A highly novel dinickel complex: A hydrogen-bonded anti-skew carboxylate bridge and a 2D supramolecular structure

Inorganic Chemistry Communications 12 (2009) 1204–1208

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A highly novel dinickel complex: A hydrogen-bonded anti-skew carboxylate bridge and a 2D supramolecular structure Neslihan Korkmaz a, Aytaç Gürhan Gökçe b, Stephen T. Astley a,*, Muhittin Aygün b,**, Demet Astley a, Orhan Büyükgüngör c _ Department of Chemistry, Faculty of Science, Ege University, 35100-Bornova, Izmir, Turkey _ Department of Physics, Faculty of Arts and Science, Dokuz Eylül University, 35160-Buca, Izmir, Turkey c Department of Physics, Faculty of Arts and Science, Ondokuz Mayıs University, 55139-Kurupelit, Samsun, Turkey a

b

a r t i c l e

i n f o

Article history: Received 1 July 2009 Accepted 21 September 2009 Available online 26 September 2009 Keywords: Dinickel complexes Bridging carboxylate Supramolecular structure Tridentate Schiff base

a b s t r a c t Reaction of the tridentate Schiff base ligands obtained from 2,4-dihydroxybenzaldehyde and either L-isoleucine or L-tert-leucine with Ni(NO3)2 in methanol/water solution in the presence of base afforded dinickel complexes. The crystal structure of the product derived from L-tert-leucine has been determined by X-ray crystallography. The octahedrally coordinated two Ni centers were found to be bridged by a single carboxylate group in an extremely unusual non-planar fashion. A 2D supramolecular structure, constructed by infinite hydrogen-bonded complex sheets parallel to the ab-plane of the unit cell, arises from intermolecular O–H  O hydrogen bonds. Ó 2009 Elsevier B.V. All rights reserved.

There are two major reasons why the preparation of compounds containing bridging carboxylate ligands is of current importance. The first reason is that the active site of metalloenzymes are often found to contain carboxylate-containing ligands bridging two metals [1]. As a consequence, there have been a number of recent reports of the preparation of dimetallic compounds which have been designed to mimic the active sites of metalloenzymes. In the majority of these cases, including dinickel urease mimics [2], additional bridging ligands such as H2O or OH are usually present and it is common for the bridging carboxylate group to adopt either a syn–syn [1,2] or a monoatomic [3] bridging conformation (Fig. 1). These conformations are suitable for metal– metal bond distances of around 3.5 Å or less [1c]. One notable exception is a diiron model for the diferrous core of ribonucleotide reductase in which the two iron centers are doubly bridged by syn–anti carboxylate groups [4]. In this particular case there were no additional bridging ligands and the authors suggested that this could explain why the carboxylate groups adopt the sterically less demanding syn–anti mode. The second major reason is that the carboxylate group is a very versatile ligand [5] and this allows for the preparation of novel 1D or 2D structures which are expected to have interesting properties [6]. In these cases, the alternative coplanar conformations anti–anti

* Corresponding author. Tel./fax: +90 232 3881036. ** Corresponding author. E-mail addresses: [email protected] (S.T. Astley), [email protected] (M. Aygün). 1387-7003/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2009.09.021

[5b,6] and the more prevalent syn–anti [7] are more likely to be observed as their geometry is particularly suitable for chain [7] or sheet [8] structures. Thus, metal carboxylate chemistry remains an area of intense research and as a result, recent years have seen a number of reports of more unusual bonding modes and structures, such as semicoordination [5c], hydrogen-bonded dimers [9] or non-planar conformations. Concerning, non-planar conformations, if deviation from a planar conformation is only slight, then the resultant structures may be labelled as twisted or tilted orientations of the corresponding planar conformations. Thus, there have been reports of twisted and tilted syn–syn conformations [10]. However, if substantial deviations are observed, such that the structures begin to no longer resemble the planar conformations, then alternative names for the resultant conformations may be used. Thus, bridging carboxylate group conformations have been labelled as being ‘‘syn-skew” [11], ‘‘skew-skew” [12] or simply ‘‘out-of-plane” [13]. The latter naming system was used to describe carboxylate bridges in two polymorphic forms of Cu(pyridine-2-carboxylate)2. In these structures, carboxylate group bridges occur between planar Cu(pyridine-2-carboxylate)2 moieties when a weak bond is formed between the free oxygen atom of a monodentate carboxylate group to the vacant axial position of a Cu(II) center of an adjacent plane. In work related to this study, Cai recently reported a series of dinickel complexes of amino acids in which the two metals atoms are bridged by a single carboxylate group in the syn–anti conformation [14]. Although other examples are known [15] this remains a relatively rare occurrence for simple dimetallic complexes. In this

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Fig. 1. Common conformations of bridging carboxylate groups: (a) syn–syn (common in dimetallic compounds), (b) monatomic and (c) syn–anti (common in polymeric compounds). All conformations are planar.

communication we report a second example of a dinickel structure containing a single carboxylate bridge. However, in this case the bridging carboxylate group occupies an extremely unusual hydrogen-bonded non-planar conformation. In this study, Schiff base ligands (4) were prepared from the reaction of 2,4-dihydroxybenzaldehyde with the sodium salts of tert-leucinol and iso-leucinol in methanol (Scheme 1) [16]. The ligands were isolated as their monosodium salts and were obtained as yellow solids which were stable for several days at room temperature in sealed containers. Reactions of the ligands with Ni(NO3)2 were carried out in methanol in the presence of base to remove the phenolic proton [17]. The reactions were monitored by UV. Upon complexation by Ni(II), blue shifts of the ligand bands at 368 and 303 nm to around 350 and 290 nm were observed. Solvent removal afforded pale green precipitates which were crystallised from MeOH/H2O to afford the pure products as pale green crystalline materials. Crystals suitable for structure determination could be obtained using compound 5a.

Single crystal X-ray diffraction analysis of 5a [18] revealed that the asymmetric unit consists of a binuclear [Ni2(C13H15O4N)2(H2O)5] complex and a water molecule, linked by an intermolecular O–H  O bond (Fig. 2). The Ni atoms are each in a distorted octahedral environment defined by five O atoms and one N atom comprising NiNO5 cores. The nickel(II) centers are bridged by a carboxylate group in an extremely unusual non-planar bridging 0 mode, resulting in a Ni  Ni distance of 4.8940(9) Å A. For both Ni centers, the equatorial planes are composed of the oxygen and nitrogen atoms of the tridentate ligand and an oxygen atom of a coordinated water molecule. For the Ni1 center, the axial positions are occupied by two oxygen atoms of coordinated water molecules, whereas for the Ni2 center, one oxygen atom of a coordinated water molecule and one oxygen atom from the bridging carboxylate group occupy these positions. The dihedral angles between the five- and six-membered rings are 17.64(18)° and 14.36(18)° for Ni1 and Ni2, respectively. The relative configuration at the C2 and C15 chiral centers are confirmed to be S and S, respectively. Two intramolecular hydrogen bonds are present between H atoms of coordinated water molecules and O atoms of the bridging carboxylate group and a coordinated phenolate group, generating an S22 graph-set motif [20]. This type of hydrogen bonding from a coordinated water molecule to a bridging carboxylate group has been observed before in a Mn complex containing a unidentate bridging carboxylate group [3]. The Ni–O and Ni–N bond lengths are in the range of 1.976(3)– 0 2.079(5) and 1.988(4)–2.088(4) Å A for the Ni1 and Ni2 centers, respectively, and in agreement with available data on carboxylate bridged octahedral dinickel complexes [2d,21]. By comparison, the Ni–O bond distances to the three axially coordinated water molecules involve longer separations and are in the range of

Scheme 1. Preparation of ligands 4a and 4b.

C2 C1

N1

O3

C15 N2

O1w

O6

O1

Ni1

O2

O8

C14 O4

Ni2

O7

O5 O6w

O3w O5w

O4w

O2w Fig. 2. An ORTEP3 view of 5a. Displacement ellipsoids are shown at the 30% probability level and hydrogen atoms not involved in the interactions shown have been omitted 0 for clarity. Selected bond lengths (Å A) and angles (°): Ni1–O1 1.976(4), Ni1–N1 2.006(4), Ni1–O2 2.051(3), Ni1–O2W 2.079(4), Ni1–O1W 2.094(5), Ni1–O3W 2.124(4), Ni2–N2 1.988(4), Ni2–O4 1.997(3), Ni2–O5 2.055(3), Ni2–O5W 2.088(4), Ni2–O4W 2.111(4), Ni2–O3 2.221(4), O2–C1 1.260(6), O3–C1 1.252(7), O1–Ni1–O2 171.99(14), N1–Ni1– O2W 173.3(2), O1W–Ni1–O3W 168.1(2), O4–Ni2–O5 173.24(14), N2–Ni2–O5W 170.21(17), O4W–Ni2–O3 166.80(17), C1–O2–Ni1 115.9(3), C1–O3–Ni2 112.4(3).

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Table 1 Hydrogen-bonding geometry (Å, °) for 5a. D–H  A

H  A

D  A

\D–H  A

O4W–H4WA  O6W O3W–H3WB  O4 O5W–H5WA  O2 O6W–H6WB  O1i O1W–H1WA  O7ii O8–H8  O3iii O6–H6  O7iv O2W–H2WA  O5v O5W–H5WB  O6vi O3W–H3WA  O6vi O6W–H6WA  O8vi

1.88 1.90(4) 1.87(4) 1.84(5) 1.94(3) 2.19 1.78 1.98 2.16(7) 2.21 2.10(4)

2.636(7) 2.715(6) 2.661(6) 2.686(6) 2.718(7) 2.992(5) 2.601(4) 2.683(6) 2.960(6) 2.924(5) 2.948(6)

152 171(6) 160(4) 159(6) 159(5) 167 173 142 172(9) 146 159(10)

Symmetry codes: (i) x, 1 + y, z; (ii) x, 1 + y, z; (iii) 1 + x, y, z; (iv) 1 + x, 1 + y, z; (v) 1  x, 1/2 + y, 3/2  z; (vi) x, 1/2 + y, 3/2 – z.

2.094(5)–2.124(4) Å A. The remaining bond in the axial direction is the out of plane bond between Ni2 and the 0bridging carboxylate group which has a bond distance of 2.221(4) Å A. Although considerably longer than all other Ni–O separations, it clearly represents a bonding interaction [13b]. This is despite the Ni2 center being far removed from the plane of the carboxylate group. The degree of non-planarity can be represented by the torsion angle Ni2–O3–C1– C2 of 111.0(4)° and by contrast, the torsion angle Ni1–O2–C1–C2 of 20.0(5)°. Although the latter shows a degree of non-planarity, it clearly resembles an anti conformation. The former torsion angle expresses a much greater deviation from planarity and in this case, the conformation does not resemble either a syn or an anti conformation. Thus, the conformation of the carboxylate group is best labelled as an anti-skew conformation. Finally, it is worth mentioning that there is disorder in one of the tertiary butyl

Fig. 3. Part of the crystal structure of 5a viewed along the b-axis, showing the formation of 2D supramolecular structure with intermolecular O–H  O hydrogen bonds. For the sake of clarity, H atoms not involved in the motifs shown have been omitted.

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groups. Although all three methyl carbons (C17, C18 and C19) of this group display large displacement parameters, the disorder manifests itself most clearly in C19. Therefore, the disordered model was used only for atom C19 in the final refinement with the final occupancies being 0.64(7)/0.36(7) for C19A and C19B, respectively. The supramolecular structure of 5a takes the form of sheets generated by O–H  O hydrogen bonds (Table 1) and the formation of the sheet is analyzed in terms of substructures. In the first substructure, oxygen atoms of uncoordinated and coordinated water act as hydrogen-bond donors, respectively, to coordinated phenolate and carboxylate oxygens (O6 W–H6WB  O1, O1 W– H1WA  O7), so forming a Cð10ÞC 22 ð10Þ½R33 ð12Þ chain of rings running parallel to the [0 1 0] direction. The second substructure is built using hydrogen bonds between the two uncoordinated phenolic groups (O8 and O6) and carboxylate oxygens (O3 and O7), allowing the formation of Cð8Þ and Cð14Þ chains along [1 0 0] and [1 1 0]. The combination of the first and second substructures generates a sheet parallel to the ab-plane of the unit cell. Considering the chains formed in the first substructure, a pair of chains related by the twofold screw axes along (1/2, y, 3/4) and (0, y, 3/4) are linked by hydrogen bonds between coordinated and uncoordinated water oxygens as donors and carboxylate oxygen and uncoordinated phenolic oxygens as acceptors, in the third substructure. Finally, the combination of these substructures generates a complex sheet parallel to (0 0 1) as shown in Fig. 3. The distance between adjacent sheets is c=2, but there are no interactions between them, so the supramolecular structure of 5a is two-dimensional. The products were also characterised by elemental analysis, magnetic susceptibility, UV and IR spectroscopy and TGA. All these methods of analysis indicate that the structures of the two products are closely related. Thus, elemental analysis of both complexes confirmed the products had an empirical formula of NiL(H2O)3. Magnetic susceptibility (leff) values of 3.1 and 3.24 BM for 5a and 5b, respectively, are within the expected range for octahedral Ni(II) complexes [22]. In methanol solution, d–d bands in their UV spectra at around 500, 610 and 880 nm are also characteristic of octahedral geometry at Ni(II) [23]. The solid-state IR spectra (KBr) of 5a and 5b, while maintaining small differences, are quite similar to each other. For both 5a and 5b the most intense band in their spectra can be assigned to a tasym(CO2 ) band. For 5a, this band is observed at 1532 cm1 whereas for 5b it appears at 1541 cm1. For both products two 1 can also bands assignable to tasym(CO 2 ) at ca. 1445 and 1385 cm be observed. For 5b two additional bands appear at 1637 and 1612 cm1. It can be assumed that one of these bands is due to t(C@N) whereas the second is due to a second tsym(CO2 ). In the same region, only one band can be observed for 5a which is centered at 1626 cm1. The overall similarity of these data suggests that the solid-state structure of 5b is similar to that of 5a. It is well  known that the value of tasym(CO 2 )  tsym(CO2 ) is normally less for bridging carboxylate groups than for unidentate carboxylate groups and it is also known that hydrogen bonding to carboxylate groups can also reduce this value [24]. Therefore, it would seem highly probable that the combination of bands near 1540 and  1445 cm1 for both complexes are the tasym(CO 2 ) and tsym(CO2 ) pair of bands arising from the bridging carboxylate group. Finally, TGA was used to investigate the effect of heat on the coordinated and uncoordinated water molecules. For both compounds, loss of H2O occurred in two steps with initial loss of four molecules of H2O occurring for each dinickel unit. This occurs in the range 30–95 °C for both 5a (10.6% found, 10.0% calculated) and 5b (9.7% found, 10.0% calculated). To explain this, it seems reasonable to assume that there is initial loss of the water of crystallisation and of three coordinated water molecules from axial positions. This could afford four-coordinate square planar Ni(II)

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species of which related Schiff base compounds are known [25]. Above 110 °C, loss of the remaining two water molecules occurs. For 5a, this occurs in the range 130–200 °C (4.9% found, 5.0% calc) and for 5b, it occurs in the range 110–170 °C (4.9% found, 5.0% calc). In summary, to our knowledge, the structure of 5a represents the first clear example of the anti-skew conformation of a bridging carboxylate group. Spectroscopic and analytical data indicates that 5b has a closely related structure. Thus, we would expect 5b also to occupy an anti-skew conformation. However, minor differences in their IR spectra and TGA behaviour might suggest that fine tuning of the structure occurs in order to find the most stable way to accommodate the different alkyl groups. We suspect that the reason why this type of conformation is favoured in these particular examples is to a large part because the resultant geometry of the dinickel complex is favourable for formation of intra- and intermolecular O–H  O hydrogen bonds utilising the carboxylate and phenolic groups along with H atoms of coordinated and uncoordinated water molecules and that these hydrogen bonds play an important role in contributing to the overall stability of this unusual dinickel structure. Acknowledgements The authors are grateful to Ege University Faculty of Science and _ to TÜBITAK (Grant 107T778) for financial support of this work. The authors also acknowledge the Faculty of Arts and Sciences, Ondokuz Mayıs University, Turkey, for the use of the Stoe IPDS-II diffractometer (purchased under Grant No. F.279 of the University Research Fund). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.inoche.2009.09.021. References [1] (a) E. Jabri, M.B. Carr, R.P. Hausinger, P.A. Karplus, Science 268 (1995) 998– 1004; (b) C. He, S.J. Lippard, J. Am. Chem. Soc. 120 (1998) 105–113; (c) J. Kuzelka, B. Spingler, S.J. Lippard, Inorg. Chim. Acta 337 (2002) 212–222; (d) U.P. Singh, P. Babbar, A.K. Sharma, Inorg. Chim. Acta 358 (2005) 271–278; (e) T. Tanase, J.W. Yun, S.J. Lippard, Inorg. Chem. 35 (1996) 3585–3594. [2] (a) R.M. Buchanan, M.S. Mashuta, K.J. Oberhausen, J.F. Richardson, Q. Li, D.N. Hendrickson, J. Am. Chem. Soc. 111 (1989) 4497–4498; (b) K. Yamaguchi, S. Koshino, F. Akagi, M. Suzuki, A. Uehara, S. Suzuki, J. Am. Chem. Soc. 119 (1997) 5752–5753; (c) S. Buchler, F. Meyer, E. Kaifer, H. Pritzkow, Inorg. Chim. Acta 337 (2002) 371–386; (d) W.-Z. Lee, H.-S. Tseng, M.-Y. Ku, T.-S. Kuo, Dalton Trans. (2008) 2538–2541; (e) H. Carlsson, M. Haukka, A. Bousseksou, J.-M. Latour, E. Nordlander, Inorg. Chem. 43 (2004) 8252–8262; (f) H. Adams, S. Clunas, D.E. Fenton, S.E. Spey, Dalton Trans. (2003) 625–630; (g) B.-H. Ye, T. Mak, I.D. Williams, X.-Y. Li, Chem. Commun. (1997) 1813–1814. [3] D. Moon, J. Kim, M. Oh, B.J. Suh, M.S. Lah, Polyhedron 27 (2008) 447–452. [4] S. Menage, Y. Zang, M.P. Hendrich, L. Que Jr., J. Am. Chem. Soc. 114 (1992) 7786–7792. ˇ ák, Z. Vargová, K. Györyová, Spectrochim. Acta Part A: Mol. Biomol. [5] (a) V. Zelen Struct. 66 (2007) 262–272; (b) J. Boonmak, S. Youngme, T. Chotkhun, C. Engkagul, N. Chaichit, G.A. van Albada, J. Reedijk, Inorg. Chem. Commun. 11 (2008) 1231–1235; ˇ ák, I. Císarˇová, P. Llewellyn, Inorg. Chem. Commun. 10 (2007) 27– (c) V. Zelen 32. [6] (a) M.R. Montney, R.L. LaDuca, Inorg. Chem. Commun. 10 (2007) 1518–1522; (b) F.S. Delgado, C. Ruiz-Perez, J. Sanchiz, F. Lloret, M. Julve, CrystEngComm 8 (2006) 507–529. [7] (a) C. R Choudhury, A. Datta, V. Gramlich, G.M.G. Hossain, K.M.A. Malik, S. Mitra, Inorg. Chem. Commun. 6 (2003) 790–793; (b) K.-Y. Choi, Y.-M. Jeon, H. Ryu, J.-J. Oh, H.-H. Lim, M.-W. Kim, Polyhedron 23 (2004) 903–911; (c) Y.-H. Wang, R.-F. Song, F.-Y. Zhang, J. Mol. Struct. 752 (2005) 104–109; (d) M. Estrader, C. Diaz, J. Ribas, X. Solans, M. Font-Bardía, Inorg. Chim. Acta 361 (2008) 3963–3969;

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[18]

[19]

[20] [21]

[22] [23] [24] [25]

analysis (%) calcd. for C26H42N2O14Ni: C, 43.13; H, 5.85; N, 3.87. Found: C, 42.71; H, 5.55; N, 4.02. IR (KBr): 1626 cm1, 1532, 1445, 1385, 1216. Crystallographic data for [Ni2(C13H20O4N)2(H2O)5]H2O: Orthorhombic P 212121, a = 9.9199(6) Å, b = 12.0799(8) Å, c = 26.5162(15) Å, V = 3177.5(3) Å3, Z = 4, F(000) = 1520, l = 1.254 mm1. A green suitable crystal with dimensions of 0.60  0.34  0.07 mm3 was selected for the crystallographic study. The diffraction intensity data were collected at room temperature (293 K) on a STOE IPDS 2 diffractometer using graphite monochromatized Mo Ka radiation (k = 0.71073 Å). The cell parameters were determined by using X-AREA software [19a] on the setting angles of 18,926 reflections [1.54 < h < 27.27°]. Absorption correction was achieved by the integration method via X-RED32 software [19a]. A total of 14,289 reflections were collected for 12 6 h 6 12, 10 6 k 6 15 and 33 6 l 6 33. The structure was solved by direct methods using SHELXS-97 [19b]. The refinement was carried out by full-matrix leastsquares method on the positional and anisotropic temperature parameters of the non-hydrogen atoms with SHELXL-97 [19b]. The methyl group (C19A and C19B) was disordered over two different orientations and accordingly was refined with a geometrically restrained model. The refinement converged to final occupancies of 0.64(7)/0.36(7). All hydrogen atoms except water hydrogen atoms were treated as riding atoms, with C–H distances of 0.93, 0.96 and 0.98 Å, and O–H distances of 0.82 Å. The H atoms of water molecules were found in difference Fourier map, and the O–H distances were restrained to 0.82(2) for coordinated and 0.89(2) for uncoordinated water molecules during refinement with DFIX instruction. The structure was refined to R1 = 0.038 and xR2 = 0.076 for observed 3763 reflections and GOF = 0.935. The maximum peaks and deepest hole observed in the final Dq map were 0.59 and 0.32 e Å-3, respectively. Molecular graphics are prepared using ORTEP [19c] and PLATON [19d] software. (a) Stoe & Cie, X-AREA (Version 1.18) and X-RED32 (Version 1.04), Darmstadt, Germany, 2002.; (b) G.M. Sheldrick, Acta Crystallogr. A 64 (2008) 112–122;; (c) L.J. Farrugia, J. Appl. Crystallogr. 30 (1997) 565;; (d) A.L. Spek, J. Appl. Cryst. 36 (2003) 7–13. J. Bernstein, R.E. Davis, L. Shimoni, N.-L. Chang, Angew. Chem., Int. Ed. Engl. 34 (1995) 1555–1573. (a) V. Lozan, A. Buchholz, W. Plass, B. Kersting, Chem. Eur. J. 13 (2007) 7305– 7316; (b) H.-K. Fun, A. Sinthiya, S.R. Jebas, B.R.D. Nayagam, S.A.C. Raj, Acta Crytallogr. E 64 (2008) m1436–m1437; (c) J. Kong, H. Zhou, Z.-Q. Pan, Acta Crytstallogr. E 64 (2008) m18. F.A. Cotton, G. Wilkinson, Advanced Inorganic Chemistry, fourth ed., Wiley, New York, 1980. p. 787. A.S. El-Tabl, F.A. El-Saied, A.N. Al-Hakimi, Trans. Met. Chem. 32 (2007) 689– 701. G.B. Deacon, R.J. Phillips, Coord. Chem. Rev. 33 (1980) 227–250. D. Pawlica, M. Marszałek, G. Mynarczuk, L. Sieron´, J. Eilmes, New J. Chem. 28 (2004) 1615–1621.