Electronic and steric effects of triarylphosphines on the synthesis, structure and spectroscopical properties of mononuclear rhodium(I)-chloride complexes

Article abstract of DOI:10.1016/j.poly.2005.10.011

A systematic study of the reaction between the dinuclear complex [{Rh(μ-Cl)(η⁴-COD)}₂], (COD = 1,5-cyclooctadiene) with 14 different triarylphosphines is presented. When two equivalents of the phosphine are employed, the main product is the mononuclear complex [RhCl(η⁴-COD)(PR₃)], with R = 4-(OCH₃)C₆H₄ (1), 4-(CH₃)C₆H₄ (2), C₆H₅ (3), 4-FC₆H₄ (4), 4-(CF₃)C₆H₄ (5), 4-ClC₆H₄ (6), 3-(OCH₃)C₆H₄ (7), 3-(CH₃)C₆H₄ (8), 3-ClC₆H₄ (9), 2-(OCH₃)C₆H₄ (10), 2-(CH₃)C₆H₄ (11) and R₃ = (C₆H₅)₂(C₆F₅) (12). No mononuclear complex could be isolated with the electron poor phosphines P(C₆H₅)(C₆F₅)₂ and P(C₆F₅)₃. A chemical equilibrium in solution was observed between the dinuclear and mononuclear species, with the formation of the mononuclear being disfavoured by the use of bulky and electron poor phosphines. The mononuclear complex 11, with the extremely bulky phosphine P(2-CH₃C₆H₅)₃, was undetected in solution by NMR, however could be crystallized and its molecular structure determined by X-ray diffraction and compared with the previously reported structures for 3 and 4. For complexes with isosteric phosphines (1-6), an inverse relationship between the coupling constant ¹J_Rh-P and the electronic parameter (χ) of the phosphine was observed.

Full text of DOI:10.1016/j.poly.2005.10.011

Polyhedron 25 (2006) 1549–1554
www.elsevier.com/locate/poly
Electronic and steric effects of triarylphosphines on the
synthesis, structure and spectroscopical properties of
mononuclear rhodium(I)–chloride complexes
,1
2
*
`
Jorge Tiburcio , Sylvain Bernes , Hugo Torrens
DEPg., Facultad de Quı´mica, UNAM, Cd. Universitaria, 04510 Me´xico D.F., Mexico
Received 14 June 2005; accepted 17 October 2005
Available online 28 November 2005
Abstract
A systematic study of the reaction between the dinuclear complex [{Rh(l-Cl)(g4-COD)}2], (COD = 1,5-cyclooctadiene) with 14
different triarylphosphines is presented. When two equivalents of the phosphine are employed, the main product is the mononuclear
complex [RhCl(g4-COD)(PR3)], with R = 4-(OCH3)C6H4 (1), 4-(CH3)C6H4 (2), C6H5 (3), 4-FC6H4 (4), 4-(CF3)C6H4 (5), 4-ClC6H4
(6), 3-(OCH3)C6H4 (7), 3-(CH3)C6H4 (8), 3-ClC6H4 (9), 2-(OCH3)C6H4 (10), 2-(CH3)C6H4 (11) and R3 = (C6H5)2(C6F5) (12). No mono-
nuclear complex could be isolated with the electron poor phosphines P(C6H5)(C6F5)2 and P(C6F5)3. A chemical equilibrium in solution
was observed between the dinuclear and mononuclear species, with the formation of the mononuclear being disfavoured by the use of
bulky and electron poor phosphines. The mononuclear complex 11, with the extremely bulky phosphine P(2-CH3C6H5)3, was undetected
in solution by NMR, however could be crystallized and its molecular structure determined by X-ray diffraction and compared with the
previously reported structures for 3 and 4. For complexes with isosteric phosphines (1–6), an inverse relationship between the coupling
1
constant JRh–P and the electronic parameter (v) of the phosphine was observed.
ꢀ 2005 Elsevier Ltd. All rights reserved.
Keywords: Rhodium complexes; Dinuclear mononuclear complexes; Stereoelectronic properties; Phosphines
1. Introduction
These coordinatively unsaturated Rh(I) complexes have
also been studied as models to estimate the electronic and
steric influence that different ligands exhibit on their chem-
ical reactivity, selectivity and catalytic activity [3].
Phosphines in particular, can be modified in size and
basicity with relatively ease, making possible to tune the
stereo-electronic properties of their complexes and a num-
ber of quantitative parameters (e.g., pKa basicity constants
[4], Tolmanꢀs m electronic parameter and h cone angle [5],
Gieringꢀs v electronic parameter and QALE method [6],
and Dragoꢀs ECW model [7]) are well established.
Recently, some electronic and steric effects have been
investigated for Rh and Ir with P–S, P–P and P–O chelate
ligands [8], P–Se [9], and P–Pt [10].
A better understanding of the exact nature of ligand
effects can help direct synthetic and catalytic research
efforts. Herein, we present a systematic study on the effect
The reactivity of the dinuclear chloro-bridged Rh(I)
complexes [{Rh(l-Cl)(Ln)}2] (n = 1 for chelates or 2 for
monodentate ligands), towards nucleophilic ligands is of
interest [1] since these compounds are frequently involved
in catalytic processes, especially hydrogenation and hydro-
formylation reactions in which phosphines are commonly
used as a co-catalysts [2].
*
Corresponding author. Tel.: +52 55 50613721; fax: +52 55 50613389.
E-mail address: jtiburcio@cinvestav.mx (J. Tiburcio).
Present address: Departamento de Quımica, Centro de Investigacion y
1
´
´
´
de Estudios Avanzados del Instituto Politecnico Nacional, Apartado
´
´
Postal 14-740, Mexico, D.F. 07000, Mexico.
2
´
Present address: Centro de Quımica, IC-UAP, Blvd. 14 Sur 6303, San
Manuel, 72570 Puebla, Pue., Mexico.
0277-5387/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2005.10.011

1550
J. Tiburcio et al. / Polyhedron 25 (2006) 1549–1554
of the electronic and steric properties of the triarylphos-
phines on the synthesis, spectroscopical properties and
structure of mononuclear Rh(I)–chloride complexes.
2.35 (s, 9H, phosphine-CH3), 2.10–1.90 (m, 4H, COD-
1
CH2). 31P NMR, CDCl3 d = 29.3 ppm, d, JRh–P
=
149.3 Hz.
2. Experimental
2.3.3. [RhCl(g4-COD){P(C6H5)3}] (3)
Yield 94%; yellow. Anal. Calc. for C26H27ClPRh: C,
61.41; H, 5.36. Found: C, 61.74; H, 5.33%. 1H NMR,
CDCl3 d (ppm); 7.75 (m, 6H, CH-phosphine), 7.40 (m,
9H, CH-phosphine), 5.58 (m, 2H, COD-CH trans-P),
3.15 (m, 2H, COD-CH trans-Cl), 2.40 (m, 4H, COD-
CH2), 2.15–1.85 (m, 4H, COD-CH2). 31P NMR, CDCl3
2.1. Chemicals and physical measurements
Phosphines PR3 with R = 4-(OCH3)C6H4, 3-
(OCH3)C6H4, 2-(OCH3)C6H4, 4-(CH3)C6H4, 3-(CH3)C6H4,
2-(CH3)C6H4, 4-ClC6H4, 3-ClC6H4, 4-FC6H4, 4-
(CF3)C6H4, C6H5, and R3 = (C6H5)2(C6F5), (C6H5)(C6F5)2
and (C6F5)3 and RhCl3 Æ 3H2O were purchased from Strem
Chemicals and used as received. The complex [{Rh-
(l-Cl)(g4-COD)}2] was prepared as previously reported
[11]. Solvents were dried and degassed prior to use. All
manipulations were carried out under dry nitrogen atmo-
spheres using Schlenk techniques. Thin layer chromatogra-
phy (Merck, 5 · 7.5 cm2 Kiesegel 60 F254) was used to
monitor the progress of the reactions. Elemental analyses
were determined by Galbraith Labs. Inc., USA. NMR spec-
tra were recorded by Spectral Data Services (IL, USA) on a
SDS-360 spectrometer working at 360 MHz (1H), 282 MHz
(19F) and 202 MHz (31P) in CDCl3 solutions at ambient tem-
perature. Chemical shifts are given in ppm relative to TMS
(1H, d = 0), H3PO4 (31P, d = 0) and CFCl3 (19F, d = 0). Since
all reactions were conducted similarly, only a general proce-
dure is described.
1
d = 31.3 ppm, d, JRh–P = 150.1 Hz.
2.3.4. [RhCl(g4-COD){P(4-FC6H4)3}] (4)
Yield 93%; yellow. Anal. Calc. for C26H24ClF3PRh: C,
55.49; H, 4.30. Found: C, 55.84; H, 4.29%. 1H NMR,
CDCl3 d (ppm); 7.70 (m, 6H, CH-phosphine), 7.10 (m,
6H, CH-phosphine), 5.60 (m, 2H, COD-CH trans-P),
3.08 (m, 2H, COD-CH trans-Cl), 2.45 (m, 4H, COD-
CH2), 2.20–1.90 (m, 4H, COD-CH2). 31P NMR, CDCl3
1
d = 29.5 ppm, d, JRh–P = 152.6 Hz. 19F NMR, CDCl3,
d = ꢀ109.4 ppm.
2.3.5. [RhCl(g4-COD){P(4-CF3C6H4)3}] (5)
Yield 97%; yellow. Anal. Calc. for, C29H24ClF9PRh: C,
48.86; H, 3.39. Found: C, 48.56; H, 3.08%. 1H NMR,
CDCl3 d (ppm); 7.84 (m, 6H, CH-phosphine), 7.68 (m,
6H, CH-phosphine), 5.69 (m, 2H, COD-CH trans-P),
3.11 (m, 2H, COD-CH trans-Cl), 2.44 (m, 4H, COD-
CH2), 2.20–1.95 (m, 4H, COD-CH2). 31P NMR, CDCl3
2.2. General procedure
1
d = 32.2 ppm, d, JRh–P = 154.1 Hz. 19F NMR, CDCl3, 4-
To a solution of the complex [{Rh(l-Cl)(g4-COD)}2]
(200 mg, 0.406 mmol) in acetone (10 cm3) were added two
equivalents of the corresponding phosphine. The color of
the solution quickly changed from orange to yellow. The
mixture was stirred at room temperature and checked for
completion by TLC. After that the solvent was removed
in vacuo. All complexes were isolated as crystalline solids.
CF3C6H4, d = ꢀ63.6 ppm, s.
2.3.6. [RhCl(g4-COD){P(4-ClC6H4)3}] (6)
Yield 82%; yellow. Anal. Calc. for C26H24Cl4PRh: C,
51.01; H, 3.95. Found: C, 51.62; H, 4.06%. 1H NMR,
CDCl3 d (ppm); 7.65 (m, 6H, CH-phosphine), 7.38 (m,
6H, CH-phosphine), 5.61 (m, 2H, COD-CH trans-P),
3.07 (m, 2H, COD-CH trans-Cl), 2.42 (m, 4H, COD-
CH2), 2.20–1.90 (m, 4H, COD-CH2). 31P NMR, CDCl3
2.3. Compounds characterization
1
d = 30.3 ppm, d, JRh–P = 152.8 Hz.
2.3.1. [RhCl(g4-COD)(P(4-OCH3C6H4)3)] (1)
Yield
89%;
orange-yellow.
Anal.
Calc.
for
2.3.7. [RhCl(g4-COD){P(3-OCH3C6H4)3}] (7)
C29H33ClO3PRh: C, 58.16; H, 5.55. Found: C, 58.06; H,
5.17%. H NMR, CDCl3 d (ppm); 7.64 (m, 6H, CH-phos-
Yield
87%;
orange-yellow.
Anal.
Calc.
for
1
C29H33ClO3PRh: C, 58.16; H, 5.55. Found: C, 58.66; H,
1
phine) 6.89 (m, 6H, CH-phosphine), 5.53 (m, 2H, COD-
CH trans-P), 3.81 (s, 9H, phosphine-OCH3), 3.12 (m, 2H,
COD-CH trans-Cl), 2.20 (m, 4H, COD-CH2), 2.10–1.90
(m, 4H, COD-CH2). 31P NMR, CDCl3 d = 27.7 ppm, d,
1JRh–P = 148.7 Hz.
5.57%. H NMR, CDCl3 d (ppm); 7.40 (m, 3H, CH-phos-
phine), 7.25 (m, 6H, CH-phosphine), 6.70 (m, 3H, CH-
phosphine), 5.57 (m, 2H, COD-CH trans-P), 3.77 (s, 9H,
phosphine-OCH3), 3.21 (m, 2H, COD-CH trans-Cl), 2.40
(m, 4H, COD-CH2), 2.15–1.85 (m, 4H, COD-CH2). 31P
1
NMR, CDCl3 d = 32.8 ppm, d, JRh–P = 149.9 Hz.
2.3.2. [RhCl(g4-COD){P(4-CH3C6H4)3}] (2)
Yield 99%; yellow. Anal. Calc. for C29H33ClPRh: C,
63.23; H, 6.04. Found: C, 64.00; H, 6.07%. 1H NMR,
CDCl3 d (ppm); 7.71 (m, 6H, CH-phosphine), 7.53 (m,
6H, CH-phosphine), 5.54 (m, 2H, COD-CH trans-P),
3.13 (m, 2H, COD-CH trans-Cl), 2.40 (m, 4H, COD-CH2),
2.3.8. [RhCl(g4-COD){P(3-CH3C6H4)3}] (8)
Yield 90%; orange-yellow. Anal. Calc. for C29H33ClPRh:
C, 63.23; H, 6.04. Found: C, 62.90; H, 5.95%. H NMR,
CDCl3 d (ppm); 7.54 (m, 6H, CH-phosphine), 7.25 (m, 6H,
CH-phosphine), 5.56 (m, 2H, COD-CH trans-P), 3.14
1

J. Tiburcio et al. / Polyhedron 25 (2006) 1549–1554
1551
Table 1
(m, 2H, COD-CH trans-Cl), 2.40 (m, 4H, COD-CH2), 2.35
(s, 9H, phosphine-CH3), 2.15–1.85 (m, 4H, COD-CH2).
31P NMR, CDCl3 d = 30.8 ppm, d, 1JRh–P = 149.4 Hz.
Crystal data for [RhCl(g4-COD){P(2-CH3C6H4)3}] (11)
Empirical formula
Formula weight
Color, habit
C29H33ClPRh
550.88
yellow regular prism
0.5 · 0.2 · 0.1
P21/n
8.2231(11)
17.575(3)
17.058(2)
91.334(11)
2464.5(6)
4
2.3.9. [RhCl(g4-COD){P(3-ClC6H4)3}] (9)
Crystal size (mm)
Space group
Yield 83%; yellow. Anal. Calc. for C26H24Cl4PRh: C,
51.01; H, 3.95. Found: C, 50.65; H, 4.02%. 1H NMR,
CDCl3 d (ppm); 7.63 (m, 6H, CH-phosphine), 7.40 (m,
6H, CH-phosphine), 5.66 (m, 2H, COD-CH trans-P),
3.11 (m, 2H, COD-CH trans-Cl), 2.45 (m, 4H, COD-
CH2), 2.20–1.95 (m, 4H, COD-CH2). 31P NMR, CDCl3
˚
a (A)
˚
b (A)
˚
c (A)
˚
b (A)
3
˚
V (A )
Z
1
Absorption coefficient (mmꢀ1
Radiation
)
0.882
d = 32.4 ppm, d, JRh–P = 153.2 Hz.
˚
Mo Ka (k = 0.71073 A)
4.64–45.98
4650
3417 (Rint = 3.01%)
0.804, 0.917
R1 = 4.09, wR2 = 8.47
R1 = 6.49, wR2 = 9.43
1.026
2.3.10. [RhCl(g4-COD){P(2-OCH3C6H4)3}] (10)
2h Range (ꢁ)
Reflections collected
Independent reflectionsa
Transmission factors (minimum, maximum)
Final R indices [2610 I > 2r(I)] (%)a
Final R indices [all data] (%)a
Goodness-of-fit, Sa
Yield 87%; yellow. Anal. Calc. for, C29H33ClO3PRh: C,
58.16; H, 5.55. Found: C, 57.89; H, 5.48%. 1H NMR,
CDCl3 d (ppm); 7.38 (m, 3H, CH-phosphine), 7.20 (m,
3H, CH-phosphine), 6.90 (m, 3H, CH-phosphine), 6.81
(m, 3H, CH-phosphine), 5.23 (m, 2H, COD-CH trans-P),
3.84 (m, 2H, COD-CH trans-Cl), 3.52 (s, 9H, phosphine-
Data-to-parameters ratio
3417/289
a
Rint, R1, wR2 and S are defined as follows:
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
OCH3), 2.35 (m, 4H, COD-CH2), 1.98 (m, 4H, COD-
P
P
P
2
jF 2o ꢀ hF 2oij
kF oj ꢀ jF ck
wðF 2o ꢀ F 2c Þ
1
CH2). 31P NMR, CDCl3 d = 16.6 ppm, d, JRh–P
=
P
Rint
¼
P
; R1 ¼
; wR2 ¼
;
P
F o2
2
wðF 2oÞ
jF oj
147.2 Hz.
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
wðF 2o ꢀ F 2cÞ
S ¼
:
2.3.11. [RhCl(g4-COD){P(2-CH3C6H4)3}] (11)
m ꢀ n
Yield 95%; yellow. Anal. Calc. for C29H33ClPRh: C,
1
63.23; H, 6.04. Found: C, 63.41; H, 5.92%. H and 31P
3. Results and discussion
3.1. Synthesis and characterization
NMR, not observed in solution, see text.
2.3.12. [RhCl(g4-COD){P(C6H5)2(C6F5)}] (12)
Yield 64%; yellow. Anal. Calc. for C26H22ClF5PRh: C,
52.15; H, 3.70. Found: C, 52.97; H, 3.72%. 1H NMR,
CDCl3 d (ppm); 7.75 (m, 4H, CH-phosphine), 7.45 (m,
6H, CH-phosphine), 5.55 (m, 2H, COD-CH trans-P),
3.38 (m, 2H, COD-CH trans-Cl), 2.40 (m, 4H, COD-
CH2), 2.15–1.80 (m, 4H, COD-CH2). 31P NMR, CDCl3
The reactions of the chloro-bridged complex [{Rh-
(l-Cl)(g4-COD)}2] with 12 different triarylphosphines in
acetone solutions afforded the mononuclear complexes
[RhCl(g4-COD)(PR3)], with R = 4-(OCH3)C6H4 (1), 4-
(CH3)C6H4 (2), C6H5 (3), 4-FC6H4 (4), 4-(CF3)C6H4 (5),
4-ClC6H4 (6), 3-(OCH3)C6H4 (7), 3-(CH3)C6H4 (8), 3-
ClC6H4 (9), 2-(OCH3)C6H4 (10), 2-(CH3)C6H4 (11) and
R3 = (C6H5)2(C6F5) (12) (Scheme 1).
1
d = 22.5 ppm, d, JRh–P = 151.9 Hz. 19F NMR, CDCl3,
d = ꢀ124.9 (m, 2F, ortho), ꢀ149.4 (dd, 1F, para), ꢀ160.3
(m, 2F, meta).
An equilibrium between mononuclear and dinuclear
species is rapidly achieved, with the mononuclear complex
existing as the predominant species [14].
2.4. X-ray crystallography
Compounds 1–12 are yellow or orange-yellow, stable
crystalline solids which are soluble in common solvents –
acetone, chloroform and dichloromethane. Analytical data
and NMR parameters are consistent with the given formu-
lae. IR spectra show absorptions characteristic of the 1,5-
cyclooctadiene and the substituted triarylphosphines
although they are not significant to the following discus-
sion. A summary of spectroscopic data for complexes
1–12 is presented in Table 2.
Single crystals of 11 were obtained by slow evaporation
of a saturated acetone solution. X-ray diffraction data were
measured at 298 K on a Siemens P4 diffractometer using
common procedures [12] and were corrected for absorption
effect by means of w-scans data. The structure was solved
and refined using standard procedures [13] without
restraints or constraints. H atoms were placed on idealised
positions and refined using a riding model with fixed isotro-
pic U. Crystal data are collected in Table 1.
Cl
Cl
Rh
2
Rh
Rh
+
2 PR 3
PR 3
Cl
Scheme 1. Reaction between [{Rh(l-Cl)(g4-COD)}2] and PR3.

1552
J. Tiburcio et al. / Polyhedron 25 (2006) 1549–1554
of a AA0BB0C magnetic system corresponding to ortho,
Table 2
Summary of spectroscopic data for complexes 1–12
meta and para fluorine nuclei of the PC6F5 moiety.
31P NMR spectra of compounds 1–12 exhibit signals
with chemical shifts spanning over ca. 5 ppm from 27.7
to 32.8 ppm except for compounds 10 (d = 16.63 ppm)
and 12 (d = 22.50 ppm). Each signal is a doublet due to
Complex
Phosphine
1H d (ppm)
CH
31P d
(ppm)
1JRh–P
(Hz)
1
2
P(4-OCH3C6H4)3
P(4-CH3C6H4)3
P(C6H5)3
P(4-FC6H4)3
P(4-CF3C6H4)3
P(4-ClC6H4)3
P(3-OCH3C6H4)3
P(3-CH3C6H4)3
P(3-ClC6H4)3
P(2-OCH3C6H4)3
P(2-CH3C6H4)3
P(C6H5)2(C6F5)
5.53, 3.12
5.54, 3.13
5.58, 3.15
5.60, 3.08
5.69, 3.11
5.61, 3.07
5.57, 3.21
5.56, 3.14
5.66, 3.11
27.7
29.3
31.3
29.5
32.2
30.3
32.8
30.8
32.4
148.7
149.3
150.1
152.6
154.1
152.8
149.9
149.4
153.2
3
1
103Rh–31P coupling with JRh–P between 147.7 and
4
154.05 Hz.
5
Although all complexes studied display expected simi-
larities, there is a feature which is worth noting. Despite
the close resemblance among the series of phosphines bear-
ing 4-, 3- and 2-OCH3C6H4 and 4-, 3- and 2-CH3C6H4 sub-
stituents, only [RhCl(g4-COD)(P(2-CH3C6H4)3)2] (11)
seems unable to remain as stable mononuclear complex
in solution and rearranges to the starting materials
[{Rh(l-Cl)(g4-COD)}2] and P(2-CH3C6H4)3. The differ-
ence between both ortho-substituted phosphines with 2-
CH3C6H4 and 2-OCH3C6H4, seems the larger flexibility
of the methoxy group compared with the methyl moiety.
Molecular models indicate a considerable steric hindrance
with 2-CH3C6H4 which is released when the methyl group
is free to rotate around the oxygen atom. A previous work
has proved the difference in reactivity between iridium
complexes when two of these phosphines are bounded to
the metal centre [16].
6
7
8
9
10
11
12
a
5.23, 3.84
a
16.6
a
147.2
a
5.55, 3.38
22.5
151.9
Not observed in solution.
Equivalent experiments using P(C6H5)(C6F5)2 or
P(C6F5)3 failed to react, even when a reflux or excess of
phosphine were used, probably reflecting a considerable
reduction of the basicity of the phosphorous atom impair-
ing its ability to act as a base (see Table 3).
Proton NMR spectra of compounds 1–12 show the
expected different resonances for methine (CH) and meth-
ylene (CH2) protons of the cyclooctadiene ring, the aro-
matic protons and, for complexes 1, 2, 7, 8 and 10, the
methyl protons of the phosphine.
3.2. 31P NMR spectroscopic analysis
Two different sets for the CH protons of the cyclooctadi-
ene were observed, arising from two different trans substit-
uents at the metallic centre [15]. The overall range for
complexes 1–12 is Dd = 3.84–3.07 ppm for the trans-Cl
CH absorptions and Dd = 5.69–5.23 ppm for the trans-P.
However, these resonances are rather insensitive to the nat-
ure of the phosphine in trans position.
Fluorine NMR spectra of compounds 4 (P(4-FC6H4)3
with a AA0BB0X magnetic system), and 5 (P(4-CF3C6H4))
exhibit single absorptions. 19F NMR spectrum of
[RhCl(g4-COD)(P(C6H5)2(C6F5))] (12), show three signals
1
Correlations between experimental JRh–P or 31P chemi-
cal shifts from the coordination compounds and electronic
properties of the phosphine ligands could be useful to ana-
lyse the metallic environment of these complexes. Chemical
shifts of free phosphines P(4-XC6H4)3 (X = OCH3, CH3,
H, F, Cl and CF3) show a tendency almost parallel to the
corresponding 31P d in their rhodium compounds, suggest-
ing that electronic shielding is affected similarly in all com-
pounds upon complexation.
As shown in Fig. 1, isosteric phosphines, according to
their cone angle (h = 145ꢁ), show an inverse relation
between their electronic properties, as accounted by Gier-
ingꢀs electronic parameter v, and the experimental magnetic
coupling constant 1JP–Rh. This is a similar behaviour to the
previously reported for Pt(II)–P [9], and Pt(0)–P complexes
[10], and it can be rationalized in terms of a reduction on
the phosphorus covalent radius upon the addition of elec-
tron-withdrawing groups on the phenyl rings, considering
these triarylphosphines as exclusively r-donors [6a].
Table 3
Summary of electronic and steric parameters for the triphenylphosphines
employed in this study [6]
Phosphine
v
h (ꢁ)
P(4-OCH3C6H4)3
P(4-CH3C6H4)3
P(C6H5)3
10.5
11.5
13.3
15.7
20.5
145
145
145
145
145
P(4-FC6H4)3
P(4-CF3C6H4)3
P(4-ClC6H4)3
P(3-OCH3C6H4)3
P(3-CH3C6H4)3
P(3-ClC6H4)3
P(2-OCH3C6H4)3
P(2-CH3C6H4)3
P(C6H5)2(C6F5)
P(C6H5)(C6F5)2
P(C6F5)3
3.3. Crystal structure
16.8
a
145
a
11.3
19.6
a
148
148
a
Relevant bond distances and angles for complex 11 are
displayed in Table 4. The asymmetric unit contains one
molecule with all atoms on general positions (Fig. 2). Con-
sidering the COD molecule as a bidentate ligand, the
metallic center displays a slightly distorted square planar
coordination geometry found in numerous 16-electrons
Rh(I) complexes. The COD ligand approximates a tub
10.7
20.4
27.6
34.8
178
158
171
184
a
Data not available.

J. Tiburcio et al. / Polyhedron 25 (2006) 1549–1554
1553
155
154
153
152
151
150
149
148
R2 = 0.954
10
12
14
16
18
20
Electronic Parameter
χ
Fig. 1. Graph of the electronic parameter v for the isosteric phosphines
1
P(4-XC6H4)3 (X = OCH3, CH3, H, F, Cl, and CF3) and JP–Rh of
[RhCl(g4-COD)(P(4-XC6H4)3)] (1–6).
Fig. 2. X-ray structure of 11, showing displacement ellipsoids at the 30%
probability level. H atoms have been omitted for clarity.
Table 4
Selected bond lengths (A) and angles (ꢁ) for [RhCl(g4-COD)-
˚
(P(2-CH3C6H4)3)] (11)
phine, complex 4 [20]. In the former case, the trans
˚
Bond length (A)
influence induced by the triphenylphosphine is character-
Rh1–C22
Rh1–C26
Rh1–P1
P1–C15
P1–C8
2.124(5)
2.182(5)
2.3607(14)
1.838(5)
1.849(5)
1.362(8)
Rh1–C29
Rh1–C25
Rh1–Cl1
P1–C1
2.138(5)
2.206(6)
2.3773(14)
1.840(5)
1.384(8)
˚
ized by a lengthening of coordination of ca. 0.10 A for
Rh–C trans to the phosphine, and for the latter, the calcu-
˚
lated effect is of ca. 0.11 A. These strong trans influences
yield, as in the previous case, unbalanced double bond
lengths for the COD moiety, for example 1.363 and
C22–C29
C25–C26
˚
1.440 A in 4.
Bond angle (ꢁ)
C22–Rh1–C29
C29–Rh1–C26
C29–Rh1–C25
C22–Rh1–P1
C26–Rh1–P1
C22–Rh1–Cl1
C26–Rh1–Cl1
P1–Rh1–Cl1
The bulkiness of the phophine is evident from the Rh–P
37.9(2)
80.5(2)
87.6(2)
95.32(16)
160.13(19)
155.73(18)
88.64(17)
88.15(5)
C22–Rh1–C26
C22–Rh1–C25
C26–Rh1–C25
C29–Rh1–P1
C25–Rh1–P1
C29–Rh1–Cl1
C25–Rh1–Cl1
95.6(2)
79.8(2)
36.2(2)
98.40(16)
163.37(17)
164.61(18)
90.02(16)
˚
distance, significantly larger in the case of 11 [2.3607(14) A]
˚
compared to 3 and 4 [2.308 and 2.297 A, respectively]. It is
obvious that 3 and 4 present similar Tolmanꢀs cone angles,
h ꢁ 145ꢁ, while 11, for which includes an ortho substituent,
should have a significantly larger cone angle (h ꢁ 178ꢁ).
Assuming, from purely steric considerations, that a smaller
cone angle should result in a better ligand, a stronger phos-
phine coordination is expected for 3 and 4 compared to 11,
as actually observed in Rh–P bond distances. This also
explains the lability of complex 11 in solution, which rap-
idly reorganizes to the dinuclear complex and the free
phosphine.
conformation (D2d local symmetry) also found in the vast
majority of COD-containing complexes. The geometry
for the tri-o-tolylphosphine is unaffected by coordination
and remains very close to that of the free ligand [17]. The
main structural feature for the solid state structure of 11
is the significant trans influence of the phosphine: Rh–C
bond lengths trans to the P atom are lengthened by ca.
4. Conclusions
˚
The addition of two equivalents of twelve different tria-
rylphosphines to solutions of the dinuclear complex
[{Rh(l-Cl)(g4-COD)}2] affords the mononuclear species
[RhCl(g4-COD)(PR3)]. However, no mononuclear species
could be detected in solution when electron-poor phos-
phines, like P(C6H5)(C6F5)2 and P(C6F5)3, or bulky phos-
phines, such as P(2-CH3C6H4)3, were employed. The
solid state structure for the complex [RhCl(g4-COD)-
(P(2-CH3C6H4)3)] (11) displays a long Rh–P bond distance,
compared to [RhCl(g4-COD){P(C6H5)3}] (3) and
0.06 A by comparison of Rh–C bond lengths trans to the
Cl atom (see Table 4). This effect also induces non-equiva-
lent double bonds in the COD moiety, the shortest bond
length, C25–C26 = 1.362(8) A, being trans to the phos-
phine, while the C22–C29 bond, 1.384(8), which is located
trans to the chloride. However, both double bonds are still
shorter than in free COD [18].
A similar behaviour was previously observed in two
directly related structures, namely those containing triphe-
nylphosphine, complex 3 [19], and tri(p-fluorophenyl)phos-
˚

1554
J. Tiburcio et al. / Polyhedron 25 (2006) 1549–1554
[RhCl(g4-COD){P(4-FC6H4)3}] (4), which accounts for the
lability of this complex in solution. Solution 31P NMR data
for complexes with isosteric phosphines 1–6, reveals that
electron-withdrawing groups on the phenyl rings increases
the 1JP–Rh, in a similar fashion to that observed for Pt com-
pounds indicating comparable bonding characteristics.
(b) V. Cˆırcu, M.A. Fernandes, L. Carlton, Polyhedron 22 (2003)
3293;
(c) S.A. Serron, L. Luo, C. Li, M.E. Cucullu, E.D. Stevens, S.P.
Nolan, Organometallics 14 (1995) 5290;
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(1990) 1758;
5. Supplementary data
(b) M.R. Wilson, H. Liu, A. Prock, W.P. Giering, Organometallics
12 (1993) 2044;
Crystallographic data for compound 11 has been depos-
ited with the Cambridge Crystallographic Data Centre;
CCDC 273550. Copy of this information can be obtained
free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
(c) M.R. Wilson, A. Prock, W.P. Giering, A.L. Fernandez, C.M.
Haar, S.P. Nolan, B.M. Foxman, Organometallics 21 (2002) 2758.
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Madison, WI, USA, 1991.
Acknowledgements
We are grateful to DGAPA-UNAM-IN116001 for
´
financial support. J.T. thanks CONACYT-Mexico for a
[13] G.M. Sheldrick, SHELXTL-Plus, Release 5.10, Siemens Analytical X-
ray Instruments Inc., Madison, WI, USA, 1998.
scholarship.
[14] (a) C.A. Ogle, T.C. Masterman, J.L. Hubbard, J. Chem. Soc., Chem.
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