containing complex stabilized by an aryldiimino NCN-pincer ligand
(Inset of Figure 2), as exists in [2,6-(2,6-C6H3R2N5CH)2-C6H3]-
DyCl2(THF)2 (DyNCN). More striking still, this unique Dy-C bond
occupies the crystallographic C2 axis in DyNCN. The combination of
ab initio calculations and detailed magnetization dynamics studies
reveals the two lowest Kramers doublets being extremely axial in
nature and possessing parallel anisotropy axes along C2 direction,
which results in the unprecedented relaxation going via the second
excited state in compound DyNCN.
Results and discussion
X-Ray structure analysis. Compound DyNCN was synthesized via
transmetalation between 2,6-(2,6-C6H3R2N5CH)2-C6H3Li and
LnCl3(THF)1,3.5
25, and crystallizes in the space group P43212, with
Z54. The whole molecule exhibits C2 crystallographic symmetry, in
which the Dy–C bond is located on a 2-fold rotation axis [ ffi2ffiffi
p
2
ffi2ffiffi
p
2
0].
The monoanionic NCN-pincer ligand coordinates to the central
metal ion in a kC: kN: kN’ tridentate mode to form a meridional
conformation. The two N-aryl rings dispose the vertical positions
against the NCN plane, which are parallel to the two cis-located THF
rings, respectively. The chloride groups arrange in trans-positions to
form a large Cl–Dy–Cl angle (176.500(23)u) bisected by the NCN
plane. Therefore, if one focuses only on the atoms of the first
coordination sphere two nearly exactly perpendicular planes (C1/
N1/O1/O1A/N1A) and (C1/Cl1/Cl1A) are formed. Then the
effective CF is of C2v symmetry.
Electronic structure calculations. The interaction of the 4f orbitals
involved in the formation of the robust organometallic surroundings
seems to play a key role in the LF thereby enhancing the magnetic
properties. A few carbon ring coordinated Ln-based SMMs, that
include ligands such as cyclopentadienide and cyclooctatetraenide,
have generated profound thoughts and enlightenment22,26–30. The
discovery of these bona fide organometallic SMMs underscores the
significance of making use of the simple Ln-C bond to design
molecules with strong axiality31. The title compound with a strong
single Dy–C bond combining the huge conjugation effect of a
carbanion offers a new paradigm for single ion anisotropy as
shown from the ab initio calculated electronic structure of lowest
eight Kramers doublets of DyNCN in Table 1 (see Experimental
Section and the ESI of Figure S1–S7 and Table S1–S8 for details).
Table 1 shows the low lying spin-orbit energy spectrum of the
DyNCN molecule in the largest computational model. We notice a
large energy separation of the ground and first exctied Kramers
doublets. As shown in Figure 3, the main anisotropy axis of the
complex passes along the main symmetry axis of the complex, i.e.
it is parallel to the Dy-C chemical bond. The reason for the strong
effect of the ligating C atom is the fact that it holds a negative charge
as the ligand is a carbanion. The calculated Mulliken charge on the
carbon atom revealed a negative charge of 21.17 on the ligating
carbon atom. The localized character of the negative charge is probably
due to intramolecular electrostatic effects: the negative charge of
the carbanion is attracted by positively charged DyIII ion, and the
ligating C atom is the closest point to the DyIII ion. This is also the
reason why the Dy-C bond is unusually short (<2.39 A˚ ). Even
though the computed Mulliken charges on both chlorine atoms are
slightly larger (21.60), the longer Dy-Cl distances (<2.60A˚ ) make
the cumulative (total) effect of both chlorine atoms less important for
the magnetic anisotropy of the two low lying Kramers doublets than
the influence of the much closer carbon atom. In fact, we have already
seen for other complexes that the main anisotropy axis is mainly
oriented by the effect of the closest ligand21.
As a result of the above described effects, we notice an interesting
fact of collinearity of the main anisotropy axes in the ground (almost
pure615/2) and first excited (almost pure 613/2) doublets. We will
see later that, the collinearity of anisotropy axes is a key factor for
enhancing the magnetic blocking performance of this compound.
Analysing further the magnetic anisotropy of the second excited
KD (262 cm21) we notice that it is again very axial, i.e. of615/2 type,
but its main magnetic axis is perpendicular to the main symmetry
axis of the complex, passing through the two chlorine atoms. The
third, fourth and fifth excited KDs are less axial, while the sixth
and seventh excited KDs are again highly axial, with their main
การแปล กรุณารอสักครู่..