Structural systematics for o-C6H4XY ligands with X,Y= O, NH, and S donor atoms. o-Iminoquinone and o-iminothioquinone complexes of ruthenium and osmium

Article abstract of DOI:10.1021/ic025766+

The structural features of quinone ligands are diagnostic of charge. The o-benzoquinone, radical semiquinonate, and catecholate electronic forms have C-O bond lengths and a pattern of ring C-C bond lengths that point to a specific mode of coordination. This correlation between ligand charge and structure has been extended to iminoquinone and iminothioquinone ligands, giving a charge-localized view of electronic structure for complexes of redox-active metal ions. The radical semiquinonate form of these ligands has been found to be a surprisingly common mode of coordination; however, the paramagnetic character of the radical ligand is often obscured in complexes containing paramagnetic metal ions. In this report, diamagnetic iminosemiquinonate (isq) and iminothiosemiquinonate (itsq) complexes of Is-d⁵ Ru(III) with related complexes of osmium are reported. With osmium, the Os(IV)-amidophenolate (ap) redox isomer is formed. Electrochemical and spectral properties are described for Ru(PPh₃)₂(isq)Cl₂, Ru(PPh₃)₂(itsq)Cl₂, Os(PPh₃)₂(ap)Br₂, Os(PPh₃)₂(atp)Br₂, and Os(PPh₃)₂(ap)H₂. Crystallographic characterization of Ru(PPh₃)2(isq)Cl₂, Ru(PPh₃)₂(itsq)Cl₂, and Os(PPh₃)₂(ap)H₂ was used to assign charge distributions.

Full text of DOI:10.1021/ic025766+

Inorg. Chem. 2002, 41, 5810−5816
Structural Systematics for o-C H XY Ligands with X,Y) O, NH, and S
6 4
Donor Atoms. o-Iminoquinone and o-Iminothioquinone Complexes of
Ruthenium and Osmium
Samaresh Bhattacharya,* Parna Gupta, and Falguni Basuli
Inorganic Chemistry Section, Department of Chemistry, JadaVpur UniVersity,
Kolkata 700032, India
Cortlandt G. Pierpont*
Department of Chemistry and Biochemistry, UniVersity of Colorado, Boulder, Colorado 80309
Received June 3, 2002
The structural features of quinone ligands are diagnostic of charge. The o-benzoquinone, radical semiquinonate,
and catecholate electronic forms have C−O bond lengths and a pattern of ring C−C bond lengths that point to a
specific mode of coordination. This correlation between ligand charge and structure has been extended to
iminoquinone and iminothioquinone ligands, giving a charge-localized view of electronic structure for complexes of
redox-active metal ions. The radical semiquinonate form of these ligands has been found to be a surprisingly
common mode of coordination; however, the paramagnetic character of the radical ligand is often obscured in
complexes containing paramagnetic metal ions. In this report, diamagnetic iminosemiquinonate (isq) and
5
iminothiosemiquinonate (itsq) complexes of ls-d Ru(III) with related complexes of osmium are reported. With osmium,
the Os(IV)−amidophenolate (ap) redox isomer is formed. Electrochemical and spectral properties are described for
Ru(PPh3)2(isq)Cl2, Ru(PPh3)2(itsq)Cl2, Os(PPh3)2(ap)Br2, Os(PPh3)2(atp)Br2, and Os(PPh3)2(ap)H . Crystallographic
2
characterization of Ru(PPh3)2(isq)Cl2, Ru(PPh ) (itsq)Cl2, and Os(PPh ) (ap)H was used to assign charge distributions.
3 2
3 2
2
Introduction
a radical semiquinonate (SQ) anion or as a catecholate (Cat)
dianion.1,3 Ligand C-O bond lengths increase from a value
of 1.22 Å for BQ ligands to 1.34 Å for a chelated catecholate,
a difference in bond length that is well within the significance
level of current crystallographic methods of structure deter-
mination. Studies on complexes containing iminoquinone
ligands have also been found to conform to specific charged
forms with characteristic structural features. Complexes of
the phenoxazinolate (PhenoxSQ) radical
As the coordination chemistry of complexes containing
quinone ligands has developed, the radical anion semi-
quinonate (SQ) electronic form has emerged as a surprisingly
common mode of coordination.1 In cases where the radical
character is retained upon coordination, magnetic measure-
ments and EPR spectroscopy provide useful information
about ligand-based paramagnetism.2 Quite commonly ligand
spins couple with unpaired electrons on the metal or with
other radical spins within the coordination sphere, obscuring
the intrinsic SQ radical character of the ligand. In these cases,
the structural features of quinone ligands have been found
to provide insight into ligand charges and intramolecular
charge distributions. Clear systematic differences in C-O
and ring C-C bond lengths are diagnostic of ligand
coordination in the form of either o-benzoquinone (BQ) or
* To whom correspondence should be addressed. E-mail: pierpont@
colorado.edu (C.G.P.).
(1) (a) Pierpont, C. G.; Lange, C. W. Prog. Inorg. Chem. 1994, 41, 331.
(b) Pierpont, C. G. Coord. Chem. ReV. 2001, 219-221, 415.
(2) Pierpont, C. G.; Attia, A. S. Collect. Czech. Chem. Commun. 2001,
66, 33.
(3) See for example: Attia, A. S.; Conklin, B. J.; Lange, C. W.; Pierpont,
C. G. Inorg. Chem. 1996, 35, 1033.
5810 Inorganic Chemistry, Vol. 41, No. 22, 2002
10.1021/ic025766+ CCC: $22.00 © 2002 American Chemical Society
Published on Web 10/04/2002

o-C6H4XY Ligands with X,Y) O, NH, and S Donor Atoms
Table 1. Average Bond Lengths for Quinoid Ligands Containing
Oxygen, Nitrogen, and Sulfur Donor Atoms
have been characterized magnetically, spectroscopically, and
structurally,4 and structural characterization of the free and
complexed phenoxazin-1-one (PhenoxBQ) form of the ligand
has provided structural features for the unreduced iminoben-
zoquinone (ibq).4c,5 Wieghardt has investigated complexes
containing the N-phenyl iminoquinone ligand coordinated
as the radical iminosemiquinonate (isq)
C-X
X ) O X ) NH X ) S C1-C2a C2-C3 C3-C4 C4-C5
BQb 1.22
1.31
1.35
1.38
1.69
1.72
1.75
1.48
1.43
1.42
1.43
1.42
1.41
1.34
1.36
1.39
1.45
1.42
1.41
SQ
Cat
1.30
1.34
a Atoms of the ring follow the numbering scheme.
b The abbreviation BQ refers to the unreduced o-benzoquinone form of the
ligand, SQ to the partially reduced radical o-semiquinonate electronic form,
and Cat to the reduced dianionic catecholate form.
and as a fully reduced amidophenolate (ap) dianion.6
Structural characterization of complexes of iminoquinone
ligands in all three charged forms has shown a pattern in
C-O, C-N, and ring C-C bond lengths consistent with the
correlation between structure and ligand charge established
for the quinone ligands. The results of these structure
determinations show that the C-O bond lengths resemble
those of BQ, SQ, and Cat ligands, but the range of C-N
bond lengths is more narrow. The neutral PhenoxBQ ligand
of [CuI(PhenoxBQ)(µ-Cl)]2 was found to have C-O and
C-N bond lengths of 1.219(14) and 1.331(14) Å,5 respec-
tively, whereas the amidophenolate (ap) ligands of Ir(PPh3)2-
(CO)Cl(LAP),7 V(LAP)(LAP-H)2,6c Mn(LISQ)2(LAP),6e and Pd-
(bpy)(LAP)6f have C-O and C-N bond lengths that average
1.35 and 1.39 Å, respectively.6 Wieghardt has extended this
analysis to complexes of iminothiosemiquinone (itsq) ligands.8
Values for C-N and C-S bond lengths for itsq complexes
are typically 1.35 and 1.72 Å, respectively, and the pattern
of ring C-C bonds shows slight contraction at positions that
would contain double bonds for a benzoquinone.8 In contrast,
complexes of fully reduced amidothiophenolate ligands have
C-N and C-S bond lengths that average 1.38 and 1.76 Å,
respectively. A list of diagnostic bond lengths for O, NR,
and S donor ligands in the three electronic forms is given in
Table 1. These values are based on a survey of structural
data that included hundreds of independent structure deter-
minations since 1985 that will appear later in the form of a
review. Deviations from these values are approximately
(0.01 Å, with allowance for the esd of the measurement.
In this report, we explore this pattern further with
unsubstituted iminoquinone and iminothioquinone complexes
of Ru and Os. The complexes investigated contain the general
Ru(PPh3)2Cl2(iminoquinone) unit and can exist in one of
three charge distributions, RuII-ibq, RuIII-isq, or RuIV-ap,
related by transfer of charge between metal and ligand-
localized electronic levels. Structural characterization was
used to assign ligand and metal charges.9
Experimental Section
Materials. Reagent-grade chemicals were used in all experi-
ments, and solvents were purified using standard procedures. Ru-
(PPh3)3Cl2 and Os(PPh3)3Br2 were prepared using procedures
described previously.10,11 2-Aminophenol and 2-aminothiophenol
were purchased from Loba Chemie, Mumbai, India, and recrystal-
lized before use.
Preparation of trans-Ru(PPh3)2(isq)Cl2. Ru(PPh3)3Cl2 (100 mg,
0.10 mmol) and 2-aminophenol (11 mg, 0.10 mmol) were dissolved
in dichloromethane (30 mL), and the mixture was stirred for 6 h.
During this time, the solution turned purple in color. Hexane was
layered over the solution, producing purple crystals of Ru(PPh3)2-
(isq)Cl2 in 75% yield.
(4) (a) deLearie, L. A.; Haltiwanger, R. C.; Pierpont, C. G. Inorg. Chem.
1989, 28, 644. (b) Karsanov, I. V.; Ivakhnenko, Y. P.; Khandkarova,
V. S.; Prokof’ev, A. I.; Rubezhov, A. Z.; Kabachnik, M. I. J.
Organomet. Chem. 1989, 379, 1. (c) Bhattacharya, S.; Pierpont, C.
G. Inorg. Chem. 1992, 31, 2020. (d) Bhattacharya, S.; Pierpont, C. G.
Inorg. Chem. 1994, 33, 6038. (e) Whalen, A. M.; Bhattacharya, S.;
Pierpont, C. G. Inorg. Chem. 1994, 33, 347. (f) Speier, G.; Whalen,
A. M.; Csihony, J.; Pierpont, C. G. Inorg. Chem. 1995, 34, 1355.
(5) Speier, G.; Csihony, J.; Whalen, A. M.; Pierpont, C. G. Inorg. Chim.
Acta 1996, 245, 1.
(6) (a) Verani, C. N.; Gallert, S.; Bill, E.; Weyhermu¨ller, T.; Wieghardt,
K.; Chaudhuri, P. Chem. Commun. 1999, 1747. (b) Chaudhuri, P.;
Verani, C. N.; Bill, E.; Bothe, E.; Weyhermu¨ller, T.; Wieghardt, K.
J. Am. Chem. Soc. 2001, 123, 2213. (c) Chun, H.; Verani, C. N.;
Chaudhuri, P.; Bothe, E.; Bill, E.; Weyhermu¨ller, T.; Wieghardt, K.
Inorg. Chem. 2001, 40, 4157. (d) Chun, H.; Weyhermu¨ller, T.; Bill,
E.; Wieghardt, K. Angew. Chem., Int. Ed. Engl. 2001, 40, 2489. (e)
Chun, H.; Chaudhuri, P.; Weyhermu¨ller, T.; Wieghardt, K. Inorg.
Chem. 2002, 41, 790. (f) Sun, X.; Chun, H.; Hildenbrand, K.; Bothe,
E.; Weyermu¨ller, T.; Wieghardt, K. Inorg. Chem. 2002, 41, 4295.
(7) Brothers, P. J.; Clark, G. R.; Rickard, C. E. F.; Heine, H. W. J.
Organomet. Chem. 1992, 433, 203.
Anal. Calcd for C42H35NOP2Cl2Ru: C, 62.7; H, 4.36; N, 1.74.
Found: C, 62.4; H, 4.12; N, 1.52.
UV-vis (CH2Cl2): 556 nm (5.9 × 103 M-1cm-1), 466 (5.7 ×
103), 348 (8.0 × 103), 288 (54.5 × 103), 232 (86.5 × 103).
CV [E1/2, V vs SCE (∆Ep, mV)]: -0.55 (Epc), 0.66 (70).
Preparation of Os(PPh3)2(ap)Br2. Os(PPh3)3Br2 (114 mg, 0.10
mmol) and 2-aminophenol (11 mg, 0.10 mmol) were dissolved in
dichloromethane (30 mL), and the mixture was stirred for 6 h.
During this time, the solution turned red-violet in color. Hexane
(9) Abbreviations for ligands: ibq, iminobenzoquinone; isq, iminosemi-
quinonate; ap, amidophenolate; itsq, iminothiosemiquinonate; atp,
amidothiophenolate.
(10) Stephenson, T. A.; Wilkinson, G. J. Inorg. Nucl. Chem. 1966, 28,
945.
(8) Herebian, D.; Bothe, E.; Bill, E.; Weyhermu¨ller, T.; Wieghardt, K, J.
Am. Chem. Soc. 2001, 123, 10012.
(11) Hoffman, P. R.; Caulton, K. G. J. Am. Chem. Soc. 1975, 97, 4221.
Inorganic Chemistry, Vol. 41, No. 22, 2002 5811

Bhattacharya et al.
a
Table 2. Crystallographic Data for Ru(PPh3)2(isq)Cl2, Ru(PPh3)2(itsq)Cl2, and Os(PPh3)2(ap)H2
Ru(PPh3)2(isq)Cl2
Ru(PPh3)2(itsq)Cl2
Os(PPh3)2(ap)H2
fw
803.6
819.7
438.0
color
red
red
orange
cryst syst
space group
a (Å)
b (Å)
c (Å)
monoclinic
C2/c
orthorhombic
Abm2
9.6958(3)
24.4220(6)
15.6173(4)
90.00
3698.0(2)
160
4
1.472
0.743
0.873, 0.950
0.019, 0.045
monoclinic
P21/n
17.534(2)
10.675(2)
20.241(2)
103.77(1)
3679.7(9)
296
9.2386(3)
18.4140(6)
20.3305(7)
97.225(1)
3431.2(2)
143
â (deg)
V (Å3)
T (K)
Z
4
4
D
calcd (g cm-3
)
1.451
0.683
0.805, 0.969
0.030, 0.047
1.595
3.845
0.634, 0.899
0.043, 0.095
µ (mm-1
)
Tmin, Tmax
b
R, Rw
a Data for all three compounds were collected using Mo KR radiation (λ ) 0.710 73 Å). b Discrepancy indices are defined as R ) Σ||Fo| - |Fc|/Σ|Fo| and
Rw ) [Σw(|Fo| - |Fc|)2/Σw(Fo)2]1/2
.
was layered over the solution, producing red-violet crystals of Os-
(PPh3)2(ap)Br2 in 65% yield.
Anal. Calcd for C42H35NOP2Br2Os: C, 51.4; H, 3.57; N, 1.43.
Found: C, 50.0; H, 3.81; N, 1.42.
UV-vis (CH2Cl2): 522 nm (5.6 × 103 M-1cm-1), 454 (8.1 ×
103), 368 (7.1 × 103), 263 (46.1 × 103), 232 (82.0 × 103).
CV [E1/2, V vs SCE (∆Ep, mV)]: -0.74 (Epc), 0.52 (80).
Preparation of trans-Ru(PPh3)2(itsq)Cl2. Ru(PPh3)3Cl2 (100
mg, 0.10 mmol) and 2-aminothiophenol (13 mg, 0.10 mmol) were
dissolved in dichloromethane (30 mL), and the mixture was stirred
for 4 h. During this time, the solution turned red in color. Hexane
was layered over the solution, producing red crystals of Ru(PPh3)2-
(itsq)Cl2 in 65% yield.
vibrating sample magnetometer interfaced with a Walker L75FBAL
magnet. 1H NMR spectra were obtained on a Bruker Avance
DRX300 spectrometer using TMS as the internal standard and are
included as figures in the Supporting Information. Electrochemical
measurements were made using CH Instruments 600A potentiostat.
A platinum disk working electrode, a platinum wire auxiliary
electrode, and an aqueous saturated calomel reference electrode
(SCE) were used in a three-electrode configuration. All electro-
chemical measurements were recorded under an atmosphere of dry
N2 in a dichloromethane solution containing TBAP as the supporting
electrolyte. The scan rate used was 50 mV/s, and E1/2 [E1/2 ) 0.5-
(Epa + Epc)] was calculated from the anodic (Epa) and cathodic (Epc)
peak potentials. All electrochemical data were recorded at 298 K
and are uncorrected for the junction potential.
Anal. Calcd for C42H35NSP2Cl2Ru: C, 61.5; H, 4.27; N, 1.71.
Found: C, 60.3; H, 4.22; N, 1.65.
Crystallographic Structure Determinations. Crystals of Ru-
(PPh3)2(isq)Cl2 and Ru(PPh3)2(itsq)Cl2 were obtained by slow
diffusion of hexane into a dichloromethane solution of the complex.
Crystals of Os(PPh3)2(ap)H2 were obtained directly from the
reaction. Crystallographic information for all three molecules are
given in Table 2. Data collection for Ru(PPh3)2(isq)Cl2 was carried
out using a Siemens P3/F diffractometer; data on Ru(PPh3)2(itsq)-
Cl2 and Os(PPh3)2(ap)H2 were collected on a Siemens Smart CCD
diffractometer.
UV-vis (CH2Cl2): 490 nm (6.6 × 103 M-1cm-1), 290 (45.1 ×
103), 232 (91.6 × 103).
CV [E1/2, V vs SCE (∆Ep, mV)]: -0.53 (Epc), 0.90 (80).
Preparation of Os(PPh3)2(atp)Br2. Os(PPh3)3Br2 (114 mg, 0.10
mmol) and 2-aminothiophenol (13 mg, 0.10 mmol) were dissolved
in dichloromethane (30 mL), and the mixture was stirred for 6 h.
During this time, the solution turned red in color. Hexane was
layered over the solution, producing red crystals of Os(PPh3)2(atp)-
Br2 in 60% yield.
Tables containing positional and thermal parameters for atoms,
bond lengths and angles, and hydrogen atom locations for Ru-
(PPh3)2(isq)Cl2 and Ru(PPh3)2(itsq)Cl2 are available in the Sup-
porting Information. A CIF file for Os(PPh3)2(ap)H2 is also
available.
Anal. Calcd for C42H35NSP2Br2Os: C, 50.6; H, 3.51; N, 1.40.
Found: C, 50.1; H, 3.72; N, 1.43.
UV-vis (CH2Cl2): 488 nm (6.2 × 103 M-1cm-1), 444 (sh), 326
(6.6 × 103), 282 (85.3 × 103).
CV [E1/2, V vs SCE (∆Ep, mV)]: -0.72 (Epc), 0.67 (60).
Preparation of Os(PPh3)2(ap)H2. Os(PPh3)3Br2 (114 mg, 0.10
mmol), 2-aminophenol (23 mg, 0.21 mmol), and triethylamine (21
mg, 0.21 mmol) were added to ethanol (40 mL), and the mixture
was refluxed under N2 for 4 h. Brownish-orange crystals of Os-
(PPh3)2(ap)H2 separated from the solution during reflux. The crystals
were collected by filtration and washed with ethanol to give Os-
(PPh3)2(ap)H2 in 65% yield.
Results
Interest in the coordination chemistry of quinone ligands
has turned to complexes of the iminoquinone ligands. Of
particular interest in the context of metal-stabilized radical
ligands are complexes containing the iminosemiquinone (isq)
ligands. Identification of the radical ligand is generally
uncomplicated in cases where it is the only paramagnetic
center of the complex molecule. Characterization becomes
more complicated for complexes containing multiple para-
magnetic centers where spin coupling results in cooperative
magnetic properties.2 As with the quinone complexes,
questions concerning charge distribution have become of
interest for the iminoquinone complexes containing redox-
active metal ions. The iminoquinone and iminothioquinone
Anal. Calcd for C42H37NOP2Os: C, 61.2; H, 4.49; N, 1.70.
Found: C, 60.1; H, 4.52; N, 1.62.
UV-vis (CH2Cl2): 390 nm (8.8 × 103 M-1cm-1), 286 (7.9 ×
103), 234 (26.0 × 103).
CV [E1/2, V vs SCE (∆Ep, mV)]: 0.45 (Epa), 0.83 (Epa).
Physical Measurements. Microanalyses (C, H, N) were per-
formed using a Perkin-Elmer 240C elemental analyzer. Electronic
spectra were recorded on a Jasco V-570 spectrophotometer.
Magnetic susceptibility measurements were made using a PAR 155
5812 Inorganic Chemistry, Vol. 41, No. 22, 2002

o-C6H4XY Ligands with X,Y) O, NH, and S Donor Atoms
Table 3. Selected Bond Lengths and Angles for Ru(PPh3)2(isq)Cl2
bond lengths (Å)
Ru-O(N)
2.003(2)
2.380(1)
2.416(1)
1.308(4)
C1-C1′
C1-C2
C2-C3
C3-C3′
1.431(7)
1.415(5)
1.358(6)
1.402(12)
Ru-Cl
Ru-P
C1-O(N)
angles (deg)
O(N)-Ru-O(N)′
Cl-Ru-Cl′
P-Ru-P′
78.3(1)
93.7(1)
171.7(1)
O(N)-Ru-Cl
94.0(1)
83.7(1)
91.4(1)
P-Ru-Cl
P-Ru-O(N)
for the average CdO and CdN double bonds of RuII(ibq).
The assignment as Ru(PPh3)2(isq)Cl2 with an iminosemi-
quinonate ligand is consistent with the charge distributions
in Ru(PPh3)2(PhenoxSQ)Cl2 and Ru(PPh3)2(3,5-DBSQ)-
Cl2.4c,14 As complexes of Ru(III), all three complex molecules
consist of interacting paramagnetic centers at the metal and
the radical ligand. Spins at both centers lie in codirectional
orbitals of π symmetry, resulting in strong antiferromagnetic
coupling.2 Magnetic measurements show that the compounds
Figure 1. View of the Ru(PPh3)2(isq)Cl2 complex molecule. The molecule
is located along a crystallographic 2-fold axis that bisects the isq ligand,
disordering the O and NH donor atoms.
1
are diamagnetic, and they all give sharp H NMR spectra
that show no effect from residual low-level paramagnetism.
They all have spin singlet ground states with no evidence
for thermal population of a triplet state at higher energy.
The closely related osmium complex Os(PPh3)2(ap)Br2 has
been prepared by treating Os(PPh3)3Br2 with 2-aminophenol
in dichloromethane. The similarity in CV electrochemical
potential and in electronic spectrum with Ru(PPh3)2(isq)Cl2
might point to a similar charge distribution. However, the
solution spectra and redox potentials of Os(PPh3)2(3,5-
DBCat)Cl2 were observed to be similar to those of Ru(PPh3)2-
(3,5-DBSQ)Cl2,12,14 but the structural features of the quinone
ligands in the solid state were consistent with different redox
isomers. This observation was also consistent with differ-
ences in charge distribution for the neutral tris(quinone)
complexes of Ru and Os, where structural and dynamic
properties point to a Os(3,5-DBCat)3 charge distribution and
a lower oxidation state for the Ru center of Ru(3,5-DBQ)3.
Consequently, the Os(IV) redox isomer has been assumed
for OsIV(PPh3)2(ap)Br2 with a reduced amidophenolate ligand,
although a shift in charge distribution to the OsIII(PPh3)2-
(isq)Br2 redox isomer might occur in solution.
Os(PPh3)2(ap)H2. The reaction between Os(PPh3)3Br2 and
2-aminophenol carried out in ethanol gives the dihydrido
complex Os(PPh3)2(ap)H2. Decomposition occurs slowly in
solution, but electrochemical characterization shows two
irreversible oxidations, and the electronic spectrum fails to
show the intense low-energy transitions that appear charac-
teristically for the iminosemiquinonate ligand. Crystal-
lographic characterization has provided insight into the
charge distribution (Figure 2). Using the C-O and C-N
bond lengths as a measure of ligand charge, values of 1.337-
(9) and 1.371(9) Å, respectively, (Table 4) point to an
amidophenolate electronic structure for the ligand. Further,
the bond lengths between carbon atoms of the ring reflect
an aromatic structure rather than the contracted lengths at
the C2-C3 and C4-C5 bonds that are generally seen for
semiquinonate and iminosemiquinonate ligands. The phos-
complexes of Ru and Os are of this type. Structural
characterization of complexes containing the phenoxazinolate
ligand in its iminobenzoquinone and iminosemiquinone
electronic forms,4,5 with recent studies from the Wieghardt
group on complexes of N-phenyl iminosemiquinone and
amidophenolate ligands,6 have provided metrical features that
can be used to assign ligand charges and, indirectly, the
charge on the metal ion.
Ru(PPh3)2(isq)Cl2. The addition of 2-aminophenol to Ru-
(PPh3)3Cl2 has been found to give Ru(PPh3)2(isq)Cl2. Redox
isomers for this product include, in order of increasing ligand
charge, RuII(ibq), RuIII(isq), and RuIV(ap) species. Electro-
chemical characterization shows a reversible couple at 0.66
V (vs SCE) and an irreversible reduction at -0.55 V.
Uncoordinated iminoquinones show similar electrochemical
processes, but shifted to more negative potentials, and we
assign these as a ligand-based oxidation and reduction
processes. This is in agreement with Lever’s assignment for
redox processes that appear at similar potentials for [Ru-
(bpy)2(isq)]2+.13 The electronic spectrum of Ru(PPh3)2(isq)-
Cl2 shows an absorption in the 550-nm region that is
commonly found for complexes containing iminosemi-
quinonato ligands,4,6 although low-intensity bands at lower
energy that are also associated with isq ligands have not been
observed.
Crystallographic characterization of Ru(PPh3)2(isq)Cl2
(Figure 1) has shown that the molecule lies along a
crystallographic two-fold axis of the monoclinic space group
C2/c. The two halves of the isq ligand are related by
symmetry, disordering the C-O and C-N bonds. Conse-
quently, detailed features of the ligand that would provide
insight into the charge distribution are imprecise. However,
the C1-O(N) bond length of 1.308(4) Å (Table 3) is clearly
too low for the RuIV(ap) charge distribution and too high
(12) Bhattacharya, S.; Pierpont, C. G. Inorg. Chem. 1991, 30, 2906.
(13) Masui, H.; Lever, A. B. P.; Auburn, P. R. Inorg. Chem. 1991, 30,
2402.
(14) Bhattacharya, S.; Pierpont, C. G. Inorg. Chem. 1991, 30, 1511.
Inorganic Chemistry, Vol. 41, No. 22, 2002 5813

Bhattacharya et al.
Figure 2. View of Os(PPh3)2(ap)H2. Hydride ligands are shown in their
refined locations.
Figure 3. View of the Ru(PPh3)2(itsq)Cl2 complex molecule. The
ruthenium atom, the chloride ligands, and the itsq ligand lie on a
crystallographic mirror plane.
Table 4. Selected Bond Lengths and Angles for Ru(PPh3)2(itsq)Cl2 and
Os(PPh3)2(ap)H2
Ru(PPh3)2(itsq)Cl2
Os(PPh3)2(ap)H2
ap electronic structure. However, it is more likely that the
OsIV(ap) charge distribution results from the difference in
d-orbital energy for the two metals and that the structural
distortion of OsIV(PPh3)2(ap)H2 is mainly a consequence of
the higher oxidation state of the metal, together with the
smaller radii of the hydride ligands.
bond lengths (Å)
1.946(2)
2.2967(5)
2.4072(5)
2.4560(6)
2.3851(3)
-
M-N
1.972(6)
2.037(4)
1.49(2)
M-O(S)
M-X1
M-X2
M-P1
1.52(2)
2.264(2)
2.308(2)
1.337(9)
1.371(9)
1.409(10)
1.391(10)
1.402(11)
1.370(12)
1.393(12)
1.400(10)
M-P2
C1-O(S)
C2-N
C1-C2
C2-C3
C3-C4
C4-C5
C5-C6
C6-C1
1.706(2)
1.326(3)
1.432(3)
1.430(3)
1.354(3)
1.430(4)
1.372(4)
1.415(3)
Ru(PPh3)2(itsq)Cl2. The iminothioquinone ligands have
the potential to coordinate with transition metals as either a
neutral iminothiobenzoquinone (itbq), an iminothiosemi-
quinonate (itsq) radical anion, or the amidothiophenolate (atp)
dianion. In a recent survey of complexes containing imi-
nothioquinone ligands, Wieghardt concluded from ligand
metrical values that, in many cases, the ligand coordinated
in the radical itsq form.8 This analysis was based on C-S
bond lengths that were found to be close to a value of 1.724
Å, C-N bond lengths of 1.356 Å, and a pattern of C-C
bond lengths for the ring that shows slight contraction at
the bonds that would be double bonds in the benzoquinone
structure. Strong M-itsq and/or intramolecular itsq-itsq spin
coupling often results in magnetic properties that obscure
the radical character of the ligands. However, the difference
in structural features relative to more oxidized or reduced
forms of the ligand is clear (Table 1).
The addition of o-aminothiophenol to Ru(PPh3)3Cl2 has
been found to give Ru(PPh3)2(itsq)Cl2 (Figure 3). Electro-
chemical potentials for the complex are similar to those of
Ru(PPh3)2(isq)Cl2, but with a slight positive shift for the
reversible oxidation at 0.90 V. Structural characterization
revealed features that are similar to those of Ru(PPh3)2(isq)-
Cl2, but free of the disorder resulting from crystallographi-
cally imposed two-fold symmetry. However, in the centered
orthorhombic space group Abm2, the complex molecule is
located on a crystallographic mirror plane that relates the
two triphenylphosphine ligands. Features of the iminothio-
quinone ligand were resolved with high precision (Table 4).
angles (deg)
81.79(5)
170.09(2)
92.633(8)
-
94.527(10)
-
95.79(2)
N-M-O(S)
P1-M-P2
P1-M-O(S)
P2-M-O(S)
P1-M-N
P2-M-N
X1-M-X2
78.8(2)
115.13(7)
146.25(13)
88.58(14)
94.13(16)
140.88(16)
110.7(5)
phine ligands are bent toward the cis-dihydrido ligands, away
from the amidophenolate ring. The P1-Os-P2 bond angle
is 115.13(7)°, and the dihedral angle between the Os-P1-
P2 plane and the plane of the ap ligand is twisted with a
value of 136.4°. Both hydrido ligands of the molecule were
refined to give Os-H bond lengths of 1.49(3) and 1.52(3)
Å and a H1-Os-H2 bond angle of 110.7°. The amidophe-
nolate ring is twisted out of the Os-H1-H2 plane with a
dihedral angle of 40.7°.
The difference in charge distribution between RuIII(PPh3)2-
(isq)Cl2 and OsIV(PPh3)2(ap)H2 might be associated with the
distortion of the phosphine ligands from axial coordination
sites to positions where they would interact strongly with
the partially filled octahedral dπ orbital. Ligand field
destabilization could induce electron transfer from the
osmium to the iminoquinone ligand, resulting in the Os(IV)-
5814 Inorganic Chemistry, Vol. 41, No. 22, 2002

o-C6H4XY Ligands with X,Y) O, NH, and S Donor Atoms
The C-S and C-N bond lengths of 1.706(2) and 1.326(3)
Å, respectively, fall among the shorter values noted by
Wieghardt for radical iminothiosemiquinonate ligands.7
Carbon-carbon bond lengths within the ring show the short/
long pattern associated with semiquinonate and iminosemi-
quinonate ligands. Contracted C-C bond lengths of 1.354(3)
and 1.372(4) Å at positions that would have double bond
values for a benzoquinone are significantly smaller than other
ring C-C bond lengths. Features point clearly to the RuIII-
itsq charge distribution, further illustrating the utility of
structural data in analysis of charge distribution for cases
where magnetic and spectroscopic properties provide little
insight. Ru(PPh3)2(itsq)Cl2 is diamagnetic at all temperatures
and gives a clearly resolved 1H NMR spectrum in the region
for aromatic proton resonances.
on complexes of the phenoxazinylate radical (an iminosemi-
quinonate);4,5 on investigations of the SQ-N-SQ and Cat-
N-SQ ligands;19 and, most recenty, on comprehensive
studies on iminosemiquinonate and amidophenolate com-
plexes by the Wieghardt group.6 The variation in the C-N
bond length with the reduced state of the imino- or
diiminoquinone ligands is smaller than the range of C-O
bond lengths. A similar pattern appears for quinoid ligands
with sulfur in place of the oxygen donors. Precise structural
data are available for complexes containing both iminothi-
osemiquinonate (itsq) and amidothiophenolate (atp) ligands.8
The structural difference between these ligands is clear, as
seen for the [TcVCl4(atp)]- anion (C-S 1.751, C-N 1.45,
C3-C4 1.40)20 relative to NiII(itsq)2 (C-S 1.724, C-N
1.348, C3-C4 1.364)8 and RuIII(PPh3)2(itsq)Cl2.
Conflicting views of ligand charge for the 1,2-dithiolate/
1,2-dithiolene complexes become clear for complexes pre-
pared with the 1,2-benzenedithiolate ligands upon recognition
of the potential for coordination as a 1,2-dithiosemiquinonate
ligand. For example, the ligands of Ni(S2-dtbsq)2 have
dithiosemiquinonate features (C-S 1.72, C3-C4 1.36),
whereas the ligands of the dianion [Ni(S2-dtcat)2]2- (C-S
1.76, C3-C4 1.40) are clearly dithiocatecholate.21 In both
complexes, the metal is Ni(II), and the redox process relating
members of the redox series is ligand-based. The benzene-
dithiolate ligands of [W(bdt)3]2- (C-S 1.752, C3-C4 1.387)
are clearly dithiocatecholate in charge,22 whereas the ligands
of Mo(bdt)3 (C-S 1.727, C3-C4 1.359)23 are dithiosemi-
quinonate with strong spin coupling, resulting in diamagne-
tism for the complex of Mo(III).
Electronic Coupling Between Mixed-Charge Quinone
Ligands. Electronic coupling between ligands of SQ-M-
Cat complexes can be viewed as resembling coupling in
ligand-bridged bimetallic mixed-valence (M-L-M′) com-
plexes. The strength of exchange in bimetallic systems is
commonly referenced to the Robin-Day classification
scheme, where type 2 refers to weakly coupled systems that
appear charge-localized on some observable time scale and
type 3 systems are delocalized. Differences in ligand structure
place members of the CoIII(N-N)(SQ)(Cat) series in the type
2 classification,24 for example, and the absence of a difference
between ligands in the RuIII(N-N)(SQ)(Cat) series might
place these complexes in the type 3 classification.1a,4d
Members of the CoIII(N-N)(SQ)(Cat) series have intense
Cat f SQ LL′IT bands in the 2500-nm region of the IR
spectrum, whereas the RuIII(N-N)(SQ)(Cat) complexes fail
to show electronic transitions that can be assigned as
intervalence transitions between charge-localized SQ and Cat
ligands. This distinction is relevant to the assignment of
The reaction between o-aminothiophenol and Os(PPh3)3-
Br2 gives the o-iminothiophenolate complex Os(PPh3)2(atp)-
Br2. The electrochemistry and solution electronic spectrum
for the complex are quite similar to those of Os(PPh3)2(ap)-
Br2, but a distinction between the OsIII(PPh3)2(itsq)Br2 and
OsIV(PPh3)2(atp)Br2 redox isomers in the solid state will
require further crystallographic characterization.
Structural Features of Quinoid Ligands with Oxygen,
Nitrogen, and Sulfur Donor Atoms. Structural reports have
appeared for complexes containing chelated quinoid ligands
that contain information on bond lengths for oxygen,
nitrogen, and sulfur donor atoms. These studies include early
reports on aromatic dithiolate/dithiolene complexes,16 ben-
zene-1,2-diimine complexes,17 catecholate/semiquinonate
complexes,1 as well as complexes containing mixed-donor
ligands. Many of the structures are, for various reasons,
imprecise or disordered, and an unambiguous structural
distinction between ligands of different charge is unclear.
However, the view of ligand coordination that emerges from
an analysis of precisely determined structures is one of
localized charge; quinoid ligands bound with defined BQ,
SQ, or Cat electronic structures to metal ions of well-defined
charge.
The difference in carbon-oxygen bond lengths for ligands
in the three charged states represents the greatest variation
for the series (Table 1). Precise structure determinations have
been reported for complexes containing o-benzoquinone,
o-semiquinonate, and catecholate ligands chelated with
transition metal ions. Systematic differences in C-O and
ring C-C bond lengths had been noted much earlier and
used as a diagnostic of ligand charge.1,18 In many cases,
magnetic and spectroscopic properties supported the charge
distribution between ligand and metal inferred from ligand
structure. Structural features of the iminoquinone ligands in
the three charged forms given in Table 1 are based on reports
(19) (a) Simpson, C. L.; Boone, S. R.; Pierpont, C. G. Inorg. Chem. 1989,
28, 4379. (b) Bruni, S.; Caneschi, A.; Cariati, F.; Delfs, C.; Dei, A.;
Gatteschi, D. J. Am. Chem. Soc. 1994, 116, 1388.
(20) Cook, J.; Davis, W. M.; Davison, A.; Jones, A. G. Inorg. Chem. 1991,
30, 1773.
(21) Sellmann, D.; Binder, H.; Haussinger, D.; Heinemann, F. W.; Sutter,
J. Inorg. Chim. Acta 2000, 300, 829.
(22) Lorber, C.; Donahue, J. P.; Goddard, C. A.; Nordlander, E.; Holm, R.
H. J. Am. Chem. Soc. 1998, 120, 8102.
(15) Bhattacharya, S.; Boone, S. R.; Fox, G. A.; Pierpont, C. G. J. Am.
Chem. Soc. 1990, 112, 1088.
(16) (a) McCleverty, J. A. Prog. Inorg. Chem. 1968, 10, 49. (b)Eisenberg,
R. Prog. Inorg. Chem. 1970, 12, 295. (c) Burns, R. P.; McAuliffe, C.
A. AdV. Inorg. Chem. Radiochem. 1979, 22, 303.
(17) Carugo, O.; Djinovic´, K.; Rizzi, M.; Castellani, C. B. J. Chem. Soc.,
Dalton Trans. 1991, 1551.
(18) Carugo, O.; Castellani, C. B.; Djinovic´, K.; Rizzi, M. J. Chem. Soc.,
Dalton Trans. 1992, 837.
(23) Cowie, M.; Bennett, M. J. Inorg. Chem. 1976, 15, 1584.
(24) Pierpont, C. G. Coord. Chem. ReV. 2001, 216-217, 99.
Inorganic Chemistry, Vol. 41, No. 22, 2002 5815

Bhattacharya et al.
ligand charge based on structural features. Members of the
RuIII(N-N)(SQ)(Cat) series typically have C-O bond
lengths that are intermediate between the localized SQ and
Cat values. The members of this series and a few other
complexes of platinum-group metals that contain two or three
quinone ligands stand as unique exceptions to the structural
pattern that associates features with localized ligand charge
and a defined formal charge for the coordinated metal ion.1b
ment of Science and Technology, New Delhi, India. P.G.
thanks the Council of Scientific and Industrial Research, New
Delhi, India, for a research fellowship.
Supporting Information Available: 1H NMR spectra for the
five complexes described in the Experimental Section. Complete
tables of crystallographic data, final atomic coordinates, bond
lengths and angles, and hydrogen atom coordinates for Ru(PPh3)2-
(isq)Cl2 and Ru(PPh3)2(itsq)Cl2 are available as Supporting Infor-
mation. A CIF file for Os(PPh3)2(ap)H2 has been deposited. This
material is available free of charge via the Internet at http://
pubs.acs.org.
Acknowledgment. Support for research carried out at the
University of Colorado was provided by the National Science
Foundation (CHE9985970). Research at Jadavpur University
was supported by a grant (SP/S1/F33/98) from the Depart-
IC025766+
5816 Inorganic Chemistry, Vol. 41, No. 22, 2002