Acid-base equilibrium of aqua-chromium-dioxolene complexes aimed at formation of oxo-chromium complexes

Article abstract of DOI:10.1021/ic020308m

A series of aqua-Cr(III)-dioxolene complexes, [Cr(OH₂)(3,5-Bu₂SQ)(trpy)](ClO₄)₂ (1s), [Cr(OH₂)(3,5-Bu₂Cat)(trpy)]-ClO₄ (1c), [Cr(OH₂)(3,6-Bu₂SQ)(trpy)](ClO₄)₂ (2), [Cr(OH₂)(Cat)(trpy)]ClO₄ (3), [Cr(OH₂)(Cl₄Cat)(trpy)]ClO₄ (4), [Cr-(OH₂)(3,5-Bu₂SQ)(Me₃-tacn)] (ClO₄)₂ (5), [Cr(OH₂)(Cat)(Me₃-tacn)]ClO₄ (6), and [Cr(OH₂)(Cl₄Cat)(Me₃-tacn)] ClO₄ (7) (Bu₂SQ = di-tert-butyl-o-benzosemiquinonate anion, Bu₂Cat = di-tert-butylcatecholate dianion, Cat = catecholate dianion, Cl₄Cat = tetrachlorocatecholate dianion, trpy = 2,2′:6′,2″-terpyridine, and Me₃-tacn = 1,4,7-trimethyl-1,4,7-triazacyclononane), were prepared. On the basis of the crystal structures, redox behavior, and elemental analyses of these complexes, dioxolene in 1c, 3, 4, 6, and 7 coordinated to Cr(III) as the catechol form, and the ligand in 1s, 2, and 5 was linked to Cr(III) with the semiquinone form. All the aqua-Cr(III) complexes reversibly changed to the hydroxo-Cr(III) ones upon dissociation of the aqua proton, and the pK_a value of the aqua-Cr(III) complexes increased in the order 6 > 3 ≈ 1c > 7 > 5 ≈ 4 > 1s. Hydroxo-Cr(III)-catechol complexes derived from 1c, 3, 4, 6, and 7 did not show any signs of dissociation of their hydroxy proton. On the other hand, hydroxo-Cr(III)-semiquinone complexes were reduced to hydroxo-Cr(III)-catechol in H₂O/THF at pH 11 under illumination of visible light.

Full text of DOI:10.1021/ic020308m

Inorg. Chem. 2002, 41, 5912−5919
Acid−Base Equilibrium of Aqua−Chromium−Dioxolene Complexes
Aimed at Formation of Oxo−Chromium Complexes
Kazushi Shiren and Koji Tanaka*
Institute for Molecular Science and CREST, Japan Science and Technology Corporation (JST),
38 Nishigonaka, Myodaiji, Okazaki, Aichi 444-8585, Japan
Received May 1, 2002
A series of aqua−Cr(III)−dioxolene complexes, [Cr(OH )(3,5-Bu2SQ)(trpy)](ClO ) (1s), [Cr(OH )(3,5-Bu2Cat)(trpy)]-
2
4 2
2
ClO (1c), [Cr(OH )(3,6-Bu2SQ)(trpy)](ClO ) (2), [Cr(OH )(Cat)(trpy)]ClO (3), [Cr(OH )(Cl4Cat)(trpy)]ClO (4), [Cr-
4
2
4 2
2
4
2
4
(OH )(3,5-Bu2SQ)(Me3-tacn)](ClO ) (5), [Cr(OH )(Cat)(Me3-tacn)]ClO (6), and [Cr(OH )(Cl4Cat)(Me3-tacn)]ClO (7)
2
4 2
2
4
2
4
(Bu2SQ ) di-tert-butyl-o-benzosemiquinonate anion, Bu2Cat ) di-tert-butylcatecholate dianion, Cat ) catecholate
dianion, Cl4Cat ) tetrachlorocatecholate dianion, trpy ) 2,2′:6′,2′′-terpyridine, and Me3-tacn ) 1,4,7-trimethyl-
1,4,7-triazacyclononane), were prepared. On the basis of the crystal structures, redox behavior, and elemental
analyses of these complexes, dioxolene in 1c, 3, 4, 6, and 7 coordinated to Cr(III) as the catechol form, and the
ligand in 1s, 2, and 5 was linked to Cr(III) with the semiquinone form. All the aqua−Cr(III) complexes reversibly
changed to the hydroxo−Cr(III) ones upon dissociation of the aqua proton, and the pK value of the aqua−Cr(III)
a
complexes increased in the order 6 > 3 ≈ 1c > 7 > 5 ≈ 4 > 1s. Hydroxo−Cr(III)−catechol complexes derived from
1c, 3, 4, 6, and 7 did not show any signs of dissociation of their hydroxy proton. On the other hand, hydroxo−
Cr(III)−semiquinone complexes were reduced to hydroxo−Cr(III)−catechol in H O/THF at pH 11 under illumination
2
of visible light.
Introduction
plexes. The structural conversion between Cu2(µ-O)2 and Cu2-
(µ-η2:η2-O2) cores is also confirmed at low temperatures.3
These studies greatly contribute to the elucidation of the
chemical properties and reactivity of the M2(µ-O)2 cores. On
the other hand, a rational preparative route to metal com-
plexes with terminal oxo ligands through O-O bond
cleavage of O2 has not been presented so far despite the
Regulation of reactivity of oxo groups ligated on metals
has been a fascinating subject in biological and catalytic
oxidation reactions. High-valent M2(µ-O)2 complexes (M )
Cu,1-16 Fe,17-21 Co,22,23 and Ni22,24) have been obtained by
2-
the O-O bond scission of O2 on dinuclear metal com-
* To whom correspondence should be addressed. E-mail: ktanaka@
ims.ac.jp. Phone: +81-564-55-7241. Fax: +81-564-55-5245.
(1) Mahapatra, S.; Halfen, J. A.; Wilkinson, E. C.; Pan, G.; Cramer, C.
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5912 Inorganic Chemistry, Vol. 41, No. 22, 2002
10.1021/ic020308m CCC: $22.00 © 2002 American Chemical Society
Published on Web 10/11/2002

Aqua-Chromium-Dioxolete Complexes
importance of those complexes in many enzymatic oxygenase
reactions.25,26 Accordingly, aqua-Ru complexes have widely
been used as precursors to oxo-Ru ones because oxidation
of those complexes is accompanied with loss of protons to
afford higher oxidation states of hydro- and oxo-Ru ones
(eq 1).27 The accessibility to the higher oxidation states of
Experimental Section
WARNING! Perchlorate salts are potentially explosives and
should only be handled in small quantities.
Materials. All reagents and solvents were commercially available
and used without further purification.
[Cr(OH2)(3,5-Bu2SQ)(trpy)](ClO4)2‚3(CH3OH) (1s). A mixture
of [CrCl3(trpy)]36 (392 mg, 1.0 mmol) and AgClO4 (622 mg, 3.0
mmol) was refluxed in methanol (30 cm3) for 6 h, and precipitated
AgCl was removed by filtration. To the brown filtrate (A) was
added 3,5-di-tert-butylcatechol (222 mg, 1.0 mmol) and sodium
hydroxide (80 mg, 2.0 mmol), and then the solution was stirred
for 6 h under N2. Acidification of the solution by an addition of
0.2 cm3 of HClO4 (70%) and the subsequent treatment with AgClO4
(207 mg, 1.0 mmol) brought about a change in color of the solution
from dark brown to green. After filtration of the solution to remove
Ag, water (10 cm3) was added to the filtrate. Slow evaporation of
the solvent yielded a green needle crystalline precipitate, which
was filtered off, washed with water, and air-dried. Yield: 539 mg
(66%). Anal. Calcd for C29H33N3O11Cl2Cr‚3(CH3OH): C, 46.95;
-
-e , -H
[RuII(OH2)(bpy)(trpy)]2+
[RuIII(OH)(bpy)(trpy)]2+
+8
-
+8 [RuIV(O)(bpy)(trpy)]2+
-e , -H
(1)
the complexes is attributable to electron donation from bound
hydroxo or oxo ligands. In addition, the higher oxidation
state oxo-Ru complexes thus formed have proven to work
as stoichiometric and catalytic oxidants.28-30 Recently, we
have shown that replacement of bpy of [Ru(OH2)(bpy)-
(trpy)]2+ with quinone as an electron acceptor greatly
enhances the acidity of the aqua ligand.31-34 Moreover,
monomeric and dimeric oxo-Ru complexes derived from
aqua-Ru ones showed high catalytic activity for some
dehydrogenation reactions32 and four-electron oxidation of
water.33,34 Along the line, we also examined the reactivity
and physical properties of aqua-, hydroxo-, and oxo-
chromium(III)-dioxolene complexes. Here, we report the
synthesis of a series of aqua-Cr-dioxolene complexes, [Cr-
(OH2)(3,5-Bu2SQ)(trpy)](ClO4)2 (1s), [Cr(OH2)(3,5-Bu2Cat)-
(trpy)]ClO4 (1c), [Cr(OH2)(3,6-Bu2SQ)(trpy)](ClO4)2 (2),
[Cr(OH2)(Cat)(trpy)]ClO4 (3), [Cr(OH2)(Cl4Cat)(trpy)]ClO4
(4), [Cr(OH2)(3,5-Bu2SQ)(Me3-tacn)](ClO4)2 (5), [Cr(OH2)-
(Cat)(Me3-tacn)]ClO4 (6), and [Cr(OH2)(Cl4Cat)(Me3-tacn)]-
ClO4 (7)35 aimed at smooth conversion to oxo-Cr(III) ones.
H, 5.54; N, 5.13. Found: C, 47.26; H, 5.18; N, 4.98. UV-vis (λmax
/
nm (ꢀ/mol-1 dm3 cm-1) in 1 mmol dm-3 HClO4 at 25 °C): 281
(17200), 328 (11500, sh), 337 (11600), 403 (5360), 432 (5070),
461 (6320), 520 (450, sh), 632 (450), 685 (950), 875 (230, br),
990 (150).
[Cr(OH2)(3,5-Bu2Cat)(trpy)]ClO4‚2(CH3OH) (1c). An aqueous
solution (10 cm3) of l(+)-ascorbic acid (88 mg, 0.5 mmol) was
added to a methanolic solution (30 cm3) of 1s (409 mg, 0.5 mmol)
with stirring at 0 °C under N2. The mixture was stirred for 5 min,
and then 3 drops of HClO4 (70%) were added. Concentration of
the solution under N2 gave 1c as a brown precipitate. Yield: 175
mg (51%). Anal. Calcd for C29H33N3O7ClCr‚2(CH3OH): C, 54.19;
H, 6.01; N, 6.11. Found: C, 53.98; H, 5.98; N, 6.21. UV-vis (λmax
/
nm (ꢀ/mol-1 dm3 cm-1) in 1 mmol dm-3 HClO4 at 25 °C): 282
(16200), 325 (11100).
Single crystals suitable for X-ray crystallography were obtained
as follows: To a methanolic solution (30 cm3) of 1c (50 mg, 0.08
mmol) was added H23,5-Bu2Cat (22 mg, 0.10 mmol), 3 drops of
HClO4 (70%), and 10 cm3 of water. Slow evaporation of the solvent
under N2 gave the 1:1 adduct of 1c with 3,5-di-tert-butylcatechol
([Cr(OH2)(3,5-Bu2Cat)(trpy)]ClO4‚H23,5-Bu2Cat‚CH3OH (1c′)) as
large brown crystals. Yield: 58 mg (83%). Anal. Calcd for
C29H33N3O7ClCr‚C14H22O2‚CH3OH: C, 60.23; H, 6.78; N, 4.79.
Found: C, 59.99; H, 6.88; N, 4.71.
[Cr(OH2)(3,6-Bu2SQ)(trpy)](ClO4)2‚CH3OH‚H2O (2). This
complex was obtained as green crystals in a manner similar to that
of 1s except that 3,6-di-tert-butylcatechol37 was used in place of
3,5-di-tert-butylcatechol. Additions of water (10 cm3), 3 drops of
HClO4 (70%) and NaClO4 (10 mg, 0.08 mmol) to a methanolic
solution (10 cm3) of 2 (100 mg, 0.13 mmol) gave green crystals
suitable for X-ray crystallography. Yield: 426 mg (55%). Anal.
Calcd for C29H33N3O11Cl2Cr‚CH3OH‚H2O: C, 46.64; H, 5.09; N,
5.44. Found: C, 46.75; H, 5.02; N, 5.38. This complex was stable
in the solid state but slowly decomposed in solutions.
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K.; Mu¨nck, E. J. Am. Chem. Soc. 1997, 119, 12683-12684.
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Soc. 1998, 120, 4699-4710.
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A.; Watanabe, Y.; Moro-oka, Y. J. Am. Chem. Soc. 2000, 122, 254-
262.
(25) Special thematic issue for metal-dioxygen complexes: Chem. ReV.
1994, 94, 567-856 and references therein.
(26) Special thematic issue for oxygen metabolism in bioinorganic
enzymology: Chem. ReV. 1996, 96, 2541-2950 and references therein.
(27) Takeuchi, K. J.; Thompson, M. S.; Pipes, D. W.; Meyer, T. J. Inorg.
Chem. 1984, 23, 1845-1851.
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Chem. Soc., Dalton Trans. 1992, 1551-1556.
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(31) Tsuge, K.; Kurihara, M.; Tanaka, K. Bull. Chem. Soc. Jpn. 2000, 73,
607-614.
[Cr(OH2)(Cat)(trpy)]ClO4‚H2O (3). To a methanolic solution
of A (same scale as 1s) was added catechol (110 mg, 1.0 mmol)
and sodium hydroxide (80 mg, 2.0 mmol) under N2. The mixture
was stirred for 6 h, and then 0.2 cm3 of HClO4 (70%) and 10 cm3
of water were added to the solution. Evaporation of the solvent
(32) Wada, T.; Tsuge, K.; Tanaka, K. Chem. Lett. 2000, 910-911.
(33) Wada, T.; Tsuge, K.; Tanaka, K. Angew. Chem., Int. Ed. 2000, 39,
1479-1482.
(34) Wada, T.; Tsuge, K.; Tanaka, K. Inorg. Chem. 2001, 40, 329-337.
(35) Abbreviations of ligands used: Bu2SQ ) di-tert-butyl-o-benzosemi-
quinonate anion, Bu2Cat ) di-tert-butylcatecholate dianion, Cat )
catecholate dianion, Cl4Cat ) tetrachlorocatecholate dianion, trpy )
2, 2′: 6′, 2”-terpyridine, Me3-tacn ) 1,4,7-trimethyl-1,4,7-triazacy-
clononane, bpy ) 2, 2′-bipyridine, acac ) acetylacetonate anion, tren
) tris(2-aminoethyl)amine, and CTH ) (()5,7,7,12,14,14-hexamethyl-
1,4,8,11-tetraazacyclotetradecane.
(36) Broomhead, J. A.; Evans, J.; Grumley, W. D.; Sterns, M. J. Chem.
Soc., Dalton Trans. 1976, 173-176.
(37) Belostotskaya, I. S.; Komissarova, N. L.; Dzhuaryan, E. V.; Ershov,
V. V. IsV. Akad. Nauk SSSR 1972, 1594-1596.
Inorganic Chemistry, Vol. 41, No. 22, 2002 5913

Shiren and Tanaka
under N2 stream afforded brown crystals suitable for X-ray
diffraction. Yield: 372 mg (70%). Anal. Calcd for C21H17N3O7-
ClCr‚H2O: C, 47.69; H, 3.62; N, 7.95. Found: C, 47.75; H, 3.74;
N, 7.82. UV-vis (λmax/nm (ꢀ/mol-1 dm3 cm-1) in 1 mmol dm-3
HClO4 at 25 °C): 288 (19500), 334 (14000), 350 (12000, sh), 650
(100).
[Cr(OH2)(Cl4Cat)(trpy)]ClO4‚H2O (4). Tetrachlorocatechol
(248 mg, 1.0 mmol) and sodium hydroxide (80 mg, 2.0 mmol) were
added to a methanolic solution of A (same scale as 1s). The mixture
was stirred for 6 h, and then 0.2 cm3 of HClO4 (70%) was added
to the suspension. After insoluble precipitate was removed by
filtration, the brown filtrate was allowed to stand for several days
to give brown crystals suitable for X-ray crystallography. Yield:
391 mg (59%). Anal. Calcd for C21H13N3O10Cl5Cr‚H2O: C, 37.84;
TUM DESIGN MPMS model. Diamagnetic correction was made
using Pascal’s constants.41
X-ray Crystallography. General Procedures. Data were col-
lected on a Rigaku/MSC Mercury CCD diffractometer using
graphite monochromated Mo KR radiation (λ ) 0.71070 Å). To
determine the cell constants and the orientation matrix, four
oscillation photographs were taken with an oscillation angle of 0.5°.
Intensity data were collected at a temperature of -100 ( 1 °C to
a maximum 2θ value of 55.0°. A total of 720 oscillation images
were collected. The first sweep of data was done using ω scans
from -70.0 to 110.0° in 0.5° step, at ø ) 45.0° and φ ) 0.0°. The
second sweep of data was done using ω scans from -70.0 to 110.0°
in 0.5° step, at ø ) 45.0° and φ ) 90.0°. The detector swing angle
was 20.0°. The crystal-to-detector distance was 45.0 mm.
The structure was solved by heavy-atom Patterson methods42
and expanded using Fourier techniques.43 The non-hydrogen atoms
were refined anisotropycally. Hydrogen atoms were included but
not refined. All calculations were performed using the teXsan44
crystallographic software package of Molecular Structure Corpora-
tion.
H, 2.27; N, 6.30. Found: C, 38.10; H, 2.27; N, 6.30. UV-vis (λmax
/
nm (ꢀ/mol-1 dm3 cm-1) in 1 mmol dm-3 HClO4 at 25 °C): 283
(13300), 327 (11100), 343 (10500, sh).
[Cr(OH2)(3,5-Bu2SQ)(Me3-tacn)](ClO4)2‚H2O (5). This com-
plex was obtained as a green polycrystalline solid in a manner
similar to that of 1s except that [CrCl3(Me3-tacn)]38 was used instead
of [CrCl3(trpy)]. Yield: 315 mg (46%). Anal. Calcd for C23H43N3O11-
Cl2Cr‚H2O: C, 40.71; H, 6.68; N, 6.19. Found: C, 40.50; H, 6.57;
N, 6.09. UV-vis (λmax/nm (ꢀ/mol-1 dm3 cm-1) in 1 mmol dm-3
HClO4 at 25 °C): 274 (7300), 338 (3000), 386 (4000), 450 (4300),
520 (330, sh), 602 (380), 685 (1200), 816 (180, br), 878 (190, br),
1000 (130, sh).
Crystallographic data are summarized in Table 1. Tables of final
atomic coordinates, thermal parameters, and full bond distances
and angles are given in Supporting Information.
Results and Discussion
Crystal Structures of 1c′-4, 6, and 7′. Figures 1 and 2
show the crystal structures of the complex cations of 1c′ and
6, respectively (those of 3, 4, and 7′ are given in Figures
S3, S4, and S6 in Supporting Information). The selected bond
distances of 1c′, 3, 4, 6, and 7′ are given in Table 2. These
complexes consist of the chromium cation unit, one counter
perchlorate, and solvated molecule(s). The chromium atoms
adopt a distorted octahedral structure with three nitrogen
atoms of trpy or Me3-tacn, two oxygen of dioxolene, and
one water oxygen. The aqua group of the complexes is
connected to water, methanol, and/or perchlorate anion with
hydrogen bonding. Complexes 3 and 6 exist as dimeric forms
in the solid state, because one of the dioxolene oxygens is
linked to the aqua ligand of the neighboring complex with
hydrogen bonding (Figure 2). The O‚‚‚O and Cr‚‚‚Cr
distances in the dimer unit are 2.572(2) and 4.954(0) Å for
3 and 2.686(2) and 5.370(1) Å for 6, respectively. For the
complex 1c′, an additional 3,5-di-tert-butylcatechol molecule
is bonded to one of the coordinated dioxolene oxygen with
a hydrogen bond, as shown in Figure 1.
[Cr(OH2)(Cat)(Me3-tacn)]ClO4‚H2O (6). The compound was
obtained as green prism crystals by a method similar to that of 3
except that [CrCl3(Me3-tacn)] was used instead of [CrCl3(trpy)].
Yield: 294 mg (63%). Anal. Calcd for C15H27N3O7ClCr‚H2O: C,
38.59; H, 6.26; N, 9.00. Found: C, 38.39; H, 6.28; N, 8.99. UV-
vis (λmax/nm (ꢀ/mol-1 dm3 cm-1) in 1 mmol dm-3 HClO4 at 25
°C): 276 (4900), 300 (2700, sh), 335 (1600), 602 (55).
[Cr(OH2)(Cl4Cat)(Me3-tacn)]ClO4 (7). This complex was pre-
pared by the same method as that of 4 except that [CrCl3(Me3-
tacn)] was used instead of [CrCl3(trpy)]. Green prism39 and/or
hexagon plate crystals40 were obtained. Yield: 284 mg. The mixture
was manually separated. Anal. Calcd for green prism crystals
(C15H23N3O7Cl5Cr‚H2O): C, 29.80; H, 4.17; N, 6.95. Found: C,
29.75; H, 4.20; N, 6.90. Anal. Calcd for green hexagon plate crystals
(C15H23N3O7Cl5Cr‚CH3OH): C, 31.06; H, 4.40; N, 6.79. Found:
C, 30.94; H, 4.36; N, 6.80. UV-vis (λmax/nm (ꢀ/mol-1 dm3 cm-1
in 1 mmol dm-3 HClO4 at 25 °C): 285 (5500), 327 (4200).
)
The crude product was allowed to stand in water/methanol (9:1
v/v), and one green needle crystal (10 × 0.2 × 0.2 mm3) as [Cr-
(OH2)(Cl4Cat)(Me3-tacn)]ClO4‚2(H2O) (7′) suitable for X-ray crys-
tallography was obtained after 2 weeks.
Physical Measurements. Electronic spectra were measured on
a Shimadzu UV-vis-NIR scanning spectrophotometer UV-
3100PC. Cyclic voltammetric experiments were carried out in a
one-compartment cell consisting of a glassy carbon working
electrode, a platinum wire auxiliary electrode, and a Ag/AgCl or
Ag/Ag+ reference electrode. An ASL model 660 electrochemical
analyzer was used to collect the electrochemical data. Magnetic
susceptibilities were measured by a SQUID susceptometer, QUAN-
Each chromium-terminal nitrogen bond distance of trpy
of 1c′, 3, and 4 (2.074-2.080 Å) is slightly longer than the
chromium-central one of those complexes (1.994-2.013 Å),
probably because of steric distortion of the chromium trpy
complexes. On the other hand, three Cr-N bond distances
(41) Mabbs, F. E.; Marchin, D. J. Magnetisms and Transition Metal
Complexes; Chapman and Hall: London, 1975; p 5.
(42) PATTY: 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 Laboratory;
University of Nijmegen, The Netherlands, 1994.
(43) DIRDIF94: 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 Laboratory;
University of Nijmegen, The Netherlands, 1994.
(38) Wieghardt, K.; Chaudhuri, P.; Nuber, B.; Weiss, J. Inorg. Chem. 1982,
21, 3086-3090.
(39) Crystal data for 7‚H2O: C15H23N3O7Cl5Cr‚H2O, monoclinic, P21/n,
a ) 8.471(5) Å, b ) 8.491(5) Å, c ) 33.23(2) Å, â ) 96.546(7)°, V
) 2374(2) Å3, Z ) 4, Dc ) 1.691 g/cm3, R ) 0.088, Rw ) 0.185.
(40) Crystal data for 7‚CH3OH: C15H23N3O7Cl5Cr‚CH3OH, monoclinic,
P21/a, a ) 16.983(2) Å, b ) 8.212(2) Å, c ) 19.873(1) Å, â )
114.186(7)°, V ) 2528.1(7) Å3, Z ) 4, Dc ) 1.625 g/cm3, R ) 0.135,
Rw ) 0.250.
(44) teXsan: Crystal Structure Analysis Package; Molecular Structure
Corporation, 1985 & 1992.
5914 Inorganic Chemistry, Vol. 41, No. 22, 2002

Aqua-Chromium-Dioxolete Complexes
Table 1. Crystallographic Data for 1c′-4, 6, and 7′
1c′
2
3
4
6
7′
formula
fw
CrC44H59N3O10Cl
877.41
CrC30H39N3O13Cl2
772.55
CrC21H19N3O8Cl
528.85
CrC21H15N3O8Cl5
666.63
CrC15H29N3O8Cl
466.86
CrC15H27N3O9Cl5
622.65
temp, °C
cryst dimensions,
mm3
-100
-100
-100
-100
-100
-100
0.25 × 0.15 × 0.50 0.15 × 0.15 × 0.50 0.40 × 0.40 × 0.50 0.20 × 0.50 × 0.50 0.30 × 0.30 × 0.40 0.45 × 0.20 × 0.20
cryst syst
space group
a, Å
b, Å
c, Å
monoclinic
P21/a (No. 14)
12.106(4)
15.625(5)
24.315(8)
100.984(4)
4514(2)
4
orthorhombic
Pna21 (No. 33)
22.058(2)
13.3259(9)
12.4739(8)
90
monoclinic
C2/c (No. 15)
24.764(2)
13.300(1)
13.915(1)
108.408(3)
4348.4(6)
8
monoclinic
P21/a (No. 14)
12.473(1)
12.2879(8)
17.138(1)
104.080(4)
2547.8(4)
4
monoclinic
P21/c (No. 14)
9.2492(8)
15.663(1)
14.005(1)
95.704(5)
2018.9(3)
4
orthorhombic
P212121 (No. 19)
8.4891(5)
15.2677(9)
19.067(1)
90
â, deg
V, Å3
3666.7(8)
4
2471.2(3)
4
Z
2θmax
55
1860
1.291
3.71
55
1608
1.399
5.22
55
2168
1.615
7.05
55
1340
1.738
10.26
55
980
1.536
7.46
55
1276
1.673
10.53
F(000)
D
calcd, g/cm3
abs coeff, cm-1
no. reflns
10088
4369
4950
5805
4482
3188
(all, 2θ < 55°)
no. variables
GOF
532
1.70
443
1.61
307
1.89
343
1.73
253
1.69
298
1.62
largest peak/
hole, e Å-3
R1a
0.75/-0.61
0.39/-0.35
0.66/-0.72
1.07/-0.63
0.68/-0.52
0.40/-0.31
0.082
0.061
0.057
0.056
0.048
0.031
[I > 2σ(I)]
Rb (all data)
Rwc (all data)
0.083
0.166
0.083
0.171
0.069
0.159
0.069
0.156
0.058
0.129
0.039
0.080
2
2
2
2
2
2
a R1 ) ∑[|Fo| - |Fc|]/∑|Fo|. b R ) ∑[Fo - Fc ]/∑Fo . c Rw ) [(∑w(Fo - Fc )2/∑w(Fo )2)]1/2
.
Table 2. Selected Bond Distances (Å) of 1c′-4, 6, and 7′
1c′
1c′a
2
3
4
6
7′
Cr1-O1 2.032(2)
Cr1-O2 1.898(2)
Cr1-O3 1.933(2)
Cr1-N1 2.080(2)
Cr1-N2 2.013(2)
Cr1-N3 2.080(2)
1.980(3) 2.015(2) 2.031(2) 2.036(2) 2.032(2)
1.906(4) 1.936(2) 1.921(2) 1.945(2) 1.936(2)
1.923(3) 1.925(2) 1.924(2) 1.935(2) 1.915(2)
2.064(5) 2.071(2) 2.071(2) 2.101(2) 2.086(3)
1.985(4) 1.994(2) 2.005(2) 2.112(2) 2.099(3)
2.065(4) 2.080(2) 2.076(2) 2.118(2) 2.110(3)
O2-C1
O3-C2
C1-C2
C1-C6
C2-C3
C3-C4
C4-C5
C5-C6
Cr1‚‚‚Cr1*
O1‚‚‚O2*
1.361(3) 1.379(3)a 1.317(5) 1.365(3) 1.331(3) 1.366(3) 1.335(4)
1.377(3) 1.387(3)a 1.289(6) 1.359(3) 1.334(3) 1.364(3) 1.331(4)
1.410(4) 1.394(4)a 1.452(6) 1.405(3) 1.423(4) 1.412(3) 1.413(5)
1.397(4) 1.393(4)a 1.419(6) 1.380(3) 1.387(4) 1.388(3) 1.391(4)
1.381(4) 1.395(4)a 1.424(7) 1.389(3) 1.395(4) 1.385(3) 1.392(4)
1.403(4) 1.399(4)a 1.380(7) 1.391(4) 1.384(4) 1.395(3) 1.400(4)
1.397(4) 1.397(4)a 1.416(8) 1.385(4) 1.407(4) 1.388(3) 1.389(5)
1.407(4) 1.389(4)a 1.389(7) 1.393(4) 1.402(4) 1.395(3) 1.402(4)
4.954(0)
2.572(2)
5.370(1)
2.686(2)
Figure 1. ORTEP views (50% probability) of complex cation and free
dioxolene of 1c′. Hydrogen atoms are omitted for clarity.
a Bond distances of an additional 3,5-dibutylcatechol molecule.
1.940 Å, and the distance is shorter than that of the Cr(III)-
triscatechol complex, K3[Cr(Cat)3] (Cr-Oav ) 1.986 Å).45
The Cr-O(aqua) bond distance of these complexes ranges
from 2.015 to 2.036 Å and is close to the Cr-OH2 bond
distance of [Cr(OH2)(acac)(Me3-tacn)]2+ (Cr-O ) 2.067-
(6) Å).46 These bond lengths apparently are longer than the
Cr-OH bond of [Cr(OH)(acac)(Me3-tacn)]+ (Cr-O ) 1.90-
(2) Å).47 Nearly identical C-C bond lengths in dioxolene
ligands (Table 2) are implications of the aromatic nature of
the C6 ring. In fact, the C-O bond length of 1c′, 3, 4, 6,
and 7′ in the range 1.333-1.352 Å is close to that of free
catechol48 and K3[Cr(Cat)3]45 (C-Oav ) 1.371 and 1.346 Å,
Figure 2. ORTEP view (50% probability) of complex cations of 6.
Hydrogen atoms are omitted for clarity.
(45) Raymond, K. N.; Isied, S. S.; Brown, L. D.; Fronczed, F. R.; Nibert,
J. H. J. Am. Chem. Soc. 1976, 98, 1767-1774.
(46) Bossek, U.; Haselhorst, G.; Ross, S.; Wieghardt, K.; Nuber, B. J. Chem.
Soc., Dalton Trans. 1994, 2041-2048.
of Me3-tacn complexes 6 and 7′ are almost the same, and
the average is 2.104 Å. The chromium-oxygen bond length
of dioxolene of 1c′, 3, 4, 6, and 7′ is in the range 1.916-
(47) Bossek, U.; Wieghardt, K.; Nuber, B.; Weiss, J. Angew. Chem., Int.
Ed. Engl. 1990, 29, 1055-1057.
(48) Wunderlich, H.; Mootz, D. Acta Crystallogr. 1971, B27, 1684-1686.
Inorganic Chemistry, Vol. 41, No. 22, 2002 5915

Shiren and Tanaka
Figure 4. Temperature dependencies of the effective magnetic moments
of 1c′ (O), 2 (0), and 3 (4). The solid line represents the best fit of the
data for 3 (see text).
Figure 3. ORTEP view (50% probability) of a complex cation of 2.
Hydrogen atoms are omitted for clarity.
respectively). The geometry of the C6 ring of free dioxolene
unit of 1c′ (C-Cav ) 1.395, C-Oav ) 1.383 Å) is also
comparable to the catechol structure.48 The dioxolene ligand
of 1c′, 3, 4, 6, and 7′, therefore, is linked to Cr(III) with the
catechol form.
Table 3. Magnetic Data for 1-7
a
complex
µ
eff/µB
g
J/cm-1 ref
[Cr(OH2)(3,5-Bu2SQ)(trpy)]2+ (1s)
[Cr(OH2)(3,5-Bu2Cat)(trpy)]+ (1c′)
[Cr(OH2)(3,6-Bu2SQ)(trpy)]2+ (2)
[Cr(OH2)(Cat)(trpy)]+ (3)
2.87
3.85
2.95
b
b
b
3.80 (50- 1.97 -0.6
b
The structure of the complex cation of 2 is shown in Figure
3. The selected bond distances of 2 are given in Table 2.
The external view of the complex cation of 2 is similar to
that of catechol complexes 1c′, 3, and 4, though 2 has two
counter perchlorate ions per one cation unit. The average
Cr-O(dioxolene) and Cr-N(terminal) bond lengths of 2 are
1.915 and 2.065 Å, respectively, which are comparable to
those of 1c′, 3, and 4. On the other hand, the distances of
the Cr-O(aqua) and Cr-N(central) bonds of 2, which are
in trans positions of oxygen atoms of dioxolene, are 1.980-
(3) and 1.985(4) Å, respectively, and these bond lengths are
slightly shorter than those of 1c′, 3, and 4. The average C-O
bond distance (1.303 Å) of 2 is different from that of free
catechol (C-Oav ) 1.371 Å)48 and o-benzoquinone (C-O
) 1.220 Å)49 and comparable to that of Cr(III) semiquinone
complexes, [Cr(tren)(3,6-Bu2SQ)](PF6)2,50 [Cr(3,5-Bu2SQ)3],51
and [Cr(Cl4SQ)3]52 (C-Oav ) 1.303, 1.285, and 1.28 Å,
respectively). Moreover, the dioxolene C6 ring has two short
C3-C4 and C5-C6 bonds (C-Cav ) 1.385 Å) and four
long C1-C2, C1-C6, C2-C3, and C4-C5 bonds (C-Cav
) 1.428 Å) which are similar to those of semiquinone com-
plexes.50-52 On the basis of these geometrical patterns, the
electronic structure of 2 is characterized as an aqua-Cr-
(III)-semiquinone complex, [CrIII(OH2)(3,6-Bu2SQ)(trpy)]2+.
300 K)
1.88 (2 K)
3.82
[Cr(OH2)(Cl4Cat)(trpy)]+ (4)
b
b
b
[Cr(OH2)(3,5-Bu2SQ)(Me3-tacn)]2+ (5) 2.96
[Cr(OH2)(Cat)(Me3-tacn)]+ (6)
3.79 (50- 1.97 -0.8
300 K)
1.69 (2 K)
3.86
2.85
3.85
[Cr(OH2)(Cl4Cat)(Me3-tacn)]+ (7)
[Cr(tren)(3,6-Bu2SQ)]2+
b
45
45
[Cr(tren)(3,6-Bu2Cat)]+
a Average value. b This work.
effective magnetic moments (µeff) of 1c′-3 are given in
Figure 4, and magnetic data for 1-7 are summarized in Table
3.
The magnetic moments of 1c′, 4, and 7 are almost constant
in the temperature region 2-300 K. The average effective
magnetic moments of 1c′, 4, and 7 were 3.85, 3.82, and 3.86
µB, respectively, which agree with the spin-only value of
3.87 µB for the Cr(III)-catechol formula (SCr ) 3/2). Complex
3 displayed temperature-dependent effective magnetic mo-
ments (Figure 4). In the temperature region 50-300 K, the
value of µeff ) 3.80 ( 0.1 µB was close to the spin-only
value of 3.87 µB for S ) 3/2. As the temperature was lowered,
the magnetic moment gradually decreased and became 1.88
µB at 2 K, indicating the presence of a weak antiferromag-
netic interaction between two Cr(III)-catechol moieties
bridged by two hydrogen bonds in the dimer unit as described
previously. The magnetic exchange coupling constant (J) was
calculated on the basis of the isotropic spin Hamiltonian H
) -2JS1‚S2. The theoretical equation of molar magnetic
susceptibility (øM) for a monomeric unit is given in eq 2
Magnetic Studies. Magnetic susceptibility data for pow-
dered samples of 1-7 were collected in the temperature
region 2-300 K. Plots of temperature dependence of the
(49) Macdonald, A. L.; Trotter, J. J. Chem. Soc., Perkin Trans. 2 1973,
476-480.
(50) Wheeler, D. E.; McCusker, J. K. Inorg. Chem. 1998, 37, 2296-2307.
(51) Sofen, S. R.; Ware, D. C.; Cooper, S. R.; Raymond, K. N. Inorg.
Chem. 1979, 18, 234-239.
øM ) [Ng2â2/kT ][(14 + 5x6 + x10)/
(7 + 5x6 + 3x10 + x12)] + TIP (2)
(52) Pierpont, C. G.; Downs, H. H. J. Am. Chem. Soc. 1976, 98, 4834-
4838.
5916 Inorganic Chemistry, Vol. 41, No. 22, 2002

Aqua-Chromium-Dioxolete Complexes
Figure 5. Magnetization M versus magnetic field H plots for 1s (]), 2
1
(0), and 5 (+) at 4.2 K: calculated curve for S )
/ (‚‚‚‚‚), calculated
2
Figure 6. Electronic absorption spectra of [Cr(OH2)(3,5-Bu2SQ)(trpy)]-
(ClO4)2 in DME/THF (9:1 v/v) containing various amounts of BuOK:
[BuOK]/[Cr(OH2)(3,5-Bu2SQ)(trpy)]2+ ) 0 - 1.0 equiv.
3
curve for S ) 1 (s), and calculated curve for S ) /2 with g ) 2.0 (- - -).
where x ) exp(-J/kT), the TIP is a temperature independent
paramagnetism which was fixed at 3 × 10-4 cgs emu mol-1,
and the other symbols have their usual meanings. The
magnetic parameters of 3 are g ) 1.97 and J ) -0.6 cm-1.
The similar coupling was observed for hydrogen bond
bridged dimer 6. The fitting led to values of g ) 1.97 and J
) -0.8 cm-1.
The magnetic moments of 1s, 2, and 5 are almost constant.
The values of µeff ) 2.87, 2.95, and 2.96 µB for 1s, 2, and
5, respectively, agree with the spin-only value of 2.83 µB
for the ST ) 1 state which is confirmed by molar magnetiza-
tion of 1s, 2, and 5 versus magnetic field plots at 4.2 K
(Figure 5). These results indicate the strong antiferromagnetic
interaction between Cr(III) (SCr ) 3/2) and semiquinone (SSQ
) 1/2). Such a strong antiferromagnetic interaction has been
reported for Cr(III)-semiquinone complexes.50,51,53,54
Electronic Absorption Spectra. The electronic spectra
of [Cr(OH2)(3,5-Bu2SQ)(trpy)]2+ (1s) show five absorption
bands at 401, 431, 463, 630, and 685 nm in the visible region
in DME/THF (9:1 v/v) (Figure 6). A similar pattern has been
reported in the electronic spectra of Cr(III)-semiquinone
complexes such as [Cr(tren)(3,6-Bu2SQ)]2+ 50 and [Cr(CTH)-
(3,6-Bu2SQ)]2+.53 An addition of small amounts of BuOK
in 2-methoxyethanol to the solution resulted in a decrease
of the absorbance of these bands and emergence of two new
bands at 490 and 735 nm with the appearance of three
isosbestic points at 475, 668, and 694 nm. The absorbance
of the latter two bands increased with an increase of the
amounts of BuOK and reached their maximum intensities
in the presence of the equimolar amount of BuOK (Figure
6). The electronic absorption spectrum of the 1:1 mixture of
1s and BuOK fully changed to that of the original 1s by an
addition of 1 equiv of CF3SO3H to the DME/THF solution.
Figure 7. Electronic spectra of [Cr(OH2)(3,5-Bu2SQ)(trpy)](ClO4)2 de-
pending on pH.
A similar spectral change was observed in aqueous
solution. The visible spectra of 1s at pH 3 in H2O/THF (9:1
v/v) exhibited several absorption bands at 403, 432, 461,
and 685 nm, which decreased with increasing pH, and new
bands emerged at 470 and 703 nm (Figure 7). The absor-
bance of the 470 and 703 nm bands grew up to pH 6 and
then became constant in the pH region higher than 7.5. On
the basis of the plots of the absorption coefficients at 403,
461, 470, and 703 nm against pH (Figure 8), the pKa of [Cr-
(OH2)(3,5-Bu2SQ)(trpy)]2+ was determined as 4.5. The pKa
values of the series of the aqua-Cr(III)-semiquinone and
-catechol complexes were similarly determined, and the
results were summarized in Table 4.
The pKa values of [Cr(OH2)(dioxolene)(trpy)]2+ are in the
range 4.5-6.8, and those of [Cr(OH2)(dioxolene)(Me3-
tacn)]2+ range from 5.0 to 7.6. Thus, the pKa of aqua-Cr-
Me3-tacn complexes is larger than the corresponding aqua-
Cr-trpy ones by 0.5-1.2. One-electron reduction of 1s gives
rise to an increase in the pKa value of the aqua ligand by
2.3. An agreement of pKa of 5.0 between [Cr(OH2)(3,5-Bu2-
SQ)(Me3-tacn)]2+ and [Cr(OH2)(Cl4Cat)(trpy)]+ apparently
results from of the compensation of the electron donor ability
(53) Benelli, C.; Dei, A.; Gatteschi, D.; Gu¨del, H. U.; Pardi, L. Inorg. Chem.
1989, 28, 3089-3091.
(54) Chang, H.-C.; Miyasaka, H.; Kitagawa, S. Inorg. Chem. 2001, 40,
146-156.
Inorganic Chemistry, Vol. 41, No. 22, 2002 5917

Shiren and Tanaka
Figure 8. Absorption coefficients at 403 (9), 461 (b), 470 (0), and 703
(O) nm of [Cr(OH2)(3,5-Bu2SQ)(trpy)](ClO4)2 at various pH values.
Table 4. pKa Values of the Aqua Complexes
[Cr(OH2)(dioxolene)(L)]n+
Figure 9. Electronic spectral change of [Cr(OH)(3,5-Bu2SQ)(trpy)]+ in
dioxolene
H2O/THF (9:1 v/v) at pH 11.
L
3,5-Bu2SQ
3,5-Bu2Cat
6.8
Cat
Cl4Cat
Table 5. Redox Potential of the [Semiquinone]/[Catechol] Redox
Couple of [Cr(OH2)(dioxolene)(L)]2+ and [Cr(OH)(dioxolene)(L)]+ in
DME/THF (9:1 v/v) and in H2O/THF (9:1 v/v)a
trpy
Me3-tacn
4.5
5.0
6.8
7.6
5.0
6.2
3,5-Bu2SQ
Cat
Cl4Cat
OH2
between Me3-tacn and trpy and between 3,5-Bu2SQ and Cl4-
Cat.
L
OH2
OH
OH2
OH
OH
trpy
Me3-tacn
0.30
(0.25)
0.35
-0.07
(0.08)
0.02
0.51
(0.51)
0.53
0.15
(0.35)
b
0.99
(0.76)
1.03
0.51
b
0.54
b
The 470 and 703 nm bands of [Cr(OH)(3,5-Bu2SQ)(trpy)]+
in H2O/THF (9:1 v/v) are stable at pH 7, while these bands
gradually weakened and finally disappeared to give a
featureless electronic absorption spectrum in the visible
region at pH 11 for 2 h (Figure 9).
(0.31)
(0.10)
(0.55)
(0.38)
(0.79)
a Values in parentheses are redox potentials in H2O/THF (9:1 v/v). b Not
determined, insoluble.
Acidification of the solution does not reproduce the
electronic spectrum of 1s, and the absorption spectrum
resembles that of 1c.55 Treatment of the resultant solution
with 1 equiv of AgClO4 at pH 4 recovered the electronic
spectrum of 1s. On the basis of the facts that the catechol
complexes, 1c, 3, 4, 6, and 7, did not show any absorption
bands in the visible region because of their small absorption
coefficients of the d-d transition bands (ꢀ ) 50-120 mol-1
dm3 cm-1) and oxidation of these catechol complexes with
Ag+ afforded the corresponding semiquinone ones quanti-
tatively, 1s is reduced to 1c in H2O/THF (9:1 v/v) at pH 11.
Similarly, [Cr(OH)(3,5-Bu2Cat)(Me3-tacn)]+ was formed in
H2O/THF solution of the semiquinone complex 5 at pH 11
for 15 h. The reduction in H2O/THF (9:1 v/v) at pH 11 was
greatly enhanced under visible light illumination from a
150 W Xe lamp,56 and the rate of the reduction proceeded
about 20 times faster than that in a cell in a spectrophotom-
eter.
plexes, an addition of less than 1 equiv of BuOK to a DME/
THF solution of [Cr(OH2)(3,5-Bu2SQ)(Me3-tacn)](ClO4)2 (5)
caused an appearance of the new [Bu2SQ]/[Bu2Cat] redox
couple at E1/2 ) -0.07 V (vs SCE) of [Cr(OH)(3,5-Bu2-
SQ)(Me3-tacn)]+ at the cost of the peak currents of the [Bu2-
SQ]/[Bu2Cat] redox one at E1/2 ) 0.30 V (vs SCE) of
[Cr(OH2)(3,5-Bu2SQ)(Me3-tacn)]2+. Similarly, all the aqua-
Cr(III)-dioxolene complexes were converted to the corre-
sponding hydroxo-Cr(III)-dioxolene ones in the presence
of a molar equivalent amount of BuOK, but the [SQ]/[Cat]
redox couple of [Cr(OH)(Cat)(Me3-tacn)] was not detected
because of extremely low solubility of the complex. The
redox potentials of the aqua- and hydroxo-Cr(III) com-
plexes thus obtained in DME/THF (9:1 v/v) and in H2O/
THF (9:1 v/v) are summarized in Table 5. It is worthy of
note that there is no difference in the patterns of the CV
between [Cr(OH2)(3,5-Bu2SQ)(L)]2+ and [Cr(OH2)(3,5-Bu2-
Cat)(L)]+ and between [Cr(OH)(3,5-Bu2SQ)(L)]+ and [Cr-
(OH)(3,5-Bu2Cat)(L)]0 (L = trpy, Me3-tacn), but the rest
potentials of the semiquinone and catechol complexes lie at
potentials more positive and negative, respectively, than the
redox potentials of the [Bu2SQ]/[Bu2Cat] couple. In fact, the
rest potential of the H2O/THF (9:1 v/v) solution of [Cr(OH)-
(3,5-Bu2SQ)(Me3-tacn)]+ at pH 11 gradually shifted from
0.15 to -0.01 V with retaining the [Bu2SQ]/[Bu2Cat] redox
couple at E1/2 ) 0.10 V under room light, as shown in Figure
10. It became constant at -0.01 V in 12 h at pH 11.
Cyclic Voltammograms. All the aqua-Cr(III)-dioxolene
complexes showed one reversible [semiquinone]/[catechol]
redox couple in the potential range from -0.8 to +0.8 V
(vs SCE) in the cyclic voltammograms in DME/THF (9:1
v/v) and in H2O/THF (9:1 v/v). As expected from the smooth
conversion between the aqua- and hydroxo-Cr(III) com-
(55) The pKa value of [Cr(OH2)(3,5-Bu2Cat)(trpy)]2+ produced by the
base-acid treatments of 1s were consistent with those of original 1c.
(56) Photoirradiation was conducted under cutting off UV light (λ < 350
nm by Toshiba UV-35 filter).
5918 Inorganic Chemistry, Vol. 41, No. 22, 2002

Aqua-Chromium-Dioxolete Complexes
lamp forced the same reduction to completion in 2 h.56
Acidification of the solution from 11 to 4 caused the
appearance of the [Bu2SQ]/[Bu2Cat] redox couple at E1/2
)
0.29 V (vs SCE) of [Cr(OH2)(3,5-Bu2Cat)(Me3-tacn)]2+
(Table 5), because the rest potential of the solution was at a
potential more negative than the redox potential of the [Bu2-
SQ]/[Bu2Cat] couple. An investigation was undertaken to
elucidate the mechanism for photochemical reduction of [Cr-
(OH)(3,5-Bu2SQ)(L)]+ in H2O at pH 11.
Supporting Information Available: Figures (S1-S6) display-
ing fully labeled ORTEP drawings for 1c′, 2, 3, 4, 6, and 7′ and
one X-ray crystallographic file, in CIF format, for 1c′, 2, 3, 4, 6,
and 7′. This material is available free of charge via Internet at
http://pubs.acs.org.
Figure 10. Change of the rest potential (Erest) of the H2O/THF (9:1 v/v)
solution of [Cr(OH)(3,5-Bu2SQ)(Me3-tacn)]+ at pH 11 for 15 h. CVs were
measured about every 3 h and started from Erest
.
Moreover, photo-irradiation of the H2O/THF solution of
[Cr(OH)(3,5-Bu2SQ)(Me3-tacn)]+ under illumination of a Xe
IC020308M
Inorganic Chemistry, Vol. 41, No. 22, 2002 5919