Oxidative atom-transfer to a trimanganese complex to form Mn 6(μ6-E) (E = O, N) clusters featuring interstitial oxide and nitride functionalities

Article abstract of DOI:10.1021/ja2066384

Utilizing a hexadentate ligand platform, a trinuclear manganese complex of the type (^HL)Mn₃(thf)₃ was synthesized and characterized ([^HL]^6- = [MeC(CH₂N(C ₆H₄-o-NH))₃]^6-). The pale-orange, formally divalent trimanganese complex rapidly reacts with O-atom transfer reagents to afford the μ⁶-oxo complex (^HL) ₂Mn₆(μ⁶-O)(NCMe)₄, where two trinuclear subunits bind the central O-atom and the (^HL) ligands cooperatively bind both trinuclear subunits. The trimanganese complex ( ^HL)Mn₃(thf)₃ rapidly consumes inorganic azide ([N₃]NBu₄) to afford a dianionic hexanuclear nitride complex [(^HL)₂Mn₆(μ⁶-N)](NBu ₄)₂, which subsequently can be oxidized with elemental iodine to (^HL)₂Mn₆(μ⁶-N)(NCMe) ₄. EPR and alkylation of the interstitial light atom substituent were used to distinguish the nitride from the oxo complex. The oxo and oxidized nitride complexes give rise to well-defined Mn(II) and Mn(III) sites, determined by bond valence summation, while the dianionic nitride shows a more symmetric complex, giving rise to indistinguishable ion oxidation states based on crystal structure bond metrics.

Full text of DOI:10.1021/ja2066384

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Oxidative Atom-Transfer to a Trimanganese Complex To Form
6
Mn (μ -E) (E = O, N) Clusters Featuring Interstitial Oxide and Nitride
6
Functionalities
Alison R. Fout, Qinliang Zhao, Dianne J. Xiao, and Theodore A. Betley*
Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge Massachusetts 02138, United States
S Supporting Information
b
Scheme 1
ABSTRACT: Utilizing a hexadentate ligand platform, a
H
trinuclear manganese complex of the type ( L)Mn (thf)
3
3
H
6ꢀ
was synthesized and characterized ([ L]
= [MeC-
6
ꢀ
(
CH N(C H -o-NH)) ] ). The pale-orange, formally di-
2
6
4
3
valent trimanganese complex rapidly reacts with O-atom
6
H
transfer reagents to afford the μ -oxo complex ( L) Mn -
(
2
6
6
μ -O)(NCMe) , where two trinuclear subunits bind the
4
H
central O-atom and the ( L) ligands cooperatively bind
H
both trinuclear subunits. The trimanganese complex ( L)Mn -
3
(
thf) rapidly consumes inorganic azide ([N ]NBu ) to
3 3 4
H
afford a dianionic hexanuclear nitride complex [( L) Mn -
2
6
6
(
μ -N)](NBu ) , which subsequently can be oxidized with
4 2
H 6
elemental iodine to ( L) Mn (μ -N)(NCMe) . EPR and
2
6
4
alkylation of the interstitial light atom substituent were used
to distinguish the nitride from the oxo complex. The oxo
and oxidized nitride complexes give rise to well-defined
Mn(II) and Mn(III) sites, determined by bond valence sum-
mation, while the dianionic nitride shows a more symmetric
complex, giving rise to indistinguishable ion oxidation states
based on crystal structure bond metrics.
1
e and others are investigating simple polynucleating
W
ligands to stabilize multinuclear reaction sites to exploit
crystals of 1, suitable for X-ray diffraction analysis, were grown
from a concentrated thf solution that was allowed to stand at
ꢀ35 °C. The solid-state molecular structure for 1 (see Figure
S11, Supporting Information [SI]) reveals a planar arrangement
of three Mn ions supported by the [ L] ligand. Each Mn
center resides in a square pyramidal coordination environment,
with four anilide nitrogens forming the basal plane and an oxygen
2
the expanded redox properties of the complexes or to mimic
3
polynuclear metallocofactors. Using a hexadentate amine-based
platform, we have observed facile construction of polynuclear
iron complexes featuring a broad range of ground-state electronic
structures and redox properties which vary as a function of the
ligand architecture. Use of the sterically unencumbered proton-
II
H
6ꢀ
II
H
6ꢀ
H
6ꢀ
6ꢀ
3
capped ligand ( L) ([ L] = [MeC(CH NPh-o-NH) ]
)
from a molecule of thf binding in the apical site. Unlike the
2
H
gives rise to low-spin iron complexes which exhibit core-deloca-
( L)Fe
3
(PMe
2
R)
3
analogues, which feature close FeꢀFe con-
1
a
1a
lized redox activity, whereas use of the bulkier silylamide ligand
tacts (2.299(2)Å), complex 1 shows much larger ion separation
and a greater deviation from an equilateral triangle (Mn1ꢀMn2
2.8347(6), Mn1ꢀMn3 2.8291(8), Mn2ꢀMn3 2.7852(7) Å).
tbs 6ꢀ tbs 6ꢀ
t
6ꢀ
2 3
(
L) ([ L] = [1,3,5-C H (NPh-o-NSi BuMe ) ] ) en-
6 9
ables stabilization of high-spin complexes capable of atom-transfer
1b
reactivity to a tri-iron core, allowing us to explore the re-
Complex 1 more closely resembles the trimagnesium complex
H
activity of the trinuclear species as a molecular unit. We posited
( L)Mg
3
(thf)
3
, which features an average Mg Mg separation
3
3 3
H
that use of the sterically less encumbered ( L) ligand platform
of 2.847(1) Å and more suggestive of the metal ions residing at a
might permit oxidative atom-transfer to a trinuclear core, which
in turn could afford products of higher nuclearity. Herein, we
present the facile, oxidative atom-transfer chemistry to a triman-
distance reflective of simple ionic radii contact as opposed to
1a
involvement of metalꢀmetal bonding. The solution magnetic
1
moment obtained for H NMR silent 1 is 5.73(12) μ
the value expected for three magnetically isolated high-spin Mn
ions (μeff = 10.25 μ ). The low moment suggests the presence of
, well below
B
II
ganese complex, giving rise to hexanuclear Mn clusters that
6
feature interstitial oxides and nitrides.
B
3
4
Reaction of / equivalents of Mn (N(SiMe ) ) with one
2
2
3 2 4
H
equivalent of LH in thawing tetrahydrofuran (thf) affords the
Received: July 16, 2011
Published: September 26, 2011
6
H
complex ( L)Mn (thf) (1) in high yield (Scheme 1). Single
3
3
r 2011 American Chemical Society
16750
dx.doi.org/10.1021/ja2066384
|J. Am. Chem. Soc. 2011, 133, 16750–16753

Journal of the American Chemical Society
COMMUNICATION
H
6
H
6
n
H
6
Figure 1. Solid-state structures for (a) ( L)
2
Mn
6
(μ -O)(NCMe)
4
(2), (b) [( L)
2
Mn
6
(μ -N)](N Bu
4
)
2
(3a), and (c) ( L)
2
Mn
6
(μ -N)(NCMe)
4
(
4) with the thermal ellipsoids set at the 50% probability level (hydrogen atoms, solvent molecules, and Bu
4
N cations in 3 omitted for clarity; Mn orchid,
0
C black, H white, N blue, O red). Bond lengths (Å) for 2: Mn1ꢀMn2, 3.0591(6); Mn1ꢀMn2 , 3.1394(6); Mn1ꢀMn3, 3.1143(6); Mn2ꢀMn3,
0
0
0
3
.0442(6); Mn2ꢀMn3 , 3.0955(6); for 3a: Mn1ꢀMn2, 3.0726(8); Mn1ꢀMn2 , 3.0192(8); Mn1ꢀMn3, 3.0058(8); Mn1ꢀMn3 , 3.0694(7);
0
Mn2ꢀMn3, 3.0120(9); Mn2ꢀMn3 , 3.0899(8); Mn1ꢀN7, 2.1443(6); Mn2ꢀN7, 2.1634(6); Mn3ꢀN7, 2.1517(5); for 4: Mn1ꢀMn2, 3.057(2);
0
0
Mn1ꢀMn2 , 3.137(2); Mn1ꢀMn3, 3.043(2); Mn1ꢀMn3 , 3.099(2); Mn2ꢀMn3, 3.116(2).
II
antiferromagnetic superexchange interactions between Mn ions
through the bridging anilide ligands, similar to the behavior observed
Table 1. Selected Bond Distances (Å)
II
5,6
complex
MnꢀX
MnꢀN
H
MnꢀNbase
in related polynuclear MN species. EPR spectra collected for 1 at
, 77, and 300 K exhibit an intense isotropic signal with g = 2.016,
consistent with the presence of an antiferromagnetically coupled
3
2
Mn1
Mn2
Mn3
Mn1
Mn2
Mn3
Mn1
Mn2
Mn3
2.2694(4)
2.1111(4)
2.2289(5)
2.1178(4)
2.1333(4)
2.2836(4)
2.111(1)
2.267(1)
2.230(2)
2.226(2)
1.997(2)
2.216(2)
2.197(2)
2.192(2)
2.217(2)
1.991(6)
2.227(6)
2.218(6)
2.245(2)
2.041(2)
2.246(2)
2.204(2)
2.226(2)
2.267(3)
2.038(6)
2.236(6)
2.248(6)
5
II
system comprised of S = / Mn ions (see Figures S4, S5, SI).
2
With 1 in hand, we canvassed its reactivity with both O- and
N-atom transfer reagents (Scheme 1). Reaction of 1 with 0.6
equivalents of pyridine N-oxide (or PhIO) results in an immedi-
ate color change from orange-red to dark brown. The oxidized
product could be crystallized from slow diffusion of diethyl ether
into concentrated acetonitrile (NCMe) solutions of the product
3
4
b
H
6
at room temperature to yield ( L) Mn (μ -O)(NCMe) (2)
2
6
4
II
III
2
which we formulate as Mn Mn , the molecular structure of
4
which is shown in Figure 1a. Use of (4-trifluoromethyl)pyridine
is nearly isostructural with its oxo analogue 2 (Figure 1c).
Although crystalline yields for this do not exceed 30%, combus-
tion analysis for the bulk material analyzed was identical to that
obtained for single-crystal samples that were crushed and dried in
7
N-oxide allowed us to confirm consumption of the N-oxide via
1
9
in situ F NMR and integration against an internal standard
(
C H CF ), suggesting the reaction is quantitative for the
6
5
3
O-atom delivery despite the isolated yield of 82% (93% isolated
yield using C H N-O).
vacuum (to remove solvent from the crystal lattice). While
6
several examples of complexes bearing the [Mn (μ -O)] motif
5
5
6
8,9
Reaction of 1 with one equivalent of tetrabutylammonium
azide in tetrahydrofuran at ꢀ35 °C results in consumption of 0.5
equivalents of azide, although a color change from 1 does not
accompany the reaction as observed in the synthesis of 2. Cooling
solutions of the reaction product in a mixture of hexane and
have been previously reported, the molecular nitrides 3 and 4
are rather unusual with only one example featuring the [Mn (μ -
6
6
10
N)] unit reported in a solid-state structure.
Each of the clusters 2-4 consist of an edge-bridged octahedral
6
arrangement of Mn ions, with a μ -oxo or nitride ligand situated
H
benzene affords crystals of the solvent-free nitride [( L) Mn -
at the center of the Mn core. This structure is similar to those
2
6
6
6
H
(
μ -N)](NBu ) (3a) (93%, Figure 1b), whereas crystallization
observed for octahedral [ L Fe ] clusters templated by the
4
2
2
6
H
6ꢀ
1c,d
in the presence of acetonitrile produces the solvated complex,
[ L] ligand, which do not feature interstitial atoms.
For
H
6
[
( L) Mn (μ -N)(NCMe) ](NBu ) (3b, see Figure S12, SI).
the neutral complexes 2 and 4, two distinct coordination
environments are apparent for the Mn ions. First, two trans-
disposed Mn sites reside in a square pyramidal coordination
environment, with nitrogen atoms of the amide ligands compris-
ing the basal plane and the interstitial bridging atom positioned in
the apical site (see Table 1 for selected bond lengths). The
remaining four Mn sites each bind a molecule of acetonitrile,
2
6
2
4 2
The presence of two NBu cations indicates the net oxidation to
4
II
5
III
the Mn core is by a single electron (Mn Mn ). Treatment
6
of 3a with one molar equivalent of iodine in NCMe rapidly
oxidizes the cluster to afford the dark-brown, neutral nitride
H
6
(
L) Mn (μ -N)(NCMe) (4) with generation of 2 equivalents
2 6 4
+ II III
ꢀ
of Bu N I , giving a formulation of Mn Mn for 4. Complex 4
4
3
3
1
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Journal of the American Chemical Society
COMMUNICATION
+
Alkylation of the nitride gave two dominant ions for Bn NH
(
3
m
+ m
z 2 2 z
/ 288.1741) and Bn NH ( / 198.1275) that match the
predicted isotope pattern and an authentic sample made from the
same reaction conditions using the camphorsulfonic acid ammo-
nium salt as a N source (see Figure S10, SI), whereas subjecting 2
to a similar treatment did not reveal those ion peaks. Furthermore,
15
alkylation of 50% isotopically enriched N 3b (synthesized using
15
the isotopically enriched azide [Na] NNN) showed the parent
1
4
15
+
m
ions for both N and N isotopologues of Bn NH ( /
2
3
z
+
m
2 z
88.1741; 289.1741) and Bn NH2 ( / 198.1275; 199.1275).
These results demonstrate the ability to extend our synthetic
protocols for the preparation of triiron complexes to prepare an
analogous trimanganese complex. Moreover, this complex ex-
hibits similar oxidative, atom-transfer reactivity to the related
triiron complexes. The trinuclear manganese complex readily
reacts with both O- and N-atom delivery agents to afford
Figure 2. X-band EPR (9.337 GHz) for (a) 3b (3 K, toluene glass) and
spectral overlay for 2 and 4 (77 K, toluene glass).
6
Mn (μ -E) (E = O, N) clusters, respectively, highlighting the
6
efficacy of using well-defined trinuclear complexes to assemble
polynuclear species of higher nuclearity. The clusters likely form
via atom transfer to a trimanganese core with subsequent
reaction with a second trinuclear complex to yield the hexa-
nuclear products. Work is currently underway to understand the
electronic structure of these complexes and canvass the scope of
reactivity of the oxidized materials.
which completes the octahedral coordination sphere. These sites
show elongation of the MnꢀX distances and expanded Mnꢀ
N
interactions, reflective of the different electronic configura-
H
L
II
1
2
1
tions for the square pyramidal Mn sites (d ) (d ,d ) (d ) -
z2
xy
xz yz
1
III
1
2
1
(
(
d
d
2
2
) and the octahedral Mn sites (d ) (d ,d ) (d ) -
z2
x ꢀy
x ꢀy
xy
xz yz
0
2
2
) . Bond valence summation corroborates this assignment
III
with the square pyramidal sites consistent with Mn , whereas
the octahedral sites are consistent with a Mn formulation.
’
ASSOCIATED CONTENT
II
11
Unlike the neutral complexes 2 and 4, there is no obvious
structural asymmetry to 3a. The average MnꢀMn separation is
S
Supporting Information. Experimental procedures and
b
spectral data for 1ꢀ4; selected crystallographic data and bond
lengths for 1ꢀ4; CIF file for 1, 2, 3, 3b, and 4; EPR spectra for
1ꢀ4. This material is available free of charge via the Internet at
http://pubs.acs.org.
3.0497(9) Å, comparable to the average distances observed in 2.
The bond metrics from Mn to both amide residues are consistent
throughout the core (MnꢀN 2.218(3), MnꢀN 2.182(3) Å),
base
H
and are similar to the divalent sites observed in 2. The Mnꢀ
nitride distances show little variance (Mn1ꢀN7 2.1443(6),
Mn2ꢀN7 2.1634(6), Mn3ꢀN7 2.1517(5) Å) indicating an equal
’ AUTHOR INFORMATION
Corresponding Author
betley@chemistry.harvard.edu
interaction with the central nitride throughout the Mn core.
6
Indeed, bond valence summation suggests the Mn ions in 3a, as
well as 3b (see Table 1), more closely resemble divalent manganese
III
than trivalent manganese, suggesting the Mn site is averaged
over all positions in 3a and within the four solvent-free sites in 3b.
’
ACKNOWLEDGMENT
The solution magnetic moments for the series of compounds:
We thank Harvard University for financial support and the
8
.9(3) μ for 2, 8.2(1) for 3b, and 6.2(1) for 4 are all indicative of
NIH for an NIH Ruth L. Kirschstein NRSA fellowship for A.R.F.
We also thank Prof. Alan Saghatelian and Nawaporn Vinayave-
khin for assistance in obtaining the MS data.
B
antiferromagnetic superexchange coupling between Mn ions
through the bridging oxo/nitride and amide ligands. A strong
isotropic signal (g = 2.17) is observed by EPR for toluene glasses
of 3b over the temperature range 77ꢀ300 K, while cooling to 3 K
reveals hyperfine coupling to the Mn ions (I = 5/2) and possibly
N (see Figures 2 and S7ꢀS8, SI). The oxidized nitride 4 also
exhibits a strong EPR signal with some hyperfine to Mn apparent,
consistent with the 27 valence electron formulation, whereas the
analogous oxo 2 (28 valence electrons) is EPR silent (Figure 2b).
Given the close structural parameters between the neutral oxo
’
REFERENCES
(1) (a) Zhao, Q.;Betley, T. A.Angew. Chem., Int. Ed. 2011, 50, 709–712.
(b) Powers, T. M.; Fout, A. R.; Zheng, S.-L.; Betley, T. A. J. Am. Chem. Soc.
2
2
011, 133, 3336. (c) Zhao, Q.; Harris, T. D.; Betley, T. A. J. Am. Chem. Soc.
011, 133, 8293. (d) Harris, T. D.; Zhao, Q.; Hern ꢀa ndez S ꢀa nchez, R.;
Betley, T. A. Chem. Commun. 2011, 47, 6344. (e) Harris, T. D.; Betley, T. A.
J. Am. Chem. Soc. 2011, 133, 13852.
(2) (a) Adams, R. D. J. Organomet. Chem. 2000, 600, 1. (b) Suzuki,
H. Eur. J. Inorg. Chem. 2002, 1009. (c) Dyson, P. J. Coord. Chem. Rev.
2
and nitride 4 and the difficulty distinguishing light atom
3c
substituents from X-ray analysis, we sought to identify unequi-
vocally the light interstitial atom via chemical assay. Mass spectral
2
004, 248, 2443. (d) Pap, J. S.; DeBeer George, S.; Berry, J. F. Angew.
Chem., Int. Ed. 2008, 47, 10102. (e) Tsui, E. Y.; Day, M. W.; Agapie, T.
Angew. Chem., Int. Ed. 2011, 50, 1668.
analysis on complexes 2ꢀ4 proved problematic due to the
H
sensitivity of the ( L)Mn units which easily hydrolyze. Thus,
3
(3) (a) Coucouvanis, D. Acc. Chem. Res. 1981, 14, 201. (b) Pecoraro,
we wanted to convert the light atom component into a species we
could identify. Protonolysis of the complexes (camphorsulfonic
acid, 25 equiv) followed by treatment with benzyl bromide (10
equiv) and base (K CO , 50 equiv) resulted in alkylation of
V. L.; Baldwin, M. J.; Gelasco, A. Chem. Rev. 1994, 94, 807. (c) Einsle, O.;
Tezcan, F. A.; Andrade, S.; Schmid, B.; Yoshida, M.; Howard, J. B.; Rees,
D. C. Science 2002, 297, 1696. (d) Venkateswara Rao, P.; Holm, R. H.
Chem. Rev. 2004, 104, 527. (e) Lee, S. C.; Holm, R. H. Chem. Rev. 2004,
104, 1135. (f) Mukhopadhyay, S.; Mandal, S. K.; Bhaduri, S.; Armstrong,
2
3
the central light atom which could be identified by ESI MS.
1
6752
dx.doi.org/10.1021/ja2066384 |J. Am. Chem. Soc. 2011, 133, 16750–16753

Journal of the American Chemical Society
COMMUNICATION
W. H. Chem. Rev. 2004, 104, 3981. (g) Mullins, C. S.; Pecararo, V. L.
Coord. Chem. Rev. 2008, 252, 416. (h) Hoffman, B. M.; Dean, D. R.;
Seefeldt, L. C. Acc. Chem. Res. 2009, 42, 609.
(
4) Anderson, R. A.; Faegri, K., Jr.; Green, J. C.; Haaland, A.;
Lappert, M. F.; Leung, W.-P.; Rypdal, K. Inorg. Chem. 1988, 27, 1782.
5) Hatnean, J. A.; Raturi, R.; Lefebvre, J.; Leznoff, D. B.; Lawes, G.;
Johnson, S. A. J. Am. Chem. Soc. 2006, 128, 14992.
6) Tsui, E. Y.; Kanady, J. S.; Day., M. W.; Agapie, T. Chem. Commun.
011, 47, 4189.
7) Jiang, B.; Xiong, W.; Zhang, X.; Zhang, F. Org. Process Res. Dev.
001, 5, 531.
8) (a) Brown, I. D.; Altermatt, D. Acta Crystallogr., Sect. B 1985,
B41, 244. (b) Thorp, H. H. Inorg. Chem. 1992, 31, 1585.
9) (a) Mondal, K. C.; Drew, M. G. B.; Mukherjee, P. S. Inorg. Chem.
007, 46, 5625. (b) Stamatatos, T. C.; Pringouri, K. V.; Abboud, K. A.;
Christou, G. Polyhedron 2009, 28, 1624.
10) Eddine, M. N.; Bertaut, E. F.; Maunaye, M. Acta Crystallogr.,
Sect. B 1977, B33, 2696.
11) (a) Cavaluzzo, M.; Chen, Q.; Zubieta, J. Chem. Commun.
993, 131. (b) Brechin, E. K.; Clegg, W.; Murrie, M.; Parsons, S.; Teat,
(
(
2
2
(
(
(
2
(
(
1
S. J.; Winpenny, R. E. P. J. Am. Chem. Soc. 1998, 120, 7365. (c) Koo,
B.-K.; Lee, U. Bull. Korean Chem. Soc. 2001, 22, 103. (d) Bagai, R;
Abboud, K. A.; Christou, G. Inorg. Chem. 2008, 47, 621.
1
6753
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