Hydrogen-Bonded Networks Based on Cobalt(II), Nickel(II), and Zinc(II) Complexes of N,N'-Diethylurea

N,N'-diethylurea (DEU) was employed as a ligand to form the octahedral complexes [M(DEU)6]2+ (M=Co, Ni and Zn). Compounds [Co(DEU)6](BF4)2 (1), [Co(DEU)6](CIO4)2 (2), [Ni(DEU)6](CIO4)2 (3), and [Zn(DMU)6](CIO4)2 (4) have been prepared from the reactions of DEU and the appropriate hydrated metal(II) salts in EtOH in the presence of 2,2-dimethoxypropane. Crystal structure determinations demonstrate the existence of [M(DEU)6]2+ cations and CIO4 − (in 2–4) or BF4 − (in 1) counterions. The [M(DEU)6]2+ cations in the solid state are stabilized by a pseudochelate effect due to the existence of six strong intracationic N-H ⋯ O(DEU) hydrogen bonds. The [M(DEU)6]2+ cations and counterions self-assemble to form hydrogen-bonded 2D architectures in 2–4 that conform to the kgd (kagome dual) network, and a 3D hydrogen-bonded rtl (rutile) network in 1. The nature of the resulting supramolecular structures is influenced by the nature of the counter-ion. The complexes were also characterized by vibrational spectroscopy (IR).


Introduction
In 1828, Wöhler attempted to synthesize ammonium cyanate by reacting silver isocyanate (AgNCO) with ammonium chloride (NH 4 Cl). The outcome of this failed attempt was urea H 2 NCONH 2 (U, Scheme 1) which represents the first organic molecule synthesized in the laboratory from purely inorganic materials [1]. Urea has also been recognized as the first organic molecule that was synthesized without the involvement of any living system [1]. Nowadays, urea represents not only an important molecule in biology [2] but also an important raw material in chemical industry [3].
Restricting further discussion to the coordination chemistry of urea and its substituted derivatives, metal-urea complexes have attracted considerable interest since the discovery of the active site of urease, a metalloenzyme that catalyzes the hydrolysis of urea into carbon dioxide and ammonia [4,5]. Considerable efforts have been devoted to devise useful bioinorganic models for the active site of urease and provide information for the intermediates and its catalytic mechanism. That in turn drove to the structural and spectroscopic characterization of many metal-urea complexes [6]. Urea usually coordinates as a monodentate ligand through the oxygen atom, forming a C=O· · · M angle considerably smaller than 180 • , in accordance with the sp 2 hybridization of the O atom (A in Scheme 2). The rare N,O-bidentate coordination mode (B in Scheme 2) has been found in a very limited number of cases [7,8], while in [Hg 2 Cl 4 U 2 ] each U molecule bridges the two Hg II atoms through the oxygen atom [9] (C in Scheme 2). Of particular chemical/biological interest is the ability of U to undergo metal-promoted deprotonation [4,10]; the monoanionic ligand H 2 NCONH − adopts the μ 2 (D in Scheme 2) and μ 3 (E in Scheme 2) coordination modes. The N,N'-alkyl symmetrically substituted derivatives of urea (RHNCONHR), such as the N,N'-dimethylurea (DMU) and N,N'-diethylurea (DEU) (Scheme 1) have only been found to coordinate as monodentate ligands through the oxygen atom (F in Scheme 2). Urea and its substituted derivatives have been extensively studied within the frame of organic crystal engineering due to their ability to form extended hydrogen bonded frameworks. In particular, symmetrically substituted ureas (i.e., RHNCONHR) form α-networks with each urea molecule donating two hydrogen bonds and "chelating" the carbonyl oxygen of the next molecule in the network. In contrast to the great number of studies concerning free ureas [11][12][13][14][15], little is known about the supramolecular structures based on hydrogen bonding interactions between simple metalureas complexes. Over the last decade, we have been studying the coordination chemistry of urea and its symmetrically substituted derivative DMU [16][17][18][19][20][21]. In all cases, ureas form stable complexes which are further connected to create extended frameworks by intermolecular/interionic hydrogen bond interactions. Despite the large number of metal-urea complexes which have been structurally characterized, the metal-DMU complexes are considerably less studied while there only three reports with crystal structures of metal-DEU complexes [22][23][24]. In this report we present our first results from the study of metal-DEU complexes, extending the known crystal structures of metal-DEU complexes to seven.

Experiments
All manipulations were performed under aerobic conditions using materials and solvents as received. IR spectra were recorded on a Perkin-Elmer PC16 FT-IR spectrometer with samples prepared as KBr pellets. C, H and N elemental analyses were performed with a Carlo Erba EA 108 analyzer. [Zn(DEU) 6 ](ClO 4 ) 2 (4). A colourless solution of Zn( ClO 4 ) 2 ·6H 2 O (0.74 g, 2.0 mmol) in EtOH (10 mL) and dimethoxy-propane (DMP) (2.5 mL) was refluxed for 20 minutes, cooled to room temperature, and then treated with solid DEU (1.40 g, 12 mmol). The colourless reaction mixture was refluxed for a further 20 minutes, cooled to room temperature, and layered with Et 2 O (25 mL). Slow mixing gave colourless crystals suitable for X-ray crystallography, which were collected by filtration, washed with cold EtOH (2 mL) and Et 2 O, and dried in vacuo over CaCl 2 . Typical yields were in the 75-85% range.

X-ray
Crystallography. X-ray data were collected at 298 K using a Crystal Logic Dual Goniometer diffractometer with graphite-monochromated Mo-K a radiation (λ = 0.71073Å). Lorentz, polarization, and Ψ-scan absorption corrections were applied using Crystal Logic software. The structures were solved by direct methods using SHELXS-86 [25] and refined by full-matrix least-squares techniques on F 2 with SHELXL-97 [26]. Details of the data collection and refinement are given in Table 1. Topological analysis of the nets was performed using TOPOS program package [27, 28].

Synthetic Comments.
The preparation of the three complexes reported here is summarized in (1):   C=O· · · M angles ranging from 127.6 • to 132.5 • . This is the usual way of coordination of urea and its derivatives and has been observed in the similar [M(DMU) 6 ]X 2 complexes [16][17][18][19][20][21]. Linearly or approximately linearly coordinated ureas are rare and have been observed only in a few cases [21]. There are six strong intramolecular (intracationic) hydrogen bonds inside each cation with atoms N(1), N (11), and N(21) (and their symmetry equivalents) as donors, and atoms  (7)    In this arrangement, a binodal (3,6)-connected network forms with Schläfli symbol (4 3 ) 2 (4 6 .6 6 .8 3 ) (Figure 6). This two-dimensional (2D) hydrogen-bonded kgd net is the dual of the kagome kgm-(3.6.3.6) net. It is worth noting that the 2D network adopted by 2-4 was not adopted by any of the [M(DMU) 6 ](ClO 4 ) 2 complexes [17,18] suggesting that the substitution of DMU by DEU substantially changes the intermolecular (interionic) interactions probably due to the larger ethyl groups (in DEU) instead of the smaller methyl groups (in DMU). Similar 2D networks have been adopted by [Zn(DMU) 6 ](ClO 4 ) 2 [17] and [Co(DMU) 6 ](BF 4 ) 2 [18]  acting as 6-connected nodes but the connections are achieved through two N-H· · · X and one C-H· · · X hydrogen bonds (and their symmetry equivalents), (X = O (perchlorate) or F (tetrafluoroborate) , resp.). The intermolecular hydrogen bonding interactions in 1 are far more interesting that those in 2-4. The [Co(DEU) 6 ] 2+ and the BF 4 − anions have assembled to create a threedimensional (3D) hydrogen-bonded framework through three crystallographically independent intermolecular (interionic) N-H· · · F (tetrafluoroborate) hydrogen bonds (and their symmetry equivalents). Each BF 4 − accepts three hydrogen bonds with the F(1), F(2) and F(3) atoms acting as hydrogen bond acceptors while each [Co(DEU) 6 ] 2+ connects to six BF 4 − anions through the remaining N-H groups (Figure 7). In this arrangement, a (3,6)-connected network forms with the [Co(DEU) 6 ] 2+ cations acting as the 6-connected nodes and the BF 4 − anions as the 3-connected nodes. Although the connectivity of each ion seems identical to that found in 2-4, the arrangement of the [Co(DEU) 6 ] 2+ and BF 4 − ions is quite different resulting in a binodal 3D hydrogen-bonded Table 6: Dimensions of the unique hydrogen bonds (distances inÅ and angles in • ) for complex 1. †    network with a rutile (rtl) topology [30,31] and Schläfli symbol (4.6 2 ) 2 (4 2 .6 10 .8 3 ) (Figure 8). It is worth noting that none of the [M(DMU) 6 ]X 2 complexes [17,18] adopts a 3D net. Table 10 gives diagnostic IR bands of the free ligand and complexes 1-4. Assignments have been given in comparison with the data obtained for the free DMU [32], the free DEU [33] and its Co(II) and Ni(II) complexes [34]. The bands with ν(CN) character are situated at higher wavenumbers in the spectra of 1-4 than for free DEU, whereas the ν(CO) band shows a frequency decrease. These shifts are consistent with oxygen coordination, suggesting the presence of + N=C-O −   resonant forms [17,18]. Upon coordination via oxygen, the positively charged metal ion stabilizes the negative charge on the oxygen atom; the NCO group now occurs in its polar resonance form and the double bond character of the CN bond increases, while the double bond character of the CO bond decreases, resulting in an increase of the CN stretching frequency with a simultaneous decrease in the CO stretching frequency [17,18] − anion appear at 1100-1000 and at 522-580 cm −1 (broad bands), respectively, in the IR spectrum of 1 [35]. The IR spectra of 2-4 exhibit strong bands at ∼1100 and 626 cm −1 due to the ν 3 (F 2 ) and ν 4 (F 2 ) vibrations, respectively, of the uncoordinated ClO 4 − [35]. The broad character and splitting of the band at ∼1100 cm −1 indicate the involvement of the ClO 4 − ion in hydrogen bonding as it was established crystallographically (see above).  (X=ClO 4 or BF 4 ) and the [M(DMU) 6 ]X 2 (X=ClO 4 or BF 4 ) we can conclude that the substitution of DMU by DEU considerably affected the nature of the hydrogen-bonded networks. We are presently pursuing our studies on the coordination chemistry of urea and its symmetrically or unsymmetrically substituted alkyl derivatives to generate a rich variety of hydrogen-bonded networks.