Synthesis and Structural Study of the Bis ( ethylenediamine ) CdFe ( CO ) 4 Monomer

The new (en) 2 CdFe(CO) 4 complex has been isolated from the crystallization of [(en)CdFe(CO) 4 ] x from ethylenediamine (en).The cadmium center bears square-pyramidal coordination, with the iron atomoccupying the apical position.The iron coordinationmay 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 P2 1 /a with a = 14.4137(3) Å, b = 7.65740(10) Å, c = 14.5465(2) Å, and β = 119.4972(9)∘ at 150(1) K.


Introduction
Since the first reports of CdFe(CO) 4 and various base adducts [1,2], primarily being of the [L 2 CdFe(CO) 4 ]  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 [5][6][7].In the [L 2 CdFe(CO) 4 ]  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 L 3 CdFe(CO) 4 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 [(neocuproin)CdFe(CO) 4 ]  ( = 2, 3) from pyridine has led to the isolation of (py)(neocuproine)CdFe(CO) 4 monomers (neocuproine = 2,9-dimethylphenanthroline) [10].Herein we report that crystallization of the known [(en)CdFe(CO) 4 ]  complex from en (en = ethylenediamine) leads to the formation of (en) 2 CdFe(CO) 4 monomers, which are especially unusual due to the presence of five-coordinate, squarepyramidal cadmium coordination.

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 [(NH 3 ) 2 CdFe(CO) 4 ]  was prepared by a published procedure [2].(en) 2 CdFe(CO) 4 (en = ethylenediamine) was synthesized by adding 20 mL of ethylenediamine to 1 g (3 mmol) of [(NH 3 ) 2 CdFe(CO) 4 ]  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  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.

X-Ray
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. 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 CdFe(CO) 4 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)CdFe(CO) 4 (1) and presumably (tren)CdFe(CO) 4 [8].

Conclusions
The incorporation of en into a CdFe(CO) 4 complex had previously been found to lead to the isolation of the oligomeric [(en)CdFe(CO) 4 ]  complex, in line with the soft acid nature of Cd(II).However, when en is used as the solvent for crystallization, the monomeric (en) 2 CdFe(CO) 4 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)CdFe(CO) 4 monomer had a coordination environment that was closer to be trigonal bipyramidal about iron, the iron coordination environment in (en) 2 CdFe(CO) 4 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.

Table 1 :
complex, whose extent of oligomerization has not been determined, has been previously prepared from the reaction of [(NH 3 ) 2 CdFe(CO) 4 ]  Crystal and experimental data.