Tying the alkoxides together: An iron complex of a new chelating bulky bis(alkoxide) demonstrates selectivity for coupling of non-bulky aryl nitrenes

Article abstract of DOI:10.1039/c9cc05319k

New chelating bis(alkoxide) ligand H₂[OO]^Ph and its iron(ii) complex Fe[OO]^Ph(THF)₂ are described. The coordination of the ligand to the metal center is reminiscent of the coordination of two monodentate alkoxides in previously reported Fe(OR)₂(THF)₂ species. Fe[OO]^Ph(THF)₂ catalyzes selective and efficient dimerization of non-bulky aryl nitrenes to yield the corresponding azoarenes.

Full text of DOI:10.1039/c9cc05319k

View Article Online
View Journal
ChemComm
Chemical Communications
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: S. Groysman, R. L.
Lord, D. Wannipurage and S. S. Kurup, Chem. Commun., 2019, DOI: 10.1039/C9CC05319K.
Volume 54
This is an Accepted Manuscript, which has been through the
Number
January 2018
Pages 1-112
1
4
Royal Society of Chemistry peer review process and has been
accepted for publication.
ChemComm
Chemical Communications
rsc.li/chemcomm
Accepted Manuscripts are published online shortly after acceptance,
before technical editing, formatting and proof reading. Using this free
service, authors can make their results available to the community, in
citable form, before we publish the edited article. We will replace this
Accepted Manuscript with the edited and formatted Advance Article as
soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes to the
text and/or graphics, which may alter content. The journal’s standard
Terms & Conditions and the Ethical guidelines still apply. In no event
shall the Royal Society of Chemistry be held responsible for any errors
or omissions in this Accepted Manuscript or any consequences arising
from the use of any information it contains.
ISSN 1359-7345
COMMUNICATION
S. J. Connon, M. O. Senge et al.
Conformational control of nonplanar free base porphyrins:
towards bifunctional catalysts of tunable basicity
rsc.li/chemcomm

Please do not adjust margins
Page 1 of 5
ChemComm
View Article Online
DOI: 10.1039/C9CC05319K
COMMUNICATION
Tying the alkoxides together: an iron complex of a new chelating bulky
bis(alkoxide) demonstrates selectivity for coupling of non-bulky aryl nitrenes
Received 00th January 20xx,
Accepted 00th January 20xx
Sudheer S. Kurup,a† Duleeka Wannipurage,a† Richard L. Lord*,b and Stanislav Groysman*a
DOI: 10.1039/x0xx00000x
New chelating bis(alkoxide) ligand H2[OO]Ph and its iron(II)
complex Fe[OO]Ph(THF)2 are described. The coordination of the
ligand to the metal center is reminiscent of the coordination of
two monodentate alkoxides in previously reported Fe(OR)2(THF)2
species. Fe[OO]Ph(THF)2 catalyzes selective and efficient
dimerization of non-bulky aryl nitrenes to yield the corresponding
azoarenes.
A
B
N3Ar
R2
P
Ar
N
iPr
iPr
Ru PR2
Si
R2P
Ni
N
Ni
N
N
iPr
N
iPr
R = iPr
2010
JACS
, 132, 4083.
2018
D
JACS
, 140, 4110.
There is a growing interest in the transition metal-catalyzed
synthesis of azoarenes.1 Azoarenes (ArN=NAr’, “azo dyes”) are
highly colored compounds that have many existing industrial
applications including as textile dyes/pigments, chemical
indicators, and food additives; many additional potential
applications are currently being pursued.2 Established
synthetic routes to azoarenes include azo coupling reaction,3
the Mills reaction,4 the Wallach rearrangement,5 and other
routes.1 These preparative routes often require stoichiometric
amounts of toxic and/or highly reactive co-reactants such as
C
Ph
THF
THF
THF
Fe
139 
THF
Fe
Ph
RO
O
R'O
O
142

tBu
tBu
tBu tBu
2018
2015
Organometallics
, 34, 2917.
Inorg. Chem.
, 57, 9485.
Figure 1. Examples of previously reported catalysts for azoarene formation
potassium dichromate, Caro’s acid (H2SO5), sodium amalgam,
3-5
lead salts and others.1,
Efficient metal-catalyzed
restricted to bulky aryl nitrenes featuring two ortho
substituents.10a Mechanistic studies suggested that alkoxide
disproportionation (to yield tris(alkoxide) Fe(OR)3 and bridging
imido species [Fe2(OR)2(μ2-NAr)]) was at least in part
responsible for this limitation.10a To overcome this problem,
we designed Fe(OR’)2(THF)2 with a bulkier alkoxide OR’
dimerization of nitrenes (derived from organoazides)
constitutes a useful alternative to the above processes.6-11
Selected transition metal catalysts for azoarene formation are
presented in Figure 1. Most of the existing catalysts
demonstrate limitations in the types of azoarenes they can
produce. Uyeda’s catalyst (B) exhibits the broadest scope of
substrates, with the only limitation being bulky substrates
(larger than mesityl).11 Our group investigates group-transfer
chemistry at [M(OR)2] platforms, generally featuring two bulky
(OR’=OCtBu2(3,5-Ph2Ph),
D).10b
Consistent
with
our
expectations, arresting the disproportionation route increased
the overall scope of substrates to single ortho as well as meta-
substituted aryl nitrenes, although the substrates lacking such
substituents still exhibited poor reactivity (generally one
turnover or less).10b A different approach to the design of the
more robust catalyst involves linking two monodentate
alkoxides into a chelating ligand. Herein we present a
preliminary report on the synthesis and reactivity of a new
chelating bis(alkoxide) ligand H2[OO]Ph (1). We demonstrate
that this ligand exhibits similar coordination chemistry to
monodentate alkoxides, allowing us to conduct a comparative
structure-activity study of different “Fe(OR)2” systems for
nitrene coupling. Most importantly, Fe[OO]Ph(THF)2 exhibits
complementary nitrene coupling reactivity to
monodentate alkoxide ligands.12 We have reported that
12a-c
Fe(OR)2(THF)2
(C, OR = OCtBu2Ph) served as an efficient
catalyst for nitrene dimerization, although its reactivity was
a. Department of Chemistry, Wayne State University, 5101 Cass Ave., Detroit, MI
48202, USA. E-mail: groysman@chem.wayne.edu.
b. Department of Chemistry, Grand Valley State University, 1 Campus Dr, Allendale,
MI 49401, USA. E-mail: lordri@gvsu.edu
† These authors contributed equally to this work.
Electronic Supplementary Information (ESI) available: Experimental procedures,
X-ray data collection/refinement details, NMR/IR/UV-vis spectra, and
computational details. CCDC 1939243-1939244. See DOI: 10.1039/x0xx00000x
Please do not adjust margins

Please do not adjust margins
ChemComm
Page 2 of 5
COMMUNICATION
Journal Name
View Article Online
DOI: 10.1039/C9CC05319K
Figure 3. Spin density isosurface (iso = 0.002 au) plot for 2.
Solution magnetic susceptibility measurements suggest the
compound has a high-spin Fe(II) center (μeff = 4.8 μB). Proton
NMR contains 16 ligand signals (ESI), suggesting restricted
rotation of the phenyl rings and an overall C2v symmetry. The
Figure 2. Top: Synthesis of 2. Bottom: structures of 1 and 2.
Fe(OR)2(THF)2/Fe(OR’)2(THF)2 (C/D, Figure 1), allowing efficient X-ray structure determination reveals
dimerization of non-bulky aryl nitrenes.
a tetra-coordinate
complex of Fe[OO]Ph(THF)2 composition. In contrast to the
Bis(alkoxide) complexes featuring two bulky monodentate structure of 1, the ligand adopts a syn conformation in 2. The
ligands generally exhibit a relatively broad angle between the structure of 2 resembles the structures of the iron complexes
alkoxides (RO-M-OR ~140 °, Figure 1), which leads to metal with monodentate alkoxides,
C and D, albeit with a
geometries intermediate between tetrahedral and seesaw. To significantly wider inter-alkoxide angle of 155.5(2) degrees (vs.
model this disposition, we decided to employ the 1,4- 138.7(1)10a and 141.5(1) for C and D,10b respectively); the
terphenyl structural motif, which was utilized by Agapie and O(THF)-Fe-O(THF) angle is 88.8(2) degrees. These structural
coworkers in the synthesis of chelating ligands with a wide parameters suggest seesaw geometry at iron. No interaction
intra-ligand angle.13 Chelating bis(alkoxide) ligand H2[OO]Ph (1) between the metal and the central phenyl ring is observed (Fe-
was synthesized in a three-step procedure (ESI) that involved phenyl distance 3.1 Å). DFT optimization of 2 as a quintet
initial synthesis of 1,4-bis(2-bromophenyl)benzene,14 followed agrees well with the experimental crystal structure and
by lithiation with tBuLi, and finally reaction with reinforces its assignment as a high-spin FeII species, as
benzophenone (72% yield, Scheme S1 in ESI). Phenyl demonstrated by the spin density isosurface plot in Figure 3
substituents were chosen for the initial foray into the chelating with most of the spin density at the Fe ion or the alkoxide O
bis(alkoxide) chemistry due to the anticipated crystallinity and atoms bonded to it. The higher energy singlet and triplet spin
slightly less steric hindrance compared with tert-butyl states have short Fe–Cphenyl distances of ~2.1 Å (see ESI Figure
substituents. 1 was characterized by NMR spectroscopy, high- S46, Table S3), which suggests [OO]Ph may allow metal-phenyl
resolution mass spectrometry, and X-ray crystallography (see interactions in some species.
ESI). X-ray crystallography confirms the overall connectivity
The reactivity of 2 in nitrene coupling was investigated and
and indicates anti geometry in the solid state with the alcohols the results are presented in Figure 4. All reactions were carried
1
on different sides of the central phenyl (Figure 2). H NMR out in C6D6 at 60 °C for 24 h using 10 mol% catalyst. The
(C6D6) indicates a species of high symmetry in solution, reactions were monitored by NMR spectroscopy, using
consistent with either rapid interconversion of syn/anti forms, trimethoxybenzene,
hexamethylbenzene,
or
predominantly C2v syn form, or predominantly C2h anti form in hexafluorobenzene as an internal standard. The products were
1
solution. Supportive of the first hypothesis, a relaxed surface identified by H NMR spectroscopy and confirmed by GC-MS;
scan using DFT about the dihedral angle between the alkoxide selected azoarenes were isolated and further characterized
arms suggests a low barrier of 9.5 kcal/mol with the syn isomer (ESI). Most notably, nitrene coupling reactivity exhibited by
only 0.4 kcal/mol higher in energy than the anti isomer (see ESI Fe[OO]Ph(THF)2 (2) is almost perfectly complementary to the
Figure S46). A VT NMR study (conducted in CD2Cl2 in -60 – 20 reactivity exhibited by Fe(OR)2(THF)2 or Fe(OR’)2(THF)2: while
ºC range) demonstrates the presence of two isomers at low the previous catalysts were selective for the coupling of bulky
temperatures, as indicated by two well-resolved OH signals aryl nitrenes, the present catalyst (2) demonstrates high to
below -50 ºC (Figure S25). Treatment of 1 with one equivalent moderate yields for non-bulky substrates featuring para alkyl,
of
Fe(N(SiMe3)2)2(THF)x
precursor,15
followed
by trifluoro, or halo substituents; the conversion is quantitative
recrystallization from CH2Cl2, leads to the isolation of light for the phenyl azide (Figure 4). The formation of halo-
brown crystals of Fe[OO]Ph(THF)2 (2) in 82% yield. 2 was substituted azoarenes is particularly noteworthy, as these
characterized by 1H NMR spectroscopy, magnetic susceptibility groups can be later functionalized via cross-coupling. Although
measurements by Evans method, IR and UV-vis spectroscopy, lower yields were observed for the p-nitro, p-acetyl, or p-
X-ray crystallography, and elemental analysis (ESI).
methoxy groups, these are uncommon functional groups for
metal-mediated formation of azoarenes.
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 3 of 5
ChemComm
Please do not adjust margins
Journal Name
COMMUNICATION
R
Fe[OO]Ph(THF)2
10 mol%
View Article Online
DOI: 10.1039/C9CC05319K
R
N
N3
1/2
N
C6D6, 60 oC, 24 h
R
iPr
Me
Et
N
N
N
80%
N
N
N
N
N
46%
100%
85%
iPr
Me
Et
Cl
Br
F3C
F
N
N
N
N
N
N
N
N
69%
75%
64%
83%
Cl
Br
CF3
F
MeO
Me(O)C
O2N
CF3
Figure 5. Optimized structures of putative iron nitrenes [Fe(L)(NAr)] supported by
chelating [OO]Ph ligand (top) or untethered OR ligands (bottom), using Ar=Ph (left) and
Ar=2,6-Me2Ph (right). All species were modeled as quintets.
N
N
N
N
N
N
N
N
37%
64%
28%
39%
OMe
C(O)Me
NO2
Me
F3C
accessible for both aryl substituents when FeNAr is supported
instead by two OR groups (Figure 5, bottom). Therefore, we
hypothesize that the inaccessibility of the nitrene when ortho-
substitutents are present may be responsible for the lack of
diazoarene formation in the presence of our new chelating
ligand. Moreover, the Fe–N bond is mostly accessible by an
incoming substrate from either face of the plane define by the
alkoxides and nitrene in Fe(OR)2(NAr), whereas the back face is
blocked by the bridging terphenyl group and the front face is
at least partially blocked by the alkoxide phenyl groups in
Fe[OO]PhNAr. Even though these ligands give a similar trigonal
planar coordination environments in both bis(alkoxide)
environments, the spatial constraints imposed by the chelating
ligand may provide an opportunity to control reactivity by
limiting the approach of reactants to the nitrene moiety.
In conclusion, we have designed new chelating
bis(alkoxide) ligand H2[OO]Ph with the 1,4-terphenyl bridge
linking [OCPh3]-like donors. Reaction of the ligand with an iron-
amide precursor led to the formation of a mononuclear
complex Fe[OO]Ph(THF)2 closely related to the previously
reported Fe(OR)2(THF)2 complexes with bulky monodentate
alkoxides (OR = OCtBu2Ph and OCtBu2(3,5-Ph2Ph)). Whereas
the overall donors disposition at the iron(II) center is
maintained, tying the alkoxides with the terphenyl bridge led
to a pronounced difference in the steric nature at the metal
site, which is illustrated by a nearly linear OAr-Fe-OAr angle
(156°), as compared with ~140° in the more relaxed structures
with monodentate ligands. Whereas the present “OCPh3”
motif is not bulkier than the previously utilized OCtBu2Ar, it is
likely the rigidity of the chelating system that prevents the
rearrangement (i.e. rotation or geometry change) of the
alkoxides. This difference in the nature of the active site is
manifested in the reactivity of the resulting nitrene coupling
catalysts. While Fe(OR)2(THF)2 complexes were able to couple
bulky aryl nitrenes only (for OR = OCtBu2Ph) or mostly (for OR
= OCtBu2(3,5-Ph2Ph), Fe[OO]Ph(THF)2 is selective for coupling
Me
N
Me
Me
N
N
N
N
Me
Me
N
Me
Me
69%
Me
0%
Me
0%
Me
Me
Figure 4. Nitrene coupling reactivity exhibited by 2.
In contrast, Fe(OR)2(THF)2 did not exhibit any reactivity with
phenyl azide or para-substituted azides, and Fe(OR’)2(THF)2
exhibited low reactivity at best. Meta-substituted 3,5-
dimethylphenyl azide and 3-trifluoromethylphenyl azide
exhibited relatively high yields (69% and 70% yields,
respectively). In a sharp contrast, no reactivity with ortho-
substituted (e.g. mesityl) azides was observed for
Fe[OO]Ph(THF)2 (2), whereas both Fe(OR)2(THF)2 and
Fe(OR’)2(THF)2 demonstrated quantitative yield with mesityl
azide.
What is the origin of the observed reactivity differences
between Fe[OO]Ph and Fe(OR)2/Fe(OR’)2 (C/D) systems? Due to
the overall similarity of the three iron bis(alkoxide) systems, it
is probable that the reaction of Fe[OO]Ph with aryl azides also
proceeds via a metal-nitrene intermediate, whose formation
and reactivity were computationally interrogated for the
Fe(OR)2/Fe(OR’)2 systems. Putative iron nitrenes with Ar = Ph
and Ar = 2,6-Me2Ph (Mes surrogate) were optimized as
quintets, based on our earlier modeling efforts,10 for [OO]Ph
and OR (Figure 5). In the less sterically bulky NPh species, the
N of the nitrene is exposed for reactivity whereas it is blocked
by one of the alkoxide phenyl groups for the ortho-substituted
aryl substituent when the iron nitrene is supported by the
[OO]Ph ligand (Figure 5, top). In contrast, the nitrene N remains
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
ChemComm
Page 4 of 5
COMMUNICATION
Journal Name
11 I. G. Powers, J. M. Andjaba, X. Luo, J. Mei and C.VUiewyeAdrtaic,leJO. nline
Am. Chem. Soc., 2018, 140, 4110-4118D. OI: 10.1039/C9CC05319K
12 (a) J. A. Bellow, M. Yousif and S. Groysman, Comments Inorg.
Chem., 2015, 36, 92-122. (b) J. A. Bellow, P. D. Martin, R. L.
Lord and S. Groysman, Inorg. Chem., 2013, 52, 12335 -12337.
(c) J. A. Bellow, D. Fang, N. Kovacevic, P. D. Martin, J.
Shearer, G. A. Cisneros and S. Groysman, Chem. Eur. J., 2013,
19, 12225-12228. (d) J. A. Bellow, M. Yousif, D. Fang, E. G.
Kratz, G. A. Cisneros and S. Groysman, Inorg. Chem., 2015,
54, 5624- 5633. (d) M. Yousif, D. J. Tjapkes, R. L. Lord and S.
Groysman, Organometallics, 2015, 34, 5119 - 5128. (e) J. A.
Bellow, S. A. Stoian, J. Van Tol, A. Ozarowski, R. L. Lord and S.
Groysman, J. Am. Chem. Soc., 2016, 138, 5531-5534. (f) A.
Grass, N. S. Dewey, R. L. Lord and S. Groysman,
aryl nitrenes lacking ortho substituents; no reactivity with
ortho-substituted (i.e. mesityl) azide took place. Successful
formation of 13 different azobenzenes is presented. The
difference in the reactivity is hypothesized to be due to the
sterically congested active site of Fe[OO]Ph, which interferes
with the reactivity of putative “Fe[OO]Ph(=NMes)” species.
Further mechanistic investigations of the present system, and
studies on the reactivity of H2[OO]Ph with other transition
metals, are currently underway.
S. G. and R. L. L. are grateful to the National Science
Foundation (NSF) for current support under grant number
CHE-1855681. Experimental characterization was carried out
at Lumigen Instrument Center of Wayne State University.
Organometallics, 2019, 38, 962-972. (g) A. Grass, S. A. Stoian,
R. L. Lord and S. Groysman, Chem. Commun. 2019, 55, 8458-
8461
13 (a) J. A. Buss, M. Hirahara, Y. Ueda and T. Agapie, Angew.
Chem. Int. Ed., 2018, 57, 16329-16333; (b) J. A. Buss, D. G.
VanderVelde and T. Agapie, J. Am. Chem. Soc., 2018, 140,
10121-10125; (c) J. A. Buss, C. Cheng and T. Agapie, Angew.
Chem. Int. Ed. 2018, 57, 9670-9674. (d) J. T. Henthorn, S. Lin
and T. Agapie, J. Am. Chem. Soc. 2015, 137, 1458-1464.
14 (a) A. Velian, S. Lin, A. J. M. Miller, M. W. Day and T. Agapie,
J. Am. Chem. Soc., 2010, 132, 6296-6297. (b) X. Feng, W.
Pisula and K. Mullen, J. Am. Chem. Soc., 2007, 129, 14116-
14117.
15 (a) M. M. Olmstead, P. P. Power and S. C. Shoner, Inorg.
Chem., 1991, 30, 2547−2551. (b) T. Deschner, K. W. Törnroos
and R. Anwander, Inorg. Chem., 2011, 50, 7217−7228. (c) D.
L. J. Broere, I. Čorić, A. Brosnahan and P. L. Holland, Inorg.
Chem. 2017, 56, 3140−3143.
Conflicts of interest
There are no conflicts to declare.
Notes and references
1
(a) E. Merino, Chem. Soc. Rev., 2011, 40, 3835−3853; (b) F.
Hamon, F. Djedaini-Pilard, F. Barbot and C. Len,
Tetrahedron, 2009, 65, 10105−10123.
2
(a) S. Yagai and A. Kitamura, Chem. Soc. Rev. 2008, 37,
1520−1529; (b) A. A. Beharry and G. A. Woolley, Chem. Soc.
Rev. 2011, 40, 4422−4437; (c) M. Dong, A. Babalhavaeji, S.
Samanta, A. A. Beharry and G. A. Woolley, Acc. Chem. Res.,
2015, 48, 2662−2670. (d) D. Bléger and S. Hecht, Angew.
Chem., Int. Ed., 2015, 54, 11338−11349. (e) S. Erbas-Cakmak,
D. A. Leigh, C. T. McTernan and A. L. Nussbaumer, Chem.
Rev., 2015, 115, 10081−10206.
3
4
For example, see: (a) K. Haghbeen and E. W. Tan, J. Org.
Chem., 1998, 63, 4503; (b) J. Merrington, M. James and M.
Bradley, Chem. Commun., 2002, 140. (c) J. Y. Kim, G. Kim, C.
R. Kim, S. H. Lee, J. H. Lee and J. S. Kim, J. Org. Chem., 2003,
68, 1933.
For selected examples of the synthesis of nitroso compounds
and their subsequent reaction with aniline (Mills reaction),
see: (a) H. H. Davey, R. D. Lee and T. J. Marks, J. Org. Chem.,
1999, 64, 4976. (b) W. W. Wood and J. A. Wilkin, Synth
Commun., 1992, 22, 1683. (c) B. C. Yu, Y. Shirai and J. M.
Tour, Tetrahedron, 2006, 62, 10303. (d) C. Tie, J. C. Gallucci
and J. R. Parquette, J. Am. Chem. Soc., 2006, 128, 1162.
For selected examples, see: (a) E. Buncel, Acc. Chem. Res.,
1975, 8, 132-139; (b) I. Shimao and S. Matsumura, Bull.
Chem. Soc. Jpn., 1976, 49, 2294; (c) J. Yamamoto, Y. Nishigaki
and M. Umezu, Tetrahedron, 1980, 36, 3177.
5
6
7
F Ragaini, A. Penoni, E. Gallo, S. Tollari, C. Li Gotti, M.
Lapadula, E. Mangioni and S. Cenini, Chem. Eur. J., 2003, 9,
249-259.
(a) N. P. Mankad, P. Müller and J. C. Peters, J. Am. Chem.
Soc., 2010, 132, 4083−4085; (b) A. Takaoka, M.-E. Moret and
J. C. Peters, J. Am. Chem. Soc., 2012, 134, 6695−6706.
A. F. Heyduk, R. A. Zarkesh and A. I. Nguyen, Inorg. Chem.
2011, 50, 9849−9863.
8
9
W. H. Harman, M. F. Lichterman, N. A. Piro and C. J. Chang,
Inorg. Chem. 2012, 51, 10037−10042.
10 (a) J. A. Bellow, M. Yousif, A. C. Cabelof, R. L. Lord and S.
Groysman, Organometallics, 2015, 34, 2917−2923; (b) M.
Yousif, D. Wannipurage, C. D. Huizenga, E. Washnock-
Schmid, N. J. Peraino, A. Ozarowski, S. A. Stoian, R. L. Lord,
and S. Groysman, Inorg. Chem. 2018, 57, 9485-9438.
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 5 of 5
ChemComm
View Article Online
DOI: 10.1039/C9CC05319K
An iron complex with a new chelating bis(alkoxide) ligand leads to an active nitrene dimerization catalyst
for a variety of para- and meta-substituted azide precursors.
R
N
N3
N
1/2
R
R
R:
/
substituents; 13 examples
para meta