Synthesis, emission, and electrochemical properties of luminescent dinuclear Zinc(II) chalcogenolate complexes. Dynamic 1H NMR studies and x-ray crystal structure of [(bpy) Zn2(SC6H4-Cl-p) (μ-SC6H4-Cl-p)(μ-OAc)2]

Article abstract of DOI:10.1021/ic000688g

A series of novel luminescent dinuclear zinc(II) diimine complexes with bridging chalcogenolate ligands have been synthesized, in which the two zinc atoms were found to exist in different coordination environment. The luminescence and electrochemical behavior of these complexes have been studied. These complexes have also been shown to exhibit dynamic fluxional behavior in solution due to an exchange of the bridging and terminal thiolate ligands. The mechanism and kinetics of which have been investigated by variable-temperature ¹H NMR studies. The X-ray crystal structure of [(bpy)Zn₂(SC₆H₄-Cl-p) (μ-SC₆H₄-Cl-p)(μ-OAc)₂] has also been determined.

Full text of DOI:10.1021/ic000688g

Inorg. Chem. 2000, 39, 5741-5746
5741
Synthesis, Emission, and Electrochemical Properties of Luminescent Dinuclear Zinc(II)
1
Chalcogenolate Complexes. Dynamic H NMR Studies and X-ray Crystal Structure of
[(bpy)Zn2(SC6H4-Cl-p)(µ-SC6H4-Cl-p)(µ-OAc)2]
Vivian Wing-Wah Yam,* Yung-Lin Pui, and Kung-Kai Cheung
Department of Chemistry, The University of Hong Kong, Pokfulam Road,
Hong Kong SAR, People’s Republic of China
ReceiVed June 23, 2000
A series of novel luminescent dinuclear zinc(II) diimine complexes with bridging chalcogenolate ligands have
been synthesized, in which the two zinc atoms were found to exist in different coordination environment. The
luminescence and electrochemical behavior of these complexes have been studied. These complexes have also
been shown to exhibit dynamic fluxional behavior in solution due to an exchange of the bridging and terminal
thiolate ligands. The mechanism and kinetics of which have been investigated by variable-temperature 1H NMR
studies. The X-ray crystal structure of [(bpy)Zn2(SC6H4-Cl-p)(µ-SC6H4-Cl-p)(µ-OAc)2] has also been determined.
Introduction
in view of the possibility of zinc to exist as penta-, tri-, and
perhaps even hexa-coordinated structures in the functional states
Zinc(II) thiolate complexes have been used as models for
thiolate metalloproteins, in which zinc centers are frequently
coordinated through histidines and the sulfur-containing cys-
teine.1 Moreover, zinc complexes with S4, S3N, or S2N2 ligand
cores are of interest as structural models of the immediate ligand
environment of zinc in DNA-binding proteins and certain zinc-
containing enzymes.2 In view of this, an understanding of its
basic coordination properties is necessary. Although there are
numerous reports on zinc(II) thiolate systems,3 most of the
studies were limited to those of structural studies, in which the
zinc atom generally is found to assume a tetrahedral geometry
with the corresponding studies on the less common five-
coordinate zinc(II) complexes4,5 being relatively less explored.
Coordination geometries other than that of four are interesting
of the biological systems. Intrigued by the recent reports on
the observation of colored complexes of these closed-shell metal
centers, particularly those of zinc(II) coordinated to diimine and
thiolate ligands and the luminescence properties of a number
of these complexes,5 efforts have been made to explore the
thiolate chemistry and luminescence properties of zinc(II)
complexes. Since most of the luminescence studies reported so
far are confined to the mononuclear zinc(II) diimine dithiolate
systems5 with only a few reports on the polynuclear anionic
zinc thiolates,6 it would be interesting to design dinuclear
analogues by utilizing the chalcogenolate as possible bridging
ligands.
In this report, a series of novel dinuclear zinc(II) chalco-
genolate complexes have been synthesized and shown to exhibit
interesting luminescence and electrochemical properties. These
complexes have been shown to exhibit dynamic fluxional
behavior in solution due to an exchange of the bridging and
terminal thiolate ligands; the mechanism of which has been
investigated by variable-temperature 1H NMR studies. The X-ray
crystal structure of one of the complexes has also been
determined.
(1) (a) Klug, A.; Rhodes, D. Trends Biochem. Sci. 1987, 12, 464. (b) Berg,
J. M. Prog. Inorg. Chem. 1989, 37, 143. (c) Vallee, B. L.; Auld, D.
S. Biochemistry 1990, 29, 5647. (d) Vallee, B. L.; Coleman, J. E.
Proc. Natl. Acad. Sci., U. S. A. 1991, 88, 999. (e) Coleman, J. E.
Annu. ReV. Biochem. 1992, 61, 897. (f) Vallee, B. L.; Auld, D. S.
Biochemistry 1993, 32, 6493.
(2) (a) Hough, E.; Hansen, L. K.; Birkness, B.; Jynge, K.; Hansen, S.;
Hordvik, A.; Little, C.; Dodson, E.; Derewenda, Z. Nature (London)
1989, 338, 357. (b) Alsfasser, R.; Powell, A. K.; Vahrenkamp, H.
Angew. Chem., Int. Ed. Engl. 1990, 29, 898. (c) Uhlenbrock, S.; Krebs,
B. Angew. Chem., Int. Ed. Engl. 1992, 31, 1647. (d) Hansen, S.;
Hansen, L. K.; Hough, E. J. Mol. Biol. 1993, 231, 870.
Experimental Section
Materials and Reagents. Thiophenol and 4-fluorothiophenol were
obtained from Aldrich Chemical Company. Zinc acetate dihydrate,
4-methoxythiophenol, 4-chlorothiophenol, and 2,2′-bipyridine were
obtained from Lancaster Synthesis Ltd. 4,4′-Di-tert-butyl-2,2′-bipyridine
(tBu2bpy) was prepared by the modification of a literature procedure.7
Tetrabutylammonium hexafluorophosphate (nBu4NPF6) (Aldrich, 98%)
(3) (a) Dance, I. G. J. Am. Chem. Soc. 1980, 102, 3445. (b) Choy, A.;
Craig, D.; Dance, I. G.; Scudder, M. J. Chem. Soc., Chem. Commun.
1982, 1246. (c) Dance, I. G.; Choy, A.; Scudder, M. L. J. Am. Chem.
Soc. 1984, 106, 6285. (d) Watson, A. D.; Rao, C. P.; Dorfman, J. R.;
Holm, R. H. Inorg. Chem. 1985, 24, 2820. (e) Abrahams, I. L.; Garner,
C. D. J. Chem. Soc., Dalton. Trans. 1987, 1577. (f) Vittal, J. J.; Dean,
P. A. W.; Payne, N. C. Inorg. Chem. 1987, 26, 1683. (g) Ueyama,
N.; Sugawara, T.; Sasaki, K.; Nakamura, A.; Yamashita, S.; Wakatsuki,
Y.; Yamazaki, H.; Yasuoka, N. Inorg. Chem. 1988, 27, 741. (h) Dean,
P. A. W.; Vittal, J. J.; Payne, N. C. Can. J. Chem. 1992, 70, 792. (i)
Melnik, M. J. Coord. Chem. 1995, 35, 179. (j) Anjali, K. S.;
Sampanthar, J. T.; Vittal, J. J. Inorg. Chim. Acta 1999, 295, 9.
(4) (a) Lee, G. S. H.; Craig, D. C.; Ma, I.; Scudder, M. L.; Bailey T. D.;
Dance, I. G. J. Am. Chem. Soc. 1988, 110, 4863. (b) Zhang, C.;
Chadha, R.; Reddy H. K.; Schrauzer, G. N. Inorg. Chem. 1991, 30,
3865. (c) Gronlund, P. J.; Wacholtz, W. F.; Mague, J. T. Acta
Crystallogr. 1995, C51, 1540. (d) Uhlenbrock, S.; Wegner, R.; Krebs,
B. J. Chem. Soc., Dalton Trans. 1996, 3731.
(5) (a) Crosby, G. A.; Highland, R. G.; Truesdell, K. A. Coord. Chem.
ReV. 1985, 64, 41. (b) Truesdell, K. A.; Crosby, G. A. J. Am. Chem.
Soc. 1985, 107, 1781. (c) Highland, R. G.; Brummer, J. G.; Crosby,
G. A. J. Phys. Chem. 1986, 90, 1593. (d) Jordan, K. J.; Wacholtz, W.
F.; Crosby, G. A. Inorg. Chem. 1991, 30, 4588.
(6) (a) Tu¨rk, T.; Resch, U.; Fox, M. A.; Vogler, A. Inorg. Chem. 1992,
31, 1854. (b) Tu¨rk, T.; Vogler, A.; Fox, M. A. AdV. Chem. Ser. 1993,
233.
(7) (a) Sasse, W. H. F.; Whittle, C. P. J. Chem. Soc. 1961, 1347. (b)
Sasse, W. H. F. Org. Syn. 1973, V, 102.
10.1021/ic000688g CCC: $19.00 © 2000 American Chemical Society
Published on Web 11/15/2000

5742 Inorganic Chemistry, Vol. 39, No. 25, 2000
Yam et al.
was purified by recrystallization with ethanol three times before use.
Dichloromethane was purified using standard procedures before use.8
All other reagents were of analytical grade and were used as received.
analyses of the new complexes were performed on a Carlo Erba 1106
elemental analyzer at the Institute of Chemistry, Chinese Academy of
Sciences.
UV/vis spectra were obtained on a Hewlett-Packard 8452A diode
array spectrophotometer, and steady-state excitation and emission
spectra on a Spex Fluorolog 111 spectrofluorimeter. For solution
emission and excitation spectra, samples were degassed on a high-
vacuum line (limiting pressure < 10-3 Torr) in a two compartment
cell consisting of a 10-mL Pyrex bulb equipped with a sidearm 1-cm
fluorescence cuvettte and sealed from the atmosphere by a Rotaflo
HP6/6 quick-release Teflon stopper. Solutions were rigorously degassed
with no fewer than four freeze-pump-thaw cycles. The concentration
of the solutions are in the range of 10-4 to 10-5 M. Solid-state emission
and excitation spectra at room temperature were recorded with ground
solid samples loaded in a quartz tube inside a quartz-walled Dewar
flask. Solid-state spectra at low temperature (77 K) were similarly
recorded with liquid nitrogen inside the optical Dewar flask.
Cyclic voltammetric measurements were performed by using a CH
Instruments, Inc., CHI 620 electrochemical analyzer interfaced to an
IBM-compatible PC. The electrolytic cell used was a conventional two-
compartment cell. The salt bridge of the reference electrode was
separated from the working electrode compartment by a Vycor glass
bridge. A Ag/AgNO3 (0.1 mol dm-3 in CH2Cl2) reference electrode
was used. The ferrocenium-ferrocene couple (FeCp2+/0) was used as
the internal reference in the electrochemical measurements.9a The
working electrode was a glassy carbon (Atomergic Chemetals V25)
electrode with a platinum foil acting as the counter electrode. Treatment
of the electrode surfaces was as reported previously.9b
[(bpy)Zn2(SC6H5)(µ-SC6H5)(µ-OAc)2] (1). To a stirred solution of
zinc acetate dihydrate (100 mg, 0.46 mmol) in methanol (5 mL) was
added a solution of 2,2′-bipyridine (72 mg, 0.46 mmol) in methanol (5
mL). The resultant solution was stirred for 30 min under a nitrogen
atmosphere. Thiophenol (50 mg, 0.46 mmol) dissolved in dichlo-
romethane (10 mL) was then added to the reaction mixture dropwise
with stirring to produce an immediate color change from a colorless to
yellow solution. The mixture was stirred overnight under a nitrogen
atmosphere. After evaporation of the solvent, the residue was dissolved
in dichloromethane, and diffusion of diethyl ether vapor into its
concentrated solution gave 1 as white crystals. Yield: 88 mg (62%).
1H NMR (300 MHz, CDCl3, 298 K, relative to Me4Si): δ 2.05 (s, 6H,
OAc), 6.90 (m, 6H, aryl H ortho to S), 7.30 (m, 4H, aryl H meta to S),
7.45 (t, 2H, J ) 12.6 Hz, bpy H’s), 8.05 (t, 2H, J ) 16.6 Hz, bpy
H’s), 8.15 (d, 2H, J ) 7.8 Hz, bpy H’s), 8.60 (unresolved d, 2H, bpy
H’s). Anal. Calcd for C26H24N2O4S2Zn2: C, 50.10; H, 3.88; N, 4.49.
Found: C, 49.99; H, 3.89; N, 4.40.
[(bpy)Zn2(SC6H4-F-p)(µ-SC6H4-F-p)(µ-OAc)2] (2). The procedure
was similar to that described for the preparation of 1, except 4-fluo-
rothiophenol (59 mg, 0.46 mmol) was used in place of thiophenol to
1
give white crystals of 2. Yield: 90 mg (59%). H NMR (300 MHz,
CDCl3, 298 K, relative to Me4Si): δ 2.05 (s, 6H, OAc), 6.60 (d, 4H,
J ) 8.4 Hz, aryl H ortho to S), 7.20 (d, 4H, J ) 8.2 Hz, aryl H meta
to S), 7.50 (t, 2H, J ) 13.3 Hz, bpy H’s), 8.05 (t, 2H, J ) 15.4 Hz,
bpy H’s), 8.15 (d, 2H, J ) 7.9 Hz, bpy H’s), 8.60 (unresolved d, 2H,
bpy H’s). Anal. Calcd for C26H22F2N2O4S2Zn2: C, 47.36; H, 3.36; N,
4.25. Found: C, 47.37; H, 3.37; N, 4.27.
Crystal Structure Determination. Crystal data for [(bpy)Zn2-
(SC6H4-Cl-p)(µ-SC6H4-Cl-p)(µ-OAc)2] (3): [C26H22Cl2N2O4S2Zn2];
fw ) 692.26, triclinic, space group P1h (No. 2), a ) 7.997(1) Å, b )
10.297(2) Å, c ) 18.550(2) Å, R ) 105.46(1)°, â ) 93.23(1)°, γ )
97.25(1)°, V ) 1454.0(9) Å3, Z ) 2, Dc ) 1.581 g cm-3, µ(Mo KR) )
20.11 cm-1, F(000) ) 700, T ) 301 K. A colorless crystal of
dimensions 0.35 × 0.20 × 0.05 mm mounted on a glass fiber was
used for data collection at 28 °C on a Rigaku AFC7R diffractometer
with graphite-monochromated Mo KR radiation (λ ) 0.71073 Å) using
ω-2θ scans with ω-scan angle (0.73 + 0.35 tan θ)° at a scan speed of
8.0 deg min-1 (up to 6 scans for reflection with I < 15σ(I)). Unit-cell
dimensions were determined on the basis of the setting angles of 25
reflections in the 2θ range of 29.3 to 34.2°. Intensity data (in the range
of 2θmax ) 50°; h -9 to 9; k 0 to 12; l -22 to 21 and three standard
reflections measured after every 300 reflections showed decay of 1.25%)
were corrected for decay and for Lorentz and polarization effects, and
empirical absorption corrections based on the ψ-scan of five strong
reflections (minimum and maximum transmission factors 0.625 and
1.000); 5459 reflections were measured, of which 5144 were unique
and Rint ) 0.016, and 3635 reflections with I > 3σ(I) were considered
observed and used in the structural analysis. The space group was
determined based on a statistical analysis of intensity distribution and
the successful refinement of the structure solved by direct methods
(SIR92)10a and expanded by Fourier method and refined by full-matrix
least-squares using the software package TeXsan10b on a Silicon
Graphics Indy computer. One crystallographic asymmetric unit consists
of one molecule. In the least-squares refinement, all 38 non-H atoms
were refined anisotropically and 22 H atoms at calculated positions
with thermal parameters equal to 1.3 times that of the attached C atoms
were not refined. Convergence for 343 variable parameters by least-
[(bpy)Zn2(SC6H4-Cl-p)(µ-SC6H4-Cl-p)(µ-OAc)2] (3). The procedure
was similar to that described for the preparation of 1, except 4-chlo-
rothiophenol (67 mg, 0.46 mmol) was used in place of thiophenol to
1
give white crystals of 3. Yield: 80 mg (51%). H NMR (300 MHz,
CDCl3, 298 K, relative to Me4Si): δ 2.05 (s, 6H, OAc), 6.85 (d, 4H,
J ) 8.1 Hz, aryl H ortho to S), 7.20 (d, 4H, J ) 8.0 Hz, aryl H meta
to S), 7.55 (t, 2H, J ) 12.7 Hz, bpy H’s), 8.10 (t, 2H, J ) 17.1 Hz,
bpy H’s), 8.17 (d, 2H, J ) 8.0 Hz, bpy H’s), 8.62 (unresolved d, 2H,
bpy H’s). Anal. Calcd for C26H22Cl2N2O4S2Zn2: C, 45.11; H, 3.20; N,
4.05. Found: C, 44.84; H, 3.28; N, 4.05.
[(bpy)Zn2(SC6H4-OCH3-p)(µ-SC6H4-OCH3-p)(µ-OAc)2] (4). The
procedure was similar to that described for the preparation of 1, except
4-methoxythiophenol (64 mg, 0.46 mmol) was used in place of
1
thiophenol to give pale yellow crystals of 4. Yield: 86 mg (55%). H
NMR (300 MHz, CDCl3, 298 K, relative to Me4Si): δ 2.05 (s, 6H,
OAc), 3.65 (s, 6H, OCH3), 6.50 (d, 4H, J ) 6.4 Hz, aryl H ortho to S),
7.18 (d, 4H, J ) 6.1 Hz, aryl H meta to S), 7.50 (t, 2H, J ) 12.5 Hz,
bpy H’s), 8.05 (t, 2H, J ) 16.9 Hz, bpy H’s), 8.15 (d, 2H, J ) 7.7 Hz,
bpy H’s), 8.65 (unresolved d, 2H, bpy H’s). Anal. Calcd for
C28H28N2O6S2Zn2: C, 49.21; H, 4.13; N, 4.10. Found: C, 48.86; H,
4.12; N, 3.84.
[(tBu2bpy)Zn2(SC6H5)(µ-SC6H5)(µ-OAc)2] (5). The procedure was
similar to that described for the preparation of 1, except 4,4′-di-tert-
butyl-2,2′-bipyridine7 (125 mg, 0.47 mmol) was used in place of 2,2′-
1
bipyridine to give white crystals of 5. Yield: 74 mg (44%). H NMR
t
(300 MHz, CDCl3, 298 K, relative to Me4Si): δ 1.40 (s, 18H, Bu),
2.03 (s, 6H, OAc), 6.90 (m, 6H, aryl H ortho to S), 7.25 (m, 4H, aryl
H meta to S), 7.40 (m, 2H, bpy H’s), 8.00 (m, 2H, bpy H’s), 8.50 (m,
2H, bpy H’s). Anal. Calcd for C34H40N2O4S2Zn2‚H2O: C, 54.19; H,
5.62; N, 3.72. Found: C, 54.53; H, 5.49; N, 3.58.
squares refinement on F with w ) 4Fo /σ2(Fo ), where σ2(Fo ) )
2
2
2
[σ2(I) + (0.030Fo )2] for 3635 reflections with I > 3σ(I) was reached
2
at R ) 0.033 and wR ) 0.046 with a goodness-of-fit of 1.68; (∆/σ)max
) 0.05. The final difference Fourier map was featureless, with
1
maximum positive and negative peaks of 0.43 and 0.32 e Å-3
respectively.
,
Physical Measurements and Instrumentation. H NMR spectra
were recorded on a Bruker DPX-300 FT-NMR spectrometer (300 MHz)
at 298 K, and chemical shifts are reported relative to Me4Si. Variable-
temperature one-dimensional 1H NMR spectra in CDCl3 and two-
dimensional COSY and EXSY in CD2Cl2 at 223 K were recorded on
a Bruker DRX-500 FT-NMR spectrometer (500 MHz). Elemental
(9) (a) Gagne, R. R.; Koval, C. A.; Lisensky, G. C. Inorg. Chem. 1980,
19, 2854. (b) Che, C. M.; Wong, K. Y.; Anson, F. C. J. Electroanal.
Chem., Interfacial Electrochem. 1987, 226, 221.
(10) (a) Altomare, A.; Cascarano, M.; Giacovazzo, C.; Guagliardi, A.; Burla,
M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27, 435.
(b) TeXsan: Crystal Structure Analysis Package, Molecular Structure
Corp., 1985 and 1992.
(8) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
Chemicals, 3rd ed.; Pergamon Press: New York, 1988.

Luminescent Zn(II) Chalcogenolate Complexes
Inorganic Chemistry, Vol. 39, No. 25, 2000 5743
Table 2. Electronic Absorption and Emission Data for
Complexes 1-5
complex medium (T/K)
λ
abs/nm (ꢀ/dm3 mol-1 cm-1
)
λ
em/nma
1
2
3
4
5
solid (298)
solid (77)
510
525
530
glass (77)b
CH2Cl2 (298)
243 (24 350), 298 (17 810),
308 (17 530), 360sh (280)
250 (32 540), 297 (16 220),
307 (15 220), 355sh (320)
MeOH (298)
solid (298)
solid (77)
504
528
520
glass (77)b
CH2Cl2 (298)
241 (26 140), 298 (16 850),
309 (16 160), 358sh (190)
258 (27 300), 297 (15 960),
307 (15 010), 356sh (260)
MeOH (298)
solid (298)
solid (77)
537
527
514
glass (77)b
CH2Cl2 (298)
Figure 1. Perspective drawing of 3 with the atomic numbering scheme.
Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are
shown at the 40% probability level.
250 (28 480), 299 (18 800),
309 (17 930), 352sh (310)
256 (27 700), 296 (18 310),
307 (16 600), 354sh (240)
MeOH (298)
Table 1. Selected Geometric Data for 3
solid (298)
solid (77)
555
544
540
Bond Lengths (Å)
glass (77)b
CH2Cl2 (298)
Zn(1)-S(1)
Zn(1)-O(2)
Zn(1)-N(12)
Zn(2)-S(2)
Zn(2)-O(4)
2.347(1)
2.039(2)
2.121(3)
2.254(1)
1.982(3)
Zn(1)-O(1)
Zn(1)-N(1)
Zn(2)-S(1)
Zn(2)-O(3)
2.047(3)
2.153(3)
2.379(1)
1.976(3)
248 (41 700), 298 (21 550),
308 (20 460), 370sh (310)
250 (32 410), 298 (19 260),
307 (17 840), 376sh (210)
MeOH (298)
solid (298)
solid (77)
504
510
508
Bond Angles (deg)
S(1)-Zn(1)-O(1)
S(1)-Zn(1)-N(1)
O(1)-Zn(1)-O(2)
O(1)-Zn(1)-N(2)
S(1)-Zn(2)-S(2)
S(1)-Zn(2)-O(4)
S(2)-Zn(2)-O(4)
Zn(1)-S(1)-Zn(2)
104.21(8) S(1)-Zn(1)-O(2)
103.40(8)
111.37(9)
85.3(1)
glass (77)b
CH2Cl2 (298)
106.60(8) S(1)-Zn(1)-N(2)
243 (27 150), 298 (15 730),
308 (15 750), 345sh (350)
250 (30 130), 297 (13 440),
307 (11 300), 350sh (300)
94.4(1)
143.3(1)
O(1)-Zn(1)-N(1)
N(1)-Zn(1)-N(2)
76.6(1)
MeOH (298)
121.79(5) S(1)-Zn(2)-O(3)
102.33(9) S(2)-Zn(2)-O(3)
106.26(9) O(3)-Zn(2)-O(4)
106.21(8)
111.62(9)
107.5(1)
a Excitation wavelength at 350 nm. b EtOH: MeOH ) 4:1 (v/v).
86.97(4) Zn(1)-S(1)-C(15) 103.9(1)
Zn(2)-S(1)-C(15) 109.5(1)
Zn(2)-S(2)-C(21) 100.0(2)
in dichloromethane and methanol were found to show a slight
energy dependence on the nature of the chalcogenolate and the
diimine ligands. The low energy absorption shoulder for 1 was
found to occur at lower energy compared to that of 5 but at
higher energy than that of 4. Such an energy trend is suggestive
of a ligand-to-ligand charge-transfer LLCT [pπ(SR-) f
π*(N-N)] transition assignment, which is in accordance with
the higher π* orbital energy of tBu2bpy than bpy and the greater
electron-richness of 4-methoxythiophenolate than thiophenolate.
Similar findings have also been observed in other related
systems.5
Upon excitation at λ ) 350 nm, complexes 1-5 are found
to be luminescent with emission maxima at ca. 500-550 nm
both in the solid state at room temperature and 77 K, and in 77
K glass, but they are nonemissive in degassed acetonitrile and
methanol solutions. The photophysical data are summarized in
Table 2. The emission energies are found to depend on the
nature of both the chalcogenolate and the diimine ligands. The
emission energies in 77 K glass follow the order: [(bpy)Zn2-
(SC6H4-OCH3-p)(µ-SC6H4-OCH3-p)(µ-OAc)2] (4) (540 nm) <
[(bpy)Zn2(SC6H5)(µ-SC6H5)(µ-OAc)2] (1) (530 nm) < [(bpy)-
Zn2(SC6H4-F-p)(µ-SC6H4-F-p)(µ-OAc)2] (2) (520 nm) < [(bpy)-
Zn2(SC6H4-Cl-p)(µ-SC6H4-Cl-p)(µ-OAc)2] (3) (514 nm). On the
basis of purely inductive effects alone, the electron-donating
ability of the thiophenolate ligands is expected to occur in the
order SC6H4-OCH3-p > SC6H5 > SC6H4-Cl-p > SC6H4-F-p.
Therefore the LLCT emission energies would be expected to
show an energy trend in the order 4 < 1 < 3 E 2 with the
same bipyridine ligand, which is more or less in line with the
Results and Discussion
Crystal Structure Determination. All of the complexes were
1
characterized by H NMR spectroscopy and gave satisfactory
elemental analyses. Single crystals of 3 were obtained by vapor
diffusion of diethyl ether into a concentrated dichloromethane
solution of the complex. One of the zinc centers adopts a
distorted tetrahedral geometry with a S(1)-Zn(2)-S(2) angle
of 121.79(5)° while the other adopts a distorted square pyramidal
geometry with a S(1)-Zn(1)-N(1) angle of 106.60(8)°. The
two metal centers are linked together by a bridging thiolate
ligand and two bridging acetates, with a Zn(1)-S(1)-Zn(2)
angle of 86.97(4)°. The Zn-S(1) and Zn(2)-S(2) bond distances
with the bridging and terminal thiolate ligands are 2.363(1) Å
(average) and 2.254(1) Å, respectively, which are comparable
to those found in other related systems.3d,e,j,5d,11 The perspective
drawing of 3 with atomic numbering is depicted in Figure 1.
Selected bond distances and angles are summarized in Table 1.
Electronic Absorption and Emission Properties. The
electronic absorption spectra of complexes 1-5 show low-
energy absorption shoulders at ca. 340-380 nm and higher
energy absorptions at ca. 240-310 nm (Table 2). The latter
are assigned as intraligand (IL) transitions of the diimines and
chalcogenolates since the free ligands also absorb at similar
energy. The low-energy absorption shoulders at 340-380 nm
(11) Halvorsen, K.; Crosby, G. A.; Wacholtz, W. F. Inorg. Chim. Acta
1995, 228, 81.

5744 Inorganic Chemistry, Vol. 39, No. 25, 2000
Yam et al.
Figure 2. Emission spectra of 1 (‚‚‚), 3 (s) and 4 (- - -) in EtOH/
MeOH (4:1 v/v) glass at 77 K. Excitation wavelength at 350 nm. [1],
[3], and [4] ) 1 × 10-3 M.
1
Figure 4. Variable-temperature H NMR spectra of 3 in CDCl3.
phenolate ligands were inequivalent and were in slow exchange
with each other. At higher temperature in CDCl3, the two
thiophenolate ligands, the terminal and the bridging at δ 6.85
and 7.20, became equivalent on the 1H NMR time scale, possibly
as a result of their increased rate of interconversion. Therefore,
only two sets of thiophenolate protons, in the range of δ 6.85
to 7.20 ppm, were observed at 313 K in the 1H NMR spectrum
as shown in Figure 4. However, these protons appeared as broad
signals as the temperature is lowered as a result of the slow
exchange of these two thiophenolate ligands when the complex
was frozen down. At 223 K, four sets of thiophenolate signals,
δ 6.81, 6.90, 7.02, and 7.50 ppm, were observed in the region,
indicating that the exchange of the bridging and the terminal
thiophenolate ligands was slow to frozen. Using DNMR line
shape simulation program,12a the exchange rate constants (k) at
different temperature were determined. The ∆Gq value for the
fluxional process of 3 in CDCl3 obtained from the linear plot
of ln k/T versus 1/T according to the Eyring equation12b-c was
found to be ca. 24.4 ( 0.5 kJ mol-1. It is likely to be ascribed
to a terminal-to-bridging thiophenolate exchange process. An
alternative possible mechanism involves the cleavage of the Zn-
SR(bridging) bond at high temperature to give two four-coordinate
Zn environments. However, in view of the small activation
barrier obtained and the close resemblance of the activation
barrier to that observed for bridging-to-terminal thiolate ex-
change processes in related systems,13 together with the large
amount of energy required for a Zn-S cleavage process (bond
energy 37.5 kcalmol-1),14 the possibility of such a Zn-S bond
cleavage mechanism is less likely.
Figure 3. Emission spectra of 1 (- - -) and 5 (s) in EtOH/MeOH
(4:1 v/v) glass at 77 K. Excitation wavelength at 350 nm. [1] and [5]
) 1 × 10-3 M.
experimental findings. The only anomaly is seen for 2 and 3 in
which the fluoro-substituted 2 emits at lower energy than its
chloro analogue 3. This may be rationalized by the fact that
resonance effect of lone pair electron donation from the F atom
to the 3d orbitals of sulfur could be more important than its
negative inductive effect. For the same chalcogenolate ligand,
the lower emission energies for [(bpy)Zn2(SC6H5)(µ-SC6H5)-
(µ-OAc)2] (1) than that of [(tBu2bpy)Zn2(SC6H5)(µ-SC6H5)(µ-
OAc)2] (5) is again in line with the assignment of an LLCT
emission origin since a higher π* orbital energy is expected
for tBu2bpy than bpy. The emission spectra of complexes 1, 3,
and 4 in EtOH/MeOH (4:1 v/v) glass at 77 K are shown in
Figure 2. Figure 3 shows the emission spectra of complexes 1
and 5 in 77 K glass. The observation of vibronic structures on
the blue side of the emission band with vibrational progressional
spacings of ca. 1570 cm-1 in 77 K glass is attributed to the
mixing into the broad featureless LLCT emission of a high-
energy intraligand IL (diimine) emission, which becomes
prominent in 77 K glass. The lack of a trend in the solid-state
emission energies, which may result from site heterogeneity
commonly observed in the solid states, is understandable.
Dynamic 1H NMR Studies. The 1H NMR spectra of
complexes 1-5 at 298 K show very complicated pattern, with
the aromatic protons on the thiophenolate ligands showing not
a simple doublet of doublets (AA′XX′) pattern that is typical
of 1,4-disubstituted benzenes. Instead four sets of the thiophe-
nolate protons were observed, suggestive of a fluxional behavior
of the system. To provide further insight into such fluxional
behavior, a variable-temperature 1H NMR study on [(bpy)Zn2-
(SC6H4-Cl-p)(µ-SC6H4-Cl-p)(µ-OAc)2] (3) in CDCl3 in the
temperature range of 223-313 K was undertaken. The spectra
recorded at low temperatures indicated that the two thio-
At the same time, it was interesting to note that, apart from
the major species, minor species also occurred when the
(12) (a) DNMR lineform simulation, http://iris.chem.uni-potsdam.de/cgi-
bin/perl_prog/lineform.pg. (b) Oki, M. Application of Dynamic NMR
Spectroscopy to Organic Chemistry; VCH: Deerfield Beach, FL, 1985.
(c) Friebolin, H. Basic One- and Two-Dimensional NMR Spectroscopy;
VCH: Germany, 1991; p 271.
(13) Bochmann, M.; Bwembya, G.; Grinter, R.; Lu, J.; Webb, K. J.;
Williamson, D. J. Inorg. Chem. 1993, 32, 532.
(14) Quartararo, J.; Mignard, S.; Kasztelan, S. J. Catal. 2000, 192, 307.

Luminescent Zn(II) Chalcogenolate Complexes
Inorganic Chemistry, Vol. 39, No. 25, 2000 5745
Figure 5. Two-dimensional 1H NMR spectra of 3 in CD2Cl2 at 223 K
1
with the normal H NMR on top of the spectra. The boxes in COSY
highlighted the cross-peaks of vicinal aryl protons within the same
species. The cross-peaks inside the boxes in EXSY indicated the
exchange of various different conformations of 3 in solution at low
temperature (the major and the minor species).
Scheme 1
Figure 6. Cyclic voltammograms showing (a) the oxidation (scan rate,
100 mV s-1) and (b) reduction (scan rate, 100 mV s-1) of [(bpy)Zn2-
(SC6H5)(µ-SC6H5)(µ-OAc)2] (1) in dichloromethane (0.1 mol dm-3
nBu4NPF6).
enced by these protons when the complex was frozen. On
the contrary, the acetate protons appeared as a sharp singlet at
313 K.
To further understand and establish the presence of various
conformations of 3 at low temperature, signal assignment in
1
the two-dimensional H NMR spectra was achieved by means
of 2D COSY and EXSY experiments12b-c,16 at 223 K in CD2-
Cl2 as shown in Figure 5. In the 2D COSY spectrum, coupling
between vicinal aryl protons among the same species was
observed, while in the 2D EXSY spectrum, the pattern of the
cross-peaks observed that was not found in COSY spectrum is
characteristic for the mechanism of the exchange process of
thiolate ligands and possibly the different orientation of the lone
pair electrons on neighboring sulfur atom15 which further
supported the presence of various different conformations of 3
in solution at low temperature. The various possible conforma-
tions of the complex were proposed and shown in Scheme 1.
Electrochemical Properties. The electrochemical properties
of complexes 1-5 have been investigated by cyclic voltam-
metry. The electrochemical data for 1-5 in dichloromethane
are summarized in Table 3. Figure 6 depicts the cyclic
voltammograms of 1. The cyclic voltammograms of 1-5 display
two irreversible oxidation waves (+1.30 to +1.99 V vs SCE)
and one quasi-reversible reduction couple (-1.52 to -1.62 V
vs SCE). In view of the redox inactive nature of Zn(II) and the
observation that the oxidation waves appear to vary with the
nature of the thiophenolate ligands, the two oxidation waves
are tentatively assigned as the thiophenolate ligand-centered
oxidations. It is likely that the less positive oxidation wave of
the two at +1.30 to +1.7 V vs SCE is due to the oxidation at
Table 3. Electrochemical Data for Complexes 1-5 in
Dichloromethane (0.1 mol dm-3 nBu4NPF6)
oxidationa,b
reductiona,c
complex
Epa/V vs SCE
E1/2/V vs SCE
1
2
3
4
5
+1.67, +1.92
+1.30, +1.64
+1.66, +1.85
+1.44, +1.73
+1.72, +1.99
-1.52
-1.53
-1.52
-1.55
-1.62
a Glassy carbon electrode; ∆Ep of Fc+/Fc ranges from 75 to 80 mV;
scan rate, 100 mVs-1
irreversible oxidation wave. E1/2 ) (Epa + Epc)/2 where Epa and Epc
are the anodic and cathodic peak potentials of the quasi-reversible
reduction couple, respectively.
.
b Epa refers to the anodic peak potential of the
c
temperature was lowered. These sets of thiophenolate and
bipyridyl protons in the minor species probably are a result of
the inability of the lone pairs on the sulfur atoms to flip freely15
due to a restricted rotation about the Zn-S bond at low
temperatures13 and, therefore, resulted in various conformations,
each experiencing a new environment of its own. The acetate
proton signals also became broad as the temperature is lowered,
most probably attributed to the different environments experi-
(15) (a) Knotter, D. M.; Blasse, G.; van Vliet, J. P. M.; van Koten, G.
Inorg. Chem. 1992, 31, 2196. (b) Iwasaki, F.; Yoshida, S.; Kakuma,
S.; Watanabe, T.; Yasui, M. J. Mol. Sci. 1995, 352, 203.
(16) Gu¨nther, H. NMR Spectroscopy: Basic Principles, Concepts, and
Applications in Chemistry, 2nd ed.; John Wiley & Sons: Germany,
1994.

5746 Inorganic Chemistry, Vol. 39, No. 25, 2000
Yam et al.
the terminal thiophenolate ligand, while the more positive one
at ca. +1.6 to +2.0 V is that at the less electron rich bridging
thiophenolate ligand. The least positive potential for the
oxidation of 4 (+1.44 and +1.73 V vs SCE) than that of 1, 2,
and 3 is in accordance with the presence of the more electron
rich methoxy group substituted on the thiophenolate, resulting
in an increased ease of oxidation on the thiophenolate ligand.
The potentials for the oxidation of 1 and 5 are similar as both
complexes bear the same thiophenolate ligand. Similar to the
observation in UV/vis and emission data, the potentials for the
oxidation of 2 are unexpectedly found to be much less positive
relative to 1, suggestive of a domination of the resonance effect
of lone pairs on F atom donating to the thiophenolate over the
negative indicative effect of F, giving rise to an anomalous
increase in the ease of oxidation of 4-fluorothiophenolate in 2
relative to that of 1. On the other hand, the quasi-reversible
reduction couple is likely to originate from a diimine ligand-
centered reduction, as such potentials are very similar for
complexes 1-4, which have the same bpy ligand. The more
negative reduction potential observed in 5 (-1.62 V vs SCE)
than that of 1 (-1.52 V vs SCE) is in agreement with the
t
reduced ease of reduction of Bu2bpy as a result of its higher
π* orbital energy. The observation of thiophenolate-centered
oxidation and diimine-centered reduction processes further
establishes the LLCT nature of the lowest energy electronic
transition.
Acknowledgment. V.W.-W.Y. acknowledges financial sup-
port from the Research Grants Council and The University of
Hong Kong. Y.-L.P. acknowledges the receipt of a postgraduate
studentship, administered by The University of Hong Kong.
Helpful discussions with Dr. H. Z. Sun on the NMR measure-
ments are gratefully acknowledged.
Supporting Information Available: Tables giving atomic coor-
dinates, anisotropic displacement parameters, bond lengths and bond
angles for complex 3. A CIF file. This material is available free of
charge via the Internet at http://pubs.acs.org.
IC000688G