~ Pergamon 0277-5387(95)00472-6

Pohhedron Vol. 15. N o 12~ pp. 1923 1930, 1996 Copyright ~ 1996 Elsevier Science Lid

Printed in Great Britain. All rights reserved 0277 5387/96 $15.00+0.00

AQUA (IODO) (PYRIDINE- 1-OXIDE-2- CARBOXYLATE)CADMIUM(II), [ C d ( C 6 H 4 N O 3 ) I ( H 2 0 ) ] , A

C O M P O U N D W I T H B I D I M E N S I O N A L SUPRAMOLECULAR S E L F - O R G A N I Z A T I O N

ELENA BERMEJO, ALFONSO CASTINEIRAS* and RICARDO DOM[NGUEZ

Departamento de Qulmica Inorg/mica, Universidad de Santiago de Compostela, E-15706 Santiago de Compostela, Spain

and

JOACHIM STRAHLE and CACILIA MAICHLE-MOSSMER

Institut far Anorganische Chemie der Universitfit Tfibingen, Auf der Morgenstelle 18, A9, D-72076 TObingen, Germany

(Received 26 June 1995 accepted 27 September 1995)

Abstract--The compound aqua(iodo) (pyridine- l-oxide-2-carboxylate)cadmium(l I), [Cd(C6H4NO3)I(H20)], which crystallizes in the monoclinic system, was prepared by reac- tion ofcadmium(II) iodide with picolinic acid N-oxide. This compound exists as a polymeric structure in layers along the bc plane forming a supramolecular association derived from the existence of Cd--O dative bonds [2.629(7) A], and in which each cadmium atom is coordinated to the iodide ligand (Cd--I, 2.814 ,~) and to the oxygen atoms of the water molecule, the carboxylate group [Cd--O, av. 2.333(8) /~] and the N--O, which acts as a bridge [Cd--O, 2.329(8) and 2.371 (7) ,~] in such a way that if all interactions are taken into account, the cadmium atom is hepta-coordinated with a distorted pentagonal bipyramidal geometry. The existence of a hydrogen bond between the water molecule and the carboxylate group helps to achieve a supramolecular self-organization. IR and NMR (~H and 13C) spectra have also been studied.

Research in the field of supramolecular chemistry has advanced greatly in the past 20 years due to the interest in molecular recognition arising from specific molecular interactions and the relation between structure and bond sites. 1 This interest in molecular associations at the supramolecular level deals not only with the construction of pre-designed crystal structures using directional molecular inter- actions associated with hydrogen bonds,: but also the construction of coordination compounds in which molecular units are associated through secondary and/or dative bonds. This results from

* Author to whom correspondence should be addressed.

the tendency of the metal to increase its coor- dination number and from the ability of ligands containing oxygen, sulfur or halogen atoms with more than one electron pair capable of being donated, to act as bridging ligands. 34

However, the formation of supramolecular struc- tures, involving self-organization processes which go beyond a simple packing of molecules in the crystal lattice, can also be achieved using ligands derived from traditional organic compounds which contain several donor atoms which can be coor- dinated simultaneously to different metal centres, thereby behaving as bridging ligands. 56 Hence, under suitable conditions and if a suitable acceptor is chosen, it is possible to predict and obtain solid

1923

1924

compounds with mono-, di- or tri-dimensional supramolecular organization.

In the case of supramolecular compounds result- ing from the existence of hydrogen bonds and/or, above all, secondary and/or dative bonds, self- organization occurs during the crystallization pro- cess and therefore the structural differences observed in related compounds with different sub- stituents on the ligand can be explained by the role played by the packing forces] This, however, can- not be expected when such organization is due to simultaneous multi-coordination of the ligand.

As part of the study of coordination compounds having supramolecular organization, this paper presents the structure and some spectroscopic properties of the bi-dimensional supramolecular compound aqua(iodo) (pyridine- 1-oxide-2-car- boxylate)cadmium(I l) [Cd(OOC-pyO) I (H20)],,.

EXPERIMENTAL

E. BERMEJO et al.

X-ray data collection and reduction

Intensity data for a prismatic crystal with dimen- sions 0.05 x 0.25 x 0.25 mm were measured at 228 K on an Enraf-Nonius CAD-4 diffractometer fitted with graphite monochromatized Mo-K~ radiation (2 = 0.71073/~), Cell parameters were refined by a least-squares procedure on the setting angles of 25 reflections (5.0 ~< 0 ~< 15.4~'). ~ The co/20 scan tech- nique was employed to measure intensities up to a maximum Bragg angle of 28'. No decomposition of the crystal occurred during the data collection. Corrections were applied for Lorentz and polar- ization effects and for absorption (/~= 5.34 mm-~)~o A total of 2900 reflections was collected, of which 2112 were unique (R~ = 0.039), and of these 1820 satisfied with I >~ 3.0a(1) criterion of observability and were used in the subsequent analysis. A summary of the crystal data are listed in Table 1.

Pyridine-l-oxide-2-carboxylic acid (picolinic acid N-oxide) was prepared by a similar method to that reported for 2-acetylpyridine-N-oxide, ~ using the reaction of 2-acetylpyridine with excess hydro- gen peroxide (30%) in glacial acetic acid. 2-Ace- tylpyridine (Aldrich) and cadmium(II) iodide (Aldrich) were used without further purification.

Preparation of [Cd(C6H4NO3)I(H:O)],

Pyridine-l-oxide-2-carboxylic acid (0.278 g, 2 retool) in ethanol (25 cm 3) was mixed with a solu- tion of CdI2 (0.732 g, 2 retools) in the same solvent (10 cm ') at room temperature, After 8 days of stir- ring a yellow crystalline material deposited. The solid was filtered, washed with ethanol and dried. Golden crystals suitable for X-ray analysis were obtained by slow evaporation of the ethanolic solu- tion. Found: C, 18.2; H, 1.9; N, 3.3. C6H6CdlNO4 requires: C, 18.2; H, 1.5; N, 3.5%.

Physical measuremen ts

Elemental analysis was carried out on a Perkin- Elmer 240B microanalyser. ~H and 13C N M R spec- tra were recorded on a Bruker AM500 spec- trometer. Chemical shifts (8) are reported in ppm relative to Me4Si. The IR spectrum in the 4000-400 cm- ~ range was recorded in KBr pellets on a Bruker IFS 66V spectrophotometer and in the 500-100 cm-~ range as polyethylene pellets on a Mattson Cygnus 100 spectrophotometer.

Structure solution and refinement

The Cd and I atoms were located using heavy- atom Patterson methods xt and subsequent cycles of structure-factor calculation and difference synthesis were used to locate the remainder of the non-H atoms. These atoms were refined with anisotropic displacement parameters by a full-matrix least- squares procedure based on F. 9 Hydrogen atoms were located from difference Fourier synthesis and added to the structure factor calculations with iso- tropic Beq values fixed at 4.0/~2, but their positions were not refined. The weighting scheme of the form w = 1/a2(F) was introduced and the refinement pro- ceeded smoothly to convergence with a maximum A/a of 0.001 when R = 0.046, Rw = 0.056 and

Table 1. Crystal data of [Cd(OOC- pyO)l(H20)],,

Molecular formula C,H~CdlNO~ Formula mass 395.42 Crystal system Monoclinic Space group P2~/c a (~) 10.848(4) b (A) 8.488(1) c (~) 10.604(4)

( ) 90.00(-) fi (') 90.77(2) 7 ( ) 90.00(-) V (/~3) 976.3(9) Z 4 D~ (g cm ~) 2.690 F(000) 728

Aqua(iodo) (pyridine-l-oxide-2-carboxylate)cadmium(!l) 1925

GOF = 3.796 (goodness-of-fit) for 119 variables. Correction for extinction was made in the last circle of refinement.t2 Secondary-extinction coefficient refined to 9 = 5.541 × 10 s {Fo = Fc/[1 +9(Fc)- 2Lp]}.

Atomic scattering factors and anomalous-dis- persion corrections for all atoms were taken from International Tables for X-ray Cr),stallography. 13 An ORTEPII t4 drawing with the numbering scheme used is shown in Fig. 1 and a SCHAKAL ~5 drawing showing the unit cell packing is shown in Figs 2 and 3.*

RESULTS AND DISCUSSION

Aqua(iodo)(pyridine - 1 - oxide - 2 - carboxylate) cadmium(II) can be described as a polymeric struc- ture developed along the b and c axes. Figure 1 shows the asymmetric unit as well as the coor- dination around each cadmium atom. Table 2 con- tains the most significant bond distances and angles.

Configuration around the cadmium atom

The coordination polyhedron for each cadmium atom can be described as a distorted pentagonal

bipyramid. A coordination number of seven is not often found in cadmium(II) complexes, but some structures are known, most of which are also dis- torted pentagonal bipyramids where the metal atom is coordinated through the oxygen atoms of car- boxylate groups giving, as in this case, polymeric compounds.16

In the compound under study the coordination number seven was achieved by coordination through iodide and six oxygen atoms, one from a water molecule, two from different N - - O groups and three from two different carboxylate groups. In the resulting CdO6I skeleton, the axial positions are occupied by an iodide and the oxygen of the N - - O group, while the equatorial positions are occupied by an oxygen atom of another N - - O group belonging to an adjacent asymmetric unit (symmetry code: l - x , - ) , , l - z ) , the oxygen of the water molecule, the two oxygen atoms from the carboxylate group and a third oxygen atom from an adjacent carboxylate group (symmetry code: 1 - x , - 0 . 5 + y , 0 . 5 - z , Fig. 1). This results in the formation of two chelate rings around the cadmium atom, one being four membered, C d - - O ( 1 ) - - C(1)--O(2) and the other six membered, C d - - O(2) - -C(1) - -C(2) - -N(1) - -O(3) .

C (5) C(~) C(3)

~~ 0 [I~'~ ~0{3}

Edii CdV

Fig. 1. Perspective view showing the atom numbering scheme and the coordination geometry about the cadmium atom. Symmetry codes are: (i) = l - x , -0.5+), , 0 . 5 - : ; (ii) = l - x , - y , l - z ;

(iii)= l - x , 0.5+y, 0 . 5 - z ; ( i v ) = x , 0.5 )', - 0 . 5 + z ; (v) = x, 0.5-y, 0.5+z.

* Further details of the structure determination have been deposited as Supplementary Publication No. CSD- 59029. Copies may be obtained through the Fach- informationszentrum Karlsruhe, D-76344 Eggenstein- Leopoldshafen, Germany.

Most of the C d - - O distances are similar and the mean value of 2.333(8) /~ is consistent with the reported mean value of 2.318 ~ for C d - - O H 2 (ter- minal) distances in cadmium(lI) compounds with coordination numbers of 5-8. ~v The C d - - O - - H angles of 104.9 and 110.6' involving the aqua ligand clearly indicate that one of the orbitals with a non-

1926 E. BERMEJO et al.

Fig. 2. View of the bidimensional supramolecular self-organization from [Cd(OOC-pyO)l (H20)],,.

)

b

a

Fig. 3. View of the unit cell from [Cd(OOC-pyO)l(H20)],,, showing the packing along the b and c axes.

bonding electron pair on the oxygen atom is point- ing directly towards the cadmium atom.

The Cd--O(3) distance of 2.371(7) ~ can be explained by the fact that 0(3) is located at one of the vertices of the bipyramid, but particularly by the fact that this atom is bridged to another cad- mium atom forming a dimeric unit. The other dis- tance, which is significantly different to the mean value indicated above, corresponds to one of the

oxygen atoms of the carboxylate group. In fact, there are two different Cd- -O distances involving this group, Cd- -O( l ) 2.329(8) and Cd--O(2) 2.629(7) A. This is to be expected since the car- boxylate group acts as both a chelating and bridg- ing ligand, the longer bond involving the bridged oxygen. Similar behaviour has previously been observed for acetate ligands. ~

The Cd-- I distance of 2.814(2) /~ is slightly

Aqua (iodo) (pyridine- 1-oxide-2-carboxylate)cadmium(ll)

Table 2. Selected bond lengths (~) and angles ( ) for [Cd(OOC- pyO)I(H20)]S

Cd--I 2.814(2) Cd--O(2 ~) 2.350(7) Cd--O 2.325(8) Cd--O(3) 2.371(7) Cd--O(1) 2.329(8) Cd--O(3') 2.329(7) Cd--O(2) 2.629(7)

l--Cd--O 94.0(2) O(1)--Cd--O(2) 52.1(2) l--Cd--O(l) 112.1 (2) O(l)--Cd--O(2') 80.7(3) I--Cd--O(2) 92.5(2) O(1)--Cd--O(3) 82.5(3) I--Cd--O(2') 90.4(2) O(I)--Cd--O(Y ~) 139.7(3) I--Cd--O(3) 164.4(2) O(2)--Cd--O(2) 129.9(2) I--Cd--O(Y ~) 97.8(2) O(2)--Cd--O(3) 101.1 (2) O--Cd--O(1) 119.3(3) O(2)--Cd--O(3 ~) 155.6(2) O--Cd--O(2) 74.0(3) O(2~)--Cd--O(3) 86.7(3) O--Cd--O(2 ~) 155.5(3) O (2')--Cd--O(Y) 72.3(3) O--Cd--O(3) 82.6(3) O(3)--Cd--O(3 ~) 66.7(2) O--Cd--O(3 ii) 83.2(3)

"Numbers in parentheses are estimated standard deviations in the least significant digits; Symmetry codes are: (i)= l - x , -0 .5+y , 0.5 z; (ii) = l - x , - y , 1-z .

1927

longer than that usually observed in most iodo- cadmium(l I) complexes.17 This can explained by an increase in the cadmium(II) coordination number, but, above all, because the iodide is located at the opposite vertex of the bipyramid.

Of the oxygen atoms occupying the equatorial positions, four are coplanar with the metal atom, while O(1) is located 0.907 A above the plane defined by the equation: - 0 . 9 0 9 0 x + 0 . 1 4 4 2 y - 0.3910z-5.8187 = 0 , where x, y and z are the orthogonal coordinates. The degree of pentagonal bipyramid distortion is also reflected in the angles around the cadmium atom in the equatorial plane, where the angle involving the oxygen atoms of the same carboxylate group is reduced to 52.1(2)- as opposed to the ideal value of 7 2 causing enlarge- ments of the other four angles, ranging from 0.3 for O(2J)--Cd--O(Y ~) to 11.2" for O--Cd--O(Yi) . Distortion is also observed in the axial positions, where the I - -Cd- -O(3) angle is 164.5(2) as opposed to the ideal angle of 180.

The geometry of the ligand although comparable to that of the free acid J9 has two notable differences. Firstly, the C- -O bond distances in the carboxylate group are quite similar [1.277(12) and 1.231(12) ]~ as opposed to 1.32 and 1.22 A in the free ligand] as a result of the participation of these oxygen atoms in coordination. Secondly, while picolinic acid N- oxide is a planar molecule, the carboxylate group in the complex is twisted 32.1- in relation to the heterocycle.

Supramolecular association

The molecule described above is linked to adjac- ent units forming a laminar supramolecular struc- ture along the b and c axes, as can be seen in Fig. 2, as a consequence of the formation of two strong oxygen-cadmium bonds [2.329(8) and 2.371 (7) A], which give rise to chains along the c axis, forming planar four-membered Cd202 rings. These chains are interlinked in the direction of the b-axis, as a result of the formation of an additional oxygem cadmium bond, which is weak (2.629 ,~) but never- theless shorter than the sum of the van der Waal's radii (3.10/~).20

Cadmium-cadmium bond distances smaller than 6 A are given in Table 3, where it can be seen that the

Table 3. Cd--Cd distances (/~) below 6A"

Cd--Cd i 4.732(1) Cd--Cd'i 3.925(1) Cd--Cd iil 4.732(1) C d - - C d i~ 5.947(1)

C d - - C d ~ 5.947(1)

"Symmetry codes are : (i) = 1 - x , -0 .5+y , 0 .5-z ; (ii)= l - x , - y , 1 z ; ( i i i ) = l - x , 0 . 5 + y , 0 . 5 - z ; (iv) = x, 0.5--y, - 0 . 5 +z ; (v) = x, 0.5-y, 0.5+z.

1928

shortest distance [3.925(1)/~], corresponding to the Cd202 ring, is larger than the sum of van der Waal 's radii. Other structural parameters for such rings are O . . . O 2.589(1) ~ , C d - - O - - C d 113.8(2) and O - - C d - - O 66.7(2)', rather different from those found in cadmium(II) 2-pyridinecarboxylate, a centrosymmetrical dimer with Cd. - • Cd = 3.709(4) ~ , O . . . O = 2.69(3) /~, C d - - O - - C d = 108.2(6) " and O - - C d - - O = 71.8", 2z although in this last compound the cadmium atom have an octahedral arrangement.

The supramolecular association is further reinforced by the fact that symmetrically related molecules are linked by hydrogen bonds between each O - - H of the water molecule and two car- boxylate oxygen atoms from different ligand mol- ecules (Table 4). 22

It can be observed in Fig. 3 that the Cd202 rings from a channel-shaped arrangement along the b axis, with hydrogen atoms of the coordinated water molecules pointing towards the centre from top and bot tom and intertwined in the direction of such axis with the Cd:O: units. Channels arranged along the c-axis are surrounded, both above and below, by an organic skeleton consisting mainly of pyridine rings which by themselves also form channels, and iodine atoms arranged alternately in the same direc- tion.

IR spectra

Assignments for some of the free ligand IR bands 23 are given in Table 5. The spectrum of the cadmium complex shows the following changes in relation to the free acid: (a) two new bands of medium intensity appear at 3478 and 3352 cm *, corresponding to the v (O- -H) modes of the water molecule coordinated to the metal. These bands appear at slightly lower frequencies than normal due to direct participation of the water molecule in hydrogen bonding. A third band, also of medium intensity, at 699 c m - 1 which does not appear in the acid can be assigned to H:O wagging modes. (b) The bands attributed to the carboxylic group, which in the free acid are at 1720, 1680 and 1305 cm ~,

E. BERMEJO et al.

Table 5. Selected IR vibrations (cm ~) for pyridin-1- oxide-2-carboxylic acid and [Cd(OOC-pyO)I(H20)],

Assignment Acid Complex

v(O--H) 3478 m 3352m

v(C--O) ~' 1720 s 1615 s 1680 s

v(C--O)" 1305 s 1397 s v(N--O) 1255 s 1209 m (NO) bend 847 s 876 m CO2 scissors 801 s 828 m H20 wagging - - 699 m CO2 wagging 674 s 675 m CO, rocking 599 s 576 s v(Cd--O) 447 s

342 s v(Cd--I) 143 s

"In the complex v,,~:m(COO ) and v,,m(COO ), respectively.

have disappeared and the spectrum shows instead two strong bands at1615 and1397 cm ~ thelatter weaker--which have been assigned to the Va~ym ( C O O ) and %re(COO ), respectively, of the car- boxylate group. 24 The positions of these bands are in accordance with partial double-bond character for the C - - O bonds of the carboxylate group, as can be deduced from the distances for C(1)--O(1) bonds (vide supra), as well as from the participation of these oxygen atoms in the intramolecular hydro- gen bond with the water molecule. 2s This is also consistent with the value A = Va~ym(COO )-- Vsym(COO ) = 217 c m - t . 26 The bands assigned to vibrations of the N - - O group in pyridine N-oxide at 1255 and 847 cm i corresponding to the v (N- -O) and bending (N- -O) modes, 27 observed in the spectrum of the complex are shifted to higher and lower frequencies, respectively, consistent with coordination of the ligand through this oxygen atom.

Table 5 also shows the bands observed in the far- IR spectrum, together with their proposed assign-

Table 4. Intermolecular hydrogen bond in [Cd(OOC-pyO)I(H:O)],/

A - - H . . - B A--H(/~) H. . .B( /~) A . - . B ( ~ ) A - - H - - B ( )

O--H(10) . . .O(1 ~i~) 0.84(19) 2.13(19) 2.74(1) 129(15) O--H(20). . . O(2") 0.96(19) 2.21(19) 3.12(1) 157(17)

"Symmetry codes are: (iii)= 1 x, 0.5+y, 0 .5 -z : ( v ) = x , 0 .5 -y , 0.5+z.

Aqua(iodo)(pyridine- 1-oxide-2-carboxylate)cadmium(ll)

0 0

H" "H77

7.67 H Fig. 4.

ments. Despite the fact that r igorous assignments are difficult in this area due to the large number o f bands present, according to reported data, 2s at least three new bands may correspond to meta l -ha logen and metal- l igand vibrations. Two broad strong bands at 446 and 342 cm ~ have been attr ibuted to v (Cd- -O) , the higher energy band probably being associated with C d - - O stretching modes of the N- oxide group, while the lower energy band cor- responds to C d - - O stretching modes of the ca~- boxylate group. > The third band at 143 cm 1 has been assigned to v (Cd-- I ) . 3°

N M R spectra

Spectral data for IH and ~3C N M R are given in Fig. 4. Assignments are based on the spectra o f the free ligand 31 and on data reported for N-oxides of pyridines substituted in position 2 and for car- boxylic ac ids) : As expected, the ~H N M R spectrum shows that carboxylic deprotonat ion and coor- dination through oxygen causes a significant shift to higher field of the four signals due to pyridine ring hydrogens (between 0.43 and 0.53 ppm), while in the ]~C N M R spectrum such deprotonat ion and coordinat ion affect mainly to C(2) and C(1) (shift to higher field). Signals on the remaining carbon atoms are almost unchanged.

Acknowledqements We acknowledge the grant from the Direcci6n General de Investigaci6n Cientifica y Tdcnica (Project PB91-0787). We also thank Professor A. Sfin- chez for valuable discussions.

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