Formation of the tetranuclear, tetrakis-terminal-imido Mn4 IV(NtBu)8 cubane cluster by four-electron reductive elimination of tBuNNtBu. the role of the s-block ion in stabilization of high-oxidation state intermediates

Article abstract of DOI:10.1039/c3cc42165a

Mn₄^IV(μ₃-N^tBu) ₄(N^tBu)₄ is obtained from a previously reported asymmetric Mn^IV/V-Li-(NR)(N) cluster by the removal of Li from the starting cluster by ion metathesis, which triggers reductive elimination of azo-tert-butane to give a tetranuclear heterocubane cluster.

Full text of DOI:10.1039/c3cc42165a

Showcasing research from Michael J. Zdilla’s
Laboratory/Department of Chemistry, Temple University,
Philadelphia, Pennsylvania USA
As featured in:
Formation of the tetranuclear, tetrakis-terminal-imido
Mn4IV(NtBu)8 cubane cluster by four-electron reductive
elimination of tBuN=NtBu. The role of the s-block ion in
stabilization of high-oxidation state intermediates
The removal of lithium from a Li–Mn(IV/V)-imido cluster by ion
metathesis with chloride ion results in reductive elimination of
azo-tert-butane to give a Mn(IV)-imido cubane cluster.
See Michael J. Zdilla,
Chem. Commun., 2014, 50, 1061.
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Formation of the tetranuclear, tetrakis-terminal-
IV
imido Mn4 (NtBu)8 cubane cluster by four-
Cite this: Chem. Commun., 2014,
50, 1061
t
t
Q
electron reductive elimination of BuN N Bu.
The role of the s-block ion in stabilization
of high-oxidation state intermediates†
Received 25th March 2013,
Accepted 12th November 2013
DOI: 10.1039/c3cc42165a
Shivaiah Vaddypally,‡a Sandeep K. Kondaveeti,‡a John H. Roudebush,b
Robert J. Cavab and Michael J. Zdilla*a
www.rsc.org/chemcomm
Mn4IV(l3-NtBu)4(NtBu)4 is obtained from a previously reported nitrene analogue of O2) from manganese aggregates in solution,
asymmetric MnIV/V–Li–(NR)(N) cluster by the removal of Li from the but the precise structure of the active species is not known,10,11 and
starting cluster by ion metathesis, which triggers reductive elimination the four-electron reductive elimination of azoarenes by the group of
of azo-tert-butane to give a tetranuclear heterocubane cluster.
Heyduk is worthy of note.12 We report here experimental evidence
for the 4-electron reductive elimination of azo-tert-butane from the
Reductive elimination reactions from transition metal-Lewis acid imide ligands of manganese cubane clusters.
metal clusters are key to the understanding of the biological Oxygen
We have previously reported the synthesis of a cocrystalline
Evolving Complex (OEC) of photosystem II, which is a Ca–Mn–O mixture of structurally analogous manganese imide clusters
cluster that generates O2 from cluster oxide ligands by 4-electron with the general formula Li3Mn4M(m3-NtBu)6(m-NtBu)3(NtBu)(N)
reductive elimination. While a number of proposals exist for the where M = MnRN (1) or Li (1a).13 This cluster, shown in
molecular mechanism of this reaction,1–4 scarcity of experimental Scheme 1, possesses several ligand types: m3-bridging imides,
examples of this reaction at metal clusters renders the proposed m2-bridging imides, terminal imide, and nitride. Charge counting,
mechanisms experimentally inscrutable. Additionally, reductive bond valence summation calculations14 and XPS analysis reveal
elimination of light ligands from metal-based molecules is of the cluster possesses a mixed (IV/V) oxidation state. This material is
interest for the generation of precursors for chemical vapor deposi- a cocrystalline mixture of the nearly isostructural Li3Mn5 (1) and
tion of elemental solid state materials.5
Li4Mn4 (1a) dicubane clusters.
Examples of reductive elimination from manganese clusters are
In our efforts to remove the Li+ from this cluster, we have
rare. The oxygen evolving dimer from the Brudvig group releases O2 discovered that this high oxidation state is unstable in the absence
from terminal or bridging sites, but O-exchange rates are too fast to of the s-block ions. When Li+ is extracted from the cluster using ion
distinguish the ligand sites responsible for reductive elimination.6 metathesis with anhydrous Et4NCl, the result is precipitation of a
Experimental evidence for reductive elimination of core oxos has mixture of salts, of which LiCl is a primary component, and the
been reported at a synthetic Mn4O4 cubane cluster from the group isolation of a new, simple Mn4N4 heterocubane cluster (Fig. 1). This
of Dismukes.7 O2 evolution from reaction of a terminal manganese reaction is diagrammed in Scheme 1, illustrating the lithium
oxo complex with hydroxide8 supports a proposed mechanism metathesis from 1a (the dominant species).
involving attack of Ca-bound hydroxide on a terminal MnQO.
The structure of 2 is that of a tetramanganese heterocubane
The reversible oxidation of oxide to peroxide has been observed by cluster with four bridging tert-butyl imide ligands, and four
Nam and Valentine.9 We have previously reported on the two- terminal tert-butyl imide ligands. The molecule sits on a crystallo-
electron reductive elimination of azobenzene (diazenes are the graphic mirror plane (through atoms Mn(2,3), N(2,3,5,6)).
a Department of Chemistry, Temple University, 1901 N. 13th Street, Philadelphia,
PA 19122, USA. E-mail: mzdilla@temple.edu; Tel: +1-215-204-7118
b Department of Chemistry, Princeton University, Princeton, NJ, 08544, USA
† Electronic supplementary information (ESI) available: Full experimental and
synthetic procedures, NMR, UV-Vis, FT-IR, ESI mass spectra, CV of 2, full X-ray
crystallographic data tables, Bond Valence Summation calculations. CCDC
930706. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c3cc42165a
Scheme 1
‡ These authors contributed equally.
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analysis of the acid digest. However, metathesis does not explain the
overall gain of electrons by Mn. Reductive elimination of nitrogen
ligands is the most plausible explanation for the change in oxida-
tion state. The six-electron reductive elimination of nitrogen is a
possibility, but would result in a product cluster with much lower
oxidation state than 2. Though the reductive elimination of N2
cannot be strictly ruled out, the reductive elimination of azo-tert-
butane, which provides four electrons to the cluster, and
removes two tert-butyl imido ligands is most reasonable.
This proposed mechanism of cluster reduction is supported
by the detection of azo-tert-butane as the sole organic byproduct of
the metathesis reaction (see ESI† for data). The byproduct is
separated from the reaction mixture by vacuum distillation, and
analyzed by mass spectroscopy. The azo-tert-butane is also quan-
tified using in situ NMR, and forms in 80% yield based upon
eqn (1) (vide infra). The formation of azo-tert-butane as an organic
byproduct of the reaction suggests reductive elimination as the
mechanism for formation of 2. Thus, the lithium ions appear to
serve a stabilizing role in clusters 1/1a, possibly due to the hard–
hard acid–base interactions among the charge-dense lithium ions,
manganese(V) ions, nitride, and imide ligands. Lewis acidic ions
have been previously observed to affect reduction potentials20,21
or induce valence tautomerism22 in manganese compounds,
Fig. 1 X-ray crystal structure of Mn4IV(m3-NtBu)4(NtBu)4 (2). Thermal
ellipsoids shown at 30% probability level. Symmetry equivalent atoms are
generated by the (2Àx, y, z) operation.
Table 1 Interatomic distances (Å) and angles (deg) in Mn4IV(m3-NtBu)4(NtBu)4 (2)
Mn–N(m3)
Mn–N(term)
MnÁ Á ÁMn
NN
1.89–1.91
1.65
2.54–2.56
2.81–2.92
176–179
Mn–N(term)–C
The short Mn–N terminal bond lengths (1.65 Å), and linear although it has been argued based on DFT that Ca has little
nitrogen atoms indicate multiple bonding to the four terminal effect on electronic structure in the OEC.4 Thus, the stabilizing
imide ligands (Table 1). Terminal imide ligation is supported role of Li may instead be structural in nature. A similar stabilizing
by the presence of a band at 1220 cmÀ1 in the FT-IR spectrum role of Li in preventing reductive elimination of azobenzene has
consistent with a MnQNR stretch. The terminal and bridging been seen in hydrazide ligand chemistry at manganese.10
imides do not exchange in solution on the NMR time scale.
The conversion of 1 (2MnIV:3MnV) or 1a (1MnIV:3MnV) to 2
Structurally characterized examples of terminal imide ligation (4MnIV) results in the gain of three electrons, while the reductive
on manganese are rare, limited to the monomers and linear elimination of tert-butyl imide ligands to give azo-tert-butane is a
clusters of Wilkinson15,16 and the corrole-type complexes of four electron process. Thus, a balanced equation cannot be
Abu-Omar17 and Goldberg.18
written for a 1 : 1 molecular conversion, implicating a multistep
Though 2 is formed in approximately 81% yield based on mechanism. Furthermore, since the Mn-cube motif changes from
quantification by NMR (see ESI†), isolation of pure 2 in good one with imide and nitride bridges to an all-imide motif, cluster
yield is challenging due to hypersolubility in even the most rearrangement is necessarily involved. We propose that an addi-
non-polar solvents such as pentane and hexamethyldisiloxane tional equivalent of 1 provides the 4th oxidizing equivalent for the
(HMDS), resulting in yield loss during crystal isolation. Purified reductive elimination reaction, resulting in the following balanced
material is obtained in 33% yield after the filtration of salt chemical equation, which is experimentally supported by quanti-
byproducts by the slow vapor diffusion of a pentane solution tation of products 2 and azo-tert-butane, and by the detection of
with HMDS in a double vial apparatus. Rinsing of the crystals nitride and imide in the insoluble residue:
briefly with cold (À30 1C) pentane results in large, pure crystals,
but significant product loss. The 1H NMR spectrum shows two
sharp singlets in the diamagnetic region corresponding to bridging
and terminal tert-butyl groups. The sharp, diamagnetic spectrum
and magnetic measurements are indicative of a S = 0. Antiferro-
magnetic coupling has been observed in the terminal-imide-
containing iron-imido cubane cluster of Lee.19
4Li4Mn4(NtBu)10(N) (1a) + 12Et4NCl
- 4Mn4(NtBu)8 (2) + 3tBuNQNtBu
+ (12Et4N+, 16Li+, 12ClÀ, 2NtBu2À, 4N3À salts)
(1)
The cyclic voltammogram of compound 2 possesses a reversible
1-electron oxidation wave giving the formally 3-Mn(IV):1-Mn(V)
Compound 2 is formally, and based on bond valence summa- cubane cluster at À0.388 V vs. ferrocene. This oxidation state is
tion calculations,14 an all-Mn(IV) cluster, while the starting the traditional ‘‘high oxidation state paradigm’’ description of the
material 1/1a contains a mixture of Mn(IV/V), implicating an S4 state, which evolves O2. In our system, the 2+ state of this
overall reduction of the Mn centers. Furthermore, ligands are cluster is formed at low potential due to the extensive p-bonding
lost in the metathesis reaction, namely, from 1a, a nitride and which stabilizes high oxidation states via p-donation into the
two imides. Ion metathesis can explain, in part, the loss of d-orbitals. A second oxidation to the formal 2-Mn(IV):2-Mn(V)
ligands from 1a; indeed, nitride and imide ligands are present in species occurs at 0.690 V. vs. ferrocene, but is quasi-reversible,
the salt precipitate byproduct based upon ion chromatographic leading to compound decomposition (Fig. 2).
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Support of this research by the Army Research Laboratory
through contract W911NF-10-2-0098 is gratefully acknowledged.
The work reported herein was additionally supported by the
National Science Foundation, award #1254545 and #0841377,
through grants to Temple University. Work at Princeton was
supported by grant DE-FG02-08ER45644. The opinions expressed
do not necessarily reflect the position of the supporting agencies
and no official endorsement should be inferred. Special thanks to
Marek Domin (Boston College) for Mass Spectroscopic analysis.
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Chem. Commun., 2014, 50, 1061--1063 | 1063