Subtle variation of stereo-electronic effects in rhodium(I) carbonyl Schiff base complexes and their iodomethane oxidative addition kinetics

Article abstract of DOI:10.1080/00958972.2020.1809657

Rhodium(I) carbonyl complexes of the form [Rh^I(N,O-ScBa)(CO)(PR₃)] (R = Ph or Cy), with N,O-ScBaH a mono anionic bidentate Schiff base ligand, 2-(cyclopentyliminomethyl)-5-methylphenol (5-Me-Sal-CyPH), and bearing different phosphine

Full text of DOI:10.1080/00958972.2020.1809657

Journal of Coordination Chemistry
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Subtle variation of stereo-electronic effects in
rhodium(I) carbonyl Schiff base complexes and
their iodomethane oxidative addition kinetics
Pennie P. Mokolokolo , Alice Brink , Andreas Roodt & Marietjie Schutte-Smith
To cite this article: Pennie P. Mokolokolo , Alice Brink , Andreas Roodt & Marietjie Schutte-
Smith (2020): Subtle variation of stereo-electronic effects in rhodium(I) carbonyl Schiff base
complexes and their iodomethane oxidative addition kinetics, Journal of Coordination Chemistry,
DOI: 10.1080/00958972.2020.1809657
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JOURNAL OF COORDINATION CHEMISTRY
https://doi.org/10.1080/00958972.2020.1809657
Subtle variation of stereo-electronic effects in rhodium(I)
carbonyl Schiff base complexes and their iodomethane
oxidative addition kinetics
Pennie P. Mokolokolo , Alice Brink , Andreas Roodt
Marietjie Schutte-Smith
and
Department of Chemistry, University of the Free State, Bloemfontein, South Africa
ABSTRACT
ARTICLE HISTORY
Received 28 May 2020
Accepted 30 July 2020
Rhodium(I) carbonyl complexes of the form [RhI(N,O-ScBa)(CO)
(PR3)] (R ¼ Ph or Cy), with N,O-ScBaH a mono anionic bidentate
Schiff base ligand, 2-(cyclopentyliminomethyl)-5-methylphenol (5-
Me-Sal-CyPH), and bearing different phosphine ligands on Rh(I),
are reported. N,O-ScBaH was also varied to be 2-(cyclohexylimino-
methyl)phenol (Sal-CyH) and 2-(isopropyliminomethyl)-5-methyl-
phenol (5-Me-Sal-IPropH) to investigate the effects of different
substituent groups on the periphery of the Schiff bases on metal
center reactivity. The structural characterization of two sample
complexes is described and an extensive spectroscopic kinetic-
mechanistic study utilizing UV/vis, infrared and 31P NMR spectros-
copy of the iodomethane oxidative addition is presented.
Iodomethane oxidative addition led exclusively to RhIII-alkyl com-
plexes as final products, while an associative activation is inferred
KEYWORDS
Structure/reactivity relation-
ships; Schiff base ligands;
rhodium phosphine; kinetics
¼
from the large negative DS value. The relative reactivity of the
[RhI(N,O-ScBa)(CO)(PR3)] complexes when stepwise varying PR3
(PPh3, PPh2Cy, PPhCy2 and PCy3) as tertiary phosphine ligands
follow a surprising similar reactivity relationship as observed for a
range of isostructural [RhI(X,O-Bid)(CO)(PR3)] complexes, with
X ¼ O, N or S in corresponding oxinate, acetylacetonate and thio-
ureate mono anionic X,O-Bid ligands. This suggests systematic
and responsive dynamic behavior of the PR3 ligands, independent
of the bidentate chelate at the Rh(I) metal center. A variation of
more than two orders-of-magnitude in reactivity was observed.
CONTACT Alice Brink
brinka@ufs.ac.za; Andreas Roodt
roodta@ufs.ac.za
Department of Chemistry,
University of the Free State, PO Box 339, Bloemfontein 9300, South Africa
Dedicated to the memory of Professor Enbo Wang.
ß 2020 Informa UK Limited, trading as Taylor & Francis Group

2
P. P. MOKOLOKOLO ET AL.
1. Introduction
Schiff bases may be considered a sub-class of imines, thus either secondary aldimines
or secondary ketimines, depending on their structure. It is often synonymous with
azomethine when referring more specifically to secondary aldimines (i.e. R-CH ¼ NR’
where R’ ¼ H). Named after Hugo Schiff, and having been around for more than
155 years [1–3], they are still relevant in (modern) coordination chemistry [4,5], show-
ing potential for application in carbon dioxide utilization [6]. Some recent papers and
reviews (not exhaustive) detailing aspects of these are available [7–10].
Oxidative addition reactions on metal complexes represent key steps in many cata-
lytic processes, including the carbonylation of alcohols, hydroformylation of alkenes
and cross-coupling reactions [11,12]. A classic example is the rhodium/iridium-iodide
supported carbonylation of methanol to produce acetic acid, i.e. the so-called
Monsanto and Cativa processes [13,14]. Oxidative addition is suggested as the rate-
determining step in the catalytic cycles of some of these processes, and therefore any
influence on this step can determine the course of the complete catalytic cycle. Due
to the vital role of this reaction, a significant amount of research has focused on oxi-
dative addition reactions to understand its kinetic properties and responses to external
and internal ligand influences [15,16]. In the Monsanto process the metal carbonyl
species is [RhI(I)2(CO)2]-, which is relatively unstable.
On the other hand, rhodium carbonyl species containing tertiary phosphine ligands
and/or bidentate ligands are generally more stable, with interesting recent metallo-
philic/non-covalent interactions described in selected Rh(I) dicarbonyl complexes [17].
By tuning the stereo-electronic properties of both the bidentate and the tertiary phos-
phine ligands, the carbonylation process may be directed towards different outcomes.
For example, other products might arise if hydrogenation precedes the reductive elim-
ination. Numerous studies have been undertaken to improve the catalytic activity of
the rhodium-based complex by introducing ligands with different electronic and steric
properties, which also include variation of the bite angle of the bidentate ligand [15].
Increasing the electron density on the rhodium center can activate it towards

JOURNAL OF COORDINATION CHEMISTRY
3
oxidative addition since the metal is a nucleophile in these reactions, although the
steric effects associated with any changes are equally important and may counter-act
the advantages of a more nucleophilic metal.
Complexes of the type [Rh(L,L’-Bid)(CO)(PR3)], where L,L’-Bid ¼ monoanionic biden-
tate ligands and PR3 are tertiary phosphine ligands, have thus been investigated
extensively as potential catalyst precursors [18–26]. In the case where bidentate
ligands contain a sulfur donor, for example thioacetylacetone [20], the iodomethane
oxidative addition primarily led to RhIII-acyl complexes [27–29]. This was also con-
firmed for other S,O-Bid ligand systems (notably thiourea based (S,O-BidH ¼ N-ben-
zoyl-N’,N’-diphenylthiourea) [23]. In both these ligand systems, however, the overall
net reaction rate was slowed significantly due to the associated steric effect intro-
duced by the bidentate ligand.
For the present study coordinated Schiff base bidentate ligand systems and their
effect on the oxidative addition reaction are considered. We opted to investigate these
N,O-donor ligands, since they allow six-membered metallacycle non-labile bidentate
entities, and limited detailed kinetic data are available for Rh-Schiff base ligand sys-
tems [10,30]. The structural flexibility of these ligands offers prospects of controlling (i)
the denticity of the final ligands, (ii) the nature of donor atoms, (iii) the number of
chelating moieties and (iv) the ease of manipulating the (outer) periphery of the lig-
and. They therefore afford an opportunity to manipulate the activity, reactivity and
selectivity of catalyst models through ligand optimization strategies [31,32]. For the
current study, varying electronic and steric parameters as informed by the cyclopentyl,
cyclohexyl, and isopropyl substituents attached to the imine nitrogen introduced on
the targeted Schiff base ligands to investigate how these entities will affect the rate of
oxidative addition. This paper also evaluates the effect on the rate of oxidative add-
ition when systematically interchanging the phenyl groups for the cyclohexyl groups
on the tertiary phosphine ligand in conjunction with varying the Schiff bases. Since
much current ’mechanistic research’ involves purely the analysis of reactants and prod-
ucts, while the time-resolved details are much less studied, we here emphasize yet
again the importance of detailed structure-reactivity studies in fundamental reactions
to catalytic processes to ensure a more elaborate and complete analysis.
2. Results and discussion
2.1. Synthesis
A series of bidentate Schiff base ligands (N,O-ScBaH), 2-(cyclopentyliminomethyl)-5-
methylphenol (5-Me-Sal-CyPH) (I), 2-(cyclohexyliminomethyl)phenol (Sal-CyH) (II), and
2-(isopropyliminomethyl)-5-methylphenol (5-Me-Sal-IpropH) (III) were synthesized
using the procedure previously reported [31,33]. The steric and electronic subtleties in
these Schiff base ligand systems were pursued through the introduction of methyl,
cyclopentyl, isopropyl and cyclohexyl substituents on the ligand periphery. The
rhodium(I) complexes, [Rh(N,O-ScBa)(CO)(PR3)] (PR3 ¼ monophosphine ligands), were
synthesized as reported previously by heating a DMF solution of RhCl3ꢀxH2O (Scheme
1), followed by addition of the respective Schiff base ligand and precipitation of the
[Rh(N,O-ScBa)(CO)2] precursors (1, 2, 3). Complexes 1a, 2a and 3a were obtained by

4
P. P. MOKOLOKOLO ET AL.
Scheme 1. General synthesis of Rh(I)-carbonyl monophosphine Schiff base complexes, [Rh(N,O-ScBa)
(CO)(PR3)] (1a-d; 2a, 3a), via the dicarbonyl complexes (1-3) containing N,O-ScBaH: 2-(cyclopentylimino-
methyl)-5-methylphenol (I), 2-(cyclohexyliminomethyl)phenol (II) and 2-(isopropyliminomethyl)-5-methyl-
phenol (III).
reacting the different Schiff base dicarbonyl complexes with the corresponding tertiary
phosphine ligand in acetone/DCM solution.
Similarly, in order to study the effect of the different substituted tertiary phosphine
ligands (PR3) on the rate of the oxidative addition reaction, [Rh(5-Me-Sal-CyP)(CO)2] (1)
was reacted with the range of monophosphine ligands to yield the corresponding
[Rh(N,O-ScBa)(CO)(PR3)] complexes (Scheme 1; 1a-d).
2.2. IR and 31P-NMR spectroscopy
All the rhodium(I) monophosphine complexes were characterized by IR and 31P-NMR
spectroscopy and the corresponding data are presented in Table 1. The Rh-CO stretch-
ing frequency serves as an excellent measure of the electron density on the rhodium
and a way of identifying the reactants and products present. Complexes 1(a-d), 2a
and 3a showed a characteristic single carbonyl absorbance peak (1937-1957 cmꢁ1),
indicative of the exclusive formation of only one isomer, with the phosphine ligand
trans to the nitrogen of the chelating bidentate ligand as indicated by X-ray crystallo-
graphic studies. Previous studies have indicated that the trans influence for oxygen,
nitrogen and sulphur follows the reverse electronegativity range, i.e. S > N > O
[20,34,35]. This general trend can however have different variations as a variable order
of substitution has been noted for electron-withdrawing phosphite ligands that are
sterically less demanding [36–38]. The formation of only one isomer product is further
supported by the 31P-NMR, as only one doublet is observed between 53 and 41 ppm
(Supporting Information). The first-order rhodium-phosphorus coupling (1JRh-P) thus
obtained, together with the Rh-CO IR stretching frequency values of 1a, 2a and 3a,
allow concluding that the peripheral substituents on the Schiff base ligand backbone
do not have a significant effect on the electron density on the rhodium center.
The infrared data in Table 1 show a decrease in the observed Rh-CO stretching fre-
quency as one moves from PPh3 to PCy3 (1a-d). This behavior is in agreement with
the increase in pKa values of the monophosphine ligands (3.28, 5.87, 8.46 and 11.3 for

JOURNAL OF COORDINATION CHEMISTRY
5
Table 1. Summary of spectroscopic data for 1(a-d), 2a and 3a.
PR3
1a
1b
1c
1d
2a
3a
IR: mCO
(cm-1)
(ppm)
(Hz)
1955
41.5
158
1945
49.5
156
1943
53.8
156
1937
50.2
151
1957
41.6
157
1955
41.5
159
31P NMR: d
1JRh-P
PPh3, PPh2Cy, PPhCy2 and PCy3, respectively) [39] since the higher the pKa value, the
more nucleophilic the phosphorus becomes. This in turn results in an increase in elec-
tron density on rhodium upon coordination of the phosphine ligands, and translates
to an increase in p-backbonding, leading to a lower wavenumber in IR absorption, as
well as corresponding changes in the chemical shift of the 13P signals.
2.3. X-ray crystallography
Perspective diagrams of the crystallographically determined geometries for the mono-
phosphine complexes 1a and 2a showing the atom numbering scheme used are given
in Figure 1. Single crystals were obtained by slow evaporation of an acetone/dichloro-
methane solution. Despite our best efforts, no crystals suitable for X-ray diffraction
analysis could be obtained for 3a, nor for any of the alkyl products 4a-d, 4e or 4f. The
basic data collection parameters are provided while important bond distances and
angles are listed in Table 2.
The neutral rhodium(I) complex 1a contains a complete molecular entity in the asym-
metric unit, while 2a crystallizes with an acetone molecule (see Figure 1 for coordination
compounds). In both 1a and 2a, the rhodium centers are coordinated by the carbonyl
and the phosphorus ligands, and the square planar geometry is completed by the biden-
tate chelating Schiff base ligands 2-(cyclopentyliminomethyl)-5-methylphenolate and 2-
(cyclohexyliminomethyl)phenolate for 1a and 2a, respectively, forming six-membered
chelate rings. The square planar geometry around the metal center in both complexes is
distorted as illustrated by the N1-Rh-O1, O1-Rh-C01 and C01-Rh-P angles of 88.16(19),
173.6(2) and 91.7(2)ꢂ, respectively, for 1a, compared with N1-Rh-O1, O1-Rh-C01 and CO1-
Rh-P angles of 89.35(17), 175.7(2) and 86.68(19)ꢂ, respectively, for 2a.
The N1-Rh-O1 bite angles formed by the six-membered ring are within normal range
and compare well with related structures with bond angles ranging around 87-90ꢂ [25].
Other examples of this type of complexes include [Rh(Tfdmaa)(CO)(PPh3)] (TfdmaaH ¼ l,1,l-
trifluoro-5,5-dimethylpentanedione] and [Rh(Sal-NR)(CO)(PPh3)] (Sal-NRH ¼ N-o-tolylsalicy-
laldiimine) [40,41]. The bite angles in the current Schiff base study are larger than those
obtained for the five-membered chelate rings, i.e. [Rh(Ox)(CO)(PPh3)] (OxH ¼ 8-hydroxyqui-
noline), [Rh(Trop)(CO)(PR3)] (TropH ¼ tropolone) and [Rh(Ox)(CO)(DMSO)] (OxH ¼ 8-oxyqui-
noline)] as well as [Rh(C9H7ClNO)(CO)(P(C6H4Cl)3] (C9H7ClNOH
¼
5-chloro-8-
hydroxyquinoline) [42–45], exhibiting bite angles of only 77.38(3) ꢁ 81.7(2)ꢂ. This can be
attributed to the smaller five-membered metal chelate formed compared to the six-mem-
bered rings in this study. The Rh-N1 and Rh-O1 bond distances of 2.122(5) and 2.034(5) Å,
respectively, for 1a and 2.091(5) and 2.022(4) Å, respectively, for 2a are comparable to
those found in rhodium(I) complexes with N,O donors with bond distances around 2.092(7)
ꢁ 2.162(4) Å and 2.027(6) ꢁ 2.087(4) Å for Rh-N and Rh-O, respectively [34,40,41,46].

6
P. P. MOKOLOKOLO ET AL.
Figure 1. Molecular diagrams (50% probability) of [Rh(5-Me-Sal-CyP)(CO)(PPh3)] (1a) and [Rh(Sal-
Cy)(CO)(PPh3)] (2a), showing geometries and atom numbering. Hydrogens and solvent molecules
were omitted for clarity.
The rhodium is displaced from the coordination plane by about 0.0389 and 0.005(2)
Å for 1a and 2a, respectively, with Rh-C01-O01 approximately linear, with reported
angles of 178.3(6)ꢂ and 178.3(5)ꢂ for 1a and 2a, respectively. The structure determina-
tions showed that the reaction of the dicarbonyl precursor [Rh(N,O-Bid)(CO)2] with
PPh3 leads to the substitution of the carbonyl ligand trans to the nitrogen in

JOURNAL OF COORDINATION CHEMISTRY
7
Table 2. Selected bond lengths [Å] and angles [ꢂ] for 1a and 2a.
Bond/Angle
1a
2a
Rh-N1
Rh-O1
Rh-P
Rh-C01
C01-O01
N1-Rh-O1
C01-Rh-O1
O1-Rh-P
P-Rh-N1
C01-Rh-N1
CO1-Rh-P
2.122(5)
2.034(5)
2.2600(16)
1.808(7)
1.154(8)
88.16(19)
173.6(2)
82.25(13)
168.39(15)
98.1(3)
2.091(5)
2.022(4)
2.2798(14)
1.810(6)
1.150(7)
89.35(17)
175.7(2)
89.47(12)
178.53(13)
94.5(2)
91.7(2)
86.68(19)
Figure 2. (a) Infrared spectral changes observed during the oxidative addition of MeI to [Rh(5-Me-
Sal-CyP)(CO)(PPh3)] (1a) in dichloromethane at 25 ꢂC; insert illustrates (i) the growth of signal from
the Rh(III)-alkyl species (kobs ¼ 0.00105(3) sꢁ1); (ii) the disappearance of the Rh(I) reactant (kobs
¼
0.00106(2) sꢁ1) species. [Rh(5-Me-Sal-CyP)(CO)(PPh3)] ¼ 9 ꢃ 10ꢁ3 M, [MeI] ¼ 0.012 M, Dt ¼ 2 min.
Solid lines represent the least-squares fit to the usual 1st order exponential [18,47]. (b) Time
resolved UV/Vis spectral change for the iodomethane oxidative addition to 1a in dichloromethane
at 25 ꢂC, [Rh] ¼ 3.26 ꢃ 10ꢁ4 M, [MeI] ¼ 0.12 M, k ¼ 432nm, Dt ¼ 20 s, with insert illustrating (i) the
absorbance vs. time plot [18,47], (kobs ¼ 0.0102(1) sꢁ1).
agreement with the structural trans effect (N > O) in both complexes. The Rh-P bond
distances of 2.2600(16) Å and 2.2798(14) Å for 1a and 2a, respectively, are in agree-
ment with the Rh-P bond lengths found in other N,O-Bid complexes with bond distan-
ces varying from 2.256(2)-2.81(2) [23,41,42].
2.4. Solution phase study
2.4.1. Preliminary IR, 31P NMR and UV/vis spectroscopy
The reaction progress for iodomethane oxidative addition to the rhodium(I) phosphine
complexes [Rh(5-Me-Sal-CyP)(CO)(PPh3)] (1a), [Rh(5-Me-Sal-Cy)(CO)(PPh3)] (2a) and [Rh(5-
Me-Sal-IProp)(CO)(PPh3)] (3a) under pseudo first-order conditions was monitored by IR
and UV/visible spectroscopies (Figures 2 and 3). Figure 2(a) presents a series of in situ IR
spectra obtained for iodomethane oxidative addition to [Rh(5-Me-Sal-CyP)(CO)(PPh3)] (1a),

8
P. P. MOKOLOKOLO ET AL.
Figure 3. 31P(1H)-NMR spectra for the disappearance of the Rh(I) complex and the simultaneous
formation of the Rh(III)-alkyl species for iodomethane oxidative addition to [Rh(5-Me-Sal-
CyP)(CO)(PPh3)] (1a) in dichloromethane at 25 ꢂC; [Rh(I)] ¼ 6 ꢃ 10ꢁ3 M, [MeI] ¼ 0.06 M, Dt ¼ 2 min,
32 scans per spectrum (referenced to H3PO4). Top spectrum collected after 12 h.
forming the Rh(III)-alkyl product, [Rh(5-Me-Sal-CyP)(CO)(Me)(I)(PPh3)] (4a) (Scheme 2).
Figure 2(b) illustrates the corresponding UV/vis time-resolved spectra of the reaction.
The successive IR spectra clearly illustrate rapid disappearance of the Rh(I) precursor
(ꢀCO ¼ 1955 cmꢁ1, kobs ¼ (1.05 0.03)ꢃ10ꢁ3
s
ꢁ1), coupled with simultaneous appear-
ance of a new strong absorption corresponding to Rh(III)-alkyl product at (ꢀCO
¼
2054 cmꢁ1, kobs ¼ (1.06 0.02)ꢃ10ꢁ3
s
ꢁ1).
The observed rate of formation of the Rh(III)-alkyl product is identical to the observed
rate for disappearance of the Rh(I) precursor. A notable observation is that only the oxida-
tive addition step was observed with no apparent formation of the Rh(III)-acyl, typically
resulting from migratory insertion of the coordinated methyl into the Rh-CO bond
[18,20]. Also notable is the complete disappearance of the rhodium(I) precursor, indicat-
ing a significant forward reaction (large equilibrium constant) with a small reverse reac-
tion. The exclusive formation of the Rh(III)-alkyl product was also confirmed by a time
resolved 31P NMR investigation (Figure 3), yielding pseudo first-order rate constants in
agreement with those found in both the IR and UV/vis experiments.
The treatment of [Rh(Sal-Cy)(CO)(PPh3)] (2a) or [Rh(5-Me-Sal-IProp)(CO)(PPh3)] (3a)
with iodomethane displays the same behavior as 1a, exclusively forming the Rh(III)-
alkyl product with observed pseudo first-order rate constants kobs ¼ 0.00079(2) sꢁ1
and 0.00084(2) sꢁ1 for the disappearance of the Rh(I) species and kobs ¼ 0.00078(2)
sꢁ1 and 0.00085(2) sꢁ1 for the formation of the Rh(III)-alkyl products 4e and 4f,
respectively (Scheme 2; Supporting Information).
The observed pseudo first-order rate constants of the oxidative addition are com-
parable with an increase of about 1.2 times in the order of 1a > 3a > 2a. This result
suggests that there is no dramatic effect on the electron density of the rhodium
induced by the Schiff base ligands in spite of the variations of the steric and electronic
parameters of the ligand backbone: cyclopentyl (1a), cyclohexyl (2a) and isopropyl
(3a) groups, respectively, attached to the imine nitrogen. (Both 1a and 3a also have

JOURNAL OF COORDINATION CHEMISTRY
9
Scheme 2. Iodomethane oxidative addition to [Rh(N,O-ScBa)(CO)(PR3)] complexes (1a-c, 2a, 3a)
fN,O-BidH ¼ 5-Me-Sal-CyPH, Sal-CyH, 5-Me-Sal-IPropH (I-III)g.
Table 3. A Summary of the rate constants and activation parameters (UV/vis spectroscopy) for the
iodomethane oxidative addition to [Rh(N,O-ScBa)(CO)(PPh3)]; dichloromethane, [Rh] ¼ 3.26 ꢃ 10-4 M.
5-Me-Sal-CyP
N,O-ScBa 5
Parameters
Sal-Cy
5-Me-Sal-Iprop
5.1 ꢂC
14.9 ꢂC
24.3 ꢂC
35.2 ꢂC
24.8 ꢂC
25.1 ꢂC
102 k1 (M-1.s-1)
103 k-1 (s-1)
K1 (M-1)a
2.27 0.05
0.195 0.006
4.11 0.06
0.49 0.06
84 16
–
7.2 0.2
1.0 0.2
72 15
11.5 0.2
1.4 0.2
82 12
–
5.3 0.1
5.8 0.1
0.2 0.2
0.4 0.2
116
9
26b
–
130b
DH (kJ mol-1)
¼
–
–
36
1
–
–
DS (J K-1 mol)
–
ꢁ145
5
–
–
¼
aValues should be treated as estimation, as statistical effects are caused by the large standard deviation on the k-1 values.
by-intercepts (k-1) even less accurate, resulting in relative e.s.d.’s of >50%.
methyl substituents on the phenolate entity). This is further supported by the first
order coupling constant (1JRh-P) of the complexes which were 158, 157 and 159 Hz for
1a, 2a and 3a, respectively. The combination of the methyl and cyclopentyl (1a), how-
ever, introduces the highest, although limited, increased activation. The relative activa-
tion by the Schiff base ligands (I, II, and III) is further discussed below.
2.4.2. Kinetic experiments
As indicated above, when 1a-d, 2a and 3a were reacted with iodomethane in
dichloromethane under pseudo first-order conditions, in situ IR-spectroscopy (Figure
2(a)) showed that the carbonyl band of the reactant at ca. 1960 cmꢁ1 disappeared,
while a new band at ca. 2050 cmꢁ1 appeared simultaneously. This reaction is also
observed by UV/vis spectroscopy (Figure 2(b)) as well as with 31P-NMR spectroscopy
(Figure 3). UV/vis experiments were therefore further utilized to study all the variations
to complete the study. This reaction is illustrated in Scheme 2, indicating the one-step
formation of the RhIII-alkyl species B (4a-c, 4e, 4f), since no tendency of rhodium(III)-
acyl formation was observed after > 12 h (Figure 3), more than 10 h after the alkyl
species formed completely.
As indicated, the UV/visible kinetic data obtained for the iodomethane oxidative
addition to [Rh(5-Me-Sal-CyP)(CO)(PPh3)] (1a), [Rh(Sal-Cy)(CO)(PPh3)] (2a) and [Rh(5-Me-
Sal-IProp)(CO)(PPh3)] (3a) correspond very well to those obtained from the IR and 31P
NMR spectroscopy results. A summary of the data obtained for these reactions is pre-
sented in Table 3, and plots of the observed first-order rate constants kobs vs. iodome-
thane concentration all show a linear dependency on the [MeI], as predicted by Eq. 1
and illustrated previously [18] (Figure 4).
kobs ¼ k1[MeI] þ k-1
(1)

10
P. P. MOKOLOKOLO ET AL.
Figure 4. Plots of the observed rate constants (kobs) vs. iodomethane concentrations for iodome-
thane oxidative addition to [Rh(5-Me-Sal-CyP)(CO)(PPh3)] at various temperatures; [Rh] ¼
3.26 ꢃ 10ꢁ4 M, k ¼ 432 nm. The lines represent the simultaneous global fit of the data to Eq. 1
and the Eyring equation [18,48].
Figure 5. Eyring plot of the second order rate constants (k1) for the formation of [Rh(5-Me-Sal-
CyP)(CH3)(I)(CO)(PPh3)] in dichloromethane.
The relatively small intercepts (ca. zero within most e.s.d.’s) for the reverse reaction
(k-1) values obtained supports the proposed direct mechanism with no solvent
dependent pathway. This is further evidenced by the IR study, indicating a large equi-
librium constant as denoted by K1 in Scheme 2.
A temperature variation study (Figure 4) was conducted for the iodomethane oxida-
tive addition to [Rh(5-Me-Sal-CyP)(CO)(PPh3)] (1a) to enable determination of the acti-
vation parameters from which an Eyring plot (Figure 5) was obtained. In Figure 4, the
solid lines represent the least-squares fits of kobs vs. [MeI] at different temperatures,
consistent with the rate expressed in Eq. 1. A summary of the data obtained is given

JOURNAL OF COORDINATION CHEMISTRY
11

12
P. P. MOKOLOKOLO ET AL.
in Table 3. Virtually identical values for the activation parameters were obtained from
a global fit, as described previously [48].
¼
¼
The values obtained for the activation parameters, DH ¼ 36 1 kJ molꢁ1 and DS
¼ ꢁ145 5 (J Kꢁ1 mol), are characteristic of an associative type mechanism for the for-
mation of the Rh(III)-alkyl product [22,23,25], where the attack by the electron rich
Rh(I) center on the iodomethane substrate is the driving force for the reaction.
A summary of the data, illustrating the relative dependence of the rate constants
on the Schiff bases and the tertiary phosphine ligands, is given in Figure 6 and
reported in Tables 3 and 4. Figure 6(a) illustrates the relative (small) effect of the three
N,O-ScBa ligands I, II and III on the iodomethane oxidative addition rates, and as indi-
cated above, the combination of the methyl and cyclopentyl substituents in 1a indu-
ces the most activation, albeit only about 1.2 times compared to 2a and 3a.
Since rhodium(I) acts as a nucleophile in the oxidative addition reaction, naturally
one expects an increase in activation of the metal center towards oxidative addition
when replacing weaker electron-donating phenyl rings on the phosphine ligands with
more electron-rich cyclohexyl rings. However, the results obtained for the various
phosphine ligands suggest otherwise. It is clear from Figure 6(b) and Table 4 that a
relative reactivity in the iodomethane oxidative addition is observed in the order
[Rh(5-Me-SalCyP)(CO)(PPh2Cy)] (1 b) > [Rh(5-Me-SalCyP)(CO)(PCy3)] (1d) > [Rh(5-Me-
SalCyP)(CO)(PPh3)] (1a) > [Rh(5-Me-SalCyP)(CO)(PPhCy2)] (1c).
The treatment of these rhodium(I) complexes with iodomethane shows a clean and
direct conversion of the rhodium(I) precursor to the rhodium(III) alkyl species as the
final product. Thus, there is no apparent formation of the acyl product observed for all
the rhodium(I) complexes, as indicated by the IR and 31P NMR data [18,25]. In the cur-
rent case, the substituent on the N atom in the series of N,O-ScBaH ligands (CyP, Cy
and IProp) seemingly exhibits a subtle (responsive) steric effect. The assumed trans
coordination of the methyl and the iodide, following the two-electron transfer from
Rh(I), only yields an octahedral trans oxidative addition RhIII-alkyl product (4a-4f in
Scheme 2). Although reasonably well favored thermodynamically, the [RhIII(5-Me-
SalCyP)(CO)(Me)(I)(PR3)] products do not allow methyl migration to the coordinated
CO, and the reaction is effectively stopped at the alkyl species [18,25].
Moreover, as illustrated, even if the substituent on the N-atom was varied as three ali-
phatic substituents fcyclopentyl (CyP), cyclohexyl (Cy) and isopropyl (IProp)g, it is clear that
conditions have been achieved wherein still only the subtle intermediate stereo-electronic
requirements for the RhIII-alkyl species are met. Thus, the total oxidative addition is not
blocked, nor is the RhIII-alkyl species bottlenecked, to allow the RhIII-acyl species to form as
in examples above and in the literature [49–51]. The Schiff base ligands described herein
therefore seemingly have just the correct steric demand to allow alkyl formation (fairly ideal
octahedral complex), but not the methyl migraton, which, in spite of being only five-coord-
inate, requires in the Schiff base systems more steric demand at the acetyl site. Thus,
although the acetyl complexes adopt a square pyramidal geometry, the much more bulky
acetyl entity on one side of the Rh(III) is seemingly thermodynamically less favorable than
the trans Me-RhIII-I orientation, having a methyl substituent on the one side and an iodido
ligand on the other. The steric demand of the apical site (formation of the acyl complex) is
augmented by the fact that the iodido ligand moves to the square plane, increasing the

JOURNAL OF COORDINATION CHEMISTRY
13
Figure 6. Effect of (a) the N,O-ScBa ligands in [Rh(N,O-ScBa)(CO)(PPh3)] (1a, 2a, 3a) and (b) the ter-
tiary phosphine ligands in [Rh(5-Me-Sal-CyP)(CO)(PR3)] (PR3 ¼ PPh3, PPh2Cy, PPhCy2 and PCy3, 1a-
d); on the observed rate constant, kobs, for the iodomethane oxidative addition in dichloromethane
at 25 ꢂC using the UV/Vis spectroscopy. [Rh] ¼ 3 ꢃ 10ꢁ4 M, k ¼ 432 nm.
overall steric demand at the acetyl-entity site. The octahedral six-coordinate geometry of
the alkyl species (4a-4f) is therefore thermodynamically more favored in the current study
than the five-coordinate square pyramidal acetyl final product.
2.5. General correlation of [Rh(L,L’-Bid)(CO)(PR3)] complexes
The results obtained from this study may be correlated with similar isostructural Rh(I)
complexes reported in literature (see Table 4 and Figure 7). Thus, [Rh(L,L’-
Bid)(CO)(PR3)] complexes (PR3 ¼ PPh3, PPh2Cy, PPhCy2 and PCy3) are presented
wherein L,L’-Bid represents four different ligand systems, with varying N, O and S
donor atom combinations within the chelating moiety:
a. N,O-BidH representing (i) 5-Me-Sal-CyP (six-membered Schiff base chelates; this
study) and (ii) 8-hydroxyquinoline (N,O-Ox; five-membered chelates);
b. O,O-BidH ¼ acetylacetone (six-membered chelates);
c. S,O-BidH ¼ N-benzoyl-N’,N’-diphenylthiourea (six-membered chelates).

14
P. P. MOKOLOKOLO ET AL.
Figure 7. Relative rate constants of the iodomethane oxidative addition (OA) to different [Rh(L,L’-
Bid)(CO)(PR3)] complexes, illustrating [Rh(5-Me-SalCyP)(CO)(PR3)], [Rh(N,O-Ox)(CO)(PR3)], [Rh(O,O-
acac)(CO)(PR3)] and [Rh(S,O-BbiPT)(CO)(PR3)] where R ¼ Ph or Cy, at 25 ꢂC. (a) Direct second order
oxidative addition rate constants. (b) Normalized 2nd order oxidative addition rate constants [rela-
tive to PPh3¼1] of complexes.
The relative influence of the combined L,L’-Bid bidentate ligands (O, N, S donor
atoms) and PR3 as illustrated in Figure 7 (data in Table 4) for all four types of L,L’-Bid
bidentate ligands given above shows a similar, interesting phenomenon. The observed
iodomethane oxidative addition rates are presented as (i) direct 2nd order rate con-
stants (Figure 7(a)) as well as (ii) the ’normalised’ relationship (considering PPh3 ¼ 1),
Figure 7(b), for all the [Rh(L,L’-Bid)(CO)(PR3)] complexes. Surprisingly, it follows exactly
the same relative profile, albeit that there is an approximate two order-of-magnitude
overall difference in reactivity (Figure 7(a)) for the four types of bidentate ligands
given in points (a)-(c) above. It underlines the similar adaptive nature of the four PR3
ligands utilized and indicates a subtle interplay between the steric and electronic
parameters operative in these complexes, independent of donor atoms or bidentate
bite angles. Moreover, it suggests a responsive effect within the tertiary phosphine
ligands, allowing them to self-adjust depending on the substituents and biden-
tate ligand(s).

JOURNAL OF COORDINATION CHEMISTRY
15
The rates of the oxidative addition obtained for [Rh(N,O-Bid)(CO)(PR3)] with iodome-
thane appear to be generally faster than those of the corresponding [Rh(O,O-
Bid)(CO)(PR3)] (OxH ¼ oxine) and the [Rh(S,O-Bid)(CO)(PPh3)] complexes. The S,O-Bid
complexes clearly show a much larger relative influence by the bulkier thioureato lig-
and (larger chelate bite angle) compared to the acac, oxine and Schiff bases.
Moreover, as emphasized in Figure 7(b), all four L,L’-Bid ligand system complexes
exhibit exactly the same relative behavior, indicating that the PPh2Cy complex is sig-
nificantly activated, while the PPhCy2 is inhibiting the oxidative addition the most.
Assuming it to be a steric effect, it means that the PPhCy2 surprisingly generates ’in
situ’ a much larger apparent steric effect than even the PCy3 ligand, independent of
the bidentate ligand present.
Indeed, the delicate interplay between the RhIII-alkyl species and the RhIII-acyl spe-
cies may thus be manipulated with a combined N,O-ScBa/PR3 ligand configuration
that creates a responsive self-adjusting system which selectively allows formation of
the RhIII-alkyl species, yet blocking the formation of the RhIII-acyl species. In principle
it may be considered as a chemical speciation probe or for that matter, a sensor for
MeI in a responsive environment.
3. Conclusion
The synthesis and characterization of a series of [Rh(N,O-ScBa)(CO)(PR3)] (PR3 ¼ PPh3,
PPh2Cy, PPhCy2 and PCy3) complexes (N,O-ScBaH ¼ Schiff bases) including two crystal
structure determinations (1a and 2a) were performed, followed by a detailed mechan-
istic investigation into the iodomethane oxidative addition thereto. The iodomethane
oxidative addition, typically proceeding via an associatively activated mechanism (large
¼
negative DS values) is accepted to be due to the electron-rich Rh(I) metal center act-
ing as nucleophile. It is manifested in a relatively fast equilibrium for the oxidative
addition/reductive elimination in the range of [Rh(5-Me-SalCyP)(CO)(PR3)] (R ¼ phenyl
or cyclohexyl) complexes (1a-d), as well as to [Rh(Sal-Cy)(CO)(PPh3)] (2a) and [Rh(5-Me-
Sal-IProp)(CO)(PPh3)] (3a). The reaction was well-defined for all complexes,
leading exclusively to formation of the octahedral RhIII-alkyl species, [Rh(N,O-
ScBa)(CO)(Me)(I)(PR3)] as final products. The reason for this lies in the subtle steric
effect exerted primarily by the N,O-ScBa ligand, whereupon the tertiary phosphine
ligands adjust themselves to accommodate the alkyl formation, yet does not allow the
further migratory insertion to produce the square pyramidal RhIII-acyl species [Rh(N,O-
ScBa)(COMe)(I)(PR3)].
Oxidative addition rates for the different Schiff base complexes (1a, 2a, 3a) showed
only a factor of 1.2 variation, indicating that the substituents on the periphery of the
ScBaH ligands were stereo-electronically very similar and did not significantly affect
the electron density at the metal center.
In the case of the [Rh(5-Me-SalCyP)(CO)(PR3)] complexes, a significant relative effect
of the oxidative addition rate on the tertiary phosphine (about one order-of-magni-
tude) was observed, with the PPhCy2 complex the slowest, the PPh2Cy the most rapid,
and the PPh3 and PCy3 lying between the mentioned first two. This is neither in agree-
ment with the expected electron density induced by the tertiary phosphine, nor with

16
P. P. MOKOLOKOLO ET AL.
the cone angles of the phoshine ligands. Yet, it confirms the consistent behavior of
the four PR3 ligands as found for three other bidentate ligand complex systems from
literature,
i.e.
[Rh(N,O-Ox)(CO)(PR3)],
[Rh(O,O-acac)(CO)(PR3)]
and
[Rh(S,O-
BbiPT)(CO)(PR3)] (Table 4; Figure 7) where R ¼ Ph or Cy, displaying the unexpected
relative reactivity relationship.
In the N,O-ScBaH ligands described herein, three things stand out. First, the com-
bination of them and the PPh3, PPh2Cy, PPhCy2 and PCy3 allows a responsive system
which totally inhibits the formation of the final RhIII-acyl species usually encountered.
Second, ScBaH introduces significantly more electron density (clearly manifested in the
increased relative oxidative addition rates, even compared for example to the S,O-Bid
ligands described above) to the metal center by the softer nature of the net N,O-
donor atom combination, coupled with the more ideal bite angle enforced by the
Schiff bases. Third, the increased reactivity is manifested in the relative rates of oxida-
tive addition, placing the N,O-ScBa systems at the top and even two orders-of-magni-
tude larger than the S,O-Bid ligand systems of thioacetylacetone or thiourea.
4. Experimental
4.1. General procedure and instrumentation
All the reagents used were of analytical reagent grade, purchased from Sigma-Aldrich,
1
South Africa and were used as purchased without purification. The 13C and H NMR
spectra of the ligands and metal complexes were recorded at 75.48 and 300.13 MHz,
respectively, on a Bruker AXS 300 MHz or on a Bruker AXS 400 MHz (100.61 and
1
400.13 MHz, respectively). H NMR data are listed in order: chemical shift (d) reported
in ppm and referenced to the solvent peak, multiplicity, coupling constant (J, in Hz),
number of hydrogens, assignment. Hydrogen and carbon decoupled experiments 31P
NMR spectra were recorded on a 400 MHz Bruker spectrometer operating at the 31P
frequency of 161.98 MHz (2074 scans, pulse of 30ꢂ and delay time of 1.89 s). The 31P
NMR data are reported in the order: chemical shift (d) reported in ppm and referenced
to H3PO4 capillary, multiplicity, number of phosphorus atoms, coupling constants (J
in Hz).
4.2. Ligands synthesis
4.2.1. Synthesis of 5-Me-Sal-CyPH (I)
Cyclopentylamine (0.313 g, 3.676 ꢃ 10ꢁ3 mol) was added dropwise to 2-hydroxy-4-
methylbenzaldehyde (0.500 g, 3.672 ꢃ 10ꢁ3 mol) in methanol (20 ml). The solution was
heated at 80 ꢂC for 3 h. Removal of the solvent under vacuum afforded the product as
1
a yellow powder (yield: 0.697 g, 93.4%). H NMR (300 MHz, MeOD) d 8.35 (s, 1H), 7.15
(s, 1H), 6.63 (s, 2H), 3.87 (s, 1H), 2.28 (s, 3H), 2.11 ꢁ 1.56 (m, 8H). 13C NMR (101 MHz,
Methanol-d4) d 165.09, 162.91, 144.25, 131.54, 118.48, 117.92 115.56 (Ar), 67.59 (CH),
34.11, 23.68 (CH2), 20.50 (CH3).

JOURNAL OF COORDINATION CHEMISTRY
17
4.2.2. Synthesis of Sal-CyH (II)
Cyclohexylamine (0.86 g, 8.847 ꢃ 10ꢁ3 mol) was dissolved in 2 ml methanol and the
solution was added dropwise to salicylaldehyde (1.15 g, 9.412 ꢃ 10ꢁ3 mol) in methanol
(20 ml). Anhydrous MgSO4 was added and the solution heated at 80 ꢂC for 3 h. MgSO4
was filtered off and the solvent was removed under reduced pressure, yellow oil was
1
obtained which was dissolved in acetone to crystallize (yield: 1.278 g, 70.8%). H NMR
(300 MHz, MeOD) d 8.41 (s, 1H), 7.29 (s, 2H), 6.85 (s, 2H), 1.22 (s, 11H). 13C NMR
(101 MHz, Acetone-d6) d 166.49, 166.25, 145.65, 132.94, 119.88, 119.32, 116.42 (Ar),
35.15, 34.12, 33.13, 25, 15, 24.12, 18.21 (Cy).
4.2.3. Synthesis of 5-Me-Sal-IPropH (III)
Isopropylamine (0.228 g, 3.857 ꢃ 10ꢁ3 mol) was added dropwise to 2-hydroxy-4-methyl-
benzaldehyde (0.501 g, 3.680 ꢃ 10ꢁ3 mol) in methanol (20 ml). The solution was heated
at 80 ꢂC for 3 h. The solvent was removed under reduced pressure resulting in a yellow
1
powder (yield: 0.582 g, 89.2%). H NMR (300 MHz, CD2Cl2) d 8.25 (s, 1H), 7.17 (s, 1H),
6.81 (s, 1H), 6.55 (s, 1H), 4.30 (s, 1H), 2.32 (s, 3H), 1.53 (s, 6H). 13C NMR (101 MHz,
CDCl3) d 161.69, 161.51, 142.88, 130.90, 119.49, 117.44, 116.44 (Ar), 59.81 (CH), 24.23
(2 ꢃ CH3), 21.39 (CH3).
4.3. General synthesis of the rhodium(I) monophosphine complexes
The monophosphine complexes, [Rh(N,O-ScBa)(CO)(PR3)], were prepared by dissolving
the dicarbonyl complexes [18] in a minimum of acetone followed by addition of the
tertiary phosphine ligand PR3 as previously reported (Scheme 3). Due to the large
trans influence of the N-atom and the corresponding thermodynamic stability only
Isomer A was obtained.
4.3.1. Synthesis [Rh(5-Me-Sal-CyP)(CO)2] (1)
RhCl3ꢀxH2O (0.021 g, 0.079 mmol) was dissolved in 5 ml of DMF and refluxed until the
red color turned yellow (approx. 30 min). 2-(Cyclopentyliminomethyl)-5-methylphenol
¼ 5-Me-Sal-CyPH (0.019 g, 0.095 mmol) was added to the cooled solution of [Rh(l-
Cl)(CO)2]2. After two minutes of stirring, the product was precipitated by ice water and
isolated by centrifugation. The product was recrystallized by dissolving it in 1 ml acet-
one and allowing slow evaporation (yield: 0.024 g, 80%). IR (ATR): ꢀCO ¼ 2067,
1
1987 cmꢁ1. H NMR (400 MHz, CD3CN) d 8.33 (s, 1H, HC ¼ N), 7.25 (s, 1H), 6.72 (s, 1H),
6.54 (s, 1H), 3.87 (m, 1H), 2.27 (s, 3H), 1.89-1.23 (m, 8H, Cyp). 1H NMR (400 MHz,
CD3CN) d 8.37 (s, 1H), 7.34 (s, 2H), 6.86 (s, 1H), 6.68 (s, 1H), 3.88 (s, 1H), 2.26 ꢁ 2.17 (m,
2H), 1.87 (s, 1H), 1.75 (d, J ¼ 4.9 Hz, 3H), 1.48 ꢁ 1.34 (m, 2H), 1.16 (d, J ¼ 20.6 Hz, 1H).
13C NMR (101 MHz, DMSO) d 166.09, 162.91, 144.25, 131.45, 118.48, 117.92, 115.56 (Ar),
67.59 (CH), 34.11, 23.68, 20.50 (cyclopentyl).
4.3.2. Synthesis of [Rh(Sal-Cy)(CO)2] (2)
RhCl3ꢀxH2O (0.020 g, 0.076 mmol) was dissolved in 5 ml of DMF and the solution was
refluxed for approximately 30 minutes. The solution was cooled and 2-
(cyclohexyliminomethyl)phenol ¼ SalH-Cy (0.019 g, 0.091 mmol) was added to the

18
P. P. MOKOLOKOLO ET AL.
Scheme 3. General synthesis of the rhodium(I) monophosphine complexes [Rh(N,O-ScBa)(CO)(PR3)].
cooled reaction mixture. After two minutes of stirring, the product was precipitated by
ice water and isolated by centrifugation. The product was recrystallized by dissolving
it in 1 ml acetone and allowing slow evaporation (yield: 0.018 g, 64%). IR (ATR): ꢀCO
1
2052, 1982 cmꢁ1. H NMR (400 MHz, CD3CN) d 8.37 (s, 1H, HC ¼ N), 7.34 (s, 2H), 6.86 (s,
1
1H), 6.68 (s, 1H), 3.88 (m, 1H), 1.87-1.13 (m, 10H, Cyc). H NMR (300 MHz, CD2Cl2) d
8.21 (s, 2H), 7.13 (s, 2H), 6.77 (s, 2H), 6.51 (s, 2H), 4.26 (s, 2H), 2.28 (s, 6H), 1.42 (s, 13H).
13C NMR (101 MHz, DMSO) d 166.49, 164.31, 145.65, 133.94, 119.88, 119.32 116.96 (Ar),
69.89 (CH), 35.51, 25.08, 21.90 (cyclohexyl).
4.3.3. Synthesis of [Rh(5-Me-Sal-IProp)(CO)2] (3)
RhCl3ꢀxH2O (0.020 g, 0.076 mmol) was dissolved in 5 ml of DMF and the solution was
refluxed for approximately 30 minutes. The solution was cooled and then 5-methyl-2-
(isopropyliminomethyl)phenol ¼ 5-Me-Sal-IPropH (0.016 g, 0091 mmol) was added to
the reaction mixture and stirred. After two minutes of stirring, the product was precipi-
tated by ice water and isolated by centrifugation. The product was recrystallized by
dissolving it in 1 ml acetone and allowing slow evaporation (yield: 0.016 g, 63%). IR
1
(ATR): ꢀCO 2062, 1994 cmꢁ1. H NMR (300 MHz, CD2Cl2) d 8.22 (s, 1H, Ar), 7.16 (s, 1H,
Ar), 6.78 (s, 1H, Ar), 6.54 (s, 1H, Ar), 4.26 (s, 1H, CH), 2.28 (s, 3H, CH3), 1.4 (s, 6H,
2 ꢃ CH3). 1H NMR (400 MHz, CD3CN) d 7.28 ꢁ 7.24 (m, 4H), 6.73 ꢁ 6.71 (m, 4H),
6.57 ꢁ 6.52 (m, 4H), 4.34 ꢁ 4.23 (m, 2H), 2.27 (s, 6H), 1.94 (dt, J ¼ 4.9, 2.5 Hz, 28H), 1.49
(d, J ¼ 6.6 Hz, 24H), 1.30 ꢁ 1.22 (m, 5H). 13C NMR (101 MHz, DMSO) d 161.69, 161.51,
142.88, 130.90, 119.49 117.44, 116.44 (Ar), 59.81(CH), 24.23 (2 ꢃ CH3), 21.29 (CH3).
4.3.4. Synthesis of [Rh(5-Me-Sal-CyP)(CO)(PPh3)] (1a)
[Rh(5-Me-Sal-CyP)(CO)2] (0.040 g, 0.067 mmol) was dissolved in 2 ml acetone to which
PPh3 (0.01 g, 0.080 mmol) was added. The solvent was allowed to evaporate slowly
resulting in a yellow product (yield: 0.039 g, 59%). IR (ATR): ꢀCO 1955 cmꢁ1; UV/Vis (nm,
1
1
molꢁ1 cmꢁ1): kmax ¼ 394 nm, e ¼ 2856; 31Pf Hg(161.0 MHz, DMSO) d ¼ 41.5 (d, JRh-P
¼
1
158 Hz) ppm. H NMR (400 MHz, Acetone) d 8.49 (s,1H), 7.7 (s, 6H), 7.48 (s, 9H), 7.0 (s,
1H), 6.33 (s, 1H), 6.09 (s, 1H), 4.36 (s, 1H), 1.55 (s, 2 ꢃ CH3).
4.3.5. Synthesis of [Rh(5-Me-Sal-CyP)(CO)(PPh2Cy)] (1b)
[Rh(5-Me-Sal-CyP)(CO)2] (0.041 g, 0.068 mmol) was dissolved in 2 ml acetone to which
PPh2Cy (0.022 g, 0.081 mmol) was added. The solvent was allowed to evaporate slowly
resulting in a yellow product (yield: 0.038 g, 56%). IR (ATR): mCO 1945 cm21; UV/Vis (nm,
1
1
mol21 cm21): kmax ¼ 391 nm, e ¼ 2564; 31Pf Hg(161.0 MHz, DMSO) d ¼ 49.5 (d, JRh-P
1
¼ 156 Hz) ppm. H NMR (400 MHz, Acetone) d 8.43 (s,1H), 7.79 (m, 10H), 7.15 (s, 1H),
6.42 (s, 1H), 6.34 (s, 1H), 4.40 (m, 1H), 1.89-1.61 (m, 10H), 1.57-1.23 (m 10H).

JOURNAL OF COORDINATION CHEMISTRY
19
4.3.6. Synthesis of [Rh(5-Me-Sal-CyP)(CO)(PPhCy2)] (1c)
[Rh(5-Me-Sal-CyP)(CO)2] (0.042 g, 0.069 mmol) was dissolved in 2 ml acetone to which
PPhCy2 (0.023 g, 0.083 mmol) was added. The solvent was allowed to evaporate slowly
resulting in a yellow product (yield: 0.031 g, 44%). IR (ATR): mCO 1943 cm21; UV/Vis (nm,
1
1
mol21 cm21): kmax ¼ 390 nm, e ¼ 2678; 31Pf Hg(161.0 MHz, DMSO) d ¼ 53.8 (d, JRh-P
1
¼ 155 Hz) ppm. H NMR (400 MHz, Acetone) d 8.45 (s, 1H), 7.89 (s, 2H), 7.49 (s, 3H),
7.17 (s, 1H), 6.48 (s, 1H), 6.35 (s,1H), 4.46 (s,1H), 1.9-1.81 (m 9H), 1.74-1.10 (m, 22 H).
4.3.7. Synthesis of [Rh(5-Me-Sal-CyP)(CO)(PCy3)] (1d)
[Rh(5-Me-Sal-CyP)(CO)2] (0.040 g, 0.065 mmol) was dissolved in 2 ml acetone to which
PCy3 (0.022 g, 0.078 mmol) was added. The solvent was allowed to evaporate slowly
resulting in a yellow product (yield: 0.043 g, 63%). IR (ATR): mCO 1937 cm21; UV/Vis (nm,
1
1
mol21 cm21): kmax ¼ 391 nm, e ¼ 2671; 31Pf Hg(161.0 MHz, DMSO) d ¼ 50.2(d, JRh-P
1
¼ 151 Hz) ppm. H NMR (400 MHz, Acetone) d 8.45 (s,1H), 7.89 (s, 1H), 6.58 (s, 1H),
6.33 (s, 1H), 4.44 (m, 1H), 2.44 (s, 3H), 1.95-1.78 (m, 9H), 1.65-1.25 (m, 33H).
4.3.8. Synthesis of [Rh(Sal-Cy)(CO)(PPh3)] (2a)
[Rh(Sal-Cy)(CO)2] (0.044 g, 0.12 mmol) was dissolved in 2 ml acetone to which PPh3
(0.032 g, 0.15 mmol) was added. The solvent was allowed to evaporate slowly resulting
in a yellow product (yield: 0.048 g, 43%). IR (ATR): ꢀCO 1957 cmꢁ1; UV/Vis (nm,
1
1
molꢁ1 cmꢁ1): kmax ¼ 388 nm, e ¼ 2453; 31Pf Hg(161.0 MHz, DMSO) d ¼ 41.6 (d, JRh-P
¼
1
158 Hz) ppm. H NMR (400 MHz, DMSO) d 8.56 (s, 1H), 7.73 ꢁ 7.38 (m, 15H), 7.09 (s,
1H), 6.87 (s, 1H), 6.49 (s, 1H), 6.09 (s, 1H), 2.29-1.08 (m, 11H).
4.3.9. Synthesis of [Rh(5-Me-Sal-IProp)(CO)(PPh3)] (3a)
[Rh(5-Me-Sal-IProp)(CO)2] (0.040 g, 0.07 mmol) was dissolved in 2 ml acetone to which
PPh3 (0.022 g, 0.08 mmol) was added. The solvent was allowed to evaporate slowly
resulting in a yellow product (yield: 0.031 g, 70%). IR (ATR): ꢀCO 1955 cmꢁ1; UV/Vis (nm,
1
1
molꢁ1 cmꢁ1): kmax ¼ 390 nm, e ¼ 3082; 31Pf Hg(161.0 MHz, DMSO) d ¼ 41.5 (d, JRh-P
¼
1
159 Hz) ppm. H NMR (400 MHz, Acetone) d 8.48 (s,1H), 7.74(m, 6H), 7.43(m, 9H), 7.19(s,
1H), 6.34 (s, 1H), 6.07 (s, 1H), 3.96 (s, 1H), 2.30 (s, 3H), 1.92-1.24 (m 10H).
4.4. In situ characterization of Rh(III)-alkyl products [Rh([Rh(N,O-ScBa)(CO)(me)
(I)(PPh3)] (4a-4f)
The Rh(III)-alkyl products for all the Schiff base-tertiary phosphine ligands could not be
isolated as clean products and were consequently only characterized in situ. Typical in
situ characterization was by 31P NMR and infrared spectroscopy in dichloromethane, with
solutions as follows: to (6.0-10.0)ꢃ10ꢁ3 M dichloromethane solutions of 1a-d, 2a and 3a,
([Rh([Rh(N,O-ScBa)(CO)(PPh3)]) were added iodomethane to yield [MeI] of 1 ꢃ 10ꢁ2 to
5 ꢃ 10ꢁ1 M, and the reactions were monitored for 12-24 h using time resolved IR and/or
31P NMR spectroscopy. All reactions indicated only the formation of the alkyl products.
1
1
4a: IR (DCM): mCO 2054 cmꢁ1
110 Hz) ppm.
;
31Pf Hg (161.0 MHz, DCM) d ¼ 20.3 (d, JRh-P
¼

20
P. P. MOKOLOKOLO ET AL.
1
1
4b: IR (DCM): mCO 2056 cmꢁ1
112 Hz) ppm.
;
;
31Pf Hg(161.0 MHz, DCM) d ¼ 22.3 (d, JRh-P
¼
¼
¼
¼
¼
1
1
4c: IR (DCM): mCO 2040 cmꢁ1
113 Hz) ppm.
;
31Pf Hg(161.0 MHz, DCM) d ¼ 21.3 (d, JRh-P
1
1
4d: IR (DCM): mCO 2036 cmꢁ1
112 Hz) ppm.
31Pf Hg(161.0 MHz, DCM) d ¼ 19.7 (d, JRh-P
1
1
4e: IR (DCM): mCO 2054 cmꢁ1
;
31Pf Hg(161.0 MHz, DCM) d ¼ 20.6 (d, JRh-P
110 Hz) ppm.
1
1
4f: IR (DCM): mCO 2054 cmꢁ1
;
31Pf Hg(161.0 MHz, DCM) d ¼ 20.3 (d, JRh-P
111 Hz) ppm.
4.5. X-ray structure determinations
The reflection data were collected at 100(2) K on a Bruker X8 ApexII 4 K diffractometer
using MoKa radiation (k ¼ 0.71073 Å). The cell parameters were refined by SAINT-Plus
[52] while SADABS [53] was used for absorption corrections. The structures were
solved by direct methods and refined on F2 using anisotropic displacement parame-
ters for all non-hydrogen atoms. SHELXL-97 [54,55] and WinGX [56] were used for
structure solutions and refinements, respectively. The molecular graphics were pre-
pared with DIAMOND [57]. All aromatic hydrogens and methyl hydrogens were placed
in geometrically idealized positions (C-H ¼ 0.93 Å and 0.96 Å) and constrained to ride
on their parent atoms with Uiso(H) ¼ 1.2 Ueq(C) and Uiso(H) ¼ 1.5 Ueq(C), respectively.
The N-bound hydrogen atom was located from the electron density map and refined
without any constraints.
4.6. Kinetic measurements
All kinetic measurements were carried out in atmospheric conditions. The UV/visible
spectra were performed on a Varian Cary 50 Conc spectrophotometer with thermo-
stated automated multicell changers, equipped with a Julabo F1-mV temperature cell
regulator and accurate within 0.1 ꢂC in 1.000(1) cm quartz tandem cuvette cells. The
infrared data for the rhodium(I) complexes were recorded as liquid samples in dry
dichloromethane in a NaCl cell on a Bruker Tensor 27 spectrometer from 2350-
1600 cmꢁ1, equipped with a temperature cell regulator accurate within 0.3 ꢂC.
Iodomethane is highly volatile and the solutions were prepared in a fume hood and
the solution used immediately after preparation.
Acknowledgements
Financial assistance from the University of the Free State and the SA NRF is gratefully acknowl-
edged. Opinions, findings, conclusions, or recommendations expressed in this material are those
of the authors and do not necessarily reflect the views of the SA NRF. Drs. D.V. Kama and O.T.
Alexander are acknowledged for the single crystal data collections and assistance with solving
the structures.

JOURNAL OF COORDINATION CHEMISTRY
21
Disclosure statement
The authors declare no conflict of interest.
Funding
We also express our gratitude towards SASOL, Dr. Gerdus Kemp and PETLabs Pharmaceuticals,
the South African National Research Foundation (SA-NRF/THRIP), in particular funding under the
Swiss-South Africa joint research programme (SSAJRP) from the SA NRF (UID: 107802) and the
Swiss National Science Foundation (Project IZLSZ2_170856) and and the University of the Free
State for financial support of this project.
ORCID
Pennie P. Mokolokolo
http://orcid.org/0000-0002-8736-0177
Alice Brink
http://orcid.org/0000-0002-2612-359X
Andreas Roodt
http://orcid.org/0000-0002-7349-6436
Marietjie Schutte-Smith
http://orcid.org/0000-0001-8935-7601
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