Synthesis, structure and spectroscopic properties of 2,3- bis(diphenylphosphino)quinoxaline (dppQx) and its copper(I) complexes

Article abstract of DOI:10.1016/j.ica.2011.01.015

Phosphinoquinoxalines were prepared by treatment of 2,3-dichloroquinoxaline (3) with phosphorus nucleophiles. The Arbuzov reaction of 3 with PPh(O-i-Pr)₂ gave a mixture of diastereomers of 2,3-(PPh(O)(O-i-Pr)) ₂quinoxaline (6); the crystal structure of rac-6 was determined, but attempts at reduction to yield bis(phenylphosphino)quinoxaline 7 resulted in P-C cleavage and formation of phenylphosphine. The bis(secondary phosphine) 7 could be generated from 3 and LiPHPh(BH₃), but was not isolated in pure form. Copper-catalyzed coupling of PHPh₂ with 3 gave 2,3-bis(diphenylphosphino)quinoxaline (4, dppQx), whose coordination chemistry was investigated, with comparison to data for the analogous 1,2- bis(diphenylphosphino)benzene (dppBz) complexes. Reaction of dppQx with [Cu(NCMe)₄][PF₆] gave [Cu(dppQx)₂][PF ₆] (8); CuCl yielded [Cu(dppQx)Cl]₂ (9). Reaction of [Cu(NCMe)₄][PF₆] with one equiv of DPEphos, followed by one equiv of dppQx, gave [Cu(dppQx)(DPEphos)][PF₆] (10). Ligand 4 and copper complexes 8 and 9 were crystallographically characterized. The UV-Vis spectra of dppQx and its copper complexes were red-shifted from those of the dppBz analogs; in contrast to results for the dppBz complexes, those of dppQx were not luminescent in solution.

Full text of DOI:10.1016/j.ica.2011.01.015

Inorganica Chimica Acta 369 (2011) 55–61
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis, structure and spectroscopic properties of 2,3-bis(diphenylphos-
phino)quinoxaline (dppQx) and its copper(I) complexes
b
Matthew F. Cain a, Samantha C. Reynolds a, Brian J. Anderson a, David S. Glueck a, , James A. Golen ,
⇑
Curtis E. Moore b, Arnold L. Rheingold b
a 6128 Burke Laboratory, Department of Chemistry, Dartmouth College, Hanover, NH 03755, USA
b Department of Chemistry, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 19 January 2011
Phosphinoquinoxalines were prepared by treatment of 2,3-dichloroquinoxaline (3) with phosphorus
nucleophiles. The Arbuzov reaction of 3 with PPh(O-i-Pr)2 gave a mixture of diastereomers of 2,3-(PPh(O)
(O-i-Pr))2quinoxaline (6); the crystal structure of rac-6 was determined, but attempts at reduction to
yield bis(phenylphosphino)quinoxaline 7 resulted in P–C cleavage and formation of phenylphosphine.
The bis(secondary phosphine) 7 could be generated from 3 and LiPHPh(BH3), but was not isolated in pure
form. Copper-catalyzed coupling of PHPh2 with 3 gave 2,3-bis(diphenylphosphino)quinoxaline (4,
dppQx), whose coordination chemistry was investigated, with comparison to data for the analogous
1,2-bis(diphenylphosphino)benzene (dppBz) complexes. Reaction of dppQx with [Cu(NCMe)4][PF6] gave
[Cu(dppQx)2][PF6] (8); CuCl yielded [Cu(dppQx)Cl]2 (9). Reaction of [Cu(NCMe)4][PF6] with one equiv of
DPEphos, followed by one equiv of dppQx, gave [Cu(dppQx)(DPEphos)][PF6] (10). Ligand 4 and copper
complexes 8 and 9 were crystallographically characterized. The UV–Vis spectra of dppQx and its copper
complexes were red-shifted from those of the dppBz analogs; in contrast to results for the dppBz com-
plexes, those of dppQx were not luminescent in solution.
This paper is dedicated to Bob Bergman
with thanks for establishing an outstanding
environment for learning and research.
Keywords:
Phosphino(quinoxaline)
X-ray crystallography
Copper(I) complexes
NMR spectroscopy
Luminescence
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
commercially available 2,3-dichloroquinoxaline (3), which con-
trasts with the difficulty of carrying out nucleophilic substitutions
Bidentate diphosphines are valuable ligands for metal-catalyzed
reactions. Activity and selectivity of these catalysts depends
strongly on the ligand’s cone angle, bite angle, and electronic prop-
erties. Modifying the linker which connects the two P-donor groups
enables rational tuning of these properties, so a wide variety of
linkers has been employed. For example, Imamoto and coworkers
recently reported the synthesis of QuinoxP⁄ (1), a useful chiral li-
gand derived from 2,3-dichloroquinoxaline [1], while a group from
Dow described a related bis(phospholane) ligand (2) used in asym-
metric hydroformylation (Chart 1) [2]. Both are outstanding ligands
for asymmetric hydrogenation [1,2]. QuinoxP⁄ is also useful in sev-
eral other asymmetric catalytic reactions [3], and has even been
claimed as a ligand in a gold anti-cancer agent [4].
This quinoxaline scaffold has several potential advantages in
ligand design, including its rigid structure, the crystallinity of its
derivatives, and its electron-withdrawing nature; QuinoxP⁄ is air-
stable, despite its electron-rich dialkylphosphino groups [1]. No
less important is the potentially general method used to prepare
the ligands in Chart 1 by nucleophilic addition/elimination to
on halobenzene derivatives [5].
Despite these potential advantages and the impressive catalytic
performance of diphosphines 1 and 2, little is known about
phosphino(quinoxalines). Because of the widespread use of
diphenylphosphino groups, an obvious target for investigation is
bis(diphenylphosphino)quinoxaline (dppQx, 4), which was briefly
described, in Japanese, in a 2010 patent [6]. We report here a more
detailed study of the synthesis and properties of 4 and some of its
analogs. Copper complexes of the analogous o-phenylene-based
1,2-bis(diphenylphosphino)benzene (dppBz, 5) show interesting
luminescent properties [7,8]; the energy of relevant MLCT transi-
tions might be tuned by the known ability of phosphino(quinoxa-
line) ligands to be reduced [9], so the synthesis, structure, and
optical properties of Cu(dppQx) complexes is also described, with
comparison to results for the dppBz analogs (Chart 2).
2. Experimental
2.1. General experimental details
⇑
Unless otherwise noted, all reactions and manipulations were
performed in dry glassware under a nitrogen atmosphere at
Corresponding author. Tel.: +1 603 646 1568; fax: +1 603 646 3946.
E-mail address: glueck@dartmouth.edu (D.S. Glueck).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.01.015

56
M.F. Cain et al. / Inorganica Chimica Acta 369 (2011) 55–61
which contained Ph2P–PPh2 and PHPh2, it was repeatedly washed
Ph
Ph
with petroleum ether (2 ꢁ 25 mL). The yellow residue was then
recrystallized from CH2Cl2 layered with petroleum ether at
ꢀ30 °C affording yellow-orange X-ray quality crystals in 100% pur-
ity by 31P NMR spectroscopy (131 mg, 0.263 mmol, 19%). When the
reaction was scaled up (1 g of PHPh2), 314 mg of 4 was isolated
(23% yield).
Anal. Calc. for C32H24N2P2: C, 77.10; H, 4.85; N, 5.62. Found: C,
76.54; H, 4.66; N, 5.52%. HRMS m/z calcd. for C32H25N2P2 (MH)+:
499.1493. Found: 499.1480. 31P{1H} NMR (CDCl3): d ꢀ9.1. 1H
NMR (CDCl3): d 7.91 (m, 2H, Ar), 7.65 (m, 2H, Ar), 7.35–7.32 (m,
8H, PPh2), 7.30–7.23 (m, 12H, PPh2). 13C{1H} NMR (CDCl3): d
164.0 (dd, J = 11, 9, Ar), 142.4 (Ar), 135.9 (m, Ar), 134.8 (t, J = 10,
Ar), 130.2 (Ar), 130.0 (Ar), 129.1 (Ar), 128.4 (t, J = 4, Ar). These data
were consistent with the literature report [6].
t-Bu Me
P
P
P
N
N
N
N
Ph
Ph
P
Me t-Bu
QuinoxP* (1)
Ph-Quinox (2)
Cl
Cl
N
N
2,3-dichloroquinoxaline (3)
Chart 1. Quinoxaline-based chiral diphosphines and starting material 3.
2.2.2. Synthesis of 2,3-bis(phenyl-isopropoxylphosphinyl)quinoxaline
(6)
2,3-Dichloroquinoxaline (1.00 g, 5.0 mmol) and diisopropyl
phenylphosphonite (3.4 mL, 15.0 mmol) were heated with stirring
to 110 °C for 2 weeks to give a cloudy orange residue. The reaction
mixture was dissolved in methylene chloride, filtered over Celite,
and concentrated to give a deep orange oil. The oil was dissolved
in minimal methylene chloride and layered with petroleum ether.
The product precipitated, and was collected via vacuum filtration
as an off-white solid (1.45 g, 59%). Recrystallization from Et2O lay-
ered with petroleum ether gave X-ray quality colorless glassy
crystals.
Anal. Calc. for C26H28N2O4P2: C, 63.16; H, 5.71; N, 5.67. Found: C,
63.08; H, 5.74; N, 5.55%. HRMS m/z calcd. for C26H29N2O4P2 (MH+):
m/z 495.1603. Found 495.1602. 31P{1H} NMR (CDCl3): d 26.3, 26.0
(1.5:1 ratio of diastereomers). 1H NMR (CDCl3): d 8.12–8.10 (m,
2H, Ar), 8.08–8.06 (m, 2H, Ar), 8.04–8.00 (m, 4H, Ar), 7.98–7.94
(m, 4H, Ar), 7.87–7.83 (m, 4H, Ar), 7.55–7.50 (m, 4H, Ar), 7.48–
7.42 (m, 8H, Ar), 4.95 (heptet, J = 6, 2H, CHMe2), 4.81 (heptet,
J = 6, 2H, CHMe2), 1.42 (d, J = 6, 6H, Me), 1.40 (d, J = 6, 6H, Me),
1.38 (d, J = 6, 6H, Me), 1.30 (d, J = 6, 6H, Me). 13C{1H} NMR (CDCl3):
d 154.5 (dd, J = 25, 24, Ar), 153.2 (dd, J = 25, 23, Ar), 140.7–140.4
(m, 2Ar), 132.9–132.7 (m, 2Ar), 132.6 (Ar), 132.5 (Ar), 132.3 (d,
J = 6, Ar), 132.1 (d, J = 5, Ar), 131.5 (Ar), 131.4 (Ar), 130.0 (2Ar),
128.1–127.9 (m, 2Ar), 72.1 (t, J = 3, CHMe2), 72.0 (t, J = 3, CHMe2),
24.59–24.55 (m, Me), 24.2 (dt, J = 8, 3, Me).
N
N
PPh2
PPh2
PPh2
PPh2
dppQx (4)
dppBz (5)
Chart 2. Structures of dppQx and dppBz.
20 °C in a dry box or using standard Schlenk techniques. As noted
by a reviewer, the air-stability of dppQx and Cu(I) salts might also
make it possible to prepare the copper complexes in the air. Petro-
leum ether (bp 38–53 °C), CH2Cl2, ether, THF, and toluene were
dried over alumina columns similar to those described by Grubbs
[10]. NMR spectra were recorded using Varian 300 or 500 MHz
spectrometers. 1H or 13C NMR chemical shifts are reported versus
Me4Si and were determined by reference to the residual 1H or
13C solvent peaks. 31P NMR chemical shifts are reported versus
H3PO4 (85%) used as an external reference. Coupling constants
are reported in Hz, as absolute values unless noted otherwise. Un-
less indicated, peaks in NMR spectra are singlets. UV–Vis and fluo-
rescence spectra were recorded using JASCO V-630 and Shimadzu
RF-1501 instruments, respectively. Elemental analyses were pro-
vided by Quantitative Technologies Inc. Mass spectra were re-
corded at the University of Illinois, Urbana-Champaign (http://
www.scs.uiuc.edu/~msweb). The phosphine-borane PH2Ph(BH3)
[11] and the complexes [Cu(NCMe)4][PF6] [12], [Cu(dppBz)2][PF6]
(11) and [Cu(dppBz)(DPEphos)][PF6] (13) [7a], and [Cu(dppBz)
(Cl)]2 (12) [7b] were prepared by the literature methods, with
modifications for 11 and 13 (PFꢀ6 anion instead of BF4ꢀ).
2.2.3. Synthesis of 2,3-bis(phenylphosphino)quinoxaline (ppQx, 7)
A
stirring solution of phenylphosphine borane (370 mg,
3 mmol) in 5 mL of THF was cooled to ꢀ40 °C and treated with
n-butyl lithium (2.0 mL of 1.5 M solution in cyclohexane,
a
3 mmol). After 15 min, the solution was added to a stirring slurry
of 2,3-dichloroquinoxaline (200 mg, 1 mmol) in 5 mL of THF, also
cooled to ꢀ40 °C. The mixture immediately turned a deep or-
ange-red color, and was allowed to warm to room temperature.
After 2 h, the mixture was quenched with degassed brine (15 mL)
and the organic layer was extracted with Et2O (3 ꢁ 15 mL), col-
lected and dried over MgSO4, then filtered via cannula into a clean
Schlenk flask. The solvent was removed in vacuo to give an orange-
red oil which contained ca. 50% of 7, on the basis of integration of
the 31P NMR spectrum. 31P{1H} NMR (C6D6): d ꢀ56.4 and ꢀ57.4
(1:1 ratio of diastereomers; a characteristic AA0XX0 pattern was ob-
served in the 31P NMR spectrum). The major impurities were
(PHPh)2 (d ꢀ65.5, ꢀ69.1) [13] and PH2Ph (d ꢀ123.0). Attempts to
purify 7 were unsuccessful.
2.2. Synthesis and characterization
2.2.1. Synthesis of 2,3-bis(diphenylphosphino)quinoxaline (dppQx, 4)
A slurry of CuCl (30 mg, 0.27 mmol, 20 mol%) and 2,3-dichloro-
quinoxaline (3, 270 mg, 1.34 mmol) in 20 mL of THF was treated
with a solution of PHPh2 (500 mg, 2.68 mmol, 2 equiv) in 10 mL
of THF, followed by addition of NaOSiMe3 (300 mg, 2.68 mmol, 2
equiv) leading to the immediate formation of a bright yellow solu-
tion. The color of the solution grew darker within minutes, ulti-
mately becoming a deep blue-green in 15 min. The solution was
stirred for 1 h, then pumped down under vacuum. The residue
was dissolved in 100 mL of a 25% THF/petroleum ether solution
and passed through a silica plug affording a new bright yellow
solution. The solution was pumped down under vacuum giving
the desired product in 96% purity by 31P NMR spectroscopy
(268 mg, 0.538 mmol, 40%). In order to further purify the product,
2.2.4. Synthesis of [Cu(dppQx)2][PF6] (8)
A
rapidly-stirring slurry of [Cu(NCMe)4][PF6] (19 mg,
0.050 mmol) in 1 mL of THF was treated with a yellow solution
of dppQx (50 mg, 0.10 mmol) in 2 mL of THF resulting in the

M.F. Cain et al. / Inorganica Chimica Acta 369 (2011) 55–61
57
immediate formation of a homogeneous, bright orange solution.
3. Results and discussion
The reaction mixture was stirred for 1 h, then pumped down under
vacuum. The orange residue was washed with petroleum ether,
dried under vacuum, and then recrystallized from methylene chlo-
ride layered with petroleum ether at ꢀ30 °C affording orange nee-
dles (61 mg, 0.050 mmol, 100%). Crystals suitable for X-ray
diffraction (and elemental analysis) were obtained by dissolving
the solid in warm methanol and allowing it to slowly cool at
ꢀ30 °C.
3.1. Synthesis and structure of phosphinoquinoxalines
The Arbuzov reaction of commercially available PPh(O-i-Pr)2
with 3 gave bis(phosphinate) 6 as a mixture of diastereomers, as
reported for analogous substrates (Scheme 1) [14]. The crystal
structure of rac-6 is shown in Fig. 1; see Table 1 and the Supporting
Information for crystallographic details.
Elemental analysis data was consistently low in carbon, for
example: Anal. Calc. for C64H48CuN4P5F6: C, 63.76; H, 4.01; N,
4.65. Found: C, 62.94; H, 3.56; N, 4.58%. HRMS m/z calcd. for
Unfortunately, attempts to reduce bis(phosphinate) 6 to yield
the bis(secondary phosphine) 7 with LiAlH4/SiMe3Cl [15], Cp2ZrHCl
[16], or DIBAL-H [17] were unsuccessful; all resulted in P–C cleav-
age and formation of phenylphosphine (Scheme 1). Instead, 7 could
be generated from 3 on treatment with two equiv of LiPHPh(BH3),
with spontaneous borane decomplexation (Scheme 1). However,
the biphosphine PhHP–PHPh and other unidentified byproducts
were formed, and phosphine 7 could not be isolated in pure form.
During the course of this work, synthesis of dppQx (4) was re-
ported by treatment of 2,3-dichloroquinoxaline with LiPPh2 in a
mixture of hexanes and THF [6]. We found it more convenient to
prepare 4 via CuCl-catalyzed reaction of 3 with PHPh2 in the pres-
ence of NaOSiMe3 (Scheme 2) [18]. Although this reaction pro-
ceeded in low yield, it could be scaled up readily, and the
workup and purification of 4 were straightforward. Yellow crystal-
line 4 was stable to air and water; its crystal structure is shown in
Fig. 2.
C
64H48N4P4Cu (M+): 1059.2126. Found: 1059.2109. 31P{1H} NMR
(CD2Cl2): d 3.7 (broad, dppQx), ꢀ143.6 (septet, J = 711, PF6). 1H
NMR (CD2Cl2): d 8.24 (m, 4H, dppQx), 8.01 (m, 4H, dppQx), 7.33–
7.27 (overlapping, 24H, Ph), 7.01 (t, J = 7, 16H, Ph). 13C{1H} NMR
(CD2Cl2): d 158.7 (m, Ar), 142.7 (Ar), 133.6 (br m, Ar), 133.3 (Ar),
131.0 (Ar), 130.2 (Ar), 129.1 (t, J = 5, Ar).
2.2.5. Synthesis of [Cu(dppQx)(Cl)]2 (9)
A slurry of CuCl (20 mg, 0.20 mmol) in 1 mL of THF was treated
with a solution of dppQx (50 mg, 0.10 mmol, 0.5 equiv) in 2 mL of
THF resulting in a homogeneous, deep brick red solution. The solu-
tion was stirred overnight, then passed through Celite to remove
excess CuCl, and pumped down under vacuum. The crude product
was recrystallized from CHCl3 layered with petroleum ether at
ꢀ30 °C affording a red-brown solid (2 crops, 56 mg, 0.047 mmol,
93%). Crystals for X-ray analysis were obtained by dissolving the
dimer in hot acetonitrile and slow cooling at ꢀ30 °C.
O
P
3
6
Ph
N
N
Cl
Cl
N
N
Elemental analysis data was consistently low in carbon, for
example: Anal. Calc. for C64H48Cu2Cl2P4N4: C, 64.33; H, 4.05; N,
4.69. Found: C, 62.42; H, 3.54; N, 4.41%. HRMS m/z calcd. for
2 PPh(O-i-Pr)2
O-i-Pr
Δ
Ph
O-i-Pr
P
O
C
64H48N4P4Cu2Cl (MꢀCl)+: 1157.1110. Found: 1157.1085. 31P{1H)
NMR (CDCl3): d ꢀ17.7 (broad). 1H NMR (CDCl3): d 8.03 (m, 4H,
dppQx), 7.71 (m, 4H, dppQx), 7.64 (q, J = 7, 16H, Ph), 7.22 (t, J = 8,
8H, Ph), 7.13 (t, J = 8, 16H, Ph). 13C{1H} NMR (CDCl3): d 160.8 (t,
J = 36, Ar), 142.2 (Ar), 134.8 (t, J = 8, Ar), 131.5 (br m, Ar), 131.3
(Ar), 130.2 (Ar), 129.8 (Ar), 128.4 (t, J = 5, Ar).
reduction
N
N
PHPh
2 LiPHPh(BH3)
7
PHPh
Scheme 1. Approaches to bis(secondary phosphine) 7.
2.2.6. Synthesis of [Cu(dppQx)(DPEphos)][PF6] (10)
A slurry of [Cu(NCMe)4][PF6] (37.4 mg, 0.100 mmol) in 1 mL of
THF was treated with a solution of DPEphos (54 mg, 0.10 mmol)
in 2 mL of THF. After the resulting clear, homogeneous solution
was stirred for 1 h, a solution of dppQx (50 mg, 0.10 mmol) in
2 mL of THF was added, and the solution turned bright yellow.
After another h, 31P NMR (THF) showed the presence of 10 (d
ꢀ4.3, ꢀ143.5) and DPEphos (d ꢀ16.1). The solvent was
removed under vacuum and repeated washings with Et2O
extracted DPEphos. The crude product was recrystallized from
methylene chloride layered with petroleum ether at ꢀ30 °C afford-
ing an analytically pure flaky yellow solid (84 mg, 0.067 mmol,
67%).
C11A
C7
C10A
C12A
C6
C5
C9A
C13A
C2A
C2
C1A
C1
O2
C3A
C3
C8A
P1
O1
N1A
N1
C4
C4A
O1A
Anal. Calc. for C68H52CuN2OP5F6: C, 65.57; H, 4.21; N, 2.25.
Found: C, 65.75; H, 4.16; N, 2.16. HRMS m/z calcd. for
C8
C
68H52N2P4OCu (M+): 1099.2326. Found: 1099.2349. 31P{1H} NMR
(CD2Cl2): d ꢀ4.5 (broad and overlapping, dppQx and DPEphos),
143.6 (septet, J = 711, PF6). 1H NMR (CD2Cl2): d 8.16 (br, 2H,
dppQx), 8.01 (m, 2H, dppQx), 7.36 (t, J = 8, 2H, Ar), 7.29 (t, J = 7,
5H, Ar), 7.19 (t, J = 7, 4H, Ar), 7.12 (m, 3H, Ar), 7.08–7.00 (broad
and overlapping, 22H, Ar), 6.88 (br m, 10H, Ar), 6.46 (br, 2H, Ar).
13C{1H} NMR (CD2Cl2): d 159.2 (m, Ar), 157.8 (Ar), 142.3 (Ar),
134.30 and 134.25 with a shoulder (3 total Ar), 133.6 (Ar), 133.0
(Ar), 132.7 (Ar), 131.8 (br m, Ar), 130.7 (Ar), 130.6 (Ar), 130.0
(Ar), 128.7 (d, J = 20, Ar), 125.4 (Ar), 124.0 (br m, Ar), 120.8 (Ar).
C9
C6A
C5A
O2A
C13
C7A
C10
C12
C11
Fig. 1. ORTEP diagram of bis(phosphinate) rac-6.

58
M.F. Cain et al. / Inorganica Chimica Acta 369 (2011) 55–61
Table 1
Crystallographic data for 6, 4, 8 and 9.
Compounds
rac-6
dppQx (4)
[Cu(dppQx)2][PF6] (8)
[Cu(dppQx)(Cl)]2 (9)
Formula
Formula weight
Space group
a (Å)
b (Å)
c (Å)
C
26H30N2O4P2
C32H24N2P2
498.47
P1
C64H48CuF6N4P5
1205.45
P2(1)/c
11.3561(15)
19.723(3)
25.477(3)
90
100.523(4)
90
5610.2(13)
4
C64H48Cl2Cu2N4P4
1194.92
P2(1)/n
10.7662(5)
25.0666(11)
22.0856(9)
90
90.818(2)
90
5959.7(5)
4
496.46
C2/c
11.2641(11)
17.5179(16)
13.2047(12)
90
101.5560(10)
90
2552.8(4)
4
ꢀ
9.3790(6)
10.0967(6)
14.8625(9)
74.2950(10)
75.5630(10)
72.1950(10)
1268.52(13)
2
a
(°)
b (°)
c
(°)
V (Å3)
Z
Dcalc (g/cm3)
1.292
0.205
1.305
0.196
1.427
0.598
1.332
3.048
l
(Mo K
a
) (mmꢀ1
)
Temperature (K)
153(2)
3.80
9.99
100(2)
3.83
8.68
100(2)
4.47
11.18
100(2)
2.90
7.28
R(F) (%)a
Rw(F2) (%)a
Quantity minimized: Rw(F2) =
R
[w(F2o ꢀ F2c )2]/
R
[(wF2o )2]1/2; R =
R
D
/
R
(Fo),
D
= |(Fo ꢀ Fc)|, w = 1/[r
2(F2o ) + (aP)2 + bP], P = [2Fc2 + Max(F2o , 0)]/3. A Bruker CCD diffractometer
a
was used in all cases.
3
4
2 PHPh2
2 NaOSiMe3
Ph2
P
Ph2
P
9
Cl
N
Cl
Cl
N
N
PPh2
PPh2
N
N
N
N
Cu
Cu
cat. CuCl
Cl
N
P
P
Ph2
Ph2
Scheme 2. CuCl-catalyzed synthesis of dppQx (4).
CuCl
N
PPh2
4
C12
C11
N
PPh2
C13
C10
0.5 [Cu(NCMe)4][PF6]
C14
Ph2
P
Ph2
C9
N
P
N
N
Cu
C16
C3
C17
N1
N
P
P
P1
C18
C19
Ph2
Ph2
8
PF6
C4
C5
C15
C2
C7
C1
C8
C20
Scheme 3. Synthesis of [Cu(dppQx)2][PF6] (8) and [Cu(dppQx)Cl]2 (9).
C6
C25
C24
[Cu(NCMe)4][PF6]
N2
C26
C23
P2
i. DPEphos
ii. dppQx
C21
C22
C32
C31
C27
Ph2
Ph2
P
C28
C29
P
N
N
Cu
O
P
Ph2
P
PF6
Ph2
10
C30
Fig. 2. ORTEP diagram of dppQx (4).
Scheme 4. Synthesis of [Cu(DPEphos)(dppQx)][PF6] (10).
In this structure, the PPh2 groups were displaced slightly on
opposite sides of the quinoxaline plane, by 0.48 and 0.33 Å, respec-
tively. This twist was not observed in the structure of dppBz,
whose structure is otherwise very similar to that of dppQx [19].
As a result of this geometry, and the slightly longer C1–C8 distance
in dppQx vs the corresponding bond length in dppBz (1.442(2) ver-
sus 1.420(4) Å), the P–P distance in dppQx was a little longer than
in dppBz (3.194 versus 3.166(1) Å).
3.2. Synthesis, structure, and spectroscopic properties of Cu(I)
complexes of dppQx
Reaction of [Cu(NCMe)4][PF6] with two equiv of dppQx gave the
cationic copper complex [Cu(dppQx)2][PF6] (8, Scheme 3).
The reaction of dppQx with one equiv of CuCl gave a mixture of
the desired complex [Cu(dppQx)Cl]2 (9) plus some [Cu(dppQx)2]-
[Cl] (8-Cl), according to 31P NMR spectroscopy. Using an excess

M.F. Cain et al. / Inorganica Chimica Acta 369 (2011) 55–61
59
Table 2
C30
C31
Selected bond lengths and angles for homoleptic Cu(I) dppQx and dppBz complexes.
C29
C28
C43
C42
C44
Complex
Cu–P (Å)
P–Cu–P (°)
C45
[Cu(dppQx)2][PF6] (8)
2.2669(6)
2.2691(7)
2.2812(7)
2.3153(7)
87.68(2)
90.74(2)
C32
C27
C61
C46
C62
C41
C49
114.14(3)
122.21(2)
122.54(3)
122.61(3)
84.72(3)
120.94(3)
122.59(5)
127.85(5)
C50
C48
C60
C26
C8
C6
C51
N2
C5
P2
C52
C47
C7
C25
C63
C35
P3
C59
Cu1
N3
C21
C22
[Cu(dppBz)2][BF4] (11)[7a]
2.3092(8)
2.2907(9)
C34
C24
N1
C36
C37
C33
C64
C4
C2
P4
C1
C23
P1
C3
C40
C10
C11
C39
N4
C53
C9
C14
C54
C58
C38
C16
C12
C13
The distorted tetrahedral structure of 8 (Fig. 3), as expected, was
very similar to that of [Cu(dppBz)2][BF4] (11) [7a]. There was a
small difference in the bite angles: 84.72(3)° for dppBz vs an aver-
age of 89.02(2)° for dppQx.
C15
C55
C56
C17
C20
C19
C57
C18
The Cu2Cl2 plane in the structure of 9 (Fig. 4) was puckered,
with a dihedral angle between the Cu1–Cl1–Cl2 plane and the
Cu2–Cl1–Cl2 plane of 31.35°. This butterfly structure (dihedral an-
gle = 29.1°) and similar bond lengths and angles were also ob-
served in the analogous complex [Cu(dppBz)Cl]2 (12) [7b].
Similarly, the Cl–Cu–Cl angles in these complexes (for 9,
104.470(18) and 104.087(18)°; for 12, 102.87(4) and 101.67(4)°)
were about the same. Tables 2 and 3 summarize selected bond
length and angle data for the dppQx and dppBz complexes.
Fig. 3. ORTEP diagram of the cation in [Cu(dppQx)2][PF6] (8).
C37
C36
C38
C35
C34
C39
C57
N4
C48
C49
C50
C56
N3
C58
C64
3.3. Optical properties of dppQx, dppBz and their copper(I) complexes
C63
C53
C33
C42
C59
UV–Vis spectral data for dppQx, dppBz, and their Cu(I) com-
plexes 8–10, 11, and 13 in CH2Cl2 are collected in Table 4, and
the spectra are shown in Supporting Information Figs. S1 and S2.
Our observations for [Cu(dppBz)2][PF6] (11) and [Cu(dppBz)(DPE-
phos)][PF6] (13) are consistent with the literature reports for these
complexes, where the BF4 anion was used [7a].
The UV–Vis absorptions of dppQx (4) and its complexes 8–10
were red-shifted in comparison to data for dppBz (5) and its com-
plexes 11–13. The lower-energy transitions are consistent with
the expected improved acceptor properties of quinoxaline versus
benzene.
In contrast to the reported room-temperature luminescence of
dppBz complexes 11-BF4 and especially 13-BF4 in solution [7a],
the dppQx complexes 8–10 were not luminescent under compara-
ble conditions (ca. 10ꢀ5 M solutions in CH2Cl2 under nitrogen). This
different behavior was most striking in side-by-side comparison of
samples of DPEphos complexes 10 and 13 (as the PF6 salts; the
emission spectra of 8-PF6 and 13-PF6 were similar to those re-
ported for the BF4 salts) [7a]. Finally, dinuclear 12 was reported
not to show luminescence in 2-Me-THF at 298 K [7b]. Similarly,
dppQx complex 9 did not luminesce at room temperature in
CH2Cl2. We have not investigated possible luminescence of dppQx
(4) or its complexes 8–10 in low-temperature solutions or in the
solid state, but the solid samples did not appear to fluoresce under
a hand-held UV lamp.
C47
C40
P3
C51
C62
C61
P4
C60
C43
C52
C45
C54
C55
C41
C46
C44
Cu2
Cl2
Cl1
C24
C25
C23
C22
Cu1
C26
C9
C11
C12
C13
C21
C10
P2
C32
C27
C28
P1
C1
C14
C20
C19
C18
C8
C31
C30
C29
C15
C16
N2
N1
C2
C3
C7
C6
C17
C5
C4
Fig. 4. ORTEP diagram of [Cu(dppQx)Cl]2 (9).
of CuCl in the reaction avoided this problem and gave the
red-brown dinuclear complex 9 in high yield (Scheme 3). Similarly,
attempted synthesis of [Cu(dppBz)Cl]2 (12) [7b] by this method
gave a mixture of 12 and [Cu(dppBz)2][Cl] (11-Cl), which, however,
could not be converted to 12. This greater lability of dppQx versus
dppBz is consistent with weaker donor properties of dppQx, per-
haps due to the electron-withdrawing quinoxaline group.
Sequential treatment of [Cu(NCMe)4][PF6] with DPEphos
and dppQx gave the yellow heteroleptic complex [Cu(DPEphos)-
(dppQx][PF6] (10, Scheme 4), whose NMR spectroscopic properties
were similar to those of the known dppBz analog 13 [7a].
Structure–property relationships in Cu(I) complexes have been
investigated in detail; the luminescent excited states frequently
arise from MLCT transitions [20]. Such excited states were pro-
posed to be responsible for the luminescence of the cations
[Cu(dppBz)2]+ (11) and [Cu(dppBz)(DPEPhos)]+ (13) [7a] and of
[Cu(dppBz)(Cl)]2 (12) and related dinuclear halide complexes
[7b]. MLCT transitions presumably also result in excited states in
the structurally similar dppQx analogs 8–10. However, the quinox-
aline nitrogen atoms in dppQx complexes may lead to quenching
of fluorescence, as observed in related systems where aromatic
amines are effective quenchers [21]; further computational and

60
M.F. Cain et al. / Inorganica Chimica Acta 369 (2011) 55–61
Table 3
Selected bond lengths and angles for [Cu(diphos)(Cl)]2 complexes (diphos = dppQx or dppBz).
Complex
Cu–P (Å)
Cu–Cl (Å)
P–Cu–P (°)
Cl–Cu–Cl (°)
Cu–Cl–Cu
P–Cu–Cl (°)
[Cu(dppQx)(Cl)]2 (9)
2.2552(5)
2.2556(5)
2.2324(5)
2.2377(5)
2.3160(5)
2.3690(5)
2.3218(5)
2.3754(5)
88.864(18)
94.288(19)
104.470(18)
104.087(18)
72.440(15)
72.454(15)
119.66(2)
124.26(2)
109.719(19)
103.089(19)
121.258(19)
115.793(19)
113.757(19)
113.126(19)
112.02(6)
123.49(6)
121.40(5)
111.36(5)
112.21(5)
118.75(6)
122.47(6)
109.31(5)
[Cu(dppBz)(Cl)]2 (12)[7b]
2.255(2)
2.260(2)
2.246(2)
2.255(1)
2.383(1)
2.316(1)
2.324(1)
2.414(1)
87.50(5)
90.56(5)
101.67(4)
102.87(4)
74.99(4)
74.54(4)
vol. 5, Wiley, Chichester, 2007, p. 65;
Table 4
For a related ligand with P(Me)(adamantyl) groups, see:(c) T. Imamoto, A.
Kumada, K. Yoshida, Chem. Lett. 36 (2007) 500.
[2] (a) M.E. Fox, M. Jackson, I.C. Lennon, J. Klosin, K.A. Abboud, J. Org. Chem. 73
(2008) 775;
(b) A.T. Axtell, J. Klosin, G.T. Whiteker, C.J. Cobley, M.E. Fox, M. Jackson, K.A.
Abboud, Organometallics 28 (2009) 2993.
[3] Rh-catalyzed hydroacylation of alkenes with aldehydes: (a) Y. Shibata, K.
Tanaka, J. Am. Chem. Soc. 131 (2009) 12552;
Selected absorbance data for dppQx (4), dppBz (5) and their Cu(I) complexes 8–11
and 13 in CH2Cl2.
Compound
kmax (nm)
e )
(Mꢀ1 cmꢀ1
dppQx (4)
dppBz (5)
333
274a
345
272a
343
347
268
5400
26 000
19 000
25 000
35 000
13 000
46 000
[Cu(dppQx)2][PF6] (8)
[Cu(dppBz)2][PF6] (11)
[Cu(dppQx)(Cl)]2 (9)b
[Cu(dppQx)(DPEphos)][PF6] (10)
[Cu(dppBz)(DPEphos)][PF6] (13)
Rh-catalyzed tetraphenylene synthesis via cycloaddition of triynes: (b) T.
Shibata, T. Chiba, H. Hirashima, Y. Ueno, K. Endo, Angew. Chem., Int. Ed. 48
(2009) 8066;
Cu-catalyzed borylation of enones: (c) I.-H. Chen, L. Yin, W. Itano, M. Kanai, M.
Shibasaki, J. Am. Chem. Soc. 131 (2009) 11664;
Pt-catalyzed asymmetric Baeyer–Villiger oxidation: (d) A. Cavarzan, G.
Bianchini, P. Sgarbossa, L. Lefort, S. Gladiali, A. Scarso, G. Strukul, Chem. Eur.
J. 15 (2009) 7930;
Cu-catalyzed reaction of allylic carbonates with a diboron derivative: (e) H. Ito,
Y. Kosaka, K. Nonoyama, Y. Sasaki, M. Sawamura, Angew. Chem., Int. Ed. 47
(2008) 7424;
a
Shoulder, not a distinct maximum.
The UV–Vis spectrum of [Cu(dppBz)(Cl)]2 (12) was not reported in reference
b
[7b]. For UV–Vis spectra of the analogous complexes [Cu(dppBz)(X)]2 (X = Br and I),
see reference [7c].
experimental studies would be required to investigate this
possibility.
(f) H. Ito, S. Ito, Y. Sasaki, K. Matsuura, M. Sawamura, J. Am. Chem. Soc. 129
(2007) 14856;
Pd-catalyzed allylic substitution and Ru-catalyzed hydrogenation: (g) T.
Imamoto, M. Nishimura, A. Koide, K. Yoshida, J. Org. Chem. 72 (2007) 7413;
Rh-catalyzed hydrosilylation of ketones: (h) T. Imamoto, T. Itoh, Y. Yamanoi, R.
Narui, K. Yoshida, Tetrahedron: Asymmetry 17 (2006) 560.
[4] H. Kodama, K. Ohto, N. Oohara, K. Nakatsui, Y. Kaneda, Nippon Chemical
Industrial Co., US20100048894, 2010.
4. Conclusions
We have prepared the new rigid chelate diphosphine dppQx, a
potentially useful ligand in coordination chemistry and catalysis.
As expected, the structures of dppQx and its Cu(I) complexes were
very similar to those of dppBz and its analogous copper complexes.
However, electronic differences between the quinoxaline and ben-
zene backbones of these ligands gave rise to different optical prop-
erties of the diphosphines and their Cu(I) complexes.
[5] Synthesis of dppBz: see (a) B.A. Baker, Z.V. Boskovic, B.H. Lipshutz, Org. Lett. 10
(2008) 289 (and references therein);
Synthesis of o-C6H4(P(t-Bu)2)2 and related ligands: see (b) Y. Yamamoto, T.
Koizumi, K. Katagiri, Y. Furuya, H. Danjo, T. Imamoto, K. Yamaguchi, Org. Lett. 8
(2006) 6103;
Synthesis of o-C6H4(PMePh)2: see (c) N.K. Roberts, S.B. Wild, J. Am. Chem. Soc.
101 (1979) 6254.
[6] M. Nakamura, T. Hatakeyama, Y.-I. Fujiwara, University of Kyoto,
WO2010001640, 2010.
[7] (a) O. Moudam, A. Kaeser, B. Delavaux-Nicot, C. Duhayon, M. Holler, G. Accorsi,
N. Armaroli, I. Séguyd, J. Navarrod, P. Destruel, J.-F. Nierengarten, Chem.
Commun. (2007) 3077;
Acknowledgments
We thank the NSF for support and the Department of Education
for a GAANN fellowship for M.F.C.
(b) A. Tsuboyama, K. Kuge, M. Furugori, S. Okada, M. Hoshino, K. Ueno, Inorg.
Chem. 46 (2007) 1992;
(c) P. Aslanidis, P.J. Cox, A. Kaltzoglou, A.C. Tsipis, Eur. J. Inorg. Chem. (2006)
334;
Appendix A. Supplementary material
(d) A. Kaltzoglou, P.J. Cox, P. Aslanidis, Inorg. Chim. Acta 358 (2005) 3048.
[8] For photophysical and structural properties of related Ag(dppBz) complexes,
see: (a) M. Osawa, M. Hoshino, Chem. Commun. (2008) 6384;
(b) T. Tsukuda, M. Kawase, A. Dairiki, K. Matsumoto, T. Tsubomura, Chem.
Commun. 46 (2010) 1905;
(c) K. Matsumoto, T. Shindo, N. Mukasa, T. Tsukuda, T. Tsubomura, Inorg.
Chem. 49 (2010) 805;
(d) A. Kaltzoglou, P.J. Cox, P. Aslanidis, Polyhedron 26 (2007) 1634.
[9] A.K. Das, E. Bulak, B. Sarkar, F. Lissner, T. Schleid, M. Niemeyer, J. Fiedler, W.
Kaim, Organometallics 27 (2008) 218.
CCDC 788062, 788063, 788064 and 788065 contains the sup-
plementary 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. Supplemen-
tary data associated with this article can be found, in the online
version, at doi:10.1016/j.ica.2011.01.015.
[10] A.B. Pangborn, M.A. Giardello, R.H. Grubbs, R.K. Rosen, F.J. Timmers,
Organometallics 15 (1996) 1518.
[11] K. Bourumeau, A.-C. Gaumont, J.-M. Denis, J. Organomet. Chem. 529 (1997)
205.
References
[12] G.J. Kubas, Inorg. Synth. 28 (1990) 68.
[13] S. Xin, H.G. Woo, J.F. Harrod, E. Samuel, A.-M. Lebuis, J. Am. Chem. Soc. 119
(1997) 5307.
[1] (a) T. Imamoto, K. Sugita, K. Yoshida, J. Am. Chem. Soc. 127 (2005) 11934;
(b) T. Imamoto, A. Koide, in: S.M. Roberts, J. Whittall (Eds.), Catalysts for Fine
Chemical Synthesis, Regio- and Stereo-Controlled Oxidations and Reductions,

M.F. Cain et al. / Inorganica Chimica Acta 369 (2011) 55–61
Lorenz, A. Saha, C.H. Senanayake, J. Org. Chem. 73 (2008) 1524;
61
[14] (a) J.J. Mrowca, DuPont, US4234729, 1980.;
(b) E.I. Matrosov, Z.A. Starikova, A.A. Khodak, E.E. Nifant’ev, Zh. Neorg. Khim.
48 (2003) 2008.
[15] (a) E.P. Kyba, S.-T. Liu, R.L. Harris, Organometallics 2 (1983) 1877;
(b) E.P. Kyba, S.T. Liu, Inorg. Chem. 24 (1985) 1613.
(c) C.A. Busacca, T. Bartholomeyzik, S. Cheekoori, R. Raju, M. Eriksson, S.
Kapadia, A. Saha, X. Zeng, C.H. Senanayake, Synlett (2009) 287.
[18] M.F. Cain, R.P. Hughes, D.S. Glueck, J.A. Golen, C.E. Moore, A.L. Rheingold, Inorg.
Chem. 49 (2010) 7650.
[16] M. Zablocka, B. Delest, A. Igau, A. Skowronska, J.-P. Majoral, Tetrahedron Lett.
38 (1997) 5997.
[17] (a) C.A. Busacca, J.C. Lorenz, N. Grinberg, N. Haddad, M. Hrapchak, B. Latli, H.
Lee, P. Sabila, A. Saha, M. Sarvestani, S. Shen, R. Varsolona, X. Wei, C.H.
Senanayake, Org. Lett. 7 (2005) 4277;
[19] W. Levason, G. Reid, M. Webster, Acta Crystallogr., Sect. C 62 (2006) o438.
[20] (a) N. Armaroli, G. Accorsi, F. Cardinali, A. Listorti, Top. Curr. Chem. 280 (2007)
69;
(b) A. Barbieri, G. Accorsi, N. Armaroli, Chem. Commun. (2008) 2185.
[21] J.R. Lakowicz, Principles of Fluorescence Spectroscopy, third ed., Springer, New
York, 2006, pp. 278–279 and 334–339.
(b) C.A. Busacca, R. Raju, N. Grinberg, N. Haddad, P. James-Jones, H. Lee, J.C.