Rhenium chemistry of azooximes: Oxygen atom transfer, azoimine chelation, and imine-oxime contrast

Article abstract of DOI:10.1021/ic0486116

The concerned azooximes (L¹OH, 1) are of type p-X-C ₆H₄C(N₂Ph)(NOH) (X = H, Me, Cl). The reaction of [Re-(MeCN)Cl₃(PPh₃)₂] with [Ag(L ¹OH)(L¹O)] in cold dichloromethane-acetonitrile solvent has furnished the green colored ionized azoimine complex [Re^V(O) Cl(PPh₃)₂(L¹)](PF₆), 2. In effect L¹O^- has undergone oxidative addition, the oxygen atom being transferred to the metal site. Upon treatment of [Re^V(NPh) Cl₃(PPh₃)₂] with L¹OH in solution, the neutral azoimine complex [Re^V(NPh)Cl₃(L ¹H)], 3, resulted due to the spontaneous transfer of the oxime oxygen atom to a PPh₃ ligand, which is eliminated as OPPh₃. In contrast, the oxime of 2-acetylpyridine (L²OH, 4) did not undergo oxygen atom transfer and simply afforded the imine-oxime complex [Re ^V(NC₆H₄Y)Cl₂(PPh₃)(L ²O)], 5, upon reacting with [Re^V(NC₆H ₄Y)Cl₂(PPh₃)₂] (Y = H, Me, Cl). The spectral and electrochemical properties of 2, 3, and 5 and the structures of three representative compounds are reported. In the cation of 2 (X = H) the two PPh₃ ligands lie trans to each other and the equatorial plane is defined by the five-membered azoimine chelate ring and the oxo and chloro ligands. The oxo ligand which forms a model triple bond (Re-O length 1.616(6) A) lies cis to the imine-N atom. In 3 (X = Cl) the ReCl₃ fragment has meridional geometry and the imido nitrogen lies trans to the imine nitrogen of the planar azoimine chelate ring. In 5·H₂O (Y = Me), the Cl, oximato-N, and P atoms define an equatorial plane and the pyridine-N lies trans to the imido-N. The water of crystallization is hydrogen bonded to the oximato oxygen atom (O...O, 2.829(5) A). Reaction models in which chelation of the azooxime precedes oxygen atom transfer are proposed on the basis of oxophilicity of trivalent rhenium, Lewis acid activity of pentavalent rhenium, electron withdrawal by the azo group, and observed relative disposition of ligands in products.

Full text of DOI:10.1021/ic0486116

Inorg. Chem. 2005, 44, 1054−1060
Rhenium Chemistry of Azooximes: Oxygen Atom Transfer, Azoimine
Chelation, and Imine Oxime Contrast
−
Indranil Chakraborty,† Bikash Kumar Panda,† Jaydip Gangopadhyay,† and Animesh Chakravorty*,†,‡
Department of Inorganic Chemistry, Indian Association for the CultiVation of Science,
Kolkata 700 032, India, and Jawaharlal Nehru Centre for AdVanced Scientific Research,
Bangalore 560 064, India
Received October 6, 2004
The concerned azooximes (L1OH, 1) are of type p-X-C6H4C(N2Ph)(NOH) (X
(MeCN)Cl3(PPh3)2] with [Ag(L1OH)(L1O)] in cold dichloromethane
acetonitrile solvent has furnished the green colored
) H, Me, Cl). The reaction of [Re-
−
ionized azoimine complex [ReV(O)Cl(PPh3)2(L1)](PF6), 2. In effect L1O- has undergone oxidative addition, the oxygen
atom being transferred to the metal site. Upon treatment of [ReV(NPh)Cl3(PPh3)2] with L1OH in solution, the neutral
azoimine complex [ReV(NPh)Cl3(L1H)], 3, resulted due to the spontaneous transfer of the oxime oxygen atom to a
PPh3 ligand, which is eliminated as OPPh3. In contrast, the oxime of 2-acetylpyridine (L2OH, 4) did not undergo
oxygen atom transfer and simply afforded the imine
with [ReV(NC6H4Y)Cl3(PPh3)2] (Y
H, Me, Cl). The spectral and electrochemical properties of 2, 3, and 5 and the
structures of three representative compounds are reported. In the cation of 2 (X H) the two PPh3 ligands lie
trans to each other and the equatorial plane is defined by the five-membered azoimine chelate ring and the oxo
and chloro ligands. The oxo ligand which forms a model triple bond (Re O length 1.616(6) Å) lies cis to the
imine-N atom. In 3 (X Cl) the ReCl3 fragment has meridional geometry and the imido nitrogen lies trans to the
imine nitrogen of the planar azoimine chelate ring. In 5 H2O (Y Me), the Cl, oximato-N, and P atoms define an
−
oxime complex [ReV(NC6H4Y)Cl2(PPh3)(L2O)], 5, upon reacting
)
)
−
)
‚
)
equatorial plane and the pyridine-N lies trans to the imido-N. The water of crystallization is hydrogen bonded to the
oximato oxygen atom (O‚‚‚O, 2.829(5) Å). Reaction models in which chelation of the azooxime precedes oxygen
atom transfer are proposed on the basis of oxophilicity of trivalent rhenium, Lewis acid activity of pentavalent
rhenium, electron withdrawal by the azo group, and observed relative disposition of ligands in products.
Introduction
initiated search into the possible oxygen atom transfer
behavior of rhenium systems based on ligands in which one
of the donor sites is a preoxidized imine, i.e., an oxime
function.
This work stems from our interest in variable-valent
rhenium chemistry of conjugated nitrogen donor ligands
especially R-diimines and azoimines.1-4 A central concern
has been oxygen atom transfer processes either from ox-
orhenium species to oxophilic substrates,1,3b,5 eq 1, or from
solvent water to an activated imine function leading to amide
formation,1a,e,2 eq 2. As a sequel to these studies, we have
RetO + PR3 f Re-OPR3
(1)
We were encouraged in this endeavor by the fact that
rhenium-oxime coordination chemistry has so far witnessed
* Author to whom correspondence should be addressed at the Indian
Association for the Cultivation of Science. E-mail: icac@mahendra.iacs.res.in.
† Indian Association for the Cultivation of Science.
(2) (a) Bhattacharyya, S.; Banerjee, S.; Dirghangi, B. K.; Menon, M.;
Chakravorty, A. J. Chem. Soc., Dalton Trans. 1999, 155. (b) Dirghangi,
B. K.; Menon, M.; Banerjee, S.; Chakravorty, A. Inorg. Chem. 1997,
36, 3595. (c) Banerjee, S.; Dirghangi, B. K.; Menon, M.; Pramanik,
A.; Chakravorty, A. J. Chem. Soc., Dalton Trans. 1997, 2149. (d)
Dirghangi, B. K.; Banerjee, S.; Menon, M.; Chakravorty, A. Indian
J. Chem. 1997, 36A, 249. (e) Menon, M.; Choudhury, S.; Pramanik,
A.; Deb, A. K.; Chandra, S. K.; Bag, N.; Goswami, S.; Chakravorty,
A. J. Chem. Soc., Chem. Commun. 1994, 57. (f) Menon, M.; Pramanik,
A.; Bag, N.; Chakravorty, A. Inorg. Chem. 1994, 33, 403.
‡ Jawaharlal Nehru Centre for Advanced Scientific Research.
(1) (a) Das, S.; Chakraborty, I.; Chakravorty, A. Inorg. Chem. 2003, 42,
6545. (b) Gangopadhyay, J.; Sengupta, S.; Bhattacharyya, S.;
Chakraborty, I.; Chakravorty, A. Inorg. Chem. 2002, 41, 2616. (c)
Bhattacharyya, S.; Chakraborty, I.; Dirghangi, B. K.; Chakravorty,
A. Inorg. Chem. 2001, 40, 286. (d) Bhattacharyya, S.; Chakraborty,
I.; Dirghangi, B. K.; Chakravorty, A. Chem. Commun. 2000, 1813.
(e) Dirghangi, B. K.; Menon, M.; Pramanik, A.; Chakravorty, A. Inorg.
Chem. 1997, 36, 1095.
1054 Inorganic Chemistry, Vol. 44, No. 4, 2005
10.1021/ic0486116 CCC: $30.25
© 2005 American Chemical Society
Published on Web 01/19/2005

Rhenium Chemistry of Azooximes
only limited attention. Oximes have been used for analytical
separation and colorimetric estimation of rhenium.6 In recent
patents oxime complexes of rhenium have been scrutinized
for diagnostic and therapeutic values.7 However, structurally
authenticated oxime-coordinated rhenium systems are very
rare and little is known beyond a family of tris(dioximates).8
In a di-2-pyridyl ketoxime complex of known structure, the
oxime group is not coordinated to the metal.9 Rhenium
complexes have recently been shown to be potent catalysts
for aldoxime dehydration10 and to be good couplers of oximes
with polar unsaturates such as nitriles.11 But in these reactions
rhenium-oxime coordination is not involved.
In this work we explore the rhenium chemistry of
azooxime ligands characterized by the -NdN-CdNOH
function. No complex incorporating chelation of the intact
function could, however, be isolated. Instead depending on
the metal oxidation state and coligands present, the oxime
function was found to transfer the oxygen atom either to the
metal site itself or to an oxophilic substrate like PPh3
furnishing hitherto unknown azoimine chelates of pentavalent
rhenium. An imine-oxime ligand bearing the -NdC-Cd
NOH function has also been examined. In striking contrast
to the isoelectronic azooxime function it was found to engage
in straightforward oxime chelation. The structures and
properties of the new systems are reported, and attempts are
made to rationalize the observed transfer reactions.
examined with a number of rhenium reagents. Stable
complexes representing two kinds of oxygen atom transfer
from the azooxime ligands have been observed in the cases
of [ReIII(MeCN)Cl3(PPh3)2] and [ReV(NPh)Cl3(PPh3)2].
(B) Transfer to Metal: Oxidative Addition. This has
been realized in the case of the trivalent metal. Upon treat-
ment of [Re(MeCN)Cl3(PPh3)2] with [Ag(L1O)(L1OH)] taken
in slight excess of 2 mol in cold mixed dichloromethane-
acetonitrile solvent, a rapid reaction occurs. It is associated
with color change from yellow to deep green and precipita-
tion of silver chloride. From the reaction solution the green
complex of composition [ReVOCl(PPh3)2(L1)]PF6, 2, was
isolated in excellent yields upon addition of NH4PF6.
No tractable product could be isolated by reacting [Re-
(MeCN)Cl3(PPh3)2] with L1OH. Evidently Ag+ facilitates
halide displacement thus promoting ligand chelation, eq 3,
which formally represents the oxidative addition of L1O- to
the metal. Low temperature (<-5 °C) is crucial for the
success of the reaction of eq 3; otherwise only a brown
insoluble mass of uncertain nature resulted. The reaction of
eq 3 proceeded unhindered in oxygen-free inert atmosphere
(N2/Ar) as it did in air, and dry solvents were as effective as
the partially wet variety. The source of the oxo oxygen in 2
is thus the oxime function of the ligand.
Results and Discussion
(A) Azooximes and Rhenium Reagents. 2-Aryla-
zooximes are known to be versatile metal chelators.12 Three
of these (1, general abbreviation L1OH) have been employed
in this work either as such or as silver salts of type [Ag-
(L1O)(L1OH)].13
[ReIII(MeCN)Cl3(PPh3)2] + 2Ag+ + L1O- f
[ReV(O)Cl(PPh3)2(L1)]+ + 2AgCl (3)
(7) (a) Bracco International BV (The Netherlands). U.S. Patents 5608110
and 242093, 1997. (b) Bracco International BV (The Netherlands).
U.S. Patents 5627286 and 472058, 1997. (c) Bracco International BV
(The Netherlands). U.S. Patents 5387409 and 818705, 1995.
(8) (a) Jurisson, S.; Haliham, M. M.; Lydon, J. D.; Barnes, C. L.;
Nowotnik, D. P.; Nunn, A. D. Inorg. Chem. 1998, 37, 1922. (b)
Jurisson, S.; Francesconi, L.; Linder, K. E.; Treher, E.; Malley, M.
F.; Gougoutas, J. Z.; Nunn, A. D. Inorg. Chem. 1991, 30, 1820.
(9) Bakir, M. Acta Crystallogr. 2001, C57, 1154.
To observe the effect of metal oxidation state and co-
ligands, the reactions of L1OH and its silver salt have been
(10) Ishihara, K.; Furuya, Y.; Yamamoto, H. Angew. Chem., Int. Ed. 2002,
41, 2983.
(11) Wagner, G.; Pombeiro, A. J. L.; Bokach, N. A.; Kukushkin, V. Yu.
J. Chem. Soc., Dalton Trans. 1999, 4083.
(3) (a) Das, S.; Chakraborty, I.; Chakravorty, A. Polyhedron 2003, 22,
901. (b) Banerjee, S.; Bhattacharyya, S.; Dirghangi, B. K.; Menon,
M.; Chakravorty, A. Inorg. Chem. 2000, 39, 6. (c) Chakraborty, I.;
Bhattacharyya, S.; Banerjee, S.; Dirghangi, B. K.; Chakravorty, A. J.
Chem. Soc., Dalton Trans. 1999, 3747. (d) Banerjee, S.; Bhattacharyya,
S.; Chakraborty, I.; Dirghangi, B. K.; Chakravorty, A. Indian J. Chem.
1999, 38A, 857.
(4) (a) Chakraborty, I.; Sengupta, S.; Das, S.; Banerjee, S.; Chakravorty,
A. Dalton Trans. 2003, 134. (b) Sengupta, S.; Chakraborty, I.;
Chakraborty, A. Eur. J. Inorg. Chem. 2003, 1157.
(5) (a) Rowbottom, J. F.; Wilkinson, G. J. Chem. Soc., Dalton Trans.
1972, 826. (b) Bryan, J. C.; Stenkamp, R. E.; Tulip, T. H.; Mayer, J.
M. Inorg. Chem. 1987, 26, 2283. (c) Fontaine, X. L. R.; Fowles, E.
H.; Layzell, T. P.; Shaw, B. L.; Thornton-Pett, M. J. Chem. Soc.,
Dalton Trans. 1991, 1519. (d) Seymore, S. B.; Brown, S. N. Inorg.
Chem. 2000, 39, 325.
(6) (a) Khaira, S.; Kakkar, L. R. Z. Anal. Chem. 1989, 335, 404. (b) Dutt,
N. K.; Dhar, S. N. J. Inst. Chem. (India) 1974, 46, 206. (c) Lazarev,
A. I. Z. Obshch. Khim. 1955, 25, 2198.
(12) (a) Ganguly, S.; Manivannan, V.; Chakravorty, A. J. Chem. Soc.,
Dalton Trans. 1998, 461 and references therein. (b) Manivannan, V.;
Dirghangi, B. K.; Pal, C. K.; Chakravorty, A. Inorg. Chem. 1997, 36,
1526. (c) Karmakar, S.; Choudhury, S. B.; Ganguly, S.; Chakravorty,
A. J. Chem. Soc., Dalton Trans. 1997, 585. (d) Manivannan, V.; Dutta,
S.; Basu, P.; Chakravorty, A. Inorg. Chem. 1993, 32, 4807 and
references therein. (e) Pal, C. K.; Chattopadhyay, S.; Sinha, C.;
Chakravorty, A. Inorg. Chem. 1994, 33, 6140. (f) Karmakar, S.;
Chowdhury, S. B.; Chakravorty, A. Inorg. Chem. 1994, 33, 6148. (g)
Ghosh, B. K.; Mukherjee, R. N.; Chakravorty, A. Inorg. Chem. 1987,
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Chem. 2000, 39, 2954.
Inorganic Chemistry, Vol. 44, No. 4, 2005 1055

Chakraborty et al.
Known instances of oxidative addition of oximes to metal
centers are very rare.14 In rhenium chemistry the only known
example15 is the addition of acetoxime to [ReICl(N2)(dppe)2]
furnishing [ReIII(OH)(NdCMe2)(dppe)2]+, where dppe is 1,2-
bis(diphenylphosphino)ethane.
(C) Transfer to Coligand. This happened upon reacting
the pentavalent imido complex [ReV(NPh)Cl3(PPh3)2] with
L1OH in 1:1 ratio. The reaction, eq 4, could be carried out in
dichloromethane solution at room temperature or more con-
veniently (faster, better yield) in boiling toluene. The resultant
green solution afforded [ReV(NPh)Cl3(L1H)], 3, in good yield
upon chromatographic workup. The crude solid before
chromatography revealed the presence of PPh3O both in IR
(νP-O, 1180 cm-1) as well as in 31P NMR (δ, 28.918 ppm).
near 420 nm. Complexes of type 2 are characterized by single
Re-Cl and Re-O stretches near 330 and 1000 cm-1,
respectively. On the other hand, in both 3 and 5 two Re-Cl
stretches occur (near 350 and 320 cm-1 in 3 and near 320
and 300 cm-1 in 5). Complexes 2 and 3 are characterized
by an NdN stretch near 1420 cm-1 while in 5 CdN and
N-O stretches occur near 1600 and 1230 cm-1, respectively.
[ReV(NPh)Cl3(PPh3)2] + L1OH f
[ReV(NPh)Cl3(L1H)] + OPPh3 + PPh3 (4)
1
The H NMR spectra of the complexes are dominated by
complex aromatic multiplets (6.5-8.2 ppm). The N-H group
in 3 is observed both in IR (sharp band near 3250 cm-1)
1
and H NMR (singlet near 9.60 ppm).
All the complexes are electroactive in acetonitrile solution.
An irreversible oxidation peak presumably corresponding to
the ReVI/ReV couple is observed with anodic peak potential
of ∼1.2 V vs SCE in 2 and ∼1.6 V in 3. In 5 the
corresponding response is quasireversible and occurs near
1.20 V. The potentials generally follow the Hammett order
(Cl > H > Me).
(F) Structures. The X-ray structures of two azoimine
complexes, [ReOCl(PPh3)2(L1a)]PF6, 2 (X ) H), and [Re-
(NPh)Cl3(L1cH)], 3 (X ) Cl), and the imine-oxime complex,
[Re(NC6H4Me)Cl2(PPh3)(L2O)]‚H2O, 5‚H2O (Y ) Me), have
been determined. Selected bond parameters are listed in
Tables 1-3, and perspective views are shown in Figures
1-3.
In the cation of 2 (X ) H) (Figure 1, Table 1), the O1,
Cl1, N1, and N3 atoms along with the metal make a good
equatorial plane (mean deviation 0.03 Å) and the phosphine
ligands define the axis (P1-Re-P2, 174.68(7)°). The chelate
ring itself makes a nearly perfect plane (mean deviation 0.005
Å) to which the pendent phenyl rings at C37 and N1 make
the dihedral angles of 13.9 and 33.6°, respectively. The oxo
atom lie cis to the imine nitrogen.
To our knowledge the only precedence of PPh3-promoted
oxime f imine reduction in rhenium chemistry occurs in
the reaction of [ReVOCl3(PPh3)2] with salicylaldoxime,16 and
a similar reaction has been documented in the case of
ruthenium.17
(D) Imine-Oxime Chelation. With the hope of under-
standing the nature of the spontaneous atom transfer pro-
cesses involving azooximes, we have investigated the
behavior of an imine-oxime ligand, viz., the oxime of
2-acetylpyridine, 4 (L2OH). We found 4 to be unreactive
toward [Re(MeCN)Cl3(PPh3)2] in dichloromethane-metha-
nol mixture even the presence of halide scavengers (Ag+
and Tl+). On the other Hand, L2OH reacted smoothly with
[ReV(NC6H4Y)Cl3(PPh3)2] (Y ) H, Me, Cl) in dichlo-
romethane solution at room temperature resulting in straight-
forward substitution of a chloride and a phosphine ligand
via imine-oximato chelation affording the green complex
[ReV(NC6H4Y)Cl2(PPh3)(L2O)], 5, in very good yield.
(E) Spectra and Electrochemistry. The complexes (2,
3, 5) are uniformly diamagnetic (Re(V), 5d2; S ) 0). In
dichloromethane solution both 2 and 3 display two moder-
ately intense bands in the visible region (near 640 and 400
nm in 2 and near 600 and 440 nm in 3). In 5 a relatively
weak band near 700 nm is associated with a stronger band
The Re-N1 bond (2.384(6) Å) lying trans to the oxo
oxygen is much longer than the Re-N3 bond (2.022(6) Å)
consistent with the strong trans influence of the oxo group.
At 1.616(6) Å, the Re-O(oxo) bond is quite short as in only
a few other ReVO systems.18 The idealized ReVtO(σ2π4)
bond length is estimated to be 1.60 Å. In the majority of
ReVO complexes the Re-O distance however lies in the
range 1.68 ( 0.03 Å1a,3b,18a corresponding to a bond order
between two and three. The double-bonded ReVdO distance
is ∼1.76 Å.19
(14) (a) Tillack, A.; Arndt, P.; Spannenberg, A.; Kempe, R.; Rosenthal,
V. Z. Anorg. Allg. Chem. 1998, 624, 737. (b) Deeming, A. J.; Owen,
D. W.; Powell, N. I. J. Organomet. Chem. 1990, 398, 299.
(15) (a) Ferreira, C. M. P.; Guedes da Silva, M. F. C.; Kukushkin, V. Yu.;
Frausto da Silva, J. J. R.; Pombeiro, A. J. L. J. Chem. Soc., Dalton
Trans. 1998, 325. (b) Kukushkin, V. Yu.; Pombeiro, A. J. L. Coord.
Chem. ReV. 1999, 181, 147.
(16) Chen, X.; Femia, F. J.; Babich, J. W.; Zubieta, J. Inorg. Chim. Acta
2000, 306, 113.
(17) Das, A. K.; Peng, S.-M.; Bhattacharyya, S. J. Chem. Soc., Dalton
Trans. 2000, 181.
In 3 (X ) Cl) (Figure 2, Table 2), the three chloride
ligands are meridionally disposed as is usual in [ReV(NAr)-
(18) (a) Mayer, J. M. Inorg. Chem. 1988, 27, 3899. (b) Edwards, C. F.;
Griffith, W. P.; White, A. J. P.; Williams, D. J. J. Chem. Soc., Dalton
Trans. 1992, 957.
1056 Inorganic Chemistry, Vol. 44, No. 4, 2005

Rhenium Chemistry of Azooximes
Table 1. Selected Bond Distances (Å) and Angles (deg) for Compound
2+ (X ) H)
Distances
Re-O1
Re-N1
Re-N3
Re-Cl1
Re-P1
1.616(6)
2.384(6)
2.022(6)
2.375(2)
2.521(3)
Re-P2
2.490(3)
1.313(8)
1.367(10)
1.324(9)
N1-N2
N2-C37
N3-C37
Angles
O1-Re-N3
N3-Re-Cl1
N3-Re-N1
O1-Re-P2
Cl1-Re-P2
O1-Re-P1
Cl1-Re-P1
P2-Re-P1
90.5(3)
164.5(2)
68.9(3)
O1-Re-Cl1
O1-Re-N1
Cl1-Re-N1
N3-Re-P2
N1-Re-P2
N3-Re-P1
N1-Re-P1
N3-C37-N2
105.0(2)
159.2(3)
95.7(2)
92.9(2)
93.0(2)
91.8(2)
86.4(2)
118.4(7)
90.5(2)
86.12(8)
91.8(2)
88.67(8)
174.68(7)
Table 2. Selected Bond Distances (Å) and Angles (deg) for Compound
3 (X ) Cl)
Distances
Figure 1. Perspective view and atom-labeling scheme of [ReOCl(PPh3)2-
(L1a)]+, 2+ (X ) H). All non-hydrogen atoms are represented by their 30%
thermal probability ellipsoids.
Re-N4
Re-N1
Re-N3
Re-Cl1
1.691(10)
2.117(12)
1.998(11)
2.328(4)
Re-Cl2
Re-Cl3
N2-N3
2.347(4)
2.392(4)
1.293(14)
Angles
N4-Re-N3
N3-Re-N1
N3-Re-Cl1
N4-Re-Cl2
N1-Re-Cl2
N4-Re-Cl3
N1-Re-Cl3
Cl2-Re-Cl3
94.2(5)
72.3(4)
159.1(3)
95.5(4)
84.9(4)
94.9(4)
86.1(3)
168.6(2)
N4-Re-N1
N4-Re-Cl1
N1-Re-Cl1
N3-Re-Cl2
Cl1-Re-Cl2
N3-Re-Cl3
Cl1-Re-Cl3
C14-N4-Re
166.4(5)
106.5(4)
87.1(3)
94.7(3)
86.6(2)
89.3(3)
85.9(2)
176.2(10)
Table 3. Selected Bond Distances (Å) and Angles (deg) for 5‚H2O (Y
) Me)
Distances
Re-N3
Re-N1
Re-N2
Re-P
1.725(3)
2.191(3)
2.099(3)
2.4521(11)
Re-Cl1
Re-Cl2
O1-N2
2.4003(12)
2.3840(11)
1.319(4)
Angles
N3-Re-N2
N2-Re-N1
N2-Re-Cl2
N3-Re-Cl1
N1-Re-Cl1
N3-Re-P
90.6(2)
N3-Re-N1
N3-Re-Cl2
N1-Re-Cl2
N2-Re-Cl1
Cl2-Re-Cl1
N2-Re-P
163.95(14)
106.15(12)
89.89(8)
73.41(12)
162.38(10)
97.62(12)
82.82(9)
Figure 2. Perspective view and atom-labeling scheme of [Re(NPh)Cl3-
(L1cH), 3 (X ) Cl). All non-hydrogen atoms are represented by their 30%
thermal probability ellipsoids.
86.04(9)
86.34(5)
101.24(9)
83.38(5)
93.89(12)
88.15(9)
166.33(4)
the Re(L2O) fragment (excluding methyl hydrogen atom)
defines a satisfactory plane with mean deviation of 0.03 Å.
The oximato and pyridine nitrogen atoms lie trans to Cl2
and imide N3, respectively. The Cl1, Cl2, N2, and P atoms
define an approximate equatorial plane (mean deviation 0.05
Å) from which the metal is displaced toward N3 by 0.27 Å.
The Re-N1 distance, 2.191(3) Å, is significantly longer
than that of the Re-N2 length, 2.099(3) Å, consistent with
the trans influence of the imide nitrogen. The Re-N3
distance, 1.725(3) Å, is slightly longer than the corresponding
Re-N(imido) distance in [Re(NPh)Cl3(L1cH)]. The water of
crystallization is hydrogen bonded to the oximato oxygen
atom, the O‚‚‚O distance being 2.829(5) Å. Last, we note
that outside tris(dioximates),8 the present complex appears
to be the only other structurally characterized oximato
complex of rhenium.
N1-Re-P
Cl2-Re-P
Cl1-Re-P
Cl3(NN)] type complexes.1a-b,2b-c,3a-c The N1, N3, N4, and
Cl1 atoms define a good plane (mean deviation 0.03 Å) and
the metal atom lies on it. The chelate ring is planar (mean
deviation 0.03 Å), and the pendent aryl groups at C1 and
N3 made dihedral angles of 17.8 and 61.4°, respectively,
with it. The Re-N4-C14 fragment is approximately linear.
The Re-N4 distance of 1.691(10) Å nearly corresponds to
an idealized triple bond.1a,b,2b,c,3b,20 The trans influence of the
imido group is expressed in the lengthening of the Re-N1
bond by ∼0.12 Å over the Re-N3 bond.
The ligand L2O in 5‚H2O (Y ) Me) (Figure 3, Table 3)
is chelated at the oximato and pyridine nitrogen atoms, and
(19) (a) Berning, D. E.; Katti, K. V.; Barbour, L. J.; Volkert, W. A. Inorg.
Chem. 1998, 37, 334. (b) Smith, C. J.; Katti, K. V.; Volkert, W. A.;
Barbour, L. J. Inorg. Chem. 1997, 36, 3928. (c) Wang, Y. P.; Che, C.
M.; Wang, K. Y.; Peng, S. M. Inorg. Chem. 1993, 32, 5827.
(20) (a) Goeden, G. V.; Haymore, B. L. Inorg. Chem. 1983, 22, 157. (b)
Nugent, W. A.; Haymore, B. L. Coord. Chem. ReV. 1980, 31, 123.
(G) Reaction Models. Numerous light and heavy transi-
tion metal ions (3-5d) are known to undergo facile five-
membered azooxime chelation.12 It is logical to assume that
similar binding precedes the transformations of eq 3 and 4.
Inorganic Chemistry, Vol. 44, No. 4, 2005 1057

Chakraborty et al.
reaction of eq 4. The crucial feature is that the oxime and
PPh3 ligands lie cis to each othersthe necessary relative
placement for facile oxygen transfer to take place. In analogy
an intermediate incorporating the moiety 7 is proposed to
be the precursor to the azoimine complex 3. The essential
gross steps are oxygen atom transfer and displacement of
OPPh3 by Cl- present in the reaction solution furnishing 8,
which is a fragment of 3. In this transformation the rhenium-
(V) atom essentially acts as a Lewis acid activator5b and the
electron-withdrawing azo group plays its part in facilitating
oxygen atom transfer. Poor electron withdrawal by the imine
function deactivates the imine-oxime complex 5.
Figure 3. Perspective view and atom-labeling scheme of [Re(NC6H4Me)-
Cl2(PPh3)(L2O)], 5 (Y ) Me). All non-hydrogen atoms are represented by
their 30% thermal probability ellipsoids.
We consider the reaction of eq 3 first. Trivalent rhenium
is a good π-donor,1c,2a,21 and the initial azooxime attack on
Re(MeCN)Cl3(PPh3)2 is expected to proceed via displacement
of MeCN/Cl- ligands leaving the π-accepting PPh3 ligands
in place. In this model a possible reaction intermediate is 6.
Interestingly a stable 2,2′-bipyridine analogue of 6 is
known.22 The oxophilicity of rhenium(III) is well-docu-
mented.23 This combined with the augmented oxygen atom
transfer ability of the oxime function due to the pronounced
electron-withdrawing character of the azo group3b-c,24 is
believed to promote the transfer reaction which results in
the displacement of a chloride ligand from 6 by the newly
formed oxo ligand affording 2. Two observations are
significant in this context. First, in 2 the imine and the oxo
ligand lie cis to each other as expected. Second, in rhenium-
(III) chelates of N,N-coordinating azoheterocycles of type
[ReIII(N,N)Cl3(OPR3)] the Re-Cl bond lying trans to the
Re-N(azo) bond is invariably longer than the other Re-Cl
bonds.1c,d,3a-c The transformation 6 f 2 would thus have
the favorable feature that the weaker Re-Cl bond is
displaced. (A reviewer of this work has observed that an
O,N-bidentate chelate might be the intermediate instead of
6 and that the observed stereochemistry might only represent
the thermodynamic structure.)
Last, we note that the azoimine ligands as in 2 and 3
generated via metal-promoted oxygen atom transfer from
oximato precursors are not known in the free state. There is
one report on azoimine chelation of bivalent palladium
achieved via ascorbic acid reduction of otherwise stable
azooximates.25
Concluding Remarks
The main findings of this work will be briefly highlighted.
Notable instances of metal-promoted oxime reactivity have
been revealed. In reactions with rhenium reagents, the oxime
function of 2-arylazooxime (L1OH, L1O-) is found to be in
a state of inherent redox activation leading to spontaneous
oxygen atom transfer.
In the case of [ReIII(MeCN)Cl3(PPh3)2], transfer occurs to
the metal site furnishing [ReV(O)(PPh3)2(Cl)(L1)]+, which
incorporates a short RetO bond. Transfer occurs to a PPh3
ligand (eliminated as OPPh3) in the formation of [ReV(NPh)-
Cl3(L1H)] from [ReV(NPh)Cl3(PPh3)2]. Authentic instances
of azoimine (L1H, (L1)-) chelation are rare in general and
unprecedented in rhenium chemistry. In contrast to L1OH,
the imine-oxime ligand L2OH engages rhenium in stable
chelation as in [ReV(NC6H4Y)Cl2(PPh3)(L2O)].
A qualitative reaction model based on initial azooxime
chelation has been developed on the basis of the consider-
ations of oxophilicity of trivalent rhenium, Lewis acid activity
of pentavalent rhenium, electron withdrawal by the azo
group, and the observed relative dispositions of ligands in
products.
The structure of the acetylpyridine oxime complex 5
provides insight about the probable intermediate in the
Experimental Section
(21) Ghosh, P.; Pramanik, A.; Bag, N.; Chakravorty, A. J. Chem. Soc.,
Dalton Trans. 1992, 1883.
(22) Casper, J. V.; Sullivan, B. P.; Meyer, T. J. Inorg. Chem. 1984, 23,
2104.
Materials. [Re(MeCN)Cl3(PPh3)2],23a [Re(NC6H4Y)Cl3(PPh3)2],26
and 2-arylazooximes27 and their silver salts13 were prepared by
(23) (a) Rouschias, G.; Wilkinson, G. J. Chem. Soc. 1967, 993. (b)
McCleverty, J. A.; Meyer, T. J. ComprehensiVe Coordination Chem-
istry II, 1st ed.; 2004; Vol. 5, p 271.
(24) (a) Pramanik, K.; Shivakumar, M.; Ghosh, P.; Chakravorty, A. Inorg.
Chem. 2000, 39, 195. (b) Shivakumar, M.; Pramanik, K.; Bhatta-
charyya, I.; Chakravorty, A. Inorg. Chem. 2000, 39, 4332.
(25) Pal, C. K.; Chattopadhyay, S.; Sinha, C.; Chakravorty, A. Inorg. Chem.
1996, 35, 2442.
(26) Chatt, J.; Garforth, J. D.; Johnson, N. P.; Rowe, G. A. J. Chem. Soc.
1964, 1012.
(27) Kalia, K. C.; Chakravorty, A. J. Org. Chem. 1970, 35, 2231.
1058 Inorganic Chemistry, Vol. 44, No. 4, 2005

Rhenium Chemistry of Azooximes
reported methods. The oxime of 2-acetylpyridine was prepared by
reaction of the ketone with hydroxylamine hydrochloride in aqueous
alkaline solution.
pure form which was dried under vacuo over fused calcium chloride.
Yield: 43 mg (66%). Anal. Calcd for C19H16N4Cl3Re: C, 38.49;
H, 2.72; N, 9.45. Found: C, 38.40; H, 2.79; N, 9.54. UV-vis (λmax
,
nm (ꢀ, M-1 cm-1); CH2Cl2 solution): 606 (1040); 445 (2600); 324
(7900). IR (KBr, cm-1): ν(Re-Cl) 322, 350; ν(NdN) 1415; ν-
Physical Measurements. UV-vis spectral measurements were
carried out with a Shimadzu UV-1601 PC spectrophotometer. IR
spectra were measured with Nicolet Magna IR 750 series II and
550 FAR IR and Perkin-Elmer L-0100 spectrometer. Proton NMR
spectra were recorded on a Bruker FT 300 MHz spectrometer. The
atom-numbering scheme used for 1H NMR is the same as that used
in crystallography. Spin-spin structure is abbreviated as follows:
s, singlet. Electrochemical measurements were performed under
nitrogen atmosphere using a model 620A electrochemical analyzer
of CH Instruments with platinum working electrode. The supporting
electrolyte was tetraethylammonium perchlorate (TEAP), and the
potentials are referenced to the saturated calomel electrode (SCE)
without junction correction. Microanalyses (C, H, N) were per-
formed using a Perkin-Elmer 2400 series II elemental analyzer.
Synthesis of Complexes. Synthesis of [ReOCl(PPh3)2(L1)]PF6,
2. The two complexes were prepared by the same general procedure
on the basis of the reaction of [Re(MeCN)Cl3(PPh3)2] with [Ag-
(L1O)(L1OH)]. Details are given below for a representative case.
[ReOCl(PPh3)2(L1a)]PF6. To a solution of [Re(MeCN)Cl3-
(PPh3)2] (100 mg, 0.120 mmol) in 20 mL of dichloromethane-
acetonitrile (1:1) was added [Ag(L1aO)(L1aOH)] (128 mg, 0.260
mmol) at -7 °C (ice-salt mixture). The resulting solution was then
stirred for 10 min affording a deep green solution that was filtered
to remove AgCl, and to the stirred filtrate an excess of NH4PF6
(49 mg, 0.30 mmol) was added. The solvent was then removed
under reduced pressure, and the solid thus obtained was thoroughly
washed with water to free any excess of NH4PF6. The product was
dried under vacuo over fused calcium chloride and then recrystal-
lized from a dichloromethane-hexane mixture. Yield: 92 mg
(71%). Anal. Calcd for C49H40N3OP3F6ClRe: C, 52.76; H, 3.61;
N, 3.77. Found: C, 52.83; H, 3.55; N, 3.81. UV-vis (λmax, nm (ꢀ,
M-1 cm-1); CH2Cl2 solution): 640 (7760); 400 (7700); 325
(13 300). IR (KBr, cm-1): ν(Re-Cl) 330; ν(RetO) 1000; ν(Nd
N) 1416; ν(P-F) 850. 1H NMR (δ (J, Hz); CDCl3 solution): 6.76-
7.66 (aromatic multiplets). Epa (vs SCE, CH3CN solution, scan rate
50 mV s-1): 1.32 V.
1
(N-H) 3250. H NMR (δ (J, Hz); CDCl3 solution): 7.21-8.03
(aromatic multiplet); 9.63 (s, N-H). Epa (vs SCE, CH3CN solution,
scan rate 50 mV s-1): 1.62 V.
[Re(NPh)Cl3(L1cH)]. Anal. Calcd for C19H15N4Cl4Re: C, 36.38;
H, 2.41; N, 8.93. Found: C, 36.47; H, 2.47; N, 8.86. UV-vis (λmax
,
nm (ꢀ, M-1 cm-1); CH2Cl2 solution): 605 (1300); 443 (3900); 327
(9000). IR (KBr, cm-1): ν(Re-Cl) 320, 350; ν(NdN) 1416; ν-
1
(N-H) 3251. H NMR (δ (J, Hz); CDCl3 solution): 7.22-7.97
(complex multiplet); 9.59 (s, N-H). Epa (vs SCE, CH3CN solution,
scan rate 50 mV s-1): 1.66 V.
Synthesis of [Re(NC6H4Y)Cl2(PPh3)(L2O)] Complexes. The
three complexes were prepared by the same general procedure on
the basis of the reaction of [Re(NC6H4Y)Cl3(PPh3)2] with L2OH
in dichloromethane solution. Details are given below for a
representative case.
[Re(NC6H4Me)Cl2(PPh3)(L2O)]. To a solution of [Re(NC6H4-
Me)Cl3(PPh3)2] (100 mg, 0.109 mmol) in 25 mL of dichloromethane
was added 24 mg (0.174 mmol) of L2OH. The resulting solution
was stirred for 3 h at room temperature, during which the color of
the solution become yellowish green. The solution was concentrated
and then subjected to chromatography on a silica gel column (25
× 1 cm, 60-120 mesh). Excess L2OH was first eluted with
dichloromethane, and then a yellowish green band was eluted with
benzene-acetonitrile (25:1) mixture. Solvent removal from the latter
elute under reduced pressure afforded [Re(NC6H4Me)Cl2(PPh3)-
(L2O)] as a yellowish green solid, which was dried under vacuo
over fused calcium chloride. Yield: 67 mg (82%). Anal. Calcd for
C32H29N3OPCl2Re: C, 50.59; H, 3.85; N, 5.53. Found: C, 50.64;
H, 3.89; N, 5.58. UV-vis (λmax, nm (ꢀ, M-1 cm-1); CH2Cl2
solution): 690 (400); 422 (4800); 338 (9100). IR (KBr, cm-1): ν-
1
(Re-Cl) 300, 320; ν(CdN) 1600; ν(N-O) 1235. H NMR (δ (J,
Hz); CDCl3 solution): 6.19-8.02 (complex multiplet); 2.94 (s, C6-
Me); 2.11 (s, C11-Me). E1/2 (vs SCE, CH3CN solution, scan rate
50 mV s-1): 1.16 V (∆Ep ) 80 mV).
[ReOCl(PPh3)2(L1b)]PF6. Anal. Calcd for C50H42N3OP3F6-
ClRe: C, 53.17; H, 3.75; N, 3.72. Found: C, 53.25; H, 3.70; N,
3.67. UV-vis (λmax, nm (ꢀ, M-1 cm-1); CH2Cl2 solution): 640
(5700); 400 (6500); 325 (11 200). IR (KBr, cm-1): ν(Re-Cl) 330;
[Re(NC6H5)Cl2(PPh3)(L2O)]. Anal. Calcd for C31H27N3OPCl2-
Re: C, 49.93; H, 3.65; N, 5.64. Found: C, 50.00; H, 3.62; N, 5.60.
UV-vis (λmax, nm (ꢀ, M-1 cm-1) CH2Cl2 solution): 696 (430);
414 (5200); 339 (8800). IR (KBr, cm-1): ν(Re-Cl) 292, 315; ν-
(CdN) 1599; ν(N-O) 1241. 1H NMR (δ (J, Hz) CDCl3 solution):
6.20-8.02 (complex multiplet): 2.96 (s, C6-Me). E1/2 (vs SCE,
CH3CN solution, scan rate 50 mVs-1): 1.17 V (∆Ep ) 80 mV).
[Re(NC6H4Cl)Cl2(PPh3)(L2O)]. Anal. Calcd for C31H26N3OPCl3-
Re: C, 47.73; H, 3.36; N, 5.39. Found: C, 47.78; H, 3.41; N, 5.35.
UV-vis (λmax, nm (ꢀ, M-1 cm-1); CH2Cl2 solution): 692 (440);
415 (4600); 338 (7900). IR (KBr, cm-1): ν(Re-Cl) 295, 318; ν-
1
ν(RetO) 1000; ν(NdN) 1415; ν(P-F) 840. H NMR (δ (J, Hz);
CDCl3 solution): 6.72-7.72 (aromatic multiplet); C(41)-Me, 2.58
(s). Epa (vs SCE, CH3CN solution, scan rate 50 mV s-1): 1.18 V.
Synthesis of [Re(NPh)Cl3(L1H)], 3, Complexes. The two
complexes were prepared by the same general procedure on the
basis of the reaction of [Re(NPh)Cl3(PPh3)2] with L1OH. The
reaction took place in dichloromethane solution at room temper-
ature, but best yields were rapidly obtained using boiling toluene
as the solvent. Details of the reaction in toluene are given below
for a representative case.
[Re(NPh)Cl3(L1aH)]. To a suspension of [Re(NPh)Cl3(PPh3)2]
(100 mg, 0.110 mmol) in 25 mL of toluene was added 24 mg (0.110
mmol) of L1aOH. The resulting mixture was heated to reflux for 1
h affording a green solution. The solvent was then removed under
reduced pressure. The residue was extracted with toluene at room
temperature and the extract concentrated to 5 mL and then subjected
to chromatography on a silica gel column (20 × 1 cm, 60-120
mesh). Following elution with pure toluene, a green band was eluted
with a toluene-acetonitrile (25:1) mixture. Solvent removal from
the elute under reduced pressure afforded [Re(NPh)Cl3(L1aH)] in
1
(CdN) 1605; ν(N-O) 1229. H NMR (δ (J, Hz); CDCl3 solu-
tion): 6.20-8.01 (complex multiplet); 2.99 (s, C6-Me). E1/2 (vs
SCE, CH3CN solution, scan rate 50 mV s-1): 1.18 V (∆Ep ) 80
mV).
X-ray Structure Determination. Single crystals of 2 (X ) H)
(0.30 × 0.25 × 0.20 mm3), 3 (X ) Cl) (0.40 × 0.30 × 0.20 mm3),
and 5‚H2O (Y ) Me) (0.35 × 0.25 × 0.20 mm3) were grown by
slow diffusion of hexane into dichloromethane solutions at room
temperature. Cell parameters were determined by a least-squares
fit of 30 machine-centered reflections (2θ ) 14-28°). Data were
collected with the ω-scan technique on a Siemens R3m/V four-
circle diffractometer with graphite-monochromated Mo KR radiation
(λ ) 0.710 73 Å). Two check reflections measured after every 198
Inorganic Chemistry, Vol. 44, No. 4, 2005 1059

Chakraborty et al.
Table 4. Crystal Data for Complexes 2 (X ) H), 3 (X ) Cl), and 5‚H2O (Y ) Me)
param
2 (X ) H)
3 (X ) Cl)
5‚H2O (Y ) Me)
formula
M
C49H40ClF6N3OP3Re
1116.41
C19H15Cl4N4Re
627.35
monoclinic
P21/c
12.502(7)
10.023(5)
17.870(9)
90
105.13(5)
90
2162(2)
4
1.928
293(2)
C32H31Cl2N3O2PRe
777.67
system
triclinic
triclinic
space group
a/Å
b/Å
c/Å
P1h
P1h
10.928(4)
12.915(6)
17.026(10)
100.88(4)
93.53(4)
99.78(3)
2315(2)
10.785(2)
10.884(2)
14.212(3)
78.63(3)
78.53(3)
74.49(3)
1557.0(5)
2
R/deg
â/deg
γ/deg
V/Å3
Z
2
D/mg m-3
T/K
1.602
1.659
293(2)
4.159
5326
5326 (0.0000)
0.0261, 0.0666
0.0280, 0.0726
293(2)
µ/mm-1
tot. reflcns
unique reflcns (Rint
2.851
5138
6.129
3982
3779 (0.0682)
0.0629, 0.1496
0.0971, 0.1847
)
4993 (0.0307)
0.0374, 0.0801
0.0518, 0.1059
R1,a wR2b [I > 2σ(I)]
all data
2
2
2
a R ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) [∑w(Fo - Fc )2/∑(Fo )2]1/2
.
reflections showed no significant intensity reduction in any case.
All data were corrected for Lorentz-polarization effects, and an
empirical absorption correction28 was done on the basis of the
azimuthal scan of six reflections for the crystals.
The metal atom was located from Patterson maps, and the rest
of the non-hydrogen atoms emerged from successive Fourier
syntheses. The structures were refined by full-matrix least-squares
procedures. All non-hydrogen atoms were refined anisotropically,
and all the hydrogen atoms (except H1 for 3 (X ) Cl), which was
emerged from successive Fourier syntheses and refined isotropi-
cally) were added at calculated positions. Calculations were
performed using the SHELXTL, version 5.03,29 program package.
Significant crystal data are listed in Table 4.
Acknowledgment. We thank the Department of Science
and Technology, Council of Scientific and Industrial Re-
search, New Delhi, India, for financial support. Thanks are
due to Dr. Sangeeta Banerjee for performing preliminary
experiments.
Supporting Information Available: For [ReOCl(PPh3)2(L1a)]-
PF6, 2 (X ) H), [Re(NPh)Cl3(L1cH)], 3 (X ) Cl), and [Re(NC6H4-
Me)Cl2(PPh3)(L2O)]‚H2O, 5‚H2O (Y ) Me), crystallographic files
in CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
IC0486116
(28) North, A. C. T.; Phillips, D. C.; Mathews, F. A. Acta Crystallogr.,
Sect. A 1968, 24, 351.
(29) Sheldrick, G. M. SHELXTL, version 5.03; Siemens Analytical X-ray
Systems: Madison, WI, 1994.
1060 Inorganic Chemistry, Vol. 44, No. 4, 2005