Cerium–Organic Magnets DOI: 10.1002/anie.201107938 Magnetic Ordering in Self-assembled Materials Consisting of Cerium- (III) Ions and the Radical Forms of 2,5-TCNQX 2 (X = Cl, Br)** María Ballesteros-Rivas, Hanhua Zhao, Andrey Prosvirin, EricW. Reinheimer, RubØn A. Toscano, Jesffls ValdØs-Martínez, and Kim R. Dunbar* Unlike transition metals, rare earth elements do not engage in appreciable magnetic superexchange due to the highly contracted nature of the 4f orbitals and strong shielding effects of the outer 5d and 6s shells. This situation notwith- standing, the co-assembly of d- as well as p-orbital based spins with lanthanide ions has been a fruitful approach to magnetic heterospin systems with interesting properties. [1] The majority of reports involving molecular magnetism of lanthanide elements focus on trivalent Sm, Eu, Gd, Tb [2] and, especially, Dy compounds. [3] Extraordinary properties have been reported for diverse families of compounds from a number of leading research groups beginning with the pioneering results of Rey and Gatteschi who began publishing research on lanthanide nitronyl-nitroxide materials as early as 1989. [1a] In terms of recent milestones, the discovery of slow para- magnetic relaxation for single ion sandwich complexes such as (BuN 4 )[Tb(Pc) 2 ] (Pc = phthalocyanine anion) [4a] and for square, [3d] triangular [3e, 4b,d] as well as other polynuclear Dy compounds [3] is infusing molecular magnetism research with renewed optimism in regard to the potential for realizing high-temperature single molecule magnets (SMMs). The most recent report in this vein of a Dy 5 pyramidal complex with blocking temperatures as high as 40 K and a thermal barrier for magnetization reversal of ca. 540 K is a particularly promising finding. [4c] Another recent remarkable report with regard to the use of radical bridges in rare earth magnetism has also appeared, that is, that the N 2 3 radical bridge leads to the strongest magnetic coupling on record for a Gd III complex and a blocking temperature of 8.3 K at a sweep rate of 0.08 T s 1 for a Dy III analogue. [5] Among the lanthanides, examples of the use of Ce III are scarce in molecular magnetism research, presumably because there is only one spin on this ion. [6] Cerium-containing magnetic materials have been reported with Mn ions and phosphonate ligands; these include a discrete cluster domi- nated by ferromagnetic superexchange and a one-dimen- sional polymer that exhibits antiferromagnetic interactions. [6e] Of further note is the work of Christou and co-workers who prepared unusual compounds including two Mn IV 8 Ce III clus- ters that behave as SMMs, one of which is the first example of a loop-based SMM. [6c] A series of mixed chromium–lantha- nide ladder-type polymers was prepared with La III , Pr III , Nd III , Ce III and [Cr III (ox) 3 ] 3 with antiferromagnetic coupling being observed for the Ce analogue. [6a] Finally, 4f/3d compounds of general formula [Ln(DMF) 4 (H 2 O) 3 (m-CN)M(CN) 5 ]·nH 2 O 5 (Ln = La III , Ce III ;M = Fe III , Co III ) were prepared and anti- ferromagnetic coupling between the Fe III and Ce III centers was observed. [6b,d] Detailed EPR analyses of the Ce–Fe, La–Fe and Ce–Co analogues provides additional evidence for the exchange coupling. Given the success of the Ln III /nitronyl-nitroxide radical systems in yielding both discrete complexes and extended structures, [1a–d,h, 2a, 3a–c, 7] it is of obvious interest to pursue the combination of trivalent lanthanide ions with other organic radicals. To date, TCNQ (TCNQ = 7,7,8,8-tetracyanoquinodi- methane) has yielded several interesting compounds with trivalent lanthanide atoms such as [Ln(N  N) x (TCNQ) 3 ] (Ln = Pr, Nd; N  N = 1,10-phenanthroline, dipyridylamine; x = 2, 4) and [Ln(N  N) 4 (TCNQ) 4 ], [8] a 2-D material [{Gd 2 - (TCNQ) 5 (H 2 O) 9 }{Gd(TCNQ) 4 (H 2 O) 3 ·4 H 2 O} 1 ] composed of interpenetrating anionic and cationic networks that exhibits ferromagnetic ordering at 3.5 K, [1j] and a family of Ln III /[TCNQF 4 ]C  (TCNQF 4 = 2,3,5,6-tetrafluoro-7,7,8,8- tetracyanoquinodimethane) molecular magnets, [1l] among which is a Tb analog which exhibits a fascinating interplay between SMM and phonon bottleneck (PB) behavior. Of additional note is the interesting report by Miller and co- workers of magnetic ordering in the rare earth series [Ln- (TCNE) 3 ] (TCNE = tetracyanoethylene) with Gd III and Dy III ions. [1g] These previous examples notwithstanding, there is very little rare earth organocyanide radical chemistry in the literature. As part of a broad effort in our group to investigate the properties of TCNQ radicals coordinated to rare earth metal ions, we investigated the chemistry of Ce III with TCNQ and derivatives. Earlier, the compound [Ce 2 (TCNQ) 4 (H 2 O) 12 ]- [TCNQ] 2 ·MeOH·3 H 2 O was reported along with its electro- chemical properties, but no magnetic studies were included. [9] Herein we report the syntheses, structures, and magnetic properties of a slightly different phase of this compound, namely [Ce 2 {(TCNQ) 4 (H 2 O) 10 (CH 3 OH) 2 ][TCNQ] 2 ·2 H 2 O ·2 CH 3 OH (1) and the two new compounds : [Ce 2 (TCNQBr 2 ) 4 - [*] Dr. H. Zhao, Dr. A. Prosvirin, Prof. K. R. Dunbar Department of Chemistry, Texas A&M University P.O. Box 30012, College Station, TX 77842-3012 (USA) E-mail: dunbar@mail.chem.tamu.edu Homepage: http://www.chem.tamu.edu/rgroup/dunbar/ dunbar.htm M. Ballesteros-Rivas, R. A. Toscano, Prof. J. ValdØs-Martínez Instituto de Química, Universidad Nacional Autónoma de MØxico (Mexico) [**] K.R.D. thanks the National Science Foundation (CHE-0957840) and the Welch Foundation (grant A-1449) for support of this work. Financial support for M.F.B.R. with a CONACyT fellowship is gratefully acknowledged. TCNQ = 7,7,8,8-tetracyanoquino- dimethane; X = Cl, Br. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201107938. . Angewandte Communications 5124  2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2012, 51, 5124 –5128