Unusual molybdenum mediated C-N bond activation

Article abstract of DOI:10.1039/b101894i

The compounds [Mo(NPh)(η2-olefin){o-(Me₃ SiN)₂C₆H₄}] (olefin = propene 1a or isobutene 1b) react with excess pyridine affording the MO(IV) bis-pyridine complex, [Mo(N-Ph)(Py)₂{o,(Me₃ SiN)₂C₆H₄}] 2 which when heated to 90°C in toluene undergoes a C-N bond cleavage reaction and is converted to 3, a bimetallic molybdenum species; the crystal structures of both 2 and 3 are reported.

Full text of DOI:10.1039/b101894i

Unusual molybdenum mediated C–N bond activation†
Thomas M. Cameron, Khalil A. Abboud and James M. Boncella*
Department of Chemistry and Center for Catalysis, University of Florida, Gainesville, FL 32611-7200,
USA. E-mail: boncella@chem.ufl.edu
Received (in Irvine, CA, USA) 27th February 2001, Accepted 24th May 2001
First published as an Advance Article on the web 14th June 2001
The compounds [Mo(NPh)(h2-olefin){o-(Me3SiN)2C6H4}]
(olefin = propene 1a or isobutene 1b) react with excess
pyridine affording the Mo(IV) bis-pyridine complex, [Mo(N-
Ph)(Py)2{o-(Me3SiN)2C6H4}] 2 which when heated to 90 °C
in toluene undergoes a C–N bond cleavage reaction and is
converted to 3, a bimetallic molybdenum species; the crystal
structures of both 2 and 3 are reported.
are observed in the 1H NMR spectrum for these ortho protons:
one for the two ortho protons syn to the imido group and the
second for two ortho protons anti to the imido functionality.
These observations are consistent with slow rotation of the
pyridine rings about the Mo–N bond on the NMR time scale at
255 °C.
An X-ray structural analysis was carried out on a single
crystal of 2 grown at room temperature by layering a saturated
toluene solution of 2 with pentane. Selected bond lengths and
angles are listed in the legend to Fig. 1.‡ The solid-state
structure reveals a square pyramidal geometry about the Mo
atom, with the imido ligand in the apical position. The Mo–N(4)
and Mo–N(5) bond lengths are consistent with a Mo(IV)–Py
Lewis acid–base interaction. A space-filling model of 2, derived
from the X-ray study, reveals a sterically congested area around
the pyridine ligands due to the presence of the Me3Si groups.8
It is this steric crowding of the pyridine ligands by the Me3Si
groups that hinders their rotation.
Reactions providing a straightforward example of C–N single
bond activation, a most desirable transformation, are rare.1 The
metal-mediated rupture of C–N bonds is for the most part
limited to those of strained amines2 or amidines.3 Activation of
non-activated substrates such as aniline4 and the ring opening of
pyridine5 has been observed with highly reactive, trivalent,
Group 5 metal complexes. A slight variation on this theme
involves the recently reported C–N bond cleavage reactivity of
a Nb(II) cluster upon ligand replacement by anionic amides.6 An
observation related to this reaction type, made in 1985 by
Chisholm et al.,7 involved the isolation of a carbide/imide
cluster that may have arisen via degradation of an amide ligand.
We have recently been able to isolate a bimetallic molybdenum
complex 3 arising by C–N activation of the o-(Me3SiN)2C6H4
ligand in the bis-pyridine complex [Mo(NPh)(Py)2{o-(Me3-
SiN)2C6H4}] 2 (Scheme 1). We herein report the synthesis and
solid-state structures of 2 and 3 providing a rare example of C–
N activation.
The structure of the o-(Me3SiN)2C6H4 ligand in 2 differs
significantly from that observed in square pyramidal M(VI
)
complexes containing both NPh imido and o-(Me3SiN)2C6H4
ligands. In 2 the C6H4 ring, the N atoms, and the metal center are
nearly co-planar. In the related M(VI) complexes the ligand is
folded along the N–N vector. This folding has been attributed to
p-donation from the NSiMe3 lone pairs to the dxy orbital of the
d0 M(VI) metal center.9,10 In 2 such p-donation is unfavorable
since it would involve a filled–filled interaction between the
ligand and the d2 metal center. The flattened conformation of
the o-(Me3SiN)2C6H4 ligand in 2 places the SiMe3 groups in the
Addition of an excess of pyridine to a stirring pentane
solution of 1a or 1b resulted in the precipitation of 2 as a purple
solid that was isolated by filtration in high yield (Scheme 1).†
The room temperature 1H NMR spectrum of 2 displays a
significant broadening of the pyridine protons in the 2 and 6
positions. At low temperature (255 °C) two distinct resonances
Scheme 1
Fig. 1 Thermal ellipsoid plot of 2 (50% probability thermal ellipsoids).
Selected bond lengths (Å) and angles (°): Mo–N(1) 1.7476(14), Mo–N(2)
2.0779(16), Mo–N(3) 2.0637(16), Mo–N(4) 2.1247(16), Mo–N(5)
2.1460(16); C(1)–N(1)–Mo 166.35(13).
† Electronic supplementary information (ESI) available: characterisation of
complexes 2 and 3, 1H NMR spectra of 1–3 and space-filling diagram of 2.
See http://www.rsc.org/suppdata/cc/b1/b101894i/
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Chem. Commun., 2001, 1224–1225
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b101894i

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Notes and references
‡ All reactions and manipulations were carried out using standard Schlenk
techniques or in a dry-box under a nitrogen atmosphere. Complexes 1a and
1b were synthesized according to published procedures.12 A representative
synthesis of 2: to a green pentane solution of freshly generated 1b (1.07 g,
2.18 mmol) was added an excess of pyridine (0.51 g, 6.54 mmol). Upon
addition of pyridine, 2 precipitated from solution and was isolated by
filtration in 90% yield.
Synthesis of 3: a toluene solution of 2 (0.200 g, 0.336 mmol) was heated
to 90 °C in a sealed ampoule for 2 h. Concentration of the reaction mixture
under reduced pressure afforded 3 as a black solid in 92% yield.
§ Crystal data: for 2: C28H37MoN5Si2, M = 595.75, a = 15.5947(8), b =
10.3170(5), c = 18.4572(9) Å, V = 2969.6 Å3, orthorhombic, space group
Pna21, Z = 4, m(Mo-Ka) = 0.547 mm21, T = 173(2) K, final R1 =
0.0218, wR2 = 0.0517, GOF (on F2) = 0.994.
For 3: C41H59Mo2N7Si4·2CH2Cl2, M = 1124.04, a = 14.4655(8), b =
23.572(1), c
= 15.9891(9) Å, b = 104.390(1)°, V =
5280.8(5) Å3,
monoclinic, space group P21/n, Z = 4, m(Mo-Ka) = 0.805 mm21, T =
173(2) K, final R1 = 0.0384, wR2 = 0.0824, GOF (on F2) = 0.994.
The structures were solved by direct methods in SHELXTL5,13 and
refined using full-matrix least squares. The non-H atoms were treated
anisotropically, whereas the hydrogen atoms were calculated in ideal
positions and were riding on their respective carbon atoms. The asymmetric
unit of 3 consists of the complex and two dichloromethane molecules, one
of which is disordered and refined in three parts. Their site occupation
factors were dependently refined until the final cycle of refinement after
which they were fixed at 0.50, 0.35 and 0.15, respectively.
Fig. 2 Molecular structure of 3 (50% probability thermal ellipsoids). The
solvating dichloromethane molecules have been omitted for clarity.
Selected bond lengths (Å) and angles (°): Mo(1)–Mo(2) 2.5669(4), Mo(1)–
C(34) 2.179(3), Mo(2)–N(7) 2.047(2), Mo(2)–N(6) 1.745(2), Mo(2)–N(5)
2.285(3); Mo(1)–N(1)–Mo(2) 80.74(9), Mo(1)–N(2)–Mo(2) 81.48(9).
CCDC 159480 and 159481. See http://www.rsc.org/suppdata/cc/b1/
b101894i/ for crystallographic data in CIF or other electronic format.
1 CX (X = N, O) multiple bond cleavage reactions are more common.
See: K. A. Hall and J. M. Mayer, J. Am. Chem. Soc., 1992, 114, 10 402;
F.-M. Su, J. C. Bryan, S. Jang and J. M. Mayer, Polyhedron, 1989, 8,
1261; R. L. Miller, P. T. Wolczanski and A. L. Rheingold, J. Am. Chem.
Soc., 1993, 115, 10 422; K. E. Meyer, P. J. Walsh and R. G. Bergman,
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L. G. Sturgeoff, J. Am. Chem. Soc., 1982, 104, 4291.
2 G. Proulx and R. G. Bergman, J. Am. Chem. Soc., 1994, 116, 7953;
L. M. Atagi, D. E. Over, D. R. McAlister and J. M. Mayer, J. Am. Chem.
Soc., 1991, 113, 870; C. C. Cummins, R. R. Schrock and W. M. Davis,
Inorg. Chem., 1994, 33, 1448.
3 F. A. Cotton, J. H. Matonic, C. A. Murillo and X. Wang, Bull. Soc.
Chim. Fr., 1996, 133, 711; F. A. Cotton, L. M. Daniels, C. A. Murillo
and X. Wang, Inorg. Chem., 1997, 36, 896; J. R. Agadorn and J. Arnold,
Organometallics, 1994, 13, 4670.
4 J. B. Bonanno, T. P. Henry, D. R. Neithamer, P. T. Wolczanski and E. B.
Lobkovski, J. Am. Chem. Soc., 1996, 118, 5132.
5 T. S. Kleckley, J. L. Bennet, P. T. Wolczanski and E. B. Lobkowski,
J. Am. Chem. Soc., 1997, 119, 247; S. D. Gray, K. J. Weller, M. A.
Bruck, P. M. Briggs and D. E. Wigley, J. Am. Chem. Soc., 1995, 117,
10 678.
6 M. Tayebani, S. Gambarotta and G. P. A. Yap, Angew. Chem., Int. Ed.,
1998, 37, 3002; M. Tayebani, S. Gambarotta and G. P. A. Yap,
Organometallics, 1998, 17, 3639; M. Tayebani, K. Feghali, S.
Gambarotta and C. Bensimon, Organometallics, 1997, 16, 5084.
7 M. H. Chisholm, K. Folting, J. C. Huffman, J. Leonelli, N. S. Marchant,
C. A. Smith and C. E. Taylor, J. Am. Chem. Soc., 1985, 107, 3722.
8 See electronic supplementary information (ESI†).
9 A. Galindo, A. Ienco and C. Mealli, New J. Chem., 2000, 2, 73.
10 S. Wang, K. A. Abboud and J. M. Boncella, J. Am. Chem. Soc., 1997,
119, 11 990.
11 R. D. Adams, D. M. Collins and F. A. Cotton, Inorg. Chem., 1974, 13,
1086; R. J. Klingler, W. Butler and M. D. Curtis, J. Am. Chem. Soc.,
1975, 97, 3535.
basal plane of the molecule resulting in steric congestion
between the Py and SiMe3 groups which causes the hindered
rotation of the Py ligands.
Monomeric 2 is stable at room temperature under an inert
atmosphere for extended periods of time. However, when
heated to 90 °C in toluene, 2 converts cleanly to the bimetallic
compound 3 (Scheme 1) in > 90% yield over the course of 2 h.‡
This air-sensitive, diamagnetic compound is stable in solution
for extended periods of time at 90 °C.
A single crystal of 3 was grown from a pentane–dichloro-
methane solution at 230 °C. Compound 3 crystallizes with two
molecules of dichloromethane. An X-ray diffraction study
shows that 3 contains two Mo atoms bridged by two phenyl
imido groups as well as a Me3SiNC6H4 ligand (Fig. 2).§ This
unusual Me3SiNC6H4 group is apparently formed by cleavage
of one NSiMe3 group from an o-(Me3SiN)2C6H4 ligand. The
NSiMe3 group that was cleaved remains as an additional
terminal imido ligand on one of the Mo atoms. The formal
oxidation state at each metal center is best described as Mo( ).
V
The Mo(1)–Mo(2) distance of 2.5669(4) Å, although short for a
Mo–Mo single bond, indicates the existence of a metal–metal
bond, and accounts for the observed diamagnetism of 3.11 Four
upfield resonances, assigned to the four inequivalent Me3Si
groups, are observed in the 1H NMR spectrum of 3 and this is
consistent with the structure as determined by X-ray crys-
tallography.
The unusual C–N bond cleavage reaction that is observed
during the pyrolysis of 2 is presumably driven by the formation
of the Mo–N triple bond and demonstrates the reactivity of the
Mo(IV) moiety towards oxidation. Further reactivity studies of 2
and 3 are currently in progress.
12 T. M. Cameron, C. G. Ortiz, I. Ghiviriga, K. A. Abboud and J. M.
Boncella, Organometallics, 2001, 20, 2032; we have reported the
synthesis of 1b previously, T. M. Cameron, C. G. Ortiz, K. A. Abboud,
J. M. Boncella, R. T. Baker and B. L. Scott, Chem. Commun., 2000,
573.
We thank the National Science foundation (CHE 9523279)
for funding of this work. K. A. A. thanks the NSF and the
University of Florida for funding X-ray equipment
purchases.
13 SHELXTL/NT Version 5.10, Bruker Analytical X-ray Instruments,
Inc., Madison, WI, 53719, 1997.
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