Preparation and structures of dinuclear complexes containing M II-OH centers

Article abstract of DOI:10.1039/c2cc16277f

The synthesis of M^II₂ complexes (M^IICo, Mn) with terminal hydroxo ligands has been achieved utilizing a dinucleating ligand containing a bridging pyrazolate unit and appended (neopentyl) aminopyridyl groups. Structural studies on the complexes revealed that the M^II-OH units are positioned in a syn-configuration, placing the hydroxo ligands in close proximity (ca. 3 A apart), which may be a prerequisite for water oxidation. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc16277f

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COMMUNICATION
Preparation and structures of dinuclear complexes containing
MII–OH centersw
Gary K.-Y. Ng, Joseph W. Ziller and A. S. Borovik*
Received 9th October 2011, Accepted 29th December 2011
DOI: 10.1039/c2cc16277f
The synthesis of MII complexes (MII Co, Mn) with terminal
We have been preparing dinucleating ligands that also
Q
2
hydroxo ligands has been achieved utilizing a dinucleating ligand
containing a bridging pyrazolate unit and appended (neopentyl)-
aminopyridyl groups. Structural studies on the complexes revealed
that the MII–OH units are positioned in a syn-configuration,
utilize a bridging pyrazolate group to separate the metal
centers. Using the structural concepts developed by Meyer,8
we reasoned that ligands containing a (3,5-diaminomethylene)-
pyrazolate unit would provide sufficient spacing between the
metal ions (ca. 3.5–4.5 A) to allow the binding of two terminal
hydroxo ligands. In addition, our ligands include H-bond
donors within the secondary coordination sphere to form
intramolecular H-bonding networks with the M–OH units;7d,9
these types of noncovalent interactions have previously
been exploited in the isolation of mononuclear M–OH
analogues.10 Two earlier versions of this design contained urea
˚
placing the hydroxo ligands in close proximity (ca. 3 A apart),
which may be a prerequisite for water oxidation.
Multinuclear metal complexes with terminal hydroxo or oxo
ligand(s) have been proposed to participate in a variety of
different biochemical processes, including the catalytic cycle of
water oxidation in photosystem II (PSII).1 Exploring the
chemistry of related synthetic systems has provided informa-
tion into the structural and mechanistic requirements neces-
sary for catalysis, yet most artificial systems still lack the
catalytic efficiency found in metalloproteins. One synthetic
approach is to develop complexes that initially place two
M–OHn (n = 1, 2) units in close proximity.2,3 This approach
is often hampered because of the tendency for hydroxo and
aquo ligands to bridge between metal ions. Nonetheless, there
are structurally characterized examples of dinuclear complexes
containing discrete M–OHn units.4 Meyer reported that the
[(bpy)2Ru-OH2]2(m-O) complex contains a diruthenium core
([H4PRbuam]5ꢀ 11 and carboxyamido-pyridyl ([H3bppap]2ꢀ 12
)
)
groups (Fig. 1) but we were unable to prepare dinuclear
M–OH complexes. For instance, the CoII2 complex of [H3bppap]2ꢀ
had only one terminal CoII–OH center: the binding of a
second hydroxo ligand appeared to be hindered by the
coordination of the oxygen atoms of the appended carboxy-
amido groups.13 These results suggested that reduction of
carboxyamido groups to neopentylamino moieties would produce
3,5-bis[bis(N-6-neopentylamino-2-pyridylmethyl)aminomethyl]-
1H-pyrazole (H5bnppap), a compound containing H-bond
donors that cannot readily coordinate to the metal ions.10c,d,14
H5bnppap was synthesized from H5bppap in nearly quanti-
tative yield by reduction using LiAlH4 in THF (Scheme 1).
The dinuclear CoII and MnII complexes of [H4bnppap]ꢀ were
prepared according to the route outlined in Scheme 2.
H5bnppap in MeOH was treated with 3 equiv of NaH under
an argon atmosphere. After stirring for 30 min, the MII
precursors (either CoII(NO3)2ꢁ6H2O or MnII(OTf)2ꢁ2MeCN)
were added in one portion and stirred for an additional 30 min.
The reaction mixtures were then treated with 2 equiv of H2O,
followed by the addition of NaBPh4, which resulted in the
immediate formation of precipitates. The solids were isolated via
filtration and purified by recrystallization from THF/pentane.
with two terminal aquo ligands5 and Me
´
nage showed that the
[(trpy)2Fe-OH]2(m-O) has two FeIII–OH units;6 however, in
these complexes the M–OHn units have an anti-configuration
in the solid state with the two OHn groups separated by over 5 A.
Success in preparing dinuclear complexes with M–OHn units in the
syn-configuration has been achieved using dinucleating ligands.7
For instance, Llobet and Meyer have reported complexes
containing Ru2-(H3O2),7a Zn2-(H3O2)7b,c and Ni-(H3O2)7d
units respectively, utilizing a pyrazolate bridging group that
prevents the formation hydroxo or oxo bridges. In this report we
describe the preparation of a new dinucleating ligand that supports
the formation of [CoII–OH]2 and [MnII–OH]2 complexes. The
MII–OH units in these complexes adopt syn-configurations, which
are stabilized by intramolecular hydrogen bonding (H-bonding)
networks.
Department of Chemistry, University of California-Irvine,
1102 Natural Science II, Irvine, CA 92697, USA.
E-mail: aborovik@uci.edu
w Electronic supplementary information (ESI) available: crystallographic
data, FT-IR spectra, absorption spectra, ESI-MS spectra. For ESI and
crystallographic data in CIF see DOI: 10.1039/c2cc16277f
Fig. 1 CoII–OH complexes of [H4PRbuam]5ꢀ and [H3bppap]2ꢀ
.
c
2546 Chem. Commun., 2012, 48, 2546–2548
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Scheme 1 Preparative route of H5bnppap from H5bppap.
Scheme 2 Synthetic route to [MII2H4bnppap(OH)2]+ (M = CoII, MnII).
Conditions: (a)
3
equiv NaH, (b)
2
equiv Co(NO3)2ꢁ6H2O or
Mn(OTf)2ꢁ2MeCN (c) 2 equiv of H2O (d) 1 equiv NaBPh4.
Analytical and spectroscopic investigations indicated that
the salts contained the dinuclear metal complexes,
[MII2H4bnppap(OH)2]+ (MII = Co, Mn). The electrospray
ionization mass spectrum (ESI-MS) of [CoII2H4bnppap(OH)2]+
contained a large ion peak at a charge-to-mass ratio (m/z) of
981.4, which matches the expected mass and isotopic distribu-
tion of a (CoII–OH)2 complex (calcd, 981.5). Similarly the
ESI-MS spectrum for ([Mn2IIH4bnppap(OH)2]+ exhibited a
peak at a m/z of 973.5 (calcd, 973.5). Each peak shifted by
4 mass-units when H218O was used in the synthesis, indicating
that the source of the hydroxo ligands is water.z Effective magnetic
moments (meff) at 298 K of 8.01 and 5.23 mBM were obtained
for [CoII2H4bnppap(OH)2]+ and [MnII2H4bnppap(OH)2]+
respectively,y values that are close to the spin-only values for
two individual high-spin MnII and CoII centers.15 These
preliminary findings suggest weak magnetic coupling, which
is inconsistent with the presence of single atom bridge(s)
between the metal centers.
Fig. 2 Thermal ellipsoid plots of [CoII2H4bnppap(OH)2]+ (A) and
[MnII2H4bnppap(OH)2]+ (B). Thermal ellipsoids are drawn at the 50%
probability level. Only hydroxo and amino hydrogen atoms are
shown for clarity. Selected bond lengths (A) and angles (1) for
[CoII2H4bnppap(OH)2]+ and [MnII2H4bnppap(OH)2]+
:
Co1–O1,
The solid-state structures of the complexes were investigated
using X-ray diffraction methods to reveal dinuclear species in
which each metal center has a coordinated hydroxo ligand. In
[CoII2H4bnppap(OH)2]+ (Fig. 2A) both CoII centers have
trigonal bipyramidal coordination geometries as judged by
index of trigonality parameter (t) of 0.99 measured for both
metal ions.16 An N4O primary coordination sphere exists
about each CoII ion, consisting of pyrazolate and pyridyl
nitrogen atoms defining the trigonal plane, and the tertiary
amino nitrogen and hydroxo oxygen atoms in the axial posi-
tions. The Co1–O1 and Co2–O2 bond distances of 1.9379(2) A
and 1.9444(2) A are similar to the Co–O(H) bond length of
1.931(2) A observed in [{CoII(OH)}CoIIH3bppap]+, but are
significantly shorter than those in [CoII2H4PRbuam(m-OH)]2ꢀ
(greater than 2.1 A) (Fig. 1).10b The pyrazolate unit bridges
between the CoII centers with Co1–N4 and Co2–N10 bond
distances of 2.034(2) and 2.046(2) A, and a Co1ꢁ ꢁ ꢁCo2 separa-
tion of 4.286 A. The remaining Co–N bond distances and
angles are unexceptional with avg. Co–Ntrig and Co–Naxial
bond lengths of 2.108(2) A and 2.194(2) A, and avg.
1.938(2); Co1-N4, 2.034(2); Co1–N3, 2.102(2); Co1–N2, 2.107(2);
Co1–N1, 2.192(2); Co2–O2, 1.944(2); Co2–N10, 2.046(2); Co2–N9,
2.119(2); Co2–N8, 2.104(2); Co2–N7, 2.195(2); O1–Co1–N4, 106.28(8);
O1–Co1–N3, 101.94(8); N4–Co1–N3, 114.64(8); O1–Co1–N2, 100.48(2);
N4–Co1–N2, 116.10(8); N3–Co1–N2, 114.73(8); O1–Co1–N1, 176.51(8);
N4–Co1–N1, 76.99(8); N3–Co1–N1, 77.50(8); N2–Co1–N1, 76.78(8);
O2–Co2–N10, 106.61(8); O2–Co2–N8, 102.23(8); N10–Co2–N8,
115.00(8); O2–Co2–N9, 100.11(8), N10–Co2–N9, 117.25(8), N8–Co2–N9,
112.95(8); O2–Co2–N9, 176.81(8); N10–Co2–N7, 76.25(8); N8–Co2–N7,
77.51(8); N9–Co2–N7, 77.17(8). Mn1–O1, 2.003(3); Mn1-N4, 2.128(4);
Mn1–N3, 2.204(4); Mn1–N2, 2.234(4); Mn1–N1, 2.330(4); Mn2–O2,
2.006(4); Mn2–N10, 2.137(4); Mn2–N9, 2.230(5); Mn2–N8, 2.207(5);
Mn2–N7, 2.285(5); O1–Mn1–N4, 111.20(2); O1–Mn1–N3, 101.67(2);
N4–Mn1–N3, 112.52(2); O1–Mn1–N2, 100.60(1); N4–Mn1–N2,
114.75(2);
N3–Mn1–N2,
114.56(2);
O1–Mn1–N1,
174.00(2);
N4–Mn1–N1, 74.57(2); N3–Mn1–N1, 76.94(2); N2–Mn1–N1, 74.98(2);
O2–Mn2–N10, 113.07(2); O2–Mn2–N8, 102.03(2); N10–Mn2–N8,
114.23(2); O2–Mn2–N9, 99.19(2), N10–Mn2–N9, 115.2(2), N8–Mn2–N9,
111.3(2); O2–Mn2–N9, 172.13(2); N10–Mn2–N7, 74.46(2); N8–Mn2–N7,
75.94(2); N9–Mn2–N7, 74.84(2).
A
striking feature of the molecular structure of
N
trig–Co–Ntrig angle and O–Co–Naxial angles of 115.11(8)1
[CoII2H4bnppap(OH)2]+ is the syn-configuration of the two
and 176.66(8)1.
Co–OH units. Their close proximity is reflected in the
c
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relatively short O1ꢁ ꢁ ꢁO2 separation of 2.971(2) A, a distance
that is indicative of H-bonds being present between the two
ligands.17z In addition, the hydroxo ligands formed intra-
molecular H-bonds with the neopentylamino groups of
[H4bnppap]ꢀ. All the N–H vectors are positioned toward the
hydroxo ligands with N–H–O angles of greater than 1641. This
alignment produced Oꢁ ꢁ ꢁN distances that are less than 2.8 A,
which, taken together, are consistent with the formation of
strong H-bonds. FTIR measurements are also consistent with
intramolecular H-bonds being present in [CoII2H4bnppap(OH)2]+
with broad signals from the amino NH groups appearing at
3235 cmꢀ1. We were unable to observed FTIR signals from the
hydroxo ligand, presumably because they are significantly
broadened because of the H-bonds.
The molecular structure of [MnII2H4bnppap(OH)2]+ was
also determined and contains nearly the same structural
features as the cobalt analogue (Fig. 2B). Disorder in the
neopentyl groups limited the quality of the structure;8 never-
theless, it is clear at the present resolution that each MnII
center has trigonal bipyramidal coordination geometry with
L. Francas, E. Mas-Marza, L. Esrlche and A. Llobet, Inorg. Chem.,
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t
Mn1 = 0.99 and tMn2 = 0.95 and a Mn1ꢁ ꢁ ꢁMn2 separation of
4.303 A. Note that this type of coordination also promotes the
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In summary, a new dinucleating ligand, [H4bnppap]ꢀ has
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the preparation of new dinuclear complexes of cobalt and
manganese, each of which has two M–OH units. The intra-
molecular H-bonding network and the rigidity of ligand
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Notes and references
z ESI-MS data: [CoII2H4bnppap(18OH)2]+: m/z = 985.8 (calcd,
985.5); ([MnII2H4bnppap(18OH)2]+: m/z = 977.5 (calcd, 977.5).
y Magnetic moments were determined in DMSO using the Evans’
method. The calculations were done relative to the shift in the solvent
peak.
z For [CoII2H4bnppap(OH)2]+ hydrogen atoms on the amino groups
were located from a difference-Fourier map and refined (x,y,z,
and Uiso), while those of the hydroxo ligands were included using a
riding model.
8 Various attempts using different solvents, experimental conditions,
and counter anions to obtain crystals that gave a better structure of
[MnII2H4bnppap(OH)2](BPh4) were unsuccessful. The freely rotating
neo-pentyl groups impose high intrinsic disorder to the cationic
complex, which likely explains the poor diffraction of the crystals.
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c
2548 Chem. Commun., 2012, 48, 2546–2548
This journal is The Royal Society of Chemistry 2012