C-H activation by a diselenido dinickel(II) complex

Article abstract of DOI:10.1021/ja407995r

Addition of selenium to the nickel(I) complex, [Ni(Me₄[12] aneN₄)(CO)]PF₆, effects a redox reaction leading to the diselenido dinickel(II) complex, {[(Ni(Me₄[12]aneN₄)] ₂(Se₂)}(PF₆)₂, in 70% crystalline yield. The product's structure features a μ-η²: η²-Se₂ ligand with Se-Se bond length of 2.379(13) A?. Upon mild heating, {[(Ni(Me₄[12]aneN₄)] ₂(μ-η²:η²-Se₂)}(PF ₆)₂ oxidizes 9,10-dihydroanthracene or 1,4-cyclohexadiene forming the terminal hydroselenide, [Ni(Me₄[12]aneN ₄)(SeH)]PF₆, and anthracene or benzene, respectively. [Ni(Me₄[12]aneN₄)(SeH)]PF₆ cleanly converts back to the diselenido dinickel(II) adduct upon addition of a phenoxy radical.

Full text of DOI:10.1021/ja407995r

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Communication
C-H Activation by a Diselenido Dinickel(II) Complex
Jessica Wallick, Charles G. Riordan, and Glenn P. A. Yap
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja407995r • Publication Date (Web): 25 Sep 2013
Downloaded from http://pubs.acs.org on September 26, 2013
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C-H Activation by a Diselenido Dinickel(II) Complex
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Jessica Wallick, Charles G. Riordan,* and Glenn P. A. Yap
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Department of Chemistry and Biochemistry, University of Delaware, Newark, DE 19716
Supporting Information
1
-1
ABSTRACT: Addition of selenium to the nickel(I) com-
plex, [Ni(Me [12]aneN )(CO)]PF effects a redox reac-
tion leading to the diselenido dinickel(II) complex,
{[(Ni(Me [12]aneN )] (Se )}(PF , in 70% crystalline
yield. The product’s structure features a µ-η :η -Se
ligand with Se–Se bond length of 2.379(13) Å. Upon
cm ) and 690 nm (1100). The former, assigned as a
2
-
2
(Se ) →Ni charge transfer transition, contains a shoul-
der at 410 nm. The complex is paramagnetic as evi-
denced by highly dispersed H NMR signals (Figure 1S)
and a µeff = 2.71(2) µ
4
4
6
1
4
4
2
2
6
)
2
2
2
B
.
2
The cation of 1 consists of a centrosymmetric planar
2
2
2
2
mild
Se )}(PF
cyclohexadiene forming the terminal hydroselenide,
Ni(Me [12]aneN )(SeH)]PF , and anthracene or ben-
zene, respectively. [Ni(Me [12]aneN )(SeH)]PF , clean-
heating,
{[(Ni(Me
4
[12]aneN
4
)]
2
(µ-η :η -
2 2
Ni (μ-η :η –Se )ꢀ core, Figure 1, with each metal in a
distorted octahedral geometry. The Se–Se bond distance
2
6 2
) oxidizes 9,10-dihydroanthracene or 1,4-
t-
of 2.379(13)Åꢀ is shorter than that found inꢀ {[(PhTt
Bu
2
2
8
[
4
4
6
)
Ni]
[(β-diketiminato)Ni]
the only other structures available for Ni
2 2
(µ-η :η -Se )}ꢀ (2.479(1)Å) ꢀ and longer than inꢀ
2
2
2+
9
4
4
6
{
2
(µ-η :η -Se
2
)} ꢀ (2.3304(6)ꢀ Å), ꢀ
(Se ) complex-
ly converts back to the diselenido dinickel(II) adduct
upon addition of a phenoxy radical.
2
2
10
es. ꢀꢀThe intermediate bond length in the title complex
is indicative of a moderate level of Se–Se bond activa-
tion. Although, it should be noted that the metal coordi-
nation number, which can also influence the Se–Se bond
length varies from six to five to four through this series
of complexes.ꢀꢀ
1
Transition metal chalcogenides enjoy a rich history
due to their wide-ranging geometric and electronic struc-
tures resulting in novel bonding characteristics and ap-
plications. Soluble forms, which can be challenging to
prepare given the significant driving force favoring bina-
ry metal chalcogenides, display a wide array of stoichi-
ometries and structures. Seminal examples date to some
of the earliest studies of organometallic chemistry in-
2
volving metal carbonyls with chalcogenide ligands.
Their renaissance may be traced, in part, to the discov-
3
ery of these cores in metalloprotein active sites.
4
5
We have explored nickel-oxygen and –sulfur com-
plexes, uncovering a range of geometric and electronic
structures and patterns of reactivity. In this report, we
turn our attention to soluble nickel-selenium complexes,
applying the strategy of synthesis via reaction of nick-
el(I) precursors with selenium. The resulting diselenido
dinickel(II) complex exhibits the capacity to activate C–
H bonds, a transformation of potential synthetic utility.
Metal-selenide C–H activation is rarely seen across the
Figure 1. Thermal ellipsoid plot of the cation of 1. Selected
bond lengths (Å) and angles (deg): Se–SeA 2.379(13), Ni–
Se 2.566(13), Ni–SeA 2.557(13); Ni–Se–NiA 124.68(2),
Se–Ni–SeA 55.32(2).
The ability of 1 to oxidize C–H bonds was surveyed
initially by reaction with 9,10-dihydroanthracene
(DHA), a substrate with activated C–H bonds (first bond
6
periodic table and unprecedented for nickel.
1
1
dissociation enthalpy = 78 kcal/mol). In a typical reac-
tion, a THF solution of DHA was added to 1 in acetoni-
trile. The dark brown solution was heated at 55°C over-
night generating a light green solution, Scheme 1.
The reaction of elemental selenium with the nickel(I)
7
complex [Ni(Me
4
[12]aneN
4
)CO]PF
6
resulted in a rapid
color change from light green to dark brown. Recrystali-
zation lead to isolation of the diselenido dinickel(II)
1
2
2
Analysis of the solution by H NMR spectroscopy and
complex, {[(Ni(Me
4 4 2 2 6 2
[12]aneN )] (μ-η :η -Se )}(PF )
integration relative to an internal standard revealed the
consumption of 50% of the DHA and the concomitant
production of anthracene in 48% yield, Figure 2. An-
(
1) in 70% yield (See Supporting Information for syn-
thetic and spectroscopic details). The optical spectrum
-
displays two intense features at 390 nm (ε = 7070 M
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thracene production was additionally confirmed by GC- acterized (SI). Metathesis of [Ni(Me
Page 2 of 5
4
4 6 2
[12]aneN )](PF )
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MS analysis.
with Na Se in a THF/acetonitrile solution resulted in
2
conversion to a bright purple solution, from which green
crystals were deposited and identified as 2. The same
Scheme 1
compound was produced upon addition of excess PPh
to 1. In each of these reactions, a common intermediate,
[Ni(Me [12]aneN )Se](PF ) is proposed, which abstracts
3
(
PF6)2
H
H
H
H
N
Ni
N
N
Ni
N
4
4
6
N
N
Se
Se
N
a hydrogen atom yielding 2. Identifying the source of the
H atom is an objective of ongoing experiments. The
high-spin nickel(II) complex features a stereochemistry
intermediate between trigonal bipyramidal (τꢀ =ꢀ 1) and
square pyramidal (τꢀ=ꢀ0) as evidenced by its τꢀvalue of
+
N
0
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Δ
0
.53. This represents the first structure of a terminally-
1
3
ligated hydroselenide of nickel and the second of any
1
4
nickel triad metal. The Ni–Se distance is 2.389(7) Å.
Corroborating evidence for the SeH ligand was provided
by IR spectroscopy and mass spectrometry, the former
H
N
PF
6
Se
Ni
–
1 15
N
N
of which revealed a ν(Se–H) at 2346 cm . Quantita-
tion of the reaction of 1 with DHA by electronic absorp-
tion spectroscopy revealed 2 production in only 15%
yield, vide infra.
+
2
N
2
Figure 3. Thermal ellipsoid plot of the cation of 2. Selected
bond lengths (Å):ꢀNi–N1 2.186(2), Ni–N2 2.0616(16), Ni–
N3 2.189(2), Ni–Se 2.389(7).
The origin of the low yield of 2 was of concern and
consequently, investigated. 2 is somewhat unstable un-
der the reactions conditions, i.e. 2 decomposed at 55ºC
in a time course comparable to that of the reaction of 1
with DHA. Further support for this assertion derives
from the reaction of 1 with 1,4-cyclohexadiene (CHD),
which produced 2 and benzene. Despite the similar C–H
BDEs for DHA and CHD, the latter reacts with 1 at low-
er temperatures, due to the smaller size of the CHD sub-
strate. Consequently, the reaction of 1 and CHD pro-
ceeded at 35°C affording 2 in 70% yield and benzene in
66% yield.
1
Figure 2. H NMR spectra in CD
3
CN of the reaction of 1
and 9,10-dihydroanthracene (DHA) before (a) and after (b)
heating at 55 Cꢀ toꢀ produce anthracene (An) in 48% yield
relative to a hexamethylbenzene (HMB) standard. The fea-
tures at ~6 ppm (#) are attributed to a thermal decomposi-
tion product of 2. Residual solvent (*).
º
The nickel-containing product was isolated as green
crystals following pentane diffusion into a concentrated
solution. X-ray diffraction analysis established the crys-
talline material as the nickel(II) hydroselenide,
The efficacy of the reactions with DHA and CHD
point to a lower limit of the effective Se–H bond disso-
ciation enthalpy (BDE) of at least 78 kcal/mol. To de-
termine the BDE with greater accuracy, one could turn
16
[
Ni(Me
4
[12]aneN
4
)SeH](PF
6
) (2), which was prepared
to the well documented approach, in which knowledge
via two independent synthetic strategies and fully char-
a
of the pK and the oxidation potential of the conjugate
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ASSOCIATED CONTENT
base are applied in a thermodynamic analysis to calcu-
1
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1
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late the X–H BDE. This strategy can not be applied
with accuracy to 2 because the conjugate base is not ac-
cessible. Consequently, to garner insight into the magni-
Supporting Information Available: Experimental details
and characterization data including CIF files. This material
is available free of charge via the Internet at
http://pubs.acs.org.
tude of the Se–H effective BDE, reaction of 2 with the
17
2
,4,6-tri-tert-butylphenoxy radical was assayed.
The
reaction efficiently resulted in conversion of 2 to 1 and
,4,6-tri-tert-butylphenol in high yields, Scheme 2.
These results lead to the conclusion that the effective
AUTHOR INFORMATION
2
Corresponding Author
1
8
riordan@udel.edu
Se–H
Ni(Me
bond
[12]aneN
dissociation
enthalpy
of
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[
4
4
)SeH](PF ) must be intermediate be-
6
ACKNOWLEDGMENT
tween those of DHA (78 kcal/mol) and 2,4,6-tri-tert-
1
9
Joliene Trujillo is acknowledged for her initial synthesis of
butylphenol (82.3 kcal/mol), i.e. the Se–H effective
BDE is 80(2) kcal/mol. By comparison, the Se–H BDE
7
{
Ni(Me
4
[12]aneN
4
)CO}OTf. We thank the National Sci-
2
0
ence Foundation (CHE-1112035) for support of this work
and for funding for the X-ray diffractometer through a
CRIF Award (CHE-1048367).
for PhSe–H is estimated at 76-80 kcal/mol. The simi-
larity of this measurement to the value determined for 2
is striking as it points to similar stabilization energies of
+
the PhSe and [Ni(Me
4
[12]aneN
4
)Se] fragments.
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Scheme 2
(
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+
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Ni
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0.5
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N
(
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+
Electrochemically generated [Ni(Me
4
[12]aneN
4
)CO] has been re-
In summary, the diselenido dinickel(II) complex pre-
pared in high yield via the reaction of its nickel(I) pre-
cursor and elemental selenium features a symmetrically
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Gundlach, E. M.; Blinn, E. L. Inorg. Chim. Acta 1996, 246, 295.
tBu
(
8) PhTt = phenyl(tris((tert-butylthio)methyl)borate). Huang, Y.;
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2
2
2
bound µ-η :η –Se ligand. The paramagnetic compound
oxidizes the C–H bonds of DHA and CHD yielding the
aromatic organic products and the hydroselenide, 2. The
hydroselenide reacts cleanly with a phenoxy radical in a
presumed proton-coupled electron transfer (PCET) pro-
cess regenerating 1 and the phenol. The proclivity of
these two reactions provides a means to bracket the Se–
H effective BDE as 80(2) kcal/mol. Future efforts in-
clude mechanistic studies to further elucidate the details
of the C–H activation process and exploiting this ap-
proach to incorporate selenium into the organic sub-
strates, e.g. Se insertion into C–H bonds to create C–Se–
H functionality adding molecular complexity via metal-
promoted elemental selenium activation.
(
2 2
10) Ni E (E = S, Se, Te) cores with "weak bonding" between the
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(
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(
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(
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1
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
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suggesting this more clear terminology.
(19) Bordwell, F. G.; Zhang, X.-M. J. Phys. Org. Chem. 1995, 8,
529.
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Chem. 1996, 100, 6608.
(
(
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Feng, Y. H.; Wang, H. B.; Smith, J. M. J. Am. Chem. Soc. 2007, 129,
2424.
(
18) The term "effective" BDE is used to emphasize that the meas-
ured energy necessarily takes into account the making/breaking of
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H
H
H
H
Effective BDE,
80(2) kcal/mol
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2+
H
+
Se
N
Ni
N
N
Ni
N
N
Se
Se
N
N
N
N
0.5
Ni
N
N
N
OH
O•
But
But
But
But
But
But
5
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