Synthesis, Structure, and Reactivities of Iminosulfane- and Phosphane-Stabilized Carbones Exhibiting Four-Electron Donor Ability

Article abstract of DOI:10.1002/chem.201502166

Iminosulfane(phosphane)carbon(0) derivatives (iSPCs; Ar₃P→C←SPh₂(NMe); Ar=Ph (1), 4-MeOC₆H₄ (2), 4-(Me₂N)C₆H₄ (3)) have been successfully synthesized and the molecular structure of 3 characterized. Carbone 3 is the first thermally and hydrolytically stable carbone stabilized by phosphorus and sulfur ligands. DFT calculations reveal the electronic structures of 1-3, which have two lone pairs of electrons at the carbon center. First and second proton affinity values are theoretically calculated to be in the range of 286.8-301.1 and 189.6-208.3 kcal mol^-1, respectively. Cyclic voltammetry measurements reveal that the HOMO energy levels follow the order of 3>2>1 and the HOMO of 3 is at a higher energy than those of bis(chalcogenane)carbon(0) (BChCs). The reactivities of these lone pairs of electrons are demonstrated by the C-diaurated and C-proton-aurated complexes. These results are the first experimental evidence of phosphorus- and sulfur-stabilized carbones behaving as four-electron donors. In addition, the reaction of hydrochloric salts of the carbones with Ag₂O gives the corresponding Ag^I complexes. The resulting silver(I) carbone complexes can be used as carbone transfer agents. This synthetic protocol can also be used for moisture-sensitive carbone species. Getting tough on carbon: The combination of the high electron-donating ability of the triarylphosphane ligand and the strong π-acceptor ability of the iminosulfane ligand confers considerable thermal and hydrolytic stability on iminosulfane(phosphane)carbon(0) (see figure). The four-electron donor ability of these carbon complexes is revealed by the synthesis of aurated complexes.

Full text of DOI:10.1002/chem.201502166

DOI: 10.1002/chem.201502166
Full Paper
&
Donor–Acceptor Systems
Synthesis, Structure, and Reactivities of Iminosulfane- and
Phosphane-Stabilized Carbones Exhibiting Four-Electron Donor
Ability
[a]
[b]
[b]
[a]
Tomohito Morosaki, Wei-Wei Wang, Shigeru Nagase, and Takayoshi Fujii*
Abstract: Iminosulfane(phosphane)carbon(0) derivatives
levels follow the order of 3>2>1 and the HOMO of 3 is at
a higher energy than those of bis(chalcogenane)carbon(0)
(BChCs). The reactivities of these lone pairs of electrons are
demonstrated by the C-diaurated and C-proton-aurated
complexes. These results are the first experimental evidence
of phosphorus- and sulfur-stabilized carbones behaving as
four-electron donors. In addition, the reaction of hydrochlo-
!
(
(
iSPCs; Ar P!C SPh (NMe); Ar=Ph (1), 4-MeOC H (2), 4-
3
2
6
4
Me N)C H (3)) have been successfully synthesized and the
2
6
4
molecular structure of 3 characterized. Carbone 3 is the first
thermally and hydrolytically stable carbone stabilized by
phosphorus and sulfur ligands. DFT calculations reveal the
electronic structures of 1–3, which have two lone pairs of
electrons at the carbon center. First and second proton affin-
ric salts of the carbones with Ag O gives the corresponding
2
I
ity values are theoretically calculated to be in the range of
Ag complexes. The resulting silver(I) carbone complexes can
À1
2
86.8–301.1 and 189.6–208.3 kcalmol , respectively. Cyclic
be used as carbone transfer agents. This synthetic protocol
can also be used for moisture-sensitive carbone species.
voltammetry measurements reveal that the HOMO energy
[
6a–c,8]
Introduction
one phosphane with various ligands.
They concluded that
diauration of the central carbon atom could be used as a crite-
rion for the carbon(0) character of divalent carbon compounds
Carbones consist of two strong electron-donating ligands,
such as phosphanes in carbodiphosphorane (termed as bi-
s(phosphane)carbon(0) (BPC: A) in our study) or carbenes in
carbodicarbene (bis(carbene)carbon(0) (BCC)), coordinated to
the central zero-valent carbon atom, which maintains its four
valence electrons in two orthogonal lone-pair orbitals. There-
CL . Shortly after, Frenking and Esterhuysen showed that the
2
nature of carbones or allenes could be differentiated by their
[
9]
coordination mode to two gold(I) moieties. In carbones, both
1
gold(I) centers coordinate to carbon in h mode, whereas in al-
2
lenes, both gold(I) centers coordinate in h mode across each
[
1–6]
[9]
fore, it is also described as a carbon complex (Figure 1).
The
of the double bonds. Thus, the central carbon of carbones
1
carbones can be clearly distinguished by their chemical reactiv-
binds to two electrophiles or gold(I) moieties in h mode,
II
ity from divalent C carbene species or ordinary organic com-
which is convincing evidence of carbone character.
pounds, such as allenes. One difference between carbones and
carbenes is that carbones should easily bind to two electro-
philes, whereas carbenes should normally bind to only one
Recently, we reported the synthesis and characterization of
two chalcogen-stabilized carbones (bis(iminosulfane)carbon(0)
(BiSC: B), iminosulfane(sulfane)carbon(0) (iSSC: C), iminosulfa-
ne(selenane)carbon(0) (iSSeC: D); Figure 1), that were four-elec-
tron donors. Herein, we extend carbone chemistry toward
bis(chalcogenane)carbon(0) (BChCs) and suggest that the re-
placement of the iminosulfane ligand with a sulfane or sele-
nane ligand leads to an increase in the reactivity of the carbon
center.
[
1,4,7]
electrophile.
Furthermore, carbones have a very high
[
10]
second proton affinity (PA). The other difference between car-
bones and organic compounds is the binding strength and co-
[8,9]
ordination mode with gold(I).
Alcarazo et al. reported the
synthesis and characterization of a series of mono- and diau-
rated complexes that contained various carbones stabilized by
Although the nature of A and BChCs have been studied,
phosphorus- and sulfur-stabilized carbones have been much
[
a] T. Morosaki, Prof. T. Fujii
Department of Applied Molecular Chemistry
College of Industrial Technology, Nihon University
Chiba 275-8575 (Japan)
E-mail: fujii.takayoshi@nihon-u.ac.jp
[
b] Dr. W.-W. Wang, Prof. S. Nagase
Fukui Institute for Fundamental Chemistry, Kyoto University
Kyoto 606-8103 (Japan)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201502166. It contains full experimental de-
tails.
Figure 1. Molecular structures of A–F.
Chem. Eur. J. 2015, 21, 15405 – 15411
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Full Paper
less explored. In fact, there are only two reports on the synthe-
sis and reactivities of phosphane(sulfane)carbon(0) (PSCs) as
To prevent hydrolytic degradation of the iSPC, we synthe-
sized derivatives 2 and 3, which contained electron-donating
substituents at the para position of the phenyl group on the
phosphorus center (Scheme 2). The corresponding cationic
salts (2·H and 3·H) were obtained by the reaction of 4 with tri-
arylphosphonium salts in the presence of LDA (3 equiv) in
moderate to good yields (64 and 80%, respectively). Com-
pounds 2 and 3 were prepared in nearly quantitative yield by
[
11]
mixed P,S-bisylides by Baceiredo et al. (Figure 1, E, F). They
used F as an unsymmetrical atomic carbon source to create
quaternary carbon centers; however, so far research into the
nature of PSCs is limited to a monometalated copper(I) com-
[11]
plex or monocationic salts. Further studies on the carbone
character of PSCs, such as in diaurated complexes or dicationic
salts, have never been reported, although the predicted elec-
deprotonation of cationic salts 2·H and 3·H with NaNH in THF
2
[1d,11a]
tron-donating ability of E is similar to that of A.
or by passing through a column of the ion-exchange resin.
Upon exposure to air, compound 2 is more stable than 1. In
fact, the decomposition half-life at room temperature in the
solid state of 2 under air is approximately 24 h, whereas 1 com-
pletely decomposes under the same conditions. Derivative 3 is
stable in air (less than three months) and thermally stable
(m.p. 120–1218C, decomposed over 1358C). To the best of our
knowledge, compound 3 is the first air, moisture, and thermal-
ly stable phosphorus- and sulfur-stabilized carbone.
PSCs are thermally unstable, even if they are stabilized by
a directly induced diamino substituent on the phosphorus
center (E: decomposes at room temperature; F: m.p. 30–328C),
whereas other heteroatom-stabilized carbones are thermally
stable (m.p. A: 208–2108C; B: 163–1648C; C:110–111 8C; D: 95–
[
7a,10–12]
9
68C).
For catalytic applications, heat- and moisture-
[13]
stable carbones are required. We reason that to obtain ther-
mally stable phosphorus- and sulfur-stabilized carbones, imino-
sulfane and triarylphosphane compounds should be used as li-
gands. The iminosulfane ligand of B binds more strongly to
the carbon center than the sulfane ligand through n–s* inter-
13
In the C NMR spectra, the central carbon atom of 1–3 ap-
1
pears as a doublet (1: d=23.1 ppm, J(P,C)=62 Hz; 2: d=
1
1
23.0 ppm, J(P,C)=58 Hz; 3: d=23.6 ppm, J(P,C)=68 Hz; see
Table S1 in the Supporting Information), which is shifted to
a lower field compared with the cationic salts (1·H: d=
[
10b]
actions.
Furthermore, the electronic properties at the phos-
phorus center of the triarylphosphine ligand of A can be easily
tuned by choosing appropriate substituents. Herein, we report
the synthesis of this new type of iminosulfane(phosphane)car-
1
1
18.4 ppm, J(P,C)=114 Hz; 2·H: d=19.4 ppm, J(P,C)=114 Hz;
1
3·H: d=19.7 ppm, J(P,C)=112 Hz), and is intermediate be-
tween BPC (d=12.3 ppm) and BiSC (d=39.9 ppm). In the
!
[8]
[15]
bon(0) (iSPCs; Ar P!C SPh (NMe); Ar=Ph(1), 4-MeOC H (2),
3
2
6
4
31
4
-(Me N)C H (3)). Among these iSPC derivatives, compound 3
P NMR spectra, the signals of 1–3 (1: d=À2.64 ppm; 2: d=
2
6
4
is heat and moisture stable and provides the first experimental
proof of the carbone character. The synthesis of silver(I) com-
plexes and their function as carbone transfer reagents are also
described.
À3.51 ppm; 3: d=À1.39 ppm) are shifted to a higher field
than those of cationic salts (1·H: d=15.9 ppm; 2·H: d=
13.4 ppm; 3·H: d=12.7 ppm) and are comparable to that of
[
8]
BPC (d=À2.14 ppm).
Results and Discussion
Abstraction of a proton from the
corresponding protonated salts
is the general method for pre-
paring carbones; thus, protonat-
ed salt 1·H appears to be a po-
tential precursor for desired iSPC
1
. Cationic salt 1·H was obtained
in 30% yield by the reaction of
Scheme 1. Synthesis and hydrolysis reaction of 1. Reagents and conditions: a) nBuLi, THF, 08C to RT, 0.5 h; b) THF,
iminosulfonium salt 4 with tri-
phenylphosphonium methylide 17 h, then H
À788C, 17 h, then H O (30%); c) nBuLi, THF, 08C to RT, 0.5 h, then tBuLi (2 equiv), À788C, 5 h; d) 4, THF, À788C,
2
2
O (50%); e) NaNH
10 ion-exchange resin (OH form), MeOH, RT; 6 (96%), 7 (92%).
2
, THF, À788C to RT, 0.5 h (97%); f) RT, air, 24 h; 6 (95%), 7 (91%); g) Amberlite IRA-
À
4
(
Scheme 1). Alternatively, the re-
action of 4 with a-lithioylide 5
afforded protonated salt 1·H in
[14]
5
0% yield (Scheme 1). Compound 1 is cleanly generated by
deprotonation of 1·H with NaNH in THF; it is stable under an
2
inert atmosphere in the solid state (m.p. 65–668C, decom-
posed over 1208C) and in C D , but is rapidly hydrolyzed to
6
6
phosphine oxide 6 and sulfimide 7 under air in the solid state
(
Scheme 1). In addition, passing 1·H through an ion-exchange
À
Scheme 2. Synthesis of 2 and 3. Reagents and conditions: a) lithium diiso-
propylamide (LDA; 3 equiv), 4, THF, À788C, 2 h, À108C, 0.5 h, À788C, 18 h,
resin column (IRA-410, OH form) afforded the hydrolyzed
products. We assume that 1 is hydrolyzed by attack at the cat-
ionic phosphorus center by H O or OH .
2
then H O; 2·H (64%), 3·H (80%); b) Amberlite IRA-410 ion-exchange resin
À
À
2
2
(OH form), MeOH, RT; 2 (95%), 3 (97%); c) NaNH , THF, RT, 0.5 h; 2 (97%), 3
(96%).
Chem. Eur. J. 2015, 21, 15405 – 15411
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The molecular structures of 3 and 3·H were determined by
X-ray crystallography (Figure 2). Selected bond lengths and
angles of 3 and 3·H are listed in Table 1, together with data for
A, B, and F and their protonated salts A·H, B·H, and F·H for
a large negative charge (1–3: À1.38 e), which is similar to that
[
17]
of BPC (A: À1.43 e, À1.55 e ), BChCs (B: À1.24 e; C: À1.20 e;
[
1,10,11]
D: À1.25 e), and PSC (E: À1.36 e).
The Wiberg bond
index values of iSPCs for P!C (1: 1.26; 2: 1.24; 3: 1.22) are
lower than those for S!C (1: 1.30 2: 1.31 3: 1.34), which is in
contrast to the trend observed in PSC F (P!C: 1.42, S!C:
1.21) and suggests the existence of strong back donation (n–
s* interaction) from the lone pairs of the central carbon to the
[
1a,10a,11b,16]
comparison.
The P!C (1.663 Š) and S!C (1.602 Š)
bond lengths of 3 are significantly shorter than those of the
corresponding cationic salt 3·H (P!C: 1.734 Š; S!C: 1.658 Š;
D
=0.071 Š, D =0.056 Š). This observation is different
S!C
P!C
[
11b]
from that for F (P!C: 1.667 Š; S!C: 1.684 Š; D =0.023 Š,
adjacent s* orbitals of the sulfur ligand.
Both carbon lone
P!C
D
=0.003 Š). In particular, the S!C bond length is the
pairs of 3 are stabilized through interactions with s*(SÀN),
s*(SÀC ), and s*(PÀC ), as determined by NBO second-order
S!C
shortest of the chalcogen-stabilized carbones B–F (1.636–
Ph
Ar
[
10,11]
1
.684 Š).
bond in 3. The P!C bond length of 3 is longer than that in A
1.635 Š), but is similar to that of F (1.667 Š). The angle at
the central carbon increases slightly from salt 3·H (123.68) to 3
This distance indicates a relatively strong S!C
perturbation analysis (Table S5 in the Supporting Information).
À1
In particular, ns interacts with s*(SÀN) (11.5 kcalmol ), s*(SÀ
C
[11b,16]
À1
À1
(
C ) (10.8 kcalmol ), and s*(PÀC ) (19.3 kcalmol ), whereas
Ph
Ar
À1
np interacts with s*(SÀN) (12.9 kcalmol ), s*(SÀC ) (28.0 kcal
C
Ph
À1
À1
(
125.68). This observation is different from the trend for BÀD
mol ), and s*(PÀC ) (25.5 kcalmol ). The total stabilization
Ar
[1a,2,10,11]
À1
and F, but is similar to the trend for A.
Interestingly, the
energy of n–s*S (E =63.2 kcalmol ) is larger than that of
total
!
À1
P!C S fragment is intermediate between A (130.1–143.88)
and B (116.88). These parameters suggest the efficient stabiliza-
tion of electron pairs from the central carbon atom to adjacent
s* orbitals of the ligands through an n–s* interaction.
n–s*P (Etotal =44.8 kcalmol ), which is in contrast to diamido-
À1
stabilized
F
(n–s*S: Etotal =31.4 kcalmol , n–s*P: Etotal
=
À1 [11b]
66.0 kcalmol ).
These results suggest that, through the n–
s* interaction, the iminosulfane ligand is more strongly bound
to the central carbon atom than the sulfide ligand of PSCs.
To verify the carbone character, we calculated the first and
second PAs (PA(1) and PA(2)) of 1–3 and compared them with
the theoretically predicted values for the PAs of carbones A–D
(Table 2). The theoretically predicted PA(1) values of 1–3 are
quite high, which suggests their strong basic character. The
calculations predict that 1 has a much higher PA(1) than that
of B. The PA(1) and PA(2) values of 1 are slightly smaller than
To gain an insight into the electronic structures of 1–3, DFT
calculations at the B3PW91/6-311G(d,p) level were performed
(
Figures S1–S3 in the Supporting Information). The model
structures of 1 and 2 are based on that of 3. The optimized
structure of 3 closely matched experimental data. Inspection
of the frontier orbitals reveals that the central carbon atom of
1
–3 has two lone-pair orbitals in the HOMO (np ) and
C
HOMOÀ1 (ns ) (Figure 3 and Figures S4 and S5 in the Support-
C
À1
ing Information). Natural bond orbital (NBO) analyses show
that of A. The largest values of PA(1) (301.6 kcalmol ) and
À1
that nsC has an s character (1: 32%; 2: 33%; 3: 31%; Ta-
PA(2) (207.8 kcalmol ) are predicted for 3, which contains the
bles S2–S4 in the Supporting Information), which indicates hy-
tri[4-(dimethylamino)phenyl]phosphane group. The PA(2)
2
bridization close to that of sp , whereas the np lone pair has
values of 1–3 are much smaller than those of PA(1). The same
C
[
1d]
1
00% p character. The central carbon atom of 1–3 bears
observation has been reported by Frenking et al.
PA(1) is
Figure 2. Molecular structures of 3·H and 3.
Figure 3. HOMO (left) and HOMOÀ1 (right) of iSPC 3.
Table 1. Comparison of selected bond lengths [Š] and angles [8] of 3, A, B, and F, and their protonated salts 3·H, A·H, B·H, and F·H.
[
b]
[c]
[d]
[b]
[c]
3
A
B
F
3·H
A·H
B·H
F·H
P!C
S!C
E!C
1.663(3)
1.602(2)
125.59(15)
1.629(3)–1.635(5)
–
130.1(6)–143.8(6)
–
1.667(3)
1.684(3)
109.78(17)
1.734(5)
1.658(5)
123.6(3)
1.699(3)
–
130.4(3)
–
1.690(4)
1.687(3)
116.6(2)
1.635(4), 1.636(2)
116.8(2)
1.691(2), 1.695(2)
118.0(1)
!
E
[
a] Ref. [16]. [b] Ref. [10a]. [c] Ref. [11b]. [d] Ref. [1a].
Chem. Eur. J. 2015, 21, 15405 – 15411
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plexes
(A·(AuCl) ,
A·(AuPPh )(AuCl),
The C!Au distances of 10 (2.118
and
2
3
Table 2. Experimental values of the onset of the first oxidation wave
[
6,8,18]
onset
Ph PCC(OEt) ·(AuCl) ).
3
2
2
(
E
ox) of 1–3 and carbones A–D, as well as HOMO and HOMOÀ1
energy levels and PAs calculated at the B3PW91/6-311G(d,p) level.
and 2.127 Š) and 11 (2.106 Š) are similar to those of diaurated
carbone complexes (A·(AuCl) : 2.074 and 2.078 Š; A·(AuP-
2
onset
E
[
ox
HOMO
[eV]
HOMOÀ1
PA(1)
[kcalmol
PA(2)
[kcalmol ]
Ph )(AuCl): 2.080 and 2.127 Š; Ph PCC(OEt) ·(AuCl) : 2.080 and
À1
À1
3
3
2
2
V]
[eV]
]
[
6,8,18]
2
.103 Š).
The AuÀAu distance of 10 (2.910 Š) is the short-
1
2
3
A
B
C
D
À0.84
À0.97
À1.04
–
À0.45
À0.73
À0.92
À5.05
À4.86
À4.58
À4.82
À5.26
À5.08
À5.09
À5.27
À5.07
À4.73
À5.01
À5.56
À5.32
À5.36
286.8
292.7
301.6
294.8
278.8
288.0
287.1
189.6
198.4
207.8
197.7
182.2
184.4
187.0
est distance between gold atoms in structurally characterized
diaurated carbone complexes (A·(AuCl) : 3.143 Š; A·(AuP-
2
[
6,8,18]
Ph )(AuCl): 3.127 Š; Ph PCC(OEt) ·(AuCl) ; 2.952 Š).
This dis-
3
3
2
2
tance is within the range observed for AuÀAu interactions
[19]
(
2.50–3.50 Š). The P!C (10: 1.788 Š; 11: 1.817 Š) and S!C
10: 1.737 Š; 11: 1.782 Š) distances are longer than those in
(
3
·H. The P!C and S!C distances of 11 are comparable to
[20,21]
single bond lengths (PÀC: 1.87 Š, SÀC: 1.81 Š).
The
!
mainly determined by the energy level of the highest lying oc-
cupied s orbital of neutral carbones 1–3, whereas PA(2) is
mainly determined by the highest lying occupied p orbital of
protonated 1·H–3·H.
Au C!Au bond angle (86.58) in diaurated complex 10 is the
smallest of the diaurated carbone complexes (A·(AuCl) : 98.48;
2
A·(AuPPh )(AuCl): 96.18: Ph PCC(OEt) ·(AuCl) : 89.78). The P!
3
3
2
2
!
C
S angles of 10 and 11 (10: 115.68; 11: 116.38) are narrower
The donor properties of 1–3 were assessed by cyclic voltam-
metry (see Table 2 and Figures S6–S8 in the Supporting Infor-
than those in 3·H.
Having obtained diaurated and proton-aurated complexes,
we examined their intrinsic reactivity toward electrophiles. The
reaction of 3 with iodomethane resulted in methylated salt 12
(Scheme 4). The formation of 12 was confirmed by X-ray crys-
tallographic analysis and occurred through nucleophilic attack
of 3 to methyl iodide. Interestingly, the reaction of 12 with
mation). Carbones 1–3 exhibit an irreversible oxidation process
onset
and the onset potential for oxidation (E
) is observed at
ox
+
À0.84, À0.97, and À1.04 V (vs. the ferrocene couple (Fc/Fc )),
respectively. These results demonstrate that the electron-donor
ability of these carbones follow the order of 3>2>1. Thus, in-
troducing electron-donating substituents to the phenyl rings
in the phosphane ligand increases the energy levels of the
onset
HOMOs. The E
of 3 is more negative than those of BChCs
ox
[10b]
B–D (B: À0.45 V; C: À0.73 V; D: À0.92 V).
Encouraged by these results, we attempted to ascertain
whether the central carbon of iSPC could actually be employed
as a four-electron donor. The reaction of 3 with [AuCl(PPh3)]
and subsequent addition of AgSbF gave monoaurated com-
6
plex 9 (Scheme 2). Gratifyingly, the desired diaurated complex
10 was efficiently obtained by treating 3 with two equivalents
of [AuCl(PPh )]/AgSbF . This demonstrates the presence of sig-
3
6
nificant electron density at the central carbon of 3. Even more
interesting is that the cationic salt 3·H reacted smoothly with
[
AuCl(PPh )]/AgTfO to give proton-aurated species 11, despite
3
its cationic character. The generation of 11, containing cation
·H as a ligand, clearly indicates the coordination ability in the
3
3
1
second lone pair of 3. In the P NMR spectra, the signals for 9,
0, and 11 are observed as a singlet, triplet, and doublet, re-
1
spectively (9: d=12.9, 39.8 ppm; 10: d=27.5 and 34.9 ppm,
J=5.1 Hz; 11: d=17.1 and 38.6 ppm, J(P,P)=8.1 Hz). These sig-
nals are similar to those of the mono- and diaurated complex
[
8]
13
of A. In the C NMR spectra, the central carbon atom of 11
appears as a doublet of doublets (52.6 ppm, J=70.4, 37.7 Hz),
but the central carbone resonances of 9 and 10 were not ob-
served.
Single-crystal XRD analysis of 10 and 11 unambiguously con-
firmed the complexation of the carbon center of 3 to the
gold(I) moieties (Scheme 3). To the best of our knowledge,
compounds 10 and 11 are the first examples of diaurated and
proton-aurated complexes of phosphorus- and sulfur-stabilized
carbones. Table 3 lists selected bond lengths and angles of 10
and 11 and the literature values of diaurated carbone com-
Scheme 3. Syntheses of gold complexes 9–11 and the molecular structures
of 10 and 11. Hydrogen atoms (except for that on Ccenter) and SbF
anions are omitted for clarity. Reagents and conditions: a) [AuCl(PPh
6
and TfO
)]
3
(
(
1 equiv), AgSbF
2 equiv), AgSbF
6
6
(1 equiv), THF, À788C 20 h; (50%); b) [AuCl(PPh
3
)]
(2 equiv), THF, À788C, 20 h; (95%); c) Amberlite IRA-410
À
ion-exchange resin (OH form), MeOH, RT, NaTfO (1 equiv); d) [AuCl(PPh
3
)]
(
1 equiv), AgTfO (1 equiv), THF, À788C to RT, 2 h; (94%). Tf=trifluorometha-
nesulfonyl.
Chem. Eur. J. 2015, 21, 15405 – 15411
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reaction with Ag O was widely
2
Table 3. Selected bond lengths [Š] and angles [8] of 10, 11, A·(AuCl)
Ph PCC(OEt) ·(AuCl)
2
,
A·(AuPPh
3
)(AuCl), and
used in the syntheses of N-heter-
ocyclic carbene (NHC) complexes
3
2
2
.
[
2
18]
[8]
[6]
1
0
11
A·(AuCl)
A·(AuPPh
3
)(AuCl)
Ph
3
PCC(OEt)
2
·(AuCl)
2
[23]
of silver. Furthermore, the cor-
lengths[Š]
C!Au
AuÀAu
P!Au
Cl!Au
P!C
responding silver NHC com-
2.127, 2.118
2.910
2.272, 2.269
–
1.788
1.737
2.106
–
2.279
–
1.817
1.782
2.078, 2.074
3.143
–
2.297, 2.295
1.776
1.776
2.127, 2.080
3.127
2.265
2.267
1.759
2.103, 2.080
2.952
–
2.301, 2.291
1.786
1.425
plexes are often used as versatile
[23]
carbene transfer agents.
Sur-
prisingly, however, this synthetic
protocol has only been applied
to the deprotonation of the di-
E!C (E=P, S, or C)
1.754
[
4h]
cationic salt of BCC
and has
angles [8]
!
never been applied to the syn-
thesis of silver carbone com-
plexes. Thus, we carried out the
reaction of hydrochloric salts of
Au C!Au
86.5
174.3, 176.7
–
–
98.4
–
175.3, 177.0
117.3
96.1
89.7
–
177.7, 178.9
114.2
!
C!Au
P
179.4
–
116.3
174.6
179.4
119.3
!
C!Au Cl
!
E!C E (E=P, S, or C)
115.6
1
and 3 with Ag O (0.5 equiv) in
2
CH Cl under air (Scheme 5). The
2
2
reaction proceeded smoothly at
room temperature to give silver complexes 20 and 21 in good
yields. In the solid state, silver complex 20 is stable to moisture
and light, and no degradation of the solids is observed over
a period of several weeks; on the other hand, complex 21
gradually decomposes in the solid state. It is important to note
that the formation of 20 indicates that this synthetic protocol
can be used for moisture-sensitive carbone species. To test
whether silver complexes 20 and 21 function as carbone trans-
fer agents, such as silver NHC complexes, the reaction of 20
and 21 with [AuCl(PPh )]/AgSbF was performed (Scheme 5). In
3
6
line with our expectations, the transmetalation of 20 and 21
proceeded smoothly and the desired mono- (9 and 10) and
diaurated (22 and 23) complexes were obtained. The structure
of diaurated complex 23 was confirmed by single-crystal XRD
analysis (Scheme 5).
These results clearly demonstrate that the silver–carbone
complexes are easily prepared by the reaction of the hydro-
chloric salt with Ag O, and the resultant silver carbone com-
2
plexes are capable of acting as a carbone transfer reagent to
give the metal–carbone complexes. On the other hand, the
treatment of 3 with AgTfO (0.5 equiv) produced the linear co-
ordinated silver complex 24 (Scheme 6).
Scheme 4. Synthesis and reaction of 12 and molecular structures of 12 and
3. Hydrogen atoms and I and TfO anions are omitted for clarity. Reagents
and conditions: a) MeI, THF, RT, 2 h; 12 (97%); b) TfOH (1.2 equiv), dichloro-
methane, RT, 0.5 h; 13 (95%); 14 (94%).
1
The structures of silver complexes 20, 21, and 24 were pro-
vided by single-crystal XRD analysis (Schemes 5 and 6). Table 4
lists selected bond lengths and angles of 20, 21, and 24, as
[
24]
well as literature values of silver carbone complex [A ·Ag]Cl.
2
To the best of our knowledge, complexes 20, 21, and 24 are
the first isolated silver complexes of phosphorus- and sulfur-
stabilized carbones. The C!Ag distances of 20 (2.131 Š) and
TfOH in CH Cl afforded vinylphosphonium salt 13 and amino-
2
2
sulfonium salt 14 in almost quantitative yield (Scheme 4). The
structure of 13 was confirmed by single-crystal XRD analysis. A
plausible mechanism for the formation of 13 and 14 involves
dicationic intermediate 15 (Scheme 4). First, protonation of the
central carbon of 12 by TfOH gives the dicationic salt 15. The
intramolecular deprotonation of the C-methyl group by the ni-
trogen atom of the iminosulfonium ligand cleaves S!C to
21 (2.098 Š) are comparable to those of [A ·Ag]Cl (2.115–
2
2.134 Š), whereas the mean value (2.161 Š) of the distances in
24 is slightly longer than that in a related silver complex of
[
24]
[A ·Ag]Cl. The P!C distances of 20, 21, and 24 (20: 1.711 Š,
2
21: 1.728 Š, 24: 1.708 Š) are longer than that of [A ·Ag]Cl
2
(1.656–1.690 Š). The S!C distances of 21 (1.636 Š) and 24
(1.630 Š and 1.639 Š) are intermediate between 3 and 3·H. The
[22]
generate salts 13 and 14.
!
In an extension of the complexation chemistry of phospho-
rus- and sulfur-stabilized carbones, we synthesized silver com-
plexes of iSPC. Recently, an in situ deprotonation/complexation
C!Ag C angle of 24 (175.48) is nearly linear, which is similar
[
1a,24]
to that of [A ·Ag]Cl (177.58).
In contrast to silver complex
2
[A ·Ag]Cl, the central carbon atoms of 20, 21, and 24 are not
2
Chem. Eur. J. 2015, 21, 15405 – 15411
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Table 4. Selected bond lengths [Š] and angles [8] of 20, 21, and 24,
A ·AgCl.
2
[24]
20
21
24
2
[A ·Ag]Cl
lengths [Š]
C!Ag
P!C
2.131
1.711
1.648
2.098
1.728
1.636
2.157, 2.164
1.707, 1.708
1.630, 1.639
2.115–2.134
1.656–1.690
–
S!C
angles [8]
!
C!Ag
C
–
–
175.4
177.5
!
C!Ag Cl
172.1
121.9
172.6
119.1
–
–
!
E!C
E
121.8, 127.0
128.5, 129.1
Conclusion
We have successfully synthesized new iminosulfane- and phos-
phane-stabilized carbones 1–3. The combination of the high
electron-donating ability of the triarylphosphane ligand and
the strong p-acceptor ability of the iminosulfane ligand confer-
red considerable thermal and hydrolytic stability on 3. The DFT
calculations for 1–3 suggested that introducing electron-do-
nating substituents at the phosphorus center increased the
HOMO energy levels and PA values. The cyclic voltammetry
measurements showed that the HOMO energy levels followed
the order of 3>2>1. These results suggested that the elec-
tron-donating ability of the central carbon in carbones could
be easily tuned by introducing electron-donating/-withdrawing
substituents onto the ligand in a way to similar to that for con-
ventional metal complexes. The carbone character of 3 was
demonstrated by the corresponding diaurated and proton-aur-
ated complexes; this constitutes the first experimental proof of
phosphorus- and sulfur-stabilized carbones behaving as four-
electron donors. The silver–carbone complexes are easily pre-
pared by the reaction of the corresponding hydrochloric salts
Scheme 5. Synthesis and transmetalation of silver complexes 20 and 21 and
the molecular structures of 20, 21, and 23. Hydrogen atoms and SbF anions
are omitted for clarity. Reagents and conditions: a) Amberlite IRA-410 ion-ex-
6
À
change resin (Cl form), MeOH, RT; b) Ag
2
O (0.5 equiv), dichloromethane, RT,
)] (1 equiv), AgSbF (1 equiv), THF,
À788C to RT, 17 h; 22 (94%); 9 (95%); d) [AuCl(PPh )] (2 equiv), AgSbF
2 equiv), THF, À788C, 17 h; 23 (97%); 10 (97%)
2
h; 20 (97%); 21 (95%). c) [AuCl(PPh
3
6
3
6
(
with Ag O under anaerobic conditions. Furthermore, the
2
silver–carbone complexes serve as carbone transfer agents.
This study provides a new approach to the design of ther-
mally and hydrolytically stable carbones and a synthetic proto-
col for the easy preparation of metal–carbone complexes. We
believe that this approach shows vast potential for developing
and improving carbone chemistry.
Acknowledgements
Scheme 6. Synthesis and molecular structure of silver complex 24. Hydrogen
atoms and TfO anions are omitted for clarity. Reagents and conditions
This work was partially supported from the Ministry of Educa-
tion, Culture, Sports, Science, and Technology of Japan by
Grants-in-Aid for Scientific Research(c) (NO.15K05438) and
a Specially Promoted Research Grant (22000009) and research
fellow of the Japan Society for the Promotion of Science for
Young Scientists (NO.15J05047). We thank Prof. Dr. T. Sasamori
(
yields): a) AgTfO (0.5 equiv), THF, RT, 2 h; 24 (94%).
perfectly planar, and the carbon atoms exhibit a slight pyra-
midalization with the sum of the three angles (20: 353.928; 21:
(
Kyoto University) for the crystallographic analysis of 10.
[24]
!
3
55.978; 24: 347.68 and 349.78). The P!C S angle of 21
119.18) is narrower than those in 3, and that in 24 (mean
value of 124.48) is similar to that in 3.
(
Keywords: carbones · density functional calculations · donor–
acceptor systems · electronic structure · gold
Chem. Eur. J. 2015, 21, 15405 – 15411
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Received: June 3, 2015
Published online on September 7, 2015
Chem. Eur. J. 2015, 21, 15405 – 15411
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15411
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