Synthesis, X-Ray Structure, and Characterization of Catena-bis(benzoate)bis{ N,N-bis(2-hydroxyethyl)glycinate}cadmium(II)

The reaction of N, N-bis(2-hydroxyethyl)glycine (bicine; bicH3) with Cd(O2CPh)2 · 2H2O in MeOH yielded the polymeric compound [Cd2(O2CPh)2(bicH2)2]n(1). The complex crystallizes in the tetragonal space group P41212. The lattice constants are a = b = 12.737(5) and c = 18.288(7) Å. The compound contains chains of repeating {Cd2(O2CPh)2(bicH2)2} units. One CdII atom is coordinated by two carboxylate oxygen, four hydroxyl oxygen, and two nitrogen atoms from two symmetry-related 2.21111 (Harris notation) bicH2 − ligands. The other CdII atom is coordinated by six carboxylate oxygen atoms, four from two bicH2 − ligands and two from the monodentate benzoate groups. Each bicinate(-1) ligand chelates the 8-coordinate, square antiprismatic CdII atom through one carboxylate oxygen, the nitrogen, and both hydroxyl oxygen atoms and bridges the second, six-coordinate trigonal prismatic CdII center through its carboxylate oxygen atoms. Compound 1 is the first structurally characterized cadmium(II) complex containing any anionic form of bicine as ligand. IR data of 1 are discussed in terms of the coordination modes of the ligands and the known structure.


Introduction
There are many areas illustrating the importance of cadmium coordination and bioinorganic chemistry and the need for further research in this field. The mobilization and immobilization of Cd II in the environment, in the organisms, and in some technical processes can depend significantly on the complexation by chelating organic ligands [1]. For example, anthropogenic chelators released into the environment, humic acids, and several types of ligands produced by microorganisms contribute to the transfer of this metal ion between solid and aqueous phases [2]. Examples of applied cadmium coordination chemistry are found in wastewater treatment and organic separation problems [1,3]. Cadmium is also important in the interdisciplinary field of Bioinorganic Chemistry. Though Cd II probably does not have any biological function, the body of a normal human adult usually contains some milligrams of it [4], mainly in metallothioneins, where it is tightly bonded to cysteinyl sulfur atoms [5]. In special cases of cadmium poisoning, the so-called "chelation therapy" can be applied in which synthetic chelators, like EDTA 4− and 2,3-dimercapto-1-propanol (BAL), are given as antidotes [6]. A number of research groups have been also using 113 Cd NMR spectroscopy as a "spin spy" in the study of Zn IIcontaining proteins [7]. Systematic comparative studies on the coordination chemistry of Cd II and Zn II with ligands containing donor groups of biological relevance are useful in this topic. The stereochemical adaptability of this d 10 metal ion favours structural variations, and this fact makes Cd II a central "player" in the fields of Crystal Engineering and Metallosupramolecular Chemistry [8,9]. Amongst the ligands that have never been used for the preparation and study of Cd II complexes, neither in the solid state nor in solution, is N,N-bis(2-hydroxyethyl)glycine, generally known as bicine (bicH 3 , Scheme 1). This is a currently "hot" ligand in Bioinorganic Chemistry. Bicine was first prepared in 1926 by Kiprianov and subsequently became a widely used buffer substance in many biochemical studies [11]. As with its parent compound, the amino acid glycine (glyH) also shown in Scheme 1, the monoanion of bicine, that is, the bicinate (−1) ion (bicH 2 − ), forms metal complexes. The stability constants of many divalent transition metal complexes of bicinate (−1) have been determined, and it has been found that the [M(bicH 2 )(H 2 O) x ] + species is always the predominant species in solution [12]. It has repeatedly emphasized [13][14][15] that as a consequence of its strong complexation properties, the use of bicine as a pH buffer in biochemical or medical studies under the assumption that only little (or no) interaction with divalent metal ions occurs is not justified. It has been shown that not only do bicH 3 and related compounds buffer H + concentrations but also the resultant metal complexes buffer H + and metal ion concentrations; therefore the employment of bicH 3 as a buffer requires great care to avoid conflicting data and erroneous conclusions [13][14][15]. Even though bicinate metal complexes have been studied in solution for years [12][13][14][15][16][17][18], mainly through the excellent research of Sigel [12], only few metal complexes have been structurally characterized in the solid state through single-crystal, X-ray crystallography. In those structural studies it was found (see "Results and Discussion") that the anionic bicH 2 − , bicH 2− , and bic 3− ligands are versatile and behave in a variety of terminal and bridging modes. Due to this versatility, the anionic forms of bicine are promising ligands for the isolation of polynuclear transition metal complexes (clusters) [19,20]. Transition metal cluster chemistry is a currently "hot" research field in contemporary inorganic chemistry [21].
In this paper we report the amalgamation of the abovementioned two research areas by reporting the preparation, structural characterization, and spectroscopic study of the first cadmium(II) bicinate complex. This paper can be considered as a continuation of our interest in the coordination chemistry of bicine [11] and in the Cd II carboxylate chemistry [22].

Experiments
All manipulations were performed under aerobic conditions using materials and solvents as received. Cd(O 2 CPh) 2 ·2H 2 O was prepared by the reaction of Cd(O 2 CMe) 2 ·2H 2 O with an excess of PhCO 2 H in CHCl 3 under reflux. C, H, and N analyses were performed with a Carlo Erba EA 108 analyzer. IR spectra (400-450 cm −1 ) were performed with a Perkin-Elmer PC16 FT-IR spectrometer with samples prepared as KBr pellets. [

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). The appropriate crystal was mounted in air and covered with epoxy glue. Unit cell dimensions were determined and refined by using the angular settings of 25 automatically centered reflections in the range 11 < 2θ < 23 • . Intensity data were recorded using a θ-2θ scan. Three standard reflections showed less than 3% variation and no decay. Lorentz polarization and Ψ-scan absorption corrections were applied using Crystal Logic software. The structure was solved by direct methods using SHELXS-97 [23] and refined by fullmatrix least-squares techniques on F 2 with SHELX-97 [24]. Hydrogen atoms were located by difference maps and refined isotropically, except those on O(3), C(6), and C(15) which were introduced at calculated positions as riding on bonded atoms with U equal 1.3 times the U(eq) of the respective atom. All nonhydrogen atoms were refined anisotropically. CCDC 771321 contains the supplementary crystallographic data for this paper. This data can be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html [or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: ++44-1223-336 033; Email: deposit@ccdc.cam.ac.uk]. Important crystal data and parameters for data collection and refinement are listed in Table 1.

Results and Discussion
The "wrong" Cd II to bicH 3 reaction ratio (1.5 : 1) employed for the preparation of 1 did not prove detrimental to the formation of the product. With the identity of 1 established by single-crystal X-ray crystallography, the "correct" stoichiometry (1 : 1) was employed and led to the pure compound in 75% yield (see Section 2).
The PhCO − 2 group present in the reaction mixture plays a double role. It helps the deprotonation of bicH 3 and participates in the complex as ligand.   Figure 1) belongs to the carboxylate group of the bicinate(−1) ligand. c This carbon atom (not labeled in Figure 1) belongs to the carboxylate group of the benzoate ligand.
As a next step we decided to use a large excess of Cd(O 2 CPh) 2 ·2H 2 O (Cd II : bicH 3 = 3 : 1) or to add base (LiOH, Et 3 N, Bu n 4 NOH) in the reaction mixture targeting the double or/and triple deprotonation of bicine. We repeatedly isolated a powder, analyzed as Cd 2 (O 2 CPh)(bic)(H 2 O) 2 , but we could not crystallize it; thus this second product has yet to be structurally characterized.

Description of Structure.
Selected interatomic distances and angles for complex 1 are listed in Table 2. The molecular structure of the compound is shown in Figure 1.
The compound contains chains of repeating {Cd 2 (O 2 CPh) 2 (bicH 2 ) 2 } units. Each unit contains two crystallographically independent Cd II atoms [Cd(1), Cd (2)  . The increase in bond length upon bridging relative to terminal ligation has been observed previously [22] in complexes containing carboxylate ligands with one bridging oxygen atom. Based on theoretical and experimental studies which have indicated that the syn-lone pairs of the carboxylate group are more basic than the anti-lone pairs [38], one might expect the Cd(2)-O(1) distance to be shorter than the Cd(1)-O(1) distance; however, the reverse relation holds for 1 (see Table 2). This result, which is in accordance with other Cd II carboxylate complexes [22], suggests that the Cd-O bond lengths involving η 1 : η 2 : μ 2 carboxylate groups are mainly influenced by geometrical factors rather than the electronic properties of the carboxylate group. The Cd(2)-O bond lengths agree well with values found for other 6coordinate cadmium(II) carboxylate complexes [39,40]. The average value for the Cd (2) Table 2).  Table 2).
The coordination geometry of Cd(2) can be described as a very distorted trigonal prismatic (  (1) is best described as a distorted square antiprism (Figure 3). Since even the more stable of the possible 8-coordinate geometries (square antiprismatic, triangular dodecahedral, and cubic) differ slightly in energy from one another, the geometry observed may be largely a reflection of constraints placed on the complex by ligand requirements and packing considerations.
Compound 1 is hydrogen bonded. Metric parameters for the bonds are listed in − ligand from a neighbouring chain as acceptor, is responsible for the formation of a 2D network.
Compound 1 joins a family of mononuclear, polynuclear, and polymeric complexes with the mono-(bicH 2 − ), di-(bicH 2− ), and trianionic (bic 3− ) derivatives of bicine as ligands [11,19,20]. The members of this family are listed in Table 4, together with the coordination modes of the bicinate ligands for convenient comparison. The to-date crystallographically established coordination modes of bicH 2 − , bicH 2− , and bic 3− are shown in Scheme 2. Compound 1 is the first cadmium(II) bicinate complex which has been structurally characterized. The bicH 2 − ligand in 1 adopts the extremely rare coordination mode 2.21111; see Scheme 2. This ligation mode has been observed in the past only in the 1D coordination polymer {Mn 2 (bicH 2 ) 2 (H 2 O) 2 ]Br 2 ·2H 2 O} n [36], in which the Mn II ions are 7 coordinate with a slightly distorted pentagonal bipyramidal coordination geometry.

IR Spectroscopy.
IR assignments of selected diagnostic bands for bicH 3 (the free ligand exists in its zwitterionic form in the solid state with the carboxylic group being deprotonated and the tertiary nitrogen atom protonated [41]) and complex 1 are given in Table 5.

Conclusions and Perspectives
Complex 1 covers a gap in literature, because it is the first structurally characterized cadmium(II) bicinate compound. The bicinate(−1) ligand adopts the extremely rare pentadentate 2.21111 coordination mode, while the two crystallographically independent Cd II centers are found in two different stereochemistries.   The results presented here support our belief that the bicH 3 /RCO 2 − (R = various) ligand "blends" may be effective generators of interesting structural types in the chemistry of other transition metals. Reactions of CdCl 2 , CdBr 2 , CdI 2 , and Cd(NO 3 ) 2 with bicH 3 have not been studied to date, and we do believe that the structural types of the products will be dependent on the particular nature of the Cd II source. Analogues of 1 with zinc(II) have not yet been reported, but preliminary results in our laboratories indicate completely different chemistry compared with that of cadmium(II). Synthetic efforts are also in progress to "activate" the potential of bicH 2− and bic 3− to bridge more than four metal ions.