that of other literature examples.50–52 A combination of PFML 4 and

M(II) ions generated 1D chains in both cases. In both cases, such chains are further connected to the neighboring ones through Hbonding involving lattice water molecules as well as C–H…π interactions (Figures 4d – 4f). Such interactions generated a 2D sheet-like packing in both cases. As expected, topological analyses revealed an identical network topology for both HCPs: 2,2,4-connected nodal net with point symbol of {82}{8}6 – {22}{2}6. (Figures 4b and 4g). Application of 3a and 4a in Heterogeneous Catalysis Heterogeneous catalysis has made significant impact in recent time.80,81 Such a fact is due to ease in product separation and recyclability of the catalyst.13,82 In this context, CPs have been found quite effective as the heterogeneous catalysts due to their robust architecture, porous nature and crystallinity.50–54 Attempts have been made to incorporate assorted Lewis acidic as well as


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redox-active metals in CPs.83 In this direction, our research group has successfully developed various heterogeneous catalysts for noteworthy organic transformations.50–54 The present HCPs provided a noteworthy opportunity to test their heterogeneous catalytic applications. This is due to the fact that two sets of HCPs differ by the presence (3a – 3d) or absence (4a – 4b) of coordinated water molecules that may have profound effect on the heterogeneous catalysis. We selected Knoevenagel condensation reactions (KCRs)84–90 and cyanation reactions (CRs)91,92 to test the said hypothesis. Both KCRs and CRs are well-established reactions for the introduction of carbon-carbon bonds via nucleophilic addition to carbonyl compounds.84–92 Both these reactions are known to be catalyzed by the Lewis acidic metals.93– 96 However, reports are available for the utilization of bases,97–102 ionic liquids103 and external stimuli, such as, microwave irradiation, supersonic waves and grinding for driving such reactions.104–107 For such catalytic reactions, one representative HCP from each set was selected, 3a and 4a. Both 3a and 4a have Mn(II) ions as the secondary metals that are coordinated to Ocarboxylate atoms originating from PFMLs. Importantly, while Mn(II) ion in 3a is coordinated by two axial water molecules; 4a does not contain any coordinated solvent molecule. It is a well-known fact that the coordinated solvent molecules on a catalytic metal are often labile and thus facilitate their easy replacement and/or exchange by a suitable solvent and/or a substrate.52,108 Therefore, we investigated possible replacement and/or exchange of the coordinated water molecules in 3a. For such a purpose, a vacuum-dried (at 50 C) sample of 3a was sealed in an atmosphere of D2O.52 A comparison of the FTIR spectra of as-synthesized 3a and the one after D2O exchange experiment exhibited that the most of the water molecules (potentially both coordinated as well as lattice) in 3a were replaced by D2O and such a sample exhibited O-D stretches at ca. 2500 cm-1 (Figure S36, SI). This simple experiment infers that the coordinated water molecules in 3a are labile and a suitable solvent can potentially replace them.52 To further explore possible replacement of the labile water molecules by a potential substrate, benzaldehyde was selected along with HCP 3a, as a representative case. For such an experiment, vacuum-dried (at 50 C) sample of 3a was dipped in a CH2Cl2 solution of benzaldehyde. This impregnated sample was then investigated by FTIR spectrum that showed red-shifted stretches for the aldehyde group (1696 cm-1) compared to neat benzaldehyde (1707 cm-1). This experiment strongly suggests that benzaldehyde has potentially substituted the coordinated water molecule in HCP 3a (Figure S37, SI).52 We started with CRs91,92 of benzaldehyde for exploring the catalytic efficiencies of the present HCPs. For optimizing the reaction conditions, 3a was used as a representative catalyst along with benzaldehyde as a model substrate and trimethylsilyl cyanide as the cyanide source (Table 3). Such reactions were performed at 25 C using only 1-mol% of a catalyst and were completed within 2 h. As can be seen, quantitative product was noted under the solvent-free conditions (entry 5) when compared to the use of various solvents (35 – 54 %; entries 1–4). Under the identical reaction conditions, HCP 4a resulted in 68% product formation (entry 6). With these optimized reaction conditions, catalytic scope was investigated. To understand the effect of an electronic group present on the benzaldehyde; assorted substituted aldehydes were used (Table 4).92,96 Notably, CRs were quite effective both with electron-rich and electron-deficient benzaldehydes when compared to benzaldehyde (entries 1-6). However, electronic group effect was much more pronounced with 2-nitrobenzaldehyde due to the anticipated –R effect of the nitro group

(entry 2). Although, bulkier naphthaldehyde produced nearly quantitative product with HCP 3a (entry 8); much lower yield was noted with 2,4,6-trimethoxybenzaldehyde, as a substantially sterically hindered substrate (entry 8).91,92 Table 3. Optimization experiments for the cyanation reactions. O H

Ph

+

Me3SiCN

OH

Catalyst Ph

CN

S. No.a

Catalyst

Solvent

Conversion (%)b

1

3a

MeCN

40

2

3a

MeOH

35

3

3a

EtOH

37

4

3a

Toluene

54

5

3a

---

100

6

4a

---

68

aReaction

Conditions: catalyst: 1-mol%; reaction time: 2h; temperature: 25 °C. bProducts were quantified by using gas chromatograph. Table 4. Cyanation reaction of assorted aldehydes with (CH3)3SiCN in presence of catalysts 3a and 4a under the solvent free condition. O R

H

S. No.a

Me3SiCN

R

OH

Catalyst R

Conversion

CN

(%)b

3a

4a

1

-C6H5

100

68

2

2-NO2C6H4

74

35

3

4-NO2C6H4

96

77

4

4-ClC6H4

95

65

5

4-OMeC6H4

94

56

6

3-OMeC6H4

93

78

7

1-Naphthyl

98

70

8

2,4,6OMeC6H2

63

42

aReaction

Conditions: catalyst: 1-mol%; reaction time: 2h; temperature: 25 °C. bProducts were quantified by using gas chromatograph. Subsequently, HCPs 3a and 4a were used as the heterogeneous catalysts for KCRs (Tables 5 and 6).99 In these reactions, an aldehyde substrate was treated with malononitrile at 40 C in presence of only 1-mol% of HCPs as the catalyst. A screening of various solvents suggested that the quantitative results are obtained under the solvent-free conditions (entry 5;


Crystal Growth & Design Table 5); however, use of MeCN (entry 1) and toluene (entry 4) also produced the corresponding product in high yield. To explore the scope of catalysis, different aldehydes containing electronwithdrawing and electron-donating groups were used (Table 6).47,48,55,84–90 As anticipated, former substrates resulted in increased product formation (entries 2 – 4) when compared to para-methoxy benzaldehyde (entry 5).48 Use of 1-naphthaldehyde produced the respective product in good yield (entry 6) with HCP 3a when compared to sterically demanding substrate, 2,4,6trimethoxybenzaldehyde (entry 7) 48 Table 5. Optimization experiments for the Knoevenagel condensation reactions. O H

Ph

Catalyst

CH2(CN)2

CN

R

40 oC, 4h

CN

S. No.a

Catalyst

Solvent

Conversion (%)b

1

3a

MeCN

97

in 3a is coordinated by two axial water molecules; 4a does not contain any coordinated solvent molecule. In addition, Mn(II) ion in 4a exhibits an eight-coordinated geometry wherein a metal is symmetrically surrounded by eight Ocarboxylate groups and not having any open and/or vacant site. These facts strongly suggest that while a substrate may not have difficulty in accessing the Lewis acidic metal ions in 3a; in contrast, 4a does not provide substrate accessibility. Furthermore, D2O and benzaldehyde exchange experiments asserted the labile nature of coordinated water molecules in 3a. Further evidence came from the timedependent reaction profile between benzaldehyde and (CH3)3SiCN using HCPs 3a (blue trace) and 4a (red trace) as the catalysts under the solvent-free conditions (Figure 5). This experiment explicitly distinguishes between HCPs 3a and 4a and proves the better catalytic efficiency of 3a when compared to 4a, justifying the aforementioned points. 110

2

3a

MeOH

92

3

3a

EtOH

94

4

3a

Toluene

95

5

3a

---

100

6

4a

---

72

100



90



80 % Conversion

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 15





70 60 50



40 30











20 10

aReaction

Conditions: catalyst: 1-mol%; reaction time: 4h; temperature: 40 °C. bProducts were quantified by using gas chromatograph.

Table 6. Knoevenagel condensation reaction of assorted aldehydes with malononitrile in presence of catalyst 3a and 4a under the solvent free condition. O H

R

CH2(CN)2

Catalyst

CN

R

40 oC, 4h

CN

Conversion (%)b S. No.a

R

3a

4a

1

-C6H5

100

72

2

2-NO2C6H4

79

46

3

4-NO2C6H4

87

64

4

4-ClC6H4

98

72

5

4-OMeC6H4

60

48

6

1-Naphthyl

87

62



0 0

1

2

3 4 Time (Hours)

5

6

Figure 5. Cyanation reaction between benzaldehyde and (CH3)3SiCN using HCPs 3a (blue trace) and 4a (red trace) as the catalysts under the solvent-free condition as a function of time. Heterogeneous Nature of Catalysis and Reusability It is desirable for a heterogeneous catalyst to exhibit both recyclability and reusability.109 For such a purpose, HCP 3a was selected as a representative catalyst for performing filtration test during the KCR between benzaldehyde and malononitrile.53,110 As can be seen from Figure 6, the catalytic reaction is nearly ceased when HCP 3a was removed by filtration at 2 h from the reaction mixture. The re-addition of 3a at 3 h resulted in the immediate commencement of the catalysis. These observations potentially suggest that the actual catalytic metal sites have not leached out from HCPs during the catalysis.53,110 Additional evidences came from the recyclability studies in which HCP 3a was filtered from the reaction mixture and was reused for five consecutive cycles for a reaction between benzaldehyde and malononitrile (inset; Figure 6). While drop in the product yield was negligible for the first three cycles; 5-6 % decrease in the product yield was noted at the 4th and 5th cycle.53,110

7

2,4,644 31 OMeC6H2 aReaction conditions: catalyst: 1-mol%; temperature: 40 °C; reaction time: 4h. bProducts were quantified by using gas chromatograph. A comparison of catalytic results both for CRs and KCRs illustrate that HCP 3a is a much efficient catalyst when compared to 4a. The crystal structures have confirmed that while Mn(II) ion


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Crystal Growth & Design Corresponding Author E-mail: [email protected]; Web: http://people.du.ac.in/~rgupta.

ORCID Rajeev Gupta: 0000-0003-2454-6705 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

Figure 6. (a) Knoevenagel condensation reaction between benzaldehyde and malononitrile in presence of catalyst 3a. (b) Catalyst 3a was filtered off after 2 h resulting in the termination of the catalysis. (c) Re-addition of 3a at 3 h resulted in commencement of catalysis leading to completeness. Inset displays recyclability of HCP 3a for five consecutive catalytic cycles. Taking advantage of the heterogeneous nature of catalysis, both HCPs 3a and 4a were recovered from the reaction mixture of a KCR of benzaldehyde with malononitrile and characterized. The sample homogeneity of 3a and 4a was confirmed by the XRPD studies.50–55 XRPD patterns, both for 3a and 4a, exhibited a nearly identical match to that of as-synthesized samples suggesting that the crystallinity of the HCPs is maintained during the catalysis (Figures S38 and S39, SI). Similarly, FTIR spectra of the recovered HCPs 3a and 4a after the catalysis overlapped nicely to that of pristine HCPs thus further confirming structural stability during the catalysis (Figures S40 and S41, SI).

RG acknowledges Science and Engineering Research Board (SERB; EMR/2016/000888), New Delhi for the financial support. SP thanks CSIR, New Delhi for the SRF fellowship. Authors thank CIF-USIC of this university for the instrumentation facility including X-ray data collection.

REFERENCES 1. 2. 3.

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CONCLUSIONS In this work, two different post–functionalized metalloligands (PFMLs 3 and 4) have been utilized for the synthesis of 1D HCPs, 3a – 3d and 4a – 4b. These chain-like HCPs illustrated intriguing catenated architectures and differed by the presence (3a – 3d) or absence (4a – 4b) of coordinated water molecules. The secondary metals of HCPs derived from PFML 3 displayed six-coordinated geometry with two axially coordinated water molecules. In contrast, PFML 4 derived HCPs showed saturated eightcoordinated geometry of the secondary metals with no ligated solvent molecule. Representative HCPs 3a and 4a were utilized as the heterogeneous catalysts for carrying out cyanation reactions and Knoevenagel condensation reactions and illustrated profound effect of the presence of labile water molecules on the secondary metals on catalysis. These catalytic results are noteworthy and will help in designing better heterogeneous catalysts. The present design strategy has provided a noteworthy alternate procedure to synthesize post-functionalized metalloligands having extended functional groups. It is believed that such a design strategy will help in preparing desirable architectures offering open-structures for various porosity related applications.

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ASSOCIATED CONTENT Supporting Information. Figures for FTIR, NMR and absorption spectra; TGA; PXRD patterns; BET isotherms; and crystal structures; and tables for the crystallographic bonding parameters.

AUTHOR INFORMATION

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For Table of Contents Use Only

Manuscript Title: Post-functionalized Metalloligands Based Catenated Coordination Polymers: Syntheses, Structures and Effect of Labile Sites on Catalysis Authors: Saurabh Pandey, Girijesh Kumar and Rajeev Gupta*

TOC Graphic:

Synopsis: This work illustrates the use of two post-functionalized metalloligands for the synthesis of several homo- and hetero-metallic coordination polymers (HCPs) displaying intriguing one-dimensional catenated architectures. Two sets of HCPs, differing by the presence or absence of coordinated water molecules, demonstrate profound effect on the heterogeneous catalysis of Knoevenagel condensation and cyanation reactions.


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Postfunctionalized Metalloligand-Based Catenated Coordination

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