A new xantphos-type ligand and its gold(I) complexes: Synthesis, structure, luminescence

Article abstract of DOI:10.1016/j.poly.2013.02.052

A novel xantphos analog diphosphine ligand, 9,9-dimethyl-4,5- bis(diphenylphosphinomethyl)-9H-xanthene (X(CP)₂), with methylene groups inserted between the xanthene skeleton and the two diphenylphosphine units, has been synthesized. A two-coordinate and a three-coordinate gold(I) complex of the ligand, [Au₂Cl₂(X(CP)₂)] and [AuCl(X(CP)₂)], have been prepared and studied by X-ray diffraction, NMR and optical spectroscopy. In the solid state, [AuCl(X(CP)₂)] adopts a highly ordered structure with a planar xanthene skeleton that faces another plane composed of two phenyl rings and the AuCl moiety. The structure of [Au₂Cl₂(X(CP)₂)] is much less regular, the two P-Au-Cl vectors point to the opposite sides of the folded xanthene backbone. The exchange-broadened resonances in the NMR spectra of [AuCl(X(CP) ₂))] indicate that this complex exists as a mixture of various chemical species and/or conformers in solution. In contrast, the NMR spectra of [Au₂Cl₂(X(CP)₂)] exclude any medium-range exchange processes. Aurophilic interactions are absent in both X(CP)₂ complexes. X(CP)₂, as well as its two gold complexes, is phosphorescent in the solid state; the complexes emit at higher wavelengths and with longer lifetimes than the free ligand.

Full text of DOI:10.1016/j.poly.2013.02.052

Polyhedron 55 (2013) 57–66
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Polyhedron
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A new xantphos-type ligand and its gold(I) complexes: Synthesis,
structure, luminescence
Gábor Besenyei a, István Bitter b, László Párkányi c, Gábor Szalontai d, Péter Baranyai a, Éva Kunsági-Máté e,
Ferenc Faigl b, Alajos Grün b, Miklós Kubinyi a,e,
⇑
a Institute of Molecular Pharmacology, Research Centre for Natural Sciences, Hungarian Academy of Sciences, 1025 Budapest, Pusztaszeri út 59-67, Hungary
b MTA-BME Organic Chemical Technology Research Group, Department of Organic Chemistry and Technology, Budapest University of Technology and Economics, 1111
}
Budapest, Muegyetem rkp. 3., Hungary
c Institute of Organic Chemistry, Research Centre for Natural Sciences, Hungarian Academy of Sciences, 1025 Budapest, Pusztaszeri út 59-67, Hungary
d University of Pannónia, NMR Laboratory, 8200 Veszprém, Egyetem u. 10., Hungary
e
}
Department of Physical Chemistry and Materials Science, Budapest University of Technology and Economics, 1111 Budapest, Muegyetem rkp. 3., Hungary
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel xantphos analog diphosphine ligand, 9,9-dimethyl-4,5-bis(diphenylphosphinomethyl)-9H-xan-
thene (X(CP)2), with methylene groups inserted between the xanthene skeleton and the two diphenyl-
phosphine units, has been synthesized. A two-coordinate and a three-coordinate gold(I) complex of
the ligand, [Au2Cl2(X(CP)2)] and [AuCl(X(CP)2)], have been prepared and studied by X-ray diffraction,
NMR and optical spectroscopy. In the solid state, [AuCl(X(CP)2)] adopts a highly ordered structure with
a planar xanthene skeleton that faces another plane composed of two phenyl rings and the AuCl moiety.
The structure of [Au2Cl2(X(CP)2)] is much less regular, the two P–Au–Cl vectors point to the opposite sides
of the folded xanthene backbone. The exchange-broadened resonances in the NMR spectra of
[AuCl(X(CP)2))] indicate that this complex exists as a mixture of various chemical species and/or con-
formers in solution. In contrast, the NMR spectra of [Au2Cl2(X(CP)2)] exclude any medium-range
exchange processes. Aurophilic interactions are absent in both X(CP)2 complexes. X(CP)2, as well as its
two gold complexes, is phosphorescent in the solid state; the complexes emit at higher wavelengths
and with longer lifetimes than the free ligand.
Received 16 November 2012
Accepted 12 February 2013
Available online 5 March 2013
Keywords:
Gold(I) complexes
Phosphine ligands
Coordination modes
Crystal structure
Luminescence
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
Combined X-ray and catalytic studies on the complexes of xant-
phos and other diphosphines revealed that the catalytic effect de-
4,5-Bis(diphenylphosphino)-9,9-dimethyl-9H-xanthene (xant-
phos; XP2 in Scheme 1), synthesized first by Kranenburg et al. [1]
and Hillebrand et al. [2], has been applied as a phosphine ligand
in the preparation of a great number of transition metal complexes.
The xanthene skeleton is not fully rigid [3,4], the dihedral angle
characterizing its folding and the torsion angle characterizing its
twisting, and thereby the Pꢀ ꢀ ꢀP distance and the P–M–P chelating
angle, vary in xantphos complexes, depending on the central metal
atom and the counter ligands [5]. This explains the structural vari-
ety of xantphos complexes, demonstrated by studies on its Pd [6],
Pt [7], Rh [5] and Ir [8] complexes, in which it acts as a monoden-
tate, bidentate (P,P with a cis or trans coordination mode) or triden-
tate (P,O,P) ligand. Many of its Pd, Ru and Rh complexes proved to
be efficient catalysts in homogeneous catalytic reactions [9,10].
pends on the P–M–P ‘bite angle’ [6,11,12].
Xantphos is also an important ligand in gold chemistry: it can
form dinuclear complexes with intramolecular aurophilic interac-
tions [4,13–16]. In addition, a three-coordinate [17] and a four-
coordinate [13] (quasi tetrahedral) gold(I) xantphos complex have
also been reported, with a lack of any aurophilic interactions.
Many of the organometallic complexes with diphosphine li-
gands exhibit strong luminescence at room temperature, making
them attractive candidates as indicators in optical sensors and as
emitters in electroluminescent devices [18,19]. Xantphos occurs
as a diphosphine ligand in luminescent Cu(I) [20–22] and Au(I)
[4,13,14, 23,24] complexes. The quenching of the luminescence
of Cu(I) xantphos complexes by oxygen was utilized for construct-
ing optical sensors [21]. Heteroleptic Cu(I) complexes with xant-
phos and a pyrrole derivative ligand have been tested as emitting
layers in LED devices [22]. Au(I) xantphos complexes can also be
expected to show photophysical properties interesting for such
technical applications, in particular gold complexes with aurophilic
interactions have special luminescence characteristics [25,26].
⇑
Corresponding author at: Institute of Molecular Pharmacology, Research Centre
for Natural Sciences, Hungarian Academy of Sciences, 1025 Budapest, Pusztaszeri út
59-67, Hungary. Tel.: +36 1 438 1120; fax: +36 1 438 1143.
E-mail address: kubinyi.miklos@ttk.mta.hu (M. Kubinyi).
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.02.052

58
G. Besenyei et al. / Polyhedron 55 (2013) 57–66
Modification of the above mentioned methods allowed the de-
sired product to be obtained in a much higher yield. Actually,
dimetallation was carried out in pure hexane at reflux temperature
followed by the addition of DMF at room temperature and a simple
aqueous hydrolysis. Under these conditions the product precipi-
tated from the reaction mixture in 91% yield and it could be used
in the next reactions without any purification.
2.1.2. 4,5-Bis(hydroxymethyl)-9,9-dimethyl-9H-xanthene (2)
To an EtOH solution (60 cm3) of dialdehyde
1 (2.75 g,
Scheme 1.
10.4 mmol) was added NaBH4 (2.6 g, 70 mmol) and the resulting
mixture was stirred overnight at ambient temperature. It was then
evaporated to dryness, treated with water (50 cm3) and acidified
with dilute HCl, followed by extraction with ethyl acetate
(2 ꢁ 50 cm3). The combined organic phases were washed with
water (50 cm3), dried (Na2SO4) and the volatiles were removed un-
der reduced pressure. The product was crystallized with ether to
obtain 2.4 g (86%) of a white semi-solid in essentially the pure
form. TLC: RF 0.35 (toluene–MeOH = 8:2).
In order to gain further information on the effects of the ligand
flexibility on the properties of diphosphine complexes, in the pres-
ent work a new diphosphine ligand with a xanthene skeleton, 9,9-
dimethyl-4,5-bis(diphenylphosphinomethyl)-9H-xanthene (X(CP)2
in Scheme 1), has been prepared, in which the diphenyphosphine
units and the xanthene scaffold are separated by methylene
groups, and two gold(I) complexes of the new ligand – [AuCl-
(X(CP)2)] and [Au2Cl2(X(CP)2)] – have been prepared and studied
by X-ray diffraction, NMR and luminescence spectroscopy.
1H NMR d (ppm): 7.38 (dd, 2H, J 8, 1.5 Hz, ArH), 7.14 (dd, 2H, J 7,
1 Hz, ArH), 7.04 (t, 2H, J 7.5 Hz, ArH), 4.73 (s, 4H, CH2), 1.62 (s, 6H,
CH3). 13C NMR d (ppm): 149.0, 130.5, 128.1, 127.9, 126.2, 123.1
(Ar), 62.1(CH2), 34.3 (C), 32.3 (CH3).
2. Materials and methods
The reagents used in the syntheses were purchased from Al-
drich and were used without purification. Hexane was distilled
from P2O5. All moisture or air sensitive compounds were synthe-
2.1.3. 4,5-Bis(bromomethyl)-9,9-dimethyl-9H-xanthene (3)
To a CHCl3 solution (30 cm3) of diol 2 (2.0 g, 7.4 mmol) was added
dropwise PBr3 (1.75 cm3, 18 mmol) dissolved in CHCl3 (10 cm3) and
the resulting mixture was stirred for 2 h at ambient temperature.
Then saturated aqueous NaHCO3 solution (60 cm3) was added care-
fully, the organic phase was separated, washed with water (2x50
cm3), dried (Na2SO4) and evaporated to obtain 2.55 g (87%) of a
white semi-solid. TLC: RF 0.75 (hexane–EtOAc = 8:2).
sized under
a nitrogen atmosphere using standard Schlenk
techniques. For column chromatography Merck Kieselgel 60
(0.063–0.20 mm), for analytical TLC Merck Kieselgel GF254 were
used.
1H NMR d (ppm): 7.40 (dd, 2H, J 7.5, 1 Hz, ArH), 7.27 (dd, 2H, J
7.5, 1.5 Hz, ArH), 7.07 (t, 2H, J 8 Hz, ArH), 4.82 (s, 4H, CH2), 1.63 (s,
6H, CH3). 13C NMR d (ppm): 147.9, 130.2, 128.8, 127.0, 125.3, 123.2
(Ar), 34.2, (C), 32.5 (CH3), 28.8 (CH2Br).
2.1. Synthesis
2.1.1. 9,9-Dimethyl-9H-xanthene-4,5-dicarbaldehyde (1)
9,9-Dimethyl-9H-xanthene (5.0 g, 23.8 mmol) was dissolved in
dry hexane (160 cm3) under an inert nitrogene atmosphere and
N,N,N0N0-tetramethylethylenediamine (TMEDA, 6.9 g, 8.9 cm3,
59.5 mmol) was added into it at 25 °C. A solution of butyllithium
in hexane (1.59 mol/dm3, 38.0 cm3, 59.5 mmol) was added drop-
wise to the stirred reaction mixture, forming a dark red solution.
This was refluxed for 30 min, then cooled down to 25 °C before
dry N,N-dimethyl-formamide (4.4 g, 5.0 cm3, 60.2 mmol) was
poured into the reaction mixture. The dark solution was stirred
for 1 h whilst its color became yellow. Aqueous hydrochloric acid
(2 mol/dm3, 150 cm3) was added and the mixture was stirred for
15 min. The precipitate that formed was filtered off, washed with
water (2 ꢁ 20 cm3) then with hexane/ethyl acetate = 9/1 mixture
(2 ꢁ 20 cm3) and dried to yield the titled compound as an off-white
solide (5.8 g, 91%), TLC RF: 0.25, eluent hexane/ethyl acetate = 9/1.
An analytically pure sample was obtained by recrystallization of
the crude product from ethyl acetate.
2.1.4. 9,9-Dimethyl-4,5-bis(diphenylphosphinoylmethyl)-9H-
xanthene (X(CPO)2, 4)
Dibromide 3 (1.58 g, 4 mmol) was dissolved in THF (10 cm3)
and added dropwise into a potassium diphenylphosphide solution
(0.5 M, 21 cm3). The mixture was refluxed for 12 h, then the sol-
vent was evaporated in vacuo. The residue was dissolved in CHCl3
(70 cm3), washed with water and chilled, then 30% H2O2 (5 cm3)
was added to complete the oxidation. After a standard work-up,
the product was purified by column chromatography (eluent: tol-
uene–methanol = 9:1) to yield 1.1 g (43%) of
a white solid.
Mp > 230 °C. Anal. Calc. for C41H36O3P2 (4): C, 77.10; H, 5.68.
Found: C, 76.76; H, 5.83%.
1H NMR (CDCl3) d (ppm): 7.68–7.60 (m, 8H), 7.43–7.37 (m, 4H),
7.33–7.26 (m, 8H), 7.21–7.17 (m, 2H), 7.08–7.04 (m, 2H), 6.94–6.89
2
(m, 2H), 3.75 (d, 4H, JHP 14.5 Hz, CH2), 1.40 (s, 6H, CH3). 31P NMR
1H NMR d (ppm): 10.69 (2H, s, CHO), 7.81 (2H, dd, J 7.6, 1.6 Hz,
ArH), 7.71 (2H, dd, J 7.8, 1.6 Hz, ArH), 7.26 (2H, t, J 7.6 Hz, ArH), 1.70
(6H, s, CH3).1
(CDCl3, 161.97 MHz) d (ppm): 30.4 (s). 13C NMR (CDCl3, 100.6 MHz,
Note: 9,9-Dimethyl-9H-xanthene-4,5-dicarbaldehyde is known
from the literature. Dilithiation of 9,9-dimethyl-9H-xanthene fol-
lowed by DMF addition was accomplished first with 3 equivalents
of a butyllithium/TMEDA mixture in diethyl ether/hexane solution
[27] to provide the target compound in 72% yield. A similar yield
was achieved when the reaction sequence was carried out in a tet-
rahydrofuran/hexane mixture and the crude product was purified
by column chromatography [28].
1
Abbreviations used in the description of the NMR spectra are s, singlet; d,
doublet; m, multiplet; t, triplet; p.q., pseudo quintet; p.t., pseudo triplet; br, broad.
Scheme 2.

G. Besenyei et al. / Polyhedron 55 (2013) 57–66
59
(see Scheme 2) d (ppm): 148.1, (d, 5.6 Hz, C1), 133.0 (d, 1JPC 98 Hz,
C10), 131.6 (d, 2.6 Hz, C13), 131.1 (d, JPC 8.8 Hz, C11), 130,2 (d,
Calc. for C41H36Cl2OP2Au2 (7): C, 45.96; H, 3.39. Found: C, 45.50;
H, 3.43%.
2
2.5 Hz, C2), 129.4 (d, 4.5 Hz, C5), 128.3 (d, 11.6 Hz, C12), 124.3
(d, 3.2 Hz, C3), 122.8 (d, 2.8 Hz, C4), 118.9 (d, 8.0 Hz, C6), 34.2 (s,
1H NMR (CDCl3) d (ppm): 7.60–7.49 (m, 8H), 7.49–7.42 (m, 4H),
7.38–7.28 (m, 10H), 6.86–6.78 (m, 2H), 6.48–6.42 (m, 2H), 4.03 (d,
2JHP 11.8 Hz, 4H, CH2), 1.60 (s, 6H, CH3); 31P NMR (CDCl3) d (ppm):
28.5 (s).
1
C8), 31.9 (d, JPC 68.0 Hz, C7), 31.4 (s, C9).
Note: During the reaction, the majority of the phosphine formed
became oxidized to phosphine oxide, therefore the product mix-
ture was treated with H2O2 to achieve complete oxidation.
2.2. X-ray diffraction
Intensity data were collected on a Rigaku Rapid IP diffractome-
2.1.5. 9,9-Dimethyl-4,5-bis(diphenylphosphynylmethyl)-9H-xanthene,
(X(CP)2, 5)
ter with Mo Ka radiation (k = 0.71075 Å) at 293 K. Numerical
absorption corrections were applied in all cases. The structures
were solved by direct methods and refined [31] with anisotropic
least-squares for the non-hydrogen atoms. Hydrogen atomic posi-
tions were generated from assumed geometries and were not
refined.
Of the various routes suitable to convert phosphine oxides to
phosphines, that suggested in Ref. [29] was followed. A suspension
of 0.38 g (0.60 mmol) of 4 in 25 cm3 of toluene and 6 cm3 of trieth-
ylamine was cooled to 0 °C. 2.01 g (14.9 mmol) of HSiCl3 was
added under N2 and the reaction mixture was thermostated at
95 °C for 22 h. The pale yellow slurry was cooled to 0 °C and a solu-
tion of 5.0 g of NaOH in 20 cm3 of H2O was introduced from a syr-
inge. The reaction mixture was allowed to warm to ambient
temperature then was further heated at 45 °C until all the white
precipitate in the bottom (water) phase dissolved (ca. 45 min.).
To the two-phase mixture, 55 cm3 of diethyl ether was added
and the upper (organic) phase was transferred into a flask contain-
ing MgSO4. The drying agent was separated by filtration and the
solvent mixture was removed under reduced pressure to give a
pale yellow oil that slowly solidified on standing. Recrystallization
of the crude product from methanol yielded 228 mg (62.6%) of the
pure diphosphine. Evaporation of the mother liquor to ca. 2 cm3 al-
lowed the isolation of a second crop (36 mg, 10%) of the target
compound. Anal. Calc. for C41H36OP2 (5): C, 81.17; H, 5.98. Found
80.79, H, 6.00%.
2.3. NMR spectroscopy
NMR spectra were recorded on a Bruker Avance DRX 500 and a
Bruker Avance II 400 instrument using tetramethylsilane (1H, 13C)
as an internal standard and 85% H3PO4
dard, in CDCl3 or CD2Cl2 solutions.
(
31P) as an external stan-
2.4. Optical spectroscopy
The UV absorption spectra were recorded on an Agilent 8453
diode array spectrometer. The fluorescence and phosphorescence
measurements were carried out on an Edinburgh Instruments
FLSP920 fluorescence and phosphorescence lifetime and steady
state spectrometer. The excitation light source was a Xe900 steady
state xenon arc lamp when measuring the fluorescence spectra, a
nF900 ns flashlamp filled with hydrogen gas when the fluorescence
1H NMR (CDCl3) d (ppm): 7.34–7.19 (overlapping multiplets,
22H), 6.89–6.85 (m, 2H), 6.75–6.70 (m, 2H), 3.58 (s, 4H, CH2),
1.60 (s, 6H, CH3). 31P NMR (CDCl3, 161.97 MHz)
d (ppm):
decay curves measured and a
lF900H microsecond xenon flash-
ꢂ13.1 (s). 13C NMR (CDCl3, 100.6 MHz, (see Scheme 2) d (ppm):
lamp when the phosphorescence spectra and decay curves were
recorded.
1
2
148.3, (p.t., C1), 138.6 (d, JPC 15.2 Hz, C10), 132.9 (d, JPC 18.5 Hz,
C11), 130,1 (p.t., C2), 128.8 (p.t., C5), 128.4 (s, C13), 128.2 (p.t.,
C12), 124.6 (p.t, C6), 123.4 (p.t. C3), 122.3 (p.t., C4), 34.3 (s, C8),
31.8 (s, C9), 29.7 (p.q., C7).
3. Results and discussion
3.1. Synthesis
2.1.6. [AuCl(X(CP)2)], (6)
Into a solution of 63.5 mg (0.105 mmol) of 5 in 5 cm3 deoxygen-
ated CH2Cl2, 33.5 mg (0.105 mmol) of [AuCl(tht)] [30] was added
and the reaction mixture was stirred for 30 min. The solution
was then concentrated to ca. 2 cm3. Addition of 10 cm3 of ethyl
ether and further stirring for an additional 60 min resulted in a
white powder that was collected on a glass filter, washed with
ethyl ether and dried under vacuum. Yield: 66 mg (75%). Crystals
isolated upon diffusion of diethyl ether vapors into a CH2Cl2 solu-
tion of 6 were suitable for crystallographic studies. Anal. Calc. for
The sequence of the reaction steps is summarized in Scheme 3.
The ligand (X(CP)2), 5, was obtained via a multistep synthesis start-
ing from 9,9-dimethyl-9H-xanthene. In the first reaction, selective
deprotonation of the xanthene backbone in the positions 4 and 5
(activated by the heteroatom) [1,2], was accomplished by 2.5
equiv. of n-BuLi/TMEDA in hexane. The dilithiated species was re-
acted with N,N-dimethylformamide followed by aqueous hydroly-
sis, affording the 4,5-bis(aldehyde) intermediate 1 in high (>90%)
yield. Further treatment with NaBH4 in ethanolic medium followed
by acidification converted 1 to 4,5-bis(hydroxymethyl)xanthene 2
which, in turn, was transformed to 4,5-bis(bromomethyl)-9,9-di-
methyl-9H-xanthene 3 with PBr3 as a brominating agent. Exchange
of the bromine atoms for Ph2P moieties was achieved by condens-
ing 3 with KPPh2 in THF. Unfortunately, the target diphosphine ap-
peared only as a minor constituent of the reaction mixture, in
which the bis(phosphine oxide) 4, and the corresponding phos-
phine monoxide could be identified as the major components. In
order to facilitate the spectroscopic characterization of the isolated
product and also to supply a uniform starting material for the next
reaction step, the mixture of the isolated products was oxidized by
H2O2 to bis(phosphine oxide). Finally, a mixture composed of
HSiCl3 and Et3N in toluene was used to convert 4 to the target
diphosphine 9,9-dimethyl-4,5-bis(diphenylphosphinomethyl)-9H-
xanthene 5.
C
41H36ClOP2Au (6): C, 58.69; H, 4.32. Found: C, 58.38; H 4.36%.
1H NMR (CD2Cl2): 7.36–7.22 (om, 6H), 7.15–7.00 (unresolved,
16H), 6.76–6.68 (m, 2H), 6.46–6.38 (unresolved, 2H), 3.84 (br, s,
4H, CH2), 1.56 (s, 6H, CH3). 31P NMR (CD2Cl2) d (ppm): 32.6 (br, s).
2.1.7. [Au2Cl2(X(CP)2)], (7)
A mixture of 32.0 mg (0.053 mmol) of diphosphine 5 and
33.8 mg (0.105 mmol) of [AuCl(tht)] was dissolved in 6 cm3 of
deaerated dichloromethane and stirred at RT for 15 min under
N2. The solution was concentrated to ca. 2 cm3 under reduced pres-
sure and methanol (8 cm3) was added in small portions. The white
precipitate that formed was filtered off, washed with methanol and
dried under vacuum. Yield: 43 mg (76%). Crystals suitable for crys-
tallographic studies were obtained by diffusion of vapors of diethyl
ether into an acetonitrile solution of the target compound. Anal.

60
G. Besenyei et al. / Polyhedron 55 (2013) 57–66
Scheme 3.
Table 1
The mononuclear [AuCl(X(CP)2)] (6) and dinuclear [Au2Cl2
Crystal data, data collection and refinement parameters.
(X(CP)2)] (7) complexes of the novel X(CP)2 ligand were then pre-
pared via the interaction of the diphosphine ligand with [AuCl(tht)]
(tht stands for tetrahydrothiophene), supplying the reagents in a
1:1 or 1:2 M ratio.
To our knowledge, not only the target diphosphine but also the
intermediates 2–4 are novel compounds, whose spectroscopic data
are given in section Materials and methods.
X(CPO)2
41H36O3P2
638.64
triclinic
[AuCl(X(CP)2)]
[Au2Cl2(X(CP)2)]
Formula
C
C41H36AuClOP2
839.05
orthorhombic
Pnma (No. 62)
C41H36Au2Cl2OP2
1071.5
monoclinic
Formula weight
Crystal system
Space group
Unit cell
ꢀ
P21/n (No. 14)
P1 (No. 2)
a (Å)
11.670(2)
11.9096(15)
12.801(2)
83.025(6)
71.010(6)
86.093(6)
1669.1(4)
2
12.9770(9)
19.5595(15)
14.2934(10)
90
90
90
3628.0(5)
4
1.536
4.247
1664
11.8821(2)
20.0774(4)
16.1880(3)
90
102.9818(7)
90
3763.13(12)
4
1.891
b (Å)
c (Å)
a
(°)
3.2. Crystal structures
b (°)
c
(°)
Besides the structures of the two Au complexes of X(CP)2, the
structure of the oxidized ligand, X(CPO)2 has also been determined
by X-ray diffraction. Crystals of X(CP)2, suitable for such measure-
ments could not be obtained. The unit cell, data collection and
refinement parameters for X(CPO)2, [AuCl(X(CP)2)] and [Au2Cl2
(X(CP)2)] are presented in Table 1.
V (Å3)
Z
Dcalc (g/cm3)
1.271
0.169
672
l
(Mo K
a
) (mmꢂ1
)
8.047
2048
F(000)
Crystal size (mm) 0.40 ꢁ 0.52 ꢁ 0.62 0.25 ꢁ 0.51 ꢁ 0.59 0.34 ꢁ 0.38 ꢁ 0.62
hmin,
(°)
3.3, 27.5
3.1, 27.1
32833, 4099,
0.049
3.2, 28.7
134684, 9670,
0.059
max
The molecular structure of X(CPO)2 is presented in Fig. 1,
selected metric parameters for this molecule are collected in
Table S1 in the Supporting Information. The oxidized ligand
crystallizes without the inclusion of solvent molecules. Whilst
the basic structural data, such as C–C, C–P and P–O bond lengths
as well as the bond angles, do not show any peculiarities, the
planar conformation of the xanthene moiety and the extensive
(intra- and intermolecular) hydrogen bond network (Table 2)
deserve attention.
Total unique data 79375, 7596,
(Rint
Observed data
[I > 2.0 (I)]
Number of
reflection,
parameter
)
0.033
6235
3185
8173
r
7596, 417
4099, 236
9670, 432
R, wR2, S
0.0442, 0.1280,
1.06
0.0328, 0.0716,
1.14
0.0322, 0.0616,
0.99
The most striking structural feature of X(CPO)2 is the almost
perfect coplanarity of the three-ring system of the xanthene frag-
ment. While the angles formed by the planes of the two aromatic
rings in the closely related structure of xantphos [2] are 23.4°

G. Besenyei et al. / Polyhedron 55 (2013) 57–66
61
Fig. 1. Molecular diagram of X(CPO)2 with the numbering of the atoms. Hydrogen atoms are omitted for clarity. Ellipsoids are drawn at the 50% probability level.
Table 2
Strong non-classical hydrogen bond interactions (D = donor, A = acceptor).
D–H
A
D–H (Å)
Hꢀ ꢀ ꢀA (Å)
Dꢀ ꢀ ꢀA (Å)
D–Hꢀ ꢀ ꢀA (°)
X(CPO)2
C9a–H9ab
C2p–H2p
C6p–H6p
C6q–H6q
C8p–H8p
C12p–H12p
[AuCl(X(CP)2)]
C6p–H6p
C12p–H12p
[Au2Cl2(X(CP)2)]
C9a–H9d
O2
0.93
0.93
0.93
0.93
0.93
0.93
2.54
2.57
2.56
2.39
2.56
2.55
3.512(2)
3.405(3)
2.980(3)
3.212(3)
3.489(2)
2.979(3)
176
149
108
148
176
109
O2a [1 ꢂ x, ꢂy, 1 ꢂ z]
O2
O2 [1 ꢂ x, 1 ꢂ y, 1ꢂz]
O2a [1 ꢂ x, ꢂy, 1 ꢂ z]
O2
Cl1
0.93
0.93
2.72
2.75
3.640(4)
3.622(6)
171
156
Cl1[1/2 + x, 1/2 ꢂ y, 3/2 ꢂ z]
O1
0.97
2.41
2.806(6)
104
(XP2) and 20.1° (XP2.THF), and slight to moderate tilting can also be
observed in the oxidized form of this diphosphine, (X(PO)2 [32],
31.1°, 30.1° and 11.4°), the respective angle is only 3.1° in
X(CPO)2. The variations of this dihedral angle prompted us to
search the Cambridge Crystallographic Database, and 403 dimeth-
ylxanthene moietes were retrieved. It is worth noting that no crys-
tal structure of dimethyl (or diethyl) xanthene has been reported.
The distribution of the phenyl dihedral angles are shown in
Fig. S1. The values distribute over a range from 0° to 50.4°, testify-
ing the flexibility of this ring system. We believe that the remark-
able difference in coplanarity can be attributed to the dissimilar
arrangement of the Ph2P units in the molecules where they are di-
rectly bonded to the xanthene backbone, compared to our case
where this coupling is accomplished through inserted CH2 groups.
In the former group of compounds, the lone pair of electrons on the
phosphorus atoms, as well as the P@O fragments, point toward the
same side of the xanthene unit, thereby allowing different intra-
and intermolecular interactions to occur on the two sides of the
plane. This arrangement is demonstrated in Fig. 2. The net outcome
of these uncompensated effects may result in bent xanthene
backbones. The opposite orientation of the two P@O units in our
case, however, makes the two sides of the xanthene unit to be sym-
metrically affected by secondary interactions, and, consequently, a
coplanar arrangement of the three rings is preferred.
The three-coordinate, mononuclear nature of [AuCl(X(CP)2)],
was confirmed by X-ray crystallographic analysis (Fig. 3). The mol-
ecule has Cs symmetry, the mirror plane passes through the Au1,
Cl1, O1, C7, C8 and C8a atoms (and one hydrogen atom from each
methyl group). Bond distances and angles around the gold atom
are presented in Table 3. As illustrated by the right view of the
structure in Fig. 3, the rings of the xanthene moiety are arranged
in ideal coplanarity. (The mean deviation is 0.007 Å, the largest
deviations from the least square-plane are O1 0.0022 Å and C7
0.019 Å.) Surprisingly, the phosphorus, gold and chlorine atoms
also form a plane with two of the phenyl rings that is obviously
stabilized by strong intramolecular interactions. Of these, two
C–Hꢀ ꢀ ꢀCl bonds seem to make the most significant contribution
(Table 2) (d(Hꢀ ꢀ ꢀCl) = 2.72 Å,
u
(C-Hꢀ ꢀ ꢀCl) = 171.0° [intramolecular]
and 2.75 Å, 156.0° [intermolecular]; doubled by symmetry). The
two planes are linked by two P–C(H2) bonds and imitate the form

62
G. Besenyei et al. / Polyhedron 55 (2013) 57–66
Fig. 2. A side view of X(CPO)2 (left) and X(PO)2 (one of the three crystallographically independent molecules) showing the symmetrical and asymmetrical arrangement of the
substituents of the xanthene group.
Fig. 3. Molecular diagram of [AuCl(X(CP)2)]. Hydrogen atoms are omitted for clarity. Ellipsoids are drawn at the 50% probability level (left). A side view of the complex (right).
Table 3
the Auꢀ ꢀ ꢀO distances are longer (3.124–3.203 Å), although the
The geometry around the Au atom (Å and °).
phosphorus atoms are directly connected to the xanthene back-
[AuCl(X(CP)2)]
2.523(2)
[Au2Cl2(X(CP)2)]
[Au2Cl2(XP2)]a
bone in these structures. The methylene hydrogen atoms pointing
towards the plane of symmetry establish short contacts with the
Au1–Cl1
Au1a–Cl1a
Au1–P1
Au1–P1a
Au1a–P1a
2.282(1)
2.286(1)
2.232(1)
2.301(2)
2.307(2)
2.235(2)
oxygen atom (d(Oꢀ ꢀ ꢀH) = 2.46 Å,
u
(C–Hꢀ ꢀ ꢀO) 98.2°; the sum of the
2.306(1)
2.306(1)
van der Waals radii is 2.72 Å) and, together with the Auꢀ ꢀ ꢀO inter-
action, may contribute to the observed stability of the monogold
2.231(1)
2.241(2)
2.9947(4)
3.10
3.99
173.07(6)
Auꢀ ꢀ ꢀAu
species. The
u(P–Au–P) angle is wider (129.50(4)°) than that in
Au1ꢀ ꢀ ꢀO1
3.056(4)
3.136(3)
3.577(3)
176.27(5)
the mononuclear complexes [AuCl(XP2)] and [AuCl(X(Pxyl)2)]
[17] (116.7°, 117.8° and 120.6°) and approaches the value found
in [AuCl(Ph3P)2] [33] where the two phosphorus donor atoms are
not linked by a carbohydride backbone. The lengthening of the
Au–Cl bond in [AuCl(X(CP)2)] (2.523(2) Å) compared to the bond
distances of 2.462, 2.457 and 2.475 Å in the xantphos counterparts
[AuCl(XP2)] and [AuCl(X(Pxyl)2)] is attributable to steric reasons
and to some extent to hydrogen bonds established by the chlorine
atoms as outlined above.
Au1aꢀ ꢀ ꢀO1
Cl1–Au1–P1
Cl1–Au1–P1a
Cl1a–Au1a–P1a
P1–Au1–P1a
P1–Au1ꢀ ꢀ ꢀO1
P1a–Au1aꢀ ꢀ ꢀO1
P1a–Au1ꢀ ꢀ ꢀO1
114.57(3)
114.57(3)
177.20(5)
168.47(7)
129.49(4)
81.5(2)
82.0(3)
77.7(3)
65.3
48.9
81.5(2)
a
From Ref. [13].
Indeed, Au–Cl bond distances in gold(I) complexes of the type
P–Au(Cl)–P seem to depend on steric factors: the wider the P–
Au–P angle, the longer the Au–Cl bond. 62 three-coordinate gold(I)
complexes were retrieved from the Cambridge Crystallographic
Database and the Au–Cl distances show a fair correlation with
the P–Au–P angles (cf. Fig. 4). For some examples other factors
of a boat. Other short contacts are listed in Table S2. The Auꢀ ꢀ ꢀO
distance is shorter by 0.08 Å than the sum of the respective contact
radii and the O1ꢀ ꢀ ꢀAu1 vector direction coincides with the ex-
pected lone pair orientation of the oxygen atom. In the structurally
related monogold complexes [AuCl(XP2)] and [AuCl(X(Pxyl)2)] [17],
may contribute (e.g. the phosphorus atoms are part of
a

G. Besenyei et al. / Polyhedron 55 (2013) 57–66
63
2.286(1) Å) are remarkably shorter than the Au–Cl separation in
[AuCl(X(CP)2)] (2.523(2) Å). The significant decrease of the Au–Cl
distance may be attributable to the lack of any steric hindrance
and hydrogen bonds in the dinuclear complex, which dominate
the environment of the chloride ligand in the monogold complex.
There are 491 P-Au(dicoordinated)–Cl fragments in the CCDC data-
base and their distribution follow a Gaussian curve. The average
Au–Cl bond is 2.288 Å (the minimum 2.233 and the maximum is
2.337 Å). The chlorine atom in [Au2Cl2(X(CP)2)] does not partici-
pate in any short non-bonded interactions. In the related complex
[Au2Cl2(XP2)] [13],
a
strong (2.9947(4) Å) Auꢀ ꢀ ꢀAu aurophilic
interaction is observed. The Au–Cl bonds are slightly longer (see
Table 3) and the Au–P bond distances are comparable to
[Au2Cl2(X(CP)2)].
3.3. NMR spectra
The NMR spectra of diphosphine X(CP)2 are consistent with
the proposed structure. In the 1H NMR spectrum (Fig. S2 in SI),
the ortho and meta hydrogen atoms of the xanthene backbone
show up separately from the rest of the aromatic protons as mul-
tiplets at 6.74 and 6.87 ppm, respectively. The methyl protons ap-
pear at 1.60 ppm as
a sharp singlet, while the resonance
Fig. 4. Au–Cl distances (Å) vs. P–Au–P angles (°) for 62 gold(I) complexes.
attributable to the methylene hydrogens can be observed at
3.58 ppm, apparently with no detectable two-bond coupling to
the adjacent phosphorus atoms. In the 31P{1H} spectrum (Fig. S3
in SI), X(CP)2 gives a sharp singlet at ꢂ13.1 ppm. The interpreta-
tion of the 13C{1H} NMR spectrum (Figs. S4 and S5 in SI) is not
straightforward. While the methyl carbon atoms (C9, 31.8 ppm),
the bridging quaternary carbon carrying them (C8, 34.3 ppm)
and the remote para carbon atoms of the phenyl rings (C13,
128.4 ppm) appear as singlets, all other carbon nuclei can be ob-
served as doublets, pseudo triplets and pseudo quintets (for the
numbering of C atoms see Scheme 2 in the Experimental section).
This second-order feature may be attributed to a weak coupling
between the phosphorus nuclei eight bonds apart from each
other. The nature of this coupling is most probably ‘‘through-
space’’. Such through-space couplings have been identified, for
example in the spectra of bis(4R,6R)-4,6-dimethyl-1,3,2-dioxa-
phosphorinan-2-oxy-alkane derivatives [34] or in the spectra of
xantphos and its analogs [1,2]. The phenomenon was seen as a
consequence of the spatial closeness of the P-atoms and their
macrocycle). The Au–Cl distance range is 0.51 Å (2.44–2.95 Å),
while the P–Au–P angle range is 57.4° (116.1–173.5°), the correla-
tion coefficient is 0.907.
Contrary to the strict intramolecular orderliness of the individ-
ual fragments of the monogold complex, the structure of the dinu-
clear [Au2Cl2(X(CP)2)] compound (see Fig. 5) seems to be much less
regular. The molecule lacks any (even approximate) symmetry and
the P-Au–Cl vectors point to opposite sides of the best plane of the
xanthene moiety which is, in this case, bent by 25.8°. There seems
to be no intramolecular C–Hꢀ ꢀ ꢀCl hydrogen bonds, although several
C–Hꢀ ꢀ ꢀ
p interactions (2.90, 2.92 Å) can be identified. While most of
the bond lengths and bond angles can be considered as normal, the
Au-P and Au-Cl bonds deserve attention. The former in [Au2Cl2
(X(CP)2)] (2.231(1) Å) is noticeably shorter than the respective
distance in [AuCl(X(CP)2)] (2.306(1) Å). The shortening of the
Au-P distance may be ascribed to the fact that the chlorine atom
has a weaker trans influence than a phosphorus donor group.
Further, the Au–Cl bond lengths in [Au2Cl2(X(CP)2)] (2.282(1) and
Fig. 5. Molecular diagram of [Au2Cl2(X(CP)2)]. Ellipsoids are drawn at the 50% probability level, hydrogen atoms are omitted for clarity (left). Side view of the complex (right).

64
G. Besenyei et al. / Polyhedron 55 (2013) 57–66
non-bonding phosphorus electron pairs toward the central re-
gions of the molecule is energetically more advantageous than
that of the phenyl groups and therefore the time averaged value
of the Pꢀ ꢀ ꢀP separation may still allow an observable through-
space coupling to occur in solutions. Simulation of the quintet-
like splitting of the C7 methylene carbon (29.7 ppm) has shown
that a weak Pꢀ ꢀ ꢀP coupling (1.6 Hz) is a prerequisite of the ob-
served slightly unequal intensity of the inner and outer lines. This
splitting mode was transformed into a first order doublet of dou-
blets when J(Pꢀ ꢀ ꢀP) was set to zero (see Fig. S6 in SI). As a further
proof, no resonances other than singlets and doublets can be ob-
served in the 13C NMR spectrum of the oxidized diphosphine
X(CPO)2, where interaction of the phosphorus free electron pairs
is not feasible.
Our NMR studies have revealed a non-negligible sensitivity to
aerial oxidation of X(CP)2, resulting in bis(phosphine oxide) through
a phosphine/phosphine oxide intermediate. Therefore, all manipu-
lations in solution, including the potentially lengthy NMR experi-
ments, were carried out in deoxygenated solvents under
a
nitrogen atmosphere, although the pure, solid diphosphine can be
stored in a refrigerator for months. This feature of X(CP)2 contrasts
with the inertness of xantphos [1], but coincides with the behavior
of the structurally related 2,11-bis(diphenylphosphinomethyl)
benzo[c]phenanthrene, transphos [35], as both X(CP)2 and trans-
phos are formally diphenylbenzylphosphine-type compounds.
In the 1H NMR spectrum of the oxidized diphosphine, X(CPO)2
(Fig. S7 in SI), all the chemically different aromatic protons can
be observed as separate multiplets. The methylene protons are
high-frequency shifted, they appear as a doublet at 3.75 ppm,
while the methyl hydrogens suffer the opposite shift and can be
detected as a singlet at 1.40 ppm. As mentioned above, the 13C
NMR spectrum of X(CPO)2 does not indicate any through-space
coupling between the P-nuclei (the 31P and 13C NMR spectra of
the dioxide can be seen in Figs. S8–S10 in the SI).
Fig. 6. (a): 1H NMR spectra (400 MHz, RT, CD2Cl2), methylene protons only (3.74
and 4.03 ppm) and (b) 31P NMR spectra (30 and 28.5 ppm) of [AuCl(X(CP)2)] (blue
trace) and [Au2Cl2(X(CP)2)] (red trace). (Color online.)
The coordination of one equivalent of AuCl to X(CP)2, i.e. the for-
mation of a ten-membered ring, is accompanied by changes in the
NMR spectrum characteristic to exchange processes. Accordingly,
the methylene resonance as well as the peaks originating from
the aromatic hydrogen atoms are broadened in the NMR spectrum
of [AuCl(X(CP)2)], 6, compared to those of X(CP)2 and the dinuclear
species [Au2Cl2(X(CP)2)], 7 (see Fig. 6, complete 1H and 31P NMR
spectra of the complexes are presented in Figs. S11–S14 in the
SI). These features, as well as the broad phosphorus resonance,
indicate that the locked conformation of the monogold complex
observed in the solid state is not retained in solution, but instead
lone electron pairs (the values reported for the interatomic dis-
tances are 4.045, 4.080 and 4.115 Å, see the same references).
In conformity with this explanation, no J(Pꢀ ꢀ ꢀP) was observed
when the organic backbones allow (DPEphos) or force out (DBF-
phos, 5.760–5.956 Å) larger separations of the interacting P-nuclei
[1]. Although the solid state structure of X(CP)2 is not known to
us at the moment, our crystallographic study on its dioxide,
X(CPO)2, has revealed a Pꢀ ꢀ ꢀP distance of 4.836 Å. Due to the rel-
atively large spatial demand of the diphenylphosphino groups, it
seems reasonable to suppose that the orientation of the
Table 4
Data of bands observed in the optical specra of X(CP)2 and its gold(I) complexes.
Absorption spectrum in CH2Cl2
Fluorescence spectrum in CH2Cl2
Phosphorescence spectrum solid state
kmax (nm) (
e
max(Mꢂ1 cmꢂ1))
kmax (nm)a
sF (ns)a,b
kmax (nm)c
sP (l
s)c,d
X(CP)2
272 sh
285 sh
294 sh
332 (220)
277 sh
285 sh
293 sh
390
1,29 (23%), 3,34 (77%)
385 sh
424
448
480 sh
468 sh
499
150 (61%), 640 (39%)
280 (45%), 970 (55%)
2250 (54%), 5050 (46%)
[AuCl(X(CP)2)]
[Au2Cl2(X(CP)2)]
non-fluorescent
non-fluorescent
530
575 sh
455 sh
477
276 sh
293 (900)
497 sh
507 sh
552 sh
a
b
c
kex = 340 nm.
kem = 390 nm.
kex = 330 nm (X(CP)2), 320 nm ([AuCl(X(CP)2)]), 315 nm ([Au2Cl2(X(CP)2)]).
kem = 450 nm (X(CP)2), 500 nm ([AuCl(X(CP)2)]), 480 nm ([Au2Cl2(X(CP)2)]).
d

G. Besenyei et al. / Polyhedron 55 (2013) 57–66
65
1.2
1
a dynamic interconversion of various species takes place. We did
not attempt to clarify the chemical nature of the individual compo-
nents in this mixture. Note, however, that NMR spectroscopic
studies on solutions of [AuCl(diphosphine)] complexes have led
to the assumption of various equilibrium processes. Thus, with
the transphos ligand, the interconversion of the [AuCl(transphos)]
and [Au(transphos)]Cl components could be deduced [36]. On the
contrary, with the structurally more rigid xantphos diphosphine,
[Au2Cl2(l-xantphos)2] and [AuCl(xantphos)] constituents have
been identified in a multicomponent system with a AuCl:xantphos
molar ratio of 1:1 [13].
The NMR spectra of the dinuclear complex [Au2Cl2(X(CP)2)] are
consistent with the proposed structure. The sharp proton and
phosphorus resonances are indicative of the absence of medium-
range exchange processes. As expected, the two-bond 31P-1H
coupling value changes upon complexation to the Au atoms so
the methylene protons appear as a doublet (2J(HP) = 11.8 Hz) and
the methyl hydrogens give a sharp singlet.
(a)
(b)
0.8
0.6
0.4
0.2
0
250
300
350
Wavelength [nm]
400
1
3.4. Optical spectra
0.5
The data of the absorption and phosphorescence spectra of the
X(CP)2 ligand and its two gold complexes are summarized in
Table 4. The absorption spectra of these compounds in CH2Cl2
solution, and their phosphorescence spectra and phosphorescence
decay curves measured in solid state are shown in Fig. 7.
0
350
450
550
650
The absorption spectrum of the free ligand X(CP)2 involves a
multicomponent band ranging to ꢃ300 nm, which is associated
Wavelength [nm]
with the
p ?
p⁄ transitions of the 9,9-dimethyl-xanthene [37]
1000
(c)
and phenyl units. The broad band around 330 nm is characteristic
of aromatic phosphines. Fife et al. [38] assigned it to an n ? p⁄
transition (the excitation of an electron from the lone-pair orbital
of the P atom to a p⁄ orbital of the phenyl rings). It is more intense
than most other (e. g. carbonyl) n ? p⁄ bands, which can be due
partly to the coupling of this electronic transition with vibrational
ones and partly to the fact that it has some
distinguish it from clear n ? p⁄ transitions, this transition of aro-
matic phosphines is frequently denoted as ‘ ? a (‘one-pair ? an-
p ?
p⁄ character. To
p
ti-bonding p) [39]. In most cases, this band of diphosphine ligands
is blue-shifted in the spectra of their transition metal complexes,
0
0
10
Time [ms]
due to the stabilization of the lone pair electrons of their P atoms,
which are used for the formation of the P-metal
r bonds [40]. This
Fig. 7. (a) Absorption spectra (b) phosphorescence spectra and (c) phosphorescence
decay curves of X(CP)2 (black trace), [AuCl(X(CP)2] (red trace) and [Au2Cl2(X(CP)2]
(blue trace). The absorption spectra were recorded as CH2Cl2 solutions,
c = 5 ꢁ 10ꢂ5 M. The phosphorescence spectra and decay curves were measured in
the solid state, setting kex and kem to the values given under Table 4. (Color online.)
shift is also shown by our complexes. In the spectrum of a
[AuCl(X(CP)2)] solution, in which the mononuclear complex may
form an equilibrium mixture with other species (see the NMR
spectroscopic section), the
the long wavelength side of the multicomponent
the spectrum of [Au2Cl2(X(CP)2)], the band of the
is not discernible, it is probably shifted completely under the
p⁄ band. Theoretical calculations on two- and a three-coordi-
r ? a transition appears as a tail on
p
p
?
p⁄ band. In
? a transition
r
p
distances. Also, the phosphorescence bands of the complexes have
a more distinctive vibrational structure, as compared to the band of
the ligand.
p
?
nate gold(I) phosphine complexes also indicated that the HOMO–
LUMO gap is expected to be larger in the two-coordinate species
[41,42].
The free ligand is fluorescent in solution, with a band maximum
at about 390 nm, but it is not phosphorescent. The solid ligand
shows a fluorescence band at a similar position. The solution sam-
ple of [AuCl(X(CP)2)] shows a weak emission, which may be attrib-
utable to the free ligand. The binuclear complex, [Au2Cl2(X(CP)2)],
is non-emissive in solution.
The solid ligand and the solid complexes are phosphorescent.
Unlike the n ? p⁄ band of the ligand, which is blue shifted in the
absorption spectra of the complexes, the phosphorescence band
of the ligand is red shifted on coordinating the Au atom, with a lar-
ger shift in the spectrum of [AuCl(X(CP)2)]. The phosphorescence
lifetime increases on the coordination of the Au atoms, particularly
in the case of [Au2Cl2(X(CP)2] with short Auꢀ ꢀ ꢀP and Auꢀ ꢀ ꢀCl
The phosphorescence properties of gold(I) complexes are
sensitive to the coordination number of the gold centers and
to the presence of Auꢀ ꢀ ꢀAu interactions [43–45]. In the majority
of the known luminescent two-coordinate Au(I) complexes, the
Au centers are connected by an aurophilic interaction, but sev-
eral luminescent two-coordinate gold(I) phosphine complexes
with a lack of any Auꢀ ꢀ ꢀAu interaction have also been described,
like [AuCl(P(o-tolyl)3)] [46,47], [Au2Cl2(dbfphos)] [13] (dbf-
phos = 4,6-bis(diphenylphosphino)dibenzofuran), [Au2Cl2(binap)]
[23] (binap = 2,20-bis(diphenylphosphino)-1,10-binaphthyl) and
various alkynyl gold(I) complexes with PPh3, PCy3 [48] and
1,4-bis(diphenylphosphino)butane [49] as auxiliary ligands. In
the phosphorescence spectra of [Au2Cl2(dbfphos)] and [Au2Cl2
(binap)], which are structurally similar to [Au2Cl2(X(CP)2], the
position and the shape of the bands are similar to those in the

66
G. Besenyei et al. / Polyhedron 55 (2013) 57–66
spectra of their free ligands, indicating that the emissions of
these complexes originate from intra-ligand (IL) type transitions.
Three-coordinate gold(I) complexes are in general luminescent
and this is not bound to the presence of aurophilic interactions
[43]. In the phosphorescence spectrum of our three-coordinate
complex, [AuCl(X(CP)2)], the band of the free ligand is shifted more
to the red and its shape changes more markedly than in the spectrum
of the two-coordinate compound, indicating that the emission of
[AuCl(X(CP)2)] cannot be attributed to a clear IL excited state.
The emission of three-coordinate gold(I) complexes with phosphine
ligands is usually assigned to metal-centered (MC) and not to IL type
transitions [50]. Quantum chemical calculations on the luminescent
excited states of three-coordinate gold(I) complexes with bulky
phosphine ligands, like our ligand, which prevent the formation of
short AuꢀꢀꢀꢀAu contacts, were carried out by Omary and co-workers
[51,52]. It has been confirmed that the phosphorescence of [AuL3]+
complexes with L = PPh3, PPhCy2 and TPA (1,3,5-triaza-7-phospha-
adamantane) ligands, arises from metal-centered (MC) transitions,
whereas the emittant triplet state of [AuCl(PPh3)2] has metal-to-
ligand charge transfer (MLCT) character. Based on the similarity of
the coordination of the Au atom in [AuCl(PPh3)2] and in
[AuCl(X(CP)2)], it can be presumed that the luminescence of the
latter complex also arises from an MLCT state.
deposit@ccdc.cam.ac.uk. Supplementary data associated with this
article can be found, in the online version, at http://dx.doi.org/
10.1016/j.poly.2013.02.052.
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4. Conclusions
We have synthesized a new diphosphine, 9,9-dimethyl-4,5-
bis(diphenylphosphinomethyl)-9H-xanthene, X(CP)2, its dioxide,
X(CPO)2 and two gold(I) complexes of X(CP)2, [AuCl(X(CP)2)] and
[Au2Cl2(X(CP)2)], and studied their structural properties using
NMR spectroscopy and X-ray diffraction. Contrary to the related
xantphos-type counterparts, an extensive hydrogen bond network
and ideal planarity of the xanthene backbone have been revealed
in X(CPO)2 and [AuCl(X(CP)2)]. In the case of the dioxide, the pla-
narity of the xanthene ring system has been attributed to balanced
intermolecular secondary interactions affecting the two sides of
the molecule. However, the same structural feature of the mono-
nuclear complex seems to be associated with the extra conforma-
tional freedom introduced by the inserted methylene groups,
which enables the Ph2P groups to approach the gold center in
the required geometry by rotating about the C(aryl)–C(CH2) axes,
making the bending of the xanthene backbone unnecessary. The
remarkable structural dissimilarity of X(CPO)2 and [AuCl(X(CP)2)]
to their xantphos-type counterparts promises fruitful further stud-
ies on this ligand and on its metal complexes more closely associ-
ated with homogeneous catalytic reactions. The results on the
luminescence of the two gold complexes of X(CP)2 – the long phos-
phorescence lifetime of [Au2Cl2(X(CP)2)], the strongly red-shifted
phosphorescence of [AuCl(X(CP)2)] – can be helpful in the design
of new organogold(I) compounds as potential organic emitters
and for the use in sensor applications.
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The authors wish to thank Professor Gábor Pálinkás for fruitful dis-
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Appendix A. Supplementary data
CCDC 890992, 890993 and 890994 contains the supplementary
crystallographic data for 4, 6 and 7. These data can be obtained free
of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336 033; or e-mail: