Different coordination modes of dipyridyl ketimine ligands in cationic arene ruthenium complexes

Article abstract of DOI:10.1016/j.inoche.2012.12.006

2,2′-Dipyridyl-N-arylimines L (L¹ = 2,4,6-trimethyl(di-2- pyridylmethylene)aniline, L² = 2,6-diisopropyl(di-2-pyridylmethylene) aniline) react with arene ruthenium dichloride dimer in methanol to give cationic arene ruthenium complex

Full text of DOI:10.1016/j.inoche.2012.12.006

Inorganic Chemistry Communications 29 (2013) 194–196
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Different coordination modes of dipyridyl ketimine ligands in cationic arene
ruthenium complexes
⁎
Mathiyazhagan Ulaganatha Raja, Bruno Therrien, Georg Süss-Fink
Institut de Chimie, Université de Neuchâtel, Avenue de Bellevaux 51, CH-2000 Neuchâtel, Switzerland
a r t i c l e i n f o
a b s t r a c t
2,2′-Dipyridyl-N-arylimines L (L1=2,4,6-trimethyl(di-2-pyridylmethylene)aniline, L2 =2,6-diisopropyl(di-
2-pyridylmethylene)aniline) react with arene ruthenium dichloride dimer in methanol to give cationic
arene ruthenium complexes of the general type [(arene)Ru(η2-N,N-L)Cl]+ (arene=C6H6, p-MeC6H4Pri).
Two coordination modes of the chelating ligands N,N-L are observed. In the major isomer, the ketimine nitro-
gen atom and one of the two pyridine nitrogen atoms are coordinated to ruthenium, while in the minor iso-
mer the two pyridine nitrogen atoms coordinate to the metal center. In the case of L1, the minor isomer of the
p-cymene ruthenium chloro complex could be isolated as the tetrafluoroborate salt and characterized by sin-
gle crystal X-ray analysis. The molecular structure of the major isomer was determined by X-ray crystallog-
raphy in the case of the tetraphenylborate salt of the benzene ruthenium chloro derivative. In both structures,
the ruthenium atom shows the expected pseudo-tetrahedral coordination geometry.
Article history:
Received 4 October 2012
Accepted 17 December 2012
Available online 28 December 2012
Keywords:
Ruthenium
Arene
2,2′-Dipyridyl imines
Coordination isomerism
Structure
© 2012 Elsevier B.V. All rights reserved.
Schiff bases have a rich coordination chemistry which involves
almost all the metals of the periodic table [1–3]. Schiff base com-
plexes have found a wide variety of applications in organic synthesis
[4,5], for liquid crystals [6], as anticancer compounds [7] or as molec-
ular switches in memory circuits [8]. On the other hand, ligands
containing two pyridyl units, in particular 2,2′-dipyridyl [9,10], have
also been extensively studied [11–14]. Much attention has been fo-
cused on half-sandwich arene ruthenium complexes containing che-
lating N,N- or N,O-ligands, because of the organometallic arene
ruthenium unit being inert toward hydrolysis [15–22].
with arene ruthenium units: Singh and Kaminsky reported the synthesis
and characterization of the cationic complexes [(arene)-Ru(η2-N,N-
RNC(NC5H4)Cl]+ (arene=C6H6, p-MeC6H4Pri, C6Me6) [24]. This paper
prompted us to report our own findings on the reaction of aromatic
2,2′-dipyridylimines with arene ruthenium dichloride dimers.
In this paper, we describe the synthesis and characterization of
cationic arene ruthenium N-aryl 2,2′-dipyridylimine chloro com-
plexes, for which we found coordination isomers, in contrast to the
findings with aliphatic 2,2′-dipyridylimines [24].
Benzene ruthenium dichloride dimer and p-cymene ruthenium
dichloride dimer react in methanol with the 2,2′-pyridyl N-aryl imines
L1 or L2 to give cationic complexes of the type [(arene)Ru(η2-N,N-L)
Cl]+, for which two isomers are observed by NMR spectroscopy
(Scheme 1). The major isomers can be isolated and characterized in
all cases, the minor isomers only in the case of the p-cymene deriva-
tives. Experimental details are given in the supplementary information.
The isomers have been separated by column chromatography of
the crude reaction mixture on silica. The major isomers 1a, 2a, 3a
and 4a have been isolated as the chloride salts. The minor isomers
3b and 4b could also be isolated as the chloride salts. All isolated
compounds are air-stable, orange to red crystalline solids, which are
soluble in chlorinated and polar solvents such as acetone, methanol
and water.
Suitable crystals for single crystal X-ray structure analysis have
been obtained for [1a][BPh4], available from [1a]Cl and NaBPh4 in a
dichloromethane–pentane mixture and for [3b][BF4], available from
[3b]Cl and NaBF4 in a dichloromethane–pentane mixture. The molecu-
lar structures of cations 1a and 3b are shown in Fig. 1. Crystallographic
details are given in the Supplementary information (Table S1).
Dipyridyl imine ligands of the type RN=C(2,2′-NC5H4)2 have
been made available only recently. The aliphatic N-methyl [23] and
N-ethyl [24] have been prepared from 2,2′-dipyridyl ketone and the
corresponding amine. In a similar fashion, the aromatic N-2,4,6-
trimethylphenyl and N-2,6-diisopropylphenyl derivatives have been
reported by Aibing Xia and co-workers [25]. While the aromatic
2,2′-dipyridylimines have been coordinated to dichloro palladium units
[25], the coordination of aliphatic 2,2′-dipyridylimines has been studied
⁎
Corresponding author. Tel.: +41 32 718 2405; fax: +41 32 718 2511.
E-mail address: georg.suess-fink@unine.ch (G. Süss-Fink).
1387-7003/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.inoche.2012.12.006

M.U. Raja et al. / Inorganic Chemistry Communications 29 (2013) 194–196
195
+
+
arene
arene
R
Ru
N
Ru
N
N
N
Cl
Cl
N
N
R
Scheme 1. Synthesis of arene ruthenium 2,2′-dipyridyl-N-arylimine complexes.
In complex 1a (major isomer), the N,N-ligand L1 is coordinated to
the ruthenium center through one of the two pyridine groups and the
ketimine nitrogen atom in a η2-chelating fashion. The chloro ligand
and the benzene ligand complement the pseudo-tetrahedral coordina-
tion sphere of the ruthenium center. Complex 3b (minor isomer) also
shows a pseudo-tetrahedral piano-stool coordination geometry of the
ruthenium center, however, the η2-chelating N,N-ligand L1 is coordina-
ted through both pyridine nitrogen atoms, while the ketimine nitrogen
atom is free. Between the metal center and the pyridine ligands, Ru–N
bond distances are 2.044(5) Å in 1a, 2.093(3) and 2.104(3) Å in 3b,
the distance between the ruthenium atom and the ketimine nitrogen
atom in 1a is 2.052(5) Å.
these isomers have not been observed in the analogous complexes
with aliphatic substituents at the ketimine nitrogen atom [24].
In conclusion, we have synthesized and characterized arene ruthe-
nium half-sandwich complexes bearing 2,2′-dipyridyl-N-arylimine
ligands. Two coordination isomers were isolated in two cases. The
X-ray diffraction study confirms a ketimine nitrogen and pyridine ni-
trogen coordination mode in the major isomer and a two pyridine
nitrogen coordination mode in the minor isomer.
Acknowledgments
Financial support of this work by the Swiss National Science Founda-
tion and a generous loan of ruthenium(III) chloride hydrate from the
Johnson Matthey Research Centre are gratefully acknowledged. M.U.R.
thanks the Swiss Federal Government for a Scholarship (2011.0894).
The formation of the minor isomer b is presumably due to the
steric hindrance of the aromatic 2,4,6-Me3C6H2 and 2,6-Pri2C6H3
substituents of the ketimine nitrogen atom. This may explain why
Fig. 1. ORTEP diagrams of cations 1a (left) and 3b (right) (35% probability level ellipsoids) The counter anions are omitted for clarity. Selected bond lengths (Å) and angles (°): [1a]
[BPh4]: Ru(1)-Cl(1) 2.380(2), Ru(1)-N(1) 2.044(5), Ru(1)-N(2) 2.052(5), N(1)-Ru(1)-N(2) 77.2, N(1)-Ru(1)-Cl(1) 82.24(15), N(2)-Ru(1)-Cl(1) 86.00(15); [3b][BF4]: Ru(1)-Cl(1)
2.3948(11), Ru(1)-N(1) 2.093(3), Ru(1)-N(2) 2.104(3), N(1)-Ru(1)-N(2) 84.76(12), N(1)-Ru(1)-Cl(1) 85.88(9), N(2)-Ru(1)-Cl(1) 85.27(9).

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Appendix A. Supplementary material
deprotonated anion with ruthenium and osmium carbonyl complexes, Organo-
metallics 22 (2003) 5137–5140.
The experimental details and spectroscopic data of all new com-
plexes are presented in the supplementary information. CCDC 896870
[1a][BPh4] and 896871 [3b][BF4] contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, by e-mailing data_request@
ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic
Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223
336033. Supplementary data to this article can be found online at
http://dx.doi.org/10.1016/j.inoche.2012.12.006.
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