Metal-catalyzed "on-demand" production of carbonyl sulfide from carbon monoxide and elemental sulfur

Article abstract of DOI:10.1002/anie.201410353

The group 6 molybdenum(II) cyclopentadienyl amidinate (CPAM) bis(carbonyl) complex [Cp Mo{N(iPr)C(Ph)N(iPr)}(CO)₂] (Cp=η⁵-C₅Me₅) serves as a precatalyst for the high-yielding photocatalytic production of COS from CO and S₈ under near-ambient conditions (e.g., 10 psi, 25°C). Further documented is the isolation and structural characterization of several key transition-metal intermediates which collectively support a novel molybdenum(IV)-based catalytic cycle as being operative. Finally, in the presence of an excess amount of a primary amine, it is demonstrated that this catalytic system can be successfully used for the "on-demand" generation and utilization of COS as a chemical reagent for the synthesis of ureas.

Full text of DOI:10.1002/anie.201410353

Angewandte
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DOI: 10.1002/anie.201410353
Sulfur Atom Transfer
Metal-Catalyzed “On-Demand” Production of Carbonyl Sulfide from
Carbon Monoxide and Elemental Sulfur**
Wesley S. Farrell, Peter Y. Zavalij, and Lawrence R. Sita*
Abstract: The group 6 molybdenum(II) cyclopentadienyl
amidinate (CPAM) bis(carbonyl) complex [Cp*Mo{N(iPr)-
C(Ph)N(iPr)}(CO)2] (Cp* = h5-C5Me5) serves as a precatalyst
for the high-yielding photocatalytic production of COS from
CO and S8 under near-ambient conditions (e.g., 10 psi, 258C).
Further documented is the isolation and structural character-
ization of several key transition-metal intermediates which
collectively support a novel molybdenum(IV)-based catalytic
cycle as being operative. Finally, in the presence of an excess
amount of a primary amine, it is demonstrated that this
catalytic system can be successfully used for the “on-demand”
generation and utilization of COS as a chemical reagent for the
synthesis of ureas.
Finally, we document the isolation and structural character-
ization of several transition-metal complexes which collec-
tively provide key insights regarding the mechanism by which
this novel molybdenum-based catalytic cycle might operate.
Recently, we reported that group 6 molybdenum(IV)
terminal oxo and imido complexes which are supported by
the CPAM ligand set, and more specifically, [Cp*Mo{N-
(iPr)C(Me)N(iPr)}(E)], where E = O (I) and NR (II), can
function as competent catalysts for the production of
isocyanates (RNCO) through either oxygen-atom transfer
(OAT) from nitrous oxide (N2O) to isonitriles (RNC), or
through nitrene-group transfer (NGT) from organoazides
(RN3) to CO.[7,8] A third reported catalytic system involves
the degenerative OAT between CO and carbon dioxide
(CO2), a process which occurs upon photolysis of a solu-
tion containing the molybdenum bis(carbonyl) complex
[Cp*Mo{N(iPr)C(Me)N(iPr)}(CO)2] (III).[7a] Importantly, all
three of these catalytic processes proceed by a formal MoII/
MoIV oxidation state couple, whereas all biological molybde-
num-dependent oxotransferase enzymes favor thermal OAT
mechanisms based on a more oxidized MoIV/MoVI couple.[9]
With the success of these CPAM-based catalytic OAT and
NGT processes firmly established, we next focused attention
on developing comparable sulfur-atom transfer (SAT)
chemistry.
As Scheme 1 reveals, compound 1 was conveniently
prepared in high yield through the addition of CO (10 psi)
to a toluene solution of the dinuclear “end-on-bridged”
dinitrogen complex [{Cp*Mo[N(iPr)C(Ph)N(iPr)]}2(m-h1:h1-
N2)] (2), which was itself obtained in a 44% yield from sodium
amalgam (NaHg) reduction of the molybdenum(IV) dichlor-
ide [Cp*Mo{N(iPr)C(Ph)N(iPr)}(Cl)2] (3) in tetrahydrofuran
(THF) solution under a dinitrogen atmosphere according to
our previously published procedures.[10,11] Photolysis of a mix-
ture consisting of a solution of 1 in [D6]benzene, an excess of
S8 (ca. 10 equiv as a suspension), and 13C-labeled (98%) CO
(initial pressure: 10 psi), within a sealed Pyrex NMR tube, was
conducted using a Rayonet carousel of medium pressure Hg
lamps at 258C, and as the series of 13C{1H} NMR (125 MHz)
= =
Carbonyl sulfide (COS), O C S, occurs as a product of
highly energetic natural events, such as volcanism, and as
a greenhouse gas, it is thought to have kept an early Earth
warm while the young sun was still faint.[1,2] In a prebiotic
world, COS might have also played a critical role as a reagent
for the coupling of amino acids to peptides under conditions
that were physiologically favorable for life.[3,4] More recently,
COS is obtained for industrial purposes through a rather
lengthy and inefficient process of separation and purification
from a complex mixture of products arising from the reaction
of potassium thiocyanate with sulfuric acid.[5] Quite surpris-
ingly, while the direct reaction of carbon monoxide (CO) with
elemental sulfur (S8) is also known to produce COS at high
temperatures and pressures, until now, no transition-metal-
catalyzed version of this process has been discovered or
developed such that it can be conducted in high yield and
under more energy efficient conditions.[6] Herein, we now
report that the group 6 molybdenum(II) cyclopentadienyl
amidinate (CPAM) bis(carbonyl) complex, [Cp*Mo{N-
(iPr)C(Ph)N(iPr)}(CO)2] (1; Cp* = h5-C5Me5), can serve as
a precatalyst for the high-yielding photocatalytic production
of COS from CO and S8 under near ambient conditions (e.g.,
10 psi, 258C). We further demonstrate that this catalytic
system can be used for the “on-demand” in situ generation
and utilization of COS as a chemical reagent for the synthesis
of 1,3-disubstituted ureas from primary amines, CO, and S8.
[*] W. S. Farrell, P. Y. Zavalij, Prof. L. R. Sita
Department of Chemistry and Biochemistry
University of Maryland, College Park, MD 20742 (USA)
E-mail: lsita@umd.edu
[**] Funding for this work was partially provided by the Department of
Energy, Basic Energy Sciences (DE-SC0002217) and the National
Science Foundation (CHE-1361716).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201410353.
Scheme 1. Synthesis of compound 1.
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Scheme 2. Coupling of primary amines to 1,3-disubstituted ureas
using in situ generated COS.
(one equivalent relative to the amine), and 1 (5 mol%
relative to the amine) for 18 hours resulted in a near
quantitative conversion into the 1,3-disubstituted urea in
each case, as determined by isolation and spectroscopic
comparison of each product against literature data.[11] When
conducted on a preparative scale, the amount of 1 could be
reduced to as little as 0.5 mol%.
Control experiments were conducted to substantiate that
the coupling of primary amines to 1,3-disubstituted ureas was
occurring through the intermediacy of in situ generated COS.
To begin, no urea product was formed when photolysis was
conducted in the absence of either 1 or S8. Second, when
isotopically labeled 13CO (98%) was used for the coupling
of tert-butylamine, tBuNH2, the 13C NMR spectra obtained
at different time intervals of photolysis clearly showed
the coexistence of 13C-labeled COS and 13CO-labeled
(tBuNH)2CO.
While many reports of transition-metal-catalyzed oxida-
tive coupling of amines with CO to produce ureas have
appeared in the literature,[13] virtually all of these methods
require elevated temperatures and pressures, as well as the
use of co-reagents which are more exotic than S8 and which
serve as sacrificial oxidants that return the transition metal to
the starting formal oxidation state within the catalytic cycle.
These catalytic processes are also thought to involve intimate
bonding interactions between the transition metal and amine
and CO substrates within the urea-product-forming steps. In
contrast, the chemical process presented in Scheme 2 pro-
ceeds through the far simpler mechanism of transition-metal-
catalyzed in situ generation of COS which is then responsible
for production of the final urea product. It is also important to
note that the success of the chemistry of Scheme 2 requires
that all the transition-metal complexes, which are intermedi-
ates in the photocatalytic generation of COS, must be
chemically tolerant of the protic and Brønsted basic nature
of both the primary amine starting materials and of the urea
products. It can also be noted that primary amines of
Scheme 2 were degassed but not distilled from a drying
agent before use.
To further establish the structural identity of the inter-
mediates arising from photolysis of 1 with S8 and CO, the
CPAM MoII mono(carbonyl), mono(acetonitrile) complex,
[Cp*Mo{N(iPr)C(Ph)N(iPr)}(CO)(NCMe)] (4) [IR (KBr)
nCO = 1767 cmÀ1], was prepared in high yield (Scheme 3).[11]
As we have previously documented,[7,8] 4 should function as
a viable synthetic model for the reactive transient species 5,
which is presumed to be generated in situ through initial
photodissociation of CO from 1. In the present work, reaction
of 4 with an excess of S8 in benzene rapidly generated the
molybdenum(IV) mono(carbonyl), h2-persulfido complex
Figure 1. Partial 13C{1H} NMR spectra (125 MHz, [D6]benzene, 258C)
for the photolysis of 1 in the presence of 13C-labeled CO and excess S8
after different timed intervals. Labeled resonances are 13C-labeled 6 (#)
and 13C-labeled 7 (+).
spectra in Figure 1 reveal, steady production of 13C-labeled
COS was observed to occur over time at the expense of free
13CO. During this same period of time, the appearance of two
additional 13C resonances at d = 249 ppm and 208 ppm in the
13C NMR spectra (see resonances labeled with a # and + ,
respectively, in Figure 1 for photolysis between 2 h and 18 h),
1
were also shown by H NMR spectroscopy to correlate with
the rapid disappearance of 1H resonances for 1 and the
appearance of those corresponding to two new CPAM
complexes. One is a complex of C1 symmetry and the other
is of Cs symmetry.[11] Importantly, the role of 1 as a precatalyst
in this catalytic production of COS from CO and S8 is strongly
supported by both the 1H NMR spectra and the fact that the
13C resonance for 13C-labeled 1(13CO) , which occurs at d =
2
270 ppm, never appears over the time course of photolysis.
The ease with which COS is photolytically generated from
CO and S8 using 1 as a precatalyst prompted us to explore the
synthetic utility of this chemistry for the on-demand and in
situ generated delivery of COS as a chemical reagent. In this
regard, direct reaction of COS with amines has previously
been used to prepare substituted ureas in a process which
generates H2S as the only by-product.[5a] The production of
substituted ureas from mixtures containing a starting amine,
CO, and S8 has also been reported to occur at high temper-
atures and pressure.[12] As presented in Scheme 2, photolysis
of a [D6]benzene solution containing a primary amine, RNH2
(R = tert-butyl, isopropyl, n-butyl, benzyl), CO (10 psi), S8
2
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a rare example of a neutral molybdenum(IV) persulfido
À
complex, and the S1 S2 bond length of 2.0555(6) ꢀ can be
compared to the corresponding bond length value of
2.091(2) ꢀ reported for [Cp*2Mo(h2-S2)].[14,15] Finally, and
1
most significantly, the H and 13C NMR spectra for 6 could
now be used to confirm that this compound is indeed identical
to the C1-symmetric complex (marked with a # in Figure 1)
which is observed during photolysis of 1 with S8 and CO.
As presented in Scheme 4, photolysis of a dilute solution
of 6 in [D6]benzene, for a short period of time (i.e., 2.5 h),
provided a quantitative yield of the isomeric molybde-
Scheme 3. Synthesis of compounds 4 and 6.
Scheme 4. Photolytic conversion of compound 6 into 7.
[Cp*Mo{N(iPr)C(Ph)N(iPr)}(CO)(h2-S2)] (6) [IR (KBr)
nCO = 1921 cmÀ1] in high yield as an orange, crystalline
solid.[11] Figure 2a provides the solid-state molecular structure
of 6 as obtained from a single-crystal X-ray analysis, along
with selected geometric parameters. Compound 6 represents
num(IV) dithiocarbonate complex, [Cp*Mo{N(iPr)C(Ph)N-
(iPr)}{k-(S,S)S2CO)}] (7) [IR (KBr) nCO = 1673 cmÀ1], as
1
determined by H NMR spectroscopy.[11,16] The identity of 7
was further established through single-crystal X-ray analysis
which provided the solid-state molecular structure shown in
Figure 2b. More importantly, with 7 in hand, it was then
possible to unequivocally verify by NMR spectroscopy that
this compound represents the other Cs-symmetric species
(marked with a + in Figure 1) which appears during photo-
lytic generation of COS from CO and S8 using 1 as a pre-
catalyst. Finally, when photolysis of 6 was conducted under an
atmosphere of 13CO (10 psi), 13C-labeled 7(S -13
was pro-
CO)
2
duced, presumably through an associative mechanism for
initial ligand substitution of CO by 13CO, followed by a formal
intramolecular 1,2-insertion of 13CO into the sulfur–sulfur
bond of 6 to form 7.[16,17]
In contrast to the quantitative photoconversion of 6 into 7
which occurs at short periods of time in the absence of any
other chemical reagents, photolysis of a solution of 6
([D6]benzene) in the presence of an excess of S8 for
a longer period of time (e.g., 6.5 h versus 2.5 h) generated
a new Cs-symmetric product at the expense of 7, as monitored
by 1H NMR spectroscopy.[11] Furthermore, when this
extended photolysis was repeated using 13C-labeled 6(
,
13CO)
the 13C NMR spectrum confirmed that while 13C-labeled COS
had been produced, no 13CO-enriched resonances, which
could be tied to generation of the new Cs-symmetric species,
appeared. Finally, extended photolysis of a pure sample of
13C-labeled 7(S -13CO) only led to photodegradation and with no
2
evidence, by either 1H or 13C NMR spectroscopy, for the
generation of either 6 or the new Cs-symmetric species.
Working on the assumption that the new Cs-symmetric
product obtained from photolysis of 6 with S8 might be
a CPAM MoIV sulfide (e.g., E = S in I), an effort was made to
synthesize such a complex through an alternative route in
a more controlled manner. Accordingly, as Scheme 5 pres-
ents, treatment of a solution of 2 in diethyl ether (Et2O) with
S8 yielded a red crystalline product which indeed proved to be
Figure 2. Molecular structures (30% thermal ellipsoids) of a) 6 and
b) 7. Hydrogen atoms have been removed for clarity. Selected bond
lengths [ꢀ] and angles [8] for 6: Mo1–S1 2.4659(4), Mo1–S2 2.4665(4),
S1–S2 2.0555(6), Mo1–C24 1.9901(17), C24–O1 1.149(2); S1-Mo1-S2
49.257(14), S1-Mo1-C24 70.90(5) and for 7: Mo1–S1 2.3876(9), Mo1–
S2 2.3954(9), S1–C24 1.784(4), S2–C24 1.786(4), C24–O1 1.198(4);
S1-C24-S2 105.77(18).
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progress, a key finding regarding the possible role that this
species plays in the photocatalytic generation of COS was
that, upon introduction of 13CO into a solution of 8 in
[D6]benzene, non-photolytic production of 6(
(57%),
13CO)
7(S -13
(29%), and an unquantified amount of 13C-labeled
CO)
2
COS was observed to occur at room temperature. Collec-
tively, these experimental results conclusively establish that 6
and 8 are interconverted under the reaction conditions of
Figure 1, and accordingly, these compounds are reasonable
candidates as intermediates in the catalytic cycle which
generates COS from CO and S8. In contrast, 7 appears to be
a kinetic product which can be produced from either 6 or 8.
However, simultaneous photodegradation of 7 under the
reaction conditions fortuitously eliminates the buildup of this
species over time. Scheme 6 summarizes these proposed
relationships of 6–8 to photocatalytic production of COS.
Scheme 5. Synthesis of compound 8.
the dimer of the proposed molybdenum(IV) sulfide in the
solid state, as confirmed by single-crystal X-ray analyses
which were obtained for both the cis- and trans-isomers of
[{Cp*Mo[N(iPr)C(Ph)N(iPr)]}2(m-S)2] (8).[11,18,19] Figure 3
Figure 3. Molecular structure (30% thermal ellipsoids) of trans-8.
Hydrogen atoms have been removed for clarity. Selected bond lengths
[ꢀ] and angles [8]: Mo1–S1 2.3408(7); S1-Mo1-S2 91.40(2), Mo1-S1-
Mo2 88.60(2).
Scheme 6. Proposed relationship of 6–8 to photocatalytic production
of COS. Bold arrows represent catalytically-relevant pathways for COS
generation.
provides the solid-state molecular structure of trans-8 along
with selected geometric parameters. It is also important to
note, however, that the solution structure of 8 is less clear.
More specifically, despite extensive efforts, we have not been
able to obtain an analytically pure sample of 8 which provides
a satisfactory elemental analysis. Furthermore, with variable-
temperature 1H NMR spectroscopy, only a single set of
resonances for the mixture of cis and trans isomers for
diamagnetic 8 are observed down to the limiting temperature
of À908C, and this observation suggests the possibility that, in
solution, 8 is engaged in a dynamic dimer–monomer exchange
In conclusion, the present report makes the case for the
first example of metal-catalyzed synthesis of COS from CO
and S8 that proceeds under near ambient conditions. The
tolerance to protic substrates that is demonstrated by this
catalytic process further supports the synthesis of more
complex organic products through the on-demand production
of COS. Finally, it is interesting to speculate that the prebiotic
existence of a similar metal-catalyzed low-energy pathway
could have effectively lowered the free energy threshold
conditions required for the COS-facilitated genesis of life, and
perhaps more importantly, under the highly reducing atmos-
phere which is proposed to have existed for archaic Earth, and
presently, for Titan.[20]
1
equilibrium at all temperatures. Finally, H NMR spectra for
crystalline samples of 8 always present evidence for a varying
amount of a paramagnetic impurity which can account for up
to 20% by weight of the material, as based on the use of an
internal standard (durene). When further interrogated by
electrospray ionization mass spectrometry (ESI-MS), a solu-
tion of 8 provided additional molecular ion (M + H) clusters
which correspond to the dinuclear [MoV, MoV] tris(sulfide)
[{Cp*Mo[N(iPr)C(Ph)N(iPr)]}2(S)3] and the formal dinuclear
[MoIV, MoV] tris(sulfide) [Cp*Mo[N(iPr)C(Ph)N(iPr)]-
(S)3Cp*Mo].[11]
Received: October 22, 2014
Revised: December 12, 2014
Published online: && &&, &&&&
While additional efforts to quantify sample composition
and the dynamic solution behavior displayed by 8 remain in
Keywords: molybdenum · oxidation · structure elucidation ·
sulfur · synthetic methods
.
4
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Angew. Chem. Int. Ed. 2015, 54, 1 – 6
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
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.
Angewandte
Communications
Communications
Sulfur Atom Transfer
BeCOS: Carbonyl sulfide (COS) is con-
veniently generated in situ in high yield
and chemical purity directly from CO and
S8 in solution and under near-ambient
conditions upon photolysis of the preca-
talyst [Cp*Mo{N(iPr)C(Ph)N(iPr)}(CO)2].
The tolerance of this catalytic process to
protic substrates supports the facile syn-
thesis of more complex organic products
through the “on-demand” production of
COS.
W. S. Farrell, P. Y. Zavalij,
L. R. Sita*
&&&& — &&&&
Metal-Catalyzed “On-Demand”
Production of Carbonyl Sulfide from
Carbon Monoxide and Elemental Sulfur
6
www.angewandte.org
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 1 – 6
These are not the final page numbers!