Syntheses and structures of Li, Fe, and Mo derivatives of N,N′-bis(2,6-diisopropylphenyl)-o-phenylenediamine

Article abstract of DOI:10.1039/c3dt51063h

Double deprotonation of N,N′-bis(2,6-diisopropylphenyl)-o- phenylenediamine (H₂L) with n-BuLi in THF yields the dilithium complex Li₂L(thf)₃. The reaction of Li₂L(thf) ₃ with FeBr₂(thf)₂ and subsequent replacement of the solvent with toluene yield LFe(η⁶-toluene), which can also be synthesized from Fe(HMDS)₂(thf) and H₂L in toluene. The NMR data, bond lengths obtained from an X-ray diffraction study, and DFT calculations indicate that the diamide ligand L^2- undergoes oxidization to a radical ligand L^1-. Reaction of LFe(η ⁶-toluene) with 1 atm CO yields the tricarbonyl complex LFe(CO) ₃. MoCl₄(thf)₂ reacts with two equivalents of Li₂L(thf)₃ to yield (LiL)₂MoCl ₂(thf)₄ in which the phenylene backbone of the ligand has been dearomatized. One-electron oxidation of Li₂L(thf)₃ by EuCl₃(dme)₂ yields the open-shell species LiL(OEt ₂), which was characterized by X-ray crystallography and EPR spectroscopy.

Full text of DOI:10.1039/c3dt51063h

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Syntheses and structures of Li, Fe, and Mo
derivatives of N,N’-bis(2,6-diisopropylphenyl)-
o-phenylenediamine†
Cite this: DOI: 10.1039/c3dt51063h
Trevor Janes,a Jeremy M. Rawsonb and Datong Song*a
Double deprotonation of N,N’-bis(2,6-diisopropylphenyl)-o-phenylenediamine (H2L) with n-BuLi in THF
yields the dilithium complex Li2L(thf)3. The reaction of Li2L(thf)3 with FeBr2(thf)2 and subsequent
replacement of the solvent with toluene yield LFe(η6-toluene), which can also be synthesized from
Fe(HMDS)2(thf) and H2L in toluene. The NMR data, bond lengths obtained from an X-ray diffraction study,
and DFT calculations indicate that the diamide ligand L2− undergoes oxidization to a radical ligand L1−
.
Received 22nd April 2013,
Accepted 20th May 2013
Reaction of LFe(η6-toluene) with 1 atm CO yields the tricarbonyl complex LFe(CO)3. MoCl4(thf)2 reacts
with two equivalents of Li2L(thf)3 to yield (LiL)2MoCl2(thf)4 in which the phenylene backbone of the
ligand has been dearomatized. One-electron oxidation of Li2L(thf)3 by EuCl3(dme)2 yields the open-shell
species LiL(OEt2), which was characterized by X-ray crystallography and EPR spectroscopy.
DOI: 10.1039/c3dt51063h
www.rsc.org/dalton
unsubstituted,4–11 N-monosubstituted,12–17 and N,N′-disubsti-
tuted variants of this family of ligands have been synthesized
Introduction
The study of redox-active ligands has led to clearer understand- and employed as ancillary ligands for main group18,19 and
ing of fundamental coordination chemistry1 as well as exciting transition metals.20–26
new examples of catalysis including the use of abundant first-
The 2,6-diisopropylphenyl (dipp) group has found use in
row metals.2,3 o-Phenylenediamine (pda) derivatives are classic chelating ligand design because it can provide unique sterics
examples of redox-active ligands. Scheme 1 depicts the three around the metal centre, which can stabilize coordinatively
possible redox forms of this ligand series with o-phenylenedi- unsaturated metal centres. Despite the popularity of the dipp
amide (pda2−) as its most reduced form, and o-diimino- group in ligand design and the ease of preparation of the
semiquinonate (pda1−) and o-benzoquinonediimine (pda0) as dipp-substituted pda,27 the coordination chemistry of a dipp-
intermediate and most oxidized forms, respectively. Early substituted pda ligand is underexplored compared to other
reports from Holm emerged in the 1960s on the synthesis pda derivatives. Only recently the dipp-substituted pda2−
and electrochemical behaviour of late-transition metal com- ligand has found use in the isolation of boryl anions.28 Our
plexes of the various redox forms of pda.4,5 To date the group is interested in exploring the coordination chemistry of
this ligand toward transition metals. Herein we report the syn-
thesis and characterization of the dilithium salt of this ligand,
its coordination behaviour towards Fe and Mo, as well as the
synthesis and characterization of the monolithium salt of the
corresponding dipp-substituted pda1− ligand.
Results and discussion
Scheme 1
Synthesis of dilithium complex 2
aDavenport Chemical Research Laboratories, Department of Chemistry, University of
Toronto, 80 St. George Street, Toronto, Ontario, Canada, M5S 3H6.
As shown in Scheme 2, diamine 1 can be doubly deprotonated
at −70 °C with two equiv. of n-butyl lithium in THF and the
dilithium complex 2 forms as a precipitate. The solid state
structure of 2 has been confirmed by X-ray crystallography. As
shown in Fig. 1, Li1 adopts a highly distorted tetrahedral
coordination geometry with two amido nitrogen donor atoms,
one oxygen donor atom from a terminal THF ligand, and
E-mail: dsong@chem.utoronto.ca; Fax: +1 416 978 7013; Tel: +1 416 978 7014
bDepartment of Chemistry and Biochemistry, University of Windsor, 401 Sunset
Avenue, Windsor, ON, Canada, N9B 3P4
†Electronic supplementary information (ESI) available: For crystallographic data
in CIF and further computation details. CCDC 935013–935017. For ESI and crys-
tallographic data in CIF or other electronic format see DOI: 10.1039/c3dt51063h
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peak in the solution 7Li NMR spectrum at 2.62 ppm also
suggests that the two lithium centres are equivalent in solu-
tion. The N2–C30 and N1–C25 bond lengths are 1.396(2) and
1.395(2) Å, respectively, consistent with the typical C–N single
bond in phenylenediamide complexes. The bridging coordi-
nation mode and the distorted tetrahedral local geometry of
the two nitrogen atoms also suggest that both nitrogen atoms
are sp3 hybridized amido nitrogen with two lone pairs on each.
Scheme 2 Synthesis of 2.
Fe coordination chemistry of 2
Compound 2 reacts with FeBr2(thf)2 in THF at −60 °C to afford
a navy blue solution. Our efforts to determine the identity of
the blue species have been unsuccessful. However, the
addition of toluene to the reaction mixture after the removal of
solvents yields compound 3 (Scheme 3), the formation of
which is accompanied by a rapid colour change to purple.
Compound 3 can also be synthesized by reacting 1 with
1
[Fe(HMDS)2(thf)] in toluene at elevated temperatures. In the H
NMR spectrum, three resonances at 5.41, 5.19, and 4.91 ppm
(triplet, doublet, triplet) suggest the presence of highly
shielded arene protons, consistent with an η6-toluene adduct
of iron.30,31 An X-ray diffraction study revealed the two-legged
‘piano stool’ structure of 3 (Fig. 2). The C30–N2 and C25–N1
bond lengths are 1.361(2) and 1.366(2) Å, respectively, shorter
than those in 2, but longer than a typical C–N double bond.
The C25–C30 bond length (1.419(2) Å) in 3 is shorter than that
in 2 (1.439(2) Å). The bond lengths in the phenylene backbone
reveal subtle dearomatization: the average Cβ–Cγ bond length in
3 is 1.377(2) Å, and the average Cα–Cβ bond length is 1.411(2) Å.
The metric parameters suggest that the N,N-chelating
ligand may contain radical character with the unpaired elec-
tron antiferromagnetically coupled with that of the low spin
iron(I) centre. Wieghardt and coworkers have demonstrated
that when ambiguous, the oxidation states of metal and bis o-
substituted N or O-donor ligands may be assigned based on
high-quality single crystal X-ray diffraction data.32 The metric
parameters of the chelating ligand in 2 compare well with lit-
erature examples of o-diiminosemiquinonate complexes
characterized crystallographically.15,17,20–25,33 To obtain further
information on the electronic structure of compound 2, we
carried out DFT calculations. Our calculations showed that the
model with antiferromagnetically coupled Fe(I) and open-shell
ligand is more stable than the closed shell model with Fe(0)
and a neutral diimine ligand. The spin density on the Fe
Fig. 1 Molecular structure of 2 (50% probability thermal ellipsoids). All hydro-
gen atoms and the iso-propyl groups are omitted and only one orientation of
the disordered portion is shown for clarity. Selected bond lengths (Å): C30–N2
1.396(2), C25–N1 1.395(2), C30–C25 1.439(2), C25–C26 1.399(3), C26–C27
1.400(3), C27–C28 1.384(3), C28–C29 1.397(3), C29–C30 1.394(3); selected
bond angles (°): O1–Li1–O2 114.09(16), O1–Li1–N2 122.92(17), O2–Li1–N2
106.25(15), O1–Li1–N1 128.91(17), O2–Li1–N1 99.46(15), N1–Li1–N2 78.65(12),
O3–Li2–N2 121.89(18), O3–Li2–N1 154.4(2), N1–Li2–N2 82.72(13), O3–Li2–O2
95.62(15), N2–Li2–O2 94.64(15), N1–Li2–O2 88.57(13).
another oxygen donor atom from a bridging THF ligand occu-
pying the four coordination sites. The coordination geometry
around Li2 is better described as trigonal pyramidal with N1
and N2 from the chelating ligand and O3 from the terminal
THF ligand occupying the trigonal base and O2 from the brid-
ging THF ligand occupying the apical position. Within the tri-
gonal base, the N1–Li2–O3, N1–Li2–N2, and N2–Li2–O3 angles
are 154.4(2), 82.7(1), and 121.9(2)°, respectively. The dihedral
angle between the phenylenediamide plane and the Li1–N1–
N2 plane is ∼74°, while that between the phenylenediamide
plane and the Li2–N1–N2 plane is ∼24°. The analogous mono-
meric dilithium complexes of N,N′-disilyl-o-phenylenediamide
derivatives reported by Lappert and coworkers29 have both a
three-coordinate and a four-coordinate lithium centre, with
one and two terminal THF ligands, respectively. The solution
1H NMR spectrum of 2 reveals a symmetrical molecule in solu-
tion, i.e., the protons of the dipp groups on both sides of the
1
phenylenediamide plane show only one set of H resonances.
The three THF ligands only show one set of resonances at 1.17
and 3.25 ppm, similar to those reported by Lappert. The single
Scheme 3 Two synthetic routes to 3.
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Scheme 4 Synthesis of 4.
Fig. 2 Molecular structure of 3 (50% probability thermal ellipsoids). All hydro-
gen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Fe–N1
1.880(1), Fe1–N2 1.882(1), C25–N1 1.366(2), C30–N2 1.361(2), C25–C26
1.409(2), C26–C27 1.379(2), C27–C28 1.407(2), C28–C29 1.374(2), C29–C30
1.413(2), C30–C25 1.419(2), C32–C33 1.419(2), C33–C34 1.409(3), C34–C35
1.414(3), C35–C36 1.415(2), C36–C37 1.409(3), C37–C32 1.415(2); N2–C30–
C25 113.2(1), N1–C25–C30 112.8(1), N2–Fe1–N1 82.68(5).
centre is 0.734 in the antiferromagnetically coupled model,
smaller than 1.00, which is reasonable due to covalent
bonding and orbital mixing between the metal and the
ligand.21 The simplified orbital spin density analysis (i.e., if
the spin density is lower than 0.5, the orbital is considered
empty; if the spin density is greater than 0.5, the orbital is con-
sidered occupied) showed the following electronic configur-
ation on Fe: empty dyz, singly occupied dxy, and doubly
occupied dxz, dx2−y2, and dz2 orbitals. Although the actual elec-
tronic structure is somewhere in between the two extremes,
i.e., Fe(0) with a neutral diimine ligand and Fe(I) with a mono-
Fig. 3 Molecular structure of 4 (50% probability thermal ellipsoids). All hydro-
gen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Fe1–
N2 1.914(2), Fe1–N1 1.911(2), Fe1–C31 1.799(2), C31–O1 1.132(3), C30–N2
1.353(3), C25–N1 1.354(3), C25–C26 1.414(3), C26–C27 1.367(4), C27–C28
1.416(3), C28–C29 1.369(4), C29–C30 1.414(3), C25–C30 1.425(3); N2–C30–
C25 113.2(2), N1–C25–C30 113.1(2), N1–Fe–N2 81.15(8).
in Fig. 3. The C25–N1 and C30–N2 bond lengths are 1.354(3)
anionic radical ligand, such an approximation in orbital spin and 1.353(3) Å, respectively, and are marginally shorter than
density analysis indicates that the actual structure is closer to those in 3 but not statistically different. To elucidate the elec-
tronic structure of 4, we performed DFT calculations. The
the latter extreme in the continuum. Overall, the transform-
ation from 2 to 3 involves reduction of Fe(II) to Fe(I) by the elec- spin-unrestricted and restricted models from DFT are similar
tron-rich dilithium diamide complex, accompanied by in energy. In the spin-unrestricted model, the spin density on
the Fe centre is 0.391. The simplified orbital spin density ana-
coordination of the oxidized ligand. Chirik et al. have charac-
terized a series of complexes with a general formula [ArNvC- lysis on such a structure showed the following electronic con-
(Me)C(Me)vNAr]Fe(η6-arene), via the reduction of dichloro figuration on Fe: empty dyz, and doubly occupied dxz, dxy,
d
x2−y2, and dz2 orbitals, consistent with Fe(0). Such an approxi-
[ArNvC(Me)C(Me)vNAr]Fe(II)
complexes
with
sodium
amalgam in the presence of various arenes.31 When the Ar mation indicates that the actual structure is closer to the
group in Chirik’s complexes is 2,6-diisopropylphenyl and the Fe(0) with a neutral diimine ligand extreme in the continuum.
η6-arene is toluene, the average CvN bond length is 1.359(3) Å,
Presumably the better π-acceptor ligand, CO induced fur-
which is similar to that in 3. The average C–C bond length ther electron transfer from the chelating ligand to the metal
of the η6-toluene in 3 is 1.414(3) Å, nearly identical to that in centre.
Chirik’s complex (1.412(5) Å).
Mo coordination chemistry of 2
When a THF solution of 3 is stirred under an atmosphere
of CO, the toluene ligand can be displaced to yield 4, in which
three CO ligands are coordinated to the iron centre
(Scheme 4). The diagnostic NMR data for 4 are the absence of
the shielded η6-toluene peaks from the 1H NMR spectrum and
the presence of a highly deshielded carbonyl peak at 210 ppm
in the 13C NMR spectrum. The crystal structure of 4 is shown
The reaction of 2 with 0.5 equiv. of MoCl4(thf)2 afforded
emerald green compound 5 (Scheme 5), which is diamagnetic
as evidenced by its sharp NMR spectra within the normal
chemical shift range. The 7Li NMR experiment reveals the pres-
ence of lithium in the product: one singlet at 1.37 ppm, which
is upfield shifted compared to the singlet at 2.62 ppm present
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diamide complexes (∼1.38–1.42 Å).35 The short C–N and long
C–C bond (C25–C30 1.498(6) Å) lengths are consistent with a
neutral diimine ligand.
The molybdenum centre is sandwiched between two central
C6 rings of two diimine ligands. Both C6 rings coordinate to
the molybdenum centre in an η4 fashion. Such a coordination
mode for a pda-type ligand is complementary to one mode
reported for an MoRu2 trimetallic complex by Mealli et al., in
which Mo is coordinated η4 to pda through both amide nitro-
gens and both α-carbons.36 A more similar coordination mode
has been observed in Mo–Al bimetallic complexes of the
[o-C6H4-(NSiMe3)2]2− ligand reported by Boncella and co-
workers.37 Distinct from this literature example, the η4-C6 ring
in compound 5 is far from planar; the ring folds about the
C26–C29 vector with a dihedral angle of ∼30° into a boat con-
formation, suggesting the diminishing conjugation between
the coordinating portion (C26, C27, C28 and C29) and the
remaining portion of the C6 ring. Such folding is similar to
that observed in the Mo-(η4-phenazine) type of complexes
reported by Parkin and coworkers.38 The coordination environ-
ment around the molybdenum centre in 5 is reminiscent of
the pseudo-C2 symmetric (Me3P)2Mo(η4-butadiene)2,39 in that
both compounds bear two syn diene ligands and two mutually
cis monodentate ligands, and have an overall coordination
number of six. The P–Mo–P angle in (Me3P)2Mo(η4-butadiene)2
is 98°, slightly larger than the 91° Cl–Mo–Cl angle in com-
pound 5, likely due to sterics. An additional structural para-
meter that has been used to describe butadiene ligand
geometry is Φ, the dihedral angle between the butadiene plane
and the plane containing both outer carbons and the metal
Scheme 5 Synthesis of 5.
1
in the starting material 2. In the H NMR spectrum, the con-
version of 2 to 5 is accompanied by the disappearance of
multiplets at 6.57 and 6.33 ppm corresponding to the phenylene
protons of the starting material, and the appearance of reson-
ances at 5.30, 4.88, 2.69, and 2.30 ppm hinting the vanished
aromaticity of the phenylene ring.
The solid state structure of 5 was confirmed by a single-
crystal X-ray diffraction study. As shown in Fig. 4, the molecule
is C2 symmetric with a crystallographically imposed C2 axis
bisecting the Cl–Mo–Cl′ angle, relating a pair of chloride
ligands and a pair of organic ligands, respectively. Interest-
ingly, instead of coordinating to the molybdenum centre, the
two nitrogen donor atoms of each organic ligand are coordi-
nating to a lithium centre, which is further ligated by two THF
molecules via the oxygen donor atoms. The C25–N1 and C30–
N2 bond lengths are 1.314(7) and 1.309(7) Å, respectively,
which are more similar to C–N bond lengths found in typical
diimine complexes (∼1.28 Å)34 than those found in typical
centre (Co–M–C′ ).40 In this respect, 5 (∼78°) closely resembles
o
(Me3P)2Mo(η4-butadiene)2 (∼83°).
Butadiene complexes are most often compared to the two
canonical resonance forms as shown in Chart 1, the neutral
diene (L2) on a metal and the metallacyclopentene (LX2) on a
metal formally oxidized by two electrons.41 Metallacyclo-
pentenes feature a negative Δd, where Δd = avg. M–Co bond
length – avg. M–Ci bond length. Another characteristic feature
of metallacyclopentenes is that Ci–Ci bonds are shorter than
Ci–Co bonds. In L2 butadiene complexes however, Δd is positive
and all butadiene C–C bond lengths are similar.41 The Δd para-
meter for 5 is 0.142(6) Å and the Ci–Ci (1.405(6) Å) and Ci–Co
(avg. 1.440(7) Å) bond lengths in 5 are statistically similar.
Similar η4-butadiene C–C bond lengths and the positive
Δd are indicative of an L2 butadiene complex. Similarly, in
[(Me3P)2Mo(η4-butadiene)2] the Δd (0.065 Å average of both
Fig. 4 Molecular structure of 5 (50% probability thermal ellipsoids). All hydro-
gen atoms and the isopropyl groups are omitted and only one orientation of
the disordered portion is shown for clarity. Selected bond lengths (Å) and
angles (°): Mo1–Cl1 2.522(1), Mo1–C26 2.342(5), Mo1–C27 2.214(5), Mo1–C28
2.207(6), Mo1–C29 2.363(6), C25–N1 1.314(7), C30–N2 1.309(7), N1–Li1 2.032(8),
N2–Li1 2.043(9), C25–C26 1.425(6), C26–C27 1.437(7), C27–C28 1.405(6),
C28–C29 1.444(7), C29–C30 1.428(9), C25–C30 1.498(6); Cl1–Mo–Cl1’ 90.60(4).
Chart 1 A: Labelling of inner (Ci) and outer (Co) butadiene carbons; B: two
extreme resonance forms of a generic diene complex.
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ligands) is positive, and the average Ci–Ci (1.398(5) Å) and
Ci–Co (1.413(5) Å) distances are statistically indistinguishable.
Although the actual structure of the η4-ligand in 5 still lies
between the two extreme resonance forms, all the metric para-
meters from the crystal structure of 5 suggest that the L2 reson-
ance form of the η4-ligand contributes more to the overall
structure. In the resonance structure with an L2 type η4-ligand,
the molybdenum centre has a formal oxidation state of zero.
The reaction between 2 and MoCl4(thf)2 could then be ration-
alized as the reduction of Mo(IV) to Mo(0) by 2 equiv. of 2
where the diamido ligand is oxidized into diimine.
When the reaction between 2 and MoCl4(thf)2 is quenched
within 2 h, an emerald green paramagnetic species 6 can be
isolated by a rapid crystallization. Alternatively, 6 can be syn-
thesized by reacting 1 with EuCl3(dme)2. The crystal structure
of 6 is shown in Fig. 5. The molecule of 6 has a crystallographi-
cally imposed C2 symmetry along the Li1–O1 direction. The
lithium centre adopts a trigonal planar coordination geometry
with two nitrogen donor atoms from the chelating ligand and
an oxygen donor atom from an ether ligand occupying the
three coordination sites. The N,N-chelate ligand has a net
charge of −1, i.e., a radical bound to a relatively innocent Li
ion. Interestingly, the N1–C13 and C14–C15 bond lengths
(1.341(3) and 1.364(4) Å, respectively) are shorter than their
counterparts in 3, while the C13–C14 and C13–C13′ bond
lengths (1.429(3) and 1.471(5) Å, respectively) are longer than
their counterparts in 3. The C15–C15′ bond length (1.400(5) Å)
is similar to its counterpart in 3.
Fig. 6 EPR spectra of 6 in the solid state (top) and in toluene solution (bottom)
at ambient temperature.
toluene at room temperature revealed a multiplet pattern con-
sistent with delocalisation over the phenylenediamine frame-
work as well as the Li atom. The observation of hyperfine
7
coupling to Li is consistent with ion-pairing in solution and
retention of the molecular structure determined from X-ray
EPR data for 6
diffraction. In contrast, the Zn complex of the open-shell neo-
The solid state EPR spectrum of compound 6 (Fig. 6) revealed
a sharp singlet (ΔH = 2.8 G, with a Lorentzian profile) at g =
2.0030, close to the free electron value of 2.0036 consistent
with an organic radical with little anisotropy and where spin–
orbit coupling is negligible. The deep green solution of 6 in
pentyl-substituted phenylenediamide derivative showed
a
broad EPR signal at room temperature in a THF solution with
unresolved hyperfine coupling.33
The spin density distribution based on coupling to two
chemically equivalent 14N nuclei and two pairs of chemically
1
7
distinct H nuclei (along with Li coupling) is consistent with
a π-delocalised radical based on the ortho-phenylene frame-
work and in good agreement with the computationally deter-
mined spin distribution. An estimate of the spin density
distribution at C can be made based on McConnell’s empirical
relationship42 (aH = QCH
ρC where QCH = 22.5 G). Whilst similar
relationships are proposed to exist for N (aN = QNρN and QN
=
26.9 G),43 the behaviour for N appears more complex.43 Never-
theless the EPR data are consistent with a ligand-based
π-radical (Fig. 7).
Conclusion
A dilithium complex (2) of N,N′-bis(2,6-diisopropylphenyl)-
o-phenylenediamide was synthesized and its coordination
behaviour towards halides of iron and molybdenum was inves-
tigated. Rather than forming mid-valent transition metal
complexes with π-donating diamido ligands, the dilithium
diamide complex 2 reduced both iron and molybdenum. In
Fig. 5 Molecular structure of 6 (30% probability thermal ellipsoids). Hydrogen
atoms are omitted and only one orientation of the disordered portions is shown
for clarity. Selected bond lengths (Å) and angles (°): Li1–O1 1.882(7), Li1–N1
1.974(6), N1–C13 1.341(3), C13–C13’ 1.478(5), C13–C14 1.422(4), C14–C15
1.364(4), C15–C15’ 1.400(6); O1–Li1–N1 136.7(2), N1–Li1–N1’ 86.7(3), C13–N1–
C7 117.8(2), C13–N1–Li1 109.4(2), C7–N1–Li1 132.9(2).
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Li2L(thf)3, 2
N,N′-Bis(2,6-diisopropylphenyl)-o-phenylenediamine (0.50 g,
1.2 mmol) was dissolved in THF (5 mL) and cooled to −70 °C.
A hexanes solution of n-butyl lithium (1.5 mL, 1.6 M) was
added dropwise. The reaction mixture was removed from the
cooling bath and stirred for 90 min, during which a precipitate
formed. Hexane (10 mL) was added and the mixture cooled to
−25 °C. The precipitate was collected on a frit and was washed
with cold hexanes. Drying in vacuo left a light yellow powder
(0.528 g, 67%), which was stored at −25 °C. Not only is this
material extremely air-sensitive, but it also decomposes under
prolonged vacuum at room temperature. Consequently no satis-
factory analytical data were obtained. Single crystals were
obtained from a C6D6 solution sitting at room temperature for
days. 1H NMR (400 MHz, 25 °C, C6D6) δ 7.39 (d, J = 7.6 Hz,
4H), 7.23 (t, J = 7.6 Hz, 2H), 6.57 (m, 2H), 6.33 (m, 2H), 3.43
(sept, J = 7.1 Hz, 4H), 3.25 (m, 12H), 1.38 (d, J = 6.8 Hz, 12H),
Fig. 7 Singly occupied molecular orbital (left) for model compound
[C6H4(NPh)2LiOMe2] (B3LYP/6-311G**) and estimated (right, top) and com-
puted (B3LYP/6-31G*) spin density (right bottom) for 6.
1.29 (d,
J = 7
Hz, 12H), 1.17 (m, 12H). 13C{1H}NMR
the case of Fe, the addition of external π-acceptor ligands,
toluene and CO, facilitated the formation and isolation of low-
valent Fe complexes 3 and 4, respectively. In the case of Mo,
the C6 ring of the resulting pda0 ligand lost its planarity and
acted as a π-acceptor ligand that coordinated to the resulting
low-valent Mo centre in an η4 fashion to form 5. The 1e− oxi-
dation of 2 by Mo(IV) or Eu(III) yielded complex 6 featuring an
open-shell ligand supported by a lithium cation. Complex 6
showed typical ligand-based radical character with delocalized
spin density. The coordination chemistry of 2 towards other
metals is underway in our laboratory.
(100.58 MHz, C6D6, 25 °C) δ 153.56, 148.72, 144.36, 121.20,
115.02, 112.14, 68.86, 29.92, 25.78, 25.69, 25.23. 7Li NMR
(233 MHz, 25 °C, C6D6): δ 2.62.
LFe(η6-toluene), 3, method A
Li2L(thf)3 (0.10 g, 0.15 mmol) was dissolved in THF (2 mL)
and cooled to an internal temperature of −60 °C. The resulting
yellow solution was added smoothly to a −60 °C suspension of
FeBr2(thf)2 (0.055 g, 0.15 mmol) in THF (1 mL). The resulting
mixture was allowed to warm to room temperature with stirring
for 24 h. Volatiles were removed under vacuum, leaving a navy
blue oil. The addition of toluene (3 mL) resulted in a purple
solution which was stirred for 1 h. The mixture was filtered
through Celite and dried under vacuum. The residue was dis-
solved in hexanes (1 mL) and cooled to −25 °C, resulting in
the precipitation of 3. The supernatant was decanted, leaving a
purple solid (0.054 g, 61%) which was dried in vacuo. Trace
LiBr could be removed by recrystallization from hexanes. NMR
data for 3 obtained this way are identical to those obtained by
method B.
Experimental section
General considerations
All operations were performed using standard Schlenk tech-
niques under a nitrogen atmosphere, or in a nitrogen-filled
MBraun
glovebox.
FeBr2(thf)2,44
[Fe(HMDS)2(thf)],45
MoCl4(thf)2,46 EuCl3(dme)2,47 and 127 were prepared according
to literature procedures. Glassware was either flame-dried or
dried overnight in a 160 °C oven prior to use except for NMR
tubes which were dried overnight in a 60 °C oven. Carbon
LFe(η6-toluene), 3, method B
monoxide was purchased from Linde and used without further Solid H2L (0.47 g, 1.1 mmol) was added to a solution of Fe
purification, n-BuLi (1.6 M in hexanes) was purchased from (HMDS)2(thf) (0.45 g, 1.0 mmol) in toluene (15 mL). The
Aldrich. THF, toluene, ether, and benzene-d6 were dried over mixture quickly turned purple and was refluxed for 16 h. After
Na/benzophenone, distilled under nitrogen, and stored over cooling to room temperature, volatiles were removed under
activated molecular sieves. Room temperature solid state and vacuum at 50 °C, leaving a purple solid which was dissolved in
toluene solution EPR spectra of 6 were recorded on a Bruker hexanes and filtered through Celite. The filtrate was concen-
EMXplus X-band EPR spectrometer and spectra simulated trated and cooled to −25 °C, which caused formation of purple
using Winsim.48 The 1H, 7Li, and 13C NMR spectra were crystals which were collected on a frit and washed with cold
recorded on either a Varian 400 MHz, an Agilent 500 MHz, or hexanes, yielding 3 (0.29 g, 0.50 mmol, 50%). Single-crystals
an Agilent 600 MHz NMR spectrometer. All chemical shifts are suitable for XRD studies were grown by slow evaporation of a
reported in ppm relative to the residual protio-solvent peaks; hexanes solution at room temperature. Anal. Calcd for
7Li NMR spectra are referenced externally using 9.7 M LiCl in C37H46N2Fe: C, 77.34; H, 8.07; N, 4.88. Found: C, 77.09;
1
D2O. FT-IR spectra were recorded on a Perkin Elmer Spectrum H, 7.94; N, 4.88. H NMR (500 MHz, 25 °C, C6D6): δ 7.49 (t, J =
One FT-IR spectrometer. Elemental analysis was performed by 7.5 Hz, 2H), 7.41 (d, J = 7.5 Hz, 4H), 6.60 (m, 2H), 6.44 (m, 2H),
ANALEST at the University of Toronto.
5.41 (t, J = 5.7 Hz, 1H), 5.19 (d, J = 6.1 Hz, 2H), 4.91 (t, J = 6 Hz,
Dalton Trans.
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2H), 3.28 (sept, J = 6.9 Hz, 4H), 2.10 (s, 3H), 1.46 (d, J = 6.9 Hz, LiL(Et2O), 6
12H), 0.88 (d, J = 6.9 Hz, 12H). 13C{1H} NMR (125.72 MHz,
6 was first isolated as a minor product in the reaction of
MoCl4(thf)2 with two equivalents of Li2L(thf)3 when the reac-
tion time was shortened from 18 h to 2 h. However, 6 could be
synthesized more reliably and in better yield from the reaction
of Li2L(thf)3 with EuCl3(dme)2. In a glove box cold well, a solu-
tion of Li2L(thf)3 (69 mg, 0.11 mmol) in THF (2 mL) was
cooled to an internal temperature of −70 °C. This solution was
25 °C, C6D6): δ 153.62, 152.01, 143.84, 126.10, 124.20, 119.14,
115.17, 97.58, 83.43, 82.77, 81.37, 28.72, 26.46, 24.74, 19.81.
LFe(CO)3, 4
LFe(η6-toluene) (70 mg, 0.12 mmol) was dissolved in THF
(2 mL) in a Schlenk bomb. The solution was degassed before
one atmosphere of carbon monoxide was introduced. The solu-
tion was stirred for 16 h at room temperature. The solvent was
removed under vacuum and the purple residue was extracted
into hexanes and filtered through Celite. Concentration of the
filtrate and storage at −25 °C yielded 4 as purple crystals. The
supernatant was decanted and the solid dried in vacuo.
(22 mg, 32%). Single crystals of 4 were grown by slow evapor-
ation of a room temperature hexanes solution. Anal. Calcd for
C33H38N2O3Fe: C, 69.96; H, 6.76; N, 4.94. Found: C, 70.01; H,
smoothly added to
a similarly cooled suspension of
EuCl3(dme)2 (46 mg, 0.11 mmol) in THF (2 mL). The mixture
turned emerald green instantly, was allowed to warm to 28 °C,
and was then stirred for 18 h. Volatiles were removed in vacuo,
ether (5 mL) was added, and the mixture was filtered. After
removal of ether and recrystallization from cold pentane, green
crystals of LiL(Et2O) (18 mg, 34%) were isolated, and stored at
−25 °C. Anal. Calcd for C34H48N2OLi: C, 80.43; H, 9.53; N,
5.52. Found: C, 79.78; H, 9.46; N, 5.49.
1
6.97, N, 4.91. IR(nujol, cm−1) νCO: 2049, 1985, 1977. H NMR
X-ray crystallography
(600 MHz, 25 °C, C6D6): δ 7.29 (t, J = 7.9 Hz, 2H), 7.22 (d, J =
7.7 Hz, 4H), 6.62 (m, 4H), 2.86 (sept, J = 7.1 Hz, 4H), 1.34 (d,
J = 7 Hz, 12H), 0.94 (d, J = 6.7 Hz, 12H). 13C{1H} NMR
(150.9 MHz, C6D6, 25 °C): δ 209.99, 155.00, 150.73, 142.42,
128.04, 124.42, 123.31, 117.20, 28.62, 25.62, 24.69.
All crystals were mounted on the tip of a MiTeGen MicroMount
and the single-crystal X-ray diffraction data were collected on a
Bruker Kappa Apex II diffractometer. All data were collected
with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å)
at 150 K controlled by an Oxford Cryostream 700 series low
temperature system. The diffraction data were processed with
the Bruker Apex 2 software package.49 Raw data were processed
and multiscan absorption corrections were applied using the
Bruker Apex 2 software package.49 All structures were solved by
the direct methods and refined using SHELXTL V6.14.50 Com-
pounds 2, 3, 4, 5 crystallized in the monoclinic space groups
P21, P21/n, P21/c, and C2/c, respectively, while 6 crystallized in
the tetragonal space group P43212. The disordered THF ligand
in 5 and the ether ligand and isopropyl groups in 6 were mod-
elled successfully. The residual diffuse electron density from
disordered, unidentified solvent molecules in the lattice of 5
was removed with the SQUEEZE function of the PLATON
program51 and their contributions were not included in the
formula. All non-hydrogen atoms were refined anisotropically,
except for the disordered portions. In all structures hydrogen
atoms bonded to carbon atoms were included in calculated
positions and treated as riding atoms. The crystallographic
data are summarized in Table 1. Selected bond lengths of the
N,N-chelating ligand in 2–6 are listed in Table 2, where Cα, Cβ,
and Cγ are defined in Chart 2.
(LiL)2MoCl2(thf)4, 5
To a suspension of MoCl4(thf)2 (36 mg, 0.095 mmol) in THF
(1 mL) which was precooled to −60 °C was slowly added a solu-
tion of Li2L(thf)3 (0.125 g, 0.19 mmol) in THF (2 mL) which
was also precooled to −60 °C. The mixture immediately turned
emerald green and was removed from the cold well and stirred
for 18 h at ambient temperature. Volatiles were removed
in vacuo and toluene (4 mL) was added. The solution was filtered
through Celite and dried under vacuum. The residue was
recrystallized from a mixture of THF and hexanes at −25 °C to
yield green crystals, which were collected by vacuum filtration
and dried under vacuum yielding 5 as a green powder (61 mg,
56%). Single crystals for XRD studies were grown by cooling a
THF solution to −25 °C. 1H NMR (400 MHz, 25 °C, C6D6) δ
7.33 (dd, J = 7.6 Hz, J = 1.4 Hz, 2H), 7.24 (dd J = 5.6 Hz, J = 1.4
Hz, 2H), 7.22 (dd, J = 5.6 Hz J = 1.6 Hz, 2H), 7.15 (dd over-
lapped with solvent peak, J = 1.8 Hz, 2H), 7.10 (td, J = 7.6 Hz
J = 2.9 Hz, 4H), 5.30 (td, J = 6.8 Hz, J = 1.4 Hz, 2H), 4.88 (dd, J =
6.8 Hz, J = 2.4 Hz, 2H), 4.20 (sept, J = 6.8 Hz, 2H), 3.40 (m, 6H),
3.31 (m, 16H), 2.69 (dt, J = 7.4 Hz, J = 2.6 Hz, 2H), 2.30 (dd, J =
DFT calculations
6.6 Hz, J = 1.0 Hz, 2H), 1.90 (d, J = 6.7 Hz, 6H), 1.51 (d, J = 6.8 All DFT calculations were performed with B3LYP func-
Hz. 6H), 1.43 (d, J = 6.8 Hz, 6H), 1.30 (m, 18H), 1.17 (d, J = 6.8 tionals52,53 using the Gaussian 09 software package54 and
Hz, 6H), 1.04 (d, J = 6.8 Hz, 6H). 13C{1H} NMR (100.58 MHz, NBO55 analysis was performed on the optimized structures.
C6D6, 25 °C): 167.91, 164.86, 147.03, 146.61, 142.32, 141.92, The 6-311G** basis set was used for calculations of 3 and 4. All
140.20, 139.80, 124.95, 124.70, 124.07, 123.87, 123.70, 123.59, geometry optimizations were performed starting from X-ray
86.76, 86.38, 76.56, 68.59, 61.12, 28.94, 28.78, 28.17, 27.78, crystal structures. Both the 6-311G** and 6-31G* basis sets
26.22, 25.65, 25.05, 24.83, 24.76, 24.73, 24.55, 24.11, 23.07, were used for the model complex simplified from compound 6
14.69. 14.62. 7Li NMR (233 MHz, 25 °C, C6D6): δ 1.37. Anal. for spin density distribution. The coordinates of the model
Calcd for C76H108N4O4MoLi2Cl2: C, 69.02; H, 8.23; N, 4.23. compound were derived from the crystal structure of 6 without
Found: C, 68.78; H, 8.00; N, 4.41.
geometry optimization.
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Table 1 Crystallographic data
2
3
4
5
6
Formula
FW
T (K)
Space group
a (Å)
b (Å)
c (Å)
C
42H62Li2N2O3
C37H46FeN2
574.61
150(2)
C33H38FeN2O3
566.50
150(2)
C76H108Cl2Li2MoN4O4
1494.81
150(2)
C34H48LiN2O
507.68
150(2)
656.82
150(2)
P21
10.9983(4)
17.1078(7)
11.1127(4)
90
P21/n
P21/c
C2/c
P43212
10.5211(6)
17.3327(11)
17.7438(17)
90
16.3433(8)
11.3928(6)
16.7688(8)
90
21.8154(18)
25.039(3)
16.3773(13)
90
10.4120(5)
10.4120(5)
29.2952(15)
90
α (°)
β (°)
110.887(2)
90
1953.52(13)
2
1.117
0.068
91.243(4)
90
3235.0(4)
4
1.180
0.493
106.838(2)
90
2988.4(3)
4
1.259
0.539
118.185(4)
90
7885.3(14)
4
1.114
0.280
90
90
3175.9(3)
4
1.062
γ (°)
V (Å3)
Z
Dc (g cm−3
)
μ (mm−1
)
0.062
No. reflns collcd
No. indept reflns
GOF on F2
11 742
3461
1.027
R1 = 0.0296
wR2 = 0.0779
R1 = 0.0310
wR2 = 0.0791
28 548
7423
1.020
R1 = 0.0350
wR2 = 0.0855
R1 = 0.0493
wR2 = 0.0927
60 497
9186
1.005
R1 = 0.0570
wR2 = 0.1111
R1 = 0.1224
wR2 = 0.1328
23 017
6890
1.061
R1 = 0.0820
wR2 = 0.1700
R1 = 0.1277
wR2 = 0.1829
28 934
2193
1.016
R1 = 0.0587
wR2 = 0.1633
R1 = 0.0823
wR2 = 0.1855
R [I > 2σ (I)]
R (all data)
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Table 2 Selected bond lengths (Å) of the chelating ligand
2
3
4
5
6
Cα–N
1.395(2)
1.396(2)
1.439(2)
1.399(3)
1.394(3)
1.400(3)
1.397(3)
1.384(3)
1.3663(19)
1.3605(19)
1.419(2)
1.409(2)
1.413(2)
1.379(2)
1.374(2)
1.407(2)
1.353(3)
1.354(3)
1.425(3)
1.414(3)
1.414(3)
1.366(3)
1.369(3)
1.417(4)
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1.310(6)
1.498(7)
1.425(7)
1.428(6)
1.437(7)
1.444(7)
1.404(7)
1.341(3)
Cα–Cα
Cα–Cβ
1.478(5)
1.422(4)
Cβ–Cγ
Cγ–Cγ
1.364(4)
1.400(6)
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Acknowledgements
We thank NSERC, the Canadian Foundation for Innovation
and the Ontario Research Fund for funding. We also acknow-
ledge Digital Specialty Chemicals for the gift of Pt-Bu3.
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