Synthesis and reactivity of molybdenum(IV) complexes with alkyl and aryl isocyanides

Article abstract of DOI:10.1021/om0502436

The synthesis and characterization of the bis(isocyanide) complexes (RNC)₂Mo(NPh)(o-(Me₃SiN)₂C₆H ₄) (2: 2a, R = ^tBuNC; 2b, R = 2,6-dimethylphenyl) and the subsequent reactivity of these complexes with excess isocyanide have been reported. An X-ray crystal structure of (RNC)₂Mo(NPh)(o-(Me ₃SiN)₂C₆H₄) (R = 2,6-dimethylphenyl) is reported. Treatment of 2a with excess teri-butyl isocyanide resulted in the formation of the tris(isocyanide) complex (^tBuNC)₃Mo(NPh) (o-(Me₃SiN)₂C₆H₄) (3). Complex 3 exists in solution in equilibrium with 2a. Using a two-site exchange mechanism the activation barrier for the dissociative exchange of ^tBuNC has been calculated. The X-ray crystal structure of (^tBuNC) ₃Mo(NPh)(o-(Me₃SiN)₂C₆H₄ is reported. Computational studies (ONIOM) performed on 3 reveal that π conflicts in this complex results in the lengthening of the Mo-N(amido) _cis bond relative to the Mo-N(amido)_trans bond. Treatment of 2b with excess 2,6-dimethylphenyl isocyanide results in the slow insertion of the isocyanide ligands into the Mo-N bond of the chelating diamide ligand, resulting in a novel chelating iminocarbamoyl bis(isocyanide) complex, 4. An X-ray crystal structure of 4 is reported.

Full text of DOI:10.1021/om0502436

6310
Organometallics 2005, 24, 6310-6318
Synthesis and Reactivity of Molybdenum(IV) Complexes
with Alkyl and Aryl Isocyanides
Elon A. Ison, Carlos O. Ortiz, Khalil Abboud, and James M. Boncella*
Department of Chemistry and Center for Catalysis, University of Florida, P.O. Box 117200,
Gainesville, Florida 32611-7200, and Chemistry Division, Los Alamos National Laboratory,
Los Alamos, New Mexico 87545
Received March 31, 2005
The synthesis and characterization of the bis(isocyanide) complexes (RNC)2Mo(NPh)-
t
(o-(Me3SiN)2C6H4) (2: 2a, R ) BuNC; 2b, R ) 2,6-dimethylphenyl) and the subsequent
reactivity of these complexes with excess isocyanide have been reported. An X-ray crystal
structure of (RNC)2Mo(NPh)(o-(Me3SiN)2C6H4) (R ) 2,6-dimethylphenyl) is reported. Treat-
ment of 2a with excess tert-butyl isocyanide resulted in the formation of the tris(isocyanide)
complex (tBuNC)3Mo(NPh)(o-(Me3SiN)2C6H4) (3). Complex 3 exists in solution in equilibrium
with 2a. Using a two-site exchange mechanism the activation barrier for the dissociative
t
exchange of BuNC has been calculated. The X-ray crystal structure of (tBuNC)3Mo(NPh)-
(o-(Me3SiN)2C6H4) is reported. Computational studies (ONIOM) performed on 3 reveal that
π conflicts in this complex results in the lengthening of the Mo-N(amido)cis bond relative to
the Mo-N(amido)trans bond. Treatment of 2b with excess 2,6-dimethylphenyl isocyanide
results in the slow insertion of the isocyanide ligands into the Mo-N bond of the chelating
diamide ligand, resulting in a novel chelating iminocarbamoyl bis(isocyanide) complex, 4.
An X-ray crystal structure of 4 is reported.
Introduction
stability, and reactivity.9,10 We have demonstrated that
π donation by these ligands is important in stabilizing
the metal center and influencing the reactivity of these
complexes.11-14 Alkyl isocyanide insertion into W-alkyl
bonds has been shown to occur readily at room temper-
ature, affording η2-imino-acyl complexes that subse-
quently react via C-C coupling reactions of the η2-
imino-acyl group, resulting in the formation of diamide
ligands.15
Until now we have not seen any evidence for isocya-
nide insertion into the metal-amide bonds of the
(TMS)2pda ligand. Isocyanide insertion has been shown
to occur preferentially at metal-alkyl bonds as opposed
to metal-amide bonds, presumably because of the
stronger metal-amide bond.5-7,16-19 In this paper, we
In comparison to the well-known insertion of CO and
isocyanides, RNC, into metal-alkyl bonds,1-3 there are
fewer examples of the insertion reaction into metal-
amide bonds.4-7 The resulting metal-iminocarbamoyl
derivatives that are formed are important in the general
context of amination of organic substrates.8 Alkyl iso-
cyanides (RNC) can be regarded as isoelectronic with
CO, and their increased ability to act as Lewis bases
makes them good candidates to interact with electro-
philic metal centers.
Recent work in our group has focused on the use of
the chelating disubstituted phenylenediamide group {o-
(Me3SiN)2C6H4}2- ((TMS)2pda) as an ancillary ligand.
The facile high-yield synthesis of several group 6 dialkyl
complexes of Mo and W has allowed us to investigate
the properties of these metal complexes and determine
the influence of this ancillary ligand on their structure,
(9) Ortiz, C. G.; Abboud, K. A.; Boncella, J. M. Organometallics 1999,
18(21), 4253-4260.
(10) Boncella, J. M.; Wang, S. Y. S.; Vanderlende, D. D.; Huff, R.
L.; Abboud, K. A.; Vaughn, W. M. J. Organomet. Chem. 1997, 530(1-
2), 59-70.
* To whom correspondence should be addressed at Los Alamos
National Laboratory, P.O. Box 1663, Mail Stop J-582, Los Alamos, NM
87545. E-mail: boncella@lanl.gov. Tel: (505) 665-0795. Fax: (505) 667-
9905.
(11) Mills, R. C.; Abboud, K. A.; Boncella, J. M. Organometallics
2000, 19(16), 2953-2955.
(12) (a) Cameron, T. M.; Ghiviriga, I.; Abboud, E. A.; Boncella, J.
M. Organometallics 2001, 20(21), 4378-4383. (b) For a comparison of
diamide ligand folding in d2 and d0 complexes see: Ison, E. A.;
Cameron, T. M.; Abboud, K. A.; Boncella, J. M. Organometallics 2004,
23, 4070.
(1) Strauch, H. C.; Wibbeling, B.; Frohlich, R.; Erker, G. Organo-
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J. A. Organometallics 1984, 3(9), 1325.
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Chem. Commun. 2001(13), 1224-1225.
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(5) Martins, A. M.; Ascenso, J. R.; De Azevedo, C. G.; Dias, A. R.;
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Organometallics 2003, 22(21), 4218-4228.
(15) Huff, R. L.; Wang, S. Y. S.; Abboud, K. A.; Boncella, J. M.
Organometallics 1997, 16(8), 1779-1785.
(16) Crociani, B. In Reactions of Coordinated Ligands; Braterman,
P. S., Ed.; Plenum Press: New York, 1986.
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Organometallics 2000, 19(16), 3161-3169.
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10.1021/om0502436 CCC: $30.25 © 2005 American Chemical Society
Publication on Web 11/15/2005

Mo(IV) Complexes with Alkyl and Aryl Isocyanides
Organometallics, Vol. 24, No. 26, 2005 6311
Scheme 1
Figure 1. Thermal ellipsoid plot for 2b (50% probability
thermal ellipsoids). Selected bond lengths (Å) and angles
(deg): Mo-N(1) ) 1.746(2), Mo-N(2) ) 2.064(2), Mo-N(3)
) 2.093(2), Mo-C(19) ) 2.088(3), Mo-C(26) ) 2.085(3),
C(19)-N(4) ) 1.160(3), C(26)-N(5) ) 1.166(3); C(19)-
N(4)-C(20) ) 173.4(3), C(27)-N(5)-C(26) ) 176.9(2).
report the synthesis and characterization of bis(iso-
cyanide) complexes of Mo and the subsequent reactivity
of these complexes with excess isocyanide, yielding tris-
(∆ν(CN) ) -31 and -92, respectively). The larger
∆ν(CN) values observed for 2b are reflective of the
greater propensity for 2,6-dimethylphenyl isocyanide to
act as a π-acceptor ligand. This is in agreement with
the observation that aryl isocyanides are better π-ac-
ceptor ligands than alkyl isocyanides.16
X-ray Crystal Structure of 2b. Single crystals of
2b were obtained by slow diffusion of pentane into a
toluene solution of the complex. In addition to the
complex, the asymmetric unit contains half of a toluene
molecule located on an inversion center. The thermal
ellipsoid plot of 2b is depicted in Figure 1.
The geometry of 2b is best described as a distorted-
square-pyramidal structure with the phenyl imido
ligand occupying the axial position. The Mo-N(imido)
bond (1.746(2) Å) and the Mo-N(amido) bonds (2.064-
(2) and 2.093(2) Å) are in the normal range for Mo-N
triple bonds and Mo-N single bonds that have been
reported in our laboratories.10,13,21,22 The Mo-C(iso-
cyanide) bonds (2.088(3) and 2.085(3) Å) are shorter
than typical Mo-C(sp) (2.2 Å) bonds in related com-
plexes seen in our group, suggestive of π back-bonding
from the metal to the π* orbital of the isocyanide ligand.
The isocyanide ligands are linear, with C-N-C angles
of 173 and 177°. As has been reported for other terminal
isocyanide complexes, there is no significant increase
in the CtN bond distance (average 1.17 Å).23-26
t
(isocyanide) complexes in the case of BuNC and an
unusual chelating iminocarbamoyl bis(isocyanide) com-
plex in the case of 2,6-dimethylphenyl isocyanide. X-ray
structures of a bis(isocyanide) complex, a tris(iso-
cyanide) complex, and the iminocarbamoyl bis(iso-
cyanide) complex are reported.
Results and Discussion
Synthesis of Bis(isocyanide) Complexes. Treat-
ment of a pentane solution of the isobutylene complex
(η2-isobutylene)Mo(NPh)(o-(Me3SiN)2C6H4) (1) with 2
equiv of tert-butyl isocyanide resulted in the precipita-
tion of ((tBuNC))2Mo(NPh)(o-(Me3SiN)2C6H4) (2a) as
green microcrystals (Scheme 1). The structure of 2a has
1
been assigned by H, 13C, and IR spectroscopy and is
consistent with a square-pyramidal structure, with the
imido ligand occupying the apical position as depicted
in Scheme 1.
Similarly, treatment of 1 with 2 equiv of 2,6-dimeth-
ylphenyl isocyanide afforded ((xylNC))2Mo(NPh)(o-(Me3-
SiN)2C6H4) (2b; xyl ) 2,6-dimethylphenyl), as a dark
red crystalline material. Resonances for the SiMe3
protons are observed at 0.73 ppm (18H), while the RNC
(R ) 2,6-dimethylphenyl) protons are observed at 2.11
ppm (12H). Two isocyanide stretches in the IR spectrum
of 2a are observed at 2122 (CN symmetric stretch) and
2082 cm-1 (CN asymmetric stretch). The appearance of
these stretches at a lower frequency than for the free
isocyanide ligand (∆ν(CN) ) -9 and -50 cm-1, respec-
tively, where ∆ν(CN) ) (stretching frequency of com-
plex) - (stretching frequency of free isocyanide)) is
indicative of back-bonding from the metal to the iso-
cyanide ligand.16,20 By comparison, the isocyanide
stretches for 2b are observed at 2083 and 2022 cm-1
(20) Ghebreyessus, K. Y.; Nelson, J. H. Inorg. Chim. Acta 2003, 350,
12-24.
(21) Cameron, T. M.; Ortiz, C. G.; Ghiviriga, I.; Abboud, K. A.;
Boncella, J. M. Organometallics 2001, 20(10), 2032-2039.
(22) Ortiz, C. G.; Abboud, K. A.; Cameron, T. M.; Boncella, J. M.
Chem. Commun. 2001(3), 247-248.
(23) Pombeiro, A. J. L.; Da Silva, C. G.; Michelin, R. A. Coord. Chem.
Rev. 2001, 218, 43-74.
(24) Michelin, R. A.; Pombeiro, A. J. L.; Fatima, M.; Da Silva, C. G.
Coord. Chem. Rev. 2001, 218, 75-112.
(25) Hahn, F. E.; Lugger, T. J. Organomet. Chem. 1995, 501(1-2),
341-346.
(19) Thirupathi, N.; Yap, G. P. A.; Richeson, D. S. Organometallics
2000, 19(13), 2573-2579.
(26) Carofiglio, T.; Floriani, C.; Chiesivilla, A.; Guastini, C. Inorg.
Chem. 1989, 28(24), 4417-4419.

6312 Organometallics, Vol. 24, No. 26, 2005
Ison et al.
Scheme 2
Scheme 3
Like the d2 complexes previously reported in our
laboratories, the diamide ligand in this complex is flat
(fold angle 173.7°).13 The fold angle results from the fact
that π donation of the diamide lone pair electrons into
the metal dxy orbital is prevented because of the pres-
ence of a pair of d electrons in this orbital. The diamide
lone pair electrons interact to a lesser extent with the
cyanide ligands were found to occupy meridional posi-
tions. The imido nitrogen was found to be trans to one
of the (TMS)2pda nitrogens, while the other (TMS)2pda
nitrogen was trans to one of the isocyanide groups. The
shorter bond length of Mo-C(24) is attributed to π back-
bonding from the metal d orbitals to this isocyanide
ligand. Angles around the metal center deviate only
slightly from idealized octahedral angles. The smallest
angle was the N(2)-Mo-N(3) angle of 77.08°, while the
largest angle was N(1)-Mo-N(2) at 106.92°. The acute
N(2)-Mo-N(3) angle can be attributed to the small bite
angle of the (TMS)2pda chelate ring.
Complexes 2 and 3 represent rare examples of iso-
cyanide complexes incorporating the isocyanide ligands
and the M-N(imido) moiety. While many transition-
metal isocyanide complexes have been reported,25,26,30
a search of the Cambridge Structural Database revealed
only two other examples of complexes incorporating the
2
metal dyz and dz orbitals because they are destabilized
by π donation from the imido lone pair electrons.14,27-29
Synthesis of a Tris (isocyanide) Complex. When
1 was allowed to react with 4 equiv of tert-butyl
isocyanide, a fast color change from green to purple
occurred, and purple microcrystals of the tris(iso-
cyanide) complex (tBuNC)3Mo(NPh)(o-(Me3SiN)2C6H4)
(3) precipitated from solution and were easily isolated
by filtration.
The pseudo-octahedral structure of 3, as shown in
Scheme 2, was proposed on the basis of the observed
NMR data and confirmed by X-ray crystallography.
Resonances corresponding to inequivalent SiMe3 pro-
tons were observed at 0.58 (br s, 9H) and 0.77 ppm (br
s, 9H). Broad singlets at 0.93 and 1.17 ppm were
assigned to inequivalent tert-butyl isocyanide peaks. A
plane of symmetry containing the imido nitrogen,
(TMS)2pda nitrogens, and one isocyanide ligand makes
the remaining isocyanide ligands chemically equivalent.
1
The broadening observed in the H NMR spectrum
of 3 has been correlated to an equilibrium involving fast,
reversible ligand dissociation. As shown in Scheme 3,
loss of one RNC ligand induces a plane of symmetry in
the bis(isocyanide) complex, 2a, that is formed. Thus,
the remaining isocyanide ligands and the SiMe3 ligands
become equivalent in 2a. Using the two-site exchange
approximation, an activation barrier of ∆Gq ) 15.0 kcal/
mol for the ligand dissociation was calculated.
X-ray Crystal Structure of 3. Single crystals of 3
were obtained from slow evaporation of a concentrated
pentane solution of 3. As shown in Figure 2, 3 was found
to have a distorted-octahedral geometry. Three iso-
Figure 2. Thermal ellipsoid plot for 3 (30% probability
thermal ellipsoids). Selected bond lengths (Å) and angles
(deg): Mo-N(1) ) 1.804(4), Mo-N(2) ) 2.166(5), Mo-N(3)
) 2.130(5), Mo-C(24) ) 2.067(5), Mo-C(29) ) 2.158(5),
Mo-C(19) ) 2.137(2), -N(2) ) 1.182(3); C(2)-N(2)-C(8)
) 152.8(5).
(27) Del Rio, D.; Galindo, A. J. Organomet. Chem. 2002, 655(1-2),
16-22.
(28) Galindo, A.; Ienco, A.; Mealli, C. New J. Chem. 2000, 24(2), 73-
75.
(29) Galindo, A.; Ienco, A.; Mealli, C. Comments Inorg. Chem. 2002,
23(6), 401-416.

Mo(IV) Complexes with Alkyl and Aryl Isocyanides
Organometallics, Vol. 24, No. 26, 2005 6313
Table 1. Selected Bond Lengths (Å) and Angles
(deg) for 3
imido and isocyanide moieties: the cationic Re complex
[ReCl(NO6H4Me-4)(OMe)(tBuNC)2(PPh3)]31 and the neu-
tral Cr(IV) complex Cr(Nmes)(Smes)2(CNxyl)2.32 In ad-
dition, the complex [PhNVCl3(tBuNC)3] was reported by
Floriani and co-workers.26
The Mo-N(diamide) bonds also reveal an interesting
illustration of the π conflicts that arise in complexes
incorporating the imido and diamide moieties. The
Mo-N(diamide) bonds Mo-N(2) and Mo-N(3) are
longer than typical Mo-N(diamide) bonds that have
been observed in our group. For example, the Mo-
N(diamide) bonds in 3 are significantly longer than the
corresponding bonds in both the five-coordinate complex
2b (vide supra) and the previously reported six-
bond
ONIOM
X-ray
Mo-N(1)
1.824
2.178
2.141
2.149
2.013
2.131
1.805(3)
2.166(3)
2.130(3)
2.137(5)
2.067(5)
2.158(5)
Mo-N(2)
Mo-N(3)
Mo-C(1)
Mo-C(2)
Mo-C(3)
C(4)-N(5)-C(2)
175.5
153.0(4)
the mixing of the metal dxy orbital with the dyz orbital;
this brings the dxy orbital slightly out of the xy plane.
This repulsive interaction is also minimized in 3,
because of the presence of a π-acceptor ligand trans to
the diamide lone pair that can accept electron density
from the metal d orbital. In the tris(phosphine), π-ac-
ceptor orbitals are not present; consequently the
Mo-N(amido)cis bond is significantly longer.
coordinate
complex
(THF)Mo(NPh)(Cl2)(o-(Me3-
SiN)2C6H4).9 In addition, the Mo-N(amido)cis bond, Mo-
N(2), is longer the Mo-N(amido)trans bond, Mo-N(3),
at 2.166(5) and 2.130(5) Å, respectively. This trend is
also seen in other d2 six-coordinate complexes observed
in our group. For example, in the previously reported
tris(phosphine)
complex
(PMe3)3Mo(NPh)(o-(Me3-
SiN)2C6H4),33 the Mo-N(amido)cis bond is also longer
than the Mo-N(amido)trans bond, 2.233(1) and 2.141(1)
Å, respectively. These results at first glance appear to
be counterintuitive, given the expected trans influence
of the imido ligand. However, the elongation of the
amido Mo-Ncis bond in these complexes results from a
filled-filled interaction between an occupied metal dxy
orbital and a lone pair on the cis diamide nitrogen.
The interaction between the lone pair electrons in the
(TMS)2pda ligand and Mo was explored by the hybrid
DFT method ONIOM, developed by Morukuma and co-
workers.34-36 For these calculations a two-layer ONIOM
method was employed. For the inner layer the o-
phenylene group (o-C6H4) that links the two N atoms
of the diamido ligand was simplified to a -CHdCH-
carbon chain. The organic groups on the nitrogen atoms
(SiMe3 of the amido and phenyl group of the imido) were
replaced by hydrogen atoms for simplicity, and the
substituents on the isocyanide ligand in 3 were replaced
by hydrogen atoms. For this layer the B3LYP/LANL2DZ
method was employed. The outer layer consisted of the
entire complex, including all substituents on the imido,
phenylene diamide, and isocyanide ligands, and was
modeled by the B3LYP/LANL2MB method. As shown
in Table 1, there is good agreement between the
calculated geometries and the crystal structure of 3.
Figure 3. Interaction of diamide lone pairs and isocyanide
ligand with metal dxy orbitals. (Orbitals are displayed at
the 0.05 au isocontour level.)
These results are a further illustration of the conse-
quences of π loading in group 6 imido-diamido com-
plexes that have been emphasized in our group. The
competition among π-donor ligands for empty metal
π-acceptor orbitals results in the unusual reactivity of
these complexes. We believe the weakening of the
diamide bonds by the repulsive interaction between the
metal and the diamide lone pairs leads to increased
reactivity of the diamide ligand (vide infra).
Synthesis, Structure, and Dynamics of a Chelat-
ing Iminocarbamoyl Complex. Treatment of a C7D8
solution of 2b with 2 equiv of 2,6-dimethylphenyl
isocyanide results in slow conversion (24 h) to the
iminocarbamoyl complex 4 (Scheme 4). Complex 4 may
be obtained on a preparative scale by treating the
metallocyclopentane complex 5, (CH2)4Mo(NPh)(o-(Me3-
SiN)2C6H4), with excess 2,6-dimethylphenyl isocyanide
(vide supra). The chemical shifts of the SiMe3 protons
occur at 0.44 (s, 9H) and 0.59 ppm (bs, 9H). Signals for
the methyl groups of the 2,6-dimethylphenyl isocyanide
ligands are observed at 1.94 ppm (s, 12H), and two
signals were observed for the 2,6-dimethylphenyl pro-
tons at 2.45 (s, 6H) and 1.86 ppm (s, 6H). As shown in
Scheme 4, the iminocarbamoyl ligand is coordinated in
an η1 and η2 fashion to Mo. The chemical shifts for the
quaternary carbons of the iminocarbamoyl ligands are
observed at 208.2 and 181.4 ppm for the η2 and η1
carbons, respectively. One absorption was observed in
the IR spectrum for the asymmetric CN stretch (2088
The presence of an unoccupied orbital on the metal
limits the π orbital donation by the diamide ligand.
Thus, in the HOMO for 3 (Figure 3) the diamide lone
pairs are primarily ligand centered. The energetically
unfavorable filled-filled interaction between the dia-
mide ligand and the metal is attenuated somewhat by
(30) Jameson, G. B.; Ibers, J. A. Inorg. Chem. 1979, 18(5), 1200-
1208.
(31) Ahmet, M. T.; Coutinho, B.; Dilworth, J. R.; Miller, J. R.;
Parrott, S. J.; Zheng, Y. F. Polyhedron 1996, 15(12), 2041-2050.
(32) Danopoulos, A. A.; Wilkinson, G.; Sweet, T. K. N.; Hursthouse,
M. B. Polyhedron 1996, 15(5-6), 873-879.
(33) Cameron, T. M.; Ortiz, C. G.; Ghiviriga, I.; Abboud, K. A.;
Boncella, J. M. J. Am. Chem. Soc. 2002, 124(6), 922-923.
(34) Vreven, T.; Morokuma, K. J. Comput. Chem. 2000, 21(16),
1419-1432.
(35) Vreven, T.; Morokuma, K. J. Chem. Phys. 2000, 113(8), 2969-
2975.
(36) Vreven, T.; Morokuma, K. J. Chem. Phys. 1999, 111(19), 8799-
8803.

6314 Organometallics, Vol. 24, No. 26, 2005
Ison et al.
Scheme 4
Figure 4. Thermal ellipsoid plot for 4 (30% probability
thermal ellipsoids). Selected bond lengths (Å) and angles
(deg): Mo-N(1) ) 1.767(2), Mo-N(2) ) 2.108(2), Mo-C(57)
) 2.110(3), Mo-C(48) ) 2.131(3), Mo-C(14) ) 2.147(3),
Mo-C(7) ) 2.052(3), N(2)-C(7) ) 1.274(3), N(3)-C(14) )
1.318(4), N(4)-C(7) ) 1.365(3), N(4)-C(15) ) 1.454(3);
C(21)-N(5)-C(14) ) 124.4(3), C(14)-N(5)-Si(2) ) 113.4-
(2), Si(2)-N(5)-C(21) ) 122.1, C(57)-N(7)-C(58) ) 166.9,
C(48)-N(6)-C(49) ) 171.8(3).
cm-1) at a lower frequency than for the free isocyanide
(∆ν(CN) ) -25 cm-1), again suggestive of net π back-
bonding in these complexes.
species is rapidly trapped by two molecules of 2,6-
dimethylphenyl isocyanide (Scheme 5).
Isocyanide insertion into metal-amide bonds has
been less extensively studied than the related insertion
reactions into metal-alkyl bonds. In recent years,
studies of isocyanide insertions into Ta-amide bonds
have been reported.6,18,37,38 We propose that the forma-
tion of 4 from 2b occurs by initial coordination of two
isocyanide ligands, followed by the rapid insertion of the
isocyanides into the metal-amide bond. A 1,3-silatropic
shift then follows, and the resulting iminocarbamoyl
X-ray Crystal Structure of 4. Single crystals of 4
were obtained by cooling a concentrated pentane solu-
tion of 4 to 0 °C (Figure 4). The geometry about Mo may
best be described as a distorted trigonal bipyramid, with
the isocyanide ligands occupying the axial position and
the iminocarbamoyl fragments and the imido ligand
occupying the equatorial plane. A plane of symmetry
bisects the two trans isocyanide ligands and passes
Scheme 5

Mo(IV) Complexes with Alkyl and Aryl Isocyanides
Organometallics, Vol. 24, No. 26, 2005 6315
Figure 5. Space-filling diagram generated from the crystal structure of 4. The Si-N bond distances (1.78 and 2.53 Å) are
within the sum of the van der Waals radii of N and Si atoms (3.60 Å).
through the imido and iminocarbamoyl fragments. The
iminocarbamoyl fragment is flat (rms ) 0.02); conse-
quently, the substituents on the amido nitrogens (N(5)
and N(4)) are sterically encumbered (vide infra).
The Mo-N(1) bond length (1.767(2) Å) is within the
normal range for a molybdenum-nitrogen triple bond.
One iminocarbamoyl fragment binds in an η1 fashion
to the Mo center and shows localized C-N bonds: i.e.,
one short CdN bond, (C(14)-N(3), 1.318(4) Å) and one
long C-N single bond (C(14)-N(5), 1.454(3) Å). The η2-
iminocarbamoyl fragment exhibits a very short C-N
Figure 6. (a) Variable-temperature 1H NMR spectra
bond (C(7)-N(2), 1.274(3) Å) and a long C-N bond
(C7D8) of 4. Broadening of the SiMe3 protons as the
(C(7)-N(4), 1.365(3) Å). Both of the amido nitrogens
temperature is lowered. At 348 K two peaks are observed
N(4) and N(5) are sp2 hybridized, with carbon-nitrogen
for the SiMe3 protons labeled Si(1) and Si(2). When the
bond lengths shorter than those typically seen for C-N
sample is cooled, the protons for Si(1) broaden into the
single bonds (1.48 Å), and the sum of the angles around
each nitrogen is equal to 359.9°. This suggests delocal-
baseline. (b) 1H NMR spectrum (C7D8) of 4 at the low-
temperature limit (188 K). Below the coalescence temper-
ization within the CNC framework of the iminocarbam-
oyl group. The bond lengths for the isocyanide ligands
(average 2.12 Å) are shorter than typical Mo-C(sp2)
bonds, suggestive of π back-bonding to isocyanide in
these complexes. The C-N-C isocyanide bond angles
deviate only slightly from linearity (average 169.4°).
The space-filling model of 4, generated from the X-ray
study, reveals a very sterically congested region in the
plane of the iminocarbamoyl fragment (Figure 5). This
suggests that free rotation about the carbon-nitrogen
bonds C(7)-N(4) and C(14)-N(5) is restricted because
of steric hindrance around the nitrogen atoms. For this
same reason free rotation of the xylyl fragments about
N(4) and N(5) is restricted because of steric interactions.
The silicon atom Si(2) is located close to two nitrogens;
this atom is directly bonded to N(5) (bond length 1.78
Å) but is also close to N(3) (2.53 Å). The latter distance
is less than the sum of the van der Waals radii of a
nitrogen and a silicon atom (3.60 Å), and as is evident
ature, two peaks are observed for the Si(1) protons at 1.13
and 0.09 ppm in a 5:1 ratio.
from the space-filling diagram, Si(2) interacts with both
N(3) and N(5). These close contacts may explain the
fluxionality of this molecule in solution (vide infra).
Dynamic Behavior of 4 in Solution. Complex 4 is
fluxional in solution. At room temperature two SiMe3
protons can be observed at 0.52 (s, 9H) and 0.69 ppm
(bs, 9H). When a C7D8 solution of 4 is heated to 348 K,
the broad singlet sharpens and grows in intensity. The
resonance observed at 0.69 ppm broadens into the
baseline as this sample is cooled (Figure 6). Below the
coalesence temperature (218 K) two peaks reemerge at
1.13 and 0.09 ppm in a 5:1 ratio. This fluxionality can
be explained by a 1,3-silatropic rearrangement of the
SiMe3 group that has close contacts to two nitrogen
atoms (Scheme 6).
Recall from Figure 6 that the group Si(2) has close
contacts between both N(5) and N(3). Below the coales-
cence temperature the equilibrium shown in Scheme 6
will be frozen in solution and two species will be
detected that differ in the position of SiMe3(2). In one
(37) Sanchez-Nieves, J.; Royo, P. J. Organomet. Chem. 2001,
621(1-2), 299-303.
(38) Gomez, M.; Gomezsal, P.; Jimenez, G.; Martin, A.; Royo, P.;
Sancheznieves, J. Organometallics 1996, 15(16), 3579-3587.

6316 Organometallics, Vol. 24, No. 26, 2005
Ison et al.
a
Scheme 6
and alkynes with alkyl and aryl isocyanides. In com-
parison with those for the related carbonylative cycliza-
tion, reports of related isocyanide-inserted cyclization
are quite rare. Tamoa et al. published the pioneering
work of the Ni(0)-induced cyclizations of diynes in
1989.40 Since then, isocyanide insertion into zirconacy-
clopentadiene has been demonstrated by Takahashi et
al.41 The Buchwald group42,43 and the Shibata group44
have demonstrated the only two examples of catalytic
coupling of alkynes and alkenes with alkyl isocyanides.
We are currently exploring the catalytic possibilities of
our complexes in the [2 + 2 + 1] cyclization of alkynes,
alkenes, dienes, and diynes with alkyl and aryl iso-
cyanides.
a Coaxial ligands have been removed for clarity.
Summary and Conclusions
Scheme 7
In this paper, we have synthesized rare examples of
bis- and tris(isocyanide) complexes incorporating the
isocyanide and imido moieties. We have also demon-
strated that the chelating diamide ligand can be acti-
vated toward isocyanide insertion. This work is an
extension of our ongoing studies on the importance of π
ligand interactions and their role in influencing the
outcome of chemical reactivity of group 6 complexes.
The insertion reactions seen between 2,6-dimeth-
ylphenyl isocyanide and the diamide ligand results from
the π conflict that arises between the diamide nitrogen
lone pairs and an occupied d orbital that results in the
weakening of the Mo-N(diamide) bond and ultimately
a lowering of the barrier to insertion. These results are
promising, because they suggest that these insertion
reactions may lead to the development of a catalyst for
the carbonylation of amines.8 These species have been
shown to be useful as precursors in organic synthesis.
We are currently investing the potential catalytic abili-
ties of our complexes.
position the group SiMe3(2) is bound to N(5), and in the
other position it is bound to N(3). The populations of
these sites are not equal; thus, the ratio of the two site
populations is N(5):N(3) ) 5:1. Above the coalescence
temperature the chemical shifts for the SiMe3 group are
observed as an average of the chemical shifts for the
two sites; i.e., above Tc the SiMe3 group “feels” both N(5)
and N(3). The exchange of unequally populated peaks
has been described in more detail by Sandstrom in ref
39.
Experimental Section
All operations were conducted in an inert atmosphere using
standard Schlenk techniques or in a nitrogen-filled drybox.
Diethyl ether, pentane, toluene, and tetrahydrofuran were
distilled under nitrogen from sodium or sodium benzophenone
ketyl, stored over molecular sieves, and degassed prior to use.
Ethylene was predried by passing the gas through a column
of molecular sieves. Complexes 1, 2a,b, and 5 were synthesized
according to published procedures.9-13 NMR spectra were
recorded on Varian (300 MHz) Gemini300, VXR300, and
Mercury300 instruments. Chemical shifts are reported in ppm
relative to the 1H and 13C residues of the deuterated solvents.
Elemental analyses were performed by Complete Analysis
Laboratories Inc., Parsipanny, NJ.
[2 + 2 + 1] Cycloaddition Reactions of Metalla-
cycles with Alkyl and Aryl Isocyanides RNC. Our
t
previous work with W alkyls and BuNC seemed to
suggest that isocyanide insertion in the case of W
proceeded selectively into W-alkyl bonds over the
W-diamide bond.15 We were interested in investigating
whether isocyanide insertion was also competitive in the
case of Mo-alkyl complexes; therefore, we pursued the
reactions of the Mo metallacyclopentane complex 5 with
isocyanides. Addition of 4.0 equiv of tBuNC to 5 results
in the formation of 2a and the iminocyclopentane
molecule C5H8NtBu (Scheme 7), while the addition of
5.0 equiv of 2,6-dimethylphenyl isocyanide to 5 results
in the formation of the iminocyclopentane molecule
C5H8N(xylyl) and 4. The reactions outlined in Scheme
7 present a rare example of metal-mediated intermo-
lecular [2 + 2 + 1] cycloaddition reactions of alkenes
Synthesis of Bis(isocyanide) Complexes. [(cis-tBuNC)2-
Mo(NPh)(o-(Me3SiN)2C6H4)] (2a). A pentane solution of 1
(1.00 g, 2.03 mmol) was charged with tert-butyl isocyanide
(0.24 mL, 4.06 mmol). Green crystals of 2a quickly precipi-
tated. The suspension was filtered and washed several times
with pentane. Excess solvent was removed in vacuo to afford
(40) Tamao, K.; Kobayashi, K.; Ito, Y. J. Org. Chem. 1989, 54, 3517.
(41) Takahashi, T.; Tsai, F. Y.; Li, Y. Z.; Nakajima, K. Organo-
metallics 2001, 20, 4122.
(42) Berk, S. G.; Grossman, R. B.; Buchwald, S. L. J. Am. Chem.
Soc. 1994, 116, 8593.
(43) Berk, S. G.; Grossman, R. B.; Buchwald, S. L. J. Am. Chem.
Soc. 1993, 115, 4912.
(39) Sandstrom, J. Dynamic NMR Spectroscopy; Academic Press:
London, 1982.
(44) Shibat, T.; Yamashita, K.; Katyama, E.; Takagi, A. Tetrahedron
2002, 58, 8661.

Mo(IV) Complexes with Alkyl and Aryl Isocyanides
Organometallics, Vol. 24, No. 26, 2005 6317
Table 2. Crystal Data and Data Collection
Parameters for (RNC)2Mo(NPh)(o-(Me3SiN)2C6H4)
(2b; R ) 2,6-Dimethylphenyl),
2a as a green microcrystalline material in 52% yield. Complex
2a is thermally unstable in solution and in the solid state and
disproportionates to 2a and an unidentified material at 20 °C
overnight. Because of this, elemental analysis was not at-
tempted on this compound. Complex 2a can be stored as a solid
at -30 °C. 1H NMR (C6D6): δ 0.73 (s, 18H, SiMe3), 1.06 (s,
18H, tBuNC), 6.81 (tt, 2H, phenyl imido meta proton, 7.4, 1.2
Hz), 6.93 (t, 1H, phenyl imido para proton, 7.4 Hz), 7.05 (dd,
2H, o-phenylene diamide proton, 5.8, 3.3 Hz), 7.32 (dd, 2H,
phenyl imido ortho proton, 7.5, 1.3 Hz), 7.48 (dd, 2H, o-
phenylene diamide proton, 5.6, 3.4 Hz). 13C NMR (C7D8):
234.9, 187.1, 151.7, 128.6, 124.3, 123.7, 117.2, 117.1, 58.7, 30.3,
5.5.
[(cis-2,6-dimethylphenyl-NC)2Mo(NPh)(o-(Me3SiN)2-
C6H4)] (2b). A pentane solution of 1 (1.00 g, 2.03 mmol) was
charged with 2,6-dimethylphenyl isocyanide (0.37 g, 2.82
mmol). Dark red crystals of 2b quickly precipitated. The
suspension was filtered and washed several times with pen-
tane. Excess solvent was removed in vacuo to afford 2b as a
dark red microcrystalline material in 72% yield. 1H NMR
(C6D6): δ 7.52 (dd, 2H, o-phenylene diamide proton, 5.7, 3.5
Hz), 7.46 (d, 2H, phenyl imido ortho proton, 7.4 Hz), 7.07 (dd,
2H, o-phenylene diamide proton, 5.8, 3.4 Hz), 6.99 (t, 2H,
phenyl imido meta proton, 7.2 Hz), 6.89 (t, 1H phenyl imido
para proton, 7.3 Hz), 6.64 (ov multiplet, 6H, isocyanide phenyl
protons), 2.12 (s, 12H, CH3), 0.74 (s, 18H, SiMe3), 13C NMR
(C7D8): δ 200.9, 151.4, 131.5, 128.8, 125.2, 124.3, 117.8, 117.5,
18.8, 5.5. Complex 2b loses a molecule of 2,6-dimethylphenyl
isocyanide during the elemental analysis. Anal. Calcd for
C27H36MoN4Si2: C, 57.02; H, 6.38; N, 9.85. Found: C, 57.20;
H, 6.00; N, 9.80.
(tBuNC)3Mo(NPh)(o-(Me3SiN)2C6H4) (3), and 4
2b
3
4
formula
C
.
39 50H49Mo-
C
33H54Mo-
N6Si2
C
58H73Mo-
N7OSi2
N5Si2
formula wt
cryst syst
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
745.96
triclinic
P1
686.94
orthorhombic
P212121
10.4614(6)
18.571(1)
19.715(1)
90
90
90
3830.1(4)
4
1.191
1036.35
triclinic
P1
11.5484(9)
11.8609(9)
14.8731(12)
102.802(2)
93.928(2)
94.868(2)
1971.4(3))
2
11.4653(12)
13.7990(14)
20.264(2)
90.006(2)
100.229(2)
112.571(2)
2905.1(5)
2
Z
F
calcd, Mg m-3
1.257
1.185
temp, K
radiation
173(2)
Mo KR (0.710 73 Å)
(wavelength)
R
Rw
0.0349
0.0758
0.0483
0.1086
0.0484
0.0868
detector and a graphite monochromator utilizing Mo KR
radiation (λ ) 0.710 73 Å). Cell parameters were refined using
up to 8192 reflections. A full sphere of data (1850 frames) was
collected using the ω-scan method (0.3° frame width). The first
50 frames were remeasured at the end of data collection to
monitor instrument and crystal stability (the maximum cor-
rection on I was <1%). Absorption corrections by integration
were applied on the basis of measured indexed crystal faces.
Crystal data and data collection parameters for 2b, 3, and 4
are given in Table 2.
Synthesis of a Tris(isocyanide) Complex. (tBuNC)3Mo-
(NPh)(o-(Me3SiN)2C6H4) (3). A diethyl ether solution of 1 (1.0
g, 2.03 mmol), was charged with 4 equiv of tert-butyl isocyanide
(0.48 mL, 8.12 mmol) at room temperature. The reaction
mixture quickly turned from green to purple and was stirred
for 1 h. Small crystals of 3 started forming in solution during
this time. Subsequently, the volume of the reaction was
reduced and cooled for 1 h (-78 °C). Purple microcrystals of 3
were obtained by filtration at -78 °C. The solids were then
dried overnight in vacuo, and 3 was obtained in 82% yield.
MS: calcd for C32H52N6Si2Mo ([M - CH3]+), m/e 674.285; found
[(cis-2,6-dimethylphenyl-NC)2Mo(NPh)(o-(Me3SiN)2-
C6H4)] (2b). The structure was solved by direct methods in
SHELXTL645 and refined using full-matrix least squares. The
non-H atoms were treated anisotropically, whereas the hy-
drogen atoms were calculated in ideal positions riding on their
respective carbon atoms. The asymmetric unit consists of the
Mo complex and two half-molecules of ether (all ether mol-
ecules are located on inversion centers). The latter were
disordered and could not be modeled properly; thus, the
program SQUEEZE, a part of the PLATON package of
crystallographic software, was used to calculate the solvent
disorder area and remove its contribution to the overall
intensity data. One of the Si atoms is disordered in two
positions, Si(2) and Si(3). Accordingly, their methyl groups and
the dimethylphenyl moiety are also disordered and were
refined in two parts, with their site occupation factors depen-
dently refined. A total of 621 parameters (with 36 restraints)
was refined in the final cycle of refinement, using 6995
reflections with I > 2σ(I) to yield R1 and wR2 values of 4.84%
and 9.81%, respectively. Refinement was done using F2.
(tBuNC)3Mo(NPh)(o-(Me3SiN)2C6H4) (3). The structure
was solved by direct methods in SHELXT5 and refined using
full-matrix least squares. The non-H atoms were treated
anisotropically, whereas the hydrogen atoms were calculated
in ideal positions riding on their respective carbon atoms. The
SiMe3 on N2 is disordered and was refined in two parts with
their occupation factors dependently refined. Also, the three
methyl groups on C30 were disordered and were treated
similarly. A total of 431 parameters (and 61 restraints) were
refined in the final cycle of refinement using 5197 reflections
with Ι > 2σ(I) to yield R1 and wR2 values of 4.83% and 8.54%,
respectively. Refinement was done using F2.
1
(FAB), m/e 674.280. H NMR (C6D6): δ 0.58 (s, 9H, NSiMe3),
0.77 (s, 9H, NSiMe3), 0.93 (s, 18H, CN(C(CH3)3), 1.17 (s, 9H,
CN(C(CH3)3)), 6.81 (t, 1H, imido para hydrogen), 6.92 (m, 2H,
(TMS)2pda), 7.05 (t, 2H, imido meta hydrogen), 7.31 (d, 2H,
imido ortho hydrogen), 7.42 (m, 2H, (TMS)2pda). 13C NMR
(C7D8, -10 °C): δ 4.02, 4.84, 30.02, 30.82, 56.26, 58.35, 112.63,
116.39, 116.53, 118.12, 120.52, 121.16, 122.60, 122.96, 150.86,
155.98, 157.72, 160.28.
Synthesis of a Chelating Imino Carbamoyl Complex
(4). A pentane solution of 5 (1.00 g, 2.03 mmol) was treated
with 2,6-dimethylphenyl isocyanide (1.16 g, 8.92 mmol); the
solution was stirred overnight, and a yellowish brown powder
precipitated. Washing this powder several times with cold
pentane followed by recrystallization of the solids from pentane
afforded 4 as red crystals in 46% yield. 1H NMR (C6D6): δ 8.04
(dd, 2H, phenyl imido ortho proton, 8.2, 1.6 Hz), 7.23 (t, 2H,
2,6-dimethylphenyl isocyanide para proton, 8.4, 1.3 Hz), 7.2-
6.5 (overlapping aromatic peaks, 16H), 5.43 (dd, 1H, phenyl
imido para proton, 8.2, 1.6 Hz) 2.50 (s, 6H, CH3 iminocarbam-
oyl), 1.94 (s, 12H, CH3 2,6-dimethylphenyl isocyanide), 1.87
(s, 6H, (CH3 imino carbamoyl), 0.66 (bs, 9H, SiMe3), 0.45 (s,
9H, SiMe3). 13C NMR (C7D8): 203.3, 176.6, 157.7, 146.6, 146.2,
145.3, 137.9, 137.4, 136.4, 133.4, 129.1, 128.8, 127.3, 126.6,
125.3, 125.1, 125.0, 124.7, 124.0, 121.0, 120.8, 119.3, 119.1,
115.3, 20.0, 18.8, 18.2, 5.5, 0.78. Anal. Calcd for C54H64MoN7-
Si2: C, 67.33; H, 6.70; N, 10.18. Found: C, 67.72; H, 6.95, N,
10.07.
Iminocarbamoyl Complex (4). The structure was solved
by direct methods in SHELXTL6 and refined using full-matrix
least squares. The non-H atoms were treated anisotropically,
X-ray Studies. Data were collected at 173 K on a Siemens
SMART PLATFORM instrument equipped with a CCD area
(45) Sheldrick, G. M. SHELXTL; Madison, WI, 1986.

6318 Organometallics, Vol. 24, No. 26, 2005
Ison et al.
whereas the hydrogen atoms were calculated in ideal positions
riding on their respective carbon atoms. The asymmetric unit
consists of the Mo complex and two half-molecules of ether
(all ether molecules are located on inversion centers). The
latter were disordered and could not be modeled properly; thus
the program SQUEEZE, a part of the PLATON package of
crystallographic software, was used to calculate the solvent
disorder area and remove its contribution to the overall
intensity data. One of the Si atoms is disordered in two
positions, Si2 and Si3. Accordingly, their methyl groups and
the dimethylphenyl moiety are also disordered and were
refined in two parts, with their site occupation factors depen-
dently refined. A total of 621 parameters (with 36 restraints)
were refined in the final cycle of refinement using 6995
reflections with I > 2σ(I) to yield R1 and wR2 values of 4.84%
and 9.81%, respectively. Refinement was done using F2.
Computational Studies. Density functional theory (DFT)
calculations were performed using the Gaussian 98W program
package. Compound 3 was optimized using Morokuma’s
ONIOM method.34-36 For these ONIOM calculations a two-
layer approach was used. For the inner layer, the o-phenylene
group (o-C6H4) that links the two N atoms of the diamido
ligand was simplified to a -CHdCH- carbon chain. The
organic groups on the nitrogen atoms (SiMe3 of the amido and
phenyl group of the imido) were replaced by hydrogen atoms
for simplicity. Becke’s hybrid three-parameter functional
(B3LYP)46 and the Los Alamos effective core potential (ECP)
plus a standard double-ú basis set (LANL2DZ)47,48 were used
to describe the molybdenum center, and the Dunning/Huzi-
naga full double-ú basis set (D95) was employed to describe
all other atoms. Calculations were performed with default
pruned fine grids for gradients and Hessians and default SCF
convergence for geometry optimizations (10-8).
Acknowledgment. We wish to acknowledge the
National Science Foundation (Grant No. CHE-0094404)
for the support of this work. K.A.A. wishes to acknowl-
edge the National Science Foundation and the Univer-
sity of Florida for funding of the purchase of the X-ray
equipment.
Supporting Information Available: Full details of ex-
perimental procedures and crystallographic data for com-
pounds 2b, 3, and 4, including tables of crystal data and data
collection parameters, atomic coordinates, anisotropic displace-
ment parameters, and all bond lengths and bond angles; CIF
files for these compounds are also provided. This material is
available free of charge via the Internet at http://pubs.acs.org.
OM0502436
(46) Becke, A. D. J. Chem. Phys. 1993, 98(2), 1372-1377.
(47) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82(1), 270-283.
(48) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82(1), 299-310.