A new class of single-molecule magnets: mixed-valent [Mn 4 (O 2 CMe) 2 (Hpdm) 6 ][ClO 4 ] 2 with an S = 8 ground state Euan K. Brechin, a Jae Yoo, b Motohiro Nakano, b John C. Huffman, a David N. Hendrickson* b and George Christou* a a Department of Chemistry and Molecular Structure Center, Indiana University, Bloomington, IN 47405-4001, USA. E-mail: christou@indiana.edu b Department of Chemistry-0358, University of California at San Diego, La Jolla, CA 92093-0358, USA. E-mail: dhendrickson@ucsd.edu Received (in Cambridge, UK) 9th February 1999, Accepted 25th March 1999 The reaction of pyridine-2,6-dimethanol (H 2 pdm) with [Mn 3 O(O 2 CMe) 6 (py) 3 ][ClO 4 ] gives the 2Mn II , 2Mn III title compound 1, which has an S = 8 ground state and displays strong out-of-phase signals in ac susceptibility studies that establish 1 as a new class of single-molecule magnet. The study of molecules with unusually large numbers of unpaired electrons has taken on added impetus in recent years as this area has been identified as the source of a new magnetic phenomenon of relevance to the magnetic materials arena, i.e. the ability of molecules below a critical temperature to function as magnetizable magnets. 1–10 Samples of such molecules thus function as collections of extremely small magnetic particles and ones that are of a uniform size distribution in contrast to metal oxide particles (or other magnetic materials) of nanoscale dimensions, which are prepared as a range of particle sizes. To date, [Mn 12 O 12 (O 2 CR) 16 (H 2 O) 4 ] (S = 10), 1–5 [Mn 12 O 12 - (O 2 CR) 16 (H 2 O) 4 ] 2 salts (S = 19/2), 6 [Mn 4 O 3 X(O 2 CMe) 3 - (dbm) 3 ] (S = 9/2; dbm is the anion of dibenzoylmethane), 7,8 [Fe 8 O 2 (OH) 12 (tacn) 6 ] 8+ (tacn = 1,4,7-triazacyclononane) salts (S = 10) 9 and [V 4 O 2 (O 2 CR) 7 (L–L) 2 ] z (S = 3; L–L = 2,2A- bipyridine, pyridine-2-carboxylate anion) 10 are the most well studied examples. A convenient way of detecting the slow magnetic relaxation (reorientation of the magnetic moment or magnetization vector) of a single-molecule magnet (SMM) is by the appearance of an out-of-phase signal (c M B) in ac susceptibil- ity studies showing that the relaxation is too slow to keep up with the oscillating field. We herein report access to a new class of Mn-based SMMs with an S = 8 ground state and a strong c M B signal, representing an important new addition to this small family of molecules. The potentially tridentate chelating ligand pyridine-2,6-di- methanol(H 2 pdm), or 2,6-bis(hydroxymethyl)pyridine has been little employed 11,12 in metal chemistry but offers interesting possibilities for transition metal cluster chemistry. Reaction of H 2 pdm with [Mn 3 O(O 2 CMe) 6 (py) 3 ][ClO 4 ] in a 3 : 1 molar ratio in CH 2 Cl 2 gives a red–brown solution from which [Mn 4 (O 2 C- Me) 2 (Hpdm) 6 ][ClO 4 ] 2 1 precipitates within 24 h. The solid can be recrystallized in ca. 50% total yield after three days from a MeCN/Et 2 O layering. Diffusion of Et 2 O directly into an MeCN reaction solution also gives 1 but in only 15% yield after one week. The centrosymmetric cation of 1†‡ (Fig. 1) consists of a planar, mixed-valence Mn 4 rhombus with Mn(1) and Mn(2) assigned as being Mn III and Mn II , respectively, on the basis of bond valence sum calculations and the presence at Mn(1) of a Jahn–Teller elongation axis [O(17)–Mn(1)–N(30)], as expected for high-spin Mn III . All ligands are thus only mono-deproto- nated (i.e. Hpdm) as found for this group previously. 11 Two Mn 3 triangular faces are each bridged by a m 3 -oxygen [O(17)] from a bidentate, chelating Hpdm whose protonated [O(23)] is unbound. Two of the remaining groups are also bidentate, with O(27) bridging Mn(1) and Mn(2A) and O(33) not ligated, but the remaining two Hpdm groups are tridentate with protonated O(13) terminally coordinated and O(7) bridging Mn(1) and Mn(2). Two bridging MeCO 2 2 groups complete the ligation. The Mn II metal atoms Mn(2) are thus seven-coordinate with distorted pentagonal bipyramidal geometry, whereas Mn(1) is distorted octahedral. There is an intramolecular hydrogen bond between O(13) and O(23) [2.852(9) Å]. The cation of 1 is the first example of the (H)pdm ligand in a bridging mode and suggests that other clusters might be accessible with this versatile ligand. Solid-state dc magnetization measurements were performed on 1 in the range 5.0–300 K. The effective magnetic moment (m eff ) slowly increases from 10.5 m B at 300 K to 12.8 m B at 15.0 K, and then decreases to 11.3 m B at 5.0 K, suggesting the complex to have a high spin ground state; the low-temperature decrease is assigned to zero-field splitting (ZFS) and Zeeman effects. Owing to the low symmetry of the cation of 1, it is not possible to use the Kambe approach to fit the m eff vs. T data with the three requisite J values. In order to determine the ground state, therefore, magnetization data were collected in the temperature and magnetic field ranges 2.00–4.00 K and 20.0–50.0 kG (2–5 T) (Fig. 2). Fitting of these data, assuming only the ground state is populated at T @ 4.00 K, gave S = 8, g = 1.85(3) and D = 20.25(3) cm 21 , where D is the axial ZFS parameter. Thus, 1 is a new example of a species with an unusually large spin. Fig. 1 ORTEP plot with 50% probability ellipsoids of the cation of 1. Selected interatomic distances (Å) and angles (°): Mn(1)…Mn(1A) 3.253(2), Mn(1)…Mn(2) 3.351(2), Mn(1)…Mn(2A) 3.284(2), Mn(1)–O(7) 1.879(5), Mn(1)–O(17A) 1.967(4), Mn(1)–O(17) 2.264(4), Mn(1)–O(27) 1.873(5), Mn(2)–O(7) 2.271(4), Mn(2)–O(17) 2.300(5), Mn(2)–O(27A) 2.227(4); Mn(1)–O(17)–Mn(1A) 100.27(17), Mn(1)–O(17A)–Mn(2) 103.24(20), Mn(1A)–O(17A)–Mn(2) 92.03(16), Mn(1)–O(7)–Mn(2) 107.30(23), Mn(1A)– O(27A)–Mn(2) 106.13(22). Chem. Commun., 1999, 783–784 783