Synthesis and ligand-based mixed valency of cis- and trans-CrIII(X4SQ)(X4Cat)(L)n (X = Cl and Br, n = 1 or 2) complexes: Effects of solvent media on intramolecular charge distribution and ligand dissociation of CrIII(X4SQ)3

Article abstract of DOI:10.1021/ic025643p

The treatment of Cr^III(X₄SQ)₃ (SQ = o-semiquinonate; X = Cl and Br) with acetonitrile affords trans Cr^III(X₄SQ)- (X₄Cat)(CH₃CN)₂ (X = Cl (1) and Br (2)). In the presence of 2,2′-bipyridine (bpy) or 3,4,7,8-tetramethyl-1,10-phenanthrene (tmphen), the reaction affords Cr^III(X₄SQ)(X₄Cat)(bpy) ·nCH₃CN (X = Cl, n = 1 (3); X = Br, n = 0.5 (4)) or Cr^III(X₄SQ)(X₄Cat)(tmphen) (X = Cl (5) and Br (6)), respectively. All of the complexes show a ligand-based mixed-valence (LBMV) state with SQ and Cat ligands. The LBMV state was confirmed by the presence of the interligand intervalence charge-transfer band. Spectroscopic studies in several solvent media demonstrate that the ligand dissociation included in the conversion of Cr^III(X₄SQ)₃ to 1-6 occurs only in solvents with relatively high polarity. On the basis of these results, the effects of solvent media were examined and an equilibrium, Cr^III(X₄SQ)₃ ? Cr^III(X₄BQ)(X₄SQ)(X₄Cat) (BQ = o-benzoquinone), is proposed by assuming an interligand electron transfer induced by solvent polarity.

Full text of DOI:10.1021/ic025643p

Inorg. Chem. 2002, 41, 4444−4452
Synthesis and Ligand-Based Mixed Valency of cis- and
trans-CrIII(X SQ)(X Cat)(L)n (X ) Cl and Br, n ) 1 or 2) Complexes:
4
4
Effects of Solvent Media on Intramolecular Charge Distribution and
Ligand Dissociation of CrIII(X SQ)3
4
Ho-Chol Chang, Katsunori Mochizuki, and Susumu Kitagawa*
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering,
Kyoto UniVersity, Sakyo-ku, Kyoto 606-8501, Japan
Received April 10, 2002
III
III
The treatment of Cr (X SQ)3 (SQ ) o-semiquinonate; X ) Cl and Br) with acetonitrile affords trans-Cr (X SQ)-
4
4
(X Cat)(CH CN)2 (X ) Cl (1) and Br (2)). In the presence of 2,2′-bipyridine (bpy) or 3,4,7,8-tetramethyl-1,10-
4
3
III
phenanthrene (tmphen), the reaction affords Cr (X SQ)(X Cat)(bpy)‚nCH CN (X ) Cl, n ) 1 (3); X ) Br, n ) 0.5
4
4
3
III
(4)) or Cr (X SQ)(X Cat)(tmphen) (X ) Cl (5) and Br (6)), respectively. All of the complexes show a ligand-based
4
4
mixed-valence (LBMV) state with SQ and Cat ligands. The LBMV state was confirmed by the presence of the
interligand intervalence charge-transfer band. Spectroscopic studies in several solvent media demonstrate that the
III
ligand dissociation included in the conversion of Cr (X SQ)3 to 1−6 occurs only in solvents with relatively high
4
III
polarity. On the basis of these results, the effects of solvent media were examined and an equilibrium, Cr (X SQ)3
4
III
T Cr (X BQ)(X SQ)(X Cat) (BQ ) o-benzoquinone), is proposed by assuming an interligand electron transfer
4
4
4
induced by solvent polarity.
Introduction
ancillary ligand such as 2,2′-bipyridine). For the cobalt
complexes, the tautomerism occurs between low-spin CoIII
(left) and high-spin CoII (right) isomers with mixed and
homovalent ligands, respectively (eq 1).1b,2 On the other hand,
the manganese complexes show an equilibrium (eq 2)
including three valence tautomers.3 In the equilibria 1 and 2
the metal ions and the ligands simultaneously change their
oxidation states in each step. The change in valence results
from the combined redox activity of the metal ion and the
o-quinone ligands, in other words, comes from the degree
of freedom in the electronic state of the metal (M)/ligand
(L) pair (M-L mechanism).
A metal complex containing noninnocent ligands serves
as a useful module for novel supramolecular assemblies with
intriguing physical and/or chemical properties based on
valence and spin states. This could be exemplified by a
family of o-quinone complexes that provide unique oxidation
and spin states irrespective of their simple constituent,
C6O2X4 (X ) H, halogen atom, etc.).1 The degree of freedom
of their electronic states, for instance, is ascribed to the
versatile redox forms of o-benzoquinone (BQ), o-semi-
quinonate (SQ•-), and catecholate (Cat2-).
Recent studies have demonstrated that the delicate balance
of the distribution of the charge and the spin over a whole
molecule gives rise to “valence tautomerism”.1a,b The tau-
tomerism has been observed for several transition metal
complexes, in particular, manganese and cobalt complexes
with two o-quinone ligands that chelate to the metal ions,
M(Q)2(N-N) (Q ) o-quinonate, N-N ) nitrogen-donor
ls-CoIII(SQ)(Cat)(N-N) T hs-CoII(SQ)2(N-N)
(1)
MnIV(Cat)2(N-N) T hs-MnIII(SQ)(Cat)(N-N) T
hs-MnII(SQ)2(N-N) (2)
In contrast with the M-L mechanism, when the ligands
act as dominant active sites against an inner and/or an outer
* Author to whom correspondence should be addressed. E-mail:
kitagawa@sbchem.kyoto-u.ac.jp.
(1) (a) Pierpont, C. G.; Lange, C. W. Prog. Inorg. Chem. 1994, 41, 331.
(b) Pierpont, C. G. Coord. Chem. ReV. 2001, 216-217, 99. (c)
Pierpont, C. G. Coord. Chem. ReV. 2001, 219-221, 415.
(2) (a) Jung, O.-S.; Pierpont, C. G. J. Am. Chem. Soc. 1994, 116, 2229.
(b) Adams, D. M.; Noodleman, L.; Hendrickson, D. N. Inorg. Chem.
1997, 36, 3966.
4444 Inorganic Chemistry, Vol. 41, No. 17, 2002
10.1021/ic025643p CCC: $22.00 © 2002 American Chemical Society
Published on Web 07/17/2002

cis- and trans-CrIII(X4SQ)(X4Cat)(L)n Complexes
sphere electron-transfer processes, we could expect another
mechanism, viz., the L-L mechanism, which is realized by
a combination of a redox inert metal ion and o-quinone
ligands.4 Chromium(III) complexes, CrIII(X4SQ)3 (X ) Cl
and Br),5 demonstrate good examples of the stepwise
chemical oxidation/reduction processes occurring on the
ligand moieties as shown in eq 3. Among these seven-
membered redox isomers, ligand-based mixed-valence
(LBMV) redox isomers, [CrIII(X4SQ)3-n(X4Cat)n]n- (n )
1-2), have been isolated by stepwise one- and two-electron
reduction.6
MnIV(SQ)2(Cat), based on the M-L mechanism.3a In contrast,
the charge distribution in CrIII(X4SQ)3 could be affected by
the solvent media, where intramolecular electron transfer,
the L-L mechanism, occurs between the redox-active ligand
moieties through the redox inert chromium(III) ion.
Experimental Section
Materials. All chemicals were reagent grade. 3,4,7,8-Tetram-
ethyl-1,10-phenanthroline was purchased from Aldrich and 2,2′-
bipyridine from Tokyo Kasei Co., Ltd. Complexes CrIII(X4SQ)3‚
4C6H6 were synthesized by the procedure described in the
literature.6a All of the reagents and solvents were used without
special purification.
[CrIII(BQ)3]3+ T [CrIII(BQ)2(SQ)]2+
T
Preparation of the Complexes. trans-CrIII(Cl4SQ)(Cl4Cat)-
(CH3CN)2 (1). A 350 mL carbon disulfide solution containing
CrIII(Cl4SQ)3‚4C6H6 (980 mg, 0.889 mmol) was added to 150 mL
of acetonitrile and stirred for 1 day. The resulting suspension was
filtered, and the obtained powder was washed several times with
carbon disulfide and then dried in a vacuum (481 mg, 86%). A
single crystal of 1 was grown from a layered solution of carbon
disulfide containing CrIII(Cl4SQ)3 and acetonitrile. Found: C, 30.46;
H, 1.15; N, 4.46. Required for C14H6Cl8CrN2O4: C, 30.71; H, 0.97;
N, 4.48. IR (KBr): 2325w (νCN), 2297w (νCN), 1517w, 1472w,
1439m, 1407w, 1361w, 1314s, 1259w, 1238w, 1155s, 1037m, 981s,
952m, 813w, 794s, 691m, 669w, 585w, 549w, 524m, 444s, and
[CrIII(BQ)(SQ)2]+ T CrIII(SQ)3 T [CrIII(SQ)2(Cat)]- T
[CrIII(SQ)(Cat)2]2- T [CrIII(Cat)3]3- (3)
To activate and to control the L-L mechanism, particular
attention needs to be paid to the effects of the solvent media.
Previous studies have demonstrated that the ligand-based
redox reactions smoothly proceed in a nonpolar solVent such
as dichloromethane and carbon disulfide. The reactions
proceed in support of the kinetic inertness of the Cr(III) ion
against the ligand exchange reaction in the nonpolar solvents.
Pierpont et al. have reported that the ligand exchange reaction
of CrIII(SQ)3 with 2,2′-bipyridine (bpy) is hardly discernible
in a nonpolar solvent.7 On the other hand, once CrIII(SQ)3 is
chemically oxidized to the [CrIII(BQ)(SQ)2]+ cation (eq 3),
drastic enhancement of the reaction rate is achieved to give
CrIII(SQ)(Cat)(bpy). The coordination ability of the BQ
ligand is much weaker than the coordination abilities of SQ•-
and Cat2-; therefore, the ligand exchange reaction readily
occurs by bpy. In this context, the unique charge distribution
over the three ligands in CrIII(SQ)3 provides structural
rigidity, supporting successfully the L-L mechanism on the
o-quinone ligands.
In this manuscript, we first demonstrate that CrIII(X4SQ)3
only undergoes a rapid ligand exchange reaction with the
nitrogen-donor chelate reagents or solvent molecules in polar
solVent. The reaction was carried out without any oxidizing
processes to give cis/trans-CrIII(X4SQ)(X4Cat)(L)n LBMV
complexes. On the basis of these reactions, we discuss effects
of solvent media and their polarity on the intramolecular
charge distribution together with the ligand dissociation of
CrIII(X4SQ)3. To date only one tris(o-quinone) complex,
MnIII(3,6-DTBSQ)3, is known to show a temperature-
dependent valence tautomeric equilibrium, MnIII(SQ)3 T
399s cm-1
.
trans-CrIII(Br4SQ)(Br4Cat)(CH3CN)2 (2). A 1000 mL carbon
disulfide solution containing CrIII(Br4SQ)3‚4C6H6 (3.50 g, 2.14
mmol) was added to 430 mL of acetonitrile and stirred for 1 day.
The resulting suspension was filtered, and the obtained powder was
washed with carbon disulfide several times and then dried in a
vacuum (1.20 g, 57%). Found: C, 19.60; H, 0.76; N, 2.84. Required
for C14H6Br8CrN2O4: C, 19.58; H, 0.62; N, 2.85. IR (KBr): 2322w
(νCN), 2293w (νCN), 1471w, 1464w, 1456w, 1429w, 1273s, 1199m,
1139s, 1037m, 938s, 756m, 746w, 702s, 654w, 628w, 614w, 596w,
566w, 533w, 518w, 496w, 478w, 423s, 430s, 421s, 402s, and 394s
cm-1
.
CrIII(Cl4SQ)(Cl4Cat)(bpy)‚CH3CN (3). A 130 mL acetonitrile
suspension containing CrIII(Cl4SQ)3‚4C6H6 (98 mg, 0.090 mmol)
was heated under air, and then a 20 mL acetonitrile solution
containing bpy (14 mg, 0.090 mmol) was added. The brownish
violet suspension became reddish violet in few minutes. The
suspension was filtered, and the filtrate was allowed to stand for 5
days. The violet crystals grown from the filtrate were collected by
filtration, washed with acetonitrile several times, and then dried in
a vacuum (27 mg, 41%). Found: C, 39.00; H, 1.61; N, 6.24.
Required for C24H11Cl8CrN3O4: C, 38.90; H, 1.50; N, 5.67. IR
(KBr): 1571w, 1566w, 1495m, 1471s, 1430s, 1376m, 1313m,
1272m, 1247s, 1216s, 1170w, 1157w, 1131w, 1104w, 1090w,
1069w, 1059w, 1045w, 1034w, 978s, 885w, 813w, 791s, 766s,
731m, 711w, 691m, 663w, 651w, 630w, 611w, 592m, 581m, 550m,
535w, 514w, 460w, 444w, and 423w cm-1
.
(3) (a) Attia, A. S.; Pierpont, C. G. Inorg. Chem. 1998, 37, 3051. (b)
Attia, A. S.; Pierpont, C. G. Inorg. Chem. 1997, 36, 6184. (c) Attia,
A. S.; Jung, O.-S.; Pierpont, C. G. Inorg. Chim. Acta 1994, 226, 91.
(4) Chaudhuri, P.; Hess, M.; Hildenbrand, K.; Bill, E.; Weyhermu¨ller,
T.; Wieghardt, K. Inorg. Chem. 1999, 38, 2781.
(5) (a) Pierpont, C. G.; Downs, H. H.; Rukavina, T. G. J. Am. Chem.
Soc. 1974, 96, 5573. (b) Pierpont, C. G.; Downs, H. H. J. Am. Chem.
Soc. 1976, 98, 4834.
(6) (a) Chang, H.-C.; Ishii, T.; Kondo, M.; Kitagawa, S. J. Chem. Soc.,
Dalton Trans. 1999, 2467. (b) Chang, H.-C.; Miyasaka, H.; Kitagawa,
S. Inorg. Chem. 2001, 40, 146. (c) Chang, H.-C.; Kitagawa, S. Angew.
Chem., Int. Ed. 2002, 41, 130.
(7) Buchanan, R. M.; Claflin, J.; Pierpont, C. G. Inorg. Chem. 1983, 22,
2552.
CrIII(Br4SQ)(Br4Cat)(bpy)‚1/2CH3CN (4). A hot 60 mL aceto-
nitrile suspension containing CrIII(Br4SQ)3‚4C6H6 (334 mg, 0.204
mmol) was treated in a 10 mL acetonitrile solution containing bpy
(32 mg, 0.20 mmol) under air. The brownish violet suspension
turned to violet in few minutes. The microcrystalline solid was
collected by filtration, washed with acetonitrile several times, and
then dried in a vacuum (157 mg, 72%). Found: C, 25.46; H, 0.94;
N, 3.04. Required for C23H9.5Br8CrN2.5O4: C, 25.67; H, 0.89; N,
3.25. IR (KBr): 1564w, 1494m, 1469m, 1445s, 1425s, 1349w,
1310m, 1283m, 1249s, 1208w, 1155s, 1124s, 1105s, 1081s, 1060s,
Inorganic Chemistry, Vol. 41, No. 17, 2002 4445

Chang et al.
Table 1. Crystallographic and Refinement Data for 1, 3, and 5
1044s, 1035s, 1023s, 965s, 933s, 886m, 757s, 748s, 729m, 686m,
665m, 651m, 645m, 627m, 576w, 564w, 535w, 504w, 473w, 452m,
1
3
5
404m, and 397m cm-1
.
formula
fw
C16H6Cl8N2O4Cr C24H11Cl8N3O4Cr C16H16Cl8N2O4Cr
CrIII(Cl4SQ)(Cl4Cat)(tmphen) (5). The complex was synthe-
sized by a procedure similar to that of 4 using CrIII(Cl4SQ)3‚4C6H6
(221 mg, 0.2 mmol) and tmphen (48 mg, 0.20 mmol). Recrystal-
lization from chloroform affords reddish violet single crystals (87
mg 56%). Found: C, 43.40; H, 2.45; N, 3.49. Required for C28H16-
Cl8CrN2O4: C, 43.11; H, 2.07; N, 3.59. IR (KBr): 1532m, 1432m,
1402m, 1374w, 1306s, 1248m, 1101s, 1079s, 977s, 936w, 898m,
863w, 827w, 813w, 783s, 750w, 726w, 707w, 690w, 683w, 672w,
642w, 630w, 614w, 589m, 575w, 552w, 535w, 515w, 485w, 459w,
625.85
740.99
reddish violet
triclinic
P1h
8.629(2)
13.739(2)
14.714(3)
115.71(1)
95.706(3)
92.154(4)
2
780.07
reddish violet
monoclinic
C2/c
16.128(7)
26.148(5)
8.5648(9)
90
color
reddish violet
triclinic
P1h
5.9174(8)
9.4798(8)
10.672(2)
109.517(5)
99.967(3)
92.822(2)
1
cryst syst
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
Z
102.944(2)
90
4
V, Å3
522.0(1)
1.882
1557.6(6)
1.580
3520(1)
1.472
424w, and 404w cm-1
.
F
calcd, g/cm3
CrIII(Br4SQ)(Br4Cat)(tmphen) (6). A 65 mL acetonitrile sus-
pension containing CrIII(Br4SQ)3‚4C6H6 (378 mg, 0.231 mmol) was
boiled under air, and then a 20 mL acetonitrile solution containing
tmphen (56 mg, 0.24 mmol) was added. The brownish violet
suspension turned to violet in few minutes. The suspension was
filtered, and the filtrate was allowed to stand for 2 days. The powder
obtained from the filtrate was collected by filtration, washed with
acetonitrile several times, and then dried in a vacuum (147 mg,
55%). Found: C, 29.11; H, 1.49; N, 2.71. Required for C28H16-
Br8CrN2O4: C, 29.61; H, 1.42; N, 2.47. IR (KBr): 1528m, 1469m,
1427m, 1398m, 1376m, 1308w, 1284m, 1267s, 1246m, 1232w,
1198w, 1114s, 933m, 896w, 871w, 812w, 752m, 727m, 722m,
700s, 660w, 623w, 597w, 569w, 556w, 531w, 495w, 434m, and
µ, cm-1
T, K
15.13
10.87
9.66
296
296
0.71069
3207
361
8.88
296
λ (Mo KR), Å 0.71069
0.71069
no. of reflns
2991
1334a
no. of params 154
195
6.9
refln/param
ratio
GOF
19.4
1.678
1.248
1.405
Rint
R, Rw
0.015
0.028
0.038
b
0.042, 0.084
0.053, 0.072
0.074, 0.089
a I > 2.0σ(I). b R ) ∑||Fo| - |Fc||/∑|Fo|, Rw ) [∑w(|Fo| - |Fc|)2/
2
∑w|Fo| ]1/2
.
Results and Discussion
424m cm-1
.
Molecular Structures and Crystal Packing Structures.
Figure 1 shows ORTEP drawings of 1, 3, and 5 with an
atom-numbering scheme. Table 2 lists selected average bond
distances and angles of the complexes. Each chromium ion
has a distorted octahedral coordination environment involving
four oxygen atoms from the two o-quinone ligands and two
nitrogen atoms from the acetonitriles (1), bpy (3), and tmphen
(5). The acetonitrile ligands in 1 coordinate to the chromium
ion in a trans fashion, allowing two o-quinone ligands to sit
coplanarly. The Cr-O bond distances are found to be 1.929-
(3) and 1.935(3) Å for Cr(1)-O(1) and Cr(1)-O(2) bonds,
respectively, and the average distance, 1.932(3) Å, is similar
to those of the series of [CrIII(X4SQ)3-n(X4Cat)n]n- isomers,6a,b
indicating the trivalent nature of the chromium atom. As a
complex of Cr(III), a formal combination of a X4SQ and
X4Cat can be suggested for charge distribution on the quinone
ligands. In many cases the crystallographic symmetry,
however, averages dissimilar ligands, giving bond distances
for the C-O bonds that are intermediate between SQ and
Cat values.3 The chromium atom of 1 is, in fact, on a special
position that averages the structural features of the two
o-quinone ligands. The observed C-O bond distance for 1
is 1.310(5) Å, which is intermediate between 1.28 and 1.34
Å for typical SQ and Cat forms, respectively.
Physical Measurements. Infrared spectra for KBr pellets were
recorded on a Perkin-Elmer system 2000 FT-IR spectrometer over
the range 350-7000 cm-1, and absorption spectra on a Hitachi
U-3500 spectrophotometer over the range 3130-33000 cm-1 at
296 K. Electrochemical measurements were carried out by a BAS
CV-50W polarographic analyzer. A standard three-electrode system
was used with glassy carbon working electrode, platinum-wire
counter electrode, and Ag/Ag+/CH3CN electrode as reference.
Thermogravimetric analyses were made on a Rigaku Thermo Plus
2/TG8120 over the temperature range 295-772 K. Magnetic
susceptibilities were recorded over the temperature range 1.9-350
K at 1 T with a superconducting quantum interference device
(SQUID) susceptometer (Quantum Design, San Diego, CA) inter-
faced with a HP Vectra computer system. All of the values were
corrected for diamagnetism that were calculated from Pascal’s
table.8
Crystallographic Data Collection and Refinement of Struc-
tures. All of the measurements were performed on a Rigaku
mercury diffractometer with CCD two-dimensional detector with
Mo KR radiation employing a graphite monochromator at 296 K.
The sizes of the unit cells were calculated from the reflections
collected on the setting angles of seven frames by changing ω by
0.5° for each frame. Two different ø settings were used, and ω
was changed by 0.5° per frame. Intensity data were collected in
480 frames with an ω scan width of 0.5° and exposure times 120,
70, and 180 s for 1, 3, and 5, respectively. Empirical absorption
correction using the program REQABA9a was performed for all of
the data. All of the crystal data are summarized in Table 1. The
structures were solved by direct methods9b and expanded using
Fourier techniques.9c The final cycles of the full-matrix least-squares
refinements were based on the observed reflections (I > 3σ(I) for
1 and 3 and I > 2σ(I) for 5). All of the calculations were performed
using the teXsan crystallographic software package of Molecular
Structure Corporation.9d
On the other hand, complex 3 has two crystallographically
independent o-quinone ligands, I and II, where the dihedral
(9) (a) Jacobson, R. A. REQABA Empirical Absorption Correction Version
1.1-03101998; Molecular Structure Corp.: The Woodlands, TX,
1996-1998. (b) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano,
M.; Giacovazzo, C.; Guagliardi, A.; Pilidori, G. J. Appl. Crystallogr.
1994, 27, 435. (c) Beurskens, P. T.; Admiraal, G.; Beurskens, G.;
Bosman, W. P.; de Gelder, R.; Israel, R.; Smits, J. M. M. The DIRDIF-
94 program system; Technical Report of the Crystallography Labora-
tory; University of Nijmegen: Nijmegen, The Netherlands, 1994. (d)
teXsan, Crystal Structure Analysis Package; Molecular Structure
Corporation: The Woodlands, TX, 1985, 1992.
(8) Kahn, O. Molecular Magnetism; VCH: New York, 1993.
4446 Inorganic Chemistry, Vol. 41, No. 17, 2002

cis- and trans-CrIII(X4SQ)(X4Cat)(L)n Complexes
Table 2. Selected Average Bond Distances (Å) and Angles (deg) of 1,
3, and 5
1
3
cisb
5
L
transa
av
Cr-O (Å)
I
II
1.934
1.911(4) 1.945(4)
1.943(4) 1.955(4)
1.939
1.949
1.939
1.331
1.310
1.321
2.062
1.912
av
1.934
1.311
1.912
1.34
C-O (Å)
I
II
1.327(8) 1.335(7)
1.312(6) 1.308(6)
av
1.311
2.040(4)
83.68(10)
1.34
2.060(9)
85.1(3)
Cr-N (Å)
O-Cr-O (deg)
I
II
84.4(2)
82.9(1)
83.7
av
83.68(10)
180.0(0)
85.1(3)
N-Cr-N (deg)
78.5(2) 78.3(5)
119.7(8)
N(1)-Cr(1)-O(1)-C(1) (deg)
N(1)-Cr(1)-O(2)-C(6) (deg)
N(2)-Cr(1)-O(1)-C(1) (deg)
N(2)-Cr(1)-O(2)-C(6) (deg)
Cr(1)-O(1)-C(1)-C(6) (deg)
Cr(1)-O(2)-C(6)-C(1) (deg)
Cr(1)-O(3)-C(7)-C(12) (deg)
Cr(1)-O(4)-C(12)-C(7) (deg)
99.5(1)
-99.2(4)
70(1)
-177.8(4)
-5.4(7)
4.7(6)
-117.1(7)
-17(1)
21(1)
11.9(6)
-8.0(6)
a Oxygen atom trans to the nitrogen atom. b Oxygen atom cis to the
nitrogen atom.
In contrast to complex 3, in the crystallographic symmetry
of complex 5 the two different o-quinone ligands similar to
the situation for complex 1 are unable to be distinguished.
Interestingly, the most striking feature of complex 5 is the
heavily bent geometry of the quinone ligands. The six-
membered carbon ring plane of the ligand I forms a dihedral
angle of 25.3° with its CrO2 chelate plane. This angle is larger
than 5.5° and 14.4° for ligands I and II of complex 3,
respectively. The observed bend could be attributed to the
difference of the crystal packing structures of 3 and 5. As
shown in Figure 2a the main intermolecular interaction in 3
is found for the overlap of adjacent bpy ligands with an
interplanar separation of 3.505(8) Å. The ligands I and II
also form intermolecular stacking structures, I‚‚‚I′ and II‚‚‚
II′, with the adjacent complex molecules; however, their
overlaps seem to be quite small. On the other hand complex
5 forms two independent columns of complex molecules.
The first column consists of a stack linked by overlapping
six-membered rings of the adjacent quinone ligand I‚‚‚I′
(Figure 2b) along the ac-direction with a mean interplanar
separation of 3.56(2) Å. The interaction causes the distortion
of the ligands from the ideal planar geometry so that each
ligand bends to the direction of the column.11 Another one-
dimensional column of the complexes occurs from the
overlapping of the adjacent tmphen ligands along the c-axis
(Figure 2c). Each tmphen column links the o-quinone
columns to form a three-dimensional assembled structure of
the complexes in the crystal (Figure 2d).
Figure 1. ORTEP drawings of (a) 1, (b) 3, and (c) 5 with hydrogen atoms
omitted (showing 30% isotropic thermal ellipsoids). Crystallographically
independent ligands are designated I and II. The chromium atoms of 1 and
5 have centroids coincident with crystallographic symmetry.
angle between two ligands defined by C6O2Cl4 planes is 98.8-
(1)°. The C-O bond distances of 1.326(8) and 1.344(8) Å
for ligand I are slightly longer than those of 1.301(6) and
1.314(7) Å for ligand II. These structural differences imply
a localized mixed-valence state for the two independent
ligands. In general, the ligand bite angle of the SQ ligand is
typically 2° smaller than that of the Cat ligand.10 This
tendency is also observed for two independent ligands, where
the O(1)-Cr(1)-O(2) angle of ligand I (Cat) is 84.4(2)°,
larger than the 82.9(2)° angle of ligand II (SQ).
Spectroscopic Properties. UV-Vis-NIR-IR Absorp-
tion Spectra. Infrared spectra (KBr) of all of the complexes
are shown in Figure 3. Each spectrum is depicted by two
characteristic absorption bands observed near 1450 cm-1 and
a broad, strong band near 1100 cm-1. Similar spectra have
been observed for [CrIII(X4SQ)3-n(X4Cat)n]n- (n ) 1-2)
redox isomers with three o-quinonate ligands in the LBMV
state.6a,b By comparing the spectra, the bands at 1450 and
(10) Sofen, S. R.; Ware, D. C.; Cooper, S. R.; Raymond, K. N. Inorg.
Chem. 1979, 18, 234.
(11) Pierpont, C. G.; Buchanan, R. M. J. Am. Chem. Soc. 1975, 97, 4912.
Inorganic Chemistry, Vol. 41, No. 17, 2002 4447

Chang et al.
Figure 2. (a) View showing the pairing of complex 3 within unit cell. The separation between adjacent bpy ligands is 3.505(8) Å. (b) One-dimensional
stacking structures of ligand I and II and (c) that of tmphen ligands in complex 5. (d) Whole crystal packing diagram of 5 along the a-axis.
Table 3. Absorption Spectral Parameters for Complexes 1-6 in
Solution
νmax × 10-3/cm-1 (ꢀmax × 10-3 M-1 cm-1
5.08 (3.47), 7.41 (sh, 1.15), 8.47 (0.789), 11.0 (1.54), 13.6 (1.98),
15.9 (0.824), 19.0 (4.31), 24.8 (2.32)
5.05 (4.26), 7.46 (sh, 1.35), 8.38 (0.948), 10.9 (1.61), 13.4 (2.16),
16.0 (0.808), 19.0 (0.507), 24.2 (2.58)
4.32 (2.50), 11.2 (2.42), 12.7 (2.52), 14.0 (sh, 1.95), 15.6 (1.72),
18.1 (6.50), 21.6 (sh, 2.74), 24.8 (sh, 3.07), 32.9
3.77, 11.2 (2.04), 12.6 (2.26), 13.9 (1.62), 15.7 (1.47), 18.1 (5.28),
21.4 (2.16), 23.9 (2.43), 33.2 (16.0)
4.31 (2.12), 11.2 (2.34), 12.8 (2.47), 14.1 (sh, 1.51), 15.8 (1.41),
18.3 (5.96), 21.3 (2.68), 24.9 (2.95)
11.1 (2.27), 12.6 (3.07), 14.5 (sh, 1.51), 15.9 (1.66), 18.2 (6.91),
21.4 (3.05), 24.0 (3.50)
)
1a
2a
3b
4b
5b
6b
a Measured in 1,2-dimethoxyethane. b Measured in dichloromethane.
strates the LBMV state of 1-6, although complexes 1 and
5 possess crystallographic ambiguity for distinction of the
SQ and Cat ligands due to their crystallographic symmetry.
Figure 4 shows UV-vis-NIR-IR absorption spectra of
the complexes measured in solution (solid lines; 1,2-
dimethoxyethane (1, 2) and dichloromethane (3-6)) and on
KBr pellets (dashed lines). The observed peak maxima are
summarized in Tables 3 (solution) and 4 (solid). In the UV-
visible region, all of the complexes commonly show several
intense absorption bands. For the spectra in solution the bands
near 14000 and 21000 cm-1 observed for 3-6 could be
assigned to electronic transitions related to the nitrogen
ancillary ligands, bpy and tmphen, because of the absence
of the corresponding bands in 1 and 2. The strongest
electronic transition is commonly observed around 19000
cm-1 for all of the complexes, where the absorption energies
of the bands for 1 and 2 are higher by 250 cm-1 than those
of 3-6. These bands have been observed in the complexes
containing CrIII-SQ moieties and, thus, are useful for chro-
Figure 3. FT-IR spectra (KBr pellets) of complexes (a) 1, (b) 2, (c) 3, (d)
4, (e) 5, and (f) 6.
1100 cm-1 are assigned to the vibration modes characteristic
for SQ•- and Cat2-, respectively. The data clearly demon-
4448 Inorganic Chemistry, Vol. 41, No. 17, 2002

cis- and trans-CrIII(X4SQ)(X4Cat)(L)n Complexes
Figure 4. Electronic absorption spectra (solid lines) of (a) 1, (b) 2, (c) 3,
(d) 4, (e) 5, and (f) 6 in 1,2-dimethoxyethane (1 and 2) and dichloromethane
(3-6) at 296 K. The dotted lines show electronic absorption spectra of the
complexes measured on KBr pellets at 296 K. The discontinuity near 12000
cm-1 is an instrumental artifact due to a detector change in the spectrometer.
Table 4. Absorption Spectral Parameters for Complexes 1-6 Measured
on KBr Pellets
Figure 5. Cyclic voltammograms of complexes (a) 1, (b) 2, (c) 3, (d) 4,
(e) 5, and (f) 6 measured in 1,2-dimethoxyethane (1 and 2) and dichlo-
romethane (3-6) at 296 K.
νmax × 10-3/cm-1
1
2
3
4
5
6
4.62, 5.10, 5.90 (sh), 6.60 (sh), 8.20 (sh), 12.1, 13.5 (sh), 15.6, 18.6,
23.9
4.50 (sh), 5.00, 5.70 (sh), 6.45 (sh), 8.00 (sh), 12.0, 13.4 (sh),
15.6 (sh), 18.6, 23.5 (sh)
3.43, 4.00 (sh), 5.10 (sh), 5.80 (sh), 8.50 (sh), 11.1, 12.5, 15.3 (sh),
18.0, 24.0 (sh)
3.42, 4.20 (sh), 5.10 (sh), 8.40 (sh), 10.5, 12.2 (sh), 15.7 (sh), 17.6,
23.8
4.35 (sh), 4.73, 9.40 (sh), 10.9, 12.0 (sh), 14.2, 15.3, 18.1, 21.4,
25.0 (sh)
4.40 (sh), 5.22, 9.20 (sh), 9.90 (sh), 10.6, 12.0 (sh), 13.8, 14.6, 15.4,
18.9, 21.7, 25.0 (sh)
The intensities of the IVCT bands for complexes 1 and 2,
which contain two quinone ligands in a plane, show higher
absorption intensities compared with those of 3-6. In the
solid state (KBr) all of the complexes show similar absorption
spectra with slight blue or red shifts at each absorption
maximum as shown in Figure 4 (dotted lines). The presence
of the IVCT bands indicates that in the solid state all of the
complexes possess the LBMV state.
Electrochemical Properties. Because of the redox-active
nature of the ligand moieties, ligand-based electrochemical
activity would be expected for the present complexes. The
electrochemical properties of the complexes were studied in
1,2-dimethoxyethane and dichloromethane solution for 1 and
2 and 3-6, respectively. The results are given in Figure 5
and Table 5. All of the complexes commonly undergo quasi-
reversible one-electron oxidation and reduction near 630 and
170 mV vs Ag/Ag+/CH3CN, respectively. Because the
chromium ion retains a trivalent state in this potential region,
the electrochemical processes correspond to one-electron
oxidation and reduction of the ligand moieties, respectively,
as shown in eq 4.
mophore identification.12 The most prominent spectral fea-
tures of spectra shown in the figures are the broad, intense
absorption bands, with several overtone vibrational transi-
tions, that appear in 3000-8000 cm-1 in each complex. Such
low-energy electronic transitions have been observed in the
series of tris-type [CrIII(X4SQ)3-n(X4Cat)n]n- (n ) 1, 2)
LBMV isomers.6 The bands have been assigned to interligand
intervalence CT (IVCT) transitions of π*(SQ) r π*(Cat),2b
indicating the presence of the SQ•- and Cat2- ligands in 1-6.
(12) (a) Benelli, C.; Dei, A.; Gatteschi, D.; Pardi, L. Inorg. Chem. 1989,
28, 1476. (b) Wheeler, D. E.; McCusker, J. K. Inorg. Chem. 1998,
37, 2296.
Inorganic Chemistry, Vol. 41, No. 17, 2002 4449

Chang et al.
Table 5. Electrochemical Data for Complexes 1-6
potential (mV vs Ag/Ag+/CH3CN)a
+1/0
0/-1
d
d
e
Eox2
Eox1
Eox
Ered
∆
E1/2
Eox
Ered
∆
E1/2
∆E1/2
Ered1
Ered2
1b
2b
3c,f
4c
5c
6c
1400
1370
817
826
778
730
718
707
505
493
575
550
551
537
312
333
203
190
167
170
661
660
677
640
635
622
327
333
265
276
230
229
6
-9
107
103
83
321
342
158
173
147
152
167
162
186
190
157
153
494
498
491
450
478
467
-1440
-1560
1860
1860
1170
1730
-1720
-1150
-1410
-1550
1540
1540
77
a Conditions: scan rate 100 mV/s, N2, 296 K. b Measured in 1,2-dimethoxyethane. c Measured in dichloromethane. d Separation between Eox and Ered
.
e Separation between E1/2(+1/0) - E1/2(0/-1). f Reference 7.
[CrIII(X SQ) (L) ]+ T
0 T
III
[Cr (X4SQ)(X4Cat)(L)n]
4
2
n
1-6
[CrIII(X4Cat)2(L)n]- (4)
The one-electron oxidation would provide a cationic
[CrIII(X4SQ)2(L)n]+ species (left) with two SQ ligands, while
the reduction affords an anionic [CrIII(X4Cat)2(L)n]- species
(right) with two Cat ligands. These electrochemical properties
are similar to those observed for CrIII(SQ)(Cat)(bpy) com-
plexes, as reported by Pierpont et al.7 The reduction potentials
of complexes 3 and 4 are higher than those of 5 and 6,
reflecting the electron-donating effect of methyl groups of
the tmphen ligand. Similarly, lower oxidation potentials are
observed for complexes 3 and 4 compared with complexes
5 and 6. The differences between the first oxidation and the
reduction potentials are approximately 500 mV for all of the
complexes, which is comparable to 390 mV observed for
the mixed-valence Creutz-Taube ion, [(NH3)5Ru(py)Ru-
(NH3)5]5+ (py ) pyrazine).13 All of the complexes show
irreversible reduction waves at potentials lower than -1400
mV. The process would be attributable to the reduction of
the CrIII to the CrII. Finally, one (1-4) or two (5, 6)
irreversible oxidation waves are also found. From a com-
parison with the redox properties of CrIII(X4SQ)3, the
oxidation of SQ to BQ would be attributable to these redox
process.14 The irreversible electrochemical process is forced
by the low coordination ability of the neutral BQ ligand.
Thermogravimetric Analyses. To examine the thermo-
labilities of the axially coordinated acetonitrile ligands of 1
and 2, thermogravimetric (TG-DTA) measurements were
carried out. The TG-DTA diagrams of both complexes are
given in Figure 6. Rapid weight losses of 14.2% (1) and
9.0% (2) take place up to 520 K for the two complexes. The
liberation of two acetonitrile ligands accounts for this weight
loss in 1 (13.1%) and in 2 (8.4%) (eq 5). Above this
temperature, the complexes gradually lost weight, which
indicated thermal decomposition.
Figure 6. Thermogravimetric data for 1 (solid line) and 2 (dotted line).
Figure 7. Plots of the temperature dependence of øMT for (a) 1 (O), (b)
2 (0), (c) 3 (]), (d) 4 (×), (e) 5 (+), and (f) 6 (4) measured under a field
of 1 T.
trans-CrIII(X4SQ)(X4Cat)(CH3CN)
2 f
1, 2
Temperature Dependence of Magnetic Susceptibilities.
Figure 7 shows the temperature dependence of øMT for all
of the complexes. At 300 K, the values are in the range
0.934-1.12 emu K mol-1, close to the theoretical value of
1.00 emu K mol-1 expected for an S ) 1 spin (g ) 2.00).8
The S ) 1 ground state could be attributed to the intramo-
[CrIII(X4SQ)(X4Cat)] + 2CH3CN (5)
(13) (a) Creutz, C.; Taube, H. J. Am. Chem. Soc. 1973, 95, 1086. (b) Creutz,
C. Prog. Inorg. Chem. 1983, 30, 1.
(14) Downs, H. H.; Buchanan, R. M.; Pierpont, C. G. Inorg. Chem. 1979,
18, 1736.
4450 Inorganic Chemistry, Vol. 41, No. 17, 2002

cis- and trans-CrIII(X4SQ)(X4Cat)(L)n Complexes
lecular antiferromagnetic interactions between an S ) 3/2 spin
on the Cr(III) ion and an S ) 1/2 spin on the SQ ligand. No
temperature dependence of øMT was observed in the region
of 50-300 K for all of the complexes; therefore, it must be
concluded that the coupling is strongly antiferromagnetic
with a ground state S ) 1 and no exited state thermally
populated. These results are similar to those for the complex,
[CrIII(tren)(3,6-DTBSQ)](PF6)2 (tren ) tris(2-aminoethyl)-
amine),12b containing the same spin components as the
present complexes. The øMT values exhibit temperature
independent behavior in the region of 50-300 K, while a
rapid decrease was observed below 50 K. The decrease of
øMT values is suggestive of intermolecular antiferromagnetic
interactions between the neighboring complexes, which are
attributable to the stacking interaction between the adjacent
complex molecules.
Spectroscopic Studies of CrIII(X4SQ)3 in Nonpolar and
Polar Solvents. CrIII(X4SQ)3 undergoes stepwise ligand-
based reduction as shown in eq 3 to give seven-membered
redox isomers including the LBMV complexes.6,14 The one-
and two-electron-reduced species are synthesized by charge-
transfer reactions performed in nonpolar solvents such as
dichloromethane and carbon disulfide. This is because
CrIII(X4SQ)3, with unique charge distribution over three
ligands, is fairly stable in nonpolar solvents. On the other
hand, the dissolution of CrIII(X4SQ)3 in acetonitrile gives rise
to a rapid formation of the dark purple solids of 1 and 2. As
reported by Pierpont et al., CrIII(SQ)3 shows poor ligand
exchange reactivity in a nonpolar solvent owing to the kinetic
inertness of the octahedral CrIII (d3).7 On the other hand, once
the one-electron-oxidized species of [CrIII(BQ)(SQ)2]+ forms,
a rapid ligand exchange with bpy occurs because of the weak
coordination ability of the BQ ligand (eq 6). However, we
found that in acetonitrile media the ligand exchange reaction
proceeds without oxidation of the complex.
Figure 8. Electronic absorption spectra of CrIII(Cl4SQ)3. Top: measured
in dichloromethane (solid line), carbon disulfide (dotted line), and cyclo-
hexane (bold line). Bottom: measured in acetone (dotted line), nitromethane
(dashed line), and benzonitrile (bold line) together with the spectrum of
complex 1 in 1,2-dimethoxyethane (solid line).
increase of the intensities in the region of 4000-8000 cm-1
as shown in Figure 8 (below). All of the spectra measured
in the polar solvent are close to the spectrum of 1 measured
in 1,2-dimethoxyethane, and drawn in Figure 8 (bottom) as
a solid line, rather than to those of CrIII(Cl4SQ)3 in the
nonpolar solvents. The most prominent feature is the presence
of the low-energy absorption band maxima near 4000 cm-1,
which is characteristic for the LBMV state (see above). A
similar spectral change was observed in a dilute acetonitrile
(ꢀr ) 36.00) solution of CrIII(Cl4SQ)3. These spectral features
clearly indicate that the dissolution of CrIII(Cl4SQ)3 in polar
solvents causes the formation of LBMV species.
Since CrIII(X4SQ)3 does not show spontaneous ligand
dissociation or ligand substitution reaction in nonpolar media,
the solvent polarity is significantly associated with the
intramolecular charge distribution of the complexes. On the
basis of the formation of 1-6 and observed spectral change
by solvent polarity, a mechanism drawn in Scheme 1 would
be one of the most possible mechanisms for the present
system. Dissolution of CrIII(X4SQ)3 in polar solvents would
lead to a disproportionation of the charge on the ligand
moieties, affording a tris-type LBMV species, [CrIII(X4BQ)-
(X4SQ)(X4Cat)], with three different ligand forms. The
LBMV species could be generated by an intramolecular
interligand electron-transfer process in CrIII(X4SQ)3. The
observed spectra indicate that the equilibrium lies to the left
in low-dielectric solvents, whereas in high-dielectric solvents
Since no chemical and spectroscopic studies of CrIII(SQ)3
in polar solvents have yet been reported,15 the electronic
absorption spectra of CrIII(Cl4SQ)3 were determined in
dichloromethane,5a carbon disulfide, cyclohexane, nitromethane,
acetone, and benzonitrile to investigate the conversion of
CrIII(X4SQ)3 to 1-6. As shown in Figure 8 (top), the spectra
obtained for the solvents with a low dielectric constant (ꢀr
at 293 K), cyclohexane (ꢀr ) 2.02), carbon disulfide (ꢀr )
2.64), and dichloromethane (ꢀr ) 9.02), were found to be
similar to each other. In these solvents, a strong absorption
maxima (ꢀ ) ca. 20000 M-1 cm-1) near 19000 cm-1 is found
and no absorption bands lower than 7000 cm-1 in energy
can be detected in the region. On the other hand, the
dissolution of CrIII(Cl4SQ)3 in polar solvents such as acetone
(ꢀr ) 21.36), benzonitrile (ꢀr ) 25.30), or nitromethane (ꢀr
) 36.16) causes a decrease of absorption intensities (ꢀ )
ca. 6000 M-1 cm-1) of the bands near 19000 cm-1 and an
(15) Vlcˇek, A., Jr. Inorg. Chem. 1986, 25, 522.
Inorganic Chemistry, Vol. 41, No. 17, 2002 4451

Chang et al.
Scheme 1
the equilibrium is driven to the right. Although the LBMV
species could not be isolated or detected, it seems likely that
the LMBV species is immediately followed by the dissocia-
tion of the X4BQ ligand in the polar solvents because the
X4BQ ligand possesses the weakest coordination ability
among the three redox isomers, BQ, SQ•-, and Cat2-. In fact,
the red crystals were isolated from the filtrate, and they show
an infrared band at 1680 cm-1, assigned to the CdO
stretching mode of Cl4BQ. If no chelate reagents such as
bpy or tmphen are in the solution, the acetonitrile (solvent)
molecules coordinate to the chromium(III) ion to form trans
complexes 1 and 2. While in the presence of the chelate
reagents, cis-coordinated products should be generated.
The intramolecular electron transfers between the ligand
and the metal ion occur by temperature, pressure, and light
in the valence tautomeric cobalt and manganese complexes.
Furthermore, Hendrickson et al. reported a solvent effect on
charge distribution of Fe(phenQ)2(N-N) (phenQ ) 9,10-
phenathreneSQ/Cat) (eq 7),16
(M-L mechanism), while with chromium complexes only
the ligands could change their valence states (L-L mecha-
nism), due to the thermodynamic stability of the chromium-
(III) ion. As shown in Scheme 1, we believe that in the
solvent media with relatively high dielectric constants the
homoleptic tautomer isomerizes rapidly to the tris-type
LBMV species through the intramolecular interligand electron-
transfer process. The LBMV species readily lose the X4BQ
ligand, followed by a ligand exchange reaction by the solvent
molecules or the chelate reagents to give complexes 1-6.
Conclusion
In this work, we reported the synthesis of novel LBMV
chromium complexes with two o-quinonate ligands, based
on the internal charge distribution of CrIII(X4SQ)3 in polar
solvents, followed by the ligand exchange reaction of the
LBMV tautomer, [CrIII(X4BQ)(X4SQ)(X4Cat)], by the solvent
molecules or the chelate reagents. Their structural and
spectroscopic results could lead to further progress in the
understanding of systems with comparisons to the tris-type
LBMV complexes, [CrIII(X4SQ)3-n(X4Cat)n]n-.6
FeII(phenSQ)2(N-N) T FeIII(phenSQ)(phenCat)(N-N) (7)
where the equilibrium lies to the left in a high-dielectric
solvent, and where in low-dielectric solvent the equilibrium
is shifted to the right to give the LBMV tautomer. This
tendency would be changed in the present system, where
the high-dielectric solvent medium promotes the dispropor-
tionation of the charge over the three ligands. The difference
between these two systems could be attributable to the
difference of the metal ions used. Namely, in iron complexes,
both metal ions and ligands can change their valence states
Acknowledgment. The authors acknowledge financial
support by a Grant-in-Aid for Scientific Research (Priority
Area No. 10149101) from The Ministry of Education,
Science, Sports and Culture of Japan.
Supporting Information Available: Crystal and structure
refinement data, positional parameters, anisotropic displacement
parameters, and complete listings of bond distances and angles of
1, 3, and 5, in CIF format. This material is available free of charge
via the Internet at http://pubs.acs.org.
(16) Lynch, M. W.; Valentine, M.; Hendrickson, D. N. J. Am. Chem. Soc.
1982, 104, 6982.
IC025643P
4452 Inorganic Chemistry, Vol. 41, No. 17, 2002