Quadruply bonded dimolybdenum complexes with highly unusual geometries and vacant coordination sites

Article abstract of DOI:10.1039/c2cc30394a

New quadruply bonded dimolybdenum complexes of the terphenyl ligand Ar ^Xyl2 (Ar^Xyl2 = C₆H₃-2,6-(C ₆H₃-2,6-Me₂)₂) have been prepared and structurally characterized. The steric hindrance exerted by the Ar ^Xyl2 groups causes the Mo atoms to feature unsaturated four-coordinate structures and a formal fourteen-electron count.

Full text of DOI:10.1039/c2cc30394a

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COMMUNICATION
Quadruply bonded dimolybdenum complexes with highly unusual
geometries and vacant coordination sitesw
Mario Carrasco,a Michelle Faust,b Riccardo Peloso,a Amor Rodrıguez,a
´
´
´
´
Joaquın Lopez-Serrano, Eleuterio Alvarez, Celia Maya, Philip P. Power* and
Ernesto Carmona*a
a
a
a
b
Received 17th January 2012, Accepted 25th February 2012
DOI: 10.1039/c2cc30394a
New quadruply bonded dimolybdenum complexes of the terphenyl
ligand ArXyl (ArXyl = C6H3-2,6-(C6H3-2,6-Me2)2) have been
[Mo2(O2CR)4] (R = Me, 1a; H, 1b) as metal precursors, the
mono- and bis-terphenyl derivatives [Mo2(ArXyl )(O2CMe)3], 2,
2
2
2
and [Mo2(ArXyl )2(O2CH)2], 3, respectively, can be isolated
2
prepared and structurally characterized. The steric hindrance
exerted by the ArXyl groups causes the Mo atoms to feature
2
and characterized. Somewhat surprisingly, information on
well-characterized dimeric alkyl or aryl Mo(II) compounds is
scarce.1,11 Moreover, the steric pressure exerted by the bulky
unsaturated four-coordinate structures and a formal fourteen-
electron count.
ArXyl group allows the observation in 2 and 3 of very unusual
2
four-coordinate, 14-electron structures derived from a square
pyramidal geometry that has an empty basal site. These
compounds could be useful precursors for so far unknown
low-coordinate second row diorganometal(II) species.
The recognition of the first metal–metal quadruple bond in the
[Re2Cl8]2ꢀ anion in 1964 marked the beginning of a new era of
chemistry that experienced a spectacular growth in the following
decades with the synthesis and characterization of numerous
complexes containing multiple bonds between transition metal
atoms.1 Despite the profound knowledge acquired in the time
that has elapsed, this fascinating field of chemistry is still
providing new, surprising, and highly interesting results.1,2 In this
sense, the use of sterically encumbered ligands to stabilize low-
coordinated metal complexes has allowed the isolation of species
of unusual structural properties, including into this category the
first complexes containing a quintuple metal–metal bond.3–8
Quadruply bonded dimolybdenum compounds have been
extensively studied and consequently a plethora of structural
and spectroscopic data are available.1,2 The vast majority of
these complexes exhibit a paddlewheel structure in which
the coordination number for the molybdenum centers is five
(considering the metal–metal bond). Recently Tsai and co-workers
have isolated the first three-coordinate, quadruply bonded
Mo2 dimers using bulky amido ligands.9
Reactions of [Mo2(O2CR)4] compounds with various alkylating
reagents to give quadruply bonded dimolybdenum alkyl complexes
stabilized by phosphine ligands were reported years ago by
Wilkinson, Andersen, and their co-workers.11 In our hands,
use of the bulky terphenyl ArXyl permits incorporation of one
2
or twoꢀArXyl units, depending on the nature of the carboxylate,
2
RCO2 group. Thus, treatment of the acetate dimer 1a with
1 equivalent of the lithium terphenyl LiArXyl occurred with
2
substitution of one carboxylate ligand by the ArXyl group and
2
formation of the mono(terphenyl) derivative 2 (Scheme 1).
The new compound was isolated as a moisture and
oxygen sensitive, deep-red crystalline solid, in over 65% yield.
Spectroscopic properties (see ESIw) are in agreement with the
proposed formulation. The two flanking 2,6-Me2C6H3 aryl units
of the ArXyl ligand in 2 are inequivalent at room temperature
2
and their methyl groups give well-defined 1H resonances at
2.18 and 2.35 ppm (each integrating for 6H). NOESY NMR
measurements reveal that they undergo exchange and indeed,
heating toluene-d8 solutions of 2 causes broadening of these signals
until they merge into a single resonance at temperatures over 80 1C.
Bulky terphenyl ligands have been broadly used to suppress
possible side reactions in low-coordinated complexes, and to
provide enhanced stability in a wide range of metal–metal
bonded compounds.10 We now report that choosing compounds
a
´
Instituto de Investigaciones Quımicas (IIQ), Departamento de
Quımica Inorganica, Universidad de Sevilla-Consejo Superior de
´
´
´
´
Investigaciones Cientıficas, Avenida Americo Vespucio 49,
41092 Sevilla, Spain. E-mail: guzman@us.es; Fax: +34-954460565
b Department of Chemistry, University of California,
One Shields Avenue, Davis, California 95616, USA.
E-mail: pppower@ucdavis.edu
w Electronic supplementary information (ESI) available: Experimental
procedures and characterization data for all new compounds described
herein along with CIF files for 2 and 3. CCDC 862529 and 862530. For
ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c2cc30394a
Scheme 1 Synthesis of the mono(terphenyl) derivative, 2.
c
3954 Chem. Commun., 2012, 48, 3954–3956
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Scheme 2 Synthesis of the bis(terphenyl) derivative, 3.
leaving group than acetate (corresponding pKa values for the
conjugated acids are 3.75, R = H and 4.75, R = Me). In
addition, formate is less sterically demanding than acetate.
As shown in Scheme 2, reaction of 1b with 2 equivalents of
LiArXyl gave the desired [Mo2(ArXyl )2(O2CH)2] dimer 3,
which was isolated as a very air sensitive red crystalline material.
1H and 13C{1H} NMR spectroscopic studies, performed in
C6D6, confirmed the incorporation of two terphenyl ligands
and formation of dimeric molecules with C2h symmetry. Thus
a deshielded singlet was recorded at 8.97 ppm (relative intensity 2H)
for the formate protons, along with two other singlets with
d 1.94 and 2.17, each integrating for 12H, attributable to the
2
2
Fig. 1 Molecular structure of 2 (30% probability ellipsoids). Selected
bond lengths (A) and angles (1): Mo(1)–Mo(2), 2.086(1); Mo(1)–C(7),
2.192(1); Mo(1)–O(1), 2.101(1); Mo(1)–O(2), 2.099(1); Mo(1)–O(3),
2.088(1); Mo(2)–O(4), 2.161(1); Mo(1)–O(5), 2.087 (1); Mo(2)–O(6),
2.117(1); C(7)–Mo(2)–Mo(1) 103.22(3).
This dynamic behavior can be associated with hindered rotation
of the terphenyl ligand around the Mo–C bond.
Me groups of the ArXyl ligands.
2
X-Ray studies (Fig. 1 and ESIw), demonstrated the existence in
compound 2 of a central Mo24+ unit with different coordination
environments for the two Mo atoms. Mo(2) is bonded to three
carboxylate oxygen atoms (average Mo–O = 2.13 A), the
terphenyl carbon, C(7), and the other molybdenum atom,
Mo(1), in a distorted square-pyramidal geometry. The Mo(2)
atom is ca. 0.17 A out of the mean plane of the surrounding
O and C donor atoms, whereas Mo(1) is at the vertex of the
pyramid, forming a metal–metal bond with a length of 2.086(1) A,
which can be categorized as a typical quadruple bond.12 The
Mo(2)–C(7) distance to the terphenyl ligand of 2.192(1) A is
only ca. 0.06 A longer than corresponding Cr–C distances in the
quintuply bonded dichromium compounds Cr2Ar02 (ca. 2.13 A).3
Molybdenum atom Mo(1) features instead an unsaturated
coordination environment, which, once more, may be viewed
as distorted square-pyramidal, but with one of the basal sites
unoccupied. Apart from the other metal atom, Mo(1) is only
bonded to three carboxylate oxygen atoms, with an average
Mo–O distance of 2.09 A. It is tempting to view one of the
flanking aryl rings as occupying the fifth coordination site,
similar to the weak secondary interactions observed in Power’s
Cr2Ar02 complexes.3,6c,6g But the shortest Mo–C distance to
this ring (Mo(1)–C(26) in Fig. 1) is of 2.571(1) A, too long to be
considered as representing significantly a sharing of electron
density. This distance is in fact about 0.38 A longer than the
Mo(2)–C(7) sigma bond, whereas in Cr2Ar02 species the difference
between the sigma bond and the weak secondary interaction is
ca. 0.16 A. Thus, the role of this flanking aryl ring seems to be
protective, rather than coordinating. It shields the unsaturated
molybdenum atom against interactions with the surroundings,
being forced into the position it adopts by the nature of the
terphenyl ligand and its characteristic spatial extent.
This compound was also characterized by X-ray crystallo-
graphy (Fig. 2). Each metal atom is bonded to the other, with
a separation of ca. 2.09 A, and is surrounded by two oxygen
atoms from the carboxylate ligands, with an average Mo–O
distance of 2.11 A, and by the carbon atom of the central aryl
ring of the terphenyl ligand. In compound 3, the Mo(1)–C(1)
bond length of 2.187(3) A is identical, within experimental
error, to the corresponding distance in the mono(terphenyl)
species 2. However, for each molybdenum atom the shortest
contact with the flanking aryl ring (Mo(10)–C(15) in Fig. 2),
i.e. the supposed secondary interaction, at 2.78 A, is ca. 0.21 A
longer than in compound 2 and almost 0.59 A longer than the
Mo–Caryl sigma bonds within the molecules of 3. This supports
the notion that there is very little (if any) covalent interaction
between these units. Therefore, in complex 3 each Mo center
exhibits an unsaturated structure, with a formal 14-electron
count. The coordination geometry approaches a distorted
square-pyramidal geometry, in which the basal site trans to
the aryl carbon is empty, as a consequence of the occupancy of
Somewhat surprisingly, treatment of 1a or 2 with an excess
of LiArXyl did not lead to replacement of the acetate trans to
2
Fig. 2 Molecular structure of 3. Selected bond lengths (A) and angles (1):
Mo(1)–Mo(1)0, 2.095(1); Mo(1)–C(1), 2.187(3); Mo(1)–O(1), 2.106(3);
Mo(1)–O(2), 2.110(3); C(1)–Mo(1)–Mo(10) 99.24(9); C(10)–Mo(10)–C(15)
170.8(1).
the terphenyl, even after prolonged refluxing of the reaction
mixture in THF. In view of this result, the formate
[Mo2(O2CH)4], 1b, was used. The formate ligand is better a
c
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Chem. Commun., 2012, 48, 3954–3956 3955

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this region of the space by a flanking aryl ring of the terphenyl
ligand bound to the other molybdenum atom.
Notes and references
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4 (a) Y.-C. Tsai, C.-W. Hsu, J.-S. K. Yu, G.-H. Lee, Y. Wang and
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To gain a deeper insight into the nature of the bonding in 3
computational studies were performed. Density Functional
Theory (DFT) calculations at the M0613 level afforded a gas
phase geometry for 3 in very good agreement with the X-ray
data. The shortest calculated distance between the Mo atoms
and the flanking aryls is 2.79 A but, interestingly, when the ArXyl
2
terphenyl ligand is replaced by 2-biphenyl (3calc) (see ESIw),
the shortest Mo–phenyl contact becomes 2.57 A, which is
almost identical to the X-ray data for 2. This suggests that
steric interactions in 3 play a role in elongating the Mo–Aryl
contact. The nature of the secondary interaction was addressed
by NBO analysis14 and analysis of the calculated electron
density of 3 within the Quantum Theory of Atoms in Molecules
formalism.15,16 Bond Critical Points (BCPs) were located
connecting the Mo atoms and the ipso carbons of the flanking
aryls. The charge density values at these BCPs [r(rc)] are low
(0.025 au compared to 0.103 au for the Mo–C sigma bonds
BCPs). The positive values of the laplacian of the electron
2
density at these critical points [r r(rc)] indicate closed shell
(ionic) interactions.17–19 An analogous analysis for 3calc gives
almost identical results for the two BCPs that connect each
Mo with one of the ortho carbons of the corresponding
flanking phenyls. The Wiberg Bond Orders (WBOs) for the
Mo–Aryl interactions are below 0.3 for 3 (the largest contri-
bution, 0.08, corresponding to the Mo–Cipso interactions),
while for 3calc the corresponding WBOs increase to 0.31 (with
the largest contribution corresponding now to the Mo–Cortho
interactions, 0.15). These results are consistent with weak,
primarily ionic in nature, interactions between the Mo atoms
of 3 (and 2) and the flanking aryls of the terphenyl ligands.
In summary, through electronic and steric modification of the
carboxylate platform of dimolybdenum tetracarboxylate dimers,
[Mo2(O2CR)4], and with the use of bulky terphenyl ligands, we
have developed a useful synthetic strategy that allows access to
quadruply bonded dimolybdenum complexes of terphenyl ligands.
A
bis(terphenyl) bis(formate) compound in which each
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Inorg. Chem., 2002, 41, 2466.
13 Y. Zhao and D. Truhlar, Theor. Chim. Acta, 2008, 120, 215.
14 A. E. Reed, L. A. Curtis and F. Weinhold, Chem. Rev., 1988,
88, 899.
15 R. F. Bader, Atom in Molecules: A Quantum Theory, Oxford
University Press, Oxford, UK, 1995.
16 R. F. W. Bader, Chem. Rev., 1991, 91, 893.
17 A. Sierraalta, Chem. Phys. Lett., 1994, 227, 557.
18 J. Saßmannshausen, A. Track and T. A. D. S. Dias, Eur. J. Inorg.
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molybdenum atom possesses an unsaturated four-coordinate
structure and a formal fourteen-electron count has been prepared.
Financial support (FEDER contribution and Subprogramas
Juan de la Cierva and Ramon y Cajal) from the Spanish
Ministry of Science and Innovation (Projects CTQ2010-15833
and Consolider-Ingenio 2010 CSD2007-00006) and the Junta
de Andalucıa (Grant FQM-119 and Project P09-FQM-5117) is
gratefully acknowledged. M. C. thanks the Spanish Ministry
of Education for a research grant (AP-4193). M. F. and
P. P. P. thank the U. S. National Science Foundation
(CHE-0948417) for financial support. The use of computational
facilities of the Consejo Superior de Investigaciones Cientıficas
(Cluster Trueno) and the Center of Supercomputing of Galicia
(CESGA) are thankfully acknowledged.
19 W. Scherer and G. S. McGrady, Angew. Chem., Int. Ed., 2004,
43, 1782.
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3956 Chem. Commun., 2012, 48, 3954–3956
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