versión impresa ISSN 0366-1644
Bol. Soc. Chil. Quím. v.46 n.4 Concepción dic. 2001
FIRST X-RAY CRYSTAL STRUCTURE OF A BIMETALLIC COMPLEX
CONTAINING A p -COORDINATED HYDRAZONE:
[(h 5-Cp)Fe(h 6-p-MeOC6H4)-NHN=CH-(h 5-C5H4)Fe(h 5-Cp)]+PF6-.
Avenida Brasil 2950, Valparaíso, Chile.
2Departamento de Física, Facultad de Ciencias Físicas y Matemáticas,
Universidad de Chile, Casilla 487-3, Santiago, Chile.
3UMR CNRS 6509, "Organométalliques et Catalyse: Chimie et Electrochimie Moléculaires" ,
Institut de Chimie de Rennes, Université de Rennes 1,
Campus de Beaulieu, 35042 Rennes-Cedex, France.
(Recived: June 20, 2001 - Accepted: July 9., 2001)
The dinuclear organometallic hydrazone [(h 5-Cp)Fe(h 6-p-MeO-C6H4)-NHN=CH-(h 5-C5H4)Fe(h 5-Cp)]+PF6- (Cp=C5H5) is stereoselectively prepared as its trans isomer by reacting the organometallic hydrazine precursor [(h 5-Cp)Fe(h 6-p-MeO-C6H4)-NHNH2)]+PF6- with the ferrocenecarboxaldehyde. Its crystalline and molecular structure has been determined and consists of two iron(II) sandwiches of the type [(h 5-Cp)Fe(h 6-arene)]+ and [(h 5-C5H4)Fe(h 5-Cp)], in anti position one with respect to the other, and linked by a zigzag hydrazonic spacer.
KEY WORDS: hydrazone, iron mixed sandwich, ferrocene, homobimetallic hydrazone, organometallic hydrazone, crystal structure.
La hidrazona organometálica dinuclear [(h 5-Cp)Fe(h 6-p-MeO-C6H4)-NHN=CH-(h 5-C5H4)Fe(h 5-Cp)]+PF6- es preparada estereoquímicamente como su isómero trans por reacción de la hidracina organometálica precursora [(h 5-Cp)Fe(h 6-p-MeO-C6H4)-NHNH2)]+PF6- con ferrocencarboxaldehido. Se ha determinado su estructura cristalina y molecular que consiste en dos sanwiches de hierro(II) del tipo [(h 5-Cp)Fe(h 6-areno)]+ y [(h 5-C5H4)Fe(h 5-Cp)] en posición anti uno respecto del otro, los que se encuentran enlazados por un espaciador hidrazónico en forma de zig-zag.
PALABRAS CLAVES: hidrazona, sandwich mixto de hierro, ferroceno, hidrazona, homobimetálica, hidrazona organometálica, estructura cristalina.
Considerable interest has been focused on the chemistry of the robust electron-acceptor mixed sandwiches derivatives [(h 5-Cp)Fe(h 6-arene)]+ (Cp=C5H5 ) due to their important position in the development of organometallic [1,2] and metal-assisted organic chemistry [3,4], their very useful redox ability to study electronic communication between ligand-bridged metals , and electrochemical and photochemical properties [6,7]. In particular, we have been interested in the synthesis of homodimetallic iron(II) hydrazones complexes, using the cationic [(h 5-Cp)Fe(h 6-arene)]+ moiety as an electron withdrawing building block  linked to a ferrocenyl unit through a hydrazone skeleton, in order to investigate the electronic cooperation between both metal centres . In the present work we describe the first crystalline and molecular structure of a dimetallic p -coordinated hydrazone complex (Scheme 1), namely [(h 5-Cp)Fe(h 6-p-MeOC6H4)-NHN=CH-(h 5-C5H4)Fe(h 5-Cp)]+PF6-, [1+PF6-].
RESULTS AND DISCUSSION
Complex 1+PF6- is readily prepared in 79% yield as yellow-orange crystalline solid, by treatment of the organometallic hydrazine precursor  [(h 5-Cp)Fe(h 6-p-MeOC6H4-NHNH2)]+PF6- with formylferrocene, (h 5-Cp)Fe(h 5-C5H4CHO) in refluxing ethanol . Interestingly, 1+PF6- is formed stereoselectively as indicated by its unique 1H NMR pattern. The trans arrangement about the N=C double bond is definitively assigned from the structural analysis (see below). Recrystallization from CH2Cl2:Et2O (1:1) mixture provided suitable single crystals for X-ray diffraction studies.
The ORTEP plot of the cationic organometallic moiety, [(h 5-Cp)Fe(h 6-p-MeOC6H4)-NHN=CH-(h 5-C5H4)Fe(h 5-Cp)]+, together with the atom labelling scheme, is presented in Figure 1. The cation contains two organometallic fragments: (i) a mixed sandwich of the type [(h 5-Cp)Fe(h 6-p-MeOC6H4)]+, and (ii) a classical ferrocenic sandwich [(h 5-Cp)Fe(h 5-C5H4)]. Both fragments are linked by the conjugated zigzag transmitter backbone -NH-N=C(R)-. Taking into account the N(2)=C(17) double bond, the structure of the organometallic cation can be assigned to the anti rotamer of the trans isomer.
Selected bond distances (Å) and angles (º) are listed in Table 1. High anisotropic thermal motion was observed for the carbon atoms of the Cp ligands both in the [Cp-Fe(1)-(arene)]+ and in the [(C5H4)-Fe(2)-Cp] fragments (Figure 1). The observed anisotropic displacements are presumably due to partial rotation of the C5 rings about the Fe(1)-Cp and Fe(2)-Cp centroid axes, a phenomenon which we  and others [12-15] have already observed in similar complexes. The Fe(1)-Cp and Fe(1)-Arene centroid distances are 1.64 and 1.58 Å, respectively, while in the ferrocenic fragments Fe(2)-(C5H4) and Fe(2)-Cp, the distances are, in both cases, 1.63 Å. The ring centroid-iron-ring centroid vectors are essentially collinear both in the Cp-Fe(1)-Arene (178.4º) and (C5H4)-Fe(2)-Cp (179.1º) moieties [10,16]. However, the dihedral angle between the two aromatic rings, C(21)···C(26) and C(6)···C(10), is 30.4(4)º revealing a deviation from planarity of the dinucleating hydrazone ligand, [(p-MeO-C6H4)-NHN=CH-(C5H4)]-. This departure from planarity is accounted for by a sterically-driven process, leading to a more comfortable arrangement of the H(17) atom and the C5H4 group at the sp2-hybridized C(17) carbon atom.
Fig. 1. ORTEP view of the title molecule showing the atom-labelling scheme. Displacement ellipsoids are shown at the 40% probability level.
On the other hand, one of the most salient features observed in the molecular parameters of the title compound corresponds to the Fe(1)-C(21) bond length, 2.176(8) Å, which is ca. 0.102 Å longer than the mean of the other Fe-C (C6 ring) bond lengths. Such a Fe-C bond elongation has already been reported by us for the monomeric organometallic acetone hydrazone [(h 5-Cp)Fe(h 6-p-MeC6H4-NHN=CMe2)]+PF6- (0.071 Å) and in the hydrazine [(h 5-Cp)Fe(h 6-C6H5-NHNH2)]+PF6- (0.12 Å) . Moreover, the C(21)-N(1) bond length, 1.358(10) Å, somewhat shorter than the one (1.370(6) Å) measured in the acetone hydrazone derivative , is ca. 0.079 Å shorter than the mean reported for a C-N single bond and ca. 0.069 Å longer than the mean reported for a C=N double bond . These data reveal a partial delocalization of the electron lone-pair of the N(1) atom toward the cationic organometallic fragment which involves its partial depyramidalization. The C(21)-N(1)-N(2) bond angle, 119.8(7)º, similar to those described for [(h 5-Cp)Fe(h 6-C6H5NHNH2)]+ and [(h 5-Cp)Fe(h 6-p-MeC6H4-NHN=CMe2)]+ , supports unequivocally this hypothesis.
These observations lead us to attribute (i) a partial positive charge on the N(1) atom of the organometallic cation and, consequently; (ii) a slight cyclohexadienyl-like character of the arene ring with a partial negative charge. The dihedral angle between the plane containing the five phenyl carbon atoms C(22)···C(26) and the C(22)-C(21)-C(26) plane of 10.3(1.1)º, confirms this description, in agreement with our previously reported structural data , and theoretical work . Indeed, based on the present findings and literature reports [10,14,18,19], it appears clearly that structurally characterised p -arenes bearing a benzylic-type nitrogen atom complexed by 12-electron organometallic moieties adopt the general pattern consisting of a cyclohexadienyl-like character with a small folding angles (ca. 10º) and a partial multiple bond character for the C-N linkage. Work is currently underway in our laboratories to extend this family of new dimetallic hydrazone complexes and to study their electrochemical and physical properties.
Crystallographic data for 1+PF6-: C23H23F6Fe2N2OP, Mr=600.10, unit cell dimensions: a=17.587(3), b=12.965(2), c=21.371(3) Å, a =b =g =90°, V=4872.9(13) Å 3, orthorhombic, Pbca, Z=8, crystal size (mm) : 0.30x0.12x0.10, q range 1.91 to 25.00° at 297(2) K, reflections collected 4400, independent reflections 4287 (Rint=0.0232). Convergence at conventional R1=0.060, wR2=0.100 (I>2s (I)). High thermal motion was noted for the C atoms of the C5H5 ligands; however, a disorder model could not be resolved. Intensity data were collected on a Siemens R3m/V four circle diffractometer in q -2q scan mode, using graphite-monochromated Mo-Ka radiation (l =0.71073 Å). Analytical corrections were applied for absorption.
Cell parameters were obtained from 58 reflections with 8£ 2q £ 25º. The structure was solved by direct methods and refined by least-squares procedures with SHELXL-97 . Only hydrogen bonded to atom N(1) was allowed to refine. A riding model was applied to all other H-atoms which were placed at geometrically idealized positions with C-H = 0.96 Å. Isotropic thermal parameters were considered for all H-atoms equal to 1.2 times the equivalent isotropic thermal parameters of the corresponding parent atom.
Crystallographic data for the structural analysis has been deposited with the Cambridge Crystallographic Data Centre, CCDC Nº 157189 for compound 1+PF6-. Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK. Fax: +44-1223-336-033; E-mail:deposit@ ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk.
The authors greatly appreciate financial support for this work from the Fondo Nacional de Desarrollo Científico y Tecnológico, FONDECYT, Grants N° 1000281 (C. M.) and 1980433 (D.C., C. M.), the Programme International de Coopération Scientifique, CNRS-CONICYT (PICS Nº 922 (2000-02), (C. M., D. C., J.-R. H.) and the Universidad Católica de Valparaíso, Chile. We thank also Fundación Andes for funding the purchase of the Single-Crystal Diffractometer.
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