FULL PAPER DOI:10.1002/ejic.201500395 Ionic and Neutral Half-Sandwich Guanidinatoruthenium(II) Complexes and Their Solution Behavior Ram Kishan, [a][‡] Robin Kumar, [a][‡] Sambath Baskaran, [b] Chinnappan Sivasankar,* [b] and Natesan Thirupathi* [a] Keywords: Ruthenium / Hydrogenation / Rearrangement / N ligands / N–H activation Ionic and neutral half-sandwich guanidinatoruthenium(II) complexes [(η 6 -C 10 H 14 )RuL(κ 2 (N,N'){(ArN) 2 C-N(H)Ar})]- [OTf] [Ar = 4-MeC 6 H 4 , L = 2-methylimidazole (1); Ar = 2- MeC 6 H 4 , L = 1,3,5-triaza-7-phosphaadamantane (PTA; 2)], [(ArNH) 3 C][(η 6 -C 10 H 14 )RuCl 3 ] [Ar = 2-ClC 6 H 4 (3)] and [(η 6 - C 10 H 14 )RuCl(κ 2 (N,N'){(ArN) 2 C-N(H)Ar})] [Ar = 2-ClC 6 H 4 (4), 2-FC 6 H 4 (5), 4-ClC 6 H 4 (6), and 4-(NO 2 )C 6 H 4 (7)] have been isolated and the molecular structures of all but 6 were deter- mined by single-crystal X-ray diffraction. VT 31 P{ 1 H} NMR spectroscopy of complex 2 revealed the presence of a mixture of four isomers in a ratio of appproximately 1.4:11.0:1.6:0.1. Theoretical calculations were performed on the syn-syn, syn- anti, anti-syn, and anti-anti conformers of 2, which revealed syn-anti to be the most stable conformer, both in the gas Introduction Half-sandwich amidoruthenium(II) complexes are well known for their role as pre-catalysts in numerous organic transformations, [1] as anticancer agents, [2] and as interesting scaffolds for exploring patterns of reactivity. [3] Amidin- ate(1–) and guanidinate(1–) are two classes of nitrogen do- nor ligands that differ by virtue of the latter possessing an amino substituent on the carbon atom of the NCN unit, thereby making the nitrogen atoms more strongly donating (Scheme 1). [4,5] This feature introduces a greater variety of bonding possibilities and coordination modes for guanidin- ates towards metal ions. [5] sym N,N',N''-trisubstituted guanidines, (RNH) 2 C=NR (R = alkyl and aryl), are steri- cally and electronically flexible guanidines as the substitu- ents on the nitrogen atoms can be symmetrically varied. [5,6] Furthermore, this class of guanidine can be mono- and di- deprotonated to afford guanidinate(1–) and guanidinate(2–) species, respectively (Scheme 2). [6] [a] Department of Chemistry, University of Delhi, New Delhi 110007, India E-mail: tnat@chemistry.du.ac.in thirupathi_n@yahoo.com [b] Department of Chemistry, School of Physical, Chemical and Applied Sciences, Pondicherry University, Puducherry 605014, India E-mail: siva.che@pondiuni.edu.in [‡] These two authors have contributed equally to the manuscript. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/ejic.201500395. Eur. J. Inorg. Chem. 2015, 3182–3194 © 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 3182 phase and in solution. Furthermore, the energies relating to the interactions between the lone pairs of the nitrogen atoms of the CN 3 unit and the antibonding orbital of the central carbon atom in the syn-anti conformer of 2 were investigated by NBO analysis. The 1 H NMR spectrum of 3 reveals the presence of two species and their ratio is dependent upon the concentration of 3 in CDCl 3 and on the amount of D 2 O in CD 3 CN. The two species have been ascribed to the pres- ence of a monomer and a water-bridged dimer. The new complexes 1–7 were screened as pre-catalysts in the transfer hydrogenation of acetophenone and complex 3 was tested as a pre-catalyst in the transfer hydrogenation of a variety of ketones to explore the scope of the reaction. Scheme 1. Scheme 2. Numerous amidinatoruthenium(II) complexes are known in the literature due to their interesting reactivity pattern and bonding aspects. [7] In contrast, guanidinatoru- thenium(II) complexes have been scarcely studied. [8] Bailey and co-workers and subsequently our group have reported the synthesis and characterization of half-sandwich guanid- inatoruthenium(II) complexes I [8a,8b] and II–V , [9] respec- tively, including the reactivity pattern and solution behavior of the latter complexes (Scheme 3). Recently, Gracía-Álvarez et al. reported the synthesis and characterization of half-sandwich guanidinatoruthen-