Dalton Transactions Dynamic Article Links Cite this: Dalton Trans., 2012, 41, 1675 www.rsc.org/dalton COMMUNICATION Influence of second sphere hydrogen bonding interaction on a manganese(II)-aquo complex† Sanae El Ghachtouli, a R´ egis Guillot, a Pierre Dorlet, b,d Elodie Anxolab´ eh` ere-Mallart c and Ally Aukauloo* a,d Received 1st October 2011, Accepted 31st October 2011 DOI: 10.1039/c1dt11858g We have developed a pentadentate N 4 O ligand scaffold with a benzimidazole group placed in a rigid fashion to develop hy- drogen bonding interaction with the ligand in the sixth posi- tion. The mononuclear Mn(II) complex with a water molecule was isolated and characterized. We discuss the role of the outer sphere ligand in stabilising a Mn(II)-aquo complex. During these past few years, there has been a growing recognition for the role of outer sphere ligands on the structure/reactivity re- lationship of several active sites of metalloenzymes. 1 For instance, different amino acid residues often located in the surrounding of the metal ion provide additional control in the binding and activation of small molecules and also offer ways to modulate the chemical reactivity of the metal ion through hydrogen bonding. Introducing such a degree of sophistication in synthetic models is a challenging task for a synthetic chemist. Remarkable effects have already been observed on the metal activation of dioxygen and water through the judicious positioning of a hydrogen bonding network in the second coordination sphere of both manganese and iron based complexes. 2 Collman and colleagues have been pioneers in the design of hydrogen bonded cavities on the porphyrin core to stabilise the dioxygen adduct. 3 In the course of our study on manganese complexes, we have shown that the pentadentate N 4 O monoanionic ligand LH, (Scheme 1) leads to the formation of a mono-m-oxo dinuclear man- ganese(III) complex [1] 2+ . 4 All attempts to isolate the mononuclear manganese species, either in the +II or +III oxidation states with an axially bound water molecule, have proved unsuccessful. In a more recent study, we have shown that adding a bulky tert-butyl a Institut de Chimie Mol´ eculaire et des Mat´ eriaux d’Orsay, UMR-CNRS 8182, Universit´ e de Paris-Sud XI, F-91405, Orsay, France. E-mail: ally. aukauloo@u-psud.fr; Fax: +33 (0) 169 154 756; Tel: +33 (0) 169 154 755 b Laboratoire Stress Oxydant et D´ etoxication CNRS URA 2096 F-91191 Gif- sur-Yvette, France and CEA, iBiTec-S, SB 2 SM, F-91191, Gif-sur-Yvette, France c Laboratoire d’Electrochimie Mol´ eculaire UMR 7591 Univ Paris Diderot, Sorbonne Paris Cit´ e, 15 rue Jean-Antoine de Ba¨ ıf, F-75205, Paris, Cedex 13, France d CEA, iBiTec-S, Service de Bio´ energ´ etique Biologie Structurale et M´ ecanismes (SB 2 SM), F-91191, Gif-sur-Yvette, France, and CEA, iBiTec- S, SB 2 SM, F-91191, Gif-sur-Yvette, France †Electronic supplementary information (ESI) available: Experimental section, crystallographic data and additional EPR spectrum. CCDC reference numbers 817802 and 828978. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c1dt11858g Scheme 1 group in the ortho- and para-positions of the phenol (tBuLH, Scheme 1), we could isolate a manganese(III) complex bearing a hydroxo ligand in the available sixth coordination site on the metal centre, complex [2a] + . Electrochemical study indicates a reversible Mn III/II process on a cyclic voltammogram scale at quite a negative potential (E 0 Mn III OH+H 2 O / Mn II OH 2 +OH - =-0.35 V vs. SCE) in the presence of water. 5 It has been argued that hydrogen bonding in the second coordination sphere can also help to prevent the formation of the M–O–M motif and at the same time stabilise highly oxidised Metal–Oxo units. 6–8 With the aim to understand the stepwise activation of a man- ganese bound water molecule, we reasoned that the N 4 O ligand skeleton stands as a good candidate to implement a functional group that could develop hydrogen bonding with a bound water molecule. We report here on the synthesis of a novel derivative of the N 4 O ligand covalently assembled with a benzimidazole as a potential donor for hydrogen bonding. The crystallographic structure of the mononuclear Mn(II) complex with a bound water molecule evidenced a hydrogen bonding interaction between the water and the protonated form of the benzimidazole unit. We also report on the spectroscopic characterisation and electrochemical properties of the analogous chloro Mn(II) derivative. Further comparison with compounds [2a] + and [2b] + helps to account for the contribution of the second coordination sphere. The synthetic pathway leading to the target ligand (LH 2 ) is presented in Scheme 2. The benzimidazole fragment was positioned in an ortho position of the phenol group (compound (a)) and was obtained in good yield by reaction of one equivalent of 1,2-diamino benzene with 5-tert-Butyl-2-hydroxybenzaldehyde in the presence of benzofuroxane. 9 The introduction of a formyl group on (a), was realised by the Duff procedure, treatment with This journal is © The Royal Society of Chemistry 2012 Dalton Trans., 2012, 41, 1675–1677 | 1675 Downloaded by SCD Université Paris 7 on 23 January 2012 Published on 20 December 2011 on http://pubs.rsc.org | doi:10.1039/C1DT11858G View Online / Journal Homepage / Table of Contents for this issue