Syntheses and Crystal Structures of Two Transition Metal Complexes (M = Mn and Co) Containing Malonate and Reduced Imino Nitroxide Radicals

Two novel transition metal complexes with malonate and reduced imino nitroxide radicals, [Co(mal)(Him2-py)2] (ClO4)1 and [Mn(mal)(Him2-py) 2] (H2O)2 (Him2-py = 1-hydroxy-2-(2′-pyridyl)-4,4,5,5-tetramethylimidazoline) have been synthesized and their crystal structures were determined by X-ray diffraction method. During the reaction, one-electron reduction of the N–O radical moiety in IM2py has been reviewed. The structural analyses reveal that two title complexes are isostructural and crystallize in monoclinic space group C2. For the complex 1, a = 17.004(9), b = 10.753(5), c = 9.207(5) Å with β = 113.856(8)◦. For the complex 2, a = 16.721(5), b = 10.897(5), c = 9.253(3) Å with β = 120.807(6)◦. In two complexes, the coordination number around the metal ion is six, and the coordination sphere is a distorted octahedron. Two nitrogen atoms from Him2-py and two oxygen atoms from malonate are in the basal plane, and two nitrogen atoms from pyridyl rings of Him2-py at the axial position.


Introduction
The design and synthesis of transition metal complexes with organic free radicals is one of the major challenges in the field of molecular magnetic materials [1]. Nitronyl nitroxide (NN) radicals are normally used as spin carriers to the development of molecular-based magnetic materials. However nitroxides can undergo redox reactions with transition metal ions under certain conditions [2,3]. In fact, nitronyl free radicals are in an oxidation state intermediate between those of the hydroxylamino anion and the nitrosonium cation. Up to now, relatively little work has been devoted to the study of the redox properties of metal-nitroxyl systems and only a few complexes containing metal ions bound to the reduced monoradical have been reported [4][5][6][7][8][9]. It is known that nitronyl nitroxide radicals can undergo redox reaction with transition metal ions, yielding complexes in which the IMHR reduced form of IM acts as a diamagnetic ligand [7]. In order to extend our knowledge of extremely rich chemistry of such systems, it is necessary to further explore the reactions between metal ion and nitronyl nitroxide radicals. In this paper, we will report that syntheses and structural characterization about two novel transition metal compounds with malonate and reduced imino nitroxide radicals, [Co(mal)(Him2-py) 2 ] (ClO 4 ) 1 and [Mn (mal)(Him2py) 2 ] (H 2 O) 2 (Him2-py = 1-hydroxy-2-(2 -pyridyl)-4,4,5,5tetramethylimidazoline).

Crystal Structure Determination and Refinement.
All measurements were made on a Bruker Smart 1000 diffractometer equipped with graphite-monochromated MoKα radiation (λ = 0.71073Å). The data were collected at room temperature. A summary of the crystallographic data is given in Table 1. These structures were solved by direct methods using the SHELXS97 program [11]. Full-matrix least-squares refinements on F 2 were carried out using SHELXL97 [12]. A summary of the crystallographic data and collections are listed in Table 1. These significant bond parameters for two complexes are given in Tables 2 and 3, respectively. Views of the molecular structures for compounds 1 and 2 are shown, respectively, in Figures 1 and 2. The sketch of the intermolecular hydrog bonds of two complexes are shown in Figure 3 and 4, respectively.

Results and Discussion
The data of (= ν as − ν s ) of IR reveal that each malonate dianion binds metal ions in bidentate mode, leading to a mononuclear structure. The bonds of the N-O stretching vibration appear at ca.1357 cm −1 and 1365 cm −1 for complex 1 and 2, respectively, which suggest that one Him2py are chelated metal ion by pyridyl and imino nitrogen atoms with the five-membered ring.
The crystal structures of both complexes have several features in common. The single-crystal X-ray structures of complexes 1 ( Figure 1 [13][14][15][16]. In complex 1, the C2-N1 and C2-N2 bond lengths of the IMH2py are 1.296(7)    Sketch of the intermolecular hydrogen bonds of the complex 1 and 2 are shown in Figure 2, 3, respectively. In the crystal packing of two complexes, hydrogen bonds of O-O type have been observed between the hydrogen atom from the coordinated malonate and the oxygen atom from adjacent Him2-py, thus one-dimensional structures are formed. Among the 1-D chains, the oxygen atoms from the NO groups of Him-2py and the mal form hydrogen bonds, and the distances of O(+x, +y, +z)-O(1/2 − x, 1/2 + y, −z) are 2.612Å (for complex 1) and 2.627Å (for complex 2), respectively.
As we know, there were several reports on metal complexes with a reduced nitroxide radical. However, no example in which M(mal) 2 converts the IMR radical into IMHR has been reported. The mechanismic details of the reduction of nitroxide radical are not completely clear, but it is likely that the formation of Him2-py is favored by acidic impurities and standing for a long time. The reduced radical can exist in two tautomeric forms [7], the amidino oxide and iminohydroxylamine. Since the nitrogen atom from the imino group here is involved in the coordinating metal ion, the reduced radical form should be the latter [6].
As mentioned in the experimental part of this paper, the cobalt reactant is cobalt(II) perchlorate hexahydrate. However, the reaction product is cobalt(III) complex 1 due to the oxidation of Co(II) to Co(III) in air. The result is corresponding to those lectures [17][18][19].