Efficient catalytic conversion of dinitrogen to N(SiMe3)3 Using a homogeneous mononuclear cobalt complex

Article abstract of DOI:10.1021/acscatal.7b04351

Incorporation of the tridentate phosphine-enamidoiminophosphorane onto cobalt(II) produces tetrahedral Co(NpNP^iPr)Cl, 1, which upon reduction under dinitrogen generates the T-shaped, paramagnetic Co(I) complex Co(NpNP^iPr), 2. This paramagnetic T-shaped derivative is in equilibrium with the paramagnetic dinitrogen derivative Co(NpNP^iPr)(N₂), 3, which can be detected by IR and low-temperature UV-vis spectroscopy. Both 1 and 2 act as homogeneous catalysts for the conversion of molecular nitrogen into tris(trimethylsilyl)amine (N(SiMe₃)₃) (～200 equiv, quantified as NH₄Cl after hydrolysis) in the presence of excess KC₈ and Me₃SiCl at low temperatures.

Full text of DOI:10.1021/acscatal.7b04351

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Efficient Catalytic Conversion of Dinitrogen to N(SiMe3)3
Using a Homogeneous Mononuclear Cobalt Complex
Tatsuya Suzuki, Keisuke Fujimoto, Yoshiyuki Takemoto, Yuko Wasada-Tsutsui,
Tomohiro Ozawa, Tomohiko Inomata, Michael D Fryzuk, and Hideki Masuda
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b04351 • Publication Date (Web): 05 Mar 2018
Downloaded from http://pubs.acs.org on March 6, 2018
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ACS Catalysis
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Efficient Catalytic Conversion of Dinitrogen to N(SiMe3)3 Using a
Homogeneous Mononuclear Cobalt Complex
Tatsuya Suzuki,¶,‡ Keisuke Fujimoto,‡ Yoshiyuki Takemoto,‡ Yuko WasadaꢀTsutsui,‡ Tomohiro Ozawa,‡
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Tomohiko Inomata,‡ Michael D. Fryzuk,*¶ and Hideki Masuda*‡
¶Department of Chemistry, The University of British Columbia, 2036 Main Mall, Vancouver, BC, CANADA, V6T 1Z1.
‡Department of Frontier Materials, Graduate School of Engineering, Nagoya Institute of Technology, Gokiso, Showa,
Nagoya 466ꢀ8555 JAPAN
2
~1500 equiv KC8
+
~2000 equiv Me3SiCl
Supporting Information
– 40˚C
SiMe3
N
ABSTRACT: Incorporation of the tridentate phosphineꢀenamidoiminophosphorane
onto cobalt(II) produces tetrahedral Co(NpNPiPr)Cl, 1, which upon reduction unꢀ
der dinitrogen generates the Tꢀshaped, paramagnetic Co(I) complex Co(NpNPiPr),
2. This paramagnetic Tꢀshaped derivative is in equilibrium with the paramagnetic
dinitrogen derivative, Co(NpNPiPr)(N2), 3, which can be detected by IR and low
temperature UVꢀvis spectroscopy. Both 1 and 2 act as a homogenous catalysts for
the conversion of molecular nitrogen into tris(trimethylsilyl)amine (N(SiMe3)3)
(~200 equiv, quantified as NH4Cl after hydrolysis) in the presence of excess KC8
and Me3SiCl at low temperatures.
N
N
Me3Si
SiMe3
~200 equivalents
N
N
P
P
+ N2
– N2
N
Co
N
Co
N
N
P
P
2
3
KEYWORDS. Dinitrogen, cobalt, ligand design, homogeneous catalysis, tris(trimethylsilylamine).
The only known industrial process that uses molecular
nitrogen (N2) as a feedstock is the HaberꢀBosch reaction,
which converts N2 and dihydrogen (H2) into ammonia
(NH3). The production of NH3 by this process occurs at
high pressures and temperatures over a heterogeneous
iron or ruthenium catalyst, and since its discovery over
100 years ago, the major improvements have been to
make this conversion more efficient.1 While one can
imagine many other transformations that could use N2 as
a feedstock,2 the fact that no other process has been deꢀ
veloped is likely due to the intrinsic inertness of this
readily available diatomic molecule. Without a doubt the
discovery of a new catalytic process that utilizes N2 as
reactant is a worthy goal and is being actively pursued
by numerous research groups.3
ingly, the very first report of a homogeneous process that
was shown to convert N2 to the higher value
tris(trimethylsilyl)amine, N(SiMe3)3, actually predates
the aforementioned ammoniaꢀproducing systems.5 The
conditions used for the catalytic production of
N(SiMe3)3 also involve the reaction of N2 in the presꢀ
ence of excess reducing agent, but with excess Me3SiCl
in lieu of protons. While the initial metal halide catalysts
gave very low turnovers,5a the use of the Mo(0) dinitroꢀ
genꢀphosphine complex, cisꢀMo(N2)2(PMe2Ph)4, gave
24 equivalents of N(SiMe3)3 per Mo center.5b More reꢀ
cent improvements on this catalytic process involve the
use of a chelating diphosphine with a ferrocenyl backꢀ
bone attached to generate a molybdenum dinitrogen
complex, which produced 226 equiv of N(SiMe3)3;6
even more intriguing, the use of a dicobalt system with a
ligand scaffold that involves three amido units linked to
three phosphine arms produced 196 equiv N(SiMe3)3 per
Co2 complex using similar reagents.7 There are also
mononuclear and polynuclear ironꢀbased catalyst preꢀ
cursors and other metal systems that are productive in
this reaction but generate lower numbers of equivalents
Recently, some intriguing proofꢀofꢀconcept homogeꢀ
neous catalytic processes that use dinitrogen have been
reported.4 While not industrially relevant, the producꢀ
tion of ammonia and hydrazine via the addition of exꢀ
cess reducing agents and proton sources to molecular
nitrogen in the presence of various Mo, Fe, and Co comꢀ
plexes has generated many intriguing results. Interestꢀ
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of tris(trimethylsilyl)amine (≤ 65 equiv/metal center).8
Page 2 of 8
sistent with a small contamination of the crystals of 2
with dinitrogen complex 3. Fortunately, after many atꢀ
tempts to grow crystals, we were able to isolate one
batch that could be modeled by a 90:10 disorder in 2
with 3.12, 14 The individual structures are shown in Figꢀ
ures 1 and 2.
1
2
3
4
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8
To continue the advances in the area of catalytic dinitroꢀ
gen conversion to higher value organonitrogen derivaꢀ
tives, exploration of chemical space to discover new
productive systems is critical. In this work, we document
an electronꢀrich iminophosphoraneꢀcobalt derivative that
acts extremely efficiently to produce N(SiMe3)3 (~200
equiv) via the reduction of N2 in the presence of excess
reducing agent and Me3SiCl.
To provide further evidence for this equilibrium, we
examined the solution UVꢀVis spectrum of 2 under N2
and observed changes as a function of temperature atꢀ
tributable to the formation of 3: an absorption at 500 nm
decreases as a function of temperature under N2 whereas
this same band shows no change under Ar when the
temperature is lowered.[14]
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We have previously reported9a the synthesis of a new
tridentate ligand precursor, Li(NpNPiPr) (A) (where
NpNPiPr = phosphineꢀenamidoiminophosphorane); this
ligand contains a Nacnac mimic10 decorated with an
electronꢀrich phosphine arm. Upon reaction of A with
CoCl2 in Et2O, the cobalt(II) complex Co(NpNPiPr)Cl (1)
is formed as shown in Scheme 1; 1 is isolated as brown
crystals and is paramagnetic with ꢀeff = 4.04 ꢀB conꢀ
sistent with 3 unpaired electrons (S = 3/2, Evans methꢀ
od).
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Scheme 1. Synthesis of Co(NpNPiPr)Cl (1) and reduc-
tion under N2 to Co(NpNPiPr) (2), which is in equilib-
rium with Co(NpNPiPr)(N2) (3)
Figure 1. ORTEP drawing of the solidꢀstate molecular
structure of Co(NpNPiPr) 2 (ellipsoids at 30% probability
level), which is modeled as 90% of the disorder. All hyꢀ
drogen atoms have been omitted for clarity. Selected bond
lengths (Å), angles (deg): Co1–P2 2.1560(8), Co1–N1
1.9169(19), Co1–N2 2.028(2), P1–N2 1.630(6), P1–C17
1.753(2), N1–C13 1.337(4), C13–C17 1.389(4); N1–Co1–
P2 166.13(8), P2–Co1–N2 87.20(7), N1–Co1–N2
106.34(10).
Reduction of 1 with potassium graphite (KC8) in Et2O
under dinitrogen leads to the formation of 2, which can
be isolated as red crystals. In solution, under argon, 2 is
also paramagnetic with ꢀeff = 2.40 ± 0.05 ꢀB, which is
invariant from 293ꢀ193 K and corresponds to two unꢀ
paired electrons (S = 1).11 Interestingly, as shown in
Scheme 1, 2 exists in equilibrium with a small amount
of the dinitrogen complex 3 (vide infra).
That the N2 complex 3 forms to a small extent under
dinitrogen was first evident upon taking an IR spectrum
of the crystals of 2, which revealed a weak absorption at
2071 cmꢀ1; performing the synthesis under 15N2 resulted
in a new band shifted to 2001 cmꢀ1. Reꢀexamination of
the solidꢀstate structural data did show some residual
electron density above the plane of the complex conꢀ
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Figure 2. ORTEP drawing of the solidꢀstate molecular
with KC8 in the presence of [18ꢀcrownꢀ6]. Importantly,
Fe(CAAC)2 is productive in the catalytic silylation of N2
to produce 20 – 27 equiv of N(SiMe3)3 using excess KC8
(600 equiv) and Me3SiCl (600 equiv) at room temperaꢀ
ture.8c
structure of Co(NpNPiPr)N2 3 (ellipsoids at 30% probability
level), which is modeled as a 10% disorder in 2. All hyꢀ
drogen atoms have been omitted for clarity. Selected bond
lengths (Å), angles (deg): Co2–P2 2.1851(7), Co2–N1
1.9735(19), Co2–N2 2.063(3), Co2–N3 1.99(4), N3–N4
1.12(6), P2–Co2–N1 152.91(7), P2–Co2–N2 85.57(6), P2–
Co2–N3 91.6(9), N1–Co1–N2 106.34(10).
1
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3
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We investigated the ability of the Tꢀshaped Co(I) comꢀ
plex 2 to act as a catalyst precursor for the catalytic siꢀ
lylation of dinitrogen. The general conditions are shown
in Scheme 2.
In solution (THF or toluene), we calculate via UVꢀvis
spectroscopy (Figure 3) that at room temperature and 1
atm N2, only about 15% of 3 is present at equilibrium,
whereas at ꢀ80 ˚C, the mixture contains almost 90% of
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Scheme 2. Conditions: 1 atm N2, ~2000 equiv
Me3SiCl and ~1500 equiv KC8 with respect to 1 equiv
of 2.
1
the dinitrogen complex 3. Interestingly, H NMR specꢀ
troscopy is not sensitive to the formation of 3 as the varꢀ
iable temperature NMR spectra of 2 under Ar and 2 unꢀ
der N2 do not show any differences as a function of temꢀ
perature (293 K – 193 K)14. In contrast, the related Tꢀ
shaped Co(I) complex, Co(SiPNP) (where (SiPNP) =
N(SiMe2CH2PBut2)2), is in equilibrium with the dia-
magnetic
square
planar
dinitrogen
complex
Co(SiPNP)(N2);13a other Co(PNP) systems typically genꢀ
erate diamagnetic N2 complexes upon reduction under
dinitrogen.4d, 13e
Optimization experiments showed that low temperaꢀ
tures and long reaction times (10 days) favored highest
turnover numbers; in fact, at ꢀ40 ˚C, we observed 200 ±
20 equivalents of N(SiMe3)3 with 2 as the catalyst preꢀ
cursor using excess reducing agent (KC8) and Me3SiCl;
analysis was carried after hydrolysis with excess HCl.14,
3.5
293 K
283 K
3.0
273 K
263 K
2.5
253 K
15
350 nm
The conditions indicated in the caption to Scheme 2
243 K
2.0
1.5
1.0
0.5
0
233 K
223 K
213 K
203 K
193 K
were done in triplicate and vary by ± 10%. The more
efficient catalytic turnover at low temperature is conꢀ
sistent with the aforementioned shift in the equilibrium
favoring formation of the N2 complex 3. We also find
that the choice of solvent is important; THF and DME
generate similar efficient turnovers while toluene is infeꢀ
rior.14 We also tested catalyst robustness; after 10 days
(215 equiv N(SiMe3)3 measured after hydrolysis), the
solution was filtered and a new charge of 2000 equiv
Me3SiCl and 1500 equiv KC8 was added and the mixture
stirred for 3 days at ꢀ40 ˚C. This generated an additional
55 equiv of N(SiMe3)3 after hydrolysis (total = 270
equiv).
ε
410 nm
400
300
500
600
700
800
Wavelength (nm)
Figure 3. UVꢀvis spectra of 2 in toluene under N2 as a
function of temperature.
On the basis of the above solution spectral data, the
structure of 3 is distorted tetrahedral, which was conꢀ
firmed by the disordered solidꢀstate structure of 2 (Figꢀ
ures 1 and 2).13 Additional support for the presence of
the tetrahedral paramagnetic dinitrogen complex 3 is
that the IR stretching frequency of the coordinated N2
unit of 2071 cmꢀ1 is indicative of very weak activation of
the dinitrogen moiety, which contrasts the corresponding
IR stretching frequency of the aforementioned
Co(SiPNP)(N2) at 2004 cmꢀ1; this lower stretching freꢀ
quency is consistent with a low spin Co(I) complex13a
and more back bonding from Co to the N2 unit. The twoꢀ
coordinate iron(0) complex, Fe(CAAC)2 (where CAAC
= bulky cyclic(amino)ꢀalkyl(carbene)), also coordinates
dinitrogen only at low temperatures (<ꢀ80 ˚C), which
facilitates isolation of the anionic dinitrogen complex,
[Fe(CAAC)2N2]–, upon reduction at low temperature
We have endeavored to understand this process by
performing a number of preliminary stoichiometric reacꢀ
tions. For example, addition of Me3SiCl to the Tꢀshaped
Co(I) complex Co(NpNPiPr) (2) partially regenerates14
the starting Co(II) species Co(NpNPiPr)Cl (1) as shown
at the top of Scheme 3. The fate of the Me3Si• radical is
unknown, but we suggest that it can react with the small
amount of dinitrogen complex 3 present at low temperaꢀ
ture to generate putative Co(NpNPiPr)(NNSiMe3) (4), a
likely intermediate6,7,13 in the formation of
tris(trimethylsilyl)amine. Unfortunately, our attempts to
detect this species or other intermediates have thus far
been unsuccessful.
One intriguing idea (Scheme 3) is that the equilibrium
between Tꢀshaped 2 and dinitrogen complex 3 may conꢀ
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Supporting Information. Supporting Information is availꢀ
Page 4 of 8
tribute to how this system catalytically turns over: coorꢀ
dinatively unsaturated 2 abstracts Cl• from Me3SiCl to
generate the Co(II) chloride 1, which is then reduced by
the excess KC8 back to 2; dinitrogen complex 3 traps the
generated Me3Si•. Consistent with this is that starting
Co(II) chloride complex 1 acts as an efficient catalyst
for this reaction under these conditions with comparable
turnover numbers for N(SiMe3)3 generation.14 Separate
attempts to generate another potential species in the proꢀ
cess, the anionic formally Co(0) dinitrogen complex
K[Co(NpNPiPr)N2], by subjecting 2 to excess KC8 in the
presence of N2 have been inconclusive at this point.
Such stoichiometric reactions are continuing.
1
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able free of charge on the ACS Publications Website at
DOI: 10.1021/acscatal.XXXXX.
Experimental Details, solution and solidꢀstate characterꢀ
ization of compounds, DFT studies, details of catalysis
experiments (PDF)
Xꢀray data (CIF)
AUTHOR INFORMATION
9
Corresponding Authors
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Email: fryzuk@chem.ubc.ca, masuda.hideki@nitech.ac.jp
Author Contributions
The manuscript was written through contributions of all
authors.
Scheme 3. Proposal for the formation of N(SiMe3)3;
the intermediate 4 has been confirmed computation-
ally.14
ACKNOWLEDGMENT
We gratefully acknowledge support for this work by a
GrantꢀinꢀAid from the Ministry of Education, Culture,
Sports, Science and Technology of Japan and by the “Straꢀ
tegic Young Researcher Overseas Visits Program for Accelꢀ
erating Brain Circulation” of the Japan Society for the
Promotion of Science. The NSERC of Canada is also
acknowledged for support in the form of a Discovery Grant
to M.D.F.
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Catalytic Nitrogenꢀtoꢀammonia Conversion by Osmium and Rutheniꢀ
um Complexes. J. Am. Chem. Soc. 2017, 139, 16105 – 16108; (o)
Arashiba, K.; Eizawa, A.; Tanaka, H.; Nakajima, K.; Yoshizawa, K.;
Nishibayashi, Y. Catalytic Nitrogen Fixation via Direct Cleavage of
Nitrogenꢀnitrogen Triple Bond of Molecular Dinitrogen under Ambiꢀ
ent Conditions. Bull. Chem. Soc. Jpn. 2017, 90, 1111 – 1118.
(5) (a) Shiina, K. Reductive Silylation of Molecular Nitrogen via
Fixation to Tris(trialkylsilyl)amine. J. Am. Chem. Soc. 1972, 94, 9266
– 9267; (b) Komori, K.; Oshita, H.; Mizobe, Y.; Hidai, M. Preparation
and Properties of Molybdenum and Tungsten Dinitrogen Complexes.
25. Catalytic Conversion of Molecular Nitrogen into Silylamines
Using Molybdenum and Tungsten Dinitrogen Complexes. J. Am.
Chem. Soc. 1989, 111, 1939 – 1940; (c) Mori, M. Activation of Niꢀ
trogen for Organic Synthesis. J. Organomet. Chem. 2004, 689, 4210 –
4227.
(6) (a) Tanaka, H.; Sasada, A.; Kouno, T.; Yuki, M.; Miyake, Y.;
Nakanishi, H.; Nishibayashi, Y.; Yoshizawa K. Molybdenumꢀ
catalyzed Transformation of Molecular Dinitrogen into Silylamine:
Experimental and DFT Study on the Remarkable Role of Ferroꢀ
cenyldiphosphine Ligands. J. Am. Chem. Soc. 2011, 133, 3498 –
3506; (b) Imayoshi, R.; Tanaka, H.; Matsuo, Y.; Yuki, M.; Nakajima,
K.; Yoshizawa, K.; Nishibayashi, Y. Cobaltꢀcatalyzed TransForꢀ
mation of Molecular Dinitrogen into Silylamine under Ambient Reacꢀ
tion Conditions. Chem. Eur. J. 2015, 21, 8905 – 8909; (c) Yuki, M.;
Miyake, Y.; Nishibayashi, Y.; Wakiji, I.; Hidai, M. Synthesis and
Reactivity of Tungstenꢀ and Molybdenumꢀdinitrogen Complexes
Bearing Ferrocenyldiphosphines toward Protonolysis. Organometal-
lics 2008, 27, 3947 – 3953.
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(8) (a) Prokopchuk, D. E.; Wiedner, E. S.; Walter, E. D.; Popescu, C.
V.; Piro, N. A.; Kassel, W. S.; Bullock, R. M.; Mock, M. T. Catalytic
N2 Reduction to Silylamines and Thermodynamics of N2 Binding at
Square Planar Fe. J. Am. Chem. Soc. 2017, 139, 9291 – 9301; (b)
Araake, R.; Sakadani, K.; Tada, M.; Sakai, Y.; Ohki, Y. [Fe4] and
[Fe6] Hydride Supported by Phosphines: Synthesis, Characterization,
and Application in N2 Reduction. J. Am. Chem. Soc. 2017, 139, 5596
– 5606; (c) Ung, G.; Peters, J. C. LowꢀTemperature N2 Binding to
Twoꢀcoordinate L2FeO Enables Reductive Trapping of L2FeN2ꢀ and
NH3 Generation. Angew. Chem. Int. Ed. 2015, 54, 532 – 535; (d)
Yuki, M.; Tanaka, H.; Sasaki, K.; Miyake, Y.; Yoshizawa, K.; Nishiꢀ
bayashi, Y. Iron Catalyzed Transformation of Molecular Dinitrogen
into Silylamine under Ambient Reaction Conditions. Nat. Commun.
2012, 3, 1254; (e) Imayoshi, R.; Nakajima, K.; Nishibayashi, Y. Vaꢀ
nadiumꢀcatalyzed Reduction of Molecular Dinitrogen into Silylamine
under Ambient Reaction Conditions. Chem. Lett., 2017, 46, 466 –
468; (f) Imayoshi, R.; Nakajima, K.; Takaya, J.; Iwasawa, N.; Nishiꢀ
bayashi, Y. Synthesis and Reactivity of Ironꢀ and Cobaltꢀdinitrogen
Complexes Bearing PSiPꢀtype Pincer Ligands toward Nitrogen Fixaꢀ
tion. Eur. J. Inorg. Chem., 2017, 32, 3769 – 3778; (g) Kendall, A. J.;
Johnson, S. I.; Bullock, R. M.; Mock, M. T. Catalytic Silylation of N2
and Synthesis of N2H4 by Net Hydrogen Atom Transfer Reaction
using a Chromium P4 Macrocycle. J. Am. Chem. Soc. 2018, 140, DOI:
10.1021/jacs.7b11132.
(9) (a) Suzuki, T.; WasadaꢀTsutsui, Y.; Ogawa, T.; Inomata, T.; Ozaꢀ
wa, T.; Sakai, Y.; Fryzuk M. D.; Masuda, H. N2 Activation by an Iron
Complex with a Strong Electronꢀdonating Iminophosphorane Ligand.
Inorg. Chem. 2015, 54, 9271 – 9281; (b) Hein, N. M.; Suzuki, T.;
Ogawa, T.; Fryzuk, M. D. Low Coordinate Iron Derivatives Stabiꢀ
lized by a βꢀDiketiminate Mimic. Synthesis and Coordination Chemꢀ
istry of Enamidophospinimine Scaffolds to Generate Diiron Dinitroꢀ
gen Complexes. Dalton Trans. 2016, 45, 14697 – 14708.
(10) (a) Ding, K.; Brennessel, W. W.; Holland, P. L. Threeꢀcoordinate
and Fourꢀcoordinate Cobalt Hydride Complexes That React with
Dinitrogen. J. Am. Chem. Soc. 2009, 131, 10804 – 10805 ; (b) Ding,
K.; Pierpont, A. W.; Brennessel, W. W.; LukatꢀRodgers, G.; Rodgers,
K. R.; Cundari, T. R.; Bill, E.; Holland, P. L. Cobaltꢀdinitrogen Comꢀ
plexes with Weakened NꢀN Bonds. J. Am. Chem. Soc. 2009, 131,
9471 – 9472.
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(11) Dugan, T. R.; Sun, X.; RybakꢀAkimova, E. V.; OlantunjiꢀOjo,
O.; Cundari, T. R.; Holland, P. L. A Masked Twoꢀcoordinate Coꢀ
balt(I) Complex That Activates CꢀF Bonds. J. Am. Chem. Soc. 2011,
133, 12418 – 12421.
(12) The disorder in 2 is manifested in the displacement of 10 % of
the Co atom out of the plane towards this dinitrogen moiety; see Supꢀ
porting Information.
(13) Co[SiPNP] = Co[N(SiMe2CH2PtBu2)2] reported in (a) Ingleson,
M.; Fan, H.; Pink, M.; Tomaszewski, J.; Caulton, K. G. Three Coorꢀ
dinate Co(I) Provides Access to Unsaturated DihydridoꢀCo(III) and
Sevenꢀcoordinate Co(IV). J. Am. Chem. Soc. 2006, 128, 1804 – 1805;
(b) Ingleson, M. J.; Pink, M.; Fan, H.; Caulton, K. G. Exploring the
Reactivity of Fourꢀcoordinate PNPCoX with Access to Threeꢀ
coordinate Spin Triplet PNPCo. Inorg. Chem. 2007, 46, 10321 –
10334; (c) Ingleson, M. J.; Pink, M.; Fan, H.; Caulton, K. G. Redox
Chemistry of the Triplet Complex (PNP)CoI. J. Am. Chem. Soc. 2008,
128, 4262 – 4276; for the related carbon backbone Co[PNP], where
PNP = N(CH2CH2PPri2)2, see (d) Rozenel, S. S.; Padilla, R.; Camp C.;
Arnold, J. Unusual Activation of H2 by Reduced Cobalt Complexes
Supported by a PNP Pincer Ligand. Chem. Commun., 2014, 50, 2612
– 2614.; (e) Rozenel, S. S. ; Padilla, R. M.; Arnold, J. Chemistry of
Reduced Monomeric and Dimeric Cobalt Complexes Supported by a
PNP Pincer Ligand. Inorg. Chem., 2013, 52, 11544 – 11550.
(14) See Supporting Information for analytical procedures, synthesis
details, optimization experiments, computational methods and results,
and detailed product analyses.
(7) (a) Siedschlag, R. B.; Bernales, V.; Vogiatzis, K. D.; Planas, N.;
Clouston, L. J.; Bill, E.; Gagliardi, L.; Lu, C. C. Catalytic Silylation
of Dinitrogen with a Dicobalt Complex. J. Am. Chem. Soc. 2015, 137,
4638ꢀ4641; (b) Cammarota, R. C.; Clouston, L. J.; Lu, C. C. Leveragꢀ
(15) There are a number of different protocols for determining the
yield of N(SiMe3)3 in this catalytic reaction; while some measure the
amount of N(SiMe3)3 directly by GCꢀMS,6,7 others hydrolyze with
excess HCl to generate NH4Cl and either analyze by the indophenol
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method or by H NMR spectroscopy using an internal standard.8a,b
We have used all of these methods but report the indophenol method
in the Supporting Information. In certain experiments we have found
the GCꢀMS method to give slightly lower turnovers of N(SiMe3)3.
(16) (a) AlꢀKaysi, R. O.; Goodman, J. L. Bondꢀcoupled Electron
Transfer Processes: Cleavage of SiꢀSi Bonds in Disilanes. J. Am.
Chem. Soc. 2005, 127, 1620 – 1621; (b) Kratish, Y.; Molev, G.;
Kostenko, A.; Sheberla, D.; Tumanskii, B.; Botoshansky, M.; Shiꢀ
mada, S.; BravoꢀZhivotoskii, D.; Apeloig, Y. Activation of Homolytic
SiꢀZn and SiꢀHg Bond Cleavage, Mediated by a Pt(0) Complex, via
Novel PtꢀZn and PtꢀHg Compounds. Angew. Chem. Int. Ed. 2015, 54,
11817 – 11821.
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2
~1500 equiv KC8
+
1
~2000 equiv Me3SiCl
2
3
4
5
6
– 40˚C
SiMe3
N
N
N
Me3Si
SiMe3
~200 equivalents
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N
N Co
N
P
P
+ N2
– N2
N
Co
N
N
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P
P
2
3
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2
3
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2
~1500 equiv KC8
+
~2000 equiv Me3SiCl
5
6
7
8
9
– 40˚C
SiMe3
N
SiMe3
~200 equivalents
10
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N
N
Me3Si
N
N
P
P
+ N2
– N2
N
Co
N
Co
N
N
P
P
2
3
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