Indian Journal of C he mi stry Vol. 44A. December 2005. pp. 2427--2432

Synthesis, structure and luminescence behaviour of zinc(II) complexes containing hexadentate N-donor Schiff bases

Sk Hafijur Rahaman" *. Rajarshi Ghosh', Tian-Huey Lub & Barindra Kumar Ghosh" * "Department of C he mi stry, The Univers ity o f Burdwan, Burdwan 713 104, Indi a

bDepartme nt of Physics, National T sing Hua University , Hsinchu 300, Taiwan, ROC Email: hafijur_am @yahoo.co.in ; barin_ l @yahoo.co. uk

Recei ved 29 Jllll e 2005: accepled 19 DC/ober 2005

Six hexacoord inated zinc(l l) complexes of the type [Zn(L»)(Y)2 [L = N-( I-pyridin-2-yl-formylidene)-N'-[2-({2-[( I­pyridin-2-y lformylidene)amino)ethylj amino)ethyl)ethane- I ,2-diamine (pfad), Y = CI04 ( la), Y = PF6 (lb); L = N-( l-pyridin-2-ylmethylidene)-N'-[2-( {2-I( I -pyridin-2-y lmethylidene)amino)e thylj amino)ethyl)ethane- I ,2-diamine (pmad), Y = CI04- (2a), Y = PF6' (2b); L = N-( l -pyridin-2-ylbenzy lidene)-N'-[2-( {2-[( l -pyridin-2-ylbenzylidene)amino]-ethylj amino )ethyl]ethane- I ,2-diamine (pbad), Y = CI04' (3a), Y = PF6' (3b)] have been synthesised and characterized on the basis of microanalytical, spectroscopic and other physicochemical propenies. Structural analys is of 3a reveals that zinc(ll) adopts a di storted octahedral environment with a ZnN6 chromophore ligated by two pyridine N atoms (N I, N I '), two imine N atoms (N2, N2' ) and two amine N atoms (N3, N3') in ciS-lrallS-cis orientati on. Spectral properties show that the other complex ions are iso-structu ra l with the dication in 3a.

IPC Code: Int. CI.7 C07C 25 1/02 ; C07F 3/06

Research on design and synthesis of mono- and polynuclear coordination compounds of Group 12 metals ions 1·7 continues unabated for the preparation of functional matelial s8

.io. Exploiting the veracity of

coordination geometry around metal ion templ ates different networks can be accessed using valied organic blockers. Self-assembl/ I is the most efficient approach towards preparation of such materi als. We are also interested in this fi e ld through variation of metal ion coordination environments using manganese(Il)12, nickel(lI )1 3, copper(I1)1 4. 15, zinc(II)2.3

and cadmium(lL)5a.b, organic li gands and suitable bridging units. Schiff bases l6 are useful chelators because of the ir preparational accessibilities, structural varieti es and varied dentic ities . We have isol ated six hexacoordinated mononucl ear zinc(lI) complex of the type [Zn(L)](Yh [L = N-( l-pyridin-2-ylfonnylidene)­N'-l2-( \2-l( l -pyridin-2-ylformylidene) amino ]ethyl} ­amino)elhyl]-ethane-I ,2-diamine (pfad), Y = CI04 (la), Y = PF6 (lb); L = N-( l -pyridin-2-ylmethylidene)-N'­[2-( \2-[( l-pyridin-2-ylmethylidene)amino ]ethyl} -amin 0)ethyl]ethane-I ,2-diamine (pmad), Y = CI04' (2a), Y = PF6' (2b) ; L = N-( I-pyridin-2-ylbenzylidene)-N '- [2-(\2-r( l-pyridin-2-ylbenzylidene)amino ]-ethyl) -amino) ethyl]ethane-I ,2-diamine (pbad), Y = CI04- (3a), Y = PF6- (3b)] (Scheme I). X-ray structure determination of one representative member, 3a shows that the metal

centre is in a distorted octahedral environment coordinated by six N atoms of the Schiff base. The details of synthesis, spectra, structure and luminescence behaviour of the complexes are described here.

Materials and Methods Hi gh purity pyridine-2-carboxaldehyde (Lancaster,

UK), 2-acetylpyridine (Lancaster, UK), 2-benzoyl pyridine (Lancaster, UK), triethylenetetramjne (Lancaster, UK), potass ium hexafluorophosphate (Fluka, Germany), zinc nitrate tetrahydrate (E. Merck, India) were purchased from respective concerns and used as received. Zinc perchlorate hexahydrate was

H H

R = H, pfad; R = Me, pmad; R = Ph, pbad

Scheme 1

2428 INDIA N J CHEM, SEC A, DECEMBER 2005

prepared on treatment of zinc carbonate (E. Merck, India) with perchloric acid (E. Merck, India) followed by slow evaporation on steam-bath, filtration through a fi ne glass-frit, and was preserved in a desiccator containing concentrated sulphuric acid for subsequent use. All other chemicals and solvents were of AR grade and were used as received.

Cautioll! Perchlorate compounds of metal ions are potentially explosive especially in presence of organic ligands. Only a small amount of material should be prepared and handled with care.

Elemental analyses (carbon, hydrogen and ni trogen) were performed on a Perkin-Elmer 2400 CHNS/O elemental analyzer. IR spectra (KBr discs, 4000-300 cm-I) were recorded using a Jasco FTIR model 420 spectrometer. Molar conductances were r. easured using a Systronics conductivity meter where the cell constant was calibrated with 0.01(M) KC I solution and dry MeOH was used as solvent. Ground state absorption and steady-state fluorescence measurements were made with a Jasco model V-530 UV -Vis spectrophotometer and Hitachi model F-40 I 0 . pectrofl uorimeter, respectively . Time-resolved fluorescence measurements were carried out using a time-correlated single photon counting (TCSPC) spectrometer Edi nburgh Instruments, model 199; a hydrogen filled coaxial fl ash lamp with a pulse width of 1.2 ns at FWHM and a Ph ilips XP-2020Q Photom ultiplier tube were respectively used as the, excitation source and the fl uorescence detector as described elsewhere2

.

Preparation of SchilT bases The Schiff bases pfad, pmad and pbad were

prepared '317 fo llowing reported method with a little modification . Details with pfad is describe: Pyridine-2-carboxaldehyde (0.214 g, 2 mmol) was refluxed with triethylenetetramine (0.146 g, 1 mIllol) in dehydrated alcohol. After 10 h, the reaction solution was evaporated under reduced pressure to yield a gummy mass, which was dried and stored in vaCllO over CaCI2

for subsequent use. Yi eld, 0.2 12 g (80%). Pmad/pbad was prepared si milarly usi ng 1:2 molar ratio of triethylenetetramine (0.146 g, 1 mmol) and 2-acetylpyridine (0.242 g, 2 mmol)/2-benzoylpyridine (0.366 g, 2 mmol) instead of pyridine-2-carboxaldehyde. Found: C, 66.82; H, 7.63; N, 25 .69; Calc. CI 8H24N6 (pfad): C, 66.63; H, 7.45; N, 25.90%. IR (KEr, em-I): 1590 (VC=N) ' UV-Vis (A..,,,, •. nm): 245 , 390. Found: C, 68.30; H, 8. 15 ; N, 23.95; Calc. C2oH2SN6 (pmad): C, 68 .14; H, 8.01 ; N, 23 .84%. IR

(KBr, cm' I): 1592 (VC=N). UV-Vis (A.,nu, nm): 242, 390. Found: C, 75.38; H, 6.56; N, 17.85; Calc. C30H32N6 (pbad): C, 75 .59; H, 6.76; N, 17.63%. 1R (KBr, cm' I): 1592 (VC=N). UV -Vis (A.,TIaX. nm): 240, 390.

Preparation of the complexes

Mononuclear complexes l a-3a were prepared from perchlorate sal t of zinc(II) using 1: 1 mole ratio of the metal and pfad/pmad/pbad. Complexes Ib-3b were prepared using a 1: 1:2 ratio of zinc(JI) nitrate, pfad/pmad/pbad and potassium hexafl uorophosphate; these were also isolated by metathesis of respectively l a , 2a and 3a with potassium hexafluorophosphate. Typical syntheses are described below. Synthesis oj [Zn(pjad)}(CI0412 (la)

A methanolic sol ution (5 ml ) of pfad (0.32 g, mmol) was added dropwise to a solution of Zn(Cl0 4h 6H20 (0.37 g, 1 mmol) in the same solvent (10 ml). The light yellow solution was fil tered and the supernatant liquid was kept in air for slo\o\­evaporation. After a few days, the complex la that separated out was washed wi th toluene and dried in vaCllO over silica gel indicator. Yield, 0.33 g (70%) . [Zn(pmad)](C10 4h (2a) and [Zn(pbad)](CI0 4)2 (3a) were prepared simi larly using pmad and pbad respectively instead of pfad. Yie ld, (70-72%). Found: C, 36.49; H, 3.58; N, 14.63: Calc. CI sH24N60gChZn (la): C, 36.63; H, 3.45 ; N, 14.49%. IR (KBr, cm-I):

1595 (VC=N), 1080, 620 (VCl04)' UV,.yis (A.n", •. nm):

328. Found: C, 39.46; H, 4.82; N, 14.32; Calc. C2oH2sN60 gCI2Zn (2a): C, 39.30; H, 4 .65 ; N, 14.00%. IR (KEr. cm' I): 1590 (VC=N), 1085, 622 (VCI04)' UV­

Vis (A.ma •. nm): 330. Fou nd: C, 48.75; H, 4.38; N, 11.76; Calc. C10H32N60 SChZn (3a): C, 48.60; H, 4.25; N, 11.50%. IR (KBr, cm-I): 1595 (VC=N), 1082, 620 (VCl04) ' UV -Vis (A.ma •. nm): 328.

Synthesis of[ ZI/(pj ud)](PF6h (lb)

Zn(N03h-4H20 (0.24 g, 1 mmol) was dissolved in MeOH (10 ml). To th is pfad (0.32 g, I mmol) dissolved in same solvent (5 ml) was added slowly followed by KPF6 (0.36 g. 2 mmol) in H20 (5 m!). The fi nal solution was fil tered, kept for slow evaporation and processed as in la to yield pure Ib; yield, 0.38 g (75%). Using pmad and pbad respectively instead of pfad yielded [Zn(pmad)](PF6h (2b) and [Zn(pbad)](PF6h (3b); Yield, (70-75%). lb. 2b and 3b were prepared in better yield (-80%) by metathesis of la, 2a and 3a, respectively using 1 mole of the former and 2 moles of KPF6 aqueous methanolic solution. Found: C, 52.44; H, 3.81 ; N,

RAHAMAN el at.: SCHIFF BASE COMPLEXES OF Zn(lI) 2429

10.54; Calc. C I8H24NoFI2P2Zn (lb): C, 52.30; H, 3.78; N, 10.41 %. IR (KBr, cm' I): 1590 (VC=N) 842, 540 (VPF6). UV -Vis (Amax. nm): 328. Found: C, 34.32; H, 4. 16; N, 10.92; Calc, C2oH28N6FI2P2Zn (2b): C, 34.13; H, 3.98; N, 10.34%. IR (KBr, cm' I) : 1590 (VC=N), 840, 542 (vPF6). UV-Vis (Amax. nm): 329. Found: C, 43.73; H, 3.87; N, 10.14; Calc. C30H32N6F I2P2Zn (3b): C, 43 .50; H, 3.89; N, 10.17%. IR (KBr, cm' I): 1592

(VC=N), 840, 540 (vPF6)' UV -Vis (Am.". nm): 330.

X-ray ditf raction study

Single crystals [size: 0.20 x 0.20 x 0.40 mm3] of 3a

were obtained by slow evaporation of MeOH solution of the reaction mixture. Colourless crystals sui table for X-ray crystallographic analysis were selected following examination under a microscope. Diffraction data 296(2) K were collected on a Bruker­SMART CCD diffractometer using MoKa radiation (A = 0.71073 A). Systematic absence led to the identification of space groups Pccn for 3a. Of the 7895 unique reflections, 3295 with 1>2cr(l) were used for structure solutions. The structure was solved by direct methods, and the structure solution and refinement were based on 1F12. All non-hydrogen atoms were refined with anisotropic displacement parameters whereas hydrogen atoms were placed in calculated positions when possible and given isotropic U values 1.2 times that of the atom to which they are bonded. At convergence, the final residuals were R 1 = 0.0676; wR2 = 0.1657 with 1>2cr(I), goodness-of-fit = 1.030. The final differences Fourier map showed the maximum and minimum peak heights at 0.912 and -0.331 ek3 with no chemical significance. All calculations were carried out using SHELXL-97' 8, ORTEP-3i9

• The crystal data and data collection parameters are listed in Table I .

Results and Discussion The ligands (pfad/pmad/pbad) were synthesised by

refluxing triethylenetetramine and pYlidine-2-carboxaldehyde/2-acetylpyridine/2-benzoylpyridine in 1:2 mole ratio in boiling alcohol. Colourless powders of the hexacoordinated mononuclear complexes [Zn(L)](Yh [L = N-( I-pyridin-2-ylformylidene)-N'-[2-({2-[(l-pyridin-2-ylformylidene)amino] ethyl }amino) ethyl] ethane-l ,2-diamine (pfad), Y = CI04 (la), Y = PF6(lb); L = N-(1-pyridin-2-ylmethylidene)-N'-[2-({2-[( l-pyridin-2-ylmethylidene)amino ]ethyl} amino )ethyl] ethane-I,2- diamine (pmad), Y = Cl04' (2a), Y = PF6' (2b); L = N-(I-pyridin-2-ylbenzylidene)-N'-[2-({2-[(1-

Table I-Summarized crystallographic data for [Zn(pbad)](CI04h (3a)

Empirical form ula Formula weight Temperature (K) Wavei<:ngth (A) Crystal system Space group D( calc) [g/cm'] Volume (A3

)

Z Unit cell dimensions a (A) b(A) c (A) a(O), r:W), yeO) F(OOO) Crystal size (mm3

)

I!(Mo-Ka) (mm'l)

C30H~608C I2Zn 738.87 296 0.7 1073 Orthorhombic Pccn 1.501 6538. 1 (6) 8 18.7302( I I) 19.5747(11) 17.8326( 10) 90,90,90 3040 0.20 x 0.20 x 0.40 0.974 1.5 to 28.3 40797

8 range for data collection (0) Reflections collected Independent reflections Max. and min. transmi ssion Refi nement method

7895[R(int) = 0.074] 0.823 and 0.677 Ful l-matri x least-squares on F2

Unique Data/restraints/parameters

Goodness-of-fi t on F2 Final R indices [[>20'(1)]

Largest diff. Peak and hole (ek')

7895/2/425 0.931

RI = 0.0676, wR2 = 0.1657 -0.33 and 0.91

pyridin-2-ylbenzylidene)amino ]ethyl } amino )elhyl] eth ane-I,2-diamine (pbad), Y = CI04' (3a), Y = PF6' (3b)] resulted in good yield through single-pot reaction of a I : I molar ratio of the metal perchlorate and organic blocker for la-3a in methanolic solution or a I : I :2 molar ratio of metal nitrate, L and KPF6 from methanolic aqueous solutions for 1b-3b. The latter were also isolated by metathesis of la, 2a and 3a respectively with potassium hexafluorophosphate. The synthetic procedures are summarised in Eqs (1-3):

MeOH Zn(CI04h,6H20 + L~ [Zn(L )](CI04h (la-3a)

298 K

MeOH . .. (I)

Zn(N03h.4H20 + L + 2KPF6 ~ [Zn(L)](PF6h (lb-3b) 298 K .. . (2)

MeOH [Zn(L)](CI04) 2 (la-3a) ~ [Zn(L)](PF6h (lb-3b)

2KPF6 (3)

The complexes were characterized using microanalytical, spectroscopic and physicochemical results. The air-stable moisture-insensitive complexes

2430 INDIAN J CHEM , SEC A, DECEMBER 2005

C2

Fig. I-DRTEP representation of [Zn(pbad)](CI04)2 (3a) with atom numbering scheme and 20% probability ellipsoids for all non-hydrogen atoms.

are soluble in common solvents like methanol, ethanol, acetonitrile, dimethylformamide and dimethylsulfoxide. [n methanol soluti on they behave20

1:2 electrolytes as indicated by thei r conductivity values. The presence of ionic perchlorate bands at - [090 and -620 cm-I for la-3a and ionic hexaflu orophosphate bands -840 and - 540 cm-I for Ib-3b are noticed21 refl ecting counter anionic view with no metal coordinate. The v(C=N) stretching vibrat ions of the Schiff base are seen at 1629 and 1589 cm-I

. All other characteristic L vibrations are seen in 1600-600 cm-I

. The spectra in nuj ol (e.g. 3a: A, 330 nm) and in MeOH (e.g. 3a: A, 328 nm) solutions are akin reflecting simil ar gross structure and elec tronic structure in solid state and in solution:!:!.

X-ray crystal structure of [Zn(pbad)](CI04h (3a)

In order to define the coordination sphere conspicuously, single-crys tal X-ray diffraction study was made. An ORTEP diagram with atom numbering scheme of the mononuclear unit in 3a is shown Fig. I. Selected bond distances and bond angles relevant to the Zn coordination sphere are given in Table 2. The crys tal lattice of 3a consists of [Zn(pbad)]"+ cations and CI04- ani ons. The coordination po lyhedron around zinc is best described as di storted octahedron with ZnN6 chromophore. The di storti on from ideal octahedral geometry is due to the asy mmetric nature of the bound hexadentate Schi ff base and the deviati ons of the refine angles form ed at the metal center (Table 2). The metal ion is li gated by two pyridine nitrogens (N I. N I *>. two imine nitrogens (N2, N2*) and two amine nitrogens (N3, N3*) in c is­

frans-c is ori en tati on. Two amine nitrogens (N3, N3*) and two pyridine nitrogens (N I, N 1*) "occupy the eq uatorial positions of the di storted octahedron ,

Fig. 2-Pack ing diagram of 3a involving 11:. . . r, interacti ons.

Table 2-Selected bond distances (A) and angles (0) for 3a

Bond di stances

Zn( 1 )-N(l) 2. 178(4) Zn( 1 )-N( I )' 2. 178(4) Zn( I )-N(2) 2.1 62(3) Zn( I )-N(2) ' 2.162(3) Zn( I )-N(3) 2. 187(4) Zn( I )-N(3) ' 2.1 87(4)

Bond ang les

N( I )-Zn( I )-N(2) 98.04( 13) N(2)-Zn( I )-;-.J(3) 77.86( 13) N( I )-Zn( I )-N(3) 100.4 1( 13) N( I )*-Zn( I )-01(2) 74.72( 13)

N( 1 )-Zn( I )-N( I ) 92.3 1 ( I 3) N(2)-Zn( 1)- (2)' 169.76(14) N( I )-Zn( I )-N(2J' 74.72( 13) N(2)-Zn( I )-N(3) ' 110.26( 13) N( I )-Zn( I )-N(3) ' 15 1.11 ( 13) N( I a)-Zn( I )-N(J) 151.1 1( 13) N(2)'-Zn( I )-N(3) 11 0.26( 13) N(3)-Zn( 1)- :(3)' 80.66( 13) N( I )-Zn( I )-N(2)' 98.04(13) N(2) * -Zn( 1)- N(3 >' 77.86( 13) . N( I a) -ZI1 ( I )-N(3) 100.41 ( 13)

Sy mmetry code: ' 1/2-x, 1/2-y. z

whereas the axial positions are occupied by the two imine nitrogens (N2, N2*>. The equatorial. Zn-N di stances li e within a close raIige [2.187(4)-2. 178(4) A), but there is a significant vari ation in the bond angles [100.41 ( 13)°-80.66(1 3)"] in that plane. The ax ial Zn-N distances [2. 162(3) A) are fo und to be small er th an the equatorial ones. Her, tetragonal compress ion of the octahedral geometry may be raised from the n-acceptance of imine nitrogen atoms. The axial bond angle N2-Zn-N4 [169.76( 14t J

RAHAMAN et al.: SCHIFF BASE COMPLEXES OF Zn(ll) 2431

Table 3--PholOphysical data

Sample Absorption Emission (A/n m) Lifetime (ns)

(A/nm) Fluorescence" Phosphorescenceh

1a 328 392 470 2.44 Ib 328 392 470 2.46 2a 330 390 470 2.42 2b 329 390 470 2.44 3a 328 395 470 2.42 3b 328 395 470 2.42

"In MeOH at roo m temperat ure (298 K) "In MeOH at 77 K.

deviates from the ideal 180". The sum (373.80°) of the equatorial angles NI-Znl-N3 (lOO.4n, NI-Znl-NI * (92.3 1°), Nl*-Znl-N3* (lOO.4n and N3-Znl-N3* (80.66) are very close to 360.00°. So, the atoms N I, N 1*, N3*, N3 and Zn 1 are almost in a same plane.

The mononuclear units pack alongs ide each other through face-to-face n-interactions to give a 20 supramolecular sheet (Fi g. 2) in ab plane. The terminal phenyl rings [Ring(7)-Ring(7) and Ring( 14)­Ring( 14)]: Cg-Cg separati on: 4.449(3/ and 5.042(4)ii A, vertical di splacement of Cg: 3.80 I and 3.448 A; dihedral angle 0.03 and 0.0"; sy mmetry code: i_x,_y,l_ z and ii_X, 1-y, I-z; Cg(7) = C(7)-C(8)-C(9)-C( 10)­C(II)-C(l2) and Cg(14) = C(22)-C(23)-C(24)-C(25)­C(26)-C(27)] are responsible for thi s to produce such novel archi tecture [2,5b].

Luminescence properties

The spectroscopic data in methanol solutions and glasses are li sted in Tabl e 3. The co mpl exes show emi ss ion spectra at around 395 nm at 298 K. These are ass ignable to intra ligand I(n-n* ) flu orescence2.5.2.1 .

The li fetimes are in the range 2.40-2.46 ns. The reasonabl y hi gher lifetime may be due to the presence of stronger n ... n interactions (F ig. 2) as is revealed in crysta lline state in 3a stabilizing the excimeric structure. In glassy so luti ons (77 K) a red shift is observable at -470 nm which is presumabl y due to \ n- n *) phosphorescence.

Conclusion Thi s work reports the sy nthes is, structure and

photophysical behaviour of a set o f zinc( lI ) complexes with hexacoordi nati on environment of the metal centre . The tetracoordinated/pentacoordin ated zinc(ll ) is well recogni sed24

, but hexacoordination of the metal ion is scarce. Under enfo rced conditi on the neutral N-donor hexacoord inated Schiff base directs such coordination of thi s 3d member of Group 12.

Supplementary data Crystallographic data for the structural analysis

have been deposited with the Cambridge Crystallographic Data Centre No. 258488 for 3a. Copies of this information can be had free of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2 IEZ, UK (fax: +44-1223-336033; e­mail: deposit @ccdc.cam.ac.uk or www: http:// www.ccdc.cam.ac.uk) .

Acknowledgement Financial support from the Department of Science

and Technology (DST), Council of Scientific and Industrial Research (CSIR), New Delhi , India is gratefully acknow ledged.

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2432 INDIAN J CHEM. SEC A, DECEMBER 2005

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