Facile one-pot access to a chiral at metal ruthenium pyrrolyl phosphine phosphoramidite complex

Article abstract of DOI:10.1016/j.inoche.2011.01.004

A facile one-step access to a chiral at metal ruthenium phosphoramidite tri(N-pyrrolyl)phosphine (PPyrl₃) complex is reported. In a one-pot reaction with PPyrl₃ and a chiral phosphoramidite ligand (L), the known ruthenium indenyl complex [RuCl(Ind)(PPh₃)₂] (Ind = indenyl) was converted to the chiral at metal complex [RuCl(Ind)PPyrl ₃(L)] as an optically pure single diastereomer in 59% isolated yield. The new complex was characterized spectroscopically and by X-ray crystallography, revealing a profound impact of the PPyrl₃ ligand on the geometry of the novel ruthenium complex.

Full text of DOI:10.1016/j.inoche.2011.01.004

Inorganic Chemistry Communications 14 (2011) 478–480
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Facile one-pot access to a chiral at metal ruthenium pyrrolyl phosphine
phosphoramidite complex
Stephen Costin a, Nigam P. Rath a,b, Eike B. Bauer a,
⁎
a
University of Missouri — St. Louis, Department of Chemistry and Biochemistry, One University Boulevard, St. Louis, MO 63121, USA
b
Center for Nanoscience, University of Missouri — St. Louis, MO 63121, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A facile one-step access to a chiral at metal ruthenium phosphoramidite tri(N-pyrrolyl)phosphine (PPyrl3)
complex is reported. In a one-pot reaction with PPyrl3 and a chiral phosphoramidite ligand (L), the known
ruthenium indenyl complex [RuCl(Ind)(PPh3)2] (Ind=indenyl) was converted to the chiral at metal complex
[RuCl(Ind)PPyrl3(L)] as an optically pure single diastereomer in 59% isolated yield. The new complex was
characterized spectroscopically and by X-ray crystallography, revealing a profound impact of the PPyrl3 ligand
on the geometry of the novel ruthenium complex.
Received 7 November 2010
Accepted 3 January 2011
Available online 19 January 2011
Keywords:
Chiral at metal
Pyrrolyl phosphine ligand
Phosphoramidite ligand
X-ray
© 2011 Elsevier B.V. All rights reserved.
One-pot procedure
Metal complexes exhibiting chirality at the metal center are of
great scientific interest [1]. They can be employed as efficient catalysts
in enantioselective organic transformations, as demonstrated by
several research groups working in that area [2]. Optically pure chiral
at metal complexes can be accessed from racemic starting materials
by reaction with chiral auxiliaries, resulting in a separable mixture of
diastereomers. Subsequent removal of the auxiliary gives the
corresponding enantiopure metal complexes [1c]. Their epimerization
has been documented by several authors [3], complicating subsequent
enantioselective catalytic applications. Another strategy is the
employment of chiral ligands in the synthesis of chiral at metal
complexes [2c,4]. These complexes are stereogenic at the metal and at
the ligand, and thus form diastereomers, which can be separated by
crystallization or column chromatography [4,5]. The optical purity of
these diastereomeric metal complexes can be established by NMR
spectroscopy, and in some cases one diastereomer is formed in excess
[4,5].
The ligand tri(N-pyrrolyl)phosphine (PPyrl3, 1) has recently
gained considerable use in the synthesis of novel organometallic
architectures [6]. Compared to other phosphorus based ligand
systems, it exhibits increased π-acidity [7]. Consequently, PPyrl3 has
been employed in a number of metal complex syntheses, aimed
towards electronic tuning efforts [8]. A variety of unusual organome-
tallic architectures were accessed utilizing PPyrl3 ligands or deriva-
tives thereof [9], and some of these complexes have been successfully
employed as catalysts in organic transformations [10].
We were seeking access to chiral at metal complexes bearing
PPyrl3 ligands, which are to the best of our knowledge unknown to
date. In previous research efforts, we have successfully accessed chiral
at metal complexes employing chiral phosphoramidite ligands such as
2a (Fig. 1) [4]. A thermal ligand exchange reaction from [RuCl(Ind)
(PPh3)2] (3, Ind=η5-indenyl) afforded the complex [RuCl(Ind)(PPh3)
(2a)] (4, Fig. 1), which was synthesized as an optically pure single
diastereomer and was characterized structurally [4b]. Indenyl com-
plexes are known to undergo facile ligand exchange reactions,
referred to in the literature as the “indenyl effect” [11]. We speculated
whether the precursor complex [RuCl(Ind)(PPh3)2] (3) could also be
employed in a double ligand exchange reaction employing the
chiral phosphoramidite ligands 2 and the PPyrl3 ligand 1. Herein, we
describe a quick, one-step process to the first mixed PPyrl3 phosphor-
amidite complex that is chiral at metal.
Accordingly, a mixture of the known [12a] complex [RuCl(Ind)
(PPh3)2] (3), the phosphoramidite ligand 2a [12b] and an excess of
PPyrl3 [12c] was heated to reflux for 15 h in toluene (Scheme 1).
Chromatographic workup afforded the title compound 5a as a single,
optically pure diastereomer, isolated as an orange solid in 59%
isolated yield [13].
The coordination of one phosphoramidite and one PPyrl3 ligand is
readily indicated by two distinct 31P NMR signals, which exhibit a 2JPP
coupling of 77.6 Hz, as expected for complexes with two different
phosphorus atoms in the metal coordination sphere. The 31P NMR
resonances for the phosphoramidite ligand 2a and the pyrrolyl ligand
1 appear at 170.7 and 124.8 ppm, respectively. Both values are in
accordance with structurally related ruthenium PPyrl3 [10b] and
phosphoramidite complexes [4]. As observed in the NMR data,
complex 5a was obtained as an optically pure single diastereomer,
⁎
Corresponding author. Fax: +1 314 516 5342.
E-mail address: bauere@umsl.edu (E.B. Bauer).
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.01.004

S. Costin et al. / Inorganic Chemistry Communications 14 (2011) 478–480
479
Ph
Ph
Ph
Ph
Ru
O
O
N
P
N
CI
P
O
PPh3
O
2a
4
Fig. 1. Phosphoramidite ligands and a chiral at metal phosphoramidite ruthenium
complex.
as only one set of signals was observed in the purified material. The
indenyl ligand gives very distinct 1H and 13C NMR signals for the three
protons and the five carbon atoms of its coordinated five-membered
ring [4b]. Due to the stereogenic centers at the metal and at the ligand,
all these carbons and protons are diastereotopic and give individual
signals in the NMR spectra.
In order to unequivocally establish the structure of the title compound
5a, its X-ray structure was determined (Fig. 2) [14]. Key bond angles and
distances are compiled in Table 1, and included for comparison is the
corresponding data for the previously synthesized complex 4 [4a].
The bond angles about the ruthenium center are in the range
from 86.906(19)° to 103.874(16)°, and the structure of 5a is thus
best described as a slightly distorted octahedron. Interestingly, the
nitrogen atom of the phosphoramidite ligand lies almost in a plane
described by the phosphorus, nitrogen and the two carbon atoms
connected to the nitrogen, as shown by the deviation from planarity
(0.043 Å). This planarity has also been observed in the X-ray structure
of a related non-coordinated phosphoramidite ligand [15]. We ascribe
the planarity to the resonance contributor B for complex 5a (Eq. (1)),
which might, due to the coordination of the phosphorus atom to the
ruthenium center, become the major contributor.
Fig. 2. Molecular structure of 5a depicted with 30% probability ellipsoids; H atoms are
omitted for clarity. For key bond lengths and angles see Table 1.
length of the PPyrl3 ligand (2.2051(5)Å). That difference in bond
lengths is expected as the significantly more π-acidic PPyrl3 shows a
higher degree of metal to ligand backbonding, which shortens the
Ru–P(2) bond. The extent of backbonding can be deduced from the
Ru–P(2) bond length of the PPh3 ligand in complex 4, which is 0.15 Å
longer than the corresponding bond for the PPyrl3 ligand in 5a.
Compared to complex 4, the ruthenium phosphoramidite bond length
Ru–P(1) in complex 5a is 0.09 Å longer. It appears that the stronger
back-donation from the metal to the PPyrl3 ligand weakens, in turn,
the Ru–P(1) bond for the phosphoramidite ligand. The PPyrl3 ligand
has also impacted on the P(1)–N(1) bond lengths of the phosphor-
amidite ligand, which is in complex 5a 0.03 Å shorter than in complex
4. We hypothesize that the more π-acidic PPyrl3 ligand promotes a
greater weighting to the resonance contributor B (Eq. (1)), thus
shortening the P–N bond.
Ru
P
Ru
P
(1)
O
O
N
N
R
R
O
The deviation from planarity in complex 4 is significantly larger
(0.0281 Å), which we tentatively ascribed to the lack of the π-acidic
PPyrl3 ligand in that complex. Thus, the metal center in 4 might be
somewhat more electron-rich, thereby weakening the influence of the
resonance contributor B (Eq. (1)), which promotes the planarity
about the nitrogen center.
O
R
R
A
B
The incorporation of the PPyrl3 ligand 1 in the coordination sphere
has a profound impact on the structural parameters of the complex, as
seen by a comparison of the structurally related metal complexes 4
and 5a. The Ru–P(1) bond length of the phosphoramidite ligand
(2.2801(5)Å) in 5a is significantly longer than the Ru–P(2) bond
Table 1
Selected bond lengths and angles.
[RuCl(Ind)(PPyr3)(2a)]
(5a)
[RuCl(Ind)(PPh3)(2a)]
(4)
Bond lengths (Å)a
Ru–P(1)
Ru–P(2)
2.2801 (5)
2.2051 (5)
2.4464 (5)
1.6384 (17)
1.7133 (19)
2.1961 (7)
2.3504 (8)
2.4439 (7)
1.663 (3)
–
Ru–Cl
P(1)–N(1)b
P(2)–N(2)b
Bond angles (°)
P(1)–Ru–P(2)
Cl–Ru–P(1)
94.71 (2)
103.874 (16)
86.906 (19)
98.87 (3)
89.28 (3)
86.99 (3)
Cl–Ru–P(2)
Other geometrical parameters
Ru–Cp (Å)c
1.896
1.909
Deviation from planarity at
nitrogend
0.0043 Å
0.0281 Å
a
P(1) corresponds to the phosphoramidite ligand, P(2) to PR3 (R=Ph, Pyrl).
P(1)–N(1) corresponds to the phosphoramidite ligand, and P(2)–N(2) to one of the
b
P–N bonds of PPyrl3.
c
Scheme 1. Synthesis of chiral at metal tris(N-pyrrolyl)phosphine phosphoramidite
complexes.
Distance between the Cp-centroid of the indenyl ligand and the ruthenium center.
d
Average deviation from a least squares mean plane defined by N1, P1 and C30, C37.

480
S. Costin et al. / Inorganic Chemistry Communications 14 (2011) 478–480
(d) K. Kromm, P.L. Osburn, J.A. Gladysz, Organometallics 21 (2002) 4275;
On the other hand, the PPyrl3 ligand has almost no influence on the
(e) J.W. Faller, B.J. Grimmond, Organometallics 20 (2001) 2454.
[3] H. Brunner, Angew. Chem. Int. Ed. 38 (1999) 1194.
Ru–Cl bond lengths and on the distance of the coordinated five-
membered ring of the indenyl ligand to the ruthenium center, which
are comparable for both metal complexes. The crystallographic data
for the complexes 4 and 5a clearly corroborate the π-acidity of the
PPyrl3 ligand, which seems to be stronger than that of the phosphor-
amidite ligand 2a.
[4] (a) S. Costin, N.P. Rath, E.B. Bauer, Tetrahedron Lett. 50 (2009) 5485;
(b) S. Costin, N.P. Rath, E.B. Bauer, Inorg. Chim. Acta 361 (2009) 1935.
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(1998) 3809.
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Organometallics 16 (1997) 3377.
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Dalton Trans. (2003) 3717.
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Font-Bardia, X. Solans, J. Organomet. Chem. 692 (2007) 3882;
(b) A.D. Burrows, M.F. Mahon, M. Varonne, Dalton Trans. (2004) 3321.
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(b) N.A. Foley, M. Lail, T.B. Gunnoe, T.R. Cundari, P.D. Boyle, J.L. Petersen,
Organometallics 26 (2007) 5507;
The phosphoramidite ligand 2b could also be converted to the
structurally related PPyrl3 complex 5b, employing conditions similar to
those for the synthesis of 5a (Scheme 1). The NMR data revealed the
formation of a single diastereomer for 5b, i.e. only one set of signals was
observed in the 31P NMR spectrum. However, purification efforts for 5b
still left 10% impurities in the sample. Employment of ligand 2c resulted
in a mixture of compounds (containing [RuCl(Ind)(PPh3)(Ppyrl3)]).
Complex 5c (Scheme 1) could only be obtained in 60% spectroscopic
purity and again, only one diastereomer of 5c formed (1H and 31P
spectra for 5b and 5c are given in the Supplementary information). Thus,
the methodology of double PPh3 ligand exchange in [RuCl(Ind)(PPh3)2]
presented herein allows for general access to chiral at metal ruthenium
PPyrl3 complexes in diastereopure form, and we anticipate that
modified reaction conditions and workup methods will afford a series
of ruthenium complexes similar to the title compound.
(c) A.M. Trzeciak, B. Borak, Z. Ciunik, J.J. Ziołkowski, M. Fátima, C. Guedes da Silva,
A.J.L. Pombeiro, Eur. J. Inorg. Chem. (2004) 1411;
(d) R. Jackstell, H. Klein, M. Beller, K.-D. Wiese, D. Röttger, Eur. J. Org. Chem.
(2001) 3871.
[11] M.J. Calhorda, C.C. Romão, L.F. Veiros, Chem. Eur. J. 8 (2002) 868.
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(b) A. Duursma, J.-G. Boiteau, L. Lefort, J.A.F. Boogers, A.H.M. de Vries, J.G. de
Vries, A.J. Minnaard, B.L.J. Feringa, J. Org. Chem. 69 (2004) 8045;
(c) R. Jackstell, H. Klein, M. Beller, K.-D. Wiese, D. Rottger, Eur. J. Org. Chem.
(2001) 3871.
[13] Synthesis and characterization data for 5a: To a Schlenk flask containing [(Ind)
RuCl(PPh3)2] (3, 0.227 g, 0.292 mmol), 2a (0.156 g, 0.305 mmol) and PPyrl3
(0.201 g, 0.879 mmol), toluene (9 mL) was added and the mixture was heated to
reflux for 15 h. Upon cooling to rt, the solvent was removed under high vacuum.
The resulting residue was purified by flash chromatography (2×14 cm silica),
eluted with CH2Cl2. The first of two orange bands was collected and all volatiles
removed under high vacuum to give 5a as an orange solid (0.170 g, 0.172 mmol,
59%), m.p. 154–156 °C dec. (capillary). An analytically pure sample and X-ray
quality crystals were obtained by slow diffusion of methanol into a solution of 5a
in CH2Cl2 (recovered mass 0.012 g from 0.015 g original sample, 80% recovery).
Anal. calcd. for C55H45N4O2P2ClRu: C, 67.84; H, 4.68; Found: C, 67.59; H, 4.72%.
In summary, we have synthesized and structurally characterized
the first chiral at metal tri(N-pyrrolyl)phosphine ruthenium complex
employing a facile one-pot procedure. The π-acidic tri(N-pyrrolyl)
phosphine ligand has a significant impact on the structure of the
complex. Expanding the methodology presented herein to a series of
metal complexes and investigation of potential catalytic applications
are currently underway.
3
3
NMR (δ, CDCl3) 1H 8.10 (d, JHH =8.8 Hz, 1 H, aromatic), 7.99 (d, JHH =9.4 Hz,
Acknowledgment
3
3
1 H, aromatic), 7.65 (d, JHH =8.7 Hz, 2 H, aromatic), 7.59 (d, JHH =8.2 Hz, 1 H,
3
3
aromatic), 7.51 (d, JHH =8.9 Hz, 1 H, aromatic), 7.46 (t, JHH =7.1 Hz, 1 H,
aromatic), 7.30–7.13 (m, 15 H, aromatic), 7.12–6.97 (m, 4 H, aromatic), 6.90–6.83
(m, 3 H, pyrrolyl), 6.31 (s, br, 1 H, pyrrolyl), 6.08 (s, br, 1 H, pyrrolyl), 5.90 (s, br,
7 H, pyrrolyl), 5.53 (s, br, 1 H, indenyl), 5.44 (s, br, 1 H, indenyl), 4.93 (s, br, 1 H,
indenyl), 4.51 (d, 2JHH =10.6 Hz, 1 H, CHH′), 4.46 (d, 2JHH =10.6 Hz, 1 H, CHH′), 3.15 (d,
We thank the University of Missouri — St. Louis for support.
Funding from the National Science Foundation for the purchase of the
ApexII diffractometer (MRI, CHE-0420497), the purchase of the NMR
spectrometer (CHE-9974801) and the purchase of the mass spec-
trometer (CHE-9708640) is acknowledged.
2JHH =11.4 Hz, 1 H, CHH′), 3.10 (d, JHH =11.4 Hz, 1 H, CHH′); 13C{1H–150.3 (d,
2
JCP=15.5 Hz, aromatic), 148.6 (d, JCP=7.0 Hz, aromatic), 138.3 (s, aromatic), 133.9 (s,
aromatic), 132.6 (s, aromatic), 131.7 (s, aromatic), 131.1 (s, aromatic), 130.3
(s, aromatic), 129.6 (s, aromatic), 129.5 (s, aromatic), 129.2 (s, aromatic), 128.5
(s, aromatic), 128.2 (s, aromatic), 128.1 (s, aromatic), 127.4 (s, aromatic), 126.9
(s, aromatic), 126.7 (s, aromatic), 126.3 (s, aromatic), 125.9 (s, aromatic), 125.2 (s,
aromatic), 125.0 (s, aromatic), 124.1 (s, br, aromatic), 123.3 (s, aromatic), 122.3 (s,
aromatic), 121.7 (s, aromatic), 121.5 (s, aromatic), 112.5 (s, aromatic), 112.2 (s,
indenyl), 111.4 (s, br, indenyl), 92.0 (s, indenyl), 69.7 (d, 2JCP=1.5 Hz, indenyl), 65.5 (d,
Appendix A. Supplementary data
CCDC 799337 contains the supplementary data for complex 5a.
The data can be obtained free of charge from the Cambridge
Crystallographic Center via www.ccdc.cam.ac.uk/data_request/cif.
Supplementary information associated with this article (experimental
details for the synthesis of compound 5b, 1H and 13C NMR spectra for
compound 5a, 1H and 31P NMR spectra for compounds 5b and c,
details for the solution of the X-Ray structure of 5a) can be found in
the online version, at doi:10.1016/j.inoche.2011.01.004.
2
2JCP=7.6 Hz, indenyl), 49.7 (s, CH2), 49.6 (s, CH2); 31P{1H}170.7 (d, JPP =77.6 Hz,
phosphoramidite), 124.8 (d, 2JPP =77.6 Hz, PPyrl3). HRMS calcd for C55H45N4O2P2ClRu
992.1749, found 992.1731. IR (neat solid, cm-1) 3052(w), 3028(w), 1587(m), 1455(m),
1321(m), 1228(s), 1178(s), 1056(s), 1037(s), 730(s).
[14] Crystallographic parameters: empirical formula
C55H45ClN4O2P2Ru; formula
weight 992.41; temperature 100(2)K; wavelength 0.71073 Å; crystal system
orthorhombic; space group P212121; unit cell dimensions a=10.3517(7) Å,
b=11.6479(8) Å, c=36.510(2) Å; volume 4402.2(5) Å3; Z=4; density
(calculated) 1.497 Mg/m3; absorption coefficient 0.540 mm–1; F(000)=2040;
crystal size 0.25×0.18×0.13 mm–3; theta range for data collection 1.84 to 27.59°;
index ranges −13≤h≤13, −15≤k≤15, −46≤l≤47; reflections collect-
ed128242; independent reflections 10178 [R(int)=0.0509]; completeness to
theta (=25.00°), 100.0%; absorption correction semi-empirical from equivalents;
max. and min. transmission 0.9341 and 0.8791; refinement method full-matrix
least-squares on F2; Data/restraints/parameters 10178/ 0/ 586; goodness-of-fit
on F2 1.058; final R indices [IN2sigma(I)] R1 =0.0243, wR2 =0.0542; R indices
(all data) R1 =0.0278, wR2 =0.0556; absolute structure parameter −0.020(15);
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