Breaking the 1.80 A barrier of the Cr-Cr multiple bond between CrII atoms

Article abstract of DOI:10.1002/anie.200804048

(Chemical Equation Presented) Short and sweet: By replacing one of the two guanidinate ligands in the monomeric complex [{(Me₃Si) ₂NC(NCy)₂}₂Cr] with a methyl group, the dimeric complex [{(Me₃Si)₂NC(NCy)₂CrMe}₂] (see picture) is generated. This species contains the shortest reported Cr-Cr quadruple bond. The bonding of the methyl group indicates a Cr-H-CH ₂Cr bridging agostic interaction which likely contributes to the short Cr-Cr contact.

Full text of DOI:10.1002/anie.200804048

Angewandte
Chemie
DOI: 10.1002/anie.200804048
Metal–Metal Interactions
II
À
Breaking the 1.80 ꢀ Barrier of the Cr Cr Multiple Bond Between Cr
Atoms**
Steven Horvath, Serge I. Gorelsky, Sandro Gambarotta,* and Ilia Korobkov
Since the discovery of Cr–Cr bonds of unusual shortness,[1]
this functionality has attracted considerable interest from
both the experimental and theoretical point of view.[2] The
remarkable shortness of these contacts had initially led
workers to believe that a strong Cr–Cr interaction exists in
these systems.[3] Therefore, the initial difficulties of ab initio
techniques to calculate meaningful energy profiles were
ascribed to the crudeness of the theoretical methods.[4]
However, experimental work has clearly demonstrated the
surprising weakness of these bonds.[5.6] The paradoxical
dichotomy of supershort, superweak quadruple bonds[7] can
only be reconciled if we regard the Cr–Cr interaction not as a
multiple chemical bond in its classical sense, but as a ligand
artifact instead.[8] In fact, the short contacts have been
exclusively detected, among the several dozens of Cr–Cr
“multiple bonds”, only in the presence of bridging metal–
ligand interactions, with only one exception.[9] This rule holds
true at least in complexes where the Cr–Cr distance remains
in the range of 1.90 ꢀ and higher. The recent discovery of
order (4.28)[13], also predicted a Cr–Cr distance (1.764 ꢀ)
appreciably shorter than the experimental value. Thus, we
ꢀ
investigated whether even shorter CrꢀCr bond lengths were
possible, provided that the bridging bonding interactions were
appropriately optimized. The present work was aimed at
ꢀ
testing the possibility of obtaining CrꢀCr bond lengths below
the current limit of 1.80 ꢀ.
In the search for very short contacts and following the
Hein principle[14] for the optimization of metal–ligand inter-
actions in three-center chelating geometries, two previously
described phenomena are, in our opinion, particularly infor-
mative. Firstly, a structural study carried out on a series of
chromium[15] and vanadium[16] amidinate systems has shown
that the deformation of the NCN ligand backbone, as
determined by the steric contacts between the groups
attached to the C and N atoms, determines the extent of M–
M separation and even the formation of monomeric versus
dimeric structures. Secondly, a more accurate reassessment of
the structure of [Me8Cr2Li4(thf)4],[17] initially believed to
I
I
ꢀ
À
Cr Cr formally quintuple-bonded systems of variable short-
ness (1.74–1.83 ꢀ) has marked a new milestone in this field.[10]
The monovalent state and the consequent presence of only
one counteranion makes these systems ideal for the occur-
rence of exceedingly short Cr–Cr contacts. Still the ligand
seemingly plays a decisive role in determining the existence of
short Cr–Cr contacts.[11] The divalent state remains more
incorporate a quadruple CrꢀCr bond without bridging
interactions,[18] has shown that a network of Cr-Me-Li-Me-
Cr agostic interactions is in fact responsible for holding the
dimeric structure and causing such short Cr–Cr distances.[5d,19]
In an attempt to combine these two features, we prepared
a [{(guanidinate)CrMe}2] complex. The (Me3Si)2NC(NCy)2
guanidinate monoanion has the same three-center chelating
geometry of an amidinate. As such, it is suitable for
accommodating short Cr–Cr contacts. In addition, the
ligand deformation, arising from the steric repulsion between
the trimethylsilyl (TMS) groups and the ipso H atoms of the
cyclohexyl (Cy) groups, was estimated to be only slightly
larger than that in formamidinate dimeric complexes. There-
fore, we reasoned that, should a dimeric structure still be
possible, an exceedingly short CrII–CrII distance could result.
II
II
À
challenging, and obtaining Cr Cr quadruple bond lengths
below the level of 1.87 ꢀ was traditionally regarded as an
impossible task. The ingenious use of a diimine ligand has
recently enabled the preparation of a divalent species with
only one ligand system per metal, and set a new record of
1.80 ꢀ for the CrꢀCr quadruple bond.[12] The characterization
ꢀ
of this species and its successful quantum-chemical treatment,
demonstrated that it is not only possible to achieve such short
À
À
Cr–Cr distances, but also to form a genuine quadruple Cr Cr
bond. Closed-shell density functional (DFT) calculations,
The presence of the Cr Me function was also deemed
necessary for two reasons. It allows the presence of only
one bulky guanidinate ligand per chromium center, thus
preventing prohibitive steric congestion. Also, it may play a
possible role in accommodating short Cr–Cr distances, as in
the case of [Me8Cr2Li4(thf)4].[19]
=
À
carried out on the quadruple-bonded complex [{(ArN CH
CH NAr)Cr}2], with a higher-than-expected Cr–Cr bond
=
[*] S. Horvath, Dr. S. I. Gorelsky, Prof. Dr. S. Gambarotta, I. Korobkov
Centre for Catalysis Research and Innovation
Department of Chemistry, University of Ottawa
10 Marie Curie, Ottawa ON K1N 6N5 (Canada)
Fax: (+1)613-562-5170
Accordingly, the reaction of [CrCl2(thf)2] with
(Me3Si)2NC(NCy)2Li gave the expected monomeric complex
[{(Me3Si)2NC(NCy)2}2Cr] (1), with the two anions coordinat-
ing to the Cr center with a predictable distorted square-planar
coordination geometry (Figure 1).
Unsurprisingly, the magnetic moment (meff = 4.77 mB) was
as expectd for a CrII monomeric complex in a square-planar
ligand field. An inspection of the nonbonding contacts
E-mail: sgambaro@uottawa.ca
[**] This work was supported by the Natural Science and Engineering
Council of Canada (NSERC).
Supporting information for this article (including computational
studies, the optimized structures, and other relevant DFT data) is
available on the WWW under http://dx.doi.org/10.1002/anie.
200804048.
À
between the ipso C H and the TMS groups (2.4 ꢀ) indicated
that there was indeed room for widening the bite angle of the
Angew. Chem. Int. Ed. 2008, 47, 9937 –9940
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
9937

Communications
Figure 2. ORTEP representation of 2, with thermal ellipsoids set at
50% probability and selected hydrogen atoms omitted for clarity.
À
À
Selected bond lengths [ꢀ] and angles [8]: Cr1 Cr1a 1.773(1), Cr1
À
À
À
N1 2.019(3), Cr1 C1 2.192(7), Cr1a C1 2.521(8), Cr1 C2a 2.181(7),
Cr1···H2c 1.15; C1-Cr1-Cr1a 78.2(2), N1-Cr1-Cr1a 96.51(9), C1-Cr1-
N1 91.2(4), N1-Cr1-N1a 166.0(1).
Figure 1. ORTEP representation of 1, with thermal ellipsoids set at
50% probability and hydrogen atoms omitted for clarity. Atoms labeled
with “a” are generated by the symmetry operation. Selected bond
À
À
lengths [ꢀ]: Cr N1 2.084(3), Cr N2 2.077(3).
and Me3Si groups in 2 were replaced by the Me and H3Si
groups, respectively (see Figure S1 in the Supporting Infor-
mation), yielded geometrical parameters in good agreement
with the experimental values. The calculated Cr–Cr distance
for the nontruncated model (1.782 ꢀ) is in excellent agree-
ment (0.009 ꢀ) with the experimental value (1.773 ꢀ). The
predicted Cr–Cr distance in the truncated model (1.791 ꢀ) is
only slightly longer. DFT calculations also predicted the
presence of a four-center agostic interaction and the inward
orientation of the two methyl groups. The results are
summarized in Figure 3, and show that symmetrization to
regular bridging methyl groups, as expected during the Me
exchange between the two Cr atoms, would require a
significant amount of energy (DG° = 15.3 kcalmolÀ1).
Among the possible spin states which have been used for
calculation (Figure 4), the singlet states, both open- and
closed-shell, displayed the lowest energies with a difference of
only 0.5 kcalmolÀ1 in favor of the open-shell, broken-sym-
metry configuration[20]. The lowest excited state (approxi-
mately 20 kcalmolÀ1 above the ground state), in both
truncated and nontruncated models, is a triplet state with a
slightly longer Cr–Cr bond (Figure 4). In comparison, the
similar triplet excited state of the CrII–CrII complexes with a
weaker Cr–Cr interaction lies at a much lower energy (1–
3 kcalmolÀ1), thus accounting for their ubiquitously detected
temperature-dependent paramagnetism.[2a,g] Consistent with
this indication of stronger a Cr–Cr interaction in 2, the
potential energy profile shows a well-defined minimum,
two N donor atoms of the NCN skeleton to accommodate a
dimeric structure. For that purpose, one guanidinate anion
had to be replaced by a small ligand.
Replacement of one of the two guanidinate groups by a
methyl group afforded analytically pure, diamagnetic
[{(Me3Si)2NC(NCy)2CrMe}2] (2) in crystalline form. For this
transformation, Me3Al proved to be the best alkylating agent.
The crystal structure for 2 reveals a dinuclear compound
II
II
À
incorporating the shortest Cr Cr quadruple bond reported
to date (Cr1–Cr1a 1.773(2) ꢀ), longer only than two recently
reported Cr Cr quintuple bonds.[10a,b] Two guanidinate
ligands adopt the classical three-center chelating geometry.
Each methyl group is placed in a terminal position and
disordered with equal occupancy on the two sides of each
metal (Figure 2). Although terminally bonded, the methyl
groups show a curious orientation towards the second
chromium atom, possibly suggesting the presence of a four-
I
I
À
À
center agostic, Cr–(CH2 H)–Cr interaction. Difference Four-
ier maps positioned the hydrogen atoms as expected. A half-
occupancy interstitial toluene molecule was squeezed out
from the refinement.
Variable-temperature NMR spectroscopy detected no
decoalescence of the methyl resonance at d = À0.782 ppm at
the freezing-point temperature of the solvent, indicating that,
should these agostic interactions truly exist, they do not
À
prevent fast rotation of the Cr Me bond. Different from the
vast majority of paddle-wheel Cr2 complexes, with Cr–Cr
distances above 1.9 ꢀ, we found no evidence that 2 could be
cleaved upon treatment with Lewis bases.[5,6]
À
corresponding to a Cr Cr bond energy of at least 40 kcal
molÀ1. As can be expected for the CrII dimers, the open-shell
DFT calculations (see the Supporting Information) on
both the nontruncated and truncated models, where the Cy
distances at the singlet and the nonet state (S = 4) become
À
degenerate at the long Cr Cr limit of the dimer dissociation
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À
(Figure 4). The calculated Cr Cr bond order for the ground
electronic state of 2 is 3.25. As expected, the frontier
molecular orbitals of 2 are mainly centered on Cr, with a
high level of 3d character (see Tables S1 and S2 in the
Supporting Information). The HOMO–LUMO gap (1.8 eVor
42 kcalmolÀ1) is substantial, and accounts well for the
diamagnetism. One Me group generates a four-center agostic
À
À
Cr–(CH2 H)–Cr interaction with the second Cr atom (C
À
H 1.117–1.124 ꢀ, Cr H 2.26–2.38 ꢀ, Figure 3) therefore nar-
rowing the Me-Cr-Cr bond angle to 82.28 and 78.88, in
nontruncated and truncated models, respectively (experimen-
À
tal value 78.2(2)8). The red-shifted methyl C H stretch at
2830 cmÀ1 in the IR spectrum of 2, which was predicted by the
DFT calculation to be at 2768 cmÀ1 in the truncated model,
provides evidence for this interaction. A frequency calcula-
tion for the nontruncated model was not attempted owing to
the large size of the complex.
Figure 3. The DFT optimized structures of truncated and full models
(only the Cr2Me2 fragments are shown for clarity) and their relative
energies. Of the interatomic distances [ꢀ] and bond angles [8] shown
in pairs, the upper values apply to the nontruncated model, and the
lower values to the truncated model. Bond angles are shown in italics.
For the transition state (TS) structure, the bonds and angles are only
presented for the truncated model. Arrows indicate forward and
backward movement along the potential energy surface at the TS.
In summary, by the judicious choice of ligand systems, we
II
II
À
have obtained the shortest Cr Cr multiple bond detected
to date. Interestingly, preliminary DFT calculations on the
hypothetical {(guanidinate)CrI} dimer predicted a diamag-
À
netic dinuclear complex with a Cr Cr bond length of 1.71 ꢀ.
Work towards the preparation of such a complex is currently
underway.
Experimental Section
[{(Me3Si)2NC(NCy)2}2Cr] (1): (Me3Si)2NC(NCy)2Li (786 mg, 2 mmol)
was added to a suspension of [CrCl2(thf)2] (266 mg, 1 mmol) in
toluene (20 mL). The color of the solution changed from purple to red
over 30 min. The reaction mixture was stirred at room temperature
for 24 h and centrifuged to remove LiCl. The supernatant liquid was
stored at À308C two days and the product crystallized out as red
needles (350 mg, 0.45 mmol, 45%). Crystals suitable for single-crystal
X-ray diffraction were obtained by recrystallization from hot toluene.
Additional product may be obtained by removal of solvent from the
mother liquor under reduced pressure. IR (Nujol): n˜ = 1464 , 1404,
1354, 1344, 1304, 1252, 1193, 1141, 1077, 1007, 964, 946, 886, 864,
839 cmÀ1
. Elemental analysis calcd for C38H80CrN6Si4: C 58.11,
H 10.27, N 10.70; found: C 58.09, H 10.23, N 10.68.
[{(Me3Si)2NC(NCy)2CrMe}2]·(C7H8)0.5 (2): Trimethylaluminum
(36 mg, 0.50 mmol) was added to
a solution of 1 (200 mg,
0.25 mmol) in toluene (20 mL). The solution turned dark red in
color upon addition of trimethylaluminum. The reaction mixture was
stirred at room temperature for 8 h. The resultant solution was stored
at À308C for 3 days, and complex 2 crystallized out as green blocks,
suitable for X-ray diffraction, alongside red needle-shaped crystals of
1. Crystals of 2 were isolated manually. 1H NMR ([D8]THF): d =
À
À
À0.782 (s, 6H, Cr CH3), 0.506 (s, 36H, Si CH3), 1.0–1.8 (br m,
40H, CH2), 3.972 ppm (m, 4H, CH). IR (Nujol): d = 2921, 2855, 2830,
1451, 1344, 1297, 1253, 1174, 1136, 1076, 1009, 935, 840 cmÀ1
.
Elemental analysis (after removal of the interstitial toluene) calcd
for C40H86Cr2N6Si4: C 55.38, H 9.99, N 9.69; found: C 55.03, H 9.86,
N 9.70.
CCDC 698753 (1) and 698754 (2) 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.
À
Figure 4. Potential energy surfaces (top) and Cr Cr bond orders
(bottom) for different spin states (S=0 OS: open-shell singlet, filled
circles; S=0 CS: closed-shell singlet, open circles; S=1: triplet,
triangles; S=2: pentet, squares; S=3: septet, diamonds; S=4:
nonet, hexagons) of the truncated model complex as a function of the
À
Cr Cr internuclear distance [ꢀ]. The NPA-derived atomic spin density
Received: August 15, 2008
Published online: November 10, 2008
of the broken-symmetry wave function for the open-shell singlet state
is indicated (NPA=natural population analysis). The vertical dashed
line indicates the position of the energy minimum for the ground
state.
Please note: Minor changes have been made to this manuscript since
its publication in Angewandte Chemie Early View. The Editor.
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[6] a) M. El-Kadri, M. J. Heeg, C. H. Winter, Inorg. Chem. 2006, 45,
5278; b) A. R. Sadique, M. J. Heeg, C. H. Winter, J. Am. Chem.
Soc. 2003, 125, 7774; c) S. Hao, J. J. H. Edema, S. Gambarotta, C.
Bensimon, Inorg. Chem. 1992, 31, 2676.
Keywords: agostic interactions · chromium ·
density functional calculations · metal–metal interactions ·
multiple bonds
.
[7] M. B. Hall, Polyhedron 1987, 6, 679.
[8] J. J. H. Edema, S. Gambarotta, Comments Inorg. Chem. 1991, 11,
195.
[9] J. Edema, S. Gambarotta, A. Spek, W. Smeets, Inorg. Chem.
1989, 28, 3782.
[10] a) Y. C. Tsai, C. W. Hsu, G. S. K. Yu, G. H. Lee, Y. Wang, T. S.
Kuo, Angew. Chem. 2008, 120, 7360; Angew. Chem. Int. Ed. 2008,
47, 7250; b) A. Noor, F. R. Wagner, R. Kempe, Angew. Chem.
2008, 120, 7356; Angew. Chem. Int. Ed. 2008, 47, 7246; c) G.
Frenking, Science 2005, 310, 796; d) M. Brynda, L. Gagliardi,
P. O. Widmark, P. P. Power, B. O. Roos, Angew. Chem. 2006, 118,
3888; Angew. Chem. Int. Ed. 2006, 45, 3804; e) T. Nguyen, A. D.
Sutton, S. Brynda, J. C. Fettinger, G. J. Long, P. P. Power, Science
2005, 310, 844.
[11] R. Wolf, M. Brynda, C. Ni, G. J. Long, P. P. Power, J. Am. Chem.
Soc. 2007, 129, 6076.
[12] K. A. Kerisel;, G. P. A. Yap, O. Dmitrenko, C. R. Landis, K. H.
Theopold, J. Am. Chem. Soc. 2007, 129, 14162.
[13] Our DFT calculations on this system gave a Mayer bond order
for the Cr–Cr interaction of only 3.61.
[1] For examples, see: a) E.-M. Pꢁligot, C. R. Hebd. Seances Acad.
Sci. 1844, 19, 609; b) E.-M. Pꢁligot, Ann.Chim. Phys. 1844, 12,
528; c) J. N. van Niekerk, F. R. L. Schoening, Acta Crystallogr.
1951, 4, 35; d) J. N. van Niekerk, F. R. L. Schoening, Acta
Crystallogr. 1953, 6, 501; e) J. N. van Niekerk, F. R. L. Schoening,
Nature 1953, 171, 36; f) F. A. Cotton, N. F. Curtis, C. B. Harris,
B. F. G. Johnson, S. J. Lippard, J. T. Mague, W. R. Robinson, J. S.
Wood, Science 1964, 145, 1305.
[2] a) F. A. Cotton, L. A. Murillo, R. A. Walton, Multiple Bonds
Between Metal Atoms, 3rd ed., Springer, Berlin, 2005; b) F. A.
Cotton, X. Feng, P. A. Kibala; M. Matusz, J. Am. Chem. Soc.
1988, 110, 2807; M. Matusz, J. Am. Chem. Soc. 1988, 110, 2807;
c) F. A. Cotton, S. Koch, M. Millar, Inorg. Chem. 1978, 17, 2087;
d) F. A. Cotton, S. Koch, M. Millar, Inorg. Chem. 1978, 17, 2084;
e) F. A. Cotton, S. Koch, Inorg. Chem. 1978, 17, 2021; f) A. Bino,
F. A. Cotton, W. Khaim, Inorg. Chem. 1979, 18, 3566; g) F. A.
Cotton, H. Chen, L. M. Daniels, X. Fen, J. Am. Chem. Soc. 1992,
114, 8980.
[3] a) M. Guest, C. D. Garner, I. H. Hiller, I. B. Walton, J. Chem.
Soc. Faraday Trans. 2 1978, 74, 2092; b) M. Benard, J. Am. Chem.
Soc. 1978, 100, 2354; c) F. A. Cotton, G. G. Stanley, Inorg. Chem.
1977, 16, 2671; d) R. A. Kok, M. B. Hall, Inorg. Chem. 1985, 24,
1542; e) R. D. Davy, M. B. Hall, J. Am. Chem. Soc. 1989, 111,
1268; f) M. Benard, J. Chem. Phys. 1979, 71, 2546; g) P.
Correa de Mello, W. D. Edwards, M. C. Zerner, J. Am. Chem.
Soc. 1982, 104, 1440.
[4] “Metal–Metal Multiple bonds in dinuclear clusters”: F. A.
Cotton, R. A. Walton, Struct. Bonding (Berlin) 1985, 62, 1.
[5] a) J. Edema, S. Gambarotta, F. van Bolhuis, A. L. Spek, J. Am.
Chem. Soc. 1989, 111, 2142; b) J. Edema, S. Gambarotta, A.
Meetsma, F. van Bolhuis, A. Spek, Inorg. Chem. 1990, 29, 2147;
c) J. J. Edema, S. Gambarotta, A. Meetsma, A. L. Spek, Organo-
metallics 1992, 11, 2452; d) S. Hao, J-I. Song, P. Berno, S.
Gambarotta, Organometallics 1994, 13, 1326.
[14] a) F. Hein, D. Tille, Z. Anorg. Allg. Chem. 1964, 329, 72; b) L. H.
Gade, Koordinationschemie, Wiley-VCH, Weinheim, 1998.
[15] S. Hao, S. Gambarotta, C. Bensimon, J. Edema, Inorg. Chim.
Acta 1993, 213, 65.
[16] a) S. Hao, P. Berno, R. Minhas, S. Gambarotta, Inorg. Chim. Acta
1996, 244, 37; b) P. Berno, S. Hao, R. Minhas, S. Gambarotta, J.
Am. Chem. Soc. 1994, 116, 7417.
[17] J. Krausse, G. Mark, G. Schoedl, J. Organomet. Chem. 1970, 21,
159.
[18] a) M. Benard, J. Am. Chem. Soc. 1978, 100, 2354; b) F. A.
Cotton, Chem. Soc. Rev. 1975, 4, 27; c) D. J. Brauer, C. Kruger,
Inorg. Chem. 1976, 15, 2511; d) A. P. Sattelberger, J. P. Fackler, J.
Am. Chem. Soc. 1977, 99, 1258.
[19] S. Hao, S. Gambarotta, C. Bensimon, J. Am. Chem. Soc. 1992,
114, 3556.
[20] L. Noodleman, D. A. Case, Adv. Inorg. Chem. 1992, 38, 423.
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