Oxidative group transfer to Co(I) affords a terminal Co(III) imido complex

Article abstract of DOI:10.1021/ja026852b

The synthesis of the first terminal imido complex of cobalt, [PhBP₃]CoN≡-p-tolyl, is reported. Its synthesis proceeds by oxidative group transfer from cobalt(I) upon addition of tolyl azide at room temperature. This species and a related η

Full text of DOI:10.1021/ja026852b

Published on Web 08/30/2002
Oxidative Group Transfer to Co(I) Affords a Terminal Co(III) Imido Complex
David M. Jenkins, Theodore A. Betley, and Jonas C. Peters*
DiVision of Chemistry and Chemical Engineering, Arnold and Mabel Beckman Laboratories of Chemical Synthesis,
California Institute of Technology, Pasadena, California 91125
Received May 9, 2002
Scheme 1
Atom and group-transfer reactions mediated by transition metal
centers represent a prominent and heavily scrutinized area of current
research in inorganic chemistry.1 Not only are such processes find-
ing relevance in the field of catalytic synthesis,2 but they have also
been proposed in many catalytic processes that occur in metallo-
protein active sites. First row transition metals that can accept and/
or release oxo and nitrene functionalities are particularly interesting.3
For the first row metals Fe, Co, Ni, and Cu, isolable complexes
with a terminal imido and/or oxo functionality bonded to a single
metal center, MdE or MtE (E ) O, NR), are extremely rare.4
This apparent incompatibility of later metals (groups 9, 10, and
11) with multiply bonded, strong π-donor ligands was overcome
in the third row more than 10 years ago (e.g., Cp*IrtNR and
Mes3IrtO).5,6
We recently prepared an anomalous low-spin cobalt(II) complex,
[PhBP3]CoI (1), exhibiting a distorted tetrahedral geometry.7,8 The
ground-state electronic configuration proposed for 1 (Figure 1b)
presumably arises from a strong axial distortion, geometrically
enforced by the [PhBP3] ligand, coupled with its strong ligand-
field-donor strength. These factors suggested to us that it should,
in principle, be possible to replace the iodide ligand by a divalent,
strongly π-donating ligand. This would conceptually afford an 18-
electron, closed-shell configuration similar to that of cobaltocenium
(Figure 1c). We therefore sought to install a terminal imido
functionality on the “[PhBP3]Co” unit and herein report a strategy
that proved viable.
Figure 2. Displacement ellipsoid (50%) representations of (a) complex 4,
and (b) complex 6. Bond lengths (Å) and angles (deg) for 4: Co-N,
1.658(2); Co-N-C46, 169.51(2); P1-Co-N, 115.32(6); P2-Co-N,
131.89(6); P3-Co-N, 125.64(6). For 6: Co-N1, 1.667(2); N1-N2,
1.280(2); N2-C27, 1.311(2); Co-N1-N2, 163.08(2); C27-N2-N1,
123.11(2).
cobalt(I) complex [PhBP3]Co(PMe3) (3). Complex 3 is bright green
in the crystalline state; its magnetic and EPR data establish a triplet
ground state, consistent with the qualitative splitting scheme
depicted in Figure 1a.
Delivery of a nitrene (or imido) functionality to the cobalt(I)
center was accomplished readily by addition of (p-tolyl)azide to 3
in benzene solution at 25 °C. Steady effervescence of nitrogen was
observed during the first several minutes of the reaction. This was
accompanied by a solution color change from brown to deep red.
The PMe3 consumes 1 equiv of added azide to form Me3PdN(p-
tolyl). A high isolated yield (97%) therefore requires 2 equiv of
azide. The diamagnetic, crystalline red product, [PhBP3]CotN-p-
tolyl (4), proved amenable to an X-ray diffraction study. Its solid-
state structure,9 shown in Figure 2a, reveals a pseudo-tetrahedral
complex in which the six phenyl rings of the [PhBP3] donor arms
flank the terminal imido ligand of the cobalt center. The imido
ligand is bent slightly (Co-N-C46 ) 169.51(2)°), and the very
short Co-N bond distance of 1.658(2) Å suggests strong multiple
bond character in the Co-N linkage. Complex 4 can be heated for
Figure 1. Qualitative splitting diagram assuming approximate C3V or Cs
symmetry for the frontier orbitals of (a) [PhBP3]Co-L; (b) Jahn-Teller
distorted low-spin [PhBP3]Co-X; (c) [PhBP3]CotE. The relative orbital
energies are not accurately known.
While several synthetic strategies were considered, the most
straightforward concerns a two-electron “NR” group-transfer reac-
tion to a suitable cobalt(I) derivative. Accordingly, the key cobalt(I)
precursor was prepared in two steps (Scheme 1). The addition of
PMe3 to a green solution of 1 in benzene resulted in the quantitative
formation of red [PhBP3]Co(I)(PMe3) (2). Complex 2, which was
characterized as a low-spin (SQUID, EPR), approximately trigonal
bipyramidal (X-ray) complex, then underwent smooth reduction
by sodium amalgam in THF solution to afford the pseudotetrahedral
* To whom correspondence should be addressed. E-mail: jpeters@caltech.edu.
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10.1021/ja026852b CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
days in toluene solution at 100 °C with only modest decomposition.
Moreover, the parent ion (M + H ) 850) can be observed by
electrospray MS. We also prepared an 15N-labeled phenyl complex,
[PhBP3]Co(15NPh) (5a), and its nonlabeled derivative, 5b, by an
analogous route. Difference IR spectra for 5a and 5b revealed a
band associated with coupled modes of the Co-NPh and the CoN-
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Ph linkages at 1340 cm-1. We have been unable to resolve the 15
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N
The solid-state structure of 4, its thermal stability, and its modest
reactivity (vide infra) are consistent with formulating the Co-N
interaction as a strong triple bond. This is plausible in view of
simple symmetry considerations and isolobal concepts.10 A preli-
mary DFT study on the geometry-optimized electronic structure
of 4 corroborated this suggestion and provided an orbital splitting
diagram consistent with the qualitative frontier orbital diagram
sketched in Figure 1c.11 The DFT study suggested the orbital of
2
predominantly dz parentage actually lies lower in energy than the
xy and x2 - y2 orbitals of the filled lower set. Two empty orbitals
of xz and yz parentage, strongly destabilized by a pair of orthogonal
π-bonds from the imido ligand, lie at higher energy.12,13
In a related reaction, we canvassed the ability of 3 to intercept
a diphenylcarbene unit from Ph2CN2. Rather than undergoing
carbene transfer and concomitant expulsion of N2, we found that 2
equiv of Ph2CN2 reacted with 3 to generate the phosphazine
Me3P(N2CPh2) and the thermally stable diazoalkane adduct com-
plex, [PhBP3]Co(N2CPh2) (6). Terminal diazoalkane adducts of
group 9 metals are very rare;14a,b the single cobalt diazoalkane
derivative previously reported exhibits side-on η2-coordination.14c
The X-ray structure of diamagnetic 6 was therefore of interest
(Figure 2b).15 Most prominent is that complex 6 features an η1-
ligated diazoalkane ligand.16 The Co-N bond length (1.667(2) Å)
is nearly as short as the Co-N distance observed in 4. This again
suggests strong multiple bond character at the Co-N linkage in 6
and perhaps explains the reluctance of this system to expel N2 under
mild conditions. The relatively short N1-N2 bond distance of
1.280(2) Å indicates there is still multiple bond character between
these two atoms. The two resonance contributors shown in Scheme
1 for 6 are emphasized.17
A cursory survey of the reactivity of 4 indicates that it is fairly
resistant to nitrene transfer chemistry. We did find that the imido
functionality can be transferred to carbon monoxide to produce the
isocyanate (OdCdN-p-tolyl), albeit sluggishly (14 equiv of CO,
70 °C, 12 days).18 The isolated cobalt(I) byproduct (90%) was the
diamagnetic dicarbonyl species [PhBP3]Co(CO)2 (7) (ν(CO) )
2008, 1932; KBr/THF).
The successive transformations 3 f 4 and 4 f 7 comprise a
system in which (i) a late first-row complex acts as an acceptor in
a high yielding, oxidative, two-electron-group-transfer process (3
f 4). The resulting species can then (ii) undergo a reductive two-
electron group-transfer process to deliver the accepted group to a
substrate (4 f 7). The method by which 4 is prepared suggests
that direct, two-electron group-transfer processes to cobalt, and
perhaps other later first-row metals, are possible. Maintaining
approximate 3-fold symmetry is a promising design strategy for
further developments in this area of synthesis.
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C. Chem. Commun. 2001, 2152. (b) Jenkins, D. M.; Di Bilio, A. J.; Allen,
M. J.; Betley, T.; Peters, J. C. 2002, submitted.
(8) [PhBP3] refers to anionic tridentate ligand [PhB(CH2PPh2)3]: (a) Peters,
J. C.; Feldman, J. D.; Tilley, T. D. J. Am. Chem. Soc. 1999, 121, 9871.
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2379.
(9) 4‚C6H6 (C55H51BCoNP3), MW ) 888.62, red plate, collection temperature
) 96(2) K, monoclinic, space group P21/c, a ) 14.1174(11) Å, b )
14.3252(11) Å, c ) 22.3306(18) Å, R ) 90°, â ) 96.202(1)°, γ ) 90°,
V ) 4489.6(6) Å3, Z ) 4, R1 ) 0.0415 [I > 2σ(I)], GOF ) 1.670.
(10) (a) Albright, T. A.; Burdett, J. K.; Whangbo, M. H. Orbital Interactions
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Organometallics 1992, 11, 4221.
(11) These studies will be elaborated in a forthcoming full paper. Briefly, a
geometry optimization was carried out with the program JAGUAR
(B3LYP/LACVP**) using the complete crystal coordinates of complex
4. Convergence was achieved, and the theoretical structure obtained for
4 was in fair agreement with that determined experimentally (Figure 2a).
(12) To correlate the frontier orbitals of [PhBP3]Co-Y fragments (Y ) PMe3,
I, or NR) to those of an octahedron, a redefinition of axes is required.
Using a notation in which the z-axis of an octahedral ML6 molecule
proceeds through the center of one triangular face of an octahedron, the
orbital parentages transform to the following: t2g set, z2; {(2/3)1/2(x2 - y2)
- (1/3)1/2yz}; {(2/3)1/2xy - (1/3)1/2xz}; eg set, {(1/3)1/2x2 - y2 + (2/3)1/2yz};
{(1/2)1/2xy + (2/3)1/2xz}. Under this notation, the ground-state electronic
configuration predicted for complex 4 is (z2)2 ({(2/3)1/2(x2 - y2) -
(1/3)1/2yz})2 ({(2/3)1/2xy - (1/3)1/2xz})2 ({(1/3)1/2x2 - y2 + (2/3)1/2yz})0
({(1/2)1/2xy + (2/3)1/2xz})0.
(13) Orgel, L. E. An Introduction to Transition-Metal Chemistry; Wiley: New
York, 1960; p 174.
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(15) 6 (C58H51BCoN2P3), MW ) 938.66, red plate, collection temperature )
98(2) K, monoclinic, space group P21/n, a ) 13.1284(8) Å, b )
16.5975(11) Å, c ) 21.8205(14) Å, R ) 90°, â ) 97.168(1)°, γ ) 90°,
V ) 4717.5(5) Å3, Z ) 4, R1 ) 0.0466 [I > 2σ(I)], GOF ) 1.572.
(16) An η1-hydrazido Co complex has been prepared: Korner, V.; Huttner,
G.; Vogel, S.; Barth, A.; Zsolnai, L. Chem. Ber. Recl. 1997, 130, 489.
(17) Hillhouse, G. L.; Haymore, B. L. J. Am. Chem. Soc. 1982, 104, 1537.
(18) Free isocyanate was observed (30% by 1H NMR integration; GC/MS,
133 m/z). An additional product(s) was also observed by 1H NMR (∼45%),
which presumably arises from thermal isocyanate degradation during the
course of the reaction.
Acknowledgment. We thank the NSF (CHE-0132216), the ACS
PRF, and the Dreyfus Foundation for financial support. D.M.J. is
grateful for an NSF predoctoral fellowship. The authors acknowl-
edge J. Christopher Thomas for assistance with preliminary DFT
calculations and Dr. Daniel Mindiola for insightful discussions.
Supporting Information Available: Experimental procedures
(PDF), characterization data, and crystallographic information (CIF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
JA026852B
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