Rhodium carbonyl complexes of bulky, anionic nitrogen-based ligands: A comparison of donating ability

Article abstract of DOI:10.1016/j.jorganchem.2011.03.036

Phosphinimine-imine ((C₆H₃i-Pr₂)NC(Me) CH₂PPh₂(NC₆H₃i-Pr ₂))Rh(CO)₂ - (1) and β-diketiminate CH(C(Me)(Ni-Pr₂C₆H₃))₂

Full text of DOI:10.1016/j.jorganchem.2011.03.036

Journal of Organometallic Chemistry 696 (2011) 2533e2536
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Rhodium carbonyl complexes of bulky, anionic nitrogen-based ligands:
A comparison of donating abilityq
*
Nick A. Giffin, Arthur D. Hendsbee, Jason D. Masuda
The Maritimes Centre for Green Chemistry and the Department of Chemistry, Saint Mary’s University, Halifax, NS, Canada B3H 3C3
a r t i c l e i n f o
a b s t r a c t
Article history:
Phosphinimine-imine ((C6H3i-Pr2)NC(Me)CH2PPh2(NC6H3i-Pr2))Rh(CO)2 e (1) and
b-diketiminate CH
Received 25 February 2011
Received in revised form
24 March 2011
(C(Me)(Ni-Pr2C6H3))2Rh(CO)2 e (2) rhodium dicarbonyl complexes were prepared as to elucidate any
difference among these anionic, nitrogen-based ligands regarding donating ability to the rhodium center.
Utilizing infrared spectroscopy and single crystal structural comparisons, differences in electron density
donation by the ligands to the rhodium center were not observed. The carbonyl stretching frequencies of
the aforementioned rhodium complexes were nCO ¼ 2055, 1987 and 2055, 1988 for the phosphinimine-
Accepted 25 March 2011
Keywords:
Rhodium
Metal carbonyl complexes
Anionic ligands
Donating ability
Phosphinimine-imine
imine (1) and b-diketiminate (2) respectively.
Ó 2011 Elsevier B.V. All rights reserved.
b
-Diketiminate
1. Introduction
During the preparation of this manuscript, the b-diketiminate
rhodium dicarbonyl complex CH(C(Me)(Ni-Pr2C6H3))2Rh(CO)2,
2, was published [18].
The development of sterically bulky nitrogen-based ligand
systems for use in late transition metal compounds was accelerated
by the discovery of highly active nickel-based olefin polymerization
2. Experimental
catalysts using the sterically bulky
a-diimine ligand [1]. These
ligand systems generally form five- or six-membered metal
chelates, thus a number of structural perturbations of these
systems have been developed, either by altering the composition of
the ligand backbone or by altering the electronic or steric proper-
ties of the peripheral substituents. The phosphinimine-imine
ligand [2e5] is shown to exhibit properties similar to both the
2.1. General data
All preparations were done under an atmosphere of dry, O2-free
N2 employing both Schlenk line techniques and an mBraun glove
box. Toluene, pentane, and THF were purified employing a Grubbs’
type solvent purification system. Deuterated solvents were dried
over Na/benzophenone (C6D6, toluene-d8). 1H, 31P{1H} and 13C{1H}
NMR spectra were recorded on a Bruker Avance-300. Trace
amounts of protonated solvent were used as internal references for
NacNac (b-diketiminate) [6e10] style of ligand and bisphosphini-
mine ligand systems [11e15] which have recently used in support
of catalytically active metal centers for lactide polymerization [16].
The extremely sterically bulky imidoylamidine ligand readily forms
late metal dihalide complexes, however the reactivity of these
complexes is greatly diminished compared to their less bulky
counterparts [17]. The synthesis of two, rhodium(I) complexes is
described, including structural characterization and infrared anal-
ysis as to determine potential differences in donating ability among
1H NMR spectra (C6D5H
d 7.16 ppm). The deuterated solvent was
used as an internal reference for 13C{1H} NMR spectra. The chemical
shifts for both 1H and 13C{1H} are reported relative to tetrame-
thylsilane. 31P{1H} was referenced to external 85% H3PO4. Coupling
constants are reported as absolute values. Combustion analyses
were done in house employing a PerkinElmer CHN Analyzer.
IR spectra were recorded using a Bruker VECTOR 22 infrared Fourier
transform spectrometer. n-BuLi (1.6 M in hexanes) was purchased
from the Aldrich Chemical Company and used as received. Hyflo
Super CelÒ (celite) was purchased from Aldrich Chemical Company
and dried for 24 h in a vacuum oven prior to use. Molecular sievꢀes
sterically similar phosphinimine-imine and b-diketiminate ligands.
q
Electronic Supplementary Information (ESI) available: Crystallographic data.
* Corresponding author. Tel.: þ1 9024205077.
ꢀ
E-mail address: jason.masuda@smu.ca (J.D. Masuda).
(4 A) were purchased from Aldrich Chemical Co. and dried at 140 C
0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2011.03.036

2534
N.A. Giffin et al. / Journal of Organometallic Chemistry 696 (2011) 2533e2536
Table 1
under vacuum using a rotary vacuum pump. (Rh(CO)2Cl)2 was
purchased from Strem Chemicals Inc. while NacNacH [19] and
(C6H3i-Pr2)NC(Me)CH2PPh2(NC6H3i-Pr2) [3] were prepared according
to literature methods.
Selected crystallographic data for compounds 1 and 2.
Compound
1
2
Formula
C41H48N2O2PRh
734.69
Monoclinic
P21/n
11.934(3)
14.590(3)
21.632(5)
90.972(6)
3766 (2)
4
297
1.296
0.532
18,586
5463
432
0.0409
0.08559
0.993
C31H41N2RhO2
576.57
Monoclinic
C2/c
35.240(9)
9.354(2)
20.881(5)
120.631(3)
5922(3)
8
123
1.293
0.605
18,873
5213
324
0.0297
0.0695
Formula weight
Crystal system
Space group
2.2. Synthesis of phosphinimine-imine and NacNacRh complexes
ꢀ
a (A)
2.2.1. Synthesis of ((C6H3i-Pr2)NC(Me)CH2PPh2(NC6H3i-Pr2))
Rh(CO)2 e 1
ꢀ
b (A)
ꢀ
c (A)
b
(ꢀ)
To a solution of 102 mg (0.177 mmol) of (C6H3i-Pr2)NC(Me)
CH2PPh2(NC6H3i-Pr2) in 5 mL THF was added one equivalent of
n-BuLi (0.111 mL of a 1.6 M solution in hexanes) and the mixture
was allowed to stir for 2 h. Then 34 mg (0.18 mmol) of (Rh(CO)2Cl)2
added to the solution. After stirring overnight, the solvent was
removed in vacuo. The solids were washed with 10 mL of toluene
and the yellow solution filtered through celite. Upon cooling
to ꢁ35 ꢀC overnight, 85 mg of bright yellow blocks were isolated by
decantation and washing with pentane. Yield: 65%. 1H NMR (C6D6)
3
ꢀ
V (A )
Z
Temperature (K)
Density (calc, g/cm3)
Abs coeff. (mmꢁ1
)
Reflections collected
Independent reflections
Variables
R1
Rw
GOF
0.979
0.695 and ꢁ1.211
d
: 7.80e7.86 (m, 4H, o-PPh2), 7.01e7.10 (m, 12H, m, p-PPh2, m, p-Ar),
3
ꢀ
3
2
Largest diff. peak and hole (e A )
0.465 and ꢁ0.203
4.09 (sept, 2H, JHeH ¼ 7 Hz, CH(CH3)2), 3.27 (d, 1H, JPeH ¼ 27 Hz,
3
ꢀ
Data collected with Mo K
R ¼ SkFoj ꢁ jFck/SjFoj, Rw ¼ [S[
a
radiation (
l
¼ 0.71069 A).
PCH), 3.27 (sept, 2H, JHeH ¼ 7 Hz, CH(CH3)2), 1.60 (d, 6H,
(Fo ꢁ Fc2)2]/S[
u .
(Fo2)2]]0.5
2
3JHeH ¼ 7 Hz, CH(CH3)2), 1.56 (s, 3H, Me), 1.25 (d, 6H, JHeH ¼ 7 Hz,
3
u
3
CH(CH3)2), 1.14 (d, 6H, JHeH ¼ 7 Hz, CH(CH3)2), 1.06 (d, 6H,
3JHeH ¼ 7 Hz, CH(CH3)2). 13C{1H} NMR (C6D6)
d: 185.9 (d,
angles are provided in Figs. 2 and 3. Selected crystallographic data
are included in Table 1.
1JRheC ¼ 67.5 Hz, Rh(CO)2), 167.8, 157.3, 146.2, 142.7, 137.0, 133.9,
133.8, 131.5,131.1, 127.7e128.3 (m, obscured by C6D6), 125.9, 124.6,
123.9, 70.0 (d, 1JPeC ¼ 121 Hz, PCH), 29.2, 28.2, 26.0, 24.6, 23.7. 31P
{1H} NMR (C6D6)
d: 18.1. IR (KBr pellet): 2055 (symmetric nCO), 1987
3. Results and discussion
(asymmetric nCO) cmꢁ1. Anal. Calc. for C41H48N2PRhO2: C, 67.02; H,
6.59; N, 3.81. Found: C, 67.39; H, 6.88; N, 3.82.
Reaction of (C6H3i-Pr2)NC(Me)CH2PPh2(NC6H3i-Pr2)Li (prepared
in situ) with an equimolar amount of (Rh(CO)2Cl)2 gave bright
yellow, analytically pure crystals of 1 upon workup (Fig. 1). In
a similar reaction, NacNacLi (prepared in situ from n-BuLi and
NacNacH) was reacted with (Rh(CO)2Cl)2 to give dark yellow crys-
tals of 2 (Fig. 1). 1H and 13C{1H} NMR spectral data were consistent
with the formation of the rhodium carbonyl compounds, with the
Rh-bound carbonyl peaks appearing in the 13C NMR spectrum as
2.2.2. Synthesis of CH(C(Me)(Ni-Pr2C6H3))2Rh(CO)2 e 2
To a solution of 108 mg (0.257 mmol) of NacNacH in 5 mL THF
was added one equivalent of n-BuLi (0.161 mL of a 1.6 M solution)
and the mixture was allowed to stir for 2 h. Then 50 mg
(0.257 mmol) of (Rh(CO)2Cl)2 in 2 mL THF was added dropwise to
the solution. After stirring overnight the solvent was removed in
vacuo. The solids were washed with 8 mL of toluene and the yellow
solution filtered through celite. Upon cooling to ꢁ35 ꢀC overnight,
74 mg of bright yellow blocks were isolated by decantation and
a
doublet at 185.9 ppm (1JRheC ¼ 67.5 Hz) for
1 and 184.9
(1JRheC ¼ 68 Hz) for 2, in agreement with other examples in the
literature [23]. The 1H NMR spectrum of 2 reveals the highly
symmetric nature of this molecule, featuring one septet for the
methine and one doublet for the methyl protons of the four i-Pr
groups as well as one signal for the imineeCeCH3 groups. This is in
contrast to 1 in which the dissymmetric ligand features two sets of
i-Pr signals from the imine-Ar and phosphinimine-Ar groups. In
addition, the IR spectrum of 1 showed peaks at 2055 and
1987 cmꢁ1, consistent with symmetric and asymmetric metal
dicarbonyl stretches, respectively, while the spectrum of 2 featured
peaks at 2055 and 1988 cmꢁ1. These results show that there is no
washing with pentane. Yield: 50%.1H NMR (toluene-d8)
d: 6.97e7.12
3
(m, 6H, m, p-Ar), 5.09 (s, 1H, CH), 3.39 (sept, 4H, JHeH ¼ 7 Hz,
CH(CH3)2),1.68 (s, CH3),1.46 (d,12H, 3JHeH ¼ 7 Hz, CH(CH3)2),1.15 (d,
12H, 3JHeH ¼ 7 Hz, CH(CH3)2). 13C{1H} NMR (toluene-d8)
d: 184.9 (d,
1JRheC ¼ 68 Hz, Rh(CO)2), 160.4, 155.5, 140.6, 126.7, 123.8, 97.6 (CH),
28.2, 24.1, 23.9, 22.8. IR (KBr pellet): 2055 (symmetric nCO), 1988
(asymmetric nCO) cmꢁ1. Anal. Calc. for C31H41N2RhO2: C, 64.58; H,
7.17; N, 4.86. Found: C, 64.65; H, 7.36; N, 4.79.
2.3. X-ray data collection, reduction, solution and refinement
Single crystals of 1 were mounted in a thin-walled capillary (1)
and data collected at room temperature. Single crystals of 2 were
placed in oil and mounted in a nylon loop and cooled to 123 K using
an Oxford Cryostream cooling system. The data were collected
using the Bruker APEX2 software package [20,21] on a Siemens
diffractometer equipped with an APEXII CCD detector, a graphite
ꢀ
monochromator and Mo K
a
radiation (
l
¼ 0.71073 A). A hemi-
sphere of data was collected in 1664 frames with 10 s exposure
times. Data processing and absorption corrections were applied
using APEX2 software package. The structure was solved (direct
methods) and all non-H atoms were refined anisotropically.
Hydrogen atoms were placed in calculated positions using an
appropriate riding model and coupled isotropic temperature
factors. ORTEP-III [22] renderings with selected bond lengths and
Fig. 1. Synthesis of rhodium dicarbonyl complexes.

N.A. Giffin et al. / Journal of Organometallic Chemistry 696 (2011) 2533e2536
2535
Table 2
Carbonyl stretching frequencies for selected rhodium compounds.
Compound
nCO (cmꢁ1
)
Reference
[Rh(TMEDA)(CO)2][ClO4]
[CH2((Ph2)PN(4-CH3C6H4))2Rh(CO)2][Cl]
2080e2010a
2077, 2012
[26]
[11]
2080, 2010
[27]
[CH2(C(Me)NH)2Rh(CO)2][BF4]
CH(C(Me)(Ni-Pr2C6H3))(P(Ph2)
(Ni-Pr2C6H3)Rh(CO)2 e 1
2090, 2040
2055, 1987
[24]
This work
CH(C(Me)(Ni-Pr2C6H3))2Rh(CO)2 e 2
2055, 1988
This work
a
A range is reported for both peaks.
Fig. 3. ORTEP-III rendering of 2 (50% probability). Hydrogen atoms are omitted for
clarity. Rh(1)eC(31) 1.864(2), Rh(1)eC(30) 1.867(2), Rh(1)eN(1) 2.050(2), Rh(1)eN(2)
2.052(2), N(2)eC(3) 1.329(3), N(1)eC(1) 1.332(3), C(2)eC(1) 1.395(3), C(2)eC(3)
1.400(3), C(30)eO(1) 1.142(3), C(31)eO(2) 1.142(3), C(31)eRh(1)eC(30) 85.78(10),
C(31)eRh(1)eN(1) 92.51(9), C(30)eRh(1)eN(1) 176.51(9), C(31)eRh(1)eN(2)
177.19(9), C(30)eRh(1)eN(2) 92.05(9), N(1)eRh(1)eN(2) 89.75(8), C(3)eN(2)eRh(1)
126.89(16), C(1)eN(1)eRh(1) 126.73(16), C(1)eC(2)eC(3) 128.0(2), N(2)eC(3)eC(2)
124.2(2), N(1)eC(1)eC(2) 124.4(2), O(1)eC(30)eRh(1) 176.5(2), O(2)eC(31)eRh(1)
176.0(2).
difference between the two ligand systems in their donating ability
to the rhodium metal center. This finding is intriguing considering
that phosphinimine-basedþsystems are thought to be relatively
good donors due to the R3P eNꢁR0 resonance structure that can be
formulated. However, these CO stretches are significantly lower in
energy compared to other diamine and bisphosphinimine ligands
in the literature (Table 2), making these ligands featured in 1 and 2
to be excellent choices for producing electron-rich metal centers.
A single crystal X-ray diffraction study of 1 revealed a square
planar Rh center, with two carbonyl ligands located trans to the
imine and phosphinimine nitrogen atoms that fill the coordination
sphere (Fig. 2). The phosphorus atom is highly distorted from the
those reported for [HC(CNH)2Rh(CO)2][BF4] (1.864(5) and
ꢀ
1.881(6) A)
and
(HC((1,2-(CN)2(C6H4))Rh(CO)2)2
(average
ꢀ
1.864(6) A), indicating
a
stronger back-bonding interaction
between the metal and carbonyl ligands in 1. In addition, the CeO
ꢀ
bond lengths are longer in 1 (1.144(5) and 1.145(5) A) than in
ꢀ
other atoms of the six-membered chelate, deviating 0.6944 A from
ꢀ
[HC(CNH)2Rh(CO)2][BF4] (1.119(6) and 1.104(7) A). These longer
the mean plane defined by N(1)eRh(1)eN(2)eC(4)eC(3). The
bond lengths are reflected in the lower frequency of CeO stretches
in 1 (Table 2) and agree with the stronger back bonding in 1.
Crystallographic characterization of 2 also reveals a square planar
arrangement around the Rh center (Fig. 3). In contrast to 1, the
RheNacNac six-membered chelate is essentially planar. The RheN
bond distances were shorter than in the phosphinimine-imine
ꢀ
RheN bond lengths are 2.099(3) and 2.109(3) A, slightly longer than
ꢀ
those in [HC(CNH)2Rh(CO)2][BF4] (2.036(3) and 2.045(4) A) [24]
ꢀ
and in (HC(1,2-(CN)2(C6H4))Rh(CO)2)2 (average 2.071(4) A) [25].
ꢀ
The RheC bond lengths are 1.837(5) and 1.844(5) A, shorter than
ꢀ
complex at 2.050(2) and 2.052(2) A. Conversely the RheC bond
ꢀ
lengths were found to be longer, being 1.864(2) and 1.867(2) A
respectively. These differences did not manifest themselves in
longer CeO bonds in 2. These were found to be 1.142(3) and
ꢀ
1.142(3) A, within the uncertainty of those found in 1. The similar
CeO bond distances in complexes 1e2 are supported by the
infrared data for these compounds.
In conclusion, the traditional back-bonding model for the
explanation of relative carbonyl stretching frequencies in metal
complexes may suggest differences in the donating ability from
otherwise analogous phosphinimine-imine and
b-diketiminate
anionic ligand systems however, the findings herein do not make
such a distinction indicating that both ligands have similar electron
donating capabilities.
Acknowledgements
J.D.M. thanks Saint Mary’s University and the Natural Sciences
and Engineering Research Council (NSERC) of Canada Discovery
Grant program for financial support as well as the Canadian
Foundation for Innovation, the Nova Scotia Research and Inno-
vation Trust and the NSERC Research Tools and Instruments grant
program for equipment. N.A.G. acknowledges the Office of the
Dean of Science at Saint Mary’s University for an undergraduate
Deans’ Summer Research Award and A.D.H. recognizes the
support of the Student Employment Experience Program for
funding.
Fig. 2. ORTEP-III rendering of 1 (50% probability). Hydrogen atoms are omitted for
ꢀ
ꢀ
clarity. Distances (A) angles ( ): Rh(1)eC(1) 1.837(5), Rh(1)eC(2) 1.844(5), Rh(1)eN(2)
2.099(3), Rh(1)eN(1) 2.109(3), P(1)eN(2) 1.610(3), P(1)eC(3) 1.726(4), O(1)eC(1)
1.143(5), O(2)eC(2) 1.145(5), C(1)eRh(1)eC(2) 87.01(19), C(1)eRh(1)eN(2) 88.16(16),
C(2)eRh(1)eN(2) 171.81(18), C(1)eRh(1)eN(1) 171.60(19), C(2)eRh(1)eN(1) 91.23(16),
N(2)eRh(1)eN(1) 94.49(12), N(2)eP(1)eC(3) 113.0(2), C(4)eN(1)eRh(1) 128.5(3),
P(1)eN(2)eRh(1) 113.33(16), O(1)eC(1)eRh(1) 174.1(5), O(2)eC(2)eRh(1) 173.7(5),
C(4)eC(3)eP(1) 122.4(3), C(3)eC(4)eN(1) 124.5(4).

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N.A. Giffin et al. / Journal of Organometallic Chemistry 696 (2011) 2533e2536
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