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Journal of the Chilean Chemical Society

versión On-line ISSN 0717-9707

J. Chil. Chem. Soc. v.48 n.3 Concepción sep. 2003

http://dx.doi.org/10.4067/S0717-97072003000300015 

J. Chil. Chem. Soc., 48, N 3 (2003) ISSN 0717-9324

X-RAY CRYSTAL AND MOLECULAR STRUCTURE OF AN
ORGANOMETALLIC HYDRAZONE CONTAINING A ZIGZAG p-CONJUGATED SPACER: [CpFe(h6-C6H5)-N(Me)N=CH-C 6H4-4-NMe2]+ PF6-

WALTER FIGUEROA, MAURICIO FUENTEALBA, CAROLINA MANZUR*, DAVID CARRILLO

Laboratorio de Química Inorgánica, Instituto de Química,
Pontificia Universidad Católica de Valparaíso,Valparaíso, Chile david.carrillo@pucv.cl

JOSE A. MATA

Departamento de Química Inorgánica y Orgánica, Universitat Jaume I, E-12080 Castellón, Spain

JEAN-RENÉ HAMON*

UMR 6509 CNRS-Université de Rennes 1, Institut de Chimie, Campus de Beaulieu, F-35042 Rennes-Cedex, France

ABSTRACT

The crystal and molecular structure of the complex [CpFe(h6-C6H5)-N(Me) N=CH-C6H4-4-NMe2] +PF6-, [1]+PF6-, was solved by single crystal X-ray diffraction analysis. One of the most salient features observed in this structure is the depyramidalization of both the benzylic nitrogen and the dimethylamino nitrogen atoms which reveals the delocalization of the p-electron system along the entire hydrazone skeleton from the donor to the acceptor termini, through a quinonoidal deformation and a pseudo-cyclohexadienyl conformation (folding dihedral angle of ca. 7.4°) of the free and coordinated phenyl rings, respectively. Both Cipso-N bond lengths are virtually identical (1.37 Å), lying between pure single and double bonds. These and other peculiarities are described and compared with the structures of other organometallic hydrazones.

Key Words: hydrazone, iron mixed sandwich, organometallic hydrazone, crystal structure, electronic cooperation, hydrogen bonding

INTRODUCTION

The robust mixed sandwich derivatives {CpFe(h6-arene)}+ (Cp=C5H5 ) have been widely employed in organometallic1) and metal-assisted organic2) chemistry, owing to its powerful electron-acceptor ability3), allowing inter alia studies of electronic communication between ligand-bridged metals.4) Over the last few years, we have been particularly interested in the study of the intramolecular electronic cooperation in organometallic iron(II) hydrazone-complexes5) that contain this cationic building block connected to various electron-donor groups through an asymmetric -NR-N=CR'- spacer (R,R'=H, Me). Very recently, we succeeded in the preparation of the N-methylated organoiron(II) dimethylaminobenzaldehyde-hydrazone complex [CpFe(h6-C6H5)-N(Me)N=CH-C 6H4-4-NMe2]+ PF6- ([1]+PF6-).5g) In order to investigate the influence of the Me group bonded to the benzylic nitrogen atom and of the remote strong electron-donor p-NMe2 substituent on the structure of [1]+PF6- complex (see Scheme 1), we have studied its crystal and molecular structure by single crystal X-ray diffraction analysis, and we report now on the most relevant structural features of this ionic complex and their probable electronic consequences.

RESULTS AND DISCUSSION

Complex [1]+PF6- was synthesized in a one-pot two-steps deprotonation/methylation sequence of its [CpFe(h6-C6H5)-NHN=CH -C6H4-4-NMe2] +PF6- precursor according to our published procedure.5g) Recrystallization of the N-methylated compound from a CH2Cl2 by slow diffusion of Et2O at room temperature provided suitable single crystals for X-ray diffraction studies. The ORTEP view of the cationic organometallic species, [CpFe(h6-C6H5)-N(Me)N=CH-C 6H4-4-NMe2]+ , along with the atom labelling scheme, is presented in Figure 1. Selected bond distances (Å) and angles () are listed in Table 1.


Fig. 1: ORTEP view of the cationic moiety [1]+ showing the atom-labelling scheme. Displacement ellipsoids are shown at the 50% probability level.

Table 1. Selected bond lengths (Å) and angles (°) for complex [1]+PF6- a)

a Abbreviations: Cp= C5H5; Ph= C6H5; CNT= centroid. b Average bond lengths.

Figure 1 clearly illustrates the zigzag conformation of the 4-dimethylaminobenzaldehyde 1-methyl-1-phenylhydrazone ligand, (h6-C6H5)-N(Me)-N=CH-C 6H4-4-NMe2, from C(6) to C(14) with bond angles close to 120° at N(1), N(2), and C(13) atoms (see Table 1). A typical high anisotropic thermal motion and/or disorder was observed for the carbon atoms of the Cp ligand presumably due to partial rotation of the C5 ring about the Fe-Cp centroid axis, a phenomenon which has already been described in sandwich complexes5b,c,f,g,6,7). The Fe(1)-Cp and Fe(1)-Ph centroid distances are ca. 1.666 and 1.566 Å, respectively, while the ring centroid-iron-ring centroid vectors are essentially collinear (177.41), in accordance with a h5-Fe-h6 coordination mode8,9. On the other hand, the dihedral angle between the coordinated and free phenyl rings, C(6)···C(11) and C(14)···C(19), respectively, is 7.5 revealing a good planarity of the hydrazone ligand despite the steric hindrance of the methyl group at N(1). Dihedral angles between two phenyl rings of 4.9 and 6.2° have been observed for two related N(1)-H mononuclear counterparts.5e,g) Such orientation of the phenyl planes is important for an efficient p-electron delocalization along the entire hydrazone backbone, and thus might ensure a large molecular nonlinearity in the crystalline solid.10) This contrasts with greater rotations of the N-methylpyrid-4-yl group by 43.2° out of the coordinated phenyl ring plane in [CpFe(h6-C6H5-N=C 5H4NMe)]+. This complex did not exhibit non-linear optical activity.3b)

One of the most remarkable features observed in the molecular parameters of the cationic moiety [1]+ is the lengthening of the Fe(1)-C(6) bond distance, 2.194(8) Å, which is ca. 0.118 Å longer than the mean of the other Fe(1)-C(7-11) bond lengths. Such a Fe-C bond elongation has already been reported by us for both mononuclear and dinuclear organometallic hydrazones5b-g. Moreover, the C(6)-N(1) bond length, 1.369(9) Å, is shorter than the mean reported for a C-N single bond (1.437 Å) but longer than the mean reported for a C=N double bond (1.289 Å)9). These data clearly reveal a partial delocalization of the electron lone-pair of the N(1) atom toward the cationic organometallic fragment which involves its partial depyramidalization. This is clearly evidenced by the bond angles about the N(1) atom (see Table 1). The sum of these bond angles is 359.8, which is similar to those measured for other organometallic hydrazine and hydrazone analogues5b-g. A similar depyramidalization is also observed for the N(3) atom (see Table 1) where the sum of the bond angles is 359.9°. This N(3) atom depyramidalization gives rise to an appreciable quinonoidal contribution to the free phenyl ring with an obvious bond length alternation,11) which is, however, more marked at C(17) than at C(14) (see Table 1). Thus, the C(17)-N(3) bond length is 1.368(10) Å, intermediate between a double C=N and single C-N bonds9) while the three types of phenyl bond length average for 1.400, 1.362 and 1.382 Å, respectively, in going from C(17) to C(14). Moreover, the exocyclic C(13)-C(14) single bond is also involved in this quinonoidal deformation with a relatively short bond length of 1.447(10) Å.

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(7)···C(11) and the C(7)-C(6)-C(11) plane of 7.4, confirms this description, in agreement with our previously reported structural5b-g), and theoretical data.12) Indeed, based on the present findings and literature reports3b,5b-g,6b,13) it appears clearly that structurally characterized p-arenes bearing a benzylic-type nitrogen atom complexed by 12-electron organometallic moieties adopt the general pattern consisting of a pseudo-cyclohexadienyl conformation with a small folding angle (ca. 10) and a partial multiple bond character for the C-N linkage. Other feature exhibited by [1]+ is the presence of the intramolecular C(11)-H(11)N(2) hydrogen bonding with a C(11)N(2) length/angle of ca. 2.683 Å/100°, being the H(11)N(2) bond length 2.36 Å.

Finally, the structure of the cationic moiety [1]+ is completed by two PF6- anions: the first one appertains to the asymmetric unit and shows a weak coupling to the coordinated phenyl ring of the organometallic cation through the F(1)H(10)-C(10) hydrogen-bonding interaction, with a F(1)C(10) length/angle of ca. 3.285 Å/152°, and a F(1)H(10) distance of 2.44 Å. The second PF6- anion, with a symmetry related position 1+x, 1/2-y, 1/2+z, is weakly coupled to the same phenyl ring through the F(3)H(8)-C(8) hydrogen-bonding interaction, with a F(3)C(8) length/angle of ca. 3.123 Å/129°, and a F(3)H(8) separation of 2.45 Å. Accordingly, each [1]+ cation is connected to two anionic PF6- counterions, each of them being in turn connected to two [1]+ units, thus allowing the formation of infinite chains (see Table 1 and Figure 2).


Fig. 2: Part of an infinite zigzag chain showing the molecular arrangement and the hydrogen-bonding interactions within the unit cell of complex [1]+PF6-.

Crystal data

Crystallographic data for [1]+PF6-: C21H24F6FeN3 P, Mr=519.25, unit cell dimensions: a=7.5641(19), b=19.161(5), c=15.837(4) Å, b=98.873(6)°, V=2267.9(10) Å3, monoclinic, P2(1)/c, Z=4, crystal size (mm): 0.23x0.18x0.11, q range 1.68 to 21.96° at 293(2) K, reflections collected 9608, independent reflections 2763 (Rint=0.0453). Convergence at conventional R1=0.0730, wR2=0.1790 (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 Smart CCD diffractometer, using graphite-monochromated Mo-Ka radiation (l=0.71073 Å) with a nominal crystal-to-detector distance of 4.0 cm. A hemisphere of data was collected on the basis of three w-scan runs (starting w=-28°) at values j=0, 90, 180°, with the detector at 2q=28°. In each of these runs, frames (606, 435, 230, respectively) were collected at 0.3° interval and for 30 s per frame. Space group assignments are based on systematic absences, E statistics and successful refinement of the structure. Structure was solved by direct methods with the aid of successful difference Fourier maps and were refined using the SHELXTL 5.1 software package.14) All non-hydrogen atoms were refined anisotropically. The diffraction frames were integrated using the SAINT15) package and corrected for absorption with SADABS.16)

Crystallographic data for the structural analysis has been deposited with the Cambridge Crystallographic Data Centre, CCDC N 188754 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.

ACKNOWLEDGEMENTS

The authors greatly appreciate financial support for this work from the Fondo Nacional de Desarrollo Científico y Tecnológico, FONDECYT, Grant N° 1000281 (C. M.), the Programme International de Coopération Scientifique, CNRS-CONICYT (PICS N 922), the CNRS-CONICYT Project N° 14531 (2003-04), (C. M., D. C., J.-R. H.) and the Pontificia Universidad Católica de Valparaíso, Chile.

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