Co(III) imidos exhibiting spin crossover and C-H bond activation

Article abstract of DOI:10.1021/ja307699u

The reaction of (^ArL)Co(py) with ^tBuN₃ afforded the isolable three-coordinate Co-imido complex (^ArL) Co(N^tBu), which is paramagnetic at room temperature. Variable-temperature (VT) ¹H NMR spectroscopy, VT crystallography, and magnetic susceptibility measurements revealed that (^ArL)Co(N ^tBu) undergoes a thermally induced spin crossover from an S = 0 ground state to a quintet (S = 2) state. The reaction of (^ArL)Co(py) with mesityl azide yielded an isolable S = 1 terminal imido complex that was converted into the metallacycloindoline (^ArL)Co(κ²- NHC₆H₂-2,4-Me₂-6-CH₂) via benzylic C-H activation.

Full text of DOI:10.1021/ja307699u

Communication
pubs.acs.org/JACS
Co(III) Imidos Exhibiting Spin Crossover and C−H Bond Activation
Evan R. King, Graham T. Sazama, and Theodore A. Betley*
Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138, United
States
S
* Supporting Information
the isolation of a reactive FeIII imido radical species following
t
ABSTRACT: The reaction of (ArL)Co(py) with BuN3
afforded the isolable three-coordinate Co−imido complex
(ArL)Co(NtBu), which is paramagnetic at room temper-
treatment of the FeII chloride with an aryl azide.6f The imido
radical is capable of stoichiometric C−H bond amination and
styrene aziridination, while less bulky versions of the dipyrrin
ligand render the nitrene-transfer reaction catalytic.6f We
hypothesized that use of the sterically encumbered dipyrrin
ligands with Co might similarly lead to open-shell configurations
with metal−ligand multiple-bonded functionalities, rendering
these typically inert complexes reactive for imido-group transfer
(Scheme 1).
1
ature. Variable-temperature (VT) H NMR spectroscopy,
VT crystallography, and magnetic susceptibility measure-
ments revealed that (ArL)Co(NtBu) undergoes a thermally
induced spin crossover from an S = 0 ground state to a
quintet (S = 2) state. The reaction of (ArL)Co(py) with
mesityl azide yielded an isolable S = 1 terminal imido
complex that was converted into the metallacycloindoline
(ArL)Co(κ2-NHC6H2-2,4-Me2-6-CH2) via benzylic C−H
activation.
Scheme 1. Reactions of CoI Synthon 2
he principal interest in synthesizing late, first-row transition
T
metal coordination complexes featuring metal−ligand
multiple bonds is that the terminally bound functionality may
be transferred to unreactive substrates (e.g., olefins, C−H
bonds).1 A key determining factor for the reactivity of the
multiply bonded functionality is the electronic structure of the
transition metal ion to which it is bound.2 Low-spin electronic
configurations engender stability,3−5 whereas open-shell config-
urations may populate the antibonding orbitals, thereby
producing a reactive species for atom- or group-transfer
processes.6 While the ligand-field strength of the metal−ligand
multiple bond is inherent, the use of weak-field ancillary ligands
may permit high-spin (HS) configurations to be obtainable. This
electronic structure−reactivity relationship has been most
heavily scrutinized in high-valent Fe−oxo chemistry.7 Unlike
Fe-based metal−ligand multiple bonds, which have been
observed to span a range of electronic configurations,3,6 nearly
all Co coordination compounds containing a metal−ligand
multiple bond feature low-spin ground states,4 producing inert
complexes. The lone exception is Theopold’s complex (Tp*)-
CoIIINAd, which undergoes radical decomposition at higher
temperatures where a thermally accessible open-shell config-
uration may be accessible.4e Transient cobalt imides have been
invoked in H-atom abstraction reactions on the basis of
characterization of the resulting amides, but their spin states
are unknown.8 We report herein the synthesis of CoIII imido
complexes featuring the weak-field dipyrrinato platform that
undergo thermal spin crossover and can effect C−H bond
activation.
Metalation of ArLH with Co followed directly from the
previously reported protocols for preparing the Fe congeners.6f
Reaction of (ArL)Li with CoCl2(py)210 afforded the dark-maroon
pyridine (py) complex (ArL)CoCl(py) (1), which exhibits a
1
paramagnetically shifted H NMR spectrum [μeff = 4.2(1)μB;
C6D6, 295 K] consistent with a quartet ground state. The
presence of pyridine bound to Co was confirmed by structural
elucidation [Figure S.1 in the Supporting Information (SI)].
Chemical reduction of 1 with KC8 in benzene at room
temperature cleanly produced the CoI pyridine adduct (ArL)-
Co(py) (2) as a purple solid. Like 1, 2 is paramagnetic, and it has
a triplet ground state, as corroborated by the solution [2.9(1)μB;
We have demonstrated the ability of the weak-field dipyrrin
platform to stabilize low-coordinate Fe complexes in HS
configurations.6e,f,9 The sterically encumbered derivative 5-
mesityl-1,9-(2,4,6-triphenylphenyl)dipyrromethene (ArLH) sta-
bilized a three-coordinate FeII chloride species and allowed for
Received: August 3, 2012
Published: October 8, 2012
© 2012 American Chemical Society
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Figure 1. Solid-state structures of (a) (ArL)Co(py) (2), (b, c) (ArL)CoNtBu (3) at (b) 150 and (c) 300 K, and (d) (ArL)Co(κ2-NHC6H2-2,4-Me2-6-
CH2) (5) at 100 K, with 40% probability ellipsoids. H atoms, solvent molecules, and the ligand phenyl substituents in 5 have been omitted for clarity.
Colors: Co, green; C, gray; H, white; N, blue. Selected bond lengths (Å) and angles (deg) for 2: Co−N1, 1.961(3); Co−N2, 1.961(3); Co−N3,
1.942(4). For 3 at 150 K: Co−N1, 1.926(3); Co−N2, 1.916(3); Co−N3, 1.609(3); N3−Co−C67, 177.5(3). For 3 at 300 K: Co−N1, 1.930(3); Co−
N2, 1.934(3); Co−N3, 1.632(3); N3−Co−C67, 178.2(3). For 5: Co−N1, 1.927(3); Co−N2, 1.992(3); Co−N3, 1.848(4); Co−C1M, 2.018(5).
C6D6, 295 K] and solid-state (χMT = 1.07 K cm3/mol) magnetic
moments. The molecular structure of 2 as determined by single-
crystal X-ray diffraction (XRD) (Figure 1a) revealed a trigonal-
planar geometry for the three-coordinate Co. With a suitable CoI
synthon in hand, we sought to examine its reactivity with two-
electron group-transfer reagents.
a value of 0.77 cm3 K/mol near 50 K, followed by a subsequent
decrease to 0.47 cm3 K/mol (1.94μB) at 5 K. As the susceptibility
data never achieved a value consistent with a singlet ground state,
we hypothesized that a fraction of the HS state remained at the
low-temperature extreme. VT magnetization data were collected
over the temperature range 1.8−10 K at fields of 1−7 T. The
resulting plot of reduced magnetization (Figure S.12) featured a
series of nonsuperimposable isofield curves, with the 7 T curve
reaching a maximum value of M = 0.62μB at 1.8 K. The χMT
versus T curve for 3 was fit as a spin transition (for xHS, ΔH, and
Tc) between a singlet state and either the triplet or quintet state
using eq 1,11 in which xHS is the fraction of molecules trapped in
the HS state at low T (see the SI for the expression used for
Dropwise addition of a 1% (w/w) solution of tBuN3 in diethyl
ether to a diethyl ether solution of 2 consumed the azide (as
ascertained by the disappearance of νN by IR analysis) and
3
cleanly afforded a new paramagnetic species. Crystals of this
product were obtained from a benzene/hexane solution at −35
°C. The X-ray crystal structure of (ArL)Co(NtBu) (3) revealed a
trigonal-planar three-coordinate cobalt imido (Figure 1b). The
Co−N distance of 1.609(3) Å is shorter than those previously
reported for either three- or four-coordinate cobalt imidos
[1.621(3)−1.675(2) Å].4 The Co−N3−C67 angle is nearly
linear [178.8(1)°], and the imido does not deviate significantly
from the plane formed by the dipyrrin nitrogens (N1 and N2)
and the Co atom. The reaction of 2 or imido 3 with excess tBuN3
led to the formation of the tetrazole complex (ArL)Co-
(κ2-tBuNNNNtBu) (Figure S.4).
x
HS).12 The reduced magnetization data were fit using
ANISOFIT13 and the spin Hamiltonian shown in eq 2.
⎛
⎜
⎞
1
1
c
χMT = xHS(χHST) + (1 − xHS)(χHST)/[1 + eΔRH
]
⎟
⎠
−
T
T
⎝
(1)
2
2
2
̂
̂
̂
̂
̂
H = μBgH·S + DSz + E(Sx − Sy )
(2)
A satisfactory model for the reduced magnetization and
susceptibility data could be obtained only by invoking a spin
transition from a singlet to quintet configuration with a small
fraction of the HS component present at low temperatures.
Setting xHS = 25.6% accounted for the plateau in the
susceptibility below 100 K. Thus, the total fraction of the HS
component is given by this baseline value plus the portion
suggested by the Boltzmann distribution for the remaining
fraction governed by the thermodynamic parameters ΔH = 472
cm−1 and Tc = 1397 K, which were obtained by fitting our data to
eq 1 using the xHS expression for S = 2 with D = −20.2 cm−1 and
gavg = 2.0. The very small change in the fraction of the HS
component between low temperature and room temperature
(∼10%) is manifested in the very small structural perturbations
observed crystallographically at 150 and 300 K. The 300 K
structure reveals a modest elongation of the Co−N bond length
from 1.609(3) to 1.632(3) Å (Figure 1c), consistent with
electronic population of σ* and π* molecular orbitals (MOs)
(see below). Population of Co−N antibonding MOs would
suggest weakening of the Co−imido bond, making it reactive for
H-atom abstraction or viable for imido-group transfer. While
reaction with a two-electron reductant, PMe2Ph, did facilitate
imido-group transfer to give the phosphinimide PhMe2P(NtBu)
Imido 3 displays a paramagnetically shifted 1H NMR spectrum
at room temperature [δ 23.2 to −8.4 ppm; μeff = 2.96(2)μB;
C6D6, 295 K]. However, cooling a sample of 3 in toluene-d8 in
the NMR probe resulted in a dramatic contraction of the
chemical shift range to 8.85−0.29 ppm at −80 °C (Figure S.8).
The 1H resonances for the imido tBu substituent appeared at 8.85
ppm [assigned via preparation of the deuterated analogue
(ArL)Co(NtBu-d9) (3-d9)], still downfield from those of a
t
diamagnetic Bu group, indicating that the spin transition is
incomplete at −80 °C. The recorded solution magnetic moment
decreased accordingly (Figure S.13). The observed spectral
features suggest a spin-crossover transition from an open-shell
configuration (i.e., S = 1 or 2) to a singlet state, indicating that the
ground state of 3 is diamagnetic.
To probe the magnetic behavior of 3 further, variable-
temperature (VT) direct-current (dc) susceptibility data were
collected over the temperature range 5−300 K. For 3, χMT at 300
K was found to be 1.12 cm3 K/mol (2.99μB), consistent with the
solution magnetic moment obtained at room temperature
[2.96(2)μB], but the plot of χMT was clearly rising at the
temperature limit of the experiment (Figure 2a). As the
temperature was lowered, χMT underwent a gradual decline to
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Communication
○
Figure 2. (a) VT magnetic susceptibility data for 3 in an applied dc field of 1 T ( ) and the fit described in the text (red solid line). (b−d) Frontier MO
description of CoIII(NR) featuring an ancillary ligand with (b) no ancillary π donation, (c) conjugation to the imido N-aryl π system, and (d) ancillary
ligand π-donating substituents.
quantitatively at 80 °C, we observed no evidence for C−H bond
activation using 3 with either 1,4-cyclohexadiene or 9,10-
dihydroanthracene.
The reaction of 2 with 1 equiv of mesityl azide (N3Mes) in
benzene resulted in consumption of the azide (as ascertained by
ground state with no apparent spin-crossover behavior yet
undergoes H-atom abstraction along the Co−NAr bond; and
finally, (3) all previously reported CoIII−imido complexes have
singlet ground states. These observations can be rationalized by
considering the frontier MO perturbations caused by the various
ligand substitutions. The Co−imido MOs arise from the imido
the disappearance of νN by IR analysis) and cleanly afforded a
3
2
N→Co σ interaction (N 2spz + Co 3dz ) and two Co−N π
new paramagnetic species upon sublimation of the benzene.
XRD analysis of crystals obtained from this reaction revealed not
an imido product but instead a metallacycloindoline product,
(ArL)Co(κ2-NHC6H2-2,4-Me2-6-CH2) (5) (Figure 1d). The
mesitylnitrene unit was modified via net H-atom transfer from a
mesityl benzylic group to the imido N (νN−H = 3357 cm−1),
enabling it to bind to Co as a dianionic chelate via a Co−anilido
bond and a Co−alkyl bond to a mesityl benzylic carbon. Within
the metallacycloindoline, the Co−N3 bond distance of 1.848(4)
Å and the Co−C1M distance of 2.018(5) Å are consistent with
single bonds to Co.
interactions, one (labeled π*x ) arising from N 2px−Co 3dxz
overlap and the other (labeled π*y ) from N 2py−Co 3dyz overlap
2
(Figure 2b). Symmetry-allowed mixing of the Co 4s and 3dz
2
orbitals stabilizes the 3dz orbital, mitigating the destabilizing
effect of populating the Co−N σz* interaction. For four-
coordinate imido complexes, the Co 3dxz and 3dyz orbitals are
degenerate and contribute to bonding of the three ancillary
ligand σ donors and the Co−imido π manifold, leading to a large
2
energy gap between the occupied 3dz orbital and the empty
3dxz/3dyz orbital set, favoring low-spin configurations.2c,4a−c,e−g
For three-coordinate species, the Co−imido π manifold is
composed of two nondegenerate bonding interactions, as the Co
3dxz orbital also bears σ* character with respect to the ancillary
bidentate ligand whereas the 3dyz orbital does not.
Given the stability of imido 3, we propose that 5 arises via
formation of the terminal imido product (ArL)Co(NMes)
followed by H-atom abstraction from one of the proximal o-
methylene groups by the imido with subsequent radical
recombination between Co and the pendant benzylic radical.
The intermediacy of the mesityl imido was supported by the
observation of a new species by 1H NMR spectroscopy
immediately following the addition of N3Mes to 2. Reaction of
an n-hexane slurry of 2 with N3Mes-d11 (using the deuterated
azide to suppress H-atom abstraction) over 48 h at room
temperature permitted the isolation of a paramagnetic product
that precedes the formation of metallacycloindoline 5-d11. As
isolated, the new purple species was stable in the solid state for
days but converted to 5-d11 slowly in solution and upon attempts
at crystallization. We propose the kinetic product to be
(ArL)Co(NMes-d11) (4-d11), which exhibits a paramagnetically
The dipyrrin σ-donor strength is attenuated relative to that of
non-N-heterocyclic donors,14 reducing the 3dxz destabilization
[i.e., Δ(3dxz − 3dxy) is diminished; see Figure 2b]. The N-based π
electrons in the dipyrrin ligand are incorporated into the ligand
conjugated π framework and therefore do not destabilize the 3dyz
2
orbital. As the 3dyz energy decreases, the 3dyz−3dz energy gap is
reduced to less than the mean spin-pairing energy, permitting an
open-shell configuration to be obtained. For complex 3 (Figure
2b), the combination of these two effects allows for population of
3dyz and the thermally induced spin transition between the
singlet and quintet states. Singlet−quintet spin crossover is most
commonly observed for six-coordinate FeII complexes,15 but
CoIII spin-crossover complexes are not without precedent.16
Complex 3, much like recently reported Ni17 and Fe6g imido
complexes, showcases spin-crossover behavior in a complex
bearing a metal−ligand multiple bond.
1
shifted H NMR spectrum [μeff = 3.6(2)μB] at room temper-
ature. Unlike 3, however, imido 4 does not undergo a spin-state
transition, as shown by VT 1H NMR (Figure S.9) and magnetic
moment susceptibility analysis (3.65μB at 300 K; Figure S.11d),
suggesting that the triplet configuration of aryl imido complex 4
is the likely electronic ground state.
Several unusual features of the Co−imido complexes
presented herein are unique to the (dipyrrin)Co platform: (1)
alkyl imido 3 undergoes spin crossover from a singlet to quintet
configuration; (2) aryl imido 4 features a well-isolated triplet
Substitution of NtBu by NMes decreases the Nimido 2py energy
relative to the alkyl imido via orbital conjugation with the
coplanar mesityl aryl ring (Figure 2c). Furthermore, orienting
the mesityl unit perpendicular to the dipyrrin plane to minimize
steric interactions raises the imido N 2px energy by introducing a
π* interaction with the mesityl aromatic π electrons (labeled
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Journal of the American Chemical Society
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2
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ASSOCIATED CONTENT
* Supporting Information
Experimental procedures; spectral, crystallographic, VT NMR,
and magnetic data; and a CIF. This material is available free of
charge via the Internet at http://pubs.acs.org.
■
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AUTHOR INFORMATION
Corresponding Author
betley@chemistry.harvard.edu
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank Harvard University and the NSF (CHE-
0955885) for financial support. E.R.K thanks the William
Lipscomb Memorial Fund and G.T.S. the NSF for pre-doctoral
fellowships, and T.A.B. is grateful for a George W. Merck
Fellowship. We thank Dr. Yu-Sheng Chen at ChemMatCARS,
APS, for assistance with data acquisition. ChemMatCARS Sector
15 is supported by the NSF (NSF/CHE-0822838) and the APS
by DOE (DE-AC02-06CH11357).
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