Ruthenium(iii)-promoted oxidative dimerization of aniline to N-phenyl-1,2-phenylenediimine. Definitive proof for the template reaction

[OsIV_Br 6]2– [OsIIL2Br2] (1) PhNH2 1 NH N L = [Ru(acac)3] PhNH2 [Ru(acac)2L] (2) 2 PhNH2 RuCl3•3H2O [RuCl2(PhNH2)2L] (3) 3 C(12) C(110) C(11) C(13) O(13) O(11) Ru(1) C(130) O(21) C(21) C(210) C(22) O(23) C(230) C(23) C(312) C(311) C(310) C(39) C(38) C(37) N(36) N(31) C(31) C(36) C(35) C(32) C(33) C(34)

Ruthenium(iii)-promoted oxidative dimerization of aniline to

N-phenyl-1,2-phenylenediimine. Definitive proof for the template reaction

Kedar Nath Mitra,a_{Partha Majumdar,}a_{Shie-Ming Peng,}b_{Alfonso Castin˜eiras}c_{and Sreebrata Goswami*}a a_{Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Calcutta 700 032, India}

b_{Department of Chemistry, National Taiwan University, Taipei, Taiwan, Republic of China}

c_{Departmento de Qu´ımica Inorg´anica, Facultad de Farmacia, Universidad de Santiago de Compostela, 15706-Santiago de}

Compostela, Spain

Two novel examples of ruthenium-promoted oxidative

ortho-dimerization of aniline are described which

demon-strate that the dimerization reaction proceeds via coordina-tion of anilines to the metal ion.

In a recent communication1_{we described unusual examples of}

osmium-promoted oxidative dimerization of primary aromatic amines [eqn. (1)] to N-aryl-1,2-arylenediimine (L). It was

proposed that prior cis-coordination of amines to the metal ion is the key step for the dimerization reaction.

Here, we further demonstrate two more novel examples of ruthenium-promoted oxidative dimerization reactions of aniline which established that coordination of amines takes place during the course of reaction.

The first reaction [eqn. (2)] is the reaction of [Ru(acac)]3]2

with neat aniline at 130 °C. Chromatographic work-up of the reaction mixture afforded [Ru(acac)2L], which was

recrystal-lised from an aqueous acetonitrile solution and obtained as bown needles in 30% yield. Addition of NEt3, which acts as a

proton sink, to the reaction mixture increases the yield considerably to 65%. The compound [Ru(acac)2L] is soluble in

common organic solvents and is diamagnetic. The N–H stretch occurs3_{as a sharp feature of moderate intensity in the range}

3300–3200 cm21_{. The presence of a strong CNN stretch near} 1600 cm21_{characterizes the presence of diimine chromophore} in the compound. The 1_{H NMR spectrum of the complex}

consists of three methyl resonances at d 1.76(3 H), 1.83(6 H) and 2.35 (3 H); resolved aromatic proton resonances between d 6.5 and 7.4 and the N–H resonance is observed4_{as a relatively}

broad singlet at d 10.8. Suitable crystals for an X-ray structure determination† were obtained upon slow evaporation of a saturated solution of the compound in hexane, and a view of the molecule is shown in Fig. 1. Ruthenium is coordinated by the four oxygens of two acetylacetonato ligands and by the two nitrogens of a diimine ligand in a distorted octahedral geometry. The imine, C–N bond lengths, average 1.341(8) Å, are considerably shorter than a C–N single bond, 1.433(9) Å. The

analytical, spectral and X-ray data collectively conform to the formulation of 2 as [Ru(acac)2L.]

To gain a better insight into the course of above metal-promoted oxidation reaction, we used RuCl3·3H2O as a starting

material to carry out a similar reaction. RuCl3·3H2O was

selected since first, as in [Ru(acac)3], ruthenium is trivalent and

secondly, the chloride salt is much more labile towards substitution than is Ru(acac)3. Therefore, a higher degree of

amine coordination was anticipated.

This reaction proceeded very smoothly and recrystallisation of the crude product from dichloromethane–hexane [eqn. (3)]

resulted a highly crystalline compound 3 (yield, 60%), which contains one diimine (L), two trans-anilines and two cis-chlorides in the co-ordination sphere.

Unlike 2, compound 3 is sparingly soluble in common organic solvents. The IR spectrum shows3_{multiple sharp n}

NH

between 3300 and 3100 cm21_{, sharp n}

CNCand nCNNat 1600 and

1580 cm21_{and two n}

Ru–Clstretches5at 360 and 340 cm21. The

Fig. 1 Molecular structure of [Ru(acac)2L] 2 showing the atom numbering scheme. The asymmetric unit consists of two crystallographically distinct molecules. The figure shows a view of molecule 1. Selected bond distances (Å): Ru(1)–N(31) 1.958(5), Ru(1)–N(36) 1.996(5), Ru(1)–O(11) 2.019(5), Ru(1)–O(13) 2.068(4), Ru(1)–O(21) 2.050(4), Ru(1)–O(23) 2.031(5), N(31)–C(31) 1.333(8), N(36)–C(36) 1.352(8), N(36)–C(37) 1.429(8).

C(22) C(21) C(23) C(24) C(19) C(20) N(4) C(11) C(12) C(10) Ru Cl(1) Cl(2) C(9) C(8) N(1) N(2) C(7) C(1) C(6) C(5) C(4) C(3) C(2) N(3) C(13) C(14) C(18) C(17) C(15) C(16)

1_{H NMR spectrum shows two doublet NH resonances}6_{at d 5.09}

and 4.84 assigned to NH2. Verification of the composition as

well as the geometry of the compound was ascertained by single-crystal X-ray diffraction,† and a view of the molecule is shown in Fig. 2. There are three types of Ru–N distances in the molecule. The two Ru–N(aniline) single bonds are identical and are longer than the Ru–N(imine) bonds indicating a relatively weak bond between Ru and aniline. The two diimine, CNN bonds are much shorter than the C–N bonds. To the best of our knowledge compound 3 represents the first authentic example of aniline coordinated to ruthenium. Indications are strong that it can act as a good starting compound for substitution reactions and for performing reactions at the coordinated aniline.

It is worthwhile to compare the results of the three reactions (1)–(3). For osmium [eqn. (1)] a bis(diimine) complex was obtained whereas only a monodiimine complex resulted from reaction (2). It is noteworthy that the difference of the oxidation states of the metal ions in the starting compound and the end product is two in reaction (1) but is one in reaction (2). Interestingly, in reaction (3) only two of the four coordinated anilines have undergone oxidative dimerization and notably, the

metal centre has undergone only a one-step reduction. From the above results it appears reasonable that the difference in the oxidation levels of the reactant and the product is equal to the number of diimine ligands formed in the reaction.

In conclusion, it may be stated that the above results are a clear manifestation of template dimerization of primary aro-matic amines to yield novel coordination complexes of arylene diimines which otherwise are not achievable.7

Financial support received from the CSIR, New Delhi is acknowledged. We thank Dr Samaresh Bhattacharya for his suggestions.

Footnotes

* E-mail: [email protected]

† Crystal data: [Ru(acac)2L]·0.25H2O 2: C22H25N2O4.25Ru, M = 486.51, monoclinic, space group C 2/c, a = 33.578(9), b = 9.922(2), c = 26.354(5) Å, b = 93.114(10)°, U = 8768(3) Å3_{, Z = 16, D}

c= 1.474 g cm23, crystal dimensions 0.20 3 0.25 3 0.30 mm. Intensity data were collected on Enraf-Nonius CAD4 diffractometer with graphite-monochromated Mo-Ka radia-tion (l = 0.710 73 Å) using the w–2q scan mode with 2qmax = 32.4°. 12 707 unique reflections were measured and 12 705 with I! 2s(I) were used in the refinement. The structure was solved by direct methods8_{by a} full-matrix least-squares procedure based on F2_{which smoothly converged} to R = 0.0805. The final Fourier difference map showed residual extrema at 0.62, 20.80 e Å23_.

[RuCl2(PhNH2)2L]·CH2Cl2 3: C25H26N4Cl4Ru, M = 625.38, mono-clinic, space group P 21/n, a = 9.390(5), b = 19.225(2), c = 15.631(3) Å,

b = 101.91(3)°, U = 2761(16) Å3_{, Z = 4, D}

c = 1.504 g cm23, crystal dimensions 0.25 3 0.45 3 0.60 mm. Intensity data were collected on Enraf-Nonius CAD4 diffractometer with graphite-monochromated Mo-Ka radia-tion (l = 0.710 73 Å) using the w–2q scan mode with 2qmax= 50.0°. 4848 unique reflections were measured and 3093 with I! 2s(I) were used in the refinement. Refinement of positional and anisotropic thermal parameters for all non-hydrogen atoms converged to R = 0.040. The final Fourier difference map showed residual extrema at 0.680, 20.510 e Å23_{. Atomic}

coordinates, bond lengths and angles, and thermal parameters have been deposited at the Cambridge Crystallographic Data Centre (CCDC). See Information for Authors, Issue No. 1. Any request to the CCDC for this material should quote the full literature citation and the reference number 182/486.

References

1 K. N. Mitra and S. Goswami, Chem. Commun., 1997, 49.

2 A. Endo, M. Watanbe, S. Hayashi, Shimizu and G. P. Sato, Bull. Chem.

Soc. Jpn., 1978, 51, 800.

3 A. Danopoulos, A. C. C. Wong, G. Wilkinson, M. Hursthouse and B. Hussain, J. Chem. Soc., Dalton Trans., 1990, 315.

4 L. F. Warren, Inorg. Chem., 1977, 16, 2814.

5 S. Choudhury, M. Kakoti, A. K. Deb and S. Goswami, Polyhedron, 1992, 11, 3183.

6 H. Y. Cheng and S.-M. Peng, Inorg. Chim. Acta, 1990, 169, 23. 7 J. March, Advanced Organic Chemistry, Wiley Interscience, New York,

3rd edn., 1985, p. 1034.

8 G. M. Sheldrick, SHELXS 86, Program for the solution of crystal structures, University of G¨ottingen, 1986.

Received in Cambridge, UK, 25th March 1997; Com. 7/02047C

Fig. 2 Molecular structure of [RuCl2(PhNH2)2L] 3 showing the atom numbering scheme. Selected bond distances (Å): Ru–N(1) 1.940(4), Ru– N(2) 1.997(4), Ru–N(3) 2.135(4), Ru–N(4) 2.135(4), Ru–Cl(1) 2.422(1), Ru–Cl(2) 2.429(1), N(1)–C(1) 1.319(7), N(2)–C(6) 1.343(7), N(2)–C(7) 1.445(6), N(3)–C(13) 1.451(7), N(4)–C(19) 1.449(7).