Indian Journal of Chemistry Vol. 43A, August 2004, pp. 1626- 1634

Synthesis, crystal structure and magnetic properties of polymeric tetrakis(pyrazolyl)borate copper(II) complexes

Anitha M Thomas, Arindam Mukheljee, Manas K Saha, Munirathinam Nethaji & Akhil R Chakravarty*

Departme nt of Inorgani c and Physical Chemistry, Indian Institute of Science, Bangalore 560 01 2, India

E-mai l: arc@ ipc .ii sc.ernet.in

Received 26 April 2004

Po ly nuclear copper(" ) co mpl exes [CU1 !I.l-B(PZ)4 }(~I -ox)(N03)(H20)].2H20 (1.2HzO), [Cuz {Il-B( PZ)4 }(Il-ox)( HzOh ](CI04).2 HzO (2.2 H20) and [CU2{ Il-B (PZ)4 }(1l-NO)(NO)z(H20)] (3) are prepared and structurally charac terized by single crystal X-ray crystallography (ox, oxalate Ion CzO}-). Complexes 1 and 2 have oxalate and te trakis(pyrazolyl)borate bridging li gands to form one-d imens ional ( I D) chains. The coordinating ato ms of the oxal ate and te trak is(pyrazoly l)borate in 1 and 2 occ upy the basal positi ons giving CuN20 3 square pyram idal geometry. The alternate copper centers in I are bonded to nitrate and aqua axia l ligands. Complex 2 has each copper bonded to one ax ial aqua li gand. In the I D polymeric structure of complex 3, the copper(lJ ) centers are linked th rough nitrate and te traki s(pyrazolyl)borate ligands. Each copper ato m has a di storted octahedral CuN10 4 coordinati on geometry with two long Cu-O bonds. The electron ic spectra o f the complexes 1-3 display a broad d-d band within 676-72 1 nm in DMF. Variable temperature magnetic susceptibility measure me nts in the range 20 to 300 K show strong anti fe rro magnetic behavior of the comp lexes 1 and 2 giving respec ti ve magneti c moment va lues of 1.49 and 1.52 Po (per copper) at 298 K. The coplanar

orbital topology in volving the {CU1(1l-0X)}2+ mo iety in 1 and 2 facilit ates strong anti ferromagnetic coupling bet ween two

{ d , , } magnetic orbitals (1: -320 cm- I 1; -353 cm- I 2). Complex 3 is essentially paramagneti c givi ng P eff va lue o f 1.93 x~ -y~

Po (per copper) in the te mperature range 20 to 300 K. Such a magnetic behavior ari ses due to orthogonality of the orbi tal overl ap ari si ng from the ax ial-equato ri al bridgi ng mode of the nitrate ligand. In 1-3, the tetraki s(pyrazolyi) borate bridgi ng ligand is apparen tl y ineffective in med iating magnetic exchange interaction due to no n-availability of orbitals for exchange interac tions. The complexes show th e formation o f different supramo lec ul ar net-work structures due to hydrogen bonding interactions.

IPC Code: Int. Cl 7 C07F 1/08; C07F 5/02

Poly(pyrazolyl )borates of the type [HnB(pz)4-nr (n = 0-2) form an important cl ass of li gand system in transition metal chemistry with various applications like modeling the active site structures of non-heme metalloproteins, in organometallic chemistry and catalysis, and for the development of the chemistry of polynuclear metal complexes I-10. Poly(py razolyl)­borates provide different steric and electronic controls that are essenti al for designing metal complexes with selecti ve structural and fu nctional properties. Whi Ie the chemistry of tri s(pyrazolyl)borate is extensive spanning all aspects of transition metal chemistry, the chemistry of tetraki s(pyrazolyl )borate is relatively unexplored although this li gand has a potential for use in the chemistry of supramolecular and high nuclearity transition metal complexes. In the present work, we have used this ligand for the synthesis of new copper(IJ) complexes in which {B(PZ)4} - is chosen as a linker of metal ions generating polymeric framework.

Polynuclear transltton metal complexes are of . .. I tt ·1 3 1\ iJ . lllterest as new magnettc matena s . IV agnetlc exchange between paramagnetic transi ti on metal ions often gives rise to magnetic properties that differ substan tially from those expected for a collection of non-interacting paramagneti c cen ter. Binuclear copper(II) complexes provide the simple models to study the spin-spin interactions. Among various systems, dicopper(II) tetracarboxy lates and di ( ~l ­

hydroxo/alkoxo)dicopper(II) complexes are well ex plored for deriving magneto-structural correlations 14- 17. A I koxo/hydroxo and carbo x y lato complexes with an asymmetrically dibridged [Cu2(OR)(0 2CR)f+ core present another magneti c system which shows reduced magnitude of the spin­spin exchange interaction due to 'countercompli­mentary ' nature of the orbital overlap of the two bridging li gands 18-20_ [n the present work, we have used oxalate-bridged dicopper(II) uni t, viz_ {CU2( ox) )2+ which is known to display different

THOMAS el al.: STUDIES ON TETRAKlS(PYRAZOL YL)BORATE COPPER(II) COMPLEXES 1627

magnetic properties based on the orbital topologies involving the magnetic orbitals of the metal ions21.24 . We have prepared and structurally characterized polymeric dicopper(JI) complexes [CU2 {!-\--B(PZ)4 }(!-\-­ox)(NO))(H20)].2H20 (1) and [CU2 {!-\--B(PZ)4 }(!-\-­ox)(H20h](CI04).2H20 (2) in which the (CU2(OX)) 2+

units are linked by tetrakis(pyrazolyl)borate ligands to form one-dimensional chain structure. Again, to explore the effect of the bridging {B(PZ)4 r ligand on the structure and magnetic property of the complex in the absence of any oxalate bridge, we have prepared

polymeric [CU2{ !-\--B(PZ)4 }(!-\--N03)(N03h(H20)] (3). Herein we report the synthesis, single crystal X-ray structure and magnetic properties of the complexes 1-3. The significance of this work is that the 1 D polymeric species, containing labile terminal ligands like nitrate or H20 , could be used as precursors for generating magnetically active 20 or 30 network structures using suitable linkers.

Materials and Methods Chemicals and reagents of analytical grade were

procured from commercial sources and were used as received . Solvents were purified as per standard methods. Sodium tetrakis(pyrazolyl)borate was prepared by a literature method25.

Physical measurements

Elemental analytical data were obtained using a Heraeus CHN-O Rapid instrument. Fr-IR and electronic spectral measurements were done using Bruker Equinox 55 and Hitachi U-3000 spectrometers, respectively . Variable temperature magnetic susceptibility data in the temperature range 20-300 K were measured with polycrystalline samples of the complexes using a George Associates Inc. model 300 Lewis-coil-force magnetometer system equipped with a closed-cycle cryostat (Air Products) and a Cahn balance. The susceptibility data were corrected for diamagnetic contributions and temperature independent paramagnetism (Na = 60x lO-6 cm3 mol- I per copper atom)26. The magnetic

susceptibility data were theoretically fitted for the oxalate bridged complexes assuming the polymeric species as an assembly of oxalate-bridged dimeric units with the tetrakis(pyrazolyl)borate not providing any significant exchange pathway. The Hamiltonian expression was: fI = -J(S/S2) + gf3HS - zi<Sz>s, where the Js are Heisenberg exchange constants, z is the number of nearest neighbors of the complex in the

crystal, <S:> is the mean magnetization of the I d SA h .. 16·28 comp ex an s are t e appropnate Spll1 operators- .

The intermolecular exchange interaction (1') was assumed to be significantly weaker than that of the oxalate-bridged dicopper(U) unit (1). The magnetic susceptibility expression used for a dicopper(lJ ) unit with a molecular field approximation was: X'M = 2Nifi[kT-2zJ'/{3 + exp(-JlkD }]-1[3 + exp(-JlkDr l

•

To account for the presence of small concentration of monomeric paramagnetic impurity in the polymeric sample, an additional Curie-like contribution to the susceptibility was added, to obtain a modified susceptibility expression: XM = X'M(l-P) + (Nif32/2kDp + Na. The agreement of the theoretical fit to the experimental data was determined from the R value which was 2.55xlO-2 for 1 and 3.6x 10-2 for 2,

where R = L; [(XobsCT;)-Xca/cd(T;))2/Xobs(Ti]. The g, gl, P and zj' values were 2 .02, 2.12, 0.054, 2 for 1 and 2.09, 2.19, 0.084, 20 for 2. The singlet to triplet separation energy (1) with singlet being the ground state was -320 cm- I for 1 and -353 cm- I for 2.

Synthesis of [Cu2{Il·B(pz)~)(Il·ox)(N03)(H20)].2H20 (1.2H20) and [Cu2{Il·B(pz)~)(Il·ox)(HzOh](Cl04).2H20 (2.2HzO)

Complexes 1 and 2 were prepared by a common synthetic procedure in which sodium tetrakis(pyrazolyl)borate (150 mg, 0.5 mmol) taken in MeOH (70 ml) was initially reacted with copper(II) nitrate (for 1, 240 mg, 1.0 mmol) or copper(II) perchlorate (for 2, 370 mg, 1.0 mmol) in MeOH (70 ml) for 30 min at 25°C. The resulting solution was then reacted with potassium oxalate (64 mg, 0.5 mmol) in water (10 ml) under magnetic stirring for 15 min at 25°C. The solution was filtered and the filtrate on slow concentration gave crystalline product in -90% yield. Anal. Found: C, 27.86; H, 3.07; N, 20.87; Calcd. for CI 4HI 8N90IOBCU2: C, 27.56; H. 2 .95; N, 20.65%. IR (cm- I, KBr pellet): 3424br, 1670vs, 1381vs, 1337m, 1298m, 1244m, 1198s, 1108111, 1077s, 849m, 828s, 782s, 653w, 620w, 480w (vs, very strong; s, strong; m, medium; w, weak ; br broad). UV-vis , Amax, nm (E, M- I cm- I) in OMF: 676 (140), 265 (5600). Magnetic moment, J.1err: 1.49 J.1B at 298 K. Anal. Found: C, 24.93; H, 2.82; N, 17.11 ; Ca\cd. for CI4H2oNsOI 2BCu2CI: C, 25.26; H. 3.00: N, 16.83%. IR (cm- I, KBr pellet): 3400br, 1658vs, 1396s, 1302m, 1233m, 1200m, 1111 vs, lO77vs, 848m, 831m, 808m, 772m, 620w, 474w. UV-vis, A.nax, nm (E, M- I cm- I) in OMF: 678 (145), 270 (5750). Magnetic moment, J.1err: 1.52 J.LB at 298 K.

]628 INDIAN J CHEM, SEC A, AUGUST 2004

Synthesis of [Cu2{I!-B(pz)~}(I!-NOJ)(N03h(H20)] (3)

Complex 3 was prepared in a s imil ar way as described above for 1 by reacting copper(ll) nitrate with the sodium salt of tetraki s(pyrazolyl)borate but in absence of potassium oxalate. The crystalline solid was isolated on slow concentration of the filtrate. Anal. Found: C, 23.42; H, 2.40; N, 25.43 ; Calcd . for CI 2HI4NIIOIOBCU2: C, 23 .64; H. 2.29; N, 25.24%. IR (cm- I, KBr pellet): 33 13br, 3138w, 1501 s, 1391 s, 1308s, 1266vs, 1199s, ] 117s, 1081 vs, 10 18m, 1005m, 9 19w, 845m, 828s, 806m, 768s, 6 ]6m. UV-vis , Am,x, nm (c:, M- I cm- I) in OMF: 721 ( 135), 277 (3670).

Magnetic moment, J-leff: 1.93 JIB at 298 K.

X-ray crystallographic procedul·es The crystal structures of the three complexes were

determined by si ngle crys ta l X-ray diffraction technique. Single crystal s were obtained from the mother liquor on slow evaporati on of the solvent at an ambient temperature. Crystal mounting was done using glass fibers and epoxy cement. All geometric and intensity data for 1.2H20 and 2.2 H20 were collected usi ng an automated Enraf-Non ius CA D4 diffractometer equipped with Mo-Ka radiation (A, 0.7 1073 A) . The intensity c1ata, collected using wand w-28 scan modes for 1 and 2, respectively, were corrected for Lorentz-polarizati on effects and for absorption29

. The cell parameters and the intensity data for 3 were o!:>tained using a Bruker SMART APEX CCO diffrac tometer, equipped with a fine focus 1.75 kW sealed tube Mo-Ka source (A., 0.71073 A) with increasing w (width of 0.3 deg per frame) at a scan speed of ] 5 sec/frame. The SMART softw are was used for data acquisition and the SAINT software for data extraction. Absorption cOITections on the intensity data were made3o. Structures were solved and refined using SHELX programs31

• The non­hydrogen atoms were refined ani sotropically. The hydrogen atoms attached to the carbons were fix ed and refined isotropically using the riding model. The structure solution of 2 gave hi gher residual val ues due to di sorder in the perchlorate anion . The difference Fourier map of this structure showed few peaks with electron density between 1.0 to 1.7 e A -3 located near the metal and hetero atoms, possibly due to packing disorder or minor twining effect. These peaks were neglected as they had no significant effect on the structural parameters. Selected crystallographic data are given in Table 1. Perspecti ve views of the complexes were obtained using ORTEp32.

Detailed crystallographic data in the CIF format have been del ·· ' .. ted with the Cambridge Crystallographic Out,· Centre, CCDC Nos. 235886-235888 for 1-3, respectively. Copies of thi s information may be oAai ned free of charge from The Director, CCDC, 12 Union Road, Cambridge CB2 I EZ, UK (Ly: +44- 1223-336033; E-mail: deposit@ccdc.cam.ac.uk or www:http://www.ccdc/ cam.ac .uk).

Results and Discus~.hn

Polymeric compLxes 1 and 2 containing tetrak is(pyrazolyl)boute and oxalate li gands are prepared in high yield from the reaction of these ligands with copper(Il) nitrate or perchlorate. Polymeric complex 3 is prepared in high yield from a simil ar reaction of copper(JI ) nitrate with sodi um tetrak is(pyrazolyl)borate but in absence of oxalate salt. The compl exes are characteri zed from analyt ical and spec tral data . They are soluble in OMF and display a d-d band near 700 nm. The infrared spectral features fo r the co nlplexes 1 and 2 indi cate the presence of the oxalate li gand.

The complexes 1-3 have been characterized by single crystal X-ray diffraction techn ique. ORTEP views and the polymeric nature of the complexes are show n in Figs 1-6. Selected bond distances and angles are given in Tables 2-4. Complex 1, as a dicopper(l l) unit along with two lattice water molecules in the crys tallographic asy mmetric unit, crystallizes in the non-centrosymmetric monoclinic space group Cc with Z = 4 (Fig. I). The structure consists of copper(ll ) centers bridged alternately by dianionic oxalate and monoanionic tetraki s(pyrazolyl)borate li gands (Fi g. 2) . In the squ are pyramidal geometry, the bridging ligands are bonded at the basal plane. While the elongated ax ial site of Cu(l) atom has a bound aq ua ligand , the Cu(2) atom is bonded to a monodentate nitrate. The Cu(1)-O(1 w) ax ial bond is considerably longer than the Cu(2)-O(S) bond. The Cu-O and Cu-N bond distances involving the bridging ligands vary within 1.932(12) to 2.0 12( 12) A. The CuO) and Cu(2) atoms are di spl aced by 0.176( I) and 0.192(1) A from the basal plane, respectively. The extent of distortion fro m the square pyramidal geometry is insign ifican t (!" value, 0.05 for Cu( I) ; 0.01 for Cu(2». The angles formed between two basal planes contai ning Cu(l) and Cu(2) atoms and the oxalate plane are 3.5° and 4 .3°, respectively, indicating essentially a planar structure. The dihedral angle between the planes formed by the peripheral

THOMAS et at.: STUDIES ON TETRAKIS(PYRAZOL YL)BORATE COPPER(II) COMPLEXES 1629

Table I-Crystal data and st ructure refinement for [CUlt ~-B(PZ)4 }(~-ox) }(NO))(H20)).2H20 (1.2HzO), [Cuz(~-B(PZ)4 }(~­ox) }(H20h)(CI04).2HzO (2.2HzO) and [CU2( (~l-B(pZ)4 }(~-ox) }(~-NO)(N03MH20») (3)

Identification code Empirical formula Formula weight Temperature Wavelength Crystal system Space group Unit ce ll dimensions

Volume Z Density (calculated)

Absorption coefficient

F(OOO) Crystal size Theta range for data collection I ndex ranges

Reflections collected I ndependent reflections Completeness to theta Refinement method

Datalrestrai ntslparameters 2

Goodness-of-fit on F Final R indices [1>2sigma(l))

R indices (all data)

Largest diff. peak and hole

Weight factor: w = l/[d(F02 ) + (ap)2 + bP)

1.2 H20 C'4H, gBCU2N901O 610.26 293(2) K 0.71073 A monoclinic Cc a = 14.384(3) A b = 7.339(3) A c = 21.550(4) A f3= 99.62(2 )0

2243.0(11) A 3

4 1.807 Mg/m)

1.967 mm-'

1232 0.28xO.2xO.2 mm3

1.92 to 24.97°

O<=h<= 17, 0<=k<=8, -25<=1<=25

2058 2058 [R(int) = 0.0000) 99.7 %

2 Full-matrix least-squares on F

2058/2/325 1.079

RI = 0.0384, wR2 = 0.1077

R I = 0.0507, wR2 = 0.1222

0.672 and -D.362 e A-3 a = 0.0756 b = 8.8487

2.2H2O C'4H20BCICu2NgO'2 665.72 293(2) K 0.71073 A monoclinic P2/n a = 11.620(2) A b = 7.414(2) A c = 15.913(3) A f3= 103.91(3t

1330.7(5) AJ 2 1.661 Mg/m) , 1.768 mm-672 0.32xO.30xO.15 mm) 1.96 to 24.94°

0<=h<=13, 0<=k<=8, -18<=I<=18

2538 2330 [R(int) = 0.0388) 100.0 %

2 Full-matrix least-squares on F

233010/171 1.213

RI = 0.0933, wR2 = 0.2568

RI = 0.1541 , wR2 = 0.3334

1.736 and -1.778 e ;"-3

a = 0.2000 b = 0.0000

Table 2-Selected bond distances (A) and angles (deg.) for polymeric [Cul t ~-B(PZ)4 }(~-OX)(N03)(H 20»).2H20 (1.2H I O) with e.s.d .s in the parenthesis

Cu( I )-O( I) 1.980( I 0) N(32)-Cu(2)-N( 42) 92.2(5)

Cu( I )-0(2) 1.980(10) N(32)-Cu(2)-0( 4) 167.7(5)

Cu( I )-O( I W) 2.328( II) N( 42)-Cu(2)-0( 4) 90.9(5)

Cu( I )-N( 12) 1.958( 12) N(32)-Cu(2)-0(3) 91.4(5)

Cu( I )-N(22) 1.976( 12) N (42)-Cu(2)-0(3) 168.2(6)

Cu(2)-O(3) 2.012(12) 0(4)-Cu(2)-0(3) 83.3(5)

Cu(2)-0(4) 2.010(10) N(32)-Cu(2)-0(5) 93.5(5)

Cu(2)-0(5) 2.183( II) N(42)-Cu(2)-0(5) 98.9(5)

Cu(2)-N(32) 1.932(12) 0(4)-Cu(2)-0(5) 97 .7(5)

Cu(2)-N(42) 1.966( II) 0(3)-Cu(2)-0(5) 92. 1(5)

O( I )-Cu( I )-0(2) 83.4(4) N( 12)-Cu( I )-N(22) 95.3(5)

N( 12)-Cu( I )-O( I W) 95.2(6) N( 12)-ClI( I )-O( I) 90.5(5)

N(22)-Cu( I )-O( I W) 96.3(5) N(22)-ClI( I )-O( I) 166.7(5)

O( I )-Cu( I )-O( I W) 95 .1(4) N( 12)-ClI( I )-0(2) 169.7(6)

0(2)-Cu( I )-O( I W) 93.6(5) N(22)-ClI(1 )-0(2) 89.0(4)

3 C'2H '4BCU2N ,,0'0 610.23 293(2) K 0.71073 A monoclinic P2,/c a = 15.8848(17) A b = 7.5365(8) A c = 19.926(2) A f3 = 108.434(2t Y = 90°. 2263.0( 4) A) 4

)

1.791 Mglm , 1.954 mm-1224 0.32xO.25xO.18 mm) 2.15 to 28.03°

-20<=h<=15, -9<=k<=9, -26<=1<=26 13277 5250 [R(int) = 0.0971) 96.1 % Full-matrix least-squares

2 on F 525010/319 0.890

RI = 0.0442, wR2 = 0.0972 RI = 0.0726, wR2 = 0.1053 0.509 and -0.566 e ;,.-) a = 0.0395 b = 0.0000

1630 INDIAN J CHEM, SEC A, AUGUST 2004

06

Fig. I-An ORTEP view of the dinuclear unit in polymeric [Cul l!.l-B(pz)4)(!.l-ox)(N03)(H20)].2H20 (1.2H20) showing 50% probability thermal ellipsoids and the atom numbering scheme.

""

Fig. 2-Polymeric structure and the hydrogen bonded supramole­cular network III [Cu21 !.l-B(PZ)4 )(!.l-ox)(NO])(H20)].2H20 (1.2H20).

Fig. 3-An ORTEP view of the dicopper(ll) structural motif in the polymeric complex [CUl I !.l-B(PZ)4) (!.l-ox)(H 20 h](CI04 ).2H20 (2.2H20) showing 50% probability thermal ellipsoids and the atom label ing scheme.

.•.......•

.. "

JJ~ 5~~ ...... /~ .... ••

~. ~;~. ~9 , ~~yc!;o

29--P~ ~~

Fig. 4-Polymeric structure and the supramo!ecular network III

[Cu21 !.l-B(PZ)4 )(!.l-ox)(H20h](CI04).2H20 (2.2H20) (CI04 - has positional disorder).

C21

B1

Fig . 5-An ORTEP view of the dicopper(II) motif in the polymeric complex [Cu21 ~l-B(pZ)4 )(!.l-NOJ)(NO)h(H20)] (3) showing 50% probability thermal ellipsoids and the atom numbering scheme.

Fig. 6- Hydrogen bonding interacti ons dimensional polymeric chains In NO))(N03MH20)] (3).

in vo lving the one­

[C u21 !.l-B(PZ)4 )(!.l -

THOMAS ef (/1.: STUDIES ON TETRAKlS(PYRAZOL YL)BORATE COPPER(II) COMPLEXES 1631

Table 3-Selected bond lengths (A) and angles (deg.) for the polymeric complex [Cu21 J..l-B(pz)4}(J..l-ox)(H20h](CI04).2H20 (2.2H20) with e.s.d.s in the parenthesis

Cu( I )-0(1) Cu(1 )-0(2)#2 Cu( I )-O( I W) Cu(I)-N(12) Cu(1 )-N(22)#1 N( 12)-Cu( I )-O( I) N( 12)-Cu( I )-N(22)#1 0(1 )-Cu(1 )-N(22)# I

1.970(13) 1.981 (8) 2.269( II) 1.962(9) 1.979( 10) 175.0(5) 94.2(4) 90.7(5)

N( 12)-Cu( 1 )-0(2)#2 O( I )-Cu( 1 )-0(2)#2 N(22)# I-Cu( 1 )-0(2)#2 N( 12)-Cu( 1)-0(1 W) O( I )-Cu( I )-O( 1 W) N(22)#I-Cu(1)-0(1 W) 0(2)#2-Cu( I )-O( I W) O( 1 A)-Cu( 1 )-O( 1 W)

Symmetry transformations used to generate equivalent atoms: # I -x+3/2,y,-z+312 #2 -x+2,-y+ I ,-z+ I

91.1(4) 83.9(5) 164.5(4) 93.6(4) 86.5(5) 101.5(5) 92.7(5) 99.7(8)

Table 4-Selected bond distances (A) and angles (deg.) for the polymeric complex [Cu21 J..l-B(pz)4 }(J..l-N03)(N03MH20)] (3) with e.s.d.s in the parenthesis

Cu( I )-O( I) 2.500(2) N( 12)-Cu(1 )-0(4) 165.30(10) Cu(l)-0(3) 2.008(2) N(22)-Cu( 1 )-0(5) 106.15(10) Cu(1 )-0(4) 2.014(2) O( I )-Cu( I )-N( 12) 89.44(1 I) Cu(I)-0(5) 2.403(2) O( 1 )-Cu( I )-N(22) 106.77(11) Cu(1 )-N( 12) 1.938(3) N(1 2)-Cu( I )-0(5) 108.24(10) Cu( I )-N(22) 1.934(3) 0(5)-Cu(2)-0(8) 84.37(10) Cu(2)-0(5) 2.370(2) 0(5)-Cu(2)-0(7) 138.29( 10) Cu (2)-0 (7) 2.578(2) 0(7)-Cu(2)-0( I W) 81.87(11 ) Cu(2)-0(8) 1.988(3) O( I W)-Cu(2)-0(8) 86.76(11) Cu(2)-0( I W) 1.986(3) O( 1 W)-Cu(2)-0(5) 89.19(10) Cu(2)-N(32) 1.958(3) 0(7)-Cu(2)-0(8) 54.63(11) Cu(2)-N(42) 1.964(3) N(32)-Cu(2)-N( 42) 94.00(11) O( I )-Cu( I )-0(3) 55.47(10) N(32)-Cu(2)-0( 1 W) 89.44(11) O(3)-Cu( I )-0(5) 88.86( I 0) N(42)-Cu(2)-0(1 W) 175.56(11) O( 4 )-Cu( 1 )-0(5 ) 57.28(9) N(32)-Cu(2)-0(8) 174.29(12) O( I )-Cu( I )-0(4) 100.87(10) N(42)-Cu(2)-0(8) 89.60(11) 0(1 )-Cu( I )-0(5) 140.77(9) N(32)-Cu(2)-0(5) 99.84(10) 0(3 )-Cu( I )-0(4) 87.15(11) N( 42)-Cu(2)-0(5) 92.99( 10) N(22)-Cu( I )-N( 12) 95 . 11(11) 0(7)-Cu(2)-N(32) 120.58( II) N(22)-Cu(1 )-0(3) 161.50(11) 0(7)-Cu(2)-N(42) 93 .95(12) N(12)-Cu(1 )-0(3) 90.19( 12) Cu( I )-0(5)-Cu(2) 150.70(11) N(22)-Cu( I )-0(4) 91.93(11)

Table 5-Chemically significant hydrogen bonding distances (A) in [Cu21J..l-B(pz)4}(J..l-ox)(N03)(H20)].2H20 (1.2H20), [Cu21J..l­B(pz)4} (J..l-ox)(H20)2](CI04).2H20 (2.2H20) and [Cu21 J..l-B(pz)4) (J..l-N03)(N03MH20)] (3)

1.2H2O" 2.2H20 b

0(1) .. . 0(3w) #1 3.04(2) O(1) ... OI(w) 2.91(2) O( 1 w) ... 0(6) #2 2.87(2) 0(1 w) ... 0(2w) 2.56(2) 0(1 w) ... O(7) #2 2.99(2) Ol(w) ... N(12) 3.09(2) 0 (2w) ... 0(4) #1 2.98(2) 0(2) ... 0(lw) #1 3.08(2) 0(2w) ... 0(3w) #3 2.95(2) 0(2w) . . . 0(5)#3 2.96(2) 0 (2w) ... 0(5) 2.92(3) 0(5) ... 0(2w) #2 2.96(2) 0(2w) ... N(I) 3.08(2)

Symmetry transformations used to generate equivalent atoms: "# 1 x,+y-I ,+z; #2 x+ 1I2,+y+ 1/2,+z; #3 x-1/2,+y-1I2-1 ,+Z b #1 -x+2,-y+I,-z+l; #2 x-1I2,-y,+z+1/2; #3 x+1/2,-y,+z-1I2 C # I x,+y+ I ,+z; #2 -x+2,-y+2,-z+ 1

O(lw) ... O(2) #1 0(1 w) ... 0(9) #2

2.74(1) 2.72( 1)

1632 INDIAN J CHEM, SEC A, AUGUST 2004

ligand donor atoms with the central plane are 12.8° for Cu(l) and 13.9° for Cu(2). These angles giving a measure of co-planarity are important factors for deriving orbital topologies. The separation between two copper centers linked by oxalate, i.e. Cu(l) .. . Cu(2), and by tetrakis(pyrazolyl)-borate, i.e. Cu(l) . . . Cu(l)', are 5.204(2) and 6.533(2) A, respectively. Complex 1 displays weak hydrogen bonding interactions involving the axial ligands, one oxalate oxygen atom and the lattice water molecules (Table 5). The polymeric chains are linked via hydrogen bonding to form a supramolecular network (Fig. 2).

Complex 2 crystallizes in the monoclinic space group P2/n with a Z value of 2. The crystal structure reveals the presence of a 10 polymeric chain of the complex along with two lattice water molecules per dicopper(II) motif (Figs 3, 4). The structure is similar to that of 1.2H20 except that the axial ligand is only water and there is one copper center in the crystallographic asymmetric unit. The perchlorate anion does not show any apparent interaction with the meral ion. Like 1, complex 2 has an oxalate bridged dicopper(II) unit that are linked by tetrakis(pyrazolyl)borate. The copper has a square pyramidal (4 + I) geometry in which the basal plane has pyrazolyl nitrogen and oxalate oxygen atoms giving Cu-N and Cu-O di stances in the range 1.963(9) to 1.980(10) A. The axial bond is 2.275( 12) A. The copper atom is displaced by 0 .176(1) A from the basal plane. The trigonality parameter value of 0. 13 indicates minor distortion of the square pyramidal geometry. The angle formed by the metal containing basal plane to the oxalate plane is 8.12°, suggesting essentially coplanar nature of the planes. Again, the dihedral angle between the planes formed by peripheral ligand donor atoms and the oxalate donor atoms is 15.79°. The separations between the copper atoms linked by oxalate and tetrakis(pyrazoly l)borate ligand are 5.199(2) and 6.562(3) A, respecti vely. The complex exhibits weak hydrogen bonding interactions involving aqua ligand, lattice water and the perchlorate anion to form a supramolecular network (Table 5). The hydrogen boding between the axial water and lattice water is , however, strong giving a di stance of 2.56(2) A.

The polymeric complex [Cu21 )l-B(pz)~ }()l-NOJ)(NO)h(H20)] (3) crystallizes in the monoclinic space group P2 1/c with a Z value of 4 (Fig. 5). [n the absence of any oxalate ligand, the nitrate ion acts as a Ii 11k to form the {Cu2()l-NOJ )} 3+ structu ra l moti f

which is linked by tetrakis(pyrazolyl)borate to form a 10 chain (Fig. 6). The nitrate link di splays bridging cum chelating mode of bonding. The Cu(l) atom has a basal plane consisting of the pyrazolyl two nitrogens, one terminal nitrate oxygen 0(3) and the 0(4) atom of the bridging nitrate giving Cu(l)-N and Cu(l)-O distances in the range 1.934(3) to 2.014(2) A. In the distorted octahedral geometry of Cu(1) atom, the Cu(1 )-0(1) and Cu(l )-0(5) bonds are long giving distances of 2.500(3) and 2.403(2) A, respectively. The Cu(2) atom has a similar coordination geometry as that of Cu(1) with two pyrazolyl nitrogen atoms, one terminal nitrate oxygen atom 0(8) and the water oxygen atom 0(1 w) forming the basal plane giving Cu(2)-N and Cu(2)-0 distances in the range l.958(3) to 1.988(3) A. In the Cu(2)N20~ distorted octahedral geometry, the Cu(2)-0(5) and Cu(2)-0(7) bond distances are 2.370(2) and 2.578(3) A. The separations between two copper atoms linked by nitrate and tetrakis(pyrazolyl)borate are 4.619(1 ) and 6.632(1) A, respectively. The complex is involved in moderately strong hydrogen bonding interactions involving the aqua ligand and nitrate atoms [0(2), 0(9)] to form a network structure that is different from those formed by 1 and 2 (Table 5, Fig. 6). The N-Cu-N chelate angle formed by the tetrakis(pyrazolyl)borate ligand in these three crystal structures vary within 92.2(5) to 95.3(5t. The boron atom of the tetrakis(pyrazolyl)borate has a distorted tetrahedral structure with the N-B-N angles in the range 116.1 (15) to 105.4(14)°.

Variable temperature magnetic measurements in the temperature range 20 to 300 K show antifelTomagnetic interaction in complexes 1 and 2,

0.3 0.4 ~ :::.:::

-0 0 ,.... 0.3 E c:: '" E 0.2 ..,

E u u

,-.. 0.2 ~ ....

Q) 0.. 0.. 0.. l3 __ • 0.. 0 0.1 0 u u .... 0. 1 '-Q) Q)

0.. 0.. '-' '-'

h ~ ~

0.0 0.0 ~ 0 50 100 150 200 250 300

T,K

Fig. 7- Plots of XMT V.I' . T for the oxalate bridged pol yn uclear complexes [CUl { il-B(PZ)4 }(il-ox)(NOJ)(H 20 )1.2 H10 (1.2HlO; L'»

and [Cul l il-B(PZ)4 }(il-ox)(H20hl(C104).2 HlO (2.2H 20: 0 ) . The solid lines are the theoretica l fits o f the ex perimental data.

THOMAS et ClI.: STUDIES ON TETRAKlS(PYRAZOL YL)BORATE COPPER(II) COMPLEXES 1633

while complex 3 shows paramagnetic behavior.

Figure 7 shows the XMT vs. T plots for the oxalate bridged complexes 1 and 2. The magnetic moment values for 1 are 1.49 and 0.69 I1B at 298 and 20 K, respectively. Complex 2 shows similar magnetic

behavior as of 1 giving l1err values of 1.52 I1B at 298 K and 0.73 I1B at 20 K. The magnetic behavior of the 10 chain polymeric complexes 1-3 is presumed to arise from the oxalate- or nitrate bridged dicopper(II) unit. This is because of the long copper to copper separation involving the tetrakis(pyrazolyl)borate bridge and due to the absence of any effective conjugation in this ligand to promote magnetic exchange interaction between two copper(II) centers.

The experimental magnetic susceptibility data for 1 and 2 have been theoretically fitted using a modified Bleaney-Bowers expression for a dicopper(lI) unit in a molecular field . The singlet-triplet energy separation is -320 cm- I for 1 and -353 cm- I for 2 with the singlet as the ground state. Magnetic exchange interaction between two copper(JI) centers propagated through an oxalate bridge is known to be dependent on the geometry around the metal center and the bridging mode of the oxalate ligand (Scheme 1)21-24. A coplanar orbital topology with the d x'_y2 metal orbital coplanar with the oxalate ligand promotes strong anti ferromagnetic interaction. The trigonal bipyramidal geometry of the metal centers or a perpendicular orbital topology involving d ,2_y2 and d,' metal centers reduces the magnitude of antifen·omagnetic interaction significantly in comparison to the coplanar situation. A parallel orbital topology makes the metal centers essentially non-interacting. Complexes 1 and 2 show coplanar dx '_y' orbital topology for the metal centers and the observation of strong anti ferromagnetic coupling is in agreement with the X-ray structural data . The weak ferromagnetic component of molecular field (zf) possibly arises due to the presence of supramolecular network involving elongated axial ligands along the c/z2 orbital that is orthogonal to the magnetic orbital. Significant deviations from coplanarity of the adjacent (CU2(OX) )2+ units linked by tetrakis­(pyrazolyl)borate could also be responsible for the positive value of zJ.

Complex 3 is paramagnetic and gives a magnetic

moment of 1.93 I1B per copper in the temperature range 300-20 K. A plot of XM- 1 vs. temperature shows an apparent Curie behavior and the theoretical fit gives a e value of only 0.45 K indicating essentially paramagnetic nature of the complex (Fig. 8). There is

(a)

(b)

(c)

(d)

Scheme I-Coplanar (a), trigonal-bipyramidal (b), perpendicular (c) and parallel (d) orbital topologies in oxalate bridged dicopper(ll) complexes.

700 3.0

600

"0 500 2.5 ::...'" E

'") 400 Q; E u 00

-: - 300 ::;: ~ 200

100

o 1.0 o 50 100 150 200 250 300

T, K

Fig. 8-Plots of XM- 1 (6) and effective magnetic moment (Ilclf per copper atom, 0) vs. temperature for [CU2( ~-B(PZ)4 }( ~­NO) (NO)2(H20)] (3). The solid line gives the theoretical fit of the data lI sing Curie-Weiss equation.

no apparent magnetic exchange interaction between two copper(II) centers through the tetrakis(pyrazolyl)­borate and nitrate bridges . The results could be rationalized from its crystal structure . The two copper centers are linked by a nitrate through long axial bonds. As the axial nitrate li gand is along the z-axis,

1634 INDIAN J CHEM, SEC A, AUGUST 2004

the orthogonality between the dx2-y2 magnetic orbital in the basal plane and bridging ligand along the z-axis makes the metal centers essentially non-interacting in 3. A e value of 0.45 K indicates the absence of any significant molecular field or any effect of the tetrakis(pyrazolyl)borate bridge.

Conclusion Three new 1 D-chain polymeric copper(II)

complexes containing alternate oxalate or nitrate and tetrakis(pyrazolyl)borate bridging ligands are prepared and structurally characterized by single crystal X-ray crystallography. The oxalate bridged complexes show coplanar orbital topology promoting strong anti ferromagnetic exchange interaction between the metal centers. The tetrakis(pyrazolyl)­borate ligand does not promote any significant magnetic interaction. The nitrate-bridged complex displays essentially paramagnetic behavior due to orthogonality between the metal dx2-y2 magnetic orbital and the nitrate axial bridge along the z-axis. The polymeric complexes have labile terminal sites like nitrate and/or water. The complexes could be potential precursors for building 2D or 3D network structures by substitution of weak axial ligands with suitable linkers.

Acknowledgement We thank the Department of Science and

Technology (DST), Government of India, for financial support and for the CCD diffractometer facility. We also thank the Convener, Bioinformatics Center of our Institute, for database search.

References I Trofimenko S, Chem Rev, 93 (1993) 943. 2 Santos I & Marques N, New J Chem, 19 (1995) 551. 3 Perkin G, Chem COIII/IIII11, (2000) 1971. 4 Kitano T, Sohrin Y, Hata Y, Kawakami H, Hori T & Ueda

K, J Chem Soc Daltoll Trans (2001) 3564. 5 Santini C, Pettinari C, Pellei M, Gioia Lobbia G, Pifferi A,

Camalli M & Mele A, Polyhedron, 18 (1999) 2255.

6 Effendy, Gioia Lobbia G, Pettinari C, San tini C, Skelton B W & White A H, J Chern Soc Daltoll TraIlS, (1998) 2739.

7 Paulo A, Reddy K R, Domingos A & Santos I, l110rg ClWI1, 37 ( 1998) 6807.

8 Rheingold A L, Yap G P A, Liable-Sands L M, Guzei I A & Trofimenko S, Inorg Chem, 36 (1997) 626 1.

9 Reger D L, Coord Chern Rev, 147 (1996) 571. 10 (a) Dfaz-Requejo M M, Belderrain T R. Nicasio M C &

Perez P J, Orgal1ollletallics, 19 (2000) 285; (b) Brunner H, Singh U P, Boeck T, Altmann S, Scheck T & Wrack meyer B, J Orgal1olllet Chem, 443 (1993) C16.

11 Winpenny REP, Adv l110rg Chelll , 52 (200 1) I. 12 Gatteschi D & Sessoli R, Angew Chem 1111 Ed Engl, 42

(2003) 268. 13 Wernsdoefer W, Aliaga-Alcade N, Hendrickson D N &

Christou G, Satllre, 416 (2002) 403. 14 Kato M & Muto Y, Coord Chem Rev, 92 (1 988) 45. 15 Hatfield W E, Commel1ts Illorg Chem, 1 (1981) 105. 16 Hay P J, Thibeault J C & Hoffmann R, JA m Chelll Soc, 97

(1975) 4884. 17 Kahn 0, Angew Chelllint Ed Engl, 24 (1985) 834. 18 Nishida Y & Kida S, J Chelll Soc Daltoll TrailS, (1986) 2633. 19 Fallon G D, Murray K S, Mazurek W & O'Connor M J,

Inorg Chilli Acta, 96 (1985) 153. 20 Geetha K, Nethaji M, Vasanthacharya N Y & Chakravarty A

R, J Coord Chelll, 47 (1999) 77. 21 Gleizes A, Julve M, Verdaguer M, Real J A. Faus J & Solans

X, J Chelll Soc Dalton Trans, (1992) 3209. 22 Alvarez S, Julve M & Verdaguer M, Inorg Ch elll , 29 (J 990)

4500. 23 Cano J, Alemany P, Alvarez S, Verdaguer M & Ruiz E,

Chem Eur J, 4 (1998) 476. 24 Thomas A M, Neelakantha G, Mahadevan S, Nethaji M &

Chakravarty A R, Ellr J II10rg Chem (2002) 2720. 25 Trofimenko S, JAm Chem Soc, 89 (1967) 3170. 26 Kahn 0, Mo/ecular Magnetism; (VCH: Weinheim,

Germany), 1993. 27 Bleaney B & Bowers K D, Proc Roy Soc (London) , 214A

(J 952) 451. 28 Geetha K, Tiwary S K, Chakravarty A R & Ananthak rishna

G, J Chem Soc Da/toll Trans (1999) 4463. 29 North ACT, Phillips D C & Mathews F S, Acta Cryst, A24

(1 968) 351. 30 Walker N & Stuart D, Acta Cryst, A39 (1983) 158. 31 Sheldrick G M, SHELXL-97, Program for the SO/lilian of

crystal structures, University of Gottingen, Gottingen, Germany, 1997.

32 Johnson C K, ORTEP, Report ORNL-3794, Oak Ridge National Laboratory, Oak Ridge, Tennessee, 1976.