Conformational flexibility and molecular dynamics of cationic diolefin-rhodium(I) complexes with iminophosphine ligands

Article abstract of DOI:10.1016/S0022-328X(01)01033-6

In solution, the cationic complexes [Rh(η²,η²-diolefin)(P-N)]⁺ [diolefin=1,5-cyclooctadiene (cod), bicyclo[2.2.1]hepta-2,5-diene (nbd); P-N=o-(PPh₂)C₆H₄CH=NR (R=C₆H₃(i-Pr)₂-2,6 (1), CMe₃ (2), (R)-bornyl (3)] undergo a conformational change of the iminophosphine ligand which inverts the position of the six-membered chelate ring relative to the N-Rh-P coordination plane. The inversion is fast for [Rh(η²,η²-diolefin)(1)]⁺ and [Rh(η²,η²-diolefin)(3)]⁺ in the temperature range 298-183 K, and becomes progressively slower for [Rh(η²,η²-diolefin)(2)]⁺ with increasing steric demand of the coordinated ligands. From the coalescing signals in the ¹H-NMR spectrum of [Rh(nbd)(2)]⁺ at 208 K, a ΔH^# value of 28.5 kJmol^-1 and a ΔS^# value of -60.3 JK^-1mol^-1 can be estimated. The phase-sensitive 2D ¹H-NMR ROESY spectrum of [Rh(cod)(3)]⁺ reveals the presence of another dynamic process which slowly and selectively interconverts the two olefinic protons on the same HC=CH unit of the chelate diolefin. A mechanism is proposed involving the initial dissociation of the rhodium-olefin bond trans to phosphorus. Semi-empirical calculations show that in the most stable conformers of [Rh(nbd)(2)]⁺ and [Rh(cod)(2)]⁺ the P-N chelate ring is not coplanar with the N-Rh-P plane, while the diolefin coordination mode is distorted largely by the steric interaction with the bulky N-CMe₃ group. When the P-N chelate ring is forced to be coplanar with the N-Rh-P plane, the enthalpy content increases to 20.3 and 114.7 kJmol^-1 for [Rh(nbd)(2)]⁺ and [Rh(cod)(2)]⁺, respectively. Entropy factors seem to be predominant in the displacement of 1,5-cyclooctadiene by other olefinic ligands in the complexes [Rh(cod)(P-N)]⁺.

Full text of DOI:10.1016/S0022-328X(01)01033-6

Journal of Organometallic Chemistry 631 (2001) 117–124
www.elsevier.com/locate/jorganchem
Conformational flexibility and molecular dynamics of cationic
diolefin–rhodium(I) complexes with iminophosphine ligands
Bruno Crociani *, Simonetta Antonaroli, Maria Luisa Di Vona, Silvia Licoccia
Dipartimento di Scienze e Tecnologie Chimiche, Uni6ersita’ di Roma Tor Vergata, Via della Ricerca Scientifica 1, 00133 Rome, Italy
Received 2 April 2001; received in revised form 30 April 2001; accepted 30 April 2001
Abstract
In solution, the cationic complexes [Rh(h2,h2-diolefin)(PꢀN)]+ [diolefin=1,5-cyclooctadiene (cod), bicyclo[2.2.1]hepta-2,5-diene
(nbd); PꢀN=o-(PPh2)C6H4CHꢁNR (R=C6H3(i-Pr)2-2,6 (1), CMe3 (2), (R)-bornyl (3)] undergo a conformational change of the
iminophosphine ligand which inverts the position of the six-membered chelate ring relative to the NꢀRhꢀP coordination plane.
The inversion is fast for [Rh(h2,h2-diolefin)(1)]+ and [Rh(h2,h2-diolefin)(3)]+ in the temperature range 298–183 K, and becomes
progressively slower for [Rh(h2,h2-diolefin)(2)]+ with increasing steric demand of the coordinated ligands. From the coalescing
signals in the 1H-NMR spectrum of [Rh(nbd)(2)]+ at 208 K, a DHc value of 28.5 kJ mol−1 and a DSc value of −60.3
J K−1 mol−1 can be estimated. The phase-sensitive 2D 1H-NMR ROESY spectrum of [Rh(cod)(3)]+ reveals the presence of
another dynamic process which slowly and selectively interconverts the two olefinic protons on the same HCꢁCH unit of the
chelate diolefin. A mechanism is proposed involving the initial dissociation of the rhodium–olefin bond trans to phosphorus.
Semi-empirical calculations show that in the most stable conformers of [Rh(nbd)(2)]+ and [Rh(cod)(2)]+ the PꢀN chelate ring is
not coplanar with the NꢀRhꢀP plane, while the diolefin coordination mode is distorted largely by the steric interaction with the
bulky NꢀCMe3 group. When the PꢀN chelate ring is forced to be coplanar with the NꢀRhꢀP plane, the enthalpy content increases
to 20.3 and 114.7 kJ mol−1 for [Rh(nbd)(2)]+ and [Rh(cod)(2)]+, respectively. Entropy factors seem to be predominant in the
displacement of 1,5-cyclooctadiene by other olefinic ligands in the complexes [Rh(cod)(PꢀN)]+. © 2001 Elsevier Science B.V. All
rights reserved.
Keywords: Solution dynamics; [Rh(h2,h2-diolefin)(iminophosphine)]+; 2D-exchange NMR; Semi-empirical calculations
1. Introduction
dination plane [5]. In solution, however, the PꢀN lig-
ands undergo a low-energy conformational change
involving the inversion of the chelate ring, whereby the
phosphine phenyl groups interchange their positions,
while the N-substituent and the ꢀC6H4CHꢁ unit move
above and below the NꢀPdꢀP plane [5].
Rhodium(I) complexes with P,N-donor bidentate lig-
ands have also been used in the catalytic hydroformyla-
tion [6] and asymmetric hydroboration [7] of olefins, as
well as in the catalytic hydrogenation of arenes [8].
In order to verify that conformational rearrange-
ments similar to those observed in palladium deriva-
tives would occur also in rhodium complexes and to get
more information about the influence of steric factors
on such processes, we have studied the solution be-
haviour of a series of cationic h2,h2-diolefin–rhodi-
um(I) complexes with iminophosphines carrying bulky
N-substituents. A better understanding of the molecular
Recently, palladium complexes with iminophosphine
ligands such as N-[(2-diphenylphosphino)benzylidene]-
amines (PꢀN) have been employed successfully as cata-
lysts (or catalysts precursors) in the cross-coupling of
aryl halides with organostannanes [1], carbostannyla-
tion of alkynes [1], oligomerisation of ethene [2], CO/
olefin copolymerisation [3], and asymmetric allylic
alkylation [4]. From X-ray structural studies on palladi-
um(II) and palladium(0) derivatives it appears that the
six-membered chelate ring formed by the P,N-bonded
iminophosphine is not coplanar with the NꢀPdꢀP coor-
* Corresponding author. Tel.: +39-6-7259-4389; fax: +39-6-7259-
4328.
E-mail address: crociani@stc.uniroma2.it (B. Crociani).
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(01)01033-6

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B. Crociani et al. / Journal of Organometallic Chemistry 631 (2001) 117–124
dynamics of these compounds is a prerequisite for their
application in catalysis.
(LꢀL%=P,N-bidentate ligands) [11]. Two sets of olefinic
proton resonances are observed: one at lower field
assigned to the H1 and H2 protons trans to P, and the
other at higher field assigned to the H3 and H4 protons
trans to N, in agreement with literature data [11,12].
This assignment is supported strongly by the inter-
2. Results and discussion
1
2.1. Preparation and characterisation of the complexes
ligand NOEs in the 2D H-NMR ROESY spectra of
[Rh(cod)(3)]+ (discussed later).
The complexes [Rh(h2,h2-diolefin)(PꢀN)]BF4 [dio-
lefin=1,5-cyclooctadiene (cod), norbornadiene (nbd);
PꢀN=o-(Ph2P)C6H4CHꢁNR] are obtained readily
from the reaction of [Rh(h2,h2-diolefin)2]BF4 with the
appropriate iminophosphine (Eq. (1) of Scheme 1).
Anhydrous solvents are required generally since the
products decompose slowly if trace amounts of water
are present in solution. A PꢀN/Rh molar ratio of 1:1 is
also required because an excess of iminophosphine
leads to the formation of the cationic species
[Rh(PꢀN)2]+, identified by the characteristic 31P-NMR
resonances in the range 55–51 ppm [9].
2.2. Dynamic beha6iour in solution
The solution behaviour of the tetrafluoborate salts
1
has been studied by H-NMR spectra at different tem-
1
peratures in CD2Cl2 and also by 2D H-NMR ROESY
experiments for [Rh(cod)(3)]BF4 and [Rh(cod)(3)]ClO4
in both CD2Cl2 and CDCl3 solvents. The spectral fea-
tures observed suggest that all the cationic derivatives
are fluxional. For [Rh(cod)(1)]+ and [Rh(nbd)(1)]+,
only two olefinic proton signals are detected in the
temperature range 298–183 K, indicating that the pro-
tons H1 and H2, as well as protons H3 and H4, are
isochronous at any temperature. In contrast, for
[Rh(cod)(2)]+, the four olefinic protons are non-equiva-
lent at 298 K as they appear as four 1:1:1:1, somewhat
broad, resonances which sharpen on cooling. For
[Rh(nbd)(2)]+, the two sharp signals of the olefinic
protons at ambient temperature split into four 1:1:1:1
distinct resonances at 183 K. Such a dynamic behaviour
can be rationalised by taking into account the possible
conformers A and A%, which arise from the non-copla-
narity of the chelating iminophosphines with the
NꢀRhꢀP coordination plane (Fig. 1), and their inter-
conversion through a PꢀN ring inversion analogous to
that described for iminophosphine–palladium deriva-
tives [5]. The rate of this conformational change is
affected markedly by the steric requirements of both
the NꢀR substituent and the chelating diolefin.
Only the complex [Rh(cod)(3)]+ appears to be suffi-
ciently stable, even in the presence of water, as to be
isolated as perchlorate salt from the reaction of [Rh(m-
Cl)(cod)]2 with ligand 3 and NaClO4·H2O in commer-
cial grade solvents. All the new compounds have been
characterised by elemental analysis, IR spectra, conduc-
1
tivity measurements (see Section 3), and by H- and
31P{1H}-NMR spectroscopy. Some selected NMR data
are listed in Table 1.
The upfield shift of the NꢁCH signals and the
downfield shift of the 31P resonances relative to the free
ligands [10] clearly indicate that the iminophosphines
are P,N-bound to the metal centre. The phosphorus
coordination is further confirmed by the 103Rh–31P
coupling constants in the range 152.9–155.1 and
170.2–170.5 Hz for the 1,5-cyclooctadiene and norbor-
nadiene derivatives, respectively, which are typical for
square-planar complexes [Rh(h2,h2-diolefin)(LꢀL%)]+
Scheme 1.

B. Crociani et al. / Journal of Organometallic Chemistry 631 (2001) 117–124
119
Table 1
Selected 1H a and 31P-{1H} b NMR data
Complex
Iminophosphine protons
31P resonances
H3, H4
NꢁCH Other signals
H1, H2
4.62 m
[Rh(cod)(1)]BF4
8.01 d 3.16 spt [CH(CH3)2]; 1.38 d, 0.74 d [CH(CH3)2]
3.82 m
35.21 d
(2.7) b
(6.9) c (6.7) c
(154.4) e
4.48 m d
4.60 s
3.68 m d
3.90 s
[Rh(nbd)(1)]BF4 8.03 d 3.13 spt [CH(CH3)2]; 1.46 d, 0.80 d [CH(CH3)2]
32.70 d
(170.2) e
(3.0) b
(6.9) c (6.3) c
[Rh(cod)(2)]BF4
8.16 d 1.28 s [C(CH3)3]
5.50 s (br), 5.33 s (br) 4.47 s (br), 3.78 s (br) 32.87 d
(2.2) b
(152.9) e
[Rh(nbd)(2)]BF4 8.04 s 1.22 s [C(CH3)3]
5.68 s
4.01 s
36.49 d
(170.2) e
5.82 s d, 5.58 s d
4.08 s d, 3.54 s d
3.78 m, 3.63 m
[Rh(cod)(3)]BF4
8.22 s 3.96 m [NCH]; 1.04 s [CH3]; 0.94 s [CH3], 0.79 s [CH3] 5.43 m, 5.16 m
34.07 d
(155.1) e
30.24 d
(153.2) e
35.98 d
(170.5) e
f
[Rh(cod)(3)]ClO4 8.31 s 3.95 m [NCH]; 1.02 s [CH3]; 0.91 s [CH3], 0.77 s [CH3] 5.44 m, 5.23 m
3.70 m, 3.55 m
4.06s, 3.71 s
[Rh(nbd)(3)]BF4 8.14 d 3.84 m [NCH]; 1.09 s [CH3]; 0.99 s [CH3], 0.83 s [CH3] 5.57 s, 5.28 s
(2.5) b
a In CD2Cl2 at 298 K unless otherwise stated; satisfactory integration values were obtained; coupling constants in Hz; s, singlet; d, doublet; spt,
septet; m, multiplet; see Scheme 1 for olefinic protons numbering.
b J(PH).
c J(HH).
d At 183 K.
e J(RhP).
f In CDCl3.
Fig. 1. Side view from the diolefin ligand toward the Rh atom for the conformers of the complexes [Rh(h2,h2-diolefin)(PꢀN)]+. The curved line
joining the N and P atoms represents the ꢁCHC6H4ꢀ unit lying below or above the NꢀRhꢀP plane: (i) interconversion through the chelate ring
inversion of the PꢀN ligand.
When the ring inversion is fast (on the NMR time
scale) a time-averaged plane of symmetry is generated in
the NꢀRhꢀP coordination plane, which brings about the
equivalence of proton H1 with H2 and of proton H3 with
H4. This is the case of complexes [Rh(cod)(1)]+ and
[Rh(nbd)(1)]+ throughout the temperature range ex-
plored. In these compounds, the NꢀC6H3(i-Pr)2-2,6 moi-
ety can minimise the steric interaction with the
cis-CH1ꢁCH2 olefinic unit by assuming a phenyl ring ori-
entation almost perpendicular to the coordination plane.
Such orientation is indeed supported by the shielding of
the H1 and H2 protons due to the anisotropic effect of the
phenyl ring current, and by the lack of free rotation
around the NꢀCphenyl bond. In [Rh(cod)(1)]+ and
[Rh(nbd)(1)]+ the H1 and H2 protons resonate at ca. 4.60
ppm, whereas the corresponding protons in the other
complexes are found at lower field in the range 5.68–5.16
ppm. On the other hand, the hindered rotation is evi-
1
denced by the number of the CHMe2 signals in the H-
NMR spectra (see Table 1) which show the presence of
two non-equivalent methyl groups within each of the mu-
tually equivalent 2,6-isopropyl substituents.
The ring inversion rate decreases considerably upon in-
troducing a bulky CMe3 group as the imino nitrogen sub-
stituent in the complexes [Rh(nbd)(2)]+ and
[Rh(cod)(2)]+. In the latter compound, containing also
the more sterically demanding 1,5-cyclooctadiene ligand,
the rate is reduced to such an extent that the H1 and H2
protons (as well as H3 and H4) are no longer equivalent
even at ambient temperature. In this case,

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B. Crociani et al. / Journal of Organometallic Chemistry 631 (2001) 117–124
coalescence of the resonances at 4.08 and 3.54 ppm
(olefinic protons H3, H4) at 228 K, from which a DGc
value of 43.9 kJ mol−1 can be estimated [13]. The
negative activation entropy suggests a transition state
with reduced degrees of freedom, which can be achieved
without any breaking of the rhodium–ligand bonds
when the six-membered ring of the chelating
iminophosphine becomes coplanar with the NꢀRhꢀP
coordination plane. Comparable free energies of activa-
tion (34.3–39.3 kJ mol−1) have been reported for the
conformational change involving inversion of the irid-
ium–diphosphine chelate ring in the complexes [Ir(-
cod)(LꢀL%)]+ [LꢀL%=1,1%-bis(phosphino)ferrocene] [14].
For the complexes [Rh(cod)(3)]+ and [Rh(nbd)(3)]+
with a chiral N-(R)-bornyl group, the conformers A
and A% are diasteromeric species. At ambient tempera-
Fig. 2. Low temperature 1H-NMR spectra of [Rh(nbd)(2)]BF4 in
CD2Cl2 in the range 3.0–6.0 ppm: (a) solvent signal; (b) nbd (1)H
and (4)H protons signal.
1
ture, the H and 31P spectra show a single set of proton
resonances for both the diolefin and the iminophos-
phine ligands, and a sharp l (31P) doublet (due to 103Rh
coupling) for each cationic complex, independently of
the counterion and solvent. No splitting of the signals
occurs at lower temperatures. Only a progressive
broadening of the phenyl proton resonances is observed
when the temperature is lowered down to 183 K in
CD2Cl2. These data are consistent with either the pres-
ence of one single diastereoisomer or with the two
isomeric forms A and A% in fast exchange on the NMR
time scale. That the latter is the case is shown by the
2D 1H-NMR ROESY spectra of [Rh(cod)(3)]+ (as
ClO−4 salt in CDCl3 and as BF4− salt in CD2Cl2) at
ambient temperature. In both solvents, the same intra-
and interligand NOEs are observed. However, their
number is much higher than what would be expected
for a single diastereoisomer, and is in agreement with
the presence of both forms A and A% in fast interconver-
four 1:1:1:1 olefinic proton signals are observed for
both the enantiomeric conformers A and A% which are
not distinguishable under our experimental conditions.
For [Rh(nbd)(2)]+ containing a chelate diolefin of
smaller size, the inversion rate is relatively high at
ambient temperature, but decreases at lower tempera-
tures. The spectral changes in the temperature range
208–183 K are shown in Fig. 2.
As can be seen, the resonances at 5.82 and 5.58 ppm
(183 K) of the olefinic protons H1, H2 coalesce at 208
K. A line-shape analysis of the signals [13] yields the
following activation parameters for the dynamic pro-
cess:
DHc =28.5
kJ mol−1
,
DSc = −60.3
J K−1 mol−1, and DGc =41.0 kJ mol−1. The spectra
at higher temperatures (not reported in Fig. 2) show a
Fig. 3. Side view from the 1,5-cyclooctadiene toward the Rh atom for the diastereoisomers of [Rh(cod)(3)]ClO4, showing the contact NOEs
observed in the 2D 1H-NMR ROESY spectrum in CDCl3.

B. Crociani et al. / Journal of Organometallic Chemistry 631 (2001) 117–124
121
The 2D 1H-NMR ROESY spectrum of [Rh(cod)-
(3)]+ in the phase-sensitive mode shows exchange cross-
peaks between the two olefinic protons trans to P and
exchange cross-peaks between the two olefinic protons
trans to N. No exchange cross-peaks involving the
olefinic protons trans to P and those trans to N are
present, as can be seen in Fig. 4.
These data indicate the occurrence of a slow dynamic
process which selectively interconverts the two protons
on the same HCꢁCH unit of the chelate cod ligand.
This finding is unprecedented since in the molecular
dynamics of the complexes [Rh(cod)(LꢀL%)]+ (LꢀL%=
P,N-bidentate ligand) the only exchanges observed so
far involve the olefinic protons trans to P with those
trans to N, when LꢀL% is an achiral ligand [11a], or the
protons H1 ? H3 and H2 ? H4 when LꢀL% is a chiral
ligand [11b,12]. In these cases, the dynamic process was
explained by a mechanism where the apparent rotation
of the chelate cod ligand results from a P,N-ligand site
exchange through initial rupture of the RhꢀN bond.
The stereochemical non-rigidity of the complex
[Rh{Fe(h5 - C5H4(2 - C5H4N))(h5 - C5H4PPh2)}(cod)]PF6
was also explained by a similar mechanism [15].
For the selective H1 ? H2 and H2 ? H4 exchanges
observed for [Rh(cod)(3)]+, we propose the mechanism
of Scheme 2.
Fig. 4. Phase-sensitive 2D 1H-NMR ROESY spectrum of
[Rh(cod)(3)]ClO4 in the region of the olefinic proton signals. Only
positive NOEs are shown.
This mechanism involves (i) initial dissociation of the
bond between rhodium and the CH1ꢁCH2 olefinic unit
(labilised by the high trans influence of the phosphine
and also by steric interaction with the cis-N-(R)-bornyl
group) with concomitant inversion of the olefin coordi-
nating face in the Rh(CH3ꢁCH4) bond; (ii) rotation of
the h2-cod ligand around the remaining rhodium–
olefin bond axis; (iii) inversion of the flexible h2-cod
ligand configuration and re-association to form the
chelate h2,h2-cod structure.
Scheme 2.
sion. As shown in Fig. 3 for the CDCl3 spectrum,
contact cross peaks are detected between the imino
protons NꢁCH (at 8.31 ppm) and the (3)H% and (5)H%
protons of the bornyl group (at 1.85 and 0.27 ppm,
respectively), and between the bornyl (1)Me protons (at
1.02 ppm) and the ortho and meta protons of the axial
PꢀPh group (in the range 7.5–7.3 ppm) for isomer A, in
addition to the NOEs NꢁCꢀHl(2)H (at 3.95 ppm)
and NꢁCꢀHl(1)Me for isomer A%.
Furthermore, strong NOEs are also present between
the bornyl (2)H proton and both the cod protons at
5.44 and 5.23 ppm, which confirm their assignment to
the olefinic protons H1, H2 trans to phosphorus.
Medium-to-strong NOEs are also observed between the
PPh2 ortho protons (in the range 7.4–7.2 ppm) and
both the cod protons at 3.70 and 3.55 ppm, which
confirms their assignment to the olefinic protons H3, H4
trans to nitrogen.
2.3. Diolefin exchange and displacement reactions
According to 1H-NMR spectra of the mixtures
[Rh(h2,h2-diolefin)(PꢀN)]+/diolefin at room tempera-
ture, the exchange rate between the free and coordi-
nated diolefin is slow, if any, on the NMR time scale.
No appreciable broadening of the corresponding signals
is in fact observed. However, the cod ligand in the
complexes [Rh(cod)(PꢀN)]+ is displaced rapidly and
quantitatively by norbornadiene (Rh/nbd=1:1.2 molar
ratio, Eq. (2)):
[Rh(cod)(P−N)]++ꢀꢀnꢀbꢂd[Rh(nbd)(P−N)]+
(2)
−cod
In contrast, no diolefin displacement takes place when
the complexes [Rh(cod)(PꢀN)]+ are treated with an
excess of ethylene.

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B. Crociani et al. / Journal of Organometallic Chemistry 631 (2001) 117–124
2.4. Theoretical calculations
In order to get more information about the preferred
iminophosphine conformation and the steric interac-
tions between the coordinated ligands, we have carried
out a semi-empirical calculation on the cationic com-
plexes [Rh(nbd)(2)]+ and [Rh(cod)(2)]+. In the most
stable conformer of [Rh(nbd)(2)]+ (enthalpy of forma-
tion=394.1 kJ mol−1) the PꢀN ligand assumes the
coordination mode shown in Fig. 5, whereas in the
most stable conformer of [Rh(cod)(2)]+ (enthalpy of
formation=217.8 kJ mol−1) the PꢀN coordination
mode is shown in Fig. 6.
In these structures, which correspond to conformer A
for [Rh(nbd)(2)]+ and to conformer A% for
[Rh(cod)(2)]+ (see Fig. 1), the ꢁCHC6H4ꢀ unit lies out
of the NꢀRhꢀP coordination plane, whereas the
NꢀCMe3 group and one of the PPh2 phenyl groups are
Fig. 7. Perspective view parallel to the NꢀRhꢀP plane for the most
stable conformer of the molecule [Rh(nbd)(2)]+. The ꢁCHC6H4ꢀ unit
has been omitted for clarity.
Fig. 5. Perspective view parallel to the NꢀRhꢀP plane for the most
stable conformer of the molecule [Rh(nbd)(2)]+. The diolefin ligand
has been omitted for clarity.
Fig. 8. Perspective view parallel to the NꢀRhꢀP plane for the most
stable conformer of the molecule [Rh(cod)(2)]+. The ꢁCHC6H4ꢀ unit
has been omitted for clarity.
on the opposite side. Interestingly, the calculations yield
PꢀN conformations for the isolated molecules which
are in a good agreement with those observed in the
solid state for palladium complexes containing related
iminophosphine ligands [5].
The coordination of the diolefin ligands is character-
ised by a large distortion from their idealised position
in square-planar complexes, as can be seen in Fig. 7 for
[Rh(nbd)(2)]+ and in Fig. 8 for [Rh(cod)(2)]+.
Such a distortion is essentially due to the steric
interaction with the NꢀCMe3 group, as can be inferred
from the short distances between the olefinic protons
trans to phosphorus and the protons of the CMe3
group. The shortest interligand contacts are actually
observed between one of the CMe3 protons and the H1
Fig. 6. Perspective view parallel to the NꢀRhꢀP plane for the most
stable conformer of the molecule [Rh(cod)(2)]+. The diolefin ligand
has been omitted for clarity.

B. Crociani et al. / Journal of Organometallic Chemistry 631 (2001) 117–124
123
Table 2
Elemental analysis and selected IR data
a
Complex
Analysis
C
IR bands b
H
N
w(CꢁN)
w(BꢀF)
[Rh(cod)(1)]BF4
[Rh(nbd)(1)]BF4
[Rh(cod)(2)]BF4
[Rh(nbd)(2)]BF4
[Rh(cod)(3)]BF4
[Rh(cod)(3)]ClO4
Rh(nbd)(3)]BF4
62.8 (62.67)
61.7 (62.40)
57.4 (57.87)
56.9 (57.44)
61.1 (61.43)
60.1 (60.27)
60.7 (61.12)
5.9 (5.93)
5.5 (5.51)
5.6 (5.64)
5.1 (5.14)
6.0 (6.13)
6.0 (6.02)
5.6 (5.70)
1.9 (1.87)
1.9 (1.91)
2.2 (2.18)
2.2 (2.23)
1.9 (1.94)
1.9 (1.90)
1.9 (1.98)
1602 ms
1601 ms
1608 ms
1610 ms
1621 ms
1617 ms
1623 ms
1054 vs
1053 vs
1054 vs
1061 vs
1069 vs
1089 vs c
1064 vs
a Calculated values in parenthesis.
b As nujol mulls.
c w(ClꢀO).
2
olefinic proton of [Rh(nbd)(2)]+ (1.79 A) or the H
estimated for reaction
2
involving the isolated
,
olefinic proton of [Rh(cod)(2)]+ (1.74 A). As a conse-
molecules. Since solvation effects are expected to play a
minor role for structurally related molecules, the small
DH value indicates that reaction 2 in solution is essen-
tially entropy driven. This is indirectly confirmed by the
lack of diolefin displacement when an excess of ethylene
is added to a solution of [Rh(cod)(2)]+.
,
quence, both the olefinic carbons trans to phosphorus
are shifted above or below the NꢀRhꢀP plane. Some
distortion of the coordinated diolefin has been observed
in the X-ray structural studies of complexes [Rh(h2,h2-
diolefin)(LꢀL%)]+ (LꢀL%=P,N- and P,P-bidentate lig-
and) [11b,12,16].
The presence of distorted diolefins and, consequently,
of weak rhodium–olefin bonds is likely to be responsi-
ble for the instability of the complexes studied toward
traces of water in solution.
3. Experimental
3.1. General
The changes in the enthalpy content during the
chelate ring inversion of the PꢀN ligand were also
estimated. The enthalpy reaches a maximum value
when the six-membered chelate ring is coplanar with
the NꢀRhꢀP coordination plane. The enthalpy increase
is DH=20.3 kJ mol−1 for [Rh(nbd)(2)]+ and DH=
114.7 kJ mol−1 for [Rh(cod)(2)]+. The calculated DH
value for the former complex compares well with the
activation enthalpy of 28.5 kJ mol−1 measured in
CD2Cl2 solution at 208 K. On the other hand, the large
difference in DH values for the two compounds is in
accord with the different rates of ring inversion ob-
served in solution.
When the inversion process is complete, the complex
[Rh(nbd)(2)]+ assumes the structure of conformer A%
(with an enthalpy of formation of 394.2 kJ mol−1) and
the complex [Rh(cod)(2)]+ assumes the structure of
conformer A (with an enthalpy of formation of 219.1
kJ mol−1). As expected for enantiomeric pairs, these
enthalpy values are very close to those of the starting
conformers A and A%.
All manipulations were carried out under a dry N2
atmosphere. Diethyl ether was distilled from Na/ben-
zophenone, and CH2Cl2 from LiAlH4. All other chemi-
cals and solvents were reagent grade and were used
without further purification. The iminophosphines [10]
and the complexes [Rh(m-Cl)(cod)]2 [17] and [Rh(h2,h2-
diolefin)2]BF4 [18] were prepared by literature methods.
1
The H- and 31P{1H}-NMR spectra were recorded on a
Bruker AM400 spectrometer operating at 400.13 and
161.98 MHz, respectively. Chemical shifts (ppm) are
given relative to Me4Si (1H-NMR) and 85% H3PO4
(31P-NMR). The 2D 1H-NMR ROESY spectra were
obtained in the phase-sensitive mode as described else-
where [5,10]. The IR spectra of solid samples were
recorded in the range 4000–200 cm−1 on a Perkin–
Elmer 983 G instrument with CsI windows.
3.2. Preparation of [Rh(p2,p2-diolefin)(PꢀN)]BF4
The iminophosphine (1–3) (0.5 mmol) was added to
a solution of [Rh(h2,h2-diolefin)2]BF4 (0.5 mmol) in
CH2Cl2 (30 cm3). After standing for 1 h at room
temperature (r.t.), the red–orange solution was concen-
trated at reduced pressure, and diluted with Et2O to
precipitate the products as yellow–orange microcrys-
talline solids. The yields were in the range 80–93%,
based on the theoretical amount. The complexes are
From the enthalpies of formation of the free dio-
lefins, calculated by using the same semi-empirical
method for the most stable conformer of 1,5-cycloocta-
diene (69.5 kJ mol−1) and for norbornadiene (245.0
kJ mol−1), and from the enthalpies of formation of the
most stable conformers of [Rh(nbd)(2)]+ and
[Rh(cod)(2)]+ a DH value of 0.8 kJ mol−1 can be

124
B. Crociani et al. / Journal of Organometallic Chemistry 631 (2001) 117–124
[4] (a) K. Fugami, Y. Mishiba, S. Hagiwara, D. Koyama, M.
Kameyama, M. Kosugi, Synlett (2000) 553;
uni-univalent electrolytes in CH2Cl2, with molar con-
ductivity values in the range 52–55 V−1 cm2 mol−1 for
1×10−3 mol dm−3 solutions at 25 °C. The elemental
analysis and some selected IR bands are reported in
Table 2.
(b) H.-Y. Jang, H. Seo, J.W. Han, Y.K. Chung, Tetrahedron
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Crociani, Transition Met. Chem. 25 (2000) 17.
3.3. Preparation of [Rh(cod)(3)]ClO4
[6] I.D. Kostas, C.G. Screttas, J. Organomet. Chem. 585 (1999) 1.
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The iminophosphine 3 (0.170 g, 0.4 mmol) was added
to a solution of [Rh(m-Cl)(cod)]2 (0.099 g, 0.2 mmol) in
CH2Cl2 (20 cm3). Upon addition of an excess
NaClO4·H2O (0.112 g, 0.8 mmol) dissolved in Me2CO
(20 cm3), an immediate precipitation of NaCl took
place. The solvents were evaporated to dryness and the
solid residue was extracted with CH2Cl2 (40 cm3). After
filtration, the solution was concentrated to small vol-
ume and diluted with Et2O to give an orange solid. The
product was further purified by reprecipitation from the
same solvent mixture (yield, 0.236 g, 80.2%; molar
conductivity, 46.2 V−1 cm2 mol−1 for a 1×10−3
mol dm−3 solution at 25 °C).
(b) S.U. Son, H.-Y. Jang, J.W. Han, I.S. Lee, Y.K. Chung,
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(b) B. Crociani, S. Antonaroli, L. Canovese, F. Visentin, P.
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2089 (and references therein);
(b) H. Berger, R. Nesper, P.S. Pregosin, H. Ru¨egger, M. Wo¨rle,
Helv. Chim. Acta 76 (1993) 1520.
[12] M. Valentini, K. Selvakumar, M. Wo¨rle, P.S. Pregosin, J.
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[13] B. Testa, Principles of Organic Stereochemistry, Marcel Dekker,
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3.4. Computations
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Semi-empirical calculations were carried out with
PM3 method [19] using the HYPERCHEM program [20].
Geometric optimisations were terminated when the en-
ergy difference among successive iterations was lower
than 4.184×10−3 kJ mol−1. Computations were per-
formed using as initial values the structural parameters
from crystallographic data of the complexes [Rh(h2,h2-
diolefin)(LꢀL%)]+ (diolefin=nbd, cod; LꢀL%=P,N- and
P,P-bidentate ligand) [11b,12,14–16,21].
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