Molybdenum Chalcogenobenzimidato Complexes: Radical Synthesis and Nitrile Extrusion via β-EPh (E = S, Se, and Te) Elimination

Article abstract of DOI:10.1021/ic035077d

Molybdenum chalcogenobenzimidates of formula (Ph[PhE]C=N)-Mo(N[t-Bu]Ar)₃ (Ar = 3,5-C₆H ₃Me₂) have been obtained by treatment of Mo(N[t-Bu]Ar)₃ sequentially with benzonitrile and 0.5 equiv of PhEEPh (E = S, Se, and Te). Molecular structure determinations have been carried out for the S and Se variants. The Te variant extrudes PhCN forming structurally characterized (PhTe)Mo(N[t-Bu]Ar)₃ with facility assessed via stopped-flow kinetic measurements, while the Se and S analogues exhibit increasing stability. Quantum chemical calculations and solution calorimetry have been employed as an aid to interpretation of the PhCN extrusion reaction.

Full text of DOI:10.1021/ic035077d

Inorg. Chem. 2003, 42, 8621−8623
Molybdenum Chalcogenobenzimidato Complexes: Radical Synthesis
and Nitrile Extrusion via â-EPh (E ) S, Se, and Te) Elimination
,†
Arjun Mendiratta,† Christopher C. Cummins,* Olga P. Kryatova,‡ Elena V. Rybak-Akimova,‡
James E. McDonough,§ and Carl D. Hoff§
Department of Chemistry, Massachusetts Institute of Technology, Room 2-227,
77 Massachusetts AVenue, Cambridge, Massachusetts 02139, Department of Chemistry,
Tufts UniVersity, Medford, Massachusetts 02155, and Department of Chemistry,
UniVersity of Miami, Coral Gables, Florida 33124
Received September 11, 2003
Scheme 1
Molybdenum chalcogenobenzimidates of formula (Ph[PhE]CdN)-
Mo(N[t-Bu]Ar)3 (Ar ) 3,5-C H Me2) have been obtained by
6
3
treatment of Mo(N[t-Bu]Ar)3 sequentially with benzonitrile and 0.5
equiv of PhEEPh (E ) S, Se, and Te). Molecular structure
determinations have been carried out for the S and Se variants.
The Te variant extrudes PhCN forming structurally characterized
(PhTe)Mo(N[t-Bu]Ar)3 with facility assessed via stopped-flow kinetic
measurements, while the Se and S analogues exhibit increasing
stability. Quantum chemical calculations and solution calorimetry
have been employed as an aid to interpretation of the PhCN
extrusion reaction.
The observation1 that aqueous Hg(II) promotes hydrolysis
of [H2NdC(SMe)Ph][I] to provide PhCN is suggestive that
nitrile release from the thioimidate skeleton may be facili-
tated by thiophilic metals. In general, coordination complexes
of organic chalcogenoimidate derivatives (NdC[ER1]R2,
E ) S, Se, or Te) are rare. To our knowledge the only
example of chalcogenoimidate coordination to a transition
metal is the complex Re(SAr)(NdC[SAr]Me)2(PPh3)2 (Ar
) 2-[Me3Si]C6H4),2 formed upon addition of an acetonitrile
solution of ReCl3(PPh3)2(CH3CN) to an excess of thiol HSAr
and triethylamine. A related reaction exemplifying nitrile
insertion into a metal thiolate bond is that between Et2AlSEt
and acetonitrile to provide Et2Al(NdC[SEt]Me).3 With the
present work we define a radical pathway for the formation
of molybdenum chalcogenobenzimidates, obtain the first
examples known for selenium and tellurium, and explore the
facility with which they extrude benzonitrile (Scheme 1).
Recently we characterized in detail a variety of organo-
nitrile adducts with molybdenum trisamides Mo(N[R]Ar)3
(Ar ) 3,5-C6H3Me2; R ) t-Bu (1), i-Pr (1b).4 The indicated
adduct chemistry was exceptionally rich with η1, η2, and
certain bis-η1 organonitrile adducts having proved amenable
to characterization. Herein we show that such adducts are
exploitable as reactive intermediates.
Sequential room-temperature treatment of 1 (0.05 M in
Et2O solvent) with 1.0 equiv of PhCN and 0.5 equiv of
PhEEPh (E ) S or Se) is found to furnish cleanly the
corresponding chalcogenobenzimidates (Ph[PhE]CdN)Mo-
(N[t-Bu]Ar)3 according to reactions i and ii in Scheme 1;
these dark blue compounds, 2-S and 2-Se, were isolated in
yields of 64% and 70%, respectively, and were characterized
structurally by single-crystal X-ray diffraction methods. The
molecular structure of 2-S is shown in Figure 1;5 2-Se is
found to be isomorphic (Figure S1, Supporting Information).6
* To whom correspondence should be addressed. E-mail: ccummins@
mit.edu.
† Massachusetts Institute of Technology.
‡ Tufts University.
§ University of Miami.
(1) Hall, A. J.; Satchell, D. P. N. J. Chem. Soc., Perkin Trans. 2 1976,
1274.
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1989, 156, 27.
(3) Hirabayashi, T.; Itoh, K.; Sakai, S.; Ishii, Y. J. Organomet. Chem.
1970, 21, 273.
(4) Tsai, Y.-C.; Stephens, F. H.; Meyer, K.; Mendiratta, A.; Gheorghiu,
M. D.; Cummins, C. C. Organometallics 2003, 22, 2902.
10.1021/ic035077d CCC: $25.00 © 2003 American Chemical Society
Published on Web 12/03/2003
Inorganic Chemistry, Vol. 42, No. 26, 2003 8621

COMMUNICATION
Figure 2. Spectral changes upon mixing toluene solutions containing
Mo(N[t-Bu]Ar)3 (0.3 mM) and PhCN (12 mM) with Ph2Te2 (0.15 mM) at
-40 °C. Formation of 2-Te is complete in 50 s.
Figure 1. X-ray crystal structure of 2-S drawn with thermal ellipsoids at
50% probability. One of two independent molecules in the asymmetric unit
is shown. Selected bond lengths (Å) and angles (deg): Mo1-N1 1.963(4),
Mo1-N2 1.988(4), Mo1-N3 1.965(4), Mo1-N4 1.794(5), N4-C4
1.317(6); Mo1-N4-C4 164.3(4), C4-S1-C412 103.6(3).
favor reaction ii over reaction iv. Interestingly, this procedure
when carried out in the -50 to +5 °C temperature range
produced a long-lived species assigned as tellurobenzimidate
2-Te with a diagnostic UV-vis spectral feature at 585 nm
(see Figure 2). This reaction was found to be a clean second-
order process (first order in each reactant) with activation
parameters ∆Hq ) 15.8 kJ mol-1, ∆Sq ) -121 J K-1 mol-1.
The procedure for generating 2-Te was adapted further for
preparative-scale purposes. Specifically, addition of 0.5 equiv
of PhEEPh to a solution of 1 (30 mg in THF, 0.02 M)
premixed with 60 equiv of PhCN (ca. 1 M) provided a
solution of 2-Te suitable for 125Te NMR studies; 2-Te is
associated with a single 125Te NMR signal of chemical shift
+699 ppm, which compares favorably to the value of +617
ppm determined computationally8 for (H[PhTe]CdN)Mo-
(NH2)3.9,10 In contrast, 3-Te (modeled as (NH2)3MoTePh) is
calculated to exhibit a singlet at +1572 ppm, although we
have been unable thus far to observe such a signal. While
the requirement for excess PhCN in the synthesis of 2-Te
has hampered its isolation as a solid, we feel that the
observed spectral data allow confidence in its assignment.
We next investigated the ability of compounds 2-E (E )
S, Se, Te) to extrude benzonitrile (reaction iii in Scheme 1).
Although heating a benzene-d6 solution of 2-S (80 °C, 12
h) elicited no reaction, complex 2-Se exhibited a slow
reaction at 37 °C in which known selenolate 3-Se11 was
detected by 1H NMR spectroscopy. Unfortunately, 3-Se itself
decomposes to a mixture of unknown products at tempera-
tures g37 °C, obviating a kinetic study of the 2-Se f 3-Se
+ PhCN conversion.
The possibility that 2-E (E ) S, Se) formation is a
consequence of the direct reaction of 1 with PhEEPh to
produce thiolate or selenolate complexes 3-E (reaction iv)
followed by PhCN insertion (reverse of reaction iii) is ruled
out by control experiments in which 3-S and 3-Se were
treated with 10 equiv of PhCN under the synthesis reaction
conditions and found to undergo no reaction.
Interestingly, use of PhTeTePh in the above synthesis did
not result in tellurobenzimidate 2-Te formation. Specifically,
sequential room-temperature treatment of 1 (0.05 M in Et2O)
with 1.0 equiv of PhCN and 0.5 equiv of PhTeTePh led
exclusively to phenyltellurolate PhTeMo(N[t-Bu]Ar)3 (3-Te)
formation. Nitrile was not incorporated. A possible explana-
tion was that 2-Te, formed as a short-lived intermediate,
underwent facile conversion to 3-Te via PhCN expulsion
(reaction iii in Scheme 1). Alternatively, PhTeTePh under-
went direct reaction with 1 to provide 3-Te despite the
presence of PhCN (reaction iv).
The direct reaction of 1 with PhTeTePh (reaction iv, E )
Te) was found by stopped-flow kinetic studies to be a facile
process (∆Hq ) 11.6 kJ mol-1, ∆Sq ) -141 J K-1 mol-1)
first order in each reactant. Forest green 3-Te is prepared
readily by this reaction in synthetically useful quantities and
has been the subject of a structural investigation (Figure S2,
Supporting Information).7 As a complement to these kinetic
and synthetic investigations, thermodynamic measurements
employing solution calorimetry were carried out on reaction
iv for E ) S, Se, and Te. The enthalpies of reaction were
found to be -31.6(8), -31.4(1), and -24.0(3) kcal mol-1,
respectively.
Fortunately, such complications did not arise in the case
of 2-Te. Upon heating THF/PhCN solutions of 2-Te (gener-
(7) X-ray crystallographic data for 3-Te: T ) 193(2) K, triclinic, P1h, a
) 11.1740(12) Å, b ) 18.1215(19) Å, c ) 20.134(2) Å, R ) 91.445-
(2)°, â ) 90.692(2)°, γ ) 90.663(2)°, V ) 4075.0(7) Å3, Z ) 2, R1
) 0.0566, wR2 ) 0.1521.
(8) ADF2002.03, SCM, Theoretical Chemistry, Vrije University, Am-
sterdam, The Netherlands, http://www.scm.com. (June, 2003). teVelde,
G.; Bickelhaupt, F. M.; van Gisbergen, S. J. A.; Guerra, C. F.;
Baerends, E. J.; Snijders, J. G.; Ziegler, T. J. Comput. Chem. 2001,
22, 931. Guerra, C. F.; Snijders, J. G.; te Velde, G.; Baerends, E. J.
Theor. Chem. Acc. 1998, 99, 391.
Stopped-flow kinetic studies subsequently were carried out
using premixing of 1 with a large excess (>40 equiv, 12
mM) of PhCN, followed by mixing with PhTeTePh, to push
equilibrium i far to the left (as drawn in Scheme 1) and thus
(5) X-ray crystallographic data for 2-S: T ) 193(2) K, monoclinic, P21/
n, a ) 20.534(2) Å, b ) 24.450(2) Å, c ) 21.635(2) Å, R ) 90°, â
) 111.261(2)°, γ ) 90°, V ) 10122.5(18) Å3, Z ) 4, R1 ) 0.0621,
wR2 ) 0.1505.
(6) X-ray crystallographic data for 2-Se: T ) 193(2) K, monoclinic, P21/
n, a ) 20.534(2) Å, b ) 24.450(2) Å, c ) 21.635(2) Å, R ) 90°, â
) 111.261(2)°, γ ) 90°, V ) 10122.5(18) Å3, Z ) 4, R1 ) 0.0799,
wR2 ) 0.1592.
(9) The chemical shift range for 125Te NMR is several thousand parts per
million: McFarlane, H. C. E.; McFarlane, W. In NMR of Newly
Accessible Nuclei; Laszlo, P., Ed.; Academic: New York, 1983.
(10) Lutz, O. In The Multinuclear Approach to NMR Spectroscopy;
Lambert, J. B., Riddell, F. G., Eds.; D. Reidel: Boston, 1983.
(11) Johnson, 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.
8622 Inorganic Chemistry, Vol. 42, No. 26, 2003

COMMUNICATION
HCN elimination to provide HEMoL3 complexes is an
enthalpically uphill process by ca. 15 kcal mol-1 for all three
E: S, Se, and Te.25 We find on going from S to Se and Te
that both the E-C and the E-Mo bond dissociation
enthalpies (BDEs) diminish progressively, resulting in the
cancellation that yields the same value as a function of E
for the HCN elimination reaction. Calculated E-C BDEs
are 61, 51, and 43 kcal mol-1, while calculated Mo-E BDEs
are 76, 68, and 51 kcal mol-1. Complexation of HCN by
MoL3 to provide (η2-HCN)MoL3 is a process calculated to
be enthalpically favorable by 30 kcal mol-1. We conclude,
tentatively, that the observed conversion of 2-E to 3-E (E )
Se, Te) is entropically driven.
The radical route presented here has allowed the synthesis
of molybdenum chalcogenobenzimidates featuring S, Se, and
Te, the latter two being the first of their kind. Preliminary
investigations of the ability of these compounds to extrude
benzonitrile indicate increasing facility on going from S to
Se to Te; this observation is consistent with the trend toward
weaker bond strengths to carbon as one descends the periodic
table in the p-block.26 We are currently exploiting the syn-
thetic route elaborated here in order to synthesize chalco-
genobenzimidates more amenable to detailed kinetic inves-
tigation of the nitrile extrusion process.
Figure 3. Spectral changes accompanying the 2-Te f 3-Te + PhCN
conversion in THF/PhCN solution (T ) 35 °C, total time ) 6 h).
ated as described above) to 35 °C, clean benzonitrile ex-
trusion could be observed. As shown in Figure 3, UV-vis
monitoring of this process indicated that 2-Te is quantita-
tively converted to 3-Te (note isosbestic points at 448 and
495 nm) with a first-order kinetic profile (kobs ) 1.2(1) ×
10-4 s-1). While we cannot conclusively rule out a bimo-
lecular mechanism, the observed first-order kinetics strongly
suggest that 2-Te converts to 3-Te through a unimolecular
â-X elimination process.
Relative to â-H elimination, â-X elimination remains
poorly studied, and the majority of well-defined systems
involve group 10 metals.12-17 Prominent among early
metal examples is Buchwald’s observation that Cp2ZrHCl
(Schwartz’s reagent) reacts with ethyl vinyl ether to yield
Cp2Zr(Cl)OEt and ethylene, presumably via 1,2 insertion of
the olefinic unit followed by â-OEt elimination.18 No
intermediates could be observed in this system, but Wolc-
zanski measured first-order â-OR elimination kinetics for
the more hindered (silox)3Ta(H)CH2CH2OR system.19 Al-
though, to the best of our knowledge, nitrile release from
well-defined halo- or chalcogenoimidate complexes has not
previously been documented, Glushkova and co-workers
have presented limited evidence for the formation of d0 early-
metal chloroimidate complexes through nitrile insertion into
M-Cl bonds (M ) Ti,20 Nb,21 Ta,22,23 W24).
Acknowledgment. We are grateful to the National
Science Foundation for their support of this work (CRC-
0209977, CHE-9988806, and a predoctoral fellowship to
A.M.). Joshua S. Figueroa is acknowledged for assistance
with X-ray crystallography.
Supporting Information Available: Complete synthetic pro-
cedures and characterization data for all new compounds, tables of
crystal structure data in CIF format, experimental details of the
stopped-flow kinetic measurements and solution calorimetery, and
details of the density functional calculations. This material is
available free of charge via the Internet at http://pubs.acs.org.
IC035077D
Quantum chemical calculations8 carried out on the model
system H(HE)CdNMoL3 (L3 ) (NHMe)(NH2)2) suggest that
(19) Strazisar, S. A.; Wolczanski, P. T. J. Am. Chem. Soc. 2001, 123, 4728.
(20) Chumaevskii, N. A.; Khmelevskaya, L. V.; Ershova, M. M.; Glush-
kova, M. A. Koord. Khim. 1991, 17, 463.
(21) Buslaev, Yu. A.; Glushkova, M. A.; Chumaevskii, N. A.; Ershova,
M. M.; Khmelevskaya, L. V. Koord. Khim. 1982, 8, 457.
(22) Glushkova, M. A.; Chumaeavskii, M. A.; Khmelevskaya, L. V.;
Ershova, M. M.; Buslaev, Yu. A. Koord. Khim. 1987, 13, 349.
(23) Ovchinnikova, N. A.; Glushkova, M. A. Koord. Khim. 1986, 12, 202.
(24) Glushkova, M. A.; Chumaevskii, N. A.; Ershova, M. M.; Kh-
melevskaya, L. V. Koord. Khim. 1985, 11, 193.
(25) The L3 ligand set was chosen because the presence of â-hydrogens
on one amide allows MoL3 to function as an effective model for both
1 and 1b.
(12) Albeniz, A. C.; Espinet, P.; Lin, Y.-S. Organometallics 1997, 16, 5964.
(13) Wiger, G.; Albelo, G.; Rettig, M. F. J. Chem. Soc., Dalton Trans.
1974, 2242.
(14) Zhu, G.; Lu. X. Organometallics 1995, 14, 4899.
(15) Kaneda, K.; Uchiyama, T.; Fujiwara, Y.; Imanaka, T.; Teranishi, S.
J. Org. Chem. 1979, 44, 55.
(16) Nguefack, J.-F.; Bolitt, V.; Sinou, D. J. Chem. Soc., Chem. Commun.
1995, 1893.
(17) Duan J.-P.; Cheng, C.-H. Tetrahedron Lett. 1993, 34, 4019.
(18) Buchwald, S. L.; Nielsen, R. B.; Dewan, J. C. Organometallics 1988,
7, 2324.
(26) Pauling, L. The Nature of the Chemical Bond, 3rd ed.; Cornell
University Press: Ithaca, New York, 1960.
Inorganic Chemistry, Vol. 42, No. 26, 2003 8623