Synthesis of tetranuclear, four-coordinate manganese clusters with "pinned butterfly" geometry formed by metal-mediated N-N bond cleavage in diphenylhydrazine

Article abstract of DOI:10.1021/ja110536t

The preparation of four-coordinate tetramanganese-amide-hydrazide clusters is described. Reaction of Mn(NR₂)₂ (R = SiMe₃) with N,N′-diphenylhydrazine resulted in the formation of a black intermediary mixture that converted to a four-coordinate tetranuclear "pinned butterfly" cluster, Mn₄(μ₃-N ₂Ph₂)₂(μ-N₂Ph₂)(μ- NHPh)₂(THF)₄. This compound was isolated in ～90% yield and identified by single-crystal X-ray diffraction analysis. In pyridine, the THF ligands were replaced, giving the pyridyl complex Mn₄(μ ₃-N₂Ph₂)₂(μ-N₂Ph ₂)(μ-NHPh)₂(py)₄. Charge counting considerations indicate that the clusters had gained two protons and two electrons in addition to the formative fragments. Isolation of the black mixture was achieved by extraction techniques from a reaction with a decreased loading of hydrazine run at low temperatures with decreased solvent polarity. The black mixture was characterized by FT-IR, UV-vis, and ¹H NMR spectroscopy. In addition, an isolable, colorless dimer, Mn₂(μ-NHPh) ₂(NR₂)₂(THF)₂, was present in the mixture and identified by single-crystal X-ray diffraction. These intermediates are discussed in light of possible mechanisms for formation of the tetranuclear cluster.

Full text of DOI:10.1021/ja110536t

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pubs.acs.org/JACS
Synthesis of Tetranuclear, Four-Coordinate Manganese Clusters
with “Pinned Butterfly” Geometry Formed by Metal-Mediated
N-N Bond Cleavage in Diphenylhydrazine
Clifton R. Hamilton, Regina A. Baglia, Alexander D. Gordon, and Michael J. Zdilla*
Department of Chemistry, Temple University, 1901 North 13th Street, Philadelphia, Pennsylvania 19122, United States
S Supporting Information
b
ABSTRACT: The preparation of four-coordinate tetra-
manganese-amide-hydrazide clusters is described. Reac-
tion of Mn(NR2)2 (R = SiMe3) with N,N0-diphenylhydra-
zine resulted in the formation of a black intermediary
mixture that converted to a four-coordinate tetranuclear
“pinned butterfly” cluster, Mn4(μ3-N2Ph2)2(μ-N2Ph2)(μ-
NHPh)2(THF)4. This compound was isolated in ∼90%
Figure 1. Crystallographic structural model of the OEC.
yield and identified by single-crystal X-ray diffraction anal-
ysis. In pyridine, the THF ligands were replaced, giving the
pyridyl complex Mn4(μ3-N2Ph2)2(μ-N2Ph2)(μ-NHPh)2-
(py)4. Charge counting considerations indicate that the
clusters had gained two protons and two electrons in
addition to the formative fragments. Isolation of the black
mixture was achieved by extraction techniques from a
reaction with a decreased loading of hydrazine run at low
temperatures with decreased solvent polarity. The black
mixture was characterized by FT-IR, UV-vis, and 1H NMR
spectroscopy. In addition, an isolable, colorless dimer, Mn2-
(μ-NHPh)2(NR2)2(THF)2, was present in the mixture and
identified by single-crystal X-ray diffraction. These inter-
mediates are discussed in light of possible mechanisms for
formation of the tetranuclear cluster.
Chart 1. Pairs of (top row) PS II-Relevant Species and
(bottom row) Their Nitrogen Analogues: (left to right)
Water/Amine, Hydroxide/Amide, Oxide/Imide, Peroxide/
Hydrazine, Dioxygen/Diazene
chemistry of manganese with nitrogen analogues of OEC-
relevant substrates (Chart 1) using the R group to control the
geometry and coordination number.
Lee and co-workers achieved entry into low-coordinate iron-
imido chemistry by the metal-mediated reductive cleavage of N,
N0-diarylhydrazines.6 In an attempt to mimic this entry for the
manganese systems, we explored N,N0-diphenylhydrazine as an
he oxidation of water to dioxygen represents a grand chal-
7
imide source. The reaction of a solution of colorless Mn(NR2)2
T
lenge for the development of catalytic water splitting sys-
tems that use electricity or sunlight to store energy in the bonds
of H2 and O2.1 In nature, a catalyst has evolved for the water
oxidation reaction, which is ubiquitous in all oxygenic photo-
synthetic organisms. The oxygen-evolving complex (OEC) of
photosystem II (PS II) is an oxo-bridged 4Mn-Ca cluster
(Figure 1) that collects oxidizing equivalents from photo-oxi-
dized chlorophyll, oxidizes H2O to O2 to return to its most
reduced state, and pumps four water protons to the lumen.2
Numerous examples of synthetic manganese-oxo clusters
can be found in the literature and have been reviewed.3 Almost
exclusively, these synthetic clusters are six-coordinate with multi-
dentate ligation. In the interest of generating biomimetic systems
with accessible redox reactivity, we are exploring the chemistry of
low-coordinate, unchelated manganese clusters by using imide as
a surrogate of oxide. Isolobal, comparably sized, and equally
charged, imides have been studied as oxide analogues in termin-
ally bonded, monomeric transition-metal species.4 Only a few
reports of Mn-imido clusters exist, published by the groups of
Wilkinson, Power, and Wright.5 We seek to explore the cluster
(R = SiMe3) with a pale-yellow solution containing 1.25 equiv of
PhNHNHPh in THF resulted in the formation of a black mixture
(1a). Over the course of minutes, this black mixture turned
orange, and in the presence of pentane, orange crystalline
material precipitated over the course of 2 h. Single-crystal X-ray
analysis identified this compound as the manganese-amide-
hydrazide cluster Mn4(μ3-PhNNPh)2(μ-PhNNPh)(μ-NHPh)2-
(THF)4 (2), whose structure is shown in Figure 2. The terminal
THF ligands were easily be replaced with pyridine by dissolution
of 2 into pyridine, which led to precipitation of Mn4(μ3-
PhNNPh)2(μ-PhNNPh)(μ-NHPh)2(py)4 (2a). These clusters
(as well as cluster 1b; see below) are remarkably air- and water-
sensitive, turning instantly black and often smoldering upon
exposure to air. 2 has a shelf life of weeks when stored at -30 °C
in a well-maintained dry atmosphere box; 2a is slightly more
robust, lasting months at -30 °C under inert atmosphere.
Received: November 23, 2010
Published: March 04, 2011
r
2011 American Chemical Society
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ligands indicates that reductive cleavage of a hydrazine N-N
bond has occurred. The two “wing” Mn atoms are further joined
to one another by a third bridging diphenylhydrazide ligand that
is roughly parallel to the Mn2N2 rhomb and forms a dihedral
angle of 23.2° with respect to the Mn(1) Mn(2) vector. All of
3 3 3
the Mn atoms are terminally coordinated by THF. The metal-
nitrogen contacts are longest (2.10-2.17 Å) for N(1), N(2),
N(5), and N(6) because they are shared between two metal
atoms, and the contacts for the remaining nitrogen atoms that
ligate only one metal atom are a bit shorter (2.05-2.08 Å). The
hydrazine N-N bonds are slightly elongated single bonds with
distances between 1.45 and 1.47 Å, consistent with the assign-
ment of deprotonated hydrazide.
The structural metrics of 2a are quite similar to those of 2. The
Mn-N contacts in the Mn2N2 rhomb are slightly elongated
because of the stronger σ-donation from the pyridine terminal
ligands, though the rhomb angles are similar. The planarity of the
base rhomb is slightly decreased, and the five-membered “wings”
are slightly more puckered in 2a, but there are no other major
differences in the structures. Only a few examples of Mn clusters
of primary amides have been reported.9 Mn clusters with
hydrazines are almost as rare and limited exclusively to acyl
hydrazides.10
The solution behavior of 2 and 2a is complex. Compound 2
showed no discernible 1H NMR signature, which we attribute to
a paramagnetic broadening by the high-spin Mn atoms and
1
possibly speciation, resulting in extensive broadening. H NMR
analysis of compound 2a was also uninformative.11
-
All of the NR2 ligands were removed via protolysis by the
N-H protons of PhNHNHPh. Charge counting considerations
on 2 and 2a require the formal assignment of an all-MnII cluster.
Thus, the cluster has two electrons and two protons more than
expected on the basis of a simple 1:1 reaction stoichiometry.
Therefore, the role of an additional (fifth) hydrazine molecule
must be invoked to write the balanced chemical equation for the
reaction:
4MnðNR2Þ2 þ 5PhNHNHPh þ 4THF f
2 þ PhNdNPh þ 8HNR2
In this reaction, diphenylhydrazine plays several roles: the
-
proton source for the protolysis of NR2 ligands, the ligand
source for the formation of the hydrazide and amide ligands in 2
and 2a, and the reductant, replacing the two electrons consumed
in the N-N bond cleavage concomitant with formation of
azobenzene. For example, the formation of an intermediate
Mn(III)-imido species by reductive cleavage of the N-N bond
followed by hydrogen atom transfer (HAT) from hydrazine to
generate 2 and azobenzene is a plausible mechanism. Indeed,
azobenzene and HNR2 were observed in the soluble fraction of
the reaction mixture by 1H NMR spectroscopy. A similar
mechanism has been proposed for hydrazine disproportionation
by Zr complexes of redox-active ligands.12a The high yields of 2
and 2a and the relatively low conversion (∼30%) to azobenzene
all disfavor simple disproportionation mechanisms13 for N-N
bond cleavage, though more complex, metal-mediated radical
disproportionations, such as those observed in iron chemistry,
are also possible.12b
Figure 2. Thermal ellipsoid plots of compounds 2, 2a, and 1b.
Ellipsoids are set at 50% probability. N-H hydrogen atoms were
located as Fourier difference peaks and refined, and they are shown as
open circles. Carbon atoms are represented by open ellipses. C-H
hydrogens have been omitted for clarity.
Selected metrics for the clusters are given in Table 1. The
cluster core of 2 can be described as a “pinned butterfly”
geometry with C2 point symmetry analogous to that described
by Dismukes for OEC models.8 The base is constructed from an
Mn2N2 rhomb with each N belonging to one hydrazide nitrogen.
This rhomb is nearly planar and slightly elongated along the N-
N vector (see Table 1) with bond angles of ca. 91.5 and 88.5° for
the Mn and N rhomb angles respectively. The base Mn atoms are
further bridged to two “wing” Mn atoms through the hydrazide
ligand and a bridging N-phenylamido ligand, forming a five-
membered puckered metallacycle. The presence of the phenylamido
The reaction of Mn(NR2)2 with a decreased equivalency of
PhNHNHPh (1.15:1 ratio) at low temperature and low dielectric
constant (higher pentane concentration) resulted in a solution
that maintained the black color. The use of pentane and low
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Table 1. Selected Bond Distances (Å) and Angles (deg)a for Mn4(μ3-PhNNPh)2(μ-PhNNPh)(μ-NHPh)2(THF)4 (2) and
Mn4(μ3-PhNNPh)2(μ-PhNNPh)(μ-NHPh)2(py)4 (2a)
2b
2a
Mn(1)-N(1/2/6)
Mn(2)-N(1/4/7)
2.1054(14)/2.0957(13)/2.1719(15)
2.1483(14)/2.0448(14)/2.0673(14)
3.1861(9)/2.9760(10)/4.1187(9)
1.4661(18)/1.453(2)
2.119(2)/2.140(2)/2.1699(18)
2.155(2)/2.050(2)/2.058(2)
3.2405(6)/2.9777(5)/4.0346(6)
1.470(3)/1.444(2)
Mn(1) Mn(2/3/4)
3 3 3
N(2/4)-N(3/8)
N2 N6
3.059(3)
3.089(2)
3 3 3
N(2/6/6)-Mn(1)-N(1/1/2)
N(1/1/7)-Mn(2)-N(7/4/4)
Mn(1)-N(1/2)-Mn(2/3)
121.57(5)/105.57(5)/91.59(5)
101.94(6)/111.20(6)/124.18(5)
97.01(6)/88.41(5)
126.11(8)/101.84(7)/91.58(7)
102.01(7)/110.23(8)/117.13(7)
98.63(8)/87.61(7)
Mn(1/3/4)-N(2/2/3)-N(3/3/2)
96.63(8)/105.19(9)/122.20(10)
88.68(11)/105.65(11)/119.46(13)
a Divided entries refer to separate, related atoms and their associated metrics in the order given, e.g., N(2/6/6)-Mn(1)-N(1/1/2) denotes the three
angles N(2)-Mn(1)-N(1), N(6)-Mn(1)-N(1), and N(6)-Mn(1)-N(2). b For 2, atoms Mn(3), Mn(4), N(5), N(6), N(7), and N(8) are
symmetry equivalents of Mn(1), Mn(2), N(1), N(2), N(3), and N(4), respectively. The naming scheme has been changed for consistency with 2a.
temperatures lowers the solubility of hydrazine and thus its rate
of reactivity with 1a. The maintenance of 1a by decreased
hydrazine availability supports the hypothesis that 1a reacts with
PhNHNHPh to form 2. Extraction techniques allowed the
identification of at least three compounds in the reaction mixture.
The residue after filtration of the reaction mixture was identified
by FT-IR as 2. The black intermediary mixture 1a was isolated by
extraction of the dried filtrate into pentane. This substance is
hypersoluble in organic solvents and has eluded structural
characterization to date. The electronic absorption spectrum of
this extracted mixture was identical to that of 1a generated in situ.
FT-IR analysis of 1a indicated the presence of aliphatic C-H
stretches belonging to Me3Si and aryl C-H stretches but no
THF stretches14 or N-H stretches. The 1H NMR spectrum of
1a exhibited broad, paramagnetically shifted signals (see the
Supporting Information).
After removal of 1a by extraction, crystals of the dimeric
compound Mn2(μ-NHPh)2(NR2)2(THF)2 (1b) were isolated
from the residue and identified by single-crystal X-ray diffraction
(Figure 2). This compound may be an intermediate en route to 2
that is isolable only under conditions of limiting PhNHNHPh,
precluding aggregation to form 2. Alternatively, 1b may be a
second product that does not form in large amounts in the 4:5
ratio reaction but becomes favored under reduced hydrazine
loading. 1b can be prepared directly from Mn(NR2)2 and aniline
in good yield (82%).
’ ACKNOWLEDGMENT
We gratefully acknowledge Pat Carroll of The University of
Pennsylvania for assistance with X-ray data collection and
Temple University for its generous support of this work.
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’ ASSOCIATED CONTENT
S
Supporting Information. Crystal data tables; experimen-
b
tal procedures; FT-IR and absorption spectra of 1a, 1b, 2, and 2a;
1H NMR spectra of 1a, 1b, and 2b; and crystallographic data
(CIF). This material is available free of charge via the Internet at
http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
mzdilla@temple.edu
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