Terminal NiII?OH/?OH2complexes in trigonal bipyramidal geometries derived from H2O

Article abstract of DOI:10.1016/j.poly.2016.11.015

The preparation and characterization of two Ni^IIcomplexes are described, a terminal Ni^II–OH complex with the tripodal ligand tris[(N)-tert-butylureaylato)-N-ethyl)]aminato ([H₃buea]^3?) and a terminal Ni^II

Full text of DOI:10.1016/j.poly.2016.11.015

Accepted Manuscript
Terminal NiII−OH/−OH2 complexes in trigonal bipyramidal geometries derived
from H2O
Nathanael Lau, Yohei Sano, Joseph W. Ziller, A.S. Borovik
PII:
S0277-5387(16)30595-2
DOI:
http://dx.doi.org/10.1016/j.poly.2016.11.015
POLY 12322
Reference:
Polyhedron
To appear in:
Received Date:
Revised Date:
Accepted Date:
31 August 2016
12 November 2016
12 November 2016
Please cite this article as: N. Lau, Y. Sano, J.W. Ziller, A.S. Borovik, Terminal NiII−OH/−OH2 complexes in trigonal
bipyramidal geometries derived from H2O, Polyhedron (2016), doi: http://dx.doi.org/10.1016/j.poly.2016.11.015
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Terminal NiII−OH/−OH2 complexes in trigonal bipyramidal geometries
derived from H2O
Nathanael Lau, Yohei Sano, Joseph W. Ziller, A.S. Borovik*
Department of Chemistry, University of California – Irvine, 1102 Natural Sciences II, Irvine, CA 92697-2025, United States
A B S T R A C T
The preparation and characterization of two NiII complexes are described, a terminal NiII–OH complex with the tripodal
ligand tris[(N)-tertbutylureaylato)-N-ethyl)]aminato ([H3buea]3−) and a terminal Ni II–OH2 complex with the tripodal ligand
N,N',N"-[2,2’,2”-nitrilotris(ethane-2,1-diyl)]tris(2,4,6-trimethylbenzenesulfonamido) ([MST]3−). For both complexes, the
source of the –OH and –OH2 ligand is water. The salts K2[NiIIH3buea(OH)] and NMe4[NiIIMST(OH2)] were characterized
using perpendicular-mode X-band electronic paramagnetic resonance, Fourier transform infrared, UV-visible spectroscopies,
and its electrochemical properties were evaluated using cyclic voltammetry. The solid state structures of these complexes
determined by X-ray diffraction methods reveal that they adopt a distorted trigonal bipyramidal geometry, an unusual
structure for 5-coordinate NiII complexes. Moreover, the NiII–OH and NiII–OH2 units form intramolecular hydrogen bonding
networks with the [H3buea]3− and [MST]3− ligands. The oxidation chemistry of these complexes was explored by treating the
high-spin NiII compounds with one-electron oxidants. Species were formed with S = 1/2 spin ground states that are consistent
with formation of monomeric NiIII species. While the formation of NiIII–OH complexes cannot be ruled out, the lack of
observable O-H vibrations from the putative Ni–OH units suggest the possibility that other high valent Ni species are formed.
Keywords: Nickel-hydroxido complexes, Nickel-aqua complexes
Stuky in 1969 [24], and since then many other structurally
characterized terminal NiII−OH2 complexes have been
prepared [25–27].
1. Introduction
Monomeric metal complexes of first-row transition
metal ions with terminal hydroxido and aqua ligands are
often difficult to prepare because of the strong tendency
of these ligands to bridge between metal centers [1–10].
However, hydroxido and aqua bridging may be prevented
by using steric effects, as evidenced by the ubiquity of
terminal metal hydroxido and aqua units in biology [11–
15]. Using steric effects in this manner is also effective in
synthetic systems, as most synthetic terminal nickel
hydroxido and aqua complexes use sterically encumbered
ligand frameworks around the metal center to prevent
bridging. Cámpora prepared the earliest examples of
monomeric square planar terminal NiII−OH moieties
[16,17], and a number of similar 4-coordinate terminal
NiII−OH complexes have been developed for catalysis
[18–21]. To the best of our knowledge, there are only two
Our group has also successfully used this approach to
prepare many terminal metal hydroxido and aqua
complexes, including a pair of terminal NiII−OH
complexes using sterically bulky tridentate ligands [28].
The ligands used in that work were derived from the urea
based tripodal ligand tris[(N)-tertbutylureaylato)-N-
ethyl)]aminato ([H3buea]3−), shown in Fig. 1A [29].
Various monometallic terminal metal hydroxido and
oxido complexes have been stabilized with [H3buea]3−, as
the three tert-butyl groups of [H3buea]3− protects the
hydroxido or oxido ligand by hindering access [30–32].
An additional feature of [H3buea]3− is its ability to
promote the formation of intramolecular hydrogen bonds
(H-bonds). For instance, in our previously prepared M–
OH complexes the terminal hydroxido ligand was further
stabilized through intramolecular H-bonding networks
that are formed between the urea N−H groups of
[H3buea]3− and the oxygen atom of the hydroxido ligand.
Our group has also designed systems that are capable
of accepting H-bonds from an apical exogenous ligand.
examples
of
crystallographically
characterized
monometallic 5-coordinate NiII−OH complexes. Riordan
has prepared a square pyramidal complex with a 1,4,8,11-
tetramethyl-1,4,8,11-tetraazadodecane ligand in which the
exogenous hydroxido ligand is derived from dioxygen
[22], and Levy has prepared a trigonal bipyramidal
complex with a bulky Schiff base ligand where the
exogenous hydroxido ligand is derived from adventitious
water [23]. The earliest structurally characterized 5-
coordinate terminal NiII−OH2 complex was reported by
For
example,
the
ligand
N,N',N"-[2,2’,2”-
nitrilotris(ethane-2,1-diyl)]tris(2,4,6-
trimethylbenzenesulfonamido) ([MST]3−) (Fig. 1B) is a
sulfonamide-based tripodal ligand that can stabilize
hydroxido or aqua ligands through intramolecular H-

Fig. 1. The ligands used in this study, (A) [H3buea]3− and (B) [MST]3−
.
bonds involving the O–H group and the sulfonamido
oxygen atoms [33–35]. In addition, the S=O and mesityl
groups in [MST]3− appear to be bulky enough to help
prevent the hydroxido or aqua ligand from bridging [36–
38].
2.2. Preparation of the complexes with [H3buea]3−
2.2.1. Preparation of K2[NiIIH3buea(OH)]
A solution of H6buea (300 mg, 0.69 mmol) in
anhydrous dimethylacetamide (DMA) (20 mL) was
treated with solid KH (110 mg, 2.9 mmol). The mixture
was stirred until gas evolution ceased. Ni(OAc)2 (120 mg,
0.69 mmol) was added to the clear pale yellow reaction
mixture, and the solution was stirred. After 2 h, H2O (13
µL, 0.72 mmol) was added to the red solution via
syringe, and the mixture was stirred for 15 min. After the
addition of dimethylformamide (DMF) (4 mL), the
reaction mixture was filtered through a medium porosity
glass-fritted funnel to remove the insoluble KOAc. The
green-yellow filtrate was concentrated under vacuum to
ca. 1 mL and treated with Et2O (20 mL) followed by
pentane (20 mL) to precipitate a green solid. The green
solid was collected on a fine porosity glass-fritted funnel
and dried under vacuum. After 1 h, the solid was washed
with acetonitrile (MeCN) to remove an orange filtrate.
The green solid was redissolved in DMA (20 mL) and
recrystallized by slow diffusion with Et2O. After 2 d,
green crystals were collected to give 360 mg (87%) of
The following work describes the synthesis and
characterization of a NiII−OH complex with the ligand
[H3buea]3− and a NiII−OH2 complex with the ligand
[MST]3−, where the hydroxido and aqua ligands were
both derived from water. Our previous work suggested to
us that these ligands would be logical candidates to
stabilize monomeric NiII complexes with terminal
hydroxido and aqua ligands. We also probed their
oxidative chemistry with the goal of establishing the
properties of NiIII–OH species. Such species are believed
to be a key intermediate in Ni based oxidations [39–43],
but to date, no putative NiIII−OH species has been
structurally characterized. Our findings show that while
new oxidize species can be detected, our evidence does
not conclusive show that the NiIII–OH unit has remained
intact.
2. Experimental
product.
Elemental
Anal.
Calc.
for
2.1. General Methods
K2[NiIIH3buea(OH)]·2DMF, K2NiC29H61N9O6: C, 45.31;
H, 8.00; N, 16.40. Found: C, 45.01; H, 8.19; N, 16.15%.
FTIR (KBr disc, cm−1, selected bands): 3233, 3146, 2962,
2921, 2849, 1592, 1509, 1447, 1388, 1357, 1247, 1223,
1149, 1118, 1034, 971, 796, 731. λmax (DMF, nm, ε
M−1cm−1): 324 (1400), 424 (61), 493 (35), 677 (23). µeff
(DMSO, µB): 3.1. Ea (DMF): −0.830 V versus [FeCp2]0/+.
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 sieves. The tripodal
compounds H6buea and H3MST were synthesized
following literature procedures [29,33]. The preparations
of metal complexes were conducted in a Vacuum
Atmospheres, Co. drybox under argon atmosphere.
Potassium hydride (KH) as a 60% dispersion in mineral
oil was filtered with a medium porosity glass-fritted
funnel and washed 5 times each with pentane and diethyl
ether (Et2O). Solid KH was dried under vacuum and
stored under inert atmosphere. Ni(OAc)2 was prepared by
literature procedures [44]. I2 was sublimed under vacuum
and stored under inert atmosphere. Water was degassed
by five freeze-pump-thaw cycles and stored under inert
atmosphere.
2.2.2. Oxidation of K2[NiIIH3buea(OH)]
A solution of K2[NiIIH3buea(OH)] (50 mg, 0.085
mmol) in DMF (2 mL) was treated with solid I2 (11 mg,
0.045 mmol). The yellow-green solution immediately
turned purple-red and was allowed to stir for 5 min. The
solution was concentrated under vacuum until near
dryness and Et2O (5 mL) was added to precipitate a
purple-red powder. After decanting the liquid, the powder
was redissolved in THF (2 mL). The reaction mixture was
filtered through a fine porosity glass-fritted funnel to
remove the insoluble KI. The filtrate was concentrated
under vacuum until near dryness then treated with Et2O

(5mL) and the resulting red powder was collected on a
medium porosity glass-fritted funnel. EPR (DMF, 77 K):
g = 2.29, 2.17, 2.04. FTIR (ATR, cm−1, selected bands):
3308, 2958, 2888, 1587, 1474, 1451, 1386, 1351, 1259,
1223, 1151, 1103, 1032, 961, 837, 777, 754. λmax (DMF,
nm, ε M−1cm−1): 326 (~ 2000), 502 (~ 570).
became dark blue. After 1 h of stirring at -30 °C, pentane
(20 mL) was added to precipitate a dark blue solid. The
solid was collected by filtering through a medium
porosity glass-fritted funnel, washed with pentane (20
mL), dried under vacuum to yield 211 mg (81%) dark
blue powder, and stored at -30°C. EPR (CH2Cl2, 77 K): g
= 2.00. λmax (CH2Cl2, nm, ε M−1cm−1): 309 (6.4 × 104),
367 (7.2 × 104), 725 (1.1 × 105).
2.3. Preparation of the complexes with [MST]3−
2.3.1. Preparation of NMe4[NiIIMST(OH2)]
2.3.3. Oxidation of NMe4[NiIIMST(OH2)]
A solution of H3MST (205 mg, 0.30 mmol) in
anhydrous DMA (4 mL) was treated with solid KH (37
mg, 0.91 mmol). The mixture was stirred until gas
evolution ceased. Ni(OAc)2·4H2O (73 mg, 0.29 mmol)
and NMe4OAc (40 mg, 0.30 mmol) were added to the
clear pale yellow reaction, and the solution was stirred.
After 3 h, Et2O (5 mL) was added to the green-yellow
solution to aid the precipitation of KOAc. The reaction
mixture was filtered through a medium porosity glass-
fritted funnel to remove the insoluble species. The filtrate
was concentrated under vacuum to ca. 1 mL and treated
with Et2O (10 mL) followed by pentane (40 mL) to
precipitate a yellow solid. The yellow solid was collected
on a medium porosity glass-fritted funnel and dried under
vacuum to give 207 mg (85%) of product. FTIR (KBr
disc, cm−1, selected bands): 3259, 3030, 2973, 2937,
2854, 1603, 1563, 1490, 1468, 1254, 1128, 1054, 977,
830, 815, 742, 656, 610. MS (ES-, m/z): Exact mass calcd
for NiC33H45N4O6S3: 747.2. Found: 747.2. This salt,
presumably NMe4[NiIIMST] (77 mg, 0.092 mmol) was
redissolved in CH2Cl2 (10 mL) and treated with H2O (2
µL, 0.10 mmol) in one portion via a syringe, and the
mixture was stirred. After 15 min the green solution was
filtered through a medium porosity glass-fritted funnel to
remove any insoluble species and the filtrate was layered
under pentane. After 2 d, green and yellow needle crystals
were collected via filtration and dried very briefly under
vacuum, to give 74 mg (94%) of crystalline product.
Elemental Anal. Calc. for NMe4[NiIIMST(OH2)]
NiC37H59N5O7S3: C, 52.86; H, 7.07; N, 8.33. Found: C,
52.95; H, 7.16; N, 8.18%. FTIR (KBr disc, cm−1, selected
bands): 3266, 3025, 2970, 2934, 2855, 1604, 1562, 1490,
1468, 1405, 1377, 1342, 1258, 1230, 1132, 1054, 977,
813, 742, 657, 607. (Nujol, cm−1): 3241 (OH). (CH2Cl2,
20 mM, cm−1): 3280. λmax (DMF, nm, ε M−1cm−1): 312
(3300) 431 (120), 506 (39), 724 (33). µeff (µB): 3.22. E1/2
(MeCN): 0.370 V versus [FeCp2]0/+
A solution of NMe4[NiIIMST(OH2)] (25 mg, 0.030
mmol) in CH2Cl2 (2 mL) was cooled to -30°C, then
treated with a solution of [TBPA][PF6] (21 mg, 0.033
mmol) in CH2Cl2 (1 mL) at -30°C. The yellow-green
solution immediately turned orange-red, and was allowed
to stir at -30°C for 5 min. The solution was concentrated
under vacuum until near dryness, washed with Et2O
(5mL), and collected on a medium porosity glass-fritted
funnel. EPR (CH2Cl2, 77 K): g = 2.66, 2.15, 1.99. FTIR
(ATR, cm−1, selected bands): 3250, 3021, 2968, 2934,
2853, 1602, 1579, 1485, 1415, 1380, 1311, 1265, 1184,
1152, 1098, 1071, 1054, 1006, 976, 950, 830, 734, 654,
609. λmax (CH2Cl2, nm, ε M−1cm−1): 312 (>12000), 440
(~1200) and 530 (~550)
2.4. Physical Methods
Elemental analyses were performed on a Perkin-
Elmer 2400 CHNS analyzer. 1H NMR and 13C NMR were
recorded on a Bruker DRX500 spectrometer. FTIR
spectra were collected on a Varian 800 Scimitar Series
FTIR spectrometer in air or a Thermo Scientific Nicolet
iS5 spectrophotometer with an iD5 Attenuated Total
Reflectance (ATR) attachment in a nitrogen filled
glovebox. High-resolution mass spectra were collected
using Waters Micromass LCT Premier Mass
Spectrometer. UV-vis spectra were recorded with a Cary
50 or an Agilent 8453 spectrophotometer using a 1.00 cm
quartz cuvette. Perpendicular-mode X-band EPR spectra
were collected using a Bruker EMX spectrometer at 10K
using liquid helium. Solution effective magnetic moments
were measured by the Evans’ method on a Bruker
DRX500 spectrometer using flame sealed standard cores
of 1:1 DMSO:DMSO-d6 or 1:1 CHCl3:CDCl3.[47] Cyclic
voltammetry (CV) experiments were conducted using a
CH1600C electrochemical analyzer. A 2.0 mm glassy
carbon electrode was used as the working electrode at
scan velocities 0.5 Vs−1 unless otherwise noted. A
cobaltocenium/cobaltocene couple ([CoCp2]+/0) was used
as an internal reference then scaled against the
2.3.2. Preparation of tris-(4-bromophenyl)ammoniumyl
hexafluorophosphate ([TBPA][PF6])
ferrocene/ferrocenium
couple
([FeCp2]0/+).[48]
[TBPA][PF6] was prepared according to literature
procedures with the following modifications [45,46]. A
solution of tris-(4-bromophenyl)amine (200 mg, 0.42
mmol) in CH2Cl2 (2 mL) was cooled to -30 °C. Upon
addition of nitrosonium hexafluorophosphate ([NO][PF6],
74 mg, 0.41 mmol), the clear solution immediately
Tetrabutylammonium hexafluorophosphate (TBAP) was
use as the supporting electrolyte at a concentration of 0.1
M.
2.5. Crystallography

t
Bu
2 K
tBu
NH
tBu
O
HN
N
O
NH
a-c
H
N
H
N
OH
O
t
N
Bu
3
-4 H2, -2 KOAc
N
NiII
O
N
N
H6buea
K2[NiIIH3buea(OH)]
Scheme 1. Preparation of K2[NiIIH3buea(OH)]. Conditions (a) 4 KH; (b) Ni(OAc)2; (c) H2O.
A Bruker SMART APEX II diffractometer and the
APEX2 program package was used to determine the unit-
cell parameters and for data collection. Crystallographic
details are summarized in the Supporting information, and
peaks for the ν(OH) [49]; the reason for the absence of
these signals in not known.
3.2. Preparation and properties of NMe4[NiIIMST(OH2)]
O
NMe4
O
Mes
O O
S
S
N
O O
S
a-d
H H
H
O
N
Mes
N
N
S
Mes
-3 H2, -3 KOAc
NiII
N
3
N
Mes
O
O
NMe4[NiIIMST(OH2)]
H3MST
Scheme 2. Preparation of NMe4[NiIIMST(OH2)]. Conditions (a) 3 KH, DMA, Ar, rt; (b) Ni(OAc)2·4H2O, DMA, Ar, rt; (c) NMe4OAc, DMA, Ar, rt; (d)
H2O, CH2Cl2 Ar, rt.
in Table S1.
The preparation of NMe4[NiIIMST(OH2)] also
followed a reported route from our group (Scheme 2)
[38]. Green crystals of this salt suitable for X-ray
diffraction were obtained by layering a CH2Cl2 solution
of the compound under pentane.
3. Results and Discussion
3.1. Preparation and properties of K2[NiIIH3buea(OH)]
The preparation of K2[NiIIH3buea(OH)] followed
literature methods reported for other MII–OH with
[H3buea]3− (Scheme 1) [49]. Green crystals of this salt
suitable for X-ray diffraction were obtained by slow
diffusion of Et2O vapors into a DMA or DMF solution of
the compound.
The absorbance spectrum of [NiIIMST(OH2)]− was
characterized by peaks in the visible region at λmax = 431
(ε = 110), 506 (ε = 33), and 724 nm (ε = 52) and had no
EPR features in perpendicular-mode. The solution
effective magnetic moment was 3.2 µB, supporting an S =
1 spin ground state [47]. The FTIR spectrum of
NMe4[NiIIMST(OH2)], recorded as a Nujol mull, revealed
a peak at 3241 cm−1 that is assigned to the O−H vibration
from the aqua ligand [37,38].
The absorbance spectrum of [NiIIH3buea(OH)]2−
displayed peaks in the visible region at λmax = 424 (ε =
61), 493 (ε = 35), and 677 nm (ε = 23). These absorbance
features are similar to those of other NiII centers in local
C3 symmetry [50,51]. The X-band perpendicular-mode
EPR spectrum taken at 77 K was silent, which is expected
for a d8 metal center of integer spin. Evans’ method was
used to determine the solution effective magnetic moment
of 3.1 µB [47]. This value is consistent with the spin-only
value for an S = 1 system of 2.8 µB, indicating that the
NiII metal center is high-spin. The FTIR spectra of
K2[NiIIH3buea(OH)], recorded both with ATR-IR and as a
Nujol mull, did not show peaks corresponding to an O−H
vibration from the hydroxido ligand. Note that all other
MII–OH complexes with [H3buea]3− also fail to reveal any
3.3.
Solid-state
molecular
structure
of
[NiIIH3buea(OH)]2− and [NiIIMST(OH2)]−
The molecular structure of [NiIIH3buea(OH)]2− was
determined by X-ray diffraction methods and the thermal
ellipsoid diagram of the complex is shown in Fig. 2A,
with selected metrical parameters shown in Table 1. The
[NiIIH3buea(OH)]2− complex crystallized as a monomer,
with trigonal bipyramidal (tbp) coordination geometry.
The N4O primary coordination sphere around the NiII
center is defined by a trigonal plane derived from three

deprotonated urea nitrogen atoms with the amine nitrogen
and the oxygen atoms from the exogenous hydroxido
ligand occupying the axial positions.
Riordan’s or Levy’s complexes, which have Ni1–O1
distances of 1.955(2) and 1.911(4) Å, respectively. This
difference is because [H3buea]3− contains anionic N
Fig. 2. Thermal ellipsoid diagram depicting the molecular structure of (A) [NiIIH3buea(OH)]2− and (B) [NiMST(OH2)]−. Ellipsoids are drawn at the 50%
probability level, and only urea, hydroxido, and aqua H atoms are shown for clarity.
The complex shows a distortion from idealized tbp
geometry based on the structural parameter τ5 = 0.81, in
which an ideal tbp geometry has τ5 = 1 and ideal square
pyramidal geometry has τ5 = 0 [52]. This distortion is
partially caused by the Jahn-Teller effect that should be
present in a high-spin d8 metal complex having local C3
symmetry [53]. Another possible contributor to this
distortion is the presence of intramolecular H-bonds
formed between the urea hydrogen atoms of [H3buea]3−
and O-atom from the NiII–OH unit. Two relatively short
H-bonds are formed as gauged by the N···O distances:
N6···O1 and N7···O1 distance of 2.789(1) and 2.786(1)
Å that are statistically shorter than the N5···O1 distance
of 2.829(1) Å. All three interactions may be considered
H-bonds because the N···O distances are under 3.07 Å.
However, the two shorter distances are in the region often
associated with strong H-bonding [54] and could reflect
the fact that the hydroxido ligand only has two available
lone pairs.
To our knowledge, the molecular structure of
[NiIIH3buea(OH)]2− represents only the third example of a
structurally characterized, 5-coordinate monometallic
NiII−OH. Riordan was the first report such a species but
this NiII–OH complex has a square planar primary
coordination sphere coordination with a τ5 value of 0.0
[22]. Levy’s nickel-hydroxido complex has a τ5 value of
0.86, and is thus the only other NiII–OH complex with tbp
geometry [23]. The Ni1–O1 distance of 2.018(1) Å is
significantly longer in [NiIIH3buea(OH)]2− than in either
donors and contains intramolecular H-bonds to the
hydroxido ligand, both of which are absent in the other
complexes.
Table 1
Selected metrical parameters for [NiIIH3buea(OH)]2−,
[NiIIMST(OH2)]− complexes
[NiIIH3buea(OH)]2−
[NiIIMST(OH2)]−
Atomic distances (Å)
Ni1–O1
Ni1–N1
Ni1–N2
Ni1–N3
Ni1–N4
2.018(1)
2.106(1)
2.089(1)
2.055(1)
2.059(1)
2.068(1)
2.074(2)
2.114(2)
2.027(2)
2.051(2)
2.026(2)
2.035(2)
Ave. Ni–Neq
N5···O1
N6···O1
N7···O1
O1···O2
O1···O4
O1···O6
2.829(1)
2.789(1)
2.786(1)
2.686(2)
2.679(2)
2.998(2)
Bond angles (˚)
N1–Ni1–O1
N1–Ni1–N2
N1–Ni1–N3
N1–Ni1–N4
N2–Ni1–N3
N2–Ni1–N4
N3–Ni1–N4
τ5 value
177.9(5)
80.5(5)
82.1(6)
83.0(6)
129.1(6)
104.6(5)
120.2(6)
0.81
176.8(8)
82.9(8)
82.6(8)
83.8(8)
120.5(9)
109.8(9)
125.4(9)
0.86

A
B
+
+
10 µA
1 µA
-0.6 -0.7 -0.8 -0.9 -1.0 -1.1 -1.2
Potential (V vs [FeCp2]+/0)
0.7
0.6
0.5
0.4
0.3
0.2
0.1
Potential (V vs [FeCp2]+/0)
Fig. 3. Cyclic voltammogram of (A) K2[NiIIH3buea(OH)], collected at 100 mV s−1 in a 0.1 M TBAP solution in DMF using [CoCp2]0/+ as an internal
reference, then scaled to [FeCp2]+/0, and of (B) NMe4[NiIIMST(OH2)], collected at 100 mV s−1 in a 0.1 M TBAP solution in CH2Cl2 using [FeCp2]+/0 as an
internal reference.
The molecular structure of [NiIIMST(OH2)]− was also
determined by X-ray diffraction methods (Fig. 2B, Table
1). The [NiIIMST(OH2)]− complex crystallized as a
monomer, with the N4O donors around the NiII center
adopting a tbp primary coordination sphere. The three
deprotonated sulfonamido nitrogen atoms define the
trigonal plane with the amine nitrogen atom and the
oxygen atom from the exogenous aqua ligand occupying
the apical positions. The complex shows a slight
distortion from tbp geometry based on the structural
parameter τ5 = 0.86. As with [NiIIH3buea(OH)]2−, this
distortion is expected to be caused by a combination of
Jahn-Teller effect and the intramolecular H-bonding
network that surrounds the NiII–OH2 unit. Two H-bonds
are formed between the aqua ligand to the sulfonamido
oxygen atom on two of the ligand arms, as gauged by the
O···O atom distances of 2.686(2) and 2.679(2) Å.
[NiIIMST(OH2)]− than in either Stucky’s or Yap’s
complexes, which have Ni1–O1 distances of 2.100(6) and
2.092(2) Å respectively. This difference is also attributed
to the presence of intramomolecular H-bonds involving
the aqua ligand.
3.4. Electrochemical properties of [NiIIH3buea(OH)]2−
and [NiIIMST(OH2)]−
The CV of K2[NiIIH3buea(OH)] in a 0.1 M TBAP
solution in DMF (Fig. 3A) showed an irreversible
oxidative event at −830 mV versus [FeCp2]+/0. In
addition, the CV for NMe4[NiIIMST(OH2] measured in a
0.1 M TBAP solution in CH2Cl2 showed a quasi-
reversible one-electron oxidation event at +370 mV
versus [FeCp2]+/0 (Fig. 3B). This positive shift in redox
potential for the complex made with [MST]3− versus
analogous [H3buea]3− complex is expected, as the
sulfonamido N atoms on [MST]3− are weaker donors to
the metal center than the ureido N atoms [56]. Moreover,
the [NiIIMST(OH2)]– complex is a mono-anion while the
Five-coordinate terminal NiII−OH2 complexes are
rare, with the majority of such complexes have τ5 values
closer to 0.0 and thus have distorted square pyramidal
primary coordination spheres. To the best of our
knowledge, [NiIIMST(OH2)]− has the the highest τ5 value
of 5-coordinate terminal NiII−OH2 complexes and is
therefore the closest to having tbp coordination geometry.
[NiIIH3buea(OH)]2–
is
a
di-anion,
and
thus
[NiIIMST(OH2)]– should be more difficult to oxidize.
These results suggested that the NiII complexes could be
oxidized to NiIII species and attempts to chemically
prepare these oxidized products were undertaken.
For
comparison,
the
N-methyl-1,4-
diazabicyclo[2.2.2]octane based complex of Stucky [24]
and the scorpionate based complex of Yap [55] have τ5
values of 0.73 and 0.66, representing higher limits for τ5
values. The Ni1–O1 distance of 2.074(1) Å is shorter in
3.5. Preparation and characterization of a NiIII species
with [H3buea]3−

2.29
A1.0
B
331
0.8
0.6
0.4
0.2
0.0
2.17
514
2.04
400
500
600
700
800
900 1000
2900
3100
3300
3500
Wavelength (nm)
B (G)
Fig. 4. (A) UV-vis spectrum for oxidation of a 0.4 mM DMF solution of K2[NiIIH3buea(OH)] by I2 at 25 ˚C, showing the conversion of the initial NiII−OH
species (solid black) to a NiIII species (dashed black) after 5 min. (B) Perpendicular-mode X-band EPR spectra taken at 10 K of the putative NiIII species
with [H3buea]3− prepared in DMF with I2 as the oxidant (solid black) and simulated spectrium (dashed red).
The addition of elemental iodine (I2) to a sample of
K2[NiIIH3buea(OH)] in DMF resulted in an immediate
color change from yellow-green to purple-red.
Monitoring this reaction with UV-vis spectroscopy at
room temperature showed the growth of an intense peak
at λmax = 326 nm (ε ~ 2000) and a peak at λmax = 502 nm
(ε ~ 570) (Fig. 4A). The perpendicular-mode EPR
spectrum collected at 10 K of the purple-red solution
contained a rhombic EPR signal with g-values at 2.29,
2.17, and 2.04 (Fig. 4B). This spectrum is consistent with
a complex containing a d7 NiIII center with an S=1/2 spin
ground state.[50,57,58] Nearly identical EPR spectra
were obtained when the oxidation of K2[NiIIH3buea(OH)]
was performed in DMA, MeCN, and THF, or when
ferrocenium was used as the oxidant.
of [NiIIH3buea(OH)]2−. We also probed the reactivity of
the NiIII species with external substrates including 9,10-
dihydroanthracene (DHA) and xanthene, yet no reaction
was observed with these species.
We have previously shown that the one-electron
oxidation of [MIIH3buea(OH)]2− complexes (MII = Fe,
Mn, Co) produces their corresponding MIII–OH analogs
[59]. The combined spectroscopic data for the oxidized
product of [NiIIH3buea(OH)]2− does not allow us to make
a similar assignment. While our results support the initial
formation of a NiIII species, there is no data that confirms
that the hydroxido ligand is still coordinated. In
particular, the characteristic peaks associated with the
O–H vibration have not been observed in the FTIR
spectrum. A new peak appears in the FTIR spectrum at
3320 cm−1 after oxidation but its shape and energy do not
correspond to bands for O–H vibrations we have observed
for other MIII−OH with this ligand. The energy of this
peak is also in the same region we observe signals from
H6buea, suggesting the possibility that the ligand has been
protonated, as found from our NMR studies (see above).
The NiIII species derived from [NiIIH3buea(OH)]2− is
unstable at room temperature and reacts further to form
an EPR silent species. This reaction followed first-order
kinetics with respect to the NiIII species in DMF and a
half-life of 10 h was determined for the oxidized species
at 25 ˚C. The NiIII species is more stable at lower
temperatures: solution samples stored at -30˚C retained
their rhombic EPR spectra even after several months. In
addition, this oxidized product was also stable in the solid
state and could be stored under an inert atmosphere at
3.6. Preparation and characterization of a NiIII species
with [MST]3−
We have also explored the oxidation of
room temperature.
Attempts at crystallizing the NiIII
[NiIIMST(OH2)]–
under
similar
conditions
to
species at −80 ˚C, −30 ˚C, and room temperature were
unsuccessful, with only light yellow powders being
isolated. UV-vis spectra of this species indicated that it
was not [NiIIH3buea(OH)]2−. 1H-NMR spectra of the
powder revealed was nearly identical to that of H6buea,
suggesting some amount of the ligand precursor was
present. The oxidized NiIII species also does not reconvert
to [NiIIH3buea(OH)]2− upon reduction. Treating the NiIII
complex immediately after its formation with CoCp2 led
to a product that had different optical properties to those
[NiIIH3buea(OH)]2−. The NiII–OH2 complex could be
oxidized with [TBPA][PF6] to induce a clear color change
from lime-green to orange. When the reaction was
monitored by UV-vis spectroscopy at room temperature,
the growth of a peak at λmax = 312 nm (ε > 12000) and
shoulders at 440 nm (ε ~ 1200) and 530 nm (ε ~ 550) are
observed (Fig. 5A). The perpendicular-mode EPR
spectrum (Fig. 5B) at 10 K showed a rhombic signal with
g = 2.29, 2.17, and 2.04. Attempts at crystallization

yielded only green crystals, which is likely a NiII species
as this compound is perpendicular-mode EPR silent.
determined by perpendicular-mode EPR spectroscopy.
Both oxidation reactions revealed rhombic perpendicular-
A2.5
B
2.29
2.0
2.17
1.5
440
1.0
535
0.5
0.0
2.04
400
500
600
700
800
900 1000
2400
2800
3200
3600
Wavelength (nm)
B (G)
Fig. 5. (A) UV-vis spectrum for oxidation of a 0.4 mM CH2Cl2 solution of NMe4[NiIIMST(OH2)] by [TBPA]PF6 at 25 ˚C, showing the conversion of the
initial NiII−OH2 species (solid black) to some NiIII species after 30 s. (B) Perpendicular-mode X-band EPR spectra taken at 77 K of the putative NiIII
species with [MST]3− prepared in CH2Cl2 with [TBPA]PF6 as the oxidant (solid black) and simulated spectrium (dashed red).
The properties of the NiIII species with [MST]3− are
similar to that with [H3buea]3−. It reacts further to form an
EPR silent species at room temperature, which occurs
within hours. Attempts at crystallizing the oxidized
species at −80˚C, −30˚C, and room temperature were also
unsuccessful, with only a light yellow powder again being
isolated from the reaction mixture. The UV-vis spectrum
of this EPR silent species is similar to the original
NiII−OH2 compound. However, attempts to immediately
reduce the putative NiIII species using CoCp2 led to a
species that is different from the starting NiII−OH2
compound and its formulation is still unknown.
mode EPR spectra at 10 K, consistent with an S = 1/2
spin-state derived from a low-spin NiIII metal center.
These NiIII species were characterized by UV-vis
spectroscopy, EA, and FTIR spectroscopy, but crystals
have yet to be grown for either compound. Moreover, the
lack of detectable vibration peaks for NiIIIO–H bonds
prevents the definitive assignment of these complexes as
high valent Ni–hydroxido species.
Acknowledgment
The authors than the National Institutes of Health,
USA (GM050781).
We have previously shown that [FeIIIMST(OH)]− is
prepared
by
the
one-electron
However,
oxidation
as
of
with
[FeIIMST(OH2)]−
[37].
Appendix A. Supplementary data
[NiIIH3buea(OH)]2−, the spectroscopic data for the
oxidized product of [NiIIMST(OH2)]− does not allow us to
make a similar, definitive assignment. An O−H vibration
at 3463 cm-1 was observed for [FeIIIMST(OH)]–, but a
similar feature was not found in FTIR spectra of the
oxidized [NiIIMST(OH2)]−. Moreover, we did not observe
any further reactivity with the NiIII species with external
substrates that would support a high valent Ni–OH
species.
CCDC 1501483 and 1501484 contain the
supplementary
crystallographic
data
for
K2[NiIIH3buea(OH)]
and
NMe4[NiIIMST(OH2)]
respectively. These data can be obtained free of charge
via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-
366-033;
or
e-mail:
deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be
found, in the online version.
4. Summary
This work has described the preparation and
characterization
of
K2[NiIIH3buea(OH)]
and
References
NMe4[NiIIMST(OH2)], complexes containing terminal
NiII−OH and NiII−OH2 moieties. These complexes adopt
a distorted tbp geometry as determined by the τ5 value,
which is an uncommon geometry for these types of
complexes. Both of these complexes were oxidized by
one-electron oxidants to form high-valent NiIII species, as
[1]
J. Du Bois, T.J. Mizoguchi, S.J. Lippard, Coord.
Chem. Rev. 200–202 (2000) 443.
[2]
M. Merkx, D.A. Kopp, M.H. Sazinsky, J.L.
Blazyk, J. Müller, S.J. Lippard, Angew. Chemie
Int. Ed. 40 (2001) 2782.

[3]
[4]
L. Que, Jr., W.B. Tolman, Angew. Chemie Int.
Ed. 41 (2002) 1114.
T.C. Brunold, C.G. Riordan, Angew. Chemie Int.
Ed. 43 (2004) 6716.
S. Hikichi, M. Yoshizawa, Y. Sasakura, M. Akita,
Y. Moro-oka, J. Am. Chem. Soc. 120 (1998)
10567.
[23]
[24]
[25]
[26]
[27]
[28]
D. Prema, K. Oshin, J. Desper, C.J. Levy, Dalton
Trans. 41 (2012) 4998.
F.K. Ross, G.D. Stucky, Inorg. Chem. 8 (1969)
2734.
[5]
[6]
S. Hikichi, M. Yoshizawa, Y. Sasakura, H.
Komatsuzaki, Y. Moro-oka, M. Akita, Chem. - A
Eur. J. 7 (2001) 5011.
D.A. Evans, C.W. Downey, J.L. Hubbs, J. Am.
Chem. Soc. 125 (2003) 8706.
B.S. Mandimutsira, J.L. Yamarik, T.C. Brunold,
W. Gu, S.P. Cramer, C.G. Riordan, J. Am. Chem.
Soc. 123 (2001) 9194.
G.A. Santillan, C.J. Carrano, Dalton Trans.
(2008) 3995.
A. Kunishita, Y. Doi, M. Kubo, T. Ogura, H.
Sugimoto, S. Itoh, Inorg. Chem. 48 (2009) 4997.
[7]
[8]
D.E. Benson, A.E. Haddy, H.W. Hellinga,
Biochemistry. 41 (2002) 3262.
D. Powell-Jia, J.W. Ziller, A.G. DiPasquale, A.L.
Rheingold, A.S. Borovik, Dalt. Trans. (2009)
2986.
S. Itoh, H. Bandoh, M. Nakagawa, S. Nagatomo,
T. Kitagawa, K.D. Karlin, S. Fukuzumi, J. Am.
Chem. Soc. 123 (2001) 11168.
[29]
[30]
B.S. Hammes, V.G. Young, Jr., A.S. Borovik,
Angew. Chemie Int. Ed. 38 (1999) 666.
[9]
L. Rodríguez, E. Labisbal, A. Sousa-Pedrares,
J.A. García-Vázquez, J. Romero, M.L. Durán,
J.A. Real, A. Sousa, Inorg. Chem. 45 (2006)
7903.
C.E. MacBeth, A.P. Golombek, V.G. Young Jr.,
C. Yang, K. Kuczera, M.P. Hendrich, A.S.
Borovik, Science (80-. ). 289 (2000) 938.
[10] R. Biswas, P. Kar, Y. Song, A. Ghosh, Dalt.
Trans. 40 (2011) 5324.
[31]
[32]
R. Gupta, C.E. MacBeth, V.G. Young,, A.S.
Borovik, J. Am. Chem. Soc. 124 (2002) 1136.
[11] N.E. Dixon, P.W. Riddles, C. Gazzola, R.L.
Blakeley, B. Zerner, Can. J. Biochem. 58 (1980)
1335.
D.C. Lacy, R. Gupta, K.L. Stone, J. Greaves, J.W.
Ziller, M.P. Hendrich, A.S. Borovik, J. Am.
Chem. Soc. 132 (2010) 12188.
[12] E. Jabri, M. Carr, R. Hausinger, P. Karplus,
Science (80-. ). 268 (1995) 998.
[33]
[34]
Y.J. Park, J.W. Ziller, a S. Borovik, J. Am.
Chem. Soc. 133 (2011) 9258.
http://www.journals.cambridge.org/abstract_S001
0417500020156.
[13] W.N. Lipscomb, N. Sträter, Chem. Rev. 96
(1996) 2375.
[14] D.W. Christianson, J.D. Cox, Annu. Rev.
Biochem. 68 (1999) 33.
Y.J. Park, S.A. Cook, N.S. Sickerman, Y. Sano,
J.W. Ziller, A.S. Borovik, Chem. Sci. 4 (2013)
717.
[15] D.W. Christianson, C.A. Fierke, Acc. Chem. Res.
29 (1996) 331.
[35]
[36]
[37]
[38]
[39]
[40]
[41]
[42]
Y. Sano, A.C. Weitz, J.W. Ziller, M.P. Hendrich,
A.S. Borovik, Inorg. Chem. 52 (2013) 10229.
[16] J. Cámpora, P. Palma, D. del Río, E. Álvarez,
Organometallics. 23 (2004) 1652.
D.C. Lacy, Y.J. Park, J.W. Ziller, J. Yano, A.S.
Borovik, J. Am. Chem. Soc. 134 (2012) 17526.
[17] J. Cámpora, I. Matas, P. Palma, C. Graiff, A.
Tiripicchio, Organometallics. 24 (2005) 2827.
S.A. Cook, J.W. Ziller, A.S. Borovik, Inorg.
Chem. 53 (2014) 11029.
[18] D. Adhikari, S. Mossin, F. Basuli, B.R. Dible, M.
Chipara, H. Fan, J.C. Huffman, K. Meyer, D.J.
Mindiola, Inorg. Chem. 47 (2008) 10479.
N. Lau, J.W. Ziller, A.S. Borovik, Polyhedron.
(2014).
[19] D. Huang, O. V Makhlynets, L.L. Tan, S.C. Lee,
E. V Rybak-Akimova, R.H. Holm, Inorg. Chem.
50 (2011) 10070.
V. Pandarus, D. Zargarian, Organometallics. 26
(2007) 4321.
T.J. Anderson, G.D. Jones, D.A. Vicic, J. Am.
Chem. Soc. 126 (2004) 8100.
[20] Y.-P. Zhang, W.-W. Li, B.-X. Li, H.-L. Mu, Y.-S.
Li, Dalt. Trans. 44 (2015) 7382.
S.L. Zultanski, G.C. Fu, J. Am. Chem. Soc. 133
(2011) 15362.
[21] F. Czerny, P. Döhlert, M. Weidauer, E. Irran, S.
Enthaler, Inorganica Chim. Acta. 425 (2015) 118.
W.-Z. Lee, C.-W. Chiang, T.-H. Lin, T.-S. Kuo,
[22] M.T. Kieber-Emmons, R. Schenker, G.P.A. Yap,

Chem. - A Eur. J. 18 (2012) 50.
[52]
[53]
A.W. Addison, T.N. Rao, J. Reedijk, J. van Rijn,
G.C. Verschoor, J. Chem. Soc. Dalt. Trans.
(1984) 1349.
[43] C.-W. Chiang, Y.-L. Chu, H.-L. Chen, T.-S. Kuo,
W.-Z. Lee, Chem. - A Eur. J. 20 (2014) 6283.
N.S. Sickerman, Y.J. Park, G.K.-Y. Ng, J.E.
Bates, M. Hilkert, J.W. Ziller, F. Furche, A.S.
Borovik, Dalton Trans. 41 (2012) 4358.
[44] W.P. Tappmeyer, A.W. Davidson, Inorg. Chem. 2
(1963) 823.
[45] L. Eberson, B. Larsson, C. Moberg, K.D.
Krautwurst, P. Krogsgaard-Larsen, R. Ryhage, R.
Isaksson, Acta Chem. Scand. 40b (1986) 210.
[54]
[55]
J. Emsley, Chem. Soc. Rev. 9 (1980) 91.
S.N. Oseback, S.W. Shim, M. Kumar, S.M.
Greer, S.R. Gardner, K.M. Lemar, P.R.
DeGregory, E.T. Papish, D.L. Tierney, M. Zeller,
G.P.A. Yap, Dalt. Trans. 41 (2012) 2774.
[46] L. Eberson, B. Larsson, Acta Chem. Scand. 41B
(1987) 367.
http://actachemscand.org/pdf/acta_vol_41b_p036
7-0378.pdf (accessed June 4, 2014).
[56]
[57]
F.G. Bordwell, Acc. Chem. Res. 21 (1988) 456.
[47] D.F. Evans, J. Chem. Soc. (1959) 2003.
J. Cho, R. Sarangi, J. Annaraj, S.Y. Kim, M.
Kubo, T. Ogura, E.I. Solomon, W. Nam, Nat.
Chem. 1 (2009) 568.
[48] N.G. Connelly, W.E. Geiger, Chem. Rev. 96
(1996) 877.
[58]
[59]
Z. Xiao, B.O. Patrick, D. Dolphin, Inorg. Chem.
42 (2003) 8125.
[49] C.E. MacBeth, B.S. Hammes, V.G. Young,, A.S.
Borovik, Inorg. Chem. 40 (2001) 4733.
C.E. MacBeth, R. Gupta, K.R. Mitchell-Koch,
V.G. Young, G.H. Lushington, W.H. Thompson,
M.P. Hendrich, A.S. Borovik, J. Am. Chem. Soc.
126 (2004) 2556.
[50] F.F. Pfaff, F. Heims, S. Kundu, S. Mebs, K. Ray,
Chem. Commun. (Camb). 48 (2012) 3730.
[51] H. Ma, J.L. Petersen, V.G. Young, G.T. Yee,
M.P. Jensen, J. Am. Chem. Soc. 133 (2011) 5644.

A terminal NiII--OH complex and a terminal NiII--OH2 complex, both supported by tripodal ligands, have been
prepared and characterized. The solid state structures of these complexes reveal that they adopt distorted trigonal
bipyramidal primary coordination spheres, an unusual geometry for NiII complexes. Treating these complexes with
one-electron oxidants formed species with S = 1/2 spin ground states, which are consistent with formation of
monomeric NiIII species.