Monometallic and mixed-valence bimetallic Rh(I/III) complexes: Synthesis, structure, electrochemistry and application in 1-octene hydroformylation

Article abstract of DOI:10.1016/j.jorganchem.2017.07.037

Two mononuclear rhodium complexes, Rh(I)-1,5-cyclooctadiene and Rh(III)-pentamethylcyclopentadienyl, and a bimetallic Rh(I)-Rh(III) mixed-valence complex were synthesised. The complexes were fully characterised using an array of analytical techniques which included NMR and infrared spectroscopy, single-crystal X-ray diffractometry, elemental analysis, and mass spectrometry. These complexes were evaluated as catalyst precursors in the hydroformylation of olefins. All catalyst precursors were active in the hydroformylation of 1-octene under optimised conditions. The rhodium complexes generally showed chemoselectivity towards the formation of linear aldehydes. The electrochemical behaviour of the rhodium complexes was examined using cyclic voltammetry.

Full text of DOI:10.1016/j.jorganchem.2017.07.037

Accepted Manuscript
Monometallic and mixed-valence bimetallic Rh(I/III) complexes: Synthesis, structure,
electrochemistry and application in 1-octene hydroformylation
Preshendren Govender, Siyabonga Ngubane, Bruno Therrien, Gregory S. Smith
PII:
S0022-328X(17)30467-9
10.1016/j.jorganchem.2017.07.037
JOM 20051
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 6 June 2017
Revised Date: 26 July 2017
Accepted Date: 30 July 2017
Please cite this article as: P. Govender, S. Ngubane, B. Therrien, G.S. Smith, Monometallic and mixed-
valence bimetallic Rh(I/III) complexes: Synthesis, structure, electrochemistry and application in 1-octene
hydroformylation, Journal of Organometallic Chemistry (2017), doi: 10.1016/j.jorganchem.2017.07.037.
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ACCEPTED MANUSCRIPT
Monometallic and Mixed-Valence Bimetallic Rh(I/III) Complexes:
Synthesis, Structure, Electrochemistry and Application in 1-Octene
Hydroformylation
a
a
b
Preshendren Govender, Siyabonga Ngubane, Bruno Therrien and Gregory S.
,
a
Smith*
a
b
Department of Chemistry, University of Cape Town, Rondebosch, 7701, Cape Town, South Africa.
Institute of Chemistry, University of Neuchatel, 51 Ave de Bellevaux, CH-2000 Neuchatel,
Switzerland.
_
*
_________
Corresponding author. Tel.: +27-21-6505279.
E-mail address: gregory.smith@uct.ac.za
Keywords:
Homogeneous catalysis
Hydroformylation
Mixed-valence
Rhodium
Chemoselectivity
Electrochemistry
_
__________________________________________________________________
1

ACCEPTED MANUSCRIPT
Abstract
Two mononuclear rhodium complexes, Rh(I)- 1,5-cyclooctadiene and Rh(III)-
pentamethylcyclopentadienyl, and a bimetallic Rh(I)-Rh(III) mixed-valence complex
were synthesised. The complexes were fully characterised using an array of analytical
techniques which included NMR and infrared spectroscopy, single-crystal X-ray
diffractometry, elemental analysis, and mass spectrometry. These complexes were
evaluated as catalyst precursors in the hydroformylation of olefins. All catalyst
precursors were active in the hydroformylation of 1-octene under optimised
o
conditions. Optimal reaction conditions were achieved at 75 C and 30 bar syngas
o
pressure for the Rh(III)-catalyst precursor, and at 90 C and 40 bar syngas pressure for
the Rh(I)- and the Rh(I)-Rh(III)-mixed-valence catalyst precursors, where
chemoselectivity towards aldehydes was achieved. The electrochemical behaviour of
the rhodium complexes was examined using cyclic voltammetry.
2

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1
.
Introduction
The hydroformylation reaction is an important homogeneously catalysed
industrial process for the synthesis of aldehydes from alkenes. The chemical reaction
entails the addition of a formyl group (CHO) and a hydrogen atom to a carbon-carbon
double bond, in the presence of a Co or Rh catalyst, to afford aldehydes [1]. The
process was first discovered by Otto Roelen using a carbonyl-based cobalt catalyst
[
HCo(CO) ] [2-3]. Subsequent research led to the development of highly active Rh-
4
based catalysts, which show high selectivity under milder conditions [4-7]. More
recently, water-soluble catalysts have been developed, that facilitate the separation of
the products from the catalysts [8]. Currently, the hydroformylation reaction is one of
several synthetic pathways in the production of long chain hydrocarbons, primarily
catalysed by Co(I) or Rh(I) species. In comparison to typical Rh(I) hydroformylation
catalysts, such as [RhCl(PPh ) ] [9-10] and [Rh(acac)(CO)PPh ] (where acac =
3
3
3
acetoacetonato) [11], there have been relatively few reported examples of Rh(III)
complexes used as hydroformylation catalysts [12-15], and this is attributed to their
difficult separation and poor stability [16-17]. To the best of our knowledge,
bimetallic Rh(I)-Rh(III) mixed-valence systems used as hydroformylation catalysts
have not been reported. Herein, we report the synthesis and characterisation of new
well-defined Rh(I) and Rh(III) complexes, including a mixed-valence bimetallic
Rh(I)-Rh(III) complex, and investigate their potential as hydroformylation catalyst
precursors, for the conversion of 1-octene into linear and/or branched aldehydes.
2
.
Experimental
2
.1. General remarks and Methods
All solvents, reagents and deuterated solvents were purchased from Sigma-
Aldrich and used without further purification. Rhodium(III)trichloride trihydrate was
2
2
5
obtained from Heraeus South Africa Limited. [RhCl(η :η -COD)] [18], [RhCl (η -
2
2
C Me )] [19], L1[20] and L2 [21], were prepared according to literature procedures.
5
5
2
Infrared (IR) spectra were determined in the solid state on a Perkin-Elmer
Spectrum 100 FT-IR spectrometer equipped with a SMART iTR ATR unit. The
intensity of stretching vibrations are marked as strong (s), medium (m) and weak (w).
1
NMR spectra were recorded on a Varian Mercury XR300 spectrometer ( H: 300.08
13
1
1
MHz; C{ H}: 75.46 MHz) or Bruker Biospin GmbH spectrometer ( H: 400.22
3

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13
1
MHz; C{ H}: 100.65 MHz) at ambient temperature. Chemical shifts δ in ppm
indicate a downfield shift relative to tetramethylsilane (TMS) and were referenced
relative to the signal of the solvent [22]. Coupling constants J are given in Hz.
Individual peaks are marked as singlet (s), doublet (d), doublet-of-doublet (dd), triplet
(t), or multiplet (m). Electron impact mass spectrometry (EI-MS) was carried out on a
JEOL GCmateII mass spectrometer. High-resolution electrospray ionization-mass
spectrometry (HR-ESI-MS) was carried out on a Waters Synapt mass spectrometer.
Elemental analysis (C, H, N) was carried out using a Thermo Flash 1112 Series
CHNS-O Analyzer. The hydroformylation products were analysed on a Perkin-Elmer
Clarus 580 GC.
2
.2. Synthesis of 1.
NaH (0.0400 g, 1.68 mmol) was added to a solution of L1 (0.126 g, 0.561
mmol) in DCM (50 mL) and stirred at RT for 15 min, resulting in a yellow
2
2
suspension. A solution of [RhCl(η :η -COD)] (0.138 g, 0.281 mmol) in DCM (10
2
mL) was added dropwise to the reaction mixture over a 10 min period. The reaction
mixture was stirred at RT for a further 10 min. The yellow suspension turned into an
orange solution, and the reaction mixture was quenched with distilled H O (20 mL).
2
The organic layer was isolated and washed with a further 20 mL of distilled H O.
2
Once again, the organic layer was isolated, dried over MgSO , filtered and the solvent
4
of the filtrate reduced to ca. 2 mL. A yellow precipitate was observed, filtered,
washed with Et O and dried under vacuum. The product was isolated as a yellow
2
solid.
-1
Yield: 0.193 g, 79.1%. IR (ATR): ν (cm ) = 1574 & 1597 (s, 2 broad overlapping
1
imine stretching vibrations, C=N). H NMR (CDCl ): δ (ppm) = 1.93 (m, 4H, CH
3
2
COD), 2.46 (m, 4H, CH2 COD), 4.20 (br m, 4H, CH COD), 6.60 (ddd, 1H, CH ), 6.91
Ar
(
m, 1H, CH ), 7.17 (dd, 1H, CH ), 7.34 (ddd, 1H, CH ), 7.61 (d, 2H, 2 x CH pyr),
Ar Ar Ar
13
1
7
.70 (s, 1H, CH imine), 8.00 (s, 1H, CH imine), 8.76 (d, 2H, 2 x CH pyr). C{ H} NMR
); 114.9, 122.0, 134.4, 134.8 (CH ); 121.9, 150.8
(CDCl ): δ (ppm) = 30.4 (CH
3
2 COD
Ar
(
CH pyr); 116.8, 166.9 (C ); 139.6 (C pyr); 152.8, 155.6 (CH imine). Elemental analysis
Ar
for C H N ORh (435.3348): Found C, 57.81; H, 5.14; N, 9.51%; calcd. C, 57.94; H,
2
1
22
3
+
5
.09; N, 9.65%. MS (EI, m/z): 435.0198 [M] .
4

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2
.3. Synthesis of [2][BPh ].
4
Triethylamine (0.0643 g, 0.623 mmol) was added to a stirred suspension of L2
0.102 g, 0.623 mmol) in a MeOH:DCM (50:50 (v:v), 5.00 mL) mixture. The
(
resulting yellow-suspension was stirred at room temperature for 15 min. Next,
5
[
RhCl (η -C Me )] (0.194 g, 0.312 mmol) was added to the reaction mixture and
2
5
5 2
stirred at RT for 1 h. This was followed by the addition of L1 (0.133 g, 0.623 mmol)
and the solution was stirred at RT for 2 h and filtered. NaBPh (0.213 g, 0.623 mmol)
4
was added to the filtrate at 0ºC and stirred for 30 min at RT. The reaction mixture was
filtered and 20 mL of MeOH were added to the filtrate. The filtrate was reduced
(essentially removing all of the DCM) to ca. 5 mL. A red-orange precipitate was
observed and was isolated by filtration, washed with 1 mL cold MeOH, and dried in
vacuo. The product was isolated as a red-orange solid.
-1
Yield: 0.254 g, 43.1%. IR (ATR): ν (cm ) = 1601 (m, 3 broad overlapping imine
1
stretching vibrations, C=N). H NMR (CDCl ): δ (ppm) = 0.79 (br t, 3H,
3
NCH CH CH ), 1.26 (m, 2H, NCH CH CH ), 1.30 (s, 15H, CH3 Cp*), 3.66 (m, 2H,
2
2
3
2
2
3
NCH CH CH ), 6.64 (br t, 1H, CH ), 6.87 (br m, 4H, BPh ), 7.01 (br t, 8H, BPh ),
2
2
3
Ar
4
4
7
7
8
1
.07 (m, 3H, CH ), 7.34 (m, 1H, CH ), 7.36 (m, 1H, CH ), 7.38 (m, 1H, CH ),
Ar Ar Ar Ar
.45 (br m, 9H, BPh & 1 x CH ), 7.66 (m, 2H, 2 x CH Pyr), 7.72 (s, 1H, CH imine),
4
Ar
.34 (br s, 1H, CH imine), 8.52 (m, 2H, 2 x CH pyr), 8.77 (s, 1H, CH imine), 11.36 (br s,
1
3
1
H, OH). C{ H} NMR (CDCl ): δ (ppm) = 8.48 (CH
); 11.1 (CH ); 24.4, 65.1
3
3
3 Cp*
(
CH ); 95.1 (C Cp*); 116.3, 117.3, 119.8, 123.3, 133.2, 134.2, 134.8, 135.7 (CH Ar);
2
1
20.8 (C pyr); 121.7, 125.5, 136.3 (CH BPh4); 124.5, 151.1 (CH pyr); 127.2, 134.7,
43.8, 165.0 (C Ar); 157.5, 160.4, 168.2 (CH imine); 163.5, 164.0, 164.4 (C BPh4).
1
Elemental analysis for C H BN O Rh (3388.336): Found C, 72.60; H, 6.27; N,
57
58
4
2
5
.48%; calcd. C, 72.46; H, 6.19; N, 5.93%. MS (HR-ESI-TOF, m/z): 400.1147 [M –
+
L1 – BPh ] .
4
2
.4. Synthesis of [3][BPh ].
4
Triethylamine (0.0114 g, 0.112 mmol) was added to a solution of [2][BPh4]
(0.107 g, 0.112 mmol) in DCM (10 mL) and the reaction mixture stirred at RT for 15
2
2
min. Then, [RhCl(η :η -COD)] (0.0448g, 0.0562 mmol) was added to the dark red
2
solution and stirred for a further 5 min at RT. The reaction mixture was filtered and
2
0 mL of isopropanol added to the filtrate. The filtrate was reduced (essentially
5

ACCEPTED MANUSCRIPT
removing all of the DCM) to ca. 5 mL. An orange precipitate was observed and was
isolated by filtration, washed with cold isopropanol and dried under reduced pressure.
The product was isolated as an orange solid. Yield: 0.0869 g, 66.7%. IR (ATR): ν
-1
(
cm ) = 1566 (m, imine, C=N); 1601 (s, 2 broad overlapping imine stretching
1
vibrations, C=N). H NMR (CDCl ): δ (ppm) = 1.02 (m, 3H, NCH CH CH ), 1.49 (br
3
2
2
3
s, 17H, CH3 Cp*, NCH CH CH ), 1.92 (m, 4H, CH2 COD), 2.46 (m, 4H, CH2 COD), 3.78
2
2
3
(
m, 2H, NCH CH CH ), 4.21 & 4.65 (br m, 4H, CH
), 6.59 (br t, 1H, CH ), 6.79
COD Ar
2
2
3
(
m, 1H, CH ), 6.91 (m, 4H, BPh ), 7.08 (m, 3H, 3 x CH ), 7.19 (br m, 8H, BPh ),
Ar 4 Ar 4
7
.35 (br m, 11H, BPh & 3 x CH ), 7.63 (m, 2H, 2 x CH Pyr), 7.81 (br s, 1H, CH
4 Ar
imine), 8.47 (br s, 1H, CH imine), 8.72 (m, 2H, 2 x CH pyr), 8.80 (br s, 1H, CH imine).
1
3
1
C{ H} NMR (CDCl ): δ (ppm) = 9.04 (CH
); 11.3 (CH ); 28.9, 30.9 (CH2 COD);
3
3
3 Cp*
2
4.7, 71.4 (CH ); 93.8 (C Cp*); 114.7, 117.1, 119.8, 123.5, 133.1, 134.4, 134.9, 135.8
2
(
CH ); 120.7 (C pyr); 115.3, 123.5, 135.8 (CH BPh4); 127.1, 128.7 (CH pyr); 115.7,
Ar
1
33.4, 135.8, 166.3 (C ); 156.5, 159.9, 165.2 (CH imine); 163.5, 163.9, 165.2 (C BPh4).
Ar
Elemental analysis for C H BN O Rh (1154.9171): Found C, 67.42; H, 5.99; N,
65
69
4
2
2
4
.66%; calcd. C, 67.60; H, 6.02; N, 4.85%. MS (HR-ESI-TOF, m/z): 835.1993 [M –
+
+
BPh ] , 400.1146 [M – 1 – BPh ] .
4
4
2
.5. X-ray Crystallography
Crystals of L1 and 1 were mounted on a Stoe Image Plate Diffraction system
equipped with a circle goniometer, using Mo-Kα graphite monochromated radiation
λ = 0.71073 Å) with range 0-200º. The structures were solved by direct methods
φ
(
φ
using the program SHELXS-97, while the refinement and all further calculations were
carried out using SHELXL-97 [23]. In 1, the H-atoms were included in calculated
positions and treated as riding atoms using the SHELXL default parameters, while in
L1 the H-atoms were located and refined isotropically. Otherwise, all the non-H
2
atoms were refined anisotropically, using weighted full-matrix least-square on F .
Figures 1 and 2 were drawn with ORTEP [24].
CCDC-1441154 L1 and 1441155 1 contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge at
www.ccdc.cam.ac.uk/conts/retrieving.html [or from the Cambridge Crystallographic
6

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Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: (internat.) +44-
1
223/336-033; E-mail: deposit@ccdc.cam.ac.uk].
2
.6. Electrochemical studies
Cyclic voltammetric studies were carried out at room temperature under an inert
atmosphere of N2 using a three-electrode system on a BASi EC Epsilon
potentiostat/galvanostat electrochemical workstation. The C-3 cell stand equipped
with a glassy carbon working electrode, Ag/AgCl (Sat’d NaCl) reference electrode
and a Pt wire auxiliary electrode was used to carry out electrochemical measurements.
All experiments were carried out in anhydrous CH Cl containing 0.10 M tetra-n-
2
2
butylammonium perchlorate (TBAP) as the supporting electrolyte. The reference
electrode was separated from bulk solution using a fritted glass bridge containing the
0
.10
M
TBAP/CH Cl2 electrolyte solution. Under these conditions,
a
2
ferrocene/ferrocenium (II/III) oxidation potential, measured at 0.54 V, was used as an
internal standard. All measurements were carried out at 100 mV/s, except in the case
of compound [2][BPh ] whereby a scan rate of 50 mV/s was used to generate a
4
significant voltage/current response in mA.
2
.7. Catalytic Studies
The reactions were conducted in a 90 mL stainless steel pipe reactor. The
reactor was charged with toluene (6 mL), 1-octene (0.805 g, 7.18 mmol), internal
standard n-decane (0.204 g, 1.44 mmol) and catalyst precursor (1, [2][BPh ] or
4
[
3][BPh ]). The reactor was purged with nitrogen three times and then three times
4
with syngas (1:1, CO:H ). The reactor was then pressurized and heated to the desired
2
pressure and temperature. Following optimization experiments, reactions were
performed for 4 h (for [3][BPh ] at 40 bar pressure) or 8 h (1 at 30 bar pressure or
4
[
2][BPh ] at 40 bar pressure), and samples were collected at the beginning and end of
4
each reaction and analysed using gas chromatography (GC), and products were
confirmed in relation to authentic iso-octenes and aldehydes.
7

ACCEPTED MANUSCRIPT
3
.
Results and Discussion
3
.1. Synthesis and Characterization of Complexes 1 – [3][BPh4]
The ligands L1 and L2 were prepared following published literature methods
[
20, 21]. Complex 1 was prepared via a bridge-splitting reaction using the Rh-
2
2
precursor [RhCl(η :η -COD)] (where COD = 1,5-cyclooctadiene) and reacting with
2
L1 in dichloromethane, in the presence of sodium hydride (Scheme 1). Complex
[
(
2][BPh ] was prepared in a one-pot reaction by first deprotonating ligand L2
4
prepared using literature methods)[21] with triethylamine, followed by the addition
5
of
the
Rh-Cp*
dimer
[RhCl (η -C Me )]
(where
C Me
5 5
=
2
5
5
2
pentamethylcyclopentadienyl) and L1 (Scheme 1). Complex [2][BPh ] was isolated as
4
a tetraphenylborate salt, via a metathesis reaction with NaBPh . The bimetallic Rh(I)-
4
Rh(III)-mixed-valence complex [3][BPh ] was prepared using similar methodologies
4
to 1, whereby [2][BPh ] was deprotonated using triethylamine and reacted with
4
2
2
[
RhCl(η :η -COD)] for 5 minutes (Scheme 1), and isolating [3][BPh ] as an orange
2 4
solid. Complexes 1 – [3][BPh ] were isolated in moderate to good yields, and are air-
4
and moisture-stable, soluble in a handful of solvents, such as dichloromethane,
chloroform, acetonitrile and dimethylsulfoxide, and insoluble in protic and non-polar
solvents.
Scheme 1. Synthesis of Rh-based complexes 1 – [3][BPh4].
The complexes 1 – [3][BPh ] were characterised using standard analytical and
4
1
13
1
spectroscopic techniques such as H and C{ H} NMR, infrared spectroscopy and
8

ACCEPTED MANUSCRIPT
high-resolution electrospray ionisation (ESI) mass spectrometry, to name a few. The
1
H NMR spectrum of 1 displays an upfield shift in the singlet assigned to one of the
imine protons from 8.81 ppm in L1 to 8.00 ppm in 1 and a disappearance in the broad
singlet (assigned to the phenolic proton on L1) around 12 ppm. This suggests
coordination of the Rh-COD moiety occurs in a bidentate manner via the phenolic
oxygen and imine nitrogen atoms of the salicylaldimine moiety in L1. Furthermore,
the appearance of the three multiplets (4.20, 2.46 and 1.93 ppm) in the aliphatic
region, integrating for four protons each, are assigned to the CH and CH protons of
2
the COD moiety, suggesting successful coordination of L1 to the metal centre. This
observation compares favourably with structurally similar complexes previously
1
3
1
reported in the literature [25-26]. In the C{ H} NMR spectrum, the imine carbon
signals resonate at 153 ppm and 156 ppm, whilst the aliphatic carbon atoms of the
13
1
COD moiety appear at ca. 30 ppm in the C{ H} NMR spectrum of L1. Upon
coordination of ligand L1 to the rhodium ion, the infrared spectrum of 1 displays a
distinct shift to lower wavenumbers in one of the C=Nimine stretching vibrations, from
-1
-1
1
623 cm (for L1) to ~1574 cm (for 1), and this is attributed to the metal
withdrawing electron density away from the imine bond. Furthermore, the second
-1
uncoordinated C=Nimine stretching vibration remains constant at around 1596 cm .
This confirms that coordination occurs in a bidentate manner via the N,O-atoms of the
salicylaldimine moiety, and not via the pyridylimine moiety.
1
The H NMR spectrum of [2][BPh ] shows three singlets at 7.72, 8.34 and 8.77 ppm
4
and these are assigned to the imine protons. Signals for the aliphatic protons appear as
a triplet at 0.79 ppm and as two multiplets at 1.26 ppm and 3.66 ppm. A broad singlet
is observed at 1.30 ppm for the Cp* protons, whilst the aromatic protons of the
pyridyl moiety appear as two multiplets at 7.66 ppm and 8.52 ppm, integrating for
two protons each. Two intense multiplets at 6.87 and 7.01 ppm are assigned to the
aromatic protons on the phenyl rings of the BPh counter-ion, with the phenolic
4
1
3
1
proton appears as a broad singlet at 11.36 ppm. The C{ H} NMR data for complex
[
2][BPh ] displays three singlets for the imine carbon atoms at 158, 160 and 168 ppm.
4
2
The sp carbon atoms of the BPh counter-ion appear as three intense signals at 122,
4
1
9
26 and 136 ppm, whilst the quaternary carbons appear in the range of 163 – 164

ACCEPTED MANUSCRIPT
ppm. The infrared spectrum of [2][BPh ] shows a broad stretching vibration of
4
-1
medium intensity at ca. 1601 cm and this is assigned to the three C=N
bonds.
imine
1
Broadening and coalescing of resonance signals in the H NMR spectrum of the
mixed-valence bimetallic complex [3][BPh ] was observed, and is attributed to the
4
multivalent nature of the compound, and the incorporation of metal ions of different
1
oxidation states contained in the same scaffold. A similar H NMR spectrum to that of
[
2][BPh ] was obtained for [3][BPh ], and shows the disappearance of the signal
4 4
assigned to the phenolic proton (observed at 11.36 ppm for [2][BPh ]), with the
4
appearance of three characteristic multiplets at 1.92, 2.46 and ca. 4.7 ppm, assigned to
1
3
1
the CH and CH protons of the COD moiety. The C{ H} NMR data for complex
2
[
3][BPh ] displays the expected number of signals for the proposed structure, with the
4
carbon signals for the imine moieties appearing at 157, 160 and 165 ppm.
Furthermore, infrared data for [3][BPh ] shows a shift in the stretching vibration,
4
-1
assigned to the newly coordinated C=Nimine bond, from 1601 cm (for [2][BPh ]) to
4
-1
lower wavenumbers at ca. 1566 cm . These observations suggests coordination
occurs in a bidentate manner through the N,O atoms of the free salicylaldimine moiety
on [2][BPh4].
In addition, high resolution ESI-mass spectra for 1 and [3][BPh ] contain molecular
4
+
ion peaks [M] at 435.0198 and 835.1993 respectively, whilst [2][BPh ] has a base
4
+
peak at 400.1147 corresponding to the charged adduct [M – L1 – BPh4]
3
.2. Single-crystal X-Ray Crystallography
The molecular structures of L1 and 1 were confirmed by single-crystal X-ray
diffraction analyses. Single crystals of each compound were grown from a
dichloromethane/diethyl ether mixture and Figure 1 and Figure 2 show the ORTEP
molecular structures of L1 and 1 respectively. Table 1 summarises the crystal data
and refinement parameters for the compounds.
1
0

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Fig. 1. ORTEP representation of L1, with thermal ellipsoids at the 50% probability
level. Selected bond lengths (Å) and angles (°): N1-N2 1.406(1), C7-N1 1.280(2), C8-
N2 1.265(2), C1-O1 1.354(1); C6-C7-N1 121.5(1), C7-N1-N2 113.8(1), N1-N2-C8
1
12.4(1), C9-C8-N2 122.2(1).
In the solid state, the pyridylimine and salicylaldimine moieties lie in a trans
conformation around the azine (-C=N-N=C-) motif (Figure 1). The C=Nimine bond
lengths and angles around the imine nitrogen are comparable with other structurally
similar Schiff base salicylaldimine and pyridylimine ligands reported in the literature.
[
27] There is an intramolecular O1-H1···N1 hydrogen bond (H1···N1 distance =
.75(2) Å, O1-H1···N1 angle = 149(2)°) in the structure of L1, strengthening the
1
planarity of the salicylaldimine ring with the pyridylimine ring (3.08(9)° between the
two planes). Such intramolecular hydrogen bonding was observed in other structurally
similar compounds [28].
1
1

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Fig. 2. ORTEP representation of 1, with thermal ellipsoids at the 50% probability
level. Selected bond lengths (Å) and angles (°):N1-N2 1.426(3), C8-N2 1.257(4), C7-
N1 1.292(4), C1-O1 1.313(3), N1-Rh1 2.058(2), O1-Rh1 2.029(2); N1-Rh1-O1
8
9.67(9), Rh1-N1-C7 126.0(2), C1-O1-Rh1 128.2(2).
The mononuclear structure of complex 1 shows a square-planar geometry at the Rh
metal centre (Figure 2), with the metal coordinated to the nitrogen and oxygen atoms
of the salicylaldimine moiety, and to the COD moiety. The bond angles around the Rh
°
metal centre are comprised between 87° and 95 , and they are comparable to
structurally similar compounds reported in the literature [7, 29]. Upon coordination,
the solid-state planarity of L1 is lost, the angle between the salicylaldimine and
pyridylimine rings being 64.39(9)°.
1
2

ACCEPTED MANUSCRIPT
Table 1. Crystallographic parameters for L1 and 1.
Parameters
L1
1
Chemical formula
Formula weight
Crystal system
Space group
C H N O
C H N ORh
21 22 3
1
3
11
3
225.25
435.32
Monoclinic
Triclinic
P-1
P2 /c
1
Crystal colour and shape Colorless plate
Yellow block
Crystal size
a (Å)
b (Å)
c (Å)
0.19 x 0.18 x 0.15
15.291(3)
5.6400(10)
15.291(3)
90
0.24 x 0.22 x 0.19
8.8848(7)
10.1956(7)
11.1252(8)
92.494(6)
108.052(6)
104.837(6)
917.92(12)
2
α (°)
β (°)
116.46(3)
90
γ (°)
3
V (Å )
1180.6(5)
4
Z
T (K)
293
293
-
3
D (g·cm )
c
1.267
1.575
-1
µ (mm )
0.084
0.945
Scan range (°)
Unique reflections
2.98 < ϕ < 29.26
3121
1.94 < ϕ < 29.20
4669
Reflections used [I>2σ(I)] 2127
Rint 0.0373
Final R indices [I>2σ(I)] 0.0469, wR 0.1256
4108
0.422
a
0.0383, wR 0.0921
2
2
R indices (all data)
0.0686, wR 0.1341
0.0434, wR 0.0941
2
2
Goodness-of-fit
1.025
0.986
-3
Max, Min ∆ρ (e Å )
0.329, -0.148
1.277, -1.608
3
.3. Electrochemistry
Cyclic voltammetry studies were carried out as a preliminary investigation to
try and understand the role of the Rh(III) centre and the impact this has at the Rh(I)
centre which is the more likely active centre. Experiments were performed in
anhydrous dichloromethane solutions containing 0.10 M tetra-n-butylammonium
perchlorate (TBAP) as the background electrolyte, using a three-electrode system.
The results are summarised in Table 2 below, listing values of the metal-centred redox
processes.
1
3

ACCEPTED MANUSCRIPT
Table 2. Electrochemical data measured in 0.10 M TBAP/CH Cl2
2
Potential, V vs Ag/AgCl (Sat’d NaCl)
Compound
a
Epa
1.28
-
Epc
E1/2
-1.72
-1.79
-
i /i
pa pc
1
-1.56
-1.54
-1.68
1.0
0.9
-
[
2][BPh4]
b
[
3][BPh4]
1.25
a
b
E1/2 = ½(E + E )
Scan rate = 50 mV/s
pa
pc
A Rh(I)/(III) oxidation process for compound 1 is observed as an irreversible
anodic peak at 1.28 V. A reversible Rh(I)/(0) reduction at -1.72 V is observed for the
same compound. For the cyclopentadienyl compound [2][BPh ], no metal-centred
4
oxidation potential is observed but a reversible Rh(III)/(I) reduction is seen at -1.79 V
half-wave potential which attests to its ability to undergo hydroformylation (vide
infra). When comparing compound 1 to the bimetallic mixed-valent compound
[
3][BPh ], a 30 mV cathodic shift on the Rh(III)/(I) oxidation (E = 1.25 V) is
4 pa
observed, affirming that the presence of Rh(III) affects the oxidation potential of
Rh(I). This shift can be attributed to the long-range conjugated interaction of the Rh(I)
and Rh(III) metal centres present in the compound. No metal-centred reduction
potentials for compound [3][BPh ] are visible on the cyclic voltammetric scale, as
4
they would be expected to occur at higher reduction potentials as a consequence of the
aforementioned cathodic shift.
3
.4. Rhodium-catalysed hydroformylation of 1-octene
The catalytic behaviour of complexes 1 – [3][BPh ] was investigated in the
4
hydroformylation of 1-octene (Scheme 2), yielding a mixture of aldehydes and olefin
isomers. In each experiment, the substrate (1-octene) and the catalyst precursors were
dissolved in toluene, and n-decane added as the internal standard.
Scheme 2. Hydroformylation of 1-octene.
1
4

ACCEPTED MANUSCRIPT
Previous studies reported by our group using similar Schiff-base Rh(I) catalyst
o
precursors for the hydroformylation of 1-octene,[6,30] revealed 75 C and 30 bar
syngas pressure as the optimum conditions to obtain high yields of aldehydes. Hence,
o
initial reactions were carried out at 75 C and 30 bar syngas pressure. Furthermore, it
-3
is important to note that the catalyst loading used was 2.87 x 10 mmol for 1 and
-3
[
2][BPh ] and 1.44 x 10 mmol for [3][BPh ] to maintain a constant metal
4 4
concentration. Initial experiments showed that all three catalyst precursors were active
as hydroformylation catalysts, with high conversion observed after 8 hours for
catalyst precursors 1 and [2][BPh ] (Table 3, entries 1,5). The bimetallic catalyst
4
precursor [3][BPh ] showed a near quantitative conversion after 4 hours (Table 3,
4
entry 9) using the same metal loading. This is not surprising as multinuclear
complexes have been shown to be highly efficient catalysts for a number of chemical
transformations [31-33] and have shown to have cooperative interactions between the
metal centres leading to higher reactivities and selectivities compared to their
mononuclear counterparts [34-36].
The influence of pressure on the hydroformylation activity and selectivity was
o
explored, keeping the temperature constant at 75 C for 4h (for [3][BPh ]) or 8h (for 1
4
and [2][BPh ]). Catalyst precursors 1 and [3][BPh ] (entries 2, 10) show no significant
4
4
change in the conversion (>98%) as the pressure is increased with a slight
improvement in aldehyde chemoselectivity. Complex [2][BPh ] (entry 6) showed a
4
dramatic improvement in conversion. Although the n:iso ratio remains largely
unchanged for 1, there is a dramatic increase in the n:iso ratio for [2][BPh ] and
4
[
3][BPh ] which is counter-intuitive, as an increase in pressure suggests the formation
4
of a CO-coordinated Rh complex which will more likely yield branched aldehydes.
Further experiments were carried out to investigate the effect of temperature
on the hydroformylation reaction, whilst the syngas pressure was kept constant at 30
o
bar (for 1) or 40 bar (for [2][BPh ] and [3][BPh ]). Increasing the temperature to 90 C
4
4
did not alter the catalytic performance for 1 and [3][BPh ] substantially (entry 3 and
4
1
1). In most cases, increasing the temperature had a huge impact on the linear to
branched aldehyde ratio, probably due to the isomerisation of the carbon-carbon
double bond, thus producing more branched aldehydes and hence a lower n:iso ratio.
1
5

ACCEPTED MANUSCRIPT
Catalyst [2][BPh ], in particular, shows the formation of more iso-octenes at higher
4
temperatures.
Table 3. Hydroformylation results of 1-octene using catalyst precursors 1 - [3][BPh4],
a
at different pressures.
Entry
Catalyst
Precursor
Pressure
[bar]
30
Temp.
%
%
% iso-
b
c
Conversion Aldehyde Octenes n:iso
TON
1
2
3
1
1
75
75
90
98.53
98.35
89.22
96.24
10.79
3.76
1.30
2198
2366
40
1.05
1.06
0.95
2.38
4.00
2.42
0.67
1.46
3.43
1.10
0.74
2.50
0.69
0.37
1
30
30
30
40
40
40
30
40
40
40
30
30
40
99.06
98.06
74.80
99.05
86.91
77.81
98.29
99.30
97.51
98.12
98.00
99
91.23
98.90
70.20
79.37
59.12
84.30
77.27
92.34
94.13
80.14
70
8.77
1.10
29.80
20.63
40.89
15.70
22.73
7.66
5.87
19.86
30
2259
2450
1314
1965
1280
1640
3797
4585
4616
3931
-
4
5
6
7
8
9
75
75
75
90
75
75
75
90
75
75
75
75
1
+ Hg
[
[
[
2][BPh4]
2][BPh4]
2][BPh4]
[
[
2][BPh ] + Hg
4
[
[
[
3][BPh4]
3][BPh4]
3][BPh4]
10
11
12
13
14
15
3][BPh ] + Hg
4
d
Ref. 30a
e
Ref. 39
84
16
-
f
Ref. 30b
99
99
1
-
a
Reactions carried out with syngas (1:1, CO:H ) in toluene (6 mL) with 7.175 mmol of 1-octene and
2
-3
-3
2
.87 x 10 mmol (for 1 and [2][BPh ]) or 1.44 x 10 mmol (for [3][BPh ]) of Rh catalyst. GC
4 4
conversions obtained using n-decane as an internal standard. Reaction time for 1 and [2][BPh ] = 8h or
4
for [3][BPh ] = 4h.
4
b
n:iso (normal aldehydes:iso aldehydes)
c
TON = (mol product/mol cat.)
d
Results obtained for an analogous Rh(I)-COD precursor containing a monodentate N-donor ligand
e
Results obtained for an analogous Rh(I)-COD precursor containing a bidentate N,N-donor ligand
f
Results obtained for an analogous Rh(I)-COD precursor containing a bidentate N,O-donor ligand
In general, all catalyst precursors show excellent aldehyde chemoselectivity.
Over 90% of the products formed for 1 and [3][BPh ] are aldehydes with very little
4
internal olefins formed. The Rh(III)-catalyst precursor [2][BPh ] showed the
4
formation of more internal olefins with just under 80% of aldehyde formation. Similar
observations were reported for a Rh(III)-system used in the biphasic hydroformylation
1
6

ACCEPTED MANUSCRIPT
of 1-octene [37]. Catalyst precursors 1 and [3][BPh ] slightly favour the formation of
4
linear aldehydes (>50%) over branched aldehydes. Such observations were previously
reported for Rh-based-N,O chelating systems which showed inferior regioselectivity
compared to reported N,N- and N,P-based catalytic systems [38-39], whilst catalyst
precursor [2][BPh ] displayed more than 80% linear aldehyde formation compared to
4
branched aldehydes.
Mercury poisoning experiments were performed to determine whether the
reaction is purely homogeneous or a combination of both homogeneous and
heterogeneous catalytic events taking place. Mercury was added to suppress any
unwanted Rh nanoparticles (the heterogeneous catalyst) that may have formed during
the reaction. In the presence of mercury, high conversions and good activity were
maintained for both catalyst precursors 1 and [3][BPh ] (entries 4, 12), essentially
4
indicating that the catalysis is occurring via a molecular species and not nanoparticles.
Furthermore, the bimetallic catalyst precursor [3][BPh ] (entry 12) shows a slight
4
decrease in aldehyde chemoselectivity (>80%) when mercury(0) is present. There is a
decrease in conversion seen for catalyst precursor [2][BPh ] (entry 8) suggesting that
4
both a homogeneous catalyst and that nanoparticle formation could be at play.
4
.
Conclusions
In summary, two mononuclear Rh(I)-COD 1 and Rh(III)-Cp* [2][BPh ] based
4
complexes, and a bimetallic Rh(I)-Rh(III)-mixed-valence complex [3][BPh ] were
4
prepared. All complexes were characterised using various spectroscopic and
analytical techniques. These complexes were used as catalyst precursors in the
hydroformylation of 1-octene. All catalyst precursors displayed catalytic activity in
the hydroformylation of 1-octene and are chemoselective towards aldehydes (major
o
products). The favoured reaction conditions were 75 C and 30 bar syngas pressure for
o
[
[
2][BPh ] and 90 C and 40 bar syngas pressure for 1 and [3][BPh ]. Generally, 1 and
4
4
3][BPh ] showed slight regioselectivity towards linear aldehydes, whilst catalyst
4
precursor [2][BPh ] is predominantly selective for linear aldehydes. Furthermore,
4
mercury poisoning experiments revealed that catalysis occurs primarily via a
homogenous, molecular species. Further, more extensive experiments, such as high
pressure NMR and IR spectroscopic studies, are being investigated to better
understand what the active catalyst is.
1
7

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Acknowledgements
We greatly acknowledge the financial support from the University of Cape
Town, the Department of Science and Technology South Africa and the NRF-DST
Centre of Excellence in Catalysis - c*change. Prof. Selwyn F. Mapolie (University of
Stellenbosch) is also thanked for useful and insightful discussions.
Supporting Information
1
13
H and C{1H} NMR spectra of complexes 1 – [3][BPh ].
4
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Graphical Abstract
2
0

ACCEPTED MANUSCRIPT
Highlights
‹
‹
‹
Two mononuclear rhodium complexes, and a bimetallic Rh(I)-Rh(III) mixed-
valence complex were synthesised based on salicylaldimine scaffolds.
The complexes were characterized using several spectroscopic and analytical
techniques.
Complexes were evaluated as catalyst precursors in the hydroformylation of
olefins.
‹
‹
Chemoselectivity towards aldehydes was achieved.
The electrochemical behaviour of the rhodium complexes was examined.