Cationic 16-electron half-sandwich ruthenium complexes containing asymmetric diamines: Understanding the stability and reactivity of coordinatively unsaturated two-legged piano stool complexes

Article abstract of DOI:10.1016/S0020-1693(98)00386-7

The cationic 16e complexes [RuCp*(η²(N,N)-Me₂NCH₂CH ₂N(CH₂CHMe₂)₂)]⁺ (1) and [RuCp*(η²(N,N)-Me₂NCH₂CH ₂NCH₂CH₂OCH₂CH₂] ⁺ (2) as well as the 18e complex [RuCp*(η⁶-C₆H₅-N(Me)NCH ₂CH₂NMe₂)]⁺ (3) have been synthesized as the BAr′₄ (Ar′=3,5-C₆H₃(CF₃)₂) salts in one-pot reactions starting from [RuCp*(Cl)]₄. For 1, the X-ray crystal structure has also been determined showing the absence of any agostic interactions between ruthenium and the C-H bonds of the diamine ligand, and only minor deviations from the planar geometry despite the bulky diamine ligand. Based on EH model calculations, the extraordinary kinetic inertness of the planar 16e [Cp*Ru(NN)]⁺ structure is traced to a high HOMO-LUMO gap deriving from through-bond coupling through the intervening σ skeleton of the chelating diamine (NN) ligand (in contrast to the P analogs) and further to the high π donor strength of Cp* (relative to parent Cp). Possible ligand rearrangements to increase the chemical reactivity are analyzed.

Full text of DOI:10.1016/S0020-1693(98)00386-7

Inorganica Chimica Acta 286 (1999) 114–120
Note
Cationic 16-electron half-sandwich ruthenium complexes
containing asymmetric diamines: understanding the stability and
reactivity of coordinatively unsaturated two-legged piano stool
complexes
Christian Gemel a, Valentin N. Sapunov a, Kurt Mereiter b, Mathias Ferencic c,
Roland Schmid a, Karl Kirchner a,*
a Institute of Inorganic Chemistry, Vienna Uni6ersity of Technology, Getreidemarkt 9, A-1060 Vienna, Austria
b Institute of Mineralogy, Crystallography, and Structural Chemistry, Vienna Uni6ersity of Technology, Getreidemarkt 9, A-1060 Vienna, Austria
c Institute of Organic Chemistry, Vienna Uni6ersity of Technology, Getreidemarkt 9, A-1060 Vienna, Austria
Received 30 June 1998; accepted 19 October 1998
Abstract
The cationic 16e complexes [RuCp*(h2(N,N)-Me2NCH2CH2N(CH2CHMe2)2)]+ (1) and [RuCp*(h2(N,N)-Me2NCH2-
¸¹¹¹¹¹¹¹¹º
CH2NCH2CH2OCH2CH2]+ (2) as well as the 18e complex [RuCp*(h6-C6H5-N(Me)NCH2CH2NMe2)]+ (3) have been synthesized
as the BAr%4 (Ar%=3,5-C6H3(CF3)2) salts in one-pot reactions starting from [RuCp*(Cl)]4. For 1, the X-ray crystal structure has
also been determined showing the absence of any agostic interactions between ruthenium and the C–H bonds of the diamine
ligand, and only minor deviations from the planar geometry despite the bulky diamine ligand. Based on EH model calculations,
the extraordinary kinetic inertness of the planar 16e [Cp*Ru(NN)]+ structure is traced to a high HOMO–LUMO gap deriving
from through-bond coupling through the intervening s skeleton of the chelating diamine (NN) ligand (in contrast to the P
analogs) and further to the high p donor strength of Cp* (relative to parent Cp). Possible ligand rearrangements to increase the
chemical reactivity are analyzed. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Crystal structures; Theoretical calculations; Ruthenium complexes; Half-sandwich complexes; Diamine complexes
1. Introduction
plexes containing O, S, P, and N donor ligands [2].
Incidentally, the configurational stability of a potential
catalyst is highly desirable with respect to achieving
enantiomerically pure products [1c].
In recent years, there has been growing interest in
coordinatively unsaturated two-legged piano stool
(half-sandwich) transition metal complexes as appealing
candidates for stoichiometric as well as catalytic appli-
cations in organic syntheses [1]. More specifically, com-
pounds of the type [MCp*L1L2] with a d6 electron
count and predominant s ligands have been found to
favor a planar geometry (i.e. the Cp* plane is perpen-
dicular to the L1–Ru–L2 plane) in a variety of com-
In the course of our efforts to synthesize and charac-
terize coordinatively unsaturated ruthenium complexes,
we recently reported on the first cationic 16-electron
ruthenium complex [RuCp*(Me2NCH2CH2NMe2)]+
[3], a compound that has proven to be remarkably inert
with respect to oxidative addition of H2, HSiEt3, and
MeBr. Note that Me2NCH2CH2NMe2 neither has p-
donor properties to stabilize the electron deficient
ruthenium center nor is particularly bulky so as to
prevent attack of incoming reagents. Beyond that, not
* Corresponding author. Tel.: +43-1-58801-15341; fax: +43-1-
58801-15399; e-mail: kkirch@mail.zserv.tuwien.ac.at
0020-1693/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(98)00386-7

C. Gemel et al. / Inorganica Chimica Acta 286 (1999) 114–120
115
even agostic interactions are apparent between
Me2NCH2CH2NMe2 and the ruthenium center.
In this work we describe the synthesis of further
cationic 16-electron complexes of the type [RuCp*-
(h2(N,N)-diamine)]+ using diamine ligands with pen-
dant alkyl substituents to facilitate agostic interactions.
For bulky phosphines, such interactions have been
demonstrated indeed [4]. In addition, we are undertak-
ing comparative extended Hu¨ckel (EH) calculations so
as to rationalize the differences in behavior between N
and P donor ligands in RuCp* chemistry on a qualita-
tive level.
(CH), 24.7 (CH), 23.6 (CHMe), 19.7 (CHMe), 10.6
(C5Me5).
2.2. Synthesis of [RuCp*(p2(N,N)-Me2NCH2CH2-
¸¹¹¹¹¹¹¹¹¹º
NCH2CH2OCH2CH2)]BAr% (2)
4
This compound was prepared analogously to 1 with
¸¹¹¹¹¹¹¹¹¹º
[RuCp*(Cl)]4 and Me2NCH2CH2NCH2CH2OCH2CH2)
as the starting materials. Yield: 90%. Anal. Calc. for
C50H45BF24N2ORu: C, 47.75; H, 3.61; N, 2.22. Found:
C, 47.77; H, 3.73; N, 2.14%. 1H NMR (l, CD2Cl2,
20°C): 7.75 (m, 8H), 7.58 (s, 4H), 4.61 (ddd, 2H, OCH2,
J=13.4 Hz, J=12.4 Hz, J=4.1 Hz), 3.87 (dd, 2H,
OCH2, J=12.4 Hz, J=12.8 Hz), 3.89 (m, 2H, NCH2),
2.87 (m, 2H, NCH2), 2.85 (s, 6H, NMe2), 2.30 (m, 2H,
NCH2CH2N), 1.86 (m, 2H, NCH2CH2N), 1.45 (s, 15H,
C5Me5). 13C{1H} NMR (l, CD2Cl2, 20°C): 161.5 (q,
2. Experimental
All reactions were performed under an inert atmo-
sphere of purified argon by using Schlenk techniques
unless otherwise stated. All chemicals were standard
reagent grade and used without further purification.
The solvents were purified according to standard proce-
dures. The deuterated solvents were purchased from
J
BC=49.7 Hz, BAr4%), 135.2 (BAr%4), 127.1 (q, JCF=31.3
Hz, BAr%4), 123.1 (q, JCF=272.6 Hz, BAr4%), 117.9
(BAr4%), 71.1 (C5Me5), 66.6, 60.5, 58.5, 56.6, 49.9, 49.2,
45.1, 10.4 (C5Me5).
˚
Aldrich and dried over
4 A molecular sieves.
2.3. Synthesis of [RuCp*(p6-C6H5-N(Me)CH2CH2-
[RuCp*(Cl)]4 and NaBAr% (Ar%=3,5-C6H3(CF3)2) were
4
prepared according to the literature [5,6]. 1H and
13C{1H} NMR spectra were recorded on a Bruker
AC-250 spectrometer operating at 250.13 and 62.86
MHz, respectively, and were referenced to SiMe4. Mi-
croanalysis were done by the Microanalytical Labora-
tories, University of Vienna.
NMe2)]BAr% (3)
4
This compound was prepared following the protocol
above with [RuCp*(Cl)]4 and Ph(Me)NCH2CH2NMe2)
as the starting materials. Yield: 87%. Anal. Calc. for
C53H45BF24N2Ru: C, 49.82; H, 3.55; N, 2.19. Found: C,
1
49.79; H, 3.43; N, 2.14%. H NMR (l, CDCl3, 20°C):
7.79 (m, 8H), 7.60 (s, 4H), 5.37 (m, 2H), 5.16 (m, 3H),
3.31 (t, 2H), 2.89 (s, 3H, Me), 2.46 (t, 2H), 2.25 (s, 6H,
NMe2), 1.86 (s, 15H, C5Me5). 13C{1H} NMR (l,
CDCl3, 20°C): 161.2 (q, JBC=49.6 Hz, BAr4%), 135.1
2.1. Synthesis of [RuCp*(p2(N,N)-Me2NCH2CH2N-
(CH2CHMe2)2]BAr% (1)
4
A suspension of [RuCp*(Cl)]4 (300 mg, 0.276 mmol)
in Et2O (5 ml) was treated with Me2NCH2CH2N-
(CH2CHMe2)2 (222 mg, 1.104 mmol) and stirred for 1 h
(BAr%4), 127.3 (q, JCF=31.5 Hz, BAr4%), 123.1 (q, JCF
=
272.8 Hz, BAr%4), 118.1 (BAr4%), 95.6, 85.1, 82.3(C5Me5),
69.6, 57.0, 51.4, 46.2, 38.4, 11.2 (C5Me5).
at room temperature. After that time, NaBAr% (0.978
4
mg, 1.104 mmol) was added and the solution stirred for
an additional 5 min. After removal of the solvent, the
residue was dissolved in Et2O (5 ml), insoluble materi-
als were removed by filtration and the blue product was
precipitated by addition of n-hexane. Yield: 1.30 g
(91%). Anal. Calc. for C54H55BF24N2Ru: C, 49.89; H,
2.4. X-ray structure determination for 1
Crystal data and experimental details are given in
Table 1. X-ray data were collected on a Siemens Smart
CCD area detector diffractometer (graphite monochro-
1
˚
4.26; N, 2.15. Found: C, 49.77; H, 4.28; N, 2.24%. H
mated Mo Ka radiation (u=0.71073 A), a nominal
NMR (l, CD2Cl2, 20°C): 7.72 (m, 8H), 7.57 (s, 4H),
3.41 (dd, 2H, NCH2, J=13.4 Hz, J=5.7 Hz), 3.11 (dd,
2H, NCH2, J=13.4 Hz, J=5.7 Hz), 2.90 (s, 6H,
NMe2), 2.20 (m, 2H, NCH2CH2N), 1.94 (m, 2H, CH),
1.89 (m, 2H, NCH2CH2N), 1.38 (s, 15H, C5Me5), 1.02
(d, 6H, Me, J=6.7 Hz), 0.88 (d, 6H, Me, J=6.7 Hz).
13C{1H} NMR (l, CD2Cl2, 20°C): 161.5 (q, JBC=49.7
Hz, BAr%4), 135.2 (BAr4%), 127.1 (q, JCF=31.3 Hz,
BAr4%), 123.1 (q, JCF=272.6 Hz, BAr4%), 117.9 (BAr4%),
71.3 (C5Me5), 69.1, 62.9, 61.5, 51.7, 45.1 (NMe2), 26.4
crystal-to-detector distance of 3.85 cm, 0.3° ꢀ-scan
frames). Corrections for Lorentz and polarization ef-
fects, for crystal decay, and for absorption were ap-
plied. The structure was solved by Patterson methods
using the program SHELXS86 [7]. Structure refinement
on F2 was carried out with the program SHELXL93 [8].
All non-hydrogen atoms were refined anisotropically.
Hydrogen atoms were inserted in idealized positions
and were refined riding with the atoms to which they
were bonded.

116
C. Gemel et al. / Inorganica Chimica Acta 286 (1999) 114–120
Table 1
tion with [RuCp*(Cl)]4 as the starting material. The
intermediarily formed complexes RuCp*(h2(N,N)-di-
amine)Cl have not been isolated. When [RuCp*(Cl)]4 is
treated with the respective diamine (one equivalent) in
Et2O and the resulting orange–red solid was reacted
Crystallographic data for [RuCp*(h2(N,N)-Me2NCH2CH2N(CH2-
CHMe2)2]BAr4% (1)
Formula
C54H55BF24N2Ru
Formula weight
Crystal size (mm)
Space group
1299.88
0.60×0.40×0.40
P21/c (no. 14)
12.646(6)
18.798(8)
25.147(11)
98.93(3)
5906(5)
2632
with NaBAr%, complexes 1–3 are, on work-up, ob-
4
tained in 91, 90 and 87% isolated yields, respectively
(Scheme 1). Characterization of all complexes was
achieved by elemental analysis, and 1H and 13C{H}
NMR spectroscopies.
˚
a (A)
˚
b (A)
˚
c (A)
i (°)
V (A )
3
˚
Complexes 1 and 2 are blue complexes which on
exposure to air decompose to severeal intractable mate-
rials (cf. the isoelectronic complex [RuCp*(h2(N,N)-
Me2NCH2CH2NMe2)]+ reacts with dioxygen to give
the hydroxo tetramethylfulvene complex [Ru(h6-
F(000)
Z
4
zcalc (g cm−3
T (K)
)
1.462
293
0.378
empirical
0.4/0.8
v (mm−1) (Mo Ka)
Absorption correction
Transmission factor
min./max.
1
C5Me4CH2)(Me2NCH2CH2NMe2)(OH)]+ [3]). The H
NMR spectrum of 1 exhibits a singlet for the Cp* ring
at 1.38 ppm. The CH2CHMe2 groups of the
Me2NCH2CH2N(CH2CHMe2)2 ligand display two dou-
blets of doublets centered at 3.41 and 3.11 ppm (CH2),
a multiplet at 1.94 ppm (CH), and two doublets cen-
tered at 1.06 and 0.88 ppm (Me), i.e. the CH2CHMe2
groups are diastereotopic. In the 13C{1H} NMR spec-
trum of 1 the resonances of the ring carbon atoms of
the Cp* ligand give rise to a singlet at 71.3 ppm.
Similar NMR spectra are obtained for 2. Complex 3,
on the other hand, is a pale yellow air stable 18-electron
sandwich complex with the C6H5-N(Me)CH2CH2NMe2
coordinated in h6-fashion. This reaction is perhaps not
surprising taken into account the high affinity of the
RuCp* fragment for arene ligands. Both 1H and
13C{1H} NMR spectra of 3 are unremarkable and are
not discussed here.
qmax (°)
Index ranges
21
−155h514, −215k521,
−155l530
16257
6307
3605
213/426
0.094
0.158
No. reflections measured
No. unique reflections
No. reflections F\4|(F)
No. restraints/parameters
R(F) (F\4|(F))
R(F) (all data)a
wR(F2) (all data)b
0.296
Difference Fourier peaks
min./max. (e A
−0.55/0.70
−3
˚
)
a R=SꢀꢀFoꢀ−ꢀFcꢀꢀ/SꢀFoꢀ.
b wR=[S(w(Fo2−Fc2)2)/S(w(Fo2)2)]1/2
.
2.5. Extended Hu¨ckel calculations
The structure of 1 has been determined by X-ray
crystallography. A structural view of 1 is shown in Fig.
1 with selected bond distances and angles given in the
caption. The average Ru–C(Cp*) distance is 2.134(10)
The extended Hu¨ckel molecular orbital calculations
were conducted by using the program developed by
Hoffmann and Lipscomb [9]. The atomic parameters
used in this study were taken from the CACAO program
[10]. All the bond lengths and angles of the complexes
analyzed were those determined crystallographically.
˚
A. The Ru–N(1) and Ru–N(2) distances are 2.177(13)
˚
and 2.213(13) A, respectively. The angle between the
planes defined by the Cp* ring and the atoms N(1), Ru,
and N(2) is 78° indicating some pyramidalization at the
metal center, compared with 89.3° found for the sym-
metrical analog [3]. However, this could well be a solid
state effect since no agostic interactions between ruthe-
nium and one of the C–H bonds of the diamine ligand
are observed; the shortest distance between the
ruthenium center and the carbon atoms of the
3. Results and discussion
3.1. Syntheses
The complexes 1, 2 and 3, as the BAr% (Ar%=3,5-
4
˚
C6H3(CF3)2) salts, were synthesized in a one-pot reac-
Me2NCH2CH2N(CH2CHMe2)2 ligand is about 3.00 A.
Scheme 1.

C. Gemel et al. / Inorganica Chimica Acta 286 (1999) 114–120
117
extend these studies to chelating ligands, thus allowing
for through-bond effects in addition to through-space
coupling [12]. Through-bond coupling is a result primar-
ily of the filled–filled interaction between the symmetric
combination of p or p orbitals with the s bonding
orbital, whereas little to no mixing occurs between the
antisymmetric p or p combination with s* [12a]. Since
the s-bonding orbital of C–C is more similar in size to
that of the p orbitals of N relative to P, differences in
behavior might be expected. The complexes [Cp*Ru(L1–
L2)]+ have been analyzed where L1–L2=Me2NC2H4-
NMe2 (NN), Me2PC2H4PMe2 (PP), and Me2NC2H4-
PMe2 (NP).
The relevant MOs of the {RuCp*} fragment are
well-known [11]. Thus, the interaction of the Cp* ligand
and Ru occurs mainly through donation of the two filled
p orbitals e1¦ of Cp* into the degenerate Ru d(p) orbitals
dyz and dxz. The resulting antibonding p* orbitals are
perpendicular to and pointing away from the Cp* plane.
Further, the a¦2 orbital of Cp* interacts weakly with the
Fig. 1. Structural view of [RuCp*(h2(N,N)-Me2NCH2-CH2N(CH2-
˚
CHMe2)2]BAr4% (1). Selected bond lengths (A) and angles (°): Ru–
C(1–5)av 2.134(10), Ru–N(1) 2.177(13), Ru–N(2) 2.214 (13),
N(1)–Ru–N(2) 78.1(5).
A planar ground-state structure is consistent with MO
calculations for diamagetic d6 complexes of the types
CpML2 and CpMLL% when L, L% are pure or predomi-
nant s-donor ligands. For instance, the planar geometry
of monomeric [CpRu(acac)] is calculated to be favored
by 6.9 kcal mol−1 (0.30 eV) over the bent one [1c].
Ru dz orbital which, therefore, is slightly destabilized
2
relative to the d-symmetric dx
and dxy orbitals that
are both coplanar to the Cp* ring plane.
2
2
–y
In free bidentate L1–L2, the two lone pair s orbitals
interact through-space and couple to the central s and
s* orbitals of the C–C bond in L1–L2, forming the new
group orbitals 8%a and 8s% [12]. As is seen in Fig. 2, this
p–s–p through-bond coupling is strong in the case of
(NN), but weak for (NP) and (PP), as expected, giving
rise to a different order in energy levels.
3.2. Extended Hu¨ckel analyses
MO studies of half-sandwich CpML1L2 have been
conducted in detail by several authors [11]. We here
Fig. 2. Comparative orbital interaction diagram resulting from through-space and through-bond effects for free PP and NN.

118
C. Gemel et al. / Inorganica Chimica Acta 286 (1999) 114–120
Fig. 3. Qualitative interaction diagram between {Cp*Ru}+ and PP and NN.
Fig. 3 shows a simplified interaction diagram be-
bending of the Cp* and L1–M–L2 planes (from −20
to +20°) virtually does not activate the molecule,
despite the rehybridization of the LUMO to point out
toward the developing vacant site (Fig. 4). The reason
is that the LUMO is pushed up in energy because 8%s of
tween {RuCp*}+ fragment and L1–L2. The 8a% orbital
interacts strongly with the d*yz orbital, removing the
degeneracy of the LUMO of {RuCp*}+ leaving behind
the dx*z MO as the new LUMO and forming SLUMO.
According to the 8%a orbital energies, the SLUMO
energy increases strongly in the series (NN)B(NP)B
(PP), reflecting the force of coupling between the
{RuCp*}+ fragment and L1–L2. On the other hand,
the changes in LUMO are negligible. A slight destabi-
L1–L2 now better interacts with dx*z. Conversely, the dz
2
MO is no longer destabilized as in the planar configura-
tion rendering the HOMO going down. The resulting
increase of the HOMO–LUMO gap (Fig. 4) is well-
known [11]. We suggest that a raise in chemical reactiv-
ity may be achieved if there is additional h5“h3 Cp
ring slippage. Such a process is known to have a low
barrier [13] but pushes down the LUMO effectively.
Along these lines the complex gradually not only be-
comes a better s acceptor, but also the donor abilities
are altered through a concomitant change in the (filled)
d-orbital energetic ordering. In the quite general case of
a two-legged piano stool metal(d6) complex (Cp*MLX)
lization of the HOMO (as d ) and d
via the
2
2
2
x
interaction with 8s% is effective zonly in the –cyase of the
NN ligand. Because of the geometries of LUMO and
HOMO (which is located in the Cp(O)–Ru–L1–L2
line) the complex RuCp*(L1–L2)+ is neither a good
acceptor (neither s nor p) nor a good donor (unless the
geometry is changed).
What modes of activation of planar [RuCp*(L1–L2)]
can be envisaged? A pyramidal distortion through
with a chiral conformation, each of the orbitals d ,
2
z

C. Gemel et al. / Inorganica Chimica Acta 286 (1999) 114–120
119
Fig. 4. Computed changes in energies of the HOMO and LUMO in [Cp*Ru(NN)]+ (upper part) and in the total energies for (i) [Cp*Ru(NN)]+
and (ii) [Cp*Ru(PP)]+ (lower part) for stepwise planar/pyramidal distortion, and h5“h3 Cp* ring slippage.
dx 2, and d can become the HOMO, depending on
complexes over the phosphorus analogs. The smaller
inversion barriers for (NP) and (PP) can be traced to
the stronger interaction of 8%a with p*-FMO of
{RuCp*} fragment in the pyramidal configuration and
the relatively small decrease in the corresponding
Že1¦ ꢀ 8a% overlap. Furthermore, because of p–s–p
through-bond coupling, the loss in energy due to the
interaction of dxy with 8%s is greater in the NN case
compared with PP. It should be mentioned here that
this overlap is also diminished when replacing Cp* with
Cp. In fact, the related complexes of Cp are found to be
much more reactive and detailed investigations in this
respect are under way in our laboratories.
2
xy
the–ynature of the ligands L and X, with eventually
dramatic differences in chemical behavior [11c,14].
Thus, if dz remains to be the HOMO, the complex will
2
be a (weak) s donor; but if dxy becomes the HOMO,
the complex will be more a p donor.
Independent of the subsequent reactions, a prior
planar/pyramidal inversion appears to be mandatory
implying that the barrier of this process would be part
of the overall activation enthalpies. Planar/pyramidal
rearrangement barriers may be high or low depending
on the relative contributions of the gain in energy from
the increased participation of 8%s and loss in energy
from the worsened conjugation between 8%a and one of
the e1¦ orbitals of Cp*. Although all three complexes
under consideration are notably destabilized upon
pyramidal distortion, this destabilizing effect is about
twice as great for the (NN) complex compared with
that of (PP) explaining the higher stability of the amine
4. Supplementary material
X-ray structural data, including positional and ther-
mal parameters, and bond distances and angles for

120
C. Gemel et al. / Inorganica Chimica Acta 286 (1999) 114–120
[7] G.M. Sheldrick, SHELXS86, Program for the Solution of Crystal
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Refinement, University of Go¨ttingen, Go¨ttingen, Germany,
1993.
complex 1 may be ordered from the Fachinformations-
zentrum Karlsruhe, Gesellschaft fu¨r wissenschaftlich-
technische Information mbH, D-76344 Eggenstein-
Leopoldshafen, Germany, referring to the deposition
number CSD 408817, the names of the authors, and the
citation of the present paper.
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Acknowledgements
Financial support by the ‘Jubila¨umsfond der
Osterreichischen Nationalbank’ is gratefully acknowl-
edged (Project no. 6474).
8
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