A structure and reactivity analysis of monomeric Ni(ii)-hydroxo complexes prepared from water

Article abstract of DOI:10.1039/b820209e

The nickel(ii) chemistry with the tridentate ligands bis[(N′-R- ureido)-N-ethyl]-N-methylamine (H₄1^R, R = isopropyl, tert-butyl) is described. The Ni(ii)-OH complexes, [Ni^IIH ₂1^R(OH)]^- were

Full text of DOI:10.1039/b820209e

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A structure and reactivity analysis of monomeric Ni(II)–hydroxo complexes
prepared from water†
Darla Powell-Jia,a Joseph W. Ziller,a Antonio G. DiPasquale,b Arnold L. Rheingoldb and A. S. Borovik*a
Received 18th November 2008, Accepted 2nd February 2009
First published as an Advance Article on the web 27th February 2009
DOI: 10.1039/b820209e
The nickel(II) chemistry with the tridentate ligands bis[(N¢-R-ureido)-N-ethyl]-N-methylamine (H41R,
R = isopropyl, tert-butyl) is described. The Ni(II)–OH complexes, [NiIIH21R(OH)]- were generated
using water as the source of the hydroxo ligand. These complexes are pseudo-square planar, in which
the primary coordination sphere contains three nitrogen donors from [H21R]2- and the oxygen atom
˚
from the hydroxide (Ni–O(H), 1.857(1) A). The Ni(II)–OH unit also is involved in two intramolecular
hydrogen bonds between the urea groups of the [H21R]2- and the hydroxo oxygen atom. Attempts to
deprotonate the Ni(II)–OH unit to produce Ni(II)–oxo complexes were unsuccessful. A variety of bases
with pKa of less than 15 (in DMSO) were unable to deprotonate the hydroxo ligand. Treating the
Ni(II)–OH complexes with KOBut (pKa ~ 29) afforded the ligand substitution product,
[NiIIH21R(OBut)]-. Ni(II)–siloxide complexes were isolated when the [NiIIH21R(OH)]- complexes were
allowed to react with K[N(TMS)2].
Introduction
Monomeric metal complexes of the middle and late 3d transition
metals with terminal hydroxo ligands are rare because the OH-
ligand tends to bridge between metal centers.1 In contrast, the
active sites of many metalloproteins contain M–OH units, which
are essential for function.2 These sites are often distant from the
protein surface, preventing interactions between M–OH centers.
A similar “site isolation” design has been used to make synthetic
M–OH complexes, in which bulky ancillary ligands are employed
to prevent formation multi-nuclear species.
Fig. 1 Ligand [H21R]2- used in this investigation.
available to bind a hydroxo ligand. There are only four examples of
structurally-characterized mononeric Ni(II) complexes with termi-
nal hydroxo ligands. Riordan has isolated a five-coordinate Ni(II)–
OH complex for the oxidation of [NiIcyclam]- with dioxygen.7
Ca´mpora has reported two square planar Ni(II)–OH complexes
prepared with hydroxide salts (e.g. KOH and LiOH)8 and Mindiola
has shown that a Ni(I) complex can reductively split water, forming
four-coordinate Ni(II)–OH species.9 Our investigations showed
that [NiIIH21R(OH)]- is formed from water and is stabilized via
an intramolecular H-bonding network. In addition, our efforts to
deprotonate the hydroxo ligand are described.
Our group3 and others4 have been exploring the chemistry of
monomeric M–OH complexes using tripodal ligands containing
hydrogen bond (H-bond) donors. We use ligands containing an
apical nitrogen donor and three ethyleneureayl groups that form
intramolecular H-bond networks between the NH groups and
the M–OH units. One outcome of the H-bond networks is that
stable cavities are formed around the metal centers, which aid in
averting bridged species from occurring. We recently reported5 the
2-
synthesis of the [H21tBu
]
ligand that has one of the tripodal arms
replaced with a methyl group (Fig. 1). [H21tBu
]
coordinates in a
2-
Experimental section
facial manner to Fe(II), Mn(II), and Co(II) centers, producing more
open structures that can lead to dimeric species.6
Herein we report the preparation of Ni(II)–OH complexes with
[H21R]2- (R = iPr, tBu). We reasoned that [H21R]2- would coordi-
nate in a meridional fashion to a d8 metal center, leaving one site
Preparative methods
All reagents were purchased from commercial sources and used
as received, unless otherwise noted. Solvents were sparged with
argon and dried over columns containing Q-5 and molecular sieve.
Anhydrous solvents were purchased from Aldrich (Milwaukee,
WI). Potassium hydride (KH) as a 30% dispersion in mineral oil
was filtered with a medium porosity glass frit, washed five times
each with pentane and Et2O, dried under vacuum, and stored
under an inert atmosphere. The syntheses of all metal complexes
were conducted in a Vacuum Atmospheres, Co. (Hawthorne, CA)
drybox under an argon atmosphere. Elemental analyses were
accomplished at Robertson Microlit (Madison, NJ). Compound
H41tBu was prepared following the literature procedure.5
aDepartment of Chemistry, University of California Irvine, 1102 Natural
Science II, Irvine, CA 92697, USA. E-mail: aborovik@uci.edu
bDepartment of Chemistry and Biochemistry, University of California San
Diego, 9500 Gilman Drive, MC 0358, La Jolla, California 92093, USA
† Electronic supplementary information (ESI) available: Crystallographic
data for K[NiIIH21But(OH)] and [NMe4][NiIIH21iPr(OH)], and selected
metrical parameters for the K[NiIIH21But(OH)], [NMe4][NiIIH21iPr(OH)],
and [NMe4][NiIIH21iPr(OBut)]. CCDC reference numbers 708859–708863.
For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/b820209e
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Ligand synthesis
84%). Purple crystals of the salt were obtained by diffusing
a 1 : 1 diethyl ether–pentane mixture into a DMA solution of
K[NiIIH21tBu(OH)] (118 mg, 87%). Found: C, 44.28; H, 8.19; N,
16.08%. K[NiIIH21tBu(OH)]·DMA (C19H41N5KNiO4) requires C,
44.28; H, 8.02; N, 16.31%. FT-IR/cm-1 (Nujol): n(OH) = 3612,
n(NH) = 3157, n(CO) = 1638, 1584, 1542; lmax (DMA, nm
(e, M-1 cm-1)) 500 (130).
Bis[(N¢-isopropylureido)-N-ethyl]-N-methylamine
(H41iPr).
Freshly distilled N¢-methyl-2,2¢-diaminodiethylamine (2.0 g,
0.017 mmol) was dissolved in 50 mL of anhydrous THF under
a N2 atmosphere, cooled to 0 ◦C, and treated with isopropyl
isocyanate (3.1 g, 0.36 mmol) via dropwise addition. The mixture
was allowed to warm to room temperature and was stirred
overnight. The solid that was formed was filtered, washed
with 10 mL of diethyl ether three times, dried under vacuum
overnight, and stored under an argon atmosphere. The yield of
H41iPr was 3.8 g (78%). dH (400 MHz; solvent DMSO-d6): 5.78
(2 H, s, CO-NH-iPr), 5.66 (2 H, t, CH2-NH-CO), 3.05 (4 H, q,
CH2–CH2-NH), 2.32 (4 H, t, N–CH2-CH2), 2.16 (3 H, s, MeH),
3.65 (2 H, m, NH–CH-Me2), 1.02 (12 H, s, iPrH); dC (400 MHz;
solvent DMSO-d6): 158.09, 57.71, 42.72, 41.48, 37.74, 23.93.
FT-IR (Nujol, cm-1) n(NH) = 3329, n(CO) = 1567, 1621; Mp
118–120 ◦C; HRMS (ES+): Exact mass calcd for C13H30N5O2
[M + H], 288.2400. Found 288.2405.
K[NiIIH21tBu(18OH)]. This was prepared using Method A and
H218O—the quantities of reagents were identical to those described
above. The spectroscopic for this complex were identical to those
of [NiIIH21tBu(16OH)]- with the following exception: FT-IR/cm-1
(Nujol): n(18OH) = 3601.
[NMe4][NiIIH21iPr(OtBu)].
Method C. A solution of H41iPr (100 mg, 0.348 mmol) in 3 mL
of DMA was treated with solid KH (29 mg, 0.73 mmol). After
gas evolution ceased, solid Ni(OAc)2 (61 mg, 0.35 mmol) was
added slowly. The solution turned dark red and was stirred for
45 min. Solid KOtBu (39 mg, 0.35 mmol) was added and the
solution turned blue-purple. [NMe4][OAc] (51 mg, 0.38 mmol) was
added, the mixture was allowed to stir for 1 h, and then filtered
to remove KOAc (91 mg, 89%). Volatiles were removed under
reduced pressure and the resulting blue-purple oil was triturated
with diethyl ether to afford a blue-purple powder (156 mg, 91%).
Found: C, 51.42; H, 10.23; N, 17.30%. [NMe4][NiIIH21iPr(OtBu)]
(C21H48N6NiO3) requires C, 51.33; H, 9.85; N, 17.10%. FT-IR
(Nujol, cm-1): n(NH) = 3157, n(CO) = 1665, 1575, 1520. lmax
(DMA, nm (e, M-1 cm-1)) 495 (210).
Preparation of metal salts
K[NiIIH21iPr(OH)]·DMA.
Method A. A solution of H41iPr (100 mg, 0.348 mmol) in 3 mL
of DMA was treated with solid KH (44 mg, 1.1 mmol). After
gas evolution ceased, Ni(OAc)2 (62 mg, 0.35 mmol) was added
slowly as a solid. The solution was dark red and was stirred for
45 min, after which H2O (6.3 uL, 0.35 mmol) was added via syringe,
causing the solution to turn purple. The solution was stirred for
an additional 20 min and then filtered through a medium glass frit
to remove KOAc (67 mg, 98%). Purple crystals were obtained by
1 : 1 diethyl ether–pentane vapor diffusion into a DMA solution
of K[NiIIH21iPr(OH)] (88 mg, 64%). Found: C, 41.76; H, 7.32; N,
17.09%. K[NiIIH21iPr(OH)]·DMA (C17H37N6KNiO4) requires C,
41.90; H, 7.65; N, 17.25%. FT-IR/cm-1 (Nujol): n(NH) = 3157,
n(CO) = 1572, 1518; lmax (DMA, nm (e, M-1 cm-1)) 500 (158). Note
that the peak associated with the n(OH) was not present in FTIR
spectra of the potassium salt of the complex. However, this peak
was observed at n(OH) = 3612 cm-1 in the tetramethyammonium
salt, prepared by metathesis with [NMe4][OAc] (see below for
similar method of metathesis).
[NMe4][NiIIH21iPr(OtBu)].
Method D. K[NiIIH21iPr(OH)] (0.35 mmol), prepared by
Method A, in 3 mL of DMA was treated with solid KOtBu
(39 mg, 0.35 mmol) and stirred for 30 min, causing a subtle
change in color from purple to bluish-purple. [NMe4][OAc] (46 mg,
0.35 mmol) was added and, after 1 h of stirring, the reaction
mixture was filtered to remove the KOAc (91 mg, 89%). Volatiles
were removed under reduced pressure, producing a blue-purple oil
that was triturated with diethyl ether to give a blue-purple powder.
The product was obtained in pure form via recrystallization
by diffusing a 1 : 1 diethyl ether–pentane mixture into a DMA
solution of [NMe4][NiIIH21iPr(OtBu)] to give 156 mg (91%) of the
salt. Spectroscopic properties of the blue-purple powder prepared
by this method are the same as those of the blue-purple powder
prepared in Method C.
Method B. A solution of H41iPr (100 mg, 0.348 mmol) in 3 mL
of DMA was treated with solid KH (29 mg, 0.73 mmol). After gas
evolution ceased, Ni(OAc)2 (62 mg, 0.35 mmol) was added slowly
as a solid. The dark red solution was then filtered to remove KOAc
(61 mg, 99%) and characterized: FT-IR/cm-1 (Nujol): n(NH) =
3355; lmax (DMA, nm) 350. The solution was further treated with
one equivalent of KH (14 mg, 0.35 mmol). After gas evolution
ceased, H2O (6.3 mL, 0.35 mmol) was added via syringe, causing
the solution to turn purple. Volatiles were removed under reduced
pressure and the resulting purple oil was triturated with diethyl
ether to afford a purple powder. The spectroscopic properties of
the purple powder prepared by this method are the same as those
obtained for K[NiIIH21iPr(OH)] prepared by Method A.
[NMe4][NiIIH21tBu(OtBu)]. This was prepared following
Method D using K[NiIIH21tBu(OH)] (0.32 mmol), KOtBu (36 mg,
0.32 mmol), and NMe4OAc (63 mg, 0.48 mmol). The amount of
KOAc isolated was 147 mg (96%). The salt was recrystallized by
diethyl ether diffusion in to a DMA solution of crude product,
resulting in blue-purple crystals (128 mg, 78%). FT-IR/cm-1
(Nujol): n(N–H) = 3157. lmax (DMA, nm (e, M-1 cm-1)) 495
(166). Repeated elemental analysis gave consistently low values.
[NMe4]0.5K0.5[NiIIH21iPr(OSiMe3)]·DMA. This was prepared
following Method D using K[NiIIH21iPr(OH)] (0.35 mmol), solid
KN(SiMe3)2 (70 mg, 0.35 mmol), and NMe4OAc (55 mg,
0.42 mmol). The amount of KOAc isolated was 106 mg (98%).
The solution was set up for recrystallization via 1 : 1 diethyl ether–
pentane vapor diffusion into a DMA solution and purple crystals
K[NiIIH21tBu(OH)]·DMA. This was prepared following the
procedure as outlined above for K[NiIIH21iPr(OH)] (Method A)
with H41tBu (100 mg, 0.317 mmol), KH (39 mg, 0.98 mmol),
Ni(OAc)2 (56 mg, 0.32 mmol), and H2O (5.7 mL, 0.32 mmol).
The solution was filtered to remove KOAc (52 mg, 0.53 mmol,
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Table 1 Crystallographic data for K[NiIIH21iPr(OH)]·DMA, [NMe4]0.5K0.5[NiIIH21iPr(OSiMe3)]·DMA, and [NMe4][NiIIH21iPr(OBut)]
Salt
K[NiIIH21iPr(OH)]·DMA
[NMe4]0.5K0.5[NiIIH21iPr(OSiMe3)]·DMA
[NMe4][NiIIH21iPr(OBut)]
Molecular formula
Formula weight/g mol-1
T/K
C13H28KN5NiO3
400.12
C24H55K1/2N6.5NiO4Si
603.83
C23H52N6NiO3
519.39
148(2)
P43212
17.6716(10)
17.6716(10)
21.525(3)
90
90
90
8
6722.1(9)
0.971
0.0996
0.3029
1.471
163(2)
158(2)
¯
¯
Space group
P1
I42d
˚
a/A
9.6492(9)
9.8057(9)
10.2370(9)
86.824(2)
88.467(2)
74.471(2)
2
931.73(15)
1.427
0.0286
24.6663(11)
24.6663(11)
17.6314(16)
90
90
90
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
Z
16
3
˚
V/A
10727.4(12)
1.213
0.0258
0.0610
1.120
d
calcd/Mg m-3
R1
a
b
wR2
0.716
1.050
GOFc
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
a R1 = ꢀFo| - |Fcꢀ/ |Fo|. b wR2 = [ [w(Fo - Fc2)2]/ [w(Fo2)2]]1/2
.
c GOF = [ [w(Fo - Fc2)2]/(n - p)]1/2 where n is the number of reflections and
2
2
p is the total number of parameters refined.
resulted (161 mg, 94%). Found: C, 45.72; H, 8.95; N, 17.34%.
[NMe4]0.5K0.5[NiIIH21iPr(OSiMe3)]·DMA, (C22H51K0.5N6.5NiO4Si)
requires C, 45.79; H, 8.91; N, 17.78%. FT-IR/cm-1 (Nujol):
n(N–H) = 3157. lmax (DMA, nm (e, M-1 cm-1)) 495 (166).
Subsequent calculations were carried out using the SHELXTL13
program. Partial crystal data, data collection and refinement
parameters for K[NiIIH21iPr(OH)], [NMe4][NiIIH21iPr(OBut)] and
(NMe4)0.5K0.5[NiIIH21iPr(OSiMe3)]·DMA are listed in Table 1.
A purple crystal of K[NiIIH21iPr(OH)]·DMA of approximate
dimensions 0.14 ¥ 0.19 ¥ 0.26 mm was mounted on a glass fiber and
transferred the diffractometer. There were no systematic absences
or any diffraction symmetry other than the Friedel condition.
[NMe4][NiIIH21tBu(OSiMe3)]. This
was
prepared
as
described above for [NMe4]0.5K0.5[NiIIH21iPr(OSiMe3) with
K[NiIIH21tBu(OH)] (0.32 mmol), solid KN(SiMe3)2 (63 mg,
0.32 mmol), and NMe4OAc (63 mg, 0.48 mmol). The solution
was then filtered to remove the insoluble KOAc (110 mg,
1.1 mmol) and recrystallization via 1 : 1 diethyl ether–pentane
vapor diffusion into a DMA solution and purple crystals resulted
(161 mg, 95%). FT-IR/cm-1 (Nujol): n(N–H) = 3157. lmax (DMA,
nm (e, M-1 cm-1)) 495 (158). Repeated elemental analysis gave
consistently low values.
¯
The centrosymmetric triclinic space group P1 was assigned and
later determined to be correct. The structure was solved by
direct methods and refined on F2 by full-matrix least-squares
techniques. The analytical scattering factors14 for neutral atoms
were used throughout the analysis. Hydrogen atoms were located
from a difference-Fourier map and refined (x,y,z and Uiso). At
convergence, wR2 = 0.0757 and GOF = 1.050 for 320 variables
˚
refined against 4501 data (0.75 A), R1 = 0.0286 for those 3931 data
Physical methods
with I > 2.0(I).
A red crystal of [NMe4]0.5K0.5[NiIIH21iPr(OSiMe3)]·DMA of
approximate dimensions 0.24 ¥ 0.32 ¥ 0.37 mm was mounted
on a glass fiber and transferred to the diffractometer. The
diffraction symmetry was 4/mmm and the systematic absences
NMR spectra were obtained on either Bruker DRX-400 MHz
or Bruker Avance 500 MHz spectrometers. NMR solvents were
purchased in ampules and stored over molecular sieves. Electronic
spectra were recorded with a Cary 50 spectrophotometer or an
Agilent 8453 spectrophotometer. FT-IR spectra were collected
on a Mattson Genesis series FT-IR spectrometer or Varian 800
Scimitar series FT-IR spectrophotometer and are reported in
wavenumbers. Mass spectra were recorded on a Termo Trace MS+
GC-MS operated in EI mode.
¯
were consistent with the tetragonal space groups I42d and I41md.
¯
It was determined that space group I42d was correct. The structure
was solved by direct methods and refined on F2 by full-matrix least-
squares techniques. The analytical scattering factors14 for neutral
atoms were used throughout the analysis. Hydrogen atoms H4
and H5 were located from a difference-Fourier map and refined
(x,y,z and Uiso); all other hydrogen atoms were included using
a riding model. The potassium and tetramethylammonium ions
were each assigned a site-occupancy value of one-half. Carbon
atoms C(11) and C(12) were disordered and included using
multiple components, partial site-occupancy factors (50%) and
isotropic thermal parameters. At convergence, wR2 = 0.0647 and
X-Ray crystallographic data collection and refinement
of the structures
Intensity data for K[NiIIH21iPr(OH)]·DMA, [NMe4][NiIIH21iPr-
(OBut)], and (NMe4)0.5K0.5[NiIIH21iPr(OSiMe3)]·DMA were col-
lected using a Bruker CCD platform diffractometer using
˚
GOF = 1.120 for 289 variables refined against 6158 data (0.77 A),
˚
graphite-monochromated Mo Ka radiation (l = 0.71073 A). The
R1 = 0.0258 for those 5709 data with I > 2.0(I). The absolute
structure was assigned by refinement of the Flack15 parameter.
An orange crystal of [NMe4][NiIIH21iPr(OBut)] of approximate
dimensions 0.38 ¥ 0.39 ¥ 0.41 mm was mounted on a glass fiber
SMART10 program package was used to determine the unit-cell
parameters and for data collection (25 s/frame scan time for a
sphere of diffraction data). The raw frame data was processed
using SAINT11 and SADABS12 to yield the reflection data file.
2988 | Dalton Trans., 2009, 2986–2992
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Table 2 Selected metrical parameters for the [NiIIH21iPr(X)]-
and transferred to the diffractometer. The diffraction symmetry
was 4/mmm and the systematic absences were consistent with
the tetragonal space groups P43212 and P41212. Unfortunately,
it was not possible to determine which space group was correct.
The structure was solved and refined in both of the above space
groups. The quality of the refinement was the same for both space
groups—the results reported here were based on refinement under
space group P43212. Attempts to refine the structure as a racemic
twin did not improve the refinements. In addition, there was an
undetermined solvent molecule present that was not included in
the refinement. Because of these difficulties in refinement, only a
brief description of structure is included.
X
OH-
OTMS-
˚
Distances (A)
Ni–O1
Ni–N1
Ni–N2
Ni–N3
O1 ◊ ◊ ◊ N4
O1 ◊ ◊ ◊ N5
O1 ◊ ◊ ◊ K1
1.857(1)
1.928(1)
1.917(1)
1.914(2)
2.649(2)
2.757(2)
3.251(2)
1.876(1)
1.925(2)
1.913(1)
1.913(2)
2.719(2)
2.760(2)
—
Angles (◦)
N1–Ni–O1
N1–Ni–N2
N1–Ni–N3
N2–Ni–N3
N2–Ni–O1
N3–Ni–O1
172.49(6)
86.21(6)
84.71(6)
168.91(6)
94.59(6)
95.24(6)
171.27(6)
85.84(7)
84.24(7)
169.18(2)
96.01(6)
94.44(6)
Results and discussion
Preparation of [NiIIH21R(OH)]-
Scheme 1 outlines the two methods that have been developed for
the isolation of the [NiIIH21R(OH)]- complexes. Method A follows
from procedures we have used previously in synthesizing M–OH
complexes with our tripodal ureayl ligands. Three equiv. of base
are used to deprotonate H41R, which deprotonates both aNH
groups and one of the a¢NH groups. Treating this salt with one
equiv. of both Ni(OAc)2 and H2O affords purple solution of
[NiIIH21R(OH)]-, from which crystalline product was obtained
in yields of greater than 60%. In addition, we recover KOAc,
which precipitates from the reaction mixture in nearly quantitative
amounts.
Both [NiIIH21iPr(OH)]- and [NiIIH21tBu(OH)]- were stable solid-
state for weeks under a dry, inert atmosphere.
Isotopic labeling studies confirmed that water was the source of
t
the hydroxo ligands in the [NiIIH21Bu (OH)]- complexes. Solid-state
t
FTIR spectra on isolated [NiIIH21Bu (16OH)]- complexes showed
a prominent band at frequency of 3612 cm-1, which is assigned
n(OH) from the terminal hydroxo ligand.3 Preparing the com-
t
plexes with H218O afforded the corresponding [NiIIH21Bu (18OH)]-
species, whose n(OH) shifted to 3601 cm-1. These observed
shifts are expected based on a harmonic O–H oscillator model
(n(16OH)/n(18OH) = 1.003; calcd. 1.003).
Structural studies on [NiIIH21iPr(OH)]-
The solid-state structures of the [NiIIH21R(OH)]- were probed
using single crystal X-ray diffraction methods. Selected data for
K[NiIIH21iPr(OH)] are found in Tables 1 and 2; similar information
for [Me4N][NiIIH21iPr(OH)] and K[NiIIH21tBu(OH)] is found in the
ESI.† Because the molecular structures of anions in the three salts
are nearly identical, only the properties of [NiIIH21iPr(OH)]- will
be discussed in detail. See the ESI for detail on the structures of
the other Ni(II)–OH complexes.
The Ni(II) center in [NiIIH21iPr(OH)]- has a distorted square
planar coordination geometry (Fig. 2), in which three of the sites
are occupied by nitrogen donors from [H21R]2-. The ureayl aN
atoms are nearly trans to each other with an average Ni–Nurea
Scheme 1 Preparative routes to Ni(II)–OH complexes.
˚
distance of 1.916(1) A, whereas the Ni–N1 bond length is slightly
˚
Method B began with the preparation of a new nickel complex,
denoted Ni-350, that was obtained from treating [H21R]2- with
one equiv. of Ni(OAc)2. This red species has a characteristic
absorbance band at lmax = 350 nm and was stable in solution for
approximately 6 h. We were not able to determine either its formula
or structure—removal of solvent produced a red powder that
formed a purple solution (i.e. [NiIIH21R(OH)]-) when dissolved.
Note that we obtained greater than 95% of the expected KOAc
precipitated from the reaction mixture after addition of Ni(OAc)2,
suggesting that acetate ions were not coordinated. We found that
Ni-350 was a good synthon to produce isolatable four-coordinate
complexes. For instance, treating in situ generated Ni-350 with
one equiv. of KH and H2O produced [NiIIH21R(OH)]- in yields
that are comparable to those obtained with Method A. This
method was also used prepare alkoxide complexes (see below).
longer at 1.928(1) A. The Ni(II) ion is nearly in the plane formed
by the three aN atoms (N1/N2/N3-plane), displaced out by
0.11 A. The final coordination site is taken up by the hydroxo
˚
˚
ligand, with a Ni–O1 distance of 1.857(1) A and an O1–Ni–N1
angle of 172.49(6)◦. The hydroxo ligand also interacts with the
potassium cation within the lattice, with a K1 ◊ ◊ ◊ O1 distance of
3.251(2) A. It does not appear that this intermolecular interaction
˚
affects the coordination properties of the complex. We have also
prepared and structurally characterized [Me4N][NiIIH21iPr(OH)]
and K[NiIIH21tBu(OH)], salts in which there are no additional
intermolecular interactions to the hydroxo ligand.16 The molecular
structures of the anions for the all salts are effectively the same:
for instance, the Ni1–O1 distance of 1.862(3) A in the tetram-
ethylammonium salt is statistically identical to that observed with
K[NiIIH21iPr(OH)] (see above). The overall coordination geometry
˚
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in [NiIIH21iPr(OH)]- is similar to that reported for NiII(PCP)(OH)
(PCP, 2,6-bis((diisopropylphosphino)methyl)phenyl);8 this struc-
˚
ture also has a short Ni–O(H) distance of 1.865(2) A. These Ni–
O(H) bond lengths in [NiIIH21iPr(OH)]- and NiII(PCP)(OH) are
˚
significantly shorter than the 1.955(2) A distance found in the
five-coordinate [NiIIcyclam(OH)]+.7
One distinguishing feature in the molecular structure of
[NiIIH21iPr(OH)]- is the H-bonding network surrounding the
hydroxo ligand. Two intramolecular H-bonds are formed between
the a¢NH groups and O1, as indicated by the N ◊ ◊ ◊ O1 distances
˚
of 2.649(2) and 2.757(2) A. This assignment is support by FTIR
studies that showed a broadening of the peaks associated with
the N–H vibrations, which is a common occurrence for H-bonded
systems. Notice the placement of the urea groups of the [H21iPr]2-
relative to the N1/N2/N3-plane: both urea arms are tilted from
the plane, positioned on the same side such that the a¢N–H vectors
are directed toward the O1. However, their relative displacements
from the N1/N2/N3-plane are not equivalent; for instance, the
differences in the distances of N4 and N5 to the plane differ
Scheme 2 Reaction of Ni(II)–OH complexes with bases.
˚
˚
˚
by more than 0.7 A (0.306 A for N4 and 1.07 A for N5). An
unsymmetrical H-bonding network is observed, with N ◊ ◊ ◊ O1
distances that differ by greater than 0.1 A. Note also that O1
is displaced from the N1/N2/N3-plane by 0.467 A, on the side
hydroxo ligand. These alkoxide complexes, [NiIIH21R(OBut)]- were
also prepared independently by treating Ni-350 with 1 equiv. of
KOBut in DMA.
˚
˚
opposite to that of the urea arms.
The molecular structure of [NiIIH21iPr(OBut)]- (Fig. 3A) de-
termined by X-ray diffraction closely resembled that found
for [NiIIH21iPr(OH)]- (Fig. 2). The X-ray data collected for
[Me4N][NiIIH21iPr(OBut)] was of insufficient quality to obtain a
complete description of the lattice, yet the connectivity of the
anion was clearly resolved. The Ni(II) center in [NiIIH21iPr(OBut)]-
is coordinated by three nitrogen atoms and the oxygen atom from
the OBut ligand, producing a nearly square planar geometry. The
complex can accommodate the bulky tert-butyl group by placing it
out of the equatorial plane, away from the H-bonding pocket that
surrounds the alkoxy oxygen, O1. In addition, the tert-butyl group
is arranged anti to the methyl group containing C13, presumably
to avoid unfavorable steric interactions.
The deprotonation of [NiIIH21R(OH)]- was further investigated
with [N(TMS)2]- because we thought this base would not displace
the hydroxo ligand as seen with alkoxides. Our previous work
on related complexes showed that external species containing
nitrogen-donors are unable to displace oxo or hydroxo ligands,
results attributed to oxygen atoms being significantly better
H-bond acceptors. Thus treating the Ni(II)–OH complexes with
[N(TMS)2]- in DMA produced spectroscopic changes consis-
tent with loss of the hydroxo ligand. However, the desired
Ni(II)–oxo complexes were not isolated; instead complexes were
produced in which the hydroxo ligand was converted into a siloxyl
group.
Reactivity of [NiIIH21R(OH)]- with bases
The isolation of the [NiIIH21R(OH)]- complexes allowed us to
explore the formation of a Ni(II)–oxo species via deprotonation of
the hydroxo ligand (eqn (1)). We have used this method previously
in preparing M(III)–O complexes (M(III) = Fe, Mn). Initial
studies involved treating [NiIIH21R(OH)]- with alkali hyrides. The
product(s) of these reactions did not contain the characteristic
n(NiO–H) peak at 3612 cm-1, yet these findings alone are
not proof of formation of the desired N(II)–O complexes. For
instance, there were no changes in the absorbance spectra of these
produces compared to those of the starting Ni(II)–OH complexes.
Unfortunately, several attempts to isolate single crystals of these
species were unsuccessful.
(1)
The reactivity of [NiIIH21R(OH)]- with weaker bases than
hydrides has also be examined. No reactions were observed be-
tween [NiIIH21R(OH)]- and quinuclidine, 1,8-diazabicyclo(5.4.0)-
undec-7-ene (DBU), 1,5-diazabicyclo[4.3.0]non-5-ene (DBN),
and triethylamine, bases with pKa values of less than 13 in
DMSO. We were able to isolate pure products for the reaction
of [NiIIH21R(OH)]- with KOBut, a base with a pKa ~ 29 in
DMSO (Scheme 2). These complexes lacked the n(NiO–H) peaks
in their FTIR spectra and showed ~5 nm blue shifts in their
visible absorbance features. Analytical and X-ray diffraction data
confirmed that rather than deprotonation, a ligand substitution
reaction occurred, whereby the tert-butoxide anion replaced the
Results from X-ray diffraction experiments confirmed the for-
mation of the siloxide (Fig. 3B, Table 2). The primary coordination
sphere in [NiIIH21iPr(OTMS)]- includes a relatively short Ni–O1
˚
bond of 1.876(1) A with Ni–N distances that are nearly identical
to those observed in [NiIIH21iPr(OH)]-. The Ni center and O1 are
˚
positioned out of the N1/N2/N3 plane by 0.079 and 0.434 A
respectively. Short N4 ◊ ◊ ◊ O1 and N5 ◊ ◊ ◊ O1 distances of 2.719(2)
˚
and 2.760(2) A suggest the presence of intramolecular H-bonds
in [NiIIH21iPr(OTMS)]-. The TMS group of the siloxide is pointed
away from the square plane in an analogous manner to that tert-
butyl group in Ni(II)–OBut complex.
2990 | Dalton Trans., 2009, 2986–2992
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Fig. 2 Thermal ellipsoid plots illustrating two views of the molecular structure of [NiIIH21iPr(OH)]-. Thermal ellipsoids are drawn at the 50% level and
for clarity only urea and hydroxo hydrogen atoms are shown.
Fig. 3 Thermal ellipsoid plots illustrating the molecular structure of [NiIIH21iPr(OBut)]- (A) and [NiIIH21iPr(OTMS)]- (B). Thermal ellipsoids are drawn
at the 50% level and for clarity only urea atoms are shown. Only one of the disorder fragments of the isopropyl group appended from N5 is shown for the
structure in B.
m-(OH)–Ni(II) species through placement of bulky R groups
proximal to the hydroxide.
Summary
We have described the preparation of monomeric Ni(II) complexes
with terminal hydroxo ligands. The isolation of such complexes
is rare and was achieved using [H21R]2-, a tridentate ligand
containing two ureayl groups. This chelating ligand provided three
nitrogen donors to the Ni(II) center, leaving one coordination
site available to bind an external ligand. Water was used as
the source of hydroxide, which was produced via deprotonation
by [H1R]3- as shown in Fig. 4. The [NiIIH21R(OH)]- complexes
have intramolecular H-bonding networks surrounding the Ni(II)–
OH unit, involving the hydroxo oxygen atom and the ureayl
groups of [H21R]2-. The H-bonds help stabilize the complexes
and undoubtedly prevent formation of the more common Ni(II)–
Attempts to deprotonate the hydoxo ligand were undertaken
with a variety of bases but definitive evidence for the formation
of a Ni(II)–oxo complex was not obtained. This type of oxometal
complex has yet to be observed—orbital arguments involving the
inability of the Ni(II) center to form p-bonds are often used to
rationalize the lack of examples. However, we have found that
H-bonds can, in some systems, replace p-bonds in stabilizing with
M–oxo unit, leading to the isolation of late 3d oxometal complexes.
Our failure to detect a Ni(II)–oxo complex is most likely because
of their strong basicity and nucleophilicity. We did observed
ligand substitution reactions with KOBut to prepare Ni(II)–OBut
complexes and the formation of Ni(II)–siloxyl complexes when
[NiIIH21R(OH)]- was treated with K[N(TMS)2].
Acknowledgements
We thank the NIH (050781) for financial support.
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Dalton Trans., 2009, 2986–2992 | 2991
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˚
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2992 | Dalton Trans., 2009, 2986–2992
This journal is
The Royal Society of Chemistry 2009
©