Quintuply-bonded dichromium(I) complexes featuring metal-metal bond lengths of 1.74 A

Article abstract of DOI:10.1002/anie.200803859

Closer than most: A series of four dichromium bis(amidinate) complexes of the type [Cr₂{μ-η²-ArNC(R)NAr}₂] display an extremely short metal-metal bond of 1.74 A, independent of the steric hindrance of the ligands. DFT calculations of Cr-Cr bond lengths agree well with the experimental values and computational analysis of their electronic structures indicates quintuple bonding between the two Cr centers. (Chemical Equation Presented)

Full text of DOI:10.1002/anie.200803859

Angewandte
Chemie
DOI: 10.1002/anie.200803859
Quintuple Bonds
Quintuply-Bonded Dichromium(I) Complexes Featuring Metal–Metal
Bond Lengths of 1.74 ꢀ**
Chia-Wei Hsu, Jen-Shiang K. Yu, Chun-Hsu Yen, Gene-Hsiang Lee, Yu Wang, and
Yi-Chou Tsai*
The construction of a metal–metal quintuple bond has long
been a challenge for chemists, since a large number of
quadruple-bonded dinuclear complexes have been reported
and their bonding and electronic structures extensively
investigated and well understood.[1–4] On the basis of theo-
retical work,[5–10] many possible structures could be capable of
accommodating a metal–metal quintuple bond, in contrast to
the strict requirement of having two metal atoms embraced by
eight ligands in a tetragonal geometry for a metal–metal
quadruple bond.[1] Of particular interest is that all of these
model structures display a common feature: a low-coordinate
environment around metal centers. From a practical point of
view, the first quintuple-bonded dichromium complex [Ar’Cr-
exhibited the shortest metal–metal bond length of
1.7397(9) ꢀ.[16] Qualitatively, the latter is believed to incor-
À
porate a Cr Cr quintuple bond. In view of Powerꢁs and
Theopoldꢁs complexes, wherein both CrI centers were coor-
dinated by two donor atoms, we set out to prepare dichro-
mium bis(amidinato) complexes, from which metal–metal
quintuple bonds are expected.[17] Herein we report a series of
complexes of the form [Cr2{m-h2-ArNC(R)NAr}2], which all
À
exhibit very short Cr Cr quintuple-bond lengths of approx-
imately 1.74 ꢀ. Amidinate ligands, featuring substituents of
À
different bulk, are used to stabilize these Cr Cr quintuple
bonds.
Prior to the synthesis of the target molecules, four green
mononuclear complexes, 1a–d, of the form [CrCl2(thf)2{h2-
ArNC(R)NAr}] were prepared in good yields by treatment of
[CrCl3(thf)3] or CrCl3 with lithiated amidines (Scheme 1, see
À
CrAr’]
(Ar’ = C6H3-2,6-(C6H3-2,6-iPr2)2,
Cr Cr=
1.8351(4) ꢀ), which adopts a trans-bent geometry, was
reported by Power and co-workers in 2005.[11–13] More
recently, in 2007, Theopold and co-workers reported an
interesting dichromium complex supported by a-diimines,
[Cr2(m-h2-{C(H)N(C6H3-2,6-iPr2)}2)2], which was shown by
À
computations to exhibit some degree of Cr Cr quintuple-
bond character.[14]
Since our first report on the characterization of an
unconventional quadruple-bonded dimolybdenum complex
[Mo2{m-h2-(DippN)2SiMe2}2], where each Mo atom is ligated
by only two nitrogen donors,[15] we have been interested in the
pursuit of low-coordinate and multiply-bonded dinuclear
complexes. We recently characterized a mixed-valent dichro-
mium complex stabilized by three amidinate ligands, [Cr2-
{ArXylNC(H)NArXyl}3] (ArXyl = 2,6-C6H3(CH3)2), and its one-
electron reduction partner [Cr2{ArXylNC(H)NArXyl}3]À, which
[*] C.-W. Hsu, C.-H. Yen, Prof. Dr. Y.-C. Tsai
Department of Chemistry, National Tsing Hua University
Hsinchu 30013 (Taiwan)
Fax: (+886)3571-1082
E-mail: yictsai@mx.nthu.edu.tw
Scheme 1. Synthesis of complexes [CrCl2(thf)2{h2-ArNC(R)NAr}] (1a–d)
and [Cr2{m-h2-ArNC(R)NAr}2] (2a–d).
Prof. Dr. J.-S. K. Yu
Institute of Bioinformatics and Systems Biology and Department of
Biological Science and Technology
National Chiao Tung University, Hsinchu, 30010 (Taiwan)
the Supporting Information for experimental details). Com-
plexes 1a–d all display paramagnetism, and each of them has
three unpaired electrons. The molecular structure of complex
1c was determined by X-ray crystallography.[18] The Cr atom
in 1c adopts a distorted octahedral geometry with two
chloride ligands arranged in a trans orientation (see the
Supporting Information, Figure S1). Subsequent reduction of
1a–d with two equivalents of KC8 in THF elicited a quick
color change to orange brown or reddish purple. After
workup, two products were detected by 1H NMR spectrosco-
Dr. G.-H. Lee, Prof. Dr. Y. Wang
Department of Chemistry, National Taiwan University
Taipei 10617 (Taiwan)
[**] J.-S.K.Y. and Y.-C.T. 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. The computational
facility is supported by NCTU under the grant from MoE ATU Plan.
Supporting Information for this article, including experimental
details and analytical data, is available on the WWW under http://
dx.doi.org/10.1002/anie.200803859.
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py. One diamagnetic species (the major product) gave sharp
coplanar, with N-Cr-Cr-N torsion angles of either 08 or 1808.
In this bridging mode, metric parameters of the ligands are
similar to those when the ligands chelate a metal center.
resonance signals in the range d = 0–8 ppm and solid state
SQUID measurements < 1.0 BM.[19] The minor product was a
paramagnetic species, speculated to be the mononuclear
complex [Cr{h2-ArNC(R)NAr}2]. The major product of each
reduction was isolated by recrystallization of the mixture at
À358C or room temperature in appropriate hydrocarbon
solvents, to give moderate yields of orange-brown crystals of
quintuple-bonded compounds of the form [Cr2{m-h2-
ArNC(R)NAr}2] (2a–d, Scheme 1). The UV/Vis spectra of
complexes 2a–d display almost identical absorption profiles
in which four absorption bands are detected in the visible
region, indicating that complexes 2a–d have similar molec-
ular structures.
À
À
Furthermore, the C1 N1 and C1 N2 bond lengths of the
three-atom ligand backbone are 1.324(3) and 1.320(3) ꢀ,
À
respectively, indicating regular amidinato ligands. The Cr N
bond lengths of 2.000(2) and 1.999(2) ꢀ are slightly shorter
than those dichromium complexes bearing bridging amidi-
nates.[1]
As is the case for most multiply-bonded metal–metal
dimers, the most interesting metric parameter is the metal–
metal bond length. The value of 1.7404(8) ꢀ for 2a is close to
the shortest reported metal–metal bond length (1.7397(9) ꢀ)
of the complex [Cr2{ArXylNC(H)NArXyl}3]À (ArXyl = 2,6-C6H3-
(CH3)2).[16] In terms of Cottonꢁs “formal shortness ratio”
(FSR), complex 2a has a FSR of 0.733, whereas the FSR of
dinitrogen is 0.786.[1]
The solid-state molecular structure of complex 2a was
deciphered by X-ray crystallography (Figure 1a).[18] Com-
À
We then investigated whether the extremely short Cr Cr
distance of 2a is a consequence of the bridging fashion of the
amidinato ligands, which push two chromium atoms inward
and close together. Interestingly, a survey of the literature
À
reveals that the Cr Cr distances in paddlewheel complexes
supported by amidinates are greater than 1.84 ꢀ.[1,20–25] Cr Cr
À
separations would be expected to shorten as the steric
repulsion between R and Ar substituents of the ligands
increases (Figure 2). To further examine the relationship
Figure 2. a) bridging and b) chelating modes of amidinates.
À
between Cr Cr bond lengths and steric hindrance of the
bridging amidinates, three complexes 2b–d with increasing
bulk of ligands were prepared and their molecular structures
were elucidated by X-ray crystallography. Interestingly, com-
plexes 2a–c (Figure 1a and Figures S2 and S3 in the
Supporting Information) feature a planar skeleton, whereas
the core structure of 2d (Figure 1b) is nonplanar, with torsion
angles of 171.128 (N2-Cr2-Cr1-N3) and 156.058 (C29-N3-Cr1-
Cr2), as a result of interligand steric repulsion between
À
adjacent 2,6-iPr2C6H3 substituents. The Cr N bond lengths of
complexes 2a–d were all roughly equal (approximately 2 ꢀ).
Figure 1. Molecular structures of 2a (a) and 2d (b) with thermal
ellipsoids set at 30% probability. Hydrogen atoms have been omitted
for clarity.
À
Surprisingly, Cr Cr separations in compounds 2b–d and 2a
are also roughly equal, at 1.7454(1), 1.7472(10), and
1.7395(7) ꢀ, respectively. These measurements suggest that
À
the Cr Cr bond lengths of dichromium bis(amidinato)
complexes are independent of the steric constraints imposed
pound 2a incorporates two amidinato ligands spanning the
metal–metal bond. The solid-state structure exhibits approx-
imate C2h symmetry, which is consistent with the single set of
À À
by the three-atom N C N bridges.
The geometries of the theoretically optimized structures
were found to be in good agreement with those of the crystal
1
alkyl resonances in the H NMR spectrum. Interestingly, the
À À
À
two Cr centers, the two three-atom N C N backbones, and
the four ipso carbon atoms of the phenyl rings are completely
structures of 2a–d. Notably, the calculated Cr Cr bond
lengths, 1.7216 ꢀ (2a), 1.7230 ꢀ (2b), 1.7250 ꢀ (2c), and
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Chemie
À
Figure 3. Frontier orbitals displaying Cr Cr bonding in 2b.
1.7188 ꢀ (2d), are very close to the experimental values. The
bonding schemes of 2a–d were further corroborated by
unrestricted density functional (DFT) calculations
(UBP86).[26] Taking 2b as a representative example, the
calculations showed that considerable metal–metal bonding
character can be found through HOMO to HOMOÀ3, and
HOMOÀ7 (Figure 3), whereas metal–metal antibonding
orbitals (d*) are displayed by LUMO and LUMO + 1 (see
the Supporting Information), and ligand-based orbitals can be
found from HOMOÀ4 to HOMOÀ6. HOMOÀ2 corresponds
quintuple-bonded
dichromium
complexes
[Cr2-
{ArXylNC(H)NArXyl}3] and [Cr2{ArXylNC(H)NArXyl}3]À,[16]
presumably as a result of the weak d-bonding. For complexes
À
2a–d, the Cr Cr bond lengths are thus predicted to be 1.7409,
1.7486, 1.7451, and 1.7735 ꢀ, respectively, amazingly close to
the experimental values. Notably, the pyramidality effect
suggests that 2d has the longest metal–metal separation
among complexes 2a–d. This discrepancy is presumably
ascribed to the nonplanar core structure of 2d. The above
results substantiate the fact that quadruple and quintuple
bonds have essentially the same behavior.
À
to the Cr Cr s bond (dz2 + dz2), and HOMOÀ3 (dxz + dxz) and
À
HOMOÀ7 (dyz + dyz) display the Cr Cr dp bonding interac-
In summary, we have demonstrated a systematic prepa-
ration and characterizations of four dichromium complexes
stabilized by two amidinate ligands incorporating various
À
tions. Two Cr Cr d bonding characters are displayed by
HOMO (dxy + dxy) and HOMOÀ1. HOMOÀ1 is a side-on sd
d bond,[14,15,27] which results from hybridization of s (24.6%)
and dx2Ày2 (75.4%), oriented such that the main hybrid orbital
axes are parallel to one another. In contrast to the significant
À
bulky substituents, and from which Cr Cr quintuple bonds
with extremely short metal–metal separations of approxi-
mately 1.74 ꢀ are identified. Measurements for these remark-
ably short metal–metal bonds also show good agreement with
À
Cr N p bonding (HOMOÀ2) in Theopoldꢁs complex [Cr2(m-
h2-{C(H)N(C6H3-2,6-iPr2)}2)2],[14] there is no p bonding
computed Cr Cr quintuple bond lengths. We anticipate that
À
between Cr atoms and N donors. As a result, complexes
this stimulating report may trigger extensive exploration of
metal–metal multiple bonds with higher bond order. Exten-
sive reactivity studies of complexes 2a–d are currently
underway in this laboratory.
À
2a–d have stronger d bonding and, consequently, shorter Cr
Cr bond lengths.
À
Accordingly, we conclude that the extremely short Cr Cr
distances in 2a–d are a consequence of strong interactions
between two d5 CrI centers, rather than the constraint of the
ligands. Thus, the five bonding orbitals of 2a–d (1 s, 2 p, and
2 d orbitals) are filled with five pairs of electrons.
Received: August 5, 2008
Published online: November 17, 2008
Mota et al.[28] proposed that, in quadruply bonded dinu-
clear complexes, the metal–metal distance is correlated to the
average metal–metal–ligand pyramidality angle, which has
been termed pyramidality effect. We have recently demon-
strated that the pyramidality effect also works very well in the
Keywords: chromium · density functional calculations · metal–
metal interactions · multiple bonds · N ligands
.
Angew. Chem. Int. Ed. 2008, 47, 9933 –9936
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
9935

Communications
0.1515; R indices (all data): R1 = 0.0766, wR2 = 0.1686. 2b:
C42H54N4Cr2: Mr = 718.89; T= 200(2) K; triclinic; space group
P1, a = 9.6922(4), b = 10.0189(4), c = 10.8279(4) ꢀ; a =
[1] F. A. Cotton, C. A. Murillo, R. A. Walton, Multiple Bonds
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Between Metal Atoms, 3rd ed., Springer, Berlin, 2005.
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6, 705.
73.440(2), b = 83.428(2), g = 73.584(2)8; V= 966.09(7) ꢀ3; Z =
1; 1calcd = 1.236 MgmÀ3; m = 0.596 mmÀ1; reflections collected:
10851; independent reflections: 3384 (Rint = 0.0531); final R
indices [I > 2s(I)]: R1 = 0.0568, wR2 = 0.1556; R indices (all
data): R1 = 0.0727, wR2 = 0.1658. 2c: C50H70N4Cr2: Mr = 831.10;
T= 150(2) K; monoclinic; space group C2/c, a = 46.9512(4), b =
[5] F. Weinhold, C. R. Landis, Chem. Educ. Res. Pract. 2001, 2, 91.
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[17] A similar quintuple-bonded dichromium complex, supported by
two sterically encumbered amidopyridines, was reported online
by Kempe et al. seven days after we submitted this manuscript
for publication. A. Noor, F. R. Wagner, R. Kempe, Angew.
Chem. 2008, 120, 7356; Angew. Chem. Int. Ed. 2008, 47, 7246.
[18] Crystallographic data for 1c: C33H51Cl2N2O2Cr, Mr = 630.66; T=
200(2) K; monoclinic; space group C2/c, a = 17.6101(4), b =
10.3920(1),
c = 21.5457(2) ꢀ;
b = 115.7385(5)8;
V=
9469.51(15) ꢀ3; Z = 8; 1calcd = 1.166 MgmÀ3
;
m = 0.495 mmÀ1
;
reflections collected: 29262; independent reflections: 10817
(Rint = 0.0371); final R indices [I > 2s(I)]: R1 = 0.0464, wR2 =
0.1266; R indices (all data): R1 = 0.0729, wR2 = 0.1379. 2d:
C52H74N4Cr2: Crystals of 2d contained a minor component of
mononuclear species [Cr(h2-{C(CH3)N(C6H3-2,6-iPr2)}2)2], so a
disordered model of two molecules was made. The percentage of
the two molecules is 95% and 5%, respectively; site occupancy
factors (sof) of the Cr1, Cr2, and Cr3 atoms are, accordingly,
0.95, 0.95, and 0.1, respectively. Mr = 853.95; T= 150(2) K;
monoclinic; space group P21/c, a = 14.4343(8), b = 17.6732(10),
c = 20.4374(12) ꢀ; b = 106.330(1)8; V= 5003.3(5) ꢀ3; Z = 4;
1calcd = 1.134 MgmÀ3
; ; reflections collected:
m = 0.450 mmÀ1
30132; independent reflections: 8808 (Rint = 0.0553); final R
indices [I > 2s(I)]: R1 = 0.0587, wR2 = 0.1338; R indices (all
data): R1 = 0.0780, wR2 = 0.1461. CCDC 694478 (1c), 694479
(2a), 695882 (2b), 694480 (2c), and 694481 (2d) contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[19] These deviations from diamagnetism are due to the presence of a
trace amount of [Cr{h2-ArNC(R)NAr}2], which is difficult to
completely remove from [Cr2{h2-ArNC(R)NAr}2].
[20] F. A. Cotton, L. M. Daniels, C. A. Murillo, P. Schooler, J. Chem.
Soc. Dalton Trans. 2008, 2007.
[21] F. A. Cotton, T. Ren, J. Am. Chem. Soc. 1992, 114, 2237.
[22] F. A. Cotton, C. A. Murillo, I. Pascual, Inorg. Chem. 1999, 38,
2182.
8.9222(2),
c = 22.4384(6) ꢀ;
b = 108.3720(10)8;
V=
[23] S. Hao, S. Gambarotta, C. Bensimon, J. J. H. Edema, Inorg.
Chim. Acta 1999, 213, 65.
3345.84(14) ꢀ3; Z = 4; 1calcd = 1.252 MgmÀ3
;
m = 0.532 mmÀ1
;
reflections measured: 13450 ; independent reflections: 3037
(Rint = 0.0992); final R indices [I > 2s(I)]: R1 = 0.0672, wR2 =
0.1806; R indices (all data): R1 = 0.0858, wR2 = 0.1987. 2a·0.5 n-
hexane: C41H53N4Cr2, Mr = 705.87; T= 150(2) K; trigonal; space
[24] F. A. Cotton, C. A. Murillo, I. Pascual, Inorg. Chem. Commun.
1999, 2, 101.
[25] F. A. Cotton, L. M. Daniels, C. A. Murillo, P. Schooler, J. Chem.
Soc. Dalton Trans. 1999, 2001.
¯
group R3c, a = 33.8181(5), b = 33.8181(5), c = 19.1232(2) ꢀ; V=
[26] See the Supporting Information.
18940.4(4) ꢀ3; Z = 18; 1calcd = 1.114MgmÀ3 m = 0.546 mmÀ1
;
;
[27] C. R. Landis, F. Weinhold, J. Am. Chem. Soc. 2006, 128, 7335.
[28] F. Mota, J. J. Novoa, J. Losada, S. Alvarez, R. Hoffmann, J.
Silvestre, J. Am. Chem. Soc. 1993, 115, 6216.
reflections collected: 30207; independent reflections: 4836
(Rint = 0.0411); final R indices [I > 2s(I)]: R1 = 0.0484, wR2 =
9936 www.angewandte.org
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Angew. Chem. Int. Ed. 2008, 47, 9933 –9936