A cluster model for the catalytic hydrogenation of CFCs and the synthesis and structural characterisation (when X = Br) of [Pd4(μ3-CF)(μ-X)3(PBut 3)4] (X = Cl, Br)

Article abstract of DOI:10.1039/a607048e

The syntheses, characterisation and crystal structure (when X = Br) of [Pd₄(μ₃-CF)(μ-X)₃(PBu^t ₃)₄] (X = Cl, Br) are reported; the μ₃-CF moiety is hydrogenated to CFH₃ under mild conditions and serves as a model for the heterogeneous hydrogenation of CFCs to HCFCs.

Full text of DOI:10.1039/a607048e

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A cluster model for the catalytic hydrogenation of CFCs and the synthesis and
structural characterisation (when X = Br) of [Pd4(m3-CF)(m-X)3(PBut3)4]
(X = Cl, Br)
Ramo´n Vilar, Simon E. Lawrence, D. Michael P. Mingos* and David J. Williams
Department of Chemistry, Imperial College of Science, Technology and Medicine, South Kensington, London, UK SW7 2AY
The syntheses, characterisation and crystal structure (when
3 4
reported; the m3-CF moiety is hydrogenated to CFH3 under
mild conditions and serves as a model for the heterogeneous
hydrogenation of CFCs to HCFCs.
that palladium black and palladium supported in alumina may
be used as heterogeneous catalysts for the hydrogenation of
chloroflurocarbons (CFCs) to hydrochlorofluorocarbons
(HCFCs) and hydrofluorocarbons (HFCs).11 Recently these
reactions have taken on considerable industrial importance
because of the need for HFCs as possible substitutes for the
ozone-damaging CFCs. However, to our knowledge only
heterogeneous catalytic processes for converting CFCs to
HCFCs have been reported.12 [Pd4(m3-CF)(m-Cl)3(PBut3)4]
derived from CFCl3 may be converted to CFH3 homogeneously
under mild conditions and therefore provides an effective model
for such a process.
X = Br) of [Pd4(m3-CF)(m-X)3(PBut ) ] (X = Cl, Br) are
The cluster–surface analogy has a long history which can be
traced back to Muetterties pioneering work.1 Molecular clusters
with coordinated organic fragments have proved to be invalu-
able for providing definitive spectroscopic data which have
been used to identify analogous species on metal surfaces.2 The
study of such systems on clean and well defined metal surfaces
has implications for understanding the heterogeneous catalysed
transformations of organic feedstocks. The lack of reactivity of
molecular clusters, specifically catalytic reactivity, has, how-
ever, limited the development of the cluster–surface analogy.
Alkylidyne ligands have been extensively studied when
coordinated to group 8 and 9 metals.3,4 However, fluoro-
methylidyne cluster compounds have been structurally charac-
terised only for iron5–7 and molybdenum.8 Recently, [Pd4(m3-
A solution of [Pd4(m3-CF)(m-Cl)3(PBut3)4] in toluene was
reacted with H2–NEt3 and 4 mol equiv. of PBut3 under mild
conditions (at STP). The products formed in this reaction were
[Pd(PBut3)2], [HNEt3]Cl and CFH3 (in > 50% yield after 2 h as
detected by gas chromatography).¶ No other volatile fluoro-
carbon compounds were detected by gas chromatography.
[Pd(PBut3)2] was detected using 31P{1H} NMR, by its charac-
teristic resonance at d 85.513 and monitoring its signal indicated
that the reaction was complete after 4 h. This confirms that a m3-
CF moiety, derived by an oxidative-addition process from
CFCl3, may be hydrogenated to CFH3 under mild conditions.
When the hydrogenation reaction was repeated using the
mononuclear compound [Pd(PBut3)2], generated in situ from
[Pd2(dba)3] and PBut3, with an excess of CFCl3 {50:1 relative
CH)(m-Cl)3(PBut3)4], the first example of
a palladium
methylidyne cluster, has been described and structurally
characterised.9 Herein we report the syntheses of [Pd4(m3-
CF)(m-X)3(PBut3)4] (X = Cl 1, Br 2) and the first structural
characterisation of a palladium m3-CF cluster. Interestingly the
CF group of [Pd4(m3-CF)(m-Cl)3(PBut3)4] is readily cleaved
when the cluster is reacted with H2 under mild conditions.
When 1 mol equiv. of [Pd2(dba)3]·C6H6, 2 mol equiv. of
PBut3 and 0.5 mol equiv. of CFCl3 were reacted in toluene for
6 h the reaction mixture turned dark brown. The solvent was
removed under reduced pressure and the crude product was
recrystallised from toluene–acetone at 4 °C to give [Pd4(m3-
CF)(m-Cl)3(PBut3)4] 1 as dark green crystals. The formulation
of this product was confirmed on the basis of IR, 31P{1H} NMR,
19F NMR, FABMS and elemental analyses.†
P(10)
F(8)
Pd(3)
C(8)
A solution of [Pd4(m3-CF)(m-Cl)3(PBut3)4] in toluene was
treated with 3 mol equiv. of LiBr in acetone for 4 h and [Pd4(m3-
CF)(m-Br)3(PBut3)4] 2 was obtained. After removing the
solvents under reduced pressure the remaining dark green solid
was extracted with benzene and crystals of [Pd4(m3-CF)(m-
Br)3(PBut3)4] were obtained from benzene–acetone after cool-
ing to 4 °C overnight. The formulation was confirmed by the IR,
31P{1H} NMR, 19F NMR, FABMS and analytical data.‡ A
single-crystal X-ray analysis§ of compound 2 has confirmed
that the cluster has a structure similar to that previously reported
for [Pd4(m3-CH)(m-Cl)3(PBut3)4].9 It consists of a tetrahedral
cluster with a face-capping fluoromethylidyne ligand. The
molecular structure (Fig. 1) demonstrates that the chlorine
atoms of the starting material have been substituted by three
bromine atoms, whilst retaining the effective C3n symmetry of
the metal cluster skeleton. The Pd–Pd bonds in the cluster are in
the range 2.727(2)–3.123(2) Å compared with 2.668(1)–
3.143(1) Å in the analogous [Pd4(m3-CH)(m-Cl)3(PBut3)4]
cluster compound.
Pd(1)
P(23)
Br(6)
Pd(4)
P(36)
Br(7)
Pd(2)
Br(5)
P(49)
Fig. 1 The molecular structure of the cluster [Pd4(m3-CF)(m-Br)3(PBut3)4]
showing the numbering used (the phosphine substituents are omitted for
clarity). Important bond lengths (Å): Pd(1)–C(8) 1.94(2), Pd(3)–C(8)
1.96(2), Pd(4)–C(8) 1.96(2), Pd(1)–Br(5) 2.524(3), Pd(2)–Br(5) 2.668(3),
Pd(2)–Br(6) 2.666(3), Pd(2)–Br(7) 2.639(3), Pd(3)–Br(6) 2.531(3), Pd(4)–
Br(7) 2.529(2), Pd(1)–P(23) 2.397(5), Pd(2)–P(49) 2.361(5), Pd(3)–P(10)
2.403(6), Pd(4)–P(36) 2.392(5), Pd(1)–Pd(2) 3.112(2), Pd(1)–Pd(3)
2.727(2), Pd(1)–Pd(4) 2.729(2), Pd(2)–Pd(3) 3.097(2), Pd(2)–Pd(4)
3.123(2), Pd(3)–Pd(4) 2.731(2), C(8)–F(8) 1.36(2).
The presence of the m3-CF capping ligand in these clusters is
interesting in terms of the cluster–surface analogy. It is known
Chem. Commun., 1997
285

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to [Pd2(dba)3]}, the only hydrogenation product observed was
CFCl2H. Complete conversion of the CFCl3 to CFCl2H required
24 h. No traces of CFH3 or any other gas were detected,
suggesting that the cluster [Pd4(m3-CF)(m-Cl)3(PBut3)4] plays
an essential role in the hydrogenation of CFCl3 to CFH3.
Although the formation of [Pd4(m3-CF)(m-Cl)3(PBut3)4] from
CFCl3 and its subsequent hydrogenation provides an illustration
of how CFCl3 may be hydrogenated to CFH3 on a metal surface,
the cluster compound itself is not an effective homogeneous
catalyst. When H2 was bubbled through a solution of [Pd4(m3-
CF)(m-Cl)3(PBut3)4] in toluene in the presence of NEt3 and with
a 50-fold excess of CFCl3, the gases produced were identified as
CFH3 and CFCl2H with a ratio of CFCl3H/CFH3 of ca. 2.3.
31P{1H} NMR data confirmed that the cluster had fragmented
during this reaction.
Scheme 1 summarises plausible mechanistic cycles involving
the reactions for the mono- and tetra-nuclear palladium
compounds. Although in the absence of hydrogen the formation
of the tetranuclear cluster with the m3-CF group proceeds at a
sufficient rate to compete with the formation of the simple
oxidative-addition products, the hydrogenation step proceeds
more quickly for the mononuclear product than for the
tetranuclear compound leading to the formation of CFCl2H in
addition to CFH3. Attempts are currently being made to use
variations in the ligands to change the relative rates of the
competing cycles and thereby improve the selectivity of the
catalytic hydrogenation process.
Footnotes
† Selected data for 1: yield 18%. (Found C, 42.4; H, 7.4. C49H108Cl3FP4Pd4
requires C, 42.9; H, 7.9%). 31P{1H} NMR ([2H6]benzene) d 103.4 (q), 68.6
(dd); 19F NMR ([2H6]benzene) d 111.0 [dq, 3J(PP) 132.2, 3J(PF) 39.7,
4J(PF) 8.5 Hz]. IR n(C–F) 1052 cm21
.
‡
Selected data for 2: yield 61%. (Found C, 40.9; H, 6.8
C49H108Br3FP4Pd4·0.75C6H6·0.75Me2CO requires C, 41.0; H, 7.1%).
31P{1H} NMR ([2H6]benzene) d 117.0 (q), 70.5 (dd); 19F NMR ([2H6]ben-
zene) d 116.0 [dq, 3J(PP) 173.0, 3J(PF) 40.1, 4J(PF) 11.0 Hz]. The
metathesis reaction of 1 to 2 was monitored at different reaction times by
31P{1H} and 19F NMR spectroscopy. The asymmetric mixed-halogen
clusters [Pd4(m3-CF)(m-Br)(m-Cl)2(PBut3)4] 3 and [Pd4(m3-CF)(m-Br)2(m-
Cl)(PBut3)4] 4 were identified as intermediates, suggesting that the
substitution reaction takes place sequentially.
§
Crystal data for 1: C49H108Br3FP4Pd4·0.75Me2CO·0.75C6H6,
M = 1607.7, monoclinic, space group C2/c; a = 25.989(4), b = 25.529(4),
c = 24.216(4) Å, b = 118.77(1)°; U = 14083(4) Å3, Z = 8, Dc = 1.517
g cm23; m(Mo-Ka) = 2.833 mm21; F(000) = 6524. Data were measured
on a Siemens P4/PC diffractometer with graphite-monochromated Mo-Ka
radiation using w scans. The structure was solved by direct methods and the
non-hydrogen atoms of the cluster refined anisotropically by full-matrix
least-squares based on F2 to give R1
= 0.0783, wR2 = 0.1446
(SHELXTL)10 for 3824 unique observed reflections (ıFı > 4sıFoı, 2q @
50°) and 609 parameters. Atomic coordinates, bond lengths and angles, and
thermal parameters have been deposited at the Cambridge Crystallographic
Data Centre (CCDC). See Information for Authors, Issue No. 1. Any request
to the CCDC for this material should quote the full literature citation and the
reference number 182/330.
¶ Selected data for chromatographic studies: the chromatographic studies
were performed in a Perkin Elmer 8500-GC with an FID detector and a
Carbopack B/5% Fluorocol column special to detect fluorinated com-
pounds. The oven temperature was set to 60 °C, the detector to 250 °C and
the injection temperature to 200 °C. The carrier gas (N2) flow rate was
adjusted to 25 ml min21. Samples: 200 ml of sample (gases produced in the
reaction) was injected using a gas-tide syringe. The assignment of the peaks
was made by comparison with standards of CFH3, CFCl2H and CFCl3 at the
same temperature and flow rate conditions.
We thank CONACYT and ORS for scholarships to R. V.,
EPSRC for financial support to S. E. L., BP plc for endowing
D. M. P. M.’s chair and ICI for providing reference samples of
CFCl2H and CFH3.
[Pd4(µ3-CF)(µ-Cl)3(PBut3)4]
H2–NEt3
References
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CFCl3
[HNEt3]+Cl– + CFH3
[Pd(PBut3)]
+PBut3
–PBut3
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CFCl3
[Pd(PBut3)2]
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H2
CFCl2H
Scheme 1 Reactions involved in the palladium-assisted hydrogenation of
CFCl3
Received, 16th October 1996; Com. 6/07048E
286
Chem. Commun., 1997