Chelating versus bridging bonding modes of N-substituted bis(diphenylphosphanyl)amine ligands in Pt complexes and Co2Pt clusters

Article abstract of DOI:10.1039/b514787e

N-Substituted dppa ligands Ph₂P-NR-PPh₂ [R = -CH ₂CH₂SCH₂C₆H₅ (1), -CH₂CH₂S(CH₂)₅CH₃ (2), -(CH₂)₉CH

Full text of DOI:10.1039/b514787e

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Chelating versus bridging bonding modes of N-substituted
bis(diphenylphosphanyl)amine ligands in Pt complexes and Co2Pt clusters
Vito Gallo,a,b Piero Mastrorilli,*a Cosimo F. Nobile,a Pierre Braunstein*b and Ulli Englertc
Received 18th October 2005, Accepted 16th January 2006
First published as an Advance Article on the web 30th January 2006
DOI: 10.1039/b514787e
N-Substituted dppa ligands Ph2P–NR–PPh2 [R = –CH2CH2SCH2C6H5 (1), –CH2CH2S(CH2)5CH3 (2),
–(CH2)9CH3 (3), –C6H5 (4)] were used for the synthesis of cis-[PtCl2{Ph2PN(R)PPh2}] complexes [R =
–CH2CH2SCH2C6H5 (5), –CH2CH2S(CH2)5CH3 (6), –(CH2)9CH3 (7), –C6H5 (8)] and heterotrinuclear
clusters of formula [PtCo2(CO)7{Ph2PN(R)PPh2}] [R = –CH2CH2SCH2C6H5 (9), –CH2CH2S(CH2)5CH3
(10), –(CH2)9CH3 (11), –C6H5 (12)]. The presence of relatively bulky substituents on N resulted in a
higher chelating power of the ligands. The thermodynamic study of the equilibrium between the chelate
and the bridged forms of clusters 9–11 showed that the bridged form is favoured by enthalpic factors
whereas entropic factors favour chelation. The structures of 5 and 9 were determined by single crystal
X-ray diffraction.
interactions that occur on chelation: N-alkylated ligands undergo
Introduction
stronger interactions than unsubstituted dppa. The entropy factor
Diphosphanes containing a single atom as spacer between
is explained by comparing the degrees of freedom associated with
the donor atoms, such as dppm [dppm = bis(diphenylphos-
rotation about the P–N bond in the free ligand with those in the
phanyl)methane] and dppa [dppa = bis(diphenylphosphanyl)-
coordinated ligand. In the free ligand, N-substituted dppa has
amine] belong to the category of short-bite ligands and their co-
a more hindered rotation about the P–N bond due to the steric
ordination chemistry has attracted much interest.1 Such bidentate
interaction of the substituent on nitrogen with the phenyl groups
ligands can coordinate to metal centres in monodentate, chelating
on the P atoms. Consequently, the decrease of internal rotational
or bridging modes and interconversions between them are often
entropy that occurs on chelation is less in the N-substituted dppa
observed as a result of seemingly minor changes in the ligand
than in dppa.
substituents or in the coligands bound to the metal centre(s). These
N-substituted dppa-type ligands represent an attractive class
aspects and, in particular, their bridging vs. chelating behaviour,
are still a matter of discussion. For instance, it has been reported
of ligands for at least two reasons: they are isoelectronic to
dppm but possess slightly different electronic properties and they
that dppa exhibits a strong chelating ability in Pt(II) complexes,2
can be easily derivatized on the nitrogen. This latter feature
fuels the stimulating research on dppa-derivatives8 which already
covers interesting applications in the fields of catalysis9 and
nanomaterials.10 Funtionalized dppa ligands were shown to be
suitable for the anchoring of carbonyl clusters on supports owing
to their propensity to bridge an edge of the cluster.10
Herein we report on the behaviour shown by N-substituted
dppa ligands Ph2PN(R)PPh2 (1–4) when used for the synthesis of
new Pt(II) complexes of formula cis-[PtCl2{Ph2PN(R)PPh2}] [R =
–CH2CH2SCH2C6H5 (5), –CH2CH2S(CH2)5CH3 (6), –(CH2)9CH3
(7), –C6H5 (8)] and of heterotrinuclear clusters of formula
[PtCo2(CO)7{Ph2PN(R)PPh2}] [R = –CH2CH2SCH2C6H5 (9),
–CH2CH2S(CH2)5CH3 (10), –(CH2)9CH3 (11), –C6H5 (12)]. The
obtained results allowed to correlate the bulkiness of R with
but no chelation is observed when it is used in the synthesis
of dinuclear Pd(I) complexes.3 The coordination mode of these
ligands depends on the nature of both the substituents on the
phosphorus atom and the bridging backbone. The propensity to
form chelate complexes increases with the bulkiness of the groups
on phosphorus.4 The replacement of one or two hydrogen atoms
of the methylene carbon of dppm with alkyl groups results in
a greater tendency to form stable four-membered chelate rings.5
A similar effect has been observed upon replacement of the N–
H hydrogen atom of dppa. Such results were readily explained
invoking the Thorpe–Ingold (gem-dimethyl) effect in small organic
rings,6 i.e. the accelerated rate of cyclization due to the “angle
compression” generated by the alkyl substitution. In studies on
Ag complexes, Hill7 reported that the effect of an alkyl group
bound to the N atom of the ligand on the cyclization has steric
origins and can be rationalized by evaluating two factors, one
enthalpic and one entropic. The first one involves the gauche
aDipartimento di Ingegneria delle Acque e di Chimica del Politecnico di
Bari, via Orabona 4, I-70125 Bari, Italy. E-mail: p.mastrorilli@poliba.it;
Fax: +39 080 5963611
bLaboratoire de Chimie de Coordination, UMR 7513 CNRS Universite´
Louis Pasteur 4, rue Blaise Pascal, F-67070 Strasbourg, France. E-mail:
braunst@chimie.u-strasbg.fr; Fax: +33 390241322
cInstitut fu¨r Anorganische Chemie der RWTH, Landoltweg 1, D-52074
Aachen, Germany
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the chelating ability of the ligand. Moreover thermodynamic
determinations clarified the relative importance of enthalpic and
entropic factors in the chelating vs. bridging bonding mode of the
N-substituted dppa ligands.
Results and discussion
N-substituted dppa ligands can be synthesised following two
different procedures: (i) the deprotonation of the acidic NH proton
of dppa and the subsequent derivatization of the nitrogen atom
by the required electrophile; (ii) the reaction of the suitable amine
and two equivalents of chlorodiphenylphosphane. In the present
work, we have chosen the second method since the starting amines
were easily available: decylamine and aniline are commercially
available and the amines needed for ligands 1 and 2 can be
obtained by reaction of 2-aminoethanethiol with the relevant alkyl
halide. All pure ligands used for the synthesis of the complexes 5–8
were characterized by IR and multinuclear spectroscopic methods.
Scheme 2 Isomerization of 13.
1/Pt molar ratio = 1 : 1) lasted for at least 2 h. 31P{ H} and
1
195Pt{ H} NMR spectroscopic features were fully consistent with
1
the bidentate ligand chelating the Pt atom. The 31P{ H} NMR
1
spectrum contained one singlet flanked with satellites centred at d
1
17.8 (1JP,Pt 3304 Hz) while the 195Pt{ H} NMR spectrum showed a
triplet at d −4037 (1JP,Pt 3304 Hz). The value of the 1JP,Pt coupling
constant is consistent11 with the presence of Cl atoms trans to
P. The IR bands due to the Pt–Cl stretching vibrations were found
at 305 and 288 cm−1.12
1
The 31P{ H} NMR spectra of 1–4 showed the expected signals
The crystal structure of complex 5 is shown in Fig. 1 and selected
bond distances and angles are collected in Table 1. The geometry
around the platinum atom is distorted square planar in order to
fulfil the constraints imposed by ligand 1. The plane defined by
the Pt, P(1) and P(2) atoms is almost coincident with that defined
by N, P(1) and P(2) (t◦he sum of the internal angles in the Pt–P(1)–
downfield shifted with respect to free dppa (d 43.5). In particular,
1–3 gave a singlet in the range d 62.2–62.9 while a singlet centred
at d 68.6 was found for 4.
Although the synthesis of dichloro-bisphosphanyl Pt(II) com-
plexes is generally easy and fast, a clean preparation of complexes
5–8 was not straightforward, due to complications originating
from concomitant formation of undesired bis-chelated species.
Taking the synthesis of 5 as a model for those of 5–8 (Scheme 1),
it was observed that when the addition of 1 was not sufficiently
slow, the reaction mixture contained at the end of reaction the
cationic bis-chelated Pt(II) complex [Pt(1)2]Cl2 (13) along with
5 and unreacted [PtCl2(cod)]. Complex 13 is characterized by
˚
N–P(2) ring is 359.92 ). The average Pt–P bond length [2.2115 A]
as well as the P–Pt–P [72.06(3)◦] and P–N–P [99.34(13)◦] angles in
5 are comparable to those found for [PtCl2(dppaMe)] [dppaMe =
˚
bis(diphenylphosphanyl)methylamine, average Pt–P = 2.206(4) A;
P–N–P = 71.8(1)◦ and 100.0(6)◦].13 Interestingly, the sum of angles
at nitrogen (Rh = 354.4◦) is lower than that in [PtCl2(dppaMe)]
1
a singlet flanked with satellites (d 29.7, JP,Pt 2373 Hz) in the
1
31P{ H} NMR spectrum and a quintet (d −4383, 1JP,Pt 2373 Hz) in
1
the 195Pt{ H} NMR spectrum. The absence of absorption bands
assignable to Pt–Cl stretching modes in the IR spectrum confirms
the ionic nature of the complex. Leaving complex 13 in CD2Cl2 at
room temperature for one week resulted in the slow isomerization
to the PhCH2SCH2CH2N(PPh2)2 bridged dinuclear complex 14
1
1
depicted in Scheme 2. The 31P{ H} and 195Pt{ H} NMR spectra
of the aged solution showed the progressive disappearance of
the signals ascribable to 13 with a contemporary growing of the
AAꢀXXꢀ spin system for 14 where A (d 65.6) is the P nucleus of
the bridging ligands and X (d 42.4) that of the chelating ligands.
1
In agreement with the proposed structure, the 195Pt{ H} NMR
spectrum of 14 showed a triplet of triplets centred at d −4682
[1JP(A),Pt 2698 Hz, 1JP(X),Pt 1908 Hz].
Fig. 1 ORTEP view of the crystal structure of 5.
Pure 5 could be obtained in 89% yield when the dropwise
addition of 1 to [PtCl2(cod)] (both as dichloromethane solutions,
Table 1 Selected bond distances and angles for 5
Bond angles/◦
˚
Bond lengths/A
Pt(1)–P(2)
Pt(1)–P(1)
Pt(1)–Cl(1)
Pt(1)–Cl(2)
P(1)–N(1)
P(2)–N(1)
2.2099(9)
2.2131(9)
2.3637(9)
2.3636(10)
1.710(3)
P(2)–Pt(1)–P(1)
P(2)–Pt(1)–Cl(1)
P(1)–Pt(1)–Cl(2)
Cl(1)–Pt(1)–Cl(2)
N(1)–P(1)–Pt(1)
N(1)–P(2)–Pt(1)
P(2)–N(1)–P(1)
C(13)–N(1)–P(2)
C(13)–N(1)–P(1)
72.06(3)
96.28(3)
98.98(3)
92.44(3)
94.11(9)
94.41(9)
99.34(13)
126.4(2)
128.7(2)
1.703(3)
Scheme 1 Synthesis of Pt(II) complexes.
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Table 2 Percentage of pyramidal character (P.P.C.%) and geometrical
12a and 9b–12b of Scheme 3. This was rather surprising, given that
analogous clusters bearing dppa ([PtCo2(CO)7{Ph2PN(H)PPh2}],
15) or dppaMe ([PtCo2(CO)7{Ph2PN(CH3)PPh2}], 16) are known
to have, both in the solid state and in solution, exclusively the
bridged structure a.8a,b Furthermore, the strong propensity of the
PCP ligand dppm to bridge the edge of an analogous PtCo2 cluster
has also been demonstrated.15
parameters for dppa-based ligands in their Pt complexes
Complex
Rh/◦
P. P. C. /%
PtCl2(dppaMe) 14
357.1
355.0
354.4
9.2
14.6
17.7
14
[Pt(dppaMe)2]2+
5
(Rh = 357.1◦) indicating a higher pyramidal character of the
nitrogen atom (compare with Rhplan = 360◦ for an ideally planar
nitrogen centre and Rhtetr = 328.5◦ for an ideally tetrahedral
nitrogen centre).
According to Farrar,14 the pyramidal character of the nitrogen
atom can be evaluated by means of the percentage pyramidal
character (P.P.C.%, defined as in eqn (1) where Rh is the sum of the
three angles at nitrogen) and can be taken as a measure of the Pt–P–
N–P ring strain (the higher the pyramidal character of N the higher
the Pt–P–N–P ring strain). In fact, on going from [PtCl2(dppaMe)]
to the higher ring strained complex [Pt(dppaMe)2]2+ the P.P.C.%
went from 9.2% to 14.6%.14 In our case, the P.P.C.% of 17.7%
calculated for 5 (Table 2) indicates a ring strain which is higher
than those of both [PtCl2(dppaMe)] and [Pt(dppaMe)2]2+. The
reason for the increased ring strain has to be associated with the
nature of the N-substituent.
Scheme 3 Structures of clusters 9–12.
In order to identify the most stable structure in the solid state, an
XRD analysis of the crystals obtained by slow diffusion of hexane
into a dichloromethane solution of 9 at room temperature was
undertaken. The crystal structure (Fig. 2) shows that in the solid
state the complex crystallises as 9b, thus confirming the unusual
stability of the four membered ring formed by ligand 1 in these
[PtCo2] clusters.
[360◦ − (Rh)]
31.5◦
[(Rh)plan − (Rh)]
[(Rh)plan − (Rh)tetr
P.P.C. (%) =
· 100 =
· 100
(1)
]
The bis-chelate complex 13 is expected to show an even higher ring
strain with respect to 5, therefore its tendency to give 14 could be
conceived as a way to reduce the strain while retaining (for one
ligand) chelation. In other words, the thermodynamically stable
species 14 represents a compromise between the strong tendency
of ligand 1 to chelate (as demonstrated by the inevitable formation
of 13 unless the reaction was carried out by slow addition of the
reagent), and the high ring strain imposed by a bis-chelation.
The procedure which gave 5 selectively, i.e. the slow, dropwise
addition of one equivalent of the relevant ligand to [PtCl2(cod)]
in dichloromethane, allowed the synthesis of 6–8 in good yields.
The spectroscopic data for complexes 5–8 are collected in Table 3
from which it is apparent the similarity between all complexes
bearing the N-alkyl substituted ligand 5–7. Complex 8 showed the
1
1
highest 31P{ H}NMR chemical shift, the highest JP,Pt coupling
and the lowest IR wavenumbers for the Pt–Cl stretching vibrations,
all indicative of a stronger bond between the phosphorus atoms
and Pt.
Fig. 2 ORTEP view of the crystal structure of 9b.
All the heterotrinuclear clusters with a PNP short-bite ligand
and the [PtCo2] core8a,b were prepared by reaction of the Pt(II) com-
plexes 5–8 with two equivalents of NaCo(CO)4 in dichloromethane
at room temperature. The resulting brown solids obtained after
work-up gave, in solution, NMR signals ascribable to species 9a–
Selected bond distances and angles are collected in Table 4. The
metal core forms a nearly equilateral triangle with the Co atoms
bridged by the C(1)O(4) ligand. This confers an 18 electron and
a 16 electron environment to the Co and Pt atoms, respectively.
˚
The Pt–C(1) distance of 2.733(4) A is longer than those found for
1
1
Table 3 Selected 31P{ H} NMR, 195Pt{ H} NMR and IR spectroscopic
˚
clusters 15 [2.55(1) A] and [Co2Pt(l3-CO)(CO)7(PPh3)] [2.57(1)
16
features of complexes 5–8
˚
A] thus indicating that the l2-CO ligand does not interact with
platinum. The geometry around the platinum is distorted square
planar (sum of the angles at Pt = 359.0◦). The P–Pt bond distances
are slightly longer than those of the Pt(II) precursor 5 while the
P–N–P angle is slightly larger.
The mixture of clusters having structures a and b formed
also varying the experimental procedure, namely carrying out
Complex
dP/ppm
1JP,Pt /Hz
dPt/ppm
mPt–Cl/cm−1
5
6
7
8
17.8
17.7
17.1
20.7
3304
3308
3296
3330
−4037
−4038
−4038
−4060
288
305
308
306
300
289
286
276
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Table 4 Selected bond distances and angles for 9b
Bond angles/◦
˚
Bond distances/A
Pt(1)–P(2)
Pt(1)–P(1)
Pt(1)–Co(1)
Pt(1)–Co(2)
Co(1)–Co(2)
Co(1)–C(1)
Co(1)–C(5)
Co(1)–C(6)
Co(1)–C(7)
Co(2)–C(1)
Co(1)–C(2)
Co(1)–C(3)
Co(1)–C(4)
Pt(1)–C(1)
P(1)–N(1)
2.2351(9)
2.2427(9)
2.5587(5)
2.5426(5)
2.5511(7)
1.922(4)
1.807(4)
1.792(4)
1.772(4)
1.928(4)
1.759(4)
1.823(4)
1.784(4)
2.733(4)
1.710(3)
1.702(3)
1.487(4)
P(2)–Pt(1)–P(1)
P(1)–Pt(1)–Co(1)
P(2)–Pt(1)–Co(1)
P(1)–Pt(1)–Co(2)
P(2)–Pt(1)–Co(2)
Co(1)–Pt(1)–Co(2)
Co(2)–Co(1)–Pt(1)
Pt(1)–Co(2)–Co(1)
N(1)–P(1)–Pt(1)
N(1)–P(2)–Pt(1)
P(2)–N(1)–P(1)
71.53(3)
114.99(3)
170.07(2)
171.10(2)
112.44(3)
60.010(16)
59.681(16)
60.309(15)
93.28(10)
93.77(10)
100.21(15)
126.1(2)
C(20)–N(1)–P(2)
C(20)–N(1)–P(1)
133.2(2)
P(2)–N(1)
N(1)–C(20)
the reactions (i) in different solvents (toluene, acetone or THF);
(ii) at different temperatures (in the range 25–50 ◦C); (iii) using
a NaCo(CO)4/Pt(II) molar ratio higher than 2. The increase
of temperature or of the NaCo(CO)4/Pt(II) molar ratio had an
accelerating effect on the reaction.
1
The 31P{ H} NMR spectra of complexes 9a–12a contain a broad
signal around d 100 ascribable to P bound to Co and a higher field
doublet flanked with satellites ascribable to P bound to 195Pt. The
Fig. 3 31P NMR spectrum of 11 after dissolution in CH2Cl2.
1
corresponding 195Pt{ H} NMR signals are doublets of doublets
centred near d −4100. Structures b, where the two phosphorus
obtained in dichloromethane at 295 K for clusters 9–12. It is
apparent that clusters 9–11 bearing alkyl substituents on the
nitrogen atom of the ligand have similar Ke while that bearing
the phenyl group has a higher Ke indicating, for 12, a stronger
preference for the chelate structure b.
It has already been mentioned that the existence of an equilib-
rium between the chelating and the bridging forms a and b for
clusters bearing alkyl or phenyl substituents on N of the dppa
ligand was not observed in related clusters bearing dppa or dppa-
Me. In order to gain a deeper insight into the contributions of
enthalpy and entropy to DG◦, we have calculated DG◦ of reaction
(1) (DG◦ = −RT lnKe) in the temperature range 295–375 K in 1,2-
dichlorobenzene. The plot of DG◦ vs. T (Fig. 4) was found linear
for clusters 9–11 and allowed to calculate DH◦ as the intercept
and −DS◦ as the slope of the line (DG◦ = DH◦ − TDS◦). Accurate
atoms are equivalent, gave a singlet flanked with satellites in the
1
1
31P{ H} NMR and a triplet in the 195Pt{ H} NMR spectra.
The relative amount of compounds a and b was found to
depend on both the solvent in which the reaction products were
dissolved, and the temperature, hence suggesting the existence
of an equilibrium between a and b (Reaction 1, Fig. 3). As
to the role of the solvent, the b/a molar ratio assessed on
the basis of the 31P NMR integrals17 of the acetone solution
of 11 at 295 K was 2.9 (sample a), while those calculated in
1,2-dichlorobenzene and in THF were 2.1 and 1.8, respectively.
The b/a value calculated for sample a after acetone evaporation
and redissolution in dichloromethane was 1.5 (Fig. 3). Further
evaporation of dichloromethane and redissolution in acetone
brought the b/a molar ratio back to 2.9.
A similar effect was observed performing 31P VT-NMR experi-
ments. When a 1,2-dichlorobenzene solution of 11 was heated to
375 K, the b/a ratio became 5.5. Upon cooling the solution down
to 295 K the b/a ratio was 2.1 again.
Considering that the volume of the solution is the same for b and
a, the b/a molar ratio represents the thermodynamic equilibrium
constant Ke = [b]/[a] = b/a. Table 5 reports the values of Ke
Table 5 Thermodynamic data of clusters 9–11
DH◦b/kJ mol−1
DS◦b/J K−1 mol−1
a
Cluster
Ke
9
1.4
1.0
1.5
5.9
7.8 0.5
12.8 0.6
11.3 0.6
n.d.
31
47
44
n.d.
2
2
2
10
11
12
a Solvent: CH2Cl2, T = 295 K. b Solvent: 1,2-dichlorobenzene, temperature
range 295–375 K.
Fig. 4 DG◦ vs. T plot. 9, R = 0.995; 10, R = 0.996, 11, R = 0.997.
Dalton Trans., 2006, 2342–2349 | 2345
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measurements of DG◦ vs. T (and obviously of DH◦ and DS◦) for 12
were prevented by the relatively large errors made when integrating
the weak NMR signal of the Pt-bound P of 12a.
of sodium (overall amount: 2.05 g) were added. After 24 h, the
resulting black suspension was filtered (sintered glass filter no. 4)
and the resulting colorless solution gave NaCo(CO)4 as a white
solid (1.47 g, 84%) after evaporation of the solvent under reduced
pressure. mmax(THF)/cm−1 1890 (CO).
The positive values of DH◦ found for the isomerization of a
into b (reaction 1) indicate that the bridged structure a is favoured
with respect to the chelate structure b in terms of bond energy
balance. This is rationalized by considering the higher stability
of five membered cycles with respect to four membered cycles
and can explain that clusters 15 and 16 only exist in the bridged
form. However, the positive DS◦ values found for clusters 9–11
(Table 5) render the TDS◦ term predominant with respect to DH◦,
resulting in a negative DG◦ which accounts for equilibrium (1). A
comparison with clusters 15 and 16 tends to indicate a relationship
between the bulkiness of the substituent on nitrogen and the
entropic contribution to DG◦ of reaction (1). In fact the number
of microstates of coordinated dppa(s) bearing bulkier substituents
on N (such as those of ligands 1–4) should increase when the P∧P
bite angle decreases (entropic factor). In the case of clusters 15
(H on N) and 16 (CH3 on N) the number of microstates could be
thought independent of the P∧P bite angle due to the less steric
demand of the substituents on N, so that the higher stability of
the less strained five-membered cycle (enthalpic factor) renders
unfavorable the isomerization a → b.
Synthesis of PhCH2SCH2CH2NH2
To a solution of EtONa in ethanol, obtained by gradual addition
of sodium metal (overall amount: 2.29 g, 99.5 mmol in 200 cm3),
2-aminoethanethiol hydrochloride (8.52 g, 49.8 mmol) was added.
After the resulting solution was boiled at reflux for 1 h, benzyl
bromide (5.66 g, 49.8 mmol) was added drop-wise, and the result-
ing solution was refluxed overnight. Cooling to room temperature,
filtration, and evaporation of the solvent led to an off-white solid
from which the waxy white amine was obtained by extraction
with dichloromethane (3 × 100 cm3); dH(CDCl3): 7.60–7.35 (5H,
m, C6H5), 3.91 (2H, s, Ph–CH2–S), 3.01 (2H, m, S–CH2–CH2–
NH2), 2.72 (2H, m, S–CH2–CH2–NH2) and 1.85 (2H, s, –NH2);
dC(CDCl3) 138.4 (C6H5, Cipso), 128.8 (C6H5, Cortho/meta), 128.5 (C6H5,
Cmeta/ortho), 127.0 (C6H5, Cpara), 40.7 (Ph–CH2–S), 35.8 (s, S–CH2–
CH2–NH2), 35.3 (s, S–CH2–CH2–NH2).
In conclusion, the presence of relatively bulky substituents on
N results in a higher chelating power of dppa(s), due to entropic
factors. In the case of mononuclear Pt complexes it justifies the
tendency to give bis-chelate species, whereas in the case of PtCo2
clusters it renders the energy of the chelate form b comparable
with that of the bridged form a, resulting in equilibrium (1).
Synthesis of CH3(CH2)5SCH2CH2NH2
To a solution of EtONa in ethanol, obtained by gradual addition
of sodium metal (overall amount: 1.93 g, 84.1 mmol in 80 cm3),
2-aminoethanethiol hydrochloride (4.76 g, 41.9 mmol) was added.
After the resulting solution was boiled at reflux for 1 h, a THF
solution of hexyl bromide (6.91 g, 41.9 mmol in 50 cm3) was added
drop-wise, and the resulting solution was refluxed overnight.
Cooling to room temperature and evaporation of the solvent
led to a pale yellow suspension. Addition of dichloromethane
(50 cm3), filtration on Celite and evaporation of the solvent
gave the amine as a dense pale yellow liquid (5.27 g, 78%);.
m/z (EI) 161 (M+, 11%), 144 (9), 132 (100), 117 (97) and 84
(46); mmax(CsI)/cm−1 3358br, 3287br, 2954s, 2926s, 2857s, 1592w,
1467w, 1378w, 1260w, 1068w, 1014w, 813w, 721w. dH(CDCl3):
0.65 (3H, t, –CH3), 1.00–1.10 (6H, –NH2 and CH3CH2CH2–),
1.14 (2H, m, –CH2CH2CH2S–), 1.33 (2H, m, –CH2CH2CH2S–),
2.25 (2H, t, –CH2SCH2CH2NH2), 2.36 (2H, t, –SCH2CH2NH2)
and 2.61 (2H, br, –CH2NH2); dC(CDCl3): 41.0 (–CH2NH2), 36.2
(–SCH2CH2NH2), 31.6 (–CH2SCH2CH2NH2), 31.2 (CH3CH2-
CH2–), 29.6 (–CH2CH2CH2S–), 28.4 (–CH2CH2CH2S–), 22.4
(CH3CH2CH2–) and 13.8 (–CH3).
Experimental
General
All the manipulations were conducted under an inert gas (ar-
gon) using standard Schlenk techniques. Flash-chromatography
was performed on SiO2 Kieselgel 230–400 mesh. Multinuclear
NMR spectra were recorded with a Bruker AX400 spectrometer
(400 MHz for 1H) at 295.0 K; chemical shifts are reported
in ppm referenced to SiMe4 for 1H and 13C, H3PO4 for 31P
and H2PtCl4 for 195Pt. J values are given in Hz. C, H, N
elemental analyses were carried out with a Eurovector CHNS–
O Elemental Analyser. Pt and Co analyses were performed with
a Perkin Elmer SIMAA6000 Atomic Absorption spectrometer
after mineralization. The chloride content of the complexes
was determined by a potentiometric automatic titration using a
Metrohm 716 DMS Titrino. The IR spectra were recorded with
a Bruker Vector 22 FT-IR spectrometer. GC-MS analyses were
performed with an HP6890-HP5973MSD instrument equipped
with an HP-5MS capillary column (30 m × 250 lm × 0.25 lm).
All commercial reagents were used as received without further
purification. Solvents were dried and distilled under nitrogen
according to standard procedures. PtCl2(cod)18 was synthesised
according to literature procedures.
Synthesis of 1 and 2
To
a
vigorously stirred suspension of the amine
PhCH2SCH2CH2NH2 (7.14 g, 42.8 mmol) and triethylamine
◦
(11.91 g, 117.9 mmol) in diethyl ether (100 cm3) kept at 0 C, a
solution of chlorodiphenylphosphane in diethyl ether (23.54 g,
107.0 mmol in 100 cm3) was added dropwise in 90 min. After
the mixture was stirred for 2 days at room temperature, the
solvent and the excess triethylamine were removed under reduced
pressure. The resulting solid was washed with water (5 × 60 cm3),
methanol (5 × 20 cm3) and hexane (5 × 20 cm3) and dried under
reduced pressure.
Synthesis of NaCo(CO)4
To a stirred deep brown solution of dicobalt octacarbonyl in THF
(1.53 g, 4.5 mmol in 80 cm3), a little drop of mercury and pieces
The same procedure was followed for the synthesis of 2.
2346 | Dalton Trans., 2006, 2342–2349
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1. (14.18g, 62%). (Found C, 74.45;H, 6.1;N, 2.75. C33H31NP2S
requires C, 74.0; H, 5.8; N, 2.6);. mmax(KBr)/cm−1 3053w, 2919w,
2903w, 1584w, 1493w, 1478w, 1452w, 1433s, 1091s, 1058vs,
1024s, 1008w, 998w, 867s, 833vs, 743vs, 722w, 695vs, 677s, 626s,
615w, 562w, 539w, 518w, 510w, 493s, 459s, 435w, 415w, 358w;
dH(CDCl3): 7.54–7.11 (25H, –N[P(C6H5)2]2 and –SCH2C6H5),
3.48 (2H, m, –SCH2CH2N–), 3.44 (2H, s, PhCH2S–) and 2.12
(2H, m, –SCH2CH2N–); dC(CDCl3): 139.1 [(C6H5)2P, Cipso], 138.4
[(C6H5)CH2S, Cipso], 132.7 [(C6H5)2P, Cortho], 129.0 [(C6H5)2P, Cpara],
128.8 [(C6H5)CH2S, Cortho/meta], 128.5 [(C6H5)CH2S, Cmeta/ortho], 128.2
[(C6H5)2P, Cmeta], 126.9 [(C6H5)CH2S, Cpara], 52.6 (t, –SCH2CH2N–,
with MgSO4 and the solvent removed under reduced pressure.
The solid was purified by flash chromatography (petroleum ether
40–60 ◦C : ethyl ether, 6 : 4) and further washed with hexane
(4 × 20 cm3). The product was obtained as a white solid after
evaporation of the solvent (4.79 g, 41%). (Found C, 78.3; H, 5.6;
N, 3.0; C30H25NP2 requires C, 78.1; H, 5.5; N, 3.0); mmax(KBr)/cm−1
3084w, 3056w, 3041w, 3028w, 3013w, 2999w, 1493s, 1438s, 1209vs,
937vs, 908w, 866vs, 737vs, 695vs, 524s, 511s, 465w; dH(CDCl3):
7.45–7.28 (20H, [(C6H5)2P]2N–), 7.05–6.95 (m, 3H, –NC6H5) and
2
6.71 (m, 2H, –NC6H5); dC(CDCl3): 147.5 (C6H5N–, t, Cipso, JC,P
2.6), 139.3 [(C6H5)2P, Cipso], 133.3 [(C6H5)2P, Cortho], 129.1 [(C6H5)2P,
3
2JC,P 10.4), 36.4 [(C6H5)CH2S] and 32.4 (t, –SCH2CH2N–, JC,P
Cpara], 129.0 [(C6H5)N–], 128.3 [(C6H5)N–], 128.1 [(C6H5)2P, Cmeta
and 125.1 [(C6H5)N–]; dP(CDCl3): 68.6 (s).
]
3.4); dP(CDCl3): 62.9 (s).
2. (6.79 g, 47%); CH3(CH2)5SCH2CH2NH2, 4.40 g, 27.3 mmol.
(Found C, 73.1; H, 7.6; N, 2.4. C32H37NP2S requires C, 72.6;
H, 7.0; N, 2.6); found mmax(KBr)/cm−1 3069w, 3057w, 2920s,
2852w, 1484s, 1427s, 1091s, 1055s, 846s, 742vs, 697vs, 508s, 452s;
dH(CDCl3): 7.50–7.30 (20H, –N[P(C6H5)2]2), 3.47 (2H, m, S–CH2–
CH2–N), 2.21–2.11 (4H, m, CH2–S–CH2–CH2–N), 1.36–1.25 (8H,
m), 0.91 (3H, t, –CH3); dC(CDCl3): 139.2 [(C6H5)2P–, Cipso], 132.7
[(C6H5)2P–, Cortho], 129.0 [(C6H5)2P–, Cpara], 128.3 [(C6H5)2P–, Cmeta],
Synthesis of 5, 6, 7 and 8
To a vigorously stirred solution of [PtCl2(cod)] in dichloromethane
(1.01 g, 2.7 mmol in 60 cm3) a solution of 1 (1.45 g, 2.7 mmol
in 20 cm3) was added dropwise in 2 h. After 5 h, the solution
was concentrated (to 30 cm3) and the product was obtained by
precipitation with diethyl ether (60 cm3). The white solid was
washed with diethyl ether (3 × 20 cm3) and the solvent evaporated
to dryness.
2
3
53.1 (t, –SCH2CH2N–, JC,P 10.2), 32.5 (t, –SCH2CH2N–, JC,P
The same procedure was followed for the synthesis of 6, 7 and
8.
3.5), 32.1, 31.4, 29.8, 28.5, 22.6 and 14.1 (–CH3); dP(CDCl3): 62.9
(s).
5. (89%). (Found C, 49.65; H, 4.0; Cl, 8.55; N, 1.6; Pt, 23.5.
C33H31Cl2NP2PtS requires C, 49.45; H, 3.9; Cl, 8.85; N, 1.75;
Pt 24.3); mmax(KBr)/cm−1 3050w, 2928w, 1480vs, 1435w, 1299w,
1103vs, 1018w, 999w, 878w, 848s, 749s, 694vs, 665s, 579s, 510vs,
492s, 454w, 305w (Pt–Cl), 288w (Pt–Cl); dH(CDCl3): 7.85–7.48
(20H, [(C6H5)2P]2N–), 7.25–6.87 (5H, C6H5CH2S–), 3.40 (2H, s,
PhCH2S–), 2.95 (2H, m, S–CH2–CH2–N) and 2.00 (2H, m, S–
CH2–CH2–NH2); dC(CDCl3): 137.4 (C6H5CH2S–, Cipso), 133.4
[(C6H5)2P–], 133.3 [(C6H5)2P–, Cpara], 129.4 [(C6H5)2P–], 128.6
(C6H5CH2S–), 128.5 (C6H5CH2S–), 127.3 (C6H5CH2S–, Cpara),
Synthesis of 3
To a vigorously stirred suspension of the decylamine (1.98 g,
12.6 mmol) and triethylamine (2.54 g, 25.2 mmol) in diethyl ether
(50 cm3) kept at 0 C, a solution of chlorodiphenylphosphane in
◦
diethyl ether (5.43 g, 24.6 mmol in 50 cm3) was added dropwise in
90 min. After 24 h stirring at room temperature, the solvent and
the excess triethylamine were removed under reduced pressure. The
resulting solid was dissolved in dichloromethane (30 cm3) and the
resulting solution was washed with deionized water (30 cm3), dried
with MgSO4 and purified by flash chromatography (petroleum
ether 40–60 ◦C : methylene chloride, 7 : 3). The product was
obtained as a white solid after evaporation of the solvents under
reduced pressure (3.89 g, 59%). (Found C, 77.9; H, 8.0; N, 2.6.
C34H41NP2 requires C, 77.7; H, 7.9; N, 2.7); mmax(KBr)/cm−1 2960w,
2924w, 2853w, 1619w, 1462s, 1385s, 1261w, 1084s, 1032s, 873w,
801w, 697w, 533w; dH(CDCl3): 7.47–7.32 (20H, [(C6H5)2P]2N),
3.27 (2H, m, –CH2N), 1.35–0.95 (16H, m, –(CH2)8CH3) and 0.92
(3H, t, –CH3, 3JH,H 7.0); dC(CDCl3): 139.7 [(C6H5)2P, Cipso], 132.8
[(C6H5)2P, Cortho], 128.7 [(C6H5)2P, Cpara], 128.1 [(C6H5)2P, Cmeta],
2
126.9 [(C6H5)2P–, Cipso], 49.1 (–SCH2CH2N–, t, JC,P 9.0), 36.3
[(C6H5)CH2S–] and 29.6 (–SCH2CH2N–); dP(CDCl3): 17.8 (s, 1JP,Pt
3304); dPt(CDCl3): −4037 (t, 1JP,Pt 3304).
6. (87%). (Found C, 48.5; H, 4.8; Cl, 8.5; N, 1.7; Pt, 23.8.
C32H37Cl2NP2PtS requires C, 48.3; H, 4.7; Cl, 8.9; N, 1.8; Pt
24.5); mmax(KBr)/cm−1 3056w, 2951w, 2924w, 2858w, 1437s, 1260s,
1129s, 1101vs, 1025s, 804s, 747s, 719s, 691s, 577w, 520s, 511s,
492w, 381w, 308w (Pt–Cl), 289w (Pt–Cl); dH(CDCl3): 7.90–7.25
(20 H, [(C6H5)2P]2N–), 3.13 (2 H, m), 2.16 (2 H, m), 2.01 (2 H,
m), 1.35–1.10 (8 H, m) and 2.01 (3 H, –CH3); dC(CDCl3): 133.4
[(C6H5)2P–], 133.3 [(C6H5)2P–, Cpara], 129.5 [(C6H5)2P–], 127.1
[(C6H5)2P–, Cipso], 49.4 (–SCH2CH2N–), 32.2, 31.3, 30.6, 29.3,
2
3
53.1 (–CH2N, t, JC,P 10.8), 31.9, 31.4 (–CH2CH2N, t, JC,P 3.2),
29.5, 29.4, 29.3, 29.1, 26.8, 22.7 and 14.2 (–CH3); dP(CDCl3):
62.2 (s).
1
28.3, 22.5 and 14.0; dP(CDCl3): 17.7 (s, JP,Pt 3308); dPt(CDCl3):
−4038 (t, 1JP,Pt 3308).
Synthesis of 4
7. (84%). (Found C, 51.4; H, 5.4; Cl, 8.5; N, 1.6; Pt, 24.0.
C34H41Cl2NP2Pt requires C, 51.6; H, 5.2; Cl, 9.0; N, 1.8; Pt
24.6); mmax(KBr)/cm−1 3053w, 2933w, 1480w, 1435vs, 1306w,
1102vs, 878w, 848s, 749s, 721w, 694vs, 664s, 575s, 522vs, 508vs,
492s, 306w (Pt–Cl), 286w (Pt–Cl); dH(CDCl3): 7.87–7.56 (20H,
[(C6H5)2P]2N–), 2.93 (2H, m, –CH2N–) and 1.20–0.80 (19H,
m, –(CH2)8CH3); dC(CDCl3):128.9 [(C6H5)2P–], 128.7 ((C6H5)2P–,
Cpara), 124.8 [(C6H5)2P–], 122.8 [(C6H5)2P, Cipso], 45.3 (t, –CH2N–,
2JC,P 8.1), 27.3, 26.4, 24.9, 24.7, 24.6, 24.2, 22.2, 22.1 and 18.1
To a vigorously stirred suspension of aniline (2.36 g, 25.3 mmol)
and triethylamine (5.26 g, 52.1 mmol) in diethylether (50 cm3)
kept at 0 ◦C, was added dropwise over 90 min to a solution of
chlorodiphenylphosphane in diethyl ether (11.28 g, 51.1 mmol in
70 cm3). After stirring for 48 h at room temperature, the solvent
and excess triethylamine were removed under reduced pressure.
The resulting solid was dissolved in dichloromethane (30 cm3)
and the solution was washed with deionized water (30 cm3), dried
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3
(–CH3); dP(CDCl3): 17.1 (s, 1JP,Pt 3296); dPt(CDCl3): −4038 (t, 1JP,Pt
9b); dPt(THF): −4120 (dd, 1JP,Pt 3589, |2JP,Pt + JP,Pt | 78, 9a), −4325
3296).
(t, 1JP,Pt 3006, 9b).
10. (81%). (Found C, 45.3; H, 3.9; Co, 11.2; N, 1.2; Pt, 18.1.
C39H37Co2NO7P2PtS requires C, 45.1; H, 3.6; Co, 11.35; N, 1.35; Pt
18.8); mmax(KBr)/cm−1 (CO): 2047s, 2033vs, 2006s, 1955vs, 1913vs,
1882s, 1761w; dP(CH2Cl2): 98.4 (br, Ph2P–Co, 10a), 69.5 (d, Ph2P–
Pt, JP,P 28, 1JP,Pt 3604, 10a) and 54.3 (s, Ph2P–Pt, 1JP,Pt 3015, 10b);
8. (88%). (Found C, 49.8; H, 3.7; Cl, 9.6; N, 2.0; Pt, 27.0.
C30H25Cl2NP2Pt requires C, 49.5; H, 3.5; Cl, 9.75; N, 1.9; Pt,
26.8); mmax(KBr)/cm−1 3084w, 3059w, 3042w, 3029w, 3012w, 2999w,
1591w, 1580w, 1485s, 1479s, 1439s, 1431s, 1207s, 1100w, 1023w,
930vs, 902s, 871vs, 735vs, 692vs, 659w, 529s, 517s, 300w (Pt–
Cl), 276w (Pt–Cl); dH(CDCl3): 7.88 (8H, [(C6H5)2P]2N–), 7.63
(4H, [(C6H5)2P]2N–, Hpara), 7.51 (8H, [(C6H5)2P]2N–), 7.21 (1H,
(C6H5)N, Hpara), 7.09 (2H, (C6H5)N, Hmeta) and 6.48 (2H, (C6H5)N,
Hortho); dP(CDCl3): 20.7 (s, 1JP,Pt 3330); dPt(CDCl3): −4060 (t, 1JP,Pt
3330).
3
dPt(CH2Cl2): −4136 (dd, 1JP,Pt 3604, |2JP,Pt + JP,Pt | 80, 10a), −4333
(t, 1JP,Pt 3015, 10b).
11. (83%). (Found C, 48.0; H, 4.3; Co, 11.1; N, 1.2; Pt, 18.3.
C41H41Co2NO7P2Pt requires C, 47.6; H, 4.0; Co, 11.4; N, 1.35;
Pt, 18.85); mmax(KBr)/cm−1 (CO): 2048vs, 2004vs, 1976vs, 1969vs,
1757s; dP(CH2Cl2): 96.6 (br, Ph2P–Co, 11a), 68.1 (d, Ph2P–Pt,
2 + 3
28, 1JP,Pt 3603, 11a) and 52.2 (s, Ph2P–Pt, 1JP,Pt 3013, 11b);
Synthesis of the clusters 9–12
J
P,P
dPt(CDCl3): −4149 (dd, JPt,Pt 3612, |2JP,Pt + JP,Pt | 77, 11a) and
1
3
To a stirred suspension of NaCo(CO)4 in dichloromethane
(180 mg, 0.93 mmol in 80 cm3), 5 (365 mg, 0.46 mmol) was added
in one portion. The colorless mixture turned to brown and after
4 h (24 h for 12) the solvent was removed under reduced pressure.
The brown solid was purified by percolation on silica and Celite
using THF as eluent. The product was obtained after evaporation
of the solvent under reduced pressure as a brown solid affording
9 in 82% yield.
−4337 (t, 1JP,Pt 3009, 11b).
12. (73%). (Found C, 45.9; H, 2.8; Co, 11.8; N, 1.1; Pt, 19.6.
C37H25Co2NO7P2Pt requires C, 45.8; H, 2.6; Co, 12.15; N, 1.4; Pt,
20.1); mmax(KBr)/cm−1 (CO): 2048vs, 2002vs, 1986vs, 1971vs, 1934s,
1741vs; dP(CH2Cl2): 104.4 (br, Ph2P–Co, 12a), 73.9 (d, Ph2P–Pt,
1
1
JP,P 30, JP,Pt 3607, 12a) and 60.7 (s, Ph2P–Pt, JP,Pt 3042, 12b);
dPt(CH2Cl2): −4143 (dd, JPt,Pt 3607, |2JP,Pt + JP,Pt | 74, 12a) and
1
3
−4358 (t, 1JP,Pt 3042, 12b).
The same procedure was followed for the synthesis of 10, 11 and
12.
X-Ray data collection, structure solution, and refinement of 5 and
9b19
9. (Found C, 46.7; H, 3.4; Co, 11.0; N, 1.1; Pt, 18.0.
C40H31Co2NO7P2PtS requires C, 46.0; H, 3.0; Co, 11.3; N, 1.3;
Pt, 18.7); mmax(KBr)/cm−1 (CO): 2049vs, 2006vs, 1977vs, 1970vs,
1956s, 1941s, 1755s; dP(CH2Cl2): 99.5 (br, Ph2P–Co, 9a), 70.3 (d,
Ph2P–Pt, JP,P 26, 1JP,Pt 3604, 9a) and 55.2 (s, Ph2P–Pt, 1JP,Pt 3019,
Crystal data, parameters for intensity data collection and conver-
gence results are compiled in Table 6. Crystals were studied at
110 K with a BRUKER-AXS SMART APEX diffractometer.
Table 6 Crystal data and structure refinement for 5 and 9b
5
9
Empirical formula
Molecular mass
Crystal size
C33H31Cl2NP2PtS
801.58
C40H31Co2NO7P2PtS
1044.61
0.27 × 0.23 × 0.06 mm
0.38 × 0.15 × 0.12 mm
Temperature/K
110(2)
110(2)
˚
Wavelength/A
0.71073
0.71073
Crystal system
Space group
Monoclinic
P21/c
Orthorhombic
Pbca
Unit cell dimensions
˚
a/A
11.4712(19)
16.585(3)
17.909(3)
111.516(9)
3169.8(9)
4
1.680
4.786
2.44–28.10
31456
7584
20.0857(19)
19.929(2)
19.9610(19)
˚
b/A
˚
c/A
b/◦
V/A
3
˚
7990.1(13)
8
1.737
Z
D
calcd./Mg m−3
Absorption coeff./mm−1
h Range for data coll./◦
Reflections collected
Independent reflections
Indep. observed reflections
(I > 2r(I))
4.495
2.04–29.62
52751
11065
6462
7584/361
1.020
0.0289
0.0668
Data/parameters
11065/487
1.013
0.0344
0.0790
1.900, −0.764
Goodness of fit on F2
Ra (I > 2r(I))
wR2b (all data)
−3
˚
Largest diff. Peak/hole [e A
]
1.532, −0.592
2
a R = RꢁFo| − |Fcꢁ/R|Fo|. b wR2 = [Rw(Fo − Fc2)2/Rw(Fo2)2]1/2
.
2348 | Dalton Trans., 2006, 2342–2349
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©

View Article Online
5. An empirical absorption correction20 was applied before
averaging symmetry related reflections. The structure was solved
by direct methods21 and refined on F2.22 In the final refinement,
all non-hydrogen atoms were assigned anisotropic displacement
parameters and hydrogen atoms were included as riding in
standard geometry.
6 (a) R. M. Beesley, C. K. Ingold and J. F. Thorpe, J. Chem. Soc., 1915,
107, 1080; (b) M. E. Jung and J. Gervey, J. Am. Chem. Soc., 1991, 113,
224.
7 (a) E. J. Sekabunga, M. L. Smith, T. R. Webb and W. E. Hill, Inorg.
Chem., 2002, 41, 1205; (b) M. L. McKee and W. E. Hill, J. Phys. Chem.
A, 2002, 106, 6201.
8 (a) I. Bachert, I. Bartusseck, P. Braunstein, E. Guillon, J. Rose´ and
G. Kickelbick, J. Organomet. Chem., 1999, 580, 257; (b) I. Bachert,
I. Bartusseck, P. Braunstein, E. Guillon, J. Rose´ and G. Kickelbick,
J. Organomet. Chem., 1999, 588, 143; (c) I. Bachert, P. Braunstein,
M. K. McCart, F. Fabrizi de Biani, F. Laschi, P. Zanello, G. Kickelbick
and U. Schubert, J. Organomet. Chem., 1999, 573, 47 and references
therein; (d) A. Choualeb, P. Braunstein, J. Rose´, S.-E. Bouaoud and R.
Welter, Organometallics, 2003, 22, 4405; (e) I. Bachert, P. Braunstein
and R. Hasselbring, New J. Chem., 1996, 20, 993; (f) R. P. Kamalesh
Babu, S. S. Krishnamurthy and M. Nethaji, Tetrahedron: Asymmetry,
1995, 6, 427.
9b. A semi-empirical absorption correction based on equiv-
alents (min. trans. 0.280, max. trans 0.615) was applied before
averaging symmetry equivalent data [R(int) = 0.0565]. After
merging 11 065 independent reflections remained for structure
solution by direct methods.21 The structure model was completed
by Fourier difference syntheses and refined with full-matrix least-
squares on F2.22
9 G. Calabro`, D. Drommi, C. Graiff, F. Faraone and A. Tiripicchio,
Eur. J. Inorg. Chem., 2004, 1447.
10 P. Braunstein, H. P. Kormann, W. Meyer-Zaika, R. Pugin and G.
Schmid, Chem. Eur. J., 2000, 6, 4637.
11 A. Pidcock, R. E. Richards and L. M. Venanzi, J. Chem. Soc. A, 1966,
1707.
Acknowledgements
This work was partially funded by the Italian MIUR (project
COFIN2004 prot. 2004030719) and by the CNRS (Paris) and
the Ministe`re de la Recherche. We thank Mr G. Allegretta for
experimental help.
12 D. M. Adams, J. Chatt, J. Gerratt and A. D. Westland, J. Chem. Soc.
A, 1964, 734.
13 C. S. Browning, D. H. Farrar and D. C. Frankel, Acta Crystallogr.,
Sect. C, 1992, 48, 806.
14 C. S. Browning and D. H. Farrar, J. Chem. Soc., Dalton Trans., 1995,
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17 The b/a molar ratio was calculated by the formula: b/a = Ib/(2Ia),
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18 H. C. Clark and L. E. Manzer, J. Organomet. Chem., 1973, 59,
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19 CCDC reference numbers 273537 (complex 5) and 286009 (cluster 9b).
For crystallographic data in CIF or other electronic format see DOI:
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20 G. M. Sheldrick, SADABS, Program for Empirical Absorption Correc-
tion of Area Detector Data, University of Go¨ttingen, 1996.
21 G. M. Sheldrick, SHELXS97, Program for Crystal Structure Solution,
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22 G. M. Sheldrick, SHELXL97, Program for Crystal Structure Refine-
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