Cyclopentadienyl chromium diimine and pyridine-imine complexes: Ligand-based radicals and metal-based redox chemistry

Article abstract of DOI:10.1039/c2dt30160a

Paramagnetic CpCr(iii) complexes with antiferromagnetically-coupled anionic radical diimine and pyridine-imine ligands were prepared and characterized. The diimine chloro CpCr[(ArNCR)₂]Cl complexes (1: Ar = 2,6-iPr ₂C₆H

Full text of DOI:10.1039/c2dt30160a

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PAPER
Cyclopentadienyl chromium diimine and pyridine-imine complexes:
ligand-based radicals and metal-based redox chemistry†
Wen Zhou,a Linus Chiang,b Brian O. Patrick,c Tim Storr*b and Kevin M. Smith*a
Received 20th January 2012, Accepted 16th April 2012
DOI: 10.1039/c2dt30160a
Paramagnetic CpCr(III) complexes with antiferromagnetically-coupled anionic radical diimine and
pyridine-imine ligands were prepared and characterized. The diimine chloro CpCr[(ArNCR)2]Cl
complexes (1: Ar = 2,6-iPr2C6H3 (Dpp), R = H; 2: Ar = 2,6-Me2C6H3 (Xyl), R = Me; 3: Ar = 2,4,6-
Me3C6H2 (Mes), R = Me) were synthesized by treatment of previously reported Cr(diimine)(THF)2Cl2
precursors with NaCp. Reduction of 1 with Zn gives CpCr[(DppNCH)2], 4, resulting from reduction of
Cr(III) to Cr(II) with retention of the ligand-based radical. Alkoxide complexes CpCr[(DppNCH)2]-
(OCR2R′) (5: R = Me, R′ = Ph; 6: R = iPr, R′ = H) were synthesized by protonolysis of Cp2Cr with
HOCR2R′ in the presence of the neutral diimine and catalytic base. The corresponding radical pyridine-
imine complexes CpCr(PyCHNMes)Cl (9), CpCr(PyCHNMes) (8), and CpCr(PyCHNMes)(OCMe2Ph)
(11), were prepared by analogous routes. Oxidation of 8 with iodine gives CpCr(PyCHNMes)I (10) where
oxidation of Cr(II) to Cr(III) again occurs with retention of the anionic pyridine-imine radical ligand. The
molecular structures of complexes 1, 2, 4–8, 10 and 11 were determined by single-crystal X-ray
diffraction. Unusual low energy bands were observed in the UV-vis spectra of the reported complexes,
with particularly strong transitions observed for the Cr(II) complexes 4 and 8. The electronic structure of
pyridine-imine complexes 8 and 10 were investigated by theoretical calculations.
Alternatively, these ligands have been used to impart redox
activity via ligand-based electron transfer, while the coordinated
metal center maintains a constant oxidation state.8
Introduction
The tendency of certain classes of ligands to accept varying
numbers of electrons from metals has long been recognized.1
This “redox noninnocent behaviour” is particularly marked for
first-row transition metals due to the energetic proximity of
ligand and metal frontier electronic levels.2 The development of
highly active C–C bond forming catalysts for polymerization3
and organic synthesis4 using α-diimine, pyridine-imine, and 2,6-
diiminepyridine ancillary ligands has naturally led to a resur-
gence of interest in these complexes.5
As our understanding of the connection between the electronic
structure of such complexes and their reactivity has improved,6
different strategies have been developed for harnessing imines as
ancillary ligands. Redox noninnocence has been employed to
discourage radical reactivity in first row transition metal com-
plexes in favour of more familiar two-electron reactivity modes.7
Inert octahedral Cr(III) complexes display effective antiferro-
magnetic coupling between the unpaired d-electrons and ligand
based radicals, such as semiquinonates.2 Over 30 years ago, tom
Dieck prepared a neutral chromium bis(α-diimine) complex as a
pre-catalyst for isoprene dimerization by reaction of Cr(acac)3
with 2 equiv of the neutral diimine and 3 equiv of sodium
metal.9 Most of the recent syntheses of chromium α-diimine
complexes follow this route, reacting the neutral α-diimine with
a reducing metal and a suitable chromium precursor, typically
CrCl2, CrCl3, or CrCl3(THF)3.10 An interesting alternative syn-
thetic strategy employed by Theopold and co-workers uses Cr(II)
as the reducing agent by reacting the neutral α-diimine with
CrCl2 in THF.11 The resulting Cr(α-diimine)(THF)2Cl2 com-
plexes have a reduced radical anionic diimine ligand antiferro-
magnetically coupled to the Cr(III) center, and bear a striking
structural resemblance to octahedral Cr(LX)(THF)2Cl2 Cr(III)
complexes, where LX is a β-diketiminate or related non-radical
bidentate monoanionic ligand,12,13 following Green’s nomen-
clature for neutral (L) and anionic (X) donor atoms.14
aDepartment of Chemistry, University of British Columbia Okanagan,
3333 University Way, Kelowna, BC, Canada V1V 1V7. E-mail:
kevin.m.smith@ubc.ca
bDepartment of Chemistry, Simon Fraser University, Burnaby, BC,
Canada V5A 1S6. E-mail: tim_storr@sfu.ca
cDepartment of Chemistry, University of British Columbia, Vancouver,
BC, Canada V6T 1Z1
Well-defined CpCr(LX) complexes with β-diketiminate
ligands can be used to control the reactivity of carbon-based rad-
icals via reversible Cr(III)–alkyl bond formation and homolysis.15
We were interested in using ligand-based radicals for this single
electron chemistry, in order to facilitate metal-mediated radical
reactivity rather than prevent it. Despite their superficial
†Electronic supplementary information (ESI) available: Complete com-
putational data, UV-vis spectra of complexes 1–11 and crystallographic
data for complexes 1, 2, 4–8, 10, 11. CCDC 864739–864747. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c2dt30160a
7920 | Dalton Trans., 2012, 41, 7920–7930
This journal is © The Royal Society of Chemistry 2012

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structural similarities to β-diketiminate ligands, it was unclear if
chromium-bound diimine ligand radicals would support the pre-
viously observed Cr(II)/Cr(III) chemistry, or if ligand-based elec-
tron transfer would occur instead. If the desired metal-based
redox process was retained, α-diimine and pyridine-imine radical
anionic ligands could provide access to CpCr(LX) complexes
with enhanced reactivity. The direct reaction of Cr(II) precursors
with neutral α-diimine ligands provided a potentially general and
direct synthetic route to mixed-ligand Cr(III) complexes bearing
anionic radical ligands.
The alternative electronic structure consistent with the exper-
imental magnetic moments is Cr(III) (d3, S = 3/2) with strong
antiferromagnetic coupling to an anionic ligand-based
radical.11,19
The molecular structures of 1 and 2 were determined by single
crystal X-ray diffraction, and are shown in Fig. 1. The X-ray
structures display bond lengths for the diimine ligands and struc-
tural parameters at the Cr centers consistent with radical anionic
ligands bound to Cr(III). Since CpCr(acac)Br was synthesized
over half a century ago,20 related Cr(III) mono-halide complexes
bearing both cyclopentadienyl and bidentate monoanionic
ligands have been synthesized,16–18,21 including recently
reported examples as olefin polymerization catalyst precursors.22
The structures of 1 and 2 are analogous to the related Cr(III)
β-diketiminate species,16–18 although the decreased bite angles in
1 and 2 and the absence of backbone methyl groups in 1 both
lead to reduced steric conflict between the bulky N-aryl substitu-
ents and the chloro ligand. The degree of steric repulsion in
CpCr(III) β-diketiminate halide complexes has an apparent
inverse relationship with the rates of oxidative addition of alkyl
halides of the corresponding Cr(II) precursors.23 Significantly,
the α-diimine C–C bond lengths (1.387 Å for 1 and 1.397 Å for
2) and C–N bond lengths (between 1.339 Å and 1.349 Å for
both complexes) are all in the narrow ranges reported by Theo-
pold and co-workers for anionic radical diimine ligands bound to
Cr(II) and Cr(III).11
Results and discussion
Synthesis of CpCr(α-diimine)Cl complexes
The new CpCr(α-diimine)Cl complexes were prepared by the
two step synthetic route illustrated in eqn (1). Octahedral Cr(III)
intermediates bearing ligand centered radicals were generated as
previously reported by Theopold and co-workers,11 by adding a
neutral diimine ligand to a suspension of CrCl2 in THF. Treat-
ment of the brown dichloro intermediates with NaCp followed
by recrystallization from hexanes gave the CpCr[(ArNCR)2]Cl
complexes (1: Ar = 2,6-iPr2C6H3 (Dpp), R = H; 2: Ar = 2,6-
Me2C6H3 (Xyl), R = Me; 3: Ar = 2,4,6-Me3C6H2 (Mes), R =
Me) in moderate yields (eqn (1)).
ð1Þ
Reduction of CpCr[(DppNCH)2]Cl
Chemical reduction of 1 could potentially reduce either the
metal center from Cr(III) to Cr(II), the radical anionic ligand to
the spin-paired dianionic enediamide, or both. As shown in eqn
(2), reaction of CpCr[(DppNCH)2]Cl with Zn in THF followed
by recrystallization from hexanes gives CpCr[(DppNCH)2], (4),
which has a magnetic moment of 3.78 μB (Evans).
The UV-vis spectra of complexes 1–3 were similar to the cor-
responding Cr(III) chloro complexes with β-diketiminate ligands,
CpCr[(ArNCMe)2CH]Cl with Ar = Dpp,16 Xyl,17 or Mes,18 con-
sisting of a high intensity peak between 430 nm and 410 nm,
and a lower intensity peak at ∼575 nm. However, complexes 2
and 3 also exhibit an additional low intensity peak at ∼650 nm
not observed in the β-diketiminate complexes.
The three CpCr(α-diimine)Cl complexes have magnetic
moments between 2.72 and 2.95 μB (Evans), corresponding to a
S = 1 spin state. This is consistent with a paramagnetic low spin
Cr(II) d4 center with two unpaired electrons, although Wieghardt
and co-workers have recently suggested that low-spin Cr(II) com-
pounds may be much rarer than had previously been thought.19
ð2Þ
The X-ray crystal structure of 4 (Fig. 2) is quite similar to the
known CpCr(II) β-diketiminate complex CpCr[(DppNCH)2CH].16
Fig. 1 Thermal ellipsoid diagrams (50%) of 1 (left) and 2 (right). For 1, the atoms designated with an additional “a” in the atom labels are at equival-
ent position (x, y, 1/2 − z). Selected bond lengths (Å) and angles (°) for 1: Cr(1)–N(1) = 1.966(1); N(1)–C(1) = 1.339(2); C(1)–C(1a) = 1.387(3); Cr
(1)–Cl(1) = 2.2779(6); N(1)–Cr(1)–Cl(1) = 93.73(4); N(1)–Cr(1)–N(1a) = 81.55(7). Selected bond lengths (Å) and angles (°) for 2: Cr(01)–N(1) =
1.962(3); Cr(01)–N(2) = 1.962(3); N(1)–C(2) = 1.346(5); N(2)–C(3) = 1.348(5); C(2)–C(3) = 1.396(5); Cr(01)–Cl(1) = 2.301(1); N(1)–Cr(01)–Cl(1)
= 96.70(9); N(2)–Cr(01)–Cl(1) = 97.10(9); N(1)–Cr(01)–N(2) = 80.1(1).
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Fig. 2 Thermal ellipsoid diagram (50%) of 4. Only one of the six inde-
pendent molecules in the unit cell is shown. Selected bond lengths (Å)
and angles (°): Cr(1)–N(01) = 1.933(3); Cr(1)–N(02) = 1.934(3);
N(01)–C(25) = 1.362(4); N(02)–C(26) = 1.357(4); C(25)–C(26) = 1.356
(4); N(01)–Cr(1)–N(02) = 81.3(1).
Fig. 3 Thermal ellipsoid diagrams (50%) of 5 (left) and 6 (right). Only
one orientation of the disordered OCHiPr2 ligand in 6 is shown. Selected
bond lengths (Å) and angles (°) for 5: Cr(01)–N(001) = 1.990(1);
Cr(01)–N(002) = 1.994(1); N(001)–C(14) = 1.339(2); N(002)–C(13) =
1.341(2); C(13)–C(14) = 1.386(2); Cr(01)–O(001) = 1.886(1); N(001)–
Cr(01)–O(001) = 96.04(5); N(001)–Cr(01)–N(002) = 80.88(5); N(002)–
Cr(01)–O(001) = 93.61(4). Selected bond lengths (Å) and angles (°) for
6: Cr(01)–N(001) = 1.978(1); Cr(01)–N(002) = 1.985(1); N(001)–C(2)
= 1.344(2); N(002)–C(1) = 1.343(2); C(1)–C(2) = 1.386(2); Cr(01)–
O(1) = 102.2(1); N(001)–Cr(01)–O(1) = 102.2(1); N(001)–Cr(01)–
N(002) = 81.17(6); N(002)–Cr(01)–O(1) = 92.8(1).
The six independent molecules of 4 in the unit cell have average
C–C (1.358 Å) and C–N (1.361 Å) bond lengths that are again
consistent with a radical diimine ligand.11 The UV-vis spectrum
of 4 displays a very strong absorbance at 670 nm that is not
observed in related CpCr(II) β-diketiminate compounds. This
prominent feature is convenient for spectroscopic monitoring of
single-electron transfer reactions such as the chemical oxidation
with PbCl2 that cleanly converts 4 back to the Cr(III) chloride 1
(see ESI†). The conservation of the ligand-based diimine radical
as the oxidation state of the chromium is altered has previously
been noted,11 and is in contrast to the reactivity observed for
iron-bound pyridinediimine where redox processes can involve
both the metal and the ligand.24
The utility of catalytic base in the protonolysis of Cp2Cr was
also extended to anilido imine ligands.33 The synthesis of CpCr
[DppNCH(C6H4)NTol] (7) (Tol = 4-MeC6H4) was achieved by
conducting the protonolysis at room temp with 22 mol% DBU
as catalytic base (eqn (4)). The first Cr(III) anilido imine com-
plexes were recently reported by Xu, Mu and co-workers.13
Single crystal X-ray diffraction of 7 showed a similar structure to
analogous mixed aryl β-diketiminate CpCr(II) complexes, which
were demonstrated to undergo faster single-electron oxidative
addition reactions with MeI compared to the symmetric ortho
disubstituted β-diketiminate derivatives.23
Synthesis of CpCr[(DppNCH)2](OR) by protonolysis of Cp2Cr
Protonolysis reactions of Cp2Cr are a useful route to new well-
defined monomeric monocyclopentadienyl chromium deriva-
tives.25 Although chromocene reacts with tBuOH at elevated
temperatures to give [CpCr(μ-OCMe3)]2,26 the stable Cr2(OR)2
core remains intact even upon oxidation to Cr(III).27 This renders
[CpCr(μ-OCMe3)]2 unsuitable as a precursor to monomeric
Cr–OR complexes. The synthesis of CpCr[(DppNCH)2]-
(OCR2R′) 5 (R = Me, R′ = Ph) and 6 (R = iPr, R′ = H) as shown
in eqn (3) was initially attempted to intercept a monomeric
CpCr(OR) intermediate in the protonolysis of Cp2Cr. The opti-
mized protonolysis reaction to give 5 and 6 proceeds in good
yields at room temperature, but the precise mechanism remains
unclear. The accelerating effect observed upon addition of a cata-
lytic amount of base, as seen in other protonolysis reactions,28
suggests initial attack of an anionic ligand at Cp2Cr to aid in
ð4Þ
The structures of the Cr(III) alkoxide complexes 5 and 6 were
determined by X-ray diffraction and are shown in Fig. 3. As in
Cr(III) chloride 1, the average C–C (1.386 Å) and C–N (1.343 Å)
bond lengths of the α-diimine groups are consistent with a
radical anionic ligand.11 Interestingly, Cr(II) complex 4 does not
react with dicumyl peroxide at room temp to give 5, presumably
due to the steric bulk of the NDpp substituents hindering the
inner sphere electron transfer process.
cyclopentadienyl ligand displacement.29
A similar effect
may also be responsible for the halide-specific synthesis of
CpM(L)X complexes by reaction of Cp2M with substituted imi-
dazolium halides (M = Ni, Cr),25,30–32 phosphonium chlorides
(M = Ni),31 or DBU·HCl (M = Cr; DBU = 1,8-diazabicyclo-
[5.4.0]undec-7-ene).25
Reversible transfer of hydrogen from multidentate ancillary
ligands to bound substrates has been recently recognized as a
valuable ligand design feature.34 A bound secondary alkoxide
ligand, Cr–OCHR2, could potentially transfer a hydrogen to one
of the diimine carbon atoms in the ligand-based radical to
ð3Þ
7922 | Dalton Trans., 2012, 41, 7920–7930
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Table 1 Comparison of experimental and calculateda bond lengths
(in Å) for 8
C–C
C–N
(imine)
Method
Cr–Npy Cr–Nim Cr–Cp (ligand)
X-rayb
1.964
2.073
1.941
2.065
1.934 1.376
2.007 1.412
1.342
1.345
DFT (8 high
spin S = 5/2)
DFT (8 S = 3/2)
DFT (A S = 2)
2.021
2.074
1.989
1.979
1.979 1.399
2.012 1.509
1.355
1.441
a See Experimental section for details. b Average of two molecules in
unit cell.
Scheme 1 Synthesis of pyridine-imine complexes.
generate a ketone and a non-radical amido imine ligand. Com-
mercially available 2,4-dimethyl-3-pentanol is less sterically
hindered than related alkoxide ligands recently employed in
mid-valent chromium chemistry,35 and the use of bulky and elec-
tron-donating R = iPr substituents is expected to favour the
H˙ transfer process both sterically and electronically. The
Cr–OCHiPr2 ligand in 6 was found to be disordered over two
positions. In neither conformation was any interaction of the
secondary alkoxide C–H bond with the ligand radical evident.
one-half equiv of I2 gives the Cr(III) iodide complex 10, which
was structurally characterized by X-ray diffraction, as shown
in Fig. 4. Compared to the neutral pyridine-imine ligand, the
relatively long imine C–N bond (1.344(2) Å) and short C–C
bond (1.406(2) Å) in complex 10 are consistent with a singly
reduced radical ligand.37,38 As was observed with the α-diimine
complexes, single electron transfer reactions of these complexes
lead to Cr-based redox chemistry while the radical ligand retains
its unpaired electron, antiferromagnetically coupled to the para-
magnetic metal center.
Attempted synthesis of CpCr(PyCH2NMes)
As shown in eqn (6), the Cr(III) alkoxide complex CpCr-
(PyCHNMes)(OCMe2Ph), 11, was prepared by protonolysis of
chromocene, analogous to the synthesis of 5. The X-ray crystal
structure of 11 (Fig. 4) revealed a typical Cr(III) half-sandwich
CpCr(LX)X geometry.16–18,20–22 The key imine C–N (1.339(2)
Å) and C–C (1.404(2) Å) bond lengths are once again indicative
of a radical anionic ligand.37,38
To explore the potential for intramolecular H-atom transfer
further, a route to an authentic CpCr(II) complex with a redox
innocent amido imine ligand was sought. Related pyridine
amido ligands have been demonstrated to lead to highly selective
Cr-based ethylene trimerization catalysts.36 The attempted syn-
thesis of CpCr(PyCH2NMes) (A) shown in eqn (5) followed the
route previously used for CpCr[(ArNCMe)2CH] complexes, by
treating CrCl2 sequentially with first NaCp, and then the deproto-
nated ligand.18
ð6Þ
ð5Þ
Investigation of CpCr(PyCHNMes) by theoretical calculations
The crystalline product isolated from this reaction unexpect-
edly displayed a very intense peak at 704 nm, similar to that
observed for the Cr(II) radical diimine complex 4. A single
crystal X-ray structure was obtained that confirmed the overall
connectivity, although there was extensive disorder in the struc-
ture. Despite the disorder and poor data quality, the structure was
more consistent with the radical complex resulting from loss of a
ligand H atom, CpCr(PyCHNMes), 8, than the initial target pyri-
dine amido complex A.
In order to explore the electronic structure and UV-vis spectra of
CpCr(II) complexes bearing radical pyridine-imine ligands,
density functional theory (DFT) calculations were employed.
The geometry of CpCr(PyCHNMes), 8, was calculated in both
the S = 5/2 and S = 3/2 spin states. For comparison, the geometry
of the initial target pyridine amido complex CpCr(PyCH2NMes),
A, was also optimized in the expected S = 2 spin state.
Although the coordination sphere metrical parameters for A
are mostly within 0.1 Å of the experimental values, a signifi-
cant difference (+0.13 Å) between the experimental and theoreti-
cal value for the pyridine-imine C–C bond is evident for A.
These differences between calculated and observed bond lengths
are reduced substantially for both the high spin (S = 5/2) and S =
3/2 solutions for 8 (Table 1). Overall, the S = 3/2 metrical par-
ameters for 8 best fit the experimental data.
Synthesis of CpCr(PyCHNMes) complexes
Radical pyridine-imine ligands have recently been investigated
for first-row transition metals,37 and for aluminum complexes.38
An improved synthetic route to CpCr(PyCHNMes), 8, is shown
in Scheme 1, consisting of Mn reduction of CpCrCl2(THF) in
the presence of the neutral pyridine-imine ligand. The corres-
ponding Cr(III) Cl complex 9 is obtained by sequential treatment
of CrCl2 with PyCHNMes and NaCp. Oxidation of 8 with
Using a hybrid functional the S = 3/2 solution for 8 is stabi-
lized in comparison to the high spin (S = 5/2) state by ca.
8 kcal mol−1, in agreement with the experimental magnetic data.
The predicted S = 3/2 spin expectation value for 8 (<S2> = 4.47)
This journal is © The Royal Society of Chemistry 2012
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Fig. 4 Thermal ellipsoid diagram (50%) of 10 (left) and 11 (right). Selected bond lengths (Å) and angles (°) for 10: Cr(01)–N(001) = 1.992(1);
Cr(01)–N(002) = 1.968(1); N(001)–C(5) = 1.382(2); N(002)–C(6) = 1.344(2); C(5)–C(6) = 1.406(2); Cr(01)–I(001) = 2.6865(3); N(001)–Cr(01)–
I(001) = 91.83(4); N(001)–Cr(01)–N(002) = 80.00(5); N(002)–Cr(01)–I(001) = 102.43(4). Selected bond lengths (Å) and angles (°) for 11: Cr(01)–
N(001) = 1.975(1); Cr(01)–N(002) = 1.988(1); N(001)–C(1) = 1.339(2); N(002)–C(2) = 1.388(2); C(1)–C(2) = 1.404(2); Cr(01)–O(001) = 1.880(1);
N(001)–Cr(01)–O(001) = 103.35(5); N(001)–Cr(01)–N(002) = 79.54(5); N(002)–Cr(01)–O(001) = 100.75(4).
Table 2 TD-DFTa predicted low energy bands for 8
Transition
(MO number)
Predicted
transitions (nm) (f) (predicted)
Oscillator strength
Assignmentb
MLCT
α-HOMO →
α-LUMO
604
684
0.0947
0.0251
β-HOMO →
β-LUMO
Intra-ligand
a S = 3/2 solution, see text for details. b Assignment by AOMIX
decomposition of relevant MOs into constituent components.43–45
Fig. 5 Spin density plot for 8 showing the antiferromagnetic coupling
between the Cr(II) ion and the imino-pyridine ligand radical. Mulliken
spin densities shown in red.
UV-vis spectroscopy of compound 8 in hexane displays a rela-
tively intense transition at low energy (Fig. 6 and Table 2). We
employed time-dependent DFT (TD-DFT)41,42 to better under-
stand the nature of the low energy band, and in addition delineate
between the possible structural and electronic forms of com-
pound 8, although it should be noted that TD-DFT computed
intensities using a mixed spin state solution should be treated
with caution. TD-DFT calculations on the high-spin (S = 5/2)
electronic structure for 8 (as well as the target molecule A)
do not predict any low energy transitions of appreciable intensity
(ƒ > 0.02) at wavelengths longer than 400 nm (see ESI†). The
S = 3/2 solution predicts two bands at low energy as shown in
Fig. 5. Even though the most intense band is blue-shifted in com-
parison to the experimental transition, this result provides further
support for an antiferromagnetic (S = 3/2) ground state for 8.
Analysis of the predicted bands in terms of the molecular orbi-
tals involved, allows for the assignment of the most intense low
energy band as a metal to ligand charge transfer (MLCT) band
(Table 2). This is predominantly the α-HOMO → α-LUMO tran-
sition (Fig. 7), in which the chromium metal character decreases
substantially in the α-LUMO based on analysis of the compo-
sitions of the MOs.43–45 The α-HOMO → α-LUMO transition is
thus predicted to involve the transfer of electron density from Cr
to the pyridine-imine ligand radical. The less intense β-HOMO
→ β-LUMO transition is not as straightforward to assign and we
have tentatively labeled as an intra-ligand transition of the pyri-
dine-imine ligand radical based on orbital analysis, similar to
related transitions observed by the groups of Wieghardt and
Berben.37,38
Fig. 6 UV-vis absorption spectra of 8 (black line), 1.32 × 10−4 M in
hexanes. Calculated low energy transitions are shown in green for the
S = 3/2 solution.
indicates ca. 15% spin contamination by the high spin state.39
The computed spin expectation value demonstrates a consider-
able bonding interaction between the metal centre and the ligand
radical, providing further evidence for antiferromagnetic coup-
ling in the ground state.40 A spin density plot for 8 is shown in
Fig. 5 and displays the predicted antiferromagnetic coupling
interaction between Cr(II) d4 ion and the pyridine-imine ligand
radical.
7924 | Dalton Trans., 2012, 41, 7920–7930
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Fig. 7 Partial Kohn–Sham molecular orbital diagram for the S = 3/2 solution for 8 and TD-DFT assignment of the low energy transitions at 604 nm
(α-HOMO → α-LUMO) and 684 nm (β-HOMO → β-LUMO). Molecular orbital compositions calculated using AOMIX.43–45 See Experimental
section for details.
Table 3 Comparison of experimental and calculateda bond lengths
(in Å) for 10
C–C
C–N
Method
X-ray
Cr–Npy Cr–Nimine Cr–Cp Cr–I (ligand) (imine)
1.992
2.048
1.968
2.057
1.887 2.686 1.406
1.938 2.722 1.406
1.344
1.351
DFT (high
spin S = 2)
DFT (broken
symmetry
S = 1)
2.017
1.999
1.931 2.736 1.406
1.346
a See Experimental section for details.
Fig. 9 UV-vis absorption spectra of 10 (black line), 1.24 × 10−4 M in
hexanes. Calculated low energy transitions 470 nm, 524 nm, and
617 nm are shown in green for the S = 1 solution.
solutions are within 0.06 Å of the experimental values. Overall,
the S = 1 metrical parameters for 10 are a slightly better fit with
the experimental metrical data.
Using a hybrid functional the S = 1 solution for 10 is stabi-
lized in comparison to the high spin (S = 2) state by ca.
5 kcal mol−1, in agreement with experimental magnetic data.
This energy difference is less than that predicted for compound
8. The predicted S = 1 spin expectation value for 10 (<S2> =
2.78) indicates ca. 20% spin contamination by the high spin
state.39 The computed spin expectation value demonstrates a
considerable bonding interaction between the metal centre and
the ligand radical, providing further evidence for antiferromag-
netic coupling in the ground state.40 A spin density plot for 10 is
shown in Fig. 8 and displays the predicted antiferromagnetic
coupling interaction between the Cr(III) d3 ion and the pyridine-
imine ligand radical in the ground state.
Fig. 8 Spin density plot for 10 showing the antiferromagnetic coupling
between the Cr(III) ion and the pyridine-imine ligand radical. Mulliken
spin densities shown in red.
Investigation of CpCr(PyCHNMes)I by theoretical calculations
DFT calculations on 10 allowed for a comparison of the X-ray
metrical parameters with the optimized geometries in both spin
states (Table 3). The predicted coordination sphere metrical par-
ameters for both the high spin (S = 2) and triplet (S = 1)
UV-vis spectroscopy of compound 10 in hexane displays a
series of weak transitions at low energy (Fig. 9 and Table 4), in
contrast to the relatively intense band present at 700 nm for 8.
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TD-DFT analysis of the S = 1 solution for 10 predicts three tran-
sitions >400 nm of appreciable intensity at 470 nm, 524 nm, and
617 nm as detailed in Table 4. Analysis of the predicted bands in
terms of the molecular orbitals involved allows for the assign-
ment of the transitions at 524 nm and 617 nm as ligand to metal
charge transfer (LMCT) bands. In both cases electron density is
transferred from the pyridine-imine ligand radical to the Cr(III)
ion (Fig. 10). The higher energy transition at 470 nm is predicted
to be a LLCT band with the transfer of electron density from the
I− ligand to the pyridine-imine ligand radical. The higher charge
of the Cr(III) ion in 10 eliminates the intense MLCT transition
predicted for 8 (containing Cr(II)), matching the experimental
UV-vis spectra for the two compounds.
Table 4 Comparison of the experimental and predicted TD-DFTa low
energy transitions for 10
Transition (MO
number)
Predicted
transitions
Oscillator strength
(f) (predicted)
Conclusions
Assignmentb
LLCT
The reaction of Cr(II) species with neutral α-diimine or pyridine-
imine ligands is a useful synthetic route to mixed-ligand com-
plexes bearing radical anionic ligands with antiferromagnetic
coupling to the Cr(III) center. Chemical reduction and re-oxi-
dation of CpCr(III) halide complexes results in metal-based redox
while retaining an unpaired ligand-based electron. Structural
characterization of the new complexes by single-crystal X-ray
diffraction reveal half-sandwich chromium complexes that at
least superficially resemble CpCr(LX) and CpCr(LX)X com-
pounds with β-diketiminate ligands. The crystal structures also
α-HOMO →
α-LUMO
470
524
617
0.0316
0.0188
0.0299
β-HOMO →
β-LUMO + 3
LMCT
LMCT
β-HOMO →
β-LUMO + 2
a S = 1 solution, see text for details. b Assignment by AOMIX
decomposition of relevant MOs into constituent components.43–45
Fig. 10 Partial Kohn–Sham molecular orbital diagram for the S = 1 solution for 10 and TD-DFT assignment of the low energy transitions at 470 nm
(α-HOMO → α-LUMO), 524 nm (β-HOMO → β-LUMO + 3), and 617 nm (β-HOMO → β-LUMO + 2). Molecular orbital compositions calculated
using AOMIX.43–45 See Experimental section for details.
7926 | Dalton Trans., 2012, 41, 7920–7930
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show the shortened C–C bonds and elongated C–N bonds
typical of α-diimine or pyridine-imine ligand radicals. The
new complexes display distinctive bands at higher wavelengths
in their UV-vis spectra than the corresponding compounds
with non-radical β-diketiminate ligands. The low energy band
is particularly intense for the CpCr(II) complexes with α-diimine
or pyridine-imine ligand radicals, and provides a convenient
peak for monitoring reactions by UV-vis spectroscopy. Density
functional theory calculations support the proposed electronic
structure of the Cr(II) and Cr(III) pyridine-imine complexes, and
TD-DFT calculations demonstrate the involvement of the ligand-
based radicals in the distinctive high intensity, lower energy
bands in the UV-vis spectra.
6-31G(d) basis set on all atoms except iodine which was
modelled with the LanL2DZ56–58 basis set. Frequency calcu-
lations at the same level of theory confirmed that the optimized
structures were located at a minimum on the potential energy
surface. Single point calculations were performed using the
B3LYP54,55 functional and the TZVP basis set of Ahlrichs59,60
on all atoms except iodine which was modeled with the
LanL2DZ basis set. The intensities of the 30 lowest-energy elec-
tronic transitions were calculated by TD-DFT40,41 at the B3LYP/
TZVP/LanL2DZ level with a polarized continuum model (PCM)
for hexane.61–64 AOMIX43–45 was used for determining atomic
orbital compositions employing Mulliken Population Analysis.
The inadvertent conversion of protonated PyCH2NHMes to
a radical pyridine–imine complex may have implications for
the nature of the catalytic species in the selective Cr-catalyzed
trimerization of ethylene using related ligand precursors.36
An isolated complex with a bulky Cr-bound secondary alkoxide
appeared to be stable with respect to intramolecular H-atom
transfer to the diimine radical ligand. However, preliminary
attempts in this laboratory to use these complexes as synthetic
precursors to Cr(III) alkyl complexes have been complicated
by unwanted ligand alkylation reactions. These initial results
support the recent observations of Theopold and co-workers
that their propensity for alkylation may limit the efficacy of
α-diimines as ancillary ligands in organochromium chemistry.46
Synthesis of CpCr[(DppNCH)2]Cl (1)
To a suspension of CrCl2 (1.324 g, 10.76 mmol) in 30 mL THF,
diimine (DppNCH)2 (3.7949 g, 10.077 mmol) was added and
stirred at room temperature for 20 h, during which time the
colour changed from green to red. NaCp (5.5 mL, 2.0 M in THF,
11.0 mmol) was then added, causing the solution to become a
dark green. After stirring for an additional 20 h, the solvent was
removed in vacuo, the residue was extracted with 80 mL
hexanes, and the dark green extracts were filtered through Celite,
and cooled to −35 °C. Black crystals of 1 were isolated in
two different fractions (2.322 g, 44%). (Evans, C6D6): 2.75 μB.
Anal. Calcd for C31H41N2CrCl: C, 70.37; H, 7.81; N, 5.29.
Found: C, 70.84; H, 7.98; N, 5.24. UV/vis (hexanes; λmax, nm
(ε, M−1 cm−1)): 412 (8700), 567 (1300).
Experimental section
General considerations
Synthesis of CpCr[(XylNCMe)2]Cl (2)
All reactions were carried out under nitrogen using standard
Schlenk and glove box techniques. Solvents were dried by using
the method of Grubbs.47 Celite (Aldrich) was dried overnight at
120 °C before being evacuated and then stored under nitrogen.
Iodine was purified by sublimation, and 2,4-dimethyl-3-pentanol
(99%, Aldrich) and DBU (1,8-diazabicyclo[5.4.0]undec-7-ene,
98%, Aldrich) were freeze–pump–thaw degassed before use.
p-Toluenesulfonic acid monohydrate, 2-phenyl-2-propanol (97%),
CrCl2 (99% anhydrous), CrCl3 (anhydrous), Zn (99% powder), Mn
(99% powder), NaN(SiMe3)2, n-BuLi (1.6 M in hexanes), NaCp
(2.0 M in THF), and 1,4-dioxane (anhydrous) were purchased from
Aldrich and used as received. (ArNCR)2,48 PyCHNMes,49
PyCH2NHMes,50 TolNHC6H4CHNDpp,33 CpCr2,51 and CpCr-
(THF)Cl2 52 were prepared according to the literature procedures.
UV/vis spectroscopic data were collected on a Varian Cary
100 Bio UV-visible or a Shimadzu UV 2550 UV-vis spectropho-
tometer in hexanes solution in a specially constructed cell for
air-sensitive samples: a Kontes Hi-Vac Valve with PTFE plug
was attached by a professional glassblower to a Hellma 10 mm
path length quartz absorption cell with a quartz-to-glass graded
seal. Elemental analyses were performed by Guelph Chemical
Laboratories, Guelph, ON, Canada or by the UBC Department
of Chemistry microanalytical services.
To a suspension of CrCl2 (171.0 mg, 1.390 mmol) in 20 mL
THF, diimine (XylNCMe)2 (405.7 mg, 1.387 mmol) was added
and stirred at room temperature for 20 h, during which time the
colour changed from green to red. NaCp (0.80 mL, 2.0 M in
THF, 1.6 mmol) was then added, causing the solution to become
a dark green. After stirring for an additional 20 h, the solvent
was removed in vacuo, the residue was extracted with 9 mL
hexanes, and then the dark green extracts were filtered through
Celite and cooled to −35 °C. Black crystals of 2 were isolated in
three different fractions (351.3 mg, 57%). (Evans, C6D6): 2.72
μB. Anal. Calcd for C25H29N2CrCl: C, 67.48; H, 6.57; N, 6.29.
Found: C, 68.06; H, 6.77; N, 6.13. UV/vis (hexanes; λmax, nm
(ε, M−1 cm−1)): 430 (6200), 580 (1000).
Synthesis of CpCr[(MesNCMe)2]Cl (3)
To a suspension of CrCl2 (151.2 mg, 1.229 mmol) in 20 mL
THF, diimine (MesNCMe)2 (403.8 mg, 1.260 mmol) was added
and stirred at room temperature for 20 h, during which time the
colour changed from green to red. NaCp (0.70 mL, 2.0 M in
THF, 1.4 mmol) was then added, causing the solution to become
a dark green. After stirring for an additional 20 h, the solvent
was removed in vacuo, the residue was extracted with 7.5 mL
hexanes, and then the dark green extracts were filtered through
Celite and cooled to −35 °C. Black crystals of 3 were isolated in
four different fractions (293.9 mg, 51%). (Evans, C6D6):
2.95 μB. Anal. Calcd for C27H33N2CrCl: C, 68.56; H, 7.03; N,
Computational details
Geometry optimizations were performed using the Gaussian 09
program (Revision A.02),53 the B3LYP54,55 functional, and the
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5.92. Found: C, 69.18; H, 7.17; N, 6.00. UV/vis (hexanes; λmax
nm (ε, M−1 cm−1)): 420 (5500), 572 (900).
,
C31H34N2Cr: C, 76.52; H, 7.04; N, 5.76. Found: C, 76.14; H,
7.34; N, 5.80. UV/vis (hexanes; λmax, nm (ε, M−1 cm−1)): 373
(6400), 493 (3600).
Synthesis of CpCr[(DppNCH)2] (4)
Synthesis of CpCr(PyCHNMes) (8)
To a solution of CpCr[(DppNCH)2]Cl (101.6 mg, 0.206 mmol)
in 15 mL THF, Zn (26.3 mg, 0.402 mmol) was added and stirred
at room temperature for 20 h, during which the colour changed
from green to blue. The solvent was removed in vacuo, the
residue was extracted with 1 mL hexanes then filtered through
Celite, and cooled to −35 °C. Dark green crystals of 4 were iso-
lated in one fraction (55.4 mg, 59%). (Evans, C6D6): 3.78 μB.
Anal. Calcd for C31H41N2Cr: C, 75.42; H, 8.37; N, 5.67.
Found: C, 75.55; H, 8.43; N, 5.70. UV/vis (hexanes; λmax, nm
(ε, M−1 cm−1)): 406 (5500), 532 (1700), 649 (4800).
To a solution of CpCr(THF)Cl2 (85.2 mg, 0.327 mmol) in
15 mL THF, a solution of the pyridine-imine PyCHNMes
(90.6 mg, 0.404 mmol) in 3 mL of THF and Mn (430.1 mg,
7.828 mmol) were added. The suspension was stirred for 20 h,
after which the solvent was removed in vacuo. The residue was
extracted with 12 mL hexanes, filtered through Celite, and
cooled to −35 °C. Black crystals of 8 were isolated in two differ-
ent fractions (51.2 mg, 56%). (Evans, C6D6): 3.63 μB. Anal.
Calcd for C20H21N2Cr: C, 70.37; H, 6.20; N, 8.20. Found:
C, 70.56; H, 6.18; N, 8.15. UV/vis (hexanes; λmax, nm (ε, M−1
cm−1)): 445 (11 000), 502 (4100), 595 (1800), 704 (6300).
Synthesis of CpCr[(DppNCH)2]OCMe2Ph (5)
To a solution of Cp2Cr (50.5 mg, 0.277 mmol) in 5 mL THF,
diimine (DppNCH)2 (116.2 mg, 0.308 mmol) was added. After
stirring for 1 h, HOCMe2Ph (44.3 mg, 0.325 mmol) and a cata-
lytic amount of NaN(SiMe3)2 (<5 mg, <10 mol%) were dis-
solved in 3 mL THF and added to the Cp2Cr solution. The
resulting solution was stirred for 20 h, after which the solvent
was removed from the dark green solution in vacuo. The residue
was extracted with 3 mL of hexanes, filtered through Celite, and
cooled to −35 °C. Dark green crystals of 5 were isolated in three
different fractions (113.8 mg, 65%). (Evans, C6D6): 2.65 μB.
Anal. Calcd for C40H52N2CrO: C, 76.40; H, 8.33.; N, 4.45.
Found: C, 76.14; H, 8.50; N, 4.44. UV/vis (hexanes; λmax, nm
(ε, M−1 cm−1)): 437 (5700), 584 (800), 688 (600).
Synthesis of CpCr(PyCHNMes)Cl (9)
To a suspension of CrCl2 (169.0 mg, 1.374 mmol) in 10 mL
THF, the pyridine-imine PyCHNMes (300.5 mg, 1.340 mmol)
was added and stirred at room temperature for 20 h. NaCp
(0.70 mL, 2.0 M in THF, 1.4 mmol) was added, and the resulting
black solution was stirred for 20 h. The solvent was then
removed in vacuo, the residue was extracted with 15 mL
hexanes, filtered through Celite, and cooled to −35 °C. Black
crystals of 9 were isolated in four different fractions (305 mg,
61%). (Evans, C6D6): 2.60 μB. Anal. Calcd for C20H21N2CrCl:
C, 63.74; H, 5.62; N, 7.43. Found: C, 63.70; H, 5.65; N, 7.24.
UV/vis (THF; λmax, nm (ε, M−1 cm−1)): 471 (7000).
Synthesis of CpCr[(DppNCH)2]OCHiPr2 (6)
Synthesis of CpCr(PyCHNMes)I (10)
To a solution of Cp2Cr (48.2 mg, 0.265 mmol) in 7.5 mL THF,
diimine (DppNCH)2 (116.5 mg, 0.309 mmol) was added. After
stirring for 30 min, HOCHiPr2 (45 μl, 0.321 mmol) and a cataly-
tic amount of NaN(SiMe3)2 (<5 mg, <10 mol%) were dissolved
in 3 mL of THF and added to the Cp2Cr solution. The resulting
solution was stirred for 20 h, after which the solvent was
removed from the dark green solution in vacuo. The residue was
extracted with 3 mL hexanes, filtered through Celite, and cooled
to −35 °C. Dark green crystals of 6 were isolated in three differ-
ent fractions (82.8 mg, 51%). Anal. Calcd for C38H56N2CrO: C,
74.96; H, 9.27; N, 4.60. Found: C, 75.23; H, 9.44; N, 4.73.
UV/vis (hexane; λmax, nm (ε, M−1 cm−1)): 444 (5500), 568
(900), 702 (1000).
To a solution of CpCr(PyCHNMes) (45.8 mg, 0.134 mmol) in
7.5 mL Et2O, was added I2 (20.5 mg, 0.080 mmol) dissolved in
3 mL of Et2O. After stirring for 20 h, the solvent was removed
in vacuo, the residue was extracted with 3 mL Et2O and 2 mL
toluene filtered through Celite, and cooled to −35 °C. Black
crystals of 10 were isolated in two fractions (25.6 mg, 41%).
Anal. Calcd for C20H21N2CrI: C, 51.30; H, 4.52; N, 5.98.
Found: C, 53.44; H, 5.36; N, 5.45. UV/vis (THF; λmax, nm
(ε, M−1 cm−1)): 478 (7000), 591 (2100), 692 (1100), 743
(1100).
Synthesis of CpCr(PyCHNMes)OCMe2Ph (11)
To a solution of Cp2Cr (50.1 mg, 0.275 mmol) in 10 mL THF,
the pyridine-imine PyCHNMes (68.3 mg, 0.304 mmol) was
Synthesis of CpCr(TolNC6H4CHNDpp) (7)
added. After stirring for
1 h, HOCMe2Ph (43.2 mg,
To a solution of Cp2Cr (109.6 mg, 0.602 mmol) in 7.5 mL THF,
the aniline imine TolNHC6H4CHNDpp (251.42 mg,
0.680 mmol) and a catalytic amount of DBU (20 μl,
0.134 mmol) were added. The resulting dark red solution was
stirred for 20 h, after which the solvent was removed in vacuo.
The residue was extracted with 3 mL hexanes, filtered through
Celite, and cooled to−35 °C. Black crystals of 7 were isolated in
two different fractions (131.0 mg, 45%). Anal. Calcd for
0.317 mmol) and catalytic amount of NaN(SiMe3)2 (<5 mg,
<10 mol%) dissolved of 3 mL THF were added. The solution
was stirred for 20 h, after which the solvent was removed from
the dark green solution in vacuo. The residue was extracted with
6 mL hexanes, filtered through Celite, and cooled to −35 °C.
Dark green crystals of 11 were isolated in one fraction (95.3 mg,
73%). (Evans, C6D6): 2.75 μB. Anal. Calcd for C29H32N2CrO:
C, 73.09; H, 6.77; N, 5.88. Found: C, 72.96; H, 7.00; N, 5.91.
7928 | Dalton Trans., 2012, 41, 7920–7930
This journal is © The Royal Society of Chemistry 2012

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UV/vis (hexanes; λmax, nm (ε, M−1 cm−1)): 413 (5500), 471
financial support. Compute Canada and Westgrid are thanked for
(6400), 612 (1800).
access to computational resources.
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X-ray crystallography
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Acknowledgements
We are grateful to the Natural Sciences and Engineering
Research Council of Canada (NSERC), the Canadian Foundation
for Innovation, and the University of British Columbia for
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