Remarkably short metal-metal bonds: A lantern-type quintuply bonded dichromium(I) complex

Article abstract of DOI:10.1002/anie.200801286

(Chemical Equation Presented) The shortest metal-metal bond so far is found in complex anion 2 (Cr-Cr 1.7397(9) A), which is the one-electron reduction product of thermally stable mixed-valent complex 1 (see scheme). Experimental data unambiguously show that the electronic configuration of 1 is σ²π⁴δ³, and theoretical studies confirm the presence of a Cr-Cr quintuple bond in 2. Ar=2,6-C₆H ₃(CH₃)₂.

Full text of DOI:10.1002/anie.200801286

Communications
DOI: 10.1002/anie.200801286
Quintuple Bonds
Remarkably Short Metal–Metal Bonds: A Lantern-Type Quintuply
Bonded Dichromium(I) Complex**
Yi-Chou Tsai,* Chia-Wei Hsu, Jen-Shiang K. Yu, Gene-Hsiang Lee, Yu Wang, and
Ting-Shen Kuo
In memory of F. Albert Cotton
The field of quadruply bonded dinuclear complexes in which
two metal atoms are embraced by eight ligands has been
considered mature. The bonding and electronic structures of
these compounds have been well understood,[1–4] ever since
the discovery of the first dimetal species containing a
quadruple bond, [Re2Cl8]2À, over 40 years ago.[5] The quest
for thermally stable and isolable dinuclear complexes with
higher bond orders is one aim of chemists in this field. From
the synthetic point of view, quintuply bonded dinuclear
complexes have become the focus in the past few years. On
the theoretical side, the hypothetical RMMR molecules (M =
Cr, Mo, W, U; R = H, F, Cl, Br, CN, Me) have been
highlighted, particularly the trans-bent structures. These
feature a quintuple bond between two metal atoms, and for
RCrCrR, the calculated Cr–Cr bond lengths are in the range
diene) was recently reported to feature a very short Cr–Cr
distance of 1.8028(9) Š and calculated to display some degree
of fivefold bonding.[15]
We have been interested in the pursuit of low-coordinate
and multiply bonded dinuclear complexes since our first
report on the unconventional quadruply bonded dimolybde-
num complex [Mo2{m-h2-(DippN)2SiMe2}2] (Dipp = 2,6-
iPrC6H3), in which each Mo atom is ligated by only two
nitrogen atoms.[16] Accordingly, we were interested in the
preparation of multiply bonded dinuclear complexes sup-
ported by ancillary ligands which can minimize the metal–
ligand p-bonding interaction and maximize the metal–metal
interaction. Inspired by the hypothetical eclipsing molecule
M2L6, proposed by Hoffmann et al.,[17] in which M M could
À
be a quintuple bond, we set out to explore the possibility of
synthesizing such complexes. Here we report the use of
of 1.64–1.78 Š.[6–11] Experimentally,
a recent landmark
advance was the isolation of a quintuply bonded dichro-
mium(I) complex supported by two monodentate bulky
carbyl ligands [Ar’CrCrAr’] (Ar’ = C6H3-2,6-(C6H3-2,6-
iPr2)2),[12] in which, coincidentally, a trans-bent (C2h) geometry
is adopted and the two chromium atoms share five electron
amidinate ligand ArXylNC(H)NArXyl (ArXyl = 2,6-C6H3-
3+
(CH3)2) to stabilize mixed-valent Cr2
complex [Cr2-
(ArXylNC(H)NArXyl)3] (2) with formal Cr Cr bond order of
4.5 and its one-electron reduced Cr2
À
2+
species [Cr2-
(ArXylNC(H)NArXyl)3]À (3 with formal Cr Cr bond order of
À
À
pairs in five bonding molecular orbitals according to compu-
5.0). Both of these species have very short Cr Cr bonds; the
14]
tational analyses.[13,
Moreover, D2h-symmetric [Cr2(m-h2-
bonding in these two compounds was studied theoretically.
Reduction of dichromium bis(amidinate) dichlorido com-
plex [{Cr(thf)}2(m-Cl)2{m-h2-(ArXylNC(H)NArXyl)}2] (1),[18] in
which the Cr–Cr distance is 2.612(1) Š, with KC8 gave red-
brown mixed-valent dichromium tris(amidinate) compound 2
in 47% yield (Scheme 1). Compound 2 is paramagnetic, as
HLiPr)2]
(HLiPr = N,N’-bis(2,6-diisopropylphenyl)-1,4-diaza-
[*] Prof. Dr. Y.-C. Tsai, C.-W. Hsu
Department of Chemistry
National Tsing Hua University, Hsinchu 30013 (Taiwan)
Fax: (+886)3-571-1082
E-mail: yictsai@mx.nthu.edu.tw
Prof. Dr. J.-S. K. Yu
Institute of Bioinformatics and Department of Biological Science
and Technology
National Chiao Tung University, Hsinchu, 30010 (Taiwan)
Dr. G.-H. Lee, Prof. Dr. Y. Wang
Department of Chemistry
National Taiwan University, Taipei 10617 (Taiwan)
T.-S. Kuo
Department of Chemistry
National Taiwan Normal University, Taipei 11677 (Taiwan)
[**] Y.C.T. and J.S.K.Y. are indebted to the National Science Council,
Taiwan for support under Grant NSC 96-2113M-007-019-MY3 and
NSC 97-2113M-009-001-MY2, respectively. We thank Prof. Roald
Hoffmann and Dr. Ramadas Balakumar (NTHU) for useful
discussions. The computational facility is supported by NCTU
under the grant from MoE ATU Plan.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200801286.
Scheme 1. Synthesis of 2 and 3.
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evidenced by its solid-state magnetic moment of about
2.21 mB, and accordingly has one unpaired electron. The
electronic absorption spectrum of 2 displays a strong absorp-
tion at 250 nm arising from the ligands and two absorptions at
389 and 572 nm with e = 2055 and 2730 molÀ1 LcmÀ1.
is perpendicular to the z axis, which is clearly consistent with
the anisotropic EPRspectra of 2. Fivefold bonding between
two Cr atoms, arising from a s2p4d3 configuration for the Cr2
core is consequently implied. The bonding scheme of 2 was
further corroborated by unrestricted DFT calculations
(BP86), and the metal–metal orbital surfaces are shown in
Figure S2 (Supporting Information). The symmetries of
SOMO and SOMOÀ1, mainly dx2Ày2 + dx2Ày2 and dxy + dxy,
correspond to d bonds. The SOMOÀ2, dz2 + dz2, corresponds
to Cr–Cr s bonding, while SOMOÀ3 and SOMOÀ5, dxz + dxz
The solid-state molecular structure of compound 2
(Figure 1) was deciphered by X-ray crystallography.[19] In
À
and dyz + dyz, are Cr Cr p bonds. The nondegeneracy of
SOMO and SOMOÀ1 is consistent with the Jahn–Teller
structural distortion observed in complex 2.[23]
The cyclic voltammogram of a solution of 2 (THF/
tetrabutylammonium phosphate) shows a reversible reduc-
tion wave at E1/2 = À2.14 V (relative to Fc/Fc+) over the
course of the cathodic sweep. Accordingly, reduction of THF
solutions of 2 with KC8 in the presence of 4,7,13,16,21,24-
hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane
(crypt[222])
gave extremely air- and moisture-sensitive dark purple
species K(crypt[222])[Cr2{ArXylNC(H)NArXyl}3] (K(crypt-
[222])[3]) in 83% yield. After workup the product showed a
diamagnetic NMRspectrum in [D 8]THF with sharp resonan-
1
ces between d = 0 and 8 ppm for H, and d = 0 and 230 ppm
for 13C. The electronic absorption spectrum of 3 has a strong
absorption at 250 nm and a broad absorption at 488 nm, with
e = 3200 molÀ1 LcmÀ1.
The solid-state molecular structure of 3 (Figure 2) was
elucidated by X-ray crystallography.[19] In comparison with
the distorted structure of 2, the lantern structure is still
conserved, except that it displays a more symmetrical D3-
symmetric structure (Figure 2b). The most remarkable fea-
ture of 3 is that the Cr–Cr bond contracts significantly to a
distance of 1.7397(9) Š, a difference of about 0.08 Š when
compared to 2. The FSRof the Cr–Cr bond in 3 is 0.733. The
magnitude of this change suggests that the additional electron
resides in the dx2Ày2- or dxy-based molecular orbital if the z axis
is coincident with the Cr–Cr bond, and therefore the dimetal
core has a s2p4d4 configuration. The increase in average Cr–N
distance from 2.044(3) in 2 to 2.092(3) Š in 3 on one-electron
reduction indicates substantially weaker interactions between
Cr and ligands compared to quadruply bonded
Figure 1. Solid-state structure of [Cr2(ArXylNC(H)NArXyl)3] (2) viewed
a) normal to and b) along the metal–metal axis with thermal ellipsoids
at 30% probability.
contrast to the D3h-symmetric trigonal paddlewheel Fe and
Co amidinates,[20,21] in which all degenerate molecular orbitals
are evenly occupied, 2 is substantially distorted from D3
symmetry (Figure 1b), as evidenced by significant deviation
from trigonal geometry at both Cr atoms with N-Cr1-N bond
angles of 106.96(10), 123.15(11), and 125.53(11)8, and N-Cr2-
N
bond angles of 107.57(10), 126.00(10), 122.73(11)8.
Cr2(amidinate)4 complexes,[4] which, as
a consequence,
Although contributions from packing forces can not be
completely ignored,[20] the structural distortion of 2 is
essentially attributed to the Jahn–Teller effect[22] (vide
infra). The metal–metal bond length of 1.8169(7) Š for 2 is
significantly shorter than the Cr–Cr quintuple-bond length of
1.8351(4) Š of complex [Ar’CrCrAr’].[12] Complex 2 has a
formal shortness ratio (FSR) of 0.766 (cf. FSR(N2) = 0.786).[4]
Since 2 has one unpaired electron, we subjected it to a
detailed EPRstudy. In toluene 2 is EPR-silent at room
temperature, but axial symmetry displayed at low temper-
ature (77 K) suggests that the unpaired electron resides in an
orbital perpendicular to the z axis (coincident with the Cr–Cr
axis), namely, dx2Ày2 or dxy (Figure S2, Supporting Informa-
tion). In fact, DFT calculations on complex 2 reproduced the
geometrical features of the solid-state structure of 2. The
calculated SOMO has mostly metal dx2Ày2 character and hence
enhance the overlap of dx2Ày2 and dx2Ày2, and dxy and dxy, to
form two d bonds. The extremely short Cr–Cr bond of 3 is the
shortest known metal–metal bond in an isolable compound. It
is also the first metal–metal bond shorter than 1.8 Š, and it
lies in the range of those of hypothetical quintuply bonded
Cr–Cr complexes.[11]
We examined the nature of the bonding in 3 by DFT
calculations (BP86). The geometry of the optimized structure
was in close agreement with the crystal structure of 3.
Considerable metal–metal bonding can be found from
HOMO to HOMOÀ4 (Figure 3). Orbital HOMOÀ2 corre-
sponds the Cr–Cr s bond (dz2 + dz2), while the degenerate
HOMOÀ3 and HOMOÀ4 display Cr–Cr p-bonding inter-
actions (dxz + dxz and dyz + dyz). Almost degenerate Cr–Cr d-
bonding character is apparently displayed by HOMO (dx2Ày2
+
d
x2Ày2) and HOMOÀ1 (dxy+dxy). In contrast to the significant
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Figure 2. Molecular structure of the anion in K(crypt[222])[3]·3THF
viewed a) normal to and b) along the metal–metal axis with thermal
ellipsoids at 30% probability. The cation, THF solvate, and hydrogen
atoms have been omitted for clarity.
Cr–N p bonding in [Cr2(m-h2-HLiPr)2],[15] no p bonding is
observed between dx2Ày2 and dxy orbitals of Cr atoms and N
donors. These observations suggest two strong Cr–Cr d bonds
and consequently an extremely short Cr–Cr bond.
Figure 3. Frontier orbitals of 3.
metal bond length of 3 is associated with the largest Cr-Cr-L
bond angle found so far.
It could be argued that the very short Cr–Cr distances of 2
and 3 are due to the bridging nature of the amidinate ligand,
which pushes the Cr atoms close together. However, the
reported amidinate-bridged paddlewheel Cr2 compounds
have Cr–Cr bond lengths that range from 1.844(2) to
2.612(1) Š.[4,18,24–28] Accordingly, the amidinate ligands can
accommodate Cr–Cr distances that vary by almost 1 Š. As a
result, we conclude that the extremely short Cr–Cr distances
in 3 is a consequence of interactions of the d5 Cr atoms, and
not due to the constraint of the ligands; thus, the five bonding
orbitals (1 s, 2 p, and 2 d) of 3 are filled with five pairs of
electrons.
In summary, we have shown that associating two Cr atoms
with three formamidinate ligands allows the isolation of a
lantern-type dichromium compound with unambiguous quin-
tuple bonding. We anticipate that the discovery of the shortest
metal–metal bond may aid the development of systematic
syntheses of more compounds with very short metal–metal
bonds. Reactivity studies on complexes 2 and 3, in which each
Cr atom is ligated by three nitrogen donors, are also
underway.
Experimental Section
Experimental details for the synthesis and characterization of and
computations on complexes 2 and 3 are provided in the Supporting
Information.
Mota et al.[29] proposed a pyramidality effect in quadruply
bonded Cr2 complexes, whereby the Cr–Cr distance is
correlated to the average pyramidality angle (Cr-Cr-L).
They explained the pyramidality effect in terms of major
changes in hybridization affecting the metal–metal s and p
bonding. Accordingly, the pyramidality angles of compounds
2 and 3 would predict Cr–Cr distances of 1.808 and 1.740 Š,
remarkably close to the experimental values of 1.817 and
1.740 Š, respectively. Given the weak d bonding, it seems
reasonable to assume that quadruple and quintuple bonds
have essentially the same behavior, and the shortest metal–
Received: March 17, 2008
Revised: May 5, 2008
Published online: August 6, 2008
Please note: Minor changes have been made to this manuscript since
its publication in Angewandte Chemie Early View. The Editor
Keywords: chromium · density functional calculations ·
.
metal–metal interactions · multiple bonds · N ligands
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Chemie
space group P21/c, a = 23.1962(4), b = 26.1563(6), c =
26.4577(6) Š, b = 97.2449(12)8, V= 15924.4(6) Š3, Z = 8,
1calcd = 1.243 MgmÀ3 m = 0.385 mmÀ1
73852 reflections col-
lected, 27954 independent reflections (Rint = 0.1078), final R
indices [I > 2s(I)]: R1 = 0.0596, wR2 = 0.1136, R indices (all
data): R1 = 0.1693, wR2 = 0.1468. CCDC-675770 (2·Et2O) and
-675771 (K(crypt[222])[3]·3THF) contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
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[19] Crystallographic data for 2·Et2O: C55H67N6OCr2, Mr = 932.15,
T= 150(2) K, monoclinic, space group P21/n, a = 15.8060(6), b =
17.3373(9), c = 18.8494(8) Š, b = 106.617(2)8, V= 4949.6(4) Š3,
Z = 4, 1calcd = 1.251 MgmÀ3, m = 0.484 mmÀ1, 20355 reflections
collected, 8686 independent reflections (Rint = 0.0696), final R
indices [I > 2s(I)]: R1 = 0.0475, wR2 = 0.1005, R indices (all
data): R1 = 0.1021, wR2 = 0.1182. K(crypt[222])[3]·3THF:
C81H117N8KO9Cr2, Mr = 1489.93, T= 150(2) K, monoclinic,
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