Synthesis and reactivity of a nickel(ii) thioperoxide complex: Demonstration of sulfide-mediated N2O reduction

Article abstract of DOI:10.1039/c8sc02536c

The thiohyponitrite ([SNNO]^2-) complex, [K(18-crown-6)][L^tBuNi^II(κ²-SNNO)] (L^tBu = {(2,6-ⁱPr₂C₆H₃)NC(^tBu)}₂CH), extrudes N₂ under mild heating to yield [K(18-crown-6)][L^tBuNi^II(η²-SO)] (1), along with minor products [K(18-crown-6)][L^tBuNi^II(η²-OSSO)] (2) and [K(18-crown-6)][L^tBuNi^II(η²-S₂)] (3). Subsequent reaction of 1 with carbon monoxide (CO) results in the formation of [K(18-crown-6)][L^tBuNi^II(η²-SCO)] (4), [K(18-crown-6)][L^tBuNi^II(S,O:κ²-SCO₂)] (5), [K(18-crown-6)][L^tBuNi^II(κ²-CO₃)] (6), carbonyl sulfide (COS) (7), and [K(18-crown-6)][L^tBuNi^II(S₂CO)] (8). To rationalize the formation of these products we propose that 1 first reacts with CO to form [K(18-crown-6)][L^tBuNi^II(S)] (I) and CO₂, via O-atom abstraction. Subsequently, complex I reacts with CO or CO₂ to form 4 and 5, respectively. Similarly, the formation of complex 6 and COS can be rationalized by the reaction of 1 with CO₂ to form a putative Ni(ii) monothiopercarbonate, [K(18-crown-6)][L^tBuNi^II(κ²-SOCO₂)] (11). The Ni(ii) monothiopercarbonate subsequently transfers a S-atom to CO to form COS and [K(18-crown-6)][L^tBuNi^II(κ²-CO₃)] (6). Finally, the formation of 8 can be rationalized by the reaction of COS with I. Critically, the observation of complexes 4 and 5 in the reaction mixture reveals the stepwise conversion of [K(18-crown-6)][L^tBuNi^II(κ²-SNNO)] to 1 and then I, which represents the formal reduction of N₂O by CO.

Full text of DOI:10.1039/c8sc02536c

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Synthesis and reactivity of a nickel(II) thioperoxide complex:
Demonstration of sulfide-mediated N2O reduction
Received 00th January 20xx,
Accepted 00th January 20xx
Nathaniel J. Hartmann, Guang Wu, and Trevor W. Hayton*
The thiohyponitrite ([SNNO]2-) complex, [K(18-crown-6)][LtBuNiII(κ2-SNNO)] (LtBu = {(2,6-iPr2C6H3)NC(tBu)}2CH), extrudes N2
under mild heating to yield [K(18-crown-6)][LtBuNiII(η2-SO)] (1), along with minor products [K(18-crown-6)][LtBuNiII(η2-
OSSO)] (2) and [K(18-crown-6)][LtBuNiII(η2-S2)] (3). Subsequent reaction of 1 with carbon monoxide (CO) results in the
formation of [K(18-crown-6)][LtBuNiII(η2-SCO)] (4), [K(18-crown-6)][LtBuNiII(S,O:κ2-SCO2)] (5), [K(18-crown-6)][LtBuNiII(κ2-CO3)]
(6), carbonyl sulfide (COS) (7), and [K(18-crown-6)][LtBuNiII(S2CO)] (8). To rationalize the formation of these products we
propose that 1 reacts with CO to form [K(18-crown-6)][LtBuNiII(S)] (I) and CO2, via O-atom abstraction. Subsequently,
complex I reacts with CO or CO2 to form 4 and 5, respectively. Similarly, the formation of complex 6 and COS can be
rationalized by the reaction of 1 with CO2 to form a putative Ni(II) monothiopercarbonate, [K(18-crown-6)][LtBuNiII(κ2-
SOCO2)] (11). The Ni(II) monothiopercarbonate subsequently transfers a S-atom to CO to form COS and [K(18-crown-
6)][LtBuNiII(κ2-CO3)] (6). Finally, the formation of 8 can be rationalized by the reaction of COS with I. Critically, the
observation of complexes 4 and 5 in the reaction mixture reveal the stepwise conversion of [K(18-crown-6)][LtBuNiII(κ2-
SNNO)] to 1 and then I, which represents the formal reduction of N2O by CO.
DOI: 10.1039/x0xx00000x
www.rsc.org/
reaction, which results in irreversible formation of
Cp*Mo(NCN)(NCO)(NO), was found to be competitive with oxo
Introduction
formation.
Similarly, Limberg and co-workers reported the
Nitrous oxide (N2O) features a long atmospheric lifetime and
large global warming potential (ca. 300 times larger than CO2),
making it an important greenhouse gas.1-4 Anthropogenic sources of
N2O include agriculture, fossil fuel combustion, adipic acid synthesis,
and nitric acid production.1,5 The latter two sources use on-site N2O
mitigation to remove N2O from the effluent stream, either by
decomposition to the elements6 or reduction to N2 and H2O, but
neither of these methods is completely effective and some N2O is
still released into the atmosphere.7
stoichiometric oxidation of a Ni(0) CO complex, [K]2[LtBuNi0(CO)]2,
with N2O to form a carbonate complex, [K]6[LtBuNiII(CO3)]6, and N2.11
Subsequent release of carbonate from the metal center was not
discussed. The homogeneous hydrogenation of N2O has also been
explored.12,13 For example, in 2015 Piers and co-workers reported
an Ir(III) pincer carbene complex that could hydrogenate N2O;14
however, this system was not reported to be catalytic. More
recently, Milstein and co-workers reported that the Ru pincer
complex, (PNP)RuH(CO)(OH) (PNP = 2,6-[CH2PiPr2]2(C5H3N)), was an
Given the above considerations, the development of new
catalysts for N2O reduction could help reduce its impact on global
temperatures.1,8 Not surprisingly, a large number of heterogeneous
systems have been developed to catalyze this reaction.9 Of most
relevance to the current study are the catalyst systems used for
automotive applications, which consist of nanoparticulate Pt and Rh
on a ceramic support. This process uses partially oxidized fuel (i.e.,
CO) to reduce N2O, forming N2 and CO2.9 Sita and co-workers
developed a homogeneous version of this transformation, mediated
effective catalyst for the hydrogenation of N2O, achieving
turnover number of ca. 400.15
a
Recently, we reported the activation of N2O by the "masked"
terminal nickel sulfide complex, [K(18-crown-6)][LtBuNiII(S)] (I) (LtBu
=
{(2,6-iPr2C6H3)NC(tBu)}2CH), which yielded an unprecedented
thiohyponitrite complex, [K(18-crown-6)][LtBuNiII(κ2-SNNO)] (II) (eq
1).16 Given the challenge of activating N2O,17 and the novelty of the
[SNNO]2- ligand in II, we endeavored to explore its reactivity in
greater detail. Herein, we describe the first reactivity study of the
[SNNO]2- ligand in an effort to uncover new routes to N2O reduction.
i
by the Mo(II) complex, Cp*Mo(NCN)(CO)2 (NCN = PrNC(Me)NiPr).10
In this process, N2O oxidizes Cp*Mo(NCN)(CO)2 to form a Mo(IV)
oxo, Cp*Mo(NCN)(O), which then reacts with CO to form CO2 and
regenerate Cp*Mo(NCN)(CO)2. However, an N-N bond cleavage
tBu
Dipp
NiII
tBu
Dipp
NiII
O
K
O
O
N
N
S
N
N
O
N2O
O
(1)
O
N
S
C6H6
K
N
O
O
Department of Chemistry and Biochemistry, University of California, Santa Barbara,
California, 93106 United States
E-mail: hayton@chem.ucsb.edu
Electronic Supplementary Information (ESI) available: [Experimental and
crystallographic details and spectral data]. See DOI: 10.1039/x0xx00000x
O
O
O
tBu
Dipp
I
tBu
Dipp
O
II
O
Dipp = 2,6-iPr2C6H3
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RESULTS AND DISCUSSION
Synthesis of an [η2-SO]2- Complex. Gentle heating of a toluene-d8
solution of [K(18-crown-6)][LtBuNiII(κ2-SNNO)] (II) at 45 °C results in
1
the complete dissapearance of II over the course of 6 d. A H NMR
spectrum of this reaction mixture reveals the presence of a new γ-
CH resonance at 5.43 ppm (Figures S2-3), which we have assigned
to the thioperoxide complex, [K(18-crown-6)][LtBuNiII(η2-SO)] (1). A
preliminary kinetic analysis suggests that the formation of 1 is first-
order with respect to complex II, indicating that this transformation
is unimolecular (Figure S25). Also present in these spectra are two
minor γ-CH resonances. The first, observed at 5.53 ppm, has been
tentatively assigned to the disulfur dioxide complex, [K(18-crown-
6)][LtBuNiII(η2-OSSO)] (2), and the second resonance at 5.47 ppm,
has been assigned to the disulfide complex, [K(18-crown-
6)][LtBuNiII(η2-S2)] (3). Work-up of the reaction mixture affords
[K(18-crown-6)][LtBuNiII(η2-SO)] (1) as an orange crystalline solid in
82% yield (eq 2). The solid state molecular structure of 1 is shown in
Figure 1. Complex 1 features a rare example of an η2-thioperoxide Figure 1. ORTEP drawing of [K(18-crown-6)][LtBuNiII(η2-SO)]·C7H8
([η2-SO]2-) ligand, and is formed via N2 extrusion from the (1·C7H8) shown with 50% thermal ellipsoids. Hydrogen atoms, a
thiohyponitrite fragment. The [η2-SO]2- ligand in 1 is disordered C7H8 solvate molecule, and one orientation of the disordered [η2-
over two positions in a 97:3 ratio, which are related by a C2 rotation SO]2- ligand have been omitted for clarity. Selected metrical
about the Ni-K axis. It possesses an S-O bond length of 1.656(3) Å, parameters: S1-O1 1.656(3) Å, Ni1-S1 2.127(1) Å, Ni1-O1 1.954(3) Å,
consistent with an S-O single bond.18 For comparison, the S-O Ni1-N1 1.881(4) Å, Ni1-N2 1.900(4) Å, S1-K1 3.162(2) Å, O1-K1
distance in free S=O is substantially shorter (1.48108(8) Å), due to 2.881(3) Å, N1-Ni1-N2 99.2(2)°, N1-Ni1-S1 110.0(1)°, N2-Ni1-O1
its higher bond order.19 The Ni-S (2.127(1) Å) and Ni-O (1.954(3) Å) 103.2(1)°, S1-Ni1-O1 47.65(9)°.
distances in 1 are both consistent with single bonds and are
comparable with those found in the starting material (II), while the
Ni-N bond lengths (1.881(4) and 1.900(4) Å) are similar to those
observed in other square planar LRNiII complexes.16,20
As mentioned above, we also observe formation of [K(18-crown-
6)][LtBuNiII(η2-OSSO)] (2), as a minor side product, during the
conversion of [K(18-crown-6)][LtBuNiII(κ2-SNNO)] to 1. Despite its
presence in trace amounts, we have been able to obtain a few
single crystals of 2 as orange plates from the reaction mixture. The
solid state molecular structure of 2 is shown in Figure 2. It features
the first example of a co-planar [OSSO]2- ligand (OSSO dihedral
angle = 2°). The [η2-OSSO]2- ligand in 2 is bound to the Ni center in
an η2 fashion, via both sulfur atoms, while the O atoms are bound
to the [K(18-crown-6)]+ cation in a κ2 fashion. Its S-S distance is
2.093(3) Å, while the S-O distances are 1.485(5) and 1.496(7) Å. For
comparison, the S-S (2.0245(6) Å) and S-O (1.458(2) Å) distances in
free S2O2 are shorter than those observed for 2,31-33 consistent with
the reduced S-S and S-O bond orders anticipated for the [OSSO]2-
fragment in the former.31,34,35 Notably, complex 2 is only the third
OSSO complex to be reported and only second to be structurally
characterized.29,36-38
The 1H and 13C{1H} NMR spectra of 1 are consistent with its
formulation as a Cs symmetric, diamagnetic, square planar NiII
1
complex. The H NMR spectrum of 1 in C6D6 features two tert-butyl
resonances at 1.32 and 1.37 ppm and a single γ-CH resonance at
5.54 ppm. The IR spectrum (KBr pellet) of 1 reveals a strong νSO
mode at 902 cm-1, which is consistent with values reported for
other [η2-SO]2- ligands (883, 873 cm-1).21,22 Only a handful of
thioperoxide complexes are known,23-26 including [(triphos)Rh(µ-
η2,η1-SO)2Rh(triphos)][BPh4]2 (triphos = CH3C(CH2PPh2)3), [{RhCl(μ-
η2,η1-SO)(PPh3)2}2], and Fe3(ꢀ3-SO)(S)(CO)9.21,27,28 The iron example
is notable because it can be prepared by O-atom transfer to
Fe3(S)2(CO)9,29 a manner of preparation that is similar to that of 1.
Interestingly, Mankad and co-workers suggest that a transient SO
To account for the presence of 2 in the reaction mixture, we
hypothesize that complex 1 undergoes a formal disproportionation,
complex is formed upon reaction of [(IPr*)Cu]2(ꢀ-S) with N2O,30
transformation that parallels our conversion of I to II and then 1.
a
forming
2 and an equivalent of unobserved “[K(18-crown-
6)][LtBuNi0]”. However, because of the low yield (typically less than
3% relative to complex 1, as assessed by 1H NMR spectroscopy), this
transformation must be very inefficient. The low yield has also
impeded our ability to fully characterize this complex.
2 | J. Name., 2012, 00, 1-3
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tBu
Dipp
NiII
tBu
Dipp
tBu
Dipp
NiII
O
tBu
Dipp
O
K
O
K
O
O
O
O
N
N
S
O
O
N
N
N
N
O
O
N
N
O
O
O
O
O
S
S
toluene
45 o
S
S
S
O
II
N
C
II
K
O
Ni
(2)
Ni
+
+
6 d
N
O
O
K
O
O
- N2
O
3
tBu
Dipp
tBu
1
O
tBu
O
O
O
tBu
Dipp
Dipp
2
Dipp
II
O
(trace)
(trace)
present in the in situ 1H NMR spectrum of the thermolysis of II
(Figure S3), confirming the formation of 3 during that reaction, via
an as-yet-unknown mechanism.
tBu
Dipp
tBu
Dipp
NiII
O
O
K
O
O
O
N
N
N
N
O
O
S
S
O
0.125 S8
toluene
(3)
II
K
Ni
S
O
O
3
I
tBu
O
tBu
Dipp
O
Dipp
Figure 2. ORTEP drawing of [K(18-crown-6)][LtBuNiII(η2-
OSSO)]·2C6H14 (2·2C6H14 shown with 50% thermal ellipsoids.
)
Hydrogen atoms and C6H14 solvate molecules have been omitted for
clarity. Selected metrical parameters: S1-S2 2.093(3) Å, S1-O1
1.485(5) Å, S2-O2 1.496(7) Å, Ni1-S1 2.181(2) Å, Ni1-S2 2.173(2) Å,
Ni1-N1 1.920(4) Å, Ni1-N2 1.925(4) Å, O1-K1 2.747(4) Å, O2-K1
2.777(6) Å, N1-Ni1-N2 97.3(2)°, N1-Ni1-S1 102.1(1)°, N2-Ni1-S2
102.9(1)°, O1-S1-S2 107.4(2)°, O2-S2-S1 107.4(2)°.
Synthesis of an [η2-S2]2- Complex. To further support the
formation of the disulfide ([η2-S2]2-) complex, [K(18-crown-
6)][LtBuNiII(η2-S2)] (3), during the synthesis of 1, we endeavored to
independently synthesize 3. We, and others, have previously
shown that terminal metal sulfides can react with S8 to form metal
disulfides.39-41 Thus, we explored the reaction of [K(18-crown-
6)][LtBuNiII(S)] (I) with elemental sulfur. Addition of 0.125 equiv of S8
to a toluene solution of [K(18-crown-6)][LtBuNiII(S)] results in a rapid
color change from brown to orange. Work-up of the reaction
mixture affords [K(18-crown-6)][LtBuNiII(η2-S2)] (3), as an orange
crystalline solid in 81% yield (eq 3). The solid state molecular
structure of 3 is shown in Figure 3. The disulfide (S22-) ligand in 3 has
a S-S distance of 2.050(2) Å, consistent with a single bond.18 This
distance is comparable to those reported for other NiII(η2-S2)
complexes.42-49 The Ni-S distances (2.202(2) and 2.199(2) Å) in 3 are
consistent with single bonds, and are much longer than the Ni-S
bond length in the starting material (I, 2.064(2) Å). Finally, the Ni-N
bonds in 3 are similar to those found in other square planar NiII β-
diketiminate complexes.16,49,50
Figure 3. ORTEP drawing of [K(18-crown-6)][LtBuNiII(η2-S2)]·2C7H8
(3·2C7H8) shown with 50% thermal ellipsoids. Hydrogen atoms and
C7H8 solvate molecules have been omitted for clarity. Selected
metrical parameters: S1-S2 2.050(2) Å, Ni1-S1 2.202(2) Å, Ni1-S2
2.199(2) Å, Ni1-N1 1.900(4) Å, Ni1-N2 1.906(4) Å, S1-K1 3.248(2) Å,
S2-K1 3.249(2) Å, N1-Ni1-N2 98.0(2)°, N1-Ni1-S2 103.1(1)°, N2-Ni1-
S1 103.4(1)°.
Reactivity of the [η2-SO]2- Ligand. While the reactivity of the SO
ligand has not been well established, it is known to react with
phosphines. For example, Schmid and co-workers reported that
[(diphos)2Ir(η2-OSSO)][Cl] reacted with PPh3 to form Ph3PO, Ph3PS,
and [(diphos)2IrCl].36
Similarly, Rauchfuss and co-workers
demonstrated that Cp2Nb(S2O)Cl reacted with Ph3P to form
Cp2Nb(O)Cl and two equiv of Ph3PS.29 Both transformations were
presumed to proceed through an unobserved SO intermediate.
More recently, Mizobe et al. reported that PPh3 could abstract an
O-atom from the thioperoxide ligand in [(Cp’RuCl)2(SbCl2)(ꢀ-
Cl)(ꢀ3:κ2-SO)] (Cp’ = C5Me4Et).25 In contrast, the reactivity of the SO
ligand with CO has not been studied. Accordingly, we explored the
reactivity of [K(18-crown-6)][LtBuNiII(η2-SO)] (1) with this substrate.
Thus, exposure of a C6D6 solution of complex 1 to an atmosphere of
The 1H NMR spectrum of 3 in toluene-d8 (Figure S7) is consistent
with a C2v symmetric, diamagnetic, square planar NiII complex and
features one tert-butyl resonance at 1.30 ppm and a single γ-CH
resonance at 5.46 ppm. Importantly, this latter resonance is also
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13CO results in complete consumption of 1 after 6 h. A 13C{1H} NMR respectively. For comparison, these resonances appear at -115.21, -
spectrum (Figure S11) of the reaction mixture reveals the formation 0.88, and 6.56 ppm, respectively, for authentic I.16 This
of several 13C-enriched products, indicating the incorporation of intermediate is quickly formed upon addition of 13CO, but its signals
13CO. Specifically, this spectrum features resonances at 214.7, immediately begin to decay, and they are completely absent after 6
177.3, 165.3, and 152.9 ppm, which are assignable to [K(18-crown- h (Figure S9).
6)][LtBuNiII(η2-SCO)] (4),51 [K(18-crown-6)][LtBuNiII(S,O:κ2-SCO2)] (5),
We also characterized the products of the reaction of 1 and CO
[K(18-crown-6)][LtBuNi(κ2-CO3)] (6), and SCO (7),52 respectively by IR spectroscopy. An IR spectrum of the reaction residue,
(Scheme 1). This spectrum also features a minor 13C-enriched dissolved in hexanes, reveals the presence of νCO modes at 2021,
resonance at 206.9 ppm, which we have tentatively assigned to 1666, and 1620 cm-1 (Figure S24), which are assignable to the νCO
[K(18-crown-6)][LtBuNiII(S2CO)] (8), on the basis of the similarity of modes of [LtBuNiI(CO)] (9),55 [K(18-crown-6)[LtBuNiII(η2-SCO)] (4),51
its dithiocarbonate ([S2CO]2-) chemical shift with those reported for and [K(18-crown-6)][LtBuNiII(κ2-CO3)] (6), respectively. Curiously,
other dithiocarbonate complexes.52-54
though, we do not observe any signals in the H NMR spectrum of
1
A
1H NMR spectrum of the reaction mixture further supports the reaction mixture that could be assigned to paramagnetic 9,
these assignments. Specifically, an examination of the γ-CH region suggesting that it is only a minor product of the reaction.
of this spectrum reveals overlapping resonances at 5.48 ppm
The 13C NMR spectrum of the in situ reaction mixture also
(Figure S10), which are assignable to [K(18-crown-6)][LtBuNiII(η2- features a minor 13C-enriched resonance at 178.5, as well as a major
SCO)] (4)51 and [K(18-crown-6)][LtBuNiII(S,O:κ2-SCO2)] (5), and a resonance at 191.4 ppm (Figure S11). While these two resonances
resonance at 5.42 ppm, assignable to [K(18-crown-6)][LtBuNi(κ2- remain unassigned, we know that neither of the peaks is assignable
CO3)] (6). This spectrum also contains a minor resonance at 5.57 to [K(18-crown-6)][LtBuNiII(η2-CO2)] (10), as we have performed the
ppm that has been tentatively assigned to [K(18-crown- independent synthesis of this complex for spectroscopic
6)][LtBuNiII(S2CO)] (8). Interestingly, at short reaction times, we comparison (see below). We also do not observe a resonance that
observe the presence of a paramagnetic intermediate in the could be assignable to free CO2.
reaction mixture (Figure S9). We have identified this intermediate
Finally, we observe no reaction between [K(18-crown-
as the NiII sulfide, [K(18-crown-6)][LtBuNiII(S)] (I), on the basis of the 6)][LtBuNiII(η2-SO)] (1) and PPh3 in C6D6, according to 1H and 31P NMR
1
similarity of its H NMR resonances with those of the previously spectroscopies. The lack of reactivity of the [SO]2- ligand in 1 with
characterized material.16 For example, this intermediate features PPh3 is somewhat surprising on the basis of thermodynamic
diagnostic resonances at -130.25, -0.63, and 5.87 ppm, which are considerations,56 and could reflect steric shielding of the [SO]2-
t
assignable to the γ-proton of the LtBu ligand, its Bu substituents, ligand by the bulky Dipp substituents
.
i
and one environment of its diastereotopic Pr methyl groups,
Scheme 1
Synthesis of an [S,O:κ2-SCO2]2- Complex. To further support the disordered over two positions, which are related by a C2 rotation
presence of [K(18-crown-6)][LtBuNiII(S,O:κ2-SCO2)] (5) in the reaction about the Ni-K vector, in a 87:13 ratio. The S-C (1.756(4) Å) and O-C
of 1 with CO, we pursued its synthesis via an independent route. (1.279(5) and 1.238(4) Å) bond lengths in 5 are consistent with
Thus, exposure of a C6D6 solution of [K(18-crown-6)][LtBuNiII(S)] (I) to those observed for previously reported [SCO2]2- complexes,57,58
excess carbon dioxide (CO2) results in a rapid color change from while the Ni-S and Ni-O distances in 5 are 2.234(1) Å and 1.922(3) Å,
1
deep brown to gold. The H NMR spectrum of the reaction mixture respectively. Moreover, the K-S and K-O distances are 3.531(1) Å
taken 15 min after addition of CO2 reveals full consumption of the and 2.715(3) Å, respectively, which are comparable to other K-S and
starting material and formation of a new diamagnetic product K-O dative interactions.59,60 Finally, the Ni-N distances in 5 are
whose spectroscopic signature is consistent with a square planar NiII comparable to those found in the starting material.16 To the best of
complex.51 Work-up of the reaction mixture provides 5 as a pale our knowledge, complex 5 is the first structurally characterized
brown crystalline solid in 57% yield (eq 4). The solid state molecular transition metal complex containing the [SCO2]2- ligand. Other
structure of 5 is shown in Figure 4. The thiocarbonate ([S,O:κ2- structurally characterized thiocarbonate complexes include
SCO2]2-) ligand in 5 features a ꢀ:κ2,κ2 binding mode and is [{((AdArO)3N)U}2(ꢀ-η1,(O):κ2(O',S)SCO2)], prepared by reaction of
4 | J. Name., 2012, 00, 1-3
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[{((AdArO)3N)U}2(μ-S)] with CO2, and [Cp*2Sm(ꢀ-η1:κ2-SCO2)SmCp*2], ꢀ:κ2,η1 binding mode, identical to that observed for the
prepared via reaction of [(Cp*2Sm)2(ꢀ-O)] with COS.57,58
trithiocarbonate (CS32-) ligand in [K(18-crown-6)][LtBuNi(κ2-CS3)].50
The O1-C1 (1.306(7) Å), O2-C1 (1.309(7) Å), and O3-C1 (1.242(7) Å)
bond lengths in 6 are consistent with those reported for
[K]6[LtBuNiII(κ2-CO3)]6,11 while the Ni-O1 and Ni-O2 distances in 6 are
1.882(4) and 1.901(4) Å, respectively, which are similar to those
reported for the starting material.
O
tBu
Dipp
NiII
t
Bu
Dipp
O
O
K
O
O
N
N
N
N
S
O
K
CO2
(4)
II
O
Ni
S
C
C6D6
O
O
O
O
O
O
5
I
tBu
O
t
O
Dipp
Bu
Dipp
O
t
tBu
Dipp
NiII
O
Bu
Dipp
NiII
O
K
The 13C{1H} NMR spectrum of 5 in benzene-d6 features a
resonance at 177.3 ppm, which we have assigned to the [S,O:κ2-
SCO2]2- moiety (Figure S13). This chemical shift is identical to the
resonance assigned to this complex in the in situ 13C NMR spectrum
O
N
N
N
N
O
O
O
O
K
6 18-c-6
toluene
O
(5)
6
O
C
O
C
O
O
6
t
K
tBu
Dipp
Bu
Dipp
O
1
of the reaction of 1 with CO (Figure S11). Moreover, the H NMR
O
6
spectrum of 5 in C6D6 features a γ-CH resonance at 5.48 ppm. This
The 13C{1H} NMR spectrum of 6 in C6D6 features a resonance at
165.3 ppm, which is assignable to the [CO3]2- moiety (Figure S15).
This chemical shift matches the resonance assigned to this complex
in the in situ 13C NMR spectrum of the reaction mixture of 1 and
1
resonance is also present in the in situ H NMR spectrum of the
reaction mixture of 1 and 13CO (Figure S10), further confirming its
formation in that transformation. Overall, these data conclusively
demonstrate that complex 5 is formed during reduction of [K(18-
crown-6)][LtBuNiII(η2-SO)] (1) with CO.
1
13CO (Figure S11). In addition, the H NMR spectrum of 6 in C6D6
features a γ-CH resonance at 5.42 ppm, which is present in the in
situ 1H NMR spectrum of the reaction mixture of 1 and 13CO (Figure
S10). The IR spectrum (hexanes solution) of 6 features a strong νCO
mode at 1620 cm-1, which is also present in a solution IR spectrum
of the reaction mixture formed upon addition of CO to 1 (Figure
S24). Overall, these data conclusively demonstrate that complex 6
is formed during reduction of [K(18-crown-6)][LtBuNiII(η2-SO)] (1)
with CO.
Figure 4. ORTEP drawing of [K(18-crown-6)][LtBuNiII(S,O:κ2-
SCO2)]·1.5C7H8 (5·1.5C7H8) shown with 50% thermal ellipsoids.
Hydrogen atoms, C7H8 solvate molecules, and one orientation of
the disordered [S,O:κ2-SCO2]2- ligand have been omitted for clarity.
Selected metrical parameters: S1-C1 1.756(4) Å, O1-C1 1.279(5) Å,
O2-C1 1.238(4) Å, Ni1-S1 2.234(1) Å, Ni1-O1 1.922(3) Å, Ni1-N1
1.904(3) Å, Ni1-N2 1.899(3) Å, S1-K1 3.531(1) Å, O2-K1 2.715(3) Å,
S1-C1-O1 108.0(3)°, S1-C1-O2 126.2(3)°, O1-C1-O2 125.9(4)°, N1-
Ni1-N2 96.7(1)°, N1-Ni1-O1 91.5(1)°, N2-Ni1-S1 99.22(9)°.
Synthesis of an [κ2-CO3]2- Complex. To further support the
presence of [K(18-crown-6)][LtBuNi(κ2-CO3)] (6) in the reaction of 1
with CO, we pursued its synthesis via an independent route. The
hexameric nickel carbonate complex, [K]6[LtBuNiII(κ2-CO3)]6,11 first
reported by Limberg and coworkers in 2012, was found to serve as
a convenient starting material for the synthesis of [K(18-crown-
6)][LtBuNiII(κ2-CO3)] (6). Addition of 6 equiv of 18-crown-6 to a
suspension of [K]6[LtBuNiII(κ2-CO3)]6 results in the formation of
complex 6 in 52% yield (eq 5). Its solid state molecular structure is
shown in Figure 5. The carbonate (CO32-) ligand in 6 features a
Figure 5. ORTEP drawing of [K(18-crown-6)][LtBuNiII(κ2-
CO3)]·0.5C5H12 (6·0.5C5H12) shown with 50% thermal ellipsoids.
Hydrogen atoms,
a C5H12 solvate molecule, and a second
independent molecule of [K(18-crown-6)][LtBuNiII(κ2-CO3)] have
been omitted for clarity. Selected metrical parameters: C1-O1
1.306(7) Å, C1-O2 1.309(7) Å, C1-O3 1.242(7) Å, Ni1-O1 1.882(4) Å,
Ni1-O2 1.901(4) Å, Ni1-N1 1.883(5) Å, Ni1-N2 1.879(5) Å, O3-K1
2.510(4) Å, O1-C1-O2 110.8(5)°, O1-C1-O3 125.0(6)°, N1-Ni1-N2
97.9(2)°, N1-Ni1-O1 96.6(2)°, N2-Ni1-O2 96.5(2)°.
Synthesis of an [η2-CO2]2- Complex. In an effort to assign the
resonance at 191.4 ppm in the in situ 13C{1H} NMR spectrum of the
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reaction of 1 and 13CO, we endeavored to independently synthesize
the carbon dioxide complex, [K(18-crown-6)][LtBuNiII(η2-CO2)] (10).
We rationalized that 10 was a plausible reaction product, given the
formation of CO2 during the reaction (see below). Several previously
reported Ni(CO2) complexes have been synthesized by reaction of
CO2 with a Ni0 precursor.61-64 In a similar vein, the Ni(0) N2 complex,
[K]2[LtBuNi0(μ-η1:η1-N2)Ni0LtBu], previously reported by Limberg and
co-workers in 2009,65 was found to serve as an effective Ni0 source
for the synthesis of 10. Thus, exposure of [K]2[LtBuNi0(μ-η1:η1-
N2)Ni0LtBu] to two equiv of CO2, followed by addition of 18-crown-6,
resulted in the formation of 10 (eq 6), which was isolated as pale
orange plates in 41% yield after work-up. Its formulation was
confirmed by X-ray crystallography and its solid state molecular
structure is shown in Figure 6.
Dipp
Ni0
tBu
tBu
Dipp
tBu
Dipp
Ni0
O
K
K
1) CO2
N
N
N
N
N
N
O
Figure 6. ORTEP drawing of [K(18-crown-6)][LtBuNiII(η2-CO2)]·2C6H6
(10·2C6H6) shown with 50% thermal ellipsoids. Hydrogen atoms,
C6H6 solvate molecules, and second orientations of the CO2 and 18-
crown-6 fragments have been omitted for clarity. Selected metrical
parameters: C1-O1 1.231(9) Å, C1-O2 1.22(1) Å, Ni1-C1 1.890(6) Å,
Ni1-O1 1.897(6) Å, Ni1-N1 1.901(6) Å, Ni1-N2 1.896(5) Å, O1-K1
2.980(6) Å, O2-K1 2.71(1) Å, O1-C1-O2 144.0(8)°, N1-Ni1-N2
99.2(2)°, N1-Ni1-C1 112.2(3)°, N2-Ni1-O1 110.7(3)°.
O
O
2) 18-c-6
C
II
N
N
Ni
0.5
(6)
K
hexane
- N2
O
O
O
10
Dipp
tBu
tBu
tBu
Dipp
Dipp
O
Complex 10 features a square planar NiII center ligated by the β-
diketiminate ligand and a [CO2]2- ligand. The [CO2]2– ligand in 10
features a μ:η2,κ2 binding mode, similar to that observed for the
[COS]2- ligand in complex 4. The [CO2]2– ligand in 10 is disordered
over two positions, in a 76:24 ratio, which are related by a C2
rotation axis about the Ni-K vector. The Ni1-O1 (1.897(6) Å) and
Ni1-C1 (1.890(6) Å) distances are consistent with those previously
reported for the Ni(η2-CO2) fragment.61,62,64,66,67 Additionally, the Ni-
N bonds in 10 are consistent with those found in other square
planar NiII β-diketiminate complexes.16,49,50
Mechanistic Considerations. To rationalize the formation of
complexes 4 and 5, we propose that CO initially reacts with 1 to
form CO2 and [K(18-crown-6)][LtBuNiII(S)] (I) (Scheme 2). Complex I
then reacts with either CO or CO2 to yield [K(18-crown-
6)][LtBuNiII(η2-SCO)] (4) or [K(18-crown-6)][LtBuNiII(S,O:κ2-SCO2)] (5),
respectively. Significantly, their presence, along with the
observation of [K(18-crown-6)][LtBuNiII(S)] (I) in the reaction mixture,
demonstrates the formal reduction of N2O by CO, as originally
envisioned. That said, the reaction rates of I with CO and CO2 are
qualitatively similar to the reaction rate of I with N2O. As a
consequence, I is unlikely to be an effective catalyst for N2O
reduction because off-cycle reactions with CO and CO2 would be
competitive with the desired N2O capture reaction (Scheme 2).
To rationalize the formation of complex 6 and COS, we propose
that reaction of the newly formed CO2 with unreacted 1 results in
The 1H NMR spectrum of 10 in C6D6 is consistent with a Cs
symmetric, square planar NiII complex. It features two tert-butyl
resonances at 1.42 and 1.34 ppm, and a single γ-CH resonance at
5.42 ppm. Its 13C{1H} NMR spectrum in C6D6 features a resonance at
167.2 ppm, which we have assigned to the [η2-CO2]2- ligand. This
chemical shift is consistent with those reported for previously
isolated Ni(η2-CO2) complexes.62-64 Most importantly, however,
these resonances are not observed in the in situ 13C{1H} and 1H NMR
spectra of the reaction between 1 and 13CO (Figure S10-11). Thus,
we can definitively conclude that complex 10 is not being formed in
that reaction. Finally, complex 10 features a νCO mode at 1664 cm-1
in its IR spectrum (KBr pellet), which is similar to those reported for
other nickel CO2 complexes.62-64 This vibration is also not present in
the in situ IR spectrum (recorded in hexanes) of the reaction residue
formed upon reaction of 1 with CO (Figure S24).
the
formation
of
a
transient,
unobserved
nickel
monothiopercarbonate complex, [K(18-crown-6)][LtBuNiII(κ2-SOCO2)]
(11). Complex 11 then transfers a sulfur atom to CO to form [K(18-
crown-6)][LtBuNiII(κ2-CO3)] (6) and COS (7) (Scheme 2), both of which
were confirmed to be present in the in situ reaction mixture. This
hypothesis also nicely explains the presence of [K(18-crown-
6)][LtBuNiII(κ2-S2CO)] (8), which could be formed via the reaction of 7
with I (Scheme 2). While a monothiopercarbonate complex has not
2-
been previously reported, the reaction of metal peroxides (O2
)
with CO2 is known to yield peroxocarbonate ([OOCO2]2-)
complexes.68-70 Similarly, metal disulfides (S22-) are known to react
with CS2 to form perthiocarbonates ([SSCS2]2-).71,72 Moreover,
peroxocarbonates are known to be very effective O-atom
donors.68,73-76
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Scheme 2
O
C
LtBuNiII
S
4
tBu
Dipp
NiII
O
K
CO
O
O
S
N
N
S
O
O
off cycle
CO2
LtBuNiII
O
C
S
C
LtBuNiII
O
O
SCO
S
CO2
5
CO
N2O
O
O
tBu
Dipp
O
11
I
O=C=S
O=C=O
L
tBuNiII
O
C
off cycle
7
S
8
tBu
Dipp
NiII
CO
O
O
N
N
S
LtBuNiII
tBu
Dipp
NiII
O
C
O
K
O
O
N
O
O
O
N
N
K
6
O
O
N
O
S
O
O
tBu
Dipp
O
O
II
tBu
Dipp
O
1
ꢁ
N2
Consistent with this hypothesis, reaction of [K(18-crown- Gentle thermolysis of the thiohyponitrite complex, [K(18-crown-
6)][LtBuNiII(η2-SO)] (1) with CO2 in C6D6 results in the rapid formation 6)][LtBuNiII(κ2-SNNO)], results in extrusion of N2 and formation of
of a new diamagnetic NiII complex, as evidenced by the appearance [K(18-crown-6)][LtBuNiII(η2-SO)], a rare example of a structurally
of diagnostic resonances at 4.49 ppm (γ-CH) and 1.20 ppm (tBu) in characterized SO complex, along with trace amounts of [K(18-
1
the in situ H NMR spectrum of the reaction mixture (Figure S18). crown-6)][LtBuNiII(η2-OSSO)] and [K(18-crown-6)][LtBuNiII(η2-S2)].
We have assigned these resonances to the monothiopercarbonate [K(18-crown-6)][LtBuNiII(η2-SO)] reacts rapidly with CO, forming the
complex [K(18-crown-6)][LtBuNiII(κ2-SOCO2)] (11) (eq 7). Complex 11 “masked” terminal Ni(II) sulfide intermediate, [K(18-crown-
is the only product observed in the reaction mixture. These results 6)][LtBuNiII(S)], along with CO2, via O-atom abstraction. This Ni(II)
provide further support for the overall reaction mechanism sulfide intermediate then reacts with CO or CO2 to form [K(18-
proposed in Scheme 2 and suggest that (SOCO2)2- could function as crown-6)][LtBuNiII(η2-SCO)] and [K(18-crown-6)][LtBuNi(S,O:κ2-SCO2)],
a very effective a S-atom transfer reagent.77
respectively. [K(18-crown-6)][LtBuNiII(η2-SO)] can also react with the
newly formed CO2 to form a putative monothiopercarbonate
complex, [K(18-crown-6)][LtBuNiII(κ2-SOCO2)], which can then
transfer an S atom to CO, forming COS and [K(18-crown-
6)][LtBuNiII(κ2-CO3)].
O
tBu
Dipp
NiII
O
S
N
N
O
O
C
K
Significantly, the observation of [K(18-crown-6)][LtBuNiII(S)] in the
reaction mixture, along with the formation of [K(18-crown-
CO2
(7)
1
O
C6D6
O
O
O
6)][LtBuNiII(η2-SCO)]
and
confirms that the SO ligand is susceptible to O-atom abstraction by
CO, which had not been previously demonstrated. More
[K(18-crown-6)][LtBuNiII(S,O:κ2-SCO2)],
O
tBu
11
Dipp
importantly, these reaction products reveal the stepwise
conversion of [K(18-crown-6)][LtBuNiII(κ2-SNNO)] to [K(18-crown-
6)][LtBuNiII(η2-SO)] and then [K(18-crown-6)][LtBuNiII(S)], which
Conclusions
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represents a formal reduction of N2O by CO, forming N2 and CO2. 15. R. Zeng, M. Feller, Y. Ben-David and D. Milstein, J. Am. Chem.
Significantly, this transformation parallels the chemistry mediated Soc., 2017, 139, 5720-5723.
by nano-particulate Pt/Rh in catalytic converters. In contrast to the 16. N. J. Hartmann, G. Wu and T. W. Hayton, Angew. Chem. Int.
metal-centered redox of the catalytic converter example, however, Ed., 2015, 54, 14956-14959.
the redox chemistry in our system occurs at the sulfide ligand, while 17. K. Severin, Chem. Soc. Rev., 2015, 44, 6375-6386.
the nickel center remains in the 2+ oxidation state at every step. 18. P. Pyykkö, J. Phys. Chem. A, 2015, 119, 2326-2337.
The use of ligand-centered redox is an intriguing strategy for N2O 19. F. X. Powell and D. R. Lide Jr., J. Chem. Phys., 1964, 41, 1413-
reduction and we suggest that the study of model systems, such as 1419.
the one presented in this manuscript, could inspire the design of a 20. B. Horn, C. Limberg, C. Herwig and B. Braun, Inorg. Chem.,
new generation of homogeneous and heterogeneous N2O reduction 2014, 53, 6867-6874.
catalysts.
21. C. Bianchini, C. Mealli, A. Meli and M. Sabat, J. Chem. Soc.,
Chem. Commun., 1985, 1024-1025.
22. A. Neher, O. Heyke and I. P. Lorenz, Z. Anorg. Allg. Chem.,
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Conflicts of interest
23. R. Huang, I. A. Guzei and J. H. Espenson, Organometallics,
1999, 18, 5420-5422.
The authors declare no competing financial interests.
24. C. Minh Tuong, W. K. Hammons, A. L. Howarth, K. E. Lutz, A.
D. Maduvu, L. B. Haysley, B. R. T. Allred, L. K. Hoyt, M. S.
Mashuta and M. E. Noble, Inorg. Chem., 2009, 48, 5027-5038.
25. K. Oya, H. Seino, M. Akiizumi and Y. Mizobe,
Organometallics, 2011, 30, 2939-2946.
Acknowledgements
We thank the National Science Foundation (CHE 1361654) for
financial support of this work. We would also like to thank the
Scott Group at UCSB for the use of their 13CO. This research
made use of the 400 MHz NMR Spectrometer in the
Department of Chemistry and Biochemistry, an NIH SIG
(1S10OD012077-01A1).
26. R. Wang, M. S. Mashuta, J. F. Richardson and M. E. Noble,
Inorg. Chem., 1996, 35, 3022-3030.
27. L. E. Longobardi, V. Wolter and D. W. Stephan, Angew. Chem.
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29. J. E. Hoots, D. A. Lesch and T. B. Rauchfuss, Inorg. Chem.,
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Chemical Science
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DOI: 10.1039/C8SC02536C
Graphical abstract:
Explanatory text: The "masked" terminal nickel sulfide [K(18-crown-6)][LtBuNiII(S)]
mediates the reduction of N2O by CO, via the thioperoxide complex [K(18-crown-
6)][LtBuNiII(η2-SO)].