Mono(dinitrogen) and carbon monoxide adducts of bis(cyclopentadienyl) titanium sandwiches

Article abstract of DOI:10.1021/ja061213c

The silyl-substituted titanocene complex, (η⁵-C₅Me₄SiMe₂Ph)₂Ti, coordinates dinitrogen upon cooling to -35 °C to yield an unprecedented example of a mono(dinitrogen) complex of a substituted bis(cyclopentadienyl) titanium compound, (η⁵-C₅Me₄SiMe₂Ph)₂Ti(N₂). Analogous monocarbonyl derivatives, (η⁵-C₅Me₄R)₂Ti(CO) (R = SiMe₃, SiMe₂Ph, CHMe₂), have been prepared by mixing the dicarbonyl compounds with the corresponding sandwiches. Both (η⁵-C₅Me₄SiMe₂Ph)₂Ti(N₂) and (η⁵-C₅Me₄SiMe₂Ph)₂Ti(CO) have been characterized by X-ray diffraction, and mixed N₂-CO titanocene complexes have also been observed by in situ IR spectroscopy. Copyright

Full text of DOI:10.1021/ja061213c

Published on Web 04/15/2006
Mono(dinitrogen) and Carbon Monoxide Adducts of Bis(cyclopentadienyl)
Titanium Sandwiches
Tamara E. Hanna, Emil Lobkovsky, and Paul J. Chirik*
Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell UniVersity, Ithaca, New York 14853
Received February 20, 2006; E-mail: pc92@cornell.edu
The unique sandwich structure of ferrocene has inspired the
preparation of related bis(cyclopentadienyl) complexes with transi-
tion metals from across the periodic table.1 Introduction of sterically
demanding cyclopentadienyl rings by Lawless and Mach has
provided the first examples of the long sought after titanium
sandwiches, (η5-C5Me4R)2Ti (R ) alkyl, silyl),2 allowing explora-
tion of the electronic structure and reactivity of these unique
molecules. Investigations from our laboratory have demonstrated
that many of these compounds coordinate dinitrogen at low
temperature, providing monomeric (η5-C5Me4R)2Ti(N2)2 com-
pounds.3 Here we describe a new entry into the coordination
chemistry of group 4 sandwich compounds with the synthesis and
characterization of thermally stable mono(dinitrogen) and carbonyl
adducts of substituted bis(cyclopentadienyl) titanium derivatives
along with mixed dinitrogen-carbon monoxide compounds.
We previously observed that cooling (η5-C5Me4SiMe3)2Ti to -45
°C produced a single new NtN stretch centered at 1983 cm-1 in
the solution infrared spectrum. This band was tentatively assigned
to the mono(dinitrogen) compound, (η5-C5Me4SiMe3)2Ti(N2) (eq
1).3 Warming the sample above this temperature resulted in N2 loss,
complicating both the isolation and further characterization of this
unique compound. Alkyl-substituted titanium sandwiches, such as
(η5-C5Me4CHMe2)2Ti (3), do not yield observable mono(dinitrogen)
complexes upon cooling, forming only the bis(dinitrogen) complex,
(η5-C5Me4CHMe2)2Ti(N2)2 (3-(N2)2), at -35 °C. As a result,
titanocenes bearing more sterically demanding and electron-
withdrawing silyl substituents were prepared with the goal of
isolating the mono(dinitrogen) compound.
corresponding bis(dinitrogen) compound, 2-(N2)2 (νN2 ) 2003, 2096
cm-1), indicated a more activated N2 ligand in the former, consistent
with increased back-bonding when only one π-acid ligand is present.
The isolation and crystallographic characterization of 2-(N2)
prompted exploration into the synthesis of related titanocene
monocarbonyl derivatives. Unlike the dicarbonyl compounds, which
have been known for decades,4 titanocene monocarbonyls have
eluded isolation despite being implicated as intermediates in both
ligand substitution5 and catalytic reactions.6 The parent compound,
(η5-C5H5)2Ti(CO) (νCO ) 1890 cm-1), has only been observed by
IR spectroscopy in an argon matrix at 12 K7 and examples of mono
CO adducts of titanium fulvalene compounds are known.8 Isolobal
(η5-C5Me5)2Si(N2) (νN2 ) 2046 cm-1) and (η5-C5Me5)2Si(CO) (νCO
) 2065 cm-1) have also been observed in situ in liquid Xe.9
Addition of a stoichiometric quantity of CO gas to 2 furnished
the desired monocarbonyl complex, 2-CO, although the product
was contaminated with significant amounts of 2-(CO)2 and unre-
acted 2. An improved synthetic method has been devised whereby
an equimolar mixture of 2 and 2-(CO)2 was mixed for 18 h and
formed 2-CO as the predominant product (eq 2).
In contrast to the dinitrogen compound, 2-CO is isolable at 23
°C. The solid-state structure (Figure 1) is similar to that of 2-N2
with idealized C2 symmetry. The Ti(1)-C(10) and C(10)-O(1)
bond lengths of 1.9786(19) and 1.151(2) Å, respectively, are
comparable to the values of 2.0065(18) and 1.150(2) Å found in
1-(CO)2. The synthetic procedure used to isolate 2-CO appears
general as 1-CO, and the alkyl-substituted compound, 3-CO, were
prepared by mixing the sandwich and the corresponding dicarbonyl
(eq 2). For these examples, isolation of the titanocene monocar-
bonyls in pure form has proven difficult as the product is in
equilibrium with the starting materials. Equilibrium constants for
each reaction were determined at 23 °C by 1H NMR spectroscopy
and are reported in eq 2. In general, introduction of larger, more
electron-withdrawing silyl substituents into the cyclopentadienyl
ligands favors the monocarbonyl more than in the purely alkylated
case.
Cooling a pentane solution of (η5-C5Me4SiMe2Ph)2Ti (2) to -35
°C produced a single NtN infrared stretch centered at 1976 cm-1
.
Notably, bands for the bis(dinitrogen) complex, 2-(N2)2, were not
observed until the solution was cooled to -60 °C. In preparative
experiments, reduction of (η5-C5Me4SiMe2Ph)2TiCl with excess
0.5% Na(Hg) followed by filtration and recrystallization from
pentane at -35 °C under an atmosphere of N2 furnished red-purple
1
dichroic crystals of 2-N2 in 83% yield. In addition to H NMR
spectroscopy, diamagnetic 2-N2 was characterized by single-crystal
X-ray diffraction (Figure 1). In the solid state, the molecule is C2
symmetric with the principal axis containing the Ti-N2 bonds. The
Ti(1)-N(1) and N(1)-N(2) distances of 2.016(1) and 1.119(2) Å
are consistent with a weakly activated dinitrogen ligand and
comparable to the distances found in 3-(N2)2.3 Comparison of the
IR stretching frequencies of 2-(N2) (νN2 ) 1976 cm-1) to the
To probe the pathway of formation for the titanocene monocar-
bonyl compounds, a series of isotopic labeling studies were
conducted. Treatment of 3-(13CO)2 with 1 atm of CO produced
isotopic exchange over the course of hours, furnishing a near
statistical mixture of the isotopologs of 3-(CO)2 and free 13CO as
judged by 13C NMR spectroscopy. Mixing equimolar quantities of
2-(13CO)2 and 3-(CO)2 also resulted in isotopic exchange, forming
9
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J. AM. CHEM. SOC. 2006, 128, 6018-6019
10.1021/ja061213c CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Figure 2. Mixed titanocene N2-CO compounds and detection of 3-(N2)-
(CO) by IR spectroscopy.
Figure 1. Molecular structures of 2-N2 (left) and 2-CO (right) at 30%
probability ellipsoids with top views shown below. Hydrogen atoms and
silyl substituents in the top views are omitted for clarity.
control the nature of N2 coordination by careful manipulation of
cyclopentadienyl substituents.11 In the present case, introduction
of extreme steric bulk has opened a new chapter in group 4
metallocene dinitrogen and carbonyl chemistry with the isolation
of unique examples of mono(dinitrogen) and carbonyl compounds
that were computationally predicted nearly 3 decades ago.
a near statistical mixture of labeled and natural abundance titanocene
dicarbonyl compounds. The converse experiment, mixing 3-(13CO)2
and 2-(CO)2, produced the same outcome. These results, in
combination with previously reported kinetic data for substitution
of (η5-C5H5)2Ti(CO)2 with 13CO,5 are consistent with a pathway
whereby the monocarbonyl derivatives are formed by initial CO
dissociation from the dicarbonyl compounds followed by trapping
of the free ligand by the titanium sandwich.
Acknowledgment. We thank the National Science Foundation
and the Director, Office of Basic Energy Sciences, Chemical
Sciences Division, of the U.S. Department of Energy (DE-FG02-
05ER15659) for financial support. P.J.C. is a Cottrell Scholar
sponsored by the Research Corporation and a David and Lucile
Packard Fellow in science and engineering. We also thank the
Collum group for access to a React IR spectrometer.
As originally predicted by Lauher and Hoffmann,10 the HOMO
of the monocarbonyl (and likewise the mono(dinitrogen)) derivative
is an essentially linear combination of a titanocene b2 molecular
orbital with the in-plane π* orbital of the CO ligand (Figure S7).
Despite an energetically accessible LUMO (∆EHOMO/LUMO ) 20-
22 kcal) of principally titanocene 1a1 character, addition of
σ-ligands, such as PMe3, tetrahydrothiophene, and N,N-dimethyl-
aminopyridine, to either 2-CO or 3-CO produced no change in
Supporting Information Available: Experimental procedures, DFT
results, and crystallographic data for 2-N2, 2-CO, and 1-(CO)2. This
material is available free of charge via the Internet at http://pubs.acs.org.
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