Terphenyl complexes of molybdenum and tungsten with quadruple metal-metal bonds and bridging carboxylate ligands

Article abstract of DOI:10.1021/ja503750a

Mono- and bis-terphenyl complexes of molybdenum and tungsten with general composition M₂(Ar′)(O₂CR)₃ and M ₂(Ar′)₂(O₂CR)₂, respectively (Ar′ = terphenyl ligand), that contain carboxylate groups bridging the quadruply bonded metal atoms, have been prepared and structurally characterized. The new compounds stem from the reactions of the dimetal tetracarboxylates, M₂(O₂CR)₄ (M = Mo, R = H, Me, CF₃; M = W, R = CF₃) with the lithium salts of the appropriate terphenyl groups (Ar′ = Ar Xyl₂, ArMes₂, Ar Dipp₂, and Ar Trip₂). Substitution of one bidentate carboxylate by a monodentate terphenyl forms a M-C σ bond and creates a coordination unsaturation at the other metal atom. Hence in M₂(Ar′) ₂(O₂CR)₂ complexes the two metal atoms have formally a low coordination number and an also low electron count. However, the unsaturation seems to be compensated by a weak M-C_arene bonding interaction that implicates one of the aryl substituents of the terphenyl central aryl ring, as revealed by X-ray studies performed with some of these complexes and by theoretical calculations.

Full text of DOI:10.1021/ja503750a

Subscriber access provided by University at Buffalo Libraries
Article
Terphenyl Complexes of Molybdenum and Tungsten with
Quadruple Metal-Metal Bonds and Bridging Carboxylate Ligands
Mario Carrasco, Irene Mendoza, Michelle Faust, Joaquín López-Serrano, Riccardo Peloso,
Amor Rodriguez, Eleuterio Álvarez, Celia Maya, Philip P. Power, and Ernesto Carmona
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja503750a • Publication Date (Web): 29 May 2014
Downloaded from http://pubs.acs.org on June 1, 2014
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a free service to the research community to expedite the
dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts
appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been
fully peer reviewed, but should not be considered the official version of record. They are accessible to all
readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered
to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published
in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just
Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor
changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors
or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Journal of the American Chemical Society is published by the American Chemical
Society. 1155 Sixteenth Street N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 11
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
Terphenyl Complexes of Molybdenum and Tungsten with Quadru-
ple Metal-Metal Bonds and Bridging Carboxylate Ligands
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Mario Carrasco,† Irene Mendoza,† Michelle Faust,‡ Joaquín López-Serrano,† Riccardo Peloso,†
Amor Rodríguez,† Eleuterio Álvarez,† Celia Maya,† Philip. P. Power*,‡ and Ernesto Carmona*,†
†Instituto de Investigaciones Químicas (IIQ), Departamento de Química Inorgánica, Universidad de Sevilla-Consejo Superior de
Investigaciones Científicas, Avenida Américo Vespucio 49, 41092 Sevilla, Spain.
‡Department of Chemistry, University of California, Davis, One Shields Avenue, Davis, California, 95616, United States
KEYWORDS: molybdenum, tungsten, quadruple bond, carboxylate, terphenyl ligand.
ABSTRACT: Mono- and bis-terphenyl complexes of molybdenum and
tungsten
with
general
composition
M2(Ar’)(O2CR)3
and
M2(Ar’)2(O2CR)2, respectively (Ar’ = terphenyl ligand), that contain
carboxylate groups bridging the quadruply bonded metal atoms, have
been prepared and structurally characterized. The new compounds
stem from the reactions of the dimetal tetracarboxylates, M2(O2CR)4
(M = Mo, R = H, Me, CF3; M = W, R = CF3) with the lithium salts of the
appropriate terphenyl groups (Ar’ = ArXyl2, ArMes2, ArDipp and ArTrip2).
2
Substitution of one bidentate carboxylate by a monodentate terphenyl
forms a M−C σ bond and creates a coordination unsaturation at the
other metal atom. Hence in M2(Ar’)2(O2CR)2 complexes the two metal
atoms have formally a low coordination number and an also low electron count. However, the unsaturation seems to be
compensated by a weak M−Carene bonding interaction that implicates one of the aryl substituents of the terphenyl central
aryl ring, as revealed by X-ray studies performed with some of these complexes and by theoretical calculations.
■ INTRODUCTION
C6H3-2,6-(C6H3-2,6-iPr2)2, allowed the observation of a
formally two coordinated geometry, complemented for
each chromium atom by a weaker Cr−Carene secondary
interaction.10,14d This involves the ipso carbon of a flanking
aryl ring belonging to the terphenyl ligand bonded to the
other metal atom. With a related approach, Tsai and co-
workers succeeded in the isolation of a three coordinated
Fifty years after the recognition by Cotton and co-workers
of the first metal-metal quadruple bond,1 the study of
compounds with multiple metal-metal bonds has become
a mature field of research. However, in many cases intri-
guing structural and electronic properties of these com-
plexes, their notable applications in inorganic, bio-
inorganic and organometallic chemistry, along with some
fascinating aspects of their bonding characteristics, con-
tinue to attract the interest of experimentalists and theo-
reticians alike.2−9 Besides, renewed impetus to the field
was provided recently by the remarkable discovery of
their quintuply bonded analogues made possible by the
use of bulky ligands, and by other interesting develop-
ments.10−14
Mo Mo
complex using a sterically encumbered silyldi-
amido ligand.15
The existence of a multiply M−M bonded Ar’CrCrAr’
family of complexes prompted us to investigate related
Mo2 and W2 complexes stabilized also by terphenyl
groups. We envisaged that the use as precursors of some
members of the well-known series of M2(O2CR)4 com-
plexes with quadruple metal-metal bonds,2 could lead to
low-coordinate terphenyl derivatives with interesting
structural properties, as a result of the replacement of
bidentate carboxylate by monodentate terphenyl ligands.
We also surmised that some of the new complexes might
prove useful as precursors for the (Mo2)2+ and (W2)2+ ana-
logues of the above dichromium molecules. While the
latter objective has proved elusive and the targeted (M2)2+
Mo
Mo complexes feature a five-
The majority of
coordinate (considering the metal-metal bond) paddle-
wheel structure. Nevertheless, in the dichromium com-
plex Ar’CrCrAr’ reported by Power and co-workers as the
first stable molecule with quintuple metal-metal bond-
ing10 (Ar’ is utilized in this paper as general abbreviation
for a terphenyl ligand) the bulkiness of the aryl group,
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 2 of 11
complexes have not been obtained, several mono- and expected Mo2(Ar’)2(O2CMe)2 derivatives. Nevertheless,
1
2
3
4
5
6
7
8
bis-terphenyl complexes of composition M2(Ar’)(O2CR)3
and M2(Ar’)2(O2CR)2 have been isolated and structurally
authenticated. Binuclear multiply bonded alkyl or aryl
(Mo2)4+ complexes are scarce.2−4,16 Moreover, the new
compounds reported present unusual four-coordinate
geometries with a formal 14-electron count, and are useful
precursors for other low-coordinate second and third row
diorganometal(II) species that will be reported in due
course. Part of this work (the molybdenum complexes 1a
and 2a) has been communicated.17
neither the reaction of Mo2(O2CMe)4 with an excess of
LiAr’ (ca. 2.2 equiv) nor the interaction of the isolated
monoterphenyl complexes 1 with another equivalent of
the corresponding LiAr’ gave the desired complexes, even
after prolonged refluxing in THF. Most probably, this is
due to the high steric protection provided by the incorpo-
rated Ar’ and the remaining acetate ligands.
9
Scheme 1. Synthesis of mono-terphenyl complexes 1a-
1d.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
■ RESULTS AND DISCUSSION
O
O
Bulky terphenyl ligands have been used widely to sta-
bilize new types of metal-metal bonds between main
group elements of groups 1, 2 and from group 12 to 15.18−21
Related complexes of some 3d elements have been studied
too,22 and include, among others, the already cited
Ar’CrCrAr’ molecules.10 Arguably, use of these and other
very bulky ligands11−13,23 aims to provide kinetic stabiliza-
tion to the low coordination number complexes that they
generate.
LiAr'
O
O
O
Mo2(O2CCH3)4
Mo
Mo
R'
O
R'
R'
R'
R'
R'
Dipp
Trip
(Ar' = ArXyl2, 1a; ArMes , 1b; Ar
, 1c; Ar 2, 1d)
2
2
As mentioned briefly, this paper discusses the synthe-
sis and structural characterization of molybdenum and
tungsten complexes with quadruple M−M bonds stabi-
lized by coordination to bulky terphenyl ligands and car-
boxylate groups. As shown in Figure 1, three Mo2(O2CR)4
complexes (R = H, Me, CF3), as well as the ditungsten
compound W2(O2CCF3)4, were used as metal precursors
and were reacted with the LiAr’ reagents that are also
presented in Figure 1.
Compounds 1, show good solubility properties in or-
ganic solvents of low polarity like benzene or toluene, are
highly sensitive to moisture and oxygen and must be
carefully handled under an inert atmosphere of Ar or N2.
Their UV-Vis spectra (Figure 2) show an intense absorp-
tion band centred at ca. 530 nm (εmax in the range
1080−1260 M-1cm-1) together with a somewhat weaker one
in the proximity of 440 nm (εmax ~ 540 M-1cm-1). These
absorptions can be attributed to δ2→δδ* transitions. DFT
(M06, 6-31g(d,p)/SDD) and time-dependent DFT (TD-
DFT)24 calculations identify the lower energy excited state
of 1a as a singlet related to the excitation of an electron
from the HOMO (δ) to the LUMO (δ∗). The calculated
(gas phase) excitation energy for this transition is 1.97 eV
(or 629.71 nm), ca. 0.37 eV lower than the experimental
values. According to the calculations, the weaker band
centred around 440 nm can be assigned to d-d transi-
tions, but with some metal to terphenyl contribution (see
SI).
R
R
O
M
O
O
M
O
O
O
R'
R'
R'
R'
R'
R'
O
R'
R'
R'
R'
O
R
R
M = Mo, R = H, Me, CF3
M = W, R = CF3
R' = Me, ArMes
2
R' = Me, ArXyl
2
R' = iPr, ArTrip
R' = iPr, ArDipp
2
2
Figure 1. M2(O2CR)4 complexes and terphenyl ligands em-
ployed in this work.
1
The new compounds are diamagnetic. In the H NMR
spectrum they exhibit similar features that are consistent
with the Cs symmetry of their molecules. Thus, two sig-
nals assigned to the methyl protons of the acetate groups
with relative intensity 6H : 3H can be recorded with
chemical shifts close to 2.5 and 2.0 ppm, respectively. The
flanking aromatic rings of the terphenyl ligand give rise to
two sets of signals when the NMR experiment is per-
formed at room temperature, thereby indicating that
neither rotation of the central aryl ring around the Mo−C
bond (Figure 3 top) nor interchange of Ar’ between the
two Mo atoms through a terphenyl-bridged structure
(Figure 3) occur at the NMR time scale. Nonetheless,
Terphenyl-acetate and -formate dimolybdenum
complexes. Reacting Mo2(O2CMe)4 with 1 equiv of LiArX-
yl
2
in THF (−40 to 20 ºC) yielded a deep red-burgundy
solution from which red crystals of the mono-terphenyl
complex Mo2(ArXyl2)(O2CMe)3, 1a, were isolated. As shown
in Scheme 1, extension of this reactivity to other LiAr’
reagents was straightforward and afforded analogous
complexes 1b (ArMes2), 1c (ArDipp2) and 1d (ArTrip2), as dark
red crystalline solids too. A number of attempts were
made to replace the acetate group of complexes 1 trans to
the terphenyl ligand by a second terphenyl to form the
2
ACS Paragon Plus Environment

Page 3 of 11
Journal of the American Chemical Society
upon heating at higher temperatures the solutions of 1a in
toluene-d8, the two Ar’ methyl resonances (2.35 and 2.18
1
2
3
4
5
6
7
8
Xyl
Xyl'
Xyl
Xyl
Xyl'
Xyl'
ppm at 25 ºC) coalesce (75 ºC) and convert at 100 ºC into a
singlet centred at ca. 2.20 ppm. Room temperature
NOESY experiments provide additional evidence for this
dynamic behaviour, which in accordance with DFT calcu-
lations, can be associated with a 1,2-Ar’ shift from one
molybdenum atom to the other. The calculations yield an
energy barrier for 1,2-Ar’ shift of 19.7 kcal·mol-1, whereas a
Relaxed Potential Energy Scan of the rotation of the ter-
phenyl ligand (ArXyl2) about the Mo−C bond gives a barri-
er 10 kcal·mol-1 higher.
Mo
Mo
Mo
Mo
Mo
Mo
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1
Figure 3. Possible fluxionality (top), variable temperature H
NMR spectra for compound 1a (400 MHz, C7D8), and calcu-
lated geometry of the transition state for 1,2-Ar’ shift.
Figure 2. UV-Vis spectra of mono- and bis-terphenyl com-
plexes 1a, 2a in Et2O and 4a, 5a in THF solution (ca. 10-4 M).
At variance with the above results, the comparable re-
actions of the formate dimer Mo2(O2CH)4 with 2 equiv of
LiArXyl yielded a bis(terphenyl) bis(formate) complex
2
Mo2(ArXyl2)2(O2CH)2, 2a, with the expected trans distribu-
tion of the terphenyl ligands. No attempts were made to
obtain and isolate the mono-terphenyl analogues of the
tris-acetates 1. Compound 2a features a deep-red colour
too (absorption band at 550 nm, with εmax 3100 M-1cm-1)
and is also very reactive toward air, both in solution and
1
in the solid state. H and 13C{1H} MNR data17 are in full
Figure 4. The solid-state molecular structure of
Mo2(ArTrip2)(O2CMe)3, 1d, with thermal ellipsoids set at 30%
probability. All hydrogen atoms and the Mo−Carene secondary
interaction have been omitted for clarity. Selected bond
lengths (Å): Mo(1)−Mo(2), 2.086(1); Mo(1)−C(1), 2.211(4);
Mo(1)−O(1), 2.101(3); Mo(1)−O(3), 2.119(4); Mo(1)−O(5),
2.150(3); Mo(2)···C(8), 2.57(1).
agreement with the proposed structure.
Compounds 1a, 1b, 1d, and 2a were characterized by
X-ray crystallography and their molecular structures are
depicted in Figures 4 and 5 (complexes 1d and 2a) and in
Figures S1 and S2 (Supporting Information, compounds 1a
and 1b). In the four compounds studied there is a (Mo2)4+
core characterized by a Mo−Mo bond length of ca. 2.09 Å.
This is a median value of the lower range 2.06−2.13 Å
compiled for quadruply bonded Mo−Mo compounds with
four bidentate ligands.25 Therefore, replacing one or two
bridging carboxylate ligands by monodentate terphenyl
groups has no effect in the length of the quadruple
Mo−Mo bond. The Mo−C distances to the terphenyl lig-
and(s) are also identical or nearly identical in the four
compounds analysed (2.19−2.21 Å).
An interesting structural peculiarity of these complex-
es is the coordinative and electronic unsaturation of one
of the Mo atoms in complexes 1 and of both molybdenum
atoms in 2a, partly offset by the existence of a Mo−Carene
secondary interaction. In complexes 1, Mo1 exhibits coor-
dination number five and a sixteen electron count, similar
Mo
to the vast majority of complexes with a
Mo bond.2
Instead and leaving aside for the time being the Mo−Carene
secondary interaction, Mo2 in compounds 1 and the two
3
ACS Paragon Plus Environment

Journal of the American Chemical Society
metal atoms of 2a are four-coordinate and have an effec-
Page 4 of 11
the Mo−Cipso critical points are indicative of weak, closed
shell or ionic interactions.28 To complete the analysis of
the secondary interaction, we are presently carrying out a
detailed experimental and computational investigation of
a series of newly prepared terphenyl complexes of the
1
2
3
4
5
6
7
8
tive valence electron count of fourteen. For the two situa-
tions, the basic coordination polyhedron surrounding
each Mo atom is a square pyramid, with the other metal
atom at the apex of the pyramid. In complexes 1 Mo1 is
bound to the terphenyl carbon atom C1 and to the oxygen
atoms O1, O3 and O5, each from a bridging acetate group,
and it lays slightly above the mean basal plane (ca. 0.17
Å). In turn, Mo2 in complexes of type 1 has only three
oxygen atoms within bonding distance, while the two
metal atoms of 2a are bound to a terphenyl carbon atom
and two oxygen atoms (one from each formate). Hence,
one of the basal coordination sites of these polyhedra is
empty. However, hovering over the unoccupied position
there is a flanking aryl ring of the terphenyl ligand, which
suggests that its ipso (or one of the ortho) carbon atom is
participating in a secondary interaction alike that found
by Power and co-workers in the Ar’CrCrAr’ molecules.10 In
complexes 1 the shortest Mo−Carene separation is of ca.
Mo Mo
core, in which the nature of the ligand in trans
with respect to the secondary interaction is varied in a
systematic manner.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
2.58 Å,
a value that increases to 2.78 Å in the
bis(terphenyl) complex 2a, due possibly to the high trans
influence of the aryl ring and also to steric hindrance.
While these metrics are too long to support significant
electron communication between the implicated Carene
carbon atom and the unsaturated molybdenum atom,
they nevertheless underpin the existence of a weakly
bonding secondary interaction that balances the metal
atom unsaturation. This was addressed by means of a
computational analysis.17 The geometry of 2a was opti-
mized in the gas phase by DFT methods. The resulting
structural parameters are in good agreement with the X-
ray data, the shortest calculated Mo−Carene being 2.79 Å.
NBO analysis26 of 2a is consistent with a quadruple
Mo−Mo bond (comprised of one localized bonding orbital
of σ symmetry, two of π and one of δ symmetry). Wiberg
Bond Orders (WBOs) indicate that the interactions of the
Mo atoms with the flanking aryls are weak. The overall
WBO for each set of Mo−ArXyl interactions amounts to
0.3, with the largest contribution corresponding to the
Mo−Cipso interactions (WBO = 0.08). Interestingly when a
model is used in which the ArXyl fragments of the ter-
phenyl ligands are replaced by less bulky ArPh, the short-
est calculated Mo−Carene separation (now Mo−Cortho) is 2.57
Å, which is remarkably similar to that measured by X-ray
in complexes 1. This result agrees with steric hindrance
having a role in the elongation of the Mo−Aryl separa-
tions in the bis(terphenyl) complexes. Nevertheless the
overall WBOs for the Mo−ArPh interactions in the model
remain unchanged (0.31), although the largest contribu-
tion, corresponding to the Mo−Cortho, increases to 0.15. In
addition the electron density of 2a has been analyzed
within Bader’s theory of Atoms in Molecules.27 While
Bond Critical Points (BCPs) have been found that connect
the Mo and the Cipso of the flanking rings, the charge den-
sities [ρ(rc)] at these points are low (0.025 au vs 0.103 au at
the BCPs of the Mo−C sigma bonds). Also, the positive
values of the Laplacian of the electron density [∇2ρ(rc)] at
Figure 5. The solid-state molecular structure of
Mo2(ArXyl2)2(O2CH)2, 2a, with thermal ellipsoids set at 30%
probability. All hydrogen atoms and the Mo−Carene secondary
interactions have been omitted for clarity. Selected bond
lengths (Å): Mo(1)−Mo(1’), 2.095(1); Mo(1)−C(1), 2.187(3);
Mo(1)−O(1), 2.106(3); Mo(1)−O(2), 2.110(3); Mo(1’)···C(15),
2.78(1).
Trifluoroacetate complexes. The stronger acidity of
HO2CCF3 relative to HO2CMe (difference in pKa of ca. 5)
makes trifluoroacetate much better a leaving group than
acetate. This observation and the isolation of the
bis(terphenyl) complex 2a, prompted us to utilize the
well-known Mo2(O2CCF3)4 compound as starting material
and to extend these investigations to the tungsten ana-
logue W2(O2CCF3)4. For the latter, its reactivity toward
LiArXyl was tested while for molybdenum two terphenyl
2
ligands were essayed, namely ArXyl2 and ArDipp
.
2
In agreement with expectations, the reaction of
Mo2(O2CCF3)4 and LiAr’ proceeded in a step-wise manner
to furnish first mono(terphenyl) complexes 3 and subse-
quently their bis(terphenyl) counterparts 4 (Scheme 2).
The reactions were begun at −40 ºC in Et2O as the solvent
and were then permitted to reach room temperature over
a period of a few hours. They could be followed readily by
19F NMR spectroscopy, as complexes 3 exhibit two reso-
nances with chemical shift −72.5 and −73.3 ppm (2:1 ratio;
data for 3a) whereas solutions of 4a give rise to a singlet
at −70.8 ppm. Interestingly, the perfluorocarboxylate
ligands impart enhanced solubility in non-polar solvents
including hexanes. Compounds 3 and 4 were isolated as
pink or red crystalline materials.
4
ACS Paragon Plus Environment

Page 5 of 11
Journal of the American Chemical Society
Scheme 2. Synthesis of mono- and bis-terphenyl
complexes 3 and 4, respectively, with bridging tri-
Similarly to Mo2(O2CCF3)4, W2(O2CCF3)4 reacted with
equiv of LiArXyl to form the green complex
2
1
2
3
4
5
6
7
8
2
W2(ArXyl2)2(O2CCF3)2, 5a, whose solutions are character-
O O
fluoroacetate groups (
= O2CCF3).
ized by three bands in the visible region of the spectrum
1
(Figure 2), with maxima at 430, 485 and 600 nm. Its H,
Mo2(O2CCF3)4
13C{1H} and 19F NMR spectra are similar to those of 4a and
LiAr'
2 LiAr'
need no further discussion.
The solid-state molecular structures of compounds 3a,
3c, 4a, 4c and 5a have been determined by X-ray crystal-
lography and are presented in Figures 6 and 7. The
tris(trifluoroacetate) complexes 3 (Figure 6) are structur-
ally similar to the acetate analogues 1, and are character-
R'
R'
9
O
O
R'
O
O
O
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
R'
LiAr'
O
O
Mo
Mo
Mo
Mo
O
R'
O
R'
Mo
Mo bond of length ca. 2.10 Å and by a
ized by a
O
R'
R'
Mo−Caryl distance of about 2.16 Å. Also in analogy with
compounds 1, the aryl bonded molybdenum atom Mo1
has coordination number five while Mo2 presents a four-
R'
R'
R'
R'
coordinate structure plus
a long, weakly bonding,
R' = Me, 3a; iPr, 3c
R' = Me, 4a; iPr, 4c
Mo−Carene interaction of 2.56 Å (to C ortho).
In the same manner, the M2(Ar’)2(O2CCF3)2, complexes
4 and 5 have structures with coordination properties akin
to those of Mo2(ArXyl2)2(O2CH)2, 2a. For example, in com-
1
13
The H and C{1H} NMR spectra of complexes 3 and 4
1
are also very informative. For instance, the H NMR spec-
pounds of type 4 the Mo−Mo bond has a length of ca. 2.11
Å, the Mo−Caryl sigma bonds exhibit distances close to 2.19
Å and the Mo−Carene secondary interactions are in the
range 2.76−2.80 Å and may therefore be viewed as weakly
bonding. The separation of 2.21 Å between the tungsten
atoms of complex 5a, while longer than the Mo−Mo dis-
tance in 4, is comparable with values reported for other
trifluoroacetate complexes of the (W2)4+ core.2 However,
both the W−Caryl and W−Carene separation at 2.15 and 2.67
Å, respectively, are slightly shorter than in the molyb-
denum analogue 4a (2.18 and 2.76 Å), despite the some-
what larger covalent radius of tungsten29 relative to mo-
trum of 3c that possesses flanking 2,6-iPr2C6H3 substitu-
ents, features two CHMe2 septets and four doublet reso-
nances, the latter associated with the CHMe2 protons.
This is in agreement with the foreseen non-equivalency of
the two lateral aryl rings and with the proposed formula-
tion with Cs symmetry (Scheme 2). Of the
bis(terphenyl)bis(trifluoroacetate) derivatives 4, complex
4c of the ArDipp ligand was not obtained with microana-
2
lytical purity except for a few crystals used for X-ray stud-
ies (see below). This was due to its high reactivity toward
oxygen and moisture that causes its partial decomposition
under our experimental conditions.
Figure 6. The solid-state molecular structures of Mo2(ArXyl2)(O2CCF3)3, 3a (left) and Mo2(ArDipp2)(O2CCF3)3, 3c (right), with
thermal ellipsoids set at 30% probability. All hydrogen atoms and the Mo−Carene secondary interactions have been omitted for
clarity. Selected bond lengths in Å. 3a: Mo(1)−Mo(2), 2.103(1); Mo(1)−C(1), 2.164(3); Mo(1)−O(1), 2.112(2); Mo(1)−O(3), 2.200(2);
Mo(1)−O(5), 2.126(2); Mo(2)···C(12), 2.55(1). 3c: Mo(1)−Mo(2), 2.100(1); Mo(1)−C(1), 2.162(4); Mo(1)−O(1), 2.118(2); Mo(1)−O(3),
2.182(3); Mo(1)−O(5), 2.119(3); Mo(2)···C(24), 2.56(1).
5
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 6 of 11
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 7. The solid-state molecular structures of Mo2(ArXyl2)2(O2CCF3)2, 4a (left), Mo2(ArDipp2)2(O2CCF3)2, 4c (centre) and
W2(ArXyl2)2(O2CCF3)2, 5a (right) with thermal ellipsoids set at 30% probability. All hydrogen atoms and the M−Carene secondary
interactions have been omitted for clarity. Selected bond lengths in Å. 4a: Mo(1)−Mo(2), 2.107(3); Mo(1)−C(1), 2.175(10);
Mo(1)−O(1), 2.123(8); Mo(1)−O(3), 2.124(8); Mo(2)−O(2), 2.115(8); Mo(2)−O(4), 2.110(8); Mo(2)···C(12), 2.76(1). 4c: Mo(1)−Mo(1A),
2.112(1); Mo(1)−C(1), 2.200(3); Mo(1)−O(1), 2.119(2); Mo(1)−O(2A), 2.114(2); Mo(1A)···C(20), 2.80(1). 5a: W(1)−W(2), 2.213(1);
W(1)−C(1), 2.157(5); W(1)−O(1), 2.093(4); W(1)−O(3), 2.092(4); W(2)−O(2), 2.092(4); W(1)−O(4), 2.078(4); W(2)···C(15), 2.67(1).
lybdenum (1.62 and 1.54 Å, respectively).
plexes M2(O2CR)4 (M = Mo, R = H, Me, CF3; M = W, R =
CF3),30−33 terphenyl iodides Ar’I (Ar’ = ArXyl2, ArMes2, ArDipp
,
2
As mentioned in the introductory paragraphs of this
paper, one of the objectives of this work was the use of
the mixed M2(Ar’)2(O2CR)2, complexes with an (M2)+2
central unit as precursors for the synthesis of the corre-
sponding Ar’MMAr’ molecules with a quintuple metal-
metal bond. Many trials were realized to reduce the newly
prepared complexes M2(Ar’)2(O2CR)2, i.e. 2, 4 and 5, either
at room temperature or above (boiling THF), by action of
a variety of reducing agents: Na, Na-Hg, K, KC8, KH and
others. However, all of these endeavours proved fruitless,
the corresponding reactions leading either to decomposi-
tion products or to unreacted starting materials. In a
similar fashion, the reduction of the complexes
Mo2(Ar’)(O2CMe)3, 1a-1d with the above reductants in the
presence of a second equivalent of the appropriate LiAr’
were unsuccessful too. It is possible that the reluctance of
these terphenyl-carboxylate complexes to undergo reduc-
tion to Ar’MMAr’ be due to kinetic reasons (rather than
thermodynamic) associated with the high steric hin-
drance, and the consequent metal protection, exerted by
the terphenyl and carboxylate ligands of these complexes.
and ArTrip2),34 and the corresponding lithium salts, LiAr’,35
were prepared according to literature methods. All other
compounds were commercially available and were used as
received. Benzene-d6 was distilled under argon over sodi-
um/benzophenone and toluene-d8 was distilled under
argon over sodium. Both were then degassed and dried
over 4 Å molecular sieves. Solution NMR spectra were
recorded on Bruker AMX-300, DRX-400, 400R, and DRX-
500 spectrometers. The resonances of the solvents were
used as the internal standard and the chemical shifts are
reported relative to TMS. UV-Visible spectra were record-
ed on a PerkinElmer Lambda 750 spectrometer and ele-
mental analysis was carried out with a LECO TruSpec
CHN elementary analyzer. Synthetic details and charac-
terization data for complexes 1a and 2a can be found in
the SI of reference 17.
General
preparation
of
mono(terphenyl)
tris(acetate) complexes Mo2(Ar’)(O2CMe)3, (1b-d).
Solid samples of Mo2(O2CCH3)4 (1.46 g, 3.42 mmol) and
the appropriate LiAr’ (3.42 mmol) were mixed in a glove-
box and cooled to −40 ºC. Tetrahydrofuran (20 mL) was
added and the resulting suspension was stirred for 12
hours, while reaching slowly the room temperature. The
solvent was then removed under reduced pressure and
the solid residue was extracted with toluene (20 mL). The
resulting red-wine-coloured suspension was centrifuged
and the solution was transferred to a Schlenk tube, con-
centrated to ca. 10 mL, and stored in a refrigerator at −23
ºC overnight. A bright red crystalline solid separated out,
■ EXPERIMENTAL SECTION
General Procedures. All manipulations were carried
out using standard Schlenk and glove-box techniques
under an atmosphere of argon and of high purity nitro-
gen, respectively. All solvents were dried and degassed
prior to use. Toluene, n-hexane, and n-pentane were dis-
tilled over sodium. Diethylether and tetrahydrofuran were
distilled under nitrogen over sodium/benzophenone.
Quadruply bonded dimolybdenum and ditungsten com-
6
ACS Paragon Plus Environment

Page 7 of 11
Journal of the American Chemical Society
which was isolated by filtration and dried under vacuum
for 3 hours.
Visible (10-4 M in benzene): λ = 530 nm (ε = 1260 mol-1 L
cm-1). Anal. Calcd. For C42H58Mo2O6: C, 59.3; H, 6.9;
Found: C, 58.6; H, 6.2.
1
2
3
4
5
6
7
8
1
Mo2(ArMes2)(O2CMe)3, (1b). Yield: 66%. H NMR (400
MHz, C6D6, 25 ºC): δ = 1.40 (s, 3H, p-MeMes’), 1.93 (s, 3H, p-
MeMes), 2.00 (s, 3H, trans-MeOAc), 2.21 (s, 6H, o-MeMes’),
2.37 (s, 6H, o-MeMes), 2.53 (s, 6H, MeOAc), 6.36 (s, 2H, m-
General
preparation
of
mono(terphenyl)
tris(trifluoroacetate) complexes Mo2(Ar’)(O2CF3)3, (3a,
3c). Solid samples of Mo2(O2CCF3)4 (0.50 g, 0.77 mmol)
and 1 equiv of the appropriate LiAr’ were mixed in a
glovebox and cooled to −40 ºC. Diethylether (15 mL, for
3a) or pentane (15 mL, for 3c) was added and the resulting
suspension was stirred for 24 hours, while reaching slowly
the room temperature. The solvent was then removed
under reduced pressure and the solid residue was extract-
ed with hexane (15 mL). The resulting red-wine-coloured
suspension was centrifuged and the solution was trans-
ferred to a Schlenk tube, concentrated to ca. 5 mL, and
stored in a refrigerator at −23 ºC overnight. A pink crystal-
line solid separated out, which was isolated by filtration
and dried under vacuum for 3 hours.
3
Mes’), 6.66 (s, 2H, m-Mes), 6.95 (d, 1H, JHH = 7.6 Hz, m’-
C6H3), 7.12 (d, 1H, 3JHH = 7.6 Hz, m-C6H3), 7.34 (t, 1H, 3JHH
=
7.6 Hz, p-C6H3) ppm. 13C{1H} NMR (100 MHz, C6D6, 25 ºC):
δ = 20.2 (p-MeMes’), 20.9 (p-MeMes), 21.2 (o-MeMes), 22.4 (o-
MeMes’), 23.0 (trans-MeOAc), 23.8 (MeOAc), 123.6 (m-C6H3),
125.7 (m’-C6H3), 126.4 (p-C6H3), 128.1 (m-Mes), 129.8 (m-
Mes’), 135.6 (p-Mes), 135.9, 136.0 (o-Mes’ and o-Mes), 138.7
(p-Mes’), 140.3 (ipso-Mes), 141.3 (ipso-Mes’), 147.4 (o-
C6H3), 147.9 (o-C6H3), 181.5 (Mo−Car), 183.4 (O2CMe), 183.5
(trans-O2CMe) ppm. UV-Visible (10-4 M in benzene): λ =
530 nm (ε = 1080 mol-1 L cm-1); (10-4 M in diethylether): λ =
280, 445, 530 nm (ε = 17400, 700, 1400 mol-1 L cm-1, respec-
tively). Anal. Calcd. for C30H34Mo2O6: C, 52.8; H, 5.0;
Found: C, 53.1; H, 5.3.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1
Mo2(ArXyl2)(O2CF3)3, (3a). Yield: 63%. H NMR (500
1
Mo2(ArDipp2)(O2CMe)3, (1c). Yield: 50%. H RMN (500
MHz, C6D6, 25 ºC): δ = 1.99 (s, 6H, MeXyl), 2.11 (s, 6H,
MeXyl’), 5.96 (t, 1H, 3JHH = 7.6 Hz, p-Xyl), 6.24 (d, 2H, 3JHH
=
MHz, C6D6, 25 ºC): δ = 1.07, 1.12, 1.31, 1.55 (d, 6H each,
MeDipp), 1.96 (s, 3H, trans-MeOAc), 2.56 (s, 6H, MeOAc), 3.13,
3.21 (sept, 4H each, CHMe2Dipp), 6.48 (t, 1H, p-Dipp’), 6.58
(d, 2H, m-Dipp’), 7.02 (m, 3H, m-Dipp and p-Dipp), 7.10
(d, 1H, m-C6H3), 7.20 (t, 1H, p-C6H3), 7.36 (d, 1H, m’-C6H3)
ppm. All 3JHH constants are about 7.0 Hz. 13C{1H} NMR (125
MHz, C6D6, 25 ºC): δ = 21.7, 22.6 (MeDipp), 22.7 (trans-
MeOAc), 23.6 (MeOAc), 26.7, 26.8 (MeDipp), 30.3, 31.3
(CHMe2Dipp), 121.9 (m-Dipp), 122.5 (m-C6H3 or m’-C6H3),
123.7 (m-C6H3 or m’-C6H3), 123.7 (m-Dipp), 126.0 (p-C6H3),
127.0 (p-Dipp), 129.4 (p-Dipp’), 140.2 (ipso-Dipp), 144.8 (o-
Dipp’), 145.1 (o-C6H3 or o’-C6H3), 145.2 (o-C6H3 or o’-C6H3),
146.5 (ipso-Dipp’), 146.6 (o-Dipp), 182.3 (Mo−Car), 182.7
7.6 Hz, m-Xyl), 6.67 (m, 3H, m-Xyl’ and p-Xyl’), 6.73 (d,
3
3
1H, JHH = 7.6 Hz, m-C6H3), 6.92 (d, 1H, JHH = 7.5 Hz, m’-
C6H3), 7.21 (t, 1H, 3JHH = 7.6 Hz, p-C6H3) ppm. 13C{1H} NMR
(125 MHz, C6D6, 25 ºC): δ = 20.6 (MeXyl’), 21.9 (MeXyl), 124.0
(m-C6H3), 126.6 (m’-C6H3), 127.7 (m-Xyl’), 127.9 (p-Xyl’),
128.4 (p-C6H3), 129.9 (p-Xyl), 130.3 (m-Xyl), 135.5 (o-Xyl),
135.8 (o-Xyl), 140.1 (ipso-Xyl’), 145.6 (ipso-Xyl’), 146.2 (o-
C6H3), 146.7 (o-C6H3), 178.1 (Mo−Car) ppm. Resonances
due to the trifluoroacetate groups were not detected. 19F
NMR (100 MHz, 25 ºC): δ = −72.2 (s, 6F, cis-O2CCF3), −72.9
(s, 3F, trans-O2CCF3) ppm. UV-Visible (10-5 M in diethy-
lether): λ = 525 nm (ε = 2160 mol-1 L cm-1, respectively).
Anal. Calcd. for C28H21F9Mo2O6: C, 41.20; H, 2.59. Found:
C, 41.0; H, 2.8.
(O2CMe), 183.4 (trans-O2CMe) ppm. UV-Visible (10-4
M
in benzene): λ = 535 nm (ε = 1300 mol-1 L cm-1). Anal.
Calcd. For C36H46Mo2O6: C, 56.40; H, 6.05; Found: C, 56.0;
H, 6.3.
1
Mo2(ArDipp2)(O2CF3)3, (3c). Yield: 33%. H NMR (500
MHz, C6D6, 25 ºC): δ = 0.91 (d, 6H, MeDipp’), 1.00 (d, 6H,
MeDipp), 1.14 (d, 6H, MeDipp’), 1.48 (d, 6H, MeDipp), 2.86
(sept, 2H, CHMe2Dipp’), 3.06 (sept, 2H, CHMe2Dipp), 6.30 (t,
1H, p-Dipp’), 6.39 (d, 2H, m-Dipp’), 6.92 (m, 3H, m-Dipp
and p-Dipp), 7.20−7.26 (m, 3H, m-C6H3, m’-C6H3 and p-
1
Mo2(ArTrip2)(O2CMe)3, (1d). Yield: 48%. H NMR (500
MHz, C6D6, 25 ºC): δ = 0.84 (d, 6H, p-MeTrip’), 1.16 (m, 12H,
o-MeTrip’ and p-MeTrip), 1.20 (d, 6H, o-MeTrip), 1.42 (d, 6H,
o-MeTrip’), 1.64 (d, 6H, o-MeTrip), 1.98 (s, 3H, trans-MeOAc),
2.23 (sept, 1H, p-CHMe2Trip’), 2.55 (s, 6H, MeOAc), 2.68
(sept, 1H, p-CHMe2Trip), 3.18 (sept, 2H, o-CHMe2Trip), 3.23
(sept, 2H, o-CHMe2Trip), 6.76 (s, 2H, m-Trip’), 7.04 (s, 2H,
m-Trip), 7.29 (m, 2H, p-C6H3 and m’-C6H3), 7.38 (d, 1H, m-
3
C6H3) ppm. All JHH constants are about 7.0 Hz. 13C{1H}
NMR (125 MHz, C6D6, 25 ºC): δ = 21.6 (MeDipp’), 21.9
(MeDipp), 26.6 (MeDipp’), 27.0 (MeDipp), 30.7 (CHMe2Dipp),
31.6 (CHMe2Dipp’), 122.8 (m-Dipp), 125.5 (m-Dipp’), 125.7
(m-C6H3 or m’-C6H3), 126.8 (m-C6H3 or m’-C6H3), 129.1 (p-
C6H3), 129.2 (p-Dipp), 131.1 (p-Dipp’), 137.7 (ipso-Dipp),
144.4 (o-C6H3 or o’-C6H3), 144.5 (o-C6H3 or o’-C6H3), 145.4
(o-Dipp’), 146.7 (o-Dipp), 148.0 (ipso-Dipp’), 180.1
(Mo−Car) ppm. Resonances due to the trifluoroacetate
groups were not detected. 19F NMR (100 MHz, 25 ºC): δ =
−72.2 (s, 6F, cis-O2CCF3), −74.1 (s, 3F, trans-O2CCF3) ppm.
UV-Visible (10-4 M in diethylether): λ = 535 nm (ε = 1300
mol-1 L cm-1). Anal. Calcd. for C36H37F9Mo2O6: C, 46.57; H,
4.02. Found: C, 46.4; H, 4.4.
3
C6H3) ppm. All JHH constants are about 7.0 Hz. 13C{1H}
NMR (125 MHz, C6D6, 25 ºC): δ = 21.8 (o-MeTrip), 22.4 (o-
MeTrip), 22.7 (trans-MeOAc), 23.5 (p-MeTrip), 23.6 (p-MeTrip),
24.0 (MeOAc), 26.8 (o-MeTrip), 26.9 (o-MeTrip), 30.4 (o-
CHMe2Trip), 31.6 (o-CHMe2Trip), 33.9 (p-CHMe2Trip), 34.3 (p-
CHMe2Trip), 119.7 (m-Trip), 121.6 (m-Trip), 123.4 (p-C6H3),
125.9 (m-C6H3), 128.3 (m-C6H3), 137.9 (ipso-Trip), 144.2
(ipso-Trip), 145.1 (o-Trip), 145.2 (o-C6H3), 145.4 (o-C6H3),
146.6 (o-Trip), 147.3 (p-Trip), 150.5 (p-Trip), 182.5
(O2CMe), 183.1 (Mo−Car), 183.3 (trans-O2CMe) ppm. UV-
7
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 8 of 11
General
preparation
of
bis(terphenyl)
were performed with the same functional and basis set as
the geometry optimization. The solution for the 10 first
singlet and the 10 first triplet excited states was requested
in one calculation, which yielded zero oscillator strength
for all triplet excited states.
1
2
3
4
5
6
7
8
bis(trifluoroacetate) complexes M2(Ar’)2(O2CF3)2, (4a,
5a). Solid samples of LiAr’ (2 equiv) and Mo2(O2CCF3)4
(1.00 g, 1.55 mmol) (in the case of 4) and W2(O2CCF3)4
(0.40 g, 0.49 mmol) (for 5) were mixed in a glovebox and
cooled to −40 ºC. Diethylether (20 mL) was added and the
resulting suspension was stirred for 12 hours, while reach-
ing slowly the room temperature. A pink (for compound
4a) or green (for compound 5a) crystalline solid separated
out, which was isolated by filtration and dried in vacuum.
The mother liquor was concentrated under reduced pres-
sure to ca. 5 mL and stored in a refrigerator overnight to
induce further precipitation of the product.
X-ray diffraction analyses. A single crystal of suita-
ble size, coated with dry perfluoropolyether, was mount-
ed on a glass fiber and fixed in a cold nitrogen stream [T =
173(2) K for compounds 1b, 1d, 3a, 3c, 4a, 4c and 5a] to
the goniometer head. Data collection was performed on
Bruker-Nonius X8APEX-II CCD diffractometer, using
monochromatic radiation λ(Mo Kα1) = 0.71073 Å, by
means of ω and φ scans. The data were reduced (SAINT)42
and corrected for Lorentz polarisation effects and absorp-
tion by multiscan method applied by SADABS.43 The
structures were solved by direct methods (SIR-2002)44 and
refined against all F2 data by full-matrix least-squares
techniques (SHELXTL-6.12).45 All the non-hydrogen at-
oms were refined with anisotropic displacement parame-
ters. The hydrogen atoms were included from calculated
positions and refined riding on their respective carbon
atoms with isotropic displacement parameters.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Mo2(ArXyl2)2(O2CCF3)2, (4a). Yield: 62%. H NMR (500
1
MHz, C6D6, 25 ºC): δ = 1.95 (s, 12H, MeXyl’), 2.13 (s, 12H,
MeXyl), 6.07 (d, 4H, JHH = 7.5 Hz, m-Xyl’), 6.16 (t, 2H, JHH
3
3
3
= 7.5 Hz, p-Xyl’), 6.52 (d, 2H, JHH = 7.5 Hz, m’-C6H3), 6.69
3
(d, 2H, JHH = 7.5 Hz, m-C6H3), 6.82 (m, 6H, m-Xyl and p-
3
Xyl, AB2 system), 7.10 (t, 2H, JHH = 7.5 Hz, p-C6H3) ppm.
13C{1H} NMR (125 MHz, C6D6, 25 ºC): δ = 20.8 (MeXyl), 22.3
(MeXyl’), 125.1 (m’-C6H3), 127.2 (m-Xyl, p-Xyl and m-C6H3),
127.5 (m-Xyl and p-C6H3), 128.6 (m-Xyl’), 129.7 (p-Xyl’),
135.5 (o-Xyl’), 137.8 (o-Xyl), 142.6 (ipso-Xyl), 144.4 (ipso-
Xyl’), 146.0 (o-C6H3), 146.5 (o-C6H3), 172.5 (Mo−Car) ppm.
Resonances due to the trifluoroacetate groups were not
detected. 19F NMR (100 MHz, 25 ºC): δ = −70.8 ppm. UV-
Visible (10-5 M in diethylether): λ = 555, 404 nm (ε = 1660,
■ CONCLUSION
In conclusion, incorporation of one or two terphenyl
groups to the coordination sphere of the quadruple
Mo−Mo and W−W bonds of the well-known Mo2(O2CR)4
family of compounds is possible, which adds extra value
to this versatile family of ligands.10,18−22 Since the carbox-
ylate groups of the binuclear precursors bridge the two
metal atoms, their replacement by the monodentate ter-
phenyls creates a vacant coordination site and generates
an uncommon four-coordinate geometry that derives
from a square pyramid with an empty basal site. In the
mono(terphenyl) complexes 1 and 3 there is one such an
unsaturated, metal centre and the other exhibits the five-
coordinate, 16-electron structure commonly found in
complexes of this type.2 For the bis(terphenyl) complexes
2, 4 and 5, the two metal atoms feature a low-coordinate
structure. However, it is noteworthy that in all com-
pounds studied there exists a weak M−Carene bonding
interaction that implicates either the ipso or an ortho
carbon atom of a flanking aryl ring. The corresponding
M−Carene distance (in the range ca. 2.58−2.80 Å) is signifi-
cantly longer than the existing σ M−Caryl bond (2.16−2.21
Å) but the interaction counterbalances in some degree
the electronic unsaturation and offers at the same time
steric protection to the unsaturated metal centre.
1500 mol-1
L
cm-1, respectively). Anal. Calcd. for
C48H42F6Mo2O4: C, 58.31; H, 4.28. Found: C, 57.9; H, 4.2.
W2(ArXyl2)2(O2CCF3)2, (5a). Yield: 70%. H NMR (500
1
MHz, C6D6, 25 ºC): δ = 1.84 (s, 12H, MeXyl), 2.19 (s, 12H,
MeXyl’), 5.93 (d, 4H, JHH = 7.6 Hz, m-Xyl), 6.35 (m, 4H, p-
3
Xyl and m-C6H3), 6.64 (dd, 2H, 3JHH = 7.5 Hz, 4JHH = 1.4 Hz,
m’-C6H3), 6.82−6.92 (m, 6H, m-Xyl’ and p-Xyl’, AB2 sys-
tem), 6.99 (t, 2H, 3JHH = 7.5 Hz, p-C6H3) ppm. 13C{1H} NMR
(500 MHz, C6D6, 25 ºC): δ = 21.0 (MeXyl’), 22.2 (MeXyl), 126.2
(m-C6H3), 126.7 (p-C6H3), 127.4 (m-Xyl’), 127.5 (p-Xyl’),
127.7 (m’-C6H3, under signal C6D6), 128.7 (m-Xyl), 130.1 (p-
Xyl), 134.4 (o-Xyl), 138.4 (o-Xyl’), 141.1 (ipso-Xyl’), 145.1 (o-
C6H3), 145.9 (ipso-Xyl), 147.5 (o-C6H3), 172.3 (Mo−Car). 19F
NMR (100 MHz, 25 ºC): δ = −67.20 ppm. UV-Visible (10-4
M in diethylether): λ = 600, 485, 430 nm (ε = 5100, 4500,
2300 mol-1
L
cm-1, respectively). Anal. Calcd. for
C48H42F6W2O4: C, 49.51; H, 3.64. Found: C, 49.6; H, 3.7.
Computational details. DFT calculations were per-
formed with the Gaussian 09 package36 using the hybrid
meta-GGA functional M0637. The Mo atoms were repre-
sented by the Stuttgart/Dresden Effective Core Potential
and the associated basis set38 as implemented in Gaussian
09 (SDD). The remaining H, C and O atoms were repre-
sented by means of the 6-31G(d,p) basis set.39−41 The ge-
ometry of 1a was optimized in the gas phase without
symmetry restrictions, and was characterized as a mini-
mum in the potential energy surface by the absence of
imaginary frequencies in a harmonic frequency calcula-
tion at the same level of theory. TD-DFT calculations
■ ASSOCIATED CONTENT
Supporting Information
Electronic supplementary information (ESI) available:
Experimental procedures and characterization data for
other new compounds described herein, additional com-
putational details along with CIF files for 1b, 1d, 3a, 3c, 4a,
8
ACS Paragon Plus Environment

Page 9 of 11
Journal of the American Chemical Society
(e) Chisholm, M. H. Phil. Trans. R. Soc. A. 2008, 366,
101−112.
(6) (a) Hicks, J.; Ring, S. P.; Patmore, N. J. Dalton.
4c, and 5a CCDC 997253−997259. This information is
available free of charge via the Internet at
http://pubs.acs.org/.
1
2
3
4
5
6
7
8
Trans. 2012, 41, 6641−6650. (b) Wilkinson, L. A.; McNeill,
L.; Meijer, A. J. H. M.; Patmore, N. J. J. Am. Chem. Soc.
2013, 135, 1723−1726. (c) Wilkinson, L. A.; McNeill, L.; Scat-
tergood, P. A.; Patmore, N. J. Inorg. Chem. 2013, 52,
9683−9691. (d) Chisholm, M. H.; Clark, R. J. H.; Gallucci,
J.; Hadad, C. M.; Patmore, N. J. J. Am. Chem. Soc. 2004,
126, 8303−8313.
■ AUTHOR INFORMATION
Corresponding author
guzman@us.es.
pppower@ucdavis.edu.
9
Present addressed
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
†Instituto de Investigaciones Químicas (IIQ), Departa-
mento de Química Inorgánica, Universidad de Sevilla-
Consejo Superior de Investigaciones Científicas, Avenida
Américo Vespucio 49, 41092 Sevilla, Spain.
‡Department of Chemistry, University of California, Da-
vis, One Shields Avenue, Davis, California 95616, USA.
(7) Collman, J. P.; Boulatov, R.; Jameson, G. B. An-
gew. Chem. Int. Ed. 2001, 40, 1271−1274.
(8) Krapp, A.; Lein, M.; Frenking, G. Theor. Chem.
Acc. 2008, 120, 313−320.
(9) (a) Rudd, P. A.; Liu, S.; Planas, N.; Bill, E.;
Gagliardi, L.; Lu, C. C. Angew. Chem. Int. Ed. 2013, 52,
4449−4452. (b) Clouston, L. J.; Siedschlag, R. B.; Rudd, P.
A.; Planas, N.; Hu, S.; Miller, A. D.; Gagliardi, L.; Lu, C. C.
J. Am. Chem. Soc. 2013, 135, 13142-13148.
■ ACKNOWLEDGEMENTS
Financial support (FEDER contribution and Subpro-
gramas Juan de la Cierva) from the Spanish Ministry of
Science and Innovation (Projects CTQ2010-15833,
CTQ2011-23862-C02-01 and Consolider-Ingenio 2010
CSD2007-00006), and the Junta de Andalucía (Grant
FQM-119 and Project P09-FQM-5117) is gratefully
acknowledged. J. L.-S. also thanks the MICINN and the
European Social Fund for the award of a “Ramón y Cajal”
contract. The use of computational facilities of the Super-
computing Center of Galicia (CESGA) is also acknowl-
edged. P. P. P. and M. F. thank the U.S. National Science
Foundation (CHE-1263760) for financial support.
(10) (a) Nguyen, T.; Sutton, A. D.; Brynda, M.;
Fettinger, J. C.; Long, G. J.; Power, P. P. Science, 2005, 310,
844−847. (b) Wolf, R.; Ni, C.; Nguyen, T.; Brynda, M.;
Long, G. J.; Sutton, A. D.; Fischer, R. C.; Fettinger, J. C.;
Hellman, M.; Pu, L.; Power, P. P. Inorg. Chem. 2007, 46,
11277−11290.
(11) (a) Kreisel, K. A.; Yap, G. P. A.; Dmitrenko, O.;
Landis, C. R.; Theopold, K. H. J. Am. Chem. Soc. 2007, 129,
14162−14163. (b) Tsai, Y.-C.; Chen, H.-Z.; Chang, C.-C.; Yu,
J.-S. K.; Lee, G.-H.; Wang, Y.; Kuo, T.-S. J. Am. Chem. Soc.
2009, 131, 12534−12535. (c) Tsai, Y.-C.; Hsu, C.-W.; Yu, J.-S.
K.; Lee, G.-H.; Wang, Y.; Kuo, T.-S. Angew. Chem. Int. Ed.
2008, 47, 7250−7253. (d) Hsu, C.-W.; Yu, J.-S. K.; Yen, C.-
H.; Lee, G.-H.; Wang, Y.; Tsai, Y.-C. Angew. Chem. Int. Ed.
2008, 47, 9933−9936. (e) Huang, Y.-L.; Lu, D.-Y.; Yu, H.-
C.; Yu, J.-S. K.; Hsu, C.-W.; Kuo, T.-S.; Lee, G.-H.; Wang,
Y.; Tsai, Y.-C. Angew. Chem. Int. Ed. 2012, 51, 7781-7785. (f)
Liu, S.-C.; Ke, W.-L.; Yu, J.-S. K.; Kuo, T.-S.; Tsai, Y.-C.
Angew. Chem. Int. Ed. 2012, 51, 6394−6397.
■ REFERENCES
(1) Cotton, F. A.; Curtis, N. F.; Harris, C. B.; Johnson,
B. F. G.; Lippard, S. J.; Mague, J. T.; Robinson, W. R.;
Wood, J. S. Science, 1964, 145, 1305−1307.
(2) Cotton, F. A.; Murillo, C. A.; Walton, R. A. Multi-
ple Bonds Between Metal Atoms, Springer, Berlin, 3rd ed.
2005.
(12) (a) Noor, A.; Glatz, G.; Müller, R.; Kaupp, M.;
Demeshko, S.; Kempe, R. Z. Anorg. Allg. Chem. 2009, 635,
1149-1152. (b) Noor, A.; Glatz, G.; Müller, R.; Kaupp, M.;
Demeshko, S.; Kempe, R. Nat. Chem. 2009, 1, 322−325. (c)
F. R. Wagner, A. Noor and R. Kempe, Nat. Chem. 2009, 1,
529−536. (d) Noor, A.; Wagner, F. R.; Kempe, R. Angew.
Chem. Int. Ed. 2008, 47, 7246−7249. (e) Noor, A.; Bauer,
T.; Todorova, T. K.; Weber, B.; Gagliardi, L.; Kempe, R.
Chem. Eur. J. 2013, 19, 9825−9832.
(3) (a) Collman, J. P.; Boulatov, R. Angew. Chem. Int.
Ed. 2002, 41, 3948−3961. (b) Krogman, J. P.; Thomas, C. M.
Chem. Comm. 2014, 50, 5115−5127.
(4) (a) Cotton, F. A.; Daniels, L. M.; Guimet, I.; Hen-
ning, R. W.; Jordan, G. T.; Lin, C.; Murillo, C. A.; Schultz,
A. J. J. Am. Chem. Soc. 1998, 120, 12531−12538. (b) Cotton,
F. A.; Daniels, L. M.; Jordan, G. T.; Lin, C.; Murillo, C. A. J.
Am. Chem. Soc. 1998, 120, 3398−3401. (c) Cotton, F. A.;
Hillard, E. A.; Murillo, C. A. J. Am. Chem. Soc. 2002, 124,
5658−5660.
(13) (a) Carrasco, M.; Curado, N.; Maya, C.; Peloso, R.;
Rodríguez, A.; Ruiz, E.; Álvarez, S.; Carmona, E. Angew.
Chem. Int. Ed. 2013, 52, 3227-3231. (b) Carrasco, M.; Cura-
do, N.; Álvarez, E.; Maya, C.; Peloso, R.; Poveda, M. L.;
Rodríguez, A.; Ruiz, E.; Álvarez, S.; Carmona, E. Chem.
Eur. J. 2014, 20, 6092–6102.
(5) (a) Chisholm, M. H.; Macintosh, A. M. Chem.
Rev. 2005, 105, 2949−2976. (b) Chisholm, M. H. Proc. Natl.
Acad. Sci. U.S.A. 2007, 104, 2563−2570. (c) Chisholm, M.
H.; Lear, B. J. Chem. Soc. Rev. 2011, 40, 5254−5265. (d)
Chisholm, M. H. Coord. Chem. Rev. 2013, 257, 1576−1583.
9
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 10 of 11
(14) (a) Merino, G.; Donald, K. J.; D’Acchioli, J. S.;
Hoffmann, R. J. Am. Chem. Soc. 2007, 129, 15295−15302. (b)
Frenking, G. Science, 2005, 310, 796-797. (c) Roos, B. O.;
Borin, A. C.; Gagliardi, L.; Angew. Chem. Int. Ed. 2007, 46,
1469−1472. (d) La Macchia, G.; Gagliardi, L.; Power, P. P.;
Brynda, M. J. Am. Chem. Soc. 2008, 130, 5104−5114. (e)
Brynda, M.; Gagliardi, L.; Widmark, P.-O.; Power, P. P.;
Roos, B. O. Angew. Chem. Int. Ed. 2006, 45, 3804−3807. (f)
Radius, U.; Breher, F. Angew. Chem. Int. Ed. 2006, 45,
3006-3010. (g) La Macchia, G.; Manni, G. L.; Todorova, T.
K.; Brynda, M.; Aquilante, F.; Roos, B. O.; Gagliardi, L.
Inorg. Chem. 2010, 49, 5216-5222.
(27) (a) Bader, R. F. W. Atoms in Molecules: A Quan-
tum Theory. Oxford University Press, Oxford, UK, 1995.
(b) Bader, R. F. W. Chem. Rev. 1991, 91, 893−928.
1
2
3
4
5
6
7
8
(28) (a) Sierraalta, A. Chem. Phys. Lett. 1994, 227,
557−560. (b) Saßmannshausen, J.; Track, A; Dias, T. A. D.
S. Eur. J. Inorg. Chem. 2007, 2327−2333. (c) Scherer, W.;
McGrady, G. S. Angew. Chem. Int. Ed. 2004, 43, 1782−1806.
(29) Cordero, B.; Gómez, V.; Platero-Prats, A. E.; Re-
vés, M.; Echeverría, J.; Cremades, E.; Barragán, F.; Alvarez,
S. J. Chem. Soc., Dalton Trans. 2008, 2832−2838.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(30) Cotton, F. A.; Norman Jr., J. G. J. Coord. Chem.
1976, 5, 217−223.
(15) Tsai, Y.-C.; Lin, Y.-M.; Yu, J.-S. K.; Hwang, J.-K. J.
Am. Chem. Soc. 2006, 128, 13980−13981.
(31) Brignole, A. B.; Cotton, F. A.; Dori, Z.; Dori, Z.;
Dori, Z.; Wilkinson, G. Rhenium and Molybdenum Com-
pounds Containing Quadruple Bonds, in Inorganic Synthe-
ses, Vol. 13 (ed F. A. Cotton), John Wiley & Sons, Inc.,
Hoboken, NJ, USA, 2007.
(16) (a) Andersen, R. A.; Jones, R. A.; Wilkinson, G. J.
Chem. Soc., Dalton Trans. 1978, 446−453. (b) Girolami, G.
S.; Mainz, V. V.; Andersen, R. A.; Vollmer, S. H.; Day, V.
W. J. Am. Chem. Soc. 1981, 103, 3953−3955. (c) Girolami, G.
S.; Mainz, V. V.; Andersen, R. A. J. Am. Chem. Soc. 1982,
104, 2041−2042.
(32) Cotton, F. A.; Norman Jr., J. G. J. Coord. Chem.
1971, 1, 161−172.
(33) (a) Santure, D. J.; McLaughlin, K. W.; Huffman, J.
C.; Sattelberger, A. P. Inorg. Chem. 1983, 22, 1877−1883. (b)
Santure, D. J.; Sattelberger, A. P. Inorg. Synth. 1989, 26,
219−225.
(17) Carrasco, M.; Faust, M.; Peloso, R.; Rodríguez, A.;
López-Serrano, J.; Álvarez, E.; Maya, C.; Power, P. P.;
Carmona, E. Chem. Commun. 2012, 48, 3954−3956.
(18) (a) Power, P. P. Nature, 2010, 463, 171−177. (b)
Power, P. P. J. Organomet. Chem. 2004, 689, 3904−3919.
(c) Twamley, B.; Haubrich, S. T.; Power, P. P. Adv. Organ-
omet. Chem. 1999, 44, 1−65.
(34) (a) Power, P. P. Main Group Chem. 1998, 2,
275−283. (b) Rivard, E.; Power, P. P. Inorg. Chem. 2007,
46, 10047−10064.
(35) Schiemenz, B.; Power, P. P. Organometallics
1996, 15, 958−964.
(19) Zhu, Z.; Wright, R. J.; Olmstead, M. M.; Rivard,
E.; Brynda, M.; Power, P. P. Angew. Chem. Int. Ed. 2006,
45, 5807−5810.
(36) Gaussian 09, Revision A.02, Frisch, M. J.; Trucks,
G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.;
Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.;
Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.;
Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.;
Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fuku-
da, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.;
Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, Jr., J. A.;
Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Broth-
ers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.;
Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.;
Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.;
Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.;
Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.;
Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G.
A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels,
A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski,
J.; Fox, D. J. Gaussian, Inc., Wallingford CT, 2009.
(20) Clyburne, J. A. C.; McMullen, N. Coord. Chem.
Rev. 2000, 210, 73−99.
(21) Su, J.; Li, X.-W.; Crittendon, R. C.; Robinson, G.
H. J. Am. Chem. Soc. 1997, 119, 5471−5472.
(22) Ni, C.; Power, P. P. Struct. Bonding 2010, 136,
59−112.
(23) (a) Hadlington, T. J.; Jones, C. Chem. Commun.
2014, 50, 2321−2323. (b) Hadlington, T. J.; Hermann, M.;
Li, J.; Frenking, G.; Jones, C. Angew. Chem. Int. Ed. 2013,
52, 10199-10203. (c) Li, J.; Schenk, C.; Goedecke, C.; Fren-
king, G.; Jones, C. J. Am. Chem. Soc. 2011, 133, 18622−18625.
(d) Li, J.; Stasch, A.; Schenk, C.; Jones, C. J. Chem. Soc.,
Dalton Trans. 2011, 40, 10448-10456.
(24) (a) Bauernschmitt, R.; Ahlrichs, R. Chem. Phys.
Lett. 1996, 256, 454−464. (b) Stratmann, R. E.; Scuseria, G.
E.; Frisch, M. J. J. Chem. Phys. 1998, 109, 8218−8224. (c)
Casida, M. E.; Jamorski, C.; Casida, K. C.; Salahub, D. R. J.
Chem. Phys. 1998, 108, 4439−4449.
(37) Zhao, Y.; Truhlar, D. G. Theor. Chem. Acc. 2008,
120, 215−241.
(25) Cotton, F. A.; Daniels, L. M.; Hillard, E. A.; Muril-
(38) Andrae, D.; Haeussermann, U.; Dolg, M.; Stoll,
lo, C. A. Inorg. Chem. 2002, 41, 2466−2470.
H.; Preuss, H. Theor. Chem. Acc. 1990, 77, 123−141.
(26) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem.
Rev. 1988, 88, 899−926.
(39) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Phys.
Chem. 1972, 56, 2257−2261.
10
ACS Paragon Plus Environment

Page 11 of 11
Journal of the American Chemical Society
(40) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta
1
2
3
4
5
6
7
1973, 28, 213−222.
TOC graphic page
LAYOUT
(41) Francl, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley,
J. S.; Gordon, M. S.; DeFrees, D. J.; Pople, J. A. Chem. Phys.
1982, 77, 3654−3665.
(42) SAINT 6.02, BRUKER-AXS, Inc., Madison, WI
53711-5373 USA, 1997–1999.
8
9
(43) SADABS George Sheldrick, Bruker AXS, Inc.,
Madison, Wisconsin, USA, 1999.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(44) Burla, M. C.; Camalli, M.; Carrozzini, B.; Cas-
carano, G. L.; Giacovazzo, C.; Polidori, G.; Spagna, R.
SIR2002: the program; J. Appl. Cryst. 2003, 36, 1103.
(45) SHELXTL 6.14, Bruker AXS, Inc., Madison, Wis-
consin, USA, 2000–2003.
11
ACS Paragon Plus Environment