S-nitrosothiol and nitric oxide reactivity at β-diketiminato zinc thiolates

Article abstract of DOI:10.1016/j.ica.2006.07.049

The β-diketiminato zinc halide [Me₂NN]ZnCl₂Li(THF)₃ (1) is prepared in 51% isolated yield by addition of the lithium β-diketiminate Li[Me₂NN] to ZnCl₂ in THF. Reaction of 1 with 2 equiv. of the thallium thiolate TlSCy provides {[Me₂NN]Zn(μ-SCy)₂Tl}₂ (2), a TlSCy adduct of [Me₂NN]ZnSCy, as colorless crystals in 51% yield. Reaction of 1 with 1 equiv. TlSR provides the dinuclear {[Me₂NN]Zn(μ-SR)}₂ (R = Cy (3), ^tBu (4)) which possess unsymmetrically bridging thiolate ligands with pairs of dissimilar Zn-S distances in the solid state (2.350(3) and 2.417(3) A? for 3; 2.312(1) and 2.415(1) A? for 4). Reaction of 1 with LiSCPh₃ results in the mononuclear zinc thiolates [Me₂NN]ZnSCPh₃(THF) (5) and [Me₂NN]ZnSCPh₃ (6) with shorter, but similar Zn-SR distances of 2.225(2) and 2.214(1) A?. Variable temperature ¹H NMR studies of 3 and 4 in CDCl₃ suggest that the aliphatic thiolates exist predominately as monomeric species in solution near room temperature, though at -50 °C two different β-diketiminato species are observed for 3. Thiolate exchange among 3, 4, and 6 also takes place on the NMR timescale near room temperature. Both 4 and 6 undergo transnitrosylation with CySNO in CDCl₃ to give {[Me₂NN]ZnSCy}₂ (3) and the corresponding S-nitrosothiol ^tBuSNO or Ph₃CSNO. Nitric oxide does not react with 4 or 6 under anaerobic conditions, but in the presence of O₂, NO cleaves the zinc-thiolate bond of 4 to rapidly give ^tBuSNO. Similarly, anaerobic NO₂ reacts with 4 to give ^tBuSNO providing insight into the active nitrogen oxide species capable of cleaving Zn-SR bonds.

Full text of DOI:10.1016/j.ica.2006.07.049

Inorganica Chimica Acta 360 (2007) 317–328
www.elsevier.com/locate/ica
S-nitrosothiol and nitric oxide reactivity at b-diketiminato zinc thiolates
*
Matthew S. Varonka, Timothy H. Warren
Department of Chemistry, Georgetown University, Box 571227, Washington, DC 20057-1227, USA
Received 26 June 2006; received in revised form 10 July 2006; accepted 10 July 2006
Available online 29 July 2006
Inorganic Chemistry – The Next Generation.
Abstract
The b-diketiminato zinc halide [Me2NN]ZnCl2Li(THF)3 (1) is prepared in 51% isolated yield by addition of the lithium b-diketiminate
Li[Me2NN] to ZnCl2 in THF. Reaction of 1 with 2 equiv. of the thallium thiolate TlSCy provides {[Me2NN]Zn(l-SCy)2Tl}2 (2), a TlSCy
adduct of [Me2NN]ZnSCy, as colorless crystals in 51% yield. Reaction of 1 with 1 equiv. TlSR provides the dinuclear {[Me2NN]Zn(l-
t
SR)}2 (R = Cy (3), Bu (4)) which possess unsymmetrically bridging thiolate ligands with pairs of dissimilar Zn–S distances in the solid
˚
˚
state (2.350(3) and 2.417(3) A for 3; 2.312(1) and 2.415(1) A for 4). Reaction of 1 with LiSCPh3 results in the mononuclear zinc thiolates
˚
[Me2NN]ZnSCPh3(THF) (5) and [Me2NN]ZnSCPh3 (6) with shorter, but similar Zn–SR distances of 2.225(2) and 2.214(1) A. Variable
temperature 1H NMR studies of 3 and 4 in CDCl3 suggest that the aliphatic thiolates exist predominately as monomeric species in solu-
tion near room temperature, though at ꢀ50 ꢀC two different b-diketiminato species are observed for 3. Thiolate exchange among 3, 4,
and 6 also takes place on the NMR timescale near room temperature. Both 4 and 6 undergo transnitrosylation with CySNO in CDCl3 to
t
give {[Me2NN]ZnSCy}2 (3) and the corresponding S-nitrosothiol BuSNO or Ph3CSNO. Nitric oxide does not react with 4 or 6 under
anaerobic conditions, but in the presence of O2, NO cleaves the zinc-thiolate bond of 4 to rapidly give tBuSNO. Similarly, anaerobic NO2
reacts with 4 to give BuSNO providing insight into the active nitrogen oxide species capable of cleaving Zn–SR bonds.
t
ꢁ 2006 Elsevier B.V. All rights reserved.
Keywords: Zinc thiolates; S-nitrosothiols; Nitric oxide; b-Diketiminates; Bioinorganic
1. Introduction
teine-rich proteins which normally contain 7 Zn2+ ions
each tetrahedrally coordinated by cysteine donors [7].
Among their many functions [1,8], metallothioneins partic-
ipate in the homeostasis of the Zn2+ ion [1,9,10], releasing
Zn2+ under the influence of mild oxidants such as glutathi-
one disulfide (GSSG) [11,12].
Nitric oxide is closely connected to the activity of zinc-
thiolate based metalloproteins, effecting the release of
Zn2+ or otherwise causing structural changes of functional
consequence. Nitric oxide releases Zn2+ from metallothi-
oneins [9,13–15] and can result in the formation disulfide
bonds [16]. Exposure of whole cells to nitric oxide also gen-
erates free intracellular zinc [17]. Zinc fingers pose an addi-
tional target for nitric oxide attack [16,18–23].
Modification of zinc fingers by nitric oxide can regulate cel-
lular gene expression via inhibition of DNA binding
[18,19], which may be reversible in some cases [19]. Due
Zinc thiolate linkages form important structural compo-
nents in metalloproteins and confer redox activity to sites
containing the non-redox active Zn2+ ion [1]. For instance,
zinc fingers which comprise the most common family of
transcription factors representing ca. 2% of the human gen-
ome [2,3] are highly conserved proteins of approximately
30 amino acids that form two b-sheet and one a-helix held
together by a Zn2+ ion most commonly coordinated to two
Cys residues and two His residues (Fig. 1) [4–6]. The a-
helix of the Zn finger typically resides in the major grove
of the DNA double helix, allowing for recognition of
between 3 and 4 bp. Metallothioneins are an especially cys-
*
Corresponding author. Tel.: +1 202 687 6362; fax: +1 202 687 6209.
E-mail address: thw@georgetown.edu (T.H. Warren).
0020-1693/$ - see front matter ꢁ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2006.07.049

318
M.S. Varonka, T.H. Warren / Inorganica Chimica Acta 360 (2007) 317–328
N
N
N
R
N
N
S
Zn SR
N
Zn
Zn
S
R
{[Me2NN]ZnSR}2
[Me2NN]ZnSR
Fig. 2. Target mononuclear and dinuclear b-diketiminato zinc thiolates.
In order to gain insight into the molecular pathways by
which nitric oxide and its organic derivatives interact with
ubiquitous zinc thiolate linkages, we targeted the synthesis
of complexes of the type [Me2NN]Zn–SR (Fig. 2). Such
species possess one zinc thiolate functional group and a
b-diketiminate ancillary ligand which very roughly approx-
imates double His ligation. Owing to the high bridging pro-
pensity of zinc thiolates, we felt that dinuclear species
would be likely with this ‘‘smaller’’ b-diketiminate; how-
ever, steric tuning through choice of the b-diketiminate
N-aryl groups or the thiolate substituent R could result
in mononuclear, three-coordinate zinc thiolates. While b-
diketiminato zinc alkoxides have been studied for the co-
polymerization of CO2 and epoxides [51–54] as well as
anionic lactam polymerization [55–58], we were unaware
of related zinc thiolates complexes. Herein we describe
the synthesis and characterization of some b-diketiminato
Zn thiolates as well as preliminary account of their differen-
tial reactivity with S-nitrosothiols and NO gas.
Fig. 1. Solution structure of the His2Cys2 zinc finger motif of the human
enhancer binding protein Zif268 [24–26].
to the important roles of zinc fingers in gene regulation as
well as signal transduction, NO and NO-containing species
may be at the molecular basis of a number of cellular dys-
function mechanisms resulting from modification of the
zinc coordination environment [23].
Of the many NO-donors such as diazeniumdiolates
[N2O2NR2]ꢀ [27] used in biological studies of zinc metal-
loproteins discussed above, S-nitrosothiols [28,29] are
perhaps the most biologically relevant [8,30–33], as S-
nitrosocysteine is present at concentrations of 0.2–0.3 lM
in human plasma [34]. In contrast to NOgas, S-nitrosothiols
are O2-stable biological carriers of NO [35–37]. RSNOs
may serve as either sources of NO+ or NOÆ depending
on the reaction conditions, yielding the thiolate anion
RSꢀ or thiyl radical RSÆ subject to dimerization to give
disulfides RSSR (Scheme 1) [28]. The latter pathway
which generates NO gas is very efficiently catalyzed by
Cu+ both in vitro [28,38,39] and in vivo [40]. Maret has
shown that Zn–SR linkages in metallothionein are subject
to cleavage by S-nitrosothiols, releasing Zn2+ ions via
transnitrosylation of the NO+ moiety to metallothionein
cysteine residues (Scheme 1) [41]. This is to be contrasted
with the sparse use of S-nitrosothiols in coordination
chemistry as reagents to deliver NO to lower-valent redox
active metals to form metal-nitrosyls [42–49], though an
intriguing structure of a relatively unreactive S-nitroso-
thiol complex of Ir(III) has recently been reported [50].
2. Results and discussion
2.1. Synthesis and characterization of a b-diketiminato zinc
chloride
b-Diketiminato zinc halides similar to Roesky’s
[iPr2NN]ZnI2Li(OEt2)2 [59], [iPr2NN]ZnCl2Li(OEt2)2 [60],
and {[Me2NN]Zn}2(l-F)2 [61] offer the potential to provide
the corresponding Zn thiolates via salt metathesis reac-
tions. We find that deprotonation of the b-diketimine
H[Me2NN] with BuLi in THF to generate Li[Me2NN] fol-
lowed by addition to a stirring solution of ZnCl2 in THF
gives the colorless [Me2NN]ZnCl2Li(THF)3 (1) in 51%
yield after crystallization (Scheme 2). The X-ray structure
of 1 contains two independent molecules of 1 in a 2:1 ratio
(Figs. 3 and 4; Tables 1 and 2). While zinc shows tetrahe-
dral coordination in each of these two molecules, the nat-
ure of the Li–Cl interactions differs. Molecule 1 (Fig. 3)
exhibits distinct Zn–Cl distances (Zn1–Cl1 = 2.312(1) and
Cu+
RS- + NO+
RS-NO
1/2 RSSR + NO
˚
Zn1–Cl2 = 2.238(1) A) which reflect coordination of the
NO
S
S
S
S
2 RSNO
Zn
+ Zn2+ + 2 RS-
NO
1) BuLi / THF
H[Me2NN]
[Me2NN]ZnCl2Li(THF)3
2) ZnCl2
1
Scheme 1. Reactivity patterns of S-nitrosothiols (top) and schematic
depiction of transnitrosylation with metallothionein (bottom) [41].
Scheme 2. Synthesis of [Me2NN]ZnCl2Li(THF)3 (1).

M.S. Varonka, T.H. Warren / Inorganica Chimica Acta 360 (2007) 317–328
319
˚
by more than 3.2 A. The second molecule of 1 (Fig. 4) is
located on a plane of symmetry containing Zn2, Cl3, Cl4,
and Li2, relating the ‘‘top’’ and ‘‘bottom’’ halves of the
molecule. This molecule contains similar Zn–Cl distances
˚
(Zn2–Cl3 = 2.268(2) and Zn2–Cl4 = 2.288 (2) A) reflecting
interaction of the Li(THF)3 cation (also containing disor-
dered THFs) with both Cl atoms (Li2–Cl3 = 2.780(11)
1
˚
and 2.471(12) A). H NMR spectroscopy of 1 in CDCl3
confirmed the presence of THF in 1, though in the solid
state THF is easily lost in vacuo, perhaps to reflecting for-
mation of a species related to [iPr2NN]ZnCl2Li(OEt2)2 [60].
It should be noted that prolonged standing of 1 in vacuo
gives a solid insoluble in pentane, toluene, and CH2Cl2.
2.2. Preparation and structure of a Tl-thiolate adduct of
[Me2NN]ZnSR
Owing to the affinity that the b-diketiminato zinc frag-
ment of 1 has for LiCl, we attempted to prepare zinc thio-
lates via thallium reagents TlSR which are easily prepared
as yellow solids from the reaction of the corresponding
thiol HSR with TlOEt. Due to the relative insolubility of
TlSCy in THF, initial exploration of the reactivity of 1
employed excess TlSCy. The addition of 2 equiv. TlSCy
to 1 in THF results in the isolation of {[Me2NN]Zn(l-
SCy)2Tl}2 (2) from dichloromethane in 51% yield (Scheme
3). Recrystallization from pentane provides crystals of 2 Æ 2
pentane suitable for X-ray diffraction. The X-ray structure
Fig. 3. X-ray structure of [Me2NN]ZnCl2Li(THF)3 (1) – molecule 1.
Table 2
Crystallographic data, collection parameters, and refinement details for 1
and 2
Compound
1
2
Formula
C49.5H68.5Li1.5N3O4.5Zn1.5 C33H47N4S2TlZn
Molecular weight
Temperature (K)
Crystal description
Crystal color
Crystal size (mm)
System
992.39
173(2)
block
colorless
0.42 · 0.40 · 0.39
orthorhombic
Pnma
805.59
173(2)
chunk
colorless
0.36 · 0.20 · 0.16
triclinic
Fig. 4. X-ray structure of [Me2NN]ZnCl2Li(THF)3 (1) – molecule 2.
ꢀ
Space group
P1
˚
Table 1
a (A)
14.539(1)
58.924(4)
12.252(1)
90.00
90.00
90.00
11.479(12)
12.973(13)
14.569(15)
88.611(16)
79.615(16)
64.265(14)
2
2.00–28.29
8714
6649
˚
˚
Bond distances (A) and angles (ꢀ) in 1
b (A)
˚
c (A)
Molecule 1
Molecule 2
a (ꢀ)
b (ꢀ)
c (ꢀ)
Z
Bond distances
Zn1–N1
Zn1–N2
Zn1–Cl1
Zn1–Cl2
Cl1–Li1
Cl2–Li1
1.981(3)
1.988(3)
2.312(1)
2.238(1)
2.372(8)
Zn2–N3
Zn2–N30
Zn2–Cl3
Zn2–Cl4
Cl3–Li2
Cl4–Li2
1.973(3)
1.973(3)
2.268(2)
2.288(2)
2.780(1)
2.471(1)
8
h Range (ꢀ)
Measured reflections
Unique reflections
Rint
1.96–28.30
13069
8209
0.0953
0.0546
Goodness-of-fit of F2
R1 (I > 2r(I))
wR2 (all data)
Largest difference in
1.085
0.0725
0.1674
0.735 and ꢀ2.373
0.960
0.0475
0.1104
3.789 and ꢀ1.390
Bond angles
N1–Zn1–N2
Cl1–Zn1–Cl2
98.19(13)
103.99(4)
N3–Zn2–N30
Cl3–Zn2–Cl4
96.9(2)
100.23(6)
ꢀ3
˚
A )
peak and hole (eꢀ
Li(THF)3 cation (possessing disordered THFs) to Cl1 with
a Li1–Cl1 distance of 2.372(8) (Table 1). No significant
interaction exists between Li1 and Cl1 which are separated
SQUEEZE subroutine of Platon (A.L. Spek, Acta Crystallogr., Sect. A 46
(1990) C-34) employed in refinement of 2 which possesses 2 equiv. of
disordered pentane per molecule of dinuclear 2.

320
2 [Me2NN]ZnCl2Li(THF)3
M.S. Varonka, T.H. Warren / Inorganica Chimica Acta 360 (2007) 317–328
[62]. Completing pseudo four-coordination at Tl in 2 is an
interaction with one b-diketiminato N-aryl ring attached to
N2 with a Tl-centroid distance of 3.449(8) A [63]. A long
THF
+
1
{[Me2NN]Zn(μ-SCy)2Tl}2
˚
4 TlSCy
2
˚
Tlꢁ ꢁ ꢁTl separation of 4.433(3) A rules out any direct inter-
Scheme 3. Formation of the Zn–Tl bis(thiolate) {[Me2NN]Zn(l-
SCy)2Tl}2 (2).
action between the two Tl centers [63–65].
2.3. Preparation and structures of dinuclear Zn thiolates
t
{[Me2NN]ZnSR}2 (R = Cy, Bu)
of 2 may be conceptually viewed as an equivalent of TlSCy
docked up to the desired three-coordinate [Me2NN]ZnSCy
fragment to give a tetrahedral Zn center (Fig. 5; Table 2).
Addition of 1 equiv. of the thallium thiolates TlSR to 1
˚
The Zn–N distances (2.007(5) and 2.029(5) A) in 2 are
in THF or dichloromethane results in the isolation of the
t
longer than in 1, consistent with the stronger donor ability
of the thiolate ligand as compared to the chlorides in 1
(Table 3). These thiolate ligands are symmetrically bound
to the zinc center with Zn–S distances of 2.369(3) and
dinuclear {[Me2NN]ZnSR}2 (R = Cy (3) or Bu (4)) in
44% and 36% yield as colorless crystals from dichlorometh-
ane (Scheme 4). The X-ray crystal structures of the dinu-
clear 3 and 4 (Figs. 6 and 7; Table 6) each show
tetrahedral coordination at zinc through bridging of the
thiolate ligand. Both 3 and 4 possess unsymmetrical Zn–
SR linkages. The cyclohexyl derivative 3 displays two dis-
˚
2.370(2) A. Each thiolate bound to Zn also bridges to the
Tl atom with Tl–S1 and Tl–S2 distances 2.943(2) and
˚
3.132(3) A. The Tl ion also engages another thiolate bound
to a symmetry-related {[Me2NN]Zn(SCy)2}ꢀ fragment
tinct Zn–S bond lengths of 2.350(3) and 2.417(3) A while
˚
0
˚
with a Tl–S2 distance of 3.088(2) A. The Tl–SCy coordina-
tion environment in 2 compares favorably that in TlSCy
itself which possesses three-coordinate, trigonal pyramidal
4 exhibits an even greater disparity between the two
˚
Zn–S distances of 2.312(1) and 2.415(1) A. Coupled with
the wider S–Zn–S0 angle in 4 (109.18(6)ꢀ) versus 3
(90.22(10)ꢀ), this may suggest a looser interaction between
the two [Me2NN]Zn–SR units in 4 than 3 (Table 4). Bridg-
˚
Tl centers with Tl–S distances of 2.826(3)–3.034(2) A and
2.965(2)–3.133(2) A for l3- and l4-SCy groups, respectively
˚
{[Me NN]ZnSR}
2
32R = Cy
TlSR
THF
t
4 R = Bu
[Me NN]ZnCl Li(THF)
3
[Me NN]ZnSCPh (THF)
2
2
2
3
LiSCPh
3
1
5
recryst
ether
[Me NN]ZnSCPh
2
3
6
Scheme 4. Synthesis of dinuclear and mononuclear zinc thiolates 3–6.
Fig. 5. X-ray structure of {[Me2NN]Zn(l-SCy)2Tl}2 (2).
Table 3
˚
Bond distances (A) and angles (ꢀ) in 2 Æ 2 pentane
Bond distances
Zn–N1
Zn–N2
Zn–S1
Zn–S2
S1–C22
2.007(5)
2.029(5)
2.370(2)
2.369(3)
1.859(6)
S2–C28
Tl–S1
Tl–S2
Tl–S20
1.865(6)
2.943(2)
3.088(2)
3.132(3)
Bond angles
N1–Zn–N2
S1–Tl–S2
95.7(2)
94.34(8)
89.10(7)
S1–Zn–S2
S1–Tl–S20
71.37(7)
96.94(8)
Fig. 6. X-ray structure of {[Me2NN]ZnSCy}2 (3) – major occupancy of
disordered cyclohexane ring shown.
S2–Tl–S20

M.S. Varonka, T.H. Warren / Inorganica Chimica Acta 360 (2007) 317–328
321
˚
2.187(3) A is at the high end of the range of Zn–O bonds
˚
in a 4-coordinate Zn center bound to THF of 2.12(9) A
˚
(n = 70) [70,71]. The Zn1–S1 bond length of 2.225(2) A is
significantly shorter than any Zn–S bond in the dinuclear
2–4.
If instead crystallization is performed in ether rather
than in THF during the synthesis of 5, the three-coordi-
nate [Me2NN]ZnSCPh3 (6) may be isolated (Scheme 4;
Fig. 9; Table 6). While the Zn–N distances are contracted
˚
to 1.936(2) and 1.949(2) A as a reflection of the lower
coordination number as compared to 5, only a minimally
˚
shortened Zn–S distance Zn–S1 2.2141(7) A results which
is a testament to the weak interaction of the THF in 5
(Table 5). The three-coordinate [Me2NN]ZnSCPh3 pos-
sesses a sum of angles about Zn of 355.46(9)ꢀ with the
˚
Zn atom out of the N1–N2–S1 plane by 0.245 A. This
departure from idealized trigonal planar geometry is
probably a reflection of the steric demands of the trityl
group exacerbated by the Zn–S–C22 angle of 111.22(8)ꢀ
resistant to further widening owing to the large S lone
pairs. [Me2NN]ZnSCPh3 represents a rare example of a
mononuclear, three-coordinate zinc thiolate alongside
Fig. 7. X-ray structure of {[Me2NN]ZnStBu}2 (4).
ing thiolates in related four-coordinate complexes may
either be2ꢀsymmetrical such as in ½ZnCl2ðl-SCH2-
˚
CH2OHÞꢂ2 (Zn–S = 2.376 and 2.387 A) [66] or unsym-
metrical as in the dimeric {Zn[2,6-(SCPh2CH2)2py]}2
˚
(Zn–S = 2.349 and 2.423 A for l-SR groups) [67]. The
cyclohexyl rings of 3 are disordered in the solid state such
that the major conformer contains the S atom in an axial
site (72%) and the minor conformer with the S atom in
an equatorial site (28%).
2.4. Preparation and structures of mononuclear Zn thiolates
In an attempt to isolate a mononuclear, three-coordi-
nate Zn thiolate, we employed the sterically demanding
trityl group [68,69]. Addition of 1 equiv. LiSCPh3 to 1 in
THF results in isolation of the colorless [Me2NN]-
ZnSCPh3(THF) (5) in 37% yield as colorless crystals from
THF/pentane (Scheme 4). The X-ray crystal structure of
5 (Fig. 8; Table 6) contains a Zn center best described
as ‘‘flattened’’ trigonal pyramidal – the THF ligand in
5 coordinates to a [Me2NN]ZnSCPh3 moiety which
approaches trigonal planar coordination. The sum of the
angles about Zn to N1, N2, and S1 is 349.4(1)ꢀ and the
˚
Fig. 8. X-ray structure of [Me2NN]ZnSCPh3(THF) (5).
Zn center is out of the N1–N2–S1 plane by only 0.356 A
(Table 5). Additionally, the Zn1–O1 bond length of
Table 5
˚
Table 4
Bond distances (A) and angles (ꢀ) in 5 and 6
˚
Bond distances (A) and angles (ꢀ) in 3 and 4
5
6
3
4
Bond distances
Zn–N1
Zn–N2
Zn–S
S–C22
Zn–O
Bond distances
Zn–N1
Zn–N2
Zn–S
Zn–S0
1.991(2)
1.956(2)
2.2248(16)
1.866(3)
2.192(2)
1.936(2)
1.949(2)
2.2142(6)
1.874(2)
1.996(6)
2.001(6)
2.350(3)
2.417(3)
1.890(11)
1.993(2)
1.985(2)
2.312 (1)
2.415(1)
1.860(3)
S–C22(A)
Bond angles
N1–Zn–N2
N1–Zn–S
Bond angles
N1–Zn–N2
S–Zn–S0
97.70(10)
105.52(8)
146.16(7)
98.70(9)
134.13(6)
122.63(6)
97.1(3)
96.21(8)
90.22(10)
109.18(6)
N2–Zn–S

322
M.S. Varonka, T.H. Warren / Inorganica Chimica Acta 360 (2007) 317–328
2.271, and 1.686 ppm for the backbone-CH, Ar–Me, and
backbone-Me groups. The cyclohexyl moiety gives rise to
many aliphatic signals, the most characteristic of which is
a multiplet at d 2.596 ppm corresponding to the Zn–SCH
methine resonance. The sharpness of the b-diketiminato
resonances, coupled with the insolubility of TlSCy in chlo-
roform, renders it unlikely that a TlSCy moiety dissociates
from a monomeric [Me2NN]Zn(l-SCy)2Tl unit in solution.
The 1H NMR spectra of zinc thiolates 3 and 4 in CDCl3
are particularly informative. While the b-diketiminato
1
backbone-CH, Ar–Me, and backbone-Me H NMR sig-
nals at d 5.05, 2.17, and 1.74 ppm for {[Me2NN]ZnStBu}2
(4) are sharp at room temperature, the corresponding
resonances at d 4.97, 2.15, and 1.69 ppm for {[Me2NN]-
ZnSCy}2 (3) are broad at room temperature. Cooling a
solution of 3 to ꢀ50 ꢀC results in the identification of
two separate backbone-CH resonances at d 4.97 (major)
and 4.70 (minor) ppm. Moreover, the ratio of the major
and minor peak increases with increasing temperature,
coalescing near 10 ꢀC to give a broad signal near d
5.01 ppm which sharpens with increasing temperature.
We interpret this behavior as stemming from the revers-
ible dissociation of the dinuclear 3 into mononuclear
[Me2NN]ZnSCy fragments which predominate in solution
at room temperature (Scheme 5). This process is also con-
centration dependent, giving qualitatively broader back-
Fig. 9. X-ray structure of [Me2NN]ZnSCPh3 (6).
the anion [Zn(SAr0)3]ꢀ (Ar0 = 2,3,5,6-Me4C6H1) [72] and
the neutral Zr(SAr*)2L (Ar* = 2,4,6-tBu3C6H2; L = ether
[73], 2,6-lutidine, PPh2Me) [74] with Zn–S distances of
˚
2.194–2.242 A.
1
2.5. Solution behavior of b-diketiminato zinc thiolates
bone-CH H NMR signals at room temperature in more
dilute solutions of 3. In contrast, cooling CDCl3 solutions
of 4 to ꢀ50 ꢀC results in no change in the shape or number
as the mononuclear [Me2NN]ZnStBu in solution over a
broader temperature range. This greater tendency towards
1H NMR spectra of the zinc thiolates 2–6 give impor-
tant clues to their solution behavior. Solutions of
{[Me2NN]Zn(l-SCy)2Tl}2 (2) in CDCl3 show sharp 1H
NMR signals for the b-diketiminato fragment at d 4.832,
1
of backbone-CH H NMR signals suggesting that 4 exists
Table 6
Crystallographic data, collection parameters, and refinement details for 3–6
Compound
3
4
5
6
Formula
C27H36N2SZn
486.01
173(2)
C25H34N2SZn
459.97
173(2)
C44H48N2OSZn
718.27
173(2)
chunk
colorless
C40H40N2SZn
646.17
173(2)
Molecular weight
Temperature (K)
Crystal description
Crystal color
Crystal size (mm)
System
plate
block
needle
colorless
0.16 · 0.10 · 0.10
monoclinic
P2(1)/n
13.834(11)
12.493(10)
14.802(12)
90.00
colorless
0.24 · 0.22 · 0.22
triclinic
colorless
0.36 · 0.19 · 0.16
monoclinic
P2(1)
8.4712(8)
19.2347(18)
10.7615(10)
90.00
0.38 · 0.22 · 0.14
triclinic
ꢀ
ꢀ
Space group
P1
P1
˚
a (A)
10.606(5)
10.943(5)
12.117(6)
76.916(7)
70.045(7)
66.813(7)
2
10.248(9)
13.089(11)
14.838(13)
98.641(13)
94.487(14)
107.709(13)
2
˚
b (A)
˚
c (A)
a (ꢀ)
b (ꢀ)
c (ꢀ)
Z
103.487(1)
90.00
4
106.650(1)
90.00
4
h Range (ꢀ)
1.90–25.00
4408
2087
0.1361
0.908
1.80–27.00
5224
4105
0.0406
1.048
1.97–26.00
7154
4865
0.0611
1.002
1.98–28.31
7859
6203
0.0407
0.962
Measured reflections
Unique reflections
Rint
Goodness-of-fit of F2
R1 (I > 2r(I))
0.0796
0.0396
0.0384
0.0366
wR2 (all data)
Largest difference in peak and hole (eꢀ
0.2309
1.213 and ꢀ1.243
0.0996
0.783 and ꢀ0.490
0.1035
0.957 and ꢀ1.301
0.0716
0.577 and ꢀ0.302
ꢀ3
˚
A
)

M.S. Varonka, T.H. Warren / Inorganica Chimica Acta 360 (2007) 317–328
323
t
t
CDCl3
{[Zn]-SCy}
2 [Zn]-SCy
{[Zn]-S Bu}
2 [Zn]-S Bu
2
2
TlSR + [NO]BF4
RSNO + TlBF4
R = Cy or tBu
3
4
Scheme 5. Dissociation of dinuclear thiolates 3 and 4 in CDCl3.
t
Scheme 7. In situ generation of S-nitrosothiols CySNO and BuSNO.
tBu
[Zn] StBu
+
[Zn] SR
+
[Zn'] StBu
{[Me2NN]ZnSCy}2
CDCl3
S
2 [Me2NN]ZnSCPh3 + 2 CySNO
+
3
[Zn]
[Zn']
2 Ph3CSNO
6
S
[Zn'] SR
R
R = Cy or CPh3
Scheme 8. Transnitrosylation of 6 with CySNO favors formation of 3.
Scheme 6. Thiolate exchange between [Me2NN]Zn fragments in CDCl3.
{[Me2NN]ZnStBu}2
{[Me2NN]ZnSCy}2
CDCl3
+
3
+
4
dissociation to the mononuclear zinc thiolate is consistent
with the greater steric bulk of the thiolate in 4 over that
in 3. Moreover, this solution behavior mirrors that of the
b-diketiminato zinc alkoxide [iPr2NN]Zn–OiPr possessing
o-iPr substituents on the N-aryl rings which is dimeric in
the solid state [55] but exists predominately in its mono-
meric form in solution [57].
2 tBuSNO
2 CySNO
Scheme 9. Transnitrosylation of 4 with CySNO reversibly generates 3 and
tBuSNO.
CDCl3 in the presence of an internal standard (naphtha-
1
lene) and used immediately (Scheme 7). Characteristic H
In CDCl3, the 1H and 13C NMR spectra for
[Me2NN]ZnSCPh3(THF) (5) and [Me2NN]ZnSCPh3 (6)
are indistinguishable except for the presence of free THF
in samples of 5. For instance, each gives a sharp back-
bone-CH resonance at d 4.96 ppm at room temperature.
This strongly suggests that 6 exists as the three-coordinate
[Me2NN]ZnSCPh3 in chloroform, not surprising since crys-
tals of the three-coordinate 6 may be isolated by recrystal-
lizing 5 from ether.
NMR signals for these S-nitrosothiols in this solvent con-
sist of a broad multiplet at d 4.34 ppm for the S-CH group
t
of CySNO and a sharp singlet at d 1.91 ppm for BuSNO.
t
Qualitatively, BuSNO is more stable against loss of spon-
taneous loss of NO to give the corresponding disulfide than
CySNO, consistent with the previously observed trend that
the kinetic stability of S-nitrosothiols increases with
increasing substitution at the a-C atom [76,77].
The addition of 1 equiv. red CySNO to colorless
[Me2NN]ZnSCPh3 (6) in CDCl3 leads to a color change
to green and the formation of {[Me2NN]ZnSCy}2 (3) with
Furthermore, thiolate exchange between [Me2NN]-
ZnStBu and [Me2NN]ZnSCy as well as [Me2NN]ZnStBu
and [Me2NN]ZnSCPh3 is facile in CDCl3 near room tem-
1
less than 5% of 6 remaining as monitored by H NMR
1
perature (Scheme 6). Even though the H NMR spectrum
spectroscopy (Scheme 8). The new S-nitrosothiol formed
was Ph3SNO, isolated as green crystals from the solution
that were identified by X-ray crystallography [78]. Reaction
of {[Me2NN]ZnStBu}2 (4) with 1 equiv. CySNO also results
in the predominate formation of {[Me2NN]ZnSCy}2 (3)
of {[Me2NN]ZnStBu}2 (4) exhibits a single, sharp backbone
C–H resonance at room temperature, mixture of 3 and 4
(each ca. 0.06 mM) gives rise to a single broad backbone-
CH 1H NMR signal at d 5.03 ppm indicating that exchange
of thiolate ligands between these two types of Zn centers
takes place on the NMR timescale. Mixture of 4 and 6 (each
ca. 0.06 mM) gives rise to two broadened signals at d 5.05
and 4.97 ppm, near the expected positions for the backbone
C–H resonances of 4 and 6. Warming to 50 ꢀC, however,
results in coalescence of these signals as well as further
broadening of the separate pairs of Ar–Me and backbone-
Me resonances for 4 and 6. Thus, thiolate exchange also
takes place between the [Me2NN]Zn fragments of
[Me2NN]ZnStBu and [Me2NN]ZnSCPh3, perhaps some-
what hindered by the bulky nature of these two thiolates.
t
along with BuSNO, though the extent of exchange is not
complete as in the reaction of CySNO with 6; some unre-
acted CySNO remains after 15 min indicating the presence
of an equilibrium (Scheme 9). These scouting reactions
suggest that one driving force in the reaction between these
zinc thiolates and S-nitrosothiols is the formation of the less
hindered Zn thiolates. We unfortunately cannot further test
this thermodynamic preference via similar reactions of these
zinc thiolates with free thiols HSR (R = Cy, tBu, CPh3). All
thiols we have explored rapidly and irreversibly protonate
the b-diketiminate ligand of 3–6 to give the free b-diketimine
H[Me2NN].
2.6. Reactions of b-diketiminato Zn thiolates with S-
nitrosothiols
2.7. Reactions of b-diketiminato Zn thiolates with nitric
oxide
Owing to the thermally sensitive nature of the CySNO
t
and BuSNO which are red and green liquids, respectively,
Bubbling several equivalents of NO gas into CDCl3
solutions of the b-diketiminato Zn thiolates 4 and 6 under
anaerobic conditions leads to no reaction as monitored by
1H NMR. No new signals appear, and no significant
each of these aliphatic S-nitrosothiols was generated imme-
t
diately prior to use [28,29,75]. CySNO and BuSNO were
prepared in situ from [NO]BF4 and TlSCy or TlStBu in

324
M.S. Varonka, T.H. Warren / Inorganica Chimica Acta 360 (2007) 317–328
NO
nately as the monomeric [Me2NN]ZnStBu in solution, sim-
no reaction
t
{[Me NN]ZnS Bu}
CDCl
ilar to [Me2NN]ZnSCPh3 (6) which is three-coordinate in
the solid state. Even for the bulkier thiolates
[Me2NN]ZnStBu and [Me2NN]ZnSCPh3, formation of
mixed dinuclear structures in solution is facile, as thiolate
exchange among 3, 4, and 6 take place on the NMR time-
scale near room temperature.
2
2
3
4
t
BuSNO
NO / O
or
2
NO
2
Scheme 10. Reaction of zinc thiolate 4 with NO (aerobic) and NO2
t
(anaerobic) forms BuSNO.
These b-diketiminato zinc thiolates undergo rapid trans-
nitrosylation with S-nitrosothiols, favoring the formation
of the least hindered thiolate [Me2NN]ZnSCy when
[Me2NN]ZnStBu or [Me2NN]ZnSCPh3 are exposed to an
equivalent of CySNO in CDCl3. Under anaerobic condi-
tions, nitric oxide does not react with [Me2NN]ZnStBu or
[Me2NN]ZnSCPh3 in CDCl3, though addition NO to diox-
ygen-saturated solutions of 4 rapidly results in the cleavage
of the Zn–StBu linkage to give tBuSNO. Reaction of 4 with
anaerobic NO2 also results in the rapid formation of
tBuSNO, suggesting that NO2 (or N2O4) may be the active
electrophilic nitrosating agent in the aerobic reactions of
NO with zinc thiolates. Thus, an oxidant is required for
zinc thiolates to react with nitric oxide – an important con-
sideration in evaluating the NO reactivity of zinc enzymes.
Moreover, transnitrosylation and nitrosation at these puta-
tively three-coordinate species in solution is essentially
instantaneous at room temperature, effecting heterolytic
cleavage of the Zn–SR linkage much faster than with alkyl-
ating reagents such as (MeO)3PO and MeI which often
require reaction times of several hours at room tempera-
ture and above [84–86].
changes occur in the appearance of the sharp resonances
for 4 and 6. First saturating CDCl3 solutions of 4 with
dioxygen, however, followed by addition of 2 equiv. NO
results in immediate coloration of the solution to green
and formation of a colorless precipitate (Scheme 10). The
t
green color results from BuSNO as shown by its 1H
NMR signal at d 1.89 ppm.
Colorless NO rapidly reacts with dioxygen to give NO2
[79], a brown, gaseous radical that often exhibits nitroso-
nium-like reactivity owing to its dimerization to N2O4
which may disproportionate to [NO]+[NO3]ꢀ in its reac-
tions with inorganic [80] and organic [81–83] molecules.
The addition of anaerobic NO2 gas to a CDCl3 solution
of 4 under nitrogen gives similar results; rapid formation
t
of green BuSNO occurs with concomitant formation of a
colorless precipitate (Scheme 10). The reaction of NO2 with
t
zinc thiolate 4 to generate BuSNO bears semblance to the
stoichiometric addition of N2O4 to thiols HSR which is an
efficient means to generate S-nitrosothiols RSNO [75]. In
either reaction of 4 with aerobic NO or anaerobic NO2,
we do not know the fate of the [Me2NN]Zn moiety, ham-
pered by the insolubility of the precipitate that forms. This
is unfortunate, for it could offer insight into the nature of
the active nitrosating reagent as the {[Me2NN]Zn}+ frag-
The above reactions represent rare examples of nitric
oxide reactivity at zinc thiolates in model complexes. While
thiols RSH and thiolate anions RSꢀ are known to undergo
transnitrosylation with S-nitrosothiols [87–91], well-
defined transnitrosylation reactions of RSNOs have little
precedent in transition metal chemistry. Instead, RSNOs
react with lower-valent, redox-active metal complexes to
give metal nitrosyls [42–49] as in the case of Fe2(CO)9
which leads to Fe(CO)2(NO)2 as well as [Fe(NO)2(SR)2]2
[43]. The reversible transnitrosylation observed between
[Me2NN]ZnStBu and CySNO to give [Me2NN]ZnSCy
ꢀ
ments might be expected to combine with NOx anions
generated in this reaction. Nonetheless, this simple reaction
unambiguously demonstrates that nitric oxide alone does
not cleave the Zn–SR bonds in these simple model com-
plexes – an oxidant such as dioxygen is required.
3. Conclusion
t
and BuSNO suggest that S-nitrosothiols can rapidly and
b-Diketiminato zinc thiolates 3–6 which are either dinu-
clear or mononuclear in the solid state may be prepared by
addition of 1 equiv. thallium or lithium thiolates to
[Me2NN]ZnCl2Li(THF)3. The use of excess TlSCy results
in the formation of {[Me2NN]Zn(l-SCy)2Tl}2 (2), a TlSCy
adduct of [Me2NN]ZnSCy which dimerizes via Tl–SR
bridging interactions. While the aliphatic thiolates
reversibly modify zinc thiolate linkages in other systems,
provided that the new S-nitrosothiol formed is stable
towards decomposition under the conditions employed.
While this family of b-diketiminato zinc thiolates illus-
trates fundamental features of S-nitrosothiol and nitric
oxide reactivity at well-defined zinc thiolates, this system
possesses limitations that deserve note. The tendency of
the aliphatic thiolates, particularly the cyclohexyl derivative
[Me2NN]ZnSCy, to undergo dimerization in the solid state
and solution makes direct comparisons difficult to the
mononuclear four-coordinate zinc centers found in zinc fin-
gers and many other zinc metalloproteins. For instance, the
use of a primary S-nitrosothiol R0SNO with [Me2NN]ZnSR
(R = tBu or CPh3) might be expected to generate a more
tightly bound dinuclear species {[Me2NN]ZnSR0}2 which
would cloud assessment of thermodynamic preferences for
t
{[Me2NN]ZnSR}2 (R = Cy (3) or Bu (4)) are dimeric in
1
the solid state, H NMR studies of 3 and 4 in CDCl3 sug-
gest that dissociation to the monomeric thiolates
[Me2NN]ZnSR occurs in solution. Only one b-diketimi-
nato zinc species is observed for the t-butyl thiolate 4 from
ꢀ50 ꢀC to RT, but two separate b-diketiminato species are
observed at ꢀ50 ꢀC for the less hindered cyclohexyl thio-
late 3 which interconvert on the NMR timescale at room
temperature. The findings suggest that 4 exists predomi-

M.S. Varonka, T.H. Warren / Inorganica Chimica Acta 360 (2007) 317–328
325
1
transnitrosylation. While it is clear that aerobic NO and
anaerobic NO2 rapidly cleave the zinc thiolate bond in
[Me2NN]ZnStBu to give tBuSNO, the fate of the zinc frag-
ment is unknown, its characterization hampered by poor
solubility. Perhaps the use of larger ancillary ligands would
allow the isolation of the resulting zinc complex which
would give important insight into the actual nitrogen oxide
species responsible for zinc thiolate cleavage. For example,
a zinc nitrate might be expected if the actual nitrosating spe-
cies is N2O4 reacting as [NO]+[NO3]ꢀ. We are currently
addressing these considerations in our laboratory through
the use of more sterically demanding model systems which
will be reported in due course.
(51%) of the product. H NMR (CDCl3): d 6.963–6.883
(m, 6, Ar-H), 4.738 (s, 1, backbone-CH), 3.567 (m, 8–12,
THF-CH2), 2.148 (s, 15, Ar-CH3), 1.688 (m, 8–12, THF-
CH2). 1.525 (s, 6, backbone-CH3); 13C{1H} NMR
(CDCl3): d 166.83, 147.25, 132.40, 127.75, 123.90, 92.27,
67.97, 25.67, 22.98, 18.85. Solid samples of 1 after drying
in vacuo contain a variable amount of THF (1.5–3 mole-
cules THF/Li) leading to irreproducible elemental analyses.
4.1.2. Synthesis of {[Me2NN]Zn(l-SCy)2Tl}2 (2)
A suspension of TlSCy (0.433 g, 1.51 mmol) in 10 mL
THF was added with stirring to
a
solution of
[Me2NN]ZnCl2Li(THF)3 (0.500 g, 0.754 mmol) in 10 mL
THF. After stirring overnight, the solution was filtered
over Celite and then the solvent was removed in vacuo.
The resulting solid was taken up in 10 mL of CH2Cl2 and
filtered over Celite. The solvent was removed and the solid
was washed with 5 mL of pentane to yield 0.310 g (51%) of
white powder. Recrystallization from pentane affords crys-
tals of 2 Æ 2 pentane used for X-ray and elemental analyses.
1H NMR (CDCl3): d 7.062–6.979 (m, 6, Ar-H), 4.832 (s, 1,
backbone-CH), 2.596 (br, 2, Cy-H), 2.271 (s, 12, Ar-CH3),
1.686 (s, 6, backbone-CH3), 1.530 (br, 10, Cy-CH2), 1.050
(br, 10, Cy-CH2); 13C{1H} NMR (CDCl3): d 167.10,
147.82, 132.94, 128.71, 124.75, 93.80, 40.96, 40.55, 27.47,
25.65, 23.21, 19.49. Anal. Calc. for 2 Æ 2 pentane (1 pen-
tane/Zn) C38H59N2S2TlZn: C, 51.99; H, 6.77; N, 3.99.
Found: C, 51.96; H, 6.49; N, 3.77%.
4. Experimental
4.1. General experimental details
All experiments were carried out in a dry nitrogen atmo-
sphere using an MBraun glovebox and/or standard Schlenk
techniques. 4A molecular sieves were activated in vacuo at
180 ꢀC for 24 h. Dry toluene and dichloromethane were pur-
chased from Aldrich and was stored over activated 4A
molecular sieves under nitrogen. Diethyl ether, tetrahydro-
furan (THF) and pentane were sparged with nitrogen and
purified by passage through activated alumina columns
before use [92]. All deuterated solvents were sparged with
nitrogen, dried over activated 4A molecular sieves and
1
stored under nitrogen. H and 13C spectra were recorded
on a Inova Varian 300 MHz spectrometer (300 and
75.4 MHz, respectively) at 25 ꢀC unless otherwise noted.
1H and 13C NMR spectra were indirectly referenced to
TMS using residual solvent signals as internal standards.
All thiols were obtained from Acros, anhydrous ZnCl2 from
Strem, TlOEt from Aldrich, and NO and NO2 gases from
MG Industries; all were used as received. 2,4-Bis(2,6-dimeth-
ylphenyllimido)pentane, (H[Me2NN]) was synthesized
according to the literature procedure for H[Me3NN]
employing 2,6-dimethylaniline in place of 2,4,6-dimethylan-
iline [93]. The thallium thiolates TlSCy and TlStBu were syn-
thesized following a modified literature procedure by the
reaction of free thiol with TlOEt in ether followed by wash-
ing of the yellow solids with pentane [94]. The lithium thio-
late LiSCPh3 was prepared by deprotonation of HSCPh3
4.1.3. Synthesis of {[Me2NN]ZnSCy}2 (3)
A suspension of TlSCy (0.144 g, 0.452 mmol) in 5 mL
THF was added with stirring to
a
solution of
[Me2NN]ZnCl2Li(THF)3 (0.300 g, 0.452 mmol) in 10 mL
THF. The solution was stirred for 2 h and the solvent was
removed in vacuo. The resulting solid was taken up in
15 mL of CH2Cl2 and filtered over Celite. The solution was
concentrated kept at ꢀ35 ꢀC overnight to yield 0.096 g
1
(44%) of colorless crystals. H NMR (CDCl3, 25 ꢀC): d
7.092–6.921 (br, 6, Ar-H), 4.972 (br, 1, backbone-CH),
3.746 (br, 1, S-CH), 2.151 (br, 12, Ar-CH3), 1.691 (br, 6,
backbone-CH3), 1.402 (br, 5, Cy-CH2), 0.810 (br, 5,
1
Cy-CH2); H NMR (CDCl3, ꢀ30 ꢀC, partial) d 4.974 (br,
backbone-CH), 4.698 (s, backbone-CH) consistent with
monomer and dimer. Poor solubility coupled with fluxional
behavior limited the number of 13C NMR signals obtained in
room temperature in CDCl3. Anal. Calc. for C27H36N2SZn:
C, 66.72; H, 7.47; N, 5.76. Found: C, 67.05; H, 7.61; N,
5.65%.
t
by BuLi in toluene. Caution! BuSH is extremely volatile
and possesses a pungent odor essentially indistinguishable
from EtSH contained in natural gas.
4.1.1. Synthesis of [Me2NN]ZnCl2Li(THF)3 (1)
A solution of n-BuLi in hexanes (12 mL, 1.6 M) was
added to a solution of [Me2NN]H (6.00 g, 19.4 mmol) in
50 mL THF and stirred for 1 h. The resulting solution
was added with stirring to a solution of ZnCl2 (2.65 g,
19.4 mmol) in 10 mL THF. After stirring for 2 h, the solu-
tion was filtered over Celite and concentrated in vacuo. The
resulting solution was kept at ꢀ35 ꢀC overnight producing
crystals that were collected and dried to afford 5.850 g
4.1.4. Synthesis of {[Me2NN]ZnStBu}2 (4)
A solution of TlStBu (0.221 g, 0.754 mmol) in 5 mL
CH2Cl2 was added with stirring to
a solution of
[Me2NN]ZnCl2Li(THF)3 (0.500 g, 0.754 mmol) in 10 mL
CH2Cl2. The solution was stirred for 1 h and then filtered
over Celite. The resulting solution was concentrated and
kept at ꢀ35 ꢀC for 2 h to yield 0.123 g (36%) of colorless crys-
tals. 1H NMR (CDCl3): d 7.080–6.989 (m, 6, Ar-H), 5.048 (s,

326
M.S. Varonka, T.H. Warren / Inorganica Chimica Acta 360 (2007) 317–328
1, backbone-CH), 2.172 (s, 12, Ar-CH3), 1.735 (s, 6, back-
bone-CH3), 0.891 (s, 9, tBu-CH3); 13C{1H} NMR (CDCl3):
d 168.25, 147.05, 131.79, 128.72, 125.28, 95.55, 40.98,
37.51, 23.35, 19.11. Anal. Calc. for C25H34N2SZn: C,
65.27; H, 7.45; N, 6.09. Found: C, 64.90; H, 7.50; N, 6.03%.
4.1.8. Reaction of {[Me2NN]ZnStBu}2(4) with CySNO
Ca. 1 equiv. CySNO was generated in situ from the addi-
tion of a suspension of TlSCy (0.013 g, 0.044 mmol) in
CDCl3 to a stirring suspension of NOBF4 (0.006 g,
0.05 mmol) in CDCl3. After stirring for 15 minutes, the red
solution was filtered into a solution of {[Me2NN]ZnStBu}2
(0.010 g, 0.0218 mmol) in 1 mL CDCl3. As the red CySNO
solution was added to the colorless solution of 4, the reaction
mixture turned green in color, consistent with the formation
4.1.5. Synthesis of [Me2NN]ZnSCPh3(THF) (5)
A solution of LiSCPh3 (0.062 g, 0.4219 mmol) in 4 mL
THF was added with stirring to
a
solution of
t
1
[Me2NN]ZnCl2Li(THF)3 (0.145 g, 0.219 mmol) in 10 mL
THF. The solution was stirred for 3 h and then the volatiles
were removed in vacuo. The light yellow residue was taken
up in 10 mL toluene and filtered over Celite. The solvent
was removed in vacuo and the residue was crystallized
from THF/pentane solution. Upon standing overnight at
ꢀ35 ꢀC, 0.058 g (37%) of colorless crystals were collected
and dried. 1H and 13C NMR data for 5 in CDCl3 are iden-
tical for 6 with the exception of resonances for free THF.
Elemental analyses were not attempted owing to the facile
loss of THF in solution and in the solid state.
of BuSNO. Analysis by H NMR spectroscopy indicates
tBuSNO formation by its tBu resonance at d 1.89 ppm and
formation of {[Me2NN]ZnSCy}2 was confirmed by the
appearance of a broad peak at d 4.98 ppm corresponding
to the backbone proton resonance and a broad peak at d
3.74 ppm corresponding to the a-CH resonance in the cyclo-
hexyl groups. In addition, some unreacted CySNO and
{[Me2NN]ZnStBu}2 were still present after 30 minutes sug-
gesting an equilibrium that favors transnitrosylation to form
tBuSNO and {[Me2NN]ZnSCy}2.
4.1.9. Reaction of {[Me2NN]ZnStBu}2 with NO/O2
Bubbling ca. 2 equiv. NO (5.3 mL, 0.218 mmol) gas
through an oxygen saturated solution of {[Me2NN]-
ZnStBu}2 (0.050 g, 0.109 mmol) resulted in the solution
turning green in color as well as the formation of a colorless
4.1.6. Synthesis of [Me2NN]ZnSCPh3 (6)
A solution of LiSCPh3 (0.127 g, 0.452 mmol) in 4 mL
THF was added with stirring to
a
solution of
[Me2NN]ZnCl2Li(THF)3 (0.300 g, 0.452 mmol) in 10 mL
THF. The solution was stirred overnight and then the vol-
atiles were removed in vacuo. The light yellow residue was
taken up in 10 mL toluene and filtered over Celite. The sol-
vent was removed in vacuo and the residue was recrystal-
lized from Et2O. Upon standing overnight at ꢀ35 ꢀC,
0.145 g (50%) of colorless crystals were collected and dried.
1H NMR (CDCl3): d 7.036–6.888 (m, 21, Ar-H), 4.955 (s, 1,
backbone-CH), 1.980 (s, 12, Ar-CH3), 1.636 (s, 6, back-
bone-CH3); 13C{1H} NMR (CDCl3): d 168.61, 151.75,
146.63, 131.87, 129.28, 128.64, 127.55, 125.67, 125.28,
95.67, 23.34, 19.09. Anal. Calc. for C40H40N2SZn: C,
74.35; H, 6.24; N, 4.34. Found: C, 73.91; H, 6.01; N, 4.28%.
1
precipitate. H NMR analysis confirmed the formation of
tBuSNO by a sharp resonance at d 1.89 ppm.
4.1.10. Reaction of {[Me2NN]ZnStBu}2 with NO2
Bubbling ca. 2 equiv. NO2 (5.3 mL, 0.218 mmol) gas
through a nitrogen-saturated solution of {[Me2NN]-
ZnStBu}2 (0.050 g, 0.109 mmol) resulted in the solution
turning green in color as well as the formation of a color-
less precipitate. 1H NMR analysis confirmed the formation
t
of BuSNO by a sharp resonance at d 1.89 ppm.
Acknowledgments
4.1.7. Reaction of [Me2NN]ZnSCPh3 (6) with CySNO
Ca. 1 equiv. CySNO was generated in situ from the
addition of a suspension of TlSCy (0.013 g, 0.044 mmol)
in CDCl3 to a stirring suspension of NOBF4 (0.006 g,
0.05 mmol) in CDCl3. After stirring for 15 minutes, the
T.H.W. thanks the donors of the American Chemical
Society for a PRF-AC grant, the NSF CAREER program,
and the Georgetown University Pilot Grant Program.
red solution was filtered into
a
solution of
Appendix A. Supplementary data
{[Me2NN]ZnStBu}2 (0.010 g, 0.0218 mmol) in 1 mL
CDCl3. As the red CySNO solution was added to the col-
orless solution of 6, the reaction mixture turned green in
Crystallographic data for the structural analyses have
been deposited with the Cambridge Crystallographic Data
Centre, CCDC Nos. 617684–617689 for compounds 1–6.
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.ica.2006.
07.049.
t
color, consistent with the formation of BuSNO. Analysis
by 1H NMR spectroscopy indicated formation of
{[Me2NN]ZnSCy}2 by the appearance of a broad peak at
d 4.96 ppm corresponding to the backbone proton reso-
nance and a broad peak at d 3.73 ppm corresponding to
1
the a-CH resonance in the cyclohexyl groups. H NMR
References
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corroborated with the spectra of independently synthesized
Ph3CSNO. In addition, a single crystal X-ray study con-
firmed that Ph3CSNO was present in the mixture.
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