Inorganic Chemistry
Article
Sm(III) complex [{Sm(N††)2}2(μ-η2:η2-Se2)] (2-Se2) was also
synthesized by treating [Sm(N††)2] with 1/8 of an equivalent of
Se8 in similar yield to 2-S2 (Scheme 2).
temperature independent paramagnetism from second-order
mixing with low-lying excited states.39,40 On cooling, χMT
S47, and S49). Magnetization traces recorded at 2 and 4 K are
distinct, though they fail to saturate as a function of the applied
field up to 7 T. As was the case for the monometallic
compounds, the magnetic moments for 2-E2 are higher than
would be expected for two isolated Sm(III) centers (Table 1).
The discrepancy between the values for 2-S2 and 2-Se2 could
be due to differences in the additive coupling of the two
magnetic moments facilitated by the dichalcogenide bridge, as
well as the significantly larger different spin−orbit contribution
from selenium compared with sulfur.
Reactions of [Sm(N††)2] with P(V) chalcogen atom transfer
reagents EPR3 (R = Ph, E = S, Se; R = Et, E = S, Se, Te)37
were uniformly sluggish, despite heating at 100 °C for several
prolonged heating of [Sm(N††)2] and TePEt3 gave single
crystals of the monochalcogenido-bridged [{Sm(N††)2}2(μ-
Te)] (3) after standing for 2 weeks (Scheme 3).
Scheme 3. Synthesis of [{Sm(N††)2}2(μ-Te)]
Structural Characterization. The crystal structures of 1-E
have been determined by single crystal X-ray crystallography at
150 K. The molecular structures are presented in Figure 2;
selected bond distances and angles are summarized in Table 2.
Complexes 1-E are all monomeric in the solid state, with
approximate trigonal planar geometry about the Sm(III) ion
(range Sm···N2E plane distance 0.065−0.082 Å). The Sm(III)
centers are coordinated by the corresponding chalcogenide
and two monodentate N†† ligands. The average Sm−N
distance shortens as the Sm−E distance increases, which is
governed by the size of the chalcogenide (S2−, 1.84 Å; Se2−,
1.98 Å; Te2−, 2.21 Å). The incremental lengthening of the
Sm−E bond on descent of group 16 is in excellent agreement
with the related monomeric complexes [Sm(Cp*)2(EPh)-
(thf)],21 although the analogous distances in 1-E are slightly
shorter on account of their coordination number. The N−
Sm−N angle is shifted significantly from 120° in all three
complexes, though the magnitude decreases on descending the
group (1-S, 138.33(7)°; 1-Se, 132.7(2)°; 1-Te, 128.6(1)°).
The Sm−E−C angles also decrease from S to Te because of
attenuated s-p hybridization in the frontier orbitals of the
heavier chalcogens pushing the angle closer to 90°.41 The
coordination spheres of 1-E are completed by several short C−
H···Sm contacts, with the shortest Sm−C distances usually
∼3.1 Å. These interactions are ubiquitous in f-element
complexes of these bulky bis(silyl)amide ligands and have
been discussed in detail previously.26,30,42−44
1
NMR Spectroscopy. The H NMR spectra of 1-E each
show two broad signals corresponding to the N†† ligand and
were assigned based on integration. The methine peak
positions shift downfield as the chalcogen electronegativity
decreases (δH = 1-S, −8.61; 1-Se, −8.47; 1-Te, −8.30 ppm),
whereas the methyl peak positions are invariant (δH = 1-S,
0.24; 1-Se, 0.22; 1-Te, 0.21 ppm). The proton resonances for
the −EPh group in 1-E were not noticeably broadened by the
paramagnetic Sm(III) center, with three peaks discernible for
1-S and 1-Te and the meta-protons in 1-Se masked by the
solvent residual. The solution phase dynamics in 1-S were
examined by monitoring the temperature dependence of the
SPh proton resonances (Figure S40). The 13C{1H} NMR
spectra show two sharp peaks for the N†† ligand with the
methine peak shifted upfield in the order of decreasing
chalcogen electronegativity (δC = 1-S, 15.66; 1-Se, 15.55; 1-
Te, 15.39 ppm). The methyl peak was again invariant (δC = 1-
S, 19.82; 1-Se, 19.77; 1-Te, 19.83 ppm). The nonquaternary
EPh resonances for 1-E were observed (δC = 1-S, 123.77,
128.68, 130.57; 1-Se, 123.77, 130.22, 134.97; 1-Te, 124.60,
129.91, 140.76 ppm), with the peaks assigned to the ortho-C
shifted upfield with decreasing chalcogen electronegativity.
This trend has been noted in the related Sm(III) complexes
[Sm(Cp*)2(EPh)(thf)].21 The 1H NMR spectra of 2-E2
contained multiple broad signals and could only be tentatively
assigned due to their integrals being affected by the presence of
diamagnetic HN††.
Complexes 2-S2 and 2-Se2 both crystallize in the P21/n
space group with one molecule in the asymmetric unit. Both
complexes feature two {Sm(N††)2} fragments linked by a μ-
η2:η2-E2 ligand (Figure 3), with E−E distances typical for
2−
S22− of 2.1075(10) Å and Se22− of 2.3662(7) Å consistent with
their dianionic formulation.18,19 For example, in [{Y-
(N″)2(thf)}2(μ-η2:η2-E2)] (N″ = {N(SiMe3)2}; E = S, Se),
the S−S distance is 2.118(2) Å and the Se−Se distance is
2.399(5) Å.33 The monodentate N†† ligands have Sm−N
lengths that are typical for Sm(III) bound to this ligand (Table
3).26,30 The SmN2 units are canted relative to each other, with
a dihedral angle (θ) between the mean planes of the terminal
SmN2 groups residing at 45.8(1)° and 45.3(1)° for 2-S2 and 2-
Se2, respectively. To the best of our knowledge 2-S2 represents
the first structurally characterized example of a Sm2S2 unit
despite several examples of Yb2S2 species generated from
similar reagents.23,45,46 The distinguishing features in 2-S2 are
the long Sm−S distances, which are ca. 0.1 Å longer than in
any related Ln2S2 species.23,33,34,45−47 A recently reported
Nd2S2 complex has the same dimensions as the Sm2S2 core in
2-S2,48 despite Nd(III) having a larger ionic radius.49 The long
Sm−S bonds are the result of the steric demands of the N††
ligand, which limits the Sm···Sm distance to 5.273(1) Å. In
Magnetometry. Room temperature solution phase mag-
netic moments determined for 1-E and 2-E2 using the Evans
method38 are consistent with other Sm(III)-N†† species (Table
1).24−26,30 The solution moments for 1-E are higher than the
room temperature χMT products that range 0.14−0.19 cm3 K
mol−1 from magnetometric measurements on powder samples,
though both are noticeably larger than the free-ion value for a
6H5/2 multiplet at 0.09 cm3 K mol−1.39 This is characteristic of
Sm(III), where the observed moment is dominated by
Table 1. Room Temperature χMT Values (cm3 K mol−1)
Determined by Evans Solution NMR and Solid-State
SQUID Magnetometry
1-S
1-Se
1-Te
2-S2
2-Se2
Evans
SQUID
0.37
0.18
0.34
0.14
0.40
0.19
0.48
0.35
0.64
0.46
C
Inorg. Chem. XXXX, XXX, XXX−XXX