Catalytic Silylation of Dinitrogen with a Dicobalt Complex

Article abstract of DOI:10.1021/jacs.5b01445

A dicobalt complex catalyzes N₂ silylation with Me₃SiCl and KC₈ under 1 atm N₂ at ambient temperature. Tris(trimethylsilyl)amine is formed with an initial turnover rate of 1 N(TMS)₃/min, ultimately reaching a turnover number of ～200. The dicobalt species features a metal-metal interaction, which we postulate is important to its function. Although N₂ functionalization occurs at a single cobalt site, the second cobalt center modifies the electronics at the active site. Density functional calculations reveal that the Co-Co interaction evolves during the catalytic cycle: weakening upon N₂ binding, breaking with silylation of the metal-bound N₂ and reforming with expulsion of [N₂(SiMe₃)₃]^-.

Full text of DOI:10.1021/jacs.5b01445

Communication
pubs.acs.org/JACS
Catalytic Silylation of Dinitrogen with a Dicobalt Complex
Randall B. Siedschlag,† Varinia Bernales,†,‡ Konstantinos D. Vogiatzis,†,‡ Nora Planas,†,‡
Laura J. Clouston,† Eckhard Bill,§ Laura Gagliardi,*,†,‡ and Connie C. Lu*,†
‡
†Department of Chemistry and Supercomputing Institute & Chemical Theory Center, University of Minnesota, Minneapolis,
Minnesota 55455-0431, United States
§Max Planck Institut fur Chemische Energiekonversion, Stiftstraße 34-36, 45470 Mulheim an der Ruhr, Germany
̈
̈
S
* Supporting Information
ABSTRACT: A dicobalt complex catalyzes N2 silylation
with Me3SiCl and KC8 under 1 atm N2 at ambient
temperature. Tris(trimethylsilyl)amine is formed with an
initial turnover rate of 1 N(TMS)3/min, ultimately
reaching a turnover number of ∼200. The dicobalt species
features a metal−metal interaction, which we postulate is
important to its function. Although N2 functionalization
occurs at a single cobalt site, the second cobalt center
modifies the electronics at the active site. Density
functional calculations reveal that the Co−Co interaction
evolves during the catalytic cycle: weakening upon N2
binding, breaking with silylation of the metal-bound N2
and reforming with expulsion of [N2(SiMe3)3]−.
he N−Si bond has long been recognized as a useful linkage
Tin organic synthesis for masking primary and secondary
amines. Catalytic schemes to construct such bonds directly from
N2 are underdeveloped and could revolutionize the manufactur-
ing of silylamines, which are increasingly important as industrial
chemicals. For example, trisilylamines are used to fabricate
silicon-nitride semiconductors in front-end electronic applica-
tions, and Si−N based polymers are incorporated into ceramic
materials to impart thermal resistance.1 Developing N2 silylation
catalysts2 also complements ongoing research in N2 fixation to
ammonia, as they share the challenge of functionalizing N2, a
molecule that is both thermodynamically stable and kinetically
inert, in an efficient and selective manner.3
Figure 1. (a) Reduction of Co2L (1) to generate K(crypt-222)-
[Co2(N2)L] (2). (b,c) Molecular structures of 1 and 2, respectively,
shown with 50% thermal ellipsoids. Hydrogen atoms (and counterion
for 2) were removed for clarity.
(C) following the first electron transfer (E) is most likely
dinitrogen binding.6 Indeed, reduction of 1 with 1 equiv KC8 in
the presence of N2 and 2.2.2-cryptand (abbreviated as crypt-222)
led to the isolation of K(crypt-222)[Co2(N2)L] (2) containing
an end-on N2 ligand (Figure 1). Infrared spectroscopy of 2
revealed an N−N stretching frequency of 1994 cm−1, which is
consistent with slight weakening of the N2 triple bond.7
The molecular structures of 1 and 2 are shown in Figure 1b,c,
respectively (SI Tables 1−2). The Co−Co bond distance
elongates substantially from 2.32 to 2.68 Å upon reduction of
Co23+ 1 to Co22+ 2. As the sum of the metallic radii of two cobalt
atoms is 2.314 Å, the former is consistent with a Co−Co single
bond, while the latter is a weak Co−Co interaction.8 The
magnetic susceptibilities of 1 and 2 were measured from 2 to 290
K and are consistent with S = 5/2 and S = 1 ground states,
respectively (SI Figures 9−11). The effective magnetic moment
for sextet 1 decreases slightly from 6.1 to 5.8 μB from 50 to 290 K.
To fit the data, a two-spin model is needed with ferromagnetic
coupling (J = 60 cm−1) between Co(II), S = 3/2, and Co(I), S =
1, centers. The triplet state of 2 is energetically isolated, with a g-
A known catalyst for N2 silylation is Mo(depf)2(N2)2 (depf
=1,1′-bis(diethylphosphino)ferrocene), which produces N-
(SiMe3)3 from N2 (1 atm), Me3SiCl, and Na(s), with a turnover
number (TON) of 150.2d Efforts to surpass the Mo catalyst with
a first-row transition metal have met with limited success.2b
A
survey of iron coordination complexes showed subdued TONs,
attaining a maximum of 34.2e,g We report a dicobalt catalyst that
achieves relatively high TON of N(SiMe3)3 at 299 K. The catalyst
features a hemilabile metal−metal interaction, which is an
unusual hallmark in catalytic N2 functionalization.3g,4
The precatalyst, Co2L (1) comprises a (Co2)3+ core within the
trisphosphino(triamido)amine ligand (L3−) framework (Figure
1, SI).5 Under N2, the cyclic voltammogram of 1 showed two
one-electron transfer processes at −2.11 and −2.54 V (vs Fc+/Fc,
SI Figure 6). The first reduction is irreversible with ΔEp of 0.4 V
and an ipc/ipa ratio of 0.64. The electrochemical behavior is
consistent with an ECE mechanism, where the chemical reaction
Received: February 13, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.5b01445
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Journal of the American Chemical Society
Communication
value of 2.14 and a zero-field splitting of 42 cm−1, and can
formally arise from antiferromagnetic coupling between S = 3/2
Co(II) and S = 1/2 Co(0) spins.
Since the anionic dicobalt N2 adduct is stable to vacuum, we
postulated that 2 could mediate the catalytic reduction of
dinitrogen by using 1 as the precatalyst. Complex 1 was tested in
catalytic N2 silylation using a large excess of Me3SiCl and
reductant, KC8, under an atmosphere of N2. Standard catalytic
conditions use a low catalyst loading (0.13 mM, 0.05 mol %) at
299 K for 12 h in THF. Under these conditions, 1 generated
N(SiMe3)3 in 30% yield, with a TON of 195 25 (Table 1).
reach ∼100 for the trialkylphosphines, PMe3 and i-Pr2PMe
(entries 6−7). While the ligand (LH3) does not generate any
amine under standard catalytic conditions, mixing the ligand with
two equivalents of CoCl2 is as effective as 1 (entry 8). Of note, a
1:1 ratio of the ligand to CoCl2 halves the TON relative to 1
(entry 9). Finally, the only cobalt precursor that did not generate
any detectable amine was cobalt nanopowder (entry 10).
Catalytic N2 functionalization can be highly sensitive to the
reductant.2c,d,3e−g Thus, we investigated catalysis by 1 with
various alkali metals: Li(s), Na(s), and K(s). Using alkali metals
significantly depressed the yield of N(SiMe3)3 (Table 2, entries
1−3). However, by prolonging the reaction time with K(s) from
12 to 95 h, the TON increases to 135 29 (entry 4).
Table 1. Reaction of N2 (1 atm), Me3SiCl (10.5 mmol), and
KC8 (10.4 mmol) Using Different Cobalt Precatalysts (5.27
μmol, 0.13 mM) in 40 mL THF at 299 K for 12 h
Table 2. Variation of Reductants or Additives with Complex 1
under Standard Catalytic Conditions (see Table 1)
a
a
entry
reductant
Li(s)
% yield
TON
1
3
1
4
19
7
6
5
3
a
b
2
3
Na(s)
K(s)
entry
precatalyst
% yield
TON
29
1
2
Co2L (1)
30
27
4
195 25
178 37
4
K
(s) (t = 95 h)
21
135 29
K(crypt-222)[Co2(N2)L] (2)
Co(N2)AlL
b
a
a
entry
additive
PMe3
% yield
TON
3
30
6
9
2
5
6
27
18
178
4
4
CoCl2
1
t-BuNC
120 13
5
Co(PPh3)3Cl
7
44 11
86
a
6
CoCl2 + 3 PMe3
CoCl2 + 3 PMe(i-Pr)2
2 CoCl2 + LH3
CoCl2 + LH3
13
14
25
16
0
6
Calculation of % yield and TON is the same as for data in Table 1.
Additives are exogenous ligands. KC8 is the reductant.
b
7
94 19
172 16
103 20
0
8
9
A difficult problem in catalysis is pinpointing the speciation of
c
10
Co nanopowder
the active species, whether it be homogeneous or heteroge-
neous.11 Though cobalt nanoparticles (NPs) were inactive for N2
silylation, the elusive nature of active species and the ambiguity
surrounding any single speciation tests prompted us to
investigate this problem. To this end, we have probed the
speciation of the catalyst through selective poisoning, a filtration
test, initial rate studies, and in operando studies.
Since catalytic NPs have a smaller fraction of active metal sites
relative to the bulk metal, they are readily poisoned by
substoichiometric, exogenous ligands (per metal). For late
metal NPs, phosphine ligands can be effective poisons.11 In
contrast, homogeneous metal catalysts require at least 1 equiv of
phosphine to inhibit activity. The addition of 1 equiv PMe3 per 1
effected no change in TON (Table 2, entry 5). The use of a π-
acceptor ligand, t-BuNC, however, did lower the TON to 120
13, but the preservation of significant activity argues against a
heterogeneous catalyst.
We also conducted a filtration test, whereby soluble and
insoluble fractions are separated by filtration and then
independently assayed for catalytic activity. In our variation of
this test, one catalytic cycle is completed prior to filtering through
graphite (SI Figure 13). The resultant filtrate is split into two
equal parts, so that one half serves to exactly quantify the amine
formed in the first cycle, TON 166.12 The resultant precipitate
and the other filtrate half are tested in a second catalytic cycle. If
the active species is insoluble, then the overall TON will be ∼166
for both the precipitate and the filtrate, as the latter carries the
amine generated in the first cycle. However, if the active species is
soluble, then the overall TON for the filtrate will double (for
precipitate, TON 0). The overall TON for the filtrate was 316
versus 30 for the precipitate. Hence, the active species is soluble.
Although insoluble aggregates are discredited as active species,
cobalt nanoclusters, by virtue of their smaller size, can be soluble
and more challenging to detect or to exclude. Formation of active
a
% yield (avg of 3 trials) is calculated for N(SiMe3) relative to
Me3SiCl. Amine was worked up and quantified by GC-MS (see SI).
TON (avg of 3 trials) = [N(SiMe3)3]/[precatalyst]. Co nanopowder
b
c
(carbon-coated, <50 nm particle size).
Based on the TON, 1 is one of the most active N2 silylation
catalysts, with comparable activity to Mo(depf)2(N2)2. Besides
the desired product, N(SiMe3)3, other side-products are typically
formed under these conditions, including Me3SiSiMe3, and
mono- and bis-silylated THF, i.e., Me3SiO(CH2)3CH2R, where
R = H or SiMe3. The latter two byproducts can be suppressed by
performing the catalysis in DME, but the TON for N(SiMe3)3 is
slightly lower at 140 9 (SI Table 5). Complex 2 was also
catalytically competent and gave an identical TON as 1 (entry 2).
To test catalyst robustness, we performed two consecutive
catalytic cycles using a 0.16 mM solution of 1. In each cycle,
Me3SiCl and KC8 (2000 equiv each) were added, and the
reaction stirred for 12 h. The overall TON was 320 18. If the
first cycle generates TON 195, then ∼65% of the activity was
retained in the second cycle (SI Table 5). We tested an
isostructural Co−Al compound, Co(N2)AlL, to gauge the effect
of the supporting metal.9 Under standard conditions, Co(N2)AlL
produced substantially less amine with a TON of 30 9 (entry
3).
The catalytic functionalization of N2 by cobalt complexes is
unconventional, though low-valent cobalt complexes are known
to bind N2.10 To investigate whether simple cobalt complexes
can perform N2 silylation, we screened some cobalt precursors
(Table 1, entries 4−7). Even CoCl2 is catalytic, although its
activity is limited to a few turnovers (entry 4).2b The Co(I)
complex, CoCl(PPh3)3, is modestly active, with a TON of 44
11 (entry 5). Increasing the Lewis basicity of the phosphine
ligands dramatically raises the yield of N(SiMe3)3, and the TONs
B
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Communication
nanoclusters may manifest in an induction period and/or
irreproducible kinetic data. Initial rate experiments were
conducted to determine the reaction order with respect to 1.
Amine formation was monitored by sampling the reaction
mixture at discrete time points (t = 2, 5, 10, 15, and 20 min). Per
sample, the amount of N(SiMe3)3 was determined by converting
N(SiMe3)3 into NH4Cl with acid and then quantifying the
ammonium by the indophenol method (SI).13 Rates of
N(SiMe3)3 production were measured at four different
concentrations of 1 (0.026, 0.13, 0.32, and 0.65 mM), and the
initial rates show a pseudo-first-order dependence on catalyst
concentration (Figure 2). The linear dependence of initial rates
(SI). The precatalyst, Co2L, and potential active species, [Co2L]−
(A) and [Co2(N2)L]− (B), were found to have spin ground-
states of S = 5/2, 2, and 1, respectively. The ground states of Co2L
and B match that of their experimental counterparts, 1 and 2. In
all three structures, the Co−Co interaction is weak: the only
delocalized molecular orbitals are (σ)2 and (σ*)1, and their
occupancies predict a bond order of 0.5. The precatalyst, Co2L,
contains an S = 3/2 CoN(II) and S = 1 CoP(I) (where CoN and
CoP denote the Co sites in the N3- and P3-plane, respectively), in
agreement with the magnetic data. In the “naked” anion, A, CoP is
reduced to S = 1/2 CoP(0), while the S = 3/2 CoN(II) center
remains unchanged. One effect of N2-binding (A → B) is to
change the nature of the magnetic coupling, leading to a lower
overall spin state.
A catalytic mechanism and its energy profile are presented in
Figure 3 (SI Figure 22). As proposed previously,2a,d the SiMe3
radical is the active silyl reagent under reducing conditions. The
overall mechanism begins with N2-binding (A → B), followed by
three sequential additions of •SiMe3 to the N2 ligand, and then
expulsion of [N2(SiMe3)3]− to regenerate Co2L. It has been
calculated that [N2(SiMe3)3]− converts spontaneously to two
N(SiMe3)3 via an uncatalyzed pathway.2d Likewise, the Co2L is
easily reduced to A with KC8, closing the catalytic cycle.
Alternative pathways, e.g. via a dicobalt nitride intermediate,
were ruled out due to high activation barriers (SI Figure 23).
The notable section of the catalytic cycle is the cobalt-
mediated reduction of N2 by a total of four electrons to [N2(Si
Me3)3]−. The three •SiMe3 equivalents add to the cobalt-bound
N2 in a distal−distal−proximal sequence, which is similar to the
DFT-calculated mechanism for Mo(depf)2(N2)2.2d The first
•SiMe3 reacts with B to form the silyldiazenido(1-) intermediate
C, with a barrier of 8.6 kcal/mol. The second addition of •SiMe3
generates the disilylhydrazido(2-) species D, with ΔG⧧ = 16.2
kcal/mol, making this the rate-determining step. D then
undergoes an exergonic dissociation of one phosphine donor,
presumably to lessen the steric repulsion between the phosphine
substituents and the disilylhydrazido ligand to give intermediate
D*, where the asterisk denotes one dangling phosphine arm. The
third •SiMe3 preferentially attacks the proximal N atom, to form
trisilylhydrazido E* with a low ΔG⧧ of 4.5 kcal/mol. Finally,
phosphine association expels [N2(SiMe3)3]− and regenerates
Co2L. No transition state could be found for this step (E* →
Co2L), so we presume that this step is essentially barrierless.2d
Notably, the Co···Co distance changes dramatically in this
elementary step, from 3.547 Å in E* to 2.522 Å in Co2L, whereby
Figure 2. Plot of initial rates (Δ[N(SiMe3)3]/Δt, mM min−1) versus [1]
(0.026−0.65 mM) in reactions with N2 (1 atm), Me3SiCl (21.3 mmol),
and KC8 (20.7 mmol).
on 1, as well as the absence of a lag period, is consistent with a
well-behaved homogeneous catalyst. We can approximate a
turnover frequency of one N(SiMe3)3 molecule per minute.14
In operando studies were conducted to assess whether the
active species is truly bimetallic. Complex 1 was mixed with a
small excess of Me3SiCl and KC8 (10 equiv) under N2 in THF-d8
for 90 min. 1H NMR analysis showed the presence of N(SiMe3)3
and 3 paramagnetic species, one of which is complex 2 (SI Figure
18a,b). If the reaction is quenched with a drop of CD2Cl2, then
only one paramagnetic species is observed, namely Co2(Cl)L.
These observations are consistent with bimetallic active species.
Theoretical calculations were performed to gain insight into
the catalytic mechanism and the electronic structures of 1 and 2
Figure 3. (a) DFT-calculated mechanism for the dicobalt-mediated silylation of N2. Intermediates with a dangling phosphine are labeled with an
asterisk, e.g., D*. (b) Energy profile (kcal/mol) for the proposed mechanism with activation energies in red. Inset shows TSC→D, the transition-state
structure between C and D with interatomic distances in Å.
C
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Journal of the American Chemical Society
Communication
formation of the Co−Co bond may assist in releasing the
[N2(SiMe3)3]− product.
ACKNOWLEDGMENTS
■
The authors thank Dr. Maria Miranda and Prof. Bill Tolman for
GC-MS access, Andreas Goebels for magnetic data, and Yuxuan
Chen for DFT help. This work was supported as part of the
Inorganometallic Catalyst Design Center, an EFRC funded by
the DOE, Office of Basic Energy Sciences (DE-SC0012702).
C.C.L. is a Sloan Fellow. R.B.S. was supported by an NSF
graduate fellowship. XRD experiments were performed using a
crystal diffractometer (NSF-MRI, CHE-1229400) under the
direction of Dr. Vic Young, Jr.
Considering the bimetallic nature of the catalyst, it is intriguing
to examine the role of the second cobalt, CoN. The supporting
metal clearly affects the overall catalysis, as substituting Co with
Al yields an isostructural Co−Al bimetallic that is less active by
6.5 times. The Co oxidation states and the nature of the Co−M
bonds are quite different between these two systems. For the
Co−Al system, the (CoM)3+ and (CoM)2+ species are consistent
with Co(0)Al(III) and Co(−I)Al(III),5a whereas in dicobalt,
they are better described as Co(I)Co(II) and Co(0)Co(II). In
[Co(N2)AlL]−, a strong inverse dative bond is present, Co(−I)
→ Al(III), whereas in [Co2(N2)L]−, the cobalt centers are
weakly interacting. The Lewis acidic Al(III) metalloligand
suppresses the catalytic activity at Co, and this is consistent
with the electronic trend that increasingly basic phosphine
ligands increase TON (Table 1, entries 4−6). Hence, the
supporting metal can effectively tune the electron density at the
active cobalt center, where Al(III) and Co(II) represent two
electronic extremes of a metalloligand.
Though CoN is potentially redox-active, our calculations show
that this is not the case here (SI Figure 24). In the mechanism,
CoP cycles between Co(0) and Co(II) in discrete one-electron
steps, while CoN maintains a constant oxidation state of +2 and S
= 3/2. Although the reducing equivalents are stored only at the
active cobalt, CoN plays a significant role in stabilizing various
CoP(N2(SiMe3)x) (x = 0 to 2) intermediates, similar to the
postulated mechanism of Rh2-catalyzed diazo-transfer reac-
tions.15 Indeed, the Co−Co interaction increasingly weakens
as N2 binds and is functionalized to disilylhydrazido D, wherein
the metal−metal bond is fully cleaved (SI Table 11). In the final
step, the release of trisilylhydrazide is concomitant with Co−Co
and Co−P bond formations.
Our study demonstrates that cobalt compounds can be
effective catalysts for N2 functionalization to N(SiMe3)3. The
dicobalt system gives high TON while operating at 299 K and at
low-catalyst loading. Additional experiments support the
homogeneity of the active species. The catalysis is nearly 7-fold
faster using KC8 as a reductant, relative to K metal, likely because
of the larger surface area in the former. We further demonstrate
an interesting bimetallic strategy to electronically tune the active
center through variation of the supporting metal atom. The
Co(II) metalloligand was critical to achieve high TONs,
suggesting that the traditional mode of tuning activity through
the ancilliary ligands (phosphines) may have a more limited
effect than changing an ancillary metal (Al for Co). In future
research, we will explore this idea by developing an isostructural
family of Co-M bimetallics for catalytic N2 silylation, where the
ancilliary metal is systematically varied.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental/theoretical methods and data are provided
(CCDC 962873−962875). This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Authors
*gagliard@umn.edu
*clu@umn.edu
Notes
The authors declare no competing financial interest.
D
DOI: 10.1021/jacs.5b01445
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