Reactions of organic nitriles with a three-coordinate molybdenum (III) complex and with a related molybdaziridine-hydride

Article abstract of DOI:10.1021/om030124v

Reactions of nitriles RCN with the sterically encumbered Mo(N[t-Bu]Ar)₃ (1, Ar = 3,5-C₆H₃Me₂) or the somewhat less hindered Mo(H)(η²-Me₂CNAr)(N[i-Pr]Ar)₂ (2) have been investigated. Where R = Me or Ph, reaction with 1 results in reductive nitrile coupling and the formation of a diiminato product [μ-NC(R)C(R)N][1]₂. In contrast, reaction of 1 with Me₂NCN surprisingly results in a stable, albeit highly congested, η² adduct of the nitrile. When the less sterically hindered 2 is used, reaction with PhCN gives the diiminato product analogous to the one mentioned for the tert-butyl system, [μ-NC(Ph)C(Ph)N] [Mo(N[i-Pr]Ar)₃]₂, where molybdaziridine-hydride 2 has provided access to the three-coordinate Mo(N[i-Pr]Ar)₃ (3) moiety. Use of a more bulky nitrile such as MesCN (Mes = 2,4,6-C₆H₂Me₃) results in formation of a bis-η¹ compound, (η¹-MesCN)₂[3]. Use of 9-anthracenylcarbonitrile results in head-to-tail C-C coupling of two monomers via the anthracenyl moiety. Detailed variable-temperature EPR and ²H NMR data are included for both molybdenum-containing starting materials and selected reaction intermediates and products.

Full text of DOI:10.1021/om030124v

2902
Organometallics 2003, 22, 2902-2913
Rea ction s of Or ga n ic Nitr iles w ith a Th r ee-Coor d in a te
Molybd en u m (III) Com p lex a n d w ith a Rela ted
Molybd a zir id in e-Hyd r id e
Yi-Chou Tsai, Frances H. Stephens, Karsten Meyer, Arjun Mendiratta,
Mircea D. Gheorghiu, and Christopher C. Cummins*
Department of Chemistry, Room 2-227, Massachusetts Institute of Technology,
77 Massachusetts Avenue, Cambridge, Massachusetts 02139
Received February 21, 2003
Reactions of nitriles RCN with the sterically encumbered Mo(N[t-Bu]Ar)3 (1, Ar ) 3,5-
C6H3Me2) or the somewhat less hindered Mo(H)(η2-Me2CNAr)(N[i-Pr]Ar)2 (2) have been
investigated. Where R ) Me or Ph, reaction with 1 results in reductive nitrile coupling and
the formation of a diiminato product [µ-NC(R)C(R)N][1]2. In contrast, reaction of 1 with
Me2NCN surprisingly results in a stable, albeit highly congested, η2 adduct of the nitrile.
When the less sterically hindered 2 is used, reaction with PhCN gives the diiminato product
analogous to the one mentioned for the tert-butyl system, [µ-NC(Ph)C(Ph)N][Mo(N[i-Pr]Ar)3]2,
where molybdaziridine-hydride 2 has provided access to the three-coordinate Mo(N[i-Pr]Ar)3
(3) moiety. Use of a more bulky nitrile such as MesCN (Mes ) 2,4,6-C6H2Me3) results in
formation of a bis-η1 compound, (η1-MesCN)2[3]. Use of 9-anthracenylcarbonitrile results in
head-to-tail C-C coupling of two monomers via the anthracenyl moiety. Detailed variable-
2
temperature EPR and H NMR data are included for both molybdenum-containing starting
materials and selected reaction intermediates and products.
In tr od u ction
center can expand to a maximum coordination number
of four,7 either through the binding of terminal func-
tionality or through the addition of a single atom, as in
the system’s terminal nitride,1-3,12,13 phosphide,4,14,15
and carbide8,16,17 complexes. On the other hand, due to
its more open nature, the iso-propyl-substituted system
can not only expand its coordination number to five, it
may coordinate substrates in an η2 fashion.9-11 An
example of this is the ready complexation of benzo-
phenone giving (η2-OCPh2)[3] for the isopropyl system
while, in contrast, the tert-butyl system exhibits no
reaction with benzophenone when an equimolar solution
of the two is prepared. Generally speaking, reaction
chemistry is simplified for the tert-butyl system in
relation to the more open isopropyl derivative.
For several years, we have investigated molybdenum
trisamide fragments Mo(N[t-Bu]Ar)3 (1, Ar ) 3,5-
C6H3Me2) and Mo(N[i-Pr]Ar)3 (3) because of their re-
markable capacity for informative small-molecule acti-
vation; while the former tert-butyl-substituted complex
is isolable as a three-coordinate monomer with a quartet
ground state,1-8 the latter isopropyl-substituted deriva-
tive reacts as though it were three-coordinate but is
obtained as its doublet ground-state molybdaziridine-
hydride tautomer Mo(H)(η2-Me2CNAr)(N[i-Pr]Ar)2 (2).9-11
An operating hypothesis has been that the tert-butyl-
substituted variant is sufficiently crowded that its Mo
* Address correspondence to this author. Fax: (617) 258-5700.
E-mail: ccummins@mit.edu.
While originally begun as a comparative study of the
reactivity of these two systems with organic nitriles, this
work became substantially enriched due to the many
unexpected phenomena encountered. Discussed are
examples of nitriles capable of coordinating η2 to the
crowded tert-butyl-substituted system, nitriles that
couple head-to-head or head-to-tail upon treatment with
either Mo system, and nitriles that bind η1 to the
isopropyl-substituted system.
(1) Laplaza, C. E.; Odom, A. L.; Davis, W. M.; Cummins, C. C.;
Protasiewicz, J . D. J . Am. Chem. Soc. 1995, 117, 4999.
(2) Laplaza, C. E.; Cummins, C. C. Science 1995, 268, 861.
(3) Laplaza, C. E.; J ohnson, M. J . A.; Peters, J . C.; Odom, A. L.;
Kim, E.; Cummins, C. C.; George, G. N.; Pickering, I. J . J . Am. Chem.
Soc. 1996, 118, 8623.
(4) Laplaza, C. E.; Davis, W. M.; Cummins, C. C. Angew. Chem.,
Int. Ed. Engl. 1995, 34, 2042.
(5) Cummins, C. C. Chem. Commun. 1998, 1777.
(6) Peters, J . C.; Cherry, J . P. F.; Thomas, J . C.; Baraldo, L.;
Mindiola, D. J .; Davis, W. M.; Cummins, C. C. J . Am. Chem. Soc. 1999,
121, 10053.
(7) Cherry, J . P. F.; J ohnson, A. R.; Baraldo, L. M.; Tsai, Y.-C.;
Cummins, C. C.; Kryatov, S. V.; Rybak-Akimova, E. V.; Capps, K. B.;
Hoff, C. D.; Haar, C. M.; Nolan, S. P. J . Am. Chem. Soc. 2001, 123,
7271.
(8) Agapie, T.; Diaconescu, P. L.; Cummins, C. C. J . Am. Chem. Soc.
2002, 124, 2412.
(9) Tsai, Y.-C.; J ohnson, M. J . A.; Mindiola, D. J .; Cummins, C. C.;
Klooster, W. T.; Koetzle, T. F. J . Am. Chem. Soc. 1999, 121, 10426.
(10) Tsai, Y.-C.; Diaconescu, P. L.; Cummins, C. C. Organometallics
2000, 19, 5260.
(12) Cui, Q.; Musaev, D. G.; Svensson, M.; Sieber, S.; Morokuma,
K. J . Am. Chem. Soc. 1995, 117, 12366.
(13) Neyman, K. M.; Nasluzov, V. A.; Hahn, J .; Landis, C. R.; Rosch,
N. Organometallics 1997, 16, 995.
(14) J ohnson, M. J . A.; Odom, A. L.; Cummins, C. C. Chem.
Commun. 1997, 1523.
(15) Wu, G.; Rovnyak, D.; J ohnson, M. J . A.; Zanetti, N. C.; Musaev,
D. G.; Morokuma, K.; Schrock, R. R.; Griffin, R. G.; Cummins, C. C. J .
Am. Chem. Soc. 1996, 118, 10654.
(16) Peters, J . C.; Odom, A. L.; Cummins, C. C. Chem. Commun
1997, 1995.
(11) Cherry, J . P. F.; Stephens, F. H.; J ohnson, M. J . A.; Diaconescu,
P. L.; Cummins, C. C. Inorg. Chem. 2001, 40, 6860.
(17) Greco, J . B.; Peters, J . C.; Baker, T. A.; Davis, W. M.; Cummins,
C. C.; Wu, G. J . Am. Chem. Soc. 2001, 123, 5003.
10.1021/om030124v CCC: $25.00 © 2003 American Chemical Society
Publication on Web 06/10/2003

Reactions of Organic Nitriles
Organometallics, Vol. 22, No. 14, 2003 2903
now juxtaposed and conjugated. Back-bonding from the
d2 molybdenum centers to the nitrile-derived NC π*
orbitals is permitted in the plane perpendicular to that
defined by the C2h-symmetric Mo-N-C-C-N-Mo mo-
lecular core, as part of a topologically linear 8-electron
π system spanning the six core atoms. With reference
to the C2h molecular core, π donation from the two N
lone pairs to otherwise empty Mo d orbitals is permitted
by symmetry within the σh plane.
Due to acetonitrile’s frequent use as a solvent in
coordination chemistry, there are several previous
reports of its participation in both 2e and 4e reductive
coupling leading to respectively bridging diiminato and
enediimido moieties. McCarley and co-workers showed
in 1975 that, under reducing conditions involving added
zinc, tantalum and niobium halides effect the reductive
coupling of acetonitrile, with {[µ-NC(Me)C(Me)N][NbCl4-
(NCMe)]2}2- having been characterized crystallographi-
cally.21 An enediimido formulation for the inversion-
symmetrical diamagnetic dimer is suggested by the
pertinent bond distances CdC, 1.35(1) Å, and C-N,
1.378(8) Å. Such a formulation was embraced by Cotton
and Hall, who carried out a crystal structure determi-
nation on the related ditantalum complex, [µ-NC(Me)d
C(Me)N][TaCl3(THF)2]2.22 The latter evinced a central
CdC distance of 1.347(16) Å and an N-C distance of
1.405(10) Å. As shown by de Bier and Teuben,23 a pair
of titanium(III) centers can be used in combination to
effect 2e reductive coupling of organic nitriles; in this
way they isolated diiminato derivatives of formula
[µ-NC(R)C(R)N][Cp2TiR]2, structures for which have not
been reported. Gambarotta and co-workers isolated the
diamagnetic enediimido complex [µ-trans-NC(Me)d
C(Me)N][TiCl2(TMEDA)]2 upon dissolution of their ti-
tanium(II) precursor in neat acetonitrile,24 postulating
carbene-like character for the unobserved η1-coordinated
nitrile complex intermediate. Their enediimido deriva-
tive exhibited a central CdC bond distance of 1.337(10)
Å and an N-C distance of 1.384(5) Å. In contrast,
Templeton and co-workers found that treatment of
{HB(Me2pz)3}W(CO)3Br with NaS-i-Pr in refluxing
acetonitrile led to formation of a 2,3-butanediiminato
derivative, [µ-NC(Me)C(Me)N][W(CO)2{HB(Me2pz)3}]2.25
The latter complex is characterized by a central C-C
bond distance of 1.40(4) Å and a CdN bond distance of
1.28(3) Å; interestingly, the bridging diiminato func-
tionality was found in this case to deviate strongly from
planarity (dihedral angle 54.4 °).
F igu r e 1. Acetonitrile coupling to form (µ-NC(CH3)C-
(CH3)N)[1]2.
It has been possible to study many reactions of the
molybdenum trisamides by 2H NMR and EPR spec-
troscopies. In certain cases, analogues of the intermedi-
ates in the nitrile coupling reactions could be rendered
isolable by judicious substitution; such isolable ana-
logues served as models and as aids to the interpreta-
tion of the spectra of reacting systems. Since the
molybdaziridine-hydride system has a doublet ground
state and a very characteristic EPR spectrum, a careful
EPR study of it and several deuterated variants was
carried out before attempting to interpret EPR spectra
obtained upon interaction with organic nitriles. Thus,
this work has provided not only a wealth of information
concerning available modes of organonitrile manipula-
tion by reducing, low-coordinate metal fragments, but
also some fundamental characterization data pertaining
to such fragments.
Resu lts a n d Discu ssion
Dim er iza tion of Nitr iles by Mo(N[t-Bu ]Ar )3. A
new, facile, inexpensive synthesis of N-tert-butyl-3,5-
dimethylaniline has been developed. Reaction of 2,4,6-
trimethylpyrillium tetrafluoroborate with tert-butyl-
amine in acetonitrile yields HN[t-Bu]Ar on scales greater
than 100 g. Subsequent synthesis of 1 via addition of
MoCl3(THF)3 to LiN[t-Bu]Ar‚OEt2 proceeds as reported
previously.1 Mixing 1 with 1 equiv of acetonitrile in
ethereal solution leads to 1:1 binding and dimerization
as depicted in Figure 1. The green dimer so obtained
was isolated in 77% yield after it precipitated from the
reaction mixture; the dimer may be considered the
product of reductive coupling where each nitrile has
been reduced by one electron in a fashion reminiscent
of pinacol coupling. The X-ray crystal structure of this
diiminato product, [µ-NC(Me)C(Me)N][1]2, has been
reported previously in brief.18 Important structural
parameters are included in Figure 1. A crystallographic
center of inversion resides at the midpoint of the newly
furnished C-C bond, such that the bridging NC(Me)-
C(Me)N ligand is, neglecting hydrogen atoms, perfectly
planar.
To summarize, it is clear from the foregoing discussion
of structural parameters that enediimido bridging ligands
are readily distinguished from their diiminato counter-
parts in that the central C-C bond is shorter than the
flanking C-N bonds in the enediimido case, but longer
in the diiminato formulation.
As discussed above, reductive dimerization of organic
nitriles (in a fashion reminiscent of pinacol coupling)
The nitrile-derived nitrogen atoms in this dimer may
be thought to bind to Mo essentially as would those in
typical monodentate ketimide (NdCR1R2) ligands,19,20
with the perturbation that two such functionalities are
(21) Finn, P. A.; King, M. S.; Kilty, P. A.; McCarley, R. E. J . Am.
Chem. Soc. 1975, 97, 220.
(22) Cotton, F. A.; Hall, W. T. Inorg. Chem., 1978, 17, 3525. For a
related piece of work by the same authors, see also: Cotton, F. A.;
Hall, W. T. J . Am. Chem. Soc. 1979, 101, 5094.
(23) De Boer, E. J . M.; Teuben, J . H. J . Organomet. Chem. 1978,
153, 53.
(18) See Supporting Information for ref 9.
(19) Zhang, S.; Piers, W. E.; Gao, X.; Parvez, M. J . Am. Chem. Soc.
2000, 122, 5499.
(24) Duchateau, R.; Williams, A. J .; Gambarotta, S.; Chiang, M. Y.
Inorg. Chem. 1991, 30, 4863.
(20) Diaconescu, P. L.; Cummins, C. C. J . Am. Chem. Soc. 2002,
124, 7660.
(25) Young, C. G.; Philipp, C. C.; White, P. S.; Templeton, J . L. Inorg.
Chem. 1995, 34, 6412.

2904 Organometallics, Vol. 22, No. 14, 2003
Tsai et al.
F igu r e 2. Thermolysis of the diiminato does not result in
the formation of an alkyne and 2 equiv of molybdenum
nitride.
F igu r e 3. Thermal ejection of two tert-butyl radicals from
1 thereby generating the enediimido (µ-N-CPhdCPh-N)-
[Mo(NAr)(N[t-Bu]Ar)2]2.
F igu r e 4. 2H NMR spectra of a mixture of 1-d18 and 1
(top) and 2 (bottom) equiv of PhCN recorded in Et2O.
has been effected previously by well-defined coordina-
tion complexes and organometallic systems, but not by
any that result in a d2/d2 coupling product. This is
significant because, as portrayed in Figure 2, double
C-N homolysis from the dimer could in principle occur
to provide 2 equiv of the known d0 terminal nitrido
complex NMo(N[t-Bu]Ar)3 (DMoN ) 155 kcal/mol)7,12,13
along with diphenylacetylene. Such a transformation
would be a 3-electron aza-analogue of the 2-electron
McMurry reaction.26 However, the type of fragmentation
of (µ-NC(Ph)C(Ph)N)[1]2 suggested in Figure 2 was
observed neither at room temperature nor upon ex-
tended heating (2 h in toluene-d8 at 100 °C). Instead,
upon thermolysis a double tert-butyl radical ejection was
observed, as illustrated in Figure 3. This unusual
reaction provides a pathway for conversion of molybde-
num from its +4 to its +6 oxidation state, the redox
being coupled with Mo-N multiple bond formation. In
fact, the bridging diiminato ligand formally accepts 2
electrons and becomes an enediimido function, while 1e
reduction each of two N-C bonds accounts for expulsion
of two tert-butyl radicals; tert-butyl radical dispropor-
tionation and combination products isobutylene, iso-
butene, and hexamethylethane were observed by 1H
NMR spectroscopy. Although the N(t-Bu)(3,5-C6H3Me2)
ancillary ligand is generally quite robust and resistant
to intramolecular decomposition reactions, tert-butyl
radical ejection from itscoupled with oxidation of the
metal centershas been seen previously.27
the nature of intermediates involved in nitrile reductive
coupling reactions. Reactions of transition metal-
coordinated nitriles were reviewed in 1996,28 while a
more recent (2002) review of additions to metal-
activated organonitriles provides little new information
2
on nitrile reductive coupling.29 In the present work, H
NMR and EPR studies of reaction mixtures have
provided substantial mechanistic insight.
Monitoring in situ the progress of paramagnetic
reaction mixtures by 2H NMR spectroscopy is facilitated
by the use of the d18 variant Mo(N[C(CD3)2CH3]Ar)3 (1-
d
18); this complex exhibits a diagnostic 2H NMR signal
at 64 ppm.3 Interrogation of a 1:1 or 1:2 mixture of 1-d18
2
and PhCN in ether by H NMR spectroscopy revealed
complete conversion upon mixing to a single new species
exhibiting a peak at 9.5 ppm (w1/2 ) 65 Hz, Figure 4).
Because the extra added equivalent of benzonitrile had
2
no effect on the observed H NMR spectrum, the new
complex can be assigned as the mononuclear 1:1 adduct
(PhCN)[1-d18]. A small peak close to 2.0 ppm corre-
sponds to the diamagnetic dimeric coupling product
[µ-NC(Ph)C(Ph)N][1-d18]2, which is formed quantita-
tively and irreversibly upon concentration of the mix-
ture. Solutions containing the intermediate 1:1 adduct
develop an intense blue-purple color, whereas upon
workup and isolation, solutions of the dimeric coupling
product are a very dark green color. Providing a stark
contrast with the prompt dimerization of MeCN de-
scribed above, it is most intriguing that the 1:1 adduct
with benzonitrile displays a long lifetime in dilute
solution at 25 °C.
2H NMR In vestiga tion of Nitr ile Cou p lin g Rea c-
tion s of Mo(N[t-Bu ]Ar )3. In no case to date has a
detailed mechanistic study been carried out to address
(28) Michelin, R. A.; Mozzon, M.; Bertani, R. Coord. Chem. Rev.
1986, 147, 299.
(26) McMurry, J . E. Chem. Rev. 1989, 89, 1513.
(27) J ohnson, A. R.; Davis, W. M.; Cummins, C. C.; Serron, S.; Nolan,
S. P.; Musaev, D. G.; Morokuma, K. J . Am. Chem. Soc. 1998, 120, 2071.
(29) Kukushkin, V. Y.; Pombeiro, A. J . L. Chem. Rev. 2002, 102,
1771.

Reactions of Organic Nitriles
Organometallics, Vol. 22, No. 14, 2003 2905
Sch em e 1
(Figure 5) exhibits a similar N-C bond distance for the
coordinated nitrile functionality, namely 1.267(8) Å,
which is consistent with the strong π-donor nature of
the molybdenum trisamide fragment.
Perhaps most relevant to the discussion of (η2-
Me2NCN)[1] is Chisholm’s study of dimethylcyanamide
complexation by Mo2(ONp)6, where Np is neopentyl.33
A crystal structure determination showed that the
dimethylcyanamide ligand binds in a µ-η1,η2 fashion in
the complex Mo2(µ-η1,η2-Me2NCN)(µ-ONp)(ONp)5, which
also exhibits a dramatically lengthened N-CNMe2 bond
distance of 1.333(4) Å (longer by ca. 0.2 Å than that of
the nitrile functionality in free dimethylcyanamide). As
found for our complex, the dimethylcyanamide ligand
is planar (ignoring hydrogen atoms) such that the NMe2
lone pair is conjugated with the π system of the
azamolybdacyclopropene moiety.
F igu r e 5. Thermal ellipsoid plot (35% probability) of (η2-
Me2NCN)[1]. Selected bond distances (Å): Mo-N5, 2.030(5);
Mo-C3, 2.037(6); N5-C3, 1.267(8); C3-N4, 1.346(8). Se-
lected bond angles (deg): Mo-N5-C3, 72.1(4); Mo-C3-
N5, 71.5(3); N5-Mo-C3, 36.3(2); N5-C3-N4, 130.9(6).
Rea ction of Mo(N[t-Bu ]Ar )3 w ith Dim eth ylcya n -
a m id e. Interestingly, the reaction of Me2NCN (di-
methylcyanamide) with 1-d18 permitted not only the
spectroscopic observation of a 1:1 adduct (2H NMR, δ )
7.1 ppm), but also its isolation in 63% yield. The 1:1
adduct was obtained as a green crystalline solid upon
mixing of the reagents in n-hexane followed by concen-
tration and cooling to -35 °C. An X-ray diffraction study
(Figure 5) revealed an extremely surprising η2 binding
mode for the cyano functionality of the dimethylcyan-
amide ligand in the complex, (η2-Me2NCN)[1]. This is
the first evidence that 1 can expand its binding pocket
sufficiently to accommodate an η2 interaction with
unsaturated functionality. Normally, three N[t-Bu]Ar
ligands pack in a pseudo-C3 three-bladed propeller
fashion in complexes of 1;3 here we are confronted with
an unusual, splayed, low-symmetry ancillary ligand
arrangement that presumably is forced by the strain of
having to accommodate the Me2NCN ligand in an η2
binding mode.
2
Based on the similar H NMR chemical shift and line
width properties for both the Me2NCN and PhCN
adducts of 1-d18, it is probable that the long-lived 1:1
PhCN adduct has an η2-bonded structure as demon-
strated in Figure 5 for the Me2NCN adduct. If this is
so, then the persistent nature of the benzonitrile adduct
in solution can be explained as outlined in Scheme 1.
As also depicted in Scheme 1, a fruitless attempt was
made to intercept the putative η1-PhCN adduct: dimer-
ization to the corresponding diiminato derivative was
not interrupted by addition of n-Bu3SnH to the system.
The idea behind the n-Bu3SnH experiment was that the
putative η1-PhCN adduct central to Scheme 1 would
have radical character at the nitrile carbon atom. It
might therefore be able to abstract an H atom from
n-Bu3SnH to give the ketimide derivative (PhCHdN)[1].
Ketyl complex intermediates have been intercepted and
converted to alkoxides in such a fashion.34-36 Evidently
the η1-PhCN adduct is not a potent H atom abstractor.
Electr on P a r a m a gn etic Reson a n ce In vestiga -
tion of Mo(H)(η2-Me2CNAr )(N[i-P r ]Ar )2 a n d Its
Isotop om er s. Prior to investigation of its reactions with
nitriles, molybdaziridine-hydride 2 was subjected to a
detailed EPR study. To aid the interpretation of the EPR
Complexation of organonitriles in an η2 fashion is
fairly well-documented. The first structurally character-
ized member of this class, described in 1986 by Wilkin-
son and co-workers, was Cp2Mo(η2-NCCH3).30 This
compound was found to have an N-C distance of
1.200(10) Å. As has been shown by Harman and
co-workers,31 the strong π-donating metal fragment
[W(bipy)(PMe3)2Cl]+ binds acetonitrile in an η2 fashion
and also gives rise to lengthening of the N-C bond, the
observed bond distance being 1.267(7) Å, which is longer
by ca. 0.12 Å than in free acetonitrile. Harman and co-
workers subsequently prepared the neutral tungsten(II)
η2 acetonitrile complex, W(PMe3)3Cl2(η2-NCCH3), which
was found by X-ray crystallography to have a C-N bond
distance of 1.27(1) Å.32 Our complex (η2-Me2NCN)[1]
(32) Barrera, J .; Sabat, M.; Harman, W. D. Organometallics 1993,
12, 4381.
(33) Chisholm, M. H.; Huffman, J . C.; Marchant, N. L. J . Am. Chem.
Soc. 1983, 105, 6162.
(34) Covert, K. J .; Wolczanski, P. T. Inorg. Chem. 1989, 28, 4565.
(35) Covert, K. J .; Wolczanski, P. T.; Hill, S. A.; Krusic, P. J . Inorg.
Chem. 1992, 31, 66.
(36) Agapie, T.; Diaconescu, P. L.; Mindiola, D. J .; Cummins, C. C.
Organometallics 2002, 21, 1329.
(30) Wright, T. C.; Wilkinson, G.; Motevalli, M.; Hursthouse, M. B.
J . Chem. Soc., Dalton Trans. 1986, 2017.
(31) Barrera, J .; Sabat, M.; Harman, W. D. J . Am. Chem. Soc. 1991,
113, 8178.

2906 Organometallics, Vol. 22, No. 14, 2003
Tsai et al.
characteristic of 2 are lost completely upon nitrile
binding. EPR characterization data for the intermediate
η2-PhCN adduct have been obtained and are compared
in Figure 938 with those for the structurally character-
ized η2-Me2NCN adduct of 1, as well as with those for
the previously described9 and structurally characterized
η2-OCPh2 adduct of 3. The EPR data depicted in Figure
1
938 reveal that S ) /2 systems consisting of a molyb-
denum trisamide fragment bonded to a η2 ketone or
nitrile functionality have characteristic rhombic spectra.
Confirmation that this should be so has been obtained
with the aid of DFT calculations carried out on the full
(η2-Me2NCN)[1] molecule, using structural data from
the X-ray diffraction study. Observed and calculated
(the latter in parentheses) g values for this system are
as follows: g11, 1.972 (1.987); g22, 1.954 (1.972); g33, 1.896
(1.916).
As in the case of PhCN coupling by the tert-butyl
derivative, concentration of solutions 1:1 in PhCN and
compound 2 gives rise irreversibly to an inversion-
symmetrical coupling product. Due to its oily nature in
this case the dimer was characterized most easily by
conversion to its maroon monocation (triiodide salt).
[(PhCN)(3)]2[I3] was isolated in 86% yield following
treatment of 2 with benzonitrile (1.1 equiv) in ether,
solvent removal and replacement, and treatment of the
resulting green solution with 1.5 equiv of molecular
iodine. While the neutral coupled product [(PhCN)(3)]2
is seen by cyclic voltammetry (Figure 10)38 to be difficult
to reduce, the complex displays two reversible oxidation
waves -0.6 and -1.1 V versus the ferrocene/ferro-
cenium couple. Disproportionation of the monocation to
equivalent amounts of dication and neutral is not an
issue due to the disparity in E1/2 values for the two
oxidation products. Accordingly, the salt [(PhCN)(3)]2[I3]
was amenable to manipulation and could be crystallized
from a methylene chloride/n-hexane mixture at -35 °C
in a form suitable for study by X-ray diffraction.
F igu r e 6. Isotopomers of the isopropyl ligand.
data, we prepared five isotopomers of the N(i-Pr)(3,5-
C6H3Me2) ligand as shown in Figure 6. In addition to
the molybdaziridine-hydride/deuteride complexes de-
rived from ligands d0 and d1, we have prepared ad-
ditionally those derived from ligands d4, d6, and d7; EPR
spectra of the latter three derivatives contained no new
or additional information. Incorporation of deuterium
into the isopropyl methine position was accomplished
by LiAlD4 reduction of the appropriate acetone imine
precursor, Me2CdNAr, while exchange of deuterium
into the aryl ring 2,4,6 positions as in the d4 ligand
variant employed conditions described in the litera-
ture.37
The most obvious feature in the EPR spectrum of 2
(Figure 7)38 is the doublet splitting (Aiso[1H] ) 16.4 G)
attributable to the presence of the terminal hydride
ligand. Successful simulation of the observed spectrum
was achieved by specifying four hyperfine couplings:
Aiso[
95/97Mo (25.5%)] ) 32.6 G, Aiso[1H (99.99%)] ) 16.4
Reported previously9 was the X-ray structure of
the neutral acetonitrile-coupled product (µ-NC(Me)C-
(Me)N)[1]2. Important structural parameters are the
interatomic distances 1.814(8), 1.325(12), and 1.43(2) Å
that correspond respectively to the MoN, NC, and CC′
distances in the molecule’s central Mo-N-C(Me)-
C(Me) core; a crystallographic center of inversion was
situated at the midpoint of the newly furnished C-C
bond (Figure 1). Note that 1.82 Å can be taken as the
expectation value for a MoN double bond since this is
the distance found for Ci-symmetric (µ-N)[3]2,9 the
molecular orbital picture for which entails a molybde-
num-nitrogen bond order of exactly two. Thus, the
valence bond picture that best describes the structurally
characterized acetonitrile-coupled system is ModNd
C(Me)sC(Me)dNdMo.
The structure of [(PhCN)(3)]2+ (Figure 11) also incor-
porates a crystallographic center of inversion at the
midpoint between the two coupled nitrile carbon atoms.
The C-C interatomic distance at the point of coupling
is 1.368(12) Å, and is sandwiched between two C-N
bonds associated with an 1.393(8) Å interatomic dis-
tance. These values, along with a Mo-N interatomic
distance of 1.740(5) Å, suggest that a substantial
electronic reorganization has occurred upon 1e oxidation
of the neutral coupled dimer. Most intriguing is the
G, Aiso[14N (99.63%)] ) 2.7 (2 amido nitrogens) and 1.35
G (molybdaziridine nitrogen). To confirm our assign-
ment of the doublet feature, the d3-isotopomer Mo(D)(η2-
(CH3)2CNAr)(N[CD(CH3)2]Ar)2 (2-d3) was prepared and
likewise studied by EPR spectroscopy (Figure 8).38
Successful simulation of the EPR spectrum of 2-d3 was
achieved by using the same Aiso 95/97Mo and 14N hyper-
fine coupling constants given above for the natural
abundance isotopomer, substituting the value Aiso[2H
(99.63%)] ) 2.4 G for the hydride coupling. Clearly, upon
replacement of the terminal hydride with deuteride, the
doublet splitting collapses because the gyromagnetic
ratio of 2H is approximately one-seventh that of 1H. The
assignment is thus confirmed.
Rea ction of Molybd a zir id in e-Hyd r id e w ith Ben -
zon itr ile. Upon treatment with 1 equiv of benzonitrile,
the molybdaziridine-hydride complex 2 provides access
to the three-coordinate trisamide Mo(N[i-Pr]Ar)3 (3). A
benzonitrile adduct of 3, presumed to be η2 bonded, is
formed that undergoes subsequent dimerization to a
µ-diiminato dimolybdenum complex. Noteworthy is the
fact that the very distinctive EPR signals (Figure 7)38
(37) Swain, C. G.; Sheats, J . E.; Harbison, K. G. J . Am. Chem. Soc.
1975, 97, 783.
(38) Tsai, Y.-C. Ph.D. Thesis, Massachusetts Institute of Technology,
2002.

Reactions of Organic Nitriles
Organometallics, Vol. 22, No. 14, 2003 2907
F igu r e 7. X-band EPR spectrum of 2 with inset of enlarged low-field signal and enlargement of main signal recorded in
Et2O/n-C5H12 (1:1) at 300 K. Experimental conditions: microwave frequency, ν ) 9.697 GHz; power, 20.1 mW; modulation
amplitude, 0.1 G. Simulated parameters: giso ) 1.9825; Aiso[
95/97Mo (25.5%)] ) 32.6 G (90.41 MHz); Aiso[1H (99.99%)] )
16.4 G (45.48 MHz); Aiso[14N (99.63%)] ) 2.75 (2 amido ligands, 7.63 MHz) and 1.35 G (imino ligand, 3.74 MHz); line
width, 1.94 G.
F igu r e 8. X-band EPR spectrum of 2-d3 with inset of enlarged low-field and enlargement of main signal recorded in
Et2O/n-C5H12 (1:1) at 300 K. Experimental conditions: microwave frequency, ν ) 9.707 GHz; power, 20.1 mW; modulation
amplitude, 0.1 G. Simulated parameters: giso ) 1.9825; Aiso[
95/97Mo (25.5%)] ) 32.6 G (90.41 MHz); Aiso[2H (99.63%)] )
2.4 G (6.66 MHz); Aiso[14N (99.63%)] ) 2.75 (2 amido ligands, 7.63 MHz) and 1.35 G (imino ligand, 3.74 MHz); line width,
1.94 G.
2
contraction of the central C-C bond length upon oxida-
tion; in effect, removal of an electron from the system
converts the bridging ligand from the diiminato to the
enediimido form. DFT studies carried out on the neutral
model system (µ-NC(H)C(H)N)[1]2 in Ci point symmetry
reveal, as expected, that the HOMO belongs to the Ag
irreducible representation and is strongly C-C π anti-
bonding in character. An illustration is provided in
Figure 12. This explains the ready 1- and 2-electron
oxidations shown in Figure 10,38 as well as the observed
contraction in C-C bond distance upon oxidation.
Va r ia ble-Tem p er a tu r e H NMR Stu d y of Molyb-
d a zir id in e-Hyd r id e. Further characterization data for
molybdaziridine-hydride include variable-temperature
2H NMR spectra of the d18-isotopomer Mo(H)(η2-
(D3C)2CNAr)(N[CH(CD3)2]Ar)2 (2-d18) over the range
20-80 °C (see the Supporting Information). At ambient
temperature are observed two relatively sharp peaks
at 6 and 8 ppm, along with a rather broad peak at -18
ppm, consistent with the complex’s time-averaged Cs
point group symmetry. Assignment of the signals can
be made based on chemical shift and line width consid-

2908 Organometallics, Vol. 22, No. 14, 2003
Tsai et al.
F igu r e 11. Thermal ellipsoid plot (35% probability) of
[(PhCN)(3)]2[I3] (hydrogen atoms and counterion removed
for clarity). Selected distances (Å): Mo1-N1, 1.921(5);
Mo1-N2, 1.918(5); Mo1-N3, 1.929(5); Mo1-N4, 1.740(5);
N4-C4, 1.393(8); C4-C4’, 1.368(12); I1-I2, 2.938(1); I2-
I3, 2.896(1). Selected angles (deg): N1-Mo1-N2, 110.7(2);
N2-Mo1-N3, 110.2(2); N3-Mo1-N1, 111.3(2); N4-Mo1-
N1, 107.5(2); N4-Mo1-N2, 107.4(2); N3-Mo1-N4, 109.6(2);
C4-N4-Mo1, 175.6(5); N4-C4-C4’, 120.6(7); I3-I2-I1,
177.22(3).
F igu r e 9. X-band EPR spectra of (η2-OCPh2)[3] (top):
microwave frequency, ν ) 9.351 GHz; power, 1 mW; mod-
ulation amplitude, 5 G. (η2-Me2NCN)[1] (middle): micro-
wave frequency, ν ) 9.707 GHz; power, 1 mW; modulation
amplitude, 5G. (η2-NCPh)[3] (bottom): microwave fre-
quency, ν ) 9.351 GHz; power, 1 mW; modulation ampli-
tude, 5 G. All spectra were recorded in frozen toluene
solution at T ) 110 K. The inset shows the isotropic
spectrum of (η2-NCPh)[3] in liquid solution at T ) 260 K.
Experimental conditions: microwave frequency, ν ) 9.351
GHz; power, 1 mW; modulation aplitude, 5 G.
F igu r e 12. Contour plot (xz plane) of the Ag HOMO,
passing through the two carbon atoms, in the model
structure (µ-NC(H)-C(H)N)[Mo(NH2)3]2. The molecule is
oriented such that the planar diiminato bridging ligand lies
in the yz plane with the carbon atoms defining the z axis.
Nodal surfaces are indicated by dash-dot-dash contours.
F igu r e 10. Cyclic voltammogram of [(PhCN)(3)]2 recorded
pushed the highly reactive/sensitive system to still more
extreme temperatures. The most plausible mechanism
accounting for the observed coalescence is reversible â-H
elimination as depicted in Figure 13: i.e., equilibration
of 2 with its three-coordinate isomer 3. Since the latter
has in no case been observed, no data are available to
determine if putative 3 would (like its tert-butyl-
substituted counterpart) have a 3-fold symmetrical
structure and a quartet ground state, or else perhaps
one or more Mo-H-C agostic interactions and a doublet
ground state.
in THF solution containing 0.50 M TBAP. Scan rate is 0.10
V s-1
.
erations: the two sharp peaks are for the diastereotopic
CH(CD3)2 groups while the broad signal signifies the
η2-bound (D3C)2CdNAr ligand. Upon raising the tem-
perature to 80 °C, coalescence of the two sharp signals
into a single peak is observed, and the -18 ppm signal
is seen to broaden further into the baseline and to move
downfield by approximately 5 ppm. Presumably, at
temperatures of ca. 150-200 °C, coalescence of all three
resonances would be observed. While the molybdaziri-
dine-hydride exhibits thermal stability as judged by
reversibility of the coalescence observed at 80 °C upon
cooling back to room temperature, we have not yet
2H NMR spectroscopy is a convenient way to monitor
the reaction between the deuterated molybdaziridine-
hydride complex 2-d18 and 1 or 2 equiv of benzonitrile.
Upon mixing with nitrile, the three peaks in a 1:1:1 ratio

Reactions of Organic Nitriles
Organometallics, Vol. 22, No. 14, 2003 2909
F igu r e 13. Reversible â-H elimination.
F igu r e 14. Thermal ellipsoid plot (35% probability) of (η1-
MesCN)2[3]. Selected distances (Å): Mo-N1, 2.050(4); Mo-
N2, 1.992(4); Mo-N3, 1.979(4); Mo-N4, 2.097(4); Mo-N5,
2.081(4); N4-C41, 1.158(6); N5-C51, 1.153(6). Selected
angles (deg): N1-Mo-N2, 124.9(2); N2-Mo-N3, 117.5(2);
N3-Mo-N1, 117.6(2); N1-Mo-N4, 86.9(2); N2-Mo-N4,
93.5(2); N3-Mo-N4, 89.4(2); N1-Mo-N5, 88.3(2); N2-
Mo-N5, 89.6(2); N3-Mo-N5, 92.4(2).
that correspond to the starting material are lost.
Dominating the spectrum when 1 equiv of PhCN is used
is a peak at 8.2 ppm (w1/2 ) 38 Hz) that may be assigned
to the η2 adduct, (η2-PhCN)[3-d18]. Present in both cases,
but far more abundant when 2 equiv of benzonitrile are
employed, is a complex with a sharp (w1/2 ) 10 Hz)
signal at 4.4 ppm. This complex is formulated as the
bis-η1 adduct, (η1-PhCN)2[3-d18].
xz and yz orbitals, giving a doublet ground state and
allowing for back-bonding into the nitrile CN π* orbitals.
That such back-bonding is in effect is indicated by the
Mo-N(nitrile) distances of 2.097(4) and 2.081(4) Å, just
slightly (ca. 0.1 Å) longer than the Mo-N(anilide)
distances in the same complex. On the other hand, the
nitrile CN interatomic distances (1.158(6) and 1.153(6)
Å) and infrared stretching frequency (2218 cm-1, Nujol
mull) are not indicative of substantial reduction of the
nitrile CN bond order, consistent with the fact that in
dilute solution, the complex is observed to lose cyano-
mesitylene and revert to the molybdaziridine-hydride
starting material. In other words, and by virtue of
The above assignments are supported by comparison
with the 2H NMR spectra involving the previously
described t-Bu system. In particular, the isolated and
structurally characterized adduct (η2-Me2NCN)[1] ex-
2
hibited a H NMR signal similar in chemical shift and
line width to that for the sole adduct observed en route
to benzonitrile coupling. Fitting this profile both in
terms of line width and chemical shift is the dominant
species observed upon mixing 1 equiv of PhCN with the
isopropyl-substituted molybdenum trisamide.
Rea ction of Cya n om esitylen e w ith Molybd a zir i-
d in e-Hyd r id e. In further defense of the foregoing
assignments, reference can be made to the fact that it
has been possible to isolate and characterize a stable
example of an (η1-nitrile)2[3] complex by employing a
nitrile so substituted as to disfavor both η2-binding and
coupling via dimerization. Thus, addition of 2,4,6-
trimethylbenzonitrile (cyanomesitylene, MesCN), either
2 equiv or in excess, to 2-d18 in hexane produced a blue,
2
careful H NMR spectroscopic monitoring, it was pos-
sible to conclude that cyanomesitylene coordination to
molybdenum is reversible.
To recap the observations thus far, we find that while
a single intermediate (or possible kinetic cul-de-sac) is
observed en route to benzonitrile coupling by the tert-
butyl-substituted molybdenum trisamide, two species
are observed en route to coupling in the case of the
isopropyl-substituted variant. By virtue of similarities
in both 2H NMR and X-band EPR profiles to the
structurally characterized dimethylcyanamide complex,
the species observed in the case of N[t-Bu]Ar ligation
is assigned to be (η2-PhCN)[1]. On the other hand, when
N[i-Pr]Ar ligation is in effect, both an EPR-active mono-
η2 adduct and an EPR-silent bis-η1 adduct are seen, and
these are presumably in a state of equilibrium due to
reversible formation of the trigonal-bipyramidal bis-η1
adduct. Thus, while it is still our working hypothesis
that 1 is reluctant to attain coordination number 5
complexes,40 we must now recognize that η2-binding, at
least of organic nitriles, is a favorable phenomenon. Not
observed are what are possibly the most interesting
reactive intermediates involved in the molybdenum/
nitrile system, namely the mono-η1 adducts, direct
dimerization of which would lead with little reorganiza-
2
EPR-silent species exhibiting a single H NMR signal
at 3.2 ppm (w1/2 ) 22 Hz) and an effective magnetic
moment of 1.6 µB.39 The new complex, (η1-MesCN)2[3-
d
18] was collected by filtration in 52% yield as a dark
blue powder following concentration of the spectroscopi-
cally pure solution.
Crystals of the bis-cyanomesitylene complex suitable
for X-ray diffraction were grown at 25 °C from hexane
solution in the presence of excess MesCN; a thermal
ellipsoid plot is provided in Figure 14, and it illustrates
several important points. First, the structure of (η1-
MesCN)2[3] illustrates dramatically the manner in
which isopropyl-substitution makes five-coordination
available to the molybdenum center. The two cyano-
mesitylene ligands are disposed in a mutually trans
arrangement at the axial positions (defining the z axis)
of a trigonal bipyramid. This geometry is expected to
place the three d electrons in the pair of near-degenerate
(39) Evans, D. F. J . Chem. Soc. 1959, 2003.
(40) Tsai, Y.-C.; Cummins, C. C. Inorg. Chim. Acta 2003, 345, 63.

2910 Organometallics, Vol. 22, No. 14, 2003
Tsai et al.
equilibrium formation of which can be driven by the
addition of excess nitrile. J udicious choice of nitrile and
ligand set has allowed for the isolation and X-ray
structural characterization of a representative 1:1 η2
adduct, in the form of (η2-Me2NCN)[1], and also a
representative 2:1 η1 adduct, in the form of (η1-
MesCN)2[3]. Finally, treatment of 2 with 9-anthracenyl
carbonitrile results in the formation of a head-to-tail
dimer.
In addition to comprising essential data on the
chemical consequences of the differing steric demands
manifested by the tert-butyl- and isopropyl-substituted
molybdenum tris-amide fragments, these results pro-
vide valuable insights into the mechanism of reductive
nitrile coupling by low-valent early metals. Future work
will utilize kinetic measurements to further develop the
mechanistic framework of nitrile reductive coupling now
that several of the key players have been identified.
Exp er im en ta l Section
Gen er a l Con sid er a tion s. Unless otherwise stated, all
operations were performed in a Vacuum Atmospheres drybox
under an atmosphere of purified dinitrogen or by using
Schlenk techniques under an argon atmosphere. Complexes
Mo(H)(η2-Me2CNAr)(N[i-Pr]Ar)2, Mo(H)(η2-(CD3)2CNAr)(N[CH-
(CD3)2]Ar)2, and Mo(N[t-Bu]Ar)3 were prepared according to
the literature.3,9 9-Anthracenylcarbonitrile, benzonitrile, and
2,4,6-trimethylbenzonitrile were used as purchased from Lan-
caster Synthesis and dimethylcyanamide was obtained from
Aldrich. Diethyl ether, benzene, n-pentane, n-hexane, and
toluene were dried and deoxygenated by the method of
Grubbs.41 THF was distilled from purple Na/benzophenone
ketyl and collected under nitrogen. C6D6 was degassed and
dried over 4 Å molecular sieves. Celite, alumina, and 4 Å
molecular sieves were dried in vacuo overnight at a temper-
ature above 200 °C. X-band EPR spectra were recorded on a
Bruker EMX spectrometer equipped with an Oxford Instru-
ments Helium flow cryostat (Model ESP 310) in the temper-
ature range 20-300 K. Spectra were recorded in liquid and
frozen toluene solution as well as mixtures of pentane/Et2O.
Room-temperature spectra were simulated by using Bruker’s
perturbation-theory based software packages SimFonia and
WinEPR.42 Frozen solution EPR data were analyzed and
simulated by using a program provided by Frank Neese, MPI
Mu¨lheim, Germany.43 Cyclic voltammetry measurements were
carried out with an Eco-Chemie Autolab potentiostat
(pgstat20) and the GPES 4.3 software in conjunction with a
three-electrode cell, using 0.5 M [NnBu4][PF6] solutions in
F igu r e 15. Thermal ellipsoid plot (35% probability) of
[(AnthCN)(3)]2. Selected distances (Å): Mo1-N1, 1.961(5);
Mo1-N2, 1.948(5); Mo1-N3, 1.941(5); Mo1-N4, 1.917(5);
Mo2-N5, 2.008(5); Mo2-N6, 1.947(5); Mo2-N7, 1.931(5);
Mo2-N8, 1.833(5); N4-C4, 1.213(7); C4-C49, 1.347(7);
C8-N8, 1.291(7). Selected angles (deg): N1-Mo1-N2,
111.6(2); N2-Mo1-N3, 115.3(2); N3-Mo1-N1, 118.7(2);
N4-Mo1-N1, 104.3(2); N2-Mo1-N4, 97.1(2); N3-Mo1-
N4, 106.5(2); C4-N4-Mo1, 168.4(5); N4-C4-C49, 178.0(6).
tion to the observed thermodynamic sink in the sys-
tem: inversion-symmetrical nitrile-coupled products.
R ea ct ion of 9-An t h r a cen ylca r b on it r ile w it h
Molyb d a zir id in e-H yd r id e. In contrast to the cases
with PhCN and MesCN, when 2 is treated with 1 equiv
of 9-anthracenylcarbonitrile, a head-to-tail dimer is
formed, the X-ray crystal structure of which is depicted
in Figure 15. Presumably, the increased steric demands
of 9-anthracenylcarbonitrile disfavor a head-to-head
coupled product. Structural features of note include the
nearly linear N(4)-C(4)-C(49) keteneimine moiety
(νNCC ) 1944 cm-1) as well as the pyramidalization at
C(410), a manifestation of the electronic reorganization
of the anthracene unit upon reductive coupling.
tetrahydrofuran. H, H, and 13C NMR spectra were recorded
on a Varian VXR-500 or a Varian XL-301 spectrometer. 1H,
2H, and 13C chemical shifts are reported with respect to
internal solvent (7.15 ppm and 128.38 (t) ppm (C6D6)). Ele-
mental analyses (C, H, N) were conducted by H. Kolbe
Mikroanalytisches Laboratorium, Mu¨lheim an der Ru¨hr,
Germany.
1
2
Syn th esis of HN[t-Bu ]Ar .45 (1) Syn th esis of 2,4,6-Tr i-
m eth ylp yr illiu m Tetr a flu or obor a te. Acetic anhydride (1
kg, 9.79 mol, 10 equiv) was placed in a 2-L, two-necked round-
bottom flask. At room temperature, 4-hydroxy-4-methyl-2-
pentanone (122 mL, 0.98 mol, 1 equiv) was added over 5 min,
using a pressure equalizing dropping funnel, and the solution
was stirred for 5 min. HBF4 (127 mL, 48% solution in water,
0.97 mol, 1 equiv) was added dropwise, using a pressure
equalizing dropping funnel, over a period of 1 h. The temper-
Con clu sion s
In conclusion, we have illustrated the ability of both
tert-butyl and isopropyl-substituted variants of the
molybdenum trisamide fragment to engage in the
reductive coupling of simple organic nitriles to form
bridging diiminato complexes. NMR and EPR spectro-
scopic monitoring of this process with benzonitrile
indicate that the initial product is a 1:1 η2 bound nitrile
complex; while this is the sole observable intermediate
in the case of 1, the reduced steric bulk of 2 allows for
the formation additionally of a bis-η1 complex, the
(41) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J . Organometallics 1996, 15, 1518.
(42) www.bruker-biospin.de/EPR/software/index.html
(43) Neese, F. QCPE Bull. 1995, 15, 5.

Reactions of Organic Nitriles
Organometallics, Vol. 22, No. 14, 2003 2911
ature of the reaction mixture was ∼90 °C during the addition.
The color of the solution gradually changed from colorless to
light orange to dark brown. The reaction mixture was heated
at ∼80 °C with stirring for another hour, then was allowed to
cool to room temperature. The light-colored precipitate was
filtered by suction and washed with a total of 1 L of diethyl
ether to give 135 g (0.64 mol, 66%) of pale yellow, crystalline
2,4,6-trimethylpyrillium tetrafluoroborate. Melting point (un-
calibrated): 219.6-220.3 °C dec. 1H NMR (CD3CN, 300 MHz,
20 °C): δ 7.72 (s, 2H, m-methyl), 2.80 (s, 6H, o-methyl), 2.65
(s, 3H, p-methyl) ppm.
Anal. Calcd for C39H60N5Mo: C, 67.41; H, 8.70; N, 10.08.
Found: C, 67.46; H, 8.62; N, 10.06.
Syn th esis of [(P h CN)(1)]2. To a diethyl ether solution of
1 (2.5422 g, 4.067 mmol) was added a diethyl ether solution
of benzonitrile (0.4228 g, 4.105 mmol, 1.01 equiv). Upon mixing
at 20 °C, a purple color was observed. This purple color
persisted until concentration of the solvent to dryness 72 h
later at which time a blue-green color was observed. Extraction
and subsequent crystallization from cold THF yielded 1.833 g
of dark green microcrystalline solid (70.3% yield). 1H NMR
(C6D6, 20 °C, 500 MHz): δ 8.05 (br s, 2H, phenyl para), 7.50
(t, 4H, phenyl ortho, J ) 8 Hz), 6.94 (q, 4H, phenyl meta, J )
7 Hz), 6.67 (s, 6H, aryl para), 6.41 (br s, 12H, aryl ortho), 2.18
(2) Syn t h esis of N-ter t-Bu t yl-3,5-d im et h yla n ilin e.
Acetonitrile (100 mL) and tert-butylamine (200 mL, 1.90 mol,
4.40 equiv) were placed in a 4-L, two-necked round-bottom
flask. The flask was purged with nitrogen and kept under
nitrogen during the course of the following operations. 2,4,6-
Trimethylpyrillium tetrafluoroborate (40 g, 0.19 mol, 0.44
equiv) was dissolved in acetonitrile (600 mL) and added
dropwise over 2.5 h, using a pressure equalizing dropping
funnel. Upon addition, the reaction mixture turned red. To the
same reaction mixture was added a second fraction of pyrillium
salt (50 g, 0.24 mol, 0.55 equiv) in acetonitrile (700 mL) over
5 h. Preserving the ratio of tert-butylamine to pyrillium salt
at 4.4, the procedure was repeated in a second flask. Aceto-
nitrile (35 mL) and tert-butylamine (70 mL, 0.66 mol, 1.54
equiv) were placed in a 2-L, two-necked round-bottom flask,
as above. 2,4,6-Trimethylpyrillium tetrafluoroborate (30 g, 0.15
mol, 0.35 equiv) was dissolved in acetonitrile (600 mL) and
added dropwise over 2.5 h. The volatiles were removed by
rotary evaporation and the residues were combined. Water
(400 mL) was added and the mixture was extracted three times
with petroleum ether (300, 300, 200 mL). The organic fractions
were dried over anhydrous MgSO4 and filtered. The volatiles
were removed by rotary evaporation to give a dark brown oil.
GC-MS of the crude product showed ∼90% content of the
desired amine. Vacuum distillation (44 °C at 15 mTorr) allowed
isolation of the desired N-tert-butyl-3,5-dimethylaniline (63.0
g, 0.36 mol, 62%) as a pale yellow oil. 1H NMR (C6D6, 300 MHz,
20 °C): δ 6.46 (s, 1H, para), 6.37 (s, 2H, ortho), 2.93 (s, 1H,
N-H), 2.21 (s, 6H, m-methyl), 1.19 (s, 9H, tert-butyl methyl)
ppm. By selective irradiation of the aryl methyl signal, Hpara
becomes a triplet (J ) 1.65 Hz) and Hortho becomes a doublet
(J ) 1.65 Hz). 13C NMR (C6D6, 300 MHz, 20 °C): δ 147.84
(ipso), 138.59 (meta), 121.03 (para), 116.35 (ortho), 51.63 (t-
Bu quaternary), 30.77 (t-Bu methyl), 22.29 (Aryl methyl) ppm.
Syn th esis of (µ-NC(CH3)C(CH3)N)[1]2. To a solution of
1 (331 mg, 530 µmol) in 5 mL of Et2O contained in a 20-mL
glass vial was added acetonitrile (26.1 mg, 636 µmol). The
solution became green quickly, and green solid precipitated
out in 5 min. After an hour, the green precipitate was collected
by filtration, washed with cold Et2O, and dried under vacuum
(272 mg, 204 µmol, 77%). 1H NMR (C6D6, 22 °C): 6.7 (9 H,
overlapping aryl ortho and para), 2.3 (21 H, overlapping aryl
CH3 and µ-NC(CH3)-C(CH3)N), 1.5 (27 H, br, NC(CH3)3). Anal.
Calcd for C76H114Mo2N8: C, 68.54; H, 8.63; N, 8.41. Found: C,
68.04; H, 8.55; N, 8.36.
(s, 36H, aryl methyl), 1.32 (s, 54H, tert-butyl methyl) ppm. 13
C
NMR (C6D6, 20 °C, 125 MHz): δ 153.32 (bridge), 137.09, 129.15
(aryl ortho), 128.42 (phenyl ortho), 127.92 (phenyl para), 127.34
(aryl para), 125.49 (phenyl meta), 116.30, 64.56 (tert-butyl
quaternary), 32.47 (tert-butyl methyl), 22.02 (aryl methyl) ppm.
Anal. Calcd for C86H118N8Mo2: C, 70.95; H, 8.17; N, 7.69.
Found: C, 70.10; H, 8.39; N, 7.24.
Th er m olysis of [(P h CN)(1)]2 w ith Con com ita n t F or -
m a tion of th e En ed iim id o [(P h CN)Mo(NAr )(N[t-Bu ]-
Ar )2]2. A toluene solution of [(PhCN)(1)]2 (ca. 50 mg in 7 mL
toluene) was sealed in a Schlenk tube and thermolyzed in a
105 °C oil bath for 1.5 h. During this time the solution changed
color from green to red-purple, and bubbling was observed.
There was evidence in the 1H NMR spectrum of isobutene,
isobutylene, and hexamethylethane consistent with tert-butyl
radical ejection.27,46-48 1H NMR (C6D6, 20 °C, 500 MHz): δ 7.55
(d, 4H, phenyl ortho, J ) 7.5 Hz), 7.16 (t, 4H, phenyl meta),
7.07 (t, 2H, phenyl para), 6.74 (s, 8H, amido aryl ortho), 6.71
(s, 4H, amide aryl para), 6.45 (s, 2H, imido aryl para), 5.75 (s,
4H, imido aryl ortho), 2.22 (s, 24H, amido aryl methyl), 2.10
(s, 12H, imido aryl methyl), 1.24 (s, 36H, tert-butyl) ppm. 13C
NMR (C6D6, 20 °C, 125 MHz): δ 157.85, 154.35, 148.69, 142.31,
137.57, 130.97 (phenyl ortho), 128.94, 128.88 (amido aryl
ortho), 128.68 (phenyl meta), 126.92 (amido aryl para), 126.55,
125.94 (imido aryl para), 121.48 (imido aryl ortho), 60.72 (tert-
butyl quaternary), 32.49 (tert-butyl methyl), 21.91 (amido aryl
methyl), 21.68 (imido aryl methyl) ppm. Anal. Calcd for
C
78H100N8Mo2: C, 69.83; H, 7.51; N, 8.35. Found: C, 69.29; H,
7.08; N, 7.91.
Syn th esis of DN[CDMe2]Ar a n d DN[CDMe2](3,5-
C6D3Me2) (Ar ) 3,5-C6H3Me2). Since the preparations of these
two anilines were carried out in essentially the same way, the
preparation of DN(CDMe2)Ar is given in detail as an example.
To a frozen solution of 9.377 g (0.223 mol) of lithium aluminum
deuteride suspended in 250 mL of tetrahydrofuran was added
dropwise a solution of 30 g (0.186 mol) of the imine Me2CNAr
in 50 mL of tetrahydrofuran. Upon complete addition, the
reaction mixture was stirred at room temperature and became
light gray-green overnight. The reaction vessel was cooled in
an ice water bath and D2O was slowly added, followed by
several portions of a dilute solution of sodium potassium
tartrate in D2O, followed by two more portions of D2O. All
organic materials were extracted with 300 mL of petroleum
ether. The organic layer was separated from the aqueous layer
and dried over anhydrous MgSO4. A pale yellow oil was
obtained upon removing all volatiles in vacuo. DN[CDMe2]Ar
was obtained by vacuum distillation (23.552 g, 0.143 mol, 77%).
1H NMR of DN[CDMe2]Ar (C6D6, 300 MHz, 20 °C): δ 6.40 (s,
1H, para), 6.15 (s, 2H, ortho), 2.21 (s, 6H, m-methyl), 0.92 (6H,
-CD(CH3)2) ppm. 1H NMR of DN[CDMe2](3,5-C6D3Me2) (C6D6,
300 MHz, 19.8 °C): δ 2.21 (s, 6H, m-methyl), 0.93 (6H,
-CD(CH3)2) ppm.
Syn th esis of (η2-Me2NCN)[1]. In 8 mL of n-hexane was
dissolved 1.278 g of compound 1. To this stirring solution was
added 0.168 g of dimethylcyanamide in 2 mL of n-hexane.
Immediately the reaction mixture turned green. After 10 min,
the reaction mixture volume was reduced in vacuo to 3 mL
and the solution was stored at -35 °C. Crystalline product
(1.097 g, 77%) was collected by filtration and rinsed with cold
n-pentane. 1H NMR (C6D6, 20 °C): δ ca. 6.5 (br s), 0.762 ppm.
2H NMR of d18 isotopomer (diethyl ether, 20 °C): δ 7.06 (∆ν1/2
) 76.6 Hz) ppm. µeff ) 1.83 µB (Evans’ method, C6D6, 19.9 °C).39
Syn th esis of Li(N[CDMe2]Ar )‚Et2O a n d Li(N[CDMe2]-
(3,5-C6D3Me2))‚Et2O (Ar ) 3,5-C6H3Me2). Since the prepara-
(44) Vernaudon, P.; Rajoharison H. G.; Roussel, C. Bull. Soc. Chim.
Fr. 1987, 205.
(45) The synthesis of N-tert-butyl-3,5-dimethylaniline given here is
adapted from a literature procedure: Vernaudon, P.; Rajoharison, H.
G.; Roussel, C. Bull. Soc. Chim. Fr. 1987, 205-211.
(46) Terry, J . O.; Futrell, J . H. Can. J . Chem. 1968, 46, 4.
(47) Weydert, M.; Brennan, J . G.; Andersen, R. A.; Bergman, R. G.
Organometallics 1995, 14, 3942.
(48) Pryor, W. A. Free Radicals; McGraw-Hill: New York, 1966.

2912 Organometallics, Vol. 22, No. 14, 2003
Tsai et al.
Ta ble 1. Cr ysta llogr a p h ic Da ta for (η2-Me2NCN)[1], (η1-MesNC)2[3], [(P h CN)(3)]2[I3], a n d [(An th CN)(3)]2
(η2-Me2NCN)[1]
(η1-MesNC)2[3]
[(PhCN)(3)]2[I3]
[(AnthCN)(3)]2
formula
fw
space group
a, Å
b, Å
c, Å
C39H60MoN5
694.86
P21/n
12.9183(13)
19.099(2)
15.748(2)
90
95.313(2)
90
3868.7(7)
4
C53H70MoN5
873.08
P21/c
23.7364(2)
11.0300(2)
19.2358(4)
90
93.6860(10)
90
5025.75(14)
4
C83H110Cl6I6Mo2N8
2385.77
C2/c
20.0607(5)
19.6646(5)
24.8443(7)
90
95.3800(10)
90
9757.5(4)
4
C192H228Mo4N16
3143.66
P1h
18.6762(2)
21.63490(10)
21.8544(3)
85.1420(10)
85.3770(10)
76.6580(10)
8544.2(2)
2
R, deg
â, deg
γ, deg
V, Å3
Z
cryst description
green block
1.193
0.371
1484
1.129
blue rod
1.154
0.299
1860
1.322
red block
1.624
0.236
4672
1.103
red plate
1.222
0.343
3320
1.328
D
calcd, g‚cm-3
µ, mm-1
F(000)
GOF on F2
R1 (Fo) for I > 2σI
0.0750
0.1481
0.0663
0.1294
0.0516
0.1265
0.0736
0.1661
wR2 (Fo2) for I > 2σI
tions of these two lithium anilides were carried out in
essentially the same way, the preparation of LiN[CDMe2]Ar‚
Et2O is given in detail. To a frozen solution of 23.0 g (0.139
mol) of DN[CDMe2]Ar in 100 mL of n-pentane were added cold
n-BuLi (105 mL, 1.6 M in hexanes, 0.168 mol). The resulting
yellow solution was stirred for 60 min at room temperature
and white precipitate formed. Upon reducing the volume of
the slurry to one-third in vacuo, 18 mL of diethyl ether was
added. After the resulting mixture was stirred for 1 h, the
mixture was placed in the cold well of the glovebox for 30 min.
A fine white powder (29.886 g, 0.233 mol, 87.7%) was collected
by filtration, rinsed with cold n-pentane, and dried in vacuo.
NMR (diethyl ether, 20 °C): δ 3.21 (∆ν1/2 ) 22.1 Hz) ppm. µeff
) 1.58 µB (SQuID). Anal. Calcd for C53H70N5Mo: C, 72.91; H,
7.93; N, 7.86. Found: C, 73.05; H, 7.96; N, 8.04.
Syn th esis of [(P h CN)(3-d 18)]2[I3]. To 0.313 g (0.537 mmol)
of 2-d18 in 10 mL of diethyl ether was added 0.061 g (0.591
mmol) of PhCN. The resulting solution quickly turned to dark
blue-green. An oily material was obtained upon removal of all
volatiles in vacuo. The oily residue was redissolved in 5 mL of
diethyl ether to yield a green solution. To this solution was
added 0.103 g (0.812 mmol) of I2 in 5 mL of diethyl ether.
Maroon solid precipitated rapidly, was isolated by filtration,
and was rinsed with n-hexane until the filtrate became
colorless (0.406 g, 86.2%). 2H NMR (THF, 20 °C): δ 0.81 (∆ν1/2
) 16.0 Hz) ppm. µeff ) 1.31 µB (Evans’ method, C5D5N, 19.9
°C).39 Anal. Calcd for C80H106N8I3Mo2: C, 54.83; H, 6.10; N,
4.80. Found: C, 54.91; H, 6.16; N, 4.73.
1
For Li(N[CDMe2]Ar)‚Et2O, H NMR (C6D6, 300 MHz, 20 °C):
δ 6.49 (s, 2H), 6.13 (s, 1H), 2.97 (q, 4H), 2.33 (s, 6H), 1.43 (s,
6H), 0.77 (t, 6H) ppm. 13C NMR (C6D6, 75 MHz, 20 °C): δ
160.38 (aryl ipso), 139.31 (aryl meta), 113.16 (aryl para),
111.00 (aryl ortho), 65.16 (OCH2CH3), 47.34 (CD(CH3)2), 29.93
(CD(CH3)2), 22.10 (Ar-CH3), 14.31 (OCH2CH3) ppm. For LiN-
[CDMe2](3,5-C6D3Me2)‚Et2O, 1H NMR (C6D6, 300 MHz, 19.9
°C): δ 2.97 (q, 4H), 2.33 (s, 6H), 1.43 (s, 6H), 0.77 (t, 6H) ppm.
2H NMR (C6H6, 19.6 °C): δ 6.49 (s, 2D), 6.13 (s, 1D), 3.37 (s,
1D) ppm.
Syn th esis of [(An th CN)(3)]2. To 0.680 mmol of 2 in 6 mL
of diethyl ether was added 0.646 mmol of 9-anthracenecarbo-
nitrile suspended in 10 mL of diethyl ether. The resulting
mixture became purple quickly, and a purple precipitate
formed in approximately 1 min. After the solution was stirred
for 1 h, the purple solid was collected by filtration, rinsed with
diethyl ether, and dried in vacuo (0.462 g, 0.294 mmol, 91%).
1H NMR (C6D6, 19.9 °C): δ 8.21 (s, 1H), 8.16 (t, 2H), 7.76 (t,
2H), 7.22 (m, 4H), 7.06 (m, 4H), 7.00 (d, 2H), 6.56 (s, 6H), 6.37
(s, 6H), 6.32 (s, 6H), 4.62 (q, 3H), 4.21 (q, 3H), 2.10 (s, 18H),
2.08 (s, 18H), 1.14 (d, 18H), 1.06 (d, 18H) ppm. IR (Et2O): νNCC
1944 cm-1. Anal. Calcd for C96H114N8Mo2: C, 73.35; H, 7.31;
N, 7.13. Found: C, 73.24; H, 7.46; N, 7.02.
Syn th eses of Molybd a zir id in e Deu ter id e Com p lexes
2-d 3 a n d 2-d 4. Since the preparations of these two complexes
were carried out in essentially the same way, that for 2-d3 is
given in detail. To 22.0 g of Li(N[CDMe2]Ar)‚Et2O (0.090 mol)
in 350 mL of diethyl ether was added orange solid MoCl3-
(THF)3 (18.84 g, 0.045 mol). The flask containing the resulting
suspension was evacuated for 3 min and then stirred at room
temperature for 5.5 h. The resulting orange-brown suspension
was filtered through a plug of Celite to remove LiCl and excess
MoCl3(THF)3. A black solid was obtained upon removing all
volatiles of the filtrate. The resulting solid was redissolved in
200 mL of diethyl ether and the resulting solution was
concentrated to 180 mL in vacuo in a 300-mL Schlenk flask.
Big blocks of black crystals formed upon storing the solution
under vacuum at -35 °C overnight. The desired product was
isolated by filtration (2 crops totaling 10.67 g, 0.018 mol,
X-r a y Cr ysta l Da ta : Gen er a l P r oced u r e. Crystals grown
from concentrated solutions at -35 °C were coated with
Paratone N oil (an Exxon product) on a microscope slide.
Samples were selected and mounted with wax on a glass fiber.
Data collection was carried out at a temperature of 183(2) K
on a three-circle goniometer Siemens Platform with a CCD
detector with Mo KR (λ ) 0.71073 Å, µ ) 2.362 mm-1). The
data were processed and refined by direct methods (SHELXTL
V5.0, G. M. Sheldrick and Siemens Industrial Automation,
Inc., 1995) in conjunction with standard difference Fourier
techniques. All non-hydrogen atoms were refined anisotropi-
2
60.0%). H NMR of 2-d3 (C6H6, 19.6 °C): δ 5.84 (∆ν1/2 ) 41.8
Hz, 2D, amido isopropyl), -12.3 (∆ν1/2 ) 412.6 Hz, 1D,
deuteride) ppm. µeff ) 2.04 µB (Evans’ method, C6D6, 20 °C).39
For 2-d4, 2H NMR (C6H6, 19.6 °C): δ 9.48 (∆ν1/2 ) 43.2 Hz),
cally and hydrogen atoms were placed in caluculated (dCH
)
0.96 Å) positions. Some details regarding refined data and cell
parameters are available in Table 1. Selected bond distances
and angles are supplied in the captions of Figures 5, 11, 13,
and 15.
3.45 (∆ν1/2 ) 95.7 Hz), -2.91 (∆ν1/2 ) 401.5 Hz) ppm. µeff
)
2.11 µB (Evans’ method, C6D6, 19.9 °C).39
Syn th esis of (η1-MesCN)2[3-d 18]. To 0.756 mmol of 2-d 18
in 10 mL of n-hexane was added 0.220 g (1.51 mmol) MesCN
in 5 mL of n-hexane. The resulting mixture became dark blue
quickly. The solution was concentrated to approximately 3 mL
in vacuo. A crop of spectroscopically pure solid was collected
by filtration as a dark blue powder (0.394 mmol, 52.1%). 2H
DF T Ca lcu la tion s. Calculations were performed with
version 2002.02 of the Amsterdam DFT program package,
ADF.49-51 Atomic coordinates from the X-ray diffraction study
of (η2-Me2NCN)[1] were employed, adjusting the CsH bond

Reactions of Organic Nitriles
Organometallics, Vol. 22, No. 14, 2003 2913
distances all to a value of 1.096 Å. The calculation was carried
out with all the electrons considered explicitly, i.e., no frozen
core approximation was utilized. The atomic basis sets em-
ployed were the ZORA/TZ2P bases supplied with the program,
and accordingly, scalar relativistic effects were included with
the ZORA treatment. The calculation was carried out in spin-
restricted mode as required for g-tensor calculation, using the
ESR block key within the ADF program.52,53 The local density
approximation functional used was that of Vosko, Wilk, and
Nusair, while the functionals for the generalized gradient
approximations took the form of Becke (exchange) and Perdew
(correlation).54-56
Ack n ow led gm en t. For support of this work, the
authors are grateful to the National Science Foundation
(CHE-9988806). The authors are grateful to Theodor
Agapie, J ane B. Greco, and Daniel J . Kramer for help
with the revised ligand synthesis.
Su p p or tin g In for m a tion Ava ila ble: 2H NMR spectra of
2-d18 in toluene at various temperatures and the simulation
of these spectra; tables with bond lengths, bond angles, atomic
coordinates, and anisotropic displacement parameters for the
structures of (η2-Me2NCN)[1], [(PhCN)(3)]2[I3], (η1-MesCN)2[3],
and [(AnthCN)(3)]2; details of the DFT calculations; X-ray
crystallographic data. This material is available free of charge
via the Internet at http://pubs.acs.org.
(49) te Velde, G.; Bickelhaupt, F. M.; van Gisbergen, S. J . A.; Fonseca
Guerra, C.; Baerends, E. J .; Snijders, J . G.; Ziegler, T. J . Comput.
Chem. 2001, 22, 931.
(50) Fonseca Guerra, C.; Snijders, J . G.; te Velde, G.; Baerends, E.
J . J . Theor. Chem. Acc. 1998, 99, 391.
OM030124V
(51) ADF2002.02, SCM, Theoretical Chemistry, Vrije Universiteit,
Amsterdam, The Netherlands, http://www.scm.com.
(52) Schreckenbach, G.; Ziegler, T. J . Phys. Chem. A 1997, 101, 3388.
(53) van Lenthe, E.; van der Avoird, A.; Wormer, P. E. S. J . Chem.
Phys. 1997, 107, 2488.
(54) Baerends, E. J .; Ellis, D. E.; Ros, P. Chem. Phys. 1973, 2, 41.
(55) Versluis, L.; Ziegler, T. J . Chem. Phys. 1988, 88, 322.
(56) te Velde, G.; Baerends, E. J . J . Comput. Phys. 1992, 99, 84.