New N-heterocyclic carbene mercury(II) complexes: Close mercuryearene interaction Rosenani A. Haque * , Abbas Washeel Salman, Teoh Siang Guan, Hassan Hadi Abdallah School of Chemical Sciences, Universiti Sains Malaysia, 11800 USM Penang, Malaysia article info Article history: Received 23 April 2011 Received in revised form 21 July 2011 Accepted 22 July 2011 Keywords: Mercury-N-heterocyclic carbenes Imidazolium salts Nitrogen heterocycles Transmetallation X-ray abstract Mononuclear mercury complexes (1 , 2, and 3) bearing bis-N-heterocyclic carbene (NHC) ligands of the form [(NHC) 2 -m-Hg] þ2 have been prepared and structurally characterised. The complexes were derived from three bis-imidazolium salts as precursors to NHC; either 1,3-bis(N-methylimidazolium-1-ylmethyl) benzene bis(hexafluorophosphate) (I$2PF 6 ), 1,3-bis(N-butylimidazolium-1-ylmethyl)benzene bis(hexa- fluorophosphate) (II$2PF 6 ) or 3,5-bis(N-butylimidazolium-1-ylmethyl)toluene bis(hexafluorophosphate) (III$2PF 6 ) treated with mercury(II) acetate. Interestingly X-ray crystal structure analysis revealed a close interaction between the Hg metal centre with one carbon atom of the aryl linker in addition to coor- dination with two NHCs. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction The use of N-heterocyclic carbene (NHC) ligands is now wide- spread in modern organometallic synthesis [1]. NHC has attractive properties such as stability, ease of handling, compatibility with metals in low- and high-oxidation states, and stability of its complexes because of a strong metal-ligand binding. Studies have reported NHC complexes with a large number of transition metals [2] showing several interesting applications, most importantly in the area of catalysis [3], as well as in antimicrobial and anti- mitochondrial biological studies [4]. The work in our laboratory is inspired by previous work in the Baker’s laboratory [5] where the corresponding author was part of the group. The group is actively working with xylyl-linked NHCs and their metal complexes, both in NHCs forming part of a macro- cyclic structures (cyclophanes) and in their analogues, wherein bis(NHC) ligands contain only one xylyl linker (non-cyclophane structures) [5]. Baker et al. reported a series of well-defined NHC-metal complexes with these ligands bearing a broad range of coordination modes, Chart 1 [5,6]. For example, two NHCs chelating a single metal forming either a dimeric (4 and 5) or a monomeric structure (6e9); two NHCs and an arene, which constitute part of a single cyclophane macrocyclic structure, simultaneously binding to one metal center (10); and a pincer complex where binding involves the NHCs and the additional groups when an additional donor atom is included (11). In 1968, Wanzlick and Schönherr successfully synthesised the first mercury-NHC complex via direct reaction of an imidazolium salt with mercury(II) acetate [7]. Despite being the earliest example of NHC-metal complexes, mercury-NHC complexes have received little attention compared with other metals. Similarly, their appli- cations have not been widely explored. Baker et al. reported the first syntheses of mercury-NHC complexes derived from imidazolium-linked cyclophanes (e.g., 6 and 7 , Chart 1), as well as their use for redoxetransmetallation chemistry, wherein the reaction of an NHC-mercury(II) complex with a palladium(0) source results in an NHC-palladium(II) complex [5c]. Although transmetallation involving silver(I)-NHC complexes is now a routine method for the synthesis of other metal-NHC complexes, this is the first reported example of a (redox) trans- metallation reaction involving a mercury-NHC complex [5c]. We have been interested in the chemistry of mercury(II)-NHC complexes due to their potential use as carbene transfer reagents. In actively dealing with both mercury- and silver-NHC complexes in our laboratory, we found that obtaining different coordination geometry of silver(I) and mercury(II) complexes starting from the same ligand is possible. For example, silver(I) and mercury(II) complexes made from an NHC-linked meta-xylyl cyclophane produce different geometry of complexes [5b]. We have shown that * Correspondence author. Tel.: þ604 653 3578. E-mail address: rosenani@usm.my (R.A. Haque). Contents lists available at ScienceDirect Journal of Organometallic Chemistry journal homepage: www.elsevier.com/locate/jorganchem 0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jorganchem.2011.07.032 Journal of Organometallic Chemistry 696 (2011) 3507e3512