[BArF4]: A well characterised transition metal dichloroethane complex

Article abstract of DOI:10.1039/b702399e

Reaction of [Ir(PPh₃)₂(COD)][BAr^F ₄] with H₂ in dichloroethane solution results in [Ir(PPh₃)₂(H)₂(ClCH₂CH ₂Cl)][BAr^F₄], which has been fully characterised by X-ray crystallography, NMR spectroscopy and ESI-MS. Its activity towards alkene hydrogenation has been compared with analogous CH ₂Cl₂ complexes. The Royal Society of Chemistry.

Full text of DOI:10.1039/b702399e

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[Ir(PPh3)2(H)2(ClCH2CH2Cl)][BArF ]: a well characterised transition metal
4
dichloroethane complex†
Gemma L. Moxham, Simon K. Brayshaw and Andrew S. Weller*
Received 15th February 2007, Accepted 13th March 2007
First published as an Advance Article on the web 26th March 2007
DOI: 10.1039/b702399e
Reaction of [Ir(PPh3)2(COD)][BArF4] with H2 in dichloro-
ethane solution results in [Ir(PPh3)2(H)2(ClCH2CH2Cl)]-
[BArF4], which has been fully characterised by X-ray crys-
tallography, NMR spectroscopy and ESI-MS. Its activity
towards alkene hydrogenation has been compared with anal-
ogous CH2Cl2 complexes.
Cationic late transition metal complexes partnered with weakly
coordinating anions such as [BArF ]− {ArF = C6H3(CF3)2} play
4
an important part in the landscape of catalysis,1 providing systems
that are useful for a number of important reactions such as
hydrogenation,2 cyclisations3 and cycloaddition reactions.4 As well
as depending on the weakly coordinating properties of the anion,
the success of such systems also lies in the choice of solvent.
Strongly coordinating solvents (such as acetonitrile or THF) can
attenuate activity as they preferentially bind over substrates, while
the charged nature of the catalysts means that a solvent of a
suitably high dielectric constant has to be used.2a,g Thus weakly co-
ordinating chlorinated solvents such as CH2Cl2 and ClCH2CH2Cl
are most often used in these systems. Even though solvent
Fig. 1 Solid-state structure of [Ir(PPh3)2H2(ClCH2CH2Cl)][BArF4] (an-
bound complexes are often implicated in catalytic cycles definitive
ion and selected hydrogen atoms omitted for clarity). Ellipsoids are shown
characterisation of such species is often difficult, a consequence
of the weak binding of solvent ligands to metal centres. Although
well characterised transition metal complexes CH2Cl2 are known;5
surprisingly, to our knowledge, no transition metal complexes of
ClCH2CH2Cl (DCE) have been definitively characterised although
silver(I),6 Tl(I)7 and Cs(I)8 complexes have been reported.
◦
˚
at the 50% probability level. Selected bond lengths (A) and angles ( ):
Ir–P 2.3136(4), 2.3145(4), Ir–H 1.43(2), 1.49(2), Ir–Cl 2.5289(5), 2.5329(5),
H–Ir–H 87.2(12), P–Ir–P 165.577(15), Cl–Ir–Cl 81.437(17).
complex [(PNP)IrH(C6H4Cl)]+ (where PNP = 2,6-bis-(di-tert-
butyl phosphino methyl)pyridine and in which the chloro is ortho
to the Ir–C bond),13 possibly due to less ring strain in complex 1.
It is, however, comparable to the Rh–Cl bond length of 2.488(1)
Given that DCE is a common solvent for catalysis by late
transition metals, often as a substitute for CH2Cl2 where a higher
temperature is required (such as in hydrogenation,9 oxidation10 and
hydroacylation11) the characterisation of a complex interacting
with this solvent would be useful. Herein we report the synthesis
and full characterisation of such a complex, [Ir(PPh3)2(H)2-
F
5d
˚
A in the compound [Cp*(PMe3)RhMe(CH2Cl2)][BAr ].
4
Complex 1 in DCE solution at 298 K shows a single resonance
in the 31P{ H} NMR spectrum at d 19.1 ppm. In the H NMR
spectrum a hydride resonance is observed at d −23.84 ppm as a
triplet [J(PH) 15 Hz]. There is no observed change to the NMR
spectra on cooling the sample to 240 K. Similar compounds of the
formula [(PPh3)2IrH2(C6H4X2)]+,12 where X = Cl, Br or I, show
that the hydride chemical shift varies according to the identity
of the trans halogen. For comparison, in the dichlorobenzene
complex the hydride chemical shift is d −20.8 ppm compared to d
−16.5 ppm for the diiodobenzene complex.
1
1
(ClCH2CH2Cl)][BArF ] including a solid-state structure (Fig. 1).
4
The complex [Ir(PPh3)2(H)2(ClCH2CH2Cl)][BArF ], 1, was
4
synthesised by hydrogenation of [Ir(PPh3)2(COD)][BArF ] in
4
dichloroethane solution and has been fully characterised by X-
ray crystallography‡, NMR spectroscopy and ESI-MS.§ In the
solid state, complex 1 has a six coordinate, octahedral, iridium
centre with the P–Ir–P angle of 165.577(15)◦ comparable to
166.5(2)◦ in the analogous complex [(PPh3)2IrH2(C6H4I2)]+.12 The
dichloroethane is bound to the iridium centre via both chlorine
When crystals of 1 are dissolved in CD2Cl2 solution at 298 K the
observed 1H NMR spectrum suggests that a fluxional process or
equilibrium is taking place as the hydride resonance is broadened
considerably and shifted upfield to d −25.08 ppm. There is a
broad signal observed at d 3.23 ppm corresponding to DCE. In
˚
atoms with Ir–Cl distances of 2.5289(5) and 2.5329(5) A. These
˚
are shorter than the Ir–Cl bond length of 2.816 A for the
Department of Chemistry, University of Bath, Bath, UK. E-mail: a.s.weller@
bath.ac.uk
† Electronic supplementary information (ESI) available: Full experimental
the 31P{ H} NMR spectrum the phosphorus resonance is now
1
observed at d 21.4 ppm and is also broad. On cooling to 270 K
1
and spectroscopic data for compound (1). See DOI: 10.1039/b702399e
in the H NMR spectrum the hydride resonance splits into two
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Scheme 1 Temperature dependant equilibrium between complexes 1 and 2 in CH2Cl2 solution.
broad peaks as does the DCE resonance. Further cooling to
220 K resolves the two hydrides into triplet resonances at d −23.12
and −25.29 ppm [J(PH) 15 Hz] in approximately a 12 : 1 ratio.
The signal for the DCE methylene protons resolves into two
sharp singlets at this temperature which we assign to free and
bound CD2Cl2 molecule. The substrate used was methylcyclohex-
1-ene.
As shown in Fig. 3 complex 1 is active towards methylcyclo-
hexene hydrogenation, but very much slower than complex 2. This
indicates that the DCE binds too strongly with the cationic iridium
centre to allow the catalysis to take place at an appreciable rate.
This is similar to the effect previously observed by partnering the
[(PPh3)2IrH2]+ cation with the [closo-CB11H6I6]− anion.14 In this
case it is the anion which binds too strongly to the iridium for
catalysis to occur rather than the solvent.
1
metal-bound DCE. The 31P{ H} NMR spectrum also shows two
peaks at d 23.2 and 19.7 ppm in the same ratio. We assign the
major compound in solution at low temperature as the DCE
complex (1) (Scheme 1). The minor compound in solution is
assigned as the CH2Cl2 complex [(PPh3)2IrH2(CH2Cl2)n][BArF ], 2,
4
previously observed on hydrogenation of [Ir(PPh3)2(COD)][BArF ]
4
14
in CD2Cl2 (n = 1 or 2, see ESI† also). The relative integrals for
both compounds change reversibly with temperature indicating a
dynamic equilibrium.
In order to determine the relative binding strengths to the
+
{Ir(PPh3)2H2} fragment of CH2Cl2 versus DCE the relative ratios
of complexes 1 and 2 were measured over the temperature range
250–210 K. A van’t Hoff plot (Fig. 2) was constructed, from which
DHo = +18.5 1.5 kJ mol−1, DSo = +8.6 6.7 J K−1 mol−1 and
DGo(298) = +16.0
3.5 kJ mol−1 were derived.15 The positive
value for DHo indicates that complex 1 is enthalpically favoured
over complex 2, i.e. DCE binds more strongly than DCM. DSo is
positive but small, as might be expected.
Fig. 3 Relative rates of hydrogenation of methylcylcohex-1-ene using
[Ir(PPh3)2(COD)][BArF4] precatalyst in CH2Cl2 (complex 2) and DCE
(complex 1) solutions. Conversions measured by GC. See ESI† for
conditions.
In conclusion, we have presented a well characterised complex
of dichloroethane and have also demonstrated that this solvent
forms stronger adducts with a cationic transition metal fragment
than does CH2Cl2. This has a negative impact on the catalytic
rate in the hydrogenation of a hindered olefin, but this is
counterbalanced with the improved resistance to decomposition.
This further underlines the inverse correlation that can often
exist between catalytic activity and catalyst robustness and makes
dichloroethane a sensible choice of solvent over CH2Cl2 when
activity is less a concern than catalyst longevity.
Fig. 2 Van’t Hoff plot for the equilibrium between 1 and 2.
Complex 1 is more stable to decomposition than complex 2.
Previous attempts to crystallise complex 2 from CH2Cl2 solvent
have resulted in the isolation of the hydride-bridged dimer
[(PPh3)2HIrH3IrH(PPh3)2][BArF ]14 whereas crystals of complex 1
4
Notes and references
were obtained from a solution of the complex in DCE, suggesting
the complex is significantly more stable in this solvent. There has
also been no evidence for dimer formation from the NMR studies
on 1 undertaken in CH2Cl2. With this in mind it was of interest
to see if complex 1 was an active hydrogenation catalyst16 and if
so how its activity compared to complex 2, which has the weaker
‡ Crystallographic data for complex 1. Intensity data were collected at
150 K on a Nonius Kappa CCD, using graphite monochromated MoKa
˚
radiation (k = 0.71073 A). C70H48BCl2F24IrP2, M = 1680.93, triclinic, space
¯
group P1 (no. 2), Z = 2, a = 14.2336(1), b = 14.2460(1), c = 17.5082(1)
◦
˚
A, a = 95.9592(4), b = 99.1182(4), c = 100.7170(4) , V = 3411.50(4)
A , l = 2.190 mm−1, Tmin/Tmax = 0.84, 2hmax = 70.0◦, 96 150 reflections
3
˚
1760 | Dalton Trans., 2007, 1759–1761
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collected, 29 864 unique [R(int) = 0.0514]. wR2 0.0687 (all data). R1
=
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data in CIF or other electronic format see DOI: 10.1039/b702399e The
hydrogen atoms H40 and H50 were located and freely refined.
1
§ Spectroscopic data for complex 1. H NMR (d C2H4Cl2 298 K) 7.67–
1
6.85 (m, 42H, ArH), −23.84 (t, J(PH) 15 Hz, 2H, IrH). 31P{ H} (d/ppm
C2H4Cl2 298 K) 19.1 (s). 1H NMR (d CD2Cl2 298 K) 7.74-6.88 (m, 42H,
1
ArH), 3.23 (br s, 4H, C2H4Cl2), −25.08 (br s, 2H, IrH). 31P{ H} (d/ppm
CD2Cl2 298 K) 21.4 (br s). 1H NMR (d CD2Cl2 270 K selected) 3.71 (br s,
1.8H, free C2H4Cl2), 2.27 (br s, 2.1H, bound C2H4Cl2), −23.47 (br s, 0.7H,
1
IrH (1)), −25.69 (br s, 0.5H, IrH (2)). 31P{ H} (d CD2Cl2 270 K) 22.8
1
(br s, (2)) 19.2 (br s, (1)). H NMR (d/ppm CD2Cl2 250 K selected) 3.75
(s, 1.2H, free C2H4Cl2), 2.05 (2.3H, bound C2H4Cl2), −23.29 [t, J(PH)
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1
(2)). 31P{ H} (d CD2Cl2 250 K) 23.1 (br s, (2)) 19.3 (br s, (1)). 1H NMR (d
CD2Cl2 220 K selected) 3.85 (s, 0.7H, free C2H4Cl2), 1.98 (s, 2.8H, bound
C2H4Cl2), −23.12 [t, J(PH) 15 Hz, 1.3H, IrH (1)], −25.29 [t, J(PH) 15 Hz,
1
0.1H, IrH (2)]. 31P{ H} (d CD2Cl2 220 K) 23.2 (br s, (2)) 19.7 (br s, (1)).
ESI-MS (CH2Cl2) m/z 817.1 (exp) 817.1 (calc.).
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15 These figures arise from the assumption that one CH2Cl2 is bound. If
there are two bound then DHo +18.5 1.5 kJ mol−1, DSo = −14.3
6.6 J K-1 mol−1 and DGo(298) = +22.8 2.1 kJ mol−1. The conclusions
about the qualitative relative strength of binding of DCE over CH2Cl2
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This journal is
The Royal Society of Chemistry 2007
Dalton Trans., 2007, 1759–1761 | 1761
©