Monomerisation of [Rh2(1,3-bis-(diphenylphosphino)-propane)2(μ2-Cl)2] detected by pulsed gradient spin echo spectroscopy and 31P NMR monitoring of metathesis experiments

Article abstract of DOI:10.3184/174751918X15333065984578

A pulsed gradient spin echo experiment on [Rh₂(1,3-bis-(diphenylphosphino)-propane)₂(μ₂-Cl)₂] complex has been conducted in order to shed light on the supposed monomerisation process of [Rh₂(diphosphine)₂(μ₂-Cl)₂] complexes in solution. Such a process should generate a 14-electron [Rh(diphosphine)Cl] complex, which has only been postulated to date. Metathesis experiments on [Rh₂(1,3-bis-(diphenylphosphino)-propane)₂(μ₂-Cl)₂] and [Rh₂(bis[2-(diphenylphosphino)phenyl]ether)₂(μ₂-Cl)₂] complexes, analysed by ³¹P NMR, reveal that monomerisation of [Rh₂(diphosphine)₂(μ₂-Cl)₂] complexes is not restricted to the case of 1,3-bis-(diphenylphosphino)propane.

Full text of DOI:10.3184/174751918X15333065984578

402 RESEARCH PAPER
VOL. 42 AUGUST, 402–404
JOURNAL OF CHEMICAL RESEARCH 2018
Monomerisation of [Rh2(1,3-bis-(diphenylphosphino)-propane)2(µ2-Cl)2]
detected by pulsed gradient spin echo spectroscopy and 31P NMR monitoring
of metathesis experiments
Alberto Mannua*, Monica Ferrob, Saskia Möllera and Detlef Hellera
aLeibniz-Institut für Katalyse e.V. an der Universität Rostock (LIKAT Rostock), Albert-Einstein-Straße 29a, 18059 Rostock, Germany
bDepartment of Chemistry, Materials and Chemical Engineering ‘G. Natta’, Politecnico di Milano, Piazza L. da Vinci 32, 20133 Milan, Italy
A pulsed gradient spin echo experiment on [Rh (1,3-bis-(diphenylphosphino)-propane) (µ2-Cl)2] complex has been conducted in order
to shed light on the supposed monomerisation2process of [Rh (diphosphine)2(µ -Cl)2]2complexes in solution. Such a process should
generate a 14-electron [Rh(diphosphine)Cl] complex, which has2only been postul2ated to date. Metathesis experiments on [Rh2(1,3-bis-
(diphenylphosphino)-propane) (µ2-Cl) ] and [Rh (bis[2-(diphenylphosphino)phenyl]ether)2(µ2-Cl)2] complexes, analysed by 31P NMR,
reveal that monomerisation of2[Rh2(di2phosphine)22(µ2-Cl)2] complexes is not restricted to the case of 1,3-bis-(diphenylphosphino)propane.
Keywords: diphosphine, Rh(I), diffusion-ordered spectroscopy, pulsed gradient spin echo spectroscopy
Neutral dimeric Rh(I) complexes stabilised by diphosphine
ligands [Rh2(PP)2(µ2-Cl)2] (PP = diphosphine) have interesting
applications, mainly in catalysis. Recent examples include the
hydrogenation of prochiral olefins,1–3 the formation of C–C
bonds mediated by hydrogen,4–6 the addition of carboxylic
acids to alkynes,7–10 CO gas-free hydroformylation,11–12
carbonylations13 and many others. As suggested by several
authors, the reactivity of these complexes should be strongly
dependent on a dissociative process, which should generate a
monomeric, highly reactive 14-electron complex of the type
[Rh(PP)Cl] (Scheme 1).
experiments can be complicated by the rate of the equilibrium
reported in Scheme 1, currently unknown. Attempts to quantify
such an equilibrium have been made by flash photolysis of the
carbonyl complex [Rh(PPh3)2Cl(CO)].20
The only NMR characterisation of a tri-coordinate complex
of this type was the proposed synthesis and characterisation
of [Rh(PCy3)2Cl] tri-coordinated complexes.21 Comparison
with the 31P NMR spectrum of the corresponding dimer
[Rh2(PCy3)4(µ2-Cl)2] revealed a slight change in the value of
JP–Rh. However the monomerisation of the [Rh2(PCy3)4(µ2-Cl)2]
complex was not observed or proved.
The formation of this kind of intermediate has been
proposed by us in the case of bis-[2-(diphenylphosphino)
phenyl]ether14 (DPEPhos) and by other authors in the case
of 1,4-bis-(diphenylphosphino)ethane (DPPE) and 1,4-bis-
A possible way to gain more information about the existence
of the equilibrium reported in Scheme 1 and to collect direct
evidence of the in situ formation of DPPP-II is by means of
pulsed gradient spin echo (PGSE) diffusion NMR spectroscopy.
Diffusion-ordered spectroscopy (DOSY) experiments have
already been used in the past for the characterisation of reactive
organometallic intermediates in solution.22,23
In this regard, when an evolving system is considered, the
interpretation of the DOSY map can be difficult. In particular,
in the case of fast exchange between two species in solution,
the distinction between the two different coefficients of
diffusion (D) is not guaranteed.24 Having this limitation in
mind, a 31P DOSY experiment has been performed on a solution
of DPPP-I in deuterated THF, with the aim of distinguishing
between DPPP-I and DPPP-II on the basis of their different
hydrodynamic radii (Fig. 1).25
(diphenylphosphino)butane15
(DPPB),
MeO-2,2′-bis-
(diphenylphosphanyl)-1,1’-biphenyl16 (MeO-BIPHEP) and
PPh3.17 However, to the best of our knowledge, the generation
of [Rh(PP)Cl] has been only postulated or even only calculated,
but never observed directly.
The aim of the present study is to present some insights about
the existence of the equilibrium reported in Scheme 1 in the
case of PP = DPPP and DPEPhos through a diffusion-ordered
NMR spectroscopy experiment and metathesis experiments
followed by 31P NMR.
Results and discussion
Complex [Rh2(DPPP)2(µ2-Cl)2] (DPPP-I) was prepared by
a variation of Bosnich´s procedure.18 A solution containing
DPPP-I (10 mg) in deuterated tetrahydrofuran (THF) (1 mL)
was used for the NMR study. 31P NMR experiments showed
The DOSY map relative to DPPP-I in THF reveals two different
species with distinct values of D (D1 = 1.24 × 10−9 m2 s−1 and
D = 1.42 × 10−9 m2 s−1), which give the same signal in the 31P NMR
(δ231.9 ppm). Due to the supposed fast equilibrium between DPPP-I
and DPPP-II, the values for D reported in the DOSY map could
be shifted with respect to the real ones, as previously reported.26
In fact, the contribution of a fast equilibrium to the variation of I0
(the signal amplitude in the absence of an external gradient), which
will affect the increasing gradient strength, is not considered. As
1
a doublet at δ 31.9 ppm with a JP–Rh of 184 Hz, while H and
13C NMR spectroscopy indicated the presence of only one
kind of species in solution. For a detailed characterisation
of [Rh2(DPPP)2(µ2-Cl)2], see ref.19. The detection of the
supposed monomer [Rh(DPPP)Cl] (DPPP-II) by routine NMR
k1
Cl
P
P
Cl
P
P
P
P
Rh
II
DPPP-II
2
Rh
Rh
Cl
k-1
I
=
DPPP-I
When PP DPPP
Scheme 1 Monomerisation of [Rh2(DPPP)2(μ2-Cl)2] complex.
* Correspondent. E-mail: alberto.mannu@catalysis.de

JOURNAL OF CHEMICAL RESEARCH 2018 403
Fig. 1 DOSY map of a solution of complex DPPP-I in deuterated THF at room temperature.
Ph
Ph
P
Ph
Ph
Ph
Ph
Ph
Ph
P
P
Cl
Cl
P
Cl
Cl
P
Rh
Rh
Rh
Rh
O
O
P
Ph
P
P
Ph
Ph
Ph
Ph
Ph
Ph
Ph
δ
31.9 ppm
JRhP = 184.0 Hz
Ph
Ph
Ph
Ph
P
P
P
P
Cl
2
Rh
2
Rh
O
+
Cl
Ph
Ph
δ
δ
38.7 ppm
JRhP = 198.1 Hz
Ph
Ph
Ph
Ph Ph
P
δ
31.7 ppm
JRhP = 184.0 Hz
38.6 ppm
JRhP = 202.4 Hz
Ph
Rh
Ph
P
P
Cl
Rh
2
O
P
Cl
Ph Ph
Ph
Fig. 2 Metathesis between [Rh2(DPPP)2(μ2-Cl)2] and [Rh2(DPEPhos)2(μ2-Cl)2]; 31P NMR in THF/benzene-d6.
a consequence of this, when the kinetic rate of the equilibrium
considered is fast enough to be comparable with the time of the
DOSY measurements, the detection of intermediate reactive
species can fail. Fortunately, in the case of the DPPP-I complex, it
was possible to distinguish between two different species present
in solution with different thermodynamic radii.
(µ2-Cl)2] (DPPP-DPPE-I) was observed by 31P NMR.27 The
experiment represented indirect evidence of the monomerisation
of [Rh2(PP)2(µ2-Cl)2] (PP = DPPP or DPPE) in aprotic solvents.
In order to increase the number of examples of such
transformations and to expand the synthetic possibilities of the
[Rh(DPPP)Cl] (DPPP-II) complex, an analogous experiment
with DPPP-I and [Rh2(DPEPhos)2(µ2-Cl)2] (DPEPhos-I) has been
performed. Equimolar quantities of DPPP-I and DPEPhos-I, mixed
and dissolved in a mixture of THF/benzene-d6, were analysed by
31P NMR, which revealed the in situ formation of the metathesis
complex DPPP-DPEPhos-I (Fig. 2). (Note: A combination of THF
and benzene was used to enhance the solubility of the mixture of
complexes [Rh2(DPEPhos)2(µ2-Cl)2] and [Rh2(DPPP)2(µ2-Cl)2].)
The results reported point to an equilibrium dimer–monomer
even in the case of complex DPEPhos-I, as previously calculated
by density functional theory (DFT) computations.14
Metathesis experiment
If the DOSY map shows the presence of two species when
[Rh2(DPPP)2(µ2-Cl)2] is dissolved in THF, it does not give us
any information about the monomeric or dimeric nature of
the species. Very recently, an easy experiment to demonstrate
the in situ monomerisation of dimeric [Rh2(PP)2(µ2-Cl)2]
complexes has been designed. [Rh2(DPPP)2(µ2-Cl)2] (DPPP-I)
and [Rh2(DPPE)2(µ2-Cl)2] (DPPE-I) were dissolved in THF and
the in situ formation of the mixed complex [Rh2(DPPP)(DPPE)

404 JOURNAL OF CHEMICAL RESEARCH 2018
Conclusions
1H NMR (300 MHz, benzene-d6): δ (ppm) 6.44–7.94 (m, 28H); 31P NMR
(300 MHz, benzene-d6): δ (ppm) 38.7 (d, JP–Rh = 198 Hz).
For the first time, a 31P DOSY experiment has been employed
to detect the formation of the complex [Rh(DPPP)Cl], a highly
reactive intermediate considered responsible for the catalytic
activity of [Rh2(DPPP)2(µ2-Cl)2] but never before directly
observed. Metathesis experiments involving [Rh2(DPPP)2(µ2-
Cl)2] and [Rh2(DPEPhos)2(µ2-Cl)2] showed the formation
of [Rh2(DPPP)(DPEPhos)(µ2-Cl)2], confirming the in situ
monomerisation of both starting complexes.
Metathesis experiment
THF (2 mL) was added to
a
Schleck tube containing
[Rh2(DPPP)2(µ2-Cl)2] (0.02 mmol) and [Rh2(DPEPHOS)2(µ2-Cl)2]
(0.02 mmol). To a portion of the THF solution (0.5 mL), benzene-d6
(0.5 mL) was added, and the resulting mixture was monitored by
31P NMR.
Received 4 June 2018; accepted 19 July 2018
Paper 1805455
https://doi.org/10.3184/174751918X15333065984578
Published online: 9 August 2018
Experimental
All manipulations were carried out under argon using standard
Schlenk techniques. THF, dichloroethane, benzene and xylene were
distilled from sodium and MeOH was distilled from magnesium.
Subsequent removal of traces of oxygen from deuterated THF was
carried out through the application of six freeze–thaw cycles. The
rhodium precursor [Rh2(cod)2(µ2-Cl)2] (98%) was purchased from
Strem. DPPP and DPEPhos were purchased from Sigma Aldrich (98%)
and were recrystallised from MeOH.
References
1
2
3
W. Gao, H. Lv and X. Zhang, Org. Lett., 2017, 19, 2877.
C.S. Shultz and S.W. Krska, Acc. Chem. Res., 2007, 40, 1320.
Y. Hsiao, N.R. Rivera, T. Rosner, S.W. Krska, E. Njolito, F. Wang, Y.K. Sun,
J.D. Armstrong III, E.J.J. Grabowski, R.D. Tillyer, F. Spindler and C. Malan,
J. Am. Chem. Soc., 2004, 126, 9918.
NMR experiments
4
5
G. Tran, K.D. Hesp, V. Mascitti and J.A. Ellman, Angew. Chem. Int. Ed.,
2017, 56, 5899; Angew. Chem., 2017, 129, 5993.
T.M. Beck and B. Breit, Angew. Chem. Int. Ed., 2017, 56, 1903; Angew.
Chem., 2017, 129, 1929.
1H NMR and 31P NMR spectra were obtained on a Bruker AV-300 or
AV-400 spectrometer at 297–298 K and were referenced internally
to the deuterated solvent. For 31P{1H} NMR spectra, 85% H3PO4 was
used as external standard. Data processing was performed by Topspin
software (version 3.5) (Bruker). All of the NMR spectra were recorded
at 297 K (24 °C) unless otherwise indicated.
6
7
8
9
L.A. Schwartz and M.J. Krische, Isr. J. Chem., 2017, 57, 1.
A.M. Haydl, B. Breit and T. Liang, Angew. Chem. Int. Ed., 2017, 56, 8422.
P. Koschker, M. Kahny and B. Breit, J. Am. Chem. Soc., 2015, 137, 3131.
C. Li and B. Breit, J. Am. Chem. Soc., 2014, 136, 862.
10 S. Wei, J. Pedroni, A. Meißner, A. Lumbroso, H.-J. Drexler, D. Heller and B.
Breit, Chem. Eur. J., 2013, 19, 12067.
11 T. Furusawa, T. Morimoto, Y. Nishiyama, H. Tanimoto and K. Kakiuchi,
Chem. Asian J., 2016, 11, 2312.
12 G. Makado, T. Morimoto, Y. Sugimoto, K. Tsutsumi, N. Kagawa and K.
Kakiuchi, Adv. Synth. Catal., 2010, 352, 299.
13 T. Furusawa, H. Tanimoto, Y. Nishiyama, T. Morimoto and K. Kakiuchi,
Adv. Synth. Catal., 2017, 359, 240.
14 U. Gellrich, A. Meißner, A. Steffani, M. Kahny, H.-J. Drexler, D. Heller, D.A.
Plattner and B. Breit, J. Am. Chem. Soc., 2014, 136, 1097.
15 S.M. Jackson, C.E. Hughes, S. Monfette and L. Rosenberg, Inorg. Chim.
Acta, 2006, 359, 2966.
Diffusion-ordered spectroscopy
For the DOSY experiment, 32 spectra with a TD = 32 k were collected.
For each experiment, 64 scans were acquired using a ∆ = 0.07 s and
a δ = 1 ms. The pulse gradients were incremented from 2% to 98%
of the maximum gradient strength in a linear ramp. The temperature
was set at 297 K (24 °C). In the PGSE diffusion experiment,
[Rh2(DPPP)2(µ2-Cl)2] (0.01 mmol) was dissolved in deuterated THF
(1 mL) and the resulting solution was subjected to a DOSY experiment.
Synthesis of [Rh2(DPPP)2(μ2-Cl)2]
A solution of DPPP (29.7 mg, 0.072 mmol) in xylene (1.5 mL) was added
dropwise to a solution of [Rh2(cod)2(µ2-Cl)2] (35.5 mg, 0.036 mmol) in
xylene (1 mL) at 70 °C. The temperature was raised to 110 °C and the
resulting red mixture was stirred for 3 h. The solvent was removed under
vacuum at 110 °C and the residue dried under vacuum overnight in order
to remove traces of free cod. The product was recrystallised from THF/
nhexane to give a red powder: Yield 95%; 1H NMR (300 MHz, THF-d8):
δ (ppm) 1.65–1.78 (m, 4H), 2.15 (s, br, 8H), 7.05 (t, J = 7.20 Hz, 16H),
7.15 (t, J = 7.25 Hz, 8H), 7.64–7.70 (m, 16H); 31P NMR (300 MHz,
THF-d8): δ (ppm) 31.9 (d, JP–Rh = 184.0 Hz).
16 T. Korenaga, R. Sasaky, T. Takemoto, T. Yasuda and M. Watanabe, Adv.
Synth. Catal., 2018, 360, 322.
17 J.M. Brown, P.L. Evans and A.R. Lucy, J. Chem. Soc. Perkin Trans. II, 1987,
1589.
18 D.P. Fairlie and B. Bosnich, Organometallics, 1988, 7, 936.
19 A. Meißner, A. König, H.-J. Drexler, R. Thede, W. Baumann and D. Heller,
Chem. Eur. J., 2014, 20, 14721.
20 D. Wink and P.C. Ford, J. Am. Chem. Soc., 1987, 109, 436.
21 H.L.M. van Gaal and F.L.A. van der Bekerom, J. Organomet. Chem., 1977,
134, 237.
22 D. Li, I. Keresztes, R. Hopson and P.G. Williard, Acc. of Chem. Res., 2009,
42, 270.
Synthesis of [Rh2(DPEPhos)2(μ2-Cl)2]
23 A. Macchioni, G. Ciancaleoni, C. Zuccaccia and D. Zuccaccia, Chem. Soc.
Rev., 2008, 37, 479.
24 C.S. Johson Jr, J. Magn. Reson., Series A, 1993, 102, 214.
25 J.S. Gounarides, A. Chen and M.J. Shapiro, J. Chromatogr. B, 1999, 725, 79.
26 M. Oikonomou, J. Asencio-Hernández, A.H. Velders and M.A. Delsuc, J.
Magn. Reson., 2015, 258, 12.
A dry, argon-flushed Schlenk tube was charged with [Rh2(COD)2(µ2-Cl)2]
(23.6 mg, 0.0479 mmol), DPEPhos (51.5 mg, 0.0957 mmol) and
1,2-dichloroethane (DCE) (1 mL) under argon. The resulting solution
was stirred at r.t. for 30 min and then layered with diethyl ether and stored
under argon. A red crystalline precipitate formed in a few hours. The
solvent was removed and the crystals were washed with diethyl ether:
27 A. Mannu, H.-J. Drexler, R. Thede, M. Ferro, W. Baumann, J. Rüger and D.
Heller, J. Organomet. Chem., 2018, 871, 178.