Characterization and electronic structures of five members of the electron transfer series [Re(benzene-1,2-dithiolato)3]z(z= 1 +, 0,1 -, 2-, 3-): A spectroscopic and density functional theoretical study

Article abstract of DOI:10.1021/ic9010532

The reaction of ReCl₅ with 3 equiv of a benzene-1,2-dithiolate derivative in CH₃CN produced, after the addition of [C ₈H₁₆N]Br ([C₈H₁₆N]⁺ is 5-azonia-spiro[4,4]nonane), brownish-

Full text of DOI:10.1021/ic9010532

10926 Inorg. Chem. 2009, 48, 10926–10941
DOI: 10.1021/ic9010532
Characterization and Electronic Structures of Five Members of the Electron
Transfer Series [Re(benzene-1,2-dithiolato)3]z (z = 1+, 0, 1-, 2-, 3-): A Spectroscopic
and Density Functional Theoretical Study
::
,†
†
†
†
‡
ꢀ
Stephen Sproules,* Flavio Luiz Benedito, Eckhard Bill, Thomas Weyhermuller, Serena DeBeer George, and
Karl Wieghardt*,†
::
::
†Max-Planck-Institut fur Bioanorganische Chemie, Stiftstrasse 34-36, D-45470 Mulheim an der Ruhr,
Germany, and ‡Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca,
New York 14853
Received June 1, 2009
The reaction of ReCl5 with 3 equiv of a benzene-1,2-dithiolate derivative in CH3CN produced, after the addition of
[C8H16N]Br ([C8H16N]+ is 5-azonia-spiro[4,4]nonane), brownish-green crystals of [C8H16N][Re(tms)3] (1c) and
[C8H16N][Re(Cl2-bdt)3] (2c), where (tms)2- represents 3,6-bis(trimethylsilyl)benzene-1,2-dithiolate and (Cl2-bdt)2-
is 3,6-dichlorobenzene-1,2-dithiolate. Chemical reduction of [Re(bdt)3] (3b) with n-butyllithium in the presence of
PPh4Br yielded [PPh4][Re(bdt)3] (3c), where (bdt)2- is benzene-1,2-dithiolate. The three monoanionic complexes
possess a diamagnetic ground state (Re(V), d2, S = 0). The crystal structures of 1c 2CH3CN and 2c C3H6O have
3
3
been determined by X-ray crystallography. The electrochemistry establishes that the complexes are members of
electron transfer series involving a monocation [ReV(L•)2(L)]+ (S = 0(?)), a neutral [ReV(L•)(L)2]0 (S = /2), a
1
monoanion [ReV(L)3]1- (S = 0), a dianion [ReIV(L)3]2- (S = 1/2), and a trianion [ReIII(L)3]3- (S = 1(?)). The unique
X-band EPR spectrum of the neutral species clearly describes a diamagnetic Re(V) d2 central ion with the unpaired
electron located in a purely ligand-centered molecular orbital, whereas it is metal-centered in the dianionic form: a
Re(IV) d3 ion with three dithiolate(2-) ligands. S K-edge and Re L-edge X-ray absorption spectroscopy confirms
these assignments and furthermore shows that the monoanion has a Re(V) central ion with three dianionic ligands.
The geometrical and electronic structures of all members of the electron transfer series have been calculated by
density functional theoretical methods, and the S K-pre-edge spectra have been simulated and assigned using a time-
dependent DFT protocol.
Introduction
authors1,4 showed that each of these neutral complexes is a
member of an electron transfer series which involves a
monocation; the above neutral species; and a mono-, a di-,
and-at times-a trianion, eq 1.
In their classic paper of 1966 Stiefel et al. reported1 the
synthesis and spectroscopic characterization in conjunction
with electro- and magnetochemistry of three neutral tris-
(dithiolene)rhenium complexes, namely, [Re(pdt)3],2
[Re(tdt)3], and [Re(bdt)3], where the ligands are 1,2-diphen-
yl-1,2-dithiolate(2-), (pdt)2-, and toluene-3,4-dithiolate-
(2-), (tdt)2-, and (bdt)2- is the unsubstituted benzene-1,2-
dithiolate(2-). [Re(pdt)3] has been characterized by single
crystal X-ray crystallography to be the first authenticated
molecular example of a trigonal prismatic coordination
compound in the solid state and solution.3 At the time, these
1
The neutral complexes possess an S = /2 ground state,
and their X-band electron paramagnetic resonance (EPR)
spectra have been reported,1,5 though controversially in-
terpreted. Porte and Al-Mowali5 have concluded from the
observed g tensor anisotropy that a sulfur-centered radical
(L•)- is present rather than a metal-centered paramagnet
(Re(VI), d1).
*To whom correspondence should be addressed. E-mail: sproules@
mpi-muelheim.mpg.de (S.S.), wieghardt@mpi-muelheim.mpg.de (K.W.).
(1) Stiefel, E. I.; Eisenberg, R.; Rosenberg, R. C.; Gray, H. B. J. Am.
Chem. Soc. 1966, 88, 2956.
(2) Schrauzer, G. N.; Mayweg, V. P. J. Am. Chem. Soc. 1966, 88, 3235.
(3) (a) Eisenberg, R.; Ibers, J. A. J. Am. Chem. Soc. 1965, 87, 3776. (b)
Eisenberg, R.; Ibers, J. A. Inorg. Chem. 1966, 5, 411. (c) Eisenberg, R.;
Brennessel, W. W. Acta Crystallogr. 2006, C62, m464.
(4) Wharton, E. J.; McCleverty, J. A. J. Chem. Soc. A 1969, 2258.
(5) Al-Mowali, A. H.; Porte, A. L. J. Chem. Soc., Dalton Trans. 1975,
250.
r
pubs.acs.org/IC
Published on Web 10/15/2009
2009 American Chemical Society

Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 10927
Figure 1. Qualitative MO schemes for trigonal prismatic [Re(pdt)3]0 as proposed by (A) Gray et al. in ref 1 with a 3a1 SOMO (A), by Al-Mowali
0
and Porte in ref 5 with a 2a2 SOMO (B), and Schrauzer and Mayweg in ref 2 with a 5e0 SOMO (C). Metal d orbitals are depicted in blue; 2a2 MO
in red.
0
0
Surprisingly few charged members of the above electron
transfer series have been isolated, and none has been struc-
turally characterized to date. Gray et al.6 and McCleverty
et al.7 describe the paramagnetic (S = 1/2) dianion in [As-
Ph4]2[Re(mnt)3] where (mnt)2- represents maleonitriledi-
thiolate(2-). Despite its S = 1/2 ground state, it did not
provide an EPR spectrum at room temperature. Only one
isolated monoanionic species has been reported in [NEt4]-
[Re(Cl4-bdt)3], where (Cl4-bdt)2- represents tetrachloroben-
zene-1,2-dithiolate(2-).4 The complex is diamagnetic; its
structure is not known. No trianionic [Re(L)3]3- species
has been isolated to date. Therefore, with the notable excep-
tion of the neutral complexes,1,2,5 the molecular and electro-
nic structures of the electrochemical series comprising the
monocationic and those of the mono-, di-, and trianionic
species are not adequately described to date.
[ReIV(L•)2(L)].8 Thus, the (pdt)2- ligands would show some
degree of oxidation (two valence holes in the S 3p orbitals).
3. Finally, Al-Mowali and Porte5 suggested, in 1975, the
MO scheme shown in Figure 1B, where the unpaired electron
resides in a ligand-centered orbital (ground state configura-
2
4
1
0
0
tion (3a1 ) (4e0) (2a2 ) ). This may be described as
[ReV(L•)(L)2] with a single valence hole in the ligands.8 The
central rhenium(V) ion possesses a diamagnetic ground state
configuration (dz2)2; the unpaired electron resides in the 2a2
0
orbital.
In order to resolve this question regarding the correct
description of these neutral species experimentally, we have
synthesized new monoanionic complexes containing the
monoanion [Re(tms)3]1- in 1c, [Re(Cl2-bdt)3]1- in 2c, and
[Re(bdt)3]1- in 3c. The neutral complex 3b has been prepared
as described in ref 1. Scheme 1 shows the complexes, ligands,
and their abbreviations, which are as follows: a = mono-
cation, b = neutral, c = monoanion, d = dianion, and
e=trianion.
We have measured the S K-edge X-ray absorption spectra
(XAS) of these complexes and their corresponding analogues
from the respective electron transfer series. This spectroscopy
has been developed by the Solomon group9 and has in recent
years been successfully applied to detect π radical character
in S,S0-coordinated dithiolene ligands.10-15 We have also
For the above neutral species, early calculations have
resulted in three different proposals for the electronic struc-
ture of these trigonal prismatic compounds:
1. Gray et al.1 proposed the molecular orbital (MO)
scheme shown in Figure 1A where the unpaired electron
0
resides in a predominantly metal-centered dz2 orbital (3a1 ;
4
2
1
ground state: (4e0) (2a2 ) (3a1 ) ), which can be depicted as
trigonal prismatic [ReVI(L)3] containing a central Re(VI) ion
(d1) and three closed-shell dianionic ligands.
0
0
2. Schrauzer and Mayweg,2 on the other hand, concluded
2
4
1
that (3a1 ) (4e0) (5e0) is the ground state (Figure 1C), which
would encompass a filled dz2 metal orbital and a half-
filled dx2-y2/dxy orbital (d3 electron configuration) as in
0
(9) Glaser, T.; Hedman, B.; Hodgson, K. O.; Solomon, E. I. Acc. Chem.
Res. 2000, 33, 859.
(10) (a) Sarangi, R.; DeBeer George, S.; Jackson Rudd, D.; Szilagyi,
R. K.; Ribas, X.; Rovira, C.; Almeida, M.; Hodgson, K. O.; Hedman, B.;
Solomon, E. I. J. Am. Chem. Soc. 2007, 129, 2316. (b) Szilagyi, R. K.; Lim, B.
S.; Glaser, T.; Holm, R. H.; Hedman, B.; Hodgson, K. O.; Solomon, E. I. J. Am.
Chem. Soc. 2003, 125, 9158.
(6) Stiefel, E. I.; Bennett, L. E.; Dori, Z.; Crawford, T. H.; Simo, C.; Gray,
H. B. Inorg. Chem. 1970, 9, 281.
(7) Connelly, N. G.; Jones, C. J.; McCleverty, J. A. J. Chem. Soc. A 1971,
712.
(11) Tenderholt, A. L.; Szilagyi, R. K.; Holm, R. H.; Hodgson, K. O.;
Hedman, B.; Solomon, E. I. Inorg. Chem. 2008, 47, 6382.
(8) It is most important to note that our use of the “L•” notation to
represent a “ligand radical” or “ligand hole” is not meant to imply that the
radical or hole is localized on one ligand. For analogous bis(dithiolene)
complexes and octahedral complexes of Cr (ref 41), it is highly possible that
the radical is localized, however, in a trigonal prismatic ligand field, the three
dithiolene ligands together form one redox center, and that the radical or
hole is shared equally amongst all three dithiolene ligands. Although the
ligand system is better represented as [(L)3]5-, we continue to use our “L•”
notation because it simplifies both electron counting and complex formula-
tion.
::
(12) Kapre, R. R.; Bothe, E.; Weyhermuller, T.; DeBeer George, S.;
Muresan, N.; Wieghardt, K. Inorg. Chem. 2007, 46, 7827.
::
(13) Kapre, R. R.; Bothe, E.; Weyhermuller, T.; DeBeer George, S.;
Wieghardt, K. Inorg. Chem. 2007, 46, 5642.
::
(14) (a) Kapre, R. R.; Ray, K.; Sylvestre, I.; Weyhermuller, T.; DeBeer
George, S.; Neese, F.; Wieghardt, K. Inorg. Chem. 2006, 45, 3499. (b) Ray, K.;
DeBeer George, S.; Solomon, E. I.; Wieghardt, K.; Neese, F. Chem.;Eur. J.
2007, 13, 2783.
(15) Pap, J. S.; Benedito, F. L.; Bothe, E.; Bill, E.; DeBeer George, S.;
::
Weyhermuller, T.; Wieghardt, K. Inorg. Chem. 2007, 46, 4187.

10928 Inorganic Chemistry, Vol. 48, No. 23, 2009
Scheme 1. Ligands and Complexes
Sproules et al.
Table 1. Crystallographic Data for 1c 2CH3CN and 2c C3H6O
3
3
1c 2CH3CN
3
2c C3H6O
3
chem. formula
fw
space group
C48H82N3Si6S6Re
1248.27
C29H28NOS6Cl6Re
997.78
Cmca, No. 64
17.6026(5)
22.7323(5)
31.6701(8)
90
12672.7(6)
8
100(2)
P21/n, No. 14
12.9802(3)
13.9055(3)
20.4137(5)
107.925(3)
3505.74(14)
4
100(2)
1.890
91435/65.00
12678/11018
399/0
˚
a, A
˚
b, A
˚
c, A
β, deg
˚
V, A
Z
T, K
F calcd, g cm-3
reflns collected/2Θmax
unique reflns/I > 2σ(I)
No. of params/restr.
1.309
80046/60.00
9464/8441
326/6
0.71073/22.61
0.0736/1.347
0.1674
-1
˚
λ, A/μ(KR), cm
0.71073/43.09
0.0294/1.041
0.0647
R1a/goodness of fitb
wR2c (I > 2σ (I))
residual density, eA
-3
˚
+2.60/-5.60
+1.20/-2.17
measured and, for the first time, interpreted the X-band EPR
spectra of the neutral and dianionic species, both with S = 1/2
ground states. Finally, we have calculated the geometry and
electronic structures of the five members of the electron
transfer series and explored the stability of a trigonal pris-
matic structure over an octahedral one.
P
P
a Observation criterion: I > 2σ(I). R1 =
Fo| - |Fc
/
|Fo|. b GoF=
P
P
P
[
w(Fo2 - Fc2)2]/(n - p)]1/2
.
c wR2 = [ [w(Fo2 - Fc2)2]/ [w(Fo2)2]]1/2
,
where w = 1/σ2(Fo2) + (aP)2 + bP, P = (Fo2 + 2Fc2)/3.
in vacuo. Yield: 114 mg (73%). Anal. Calcd for C42H32PS6Re:
C, 53.31; H, 3.41. Found: C, 52.71; H, 3.42.
X-Ray Crystallographic Data Collection and Refinement of the
Structures. Single crystals of compounds 1c 2CH3CN and
3
Experimental Section
2c C3H6O were coated with perfluoropolyether, picked up with
3
Syntheses of Complexes. All air-sensitive materials were
manipulated using standard Schlenk techniques or a glovebox.
The neutral species [Re(bdt)3] (3b) has been prepared as
described in ref 1, using chloroform in place of carbon tetra-
chloride, and the ligand 3,6-bis(trimethylsilyl)benzene-1,2-
dithiol has been synthesized as described in ref 15. The salt
5-azonia-spiro[4,4]nonane bromide ([C8H16N]Br) was prepared
according to the method of Schmidbaur et al.16 The ligand
3,6-dichlorobenzene-1,2-dithiol was commercially obtained and
used without further purification.
nylon loops, and immediately mounted in the nitrogen cold
stream of a Bruker-Nonius Kappa CCD diffractometer
equipped with a Mo-target rotating-anode X-ray source (Mo
˚
KR radiation, λ=0.71073 A) and a graphite monochromator.
Final cell constants were obtained from least-squares fits of all
measured reflections. Intensity data were corrected for absorp-
tion using intensities of redundant reflections with the program
SADABS.17 The structures were readily solved by Patterson
methods and subsequent difference Fourier techniques. The
Siemens ShelXTL18 software package was used for solution
and artwork of the structures; ShelXL9719 was used for the
refinement. All non-hydrogen atoms were anisotropically re-
fined, and hydrogen atoms were placed at calculated positions
and refined as riding atoms with isotropic displacement para-
meters. Crystallographic data of the compounds are listed
in Table 1.
[C8H16N][ReV(tms)3] 2CH3CN (1c). The dipotassium salt,
3
K2(tms), was produced by treating a suspension of 3,6-bis-
(trimethylsilyl)benzene-1,2-dithiol (100 mg; 0.35 mmol) in 4
mL of dry acetonitrile with potassium tert-butoxide (KOtBu;
76 mg; 0.68 mmol) and stirring (30 min) at ambient temperature.
To the resulting yellow solution was added ReCl5 (33 mg;
0.09 mmol) and [C8H16N]Br ((C8H16N)+ is 5-azonia-spiro-
[4,4]nonane; 20 mg; 0.18 mmol). After stirring for 60 min at
20 °C, a brown solution was obtained which was filtered through
Celite and exposed to the air. Brownish-green crystals of 1c were
grown from concentrated CH3CN solutions. Yield: 57 mg
(55%). Anal. Calcd for C48H82N3Si6S6Re: C, 49.39; H, 7.03;
N, 3.60. Found: C, 49.40; H, 7.06; N, 3.56.
Several crystals and crystal fragments of 1c were examined on
the diffractometer. The best specimen was used for data collec-
tion, but the quality of the resulting data set was found to be
quite low. This is due to an unresolved nonmerohedral twinning
problem, which is further reflected in a high mosaicity, elevated
standard deviations, moderate intensities of reflections which
should be systematically absent, and an unusually high second
weighting parameter. Careful attempts to refine the structure in
lower symmetry space groups did not yield better results. Two
acetonitrile solvent molecules are located on crystallographic
mirror planes and are not well-defined. N-C and C-C dis-
tances were fixed with the DFIX instruction of ShelXL97.
X-Ray Absorption Spectroscopy. All data were measured at
the Stanford Synchrotron Radiation Lightsource under ring
conditions of 3.0 GeV and 60-100 mA. S K-edge data were
measured using the 54-pole wiggler beamline 6-2 in a high-
magnetic field mode of 10 kG with a Ni-coated harmonic
rejection mirror and a fully tuned Si(111) double-crystal mono-
chromator. Details of the optimization of this setup for low
[C8H16N][ReV(Cl2-bdt)3] C3H6O (2c). The complex was
3
synthesized as described above for 1c using 3,6-dichloroben-
zene-1,2-dithiol as a starting material. Recrystallization of the
crude product from acetone afforded crystals of 2c. Yield: 80 mg
(53%). Anal. Calcd for C29H28NOS6Cl6Re: C, 37.07; H, 2.98; N,
1.49. Found: C, 37.18; H, 2.87; N, 1.54.
[PPh4][ReV(bdt)3] (3c). To a tetrahydrofuran (10 mL) solution
of [Re(bdt)3] (3b) (100 mg; 0.165 mmol) was added n-butyl-
lithium (1.6 M in hexanes; 103 μL; 0.165 mmol) under anaerobic
conditions (argon), and the solution was stirred for 10 min. The
resulting dark brown solution was filtered and added to a
solution of methanol (25 mL) containing [PPh4]Br (70 mg;
0.165 mmol). A brown crystalline precipitate formed which
was collected by filtration, washed with methanol, and dried
:: ::
(17) Sheldrick, G. M. SADABS, version 2006/1; Universitat Gottingen:
::
Gottingen, Germany, 2006.
(18) ShelXTL 6.14; Bruker AXS Inc.: Madison, WI, 2003.
::
::
::
(16) Schmidbaur, H.; Wohlleben, A.; Schubert, U.; Frank, A.; Huttner,
(19) Sheldrick, G. M. ShelXL97; Universitat Gottingen: Gottingen,
G. Chem. Ber. 1977, 110, 145.
Germany, 1997.

Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 10929
energy have been previously described.20 All samples were
measured at room temperature as fluorescence spectra using a
Lytle detector. Samples were ground finely and dispersed as
thinly as possible on Mylar tape to minimize the possibility
of fluorescence saturation effects or run in dichloromethane
solutions of ∼2 mM concentration. Data represent 2-3 scan
averages. All samples were monitored for photoreduction
throughout the course of data collection. The S K-edge energy
optimizations were carried out using the B3LYP functional.24
A segmented all-electron relativistically contracted basis set of
triple-ζ-quality (TZVP) was used for rhenium.25 A scalar rela-
tivistic correction was applied using the zeroth-order regular
approximation (ZORA) method.26 In the context of ZORA, a
one-center approximation has been shown to introduce only
minor errors to the final geometries.26b An all-electron polarized
triple-ζ-quality (TZVP) basis set of Ahlrich’s group was used for
sulfur.27 The remaining atoms were described by slightly smaller
polarized split-valence SV(P) basis sets that are double-ζ-quality
in the valence region and contain a polarizing set of d functions
for the non-hydrogen atoms.27c Auxiliary basis sets for all
complexes used to expand the electron density in the calcula-
tions were chosen to match the orbital basis. The self-consistent
field calculations were tightly converged (1 ꢀ 10-8 Eh in energy,
1 ꢀ 10-7 Eh in the density change, and 1 ꢀ 10-7 in the maximum
element of the DIIS28 error vector). The geometry search for all
complexes was carried out in redundant internal coordinates
without imposing geometry constraints. The coordinates for
the sequentially (∼3.5°) twisted molecules for the [Re(bdt)3]z
(z = 1+, 0, 1-, 2-) were generated with the Z-matrix editor in
Molden.29 A broken symmetry BS(1,1) state was calculated
for [Re(bdt)3]+ using the method of Noodleman and co-
workers.12,30 We adopted the following notation: The given
system was divided into two fragments. The notation BS(m,n)
refers then to a broken symmetry state with m unpaired R-spin
electrons essentially on fragment 1 and n unpaired β-spin
electrons localized on fragment 2. In most cases, fragments
1 and 2 correspond to the metal and the ligands, respectively.
In this notation, the standard high-spin, open-shell solution is
written as BS(m + n,0). The BS(m,n) notation refers to
the initial guess of the wave function. The variational process
does, however, have the freedom to converge to a solution of
the form BS(m - n,0) in which effectively the n β-spin
electrons pair up with n < m R-spin electrons on the
was calibrated using the S K-edge spectra of Na2S2O3 5H2O,
3
run at intervals between sample scans. The maximum of the first
pre-edge feature in the spectrum was fixed at 2472.02 eV. A step
size of 0.08 eV was used over the edge region. Data were
averaged, and a smooth background was removed from all
spectra by fitting a polynomial to the pre-edge region and
subtracting this polynomial from the entire spectrum. Normal-
ization of the data was accomplished by fitting a flattened
polynomial or straight line to the postedge region and normal-
izing the postedge to 1.0.
Re L-edge XAS data were measured using the focused 16-pole
wiggler beamline 9-3. A Si(220) monochromator was utilized for
energy selection, and a harmonic rejection mirror was present to
minimize higher harmonic components in the X-ray beam. The
solid samples were prepared as a dilute matrix in boron nitride,
pressed into a pellet, and sealed between 38 μm Kapton tape
windows in a 1 mm aluminum spacer. The sample of 3d was
prepared by controlled potential coulometry at -25 °C in a
dichloromethane solution containing 0.10 M [N(n-Bu)4]PF6.
Samples were maintained at 10 K during data collection by
using an Oxford Instruments CF1208 continuous flow liquid
helium cryostat. Data were measured in the transmission mode.
Internal energy calibrations were performed by simultaneous
measurement of the Re reference foil placed between a second
and third ionization chamber with inflection points assigned as
12 527, 11 959, and 10 535 eV for the L1-, L2-, and L3-edges,
respectively. Data represent 2-4 scan averages and were pro-
cessed by fitting a second-order polynomial to the pre-edge
region and subtracting this background from the entire spec-
trum. A three-region cubic spline was used to model the smooth
background above the edge. The data were normalized by
subtracting the spline and normalizing the postedge to 1.0. Fits
to the pre-edges modeled by pseudo-Voigt lines were carried out
using the program EDG_FIT21 with a fixed 1:1 ratio of Lor-
entzian to Gaussian contributions.
partner fragment. Such a solution is then a standard MS
=
(m - n)/2 spin-unrestricted Kohn-Sham solution. As ex-
plained elsewhere,31 the nature of the solution is investigated
from the corresponding orbital transformation, which, from
the corresponding orbital overlaps, displays whether the
system should be described as a spin-coupled or a closed-shell
solution. Single point energies were run using the same
conditions detailed for the optimization. Canonical orbitals
and density plots were constructed using the program
Molekel.32
Time-dependent (TD-DFT) calculations of the sulfur K-pre-
edges using the BP86 functional were conducted as previously
described.15,33 A single point, spin-unrestricted ground state
DFT calculation, starting from the optimized coordinates
and using the same basis sets described above, was performed.
Other Physical Measurements. Cyclic voltammograms (CVs)
and coulometric measurements were performed with an EG&G
potentiostat/galvanostat. Electronic absorption spectra from
the spectroelectrochemical measurements were obtained on a
Hewlett-Packard 8453 diode-array spectrophotometer (range
200-1100 nm). X-band EPR spectra were recorded with a
Bruker ESP 300 spectrometer. The spectra were simulated on
the basis of a spin-Hamiltonian description of the electronic
ground state with S = 1/2:
(25) Pantazis, D. A.; Chen, X.-Y.; Landis, C. R.; Neese, F. J. Chem.
Theory Comput. 2008, 4, 908.
!^ !^
!^ !^ !^
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1996, 105, 6505. (b) van Lenthe, E.; Faas, S.; Snijders, J. G. Chem. Phys. Lett.
2000, 328, 107. (c) van Lenthe, E.; van der Avoird, A.; Wormer, P. E. S. J. Chem.
Phys. 1998, 98, 5648.
ꢁ
H ¼ μBBB g S þ S A I þ I P I
3
3
3
3
3
3
by using the simulation package XSOPHE.22
Calculations. All calculations in this work were performed
(27) (a) Ahlrichs, R.; May, K. Phys. Chem. Chem. Phys. 2000, 2, 943. (b)
::
::
with the electronic structure program ORCA.23 Geometry
Schafer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992, 97, 2571. (c) Schafer,
A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1994, 100, 5829.
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Hodgson, K. O. J. Am. Chem. Soc. 1988, 110, 3798.
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Germany, 2008.
(24) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C. T.; Yang,
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361, 965.

10930 Inorganic Chemistry, Vol. 48, No. 23, 2009
Sproules et al.
The six sulfur 1s orbitals were localized using the Pipek-Mezey
criteria,34 and the TD-DFT equations were solved35 individu-
ally for each sulfur atom, excluding all but excitations originat-
ing from the sulfur 1s orbital.14,33,36 Since the absolute
calculated transition energies show large errors due to the
shortcomings in the treatment of relativity and DFT potentials
in the core regions,33,37 it was established that a constant shift of
+61.45 eV is required for this regime of basis sets and applied to
the transition energies. Plots were obtained using a line broad-
ening of 0.8 eV.
Table 2. Redox Potentials (V) of Complexes versus Fc+/Fc
complexa
E1/2
E1/2
E1/2
1 b 2 c
E1/2
3 d
4 e
1c
2c
3b
+0.27
-0.30
+0.45
-0.01
+0.33
-1.89
-1.14
-1.43
-1.06
+0.43
+0.96
-2.25
-1.73
[Re(Cl4-bdt)3]1-f
a Recorded in CH2Cl2 solutions containing 0.1 M [N(n-Bu)4]PF6 at
25 °C using a scan rate of 100 mV s-1 b 1+/0 couple. c 0/1- couple.
.
d 1-/2- couple. e 2-/3- couple. f Ref 4. The original data recorded in
CH2Cl2 were referenced versus an aqueous standard calomel electrode
(SCE) with LiCl; here, they are converted by adding -0.38 V.
Results and Discussion
a. Syntheses. The neutral complex [Re(bdt)3] (3b) has
been obtained from the reaction of ReCl5 with 3 equiv of
the ligand benzene-1,2-dithiol in CHCl3 solutions.1
Correspondingly, the reaction of ReCl5 with 3 equiv of
the ligand dipotassium[3,6-bis(trimethylsilyl)benzene-
1,2-dithiolate]15 in acetonitrile in the presence ofpotassium
tert-butyloxide afforded, upon the addition of 5-azo-
niaspiro[4,4]nonane bromide, [C8H16N]Br,16 brown-green
crystals of [C8H16N][Re(tms)3] 2CH3CN (1c). The
complex [C8H16N][Re(Cl2-bdt)3] C3H6O (2c) has been
3
3
obtained in an analogous fashion using 3,6-dichloro-
benzene-1,2-dithiol as a ligand. Reduction of the neu-
tral complex 3b with 1 equiv of n-butyllithium under
anaerobic conditions in tetrahydrofuran solution yielded,
upon addition of [PPh4]Br, the complex [PPh4][Re-
(bdt)3] (3c).
It has been established by 1H NMR and temperature-
dependent (3-300 K) magnetic susceptibility measure-
ments1 that the neutral complex 3b possesses a paramag-
netic S = 1/2 ground state, whereas the monoanions in 1c,
2c, and 3c are diamagnetic (S = 0). The dianion in 3d
possesses again an S=1/2 ground state. The spin ground
states of the monocations [Re(L)3]+ and of the trianion [Re-
(Cl4-bdt)3]3- 4 are not known, but-as shown below-the
former is most likely diamagnetic (S = 0) whereas the latter
is a [ReIII(Cl4-bdt)3]3- species with an S = 1 ground state.
b. Spectro- and Electrochemistry. Cyclic voltammo-
grams of 1c, 2c, and 3b have been recorded in CH2Cl2
solutions containing 0.10 M [N(n-Bu)4]PF6 at 25 °C. All
potentials are referenced versus the ferrocenium/ferro-
cene (Fc+/Fc) couple (ferrocene internal standard). The
results are summarized in Table 2. Figure 2 displays the
CV of 1c and 3b, where there are three and four reversible
one-electron transfer waves observed, respectively, in the
potential range +0.5 to -2.4 V. From coulometric
measurements at fixed potentials, it has been established
that the monoanion in 1c is one-electron reduced at -1.9
V, affording a dianion but successively, twice one-elec-
tron oxidized at -0.30 V and +0.27 V, generating a
neutral and then a monocationic species.
Figure 2. Cyclic voltammograms of 1c and 3b in CH2Cl2 solution (0.10
;
M [N(n-Bu)4]PF6 supporting electrolyte; 25 °C; scan rate, 100 mV s-1
glassy carbon working electrode). Potentials are referenced versus Fc+/Fc.
potentials (Figure 2). An effect of the aromatic ring
substituent in complexes 1, 2, and 3 is clearly reflected
in the redox potentials E1/22, where the potential of the
0/1- couple increases in the order tms < bdt < Cl2-bdt.
Thus, electron-withdrawing chloro substituents in Cl2-
bdt ligands render its oxidation the most difficult process.
We have been able to generate electrochemically the
monocation (1a), the neutral (1b), the monoanion (1c),
and dianion (1d) forms of the series [Re(tms)3]z and
record their electronic spectra in a CH2Cl2 solution at
-25 °C; they are shown in Figure 3 (top). The electro-
chemically generated species monoanion (3c) and dianion
(3d) of 3b are shown in Figure 3 (bottom). The oxidation
of 3b to a monocationic species (3a) is not reversible by
controlled potential coulometry, as is the formation of the
trianionic species, 3e; hence, no electronic absorption
spectra could be collected for these complexes. The data
for all of the electrochemically isolated species are sum-
marized in Table 3. Electrochemical oxidation of 2c to
form the neutral species, 2b was incomplete, with only
16% of the desired product forming (Figure S2, Support-
ing Information). It is worthwhile to note that the
electronic absorption spectra collected for each of the res-
pective complex charges (1+, 0, 1-, 2-) are very similar,
indicating that the electronic structure for each is inde-
pendent of the ligand substitution.
Similar results have been obtained for 2c, where the
second oxidation wave is not observed in the above
potential range (Figure S1, Supporting Information).
The CV of 3b depicts the entire five-membered electron
transfer series, where four reversible one-electron transfer
4
processes are seen with E1/2 occurring at very negative
(34) Pipek, J.; Mezey, P. G. J. Chem. Phys. 1989, 90, 4916.
(35) Neese, F.; Olbrich, G. Chem. Phys. Lett. 2002, 362, 170.
The electronic spectra of the monocation and of the
neutral species display an extremely intense (ε > 104 M-1
cm-1) charge transfer transition in the range 600-
800 nm. This transition has been observed in previously
::
(36) Milsmann, C.; Patra, G. K.; Bill, E.; Weyhermuller, T.; DeBeer
George, S.; Wieghardt, K. Inorg. Chem. 2009, 48, 7430.
(37) DeBeer George, S.; Petrenko, T.; Neese, F. J. Phys. Chem. A 2008,
112, 12936.

Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 10931
dianions and a rhenium(V) and rhenium(IV) central metal
ion, respectively. The final member of these electron trans-
fer series, [ReIII(bdt)3]3- (3e), is generated at very negative
potentials (-2.25 V) and is only reversible at -25 °C on the
CV time scale. Similarly, the trianions [Re(tdt)3]3- (tdt2-
=
toluene-3,4-dithiolate) and [Re(Cl4-bdt)3]3- are also only
observed electrochemically,1,4 at potentials of -2.34 V40 and
-1.73 V versus Fc+/Fc, respectively. No other spectroscopic
data are available, but DFT calculations (see below) suggest
a [ReIII(bdt)3]3- electronic structure with a central Re(III)
ion (d4, S=1) and three closed-shell dithiolate(2-) anions.
c. Crystal Structures of 1c and 2c. The crystal struc-
tures of 1c and 2c have been determined by X-ray crystal-
lography. Both species consist of well-separated
[C8H16N]+ cations and [Re(tms)3]1- and [Re(Cl2-bdt)3]1-
monoanions, respectively. Figure 4 exhibits the structures
of the monoanions in 1c and 2c. Table 4 summarizes
selected bond distances.
The structure of neutral [Re(pdt)3] is also trigonal
prismatic (Θ=3.8°), and Gray et al.1 have suggested that
[Re(bdt)3] (3b) has a similar structure. From our previous
investigation12,41 of [Cr(L)3]3-, containing an octahedral
CrS6 moiety with a central Cr(III) ion (d3, S = 3/2) and
three closed-shell dithiolato(2-) anions, we propose
that [Re(bdt)3]3- also contains a distorted octahedral
ReS6 polyhedron with a central Re(III) ion (d4,
S = 1) and three closed-shell bdt2- ligands. No structural
data are available for dianionic [Re(bdt)3]2- species, but
a recent X-ray crystal structure determination of
[PPh4]2[Re(mnt)3] (S = 1/2; mnt2- = maleonitriledithio-
late) revealed a structure intermediate between a trigonal
prism and an octahedron (Θ = 38.3°).42 The average fold
angle is again very small.
Figure 3. Electronic spectra of electrochemically generated monocation
(purple), neutral molecules (black), monoanions (red), and dianions (green)
of the series [Re(tms)3]z (z = 1+, 0, 1-, 2-) (top) and the series [Re(bdt)3]z
(z = 0, 1-, 2-) (bottom) in CH2Cl2 (0.10 M [N(n-Bu)4]PF6) at -25 °C.
Table 3. Electronic Spectra of Electrochemically Generated Species in CH2Cl2
Solutions (0.10 M [N(n-Bu)4]PF6) at -25 °C
complex
λ )
max, nm (ε, 104 M-1 cm-1
1a
1b
1c
409(2.45), 454(1.67), 661(4.67), 811(0.27), 905(0.22)
338(2.54), 459(sh, 0.87), 684(3.89), 841(0.23), 1026(0.19)
376(2.54), 434(1.26), 437(1.15), 548(0.87), 599(0.85), 712(1.09),
941(0.24)
d. Electron Paramagnetic Resonance (EPR) Spectros-
copy. Neutral tris(dithiolene)rhenium complexes having a
S = 1/2 ground state have been analyzed by EPR spec-
troscopy and proved to be the key evidence that moti-
vated Al-Mowali and Porte5 to revise the published
orbital manifolds of Gray et al.1 and Schrauzer and
Mayweg.2 Gray et al.’s and Schrauzer and Mayweg’s
electronic structures depicted a Re(VI) d1 and Re(IV) d3
ion, respectively (Figure 1), placing the unpaired spin on
the metal. Porte and Al-Mowali keenly observed that the
frozen solution spectrum of [Re(tdt)3] was drastically
different to that of [ReOCl4],43 containing a Re(VI)
central ion.5 The EPR spectrum of the latter is character-
ized by a pronounced g anisotropy resulting from the
large spin orbit coupling of the third row transition metal.
Additionally, there is an even larger magnetic hyperfine
interaction from the 185Re (37.4% natural abundance)
and 187Re (62.6% natural abundance) nuclei that are
both I=5/2, affording a spectrum that spans 500 mT, as
typified by Re(VI)-nitrido species investigated by Abram
and co-workers.44
1d
377(2.42), 434(1.20), 473(1.15), 548(0.87), 599(0.85), 729(1.01),
941(0.21)
2c
2d
3b
3c
313(5.90), 350(2.07), 411(2.22), 507(0.56), 730(1.81), 921(0.57)
315(5.85), 360(3.30), 429(1.72), 603(0.73), 658(0.84), 737(0.38)
390(2.50), 442(sh, 1.54), 674(3.89), 1062(0.22)
304(3.19), 346(sh, 1.71), 429(2.36), 502(0.86), 742(1.49),
942(0.46)
3d
320(4.28), 393(sh, 2.73), 429(sh, 2.36), 605(0.83), 670(0.90),
765(0.44), 956(0.12)
reported neutral tris(dithiolene)molybdenum com-
plexes,1,2,13,38,39 which have recently been characterized
as [MoV(tbbdt•)(tbbdt)2] (tbbdt = 3,5-di-tert-butylben-
zene-1,2-dithiolate)13 or [MoIV(mdt•)2(mdt)] (mdt=1,2-
dimethyl-1,2-dithiolate) species.11 This band has been
identified as a marker for the presence of an oxidized
dithiolene ligand(s). From the DFT-derived qualitative
MO scheme presented in Figure 12 (vide infra), we
tentatively classify this as the 4e0 f 2a2 transition since
0
it has ∼90% ligand-to-ligand charge transfer (LLCT)
character with only a small amount of metal-to-ligand
charge transfer (MLCT) character resulting from ∼8%
Re 5dx2-y2,xy contribution to the 4e0 orbitals.
This band is absent in the spectra of the mono- and dia-
nionic forms, [ReV(L)3]1- and [ReIV(L)3]2-. This feature
indicates the presence of three closed-shell dithiolate
::
(41) Banerjee, P.; Sproules, S.; Weyhermuller, T.; DeBeer George, S.;
Wieghardt, K. Inorg. Chem. 2009, 48, 5829.
::
(42) Sproules, S.; Weyhermuller, T.; Wieghardt, K. Unpublished results.
(43) Al-Mowali, A. H.; Porte, A. L. J. Chem. Soc., Dalton Trans. 1975, 50.
(44) (a) Abram, U.; Braun, M.; Abram, S.; Kirmse, R.; Voigt, A. J. Chem.
(38) Formitchev, D.; Lim, B. S.; Holm, R. H. Inorg. Chem. 2001, 40, 645.
(39) Lim, B. S.; Donahue, J.; Holm, R. H. Inorg. Chem. 2000, 39, 263.
(40) The potential was determined in N,N0-dimethylformamide and
referenced to Ag/AgCl. We arrived at an empirical conversion factor of
-0.03 V to convert this potential to our scale.
Soc., Dalton Trans. 1998, 231. (b) Abram, U.; Hagenbach, A.; Voigt, A.; Kirmse,
::
R. Z. Anorg. Allg. Chem. 2001, 627, 955. (c) Abram, U.; Schmidt-Brucken, B.;
Hagenbach, A.; Hecht, M.; Kirmse, R.; Voigt, A. Z. Anorg. Allg. Chem. 2003,
629, 838.

10932 Inorganic Chemistry, Vol. 48, No. 23, 2009
Sproules et al.
˚
Table 4. Selected Bond Distances (A) in 1c and 2c
1c
2c
Re(1)-S(1)
Re(1)-S(2)
Re(1)-S(3)
Re(1)-S(4)
Re(1)-S(5)
Re(1)-S(6)
S(1)-C(1)
2.341(2)
2.358(2)
2.330(2)
2.345(2)
2.330(2)
2.345(2)
1.762(9)
1.739(1)
1.742(7)
1.743(7)
1.742(7)
1.743(7)
1.398(1)
1.434(1)
1.387(1)
1.430(1)
1.384(1)
1.429(1)
1.398(9)
1.426(1)
1.386(1)
1.381(1)
1.391(1)
1.414(1)
1.398(1)
1.426(1)
1.386(1)
1.381(1)
1.391(1)
1.414(9)
2.3513(6)
2.3373(6)
2.3279(6)
2.3390(6)
2.3371(6)
2.3476(6)
1.745(3)
1.737(3)
1.738(2)
1.740(2)
1.740(2)
1.735(2)
1.400(4)
1.407(4)
1.382(4)
1.387(5)
1.381(4)
1.403(4)
1.401(3)
1.406(3)
1.377(4)
1.394(4)
1.381(4)
1.402(3)
1.401(3)
1.406(3)
1.377(4)
1.394(4)
1.381(4)
1.402(3)
S(2)-C(2)
S(3)-C(7)
S(4)-C(12)
S(5)-C(13)
S(6)-C(18)
C(1)-C(2)
C(2)-C(3)
C(3)-C(4)
C(4)-C(5)
C(5)-C(6)
C(6)-C(1)
C(7)-C(8)
C(8)-C(9)
C(9)-C(10)
C(10)-C(11)
C(11)-C(12)
C(12)-C(7)
C(13)-C(14)
C14-C(15)
C(15)-C(16)
C(16)-C(17)
C(17)-C(18)
C18-C(13)
central signal at g ∼ 2.013 flanked on either side by two
features that were incorrectly assigned as g tensor com-
ponents.5 Importantly, the lower field feature exhibits a
crossing point such that it cannot arise from the g tensor
alone.45 In fact, these signals stem from the combination
of a small Re superhyperfine coupling of ∼63 MHz
with a very large quadrupole interaction, P = 73 MHz,
where P = [Pzz - (Pxx + Pyy)/2]/3. Nuclear quadru-
pole interactions can result in perturbations of the in-
tensity and spacing of the ΔmI = 0 transitions.46,47 Such
an effect is operative here, with the expected “six-line”
magnetic hyperfine interaction (given the isotropic g
tensor) comprising ΔmI = 0 transitions that are com-
pressed into the two flanking features of the central
resonance. Additionally, we find a small pair of satellite
signals 15 mT away from the central resonance (inset,
Figure 5) that are quadrupole-allowed transitions tradi-
tionally not observed in EPR spectra. These are assigned
Figure 4. Structure of the monoanions in 1c (top) and 2c (bottom).
In contrast, [Re(tdt)3] has a narrow signal spanning
2
4
1
0
0
only 15 mT, and a (3a1 ) (4e0) (2a2 ) electronic con-
figuration was proposed with the unpaired electron
located in a pure ligand orbital. While this is the cor-
rect electronic structure, as reiterated by our DFT
calculations, Porte provided an erroneous simulation
of the experimental spectrum. We have discovered
that this complicated X-band spectrum results from
a strong quadrupole interaction derived from a very
large valence contribution to the electric field gradient
(EFG) in conjunction with the sizable 185,187Re nuclear
quadrupole moments (2.8 and 2.6 ꢀ 10-24 cm2, respec-
tively).
(45) The central line and its nearest satellites cannot be mistaken as the
derivatives of an anisotropic powder spectrum with three separate g values
because the corresponding absorption spectrum (the first integral), being a
broad powder distribution, would have a unique maximum and two distinct
shoulders. That pattern could exhibit only one unique zero crossing point in
the derivative spectrum (at g2), whereas the experimental traces show two
crossing points at 335 and 330 mT (and a third on the right of the center is
blurred by line broadening; see simulation). Furthermore, the low intensities
of the satellites also do not fit to a powder distribution. Moreover, the remote
satellites were not included in the published spectrum by Al-Mowali and
Porte (ref 5).
(46) (a) Liczwek, D. L.; Belford, R. L.; Pilbrow, J. R.; Hyde, J. S. J. Phys.
Chem. 1983, 87, 2509. (b) Connelly, N. G.; Emslie, D. J. H.; Klangsinsirikul, P.;
Rieger, P. H. J. Phys. Chem. A 2002, 106, 12214. (c) Shaw, J. L.; Wolowska, J.;
The frozen glass spectra of 1b, 2b, and 3b displayed in
Figure 5 are almost indistinguishable. The simulation
parameters are listed in Table 5, and given their similarity,
one simulation is shown (in red, top) in Figure 5. They are
identical to the published spectrum of [Re(tdt)3], with a
Collison, D.; Howard, J. A. K.; McInnes, E. J. L.; McMaster, J.; Blake, A. J.;
::
Wilson, C.; Schroder, M. J. Am. Chem. Soc. 2006, 128, 13827.
(47) Huang, D.; Zhang, X.; McInnes, E. J. L.; McMaster, J.; Blake, A. J.;
::
Davies, E. S.; Wolowska, J.; Wilson, C.; Schroder, M. Inorg. Chem. 2008, 47,
9919.

Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 10933
Scheme 2. Twist Angle Θ and Ligand Fold Angles R, R0, and R00
as the (1/2 f (3/2 (ΔmI = 1) transitions since these have
the smallest energy separation at 438 MHz (6P). This very
unique situation where the quadrupole interaction is larger
than the magnetic hyperfine interaction can only arise if the
spin is located on the ligand and the diamagnetic Re ion has
a (dz2)2 electronic configuration. This generates the very
large valence contribution to the EFG producing the
observable quadrupole-allowed transitions in the spec-
trum. Large quadrupole couplings that exceed the mag-
netic hyperfine interaction have only been observed for
neutral square planar bis(dithiolene)gold(III).48 There-
fore, we can definitively affirm the electronic structure of
these neutral tris(dithiolene)rhenium complexes as
[ReV(L•)(L)2]0.8
The doubly reduced dianionic species also has a S = 1/2
ground state; however, there is no recorded EPR spec-
trum for such a complex. McCleverty et al. commented
that [PPh4]2[Re(mnt)3] (mnt=maleonitriledithiolate) did
not yield a signal at room temperature.49 The EPR
spectrum of 3d at 20 K is shown in Figure 6 and is
distinctly different from the spectra of the neutral species
described above. The spectrum spans 450 mT, akin to the
Re(VI) d1 complexes of Abram et al.,44 and clearly depicts
a metal-centered paramagnet.
A very concentrated sample (∼5 mM) was generated by
controlled potential coulometry. A reasonably large
modulation amplitude (1.5 mT) and high power (6.3
mW) were necessary to record the spectrum of 3d at 20
K. The spectrum is characterized by six broad hyperfine
lines, stemming from the magnetic hyperfine interaction
of the 185,187Re I = 5/2 nuclei. There is a sharp signal at
g = 2.003 that results from a small organic impurity,
possibly from the solvent stabilizers, that accounts for
<1% of the total absorption. Quadrupole effects pro-
duce the characteristic uneven splitting of the six hyper-
fine lines, and quadrupole-allowed transitions are seen in
the low-field portion of the spectrum. These features have
only been sparingly observed at X-band frequencies pre-
viously.46 The simulation parameters for 3d listed in
Table 5 show rhombic g and A tensors, and an axial
P=(20, 20, -40) ꢀ 10-4 cm-1 that gives a quadrupole
coupling value of P = 60 MHz. It is important to note
that the A tensor is rotated away from the P tensor (Euler
angles: χ = 82°; ς = 25°; τ=-90°); the g and P tensors are
Figure 5. X-band EPR spectra of neutral S=1/2 complexes 1b, 2b, and3b.
A representative simulation is shown in red (top), and parameters are given in
the text and Table 5. Experimental spectra (black) in descending order: 1b in
CH2Cl2 at 10 K (conditions: frequency, 9.45 GHz; power, 0.2 mW;
modulation, 0.7 mT); 2b in CH2Cl2 at 30 K (conditions: frequency, 9.45
GHz; power, 0.01 mW; modulation, 1.0 mT); and 3b in THF at 77 K
(conditions: frequency, 9.24 GHz; power, 1.0 mW; modulation, 1.0 mT).
The inset shows magnification of the satellite features found in each spec-
trum due to the large quadrupole interaction inherent to this electronic
configuration.
Table 5. EPR Spin Hamiltonian g Tensor, Re Magnetic Hyperfine Tensor (A ꢀ
10-4 cm-1),a and Re Electric Quadrupole Tensor (P ꢀ 10-4 cm-1),b Derived from
Simulation of the Neutral S = 1
/
2 Complexes
gxx
gyy
gzz
Axx Ayy
Azz
Pxx Pyy
Pzz
1b 2.016 2.015 2.014 -31.0 -27.0
2b 2.015 2.015 2.015 -32.0 -25.0
3b 2.014 2.012 2.012 -29.0 -30.0
-4.0 24.2 24.2 -48.4
-6.0 24.2 24.2 -48.4
-4.5 24.3 24.3 -48.6
3dc 1.701 1.680 1.515 -310
-690
-100
20.0 20.0 -40.0
a The sign of the A tensor components is not unique and is assigned as
negative assuming a dominating Fermi-contact contribution. b The sign
of the P tensor components is not known from the simulation, so we
assume the negative component to be the z direction. c A is rotated with
respect to P, with angles χ = 82°, ζ=25°, and τ=-90°.
(48) Kokatam, S.; Ray, K.; Pap, J.; Bill, E.; Geiger, W. E.; LeSuer, R. J.;
Rieger, P. H.; Weyhermuller, T.; Neese, F.; Wieghardt, K. Inorg. Chem.
2007, 46, 1100.
(49) McCleverty, J. A.; Locke, J.; Wharton, E. J.; Gerloch, M. J. Chem.
Soc. A 1968, 816.
::

10934 Inorganic Chemistry, Vol. 48, No. 23, 2009
Sproules et al.
Figure 7. Normalized Re L1-edge XAS data of [Re(bdt)3] (black),
[Re(bdt)3]1- (red), and [Re(bdt)3]2- (green) in 3b, 3c, and 3d.
exists in 3d while the monoanion and neutral species
possess a Re(V) center, in line with the outcome from
the EPR measurements.
We find pre-edge region featureless despite the pre-
sumed trigonal prismatic geometry of 3b. Pre-edge fea-
tures have been observe at the Re L1-edge for tetrahedral
rhenium oxides,51 which are apparently larger than the
symmetry-derived 5d-6p hybrization in our system
(DFT calculations estimate a 5.4% Re 6p mixing into
the 5e0 orbitals). We have observed an intense pre-edge
due to significant 4p mixing into 3d orbitals in V K-edge
spectra52 and calculated Cr K-pre-edge spectra41 where
the pre-edge and near-edge regions of the spectrum are
well-resolved, owing to the short core-hole lifetime for
first row transition metals.
The Re L3- and L2-edge spectra for 1c, 2c, 3b, 3c, and 3d
are overlaid in Figure 8 and Figure S3 (Supporting
Information), respectively, and contrast the absorption
at the L1-edge. Re L3,2-edges result from the dipole-
allowed 2p f 5d transitions, where spin-orbit coupling
of the 2p hole generates 2P3/2 (L3-edge) and 2P1/2 (L2-edge)
states, separated by ∼1424 eV. The energy of these
transitions depends on the electronic structure of the
metal site, and changes in the oxidation state, coordina-
tion number, and geometry will affect the energy of both
the 2p and 5d orbitals. However, the 2p and 5d spin-orbit
and the 2p core-hole-5d multiplet interactions may affect
the intensities and energy distributions of these transi-
tions and must be considered in any analysis of these
edges.53 Each compound exhibits a single intense absorp-
tion peak (white line); the transition energies are pre-
sented in Table 6. In both edges, the data display no
obvious trend that reflects a different metal oxidation
state, instead showing a ligand field splitting dominance
of the edge energy.
Figure 6. X-band EPR spectrum of 3d in CH2Cl2 at 20 K. The simula-
tion is shown in red with the experimental spectrum in black (conditions:
frequency, 9.41 GHz; power, 6.3 mW; modulation, 1.5 mT).
coaxial. We have been unable to reproduce a number of
the small quadrupole-allowed transitions at low field in
the experiment spectrum, and these could result from a
Gaussian distribution of the magnetic hyperfine with
respect to the P tensor, such that we would need to
perform single-crystal measurements to resolve such a
complicated tensor orientation. However, it is clear
from this spectrum that we can unambiguously assign a
[ReIV(L)3]2- electronic structure to dianionic tris-
(dithiolene)rhenium complexes, where the Re(IV) ion
possesses a (dz2)2(dx2-y2,xy)1 electron configuration, with
the unpaired spin centered on the Re atom.
e. X-Ray Absorption Spectroscopy (XAS). i. Re
L-Edge XAS. The Re L1-edge spectra for 3b, 3c, and 3d,
where the latter was prepared electrochemically at -25 °C
in dichloromethane solutions containing 0.10 M [N(n-
Bu)4]PF6, are presented in Figure 7. We have measured at
this edge since it is dominated by the Re 2s f 6p dipole-
allowed transition; it roughly gives the same information
as the K-edge because it measures the density of projected
p states. Also, the pre-edge is sensitive to local geometry
where dipole-forbidden 2s f 5d transitions gain intensity
through 5d/6p mixing that is maximized in systems with
noninversion centers.50 For six-coordinate complexes,
d-p hybridization occurs most strongly in trigonal pris-
matic geometry, while it is absent in octahedral symme-
try. The spectra shown in Figure 7 show a clear 1 eV
difference in the rising edge energy for 3d compared with
3b and 3c. This clearly demonstrates that a Re(IV) ion
In the Mo L3-edge XAS spectra of analogous molyb-
denum compounds,13 two peaks are observed, where the
difference in the peak energy can be used to estimate
the crystal field splitting and, hence, the coordination
(51) Lytle, F. W.; Greegor, R. B. Appl. Phys. Lett. 1990, 56, 192.
::
(52) Sproules, S.; Bill, E.; Weyhermuller, T.; DeBeer George, S.;
Wieghardt, K. Manuscript in preparation.
(50) Shulman, R. G.; Yafet, Y.; Eisenberger, P.; Blumberg, W. E. Proc.
Natl. Acad. Sci. U. S. A. 1976, 73, 1384.
(53) de Groot, F. M. F.; Hu, Z. W.; Lopez, M. F.; Kaindl, G.; Guillot, F.;
Trone, M. J. Chem. Phys. 1994, 101, 6570.

Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 10935
Figure 8. Normalized Re L3-edge XAS data of [Re(tms)3]1- (1c), [Re(Cl2-
bdt)3]1- (2c), [Re(bdt)3] (3b), [Re(bdt)3]1- (3c), and [Re(bdt)3]2- (3d).
Figure 9. Comparison of the normalized S K-edge spectra of 1b (black)
and 1c (red), recorded at room temperature in dichloromethane solutions.
The dashed line is the spectrum of solid 1c.
Table 6. Experimental Re L-Edge Transition Energies (eV) for the Complexes
due to 1e- oxidation, and it has been assigned here as a
transition to the 2a2 and confirms the assignment of a
0
Re L-edge
one-electron ligand oxidation of the monoanion generat-
ing the neutral species. The peaks at higher energy
(>2472 eV) are transitions to C-S π* and C-S
σ* orbitals frequently observed in the spectra of dithio-
lene10,12,13,41 and thiolate55 complexes and are best as-
signed as “rising” edge features because they are followed
by transitions to the sulfur 4p levels and finally into the
continuum. The shift in the rising edge (comparing the
peak at ∼2474 eV in each spectrum) of ∼0.25 eV to lower
energy for 1c compared to 1b indicates more oxidized
sulfur in the latter complex. The S K-edge spectrum of
solid 1c is overlaid with the solution spectrum in Figure 9
and shows the same peak energies. It is difficult to
speculate whether the geometry is retained in solution
since the difference in the total energy of these structures,
at least in the gas phase, is miniscule (vide infra). How-
ever, the small increase in the pre-edge peak intensity
could reflect a slight structural rearrangement in solution.
The S K-edge XAS spectra of 2c, 3b, and 3c are
presented in Figure S4 (Supporting Information). While
their peak energies are consistent with the formulation
discussed above, there are noticeable intensity differences
when compared with 1b and 1c due to decomposition in
the beam that precludes their inclusion in a quantitative
analysis. The coulometrically generated sample of 3d was
not amenable to measurement at room temperature.
g. Calculations. In this section, a picture of the electro-
nic structures of [Re(tms)3]+,0,1-,2-,3- (3a, 3b, 3c, 3d, and
3e) as well as for [Re(tms)3]1- (1b), [Re(tms)3]- (1c), and
[Re(Cl2-bdt)3]1- (2c) is derived from DFT calculations by
using the B3LYP functional for geometry optimizations,
the MO descriptions, and spin distributions in these
complexes. Spin-unrestricted Kohn-Sham (UKS) DFT
calculations were carried out with inclusion of scalar
relativistic effects using the ZORA method. In general,
the calculated geometries and metrical parameters of the
complexes 1b, 1c, 2c, and 3a-3e presented in Table 7 were
a
a
b
L3
L2
L1
1c
2c
3b
3c
3d
10541.8
10541.8
10542.1
10541.9
10541.6
11964.5
11964.9
11966.2
11965.6
11965.1
nmc
nm
12533.6
12533.9
12532.6
a Transition energies determined from peak maxima. b Transition
energy determined from the mid point of the rising edge. c nm=not
measured.
geometry. This feature has been utilized in previous
studies at the Re L3-edge on rhenium-oxide-containing
materials,54 where two closely spaced maxima are ob-
served for a Re ion in (distorted) tetrahedral or octahedral
geometry. Here, linewidths of ∼5 eV highlight the similar
ligand field splitting for these complexes calculated to be
trigonal prismatic (1c and 3b) and distorted trigonal
prismatic (2c, 3c, and 3d). The ligand field splitting is
too small to be observed experimentally, which is a
consequence of the longer core-hole lifetime for Re
L-edges compared with Mo L-edges. The Re L2-edge
XAS spectra are also characterized by a single peak, and
here again the peak energies do not present any trend that
can be correlated with the effective nuclear charge alone.
ii. S K-edge XAS. Figure 9 displays the S K-edge X-ray
absorption spectra of two members of the electron trans-
fer series [Re(tms)3]0,1- (1b, 1c) recorded in dichloro-
methane solutions at room temperature. The neutral
complex 1b was prepared via in situ oxidation of 1c by
molecular iodine.
Both the neutral 1b and monoanionic 1c species have a
resolved pre-edge feature at ∼2471.0 eV. The spectrum of
1b exhibits an additional lower energy pre-edge peak at
2470.05 eV. It has been established11,13 that this transition
is consistent with an extra hole in the dithiolene ligands
(54) (a) Enderle, B.; Gates, B. C. Phys. Chem. Chem. Phys. 2004, 6, 2484.
ꢀ
(b) Herro-Martín, J.; Subías, G.; Blasco, J.; García, J.; Sanche, M. C. J. Phys.:
Condens. Matter 2005, 17, 4963. (c) Lukens, W. W.; McKeown, D. A.; Buechele,
A. C.; Muller, I. S.; Shuh, D. K.; Pegg, I. L. Chem. Mater. 2007, 19, 559. (d)
Nunes, C. D.; Pillinger, M.; Hazell, A.; Jepsen, J.; Santos, T. M.; Madureira, J.;
Lopes, A. D.; Gonc-alves, I. S. Polyhedron 2003, 22, 2799.
(55) (a) Dey, A.; Glaser, T.; Couture, M. M.-J.; Eltis, L. D.; Holm, R. H.;
Hedman, B.; Hodgson, K. O.; Solomon, E. I. J. Am. Chem. Soc. 2004, 126,
8320. (b) Glaser, T.; Rose, K.; Shadle, S. E.; Hedman, B.; Hodgson, K. O.;
Solomon, E. I. J. Am. Chem. Soc. 2001, 123, 442.

10936 Inorganic Chemistry, Vol. 48, No. 23, 2009
Sproules et al.
Figure 10. Qualitative MO scheme depicting the ordering of the frontier orbitals for the [Re(L)3]z (z = 1+, 0, 1-, 2-, 3-) electron transfer series derived
from B3LYP DFT calculations. The MOs shown on the left are derived from the [Re(bdt)3]+ calculation with D3h symmetry labels.56
˚
Table 7. Averaged Calculated Bond Distances (A) and Angles (deg) Compared with Experimental Values in Parentheses
complex
1b
1c
2c
3a
3b
3c
3d
3e
Re-S
C-S
S-Re-S
Θ
2.361
1.761
81.9
0.37
2.372(2.348)
1.763(1.745)
80.9(81.0)
2.365(2.340)
1.757(1.743)
83.1(83.1)
2.354
1.735
83.7
0.05
2.371
1.741
82.7
0.34
2.377
1.758
82.7
24.9
2.418
1.764
83.0
33.5
2.466
1.765
82.9
41.4
0.02(0.46)
22.9(24.8)
found to be in very good agreement with experimental
values where available, and moreover, the trend in the
Re-S and intraligand bond distances is representative of
the metal and ligand oxidation levels. While the mono-,
di-, and trianions have structures ranging from distorted
trigonal to near-octahedral, we will express their ground-
state electron configurations in D3h rather than D3 sym-
metry, using the orbital terms coined originally by Gray
et al.,1 and Schrauzer and Mayweg2 for a simple compar-
ison of this electron transfer series.
orbitals defining the electronic structures of the five
members of this electron transfer series are shown in
Figure 10.
Doublet State Dianion. The calculated structure of the
dianion [Re(bdt)3]2- (S = 1/2; 3d) shows an absence of
quinoidal distortion in the aromatic ring and long C-S
˚
bonds (av. 1.764 A) consistent with three closed-shell
(bdt)2- ligands coordinated to a Re(IV) (d3) central ion.
The geometry is less octahedral than the trianion (3e),
with the twist angle calculated as Θ = 33.5°, similar to the
angle witnessed in the only crystallographically charac-
terized dianion, [PPh4]2[Re(mnt)3] (Θ=38°).42 The Re-S
Triplet State Trianion. A rhenium tris(dithiolene) tri-
anion has only been observed by cyclic voltammetry,
where at very negative potentials [Re(bdt)3]3- (3e), [Re-
˚
bonds are shortened to 2.419 A compared to 3e, con-
(tdt)3]3- 1
,
and [Re(Cl4-bdt)3]3- 4
,
are generated. [Re-
sistent with a metal-centered oxidation. The dianion
(bdt)3]3- was too unstable to be generated by controlled
potential coulometry, and thus we have no experimental
data to corroborate our calculations for this species. A
UKS solution for the trianion, [Re(bdt)3]3- (3e) was
calculated using the B3LYP functional beginning with
the coordinates from the optimized structure of the
dianion (3d). The geometry optimized structure is near-
octahedral with Θ = 41.4°, with long Re-S bonds at
carries a (4e0) (2a2 ) (3a1 ) (5e0) electron configuration
(Figure 10) and is formulated as [ReIV(L)3]2-. The Mulli-
ken spin density plot (Figure 11a) shows one unpaired
electron centered on Re, indicative of a low-spin ReIV d3
ion, in accord with its EPR spectrum. We have incremen-
tally increased the twist angle (0-56°) for 3d and per-
formed a single point calculation at each step. The total
energy was then plotted versus the twist angle, as shown in
Figure 12. While there is an energy minimum at Θ =
30.4°, a trigonal prismatic structure is only 4.3 kcal mol-1
less stable (Figure 12).
4
2
2
1
0
0
˚
2.466 A (Table 7) and intraligand bond distances consis-
tent with three closed-shell (bdt)2- ligands coordinated to
a Re(III) d4 central ion. This species carries a
4
2
2
2
0
0
(4e0) (2a2 ) (3a1 ) (5e0) electron configuration where the
large twist angle results from populating the 5e0 MOs,
which are antibonding with respect to a trigonal prism
(Scheme S8, Supporting Information). The complex pos-
sesses D3 symmetry with an S=1 ground state, as exem-
plified by the Mulliken spin density plot (Figure S13,
Supporting Information). The ordering of the principle
Singlet State Monoanion. We have isolated three salts
containing monoanions, two of which (1c and 2c) are
characterized crystallographically. Gratifyingly, the cal-
culated Re-S, C-S, and Carom-Carom bond lengths of 1c
and 2c are in excellent agreement with the corresponding
experimental values (Table 7); the difference in calculated
and experimental values of C-S and Carom-Carom bonds

Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 10937
Figure 11. Spin density plots of (a) the dianion in 3d and (b) neutral 3b,
from spin unrestricted DFT calculations together with values from the
Mulliken analyses (red, R-spin; yellow, β-spin). A complete Mulliken spin
density listing is available in the Supporting Information.
˚
never exceeds 0.02 A. The Re-S bond distances are
overestimated by maximally 0.03 A, which is a typical
˚
Figure 12. Walsh-type plot of the total electronic energies of 3a-3d as a
function of the trigonal twist angle (Θ). The relative total energy from the
spin-unrestricted Kohn-Sham calculations for 3a-3d is depicted by the
filled circles. In each case, the energy for Θ = 0° has been set to zero. For
3a (top), the total energy of the low-spin S=0 (squares) and high-spin S =
1 (triangles) solutions obtained from BS(1,1) DFT calculations are also
shown.
aspect of the B3LYP functional. The calculated av. C-S
bond length of 1.763 A in 1c and 1.758 in 2c (experimental
av. C-S values of 1.745 and 1.743 A, respectively)
˚
˚
indicate the presence of closed-shell, dianionic dithiolate
ligands as does the absence of quinoid-type distortions in
the aromatic ring. The same observations hold for 3c,
where the calculations also indicate the presence of three
(bdt)2- ligands. More interestingly, the calculated geo-
metry of the monoanion in 1c is trigonal prismatic; the
average twist angle Θ is calculated to be 0.02° and
observed (experimentally) at 0.46°. In contrast, the struc-
ture of 2c is intermediate between trigonal prismatic and
octahedral, with a calculated (and experimental) twist
angle of 22.9° (24.8°). Both optimizations were initiated
from the crystallographic coordinates. The calculated
structure of [Re(bdt)3]1- (3c) containing three unsubsti-
tuted benzene-1,2-dithiolate(2-) ligands is also distorted
trigonal prismatic (Θ = 24.9°). Note that, with a decreas-
ing number of 5dn electrons (n = 4 in the trianion, 3 in the
dianion, and 2 in the monoanion), the distortion of the
near-octahedral ReS6 polyhedron in 3e to an intermediate
structure in 3c decreases, as is exemplified by the increas-
ing calculated twist angle Θ, which is calculated to be
41.4° in 3e, 33.5° in 3d, and 24.9° in 3c. Furthermore, the
oxidations of the tri- to the monoanion are metal-cen-
tered processes, as displayed in Figure 13. The Walsh-
type plot for the incremental twisting of 3c, as shown in
Figure 12, identifies an energy minimum at Θ=24° (the
angle from optimization); however, most importantly,
there is only a 2 kcal mol-1 difference in the total energy
from 0° to 34°. A similar result has been obtained for 1c,
where the most energetically stable geometry is inter-
mediate between a trigonal prism and an octahedron
(25°) despite the crystallographic geometry being a per-
fect trigonal prism. It is important to note that the
geometry did not relax at each specific twist angle (the
bond distances and angle were fixed); however, we do
obtain a trigonal prismatic structure when the optimiza-
tion begins with trigonal prismatic coordinates (i.e., from
the neutral molecule) and an intermediate structure when
beginning with distorted octahedral coordinates (i.e.,
from the dianionic species). The energy difference be-
tween these two structures is very similar to the difference
obtained from the single point calculations, underscoring
˚
average calculated Re-S distances increase from 2.378 A
˚
in 3c to 2.419 A in 3d to 2.466 A in 3e. Thus, these struc-
˚
tural parameters imply that the successive one-electron

10938 Inorganic Chemistry, Vol. 48, No. 23, 2009
Sproules et al.
˚
shortening of the average C-S bond length to 1.741 A
and a small quinoid-type distortion of the benzene ring.
This is classified as a ligand-centered oxidation where
one-electron is removed from the 2a2 MO in 3c, produ-
cing the neutral species 3b. A similar scenario exists when
1c is oxidized to 1b. The changes in the average Re-S and
C-S bond distances are charted in Figure 13, where
oxidation of 3c sees a pronounced shortening of the
C-S bond length, whereas the Re-S bond length remains
almost the same. Interestingly, the ReS6 polyhedron in
[Re(bdt)3] is now calculated to be trigonal prismatic
(Θcalcd = 0.34°), with a structure that closely resembles
crystallographically characterized [Re(pdt)3] containing
three cis-1,2-diphenyl-1,2-dithiolene ligands.3 Its electro-
nic structure is defined as [ReV(L•)(L)2]0,8 with the elec-
2
4
1
tron configuration of (3a1 ) (4e0) (2a2 ) (Figure 10)
correctly issued by Al-Mowali and Porte.5 The Mulliken
spin plot displayed in Figure 11b shows that a small
amount of spin density is found at the Re center (F =
-0.18) through spin polarization of the Re-S π bonds of
the 4e0 MOs. This generates the observed quadrupole-
allowed transitions in the EPR spectrum of 3b. The
Walsh-type diagram for 3b presented in Figure 12 shows
Θ = 0° to be the most stable geometry. There is an abrupt
increase in the total energy at Θ = 17.7° as twisting
0
0
Figure 13. Plot of the av. calculated Re-S (squares) and av. C-S
(circles) bond distances from the geometry optimization of 3a-3e versus
the complex charge. The two purple solid lines emphasize that reduction
from 3a to 3c is ligand-centered, whereas 3c-3e are linked by successive
metal reduction. The splinecurve (dot0tedline) highlights the shorteningof
the C-S bond distance when the 2a2 MO is depopulated for neutral 3b
and monocationic 3a.
the flat potential energy surface that encompasses a large
range of twist angles. Therefore, in rhenium tris-
(dithiolene) monoanions, where the energy difference is
<2.2 kcal mol-1 for twist angles ranging from 0 to 35°,
crystal packing is highly influential in the observed crys-
tallographic twist angles in 1c and 2c.
0
destabilizes the 3a1 (now 3a1 in D3 symmetry) orbital
0
relative to the 2a2 MO due to the aforementioned con-
figurational interaction. Thus, the distorted octahedral
4
2
1
0
0
structure has a (4e0) (2a2 ) (3a1 ) electron configuration,
that is, a Re(VI) ion, originally proposed by Gray et al.1
This second potential energy surface has a local minimum
at Θ =36.5° that is 10 kcal mol-1 higher in energy than
the perfect trigonal prism, and recent calculations suggest
this octahedral structure is a transition state.57 We can
geometry-optimize, starting from octahedral coordi-
nates, and obtain distorted octahedral structure for 3b.
This species possesses a Re(VI) central ion, three dianio-
nic dithiolate ligands, and a twist angle, Θ = 36.8°. The
energy of this structure is 8.2 kcal mol-1 higher than the
trigonal prismatic solution and, reassuringly, is approxi-
mately the same energy as that of the single point mole-
cule with the same twist angle.
The qualitative MO scheme for 3c displayed in Scheme
S6 (see the Supporting Information) possesses the same
MO ordering as that tendered by Al-Mowali and Porte,5
though with twice as many orbitals owing to the different
combinations of π orbitals from the aromatic ring. The
highest occupied molecular orbital (HOMO) is the 2a2
orbital that is 100% ligand in composition. Populating
this orbital should destabilize a trigonal prism because it
increases the interligand repulsion since the phases of the
sulfur 3p orbitals are antibonding with respect to each
other. However, as seen both crystallographically and in
the Walsh-type plot of 3c, this is a very small energy value,
which leads to the conclusion that the occupancy of this
orbital has little bearing on the observed geometry.
Beneath this is the 3a1 MO which has 31% Re dz2
Singlet-State Monocation. The monocation (3a) species
of this series displays the shortest average C-S bonds at
0
00
character. The a1 and a1 orbitals in D3h symmetry both
transform as a1 in D3 symmetry (trigonal twisted) such
that there are two MOs with ∼35% Re dz2 character
derived from the mixing of the purely 3a100 ligand with the
˚
˚
1.725 A and a longest Carom-Carom distance of 1.424 A
and a significant quinoid-type distortion of the benzene
ring, indicating a ligand-centered oxidation from 3b to 3a
(Figure 13). The ReS6 polyhedron remains trigonal pris-
matic (Θcalcd = 0.05°). With limited spectroscopic char-
acterization of this complex, we must explore all plausible
electronic structures that could give a trigonal prismatic
solution, namely, [ReV(L•)2(L)]+, [ReVI(L•)(L)2]+, and
[ReVII(L)3]+. The Re(V) solution results from the UKS
calculation, and, as shown in the Walsh-type diagram, is
the most energetically favorable (Figure 12). Again, we
observe an abrupt rise in the total energy at Θ = 31°
where, at this angle, the configurational interaction (in D3
0
3a1 orbital comprising 70% dz2. This configurational
interaction destabilizes the Re dz2 orbital in these mono-
anionic species and has been recently implicated in pro-
ducing different electronic grounds in molybdenum
tris(dithiolene) monoanions.11 However, the rhenium 5d
orbital manifold is more stable than the Mo 4d mani-
fold, and all of the monoanions possess a Re(V) d2 central
ion irrespective of the ligand substituents and complex
geometry. The lowest unoccupied molecular orbitals
(LUMOs) are degenerate combinations of the dx2-y2,xy
and dxz,yz orbitals, and C-S π* MOs. Overall, these
monoanionic complexes have an electronic structure
(56) For consistency, D3h symmetry labels are used to describe the
electron configuration for all members of this electron transfer series despite
the reduction in symmetry to D3 when the molecule twists towards octahe-
dral.
4
2
described as [ReV(L)3]1- (S = 0) with a (4e0) (3a1 ) -
0
2
(2a2 ) electronic configuration.
0
Doublet State Neutral Species. The optimized structure
ꢀ
(57) Chandrasekaran, P.; Arumugam, K.; Jayarathne, U.; Perez, L. M.;
of neutral [Re(bdt)3] (S = 1/2) (3b) reveals a small
Mague, J. T.; Donahue, J. P. Inorg. Chem. 2008, 48, 2103.

Article
Inorganic Chemistry, Vol. 48, No. 23, 2009 10939
0
symmetry) destabilizes the dz2 orbital relative to the 2a2
ligand facilitates HOMO-LUMO mixing through a
D3h f C3h symmetry reduction that stabilizes the 3a1
MO relative to the 2a2 MO.
0
MO. This distorted octahedral structure from the UKS
calculation (purple circles, Figure 12, top) is formulated
as [ReVII(L)3]+ and is 29.2 kcal mol-1 less stable than the
trigonal prismatic solution. The [ReVI(L•)(L)2]+ electro-
nic structure is a singlet diradical, with one unpaired
electron on the Re ion and one hole in the dithiolene
ligands. To investigate this, and the high-spin MS = 3, we
have used a broken symmetry BS(1,1) approach. As
shown in Figure 12, the low-spin solution from the BS-
(1,1) calculation converges to the [ReV(L•)2(L)]+ state
found in the UKS calculation. This solution is 17.1 kcal
mol-1 more stable than the high-spin (S = 1) state, when
Θ = 0°. At Θ > 33°, there is a change to a new potential
energy surface with a local minimum that lies 14 kcal
mol-1 above the trigonal prismatic structure. This state
11,61
0
h. Time-Dependent DFT Calculations. In order to ob-
tain a deeper understanding of the pre-edge transitions in
the S K-edge X-ray absorption spectroscopy, TD-DFT
calculations have been used to assign the transitions seen
in the S K-edge spectra of 1b and 1c. Additionally, these
calculations also serve to calibrate the computed electro-
nic structure with experimental results. Using a recently
devised method for the calculation of K-pre-edge transi-
tions,34 we have simulated the S K-pre-edge region of 1b
and 1c using the BP86 functional. For this system, there
is a negligible difference in the electronic structure
calculated by the BP86 and B3LYP functionals; hence,
the former functional is preferred since it is considerably
faster at computing the pre-edge. Additionally, due to
the limitations in the accurate treatment of excited states
in DFT, absolute transition energies cannot be obtained
by this method. Nevertheless, the relative transition
energies and the relative intensities are, in general,
reliably modeled. For a given theoretical method,
that is, combination of functional, basis sets, relati-
vistic treatment, and so forth, an empirical correction
for the energy and intensity can be introduced. For
the present study, the calculated spectra are shifted
by +61.45 eV.
4
1
1
carries a (4e0) (3a1 ) (2a2 ) electron configuration, where
the S = 0 and S = 1 states are degenerate because the two
SOMOs cannot mix in D3 symmetry. Thus, we are con-
fident that a one-electron oxidation of the neutral rhe-
nium tris(dithiolene) is ligand-centered, producing a
0
0
trigonal prismatic monocationic complex with
0
a
4
2
0
(4e0) (3a1 ) (2a2 ) electron configuration, wherein the
0
0
depopulation of the 2a2 MO gives rise to a more intense
LLIVCT band, as seen in the electronic absorption
spectrum of 1a. This is an interesting molecule to study
owing to the fact that it is isoelectronic to the neutral
tris(dithiolene)molybdenum complexes. Recent XAS stu-
dies have produced two different electronic structures,
either [MoIV(mdt•)2(mdt)] or [Mov(tbbdt•)(tbbdt)2],11,13
where it appears that the redox potential of the ligand,
The calculated S K-pre-edge spectra are shown in
Figure 14 and afford an excellent fit of the experimental
data. The peak energies and transition assignment are
presented in Table 8. For the 1b, two pre-edge peaks are
calculated at 2470.09 and 2471.31 eV. The monoanion in
1c has only one pre-edge feature calculated at 2471.05 eV.
All of these values are within 0.12 eV of the experimental
energies. It is significant that 1c lacks the low-energy pre-
edge transition found in 1b, since this transition is char-
acteristic of excitation to a hole in the S 3p orbitals. Here
in trigonal prismatic geometry, it is defined as the 1s f
0
which is reflected in the energy of the 2a2 orbital with
0
respect to the 4dz2 (3a1 ), governs the oxidation state of the
Mo ion. Most intriguingly, both complexes have very
similar sulfur K-edge spectra,11,13 which are difficult to
justify with this apparent difference in the Mo oxidation
state. Our calculation of the monocation shows, for the
Re atom, that a d2 system with two ligand holes is
operative and is derived from the larger effective nuclear
charge of Re that lowers the energy of the 5d orbitals,
relative to the Mo 4d manifold. Molybdenum appears to
be positioned at just the right energy that subtle changes
in the π donor capabilities of the dithiolene ligand,
through modifying the substituents, affects the ordering
of the 3a1 and 2a2 orbitals such that we apparently have
both Mo(IV) and Mo(V) centers in different neutral
tris(dithiolene)molybdenum complexes. Importantly,
the optimized structure of 3a contains three very flat
dithiolene ligands (av. R = 0.03°) despite starting from
the [Mo(bdt)3] crystal structure coordinates.58 Dithiolene
folding is present in all neutral tris(dithiolene) complexes
of molybdenum13,39,58,59 and the isoelectronic mono-
anionic vanadium compounds.60 This is a second-order
Jahn-Teller distortion whereby folding the dithiolene
0
2a2 transition: one hole distributed over the three ligands.
The second feature is assigned as the 1s f 5e0 transition,
which is empty for both 1b and 1c, and thus has approxi-
mately the same intensity. The relative intensity of the two
calculated pre-edge peaks for 1b has exactly the same
ratio (1.0:2.5) as the experiment. The second pre-edge
peak is considerably broader experimentally (fwhm =
1.20 eV) than the first pre-edge peak (0.88 eV) and that
used in the simulation (fixed at 0.8 eV).
0
0
Although there is no crystal structure for 1b, the
optimized Re-S bond distances do not vary by more
˚
than 0.017 A. The only crystallographically defined rhe-
nium tris(dithiolene) complex, [Re(pdt)3],3 has six near-
equivalent Re-S bond distances that implies that the
three-fold axis is retained, and therefore, broadening
of this band is not a consequence of nondegenerate
5e0 MOs through a lowering of the symmetry. This
broadening is caused by a small splitting of the spin-up
(R-spin) and spin-down (β-spin) set of 5e0 MOs by spin
polarization rather than a reduction of the symmetry
due to a molecular distortion. The TD-DFT calcu-
lates a splitting (between the 5e0(R) and 5e0(β) orbitals)
(58) Cowie, M.; Bennett, M. J. Inorg. Chem. 1975, 15, 1584.
(59) (a) Friedle, S.; Partyka, D. V.; Bennett, M. V.; Holm, R. H. Inorg.
Chim. Acta 2006, 359, 1427. (b) Smith, A. E.; Schrauzer, G. N.; Mayweg, V. P.;
Heinrich, W. J. Am. Chem. Soc. 1965, 87, 5798. (c) Wang, K.; McConnachie,
J. M.; Stiefel, E. I. Inorg. Chem. 1999, 38, 4334.
ꢀ
(60) (a) Livage, C.; Fourmigue, M.; Batail, P.; Canadell, E.; Coulon, C.
Bull. Soc. Chim. Fr. 1993, 130, 761. (b) Broderick, W. E.; McGhee, E. M.;
Godfrey, M. R.; Hoffman, B. M.; Ibers, J. A. Inorg. Chem. 1989, 28, 2902. (c)
Welch, J. H.; Bereman, R. D.; Singh, P. Inorg. Chem. 1988, 27, 2862. (d) Chung,
G.; Bereman, R.; Singh, P. J. Coord. Chem. 1994, 33, 331.
(61) Campbell, S.; Harris, S. Inorg. Chem. 1996, 35, 3285.

10940 Inorganic Chemistry, Vol. 48, No. 23, 2009
Sproules et al.
and calculated by DFT. For 3c containing unsubstituted
benzene-1,2-dithiolate(2-) ligands, a twist angle of 24.9° has
been calculated. The calculations show that the energetic
profile for twist angles between 0° and ∼34° is very shallow
(within 2 kcal mol-1) with packing forces being very influen-
tial on the observed twist angles. There is no clear electronic
preference for an ideal trigonal prismatic versus a distorted
trigonal prismatic structure.
1
The corresponding dianions [Re(L)3]2- (S = /2) consist
again of three closed-shell benzene-1,2-dithiolate(2-)
ligands and a low-spin central Re(IV) ion (d3, SRe = 1/2).
This is clearly shown by the X-band EPR spectrum
that is dominated by the magnetic hyperfine of the Re
nucleus. The calculated twist angle of 33.5° of the ReS6
polyhedron indicates a geometry intermediate between
trigonal prismatic and octahedral. Reduction of the
dianion to the trianion [Re(L)3]3- (S = 1) is again a metal-
centered process yielding a central Re(III) ion (d4, SRe=1)
and three S,S0-coordinated dithiolate(2-) ligands. The ReS6
polyhedron now approaches an octahedral geometry (Θ =
41.4°).
Figure 14. Experimental (top) and calculated (bottom) S K-edge XAS
spectra of 1b and 1c obtained from BP86 TD-DFT calculations. Calcu-
lated intensity in arbitrary units.
The most salient feature of this study is the observation
that the neutral species [Re(L)3] (S=1/2) and the monocation
[Re(L)3]+ (S = 0) contain both a trigonal prismatic ReS6
polyhedron and a central low-spin Re(V) ion (d2, SRe =0)
Table 8. Comparison of Calculated and Experimentally Determined S K-edge
XAS Spectra for 1b and 1c
2
0
with two electrons in the 5dz2 metal orbital, namely, (3a1 ) in
D3h symmetry. The unpaired electron in neutral 3b occupies a
transition energiesa
1
0
pure ligand MO, (2a2 ) . This electron configuration is
unequivocally confirmed by the EPR spectrum where a
unique situation exists, in that the quadrupole interaction is
larger than the magnetic hyperfine interaction. The electronic
transition
exptlb
calcdc
areab
0
1b
1c
1s f 2a2
1s f 5e0
1s f 5e0
2470.05
2471.20
2470.94
2470.09
2471.32
2471.05
0.549
1.389
1.405
spectrum of 1b and 3b displays an intense (>104 M-1 cm-1
)
LLIVCT band at 684 and 674 nm, respectively, that is absent
in the spectra of the mono-, di-, and trianions. The spectrum
of 1a shows this band at 661 nm. Thus, both the neutral and
monocationic forms possess oxidized forms of the ligands
(formally one and two ligand holes, respectively) and a
cent0ral Re(V) ion. In the monocation, the ligand orbital
(2a2 ) is empty (LUMO), whereas in the neutral form it is the
SOMO and in the monoanion it is the HOMO (filled with
two electrons). X-ray absorption spectroscopy at the S K
edgeand Re L edge providesdirect spectroscopic evidence for
the above interpretation. In summary, we have shown
that the electronic structures of the five-membered electron
transfer series [Re(L)3]z (z=1+, 0, 1-, 2-, 3-) is as follows:
a In eV. b Derived from pseudo-Voigt deconvolution of the experi-
mental spectrum. c Energy shifted +61.45 eV.
of 0.05 eV for 1b, which is an underestimate of the
experiment. The spin polarization in neutral rhenium
tris(benzene-1,2-dithiolato) complexes is crucial in gene-
rating the large quadrupole coupling observed in the
X-band EPR spectra. Diamagnetic 1c has negligible
spin polarization (equivalent 5e0(R) and 5e0(β) orbitals)
and has a sharper pre-edge peak (fwhm=1.02 eV) that
is of a similar width to the first pre-edge peak of 1b
(0.88 eV).
[ReV(L•)2(L)]+, [ReV(L•)(L)2]0, [ReV(L)3]1-, [ReIV(L)3]2-
[ReIII(L)3]3-
,
Conclusions
.
We have investigated each member of the electron transfer
series [tris(benzene-1,2-dithiolene)rhenium]z (z=1+, 0, 1-,
2-, 3-) using a host of spectroscopies (UV-vis, EPR, XAS)
and DFT calculations. Electrochemical measurements reveal
that they are related by reversible one-electron transfer steps
in solution. The most important result of this study is the fact
that it is possible to unequivocally determine the spectro-
scopic oxidation state of the central rhenium ion (the 5dn
electron configuration) and the oxidation level of the dithio-
lene ligands.
It is clearly established that the diamagnetic monoanions
[Re(L)3]1- (S = 0) consist of three closed-shell dithiolato(2-)
ligands and a central Re(V) ion (d2, SRe = 0). The structure
of the ReS6 polyhedron is (distorted) trigonal prismatic with
twist angles Θ varying from 0.46° in 1c to 24.8° in 2c,
as determined experimentally by X-ray crystallography
Acknowledgment. This work is dedicated to Professor
Wolfgang Lubitz on the occasion of his 60th birthday. We
thank Dr. Eberhard Bothe and Ms. Petra Hofer for their
::
technical expertise with electrochemical measurements.
We are grateful for financial support from the Fonds der
Chemischen Industrie. S.S. and F.L.B. thank the Max-
Planck-Society for a postdoctoral fellowship and doctor-
al stipend, respectively. SSRL operations are funded by
the Department of Energy, Office of Basic Energy
Sciences. The Structural Molecular Biology program is
supported by the National Institutes of Health (grant 5
P41 RR001209), National Center for Research Re-
sources, Biomedical Technology Program and by the
Department of Energy, Office of Biological Environmen-
tal Research.

Article
Supporting Information Available: X-ray crystallographic
Inorganic Chemistry, Vol. 48, No. 23, 2009 10941
for the S K-edge spectra of 1b and 1c. Tables of geometric and
electronic structures details (qualitative MO schemes and Mul-
liken spin density plots) of DFT calculated structures of 1b, 1c,
2c, and 3a-3e. This material is available free of charge via the
Internet at http://pubs.acs.org.
files in CIF format for 1c and 2c. Cyclic voltammogram for
2c; electronic absorption spectra of [Re(Cl2-bdt)3]z (z = 0, 1-,
2-); Re L2-edge spectra of 1c, 2c, 3b, 3c, and 3d; S K-edge
spectra of 2c, 3b, and 3c; and pseudo-Voigt fitting parameters