Nitrosonium reactivity of (NHC)Copper(I) sulfide complexes

Article abstract of DOI:10.1021/acs.inorgchem.9b01676

This study examines the reactivity of a series of copper(I) sulfide complexes stabilized by the expanded-ring N-heterocyclic carbene (NHC) 1,3-bis(2,6-diisopropylphenyl)-4,5,6,7-tetrahydro-1,3-diazepin-2-ylidene (7Dipp) toward the nitrosonium ion (NO⁺). 7Dipp is shown to support neutral sulfide- and disulfide-bridged dicopper(I) complexes, as well as mononuclear copper(I) hydrosulfide. The addition of NO⁺ to each of these results in the formation of NHC-supported copper(I) cations and elemental sulfur. Reduction of copper(I) to copper(0) is observed upon reaction of NO⁺ with dicopper(I) sulfide or disulfide, whereas ammonium ion formation is observed upon reaction of copper(I) hydrosulfide with NO⁺. Ammonium ion formation is likewise observed upon reaction of NO⁺ with (7Dipp)copper(I) hydride.

Full text of DOI:10.1021/acs.inorgchem.9b01676

Communication
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Nitrosonium Reactivity of (NHC)Copper(I) Sulfide Complexes
Abraham J. Jordan, Rebecca K. Walde, Kelly M. Schultz, John Bacsa, and Joseph P. Sadighi*
†
†
‡
†,§
,†
†
School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States
‡
Murdock Hall, Department of Chemistry, Linfield College, McMinnville, Oregon 97128, United States
§
X-ray Crystallography Center, Department of Chemistry, Emory University, 1515 Dickey Drive, Atlanta, Georgia 30322, United
States
*
S Supporting Information
Scheme 1. Thionitrite-Related Compounds of Transition
Metals
ABSTRACT: This study examines the reactivity of a
series of copper(I) sulfide complexes stabilized by the
expanded-ring N-heterocyclic carbene (NHC) 1,3-bis-
(
2,6-diisopropylphenyl)-4,5,6,7-tetrahydro-1,3-diazepin-2-
+
ylidene (7Dipp) toward the nitrosonium ion (NO ).
Dipp is shown to support neutral sulfide- and disulfide-
7
bridged dicopper(I) complexes, as well as mononuclear
+
copper(I) hydrosulfide. The addition of NO to each of
these results in the formation of NHC-supported
copper(I) cations and elemental sulfur. Reduction o+f
copper(I) to copper(0) is observed upon reaction of NO
with dicopper(I) sulfide or disulfide, whereas ammonium
ion formation is observed upon reaction of copper(I)
+
hydrosulfide with NO . Ammonium ion formation is
+
likewise observed upon reaction of NO with (7Dipp)-
copper(I) hydride.
•
itric oxide (NO ) and dihydrogen sulfide (H S) are
2
N
important biological signaling molecules whose chem-
istries are often linked through the proposed “cross-talk”
1
,2
−
products
HNO), and perthionitrite (SSNO ).
have shed light on the properties and reactivity of these
thionitrous acid (HSNO or SNO ), nitroxyl
−
3−5
(
Many recent studies
4
−6
transient species, but much still remains unknown.
Of
particular interest is their interaction with transition metals, but
there are few studies relating well-defined transition-metal
complexes and “cross-talk” products. The addition of HS to
iron nitrosyl complexes (sodium nitroprusside or iron
porphyrin) initially forms an iron-bound HSNO complex,
observable by mass spectrometry in the case of the iron
porphyrin system, before HSNO decomposition products are
Copper sulfides tend to form higher-order complexes, featuring
−
15−25
three or more copper centers; 2
copper/sulfur clusters with
6
two metal centers are rare but include a number of
2
7−40
dicopper(II) disulfide complexes
and a single dicopper(I)
26,41
sulfide complex.
We envisioned that similar low-nuclearity
sulfur-bound copper(I) complexes could shed light on the
7
−9
observed (Scheme 1a).
A “masked” terminal nickel sulfide
reactivity of various “cross-talk” products. Considering the
•
−
2−
2−
reacts with NO to yield a nickel nitrosyl and the SNO anion,
which reacts with further NO to yield the SSNO anion
Scheme 1b). An ethylenediaminetetraacetate (EDTA)-
supported diruthenium disulfide reacts with NO to yield the
Ru−SNO complex, observed by mass spectrometry (Scheme
c). Last, SSNO and SNO anions have been isolated as
bis(triphenylphosphine)iminium salts. Few studies have
examined the reactivity of “cross-talk” products with copper.
Copper sulfide clusters have garnered much attention
because of their prevalence in biological systems, particularly
sulfur atom(s) to be S (or SS ), the addition of nitrosonium
•
−
+
−
−
(NO ) would formally lead to HSNO, SNO , SSNO , and
HNO products.
1
0
(
•
Warren and co-workers have shown that a copper(II)
•
thiolate reacts reversibly with NO at low temperature to yield
42
1
1
−
−
a bound alkyl thionitrite. They have also shown that N-
heterocyclic carbene (NHC)-supported copper(I) thiolates
react with NO+ to cleanly generate a thiolate-bridged
dicopper(I) cation and free organic thionitrites (Scheme
1
1
2
1
3
in the active sites of nitrous oxide reductase
molybdenum/copper carbon monoxide dehydrogenase.
and
Received: June 5, 2019
1
4
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b01676
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
4
3
1
d,e). In this study, we have examined the reactivity of
Scheme 3. Synthesis of a Disulfide-Bridged Dicopper
Complex
NHC-supported copper(I) sulfide, disulfide, and hydrosulfide
complexes with NO .
+
We initially set out to determine whether sterically
encumbering expanded-ring NHCs stabilize a dicopper(I)
sulfide core. We have previously shown that the expanded-ring
NHC 1,3-bis(2,6-diisopropylphenyl)-4,5,6,7-tetrahydro-1,3-di-
4
4,45
azepin-2-ylidene (7Dipp)
supports a stable copper(I)
4
6
hydride dimer. Gratifyingly, (7DippCu) S (1) could be
2
generated in good yield from the reaction of 7DippCuCl with
excess Na S following an analogous procedure from Hillhouse
2
26
and co-workers (Scheme 2). The 7Dipp-supported copper-
Scheme 2. Synthesis of a Sulfide-Bridged Dicopper Complex
(
I) hydrosulfide (2) was readily synthesized from 7DippCuCl
and potassium hydrosulfide. Another route to sulfide-bridged
dicopper(I) shown by Hillhouse and co-workers, deprotona-
tion of a hydrosulfide complex by copper(I) alkoxide, also
proved viable with the 7Dipp ligand. Interestingly, deprotona-
tion of 2 by sodium tert-butoxide resulted in the formation of 1
Figure 1. Thermal ellipsoid depiction (50% probability) of 1 (a) and
4 (b). Selected interatomic distances (Å) and angles (deg) for 1 (one
molecule shown of four in the asymmetric unit): C1_10−Cu3
1.909(3), Cu3−S2 2.117(1), C1_11−Cu4 1.892(3), Cu4−S2
as the major species, with a presumed loss of Na S, offering
another avenue to the bridging sulfide. Rapid conversion of 2
2
2
.108(2), Cu3−Cu4 3.475(2); C1_10−Cu3−S2 163.94(9),
C1_11−Cu4−S2 168.60(9), Cu3−S2−Cu4 110.66(6). Selected
by either deprotonation route was evident from the bright-
1
interatomic distances (Å) and angles (deg) for 4: Cu1−C1
yellow color of 1 and confirmed by H NMR spectroscopy.
1
1
.906(3), Cu1−S1 2.132(1), S1−S1 2.113(2); C1−Cu1−S1
78.02(9), Cu1−S1−S1 99.80(7).
While there are numerous examples of dicopper(II)
disulfides (vide supra), we have found no examples of
dicopper(I) disulfides. Initial attempts to synthesize a 7Dipp-
supported dicopper(I) disulfide by the addition of elemental
sulfur to 1 resulted in the formation of numerous products,
regardless of stoichiometry, suggesting a tendency to form
mixtures of higher-order copper oligosulfides. As an alternative,
reported by Karlin and co-workers for a dicopper(II)
29
disulfide. Many side-on dicopper(II) disulfides have also
2
7−30,33,34,36,37,39,40
been reported.
Complex 4 features a Cu−S
distance of 2.132(1) Å, an S−S distance of 2.113(2) Å, and a
3
reacts with bis(phenylacetyl) disulfide to give the 7Dipp-
Cu−S−S angle of 99.80(7)°.
+
supported dicopper(I) disulfide (4), isolated as an orange-
The addition of NO to a solution of 2 in CD CN results in
3
the formation of 7DippCu+ as its acetonitrile adduct (5;
yellow solid (Scheme 3). The addition of PPh to a solution of
3
+
4
4
in C D resulted in conversion to 1 by sulfur abstraction with
Scheme 4a). Ammonium ion (NH ) formation was evident by
the appearance of a 1:1:1 triplet resonance, due to H− N
coupling, at 5.91 ppm in the H NMR spectrum (Scheme 4a
6
6
1
1
14
concomitant formation of Ph PS, as confirmed by H and
3
3
1
1
P NMR.
Recrystallization of 1 from CH CN (−35 °C) resulted in
and Figure S15). Notably, NH3 is observed from the
3
crystals suitable for X-ray diffraction. The structure (Figure 1a)
is similar to that of the dicopper(I) sulfide reported by
decomposition of an iron-bound HSNO complex in a buffered
7
aqueous solution, whereas ammonium ion formation, shown
26
Hillhouse and co-workers. Only one molecule of four in the
herein, occurred in a dried, deuterated solvent. The sample
after the addition of NO+ appeared cloudy, suggesting
precipitation of elemental sulfur. The subsequent addition of
PPh generated a clear solution, and formation of Ph PS was
asymmetric unit is shown. The Cu···Cu distances range from
3
1
.475(2) to 3.677(2) Å and the Cu−S−Cu angles from
10.66(6) to 120.68°. Crystals of 4 suitable for X-ray
3
3
3
1
diffraction resulted after recrystallization from Et O (−35
confirmed by P NMR. We thus suggest the formation of
HSNO (copper-bound or free), which releases elemental
sulfur, generating HNO. Subsequent decomposition of HNO
2
°
C). The structure of 4 (Figure 1b) reveals end-on binding of
the disulfide unit to the copper(I) centers, analogous to that
B
DOI: 10.1021/acs.inorgchem.9b01676
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
Scheme 4. Nitrosylation of Copper(I) Hydrosulfide
Scheme 5. Nitrosylation of Dicopper(I) Sulfides
would form NH4+ and should generate N O. An analysis of the
2
reaction headspace by gas chromatography−mass spectrome-
try shows a peak at m/z 44, corresponding to N O (Supporting
2
Information p S20). To balance these observed products, we
thus infer the formation of NO3− (Scheme 4). On the basis of
+
this hypothesis, the reaction of NO with copper(I) hydride
should generate HNO, with the Cu−H acting as a source of
−
+
1
H , and likewise lead to NH formation. The H NMR
4
+
spectrum following dissolution of (7DippCuH) and NO in
CD CN reveals the formation of [7DippCu−NCCD ] , with a
:1:1 triplet resonance corresponding to NH (Scheme 4b).
2
+
3
3
+
4
1
+
Last, we investigated whether NO would act as a Lewis acid
or an oxidant toward 1 and 4. Furthermore, we wondered
whether oxidation would occur at the copper centers or the
sulfur atom(s). Calculations by Hillhouse and co-workers show
the highest occupied molecular orbital consisting mainly of a
nonbonding S p orbital but with some copper character (see
the Supporting Information of ref 26). Furthermore, oxidation
of the dicopper(I) sulfide complex by CO or N O to
1
31
to the NMR sample, the H and P NMR spectra were
+
31
consistent with the formation of [7DippCu−PPh ] . The
P
3
NMR spectrum also showed the resonance for Ph PS,
3
confirming that elemental sulfur had been formed. Further-
1
9
more, the F NMR spectrum following Ph P showed a sharp
BF4 resonance, no longer shifted upfield from that of the free
ion. This finding suggests that weakly coordinating BF can
interact with the copper(I) center. Labile BF4 complexes of
(NHC)gold(I) cations have likewise been reported. The
reaction of 7DippCuCl and AgBF in a fluorobenzene solution
resulted in identical H NMR shifts in addition to a broadened,
upfield shift of the BF4 resonance. H NMR spectra taken of
3
−
2
2
4
1
−
dicopper(I) sulfate was recently reported. Interestingly,
4
+
−
dicopper(I) sulfide and disulfide react similarly with NO (1
48
equiv). Reactions in CD CN result in decomposition to
3
1
copper(0), while the H NMR spectra show the presence of 5
4
1
(Scheme 5). We have been unable to determine the amount of
−
1
copper(0) formed, but the mass balance suggests about 50%
conversion of 1 to 5. On the basis of these findings, we inferred
the formation of elemental sulfur, which was confirmed by the
+
mixtures kept at or below −35 °C showed 7DippCu
fragments as the major or the only detectable species.
Additionally, we saw no evidence for the formation of N-
heterocyclic thiourea [(7Dipp)S] or for nitrosylation of the
7Dipp ligand.
3
1
resonance for Ph PS in the P NMR spectra after the
3
addition of Ph P. We thus conclude that an NHC-coordinated
3
Cu−SNO (or Cu−SSNO) complex is unstable if formed,
•
decomposing to form copper(0), elemental sulfur, and NO .
We have outlined the nitrosonium reactivity of a series of
NHC-supported copper(I) sulfide complexes. The syntheses
and structures of new neutral sulfide- and disulfide-bridged
dicopper(I) complexes, in addition to their related hydro-
sulfide complex, show the expanded-ring NHC 7Dipp to be a
capable supporting ligand. The reaction with NO results in
oxidation at the sulfur atom(s) to elemental sulfur and the
formation of (NHC)copper(I) cations. Despite the high
oxidation potential of NO (0.87 V vs Fc in CH CN), this
oxidation leads to a net reduction of a copper(I) center. The
reaction of 7Dipp-supported copper(I) hydrosulfide leads to
elemental sulfur, (NHC)copper(I) cation, N O, and NH ,
Knowing that organic thiols react with t-BuONO to yield
47
organic thionitrites, we added t-BuONO to 2 but saw no
appreciable reaction after 24 h. After heating at 50 °C,
significant decomposition was evident. We also cannot rule out
the possibility of outer-sphere electron transfer from 1, or 4, to
+
+
NO with subsequent decomposition. Although the reaction
•
+
between 1 and [thianthrene] gave a complex mixture,
oxidation of sulfide to elemental sulfur was confirmed by the
+
49
3
3
1
resonance for Ph PS in the P NMR spectrum after the
3
addition of Ph P.
3
+
+
4
The reaction of 1 with NO in weakly coordinating
2
fluorobenzene at −35 °C, with subsequent warming to room
possibly via decomposition of HSNO. Furthermore, the
7Dipp-supported copper(I) hydride dimer reacts with NO
to yield an ammonium ion, suggesting the formation and
decomposition of HNO. These reactions model possible
pathways in the generation and subsequent reactivity of
+
temperature, also resulted in decomposition to copper(0) and
1
a single set of resonances in the H NMR spectrum (CD Cl ).
2
2
1
9
The F NMR spectrum revealed a very broadened and
−
upfield-shifted BF4 resonance. Following the addition of Ph3P
C
DOI: 10.1021/acs.inorgchem.9b01676
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
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AUTHOR INFORMATION
Corresponding Author
E-mail: joseph.sadighi@chemistry.gatech.edu.
ORCID
Joseph P. Sadighi: 0000-0003-1304-1170
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
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We thank the Georgia Institute of Technology School of
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generous support of this research. We thank the U.S. National
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DOI: 10.1021/acs.inorgchem.9b01676
Inorg. Chem. XXXX, XXX, XXX−XXX