Mononuclear and dinuclear molybdenum and tungsten complexes of p-tert-butyltetrathiacalix[4]arene and p-tert-butyltetrasulfonylcalix[4]arene: Facile cleavage of the calixarene ligand framework by nickel

Article abstract of DOI:10.1021/ja803215x

The reactivity of p-tert-butyltetrathiacalix[4]arene, [S₄Calix^But(OH)₄], and p-tert-butyltetrasulfonylcalix[4]arene, [(SO₂)₄Calix^But(OH)₄], toward Mo(PMe₃)₅H₂, Mo(PMe₃)₆, and W(PMe₃)₄(η²-CH₂PMe₂)H has been used to synthesize a series of mononuclear molybdenum and tungsten calixarene compounds that feature both coordinatively saturated and unsaturated metal centers, such as [S₄Calix^But(OH)₂(O)₂]M(PMe₃)₃H₂ (M = Mo, W), [(SO₂)₄Calix^But(OH)₂(O)₂]M(PMe₃)₃H₂, [S₄Calix^But(OH)₂(O)₂]Mo(PMe₃)₃, [(SO₂)₄Calix^But(OH)₂(O)₂]Mo(PMe₃)₃, and [(SO₂)₄Calix^But(OH)(O)₃]M(PMe₃)₃H. Comparison with the related {[Calix^But(OH)₂(O)₂]M} complexes indicates that the chemistry of the system is strongly influenced by the nature of the calixarene linker, that is, CH₂, S, and SO₂. For example, in contrast to the methylene-bridged calixarene system, the thiacalixarene and sulfonylcalixarene systems readily coordinate a second metal center to form homo- and heterodinuclear complexes, namely {[S₄Calix^But(O)₄]}[M(PMe₃)₃H₂]₂, {[(SO₂)₄Calix^But(O)₄]}[Mo(PMe₃)₃H₂]₂ and {[S₄Calix^But(O)₄]}[Mo(PMe₃)₃H₂][W(PMe₃)₃H₂]. Of most interest, incorporation of nickel into [S₄Calix^But(OH)₂(O)₂]M(PMe₃)₃H₂ using Ni(PMe₃)₄ results in cleavage of a C-S bond to give [(SAr^ButOH)(SAr^ButO)₃][M(PMe₃)₃H₂][Ni(PMe₃)₂], an observation that is of relevance to the role that nickel plays in hydrodesulfurization catalysis. Copyright

Full text of DOI:10.1021/ja803215x

Published on Web 06/17/2008
Mononuclear and Dinuclear Molybdenum and Tungsten Complexes of
p-tert-Butyltetrathiacalix[4]arene and p-tert-Butyltetrasulfonylcalix[4]arene:
Facile Cleavage of the Calixarene Ligand Framework by Nickel
Daniela Buccella and Gerard Parkin*
Department of Chemistry, Columbia UniVersity, New York, New York 10027
Received April 30, 2008; E-mail: parkin@columbia.edu
Scheme 1
As part of our efforts to model species relevant to hydrodesulfur-
ization (HDS) and hydrodenitrogenation (HDN),1 we are particularly
interested in synthesizing heterodinuclear complexes that incorporate
combinations of molybdenum and tungsten with promoter metals such
as cobalt and nickel.2 Calixarenes are a class of multidentate macro-
cyclic phenols3 that offer considerable potential for achieving this
objective, especially if the linker between the phenolic groups also
contains donor atoms. For this reason, we have focused our attention
on the applications of calixarenes that feature S and [SO2] linkers,
t
specifically p-tert-butyltetrathiacalix[4]arene, [S4CalixBu (OH)4],4,5 and
t
p-tert-butyltetrasulfonylcalix[4]arene, [(SO2)4CalixBu (OH)4].6 An in-
vestigation of these calixarenes is also pertinent since we have recently
demonstrated that the methylene linker of p-tert-butylcalix[4]arene,
t
[CalixBu (OH)4], can serve as a binding functionality in molybdenum
and tungsten chemistry.7 Here we report that the nature of the linker
has a profound effect on the chemistry of the system and influences
the ability of the calixarene to coordinate a second metal center.
The p-tert-butyltetrathiacalix[4]arene molybdenum dihydride com-
t
plex [S4CalixBu (OH)2(O)2]Mo(PMe3)3H2 is conveniently obtained by
t
treatment of the calixarene [S4CalixBu (OH)4] with either Mo(PMe3)6
or Mo(PMe3)5H2 (Scheme 1). The molecular structure of
t
[S4CalixBu (OH)2(O)2]Mo(PMe3)3H2 has been determined by X-ray
diffraction, thereby demonstrating that the calixarene coordinates to
the metal with a κ3-mode via one sulfur and two oxygen atoms. As
such, the molybdenum coordination environment is similar to that of
t
[CalixBu (OH)2(O)2]Mo(PMe3)3H2,7 with the principal difference being
Interestingly, and in contrast to the reaction of Mo(PMe3)5H2,
t
that the thioether coordination replaces the agostic interaction involving
treatment of Mo(PMe3)6 with [(SO2)4CalixBu (OH)4] yields the zwit-
t
the methylene linker of [CalixBu (OH)2(O)2]Mo(PMe3)3H2. Evidence
t
terionic monohydride, [(SO2)4CalixBu (OH)(O)3]Mo(PMe3)3H (Scheme
t
that the Mo-S interaction within [S4CalixBu (OH)2(O)2]Mo(PMe3)3H2
t
2),11 which is a tautomer of octahedral [(SO2)4CalixBu (OH)2-
t
is stronger than the agostic interaction within [CalixBu (OH)2(O)2]-
Mo(PMe3)3H2 is provided by the observation that the metal center of
t
(O)2]Mo(PMe3)3. The zwitterionic complex [S4CalixBu (OH)(O)3]Mo-
(PMe3)3H has not, however, been observed for the thiacalixarene
system. In this regard, DFT calculations are in accord with the nature
of the product being dependent on the calixarene linker. Thus, while
[(X)4CalixH(OH)2(O)2]Mo(PMe3)3 and [(X)4CalixH(OH)(O)3]Mo-
(PMe3)3H have comparable energies for X ) SO2 (∆H ) 1.6 kcal
mol-1), the zwitterion is strongly disfavored for X ) S (∆H ) 29.5
kcal mol-1).12 As such, the nature of the calixarene linker has a
the latter migrates rapidly around the calixarene ring,7 whereas that
t
of [S4CalixBu (OH)2(O)2]Mo(PMe3)3H2 appears static with respect to
this process on the NMR time-scale at room temperature.8
t
The dihydride [S4CalixBu (OH)2(O)2]Mo(PMe3)3H2 is subject to
photochemically induced reductive elimination of H2, thereby
generating
the
six-coordinate
thiacalixarene
complex
t
[S4CalixBu (OH)2(O)2]Mo(PMe3)3 (Scheme 1). The formation of
profound effect.
t
[S4CalixBu (OH)2(O)2]Mo(PMe3)3 is reversible and addition of H2
t
The molecular structures of [(SO2)4CalixBu (OH)2(O)2]Mo-
t
regenerates [S4CalixBu (OH)2(O)2]Mo(PMe3)3H2 over a period of
t
(PMe3)3H2 and [(SO2)4CalixBu (OH)(O)3]Mo(PMe3)3H have been
determined by X-ray diffraction, thereby demonstrating that the
calixarene coordinates to the metal with a κ3-mode via two phenolic
oxygen atoms and one oxygen of the [SO2] linker. While both the
thiacalixarene and sulfonylcalixarene ligands coordinate in a κ3-mode,
a significant difference pertains to the conformations of the calixarenes.
Specifically, the thiacalixarene ligands adopt a cone conformation that
is supported by O-H· · ·O hydrogen bonding interactions which span
the rim of the calixarene, whereas the sulfonylcalixarene compounds
adopt 1,2-alternate conformations in which the phenolic OH groups
days at room temperature.
t
The sulfonyl calixarene [(SO2)4CalixBu (OH)4] has so far been little
exploited in transition metal chemistry.9,10 It is, therefore, noteworthy
t
that [(SO2)4CalixBu (OH)4] reacts with Mo(PMe3)5H2 to yield [(SO2)4-
t
CalixBu (OH)2(O)2]Mo(PMe3)3H2 (Scheme 2). As observed for
t
t
[S4CalixBu (OH)2(O)2]Mo(PMe3)3H2, [(SO2)4CalixBu (OH)2(O)2]Mo-
(PMe3)3H2 also undergoes photochemically induced reductive elimina-
t
tion of H2 thereby generating 16-electron [(SO2)4CalixBu (OH)2(O)2]-
Mo(PMe3)3, a process that may be reversed under thermal conditions.
9
10.1021/ja803215x CCC: $40.75  2008 American Chemical Society
J. AM. CHEM. SOC. 2008, 130, 8617–8619 8617

C O M M U N I C A T I O N S
Scheme 2
Scheme 3
participate in hydrogen bonding interactions with either the sulfonyl
or phenoxide groups.
t
The tungsten counterparts, [S4CalixBu (OH)2(O)2]W(PMe3)3H2,
t
t
[(SO2)4CalixBu(OH)2(O)2]W(PMe3)3H2, and [(SO2)4CalixBu(OH)(O)3]W-
(PMe3)3H, have also been synthesized employing W(PMe3)4(η2-
CH2PMe2)H and possess analogous structures to the molybdenum
derivatives.
The metal centers in the mononuclear thiacalixarene complexes
are readily derivatized. For example, the reactions of
t
[S4CalixBu (OH)2(O)2]M(PMe3)3H2 (M ) Mo, W) with Ph2C2 give
t
t
[S4CalixBu (OH)2(O)2]Mo(PMe3)2(Ph2C2) and [S4CalixBu (OH)2(O)2]-
W(PMe3)(Ph2C2)2. The formation of the tungsten diphenylacetylene
t
compound [S4CalixBu (OH)2(O)2]W(PMe3)(Ph2C2)2 is noteworthy be-
t
cause the corresponding reaction of {[CalixBu (OH)2(O)2]W(PMe3)3H2}
t
with Ph2C2 yields the alkylidene complex [CalixBu (O)4]WdC(Ph)R
[R ) C(Ph)C(Ph)CH2Ph].7 This observation provides a further
illustration of how the nature of the calixarene linker (agostic C-H
versus thioether S) may influence the reactivity of the metal center.
t
An interesting feature of the reactivity of [S4CalixBu (OH)2(O)2]-
M(PMe3)3H2 pertains to the ability to functionalize the unused
coordination sites of the calixarene with other metal centers. For
t
example, the dinuclear complexes {[S4CalixBu (O)4]}[M(PMe3)3H2]2
t
may be obtained Via reaction of [S4CalixBu (OH)2(O)2]M(PMe3)3H2
with the respective low valent metal trimethylphosphine compound,
Mo(PMe3)6 or W(PMe3)4(η2-CH2PMe2)H (Scheme 3). In addition, the
sulfonylcalixarene ligand also enables isolation of the dinuclear
t
complex {[(SO2)4CalixBu (O)4]}[Mo(PMe3)3H2]2, but a corresponding
dinuclear complex is yet to be isolated for the methylene-bridged
calixarene.13 Complementing these homonuclear compounds
t
{[S4CalixBu (O)4]}[M(PMe3)3H2]2, the heteronuclear counterpart
t
{[S4CalixBu (O)4]}[Mo(PMe3)3H2][W(PMe3)3H2] may be synthe-
t
sized by treatment of [S4CalixBu (OH)2(O)2]Mo(PMe3)3H2 with
W(PMe3)4(η2-CH2PMe2)H (Scheme 3).14
X-ray diffraction studies indicate that the dinuclear complexes
t
{[S4CalixBu (O)4]}[M(PMe3)3H2]2 possess a 1,2-alternate conformation
of the calixarene backbone, such that the two metal centers adopt an
anti disposition. H NMR spectroscopic studies also suggest this is
t
1
[S4CalixBu (OH)2(O)2]M(PMe3)3H2 would be able to access more
readily a non-cone conformation, thereby facilitating the formation of
the dinuclear complex. In support of the notion that thiacalixarene metal
the only conformation obtained in solution, presumably as a conse-
quence of the steric requirements of the [M(PMe3)3H2] fragments.15
t
The observation that dinuclear {[S4CalixBu(O)4]}[M(PMe3)3H2]2 adopts
compounds are more conformationally flexible, the mononuclear
t
a 1,2-alternate conformation of the calixarene provides a possible
thiacalixarene complex [S4CalixBu (OH)2(O)2]W(PMe2Ph)3H2 has been
t
explanation for the fact that [CalixBu (OH)2(O)2]M(PMe3)3H2 com-
pounds with methylene linkers do not form corresponding dinuclear
complexes upon treatment with a second equivalent of Mo(PMe3)6 or
isolated with both 1,2-alternate and partial cone conformations in the
solid state.17
t
The unused coordination sites in [S4CalixBu (OH)2(O)2]M(PMe3)3H2
W(PMe3)4(η2-CH2PMe2)H under comparable conditions. Specifically,
may also be employed to bind metals other than molybdenum and
t
t
the uncoordinated thiacalixarene [S4CalixBu (OH)4] is known to (i) be
tungsten. For example, [S4CalixBu (OH)2(O)2]M(PMe3)3H2 incorporates
nickel upon treatment with Ni(PMe3)4; however, rather than simply
coordinate, the nickel cleaves one of the C-S bonds of the calixarene
t
more flexible than [CalixBu (OH)4] and (ii) exhibit a reduced tendency
to adopt a cone conformation;16 as such, it is expected that
9
8618 J. AM. CHEM. SOC. VOL. 130, NO. 27, 2008

C O M M U N I C A T I O N S
Class of Macrocyclic Compounds; Kluwer: Dordrecht, The Netherlands,
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Miyanari, S.; Miyashi, T.; Miyano, S. Tetrahedron 2000, 56, 1437–1443.
(d) Akdas, H.; Bringel, L.; Graf, E.; Hosseini, M. W.; Mislin, G.; Pansanel,
J.; De Cian, A.; Fischer, J. Tetrahedron Lett. 1998, 39, 2311–2314.
(5) For reviews of thiacalixarenes, see: (a) Morohashi, N.; Narumi, F.; Iki, N.;
Hattori, T.; Miyano, S. Chem. ReV. 2006, 106, 5291–5316. (b) Iki, N.;
Miyano, S. J. Incl. Phenom. Macrocycl. Chem. 2001, 41, 99–105. (c)
Lhotak, P. Eur. J. Org. Chem. 2004, 167, 5–1692. (d) Shokova, E. A.;
Kovalev, V. V.; Russ, J. Organ. Chem. 2003, 39, 1–28. (e) Morohashi, N.;
Iki, N.; Miyano, S. J. Synth. Org. Chem. Jpn. 2002, 60, 550–562.
(6) Morohashi, N.; Iki, N.; Sugawara, A.; Miyano, S. Tetrahedron 2001, 57,
5557–5563.
(7) Buccella, D.; Parkin, G. J. Am. Chem. Soc. 2006, 128, 16358–16364.
(8) Specifically, no magnetization transfer is observed between adjacent phenoxy
t
groups. Furthermore, treatment of [S4CalixBu (OH)2(O)2]Mo(PMe3)3 with D2
t
t
Figure 1. Molecular structure of [(SArBu OH)(SArBu O)3][Mo(PMe3)3H2]-
[Ni(PMe3)2].
t
gives [S4CalixBu (OH)2(O)2]Mo(PMe3)3D2 in which deuterium is incorporated
into the hydride sites and not the hydroxy groups, thereby suggesting that
reductive elimination of O-H(D) and migration around the calixarene is not
facile.
t
t
to give [(SArBu OH)(SArBu O)3][M(PMe3)3H2][Ni(PMe3)2] (Figure 1
(9) Structurally characterized transition metal compounds derived from
and Scheme 3).18 Since this type of reactivity is not observed upon
t
[(SO2)4CalixBu (OH)4] are presently limited to metals of the 1st transition series.
A zinc complex is also known. (a) See : Kajiwara, T.; Yokozawa, S.; Ito, T.;
Iki, N.; Morohashi, N.; Miyano, S. Chem. Lett. 2001, 6–7. (Co, Ni). (b)
Kajiwara, T.; Yokozawa, S.; Ito, T.; Iki, N.; Morohashi, N.; Miyano, S.
Angew. Chem., Int. Ed. Engl. 2002, 41, 2076–2078. ( Zn). (c) Guo, Q.-L.;
Ma, S.-L.; Zhu, W.-X.; Liu, Y.-C.; Zhang, J. Chin. J. Chem. 2005, 23,
1387–1390. (Cu). (d) Kajiwara, T.; Kobashi, T.; Shingawa, R.; Ito, T.;
Takaishi, S.; Yamashita, M.; Iki, N. Eur. J. Inorg. Chem. 2006, 1765–
t
treatment of [S4CalixBu (OH)2(O)2]M(PMe3)3H2 with either Mo(PMe3)6
or W(PMe3)4(η2-CH2PMe2)H, the C-S bond cleavage achieved by
nickel is of particular significance. Specifically, the observation supports
the notion that the role of nickel in HDS may be concerned with the
C-S cleavage step. In this regard, Jones has demonstrated that low
valent tertiary phosphine compounds of nickel may cleave the C-S
bond of thiophene derivatives,19 and Garc´ıa has reported that similar
compounds are capable of achieving catalytic desulfurization of
thiophenes in the presence of RMgX.20
1770. (Mn, Co, Ni)
t
(10) For applications of [(SO2)4CalixBu (OH)4] in lanthanide chemistry, see: (a)
Kajiwara, T.; Katagiri, K.; Hasegawa, M.; Ishii, A.; Ferbinteanu, M.;
Takaishi, S.; Ito, T.; Yamashita, M.; Iki, N. Inorg. Chem. 2006, 45, 4880–
4882. (b) Kajiwara, T.; Wu, H.; Ito, T.; Iki, N.; Miyano, S. Angew. Chem.,
Int. Ed. Engl. 2004, 43, 1832–18356. (c) Kajiwara, T.; Katagiri, K.;
Takaishi, S.; Yamashita, M.; Iki, N. Chem. Asian J. 2006, 1, 349–351.
In summary, the nature of the calixarene linker can have a subs-
tantial impact on the chemistry of the system, as illustrated by the
ability of the S and [SO2] linkers to facilitate coordination of a sec-
t
(11) Since the formation of [(SO2)4CalixBu (OH)(O)3]Mo(PMe3)3H is accompanied
by elimination of H2, which reacts with Mo(PMe3)6 to give Mo(PMe3)5H2,
t
the synthesis of [(SO2)4CalixBu (OH)(O)3]Mo(PMe3)3H is best performed by
periodically evacuating the sample.
t
ond metal center. Furthermore, the incorporation of nickel into
(12) X-ray diffraction studies provide evidence that [(SO2)4CalixBu (OH)-
(O)3]Mo(PMe3)3H adopts more than one possible conformation in the solid
state, pertaining to different hydrogen bonding arrangements of the phenol/
phenoxide groups. The different conformations were considered in the
calculations, and the values reported here correspond to the lowest energy
conformation in which the phenolic hydroxy group participates in hydrogen
bonding with the uncoordinated phenoxide.
t
[S4CalixBu (OH)2(O)2]M(PMe3)3H2 is accompanied by C-S bond
cleavage of the calixarene ligand, an observation that is of relevance
to the role that nickel plays in HDS catalysts.
Acknowledgment. We thank the U.S. Department of Energy,
Office of Basic Energy Sciences (DE-FG02-93ER14339) for support
of this research and the National Science Foundation for acquisition
of the X-ray diffractometer (Grant CHE-0619638).
(13) Dinuclear methylene-bridged calixarene complexes with a cone conformation
are, nevertheless, known. See, for example: Petrella, A. J.; Roberts, N. K.;
Craig, D. C.; Raston, C. L.; Lamb, R. N. Chem. Commun. 2004, 64–65.
(14) For an example of a heterodinuclear thiacalixarene complex that features Mo
and Ti, see: Takemoto, S.; Tanaka, S.; Mizobe, Y.; Hidai, M. Chem. Commun.
Supporting Information Available: Experimental details, compu-
tational data and crystallographic data in CIF format. This material is
available free of charge via the Internet at http://pubs.acs.org.
2004, 838–839.
t
(15) In contrast, the titanium complex {[S4CalixBu (O)4]}[TiCl2]2 exists as both syn
and anti isomers which maintain their integrity in solution : Morohashi, N.;
Hattori, T.; Yokomakura, K.; Kabuto, C.; Miyano, S. Tetrahedron Lett. 2002,
43, 7769–7772.
(16) Bernardino, R. J.; Cabral, B. J. C. J. Mol. Struct. (Theochem) 2001, 549,
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