Crystallization and Structural Characterization of Dimeric and Trimeric Forms of (Neocuproine)CdFe(CO)4

The previously reported (neocuproine)CdFe(CO)4 has been found to crystallize as either a dimer or a trimer depending on the procedure used. These species exist as nearly planar four- and six-membered rings composed of alternating cadmium and iron atoms. The trimeric form is reasonably similar to its 2,2′-bipyridyl analogue, while the dimeric form had not been observed previously for a CdFe(CO)4 complex. The space group for the dimeric isomer is C2/c, with (3), (2), (2)？ ？, and (11)°, while for the trimeric isomer the space group is P , with (1), (1), (2)？ ？, (3)°, (4)°, and (5)°. 1. Introduction The complex CdFe(CO)4 (1) was first reported in 1933 and due to its insolubility was proposed to be a polymeric species [1, 2]. A subsequent study revealed it to be tetrameric, composed of a square, eight-membered ring having alternating cadmium and iron atoms, bearing nearly linear and cis-disubstituted octahedral coordination geometries, respectively [3]. In contrast to its mercury analogue [4], 1 has been found to readily incorporate ligands into its coordination sphere, generating a variety of [LxCdFe(CO)4]n complexes (e.g., L = NH3 or py, ; L2 = 2,2′-bipyridine or 1,10-phenanthroline [2]). As can be seen, these most commonly involve two coordination sites on cadmium being occupied by amines, though THF coordination is also possible, and at least one [LCdFe(CO)]n species (L = 4-phenylpyridine) [5] and at least one monomeric L3CdFe(CO)4 complex, in which L3 represents a combination of one pyridine and one bidentate neocuproine (2,9-dimethylphenanthroline) ligand [6], are known. The isolation of the monomeric species appears to have been promoted by the use of pyridine as the solvent for crystallization. Further, the use of the chelating tren ligand appears, from spectroscopic data, also to have led to a monomeric complex [5]. In comparison, analogous complexes with the harder zinc ion are typically found as monomeric L3ZnFe(CO)4 species (L = NH3 or pyridine [2] or L3 = (NH3)(2,2′-bpy) [5, 6]). An unusual dianionic species, {[CdFe(CO)4]6Cl2(DME)2}2？, has also been reported [7] and found to contain a twelve-membered ring, thus far the largest known. The use of larger iron carbonyl anions, such as or , has thus far not led to oligomers but instead to species possessing CdFe2 rings [8, 9]. To date, the only [L2CdFe(CO)4]n species to have been crystallographically characterized have been the L = THF or pyridine (py) or L2 = 2,2′-bipyridine species [10], which have all been found to exist as trimeric complexes. Notably, the (2,2′-bipyridine)zinc analogue (2) has