Reactions of [Cp*RuCl]4 and [(p-cymene)RuCl 2]2 with the tridentate ligand [Ph(pz)B(μ-O)(μ-pz) B(pz)Ph]-

Article abstract of DOI:10.1021/om901022d

The reaction of the tridentate [N,O,N] (pyrazol-l-yl)borate ligand [Ph(pz)B(μ-O)(μ-pz)B(pz)Ph]^- ([L¹]") with [Cp*RuCl]₄ and [(p-cym)RuCl₂]₂ gives the Ru^II complexes [Cp*Ru(L¹)] and [(p-cym)Ru(L ¹)]Cl, respectively (pz = pyrazolyl, Cp* = pentamethylcyclopentadienyl, p-cym = p-cymene). In order to avoid degradation of the [(p-cym)Ru(L¹)]+ complex in solution, its Cl- counterion has been exchanged for PF₆-, [B(C₆F₅) ₄]-, tosylate, and triflate. When the reaction between [L ¹]- and [(p-cym)RuCl₂]₂ is carried out in the presence of 4 equiv Of TlPF₆, the dinuclear pyrazolyl-bridged complex [(p-cym)Ru(μ-Cl)(μ-pz)₂Ru(p-cym)]PF₆ and the mononuclear species [(p-cym)Ru(L²)] are obtained ([L ²]²- = [Ph(pz)B(μ-O)(μ-OB(Ph)O)B(pz)Ph] ²-). In a targeted synthesis, the lithium salt of the novel ligand [L²]²was prepared from 2 equiv of Lipz and 2,4,6-Ph ₃B₃O₃ and successfully transformed into [(p-cym)Ru(L²)]. [Cp*Ru(L¹)], [(p-cym)Ru(L ¹)]PF₆, and [(p-cym)Ru(L²)] have been characterized by NMR spectroscopy, X-ray crystallography, and (spectro)electrochemistry. One-electron oxidation of [Cp*Ru(L ¹)] by electrochemical or chemical ([Cp₂F _e]PF₆) means leads to the Ru^III species [Cp*Ru(L¹)]PF₆, which has been isolated and fully characterized (E₁/₂(Ru^II/Ru^III) = -0.39 V; CH₂Cl₂, vs FcH/FcH+). A comparison of the solid-state structures of [Cp*Ru(L¹)] and [Cp*Ru(L ^l)]PF₆ reveals that oxidation of the ruthenium center results in a lengthening of the average Ru-Cp* distances and a shortening of all Ru-L¹ bond lengths. According to the X-ray data, the angle strain within [p-cym)Ru(L¹)]PF₆ is higher than in [(P-cym)Ru(L²)], which could account for the fact that [L ¹]- is apparently less stable than [L²]²-.

Full text of DOI:10.1021/om901022d

966 Organometallics 2010, 29, 966–975
DOI: 10.1021/om901022d
Reactions of [Cp*RuCl]4 and [(p-cymene)RuCl2]2 with the Tridentate
Ligand [Ph(pz)B(μ-O)(μ-pz)B(pz)Ph]-
€
Elena V. Mutseneck, Christian Reus, Frauke Schodel, Michael Bolte,
Hans-Wolfram Lerner, and Matthias Wagner*
€
Institut fu€r Anorganische und Analytische Chemie, Goethe-Universitat Frankfurt, Max-von-Laue-Strasse 7,
D-60438 Frankfurt (Main), Germany
Received November 25, 2009
The reaction of the tridentate [N,O,N] (pyrazol-1-yl)borate ligand [Ph(pz)B(μ-O)(μ-pz)B(pz)Ph]-
([L1]-) with [Cp*RuCl]4 and [(p-cym)RuCl2]2 gives the RuII complexes [Cp*Ru(L1)] and [(p-cym)-
Ru(L1)]Cl, respectively (pz = pyrazolyl, Cp* = pentamethylcyclopentadienyl, p-cym = p-cymene). In
order to avoid degradation of the [(p-cym)Ru(L1)]þ complex in solution, its Cl- counterion has been
exchanged for PF6-, [B(C6F5)4]-, tosylate, and triflate. When the reaction between [L1]- and [(p-cym)Ru-
Cl2]2 is carried out in the presence of 4 equiv of TlPF6, the dinuclear pyrazolyl-bridged complex [(p-cym)-
Ru(μ-Cl)(μ-pz)2Ru(p-cym)]PF6 and the mononuclear species [(p-cym)Ru(L2)] are obtained ([L2]2-
=
[Ph(pz)B(μ-O)(μ-OB(Ph)O)B(pz)Ph]2-). In a targeted synthesis, the lithium salt of the novel ligand [L2]2-
was prepared from 2 equiv of Lipz and 2,4,6-Ph3B3O3 and successfully transformed into [(p-cym)Ru(L2)].
[Cp*Ru(L1)], [(p-cym)Ru(L1)]PF6, and [(p-cym)Ru(L2)] have been characterized by NMR spectroscopy,
X-ray crystallography, and (spectro)electrochemistry. One-electron oxidation of [Cp*Ru(L1)] by electro-
chemical or chemical ([Cp2Fe]PF6) means leads to the RuIII species [Cp*Ru(L1)]PF6, which has been
isolated and fully characterized (E1/2(RuII/RuIII) =-0.39 V; CH2Cl2, vs FcH/FcHþ). A comparison of the
solid-state structures of [Cp*Ru(L1)] and [Cp*Ru(L1)]PF6 reveals that oxidation of the ruthenium center
results in a lengthening of the average Ru-Cp* distances and a shortening of all Ru-L1 bond lengths.
According to the X-ray data, the angle strain within [(p-cym)Ru(L1)]PF6 is higher than in [(p-cym)Ru(L2)],
which could account for the fact that [L1]- is apparently less stable than [L2]2-
.
Introduction
acceptor properties of scorpionate ligands is to replace
one or more pyrazolyl rings by phosphorus-,4-8 oxygen-,9-23
Tris(pyrazol-1-yl)borates (“scorpionates”) are among the
most prominent ligands in coordination chemistry, which is
due to the fact that they are not only readily accessible but
also very versatile.1,2 For example, introduction of appro-
priate substituents into the 3-positions of the pyrazolyl rings
allows extensive control over the steric demand of the ligands
and thus over their ability to kinetically stabilize reactive
complex fragments. In contrast, an adjustment of the ligand
field strengths of tris(pyrazol-1-yl)borates is much harder to
achieve, because electronic substituent effects on the donor
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Published on Web 01/25/2010
2010 American Chemical Society

Article
Organometallics, Vol. 29, No. 4, 2010 967
Scheme 1. Synthesis of [Li(thf)L1]a
Figure 1. Mixed-donor (pyrazol-1-yl)borates: classic scorpio-
nate A and modified ligand architecture B. Do = N-, P-, O-, or
S-containing donor group.
or sulfur-containing24-41 groups. For a systematic tuning of
metal complex properties, it is particularly desirable to have
complete homogeneous series of closely related homo- and
-
heteroleptic scorpionates of the form [RB(Do1)x(Do2)3-x
]
(x = 0-3). Unfortunately, this is where scorpionate chemistry
exhibits its weaknesses, because (i) the selective preparation of a
specific mixed-donor borate is often difficult to achieve and (ii)
borate ions have a tendency for substituent scrambling.
Given this background, we became interested in the ques-
tion how the molecular framework of “classic” mixed-donor
scorpionates A (Figure 1) has to be modified if we want to
avoid the problems mentioned above but still take advantage
of (pyrazol-1-yl)borate chemistry. For the following reasons,
we came to the conclusion that B-type molecules (Figure 1)
would be promising lead structures for future investigations:
(i) many of the established design principles of scorpionate
chemistry are still valid for B; (ii) the bonding situation of the
central donor moiety (Do) is distinctly different from the way
the pyrazolyl rings are attached to the molecule, which
should help to deal with selectivity and substituent scram-
bling issues; (iii) molecular modeling studies indicate that
ligands B are able to adopt both facial and (distorted)
meridional coordination modes.
a Legend: (i) toluene, reflux, 2 h; (ii) toluene/THF, reflux, 8 h.
R = Ph) toward FeII, FeIII, and CuII.42 The three correspond-
ing complexes [(pyridine)(Cl)Fe(L1)], [Cl2Fe(L1)], and
[ClCu(L1)] have been structurally characterized by X-ray
crystallography. Both iron complexes possess a distorted-
trigonal-bipyramidal configuration with the pyrazolyl rings
occupying equatorial positions and the oxygen donor being
located at an apical position. The copper complex crystallizes
as the chloro-bridged dimer [ClCu(L1)]2, in which the ligand
environments are intermediate between a square-planar and a
trigonal-bipyramidal geometry. This leads to the conclusion
that [L1]- has a higher tendency to act as a trans chelator than
scorpionates A, which are always facially coordinating.
The purpose of this paper is to explore possible applica-
tions of [L1]- in organometallic chemistry. We have chosen
RuII as the central metal ion, because ruthenium scorpionate
complexes have already been shown to be useful in such
diverse areas as C-H activation reactions,43 the catalytic
hydroarylation of olefins,44,45 C-C coupling reactions,46,47
In a previous publication, we reported on the ligand
behavior of [L1]- (Scheme 1; B-type ligand with Do = O,
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968 Organometallics, Vol. 29, No. 4, 2010
Mutseneck et al.
Scheme 2. Synthesis of the RuII Complex [Cp*Ru(L1)] and Its
should be stored under nitrogen. It is readily soluble in
hexane, CH2Cl2, and THF but rapidly decomposes in
strongly coordinating solvents such as CH3CN and acetone.
In contrast, the related complexes [Cp*Ru(pz3BH)]55 and
[Cp*Ru(pz3CH)]PF656 are relatively stable in CH3CN.
Oxidation to the RuIII Complex [Cp*Ru(L1)]PF6
a
1
The H NMR spectrum (CDCl3) of [Cp*Ru(L1)] is con-
sistent with the presence of one Cp* and one tridentate [L1]-
ligand in the molecule. A sharp singlet at 1.26 ppm clearly
arises from the Cp* ring. A virtual triplet at 6.24 ppm and a
triplet at 6.44 ppm (integral ratio 2:1) can be assigned to
magnetically inequivalent pzH-4 protons and thus confirm
the presence of two types of pyrazolyl rings in the molecule.
The pzH-3,5 resonance of the bridging pyrazolyl ring ap-
pears as a doublet at 7.51 ppm. The pzH-3,5 protons of the
pyrazolyl rings coordinated to the RuII center give two
signals at 7.24-7.28 and 7.61 ppm. The first multiplet
partially overlaps with resonances of the phenyl protons.
The cyclic voltammogram of the complex [Cp*Ru(L1)] in
CH2Cl2 shows a reversible one-electron wave at E1/2 = -0.39
V arising from the Ru(II)/Ru(III) transition (ΔEp = 126 mV;
vs FcH/FcHþ; cf. the Supporting Information for more
details). This redox potential is comparable to the potential
of the related complex [Cp*Ru(pz3BH)], which undergoes one
quasi-reversible oxidation.55,57,58 Moreover, the fact that the
oxidation of [Cp*Ru(L1)] is a one-electron process resembles
the electrochemical behavior of ferrocene but is different from
the behavior of ruthenocene, which undergoes a one-step,
two-electron transition upon oxidation at a Pt electrode.59,60
In line with our electrochemical studies, chemical oxidation
of [Cp*Ru(L1)] with[Cp2Fe]PF6 (0.8 equiv) in CH2Cl2 gave the
RuIII complex [Cp*Ru(L1)]PF6 in good yield (88%; Scheme 2).
[Cp*Ru(L1)]PF6 was isolated as a dark red crystalline com-
pound by slow diffusion of pentane into its CH2Cl2 solution.
The molecular structures of [Cp*Ru(L1)] and [Cp*Ru-
(L1)]PF6 are shown in Figures 2 and 3S (cf. the Supporting
Information; the numbering scheme is the same in both
structure plots); selected crystallographic data are summar-
ized in Table 3S (cf. the Supporting Information).
a Legend: (i) THF, room temperature, 12 h; (ii) CH2Cl2, room
temperature, 12 h.
the radical polymerization of electron-deficient olefins,48
the transfer hydrogenation of ketones,49 and the catalytic cis-
trans isomerization of functionalized epoxides.50 Penta-
methylcyclopentadienide (Cp*) and p-cymene (p-cym) will
be employed as organic ligands, because they will force [L1]-
into as much of a facial coordination mode as possible, which,
together with our earlier investigations, will allow important
insights into the conformational flexibility of [L1]-.
Results and Discussion
For the synthesis of [Li(thf)L1] (Scheme 1), the literature
protocol42 has been slightly modified. Instead of preparing
the 1,3-diboroxane intermediate (Me2N(Ph)B)2O via the
controlled hydrolysis of Me2N(Ph)BBr,42 we now allowed
bis(dimethylamino)phenylborane, (Me2N)2(Ph)B,51 to react
with 2,4,6-triphenylboroxine, Ph3B3O3,52 in refluxing to-
luene53 and obtained (Me2N(Ph)B)2O in 65% yield.
Further treatment of (Me2N(Ph)B)2O with Lipz and Hpz
(1:2) in a toluene/THF mixture at reflux temperature
(approximately 80 °C) led to [Li(thf)L1] in 80% yield.
Synthesis, Characterization, and Reactivity of [Cp*Ru(L1)].
The room-temperature reaction of [Li(thf)L1] with [Cp*Ru-
Cl]454 in THF resulted in the formation of complex [Cp*Ru-
(L1)] in good yield (60-70%; Cp* = [C5Me5]-; Scheme 2).
Recrystallization of the crude product from hexane led to
yellow-orange crystals suitable for X-ray analysis. [Cp*Ru-
(L1)] is only moderately air-stable, even in the solid state, and
The crystal lattice of [Cp*Ru(L1)] contains two indepen-
dent molecules in the asymmetric unit ([Cp*Ru(L1)]A and
[Cp*Ru(L1)]B); since the key structural parameters of both
molecules are the same within the experimental error margins,
only the structure of [Cp*Ru(L1)]A will be discussed further.
The Ru atom binds to ligand [L1]- via Ru-O and Ru-N
bonds with bond lengths of Ru(1)-O(1) = 2.227(3) A,
Ru(1)-N(12) = 2.139(4) A, and Ru(1)-N(22) = 2.140(5)
A. The Ru-N bond lengths are similar to the Ru-N dis-
tances in related complexes (e.g., (Ru-N)av = 2.128(3) A in
[CpRu(pz3BH)]55 and 2.145(3) A in [Cp*Ru(pz3CH)]PF6).56
The distance between the RuII ion and the centroid (COG) of
the Cp* ring in [Cp*Ru(L1)]A equals 1.772 A, which is in
good agreement with the case for other Cp*Ru complexes
(e.g., 1.782 A for [Cp*Ru(pz3CH)]PF6).56
In [Cp*Ru(L1)]PF6, we observe a Ru(1)-O(1) bond
length of 2.173(2) A and Ru-N bond lengths of 2.078(2)
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Organometallics, Vol. 29, No. 4, 2010 969
Scheme 3. Synthesis of [(p-cym)Ru(L1)]X (X = Cl, PF6) and
Formation of [(p-cym)Ru(L2)] and [(p-cym)Ru(μ-Cl)(μ-pz)2Ru-
(p-cym)]PF6
a
Figure 2. Molecular structure of [Cp*Ru(L1)]A. H atoms are
omitted for clarity; displacement ellipsoids are drawn at the 30%
probability level. Selected bond lengths (A) and bond angles (deg):
Ru(1)-O(1)=2.227(3), Ru(1)-N(12)=2.139(4), Ru(1)-N(22)=
2.140(5), Ru(1) COG(Cp*)=1.772; O(1)-Ru(1)-N(12)=76.8-
3 3 3
(1), O(1)-Ru(1)-N(22)=76.7(2), N(12)-Ru(1)-N(22)=85.1(2),
O(1)-B(1)-N(11) = 108.7(4), O(1)-B(1)-N(31) = 101.2(4),
O(1)-B(2)-N(21) = 108.9(4), O(1)-B(2)-N(32) = 100.8(4),
B(1)-O(1)-B(2)=115.6(4). Corresponding structure parameters
of [Cp*Ru(L1)]PF6: Ru(1)-O(1) = 2.173(2), Ru(1)-N(12) =
2.078(2), Ru(1)-N(22) = 2.086(2), Ru(1) COG(Cp*) =1.830;
3 3 3
O(1)-Ru(1)-N(12) = 78.2(1), O(1)-Ru(1)-N(22) = 76.9(1),
N(12)-Ru(1)-N(22) = 95.8(1), O(1)-B(1)-N(11) = 106.5(2),
O(1)-B(1)-N(31) = 101.1(2), O(1)-B(2)-N(21) = 105.8(2),
O(1)-B(2)-N(32)=100.9(2), B(1)-O(1)-B(2)=115.6(2). COG-
(Cp*) denotes the centroid of the Cp* ring.
and 2.086(2) A. The Ru COG(Cp*) distance amounts
3 3 3
to 1.830 A. Oxidation of [Cp*Ru(L1)] thus leads to a short-
ening of all Ru-L1 contacts (Δ(Ru-O) = -0.054 A;
Δ(Ru-N)av = -0.058 A with respect to [Cp*Ru(L1)]A)
and to a lengthening of the Ru-Cp* bonds (Δ(Ru
3 3 3
COG(Cp*)) = þ0.058 A with respect to [Cp*Ru(L1)]A).
The contraction of the Ru-L1 contacts in [Cp*Ru(L1)]PF6
results in a widening of the N(12)-Ru(1)-N(22) angle by a
value of 10.7° ([Cp*Ru(L1)]PF6, 95.8(1)°; [Cp*Ru(L1)]A,
85.1(2)°). In related tris(pyrazol-1-yl)borate complexes, the
average N-Ru-N0 angles are 83.8(1)° ([CpRu(pz3BH)])55
and 81.7(1)° ([Cp*Ru(pz3CH)]PF6).56
a Legend: (i) THF, room temperature, 2 h; (ii) CH2Cl2, room tem-
perature, 2 h; (iii) THF, room temperature, 12 h.
The 11B NMR spectra of [(p-cym)Ru(L1)]X (X = PF6,
B(C6F5)4, Tos, Tfl) are characterized by signals in the range
between 5.9 and 8.1 ppm, testifying to the presence of four-
coordinate boron atoms.61,62 The 1H and 13C NMR features of
the [L1]- part of [(p-cym)Ru(L1)]PF6 are similar to those of
[Li(thf)L1], and the chemical shifts of the p-cymene resonances
also do not show any peculiarities. We note, however, that one
pyrazolyl resonance and the signals arising from the aromatic
p-cymene protons are quite sensitive to changes in the counter-
ion X- (cf. the Supporting Information for synthesis protocols
and NMR data of [(p-cym)Ru(L1)]X (X=B(C6F5)4, Tos, Tfl)).
The reduction part of the cyclic voltammogram of the
complex [(p-cym)Ru(L1)]PF6 in CH2Cl2/NBu4PF6 exhibits a
Synthesis, Characterization, and Reactivity of [(p-cym)Ru-
(L1)]X Complexes (X = Cl, PF6, B(C6F5)4, Tos, Tfl). The
reaction of [(p-cym)RuCl2]2 with [Li(thf)L1] (1:2; THF) gave
the cationic complex [(p-cym)Ru(L1)]Cl in 70% yield
(Scheme 3). Within a time span of 2 h, [(p-cym)Ru(L1)]Cl
precipitated from the reaction mixture as a yellow solid
which should be isolated without further delay, since pro-
longed stirring of the reactants leads to significantly lower
yields (ca. 20% after 24 h, close to 0% after 72 h).
An exchange of the counteranion can be achieved quanti-
tatively using NH4PF6 or K[B(C6F5)4] in CH2Cl2; the corre-
sponding tosylate (Tos) or triflate (Tfl) salts have been
prepared using Ag[Tos] or Ag[Tfl] in THF. In the solid state,
the complexes [(p-cym)Ru(L1)]X (X = PF6, B(C6F5)4, Tos,
Tfl) are inert toward air and moisture over a period of several
months. They are readily soluble in CHCl3, CH2Cl2,
CH3CN, and acetone, and the solutions were found to be
stable at least for several hours under inert conditions
(1H and 11B NMR spectroscopic control).
€
(61) Noth, H.; Wrackmeyer, B. Nuclear Magnetic Resonance Spec-
troscopy of Boron Compounds. In NMR Basic Principles and Progress;
Diehl, P., Fluck, E., Kosfeld, R., Eds.; Springer: Berlin, Heidelberg,
New York, 1978.
(62) Mason, J., Ed. Multinuclear NMR; Plenum Press: New York,
London, 1987.

970 Organometallics, Vol. 29, No. 4, 2010
Mutseneck et al.
example, it has been established that treatment of [(p-cym)-
RuCl2]2 with AgPF6 (1:1) in acetone leads to the formation
of the cationic dinuclear complex [(p-cym)Ru(μ-Cl)3Ru-
(p-cym)]PF6.65 When the ratio Agþ: [(p-cym)RuCl2]2 is
increased to 4:1, even the dicationic species [(p-cym)Ru-
(solv)3]2þ can be obtained (solv = acetone, H3CCN).66,67
In the course of our own studies, we confirmed that
[(p-cym)Ru(μ-Cl)3Ru(p-cym)]PF6 also forms upon stirring
of [(p-cym)RuCl2]2 with TlPF6 in THF. In contrast to the
reaction with Agþ ions, however, the reaction with TlPF6
stops at the stage of the dinuclear complex, even when a
4-fold excess of the Tlþ salt is added (1H NMR spectroscopic
control; note that the use of THF as solvent is essential).
Given this background, we have treated a THF solution of
[(p-cym)RuCl2]2 with [Li(thf)L1] in the presence of TlPF6
1
(1:2:4 equiv, respectively; Scheme 3). A H NMR spectrum
of the crude reaction mixture revealed the formation of new
complexes; however, the target compound [(p-cym)Ru-
(L1)]PF6 could not be detected. Removal of TlCl, followed
by layering of the concentrated filtrate with hexane, gave two
kinds of crystals, yellow plates and light orange plates, which
were identified by X-ray analysis as [(p-cym)Ru(L2)] and
[(p-cym)Ru(μ-Cl)(μ-pz)2Ru(p-cym)]PF6, respectively (cf. the
Supporting Information for details of the X-ray crystal struc-
ture analysis of [(p-cym)Ru(μ-Cl)(μ-pz)2Ru(p-cym)]PF6
(Figure 5S) and for a compilation of related complexes). We
were able to isolate the complex [(p-cym)Ru(L2)] in pure form
(yield 20%) by column chromatography of the product mix-
ture on silica gel with CH2Cl2 as eluent (for a targeted synthe-
sis and full characterization of [(p-cym)Ru(L2)], see below).
There are two likely scenarios of how the boroxine back-
bone of the complex [(p-cym)Ru(L2)] might have formed: (i)
hydrolysis of the ligand [L1]- by water possibly originating
from the hygroscopic TlPF6; (ii) degradation of 3 equiv of
[L1]- and reassembly of the new ligand molecule. Both
scenarios can also explain the source of the bridging pyr-
azolide ligands in the byproduct [(p-cym)Ru(μ-Cl)(μ-pz)2-
Ru(p-cym)]PF6.
Figure 3. Molecular structure of the complex [(p-cym)Ru-
(L1)]PF6. The anion is not shown, and all H atoms are omitted
for clarity. Displacement ellipsoids are drawn at the 50%
probability level. Selected bond lengths (A) and bond angles
(deg): Ru(1)-O(1) = 2.135(3), Ru(1)-N(2) = 2.070(4), Ru-
(1)-N(12) = 2.079(4), Ru(1) COG(p-cym) = 1.689; O-
3 3 3
(1)-Ru(1)-N(2) = 78.1(2), O(1)-Ru(1)-N(12) = 78.1(2),
N(2)-Ru(1)-N(12) = 86.5(2), O(1)-B(1)-N(1) = 108.9(5),
O(1)-B(1)-N(21) = 101.2(4), O(1)-B(2)-N(11) = 106.9(4),
O(1)-B(2)-N(22) = 100.9(4), B(1)-O(1)-B(2) = 115.5(4).
COG(p-cym) denotes the centroid of the p-cymene ring.
partially reversible wave at E1/2 = -1.78 V (vs FcH/FcHþ;
cf. the Supporting Information for more details).
The molecular structure of [(p-cym)Ru(L1)]PF6 is pre-
sented in Figure 3; selected crystallographic data are sum-
marized in Table 3S (cf. the Supporting Information). The
RuII ion, which possesses a distorted-octahedral ligand
sphere, is η6-coordinated to the p-cymene ring and estab-
lishes Ru-N and Ru-O bonds to the [L1]- ligand. The
distance between the RuII ion and the centroid of the
p-cymene ring amounts to Ru(1) COG(p-cym) = 1.689
3 3 3
A, in accordance with other (arene)RuII complexes (e.g.,
In order to exclude adventitious traces of water in our
setup, we repeated the reaction with preformed [(p-cym)Ru-
[(p-cym)Ru(C5H4R)]PF6, Ru COG(p-cym) = 1.70 A;63
3 3 3
68
(μ-Cl)3(p-cym)]PF6 and [Li(thf)L1] (1:2) in THF under
[(p-cym)Ru(pz3BH)]PF6, Ru COG(p-cym) = 1.70 A).64
3 3 3
strictly anhydrous conditions. This modified protocol again
afforded a complicated product mixture; however, the com-
plex [(p-cym)Ru(L2)] was absent in this mixture according to
1H and 11B NMR spectroscopy.
The Ru-O and Ru-N bond lengths are equal to 2.135(3)
and 2.070(4)/2.079(4) A, respectively. These values are signi-
ficantly smaller than in the corresponding Cp* complex
[Cp*Ru(L1)]A (Ru-O = 2.227(3) A; Ru-N = 2.139(4)/
2.140(5) A) but comparable to those of the cationic RuIII
congener [Cp*Ru(L1)]PF6 (Ru-O = 2.173(2) A; Ru-N =
2.078(2)/2.086(2) A).
In view of these results, we abandoned TlPF6 and switched
to AgBF4. In contrast to a priori expectations, treatment of
[(p-cym)RuCl2]2 with AgBF4 (1:4, THF) followed by addi-
tion of [Li(thf)L1] gave neither [(p-cym)Ru(L1)]BF4 nor
[(p-cym)Ru(L2)] but an inseparable mixture of other pro-
ducts (1H and 11B NMR spectroscopic control). The situa-
tion changed when Ag[Tos] (4 equiv, THF) was used instead
of AgBF4, because now [(p-cym)Ru(L1)][Tos] could be iso-
lated in 67% yield (cf. the Supporting Information). The
analogous reaction with Ag[Tfl] led to [(p-cym)Ru(L1)][Tfl]
in 28% yield (cf. the Supporting Information).
Hoping to circumvent problems arising from the delete-
rious effects of chloride ions on [(p-cym)Ru(L1)]þ (see
above), we decided to test the possibility of improving our
synthesis protocol by adding appropriate Tlþ or Agþ salts
already at the beginning of the reaction. Because of the good
solubility of LiCl in THF, 4 equiv of Tlþ/Agþ ions has to be
employed per equiv of [(p-cym)RuCl2]2 in order to achieve
complete removal of chloride. However, the presence of
larger quantities of Tlþ/Agþ salts at an early stage of the
reaction sequence may influence the outcome also by chang-
ing the chemical composition of the starting material. For
(65) Arthur, T.; Stephenson, T. A. J. Organomet. Chem. 1981, 208,
369–387.
(66) Bennett, M. A.; Smith, A. K. J. Chem. Soc., Dalton Trans. 1974,
233–241.
(67) Bennett, M. A.; Matheson, T. W.; Robertson, G. B.; Steffen,
W. L.; Turney, T. W. J. Chem. Soc., Chem. Commun. 1979, 32–33.
(68) Robertson, D. R.; Stephenson, T. A.; Arthur, T. J. Organomet.
Chem. 1978, 162, 121–136.
(63) Abbenhuis, H. C. L.; Burckhardt, U.; Gramlich, V.; Martelletti,
A.; Spencer, J.; Steiner, I.; Togni, A. Organometallics 1996, 15, 1614–
1621.
(64) Bhambri, S.; Tocher, D. A. Polyhedron 1996, 15, 2763–2770.

Article
Our findings are most likely attributable to the different
Organometallics, Vol. 29, No. 4, 2010 971
Scheme 4. Targeted Synthesis of Li[Li(thf)L2] and [(p-cym)-
Ru(L2)]a
natures of the intermediate species formed in the mixture
after the reaction between [(p-cym)RuCl2]2 and Agþ has
occurred. We assume that addition of AgBF4 generates [(p-
cym)Ru(thf)x]2þ (with noncoordinated [BF4]- counterions),
which in turn induces degradation of the ligand [L1]-. Since
the [Tos]- ion is more prone to metal coordination than
[BF4]-, the use of Ag[Tos] leads to more stable complexes,
among them the dimer [(p-cym)Ru(Tos)(μ-Cl)2Ru(Tos)-
(p-cym)], which we have identified in the reaction mixture
by comparison of its NMR data with those of an authentic
sample (cf. the Supporting Information for the synthesis,
elemental analysis, and X-ray crystal structure analysis
of [(p-cym)Ru(Tos)(μ-Cl)2Ru(Tos)(p-cym)] (Figure 6S)).
To find out whether [(p-cym)Ru(Tos)(μ-Cl)2Ru(Tos)-
(p-cym)] is indeed a viable precursor of [(p-cym)Ru(L1)]-
[Tos], we treated a THF solution of the dinuclear complex
with 2 equiv of [Li(thf)L1] and isolated [(p-cym)Ru(L1)][Tos]
in 70% yield. In contrast to [(p-cym)Ru(L1)]Cl, [(p-cym)Ru-
(L1)][Tos] does not show signs of degradation upon pro-
longed stirring of the reaction mixture.
Synthesis and Characterization of Li[Li(thf)L2] and
[(p-cym)Ru(L2)]. The serendipitous discovery of the complex
[(p-cym)Ru(L2)] illustrates only one facet of a more serious
problem: on several occasions we already had to recognize
that the stability of [Li(thf)L1] toward water and/or strongly
Lewis acidic transition-metal ions is less than desirable for a
universally applicable new ligand system. Looking at the
structural parameters of ligand [L1]-, it is reasonable to
assume that the μ-pz linker is a little too small to span the
distance between the two boron atoms without forcing
the molecule to adopt nonideal bond angles (cf. O-B-N-
(μ-pz) = 101.6(2), 101.7(2)° as opposed to 109.5°).42
a Legend: (i) THF, room temperature, 30 min; (ii) THF, room
temperature, 12 h.
a neat mixture of Ph3B3O3 and Hpz affords the pyrazabole
Ph2B(μ-pz)2BPh2.53
Since a pyrazolide ion should have an even stronger tendency
to bridge adjacent boron atoms of Ph3B3O3 than the parent
pyrazole, we reasoned that the entire project of a targeted
synthesis of ligand [L2]2- would be put into jeopardy if
The ligand backbone in [(p-cym)Ru(L2)] consists of a
three-atom O-B-O bridge instead of the two-atom N-N
bridge, which should alleviate the angle strain. We therefore
decided to work out a targeted synthesis of the free ligand
[L2]2- (Scheme 4) in order to arrive at a chelator that is
potentially more stable but still features the main coordina-
Niedenzu’s suggestion of the molecular structure of Ph3B3O3
Hpz was indeed correct. We therefore investigated Ph3B3O3
Hpz by X-ray crystallography (Figure 4; cf. also Table 4S in the
Supporting Information), and thereby confirmed that the
compound possesses an ordinary adduct structure with only
3
3
tion characteristics of [L1]-.
Niedenzu53 and Koster already reported on the room-
69
€
temperature reaction of 2,4,6-triphenylboroxine, Ph3B3O3,
€
one boron-nitrogen bond, as proposed by Koster and as
required for further transition-metal complexation.
with pyrazole (1:1), which gives the monoadduct Ph3B3O3
Hpz. Both authors, however, had conflicting opinions about
3
The N(1)-B(1) bond length amounts to 1.613(4) A and is
thus slightly shorter than the N-B bond in the pyridine
the molecular architecture of Ph3B3O3 Hpz: Niedenzu pro-
3
posed the unusual tricyclic structure (PhB(μ-OB(Ph)O)-
(μ-OH)(μ-pz)BPh) in which both pyrazolyl nitrogen atoms
monoadduct Ph3B3O3 Py (1.635(4) A70/1.640(3) A71).
3
Ph3B3O3 Hpz is further stabilized by an O(2)-H-N(2)
€
are involved in boron coordination, while Koster suggested
3
hydrogen bond (O(2) N(2) = 2.835(3) A, O(2)-H(2) =
that only one B-N adduct bond was present in the molecule.
Niedenzu et al. further claimed the formation of the triad-
3 3 3
2.52(3) A). Such an intramolecular hydrogen-bonding motif
has previously been reported by Wang et al. for the
duct Ph3B3O3 3Hpz after they had increased the stoichio-
3
7-azaindole adduct of Ph3B3O3 (O N distance within the
3 3 3
metric ratio Ph3B3O3:Hpz to 1:3. They described Ph3B3O3
3Hpz as being unstable in time and reverting slowly to the
3
hydrogen bond in Ph3B3O3 (7-azaind) = 2.792 A; these
3
OBO-bridged pyrazabole PhB(μ-OB(Ph)O)(μ-pz)2BPh.53
researchers already suggested that Ph3B3O3 Hpz should also
3
have a hydrogen bond structure in the solid state).71 Similar
to Ph3B3O3 Py, the B3O3 core in Ph3B3O3 Hpz adopts an
€
This view was again questioned by Koster, who provided
NMR evidence that even a 3-fold excess of pyrazole leads to
coordination of only one nitrogen atom per boroxine mole-
cule. Both authors further conclude that Ph3B3O3 Hpz is in a
3
3
almost planar conformation; all bond lengths and angles
within the boroxine ring possess comparable values in both
compounds and therefore do not merit further discussion.
3
multistep equilibrium with the OBO-bridged pyrazabole.
The dehydration sequence to PhB(μ-OB(Ph)O)(μ-pz)2BPh
can be driven to completion by heating (145 °C, mesitylene)
or, more efficiently, by azeotropic distillation.53,69 Refluxing
(70) Beckmann, J.; Dakternieks, D.; Duthie, A.; Lim, A. E. K.;
Tiekink, E. R. T. J. Organomet. Chem. 2001, 633, 149–156.
(71) Wu, Q. G.; Wu, G.; Brancaleon, L.; Wang, S. Organometallics
1999, 18, 2553–2556.
€
(69) Yalpani, M.; Koster, R. Chem. Ber. 1988, 121, 1553–1556.

972 Organometallics, Vol. 29, No. 4, 2010
Mutseneck et al.
Figure 4. Molecular structure of Ph3B3O3 Hpz. Displacement
3
ellipsoids are drawn at the 50% probability level. Selected bond
lengths (A), bond angles (deg), and torsion angles (deg): O(1)-
B(1)= 1.463(4), O(1)-B(3) = 1.352(3), O(2)-B(1) = 1.466(3),
O(2)-B(2) = 1.360(3), O(3)-B(2) = 1.388(4), O(3)-B(3) =
1.400(4), N(1)-B(1) = 1.613(4); O(1)-B(1)-O(2) = 114.3(2),
O(2)-B(2)-O(3)=121.0(2), O(1)-B(3)-O(3)=120.5(3), B(1)-
O(1)-B(3)=121.9(2), B(1)-O(2)-B(2)=121.4(2), B(2)-O(3)-
B(3)=120.2(2); N(1)-B(1)-O(1)-B(3)=104.3(2), N(1)-B(1)-
O(2)-B(2)=-105.1(2), O(2)-B(1)-O(1)-B(3)=-10.0(3).
Figure 5. Molecular structure of the [Li(thf)L2]- anion of Li-
(thf)([12]-c-4)[Li(thf)L2]. H atoms are omitted for clarity; displa-
cement ellipsoids are drawn at the 30% probability level. Selected
bond lengths (A) and angles (deg): O(1)-Li(1) = 1.892(7), O-
(61)-Li(1)=1.939(7), N(12)-Li(1)=2.035(8), N(22)-Li(1)=
2.055(8), O(1)-B(1)=1.443(4), O(1)-B(2)=1.447(4), O(2)-B-
(2) = 1.464(4), O(2)-B(3) = 1.347(4), O(3)-B(1) = 1.469(4),
O(3)-B(3) = 1.353(4), N(11)-B(1) = 1.605(4), N(21)-B(2) =
1.611(4); N(12)-Li(1)-N(22) = 125.3(4), O(1)-B(1)-O(3) =
114.5(3), O(1)-B(2)-O(2)=115.1(2), O(2)-B(3)-O(3)=123.2-
(3), B(1)-O(1)-B(2) = 123.1(2), B(1)-O(3)-B(3) = 121.7(3),
B(2)-O(2)-B(3) = 121.4(2), O(1)-B(1)-N(11) = 106.7(3),
O(1)-B(2)-N(21)=106.7(2).
Literature has it that the formation of triorganoboroxine
rings, R3B3O3, via condensation of organylboronic acids,
RB(OH)2, is more facile in the presence of pyridine.72,73 In
line with that, hydrolysis of Ph3B3O3 occurs readily in
organic solvents at room temperature already with traces
of water, whereas solutions of Ph3B3O3 Py remain un-
3
changed under the same conditions.70 DFT calculations
indicate the high stability of 1:1 boroxine-Lewis base com-
plexes to be due to a relief of ring strain upon adduct
formation.70 A computational study by Kua and Iovine on
the formation of para-substituted triphenylboroxines, how-
ever, revealed that not only ring-strain effects but also
electronic effects are likely to play a role in stabilizing or
destabilizing the amine adducts.74,75 Most importantly, all
experimentally and theoretically obtained results are consis-
tent with the view that the formation of 1:1 adducts, but not
1:2 adducts, between boroxine and amines is thermodyna-
mically favorable.
substituents, whereas the two pyrazolyl rings give rise to only
one signal set. Thus, in contrast to various complexes
Ph3B3O3 L, where L is a simple terminal ligand,71 Li[Li-
3
(thf)L2] shows no signs of a dynamic B-N association/
dissociation equilibrium at room temperature on the NMR
time scale.
Compound Li[Li(thf)L2] was recrystallized from THF/
hexane in the presence of [12]-crown-4 as ether solvate
Li(thf)([12]-c-4)[Li(thf)L2]. Selected crystallographic data
are summarized in Table 4S (cf. the Supporting In-
formation). The molecular structure of the anionic subunit
[Li(thf)L2]- is presented in Figure 5.
The Liþ ion of [Li(thf)L2]- binds to the [Ph3B3O3 2pz]2-
In spite of these discouraging reports, we have now shown
that stirring a mixture of 2 equiv of Lipz and 1 equiv of
boroxine in THF affords the diadduct Li[Li(thf)L2]
(Scheme 4) in virtually quantitative yield.
3
ligand via Li-N and Li-O bonds. The bond lengths
N(12)-Li(1) = 2.035(8) A and N(22)-Li(1) = 2.055(8) A
are comparable to those in the related compound [Li(thf)L1]
(N-Li = 2.023(4), 2.034(4) A).42 However, we note that a
change of the bridging element from [pz]- to [OB(Ph)O]2-
causes a widening of the N-Li-N0 angle from 115.9(2)° to
125.3(4)°. The opposite is true for the O-Li bonds and the
corresponding Li-O-B angles: while the bond is signifi-
cantly shorter in [Li(thf)L2]- than in the pyrazolide-bridged
[Li(thf)L1] (1.892(7) A vs 1.940(4) A), the angles remain
largely the same in both molecules.42 The B-O-B0
angle increases by 7.2(2)° upon going from [Li(thf)L1] to
the boroxine derivative [Li(thf)L2]- (B(1)-O(1)-B(2) =
123.1(2)°).
The 11B NMR spectrum (THF-d8) of Li[Li(thf)L2] is
characterized by resonances at ca. 30 and 5.5 ppm (integral
ratio 1:2), corresponding to three- and four-coordinate
boron atoms, respectively. In line with the proposed struc-
1
ture of Li[Li(thf)L2], two sets of H and 13C NMR reso-
nances can be assigned to magnetically inequivalent phenyl
(72) Perttu, E. K.; Arnold, M.; Iovine, P. M. Tetrahedron Lett. 2005,
46, 8753–8756.
(73) Iovine, P. M.; Gyselbrecht, C. R.; Perttu, E. K.; Klick, C.;
Neuwelt, A.; Loera, J.; DiPasquale, A. G.; Rheingold, A. L.; Kua, J.
Dalton Trans. 2008, 3791–3794.
(74) Kua, J.; Iovine, P. M. J. Phys. Chem. A 2005, 109, 8938–8943.
(75) Kua, J.; Fletcher, M. N.; Iovine, P. M. J. Phys. Chem. A 2006,
110, 8158–8166.
As in the case of [Li(thf)L1], the coordination sphere
around the Liþ ion of [Li(thf)L2]- is completed by one
THF molecule (O(61)-Li(1) = 1.939(7) A). The geometry

Article
Organometallics, Vol. 29, No. 4, 2010 973
index τ4 = 0.7776 for the corresponding coordination poly-
hedron indicates that the ligand sphere of the Liþ ion more
closely approaches a trigonal-pyramidal (τ4 = 0.85) than a
tetrahedral (τ4 = 1.00) configuration (cf. [Li(thf)L1]: τ4 =
0.83).
In comparison to Ph3B3O3 Hpz, it is interesting to see that
3
the N-B bond lengths of [Li(thf)L2]- (N(11)-B(1) =
1.605(4) A, N(21)-B(2) = 1.611(4) A) lie in the same range
as in the pyrazole monoadduct (1.613(4) A) and that the
boroxine ring of [Li(thf)L2]- is also practically planar.
As expected, the shortest O-B bonds in [Li(thf)L2]- are
those involving the sp2-hybridized boron atom (O(2)-B-
(3) = 1.347(4) A, O(3)-B(3) = 1.353(4) A; cf. Ph3B3O3
(O-B)av = 1.382 A).77 The longest bond lengths are ob-
served for O(2)-B(2) = 1.464(4) A and O(3)-B(1) =
1.469(4) A, while the O-B bonds between the Liþ-coordi-
nated oxygen atom O(1) and the sp3-hybridized boron
centers possess intermediate lengths (O(1)-B(1) = 1.443-
(4) A, O(1)-B(2) = 1.447(4) A).
For the targeted synthesis of [(p-cym)Ru(L2)] (Scheme 4),
we treated a mixture of Lipz and Ph3B3O3 (2:1) in THF with
0.5 equiv of [(p-cym)RuCl2]2. Importantly, the yield of
[(p-cym)Ru(L2)] (33%) depends strongly on the amount of
Lipz used and reduces to 0% when 3 equiv of Lipz are
employed.
Figure 6. Molecular structure of [(p-cym)Ru(L2)]. H atoms are
omitted for clarity; displacement ellipsoids are drawn at the 50%
probability level. Selected bond lengths (A) and angles (deg):
Ru(1)-O(1)=2.142(1), Ru(1)-N(2)=2.078(2), Ru(1)-N(12)=
2.079(2), Ru(1) COG(p-cym)=1.690, O(1)-B(1)=1.502(3),
3 3 3
[(p-cym)Ru(L2)] is readily soluble in THF and CH2Cl2 but
only moderately soluble in acetone and CH3CN, the solu-
tions being air-stable at least for several hours (NMR
spectroscopic control).
O(1)-B(2) = 1.500(3), O(2)-B(2) = 1.453(3), O(2)-B(3) =
1.365(3), O(3)-B(1)=1.455(3), O(3)-B(3)=1.364(3); O(1)-Ru-
(1)-N(2) = 78.3(1), O(1)-Ru(1)-N(12) = 79.5(1), N(2)-Ru-
(1)-N(12) = 82.7(1), B(1)-O(1)-B(2) = 121.3(2), O(1)-
B(1)-O(3)=113.9(2), O(1)-B(2)-O(2)=113.9(2). COG(p-cym)
denotes the centroid of the p-cymene ring.
Similar to the case for Li[Li(thf)L2], the 11B NMR spec-
trum (CDCl3) of [(p-cym)Ru(L2)] is characterized by a broad
signal at ca. 28 ppm and a sharp signal at 5.0 ppm, confirm-
ing the presence of three- and four-coordinate boron atoms
in the complex61,62 (note that the integral ratio of the two
signals is less than 1:2, which we attribute to difficulties in
the determination of the integral value of the broad sig-
in which a transition-metal ion is coordinated to a discrete
boroxine ring. The Ru-O interaction is stabilized by two
bridging pyrazolide rings. All bond lengths and angles within
the coordination environment of the RuII ion are almost
identical with those in [(p-cym)Ru(L1)]PF6; the largest
difference is observed for the N(2)-Ru(1)-N(12) angle,
which is more acute in [(p-cym)Ru(L2)] (82.7(1)°) than in
[(p-cym)Ru(L1)]PF6 (86.5(2)°).
1
nal (h1/2 = 870 Hz)). The H and 13C NMR spectra of
[(p-cym)Ru(L2)] show resonances of two magnetically
equivalent pyrazolyl rings (δ(1H) 6.20 (H-4), 7.48, 7.62
(H-3,5)) and two sets of signals for the three phenyl groups.
According to CV measurements, [(p-cym)Ru(L2)] under-
Most of the key structural parameters of the [L2]2- part of
[(p-cym)Ru(L2)] closely resemble those of [Li(thf)L2]-, with
the remarkable exception of the O-B bonds involving the
metal-coordinated oxygen atom, which are, on average,
0.056 A longer in [(p-cym)Ru(L2)] (O(1)-B(1) = 1.502(3)
A, O(1)-B(2) = 1.500(3) A) than in the lithium complex.
Moreover, the boroxine ring of [(p-cym)Ru(L2)] is slightly
nonplanar and adopts a shallow chair conformation with
dihedral angles B(1)O(1)B(2)//B(1)B(2)O(2)O(3) and O(2)B-
(3)O(3)//B(1)B(2)O(2)O(3) of 14.7 and 12.4°, respectively
(in [Li(thf)L2]-, the corresponding angles amount to 9.5
and 2.5°).
goes one partially reversible reduction process at E1/2
=
-2.09 V (vs FcH/FcHþ, ia/ic = 0.64; cf. the Supporting
Information for more details). This electrochemical feature
is observed at a more cathodic potential value than in the
case of [(p-cym)Ru(L1)]PF6 (E1/2 = -1.78 V vs FcH/FcHþ).
Given that [(p-cym)Ru(L2)] and [(p-cym)Ru(L1)]PF6 have
practically the same coordination environment at the ruthe-
nium center, these differences in the redox potentials are
most likely due to the fact that [(p-cym)Ru(L2)] contains a
dianionic and [(p-cym)Ru(L1)]PF6 a monoanionic (pyrazol-
1-yl)borate ligand. In contrast to [(p-cym)Ru(L1)]PF6,
[(p-cym)Ru(L2)] did not undergo oxidation under the mea-
surement conditions applied.
Similar to the case for [Li(thf)L2]- vs [Li(thf)L1], the
B(1)-O(1)-B(2) angle of [(p-cym)Ru(L2)] (121.3(2)°) is
larger by 5.8(2)° than that of [(p-cym)Ru(L1)]PF6
(115.5(4)°).
The molecular structure of [(p-cym)Ru(L2)] is shown in
Figure 6 (cf. also Table 4S in the Supporting Information).
[(p-cym)Ru(L2)] is one of very few examples78 of complexes
Conclusion
The reaction of the tripodal [N,O,N] (pyrazol-1-yl)borate
ligand [Ph(pz)B(μ-O)(μ-pz)B(pz)Ph]- ([L1]-) with [Cp*Ru-
Cl]4 and [(p-cym)RuCl2]2 leads to the pseudo-octahedral
RuII complexes [Cp*Ru(L1)] and [(p-cym)Ru(L1)]Cl, res-
pectively (pz = pyrazolyl, Cp* = pentamethylcyclopenta-
dienyl, p-cym = p-cymene). [(p-cym)Ru(L1)]Cl slowly
(76) Yang, L.; Powell, D. R.; Houser, R. P. Dalton Trans. 2007, 955–
964. The geometry index τ4 = {360° - (R þ β)}/141° is defined for four-
coordinate complexes, with R and β being the two largest bond angles around
the central atom.
(77) Boese, R.; Polk, M.; Blaser, D. Angew. Chem., Int. Ed. Engl.
1987, 26, 245–247.
€
(78) Behm, H. Acta Crystallogr. 1988, C44, 1348–1351.

974 Organometallics, Vol. 29, No. 4, 2010
Mutseneck et al.
decomposes in solution but can be stabilized by exchanging
-
1.26 (s, 15H; CH3), 6.24 (vt, 2H; pzH-4), 6.44 (t, 1H, 3JHH = 2.4
Hz; μ-pzH-4), 7.13-7.17 (m, 4H; PhH), 7.24-7.28 (m, 8H;
the Cl- anion for PF6
.
3
Attempts to remove the Cl- ions by addition of 4 equiv of
TlPF6 already at the initial stage of the reaction did not lead
to [(p-cym)Ru(L1)]PF6, but resulted in a mixture of other
products. The most abundant constituents of this mixture
were the dinuclear complex [(p-cym)Ru(μ-Cl)(μ-pz)2Ru-
(p-cym)]PF6 and the mononuclear species [(p-cym)Ru-
(L2)] ([L2]2- = [Ph(pz)B(μ-O)(μ-OB(Ph)O)B(pz)Ph]2-).
[(p-cym)Ru(L2)] contains a promising new [N,O,N] (pyra-
zol-1-yl)borate ligand with a phenylboroxine backbone. In a
subsequent targeted synthesis, we prepared the lithium salt
Li[Li(thf)L2] from Ph3B3O3 and 2 equiv of Lipz and success-
fully transformed it into [(p-cym)Ru(L2)] by treatment with
[(p-cym)RuCl2]2.
PhH/pzH-3 or 5), 7.51 (d, 2H, JHH = 2.4 Hz; μ-pzH-3,5),
7.61 (br, 2H; pzH-3 or 5). 11B{1H} NMR (96.3 MHz, CDCl3): δ
5.7. 13C{1H} NMR (100.6 MHz, CDCl3): δ 9.9 (CH3), 73.3
(CCH3), 106.2 (pzC-4), 109.7 (μ-pzC-4), 127.1 (PhC), 127.3
(PhC-p), 130.2 (br, pzC-3 or 5), 130.4 (μ-pzC-3,5), 132.8
(PhC), 139.2 (br, pzC-3 or 5). Anal. Calcd for C31H34B2N6ORu
[629.33]: C, 59.16; H, 5.45; N, 13.35. Found: C, 59.11; H, 5.53;
N, 13.21.
Synthesis of [Cp*Ru(L1)]PF6. A mixture of [Cp*Ru(L1)]
(82 mg, 0.13 mmol) and [Cp2Fe]PF6 (36 mg, 0.11 mmol) was
stirred in CH2Cl2 (10 mL) overnight. The solvent was evapo-
rated under vacuum to give a dark red solid, which was extracted
with pentane to remove ferrocene and excess [Cp*Ru(L1)]. The
crude product was redissolved in CH2Cl2 (3 mL) and layered
with pentane (15 mL), whereupon dark red crystals grew near
the interface of the two liquids. The crystals were suitable for
X-ray analysis. Yield: 75 mg (88%). Anal. Calcd for C31H34-
B2F6N6OPRu [774.30]: C, 48.09; H, 4.43; N, 10.85. Found: C,
48.35; H, 4.48; N, 10.99.
Our preliminary experience with [L1]- and [L2]2- indicates
the latter ligand to be less prone to hydrolysis and more
tolerant to strongly Lewis acidic transition metal complex
fragments. Since the substitution pattern of the boroxine
backbone and/or the pyrazolyl rings can easily be varied, the
straightforward synthesis procedure developed for [L2]2-
should also make a broad selection of custom-tailored
derivatives readily accessible. Moreover, [L1]- and [L2]2-
provide very similar coordination environments but differ
in their electronic charge, thereby offering another set screw
for the gradual adjustment of the properties of a chelated
metal center (cf. the electrode potential required for the
partly reversible reduction of [(p-cym)Ru(L2)], which is more
cathodic by -0.31 V than that of [(p-cym)Ru(L1)]PF6).
In summary, we suggest L1- and L2-type ligands as viable
alternatives to more conventional mixed-donor bis(pyrazol-
1-yl)borates [RB(OR0)pz2]-, because they are comparatively
easy to prepare and show a higher conformational flexibility
together with a lower tendency to substituent scrambling.
Synthesis of [(p-cym)Ru(L1)]PF6. A mixture of [Li(thf)L1] (154
mg, 0.33 mmol) and [(p-cym)RuCl2]2 (100 mg, 0.16 mmol) was
stirred in THF (5 mL) for 2 h. A yellow precipitate formed,
which was isolated by filtration, washed with Et2O, redissolved
in CH2Cl2 (5 mL), and treated with NH4PF6 (53 mg, 0.33
mmol). After 2 h, the mixture was filtered and the filtrate
concentrated under reduced pressure to a volume of 2 mL.
Addition of Et2O (20 mL) resulted in the precipitation of a
yellow solid, which was collected on a frit, washed with Et2O,
and dried under vacuum. Yield: 171 mg (69%). X-ray-quality
crystals were obtained by slow diffusion of hexane into a CH2Cl2
1
solution of the complex. H NMR (300 MHz, CDCl3): δ 0.72
(d, 6H, 3JHH = 6.9 Hz; (CH3)2CH), 1.63 (s, 3H; CH3), 1.83 (sept,
3
1H, JHH = 6.9 Hz; (CH3)2CH), 5.31, 5.54 (2 ꢀ d, 2 ꢀ 2H,
3JHH = 6.0 Hz; p-cymH), 6.31 (vt, 2H; pzH-4), 6.58 (t, 1H,
3JHH = 2.4 Hz; μ-pzH-4), 7.27-7.29, 7.39-7.41 (2 ꢀ m, 4H, 6H;
3
PhH), 7.44 (d, 2H, JHH = 2.4 Hz; pzH-3 or 5), 7.62 (d, 2H,
3JHH = 2.4 Hz; μ-pzH-3,5), 8.07 (d, 2H, 3JHH = 2.1 Hz; pzH-3
or 5). 11B{1H} NMR (96.3 MHz, CDCl3): δ 5.9. 13C{1H} NMR
(100.6 MHz, CDCl3): δ 18.2 (CH3), 21.8 ((CH3)2CH), 30.6
((CH3)2CH), 81.3, 81.9, 101.1, 106.1 (p-cymC), 109.2 (pzC-4),
111.2 (μ-pzC-4), 128.1 (PhC), 129.0 (PhC-p), 131.8 (PhC), 132.1,
132.3 (μ-pzC-3,5/pzC-3 or 5), 143.6 (pzC-3 or 5). Anal. Calcd for
C31H33B2F6N6OPRu [773.29]: C, 48.15; H, 4.30; N, 10.87.
Found: C, 47.89; H, 4.30; N, 10.63.
Experimental Section
General Considerations. All reactions were carried out under
nitrogen using standard Schlenk techniques. All solvents were
dried and distilled prior to use. Starting materials [Cp*RuCl]4,54
PhB(NMe2)2,51 2,4,6-Ph3B3O3,52 and [Li(thf)L1]42 were pre-
pared as published in the literature. [(p-cym)Ru(μ-Cl)3-
(p-cym)]PF6 was synthesized by adapting a synthesis protocol
previously reported for the synthesis of the analogous benzene
derivative.68 Compounds [Cp2Fe]PF6 and [(p-cym)RuCl2]2 were
Synthesis of [(p-cym)Ru(μ-Cl)3Ru(p-cym)]PF6. A mixture of
[(p-cym)RuCl2]2 (100 mg, 0.16 mmol) and TlPF6 (230 mg, 0.65
mmol) in THF (15 mL) was stirred for 2 h. TlCl was removed by
filtration, the filtrate was evaporated to dryness under reduced
pressure, and the orange solid residue was carefully dried. Yield:
1
obtained from commercial sources. H, 13C{1H}, and 11B{1H}
NMR spectra were recorded with Bruker AMX 300 or Avance
400 spectrometers at room temperature. Abbreviations: s =
singlet, d = doublet, t = triplet, sept = septet, vt = virtual
triplet, dd = doublet of doublets, m = multiplet, br = broad;
pz = pyrazolide, Tos = tosylate; Tfl = triflate; p-cym =
p-cymene. Cyclic voltammograms were recorded using an
EG&G Princeton Applied Research 263A potentiostat. UV/
vis spectra were recorded on a Varian Cary 50 UV/vis spectro-
photometer. For spectroelectrochemical measurements, the
spectrometer was equipped with a Hellma 661.500 quartz
immersion probe. Elemental analyses were performed by the
microanalytical laboratory of the Goethe University Frankfurt.
Synthesis of [Cp*Ru(L1)]. A mixture of [Li(thf)L1] (96 mg,
0.20 mmol) and [Cp*RuCl]4 (55 mg, 0.05 mmol) was stirred in
THF (10 mL) for 12 h. The solvent was removed under vacuum
and the orange residue extracted into hexane (20 mL). The
extract was concentrated to a volume of 3 mL under reduced
pressure and kept at -30 °C overnight, whereupon yellow-
orange crystals precipitated that were suitable for an X-ray
1
111 mg (95%). H NMR (250.0 MHz, CDCl3): 1.31 (d, 6H,
3JHH = 7.0 Hz; (CH3)2CH), 2.24 (s, 3H; CH3), 2.78 (sept, 1H,
3JHH = 7.0 Hz; (CH3)2CH), 5.48, 5.65 (2 ꢀ d, 2 ꢀ 2H, 3JHH
=
6.3 Hz; p-cymH).
Synthesis of Li(thf)([12]-c-4)[Li(thf)L2]. A mixture of Lipz
(47 mg, 0.64 mmol), Ph3B3O3 (100 mg, 0.32 mmol), and [12]-
crown-4 (113 mg, 0.64 mmol) was stirred in THF (5 mL) for
30 min. Hexane (3 mL) was added, and the solution was cooled
to -5 °C overnight. Colorless X-ray-quality crystals formed,
which were isolated on a frit, rinsed with hexane (2 ꢀ 5 mL), and
briefly dried under reduced pressure. Yield: 260 mg (95%). Since
the crystals tend to lose THF, most of the crop was kept under
vacuum for several hours in order to obtain a well-defined
sample for elemental analysis. 1H NMR (300.0 MHz, THF-
d8): δ 3.58 (s, 16H; [12]-c-4), 5.96 (vt, 2H; pzH-4), 6.80-6.87,
7.07-7.10, 7.25-7.27 (3 ꢀ m, 6H, 4H, 3H; PhH), 7.28, 7.60 (2 ꢀ
d, 2 ꢀ 2H, 3JHH = 1.8 Hz; pzH-3,5), 7.97-8.00 (m, 2H; PhH).
11B{1H} NMR (96.3 MHz, THF-d8): δ 5.5 (s, 2B), ca. 30 (h1/2
=
1
analysis. Yield: 81 mg (64%). H NMR (300 MHz, CDCl3): δ
720 Hz, 1B). 13C{1H} NMR (100.6 MHz, CDCl3): δ 71.6

Article
Organometallics, Vol. 29, No. 4, 2010 975
([12]-c-4), 103.2 (pzC-4), 124.8, 126.6, 127.7, 129.4, 132.4, 132.7,
135.0, 136.5 (PhC/pzC-3,5). Anal. Calcd. for C36H45B3Li2N4O8
[708.06] ꢀ 2 H2O [18.02]: C, 58.11; H, 6.64; N, 7.53. Found: C,
56.83; H, 6.30; N, 7.84.
against F2 with full-matrix least-squares techniques using the
program SHELXL-97.82 All non-hydrogen atoms (except the
disordered atoms of Li(thf)([12]-c-4)[Li(thf)L2]) were refined
with anisotropic displacement parameters. The hydrogen atoms
were geometrically positioned and treated as riding on the
Synthesis of [(p-cym)Ru(L2)]. Method 1. A mixture of
[Li(thf)L1] (200 mg, 0.42 mmol), [(p-cym)RuCl2]2 (130 mg,
0.21 mmol), and TlPF6 (300 mg, 0.86 mmol) in THF (10 mL)
was stirred for 12 h. After TlCl had been removed by filtration,
the filtrate was first concentrated to a volume of 1 mL under
reduced pressure and then layered with hexane (15 mL). Two
kinds of crystals, orange plates and yellow plates, were obtained.
The crystals were identified by X-ray analysis as [(p-cym)Ru-
(μ-Cl)(μ-pz)2Ru(p-cym)]PF6 and [(p-cym)Ru(L2)], respectively.
The crystal mixture was dissolved in CH2Cl2 and filtered
through a thin layer of silica gel (1 ꢀ 0.5 cm) using CH2Cl2
as the eluent. Evaporation of the solvent from the eluate left
[(p-cym)Ru(L2)] as a yellow solid. Yield: 57 mg (20%).
carbon atoms. The Hpz-atom in Ph3B3O3 Hpz was freely
3
refined.
The crystal of [Cp*Ru(L1)] looks monoclinic but is actually a
triclinic twin (twin law: -100/0-10/001) with a ratio of 0.772(1)/
0.228(1) for the two twin components.
In the structure [(p-cym)Ru(L1)]PF6 there are channels con-
taining disordered solvent molecules. However, no reasonable
model could be found for refinement. Thus, the contribution of
the solvent to the reflections has been suppressed with the
SQUEEZE83 option in PLATON.
The crystal lattice of Li(thf)([12]-c-4)[Li(thf)L2] contains
1 equiv of noncoordinating THF molecules, i.e. Li(thf)([12]-
Method 2. A mixture of Lipz (49 mg, 0.66 mmol) and Ph3B3O3
(102 mg, 0.33 mmol) was stirred in THF (10 mL) for 30 min.
[(p-cym)RuCl2]2 (100 mg, 0.16 mmol) was added, and stirring
was continued for 12 h. The solvent was removed under vacuum
to give a yellow-orange solid residue, which was extracted into
CH2Cl2. The extract was filtered through a thin layer of silica gel
(1 ꢀ 0.5 cm) using CH2Cl2 as the eluent. The yellow band was
collected and the eluate evaporated to dryness under vacuum.
c-4)[Li(thf)L2] THF. Two THF molecules (marked with # in
the structure below) and the crown ether ring of Li(thf)([12]-
3
c-4)[Li(thf#)L2] THF# are disordered over two sites with site
3
occupation factors of the major occupied site of 0.63(1)/0.61(1)
for the two THF molecules and 0.62(1) for the crown ether ring.
Equivalent bond lengths and angles in the disordered THF
molecules were restrained to be equal in order to keep the
geometric parameters within a reasonable range.
1
Yield: 72 mg (33%). H NMR (300.0 MHz, CDCl3): δ 1.05
(d, 6H, 3JHH = 6.9 Hz; (CH3)2CH), 1.13 (s, 3H; CH3), 2.09 (sept,
Acknowledgment. M.W. is grateful to the “Deutsche
Forschungsgemeinschaft” (DFG) and the “Fonds der
Chemischen Industrie” (FCI) for financial funding.
3
1H, JHH = 6.9 Hz; (CH3)2CH), 4.17, 5.25 (2 ꢀ d, 2 ꢀ 2H,
3JHH = 6.0 Hz; p-cymH), 6.20 (vt, 2H; pzH-4), 7.16-7.30
3
(m, 13H; PhH), 7.48, 7.62 (2 ꢀ 2H, 2 ꢀ d, JHH = 1.5 Hz;
pzH-3,5), 7.97 (m, 2H; PhH). 11B{1H} NMR (96.3 MHz,
CDCl3): δ 5.0 (s, 2B), ca. 28 (h1/2 = 870 Hz, 1B). 13C{1H}
NMR (75.4 MHz, CDCl3): δ 17.4 (CH3), 22.7 ((CH3)2CH), 29.9
((CH3)2CH), 80.2, 84.1, 100.2, 104.6 (p-cymC), 106.4 (pzC-4),
126.1, 126.9, 127.0, 129.1, 131.8 (PhC), 133.2 (pzC-3 or 5), 134.8
(PhC), 138.9 (pzC-3 or 5). Anal. Calcd for C34H35B3N4O3Ru
[681.16]: C, 59.95; H, 5.18; N, 8.23. Found: C, 60.07; H, 5.23;
N, 7.99%.
Supporting Information Available: CIF files giving crystal-
lographic data for [Cp*Ru(L1)] (CCDC-735031), [Cp*Ru(L1)]PF6
(CCDC-735033), [(p-cym)Ru(L1)]PF6 (CCDC-735035), [(p-cym)-
Ru(μ-Cl)(μ-pz)2Ru(p-cym)]PF6 (CCDC-735032), [(p-cym)Ru-
(Tos)(μ-Cl)2Ru(Tos)(p-cym)] (CCDC-735036), Ph3B3O3 Hpz
3
(CCDC-735030), Li(thf)([12]-c-4)[Li(thf)L2] (CCDC-735037), and
[(p-cym)Ru(L2)] (CCDC-735034), structure plot of [Cp*Ru(L1)]-
PF6, structure plots and a compilation of key structural parameters
of [(p-cym)Ru(μ-Cl)(μ-pz)2Ru(p-cym)]PF6 and [(p-cym)Ru(Tos)-
(μ-Cl)2Ru(Tos)(p-cym)], UV/vis spectra of [Cp*Ru(L1)] and
[Cp*Ru(L1)]PF6, synthesis and NMR data of [(p-cym)Ru(L1)]X
(X=[B(C6F5)4], [Tos], [Tfl]), and details of the electrochemical
investigations of [Cp*Ru(L1)], [(p-cym)Ru(L1)]PF6, and [(p-cym)-
Ru(L2)]. This material is available free of charge via the Internet at
http://pubs.acs.org. Crystallographic data can also be obtained
free of charge from the Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
X-ray Crystal Structure Analysis. Single crystals were ana-
lyzed with a STOE IPDS II two-circle diffractometer with
graphite-monochromated Mo KR (0.710 73 A) radiation. Em-
pirical absorption corrections were performed for all structures,
except Ph3B3O3 Hpz and Li(thf)([12]-c-4)[Li(thf)L2], using the
3
MULABS79 option in PLATON.80 The structures were solved
by direct methods using the program SHELXS81 and refined
(79) Blessing, R. H. Acta Crystallogr. 1995, A51, 33–38.
(80) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7–13.
(81) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467–473.
(82) Sheldrick, G. M. SHELXL-97. A Program for the Refinement of
(83) van der Sluis, P.; Spek, A. L. Acta Crystallogr. 1990, A46, 194–
€
€
€
Crystal Structures; Universitat Gottingen, Gottingen, Germany, 1997.
201.