Nitric oxide release from a nickel nitrosyl complex induced by one-electron oxidation

Article abstract of DOI:10.1021/ic302697v

Reaction of [Ni(NO)(bipy)][PF₆] (2) with AgPF₆ or [NO][PF₆] in MeCN results in formation of [Ni(bipy) _x(MeCN)_y]²⁺ and release of NO gas in moderate yields. In contrast, the addition of the inner sphere oxidant Ph ₂S₂ to 2 does not result in denitrosylation. Instead, the diphenyldisulfide adduct [{(bipy)(NO)Ni}₂(μ-S₂Ph ₂)][PF₆]₂ (3) is formed in good yield. However, oxidation of 2 with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) does results in cleavage of the Ni-NO bond and generation of NO. The metal-containing product, [(bipy)Ni(η²-TEMPO)][PF₆] (4), can be isolated as an orange-brown solid in excellent yields. In the solid state, complex 4 contains a side-on bound TEMPO^- ligand, which is characterized by a long N-O bond length [1.383(2) A]. The contrasting reactivity of Ph₂S₂ and TEMPO likely relates to their different redox potentials, as Ph₂S₂ is a relatively weak oxidant. Finally, the addition of pyridine-N-oxide to 2 results in the formation of the adduct, [(bipy)Ni(NO)(ONC₅H₅)][PF₆] (5). No evidence of NO release is observed in this reaction, probably because of the low one-electron (1e^-) reduction potential of pyridine-N-oxide.

Full text of DOI:10.1021/ic302697v

Article
pubs.acs.org/IC
Nitric Oxide Release from a Nickel Nitrosyl Complex Induced by One-
Electron Oxidation
Ashley M. Wright, Homaira T. Zaman, Guang Wu, and Trevor W. Hayton*
Department of Chemistry and Biochemistry, University of CaliforniaSanta Barbara, Santa Barbara, California 93106, United States
S
* Supporting Information
ABSTRACT: Reaction of [Ni(NO)(bipy)][PF6] (2) with
AgPF6 or [NO][PF6] in MeCN results in formation of
[Ni(bipy)x(MeCN)y]2+ and release of NO gas in moderate
yields. In contrast, the addition of the inner sphere oxidant
Ph2S2 to 2 does not result in denitrosylation. Instead, the
diphenyldisulfide adduct [{(bipy)(NO)Ni}2(μ-S2Ph2)][PF6]2 (3) is formed in good yield. However, oxidation of 2 with 2,2,6,6-
tetramethylpiperidine-1-oxyl (TEMPO) does results in cleavage of the Ni−NO bond and generation of NO. The metal-
containing product, [(bipy)Ni(η2-TEMPO)][PF6] (4), can be isolated as an orange-brown solid in excellent yields. In the solid
state, complex 4 contains a side-on bound TEMPO− ligand, which is characterized by a long N−O bond length [1.383(2) Å].
The contrasting reactivity of Ph2S2 and TEMPO likely relates to their different redox potentials, as Ph2S2 is a relatively weak
oxidant. Finally, the addition of pyridine-N-oxide to 2 results in the formation of the adduct, [(bipy)Ni(NO)(ONC5H5)][PF6]
(5). No evidence of NO release is observed in this reaction, probably because of the low one-electron (1e−) reduction potential
of pyridine-N-oxide.
except when X was a strong donor ligand, such as a
dialkylamide.35 Importantly, in both cases, a stable nitrosyl
complex with an Enemark and Feltham notation of {CrNO}5 is
converted into an unstable {CrNO}4 complex upon oxida-
tion.34−37 This suggests that further examples of oxidative
denitrosylation could be discovered by exploiting the
{MNO}5/4 couple.37 Another {MNO}x/x−1 couple that may
also be susceptible to oxidative denitrosylation is the
{MNO}10/9 couple. While there are relatively few examples of
metal nitrosyls with an {MNO}10 configuration,38−44 the
number of {MNO}9 species is even smaller, suggesting that
the latter configuration is particularly unstable. To our
knowledge, there are only two examples of mono nitrosyls
with a {MNO}9 configuration,45−47 namely, TptBu,MeCo(NO)
(Tp = tris-pyrazolyl borate) and [Co(NO)(np3)][BPh4] (np3 =
tris(2-diphenylphosphinoethyl)amine). Both compounds con-
tain Co as the metal center and are supported by strongly
donating tripodal scaffolds.46,47 Furthermore, there are no
{NiNO}9 complexes in the literature. Therefore, we reasoned
that oxidation of a {NiNO}10 complex, to generate a {NiNO}9
species, would likely result in denitrosylation. To this end, we
have investigated the 1e− oxidation of [Ni(NO)(bipy)][PF6], a
{NiNO}10 species, in an attempt to effect controlled NO
release.
INTRODUCTION
■
The reactivity of nitric oxide (NO) remains a fundamentally
important area of research. This is due in part to the extensive
roles of NO in biology and in the environment.1−9 Nitric oxide
is an integral part of immune response, vasodilatation, and
platelet aggregation,5,10,11 and compounds that release NO
have been sought for their potential use as therapeutics.11−14
Several organic reagents, including alkyl nitrates (glyceryl
trinitrate),15 alkyl nitrites (iso-amyl nitrite),5 and NON-
Oates16−18 have been utilized as NO-releasing compounds.5,19
Metal nitrosyl complexes have also been used as NO delivery
agents,19 the classic example being sodium nitroprusside
(Na2[FeNO(CN)5], SNP).20,21 Photodissociation,12,13,22−25
reduction,26 and oxidation27−32 have all been employed to
effect NO release from a metal nitrosyl complex. The latter two
approaches, in particular, require a clear understanding of the
redox chemistry of metal nitrosyl complexes. For instance, the
mechanism of NO release from SNP is thought to involve an
initial reduction event at the Fe center; however, this
mechanism has not been fully elucidated.5,33
There are only a few well-defined examples of NO
dissociation upon oxidation of a metal nitrosyl complex.27−32
For instance, Bakac and co-workers reported that the one-
electron (1e−) oxidation of [CrNO(H2O)5]2+ or [L1(H2O)-
CrNO]2+ (L1 = 1,4,8,11-tetraazacyclotetradecane) resulted in
rapid denitrosylation.27−29 The putative 1e− oxidation products
([CrNOL5]3+) were not detected, suggesting that denitrosyla-
tion occurred immediately upon electron transfer. Similarly,
Ledgzins and co-workers investigated the reactivity of [CpCr-
(NO)X2]− (X = Cl, Br, I, OTf) with a variety of oxidants.34
They also observed that the nitrosyl ligand in the {CrNO}4
oxidation product was unstable with respect to dissociation,
RESULTS AND DISCUSSION
■
Synthesis and Characterization of [Ni(NO)(bipy)][PF6].
Addition of bipy to THF solutions of Ni(NO)I(THF)n
48
results in the precipitation of (bipy)Ni(NO)(I) (1) as a green
Received: December 7, 2012
Published: February 22, 2013
© 2013 American Chemical Society
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microcrystalline powder. Subsequent addition of 1.05 equiv of
AgPF6 to 1 in CH2Cl2 provides [Ni(NO)(bipy)][PF6] (2) in
high yields (70%−80%) (Scheme 1). Complexes 1 and 2 have
and the electronically unsaturated Ni center. In accord with this
hypothesis, the addition of a coordinating solvent, such as
MeCN, results in a sharpening of these resonances, as
formation of the solvento species, [(bipy)Ni(NO)(NCMe)]-
[PF6], forces the PF6 anion out of the Ni coordination sphere.
The solution IR spectrum of the MeCN adduct exhibits an NO
stretch at 1828 cm−1, lower than that observed for 2 by 41
cm−1.
Scheme 1
Complex 2 crystallizes in the triclinic space group P1 and its
̅
solid-state molecular structure is shown in Figure 2. There are
been characterized by X-ray crystallography, elemental analysis,
and IR spectroscopy. Both complexes have been reported
previously, but their characterization and solid-state structures
have not been discussed.39
Complex 1 is sparingly soluble in THF and CH2Cl2, but is
completely insoluble in nonpolar solvents, such as hexanes and
diethyl ether. Its solution IR spectrum in CH2Cl2 exhibits a νNO
1
stretch at 1792 cm−1, while its H NMR spectrum in CD2Cl2
contains three resonances at 7.76, 8.03, and 10.31 in a 2:4:2
ratio, assignable to aryl protons on the bipy ligand. Complex 1
crystallizes in the monoclinic space group C2/m and its solid-
state molecular structure is depicted in Figure 1. Complex 1
Figure 2. ORTEP drawing of [Ni(NO)(bipy)][PF6] (2) at 50%
thermal ellipsoids. Hydrogen atoms and one PF6 anion have been
omitted for the sake of clarity. Selected bond lengths (Å) and bond
angles (deg): Ni1−N1 = 1.617(3), N1−O1 = 1.152(4), Ni1−N3 =
1.976(3), Ni1−N4 = 1.963(2), Ni1···F12 = 2.697(2), ∠Ni1−N1−O2
= 176.2(3), Ni2−N2 = 1.611(3), N2−O2 = 1.153(4), Ni2−N5 =
1.970(3), Ni2−N6 = 1.956(3), Ni2···F8 = 2.947(2), ∠Ni2−N2−O2 =
174.7(3).
Figure 1. ORTEP drawing of (bipy)Ni(NO)(I) (1) shown with 50%
thermal ellipsoids. Hydrogen atoms have been omitted for clarity.
Selected bond lengths and bond angles: Ni1−N1 = 1.653(3) Å, N1−
O1 = 1.153(4) Å, Ni2−I1 = 2.6013(4) Å, ∠N1−N1−O1 = 167.0(2)°.
two independent nickel nitrosyl cations in the asymmetric cell.
Each cationic Ni center is ligated by one bipy and one NO in an
approximately trigonal planar environment. The metrical
parameters of the Ni(NO) moieties are characterized by
short Ni−N (1.617(3) Å; 1.611(3) Å) and N−O (1.152(4) Å;
1.153(4) Å) bond lengths and linear Ni−N−O bond angles
(176.2(3)°; 174.7(3)°). Both independent [Ni(NO)(bipy)]+
molecules exhibit long Ni···F contacts with a single PF6 anion;
however, the Ni−F distance varies considerably between the
two molecules [Ni1−F12 = 2.697(2) Å; Ni2−F8 = 2.947(2)
Å], and is likely quite weak in both instances. The second PF6
anion does not interact with either Ni center. For comparison,
the nickel complex [Ni(L)2(η1-F-PF5)][PF6] (where L = 2-
phenyl-1,4-bis(isopropyl)-1,4-diazabutadiene) contains a much
shorter Ni···F contact of 2.453(2) Å.53 Structurally charac-
terized three coordinate nickel nitrosyls are quite rare.50,54−56
For example, Storr and co-workers reported the neutral three
coordinate nickel nitrosyl, (μ-3,5-dimethylpyrazolyl-
N,N′)2(NiNO)2,54 while Hillhouse and co-workers isolated
features a severally distorted tetrahedral geometry about the Ni
center (∠N2−N1−I1 = 99.74(5)°, ∠N1−Ni1−N2 =
132.30(6)°, ∠N1−Ni1−I1 = 105.12(9)°), a consequence of
the small bite angle of the bipy ligand. In addition, the metrical
parameters of the nitrosyl moiety (Ni1−N1 = 1.653(3) Å, N1−
O1 = 1.153(4) Å, ∠Ni1−N1−O1 = 167.0(2)°) are consistent
with other four-coordinated nickel nitrosyl complexes.43
Complex 2 is quite soluble in THF and CH2Cl2 but
completely insoluble in nonpolar solvents, such as hexanes and
diethyl ether. Its solution IR spectrum in CH2Cl2 exhibits a νNO
value at 1869 cm−1, which is toward the high end of values
reported for other nickel nitrosyl complexes (range: 1568−
1915 cm−1).38,40,43,49−52 The 1H NMR spectrum of 2 in
CD2Cl2 consists of four sharp resonances at 7.87, 7.95, 8.17,
and 10.26 ppm, in a 2:2:2:2 ratio, which are assignable to the
four proton environments of the bipy ligand. The 19F and 31P
NMR spectra display significantly broadened resonances, at
−72.4 ppm (full width at half maximum (fwhm) = 240 Hz) and
−144.0 ppm (fwhm = 97 Hz), respectively. The broadening is
ascribed to a weak dative interaction between the PF6 anion
the cationic {NiNO}10 species, [(dtbpe)Ni(NO)][BArF ].55
4
Not surprisingly, its Ni−N (Ni1−N1 =1.649(5) Å) and N−O
(N−O = 1.141(8) Å) bond parameters are similar to those of 2.
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More recently, Warren and co-workers have reported the
neutral three-coordinate nickel nitrosyls, [Ar2nacnac]NiNO
(Ar2nacnac = ArNC(Me)CHC(Me)NAr; Ar = 2,6-iPr2C6H3)
and (IPr)NiNO(X) (IPr = N,N′-bis(2,6-diisopropylphenyl)-
imidazole; X = I, OTf, SCPh3).50,56
mixture of paramagnetic products, tentatively formulated as
[Ni(bipy)x(CD3CN)y]2+. The addition of 2 equiv of bipy to this
reaction mixture results in complete conversion to [Ni(bipy)3]-
[PF6]2,39 which can be isolated in 60% yield after workup. We
also explored the reactivity of 2 with [NO][PF6] (1.00 V vs
[Cp2Fe]0/+ in CH2Cl2; 0.87 V vs [Cp2Fe]0/+ in MeCN).58 The
reaction of [NO][PF6] with 2 in CH2Cl2/MeCN results in a
rapid color change from dark blue to light purple, concomitant
with gas evolution (see Scheme 2). An IR spectrum of the
reaction mixture reveals the loss of the νNO stretch assignable to
2. Moreover, N2O was not observed in the IR spectrum, nor
was the nitrosylation product of 2, [(bipy)Ni(NO)2][PF6]2.39
Analysis of the head space by GC/MS reveals the presence of
NO gas, and the yield of NO was determined to be 51% using
the NOA (based on 2 equiv of NO being evolved during the
reaction). Oxidation of 2 with [NO][PF6] in MeCN, followed
by the addition of 2 equiv of 2,2-bipyridine, also results in the
formation of [Ni(bipy)3][PF6]2, which can be isolated in 63%
yield.39 Importantly, the release of NO upon oxidation of 2
with either AgPF6 or [NO][PF6] confirms our initial hypothesis
that the {MNO}10/9 couple is susceptible to oxidative
denitrosylation. Finally, we observed no reaction between 2
and [Cp2Fe][PF6] in CD2Cl2 over the course of 8 h. This is not
surprising, given that the ferrocenium redox potential is lower
than that of AgPF6 or [NO][PF6] in either MeCN or CH2Cl2.
Oxidation of [Ni(NO)(bipy)][PF6] with Inner Sphere
Oxidants. We also explored the reactivity of complex 2 with
inner sphere oxidants, including disulfides, an important class of
biological oxidants. Many sulfur-containing molecules, such as
thiols and disulfides, are known to interact with nitric oxide.59
For example, sulfhydryl-containing molecules, such as cysteine
and glutathione, are thought to induce the release of NO from
SNP upon reduction of the Fe center.33 In contrast, disulfides
are potential oxidants, and low-valent Group 10 metal
complexes have been shown to induce disulfide bond
cleavage.60,61 With this in mind, we investigated the reactivity
of diphenyl disulfide (Ph2S2) with 2. Thus, the addition of 1
equiv of Ph2S2 to 2 at room temperature in CH2Cl2 results in a
color change from dark green to blue-green. The solution IR
spectrum of the reaction mixture reveals two νNO values at 1832
cm−1 and 1868 cm−1, in a 1:4 ratio. The former stretch is
assignable to [{(bipy)(NO)Ni}2(μ-S2Ph2)][PF6]2 (3), whereas
the latter stretch is assignable to unreacted 2. The reaction
equilibrium can be shifted completely to the product side,
according to IR spectroscopy, via the addition of 15 equiv of
Ph2S2 to solutions of 2 in CH2Cl2. However, complex 3 can be
cleanly isolated on preparative scale in 59% yield as a blue
crystalline material by addition of only 2.3 equiv of Ph2S2 to a
CH2Cl2 solution of 2 (Scheme 3).
The electronic structure of nickel nitrosyl complexes has
come under recent scrutiny.45,57 Historically, nickel nitrosyl
complexes were assigned a Ni0(NO+) electronic structure.2
However, an extensive spectroscopic and theoretical study on
Tp′M(NO) (where M = Co, Ni, Cu) suggested that, for the
nickel analogue, the NiII(NO−) electronic structure is the best
description of its ground state. However, this description does
not appear to be entirely adequate for complex 2. For instance,
its high NO stretching frequency [1868 cm−1] and the short
N−O bond length [1.152(4) Å vs 1.15 Å for NO gas] are more
consistent with that expected for a NiI(NO•) or Ni0(NO+)
configuration. In addition, the related nickel nitrosyl, [Ni-
(NO)(bipy)2][PF6], which clearly exhibits a NiII(NO−)
electronic structure, features a much lower NO stretching
frequency (1567 cm−1) and substantially longer N−O bond
length (1.207(3) Å).39 Accordingly, we prefer the NiI(NO•) or
Ni0(NO+) configurations for complex 2, as these better reflect
its characterization data.
Cyclic Voltammetry of [Ni(NO)(bipy)][PF6]. The solution
phase redox properties of 2 were investigated by cyclic
voltammetry. In MeCN at room temperature, the cyclic
voltammogram of 2 displays an oxidation feature at +0.11 V
(vs [Cp2Fe]0/+ at 100 mV/s) (see the Supporting Information
(SI)), which is irreversible at all scan rates studied (up to 2 V/
s). We propose that this feature reflects the formation of
[Ni(NO)(bipy)]2+, which is unstable and rapidly loses NO
under the conditions of the experiment. Also observed in the
cyclic voltammogram of 2 is a second irreversible oxidation
feature at +0.90 V (vs [Cp2Fe]0/+ at 200 mV/s), which we have
been unable to assign. No reduction features are observed
within the solvent window at all scan rates. The cyclic
voltammogram of 2 was also recorded in CH2Cl2; however,
this trace is relatively complicated and consists of several
indistinct oxidation features that we could not assign.
Oxidation of [Ni(NO)(bipy)][PF6] with Outer Sphere
Oxidants. With the electrochemical data for 2 in hand, we
investigated its reactivity with a series of outer sphere oxidants.
Gratifyingly, the one-electron (1e−) oxidant AgPF6 (0.65 V vs
[Cp2Fe]0/+ in CH2Cl2; 0.04 V vs [Cp2Fe]0/+ in MeCN)58 reacts
with 1 in CH2Cl2 over the course of 2 h, resulting in the
deposition of a silver mirror, the formation of a brown
precipitate, and gas evolution (Scheme 2). The evolved gas was
determined to be NO by gas chromatography−mass spectros-
copy (GC/MS) and the yield was quantified to be 56% using a
The formulation of 3 was established by X-ray diffraction
(XRD) analysis of a blue block grown by slow diffusion of
hexanes into a concentrated CH2Cl2 solution of 3 at −25 °C.
1
nitric oxide analyzer (NOA). Following this reaction via H
NMR spectroscopy in MeCN-d3 reveals the formation of a
Complex 3 crystallizes in the triclinic space group P1, and its
̅
Scheme 2
solid-state molecular structure is shown in Figure 3. The cation
in complex 3 consists of a diphenyl disulfide ligand bridging
two Ni metal centers. In addition, each nickel center is
coordinated by a bipy ligand and a nitric oxide moiety in an
overall distorted tetrahedral environment (e.g., ∠N3−Ni1−S1
= 111.90(11)°, ∠N3−Ni1−N1 = 130.90(14)°, ∠N3−Ni1−N2
= 129.76(14)°, τ4 = 0.70).62 The [NiNO]+ moiety in 3 contains
short Ni−N and N−O bond lengths [Ni1−N3 = 1.648(3) Å;
N3−O1 = 1.153(4) Å], characteristic of cationic four-
coordinated nickel nitrosyls.2 The Ni−N−O bond angle
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Scheme 3
release of NO. Instead, complete decomposition of complex 3
is observed after 24 h, as determined by 1H NMR spectroscopy,
and no signals assignable to a paramagnetic Ni(II) complex
were present in the spectrum (see Figure S37 in the SI).
Nonetheless, it is clear that the addition of Ph2S2 to 2 does not
result in oxidative denitrosylation under ambient conditions.
This can be explained by examining its redox potential, as Ph2S2
is not an especially good oxidant (−2.00 V vs [Cp2Fe]+/0 in
MeCN).66,67 While coordination of Ph2S2 to a metal center is
anticipated to change this potential, this effect is clearly not
large enough to result in electron transfer from 2.
We also explored the reactivity of 2 with TEMPO (TEMPO
= 2,2,6,6-tetramethylpiperidine-1-oxyl). Like Ph2S2, TEMPO is
also weak oxidant (−1.99 V vs [Cp2Fe]+/0 in MeCN), but its
oxidation potential can be modulated greatly by coordination to
a Lewis acid.68 For example, TEMPOH+ has a much higher
oxidation potential (0.71 V vs [Cp2Fe]+/0 in MeCN) than free
TEMPO.67,69−71 Thus, the addition of TEMPO to a CH2Cl2
solution of 2 results in vigorous gas evolution and an immediate
color change from dark green to orange. The IR spectrum of
the reaction mixture reveals the loss of the νNO stretch for 2.
Furthermore, the appearance of a peak assignable to a new
nitrosyl complex, or to N2O, was not observed.39 The evolved
gas was identified as NO by GC/MS and the yield determined
to be 59% using the NOA. In addition, monitoring the reaction
Figure 3. ORTEP drawing of [{Ni(NO)(bipy)}2(μ-S2Ph2)][PF6]2
(3·2CH2Cl2) shown at 50% thermal ellipsoids. Hydrogen atoms,
CH2Cl2 solvate, and PF6 anions have been excluded for clarity.
Selected bond lengths (Å) and bond angles (deg): Ni1−N1 =
1.648(3), N1−O3 = 1.153(4), Ni1−S1 = 2.3071(9), S1−S1* =
2.1306(17), Ni1−N1−O1 = 167.4(3).
(167.4(3)°) is slightly bent, the result of a non-C3 symmetric
ligand set.63 The Ni−S bond length (2.3071(9) Å) is short, in
comparison to other Ni−S dative bonds [2.42−2.55 Å];
however, in these examples, the sulfide donor is contained
within a larger ligand scaffold that likely dictates the Ni−S bond
distance. The S−S bond length in 3 (S1−S1* = 2.1306(17) Å)
is elongated by 0.1 Å relative to free Ph2S2 (S−S = 2.03 Å).64
To our knowledge, only two other complexes, [ReBr-
(CO)3]2(Ph2S2) and [Ag2(Ph2S2)4][AsF6]2, exhibit a bridging
disulfide similar to that found in 3.65
1
by H NMR spectroscopy reveals clean generation of a single
metal-containing product. Crystallization from CH2Cl2/hex-
anes yields the diamagnetic Ni(II) complex, [(bipy)Ni(η2-
TEMPO)][PF6] (4), as a yellow-orange crystalline solid in 88%
yield (see Scheme 3).
The composition of 4 was established by XRD analysis of a
crystal grown from a dilute CH2Cl2/hexanes solution. Complex
4 crystallizes in the monoclinic space group C2/c, and the
cationic portion of the molecule is shown in Figure 4. The
nickel center is ligated by a single bipy ligand and a η2-O,N-
bound TEMPO ligand, in an overall square planar geometry.
The Ni1−O1 and Ni1−N1 bond distances are 1.804(2) and
1.898(2) Å, and are comparable to previously characterized
Ni(η2-TEMPO) complexes.72,73 The N1−O1 bond distance
[1.383(2) Å] is consistent with the presence of a reduced
TEMPO− ligand. For comparison, this distance is similar to
those found in Ni(η2-TEMPO)2 [1.3850(10) Å] and (κ1-P-
dtpbe)NiCl(TEMPO) [1.385(2) Å], which also feature
reduced TEMPO− ligands.72,73 In contrast, the neutral
TEMPO ligand in CuBr2(η2-TEMPO), contains a much
shorter N−O bond distance of 1.304(8) Å.74 The Ni1−N2
and Ni1−N3 bond lengths are 1.896(2) and 1.881(2) Å,
respectively, and are comparable with previously reported
square planar nickel(II) bipyridine complexes.39
1
The H NMR spectrum of a crystalline sample of 3 in
CD2Cl2 confirms the 2:1 ratio of Ni to Ph2S2 that was observed
in the solid state. In addition, the 31P and 19F NMR spectra
−
(CD2Cl2) feature broad PF6 signals, suggestive of a fluxional
−
process involving displacement of the Ph2S2 ligand by PF6 .
This is further evidenced by the solution IR spectrum
(CH2Cl2) of isolated crystals of 3, which exhibits two NO
stretching values at 1868 cm−1 and 1832 cm−1, assignable to 2
and 3, respectively. The weak coordination of Ph2S2 to Ni is
also apparent upon addition of a stronger donor ligand, such as
MeCN, to a sample of 3 in CH2Cl2, which immediately
generates [(bipy)NiNO(MeCN)][PF6] (νNO = 1828 cm−1).
Complex 3 is stable at room temperature for at least 24 h in
1
CD2Cl2, according to H NMR spectroscopy. Furthermore,
thermolysis of 3 at 60 °C does not result in any apparent
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acid.68 Thus, while TEMPO is not a strong enough oxidant to
effect the outer sphere oxidation of 2, according to cyclic
voltammetry, it is likely that upon binding to Ni, the redox
potential of TEMPO is sufficiently shifted to induce electron
transfer.
To evaluate the validity of the putative [(bipy)Ni(NO)(η1-
TEMPO)][PF6] intermediate, we explored the reactivity of 2
with pyridine-N-oxide (C5H5NO). Previously, Drago and co-
workers determined that the Lewis basicities of TEMPO and
pyridine-N-oxide were comparable, as revealed by the heats of
formation of their hydrogen-bonded adducts with phenol
(TEMPO: −ΔH = 6.1 kcal/mol; C5H5NO: −ΔH = 8.1 kcal/
mol).75−77 However, the 1e− reduction potential of C5H5NO
(−2.747 V vs Cp2Fe+/0 in DMF)78 is much lower than that of
TEMPO, making it less likely to induce oxidative denitrosy-
lation by a 1e− oxidation event. Thus, treatment of a green
CH2Cl2 solution of 2 with C5H5NO results in a rapid color
change to deep blue. Crystallization from CH2Cl2/hexanes at
−25 °C results in the isolation of [(bipy)Ni(NO)(ONC5H5)]-
[PF6] (5) as a blue crystalline solid in 81% isolated yield (eq 1).
Figure 4. ORTEP drawing of [Ni(bipy)(η2-TEMPO)][PF6]
(4·0.5CH2Cl2) shown at 50% thermal ellipsoids. Hydrogen atoms,
CH2Cl2 solvate, and PF6 anion have been excluded for the sake of
clarity. Selected bond lengths: Ni1−O1 = 1.804(2) Å, Ni1−N1 =
1.898(2) Å, N1−O1 =1.382(2) Å.
The 1H NMR spectrum of 4 in CD2Cl2 exhibits two
inequivalent methyl environments at 1.37 ppm and 2.60 ppm,
which is consistent with the η2 coordination mode observed in
the solid state, and suggests the TEMPO coordination mode is
static on the NMR time scale. In contrast, the 1H NMR
spectrum of [Ni(η2-TEMPO)2] exhibits broad spectral features,
likely due to the fluxional coordination of the TEMPO ligand.72
In addition, there are eight well-resolved bipy resonances in the
aromatic region, also consistent with the η2-TEMPO
coordination mode. Finally, the 19F and 31P NMR spectra
exhibit a doublet at −73.3 ppm (JFP = 711 Hz) and a septet at
−144.5 ppm (JPF = 745 Hz), respectively, consistent with the
presence of a PF6 anion.
We propose that the reaction of 2 with TEMPO results in
the formation of a transient {NiNO}9 intermediate, [(bipy)-
Ni(NO)(η1-TEMPO)][PF6] (Scheme 4). This species rapidly
The observation that pyridine-N-oxide cannot effect oxidation
denitrosylation of 2 is consistent with its low 1e− redox
potential, while the lack of O-atom transfer to the NO ligand
suggests a reasonably large kinetic barrier to N−O bond
cleavage. In addition, the isolation of a stable pyridine-N-oxide
adduct of the [(bipy)Ni(NO)]+ fragment adds credence to the
proposed mechanism of NO release described in Scheme 4.
The solid-state IR spectrum (Nujol mull) of 5 features a νNO
stretch at 1801 cm−1, which is among the lowest values yet
observed for a cationic nickel nitrosyl,39 demonstrating the
strong donor ability of pyridine-N-oxide. Its 1H NMR spectrum
exhibits four well-defined resonances at 7.88, 8.06, 8.14, and
10.29 ppm, in a 2:2:2:2 ratio, assignable to the four bipy proton
environments. In contrast, the resonances of the pyridine-N-
oxide ligand, observed at 7.47, 7.61, and 8.26 ppm (2:1:2 ratio),
are significantly broadened.79 Moreover, the 19F and 31P NMR
spectra of 5 feature a broadened doublet and septet, at −72.5
ppm (fwhm = 90 Hz) and −144.2 ppm (fwhm = 120 Hz),
respectively. A plausible explanation for this phenomenon
would be an equilibrium between 2 and 5; however, this
hypothesis is not supported by the solution IR spectrum in
CH2Cl2, which features a single νNO stretch at 1817 cm−1.
While this is slightly higher than the NO stretch observed in
Nujol, it is still much lower than the νNO stretch of 2 (1869
cm−1). Furthermore, the deep blue color of 5 in CH2Cl2
suggests the presence of a four coordinate [Ni(NO)L3]+
species, as complexes of this type are typically blue.43 Given
these considerations, we suggest that there is a close-contact
ion pair between the PF6 anion and the Ni complex in solution,
which restricts rotation of both the PF6 moiety and the
pyridine-N-oxide coligand, thereby explaining the broadened
NMR signals.
Scheme 4
releases NO to generate the final product (4). As evidence, we
note that the reaction of 2 with TEMPO in the presence of
MeCN is substantially slower, suggesting that TEMPO must
displace MeCN in the Ni-coordination sphere before oxidation
can occur. The proposed inner sphere electron transfer is
further evidenced by the lack of reactivity observed between
TEMPO and the four-coordinated nitrosyl complex, (bipy)Ni-
(NO)(I) (1). As mentioned above, the redox potential of
TEMPO can be greatly modulated by coordination to a Lewis
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The solid-state molecule structure of 5 has been determined
by X-ray crystallography and the cationic portion of the
molecule is shown in Figure 5. The cation in complex 5 consists
EXPERIMENTAL SECTION
■
General. All reactions and subsequent manipulations were
performed under anaerobic and anhydrous conditions under an
atmosphere of nitrogen or argon. Nitromethane was recrystallized
from Et2O three times under a nitrogen atmosphere and then distilled
over CaH2; dichloromethane was degassed and stored over 3 Å sieves
for 24 h before use. Diethyl ether, hexanes, and tetrahydrofuran
(THF) were dried using a Vacuum Atmospheres DRI-SOLV solvent
purification system. Deuterated NMR solvents (dichloromethane-d2,
1,1,2,2-tetrachloroethane-d2, nitromethane-d3, acetonitrile-d3) were
dried over activated 3 Å molecular sieves for 24 h. Ni(NO)I(THF)n
was prepared using previously published methods.48 All other reagents
were purchased from commercial suppliers and used as-received.
NMR spectra were recorded on a Varian UNITY INOVA 400
1
spectrometer or a Varian UNITY INOVA 500 spectrometer. The H
and 13C NMR spectra were referenced to external SiMe4, using the
residual protio solvent peaks as an internal standard. The 19F{1H}
NMR spectra were referenced to external CFCl3 and 31P{1H} NMR
spectra were referenced to external 85% phosphoric acid. IR spectra
were recorded on a Thermo Scientific Nicolet 6700 FTIR
spectrometer using NaCl plates for Nujol mulls or CaF2 plates for
solution-phase experiments. UV-vis experiments were performed with
an Ocean Optics USB4000 UV-vis spectrometer equipped with a USB-
DT light source or using a Shimadzu dual-beam Model UV-2401 PC
spectrophotometer. Elemental analyses were performed by the Micro-
Mass Facility at the University of California, Berkeley.
Gas Chromatography. Gas chromatography (GC) experiments
were performed on an Agilent model 6890 gas chromatograph
equipped with a thermal conductivity detector. The constituent gases
were separated by passage through a 10-ft Carbosieve packed column
using helium as the carrier gas. A typical experiment used the following
procedure: the reaction components were loaded into a gas-tight piece
of glassware and outfitted with a rubber septum in a glovebox. The
reaction flask was stored inside the glovebox until the reaction was
completed. The headspace was then sampled using a gas-tight syringe.
Cyclic Voltammetry Measurements. Cyclic voltammetry (CV)
experiments were performed using a CH Instruments Model 600c
potentiostat, and the data were processed using CHI software (Version
6.29). All experiments were performed in a glovebox using a 20-mL
glass vial as the cell. The working electrode consisted of a platinum
disk embedded in glass (2 mm in diameter), and both the counter and
reference electrodes consisted of platinum wire. Solutions employed
during the CV study were 1 mM in the nickel complex and 0.1 M in
[Bu4N][PF6]. All potentials are reported versus the [Cp2Fe]0/+ couple.
For all trials, ip,a/ip,c = 1 for the [Cp2Fe]0/+ couple, while ip,c increased
linearly with the square root of the scan rate (i.e., v1/2).
Nitric Oxide Analyzer. Nitrogen monoxide was detected using a
GE Nitric Oxide Analyzer (model NOA-280i). The head space of a
gas-tight reaction flask was sampled using a Hamiliton gas-tight
syringe. The samples were injected into the manifold setup using
helium as the flow gas. The NOA was calibrated by injection of
standard NaNO2 solutions from 5 μM to 100 μM into a potassium
iodide solution in glacial acetic acid. Calculations determining the
amount of NO present in the reaction mixtures were performed by
assuming that no nitric oxide remained dissolved in solution.87
Ni(NO)I(bipy) (1). To a stirring solution of Ni(NO)(I)(THF)n
(153 mg, 0.424 mmol) in THF (2 mL) was added an excess of 2,2-
bipyridine (72 mg, 0.46 mmol) in THF (1 mL). Immediately, a green
microcrystalline precipitate formed. After stirring for 30 min, the
product was collected on a medium porosity frit and washed with Et2O
(3 × 5 mL). 110 mg, 69% yield. Anal. Calcd. for C10H8IN3NiO: C,
32.30; H, 2.17; N, 11.30. Found: C, 32.48; H, 2.24; N, 11.19. 1H NMR
(CD2Cl2, 22 °C, 400 MHz): δ 7.76 (m, 2H), 8.03 (m, 4H), 10.31 (d,
2H, JHH = 4 Hz). IR (CH2Cl2 solution, cm−1): 1792 (s, νNO). IR
(Nujol mull, cm−1): 1764 (s, νNO).
Figure 5. (a) ORTEP drawing of [(bipy)Ni(NO)(ONC5H5)][PF6]
(5·0.5CH2Cl2) shown at 50% thermal ellipsoids. CH2Cl2 solvate,
hydrogen atoms, and PF6 anion have been excluded for the sake of
clarity. Selected bond lengths (Å) and bond angles (deg): Ni1−N1 =
1.6300(18) Å, Ni1−O2 = 2.0323(15) Å, Ni1−N3 = 1.9983(16) Å,
Ni1−N4 = 2.0084(17) Å, O1−N1 = 1.167(2) Å, O2−N2 = 1.337(2)
Å; ∠Ni1−N1−O1 = 176.0(2)°.
of a Ni center ligated by a bipy ligand, an NO ligand, and an
C5H5NO ligand, in an overall distorted tetrahedral geometry
(τ4 = 0.73).62 The metrical parameters of the Ni−N−O moiety
(Ni1−N1 = 1.6300(18) Å, N1−O1 = 1.167(2) Å; ∠Ni1−N1−
O1 = 176.0(2)°) are characteristic of four-coordinated nickel
nitrosyl complexes.2,43,80,81 Furthermore, the Ni−Opy bond
distance is 2.0323(15) Å and the N−O bond length of the
C5H5NO ligand is 1.337(2) Å. For comparison, the N−O bond
−
length in [Ni(ONC5H5)6][X]2 (X = BrO3− or BF4 ) is 1.32 Å,
while the N−O bond length for free pyridine-N-oxide is
1.330(9) Å, suggesting little activation of the N−O bond in
5.82−84
CONCLUSIONS
■
One-electron (1e−) oxidation of [Ni(NO)(bipy)][PF6] with
AgPF6, [NO][PF6], or TEMPO results in oxidative homolysis
of the Ni−NO bond and subsequent evolution of NO gas, with
NO yields ranging from 51 to 59%. In the case of the TEMPO
reaction, the Ni oxidation product, [(bipy)Ni(η2-TEMPO)]-
[PF6] can be isolated in 81% yield. Interestingly, the reaction of
[Ni(NO)(bipy)][PF6] with Ph2S2 or pyridine-N-oxide does not
lead to oxidative denitrosylation; instead, it results in the
formation of the adducts [{Ni(NO)(bipy)}2(μ-S2Ph2)][PF6]2
and [(bipy)Ni(NO)(ONC5H5)][PF6], respectively. The lack of
NO release in these examples can be rationalized by the low
1e− reduction potentials observed for both diphenyl disulfide
and pyridine-N-oxide. Overall, these results support our
hypothesis that the {NiNO}9 configuration is unstable, with
respect to denitrosylation, and suggests that the {MNO}10/9
couple could be harnessed in the design of a controlled NO
release platform. This idea is further buttressed by the recent
observation that dinitrosyliron complexes (DNICs) with a
{Fe(NO)2}9 configuration are known to be more active toward
NO loss than those with a {Fe(NO)2}10 configuration.85,86
[Ni(NO)(bipy)][PF6] (2). A CH2Cl2 (3 mL) suspension of
[Ag][PF6] (195 mg, 0.77 mmol) was added to a stirring suspension
of [Ni(I)NO(bipy)] (280 mg, 0.75 mmol) in CH2Cl2 (5 mL). The
mixture was stirred for 30 min, during which time the solution
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changed to a dark green color, concomitant with precipitation of a
flocculent white solid (AgI). The solution was filtered through a Celite
column (2 cm × 0.5 cm) supported on glass wool, concentrated under
reduced pressure, and layered with hexanes (3 mL). Subsequent
storage at −25 °C for 24 h resulted in the deposition of dark green
crystals. The supernatant was decanted and the crystals were washed
with hexanes (1 × 3 mL) and dried in vacuo. 253 mg, 86% yield. Anal.
Calcd. for C10H8F6N3NiOP: C, 30.81; H, 2.07; N, 10.78. Found: C,
31.13; H, 1.70; N, 10.47. 1H NMR (CD2Cl2, 22 °C, 500 MHz): δ 7.87
(d, 2H, JHH = 8 Hz) 7.95 (m, 2H) 8.17 (t of d, 2H, JHH = 7.5 Hz and
Nitric Oxide Analyzer. Stock solutions of [Ni(NO)(bipy)][PF6]
and [NO][PF6] in MeCN were prepared with concentrations of 3.22
mM and 17.9 mM, respectively. An aliquot of the [Ni(NO)(bipy)]-
[PF6] solution (0.50 mL, 1.6 × 10−6 mol) was transferred via syringe
to a gas-tight reaction flask. Subsequently, [NO][PF6] (0.10 mL, 1.8 ×
10−6 mol) was injected through a rubber septum into the reaction
flask. The solution color changed from deep blue to light purple with
concomitant gas evolution. After 5 min at room temperature, a volume
of 10 μL in the head space was sampled using a gas-tight syringe and
injected into the NOA. The area of the peak was compared to a
calibration curve and the amount of NO formation was determined to
be 51% (based on the formation of 2 equiv of NO).
Preparative Scale. [NO][PF6] (47 mg, 0.27 mmol) was added to a
MeCN solution (2 mL) of [Ni(NO)(bipy)][PF6] (90 mg, 0.23
mmol). Gas evolution was observed immediately upon the addition,
and the reaction color changed from deep purple to light purple. The
reaction mixture was stirred for a total of 30 min, whereupon 2,2-
bipyridine (87 mg, 0.55 mmol, 2.4 equiv) was added to the reaction
mixture. The solution was filtered through a Celite column (2 cm × 1
cm) supported on glass wool and then reduced in vacuo to 1.5 mL.
The solution was layered with Et2O (5 mL) and subsequent storage at
−25 °C for 6 h resulted in the deposition of a red-purple
microcrystalline powder of [Ni(bipy)3][PF6]2. 119 mg, 63% yield.
1H NMR (CD3CN, 22 °C, 400 MHz): δ 14.6 (br s, 12H), 46.3 (br s,
6H), 63.3 (br s, 6H). This material was spectroscopically identical to
an authentic sample of [Ni(bipy)3][PF6]2.39
1
JHH = 1 Hz) 10.26 (d, 2H, JHH = 5 Hz). H NMR (CD3NO2, 22 °C,
400 MHz): δ 8.00 (m, 2H) 8.20 (d, 2H, JHH = 8 Hz) 8.25 (t of d, 2H,
JHH = 8 Hz and JHH = 4 Hz) 10.41 (d, 2H, JHH = 4 Hz). 19F{1H} NMR
−
(CD2Cl2, 22 °C, 470 MHz): δ −72.42 (br d, PF6 , JPF = 724 Hz). 31
P
−
NMR (CD2Cl2, 22 °C, 202 MHz): δ −144.0 (br sept, PF6 , JPF = 715
Hz). 13C{1H} NMR (CD2Cl2, 22 °C, 125 MHz): δ 121.88, 128.44,
142.48, 151.43. 153.19. IR (CH2Cl2 solution, cm−1): 1869 (s, νNO). IR
(Nujol mull, cm−1): 1871 (s, νNO). UV−vis (CH2Cl2, 1.34 mM, 25
°C): 407 nm (ε = 225 L mol−1 cm−1), 670 nm (ε = 291 L mol−1
cm−1).
Oxidation of 2 with [Ag][PF6]. NMR spectroscopy. An NMR
tube was charged with [Ni(NO)(bipy)][PF6] (34 mg, 0.088 mmol)
and CD2Cl2 (0.5 mL), and an initial NMR spectrum was recorded.
AgPF6 (23 mg, 0.093 mmol) was then added as a CD2Cl2 suspension
1
(0.5 mL). The reaction was monitored by H NMR spectroscopy.
After 24 h, a large amount of precipitate had deposited in the NMR
tube. In addition, a silver mirror had formed on the walls of the NMR
tube (see Figure S31 in the SI).
[{(NO)(bipy)Ni}2(μ-S2Ph2)][PF6]2 (3). A CH2Cl2 solution (1 mL)
of [Ni(NO)(bipy)][PF6] (71 mg, 0.18 mmol) was treated with a
CH2Cl2 solution (1 mL) of diphenyl disulfide (91 mg, 0.42 mmol).
The solution transformed from a green color to a dark blue color. The
solution was subsequently layered with hexanes (5 mL), and storage at
−25 °C for 24 h resulted in the deposition of blue blocks. 80 mg, 88%
yield. Anal. Calcd for C32H26F12N6Ni2O2P2S2: C, 38.51; H, 2.63; N,
Nitric Oxide Analyzer. A stock solution of [Ni(NO)(bipy)][PF6] in
CH2Cl2 was prepared with a concentration of 5.15 mM. An aliquot of
[Ni(NO)(bipy)][PF6] (1.00 mL, 5.15 × 10−6 mol) was transferred
using a syringe to a gas-tight reaction flask. Subsequently, AgPF6 (1.6
mg, 6.3 × 10−6 mol), added as a suspension in CH2Cl2 (0.3 mL), was
transferred to the reaction flask. Over the course of 3 h at room
temperature, the reaction mixture faded from an intense green color to
a light green color, concomitant with formation of a precipitate. A
volume of 10 μL in the head space was sampled using a gas-tight
syringe and injected into the NOA. The area of the resulting peak was
compared to a calibration curve, and the amount of NO formation was
determined to be 56% (average based on three injections).
1
8.42. Found: C, 38.16; H, 2.75; N, 8.06. H NMR (CD2Cl2, 22 °C,
400 MHz): δ 7.18−7.30 (m, 10H, Ph), 7.88 (dd, 4H, bipy, JHH = 5.6
and 7.2 Hz), 7.92 (d, 4H bipy, JHH = 8.4 Hz), 8.14 (t, 4H bipy, JHH = 8
Hz), 10.06 (d, 4H bipy, JHH = 8.4 Hz). 19F NMR (CD2Cl2, 22 °C, 376
MHz): δ −66.21 (br s). 31P NMR (CD2Cl2, 22 °C, 170 MHz): δ
−140.1 (v br). IR (CH2Cl2, cm−1): 1832 (νNO, s). IR (Nujol mull,
cm−1): 1836 (νNO, s).
[Ni(bipy)(η2-TEMPO)][PF6] (4). A CH2Cl2 (1 mL) solution of
TEMPO (27 mg, 0.17 mmol) was added dropwise to a stirring
solution of [Ni(NO)(bipy)][PF6] (48 mg, 0.12 mmol) in CH2Cl2 (2
mL). Upon addition, the solution color changed from green to bright
orange, with concomitant gas evolution. After 5 min of stirring, the
solution volume was reduced in vacuo to 1.5 mL and filtered through a
Celite column (0.5 cm × 0.5 cm) supported on glass wool. Addition of
hexanes (4 mL) to the supernatant, followed by storage at −25 °C for
24 h, resulted in the precipitation of orange-yellow crystalline material.
52 mg, 81% yield. Anal. Calcd for C19H26F6N3NiOP·CH2Cl2: C, 39.97;
H, 4.70; N, 6.99. Found: C, 40.13; H, 4.83; N, 6.97. 1H NMR
(CD2Cl2, 400 MHz, 22 °C): δ 1.37 (s, 6H, Me), 1.58−1.99 (m 6H,
CH2), 2.60 (s, 6H, Me), 7.54 (m, 1H, bipy), 7.61 (m, 1H, bipy), 8.04
Preparative Scale. A 20-mL scintillation vial was charged with
[Ni(NO)(bipy)][PF6] (83 mg, 0.212 mmol) and MeCN (2 mL).
AgPF6 (65 mg, 0.260 mmol) was added to the solution and the
reaction mixture was stirred at room temperature. Over the course of 2
h, the solution changed from an intense purple to blue, concomitant
with the formation of a brown precipitate. The solution was filtered
through a Celite column (2 cm × 1 cm) supported on glass wool into
a new scintillation vial containing bipy (76 mg, 0.49 mmol, 2.3 equiv),
whereupon the solution became dark orange. The supernatant was
layered with Et2O (5 mL) and stored at −25 °C for 24 h, resulting in
the deposition of red crystals of [Ni(bipy)3][PF6]2. 104 mg, 60% yield.
This material was spectroscopically identical to an authentic sample of
[Ni(bipy)3][PF6]2.39
Oxidation of 2 with [NO][PF6]. IR Spectroscopy. [NO][PF6] was
added to a stirred CH2Cl2 solution of [Ni(NO)(bipy)][PF6] (16 mg,
0.042 mmol), to which acetonitrile (10 μL, 0.19 mmol, 4.5 equiv) had
been added. The dark blue solution converted to light purple over the
course of 20 min. The in situ IR spectrum of this solution revealed the
complete loss of the NO stretch of 2. In addition, no peaks were
observed that could be assigned to either a new NO complex or to
N2O. IR (CH2Cl2, cm−1): 2057 (w, νCN) (see Figure S29 in the SI).
Gas chromatography. A gas-tight reaction flask was charged with
[Ni(NO)(bipy)][PF6] (38 mg, 0.097 mmol) and MeCN (0.73 mL).
To this solution was added an MeCN solution (0.42 mL) of
[NO][PF6] (19 mg, 0.097 mmol) via syringe. The deep blue solution
turned pale purple and gas evolution was observed. After 5 min at
room temperature, a volume of 300 μL in the head space was sampled,
using a gas-tight syringe, and a gas chromatogram was recorded,
revealing the presence of N2 and NO. No other products were
observed in the chromatogram (see Figure S28 in the SI).
(d, 1H, JHH = 8 Hz, bipy), 8.11 (d, 1H, 4 Hz, bipy), 8.13 (d, 1H, JHH
=
4 Hz, bipy), 8.16−8.22 (m, 2H, bipy), 8.36 (d, 1H, 8 Hz, bipy).
31P{1H} NMR (CD2Cl2, 170 MHz, 22 °C): δ −144.3 (sept, PF6, JPF
745 Hz). 19F NMR (CD2Cl2, 376 MHz, 22 °C): δ −73.3 (d, PF6, JPF
=
=
711 Hz). 13C NMR (CD2Cl2, 125 MHz, 22 °C): δ 16.69 (Me), 23.82
(Me), 34.09 (γ-CH2 TEMPO), 39.38 (β-CH2 TEMPO), 67.20 (α-C
TEMPO), 122.82 (aryl H bipy), 123.19 (aryl H bipy), 127.86 (aryl H
bipy), 128.84 (aryl H bipy), 141.16 (aryl H bipy), 141.32 (aryl H
bipy), 151.42 (aryl H bipy), 153.35 (aryl H bipy), 154.17 (aryl H
bipy), 154.51 (aryl H bipy). IR (Nujol mull, cm−1): 1607 (w), 1460
(w), 1378 (m), 1315 (w), 1273 (w), 1250 (w), 1219 (w), 1169 (w),
1130 (w), 1076 (w), 1022 (w), 976 (w), 954 (w), 842 (s), 772 (m),
746 (w), 730 (m), 649 (w), 558 (w). UV-vis (0.45 mM, CH2Cl2, 25
°C): 394 nm (ε = 4400 L mol−1 cm−1), 415 nm (ε = 3900 L mol−1
cm−1), 464 nm (sh, ε = 1400 L mol−1 cm−1).
[(bipy)Ni(NO)(ONC5H5)][PF6] (5). A CH2Cl2 solution (1 mL) of
pyridine-N-oxide (17 mg, 0.18 mmol) was added to a stirred CH2Cl2
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Table 1. X-ray Crystallographic Data for Complexes 1−5
1
2
3·2CH2Cl2
4·0.5CH2Cl2
5·0.5CH2Cl2
empirical formula
crystal habit, color
crystal size (mm)
crystal system
space group
vol (Å3)
C10H8IN3NiO
block, green
0.25 × 0.20 × 0.18
monoclinic
C2/m
C10H8F6N3NiOP
rod, green
C34H30Cl4F12N6Ni2O2P2S2
brick, dark blue
0.20 × 0.20 × 0.03
triclinic
C19.50H27ClF6N3NiOP
needle, yellow
0.20 × 0.05 × 0.05
monoclinic
C2/c
C15.50H14ClF6N4NiO2P
block, blue
0.30 × 0.15 × 0.15
monoclinic
C2/c
0.60 × 0.20 × 0.02
triclinic
P1
̅
P1
̅
1181.73(13)
13.9792(9)
11.8611(8)
7.7282(5)
90
1358.35(8)
7.9403(3)
10.4457(3)
16.9595(6)
81.625(2)
86.498(2)
77.562(2)
4
1107.76(5)
10.1213(3)
10.6254(3)
12.2737(3)
75.043(2)
66.164(2)
67.598(2)
1
4703(3)
21.240(7)
9.737(3)
22.745(8)
90
3994.1(6)
17.7511(15)
10.5069(9)
21.4383(19)
90
a (Å)
b (Å)
c (Å)
α (deg)
β (deg)
112.748(4)
91
90.718(6)
90
92.663(2)
90
γ (deg)
Z
4
8
8
formula weight, fw (g mol−1
)
371.80
389.87
1167.92
1.751
558.57
527.44
density (calcd) (Mg M−3
)
2.090
1.906
1.578
1.754
abs coeff (mm−1
)
4.235
3.933
1.349
1.071
1.261
F000
712
776
586
2296
2120
total no. reflections
unique reflections
Rint
5461
4976
10462
33969
28805
1883
3382
4533
4815
4086
0.0257
0.046
0.0277
0.0471
0.0352
final R indices [I > 2σ(I)]
R1 = 0.0199, wR2
0.0442
R1 = 0.0498, wR2 =
0.1483
R1 = 0.0450, wR2 = 0.1256 R1 = 0.0297, wR2 =
R1 = 0.0288, wR2 =
0.0587
0.0640
largest diff peak and hole
0.848 and −0.608
0.602 and −0.516
1.164 and −1.412
0.356 and −0.278
0.760 and −0.424
(e− Å−3
)
goodness of fit, GOF
1.041
1.062
1.102
1.005
1.139
(2 mL) solution of [Ni(NO)(bipy)][PF6] (50 mg, 0.13 mmol).
Immediately, the solution changed from green to royal blue. After 15
min of stirring, the reaction mixture was filtered through a Celite
column supported on glass wool (0.5 cm × 2 cm). The filtrate was
concentrated under reduced pressure to 1 mL and then layered with
hexanes (4 mL). Subsequent storage at −25 °C for 24 h resulted in the
precipitation of blue crystalline needles. 51 mg, 81% yield. Anal. Calcd
for C15H13F6N4NiO2P: C, 37.15; H, 2.70; N, 11.55. Found: C, 36.85;
H, 2.86; N, 11.20. 1H NMR (CD2Cl2, 400 MHz, 22 °C): δ 7.47 (br s,
pyridine 2H), 7.61 (br s, pyridine 1H), 7.88 (t, 2H, bipy, JHH = 8 Hz),
8.06 (d, 2H, bipy, JHH = 8 Hz), 8.14 (t, 2H, bipy, JHH = 8 Hz), 8.26 (br
s, 2H, pyridine), 10.29 (d, 2H, bipy, JHH = 4 Hz). 13C NMR (CD2Cl2,
125 Hz, 22 °C): δ 122.2 (aryl H bpy), 127.8 (aryl H bpy), 140.2 (aryl
H bpy), 152.2 (aryl H bpy). 152.7 (aryl H bpy). At 22 °C, only signals
attributable to the bipy ligand are observed. 31P{1H} NMR (CD2Cl2,
170 MHz, 22 °C): δ −144.2 (br sept, PF6, JPF = 755 Hz, fwhm = 90
creation of publication materials were performed using SHELXTL.91 A
PF6 anion in complex 2 was disordered over two orientations in a
51:49 ratio.
Data for 1, 3, 4, and 5 were collected on a Bruker KAPPA APEX II
diffractometer equipped with an APEX II CCD detector using a
TRIUMPH monochromator with a Mo Kα X-ray source (α = 0.71073
Å). The crystals were mounted on a cryoloop under Paratone-N oil,
and all data were collected at 100(2) K, using an Oxford nitrogen gas
cryostream system. Data were collected using ω scans with 0.5° frame
widths. The frame exposures were as follows: 10 s for 1, 5 s for 3, 10 s
for 4, and 15 s for 5. Data collection and cell parameter determination
were conducted using the SMART program.88 Integration of the data
frames and final cell parameter refinement were performed using
SAINT software.89 Absorption correction of the data was carried out
using the multiscan method SADABS.90 Subsequent calculations were
carried out using SHELXTL.91 Structure determination was done
using direct or Patterson methods and difference Fourier techniques.
All hydrogen atom positions were idealized, and rode on the atom of
attachment. Structure solution, refinement, graphics, and creation of
publication materials were performed using SHELXTL. The PF6 anion
in 4 exhibits rotational disorder and was modeled over two
orientations in a 63:37 ratio. The CH2Cl2 solvate molecule in 5
exhibits positional disorder, the Cl atom was modeled over two
positions in a 55:45 ratio.
Hz). 19F NMR (CD2Cl2, 376 MHz, 22 °C): δ −72.5 (br d, PF6, JPF
=
711 Hz, fwhm = 120 Hz). IR (CH2Cl2, cm−1): 1817 (s, νNO). IR
(Nujol mull, cm−1): 1802 (m, νNO), 1603 (m), 1574 (w), 1491 (w),
1466 (s), 1377 (m), 1318 (w), 1250 (w), 1202 (m), 1178 (m), 1160
(m), 1104 (w), 1072 (w), 1045 (w), 1025 (m), 935 (w), 835 (s), 767
(s), 733 (m), 673 (m), 651 (w), 639 (w), 557 (w). UV-vis (0.35 mM,
CH2Cl2, 25 °C): 618 nm (ε = 275 L mol−1 cm−1).
X-ray Crystallography. Data for 2 were collected on a Bruker
Proteum2 diffractometer equipped with a PLATINUM CCD detector
using multilayer optics with a Cu Kα X-ray source (α = 1.4178 Å). The
crystals were mounted on a cryoloop under Paratone-N oil, and all
data were collected at 100(2) K using an Oxford nitrogen gas
cryostream system. Data were collected using ω scans with 0.5° frame
widths. Frame exposures of 5 and 20 s were used for low-angle and
high-angle diffractions, respectively. Data collection and cell parameter
determination were conducted using the SMART program.88
Integration of the data frames and final cell parameter refinement
were performed using SAINT software.89 Absorption correction of the
data was carried out using the multiscan method SADABS.90
Subsequent calculations were carried out using SHELXTL.91 Structure
determination was done using direct methods and difference Fourier
techniques. All hydrogen atom positions were idealized, and rode on
the atom of attachment. Structure solution, refinement, graphics, and
A summary of the relevant crystallographic data for complexes 1−5
can be found in Table 1.
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures, crystallographic details (as CIF files)
and spectral data for compounds 1−5. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: hayton@chem.ucsb.edu.
3214
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Inorganic Chemistry
Article
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(37) Legzdins, P.; McNeil, W. S.; Smith, K. M.; Poli, R.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We thank the University of California, Santa Barbara and the
Alfred P. Sloan Foundation for financial support of this work.
We also thank Prof. Peter Ford and Meredith Crisalli for
assistance with NOA measurements.
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