COMMUNICATION
Coordination chemistry of the bis(trifluoromethylsulfonyl)imide anion:
molecular interactions in room temperature ionic liquids
D. Bridget Williams,{a Michael E. Stoll,a Brian L. Scott,b David A. Costaa and Warren J. Oldham, Jr.*b
Received (in Cambridge, UK) 4th November 2004, Accepted 23rd December 2004
First published as an Advance Article on the web 21st January 2005
DOI: 10.1039/b416830e
displays n(CO) bands in the solution phase IR spectrum shifted to
significantly higher energy compared to the parent chloride
complex, consistent with the weak donor ability of the 2NTf2
anion.12,13
Room temperature ionic liquids composed of bis(trifluoro-
methylsulfonyl)imide anions and 1,3-ethylmethylimidazolium
(EMI) cations are shown to stabilize monomeric ligand
deficient transition metal complexes via four distinct binding
modes: monodentate nitrogen or oxygen coordination and/or
bidentate oxygen–oxygen9 or nitrogen–oxygen coordination
(g1–N, g1–O, g2–O,O9 and g2–N,O).
Reactions carried out in weakly coordinating molecular solvents
such as dichloromethane or benzene have allowed the isolation of
single crystals suitable for structural determination. We note that
analysis of these solutions by IR spectroscopy gives identical n(CO)
bands as compared to the ionic liquid reactions. A thermal
ellipsoid plot of 1 is shown in Fig. 1(a). The (C5H5)(CO)2Fe
fragment, characterized by typical bond lengths and angles, is
directly coordinated to the nitrogen atom of the 2NTf2 anion with
Room temperature ionic liquids (RTILs) are being actively
investigated as alternative solvent media in synthesis,1 catalysis,2
separations,3 and electrochemistry.4 A primary motivation of these
studies is the desire to develop clean chemical processes that take
advantage of the unique properties of RTILs such as their non-
volatility and electrical conductivity. These solvent systems also
offer a novel chemical environment that may uniquely influence
the course of chemical reactions compared to traditional solvents.5
In applications involving dissolved metal species, interaction of
either the ionic liquid cation6 or anion7 with the metal complex
may in fact control or strongly influence its chemical behaviour.
However, the coordination chemistry of ligand deficient metal
complexes dissolved in ionic liquid solvents remains almost
completely unknown.
˚
an Fe–N bond length of 2.084(4) A. Comparison of the structural
parameters for simple 2NTf2 salts14 and the 2NTf2 ligand in 1
reveals that the N–S bond lengths become significantly longer
upon nitrogen coordination (N(1)–S(1) and N(1)–S(2) are 1.630(4)
˚
and 1.643(4) A, respectively versus 1.56–1.57 in the free anion).
Further, the S–N–S angle becomes more acute in the complex
(117u vs. 125u), although the S–O bond lengths remain short at ca.
˚
1.42 A. Similar changes in metrical parameters are seen in the
comparison of HNTf2 and the uncomplexed anion.14 Using
analogous procedures, complexes (C5H5)2Ti(NTf2)2 (2),
Recently, organic salts of the bis(trifluoromethylsulfonyl)imide
anion (2N(SO2CF3)2; abbreviated as 2NTf2) have emerged as
perhaps the most generally useful RTILs because of their relatively
low viscosity, excellent thermal and chemical stability, and their
ease of synthesis.8 Although 2NTf2 typically behaves as a weakly
or non-coordinating anion,9,10 in the absence of more competent
ligands, discreet complexes may be formed. This work describes
the preparation of four new transition metal complexes of the
2NTf2 anion. These complexes were specifically chosen for
presentation in order to illustrate the range of 2NTf2–metal
binding modes identified to date and to address the more general
question of how ionic liquid solvents might influence the chemical
and electrochemical behaviour of ligand deficient metal species.
Ligand deficient metal complexes may be accessed in ionic liquid
solvents either by treatment of metal alkyl or hydride complexes
with HNTf2 (liberating alkane or hydrogen) or by metathesis of
metal halide complexes with AgNTf2.11 For example treatment of
a bright yellow solution of (C5H5)Fe(CO)2Me in [EMI]NTf2 with
HNTf2 rapidly yields a deep red solution from which bubbles of
methane are liberated. The same species is identified by ionic liquid
phase IR spectroscopy upon reaction of (C5H5)Fe(CO)2I with
AgNTf2. The new complex, assigned as (C5H5)Fe(CO)2(NTf2) (1)
[(Me2Si(g-C5Me4)(N-t-Bu)]Ti(NTf2)2
(3)
and
(cymene)–
Ru(NTf2)2 (4) were each prepared and structurally characterized.{
Fig. 1(b) depicts a thermal ellipsoid plot of 2, illustrating a
typical titanocene fragment coordinated to two 2NTf2 ligands
through
a monodentate metal–oxygen binding mode. The
˚
titanium–oxygen bond lengths of 2.050(3) and 2.067(3) A are
within the limits of normal Ti–O s-bonds. Significant structural
changes are observed in the 2NTf2 ligands compared to the free
anion, though these changes are of a different nature compared to
1 (e.g. A below). The S–O bond lengths of the titanium bound
oxygen atoms, O(1) and O(5), are lengthened to 1.467(4) and
˚
1.468(3) A compared to the remaining S–O bond lengths, which
˚
remain short (ca. 1.42 A). The N–S bond lengths in 2 reveal
significant distortion indicated by a pair of short (N(1)–
˚
S(1) 5 1.523(5) and N(2)–S(3) 5 1.542(4) A) and a pair of long
˚
(N(1)–S(2) 5 1.613(5) and N(2)–S(4) 5 1.605(4) A) distances that
are represented by structure B below.15
{ Currently at the Department of Chemistry, University of Washington,
Box 351700, Seattle, Washington 98195-1700, USA.
*woldham@lanl.gov
Thermal ellipsoid plots of 3 and 4 are shown in Figs. 1(c) and
(d). The ‘‘constrained geometry’’ ligand in 3 serves to reduce steric
1438 | Chem. Commun., 2005, 1438–1440
This journal is ß The Royal Society of Chemistry 2005
Williams, D. Bridget
