Solvate-Dependent Spin Crossover and Exchange in Cobalt(II) Oxazolidine Nitroxide Chelates Ian A. Gass, †,‡ Subrata Tewary, § Gopalan Rajaraman, § Mousa Asadi, † David W. Lupton, † Boujemaa Moubaraki, † Guillaume Chastanet, ∥ Jean-Francois Le ́ tard, ∥ and Keith S. Murray* ,† † School of Chemistry, Monash University, Clayton, Victoria 3800, Australia ‡ School of Pharmacy and Biomolecular Sciences, University of Brighton, Brighton BN2 4GJ, U.K. § Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India ∥ CNRS, Universite ́ de Bordeaux, ICMCB, 87 avenue du Dr. A. Schweitzer, 33608 Pessac, France *S Supporting Information ABSTRACT: Two oxazolidine nitroxide complexes of cobalt(II), [Co II (L • ) 2 ]- (B(C 6 F 5 ) 4 ) 2 ·CH 2 Cl 2 (1) and [Co II (L • ) 2 ](B(C 6 F 5 ) 4 ) 2 ·2Et 2 O(2), where, L • is the tridentate chelator 4,4-dimethyl-2,2-bis(2-pyridyl)oxazolidine N-oxide, have been investigated by crystallographic, magnetic, reflectivity, and theoretical (DFT) methods. This work follows on from a related study on [Co II (L • ) 2 ] - (NO 3 ) 2 (3), a multifunctional complex that simultaneously displays magnetic exchange, spin crossover, and single molecule magnetic features. Changing the anion and the nature of solvation in the present crystalline species leads to significant differences, not only between 1 and 2 but also in comparison to 3. Structural data at 123 and 273 K, in combination with magnetic data, show that at lower temperatures 1 displays low-spin Co(II)-to-radical exchange with differences in fitted J values in comparison to DFT (broken symmetry) calculated J values ascribed to the sensitive influence of a tilt angle (θ) formed between the Co(d z 2 ) and the trans-oriented O atoms of the NO radical moieties in L • . Spin crossover in 1 is evident at higher temperatures, probably influenced by the solvate molecules and crystal packing arrangement. Complex 2 remains in the high-spin Co(II) state between 2 and 350 K and undergoes antiferromagnetic exchange between Co−radical and radical−radical centers, but it is difficult to quantify. Calculations of the magnetic orbitals, eigenvalue plots, and the spin densities at the Co and radical sites in 1 and 2 have yielded satisfying details on the mechanism of metal−radical and radical−radical exchange, the radical spins being in π* NO orbitals. ■ INTRODUCTION The “metal−radical” approach was pioneered by Gatteschi and co-workers 1 through the use of nitroxide radicals 2 whose weak Lewis base character means they are not expected to coordinate directly to the metal center unless the Lewis acidity of the metal itself is increased by the addition of electron-withdrawing groups such as hexafluoroacetylectonate (hfac − ), examples of which include the first single-chain magnet, [Co II (hfac) 2 (rad)], 3 and the ferrimagnetically ordered compound [Mn II (hfac) 2 (rad)]. 4 An alternative approach used to coordinate nitroxides to metal centers involves incorporation of a suitable coordinating group adjacent to the nitroxide radical such as pyrazine, 2,2′-bipyridine, or imidazole. 5 Such metal−radical systems have been extensively studied to gain information on their electronic structure, electron transfer properties, reactivity, and catalytic properties 6 as well as fundamental studies on the type and magnitude of the magnetic ex- change interactions to and via a range of transition-metal centers. 1,2 Multifunctional approaches to spin crossover (SCO) involve the study of the interplay between the spin-crossover properties of a material with a secondary function: for example liquid crystal properties, 7 porosity, 8 and ferromagnetic ordering. 9 Our interest in studying metal complexes using nitroxides stems from our interest in studying the effect of radical−M II (where M = Fe, Co) exchange on the potential of Fe II or Co II to undergo a thermally induced spin transition in a vein similar to the simultaneous spin crossover and exchange seen in a radical Fe III (SO 4 ) complex 10 and in the cobalt-based dimer [Co II 2 L- (NCS) 2 (SCN) 2 ], where L is a dinucleating pyridazine-based chelator. 11 We have recently reported simultaneous exchange interactions, spin crossover, reductively induced oxidation, and field-induced slow magnetic relaxation in [Co II (L • ) 2 ](NO 3 ) 2 (3), 12 where L • is 4,4-dimethyl-2,2-bis(2-pyridyl)oxazolidine N-oxide (Figure 1), and have extended this system to a different anion, tetrakis(pentafluorophenyl)borate. Here we report on the synthesis of the two solvated analogues [Co II (L • ) 2 ](B- (C 6 F 5 ) 4 ) 2 ·CH 2 Cl 2 (1) and [Co II (L • ) 2 ](B(C 6 F 5 ) 4 ) 2 ·2Et 2 O(2). Complex 1 was formed in a manner similar to that for [Co III (L − ) 2 ](BPh 4 ) 12 using potassium tetrakis(pentafluorophenyl)- borate instead of sodium tetraphenylborate. Complex 1 was Received: January 15, 2014 Published: May 7, 2014 Article pubs.acs.org/IC © 2014 American Chemical Society 5055 dx.doi.org/10.1021/ic5001057 | Inorg. Chem. 2014, 53, 5055−5066 Downloaded by INDIAN INST OF TECHNOLOGY BOMBAY on August 31, 2015 | http://pubs.acs.org Publication Date (Web): May 7, 2014 | doi: 10.1021/ic5001057