The new complex has been isolated from the crystallization of from ethylenediamine (en). The cadmium center bears square-pyramidal coordination, with the iron atom occupying the apical position. The iron coordination may be described either as trigonal bipyramidal, with the cadmium atom being in an axial position, or as monocapped (by cadmium) tetrahedral. The Cd-Fe distance was found to be 2.6250(2) Å. The complex crystallizes in the monoclinic space group with  Å,  Å,  Å, and at 150(1) K.

1. Introduction

Since the first reports of and various base adducts [1, 2], primarily being of the type with L representing chelating or nonchelating amine or ether ligands, surprisingly little effort has been expended toward the investigation of their structural natures, including their degrees of association. To date, most of the ligand adduct complexes have been found to be trimeric, as observed for THF, pyridine, and 2,2′-bipyridine [3, 4], though dimeric species have also been isolated (vide infra), as well as more complicated ring systems [57]. In the complexes, oligomerization occurs through the formation of metal-metal bonded rings composed of alternating cadmium and iron atoms. Under the right conditions, additional base coordination can be used to generate monomeric species (Figure 1) [8], which are actually more typical of analogous zinc complexes [2, 8, 9]. For the cadmium complexes, either the use of special chelating ligands, such as tren (2,2′,2′′-triaminotriethylamine), or crystallization from strongly coordinating solvents such as pyridine, can be used to favor the isolation of such monomers [8]. As an example of the latter approach, crystallization of () from pyridine has led to the isolation of (py)(neocuproine) monomers (neocuproine = 2,9-dimethylphenanthroline) [10]. Herein we report that crystallization of the known complex from en (en = ethylenediamine) leads to the formation of monomers, which are especially unusual due to the presence of five-coordinate, square-pyramidal cadmium coordination.

2. Materials and Methods

2.1. Synthesis

All reactions were carried out in Schlenk apparatus under a nitrogen atmosphere. All nonmetallic reagents were obtained commercially, and was prepared by a published procedure [2]. (en = ethylenediamine) was synthesized by adding 20 mL of ethylenediamine to 1 g (3 mmol) of while stirring, leading to complete dissolution of the compound. In order to form crystals, water was added at room temperature until a small amount of precipitate had formed. The solution was then heated with stirring to 90°C until the precipitate was completely redissolved and allowed to cool slowly back to room temperature. The collected crystals were subsequently subjected to an X-ray diffraction study.

Anal. Calc. for : C, 23.99; H, 4.03; N, 13.99. Found: C, 24.4; H, 4.58; N, 14.1.

2.2. X-Ray Crystallography

Single crystals of the compound were examined under Paratone oil, and a suitable crystal was selected for data collection. It was transferred to a Nonius Kappa CCD diffractometer where it was held in place on a glass fiber through the use of a cold nitrogen stream. The programs COLLECT, DENZO-SMN, and SCALEPAC [11] were used for unit cell determination and data collection and processing. SIR97 [12] was used for the initial structural solution, while SHELXL-97 [13] was used for subsequent refinements, based on published scattering factors [14]. All of the nonhydrogen atoms could be located and refined anisotropically. Key experimental details are given in Table 1, while pertinent bonding parameters are listed in Table 2. An ORTEP representation of the molecule is given in Figure 2.

The CCDC deposition 978825 contains the full crystallographic information for this structure. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge Crystallographic Data Centre, Cambridge, UK.

3. Results and Discussion

3.1. Synthesis

The complex, whose extent of oligomerization has not been determined, has been previously prepared from the reaction of with ethylenediamine. Due to the compound’s low solubility in noncoordinating solvents, crystallization was attempted from ethylenediamine, to which water had been added to promote crystallization. Such a strategy had earlier been utilized to crystallize the tetramer [15], which also is insoluble in noncoordinating solvents. For the tetramer, acetone was used as the solvent and was mostly removed from the metal through the addition of water to the acetone. For the en complex, however, the more strongly coordinating nature of the en ligand did not allow for its removal, and in fact a second equivalent was even found to remain coordinated in the isolated crystalline material, leading to a monomeric species, one of few known for cadmium [8, 10]. One can expect that this will become part of a general trend through which crystallization from strongly coordinating solvents such as amines will allow for the isolation of monomeric species, as has previously been demonstrated for (py)(neocuproine) (1) and presumably (tren) [8].

3.2. Structural Description

The structure of (2) can be seen in Figure 2. As in the case of complex 1, the iron center may be viewed as having approximate trigonal bipyramidal coordination, with cadmium in an axial position. Alternatively, the iron coordinations in these two species can be regarded as involving the cadmium center capping a tetrahedral face. In fact, the C(1)-Fe-C(2,3,4) angles in 2 average about 101°, while the Cd-Fe-C(2,3,4) angles average about 78°, and these values are slightly closer to expectations for the capped tetrahedral (109.5°, 70.5°) as opposed to the trigonal bipyramidal (90°, 90°) extreme. On the other hand, the C-Fe-C angles for the equatorial carbonyls (C(2–4)) fall in the narrow range of 115.53(8)–116.88(8)°, which is slightly closer to the trigonal bipyramidal expectation of 120°, as compared to the tetrahedral value of 109.5°. In comparison, for 1, having four-coordinate cadmium, the respective C(1)-Fe-C(2,3,4), Cd-Fe-C(2,3,4), and equatorial C-Fe-C angles average 97.5°, 82.5°, and 118.3°, being closer to the trigonal bipyramidal model. The greater contribution of the more ionic capped tetrahedral geometry for 2 is reasonable, given the greater basicity of the alkyl amine ligands, and the presence of four rather than three nitrogen coordinations.

The Cd-Fe distance of 2.6250(2) Å in 2 may be compared to that of 2.5380(5) Å in 1, consistent with the higher cadmium coordination number in the former, as well as the presence of the more strongly basic alkyl amines. The Fe-C and C-O distances and the Fe-C-O angles (ranges, 1.7558(17)–1.7642(16) Å, 1.162(2)–1.177(2) Å, and 176.35(16)–178.75(16)°) are similar to those in 1 (Table 2). The Cd-N distances range from 2.3819(15) to 2.4163(14) Å, as compared to 2.344(2)–2.360(2) Å in 1. The longer Cd-N bonds in 2 match expectations based upon the greater cadmium coordination number in the en complex, and its formal nitrogen hybridization of versus in 1. The Fe-Cd-N angles are similar for N(2,3,4), ranging from 113.20(4) to 113.97(4)°, while the Fe-Cd-N1 angle is somewhat larger at 120.81(4)°, for reasons that are not clear, though perhaps the slightly nonlinear Cd-Fe-C1 angle of 174.29(6)° may play a role. The N-Cd-N′ angles fall into three types, one within a given ligand (N1-Cd-N2, 73.50(5)°; N3-Cd-N4, 73.39(5)°), the “cis” values between different ligands (N1-Cd-N4, 83.47(5)°; N2-Cd-N3, 87.46(5)°), and the “trans” values (N1-Cd-N3, 125.80(6)°; N2-Cd-N4, 132.82(6)°).

There are short intramolecular contacts between the cadmium center and the three adjacent carbonyl carbon atoms (Cd-C2, 2.871 Å, Cd-C3, 2.878 Å, and Cd-C4, 2.809 Å). Comparable contacts have been observed in other species, such as complexes [1618], as well as complexes [4], and have been attributed to weak attractive interactions between the carbonyl ligands and the cadmium center. In addition, there are some short (N)H-O contacts, the most notable involving O2 with H1b (2.394 Å), O3 with H4b (2.379 Å), and O4 with H2a (2.430 Å) and H3a (2.324 Å).

4. Conclusions

The incorporation of en into a complex had previously been found to lead to the isolation of the oligomeric complex, in line with the soft acid nature of Cd(II). However, when en is used as the solvent for crystallization, the monomeric is instead obtained. This species is particularly unusual in having square-pyramidal coordination about the cadmium center, with the iron atom in the axial position. Although the related (py)(neocuproine) monomer had a coordination environment that was closer to be trigonal bipyramidal about iron, the iron coordination environment in has been found to be closer to be monocapped tetrahedral. It can be expected that numerous related monomeric species can be isolated when oligomeric complexes are crystallized from strongly coordinating solvents.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.