Job/Unit: I30576 /KAP1 Date: 30-07-13 17:12:20 Pages: 10 FULL PAPER DOI:10.1002/ejic.201300576 Dinuclear Cu II –Cu II and Cu I –Cu II Complexes of a Compartmental Ligand – Syntheses, Structures, Magnetic, and Catalytic Studies Surajit Biswas, [a] Arpan Dutta, [a] Malay Dolai, [a] Indrani Bhowmick, [b,c] Mathieu Rouzières, [b,c] Hon Man Lee, [d] Rodolphe Clérac, [b,c] and Mahammad Ali* [a] Keywords: Schiff bases / Copper / Structure elucidation / Magnetic properties / Epoxidation Dinuclear Cu II –Cu II (1) and Cu I –Cu II (2) complexes were de- rived from a new N 4 O 2 donor compartmental ligand (H 2 L) by changing the nature of the Cu precursors used. Single-crystal X-ray diffraction studies revealed that the Cu1 site in 1 has octahedral geometry and is in the inner compartment of the ligand axially coordinated by two ClO 4 – anions, whereas the outer Cu2 ion has square-pyramidal geometry. In 2, there are two copper dinuclear complexes (A and B) in the asymmetric unit; the inner core is occupied by Cu1/Cu3 in the +2 oxi- dation state in a square-planar geometry. The Cu2/Cu4 ion occupies the outer sites and has distorted tetrahedral geome- try with a +1 oxidation state. Complexes 1 and 2 were ob- tained simply by changing Cu(ClO 4 ) 2 in 1 to Cu(ClO 4 ) 2 and Cu(bipy)(NO 3 ) 2 in 2; the bipyridyl (bipy) ligand induces the reduction of Cu II to Cu I , which is trapped in the Cu I –Cu II di- nuclear product. The oxidation states of the metal ions were ascertained from charge-balance considerations as well as Introduction The synthesis and investigation of dinuclear complexes have attracted much attention because of their widespread application in the design of molecular materials (e.g., mole- cule-based magnets) [1] and as models for the active sites of many metalloproteins. [2] Many metalloenzymes such as hemocyanin (2Cu), [3] hemerythrin (2Fe), [4] tyrosinase (2Cu), [5] and methane monooxygenase (2Fe) [6] possess active sites with homodimetallic cores and involve either activation or transport of dioxygen in biological systems. Compounds containing metal atoms in two valence states [a] Department of Chemistry, Jadavpur University, Kolkata 700 032, India E-mail: m_ali2062@yahoo.com Homepage:http://www.jaduniv.edu.in/profile.php?uid=30 [b] CNRS, CRPP, UPR 8641, 33600 Pessac, France [c] Univ. Bordeaux, CRPP, UPR 8641, 33600 Pessac, France [d] National Changhua University of Education, Department of Chemistry, Changhua 50058, Taiwan Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/ejic.201300576. Eur. J. Inorg. Chem. 0000, 0–0 © 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1 from bond valence sum (BVS) calculations. No signature of intervalence charge transfer (IVCT) was observed by spec- troscopy (UV/Vis/NIR) as well as from the low-temperature magnetic studies. This might be because of the presence of two copper centers in two different geometries with a wide separation between them (ca. 3.27 Å). For 1, the best fit of the χT vs. T data to the dimer model gives J/k B = –262(1) K and g av = 2.05(5), which indicates that there is a strong anti- ferromagnetic coupling between the two Cu atoms. In 2, the Cu I center is diamagnetic and, thus, the remaining S = 1/2 Cu II magnetic center follows a Curie law with g = 2.06(5). Under homogeneous conditions, both complexes showed catalytic epoxidation of cyclooctene, styrene, and norbor- nene to the corresponding epoxides with high selectivities and turnover numbers (TONs), which seem to be slightly bet- ter than the reported values for Cu II Schiff base complexes. are of special interest because of the possible electron trans- fer between these states; [7] this is the basic activity mecha- nism of many important chemical and biochemical catalytic reactions. [8] The most commonly studied mixed-valence transition-metal system incorporates Cu I /Cu II pairs because copper can adopt different coordination numbers and stereochemistry in its different oxidation states, which are both highly labile and stereochemically flexible [9,10] unlike other couples such as iron, cobalt, and ruthenium. Al- though a pronounced geometrical change is usually ob- served during the Cu I /Cu II interconversion, allosteric effects can also be observed. [11] Moreover, the control of the geo- metric environment around the copper centers allows the redox potential of the Cu I /Cu II couple to be tuned; this is the basis of the redox properties of type 1 copper pro- teins [12] and many synthetic analogues. [13,14] Nevertheless, it is very difficult to control the final product with two dif- ferent oxidation states, which is reflected by the very limited number of reports on mixed-valence copper coordination compounds. [15–17] On the other hand, compartmental ligands of the “end- off” type with a phenolic or alcoholic oxygen atom as an