Nitrene Insertion into C-C and C-H Bonds of Diamide Diimine Ligands Ligated to Chromium and Iron

Article abstract of DOI:10.1002/anie.201507463

The impact of redox non-innocence (RNI) on chemical reactivity is a forefront theme in coordination chemistry. A diamide diimine ligand, [{-CH￡N(1,2-C₆H₄)NH(2,6-iPr₂C₆H₃)}₂]ⁿ (n=0 to -4), (dadi)ⁿ, chelates Cr and Fe to give [(dadi)M] ([1Cr(thf)] and [1Fe]). Calculations show [1Cr(thf)] (and [1Cr]) to have a d⁴ Cr configuration antiferromagnetically coupled to (dadi)^2-, and [1Fe] to be S=2. Treatment with RN₃ provides products where RN is formally inserted into the C-C bond of the diimine or into a C-H bond of the diimine. Calculations on the process support a mechanism in which a transient imide (imidyl) aziridinates the diimine, which subsequently ring opens.

Full text of DOI:10.1002/anie.201507463

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201507463
German Edition: DOI: 10.1002/ange.201507463
Redox Chemistry
À
À
Nitrene Insertion into C C and C H Bonds of Diamide Diimine
Ligands Ligated to Chromium and Iron
Spencer P. Heins, Wesley D. Morris, Peter T. Wolczanski,* Emil B. Lobkovsky, and
Thomas R. Cundari*
In memory of Gregory L. Hillhouse
Abstract: The impact of redox non-innocence (RNI) on
chemical reactivity is a forefront theme in coordination
capability can enable reactivity by modulating electron
density at the metal.
=
chemistry.
A
diamide diimine ligand, [{-CH N(1,2-
RNI ligands for first-row transition metals have focused
on the diimine[50,51] or imine functionality, mostly in con-
junction with pyridine,[6,12,52–54] while amides have been mostly
featured in second- and third-row applications.[55,56] In a new
thrust targeting the first row,[57] imine and amide function-
C6H4)NH(2,6-iPr2C6H3)}2]n (n = 0 to À4), (dadi)n, chelates Cr
and Fe to give [(dadi)M] ([1Cr(thf)] and [1Fe]). Calculations
show [1Cr(thf)] (and [1Cr]) to have a d4 Cr configuration
antiferromagnetically coupled to (dadi)2À*, and [1Fe] to be S =
2. Treatment with RN3 provides products where RN is formally
alities have been combined within a tetradentate framework
n
À
À
=
inserted into the C C bond of the diimine or into a C H bond
of the diimine. Calculations on the process support a mecha-
nism in which a transient imide (imidyl) aziridinates the
diimine, which subsequently ring opens.
to afford the [{-CH N(1,2-C6H4)NH(2,6-iPr2C6H3)}2] ligand,
(dadi)n. Due to extensive delocalization, (dadi)n has several
potential redox states, five of which are illustrated in
Scheme 1. Condensation of glyoxal with two equivalents 1-
NH2,2-N(2,6-iPr2C6H3)C6H4 afforded (dadi)H2 (1H2) in 24%
yield, and this can be deprotonated to produce (dadi)n.
R
eactions of organoazides[1,2] with transition-metal centers
provide a historically important[3,4] and useful means to
prepare first-row transition-metal imido complexes.[5–19] For
certain metals, nitrene-like activity is inferred by the products
À
derived from inter- and intramolecular insertions into C H
bonds,[20–27] aziridinations,[28–31] and related reactions.[32–40] In
many instances, imide radical character is inferred from the
reactivity and supported by calculations,[41,42] and the chemis-
try can be related to biological transformations such as the
oxygenations by cytochrome P450.[43–49]
During the course of examining chromium and iron
complexes chelated by a diamide diimine tetradentate ligand,
À
À
unusual azide-dependent reactivity featuring C C and C H
bond activations was discovered in conjunction with redox
non-innocence (RNI). RNI is found when ligand and metal
d orbitals are close in energy, and electron density can be
shuttled back and forth. In principle, ligands with RNI
Scheme 1. Five plausible redox states of the (dadi)n (n=0 to À4)
ligand, with the total number of p electrons given for each.
Treatment of (1H2) with [M{N(SiMe3)2}2(thf)n] (M = Cr,
n = 2;[58,59] Fe, n = 1)[60] in benzene produced 2 equivalents of
HN(SiMe3)2 and [(dadi)M(thf)y] ([1M]; for M = {Cr(thf)},
[1Cr(thf)]: 76%; for M = Fe, [1Fe]: 75%) as maroon and dark
green crystals, respectively (Scheme 2). Measurements using
Evansꢀ method[61] were consistent with an intermediate spin
system (S = 1) for [1Cr(thf)], while [1Fe] is a high-spin (S = 2)
complex.
[*] S. P. Heins, W. D. Morris, Prof. P. T. Wolczanski, E. B. Lobkovsky
Department of Chemistry & Chemical Biology, Baker Laboratory,
Cornell University
Ithaca, NY 14850 (USA)
E-mail: ptw2@cornell.edu
Prof. T. R. Cundari
Department of Chemistry, Center for Advanced Scientific Computing
and Modeling (CASCaM), University of North Texas
Box 305070, Denton, TX 76203-5070 (USA)
E-mail: Thomas.Cundari@unt.edu
Figure 1 provides a molecular view of [1Fe], thus reveal-
ing its pseudo-square-planar conformation, with a dihedral
twist between the diamide-iron and diimine-iron planes of
13.27(10)8. Metric parameters indicate a conventional di-
Supporting information for this article (including experimental
procedures, NMR spectra, UV/Vis spectra, crystallographic infor-
mation, computational methods, and computational geometric
coordinates) is available on the WWW under http://dx.doi.org/10.
1002/anie.201507463.
amide (dadi)2À ligand with an intact diimine [d(C N) =
=
À
1.294(3) Š (ave), d(CC) = 1.437(3) Š], and Fe Nimine bond
lengths of 2.131(4) (ave) Š, which are longer than the
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Precedent suggested that the imido derivatives of [1Cr-
(thf)][12] and [1Fe][5–9] could be prepared from organoazides,
but surprising results ensued from such attempts. Treatment
of [1Cr(thf)] with RN3 [R = adamantyl (Ad), or 2,6-iPr2C6H3
(Ar)] afforded products derived from formal insertion of
À
a nitrene into the C C bond of the diimine unit. Purple and
=
blue [{RN(-CH N(1,2-C6H4)NH(2,6-iPr2C6H3))}2Cr] (R =
Ad, [2CrAd]; R = Ar, [2CrAr]) are synthesized in good
yields as illustrated in Scheme 3. Related maroon iron species,
=
[{RN(-CH N(1,2-C6H4)NH(2,6-iPr2C6H3))}2Fe]
(R = Ad,
[2FeAd]; R = Ar, [2FeAr]) were prepared from [1Fe] in
slightly lower yields.
Scheme 2. Syntheses of [(dadi)Fe] ([1Fe]) and [(dadi)Cr(thf)]
([1Cr(thf)]). thf=tetrahydrofuran.
Figure 1. Molecular views obtained from single-crystal X-ray crystallog-
raphy, with pertinent distances (Š) and angles. Thermal ellipsoids
shown at 50% probability. a) [1Fe]: ]N1-Fe-N3, 147.53(7)8; ]N2-Fe-
N4, 151.53(7)8. b) One of two independent [1Cr(thf)] molecules (thf
methylenes removed for clarity): ]N1-Cr-N2, 79.55(16)8; ]N1-Cr-N3,
147.4(18)8; ]N1-Cr-N4, 119.1(2); ]N2-Cr-N3, 77.80(7)8; ]N2-Cr-N4,
157.35(12)8; ]N3-Cr-N4, 79.83(7)8; ]N-Cr-O, 98.17(6)8 (N1),
90.80(3)8 (N2), 105.2(22)8 (N3), 98.44(6)8 (N4).
À
À
Scheme 3. Azide-derived nitrene insertions into C C and C H bonds.
Use of an electron-withdrawing azide, tosyl azide, gen-
erated a product consistent with initial nitrene insertion into
À
À
Fe Namide [1.996(4) (ave) Š] distances. In contrast, the
a C H bond of the diamine (Scheme 3). A dimer, [3Cr] (ca.
À
pseudo-square-pyramidal [1Cr(thf)] displays an apparently
reduced diimine fragment with d(CC)ave = 1.361(3) Š, and
d(CN)ave of 1.376(3) and 1.378(5) Š, which are about 0.08 Š
longer and shorter, respectively, than the corresponding
distances in [1Fe]. The diamide portion of the ligand is
60% crude; 25% isolated), results from C C bond formation
at one side of the original diimide unit, as the adjacent carbon
=
atom has lost its hydrogen to the formation of a C N bond
within an h1-amidinate. Dinitrogen (1.0 equiv) was detected
by Toepler pump, but no evidence of dihydrogen was noted.
À
À
normal, but now the Cr Namide bond lengths of 1.998(16)
No NH absorption is observed in its IR spectrum and a C
À
À
(ave) Š are longer than those of the Cr Nimine bonds [1.960-
NH S unit is ruled out because of spatial H overlap with the
À
(17) (ave) Š]. The dihedral twist angle of 13.2(4)8 is similar,
despite the Cr residing slightly above the ligand plane, as ]N-
Cr-O = 98(6)8(ave).
coplanar arene C H, hence the hydrogen is likely to be found
in byproducts of the reaction. A meff of 3.7 mB for the dimer is
consistent with two non-interacting S = 1 centers.
=
Calculations on [1Fe] (DFT and MCSCF) support the
(dadi)2À quintet d6 electronic structure implicated by its
crystal structure and magnetism, but two plausible descrip-
tions exist for [1Cr(thf)]: a) antiferromagnetic (AF) coupling
between an S = 1 excited state of [(dadi)2À]*, containing
electrons in p and p* orbitals, and a high-spin chromous
center, and b) AF coupling of CrIII and (dadi)3À. Since the
Figure 2 illustrates [{2,6-iPr2C6H3-N(-CH N(1,2-C6H4)-
=
NH(2,6-iPr2C6H3))}2Cr] [2CrAd] and [{AdN(-CH N(1,2-
C6H4)NH(2,6-iPr2C6H3))}2Fe] [2FeAd], thus showing their
pseudo-square-planar geometries.[62] The 26.58 twist in the
latter is substantially greater than the 10.38 distortion in the
{CrN4} frame, a distortion that is also evident in the core
angles listed. Elongated Fe Nimine distances [2.129(4) Š (ave)]
relative to 1.993(6) Š (ave) Fe Namide bond lengths are in
À
computed spin density on the Cr center in [1Cr(thf)] is 3 3= eÀ,
À
4
(a) is the dominant description of the complex, albeit with
a significant admixture of (b).
contrast to the 2.030(14) Š (ave) Cr–N distances. Occupation
of the M N s* orbital (dxy, x-axis defined by Fe1–N5) in the
À
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Figure 4. A core view of [3Cr] and selected metric parameters [8]: ]N6-
Cr2-N7, 80.60(12); ]N6-Cr2-N8, 155.92(13); ]N6-Cr2-N9, 78.40(12);
]N7-Cr2-N8, 111.99(12); ]N7-Cr2-N9, 146.24(13); ]N8-Cr-N9,
80.52(12); ]O5-Cr2-N6, 93.32(11); ]O5-Cr2-N7, 106.70(12); ]O5-
Cr2-N8, 101.92(12); ]O5-Cr2-N9, 100.65(12); similar angles describe
Cr3. Thermal ellipsoids shown at 50% probability.
Figure 2. Molecular views obtained from single-crystal X-ray crystallog-
raphy (iPr groups have been removed for clarity), with pertinent
distances [Š] and core angles [8] listed. Thermal ellipsoids shown at
=
50% probability. a) One of two independent [{2,6-iPr2C6H3-N(-CH
N(1,2-C6H4)NH(2,6-iPr2C6H3)}2Cr] [2CrAr] molecules: ]N1-Cr-N2,
80.22(13); ]N1-Cr-N3, 163.82(13); ]N1-Cr-N4, 113.28(13); ]N2-Cr-
N3, 86.75(13); ]N2-Cr-N4, 164.64(12); ]N3-Cr-N4, 81.02(13).
=
b) {AdN(-CH N(1,2-C6H4)NH(2,6-iPr2C6H3)}2Fe (2-FeAd): ]N1-Fe-N2,
79.94(9)8; ]N1-Fe-N3, 149.39(9)8; ]N1-Fe-N4, 127.03(9); ]N2-Fe-
N3, 80.77(8)8; ]N2-Fe-N4, 148.03(9)8; ]N3-Fe-N4, 79.85(8)8. [2CrAd]
is not illustrated (see the Supporting Information).
Figure 3. A molecular view of the dimer [3Cr] (an additional 1/2 dimer
in the asymmetric unit is not shown) obtained from a single crystal
X-ray study, with iPr groups, tolyl, and sulfoxide oxygen removed for
clarity. Thermal ellipsoids shown at 50% probability.
case of [2FeAd], but not in [2CrAr], undoubtedly contributes
to the angular and bond distortions in the former.
Metric parameters affiliated with [3Cr] reflect a subtle
difference between the amide amidinate arene and the
Scheme 4. Proposed imidyl (aziridination ring opening to [2Cr], black;
calculated free energies shown in red) and nitrene reactivity (to give
[3Cr]; blue) from [1Cr].
À
À
diamide arene, as Figure 3 reveals. The average C N, C(N)
À
C(N), and C C distances affiliated with the latter are
formulation. The sulfur bond distances are consistent with
[63]
À
À
=
1.364(9), 1.428(5), and 1.411(10) Š, respectively, while the
corresponding distances of the former average 1.408(12),
1.410(11), and 1.397(5) Š, thus suggesting 1eÀ reduction of
the diamide arene.[63] Calculations suggest that each chro-
mium is best considered to be CrIII ligated by amide,
amidinate, and an AF coupled arene diimine radical anion,
S N, S C, and S O bonds.
Scheme 4 illustrates a plausible sequence of reactions
rationalizing the azide chemistry of [1Cr] or [1Cr(thf)], and by
inference, [1Fe]. Azide treatment of [1Cr] affords a CrIV
center ligated by a dadi3À ligand containing an azaallyl radical
AF coupled to an imidyl radical,[41] that is, [(CrIV)››(dadiC3À)›-
(NRCÀ)fl] (DG8calcd = À32.5 kcalmolÀ1). This assignment is
based on calculations of the truncated model {(dadi’)CrNMe}
(H in place of 2,6-iPr2C6H3) illustrated in Figure 5. Lying
3 kcalmolÀ1 above the triplet is an S = 0 broken symmetry
solution with like-spin imidyl and dadi3À ligands, AF coupled
to d2 CrIV. Both solutions stand in contrast to Wieghardtꢀs
À
thus resulting in two S = 1 centers. The new C46 C92 bond is
a lengthy 1.60(2) Š (ave), thus hinting at possible reversible
formation such as those seen in reversible 2-azaallyl cou-
plings.[62] C N distances for the h -amidinate average 1.323-
1
À
(2) Š, and the square-pyramidal cores, shown in Figure 4,
have varied d(CrN) bond lengths which support a CrIII
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Acknowledgements
Support from the National Science Foundation (PTW, CHE-
1402149; TRC, CHE-1057785) is gratefully acknowledged, as
is Cornell University.
Keywords: chelates · chromium · iron · nitrene · redox chemistry
Figure 5. Spin density (IsoValue=0.01) of triplet (left) and singlet
=
(right) [(dadi’)Cr NMe]. Note for the triplet the positive spin density
on the metal (1spin ꢀ +2eÀ) and dadi ligand (1spin ꢀ +1eÀ), and the
negative spin density on the imidyl nitrogen (1spin ꢀÀ1eÀ). For the
broken-symmetry singlet, the spin density on the imidyl (1spin ꢀ +1eÀ)
and dadi (1spin ꢀ +1eÀ) ligand are positive, while the Cr has negative
spin density (1spin ꢀÀ2eÀ).
How to cite: Angew. Chem. Int. Ed. 2015, 54, 14407–14411
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À
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Angew. Chem. Int. Ed. 2015, 54, 14407 –14411
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