Rhenium chemistry of diazabutadienes and derived iminoacetamides spanning the valence domain II-VI. Synthesis, characterization, and metal-promoted regiospecific imine oxidation

Article abstract of DOI:10.1021/ic034586c

The reaction of diazabutadienes of type R′N=C(R)-C(R)=NR′, L (R = H, Me; R′ = cycloalkyl, aryl) with Re^V-OCl ₃(AsPh₃)₂ has furnished Re^VOCl ₃(L), 1, from which Re^III(OPPh₃)Cl ₃(L), 2, and Re^V(NAr)Cl₃(L), 3, have been synthesized. Chemical oxidation of 2(R = H) by aqueous H₂O ₂ and of 3(R = H) by dilute HNO₃ has yielded Re ^IV(OPPh₃)Cl₃(L′), 5, and Re ^VI(NAr)Cl₃(L′), 4, respectively, where L′ is the monoionized iminoacetamide ligand R′N=C(H)-C(=O)-NR′ ^-. Finally, the reaction of Re^VO(OEt)X ₂(PPh₃)₂ with L has furnished bivalent species of type Re^IIX₂(L)₂, 6(X = Cl, Br). The X-ray structures of 1 (R = Me, R′ = Ph), 3 (R = H, R′ = Ph, Ar = Ph), and 4 (R = H, R′ = cycloheptyl, Ar = C₆H₄Cl) are reported revealing meridional geometry for the ReCl₃ fragment and triple bonding in the ReO (in 1) and ReNAr (in 3 and 4) fragments. The cis geometry (two Re-X stretches) of ReX₂(L)₂ is consistent with maximized Re^II-L back-bonding. Both ReX₂(L) ₂ and Re(NAr)Cl₃(L′) are paramagnetic (S = 1/2) and display sextet EPR spectra in solution. The g and A values of Re(NAr)Cl ₃(L′) are, respectively, lower and higher than those of ReX₂(L)₂. All the complexes are electroactive in acetonitrile solution. The Re(NAr)Cl₃(L) species display the Re ^VI/Re^V couple near 1.0 V versus SCE, and coulometric studies have revealed that, in the oxidative transformation of 3 to 4, the reactive intermediate is Re^VI(NAr)Cl₃(L)⁺ which undergoes nucleophilic addition of water at an imine site followed by induced electron transfer finally affording 4. In the structure of 3 (R = H, R′ = Ph, Ar = Ph), the Re-N bond lying trans to the chloride ligand is ～0.1 A shorter than that lying trans to NPh. It is thus logical that the imine function incorporating the former bond is more polarized and therefore subject to more facile nucleophilic attack by water. This is consistent with the regiospecificity of the imine oxidation as revealed by structure determination of 4 (R = H, R′ = cycloheptyl, Ar = C ₆H₄Cl).

Full text of DOI:10.1021/ic034586c

Inorg. Chem. 2003, 42, 6545−6555
Rhenium Chemistry of Diazabutadienes and Derived Iminoacetamides
Spanning the Valence Domain II−VI. Synthesis, Characterization, and
Metal-Promoted Regiospecific Imine Oxidation
,†,‡
Samir Das,† Indranil Chakraborty,† 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 May 29, 2003
The reaction of diazabutadienes of type R′NdC(R)sC(R)dNR′, L (R ) H, Me; R′ ) cycloalkyl, aryl) with ReV-
III
OCl3(AsPh3)2 has furnished ReVOCl3(L), 1, from which Re (OPPh3)Cl3(L), 2, and ReV(NAr)Cl3(L), 3, have been
synthesized. Chemical oxidation of 2(R ) H) by aqueous H O and of 3(R ) H) by dilute HNO has yielded
2
2
3
IV
Re (OPPh3)Cl3(L′), 5, and ReVI(NAr)Cl3(L′), 4, respectively, where L′ is the monoionized iminoacetamide ligand
R′NdC(H)sC(dO)sNR′-. Finally, the reaction of ReVO(OEt)X (PPh3)2 with L has furnished bivalent species of
2
II
type Re X (L)2, 6(X ) Cl, Br). The X-ray structures of 1 (R ) Me, R′ ) Ph), 3 (R ) H, R′ ) Ph, Ar ) Ph), and
2
4 (R ) H, R′ ) cycloheptyl, Ar ) C H Cl) are reported revealing meridional geometry for the ReCl3 fragment and
6
4
triple bonding in the ReO (in 1) and ReNAr (in 3 and 4 ) fragments. The cis geometry (two Re−X stretches) of
II
ReX (L)2 is consistent with maximized Re −L back-bonding. Both ReX (L)2 and Re(NAr)Cl3(L′) are paramagnetic
2
2
(S ) 1/2) and display sextet EPR spectra in solution. The g and A values of Re(NAr)Cl3(L′) are, respectively, lower
and higher than those of ReX (L)2. All the complexes are electroactive in acetonitrile solution. The Re(NAr)Cl3(L)
2
species display the ReVI/ReV couple near 1.0 V versus SCE, and coulometric studies have revealed that, in the
oxidative transformation of 3 to 4, the reactive intermediate is ReVI(NAr)Cl3(L)+ which undergoes nucleophilic addition
of water at an imine site followed by induced electron transfer finally affording 4. In the structure of 3 (R ) H, R′
) Ph, Ar ) Ph), the Re−N bond lying trans to the chloride ligand is ∼0.1 Å shorter than that lying trans to NPh.
It is thus logical that the imine function incorporating the former bond is more polarized and therefore subject to
more facile nucleophilic attack by water. This is consistent with the regiospecificity of the imine oxidation as revealed
by structure determination of 4 (R ) H, R′ ) cycloheptyl, Ar ) C H Cl).
6
4
Introduction
anions,3 redox-promoted transformations,4 and polymeriza-
tion catalysis,5 as well as photophysical and photochemical
phenomena.6 The present work has originated from our
interest in the rhenium chemistry of conjugated nitrogen-
donor ligands.7-12 As for 1, 4-diazabutadienes, the activities
reported so far from different laboratories have been mostly
limited to tricarbonyl species incorporating monovalent
rhenium.6e,13-15 This has prompted us to probe the scope of
variable-valent diazabutadiene chemistry of the metal with
special reference to higher oxidation states.
Prototypic of a large family of conjugated nitrogen-donor
ligands, 1,4-diazabutadienes1 are of abiding interest in
transition metal chemistry. Recent studies have concerned
topics such as unusual compound types,2 coordinated radical
* To whom correspondence should be addressed, at the Indian Associa-
tion for the Cultivation of Science. E-mail: icac@mahendra.iacs.res.in.
† Indian Association for the Cultivation of Science.
‡ Jawaharlal Nehru Centre for Advanced Scientific Research.
(1) Vrieze, K.; Van-Koten, G. AdV. Organomet. Chem. 1982, 21, 153.
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2001, 20, 3351. (b) McInnes, J. M.; Blake, A. J.; Mountford, P. J.
Chem. Soc., Dalton Trans. 1998, 3623. (c) Haddleton, D. M.; Clark,
A. J.; Duncalf, D. J.; Heming, A. M.; Kukulj, D.; Shooter, A. J. J.
Chem. Soc., Dalton Trans. 1998, 381. (d) Yang, K.; Lachicotte, R. J.;
Eisenberg, R. Organometallics 1997, 16, 5234.
Herein we report the first diazabutadiene monochelates
incorporating the ReVO core from which the corresponding
ReVNAr and ReIIIOPPh3 species have been synthesized.
Regiospecific metal-promoted ligand oxidation of the ReV-
10.1021/ic034586c CCC: $25.00 © 2003 American Chemical Society
Published on Web 09/04/2003
Inorganic Chemistry, Vol. 42, No. 20, 2003 6545

Das et al.
NAr and ReIIIOPPh3 complexes have, respectively, afforded
the ReVINAr and ReIVOPPh3 cores monochelated by imi-
noacetamide ligands. Finally, bis diazabutadiene chelates of
bivalent rhenium have been obtained via reduction of ReIII-
OPPh3 species. Representative compounds have been struc-
turally characterized. The spectral and electrochemical
features of this remarkable variable-valent family are scru-
tinized, and rationales are proposed for the transformations
observed.
Scheme 1
Results and Discussion
(A) Synthesis. The diazabutadiene ligands L1- L7 (general
abbreviation, L) used in the present work and the types of
complexes (1-3, 6) synthesized are listed in Scheme 1. Also
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Organometallics 2001, 20, 5492. (d) Grevels, F. W.; Kerpen, K.;
Klotzbu¨cher, W. E.; Schaffner, K.; Goddard, R.; Weimann, B.; Kayran,
C.; Oezkar, S. Organometallics 2001, 20, 4775. (e) Guillaumont, D.;
Finger, K.; Hachey, M. R.; Daniel, C. Coord. Chem. ReV. 1998, 171,
439. (f) Aarnts, M. P.; Wilms, M. P.; Stufkens, D. J.; Baerends, E. J.;
Vleck, A., Jr. Organometallics 1997, 16, 2055.
a
Scheme 2
(7) Dirghangi, B. K.; Menon, M.; Pramanik, A.; Chakravorty, A. Inorg.
Chem. 1997, 36, 1095.
(8) Dirghangi, B. K.; Menon, M.; Banerjee, S.; Chakravorty, A. Inorg.
Chem. 1997, 36, 3595.
(9) Banerjee, S.; Dirghangi, B. K.; Menon, M.; Pramanik, A.; Chakravorty,
A. J. Chem. Soc., Dalton Trans. 1997, 2149.
(10) (a) Bhattacharyya, S.; Banerjee, S.; Dirghangi, B. K.; Menon, M.;
Chakravorty, A. J. Chem. Soc., Dalton Trans. 1999, 155. (b) Dirghangi,
B. K.; Banerjee, S.; Menon, M.; Chakravorty, A. Indian J. Chem. 1997,
36A, 249. (c) Menon, M.; Pramanik, A.; Bag, N.; Chakravorty, A.
Inorg. Chem. 1994, 33, 403. (d) 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.
(11) (a) Banerjee, S.; Bhattacharyya, S.; Dirghangi, B. K.; Menon, M.;
Chakravorty, A. Inorg. Chem. 2000, 39, 6 and references therein. (b)
Chakraborty, I.; Bhattacharyya, S.; Banerjee, S.; Dirghangi, B. K.;
Chakravorty, A. J. Chem. Soc., Dalton Trans. 1999, 3747. (c) Das,
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(12) (a) Bhattacharyya, S.; Chakraborty, I.; Dirghangi, B. K.; Chakravorty,
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Chem. 1979, 175, 73.
a Summary of syntheses: (i) L, benzene; (ii) ArNH2, toluene; (iii) dil
HNO3, acetonitrile; (iv) PPh3, benzene; (v) H2O2, acetonitrile; (vi) L/PPh3,
benzene.
listed in the scheme are the monoionized iminoacetemide
ligands L′1-L′4 (general abbreviation, L′) which have been
isolated only in their coordinated form as complexes of type
4 and 5. A summary of the syntheses of complexes arranged
according to metal oxidation state is given in Scheme 2.
6546 Inorganic Chemistry, Vol. 42, No. 20, 2003

Rhenium Chemistry of Diazabutadienes
The golden yellow oxo complexes of type ReVOCl3(L),
1, were prepared in excellent yields by reacting ReOCl3-
(AsPh3)2 with L in boiling benzene, eq 1. Use of the arsine
be prepared by simply reacting ReOCl3(PPh3)2 with excess
L in boiling toluene, the imido group originating from partial
break-up of L. Indeed, this is the best method for the
precursor is crucial. The reaction of the more common
phosphine analogue ReOCl3(PPh3)2 with L yielded only the
phosphine oxide complex ReIII(OPPh3)Cl3(L), 2, evidently
due to facile oxygen atom transfer between the initially
generated ReOCl3(L) and liberated PPh3. Indeed, preformed
synthesis of Re(NC6H5)Cl3(L4) for which the oxo precursor
was difficult to isolate.
ReOCl3(L) + ArNH2 f Re(NAr)Cl3(L) + H2O (3)
The type 3 complexes having R ) H (ligands L1-L4)
undergo facile oxidation of one of the imino groups to an
amide function upon treatment with dilute nitric acid. The
product is the iminoamide rhenium(VI) system, 4. The net
reaction involves a molecule of water as stated in eq 4 (the
electrons are consumed in the reduction of nitric acid). The
nature of the reaction will be scrutinized in a latter sec-
tion.
Re(NAr)Cl3(L) + H2O f Re(NAr)Cl3(L′) + 3e- + 3H+
(4)
The phosphine oxide complexes of type 2 having R ) H
also undergo a similar reaction furnishing ReIV(OPPh3)Cl3-
(L′), 5. Here, hydrogen peroxide is the most convenient
oxidizing agent.
ReOCl3(L) was found to rapidly oxidize PPh3 in solution,
eq 2. Under the same conditions AsPh3 is unreactive to
ReOCl3(L).
ReOCl3(AsPh3)2 + L f ReOCl3(L) + 2AsPh3
ReOCl3(L) + PPh3 f Re(OPPh3)Cl3(L)
(1)
(2)
In this context, we note that pyridylaldimines7 and
pyridylazoles16 which bear a formal analogy of L in terms
of the R-diimine function react relatively slowly with PPh3,
and their oxo complexes are readily accessible from ReOCl3-
(PPh3)2.
The reaction of ReOCl3(L) with excess primary aromatic
amines (ArNH2) yielded pink imido species of type Re(NAr)-
Cl3(L), 3, via the acid-base reaction of eq 3. In the case of
R′ ) aryl ligands (L4-L7), the imido complexes could also
Transition metal chelates of ligands of type L′ are rare,
and to our knowledge, only a few iron17 and ruthenium4b
systems have been reported so far.
In the presence of free L, Re(OPPh3)Cl3(L) is reduced by
PPh3, furnishing the bis chelates ReIICl2(L)2, 6(X ) Cl). Both
the X ) Cl and X ) Br analogues are also obtainable from
(14) (a) Rossenaar, B. D.; Stufkens, D. J.; Vleck, A., Jr. Inorg. Chem. 1996,
35, 2902. (b) Andrea, R. R.; Stufkens, D. J.; Oskam, A. J. Organomet.
Chem. 1985, 290, 63. (c) Kokkes, M. W.; Stufkens, D. J.; Oskam, A.
Inorg. Chem. 1985, 24, 4411. (d) Balk, R. W.; Stufkens, D. J.; Oskam,
A. J. Chem. Soc., Dalton Trans. 1982, 275. (e) Balk, R. W.; Stufkens,
D. J.; Oskam, A. J. Chem. Soc., Dalton Trans. 1981, 1124.
(15) (a) Rossenaar, B. D.; Hartl, F.; Stufkens, D. J. Inorg. Chem. 1996,
35, 6194. (b) Andrea, R. R.; de-Lange, W. G. J.; Graaf, T. V.; Rijkhoff,
M.; Stufkens, D. J.; Oskam, A. Organometallics 1988, 7, 1100. (c)
Alberti, A.; Hudson, A. J. Organomet. Chem. 1983, 248, 199.
(16) Gangopadhyay, J.; Sengupta, S.; Bhattacharyya, S.; Chakraborty, I.;
Chakravorty, A. Inorg. Chem. 2002, 41, 2616 and references therein.
(17) (a) Chum, H. L.; Krumholtz, P. Inorg. Chem. 1974, 13, 519. (b) Chum,
H. L.; Helene, M. E. M. Inorg. Chem. 1980, 19, 876 and references
therein.
Inorganic Chemistry, Vol. 42, No. 20, 2003 6547

Das et al.
ReO(OEt)X2(PPh3)2. Bivalent rhenium is a relatively rare 5d5
oxidation level.18,19 The first examples of the ReX2(NN)2
coordination sphere to which 6 also belongs were realized
only recently.19,20
Meridionally disposed ReCl3 is expected to give rise to
three Re-Cl stretches of which all or at least two are
observed (300-360 cm-1) in the compounds of type 1-5.
On the other hand, the bis chelate ReCl2(L6)2 and its bromo
analogue display two well-resolved Re-X stretches at 317,
304 and 229, 213 cm-1, respectively, consistent with cis
geometry. The RetO stretch in 1 and PdO stretch in 2 and
5 occur as strong bands near 1000 and 1120 cm-1, respec-
tively. Complexes 1-3 and 6 generally show two CdN
stretches (1450-1500 and 1570-1600 cm-1). Two amide
bands in the regions 1550-1580 and 1610-1630 cm-1 occur
in 4 and 5. The diamagnetic (5dxy2) ReOCl3(L) and Re(NAr)-
Figure 1. Perspective view and atom-labeling scheme of ReOCl3(L5). All
non-hydrogen atoms are represented by their 30% thermal probability
ellipsoids.
1
Cl3(L) species exhibit normal H NMR spectra while the
spectra of Re(OPPh3)Cl3(L) are paramagnetically shifted.11,16
Spectral data of the complexes are listed in the Experimental
Section.
(B) Structures. Three compounds furnished single crystals
suitable for structure determination: ReOCl3(L5), Re(NC6H5)-
Cl3(L4), and Re(NC6H4Cl)Cl3(L′3). Molecular views are
shown in Figures 1-3, and selected bond parameters are
listed in Tables 1 and 2.
In all three compounds, the distorted octahedral coordina-
tion sphere incorporates meridional disposition of the chloride
ligands. The Cl1, Cl2, Cl3, and N2 atoms define a nearly
perfect plane in Re(NC6H4Cl)Cl3(L′3) and good planes (mean
deviation ∼0.02 Å) in both ReOCl3(L5) and Re(NC6H5)Cl3-
(L4). The metal atom is uniformly displaced from the plane
by 0.28-0.30 Å toward the O/N3 atom. In both ReOCl3-
(L5) and Re(NC6H5)Cl3(L4), the five-membered chelate ring
is excellently planar (mean deviation ∼0.02 Å), and the
pendent phenyl groups of the L ligand make dihedral angles
in the range 50-80° with the chelate planes. On the other
hand, in Re(NC6H4Cl)Cl3(L′3) the chelate ring is only roughly
planar, primarily due to a 0.26 Å shift of the amide N2 atom
Figure 2. Perspective view and atom-labeling scheme of Re(NC6H5)Cl3-
(L4). All non-hydrogen atoms are represented by their 30% thermal
probability ellipsoids.
from the otherwise good plane (mean deviation, 0.02 Å)
defined by the remaining four atoms. The two cycloheptyl
rings are puckered in two different ways (inset in Figure 3).
The amide atoms C15, C16, O, and N2 make a perfect plane.
The C15-C16 and C16-N2 bonds are both longer by ∼0.07
(18) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. AdVanced
Inorganic Chemistry, 6th ed.; John Wiley and Sons: New York, 1999.
(19) Chakraborty, I.; Sengupta, S.; Das, S.; Banerjee, S.; Chakravorty, A.
Dalton Trans. 2003, 134.
(20) Sengupta, S.; Chakraborty, I.; Chakravorty, A. Eur. J. Inorg. Chem.
2003, 1157.
6548 Inorganic Chemistry, Vol. 42, No. 20, 2003

Rhenium Chemistry of Diazabutadienes
Table 2. Selected Bond Distances (Å) and Angles (deg) for
Compounds Re(NC6H5)Cl3(L4) and Re(NC6H4Cl)Cl3(L′3)
[Re(NC6H5)Cl3(L4)]
[Re(NC6H4Cl)Cl3(L′3)]
Distances
1.704(6)
2.194(6)
2.032(6)
2.343(2)
2.377(3)
2.347(2)
1.306(10)
1.294(9)
1.406(10)
Re-N3
1.717(4)
2.214(3)
2.035(4)
2.3460(13)
2.3397(13)
2.3409(14)
Re-N1
Re-N2
Re-Cl1
Re-Cl2
Re-Cl3
N1-C13
N2-C14
C13-C14
N1-C15
N2-C16
C15-C16
1.259(6)
1.345(6)
1.492(6)
Angles
90.8(3)
73.4(2)
166.8(2)
97.8(2)
82.5(2)
97.8(2)
83.3(2)
164.07(8)
164.2(3)
102.4(2)
93.4(2)
N3-Re-N2
N2-Re-N1
N2-Re-Cl1
N3-Re-Cl3
N1-Re-Cl3
N3-Re-Cl2
N1-Re-Cl2
Cl3-Re-Cl2
N3-Re-N1
N3-Re-Cl1
N1-Re-Cl1
N2-Re-Cl3
Cl1-Re-Cl3
N2-Re-Cl2
Cl1-Re-Cl2
94.6(2)
75.85(13)
164.12(10)
96.33(13)
81.04(10)
97.76(13)
85.51(10)
165.66(5)
169.8(2)
100.65(14)
89.18(10)
93.90(12)
88.88(6)
91.1(2)
86.46(9)
91.6(2)
Figure 3. Perspective view and atom-labeling scheme of Re(NC6H4Cl)-
Cl3(L′3). All non-hydrogen atoms are represented by their 30% thermal
probability ellipsoids. The inset shows conformations of the two cycloheptyl
rings.
87.58(12)
85.97(5)
87.25(9)
Cl3(L4), the Re-N3 distance, 1.704(6)Å, lies close to the
idealized triple bond (RetNAr) length of 1.69 Å.8,9,16,23 The
Re-N3-C15 fragment is approximately linear. The trans
influence of the oxo and the NAr groups is evident in the
lengthening of the Re-N1 bond compared to the Re-N2
bond by ∼0.10 Å in both ReOCl3(L5) and Re(NC6H5)Cl3-
(L4).
Table 1. Selected Bond Distances (Å) and Angles (deg) for Compound
ReOCl3(L5)
Distances
Re-O
1.646(13)
2.171(14)
2.07(2)
2.257(5)
2.335(5)
Re-Cl3
2.303(5)
1.29(2)
1.26(2)
1.43(2)
Re-N1
Re-N2
Re-Cl1
Re-Cl2
N1-C13
N2-C14
C13-C14
Angles
An increase of the metal oxidation state in going from
Re(NC6H5)Cl3(L4) to Re(NC6H4Cl)Cl3(L′3) only marginally
affects metal-ligand bond lengths: the average Re-Cl
distance decreases by ∼0.02 Å while the Re-N3 and Re-
N1 lengths increase by ∼0.02 Å. The Re-N3 bond length,
1.717(4)Å, and the linearity of the ReNAr fragment in Re-
(NC6H4Cl)Cl3(L′3) imply triple bonding, ReVItNAr. Very
few imido complexes of rhenium(VI) have so far been
structurally characterized.8,9,24,25
The compounds of type Re(OPPh3)Cl3(L), Re(OPPh3)Cl3-
(L′), and ReX2(L)2 did not afford single crystals. The former
two are believed to retain the meridional geometry of the
parent oxo complex on the basis of previous results on other
species.7,11,16 For ReX2(L)2, the cis geometry for 6 is expected
to be more stable than the trans form due to superior Re-L
back-bonding.19,20 IR spectral data are indeed consistent (vide
supra) with the cis configuration of the isolated compounds.
O-Re-N2
88.9(6)
71.6(6)
163.4(4)
99.2(5)
82.7(4)
95.0(5)
84.1(4)
165.7(2)
O-Re-N1
160.4(6)
107.7(5)
91.8(4)
90.1(4)
87.2(2)
91.0(4)
87.7(2)
N2-Re-N1
N2-Re-Cl1
O-Re-Cl3
N1-Re-Cl3
O-Re-Cl2
N1-Re-Cl2
Cl3-Re-Cl2
O-Re-Cl1
N1-Re-Cl1
N2-Re-Cl3
Cl1-Re-Cl3
N2-Re-Cl2
Cl1-Re-Cl2
Å, and the C15-N1 bond is shorter by ∼0.05 Å than the
corresponding bonds in Re(NC6H5)Cl3(L4). This is consistent
with the loss of conjugation in going from the diimine to
the iminoamide function.
In ReOCl3(L5), the Re-O length, 1.646(13) Å, lies within
the range 1.68 ( 0.03 Å observed in most structurally
characterized ReVO species.7,11a,21 Idealized ReVtO and
ReVdO distances have been estimated to be approximately
1.60 and 1.76 Å, respectively.11a,21,22 The observed bond
length in the present complex is thus indicative of a bond
order lying close to three as idealized in 1. In Re(NC6H5)-
(23) (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.
(24) Weiher, U.; Dehnicke, K.; Fenske, D. Z. Anorg. Allg. Chem. 1979,
457, 115.
(25) (a) Danopoulos, A. A.; Longley, C. J.; Wilkinson, G.; Hussain, B.;
Hursthouse, M. B. Polyhedron 1989, 8, 2657. (b) Danopoulos, A. A.;
Wilkinson, G.; Sweet, T. K. N.; Hursthouse, M. B. J. Chem. Soc.,
Dalton Trans. 1996, 2995. (c) Abram, U.; Voight, A.; Kirmse, R.
Polyhedron 2000, 19, 1741.
(21) (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.
(22) (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.
Inorganic Chemistry, Vol. 42, No. 20, 2003 6549

Das et al.
Table 3. EPR Spectral Data at 300 K in Dichloromethane Solution
compd
ga
Ab
Re(NC6H4Cl)Cl3(L′1)
Re(NC6H4Cl)Cl3(L′2)
Re(NC6H4Cl)Cl3(L′3)
Re(NC6H4Cl)Cl3(L′4)
ReCl2(L2)2
1.917
1.910
1.912
1.899
2.090
2.085
2.091
484
483
484
483
330
345
305
ReCl2(L6)2
ReBr2(L6)2
a At center field. b Average values.
Figure 4. X-band EPR spectrum of (a) Re(NC6H4Cl)Cl3(L′3) and (b)
ReBr2(L6)2 in dichloromethane solution (300 K). Instrument settings: power,
30dB; modulation, 100 kHz; sweep center, 3200 G.
(C) Magnetism and EPR Spectra of ReVI(NAr)Cl3(L′)
and ReIIX2(L)2. The Re(NAr)Cl3(L′) complexes have one
1
unpaired electron (idealized t2g ) although the observed
magnetic moment is smaller than the spin-only value due to
strong orbital coupling.8,26 Representative room temperature
(300 K) magnetic moment values are ReVI(NC6H4Cl)Cl3(L′1),
1.50 µB, and ReVI(NC6H4Cl)Cl3(L′3), 1.45 µB. The EPR-
active complexes are unusual8,9,26b in displaying well-resolved
hyperfine lines in solution at room temperature (Figure 4).
The sextet spectrum (I ) 5/2) is due to both 185Re (37.07%)
and 187Re (62.93%), the nuclear moments of which differ
only slightly (1%), leaving isotopic fine structure unresolved.
The separation between adjacent hyperfine lines is variable
(320-640 G) due to second order effects.27 Center-field g
values and average hyperfine splittings are listed in Table
3. The magnetic moment of ReCl2(L6)2 is 1.89 µB (300 K)
Figure 5. Cyclic voltammograms of ∼10-3 M solutions of (a) ReOCl3-
(L7), (b) Re(OPPh3)Cl3(L1), (c) Re(OPPh3)Cl3(L′2), and (d) ReCl2(L6)2 in
acetonitrile solution.
(D) Ligand Control of Valence and Reduction Poten-
tials. All the complexes are electroactive in acetonitrile
solution. Cyclic voltammetric reduction potential data are
given in the Experimental Section, and representative vol-
tammograms are shown in Figures 5 and 6.
The strong donor (σ and π) O2- and NAr2- coligands are
suited for stabilizing (multiple dative bonds) higher oxidation
states, and in combination with L, the state isolated is
rhenium(V). In ReOCl3(L), an anodic response occurs near
1.7 V, believed to be due to ReVO f ReVIO oxidation, the
oxidized complex being too reactive to survive for the
cathodic scan. The NAr2- ligand imparts better redox stability
to the hexavalent state (see also the next section), and Re-
(NAr)Cl3(L) displays a quasireversible ReVI/ReV couple near
1.0 V. The OPPh3 ligand is a much weaker donor (σ only)
than the oxo group and is suited for the stable binding of
rhenium(III). The quasireversible ReIV/ReIII couple in Re-
(OPPh3)Cl3(L) occurs near 0.6 V. The L ligand is a good σ
donor, and it is also a π acceptor.3 In the absence of strongly
donor coligands, it can thus support low oxidation states as
in the bis chelate, ReX2(L)2, which displays the ReIII/ReII
couple near 0.4 V. On the other hand, the anionic iminoamide
ligand L′ is a stronger σ donor and much weaker π acceptor
5
corresponding to the idealized t2g configuration. The bis
chelates also show sextet EPR spectra in solution with an
average hyperfine splitting of ∼330 G (variable splitting
range 260-440 G). A representative spectrum is shown in
Figure 4, and the relevant parameters are listed in Table 3.
Significantly, the g and A values of the lower valent complex
5
(t2 ) are, respectively, higher and lower than those of the
1
higher valent complex (t2 ).
(26) (a) Gardner, J. K.; Pariyadath, N.; Corbin, J. L.; Stiefel, E. I. Inorg.
Chem. 1978, 17, 897. (b) de-Learie, L. A.; Haltiwanger, R. C.;
Pierpont, C. G. Inorg. Chem. 1987, 26, 817.
(27) Abragam, A.; Bleaney, B. Electron Paramagnetic Resonance of
Transition Ions; Clarendon: Oxford, England, 1970; p 163.
6550 Inorganic Chemistry, Vol. 42, No. 20, 2003

Rhenium Chemistry of Diazabutadienes
consistent with that of a 1:2 mixture of the two compunds.
The other Re(NAr)Cl3(L) complexes behaved similarly, and
the formation of the amide complex from the reactive
intermediate Re(NAr)Cl3(L)+ can be logically represented
by the reaction of eq 5. When the oxidation is carried out
by an external oxidant like aqueous nitric acid the regenerated
Re(NAr)Cl3(L) is reoxidized to react again as in eq 5, thus
increasing the net yield of the amide complex.
3Re(NAr)Cl3(L)+ + H2O f Re(NAr)Cl3(L′) +
2Re(NAr)Cl3(L) + 3H+ (5)
It is logical to propose4b,7-9,10a that the crucial step in the
reaction is the nucleophilic addition of water to Re(NAr)-
Cl3(L)+, the site of addition being an imine function polarized
by the oxidized metal. The R-hydroxylamine intermediate,
7, thus generated can react via an induced electron transfer
route28 finally furnishing the Re(NAr)Cl3(L′). Instances of
stable aqua adduct formation by aldimines have been
documented.29
Figure 6. Cyclic voltammograms (in dry acetonitrile) of (a) coulometrically
generated Re(NC6H4Cl)Cl3(L2)+, (b) the product obtained after solvent
removal following the reaction of Re(NC6H4Cl)Cl3(L2)+ with water (see
text), (c) Re(NC6H4Cl)Cl3(L′2).
than L. Consistent with this, the isolated oxidation level
increases by one unit in going from L to L′ chelation for
both the Re(NAr) and Re(OPPh3) species. In Re(NAr)Cl3-
(L′), quasireversible ReVI/ReV and ReVII/ReVI couples are
observed near 0.2 and 1.5 V, respectively. Compared to the
parent L complex, there is a remarkable ∼0.8 V decrease in
the ReVI/ReV reduction potential. In Re(OPPh3)Cl3(L′), the
ReIV/ReIII and ReV/ReIV couples occur near -0.4 and 1.7 V,
respectively.
In the observed structure of Re(NC6H4Cl)Cl3(L′3), the
amide nitrogen lies trans to a chloride ligand and not to the
imide ligand. The close similarity of spectral, magnetic, and
electrochemical properties of all the four isolated Re(NAr)-
Cl3(L′) complexes strongly suggests that the whole family
belongs to the same structural type, viz. 4. During synthesis,
we found no evidence indicating that the possible isomer
incorporating the amide nitrogen trans to the imide function
is formed at all. This regiospecificity of the oxidation reaction
is fully consistent with the proposed pathway. We recall that
in Re(NC6H5)Cl3(L4) the Re-N1 bond is significantly longer
than the Re-N2 bond due to the trans influence of the
NC6H5 group. This trend is expected to continue even after
metal oxidation. The imine function incorporating the N2
atom will thus be more polarized and subject to more facile
nucleophilic attack by water. It is, indeed, this imine function
that is selectively oxidized to the amide function as is evident
in the structure of Re(NC6H4Cl)Cl3(L′3).
In summary, the increase of metal reduction potentials with
ligands/coligands can be categorized in three groups: L′ ,
L; L , (L)2; NAr2- < O2- , OPPh3. These trends are
generally consistent with donor-acceptor properties of the
ligands/coligands and with the observed stabilization of the
different oxidation states under ambient conditions.
(E) Imine Oxidation and Its Regiospecificity. The
representative case of ReV(NC6H4Cl)Cl3(L2) will be consid-
ered first. Constant potential coulometry of this complex at
1.2 V in dry acetonitrile furnished the cation ReVI(NC6H4-
Cl)Cl3(L2)+ whose cyclic voltammogram (Figure 6a; initial
scan cathodic) is the same as that of the parent complex
(initial scan anodic). Upon making the solvent moist, the
cation is transformed to the amide complex ReVI(NC6H4Cl)-
Cl3(L′2) but with regeneration of a part of the parent complex.
The residue obtained by removal of the solvent at the end
of the transformation was subjected to cyclic voltammetry,
and current height data revealed that ReVI(NC6H4Cl)Cl3(L′2)
(E1/2; 0.19 V) and ReV(NC6H4Cl)Cl3(L2) (E1/2; 1.08 V) are
present in 1:2 proportion (Figure 6b). The absorption
spectrum of the product in the visible region was also
Since complexes of types Re(OPPh3)Cl3(L) and Re-
(OPPh3)Cl3(L′) failed to afford single crystals, the site of
(28) Taube, H. Electron-Transfer Reactions of Complex Ions in Solution;
Academic Press: New York, 1973; p 73.
(29) (a) Katovic, V.; Vergez, S. C.; Busch, D. H. Inorg. Chem. 1977, 16,
1716. (b) Bandoli, G.; Gerber, T. I. A.; Jacobs, R.; du Preez, J. G. H.
Inorg. Chem. 1994, 33, 178.
Inorganic Chemistry, Vol. 42, No. 20, 2003 6551

Das et al.
ReOCl3(L5). To a suspension of ReOCl3(AsPh3)2 (100 mg, 0.109
mmol) in 25 mL of benzene was added 26 mg (0.109 mmol) of
L5. The resulting mixture was heated to reflux for 15 min producing
a pink solution. The reaction mixture was then cooled, and the
solvent was removed under reduced pressure. The solid mass thus
obtained was repeatedly washed with n-hexane to remove the
liberated AsPh3. The solid was then dissolved in 5 mL of
dichloromethane and subjected to chromatography on a silica gel
column (10 cm × 1 cm, 60-120 mesh) prepared with toluene. The
pink complex was eluted out with pure toluene. Solvent removal
from the eluate under reduced pressure afforded ReOCl3(L5) in pure
form which was dried under vacuum over fused calcium chloride.
Yield: 50 mg (85%). Anal. Calcd for C16H16N2Cl3ORe: C, 35.27;
oxidation could not be established directly. However,
structural data on other Re(OPPh3)Cl3(NN) type complexes
have revealed that the Re-N bond trans to chloride is
generally shorter than that trans to OPPh3 by 0.01-0.05
Å.11,12 It is therefore very probable that the oxidation is
regiospecific here as well, as implied in drawing 5.
Concluding Remarks
The diazabutadiene chemistry and the corresponding
iminoamide chemistry of rhenium have been developed in
the valence domain II-VI. The compounds isolated and
characterized belong to the types ReIIX2(L)2, ReIII(OPPh3)-
Cl3(L), ReIV(OPPh3)Cl3(L′), ReVOCl3(L), ReV(NAr)Cl3(L),
and ReVI(NAr)Cl3(L′). The ultimate parent of the family is
ReVOCl3(L) from which the other members are derived. The
observed oxidation states and reduction potentials are
consistent with the combined donor-acceptor potency of the
ligands and coligands. The iminoamide species are formed
by metal-promoted ligand oxidation of the corresponding
diazabutadiene complexes via nucleophilic aquation of the
more tightly bound imine function followed by induced
electron transfer.
H, 2.96; N, 5.14. Found: C, 35.37; H, 2.91; N, 5.21. UV-vis (λmax
,
nm (ꢀ, M-1 cm-1) CH2Cl2 solution): 278 (5770); 505 (3200); 746
(540). IR (KBr, cm-1): ν(ResCl) 304, 329, 354; ν(RetO) 998;
1
ν(CdN) 1485, 1587. H NMR (δ (J, Hz) CDCl3): 2.20 and 4.57
(C(13)-Me, s and C(14)-Me, s); 7.25-7.50 (aromatic multiplet).
Epa (ReVI/ReV couple): 1.70 V.
ReOCl3(L1). Anal. Calcd for C12H20N2Cl3ORe: C, 28.78; H, 4.02;
N, 5.59. Found: C, 28.84; H, 4.10; N, 5.65. UV-vis (λmax, nm (ꢀ,
M-1 cm-1) CH2Cl2 solution): 260 (5720); 503 (2370); 737 (260).
IR (KBr, cm-1): ν(ResCl) 320, 336, 357; ν(RetO) 992; ν(CdN)
1480, 1587. 1H NMR (δ (J, Hz) CDCl3): 6.62 and 8.24 (C(13)-H,
s and C(14)-H, s); 1.42-5.06 (aliphatic multiplet). Epa (ReVI/ReV
couple): 1.70 V.
Ongoing studies include oxidation of chelated diacetyl-
diimine ligands and photophysical and photochemical be-
havior of ReOCl3(L), Re(NAr)Cl3(L), and Re(NAr)Cl3(L′)
which have been found to fluoresce in the visible region.
ReOCl3(L2). Anal. Calcd for C14H24N2Cl3ORe: C, 31.79; H, 4.57;
N, 5.30. Found: C, 31.86; H, 4.51; N, 5.36. UV-vis (λmax, nm (ꢀ,
M-1 cm-1) CH2Cl2 solution): 263 (5140); 500 (3300); 745 (360).
IR (KBr, cm-1): ν(ResCl) 317, 333, 355; ν(RetO) 993; ν(CdN)
1481, 1583. 1H NMR (δ (J, Hz) CDCl3): 6.58 and 8.27 (C(13)-H,
s and C(14)-H, s); 1.01-4.52 (aliphatic multiplet). Epa (ReVI/ReV
couple): 1.72 V.
ReOCl3(L3). Anal. Calcd for C16H28N2Cl3ORe: C, 34.50; H, 5.07;
N, 5.03. Found: C, 34.57; H, 5.12; N, 5.07. UV-vis (λmax, nm (ꢀ,
M-1 cm-1) CH2Cl2 solution): 262 (5870); 498 (2250); 737 (250).
IR (KBr, cm-1): ν(ResCl) 325, 340, 360; ν(RetO) 995; ν(CdN)
1480, 1580. 1H NMR (δ (J, Hz) CDCl3): 6.52 and 8.22 (C(13)-H,
s and C(14)-H, s); 1.10-4.58 (aliphatic multiplet). Epa (ReVI/ReV
couple): 1.71 V.
Experimental Section
Materials. ReOCl3(AsPh3)2,30 ReOCl3(PPh3)2,30 ReO(OEt)X2-
31
(PPh3)2 (X ) Cl, Br), and diazabutadienes32 were prepared by
reported methods. For electrochemical work, HPLC grade aceto-
nitrile was used. All other chemicals and solvents were of reagent
grade and were used as received.
Physical Measurements. UV-vis spectral measurements were
carried out with a Shimadzu UVPC 1601 spectrophotometer. IR
spectra were measured with Nicolet Magna IR 750 series II and
550 FAR IR and Perkin-Elmer L-0100 spectrophotometers. 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 structures are abbreviated as
follows: s, singlet; d, doublet; t, triplet. EPR spectra were recorded
on a Varian E-109C X-band spectrometer. Magnetic susceptibilities
were measured on a PAR 155 vibrating-sample magnetometer. A
Perkin-Elmer 2400 series II elemental analyzer was used for
microanalysis (C,H,N). Electrochemical measurements were per-
formed under nitrogen atmosphere using a CH 620A electrochemi-
cal analyzer, 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.
ReOCl3(L6). Anal. Calcd for C18H20N2Cl3ORe: C, 37.73; H, 3.52;
N, 4.91. Found: C, 37.61; H, 3.58; N, 4.85. UV-vis (λmax, nm (ꢀ,
M-1 cm-1) CH2Cl2 solution): 279 (4900); 507 (3500); 747 (570).
IR (KBr, cm-1): ν(ResCl) 310, 330, 357; ν(RetO) 990; ν(CdN)
1
1478, 1586. H NMR (δ (J, Hz) CDCl3): 2.23 and 4.55 (C(13)-
Me, s and C(14)-Me, s); 7.27-7.56 (aromatic multiplet). Epa (ReVI/
ReV couple): 1.60 V.
ReOCl3(L7). Anal. Calcd for C16H14N2Cl5ORe: C, 33.90; H, 2.21;
N, 4.39. Found: C, 34.00; H, 2.27; N, 4.49. UV-vis (λmax, nm (ꢀ,
M-1 cm-1) CH2Cl2 solution): 275 (5200); 510 (3700); 745 (630).
IR (KBr, cm-1): ν(ResCl) 315, 328, 352; ν(RetO) 994; ν(CdN)
1
1480, 1588. H NMR (δ (J, Hz) CDCl3): 2.21 and 4.58 (C(13)-
Me, s and C(14)-Me, s); 7.26-7.54 (aromatic multiplet). Epa (ReVI/
ReV couple): 1.75 V.
Preparation of Complexes. Preparations of ReOCl3(L), 1.
These complexes were prepared in 80-85% yield by the same
general procedure based on the reaction of ReOCl3(AsPh3)2 with
L in benzene. Details are given here for a representative case.
Preparation of Re(OPPh3)Cl3(L), 2. The same general method
was used for the four complexes (yield, 80-85%). The case of
Re(OPPh3)Cl3(L1) is detailed here.
Re(OPPh3)Cl3(L1). To a solution of ReOCl3(L1) (100 mg, 0.200
mmol) in benzene (15 mL) was added 105 mg (0.400 mmol) of
PPh3. The pink solution turned brown instantly, and the reaction
mixture was stirred for 20 min at room temperature. The solvent
was then removed under reduced pressure. The solid thus obtained
was washed several times with n-hexane, and finally, it was
(30) Chatt, J.; Rowe, G. A. J. Chem. Soc. 1962, 4019.
(31) Francociani, G.; D’Alfonso, G.; Romiti, P.; Sironi, A.; Freni, M. Inorg.
Chim. Acta 1983, 72, 29.
(32) (a) Kliegman, J. M.; Barnes, R. K. Tetrahedron 1970, 26, 2555. (b)
Tom Dieck, H.; Svoboda, M.; Greiser, T. Z. Naturforsch. Anorg.
Chem., Org. Chem. 1981, 36B, 823.
6552 Inorganic Chemistry, Vol. 42, No. 20, 2003

Rhenium Chemistry of Diazabutadienes
dissolved in 5 mL of dichloromethane and subjected to chroma-
tography on a silica gel (15 cm × 1 cm, 60-120 mesh) prepared
in benzene. Following elution with pure benzene, a yellowish brown
band was eluted out with a benzene-acetonitrile (50:3) mixture.
Solvent removal from the eluate under reduced pressure afforded
Re(OPPh3)Cl3(L1) in pure form. Yield: 122 mg (80%). Anal. Calcd
for C30H35N2Cl3OPRe: C, 47.22; H, 4.62; N, 3.67. Found: C, 47.30;
H, 4.56; N, 3.77. UV-vis (λmax, nm (ꢀ, M-1 cm-1) CH2Cl2
solution): 408 (600); 459 (1100). IR (KBr, cm-1): ν(ResCl) 310,
320, 335; ν(OdP) 1123; ν(CdN) 1472, 1595. E1/2 (ReIV/ReIII
couple): 0.66 V (∆Ep, 62 mV).
H, 3.96; N, 6.88. Found: C, 35.50; H, 3.92; N, 6.98. UV-vis (λmax,
nm (ꢀ, M-1 cm-1) CH2Cl2 solution): 308 (4500); 512 (2700); 743
(400). IR (KBr, cm-1): ν(ResCl) 320, 330; ν(CdN) 1482, 1574.
1H NMR (δ (J, Hz) CDCl3): 5.93 and 8.06 (C(13)-H, s and C(14)-
H, s); p-ClC6H4N protons, 7.42 (d, 8.4), 7.64 (d, 8.2); 1.02-5.10
(aliphatic multiplet). E1/2 (ReVI/ReV couple): 1.02 V (∆Ep, 63 mV).
Re(NC6H4Cl)Cl3(L3). Anal. Calcd for C22H32N3Cl4Re: C, 39.64;
H, 4.84; N, 6.30. Found: C, 39.57; H, 4.89; N, 6.37. UV-vis (λmax
,
nm (ꢀ, M-1 cm-1) CH2Cl2 solution): 307 (5400); 507 (2900); 739
(730). IR (KBr, cm-1): ν(ResCl) 300, 315, 336; ν(CdN) 1486,
1
1571. H NMR (δ (J, Hz) CDCl3): 5.87 and 8.03 (C(13)-H, s and
Re(OPPh3)Cl3(L2). Anal. Calcd for C32H39N2Cl3OPRe: C, 48.58;
C(14)-H, s); p-ClC6H4N protons, 7.40 (d, 8.7), 7.66 (d, 8.4); 1.04-
5.15 (aliphatic multiplet). E1/2 (ReVI/ReV couple): 1.07 V (∆Ep,
75 mV).
H, 4.97; N, 3.54. Found: C, 48.65; H, 5.02; N, 3.47. UV-vis (λmax
,
nm (ꢀ, M-1 cm-1) CH2Cl2 solution): 388 (900); 449 (1700). IR
(KBr, cm-1): ν(ResCl) 320, 336; ν(OdP) 1121; ν(CdN) 1470,
1596. E1/2 (ReIV/ReIII couple): 0.69 V (∆Ep, 66 mV).
Re(NC6H5)Cl3(L5). Anal. Calcd for C22H21N3Cl3Re: C, 42.62;
H, 3.41; N, 6.78. Found: C, 42.69; H, 3.46; N, 6.83. UV-vis (λmax
,
Re(OPPh3)Cl3(L5). Anal. Calcd for C34H31N2Cl3OPRe: C, 50.59;
nm (ꢀ, M-1 cm-1) CH2Cl2 solution): 344 (8460); 517 (4300); 751
(1500). IR (KBr, cm-1): ν(ResCl) 300, 316, 332; ν(CdN) 1485,
1595. 1H NMR (δ (J, Hz) CDCl3): 2.36 and 4.57 (C(13)-Me, s and
C(14)-Me, s); 6.29-7.47 (aromatic multiplet). E1/2 (ReVI/ReV
couple): 0.97 V (∆Ep, 70 mV).
H, 3.87; N, 3.47. Found: C, 50.62; H, 3.92; N, 3.52. UV-vis (λmax
,
nm (ꢀ, M-1 cm-1) CH2Cl2 solution): 352 (4200); 453 (2000). IR
(KBr, cm-1): ν(ResCl) 319, 340; ν(OdP) 1121; ν(CdN) 1487,
1
1590. H NMR (δ (J, Hz) CDCl3): -27.96 and -30.64 (C(13)-
Re(NC6H4Me)Cl3(L6). Anal. Calcd for C25H27N3Cl3Re: C, 45.35;
Me, s and C(14)-Me, s); ligand protons, 21.68 (d, 7.2), 15.54 (d,
7.2), 12.53 (t, 7.1), 11.65 (t, 7.2), 8.26 (t, 7.5), 6.14 (t, 7.2); PPh3
protons, 6.08-7.38. E1/2 (ReIV/ReIII couple): 0.54 V (∆Ep, 80 mV).
Re(OPPh3)Cl3(L6). Anal. Calcd for C36H35N2Cl3OPRe: C, 51.77;
H, 4.11; N, 6.34. Found: C, 45.39; H, 4.17; N, 6.27. UV-vis (λmax
,
nm (ꢀ, M-1 cm-1) CH2Cl2 solution): 341 (9500); 515 (5100); 749
(1600). IR (KBr, cm-1): ν(ResCl) 317, 331; ν(CdN) 1505, 1592.
1H NMR (δ (J, Hz) CDCl3): 2.34 and 4.53 (C(13)-Me, s and C(14)-
Me, s); 6.79-7.23 (aromatic multiplet). E1/2 (ReVI/ReV couple): 0.93
V (∆Ep, 65 mV).
H, 4.22; N, 3.35. Found: C, 51.85; H, 4.26; N, 3.42. UV-vis (λmax
,
nm (ꢀ, M-1 cm-1) CH2Cl2 solution): 358 (3100); 456 (1550). IR
(KBr, cm-1): ν(ResCl) 310, 318, 335; ν(OdP) 1122; ν(CdN)
1
Re(NC6H4Cl)Cl3(L7). Anal. Calcd for C22H18N3Cl6Re: C, 36.54;
1504, 1600. H NMR (δ (J, Hz) CDCl3): -27.00 and -29.80
(C(13)-Me, s and C(14)-Me, s); ligand protons, 21.27 (d, 9.0), 15.65
(d, 6.0), 12.11 (d, 9.0), 11.22 (d, 6.0); 2.34 and 2.39 (C(4)-Me, s
and C(10)-Me, s); PPh3 protons, 6.42-7.54. E1/2 (ReIV/ReIII
couple): 0.49 V (∆Ep, 100 mV).
The Re(OPPh3)Cl3(L1) complex was also synthesized by the
reaction of ReOCl3(PPh3)2 with L1. To a suspension of ReOCl3-
(PPh3)2 (100 mg, 0.120 mmol) in 15 mL of benzene was added 15
mg (0.176 mmol) of L1, and the mixture was stirred for 30 min at
room temperature. The solvent was then removed under reduced
pressure. The solid thus obtained was processed in the same manner
as described in previous paragraphs. Yield: 64 mg (70%). All the
phosphine oxide complexes of L ligands reported in this work can
be prepared by this method in similar yields.
H, 2.51; N, 5.81. Found: C, 36.60; H, 2.55; N, 5.75. UV-vis (λmax
,
nm (ꢀ, M-1 cm-1) CH2Cl2 solution): 347 (8660); 521 (4750); 750
(1450). IR (KBr, cm-1): ν(ResCl) 320, 340; ν(CdN) 1492, 1598.
1H NMR (δ (J, Hz) CDCl3): 2.40 and 4.69 (C(13)-Me, s and C(14)-
Me, s); 6.95-7.42 (aromatic multiplet). E1/2 (ReVI/ReV couple): 1.07
V (∆Ep, 70 mV).
Preparation of Re(NC6H5)Cl3(L4) from ReOCl3(PPh3)2. To a
suspension of ReOCl3(PPh3)2 (100 mg, 0.120 mmol) in 15 mL of
toluene was added 62 mg (0.300 mmol) of L4, and the mixture
was heated to reflux for 1 h. During this time, the color changed
to reddish pink. The solvent was then removed under reduced
pressure, and the solid thus obtained was subjected to chromato-
graphic workup as in the case of Re(NC6H4Cl)Cl3(L2). Yield: 46
mg (65%). Anal. Calcd for C20H17N3Cl3Re: C, 40.58; H, 2.89; N,
7.10. Found: C, 40.50; H, 2.94; N, 7.18. UV-vis (λmax, nm (ꢀ,
M-1 cm-1) CH2Cl2 solution): 335 (19200); 514 (5250); 749 (2400).
IR (KBr, cm-1): ν(ResCl) 305, 315, 335; ν(CdN) 1503, 1590.
1H NMR (δ (J, Hz) CDCl3): 6.01 and 7.89 (C(13)-H, s and C(14)-
H, s); 7.08-7.51 (aromatic multiplet). E1/2 (ReVI/ReV couple): 1.10
V (∆Ep, 120 mV).
Preparation of Re(NAr)Cl3(L′), 4. The R′ ) alkyl complexes
were synthesized in 55-65% yield by the same general method
based on the reaction of Re(NAr)Cl3(L) with dilute nitric acid in
acetonitrile. Details are given here for a representative case.
Re(NC6H4Cl)Cl3(L′3). The complex Re(NC6H4Cl)Cl3(L3) (100
mg, 0.150 mmol) was dissolved in acetonitrile (20 mL), and 0.5 N
nitric acid (2 mL) was added. The solution was then stirred for 10
h at room temperature during which time the color turned brown.
Solvent evaporation under reduced pressure afforded a dark solid
which was repeatedly washed with water. The product was then
dried in vacuo over P4O10 and finally extracted with n-hexane. The
extract was evaporated to dryness under reduced pressure, and the
solid so obtained was further dried in vacuo. Yield: 61 mg (60%).
Anal. Calcd for C22H31N3Cl4ORe: C, 38.77; H, 4.58; N, 6.17.
Preparation of Re(NAr)Cl3(L), 3. These complexes were
synthesized in 80-85% yield by the same general procedure, and
details are given here for a representative case.
Re(NC6H4Cl)Cl3(L2). To a solution of ReOCl3(L2) (100 mg,
0.190 mmol) in 15 mL of toluene was added an excess of
p-chloroaniline (121 mg, 0.950 mmol), and the mixture was heated
to reflux for 5 h. The solvent was then removed under reduced
pressure, and the solid thus obtained was subjected to column
chromatography as for ReOCl3(L5). Following elution with pure
toluene (to remove the excess p-ClC6H4NH2), a reddish pink band
was eluted out with a toluene-acetonitrile (25:1) mixture. Solvent
removal from the eluate under reduced pressure afforded the imido
complex which was dried under vacuo over fused calcium chloride.
Yield: 97 mg (80%). Anal. Calcd for C20H28N3Cl4Re: C, 37.23; H,
4.42; N, 6.58. Found: C, 37.15; H, 4.46; N, 6.63. UV-vis (λmax
,
nm (ꢀ, M-1 cm-1) CH2Cl2 solution): 306 (4250); 511 (2200); 742
(600). IR (KBr, cm-1): ν(ResCl) 317, 338; ν(CdN) 1481, 1572.
1H NMR (δ (J, Hz) CDCl3): 5.96 and 8.04 (C(13)-H, s and C(14)-
H, s); p-ClC6H4N protons, 7.40 (d, 8.7), 7.65 (d, 8.7); 0.84-5.06
(aliphatic multiplet). E1/2 (ReVI/ReV couple): 1.08 V (∆Ep, 70 mV).
Re(NC6H4Cl)Cl3(L1). Anal. Calcd for C18H24N3Cl4Re: C, 35.42;
Inorganic Chemistry, Vol. 42, No. 20, 2003 6553

Das et al.
Found: C, 38.87; H, 4.64; N, 6.28. UV-vis (λmax, nm (ꢀ, M-1 cm-1
)
Table 4. Summary of the Crystal and Refinement Data for
ReOCl3(L5), Re(NC6H5)Cl3(L4), and Re(NC6H4Cl)Cl3(L′3)
CH2Cl2 solution): 293 (3900); 363 (6700); 579 (1260). IR (KBr,
cm-1): ν(ResCl) 304, 326, 342; ν(amide) 1566, 1621. E1/2 (ReVI/
ReV couple): 0.20 V (∆Ep, 72 mV). E1/2 (ReVII/ReVI couple): 1.58
V (∆Ep, 90 mV).
Re(NC6H4Cl)Cl3(L′1). Anal. Calcd for C18H23N3Cl4ORe: C,
34.56; H, 3.71; N, 6.72. Found: C, 34.62; H, 3.76; N, 6.67. UV-
vis (λmax, nm (ꢀ, M-1 cm-1) CH2Cl2 solution): 290 (4000); 359
(6200); 577 (1200). IR (KBr, cm-1): ν(ResCl) 310, 325, 340; ν-
(amide) 1567, 1622. E1/2 (ReVI/ReV couple): 0.18 V (∆Ep, 62 mV).
E1/2 (ReVII/ReVI couple): 1.55 V (∆Ep, 100 mV).
Re(NC6H4Cl)Cl3(L′2). Anal. Calcd for C20H27N3Cl4ORe: C,
36.76; H, 4.16; N, 6.43. Found: C, 36.66; H, 4.21; N, 6.49. UV-
vis (λmax, nm (ꢀ, M-1 cm-1) CH2Cl2 solution): 294 (3600); 364
(6100); 580 (1100). IR (KBr, cm-1): ν(ResCl) 325, 342; ν(amide)
1569, 1624. E1/2 (ReVI/ReV couple): 0.19 V (∆Ep, 74 mV). E1/2
(ReVII/ReVI couple): 1.64 V (∆Ep, 80 mV).
Re(NC6H4Cl)-
ReOCl3(L5)
Re(NC6H5)Cl3(L4)
C20H17Cl3N3Re
Cl3(L′3)
formula
C16H16Cl3N2-
ORe
C22H31Cl4N3ORe
fw
544.86
orthorhombic
Pbca
20.196(8)
8.227(6)
20.880(11)
90
591.92
monoclinic
P21/c
14.991(11)
7.139(3)
19.717(11)
90
681.50
monoclinic
P21/n
11.257(2)
12.922(3)
18.295(4)
90
system
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
90
90
104.72(5)
90
104.99(3)
90
3470(4)
8
2041(2)
4
2570.8(9)
4
D
calcd (mg m-3
)
2.086
1.926
1.761
temp (K)
293
293
293
µ (mm-1
)
7.471
6.356
5.162
R1,a wR2b
0.0673, 0.1648 0.0372, 0.0903
0.0306, 0.0703
The syntheses of R′ ) aryl complexes require certain modifica-
tions. A representative case is given here.
[I > 2σ(I)]
GOF on F2
1.137
1.101
1.026
Re(NC6H5)Cl3(L′4). The complex Re(NC6H5)Cl3(L4) (100 mg,
0.169 mmol) was dissolved in acetonitrile (20 mL), and 0.5 N nitric
acid (2 mL) was added. The solution was then stirred for 3 h during
which the color turned brown. Solvent evaporation afforded a dark
product which was repeatedly washed with water and dried in vacuo
over P4O10 followed by recrystallization from dichloromethane.
Yield: 62 mg (60%). Anal. Calcd for C20H16N3Cl3ORe: C, 39.58;
2
2
2
a R1 ) ∑|Fo| - |Fc|/∑|Fo|. b wR2 ) [∑w(Fo - Fc )2/∑w(Fo )2]1/2
.
reduced pressure afforded ReCl2(L2)2 in pure form. Yield: 57 mg
(65%). Anal. Calcd for C28H48N4Cl2Re: C, 48.19; H, 6.93; N, 8.03.
Found: C, 48.24; H, 6.87; N, 8.09. UV-vis (λmax, nm (ꢀ, M-1 cm-1
)
CH2Cl2 solution): 449 (3800). IR (KBr and polyethylene, cm-1):
ν(ResCl) 305, 321; ν(CdN) 1454, 1627. E1/2 (ReIII/ReII couple):
0.41 V (∆Ep, 100 mV).
H, 2.66; N, 6.92. Found: C, 39.63; H, 2.71; N, 6.97. UV-vis (λmax
,
nm (ꢀ, M-1 cm-1) CH2Cl2 solution): 328 (14300); 511 (4600). IR
(KBr, cm-1): ν(ResCl) 306, 323, 338; ν(amide) 1574, 1620. E1/2
(ReVI/ReV couple): 0.16 V (∆Ep, 60 mV). E1/2 (ReVII/ReVI
couple): 1.52 V (∆Ep, 70 mV).
ReCl2(L6)2. To a suspension of ReO(OEt)Cl2(PPh3)2 (100 mg,
0.119 mmol) in 25 mL of benzene was added 79 mg (0.298 mmol)
of L6. The resulting mixture was heated to reflux for 1 h affording
a yellowish brown solution. The solvent was then removed under
reduced pressure, and chromatographic workup in the same manner
as described for ReCl2(L2)2 afforded ReCl2(L6)2 in pure form. Yield:
61 mg (65%). Anal. Calcd for C36H40N4Cl2Re: C, 55.02; H, 5.14;
N, 7.13. Found: C, 54.97; H, 5.09; N, 7.17. UV-vis (λmax, nm (ꢀ,
M-1 cm-1) CH2Cl2 solution): 464 (5000). IR (KBr and polyethylene,
cm-1): ν(ResCl) 304, 317; ν(CdN) 1454, 1627. E1/2 (ReIII/ReII
couple): 0.40 V (∆Ep, 66 mV).
Preparation of Re(OPPh3)Cl3(L′), 5. The two complexes
reported were prepared in 70% yield by the same general procedure.
Details are given here for a representative case.
Re(OPPh3)Cl3(L′1). The complex Re(OPPh3)Cl3(L1) (100 mg,
0.131 mmol) was dissolved in 20 mL of acetonitrile, and 0.5 mL
of H2O2 (30%) was added. The solution was stirred for 2 h at room
temperature, during which time the color become yellowish orange.
Evaporation of the solution under reduced pressure gave a brown
solid, which was repeatedly washed with water and dried in vacuo
over P4O10. The product was recrystallized from dichloromethane.
Yield: 71 mg (70%). Anal. Calcd for C30H34N2Cl3O2PRe: C, 46.31;
ReBr2(L6)2. This complex was synthesized by the same proce-
dure as described from ReO(OEt)Br2(PPh3)2 in similar yield. Anal.
Calcd for C36H40N4Br2Re: C, 49.43; H, 4.61; N, 6.40. Found: C,
49.37; H, 4.65; N, 6.34. UV-vis (λmax, nm (ꢀ, M-1 cm-1) CH2Cl2
solution): 476 (7950). IR (KBr and polyethylene, cm-1): ν(Res
Br) 213, 229; ν(CdN) 1454, 1632. E1/2 (ReIII/ReII couple): 0.38 V
(∆Ep, 70 mV).
X-ray Structure Determination. Single crystals of the com-
plexes ReOCl3(L5) and Re(NC6H5)Cl3(L4) were grown by slow
diffusion of hexane into dichloromethane solutions of the respective
compounds. Single crystals for the complex Re(NC6H4Cl)Cl3(L′3)
were grown by slow evaporation of a solution of dichloromethane-
heptane (1:2). Data for the compounds ReOCl3(L5) and Re(NC6H5)-
Cl3(L4) were collected on a Nicolet R3m/V four circle diffractometer
with graphite monochromated Mo KR radiation (λ ) 0.71073 Å)
by the ω-scan technique in the 2θ ranges 3-50° and 3-47°,
respectively. Data for the compound Re(NC6H4Cl)Cl3(L′3) were
collected on a Bruker SMART diffractometer having CCD area
detector by φ- and ω-scan technique in the 2θ range 3-55°. All
the data were corrected for Lorentz-polarization and absorption.
The metal atoms were 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
procedure on F2. All the non-hydrogen atoms were refined
anisotropically. All the hydrogen atoms were included in calculated
H, 4.40; N, 3.60. Found: C, 46.38; H, 4.46; N, 3.55. UV-vis (λmax
,
nm (ꢀ, M-1 cm-1) CH2Cl2 solution): 492 (1000). IR (KBr, cm-1):
ν(ResCl) 325, 338; ν(OdP) 1120; ν(amide) 1552, 1612. Epc (ReIV/
ReIII couple): -0.40. E1/2 (ReV/ReIV couple): 1.67 V (∆Ep, 100
mV).
Re(OPPh3)Cl3(L′2). Anal. Calcd for C32H38N2Cl3O2PRe: C,
47.67; H, 4.75; N, 3.47. Found: C, 47.62; H, 4.81; N, 3.43. UV-
vis (λmax, nm (ꢀ, M-1 cm-1) CH2Cl2 solution): 495 (600). IR (KBr,
cm-1): ν(ResCl) 323, 334; ν(OdP) 1121; ν(amide) 1558, 1613.
Epc (ReIV/ReIII couple): -0.38 V. E1/2 (ReV/ReIV couple): 1.66 V
(∆Ep, 100 mV).
Preparation of ReX2(L)2, 6. ReCl2(L2)2. To a solution of Re-
(OPPh3)Cl3(L2) (100 mg, 0.126 mmol) in 25 mL of benzene was
added 42 mg (0.190 mmol) of L2 and 3 mg (0.011 mmol) of PPh3.
The resulting mixture was heated to reflux for 1 h affording a brown
solution. The solvent was then removed under reduced pressure,
and the mass thus obtained was dissolved in 5 mL of dichlo-
romethane and subjected to chromatography on a silica gel column
(15 cm × 1 cm, 60-120 mesh). Following elution with pure
benzene, a brown band was eluted out with a benzene-acetonitrile
(25:2) mixture, and solvent removal from the latter eluate under
6554 Inorganic Chemistry, Vol. 42, No. 20, 2003

Rhenium Chemistry of Diazabutadienes
positions [U ) 0.08Å2]. Calculations were performed using the
SHELXTL V5.0333 program package. Significant crystal data are
listed in Table 4.
to Professors S. Chattopadhyay, G. K. Lahiri, and T. N. Guru
Row for help.
Supporting Information Available: For ReOCl3(L5), Re-
(NC6H5)Cl3(L4), and Re(NC6H4Cl)Cl3(L′3), crystallographic data,
in CIF and PDF formats. This material is available free of charge
via the Internet at http://pubs.acs.org.
Acknowledgment. We thank the Department of Science
and Technology and Council of Scientific and Industrial
Research, New Delhi, for financial support. We are thankful
(33) Sheldrick, G. M. SHELXTL, version 5.03; Siemens Analytical Instru-
ments, Inc: Madison, WI, 1994.
IC034586C
Inorganic Chemistry, Vol. 42, No. 20, 2003 6555