Wide bite angle diphosphine rhodium complexes: Synthesis, structure, and catalytic 1,4-addition of arylboronic acids to enones

Article abstract of DOI:10.1016/j.jorganchem.2007.09.034

A rhodium complex [ClRh(CO)(L1)] featuring a wide bite angle diphosphine ligand (L1 = 1,3-bis(2-diphenylphosphinomethylphenyl)benzene) has been synthesized and structurally characterized. L1 supports a bite angle (P-M-P angle, β) of 171.4° in the trans-square planar complex. L1 was tested in Rh-catalyzed 1,4-addition reactions of arylboronic acids (six examples) to α,β-unsaturated ketones (five examples). In mixed aqueous/cyclohexane solution at 60 °C, addition reactions proceed in up to quantitative yield with a 1:1 arylboronic acid/enone ratio. Yields as high as 77% can be acquired even when one of the coupling partners is sterically encumbered 2,4,6-trimethylphenylboronic acid.

Full text of DOI:10.1016/j.jorganchem.2007.09.034

Available online at www.sciencedirect.com
Journal of Organometallic Chemistry 693 (2008) 11–16
www.elsevier.com/locate/jorganchem
Wide bite angle diphosphine rhodium complexes: Synthesis,
structure, and catalytic 1,4-addition of arylboronic acids to enones
*
Brad P. Morgan, Rhett C. Smith
Department of Chemistry and Center for Optical Materials Science and Engineering Technologies (COMSET),
Clemson University, Clemson, SC 29634, United States
Received 13 July 2007; received in revised form 27 September 2007; accepted 28 September 2007
Available online 12 October 2007
Abstract
A rhodium complex [ClRh(CO)(L1)] featuring a wide bite angle diphosphine ligand (L1 = 1,3-bis(2-diphenylphosphinomethylphe-
nyl)benzene) has been synthesized and structurally characterized. L1 supports a bite angle (P–M–P angle, b) of 171.4ꢀ in the trans-square
planar complex. L1 was tested in Rh-catalyzed 1,4-addition reactions of arylboronic acids (six examples) to a,b-unsaturated ketones (five
examples). In mixed aqueous/cyclohexane solution at 60 ꢀC, addition reactions proceed in up to quantitative yield with a 1:1 arylboronic
acid/enone ratio. Yields as high as 77% can be acquired even when one of the coupling partners is sterically encumbered 2,4,6-
trimethylphenylboronic acid.
ꢁ 2007 Elsevier B.V. All rights reserved.
Keywords: Wide bite angle diphosphine; Rhodium; Conjugate addition; Arylboronic acid
1. Introduction
dium-catalyzed hydroformylation, where wide bite angles
(greater than ꢀ110ꢀ) can enhance activity and regioselectiv-
ity [16–18]. A number of scaffolds have been subsequently
pursued to support wide bite angle diphosphines for use
in catalysis [13,15,16,19–26]. Among the most well known
wide bite angle diphosphines are those of the xantphos
family (Fig. 1a). These ligands support high activity rho-
dium catalysts for hydroformylation, even with challenging
substrates such as internal olefins. Xantphos has a natural
bite angle of 111.7ꢀ [18], very close to the average P–M–P
angle for all Xantphos complexes crystallographically char-
acterized (104.6ꢀ), although a trans-spanning mode (P–M–
P angle = 150.7ꢀ) is also attainable [27].
We are interested in flexible scaffolds for wide bite angle
phosphines. Flexibility may be beneficial when a catalytic
process requires a ligand to accommodate different geome-
tries as its metal complex rearranges to form intermediates
in the course of the mechanistic cycle. The trans-spanning
‘‘terphspan’’ diphosphine L1 (Fig. 1b), for example, is built
on a m-terphenyl scaffold capable of accommodating a
range of bite angles. Crystallographically characterized
pseudo-square planar palladium and nickel complexes
Catalytic processes mediated by transition metals are
dramatically impacted by supporting ligands. Phosphines
have been subject to particular scrutiny in this regard due
to their utility in numerous functional group tolerant
C–C bond-forming reactions. Consequently, a number of
parameters are used to assist in the classification of phos-
phine properties most pertinent to their role in catalysis.
Tolman cone angles [1] and molecular mechanics are used
to quantify phosphine sterics [2,3] and various scales have
been forwarded to describe phosphine electronics [4–11].
More recently, widespread application of bidentate
P-donor ligands resulted in the introduction of a bite
angle (P–M–P angle, b) [12] parameter that is applied to
predict coordination modes. A definite correlation between
bite angle and catalytic activity has been observed for
numerous industrially important reactions [13–15]. This
relationship has been particularly well studied for rho-
*
Corresponding author. Tel.: +1 864 656 6112.
E-mail address: rhett@clemson.edu (R.C. Smith).
0022-328X/$ - see front matter ꢁ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2007.09.034

12
B.P. Morgan, R.C. Smith / Journal of Organometallic Chemistry 693 (2008) 11–16
Table 1
Crystal data and structure refinement details for 1 Æ DMF
Empirical formula
Formula weight (g/mol)
Temperature (K)
Wavelength (A)
Crystal system
Space group
C45H36ClOP2Rh Æ C3H7NO
866.13
153 (2)
0.71073
Monoclinic
P21/c
˚
Unit cell dimensions
˚
a (A)
18.623(4)
11.779(2)
18.629(4)
90.00
98.05(3)
90.00
4046.1(14)
4
1.422
Fig. 1. Examples of wide bite angle diphosphines.
˚
b (A)
˚
c (A)
a (ꢀ)
b (ꢀ)
c (ꢀ)
feature L1 in a trans-spanning binding mode [28]. These
complexes show high activity and regioselectivity in Suzuki
and Heck coupling reactions, and cyclometallated pallada-
cycle variations with C2 symmetry are potential candidates
for use in asymmetric catalysis [29,30].
3
˚
Volume (A )
Z
Calculated density (Mg/m3)
Absorption coefficient (mmÀ1
F(000)
)
0.608
1784
Herein we describe the first rhodium complex employing
this new class of wide bite angle phosphine and subsequent
use of L1 to support Rh-catalyzed 1,4-conjugate addition
of aryl boronic acids to a,b-unsaturated ketones (Table 3).
Crystal size (mm)
Crystal color and shape
h range for data collection (ꢀ) 2.05–25.10
0.36 · 0.29 · 0.14
Yellow plate
Limiting indices
À22 < h < 20, À14 < k < 14,
À22 < l < 22
2. Results and discussion
Reflections collected
Independent reflections
Completeness to h
Maximum transmission
Minimum transmission
Refinement method
Data/restraints/parameters
Goodness of fit on F2
Final R indices (I > 2r(I))
R1
7182
6016
25.10 (99.6%)
2.1. Synthesis and characterization
0.9197
0.8108
The first terphspan rhodium complex targeted for syn-
thesis was [ClRh(CO)(L1)] (1). A convenient one-pot
method, previously described for the preparation of
trans-[ClRh(CO)(P(t-Bu)2H)2] [31], was selected for the
preparation of 1. The complex 1 was readily prepared in
43% yield by reacting RhCl3 Æ 3H2O with N,N-dimethyl-
formamide (DMF) in the presence of L1, requiring gener-
ation of CO by thermal decomposition of the DMF.
Compound 1 exhibits a C–O stretch in the IR spectrum
at mCO = 1966 cmÀ1, which compares well with the values
observed in trans-[ClRh(CO)(PR3)2] complexes such as
Full-matrix least-squares on F2
7182/1/666
1.144
0.0489
0.1182
wR2
R indices (all data)
R1
wR2
0.0604
0.1273
1
The H NMR spectrum of [ClRh(CO)(L1)] Æ DMF in
trans-[ClRh(CO)(PPh3)2] (1960 cmÀ1
[ClRh(CO)(P(t-Bu)2H)2] (1965 cmÀ1) [32].
) [31] and trans-
CDCl3 at room temperature is well-resolved, in contrast
1
to the H NMR spectra of [PdCl2(L1)], which exhibited
broad peaks due to a fluxional process [28]. This observa-
tion indicates that L1 is more rigid (on the NMR timescale)
in the Rh complex than in the Pd complex, despite very
similar geometries about the metal centers in the two cases.
Another noteworthy feature of the 1H NMR spectrum of 1
is the distribution of aromatic resonances corresponding to
protons on the central terphenyl ring (d 6.18–8.04 ppm) as
a result of interaction between the ring’s p-electron cloud
and the metal center. A similar effect was noted in the Pd
and Ni complexes of L1 [28]. The metal center for 1 sites
˚
˚
3.37 A from H(1), even closer than in Pd (3.48 A) and Ni
˚
(3.51 A) complexes, but is not within the sum of van der
Waals radii for C (1.20 A) and Rh (2.00 A).
˚
˚
The difference in 31P NMR chemical shift (Dd) between
free and complexed states can also be used to assess
whether the proposed complex has formed in the desired
coordination environment [33]. The Dd between L1
(À8.8 ppm) and 1 (27.3 ppm) is 36.1 ppm, in close agree-
ment with Dd (37.3 ppm) observed for PPh3 (À4.8 ppm
Fig. 2. ORTEP drawing (30% probability ellipsoids) of the molecular
structure of 1. Hydrogen atoms and the cocrystallized DMF solvent
molecule are omitted for clarity.

B.P. Morgan, R.C. Smith / Journal of Organometallic Chemistry 693 (2008) 11–16
13
Table 2
and coworkers in 1997 [40]. Among the phosphines screened
in Miyaura’s study, the greatest activity was observed
with dppb (1,4-bis(diphenylphosphino)butane), a flexible
diphosphine capable of supporting large bite angle com-
plexes. This observation suggests that a similarly disposed
phosphine such as L1 could exhibit similarly high activity.
We thus selected a set of six arylboronic acids and five
enones (Table 3) that would provide a breadth of steric
and electronic diversity upon which future studies could
be based.
Reaction conditions were initially selected to mimic
those in the aforementioned study by Miyaura [40] to facil-
itate 1:1 correlation of results. The results of these trials,
carried out at 60 ꢀC in 6:1 cyclohexane/water with a 2:1
arylboronic acid/enone ratio, are summarized in Table 3.
Under these conditions, reaction yields (determined by
GC) are as good as or better than those observed with
dppb [40]. An isolated yield of 58% was also determined
for coupling of 2-cyclohexene-1-one and phenylboronic
acid. Although stereogenic centers are present in all cou-
pling products, the yields provided in Table 3 refer to mix-
tures of isomers; no stereoselectivity is expected employing
achiral L1 complexes.
˚
Select bond lengths (A) and angles (ꢀ) for 1 Æ DMF
Rh–Cl
Rh–P(1)
Rh–P(2)
Rh–C(45)
C(45)–O(1)
P(1)–Rh–P(2)
P(1)–Rh–Cl
P(1)–Rh–C(45)
P(2)–Rh–Cl
P(2)–Rh–C(45)
Cl–Rh–C(45)
2.3839 (5)
2.3146 (4)
2.3228 (4)
1.7978 (4)
1.1515 (3)
171.372 (10)
87.887 (10)
91.104 (11)
89.332 (9)
90.819 (11)
174.048 (16)
free) vs. trans-[ClRh(CO)(PPh3)2] (32.5 ppm) [32]. The
P–Rh coupling constant for 1 (137 Hz) is fairly low and
diagnostic for the expected trans-diphosphine complex.
The JRhÀP for trans-diphosphine Rh(I) complexes
(ꢀ140–145 Hz) are typically smaller than in corresponding
cis-isomers (ꢀ190–195 Hz), an effect that has been
attributed to the trans influence, though the electronegativ-
ity of the Cl and CO also contribute to a lower coupling
constant [34–39].
2.2. Structure
One of the drawbacks of the 1,4-addition reaction has
been that excess arylboronic acid is often necessary to reach
acceptable yields. Having accomplished high yields in some
of the initial trials, we attempted the same set of reactions
with a 1:1 arylboronic acid/enone ratio and expanded our
reactions to include the sterically encumbered 2,4,6-trim-
ethyphenylboronic acid as a coupling partner (Table 3).
Similar yields (within 11%) were observed in 10 out of the
25 trials as for those employing the 2:1 ratio, with depressed
yields generally limited to the more sterically encumbered
enones 4-phenyl-3-butene-2-one and phenyl styryl ketone.
Coupling of encumbered 2,4,6-trimethyphenylboronic acid
was moderately successful only with 3-octene-2-one (47%)
and 2-octene-4-one (77%), while yields were low (6–31%)
for the other enones. These data indicate that, for less steri-
cally encumbered enones, the terphspan scaffold holds
promise for elaboration to more commercially important
asymmetric coupling reactions by modification of the terph-
span scaffold with chiral phosphine moieties.
X-ray quality crystals of [ClRh(CO)(L1)] (Fig. 2) were
obtained by slowly cooling a saturated DMF solution from
60 ꢀC to room temperature overnight. Somewhat lower
quality, DMF-free crystals were obtained by diffusion of
tetramethylsilane into a saturated CH2Cl2 solution of 1.
Crystal data and structure refinement details for 1 Æ DMF
are provided in Table 1, and select bond lengths and angles
are provided in Table 2. The salient feature of interest, the
P–M–P angle of 171.4ꢀ, is quite similar to those observed in
[PdCl2(L1)] and [NiCl2(L1)] (173.0ꢀ and 174.8ꢀ, respec-
tively) [28]. Another feature of note is the selective forma-
tion of the regioisomer in which the carbonyl ligand is
directed into the more sterically crowded cleft under the
C(1) site of the terphenyl scaffold, while the more volumi-
nous chloride substituent resides in the open space under
the central terphenyl ring.
2.3. Catalysis
2.4. Concluding remarks
Having characterized a thermally stable rhodium com-
plex supported by L1 and reaffirmed a trans-spanning bind-
ing mode, we sought to investigate the use of L1 to support
Rh-catalyzed C–C bond formation. The first catalytic reac-
tion investigated was the 1,4-conjugate addition of aryl
boronic acids to a,b-unsaturated ketones (Scheme 1)
[41–66]. The use of Rh complexes to catalyze this reaction
has been aggressively pursued since the report by Miyaura
We have prepared and characterized the first terphspan
rhodium complex (1) in which a trans-spanning mode sim-
ilar to that observed in Ni and Pd complexes was observed
by X-ray diffraction. The utility of terphspan ligand L1 in
rhodium-catalyzed 1,4-addition of aryl boronic acids to
a,b-unsaturated ketones was examined, and moderate to
Scheme 1.

14
B.P. Morgan, R.C. Smith / Journal of Organometallic Chemistry 693 (2008) 11–16
Table 3
Catalytic 1,4-addition of boronic acids to a,b-unsaturated enones
Boronic acid:Enone
2:1
89
1:1
88
2:1
68
1:1
32
2:1
95
1:1
41
2:1
63
1:1
25
2:1
23
1:1
20
1:1
77
91
84
98
90
58
90
46
83
98
98
ꢀ100
ꢀ100
ꢀ100
32
46
ꢀ100
13
38
47
24
9
97
98
93
77
70
34
41
23
36
83
98
36
40
62
41
30
24
36
36
27
26
6
31
Conditions are described in the Section 2.
excellent yields were observed with many substrate pairs
even when a 1:1 arylboronic acid/enone ratio was
employed. Efforts are currently underway to modify L1
for asymmetric catalysis and to extend the use of terphspan
phosphines to other catalytic C–C bond-forming reactions.
compound (0.196 g, 43.1%). An analytically pure sample
of 1 Æ DMF was obtained by crystallization via slowly cool-
ing a saturated DMF solution from 60 ꢀC to room temper-
1
ature overnight. IR: (CH2Cl2) mCO = 1966 cmÀ1. H NMR
(500 MHz, CDCl3): d 2.88 (s, 3H, from cocrystallized
DMF), 2.94 (s, 3H, from cocrystallized DMF), 3.36–3.39
(m, 2H), 4.82 (d, 2H, J = 12 Hz), 6.16 (d, 2H, J = 8 Hz),
6.79–6.83 (m, 6H), 7.05 (t, 4H, J = 8 Hz), 7.20–7.45 (m,
15H), 7.78–7.79 (m, 4H), 8.02 (s, 1H), 8.04 (s, 1H from
cocrystallized DMF). 31P NMR (121.4 MHz, CDCl3): 27.4
(d, JRh–P = 137 Hz). 13C NMR (125.7 MHz, CDCl3): 31.4
(m, CH2 partially overlapping cocrystallized DMF), 36.4
(from cocrystallized DMF), 126.2, 126.6, 127.1 (t,
J = 5 Hz), 127.7 (t, J = 5 Hz) 128.5, 128.9, 129.5, 129.7,
130.1, 131.6, 132.3, 133.5 (t, J = 6 Hz), 134.1 (t, J = 6 Hz),
142.0, 143.8, 162.5 (from cocrystallized DMF), (carbonyl
not observed). Anal. Calc. for C45H36ClOP2Rh: C, 68.15;
H, 4.58; N, 0.00. Found: C, 67.84; H, 4.36; N < 0.5%.
3. Experimental
3.1. General
All synthesis was carried out in an inert atmosphere of
nitrogen using an MBraun dry box or with standard
Schlenck techniques. Ligand L1 [28] and Rh(CO)2(dipiva-
loylmethanoate) [67] were prepared as reported previously.
Solvents were purified by passage through alumina col-
umns under a N2 atmosphere employing an Mbraun sol-
vent purification system. All other reagents were used as
received from TCI and Alfa Aesar. 31P NMR spectra were
collected using a Bruker Avance 300 instrument operating
at 121.4 MHz, while proton and 13C spectra were collected
using a Bruker Avance 500 instrument operating at
500 MHz for proton and 125.7 MHz for carbon. Infrared
spectra were collected using a Thermo Nicolet IR-100
FT-IR utilizing the EZ OMNIC analysis program. Gas
Chromatographs and Mass Spectra were collected using
a Shimadzu GCMS-QP2010 and analyzed using the
GCMS solution software.
3.3. General conditions for catalysis
To a preloaded conical vial containing the arylboronic
acid (0.50 or 1.0 mmol) were sequentially added a solution
of Rh(dpm)(CO)2 (0.015 mmol) in 1 mL cyclohexane and
L1 (0.015 mmol in 1 mL 9:1 cyclohexane/CH2Cl2). This
mixture was allowed to stir at room temperature for
15 min prior to addition of the enone (0.50 mmol) solution
in 1 mL cyclohexane followed by 0.5 mL of water. The
mixture was heated to 60 ꢀC with stirring for 16 h. The
mixture was collected and diluted with 10 mL THF for
immediate GC/MS analysis.
3.2. Preparation of [ClRh(CO)(L1)] (1)
A solution of RhCl3 Æ 3H2O (0.150 g, 0.570 mmol) and
L1 (0.393 g, 0.627 mmol) in 15 mL of N,N-dimethylform-
amide (DMF) was heated to 120 ꢀC under nitrogen for
2.5 h. The resultant yellow solution was reduced to half its
volume by vacuum distilling away some of the solvent.
The solution was then cooled to 0 ꢀC and 20 mL of cold
methanol was added to produce a yellow precipitate. The
precipitate was collected by filtration, washed with cold
methanol three times and dried in vacuo to yield the target
3.4. X-ray crystallography
Intensity data were collected using a Rigaku Mercury
CCD detector and an AFC8S diffractometer. Data reduc-
tion and absorbance corrections were achieved using Crys-
talClear [68]. The structure was solved by direct methods
and subsequent Fourier difference techniques, and refined

B.P. Morgan, R.C. Smith / Journal of Organometallic Chemistry 693 (2008) 11–16
15
anisotropically, by full-matrix least squares, on F2 using
SHELXTL 6.10 [69]. Hydrogen atom positions were calcu-
lated from the Fourier difference and then refined. C(13)–
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metallics 24 (2005) 335.
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Protasiewicz, Inorg. Chem. 46 (2007) 5220.
˚
H(13B) was constrained to 1.083 A.
[30] L. Ma, R.A. Woloszynek, W. Chen, T. Ren, J.D. Protasiewicz,
Organometallics 25 (2006) 3301.
Acknowledgement
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This work was funded by ACS-PRF Grant #45992-G3.
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Appendix A. Supplementary material
CCDC 653769 contains the supplementary crystallo-
graphic data for the compound 1 Æ DMF. These data can
be obtained free of charge from The Cambridge Crystallo-
graphic 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.jorganchem.2007.09.034.
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