Y.D. Boyko et al. / Polyhedron 71 (2014) 24–33
31
Interaction of hexa-oxime ligand Ox
amounts produced a violet complex Ni(Ox
to elemental analysis data. In high-resolution mass spectra (micrO-
6
H
6
with NiCl
)Cl (H
2
in equimolar
O) according
O
2
, KOH, complex
6
H
6
2
2
4
MeOH, r.t., 72 h
Ph
P
Ph PO
3
3
TOF, electrospray ionization), only one ion (m = 629.3036) corre-
2+
3 n n
Scheme 3. Aerobic oxidation of Ph P in presence of Ni(Ox H )mX2 complexes.
sponding to Ni(Ox
6
6
H )
(/d/ = 1.2 ppm) was observed.
Unfortunately, crystals of the complex suitable for X-ray crystal-
lography could not be obtained. Changing of counterion to perchlo-
rate also did not furnish the crystalline complex. In HRMS spectra
Table 3
Results of Ni(II)-promoted Ph
2+
of perchlorate complex, only ion Ni(Ox
2.7 ppm) was detected as well. This indicates that in the complex
Ni(Ox )Cl (H O) nickel atom is coordinated only by nitrogen
atoms of the ligand with chlorine atoms located out of the metal’s
coordination sphere. Reaction of hexaoxime Ox with two equiv-
6
6
H )
(629.3011, /d/
3
P aerobic oxidation experiments.
=
Nc
Entry Complex
Amount of KOH
(equivalents to
complex)
Yield of
Ph PO,
%
6
H
6
2
2
4
3
a,b
6
H
6
1
Ni(Ox H )(NO ) (H O)
3
3
3
2
2
2
2
3
41
2.05
(2.0 )
0 (0 )
0
2.25
0.4
1.85
0.75
0.95
0.5
d
alents of nickel perchlorate does not furnish the dinuclear complex,
though several non-coordinated donor groups should be present in
0e
0
d
2
3
4
5
6
7
8
9
Ni(Ox
none
Ni(Ox
Ni(Ox
3
H
3
)(NO
3
)
2
(H
2
O)
0
–
f
g
2
+
Ni(Ox
6
6
H )
cation (for the synthesis of dinuclear nickel complexes
3
H
H
3
)Cl
Cl
2
3
2
4
4
6
6
45
8
37
15
19
10
with some b-oximinoalkylamine ligands see Ref. [16]). In HRMS
spectra of the resulting product, only ion of mononuclear complex
1
1
)
2
2
2
Ni(Ox H ) Cl
2
4
6
2
4
6
2
2
+
d
Ni(Ox
In FTIR spectra of the complexes Ni(Ox
Ni(Ox )Cl (H O) and Ni(Ox )Cl (H O)
corresponding to oxime fragments are present, i.e. broad bands
6
H
6
)
(629.3028, /d/ = 0 ppm) was observed.
Cl , [Ni(Ox
, characteristic bands
Ni(Ox
Ni(Ox
H
H
)Cl
2
(H
(H
2
O)
O)
2
)Cl
2
2
4
1
H
1
)
2
2
2 2 2 2
H ) ]Cl ,
NiCl
2
ꢁ6H
2
O
4
H
4
2
2
2
H
6 6
2
2
4
a
b
c
d
e
f
Yield of Ph
Average of two experiments.
3
PO isolated by flesh chromatography from reaction mixture.
ꢀ1
ꢀ1
at 3400–3600 cm (O–H bond stretch), 1600–1700 cm (C@N
bond stretch), 950–1100 cm (N–O bond stretch) and characteris-
tic bands of C–N bond stretch in 1210–1250 cm region. These
data are in agreement with experimental and calculated (ub3lyp)
ꢀ1
N – equivalents of Ph
Lit. data [6].
Only starting material was recovered (92%).
1 Equivalent of KOH to PPh was used.
Only starting material was recovered (99%).
3
PO formed per 1 equivalent of nickel complex.
ꢀ1
3
g
IR spectra of Ni(Ox
3 3 2
H )Cl , which is the most structurally related
to the obtained complexes [13a]. In nickel complexes of Ox
and Ox H , several bands in region 1600–1700 cm are observed,
6 6
4
H
4
ꢀ
1
which presumably correspond to a coordinated and non-coordi-
nated oxime C@N bond stretching.
with previously reported data [6] (Table 3, entry 1). In the absence
of KOH, no oxidation of Ph P to Ph PO was detected (Table 3, entry
2). Similarly, in the absence of nickel complexes no oxidation of
Ph P with dioxygen occurred, and the starting material was quan-
titatively recovered in this experiment (Table 3, entry 3). Another
tris-oxime complex Ni(Ox )Cl [11] promoted Ph P oxidation
to a similar extent as Ni(Ox )(NO (H O) did (cf. entries 1
and 4 in Table 3) demonstrating no significant counterion effect
3
3
Thus, in spite of significant distinctions in structures of Ox
, Ox , Ox and Ox nickel complexes, the metal–li-
gand complexation in these series shares some general features.
First, all Ox complexes have a six-coordinated pseudo-octahe-
dral structure. Second, ligands Ox coordinate nickel by both
1 1
H ,
2
Ox H
2
H
3 3
H
4 4
H
6 6
3
H
n n
3
H
3
2
3
H
n n
3
H
3
3
)
2
2
2
amino group’s and oxime groups’ nitrogen atoms forming five-
membered chelate rings, which have similar geometrical parame-
ters for all considered ligands. Third, OH-groups of coordinated
oxime fragments are involved in hydrogen bonding with counteri-
2
on dioxygen reactivity of nickel Ox
3
H
3
complexes.
(H O) was found for bis-
, which promotes formation of al-
A similar activity to Ni(Ox
oxime complex Ni(Ox Cl
most 2 equivalents Ph
try 6). Nickel complexes of tetra-oxime Ox
Ox proved to be much less active promoters leading to oxida-
tion of less than 20% of initial triphenylphosphine (less than 1
equivalent of Ph PO per complex was formed, Table 3, entries 7
and 8). The lowest yield of Ph PO (less than 10%) was observed
in Ph P oxidation reaction promoted by mono-oxime complex
Ni(Ox Cl (Table 3, entry 5).
Interestingly, NiCl in the presence of KOH led to oxidation of
P with dioxygen, though to a small extent (yield of Ph PO c.a.
10%, Table 3, entry 9). Apparently, nickel hydroxide serves as a
3
3
H )(NO
3
)
2
2
2
2 2 2
H )
2
ꢀ
ons (Cl ), which can be located in the inner or outer coordination
3
PO per 1 equivalent of complex (Table 3, en-
and hexa-oxime
sphere of nickel. Intramolecular hydrogen bonding effects may en-
hance the catalytic activity due to additional stabilization of inter-
mediate oxygen-containing species and complexes with oxidation
substrate [17].
4 4
H
6 6
H
3
3
3.3. Study of oxygen reactivity of nickel complexes
3
H
1 1
)
2
2
Each of the obtained complexes was studied in a model oxida-
2
tion reaction of triphenylphosphine in molecular oxygen atmo-
sphere in the presence of potassium hydroxide as base
Scheme 3, Table 3). In these experiments, 5 equivalents of triphen-
Ph
3
3
a
3
(
3
Ph P oxidation promoter in this reaction. Examples of aerobic oxi-
ylphosphine with respect to nickel complex were used. The
amount of base (KOH) per nickel complex was taken equal to the
number of NOH-fragments in a complex for an exhaustive deproto-
nation of all ligand’s oxime groups. For comparison purposes, aer-
dation of some organic substrates (primary and secondary alcohols)
employing nickel hydroxide as hetegeneous catalyst at elevated
temperatures are precedent in literature [18].
According to Baldwin’s hypothesis [6,7], the ability of complex
3 3 3 2 2 2
Ni(Ox H )(NO ) (H O) to promote aerobic oxidation is governed
3
obic oxidation of Ph P in presence of Ni(Ox
3 3 3
H )(NO )
(H
2 2
O)
2
previously reported by Baldwin and co-workers [6] was performed
by stabilization of Ni(III) ion by coordination with three deproto-
nated oximate groups (lowering the oxidation potential of Ni(II)
to Ni(III) transfer) and by formation of oximate-bridged dinuclear
(Table 3, entry 1).
3
In all cases, exposure of the solution of Ph P and deprotonated
nickel complex to oxygen atmosphere resulted in deep color
changes (up to deep-brown after 48 h of exposure, for changes in
UV–Vis spectra see Supporting information). In argon atmosphere,
no color change was observed. Baldwin’s tris-oxime complex
2
3 3 2
Complex Ni(Ox H )Cl was not studied for the ability to promote in aerobic
oxidation reactions before.
3
Nickel hydroxide is a well-known redox-active compound. In particular, in
Ni(Ox
3
H )(NO
3
3
)
2 2
(H O)
2
under the studied conditions promotes
P, which is in good agreement
alkaline conditions it is oxidized by strong oxidants (H
oxides.
2 2
O , NaClO) to higher nickel
oxidation of 2 equivalents of Ph
3