Neutral and cationic gold(I) complexes with π-conjugated phosphasilene ligands

Article abstract of DOI:10.1021/om2003442

A series of neutral and cationic Au(I) complexes incorporating π-conjugated phosphasilene ligands stabilized by the bulky 1,1,3,3,5,5,7,7-octa-R-substituted s-hydrindacen-4-yl (Rind) groups have been synthesized. X-ray crystallographic studies show the ?¹-coordination mode of the phosphasilene ligand, accommodating the linear trans two-coordinate geometry around the gold center. While the metal coordination slightly enhances the intensity of the strong π-π* absorption of the phosphasilene chromophores, the phosphashilene π-conjugation is not extended through the Au atom.

Full text of DOI:10.1021/om2003442

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pubs.acs.org/Organometallics
Neutral and Cationic Gold(I) Complexes with π-Conjugated
Phosphasilene Ligands
Baolin Li,† Tsukasa Matsuo,*,† Takeo Fukunaga,‡ Daisuke Hashizume,‡ Hiroyuki Fueno,§
Kazuyoshi Tanaka,§ and Kohei Tamao*,†
†Functional Elemento-Organic Chemistry Unit and ‡Advanced Technology Support Division, RIKEN Advanced Science Institute,
2-1 Hirosawa, Wako, Saitama 351-0198, Japan
§Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan
S Supporting Information
b
ABSTRACT: A series of neutral and cationic Au(I) complexes incor-
porating π-conjugated phosphasilene ligands stabilized by the bulky
1,1,3,3,5,5,7,7-octa-R-substituted s-hydrindacen-4-yl (Rind) groups have
been synthesized. X-ray crystallographic studies show the η1-coordina-
tion mode of the phosphasilene ligand, accommodating the linear trans
two-coordinate geometry around the gold center. While the metal
coordination slightly enhances the intensity of the strong πꢀπ* absorp-
tion of the phosphasilene chromophores, the phosphashilene π-conjuga-
tion is not extended through the Au atom.
ince stable compounds comprising PdC,1 PdP,2 SidC,3 and
SidSi4 fragments were synthesized by introducing the con-
rational design and synthesis of neutral and cationic Au(I)
complexes of phosphasilenes 1 and 2 with the bulky Eind and
the less bulky EMind16 groups (Chart 1). Our research interests
are directed toward photophysical properties of the phosphasi-
lene complexes: whether the π-conjugation can be extended
through the Au atom.
S
cept of kinetic stabilization with bulky substituents, many
experimental and computational efforts have been directed
toward the study of a variety of unsaturated compounds of the
heavier main group elements in the last three decades.5 Transi-
tion metal complexes of phosphaalkenes6 and diphosphenes7
have also been extensively investigated, thus demonstrating
various coordination modes.
The first stable SidP compound, phosphasilene, was reported
by Bickelhaupt et al. in 1984, as a rare case of a heavy hetero-
nuclear double bond species between group 14 and 15 elements.8
After that, a number of phosphasilene derivatives have been
successfully synthesized by taking advantage of the steric protec-
tion with appropriate bulky groups on the phosphorus and
silicon atoms.9ꢀ12 A half-parent phosphasilene with a PH group
and its metalated compounds were also isolated and structurally
characterized by Driess et al.11 Very recently, a cyclic compound,
2,4-disila-1,3-diphosphacyclobutadiene, has been synthesized by
two independent groups.13 However, transition metal complexes
having a neutral phosphasilene ligand remain unknown, which
would provide new aspects of coordination modes and homo-
geneous catalysis.
In 2009, we reported the synthesis of two π-conjugated
phosphasilenes, 1a and 2a, stabilized by the fused-ring bulky
1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl (Eind) groups
(Chart 1).14 The Eind groups encapsulate the reactive SidP
moieties and produce highly coplanar π-conjugated frameworks
by their orthogonal arrays to exhibit a strong πꢀπ* absorption
on the basis of the efficient π-delocalization over the skeleton,15
which is in sharp contrast to the previously reported phospha-
silenes with a weak nꢀπ* absorption. Herein, we report the
Scheme 1 shows the synthesis of mononuclear neutral com-
plexes 3 and cationic complex 4 bearing phenylphosphasilene
ligands 1a and 1b. The treatment of 1 with AuCl(tht) (tht =
tetrahydrothiophene) in benzene afforded AuCl complexes 3 as a
yellow powder. The formation of 3 has been clearly confirmed by
29Si and 31P NMR spectra. The 29Si NMR signal of 3a (144.4
ppm) is shifted upfield relative to 1a (161.7 ppm) with a
1
decreasing JSiP coupling constant (39 Hz for 3a and 171 Hz
for 1a). The 31P NMR signal of 3a (43.2 ppm) is also shifted to a
higher field with respect to 1a (89.2 ppm), which is similar to
phosphaalkene Au(I) complexes.17 The Eind-based 3a is stable
in air for months in the solid state and for weeks as a dilute
solution, whereas the less bulky EMind-based 3b decomposes in
air within a few weeks in the solid state and a few hours in
solution.
The molecular structure of 3a was determined by X-ray
crystallography (Figure 1a).18 The phosphasilene ligand links
to the gold atom through the phosphorus atom in an η1-fashion
with a virtually linear two-coordinate geometry at the gold atom
with a PꢀAuꢀCl angle of 178.828(6)°. The SidP bond length in
3a [2.0924(2) Å] is comparable to that in 1a [2.0917(5) Å].
Compounds 3 are discrete metal chloride complexes and can be
regarded as good precursors for the cationic Au(I) complexes.
Received: April 26, 2011
Published: June 14, 2011
r
2011 American Chemical Society
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|

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In fact, as shown in Scheme 1, the chlorine atom in 3b can be
removed, in the presence of another molecule of phosphasilene
1b, by the action of [Ph3C][B(C6F5)4] in CH2Cl2 to produce the
cationic complex 4 with two EMind-based phosphasilene ligands
as yellow crystals in 81% yield. The 31P NMR spectrum of 4
shows only one signal appearing at 48.1 ppm, which indicates a
highly symmetric structure in solution. A similar reaction of the
bulkier Eind-based 3a with 1a was also examined, but no forma-
tion of such a cationic complex was observed due to the steric
repulsion between the bulky Eind groups. A cross-combination
between 3b and 1a afforded a mixture of complex 4 and a cationic
complex bearing both 1a and 1b ligands due to the ligand
exchange, as observed by the NMR studies; however, none of
them were isolated.
The X-ray molecular structure of the cation in 4 has C2-
symmetry in the crystal with the 2-fold axis passing through the
Au atom (Figure 1b). A linear two-coordination is found at the
gold center with a PꢀAuꢀP angle of 178.13(6)°. The PꢀAu
distance in 4 [2.2807(13) Å] is slightly longer than that in 3a
[2.2295(3) Å], which indicates the sterically overcrowded metal
center. While the phenyl groups on the Si atoms are nearly
coplanar with the SidP bonds with a PꢀSiꢀCꢀC torsion angle
of 6.7(4)°, the dihedral angle between the two terminal benzene
rings is 39.3°, thus producing a twisted arrangement between the
two phenylphosphasilene skeletons in the crystal.
Chart 1. Phosphasilenes and Rind Groups
Dinuclear neutral AuCl complexes 5a and 5b were also
synthesized employing 1,4-bis(phosphasilenyl)benzenes 2a and
2b as ligands (Scheme 2). The Eind-based complex 5a and the
less bulky EMind-based 5b exhibit air stability similar to that of
the corresponding mononuclear complexes 3a and 3b. The X-ray
structural analysis of 5a demonstrates the maintenance of the
high coplanarity of the 1,4-bis(phosphasilenyl)benzene skeleton
containing two AuCl fragments, as shown in Figure 1c.
The synthesis of the dinuclear dicationic complexes was not
easy. After several attempts with possible combinations of two
different phosphasilene ligands 1 and 2, we have finally succeeded
in the synthesis of a mixed dinuclear dicationic complex 6,
consisting of the Eind-based 2a and the less bulky EMind-based
1b. It should be noted that the use of bulky Eind-based 2a was
essential as the central ligand for 6: a combination of less bulky 2b
instead of 2a as the central ligand with 1b resulted in a complex
mixture containing oligonuclear compounds. In the successful case,
5a and 1b in a 1:2 ratio were allowed to react with two equivalents
of [Ph3C][B(C6F5)4] to afford 6, together with a small amount of
4. After washing of the precipitates with CH2Cl2/hexane, 6 was
isolated as orange solids in 81% yield. Complex 6 is highly air-
sensitive but thermally stable. No ligand redistribution occurs in
solution at room temperature, as confirmed by NMR studies. Thus,
in the 31P NMR spectrum of 6 in CDCl3, a pair of doublets appear
at 52.6 and 65.9 ppm with a large 2JPP coupling constant of 388 Hz;
Scheme 1. Synthesis of Complexes 3 and 4
2
such a large JPP is characteristic of the linear two-coordinate
geometry around the metal center.19 Also in the ESI-HRMS
spectrum, the observed ion peak at m/z 1843.2043 is in good
agreement with the counterion-free molecular ion, [M ꢀ 2B
(C6F5)4]2þ (calcd for [C226H342Au2Si4P4]2þ: m/z 1843.2060).
Figure 1. Molecular structures of (a) 3a, (b) 4, and (c) 5a. Hydrogen atoms and counteranion of 4 are omitted for clarity.
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COMMUNICATION
Scheme 2. Synthesis of Complexes 5 and 6
Table 1. UVꢀVis Absorption Dataa of Phosphasilenes 1 and 2
and Their Gold Complexes 3ꢀ6 Measured at Room
Temperature
Figure 2. Frontier molecular orbitals of 3a (left) and 40 (right).
[PhSiHdPH]AuCl and [PhSiHdPH]2Auþ, at the B3LYP/
6-31G(d,p) level (B3LYP/cc-pVDZ-pp level for the Au atom)
using the Gaussian 03 suite of programs.21 The optimized
structures of 3a and 40 reproduced well the X-ray molecular
structures, while the parent counterparts were found to have an
η2-coordination of the phosphasilene ligand(s) in the equilibrium
geometries. Thus, the experimentally observed η1-coordination
mode should be ascribed to the steric hindrance by the bulky
Rind groups, covering the SidP double bond.
In addition to the σ-donor through the phosphorus atom, the
phosphasilene ligands would act as a π-acceptor for transition
metals on the basis of their low-lying LUMO.14 However, as
shown in Figure 2, the LUMOs of 3a and 40 consist mainly of the
SiꢀP π*-orbitals with essentially no contribution of the Au
atomic orbital, although the LUMO energy levels are lowered
by the complexation [ꢀ1.552 (1a) vs ꢀ2.256 (3a) and ꢀ4.143
(40) eV]. The HOMOs of 3a and 40 are primarily represented by
the SiꢀP π-orbital(s). Thus, the Au atom may interrupt the
extended π-conjugation in 40 as a defect, which is consistent with
the experimental results. This is most probably due to the
mismatching of the energy levels between the 3p-based frontier
MOs of the phosphasilene ligands and the filled 5d-orbitals of the
Au(I) ion. The TD-DFT calculations indicate that the absorp-
tions observed in both 3a and 40 are mainly assigned to the
HOMO f LUMO (πꢀπ*) transition. The nꢀπ* transition of 1
is hampered by the Au coordination through the nonbonding
pairs of electrons on the phosphorus atom.22
cmpd
λmax/nm
ε/cmꢀ1 Mꢀ1
phosphasilenes
1ab
385c
378
449c
436
1.2 ꢁ 104
1.0 ꢁ 104
2.2 ꢁ 104
2.0 ꢁ 104
1b
2ab
2b
Au(I) complexes
3a
3b
4
395c
386
408
460
445
383
465
1.8 ꢁ 104
1.7 ꢁ 104
4.0 ꢁ 104
3.3 ꢁ 104
3.0 ꢁ 104
2.6 ꢁ 104
5.3 ꢁ 104
5a
5b
6
a Measured in CH2Cl2. b See ref 14. c Measured in THF.
Table 1 summarizes the UVꢀvis absorption data for the
phosphasilenes and their gold complexes. The mononuclear AuCl
complexes 3a and 3b in CH2Cl2 show the πꢀπ*absorptionpeakat
395 and 386 nm, which are 10 and 8 nm red-shifted from 1a and 1b,
respectively, together with the slightly enhanced absorption in-
tensity. The strong πꢀπ* absorption for the cationic bis-phospha-
silene complex 4 at 408 nm is red-shifted by 22 nm relative to that
of neutral 3b. The latter data suggest a certain π-conjugation
between the two phenylphosphasilene ligands through the Au
atom. However, the UVꢀvis spectrum of 6, which has two different
phosphasilene chromophores, shows two distinct absorption peaks
at 383 and 465 nm; each of them is essentially the same as that of
the corresponding free ligands [378 (1b) and 460 (5a) nm]. This
fact indicates a simple electronic accumulation of the two different
phosphasilene chromophores in 6; thus, the π-conjugation is not
extended through the Au atoms.20
We have demonstrated a rational approach for producing
phosphasilene gold(I) complexes. The steric bulkiness of the
Rind groups greatly affects the coordination mode from
π-coordination to σ-coordination, air stability, and reactivity of
the resulting complexes. An extension of the phosphasilene
π-conjugation through the Au atom has not been found in the
bis-phophasilene cationic complexes; these observations are theore-
tically supported by DFT calculations. Investigations on the use
of the new complexes as a catalyst in organic synthesis are now in
progress.
DFT computations were performed for 3a and 40 (a counter-
ion-free form of 4) and their parent phosphasilene complexes,
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COMMUNICATION
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*Phone: þ81-48-462-4994. Fax: þ81-48-462-4995. E-mail:
matsuo@riken.jp; tamao@riken.jp.
’ ACKNOWLEDGMENT
We thank the Ministry of Education, Culture, Sports, Science,
and Technology of Japan for the Grant-in-Aid for Specially
Promoted Research (No. 19002008). Numerical calculations
were partly performed at the Supercomputer Laboratory, In-
stitute for Chemical Research, Kyoto University. We thank
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