Chiral [NaMnIIMnIII3] and [Na2MnII2MnIII6] clusters constructed by chiral multidentate Schiff-base ligands: synthesis, structures, CD spectra and magnetic properties

Abstract

Two pairs of novel enantiomerically chiral clusters R/S-[NaMn(II)Mn(III)3L3(μ3-O)] (R/S-1) and R/S-[Na2Mn(II)2Mn(III)6L6(μ3-O)2] (R/S-2) have been obtained via the self-assembly of R/S-H2L Schiff base ligands and different auxiliary ligands (N3(-), dca(-)) with divalent manganese salt in an air-exposed methanol-ethanol solution. The structures of R/S-1 and R/S-2 were characterized by single-crystal X-ray diffraction analysis and powder X-ray diffraction. When the dicyanamide anion serves as an auxiliary ligand in the assembling reaction system, a pair of enantiomeric clusters R/S-[NaMn(II)Mn(III)3L3(μ3-O)] (R-1 and S-1) with a trigonal bipyramid configuration were obtained, while another pair of enantiomeric clusters R/S-[Na2Mn(II)2Mn(III)6L6(μ3-O)2] (R-2 and S-2) were formed in the case of the azide. Interestingly, the skeleton configuration of R/S-2 can be described as a 3-fold EO-azide bridging double trigonal bipyramid of [NaMn(II)Mn(III)3L3(μ3-O)]2via Mn(II) vertices. Circular dichroism (CD) spectra demonstrated the enantiomeric nature of the two pairs of clusters. Detailed direct current (DC) magnetic susceptibility studies in the temperature range 2-300 K suggested that R-1 and R-2 showed predominantly antiferromagnetic interactions between the manganese centers.

Chiral [MnIIMnIII3M′] (M′=NaI, CaII, MnII) and [MnIIMnIII6NaI2] Clusters Built from an Enantiomerically Pure Schiff Base: Synthetic, Chiroptical, and Magnetic Properties

“…The calculated energy of the spin levels shows a well‐isolated S =7/2 ground state with a gap of 27 cm −1 to the first S= 9/2 excited state. This ground state, which comes from the antiferromagnetic interaction of the Mn II ( S= 5/2) with the local S =6 from the Mn III 3 triangle, is confirmed by the magnetization measurement that tends to an unsaturated value of 5.7 Nμ B under the maximum applied field of 5 T, in agreement with data reported for related [Mn II Mn III 3 Na] systems with S T =7/2 . This value is close to that expected for seven electrons, assuming the calculated g value from the susceptibility fit (6.5 Nμ B ).…”

“…On the contrary, the H 2 L ligands derived from 2‐amino‐1‐ethanol with only three O donors keep the octahedral cavity around the Na + cation, whereas they generate an open face on the Mn II cation that usually fulfills its coordination sphere with anions or solvent molecules (Scheme ‐ II ). In addition, this coordination gives the possibility to share the central cation or the open faces by means of bridging ligands that can join the pentanuclear units, resulting in larger nuclearities . In these systems, the three ligands are parallel, and only one example with the divalent cation Ca 2+ instead of Na + has been reported as the OEC model with one of the ligands inverted with respect to the other two (Scheme ‐ III ).…”

“…Condensation of o-vanillin and aminoalcohols yields multidentate Schiff bases for which ac haracteristicf amilyo f [Mn II Mn III 3 M n + ]c lusters with M n + = Na I and at rigonal bipyramidal arrangement of the cations has been reported, [3][4][5][6][7][8][9][10] together with af ew examples in which M n + = Ca 2 + or Ln 3 + . [3,11] All of these clustersa re characterized by the triangular arrangement of the trivalent manganese cations and the other two cations in apical positions resulting in at rigonal bipyramidal cage,i n whicht he cations are held by three dianionicl igands.…”

“…Schiff bases derivedfrom the condensation of 3-amino-1,2propanediol with o-vanillin gives H 3 Ll igandst hat satisfy all the coordination sites of the apical cations,w hich are hostedi n two predetermined octahedral cavities [6][7][8][9] (Scheme 1-I). On the contrary,t he H 2 Ll igandsd erived from 2-amino-1-ethanol with only three Od onorsk eep the octahedral cavity around the Na + cation, whereas they generate an open face on the Mn II cation [3][4][5]10] that usually fulfills its coordination sphere with anionso rs olvent molecules (Scheme1-II). In addition, this coordination gives the possibility to share the centralc ation or the open faces by means of bridgingligandst hat can join the pentanuclear units, resulting in larger nuclearities.…”

“…In addition, this coordination gives the possibility to share the centralc ation or the open faces by means of bridgingligandst hat can join the pentanuclear units, resulting in larger nuclearities. [10,12,13] In these systems, the three ligandsa re parallel,a nd only one examplew ith the divalent cation Ca 2 + instead of Na + has been reported as the OEC model [3] with one of the ligands inverted with respect to the other two (Scheme 1-III). Ac ommon point in these structures is the presence of three additional bridging ligands on the elongated coordination sites of the trivalent manganese cations.…”