Inorganic-Organic Molecular Based Magnets
- PDF / 108,950 Bytes
- 7 Pages / 595 x 842 pts (A4) Page_size
- 77 Downloads / 269 Views
- O +N
- O +N
N
.N
N H
O
.N
O
N
R'
N H
R
(a)
(b) Figure 1: Scheme of the nitronyl nitroxide radicals (a) NIT-ImH, (b) NIT-Bz(R,R’)ImH (R=R’=H, CH3, Cl or R=H and R’=H, CH3, NO2)
Accordingly, 2-imidazolyl- and 2-benzimidazolyl substituted nitronyl nitroxide radicals were synthesized in which t w o symmetrical chelating sites (Figure 1) allow the synthesis o f extended compounds. DISCUSSION Manganese(II) mononuclear compounds:
O
N N
N Mn N
O
N O
N
O N
N N
N O
N
O
N
Figure 2 View of the [MnII(NIT-Bz(H,H)ImH) 3]2+ cation. Prior to deprotonation of the imidazole ring, these ligands (Figure 1) are mono-chelating. Reacting them with MnII(ClO4)2.6H2O leads to complexes of formula [MnII(L) 3](ClO 4) 2 (L = NIT-ImH 1 or NIT-
Bz(H,H)ImH 2 ) in which the coordination sphere is fully occupied b y only nitroxide ligands [4] (Figure 2). 3.0
-1 χ.T(cm.K.mol )
2.5 2.0
3
1.5 1.0 exp
0.50
fit
0.0 0
50
100
150
200
250
300
T(K)
Figure 3 Plot of χ T vs. T for [Mn (NIT-Bz(H,H)ImH) 3](ClO 4) 2. At room temperature (Figure 3) the product of the molar magnetic susceptibility with the temperature χ T (2.91cm3 K mol -1 ( 1 ) and 2.5 cm3 K mol-1 (2)) is well below the expected ones for isolated spins (5.5cm3 K mol -1). When cooling χT rapidly decreases to reach a plateau whose χT value corresponds to a S=1 ground spin state ( 1 cm 3 K mol -1). This magnetic behavior is consistent with a four-spin system where the Mn(II) ion (S=5/2) strongly interacts antiferromagnetically with the three bound free radicals (S=1/2) ( J = -53(2)cm -1 (1 ) and -79(5)cm -1 ( 2 ), H =-2 Σ J ijS iS j). Intermolecular interactions are weaker than 2 c m -1 . These mononuclear species are molecular bricks, and as we will see below, the imidazole rings o f the free radicals can be deprotonated to afford extended compounds. II
Manganese(II)
layered
compounds:
Two-dimensional (2D) compounds of general formula {[Mn II2 (L) 3 ] X} n (L=NIT-ImH or NIT-Bz(R,R’)ImH) are synthesized in methanol as powder when reacting the radical ligands (L) with MnII(OAc)2.4H2O followed by addition of any counter anion (X).
In the case of {[Mn II2 (NIT-Im) 3 ] ClO4}n 3 tiny single crystals were obtained and the structure was determined by X - r a y diffractometry. It brings evidence for a lamellar structure [5] made with of {[Mn II2 (NIT-Im) 3 ] -} n sheets separated by 10.32(1)Å, perchlorate anions intercalated in between (Figure 4). Mn
Mn
Mn Mn Mn
Mn
Mn
Mn Mn
a
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
Mn
Mn
Mn
Mn
Mn Mn Mn Mn
Mn
Mn
Mn
b
c
Mn
Mn
Figure 4: View of the layered structure {[MnII2 (NIT-Im) 3 ] ClO4 } n 3. Within the {[Mn II2 (NIT-Im) 3 ] -} n sheets each radical bridges t w o Mn(II) ions with opposite ∆ and Λ chirality. This arrangement results in a 2D distorted honeycomb structure in agreement with previous findings. It is indeed known that building blocks of formula [M m+ (L n-) 3 ]m 3n , where L is a bis-chelating ligand, may lead to 2D structures whith ∆ and Λ metal centers alternate, or to 3D species
Data Loading...
