A diamagnetic dititanium(III) paddlewheel complex with no direct metal-metal bond

Article abstract of DOI:10.1021/ic0602650

Reaction of Ti[N(Bu^t)Ar]₃ (Ar = 3,5-C ₆H₃Me₂ or Ar′ = C₆H ₅) with CO₂ at -40°C produces diamagmetic Ti ^III paddlewheel complexes with long Ti-Ti separations (>3.4 A), thus excluding direct Ti-Ti bonding. ¹H NMR spectroscopy shows that the compounds are diamagnetic in solution in the temperature range of -65 to +70°C. In the solid state, the diamagnetism was found to persist between 2 and 300 K. Calculations at the density functional theory level suggest that the diamagnetism results from antiferromagnetic coupling by superexchange through the ligand π system.

Full text of DOI:10.1021/ic0602650

Inorg. Chem. 2006, 45, 4328−4330
A Diamagnetic Dititanium(III) Paddlewheel Complex with No Direct
Metal Metal Bond
−
Arjun Mendiratta,† Christopher C. Cummins,*,† F. Albert Cotton,*,‡ Sergey A. Ibragimov,‡
Carlos A. Murillo,*,‡ and Dino Villagra´n‡
Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts AVenue,
Cambridge, Massachusetts 02139-4307, and Laboratory for Molecular Structure and Bonding,
and Department of Chemistry, Texas A&M UniVersity, P.O. Box 30012,
College Station, Texas 77842-3012
Received February 15, 2006
Reaction of Ti[N(But)Ar]3 (Ar
CO2 at 40
C produces diamagmetic TiIII paddlewheel complexes
with long Ti Ti separations (>3.4 Å), thus excluding direct Ti Ti
bonding. 1H NMR spectroscopy shows that the compounds are
diamagnetic in solution in the temperature range of 65 to 70
C. In the solid state, the diamagnetism was found to persist
) 3,5-C6H3Me2 or Ar′ ) C6H5) with
−
°
−
−
−
+
°
between 2 and 300 K. Calculations at the density functional theory
level suggest that the diamagnetism results from antiferromagnetic
coupling by superexchange through the ligand
π system.
While the insertion of CO2 into metal-amide bonds is a
well-known reaction leading to metal carbamate systems,1-4
complexes containing three of the sterically demanding
-N(But)Ar ancillary ligands, where Ar ) 3,5-C6H3Me2, have
not been shown previously to absorb CO2. Accordingly, we
have proposed that the odd-electron CO2 adduct (CO2)Ti-
[N(But)Ar]3 is generated upon exposure of the titanium(III)
trisamide Ti[N(But)Ar]3 to 1 equiv of CO2 at low temperature
in an ether solution.5,6 Carrying out a similar experiment in
the presence of the benzonitrile adduct (PhCN)Mo[N(But)-
Ar]3,7,8 employed as a radical trap, led to heterobimetallic
Figure 1. Structure of one of two crystallographically independent Ti2[µ2-
O2CN(But)Ph]4(N(But)Ph)2 molecules in 1b‚Et2O. Each molecule has an
inversion center. Displacement ellipsoids are shown at the 50% probability
level. Selected interatomic distances (Å) and angles (deg): Ti1-Ti1′,
3.515(1); Ti1-O1, 2.041(2); Ti1-N3, 1.920(3); N3-Ti1-O1, 105.7(1).
reductive cross-coupling of CO2 with PhCN.5 Now we report
9,10
that extended treatment (2.5 h) of Ti[N(But)Ar]3
with
excess CO2 (4 equiv) in Et2O at -40 °C produces a lime-
green precipitate, which has been identified as a diamagnetic
dititanium(III) paddlewheel complex Ti2[µ-O2CN(But)-3,5-
C6H3Me2]4(N(But)-3,5-C6H3Me2)2 (1a).
In the context of metal-metal bonding,11 dinuclear d1-
d1 systems of the paddlewheel variety have been elusive.
Compound 1a was not readily obtained as single crystals;
therefore, the analogue Ti2[µ-O2CN(But)Ph]4(N(But)Ph)2 (1b)
was prepared similarly from Ti[N(But)Ph]310 and crystallized
and its structure determined crystallographically (Figure 1).
There are two crystallographically independent molecules,
each possessing an inversion center. The cores are shown
* To whom correspondence should be addressed. E-mail: ccummins@
mit.edu (C.C.C.), cotton@tamu.edu (F.A.C.), murillo@tamu.edu (C.A.M.).
† Massachusetts Institute of Technology.
‡ Texas A&M University.
(1) Lappert, M. F.; Power, P. P.; Sanger, A. R.; Srivastava, R. C. Metal
and Metalloid Amides. Syntheses, Structures, and Physical and
Chemical Properties; Ellis Horwood Ltd.: Chichester, U.K., 1980.
(2) Darensbourg, D. J.; Frost, B. J.; Larkins, D. L. Inorg. Chem. 2001,
40, 1993-1999.
(3) Chisholm, M. H.; Extine, M. W. J. Am. Chem. Soc. 1977, 99, 782-
792.
(4) Chisholm, M. H.; Extine, M. W. J. Am. Chem. Soc. 1977, 99, 792-
802.
(5) Mendiratta, A.; Cummins, C. C. Inorg. Chem. 2005, 44, 7319-7321.
(6) It should be noted that CO2 insertion is probably catalyzed by the
free amine produced by hydrolysis, as has been reported in other
instances. See, for example, ref 4.
(7) Tsai, Y. C.; Stephens, F. H.; Meyer, K.; Mendiratta, A.; Gheorghiu,
M. D.; Cummins, C. C. Organometallics 2003, 22, 2902-2913.
(8) Mendiratta, A.; Cummins, C. C.; Kryatova, O. P.; Rybak-Akimova,
E. V.; McDonough, J. E.; Hoff, C. D. Inorg. Chem. 2003, 42, 8621-
8623.
(9) Wanandi, P. W.; Davis, W. M.; Cummins, C. C.; Russell, M. A.;
Wilcox, D. E. J. Am. Chem. Soc. 1995, 117, 2110-2111.
(10) Peters, J. C.; Johnson, A. R.; Odom, A. L.; Wanandi, P. W.; Davis,
W. M.; Cummins, C. C. J. Am. Chem. Soc. 1996, 118, 10175-10188.
(11) Cotton, F. A.; Murillo, C. A.; Walton, R. A. Multiple Bonds Between
Metal Atoms; Springer Science and Business Media, Inc.: New York,
2005.
4328 Inorganic Chemistry, Vol. 45, No. 11, 2006
10.1021/ic0602650 CCC: $33.50
© 2006 American Chemical Society
Published on Web 05/05/2006

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Figure 2. Overlay of the core structures of the two crystallographically
independent molecules in 1b‚Et2O. In red is the molecule containing Ti1
[Ti1-Ti1′ ) 3.515(1) Å], and in blue is the molecule containing Ti2 [Ti2-
Ti2′ ) 3.426(1) Å].
as an overlay in Figure 2. The conformer drawn in blue and
containing Ti2 shows an unsymmetrical bridging mode for
its carbamate ligands, reminiscent of a carboxylate shift,12
and its Ti2-Ti2′ interatomic distance is shorter by ca. 0.1
Å than is the Ti1-Ti1′ distance in the conformer drawn in
red, which displays greater symmetry in its carbamate
bridges. This paddlewheel system is both surprising and
interesting because it combines diamagnetism with an
intertitanium distance that surely precludes direct metal-
metal bonding [3.515(1) and 3.426(1) Å for two crystallo-
graphically independent molecules]. We provide here a
detailed account of the synthesis, solution behavior, and
bonding in this unusual dititanium(III) paddlewheel system.
The syntheses of the new lime-green titanium(III) paddle-
wheel compounds are straightforward, involving simple
exposure of the trisamide precursor molecules Ti[N(But)-
Ar]3 or Ti[N(But)Ph]3 to ca. 4 equiv of CO2 at -40 °C in
Et2O. After removal of the solvent, the solid was easily
Figure 3. 1H NMR spectra for 1a. The signals marked with asterisks are
due to protons in the methyl groups of the solvent toluene-d8 (2.089 ppm)
and in the free amine [HN(But)-3,5-C6H3Me2] arising from a small amount
of hydrolysis (singlets at 1.187 and 2.192 ppm).
structure is retained in solution. At room temperature (Figure
3b), the signals have broad bases and relatively sharp tops.
The two pairs of singlets observed at 70 °C have collapsed
into signals centered at 2.42 and 1.42 ppm. Because of the
3:2 ratio for the peak at 1.42 ppm to the peak at 2.42 ppm,
these signals can be assigned to the methyl groups located
on the tert-butyl goups [-C(CH3)3] and those in the aromatic
group (-C6H3Me2). As the temperature was then lowered
in 10 °C steps to -65 °C, the spectra became more and more
complex. As shown in Figure 3c, in the spectrum at the
lowest temperature to which we can safely cool the probe
(-65 °C), most of the signals are sharp. However, there are
still a few broad signals, indicating that a fluxional process
is still occurring. We propose that the molecule is fluxional
and that slower rotation around the Ti-N, N-C6H3Me2, and
N-But σ bonds at low temperatures accounts for the
observations, but we have not attempted to perform a detailed
analysis. However, it is evident that the compound is
diamagnetic,13 over the temperature range of -65 to
+70 °C.
1
purified by washing with pentane and then ether. The H
NMR spectrum at ambient temperature reveals broad reso-
nances for 1a and 1b, and it is complex, especially in the
aromatic region. To determine if the broadness of the signals
is due to 1 being paramagnetic, a variable-temperature study
with a 300-MHz spectrometer was undertaken. Selected
spectra in the high-field region where signals from the methyl
groups appear are shown in Figure 3. At a temperature of
70 °C (Figure 3a), there is a pair of singlets at 2.32 and 2.37
ppm in a 2:1 ratio. There is also a singlet at 1.32 ppm and
a broad signal at 1.55 ppm, again in a 2:1 intensity ratio.
Because the number of bridging groups is twice the number
of terminal ligands in Ti2[µ-O2CN(But)-3,5-C6H3Me2]4-
(N(But)-3,5-C6H3Me2)2, the integration ratios are consistent
with the solid-state structure, and we presume that this
To explain why the compound is diamagnetic despite the
long Ti-Ti distance, density functional theory (DFT)
calculations were performed on a simplified model.14 Three
different spin states were investigated. The first was a singlet
(S ) 0) model that assumes the presence of a metal-metal
bond; the second was a triplet state (S ) 1) that describes a
(13) Magnetic susceptibility measurements of 1a, using a SQUID instru-
ment, in the temperature range of 2-300 K confirmed that the
compound is diamagnetic.
(14) The DFT model was simplified to expedite the calculations by
replacing all of the aryl groups with a hydrogen atom; similar
simplifications in other systems have proved to be reasonable.
(12) Rardin, R. L.; Tolman, W. B.; Lippard, S. J. New J. Chem. 1991, 15,
417-430.
Inorganic Chemistry, Vol. 45, No. 11, 2006 4329

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ferromagnetic interaction between the Ti d1 electrons; the
third model described an antiferromagnetically coupled
unrestricted singlet state (broken symmetry). The latter gave
the lowest energy among the three models and yielded an
exchange coupling constant15 of -1266 cm-1 by Yamagu-
chi’s method.16,17 This low value is consistent with the
magnetic studies.13 The electronic structure of the anti-
ferromagnetically coupled model shows that the R and â
electrons in the HOMO are each localized on different Ti
atoms,18 as shown in Figure 4. Also, a significant amount
of ligand character is found in both the R and â HOMOs.19
This strong antiferromagnetic coupling takes place by
superexchange through the ligand π system, in a manner
similar to that in Cu2(CH3COO)4[H2O]2,20 and as in that case,
no direct metal-metal bond between the metal atoms is
present.21
Figure 4. Illustrations of the 0.04 surface contour diagrams for the R and
â HOMOs for the model of 1.
(15) The magnetic exchange coupling constant, 2J, is based on the
Heisenberg-Dirac-van Vleck spin Hamiltonian H ) -2JS1S2.
Acknowledgment. C.C.C. thanks the U.S. National
Science Foundation for funding via Grant CHE-0316823,
and F.A.C. and C.A.M. thank the Department of Chemistry,
Texas A&M University (TAMU), and the Welch Foundation.
We also thank C. R. Clough for assistance with crystal-
lography. We are grateful to the Laboratory for Molecular
Simulation at TAMU for hardware and software and the
TAMU supercomputer facility.
(16) Yamaguchi’s relationship is provided by the following equation: J )
EBS - EHS/ S2 HS
-
S2 BS, where EBS and EHS represent the total
energy values of the broken-symmetry and high-spin DFT calculations,
respectively. See: Soda, T.; Kitagawa, Y.; Onishi, T.; Takano, Y.;
Shigeta, Y.; Nagao, H.; Yoshioka, Y.; Yamaguchi, K. Chem. Phys.
Lett. 2000, 319, 223-230.
(17) The calculated antiferromagnetic exchange coupling constant, 2J, is
significantly more negative than the conventional 2J value (-800
cm-1) used for diamagnetic compounds. See: Crutchley, R. J. AdV.
Inorg. Chem. 1994, 41, 273-325.
(18) In R HOMO: Ti1, 80.0%; Ti2, 0.6%. In â HOMO: Ti1, 0.6%; Ti2,
80.0%.
(19) Ligand character in R and â HOMOs: 19.4%.
Supporting Information Available: Experimental details of
syntheses and H NMR analyses, a plot of øT vs T for 1a in the
solid state, and X-ray crystallographic files in CIF format for 1b‚
Et2O. This material is available free of charge via the Internet at
http://pubs.acs.org.
(20) Molecular Magnetism; Kahn, O., Ed.; VCH: New York, 1993.
(21) It should be noted that there are a few nonpaddlewheel compounds
containing Ti24+ units that are diamagnetic and for which Ti-Ti bonds
have been suggested. For example, see: (a) Cotton, F. A.; Wojtczak,
W. A. Gazz. Chim. Ital. 1993, 123, 499-507. (b) Hao, S.; Fegali, K.;
Gambarotta, S. Inorg. Chem. 1997, 36, 1745-1748. (c) Utko, J.;
Pryzbylak, S.; Jersykiewicz, L. B.; Mierzwicki, K.; Latajka, Z.; Sobota,
P. Inorg. Chem. 2003, 42, 267-269.
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