Dodecanuclear-ellipse and decanuclear-wheel nickel(II) thiolato clusters with efficient femtosecond nonlinear absorption

Article abstract of DOI:10.1002/anie.200907074

(Figure Presented) Crowned with success: Stepwise coordination of two kinds of thiolate ligands to a nickel (II) center affords a dodecanuclear ellipse (see structure) and two decanuclear wheel-like Ni^II-thiolato clusters. The wavelength-dependent nonlinear optical behavior of these tiara-like compounds was investigated and supported by theoretical studies. The results suggest that Ni-S π bonds play a crucial role.

Full text of DOI:10.1002/anie.200907074

Angewandte
Chemie
DOI: 10.1002/anie.200907074
Tiara Structures
Dodecanuclear-Ellipse and Decanuclear-Wheel Nickel(II) Thiolato
Clusters with Efficient Femtosecond Nonlinear Absorption**
Chi Zhang,* Tsuyoshi Matsumoto, Marek Samoc, Simon Petrie, Suci Meng,
T. Christopher Corkery, Robert Stranger, Jinfang Zhang, Mark G. Humphrey,* and
Kazuyuki Tatsumi*
Thiolate ligands have been of longstanding interest for a
variety of reasons: 1) their diverse binding modes give rise to
an array of bonding motifs that are of fundamental impor-
tance in coordination chemistry,[1] 2) metal–thiolate interac-
tions are key elements of numerous metalloproteins and play
a crucial role in the broader field of bioinorganic chemistry,[2]
and 3) thiolate-mediated magnetic coupling is an essential
component in novel molecular magnets.[3] Amongst the range
of metal thiolate-derived structures, the synthesis, structure,
and magnetic properties of toroidal (or tiara-like) architec-
tures have raised considerable interest.[4,5] However, most of
the known tiara-like [M(m-SR)]n (M = Ni, Pd) clusters to date
have been constructed by single thiolate ligands and possess
geometrically similar ring systems,[4] which has restricted, to
some extent, the abundance of examples and structural
diversity of the tiara family.
The development of high-performance molecular materi-
als with optimized nonlinear optical (NLO) properties has
also been the focus of much current research.[6,7] Previous
studies have demonstrated that the presence of large p-
electron delocalization and a symmetrical planar structure
play crucial roles in determining the properties of nonlinear
chromophores.[6a–c,e,7h,k,8] Curiously, though, despite the quasi-
aromatic nature of the bonding that has been proposed in
nickel toroidal species,[4d] their optical properties are little
explored; in particular, no study of the NLO properties is
extant.
We present here the synthesis of the largest tiara-like
nickel(II)–thiolate cluster thus far by a novel route that
employs two different thiolate bridges, structural studies that
reveal an unprecedented elliptical structure for [Ni(m-StBu)-
(m-etet)]12 (etet = 2-ethylthioethanethiolate) and two new
decanuclear-wheel nickel(II)–thiolato clusters [Ni(m-StBu)-
(m-pyet)]10 (pyet = 2-(2-mercaptoethyl)pyridine) and [Ni(m-
StBu)(m-atet)]10 (atet = 2-aminoethanethiol), and the first
NLO studies of examples from this important class of
molecules, together with time-dependent DFT studies that
shed light on the optical behavior.
[*] Prof. Dr. C. Zhang, Dr. S. C. Meng, Dr. J. F. Zhang
Molecular Materials Research Center, Scientific Research Academy,
School of Chemistry and Chemical Engineering, Jiangsu University
Zhenjiang 212013 (P.R. China)
Fax: (+86)511-8879-7815
E-mail: chizhang@ujs.edu.cn
Dr. T. Matsumoto, Prof. Dr. K. Tatsumi
Research Center for Materials Science and Department of Chemis-
try, Graduate School of Science, Nagoya University
Nagoya 464-8602 (Japan)
Fax: (+81)52-789-2943
E-mail: i45100a@nucc.cc.nagoya-u.ac.jp
Dr. S. Petrie, T. Christopher Corkery, Prof. Dr. R. Stranger,
Prof. Dr. M. G. Humphrey
The reaction of NiCl2·6H2O with 1 equivalent K(etet)
gave, after work-up, separable [(CH3C6H5)ꢀ{Ni(m-StBu)(m-
etet)}10] (1a) and [Ni(m-StBu)(m-etet)]12·(CH3C6H5)2 (1b),
whereas similar reactions with K(pyet) or K(atet), instead
of K(etet), afforded [(CH3C6H5)ꢀ{Ni(m-StBu)(m-pyet)}10]·-
(CH3C6H5)4 (2) and [(0.5CH3C6H5)ꢀ{Ni(m-StBu)(m-atet)}10]·-
(CH3C6H5)2 (3), respectively. The atomic arrangements and
stoichiometries of 1a, 1b, 2, and 3 were unequivocally
established from low-temperature CCD area-detector X-ray
diffraction studies. The single-crystal X-ray analysis of 1b
reveals a heretofore unknown dodecagonal-ellipse Ni12S24
framework, as displayed in Figure 1. The top view (Figure 1a)
of the cyclic Ni12S24 architecture shows that edge-fusion of the
12 localized planar [NiS4] subunits along opposite nonbond-
ing S–S edges gives rise to a triple-layer elliptical geometry
that approximately conforms to pseudo-D2 symmetry. The
transannular Ni···Ni distances of 1b are in the range of
11.343(6)–13.528(7) ꢀ, while the dihedral angles between
adjoining [NiS2] planes vary from 140.32 to 157.848 because of
the unsymmetrical elliptical geometry of this unique 12-
membered Ni ring. The side view (Figure 1b) of the toroid 1b
Research School of Chemistry, Australian National University
Canberra, ACT 0200 (Australia)
Fax: (+61)2-6125-0760
E-mail: mark.humphrey@anu.edu.au
Prof. Dr. M. Samoc
Institute of Physical and Theoretical Chemistry, Wroclaw University
of Technology, 50370 Wroclaw (Poland)
and
Laser Physics Centre, Research School of Physics and Engineering,
Australian National University, Canberra, ACT 0200 (Australia)
[**] This research was financially supported by the National Natural
Science Foundation of China for the Distinguished Young Scholar
Fund to C.Z. (50925207), the Ministry of Science and Technology of
China (2009DFA50620), and Grant-in-Aids for Scientific Research
(Nos. 14078211, 16350031, and 17036020) from the Ministry of
Education, Culture, Sports, Science, and Technology (Japan), and
the Commonwealth of Australia under the International Science
Linkages program (“Joint Research Centre for Functional Molecular
Materials”). M.G.H. is an Australian Research Council Australian
Professorial Fellow, and M.S. is a laureate of the Foundation for
Polish Science “Welcome” program.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200907074.
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4209

Communications
(9.974(9)–10.397(9) ꢀ in 2, and 10.04(1)–10.46(1) ꢀ in 3)). As
was the case for [Ni(m-StBu)(m-etet)]12·(CH3C6H5)2 (1b), the
tBu groups are all oriented away from the Ni10S20 toroid, and
the pyet/atet substituents project above and below the plane
of the toroid, except for two pyet/atet ligands whose
amidocyanogen arms apparently clamp the encapsulated
toluene molecule from above and below. Dihedral angles
between adjacent [NiS2] planes are similar (141.7–146.888 for
2, and 138.1–151.08 for 3), and close to the interior angle of an
idealized decagon, consistent with the pseudo-D5d symmetry
for the Ni10S20 tiara-geometry of 2 and 3.
Z-scan measurements on the toroid clusters 1b, 2, and 3
were performed in the wavelength range 530–1300 nm (see
Figures 3 and S6). The dominant NLO process is presumed to
Figure 1. Molecular view of [Ni(m-StBu)(m-etet)]12·(CH3C6H5)2 (1b): Ni
green, S red, C gray. Neighboring interatomic distances [ꢀ]: Ni–S
2.192(3)–2.248(4) (av: 2.217), Ni–Ni 3.140(1)–3.257(1) (av: 3.212).
The hydrogen atoms and two toluene solvent molecules outside the
Ni12S24 toroid are omitted for clarity.
shows a virtually planar nonbonding Ni12 ring sandwiched
between two approximately coplanar nonbonding S12 rings.
The two kinds of thiolate ligands, etet and StBu, are situated
alternately above and below the Ni12 plane, with all tBu
groups oriented away from the ring. The etet substituents
display three distinct orientations; six of the etet ligands
project upward or downward, and four etets incline at
approximately 458 to the Ni12 ring, while the remaining two
etet groups penetrate the cavity of the Ni12S24 toroid and
sustain the tiara architecture in the absence of co-crystallized
solvent molecules within the Ni12 ring. The toroidal Ni10S20
core geometries of 2 and 3 are shown in Figures 2 and S5 (see
Figure 3. Experimental two-photon absorption profiles for a) cluster 2
(1.48ꢁ10À4 molLÀ1 in DMF) and b) cluster 3 (1.56ꢁ10À4 molLÀ1 in
Figure 2. Molecular view of [(CH3C6H5)ꢀ{Ni(m-StBu)(m-pyet)}10]·-
(CH3C6H5)4 (2): Ni green, S red, N blue, C gray. Neighboring
interatomic distances [ꢀ]: Ni–S 2.1870(8)–2.2188(9) (av: 2.2067), Ni–
Ni 3.1488(2)–3.1771(2) (av: 3.1606). The hydrogen atoms and four
toluene solvent molecules outside the Ni10S20 toroid are omitted for
clarity.
&
^
DMF). s2, sex, c s.
be one-photon absorption in the tail of the absorption curve,
followed by excited-state absorption. Under such circum-
stances, the two-photon absorption cross-sections measured
in the Z-scan experiments should be considered effective
values only. Assuming that the pulse duration ( ꢁ 150 fs) is
much shorter than the lifetime of the excited state, the
effective two-photon cross-section can be approximated as
s2,eff = 0.5ssex tp, where s is the one-photon cross-section, sex
is the excited state cross-section and tp is the pulse duration.
Figures 3 and S6 illustrate the results of calculations of the
excited-state cross-section spectra. All three compounds show
similar spectra that peak at approximately 700 nm with a
the Supporting Information), with pyet (or atet) and StBu
ligands bridging adjacent Ni atoms alternately above and
below the Ni10 ring. In contrast to the Ni12S24 ellipsoid of 1b,
one toluene molecule is encapsulated in the Ni10 cavity of 2
and 3, and is situated approximately in the Ni10 plane, tilted
from coplanarity by 29.434(2)8 for 2 (18.338(1)8 for 3), which
results in the toroidal Ni10S20 core approximating a circular
shape (transannular Ni···Ni distances are nearly constant
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cross-section of approximately 10À17 cm2. This is of the same
order of magnitude as those obtained for many other cluster
compounds in nanosecond experiments,[7g–j] but further sub-
stantial comment is not warranted because ns experiments are
usually dominated by long-lived triplet excitations, whereas fs
measurements are dominated by much shorter-lived singlet
excited species. Nevertheless, a preliminary internal compar-
ison of the NLO performance of 1b, 2, and 3 with tiara-like
geometries can be undertaken. Their coplanar NinS2n (n = 10,
À
12) geometries and Ni S p bonding orbitals distributed
throughout the NinS2n (n = 10, 12) tiara-like structures should
influence the fs NLO merit of these toroidal thiolate clusters.
Although the NiII thiolate clusters 1b, 2, and 3 are constructed
from the same [NiS4] subunits and possess similar toroidal
frameworks, the observed NLO behavior of 1b, 2, and 3 are
distinct. Specifically, the nonlinear absorption maxima of 2
and 3 are greater than that of 1b at around 630 nm, suggesting
superiority of the Ni10S20-centered coplanar geometry.
DFT calculations, using the Amsterdam Density Func-
tional program suite ADF2008.01,[9] were performed on
simplified models of compounds 1b, 2, and 3, to shed light
on the structural diversity and optical behavior of these novel
species. Geometry optimization at the PBE/TZP level of
theory was undertaken on C2-, D5d- and D2-symmetric
structures analogous to 1b, 2, and 3, respectively, but with
all bridging groups replaced by m-SMe groups for the
purposes of computational expediency. The corresponding
key data for bond lengths and angles of the optimized
geometries (see Table S2 in the Supporting Information) of
1b in D2- and 2 and 3 in D5d-symmetrical models are
consistent with those of their X-ray diffraction data. Spec-
troscopy of the PBE/TZP optimized “permethylated” models
described above was computed by time-dependent DFT (TD-
DFT), using the SAOP/TZP level of theory. A summary of
Figure 4. “Synthetic” spectra obtained from TD-DFT calculations, at
the SAOP/TZP level of theory, of transition energies and relative
oscillator strengths for the PBE/TZP optimized {Ni(m-SMe)2}n models
(n=10: a, 12: c). Peak shapes were obtained by Gaussian
broadening of up to 700 summed dipole-allowed transitions across the
wavelength range of interest. Arrows and vertical lines superimposed
upon the graph show the observed wavelengths of the experimental
spectral peaks.
the results is presented in Figures 4, 5, S9, S10, and Table S3.
We find that the observed lower-energy features in the
experimental spectra (l > 400 nm) are attributable to excita-
tion from occupied Nid-character orbitals to vacant NiS s*-
type orbitals, while the spectral features at wavelengths
shorter than 400 nm arise predominantly from more deeply
submerged occupied orbitals (principally Sp in character) to
the same vacant NiS s* orbitals. These calculations necessa-
rily relate to the clusters in vacuum, because solvent
correction using triple-zeta-plus-polarization basis sets (as
above) was judged to be prohibitive in regard to the TD-DFT
computation required for the theoretical spectra. Solvent-
corrected (polarized continuum model, or PCM) TD-DFT
Figure 5. Assignment of the absorption peaks 1 and 3 of the D5d-symmetric {Ni(m-SMe)2}10 model and the D2-symmetric {Ni(m-SMe)2}12 model in
vacuum. The orbitals are obtained at the PBE/TZP level.
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Communications
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In summary, this study demonstrates that a geometrically
unprecedented dodecanuclear-ellipse [Ni(m-StBu)(m-etet)]12·
(CH3C6H5)2 and two decanuclear-wheel nickel(II)–thiolato
clusters can be constructed by stepwise introduction of two
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À
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a crucial role in both the stabilization of such dodeca- and
decanuclear tiara-like geometries, their linear optical proper-
ties, and, by implication, their strong fs nonlinear optical
behavior. An in-depth study probing the mechanism of both
the fs and ns optical nonlinearity of these NinS2n toroids is
currently underway.
Experimental Section
Details of the chemical synthesis, spectroscopic characterization,
structural, theoretical, and nonlinear optical studies are given in the
Supporting Information.
Received: December 15, 2009
Published online: April 26, 2010
Keywords: cluster compounds · nonlinear optics ·
.
TD-DFT calculations · thiolate ligands · tiara structures
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