Structural diversity in manganese, iron and cobalt complexes of the ditopic 1,2-bis(2,2′-bipyridyl-6-yl)ethyne ligand and observation of epoxidation and catalase activity of manganese compounds

Abstract

A ditopic 1,2-bis(2,2'-bipyridyl-6-yl)ethyne ligand, L, has been synthesized for the first time by consecutive Suzuki and Sonogashira coupling reactions either in a one- or two-step synthesis. Coordination of L with some first-row transition metals, Fe, Mn and Co showed a very rich structural diversity that can be obtained with this ligand. Reaction of L with Mn(II)(OAc)(2) yielded a dimanganese(II) complex, [Mn(2)L(mu-OAc)(3)]PF(6), (1) where the two somewhat inequivalent trigonal-bipyramidal Mn atoms separated by 3.381 A are bridged by L and three acetate moieties. A similar reaction of L with Mn(III)(OAc)(3) yielded a very different dimanganese complex [Mn(2)L'(OH)(OAc)(2)(DMF)(2)]PF(6) x DMF (2) where L' is a E-1,2-bis(2,2'-bipyridyl-6-yl)ethene fragment that was formed in situ. The L' ligand bridges between the two Mn centers, despite its trans configuration, which leads to a very strained ethene bridging moiety. The Mn atoms are also bridged by two acetate ligands and a hydroxy group that bridges between the Mn atoms and the ethene fragment; DMF completes the octahedral coordination around each Mn atom which are separated by 3.351 A. A comproportionation reaction of L with Mn(II)(OAc)(2) and n-Bu(4)NMnO(4) yielded a tetramanganese compound, [Mn(4)(mu(3)-O)(2)(OAc)(4)(H(2)O)(2)L(2)](PF(6))(2) x 2 CH(3)CN (3). Compound 3 has a dimer of dimers structure of the tetranuclear Mn core that consists of binuclear [Mn(2)O(OAc)(2)L](+) fragment and a PF(6) anion. BVS calculations indicate that 3 is a mixed-valent 2Mn(II) plus 2Mn(III) compound where two [Mn(II)(2)O(OAc)(2)L](+) fragments are held together by Mn(III)-O inter-fragment linkers which have a distorted octahedral geometry. The Mn atoms in the [Mn(2)O(OAc)(2)L](+) fragments have a capped square-pyramid configuration where an aqua ligand is capped on one of the faces. Although the aqua ligand is well within a bonding distance to a carbon atom of the proximal ethyne bridge, there does not appear to be an oxygen-carbon bond formation, rather the ligand is constrained in this position, as deduced by the observation that the bond lengths and angles of the ligand are essentially the same as those for the free ligand, L. Reaction of L with perchlorate or triflate salts of Fe(II), Mn(II) and Co(II) in dry acetonitrile yielded binuclear triple helicate structures (2:3 metal to L ratios) [Fe(2)L(3)](CF(3)SO(3))(4) x CH(3)CN (4), [Mn(2)L(3)](ClO(4))(4) x 1.7 CH(3)CN x 1.65 EtOEt (5) and [Co(2)L(3)](ClO(4))(4) x 2 CH(3)CN x 2 EtOEt (6) where each M(II) center with a slightly distorted octahedral geometry is bridged by three of the ditopic ligands. The M-M distances varied; 5.961 A (Mn), 6.233 A (Co) 6.331 A (Fe). Reaction of L with Co(ClO(4))(2) x 6 H(2)O in wet acetonitrile yielded a dicobalto(III) compound, [Co(2)L'(3)(O)(2)](ClO(4))(2) x H(2)O (7), with two types of L' fragments; one bridging between the two Co centers and two non-bridging ligands, each bonded to a Co atom via one bipyridyl group where the other is non-bonding. The octahedral coordination sphere arou...

Hydrogen‐Atom Transfer Oxidation with H2O2 Catalyzed by [FeII(1,2‐bis(2,2′‐bipyridyl‐6‐yl)ethane(H2O)2]2+: Likely Involvement of a (μ‐Hydroxo)(μ‐1,2‐peroxo)diiron(III) Intermediate

“…[7] Metalation of L with Fe II (CF 3 SO 3 ) 2 ·4 MeCN yielded an orange crystalline compound [Fe II ( L )](H 2 O) 2 ](CF 3 SO 3 ) 2 ( 1 ), Figure 1. The crystal structure obtained is similar to an analogous [Fe II ( L )](SCN) 2 compound recently published.…”

“…The synthesis of the catalyst precursor, Scheme , involved the preparation of 1,2‐bis(2,2′‐bipyridyl‐6‐yl)ethyne by reacting 6‐bromo‐2,2‐bipyridine with sodium acetylide through consecutive Suzuki and Sonogashira coupling reactions followed by hydrogenation over Pd/C to yield the tetradentate ligand, 1,2‐bis(2,2′‐bipyridyl‐6‐yl)ethane ( L ) . Metalation of L with Fe II (CF 3 SO 3 ) 2 ⋅ 4 MeCN yielded an orange crystalline compound [Fe II ( L )](H 2 O) 2 ](CF 3 SO 3 ) 2 ( 1 ), Figure .…”

Epoxidation of olefins with homogeneous catalysts – quo vadis?

“…33,34 Peracetic acid is used in the reaction with the dimeric Mn-catalyst 12, which quantitatively converts cyclooctene to the epoxide within 3 min (at 3 mol% catalyst loading, entry 12). 35…”

“…2). 32,35 Bhattacharyya et al reported the successful application of an ionic diperoxo tungsten complex, PPh 4 [WO(O 2 ) 2 (1-(2 0hydroxy-phenyl)ethanonoxime)] ( 13), in the epoxidation of 1-octene with H 2 O 2 . 36 A very low catalyst concentration (0.05 mol%) yields 95% 1-octene oxide within 2.25 h in acetonitrile at 40 1C.…”

Water is a key factor to alter the structure and electrochemical properties of carboxylate-bridged dimanganese(ii) complexes

“…The preparation of carboxylate-rich Mn complexes introduces synthetic complications, owing both to the kinetic lability of Mn(II) ions and the multiple binding modes of carboxylate ligands. 16,17 Significant problems include the generation of mono-, tri-, or polynuclear complexes, commonly encountered products in Mn-carboxylate chemistry. 18,19 Successful preparations of dinuclear Mn complexes with only carboxylate ligands and water molecules, rather than elaborate ligand moieties, have not yet been reported, indicating that the synthesis of dinuclear manganese complexes with O-rich coordination environments is a difficult task to achieve.…”