Boron functionalization and unusual B-C bond activation in rhodium(III) and iridium(III) complexes with diphenylbis(pyrazolylborate) ligands (Ph 2Bp)

Article abstract of DOI:10.1021/om400375j

The Rh(III) and Ir(III) pentamethylcyclopentadienyl compounds [M(Cp*)(κ²-Ph₂Bp)Cl] (M = Rh, 1; M = Ir, 4) were readily prepared from interaction of the salt K[Ph₂Bp] (Ph ₂Bp = diphenylbis(pyrazolyl)borate) and the [M(Cp*)Cl ₂]₂ dimer precursors in dichloromethane under anhydrous conditions. When the same reactions were carried out in non-anhydrous conditions by using acetonitrile as solvent, we observed, in the case of Rh, both B-N bond hydrolysis and Rh-C(Ph) bond activation with the formation of the hydroxy(pyrazolyl)borate complex [Rh(Cp*)(Ph){κ²-(pz)(OH) BPh₂}] (2). In contrast, in the case of Ir only B-N hydrolysis was observed and the ionic species [Ir(Cp*)(Hpz){κ²-(pz)(OH) BPh₂}]Cl (5) was obtained, upon coordination of the liberated Hpz. Additionally, by reaction of 1 with AgClO₄ in acetonitrile, the ionic [Rh(Cp*)(Ph){κ²-(pz)(OH)B(OH)}]ClO₄ (3) was isolated. Complexes 1-3 and 5 have been structurally characterized by X-ray crystallography. Spectral studies have been performed for all species, together with a computational DFT modeling investigation. A decomposition mechanism for the diphenylbis(pyrazolyl)borate ligand in the different complexes is proposed.

Full text of DOI:10.1021/om400375j

Article
pubs.acs.org/Organometallics
Boron Functionalization and Unusual B−C Bond Activation in
Rhodium(III) and Iridium(III) Complexes with
Diphenylbis(pyrazolylborate) Ligands (Ph2Bp)
Riccardo Pettinari,*,† Claudio Pettinari,† Fabio Marchetti,‡ Magda Monari,§ Edoardo Mosconi,∥
and Filippo De Angelis∥
†School of Pharmacy and ‡School of Science and Technology, University of Camerino, via S. Agostino 1, 62032 Camerino MC, Italy
§Dipartimento di Chimica “Giacomo Ciamician”, Universita
̀
di Bologna, Via Selmi 2, 40126 Bologna, Italy
∥Istituto CNR di Scienze e Tecnologie Molecolari (CNR-ISTM), Via Elce di Sotto 8, 06123 Perugia, Italy
S
* Supporting Information
ABSTRACT: The Rh(III) and Ir(III) pentamethylcyclopentadienyl compounds
[M(Cp*)(κ2-Ph2Bp)Cl] (M = Rh, 1; M = Ir, 4) were readily prepared from
interaction of the salt K[Ph2Bp] (Ph2Bp = diphenylbis(pyrazolyl)borate) and the
[M(Cp*)Cl2]2 dimer precursors in dichloromethane under anhydrous conditions.
When the same reactions were carried out in non-anhydrous conditions by using
acetonitrile as solvent, we observed, in the case of Rh, both B−N bond hydrolysis
and Rh−C(Ph) bond activation with the formation of the hydroxy(pyrazolyl)-
borate complex [Rh(Cp*)(Ph){κ2-(pz)(OH)BPh2}] (2). In contrast, in the case
of Ir only B−N hydrolysis was observed and the ionic species [Ir(Cp*)(Hpz){κ2-
(pz)(OH)BPh2}]Cl (5) was obtained, upon coordination of the liberated Hpz.
Additionally, by reaction of 1 with AgClO4 in acetonitrile, the ionic [Rh(Cp*)-
(Ph){κ2-(pz)(OH)B(OH)}]ClO4 (3) was isolated. Complexes 1−3 and 5 have
been structurally characterized by X-ray crystallography. Spectral studies have been
performed for all species, together with a computational DFT modeling investigation. A decomposition mechanism for the
diphenylbis(pyrazolyl)borate ligand in the different complexes is proposed.
[Rh(Cp*)(κ2-Bp)Cl] in high yield. At variance with this, two
INTRODUCTION
■
different compounds, monomeric [IrCp*Cl2(pzH)] and the
The dimeric chloro-bridged complexes [{M(η5-Cp*)(μ-Cl)-
binuclear [Ir(Cp*)Cl]2(μCl)(μ-pz), are generated from the
Cl}2] (M = Rh, Ir) have been extensively used as starting
reaction of [Ir(Cp*)Cl2]2 with K[Bp].7 From the reaction of
materials for a wide number of organometallic complexes that
the analogous diphenyl-substituted ligand Ph2Bp with
[Rh2Cl2(LL)2] (LL = cyclooctadiene, norbornadiene, or
have found applications in several fields.1 These complexes
undergo a rich variety of chemistry through chloro bridge
2CO) formation of the corresponding[Rh(LL)(Ph2Bp)]
cleavage reactions, leading to the formation of interesting
neutral and cationic mononuclear complexes.2 In addition,
derivatives is observed, for which spectroscopic data showed
that the C2v symmetry of the Ph2Bp group was preserved in the
rhodium(III) and iridium(III) complexes containing scorpio-
complexes.8 Thompson prepared the complex [Ir(N,C2′-(2-p-
tolylpyridine))2(Ph2Bp)] in good yields, provided that the
complex is chloride-free prior to the addition of the Ph2Bp
ligand.9 A report describing Mo(II) and W(II) derivatives
containing the Ph2Bp ligand showed the intrinsic instability of
B−N bonds, which readily undergo an hydrolytic cleavage with
formation of the new (OH)(pz)BPh2 ligand acting as a
chelating O,N-donor.10
nate ligands3 have recently attracted considerable attention
because of their ability to activate the aliphatic and aromatic
C−H bonds of hydrocarbons and other substrates.4
Bis(pyrazolyl)borate ligands (Bp′) are versatile anionic
chelating ligands widely used in coordination chemistry, able
to yield stable complexes with all metals across the periodic
table in both low and high oxidation states. In some cases,
however, the nucleophilicity of the B−H bond and the latent
Lewis acidity of the boron center allowed important
functionalization of the BH2 moiety5 as a consequence of
hydrolysis or alcoholysis reactions, yielding hydroxy- and
alkoxy(pyrazolyl)borate complexes, mainly when Rh or Ir
acceptors have been employed.6
As an extension of our works on scorpionates containing
Rh(III) and Ir(III),11 we have undertaken a systematic study of
the reactions of the dimers [M(Cp*)Cl2]2 (M = Rh, Ir) with
the diphenylbis(pyrazolyl)borate K[Ph2Bp] ligand. In addition
to the previously observed B−N hydrolysis, here we report also
We have previously reported that η5-pentamethylcyclopenta-
dienyl rhodium(III) containing the unsubstituted bis-
(pyrazolyl)borate ligand Bp provided the formation of
Received: April 29, 2013
Published: July 9, 2013
© 2013 American Chemical Society
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Article
Calcd for C23H36BClN2O6Rh: C, 47.16; H, 6.20; N, 4.78. Found: C,
on the unexpected B−C activation, with consequent transfer of
one phenyl substituent from the Ph2Bp moiety to the Rh(III)
center.
47.32; H, 6.12; N, 4.68. IR (cm−1): 3221 m ν(O−H), 3123 w, 3060 w
νν(C−H), 1656m ν(O−H), 1564m, 1524w, 1509w ν(C−C, C−N),
̈
̈
1074 s ν(ClO4). 1H NMR (CDCl3, 293 K): δ 1.67 (s, 15H, CH3 Cp*),
6.74 (t, 1H, H4 pz), 6.80−7.18 (m, 5H, Ph), 7.91 (d, 1H, H5 pz), 7.95
(d, 1H, H3 pz), 11.20 (br, 1H, H-OB), 12.0 (br, 1H, H-OB). 13C
NMR (CDCl3, 293 K): δ 9.0 (s, CH3 Cp*), 97.3 (d, C Cp*,
J(103Rh−13C) = 7.8 Hz), 111.5 (s, C4 pz), 123.5, 128.2, 136.0 (s, Ph),
131.3 (s, C5 pz), 143.4 (s, C3 pz). ΛM (CH3CN, 10−3 M): 134.6 Ω−1
cm2 mol−1.
EXPERIMENTAL SECTION
■
Materials and Methods. All chemicals and reagents were of
reagent grade quality and were used as received without further
purification. The potassium salt of the scorpionate ligand K[Ph2Bp]
was synthesized as previously reported.3 Dichloromethane was freshly
distilled from CaH2. Acetonitrile was used without previous
distillation. The samples were dried in vacuo to constant weight (20
°C, ∼0.1 Torr). Elemental analyses were carried out in house with a
Fisons Instruments 1108 CHNSO elemental analyzer. IR spectra were
recorded from 4000 to 600 cm−1 with a Perkin-Elmer Spectrum 100
[Ir(Cp*)(Ph2Bp)Cl] (4). To a dichloromethane solution (10 mL) of
[Cp*IrCl2]2 (0.398 g, 0.50 mmol) was added K[Ph2Bp] (0.338 g, 1.00
mmol) at −10 °C. The mixture was stirred for 1 h, while the
temperature was raised to 0 °C, and then was filtered to remove KCl.
The solution was dried under vacuum, and the orange residue
obtained was recrystallized from 1 1 CH2Cl2/light petroleum (40−60
°C) and shown to be compound 4 (0.541 g, 0.800 mmol, yield 80%).
Anal. Calcd for C29H34BClN4Ir: C, 51.44; H, 5.06; N, 8.27. Found: C,
51.33; H, 5.00; N, 8.12. IR (cm−1): 3194 w, 3116 w, 3056 w, 1602 m,
1566 m, 1543 w ν(C−C, C−N). 1H NMR (CDCl , 293 K): δ 1.78 (s,
1
FT-IR instrument. H and 13C NMR spectra were recorded on a 400
Mercury Plus Varian instrument operating at room temperature (400
MHz for 1H and 100 MHz for 13C). 1H and 13C chemical shifts (δ) are
reported in parts per million (ppm) from SiMe4 (1H and 13C
calibration by internal deuterium solvent lock). Peak multiplicities are
abbreviated as follows: singlet, s; doublet, d; triplet, t; quartet, q;
multiplet, m. Melting points are uncorrected and were taken on an
STMP3 Stuart Scientific instrument and on a capillary apparatus. The
electrical conductivity measurements (ΛM, reported as Ω−1 cm2
mol−1) of acetonitrile solutions of the complexes were taken with a
Crison CDTM 522 conductimeter at room temperature.
̈
̈
3
15H, CH3 Cp*), 6.78−6.95 (m, 6H, Ph and H4 pz), 7.04 (m, 2H, Ph),
7.09 (d, 2H, H5 pz), 7.28 (m, 4H, Ph), 7.88 (d, 2H, H3 pz). 13C NMR
(CDCl3, 293 K): δ 8.7 (s, CH3 Cp*), 88.6 (s, C Cp*), 104.16 (s, C4
pz), 122.2, 126.5, 127.5, 127.9, 128.2 (s, Ph), 137.3 (s, C5 pz), 139.1
(s, C3 pz). Λm (CH3CN, 10−3 M): 1.2 Ω−1 cm2 mol−1. After 1 h the
conductivity of the CH3CN solution increases to 16.1 Ω−1 cm2 mol−1.
After 1 day the conductivity of the acetonitrile solution reaches the
value 65.5 Ω−1 cm2 mol−1.
Syntheses of Complexes. [Rh(Cp*)(Ph2Bp)Cl] (1). To a dichloro-
methane solution (10 mL) of [Rh(Cp*)Cl2]2 (0.618 g, 1.00 mmol)
was added K[Ph2Bp] (0.676 g, 2.00 mmol) at −10 °C. The mixture
was stirred for 1 h, while the temperature was raised to 0 °C, and then
was filtered to remove KCl. The solution was dried under vacuum, and
the orange residue obtained was recrystallized from 1/1 CH2Cl2/light
petroleum (40−60 °C) and shown to be compound 1 (0.334 g, 0.568
mmol, yield 88%). Anal. Calcd for C29H34BClN4Rh, C, 59.26; H, 5.83;
N, 9.53. Found: C, 58.96; H, 5.68; N, 9.60. IR (cm−1): 3112 w, 3037
w, 1564 m, 1526 w ν(C−C, C−N). 1H NMR (CDCl , 293 K): δ 1.26
[Ir(Cp*){(pz)(OH)BPh2(pzH)}]Cl (5). To a stirred acetonitrile
solution (20 mL) of [Cp*IrCl2]2 (0.398 g, 0.5 mmol) was added
0.338 g (1.0 mmol) of K[Ph2Bp]. The red solution was stirred for 10
min, giving an orange solution, and the solvent was then removed
under reduced pressure. The resulting orange solid was vigorously
stirred with 20 mL of chloroform at room temperature for 1 h and
then the solution was slowly evaporated. Yellow-orange crystals of 5
were obtained in 15% yield. Anal. Calcd for C32H44BClIrN4O: C,
51.99; H, 6.00; N, 7.58. Found: C, 52.11; H, 5.89; N, 7.40. IR (cm−1):
3200 br ν(O−H), 3123 w, 3060 w ν(C−H), 1641 m ν(O−H), 1597
w, 1567 w ν(C−C, C−N). 1H NMR (CDCl , 293 K): δ 1.55 (s, 15H,
̈
̈
3
(s, 15H, CH3 Cp*), 6.32 (t, 2H, H4 pz), 6.62 (dd, 2H, Ph), 6.95 (dbr,
2H, Ph), 7.10−7.18 (m, 6H, Ph), 7.22 (d, 2H, H5 pz), 7.48 (d, 2H, H3
pz). 13C NMR (CDCl3, 293 K): δ 8.9 (s, CH3 Cp*), 95.6 (d, C Cp*,
J(103Rh−13C) = 7.9 Hz), 105.9 (s, C4 pz), 126.1, 126.9, 127.4, 127.5,
128.5, 132.2, 139.3 (s, Ph), 135.8 (s, C5 pz), 143.6 (s, C3 pz). ΛM
(CH3CN, 10−3 M): 1.3 Ω−1 cm2 mol−1. After 1 h the conductivity of
the CH3CN solution increases to 25.3 Ω−1 cm2 mol−1. After 1 day the
conductivity of the acetonitrile solution reaches the value 71.3 Ω−1 cm2
mol−1.
̈
̈
3
CH3 Cp*), 2.60 (br, 1H, H-OB), 6.37 (t, 1H, H4 pzH), 6.40 (t, 1H,
H4 pz), 6.70−7.10 (m, 6H, Ph), 7.20−7.45 (m, 4H, Ph), 7.60 (d, 1H,
H5 pzH), 7.79 (d, 1H, H5 pz), 7.85 (d, 1H, H3 pzH), 8.02 (d, 1H, H3
pz). 13C NMR (CDCl3, 293 K): δ 9.3 (s, CH3 Cp*), 87.7 (s, C Cp*),
106.6 (C4 pz), 122.2, 127.2, 128.3, 133.2, 139.7 (s, Ph), 137.5 (s, C5
pz), 141.8 (s, C3 pz). Λm (CH3CN, 10−3 M): 126.1 Ω−1 cm2 mol−1.
X-ray Crystallography. The X-ray intensity data for 1·H2O, 2, 3,
and 5 were measured on a Bruker SMART Apex II diffractometer
equipped with a CCD area detector using a graphite-monochromated
Mo Kα radiation source (λ = 0.71073 Å). Cell dimensions and the
orientation matrix were initially determined from a least-squares
refinement on reflections measured in three sets of 20 exposures,
collected in three different ω regions, and eventually refined against all
data. For all crystals, a full sphere of reciprocal space was scanned by
0.3° ω steps. The software SMART12 was used for collecting frames of
data, indexing reflections, and determination of lattice parameters. The
collected frames were then processed for integration by SAINT12
software, and an empirical absorption correction was applied with
SADABS.13 The structures were solved by direct methods (SIR 97)14
and subsequent Fourier syntheses and refined by full-matrix least-
squares calculations on F2 (SHELXTL),15 attributing anisotropic
thermal parameters to the non-hydrogen atoms. All hydrogen atoms
were located in the Fourier map. The aromatic and methyl hydrogen
atoms were placed in calculated positions, refined with isotropic
thermal parameters U(H) = 1.2[Ueq(C)] or U(H) = 1.3[Ueq(C)],
respectively, and allowed to ride on their carrier carbons, whereas the
hydroxy H atoms in 2, 3, and 5 were located in the Fourier map and
refined isotropically (U(H) = 1.2[Ueq(O)]). Complex 1·H2O was
found to crystallize with one water molecule of crystallization. In 3
[Rh(Cp*)(Ph){(pz)(OH)BPh2}] (2). To a stirred acetonitrile solution
(20 mL) of [Cp*RhCl2]2 (0.618 g, 1.0 mmol) was added 0.676 g (2.0
mmol) of Kpz2BPh2. The red solution was stirred for 10 min, giving an
orange solution, and the solvent was then removed under reduced
pressure. The resulting orange solid was vigorously stirred with 20 mL
of chloroform at room temperature for 1 h and then the solution was
slowly evaporated. Yellow-orange crystals of 2 were obtained in 40%
yield. Anal. Calcd for C32H37BN2ORh: C, 66.34; H, 6.44; N, 4.84.
Found: C, 66.11; H, 6.28; N, 4.91. IR (cm−1): 3221 br ν(O−H), 3123
w, 3060 w ν(C−H), 1737 m ν(O−H), 1596 w, 1564 w ν(C−C,
̈
C−N). 1H NMR (CDCl , 293 K): δ 1.27 (s, 15H, CH Cp*), 3.40 (br,
̈
3
3
1H, H-OB), 6.34 (t, 1H, H4 pz), 6.68−7.08 (m, 11H, Ph), 7.17−7.24
(m, 5H, Ph and H5 pz), 7.67 (d, 1H, H3 pz). 13C NMR (CDCl3, 293
K): δ 8.9 (s, CH3 Cp*), 95.19 (d, C Cp*, J(103Rh−13C) = 8.0 Hz),
105.92 (s, C4, pz), 126.1, 126.9, 127.1, 127.5, 128.5, 132.2, 139.3 (s,
Ph), 135.8 (s, C5 pz), 143.6 (s, C3 pz). ΛM (CH3CN, 10−3 M): 3.2 Ω
cm2 mol−1.
−1
[Rh(Cp*)(Ph){pzB(OH)2}]ClO4 (3). A 0.103 g amount of AgClO4
(0.05 mmol) was added to a CH3CN (7 mL) solution/suspension of 1
(0.293 g, 0.05 mmol) under N2. After 48 h at room temperature a
colorless precipitate formed, which was filtered off and shown to be
AgCl. The clear orange solution obtained was evaporated under
vacuum and the residue washed with light petroleum and shown to be
compound 3 (0.091 g, 0.015 mmol, yield 31%). It was recrystallized
from CH2Cl2/light petroleum (40−60 °C). Mp: 240 °C dec. Anal.
−
three oxygens of the ClO4 were found to be disordered over two
positions with occupation factors of 0.57 and 0.43% for the major and
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Scheme 1
minor components, respectively. Crystal data and details of the data
collection for 1·H2O, 2, 3, and 5 are reported in Table S1 (Supporting
Information) and relevant bond distances and angles in Table S2
(Supporting Information).
When the same reaction was carried out for longer periods
under non-anhydrous conditions by using acetonitrile as
solvent, the complex [Rh(Cp*)(Ph){κ2-(pz)(OH)BPh2}] (2)
was obtained, where the chelating ligand bears one hydroxyl
group in place of one pyrazolyl ring and, unexpectedly, a phenyl
group is directly bonded to the metal center (Scheme 1).
The source of Ph is the decomposition of a second molecule
of the Ph2Bp ligand. Accordingly, the yield of 2 is only 40%. In
solution an additional amount of not clearly identifiable
Computational Details. The B3LYP exchange-correlation func-
tional,16 as implemented in the GAUSSIAN03 program package,17 is
used for all DFT calculations. All geometry optimizations have been
performed in vacuo and in solution with a LANL2DZ basis set for all
atoms, along with a LANL218−21 pseudopotential for Rh and Ir
centers. Solvation effects and solution free energies are evaluated from
a single-point energy calculation with the6-31G*22 basis set for C, H,
B, N, and O atoms and LANL2DZ for Rh and Ir centers, by the
conductor-like polarizable continuum model (C-PCM)23−28 on the
geometries optimized in solution. As in the experimental conditions,
calculations in solution were performed in acetonitrile. Vibrational
frequency analysis was performed on the geometries optimized in
vacuo. The evaluation of Gibbs free energy was calculated by following
a procedure similar to that employed for the evaluation of absolute
pKa’s.29−31 Following a thermodynamic cycle, the Gibbs free energy in
solution of species i (Gisolv) is defined as Gisolv = Givac + ΔGsolv, where
Givac is the free energy of the species in vacuo and ΔGsolv is the free
energy of salvation. Givac is computed at the geometry optimized in
vacuo, followed by frequency calculations to take into account the
vibrational contribution to the total partition function. The transla-
tional and rotation contributions were evaluated by standard statistical
mechanics (particle in a box and rigid rotor models). ΔGsolv was
directly calculated by a C-PCM single-point calculation together with a
reference calculation in vacuo.
1
byproducts based on pz and OH is found. In the H NMR
spectrum of 2 the resonance due to the hydroxyl group bonded
to the boron atom appears at δ 3.40. The signals due to the
phenyl group directly bonded to Rh and those on B fall in the
ranges 6.80−7.08 and 7.17−7.24 ppm.
To further develop the potential reactivity of [Rh(Cp*)-
(Ph2B)Cl] (1), containing the chelating coordinated Ph2Bp, we
have performed the reaction of 1 with AgClO4 in acetonitrile: a
colorless precipitate immediately appeared, which was filtered
off and shown to be AgCl. From evaporation under vacuum of
the clear orange solution a residue was obtained and shown to
be the ionic [Rh(Cp*)(Ph){κ2-(pz)(OH)B(OH)}]ClO4 (3). 3
is a very unusual borane complex, in which the rhodium bears a
phenyl ring and is coordinated by a pyrazolyl and a hydroxyl
group from the pzB(OH)2 ligand, clearly formed in situ during
the reaction (Scheme 2), through both B−N and B−C
breaking.
RESULTS AND DISCUSSION
Scheme 2
■
Synthesis and Spectroscopic Characterization of
Complexes. The compound [Rh(Cp*)(Ph2Bp)Cl] (1; Cp*
= C5Me5) is readily obtained from a 1:1 interaction of the
potassium salt of ligand K[Ph2Bp] with the dimer [Rh(Cp*)-
Cl2]2 in anhydrous dichloromethane, under an atmosphere of
N2 (Scheme 1). The complex is air stable in the solid state but
sensitive to moisture in chlorinated solution. However, under
an inert atmosphere the solution is indefinitely stable. NMR
data for 1 suggest an η5-Cp* coordination due to a doublet
observed in the 13C{1H} NMR spectrum at δ 95.6 (1J(C−Rh)
= 7.9 Hz). The nonelectrolytic nature of 1 has been confirmed
by conductivity measurements in acetonitrile; however, on
prolonged standing of the solution in contact with air, a
progressive increase of conductivity has been observed, in
accordance with partial displacement of chloride from Rh and
substitution with acetonitrile or water from moisture.
We can reasonably hypothesize that the origin of Ph and of
pzB(OH)2 coordinated to 3 is due to the presence of the Lewis
acidic reagent AgClO4 and to water traces within the reaction
mixture. The IR spectrum of 3 shows absorptions at 3221 and
1737 cm−1 due to ν(O−H) and γ(O−H) modes and typical
strong absorptions due to ClO4 in the anionic form centered at
1074 cm−1.32 The ionic nature of 3 has also been confirmed by
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conductivity measurements in acetonitrile that give a value
typical of 1:1 electrolytes. The H NMR spectrum of 3 shows
Such evidence further supports our previous hypothesis on
the starting Ph2Bp ligand as the source of Ph in 2 and Hpz in 5
and on the catalytic role of d8 Rh(III) and Ir(III) centers on the
decomposition pattern of the starting Ph2Bp ligand.
It is worth noting that different decomposition mechanisms
of the Ph2Bp ligand occurred, depending on the coordinated
metal: a Ph is in fact captured by Rh during decomposition of
(Ph2Bpz2)− with formation of derivative 3, whereas a neutral
pzH is bonded to Ir in derivative 5. A reasonable mechanistic
hypothesis is proposed in Scheme 5, where the species isolated
and characterized are depicted in black and those hypothesized
are given in blue.
The presence of traces of moisture in the acetonitrile
reaction mixture can afford steps A → B → C → D, and the last
species has been isolated for M = Ir (derivative 5). A proof of
this decomposition path has been the isolation of 5 also from
an acetonitrile solution of 4 after standing in the atmosphere
for 1 day. Tthe additional step D → E seems favored for M =
Rh, as confirmed by the isolation of derivative 2. 2 can be
recovered also from an acetonitrile solution of 1 after 24 h. Step
D → E requires attack of a phenyl arising from a side-
decomposition reaction of the starting Ph2Bp ligand through
B−C hydrolysis. The final step E → F seems reasonable on the
basis of the fact that the same cationic species with M = Rh
(derivative 3) has been obtained by a metathesis reaction in
acetonitrile between 1 and AgClO4.
1
resonances due to Ph bonded to the Rh atom in the range
6.80−7.18 ppm, whereas the two hydroxyl groups on the boron
atom are magnetically different and afford two resonances of
equal intensity at 11.20 and 12.00 ppm. Tricoordinate borane
species are in general strong Lewis acids, requiring the donation
of an extra electron pair to complete the octet of valence
electrons. The relevant fact that boron in 3 maintains a trigonal-
planar sp2 hybridization seems to indicate a weak Lewis acidity
of the borane moiety. In the literature only a report on an
analogous free borane has been found,33 in which however the
pyrazole is C-bonded to boron (Scheme 3).
Scheme 3
In order to give further insight into the possible catalytic role
of rhodium in the degradation of the starting (Ph2Bpz2)−
ligand, we have extended our studies toward analogous
iridium(III) compounds. By interaction of [Ir(Cp*)Cl2]2 with
K[Ph2Bp] under the same reaction conditions employed for 1,
the compound [Ir(Cp*)(Ph2Bp)Cl] (4) was obtained (Scheme
4). This derivative is sufficiently air-stable in the solid state but
is sensitive to moisture in chlorinated solution. The
spectroscopic data are in accordance with the proposed
formulation and need no further comment. However, when
the same reaction was carried out for longer periods under non-
anhydrous conditions by using acetonitrile as solvent, the
different compound [Ir(Cp*){κ2-(pz)(OH)BPh2(pzH)}]Cl
(5) was isolated, containing the same chelating (pz)(OH)BPh2
ligand previously found in derivative 2. However, in 5 the Ir
coordination sphere is completed by a neutral pyrazole
(Scheme 4). The derivative is a 1:1 electrolyte in acetonitrile,
and the 1H NMR spectrum shows two different sets of pz rings,
thus confirming the presence of a neutral pyrazole bonded to Ir.
The X-ray molecular structures of 1·H2O, 2, 3, and 5 are
shown in Figures 1−4, and relevant bond lengths and angles are
reported in the captions of Figures 1−4, respectively. In the
four structures the coordination geometry around the central
metal atom is of the three-legged piano-stool type. The
complex 1·H2O (Figure 1) was found to crystallize with one
crystallization water molecule coming from humidity traces
present in the dichloromethane/light petroleum mixture. In
1·H2O only a half-molecule is present in the asymmetric unit
because of the presence of a crystallographically imposed
mirror plane passing through rhodium, chlorine, and boron
atoms and one phenyl group of the diphenylbis(pyrazolyl)-
borate ligand and bisecting the Cp* ring and the second phenyl
ring of the pz2BPh2 ligand. The coordination geometry at the
rhodium atom is pseudo-octahedral and is similar to that found
in the ruthenium complex [Ru(η6-p-cymene)(κ2-Ph2Bp)Cl],34
from which it differs in the presence of a Cp* ring in place of
the η6-p-cymene. The Rh−Cl distance of 2.423(2) Å is very
Scheme 4
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Scheme 5
close to that found in the related complex RhCp*Tp*Cl (Tp*
= hydrotris(3,5-dimethylpyrazolyl)borate) (2.429(1) Å).7 The
average Rh−C(Cp*) distance is 2.154 Å, and the Rh−
centroid(Cp*) distance is 1.80 Å. The water molecule of
crystallization whose hydrogen atoms were not located
establishes a weak nonclassical H bond (O1w···HC = 2.61 Å)
with one H atom of a phenyl ring.
Å) is longer than that found in the rhodium boronate complex
{Rh(PEt3)3[OB(OH)(p-tolyl)]}35 (Rh−O(1) = 2.102(2) Å).
The molecular structure of 3 is shown in Figure 3. The cation
differs from 2 mainly in the nature of the B(OH)2pz ligand,
which is κ2 coordinated to the metal through the N(1) and
O(1) atoms. The resulting five-membered metallacycle Rh−
N(1)−N(2)−B−O(1) is approximately flat, the boron atom
being tricoordinate. The coordination at the Rh atom is
completed by a σ-bonded phenyl ring (Rh−C(phenyl) =
2.037(4) Å) and by an η5-Cp* ring (average Rh−C(Cp*)
distance 2.177 Å, Rh−centroid distance 1.81 Å). The Rh−O(1)
distance of 2.096(4) Å is shorter than the analogous bond in 2.
One oxygen of the perchlorate anion interacts with the
hydrogen of the hydroxy group coordinated to the metal
(O(3)···O(1) = 2.963(7) Å, O(3)···H(11)−O(1) = 168(4)°).
The molecular structure of 5 is shown in Figure 4, and
relevant bond lengths and angles are reported in the figure
caption. In 5 the iridium atom achieves an octahedral geometry,
being coordinated by the B(OH)(pz)Ph2 ligand, an η5-Cp*
ring, and a terminally bound pyrazole ring. The N(1)−Ir−O(1)
bite angle of the five-membered metallacycle (75.4(2)°) is
similar to those already reported for complexes 2 and 3. The
Ir−O(1) and Ir−N(1) distances in the chelate ligand are
2.109(4) and 2.076(5) Å, respectively, and the Ir−N(3) bond
length, in which the pyrazole ring is involved, is slightly longer
(2.107(3) Å) than the other Ir−N bond. The Ir−centroid
distance of 1.77 Å is shorter than the analogous distances
The molecular structure of complex 2 is shown in Figure 2.
The coordination geometry around the Rh atom is pseudo-
octahedral, with the Rh atom coordinated by an η5-Cp* ring, an
η1 phenyl ring, and a chelate B(OH)(pz)Ph2 ligand. The most
interesting feature of 2 resides in the uncommon B(OH)(pz)-
Ph2 ligand that has been found, for example, in a pseudo-
octahedral W complex10 where the metal atom is in addition
coordinated by two CO ligands and by an η3-pyrazolyl-3-
penten-2-yl ligand showing a further interaction with the metal
through a pyrazole nitrogen. The N(1)−Rh−O(1) bite angle,
as expected for a five-membered metallacycle, is narrower
(78.1(1)°) than the N(1)−Rh−N(1′) bite angle in 1
(86.1(2)°), in which a six-membered metallacycle adopts a
boat conformation, and similar to that observed in 3 (see
below) for the analogous O(6)−Rh−N(1) bite angle
(75.8(2)°). The Rh−C(Cp*) average distance is 2.190 Å,
and the Rh−centroid distance is 1.83 Å, whereas the Rh−
C(4)(phenyl) distance of 2.028(3) Å falls in the range observed
for similar σ bonds and is almost identical with that found in 3
of 2.037(4) Å (see below). The Rh−O(1) distance (2.157(2)
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Figure 3. Molecular structure of 3 with the atom-numbering scheme.
Dotted purple lines indicate the O−H···O(perchlorate) interaction.
Selected bond lengths (Å) and angles (deg) for 3: Rh−N(1),
2.069(4); Rh−C(14), 2.037(4); Rh−O(1), 2.096(4); Rh−C(4),
2.137(5); Rh−C(5), 2.180(5); Rh−C(6), 2.146(5); Rh−C(7),
2.217(6); Rh−C(8), 2.205(5); N(1)−Rh−O(6), 75.8(2); N(1)−
Rh−C(14), 87.7(2); C(14)−Rh−O(6), 87.8(2).
Figure 1. Molecular structure of 1·H2O with the atom-numbering
scheme. Selected bond lengths (Å) and angles (deg) for 1: Rh−Cl,
2.423(2); Rh−N(1), 2.089(4); Rh−C(14), 2.16(1); Rh−C(15),
2.157(6); Rh−C(16), 2.148(5); N(1)−Rh−N(1′), 86.1(2); N(1)−
Rh−Cl, 87.1(2) (symmetry code (I): x, −y + 0.5, z).
Figure 4. Molecular structure of 5 with the atom-numbering scheme.
Dotted purple lines indicate the O−H···Cl and N−H···Cl interactions.
Selected bond lengths (Å) and angles (deg) for 5: Ir−N(1), 2.076(5);
Ir−O(1), 2.109(4); Ir−N(3), 2.107(5); Ir−C(19), 2.155(7); Ir−
C(20), 2.149(6); Ir−C(21), 2.133(7); Ir−C(22), 2.144(7); Ir−C(23),
2.169(6); N(1)−Ir−O(1), 75.4(2); N(3)−Ir−O(1), 84.9(2); N(3)−
Ir−N(1), 86.5(2).
Figure 2. Molecular structure of 2 with the atom-numbering scheme.
Selected bond lengths (Å) and angles (deg) for 2: Rh−N(1),
2.077(2); Rh−C(4), 2.028(3); Rh−O(1), 2.157(2); N(1)−Rh−O(1),
78.6(1); Rh−C(22), 2.262(3); Rh−C(23), 2.270(3); Rh−C(24),
2.119(3); Rh−C(25), 2.151(3); Rh−C(26), 2.148(3); N(1)−Rh−
O(1), 78.6(1); C(4)−Rh−O(1), 86.0(1); N(1)−Rh−C(4), 91.2(1).
ring (Cl···H(10)−O(1) = 168(5)°, Cl···O(1) = 2.987(4) Å,
N(4)−H(4N)···Cl = 170(5)°, N(4)···Cl = 3.170(6) Å).
DFT Calculations. We have undertaken DFT calculations
to investigate the competitive reaction products 2 and 5
considering both Rh and Ir metal centers. We simulated four
different minima: 2-Rh and 5-Ir, shown in Figure 5, and the
observed in the rhodium complexes. There are interactions
between the chloride anion the hydrogen of the hydroxy group
bound to Ir and the nitrogen-bound H atom of the pyrazole
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Figure 5. Optimized geometry structures of 2-Rh and 5-Ir complexes.
a
Table 1. Main Atomic Distances in Å of 2-Rh, 2-Ir, 5-Rh, and 5-Ir Complexes Calculated in Vacuo and in Solution
Rh
Ir
2
5
2
5
distance
vac
solv
2.041
exptl
vac
solv
vac
solv
vac
solv
exptl
M−C1
M−N3
M−N1
M−Cp
M−O
B−O
B−N2
B−C2
B−C3
2.039
2.028
2.054
2.054
2.300
2.064
2.301
2.182
1.553
1.584
1.617
1.626
2.121
2.085
2.262
2.143
1.571
1.599
1.608
1.614
2.106
2.072
2.247
2.162
1.544
1.592
1.620
1.621
2.098
2.076
2.256
2.148
1.576
1.596
1.607
1.613
2.088
2.070
2.259
2.144
1.551
1.588
1.620
1.621
2.108
2.077
2.150
2.109
2.075
2.318
2.155
1.553
1.592
1.615
1.622
2.079
2.318
2.178
1.547
1.589
1.619
1.627
2.077
2.190
2.157
2.065
2.281
2.165
1.558
1.588
1.613
1.620
a
M−Cp is the average distance between the metal atom and the five carbon atoms of the pentamethylcyclopentadienyl moiety.
corresponding 2-Ir and 5-Rh species. The main geometrical
parameters optimized in vacuo and in solution are reported in
Table 1, where they are compared to available X-ray data for 2-
Rh. From the calculation of the Gibbs free energy associated
with all the species involved in the process from D to E shown
in Scheme 5 (except the Cl− ion), we are able to evaluate the
different stability of 5 compared to that of 2 for Rh
(ΔG5‑2[Rh]) and for the corresponding Ir complexes
(ΔG5−2[Ir]). ΔG5‑2[M] is calculated by evaluating the Gibbs
free energy difference of the reaction from D to E reported in
Scheme 5, considering M = Rh for ΔG5‑2[Rh] and M = Ir for
ΔG5‑2[Ir], respectively. From the difference of energy differ-
ences ΔG5‑2[Ir] − ΔG5‑2[Rh] we can estimate the relative
thermodynamic stabilization of species 5-M with respect to
species 2-M when moving from M = Rh to M = Ir. Our
calculations provide a value of 1.2 kcal/mol for the ΔG5‑2[Ir] −
ΔG5‑2[Rh] difference, indicating that species 5 is more stable
than species 2 when the metal is iridium in comparison to
rhodium, in line with the experimental evidence. This is due to
the different solvation energy (ΔGsolv) of the species involved
in the reaction. In particular, while ΔGsolv of 2-Ir is 0.8 kcal/mol
higher than ΔGsolv of 2-Rh, ΔGsolv of 5-Ir is 2.1 kcal/mol lower
than ΔGsolv of 5-Rh, confirming a greater stabilization in
solution for the 5-Ir species.
CONCLUSIONS
■
Novel Rh(III) and Ir(III) pentamethylcyclopentadienyl com-
pounds 1 and 4 ,containing the diphenylbis(pyrazolyl)borate
ligand, have been isolated from dichloromethane under
anhydrous conditions. In contrast, by using acetonitrile as
solvent and non-anhydrous conditions B−N bond hydrolysis
occurs in the ligand Ph2Bp and unusual neutral and ionic
species are obtained. In detail, rhodium and iridium have been
found to be coordinated by the hydroxy(pyrazolylborate)
ligand B(OH)(pz)Ph2. The coordination sphere of the Ir
derivative is completed by a neutral pyrazole, whereas the
rhodium complex contains an η1-phenyl. An uncommon
B(OH)2(pz) ligand κ2 coordinated to rhodium has instead
been found in compound 3, the boron atom being only
tricoordinate. To give insight into the formation of these
unusual species, we have proposed a mechanism for a
decomposition pathway of the diphenylbis(pyrazolyl)borate
ligand coordinated to the metal, involving substitution of the
chloride with water in the metal environment, followed by B−
N hydrolysis and pyrazole replacement by a OH to boron.
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ASSOCIATED CONTENT
* Supporting Information
CIF files and a table giving crystallographic data for 1−3 and 5.
This material is available free of charge via the Internet at
http://pubs.acs.org.
■
S
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AUTHOR INFORMATION
Corresponding Author
*R.P.: e-mail, riccardo.pettinari@unicam.it; tel, +39
0737402338; fax, +39 0737 402338.
■
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank the University of Camerino for financial support.
■
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