Rhodium(I) iminophosphine poly(propyleneimine) dendrimers: Synthesis, characterization and molecular structure of a mononuclear analogue

Article abstract of DOI:10.1016/j.inoche.2009.05.026

First (G1)- and second-generation (G2) poly(propyleneimine) (PPI) dendrimers were reacted with 2-(diphenylphosphino)benzaldehyde, to form iminophosphine-functionalised dendritic ligands (1, 2). The reaction of the dimeric precursor [RhCl(CO)₂]<

Full text of DOI:10.1016/j.inoche.2009.05.026

Inorganic Chemistry Communications 12 (2009) 716–719
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Rhodium(I) iminophosphine poly(propyleneimine) dendrimers: Synthesis,
characterization and molecular structure of a mononuclear analogue
Nathan C. Antonels a, Bruno Therrien b, John R. Moss a, Gregory S. Smith a,
*
a Department of Chemistry, University of Cape Town, Rondebosch 7701, Cape Town, South Africa
b Service Analytique Facultaire, University of Neuchatel, Case postale 158, CH-2009 Neuchatel, Switzerland
a r t i c l e i n f o
a b s t r a c t
Article history:
First (G1)- and second-generation (G2) poly(propyleneimine) (PPI) dendrimers were reacted with
2-(diphenylphosphino)benzaldehyde, to form iminophosphine-functionalised dendritic ligands (1, 2).
The reaction of the dimeric precursor [RhCl(CO)2]2 with the heterobidentate dendritic ligands 1, 2 gave
the corresponding rhodium(I) complexes (3, 4).
Received 11 February 2009
Accepted 21 May 2009
Available online 28 May 2009
Ó 2009 Elsevier B.V. All rights reserved.
Keywords:
Dendrimers
Rhodium complexes
Poly(propyleneimine)
Iminophosphine
P,N-heterobidentate
Dendrimers are highly branched, monodisperse, macromolecu-
lar constructs, that are often used as supports for homogeneous
catalysts. Various examples of dendritic scaffolds have been used
to immobilize homogeneous catalysts, including carbosilanes [1],
polyamidoamines [2], polypropyleneimines [3] and polyarylethers
[4], to name a few.
Dendritic ligands generally have an increased local concentra-
tion of similar functional groups at their periphery. These dendri-
mers may contain donor ligands capable of coordinating to
various transition metals, thereby catalyzing various organic trans-
formations with a range of metals, such as Kharasch-type addition
reactions [5] (Ni), ethylene oligomerisations/norbornene polymer-
izations [6] (Ni), hydroformylation [7] (Rh), carbon–carbon cross-
coupling reactions [8] (Pd), hydrogen transfer reductions [9] (Ru)
and epoxidation of cyclohexene [10] (Ti).
There has been recent interest in the chemistry of polydentate
ligands, especially those which combine ‘‘soft” and ‘‘hard” donor
atoms [11]. The most used hemilabile ligands are the P,N-donor
bidentate ligands, combining the distinct behaviour of a soft phos-
phorus atom which coordinates very strongly to a soft metal centre
(e.g. palladium) and a hard nitrogen donor which is weakly bound.
Non-rigid ligands, such as the widely studied iminophosphines, of-
ten act as chelating ligands that easily form five- or six-membered
chelate rings. Iminophosphines display versatile coordination
behaviour and are potentially hemilabile, making them useful for
a variety of homogeneous processes, such as, C–C coupling reac-
tions [12], oligomerisation and polymerization of ethylene [13],
aminations [14], transfer hydrogenations [15] and hydroboration
[16]. The steric and electronic properties of such ligands can also
be fine-tuned and this can have a huge effect on the kinetics and
thermodynamics of a chemical reaction [17].
Examples of metallodendritic chelating heterobidentate com-
plexes are rare. For metallodendrimers containing P and N donor
atoms, many transition metals are coordinated in a P-monodentate
or N-monodentate fashion. In this communication, we report on
the synthesis of first- and second-generation chelating, heterobid-
entate [P, N] iminophosphine dendritic ligands and their coordina-
tion chemistry with a Rh(I) precursor. The molecular structure of a
mononuclear analogue is also presented and discussed.
The synthesis of the first- and second-generation iminophos-
phine-functionalised dendritic ligands 1 and 2 (Fig. 1) [19] was
achieved by a Schiff-base condensation reaction of the commer-
cially available DAB-dendr-(NH2)n with 2-diphenylphosphino-
benzaldehyde. Screttas et al. first reported the synthesis of a
third generation iminophosphine dendritic ligand based on a
poly(propyleneimine) scaffold [18]. These dendritic P,N ligands
were used in situ in the presence of palladium acetate in the Heck
cross-coupling of p-anisyl bromide with styrene.
The dendritic ligands (1, 2) were isolated as pale yellow solids
in moderate yields (65–68%). These compounds were characterized
by elemental analysis (C, H, N), and by IR and 1H, 13C{1H} and
31P{1H} NMR spectroscopy. The 1H NMR spectra of 1 and 2 show
the aliphatic protons for the dendritic core occurring in the range
between 1 and 4 ppm, and the aromatic and imine resonances
occurring between 6 and 9 ppm. The CH2 protons adjacent to the
* Corresponding author. Tel.: +27 21 6505279; fax: +27 21 6897499.
E-mail address: Gregory.Smith@uct.ac.za (G.S. Smith).
1387-7003/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2009.05.026

N.C. Antonels et al. / Inorganic Chemistry Communications 12 (2009) 716–719
717
of the imine functionality. The stretching frequency for the termi-
nal carbonyl is observed at about 2000 cmÀ1. The 1H NMR spec-
trum shows a singlet at 8.49 ppm for the methine proton of the
imino group, an upfield shift from 8.85 ppm for the free, uncom-
plexed dendritic ligand, providing further evidence of nitrogen
coordination of the C@N group. The 31P{1H} NMR spectrum dis-
plays a doublet at 48.5 ppm with 1J(RhP) = 163 Hz, consistent with
rhodium coupling to phosphorus and is typical of phosphorus trans
to chlorine [21,22]. This indicates the presence of a single isomer
for each generation (complexes 3 and 4). Elemental analyses corre-
lates with solvent inclusion within the dendritic structures.
To gain further insight into the coordination environment of the
Rh(I) atoms in the metallodendrimers, the analogous mononuclear
compound (5) was synthesized [20] from the known compound o-
Ph2PC6H4CH@NPrn [22] and [RhCl(CO)2]2. Spectroscopic data (1H,
13C{1H}, 31P{1H}, IR) for 5 are similar to that observed for the rho-
dium metallodendrimers (3, 4), providing evidence of metal coor-
dination for the respective metallodendrimers, indicative of the
formation of six-membered chelate rings with rhodium on the
periphery of the dendrimers.
The molecular structure of 5 was confirmed by X-ray crystallog-
raphy [23]. An ORTEP drawing [24] with the corresponding atom
labelling scheme is shown in Fig. 2 together with selected bond
lengths and angles. The molecular structure shows the rhodium
atom to be in a square-planar geometry surrounded by the imino
nitrogen atom trans to the carbonyl ligand and the chloro ligand
trans to phosphorus. The geometrical parameters around the
rhodium atom are comparable to those found in the analogous
complexes [Rh(PyP)(CO)Cl] (PyP = 1-(2-diphenylphosphino)ethyl-
pyrazole) [25], [Rh(P–N)(CO)Cl] (P–N = diphenylphosphino(phenyl
pyridine-2-yl methylene)amine) [26] and [Rh(P–N*)(CO)Cl] (P–
N* = (2S, 3S)-2-{4-(dimethylamino)benzylideneamino} + -3-meth-
ylpentyl bis(2,6-dimethylphenyl) phosphite) [27]. The formation
of a six-membered chelate ring imposes distortion around the rho-
dium atom. The P–Rh–Cl angle [170.60(11)] is smaller than the ex-
pected value of 180°. The atoms Rh, Cl, N, C(23) and O are
essentially coplanar with an average deviation of 0.0218 Å from
the mean plane, while the P atom is 0.39(1) Å away from this
plane.
PPh2
N
N
P
Ph2
N
N
N
PPh2
N
1 (First-generation)
PPh2
PPh2
N
P
Ph2
N
N
N
Ph2P
N
Ph2P
N
N
N
N
N
N
PPh2
N
Ph2P
N
N
Ph2P
Ph2P
2( Second-generation)
Fig. 1. First- and second-generation iminophosphine dendrimers.
imine moiety are observed as a triplet around 3.69 ppm, for 1 and
2. The appearance of a singlet at ca. 8.37 ppm, assigned to the
imine proton (–CH2N@CH–CH2–) establishes that the condensation
reaction has occurred. As was the case for Screttas and co-workers,
it is important to consider syn and anti isomerism about the imine
double bond [18]. One resonance is noted for the imine proton in
both ligands, assigned to the trans isomer for steric reasons. The
31P{1H} NMR spectra of 1 and 2 show a singlet at À13.35 and
À13.43 ppm, respectively. Infrared spectroscopy confirms the for-
mation of the imine functionality. In the IR spectrum of 1 and 2,
the absorption bands associated with the imine functionality
In conclusion, first- and second-generation iminophosphine
(m
(C@N)) occur at 1637 cmÀ1. This corroborates the evidence for
rhodium(I) metallodendrimers were synthesized, based on
a
imine formation observed in the 1H NMR spectra. The composition
and purity of the dendritic ligands were further confirmed by ele-
mental analysis and ESI-mass spectrometry.
Reaction of the iminophosphine dendrimers 1 and 2 with the
dimeric rhodium(I) precursor [RhCl(CO)2]2 (Scheme 1) gave the
tetranuclear (3) and octanuclear (4), six-membered chelate ring,
rhodium(I) metallodendrimers, respectively [20].
The metallodendrimers (3, 4) were isolated as orange powders
in moderate to good yields, stable at room temperature and soluble
in toluene, THF and chlorinated solvents but insoluble in hydrocar-
bons. Generally, the IR spectra for 3 and 4 show a wavelength shift
of the C@N absorption band towards lower wavenumbers, as com-
pared to the uncomplexed dendritic Schiff-base (ca. 1630 vs
1637 cmÀ1), indicative of coordination of rhodium to the nitrogen
[RhCl(CO)2]2
Gn
N
Gn
N
Ph2P
Cl Rh PPh2
CO
Fig. 2. Molecular structure of the mononuclear rhodium(I) iminophosphine com-
plex 5 showing ellipsoids at the 50% probability level with hydrogen atoms omitted
for clarity. Selected bond lengths (Å) and angles (°): Rh–P 2.188(4), Rh–Cl 2.394(4),
Rh–N 2.118(11), Rh–C(23) 1.806(14), N–C(4) 1.284(16), N–C(3) 1.468(15); P–Rh–Cl
170.60(11), P–Rh–N 86.1(3), P–Rh–C(23) 93.7(4), Cl–Rh–N 90.0(3), Cl–Rh–C(23)
90.6(4), N–Rh–C(23) 177.1(5), C(3)–N–C(4) 116.9(11).
m
m
3: n = 1, m= 4
4: n = 2, m= 8
Scheme 1.

718
N.C. Antonels et al. / Inorganic Chemistry Communications 12 (2009) 716–719
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ing the coordination of other rhodium(I) homogeneous catalysts
to similar dendrimeric scaffolds, and the use of these complexes
in alkene hydroformylation reactions.
Acknowledgements
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We would like to thank the Anglo Platinum Corporation, Uni-
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South Africa and NRF-DST Centre of Excellence in Catalysis (c
*
change) for financial support.
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Appendix A. Supplementary material
[19] General Procedure for the synthesis of 1–2:
A mixture of 2-(diphenyl-
phosphino)benzaldehyde and DAB-(NH2)n in dichloromethane/ethanol
(50:50 v/v%) (60 cm3) was stirred at room temperature (48 h). Anhydrous
MgSO4 was transferred to the stirred solution. The mixture was filtered by
gravity, and the solvent removed from the filtrate collected. The residue was
dissolved in dichloromethane (3–5 cm3) and pentane added to precipitate a
pale yellow solid. Compound 1: Yield = 1.27 g (76%). M.p. = 56–58 °C. IR
(CH2Cl2, cmÀ1): 1637 (s, C@N). 1H NMR (300 MHz, CDCl3): d (ppm) = 1.29 (br
qn, 4H, CH2CH2N((CH2)3N)2), 1.59 (br qn, 8H, CH2N(CH2CH2CH2N)2), 2.28 (br
m, 12H, CH2N(CH2CH2CH2N)2), 3.44 (br t, 8H, CH2NCH), 6.86 (m, 4H,
P(C6H5)CCH), 7.29 (br m, 48H, phenyl), 7.94 (m, 4H, NCHCCH), 8.85 (d, 4H,
CCDC 716316 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif. Supplementary data associated with this article
can be found, in the online version, at doi:10.1016/j.inoche.
2009.05.026.
4
NCHC, JPH = 4.5 Hz). 13C{1H} NMR (100 MHz, CDCl3): d (ppm) = 25.4, 28.4,
References
4
51.8, 54.3, 59.8 (aliphatic), 127.9–140.0 (aromatic), 159.5 (d, JPC = 20.7 Hz,
ÀC@N); 31P{1H} NMR (121 MHz, CDCl3):
d
(ppm) = À13.4 (s). Elemental
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Compound 2: Yield = 0.362 g (68%). M.p. = 61–63 °C. IR (CH2Cl2, cmÀ1): 1637
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4
(d, 8H, NCHC, JPH = 4.5 Hz). 13C{1H} NMR (100 MHz, CDCl3): d (ppm) = 23.7,
24.2, 27.2, 50.6, 51.3, 53.4, 58.6 (aliphatic), 126.7–138.8 (aromatic), 158.2 (d,
4JPC = 20.7 Hz, ÀC@N). 31P{1H} NMR (121 MHz, CDCl3): d (ppm) = À13.4 (s).
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6.13; Found: C, 73.05; H, 6.48; N, 6.12.
[20] General Procedure for the synthesis of 3–5: A solution of the iminophosphine
and [RhCl(CO)2]2 in THF was stirred for 2 h at room temperature. The solvent
was reduced and n-pentane added to precipitate the product as an orange
solid. Compound 3: Yield = 0.168 g (63%). M.p.: does not melt <300 °C. IR
(CH2Cl2, cmÀ1): 1630 (m, C@N); 2001 (s, C@O). 1H NMR (300 MHz, CDCl3): d
(ppm) = 1.45 (br s, 4H, CH2CH2N((CH2)3N)2), 1.84 (br s, 8H, CH2N(CH2-
CH2CH2N)2), 2.27 (br s, 12H, CH2N(CH2CH2CH2N)2), 4.07 (br s, 8H, CH2NCH),
6.82 (m, 4H, P(C6H5)CCH), 7.43 (br m, 48H, phenyl), 7.91 (m, 4H, NCHCCH),
8.49(s, 4H, NCHC). 13C{1H} NMR (100 MHz, CDCl3): d (ppm) = 25.8 (br), 62.0
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(br), 68.2, 125.6–136.6 (aromatic), 167.1 (br, C@N), 189.4. (dd, JPC = 17.0 Hz,
1JRhC = 72.0 Hz, CO). 31P{1H} NMR (121 MHz, CDCl3): d (ppm) = 48.5 (d, PPh2,
1JRhP = 163 Hz). Elemental analysis (%): Calc. for C96H92N6P4Rh4Cl4O4Á
2.5CH2Cl2: C, 51.81; H, 4.28; N, 3.68; Found: C, 51.83; H, 4.38; N, 3.61. ESI-
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d (ppm) = 1.64 (br s, 28H, CH2CH2N(CH2-
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4.11 (m, 16H, CH2NCH), 6.79 (m, 8H, NCHCCHCH), 7.92 (br m, 96H, phenyl),
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2
1
C@N), 190.0 (dd, JPC = 17.0 Hz, JRhC = 72.0 Hz, CO). 31P{1H} NMR (121 MHz,
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NCHC). 13C{1H} NMR (100 MHz, CDCl3): d (ppm) = 10.8, 24.3, 66.9, 128.8–135.4
2
1
(aromatic), 163.8 (s, C@N), 189.5 (dd, JPC = 17.0 Hz, JRhC = 72.0 Hz, CO).
31P{1H} NMR (121 MHz, CDCl3):
d
(ppm) = 48.3 (d, PPh2, JRhP = 167 Hz).
Elemental analysis (%): Calc. for C23H22NOPClRh: C, 55.49; H, 4.45; N, 2.81;
Found: C, 55.49; H, 4.10; N, 2.58.
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N.C. Antonels et al. / Inorganic Chemistry Communications 12 (2009) 716–719
719
[23] Orange crystals of 5 (0.15 Â 0.14 Â 0.10 mm) suitable for X-ray diffraction
analysis were grown by layering pentane on a concentrated solution of 5 in
dichloromethane. Crystal data for 5; C23H22ClNOPRh, triclinic space group
P À 1 (No. 2), cell parameters a = 8.7765(12) Å, b = 10.218(2) Å, c = 13.180(2) Å,
[24] L.J. Farrugia, J. Appl. Cryst. 30 (1997) 565.
[25] S. Burling, L.D. Field, B.A. Messerle, K.Q. Vuong, P. Turner, Dalton Trans. (2003)
4181.
[26] P.W. Dyer, J. Fawcett, M.J. Hanton, J. Organomet. Chem. 690 (2005) 5264.
[27] K.N. Gavrilov, O.G. Bondarev, R.V. Lebedev, A.A. Shiryaev, S.E. Lyubimov, A.I.
Polosukhin, G.V. Grintselev-Knyazev, K.A. Lyssenko, S.K. Moiseev, N.S.
Ikonnikov, V.N. Kalinin, V.A. Davankov, A.V. Korostylev, H.-J. Gais, Eur. J.
Inorg. Chem. (2002) 1367.
a
= 109.90(2), b = 101.83(2),
c
= 90.90(2)°, V = 1083.0(3) Å3, T = 173(2) K, Z = 2,
Dc = 1.526 g cmÀ3
,
F(0 0 0) 504,
k (Mo Ka) = 0.71073 Å, 8534 reflections
measured, 3962 unique (Rint = 0.0720) which were used in all calculations.
The structure was solved by direct method (SHELXS-97) and refined (SHELXL-
97) [28] by full-matrix least-squares methods on F2 with 254 parameters.
[28] G.M. Sheldrick, SHELXS-97, SHELXL-97, University of Göttingen, Göttingen,
Germany, 1999.
R1 = 0.0697 (I > 2r(I)) and wR2 = 0.2565, GOF = 1.143; max./min. residual
density 2.005/À1.185 eÅÀ3 located at less than 1 Å from the rhodium atom.