Heterocycles Derived from Generating Monovalent Pnictogens within NCN Pincers and Bidentate NC Chelates: Hypervalency versus Bell-Clappers versus Static Aromatics

Article abstract of DOI:10.1021/acs.organomet.8b00290

Generating monovalent pnictogens within NCN pincers has resulted in the isolation of three distinct types of 1,2-azaheteroles, highly aromatic nitrogen analogues like pyrazole-based 5, aromatic yet fluxional P- and As-derived bell-clappers 1 and 2, and hy

Full text of DOI:10.1021/acs.organomet.8b00290

Article
Cite This: Organometallics XXXX, XXX, XXX−XXX
Heterocycles Derived from Generating Monovalent Pnictogens
within NCN Pincers and Bidentate NC Chelates: Hypervalency versus
Bell-Clappers versus Static Aromatics
†
,#
‡,#
‡
†
§
∥
Vít Kremlac
Russell P. Hughes, Libor Dostal
†Department of General and Inorganic Chemistry, Faculty of Chemical Technology, University of Pardubice, Studentska
ek, Jakub Hyvl, Wesley Y. Yoshida, Ales Ruzicka, Arnold L. Rheingold, Jan Turek,
̊
̌
́ ̌ ̌ ̌
⊥
,†
,‡
́
,* and Matthew F. Cain*
́
573,
Pardubice 53210, Czech Republic
‡
Department of Chemistry, University of Hawai‘i at Manoa, 2545 McCarthy Mall, Honolulu, Hawaii 96822, United States
̅
§
Department of Chemistry, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093,
United States
∥
Eenheid Algemene Chemie (ALGC), Vrije Universiteit Brussel (VUB), Pleinlaan 2, B-1050 Brussels, Belgium
⊥
Department of Chemistry, Dartmouth College, 6128 Burke Laboratory, Hanover, New Hampshire 03755, United States
S Supporting Information
*
ABSTRACT: Generating monovalent pnictogens within NCN pincers has resulted in the isolation of three distinct types of
1
,2-azaheteroles, highly aromatic nitrogen analogues like pyrazole-based 5, aromatic yet fluxional P- and As-derived bell-clappers
and 2, and hypervalent Sb and Bi derivatives 3 and 4, which are supported by 3-center, 4-electron N−E−N bonds. Careful
1
analysis of the solid-state structures of 1−5/[5-Me][OTf] in combination with NICS calculations (at the GIAO/M06/
cc-pVTZ(-PP) level) and other computational methods (NBO) suggest that simpler NC chelates may support new
phosphorus- and arsenic-containing heterocycles. Indeed, reduction of ECl (E = P or As) derivatives supported by N-Dipp
2
(
1
Dipp = 2,6-diisopropylphenyl) substituted NC bidentate ligands produced 1,2-benzoazaphosphole 11 and 1,2-benzoazaarsole
2. NICS calculations revealed 11 and 12 had aromatic character on par with that of pyrazole-based 5.
INTRODUCTION
An alternative approach to maximizing π-conjugation through
■
3
1
modification of the σ -heteroles is to force the phosphorus or
Five-membered heterocyclic rings, especially those featuring
heavier main group elements such as Si, P, Se, and Te have
12
2
3
4
5
arsenic atoms directly into (p−p)π bonding. Unfortunately, the
bulky substituents often required to stabilize EC or EE
double bonds (E = P or As) result in twisting of the structure
acted as building blocks for the construction of new electronic
6
materials. Phospholes are arguably the most versatile scaffold
13
7
and reduced π-conjugation, a situation encountered during the
synthesis of poly(p-phenylenephosphaalkene) (PPP) building
as the P-substituent can be sterically and electronically varied,
the P-lone pair can be functionalized by electrophiles, oxidized,
or coordinated to a metal center,
based substituents can be introduced at the 2,5- and 3,4-positions
using the Fagan−Nugent method. Most of the tunability is
aimed at careful tweaking of its weakly aromatic character
12
1
,6,8
blocks, the phosphorus analogue of poly(p-phenylenevinylene)
and a plethora of carbon-
3
2
(
PPV) (Chart 1, top). However, low-coordinate (λ −σ )
9
phosphorus can be incorporated into ring systems like 1,3-
14
benzoxaphospholes (BOP), 2,6-substituted benzobisoxaphosp-
15
16
holes (BBOP), and heteroacene derivatives (NBOP), leading
to more extended π networks with advantageous properties
such as reversible electrochemical behavior and luminescence/
fluorescence with high quantum yields (Chart 1, bottom).
derived from σ*−π* hyperconjugation between the exocyclic
1
0
P−R bond and the butadiene unit with the goal of maximizing
1,3,6
π-conjugation throughout the organic scaffolding.
Recently,
in some of the most promising materials, phosphorus has been
replaced by arsenic affording new arsole-based building blocks,
11
permitting direct comparison with their lighter phosphole
analogues (Scheme 1).
Received: May 4, 2018
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.8b00290
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Organometallics
Article
Scheme 1. Phosphole Tunability and Aromaticity
Scheme 2. Targeted Group 15 Compounds (E = N, P, As,
Sb, or Bi)
indeed both isolable and aromatic making them interesting
targets as building blocks for future electronic materials.
RESULTS AND DISCUSSION
Preparation and Solid-State Structures of 1,2-
Azaarsoles 2a/b and Pyrazole-Derived 5. By analogy to
■
Chart 1. Comparison of PPV- and PPP-Based Building
Blocks and Benzoxazole-Derived Materials
17
previously reported chemistry with azaphosphole 1, arsenic
derivatives 2a/b were isolated in 45 and 60% yield, respec-
tively, by sequential addition of n-BuLi, AsCl , and either PMe
3
3
2
8
29
(
(
for 2a) or KC (for 2b) to aryl bromides of type A
8
30
Scheme 3). Both azaarsoles (2a/b) exhibited averaged C2v
a
Scheme 3. Synthesis and Dynamics of 2
During our efforts to stabilize a phosphinidene within an
NCN pincer, we unexpectedly isolated a related, nitrogen-
containing 5-membered heterocycle, namely, 1,2-benzoaza-
1
7
phosphole (1). Known synthetic routes to 1,2-azaphospholes
include vanadium-mediated cycloaddition of phosphaalkynes
18
and substituted acetylenes, flash vacuum pyrolysis of 1,2,3,4-
1
9
triazaphospholes, and Diels−Alder-type reactions between
1
,3,2-diazaphosphole-4,5-dicarbonitriles and various acety-
2
0
lenes, all of which are impractical. DFT calculations indicated
a
The calculated (DFT/B3LYP-D3/LACV3P**) transition state
the As derivative (2) should have a similar C symmetric ground-
s
structure of 2a with selected bond lengths is included.
1
7
state structure, potentially providing access to a new ring
21
system containing AsC (p−p)π bonding. In contrast,
22
Sb(I) and Bi(I) centers formed by reduction within an
analogous NCN pincer afforded C symmetric stibinidene
2
v
2
3
3
and bismuthinidene 4, hypervalent compounds stabilized
24
by 3-center, 4-electron N−E−N bonds (E = Sb or Bi). The
apparent redistribution of electron density observed in 1 and
predicted with 2 compared with 3 and 4 suggests that 1,2-
azaphospholes and -arsoles may feature weak aromatic char-
25
acter and high π conjugation, ideal monomer properties for
1
,3,6
the construction of macromolecules.
Generation of the corre-
sponding nitrene was expected to result in trapping by one of the
2
26
flanking sp -hybridized nitrogen donors affording pyrazole-
27
based 5, an established aromatic ring system and excellent
comparison point for compounds 1−4 (Scheme 2).
Here, we report the synthesis of NCN-supported arsenic and
nitrogen derivatives 2 and 5, a comparison of the solid-state
structures of 1−5 and evaluation of their aromaticity on the
basis of NICS calculations, NBO analysis, and by analogy with
other related compounds. In addition, we demonstrate that
unlike previously reported azaheteroles derived from Sb and Bi
Figure 1. X-ray crystal structure of 2a. Selected bond lengths (Å) and
angles (deg): As −N = 1.976(4), As −C = 1.875(4), N −C
=
=
=
1
1
1
13
1
11
1
1
8
.321(6), N −C = 1.297(6), C −C = 1.429(6), C −C
2
18
11
12
14
18
.469(6), C −C = 1.412(7), C −C = 1.422(6), N −As −C
1
3
14
13
12
1
1
13
2.65(18), As −N −C = 115.6(3). As −N contact: 2.504 Å.
1
1
11
1
2
1
,2-benzoazaphospholes and 1,2-benzoazaarsoles (11 and 12,
vide infra) can be supported by simple NC chelates lacking a
tethered Lewis base and/or flanking steric protection and are
symmetry in solution with a particularly diagnostic downfield-
shifted doublet/triplet pattern (J = ∼7.5 Hz) integrating in
B
DOI: 10.1021/acs.organomet.8b00290
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Scheme 4. Synthesis of 5 and [5-Me][OTf] and Possible
Resonance Contributor [5-Me][OTf]′
Figure 2. X-ray crystal structure of 2b. Selected bond lengths (Å) and
angles (deg): As −N = 1.925(2), As −C = 1.865(2), N −C =
1
1
1
1
1
7
1
1
1
8
.334(4), N −C = 1.282(3), C −C = 1.404(3), C −C
=
=
2
16
2
7
6
16
overall resonance hybrid. While the corresponding resonance
structure for N-analogue 5 (Scheme 4) is not the principal
contributor, it is drawn as such for consistency with its heavy
congeners, and all subsequent comparisons use this as the
reference structure. Delocalizations from these reference Lewis
structures are discussed below (Table 2).
.453(4), C −C = 1.413(3), C −C = 1.422(4), N −As −C
1
6
1
2
1
1
2.93(1), As −N −C = 115.34(2). As −N contact: 2.668 Å.
1
1
7
1
2
a 2:1 ratio corresponding to the central aromatic protons.
These NMR observations are consistent with previous DFT
1
7
predictions, which suggested azaarsole 2a would have a Cs
symmetric ground-state structure with a low lying (2.1 kcal/mol)
C symmetric transition state to rapid N −As /As −N bond-
Given the impracticality associated with handling chlori-
34
nated amine derivatives, we devised an alternative route to
nitrogen counterpart 5. Lithiation of A (R = Mes, R′ = Me)
with n-BuLi, followed by treatment with the electrophilic nitro-
2v
1
1
1
2
3
1
switching via a bell-clapper type mechanism. Azaarsole 2b
was also predicted to display a similar low barrier exchange
32
35
process (2b = 1.7 kcal/mol), neither of which can be frozen
out by routine VT NMR spectroscopy. Ultimately, their (2a/b)
unsymmetrical ground state structures were confirmed by X-ray
diffraction (Figures 1 and 2, see Table 1 for comparison with 1).
Natural Resonance Theory (NRT) indicates that the resonance
structures illustrated for compounds 2a/b and other heavy
atom analogues are the most significant contributors to the
gen source TsN led to the in situ generation of (NCN)−N ,
3
3
which extruded N upon heating (100 °C) to afford 5 as a
colorless oil in 62% yield (Scheme 4).
2
Unlike 1−4, pyrazole 5 was predicted to and did display an
33
apparent C symmetric structure in solution at room temper-
s
1
ature by NMR spectroscopy (Figure 3, left). The H NMR
spectrum showed five separate aryl resonances integrating in a
a
Table 1. Selected Bond Lengths and Angles of Compounds 1−5/[5-Me][OTf], 6, and 10
a
2
2
The CN imine bonds and exocyclic sp −sp C−C bonds listed have been labeled for clarity.
C
DOI: 10.1021/acs.organomet.8b00290
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
a
Table 2. Selected Atom Hybridizations in a Model NCN Pincer System
delocalization
delocalization
N1LP → C−Cπ*
(kcal/mol)
delocalization
N1LP → E−C π*
N2LP→ E−N σ*
Ar
1
E
ELP
N1LP N2LP E−N σ E−CAr
σ
N −E σ C −E σ
(kcal/mol)
(kcal/mol)
1
1
Ar
sp1.5
sp0.4
sp0.2
sp0.2
sp0.1
p
p
p
p
p
sp
sp
sp
sp
sp
1.7
1.8
2.0
2.0
2.0
sp2.1
sp
sp
sp
sp
sp
1.7
3.7
4.9
5.5
7.9
sp
sp
sp
sp
sp
3.5
2.1
2.0
2.3
2.4
sp
2.3
2.4
2.5
2.6
2.8
28.6
10.2
4.0
2.0
1.5
37.0
45.4
66.9
79.2
83.1
0.0
10.0
30.6
37.8
36.9
N
P
As
Sb
Bi
12.7
sp
sp
sp
sp
sp
sp
sp
sp
65
100
100
a
The lone pair on N and the orbitals forming the E−C π-bond are all 100% p.
1
Figure 3. Calculated structure of 5 and predicted TS to bell-clapper
type behavior.
1
:1:1:2:2 ratio; the central aromatic protons each appeared as
doublets of doublets and two separate singlets corresponding
to CH protons of N−Mes rings were observed. In addition, six
chemically distinct methyl signals were detected, consistent
with a structure in which the central nitrogen atom is locked to
one side. Despite heating a solution of 5 to 100 °C, the locked
structure persisted, indicating the bond-switching process
between N −N and N −N must proceed via a high-energy
Figure 4. X-ray crystal structure of [5-Me]+ (anion removed for
clarity). Selected bond lengths (Å) and angles (deg): N −C
=
=
=
2
13
12
20
1
.347(4), N −N = 1.377(3), N −C = 1.344(4), C −C
2
1
1
10
10
1.409(4), C −C = 1.431(4), N −C = 1.297(4), N −C
1
2
13
3
18
3
1.486(4), C −C = 1.473(4), N −N −C = 102.8(2), N −N −C
1
4
18
1
2
13
2
1
10
= 114.8(3).
1
2
2
3
transition state. DFT calculations located this C transition
state 39.4 kcal/mol above the ground state (Figure 3, right),
These perturbations are consistent with a contribution from
resonance structure [5-Me][OTf]′ (Scheme 4), reflecting the
electronegative nature of the newly methylated nitrogen center.
Related indazoles featuring electron-withdrawing groups at the
7-position like 2-allyl-7-nitro-2H-indazole have remarkably
2v
32
a barrier much higher than associated with analogous P (1)
and As derivatives (2a/b) and inaccessible at 100 °C.
In order to experimentally determine the structure of the
36
37
39
pyrazole ring, we quaternized the imine nitrogen (Scheme 4).
similar structures.
Addition of MeOTf to a solution of 5 in methylene chloride
resulted in crystalline salt [5-Me][OTf] with an unidentified
impurity comprising ∼10% of the reaction mixture. A combination
Selected bond lengths and angles of 1−5/[5-Me][OTf]
(and 6 and 10, vida infra) are tallied in Table 1. Consistent with
the inability of 3s/3p and larger atomic orbitals to effectively
1
40
of H NMR spectroscopy (δ 3.83 and 2.77, J = 1.4 Hz) and
hybridize, the central atoms in 1−5 are increasingly pyra-
HSQC and HMBC experiments confirmed the primary site
of methylation was at the pendant imine arm and not the
central nitrogen atom. Recrystallization of the product mixture
from dioxane layered with diethyl ether afforded crystals of
midalized descending down Group 15 with Bi displaying severely
23
acute N−Bi−C angles of 71.13° (avg). Furthermore, in line
Ar
with well-documented trends for main group compounds, the
N−E and E−C bonds lengths increase progressively, from
1.355 Å (N−N) to 2.463 Å (N−Bi average) and from 1.343 Å
(N−C) to 2.150(5) Å (Bi−C) as the size of the pnictogen
[
5-Me][OTf] suitable for elemental analysis and X-ray crystal-
lography (Figure 4). In comparison to the calculated structure
of 5 (Figure 3, above), the remote methylation altered the
structure of the pyrazole-based ring slightly. Quaternization
32
41
increased, reflecting both increasing p character in the E →
36
N/E → C sigma bonds and the poorer overlap between large p
orbitals and the pincer nitrogen and aryl donors (see Table 2
for NBO calculations using the $CHOOSE keyword along
of nitrogen is known to result in the type of C−N bond elon-
+
38
gation observed here (1.275 Å in 5 vs 1.297(4) in [5-Me] );
42
however, other minor changes were also observed, including
with the reference structure). In fact, hypervalent Sb(I) and
23
shortening of the C−N pyrazole bond (N −C ) accompanied
Bi(I) analogues 3 and 4 contain virtually no s/p mixing; the
1
10
2
2
24
by subsequent lengthening of the adjacent sp −sp C−C bond
3-center, 4-electron N−E−N bond involves overlap from
2
2
2
(
C −C ) and contracting of the second exocyclic sp −sp
two roughly sp hybridized N orbitals and a pure p orbital on
1
0
12
23
C−C bond (C −C , nearest to the quaternized nitrogen).
the heavy pnictogen. In contrast, the presumed generation of
1
4
18
D
DOI: 10.1021/acs.organomet.8b00290
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Organometallics
Article
4
3
44
45
nitrene, phosphinidene, and arsinidene species following
down group 15 not only is consistent with the transition to the
symmetrical ground state structures for Sb and Bi but also
in situ nitrogen extrusion from (NCN)−N or reduction of the
NCN)ECl intermediates (E = P or As) resulted in the iso-
lation of trivalent 5, 1, and 2, respectively. Calculated pyrazole
structure 5 and salt [5-Me][OTf] both contained an sp hybrid-
ized central nitrogen with a five-membered ring structure devi-
ating only slightly from the parent pyrazole (C H N ),
featuring significant delocalization of the N1LP into the E−CAr
π* bond (Table 2), leading to C−N bond lengths intermediate
between single and double bonds.
Results of NBO analyses of model compounds are presented
in Table 2. The resonance structure shown was selected as the
reference Lewis structure using the $CHOOSE option in NBO
3
(
provides insight into the origin of the low barrier to N −E/E−
2
1
32
N bond exchange via a bell-clapper type process observed
2
2
17
even at −60 °C by NMR spectroscopy for 1 and 2a/b.
Aromaticity and Isolability of Group 15 Heterocycles.
36
50
1,3,6,10
Pyrroles are fully aromatic, while phospholes
arsoles are weakly aromatic, and stiboles and bisboles are
increasingly labile. However, their aza-analogues (with the
exception of pyrazoles), specifically azaphospholes
and
3
4
2
51
52
17−20
(like 1)
are far less common; there-
2
1,29
and azaarsoles (like 2a/b)
fore, nuclear-independent chemical shift (NICS) calculations
(at the GIAO/M06/cc-pVTZ(-PP) level) were performed to
gain insight into their aromaticity. Computations were per-
formed at the geometric center (NICS(0)) and at 1 Å above
and below (average value NICS(1)) both the five- and six-
membered rings, including the out-of-plane component
6
.0. Second-order perturbation theory analysis of the Fock
matrix in the NBO basis was used to evaluate the stabilization
energies associated with delocalizations from this reference
structure. The s-character of the lone pair orbital hybridiza-
tion ELP increases dramatically on descending the group. The
p-character “lost” from this lone pair is relocated in the
σ-bonds from E to its flanking atoms, most significantly in that
to N , consistent with Bent’s Rule. As expected, π-delocalization
energies for the N p-lone pair electrons into the flanking E−C π*
orbital decrease dramatically on descending the group, yet
(NICS(1) ), which is often considered the best gauge of
zz
53
aromaticity. Of the series of group 15 azaheteroles (1−5),
the pyrazole unit (C NN ring) in 5 was the most aromatic,
3
42
1
displaying even more negative (aromatic) NICS values than
1
benzene (Table 3). Azaphosphole 1 (C NP) was more aromatic
3
than were azaarsoles 2a/b (C NAs), but both were less aromatic
3
azaphosphole 1 and azaarsoles 2a/b still exhibit bond lengths
than the pyrazole. In contrast, the fused nitrogen-containing
4
6
in between E−C and EC , while simultaneously
five-membered rings of hypervalent Sb (3, C NSb) and Bi
Ar
Ar
3
displaying appreciably pyramidalized N−E−C bond angles
(4, C NBi) derivatives exhibited low aromaticity, consistent with
Ar
3
of 87.81(12) and 82.65(18)° (for 2a; 2b = N −As −C =
the lack of hybridization at the central atom, weak 3-center,
4-electron N−E−N bonds (E = Sb or Bi), and negligible
redistribution of electron density between the heavy atom and
the pincer scaffold. In fact, only recently have “non-hypervalent”
1
1
1
40,42
8
2.93(1), Table 1).
This is counteracted by an increase in
delocalization of the N lone pair into the flanking C−C π*
orbital moving down the pnictogen series. In the structures of
1
47
48b
54
48a
1
and 2a/b, the exocyclic CC bond of the five-membered
azastibole derivatives 6−8
been reported (Chart 3).
and azabisboles 9 and 10
rings remains noticeably short, especially in comparison to the
2
2
sp −sp C−C bond of the pendant imine donor (Table 1).
Antimony-derived 6, featuring a five-membered C NSb ring
3
23,48
However, Sb and Bi analogues
3 and 4 feature exocyclic
with significant aromatic character is the singly hydrogenated
analogue of NCN pincer ligated 3 (with Dipp substituents on
nitrogen instead of Xyl), possessing a noticeably longer Sb−C
bond and unequally contributing nitrogen donors (see Table 1
C−C bonds of both equal length (within error) and within
2
2
42
normal range of sp −sp C−C single bonds, likely due to
substantial delocalization of the N lone pair into the CC π*
1
48b
orbital (Table 2). Additional, closely related changes were
observed at imine functionalities of the NCN pincer. In 5, 1,
and 2, the tethered imine unit maintained a short CN
double bond (∼1.28 Å), while the imine donor incorporated
into the new five-membered rings was significantly elongated
for 6 and 10).
Interestingly, despite substantial flanking
steric bulk, preparation of an Sb(I) species without the second
48b
nitrogen donor resulted in dimerization to trivalent 8. Single-
3
arm 7 containing a related Sb(III) center, an sp benzylic carbon,
and amido and phenyl donors is a nonaromatic five-membered
48b,55
(
Table 1). Again, no such structural reorganizations were observed
with 3 and 4.
WBI values highlight that Sb- and Bi-based orbitals ineffec-
tively overlap with their C- and N-based counterparts, resulting
in more significant redistribution of electron density from the
-membered arene ring into the fused 5-membered ring for 5,
, and 2 than in 3 and 4 (Chart 2). This led us to speculate
ring system and only unsupported azastibole known to date.
1
7,23,48
These observations in combination with
On the contrary, singly ligated Bi(I) species 9 can be generated,
but in the absence of trapping agents (RE−ER, E = S, Se, and
Te) to afford Bi(III) species, decomposes into unidentifiable
4
9
41
54
product mixtures. However, if considerable steric bulk is
placed at the ortho and para positions of the fused benzene
6
1
48a
ring, then 10 is isolable. Intriguingly, some structural reorga-
nization like that present in 5, 1, and 2 did occur. The
2
2
Chart 2. WBI Values for the Five-Membered Rings in 1−5
exocyclic sp −sp C−C bond was short, and the CN imine
bond was long with NICS calculations indicating appreciably
more aromatic character was present in 10 versus 4 but was
48a
still less in comparison to those in 1 and 2a/b. Furthermore,
the Bi−C bond remained composed of almost entirely p char-
48a
acter, showing insignificant multiple bonding (WBI = 1.05)
and the presence of a high-energy p-type lone pair, which may
account for its reactivity in the absence of neighboring steric
bulk. Given 1 and 2 represented lighter analogues of 9, but
with a secondary E−N contact like that in 6 (in 1, P−N
contact = 2.676 Å; in 2a, As−N contact = 2.504 Å; in 6, long
Sb−N bond = 2.534(5) Å), we speculated if aromatic character
alone in the absence of any additional steric (large t-Bu groups
that the driving force for the formation of trivalent C sym-
s
metric 5, 2, and 1 over monovalent C symmetric 3 and 4 was
2v
aromaticity. Furthermore, the increasingly significant delocal-
ization of the N lone pair into the E−N σ* orbital descending
2
1
E
DOI: 10.1021/acs.organomet.8b00290
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Table 3. GIAO/M06/cc-pVTZ(-PP) Computed NICS (ppm) for the Five- And Six-Membered Rings in Compounds 1, 2a/b,
+
3
−5, [5-Me] , and 10−12
3
C NE ring
6
C ring
compound
NICS(0)
NICS(1)
NICS(1)zz
NICS(0)
NICS(1)
NICS(1)zz
1
2
2
3
4
5
E = P
−10.3
−8.0
−9.7
−3.2, −3.2
−2.8, −2.8
−13.2
−12.7
−8.7
−9.5
−7.8
−8.9
−26.8
−20.7
−25.2
−8.8, −8.8
−7.3, −7.3
−34.4
−32.7
−21.5
−5.9
−6.0
−5.9
−6.2
−6.4
−6.8
−6.7
−5.9
−6.6
−6.8
−6.7
−8.4
−8.1
−8.2
−8.0
−8.6
−8.8
−8.3
−8.4
−8.2
−8.6
−8.8
−8.7
−10.4
−21.1
−21.6
−21.6
−22.8
−23.0
−21.6
−21.6
−22.0
−21.2
−24.0
−23.6
−29.8
a
E = As
E = As
E = Sb
E = Bi
E = N
E = N
E = Sb
E = Bi
E = P
b
48a
−4.2, −4.2
−3.9, −3.9
−11.7
−11.4
−7.9
−7.1
−10.9
−10.2
48a
+
[
5-Me]
48b
6
1
1
1
48a
0
1
2
−8.1
−12.9
−11.9
−17.5
−31.9
−29.4
E = As
benzene
reference
Chart 3. Azastibole and Azabisbole Derivatives 6−10
Scheme 5. Synthesis of 11 and 12
Figure 5. X-ray crystal structure of 12. Selected bond lengths (Å) and
angles (deg): As −C = 1.865(5), As −N = 1.883(6), N −C =
1
1
1
1
1
8
1
.452(8), N −C = 1.308(7), C −C = 1.419(9), C −C = 1.417(9),
1
7
7
2
2
1
N −As −C = 83.8(2), As −N −C = 115.5(4).
1
1
1
1
1
7
on the central ring, i.e., 10) or electronic stabilization (second
nitrogen donor, i.e., 6) would render azaphospholes and
azaarsoles isolable.
Table 4. Selected Bond Lengths (Å) and Bond Angles (deg)
of 11 and 12
56
Synthesis of Unsupported 1,2-Benzoazaphospholes
and 1,2-Benzoazaarsoles. To this end, azaphosphole 11
and azaarsole 12 featuring unsubstituted, fused benzene rings
and single imine donors to the pnictogen center were targeted.
Azaphosphole 11 was previously isolated and structurally
characterized by Tokitoh as the unexpected byproduct in the
attempted synthesis of phosphorus analogues of the β-diketiminate
1
1
12
N−EC
E−N
88.1(3)
83.8(2)
1.702(10)
1.744(4)
1.416(7)
1.426(6)
1.334(10)
1.883(6)
1.868(5)
1.419(9)
1.417(9)
1.308(7)
E−CAr
4
7
2
2
exocyclic sp −sp C−C bond
2
2
shared sp −sp C−C bond
CN imine bond
5
6
ligands. Here, its rational synthesis along with As counterpart
57
1
2 was achieved by treatment of B with n-BuLi, followed by
quenching with ECl (E = P or As) and in situ reduction with
Interestingly, the E−C bond lengths in azaphospholes 1
3
Ar
Mg or KC , affording both compounds as analytically pure
and 11 and azaarsoles 2 and 12 were the same within error
(Tables 4 and 1), indicative of similar redistribution of electron
density within the fused ring system and corroborated by
8
yellow solids in 24 and 51% yield, respectively (Scheme 5).
In solution, 11 and 12 exhibited apparent C symmetry with
s
six downfield-shifted aryl protons and a seventh imine signal
integrating in a 1:1:1:1:2:1:1 ratio. In addition, two separate
signals for the i-Pr methyl groups were observed by both
remarkably similar E−C hybridizations and delocalizations of
Ar
the N lone pair into the E−C π* orbital as determined by
NBO calculations using the reference structure illustrated in
Table 5 (compare with Table 2). However, the lack of the
1
13
1
H and C{ H} NMR spectroscopy, likely due to restricted
2
rotation about the (sp )C −CH(CH ) bonds leaving one
destabilizing N2LP → E−N σ* interaction from the flanking
Ar
3 2
1
set of methyl groups pointed above the five-membered
As-containing ring and other pair directed below. X-ray quality
crystals confirmed the structure of 12 with the inset of Figure 5
highlighting the orientation of the i-Pr methyl groups.
imine donor in 11 and 12 resulted in significantly shorter
(>0.05 Å) N−E bonds with increased s character (E → N)
versus 1 and 2. A related but less drastic N−E bond contrac-
tion (0.02 Å) was observed on comparing pincer ligated
F
DOI: 10.1021/acs.organomet.8b00290
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
a
Table 5. Selected Atom Hybridizations in a Model NC Bidentate System
E
ELP
E−N σ
E−C σ
N−E σ
C−E σ
delocalization NLP → E−Cπ* (kcal/mol)
delocalization NLP → C−Cπ* (kcal/mol)
sp1.5
sp0.4
sp0.3
sp0.2
sp0.1
sp
sp
sp
sp
3.5
sp
sp
sp
sp
1.7
sp
sp
sp
sp
2.1
sp
sp
sp
sp
2.3
27.2
11.7
6.9
4.0
3.1
38.6
40.8
48.5
54.6
59.1
N
P
As
Sb
Bi
7.7
4.0
2.1
2.5
13.6
6.0
2.2
2.6
17.7
7.9
2.2
2.8
27.2
11.1
2.3
2.9
sp
sp
sp
sp
a
The lone pair on N and the orbitals forming the E−C π-bond are all 100% p.
1
1
7,23,48
Bi-based 4 with its structurally characterized single-arm deriva-
tive 10. However, unlike the Bi case where the disparity in
N−Bi bond lengths could be attributed to 2-center, 2-electron
bonding in 10 versus notoriously weak and hypervalent
state within an NCN pincer.
These heterocycles range
from static and highly aromatic 5 (and its salt [5-Me][OTf])
to aromatic, but fluxional bell-clappers 1 and 2a/b to
hypervalent and weakly aromatic 3 and 4, containing 3-center,
4-electron N−E−N bonds (E = Sb or Bi). Previous efforts
showed that if Sb(I) or Bi(I) species were supported by
bidentate NC chelates that dimerization (8) or unidentified
decomposition pathways occurred (9) unless significant steric
protection was employed (10). Here, we demonstrated that the
analogous P(I) and As(I) intermediates undergo a redistribution
of electron density similar to those of 1 and 2a/b, affording
trivalent benzoazaheteroles 11 and 12. These heterocycles
display aromatic character on par with pyrazole-derived 5,
resulting in an apparent “line in the sand” between As and Sb
in which simple five-membered 6π electron heterocycles can or
cannot be isolated. Future investigations will explore if more
sterically and electronically diverse benzoazaphospholes and
-arsoles can be synthesized and if they can be employed as
novel building blocks for new electronic materials.
24
3
-center, 4-electron bonding in 4, the structural differences
between 1 and 11 and 2 and 12 are linked to aromaticity.
NICS calculations revealed 11 was more aromatic than 12, but
both C NE five-membered rings were almost as aromatic as
3
the pyrazole in 5, dwarfing the aromaticity of 1 and 2a/b
(
1
Table 3)! Of the remaining bonds in the azaphosphole ring in
1, the exocyclic sp −sp C−C bond shortened (1.416(7)
4
7
2
2
Å) and the CN bond lengthened (1.334(10) Å), but not
as dramatically as those in 1 (1.403(4) and 1.350(4) Å).
However, azaarsole 12 featured a C−C bond (1.419(9) Å)
intermediate between pincer derivatives 2a (1.429(6) Å) and
2
b (1.404(3) Å) with only a marginally elongated CN bond
(
1.308(7) Å versus 1.321(6) Å in 2a and 1.334(4) Å in 2b).
2
2
Finally, the shared sp −sp C−C bond joining the five- and six-
membered rings in 11 and 12 is the same length within error as
4
7
2
2
the exocyclic sp −sp C−C bond.
WBI values corroborate the widespread delocalization of
electron density and intermediacy of single and double bonds
in the five-membered rings of 11 and 12 (Chart 4, compared
EXPERIMENTAL SECTION
■
General Experimental Details. Unless otherwise specified, all
reactions and manipulations were performed under a nitrogen atmo-
sphere in a MBraun glovebox or using standard Schlenk techniques.
All glassware was oven-dried overnight (at minimum) at 140 °C prior
to use. Anhydrous solvents were purchased directly from chemical
suppliers (Aldrich or Acros), pumped directly into the glovebox and
stored over oven-activated 4 or 5 Å molecular sieves (Aldrich) or were
dried using Pure Solv−Innovative Technology equipment and stored
over molecular sieves in reservoirs equipped with Teflon Young valves.
Aryl bromides of types A and B were prepared by previously published
Chart 4. WBI Values for the Five-membered Rings in 11
and 12
30,57
methods.
AsCl , n-BuLi, MeOTf, and PCl were purchased from
3
3
commercial suppliers. PMe was obtained from Strem Chemicals and
3
stored over 4 Å molecular sieves in a Schlenk bomb equipped with a
screw-top Teflon cap. NMR spectra were obtained on either Varian
spectrometers operating at 300 or 500 MHz or Bruker 400 and
with Chart 2). This type of bond length equalization in
53
combination with the planarity of the fused ring system, the
5
00 MHz spectrometers; all spectra are displayed in the Supporting
48a
isolability and low reactivity compared with 9/10, and the
strikingly negative NICS values make a strong case for azapho-
sphole 11 and azaarsole 12 to be considered bona fide
aromatic heterocycles. Upcoming studies will ascertain if 11
and 12 exhibit reactivity consistent with a 6π electron (or 10π
including the fused benzene unit) heterocycle or a 1,3-diene.
Information. NMR chemical shifts are reported as ppm relative to
tetramethylsilane and are referenced to the residual proton or C signal
of the solvent ( H CDCl , 7.27 ppm; H C D , 7.16 ppm; C CDCl3
77.16 ppm; C C D , 128.06 ppm). Mass spectroscopic data was
13
1
1
13
3
6
6
1
3
6
6
obtained on an Agilent 6545 Accurate-Mass Q-TOF LC/MS (NSF
CHE-1532310). Analytical data were obtained from the CENTC
Elemental Analysis Facility at the University of Rochester, funded by
NSF CHE-065−456 or on a LECO−CHNS-932 analyzer. All X-ray
quality crystals were analyzed at the Small Molecular X-ray Crystal-
lography Facility located at the University of California, San Diego.
CONCLUSION
■
Using methodology closely related to the preparation of 1,
azaarsoles 2a/b and pyrazole-based 5 were synthesized and
fully characterized, adding to the growing number of com-
pounds derived from generating pnictogens in the +1 oxidation
30
Synthesis of 2a. Aryl bromide A (R = Mes, R′ = Me, 800 mg,
1.68 mmol) was dissolved in THF (100 mL) and cooled to −78 °C.
A solution of 1.6 M n-BuLi in hexanes (1.2 mL, 1.92 mmol, 1.1 equiv)
G
DOI: 10.1021/acs.organomet.8b00290
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
was added dropwise to the precooled solution and stirred at −78 °C
The title product (428 mg, 1.05 mmol) was obtained in 62% overall
yield. Anal. Calcd for C H N : C, 82.11; H, 7.63; N, 10.26. Found:
for 10 min. Subsequently, a solution of AsCl (660 mg, 3.64 mmol,
3
28 31
3
1
2.2 equiv) in THF (10 mL) was added to the reaction mixture, stirred
C, 81.73; H, 7.61; N, 9.67. H NMR (500 MHz, CDCl ): δ 8.05
3
at −78 °C for 10 min, and then stirred at r.t. for 1 h. The volatiles
were then removed from the reaction mixture under reduced pressure.
The obtained residue was washed with n-pentane (2 × 75 mL),
suspended in THF (50 mL), and treated with PMe3 (450 mg,
(dd, J = 7.0, 1.0 Hz, 1H), 7.78 (dd, J = 8.3, 1.0 Hz, 1H), 7.20 (dd, J =
8.3, 7.0 Hz, 1H), 7.04 (s, 2H), 6.89 (s, 2H), 2.43 (s, 3H), 2.41
1
3
1
(s, 3H), 2.39 (s, 3H), 2.30 (s, 3H), 2.10 (s, 6H), 1.91 (s, 6H). C{ H}
NMR (126 MHz, CDCl ): δ 168.50, 147.08, 146.52, 139.48, 135.75,
3
5.92 mmol, 3.5 equiv), leading to an intense purple reaction mixture.
135.58, 132.82, 131.68, 130.39, 129.13 (CH), 128.94, 128.55 (CH),
The reaction mixture was stirred at r.t. for 2 h, and the volatiles were
removed under reduced pressure. The obtained residue was extracted
with toluene (4 × 25 mL), the extracts were combined and filtered
through a Celite plug, and the filtrate was concentrated under reduced
126.05 (CH), 122.34 (CH), 121.99, 120.60 (CH), 21.36 (CH ), 21.30
3
(CH ), 20.90 (CH ), 18.09 (CH ), 17.34 (CH ), 9.95 (CH ).
3
3
3
3
3
Synthesis of [5-Me][OTf]. A solution of methyl triflate (156 mg,
0.95 mmol, 0.95 equiv) in DCM (4 mL) was added dropwise to a
stirring solution of 5 (410 mg, 1.00 mmol) in DCM (6 mL). The
reaction mixture was stirred at r.t. for 2 h, and the volatiles were
removed under reduced pressure. The obtained residue was triturated
pressure. The crude purple product was dissolved in CH CN (50 mL),
3
filtered through a Celite plug, and the filtrate was concentrated to
∼
20 mL and kept at −30 °C overnight. Purple crystals were then col-
lected by filtration (359 mg, 0.763 mmol, 45%). Anal. Calcd for
in Et O (6 mL) and stirred overnight affording a solid, which was
2
C H AsN : C, 71.48; H, 6.64; N, 5.95. Found: C, 71.64; H, 6.81; N,
collected by filtration, washed with Et O (3 × 10 mL), and dried
28
31
2
2
1
5
.65. H NMR (500 MHz, CDCl ): δ 8.11 (d, J = 7.6 Hz, 2H), 7.36
under vacuum. The crude product was recrystallized twice from a
3
(
(
t, J = 7.6 Hz, 1H), 6.95 (s, 4H), 2.39 (s, 6H), 2.33 (s, 6H), 2.02
solution of 1,4-dioxane layered with Et O. The title product (243 mg,
2
s, 12H). 13C{ H} NMR (126 MHz, CDCl ): δ 163.35, 157.62,
1
0.42 mmol) was obtained as yellow (X-ray quality) crystals in 42%
yield. Major compound: Anal. Calcd for C H F N O S: C, 62.81;
3
1
42.36, 134.50, 133.10, 130.10, 128.68 (CH), 128.56 (CH), 119.57
3
0
34
3
3
3
1
(
CH), 21.02 (CH ), 18.55 (CH ), 15.41 (CH ).
H, 5.97; N, 7.32. Found: C, 62.58; H, 5.89; N, 7.03. H NMR
3
3
3
Synthesis of 2b. First, 1.478 g (3.5 mmol) of aryl bromide A (R =
(500 MHz, CDCl ): δ 8.21 (dd, J = 7.1, 0.7 Hz, 1H), 8.09 (dd, J =
3
Xyl, R′ = H) was dissolved in 200 mL of diethyl ether, cooled to
8.4, 0.7 Hz, 1H), 7.42 (dd, J = 8.4, 7.1 Hz, 1H), 7.07−7.03 (m, 4H),
−78 °C, and 2.2 mL of n-BuLi (3.5 mmol, 1.6 M solution in hexanes)
3.83 (d, J = 1.4 Hz, 3H), 2.77 (d, J = 1.4 Hz, 3H), 2.45 (s, 3H), 2.39
1
3
was added. The resulting mixture was stirred for an additional 30 min
(s, 3H), 2.35 (s, 6H), 2.33 (s, 3H), 1.85 (s, 6H). C NMR
+
at −78 °C and then added to a precooled (−78 °C) solution of AsCl
(126 MHz, CDCl ): δ 188.47 (q, CN ), 143.32, 141.10, 140.21,
3
3
(
0.3 mL, 3.5 mmol) in 100 mL of diethyl ether. The reaction mixture
138.60, 135.84, 134.92, 134.85, 132.10 (CH), 131.07, 130.87 (CH),
129.35 (CH), 128.60 (CH), 121.79, 120.94 (CH), 119.69, 48.08
(CH ), 26.58 (CH ), 21.29 (CH ), 21.12 (CH ), 17.25 (two
was stirred for 24 h at r.t., resulting in the formation of a yellow pre-
cipitate. Subsequently, the mixture was concentrated (to ca. 50 mL),
the solution was filtered off, and the remaining yellow solid was dried
in vacuo. This solid was dissolved in 150 mL of THF and freshly
3
3
3
3
1
9
overlapping CH ), 10.08 (CH ). F NMR (471 MHz, CDCl ):
3
3
3
1
δ −78.88 (OSO CF ). Minor compound: H NMR (500 MHz,
2
3
prepared KC from 0.738 g (61.5 mmol, 10% excess) of graphite, and
CDCl ): δ 8.74 (d, J = 7.5 Hz, 1H), 8.38 (d, J = 8.3 Hz, 1H), 7.62−
8
3
0.333 g (8.5 mmol, 20% excess) of potassium was added. The reaction
7.54 (m, 1H), 7.07−7.05 (m, 4H), 3.69 (s, 6H), 2.88 (s, 3H), 2.54
mixture was stirred overnight at r.t. Thereafter, the mixture was evap-
orated in vacuo and extracted with 200 mL of hexane. The red extracted
solution was concentrated to ca. 20 mL and crystallized at −30 °C
giving a red microcrystalline solid characterized as {C H -2,6-[CH
(s, 3H), 2.38 (s, 3H), 2.16 (s, 6H), 1.88 (s, 6H).
5
6
57
Synthesis of 11. First, 4.111 g (11.9 mmol) of aryl bromide B
was dissolved in 250 mL of diethyl ether, cooled to −78 °C, and
7.47 mL of n-BuLi (11.9 mmol, 1.6 M solution in hexanes) was added.
The solution was stirred at −78 °C for an additional 1.5 h, resulting in
the formation of a yellow precipitate. This mixture was then added to
6
3
NC H -2′,6′-(CH ) ] }As. X-ray quality single-crystals were obtained
6
3
3 2 2
by recrystallization from a saturated hexane solution. Yield: 0.870 g
(
60%, 2.1 mmol), mp 145 °C. Anal. Calcd for C H AsN
1.04 mL of PCl (11.9 mmol) in 100 mL of diethyl ether precooled to
2
4
23
2
3
(
MW 414.37): C 69.6, H 5.5, N 6.8. Found: C 69.3, H 5.6, N 6.6%.
−78 °C. The reaction mixture was stirred at room temperature for
1.5 h, then evaporated in vacuo yielding a green precipitate, which was
subsequently dissolved in 200 mL of THF and treated with 0.319 g
(13.1 mmol, 10% excess) of magnesium, activated with a small amount
of 1,2-dibromoethane. The reaction mixture was stirred at r.t. overnight
and evaporated in vacuo. The residue was extracted with hexane
(200 mL), and the extract was concentrated and crystallized at −30 °C
affording a yellowish precipitate of {C H -2-[CHNC H -2′,6′-
1
H NMR (500 MHz, C D , 25 °C): δ 2.08 (s, 12H, CH ), 6.98
6
6
3
3
1
1
(
m, 6H, C H (CH ) −H ), 7.08 (t, J( H, H) = 7.5 Hz, 1H,
6
3
3
2
3,4,5
3 1 1
C H −H ), 7.57 (d, J( H, H) = 7.5 Hz, 2H, C H −H ), 7.99
(
6
3
4
6
3
3,5
1
3
1
s, 2H, CHN) ppm. C{ H} NMR (126 MHz, C D , 25 °C):
6
6
δ 18.61 (s, C H (CH ) ), 121.2 (s, C H −C ), 126.3 (s, C H −C ),
1
(
C ), 152.7 (s, CHN), 165.7 (s, C H −C ) ppm.
6 3 3 2 6 3 4 6 3 3,5
28.7 (s, C H (CH ) −C ), 130.7 (s, C H (CH ) −C ), 131.2
6
3
3
2
3,5
6
3
3
2
4
s, C H (CH ) −C ), 134.3 (s, C H (CH ) −C ), 148.3 (s, C H −
6
3
3
2
2,6
6
3
3
2
1
6
3
6
4
6
3
(i-Pr) ]}P. Yield: 0.844 g (24%, 2.9 mmol), mp 113 °C. Anal. Calcd
2,6
6
3
1
2
Synthesis of 5. Aryl bromide A (R = Mes, R′ = Me, 800 mg,
for C H NP (M 295.358): C 77.3, H 7.5, N 10.5; Found: C 77.5,
1
9
22
W
3
1
1
1
.68 mmol) was dissolved in THF (100 mL) and cooled to −78 °C.
H 7.8, N 10.8%. P{ H} NMR (161.9 MHz, C D , 25 °C): δ 182.8
6 6
1 3 1 1
A 1.6 M n-BuLi solution in hexanes (1.1 mL, 1.77 mmol, 1.05 equiv)
ppm. H NMR (500 MHz, C D , 25 °C): δ 0.98 (d, J( H, H) =
6 6
3 1 1
was added dropwise to the solution and stirred at −78 °C for 10 min.
6.9 Hz, 6H, CH ), 1.12 (d, J( H, H) = 6.9 Hz, 6H, CH ), 2.38
3
3
3
1
1
Subsequently, a solution of tosylazide (TsN , 400 mg, 2.03 mmol,
(septet, J( H, H) = 6.9 Hz, 2H, (CH ) CH), 6.93 (m, 1H, C H −
3
3
2
6
4
3 1 1
1.2 equiv) in 10 mL of THF was added dropwise to the reaction
H ), 7.00 (m, 1H, C H −H ), 7.07 (d, J( H, H) = 7.8 Hz, 2H,
5
6
4
4
3 1 1
mixture, stirred at −78 °C for 10 min, and then at r.t. for 1 h. The
reaction was quenched by addition of water (2 mL). The volatiles
were removed under reduced pressure, and the resulting intermediate
C H −H ), 7.22 (t, J( H, H) = 7.8 Hz, 1H, C H −H ), 7.58
6
3
4
6
3
3,5
n 31 1 3 1 1
(d, J( P, H) = 3.9 Hz, 1H, CHN), 7.70 (d, J( H, H) = 8.6 Hz,
3
1
1
3
31
1
1H, C H −H ), 7.99 (dd, J( H, H) = 8.5 Hz, J( P, H) = 3.3 Hz,
6
4
3
1
3
1
azide (NCN)−N was purified by column chromatography (silica gel,
1H, C H −H ) ppm. C{ H} NMR (125.8 MHz, C D , 25 °C):
3
6
4
6
6
6
mobile phase with polarity gradient from ethyl acetate/n-hexane 1:10
to 1:8, R = 0.24 at 1:8 ratio). The isolated azide (NCN)−N
δ 25.2 (s, CH ), 25.8 (s, CH ), 28.7 (s, (CH ) CH), 123.1
3 3 3 2
3
31 13
4
31 13
(d, J( P, C) = 18.5 Hz, C H −C ), 123.6 (d, J( P, C) = 3.7 Hz,
f
3
6
4
5
1
3
31 13
(
(
548 mg, 1.25 mmol) was used in the following step. H NMR
C H −C ), 124.0 (s, C H −C ), 124.7 (d, J( P, C) = 1.8 Hz,
3,5
2 31 13
6
4
4
6
3
300 MHz, CDCl ): δ 7.68 (d, J = 7.6 Hz, 2H), 7.41 (m, 1H), 6.91
C H −C ), 125.5 (d, J( P, C) = 19.8 Hz, C H −C ), 129.8
3
6
4
3
6
4
6
2 31 13
(s, 4H), 2.30 (s, 6H), 2.14 (s, 12H), 2.12 (s, 6H). The azide (NCN)−N
(s, C H −C ), 131.8 (d, J( P, C) = 7.3 Hz, C H −C ), 135.9
3
6
3
4
6
4
2
n 31 13 2 31 13
(
548 mg, 1.25 mmol) was dissolved in CHCl (10 mL) and stirred
(d, J( P, C) = 7.4 Hz, CHN), 139.1 (d, J( P, C) = 10.1 Hz,
3
3
31 13
at 100 °C for 2 h in Schlenk flask sealed with a Teflon screw cap.
The organic volatiles were removed under reduced pressure, and the
resulting crude product was purified by column chromatography
C H −C ), 146.4 (d, J( P, C) = 1.8 Hz, C H −C ), 159.1
6 3 1 6 3 2,6
1 31 13
(d, J( P, C) = 40.9 Hz, C H −C ) ppm.
6
4
1
Synthesis of 12. First, 2.367 g (5.8 mmol) of {C H -2-[CH
6
4
5
8
(
silica gel, mobile phase: ethyl acetate/n-hexane 1:10; R = 0.23).
NC H -2′,6′-(i-Pr) ]}AsCl was dissolved in 150 mL of THF and
f
6
3
2
2
H
DOI: 10.1021/acs.organomet.8b00290
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
2
equiv of KC freshly prepared from 1.210 g (100.8 mmol, 10% excess)
Notes
8
of graphite, and 0.542 g (13.9 mmol, 20% excess) of potassium were
added. The resulting mixture was stirred at r.t. overnight, evaporated in
vacuo, and extracted with 200 mL of hexane. The yellow extracted
solution was concentrated to ca. 30 mL and crystallized at −30 °C.
Compound {C H -2-[CHNC H -2′,6′-(i-Pr) ]}As was obtained
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
6
4
6
3
2
M.F.C. thanks the University of Hawai‘i at Manoa (UHM) for
̅
in the form of yellow microcrystalline solid. X-ray quality single-
crystals were obtained by recrystallization from diethyl ether. Yield:
generous start-up funds and laboratory space. Mass spectro-
scopic data was obtained at UHM on an Agilent 6545
Accurate-Mass QTOF-LCMS (NSF CHE-1532310). L.D.
thanks the Czech Science Foundation for continuous support
project no. 18-10222S. J.T. acknowledges the financial support
of the Research FoundationFlanders (FWO; Project No.
12Y7718N) and the computational resources and services
provided by the Shared ICT Services Centre funded by the
Vrije Universiteit Brussel, the Flemish Supercomputer Center
(VSC) and FWO. R.P.H. thanks Prof. Eric Glendening
1
.002 g (51%, 2.9 mmol), mp 126 °C. Anal. Calcd for C H AsN
19 22
(
MW 339.306): C 67.3, H 6.5, N 4.1; Found: C 67.6, H 6.3, N 4.2%.
1
3
1
1
H NMR (500 MHz, C D , 25 °C): δ 0.98 (d, J( H, H) = 6.9 Hz,
6
1
6
3
1
6
H, CH ), 1.13 (d, J( H, H) = 6.8 Hz, 6H, CH ), 2.51 (septet,
3
3
3
1
1
3
1
1
J( H, H) = 6.9 Hz, 2H, (CH ) CH), 6.90 (t, J( H, H) = 7.3 Hz,
3
2
3
1
1
1
H, C H −H ), 7.00 (t, J( H, H) = 7.7 Hz, 1H, C H −H ), 7.08
6
4
5
6
4
4
3 1 1 3 1 1
(
1
(
d, J( H, H) = 7.8 Hz, 2H, C H −H ), 7.22 (t, J( H, H) = 7.8 Hz,
6
3
4
3 1 1
H, C H −H ), 7.80 (d, J( H, H) = 8.6 Hz, 1H, C H −H ), 7.89
6
3
3,5
6
4
3
3 1 1
s, 1H, CHN), 8.02 (d, J( H, H) = 8,4 Hz, 1H, C H −H ) ppm.
6
4
6
13
1
C{ H} NMR (125.8 MHz, C D , 25 °C): δ 25.3 (s, CH ), 25.8
6
6
3
(
Indiana State University) for help with some NBO CHOOSE
(
s, CH ), 28.5 (s, (CH ) CH), 123.0 (overlap of two signals, C H −
3
3
2
6
4
settings. Elemental analyses were conducted by William
Brennessel (University of Rochester).
C ), 124.0 (s, C H −C ), 127.3 (s, C H −C ), 127.4 (s, C H −
4,5
6
3
3,5
6
4
3
6
4
C ), 129.3 (s, C H −C ), 132.7 (s, C H −C ), 141.3 (s, C H −C ),
6
6
3
4
6
4
2
6
3
1
1
41.8 (s, CHN), 145.5 (s, C H −C ), 172.2 (s, C H −C ) ppm.
6
3
2,6
6
4
1
DFT Calculations. DFT and associated NBO calculations were
REFERENCES
59
■
run using the Jaguar suite of programs; full details are provided in
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.organomet.8b00290.
■
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AUTHOR INFORMATION
Corresponding Authors
E-mail: libor.dostal@upce.cz.
E-mail: mfcain@hawaii.edu.
■
(
*
*
3
2, 7116−7121.
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Libor Dostal: 0000-0002-5982-1239
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Matthew F. Cain: 0000-0002-0673-5839
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V.K. and J.H. contributed equally to this work.
I
DOI: 10.1021/acs.organomet.8b00290
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DOI: 10.1021/acs.organomet.8b00290
Organometallics XXXX, XXX, XXX−XXX