Journey from Mo-Mo quadruple bonds to quintuple bonds

Article abstract of DOI:10.1021/ja905035f

(Chemical Equation Presented) Heating K₄Mo₂Cl ₈ and 2 equiv of Li[RC(NDipp)₂] (R = H, Ph; Dipp = 2,6-i-Pr₂C₆H₃) in tetrahydrofuran (THF) at 60°C gives two paddlewheel type quadruply bonded dimolybdenum complexes, Mo₂(μ -Cl)[Cl₂Li(OEt₂)][μ- η²-RC(N-2,6-i-Pr₂C₆H₃) ₂]₂ (R = H (1), Ph (2)). The Mo-Mo bond lengths of 1 and 2 are 2.0875(4) and 2.0756(8) A, respectively, indicating typical Mo-Mo quadruple bonds. Reduction of 1 and 2 by two electrons results in the isolation of their corresponding Mo-Mo quintuple bonded complexes, Mo₂[μ- η²-RC(N-2,6-i-Pr₂C₆H₃) ₂]₂ (R = H (3), Ph (4)), and the Mo-Mo bond lengths dramatically decrease to 2.0187(9) A (3) and 2.0157(4) A (4), a consequence of the formation of the second δ bond and representing the shortest metal-metal bonds beyond the first row elements. The Mo-Mo quintuple bonding characters are corroborated by DFT calculations at the level of BP86/def2-TZVP and BP86/def2-TZVPP.

Full text of DOI:10.1021/ja905035f

Published on Web 08/17/2009
Journey from Mo-Mo Quadruple Bonds to Quintuple Bonds
Yi-Chou Tsai,*,† Hong-Zhang Chen,† Chie-Chieh Chang,† Jen-Shiang K. Yu,‡ Gene-Hsiang Lee,§
Yu Wang,§ and Ting-Shen Kuo|
Department of Chemistry, National Tsing Hua UniVersity, Hsinchu 30013, Taiwan, ROC, Institute of Bioinformatics
and Systems Biology and Department of Biological Science and Technology, National Chiao Tung UniVersity,
Hsinchu, 30010, Taiwan, ROC, Department of Chemistry, National Taiwan UniVersity, Taipei 10617, Taiwan, ROC,
and Department of Chemistry, National Taiwan Normal UniVersity, Taipei 11677, Taiwan, ROC
Received June 23, 2009; E-mail: yictsai@mx.nthu.edu.tw
Scheme 1
Whereas a large number of compounds containing metal-metal
quadruple bonds has been documented over the past four decades1
since the discovery of Re2Cl82-,2 the quest for complexes containing
metal-metal quintuple bonds has just begun. As the existence of
the first quintuple Cr-Cr bond in the complex Ar′CrCrAr′ (where
Ar′ is the encumbering 2,6-(C6H3-2,6-i-Pr2)2C6H3 ligand) was
recognized by Power et al.,3 several dichromium complexes
featuring ultrashort quintuple bonds have recently been identified
by us,4 Power,5 Theopold,6 and Kempe.7
Molybdenum is notable for its ability to form strong Mo-Mo
multiple bonds. For example, Mo forms the most quadruply bonded
compounds and consequently accumulates a wealth of structural and
spectroscopic data.1 Furthermore, it must be noted that the singlet state
diatomic Mo2 molecule containing a true sextuple bond has been
observed in the gas phase at low temperatures,8 and the Mo-Mo bond
1 and 2 essentially adopt a paddlewheel structure supported by two
length was determined to be 1.93 Å.9 Overall, owing to the success in
encumbering amidinates, one bridging chloro ligand and the
Cl-Li-Cl linkage spanning the Mo-Mo bond forming the “-ate”
the recognition of it is homologous Cr-Cr quintuple bonded
complexes4-7 and being sandwiched between Mo-Mo quadruple and
complexes. The Mo-Mo bond length of 2.0875(4) Å of 1 and
2.0756(8) Å of 2 indicates two typical Mo-Mo quadruple bonds.1
sextuple bonded compounds, Mo-Mo quintuple bonded complexes
have thus been proposed to be synthesized.10
The slightly shorter Mo-Mo bond length in 2 is presumably
ascribed to the more encumbering amidinato ligand.4a
We recently reported a dimolybdenum complex supported by
only two diamido ligands, Mo2[µ-η2-Me2Si(N-2,6-i-Pr2C6H3)2]2,
obtained from reduction of the triply bonded syn-Mo2Cl2[µ-η2-
Me2Si(N-2,6-i-Pr2C6H3)2]2.11 DFT calculations indicated that this
compound features a Mo-Mo quadruple bond. Of particular
interest, moreover, is the strong N-Mo(dxy) π interactions, which
consequently results in no δ bonding between two dxy orbitals.
Inspired by these observations, the Mo-Mo quintuple bond is
expected if the said diamido ligands are replaced by the monoan-
ionic amidinates. Herein we report that the encumbering amidinates
can indeed stabilize the Mo-Mo quintuple bonds, which are
extremely short up to 2.02 Å, the shortest metal-metal bonds
beyond the first row transition metals.
Figure 1. Molecular structure of 2 with thermal ellipsoids shown at the
35% probability level.
Similar to the access of the aforementioned Mo-Mo quadruple
bonded compound Mo2[µ-η2-Me2Si(N-2,6-i-Pr2C6H3)2]2, our strat-
egy to accomplish the preparation of complexes containing the
Mo-Mo quintuple bonds started from the synthesis of compounds
having the Mo-Mo quadruple bonds. As shown in Scheme 1,
treatment of the red Mo-Mo quadruply bonded complex
Subsequent reduction of 1 and 2 with 2 equiv of KC8 gave the
desired quintuply bonded Mo2 complexes, Mo2[µ-η2-RC(N-2,6-i-
Pr2C6H3)2]2 (3, R ) H; 4, R ) Ph). The diamagnetic nature of 3
and 4 are confirmed by measuring their solid (<0.8 B. M.) and
solution (Evans method13) magnetic moments. The 1H NMR spectra
of 3 in d8-THF and 4 in C6D6 solution showed sharp one ligand set
signals in the range δ 0-8 ppm, which are consistent with the high
symmetry of 3 and 4.
The molecular structures of 3 and 4 (Figure 2) were determined
by X-ray crystallography. 3 and 4 contain two amidinato ligands
spanning the Mo-Mo quintuple bond, and the two Mo atoms, the
two N-C-N backbones, and the four ipso-carbon atoms of the
phenyl rings of 3 are completely coplanar, while the N-Mo-Mo-N
torsion angles of 4 are 2° and 172°. Therefore, 3 and 4 display
approximately C2h symmetry.14 The amidinato ligands of the
12
K4Mo2Cl8 with 2 equiv of the sterically hindered amidinates,
Li[RC(N-2,6-i-Pr2C6H3)2] (R ) H, Ph), in THF engendered the
formation of two Mo-Mo quadruply bonded complexes, Mo2(µ-
Cl)[Cl2Li(OEt2)][µ-η2-RC(N-2,6-i-Pr2C6H3)2]2 (1, R ) H; 2, R ) Ph).
The molecular structures of 1 and 2 were corroborated by X-ray
crystallography (Supporting Information (SI) and Figure 1). Both
† National Tsing Hua University.
‡ National Chiao Tung University.
§ National Taiwan University.
| National Taiwan Normal University.
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12534 J. AM. CHEM. SOC. 2009, 131, 12534–12535
10.1021/ja905035f CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
axes are parallel to one another. As a result, computations
unambiguously support that 3 and 4 possess a Mo-Mo quintuple
bond, and these two extremely short Mo-Mo bonds are due to
strong interactions between two d5 Mo(I) centers.
Note also that the Cr-Cr quintuple bond lengths4 can be
successfully predicted by the semiempirical pyramidality effect.18
However, the estimated Mo-Mo quintuple bond lengths for 3 and
4 by the pyramidality effect are 2.053 and 2.044 Å, respectively,
which significantly deviate from the experimental values 2.0187(9)
and 2.0157(4) Å, respectively. This discrepancy has also occurred
to most of the Mo-Mo quadruply bonded tetragonal complexes.17
In summary, we have demonstrated a method to construct the
Mo-Mo quintuple bonds. The second Mo-Mo δ bonds of the
quintuple bonded dimolybdenum compounds 3 and 4 were devel-
oped from reduction of their respective quadruply bonded Mo2
precursors 1 and 2. Both 3 and 4 possess an extremely short
Mo-Mo quintuple bond of 2.02 Å. The bonding schemes, 2 δ, 2
π, and 1 σ, of the Mo-Mo quintuple bonds of 3 and 4 were
corroborated by sophisticated DFT calculations. Owing to the low-
coordinate and -valent Mo-Mo centers, 3 and 4 provide a good
platform to explore their chemistry. Reactivity studies of 3 and 4
are currently underway.
Figure 2. Molecular structures of 3 (top) and 4 (bottom) with thermal
ellipsoids shown at the 35% probability level.
Acknowledgment. We are indebted to the National Science
Council, Taiwan for support under Grant NSC 96-2113-M-007-
019-MY3 and 97-2113-M-009-001-MY2. The computational facil-
ity is supported by NCTU under the grant from MoE ATU Plan.
complexes 1-4 remain intact due to the C-N bond lengths of their
backbones being 1.32-1.35 Å.
Typically for most multiply bonded metal-metal complexes, the
most interesting metrical parameter is the metal-metal bond length.
The Mo-Mo quintuple bond lengths of 3 and 4 are 2.0187(9) and
2.0157(4) Å, respectively, dramatically shorter than those in 1 and
2. Though the X-ray determined the shortest Mo-Mo quadruple
bond is 2.037(3) Å in the tetragonal dimolybdenum complex,
Mo2(µ-η2-pyNC(O)CH3)4 (Cotton et al.),15 this unusually short
quadruple bond was suggested to be unreliable. It is interesting to
note that these values suggest that the Mo-Mo quintuple bond
lengths do not or slightly correlate with the steric bulk of the
ancillary ligands. It is also important to note that the Mo-Mo bond
lengths in 3 and 4 are substantially shorter than the theoretically
predicted Mo-Mo quintuple bond lengths of 2.03-2.10 Å.10
Conclusively, the ultrashort Mo-Mo quintuple bonds herein are
unequivocally a consequence of the formations of 1 σ, 2 π, and 2
δ bonding interactions between two Mo atoms, although a δ bond
contributes only slightly to the bond shortening.1 In comparison,
in terms of Cotton’s “formal shortness ratio” (FSR), 3 and 4 has
an FSR of 0.776 and 0.775, while the FSR of N2 is 0.786.1
To understand the electronic structures and bonding schemes of
3 and 4, we carried out computations using BP8616 density
functional theory (DFT) with def2-TZVP and def2-TZVPP basis
sets.17 Geometry optimizations on diamagnetic 3 gave metrical
parameters (Table S10 in SI) that are in excellent agreement with
the X-ray structure. For example, the computed Mo-Mo bond
lengths are 2.029 (def2-TZVP) and 2.021 Å (def2-TZVPP). As for
the electronic structures, the calculations at BP86/def2-TZVP
showed that there is no N-Mo π bonding interactions, and
considerable metal-metal bonding characters can be found through
HOMO to HOMO-2, HOMO-10, and HOMO-12 as shown in
Figure S5 (SI). Those between HOMO-3 and HOMO-9 are
primarily contributed from ligands. Of these five Mo-Mo bonding
orbitals, HOMO-10 (dxz + dxz) and HOMO-12 (dyz + dyz) represent
two Mo-Mo π bonds, while the Mo-Mo σ character is found at
Supporting Information Available: Experimental details for syn-
thesis, X-ray crystallographic data of 1-4 with tables and CIF files.
This material is available free of charge via the Internet at http://
pubs.acs.org.
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2
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2
2
dx -y ) clearly indicate a pair of Mo-Mo δ bonds. Note that HOMO-
1, the side-on sd δ bond, results from hybridization of s (36.7%)
and d (63.3%) orbitals, oriented such that the main hybrid orbital
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