The shortest metal-metal bond yet: Molecular and electronic structure of a dinuclear chromium diazadiene complex

Article abstract of DOI:10.1021/ja076356t

Reduction of a diazadiene chromium halide complex, [(^HL^iPr)Cr(μ-Cl)]₂ (1, ^HL^iPr = bis(2,6-diisopropylphenyl)diazadiene), with KC₈ gave a diamagnetic, bimetallic complex, (^HL^iPr)₂Cr₂ (2). Complex 2 has been structurally characterized by X-ray crystallography and consists of a Cr₂ unit spanned by two diazadiene ligands. The very short Cr-Cr distance (1.8028(9) A) and low formal oxidation state of the Cr atoms suggest that the Cr-Cr bond order may be greater than 4. Spin-restricted and spin-unrestricted DFT calculations on a model complex both converge on a singlet spin state and accurately reproduce the metric parameters of 2. The calculations show high-order metal-metal bonding along with extensive delocalization over the diazadiene ligands. Natural resonance theory analysis shows population of resonance structures that contain quintuple bonds, and natural bond order analysis gives a bond order of 4.28 for the Cr-Cr bond. Copyright

Full text of DOI:10.1021/ja076356t

Published on Web 10/30/2007
The Shortest Metal-Metal Bond Yet: Molecular and Electronic Structure of a
Dinuclear Chromium Diazadiene Complex
Kevin A. Kreisel,‡,§ Glenn P. A. Yap,‡ Olga Dmitrenko,‡ Clark R. Landis,*,§ and Klaus H. Theopold*,‡
Department of Chemistry and Biochemistry, UniVersity of Delaware, Newark, Delaware 19716, and Department of
Chemistry, UniVersity of WisconsinsMadison, Madison, Wisconsin 53706
Received August 23, 2007; E-mail: landis@chem.wisc.edu; theopold@udel.edu
Dinuclear chromium complexes have played a special role in
the history of multiple metal-metal bonding,1 featuring “supershort”
quadruple bonds (dM-M < 1.9 Å) of questionable strength.2 Recent
synthetic work by Power et al. and computational investigations
by others have even broached the notion of metal-metal bond
orders exceeding 4, resulting from extremely sparing coordination
of low-valent metals (i.e., those with d5 and d6 configurations).3
For various reasons, we have been exploring the chemistry of
chromium coordinated by diazadienes (or R-diimines). In the course
of these investigations, we have discovered a dinuclear compound
that exhibits a very short chromium-chromium distance. Indeed,
in terms of Cotton’s “formal shortness ratio” (FSR), this compound
features the shortest chemical bond presently known (FSR(2) )
0.760, compare with FSR(N2) ) 0.786).2a
The chemistry of this system began with the synthesis of dark
green [(HLiPr)Cr(µ-Cl)]2 (1, HLiPr ) N,N′-bis(2,6-diisopropylphenyl)-
1,4-diazadiene) by slow addition of a THF solution of Na2[HLiPr]
to CrCl3(THF)3 according to Scheme 1 (see Supporting Information
for full characterization). 1 is an antiferromagnetically coupled
dimer (µeff(300 K) ) 3.4(1) µB per Cr, J ) -17 cm-1) with a Cr-
Figure 1. The molecular structure of 2 (30% probability level). Selected
interatomic distances (Å) and angles (deg): Cr(1)-Cr(1A), 1.8028(9); Cr-
(1)-N(1), 1.914(2); Cr(1)-N(2), 1.913(2); N(1)-C(13), 1.373(3); N(2)-
C(26), 1.362(3); C(13)-C(13A), 1.345(5); C(26)-C(26A), 1.354(5); N(1)-
Cr distance of 3.431(1) Å. Reduction of 1 with 2 equiv of KC8 in
Cr(1)-N(2), 150.58(9); N(1)-Cr(1)-Cr(1A), 104.78(6); N(2)-Cr(1)-
Cr(1A), 104.63(6); N(1)-Cr(1)-Cr(1A)-N(1A), 17.74(6); N(2)-Cr(1)-
Cr(1A)-N(2A), 15.82(6).
1
Et2O gave a green solution after stirring overnight. A H NMR
spectrum in toluene-d8 of the product after workup revealed a
diamagnetic spectrum with sharp resonances between 0 and 8 ppm,
which showed negligible temperature dependence of the chemical
shifts from 0 to 70 °C. Crystallization from a dilute Et2O solution
at -30 °C gave dichroic, red/green plates of (µ-η2-HLiPr)2Cr2 (2)
that were characterized by X-ray diffraction (see Figure 1). The
geometry around each chromium atom in 2 can best be described
as trigonal planar with each metal being coordinated by two N atoms
from two different diazadiene ligands as well as by the neighboring
Cr atom. The Cr-N distances are unexceptional at 1.913(2) and
1.914(2) Å with a N(1)-Cr(1)-N(2) angle of 150.58(9)°. The most
striking feature of 2 is the extremely short Cr-Cr distance of
1.8028(9) Å, making it the shortest metal-metal distance reported
to date.4
Scheme 1. Preparation of [(HLiPr)Cr(µ-Cl)]2 (1)
CH3 resonances are accidentally degenerate and show no significant
temperature dependence.
Because of the redox ambiguity of diimine and other imine
containing ligands,5 the Cr atoms in 2 could formally be assigned
as Cr(II) coordinated by dianionic enediamide ligands, Cr(I) with
monoanionic ligand-centered radical ligands, or Cr(0) with neutral
diimine ligands. The long C-N distances (1.373(3) and 1.362(3)
Å) and short C-C distances (1.345(5) and 1.354(5) Å) are
consistent with a reduced diimine ligand.6
For a more complete description of the electronic structure of 2,
DFT calculations were performed at the BLYP/6-311g level using
a model complex where the 2,6-diisopropylphenyl substituents were
replaced by hydrogen atoms (i.e., 2′).7 Geometry optimizations on
the model complex give bond distances that are in good agreement
with the X-ray structure in Figure 1 (see structure A in Figure 2).
Furthermore, spin-restricted and spin-unrestricted calculations sug-
gest a closed shell singlet ground state, consistent with the observed
diamagnetic nature of 2 (vide supra). One shortcoming of the
computational model is that it is completely planar, whereas the
Due to the nonplanar structure of the N4Cr2 core, chiral 2 is
expected to exhibit resonances of four unique methyl groups and
two unique methine protons associated with the isopropyl groups
in its 1H NMR spectrum. However, at room temperature, only two
i
iPr-CH3 and one CH Pr-CH resonances were observed. This is
presumably due to a low barrier, fluxional process whereby a twist
about the metal-metal bond interconverts the two enantiomers on
1
1
the H NMR time scale. Accordingly, low-temperature H NMR
spectra in the -70 to 25 °C temperature range showed a decoa-
lescence phenomenon resulting in three methyl resonances (in a
1:1:2 ratio) and two methine signals. Presumably, two of the iPr-
§ University of WisconsinsMadison.
‡ University of Delaware.
9
14162
J. AM. CHEM. SOC. 2007, 129, 14162-14163
10.1021/ja076356t CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
The presence of five occupied molecular orbitals with Cr-Cr
bonding character raises the question of metal-metal quintuple
bonding. Complexity arises from the highly delocalized nature of
the HOMO-2 δ-like orbital. To gain a more nuanced perspective
on the issue of effective bond order, natural resonance theory (NRT)
analysis was performed.9,10 As expected for a system that bears
strong delocalization (to the extent of resembling a dimetallanaph-
thalene), multiple resonance configurations are occupied, some
having Cr-Cr quintuple bonds. The effective bond orders are
illustrated in Figure 2 (B). This analysis gives slightly higher than
4-fold bonding, 4.28, which is less than the value of 4.64 previously
computed for trans-bent HCrCrH.8
Figure 2. Bond lengths (Å) for 2 (bold) and 2′ (italics) (A) and effective
bond orders from NRT analysis of 2′ (B).
In conclusion, we have prepared a bimetallic chromium complex
that features the shortest metal-metal distance measured to date.
DFT analysis of its electronic structure indicates high-order metal-
metal bonding. Although the highly delocalized nature of the
bonding precludes description using a single Lewis structure only,
NRT and NBO analyses indicate some degree of quintuple bonding.
Further exploration (both experimental and computational) of
molecules with metal-metal bonds of high bond order is currently
underway in these laboratories.
Acknowledgment. This research was supported by a grant from
the National Science Foundation (Grant No. CHE-0616375 to
K.H.T.).
Supporting Information Available: Synthesis, characterization,
and crystallographic details for 1 and 2 as well as the computational
details for 2′ and 2 (ONIOM). This material is available free of charge
via the Internet at http://pubs.acs.org.
References
(1) (a) Peligot, E.-M. C.R. Acad. Sci. 1844, 19, 609. (b) Peligot, E.-M. Ann.
Chim. Phys. 1844, 12, 528. (c) van Niekerk, J. N.; Schoening, F. R. L.
Acta Crystallogr. 1951, 4, 35. (d) van Niekerk, J. N.; Schoening, F. R. L.
Acta Crystallogr. 1953, 6, 501. (e) van Niekerk, J. N.; Schoening, F. R.
L. Nature 1953, 171, 36. (f) 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.
(2) (a) Cotton, F. A.; Murillo, L. A.; Walton, R. A. Multiple Bonds Between
Metal Atoms, 3rd ed.; Springer: Berlin, 2005. (b) Edema, J. J. H.;
Gambarotta, S. Comments Inorg. Chem. 1991, 11, 195.
(3) (a) Nguyen, T.; Sutton, A. D.; Brynda, M.; Fettinger, J. C.; Long, J. G.;
Power, P. P. Science 2005, 310, 844. (b) Brynda, M.; Gagliardi, L.;
Widmark, P.-O.; Power, P. P.; Roos, B. O. Angew. Chem., Int. Ed. 2006,
45, 3804. (c) Gagliardi, L. Nature 2005, 433, 848. (d) Roos, B. O. Collect.
Czech. Chem. Commun. 2003, 68, 265.
(4) (a) A search of the Cambridge Structural Database gave no hits for M-M
distances < 1.81 Å: Allen, F. H. Acta Crystallogr. 2002, B58, 380. (b)
Cotton, F. A.; Koch, S. A.; Millar, M. Inorg. Chem. 1978, 17, 2084.
(5) (a) van Koten, G.; Vrieze, K. AdV. Organomet. Chem. 1982, 21, 151. (b)
Bart, S. C.; Chlopek, K.; Bill, E.; Bouwkamp, M. W.; Lobkovsky, E.;
Neese, F.; Wieghardt, K.; Chirik, P. J. J. Am. Chem. Soc. 2006, 128, 13901.
(6) (a) Muresan, N.; Chlopek, K.; Weyhermu¨ller, T.; Neese, F.; Wieghardt,
K. Inorg. Chem. 2007, 46, 5327. (b) Gardiner, M. G.; Hanson, G. R.;
Henderson, M. J.; Lee, F. C.; Raston, C. L. Inorg. Chem. 1994, 33, 2456.
(7) The calculations were preformed using Gaussian 03 software: Frisch, M.
J.; et al. Gaussian 03, revision B.05; Gaussian, Inc.: Pittsburg, PA, 2003
(see Supporting Information for complete citation).
Figure 3. A representation of the frontier molecular orbitals of 2′.
N4Cr2 core in 2 is not. This deviation is presumably due to the
sterically demanding 2,6-diisopropylphenyl groups that are absent
in 2′. Accordingly, an ONIOM optimization of the entire molecule
resulted in an N-Cr-Cr-N angle of 12.2°, reasonably close to
the average of the two measured angles in 2 (16.8°) and a Cr-Cr
distance of 1.790 Å.
The calculations showed that the HOMO of 2′ is mostly ligand
based and corresponds to an orbital of the diimine ligand that is
both C-C π-bonding and C-N π-antibonding (see Figure 3).
HOMO-1 through HOMO-5 show considerable metal-metal bond-
ing, whereas the LUMO, LUMO+1, and LUMO+2 appear to be
metal-metal antibonding orbitals (see Supporting Information).
Cr-Cr σ-bonding character is displayed by HOMO-3, whereas
HOMO-4 and HOMO-5 exhibit dπ-bond character. The fourth Cr-
Cr bonding interaction (HOMO-1) comprises a combination of sd
hybrids oriented such that the main hybrid orbital axes are parallel
to one another (such a bonding mode has previously been termed
a side-on sd-πδ bond).8 The fifth Cr-Cr bonding interaction
(HOMO-2) is highly delocalized but otherwise takes the form of a
dδ metal-metal bond. Natural bond orbital (NBO) and natural
localized molecular orbital (NLMO) analyses of the electron density
mirror the major features of the Cr-Cr interactions determined from
the canonical MO’s.9
(8) Landis, C. R.; Weinhold, F. J. Am. Chem. Soc. 2006, 128, 7335.
(9) Glendening, E. D.; Badenhoop, J. K.; Reed, A. E.; Carpenter, J. E.;
Bohmann, J. A.; Morales, C. M.; Weinhold, F. NBO 5.0, version g;
Theoretical Chemistry Institute, University of Wisconsin: Madison, WI,
2001.
(10) Weinhold, F.; Landis, C. Valency and Bonding: A Natural Bond Order
Donor-Acceptor PerspectiVe; Cambridge University Press: Cambridge,
2005; p 32.
JA076356T
9
J. AM. CHEM. SOC. VOL. 129, NO. 46, 2007 14163