RuCl3•3H2O RuCl2(PhNH2)2(L1) (1) 1 aniline RuCl2(PhNH2)2(L1) + 3,5-di-tert-butylcatechol CH3OH RuCl2(L1)(L2) (2) 1 2 N H N Ph N O Ph L1 = L2 = O RuII N Ph O RuIII N Ph 2a 2b
Two novel reactions of ruthenated aniline. Structure and bonding in
bis-chelated ruthenium complexes of quinone related ligands
Kedar Nath Mitra,aShie-Ming Pengband Sreebrata Goswami*a†
aDepartment of Inorganic Chemistry, Indian Association for the Cultivation of Science, Calcutta, 700032, India bDepartment of Chemistry, National Taiwan University, Taipei, Taiwan, Republic of China
Two novel examples of oxidative ortho coupling of ruthen-ated aniline are described; the products are fully charac-terised by X-ray and other physicochemical data.
In a recent communication1 we reported the formation of RuCl2(PhNH2)2(L1), 1 (L1= N-phenyl-1,2-phenylenediimine), from the reaction of hydrated RuCl3and PhNH2[eqn. (1)].
Although ruthenium(II) aniline complexes are known,2 complex 1 represents the first example of a structurally characterised aniline complex of ruthenium. One of the principal interests in 1 was the possibility to study the chemical reactions of the coordinated aniline. In this report we present two such novel examples.
The reaction of RuCl2(PhNH2)2(L1) and 3,5-di-tert-butyl-catechol occurs smoothly in methanol at room temperature to produce a blue compound of composition RuCl2(L1)(L2), 2 [eqn. (2)], in 55% yield. This compound is highly soluble in
common organic solvents and the solution shows an intense transition in the visible region at 627 nm. Verification of the composition and geometry of 2 was ascertained by the
determination of its single crystal X-ray structure.‡ A view of the molecule is shown in Fig. 1 and selected bond distances are collected in Table 1. The results reveal the oxidative coupling of coordinated aniline and catechol to result in an
N-phenyliminoquinone ligand, L2, with the formation of a C–N
bond. This transformation was not known previously. The formulation of the diimine oxidation state of L1 (N,N) is evident1 from the structural data of 2. The imine C–N bond lengths, average 1.315(6) Å, are considerably shorter than the C7–N2 single bond, 1.444(6) Å, present in the same ligand. Considering the diimine oxidation state of the ligand L1, two charge distributions for the RuL2 moiety are possible in the above complex, either RuII–L2BQ (2a) or RuIII–L2 SQ (2b). Compound 2 is diamagnetic and shows two sharp tert-butyl proton resonances at d 0.993 and 1.267. The C14–O bond length
of the coordinated L2is 1.280(5) Å and the C13–N3 bond length is 1.339(6) Å. This C–O bond length is longer than the C§O
bond length [1.239(7) Å] observed3 in free
2,4,6,8-tetra-tert-butylphenoxazin-1-one (Phenox BQ) but it falls below the range observed4(1.29 Å and above) for a coordinated semiquinone ligand. For comparison, the C–O bond length3ain Ru(PPh3)2Cl2-(phenox SQ) Ru(PPh3)2Cl2-(phenox SQ = 1-hydroxy-2,4,6,8-tetra-tert-butylphenoxyzinyl radical) is 1.300(4) Å and the average C–O bond length5 in Ru(bpy)2(DBSQ)+ (bpy = 2,2A-bipyridine, DBSQ = 3,5-di-tert-butyl-1,2-semiquinone) is 1.308 Å. The C–N bond length of coordinated L2is within the range of values expected6 for imine ligands. Moreover, the C15–C16 and C17–C18 bonds have almost localised double bond character which is also consistent4bwith the iminobenzoquinone formula-tion (2a). Their average distance of 1.355(6) Å is significantly shorter than the other four bonds of the ring (Table 1). To the
Fig. 1 Molecular structure of RuCl2(L1)(L2) showing the atom numbering scheme
Table 1 Selected bond distances (Å) for RuCl2(L1)(L2)·CH2Cl2 Ru–O 2.022(3) N3–C13 1.339(6) C5–C6 1.434(6) Ru–N1 1.950(4) O–C14 1.280(5) C6–C1 1.447(6) Ru–N2 2.001(4) N2–C7 1.444(6) C13–C14 1.439(6) Ru–N3 1.974(4) N3–C19 1.446(6) C14–C15 1.439(6) Ru–Cl1 2.364(13) C1–C2 1.421(7) C15–C16 1.355(6) Ru–Cl2 2.385(13) C2–C3 1.341(7) C16–C17 1.441(7) N1–C1 1.317(6) C3–C4 1.417(8) C17–C18 1.355(7) N2–C6 1.313(6) C4–C5 1.338(8) C18–C13 1.420(6) Chem. Commun., 1998 1685
RuCl2(PhNH2)2(L1) H2O2 CH2Cl2 RuCl2(L 1 )2 (3) 1 3
best of our knowledge compound 2 represents the first example7 of a mixed ligand compound containing both 1,2-diimine (N,N) and 1,2-iminoquinone (N,O) which gives an opportunity for the direct comparison of bonding between an (N,N) and an (N,O) donor. Furthermore, we wish to note here that structurally characterised Ru 1,2-diimines (N,N) are scarce6 and there is only one example3of a ruthenium complex of 1,2-iminosemi-quinone, the structure of which has been reported. Synthetic difficulties7 have inhibited the study of iminoquinone com-plexes.
It was observed that prolonged exposure of a brown solution of 1 to air led to formation of a blue solution. A similar transformation occurs instantaneously and smoothly when a solution of 1 in CH2Cl2is treated with H2O2. Chromatographic work up followed by crystallisation of the crude product yielded crystalline RuCl2(L1)2, 3, in 45% yield [eqn. (3)]. The
three-dimensional X-ray structure of 3 authenticates the formation of the compound from reaction (3). The isomer geometry of the ruthenium complex is identical8 to that of the analogous dibromo osmium complex, OsBr2(L1)2. The bond distances of the coordinated diimine ligand in the above two ruthenium and osmium compounds are similar. (Details of the X-ray structure of 3 will be reported elsewhere.) The compound shows an intense transition at 590 nm in the visible region.
The above coupling reactions [(2) and (3)] do not occur with uncoordinated aniline. These reactions therefore may be classified as reactions of the activated coordinated aniline. In reaction (2) one of the coordinated anilines coupled with externally added 3,5-di-tert-butyl catechol to form an N-phenyl substituted derivative of o-iminobenzoquinone.7It is possible that oxidation of catechol followed by condensation with the coordinated aniline have occurred during the above transforma-tion. Reaction (3) formally involves many operations which are believed to occur simultaneously: isomerization of the starting compound to bring the two interacting aniline molecules into close proximity, oxidative coupling of two aniline molecules to
o-semidine and further oxidation of o-semidine to diimine. For
comparison, the reaction8of aniline with [OsBr6]22resulted in
formation of a bis-chelated complex, OsBr2(L1)2, directly. In contrast, a similar reaction1using hydrated RuCl3as the starting material yielded only a monochelated diimine complex, RuCl2(PhNH2)2(L1), where two out of the four coordinated
aniline molecules underwent oxidative dimerisation. Our pre-sent results have been able to demonstrate clearly that further oxidative coupling of the coordinated anilines in RuCl2(PhNH2)2(L1) is possible by the use of a suitable oxidant. Our preliminary results in the area of oxidative coupling reactions of coordinated aromatic amines2in [Ru(ArNH2)6]2+ and related substrates are highly encouraging and the scope of these reactions is very high.
We are grateful to the referees for their suggestions. Financial support received from the Council of Scientific and Industrial Research, New Delhi is gratefully acknowledged.
Notes and References
† E-mail: [email protected]
‡ Crystal data: [RuCl2(L1)(L2)]·CH2Cl22: C33H37N3OCl4Ru, M = 734.55, monoclinic, space group P21/n, a = 12.1035(20), b = 14.7953(11), c = 19.718(3) Å, b = 104.715(11)°, U = 3415.2(8) Å3, Z = 4, D
c = 1.429 g cm23, crystal dimensions 0.25 3 0.25 3 0.20 mm, T = 298 K, m = 6.259 cm21. Intensity data were collected on an Enraf-Nonius CAD4 diffractometer with graphite-monochromated Mo-Ka radiation (l =
0.7107 Å). 6007 unique reflections were measured and 3728 with I ! 2s(I)
were used in the refinement. Refinement9 of positional and anisotropic thermal parameters for all non-hydrogen atoms converged to R = 0.037. The final Fourier difference map showed residual extrema at 0.410, 20.430
e Å23. CCDC 182/919.
1 K. N. Mitra, P. Majumdar, S.-M. Peng, A. Castiñeiras and S. Goswami, Chem.Commun., 1997, 1267.
2 D. L. Key, L. F. Larkworthy and J. E. Salmon, J. Chem. Soc. (A), 1971, 2583.
3 (a) S. Bhattacharya and C. G. Pierpont, Inorg. Chem., 1992, 31, 2020; (b) S. Bhattacharya, S. R. Boone and C. G. Pierpont, J. Am. Chem. Soc., 1990, 112, 4561.
4 (a) S. Bhattacharya and C. G. Pierpont, Inorg. Chem., 1991, 30, 1511; (b) C. G. Pierpont and R. M. Buchanan, Coord. Chem. Rev., 1981, 38, 45.
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Wieghardt, Inorg. Chem., 1998, 37, 35; P. Belser, A. von. Zelewsky and M. Zehnder, Inorg. Chem., 1981, 20, 3098; C. J. da. Cunha, S. S. Fielder, D. V. Stynes, H. Masui, P. R. Auburn and A. B. P. Lever, Inorg. Chim. Acta, 1996, 242, 293.
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8 K. N. Mitra and S. Goswami, Chem. Commun., 1997, 49.
9 G. M. Sheldrick, SHELXS 86, Program for the solution of crystal structures, University of Göttingen. 1986.
Received in Cambridge, UK, 3rd April 1998; revised manuscript received 8th June 1998; 8/04794D
