Isocyano analogues of [Co(CO)4]n: AA tetraisocyanide of cobalt isolated in three states of charge

Article abstract of DOI:10.1021/ja1012382

The encumbering m-terphenyl isocyanide ligand, CNAr^Mes2 (Mes = 2,4,6-Me³C⁶H²), is used to stabilize homoleptic tetraisocyanide complexes of cobalt in the 1?, 0, and 1+ charge state. Most importantly, these complexes serve as isolable analogues of the binary carbonyl complexes [Co(CO)⁴]^?, Co(CO)⁴, and [Co(CO) ⁴]⁺. Sodium amalgam reduction of CoCl² in the presence of CNAr^Mes2 provides the salt Na[Co(CNAr^Mes2) ⁴], which can be oxidized with 1 equiv of ferrocenium triflate (FcOTf) to the neutral complex, Co(CNAr^Mes2)⁴. X-ray diffraction, FTIR spectroscopy, and low-temperature EPR spectroscopy reveal that Co(CNAr^Mes2)⁴ modulates between D^2d- and C^2v-symmetric forms. DFT calculations are used to rationalize this structural modulation in terms of thermal access to low-energy b ²-symmetric C?Co?C bending modes. Treatment of Na[Co(CNAr ^Mes2)⁴] with 2 equiv of FcOTf, followed by addition of Na[BAr^F4], provides the salt [Co(CNAr^Mes2) ⁴]BAr^F4, which contains a diamagnetic, square planar monovalent cobalt center. The molecular and electronic structures of [Co(CNAr^Mes2)⁴]BAr^F4 are compared and contrasted to the reported properties of the carbonyl cation, [Co(CO) ⁴]⁺.

Full text of DOI:10.1021/ja1012382

Published on Web 03/21/2010
Isocyano Analogues of [Co(CO)4]n: A Tetraisocyanide of Cobalt Isolated in
Three States of Charge
Grant W. Margulieux, Nils Weidemann, David C. Lacy, Curtis E. Moore, Arnold L. Rheingold, and
Joshua S. Figueroa*
Department of Chemistry and Biochemistry, UniVersity of California, San Diego, 9500 Gilman DriVe,
Mail Code 0358, La Jolla, California 92093-0358
Received February 10, 2010; E-mail: jsfig@ucsd.edu
Scheme 1
Intense interest in the unsaturated mononuclear cobalt carbonyls
(i.e., Co(CO)4 and [Co(CO)4]+) has stemmed from their presumed
role as reactive intermediates in industrial hydroformylation and
carbonylation processes.1-3 Despite such attention, the inherent high
reactivity of these species has precluded a definitive description of
their geometric and electronic structures in the condensed phase.
Based on the isolobal relationship between isocyanides (CNR) and
CO,4 we reasoned that use of encumbering R-groups might provide
access to stable unsaturated cobalt isocyanide complexes which
formally mimic their carbonyl analogues. Notably, commonly
employed isocyanide variants including CNt-Bu and CNXyl (Xyl
) 2,6-Me2C6H3) have been unsuccessful in this regard.5-7 Herein
we show that the m-terphenyl isocyanide8 CNArMes2 (Mes ) 2,4,6-
Me3C6H2) allows for the isolation of a full series of mononuclear
[Co(CNR)4]n complexes (n ) 1+, 0, 1-) relevant to the binary
carbonyls. Most importantly, the geometric and electronic structure
properties of the Co complexes reported here elucidate dn/valence
combinations where aspects of CNR for CO isolobal substitution
are valid, and others where the analogy seemingly fails.
Synthetic entry to the [Co(CNArMes2)4]n system is achieved by
the addition of sodium amalgam to a CoCl2/CNArMes2 mixture in
THF (Scheme 1). This protocol provides the dark red tetraisocya-
nometalate salt Na[Co(CNArMes2)4], crystallographically character-
ized as a contact ion pair with interactions between the Na cation
and isocyano Ct N units (Figure S4.1). Similar cation-CNiso
interactions were observed in Cooper’s salt, [K(DME)][Co-
(CNXyl)4] (Xyl ) 2,6-Me2C6H3),7b which is the only other
structurally characterized homoleptic cobalt isocyanometalate.9
FTIR analysis of Na[Co(CNArMes2)4] in C6D6 solution revealed three
intense ν(CN) stretches (1903, 1821, and 1761 cm-1), thereby
confirming that a low-symmetry local environment of the [Co-
(CNR)4]-core is retained in solution. However, the anion-cation
interaction in Na[Co(CNArMes2)4] can be readily disrupted upon
addition of [Ph3PNPPh3]Cl ([PPN]Cl) in THF solution. Structural
characterization of the resulting purple-red salt, [PPN][Co-
(CNArMes2)4] (Figure 1a), revealed a unique example of an
unperturbed tetraisocyanide anion adopting a relaxed tetrahedral
geometry (τ4 geometry index ) 0.95).10 Further support for this
notion is provided by solution FTIR analysis, which gave rise to a
single broad ν(CN) stretch (1821 cm-1) consistent with a complex
possessing local tetrahedral symmetry.
ingly, Evans method magnetic moment determination for Co-
(CNArMes2)4 resulted in a µeff value of 1.79((0.10) µB, consistent
1
with a d9 S ) /2 Co center. In addition, solution (C6D6) FTIR
analysis of Co(CNArMes2)4 at room temperature revealed ν(CN)
stretches at 2005 and 1941 cm-1, which reflect a decrease in electron
density at the Co center relative to the salts Na[Co(CNArMes2)4]
and [PPN][Co(CNArMes2)4].
With respect to the relationship between Co(CNArMes2
) and
4
Co(CO)4, it is important to note that uncertainty remains concerning
the ground state geometry of the latter. Early calculations by
Hoffman and Burdett favored a D2d-symmetric structure (i.e.,
In marked contrast to [Co(CNXyl)4]-, which readily dimerizes
to Co2(CNXyl)8 when oxidized,7b the encumbering ArMes2 units
stabilize a zerovalent cobalt tetraisocyanide monomer. Thus treat-
ment of Na[Co(CNArMes2)4] with 1 equiv of ferrocenium triflate
b2 a1 e4b1 ), with a C3V trigonal pyramidal structure (i.e., 1e42e4a1 )
lying ca. 2-3 kcal/mol higher in energy.12,13 More recent consid-
eration at the DFT level, however, reverses the stability of these
minima by comparable energies.14 In addition, spectroscopic studies
on matrix-generated Co(CO)4 have concluded that the relative
stabilities of its D2d and C3V isomers are highly dependent on the
identity of the matrix.15,16
2
2
1
1
(FcOTf) affords the neutral, paramagnetic complex Co(CNArMes2
)
4
as determined by X-ray diffraction (Figure 1b). To our knowledge,
Co(CNArMes2)4 is a unique example of an isolable zerovalent Co
monomer possessing a homoleptic π-acidic ligand field.11 Accord-
9
10.1021/ja1012382  2010 American Chemical Society
J. AM. CHEM. SOC. 2010, 132, 5033–5035 5033

C O M M U N I C A T I O N S
Table 1. Variable Temperature Crystallographic Parameters for
Co(CNArMes2
)
4
Temp
K
∠C1-Co-C1′
(deg)
∠C2-Co-C2′
(deg)
∠C1-Co-C2/
∠C1-Co-C2′ (deg)a
100
173
273
100.2(2)
101.5(2)
102.4(2)
107.8(2)
105.5(2)
103.8(2)
112.29(18)/ 112.14(18)
113.06(14)/112.00(14)
113.22(15)/ 112.32(15)
a Reflects crystallographic 2-fold symmetry.
Figure 1. Molecular structures of the cobalt-containing components of
[PPN][Co(CNArMes2)4] (A), Co(CNArMes2)4 at 100 K (B), [Co(CNArMes2)4]-
BArF (C), and [(THF)Co(CNArMes2)4]OTf (D).
4
Whereas the presence of two ν(CN) stretches in the room
temperature FTIR spectrum of Co(CNArMes2)4 is consistent with
both C3V and D2d symmetric environments, its X-ray structure clearly
indicates that a C3V geometry is not adopted. Most plausibly, the
encumbering ArMes2 groups destabilize a C3V geometry in favor of
a lower symmetry ground state. However, closer inspection of the
X-ray structure of Co(CNArMes2)4 determined at 100 K reveals that
the Co center possesses rigorous C2V point symmetry. As shown in
Figure 1b, the C1-Co-C1′ and C2-Co-C2′ angles which bisect
the molecular 2-fold rotation axis are 100.3(2)° and 107.7(2)°,
respectively, while the four remaining angles are split into two
statistically identical sets by crystallographic symmetry (i.e.,
112.29(18)° and 112.14(18)°). Remarkably however, the molecular
structure of Co(CNArMes2)4 exhibits a subtle temperature dependence
which can be probed crystallographically. Thus redetermination of
the X-ray structure at 273 K revealed that the C1-Co-C1′ and
C2-Co-C2′ angles expand and contract, respectively, while much
less variation in the remaining angles is observed (Table 1). Thus,
at higher temperatures Co(CNArMes2)4 seemingly approaches a more
symmetric, D2d geometry. Notably, a third structure determination
at 173 K revealed C1-Co-C1′ and C2-Co-C2′ angles intermedi-
ate between those of the 100 and 273 K structures (Table 1), thereby
providing further evidence for a structural progression to higher
symmetry with increasing temperature.
Figure 2. X-Band EPR spectra for toluene glass samples of Co(CNArMes2
at 100 K (top) and 4 K (bottom). A values in MHz.
)
4
2 bottom).17 Whereas a rhombic pattern at 4 K is indicated from
the simulation, the two low-field g-values of similar magnitude
reflect a system that is clearly near axial. Accordingly, such behavior
is consistent with a C2V-symmetric system that is only slightly
distorted from D2d symmetry.
We suggest the unusual structural and spectroscopic features of
Co(CNArMes2
) arise to achieve efficient delocalization of its
4
unpaired spin via π back-donation. Thus nominally D2d-symmetric
Co(CNArMes2
)
likely undergoes a C2V distortion to maximize
4
overlap of the CNArMes2 π* system with the singly occupied Co
2
2
d-orbital (i.e., dx -y in D2d symmetry). Accordingly, this mild
distortion would become an important mechanism for static
stabilization at low temperatures where thermal access to low-energy
vibrations is inhibited. Indeed, DFT vibrational frequency analysis
on the simple Co(CNMe)4 molecule in D2d symmetry mimicking
that of Co(CNArMes2
)
revealed a b2 vibration at 453 cm-1
4
representing a C-Co-C bending mode consistent with such a C2V
distortion (Figure 3). Furthermore, single-point energy calculations
on the model Co(CNArPh2)4 in the coordinates of both the 273 and
100 K structures indicate the latter to be more stable on average
by ca. 1.0 kcal/mol. Importantly, the magnitude of this thermody-
namic preference is consistent over several density functionals18
and corresponds roughly to the energy expected to excite such a
C-Co-C bending mode. Whereas C-Co-C bending as a means
of augmenting π delocalization may be more important for the CNR
function relative to CO due to its decreased π-acidity,19 we propose
that the structural perturbations of Co(CNArMes2)4 may potentially
also illustrate the reported behavior of D2d-symmetric Co(CO)4.16,20
In addition to the zero-valent state, the tetra-CNArMes2 frame-
work stabilizes monovalent Co centers. Treatment of Na[Co-
Importantly, Ozin has argued on the basis of variable-temperature
EPR studies that the D2d form of Co(CO)4 can further distort to
lower symmetry with decreasing temperature.16a In addition, it has
been suggested that temperature regimes exist where multiple low
symmetry (i.e., D2d and C2V) isomers of Co(CO)4 are in equilibri-
um.16 Therefore it is particularly noteworthy that a temperature
dependence is also manifest in the EPR signatures of Co-
(CNArMes2)4. Accordingly, the EPR spectrum of Co(CNArMes2
)
4
recorded at 100 K in toluene glass features a complicated pattern
1
that is not readily simulated as a single S ) /2 species (Figure 2
top). However, EPR acquisition at 4 K revealed a well-resolved
signal best fit with distinct g-values of 2.15, 2.14, and 2.01 (Figure
9
5034 J. AM. CHEM. SOC. VOL. 132, NO. 14, 2010

C O M M U N I C A T I O N S
reversible one-electron modulation of the 1+, 0, and 1- charge
states for the [Co(CNR)4] fragment. This latter characteristic is
expected to feature prominently in the multielectron reactivity
of the system. Accordingly, reactivity studies of these
[Co(CNArMes2)4]n complexes and more detailed analysis of their
spectroscopic and electrochemical properties are underway.
Acknowledgment. We thank UCSD, the ACS-PRF, and the
Camille and Henry Dreyfus Foundation for support. We are
indebted to Profs. Andrew S. Borovik and Michael P. Hendrich
for assistance with EPR acquisition and simulation, respectively.
Figure 3. DFT calculated geometry displacement for the origin (D2d) and
end points (C2V) of a b2 C-Co-C bending mode for D2d-symmetric
Co(CNMe)4.
Supporting Information Available: Synthetic procedures, spec-
troscopic, computational and crystallographic data (PDF and CIF). This
material is available free of charge via the Internet at http://pubs.acs.org.
(CNArMes2)4] with 2 equiv of FcOTf, followed by the addition of
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4
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Most interestingly, pyramidalization concomitant with a spin-
state change of the [Co(CNArMes2)4]+ core occurs on coordination
of Lewis bases. Thus, addition of 2 equiv of FcOTf to Na[Co-
(CNArMes2)4] in THF results in the paramagnetic THF adduct,
[(THF)Co(CNArMes2)4]OTf. Structural characterization of [(THF)Co-
(CNArMes2)4]OTf revealed a Co center best described as trigonal
bipyramidal with an equatorial THF ligand (Figure 1d). However,
only moderate pyramidalization is effected by the weak Lewis base
THF, as evinced by the C3-Co-C4 angle of 150.29(16)°. Evans
method magnetic moment determination resulted in a µeff value of
2.96((0.15) µB for [(THF)Co(CNArMes2)4]OTf, consistent with an
S ) 1 ground state. Furthermore, the paramagnetism exhibited by
[(THF)Co(CNArMes2)4]OTf is not a consequence of its preparation.
Thus, addition of 1.0 equiv of THF to a C6D6 solution of
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(17) A clear transition between these two distinct spectra is observed at 75 K.
See the Supporting Information.
(18) Single-point energies were assessed with the BP86, PW91, BYLP, and
OLYP density functionals (ADF2007.01). In all cases, the 100 K (C2V)
isomer was more stable. See the Supporting Information for details.
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(20) DFT vibrational frequency analysis of Co(CO)4 in D2d symmetry reveals a
b2 C-Co-C bending mode located at 515 cm-1 which is analogous to
that observed for Co(CNMe)4. See the Supporting Information for details.
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[Co(CNArMes2)4]BArF yields ArMes2 1H NMR shifts identical to
4
those of [(THF)Co(CNArMes2)4]OTf, thereby indicating anion
independence for ligation-induced spin-state changes. To our
knowledge, coordinatively induced S ) 0 to S ) 1 spin-state
changes are extremely rare, whereas the reverse are quite common.24
Accordingly, this coordinatively induced spin-state variation may
be reasonably expected to significantly affect the reactivity of
[Co(CNArMes2)4]+ with certain substrate molecules.
As
a
final note, both [(THF)Co(CNArMes2)4]OTf and
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[Co(CNArMes2)4]BArF4 react upon mixing with Na[Co(CNArMes2)4]
to generate Co(CNArMes2)4. These comproportionation reactions are
significant in that a seemingly large structural reorganization of
the [Co(CNR)4] fragment is accommodated along the pathway from
the mono- to zero-valent state. Furthermore, they establish fully
(24) Poli, R. Chem. ReV. 1996, 96, 2135–2204.
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