Neutral acetohydroxamic acid coordination to a mononuclear Ni(II) center stabilized by an intramolecular hydrogen-bonding interaction

Article abstract of DOI:10.1039/b413683g

Treatment of a new chelate ligand having both amide- and phenyl-appended pyridyl moieties with Ni(ClO₄)₂·6H₂O and acetohydroxamic acid in methanol solution results in the production of a novel pseudo-octahedral Ni(II) comp

Full text of DOI:10.1039/b413683g

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COMMUNICATION
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Neutral acetohydroxamic acid coordination to a mononuclear Ni(II)
center stabilized by an intramolecular hydrogen-bonding interaction{
Katarzyna Rudzka,a Magdalena M. Makowska-Grzyska,a Ewa Szajna,a Atta M. Arifb and
Lisa M. Berreau*a
Received (in Cambridge, UK) 7th September 2004, Accepted 25th October 2004
First published as an Advance Article on the web 2nd December 2004
DOI: 10.1039/b413683g
CH3OH solution, followed by recrystallization via Et2O diffusion
into a CH3CN : CH3OH solution of the complex at ambient
temperature resulted in the deposition of purple block crystals of
[(bppppa)Ni(HONHC(O)CH3)](ClO4)2 (1) in 72% yield. Complex
1 has been characterized by X-ray crystallography, elemental
analysis, FTIR, UV-vis, and a solution magnetic moment
measurement.
Treatment of a new chelate ligand having both amide- and
phenyl-appended pyridyl moieties with Ni(ClO4)2?6H2O and
acetohydroxamic acid in methanol solution results in the
production of a novel pseudo-octahedral Ni(II) complex having
a neutral acetohydroxamic acid ligand stabilized by a hydro-
gen-bonding interaction.
The acetohydroxamato monoanion (AHA2) is a well-known
inhibitor of several metalloenzymes, including urease enzymes
from plants and bacteria, which contain a binuclear Ni(II) center
within the active site.1–5 AHA2 inhibition of Klebsiella aerogenes
and Bacillus pasteurii ureases has been suggested to involve initial
formation of a weak enzyme/inhibitor complex (E–I, Scheme 1)
having coordination of a neutral acetohydroxamic acid (AHA)
molecule at a single Ni(II) ion.6–8 Stabilization of the E-I species
X-Ray crystallographic analysis of 1 revealed a mononuclear
pseudo-octahedral Ni(II) center and a tetradentate N3O-donor
coordination mode for the bppppa chelate ligand with one phenyl-
appended pyridyl moiety noncoordinated (Fig. 1).{ The N–H and
O–H hydrogen atoms of the acetohydroxamic acid ligand, as well
as the amide N–H hydrogen of the bppppa ligand, were located
and refined independently. Importantly, the noncoordinated
pyridyl nitrogen acts as a hydrogen bond acceptor for the
Ni(II)-coordinated hydroxyl group of the acid. This hydrogen
bond may be classified as moderate based on the short heteroatom
may involve formation of
a hydrogen-bonding interaction
involving the bridging hydroxyl group.8 This E–I complex is then
proposed to slowly convert to a more stable E–I* structure
(Scheme 1), which exhibits a bridging coordination mode for the
hydroxamato monoanion, a structural motif that has also been
identified in model systems.9,10 In regard to the E–I species, to our
knowledge, a structurally characterized complex having neutral
acetohydroxamic acid coordination has not previously been
reported for any transition metal ion.11 Herein we report the
preparation and characterization of a novel synthetic mononuclear
Ni(II) complex having a coordinated neutral acetohydroxamic acid
ligand, the hydroxyl proton of which forms a moderate hydrogen-
bonding interaction with a noncoordinated pyridyl nitrogen of the
supporting chelate ligand.
…
˚
distance [O(2)–H(2) N(5) 2.518(3) A] and a somewhat acute
bond angle [158(5)u].13 A similar hydrogen-bonding interaction has
been reported involving a Mn(II)-coordinated methanol ligand and
a
noncoordinated phenyl-appended pyridyl moiety in [(6-
Ph2TPA)Mn(CH3OH)3](ClO4)2 {6-Ph2TPA 5 N,N-bis[(6-phenyl-
2-pyridyl)methyl]-N-[(2-pyridyl)methyl]amine}.14
The Ni–O(H) distance involving the hydroxamic acid ligand is
˚
elongated in 1 by y0.072 A relative to that found in a structurally
A new chelate ligand, N,N-bis[(6-phenyl-2-pyridyl)methyl]-N-
[(6-pivaloylamido-2-pyridyl)methyl]amine (bppppa) having both
amide-12 and aryl-substituted pyridyl moieties was assembled as
shown in Scheme 2. Admixture of this ligand with equimolar
amounts of Ni(ClO4)2?6H2O and acetohydroxamic acid in
Scheme 1
{ Electronic supplementary information (ESI) available: Experimental
section, Figs. S1 and S2. See http://www.rsc.org/suppdata/cc/b4/b413683g/
*berreau@cc.usu.edu
Scheme 2
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3800–2600 cm21 for 1 contains a broader, more intense feature
than is found for 2 under identical conditions (see ESI{). This is
consistent with the presence of the additional hydroxamic acid
–OH and amide NH (within the bppppa ligand) moieties in 1, both
of which participate in hydrogen-bonding interactions. In
Fig. S1(b), a nCLO vibration can be identified at 1656 cm21 for
the Ni(II)-coordinated bppppa amide carbonyl group (see ESI{).
The nCLO vibration for the acetohydroxamic acid/AHA2 carbonyl
groups in 1 and 2 should be present near 1600 cm21. However, this
region in both complexes is complicated by a pyridyl ring
vibration, which precludes conclusive assignments.
3
3
The energy of the A2g
A A
3T1g(F) and A2g 3T2g(F)
Fig. 1 ORTEP representation of the cationic portion of 1. All ellipsoids
transitions differ in the electronic absorption spectra of 1 and 2 (see
ESI{, Fig. S2) in dry acetonitrile solution.15 The former transition
is shifted to slightly higher energy in 1 (570 nm vs. 585 nm in 2). In
are drawn at the 35% probability level. All hydrogen atoms except the
˚
N–H and O–H protons not shown for clarity. Selected bond lengths (A)
and angles (u): Ni(1)–N(2) 2.027(3), Ni(1)–N(3) 2.130(3), Ni(1)–N(4)
2.128(3), Ni(1)–O(1) 2.040(2), Ni(1)–O(2) 2.091(2), Ni(1)–O(3) 2.037(2),
O(2)–Ni(1)–O(3) 79.71(9).
addition, whereas in 1 the latter transition is found at y920 cm21
,
in 2 this feature is shifted into the near-IR region.
In summary, we have found that a novel mononuclear Ni(II)
complex having neutral acetohydroxamic acid coordination may
be isolated using a chelate ligand that provides an internal
hydrogen bond acceptor. The structural and spectroscopic
properties of 1 are notably different from those of a structurally-
related Ni(II) complex of the acetohydroxamato anion (2). This
work provides the first chemical precedent upon which to evaluate
acetohydroxamic acid versus acetohydroxamato anion coordina-
tion to a Ni(II) center, a topic that is important toward fully
understanding the inhibition properties of urease enzymes.
We thank the National Science Foundation (CAREER Award
CHE-0094066) and the Herman Frasch Foundation (501-HF02)
for funding this research.
related pseudo-octahedral Ni(II) hydroxamato (AHA2) complex
of the Ph2TPA ligand [(6-Ph2TPA)Ni(ONHC(O)CH3)]ClO4, [2,
14
Fig. 2(a), Ni(1)–O(2) 2.0203(15) A]. The Ni–O bond distance
˚
involving the acetohydroxamic acid carbonyl oxygen in 1 [Ni(1)–
˚
O(3) 2.037(2) A] is also slightly longer than the analogous bond in
2 [1.9964(14) A]. Within the hydroxamic acid/AHA2 units in 1 and
˚
2, the C–O and C–N bond distances are very similar, with the
˚
largest difference outside experimental error being y0.005 A in the
C–N bond. Overall, these combined structural parameters indicate
that the neutral hydroxamic acid binds more weakly to the
mononuclear Ni(II) center in 1 than does the monoanionic
acetohydroxamato ligand (AHA2) in 2. Finally, we note that
˚
the average Ni–O bond distance for 1 (2.06 A) is slightly longer
Katarzyna Rudzka,a Magdalena M. Makowska-Grzyska,a Ewa Szajna,a
Atta M. Arifb and Lisa M. Berreau*a
˚
˚
than that of 2 (2.01 A) and the symmetric Ni–O distance (2.0 A) in
acetohydroxamato-inhibited urease from Bacillus pasteurii.8,14
In regard to the structural features of 1 versus 2, it is also worth
aDepartment of Chemistry and Biochemistry, Utah State University,
0300 Old Main Hill, Logan, UT 84322-0300, USA.
˚
noting that the shorter Ni–NPhPy distance in 1 [2.128(3) A] as
˚
compared to those found in 2 [2.2630(17)/2.2292(17) A] suggests
E-mail: berreau@cc.usu.edu; Fax: 435 797 3390; Tel: 435 797 1625
bDepartment of Chemistry, University of Utah, 315 S. 1400 E., Salt
Lake City, UT 84112, USA. E-mail: arif@chemistry.utah.edu
the presence of a more Lewis acidic Ni(II) center in 1, consistent
with the coordination of the neutral acetohydroxamic ligand.
The solid state infrared spectra of 1 and 2 differ in several
ways. For example, as shown in Fig. S1(a), the region of
Notes and references
{ Crystal data: for 1: C38H44Cl2N6NiO12, M 5 906.40, orthorhombic,
˚
space group Pbca, a 5 19.2023(5), b 5 34.9478(9), c 5 12.2308(2) A,
3
V 5 8207.8(3) A , Z 5 8, m 5 0.672 mm21. Using Mo-Ka radiation
˚
˚
(0.71073 A), a total of 16542 reflections were collected (4.84 , 2h , 54.96)
of which 9172 were independent. Refinement converged to R1 5 0.0555,
wR2 5 0.1229 (I .2sI) and R1 5 0.0962, wR2 5 0.1440 (all data). Complex
1 crystallized with one molecule of noncoordinated methanol per formula
unit. CCDC 250014. See http://www.rsc.org/suppdata/cc/b4/b413683g/ for
crystallographic data in CIF or other electronic format.
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coordinated acetohydroxamato anion and a cocrystallized molecule of
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