Ditopic ligands featuring [P,S], [P,P] or [P,B] chelating pockets housed on a protected o-hydroquinone core

Article abstract of DOI:10.1016/j.jorganchem.2012.10.032

1,2-Dibromo-4,5-dimethoxybenzene was explored as a platform for the buildup of ditopic ligands. Low temperature reactions with n-butyllithium followed by quenches with various electrophiles led to the syntheses (2-bromo-4,5- dimethoxyphenyl)diphenylphosphine (1), (4,5-dimethoxy-2-(methylthio)phenyl) diphenylphosphine (2), 1,2-bis(diphenylphosphino)-4,5-dimethoxybenzene (3), and (2-(dicyclohexylboryl)-4,5-dimethoxyphenyl)diphenylphosphine (4). Oxidation of 3 produced 1,2-bis(diphenylphosphoryl)-4,5-dimethoxybenzene (5). All compounds were characterized by multinuclear (¹H/¹³C/ ³¹P{¹H}) NMR spectroscopy and by high resolution mass spectrometry. In addition, single crystal X-ray diffraction characterizations of 1, 3, 4, and 5 were also carried out. Reactivity of o-phosphinophenylborane 4 toward molecular hydrogen was also probed.

Full text of DOI:10.1016/j.jorganchem.2012.10.032

Journal of Organometallic Chemistry 724 (2013) 45e50
Contents lists available at SciVerse ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Ditopic ligands featuring [P,S], [P,P] or [P,B] chelating pockets housed on
a protected o-hydroquinone core
Rose Chuong a, Kyle A. Luck b, Rudy L. Luck b, Lillian P. Nguyen a, Diane Phan a, Louis R. Pignotti b,
,
a
Eugenijus Urnezius a , Edward J. Valente
*
a Department of Chemistry, University of Portland, 5000 N Willamette Blvd., Portland, OR 97203, USA
b Department of Chemistry, Michigan Technological University, 1400 Townsend Dr., Houghton, MI 49931, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
1,2-Dibromo-4,5-dimethoxybenzene was explored as a platform for the buildup of ditopic ligands. Low
temperature reactions with n-butyllithium followed by quenches with various electrophiles led to the
syntheses (2-bromo-4,5-dimethoxyphenyl)diphenylphosphine (1), (4,5-dimethoxy-2-(methylthio)phenyl)
diphenylphosphine (2),1,2-bis(diphenylphosphino)-4,5-dimethoxybenzene (3), and (2-(dicyclohexylboryl)-
4,5-dimethoxyphenyl)diphenylphosphine (4). Oxidation of 3 produced 1,2-bis(diphenylphosphoryl)-4,5-
dimethoxybenzene (5). All compounds were characterized by multinuclear (1H/13C/31P{1H}) NMR spec-
troscopy and by high resolution mass spectrometry. In addition, single crystal X-ray diffraction character-
izations of 1, 3, 4, and 5 were also carried out. Reactivity of o-phosphinophenylborane 4 toward molecular
hydrogen was also probed.
Received 8 August 2012
Received in revised form
18 October 2012
Accepted 18 October 2012
Keywords:
Redox-active ligands
Bisphosphine
Phosphine/hetero-donor chelates
o-Phosphinophenylborane
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
investigated, but the studies have been mostly limited to mono-
dentate o-hydroquinoneePR2 derivatives [20e23].
Polydentate phosphine ligands are ubiquitous in coordination
chemistry [1]. Phosphine groups (ePR2) are also common building
blocks for constructing chelating ligand architectures, particularly
when combined with other donor functionalities [2e5]. More
recently chelating phosphine compounds where the ePR2 group is
adjacent to electron accepting borane eBR2 functionality have been
characterized. Various derivatives of o-phosphinophenylboranes
have attractedattention asligands forcoordination chemistry [6e9],
as units for small molecule activation [10e13], and as platforms for
developing new catalysts [14e16]. The properties of the phosphine
ligand are usually set at the stage of synthesis, by varying the nature
of the substituents on the phosphorus center. Possibilities for post-
synthetic modifications of steric or electronic properties are rather
limited. One avenue for such modification is available if ePR2 group
is mounted onto suitable redox active platforms. Thus, tetrathia-
fulvalenes [17,18] and ferrocene [19] appended by phosphines have
been successfully utilized for the coordination chemistry of selected
transition metals. Another common redox-active molecular plat-
form suitable for such purposes is o-hydroquinone. Several phos-
phine derivatives bearing this functional group have been
Herein we report on the syntheses and characterizations of
several new phosphine derivatives where 1,2-dimethoxy-benzene
unit (a methyl-protected o-hydroquinone) is appended by adjacent
ePPh2/X groups (X ¼ eBr, eSCH3, eBCy2, or ePPh2). We obtained
(2-bromo-4,5-dimethoxyphenyl)diphenylphosphine (1) from 1,2-
dibromo-4,5-dimethoxybenzene. The compound was used to
synthesize (4,5-dimethoxy-2-(methylthio)phenyl)diphenylphos-
phine (2), 1,2-bis(diphenylphosphino)-4,5-dimethoxybenzene (3),
and (2-(dicyclohexylboryl)-4,5-dimethoxyphenyl)diphenylphos-
phine (4). Oxidation of 3 led to 1,2-bis(diphenylphosphoryl)-4,5-
dimethoxybenzene (5). Single crystal X-ray diffraction character-
izations of compounds 1, 3, 4, and 5$H2O are also reported.
2. Experimental
2.1. General procedures
All reactions and manipulations requiring exclusion of air were
carried out under nitrogen atmosphere, utilizing standard Schlenk
techniques involving a double manifold vacuum line, or in a glo-
vebox (Innovative Technologies). Low temperature (ꢀ100 ꢁC)
reactions were carried out in Schlenk flasks immersed into shallow
Dewar flasks filled with isopropanol/liquid N2. Once a required
temperature of the cooling mixture was attained, the flasks
* Corresponding author. Tel.: þ1 503 943 8592; fax: þ1 503 943 7784.
E-mail address: urnezius@up.edu (E. Urnezius).
0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2012.10.032

46
R. Chuong et al. / Journal of Organometallic Chemistry 724 (2013) 45e50
1
containing reaction mixtures were maintained at that temperature
for at least 1 h before the addition of other reagents. Diethyl ether,
THF, heptane, hexanes and pentane were distilled from Na/
benzophenone. Methylene chloride was distilled from CaH2.
Starting materials were purchased from commercial suppliers and
were used as received except for ClPPh2 (distilled in vacuum), and
dimethyldisulfide (dried over activated 4A molecular sieves).
NMR measurements were carried out on Varian INOVA or
Varian-MR spectrometers operating at 400 (1H), 100 (13C), and 161
1JCP ¼ 27 Hz), 128.6 (s), 120.5 (d, JCP ¼ 33 Hz), 117.4 (s), 117.3 (d,
2JCP ¼ 10 Hz), 55.6 (s), 54.9 (s). 31P NMR (acetone-d6)
ꢀ3.4 (s). HRMS
d
(ESI): 401.0315 (MHþ); calcd. for C20H19BrO2P (MHþ): 401.0306.
2.2.2. (4,5-Dimethoxy-2-(methylthio)phenyl)diphenylphosphine, 2
Compound 1 (2.690 g, 6.7 mmol) was dissolved in diethyl ether
(100 mL). The solution was cooled to 0 ꢁC, and a solution of n-BuLi
(1.6 M in hexanes, 4.2 mL, 6.72 mmol) was slowly added. The
reaction mixture became a creamy suspension, and was stirred at
0 ꢁC for 1 h. A solution of dimethyldisulfide (0.67 mL, 7.5 mmol) in
10 mL of diethyl ether was added, producing a yelloweorange
suspension. Reaction mixture was stirred overnight while warm-
ing to room temperature. Workup and purification were carried out
exposing the reaction mixture to air. (Caution! Reaction mixture
typically has very strong and unpleasant odor, thus it is best to
handle it in the fume hood.) All volatiles were removed under
reduced pressure (rotary evaporator), yielding a yellow oil. This was
dissolved in diethyl ether (20 mL); cooling the solution to ꢀ28 ꢁC
overnight produced a white solid 2 (1.109 g) which was isolated by
filtration. The volume of the filtrate was reduced in vacuum, and
repeated crystallization yielded another 0.405 g of compound 2.
Combined yield: 1.514 g (61.2%). Melting point: 67e69 ꢁC. 1H NMR
(
31P) MHz, respectively. 1H and 13C spectra were referenced to
tetramethylsilane; 31P{1H} spectra were referenced to 85% H3PO4 as
an external reference. High resolution mass-spectrometry
measurements were carried out at the Mass Spectrometry Facility
of Michigan State University (East Lansing, MI 48829-1319).
2.2. Syntheses and characterizations
2.2.1. (2-Bromo-4,5-dimethoxyphenyl)diphenylphosphine, 1
A
solution of 2.96 g (10 mmol) of 1,2-dibromo-4,5-
dimethoxybenzene in diethyl ether (100 mL) and THF (10 mL) was
cooled to ꢀ100 ꢁC, and a solution of n-BuLi (1.6 M in hexanes,
6.25 mL, 10 mmol) was slowly (over 15 min) added via syringe. The
reaction mixture (white suspension) was stirred for 1 h while the
temperature was maintained at ꢀ100 ꢁC. A solution of ClPPh2
(1.8 mL, 10 mmol) in 10 mL of diethyl ether was added over a period
of 5 min. The reaction mixture (yellow/orange solution) was stirred
while warming up to room temperature overnight. Workup and
purificationwerecarried outexposing the reaction mixtureto air. All
volatiles were removed under reduced pressure (rotary evaporator),
yielding a pale yellow waxy solid. Crystallization from boiling
ethanol (70 mL) yielded pure 1 as white needles. Selected crystals
were suitable for single crystal X-ray diffraction experiments. Yield:
(C6D6)
d
7.42 (m, 4H), 7.03 (m, 6H), 6.89 (d, 1H, 3JPH ¼ 4 Hz), 6.49 (d,
1H, JPH ¼ 3 Hz), 3.29 (s, 3H), 3.13 (s, 3H), 2.08 (s, 3H). 13C NMR
4
1
(C6D6)
d
150.9 (s), 149.2 (s), 138.4 (d, JCP ¼ 14 Hz), 135.2 (d,
1JCP ¼ 30 Hz), 133.9 (d, 2JCP ¼ 20 Hz), 133.6 (d, 2JCP ¼ 18 Hz), 131.5 (d,
2JCP ¼ 9 Hz), 128.5 (s), 128.4 (d, JCP ¼ 3 Hz), 115.4 (d, JCP ¼ 4 Hz),
3
3
55.3 (s), 55.0 (s), 19.1 (d, JCP ¼ 6 Hz). 31P NMR (C6D6)
d
ꢀ12.1 (s).
4
HRMS (ESI): 369.1074 (MHþ); calcd. for C21H22O2PS (MHþ):
369.1078.
2.2.3. 1,2-Bis(diphenylphosphino)-4,5-dimethoxybenzene, 3
Compound 3 was prepared by two methods; descriptions of
both are provided.
Method A: From 1 by single lithium-halogen exchange. The
reaction leading to LieBr exchange was carried out in an analogous
3.267 g (81.5%). Melting point: 133e135 ꢁC. 1H NMR (acetone-d6)
3
d
7.41 (m, 6H), 7.29 (m, 4H), 7.21 (d, 1H, JHP ¼ 9 Hz), 6.29 (d, 1H,
4JHP ¼ 6 Hz), 3.87 (s, 3H), 3.43 (s, 3H). 13C NMR (acetone-d6)
d
160.0
2
(s), 148.9 (s), 136.9 (s), 136.7 (s), 133.6 (d, JCP ¼ 20 Hz), 128.8 (d,
Table 1
Summary of crystallographic data for 1, 3, 4 and 5$H2O.
Compound reference
1
3
4
5$H2O
Chemical formula
Formula mass
C20H18Br1O2P1
401.24
C32H28O2P2
506.48
C32H40B1O2P1
498.42
Triclinic
9.247(5)
9.275(3)
19.243(13)
101.83(3)
93.12(5)
115.14(3)
1443.5(13)
291(2)
C32H30O5P2
556.54
Monoclinic
14.7204(5)
16.1482(7)
12.1826(5)
90.00
99.698(4)
90.00
2854.5(2)
299(2)
Crystal system
Monoclinic
11.6091(5)
7.3074(2)
22.5063(10)
90.00
101.252(4)
90.00
1872.56(13)
297(2)
Triclinic
9.5793(4)
10.5801(4)
14.0265(4)
97.646(3)
95.735(3)
109.790(3)
1309.62(8)
150(2)
ꢀ
a/A
ꢀ
b/A
ꢀ
c/A
ꢁ
ꢁ
a
/
b
/
ꢁ
g
/
3
ꢀ
Unit cell volume/A
Temperature/K
Space group
P21/n
P1
P1
P21/c
No. of formula units per unit cell, Z
4
2
2
4
Absorption coefficient,
m
/mmꢀ1
2.289
0.194
0.121
0.192
No. of reflections measured
19,376
5698
0.0435
15,295
8026
0.0213
4035
3753
14,366
6910
No. of independent reflections
a
Rint
0.0263
0.0446
0.1183f
0.0636
0.1294
1.081
0.0244
0.0899
0.2018g
0.1003
0.2070
1.011
Final R1 values (I > 2
s
(I))b
s
0.0335
0.0354
Final wR(F2) values (I > 2
Final R1 values (all data)
(I))c
0.0524d
0.1026
0.0729e
0.0513
Final wR(F2) values (all data)
0.0549
1.001
0.0753
1.005
Goodness of fit on F2 h
P
P
a
b
c
Rint
R1
¼
jFo2ꢀ < Fo2 > j= jFo2j:
P
P
¼
jjFoj ꢀ jFcjj= jFoj:
P
P
wR2 ¼ ½ ½wðFo2 ꢀ Fc2Þ2ꢂ= ½wðFo2Þ2ꢂꢂ1=2
:
w ¼ 1=½s2ðFo2Þ þ ð0:0130PÞ2 þ 0:0000Pꢂ:
w ¼ 1=½s2ðFo2Þ þ ð0:0250PÞ2 þ 0:2800Pꢂ:
w ¼ 1=½s2ðFo2Þ þ ð0:0627PÞ2 þ 0:6365Pꢂ:
d
e
f
w ¼ 1=½s2ðFo2Þ þ ð0:0250PÞ2 þ 15:0000Pꢂ where P ¼ ðFo2 þ 2Fc2Þ=3:
g
h
P
GooF ¼ S ¼ ½ ½wðFo2 ꢀ Fc2Þ2ꢂ=ðn ꢀ pÞꢂ1=2
:

R. Chuong et al. / Journal of Organometallic Chemistry 724 (2013) 45e50
47
for 14 h. This solution was filtered, and all volatiles were removed
under reduced pressure (Schlenk line). The remaining white solid
was extracted with hot heptane (w40 mL), and the solution was
kept at ꢀ28 ꢁC. Crystals of 4 (large light yellow cubes; suitable for
X-ray analysis) formed upon standing for 6 days, and were isolated
by filtration. Yield: 0.310 g (31%). Melting point: 114e116 ꢁC. 1H
O
O
Li
O
O
Br
Br
O
O
PPh2
Br
ClPPh2
n-BuLi
THF/Et2O
Br
-100oC
1
A
Scheme 1. Generation of organolithium intermediate A and synthesis of compound 1.
NMR (C6D6)
3H), 3.30 (s, 3H), 2.03 (m, 4H), 1.81e1.67 (m, 8H), 1.43e1.15 (m,
10H). 13C NMR (C6D6)
151.7 (s), 149.4 (s), 138.8 (d, JCP ¼ 9 Hz), 133.3
(d, JCP ¼ 16 Hz), 128.5 (s), 128.4 (s), 128.3 (s), 125.0 (s), 116.7 (s), 110.1
d 7.51 (m, 4H), 7.11e6.97 (m, 7H), 6.56 (m, 1H), 3.44 (s,
manner to the procedure described for the synthesis of 2 (2.106 g,
(5.25 mmol) of (2-bromo-4,5-dimethoxyphenyl)diphenylphos-
phine was used). The generated aryl lithium compound was reacted
with a solution of ClPPh2 (1.131 mL, 6.1 mmol) in 15 mL of diethyl
ether. The reaction mixture was allowed to warm to room temper-
ature while being stirred overnight. Workup and purification were
carried out byexposing the reaction mixture to air. All volatiles were
removed under vacuum (rotary evaporator), and the resulting pale
yellow paste was extracted with 100 mL of boiling ethanol.
Compound 3 precipitated in the form of white crystals upon cooling
the solution to room temperature. Selected crystals were suitable for
single crystal X-ray diffraction experiments. Yield: 2.617 g (52%).
Method B: One-step procedure via double lithiumehalogen
d
(s), 109.8 (s), 55.3 (s), 55.2 (s), 29.4 (s), 29.3 (s), 28.2 (s), 27.2 (s). 31
P
NMR (C6D6)
C
d
ꢀ6.7 (s). HRMS (APCI): 499.2959 (MHþ); calcd. for
32H41BO2P (MHþ): 499.2937.
2.2.5. 1,2-Bis(diphenylphosphoryl)-4,5-dimethoxybenzene 5
To a stirred solution of 3 (0.506 g, 1 mmol) in 10 mL of acetone
was added 1 mL of 30% H2O2 and the reaction mixture was stirred
overnight. All the volatiles were removed under reduced pressure
(rotary evaporator), and compound 5 (white solid) was obtained in
quantitative yield as a water adduct, 5$H2O. X-ray quality crystals of
5$H2O were obtained by crystallization from acetone. Melting
exchange.
A
solution of 1,2-dibromo-4,5-dimethoxybenzene
point: 135e140 ꢁC. H NMR (CDCl3)
d 7.44 (m, 12 H), 7.31 (m, 10H),
(2.96 g, 10 mmol) in 100 mL of Et2O/THF (1:1) was cooled
to ꢀ100 ꢁC, and a solution of n-BuLi (1.6 M in hexanes, 13.1 mL,
21 mmol) was added to it. The pale brown reaction mixture was
stirred at ꢀ100 ꢁC for 45 min, after which a solution of ClPPh2
(4.63 g, 21 mmol, in 15 mL of THF) was slowly added to it. The
temperature of the reaction flask was maintained at ꢀ100 ꢁC during
the addition. The reaction mixture was further stirred at ꢀ100 ꢁC
for 2 h and was then allowed to warm to room temperature over-
night. Workup of the reaction mixture and isolation of the product
were analogous to the ones described in Method A. Yield: 1.417 g
1
3.75 (s, 6H). 13C NMR (CDCl3)
d
154 (m), 132.9 (d, JCP ¼ 107 Hz),
132.1 (m), 131.5 (s), 128.7 (d, 2JPC ¼ 9 Hz), 127.8 (m), 118.7 (m), 56.0
(s). 31P NMR (CDCl3) 34.7 (s). HRMS (ESI): 539.1548 (MHþ); calcd.
d
for C32H29O4P2 (MHþ): 539.1541.
2.3. Single crystal X-ray diffraction studies
Compounds 1, 3 and 5$H2O were characterized at the University
of Portland Diffraction Facility. A typical experimental procedure is
described below. A small specimen was retrieved and affixed to
a fine glass fiber attached to a stout glass fiber mounted on a pin; the
pin was placed on a goniometer head. The crystallographic proper-
(53%). Melting point: 157e159 ꢁC. 1H NMR (C6D6)
d
7.36 (m, 8H),
149.7 (s),
6.99 (m, 12H), 6.77 (m, 2H), 3.14 (s, 6H). 13C NMR (C6D6)
d
137.6 (m), 134.8 (m), 132.8 (m), 127.2 (m), 127.1 (s), 116.3 (m), 53.7
(s). 31P NMR (C6D6)
d
ꢀ12.9 (s). HRMS (ESI): 507.1646 (MHþ); calcd.
ties and data were collected using MoKa radiation and the charge-
for C32H29O2P2 (MHþ): 507.1643.
coupled area detector (CCD) on an Oxford Diffraction Systems
Gemini S diffractometer. A preliminary set of cell constants was
calculated from reflections observed on three sets of 5 frames which
were oriented approximately in mutually orthogonal directions of
2.2.4. (2-(Dicyclohexylboryl)-4,5-dimethoxyphenyl)
diphenylphosphine, 4
reciprocal space. Data collection was carried out using MoKa radi-
A solution of compound 1 (0.802 g, 2 mmol) in ether (40 mL)
was cooled to 0 ꢁC, and a solution of n-BuLi (1.6 M in hexanes,
1.3 mL, 2.1 mmol) was added. The reaction mixture (beige
suspension) was stirred at 0 ꢁC for 1 h, and a solution (1 M in
hexanes) of ClBCy2 was added (2.1 mL, 2.1 mmol). The reaction
mixture was allowed to attain room temperature and was stirred
ation (graphite monochromator) with a crystal-to-CCD distance of
ꢀ
50 mm, and a strategy to achieve a resolution of 0.7 A. Crystals of
compounds 1 and 4 were characterized at Michigan Technological
University following previously described procedures [24]. In both
cases, in the final cycles of refinement, all non-H atoms were refined
withanisotropicthermalparameterswithall Hatoms constrained to
the atoms to which they were attached. Crystallographic data and
details about the refinements are given in Table 1.
CH3S-SCH3
O
O
PPh2
S
3. Results and discussion
2
3.1. Syntheses and characterizations
O
O
PPh2
PPh2
O
O
PPh2
Br
Chemical attributes of 1,2-dibromo-4,5-dimethoxybenzene
(4,5-dibromoveratrole) render it a useful starting material for the
design and syntheses of various ditopic ligands. The compounds
ClPPh2
ClBCy2
n-BuLi
Et2O, 0oC
3
O
O
Li
Li
O
O
PPh2
PPh2
O
O
Br
Br
2.1 ClPPh2
2.1 n-BuLi
O
O
PPh2
BCy2
THF/Et2O
-100oC
3
B
4
Scheme 3. Generation of the organodilithio intermediate B and synthesis of bis-
phosphine 3.
Scheme 2. Syntheses of compounds 2, 3, and 4.

48
R. Chuong et al. / Journal of Organometallic Chemistry 724 (2013) 45e50
Fig. 1. Thermal ellipsoid plots of compounds 1 and 3.
syntheses of the 4,5-dithiocatecholate ligand and of transition
metal complexes based on it [25]. Given our interest in phosphine-
appended hydroquinone ligands [26] we decided to explore 1,2-
dibromo-4,5-dimethoxybenzene as
a platform for phosphine
ligands containing the o-hydroquinone unit.
Reactions of 1,2-dibromo-4,5-dimethoxybenzene with an
equivalent of n-BuLi led to a single lithiumebromine exchange and
formation of the thermally unstable organolithium intermediate A
(Scheme 1). Similar o-bromo-phenyllithium intermediates derived
from 1,2-dibromo-4,5-dialkoxybenzene were reported [27]. It is
critical to maintain a low temperature of the reaction media
(wꢀ100 ꢁC), since reaction mixtures reaching wꢀ90 ꢁC and higher
temperatures led to significant darkening of the solution,
presumably due to the formation of benzyne intermediates. Reac-
tions of A with chlorodiphenylphosphine at wꢀ100 ꢁC led to the
formation of (2-bromo-4,5-dimethoxyphenyl)diphenylphosphine
1 (Scheme 1) in high yields (up to 80%). Phosphine 1 is an air-stable
crystalline solid that is soluble in common organic solvents. It
displays a singlet in 31P NMR spectrum, with a chemical shift value
(
d
ꢀ3.4) which is very close to that of (2-bromophenyl)diphenyl-
phosphine (
d
ꢀ4.5) [28]. The presence of a bromine substituent at
the position ortho to a ePPh2 group makes it a valuable starting
material for the syntheses of other phosphine derivatives. Sub-
jecting the solution of 1 in diethyl ether to n-BuLi at 0 ꢁC led to facile
lithiumebromine exchange. Quenching such reaction mixtures
with electrophiles MeSeSMe, ClPPh2, or ClBCy2 led to the formation
of 2, 3, or 4, respectively (Scheme 2). All three compounds were
isolated as air-stable crystalline solids. The 31P NMR features of
Fig. 2. Thermal ellipsoid plot of 3 illustrating intermolecular Hearyl interactions.
possess a methyl-protected o-hydroquinone functionality which
can be transformed into [O,O] chelating pocket. Another chelating
pocket can be constructed by replacing bromines with various
ligating groups. This strategy has been successfully utilized for the
compounds 2 (
d
ꢀ12.1) and 3 ( ꢀ12.9) resemble those reported for
d
Fig. 3. Thermal ellipsoid plots of 4 and 5$H2O.

R. Chuong et al. / Journal of Organometallic Chemistry 724 (2013) 45e50
49
Table 2
Selected relevant bond distances for crystallographically characterized compounds.
3.2. Structural studies
In addition to spectroscopic characterizations we have suc-
ceeded in obtaining high quality single crystals of compounds 1, 3, 4
and 5$H2O, and characterized them by single crystal diffraction
methods (Table 1). Thermal ellipsoid plots for these compounds are
displayed in Figs. 1e3 while the selected bond distances and angles
are shown in Tables 2 and 3.
1
3
4
5$H2O
C1eP 1.8384(17) C1eP1 1.8463(11) C1eP1
C1eC6 1.399(2) C2eP2 1.8391(12) C1eC6
C1eC2 1.378(2) C1eC2 1.4022(16) C6eC5
C2eBr 1.9048(16) C2eC3 1.4059(15) C6eB6
O1eC5 1.3676(19) C1eC6 1.4035(16) O3eC3
1.806(3) C1eP1 1.817(3)
1.382(4) C2eP2 1.825(3)
1.411(4) C1eC2 1.410(4)
1.589(4) P2eO3 1.495(2)
1.369(3) P1eO4 1.481(2)
C5eC6 1.3914(15) C611eB6 1.584(4) C2eC3 1.397(4)
Most of the structural parameters for 1 (Fig.1) are unremarkable,
as bond distances and angles involving phosphorus and bromine
atoms are similar to those reported for (2-bromophenyl)diphenyl-
phosphine [40]. The strain resulting from accommodating Br and e
PPh2 groups on the adjacent positions of the central phenyl ring is
rather minimal, as the torsion angle BreC2eC1eP is only 4.12ꢁ.
The most interesting structural parameters of bisphosphine 3
(Fig. 1) are those involving the eOMe and ePPh2 groups, as these
functionalities define the compound’s capacity to function as a [P,P]
and [O,O] bis-chelating ligand. The bond lengths and angles around
the central phenyl ring are typical for an aromatic ring, and the
arrangement of the ePPh2 groups is similar to that reported for o-
C6H4(PPh2)2 [30]. The phosphorus atoms are moved toward each
other, as can be judged from the values of the inner bond angles P1e
C1eC2 (117.69(8)ꢁ) and C2eC2eP2 (118.33(8)ꢁ). Peripheral phos-
phorus and oxygen atoms are also slightly bent out of the plane of
O1eC5 1.3634(14)
O2eC4 1.3773(13)
C3eC4 1.375(4)
C4eC5 1.400(4)
C4eO1 1.363(3)
o-diphenylphosphinothioanisole
bis(diphenylphosphino)benzene
(
(d
d
ꢀ14.4) [29] and 1,2-
ꢀ13.0) [30]. While bis-
phosphine 3 is air-stable in the solid state, oxidation of its phos-
phorus centers is facile in solution (acetone) when excess of
hydrogen peroxide is used. The product, bis(phosphine-oxide) 5 is
a white crystalline solid. The solid is hygroscopic, as evident from
the presence of a water signal in the 1H NMR spectrum of vacuum
dried crystals. X-ray structural analysis also revealed water present
in the lattice (5$H2O). A sharp singlet in the 31P NMR spectrum (
34.1) suggests that bulky adjacent diphenylphosphoryl groups in 5
are equivalent in solution. Similar observations have been reported
for 1,2-bis(diphenylphosphoryl)benzene [31].
A notable feature of compound 4 is the presence of adjacent
phosphine and borane groups. Structural motifs consisting of
a donor/acceptor pair mounted onto the same molecular frame have
been extensivelyexplored recently, mainly for pursuing activation of
dihydrogen or other small molecules [32]. The extent of intermo-
lecular donor/acceptor interactions in these compounds is deter-
mined by the steric bulk around the donor/acceptor atoms and by
the overall molecular geometry [32]. Spectroscopic data for
compound 4 suggests that there is little, if any, interaction between
the ePPh2 and eBCy2 groups in solution, as the chemical shift value
d
ꢀ
the central phenyl ring (deviations ranging from 0.055 to 0.129 A),
with all of the atoms bent to the same side of the parent ring. These
deviationsfromthe ideal geometries areprobablydue tothepacking
of molecules in the crystal. In the crystalline state of 3, intermolec-
ular Hearyl interactions [41] are observed, as shown in Fig. 2. The
distance between the interacting hydrogen atom and the calculated
ꢀ
centroid of the aryl ring in another molecule is estimated at 2.491 A.
Structural characterization of 4 was carried out in order to assess
the extent of phosphoruseboron interactions in the solid state.
Both groups (ePPh2 and eBCy2) are clearly tilted toward each other,
as can be judged from the values of the bond angles at their points
of attachment to the central phenyl ring (C6eC1eP1 109.01(19)ꢁ
and C1eC6eB6 116.3(2)ꢁ). Phosphorus and boron atoms are slightly
(
31P NMR,
d
ꢀ6.7 (s)) is characteristic to the derivatives of uncoor-
dinated triphenyl phosphine. The data is consistent with spectro-
scopic observations reported for other o-phosphinophenylboranes
[13,33e35], although similar compounds with pronounced PeB
interactions in solution are also known [36,37]. The spectral signa-
ture remained unchanged upon exposure of the solutions of 4 to H2,
leading us to conclude that compound 4 does not react with H2
under normal conditions. This is not surprising, as reactivity of
phosphine/borane combinations toward H2 is heavily dependent on
steric and electronic properties of the pair [38,39]. When in crys-
talline state, compound 4 is stable in air.
ꢀ
tilted out of the plane of the central phenyl ring (0.101 A for P1, and
ꢀ
0.122 A for B6), resulting in a torsion angle P1eC1eC6eB6 value of
6.98ꢁ. The geometry around the boron center is planar, as can be
judged by the sum of the bond angles around the boron center
(357.7ꢁ). There are no intermolecular PeB interactions, as well as no
interactions between the boron center and the oxygen atoms of the
methoxy groups of another molecule of 4. The phosphoruseboron
distances reported for other structurally characterized o-phosphi-
ꢀ
nophenylboranes range from 3.0 to 3.3 A (no interaction) [35,36] to
We also brieflyexplored1,2-dibromo-4,5-dimethoxybenzene for
double lithiumebromine exchange (intermediate B, Scheme 3)
reactions. Such exchange is facile when the starting material is
subjected to slight excess of n-butyllithium, and 1:1 mixture of
diethylether/THF is used as a solvent. Quenching the reaction
mixture with equimolar amount of chlorodiphenylphosphine led to
the synthesis and isolation of 3 in 52% yield.
ꢀ
2.154 A (donoreacceptor interaction) [36]. The intramolecular
ꢀ
phosphoruseboron distance in 4 was determined at 2.695 A. It is
significantly longer than the distance expected for a P(III)eB coor-
dinate covalent bond, where the sum of covalent radii of P and B
ꢀ
atoms is estimated at 1.98 A. The extent of PeB interaction in 4 is
probably limited both by the geometry of the molecule and by the
Table 3
Selected relevant bond angles for crystallographically characterized compounds.
1
3
4
5$H2O
PeC1eC2
C1eC2eBr
C1eC2eC3
C2eC3eC4
PeC1eC6
C3eC2eBr
120.54(13)
120.75(13)
122.85(15)
119.88(16)
123.19(13)
116.36(13)
P1eC1eC2
C1eC2eP2
P2eC2eC3
C2eC3eC4
P1eC1eC6
C1eC6eC5
C6eC5eO1
C5eC4eO2
117.69(8)
118.33(8)
122.95(9)
121.52(11)
122.46(9)
121.02(11)
124.80(11)
120.70(10)
C6eC1eC2
C6eC1eP1
C2eC1eP1
C1eC6eB6
C621eB6eC611
C621eB6eC6
C611eB6eC6
C5eC6eB6
121.3(2)
109.01(19)
129.6(2)
116.3(2)
120.2(2)
116.6(2)
120.9(2)
125.5(2)
P2eC2eC1
C2eC1eP1
C2eP2eO3
C1eP1eO4
P2eC2eC3
C2eC3eC4
C3eC4eC5
C3eC4eO1
129.2(2)
121.5(2)
108.68(12)
111.58(13)
112.5(2)
122.0(3)
119.7(3)
125.1(3)

50
R. Chuong et al. / Journal of Organometallic Chemistry 724 (2013) 45e50
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steric restrictions of the Ph and Cy substituents. Similar observa-
tions on donoreacceptor interactions between adjacent groups on
the same phenyl ring have been reported for structurally related o-
aminophenylboranes [42,43].
Bis(phosphine) oxide 5 crystallizes with one molecule of water
(Fig. 2), which is bound via hydrogen bonding to one of the P]O
ꢀ
groups (distance HeOeH/O]P is 1.989 A), and to the oxygen
ꢀ
atom of the methoxy group (distance HeOeH/OeCH3 is 2.318 A)
of the other molecule of 5. The phosphoryl groups are not equiva-
lent in the solid state. One of them (O3]P2) is engaged in hydrogen
bonding to water and rotated away from the central ring, whereas
the other one (O4]P1) has its oxygen atom pointing toward P2
atom. The phosphoryl group involved in hydrogen bonding also has
ꢀ
a longer P]O bond (by 0.014 A). It also displays significant
distortions from the ideal molecular geometry at the attachment
point to the central phenyl ring (angle values: C1eC2P2 129.2(2)ꢁ
and C3eC2eP2 112.5(2)ꢁ). Similar structurally characterized
compound, o-C6H4(P(O)PPh2)2$CH2Cl2 [44] shows different orien-
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mental error) P]O bond lengths. Since P]O groups in the later
compound are not engaged in hydrogen bonding, we attribute the
differences in structural features seen in 5 to the inclusion of water
in the crystal. Analysis of the difference maps for 5$H2O revealed
additional density located nearby the oxygen atom of water, but we
have not been able to model a disorder. It is also possible that the
crystals could be slightly desolvated since the data was collected at
room temperature.
4. Conclusions
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The authors are thankful to the University of Portland, M.J.
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for financial support. Support of NSF (grant MRI 0618148) (EJV) is
also gratefully acknowledged. Agatha Brzezinski and Lawaaine
Innis (University of Portland) are acknowledged for crystallo-
graphic study of 5$H2O which was carried out as a part of a class
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Appendix A. Supplementary material
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CCDC 890210, 890692, 890211, 890693 and 890212 contain the
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