Diastereoselective self-assembly of heterochiral Zn(ii) complexes of racemic Schiff bases in a chiral self-discriminating process: Effect of non-covalent interactions on solid state structural self-assembly

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Diastereoselective self-assembly of heterochiral

Zn(^II) complexes of racemic Schiﬀ bases in a chiral

self-discriminating process: eﬀect of non-covalent

interactions on solid state structural self-assembly†

Cite this: RSC Adv., 2014, 4, 3028

*

Himanshu Sekhar Jena‡

Diastereoselective self-assembly of Zn(II) heterochiral complexes of racemic Schiﬀ bases L1H and L2H

(where L1H ¼ 2-((phenyl(2-pyridyl)methylimino)-methyl)phenol; L2H ¼ 1-((phenyl(2-pyridyl)methylimino)

methyl)-2-naphthol) in a chiral self-discriminating process are reported. Complexes 1–6 are synthesized

using ligand L1H (1–3), L2H (4–6), Zn(NO₃)₂$6H₂O, and co-ligands such as N₃or NCS^ꢀand are

ꢀ

conclusively structurally characterized. Determination of molecular structures of 1–5 conﬁrmed the

presence of a di-metallic core constructed by monochelated tridentate ligands with an inversion centre

located directly between the two zinc ions. In 1–5, within one centrosymmetric dimer one of the ligands

possessed R conﬁguration whereas other possessed S conﬁguration resulting a heterochiral dimerization

of ligands around the Zn(^II) centre in a chiral self-discriminating manner. Complex 6 is mononuclear

having R conﬁguration of ligand L2 and linked through a C–H/O interaction with the nearby

mononuclear Zn(^II) centre having S conﬁguration of ligand L2 and consequently a heterochiral dimer.

The plausible mechanism for the chiral self-discrimination is revealed which makes it clear that trans

oriented anions direct the phenyl group appended to the asymmetric carbon atoms to be trans due to

steric congestion and consequently the heterochiral dimerization conﬁguration. The eﬀects of diﬀerent

non-covalent interactions to self-assemble the heterochiral dimers to 1D chain having the sequence

/RS/RS/RS/ or /RS/SR/RS/ or /R/S/R/ and to transfer the stereochemical information of

the heterochiral dimers throughout the chain are studied. Also the eﬀects of the anions on the isotactic

and syndiotactic arrangements of coordination complex are studied. The results demonstrate that ligand

modiﬁcation, anions and non-covalent interactions play a domino role on controlling the solid state

structural rearrangements and photoluminescence properties of the synthesized complexes.

Received 13th September 2013

Accepted 15th October 2013

DOI: 10.1039/c3ra45082a

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N₂O donor type ligands and are found in the active sites of

Introduction

carboxypeptidase A, carboxypeptidase

B and thermolysin

enzymes.⁴Zinc complexes in particular also have antibacterial

and antifungal activities.⁵Among the various N, O-donor

bridging ligands, Schiﬀ base ligands⁶have been proven to be

good candidates for the construction of coordination polymers

with versatile topologies and functional properties. Of partic-

ular interest are Schiﬀ base complexes of zinc, as these have

found numerous applications, for example, as uorescent

emitters and as active layers in organic light-emitting diodes

(OLEDs).⁷Moreover these complexes continue to be motivated

by their remarkable structural role in DNA binding, cleavage,

anticancer and cytotoxicity activity.⁸The aforementioned

prominence of Zn(^II) Schiﬀ base complexes in the coordination

chemistry as well as in biology motivates the study of possible

outcomes of self-assembly in coordination complexes contain-

ing racemic Schiﬀ base ligands.

Zinc(II) ion which is the second most abundant element, is

essential for all living organisms and is found to be present in

all six major classes of metallo-enzymes.¹Also it is found in

every cell in the human body and is mostly tightly bound to

proteins as a key component of numerous enzymes and tran-

scription factors.²Various model studies with ligands contain-

ing N, O and/or S donor atoms have been reported.³Most

protein residues contain a zinc ion surrounded by tridentate

Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati, Assam,

India, 781039. E-mail: hsjena@gmail.com; Fax: +91 755 4092392; Tel: +91

9425807692

† Electronic supplementary information (ESI) available: Additional gures and

PXRD plots provided. CCDC 953886–953891. For ESI and crystallographic data

in CIF or other electronic format see DOI: 10.1039/c3ra45082a

‡ Present address: Post-Doctoral Fellow, Department of Chemistry, Indian

Institute of Science Education and Research Bhopal, 462023, Madhya Pradesh,

India. Email: hsjena@iiserb.ac.in, Fax: +91 755 4092392, Phone: +91 9425807692.

Furthermore the transmission of the chirality in the self-

assembly of coordination complexes and their applications in

3028 | RSC Adv., 2014, 4, 3028–3044

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fabricating chiral luminescent materials, homochiral MOFs, aid of a crystal engineering approach.¹⁷The plausible mecha-

chiral liquid crystals, enantioselective sensors, chiroptical nism for chiral self-discrimination over chiral self-recognition

switches, and magnetochiral complexes are reported in the for the diastereoselective self-assembly of dinuclear hetero-

literature.⁹It was known that the self-assembly of coordination chiral complexes is also established.

complexes having enantiopure ligands necessarily outcomes

only homochiral complexes.^9c,d,f,h,iThe self-assembly of racemic

Schiﬀ base ligands are very diﬃcult to predict as serendipity

plays a major role in the nal structural arrangements. There

Experimental section

Materials and general methods

are three possible outcomes of the self-assembly of racemic

systems: (i) a racemic mixture of homochiral dimers (in the self-

recognition process); (ii) heterochiral dimer (in self/non-self-

discrimination process); or (iii) manual separation of homo-

chiral dimers from the racemic system in the crystalline state

(spontaneous chiral resolution).¹⁰Therefore three important

questions can be systematically addressed to understand the

above mechanism and to control the nal structural outcomes:

(i) the inuence of the relative orientation of the ligand arms on

the structural self-assembly; (ii) the ability of the anions or

bridging ligands to control the stereochemistry of the metal

centre(s); and (iii) the eﬀect of the directional nature of supra-

molecular interactions which sometimes can eﬃciently transfer

the stereo-chemical information between neighbouring homo-

or heterochiral molecules. Factors¹¹aﬀecting the diaster-

eoselective self-assembly of coordination complexes containing

chiral N-donor ligands are reported in the literature. Meanwhile

very few noteworthy results deliberating the self-assembly of

coordination complexes containing racemic Schiﬀ bases are

reported, these are by Stack et al., Mascharak et al., Kuroda

All the solvents and reagents were purchased from commercial

sources and used as received. 2-Benzoylpyridine, 2-hydroxy-1-

naphthaldehyde, NaNCO, (Aldrich, USA); Zn(NO₃)₂$6H₂O,

NaN₃, NaBH₄, hydroxylamine hydrochloride, Zn dust (Merck

India Ltd) and solvents were used as received without further

purication. (R,S)-1-(2-pyridyl)ethylamine, (R,S)-phenyl(2-pyr-

idyl)methaneamine, and (((R,S)-phenyl(2-pyridyl)methylimino)

methyl)phenol (L1H) were prepared using our earlier reported

procedure.^15b,cA Perkin-Elmer Spectrum One spectrometer

(4000–250 cm^ꢀ1), Perkin-Elmer Series II CHNS/O Analyzer 2400,

Perkin-Elmer Lambda 25 spectrometer, Cary Eclipse

EL05033882 was used for uorescence spectra and a Bruker 400

MHz spectrophotometer were used to obtain relevant NMR

data. Powder X-ray diﬀraction data were recorded using a Seifert

powder X-ray diﬀractometer (XRD 3003TT) with a Cu Ka source

˚

(l ¼ 1.54 A) on a glass surface of an air-dried sample.

Crystallographic data collection and renement

X-ray crystallographic data were collected using Bruker SMART

˚

APEX-CCD diﬀractometer with Mo Ka radiation (k ¼ 0.71073 A).

et al.¹²In particular, self-assembly of Zn(II) complexes contain-

The intensity data were corrected for Lorentz and polarization

eﬀects and empirical absorption corrections were applied using

the SAINT program.^18,19All the structures were solved by direct

methods using SHELXS-97.²⁰Non-hydrogen atoms located from

the diﬀerent Fourier maps were rened anisotropically by full-

matrix least-squares on F², using SHELXS-97.²⁰All hydrogen

atoms were included in the calculated positions and rened

isotropically using a riding model.

13

´

ing racemic Schiﬀ base is reported by Garcıa-Deibe et al.

Recently eﬀect of ligand substitution on the coordination

stereochemistry of transition metal Schiﬀ base complexes are

reported.¹⁴Thus, it is immensely important to gather precise

knowledge on how the diﬀerent factors aﬀect the coordination

stereochemistry and solid state structural self-assembly of

racemic ligands. Therefore a system is presented here that

exhibits ligand self-discrimination based solely on chirality in

which the reaction of racemic mixtures of ligand with metal

ions generates only heterochiral complexes and adds a new

example to the very scarcely reported chiral-discrimination

process.

Syntheses

Synthesis of (R,S) 1-((phenyl(2-pyridyl)methylimino)methyl)-

2-naphthol (L2H). A mixture of phenyl(2-pyridyl)methaneamine

The immense interest in coordination stereochemistry (4.5 g, 26.3 mmol) and 2-hydroxy-1-naphthaldehyde (4.2 g, 26.3

enables us to report the diastereoselective self-assembly of mmol) in methanol (100 mL) was stirred for 2 h. A yellow

racemic Schiﬀ bases around metal centres.¹⁵The results include precipitate of L2H deposited was collected by suction ltration,

the diastereoselective self-assembly of racemic Schiﬀ bases with washed with cold methanol and dried over CaCl₂. Yield: 7.2 g

transition metal ions to form homo- or heterochiral complexes (95%). IR (KBr, cm^ꢀ1): 3049(w), 2929(w), 1625(s), 1609(m),

which further self-assembled into a 1D chain with suitable 1595(m), 1571(w), 1540(s), 1519(w), 1489(s), 1474(m), 1454(m),

bridging ligands.^15a,bThe eﬀects of ligand geometry, bridging 1432(m), 1400(s), 1349(s), 1337(s), 1313(m), 1251(w), 1214(s),

ligands and non-covalent interactions on the diastereoselective 1182(s), 1140(m), 1165(w), 1097(w), 1027(s), 993(s), 943(w),

self-assembly are also reported.^15cA recent elegant example by 862(m), 837(s), 753(s), 699(s), 609(m), 562(s), 497(m), 470(w).

1

Yan and co-workers deliberated that the azide ion mediated 400 MHz H NMR (d (J, Hz), CDCl₃): 9.02 (1H, s), 8.52 (1H, d,

chiral transfer in coordination complexes containing azine 4.0), 7.80 (1H, d, 8.4), 7.64 (1H, d, 9.2), 7.58 (1H, d, 8.0), 7.569

ligands and motivated the use of bridging ligands, such as azide (2H, t, 6.6), 7.37 (2H, t, 8.0), 7.30 (1H, t, 8.4), 7.22 (1H, t, 7.4),

and thiocyanate ions, in the present article.¹⁶Herein chiral self- 7.15 (1H, t, 6.3), 7.01 (1H, t, 5.4), 6.96 (1H, d, 9.2), 5.84 (1H, s),

discriminating assembly of racemic Schiﬀ bases and Zn(^II) ion 2.0 (OH, s). 100 MHz ¹³C NMR (d, CDCl₃): 171.36, 160.5, 159.4,

into dinuclear heterochiral complexes is reported, which 149.6, 140.7, 137.3, 136.5, 133.5, 129.3, 129.1, 128.1, 128.0,

subsequently self-organise to form 3D structures through the 127.6, 127.0, 123.2, 122.94, 122.8, 121.9, 118.7, 108.1, 74.7.

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place of L1H in the same molar ratio as described for

complex 2. Yield: 0.60 g (67%). Anal. calc. for C₄₆H₃₄N₁₀O₂Zn₂:

C, 62.11; H, 3.85; N, 15.74%. Found: C, 62.10; H, 3.83; N,

15.72%. IR (KBr, cm^ꢀ1): 3434(b), 2077(m), 1615(s), 1607(m),

1569(w), 1538(m), 1482(m), 1457(m), 1414(m), 1385(s), 1365(s),

1339(s), 1295(s), 1246(m), 1184(m), 1156(m), 1020(w), 960(w),

826(s), 740(m), 702(m), 686(w), 643(w), 622(w), 558(m), 490(w),

411(w), 349(w).

Synthesis of [Zn(L1)(NO₃)]₂(1). To L1H (0.288 g, 1.0 mmol)

dissolved in methanol (10 mL), solid Zn(NO)₃$6H₂O (0.300 g,

1.0 mmol) was added and stirred for 2 h. The resulting col-

ourless solution is ltered to remove the white precipitates and

stored at room temperature for crystallization. Colourless single

crystals of 1 suitable for the X-ray diﬀraction study were

obtained from this methanol–DMF (1 : 1) solution on standing

for about one week. Yield: 0.68 g (82%). Anal. calc. for

Synthesis of [Zn(L2)(NCS)]₂(6). The same procedure was

followed for the synthesis of complex 6 by using ligand L2H in

place of L1H in the same molar ratio as described for complex 4.

Yield: 0.40 g (75%). Anal. calc. for C₂₇H₂₄N₄O₂S1Zn1: C, 60.73;

H, 4.53; N, 10.49%. Found: C, 60.71; H, 4.51; N, 10.47%. IR (KBr,

cm^ꢀ1): 3449(b), 2065(s), 1618(s), 1609(m), 1570(w), 1549(m),

1511(m), 1479(m), 1455(m), 1438(m), 1418(w), 1337(s), 1294(s),

1247(m), 1187(m), 1145(m), 1090(w), 1051(w), 994(m), 959(m),

860(w), 825(s), 748(m), 702(m), 619(w), 571(w), 495(w), 449(w),

412(w), 342(w).

C

₃₈H₃₀N₆O₈Zn₂: C, 55.02; H, 3.65; N, 10.03%. Found: C, 54.00;

H, 3.63; N, 10.00%. IR (KBr, cm^ꢀ1): 3436(b), 1625(s), 1602(w),

1571(m), 1555(w), 1470(m), 1448(m), 1407(m), 1384(s), 1286(s),

1198(m), 1150(w), 1150(w), 1045(w), 876(w), 789(w), 759(s),

746(w), 704(m), 546(w), 431(w), 324(w).

Synthesis of [Zn(L1)(N₃)]₂(2). To L1H (0.226 g, 1.0 mmol)

dissolved in methanol (10 mL), solid Zn(NO)₃$6H₂O (0.300 g,

1.0 mmol) was added and stirred for 10 min. To this colourless

solution, methanol solution of (5 mL) NaN₃(0.65 g, 1.0 mmol)

was added and stirred for another 3 h. The resulting colourless

solution is ltered to remove the white precipitates and stored

at room temperature for crystallization. Colourless single crys-

tals of 2 suitable for the X-ray diﬀraction study were obtained

from the methanol–DMF (1 : 1) solution on standing for about

one week. Yield: 0.56 g (73%). Anal. calc. for C₃₈H₃₀N₁₀O₂Zn₂: C,

57.81; H, 3.83; N, 17.74%. Found: C, 57.80; H, 3.80; N, 17.73%.

IR (KBr, cm^ꢀ1): 3436(b), 2061(s), 1625(s), 1600(w), 1571(w),

1551(w), 1470(m), 1444(m), 1407(m), 1286(s), 1198(m), 1150(w),

1128(w), 1045(w), 876(w), 789(m), 704(m), 546(w), 434(w),

321(w).

Synthesis of [Zn(L1)(NCS)]₂(3). To L1H (0.226 g, 1.0 mmol)

dissolved in methanol (10 mL), solid Zn(NO)₃$6H₂O (0.300 g,

1.0 mmol) was added and stirred for 10 min. To this colourless

solution, methanol solution of (5 mL) NaSCN (0.81 g, 1.0 mmol)

was added and stirred for another 3 h. The resulting colourless

solution was ltered to remove the white precipitates and stored

at room temperature for crystallization. Colourless single crys-

tals of 2 suitable for the X-ray diﬀraction study were obtained

from the methanol–DMF (1 : 1) solution on standing for about

one week. Yield: 0.58 g (70%). Anal. calc. for C₄₀H₃₀N₆O₂S₂Zn₂:

C, 58.47; H, 3.68; N, 10.23%. Found: C, 58.45; H, 3.66; N,

10.21%. IR (KBr, cm^ꢀ1): 3443(b), 3059(w), 3026(w), 2070(s),

1629(s), 1602(m), 1567(w), 1550(w), 1472(m), 1445(m), 1405(m),

1292(s), 1253(w), 1200(m), 1153(m), 1125(w), 1045(w), 929(m),

875(w), 789(m), 774(m), 760(m), 699(m), 621(m), 589(w),

548(m), 465(w), 411(w), 326(w).

Results and discussion

Synthesis of the complexes

It was known that ligand modications, bridging linkers and

the nature of non-covalent interactions, have a profound eﬀect

on the nal structural self-assembly. However their collective

eﬀects on the diastereoselective self-assembly of coordination

complexes having racemic Schiﬀ bases are not documented in

the literature. In order to reveal the possible self-assembly of

coordination complexes containing racemic ligands, it is

important to synthesize a wide variety of racemic Schiﬀ bases

and study their possible self-assembly²¹upon metalation. With

the help of the modern crystal engineering approach such self-

assembly can be extended to network structures with desired

applications. Recently the eﬀect of ligand geometry on the

diastereoselectivity of the chiral tridentates imine and amine

around zinc centres has been reported. From this perception, in

this work two tridentate N₂O-donor type racemic Schiﬀ bases

(L1H and L2H: Scheme 1) have been synthesized in order to

investigate their possible solid state diastereoselective self-

assembly. The isolated complexes further undergo anion

metathesis with sodium azide and sodium thiocyanate to

inspect the possible structural self-assembly upon introduction

of pseudohalogens (Scheme 2). We believe in future that this

strategy (supra) will boost the synthesis of HMOF (Homochiral

MOF)^22a–eand MOCN (metal–organic coordination network)^22f

using racemic ligands.

Synthesis of [Zn(L2)(NO₃)]₂(4). The same procedure was

followed for the synthesis of complex 4 by using ligand L2H in

place of L1H in the same molar ratio as described for complex 1.

Yield: 0.68 g (73%). Anal. calc. for C₄₆H₃₄N₆O₈Zn₂: C, 59.43; H,

3.69; N, 9.04%. Found: C, 59.41; H, 3.66; N, 9.02%. IR (KBr,

cm^ꢀ1): 3449(b), 1618(s), 1609(m), 1570(w), 1549(m), 1511(m),

1479(m), 1455(m), 1438(m), 1418(w), 1384(s), 1334(s), 1296(s),

1247(m), 1187(m), 1145(m), 1090(w), 1051(w), 994(m), 959(m),

860(w), 825(s), 751(m), 702(m), 619(w), 571(w), 495(w), 446(w),

341(w).

In IR spectra (range 4000–250 cm^ꢀ1), the characteristic

sharp peak at 1625 cm^ꢀ1corresponds to the presence of imine

(n_C^N) function in ligand L2H. The characteristic peaks of

ligand L1H are reported earlier.^15b,c,dIn complex 1–3, the

presence of a sharp peak at around 1620–1630 cm^ꢀ1can be

assigned to the imine (n_C^N) function of the Schiﬀ base ligand.

The peak at 1384 cm^ꢀ1in complex 1 is due to the presence of

nitrate ion. In complex 2 and 3, the absence of peak at 1384

cm^ꢀ1and the presence of a new sharp peak at 2061 cm^ꢀ1and

2070 cm^ꢀ1conrm the anion metathesis and correspond to

azide and thiocyanate functions, respectively. Similarly, in

Synthesis of [Zn(L2)(N₃)]₂(5). The same procedure was fol-

lowed for the synthesis of complex 5 by using ligand L2H in

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Scheme 1 Synthetic scheme of ligands L1H and L2H.

Scheme 2 Synthetic scheme of complexes 1–6.

complex 4–6, the presence of a sharp peak at around 1615– reported yield of complex 1–6 refers to the respective

1620 cm^ꢀ1can be assigned to the imine (n_C^N) function of the complexes only in the crystalline state. Due to the solubility

Schiﬀ base ligand. The peak at 1384 cm^ꢀ1in complex 4 is due problem we are at present unable to study the self-assembly of

to the presence of nitrate ion. The absence of peak at complexes in the solution state. Therefore for every complex

1384 cm^ꢀ1and the presence of a new sharp peak at 2077 cm^ꢀ1aer isolation of the crystals, the ltrate was allowed to

and 2091 cm^ꢀ1in 5 and 6 conrm the anion metathesis and evaporate and the remaining residue was subjected to powder

corresponds to azide and thiocyanate functions, respectively. X-ray analysis (XRPD) at room temperature. As shown in

The shis of the imine (n_C^N) function band towards lower Fig. S1 and S2,† each experimental XRPD pattern is in agree-

wave numbers upon coordination are indicative of the binding ment with the XRPD pattern calculated from the correspond-

of the imine nitrogen atom to the metal centres. The coordi- ing single crystal X-ray structure generated from mercury,

nation of the ligands to the metal ion is substantiated further indicating the phase purity of the products and suggestive of

by prominent bands observed at around 430–320 cm^ꢀ1in all the heterochiral self-assembly in the solid state. The diﬀer-

the complexes which can be attributed to the n_M–Nand n_M–Oences in intensity may be due to the preferred orientation of

vibrations, respectively. It is important to note that the above the crystalline powder samples.

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Table 1 Crystallographic and reﬁnement parameters of 1–6

1

2

3

4

5

6

Formula

C₃₈H₃₀N₆O₈Zn₂C₃₈H₃₀N₁₀O₂Zn₂C₄₀H₃₀N₆O₂S₂Zn₂C₄₆H₃₄N₆O₈Zn₂C₄₆H₃₄N₁₀O₂Zn₂C₂₇H₂₄N₄O₂S₁Zn₁

CCDC number

Formula weight

T (K)

953886

829.46

296(2)

0.71073

Monoclinic

C2/c

14.5074(4)

11.5459(4)

21.8496(8)

—

96.259(3)

—

3638.0(2)

4

953887

788.86

296(2)

0.71073

Monoclinic

P21/c

14.3323(3)

9.2491(2)

15.9719(3)

—

123.4570(10)

—

1766.42(6)

2

1.492

1.408

953888

821.62

296(2)

0.71073

Triclinic

ꢀ

P1

8.8651(3)

8.9933(3)

11.7540(4)

88.581(2)

80.825(2)

83.676(2)

919.46(5)

1

953889

929.57

296(2)

0.71073

Monoclinic

C2/c

22.625(3)

8.3854(8)

22.904(3)

—

114.515(13)

—

3953.5(9)

4

1.562

1.280

953890

889.61

296(2)

0.71073

Monoclinic

P21/n

11.0381(10)

13.9811(12)

13.0736(11)

—

96.529(5)

—

2004.5(3)

2

1.474

1.250

953891

533.96

296(2)

0.71073

Orthorhombic

Pbca

24.6911(18)

10.2087(8)

20.4419(15)

90.00

5152.7(7)

8

1.377

˚

Wavelength (A)

Crystal system

Space group

˚

a (A)

˚

b (A)

˚

c (A)

a (^ꢁ)

b (^ꢁ)

g (^ꢁ)

3

˚

V (A )

Z

D_calcd(g m^ꢀ3

)

1.514

1.380

1696

1.484

1.462

420

m (mm^ꢀ1

F (0 0 0)

)

1.065

2208.0

808

1904

912

Reection collected

Unique reections

q_min, q_max(^ꢁ)

4805

3307

1.88, 28.90

4035

4034

1.70, 27.45

1.372

3504

2819

1.76, 25.72

1.002

4120

2082

1.95, 26.52

1.021

3838

3060

2.14, 25.78

1.010

4793

3599

1.65, 25.50

1.034

Goodness-of-t (GOF) on F²1.004

R₁, wR₂(I $ 2s(I))

R₁, wR₂(all data)

Largest peak/hole (e A

0.0276, 0.0694

0.0306, 0.0716

0.281/ꢀ0.331

0.516, 0.1187

0.515/ꢀ0.513

0.0304, 0.0881

0.0393, 0.1026

0.412/ꢀ0.303

0.0459, 0.0844

0.0998, 0.0981

0.289/ꢀ0.450

0.0375, 0.0960

0.0552, 0.1054

0.549/ꢀ0.370

0.1044, 0.1418

0.0564, 0.1201

0.761/ꢀ0.371

ꢀ3

˚

)

diﬀraction techniques. The crystallographic data and rene-

ment parameters are listed in Table 1. Selected bond parame-

ters are listed in Table 2. The conventions followed in

Structural description

Crystal structures of the complexes 1–6. The molecular

structures of 1–6 were determined using single crystal X-ray

Table 2 Selected bond lengths (A˚) and bond angles (^ꢁ) in 1–6

1

2

3

4

5

6

Zn1–N1

Zn1–N2

Zn1–O1

Zn1–O1A

Zn1–O2

Zn1–N3

Zn1–N3A

Zn/Zn

N1–Zn1–N2

N1–Zn1–O1

N1–Zn1–O1A

N1–Zn1–O2

N1–Zn1–N3

N1–Zn1–N3A

N2–Zn1–O1

N2–Zn1–O1A

N2–Zn1–O2

N2–Zn1–N3

N2–Zn1–N3A

O1–Zn1–O1A

O1–Zn1–O2

O2–Zn1–O1A

O1–Zn1–N3

O1A–Zn1–N3

O1–Zn1–N3A

O2–Zn1–N3

N3–Zn1–N3A

2.124(3)

2.042(2)

2.065(2)

2.025(2)

2.093(2)

—

2.143(5)

2.081(4)

2.105(3)

2.002(3)

—

2.135(3)

2.053(2)

2.105(2)

1.981(2)

—

2.078(3)

2.046(3)

2.020(2)

2.121(3)

2.184(3)

—

2.122(3)

2.045(2)

1.965(2)

—

2.165(6)

2.043(7)

1.994(5)

—

2.050(6)

2.008(7)

—

1.998(4)

—

1.967(3)

—

2.172(3)

2.066(3)

3.173(1)

78.7(1)

150.9(1)

—

3.115(1)

78.0(1)

165.36(9)

101.98(9)

97.4(1)

—

3.183(1)

78.4(2)

164.0(1)

107.5(2)

—

93.9(2)

—

86.1(1)

125.1(1)

—

3.162(1)

78.00(9)

161.58(9)

103.23(9)

—

101.0(1)

—

85.04(8)

118.86(8)

—

3.09(1)

80.3(1)

167.7(1)

96.6(1)

95.9(1)

—

77.6(3)

161.5(2)

—

92.6(2)

95.3(3)

—

87.0(2)

—

136.5(2)

121.4(3)

—

101.4(2)

—

103.7(1)

92.3(1)

88.0(1)

—

87.59(9)

112.07(9)

149.8(1)

—

87.7(1)

105.3(1)

162.1(1)

—

122.0(2)

—

78.4(2)

—

123.9(1)

—

78.64(8)

—

155.0(1)

121.6(1)

—

105.3(1)

—

89.0(1)

—

80.80(8)

98.06(9)

96.40(9)

—

83.5(1)

96.4(1)

92.5(1)

—

97.6(2)

112.1(2)

—

94.3(1)

115.9(1)

—

101.5(3)

—

83.1(1)

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discussions are: L1

¼

2-((phenyl(2-pyridyl)methylimino)- rings. The Zn₂O₂unit has a planar rectangular shape having

˚

methyl)phenolate ion; L2 ¼ 1-((phenyl(2-pyridyl)methylimino)- Zn–O distances of 2.065(2) and 2.025(2) A, implying that the O_P

methyl)-2-naphtholate ion; O_P¼ phenolate-O; N_Y¼ pyridyl-N; bridge is unsymmetrical in nature having the non-bonded

˚

N_I¼ imine-N; O_N¼ nitrate-O; N_T¼ thiocyanate-N and N_Z

¼

contacts Zn/Zn, 3.115(1) A and O1/O1A, 2.651(3) A. Among

the coordination bonds, the trend Zn–N_Y> Zn–O_N> Zn–N_Iin

azide-N.

Structure of [Zn(L1)(ONO₂)]₂(1). Complex 1 crystallized in bond lengths was observable.

space group C2/c. The asymmetric unit of 1 contains a mono-

The packing diagram of 1 shows several interesting struc-

nuclear Zn(^II) centre bridged by ligand L1 having S congura- tural arrangements through the aid of diﬀerent cooperative

tion. The mononuclear unit is further bridged to another non-covalent interactions.^24aRecently Khavasi et al. substan-

mononuclear Zn(^II) centre having R conguration of ligand L1 tiate the role of cooperative non-covalent interactions on solid

through an O_Pbridge. Thus complex 1 crystalizes in centro- state structural self-assembly of coordination complexes.^24b–f

A

˚

symmetric space group with the inversion centre located C–H/O interaction of (C6–H6/O3 ¼ 3.432(5) A) between

directly between the two Zn(^II) ions. ORTEP diagram with the asymmetric carbon (C6) centre and O3 of coordinated nitrate

atom labeling scheme of the centrosymmetric dimer having ion extends the heterochiral dimer to 1D polymeric structure

Zn₂O₂unit is displayed in Fig. 1. Within one molecule one of the having the sequence /RS/RS/RS/ where isotactic arrange-

ligands possesses R conguration whereas other possesses S ment of the asymmetric carbon centres occurs (Fig. 2). The Zn/

conguration resulting in a heterochiral dimerization of Zn distance between two heterochiral dimers, linked by C–H/

˚

ligand L1 around the Zn(^II) centre in

a

chiral self- O interaction is 6.81(1) A. The aforementioned C–H/O inter-

discriminating manner. The coordination geometry around action retains the stereochemical interlinks between two nearby

the ve coordinated Zn(^II) centres are best described as dis- heterochiral dimer via a centre of inversion. Thus in a particular

torted trigonal bipyramid as inferred from the calculated s dimer S conguration of ligand L1 is linked to R conguration

(ref. 23) value of 0.24.

of L1 of the nearby dimer through C–H/O interaction. A two

In 1, the distorted trigonal bipyramid geometry around Zn(^II) centre of symmetry exits in the 1D chain; one in Zn₂O₂unit

centres are satised by two NNO-donor sets of the ligand L1 and through the O_Pbridge and another in the Zn₂(NO₃)₂unit

two trans orientated terminal coordinated nitrate ions. The through C–H/O interaction. Besides C–H/O interactions of

˚

trigonal plane is formed by the O_PN_IO_Ndonor set with its angles (C2–H2/O2 ¼ 3.375(5) A; C17–H17/O4 ¼ 3.249(5) A) between

lying in the range of 98.10(1)–149.8(1)^ꢁand the axial N1–Zn1– the nitrate ion and the ligand backbone allow the dimer

O1angle has a value of 165.40(1)^ꢁ. The cisoid angles are lying in to extend into a zigzag 1D polymer (Fig. 3). Furthermore

the range of 78.01(1)–102.10(1)^ꢁand the trans angles are in the localized C–H/p interactions of (C10–H10/C18 ¼ 3.705(5) A;

˚

range of 112.10(1)–165.40(1)^ꢁ. Ligand L1 is bound to the zinc C16–H16/C8 ¼ 3.780(1) A) and C–H/O interaction of (C9–

˚

centre by a ve-membered and a six-membered chelate ring and H9/O4 ¼ 3.623(5) A) extend the 1D chain to a 2D ower-like

both rings are puckered, resulting in an open book shape for the structure along the bc plane as shown in Fig. S3.†

ligand moiety. The asymmetric carbon atom deviates from the

Structure of [Zn(L1)(N₃)]₂(2). Complex 2 crystallized in space

puckered ve-membered chelate ring and dihedral angle group P21/n. The asymmetric unit of 2 contains a mononuclear

between the two planes C5C6N2 and C5N1Zn1N2 being 7.66^ꢁ. Zn(^II) centre bridged by ligand L1 having R conguration. The

The dihedral angle of 26.99^ꢁbetween the planes is formed by mononuclear unit is further bridged to another mononuclear

N2Zn1O1 and O1C18C14C13N2 suggesting an out of plane Zn(^II) centre having S conguration of ligand L1 through an O_P

displacement of zinc from envelope shaped six membered bridge. Thus complex 2 crystalizes in a centrosymmetric space

group with the inversion centre located directly between the two

zinc ions. ORTEP diagram with the atom labeling scheme of the

centrosymmetric dimer having Zn₂O₂unit is displayed in Fig. 4.

Within one molecule one of the ligands possesses R congu-

ration whereas other possesses S conguration resulting a

heterochiral dimerization of ligand L1 around Zn(^II) centre in a

chiral self-discriminating manner. The coordination geometry

around the ve coordinated Zn(^II) centres are best described as

distorted trigonal bipyramid as inferred from the calculated s

value of 0.65.

In 2, the distorted trigonal bipyramid geometry around Zn(^II)

centres are satised by NNO-donor set of the ligand L1 and trans

orientated terminal coordinated (m-1-N_Z) azide ions. The

trigonal planes are formed by the O_PN_IO_Ndonor set with its

angles lying in the range of 112.1(2)–125.1(2)^ꢁand both the axial

N1–Zn1–O1 angles have a value of 164.0(2)^ꢁ. The cisoid angles

lie in the range of 78.4(2)–112.1(2)^ꢁand the trans angles are in

the range of 122.1(2)–164.0(2)^ꢁ. Ligand L1 is bound to the zinc

Fig. 1 ORTEP diagram (30%) and atom labeling scheme of RS-1 (all the

H-atoms except H6 are omitted for clarity).

centre by a ve-membered and a six-membered chelate ring and

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Fig. 2 Illustration of 1D chain formed by C–H/O interaction between two adjacent centrosymmetric dimers with an isotactic arrangement of

the asymmetric carbon centres present in 1.

both rings are puckered, resulting in an envelope shape for the

ligand moiety. The asymmetric carbon atoms deviates from the

puckered ve-membered chelate rings and dihedral angle

between the two planes C5C6N2 and C5N1Zn1N2 being 5.18^ꢁ.

The dihedral angle of 26.40^ꢁbetween the planes formed by

N2Zn1O1 and O1C18C14C13N2 suggests an out of plane

displacement of Zn1 from envelope shaped six membered rings.

Similarly the dihedral angle of 26.40^ꢁbetween the planes

formed by N7Zn2O2 and O2C38C33C32N7 suggests an out of

plane displacement of Zn2 from envelope shaped six membered

rings. The Zn₂O₂unit in 2 is formed of nearly planar rectangular

shapes. The bond parameters around Zn₂O₂are: Zn–O

˚

distances (Zn1–O1 ¼ 2.105(3) and Zn1–O1A ¼ 2.002(3) A) and

the corresponding bond angles (Zn1–O1–Zn1A ¼ 101.6(1)^ꢁand

O1–Zn1–O1A ¼ 78.4(1)^ꢁ), implying that the O_Pbridge is

Fig. 4 ORTEP diagram (30%) and atom labeling scheme of 2 (all the H-

atoms except H6 and H25 are omitted for clarity).

unsymmetrical in nature having the non-bonded contacts of

˚

Zn/Zn, 3.183(1) A and O1/O1A, 2.598(4) A. Among the coor-

dination bonds, the trend Zn–N_Y> Zn–N_I> Zn–N_Zin bond

lengths was observable in 2 in contrast to the bond order of Zn–

N_Y> Zn–O_N> Zn–N_Iin 1 which is due to the change of anion

from nitrate ion to azide ion.

exemplied.²⁵The aforementioned C–H/N interaction does

not allow the centre of symmetry in between the two nearby

heterochiral dimers hence results in homochiral dimerization

in the chiral self-recognition process.

The packing diagram of 2 shows several interesting struc-

tural arrangements with assistance from diﬀerent non-covalent

interactions. Diﬀerent C–H/N interactions of (C6–H6/N4 ¼

The Zn/Zn distance between the homochiral dimers linked

˚

through C–H/N interactions is 6.853(1) A. As a result, in a

˚

particular 1D chain; heterochiral dimerization occurs in the

Zn₂O₂unit through an O_Pbridge whereas homochiral dimer-

ization occurs in Zn₂(N_Z)₂unit through C–H/N interactions

(Fig. 5) in contrast to complex 1 where both heterochiral

dimerization occurs. Also the two heterochiral dimers are

3.57(1); C8–H8/N5 ¼ 3.50(1) and C2–H2/N5 ¼ 3.58(1) A)

between coordinated azide ion and ligand backbones extends

the centrosymmetric dimer to 1D polymeric structure having

the sequence /RS/SR/RS/ where a syndiotactic arrange-

ment of the asymmetric carbon centres occurs (Fig. 5). Recently

such arrangement types in coordination complexes are

˚

linked through a localized C–H/p (C3–H3/C9 ¼ 3.63(1) A)

Fig. 3 Illustration of 1D zigzag chains formed by diﬀerent C–H/O interactions between coordinated nitrate ion and ligand moiety found in 1.

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Fig. 5 Illustration of 1D chain formed by C–H/N interactions between two adjacent heterochiral dimers with a syndiotactic arrangement of the

asymmetric carbon centres present in 2.

interaction having the sequence /RS/RS/RS/ centro- the ligand moiety. The asymmetric carbon atoms deviates from

symmetrically resulting in a 1D chain as shown in Fig. 6. In the puckered ve-membered chelate rings and dihedral angle

complex 2, the combined supports from the above mentioned between the two planes C5C6N2 and C5N1Zn1N2 being 8.04^ꢁ. It

C–H/N interactions, and localized C–H/p interactions is very curious to note that in between complex 1–3, the asym-

lengthens the 1D chain to a 2D layer structure along the ac plane metric carbon atom in complex 3 is sited more out of plane from

(Fig. S4†).

the ve membered rings than that of other two. The dihedral

Structure of [Zn(L1)(NCS)]₂(3). Complex 3 crystallized in angle of 30.61^ꢁbetween the planes formed by N2Zn1O1 and

ꢀ

space group P1. The asymmetric unit of 3 contains mono- O1C19C14C13N2 suggests an out of plane displacement of

nuclear Zn(^II) unit bridged by ligand L1 having S conguration. Zn(^II) centre from envelope shaped six membered rings. In

The mononuclear unit is further bridged to another mono- between complex 1–3, centre in complex 3 zinc is situated more

nuclear Zn(^II) centre having R conguration of ligand L1 out of plane from the six membered rings than that of other two

through an O_Pbridge. Thus complex 3 crystalizes in centro- complexes. Since the coordination geometry around the Zn₂O₂

symmetric space group with the inversion centre located units in complexes 1–3 are same, hence they can be called as

directly between the two Zn(^II) ions. ORTEP diagram with the quasi-isostructural as the trans orientated anions are diﬀerent.

atom labeling scheme of centrosymmetric dimer having Zn₂O₂The Zn₂O₂unit in 3 is nearly planar rectangular shapes. The

unit is displayed in Fig. 7. In similar to complex 1 and 2, bond parameters around Zn₂O₂are: Zn–O distances of 1.981(2)

ꢁ

˚

complex 3 contains both R- and S-conguration of L1 resulting a and 2.105(2) A and corresponding bond angles of 78.65(1) and

heterochiral dimerization (RS) of ligand L1 around Zn(^II) centre 101.4(1)^ꢁ, implying that the O_Pbridge is unsymmetrical in

˚

in a chiral self-discriminating process. The coordination nature having the non-bonded contacts Zn/Zn, 3.162(1) A and

˚

geometry around the ve coordinated Zn(^II) centre is best O1/O2, 2.591(3) A. Among the coordination bonds, the trend

described as distorted trigonal bipyramid as inferred from the Zn–N_Y> Zn–N_I> Zn–N_Tin bond lengths was observable in 3 and

calculated s value of 0.628. In 3, the distorted trigonal bipyr- is similar to complex 2.

amid geometry around Zn(^II) centres are satised by two NNO-

The packing diagram of 3 displays several interesting struc-

donor set of the ligand L1 and two trans orientated thiocyanate tural arrangements with backing from diﬀerent non-covalent

ions. The trigonal planes are formed by O_PN_IO_Ndonor set with interactions. Weak C–H/p interaction of (C15–H15/C20 ¼

26

its angles lying in the range of 115.9(1)–123.9(1)^ꢁand the axial 3.786(5) A) and parallel displaced p/p interaction of (C5/

˚

N1–Zn1–O1 angles have a value of 161.6(1)^ꢁ. The cisoid angles C16 ¼ 3.384(4) A) between the coordinated thiocyanate ion and

˚

are lying in the range of 78.1(1)–123.9 (1)^ꢁand the trans angles ligand backbones extends the centrosymmetric dimer to a 1D

are in the range of 118.9(1)–161.6(1)^ꢁ. Ligand L1 is bound to the polymeric structure having the sequence /RS/RS/RS/

zinc centre by a ve-membered and a six-membered chelate ring where isotactic arrangement of the asymmetric carbon centres

and both rings are puckered, resulting in an envelope shape for occurs (Fig. 8). The aforementioned C–H/p and parallel

Fig. 6 Illustration of 1D chain formed by a localized C–H/p interaction between two adjacent heterochiral dimers present in 2.

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structures, where Zn₂O₂units are situated in linear chains along

the ac plane as shown in Fig. S5.†

Structure of [Zn(L2)(ONO₂)]₂(4). Complex 4 crystallized in

space group C2/c. The asymmetric unit of 4 contains mono-

nuclear Zn(^II) centre bridged by ligand L2 having R congura-

tion. The mononuclear unit is further bridged to another

mononuclear Zn(^II) centre having S conguration of ligand L2

through an O_Pbridge. Thus complex 4 crystalizes in the

centrosymmetric space group with the inversion centre located

directly between the two Zn(^II) ions. ORTEP diagram with the

atom labeling scheme of centrosymmetric dimer having Zn₂O₂

unit is displayed in Fig. 10. Within one molecule one of the

ligands possesses R conguration whereas the other possesses S

conguration resulting in a heterochiral dimerization of ligand

L2 around the Zn(^II) centre in a chiral self-discriminating

manner similar to complex 1–3. In 4, the coordination geometry

around the ve coordinated Zn(^II) centres are best described as a

nearly square pyramidal shape as inferred from the calculated s

value of 0.09. The apical site is occupied by an O_patom of the L2

whereas the basal positions are occupied by N_YN_IO_P-donor set

of L2 and O2 atom of the nitrate ion. It is worth mentioning that

on modication of the ligand backbone with a naphthalene

moiety, ligand L2 becomes nearly planar in nature, which is due

to the enhancement of rigidity upon ligand modication. The

cisoid angles are lying in the range of 80.3(1)–105.3(1)^ꢁand the

trans angles are in the range of 162.1(1)–167.7(1)^ꢁ. Ligand L2 is

bound to the zinc centre by a ve-membered and a six-

membered chelate ring and both rings are nearly planar in

shape. The asymmetric carbon atom deviates from the nearly

planar ve-membered chelate ring and the dihedral angle

between the two planes C5C6N2 and C5N1Zn1N2 is 5.26^ꢁ. The

dihedral angle of 15.00^ꢁbetween the planes formed by

N2Zn1O1 and O1C23C14C13N2 suggests that the Zn(^II) centre is

sited less out of plane comparable to complex 1–3 which is due

to the enhancement of ligand rigidity on modication. The

Zn₂O₂unit has a planar rectangular shape having Zn–O

Fig. 7 ORTEP diagram (30%) and atom labeling scheme of 2 (all the H-

atom except H6 are omitted for clarity).

displaced p/p interactions retains the centres of symmetry in

between two nearby heterochiral dimers, hence results in

another heterochiral dimerization through non-covalent inter-

actions in the chiral self-recognition process. Recently Scott

et al. have shown that the p/p stacking interaction can eﬀec-

tively control the diastereoselectivity in the formation of fac

tris(diimine) complexes of Fe(^II).^11bThe distance between two Zn

centres in nearby heterochiral dimers linked through the above

˚

non-covalent interaction is 7.325(5) A. Thus in a particular 1D

chain; two heterochiral dimerization occurs in Zn₂O₂unit

through the O_Pbridge and in the Zn₂(NCS)₂unit through C–H/

p and parallel displaced p/p interactions. It is noteworthy that

the aforementioned weak interactions are quite ample to retain

and transfer the stereochemical information of the heterochiral

dimers throughout the 1D chain. Similarly a C–H/S interaction

˚

of (C17–H17/S1 ¼ 3.784(4) A) between the trans coordinated

thiocyanate ion and ligand backbone extends the heterochiral

dimers to 1D polymer as shown in Fig. 9. In the lattice, collective

support from the above mentioned C–H/S, C–H/p and

parallel displaced p/p interactions results in 3D porous like

˚

distances of 2.121(3) and 2.020(2) A, implying that the O_Pbridge

is unsymmetrical in nature having the non-bonded contacts

Fig. 8 Illustration of 1D chain formed by C–H/p and parallel displaced p/p interactions between two adjacent heterochiral dimers with an

isotactic arrangement of the asymmetric carbon centres present in 3.

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Fig. 9 Illustration of the 1D chain formed by C–H/S interaction between two adjacent heterochiral dimers present in 3.

1D polymer (Fig. 12). In the lattice, the cooperative nature of the

aforementioned weak interactions results in a 3D staircase

structure down the b axis as shown in Fig. S6.†

Structure of [Zn(L2)(N₃)]₂(5). Complex 5 crystallized in space

group P21/n. The asymmetric unit of 5 contains mononuclear

Zn(^II) centre bridged by ligand L2 having R conguration. The

mononuclear unit is further bridged to another mononuclear

Zn(^II) centre having S conguration of ligand L2 through m-1,1-

azide bridge. Thus complex 5 crystalizes in a centrosymmetric

space group with the inversion centre located directly between

the two zinc ions in Zn₂(m-1,1-N_Z)₂unit. ORTEP diagram with

the atom labeling scheme of centrosymmetric dimer is dis-

played in Fig. 13. It is essential to note that although complex 2

and 5 are synthesized using same Zn(^II) ion and azide ion, the

former results in a centrosymmetric dimer through an O_P

bridge whereas latter results in the same through a m-1,1-azide

bridge which suggests that ligand modication has a substan-

tial controlling eﬀect on the coordination geometry around the

metal centres. Similar to complex 2, one molecule of complex 5

Fig. 10 ORTEP diagram (30%) and atom labeling scheme of 4 (all the

H-atoms except H6 are omitted for clarity).

˚

Zn/Zn, 3.09(1) A and O1/O1A, 2.76(1) A. Among the coordi- contains both R- and S-conguration of ligand L2 resulting a

nation bonds, the trend Zn–O_N> Zn–N_Y> Zn–N_Iin bond lengths heterochiral dimerization (RS) around Zn(^II) centre in a chiral

was observable which contradicts the bond order in complex self-discriminating process. The coordination geometry around

1–3 and which is attributable to the more planarity of ligand L2 the ve coordinated Zn(^II) centres are best described as nearly

than L1 upon ligand modication. A packing diagram of 4 square pyramidal geometry as inferred from the calculated s

shows several interesting structural arrangements through the value of 0.068. The apical site is occupied by N3A atom of the m-

aid of diﬀerent non-covalent interactions. It is noteworthy that, 1,1-azide ion whereas the basal positions are occupied by NNO-

in 4, ligand modication marks a possible additional parallel donor set of L2 and N3 atom of azide ion. It is worth mentioning

displaced p/p interaction which was absent earlier in its that in 5 the geometry around the zinc centre is nearly square

congener complex 1. Both C–H/O interaction of (C6–H6/ pyramidal whereas in its congener complex 2 it is a distorted

˚

O3 ¼ 3.298(5); C13–H13/O3 ¼ 3.246(5) A) and p/p interac- tbp geometry. This is due to ligand modication which induces

˚

tion of (C3/C20 ¼ 3.386(7) A) extends the centrosymmetric rigidity followed by planarity around the zinc centre and

dimer to 1D polymeric structure having the sequence /RS/ reduces the distortion around zinc centre. In 5, zinc centre is

RS/RS/ where isotactic arrangement of the asymmetric situated out of the square plane as evident from the bond

˚

carbon centres occurs similar to complex 1 and 3 (Fig. 11). The distance of 0.474 A between Zn(^II) ion and the square plane

Zn–Zn distance between two adjacent heterochiral dimers containing N1N2O1N3 atoms. The cisoid angles are lying in the

through C–H/O and parallel displaced p/p interactions is range of 78.7(1)–103.7(1)^ꢁand the trans angles are in the range

6.11(1) A. The aforementioned weak interactions retains the of 121.6(1)–155.0(1)^ꢁ. Ligand L2 is bound to the zinc centre by a

˚

stereochemical interlinks between two nearby heterochiral ve-membered and a six-membered chelate ring and both rings

dimer via centre of inversion. Thus in a particular dimer S are slightly puckered.

conguration of ligand L2 is linked to R conguration of L2 of

The asymmetric carbon atoms deviates from the ve-

the nearby dimer. In result the two centre of symmetry exits in membered chelate rings and dihedral angle between the two

the 1D chain; one in Zn₂O₂unit through the O_Pbridge and planes C5C6N2 and C5N1Zn1N2 being 11.68^ꢁ. The dihedral

another in Zn₂(NO₃)₂unit through weak interactions. Similar a angle of 14.37^ꢁbetween the planes formed by N2Zn1O1 and

˚

C–H/p interaction of (C2–H2/C10 ¼ 3.730(5) A) between O123C14C13N2 suggests an out of plane displacement of Zn(^II)

ligand backbones allow the extension of the dimer into a zig-zag ion from six membered rings. The Zn₂(N_Z)₂unit in 5 is nearly

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Fig. 11 Illustration of 1D chain formed by C–H/O and parallel displaced p/p interactions between two adjacent heterochiral dimers with an

isotactic arrangement of the asymmetric carbon centres present in 4.

planar rectangular shapes. The bond parameters around

Zn₂(N_Z)₂are: Zn–N_Zdistances (Zn1–N3 ¼ 2.172(3) and Zn1–

˚

N3A ¼ 2.066(3) A) and corresponding bond angles (Zn1–N3–

Zn2 ¼ 96.9(1)^ꢁand N3–O2–N3A ¼ 83.1(1)^ꢁ), imply that the O_P

bridge is unsymmetrical in nature having the non-bonded

˚

contacts of Zn/Zn, 3.173(6) A and N_Z/N_Z, 2.812(4) A. Among

the coordination bonds, the trend Zn–N_Z> < Zn–N_Y> Zn–N_Iin

bond lengths was observable in 5.

Packing diagram of 5 shows various interesting structural

representations through diﬀerent C–H/p interaction. Like

complex 1–4, complex 5 does not display any non-covalent

interactions associated with the co-ligands. Only a T-shaped²⁶

edge to face C–H/p interaction of (C11–H11/C13 ¼ 3.666(5)

Fig. 13 ORTEP diagram (30%) and atom labeling scheme of 5 (all the

H-atom except H6 are omitted for clarity).

˚

A) assembles the heterochiral dimers to 2D zigzag structures as

shown in Fig. 14. It is essential to note that the above T-shaped

C–H/p interactions are selective in communicating chirality in

a particular zigzag chain. Thorough investigation of the 2D

zigzag structure reveal that the above weak interactions are too

selective to interlink only R or S isomer of ligand L2 in a

particular chain. This means that the isomers are clearly

segregated to diﬀerent chains and packed alternatively to retain

the overall racemic nature of complex 5.

Structure of [Zn(L2)(NCS)(DMF)] (6). Complex 6 crystallized

in space group Pbca. The asymmetric unit of 6 contains

mononuclear Zn(^II) unit bridged by ligand L2 having S cong-

uration. ORTEP diagram with the atom labeling scheme of

complex 6 is displayed in Fig. 15. The coordination geometry

around the ve coordinated Zn(^II) centre is best described as

distorted trigonal bipyramid as inferred from the calculated s

value of 0.416. In 6, the distorted trigonal bipyramid geometry

around Zn(^II) centres are satised by one NNO-donor set of the

ligand L2, one terminal coordinated thiocyanate ion and one

DMF molecule. The coordination of DMF molecule satises the

coordination geometry around the Zn(^II) centre and does not

allow the O_Patom of L2 to bridge to another Zn(II) centre to form

the centrosymmetric dimer as found earlier in complexes 1–4.

The trigonal planes are formed by O₂N₂N₃donor set with its

angles lying in the range of 101.5(3)–136.5(2)^ꢁand the axial

angles N1–Zn1–O1 have a value of 161.5(2)^ꢁ. The cisoid angles

are lying in the range of 77.6(3)–101.5(3)^ꢁand the transoid

Fig. 12 Illustration of 1D chain formed by C–H/p interaction between two adjacent heterochiral dimers present in 4.

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Fig. 14 Packing diagram of complex 5 representing a 2D zigzag structure down the a-axis originating from diﬀerent non-covalent interactions.

In lattice of 6, O_Patom of L2 (having S conguration) is

˚

involved in C–H/O interaction of (C13–H13/O1 ¼ 3.11(6) A)

with nearby mononuclear Zn(^II) centre having R conguration of

ligand L2 in a chiral discrimination manner to result in a het-

erochiral dimer (Fig. 16). The formation of heterochiral dimer is

˚

also supported by C–H/N (C16–H16/N3 ¼ 3.62(1) A) and C–

˚

H/p (C12–H12/C24 ¼ 3.61(1) A) interactions. Also a C–H/S

˚

interaction of (C26–H26/S1 ¼ 3.76(1) A) results in heterochiral

dimerization as shown in Fig. 17. The aforementioned C–H/N

and C–H/p interactions self-assembled the mononuclear

Zn(^II) complexes in diastereoselective manner (having the

sequence /R/S/R/) to 1D chain as shown in Fig. 18. The

cooperative nature of the aforementioned non-covalent inter-

actions self-assemble the heterochiral dimers to a 3D zigzag

structure down the b-axis as shown in Fig. S7.†

On closely examining the packing diagram, it was found that

Fig. 15 ORTEP diagram (30%) and atom labeling scheme of 6 (all the

H-atoms except H6 are omitted for clarity).

the Zn(^II) centres are sited in zigzag fashion. Investigation of the

packing diagram of 6 reveals zigzag shaped structural arrange-

ments of terminal coordinated thiocyanate ion to zinc centre

angles are in the range of 121.4(3)–161.5(2)^ꢁ. Ligand L2 is bound

to the zinc centre by a ve-membered and a six-membered

chelate ring and both rings are puckered. The asymmetric

carbon atom deviates from the puckered ve-membered chelate

rings and dihedral angle between the two planes C5C6N2 and

C5N1Zn1N2 being 20.27^ꢁ. The dihedral angle of 16.14^ꢁbetween

the planes formed by N2Zn1O1 and O1C23C13C14N2 suggests

an out of plane displacement of Zn(^II) centre from envelope

shaped six membered rings. It is extremely important to note

that although complex 3 and 6 are prepared from same source

of Zn(^II) ion and thiocyanate ion, the asymmetric carbon in 6 is

sited more out of plane from the ve membered ring than 3.

This is due to the ligand modication and DMF coordination in

6 which enhances the rigidity of the structure and results in a

more puckered structure in comparison to 3. Among the coor-

dination bonds, the trend Zn–N_Y> Zn–N_I> Zn–N_Tin bond

lengths was observable in 6.

Fig. 16 Capped stick model representing the self-assembly of het-

erochiral dimer through non-covalent interaction in 6.

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complex 6 is mononuclear in nature. The versatility in change in

the coordination core can be rationalized in terms of ligand

modication. Complexes 1–4 crystalizes in centrosymmetric

space group with the inversion centre located directly between

the two zinc ions in each Zn₂O₂unit whereas in 5 the inversion

centre located in Zn₂X₂unit (X ¼ azide ion). Within one mole-

cule one of the ligands possesses R conguration whereas other

possesses S conguration resulting in heterochiral dimerization

of ligands L1 and L2 around Zn(^II) centre in chiral self-

discriminating manner. Although the mechanism of such self-

assembly is not clear, the observed heterochiral self-assembly

might be the stable product over the homochiral self-assembly

in the solid state. Since the ligands used are racemic in nature,

overall all the complexes are racemic in nature.

Fig. 17 Capped stick model representing the self-assembly of heter-

ochiral dimer through C–H/S interaction in 6.

The aforementioned solid state heterochiral diaster-

down the a-axis where only zinc centres are positioned in eoselective self-assembly of Zn(^II) complexes can be rationalized

U-shaped as illustrated in Fig. 19. On comparing complexes 4–6, by the orientation of the phenyl ring attached to asymmetric

it was found that although they are prepared from the same carbon atom (C6). When two monomeric units (R and S isomer

ligand L2 and same Zn(^II) ion, they are not quasi-isostructural of L1(L2)) are brought together to form a dimeric [Zn(L1/L2)(X)]₂

like complex 1–3. This is due to the mutual eﬀect of ligand molecule where X ¼ NO₃^ꢀ, N₃^ꢀ, NCS^ꢀions, the chiral carbon

modication and non-covalent interactions on the structural centres hold the pendant phenyl ring in such a way that it

self-assembly.

minimizes the steric interaction between phenyl ring and

terminal coordinating X^ꢀions (Scheme 3). Thus results in a

stable heterochiral dimerization of the ligands around the zinc

centres. It is extremely important to mention that trans coor-

dinated X^ꢀions forces the pendant phenyl ring to remain in the

trans position opposite to the X^ꢀions and drives towards het-

erochiral dimerization of the ligands. The situation is very

complicated as one can expect for the formation of homochiral

dimeric complex. In such cases the chiral carbon atoms orient

the phenyl ring on the same side of the X^ꢀions which increases

the steric interactions between the phenyl ring and X^ꢀions and

results in unstable homochiral complexes. Similarly in the case

of complex 5 heterochiral dimerization of ligand L2 is preferred

over homochiral dimerization due to steric congestion.

However from the above mechanism one can expect that

homochiral dimerization may be possible in two ways: (i) if the

bridging linker or anions (X^ꢀions) are cis directing i.e. the

ligand should be exible and should coordinate to the metal

centre in a facial manner not in a meridional manner such as

that there will be enough space for the bridging ligands to bind

Eﬀect non-covalent interactions on the diastereoselective

solid state structural self-assembly

Herein, the eﬀect of diﬀerent non-covalent interactions on the

solid state diastereoselective self-assembly of the six coordina-

tion complexes of Zn(^II) and racemic Schiﬀ bases have been

studied. The aforementioned noteworthy results clearly indicate

that the eﬀect of ligand modication also has a signicant

controlling eﬀect on the nal structural arrangements through

cooperative non-covalent interactions. Mostly the diﬀerence in

structural arrangement can be explained by the changing of the

steric environment of the ligands, but the role of the diﬀerent

terminal anions should not be neglected. It is extremely inter-

esting to compare the coordination core between complex 1–3

and 4–6. The coordination core of 1–3 contains similar rectan-

gular Zn₂O₂unit whereas 4–6 are diﬀerent i.e. complex 4

contains rectangular Zn₂O₂unit similar to complex 1–3,

complex 5 contains rectangular Zn₂X₂unit (X ¼ azide ions) and

Fig. 18 Capped stick model representing the self-assembly of mononuclear Zn(II) complex to 1D chain in diastereoselective manner found in 6.

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Fig. 19 Illustration of 3D zigzag structure representing U-shaped arrangements of zinc centres in complex 6 down the a-axis.

to the metal centre in cis manner^15c(ii) if the bridging ligands

Photophysical study of ligand L1H, L2H and complex 1–6

hold the metal centres in an asymmetric way, there may the

possibility of homochiral dimerization over heterochiral

dimerization to minimise the steric congestion.^15bBased on the

above results it can be proposed that the most important factors

dictating the homo- or heterochiral dimerization of the racemic

Schiﬀ bases are the steric properties of the ligands, which allows

the binding of the ligand in such a way that it minimizes the

steric congestion around the metal centres. The crystal packing

pattern of complexes 1–6 are analyzed in terms of C–H/O,

C–H/N, C–H/S, C–H/p and p/p interactions. It was found

that the resulting heterochiral dimers are linked through coop-

erative non-covalent interactions with the nearby dimer to

extend the structure to network. Thus the eﬀect of diﬀerent non-

covalent interactions to assemble the heterochiral dimers to 1D

chain having the sequences /RS/RS/RS/ or /RS/SR/

RS/ or /R/S/R/ and to transfer the stereochemical infor-

mation throughout the chain enhances the worthiness of a

crystal engineering approach in coordination chemistry. The

presence of isotactic and syndiotactic arrangements of coordi-

nation complex adds new examples to the Puddephatt concept of

“supramolecular chirality” in coordination complexes.

The combination of d¹⁰metal centres and Schiﬀ base ligand

systems, have been recognized for exhibiting diverse uores-

cence properties and are used for potential applications such as

light-emitting diodes (LEDs).^6fIn this regard the luminescent

behavior of free ligands L1H, L2H and complexes 1–6 were

studied at room temperature in the solid state. As shown in

Fig. 20, intense emission bands of free L1H was observed at

about 544 nm (l_ex¼ 358 nm) resulting from the p–p* or n–p*

transitions. On complexion of L1H with the Zn(^II) ion, the

emission peak occurs at 422 and 549 nm (l_ex¼ 358 nm) for

complex 1. Upon anion metathesis, the emission peak exhibited

by complex 1 are shied to 469 nm (2), 417 and 529 nm (3) upon

same excitation at 358 nm. It can presume that these emissions

are neither ligand-to-ligand charge transfer (LLCT) nor metal-

to-ligand charge transfer (MLCT) in nature. Therefore, the

emission of 1–3 may be ascribed to a charge transfer transition

between ligands and metal centres, namely ligand-to-metal

charge transfer (LMCT).²⁷Compared to the emission spectrum

of free L1H, complexes 1–3 showed much more intense emis-

sion compared with free L1H ligand, which may be attributed to

Scheme 3 Plausible mechanism of chiral self-discrimination over chiral self-recognition of racemic Schiﬀ bases around a Zn₂O₂core.

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Fig. 20 Solid state photoluminescence spectra of L1H, L2H and complexes 1–6 at room temperature.

the chelation of the ligand to the metal centre. The chelation the cooperative association of coordination interactions as well

enhances the “rigidity” of the ligand and thus reduces the loss as non-covalent interactions which imparts the eﬀect of crystal

of energy through a radiationless pathway.²⁸It is worth noting engineering on the photoluminescence properties.

that the resultant blue shi of around 80 nm for 2 and of

around 20 nm for 3 in comparison to complex 1 demonstrates

the eﬀect of anion metathesis on photo-luminescence proper-

Conclusion

ties. These could be probably be due to the increased and Overall, the eﬀect of ligand modication, anions and diﬀerent

decreased energy gaps between the highest occupied molecular non-covalent interactions on the diastereoselective self-

orbital (HOMO) and the lowest unoccupied molecular orbital assembly of the six coordination complexes of Zn(^II) and

(LUMO) of the ligands when coordinating to the metal ions. racemic Schiﬀ bases have been studied. Complexes 1–3 are

Although complexes 4–6 are prepared similarly using ligand quasi-isostructural in nature whereas 4–6 are diﬀerent from

L2H, the emission prole is not similar to complex 1–3. As each other in terms of coordination environment. In complex

shown in Fig. 20, intense emission bands of free L2H was 1–4, the inversion centre located directly between the Zn₂O₂

observed at about 475 nm (l_ex¼ 390 nm) resulting from the unit whereas in 5 the same is located in Zn₂X₂unit (X ¼ azide

p–p* or n–p* transitions. On complexion of L2H with the ion). Within one unit one the ligands possess R conguration

Zn(^II) atom, the emission peak occurs at 465 and 513 nm (l_ex

390 nm) for complex 4. Upon anion metathesis, the emission chiral dimerization of ligands around the Zn(^II) centre in chiral

peaks exhibited by complex 4 are shied to 482 nm (5), 458 and self-discriminating manner. The plausible mechanism for the

¼

whereas the other possess S conguration resulting in hetero-

513 nm (6) upon same excitation at 390 nm.

above chiral self-discrimination process is established which

Similar to complex 1–3, the emissions of complex 4–6 are makes it clear that trans oriented anions direct the phenyl group

ligand-to-metal charge transfer (LMCT) in nature. It is worth appended to the asymmetric carbon atoms to be trans due to

noting that the resultant red shi of ꢂ40 nm for complexes 4 steric congestion and results in heterochiral dimerization. The

and 6 and of ꢂ7 nm for 5 in comparison to free ligand L2H eﬀect of diﬀerent non-covalent interactions to assemble the

could be probably be due to the ligand bridging eﬀect upon heterochiral dimers to 1D chain having the sequences /RS/

metal coordination, which can eﬀectively modify the rigidity RS/RS// or /RS/SR/RS/ or /R/S/R/ and to transfer

and conjugation, and then reduce the energy loss by radiation the stereochemical information throughout the chain enhances

less thermal vibrations. It is immense important that in the worthiness of crystal engineering strategies in coordination

complex 1–3, the emission bands are blue shied whereas in chemistry. Detailed isotactic and syndiotactic arrangements of

4–6 they are red shied in comparison to the emission peaks of the zinc coordination complexes through non-covalent inter-

L1H and L2H respectively which could be due to the collective actions are studied. Solid state photoluminescence properties

eﬀect of anion metathesis^29a–cand ligand modication^29don of free ligands as well as six zinc complexes are studied. In

photo-luminescence properties. Apparently, the emission complex 1–3 the emission bands are blue shied whereas in 4–6

spectra of complexes 1–6 resemble those of ligands origin they are red shied in comparison to the emission peaks of free

except for the emission intensity and peak positions, indicating ligands which could be due to the collective eﬀect of anion

that the uorescence of complexes 1–6 are ligand based emis- metathesis and ligand modication on photo-luminescence

sion. Thus, the enhanced uorescence intensities of the six properties. The aforesaid conclusive results suggests that

complexes are detected, which shows that they are good supramolecular self-assembly of such systems is a process that

candidate materials for diﬀerent photochemical applications. is very sensitive not only to the chirality but also to the degree of

Moreover, the diﬀerence in the uorescent intensities of the substitution, bridging linker and non-covalent interaction as

complexes 1–6 evidences that there are immense conforma- well. Thus all of these criteria need to be considered in the

tional rigidity for 3D supramolecular networks constructed by future design of similar racemic ligands in order to aﬀect the

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stereo-selectivity of the self-assembly. Further work on the dia-

stereoselective self-assembly of exible racemic ligands and

diﬀerent bridging linkers are in progress.

8 (a) Y. Shimazaki, M. Takani and O. Yamauchi, Dalton Trans.,

2009, 7854; (b) S. Roy, S. Saha, R. Majumdar, R. R. Dighe and

A. R. Chakravarty, Inorg. Chem., 2009, 48, 9501; (c)

U. Schatzschneider, Eur. J. Inorg. Chem., 2010, 1451.

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(b) I. Ojima, Catalytic Asymmetric Synthesis, VCH,

Weinheim, 1993; (c) U. Knof and A. von Zelewsky, Angew.

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Acknowledgements

HSJ is grateful to the Department of Science and Technology

(DST), New Delhi, for establishing a single crystal X-ray

diﬀractometer facility under the FIST scheme, IIT Guwahati for

infrastructural support. HSJ also thanks Dr Sanjit Konar and

IISER Bhopal for current Post-Doctoral Fellowship.

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3044 | RSC Adv., 2014, 4, 3028–3044

Article Doi

Diastereoselective self-assembly of heterochiral Zn(ii) complexes of racemic Schiff bases in a chiral self-discriminating process: Effect of non-covalent interactions on solid state structural self-assembly

DOI: 10.1039/c3ra45082a

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Article abstract of DOI:10.1039/c3ra45082a

Full text of DOI:10.1039/c3ra45082a

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