Angewandte Chemie DOI: 10.1002/anie.201202337 Quintuple Bonding Stepwise Construction of the CrÀCr Quintuple Bond and Its Destruction upon Axial Coordination** Yu-Lun Huang, Duan-Yen Lu, Hsien-Cheng Yu, Jen-Shiang K. Yu, Chia-Wei Hsu, Ting-Shen Kuo, Gene-Hsiang Lee, Yu Wang, and Yi-Chou Tsai* Although quadruple bonding in transition-metal chemistry Wurtz reductive coupling reaction of the corresponding has been considered a thoroughly studied area,[1a] the concept chloride coordinated precursors.[2,3] The previously reported of multiple bonding[1b] was reinvigorated in 2005 by the quintuple-bonded dichromium examples were obtained by seminal discovery of the first CrÀCr quintuple bond in the alkali metal reduction of the mononuclear [LCrCl2- [3c,e] [3] isolable dimeric chromium compound Ar’CrCrAr’ (Ar’ = 2,6- (THF)2] or dimeric complexes [LCr(m-Cl)]2 (L = mono- [2] (2,6-iPr2C6H3-)2C6H3) by Power and co-workers. Since then, dentate or bidentate ligand). It should be noted that all these the structures of several Group 6 homobimetallic compounds precursors lack CrÀCr bonding. Besides, we have recently with very short CrÀCr (1.73–1.75 ) and MoÀMo (2.02 ) demonstrated that the metal–metal quintuple and quadruple quintuple bonds have been characterized.[3] All these remark- bond can be constructed from the corresponding quadruple able quintuple-bonded bimetal units are supported by either and triple bond, respectively. For example, the d bonds in the 2 C- or N-based bridging ligands. Based on their structures, quintuple-bonded species [Mo2{m-h -RC(N-2,6-iPr2C6H3)2}2] [3h] 2 these quintuple-bonded dinuclear compounds can be simply (R = H, Ph) and quadruple-bonded complex [Mo2{m-h - [6] classified into two types as illustrated in Figure 1. The Me2Si(N-2,6-iPr2C6H3)2}2] are formed by alkali metal reduc- existence of the type I quintuple tion of the corresponding chloride-coordinated quadruple- bond was recently corroborated by and triple-bonded species, respectively. However, the forma- experiments,[4a] and the bonding tion mechanism of the metalÀmetal quintuple bonds has not paradigms of both types were real- been investigated. To this end, continuing our exploration in ized by theoretical investigations.[4] the field of quintuple-bond chemistry, we herein report the Preliminary reactivity studies on the construction of a complex with a CrÀCr quintuple bond by type I complexes show that they are two subsequent one-electron-reduction steps from a halide- Figure 1. Two types of reactive towards the activation of free homo-divalent dichromium complex to a mixed-valent quintuple-bonded com- small molecules and display inter- intermediate (CrI,CrII), and then to the final quintuple- plexes. esting complexation with olefins and bonded product. Structural characterization of these dichro- alkynes.[5] mium compounds is important to shed light on the formation Up to now, both type I and II compounds have been mechanism of the metal–metal quintuple bonds. Moreover, exclusively synthesized by a procedure analogous to the the metalÀmetal quadruple bonds can be dramatically elongated by intramolecular axial coordination, but such an [*] Y.-L. Huang, D.-Y. Lu, H.-C. Yu, C.-W. Hsu, Prof. Dr. Y.-C. Tsai interaction in the quintuple-bonding system has not been Department of Chemistry and Frontier Research Center on investigated. We report herein that the CrÀCr quintuple bond Fundamental and Applied Sciences of Matters can be readily cleaved by disproportionation induced by National Tsing Hua University, Hsinchu 30013 (Taiwan) intramolecular axial coordination. E-mail: [email protected] As illustrated in Scheme 1, treatment of CrCl2 in THF Prof. Dr. J.-S. K. Yu with 1 equiv of dilithiated 2,6-diamidopyridine Li [2,6-(2,6- Institute of Bioinformatics and Systems Biology and 2 i i Department of Biological Science and Technology Pr2C6H3-N)2-4-CH3C5H2N] (1) and Li2[2,6-( Pr3SiN)2-C5H3N] National Chiao Tung University, Hsinchu, 30010 (Taiwan) (2), prepared by adding 2 equiv of nBuLi to the correspond- T.-S. Kuo ing 2,6-diaminopyridine in n-hexane, yields two dark green 1 2 Department of Chemistry dimeric complexes [{(THF)Cr(m-k :k -2,6-(2,6-iPr2C6H3-N)2- 1 2 National Taiwan Normal University, Taipei 11677 (Taiwan) 4-CH3C5H2N)}2](3) and [{(THF)Cr(m-k :k -2,6-(iPr3Si- Dr. G.-H. Lee, Prof. Dr. Y. Wang N)2C5H3N)}2](4), respectively, in good yields (62% for 3 Department of Chemistry and 74% for 4). The 1H NMR spectra of 3 and 4 display broad National Taiwan University, Taipei 10617 (Taiwan) signals in the range of 20 and À10 ppm, so little useful [**] We are grateful to the National Science Council, Taiwan for financial information could be obtained. support under grants NSC 99-2113M-007-012-MY3 (Y.C.T.) and 100- The dinuclear nature of 3 and 4 was confirmed by single- 2627-B-009-001 (J.S.K.Y.), and the “Center for Bioinformatics crystal X-ray crystallography[7] and their molecular structures Research of Aiming for the Top University Program” of NCTU and MoE, Taiwan. are depicted in Figure 2 and Figure S2 in the Supporting Information). It is interesting to note that although these two Supporting information for this article (including experimental details for the synthesis and characterization of complexes 3–8)is dinuclear species bear the same number of ligands, they available on the WWW under http://dx.doi.org/10.1002/anie. exhibit very different structural conformations. In compound 201202337. 3, each Cr atom is five-coordinate, ligated by four nitrogen Angew. Chem. Int. Ed. 2012, 51, 7781 –7785 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 7781 Angewandte. Communications 1 2 [(Et2O)K[18]crown-6][Cr{m-k :k -2,6-(2,6-iPr2C6H3-N)2-4- CH3C5H2N}2Cr] (5) (43% yield) and brown [(THF)2K- 1 2 [18]crown-6][Cr{m-k :k -2,6-(iPr3Si-N)2C5H3N}2](6) (35% yield), respectively. Like 3 and 4, the 1H NMR spectra of 5 and 6 are also not diagnostic because of the extreme line broadening caused by their nondiamagnetic properties. Fortunately, the formulation of 5 and 6 was corroborated by X-ray crystallography,[7] and their molecular structures are depicted in Figure 3 ([(Et2O)K18-crown-6][5]) and Fig- Scheme 1. Synthesis of complexes 3–7. Figure 3. Molecular structure of the Cr-containing anion of 5 with thermal ellipsoids at 35% probability. The counter cation + [(Et2O)K[18]crown-6] , diethyl ether solvate, and hydrogen atoms have been omitted for clarity. Figure 2. Molecular structure of 3 with thermal ellipsoids at 35% probability. Hydrogen atoms have been omitted for clarity. ure S4 ([(THF)2K18-crown-6][6]). Unlike 3 and 4 and their striking structural difference, compounds 5 and 6 display strongly similar structures. In contrast to 3, the two 2,6- diamidopyridyl units in 5 are no longer parallel; the dihedral donors and a molecule of THF, but the Cr atoms in 4 are four- angle between these two supporting ligands is 33.4(2)8.In6, coordinate, surrounded by three nitrogen atoms and a THF the torsion angle between two pyridyl units is 52.7(3)8. Both molecule. In 3, the two 2,6-diamidopyridyl ligands are parallel compounds consistently show two chromium atoms in differ- to each other and two THF ligands are arranged in an anti ent coordination environments. One chromium atom is conformation. The two much more sterically encumbered ligated by two N donors (amido) and thus shows a linear silyl-substituted 2,6-diamidopyridyl ligands in 4, however, are geometry with the bond angle N1-Cr1-N6 of 178.00(17)8 in 5 arranged in a nearly orthogonal orientation with a torsion and N3-Cr2-N3A of 177.8(2)8 in 6, while the other chromium angle of 77.5(1)8 with two THF ligands in a syn conformation. atom is embraced by the other four nitrogen atoms and thus Each Cr atom in 3 adopts a square-pyramidal geometry, but displays a roughly planar conformation. In comparison with the geometry at each Cr atom in 4 is approximately planar. As two structurally characterized monomeric two-coordinate a result, compound 3 has a shorter CrÀCr distance of monovalent chromium complexes [(2,6-(2,4,6-iPr3C6H2)2-3,5- 2.8513(25) , while the separation between two chromium iPr2C6H1)Cr(L)] (L = THF, PMe3), in which the central Cr centers is 3.1151(7) in 4. Despite the significant structural atom has five unpaired electrons described by Power and co- differences, 3 and 4 display similar magnetic properties. The workers,[8] a + 1 oxidation state is accordingly assigned to the room-temperature solution magnetic moments of 3 and 4 are two-coordinate chromium centers, while the four-coordinate 4.22 and 3.89 mB, respectively, which are both less than the chromium centers are divalent. The distances between two Cr 2+ spin-only value expected for two uncoupled Cr ions (meff = atoms are 3.1036(11) (5) and 2.9277(15) (6), indicative of 6.93 for S1 = S2 = 2), indicative of antiferromagnetic coupling. no CrÀCr bond. Besides the structural resemblance, 5 and 6 Subsequent one-electron reduction of dark green 3 and 4 also possess similar magnetic properties. They both exhibit an in THF by 1 equiv of KC8 in the presence of 1 equiv of antiferromagnetic exchange between two Cr spin centers. The [18]crown-6 and recrystallization from diethyl ether and THF room-temperature solution magnetic moments of 5 and 6 are gives the reddish brown mixed-valent dinuclear complex 5.00 and 3.53 mB, respectively, which are less than the spin-only 7782 www.angewandte.org 2012 Wiley-VCH Verlag GmbH & Co.