Open-shell organometallic [MII(dbcot(bislutidylamine)] 2+ complexes (M = Rh, Ir): Unexpected base-assisted reduction of the metal instead of amine ligand deprotonation

Article abstract of DOI:10.1021/om101157r

Cationic rhodium and iridium complexes with dibenzo[a,e]cyclooctatetraene (dbcot) and bislutidylamine (bla) were synthesized from the respective [M(μ-Cl)(dbcot)]₂ dimers and the bla ligand and were characterized by NMR, X-ray diffraction, and cyclic voltammetry. Both [M^I(dbcot) (bla)]⁺ complexes are easily oxidized by one electron to form paramagnetic dicationic analogues [M^II(dbcot)(bla)]²⁺ (M = Rh, Ir). In the solid state, the geometry changes from a distorted trigonal bipyramid to square pyramidal upon oxidation of the metal from M^I to M^II, as evidenced by X-ray diffraction studies of the iridium complexes. The paramagnetic complexes undergo facile one-electron reduction in the presence of base, most likely involving oxidation of the solvent or hydroxide anions. A control experiment using the corresponding [Rh ^II(dbcot)(Bn-bla)]²⁺ complex, wherein Bn-bla is the N-benzyl-protected bla ligand, gives similar results, which strongly suggests that deprotonation of the N-H moiety of the [M^II(dbcot)(bla)] ²⁺ plays a minor role (if any at all) in the observed base-mediated reduction processes. Reaction of bla with [Ir(μ-Cl)(coe)]₂ (coe = cis-cyclooctene) results in oxidative addition of the vinylic C-H bond of coe to the metal center, as evidenced by X-ray diffraction.

Full text of DOI:10.1021/om101157r

ARTICLE
pubs.acs.org/Organometallics
Open-Shell Organometallic [MII(dbcot(bislutidylamine)]2þ Complexes
(M = Rh, Ir): Unexpected Base-Assisted Reduction of the Metal Instead
of Amine Ligand Deprotonation
Wojciech I. Dzik,† Luis Fuente Arruga,† Maxime A. Siegler,‡ Anthony L. Spek,‡ Joost N. H. Reek,† and
Bas de Bruin*,†
†Van ‘t Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
‡Bijvoet Center for Biomolecular Research, Utrecht University, Utrecht, The Netherlands
S Supporting Information
b
ABSTRACT:
Cationic rhodium and iridium complexes with dibenzo[a,e]cyclooctatetraene (dbcot) and bislutidylamine (bla) were synthesized
from the respective [M(μ-Cl)(dbcot)]2 dimers and the bla ligand and were characterized by NMR, X-ray diffraction, and cyclic
voltammetry. Both [MI(dbcot)(bla)]þ complexes are easily oxidized by one electron to form paramagnetic dicationic analogues
[MII(dbcot)(bla)]2þ (M = Rh, Ir). In the solid state, the geometry changes from a distorted trigonal bipyramid to square pyramidal
upon oxidation of the metal from MI to MII, as evidenced by X-ray diffraction studies of the iridium complexes. The paramagnetic
complexes undergo facile one-electron reduction in the presence of base, most likely involving oxidation of the solvent or hydroxide
anions. A control experiment using the corresponding [RhII(dbcot)(Bn-bla)]2þ complex, wherein Bn-bla is the N-benzyl-protected
bla ligand, gives similar results, which strongly suggests that deprotonation of the N-H moiety of the [MII(dbcot)(bla)]2þ plays a
minor role (if any at all) in the observed base-mediated reduction processes. Reaction of bla with [Ir(μ-Cl)(coe)]2 (coe = cis-
cyclooctene) results in oxidative addition of the vinylic C-H bond of coe to the metal center, as evidenced by X-ray diffraction.
’ INTRODUCTION
demonstrated that C-H8 and H-H8c bonds can be broken
homolytically between two metalloradicals. Ligand radicals gen-
erated on these metals are also capable of abstracting hydrogen
atoms from the reaction medium.7,9 Recently, the group of
Gr€utzmacher elegantly presented a galactose oxidase mimic
based on an iridium aminyl radical complex.10 The study of
hydrogen atom transfer at open-shell second- and third-row
transition metal complexes clearly deserves much more atten-
tion, as we are only beginning to understand such reactivity.
Recently, our group investigated the reactivity of paramagnetic
rhodium and iridium compounds with the general formula
[MII(N3-ligand)(cod)]2þ (cod = Z,Z-1,5-cyclooctadiene).
These complexes undergo interesting bimolecular HAT reac-
tions, and the rate of the HAT pathway proved to be markedly
dependent on the nature of the used bispicolylamine (bpa)-type
N3 ligand. Irrespective of the R groups introduced in the bpa
moiety (R1, R2 = H or Me, R3 = H or Bn) the paramagnetic
Hydrogen atom transfer (HAT) plays an important role in
inorganic chemistry and is of fundamental importance in many
biochemical transformations and some industrially relevant
synthetic reactions. Transfer of a hydrogen atom from the
substrate to a metal-coordinated oxygen atom is one of the key
steps in biotransformations catalyzed by galactose oxidase,1
lipoxygenase,2 cytochrome P450,3 methane monooxygenases,4
and class I ribonucleotide reductase.5 HAT reactions are essential
for chain transfer during metal-mediated radical polymerization
of olefins,6 but can also lead to catalyst deactivation in cyclopro-
panation reactions.7
Open-shell second- and third-row transition metal complexes
are excellent models to study HAT reactions, because the
products are generally well-defined NMR-characterizable dia-
magnetic compounds. This has been exploited by a number of
groups over the past years, providing information on the funda-
mental reactivity of transition metal radical complexes toward
Y-H bonds (Y = C, H, Si, S). The pioneering work of the
group of Wayland on rhodium and iridium complexes has
Received: December 8, 2010
Published: March 10, 2011
r
2011 American Chemical Society
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Scheme 1. Hydrogen Atom Transfer of Allylic Hydrogen in [MII(N3-ligand)(cod)]2þ Complexesa
a R1, R2 = H or Me, R3 = H or Bn.
Scheme 2. Possible Hydrogen Atom Transfer of the Allylic Hydrogen of the Cod Ligand via a Putative Aminyl Radical Species
complexes spontaneously transform into a 1:1 mixture of
[MIII(allyl)(N3-ligand)]2þ and protonated M(cod) complexes
(in the form of [MI(olefin)(N3-ligandþH)]2þ for M = Rh or
[MIII(H)(olefin)(N3-ligand)]2þ for M = Ir, Scheme 1).11 The rate
of decomposition depends strongly on the nature of substituents,
and only the complexes with R1, R2 = Me, R3 = Bn were stable
enough to be isolated and fully characterized. The low stability of
complexes with secondary amine bpa-type ligands (R3 = H) raises
the question if the decomposition to the allyl and hydride complexes
could proceed via the involvement of aminyl radicals (Scheme 2).
The possible involvement of aminyl radicals in the
[MII(cod)(N3-ligand)]2þ system seems plausible. Aminyl radi-
cals can be stabilized by coordination to a metal center, and
several examples of such stable species have been reported.12,13
Reversible H atom transfer (HAT) from a nitrogen atom of a
transition metal complex with a ligand possessing an amine
functionality can occur and has been extensively studied in the
past decade by the group of Mayer.14 Recently, Gr€utzmacher
et al. showed that amine-diolefin Rh(I) and Ir(I) complexes can
form stable, d8 transition metal aminyl radical complexes upon
deprotonation of the amine and subsequent one-electron
oxidation.10,15 These aminyl radical complexes abstract hydrogen
atoms from activated Y-H bonds. Moreover, bispicolylamine
(bpa) is also prone to mono- and double-deprotonation when
coordinated to rhodium or iridium diolefin species16 and can
subsequently form transient ligand16a,c,d-centered radical species
upon one-electron oxidation.
To study the possible aminyl radical reactivity of the bpa-type
ligand, we need a diene ligand that is more robust than
cyclooctadiene (innocent toward allylic C-H bond activation
or C-C coupling). Therefore we decided to synthesize rhodium
and iridium complexes with dibenzo[a,e]cyclooctatetraene
(dbcot) as the supporting diolefin.17 Since the steric shielding
of the metal center proved to have a positive effect on the stability
of [MII(cod)(N3-ligand)]2þ complexes, we chose bislutidyla-
mine (bla) as the N3-donor ligand.
’ RESULTS AND DISCUSSION
Synthesis and Characterization of [MI(bla)(dbcot)]þ Com-
plexes. Complexes [Rh(η4-dbcot)(κ3-bla)]PF6 ([1]PF6) and
[Ir(η4-dbcot)(κ3-bla)]PF6 ([2]PF6) (dbcot = dibenzo[a,
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Scheme 3. Synthesis of [Rh(η4-dbcot)(κ3-bla)]þ (1þ) and [Ir(η4-dbcot)(κ3-bla)]þ (2þ)
Scheme 4. Synthesis of [Ir(κ3-bla)(H)(σ-C8H13)(η2-C8H14)]þ (3þ)
[Rh(μ-Cl)(η4-dbcot)]2 used for the synthesis of 1þ could be
generated in situ from [Rh(μ-Cl)(η2-coe)2]2 (coe = cis-cy-
clooctene) and dbcot by exchange of the coe ligands bound to
rhodium for dbcot. A similar approach to synthesize the iridium
complex 2þ failed and resulted in unexpected formation of a new
complex, [Ir(κ3-bla)(H)(σ-C8H13)(η2-C8H14)]PF6 ([3]PF6),
by oxidative addition of a vinylic C-H bond of coe to the
iridium metal center (Scheme 4). Therefore the successful
synthesis of [2]PF6 required the use of the isolated [M(μ-
Cl)(η4-dbcot)]2.
Oxidative addition of the vinylic C-H bond of cyclooctene
18
was previously reported for iridium complexes with HB(Pz)3
and Cn19 N3-supporting ligands (HB(Pz)3 = tris(pyrazolyl)-
borate, Cn = 1,4,7-triazacyclononane). Such reactivity is in line
with comparable electronic properties of tris(pyrazolyl)borate-
and bislutidyl-amine-type ligands, which resulted in similar
reactivity of the iridium biscarbonyl complexes toward
nucleophiles.20 Other electron-rich (PNP) tridentate ligands
also allow for such reactivity on the iridium center.21 X-ray
quality crystals of [3]PF6 were grown by layering an acetone
solution of [3]PF6 with hexanes (Figure 1). The structure of 3þ
is noteworthy and is an interesting example of a metal-olefin-
hydride complex that does not undergo spontaneous olefin
insertion into the M-H bond in coordinating solvents (e.g.,
acetone). [Ir(κ3-bla)(H)(σ-C8H13)(η2-C8H14)]PF6 has a dis-
torted octahedral geometry with the bla ligand being coordinated
in a fac-mode. The most sterically hindered position trans to the
N3 amine nitrogen is occupied by the least sterically demanding
hydride anion. The cyclooctenyl group coordinates trans to the
N1 lutidyl nitrogen, and its C9-C10 bond has a clear double-
bond character (1.326(4) Å). This vinylic double bond has no
significant interaction with the metal. The cyclooctenyl group has
a large trans influence on the lutidyl N1 atom, which results in a
rather large (>2.23 Å) Ir-N1 distance. The cyclooctene ligand
coordinates trans to the N2 lutidyl nitrogen. As expected for an
olefin coordinated to an electron-deficient metal, the double
Figure 1. X-ray structure of [Ir(H)(σ-C8H13)(η2-C8H14)(κ3-bla)]þ
(3þ). Displacement ellipsoids are drawn at the 50% probability level
(hydrogen atoms and the PF6 counterion were omitted for clarity).
Selected bond distances (Å) and angles (deg): Ir-N1 2.239(3); Ir-N2
2.136(2); Ir-N3 2.174(3); Ir-C1 2.200(3); Ir-C2 2.168(3); Ir-C9
2.041(3); Ir- H1 1.490(3); C1-C2 1.387(5); C9-C10 1.326(4);
N1-Ir-N3 79.31(10); N1-Ir-N2 82.12(10); N2-Ir-N3 78.12(9);
C1-Ir-C2 37.02(12); Ir-C9-C10 123.04(2); N1-Ir1-C9
167.63(11); N2-Ir1-C9 88.13(11); N3-Ir1-C9 91.29(11); N3-
Ir1-H1 170.0(11); N2-Ir1-H1 92.2(11); N1-Ir1-H1 97.0(11);
H1-Ir1-C9 91.0(11).
-
e]cyclooctatetraene, bla = bis-N,N-(6-methyl-2-pyridylmethyl)-
amine) were synthesized from the bla ligand and the correspond-
ing dinuclear [M(μ-Cl)(η4-dbcot)]2 compounds in methanol
-
followed by precipitation as a PF6 salt by adding KPF6
(Scheme 3).
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The redox properties of 1þ and 2þ were investigated using
cyclic voltammetry. All complexes reveal reversible one-electron
oxidation waves in the scan range between 10 and 200 mV/s. The
redox potential of the bla rhodium complex is higher by 47 mV
compared to the iridium analogue, which is in line with previous
results showing that a variety of [Ir(cod)(N3-ligand)]þ (cod =
1,5-cyclooctadiene) complexes are oxidized at lower potentials
than their rhodium analogues (E1/2 lowered by 70-100 mV in
those cases).11
Synthesis and Characterization of Paramagnetic [MII(dbcot)-
(bla)]2þ Complexes. In contrast to the abundant chemistry of
d8-metal olefin complexes, well-characterized, stable d7 rhodium-
(II) and iridium(II) olefin compounds are rather scarce.25 This is
caused by the general low stability of organometallic radicals,
coupled with relatively easy allylic C-H bond activation11,26 and
Calkene-Calkene coupling27 reactions triggered by the open-shell
metal center. We expected that the use of robust dbcot diolefin
should block the undesired decomposition pathways involving
the metal center and allow for exclusive reactivity on the nitrogen
atom of the coordinated bislutidylamine.
Figure 2. X-ray structure of [Rh(η4-dbcot)(κ3-bla)]þ (1þ) (left) and
[Ir(η4-dbcot)(κ3-bla)]þ (2þ) (right). Displacement ellipsoids are
-
drawn at the 50% probability level (hydrogen atoms, the PF6 coun-
terions, and solvent molecules are omitted for clarity). The crystal of 1þ
reveals the presence of two independent but isostructural cations in the
asymmetric unit. See Table 1 for selected bond distances and angles.
The electrochemical data suggest that stable d7 MII complexes
should be obtainable from 1þ and 2þ by one-electron oxidation.
Hence we investigated their chemical oxidation in solution.
Treatment of 1þ and 2þ with appropriate oxidants28 led to
formation of 12þ and 22þ, respectively (Scheme 5). Compound
22þ proved to be sufficiently stable to obtain X-ray quality
crystals by layering an acetone solution of 22þ onto dichloro-
methane at -20 °C.
bond of the cyclooctene ligand does not reveal substantial
elongation.
Single crystals of [1]PF6 and [2]PF6 suitable for X-ray structure
determination were grown from acetone solutions layered with
hexanes or diethyl ether, respectively. The two compounds are
isostructural in the solid state (Figure 2) and are best described as
distorted trigonal bipyramids (tbpy). The axial positions of the
trigonal bipyramid are occupied by the lutidyl (N1) nitrogen atom
and a double bond of the dbcot ligand (C5-C6). The M-N1
distance is the same for both complexes within the experimental
error, while the axially coordinated double bond binds more
strongly to iridium (resulting in shorter M-C5 and M-C6
distances and longer C5-C6 distance in 2þ compared to 1þ).22
The equatorial sites are coordinated with lutidyl N2, amine N3,
and the C1-C2 double bond of dbcot. For both complexes there
is no significant difference between their M-N2 and M-N3
distances; however these nitrogen atoms are bound more strongly
to iridium with on average 0.03 Å shorter M-N bonds for 2þ
compared to the rhodium analogue 1þ. The axially coordinated
lutidyl moiety (N1) is bound more strongly to the metal than the
equatorially bound N2 and N3 donors, as expected for a d8 tbpy
complex.23 Due to strongerπ-back bonding in the equatorialplane
of tbpy complexes,23 the C1-C2 double bond is bound stronger
compared to the axially coordinated C5-C6 bond. The metal
C1-C2 interaction has a significant metalla(III)cyclopropane
character, resulting in a remarkable elongation of the C1-C2
distance (1.32, 1.44, and 1.47 Å for free dbcot,24 1þ and 2þ,
respectively). This substantial elongation of the double bonds of
coordinated dbcot is indicative of strong π-acidity of this ligand.
In solution 1þ is fluxional on the NMR time scale, leading to
averaged 1H and 13C NMR signals for both lutidyl groups of the
bla ligand and averaged signals of the two olefinic HCdCH
moieties and the aromatic rings of the dbcot ligand. Analogous
behavior was observed for 2þ. Strong π-back-bonding from the
metal to the dbcot ligand results in substantial upfield shifts of
both the proton and carbon resonance frequencies (6.78, 4.50,
and 4.37 ppm in 1H and 133.2, 74.6, and 58.3 ppm in 13C for free
dbcot, 1þ, and 2þ, respectively). Thus, both the X-ray and NMR
analyses confirm a very strong π-bonding of dbcot to the metal.
One-electron oxidation of 2þ to 22þ results in a change of the
coordination geometry from trigonal bipyramidal to a distorted
square pyramid (sqpy) with the two lutidyl donors and the two
olefinic double bonds coordinated in the basal plane and the
amine at the apical position, as evidenced by the single-crystal
X-ray diffraction (Figure 3). The Ir-N1 and Ir-N2 distances of
22þ are not significantly different (Δr < 3σ). The same is
observed for the distance between iridium and the olefinic bonds.
Lower electron density on the metal after oxidation results in
increased Ir-N and decreased Ir-olefin interactions as com-
pared to 2þ. The N1, N2 atoms and the centroids of the double
bonds are not in one plane due to a slight “twist” of the dbcot
moiety (Figure 3B). The measured crystal contained two in-
dependent molecules: the first isomer has the dbcot moiety
twisted clockwise, whereas the other counterclockwise. Optimi-
zation of the X-ray structure using DFT resulted in convergence
to a structure where N1, N2, and the centroids of the double
bonds are aligned in a plane. The slightly “twisted” geometry thus
seems to be a result of the crystal-packing forces.
The observed change of the coordination geometry from tbpy
to sqpy is more frequently observed when the electron config-
uration of the transition metal changes from d8 to d7 and is most
likely a result of the Jahn-Teller effect.29 For that reason we
expect that compound 12þ has a structure similar to 22þ
.
We used EPR spectroscopy to further investigate the
(electronic) structure of 12þ and 22þ. Measured X-band EPR
spectra of 12þ and 22þ in frozen acetone could be simulated as
rhombic spectra (i.e., g1 ¼ g2 ¼ g3) with well-resolved
(super)hyperfine interactions with the metal center and a single
nitrogen atom along the g3(z) axis (Figure 4 and Table 3). The
measured g values are indicative of metal-centered radicals. As
expected for heavier transition metals with larger spin-orbit
couplings, the g-anisotropy (rhombicity) of the iridium complex
22þ is larger than that of the rhodium analogue 12þ
.
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Table 1. Selected Bond Distances (Å) and Angles (deg) for Complexes 1þ, 2þ, and 22þ
[Rh(κ3-bla)(η4-dbcot)]þ
[Ir(κ3-bla)(η4-dbcot)]þ
[Ir(κ3-bla)(η4-dbcot)]2þ
1aþ
1bþ
2þ
2a2þ
2b2þ
M-N1
2.116(2)
2.257(3)
2.261(3)
2.103(3)
2.064(3)
2.139(3)
2.155(4)
1.440(5)
1.397(5)
40.42(13)
37.96(14)
88.97(9)
75.63(10)
76.18(10)
2.113(2)
2.259(3)
2.249(3)
2.101(3)
2.062(3)
2.147(3)
2.155(4)
1.436(5)
1.400(5)
40.34(13)
37.99(14)
89.12(10)
75.00(9)
76.46(10)
2.114(4)
2.229(3)
2.224(4)
2.066(4)
2.130(5)
2.128(4)
2.097(4)
1.472(6)
1.411(6)
41.41(17)
38.72(17)
91.48(12)
75.09(13)
76.55(12)
2.111(3)
2.095(3)
2.184(3)
2.189(3)
2.159(3)
2.191(3)
2.184(4)
1.414(5)
1.390(5)
37.94(12)
37.05(13)
92.17(11)
76.57(11)
79.97(11)
2.093(3)
2.112(3)
2.185(3)
2.189(4)
2.182(4)
2.165(3)
2.183(4)
1.410(5)
1.408(5)
37.65(13)
37.77(13)
89.91(11)
79.80(11)
76.93(12)
M-N2
M-N3
M-C1
M-C2
M-C5
M-C6
C1-C2
C5-C6
C1-M-C2
C5-M-C6
N1-M-N2
N1-M-N3
N2-M-N3
Scheme 5. Synthesis of Open-Shell Species [Rh(η4-dbcot)-
(κ3-bla)]2þ (12þ) and [Ir(η4-dbcot)(κ3-bla)]þ (22þ
)
The spectrum of the iridium(II) compound 22þ is influ-
enced by the presence of large Ir-quadrupole interactions,
causing the appearance of two weak “forbidden” transitions, as
indicated by the arrows in Figure 4.11,30 Since the spectrum
does not reveal any resolved hyperfines in this region of the
spectrum that could be influenced by these quadrupole inter-
actions, we did not take these interactions into account in the
spectral simulations.
To get a better insight in the electronic structure of both
complexes, we calculated the EPR properties with DFT
(Table 3). In line with the experiments, DFT predicts that 12þ
and 22þ are metal-centered radicals with Mulliken spin densities
of about 72% and 73% on rhodium and iridium, respectively, and
around 16.5% on the amine nitrogen atom.
The radical species 12þ and 22þ are sterically shielded
around the metal, which is reflected by their relative inertness
toward dioxygen in solution. Bubbling air through the solu-
tions of 12þ or 22þ did not give rise to formation of superoxo
species nor to a noticeable decrease of the EPR signal.
Apparently, the shielding of the metal center by the methyl
groups of the bla ligand prevents any rapid radical-type
reactivity at the metal center. However, somewhat surprisingly,
in solution the complexes slowly convert to the one-electron-
reduced diamagnetic compounds 1þ and 2þ, as evidenced by
NMR spectroscopy.31 This process is not a disproportionation
reaction (as commonly observed for RhII and IrII complexes),
Figure 3. X-ray structure of [Ir(η4-dbcot)(κ3-bla)]2þ (22þ). Displacement
ellipsoids are drawn at the 50% probability level (hydrogen atoms, the PF6
counterions, and a disordered CH2Cl2 molecule are omitted for clarity). The
crystal contains two independent cations in the asymmetric unit; only one of
them is shown. See Table 1 for selected bond distances and angles.
-
because the one-electron-reduced species 1þ and 2þ are the
only observed products.
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Table 2. Electrochemical Data for Complexes 1þ, 2þ, and 4þ
(vide supra)a
complex
Ea (V) ΔE (mV) E1/2 (V) If/Ib
[Rh(η4-dbcot)(κ3-bla)]PF6 ([1]PF6)
[Ir(η4-dbcot)(κ3-bla)]PF6 ([2]PF6)
[Rh(η4-dbcot)(κ3-Bn-bla)]PF6 ([4]PF6) 0.477
0.410
0.364
101
104
70
0.359 1.0
0.312 1.0
0.443 1.0
a Solvent: CH2Cl2; scan rate = 40 mV/s; E1/2 vs Fc/Fcþ. ΔE = peak
separation, Ea = anodic peak potential, E1/2 = half-wave potential, If/Ib =
anodic peak current/cathodic peak current.
Table 3. Simulated (expt) and Calculated (DFT) g Values
and (Super)hyperfine Interactions (MHz) for Compounds
12þ, 22þ, and 42þa
g11 (x)
DFT
g22 (y)
DFT
2.118
g33 (z)
DFT
1.982
complex property expt
expt
expt
12þ
22þ
42þ
g value 2.181
2.133 2.158
2.000
54.0
64.0
1.942
HFIRh n.r.
-7.67 n.r.
-14.2
-74.6
HFIN
n.r.
36.8
n.r.
36.9
62.1
g value 2.368
2.320 2.294
2.257
103
38.5
2.122
1.863
167
63.2
1.982
HFIIr
HFIN
n.r.
n.r.
97.0
38.4
n.r.
n.r.
122.0
60.0
g value 2.211
2.128 2.184
1.996
HFIRh n.r.
-8.67 n.r.
-16.9
58.9
61.8
-70.7
HFIN
n.r.
34.1
n.r.
34.2
63.3
a n.r. = not resolved. Directions of the principal axes of the g-tensor in the
molecular axis system of complexes are shown.
Figure 4. Experimental and simulated X-band EPR spectra of
[RhII(dbcot)(bla)]2þ (frequency: 9.379235 GHz; modulation ampli-
tude: 0.8 G; power: 0.02 mW) complex 12þ (top) and [IrII(dbcot)-
(bla)]2þ (frequency 9.377560 GHz; modulation amplitude: 4 G; power:
0.2 mW) complex 22þ (bottom) recorded in frozen acetone at 60 K.
Arrows indicate the forbidden transitions. The salt [n-Bu4N]PF6 was
added to the solution to obtain a better glass. The spectrum of 42þ is
similar to the spectrum of 12þ (see Supporting Information).
could be detected.32 Instead, quantitative formation of the one-
electron-reduced diamagnetic complexes 1þ and 2þ was ob-
served with 1H NMR. Remarkably, the NMR data further reveal
that the reactions retain the N-H hydrogen atoms for a large
part, whereas formation of the N-D deuterated analogues of 1þ
and 2þ would be expected if the reactions would proceed via the
pathway depicted in Scheme 6. This makes the aminyl radical
pathway rather doubtful, unless the reaction would be associated
with a very large kinetic isotope effect.33 Radical trapping
experiments34 did not allow for unambiguous detection of the
proposed aminyl radicals nor of solvent radicals.35
Our initial hypothesis was that the increased charge of the
metal (changing from 1þ to 2þ upon one-electron oxidation)
results in an increased acidity of the NH group of the bla ligand.
Spontaneous deprotonation of the amine group would then lead
to formation of an aminyl radical via an intramolecular redox
reaction between the metal and the nitrogen atom. Subsequently,
this aminyl radical could then abstract a hydrogen atom from the
reaction medium, thus re-forming the reduced complex 1þ or 2þ
(Scheme 6). To check whether the re-formation of compounds
1þ and 2þ indeed proceeds via the pathway shown in Scheme 6,
we studied the reactivity of 12þ and 22þ toward Brønsted bases.
Reaction of complexes 12þ and 22þ with solid K2CO3 in
acetone or acetonitrile resulted in disappearance of the charac-
teristic EPR signals, and no signal of the putative aminyl radical
Synthesis of Paramagnetic N-Protected [RhII(Bn-bla)-
(dbcot)]2þ Complex and Its Reactivity with K2CO3. Since
the above results are inconclusive about the role of the amine/
aminyl radical in the observed reduction of 12þ/22þ to 1þ/2þ,
we decided to perform a control experiment using a similar
compound in which the amine is protected with a benzyl moiety.
Hence we investigated the reactivity of the complex [Rh(dbcot)-
(Bn-bla)](PF6)2 ([4](PF6)2).
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Scheme 6. Hypothetical Deprotonation-HAT Pathway Leading to Net Reduction of the Paramagnetic Species 12þ and 22þ
ARTICLE
Scheme 7. Synthesis of [Rh(η4-dbcot)(κ3-Bn-bla)]þ (4þ)
Scheme 8. Regeneration of the Diamagnetic Complex by Base-Promoted Oxidation of Solvent or Hydroxide Anion
The precursor complex [Rh(η4-dbcot)(κ3-Bn-bla)]PF6 ([4]PF6)
was prepared using the method described for [1]PF6 (Scheme 7).
Stirring an acetone-d6 solution of 42þ (directly dissolved from
a pregenerated 1:1 mixture of 42þ and precipitated metallic
silver) with solid K2CO3 again results in a color change from dark
green to bright yellow, with quantitative formation of the reduced
species 4þ (as evidenced by NMR). This is similar to the
reactions observed for 12þ and 22þ and shows that the presence
of the N-H group is not necessary for the reduction of the MII
oxidation state radical species to their MI precursors. Hence, the
reaction does not necessarily involve the intermediacy of aminyl
radicals, and the role of the Brønsted base has to be different than
originally anticipated.
1
The H NMR signals of the dbcot moiety in compound 4þ are
broadened compared to the signals of Bn-bla at room temperature,
and contrary to 1þ and 2þ the olefinic signals of dbcot are
magnetically inequivalent, giving two broad signals at 4.75 and 4.24.
This suggests that the rotation of dbcot is much slower in the case of
4þ than in 1þ and 2þ.
CV measurements reveal that the redox potential of [Rh(Bn-
bla)(dbcot)]þ (4þ) is higher by 67 and 170 mV compared with
[Rh(bla)(dbcot)]þ (1þ) and the previously reported [Rh(Bn-
bla)(cod)]þ,11 respectively, showing that exchange of either the
bla ligand for the Bn-bla ligand or cod for dbcot results in a
considerable destabilization of the þII oxidation state of rhodium
(see Table 2). For that reason we used AgPF6 in DCM as the
Base-assisted oxidation of solvent36 seems to be a viable
pathway for the reduction of species 12þ/22þ (Scheme 8).37
Judging from the redox potentials, the dicationic radical metal
complexes 12þ/22þ and acetone must exist in a redox equilib-
rium with the closed-shell metal complexes 1þ/2þ and the
acetone radical cation.38 Without a base, this equilibrium lies
far to the left, but the acetone radical cation formed in small
quantities can be easily deprotonated by the base, shifting the
redox equilibrium between acetone and 12þ/22þ completely to
the right. Alternatively, in the presence of small quantities of
water the hydroxide anion can be generated, which is known as a
potential one-electron reducing agent in organic solvents.39
oxidizing reagent to form 42þ
.
The EPR spectrum of 42þ (see the Supporting Information) is
very similar to the spectrum of 12þ and shows a rhombic, almost axial
g-tensor and hyperfine couplings with rhodium and nitrogen (see
Table 3). As for 12þ and 22þ the DFT-calculated EPR parameters
are in good qualitative agreement with the experiment. The DFT-
calculated Mulliken spin densities at rhodium (67%) and nitrogen
(18%) are comparable to those of 12þ and 22þ
.
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ARTICLE
Table 4. Crystallographic Data for [1](PF6), [2](PF6), [2](PF6)2, and [3](PF6)
[Rh(κ3-bla)
(dbcot)]PF6 CH3COCH3
[Ir(κ3-bla)(dbcot)]PF6
[Ir(κ3-bla)(dbcot)](PF6)2
CH2Cl2 ([2](PF6)2)
[Ir(κ3-bla)(H)(σ-C8H13
(η2-C8H14)]PF6(3[PF6])
)
3
3
3
([1]PF6)
(CH3CH2)2O ([2]PF6)
empirical formula
C
30H29N3Rh, C3H6O, F6P
C30H29IrN3, C4 H10 O, F6 P
2(C30H29IrN3), 4(F6P),
CH2Cl2 þ solvent
1912.37a
110
C30H45IrN3, F6P
fw
737.52
842.87
784.88
temperature [K]
radiation
wavelength [Å]
cryst syst
space group
a [Å]
110
110
150
Mo KR
0.71073
monoclinic
Cc
Mo KR
0.71073
monoclinic
P21/c
Mo KR
Mo KR
0.71073
monoclinic
P21/c
0.71073
monoclinic
P21/c
28.5187(8)
11.2036(3)
19.4833(4)
101.351(1)
6103.4(3)
8
9.2911(4)
20.4418(9)
17.3595(7)
98.599(2)
3260.0(2)
4
21.5483(9)
16.7416(7)
20.9573(8)
110.520(2)
7080.7(5)
4
9.5523(7)
16.2517(5)
20.1842(7)
97.861(2)
3104.0(3)
4
b [Å]
c [Å]
β [deg]
volume [Å3]
Z
color
yellow
yellow
black
pale yellow
27.5
θ-max
27.5
27.5
27.5
density [Mg m-3
absorp coeff [mm-1
F(000)
]
1.605
1.717
1.794a
4.027a
3728a
1.680
]
0.682
4.212
4.414
3008
1672
1568
R1/wR2/S
0.0281/0.0600/1.047
0.0333/0.0584/1.093
0.0292/0.0759/1.101
0.0234/0.0496/1.027
a Excluding the disordered solvent contribution.
Thus, depending on the water content in the solvent (or base),
this seems to be a plausible mechanism as well.
via deprotonation of the amine followed by HAT. It is remark-
able, however, that the complexes studied are quantitatively
reduced in the presence of bases to the MI species. The reduction
is most likely caused by a hydroxide anion or a solvent molecule
that can be deprotonated in its oxidized form. We show that
generation of aminyl radicals at cationic rhodium(II) and
iridium(II) species is not as straightforward as expected. Depro-
tonation of amines coordinated to these paramagnetic metal
centers might well be intrinsically limited by the instability of the
reactive radical compounds under basic conditions. Generation
of aminyl radicals on bislutidylamine ligands (or bispicolyl-type
ligands in general) remains elusive.
At this point we cannot exclude that the pathway involving
deprotonation of the N-H group can take place, taking into
account the reactivity of related amine complexes of rhodium and
iridium. Instant hydrogen atom abstraction by an iridium-co-
ordinated aminyl radical has been recently reported by Gr€utzma-
cher et al.15c However, it is tempting to assume that base-assisted
solvent oxidation is the actual mechanism of re-formation of 1þ/
2þ from 12þ/22þ and that the aminyl radical pathway plays little
(if any) role in this reaction.
’ SUMMARY AND CONCLUSIONS
’ EXPERIMENTAL SECTION
Open-shell rhodium(II) and iridium(II) complexes with
dibenzocyclooctadiene (dbcot)- and bislutidylamine (bla)-type
ligands were successfully synthesized. They represent rare ex-
amples of isolable open-shell olefin complexes. The chosen
diolefin ligand is inert to radical reactions, which allowed for
investigations of possible radical reactivity pathways involving
the aminyl nitrogen atom of the bla ligand.
An attempt to obtain the aminyl radical complexes by depro-
tonation of the N-H group of the bla ligand using a base led to
an unexpected, selective reduction of the paramagnetic species
[MII(dbcot)(bla)]2þ to their one-electron-reduced closed-shell
analogues [MI(dbcot)(bla)]þ (M = Rh, Ir). Control experiments
with the rhodium(II) complex [RhII(dbcot)(Bn-bla)]2þ con-
taining the Bn-bla ligand (a bla derivative having the amine
protected with a benzyl group) revealed similar reactivity.
These results show that the generation of aminyl radicals from
the complexes of the general formula [MII(diolefin)(N3-
ligand)]2þ (M = Rh, Ir) is perhaps not occurring at all. At least
it is rather unlikely that the observed reduction reactions occur
X-ray Diffraction. X-ray data were collected at low temperature
under the control of the Nonius COLLECT software with a Nonius
KappaCCD on a rotating anode. Numerical details have been collected
in Table 4. The structures are shown in Figures 1, 2, and 3.40 The
intensity data were corrected for absorption with the program SADABS.
The structures were solved with the program DIRDIF and refined with
SHELXL97. The Flack parameter for [1]PF6 refined to 0.484(13) with a
BASF/TWIN refinement. The structure of [2](PF6)2 has solvent-
accessible voids filled with disordered solvent. Their contribution to
the structure factors in the refinement was taken into account with the
PLATON/SQUEEZE approach. The hydrogen atom on Ir for [3]PF6
was located in a difference density map.
DFT Calculations. Geometry optimizations were carried out with the
Turbomole program package41 coupled to the PQS Baker optimizer42 at
the ri-DFT43 level using the BP8644 functional and SV(P) basis set.45
Calculated EPR spectra46 were obtained with the ADF program47 at the
DFT, BP86,48 and TZP49 level, using the Turbomole-optimized
geometries.
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General Procedures. All manipulations were performed in an argon
atmosphere by standard Schlenk techniques or in a glovebox. Methanol,
acetonitrile, and dichloromethane were distilled under nitrogen from
CaH2. Hexanes were distilled under nitrogen from Na wire. Acetone was
deoxygenated using the freeze-pump-thaw method. NMR experi-
ments were carried out on a Bruker DRX300 (300 and 75 MHz for 1H
and 13C, respectively) or on a Varian Inova 500 (500 and 125 MHz for
1H and 13C, respectively) spectrometer. Solvent shift reference: acetone-
d6 δ = 2.05 and δ = 29.84 for 1H and 13C, CDCl3 δ = 7.26 and δ = 77.16
C4); 127.2 (Ardbcot); 126.5 (Py-C5); 126.4 (Ardbcot); 121.5 (Py-C3);
60.2 (N-CH2-Py); 58.3 (CHdCH); 29.5 (Py-CH3). Anal. Calcd for
[2]PF6 CH3OH: C, 46.50; H, 4.15; N, 5.25. Found: C: 46.58; H: 4.29;
3
N: 5.12. 1H NMR indicates the presence of cocrystallized MeOH.
Synthesis of [Rh(K3-bla)(dbcot)](PF6)2 ([1](PF6)2). [1]PF6
(94.5 mg, 0.139 mmol) and 35.6 mg of AgPF6 (0.141 mmol) were
dissolved in 3 mL of acetone and left to react for 30 min. The solution
was filtered using a syringe filter, and the filtrate was condensed to a
black oil in vacuo. An 8 mL amount of CH2Cl2 was added, causing
precipitation of a purple solid, which was centrifuged. Yield: 84 mg
(0.102 mmol, 73.4%). Unfortunately, despite many attempts, due to the
intrinsic instability of [1](PF6)2, we were not able to obtain an
analytically pure sample.
1
for H and 13C, respectively. Abbreviations used are s = singlet, d =
doublet, t = triplet, m = multiplet, br = broad. Elemental analyses (CHN)
were carried out by H. Kolbe Mikroanalytisches Laboratorium
(Germany). X-band EPR spectroscopy measurements were performed
with a Bruker EMX Plus spectrometer. Cyclic voltammograms of
∼2 mM parent compounds in 1 M Bu4NPF6 electrolyte solution were
recorded in a gastight single-compartment three-electrode cell equipped
with platinum working (apparent surface of 0.42 mm2), coiled platinum
wire auxiliary, and silver wire pseudoreference electrodes. The cell was
connected to a computer-controlled PAR model 283 potentiostat. All
redox potentials are reported against the ferrocene/ferrocenium (Fc/
Fcþ) redox couple. Ferrocene was used as internal standard. Magnetic
susceptibility was measured on a Magway MSB Mk1 magnetic balance.
[Fe(μ-C5H4COMe)Cp]PF6,28 [Ir(dbcot)(μ-Cl)]2,50 [M(coe)2(μ-
Cl)]2,51 dibenzo[a,e]cyclooctatetraene,52 N-benzyl-N,N-di[(6-methyl-2-
pyridylmethyl)]amine,53 and bis((6-methyl-2-pyridyl)methyl)amine54
have been prepared according to previously reported procedures.
Dibenzo[a,e]cyclooctadiene. Although various experimental
procedures are available for the synthesis of dbcot,52 we did not find a
full assignment of all 13C NMR signals of this compound. 1H NMR (500
MHz, CDCl3 (7.26 ppm)): δ 7.17 (m, 4H, Ardbcot-H); 7.08 (m, 4H,
Ardbcot-H); 6.78 (s, 4H, olefinic). 13C NMR (125 MHz, CDCl3 (77.23
ppm): δ 137.27 (CIV); 133.44 (olefinic); 129.31; 127.03.
Synthesis of [Ir(K3-bla)(dbcot)](PF6)2 CH2Cl2 ([2](PF6)2).
3
[2]PF6 (136 mg, 0.177 mmol) and 50 mg of [Fe(μ-C5H4CO-
Me)Cp]PF6 (0.15 mmol) were dissolved in 20 mL of dichloromethane
and stirred for 45 min, at which time a brown precipitate formed and the
characteristic absorbance of acetylferrocenium was gone. The solution
was filtered, washed with dichloromethane, and dried in vacuo. Yield: 80
mg (0.08 mmol, 53.3%). Magnetic susceptibility: μeff = 2.06 (corrected
for diamagnetism of [2]PF6, PF6, Ir2þ, and CH2Cl2; measured: 1.90).55
Anal. Calcd for [2](PF6)2 CH2Cl2: C, 37.28; H, 3.13; N, 4.21. Found:
3
C, 37.24; H, 3.08; N, 4.36.
Synthesis of [Ir(K3-bla)(H)(σ-C8H13)(η2-C8H14)]PF6 ([3]PF6).
An excess of dbcot (238, 1.17 mmol) and bla (265 mg, 1.17 mmol) were
added to a suspension of [{Ir(coe)2Cl}2] (523 mg, 1.16 mmol Ir) in dry
methanol (40 mL), and the mixture was stirred for 18 h. The reaction
mixture turned to a transparent green solution, to which NH4PF6 was
added, causing precipitation of a white solid. The product was filtered off
and recrystallized. Yield: 270 mg, 30%. Crystals suitable for X-ray
diffraction were grown by layering an acetone solution of 3[PF6] with
hexanes.
Synthesis of [Rh(K3-bla)(dbcot)]PF6 ([1]PF6). [Rh(coe)2Cl]2
(179 mg, 0.499 mmol Rh) and 107 mg of dbcot (0.524 mmol) were
dissolved in 10 mL of CH2Cl2, and the solution was stirred for 2 h and
evaporated to dryness. Next, 115 mg of bla (0.506 mmol) and a mixture
of 10 mL of MeOH and 10 mL of CH2Cl2 were added and stirred for 1 h.
The unreacted solid was filtered off, and the remaining solvent was
removed in vacuo. The thus formed solid was dissolved in 5 mL of
MeOH, and 106 mg of KPF6 was added, causing precipitation of a bright
yellow solid, which was filtered and washed with 2 mL of MeOH. Yield:
168 mg (0.247 mmol, 49.5%).
1H NMR (acetone-d6, 300 MHz): δ 7.79 (t, 3J(H,H) = 7.8 Hz, 1H;
PyA); 7.63 (t, 3J(H,H) = 7.5 Hz, 1H; PyB); 7.46 (m, 2H; PyA); 7.25 (d,
3J(H,H) = 7.5 Hz, 2H; PyB); 6.60 (s, br, 1H; N-H); 5.47 (dd[AB], 2J(H,
H) = 17.7 Hz, 3J(H,H) = 8.1 Hz, 1H; N-CH2-Py); 5.15 (dd[AB], 2J(H,
H) = 15.9 Hz, 3J(H,H) = 5.1 Hz, 1H; N-CH2-Py); 5.05 (t, 3J(H,H) = 7.5
Hz, 1H; (IrC-CH); 4.76 (dd[AB], 2J(H,H) = 18.6 Hz, 1H; N-CH2-Py);
4.68 (dd[AB], 2J(H,H) = 15.6 Hz, 1H; N-CH2-Py); 3.88 (m, 2H;
CHdCH); 3.13 (s, 3H; PyA-CH3); 3.02 (s, 3H; PyB-CH3); 2.66 (m,
1H; coe); 2.25 (m, 1H; coe); 1.5 (m, 22H; coe); -15.84 (s, 1H, Ir-H).
13C NMR (75 MHz, acetone-d6): δ 164.5 (Py); 163.4 (Py); 162.3 (Py);
161.7 (Py); 140.5 (PyA); 139.6 (PyB); 127.9 (IrCdCH); 127.5 (PyA);
127.2; (Ir-C); 125.6 (PyB); 122.1 (PyA); 121.6 (PyB); 69.3 (CHdCH);
64.6 (N-CH); 63.9 (N-CH) 63.8 (CHdCH); 40.8 (CH-CH2); 33.8
(CH-CH2); 33.1 (CH-CH2); 33.0 (CH-CH2); 32.4 (coe); 32.2
(PyACH3); 31.8 (PyBCH3); 29.9 (coe); 29.7 (coe); 28.8 (coe); 28.5
(coe); 25.5 (coe); 27.8 (coe); 27.6 (coe). Anal. Calcd (C30H45F6IrN3P):
C, 45.91; H, 5.78; N, 5.35. Found: C, 45.83; H, 5.86; N, 5.33
Synthesis of [Rh(K3-Bn-bla)(dbcot)]PF6 ([4]PF6). [Rh(coe)2-
Cl]2 (180 mg, 0.499 mmol Rh) and 102 mg of dbcot (0.499 mmol)
were dissolved in 10 mL of CH2Cl2, and the solution was stirred for 2
h and evaporated to dryness. Next, 164 mg of Bn-bla (0.517 mmol)
and a mixture of 8 mL of MeOH and 8 mL of CH2Cl2 were added and
stirred for 1 h, and the solvent was removed in vacuo. The thus formed
solid was dissolved in 6 mL of MeOH, and 126 mg of KPF6 was added,
causing precipitation of a bright yellow solid. Then 2 mL of H2O was
added, and the solution was stirred for 15 min, after which it was
filtered and washed with 2 mL of MeOH. Yield: 331 mg (0.430 mmol,
86.2%).
1H NMR (500 MHz, acetone-d6): δ 7.80 (t, 3J(H,H) = 7.5 Hz, 2H;
Py); 7.44 (d, 3J(H,H) = 7.5 Hz, 2H; Py); 7.35 (d, 3J(H,H) = 7.5 Hz, 2H;
2
Py); 6.62 (m, 8H; Ardbcot); 5.29 (d[AB] br, J(H,H) = 15.5 Hz, 2H;
N-CH2-Py); 4.50 (d, 2J(Rh,H) = 1.5 Hz, 4H; CHdCH); 4.28 (d[AB],
2J(H,H) = 16.5 Hz, 2H; N-CH2-Py); 3.39 (s br, 6H; Py-CH3). 13C NMR
(125 MHz, acetone-d6): δ 161.3 (Py); 161.2 (Py); 145.6 (Ardbcot);
139.3 (Py-C4); 126.9 (Ardbcot); 126.7 (Ardbcot); 126.0 (Py-C5); 121.3
(Py-C3); 74.6 (d, 2J(Rh,H) = 12.6 Hz; (CHdCH); 58.3 (N-CH2-Py);
28.6 (Py-CH3). Anal. Calcd: C, 53.03; H, 4.30; N, 6.18; Found: 52.77;
H, 4.67; N, 5.98.
Synthesis of [Ir(K3-bla)(dbcot)]PF6 ([2]PF6). [Ir(dbcot)Cl]2
(300 mg, 0.695 mmol Ir) was suspended in 10 mL of methanol, and 160
mg of bla (0.704 mmol) was added. The solution was stirred for 30 min
and turned transparent yellow. Then 128 mg KPF6 (0.695 mmol) was
added, causing precipitation of a bright yellow solid, which was filtered
and washed with 2 mL of MeOH. Yield: 234 mg (0.304 mmol, 43.7%).
1H NMR (500 MHz, acetone-d6): δ 7.88 (t, 3J(H,H) = 7.5 Hz, 2H;
Py); 7.52 (d, 3J(H,H) = 7.5 Hz, 2H; Py); 7.48 (d, 3J(H,H) = 7.5 Hz, 2H;
Py); 6.91 (m, 4H; Ardbcot); 6.79 (m, 4H; Ardbcot); 5.33 (d[AB], 2J(H,H)
= 17.0 Hz, 2H; N-CH2-Py); 4.52 (d[AB], 2J(H,H) = 17 Hz, 2H; N-CH2-
Py); 4.37 (s, 4H; CHdCH); 3.30 (s, 6H; Py-CH3). 13C NMR (125
MHz, acetone-d6): δ 162.9 (Py); 161.5 (Py); 148.2 (Ardbcot); 139.7 (Py-
1H NMR (300 MHz, acetone-d6): δ 7.80 (t, 3J(H,H) = 7.7 Hz, 2H;
Py); 7.66-7.41 (m, 5H; Py, Ph); 7.36 (d, 3J(H,H) = 7.6 Hz, 2H; Py);
7.03 (m, br, 4H; Ardbcot); 6.97 (m, br, 4H; Ardbcot); 5.04 (d[AB] br,
2J(H,H) = 15.8 Hz, 2H; N-CH2-Py); 4.91 (s, 2H; N-CH2-Ph); 4.75 (s,
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br, 4H; CHdCH), 4.24 (s, br, 4H; CHdCH); 3.86 (d[AB], 2J(H,H) =
16.0 Hz, 2H; N-CH2-Py); 3.60 (s, 6H; Py-CH3). 13C NMR (125 MHz,
acetone-d6): δ 162.1 (Py); 160.6 (Py); 145.0 (Ardbcot-C1); 139.8 (Py-
C4); 133.5 (Ardbcot-C1); 132.7 (Ph-C2/6); 129.7 (Ph-C3/5); 129.7
(Ph-C4); 127.3 (Ardbcot-C2/3); 127.2 (Py-C5); 122.6 (Py-C3); 63.6
(N-CH2-Ph); 61.6 (N-CH2-Py); 30.0 (Py-CH3). The 13C NMR signals
of the double bond of dbcot could not be located due to the fluxional
behavior of this moiety. FABþ-MS: calcd for [4]þ (C37H35N3Rh) m/z
624.1883; found m/z 624.1886 (Δ = -0.5 ppm).
(8) (a) Sherry, A. E.; Wayland, B. B. J. Am. Chem. Soc. 1990,
112, 1259–1261. (b) Cui, W.; Zhang, X. P.; Wayland, B. B. J. Am. Chem.
Soc. 2003, 125, 4994–4995. (c) Cui, W.; Wayland, B. B. J. Am. Chem. Soc.
2004, 126, 8266–8274. (d) Puschmann, F. F.; Gr€utzmacher, H.; de
Bruin, B. J. Am. Chem. Soc. 2010, 132, 73–75.
(9) (a) Zhang, L.; Chan, K. S. Organometallics 2007, 26, 679–684.
(b) Dzik, W. I.; Reek, J. N. H.; de Bruin, B. Chem.—Eur. J. 2008,
14, 7594–7599.
(10) K€onigsmann, M.; Donati, N.; Stein, D.; Sch€onberg, H.; Harmer,
J.; Sreekanth, A.; Gr€utzmacher, H. Angew. Chem., Int. Ed. 2007, 46,
3567–3570.
(11) Hetterscheid, D. G. H.; Klop, M.; Kicken, R. J. N. A. M.; Smits,
J. M. M.; Reijerse, E. J.; de Bruin, B. Chem.—Eur. J. 2007, 13, 3386–3405.
(12) For an overwiew see: Hicks, R. G. Angew. Chem., Int. Ed. 2008,
47, 7393–7395.
(13) Copper: (a) Mankad, N. P.; Antholine, W. E.; Szilagyi, R. K.;
Peters, J. C. J. Am. Chem. Soc. 2009, 131, 3878–3880. Ruthenium:(b)
Miyazato, Y.; Wada, T.; Muckerman, J. T.; Fujita, E.; Tanaka, K. Angew.
Chem., Int. Ed. 2007, 46, 5728–5730. Nickel:(c) Adhikari, D.; Mossin,
S.; Basuli, F.; Huffman, J. C.; Szilagyi, R. K.; Meyer, K.; Mindiola, D. J. J.
Am. Chem. Soc. 2008, 130, 3676–3682. For aminyl radicals coordinated
to iridium and rhodium see ref 15.
(14) (a) Roth, J. P.; Yoder, J. C.; Won, T.-J.; Mayer, J. M. Science
2001, 294, 2524–2526. (b) Yoder, J. C.; Roth, J. P.; Gussenhoven, E. M.;
Larsen, A. S.; Mayer, J. M. J. Am. Chem. Soc. 2003, 125, 2629–2640. (c)
Mader, E. A.; Larsen, A. S.; Mayer, J. M. J. Am. Chem. Soc. 2004,
126, 8066–8067. (d) Mader, E. A.; Davidson, E. R.; Mayer, J. M. J. Am.
Chem. Soc. 2007, 129, 5153–5166. (e) Wu, A.; Mayer, J. M. J. Am. Chem.
Soc. 2008, 130, 14745–14754. (f) Mader, E. A.; Manner, V. W.; Markle,
T. F.; Wu, A.; Franz, J. A.; Mayer, J. M. J. Am. Chem. Soc. 2009,
131, 4335–4345.
(15) (a) B€uttner, T.; Geier, J.; Frison, G.; Harmer, J.; Calle, C.;
Schweiger, A.; Sch€onberg, H.; Gr€utzmacher, H. Science 2005,
307, 235–238. (b) Maire, P.; K€onigsmann, M.; Sreekanth, A.; Harmer,
J.; Schweiger, A.; Gr€utzmacher, H. J. Am. Chem. Soc. 2006,
128, 6578–6580. (c) Donati, N.; Stein, D.; B€uttner, T.; Sch€onberg,
H.; Harmer, J.; Anadaram, S.; Gr€utzmacher, H. Eur. J. Inorg. Chem.
2008, 4691–4703.
Synthesis of [Rh(K3-Bn-bla)(dbcot)](PF6)2 ([4](PF6)2).
[4]PF6 (89.4 mg, 0.116 mmol) and 29.4 mg of AgPF6 (0.116 mmol)
were dissolved in 3 mL of CH2Cl2, causing instant precipitation of
metallic silver and [4](PF6)2. Due to a rather low stability of [4](PF6)2,
attempts to isolate an analytically pure sample were not successful.
Hence for the reaction with base we used a ca. 1:1 mol mixture of
[4](PF6)2 and Ag black obtained after decanting the green supernatant
and drying the black-green powder in vacuo.
Reaction of Radical Species 12þ, 22þ, and 42þ with K2CO3
(ref 56). In a typical experiment 0.02 mmol of the metal complex 12þ
,
22þ, or 42þ (the isolated complexes were used in the case of 12þ and
22þ; in the case of 42þ a mixture with co-precipitated metallic silver was
used) was dissolved in 1 mL of acetone-d6 and stirred with 0.4 mmol of
anhydrous solid K2CO3. After the solution turned yellow, the recorded
NMR spectra showed quantitative formation of 1þ, 2þ, or 4þ,
respectively.
’ ASSOCIATED CONTENT
S
Supporting Information. Crystallographic information
b
(cif file) containing data for complexes [1](PF6), [2](PF6),
[2](PF6)2, and [3](PF6). The EPR spectra of 42þ and the
reaction of 12þ with DMPO. This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
(16) (a) Hetterscheid, D. G. H. Ph.D. Thesis, Radboud University
Nijmegen, 2006. (b) Tejel, C.; Ciriano, M. A.; del Río, M. P.; van den
Bruele, F. J.; Hetterscheid, D. G. H.; Tsichlis i Spithas, N.; de Bruin, B. J.
Am. Chem. Soc. 2008, 130, 5844–5845. (c) Tejel, C.; Ciriano, M. A.; del
Río, M. P.; Hetterscheid, D. G. H.; Tsichlis i Spithas, N.; Smits, J. M. M.;
de Bruin, B. Chem.—Eur. J. 2008, 14, 10932–10936. (d) Tejel, C.; del
Río, M. P.; Ciriano, M. A.; Reijerse, E. J.; Hartl, F.; Zꢀaliꢁs, S.; Hetterscheid,
D. G. H.; Tsichlis i Spithas, N.; de Bruin, B. Chem.—Eur. J. 2009,
15, 11878–11889.
(17) Recently it was shown that replacement of cod ligand with
dbcot greatly improved the (air) stability of an iridium catalyst for allylic
substitutions: Spiess, S.; Welter, C.; Franck, G.; Taquet, J. -P.; Helmchen,
G. Angew. Chem., Int. Ed. 2008, 47, 7652–7655.
Corresponding Author
*Fax: (þ31) 20 5255604. E-mail: b.debruin@uva.nl.
’ ACKNOWLEDGMENT
We thank Han Peeters for measuring the FABþ-MS mass
spectrum. Financial support from the European Research Coun-
cil (ERC Grant Agreement 202886-CatCIR), NWO-CW (VIDI
grant 700.55.426), and the University of Amsterdam is gratefully
acknowledged.
(18) (a) Fernandez, M. J.; Rodriguez, M. J.; Oro, L. A.; Lahoz, F. J.
J. Chem. Soc., Dalton Trans. 1989, 2073–2076. (b) Alvarado, Y.; Boutry,
O.; Gutiꢀerrez, E.; Monge, A.; Nicasio, M. C.; Poveda, M. L.; Pꢀerez, P. J.;
Ruíz, C.; Bianchini, C.; Carmona, E. Chem.—Eur. J. 1997, 3, 860–873.
(19) Iimura, M.; Evans, D. R.; Flood, T. C. Organometallics 2003,
22, 5370–5373.
’ REFERENCES
(1) Whittaker, J. W. Chem. Rev. 2003, 103, 2347–2363.
(2) (a) Costas, M.; Mehn, M. P.; Jensen, M. P.; Que, L., Jr. Chem.
Rev. 2004, 104, 939–986. (b) Limberg, C. Angew. Chem., Int. Ed. 2003,
42, 5932–5954.
(20) Dzik, W. I.; Smits, J. M. M.; Reek, J. N. H.; de Bruin, B.
Organometallics 2009, 28, 1631–1643.
(3) Meunier, B.; de Visser, S. P.; Shaik, S. Chem. Rev. 2004,
104, 3947–3980.
(21) (a) Hermann, D.; Gandelman, M.; Rozenberg, H.; Shimon,
L. J. W.; Milstein, D. Organometallics 2002, 21, 812–818. (b) Friedrich,
A.; Ghosh, R.; Kolb, R.; Herdtweck, E.; Schneider, S. Organometallics
2009, 28, 708–718.
(4) Baik, M.-H.; Newcomb, M.; Friesner, R. A.; Lippard, S. J. Chem.
Rev. 2003, 103, 2385–2419.
(5) Stubbe, J.; Nocera, D. G.; Yee, C. S.; Chang, M. C. Y. Chem. Rev.
2003, 103, 2167–2201.
(6) (a) Gridnev, A. A.; Ittel, S. D. Chem. Rev. 2001, 101, 3611–3659.
(b) de Bruin, B.; Dzik, W. I.; Li, S.; Wayland, B. B. Chem.—Eur. J. 2009,
15, 4312–4320.
(7) Penoni, A.; Wanke, R.; Tollari, S.; Gallo, E.; Musella, D.; Ragaini,
F.; Demartin, F.; Cenini, S. Eur. J. Inorg. Chem. 2003, 1452–1460.
(22) Shorter metal-ligand bond distances are generally interpreted
to indicate stronger metal-ligand bond interactions. This however does
not always reflect the actual thermodynamic metal-ligand bond dis-
sociation (free) energies, which are also influenced by reorganization
and solvation effects. See for instance: Huang, J.; Haar, C. M.; Nolan,
1911
dx.doi.org/10.1021/om101157r |Organometallics 2011, 30, 1902–1913

Organometallics
ARTICLE
S. P.; Marshall, W. J.; Moloy, K. G. J. Am. Chem. Soc. 1998,
120, 7806–7815.
(23) Rossi, A. R.; Hoffmann, R. Inorg. Chem. 1975, 14, 365–374.
(24) Irngartinger, H.; Reibel, W. R. K. Acta Crystallogr. B 1981,
B37, 1724–1728.
(25) (a) [Rh(C6Cl5)2(cod)]: García, M. P.; Jimꢀenez, M. V.; Oro,
L. A.; Lahoz, F. J.; Casas, J. M.; Alonso, P. J. Organometallics 1993,
12, 3257–3263. (b) [Ir(C6Cl5)2(cod)]: García, M. P.; Jimꢀenez, M. V.;
Oro, L. A.; Lahoz, F. J.; Alonso, P. J. Angew. Chem., Int. Ed. 1992,
31, 1527–1529. (c) [(η5-C5Ph5)Rh(cod)]þ: Shaw, M. J.; Geiger,
W. E.; Hyde, J.; White, C. Organometallics 1998, 17, 5486–5491.
(d) [Ir(Me3tpa)(C2H4)]2þ: de Bruin, B.; Peters, T. P. J.; Thewissen,
S.; Blok, A. N. J.; Wilting, J. B. M.; de Gelder, R.; Smits, J. M. M.; Gal,
A. W. Angew. Chem. Int. Ed. 2002, 41, 2135–2138. (e) For a review see:
de Bruin, B.; Hetterscheid, D. G. H. Eur. J. Inorg. Chem.
2007, 211–230.
been captured. These species existed in a 1:0.1 ratio, which changed in
time. Simulated parameters: I (major): giso = 2.007, AN = 48.9 MHz,
AH = 56.6 MHz, ARh = 4.9 MHz; II (minor): giso = 2.0049, AN = 46.3
MHz, AH = 57.5 MHz. See Supporting Information. Species 22þ with
DMPO and K2CO3 gave a complex, very low intensity signal, which was
impossible to simulate reliably.
(36) The oxidation of CO32- to C2O62- seems to be less probable.
Although the redox potential of the C2O62-/CO32- couple in aqueous
carbonate buffer is þ0.67 V vs Fc/Fcþ (0.21 vs SCE), the low
concentration of carbonate anions in acetone makes this pathway
energetically unfavorable. Zhang, J.; Oloman, C. W. J. Appl. Electrochem.
2005, 35, 945–953.
(37) As suggested by a referee, reaction in dry dmso (which is not
susceptible to deprotonation) could give more mechanistic informa-
tion. Hence, we investigated the reactivity of radical compound 42þ in
dmso. Unfortunately, however, the radical complex proved highly
unstable in dry dmso-d6 (<1 mol % H2O per Rh), in which it
immediately decomposes to form a mixture of diamagnetic com-
pounds even in the absence of a base. Complex 42þ is stable for much
longer in pure acetone. The reaction in dmso-d6 does produce significant
amounts of the rhodium(I) compound 4þ, but in this solvent the reaction
is not clean and substantial amounts of other unidentified products and
free dbcot were observed (perhaps indicating a redox disproportionation
reaction). Hence, the results in dmso are mechanistically inconclusive for
the reactions in acetone.
(26) (a) Ogoshi, H.; Setsunu, J.; Yoshida, Z. J. Am. Chem. Soc. 1977,
99, 3869–3870. (b) Wayland, B. B.; Feng, Y.; Ba, S. Organometallics
1989, 8, 1438–1441.
(27) (a) Brammer, L.; Connelly, N. G.; Edwin, J.; Geiger, W. E.;
Orpen, A. G.; Sheridan, J. B. Organometallics 1988, 7, 1259–1265. (b)
Zhai, H.; Bunn, A.; Wayland, B. Chem. Commun. 2001, 1294–1295.
(c) Bunn, A. G.; Wayland, B. B. J. Am. Chem. Soc. 1992, 114, 6917–
6919.
(28) Both compounds could be oxidized by Agþ in acetone. 22þ was
prepared by oxidation with [Fe(η5-C5H4COMe)Cp]þ in CH2Cl2;
however this oxidant was too weak to oxidize 1þ. For a list of common
redox agents in organometallic chemistry see: Connelly, N. G.; Geiger,
W. E. Chem. Rev. 1996, 96, 877–910.
(38) This equilibrium lies on the side of 12þ/22þ and acetone (the
oxidation of acetone occurs over 1000 mV vs the Fc/Fcþ couple,11
which gives an energy difference between the two sides of the equilib-
rium of roughly 600-700 mV or 14-16 kcal/mol).
(29) (a) Hetterscheid, D. G. H.; Gr€utzmacher, H.; Koekoek,
A. J. J.; de Bruin, B. Prog. Inorg. Chem. 2007, 55, 247–354. (b)
Wassenaar, J.; de Bruin, B.; Siegler, M. A.; Spek, A. L.; Reek,
J. N. H.; van der Vlugt, J. I. Chem. Commun. 2010, 47, 1232–1234.
For examples of related [Rh(N3-ligand)(diolefin)]þ/2þ complexes see
also ref 11 and (c) Hetterscheid, D. G. H.; Smits, J. M. M.; de Bruin, B.
Organometallics 2004, 23, 4236–4246.
(30) (a) Hetterscheid, D. G. H.; Kaiser, J.; Reijerse, E. J.; Peters,
T. P. J.; Thewissen, S.; Blok, A. N. J.; Smits, J. M. M.; de Gelder, R.; de
Bruin, B. J. Am. Chem. Soc. 2005, 127, 1895–1905.
(31) In acetone-d6 a very slow rise of the signals of the diamagnetic
precursor could be observed for both complexes, in the case of the
rhodium complex 12þ accompanied by the rise of signals of the free
dbcot moiety. Kinetic measurements using UV/vis spectrometry re-
vealed that the decomposition of 12þ and 22þ proceeds according to a
(39) The redox potential of the -OH/ OH couple in MeCN is only
3
þ0.29 vs Fc/Fcþ (þ0.96 vs NHE): (a) Sawyer, D. T.; Roberts, J. K., Jr.
Acc. Chem. Res. 1988, 21, 469–476. (b) Calculated from the relationship
NHE - E1/2 Fc/Fcþ = 630 mV: Pavlishchuk, V. V.; Addison, A. W.
Inorg. Chim. Acta 2000, 298, 97–102.
(40) (a) Ortep-3 for Windows: Farrugia, L. J. J. Appl. Crystallogr.
1997, 30, 565. (b) Rendering was made with POV-Ray 3.6. Persistence of
Vision Raytracer (Version 3.6), Persistence of Vision Pty. Ltd., 2004.
Retrieved from http://www.povray.org/download/.
(41) Treutler, O.; Ahlrichs, R. J. Chem. Phys. 1995, 102, 346–354.
(42) PQS version 2.4; Parallel Quantum Solutions: Fayettevile, AR,
USA, 2001. The Baker optimizer is available separately from PQS upon
request. Baker, J. J. Comput. Chem. 1986, 7, 385–395.
(43) Sierka, M.; Hogekamp, A.; Ahlrichs, R. J. Chem. Phys. 2003,
118, 9136–9148.
first-order rate equation in the metal complex concentration with kobs
=
(44) (a) Becke, A. D. Phys. Rev. A 1988, 38, 3098–3100. (b) Perdew,
J. P. Phys. Rev. B 1986, 33, 8822–8824.
(45) Schaefer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992,
97, 2571–2577.
1 ꢀ 10-6 and 4 ꢀ 10-7 s-1, respectively. Addition of H2O increases the
rate of decomposition considerably, however, in the case of 12þ in an
aselective manner.
(32) The same results were obtained when exactly 1 equiv of
KOtBu was added. This base however proved to be less suitable,
because if used in excess, some follow-up unselective reactivity was
observed. Indeed, treatment of the iridium(I) compound 2þ with 1
equiv of KOtBu in THF produces a red precipitate (insoluble in all
common organic solvents) and the free bla ligand. This is most likely
due to double bla ligand deprotonation (see also ref 16), as double bpa
ligand deprotonation reactions of related [(bpa)M(cod)]þ (M =
Rh, Ir) complexes has been observed in our group. This will be
described in a separate paper.
(33) If the aminyl radical would abstract a hydrogen atom from the
solvent, one would expect that mostly deuterium would be incorporated.
Alternatively a very large kinetic isotope effect could explain the obtained
hydrogen atom; see: Huynh, M. H. V.; Meyer, T. J. Proc. Natl. Acad. Sci.
U. S. A. 2004, 101, 13138–13141.
(46) (a) g-tensor: van Lenthe, E.; van der Avoird, A.; Wormer,
P. E. S. J. Chem. Phys. 1997, 107, 2488. (b) A-tensor: van Lenthe, E.; van
der Avoird, A.; Wormer, P. E. S. J. Chem. Phys. 1998, 108, 4783.
(47) te Velde, G.; Bickelhaupt, F. M.; van Gisbergen, S. J. A.; Fonseca
Guerra, C.; Baerends, E. J.; Snijders, J. G.; Ziegler, T. J. Comput. Chem.
2001, 22, 931.
(48) (a) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988,
37, 785–789. (b) Becke, A. D. J. Chem. Phys. 1993, 98, 1372–1377.
(c) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652.
(49) Eichkorn, K.; Weigend, F.; Treutler, O.; Ahlrichs, R. Theor.
Chem. Acc. 1997, 97, 119–124.
(50) (a) Anton, D. R.; Crabtree, R. H. Organometallics 1983,
2, 621–627. (b) Singh, A.; Sharp, P. R. Inorg. Chim. Acta 2008,
361, 3159–3164.
(51) van der Ent, A.; Onderdenlinden, A. L. Inorg. Synth. 1990,
(34) Evans, C. A. Aldrichim. Acta 1979, 12, 23–29.
28, 90–91.
(35) Radical trapping of species 12þ in acetone solution using
DMPO (DMPO = 5,5-dimethyl-1-pyrroline-N-oxide) revealed forma-
tion of at least two species, A and B; however neither of these could be
assigned to a solvent radical and most probably a rhodium radical has
(52) Chaffins, S.; Brettreich, M.; Wudl, F. Synthesis 2002, 9,
1191–1194. Instead of Li sand (as described in the original procedure),
for performing the first step we used granular Li (0.5% Na) and obtained
dbcot in 15% overall yield.
1912
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Organometallics
ARTICLE
(53) de Bruin, B.; Brands, J. A.; Donners, J. J. J. M.; Donners, M. P. J.;
de Gelder, R.; Smits, J. M. M.; Gal, A. W.; Spek, A. L. Chem.—Eur. J.
1999, 5, 2921–2936.
(54) Komatsu, K.; Kikuchi, K.; Kojima, H.; Urano, Y.; Nagano, T. J.
Am. Chem. Soc. 2005, 127, 10197–10204.
(55) Bain, G. A.; Berry, J. F. J. Chem. Educ. 2008, 85, 532–536.
(56) The radical complexes [1](PF6)2, [2](PF6)2, and [4](PF6)2
are all well soluble in acetone and acetonitrile. In contrast to [1](PF6)2
and [2](PF6)2, complex [4](PF6)2 is also slightly soluble in CH2Cl2.
’ NOTE ADDED AFTER ASAP PUBLICATION
This paper was published ASAP on March 10, 2011. An entry
in Table 1 was updated. The revised paper was reposted on
March 16, 2011.
1913
dx.doi.org/10.1021/om101157r |Organometallics 2011, 30, 1902–1913