Rhodium complexes with the chelating and binucleating ligands P(CH2CH2Py)(n)Ph(3-n) (Py = 2-pyridyl; n = 1, 2): Structures and fluxional behavior

Article abstract of DOI:10.1021/ic990634a

Several rhodium(I) complexes of the type [RhX(CO)(PePy₂)], [Rh(diene)(PePy)]⁺, and [Rh(diene)(PePy₂)]⁺ (PePy(n) = P(CH₂CH₂Py)(n)Ph(3-n); Py = 2-pyridyl; n = 1, 2) have been prepared. The two former are square planar; the latter are pentacoordinated for diene = tetrafluorobenzobarrelene or norbornadiene (confirmed by X-ray diffraction), but an equilibrium of 4- and 5-coordinate isomers exists in solution for diene = 1,5-cyclooctadiene. The fluxional behavior of all these complexes is studied by NMR spectroscopy. The complex [Rh(NBD)(PePy₂)]PF₆·Cl₂CH₂ crystallizes in the monoclinic space group P2₁/n with a = 8.455(1) A , b = 18.068(3) A, c = 19.729(3) A, β = 99.658(3)°, and Z = 4. The complexes [Rh(diene)(PePy₂)]⁺ react with CO to give the dimeric complex [Rh₂(CO)₂{P(CH₂CH₂Py)₂Ph}₂](BF₄)₂ with the pyridylphosphine acting as P,N-chelating and P,N-bridging.

Full text of DOI:10.1021/ic990634a

Inorg. Chem. 2000, 39, 705-711
705
Rhodium Complexes with the Chelating and Binucleating Ligands P(CH2CH2Py)nPh3-n
(Py ) 2-Pyridyl; n ) 1, 2): Structures and Fluxional Behavior
M. Ara´nzazu Alonso, Juan A. Casares, Pablo Espinet,* and Katerina Soulantica
Departamento de Qu´ımica Inorga´nica, Facultad de Ciencias, Universidad de Valladolid,
E-47005 Valladolid, Spain
Jonathan P. H. Charmant and A. Guy Orpen
School of Chemistry, University of Bristol, Bristol BS8 1TS, United Kingdom
ReceiVed June 2, 1999
Several rhodium(I) complexes of the type [RhX(CO)(PePy2)], [Rh(diene)(PePy)]+, and [Rh(diene)(PePy2)]+ (PePyn
) P(CH2CH2Py)nPh3-n; Py ) 2-pyridyl; n ) 1, 2) have been prepared. The two former are square planar; the
latter are pentacoordinated for diene ) tetrafluorobenzobarrelene or norbornadiene (confirmed by X-ray diffraction),
but an equilibrium of 4- and 5-coordinate isomers exists in solution for diene ) 1,5-cyclooctadiene. The fluxional
behavior of all these complexes is studied by NMR spectroscopy. The complex [Rh(NBD)(PePy2)]PF6‚Cl2CH2
crystallizes in the monoclinic space group P21/n with a ) 8.455(1) Å, b ) 18.068(3) Å, c ) 19.729(3) Å, â )
99.658(3)°, and Z ) 4. The complexes [Rh(diene)(PePy2)]+ react with CO to give the dimeric complex
[Rh2(CO)2{P(CH2CH2Py)2Ph}2](BF4)2 with the pyridylphosphine acting as P,N-chelating and P,N-bridging.
Introduction
assignment of the solution structures. For other multidentate
ligands, the situation is less known, since less structural and
There is interest in the chemistry of Rh(I) complexes with
ligands containing pendant arms that can either coordinate or
leave a vacant position, since this may facilitate a catalytic cycle.
A common problem in the study of these complexes is that in
square-planar complexes there is usually fast exchange of the
pendant and coordinated arms of the multidentate ligand, a
situation which is difficult to distinguish from pentacoordination.
The problem has been extensively studied in substituted
hydrotris(pyrazolyl)borate (TpR) complexes, for which many
solid-state structures have been described, as well as the
existence of equilibria between square-planar and pentacoor-
dinated complexes, boat-to-boat conformational changes, and
intramolecular substitution processes.1 Recently, an extreme
example has been reported: The compound TpiPrRh(NBD)
(NBD ) norbornadiene) crystallizes in two coordination
geometries in the same unit cell.2 This case provides structural
information which is lost when the complexes are studied by
NMR spectroscopy in solution because of the fast exchange
between both structures and between coordinated and uncoor-
dinated arms in the square-planar isomer. The authors have
characterized the complexes using IR spectroscopy for the
dynamic information is available.
Related behavior is possible for 2-(phosphino)pyridines,
which have been extensively used as homo- and heterometallic
binucleating ligands. Some representative coordination modes
are shown in Chart 1. Thus, 2-(diphenylphosphino)pyridine
gives dimeric complexes of the type A, whereas 2,6-bis-
(diphenylphosphino)pyridine allows the synthesis of linear
compounds of higher nuclearity of the type B.3 In these
complexes, two binucleating ligands are coordinated to the metal
center in mutually trans positions, with the remaining ligands
also in trans positions. Budzelaar et al. have used a group of
binucleating 2-pyridyldiphosphines capable of forming metal
complexes in which the P and N atoms are in mutually cis
positions, as in type C.4
2-Pyridylphosphines with the P atom separated from the Py
group by one or two methylene links, capable of forming five-
and six-membered rings by chelation, have been much less
studied and seem to show little tendency to behave as bridging
ligands.3,5
We report here the syntheses and structures of complexes of
rhodium(I) with the ligands PePyn (PePyn ) P(CH2CH2Py)nPh3-n
;
* Corresponding author. E-mail: espinet@qi.uva.es.
Py ) 2-pyridyl; n ) 1, 2). PePy can act as a bidentate ligand,
whereas PePy2 is capable of behaving either as a bidentate or
as a tridentate chelating ligand, to give square-planar and
pentacoordinated structures (D and E), and also as a bridging
(1) (a) Trofimenko, S. Chem. ReV. 1993, 93, 3. (b) Kitajima, N.; Tolman,
W. B. Prog. Inorg. Chem. 1995, 43, 419. For recent examples, see:
(c) Purwoko, A. A.; Lees, A. J. Inorg. Chem. 1996, 35, 675. (d) Keyes,
M. C.; Young, V. G., Jr.; Tolman, W. B. Organometallics 1996, 15,
4133. (e) Chauby, V.; Le Berre, S.; Daran, J.-C.; Commenges, G.
Inorg. Chem. 1996, 35, 6345. (f) Sanz, D.; Santa Mar´ıa, M. D.;
Claramunt, R. M.; Cano, M.; Heras, J. V.; Campo, J. A.; Ruiz, F. A.;
Pinilla, E.; Monge, A. J. Organomet. Chem. 1996, 526, 341. (g)
Connely, N. G.; Emslie, D. J. H.; Metrz, B.; Orpen, A. G.; Quaile,
M. J. J. Chem. Soc., Chem. Commun. 1996, 2289. (h) Oldham, W. J.;
Heinekey, D. M. Organometallics 1997, 16, 467.
(3) (a) Newkome, G. R. Chem. ReV. 1993, 93, 2067-2089. (b) Sharp, P.
R. In ComprehensiVe Coordination Chemistry; Abel, E. W., Stone,
F. G. A., Wilkinson, G., Eds.; Pergamon Press: Oxford, U. K., 1995;
Vol. 8, Chapter 2, pp 152-158.
(4) Budzelaar, P. H. M.; Frijns, H. G.; Orpen, A. G. Organometallics
1990, 9, 1222.
(2) Akita, M.; Ohta, K.; Takahasi, Y.; Hikichi, S.; Moro-Oka, Y.
Organometallics 1997, 16, 4121.
(5) Yang, H.; Lugan, N.; Mathieu, R. Organometallics 1997, 16, 2089.
10.1021/ic990634a CCC: $19.00 © 2000 American Chemical Society
Published on Web 01/27/2000

706 Inorganic Chemistry, Vol. 39, No. 4, 2000
Alonso et al.
1
Chart 1
In the H NMR spectra, the chemical shift of H6 of the Py
groups is also very sensitive to coordination.8,9 At room
temperature, 1 and 2 exhibit fast exchange of their Py groups,
which renders them equivalent (Scheme 1). For 1, the low
exchange limit is not reached because Tc is very low (the H6
signals are still in coalescence at -82 °C), thus preventing the
evaluation of ∆Gq. Fortunately, the exchange is slower in 2,
with Tc ) -70 °C for the H6 signals (k ) 840 s-1, ∆Gq ) 37.7
kJ mol-1). The low-temperature spectrum (Table 1) indicates
that only one Py group is coordinated, again supporting a square-
planar structure with a pendant Py arm, in accordance with the
IR spectroscopic data. The variation of rate of Py exchange with
the halo ligand (Cl > I) is consistent with the expected
associative mechanism, which should be faster the more
electrophilic the Rh center, i.e., the more electronegative and
less π-donating the halo ligand.
The six-membered ring formed by P,N coordination of the
ligand is nonplanar, although the barrier to conformational
change averaging it to planar is very low, as reported for related
complexes with PePy.6,9 Complexes 1 and 2 can give rise to
two chiral diastereoisomers (A and B in Scheme 1), each with
their corresponding enantiomers (A′ and B′). Two movements
are possible in these complexes, as shown in Scheme 1: (i) the
Py exchange just discussed and (ii) an inversion of the
conformation of the nonplanar metallacycle (boat inversion for
short). It can be seen in Scheme 1 that, although racemization
(A to A′ or B to B′) needs the two movements, just any one of
the two is enough to convert one diastereoisomer into the other
with neglect of chirality (A to either B or B′; B to either or A′).
Since NMR is not sensitive to optical activity, different
diastereoisomers will be observed by NMR only if both
movements are frozen. Conversely, the fact that only one
product is observed for 2 below coalescence of the Py exchange
indicates that the boat inversion is still fast at that temperature
and is responsible for the interconversion of diastereoisomers.
ligand, giving complexes of type F. The results obtained for
PePy assist in the interpretation of the more complex behavior
of PePy2.
Results and Discussion
The compounds prepared, along with relevant spectroscopic
data, are listed in Table 1. Their syntheses are straightforward.
[RhCl(CO)(PePy2)] (1) was prepared by addition of [Rh2(µ-
Cl)2(CO)4] to a solution containing a stoichiometric amount of
PePy2. Complex 1 reacted with an excess of NaI to give [RhI-
(CO)(PePy2)] (2) and with AgBF4 or with TlBF4 to give [Rh2-
(CO)2(PePy2)2](BF4)2 (3).
The diolefin complexes, [Rh(TFB)(PePy)](PF6) (4; TFB )
tetrafluorobenzobarrelene), [Rh(COD)(PePy)](BF4) (5; COD )
1,5-cyclooctadiene), ([Rh(NBD)(PePy2)](PF6) (6; NBD ) nor-
bornadiene), [Rh(TFB)(PePy2)](PF6) (7), and [Rh(COD)(PePy2)]-
(BF4) (8), were prepared as yellow crystalline solids by reaction
of PePy or PePy2 with the corresponding dimers [Rh2(µ-Cl)2-
(diene)2] in the presence of TlBF4, TlPF6, or AgBF4. Bubbling
CO through solutions containing [Rh(diene)(PePy2)]+ is another
way of obtaining complex 3 in high yield.
Halo Carbonyl Complexes (1 and 2). For ligands containing
pyridyl groups, the analysis of the IR spectra indicates whether
they are coordinated or not, by the changes in frequency of νPy-
(CN).6,7 The uncoordinated Py groups exhibit this band at 1590
cm-1 (PePy) and 1592 cm-1 (PePy2), whereas when they are
coordinated, the band moves to 1600-1610 cm-1. The IR
spectra of 1 and 2 in the solid state show two such bands, one
due to coordinated and one to free Py. No significant changes
are observed in their IR spectra in CH2Cl2 solution (Table 1).
Thus, only one of the two Py groups is coordinated and the
compounds are square-planar. Since their IR spectra are almost
identical to that of [RhCl(CO)(PePy)], characterized previously
by Pignolet et al.,6 1 and 2 are assigned the same stereochemistry
with the CO and the phosphine ligands in mutually cis positions.
It is worth noting that the opposite order of rate for these
two movements is found in the complexes [Rh(diene){P(bzN)3}]-
(BF4) (bzN ) 2-((dimethylamino)methyl)phenyl), where the
inversion of the boat is much slower than the exchange of amine
groups, producing exchange with retention of configuration of
the boat. Thus the nature of the chelate group exerts a dramatic
influence on the ease of boat inversion, even though both ligands
give rise to six-membered chelating rings including one ortho-
substituted aromatic ring.10 The low energy associated with the
conformational change in PePy ligands is possibly associated
with the presence of three consecutive sp3 stereocenters, which
makes possible not only boat conformations but also chairlike
and twist conformations. The two extreme boat conformations
may be connected by twisted conformations which are not much
higher in energy.
[Rh2(CO)2(PePy2)2](BF4)2 (3). The solid-state IR spectrum
of 3 shows two CO stretching bands at 1978 and 2002 cm-1
separated by only 24 cm-1. This suggests a dimeric structure
in which two CO ligands are located in different metallic centers
(8) (a) Wajda-Hermanowicz, K.; Pruchnik, F. Transition Met. Chem. 1988,
13, 101. (b) Brown, D. G.; Byers, P. K.; Canty, A. J. Organometallics
1990, 9, 1231-1235. (c) Byers, P. K.; Canty, A. J.; Honeyman, R. T.
J. Organomet. Chem. 1990, 385, 417-427. (d) Casares, J. C.; Coco,
S.; Espinet, P.; Lin, Y.-S. Organometallics 1995, 14, 3058-3067. (e)
Casares, J. A.; Espinet, P.; Mart´ınez de Ilarduya, J. M.; Lin, Y. S.
Organometallics 1997, 16, 770-779.
(9) Casares, J. A.; Espinet, P.; Soulantica, K.; Pascual, I.; Orpen, A. G.
Inorg. Chem. 1997, 36, 5251.
(10) Alonso, M. A.; Casares, J. A.; Espinet, P.; Soulantica, K. Angew.
Chem., Int. Ed. Engl. 1999, 38, 533.
(6) Anderson, M. P.; Casalnuovo, A. L.; Johnson, B. J.; Mattson, B. N.;
Mueting, A. M.; Pignolet, L. H. Inorg. Chem. 1988, 27, 1649.
(7) (a) Wang, H.-H.; Casalnuovo, A. L.; Johnson, B. J.; Mueting, A. M.;
Pignolet, L. H. Inorg. Chem. 1988, 27, 325. (b) Costella, L.; Del Zotto,
A.; Mezzetti, A.; Zangrando, E.; Rigo, P. J. Chem. Soc., Dalton Trans.
1994, 3001. (c) Del Zotto, A.; Mezzetti, A.; Rigo, P. J. Chem. Soc.,
Dalton Trans. 1995, 2257. (d) Del Zotto, A.; Nardin, G.; Rigo, P. J.
Chem. Soc., Dalton Trans. 1996, 334.

Rh Complexes with Chelating and Binucleating Ligands
Inorganic Chemistry, Vol. 39, No. 4, 2000 707
Table 1. Relevant Spectroscopic Data for Complexes 1-8
IR: ν(CN)Py, cm-1
1H NMR:
1H NMR:
31P NMR:
compound
T, °C
solid
Cl2CH2
δ(Py H6), ppma
δ(olefinic signals), ppma
δ, ppm (1JRhP, Hz)
[RhCl(CO)(PePy2)] (1)b
[RhI(CO)(PePy2)] (2)c
rt
rt
-90
1606, 1587 1608, 1596 8.79 (2)
1605, 1593 1608, 1595 8.90 (2)
44.0 (161.5)
41.8 (168.4)
9.12 (1), 8.49 (1)
[Rh2(CO)2(PePy2)2](BF4)2 (3)d,e rt
1606
1608
(a) 8.76 (1), 7.81 (1)
(b) 8.66 (1), 7.72 (1)
8.67 (1) 5.70 (2), 3.65 (2)
45.7 (148.5)
47.3 (148.2)
34.0 (166.9)
34.2 (148.9)
35.5 (144.7)
[Rh(TFB)(PePy)](PF6) (4)d
[Rh(COD)(PePy)](BF4) (5)d
[Rh(NBD)(PePy2)](BF4) (6)b
rt
rt
rt
1609
1606
1600
1602
1608
1608
8.87 (1)
8.87 (2)
5.26 (2), 3.79 (2)
3.43 (4)
-90
[Rh(TFB)(PePy2)](PF6) (7)b
[Rh(COD)(PePy2)](BF4) (8)d
rt
1604
1608
8.94 (2)
10.07 (1), 7.73 (1)
3.45 (4)
43.4 (146.3)
47.9 (145.8)
24.93 (145.2)
(a) 24.0 (146)
(b) 28.2 (140)
-90
rt
-90
5.38 (1), 4.95 (1), 1.90 (1), 1.25 (1)
4.6 (4)
1606, 1593 1606, 1596 8.39 (2)
(a) 8.40 (1), 8.18 (1) (a) 5.1 (1), 4.7 (2), 4.2 (1)
(b) 8.40 (2) (b) 4.7 (2), 3.2 (2)
a The number in parentheses corresponds to the number of protons for each signal. b Acetone-d6. c CDCl3. d CD2Cl2. e Two isomers observed in
solution, being (a) the major and (b) the minor.
Scheme 1. Exchange Outline for Complex 1.
of them, the two Py groups are inequivalent, in agreement with
the proposed structures. The H6 chemical shifts indicate that
all the Py groups are coordinated. The low solubility of the
product precluded dissolution of 3 at low temperature in order
to determine whether the cis isomer, found in the solid state,
corresponds to the major or to the minor isomer in solution.
However, the IR spectrum suggests that the trans isomer (for
which only one ν(CO) is expected) is the predominant com-
pound in solution.
An X-ray study of the compound was undertaken. The
compound crystallizes in space group P21/n with unit cell
parameters a ) 12.652(3) Å, b ) 26.107(7) Å, c ) 16.753(6)
Å, â ) 93.22(3)°, and Z ) 4. Although the refinement of the
data could not be taken to acceptable values, there was no doubt
that the structure is similar to that reported for the complex
[Pd2(C6Cl2F3)2{PePy2}2](BF4)2 with Rh-CO in place of Pd-
C6Cl2F3.9 Thus the structure corresponds to that at left in Scheme
2.
Scheme 2. Equilibrium between Two Isomers of Complex
3.
Diene Complexes. The complexes with PePy must neces-
sarily be square planar, and accordingly [Rh(TFB)(PePy)](PF6)
(4) and [Rh(COD)(PePy)](BF4) (5) in the solid state show only
signals of coordinated Py groups in their IR spectra. Complexes
with PePy2 can be either square planar or pentacoordinated, and
indeed both situations are observed, depending on the diene.
The IR spectra of [Rh(NBD)(PePy2)](PF6) (6) and [Rh(TFB)-
(PePy2)](PF6) (7) show only bands due to coordinated Py; hence
the complexes are pentacoordinated. However, since the IR
spectrum of [Rh(COD)(PePy2)](BF4) (8) shows two ν(CN)
bands, one due to coordinated Py and one due to uncoordinated
Py, 8 must be a square-planar complex with a pendant Py group.
(the usual separation in cis dicarbonyl complexes is higher,
typically 50-70 cm-1).11
In the IR spectrum of 3 in the solid state, the presence of
The structure of 6 was ascertained by X-ray diffraction
methods and is shown in Figure 1. Selected bond distances and
angles are given in Table 2. The geometry of 6 can be described
as a sort of piano stool, with the norbornadiene ligand forming
the seat, or as a very distorted TBP. The P atom and one olefinic
bond are almost trans, with an angle M1-Rh-P of 161.8° [M1
being the midpoint of C(1)-C(6)]. The other olefinic bond is
coordinated in the plane defined by N(1), N(2), and the rhodium
center, the equatorial plane of the TBP structure. The π-back-
bonding in the equatorial plane is stronger than that in the axial
position, as indicated by the shorter Rh-C(olefinic) and longer
C-C(olefinic) bonds for the equatorial alkene when compared
with those in the axial position. This effect is usually found in
pentacoordinated norbornadiene complexes of rhodium(I)12,13
and agrees with calculations for pentacoordinated d8 com-
only one νPy(CN) band, at a frequency higher than 1600 cm-1
,
indicates that both Py groups of the PePy2 ligand are coordi-
nated. In CH2Cl2 solution, the IR spectrum of 3 shows a quite
broad ν(CO) band at 2011 cm-1 with a much less intense
shoulder at about 1980 cm-1, suggesting the presence of
different isomers or conformers in solution. This is confirmed
by the presence of two doublets in a 1:0.22 ratio in the 31P
spectrum at room temperature (Table 1). Their similarity in
chemical shifts and coupling constants suggests that they
correspond to the trans and cis isomers depicted in Scheme 2.
1
Both isomers are also observable in the H spectrum. In each
(11) Huges, R. P. In ComprehensiVe Coordination Chemistry; Wilkinson,
G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon Press: Oxford, U.
K., 1982; Vol. 5, Chapter 35, pp 277-540.

708 Inorganic Chemistry, Vol. 39, No. 4, 2000
Alonso et al.
known and has been extensively studied for rhodium(I) pyra-
zolylborate complexes and more recently for other bi- and
tridentate nitrogen-containing ligands.12,15 It has been proposed
to occur by a Berry mechanism in pentacoordinated intermedi-
ates formed by coordination of pendant arms of the ligands,
coordinating anions such Cl-, or coordinating solvents.
Complexes 6 and 7 show only one sharp doublet in their 31
P
NMR spectra at all temperatures. Their 1H NMR spectra show
only signals for one pentacoordinated isomer at all temperatures
studied. At room temperature they show equivalence of the
olefinic protons and the Py rings. At -90 °C, all the signals
are in coalescence for 6, but for 7, they are below coalescence
and two inequivalent Py rings, both coordinated, are observed.
1
Figure 1. Molecular structure of complex 6 showing the labeling
scheme. The PF6 anion, the molecule of CH2Cl2, and all hydrogens
have been omitted for clarity.
At the same time, all the protons (in the H spectrum) and all
-
the F atoms (in the 19F spectrum) of the TFB ligand appear
1
inequivalent. In other words, the low-temperature H and 19F
NMR spectra of 7 correspond to the situation expected from
the structure observed in the solid, while the room-temperature
spectrum requires (i) fast site exchange of the two double bonds
and (ii) fast inversion of the boat conformation of the metal-
lacycles involving the Py groups. Each movement alone cannot
produce all the equivalencies observed. For instance, the
equivalence of the fluorines by pairs is produced by any of them.
On the contrary, (i) would produce equivalence of olefinic
protons by pairs in two different double bonds, whereas (ii)
would produce equivalence by pairs of signals from the same
double bond, but total equivalence requires the occurrence of
both processes. The exchange is outlined in Scheme 3. The
rotation motion (i) is represented in a box for each conformation
and exchanges even or odd olefinic protons between them. The
conformational change means movement from one box to the
other (i.e., C to C′, D to D′, etc.) and makes equivalent the
olefinic protons of each double bond.
At higher temperatures (-88 °C), the boat inversion in 7
broadens all the signals on the spectrum and eventually leads
to the coalescence of the H6 signals of the Py groups at Tc )
-69 °C, which corresponds to a ∆Gq of 36.9 kJ mol-1 (k )
1560 s-1).16 The process is also observable by 19F NMR, where
the fluorine signals become equivalent two by two in the same
temperature range. Also in this temperature interval, the
exchange of the coordination sites of the TFB ligand by rotation
starts to be observed and the olefinic signals coalesce to one at
room temperature. For 6, the fluxional processes are faster and
only the averaged signals are observed in the temperature
window of the solvent (acetone-d6). Thus, again a fast confor-
mational change is found compared to the case of the complexes
with P(bzN)n,10 despite the apparently more crowded situation
in a pentacoordinated structure.
Table 2. Selected Bond Lengths (Å) and Angles (deg) for 6
Rh(1)-C(4)
Rh(1)-C(3)
Rh(1)-N(2)
Rh(1)-P(1)
Rh(1)-C(6)
Rh(1)-C(1)
Rh(1)-N(1)
2.072(2)
2.095(2)
2.190(2)
2.2579(5)
2.259(2)
2.279(2)
2.382(2)
C(4)-Rh(1)-N(2)
C(3)-Rh(1)-N(2)
C(4)-Rh(1)-P(1)
C(3)-Rh(1)-P(1)
N(2)-Rh(1)-P(1)
C(4)-Rh(1)-C(6)
161.74(8)
124.57(9)
100.06(7)
92.99(6)
89.48(4)
66.44(9)
C(3)-Rh(1)-C(6)
N(2)-Rh(1)-C(6)
P(1)-Rh(1)-C(6)
C(4)-Rh(1)-C(1)
C(3)-Rh(1)-C(1)
N(2)-Rh(1)-C(1)
P(1)-Rh(1)-C(1)
77.59(9)
C(6)-Rh(1)-C(1)
C(4)-Rh(1)-N(1)
C(3)-Rh(1)-N(1)
N(2)-Rh(1)-N(1)
P(1)-Rh(1)-N(1)
C(6)-Rh(1)-N(1)
C(1)-Rh(1)-N(1)
34.74(8)
104.56(8)
144.50(9)
90.73(6)
90.69(4)
91.46(7)
121.68(8)
103.89(7)
166.42(6)
77.54(9)
63.93(9)
86.01(7)
147.32(7)
pounds.14 The bite angle found for the NBD ligand is 69.3°.
Each of the pairs of the arms of the PePy2 ligand has a bite
angle of ca. 90° at rhodium, and each six-membered PN-Rh
ring adopts a boat conformation. Both boats have the same
orientation, decreasing the steric interaction between the Py
groups. In this conformation, the Py rings are forced to form
very different angles with respect to the equatorial plane of
the TBP and very different Rh-N distances (2.190(2) Å Rh-
N(2) and 2.382(2) Å Rh-N(1)). These distances are slightly
longer than others found in analogous Rh(I) complexes contain-
ing NBD as a ligand.12
Behavior of Complexes 4-8 in Solution. The solution IR
spectra of compounds 4-7 indicate that these complexes retain
in solution the same structures as in the solid state, with all the
Py groups coordinated, i.e., square planar for 4 and 5 and
pentacoordinated for 6 and 7. The IR spectrum of compound 8
in solution shows bands at 1596 cm-1 (noncoordinated Py) and
at 1608 cm-1 (broad band, coordinated Py). In fact, since the
NMR spectra show that, in solution, 8 is a mixture of two
isomers (see below), the IR data imply only that one of them is
square planar with a pendant Py.
The complex [Rh(COD)PePy2](BF4) (8) behaves quite dif-
ferently. At -90 °C, its 31P NMR spectrum reveals the existence
of two different isomers in a 1.5:1 ratio, which are also seen in
the 1H NMR spectrum (Table 1). A COSY experiment allowed
us to assign the signals, which correspond to one square-planar
(major) and one trigonal-bipyramidal (minor) isomer (Figure
2). This assignment is further supported by a 31P-1H correlation
experiment, which shows the couplings of each olefinic proton
signal with its corresponding 31P signal and also agrees with
In accord with the fast conformational change found for 1
and 2 and for related complexes,6,9 [Rh(TFB)(PePy)](PF6) (4)
shows, even at low temperature, equivalence for the protons of
each coordinated double bond, the methylenic protons of the
PePy ligand, and the two pairs of fluorine atoms. The same
applies to [Rh(COD)(PePy)](BF4) (5).
Another process is observed: the exchange of the olefinic
protons trans to P and trans to N. This phenomenon is well-
(15) (a) Cocivera, M.; Ferguson, G.; Lalor, F. J.; Szczecinski, P. Organo-
metallics 1982, 1, 1139. (b) Vrieze, K.; van Leeuwen, P. W. N. M.
Prog. Inorg. Chem. 1971, 14, 1. (c) Heitner, H. I.; Lippard, S. J. Inorg.
Chem. 1972, 11, 1447. (d) Kemmitt, R. D. W. J. Organomet. Chem.
1979, 176, 339.
(12) Haarman, H. F.; Bregman, F. R.; Ernsting, J. M.; Veldman, N.; Spek,
A. L.; Vrieze, K. Organometallics 1997, 16, 54 and references therein.
(13) Bucher, U. E.; Currao, A.; Nesper, R.; Ru¨eger, H.; Venanzi, L. M.;
Younger, E. Inorg. Chem. 1995, 34, 66.
(16) Gq has been calculated by application of the Eyring equation to the k
values obtained as k ) π∂ν/x2. See: Sandstrom, J. Dynamic NMR
Spectroscopy; Academic Press: London, 1982.
(14) Rossi, A. R.; Hoffmann, R. Inorg. Chem. 1975, 14, 365.

Rh Complexes with Chelating and Binucleating Ligands
Inorganic Chemistry, Vol. 39, No. 4, 2000 709
Figure 3. 13C-1H correlation spectrum in CD2Cl2 at -90 °C (only
olefinic signals shown): (*) solvent.
Figure 2. COSY spectrum of 8 at -90 °C in CD2Cl2: (1) olefinic
signals of the square-planar isomer; (2) olefinic signals of the trigonal-
bipyramidal isomer. (*) solvent signal.
Scheme 3. Exchanges in 7.a
Figure 4. EXSY NMR spectrum of 8 at -90 °C in CD2Cl2.
ppm correlate with signals at 104.0, 75.9, and 72.5 ppm,
respectively.17 This is again in agreement with the proposed
assignment.
In the minor isomer (pentacoordinated), the two Py groups
are equivalent, and only two olefinic signals are observed due
to the fast conformational change on the chelated rings, as
previously described for compound 6. The equilibrium between
these isomers has been studied by an EXSY experiment at -90
°C (Figure 4). It shows that each Py group of the square-planar
isomer is exchanging with the Py groups of the pentacoordinated
isomer and also that the olefinic protons of both complexes are
exchanging. Particularly informative is the fact that the olefinic
signal of the pentacoordinated complex at 3.2 ppm does not
exchange with the olefinic proton signal at 5.1 ppm while it
gives strong cross-peaks with the olefinic proton signals at 4.2
and 4.7 ppm. This means that each double bond of the square-
planar complex exchanges with just one double bond of the
trigonal-bipyramidal isomer. Since an exchange of the Py groups
on 6 is possible without moving the coordinated phosphorus
(in the trigonal-bipyramidal structure both Py groups are in
a The Rotation of the Diolefin Exchanges the Olefinic Protons 1 with
3 and 2 with 4. The simultaneous conformational changes of the
chelated rings allow for the exchange of the four olefinic protons. The
different conformations of the metallacycles are indicated by the
different orientations of the substituents at the N atoms.
the IR spectrum described before. Furthermore, a 1H-13C
correlation experiment (Figure 3) shows that the olefinic signal
at 4.7 ppm is in effect the result of three different protons
overlapping, as it is related to three 13C signals at 77.8, 103.0,
and 103.8 ppm, while the olefinic signals at 5.1, 4.2, and 3.2
(17) We are grateful to an anonymous reviewer for suggesting this
experiment.

710 Inorganic Chemistry, Vol. 39, No. 4, 2000
Alonso et al.
Hz). IR (Nujol mull, cm-1): 1974 vs, ν(CO); 1587 m, 1606 m, ν(CN).
IR (CH2Cl2, cm-1): 1991 vs, ν(CO); 1596 m, 1608 m, ν(CN).
(b) [RhI(CO)(PePy2)]‚0.5CH3COCH3 (2). To a solution of [Rh2-
(CO)2(PePy2)2](BF4)2 (300 mg, 0.28 mmol) in acetone (15 mL) was
added sodium iodide (340 mg, 2.28 mmol). After 1 h, the solution was
filtered, the acetone was evaporated to 2 mL, and ethanol (10 mL)
was added. The compound was crystallized by cooling to -20 °C as
brown crystals, which were filtered off and vacuum-dried. Yield: 275
mg (80%). Anal. Calcd for C22.5H24IN2O1.5PRh: C, 44.50; N, 4.61; H,
3.98. Found: C, 44.48; N, 4.58; H, 3.93. 1H NMR (CDCl3, rt): δ 8.90
(m, 2H); δ 7.78, (m, 2H); δ 7.60 (m, 2H); δ 7.53-7.09 (m, 7H); δ
3.15 (m, 4H); δ 2.43 (m, 2H); δ 2.24, (m, 2H). 31P NMR (CDCl3, rt):
δ 41.8 (d, JRh-P ) 168.4 Hz). IR (Nujol mull, cm-1): 1966 vs, 1730
s, ν(CO); 1605 m, 1593 s, ν(CN). IR (CH2Cl2, cm-1): 1987 vs, ν-
(CO); 1608 m, 1595 m, ν(CN).
equatorial positions), the slow fluxionality of the trigonal-
bipyramidal structure does not affect the Py exchange rate,
which is only related to the rate of exchange between the square-
planar and bipyramidal-trigonal structures. Hence, the equilib-
rium keeps one olefin trans to the phosphorus and the other
trans to the nitrogen. This implies that the rotation of the olefin
on the pentacoordinated complex is slower than the equilibrium
between square-planar and trigonal-bipyramidal isomers.
This exchange mechanism is consistent with the evolution
of the signals during the variable-temperature experiments. With
an increase in temperature, a coalescence is observed (Tc ) -50
°C). This affects all the olefinic signals in such a way that each
olefin of the square-planar isomer coalesces with just one of
the trigonal-bipyramidal isomers. Between -40 and 0 °C, two
olefinic signals are observable. When the temperature is raised
to 10 °C, the two olefinic signals that were observable at 0 °C
reach a new coalescence and become equivalent at 50 °C. This
second coalescence is related to the site exchange of the olefin
(axial-equatorial) in the trigonal-bipyramidal structure.
(c) [Rh2(CO)2(PePy2)2](BF4)2 (3). To a solution of [Rh2(µ-Cl)2-
(COD)2] (343 mg, 0.695 mmol) in acetone (20 mL) were added PePy2
(445 mg, 1.39 mmol) and TlBF4 (405 mg, 1.39 mmol). The solution
was stirred during 1 h, filtered, and allowed to stir under CO for 2 h.
During this time, the complex, a pale yellow product, crystallized. The
addition of diethyl ether (15 mL) completed the precipitation. The
product was filtered off, washed with diethyl ether, and vacuum-dried.
Yield: 626 mg (90%). Anal. Calcd for C42H42B2F8N4O2P2Rh2: C, 46.87;
1
Simultaneously with the first coalescence in H NMR, the
31P NMR spectrum shows the coalescence of the two signals
that correspond to the square-planar and the pentacoordinated
isomers. Line shape analysis of the 31P signals at -61 °C gives
a value of kex ) 70 s-1 (∆Gq ) 43.8 kJ mol-1).18
1
H, 3.77; N, 5.20. Found: C, 46.81; H, 3.72; N, 5.16. H NMR (CD2-
Cl2, rt) for major isomer, trans-[Rh2(CO)2(PePy2)2](BF4)2: δ 8.76 (m,
1H); δ 7.81 (m, 1H); δ 8.04 (m, 2H); δ 7.98 (m, 1H); δ 7.87 (m, 2H);
δ 7.56 (m, 4H); δ 7.43 (m, 1H); δ 7.14 (m, 1H); δ 4.96 (m, 1H); δ
3.73 (m, 1H); δ 3.48 (m, 2H); δ 3.06 (m, 1H); δ 2.58 (m, 2H); δ 2.28
(m, 1H). 1H NMR for minor isomer, cis-[Rh2(CO)2(PePy2)2](BF4)2: δ
8.66 (m, 1H); δ 7.72 (m, 1H); δ 8.22 (m, 2H); δ 8.07 (m, 2H); δ 7.8
(m, 1H); δ 7.64 (m, 2H); δ 7.55 (m, 1H); δ 7.45 (m, 2H); δ 7.01 (m,
1H); δ 4.43 (m, 1H); δ 4.21 (m, 1H); δ 2.34 (m, 2H); 2.94 (m, 1H);
δ 2.76 (m, 1H); δ 2.35 (m, 1H); δ 2.20 (m, 1H). 31P NMR (CD2Cl2,
rt): major isomer δ 45.7 (d, JRh-P ) 148.5 Hz); minor isomer δ 47.3
(d, JRh-P ) 148.2 Hz). IR (Nujol mull, cm-1): 2002 vs, 1978 vs, ν-
(CO); 1606 s, ν(CN). IR (CH2Cl2, cm-1): 2011 vs, 1980 sh, ν(CO).
(d) [Rh(TFB)(PePy)](PF6) (4). To a stirred suspension of [Rh2(µ-
Cl)2(TFB)2] (100 mg, 0.137 mmol) in acetone (10 mL) were added
PePy (88 mg, 0.274 mmol) and AgNO3 (48 mg, 0.274 mmol). After
24 h, the precipitate of AgCl was filtered off, the acetone was
evaporated, and ethanol (10 mL) was added. Upon the addition of NH4-
PF6 (88 mg, 0.536 mmol) dissolved in the smallest amount of ethanol,
the product precipitated as an orange powder, which was filtered off,
washed with 2 mL of CHCl3, and vacuum-dried. Yield: 153 mg (75%).
Anal. Calcd for C31H24F10NP2Rh: C, 48.65; N, 1.83; H, 3.16. Found:
C, 48.59; N, 2.02; H, 3.24. 1H NMR (acetone-d6, rt): δ 8.67 (m, 1H);
δ 7.93 (m, 2H); δ 7.72-7.41 (m, 11H); δ 5.99 (m, 2H); δ 5.70 (br,
2H); δ 3.99 (m, 1H); δ 3.90 (m, 1H); δ 3.65 (br, 2H); δ 2.66 (m, 2H).
19F NMR (acetone-d6, rt): δ -147.0 (m, 2F); δ -160.0 (m, 2F); δ
-71.0 (d, JP-F ) 708 Hz, 6F). 31P NMR (acetone-d6, rt): δ 34.0 (d,
JRhp ) 166.9). IR (Nujol mull, cm-1): 1609 s, ν(CN). IR (CH2Cl2,
cm-1): 1602 s, ν(CN).
(e) [Rh(COD)(PePy)](BF4) (5). To a solution of [Rh2(µ-Cl)2(1,5-
COD)2] (120 mg, 0.243 mmol) in THF (20 mL) was added AgBF4
(94.7 mg, 0.487 mmol). The mixture was stirred for 30 min, and the
precipitate of AgCl was filtered off. PePy (142 mg, 0.487 mmol) was
added to the solution. The mixture was stirred for 1 h. Concentration
of the clear solution yielded yellow crystals, which were filtered off
and vacuum-dried. Yield: 260 mg (90%). Anal. Calcd for C27H30BF4-
NPRh: C, 55.04; N, 2.38; H, 5.13. Found: C, 55.11; N, 2.23; H, 5.12.
1H NMR (acetone-d6, rt): δ 8.87 (m, 1H); δ 7.80 (m, 1H); δ 7.63 (m,
5H); δ 7.44 (m, 6H); δ 7.32 (m, 1H); δ 5.26, (m, 2H); δ 4.01 (m, 2H);
δ 3.79 (m, 2H); δ 2.83, (m, 2H); δ 2.55 (m, 4H); δ 2.24 (m, 4H). 31P
NMR (acetone-d6, rt): δ 34.2 (d, JRh-P ) 148.9 Hz). IR (KBr pellet,
cm-1): 1606 s, ν(CN).
This exchange pattern confirms the assignment of the isomers
and constitutes a rare example of equilibrium in which the
fluxionality in a pentacoordinated intermediate is slower than
the equilibrium between penta- and tetracoordinate complexes.
Experimental Section
General Methods and NMR Techniques. All reactions were carried
out under N2. Solvents were distilled using standard methods. The
complexes [Rh2(µ-Cl)2(1,5-COD)2]19 and [Rh2(µ-Cl)2(TFB)2],20 [Rh2-
(µ-Cl)2(CO)4], the ligand PePy2, and TlBF4 were prepared by published
methods.21-23
IR spectra were recorded on a Perkin-Elmer FT 1720 X spectro-
photometer. Combustion CHN analyses were made on a Perkin-Elmer
1
2400 CHN microanalyzer. H NMR (300.16 MHz), 19F NMR (282.4
MHz), and 31P NMR (121.4 MHz) spectra were recorded on a Bruker
ARX 300 instrument equipped with a VT-100 variable-temperature
probe. Chemical shifts are reported in ppm from tetramethylsilane (1H),
CCl3F (19F), or H3PO4 (85%) (31P), with positive shifts downfield, at
ambient probe temperature unless otherwise stated. NOESY spectra
were recorded in the phase-sensitive mode, using the average of the
relaxation times as mixing time. The 1H-13C correlation spectra were
recorded in the inverse mode, with a HMQC sequence with BIRD
selection and GARP decoupling during acquisition.
Synthesis of the Complexes. (a) [RhCl(CO)(PePy2)] (1). A solution
of PePy2 (161 mg, 0.50 mmol) in CH2Cl2 (5 mL) was saturated with
CO by bubbling for 5 min. Then [Rh2(µ-Cl)2(CO)4] (97 mg, 0.25 mmol)
was added. After 30 min of stirring, n-hexane (12 mL) was added.
The complex precipitated as a yellow powder, which was filtered off
and vacuum-dried. Attempts at washing the solid with n-hexane or
diethyl ether led to the formation of oils. Yield: 219 mg (89%). Anal.
Calcd for C21H21ClN2OPRh: C, 51.82; N, 5.76; H, 4.35. Found: C,
51.62; N, 5.68; H, 4.26. 1H NMR (acetone-d6, rt (room temperature)):
δ 8.79 (m, 2H); δ 7.99-7.96 (m, 2H); δ 7.77 (m, 2H); δ 7.45 (m,
3H); δ 7.38 (m, 2H); δ 7.26 (m, 2H); δ 3.25 (m, 4H); δ 2.59 (m, 2H);
δ 2.32 (m, 2H). 31P NMR (acetone-d6, rt): δ 44.0 (d, JRhP ) 161.5
(18) DNMR6. QCPE No. 633, 1995.
(19) Giordano, G.; Crabtree, R. H. Inorg. Synth. 1990, 28, 84.
(20) Uso´n, R.; Oro, L. A.; Valderrama, M.; Claver, C. Synth. React. Inorg.
Met.-Org. Chem. 1979, 9, 577.
(21) MacCleverty, J. A.; Wilkinson, G. Inorg. Synth. 1990, 28, 84.
(22) Arnaiz, J. J. Chem. Educ. 1997, 74, 1332.
(23) (a) Hogeveen, H.; Nusse, B. J. J. Organomet. Chem. 1979, 171, 237.
(b) Chen, M. J.; Feder, H. M. Inorg. Chem. 1979, 18, 4.
(f) [Rh(NBD)(PePy2)](PF6)‚CH2Cl2 (6). To a stirred suspension of
[Rh2(µ-Cl)2(NBD)2] (152 mg, 0.329 mmol) in acetone (15 mL) were
added PePy2 (211 mg, 0.659 mmol) and TlBF4 (192 mg, 0.659 mmol).
After 1 h, the TlCl was filtered off, the solvent was evaporated, and
ethanol (10 mL) was added. Upon addition of NH4PF6 (536 mg, 3.28
mmol) dissolved in the smallest amount of ethanol, the product

Rh Complexes with Chelating and Binucleating Ligands
Inorganic Chemistry, Vol. 39, No. 4, 2000 711
Table 3. Crystallographic Data for 6
precipitated as a yellow sticky solid. The ethanol was filtered off and
the solid was washed twice with diethyl ether (15 mL each). The yellow
powder was filtered off and vacuum-dried. The compound was
recrystallized by slow diffusion in CH2Cl2/hexane. Yield: 220 mg
(85%). Anal. Calcd for C28H31Cl2F6N2P2Rh: C, 45.12; N, 3.76; H, 4.19.
empirical formula
fw
C28H31Cl2F6N2P2Rh
745.3
temp
173 K
wavelength
space group
unit cell dimens
0.710 73 Å
P21/n
1
Found: C, 45.04; N, 3.74; H, 4.22. H NMR (acetone-d6, rt): δ 8.87
a ) 8.455(1) Å
b ) 18.068(3) Å
c ) 19.729(3) Å
â ) 99.658(3)°
V ) 2971.1(8) Å3
4
(m, 2H); δ 7.94 (m, 2H); δ 7.57-7.37 (m, 9H); δ 3.43 (m, 2H); δ
3.70 (s, 2H); δ 3.43 (m, 4H); δ 2.71 (m, 2H); δ 2.40 (m, 2H); δ 2.17
(m, 2H); δ 1.22 (s, 2H). 31P (acetone-d6, rt): δ 35.5 (d, JRh-P ) 144.7
Hz). IR (Nujol mull, cm-1): 1600 s, ν(CN).
(g) [Rh(TFB)(PePy2)](PF6) (7). To a stirred suspension of [Rh2(µ-
Cl)2(TFB)2] (100 mg, 0.137 mmol) in acetone (10 mL) were added
PePy2 (91 mg, 0.284 mmol) and TlBF4 (82 mg, 0.284 mmol). After 1
h, the TlCl was filtered off, the acetone was evaporated, and ethanol
(10 mL) was added to the yellow oil. After the addition of NH4PF6
(223 mg, 1.37 mmol) dissolved in the smallest amount of ethanol, the
product precipitated as a yellow powder, which was filtered off, washed
with ethanol, and vacuum-dried. Yield: 180 mg (83%). Anal. Calcd
for C32H27BF8N2PRh: C, 52.20; N, 3.80; H, 3.70. Found: C, 52.25;
N, 3.71; H, 3.66. 1H NMR (acetone-d6, rt): δ 8.94 (m, 2H); δ 8.00 (m,
2H); δ 7.65-7.40 (m, 9H); δ 5.46 (m, 2H); δ 3.57 (m, 2H); δ 3.45
(br, 4H); δ 2.80 (m, 2H); δ 2.54 (m, 2H); δ 2.22 (m, 2H). 19F NMR
(acetone-d6, rt): δ -148 (m, 2F); δ -161.5 (m, 2F); δ -71.5 (d, 6F,
JP-F ) 708 Hz). 31P NMR (acetone-d6, rt): δ 43.52 (d, JRh-P ) 146.32
Hz). IR (Nujol mull, cm-1): 1604 m, ν(CN). IR (CH2Cl2, cm-1): 1608
m, ν(CN).
Z
density (calcd)
µ(Mo KR)
1.666 Mg/m3
0.923 mm-1
R indices [for 5468 reflns with I > 2σ(I)] R1 ) 0.0264, wR2 ) 0.0624
R indices (for all 6793 data)
R1 ) 0.0382, wR2 ) 0.0660
empirical absorption correction (SADABS) was applied.24 The structure
was solved by direct methods and refined on all F2 data using the
SHELX suite of programs on a Silicon Graphics Indy computer.25 All
non-hydrogen atoms were assigned anisotropic displacement parameters
and refined without positional constraints. Hydrogen atoms H(1)-H(6)-
were located in the electron density difference map, assigned isotropic
displacement parameters, and refined without positional constraints.
All other hydrogen atoms were constrained to ideal geometries and
refined with fixed isotropic displacement parameters. Refinement
proceeded smoothly to give the residuals shown in Table 3. Complex
neutral-atom scattering factors were used.26
(h) [Rh(COD)(PePy2)](BF4) (8). To a stirred solution of [Rh2(µ-
Cl)2(1,5-COD)2] (194 mg, 0.393 mmol) in THF (20 mL) was added
AgBF4 (152 mg, 0.785 mmol). After 2 h, the precipitate of AgCl was
filtered off and PePy2 (265 mg, 0.827 mmol) was added. The THF
was evaporated to 2 mL, and the complex was precipitated by addition
of diethyl ether (15 mL). The yellow powder formed after repetitive
washings with ether was filtered off and vacuum-dried. Yield: 410 mg
(84%). Anal. Calcd for C27H29BF4N2PRh: C, 54.39; N, 4.53; H, 5.38.
Acknowledgment. The Direccio´n General de Investigacio´n
Cient´ıfica
y Te´cnica (Projects PB96-0363 and SB95-
BOZ67791015) and the Junta de Castilla y Leo´n (Project VA40/
96) are gratefully acknowledged for financial support and
fellowships to K.S. and M.A.A.
1
Found: C, 54.24; N, 4.51; H, 5.48. H NMR (acetone-d6, rt): δ 8.76
Supporting Information Available: An X-ray crystallographic file,
in CIF format, for 6. This material is available free of charge via the
Internet at http://pubs.acs.org.
(d, J ) 5.1 Hz, 2H); δ 7.79 (m, 2H); δ 7.62 (m, 2H); δ 7.4, (m, 5H);
δ 7.26 (m, 2H); δ 4.7 (br, 4H); δ 3.4 (br, 4H); δ 2.60-2.3 (br, 12H).
31P NMR (acetone-d6, rt): δ 26.6 (d, JRh-P ) 145.7 Hz). IR (KBr pellet,
cm-1): 1606 s, 1593 s, ν(CN). IR (CH2Cl2, cm-1): 1606 s, 1596 s,
ν(CN).
IC990634A
(24) Sheldrick, G. M. SADABS: A program for absorption correction with
the Siemens SMART system; University of Go¨ttingen: Go¨ttingen,
Germany, 1996.
(25) SHELXTL program system, Version 5.03; Siemens Analytical X-ray
Instruments: Madison, WI, 1995.
(26) International Tables for Crystallography; Kluwer: Dordrecht, The
Netherlands, 1992; Vol. C.
Crystal Structure Determination of 6‚CH2Cl2. Crystals of 6‚
CH2Cl2 suitable for X-ray diffraction were obtained through recrys-
tallization of 6 by slow diffusion in dichloromethane/hexane A yellow
crystal (0.30 × 0.25 × 0.25 mm) was coated with high-vacuum grease
and mounted on a Siemens SMART diffractometer under a stream of
N2 at 173 K. Crystallographic data are summarized in Table 3. An