A three-coordinate and quadruply bonded Mo-Mo complex

Article abstract of DOI:10.1021/ja0635884

Reaction of MoCl₃(THF)₃ with [Me₂Si{NLi(Dipp)}₂]₂ (Dipp = 2,6-i-PrC₆H₃) afforded a triply bonded dimolybdenum complex 1,2-Mo₂Cl₂[Me₂Si(NDipp)₂]₂ 1, spanned by two Me₂Si[N(Dipp)]₂ ligands, thus resulting in a syn conformation. The air- and moisture-sensitive compound 1 was characterized by NMR spectroscopic, elemental, and single-crystal X-ray crystallographic analysis. Reduction of 1 by Na/Hg yielded the quadruply bonded dimeric complex Mo₂[Me₂Si(NDipp)₂]₂ 2, which was also characterized by the aforementioned analytical methods. The Mo-Mo bond was determined to be 2.1784(12) A, which is considered a long quadruple bond. In addition, density functional theory (DFT) computations on compound 2 provided insight into the intriguing Mo-Mo quadruple bond. Copyright

Full text of DOI:10.1021/ja0635884

Published on Web 10/06/2006
A Three-Coordinate and Quadruply Bonded Mo-Mo Complex
Yi-Chou Tsai,*,† Yang-Miin Lin,† Jen-Shiang K. Yu,‡ and Jenn-Kang Hwang‡
Department of Chemistry, National Tsing Hua UniVersity, Hsinchu 30013, Taiwan, Republic of China, and
Department of Biological Science and Technology, National Chiao Tung UniVersity,
Hsinchu 30050, Taiwan, Republic of China
Received May 23, 2006; E-mail: yictsai@mx.nthu.edu.tw
Scheme 1
Since Professor Cotton’s seminal recognition of the first dirhe-
nium complex [Re2Cl8]2- featuring a quadruple bond between two
metal atoms in 1964,1 the field of metal-metal quadruple-bond
chemistry has progressed at a dramatic rate in the past decades.
Thus far, the paddlewheel motif has been one of the most prevalent
structural types observed.2 It is worth noting that in order to preserve
the δ-interaction between two metals, the four ligands coordinating
to each metal are restricted to either σ-donors or weak π-donors,
such as amines, halides, phosphines, and alkoxides.2,3
In addition to the regular quadruply bonded dimetal species,
coordination complexes having metal-metal multiple bonds can
be stabilized by good π-donor ligands. For instance, Chisholm has
pioneered the use of six amide ligands to support group VI metals
in the corresponding anti-1,2-MoCl2(NMe2)4,9b due to greater orbital
in discrete “ethane-like” dimeric forms, M2(NMe2)6 where M )
overlap between MX3 fragments in an eclipsed ligand conforma-
Mo or W.4 Moreover, Power’s group recently reported the first
tion.11 Compound 1 thus provides us a good opportunity for the
isolable dichromium complex with a five-fold bond between two
preparation of an unprecedented complex of the type Mo2X4.
Cr atoms, supported by only two bulky monodentate carbyl ligands.5
In pursuit of an unprecedented three-coordinate and quadruply
Low-coordinate metal complexes are highly attractive due to their
unsaturated coordination spheres which can serve as an efficient
platform for activating small molecules such as N2.6 In this com-
munication, we report a successful synthesis of the first symmet-
rically bridged7 and quadruply bonded dinuclear Mo(II) amido
complex of the type of M2X4.
Metathesis reaction between the dimeric dilithio salts of [Me2-
Si{NLi(Dipp)}2]2 (Dipp ) 2,6-i-Pr2C6H3)8 with 2 equiv of mon-
omeric MoCl3(THF)3 in diethyl ether at room-temperature resulted
in the formation of the corresponding triply bonded dimolybdenum
complex syn-1,2-Mo2Cl2[µ-η2-Me2Si(NDipp)2]2 1, (eq 1, Scheme
bonded Mo2 complex, we sought to chemically reduce 1 by two
electrons. The cyclic voltammogram of a solution of 1 (THF/TBAP)
shows two reversible reductions at E1/2 ) -1.83 and -2.01 V
(relative to Fc/Fc+) over the course of the cathodic sweep. Accord-
ingly, reduction of an ether solution of 1 with Na/Hg gave the dia-
magnetic Mo2[µ-η2-Me2Si(NDipp)2]2 2 in 43% yield as an ex-
tremely air- and moisture-sensitive orange solid. X-ray structure
analysis confirmed the dinuclear nature of 2 and the central Mo-
Mo bond. The structure revealed a disorder problem, in which two
molybdenum atoms were disordered over three positions. (Sup-
porting Information) One orientation of the disordered Mo2 in 2
1) which was isolated by recrystallization from n-hexane in 90%
depicted in Figure 1 shows a fused bicyclic skeleton containing a
yield as air- and moisture-sensitive orange crystals. The proton
4+
central Mo2 core spanned by two ligands, and thus exhibiting a
NMR spectrum of 1 in C6D6 showed four doublet resonance signals
(δ 0.62, 1.29, 1.47 and 1.69) corresponding to the eight i-Pr groups,
and two multiplet signals around δ 4.27 and 4.13 ppm which were
virtual C2h symmetry in which the SiC2 units lie in the horizontal
mirror plane. Each Mo atom is three-coordinate by two N atoms
of the two amides and one adjacent Mo atom. Atoms of N(1), Mo-
assigned to the eight methine protons.
(1), Mo(1A), and N(1A) are coplanar, and the dihedral angle of
Crystals of 1 were analyzed by X-ray crystallography. Interest-
ingly, in contrast to the reported staggered anti rotamers of 1,2-
M2X2(NR2)4,2,9 the structure of 1 adopted the form of the eclipsed
N-Mo-Mo-N in both the five-membered SiN2Mo2 rings are
12.08(16)°, thus providing effective steric protection for the two
Mo atoms. The bond length between Mo(1) and N(1) is 1.967(4)
syn conformation, featuring two terminal chlorides and two Me2-
Å and between Mo(1) and N(2A) is 1.958(4) Å which are fairly
Si(NDipp)2 ligands spanning a metal-metal triple bond.10 The
ORTEP drawing of 1 is shown in Figure S1 (see Supporting
Information). The coordination geometry of each metal center is
nearly trigonal pyramidal. A striking structural feature of 1 is the
short, and the sum of the bond angles around N(1), which adopts
a trigonal-planar geometry, is 359.4° and that of N(2) is 359.8°,
indicating strong π-interactions between Mo and N atoms. As is
usually the case for metal-metal dimers, the most intriguing metric
large Cl-Mo-Mo-Cl torsion angle of 30.43(8)°, which is assumed
is the metal-metal bond length, particularly in such a low-
as a consequence of the tetrahedral geometry around Si atoms. The
Mo-N distances of 1.977(5) and 1.988(5) Å in 1 are in the range
coordinate compound as 2. Interestingly, the bond length of Mo(1)-
Mo(1A) is 2.1784(12) Å and is categorized as a long Mo-Mo
of the documented Mo-N lengths.2,4,9 Interestingly, despite the
quadruple bond.12
structural difference between 1 and complexes anti-1,2-MoX2-
(NMe2)4, the Mo-Mo bond length of 2.2016(10) Å for 1 is typical
for Mo-Mo triple bonds, which is even slightly shorter than that
In addition to crystallographic data, electronic absorption and
resonance Raman spectroscopy also provided insight into the
existence of metal-metal quadruple bonds.2 Most quadruply bonded
dinuclear complexes are vividly colored due to the small separations
in energy between the δ and δ* orbitals. Indeed, an electronic
† National Tsing Hua University.
‡ National Chiao Tung University.
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J. AM. CHEM. SOC. 2006, 128, 13980-13981
10.1021/ja0635884 CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
2
2
plot of HOMO have contribution from dx -y (69.61%), it also
contains 30.30% of s orbital and 0.09% of p orbital on the basis of
natural bond orbital (NBO) analysis.13
The chemical property of 2 is consistent with the observation
made from electrochemical measurements (i.e. the oxidative addi-
tion).14 For example, exposure of 2 to organic chlorides, such as
CH2Cl2 or 1,2-C2H4Cl2, quickly gives 1, which can then be con-
verted once again to 2 upon reduction. The result provides us the
opportunity to further explore the potentially rich chemistry of inter-
conversion between M2X6 triple bonds and M2X4 quadruple bonds.
In summary, we have prepared an unusual triply bonded dimolyb-
denum complex, syn-1,2-Mo2Cl2[µ-η2-Me2Si(NDipp)2]2, 1, from
which the first three-coordinate and quadruply bonded dimolyb-
denum complex Mo2[µ-η2-Me2Si(NDipp)2]2 2 can be isolated upon
reduction of 1. Complex 2 exhibits an electronic structure different
from that of the conventional paddlewheel structures. Reactivity
studies of 2 are underway.
Figure 1. Molecular structure of 2 (thermal ellipsoids at the 30% probability
level). Selected bond distances (Å) and angles (deg): Mo(1)-Mo(1A),
2.1784(12); Mo(1)-N(1), 1.967(4); Mo(1)-N(2A), 1.958(4); Si(1)-N(1),
1.747(4); Si(1)-N(2), 1.765(4); N(1)-Mo(1)-Mo(1A), 97.16(11); N(2)-
Mo(1A)-Mo(1), 99.37(11); N(1)-Mo(1)-N(2A), 159.56(16); N(1)-
Si(1)-N(2), 104.31(19); Si(1)-N(2)-Mo(1), 114.5(2); Si(1)-N(1)-Mo(1),
113.2(2).
Acknowledgment. We are grateful to the National Science
Council of Taiwan (Grant NSC 93-2113-M-007-020) for financial
support, Mr. Ting-Shen Kuo (National Taiwan Normal University),
Professor Ju-Chun Wang (Soochow University, Taiwan, R.O.C.)
for help with crystallographic details, and the National Center for
High-performance Computing for computer time and facilities. We
also thank Professors Christopher C. Cummins (MIT) and Ching-
Han Hu (National Changhua University of Education, Taiwan,
R.O.C.) for insightful discussions.
Supporting Information Available: Experimental details for the
synthesis of 1 and 2, cyclic voltammetry, UV-vis, X-ray crystal-
lographic data, including tables and CIF files, and details of the
computational study (DFT). This material is available free of charge
via the Internet at http://pubs.acs.org.
Figure 2. Contour plots of HOMO (a) and HOMO-11 (b) of 2.
absorption band of the orange-colored complex 2 was observed at
about 17240 cm-1, which is assigned tentatively to an allowed
transition from the molecule’s δ2 (1Ag) to its δδ* (1Bu) electronic
state, assuming idealized C2h symmetry for the complex. A time-
dependent density functional theory (TDDFT) calculation of the
electronic transition gave a value well in accord with the experi-
mentally observed quantity (Table S2, Supporting Information).
Furthermore, the totally symmetric metal-metal stretch, ν(Mo-
Mo), was found at the frequency of 343 cm-1 in the resonance
Raman spectrum falling in the range of frequencies between 330
and 430 cm-1 for the documented quadruply bonded dimolybdenum
species.2
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., Eds. Multiple Bonds Between
Metal Atoms, 3rd ed.; Springer Science and Business Media, Inc.: New
York, 2005.
(3) Chisholm, M. H., Ed. Early Transition Clusters with π-Donor Ligands;
VCH Publishers: New York, 1995; p 167.
(4) (a) Chisholm, M. H.; Reichert, W. W. J. Am. Chem. Soc. 1974, 96, 1249-
1251. (b) Chisholm, M. H.; Cotton, F. A.; Frenz, B. A.; Reichert, W. W.;
Shive, L. W.; Stults, B. R. J. Am. Chem. Soc. 1976, 98, 4469-4476.
(5) (a) Nguyen, T.; Sutton, A. D.; Brynda, M.; Fettinger, J. C.; Long, G. J.;
Power, P. P. Science 2005, 310, 844-847. (b) Radius, U.; Breher, F.
Angew. Chem., Int. Ed. 2006, 45, 3006-3010. (c) Brynda, M.; Gagliardi,
L.; Widmark, P.-O.; Power, P. P.; Roos, B. O. Angew. Chem., Int. Ed.
2006, 45, 3804-3807.
With the intent of gaining an understanding of the electronic
structure and bonding of 2, we carried out electronic structure
computations using density functional theory at the BLYP level. It
is noteworthy that sterically encumbered ligand variants are
necessary to stabilize 2. Attempts to model 2 via replacing Dipp
groups with H atoms failed due to the crash of the molecular shape.
Consequently, calculations on authentic formula were engaged, and
the optimized geometry was constrained to C2h. The computed Mo-
Mo bond length is 2.168 Å, which agrees very well with the
experimental Mo-Mo value of 2.1784(12) Å. As for the electronic
(6) (a) Laplaza, C. E.; Cummins, C. C. Science 1995, 268, 861-863. (b)
Laplaza, C. E.; Johnson, M. J. A.; Peters, J. C.; Odom, A. L.; Kim, E.;
Cummins, C. C.; George, G. N.; Pickering, I. J. J. Am. Chem. Soc. 1996,
118, 8623-8638. (c) Yandulov, D. V.; Schrock, R. R. J. Am. Chem. Soc.
2002, 124, 6252-6253 (d) Yandulov, D. V.; Schrock, R. R. Science 2003,
301, 76-78.
(7) Similar structures have recently been reported on dinuclear Au(I) and Cu-
(I) amidinato complexes. (a) Abdou, H. E.; Mohamed, A. A.; Fackler, J.
P., Jr. Inorg. Chem. 2005, 44, 166-168. (b) Jiang, X.; Bollinger, J. C.;
Baik, M.-H.; Lee, D. Chem. Commun. 2005, 1043-1045.
(8) Hill, M. S.; Hitchcock, P. B. Organometallics 2002, 21, 3258-3262.
(9) (a) Schulz, H.; Folting, K.; Huffman, J. C.; Streib, W. E.; Chisholm, M.
H. Inorg. Chem. 1993, 32, 6056-6066. (b) Akiyama, M.; Chisholm, M.
H.; Cotton, F. A.; Extine, M. W.; Murillo, C. A. Inorg. Chem. 1977, 16,
2407-2411.
structure, attention should be paid to dx -y and dxy orbitals13 with
2
2
(10) (a) Manke, D. R.; Loh, Z.-H.; Nocera, D. G. Inorg. Chem. 2004, 43, 3618-
3624. (b) Su, K.; Tilley, T. D. Chem. Mater. 1997, 9, 588-595. (c)
Blatchford, T. P.; Chisholm, M. H.; Huffman, J. C. Inorg. Chem. 1987,
26, 1920-1925. (d) Armstrong, W. H.; Bonitatebus, P. J., Jr. Z.
Kristallogr. New Cryst. Struct. 1999, 214, 241-242.
2
2
two Mo atoms being defined to lie on the z axis. The dx -y orbitals
of Mo atoms in 2 are used for δ-bond formation (HOMO, Figure
2
2
2a), although half of each dx -y orbital is engaged in the formation
of two Mo-to-N σ-bonds, while two dxy orbitals are used to form
four Mo-to-N π-bonds (HOMO-11, Figure 2b). This is in contrast
to the bondings between two metals in conventional paddlewheel
structural motifs in which overlap of two dxy orbitals gives rise to
(11) Albright, T. A.; Hoffmann, R. J. Am. Chem. Soc. 1978, 100, 7736-7737.
(12) Cotton, F. A.; Daniels, L. M.; Hillard, E. A.; Murillo, C. A. Inorg. Chem.
2002, 41, 2466-2470.
(13) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. ReV. 1988, 88, 899-926
and references therein.
(14) Nocera, D. G. Acc. Chem. Res. 1995, 28, 209-217.
2
2
a δ-bond in paddlewheel structures while each metal uses dx -y to
form metal-to-ligand σ-bonds. Moreover, not only does the contour
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