Azo anion radical complexes of osmium and related nonradical species

Article abstract of DOI:10.1021/ic9908322

The reaction of [Os(H)(Br)(CO)(PPh₃)₃], 5, with 2-(phenylazo)pyridine (pap) in boiling dry heptane has afforded the azo anion radical complex [Os(pap·^-)(Br)(CO)(PPh₃)₂], 6a, as the major product and [Os(pap)(H)(CO)(PPh₃)₂]-Br, 7, as a minor byproduct. Upon replacing pap by the better π-acceptor azo-2,2'-bipyridine (abp) in the above synthesis, the radical complex [Os(abp·^-)(Br)(CO)(PPh₃)₂], 6b, becomes the sole product. It is proposed that 6 is formed via homolytic cleavage of the Os-H bond in 5; in the formation of 7, the Os - Br bond of 5 is heterolytically cleaved. The X-ray structures of 6b and 7·CH₂Cl₂ have been determined. In 6b, the N-N length is 1.35(2) A, consistent with the anion radical description; in 7·CH₂Cl₂ the length is 1.27(1) A. The spin-bearing extended Huckel HOMO in a model of 6 is found to be ~70% azo-π* in character associated with a small metal contribution. An electronic band observed in the range 600-700 nm in solutions of 6 is assigned to the HOMO → LUMO transition, the LUMO being 95% pyridine-π* in character. One-electron paramagnetic 6 displays well-defined anisotropic EPR features near g = 2.00. The anisotropy arises from the metal character of HOMO and is magnified by the large spin-orbit coupling in osmium. In a moisture-free environment 6 is indefinitely stable in the solid state, but in CH₂Cl₂-MeCN solution 6a is rapidly oxidized by air, affording [Os(pap)(Br)(CO)(PPh₃)₂]⁺, 6a⁺, which has been isolated as the diamagnetic PF₆^- salt; 6b⁺PF^6- has been similarly prepared. The voltammetric reduction potentials of the 6⁺/6 couple follow the order 6a⁺/6a < 6b⁺/6b, and the carbon monoxide stretching frequencies follow the order 6a < 6b and 6a⁺ < 6b⁺. These trends are consistent with the π-acidity order pap < abp. Crystal data are as follows: (6b, C₄₇H₃₈BrN₄OOsP₂) monoclinic, space group P2₁/c (no. 14), a = 10.215-(4) A, b = 17.634(7) A, c = 22.473(8) A, β = 97.67(3)°, Z = 4; (7·CH₂Cl₂, CH₄₉H₄₂BrCl₂N₃OOsP₂) monoclinic, space group P2₁/n (no. 14), a = 15.323(7) A, b = 15.201(6) A, c = 19.542(7) A, β = 92.51(3)°, Z = 4.

Full text of DOI:10.1021/ic9908322

Inorg. Chem. 2000, 39, 195-199
195
Azo Anion Radical Complexes of Osmium and Related Nonradical Species
Kausikisankar Pramanik, Maya Shivakumar, Prasanta Ghosh, and Animesh Chakravorty*
Department of Inorganic Chemistry, Indian Association for the Cultivation of Science,
Calcutta 700 032, India
ReceiVed July 14, 1999
The reaction of [Os(H)(Br)(CO)(PPh3)3], 5, with 2-(phenylazo)pyridine (pap) in boiling dry heptane has afforded
the azo anion radical complex [Os(pap•-)(Br)(CO)(PPh3)2], 6a, as the major product and [Os(pap)(H)(CO)(PPh3)2]-
Br, 7, as a minor byproduct. Upon replacing pap by the better π-acceptor azo-2,2′-bipyridine (abp) in the above
synthesis, the radical complex [Os(abp•-)(Br)(CO)(PPh3)2], 6b, becomes the sole product. It is proposed that 6 is
formed via homolytic cleavage of the Os-H bond in 5; in the formation of 7, the Os-Br bond of 5 is heterolytically
cleaved. The X-ray structures of 6b and 7‚CH2Cl2 have been determined. In 6b, the N-N length is 1.35(2) Å,
consistent with the anion radical description; in 7‚CH2Cl2 the length is 1.27(1) Å. The spin-bearing extended
Hu¨ckel HOMO in a model of 6 is found to be ∼70% azo-π* in character associated with a small metal contribution.
An electronic band observed in the range 600-700 nm in solutions of 6 is assigned to the HOMO f LUMO
transition, the LUMO being 95% pyridine-π* in character. One-electron paramagnetic 6 displays well-defined
anisotropic EPR features near g ) 2.00. The anisotropy arises from the metal character of HOMO and is mag-
nified by the large spin-orbit coupling in osmium. In a moisture-free environment 6 is indefinitely stable in
the solid state, but in CH2Cl2-MeCN solution 6a is rapidly oxidized by air, affording [Os(pap)(Br)(CO)(PPh3)2]+,
6a+, which has been isolated as the diamagnetic PF6- salt; 6b+PF6- has been similarly prepared. The voltammetric
reduction potentials of the 6+/6 couple follow the order 6a+/6a < 6b+/6b, and the carbon monoxide stretching
frequencies follow the order 6a < 6b and 6a+ < 6b+. These trends are consistent with the π-acidity order pap
< abp. Crystal data are as follows: (6b, C47H38BrN4OOsP2) monoclinic, space group P21/c (no. 14), a ) 10.215-
(4) Å, b ) 17.634(7) Å, c ) 22.473(8) Å, â ) 97.67(3)°, Z ) 4; (7‚CH2Cl2, C49H42BrCl2N3OOsP2) monoclinic,
space group P21/n (no. 14), a ) 15.323(7) Å, b ) 15.201(6) Å, c ) 19.542(7) Å, â ) 92.51(3)°, Z ) 4.
Introduction
orbital findings are found to converge to a predominant azo
anion radical description of the species. Nonradical congeners
formed via one-electron electrochemical/chemical oxidation
have been isolated and characterized. The structure and proper-
ties of a hydrido byproduct that throws light on the nature of
the radical-forming reaction are scrutinized.
The generally facile reduction of azo compounds in free1,2
as well as coordinated2-4 states is ensured by the low-lying
nature of the azo-π* orbital. One-electron reduction, eq 1,
...
-NdN- + e f -N N-
(1)
Results and Discussion
generates an azo anion radical. Though observable in solution,1-4
such radicals have generally eluded isolation in pure form. The
situation has just begun to change with the characterization of
a few crystalline ruthenium5,6 and copper7 chelates.
In the present work we explore the case of osmium using
azopyridines as reducible ligands. A synthetic strategy has been
developed and successfully applied for isolation of the title
complexes. Bond parametric, spectroscopic, and molecular
Synthetic Studies. The two azo ligands (general abbreviation
L) used in the present work are 2-(phenylazo)pyridine (pap)
and azo-2,2′-bipyridine (abp). The binding of pap and its
substituted derivatives to bivalent osmium in the form of the
Os(L) chelate ring, 3, is well-documented.8-10 No Os(abp)
complexes are however known. The Os(pap) type species are
invariably electroactive9 in solution and undergo one-electron
reduction, presumably forming the anion radical entity Os(L•-),
represented by the approximate valence bond structure 4.
However, all attempts to isolate such reduced species have
failed.
(1) (a) Sadler, J. L.; Bard, A. J. J. Am. Chem. Soc. 1968, 90, 1979. (b)
Jonson, C. J.; Chang, R. J. Chem. Phys. 1965, 43, 3183.
(2) Goswami, S.; Mukherjee, R.; Chakravorty, A. Inorg. Chem. 1983, 22,
2825.
(3) (a) Pal, C. K.; Chattopadhyay, S.; Sinha, C. R.; Chakravorty, A. Inorg.
Chem. 1994, 33, 6140. (b) Pal, C. K.; Chattopadhyay, S.; Sinha, C.
R.; Chakravorty, A. Inorg. Chem. 1996, 35, 2442.
This has prompted us to search for a qualitatively different
synthetic strategy that can bypass preisolation of Os(L) species.
(4) Krause, R. A.; Krause, K. Inorg. Chem. 1984, 23, 2195. (b) Kreje`´ık,
M.; Za´lisˇ, S.; Kl´ıma, J.; Sy´kora, D.; Matheis, W.; Klein, A.; Kaim,
W. Inorg. Chem. 1993, 32, 3362.
(5) Shivakumar, M.; Pramanik, K.; Ghosh, P.; Chakravorty, A. Inorg.
Chem. 1998, 37, 5968.
(6) Shivakumar, M.; Pramanik, K.; Ghosh, P.; Chakravorty, A. J. Chem.
Soc., Chem. Commun. 1998, 2103.
(7) (a) Schwach, M.; Hausen, H.-D.; Kaim, W. Inorg. Chem. 1999, 38,
2242. (b) Doslik, N.; Sixt, T.; Kaim, W. Angew. Chem. 1998, 110,
2125; Angew. Chem., Int. Ed. Engl. 1998, 37, 2403.
(8) Ghosh, B. K.; Goswami, S.; Chakravorty, A. Inorg. Chem. 1983, 22,
3358.
(9) Ghosh, B. K.; Mukhopadhyay, A.; Goswami, S.; Chakravorty, A.
Inorg. Chem. 1984, 23, 4633.
(10) (a) Mallick, T. K.; Roy, B. K.; Das, P. K.; Ghosh, B. K. Transition
Met. Chem. 1993, 18, 609. (b) Mallick, T. K.; Roy, B. K.; Das, P. K.;
Ghosh, B. K. Indian J. Chem. 1993, 32A, 580. (c) Mallick, T. K.;
Roy, B. K.; Das, P. K.; Ghosh, B. K. Polyhedron 1994, 13, 1817. (d)
Sinha, S.; Banerjee, A. K.; Ghosh, B. K. Transition Met. Chem. 1992,
17, 22.
10.1021/ic9908322 CCC: $19.00 © 2000 American Chemical Society
Published on Web 12/28/1999

196 Inorganic Chemistry, Vol. 39, No. 2, 2000
Pramanik et al.
We looked for a starting material in which the obligatory
reducing equivalent as well as certain radical-stabilizing features
are already built-in. It was hoped that such a material would
react with L, forming Os(L•-) directly. The experience with
radical complexes of ruthenium5,6 suggested the use of hydridic
starting materials. The hydride 511 was indeed found to ideally
suit our design strategy.
Figure 1. Molecular structure of [Os(abp•-)(Br)(CO)(PPh3)2] (6b)
showing 30% probability ellipsoids.
replacing pap by abp in the above synthetic procedure. Here
the abp•- anion radical complex, 6b, is the sole product (π-
acidity: abp > pap). Facile homolytic M-H cleavage also
favors radical formation. Thus, the reaction of hydridic ruthe-
nium starting materials with L furnishes only Ru(L•-) species5,6
(ease of M-H cleavage: Ru > Os).
In a moisture-free environment the type 6 complexes are
indefinitely (more than two years) stable in the solid state.
However, upon stirring CH2Cl2-MeCN solutions containing 6
and NH4PF6 in air, rapid aerial oxidation to 6+ occurs. The latter
[Os(H)(Br)(CO) (PPh3)3]
5
The reaction of 5 with excess pap (for abp, see below) in
dry boiling heptane under nitrogen afforded the desired radical
complex, green-colored 6a, as the major product. Interestingly
the similarly constituted but nonradical hydrido complex 7 was
obtained as a minor (∼10%) byproduct. The choice of a dry
hydrocarbon (here heptane) as the reaction solvent is crucial
for the synthesis of the radical product. In polar solvents there
is a strong tendency to form nonradical products.
The contrast between the reactions leading to 6a and 7 is
highlighted in eq 2 where only the relevant ligands of 5, 6a,
and 7 are shown. It is logical to assume that in both cases the
-
has been isolated as dark-colored 6+PF6 salts. The detailed
structure of 6+ could not be determined due to a lack of single
crystals. The structures of ruthenium analogues are, however,
known.5,12
Physical data of the complexes synthesized in this work are
partly set out in the tables, and the remaining information is
incorporated in the Experimental Section.
Structure. The X-ray structure of 6b has been determined.
A molecular view is shown in Figure 1, and selected bond
parameters are listed in Table 1. The coordinated azo and
pyridine nitrogen atoms of the chelated abp•- ligand lie trans
to Br- and CO, respectively, in the distorted octahedral
coordination sphere. The chelate ring is coplanar with the
coordinated pyridine ring, and the pendant pyridyl ring makes
a dihedral angle of 20.0° with it.
pap ligand first attaches itself to the metal site of 5 by displacing
a labile PPh3 ligand. In the route to 6a (eq 2a), homolytic
cleavage of the Os-H bond occurs. This provides the electron
required for in situ pap•- formation, and the vacated site is
utilized to complete pap•- chelation. In eq 2b the Os-Br bond
is cleaved heterolytically, and chelation by pap is completed,
affording 7. The relative yields of the two products show that
the path of eq 2a is much more facile than that of eq 2b.
An increase in the π-acidity of the azo ligand should favor
the route of eq 2a even further. This indeed happens upon
In uncoordinated azo compounds and hydrazines the N-N
distances are 1.25 Å13 and 1.45 Å,14 respectively. In 6b, the
(11) Ahmad, N.; Levason, J. J.; Robinson, S. D.; Utterley, M. F. Inorg.
Synth. 1975, 15, 45.
(12) The X-ray structure of [Ru(abp)Cl(CO)(PPh3)2]PF6 has been deter-
mined, and the details will be reported elsewhere.

Azo Anion Radical Complexes of Osmium
Inorganic Chemistry, Vol. 39, No. 2, 2000 197
Table 1. Selected Bond Distances (Å) and Angles (deg) and Their Estimated Standard Deviations for 6b and 7‚CH2Cl2
6b
7‚CH2Cl2
6b
7‚CH2Cl2
Os-P1
Os-P2
Os-Br
Os-N1
2.395(5)
2.389(5)
2.584(2)
2.172(16)
2.382(3)
2.377(3)
Os-N3
2.044(15)
1.914(22)
1.348(22)
2.095(10)
1.842(13)
1.274(13)
Os-C47/C48a
N2-N3
2.118(10)
P1-Os-P2
176.5(2)
176.0(7)
167.7(5)
75.5(7)
92.4(5)
100.5(7)
91.6(5)
87.9(4)
163.1(1)
174.5(5)
P1-Os-N3
89.8(5)
91.9(1)
91.6(6)
88.6(4)
89.2(5)
88.4(1)
91.9(6)
97.7(3)
N1-Os-C47/C48a
N3-Os-Br
P1-Os-Br
P1-Os-C47/C48a
P2-Os-N1
89.2(4)
93.2(3)
99.2(3)
N1-Os-N3
73.8(4)
100.7(5)
91.3(3)
N1-Os-Br
P2-Os-N3
N3-Os-C47/C48a
Br-Os-C47/C48a
P1-Os-N1
P2-Os-Br
P2-Os-C47/C48a
87.9(4)
a C47 in 6b and C48 in 7‚CH2Cl2.
The hydride ligand is not observable in difference Fourier
maps, although its presence is indirectly reflected in the other
structural features noted above such as pronounced P(1)OsP(2)
nonlinearity and Os-N(azo) lengthening. In 1H NMR of 7 the
hydride resonance occurs at -9.13 ppm as a well-resolved triplet
corresponding to coupling with two equivalent 31P nuclei (J )
22.5 Hz).
Frontier Orbitals, Magnetism, and Spectra. To gain insight
into the approximate composition of the frontier orbitals in type
6 complexes, extended Hu¨ckel calculations were performed on
a simplified model (Ph/C5H4N groups replaced by H). The half-
filled HOMO, depicted in 8, is indeed primarily azo-π* in
character (70%) but with significant pyridyl-π* (15%) and
metal-dyz (10%) contributions. The ligands not shown in 8 have
little or no contributions. The LUMO, 9, lying ∼2 eV above 8
Figure 2. Molecular structure of the cation [Os(pap)(H)(CO)(PPh3)2]+
in the crystal of 7‚CH2Cl2 showing 30% probability ellipsoids.
distance is of intermediate value, 1.35(2) Å. This is qualitatively
consistent with an anion radical description of the azo ligand
...
in the complex. In going from -NdN- (σ2π2) to -N N-
(σ2π2π*1), the bond order decreases from 2 to 1.5; in hydrazines
the bond order is 1. We note that in the reported ruthenium
anion radical species the average N-N length is also 1.35 Å.5,6
The observed Os-P, Os-Br, and Os-CO lengths are more or
less unexceptional.15-17
The structure of the solvate 7‚CH2Cl2 has also been deter-
mined (Figure 2, Table 1). The gross coordination geometry is
the same as in 6b but with replacement of Br- by H- and abp•-
by pap. Here, unlike in 6b, the P(1)OsP(2) axis deviates
considerably (∼17°) from linearity. The pendant phenyl ring
makes a dihedral angle of 35.7° with the plane of the chelate
ring. The Os-N(azo) length is longer in 7‚CH2Cl2 than in 6b,
presumably due to the strong trans influence of the hydride
ligand.
is a nearly pure (95%) pyridyl-π* orbital. The metal-t2 orbitals
have some Br- character and lie below 8. In going from 6 to
6+, the orbital 8 becomes the LUMO and the HOMO is
primarily metal-t2 in nature.
(13) (a) Mostad, A.; Rømming, C. Acta Chim. Scand. 1971, 25, 3361. (b)
Chang, C. H.; Porter, R. F.; Brauer, S. H. J. Am. Chem. Soc. 1970,
92, 5313. (c) Roy, T.; Sengupta, S. P. Cryst. Struct. Commun. 1980,
9, 965.
The room-temperature magnetic moments of the polycrys-
talline complexes (6a, 1.96 µB; 6b, 1.94 µB) correspond to the
presence of one unpaired electron (S ) 1/2). The same samples
display well-defined EPR features near g ) 2 (6a, 1.998, 1.966;
6b, 2.001, 1.971). The spectra are approximately axial, with
the perpendicular resonance showing some signs of rhombic
splitting (Figure 3). The small anisotropy, unresolved in the
analogous ruthenium species,5,6 is consistent with the presence
(14) Morino, Y. Iijima, T.; Murata, Y. Bull. Chem. Soc. Jpn. 1960, 33, 46.
(15) (a) Ghosh, P.; Bag, N.; Chakravorty, A. Organometallics 1996, 15,
3042. (b) Pramanik, K.; Ghosh, P.; Chakravorty, A. J. Chem. Soc.,
Dalton Trans. 1997, 3553.
(16) (a) Fergusson, J. E.; Robinson, W. T.; Coll, R. K. Inorg. Chim. Acta
1991, 181, 79. (b) Robinson, P. D.; Ali, I. A.; Hinckley, C. C. Acta
Crystallogr. 1991, C47, 1397.
(17) (a) Bottomley, F.; Ivan, J. B.; Peter, L.; White, S. J. Chem. Soc., Dalton
Trans. 1978, 1726. (b) Clark, G. R.; Collins, T. J.; Marsden, K.; Roper,
W. R. J. Organomet. Chem. 1978, 157, C23.

198 Inorganic Chemistry, Vol. 39, No. 2, 2000
Pramanik et al.
Concluding Remarks
The first azo anion radical complexes of osmium have been
isolated and characterized in the form of type 6 species.
Augmented back-bonding to carbon monoxide and possibly
phosphine helps in radical stabilization. The small metal
character in the spin-bearing azo-π* orbital is expressed in EPR
anisotropy. The π-acidity order abp > pap is reflected in
reduction potential and carbonyl stretching frequency data.
This work taken collectively with those on ruthenium azo
anion radical species5,6 has convincingly demonstrated the value
of homolytic M-H cleavage as a rational route for accessing
azo anion radical species. We also have here a unique op-
portunity to observe and contrast two parallel routes: homolytic
cleavage as above, leading to 6a, and heterolytic Os-Br cleav-
age, affording 7. When azo π-acidity is augmented via replace-
ment of pap by abp, the heterolytic route is no longer observable.
The starting materials used so far (5 and others5,6) have a
single hydride ligand. Materials with two hydride ligands are
under scrutiny; the purpose is to generate and isolate bis(azo
anion radical) and (azo)(azo anion radical) complexes. Prelimi-
nary results are encouraging.
Figure 3. Powder EPR spectrum of 6b in the X-band (9.1 GHz) at
298 K. Instrument settings: power, 30 dB; modulation, 100 kHz; sweep
center, 3300 G; sweep width, 500 G; sweep time, 240 s.
of the small metal contribution to the spin-bearing HOMO, 8.
The amplification of the anisotropy in the osmium complexes
is largely a spin-orbit phenomenon (coupling constant: Os .
Ru) which has been observed in other systems, e.g., in radical
complexes of the nickel triad.18 The lack of nitrogen hyperfine
structure in the spectra of 6 is not unusual since the 14N coupling
constant in azo-π* radicals is small and difficult to observe.1b,3
Experimental Section
General Information. All reactions involving radical syntheses were
carried out under an atmosphere of pure and dry nitrogen. The pap
and abp ligands were synthesized according to previously published
procedures.19 The starting material, 5, was prepared by the reported
procedure.11 The chemicals 2-aminopyridine (Aldrich), OsO4 (Arora-
Mathey, India), and NH4PF6 (Aldrich) were used as received. Heptane
used in the preparation was dried by distillation over Na/benzophenone.
The purification of dichloromethane and acetonitrile and the preparation
of tetraethylammonium perchlorate (TEAP) for electrochemical work
were done as described before.20
1
The 6+ species are diamagnetic and display well-resolved H
NMR spectra.
The electronic spectral band of 6 in the 600-700 nm region
is assigned to the HOMO (8) f LUMO (9) transition. It is
essentially an intraligand excitation of the anion radical electron
from the azo to the pyridyl function. The band near 500 nm in
6+ is assigned to HOMO (t2) f LUMO (8) charge transfer.
Physical Measurements. Electronic and IR spectra were recorded
with Hitachi 330 and Perkin-Elmer 783 IR spectrophotometers,
respectively. For 1H NMR spectra, a Bruker 300 MHz FT NMR
spectrophotometer was used (tetramethylsilane is the internal standard).
Magnetic properties were examined using a PAR-155 vibrating sample
magnetometer fitted with a Walker Scientific magnet. EPR spectra were
recorded on a Varian E-109C spectrometer. Microanalyses (C, H, N)
were done by using a Perkin-Elmer 240C elemental analyzer. Elec-
trochemical measurements were performed under nitrogen atmosphere
using a PAR 370-4 electrochemistry system as before:20b working
electrode, SCE; auxiliary electrode, platinum wire; supporting electro-
lyte, Et4NClO4 (0.1 M); scan rate, 50 mV s-1; solution concentration,
∼10-3 M. All potentials reported in this work are uncorrected for
junction contribution.
Trends of Reduction Potential and CO Stretch. In CH2-
Cl2 solution, 6 (initial scan anodic; peak-to-peak separation, ∼
130 mV) displays a quasireversible one-electron cyclic volta-
mmetric response near -0.35 V vs SCE (Table 2) due to the
couple of eq 3. Reduction potential data are collected in Table
6+ + e H 6
(3)
2. Coulometry of 6 at 0.0 V is attended with the transfer of one
electron, quantitatively affording 6+ in solution. The parent
radical 6 is fully regenerated upon coulometry of the oxidized
solution at -0.6 V.
[Os(pap•-)(Br)(CO)(PPh3)2], 6a. To an excess of pap (65 mg, 0.35
mmol) dissolved in dry heptane (30 mL) was added [Os(H)(Br)(CO)-
(PPh3)3] (100 mg, 0.09 mmol), and the mixture was heated to reflux
for 2 h. The initial orange color gradually changed to dark green. The
reaction mixture was then cooled to room temperature and filtered.
The residue thus collected was a mixture of green and red components.
The green component alone was soluble in benzene and was recovered
by evaporating the benzene solution under reduced pressure. It was
successively washed with dry acetonitrile and hexane and finally dried
in vacuo. Yield 55%. Data for 6a are as follows. Anal. Calcd (found)
for C48H39BrN3OOsP2: C, 57.31 (57.16); H, 3.92 (3.98); N, 4.18 (4.04).
It was difficult to study the voltammetry of 6 in MeCN due
to high oxygen sensitivity. However, this could be done in the
case of 6+ (initial scan cathodic) which afforded a nearly
reversible (peak-to-peak separation, 60-70 mV) voltammogram
representing the couple of eq 3. The E1/2 values (Table 2) lie
close to those determined using 6 as the electroactive substance.
The E1/2 of the 6a+/6a couple is ∼50 mV lower than that of
the 6b+/6b couple. This is consistent with the π-acidity gradation
pap < abp. There is a qualitative correlation between the E1/2
values and CO stretching frequencies (Table 2) which follow
the order 6a < 6b and 6a+ < 6b+. Superior π-acidity of abp
diminishes the extent of back-bonding to the CO ligand. For a
given azo ligand the νCO values follow the order 6 < 6+,
representing a decline of the extent of the net back-bonding to
coordinated CO in going from the electron-rich anion radical
to the nonradical species. It is believed that back-bonding to
CO (and possibly to PPh3 ligands) plays a significant role in
stabilizing the azo anion radical complexes.
(18) (a) Goodman, B. A.; Raynor, J. B. AdV. Inorg. Chem. Radiochem.
1970, 13, 135. (b) Balch, A. L.; Holm, R. H. J. Am. Chem. Soc. 1966,
88, 5201.
(19) (a) Goswami, S.; Chakravarty, A. R.; Chakravorty, A. Inorg. Chem.
1981, 20, 2246. (b) Baldwin, A.; Lever, A. B. P.; Parish, R. V. Inorg.
Chem. 1969, 8, 107.
(20) (a) Lahiri, G. K.; Bhattacharya, S.; Ghosh, B. K.; Chakravorty, A.
Inorg. Chem. 1987, 26, 4324. (b) Chandra, S. K.; Basu, P.; Ray, D.;
Pal, S.; Chakravorty, A. Inorg. Chem. 1990, 29, 2423.

Azo Anion Radical Complexes of Osmium
Inorganic Chemistry, Vol. 39, No. 2, 2000 199
Table 2. Electrochemicala and IRb Data of 6 and 6+
compd
E1/2, V (∆Epc, mV) L/L•-
ν
CO, (cm-1
)
compd
E1/2, V (∆Epc, mV) L/L•-
ν
CO, (cm-1
)
6a
6b
-0.38 (120)
-0.33 (140)
1907
1922
6a+
6b+
-0.40 (70)
-0.34 (70)
1942
1962
a 6a, 6b in CH2Cl2 and 6a+, 6b+ in MeCN. b In a KBr disk. c ∆Ep ) Epa - Epc.
Table 3. Crystallographic Data for 6b and 7‚CH2Cl2
UV-vis (C6H6) [λmax, nm (ꢀ, M-1 cm-1)]: 610 (2900), 500 (sh, 2700),
450 (sh, 3600), 380 (12 000).
6b
7‚CH2Cl2
[Os(pap)(H)(CO)(PPh3)2]Br, 7. The red residue left after removal
of green 6a via dissolution in benzene in the above preparation is 7. It
was recrystallized from a dichloromethane-hexane mixture and then
dried in a vacuum whereby the solvent of crystallization (vide infra)
was lost. Yield 12%. Data for 7 are as follows. Anal. Calcd (found)
for C48H40BrN3OOsP2: C, 57.26 (57.11); H, 4.00 (3.86); N, 4.17 (4.21).
UV-vis (CH2Cl2) [λmax, nm (ꢀ, M-1 cm-1)]: 515 (3800), 350 (14 500).
IR (KBr, cm-1): 1922 (νCO). Pap/pap•- couple (CH3CN), E1/2, V (∆Ep,
empirical formula
fw
cryst syst
space group
a, Å
b, Å
c, Å
C47H38BrN4OOsP2 C49H42BrCl2N3OOsP2
1006.9
monoclinic
P21/c
10.215(4)
17.634(7)
22.473(8)
97.67(3)
4012(3)
4
1091.8
monoclinic
P21/n
15.323(7)
15.201(6)
19.542(7)
92.51(3)
4547(3)
4
â, deg
U, Å3
1
mV): -0.56 (65). H NMR (CDCl3, 298 K, ppm from TMS): δ 8.41
(d, J ) 8.1, 1H), 8.24 (t, J ) 7.5, 1H), 7.5 (m, 2H), 6.80 (t, J ) 6.2,
1H), -9.13 (t, J ) 22.5, 1H).
Z
T, oC
293
0.710 73
1.670
293
0.710 73
1.598
λ, Å
[Os(abp•-)(Br)(CO)(PPh3)2], 6b. The green crystalline compound
was prepared in a way similar to that used for 6a, except using abp in
place of pap. Here there was no red component, and pure 6b was
deposited on cooling the reaction mixture. It was collected by filtration
and then washed and dried as in the case of 6a. Yield 90%. Anal. Calcd
(found) for C47H38BrN4OOsP2: C, 56.07 (55.89); H, 3.80 (3.68); N,
5.56 (5.42). UV-vis (C6H6) [λmax, nm (ꢀ, M-1 cm-1)]: 680 (2600),
650 (sh, 2500), 540 (2300), 385 (13 000).
Dc, g cm-3
µ(Mo KR), mm-1
F(000)
4.303
1988
3.918
2160
R1,a wR2b [I > 2σ(I)] 0.0866, 0.2012
0.0628, 0.1419
1.056
goodness of fit on F2
1.032
a R1 ) ∑||Fo| - |Fc||/∑||Fo|. b wR2 ) [∑w(Fo2 - Fc2)2/∑w(Fo )2]1/2
.
2
measured after every 98 reflections showed no significant intensity
reduction in either case. All data were corrected for Lorentz-polarization
effects, and an empirical absorption correction was done on the basis
of azimuthal scans23 of six reflections. For 6b, which was relatively
weakly diffracting, 7446 reflections were collected, 6940 were unique,
and 3862 satisfying I > 2.0σ(I) were used for structure solution. In the
case of 7‚CH2Cl2 the corresponding numbers are 8353, 7972, and 4960,
respectively.
All calculations for data reduction, structure solution, and refinement
were done using the programs of SHELXTL, Version 5.03.24 Both
structures were solved by the Patterson heavy-atom method and were
refined by full-matrix least squares on F2, making non-hydrogen atoms
(all in 6b and all except solvent in 7‚CH2Cl2) anisotropic, the hydrogen
atoms being added at calculated positions. The hydridic H atom in 7‚
CH2Cl2 was fixed at a distance of 1.60 Å25 from the metal; the thermal
parameters of the CH2Cl2 molecule are high, representing disorder.
Significant crystal data are listed in Table 3.
[Os(pap)(Br)(CO)(PPh3)2]PF6, 6a+PF6-. To a solution of 6a (50
mg, 0.04 mmol) in dichloromethane (20 mL) was added an acetonitrile
(5 mL) solution of NH4PF6 (20 mg, 0.12 mmol), and the mixture was
magnetically stirred in air until the green color changed to dark brown.
The organic solvents were removed under reduced pressure, and the
solid was thoroughly washed with water and dried in vacuo. Subsequent
chromatographic workup of the dichloromethane solution of the solid
on a silica gel column with a 1:4 acetonitrile-benzene mixture as eluent
afforded the red-brown complex. Yield 70%.. Anal. Calcd (found) for
C48H39BrF6N3OOsP3: C, 50.09 (49.84); H, 3.42 (3.27); N, 3.65 (3.71).
UV-vis (CH2Cl2) [λmax, nm (ꢀ, M-1 cm-1)]: 575 (sh, 2000), 505
(2600), 380 (8400). 1H NMR (CDCl3, 298 K, ppm from TMS): δ 8.78
(d, J ) 8.1, 1H), 8.23 (t, J ) 7.7, 1H), 7.76 (d, J ) 5.7, 1H), 6.90 (t,
J ) 7.5, 2H), 6.78(t, J ) 6.6, 1H), 6.69 (d, J ) 8.4, 2H).
[Os(abp)(Br)(CO)(PPh3)2]PF6, 6b+PF6-. This compound was
prepared in a manner similar to that used for 6a+PF6-, except for using
6b as the starting material. Yield 80%. Anal. Calcd (found) for C47H38-
BrF6N4OOsP3: C, 49.01 (49.18); H, 3.33 (3.43); N, 4.86 (4.71). UV-
vis (CH2Cl2) [λmax, nm (ꢀ, M-1 cm-1)]: 625 (sh, 1350), 535 (2200),
450 (sh, 2900), 365 (8800). 1H NMR (CDCl3, 298 K, ppm from TMS):
δ 8.79 (d, J ) 7.8, 1H), 8.15 (t, J ) 9.0, 1H), 8.14 (d, J ) 6.0, 1H),
8.00 (d, J ) 5.4, 1H), 7.63 (d, J ) 8.1, 1H), 7.52 (t, J ) 7.5, 1H), 7.37
(t, J ) 5.9, 1H), 7.02 (t, J ) 6.6, 1H).
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. Affiliation with the
Jawaharlal Nehru Center for Advanced Scientific Research,
Bangalore, is acknowledged.
Molecular Orbital Calculation. Extended Hu¨ckel calculations and
orbital plots were performed using the ICON software package
developed by Hoffmann and others.21,22 The orthogonal coordinate
system chosen for the calculations is defined in structures 8 and 9.
The experimental bond distances and angles of 6b were used in the
calculations. The C-H, N-H, and P-H distances were taken as 0.96,
0.90, and 1.42 Å, respectively.
Supporting Information Available: For 6b and 7‚CH2Cl2, text
describing the X-ray structure determination and tables of crystal data,
complete atomic coordinates and thermal parameters, bond distances
and angles, anisotropic thermal parameters, and hydrogen atom
positional and thermal parameters. This material is available free of
charge via the Internet at http://pubs.acs.org.
Crystal Structure Determination. Dark green single crystals of
6b grew directly from the reaction medium in synthesis via careful
cooling to room temperature. Single crystals of 7‚CH2Cl2 were grown
by slow diffusion of hexane into a dichloromethane solution of 7. The
sizes of the crystals of 6b and 7‚CH2Cl2 used for crystallographic
measurement were 0.38 × 0.35 × 0.24 and 0.34 × 0.25 × 0.22 mm,
respectively. Cell parameters were determined by the least-squares fit
of 30 machine-centered reflections (2θ ) 15-30°) in each case. Data
were collected by the ω-scan technique in the range 1.83° e θ e 25.05°
for 6b and 1.70° e θ e 25.05° for 7‚CH2Cl2 on a Siemens R3m/V
four-circle diffractometer equipped with a graphite crystal monochro-
mator and Mo KR (λ ) 0.710 73 Å) radiation. Two check reflections
IC9908322
(21) (a) Hoffmann, R. J. Chem. Phys. 1963, 39, 1397. (b) Ammeter, J. H.;
Bu¨rgi, H.-B.; Thibeault, J. C.; Hoffmann, R. J. Am. Chem. Soc. 1978,
100, 3686.
(22) Maelli, C.; Proserpio, D. M. CACAO, Version 4.0, Firenze, Italy, July
1994. (b) Maelli, C.; Proserpio, D. M. J. Chem. Educ. 1990, 67, 399.
(23) North, A. C. T.; Philips, D. C.; Mathews, F. S. Acta Crystallogr. 1968,
A24, 351.
(24) Sheldrick, G. M. SHELXTL, Version 5.03; Siemens Analytical
Instruments Inc.: Madison, WI, 1994.
(25) Green, M. A.; Huffman, J. C.; Caulton, K. G. J. Organomet. Chem.
1983, C78, 243.