Enhancing SMM properties in a family of [Mn 6 ] clusters{ Constantinos J. Milios, a Ross Inglis, a Rashmi Bagai, b Wolfgang Wernsdorfer, c Anna Collins, a Stephen Moggach, a Simon Parsons, a Spyros P. Perlepes, d George Christou b and Euan K. Brechin* a Received (in Cambridge, UK) 5th April 2007, Accepted 17th May 2007 First published as an Advance Article on the web 25th June 2007 DOI: 10.1039/b705170k The complex [Mn 6 O 2 (Et-sao) 6 (O 2 C 11 H 15 ) 2 (EtOH) 6 ] has U eff = 80 K. For some time, we have been attempting to make polymetallic clusters of transition metal ions with the purpose of synthesizing single-molecule magnets (SMMs). 1 Our approach, like many others, 2 was to employ flexible organic bridging ligands in self- assembly processes and was particularly focused on using manganese. One such project involved the coordination chemistry of salicylaldoxime (saoH 2 ). 3 In all of the Mn clusters we isolated and analysed we noticed that, without exception, the exchange between the metal centres was very weak and typically only a few wavenumbers (,1–2 cm 21 ) in magnitude. This is exemplified by the hexametallic SMM [Mn 6 O 2 (sao) 6 (O 2 CH) 2 (EtOH) 4 ]. 4 Normally this would be considered a disadvantage since it inevitably leads to a ground state that is not well isolated from its excited states. However, it also means that the switching from an antiferromagnetic exchange interaction to a ferromagnetic exchange interaction becomes easier to achieve—especially since only minor structural modifications can lead to major changes in | J|. 5 One possible way of achieving such a switch is to structurally distort the molecule in question, either via ‘external’ means, (i.e. the application of pressure 6 ), or ‘internally’ via deliberate chemical modification of the magnetic core. We speculated 7 that the latter strategy would bear fruit in the case of the [Mn 6 O 2 (sao) 6 - (O 2 CR) 2 (EtOH) 4 ] family (S = 4) 8 if the Mn–O–N–Mn torsion angles of the bridging salicyaldoximate ligands could be sufficiently ‘twisted’. We achieved this by derivatising the oximate carbon atom with ‘bulky’ Me (Me–saoH 2 ), Et (Et–saoH 2 ) and Ph (Ph–saoH 2 ) groups and re-making the analogous, but more sterically ‘hindered’ hexametallic clusters. For example, the complex [Mn 6 O 2 (Et–sao) 6 (O 2 CPh) 2 (EtOH) 4 (H 2 O) 2 ](1) has an S = 12 ground state, 7a while complex [Mn 6 O 2 (Et–sao) 6 - (O 2 CPh(Me) 2 ) 2 (EtOH) 6 ] (also with an S = 12 ground state) 7b — has a record value of the effective barrier to magnetization reversal (U eff ) of 86.4 K. Here we demonstrate the general applicability of this approach to the [Mn 6 O 2 (R-sao) 6 (O 2 CR) 2 L 6 ] class of SMMs by reporting new members of this family which show either similar, or larger, effective energy barriers than the [Mn 12 O 12 (O 2 CCH 2 Br) 16 (H 2 O) 4 ] (Mn 12 –BrAc) 9 member of the prototype [Mn 12 ] family. We also speculate that the antiferro- magnetic (AF) to ferromagnetic (F) transition in the exchange occurs at a Mn–N–O–Mn torsion angle of y31u, and that the bigger this angle the more positive (F) the exchange becomes. The complexes [Mn 6 O 2 (Et–sao) 6 (O 2 CPhMe) 2 (EtOH) 4 (H 2 O) 2 ] (2) and [Mn 6 O 2 (Et–sao) 6 (O 2 C 11 H 15 ) 2 (EtOH) 6 ] (3), (where HO 2 CPhMe = 4-methyl-benzoic acid; HO 2 C 11 H 15 = adamantane carboxylic acid) can be made in excellent yields from the simple combination of Mn(ClO 4 ) 2 ?4H 2 O, the corresponding derivatized oxime, Et 4 NOH (or CH 3 ONa) and the appropriate HO 2 CR in EtOH. The three complexes (1–3) are isostructural: each (Fig. 1 shows complex 3) consists of two off-set [Mn 3 O] 7+ triangles linked together via two oximate oxygen atoms from two g 1 :g 2 :g 1 :m 3 Et–sao 22 ligands and two phenolate oxygen atoms derived from two g 2 :g 1 :g 1 :m 3 Et–sao 22 ligands. The remaining two oximato(22) ligands each bridges one edge of a [Mn 3 O] 7+ triangle in an g 1 :g 1 :g 1 :m fashion thus forming a [Mn III 6 (m 3 -O) 2 (m 3 - ONR) 2 (m-ONR) 4 (m-OR9) 2 ] 6+ core. The remaining coordination sites on the Mn ions are filled by terminally coordinated carboxylates and alcohols (or a combination of alcohol/H 2 O). Each 6-coordinate Mn ion is in distorted octahedral geometry with the Jahn–Teller axes all approximately perpendicular to the [Mn 3 ] planes. There is only one major intramolecular structural differ- ence between all three complexes (1–3)—the degree of twisting in the Mn–N–O–Mn linkage along each edge of the Mn 3 triangles. These torsion angles are summarized in Table 1 and Fig. 1, and range from a minimum value of 30.4u to 47.2u. 1–3 all crystallize in the triclinic space group P1 ¯ as 1?2EtOH, 2?2EtOH, and 3. In each case the molecules have only one orientation in the crystal, with neighbouring molecules stacked directly upon each other in a head-to-tail fashion. For 1 and 2 the solvate molecules sit between a School of Chemistry, The University of Edinburgh, West Mains Road, Edinburgh, EH9 3JJ, UK. E-mail: ebrechin@staffmail.ed.ac.uk; Tel: 0131-650-7545 b Chemistry Department, University of Florida, Gainesville, Florida, 32611-7200, USA c Laboratoire Louis Ne ´el-CNRS, 38042 Grenoble, Cedex 9, France d Department of Chemistry, University of Patras, 26504, Patras, Greece { Electronic supplementary information (ESI) available: Exchange inter- action schemes for complexes 1–3 and susceptibility measurements for 3. See DOI: 10.1039/b705170k Fig. 1 Molecular structure of 3 and the [Mn 6 O 2 (NOR) 6 (OR9) 2 ] 6+ core common to 1–3 highlighting the Mn–N–O–N torsion angles (a). Atom colour code: Mn = red, O = green, N = blue, C = yellow. COMMUNICATION www.rsc.org/chemcomm | ChemComm 3476 | Chem. Commun., 2007, 3476–3478 This journal is ß The Royal Society of Chemistry 2007