carbonilos

1. Review
Synthesis, characterization and dynamic behavior of some iridium
carbonyl cluster complexes derived from Ir4(CO)12 with N-, P- and
C-donor ligands: A survey
Augusto Tassan a,⇑
, Mirto Mozzon a
, Giacomo Facchin b
, Alessandro Dolmella c
, Serena Detti d
a
Dipartimento di Ingegneria Industriale, via Marzolo 9, 35131 Padova, Italy
b
Istituto per l’Energetica e le Interfasi IENI-CNR, via Marzolo 9, 35131 Padova, Italy
c
Dipartimento di Scienze del Farmaco, via Marzolo 5, 35131 Padova, Italy
d
Institute of Ecosystem Study ISE-CNR, via Moruzzi 1, 56122 Pisa, Italy
a r t i c l e i n f o
Article history:
Received 4 August 2014
Received in revised form 9 September 2014
Accepted 14 September 2014
Available online 28 September 2014
Keywords:
Iridium clusters
Carbonyl
Intramolecular dynamic
a b s t r a c t
The synthesis of iridium dodecacarbonyl cluster derivatives Ir4(CO)12 with donor ligand such as amine,
phosphites, hydrido and cyclic mono and dioxycarbene, NMR and X-ray characterization and fluxional
behavior study in solution at variable temperature is briefly reviewed.
Ó 2014 Elsevier B.V. All rights reserved.
Augusto Tassan initiated its research activity in the Chemistry Department of the Venice University. He then moved to the University of Padova,
Industrial Chemistry Institute, under the supervision of Prof. R. Ros and R.A. Michelin, and collaborating with the Prof. R. Roulet of EPFL in Lausanne.
His research focuses on the synthesis of new organometallic platinum and iridium clusters, with particular interest on NMRcharacterization
Mirto Mozzon took a degree of Industrial Chemistry at the University of Padova with full marks. He became then CNR researcher in the group headed
by Prof. U. Belluco, and finally researcher at the University of Padova, under the supervision of Prof. R. A. Michelin. He is now Associate Professor. He
maintained a research cooperation whit Prof. A.J.L. Pombeiro at Instituto Superior Tecnico in Lisbon. He coauthored about 80 papers published on
peerreviewed international journals in Inorganic and Organometallic Chemistry. He also filled 4 International Patents and written 2 student’s book.
http://dx.doi.org/10.1016/j.ica.2014.09.014
0020-1693/Ó 2014 Elsevier B.V. All rights reserved.
⇑ Corresponding author. Tel.: +39 498275518.
E-mail address: augusto.tassan@unipd.it (A. Tassan).
Inorganica Chimica Acta 424 (2015) 91–102
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica

2. Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
2. 13
CO-enrichment of tetrairidium carbonyl clusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
3. Diamine derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
4. Monoamine derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
5. Hydrido derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
6. Cyclic mono and dioxycarbene derivatives. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
7. Phosphites. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
8. Intramolecular dynamics of [Ir4(CO)12] derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
1. Introduction
The chemistry of iridium carbonyls, notably, the chemistry of
clusters derived from Ir4(CO)12, were developed along various
research lines. Among the investigated topics, we can mention
the stereochemistry of the ligands [1], the fluxional processes
occurring in solution [2], the studies on the kinetics of carbonyl
substitution reactions [3], the modellisation of metal surfaces for
absorption reactions of unsaturated substrates [4], the use of
such materials as catalysts or precursors in the hydrogenation
processes of hydroformylation of unsaturated organic molecules
[5].
Along with these perspectives, Garlaschelli and co-workers [6]
prepared the starting complex Ir4(CO)12 from IrCl3ÁnH2O in
ethylene glycol monomethylether medium under a CO gas flow
with more than 80% yield.
The IR spectrum of the obtained mixture shows the typical
bands of terminal carbonyls in the range 2114–2000 cmÀ1
.
A few years later Pruchnik et al. [7] reported an even more effi-
cient method (ca. 95% yield) to obtain Ir4(CO)12 by reacting IrCl3Á3
H2O with formic acid in autoclave at 100 °C for 12 h.
Tri- and tetra-substituted derivatives of Ir4(CO)12 can be
obtained in good yield by means of the direct reaction of the
tetrairidium complex with different ligands (L). Further studies of
substitution reactions have identified as a process made of three
consecutive steps:
Ir4ðCOÞ12 þ L !
ÀCO
Ir4ðCOÞ11L þ L !
ÀCO
Ir4ðCOÞ10L þ L !
ÀCO
Ir4ðCOÞ9L
Giacomo Facchin studied chemistry at the Università degli Studi di Padova (Italy) and completed his PhD in 1979. After a post doc term with Prof. R.J.
Angelici at the Iowa State University he joined the Italian National Research Council (CNR) where is currently Senior Researcher at the Istituto per
l’Energetica e le Interfasi (IENI). His scientific activity mainly focuses on organometallic and coordination chemistry, nanostructurated materials and
materials containing metallic nanoparticles.
Alessandro Dolmella entered the Department of Pharmaceutical and Pharmacological Sciences in 1990 to work in the research group headed by
Prof. M. Nicolini, studying radiopharmacy and computational chemistry. He is presently interested in bioinorganic and coordination chemistry, with a
special emphasis on transition metals complexes.
Serena Detti graduated in chemistry at the University of Pisa in 1996, under the supervision of Prof. F. Calderazzo and G. Pampaloni. She received her
PhD in Chemistry at the Swiss Federal Institute of Technology of Lausanne in 2002, working in the group of Prof. R. Roulet in the field of metals
carbonyl clusters. She worked at the Italian forensic science service and later she devoted to research on nanotechnology, studying potential
interactions of metal nanostructures and the environment.
92 A. Tassan et al. / Inorganica Chimica Acta 424 (2015) 91–102

3. In the first step, the substitution reaction follows a second-
order kinetics law. It is characterized by an associative mechanism,
and three of the (previously) all terminal CO undergo a structural
rearrangement that transforms them into bridging CO units on
the basal iridium atoms. The following two steps have an associa-
tive-to-dissociative substitution mechanism. Factors as basicity
and the bulkness of the incoming ligand do not vary the process
kinetics [3].
Mono- or di-substituted derivatives of Ir4(CO)12 cannot instead
be isolated by direct synthesis from the tetrairidium complex,
because of its insolubility in common organic solvents and also
because the reaction requires high temperatures (80–120 °C).
Accordingly, alternative routes had to be sought. An opportunity,
although with low yields, is given by the reduction of Ir(CO)2(H2-
NC6H4Me-p)Cl by zinc metal in the presence of carbon monoxide
and the ligands [8]; another one is offered by the reaction of Ir4-
H(CO)11 with the appropriate ligands [9].
Still another possibility was explored by Chini et al., who pre-
pared the anionic complexes of the type [Ir4(CO)11X]À
(X = Br or
I) [10]. Since the latter are soluble and more reactive than iridium
dodecacarbonyl, they can undergo replacement reactions of a CO
with several ligands, including PF3 and SO2 [11], olefin [12], phos-
phines [13], arsines [14], yielding various complexes.
In the following sections, we provide a brief account on the syn-
thesis, characterization and dynamic behavior of new tetrairidium
clusters with H-, N-, P-, and C-donor ligands derived from Ir4(CO)12.
2. 13
CO-enrichment of tetrairidium carbonyl clusters
The molecular structure and the stereochemistry of metal car-
bonyl derivatives might be investigated by means of 13
C NMR anal-
ysis. However, a problem arises because the exchange reactions do
not always occur with free 13
CO; while for some metal carbonyl
clusters, such as Co4(CO)12 and Rh4(CO)12, this exchange is easy,
the reaction involving Os3(CO)12 and Ir4(CO)12 occur only with
more difficulty. The main obstacle is the low solubility of the clus-
ters, which forces the exchange reactions to take place in heteroge-
neous phase and makes them extremely slow, even at high
pressures and high temperatures.
Tassan and co-workers have reported [15] two simple proce-
dures to make the 13
CO exchange process easier. The first one
involves the use of anionic clusters; the second one requires the
use of the well-known decarbonylating agent trimethylamine-N-
oxide, Me3NO [16–18], in the presence of free 13
CO.
The first method (a two-step process) is illustrated in Scheme 1.
At the beginning, as reported by Chini et al. [10], the reaction of
Ir4(CO)12 with NEt4I in THF at 70 °C leads to the formation of the
anionic iridium tetracarbonyl [Ir4(CO)11I]À
. The second step, the
displacement of iodide by 13
CO, occurs in THF at room temperature
and affords the enriched Ir4(⁄
CO)12 cluster.
Both reactions occur with more than 90% yield. The degree of
the first enrichment A1 is given by the molar fraction of coordi-
nated 13
CO and can be calculated from Eq. (1) below:
A1 ¼
b þ 11A0
12
¼ 0:09266 ð1Þ
where A0 and b are, respectively, the molar fractions for natural
abundance 13
CO (0.01108) and for used 13
CO (0.99). A second
enrichment step can then be carried out, again, in THF at room tem-
perature as outlined in Eq. (2):
NEt4½Ir4ðÃ
COÞ11Š þ 13
CO ! Ir4ðÃ
COÞ12 # þNEt4I ð2Þ
i.e., by repeating the reaction path described in Scheme 1, this time
using the enriched Ir4(⁄
CO)12 as starting material. The molar frac-
tion of 13
CO can be calculated from Eq. (3) below:
A2 ¼
b þ 11A1
12
¼ 0:16743 ð3Þ
Further A3, A4,. . .Ax values can then be calculated from the fol-
lowing Eq. (4):
Ax ¼
b þ 11AkÀ1
12
ðk ¼ 1; 2; 3; . . .Þ ð4Þ
Eq. (4) has basically the form:
Ak ¼ fðAkÀ1Þ ð5Þ
where f is a linear function that converges to the point (b = 0.99), a
value which verifies the equation t = f(t).
As mentioned above, the alternative direct method uses Me3NO
as decarbonylating reagent, according to the following scheme:
The first reaction of the Scheme 2 is carried out at À30 °C in
THF, with a Me3NO/cluster stoichiometric ratio and a slight excess
of 13
CO, affording the anionic [Ir4(⁄
CO)11I]À
complex. The subse-
quent reaction is performed at room temperature and yields the
tetrairidium carbonyl complex in more than 90% yield. In general,
this kind of reaction allows the preparation of a large number of
13
CO-enriched iridium carbonyl cluster complexes with different
ligands, including monodentate phosphines, diphosphines or
arsines. The fluxional behavior of all these species can be readily
analyzed by 13
C NMR (eq. (6)).
Ir4ðCOÞ12ÀnLn þ13
CO þ Me3NO ! Ir4ðÃ
COÞ12ÀnLn þ CO2
þ NMe3 ð6Þ
The exchange rate 12
CO–13
CO was determined by following,
through IR spectrometry, the enrichment reaction of the complex
Ir4(CO)11(PPh3) in CH2Cl2. As expected, as long as 13
CO increases
there is a lowering of the CO stretching frequencies. Changing from
99% 12
CO to 98% 13
CO, the IR spectrum presents the same overall
shape and the same number of bands, however, with a shift of
47.0–48.5 cmÀ1
for terminal carbonyls, and of 40 cmÀ1
for
edge-bridging CO. These frequency shifts are in agreement with
the values shown by the 12
CO and 13
CO ligands, whose vibrations
are related only to their reduced masses and not influenced by
the coupled cluster moiety.
m 12COð Þ À m 13COð Þ
m 13COð Þ
¼ 1 À
l 13COð Þ
l 12COð Þ
!" #1=2
ð7Þ
3. Diamine derivatives
The first diamine derivatives of tetrairidium dodecacarbonyl
have been synthesized by Tassan and co-workers [19]. Initially
(Scheme 3), [Ir4(CO)11X]À
(X = Br or I) reacts with a large excess
of aromatic ligand in presence of Ag+
(one equivalent) in dichloro-
Scheme 1. Reaction path for the synthesis of 13
CO-enriched tetrairidium dodeca-
carbonyl. ⁄
Mixture of 12
CO and 13
CO.
Scheme 2. Alternative synthesis of 13
CO-enriched tetrairidium dodecacarbonyl.
A. Tassan et al. / Inorganica Chimica Acta 424 (2015) 91–102 93

4. methane at low temperature (À30 °C); the diamines are then
obtained after a disproportion reaction at room temperature.
The configuration of the complexes (Fig. 1) has been assigned
on the basis of IR and 13
C{1
H} NMR spectroscopies at low
temperature (180 K). The complexes show six resonances: for
example in the case of N-N = 1,10-phenantroline the 13
C{1
H}
NMR carbonyls a at 226.6 (relative intensity 2), b at 200.1 (r.i. 1),
d (r.i. 2), c (r.i. 2), e (r.i. 2) and g (r.i. 1). The IR spectroscopic data
show the presence of terminal carbonyls (2070s, 2043vs, 1995s)
and bridged carbonyls (1830m and 1797m).
The structure proposed in Fig. 1 has been confirmed by single
crystal X-ray diffraction analysis (Fig. 2). The complex presents a
tetrahedral cluster of iridium atoms and a distribution of CO
ligands similar to that found in many other derivatives of Ir4(CO)12
[20]. The Ir–C bond distances become shorter in presence of the
diamino groups. The diamino ligands chelate the cluster through
a basal iridium via the lone-pairs of nitrogen atoms lying in axial
and radial positions. The Ir2–Ir3 and Ir2–Ir4 distances involving
the iridium atom (Ir2) which is chelated the diamine ligand are
comparable to the unbridged Ir–Ir bond distances [20].
4. Monoamine derivatives
The work on diamino ligands was extended by preparing com-
plexes with monoamine [21], notably, with pyridine (7), 4-methyl-
pyridine (8), 4-ter-buthylpyridine (9), 3,5-dimethylpyridine (10)
and 3,4-dimethylpyridine (11). In this case, [Ir4(CO)11Br]À
reacts
readily with an excess of aromatic monoamine and one equivalent
of AgBF4 in CH2Cl2 at À25 °C. The products are obtained with
60–71% yield, after recrystallization from a CH2Cl2/MeOH mixture.
NEt4½Ir4ðCOÞ11BrŠ þ AgBF4 þ L ! Ir4ðCOÞ11L þ AgBr
ðwhere L ¼ monoamine ligandsÞ ð8Þ
The IR spectra of these compounds, in CH2Cl2 solution, show
either the presence of the characteristic terminal CO bands
(2100–1950 cmÀ1
) and also two adsorptions in the region of bridg-
ing CO. However, by replacing the CH2Cl2 solvent with cyclohex-
ane, the bands of bridging CO disappear. This can be explained
(Scheme 4) by assuming the presence of at least two species in
solution: an isomer with all terminals ligands (A), an isomer with
bridging CO and the amine in axial position (B) and another one
with the amine group in equatorial position (C).
Likewise, the 13
C NMR analysis of compound (8) enriched with
13
CO ca. 20% reveals two sets of signals in 36/73 ratio. The first set
can be attributed to the 8B isomer, with the monoamine ligand in
axial position, the second and more abundant one to isomer 8A.
The same analysis for compound 9 shows the presence of three
sets of signals originated by the three isomers (A, B, C) in 42/55/
3 ratio, respectively.
The X-ray crystal structure of Ir4(CO)11(4-methylpyridine)
(Fig. 3) shows that the molecule contains a nearly tetrahedral Ir4
core and all terminal ligands, as resulting also from the analysis
of the 13
C NMR spectra. The Ir1–Ir4 distance, in a pseudo-trans posi-
tion with respect to the amine ligand, is lower (2.659(6) Å) than
the average value found for the remaining metal–metal distances
(2.687(17) Å); this may be due to the weaker sigma-trans influence
with respect to the carbonyl ligand.
5. Hydrido derivatives
Known hydrido derivative of tetrairidium dodecacarbonyl
are [H2Ir4(CO)10]2À
[10], [HIr4(CO)11]2À
[22], and the neutral
orthometalled compound [HIr4(CO)7(Ph2PCH@CHPPh2)(PhC6H4
PCH@CHPPh2)] [18] reported by Albano et al. that shows a bridging
hydride between two iridium atoms with IrAH bond lengths of
1.71 and 1.76 Å. With respect to similar derivatives, it is worth
noting that the deprotonated form of the dppm diphosphine
ligand, bis(diphenylphosphino)methanide [(Ph2P)2CH]À
, has been
used for its ability to behave as a two-, four- or six electrons donor
[23]. In fact, simple deprotonation of dppm ligand with a base
affords the preparation of new hydride iridium cluster derivatives
[24]. The reaction underlined below (Eq. (9)) is carried out with an
excess of KOH dry powder in dichloromethane at À20 °C and gives
the product with 76% yield:
Ir4ðCOÞ10ðl-dppmÞ þ 2KOH þ PPNCl
! ½PPNŠ½Ir4ðCOÞ9ðl3-ðPh2PÞ2CHÞŠ þ KCl þ KHCO3 ð9Þ
Since in the IR spectrum there are no bands due to bridging car-
bonyls, this complex shows in solution and solid state a symmetry
with only terminals CO ligands. At 173 K the 13
C NMR shows that
the apicals CO are already slowly exchanging and at 203 K the only
fluxional mechanism observed arises from the rotation of apical
carbonyls.
The crystal structure of [PPN][Ir4(CO)10(l3-(Ph2P)2CH)] shows
Cs symmetry, with all terminals CO and with the ligand [(Ph2P)2-
[NEt4][Ir4(CO)11X]
+ (N-N) -AgX
-30°C
[Ir4(CO)11{η1
-(N-N)}]-
1/2 Ir4(CO)10{η2
-(N-N)} + 1/2 Ir4(CO)12 + 1/2 (N-N)
R.T.
Scheme 3. Synthesis of diamine derivatives of tetrairidium dodecacarbonyl. (N-N)
= 1,10-phenantroline (1); 4,7-dimetylphenantroline (2); 5,6-dimetylphenantroline
(3); 3,4,7,8-tetrametylphenantroline (4); 2,20
-dipiridine (5); 4,40
-dimetyl-2,20
-dipir-
idine (6).
a
a b
c
d
c
e
e g
d
L
L
Fig. 1. Proposed structure for the diamine derivatives. (L-L = N-N).
Fig. 2. X-ray structure of Ir4(CO)11(1,10-phentroline) 1.
94 A. Tassan et al. / Inorganica Chimica Acta 424 (2015) 91–102

5. CH]À
face bridging with the plane P(2)ACAP(3) eclipsed with
respect to the plane Ir(3)AIr(2)AIr(4). The IrAIr distance (mean
value = 2.700(1) Å) is shorter than the one found for Ir4(CO)7
(l-CO)3-(l-(Ph2P)2CHMe) (2.729(1) Å). This finding has been sup-
ported by other structural data, confirming that the CO-unbridged
metal bonds are shorter than those bridged [25]. Likewise, the
hydrido-cluster [PPN][HIr4(CO)9(l-dppm)] is obtained in 85% yield
according to the following reaction, (Eq. (10)) by using a large
excess of 1,8-diazobicylo[5,4,0]undec-7-ene (DBU) as base under
CO atmosphere in wet CH2Cl2 at À20 °C and [Ir4(CO)10(l-dppm)]
as starting material [24]:
Ir4ðCOÞ10ðl-dppmÞ þ DBU þ H2O þ PPNCl
! ½PPNŠ½HIr4ðCOÞ9ðl-ðdppmÞÞŠ þ CO2 þ ½DBUHŠCl ð10Þ
Also for this hydrido compound the geometry in solution and in
the solid state have been found to be the same, with three bridging
CO, the remaining CO terminal and the hydride ligand coordinated
in axial position. Interestingly, in the 1
H NMR spectrum the meth-
ylene CH2 protons signals of dppm originate an ABX2 spin system
due to the inequivalence of the two protons. While the proton HB
shows a chemical shift of 2.69 ppm, the HA is observed at
6.03 ppm, indicating a strongly deshielded nucleus. This cluster is
fluxional in solution. The first CO scrambling process takes place
at 200 K and involves the basal carbonyls a, b, d and f; a second
process takes place at 220 K and involves the rotation of apical
carbonyls e and g.
The crystal structure of the hydrido-cluster [PPN][HIr4(CO)9
(l-dppm)] shows Cs symmetry, with three bridging CO on the
basal plane and the bidentate diphosphine ligand taking two axial
positions. The mean value of the IrAIr distance (2.769(3) Å) is
longer than that reported for the [PPN][Ir4(CO)10((Ph2P)2CH)],
while the IrAH distance (2.08(6) Å) is longer than those found for
monometallic complexes [24]. The exact location of the hydride
ligand in the complex could not be successfully defined by
conventional X-ray diffraction analysis. Consequently, a neutron
diffraction experiment was performed at the Institute Laue
Langevin in Grenoble [26]. Fig. 4 illustrates the outcome of this
experiment. The IrAH distance found is 1.618(14) Å and it is the
first experimental determination of an iridium cluster. Comparison
with the value above reported of 2.08(6) Å proves the latter to be
incorrect and, confirms the predictive power of ab initio
calculations [26], and, at the same time, highlights once more the
limits of conventional X-ray diffraction analysis in defining the
position of light atoms in the proximity of heavy ones.
As described above, Ir4(CO)10(l-dppm) quickly reacts with an
excess of KOH to give [Ir4(CO)10(l-(PPh2P)2CH)]À
, which in turn con-
verts into the decarbonylated anion [PPN][Ir4(CO)9(l3-(PPh2P)2CH)],
whereas the hydrido-derivative Ir4H(CO)9(l-dppm) is obtained if
the same reaction is carried out with an excess of DBU. Detti and
co-workers [27] further studied this reaction in dichloromethane
with an excess of DBU and PPNCl, using different phosphines
and arsines, such as: bis(diphenylphosphino)methane, 1,1-bis
(diphenylphosphino)ethane, 1,2-bis(diphenylphosphino)ethane, 1,
3-bis(diphenylphosphino)propane and bis(diphenylarsino)meth-
ane. All the corresponding hydrido-complexes (12, 13, 14, 15, 16,
respectively) were obtained with more than 75% yield. The
proposed mechanism requires the nucleophilic attack of OHÀ
on
the metal carbonyl, as in Scheme 5:
On the contrary, the reaction of Ir4(CO)10(l-dppmMe) carried
out in presence of an excess of DBU, but without PPNCl, the hydr-
ido-compound (13a) (75% yield, see Scheme 6) with [DBUH]+
as
counterion was obtained together with a secondary derivative
(13b, 1%) with [DBUMe]+
. Using Ir4(CO)10(L) (L = dppm, dppe, dppp,
dpam) as starting materials and the same reaction conditions used
for Ir4(CO)10(l-dppmMe) does not lead to the formation of the
analogous hydrides-anions. The explanation appears to be that
the nucleophilic attack by such a strong base as DBU on a diphos-
phinic chain produces a lack of a site and this is related with the
weak acidity of the methyl group [28]. The compounds (12–16)
show the characteristic IR bands in the bridging CO region. The
31
P {1
H} NMR spectra have only one signal for the diphosphine, like
the starting complexes. The hydrido ligand is located in axial posi-
tion and presents a single 1
H NMR signal at low field (À15 ppm).
Finally, the low-temperature 13
C{1
H} NMR spectra obtained from
enriched compounds show the typical pattern of carbonyls.
The molecular structures of compounds (13) with counterion
[DBUH]+
and [DBUMe]+
respectively, and (14, 15) with [PPN]+
are
illustrated in Fig. 5. All structures show the diphosphinic and
hydride ligands in axial position with respect to the Ir1AIr2AIr3
plane that also accommodates three bridging CO units.
L
a a
b
cc
dd
ee
g
f
a a
b
cc
dd
ee
g
h
L
6 6
5
L
112 2
3 3
44
AB C
Scheme 4. Possible arrangements for amine ligands.
Fig. 3. ORTEP view of the complex Ir4(CO)11(4-methylpyridine) 8.
A. Tassan et al. / Inorganica Chimica Acta 424 (2015) 91–102 95

6. The investigation of structural data highlights that the hydride
ligand has a stronger trans-influence compared to the CO group.
As for the bond distances, the Ir3AIr4 distance is longer than the
remaining IrAIr bonds and also longer than those found in the
starting compounds. In contrast, the IrAP distances are shorter
than those observed in Ir4(CO)10(l-dppmMe) and Ir4(CO)10(l-
dppp). As expected, the determination of hydride bond length
proved difficult [26]. The IrAH bond length is 1.32(5) Å and it is
shorter than that found for [HIr4(CO)9(l-dppm)]À
(1.618(14) Å)
by neutron diffraction.
6. Cyclic mono and dioxycarbene derivatives
Tassan and co-workers [29] have reported the synthesis and the
investigation of the fluxional dynamics and the X-ray molecular
structures of a series of new dioxycarbene compounds obtained
from Ir4(CO)11(L) (L = Pt
Bu3 (17), PPh3 (18,19) and Ir4(CO)10(LAL)
(LAL = Ph2PCH2PPh2 (20), norbornadiene (21) and 1,5-cyclooctadi-
ene (22,23)). The starting phosphine derivative Ir4(CO)11(Pt
Bu3)
was obtained by reacting Ir4(CO)11(norborn-2ene) [15] with a stoi-
chiometric amount of tri(ter-butyl)phosphine (Pt
Bu3) in dichloro-
methane. The 31
P{1
H} NMR spectrum shows a single resonance
for phosphine at 65.9 ppm. The values of the coordination chemical
shift (Dd = dcoord. À dfree phosphine) [13] of 2.6 ppm suggests that the
phosphine lies in axial position. This idea is supported by the pres-
ence in the 13
C{1
H} NMR spectrum of two bands in the radial CO
domain, one of which, f, Scheme 7, shows a coupling of 8.1 Hz with
the phosphorus atom, and, in the apical ligands field, 27.1 Hz
pseudo-trans-coupling of CO g with the same atom.
The reaction of Ir4(CO)11(Pt
Bu3) with oxirane 2-bromoethanol
and sodium bromide as catalyst leads to the formation of the
monocyclic dioxocarbene derivative Ir4ðCOÞ10ðPt
Bu3Þ
ðCOCH2CH2OÞ. The IR spectrum of this complex shows the presence
of three bands (at 1862, 1819 and 1795 cmÀ1
) due to bridging CO
that are typical of complexes having a ground state C3v symmetry.
The 31
P{1
H} NMR at 230 K exhibits two resonances, d = 62.29 and
64.56 ppm, due to two different isomers, A and B (ratio = 28:72;
17; see Scheme 7); the latter may be separated by TLC.
From values of the calculated coordination chemical shifts,
Dd = 1.3 and À1.0 ppm, it is possible to infer that in isomer A the
phosphine and carbene ligands are both in axial position, while
in isomer B the ter-butyl is in axial and the carbene in radial
position. Likewise, the 13
C{1
H} NMR spectrum in CD2Cl2 at 230 K
present two sets of signals. Those relating to major isomer B are
Fig. 4. Left: ORTEP drawing of the [HIr4(CO)9(l-dppm)]À
(12) obtained from X-rays diffraction. Right: Structure obtained from neutron diffraction.
Ir
CO
H2O -H+
Ir C
O
OH
Ir H + CO2
Scheme 5. The formation of hydride-derivatives by nucleophilic attack of OHÀ
on
metal carbonyl.
C
H
CH3
P
P
Ir
Ir
+ dbu
P
P
Ir
Ir
C CH3 CH(CH3)
P
P
Ir
Ir
P
P
Ir
Ir
C H CH2
P
P
Ir
Ir
-dbuH+
-dbuCH3
+
+H+
+H+
75 %
1 %
Scheme 6. The formation of hydride-derivatives with two possible types of direct attack of dbu.
96 A. Tassan et al. / Inorganica Chimica Acta 424 (2015) 91–102

7. identified by the carbene chemical shift at 212.04 ppm (COO group
in radial position); the 198.91 ppm value of the minority (28%)
isomer indicates that in this case the carbene occupies an axial
position. The molecular structure of 17A shows that the four
iridium atoms define a regular tetrahedron and the phosphine
and carbene ligands are axially bonded to two vicinal Ir atoms of
the basal plane. The values of the dihedral angles between the tet-
rahedron base on the plane Ir1AIr2ACO12, Ir1AIr2ACO13 and
Ir2AIr3ACO23 [7.7(5)°, 0.7(7)° and 2.4(6)°, respectively] suggest
an asymmetrical bridging of the CO units. The reaction of Ir4
(CO)11(PPh3) [13] with a large excess of oxirane, NaBr and 2-bro-
moethanol gives compounds 18 and 19 with 37 and 40% yield,
respectively. The three bands at À9.68, À10.10 and 20.36 ppm
(42:39:19 ratios) in the 31
P{1
H} spectrum of 18 at 183 K identify
three possible isomers, 18A–18C. The resonances at À9.68 and
À10.10 ppm were assigned to PPh3 in axial position (18A and
18B) because they look like the starting complex (dax = À11.08
for Ir4(CO)11(PPh3)); the two isomers differ for carbene position,
as 17A and 17B above. The resonance at 20.36 ppm is coherent
with a radial coordination of the PPh3 moiety and belongs to
isomer 18C (see Scheme 7). When the 31
P{1
H} NMR spectrum is
collected at 310 K, the above three resonances coalesce into a
broad signal, an indication that the isomers undergo structural
rearrangement according to ‘‘merry-go-round’’ and ‘‘change of
basal face’’ of CO. A further confirmation of the existence of the
three isomers 18A–18C is given by the 13
C{1
H} spectrum. The latter
shows three separate sets of resonances with 42:39:19 ratios, each
one with eleven resonances in the areas typical of bridging and ter-
minal CO.
For the dioxycarbene (19) the 31
P{1
H} NMR spectrum at 183 K
has three signals at À7.08, À9.81 and 19.71 ppm with 55:34:11
ratios, thus indicating the existence of three isomeric forms
19A–19B–19C (see Scheme 8). The Dd (À0.2 and 2.9 ppm) suggest
an axial coordination of PPh3 (19A, 19B) and the value of 26.6 ppm
Fig. 5. (A,B) Molecular structures of [HIr4(CO)9(dppmMe)] 13a with [DBUH+
] and [DBUMe+
] 13b as counterion, respectively; (C,D) molecular structures of [HIr4(CO)9(dppe)]
14 and [HIr4(CO)9(dppp)] 15, respectively, both with [PPN]+
as counterion.
L
b a
b'
c
d'd
ee
g
f *C
c'
b a
b'
c
d'd
e'e
g
L *C
L
b a
b'
c
*Cd
e'e
g
f c'
17A
18A
18C 17B
18B
Scheme 7. Possible arrangements of ligands. L = Pt
Bu3 (17A–17B), PPh3 (18A–18B),
*C = COCH2CH2O.
A. Tassan et al. / Inorganica Chimica Acta 424 (2015) 91–102 97

8. a radial-coordinated of PPh3 (19C). The low-temperature 13
C{1
H}
spectrum is similar to that obtained previously (for compound
18), indicating the presence of three edge-bridging, two radial
and three apical CO units. On the other hand, the two signals
related to COOA show that in isomer 19A they hold one radial
and one axial position on two separate Ir atoms; in isomer 19B
they also are placed in radial and axial positions, but on the same
basal iridium atom; finally, in the minority isomer 19C the phos-
phine is radial, and both COOA groups take axial positions.
The compound 20, where the phosphine is dppm, can be pre-
pared in a similar manner to that reported for Pt
Bu3. The 31
P spec-
troscopic data show two signals at À51.14 and À57.25 ppm with
23/77 ratio, compared with d = À52.2 ppm for the starting cluster.
These data are consistent with the diphosphine being coordinated
in axial–axial positions [13]; hence, the two isomers differ only by
the position of the COOA group. Scheme 9 and Fig. 6 show the X-
ray crystal structure, confirming the results of the spectroscopic
analysis. The cluster has Cs symmetry, with the four iridium atoms
defining a regular tetrahedron, three bridging CO on the basal face
Ir1AIr2AIr3 and the carbene and diphosphine ligands in axial posi-
tions. The mean IrAP distance of 2.300(3) Å is in agreement with
known data [13,18,30].
Also the carbene derivatives obtained with olefinic ligands (nor-
bornadiene, 21, and 1,5-cyclooctadiene 22, 23) can be reacting
[Ir4(CO)11Br]À
with suitable olefin and the complexes have been
characterized by means of IR and 13
C NMR spectroscopy. Cluster
21 presents two isomers with 89:11 ratios, where the carbene
ligand binds to an axial and to a radial position, respectively. Com-
pounds 22 and 23 are obtained with 50% and 23% yield, respec-
tively. The IR spectra of both clusters show bands of bridging and
terminal CO. The 13
C NMR spectrum of compound 22 at 200 K
shows two sets of resonances (10 signals, relative intensities
18:82). Compound 23 shows three sets of signals. The first is given
by two carbenes holding a radial and an axial position on two
separate Ir atoms; the second refers to a couple of carbenes again
placed in radial and axial positions, but on the same Ir atom;
finally, the third one indicates two axially-bonded carbenes on
two separate basal iridium atoms.
7. Phosphites
The reaction of anionic clusters [Ir4(CO)11Br]À
with phosphite
ligands such as phenyl-dimethoxyphosphine, diphenyl-methoxy-
phosphine and diphenyl-phenoxyphosphine have been investi-
gated by Detti et al. [30,31]. The bromide is displaced by one
equivalent of phosphite at room temperature, giving the monosub-
stituted products [Ir4(CO)11{L}] [L = PPh(OMe)2 24; PPh2(OMe) 25
and PPh2(OPh) 26]. An excess of ligand affords the disubstituted
compounds [Ir4(CO)10{L2}] [L = PPh(OMe)2 27; PPh2(OMe) 28 and
PPh2(OPh) 29]. The monosubstituted complexes 24–26 can be
obtained with 35–60% yield. The IR spectra collected in dichloro-
methane solution show two m(CO) stretching bands below
1900 cmÀ1
, indicating the presence of bridging CO ligands. The
31
P{1
H} spectra obtained at 195 K in CD2Cl2 solution show only
one resonance, suggesting the presence of a single isomer. Besides,
the 13
CO-enriched (ca. 30%) 13
C NMR spectra of all compounds
point to the presence of two axial, three bridging, three radial
and three apical carbonyl groups, indicating that the phosphite
coordinates through an axial position.
The crystal structure of 26 and the selected labeling scheme are
shown in Fig. 7. The molecule contains a nearly tetrahedral Ir4 core,
with three CO units bridging to the basal face and with the phos-
phite ligands in axial position. The presence of a good rÀdonor
such as the diphenyl-phenoxyphosphine makes the Ir4AIr2, Ir4AIr3
distances (mean 2.755 Å) longer than the Ir1AIr2, Ir1AIr3 and
ca' a
b *Ch
e'e
g
f
ca' a
b
*C
h*C
e'e
g
f
c
h
a' a
b
d
e'e
g
*C
19B19A 19C
C*
PPh3
PPh3
PPh3
*C
Scheme 8. Arrangements of ligands in the dicarbene derivatives.
Ph2P
PPh2
b b
b hf
ge
g
f *C
cPh2P
PPh2
b b
bf
ge
g
f
*C
20A 20B
Scheme 9. Structure of Ir4(CO)9(dppm)(COCH2CH2O)
Fig. 6. ORTEP plot (30% of probability) of compound 20.
Fig. 7. ORTEP view of the molecular structure of [Ir4(CO)11{PPh2(OPh)}] 26. Thermal
ellipsoids at 50% probability.
98 A. Tassan et al. / Inorganica Chimica Acta 424 (2015) 91–102

9. Ir2AIr3 ones (mean 2.707 Å). The IrAC distances are comparable
with those of the other iridium clusters [32]. The IrAP distance
of 2.301(2) Å is greater than that found in Ir4(CO)11{P(OMe)3}
(2.258 Å), but shorter than those observed for phosphines (that
is, 2.311 Å for PMe3 [32], 2.335 Å for PPh3 [33]). As expected, the
differences of the bond distances between phosphites and
phosphines derive from the high p-withdrawing character of the
phosphites with respect to the phosphines.
The thermodynamic parameters for the isomerization A M B
have been determined integrating the 31
P{1
H} NMR signals
recorded at variable temperature (185–300 K) in toluene-d8 solu-
tion, because in this solvent the two isomers are present in similar
proportions. In the 185–210 K temperature range the exchange
between the two populations is slow and it is possible to calculate
the rate constant Keq = [A]/[B] at different temperatures. The linear
regression of logKeq versus 1/T allows to determine the difference
energy between the two isomers. The calculated thermodynamic
parameters are: DHeq = 2.132 ± 0.155 kJ molÀ1
, DSeq = 0.014 ±
0.005 kJ KÀ1
molÀ1
, DGeq = 1.970 ± 0.155 kJ molÀ1
.
The variable-temperature (190–300 K) 13
C{1
H} NMR spectra in
CD2Cl2 solution of compound A, the only isomer formed in this sol-
vent (Scheme 10), was carried out to investigate its fluxional
behavior. By analyzing the spectra obtained between 190 and
230 K, it was possible to identify only ‘‘merry-go-round’’ processes
of basal CO groups (bridging and radial, Scheme 10B). In the 230–
300 K range, it was not possible to define the other two processes:
face exchange and rotation of apical carbonyls, because the peaks
hinting at the two processes were overlapped (see Scheme 10A),
besides, above 300 K the compound decomposed. A simulation of
the NMR spectra by means of the Exchange program [34] allowed
to calculate the activation energy of the process at several
temperatures.
By using the Eyring linear regression equation we found for this
process: DG–
= 44.3 ± 0.8 kJ molÀ1
at 298 K; DH–
= 37.3 + 0.8 -
kJ molÀ1
; DS–
= À23.6 ± 3.5 J KÀ1
molÀ1
. In Table 1, the values of
the calculated activation energies for the ‘‘merry-go-round’’
process are compared with those experimentally obtained for
similar compounds. These data indicate the effect of the ligand
bulk, that is, the effect of increasing the angle between the basal
plane and the iridium-carbonyl bond.
The infrared spectra of compounds (27–29) collected in
dichloromethane solution show two m(CO) stretching bands below
1900 cmÀ1
, indicating the presence of bridging carbonyl ligands in
all complexes. The 31
P{1
H} spectra have been carried out in CD2Cl2
solution at 195 K and reveal two resonances due of the radial and
axial phosphorous. In addition, the analysis of 13
CO-enriched (ca.
30%) 13
C NMR spectra of all compounds point to the presence of
two axial, three bridging, two radial and three apical carbonyl
groups, indicating that two phosphite units coordinate through
an axial and a radial position.
The crystal structures of 27 and 29 and the selected labeling
schemes are shown in Fig. 8. The two molecules contain a nearly
tetrahedral Ir4 core with three CO units bridging to the basal face
and with the phosphite ligands in axial and radial positions. The
average IrAIr distance for 27 is 2.724 Å, a value consistent with
those found for related compounds such as Ir4(CO)10{P(OMe)3}2,
2.728 Å, and Ir4(CO)10(PPh3)2, 2.739 Å [33], and also in the IrAIr
distance range of dodecacarbonyl derivatives, but longer than that
of Ir4(CO)12 (2.693 Å). The Ir2AIr3 bond (2.702(10) Å) (see Fig. 8) is
considerably shorter than Ir2AIr4 (2.7399(7) Å) and Ir3AIr4
(2.7419(6) Å). Moreover, as observed for Ir4(CO)10(PPh3)2, the dis-
tances between the iridium atoms of the basal plan and the one
in apical position (Ir1) are all different: 2.7367(6) Å (Ir2AIr1),
2.7159(6) Å (Ir4AIr1) and 2.7089(6) Å (Ir3AIr1). The IrAP distances
for P4 (radial) and P2 (axial), are 2.262(2) and 2.251(2) Å, respec-
tively. They are shorter than those found in bis-diphenylphosphino
derivatives [33], because the two AOCH3 groups make the ligand a
good p-accepter.
The metal–metal bond distances in the basal plane (Ir2AIr3;
where the bound phosphorus atoms are located) for 29 are longer
than the other (2.770 versus 2.755 Ir3AIr4, and 2.762 Å Ir2AIr4),
and the Ir1AIr4 are shorter than the other (2.735 versus 2.7678
Ir1AIr3 and 2.7624 Å Ir1AIr2).
8. Intramolecular dynamics of [Ir4(CO)12] derivatives
Most studies on the fluxional behavior of the tetrahedral cluster
of iridium covers the migration of carbon monoxide. This migra-
tion has been described, in particular, with the models developed
by Cotton [35,36] and by Johnson and Benfield [37,38]. The first
is named ‘‘merry-go-round’’ and describes the exchange of sites
around the metal backbone; the second is called LPM, ‘‘Ligand
Polyhedral Model’’, and describes the exchange of the CO site as
the result of a rotation (or libration) of the metallic skeleton within
the envelope of the ligands whose donor atoms form the vertices of
a polyhedron which can deform (icosahedral M anticubeoctahe-
dral M icosahedral, for example).
The first experimental evidence, IR and NMR, of the ‘‘merry-
go-round’’ process has been obtained by the Roulet’s group during
the studies of [Ir4(CO)9(l3-1,3,5-trithiane], where the unbridged
isomer (A), which is in general the transition state of the
merry-go-round, was found both in solid state and in solution
(see Fig. 9) [39].
Over the years, a lot of monosubstituted tetrairidium deriva-
tives with Cs symmetry of general formula [Ir4(CO)11L] (L = PEt3,
PAr3, PMePh2, PHPh2, PH2Ph2, PPh3, P(OMe)3, P(OPh)3, etc.) have
been investigated [40,2]. Roulet and co-workers further deepened
the studies about the intramolecular dynamics of iridium carbonyl
clusters by analyzing the solution and the solid state behavior of
bidentate donor ligands, such as: 1,1-bis-(methylthio)ethane 30,
ethylidenebis(diphenylphosphine) 31 and propane-1,3-diyl-
bis(diphenylphosphine)] 32 [41]. The [Ir4(CO)10(l2-(MeS)2CHMe)]
(30) has a ground state geometry with only terminal CO units;
on the contrary, compounds 31 and 32 show three edge-bridging
CO groups, both in solution and solid state.
The crystallographic analysis for compound 30 shows a tetrahe-
dral metal core of Cs symmetry and only terminal CO ligands and
the S-atoms in axial–axial positions; it is one of the few Ir clusters
a
bb
ee
g
PPh2(OPh)
dd
fc c
ee
g
PPh2(OPh)
dd
fc c
d d
A B
Scheme 10. Two-isomer equilibrium for compound 26.
Table 1
Activation parameters at 298 K; h = Tolman’s cone angle [4].
L h(deg) DG–
(kJ molÀ1
)
PPh3 145 45.6 ± 0.4
PPh2(OPh) 139 44.3 ± 0.8
PPh2(OMe) 132 42.8 ± 0.8
P(OMe)3 107 37.5 ± 0.4
A. Tassan et al. / Inorganica Chimica Acta 424 (2015) 91–102 99

10. without any bridging CO. The CAO bond lengths are in the typical
range for terminal CO groups. Interestingly, complex (30) can exist
in two conformations (A and B). Upon coordination to the axial
positions, the ligand forms a five-member ring, where C1 may stay
apart from the Ir1AIr2AIr3 plane or lie beneath. Of course, each
one of these conformers may also have two isomers (a and b),
depending on whether the Me group bound to C1 is also placed
away or under the Ir1AIr2AIr3 plane (Scheme 11).
As mentioned above, complexes 31 and 32 both have a struc-
ture with three bridging CO on the basal triangle Ir1AIr2AIr3,
and a diphosphine ligand bound to axial positions (see Fig. 10).
Compound 31 has an Aa conformation, with the CAMe bond
roughly parallel to the Ir1AIr2AIr3 triangular face.
Complexes 31 and 32 both have Cs symmetry, but while for 31
the phenyl moieties P(1) and (P2) are not related by symmetry;
complex 32 shows a mirror plane passing through Ir3AIr4 and
Ir1AIr2 bond. The reason of the difference between the two com-
plexes both, must be ascribed to intramolecular steric effects and
a different hydrogen bonding network, which has already been
described [42].
The 1
H NMR spectrum of compound 30 shows one quartet and
one doublet relative to HACAMe that indicate the conformation of
the coordinated ligand; moreover, the presence of a singlet for the
two SAMe shows the mirror symmetry of the complex. The
13
CO-enriched (30%) 13
C{1
H} NMR spectrum in CD2Cl2 at 177 K of
compound 30 presents six resonances for terminal CO units at:
167.7 (a), 164.9 (b), 164.1 (c), 158.8 (d), 157.2 (g) and
154.3(e) ppm, with relative intensities 2:2:1:2:2:1. The 2D-EXSY
Fig. 8. ORTEP view of the molecular structure of 27 and 29. Thermal ellipsoids at 50% probability.
a a
a bb
ee
e
b
g g
g
cc
c c
cc
S S
S
SS
S
A B
Fig. 9. Ir4(CO)9(l3-1,3,5-trithiane in solution, unbridged (A) and bridged (B) isomer.
Ir4
Ir3
Ir1
X
Ir2
X
C1
H
CH3
H
CH3
a b
Conformers A
Ir4
Ir3
Ir1
X
Ir2
X
C1
H3C
H
CH3
H
a b
Conformers B
Scheme 11. The two conformation A and B with the two possible isomers a and b (X = SCH3, PCH3).
100 A. Tassan et al. / Inorganica Chimica Acta 424 (2015) 91–102

11. spectrum in CD2Cl2 at 215 K shows one intense cross-peak,
between 164.9 and 158.8 ppm, indicating the dynamic connectiv-
ity b M a M d (see Fig. 11), and one less intense at 164.1 and
158.8 ppm indicating c M d exchange; a third g M e exchange pro-
cess takes place at 270 K. The exchange b M a M d corresponds to
the ‘‘merry-go-round’’ of the six CO groups about the Ir1AIr2AIr3
triangular face; the second and third exchanges involve only two
sites, that is, those arising from the rotation of three CO residing
on the mirror plane Ir3 and Ir4. The free activation enthalpies
calculated by Eyring linear regression equation at 298 K are:
DG1
–
= 42.6 ± 0.4, DG2
–
= 47.0 ± 0.4 and DG3
–
= 58.0 ± 0.8 kJ molÀ1
.
The cluster 31 has the same geometry both in solution and in
the solid state. The 13
CO-enriched (30%) 13
C{1
H} NMR spectrum
of compound 31 shows seven resonances at: 223.5 (a), 203.8 (b),
179.5 (f), 171.3 (d), 163.8 (c), 162.2(e) and 157.7(g) ppm, with
relative intensities 1:2:2:1:1:1:2. The 2D-EXSY spectrum of 31 in
THF at 215 K is similar to that of 32 [18]. The lowest energy
process, again, the ‘‘merry-go-round’’ one, involves with rate
constant k1 the a, b, f and d CO units. At 247 K, the signal for CO
c starts to broaden (rate constant k2), while at 287 K emerges the
exchange between CO g and e. The free activation enthalpies, calcu-
lated by Eyring linear regression equation are: DG1
–
= 38.7 ± 0.4,
DG2
–
= 50.4 ± 0.4 and DG3
–
= 60.1 ± 0.6 kJ molÀ1
.
The unobserved intermediate of ‘‘merry-go-round’’ in
complexes 31 and 32 has a geometry with all CO in terminal posi-
tions. It may be that the transition state for the ‘‘merry-go-round’’
process has a semi-bridged geometry, as already reported [43]. The
energy barrier of ‘‘merry-go-round’’ process for compound 31 is
Fig. 10. ORTEP-like view of molecular structures of 30, 31, 32.
Fig. 11. 2D-EXSY spectrum of 30 in CD2Cl2 at 215 K.
A. Tassan et al. / Inorganica Chimica Acta 424 (2015) 91–102 101

12. lower than that calculated for 32 (DG1
–
= 38.7 kJ molÀ1
and
53.9 kJ molÀ1
respectively).
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