Large Hexadecametallic {Mn–Ln} Wheels: Synthesis, Structural, Magnetic, and Theoretical Characterization

The synthesis, gas sorption studies, magnetic properties, and theoretical studies of new molecular wheels of core type {Mn8Ln III 8} (Ln=Dy, Ho, Er, Y and Yb), using the ligand mdeaH2, in the presence of ortho-toluic or benzoic acid are reported. From the seven wheels studied the {Mn8Dy8} and {Mn8Y8} analogues exhibit SMM behavior as determined from ac susceptibility experiments in a zero static magnetic field. From DFT calculations a S=16 ground state was determined for the {Mn8Y8} complex due to weak ferromagnetic Mn–Mn interactions. Ab initio CASSCF+RASSI-SO calculations on the {Mn8Dy8} wheel estimated the Mn–Dy exchange interaction as 0.1 cm . This weak exchange along with unfavorable single-ion anisotropy of Dy/Mn ions, however, led to the observation of SMM behavior with fast magnetic relaxation. The orientation of the g-anisotropy of the Dy ions is found to be perpendicular to the plane of the wheel and this suggests the possibility of toroidal magnetic moments in the cluster. The {Mn8Ln8} clusters reported here are the largest heterometallic MnLn wheels and the largest {3d– 4f} wheels to exhibit SMM behavior reported to date. Investigations of molecular transition metal and lanthanide coordination complexes have attracted great attention over the past two decades because of their beautiful molecular architectures and interesting magnetic properties. Current hot topics in molecular magnetism include single-molecule magnets (SMMs), the magnetocaloric effect, and molecular spin structures used in electronic devices (spintronics). Research into wheel/ring-shaped molecular nanomagnetic materials has gained momentum in recent years because of their potential application as functional materials in nanotechnology, with a particular focus on SMMs, models of one-dimensional chains, and in the study of quantum-size effects. Furthermore, pioneering research by Winpenny and co-workers on polynuclear chromium wheels, incorporating a single heterometallic 3d ion aimed towards providing S=1/2 qubits, has opened up the possibility of quantum computation from such molecular systems. To date, few heterometallic Mn–Ln wheels have been documented, with only one structural example, a family of {Mn4Ln III 4} complexes. [6] Examples of other 3d–4f wheel motifs include {Fe4Dy4}, {Fe4Ln2}, {Fe5Yb3}, {Fe3Yb2}, {Fe16Ln4}, and {Fe10Ln10} complexes, all reported by Powell and co-workers. [7] It is noted that the common denominator in the synthesis of the above 3d–4f wheel-type structures is in the use of amine polyalcohol ligands such as triethanolamine and N-n-butyldiethanolamine. In recent years some of us have researched, extensively, the coordinating ability of various amine-based polyalcohol pro-ligands, successfully isolating many 3d, 3d–4f, and 4f coordination complexes, including a fascinating sixmembered Dy-metallo wheel, which displayed a toroidal magnetic moment. We have therefore used the above precedence and the versatile nature of the pro-ligand N-methyldiethanolamine (mdeaH2) towards the formation of novel heterometallic 3d-4f ring/wheel-like molecular complexes. We report herein the synthesis, gas sorption studies, magnetic properties, and theoretical studies of a new molecular wheel of core type {Mn8Ln III 8} (Ln=Dy, Ho, Er, Y and Yb), using the ligand mdeaH2, in the presence of ortho-toluic (o-tolH) or benzoic acid (benzH). To the best of our knowledge, the compounds represent the largest Mn–Ln heterometallic wheels thus far reported. The reactions of Mn(NO3)2·6H2O and Ln(NO3)3·6H2O (Ln = Dy, Ho, Y, and Er) or YbCl3·6H2O, with mdeaH2 and ortho-toluic acid (o-tol) or benzoic acid (benz) in acetonitrile, at ambient temperature, followed by the removal of the solvent and redissolution in a MeOH/iPrOH or a MeOH/EtOH mixture yielded single crystals of a family of heterometallic-ring complexes of general formulae [Mn8Ln III 8(mdea)16(o-tol)8(NO3)8] (Ln=Dy (1) ; Ho (2) ; Y (3) and Yb (4)) or [Mn8Ln III 8(mdea)16(benz)8(NO3)8] (Ln=Ho (5) ; Y (6) and Er (7)). For full experimental details, see the Supporting Information. It was found that the nature of the crystallized product was sensitive to the reaction conditions and crystallization solvent employed. If the reaction was [a] K. R. Vignesh IITB-Monash Research Academy, IIT Bombay Mumbai, 400076 (India) [b] Dr. S. K. Langley, Dr. B. Moubaraki, Prof. K. S. Murray School of Chemistry, Monash University, Victoria, 3800 (Australia) E-mail : [email protected] [c] Prof. G. Rajaraman Department of Chemistry Indian Institute of Technology Bombay Powai, Mumbai, Maharashtra, 400 076 (India) Fax: (+91)22-2576-7152 E-mail : [email protected] Supporting information, including experimental, crystallographic, and computational details (anisotropy, J and D values), SQUID (ac data and other plots) and TGA instrumentation, TGA images, and powder XRD images, for this article is available on the WWW under http://dx.doi.org/10.1002/ chem.201503424. Chem. Eur. J. 2015, 21, 1 – 7 2015 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim 1 && These are not the final page numbers! Communication DOI: 10.1002/chem.201503424