Chemistry of metal-bound anion radicals. A family of mono- and bis(azopyridine) chelates of bivalent ruthenium

Article abstract of DOI:10.1021/ic000356b

The reaction of the dihydride [Ru(II)(H)₂(CO)(PPh₃)₃], 3, with excess azo-2,2'-bipyridine (abp) in boiling dry benzene has afforded the diradical bischelate [Ru(II)(abp^.-)₂(CO)(PPh₃)], 4, and the hydridic monochelate monoradical [Ru(II)(abp^.-)(H)(CO)(PPh₃)₂], 5. A similar reaction between 3 and 2-(p-chlorophenylazo)pyridine (Clpap) did not yield a bischelate, but the hydridic monoradical [Ru(II)(Clpap^.-)(H)(CO)(Pph₃)₂], 6, has been isolated. Upon treatment of 4-6 with NH₄PF₆ in a wet dichloromethane-acetonitrile medium, the one-electron-oxidized salts 4⁺PF₆^-, 5⁺PF₆^-, and 6⁺PF₆^- are isolated, H⁺ being the oxidizing agent. The X-ray structures of 4⁺PF₆^-.CH₂Cl₂, 5⁺PF₆^-·H₂O, and 6⁺PF₆^- have been determined. In the monoradical 4⁺ the azo N-N bond lengths in the two chelate rings are 1.284(6) and 1.336(6) A, showing that the radical electron is localized in the latter ring. The half-filled extended Huckel HOMO is indeed found to be so localized, and it has a large azo character. Complexes 4-6 display radical redox couples with E(1/2) in the range -0.5 to +0.10 V vs SCE. The E(1/2) values qualitatively correlate with corresponding v(co) values (1900-2000 cm^-1). The monoradicals (S = 1/2) 4⁺, 5, and 6 uniformly display a strong EPR signal near g = 2.00. Metal-mediated magnetic interaction makes the EPR-silent diradical 4 strongly antiferromagnetic with J = -299 cm^-1. Crystal data are as follows: (4⁺PF₆^-·CH₂Cl₂, C₄₀H₃₃Cl₂F₆N₈OP₂Ru) monoclinic, space group P2₁/c (no. 14), a = 14.174(6) A, b = 16.451(4) A, c = 18.381(4) A, β = 98.00(3)°, Z = 4; (5⁺PF₆^-·H₂O, C₄₇H₄₁F₆N₄O₂P₃Ru) monoclinic, space group P2₁/n (no. 14), a = 9.433(2) A, b = 38.914(17) A, c = 13.084(3) A, β = 103.47(2)°, Z = 4; (6⁺PF₆^-, C₄₈H₃₉ClF₆N₃OP₃Ru) monoclinic, space group P2₁/n (no. 14), a = 10.496(5) A, b = 22.389(8) A, c = 19.720(6) A, β = 90.53(3)°, Z = 4.

Full text of DOI:10.1021/ic000356b

4332
Inorg. Chem. 2000, 39, 4332-4338
Chemistry of Metal-Bound Anion Radicals. A Family of Mono- and Bis(azopyridine)
Chelates of Bivalent Ruthenium
Maya Shivakumar, Kausikisankar Pramanik, Indranil Bhattacharyya, and
Animesh Chakravorty*
Department of Inorganic Chemistry, Indian Association for the Cultivation of Science,
Calcutta 700 032, India
ReceiVed April 4, 2000
The reaction of the dihydride [RuII(H)2(CO)(PPh3)3], 3, with excess azo-2,2′-bipyridine (abp) in boiling dry benzene
has afforded the diradical bischelate [RuII(abp•-)2(CO)(PPh3)], 4, and the hydridic monochelate monoradical
[RuII(abp•-)(H)(CO)(PPh3)2], 5. A similar reaction between 3 and 2-(p-chlorophenylazo)pyridine (Clpap) did not
yield a bischelate, but the hydridic monoradical [RuII(Clpap•-)(H)(CO)(PPh3)2], 6, has been isolated. Upon treatment
-
of 4-6 with NH4PF6 in a wet dichloromethane-acetonitrile medium, the one-electron-oxidized salts 4+PF6
,
5+PF6-, and 6+PF6 are isolated, H+ being the oxidizing agent. The X-ray structures of 4+PF6-‚CH2Cl2,
-
5+PF6-‚H2O, and 6+PF6 have been determined. In the monoradical 4+ the azo N-N bond lengths in the two
-
chelate rings are 1.284(6) and 1.336(6) Å, showing that the radical electron is localized in the latter ring. The
half-filled extended Hu¨ckel HOMO is indeed found to be so localized, and it has a large azo character. Complexes
4-6 display radical redox couples with E1/2 in the range -0.5 to +0.10 V vs SCE. The E1/2 values qualitatively
1
correlate with corresponding νCO values (1900-2000 cm-1). The monoradicals (S ) /2) 4+, 5, and 6 uniformly
display a strong EPR signal near g ) 2.00. Metal-mediated magnetic interaction makes the EPR-silent diradical
4 strongly antiferromagnetic with J ) -299 cm-1. Crystal data are as follows: (4+PF6-‚CH2Cl2, C40H33Cl2F6N8-
OP2Ru) monoclinic, space group P21/c (no. 14), a ) 14.174(6) Å, b ) 16.451(4) Å, c ) 18.381(4) Å, â )
98.00(3)°, Z ) 4; (5+PF6-‚H2O, C47H41F6N4O2P3Ru) monoclinic, space group P21/n (no. 14), a ) 9.433(2) Å, b
-
) 38.914(17) Å, c ) 13.084(3) Å, â ) 103.47(2)°, Z ) 4; (6+PF6 , C48H39ClF6N3OP3Ru) monoclinic, space
group P21/n (no. 14), a ) 10.496(5) Å, b ) 22.389(8) Å, c ) 19.720(6) Å, â ) 90.53(3)°, Z ) 4.
Introduction
Chart 1
The low-lying nature of the π*-orbital makes the azo function
susceptible to facile one-electron reduction, eq 1, in both free
-NdN- + e f -N-N--
(σ2π2) (σ2π2π*1)
(1)
and coordinated states. Generation of anion radical species by
such reduction in solution has been documented for a long
time,1-4 but their isolation in the pure state was achieved only
very recently.5-9 We have demonstrated that the reaction of
monohydrides of the type [MII(H)(X)(CO)(PPh3)3] (M ) Ru,
Os; X ) Cl, Br) with azo heterocyclic ligands (general
abbreviation L) in dry hydrocarbon solvents is attended with
homolytic M-H cleavage, generating the chelated anion radi-
cal ligand L•-, eq 2. Crystalline species of the type [MII(L•-)(X)-
(CO)(PPh3)2] have been isolated in this manner.5-7
(1) Jonson, C. J.; Chang, R. J. Chem. Phys. 1965, 43, 3183.
(2) Sadler, J. L.; Bard, A. J. J. Am. Chem. Soc. 1968, 90, 1979.
(3) Goswami, S.; Mukherjee, R.; Chakravorty, A. Inorg. Chem. 1983, 22,
2825.
(4) Krause, R. A.; Krause, K. Inorg. Chem. 1984, 23, 2195.
(5) Shivakumar, M.; Pramanik, K.; Ghosh, P.; Chakravorty, A. Inorg.
Chem. 1998, 37, 5968.
II
M H + L f M L•- + 1/2H2
II
(2)
(6) Shivakumar, M.; Pramanik, K.; Ghosh, P.; Chakravorty, A. Chem.
Commun. 1998, 2103.
(7) Pramanik, K.; Shivakumar, M.; Ghosh, P.; Chakravorty, A. Inorg.
Chem. 2000, 31, 195.
(8) Schwach, M.; Hausen, H.-D.; Kaim, W. Inorg. Chem. 1999, 38, 2242.
(9) Doslik, N.; Sixt, T.; Kaim, W. Angew. Chem. 1998, 110, 2125; Angew.
Chem., Int. Ed. Engl. 1998, 37, 2403.
The above development has prompted us to explore the
reaction between L and dihydridic MII(H)2 species with the
objective of observing and isolating bischelated radical moieties
such as MII(L•-)2 and MII(L)(L•-). This has been successfully
achieved in the case of bivalent ruthenium. The intermediate
10.1021/ic000356b CCC: $19.00 © 2000 American Chemical Society
Published on Web 08/25/2000

Chemistry of Metal-Bound Anion Radicals
Inorganic Chemistry, Vol. 39, No. 19, 2000 4333
monohydridic monochelate radical system RuII(H)(L•-) has also
been characterized. The structure and properties of representative
members of this remarkable family of compounds are scrutinized
in the present work.
Results and Discussion
A. Synthesis. a. Radical Species. The azo heterocyclic
ligands 1 and 2, the hydridic starting material 3, and the iso-
lated radical (4-6 and 4+) and nonradical (5+ and 6+) species
are listed in Chart 1. The structural drawings 4-6 shown below
are based on definitive structural work (vide infra) and logical
comparisons. In these drawings only the azo (Na) and pyridine
(Np) nitrogen atoms of the L•- ligand are shown.
Upon reaction of the dihydride 310 with a 3-fold excess of
abp in boiling dry benzene under nitrogen, a green solution is
formed from which the diradical bischelate 4 incorporating
Figure 1. Molecular structure of the cation [RuII(abp)(abp•-)(CO)(PPh3)]+
in the crystal of 4+PF6-‚CH2Cl2 showing 30% probability ellipsoids.
b. Oxidized Species. Upon treatment of 4 with NH4PF6 in a
wet dichloromethane-acetonitrile medium, the color of the
solution changes rapidly from dark green to brown red. In this
process ring A undergoes selective one-electron oxidation (abp•-
f abp), and the brown-colored monoradical bischelate salt
-
4+PF6 with the unpaired electron in ring B is isolated from
the reaction mixture. This salt is indefinitely stable both in the
solid state and in solution. Indeed it is the most stable anion
radical complex isolated by us so far. Similar treatment of 5
and 6 with NH4PF6 yielded the corresponding nonradical
chelate rings A and B was isolated in good yield as a dark
colored solid. In this synthesis the solvent and the reaction
environment should be free of moisture; otherwise the mono-
radical 4+ (see below) is formed as a contaminant.
When 3 and abp were reacted as above but with a smaller
excess of abp, the hydridic monochelate monoradical 5 ac-
monochelate salts 5+PF6 and 6+PF6-, respectively.
-
It has so far been presumed5-7 that the oxidation of the
chelated anion radicals in dichloromethane-acetonitrile solution
containing NH4PF6 was caused by oxygen from the air. We
have now found that the oxidation process occurs equally
smoothly in the absence of oxygen, and the crucial factor is
that the solvent mixture must be quite wet. This facilitates acid
formation via hydrolysis of NH4PF6, the proton then acting as
1
the oxidant: RuIIL•- + H+ f RuIIL + /2H2. We have indeed
observed that addition of very dilute acids to solutions of the
radicals leads to immediate oxidation. This is consistent with
the low E1/2 of the radicals, vide infra. Significantly, while 4 is
smoothly converted to 4+ by this procedure, the latter having a
higher E1/2 is not oxidized to the nonradical dication 42+
.
cumulated in solution and could be isolated, though in relatively
poor yield. The radical-forming transformations involved in the
synthesis of 4 and 5 are summarized in eq 3.
abp
abp
8
Ru(abp•-)
Ru(H)2 8 Ru(abp•-)(H)
(3)
2
5
4
3
The reaction between 3 and excess Clpap failed to afford
any tractable bischelate radical complex probably due to the
weaker π-acceptor ability of Clpap compared to abp. However,
the hydridic monoradical 6 could be isolated in which the
coordination sphere is geometrically isomeric with that in 5.
B. Structures. a. Monoradical BisChelate: Radical Local-
ization. The diradical bischelates 4 did not afford suitable single
crystals. The structure of the monoradical [RuII(abp)(abp•-)(CO)-
(PPh3)]PF6‚CH2Cl2, 4+PF6-‚CH2Cl2, has however been deter-
mined. A view of the cation is shown in Figure 1, and selected
bond parameters are listed in Table 1. The mutually cis CO
and PPh3 ligands both lie trans to pyridine nitrogen atoms while
the coordinated azo nitrogen atoms are mutually trans to each
other. The chelate ring and the pyridyl ring incorporating N1
(10) Ahmad, N.; Levison, J. J. Robinson, S. D.; Uttley, M. F. Inorg. Synth.
1975, 15, 45.

4334 Inorganic Chemistry, Vol. 39, No. 19, 2000
Shivakumar et al.
Figure 2. Molecular structure of the cation [RuII(abp)(H)(CO)(PPh3)2]+
in the crystal of 5+PF6-‚H2O showing 30% probability ellipsoids.
Figure 3. Molecular structure of the cation [RuII(Clpap)(H)(CO)-
(PPh3)2]+ in the crystal of 6+PF6- showing 30% probability ellipsoids.
Table 1. Selected Bond Distances (Å) and Angles (deg) for
4+PF6-‚CH2Cl2
6+PF6- are directly observable in 1H NMR (vide infra) but are
not resolved in difference Fourier maps. The cations 5+ and 6+
have isomeric geometries, the position of H- and CO ligands
being interchanged. The origin of this difference between 5+
and 6+ is unclear.
C. Frontier Orbitals. To gain qualitative insight into the
electronic nature of 4+, extended Hu¨ckel calculations were
performed on the model [RuII(abp)(abp•-)(PH3)(CO)]+. The
half-filled HOMO (E ) -11.04 eV) and the LUMO (E )
-10.58 eV) are 7 and 8, respectively. The former is indeed
Ru-P1
Ru-C39
Ru-N3
Ru-N7
N6-N7
2.404(2)
1.901(6)
2.013(4)
2.083(4)
1.336(6)
O1-C39
Ru-N1
Ru-N5
N2-N3
1.125(7)
2.116(4)
2.112(4)
1.284(6)
C39-Ru-P1
N1-Ru-P1
N5-Ru-P1
N1-Ru-N3
N5-Ru-N7
N3-Ru-N5
C39-Ru-N1
C39-Ru-N5
91.5(2)
O1-C39-Ru
N3-Ru-P1
N7-Ru-P1
N1-Ru-N5
N1-Ru-N7
N3-Ru-N7
C39-Ru-N3
C39-Ru-N7
173.7(5)
90.30(13)
102.27(13)
83.1(2)
92.71(12)
174.94(13)
75.1(2)
75.6(2)
91.3(2)
174.6(2)
92.9(2)
97.7(2)
165.9(2)
101.4(2)
84.8(2)
(ring A) together constitute a good plane with a mean deviation
of 0.02 Å, and the pendant pyridyl ring makes a dihedral angle
of 24.2° with it. Similarly ring B defines a satisfactory plane
(mean deviation 0.04 Å) to which the pendant pyridyl ring
incorporating N8 makes a dihedral angle of 15.4°. The dichlo-
romethane molecule does not display any unusual nonbonded
interaction in the lattice.
The most remarkable feature of the structure is the difference
in the two N-N lengths: N2-N3, 1.284(6) Å (ring A); N6-
N7, 1.336(6) Å (ring B). In nonradical and radical monochelates
of azo heterocycles, the azo N-N distances have been found
to lie in the ranges 1.28-1.29 and 1.33-1.37 Å, approximately
corresponding to -NdN- and -N-N- descriptions, respec-
tively.5-7 In the present complex both the types are present
within the same molecule, and we have a model case of ground-
state radical localization in one of the two rings.
The Ru-Np lengths, 2.116(4) and 2.112(4) Å, in chelate rings
A and B are nearly equal. On the other hand, the corresponding
Ru-Na lengths, 2.013(4) and 2.083(4) Å are well discriminated
due to radical localization in ring B. The decrease of Ru-azo
back-bonding in going from RuII(abp) to RuII(abp•-) lengthens
the Ru-Na distance in the latter.
b. Nonradical Monochelates: Isomeric Geometries. The
radical chelates 5 and 6 did not afford suitable single crystals,
but the nonradical congeners [RuII(abp)(H)(CO)(PPh3)2]PF6‚
H2O, 5+PF6 ‚H2O and [RuII(Clpap)(H)(CO)(PPh3)2]PF6, 6+PF6 ,
did. Since the radical f nonradical oxidation process occurs
under very mild conditions, it is logical to assume that the
process is stereoretentive and the gross geometries of 5 and 6
could be ascertained from those of 5+ and 6+. The latter are
shown in Figures 2 and 3, and selected bond parameters are
given in Table 2. The water molecule in 5+PF6-‚H2O is strongly
hydrogen bonded to a neighboring water molecule in the lattice
(O...O, 2.54(1) Å). The hydride ligands in 5+PF6-‚H2O and
-
-
localized in ring B and is 51% azo, 19% pyridyl, and 9% metal-d
in character. The azo group of ring A makes only a 5%
contribution in 7. On the other hand, 8 is localized on ring A

Chemistry of Metal-Bound Anion Radicals
Inorganic Chemistry, Vol. 39, No. 19, 2000 4335
Table 2. Selected Bond Distances (Å) and Angles (deg) for 5+PF6-‚H2O and 6+PF6
-
5+PF6-‚H2O
6+PF6
5+PF6-‚H2O
6+PF6
-
-
Ru-P1
Ru-P2
Ru-N1
Ru-N3
2.362(3)
2.374(3)
2.105(9)
2.106(10)
2.365(2)
2.375(2)
2.158(4)
2.127(4)
Ru-C47/C48a
O1-C47/C48a
N2-N3
1.845(12)
1.138(13)
1.282(13)
1.851(5)
1.136(6)
1.285(5)
P1-Ru-C47/C48a
P2-Ru-C47/C48a
N1-Ru-C47/C48a
N3-Ru-C47/C48a
N1-Ru-P1
87.3(4)
91.8(4)
178.1(4)
104.9(5)
91.5(2)
89.8(3)
88.3(2)
89.2(2)
105.7(2)
179.4(2)
96.54(11)
93.47(11)
N3-Ru-P1
101.3(3)
95.6(3)
162.77(11)
73.9(4)
91.14(11)
91.43(11)
169.99(5)
74.4(2)
N3-Ru-P2
P1-Ru-P2
N1-Ru-N3
O1-C47/C48-Rua
176.70(12)
176.9(5)
N1-Ru-P2
a C47 in 5+PF6-‚H2O and C48 in 6+PF6
.
-
Table 3. Electrochemicala and IRb Data
with 50% azo, 8% pyridyl, and 20% metal-d character. The
significantly larger RuII-abp interaction is no doubt an impor-
tant factor that places 8 above 7 in energy, causing radical
localization.
The orbitals 9 (E ) -8.99 eV) and 10 (E ) -8.74 eV) lying
∼2 eV above the LUMO are spectrally relevant (see below).
E1/2, V
νCO
E1/2, V
νCO
compd
(∆Ep,c mV)
(cm-1
)
compd (∆Ep,c mV) (cm-1
)
4
0.10 (100), -0.28 (100) 1960
0.10 (90), -0.25 (90) 2000
5
6
-0.46 (100) 1922
-0.52 (100) 1910
4+
a In CH2Cl2 (0.1 M Et4NClO4). b In a KBr disk. c ∆Ep ) Epa - Epc.
Table 4. Bulk Magnetic Moments and EPR g Values in Solid State
µeff (µB)
g (∆Hpp,a G)
compd
298 K
298 K
77 K
4
1.37
1.79
1.83
1.80
4+
5
2.003 (19)
2.002 (13)
2.002 (9)
1.999 (13)
2.003 (15)
2.003 (12)
6
a ∆Hpp ) peak-to-peak line width.
complexes display one or two moderately intense (ꢀ ) 2500-
5500 M-1 cm-1) and relatively broad bands in the 500-600
nm region. In the case of 4 and 4+ these are tentatively assigned
to intra/interligand transitions 7, 8 f 9, 10. The diamagnetic
1
nonradical species show well-resolved H NMR spectra. The
Ru-H proton in 5+PF6- (-8.79 ppm) and 6+PF6- (-9.71 ppm)
has the expected triplet structure due to coupling with the two
31P atoms of the PPh3 ligands, the coupling constants being 22.2
and 18.6 Hz, respectively.
The E1/2 values of the radical species in dichloromethane
solution are listed in Table 3. The monochelates display a single
couple due to radical redox RuII(L)/RuII(L•-). The bischelate
shows two successive couples, RuII(abp)2/RuII(abp)(abp•-)
(electron added to ring B) and RuII(abp)(abp•-)/RuII(abp•-)2
(electron added to ring A), the voltage gap between them being
∼0.4 V, which qualitatively correlates with the energy gap
between 7 and 8.
We have noted earlier5-7 that the PPh3 and CO ligands play
a significant role in radical stabilization, one of the factors being
back-bonding. Indeed the CO stretching frequencies of the
radicals (Table 3) qualitatively correlate with the azo reduction
potentials. The higher the energy of the azo π*-orbital (lower
E
1/2) the stronger the Ru-CO back-bonding (lower νCO).
E. Magnetism. a. Monoradicals. The room temperature
magnetic moments of the polycrystalline monoradical complexes
(Table 4) generally lie near 1.8 µB, corresponding to the presence
of one unpaired electron (S ) 1/2). The samples display a strong
EPR signal near g ) 2.00 with peak-to-peak line widths (∆Hpp)
in the range 10-20 G (Table 4). The lack of nitrogen hyperfine
structure in the EPR spectra is not unusual since the 14N coupling
constant in the azo π* radicals is small and difficult to
observe.1,5-7,12,13
These are ∼90% pyridyl in character, being localized in rings
A and B, respectively. We thus roughly have one pair of azo
orbitals (7 and 8) and one pair of pyridyl orbitals (9 and 10) in
the frontier, the energy gap within each pair being relatively
small. In monochelate monoradicals of the type [MII(L•-)(X)-
(CO)(PPh3)2] (M ) Ru, Os; X ) Br, Cl), we have single azo
and pyridyl orbitals instead.7,11
D. Spectra and Reduction Potentials. Relevant data are
listed in Table 3 and in the Experimental Section. The radical
(11) Shivakumar, M. Ph.D. Thesis, Jadavpur University, Calcutta 700 032,
India, February 2000.

4336 Inorganic Chemistry, Vol. 39, No. 19, 2000
Shivakumar et al.
material 3 was prepared by the reported procedure.10 2-Ami-
nopyridine (Aldrich), RuCl3‚xH2O (Arora-Mathey, India), and
NH4PF6 (Aldrich) were used as received. Benzene and heptane
used in synthesis were dried by distillation over Na/benzophe-
none. The purification of dichloromethane and the preparation
of tetraethylammonium perchlorate (TEAP) for electrochemical
work were done as described before.19,20
Physical Measurements. Spectral measurements were made
with the following equipment: UV-vis, Shimadzu UV-1601
PC spectrophotometer; IR (KBr disk), Perkin-Elmer 783
Figure 4. Temperature dependence of the molar susceptibility ø for
complex 4. The solid line represents the fit with J ) -299 cm-1
.
1
spectrometer; H NMR, Bruker 300 MHz FT spectrometer;
X-band EPR, Varian E-109C spectrometer fitted with a liquid
nitrogen quartz dewar, 9.1-9.2 GHz microwave frequency, 100
kHz modulation, 30 dB power, 3300 G sweep center, 1000 G
sweep width, 240 s sweep time. Electrochemical measurements
were performed under a nitrogen atmosphere using a PAR 370-4
electrochemistry system as before:20 working electrode, planar
Beckmann model 39273 platinum-inlay electrode; auxiliary
electrode, platinum wire; reference electrode, SCE; supporting
electrolyte, Et4NClO4 (0.1 M); scan rate, 50 mV s-1; solution
concentration, ∼10-3 M. All potentials reported are uncorrected
for junction contributions.
Room temperature magnetic moments were measured by
using a PAR-155 vibrating sample magnetometer. Variable
temperature susceptibility work was done with a Georges
Associates Inc. Lewis-coil force magnetometer equipped with
a closed cycle cryostat (Air Products) and a Cahn-2000 balance
(we are thankful to Professor A. R. Chakravarty for the use of
the equipment). The antiferromagnetically coupled susceptibility
in the diradical [RuII(abp•-)2(CO)(PPh3)], 4, was fitted to the
expression of eq 4, where J is the singlet-triplet separation
and the other symbols have their usual meaning.21
b. Diradical 4. The effective magnetic moment of 4 is ∼1.4
µB at room temperature. The susceptibility decreases progres-
sively with temperature, and the sample becomes virtually
diamagnetic near 50 K (Figure 4). The observed behavior is
reproduced well on the basis of antiferromagnetic interaction
between the two radical sites, the coupling constant J defining
the singlet-triplet energy gap as -299 cm-1. It is logical to
assume that the two half-filled shells in 4 are closely similar to
those in 7 and 8. The significant metal-d character (vide supra)
of these orbitals ensures a metal-mediated pathway for the
magnetic interaction. Pure samples of complex 4 are EPR-silent,
presumably due to relaxation phenomena.
We note that complex 4 can be considered as a distant ana-
logue of bissemiquinone complexes of the type [RuII(sq•-)2-
(PPh3)2]14,15 and [RuII(sq•-)2(CO)2].16 In the latter, the antifer-
romagneticinteraction is however much stronger and the species
are diamagnetic at room temperature.
Concluding Remarks
The scope of the synthesis of radical anion chelates via
heterolytic M-H bond cleavage by azo heterocyclic ligands
(L) has been augmented by using the dihydridic starting ma-
terial [RuII(H)2(CO)(PPh3)3], 3. Both steps of the conversion
RuII(H)2 f RuII(H)(L•-) f RuII(L•-)2 have been realized in
the case of L ) abp; for L ) Clpap, only the first step could be
diagnosed. The hydridic monoradicals [RuII(abp•-)(H)(CO)-
(PPh3)2], 5, and [RuII(Clpap•-)(H)(CO)(PPh3)2], 6, are curiously
isomeric in coordination geometry. The oxidations such as 4
f 4+, 5 f 5+, and 6 f 6+ are brought about by acids (H+).
The diradical bischelate [RuII(abp•-)2(CO)(PPh3)], 4, displays
strong metal-mediated antiferromagnetism, and upon oxidation
to the monoradical 4+, the unpaired electron with ∼40% azo
π*-character gets localized in one of the chelate rings. This is
consistent with simple molecular orbital computations.
ø ) 2Ng2â2/kT[3 + exp(-J/kT)]
(4)
Preparation of Complexes. [RuII(abp•-)2(CO)(PPh3)], 4.
Azo-2,2′-bipyridine (60 mg, 0.33 mmol) was dissolved in 10
mL of dry benzene, and the solution was purged with dry
nitrogen for 0.5 h. To this solution was added [RuII(H)2(CO)-
(PPh3)3], 3 (100 mg, 0.11 mmol), and the mixture was heated
to reflux under nitrogen for 40 min. The color of the reaction
mixture changed from yellow to dark green. The solution was
allowed to cool to room temperature and then evaporated to
dryness under reduced pressure, affording a dark-colored solid
which was repeatedly extracted with warm dry heptane until
the heptane extracts were no longer deep green in color. The
combined heptane extract was concentrated to a volume of 15
mL under reduced pressure. The solution was kept in the
refrigerator overnight. The precipitated solid was filtered,
washed with dry acetonitrile, and dried immediately in vacuo.
Yield: 58 mg (71%). Data for 4 are as follows. Anal. Calcd
for C39H31N8OPRu: C, 61.65; H, 4.08; N, 14.75. Found: C,
60.96; H, 4.18; N, 14.54. UV-vis (C6H6) [λmax, nm (ꢀ, M-1
cm-1)]: 568 (4250), 368 (13 950).
The role of CO in stabilizing the radical species is reflected
in the qualitative correlation between CO stretching frequencies
and radical E1/2
.
Experimental Section
Starting Materials and Solvents. The azo-2,2′-bipyridine
and 2-(p-chlorophenylazo)pyridine ligands were synthesized
according to previously published procedures.17,18 The starting
[Ru(abp•-)(H)(CO)(PPh3)2], 5. To a solution of abp (26 mg,
0.14 mmol) dissolved in dry benzene (20 mL) was added 3 (75
mg, 0.08 mmol), and the reaction mixture was heated to reflux
under nitrogen for 1 h. The resulting dark green solution was
allowed to cool somewhat and then quickly evaporated to
(12) Pal, C. K.; Chattopadhyay, S.; Sinha, C. R.; Chakravorty, A. Inorg.
Chem. 1994, 33, 6140.
(13) Pal, C. K.; Chattopadhyay, S.; Sinha, C. R.; Chakravorty, A. Inorg.
Chem. 1996, 35, 2442.
(14) Bhattacharya, S.; Pierpont, C. G. Inorg. Chem. 1991, 30, 1511.
(15) Bag, N.; Lahiri, G. K.; Basu, P.; Chakravorty, A. J. Chem. Soc., Dalton
Trans. 1992, 113.
(16) Bhattacharya, S.; Pierpont, C. G. Inorg. Chem. 1994, 33, 6038.
(17) Goswami, S.; Chakravarty, A. R.; Chakravorty, A. Inorg. Chem. 1981,
20, 2246.
(19) Lahiri, G. K.; Bhattacharya, S.; Ghosh, B. K.; Chakravorty, A. Inorg.
Chem. 1987, 26, 4324.
(20) Chandra, S. K., Basu, P.; Ray, D.; Pal, S.; Chakravorty, A. Inorg.
Chem. 1990, 29, 2423.
(18) Baldwin, A.; Lever, A. B. P.; Parish, R. V. Inorg. Chem. 1969, 8,
107.
(21) Kahn, O. Molecular Magnetism; Wiley-VCH: New York, 1993, p
104.

Chemistry of Metal-Bound Anion Radicals
Inorganic Chemistry, Vol. 39, No. 19, 2000 4337
Table 5. Crystallographic Data for 4+PF6-‚CH2Cl2, 5+PF6-‚H2O and 6+PF6
4+PF6-‚CH2Cl2
-
5+PF6-‚H2O
6+PF6
-
empirical formula
fw
C40H33Cl2F6N8OP2Ru
989.7
C47H41F6N4O2P3Ru
1001.8
C48H39ClF6N3OP3Ru
1016.6
space group
a, Å
b, Å
P21/c (No. 14)
14.174(6)
16.451(4)
18.381(4)
98.00(3)
4244(2)
P21/n (No. 14)
9.433(2)
38.914(17)
13.084(3)
103.47(2)
4670(2)
P21/n (No. 14)
10.496(5)
22.389(8)
19.720(6)
90.53(3)
4634(3)
c, Å
â, deg
V, Å3
Z
4
4
4
temp, K
λ, Å
295
0.7107
1.551
0.641
295
0.7107
1.427
0.505
295
0.7107
1.461
0.564
F
calc, g cm-3
µ, cm-1
R1,a wR2b [I>2σ(I)]
0.0578, 0.1461
0.0828, 0.2049
0.0471, 0.1170
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) [∑w(Fo - Fc2)2/∑w(Fo )2]1/2
.
2
2
dryness while still warm. The resulting solid was quickly
extracted several times with warm heptane. The heptane extract
was reduced to a volume of 10 mL under reduced pressure.
The solid, which precipitated from the concentrated solution
on standing, was filtered, washed with dry acetonitrile, and dried
immediately. Yield: 14 mg (20%). Data for 5 are as follows.
Anal. Calcd for C47H39N4OP2Ru: C, 67.30; H, 4.65; N, 6.68.
Found: C, 67.14; H, 4.73; N, 6.48. UV-vis (C6H6) [λmax, nm
(ꢀ, M-1 cm-1)]: 577 (5350), 540 (5550).
[RuII(Clpap)(H)(CO)(PPh3)2]PF6, 6+PF6-. The complex
was prepared in a manner similar to that of 5+PF6-. Chromato-
graphic workup yielded a light-green-colored eluate with a 1:2
acetonitrile-toluene mixture which on evaporation under
reduced pressure gave a green-colored crystalline solid. Yield:
34 mg (58%). Data for 6+PF6- are as follows. Anal. Calcd for
C48H39ClF6N3OP3Ru: C, 56.67; H, 3.84; N, 4.13. Found: C,
56.53; H, 3.71; N, 4.06. UV-vis (CH2Cl2) [λmax, nm (ꢀ, M-1
cm-1)]: 600 (1870), 450 (sh, 4500), 400 (18 300). IR (KBr,
[RuII(Clpap•-)(H)(CO)(PPh3)2], 6. The synthesis of this
complex is similar to that of 5, Clpap replacing abp. The reactant
stoichiometry was Clpap (32 mg, 0.15 mmol) and 3 (75 mg,
0.08 mmol). Yield: 19 mg (27%). Data for 6 are as follows.
Anal. Calcd for C48H39ClN3OP2Ru: C, 66.08; H, 4.47; N, 4.82.
Found: C, 65.87; H, 4.39; N, 4.77. UV-vis (C6H6) [λmax, nm
(ꢀ, M-1 cm-1)]: 600 (3170), 560 (3190).
cm-1): 1955 cm-1 (νCO). H NMR (CDCl3, 298 K, ppm from
1
TMS): δ 8.81 (d, J ) 5.7, 1H), 8.34 (d, J ) 7.9, 1H), 8.08 (t,
J ) 7.6, 1H), 7.82 (d, J ) 8.9, 2H), 7.43 (t, J ) 6.5, 1H), 6.91
(d, J ) 9.1, 2H), -9.71 (t, J ) 18.6, 1H).
Molecular Orbital Calculation. Extended Hu¨ckel calcula-
tions were performed using the ICON software package
developed by Hoffmann and others.22-25 The calculations were
performed using experimental atomic coordinates transformed
to rectangular axes using the program SHELXTL (Version
5.03).26 The PPh3 group in the molecule was simplified to PH3
via replacement of Ph by H, the P-H bond lengths were
adjusted to 1.42 Å, and the angles around P were left unchanged.
In a previous calculation7 on [Os(pap•-)(Br)(CO)(PPh3)2] (pap
) 2-(phenylazo)pyridine), the pendant pap Ph as well as the
phosphine Ph were replaced by H and the geometry of the
coordination sphere was also adjusted for higher symmetry;
experimental coordinates were not used. This yielded a HOMO
with 70% azo character. It is more realistic to use experimental
coordinates as has been done in the present work. On the basis
of the experimental coordinates,5 the EHMO results11 on
[Ru(pap•-)(Cl)(CO)(PH3)2] are as follows. The half-filled
HOMO is 52% azo, 7% pyridyl, 13% phenyl, and 15% metal-
d, and the LUMO is 90% pyridyl and 3% azo. Use of the
approximations as in the osmium compound7 pushes the azo
contribution to a much higher value.
[RuII(abp)(abp•-)(CO)(PPh3)]PF6, 4+PF6-. In this and
subsequent syntheses the solvents were not dry. To a solution
of [RuII(abp•-)2(CO)(PPh3)] (50 mg, 0.07 mmol) in dichlo-
romethane (10 mL) was added an acetonitrile (10 mL) solution
of NH4PF6 (20 mg, 0.12 mmol). The mixture was magnetically
stirred in air. The dark green color of the solution immediately
turned brown-red. Stirring was continued for 0.5 h. The excess
solvent was removed under reduced pressure, and the resultant
solid was washed with water repeatedly and dried in vacuo.
Subsequent chromatographic workup of the dichloromethane
solution of the solid on a silica gel column with a 1:4 aceto-
nitrile-toluene mixture as the eluant afforded the brown-colored
complex. The solid was recrystallized from a dichloromethane-
hexane mixture yielding 4+PF6-‚CH2Cl2, which was dried in
vacuo, leading to loss of the solvent of crystallization. Yield:
39 mg (65%). Data for 4+PF6- are as follows. Anal. Calcd for
C39H31F6N8OP2Ru: C, 51.76; H, 3.43; N, 12.39. Found: C,
51.68; H, 3.39; N, 12.31. UV-vis (CH2Cl2) [λmax, nm (ꢀ, M-1
cm-1)]: 556 (sh, 2660), 489 (3100), 349 (10 880).
X-ray Structure Determination. Single crystals (dimension
of sides 0.2-0.4 mm) of complexes were grown by slow
diffusion of hexane into dichloromethane solutions of the
complexes. Cell parameters were determined by the least-squares
fit of 30 machine-centered reflections (2θ ) 14-30°). Data were
collected by the ω-scan technique in the range 3° e 2θ e 48°
[RuII(abp)(H)(CO)(PPh3)2]PF6, 5+PF6-. The dark-colored
complex was prepared in the same manner as 4+PF6-. Chro-
matographic workup yielded a violet-colored eluate with a 1:2
acetonitrile-toluene mixture. On evaporation under reduced
pressure, a dark-violet-colored crystalline solid was obtained.
-
Yield: 30 mg (52%). Data for 5+PF6 are as follows. Anal.
(22) Hoffmann, R. J. Chem. Phys. 1963, 39, 1397.
(23) Ammeter, J. H.; Bu¨rgi, H.-B.; Thibeault, J. C.; Hoffmann, R. J. Am.
Chem. Soc. 1978, 100, 3686.
(24) Maelli, C.; Proserpio, D. M. CACAO, Version 4.0, Firenze, Italy, July
1994.
(25) Maelli, C.; Proserpio, D. M. J. Chem. Educ. 1990, 67, 399.
(26) Sheldrick, G. M. SHELXTL, Version 5.03, Structure Determination
Software Programs, Siemens Analytical X-ray Instruments Inc.,
Madison, WI, 1994.
Calcd for C47H39F6N4OP3Ru: C, 57.37; H, 3.97; N, 5.69.
Found: C, 57.20; H, 3.87; N, 5.61. UV-vis (CH2Cl2) [λmax
,
nm (ꢀ, M-1 cm-1)]: 555 (3300), 354 (9970). IR (KBr, cm-1):
1
1960 cm-1 (νCO). H NMR (CDCl3, 298 K, ppm from TMS):
δ 8.84 (d, J ) 4.5, 1H), 8.37 (d, J ) 7.9, 1H), 8.01 (t, J ) 7.7,
1H), 7.85 (d, J ) 7.7, 2H), 7.74 (t, J ) 8.1, 1H), -8.79 (t, J )
22.2, 1H).

4338 Inorganic Chemistry, Vol. 39, No. 19, 2000
Shivakumar et al.
for 4+PF6-‚CH2Cl2, 3° e 2θ e 45° for 5+PF6-‚H2O (relatively
poorly diffracting), and 3° e 2θ e 47° for 6+PF6- on a Siemens
R3m/V four-circle diffractometer with graphite-monochromated
Mo KR (λ ) 0.710 73 Å) radiation at 25 °C. Two check
reflections after each 198 reflections showed no intensity
reduction for any of the crystals. All data were corrected for
Lorentz-polarization effects and absorption.27 Totals of 7940
(4+PF6-‚CH2Cl2), 6754 (5+PF6-‚H2O), and 7686 (6+PF6-) were
collected, 6689, 6084, and 6877, respectively, of which were
unique; of these 5192, 4249, and 5408 were taken as observed
(I > 2σ(I)).
atoms in 6+PF6 were directly located from X-ray data. For
4+PF6-‚CH2Cl2 all the hydrogen atoms were included at
calculated positions. The N atoms of the pendant pyridyl ring
of the chelated abp ligands in 4+PF6-‚CH2Cl2 and 5+PF6-‚H2O
were identified by the bond lengths, the average C-N and C-C
distances being 1.33 and 1.37 Å, respectively. Significant crystal
data are listed in Table 5.
-
Acknowledgment. Financial support received from the
Indian National Science Academy, the Department of Science
and Technology, and the Council of Scientific and Industrial
Research, New Delhi, is acknowledged. Thanks are due to
Professor A. R. Chakravarty and Mr. Sibaprasad Bhattacharyya
for help in measurements and computations. Affiliation with
the Jawaharlal Nehru Centre for Advanced Scientific Research,
Bangalore, is acknowledged.
All calculations for data reduction and structure solution and
refinement were done using the programs of SHELXTL, Version
5.03.26 The structures were solved by full-matrix least-squares
on F2, making non-hydrogen atoms anisotropic. The hydridic
H atoms in 5+PF6-‚H2O and 6+PF6 were fixed at a distance
-
of 1.70 Å28-31 from the metal center. In 5+PF6-‚H2O, 28 of
Supporting Information Available: X-ray crystallographic files
in CIF format, for the structure determinations of 4+PF6-‚CH2Cl2,
5+PF6-‚H2O, and 6+PF6-. This material is available free of charge via
the Internet at http://pubs.acs.org.
the total 39 hydrogen atoms could be located. All the hydrogen
(27) North, A. C. T.; Philips, D. C.; Mathews, F. S. Acta Crystallogr. 1968,
24A, 351.
(28) Pramanik, A.; Bag, N.; Chakravorty, A. J. Chem. Soc., Dalton Trans.
1992, 97.
(29) Skapski, A. C.; Stephens, F. A. J. Chem. Soc., Dalton Trans. 1974,
390.
IC000356B
(30) McConway, A. C.; Skapski, A. C.; Phillips, L.; Young, R. J.;
Wilkinson, G. J. Chem. Soc., Chem. Commun. 1974, 327.
(31) Ashworth, T. V.; Nolte, M. J.; Singleton, E.; Laing, M. J. Chem. Soc.,
Dalton Trans. 1977, 1816.