The reactions of 2-hydroxypyridine ligands with Re2Cl 4(μ-dppm)2 (dppm=Ph2PCH2PPh 2) that lead to 2-pyridonate complexes

Article abstract of DOI:10.1016/j.ica.2003.06.018

2-Hydroxypyridine (Hhp), 2-hydroxynicotinic acid (HnicOH) and 6-hydroxypicolinic acid (HpicOH) react with Re₂Cl₄(μ- dppm)₂ (dppm=Ph₂PCH₂PPh₂) to afford the complexes Re₂(η²-hp)Cl₃(μ-dppm) ₂ (1), Re₂(η²-HnicO)Cl₃(μ- dppm)₂ (2) and Re₂(μ-picO)₂(μ-dppm) ₂ (3). The identities of 1 and 2 have been established by single-crystal X-ray structure determinations (Re-Re distances of 2.2602(3) and 2.2539(3) A?, respectively) and they are shown to have unsymmetrical structures with staggered rotational geometries and trans, cis coordination of the pair of μ-dppm ligands. The crystal of 2 that was used in the structure determination was found to be of composition 2Re₂(η ²-HnicO)Cl₃(μ-dppm)₂·Re ₂Cl₆(μ-dppm)₂·2.906CH ₂Cl₂. The structure of Re₂Cl ₆(μ-dppm)₂ in this crystal is compared with structures reported in the literature for other crystals that contain this edge-sharing bioctahedral dirhenium(III) complex.

Full text of DOI:10.1016/j.ica.2003.06.018

Inorganica Chimica Acta 357 (2004) 764–768
www.elsevier.com/locate/ica
The reactions of 2-hydroxypyridine ligands with Re2Cl4(l-dppm)2
(dppm ¼ Ph2PCH2PPh2) that lead to 2-pyridonate complexes
*
Swarup Chattopadhyay, Phillip E. Fanwick, Richard A. Walton
Brown Laboratory of Chemistry, Department of Chemistry, Purdue University, 425 Central Drive, West Lafayette, IN 47907-2018, USA
Received 10 May 2003; accepted 18 June 2003
Abstract
2-Hydroxypyridine (Hhp), 2-hydroxynicotinic acid (HnicOH) and 6-hydroxypicolinic acid (HpicOH) react with Re2Cl4(l-
dppm)2 (dppm ¼ Ph2PCH2PPh2) to afford the complexes Re2(g2-hp)Cl3(l-dppm)2 (1), Re2(g2-HnicO)Cl3(l-dppm)2 (2) and Re2(l-
picO)2(l-dppm)2 (3). The identities of 1 and 2 have been established by single-crystal X-ray structure determinations (Re–Re
ꢀ
distances of 2.2602(3) and 2.2539(3) A, respectively) and they are shown to have unsymmetrical structures with staggered rotational
geometries and trans, cis coordination of the pair of l-dppm ligands. The crystal of 2 that was used in the structure determination
was found to be of composition 2Re2(g2-HnicO)Cl3(l-dppm)2 ꢀ Re2Cl6(l-dppm)2 ꢀ 2.906CH2Cl2. The structure of Re2Cl6(l-dppm)2
in this crystal is compared with structures reported in the literature for other crystals that contain this edge-sharing bioctahedral
dirhenium(III) complex.
Ó 2003 Elsevier B.V. All rights reserved.
Keywords: Crystal structures; Metal–metal multiple bonds; Dirhenium(II) complexes; 2-Hydroxypyridine ligands
1. Introduction
tive strategy to access such complexes we have examined
the reactions of the synthon Re2Cl4(l-dppm)2 [3,5,6] with
ligands of this type, and herein report the results obtained
with 2-hydroxypyridine, 2-hydroxynicotinic acid and 6-
hydroxypicolinic acid. In all cases, the products have been
characterized by single-crystal X-ray structure determi-
nations. In the case of the reactions with Hhp and Hni-
cOH the products contain chelating monoanionic hp
and HnicO ligands rather than the expected l-hp and
l-HnicO modes.
In our laboratory we found that the dirhenium(III)
carboxylate
complexes
cis-Re2(l-O2CR)2Cl4L2
(R ¼ CH3 or C2H5; L ¼ H2O or 4-Mepy) react with 2-
hydroxypyridine (Hhp) and 2-hydroxy-6-methylpyridine
(Hmhp) to furnish the complexes Re2(l-hp)2Cl4 ꢀ Hhp,
Re2(l-hp)4Cl2 and Re2(l-mhp)2Cl4 ꢀ Hmhp [1]. More
recently, we have explored the extension of this chemistry
to include compounds that contain the electron-rich triple
bond as exemplified by [Re2]4þ species [2]. To this end, we
used cis-Re2(l-O2CCH3)2Cl2(l-dppm)2 (dppm ¼ Ph2
PCH2PPh2) [3] as a starting material but failed to observe
any reaction with 2-hydroxypyridine in refluxing ethanol
[4], although other conditions (e.g., the use of molten
Hhp) may well be successful. However, reaction occurs
readily with the substituted ligands 2-hydroxynicotinic
acid (HnicOH) and 6-hydroxypicolinic acid (HpicOH) to
afford the compounds Re2(l-HnicO)2Cl2(l-dppm)2 and
Re2(l-picO)2(l-dppm)2, respectively [4]. As an alterna-
2. Experimental
2.1. Starting materials, reaction procedures and charac-
terizations
The compound Re2Cl4(l-dppm)2 was prepared by
the literature method [3]. The ligands 2-hydroxypyri-
dine, 2-hydroxynicotinic acid and 6-hydroxypicolinic
acid were purchased from Aldrich Chemical Co. All
other reagents and organic solvents were purchased
from commercial sources and were used without further
* Corresponding author. Tel.: +1-765-4945464; fax: +1-765-4940239.
E-mail address: rawalton@purdue.edu (R.A. Walton).
0020-1693/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2003.06.018

S. Chattopadhyay et al. / Inorganica Chimica Acta 357 (2004) 764–768
765
purification. Solvents were deoxygenated by purging
with dinitrogen prior to use and all reactions were car-
ried out under an atmosphere of dinitrogen. Infrared
spectra, NMR spectra and cyclic voltammetric (CV)
measurements were carried out as described previously
[7]. Elemental microanalyses were done by Dr. Lee of
the Purdue University Microanalytical Laboratory.
reaction solution was then filtered, the filtrate evapo-
rated under reduced pressure, and the residue thus ob-
tained was extracted into 10 ml of dichloromethane. The
extract was filtered and then evaporated under reduced
pressure to afford a red solid; yield 70 mg. This crude
material, which contained two products (as shown by
cyclic voltammetry), was recrystallized from dichlo-
romethane/hexanes to give pure 3; yield 32%. The
identity of 3 was established by X-ray crystallography
[4]. The properties of 3 are identical to those of the
product with this same composition that is obtained in
high yield by the reaction of cis-Re2(l-O2CCH3)2Cl2(l-
dppm)2 with HpicOH [4].
2.2. Synthesis of Re2(g2-hp)Cl3(l-dppm)2 (1)
A mixture of Re2Cl4(l-dppm)2 (100 mg, 0.078 mmol)
and 2-hydroxypyridine (74 mg, 0.778 mmol) in 30 ml of
ethanol was refluxed for 2 days and then cooled to room
temperature. The orange solid was filtered off and wa-
shed with ethanol (3 ꢁ 5 ml) and diethyl ether (3 ꢁ 5 ml)
and dried under a vacuum; yield 72 mg (69%). Anal.
Calc. for C55H48Cl3NOP4Re2: C, 49.24; H, 3.61; N,
1.04. Found: C, 48.87; H, 3.70; N, 0.90%.
IR spectrum (KBr pellet, 1800–400 cmꢂ1): 1599(m),
1471(s), 1435(s), 1345(m-w), 1288(m-w), 1251(w),
1194(w), 1094(m-s), 1026(w), 1000(w), 838(w), 774(m),
736(vs), 710(m), 693(vs), 516(vs), 483(m), 423(w).
31P{1H} NMR (CDCl3, d): +6.88(m) and )3.03(m)
(centers of multiplets of an AA0 BB0 splitting pattern). 1H
NMR (CDCl3, d): 8.8–6.5 (m, 44H, hp and Ph of dppm),
6.05 and 5.98 (m, 4H, CH2 of dppm).
2.5. X-ray crystallography
Single crystals of 1–3 were obtained by the slow dif-
fusion of hexanes into dichloromethane solutions of 1–3
under a dinitrogen atmosphere. In the case of 2, the
crystal selected was found to be of composition
2 ꢀ 0.5Re2Cl6(l-dppm)2 ꢀ 1.453CH2Cl2. The crystals were
mounted on glass fibers in random orientations. The data
collections were carried out at 150(ꢃ1) K with graphite-
ꢀ
monochromated Mo Ka radiation (k ¼ 0:71073 A) on a
Nonius KappaCCD diffractometer. Lorentz and polari-
zation corrections were applied to all data sets. The key
crystallographic data for 1 and 2 are given in Table 1. The
structural details for 3 are reported elsewhere [4].
CV (0.1 M n-Bu4NPF6–CH2Cl2 at 25 °C, Pt-bead
electrode, m ¼ 200 mVsꢂ1): E1=2(ox) ¼ +0.44 V (DEp ¼ 75
mV) and Ep;a ¼ +1.30V versus Ag/AgCl.
The structures were solved using the structure solu-
tion program PATTY in DIRDIF-99 [8]. The remaining
atoms were located in succeeding difference Fourier
syntheses. Hydrogen atoms were placed in calculated
positions according to idealized geometries with
2.3. Synthesis of Re2(g2-HnicO)Cl3(l-dppm)2 (2)
An identical procedure to that described in Section
2.2 gave insoluble green microcrystals of 2; yield 60%.
Anal. Calc. for C56H48Cl3NO3P4Re2: C, 48.54; H, 3.49;
N, 1.01. Found: C, 48.83; H, 3.71; N, 1.06%.
IR spectrum (KBr pellet, 1800–400 cmꢂ1): 1746(vs),
1723(s), 1700(s), 1586(m), 1568(s), 1486(m), 1467(vs),
1435(vs), 1357(w), 1291(m-w), 1257(s), 1192(m-s),
1132(s), 1094(vs), 1027(w), 1000(w), 911(w), 838(w),
818(w), 766(vs), 710(s), 693(vs), 675(m), 626(w), 592(w),
514(vs), 480(s), 456(w), 418(w).
31P{1H} NMR (CDCl3 d):0 +5.63(m) and )3.79(m)
(centers of multiplets of an AA BB0 splitting pattern). 1H
NMR (CDCl3, d): 12.71(s, 1H, CO2H of HnicO), 9.0–
6.8 (m, 43H, HnicO and Ph of dppm), 6.02 (m, 4H, CH2
of dppm).
CV (0.1 M n-Bu4NPF6–CH2Cl2 at 25 °C, Pt-bead
electrode, m ¼ 200 mV sꢂ1): E1=2(ox) ¼ +0.69V (DEp ¼
125 mV) and Ep;a ¼ +1.60 V versus Ag/AgCl.
Table 1
Crystallographic data for Re2(g2-hp)Cl3(l-dppm)2 (1) and Re2(g2-
HnicO)Cl3(l-dppm)2 ꢀ 0.5Re2Cl6(l-dppm)2 ꢀ 1.453CH2Cl2 (2ꢀ 0.5Re2Cl6-
(l-dppm)2 ꢀ 1.453CH2Cl2)
1
2
Empirical formula
C
55H48Cl3NOP4Re2
C164:91H143:81
Cl17:81N2O6P12Re6
4370.06
Formula weight
Space group
1341.66
ꢁ
ꢁ
P1 (No. 2)
P1 (No. 2)
ꢀ
a (A)
11.1944(3)
12.1336(4)
21.1096(7)
74.1775(9)
79.1110(10)
64.2428(12)
2476.34(13)
2
13.2515(2)
15.1332(3)
23.2856(6)
71.8670(10)
82.7790(10)
70.0500(10)
4170.43(15)
1
ꢀ
b (A)
ꢀ
c (A)
a (°)
b (°)
c (°)
3
ꢀ
V (A )
Z
qcalcd (g cmꢂ3
)
1.799
5.282
1.740
4.851
l (Mo Ka) (mmꢂ1
)
R (Fo)a
wR (Fo2)b
GOF
0.036
0.069
0.927
0.046
0.123
1.036
2.4. Synthesis of Re2(l-picO)2(l-dppm)2 (3)
A mixture of Re2Cl4(l-dppm)2 (100 mg, 0.078 mmol)
and 6-hydroxypicolinic acid (28 mg, 0.201 mmol) in 30
ml of ethanol was refluxed for 2 days. The dark red
P
P
a R ¼ jjFoj ꢂ jFcjj= jFoj with Fo2 > 2rðFo2Þ.
h
.
i
1=2
ꢀ
ꢁ
P
P
2
2
b Rw
¼
w jFo2j ꢂ jFc2j
wjFo2j
.

766
S. Chattopadhyay et al. / Inorganica Chimica Acta 357 (2004) 764–768
UðHÞ ¼ 1:3Ueq (C). They were included in the refine-
ment but constrained to ride on the atom to which they
are bonded. An empirical absorption correction using
SCALEPACK [9] was applied in all cases. The final
refinements were performed by the use of the program
SHELXL-97 [10]. All non-hydrogen atoms were refined
with anisotropic thermal parameters unless indicated
otherwise. Crystallographic drawings were done using
the program ORTEP [11].
The structure solution and refinement of 1 was rou-
tine. In the case of 2, the asymmetric unit contained a
molecule of Re2(g2-HnicO)Cl3(l-dppm)2 at a general
position, a half molecule of Re2Cl6(l-dppm)2 located
about a center of inversion, and two independent
CH2Cl2 solvent molecules that were allowed to refine
freely to occupancies of 1.000 and 0.453. The C and Cl
atoms associated with the partial molecule of CH2Cl2
were refined with isotropic thermal parameters. For the
Re2(g2-HnicO)Cl3(l-dppm)2 molecule, three of the
carbon atoms of one of the dppm phenyl rings were
disordered such that three partial atom positions (C14A,
C15A and C16A) were refined anisotropically with oc-
cupancies of 0.240 and the other three (C14B, C15B and
C16B) with occupancies of 0.760. The hydrogen atom of
the uncoordinated carboxylic acid group of the HnicO
ligand was not included. The largest peaks remaining in
pic)Cl3(l-dppm)2 (E1=2(ox) ¼ +0.64 V and Ep;a ¼ +1.30
V versus Ag/AgCl) which is prepared by the reaction
between Re2Cl4(l-dppm)2 and the [(Ph3P)2N]þ salt of
picolinic acid [12]. The structure of this complex re-
sembles closely those of 1 and 2 (vide infra).
The reaction between Re2Cl4(l-dppm)2 and 6-hy-
droxypicolinic acid (HpicOH) differs from the formation
of 1 and 2 in giving a product Re2(l-picO)2(l-dppm)2
(3) that contains a pair of dianionic bridging 2-pyrido-
nate ligands which also coordinate their carboxylate
groups. All four Re–Cl bonds of Re2Cl4(l-dppm)2 have
been substituted in this reaction. Since 3 is identical to
the product that is formed in the reaction between cis-
Re2(l-O2CCH3)2Cl2(l-dppm)2 and HpicOH [4], it is not
discussed further in the present report.
Single-crystal X-ray structure determinations of 1 and
2 (Figs. 1 and 2) confirm the close structural identity of
these complexes. Although we obtained analytically and
spectroscopically pure samples of 2, the crystal chosen
for the structure determination (after recrystallization
from dichloromethane/hexanes) turned out to be of
overall composition 2Re2(g2-HnicO)Cl3(l-dppm)2 ꢀ
Re2Cl6(l-dppm)2 ꢀ 2.906CH2Cl2. This corresponds clo-
sely to one of the crystals we recently characterized for
the picolinate complex Re2(g2-pic)Cl3(l-dppm)2 [12],
which is of composition 2Re2(g2-pic)Cl3(l-dppm)2 ꢀ
the final difference maps of 1 and 2 were 2.68 and 4.31 e/
3
ꢀ
ꢀ
A , respectively; the peak remaining in 2 is at 3.12 A
from a hydrogen atom (H91A) of one of the CH2Cl2
solvent molecules. As far as we can ascertain, this peak
is of no great chemical significance.
3. Results and discussion
The reactions of the dirhenium(II) synthon Re2Cl4(l-
dppm)2 [3,5,6] with 2-hydroxypyridine (Hhp) and 2-
hydroxynicotinic acid (HnicOH) give products in which
one Re–Cl bond has been solvolyzed and the resulting
complexes Re2(g2-hp)Cl3(l-dppm)2 (1) and Re2(g2-
HnicO)Cl3(l-dppm)2 (2) contain chelating rather than
bridging 2-pyridonate ligands. Both complexes have
very similar spectroscopic and cyclic voltammetric (CV)
properties (see Sections 2.2 and 2.3), which implies a
close similarity between their molecular and electronic
structures. An important difference between the IR
spectra of 1 and 2 is a set of strong bands at ꢄ1720 cmꢂ1
in the spectrum of 2 that are associated with the m(C–O)
modes of the uncoordinated –CO2H group. The CVs of
these two complexes (recorded in 0.1 M n-Bu4NPF6–
CH2Cl2) each show a reversible one-electron process
that is associated with the [Re2]5þ/[Re2]4þ couple and an
irreversible oxidation associated with the formation of
the unstable [Re2]6þ species. This redox behavior, which
is quite typical of dirhenium(II) complexes [2], is in turn
similar to that reported for the complex Re2(g2-
Fig. 1. ORTEP [11] representation of the structure of the dirhenium(II)
complex Re2(g2-hp)Cl3(l-dppm)2 as present in crystals of 1. Thermal
ellipsoids are drawn at the 50% probability level except for the carbon
atoms of the phenyl rings of the l-dppm ligands which are circles of
ꢀ
arbitrary radius. Selected bond distances (A) and bond angles (°) are as
follows: Re(1)–Re(2) 2.2602(3), Re(1)–N(1) 2.117(4), Re(1)–O(2)
2.350(3), Re(1)–Cl(11) 2.3782(13), Re(1)–P(1) 2.4569(13), Re(1)–P(3)
2.4503(13), Re(2)–Cl(21) 2.4363(13), Re(2)–Cl(22) 2.3951(12), Re(2)–
P(2) 2.3786(12), Re(2)–P(4) 2.3623(14); Re(2)–Re(1)–O(2) 157.06(10),
N(1)–Re(1)–Cl(11) 147.48(11), P(1)–Re(1)–P(3) 171.64(5), N(1)–
Re(1)–O(2) 60.18(14), Cl(21)–Re(2)–P(4) 157.97(5), Cl(22)–Re(2)–P(2)
152.51(4), Cl(21)–Re(2)–Cl(22) 81.45(4), P(2)–Re(2)–P(4) 103.69(4).
Torsion angles: P(1)–Re(1)–Re(2)–P(2) 28.56(5)°, Cl(11)–Re(1)–Re(2)–
P(4) 39.81(5)°, P(3)–Re(1)–Re(2)–Cl(22) 38.83(5)°, N(1)–Re(1)–Re(2)–
Cl(21) 34.26(12)°.

S. Chattopadhyay et al. / Inorganica Chimica Acta 357 (2004) 764–768
767
Fig. 2. ORTEP [11] representation of the structure of the dirhenium(II)
complex Re2(g2-HnicO)Cl3(l-dppm)2 as present in the crystal of com-
position 2 ꢀ 0.5Re2Cl6(dppm)2 ꢀ 1.453CH2Cl2. Thermal ellipsoids are
drawn at the 50% probability level except for the carbon atoms of the
phenyl rings of the l-dppm ligands which are circles of arbitrary ra-
Fig. 3. ORTEP [11] representation of the structure of the molecule of
Re2Cl6(l-dppm)2 as present in the crystal of composition
2 ꢀ 0.5Re2Cl6(l-dppm)2 ꢀ 1.453CH2Cl2. This molecule has a crystallo-
graphic center of inversion. The thermal ellipsoids are drawn at the 50%
probability level except for the carbon atoms of the phenyl groups of
the l-dppm ligands which are circles of arbitrary radius. Selected bond
ꢀ
dius. Selected bond distances (A) and bond angles (°) are as follows:
Re(1)–Re(2) 2.2539(3), Re(1)–N(1) 2.110(5), Re(1)–O(2) 2.434(5),
Re(1)–Cl(11) 2.3839(15), Re(1)–P(1) 2.4507(16), Re(1)–P(3) 2.4432(16),
Re(2)–Cl(21) 2.4238(15), Re(2)–Cl(22) 2.4055(16), Re(2)–P(2)
2.3929(15), Re(2)–P(4) 2.3808(17), C(31)–O(31) 1.360(10), C(31)–O(32)
1.168(10); Re(2)–Re(1)–O(2) 158.04(11), N(1)–Re(1)–Cl(11) 148.42(14),
P(1)–Re(1)–P(3) 168.69(6), N(1)–Re(1)–O(2) 58.37(17), Cl(22)–Re(2)–
P(4) 151.38(6), Cl(21)–Re(2)–P(2) 157.81(6), Cl(22)–Re(2)–Cl(21)
81.01(6), P(2)–Re(2)–P(4) 105.65(6). Torsion angles: P(1)–Re(1)–Re(2)–
P(2) 36.25(6)°, Cl(11)–Re(1)–Re(2)–P(4) 50.24(6)°, P(3)–Re(1)–Re(2)–
Cl(21) 44.67(6)°, N(1)–Re(1)–Re(2)–Cl(22) 44.84(16)°.
0
ꢀ
distances (A) and bond angles (°) are as follows: Re(3)–Re(3)
2.5998(5), Re(3)–Cl(31) 2.3934(17), Re(3)–Cl(32) 2.3891(19), Re(3)–
Cl(33) 2.364(3), Re(3)0 –Cl(33) 2.389(3), Re(3)–P(5) 2.4774(17), Re(3)–
P(6) 2.5028(18); Cl(31)–Re(3)–Cl(32) 83.68(8), Cl(31)–Re(3)–Cl(33)
80.59(8), Cl(33)–Re(3)–Cl(33)0
113.67(7), Cl(32)–Re(3)–Cl(33)0
82.37(8), Re(3)–Cl(33)–Re(3)0 66.33(8), P(5)–Re(3)–P(6) 170.81(6).
HnicO is not coordinated but is involved in forming an
intramolecular hydrogen-bond to the coordinated oxy-
gen atom O(2) (see Fig. 2). This is supported by the
Re2Cl6(l-dppm)2 ꢀ 2.172CH2Cl2. This crystal, which
we will label as 4 in the present report, will be compared
to that obtained in the case of 2. The key structural pa-
rameters for the dirhenium compounds present in 1 and 2
are given in the captions to Figs. 1–3; data for 4 are
available as supplementary material in [12].
ꢀ
value of 2.604 A for the distance O(31)ꢀ ꢀ ꢀO(2) and the
planarity of the five atom set O(2), C(2), C(3), C(31) and
O(31) that are involved, along with the H atom bound
to O(31), in this six-membered ring.
All three complexes possess staggered rotational
geometries and have their l-dppm ligands bound in a
trans, cis fashion to the two Re atoms. The explanation
for this change from the trans, trans coordination in the
parent molecule Re2Cl4(l-dppm)2 may be similar to that
proposed in the case of Re2(g2-pic)Cl3(l-dppm)2 [12]. A
partial ‘‘merry-go-round’’ process, together with the
transfer of chlorine atoms between the two metal centers,
could occur following the formation of an unstable ki-
netic isomer in which the chelating g2-hp and g2-HnicO
ligands are bound with their N atoms in an axial position
and with the O atoms equatorially bound. The stable
thermodynamic products 1 and 2 that are isolated have
these donor atoms bound in the opposite fashion. The
binding of 2-pyridonate ligands to multiply bonded di-
metal units to form a four-membered chelate ring is
unusual and a bidentate bridging mode is commonly
encountered. The dirhenium(III) complex Re2(l-
chp)2(g2-chp)Cl3 (where chp is the anion of 6-chloro-2-
hydroxypyridine) is a rare example of a complex with
The Re–Re distances in 1 and 2, which are 2.2602(3)
ꢀ
and 2.2539(3) A, respectively, are a little shorter than the
distances reported for three different crystals that con-
tain Re2(g2-pic)Cl3(l-dppm)2
(range 2.2816(4)–
ꢀ
2.2937(4) A) [12]. These differences in bond distances
presumably reflect the smaller bite of the chelating g2-
hp and g2-HnicO ligands in 1 and 2, which is a conse-
quence of the four-membered chelate rings that are
formed; a five-membered chelate ring is present in the
analogous g2-pic complex [12]. This leads to a decrease
in the Re–Re–O angle from 168.58(12)° in the g2-pic
complex to 157.06(10)° and 158.04(11)° in 1 and 2, re-
spectively, and to a lengthening of the axial Re–O bond
2
ꢀ
from 2.234(4) A in Re2(g -pic)Cl3(l-dppm)2 to 2.350(3)
A and 2.434(5)° in 1 and 2. These changes should result
in 1 and 2 having somewhat shorter and stronger Re–Re
ꢀ
triple bonds, as we have observed.
In the Re2(g2-HnicO)Cl3(l-dppm)2 molecule that is
present in 2, the carboxylic acid substituent group in

768
S. Chattopadhyay et al. / Inorganica Chimica Acta 357 (2004) 764–768
such a chelate ring [13]. Compounds 1 and 2, which are
dirhenium(II) species, increase the number of such cases.
Another interesting feature with the chp complex [13] is
that the N atom of the g2-chp ligand is the one that is
bound axially (the Re–Re–N angle is 158.2(3)°); this may
be a consequence of minimizing intramolecular repul-
sions associated with the Cl substituent on the organic
ring. This contrasts with 1 and 2 which have an O atom
in the axial position.
recrystallization. The mechanism for the oxidation of
some [Re2]4þ species to Re2Cl6(l-dppm)2 is not obvious
in these cases and, indeed, may not be simple, although
it is known [14] that the stepwise oxidation of Re2Cl4(l-
dppm)2 to Re2Cl5(l-dppm)2 and then Re2Cl6(l-dppm)2
can occur in the presence of a suitable one-electron
oxidant and excess Clꢂ.
The crystals of composition 2Re2(g2-HnicO)Cl3(l-
dppm)2 ꢀ Re2Cl6(l-dppm)2 ꢀ 2.906CH2Cl2 and 2Re2(g2-
pic)Cl3(l-dppm)2 ꢀ Re2Cl6(l-dppm)2 ꢀ 2.172CH2Cl2 [12]
(labeled 4) are closely similar to one another in all re-
spects. In addition to the structurally related Re2(g2-
HnicO)Cl3(l-dppm)2 and Re2(g2-pic)Cl3(l-dppm)2
molecules that have already been discussed, each crystal
contains an edge-sharing bioctahedral dirhenium(III)
molecule Re2(l-Cl)2Cl4(l-dppm)2 that is located about
a crystallographic center of inversion. The structural
parameters are similar to those of a previously reported
structure of centrosymmetric Re2Cl6(l-dppm)2 [14]. Key
structural data from the three structure determinations
of Re2Cl6(l-dppm)2 are compared in Table 2. Interest-
ingly, there is some variation in the Re–Re distance
within this set of structures, the spread in values being
4. Supplementary material
Full crystallographic data for compounds 1 and 2 have
been deposited with Cambridge Crystallographic Data
Centre (CCDC Nos. 212511 and 212512). Copies of the
information can be obtained free of charge from the Di-
rector, CCDC, 12 Union Road, Cambridge CB2 1EZ,
UK (fax: +44-1223-336-033; e-mail: deposit@ccdc.ac.uk
or www: http://www.ccdc.cam.ac.uk).
Acknowledgements
We thank the John A. Leighty Endowment Fund for
support of this work.
ꢀ
0.11 A. Previous characterizations of Re2Cl6(l-dppm)2
have shown [14,15] that it is weakly paramagnetic even
though
a diamagnetic ground state configuration
r2P2d2dꢅ2, corresponding to a formal double bond, is to
be expected. The thermal population of a low-lying
paramagnetic state is possible and it may well be that the
structure of this molecule is defined by a rather shallow
potential energy minimum. Since the earlier structure
[14] was determined at room temperature, while the
structure determinations of 2 and 4 [12] were both car-
ried out at 150 K, it would be of interest to study
the structure of one of these crystals as a function of
temperature.
References
[1] A.R. Cutler, S.M.V. Esjornson, P.E. Fanwick, R.A. Walton,
Inorg. Chem. 27 (1988) 287.
[2] F.A. Cotton, R.A. Walton, Multiple Bonds Between Metal Atoms,
second ed., Oxford University Press, Oxford, 1993 (Chapter 2).
[3] A.R. Cutler, D.R. Derringer, P.E. Fanwick, R.A. Walton, J. Am.
Chem. Soc. 110 (1988) 5024.
[4] S.K. Chattopadhyay, P.E. Fanwick, R.A. Walton, Dalton Trans.
(2003) 3617.
[5] J.R. Ebner, D.R. Tyler, R.A. Walton, Inorg. Chem. 15 (1976) 833.
[6] T.J. Barder, F.A. Cotton, K.R. Dunbar, G.L. Powell, W.
Schwotzer, R.A. Walton, Inorg. Chem. 24 (1985) 2550.
[7] M. Ganesan, K.-Y. Shih, P.E. Fanwick, R.A. Walton, Inorg.
Chem. 42 (2003) 1241.
It remains unclear why 2 and its g2-pic analogue [12]
incorporate Re2Cl6(l-dppm)2 into the crystals during
[8] P.T. Beurskens, G. Beurskens, R. de Gelder, S. Garcia-Granda,
R.O. Gould, R. Israel, J.M.M. Smits, The DIRDIF-99 Program
System, Crystallography Laboratory, University of Nijmegen,
Netherlands, 1998.
Table 2
ꢀ
Comparison of important bond distances (A) and angles (°) for the
three structures that contain Re2(l-Cl)2Cl4(l-dppm)2
[9] Z. Otwinowski, W. Minor, Methods Enzymol. 276 (1996) 307.
[10] G.M. Sheldrick, SHELXL-97. A Program for Crystal Structure
Refinement, University of Gottingen, Germany, 1997.
[11] C.K. Johnson, ORTEP II, Report ORNNL-5138, Oak Ridge
National Laboratory, Tennessee, USA, 1976.
a
Re2Cl6(l-dppm)2
2b
4c
2.7101(7)
Re–Re
Re–Clt
2.616(1)
2.391(2)
2.385(2)
2.389(2)
2.392(2)
66.33(5)
2.5998(5)
2.3934(17)
2.3891(19)
2.364(3)
2.397(2)
2.366(2)
2.392(3)
2.407(3)
68.77(7)
[12] S.K. Chattopadhyay, P.E. Fanwick, R.A. Walton, Inorg. Chem.
42 (2003) 5924.
Re–Clb
2.389(3)
66.33(8)
[13] F.A. Cotton, T. Ren, Polyhedron 11 (1992) 811.
[14] T.J. Barder, F.A. Cotton, D. Lewis, W. Schwotzer, S.M. Tetrick,
R.A. Walton, J. Am. Chem. Soc. 106 (1984) 2882.
[15] J. Chantler, D.A. Kort, P.E. Fanwick, R.A. Walton, J. Organo-
met. Chem. 596 (2000) 27.
Re–Clb–Re
a Data from [14].
b This work.
c Data from [12].