Isocyanide and phosphine oxide coordination in binuclear chromium Pacman complexes

Article abstract of DOI:10.1021/om4009313

The new binuclear chromium Pacman complex [Cr₂(L)] of the Schiff base pyrrole macrocycle H₄L has been synthesized and structurally characterized. Addition of isocyanide, CiNR (R = xylyl, ^tBu), or triphenylphosphine oxide donors to [Cr₂(L)] gives contrasting chemistry with the formation of the new coordination compounds [Cr ₂(μ-CNR)(L)], in which the isocyanides bridge the two Cr(II) centers, and [Cr₂(OPPh₃)₂(L)], a Cr(II) phosphine oxide adduct with the ligands exogenous to the cleft.

Full text of DOI:10.1021/om4009313

Communication
pubs.acs.org/Organometallics
Isocyanide and Phosphine Oxide Coordination in Binuclear
Chromium Pacman Complexes
Charlotte J. Stevens, Gary S. Nichol, Polly L. Arnold, and Jason B. Love*
EaStCHEM School of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ, U.K.
S
* Supporting Information
ABSTRACT: The new binuclear chromium Pacman complex [Cr2(L)] of the
Schiff base pyrrole macrocycle H4L has been synthesized and structurally
characterized. Addition of isocyanide, CNR (R = xylyl, tBu), or
triphenylphosphine oxide donors to [Cr2(L)] gives contrasting chemistry with
the formation of the new coordination compounds [Cr2(μ-CNR)(L)], in which
the isocyanides bridge the two Cr(II) centers, and [Cr2(OPPh3)2(L)], a Cr(II)
phosphine oxide adduct with the ligands exogenous to the cleft.
he chemistry of binuclear, low-oxidation-state chromium
group, late-transition-metal, lanthanide, and actinide complexes
Tcomplexes is dominated by a tendency to form metal− of these macrocycles have been synthesized,8 of which cobalt
metal multiple bonds and an involvement in the activation of
small molecules.1 For example, quintuple M−M bond formation
was demonstrated recently in binuclear Cr(I) complexes,2 a new
side-on bridging dinitrogen chromium complex was reported,3
and dinitrogen reduction has been displayed at a Cr(0) center.4
Industrially, chromium catalysts are used in the selective
oligomerization and polymerization of olefins and there is
ongoing interest in understanding and optimizing these
processes.5 Chromium complexes have been exploited as
catalysts for other useful C−C bond forming reactions, including
the coupling of alkyl halides with aldehydes and pinacol-type
couplings.6
Strategies to define the formation and reactivity of binuclear
complexes often involve the design of ligands that control both
the primary coordination sphere of the metal and the separation
between the metals. In this context, cofacial diporphyrins and
their bimetallic complexes have displayed a diversity of small-
molecule chemistry, but their exploitation is limited due to the
complexity of the ligand synthesis.7 In recent years a class of
Schiff-base polypyrrole macrocycles (H4L, Scheme 1) has been
developed which fold upon metalation into structures reminis-
cent of cofacial or Pacman diporphyrins. A wide range of main-
complexes were found to be effective as catalysts for the
reduction of dioxygen to water.9 However, the early-transition-
metal chemistry of either H4L or cofacial diporphyrins remains a
relatively unexplored field. We reported previously the syntheses
of the Ti(III) and V(III) complexes [(MCl)2(L)], but could not
structurally characterize either complex and did not carry out
extensive investigation into their reactivity.10 Herein, we report
the synthesis and structure of the first binuclear chromium
Pacman complex and its coordination chemistry with isocyanide
and phosphine oxide donors.
The new binuclear chromium complex of the Pacman
macrocycle [Cr2(L)] can be prepared either by addition of
[Cr{N(SiMe3)2}2(THF)2] to H4L or by reaction of K4L with
CrCl2 (Scheme 1). Both reactions have comparable yields
(∼70%), but salt elimination is preferred, since the synthesis of
[Cr{N(SiMe3)2}2(THF)2] is low yielding.11 The 1H NMR
spectrum of [Cr2(L)] in d5-pyridine at 298 K shows para-
magnetically broadened and contact-shifted resonances at 16.9,
14.0, 6.7, −29.2, and −97.7 ppm, which are not assignable to
specific ligand protons. Two broad, residual protio solvent
resonances are visible in the room-temperature spectrum at 8.7
and 7.2 ppm. At 393 K, the resonance at 7.2 ppm separates into
two sharper resonances at 7.3 and 7.2 ppm, indicating that
pyridine binds transiently to the paramagnetic chromium
complex in solution.
a
Scheme 1. Synthesis of [Cr2(L)]
The X-ray crystal structure of [Cr2(L)] crystallized from
benzene reveals that the macrocycle adopts a Pacman geometry
(Figure 1). In the lattice, molecules of [Cr2(L)] are arranged in
chains alternating with benzene molecules which engage in
bonding to the exo faces of the macrocycles at a Cr−C contact
distance of 3.608(2) Å. Both Cr(II) ions are bound in equivalent
pseudo-square-planar environments comprising N4 pyrrolide
and imine donor sets with the mean Cr−N(pyrrolide) distance
a
Reagents: (A) 2 [Cr{N(SiMe3)2}2(THF)2], toluene; (B) (i) 4
Received: September 17, 2013
KN(SiMe3)2, (ii) 2 CrCl2, THF.
© XXXX American Chemical Society
A
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Figure 1. Solid-state structure of [Cr2(L)] illustrating the molecular geometry (left and center) and packing in the unit cell (right). For clarity, hydrogen
atoms and lattice solvent are omitted (where shown, displacements ellipsoids are drawn at 50% probability). Selected bond lengths (Å) and angles
(deg): Cr1···Cr1′ = 3.1221(1), Cr1−N1 = 2.1139(16), Cr1−N2 = 1.9702(15), Cr1−N3 = 1.9758(15), Cr1−N4 = 2.0780(15); N1−Cr1−N2 =
80.06(6), N2−Cr1−N3 = 85.52(6), N3−Cr1−N4 = 78.64(6), N4−Cr1−N1 = 114.02(6).
of 1.97 Å shorter than the mean Cr−N(imine) distance of 2.10 Å.
The two pockets of the macrocycle are twisted with respect to
each other in order to maximize favorable offset π−π stacking
interactions between the aryl hinges of the ligand. The sum of the
four N−Cr−N angles is 358°, and the Cr(II) ions are displaced
0.20 Å from the N4 plane into the macrocyclic cleft. The resulting
Cr···Cr separation of 3.1221(1) Å is the shortest M−M distance
observed in any [M2(L)] complex of this type.12 In structures
where two metal-metal-bonded Cr centers are supported by an
N4 donor set, Cr−Cr bond lengths range from 1.86 to 3.00 Å,
with a median value of 2.40 Å.13 The Cr−Cr separation in
[Cr2(L)] lies outside this range, and so it seems that there is no
metal-metal bonding interaction. This is supported by the
solution magnetic moment of [Cr2(L)] of 6.34 μB (C6D6/THF),
which approaches that for two independent (noncommunicat-
ing) Cr(II) ions (spin only, 6.93 μB). Full magnetic, EPR, and
computational studies to elucidate the electronic structure of
[Cr2(L)] and its adducts described below are ongoing.
Scheme 2. Reaction of [Cr2(L)] with Isocyanides and
Triphenylphosphine Oxide
a
a
t
The ligand architecture is shown in cartoon form. R = Bu, 2,6-
Me2C6H3 (Xyl).
tungsten complexes.18 To our knowledge [Cr2(μ-CNXyl)(L)]
and [Cr2(μ-CNtBu)(L)] are the first structurally characterized
first-row early-transition-metal complexes featuring bridging
isocyanide ligands.
The addition of Lewis base donors to Pacman complexes can
result in the binding and activation of small-molecule substrates
such as O2 and N2.9b,14 These donors bind to the metals in the
exo coordination sites, thereby directing substrates to the endo
intermetallic site and can also increase the electron density
available at the metal centers. Isocyanide ligands CNR are
isoelectronic with carbon monoxide but are better σ donors and
generally poorer π acceptors.15 Their electronic and steric
properties are tunable by modification of the organic substituent
R. Transition-metal isocyanide complexes have been shown to
achieve C−F bond activation and selective hydrogenation of
alkynes, nitriles, and isocyanides, as well as alkene polymer-
ization.16 Recently a coordinatively unsaturated Co(−I) complex
of bulky m-terphenyl isocyanides has been isolated and shown to
bind dinitrogen, as well as undergoing reactions with a range of
organic substrates.17 In light of these advances, reactions between
[Cr2(L)] and isocyanides were evaluated.
The solid-state structures reveal that the isocyanides bridge the
square-pyramidal Cr centers symmetrically. In [Cr2(μ-CNXyl)-
(L)], the planar xylyl ring is perpendicular to the aryl hinges of
the macrocycle, minimizing steric interactions with the endo Me
groups, C7 and C28. One of the protons bound to C7 is oriented
toward the electron-rich π system of the isocyanide ligand (C7···
C44 = 3.349(3) Å, C7···N9 = 3.568(3) Å) indicating that
intramolecular hydrogen bonding occurs, similar to that seen in
the related complex [Cu2(μ-py)(L)].12 In contrast, the three-
dimensional steric bulk of the tBu group in [Cr2(μ-CNtBu)(L)]
forces the isocyanide to protrude sideways out of the macrocycle
jaws to avoid clashing with the meso Me groups (Figure 2,
center). We reason that these steric constraints prevent the tBu
isocyanide from approaching closer to the Cr centers, resulting in
the longer Cr−C separation observed in [Cr2(μ-CNtBu)(L)] of
2.490(2) Å in comparison to 2.259(2) and 2.261(2) Å in [Cr2(μ-
CNXyl)(L)].
An excess of the isocyanides CNR (R = Xyl, tBu) was added
to solutions of [Cr2(L)] (Scheme 2). Single crystals were
obtained from the reaction carried out in fluorobenzene (R =
Xyl) and a THF/C6D6 mixture (R = tBu). Determination of the
structures reveals the 1:1 endo adduct [Cr2(μ-CNR)(L)], in
which the isocyanide adopts a bridging position within the
macrocyclic cleft instead of the anticipated 2:1 exo adduct
(Figure 2). Although bimetallic complexes with bridging
isocyanide ligands are common in late-transition-metal chem-
istry, those containing early transition metals are rare, with the
only homobimetallic examples being two molybdenum and three
Since these are the first binuclear chromium μ-CNR
complexes, comparison with later first-row transition-metal
isocyanide complexes is instructive. A number of complexes
containing the motifs {M2(μ-CNR)} and {M3(μ3-CNR)} have
been reported for both CNXyl and CNtBu for M = Fe, Co, Ni,
Cu. For the binuclear complexes, the mean M−CCNR distance is
1.98 Å.13 The longest M−CCNR bond previously reported is
2.381(4) Å in a binuclear Fe(II) compound bridged by CNMe.19
The long Cr−C distances observed in [Cr2(μ-CNXyl)(L)] and
[Cr2(μ-CNtBu)(L)] of 2.26 and 2.49 Å, respectively, are thus
likely imposed by the ligand architecture.
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Figure 2. Solid-state structures of [Cr2(μ-CNXyl)(L)] (left), [Cr2(μ-CNtBu)(L)] (center), and [Cr2(OPPh3)2(L)] (right). For clarity, hydrogen atoms
t
and lattice solvent are omitted (where shown, displacement ellipsoids are drawn at 50% probability). The Bu group in [Cr2(μ-CNtBu)(L)] was
rotationally disordered, and the major conformer is shown. Selected bond lengths (Å) and angles (deg) are as follows [Cr2(μ-CNXyl)(L)]: Cr1···Cr2 =
3.5877(5), Cr1−C43 = 2.259(2), Cr2−C43 = 2.261(2), C43−N9 = 1.169(3); Cr1−C43−Cr2 = 105.08(9), C43−N9−C44 = 175.9(2). [Cr2(μ-
CNtBu)(L)]: Cr1···Cr1′ = 3.7101(3), Cr1−C22 = 2.490(2), C22−N5 = 1.151(3); Cr1−C22−Cr1′ = 96.35(7), C22−N5−C23 = 172.8(2).
[Cr2(OPPh3)2(L)]: Cr1···Cr1′ = 4.3120(5), Cr1−O1 = 2.327(2), O1−P1 = 1.497(2); Cr1−O1−P1 = 137.6(1).
In both [Cr2(μ-CNXyl)(L)] (C43−N9−C44 = 175.9(2)°)
and [Cr2(μ-CNtBu)(L)] (C22−N5−C23 = 172.8(2)°) the
bridging isocyanide retains the linear geometry of the free ligand.
This is not uncommon for a bridging isocyanide,20 and the
frequency of the CN stretching band in the IR spectrum is
more indicative of the degree of back-donation to the isocyanide
than its geometry. In the IR spectrum (Nujol mull) of [Cr2(μ-
CNtBu)(L)], ν(CN) is 2150 cm−1. This is shifted to slightly
higher energy than in CNtBu (2132 cm−1) and indicates that a
small amount of π back-donation occurs from the Cr(II) centers
to the isocyanide. The metal−ligand interaction is dominated by
σ donation which occurs from a carbon-based orbital that is
antibonding with respect to the (CN) π system of the
isocyanide.21 In contrast, two CN stretching bands are
observed in the IR spectrum of [Cr2(μ-CNXyl)(L)] at 1990
and 1970 cm−1, a phenomenon which has been observed before
in complexes containing a single bridging isocyanide and is
attributed to solid-state effects.22 These bands are shifted to
considerably lower energy than in CNXyl (2114 cm−1),
indicating that significant π back-donation occurs. This may be
due to the greater π-acceptor ability of the conjugated aryl
isocyanide in comparison to CNtBu and the shorter Cr−CCNR
separation in [Cr2(μ-CNXyl)(L)] in comparison to [Cr2(μ-
CNtBu)(L)], allowing increased orbital overlap.
On a preparative scale [Cr2(μ-CNXyl)(L)] and [Cr2(μ-
CNtBu)(L)] may be synthesized in a number of different
solvents in good yield. The reaction between [Cr2(L)] and
CNXyl is instantaneous and is accompanied by a solution color
change from dark red-brown to dark green. [Cr2(μ-CNXyl)(L)]
is stable under dynamic vacuum and in THF solution. However,
the 1H NMR spectrum recorded in d5-pyridine shows resonances
corresponding to [Cr2(L)] and a broad resonance at 1.8 ppm
attributed to the o-Me groups of the free isocyanide. This implies
that coordination of pyridine to the Cr centers is competitive
with isocyanide. In contrast, [Cr2(L)] reacts slowly with CNtBu
at room temperature, though the reaction is complete within 48 h
at 80 °C. Once formed, the complex is stable under dynamic
vacuum, in THF or pyridine solution, and even upon addition of
the highly Lewis basic 4-dimethylaminopyridine, which suggests
that the sterically hindered isocyanide is kinetically trapped
within the macrocyclic cleft. The magnetic moment of [Cr2(μ-
CNtBu)(L)] in C6D6 solution is 4.75 μB, significantly less than
that of [Cr2(L)] (6.34 μB), indicating that the presence of the
isocyanide bridge increases the electronic communication
between the two Cr(II) centers.
The steric bulk of CNtBu is not sufficient to prevent it from
coordinating within the macrocyclic cleft and thereby blocking
the intermetallic reaction space. In light of this, an excess of
triphenylphosphine oxide was added to a toluene solution of
[Cr2(L)] (Scheme 2). This ligand is much bulkier than the
isocyanides, and furthermore, phosphine oxides do not
commonly adopt bridging modes in transition-metal complexes;
only six examples have been structurally characterized.13 Single
crystals were isolated from the toluene solution, and X-ray
analysis revealed the formation of the desired 2:1 exo adduct
[Cr2(OPPh3)2(L)], in which one phosphine oxide coordinates to
each Cr(II) ion in the exo axial coordination site (Figure 2, right).
The Cr centers adopt square-pyramidal geometries with a Cr−O
distance of 2.327(2) Å. To our knowledge this is the first
structurally characterized Cr(II) phosphine oxide complex. A few
phosphine oxide complexes of Cr(III) have been reported,
including a Cr(III) porphyrin bearing chloride and triphenyl-
phosphine oxide axial ligands.23 These compounds feature
markedly shorter Cr−O distances than in [Cr2(OPPh3)2(L)],
ranging from 1.83 Å24 to 2.03 Å,23 due to the increased
electrostatic attraction between the O donor and the Cr(III)
cation.
[Cr2(OPPh3)2(L)] precipitates as a microcrystalline solid
from toluene and may be redissolved in THF. However, the 1H
NMR spectrum recorded in THF/C6D6 shows resonances
consistent with [Cr2(L)] and broad features in the aromatic
region corresponding to the phenyl protons of free triphenyl-
phosphine oxide. No resonances were present in the 31P{1H}
NMR spectrum at 298 K, but cooling to 203 K resulted in a broad
resonance at 24 ppm corresponding to free OPPh3. Therefore, in
THF the phosphine oxide ligands are labile and an equilibrium is
C
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likely established between the THF and phosphine oxide adducts
of [Cr2(L)].
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Experimental Section. All reactions were carried out under
an atmosphere of dry N2 using dry solvents and standard Schlenk
and glovebox techniques. Isolated yields and elemental analyses
of compounds are as follows. [Cr2(L)]: 72%. Anal. Calcd for
C42H40Cr2N8: C, 66.30, H, 5.30, N, 14.73. Found: C, 66.10; H,
5.17; N, 14.68. [Cr2(μ-CNXyl)(L)]: 73%. Anal. Calcd for
C51H49Cr2N9: C, 68.67; H, 5.54; N, 14.13. Found: C, 68.45; H,
5.42; N, 13.96. [Cr2(μ-CNtBu)(L)]: 83%. Anal. Calcd for
C47H49Cr2N9: C, 66.89; H, 5.85; N, 14.94. Found: C, 66.78; H,
5.81; N, 14.86. [Cr2(OPPh3)2(L)]: 63%. Anal. Calcd for
C78H70Cr2N8P2O2: C, 71.11; H, 5.36; N, 8.51. Found: C,
70.84; H, 5.45; N, 8.40.
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ASSOCIATED CONTENT
* Supporting Information
Text, figures, tables, and CIF files giving full synthetic
procedures, 1H and 31P{1H} NMR, IR and magnetic data
(where appropriate), and details of single-crystal X-ray structure
determinations. This material is available free of charge via the
Internet at http://pubs.acs.org.
■
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H.; Hoshino, M. Inorg. Chem. 2005, 44, 6445−6455.
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AUTHOR INFORMATION
Corresponding Author
*J.B.L.: e-mail, jason.love@ed.ac.uk; tel, +44 131 6504762; fax,
+44 131 6504743.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the University of Edinburgh,
EaStCHEM School of Chemistry, and the EPSRC (U.K.). We
thank Dr. Lorna Murray for her assistance with NMR
spectroscopy.
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