Easy access to hydride chemistry on a tripodal P-based rhodium scaffold

Article abstract of DOI:10.1021/om201129j

The TBPY-5 rhodium complex [(PhBP₃)Rh(CH₂=CH ₂)(NCMe)] (1; PhBP₃ = PhB(CH₂PPh ₂)₃), which contains ethylene in the equatorial plane and a labile acetonitrile ligand at one of the axial positions, provides a simple entry into the chemistry of the fac-[P₃Rh] scaffold. DFT calculations using the model compound [(MeB(CH₂PMe₂) ₃)Rh(CH₂=CH₂)(NCMe)] (1′) reproduce well the crystallographic geometry of 1 and converge to the isolated conformer, in which the strong π back-donation from the metal to the π* orbital of ethylene fixes it coplanar with the equatorial plane. Oxidation of 1 is electrochemically irreversible (at 0.080 V vs SCE), and 1 was effectively oxidized with [Cp₂Fe]⁺ in acetonitrile to [(PhBP ₃)Rh(NCMe)₃]²⁺. Reaction of 1 with hydrogen in toluene gives the dihydride [(PhBP₃)Rh(H)₂(NCMe)] (3), which loses acetonitrile to give the insoluble hydride [{(PhBP ₃)Rh(H)(μ-H)}₂] (4), while 1 in THF with BHT (2,6-bis(1,1-dimethylethyl)-4-methylphenol) results in the mixed-valence paramagnetic hydride [{(PhBP₃)Rh}₂(μ-H)₃] (5). Complexes 1 and 3 react with oxygen to give the dinuclear complex [{(PhBP₃)Rh(μ-O₂)}₂]. The kinetic products from the protonation of complex 1 with carboxylic acids were found to be the ethyl complexes [(PhBP₃)Rh(η¹-C₂H ₅)(κ²-O₂CR)] (R = Ph, Me), which establish an equilibrium in solution with the corresponding hydride complexes [(PhBP₃)Rh(H)(κ²-O₂CR)] (R = Ph, Me) and free ethylene. These equilibria can be shifted to the desired compound working under either an atmosphere of ethylene or vacuum. Sequential protonation of 1 and 3 with HBF₄ in acetonitrile gave cleanly the cationic hydride complex [(PhBP₃)Rh(H)(NCMe)₂]BF₄ first and then [(PhBP₃)Rh(NCMe)₃](BF₄)₂ and hydrogen. Other electrophiles such as MeOTf react immediately with 1 at one phosphorus atom to give the alkylated (P-methyl) complex [{PhB(PMe)P ₂}Rh(κ²-O₂SOCF₃)].

Full text of DOI:10.1021/om201129j

Article
pubs.acs.org/Organometallics
Easy Access to Hydride Chemistry on a Tripodal P-Based Rhodium
Scaffold
Cristina Tejel,* Ana M. Geer, Sonia Jimenez, Jose A. Lopez, and Miguel A. Ciriano*
́
́
́
́ ́ ́
Instituto de Síntesis Química y Catalisis Homogenea (ISQCH), Departamento de Química Inorganica, CSIC-Universidad de
Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain
S
* Supporting Information
ABSTRACT: The TBPY-5 rhodium complex [(PhBP3)Rh(CH2CH2)(NCMe)] (1; PhBP3
= PhB(CH2PPh2)3), which contains ethylene in the equatorial plane and a labile acetonitrile
ligand at one of the axial positions, provides a simple entry into the chemistry of the fac-[P3Rh]
scaffold. DFT calculations using the model compound [(MeB(CH2PMe2)3)Rh(CH2
CH2)(NCMe)] (1′) reproduce well the crystallographic geometry of 1 and converge to the
isolated conformer, in which the strong π back-donation from the metal to the π* orbital of
ethylene fixes it coplanar with the equatorial plane. Oxidation of 1 is electrochemically
irreversible (at 0.080 V vs SCE), and 1 was effectively oxidized with [Cp2Fe]+ in acetonitrile to
[(PhBP3)Rh(NCMe)3]2+. Reaction of 1 with hydrogen in toluene gives the dihydride
[(PhBP3)Rh(H)2(NCMe)] (3), which loses acetonitrile to give the insoluble hydride
[{(PhBP3)Rh(H)(μ-H)}2] (4), while 1 in THF with BHT (2,6-bis(1,1-dimethylethyl)-4-
methylphenol) results in the mixed-valence paramagnetic hydride [{(PhBP3)Rh}2(μ-H)3] (5).
Complexes 1 and 3 react with oxygen to give the dinuclear complex [{(PhBP3)Rh(μ-O2)}2].
The kinetic products from the protonation of complex 1 with carboxylic acids were found to be the ethyl complexes
[(PhBP3)Rh(η1-C2H5)(κ2-O2CR)] (R = Ph, Me), which establish an equilibrium in solution with the corresponding hydride
complexes [(PhBP3)Rh(H)(κ2-O2CR)] (R = Ph, Me) and free ethylene. These equilibria can be shifted to the desired compound
working under either an atmosphere of ethylene or vacuum. Sequential protonation of 1 and 3 with HBF4 in acetonitrile gave
cleanly the cationic hydride complex [(PhBP3)Rh(H)(NCMe)2]BF4 first and then [(PhBP3)Rh(NCMe)3](BF4)2 and hydrogen.
Other electrophiles such as MeOTf react immediately with 1 at one phosphorus atom to give the alkylated (P-methyl) complex
[{PhB(PMe)P2}Rh(κ2-O2SOCF3)].
i
phenyl groups in PhBP3 by the more basic and bulkier Pr
groups produces [PhB(CH2PiPr2)3]− (PhBP′3), which subtly
governs the reactivity of metals in its complexes, allowing, for
example, the stabilization of η3-silane adducts of iron(II)11 or
promoting Si−H and B−C activation processes on coordina-
tion to iridium.12 Moreover, when PhBP′3 is bound to iron, it
provides support enough to accommodate this metal in an
unusually wide range of oxidation states (from Fe(0) to
Fe(IV)).13 Furthermore, pseudotetrahedral manganese com-
plexes14 or novel dinitrogen activation chemistry has been
achieved with iron and cobalt complexes.13b Related hybrid
scorpionate ligands such as [PhB(CH2PiPr2)2(Pz)]− (PhBNP′2)
and [allylB(CH2PPh2)(Pz)2]− (ABPN2) stabilize terminal
iron(IV) imides15 and show new coordination modes,16
respectively.
INTRODUCTION
■
Metal−multipodal ligand scaffolds provide a common way to
study reactions giving well-defined yet unusual and reactive
geometries. Trofimenko’s scorpionates (or tris(pyrazolyl)-
borate (Tp) ligands) are a classic example of such highly
ordered and sterically encumbered multidentate scaffolds.1 The
success of tris(pyrazolyl)borates has inspired the development
of new anionic scorpionate ligands based on other donor
groups that provide a facial array of [X3] donors and differ in
their topology, flexibility, and donor properties.2 In this respect,
interest has increased in anionic [P3] donor ligands, first
reported in 1999, which are strongly donating scorpionates.
The simplest, [PhB(CH2PPh2)3]− (henceforth referred to as
PhBP3),3 has been shown to be suitable for creating
pseudotetrahedral4 metal environments and for stabilizing
terminal imido complexes of first-row late transition metals
(LTM) with robust MNR (M = Fe, Co) triple bonds,5 as
well as unusual peroxo and hydroperoxo rhodium compounds6
and isocarbonyl nickel complexes.7 Evidence for its ability to
form reactive iridium complexes has been verified by C−H8
and double-geminal Si−H and Ge−H bond activation
processes,3a,9 as well as facile heterolytic hydrogen activation
by nitride and hydrogenolysis by imido iron complexes,
respectively, under mild conditions.10 Substitution of the
The utility of complexes of late transition metals with these
types of ligands in catalysis remained undocumented until the
use of [(PhBP′3)Fe(H)3(PR3)] (R = Me, Et) as catalyst
precursors for olefin hydrogenation.17 More recently, we have
reported the complexes [(PhBP3)Rh(cod)] and [{(PhBP3)Ru-
Special Issue: F. Gordon A. Stone Commemorative Issue
Received: November 23, 2011
Published: February 7, 2012
© 2012 American Chemical Society
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(μ-Cl)}2]18 as precatalysts for the selective hydrogenation of
the carbonyl group in α,β-unsaturated substrates,19 while the
cationic ruthenium complex [(PhBP3)Ru(NCMe)3]PF6 is a
catalyst precursor for hydrogen transfer reactions from 2-
propanol to aromatic and aliphatic ketones.20 Moreover, we
have shown that the dihydridorhodium(III) complex [(PhBP3)-
Rh(H)2(NCMe)] is an extremely efficient catalyst for the
dimerization of both aliphatic and aromatic, enolizable and
nonenolizable aldehydes under exceptionally smooth con-
ditions.21 Key steps in the catalytic cycle are an easy β-hydride
insertion into the CO bond of the aldehyde to give an
alkoxide, followed by nucleophilic attack on a new molecule of
coordinated aldehyde and a β-hydrogen elimination from the
hemiacetal intermediate to regenerate the catalyst. From our
experience, it was apparent that hydrogenation of carbonyl
groups was preferred over that of CC bonds. This prompted
us to study some basic chemistry of hydride/olefin complexes
on the “(PhBP3)Rh” scaffold. In particular, we succeeded in
preparing the monoolefin complex [(PhBP3)Rh(C2H4)-
(NCMe)] (1), which possesses a simple olefin and a labile
acetonitrile ligand, as a key compound to enter this chemistry.
Herein, we report the synthesis and full characterization of
complex 1, its reactions leading to hydride and alkyl complexes,
and their interplay as well as other related reactions.
Figure 1. Structure (ORTEP at the 50% level) of complex 1.
Hydrogen atoms have been omitted, and only the ipso carbons of the
phenyl groups are shown for clarity.
RESULTS AND DISCUSSION
■
the dxy orbital of rhodium, this interaction being responsible for
the coplanar disposition of the ethylene ligand mentioned
above. Attempts to model the conformational isomer having
apical ethylene and equatorial acetonitrile did not reach an
energy minimum, since the back-donation is maximum at the
equatorial plane in a d8-ML5 TBPY-5 geometry and the best π-
acceptor ligands occupy these positions. The calculations
converge to the isolated conformer, which in turn represents
the thermodynamically most stable conformer of complex 1.
Similarly, the conformational isomer with the ethylene
molecule perpendicular to the equatorial plane does not
reach any energy minimum and again converges to 1, with
ethylene coplanar with this plane.
Synthesis and Characterization of [(PhBP3)Rh(CH2
CH2)(NCMe)] (1). Reaction of [{Rh(μ-Cl)(C2H4)2}2] with
[Li(tmen)][PhBP3] (tmen = N,N,N′,N′-tetramethylethane-1,2-
diamine, PhBP3 = PhB(CH2PPh2)3) in acetonitrile gives the
mononuclear compound [(PhBP3)Rh(CH2CH2)(NCMe)]
(1) as the result of the coordination of the anionic tripodal
[PhBP3]− ligand to rhodium and the replacement of one
molecule of ethylene by acetonitrile. Complex 1 was isolated as
an air-sensitive yellow-orange crystalline solid in good yield and
was fully characterized by analytical and spectroscopic methods.
The isolated crystals usually contain acetonitrile of crystal-
lization. The X-ray structure of 1 (Figure 1) shows a
pentacoordinated rhodium center fac-bound to the three
phosphorus atoms of the anion [PhBP3]− and to ethylene
and acetonitrile ligands. The geometry around the rhodium
atom can be described as distorted trigonal bipyramidal (TBPY-
5) with P1 and acetonitrile at the apical positions and two P
atoms (P2 and P3) and ethylene at the equatorial plane,
forming P−Rh−P angles close to 90° (see Table 1): i.e., smaller
than the expected angles, probably due to the geometric
requirements of the tripodal ligand.
The Rh−P bond distances are in the normal range, with that
of phosphorus at the apical position (Rh−P1) being the
shortest. The ethylene molecule is coplanar with the equatorial
plane (the maximum deviation of the plane defined by C46,
C47, Rh, P2, and P3 is 0.012 Å), and shows a quite long
carbon−carbon distance (1.443(7) Å), which indicates a strong
π back-donation from the metal to the π* orbital of ethylene.
The DFT (b3-lyp, LanL2DZ, and 6-31G**) optimized
geometry of 1 using the model compound [(MeBP3)Rh-
(CH2CH2)(NCMe)] (1′; MeBP3 = MeB(CH2PMe2)3)
reproduces well the crystallographic data (Table 1), with
bond distances and angles close to those found for 1. The
experimental metal−ligand distances are slightly shorter than
those from the DFT geometry, which is quite common for the
b3-lyp functional. The HOMO of 1′ (Figure 2) clearly shows
the bonding interaction between the π* orbital of ethylene and
Solutions of 1 in CD2Cl2 at −50 °C showed well-defined 1H
and 31P{1H} NMR spectra. The two equatorial phosphorus
nuclei become equivalent by a symmetry plane, giving a doublet
of doublets, while the axial nucleus gives a doublet of triplets in
the 31P{1H} NMR spectrum. Note that an easy turnstile motion
between the two TBPY-5 conformers (with ethylene or
acetonitrile at the apical position) would produce only a
doublet in the 31P{1H} NMR spectrum. The ethylene protons
1
gave two signals in the H NMR spectrum, which correspond
to a nonrotating ethylene ligand, as expected from the strong π
interaction with the metal described above. On cooling,
broadening of the phosphorus nuclei signals was observed
and that corresponding to the equatorial ones eventually splits
into two broad resonances at −90 °C (see the Supporting
Information). This can be due to one or both of the following
processes: (i) a freezing of the free rotation of the phenyl rings
of the phosphorus atoms, in particular those at the apical
position, and (ii) a lack of flexibility of the framework of bonds
made by the methylene carbons in [PhB(CH2PPh2)3]−. Both
produce the loss of the aforementioned symmetry plane that
makes the equatorial phosphorus nuclei equivalent, and
consequently, the corresponding signal splits into two while
that of the axial phosphorus remains almost unchanged.
1
However, the H NMR at low temperature is inconclusive
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Table 1. Selected Crystallographic (Complex 1) and DFT-Calculated (Complex 1′) Bond Distances (Å) and Angles (deg)
distance
1
1′
angle
1
1′
Rh−P1
Rh−P2
Rh−P3
Rh−N1
Rh−C46
Rh−C47
2.2518(17)
2.3854(17)
2.3766(16)
2.060(5)
2.303
2.445
2.448
2.098
2.175
2.181
2.058
1.429
P1−Rh−N1
P1−Rh−P2
P1−Rh−P3
P1−Rh−Ct
P2−Rh−P3
P2−Rh−Ct
P3−Rh−Ct
N1−Rh−Ct
178.28(15)
88.13(6)
178.1
86.9
89.67(6)
88.2
93.43(16)
89.20(6)
93.5
2.139(5)
90.2
2.167(6)
137.83(15)
132.91(15)
84.9(2)
137.7
132.0
86.3
a
Rh−Ct
C46−C47
2.029(5)
1.443(7)
a
Ct represents the midpoint of C46 and C47.
dimethylpyrazolyl)borate (TpMe2) in THF at 0 °C gives mainly
the bis(ethylene)rhodium(I) compound [(TpMe2)Rh(C2H4)2],
which renders the butadiene complex [Rh(TpMe2)(η4-C4H6)]
on warming.22 Moreover, addition of acetonitrile to [(TpMe2
)
-
Rh(C2H4)2] promotes the formation of the vinylethylrhodium-
(III) complex [(TpMe2)Rh(C2H3)(C2H5)(NCMe)].23 How-
ever, no C−H bond activation reactions were detected in our
case. Thus, the reaction carried out in d8-THF gave red-brown
solutions containing mainly the highly oxygen-sensitive
complex [(PhBP3)Rh(CH2CH2)(d8-THF)], which could
not be isolated. However, the two ethylene signals at δ 2.86
and 1.38 ppm observed in the 1H NMR spectrum and the two
resonances (at δ 55.7 (dt, JRh,P = 134, JP,P = 25 Hz, 1P, PM) and
1.04 (dd, JRh,P =111, JP,P = 25 Hz, 2P, PA)) in the 31P{1H}
NMR spectrum support the proposed formulation as the THF
equivalent of 1. Moreover, warming 1 in C6D6 led to a slight
broadening of the NMR signals and further to the
decomposition of the sample if warming was continued for
prolonged times.
Figure 2. DFT-calculated HOMO of 1′.
because broad signals for the phenyl and ethylene protons are
observed. In addition, the frozen structure of the complex
shows a helical disposition of the three arms of the [PhBP3]−
ligand close to a local C3 symmetry (see Figure 3). At room
Redox Behavior of Complex 1. Cyclic voltammetry of the
complex in acetonitrile at a scan rate of 200 mV/s showed an
irreversible oxidation wave at ca. 80 mV versus SCE (Figure 4).
Figure 4. Cyclic voltammogram of complex 1 in acetonitrile (scan rate
200 mV/s).
Figure 3. View of the “(PhBP3)Rh” fragment of 1 along the B−Rh
axis. Rh is labeled in green, B in orange, P in purple, C in gray, and H
in black.
When the scan rate was raised, a slight increase of the reversal
current intensity was observed, which suggests an ECirr
mechanism: i.e., an electron transfer reaction followed by an
irreversible chemical reaction.
temperature, broadening of the 31P{1H} NMR signals was
observed at the same time that the signals of free and
coordinated acetonitrile (at δ 1.97 and 1.38 ppm, respectively)
The relatively low value of the oxidation potential makes
feasible the chemical oxidation of the complex with a mild
oxidant such as [Cp2Fe]PF6. Monitoring the reaction of 1 with
1
become a single resonance in the H NMR spectrum, which
indicates that the fluxionality observed at room temperature is
most probably due to dissociation/association of acetonitrile.
The isolation of the rhodium(I) complex 1 from the reaction
mentioned above contrasts with the results from similar
reactions with Tp ligands, which lead to easy C−H bond
activation reactions. By way of example, the analogous reaction
of [{Rh(μ-Cl)(C2H4)2}2] with hydrogentris(3,5-
1
ferrocenium by H and 31P{1H} NMR spectroscopy revealed
that 2 molar equiv of [Cp2Fe]PF6 was required to consume
fully the starting material (1), no compounds being observed
other than the starting material, the product, and free ethylene.
Hence, the wave in the CV of 1 corresponds to a two-electron-
oxidation process with release of ethylene. On a preparative
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scale, the dicationic complex [(PhBP3)Rh(NCMe)3](PF6)2
([2]2+) was isolated from this reaction as a white crystalline
solid in very good yield (Scheme 1).
Overall, the structure is quite similar to that of the recently
reported ruthenium analogue [(PhBP3)Ru(NCMe)3]+.20 The
acetonitrile ligands are squeezed (mean N−Rh−N angle of
83.9°), presumably as a result of the steric constraints imposed
by the phenyl groups on the phosphorus atoms. In addition, the
Rh−N bond distances are elongated (averaged value of 2.115
Å), which reflects the steric demands of the PhBP3 ligand and
the trans influence exerted by members of this class of
phosphine ligand.24
Scheme 1. Oxidation of [(PhBP3)Rh(CH2CH2)(NCMe)]
(1) to [(PhBP3)Rh(NCMe)3](PF6)2 ([2]2+)
Reactions of 1 with H2. The simple exposure of an
acetonitrile suspension of complex 1 to hydrogen under
atmospheric pressure caused the immediate discoloration of
the solution while a white solid separated from the reaction
media. This solid was identified as [(PhBP3)Rh(H)2(NCMe)]
(3) by comparison of its spectroscopic data with those of the
previously prepared complex by hydrogenation of [(PhBP3)-
Rh(cod)].19 Although complex 3 is stable in the solid state
under argon and in solution in toluene or benzene, leaving
these solutions for days produced the crystallization of an
orange insoluble solid further identified as the dinuclear
complex [{(PhBP3)Rh(H)(μ-H)}2] (4) from a X-ray diffrac-
tion study. The reaction involves the dissociation of the labile
acetonitrile ligand and the dimerization of the “(PhBP3)Rh-
(H)2” fragment through the formation of hydrogen bridges. A
similar behavior has been observed for the related iridium
complex [(MeCP3)Ir(C2H4)(H)2]+ (MeCP3 = MeC-
(CH2PPh2)3), which was found to dimerize to the tetrahydride
[{(MeCP3)Ir(H)(μ-H)}2]2+ with evolution of ethylene on
heating the solid.25
Analytical, spectroscopic, and structural data of [2]2+ agreed
with the proposed formulation (see the Experimental Section).
Thus, the three equivalent phosphorus nuclei give a sharp
doublet while the three acetonitrile ligands give a singlet (at δ
1
2.29 ppm) in the 31P{1H} and H NMR spectra, respectively.
Figure 5 shows the molecular structure of the dication [2]2+
Figure 6 shows the molecular structure of 4, while Table 3
collects selected bond distances and angles. As anticipated, the
X-ray structure of 4 revealed it to be a dinuclear edge-sharing
bioctahedral complex with two bridging hydrides and one
terminal hydride on each rhodium atom; the halves of the
molecule are related by an inversion center. All hydrides were
located in a difference Fourier map and refined with some
geometrical constraints.
Dinuclear rhodium complexes with the “{Rh(H)(μ-H)}2”
core are quite uncommon, the dicationic [{(Cn)Rh(H)(μ-
H)}2]2+ (Cn = 1,4,7-trimethyl-1,4,7-triazacyclononane)26 being
the single crystallographically characterized precedent found in
the Cambridge Structural Database. Nonetheless, complexes of
the type [{(MeCP3)Rh(H)(μ-H)}2]n+ (n = 2−0; MeCP3 =
MeC(CH2PPh2)3) have been previously reported by Bianchi-
ni.27 The rhodium−rhodium distance in 4 (2.8152(9) Å) was
found to be longer than in [{(Cn)Rh(H)(μ-H)}2]2+ (2.595(2)
Å), probably due to the steric overcrowding generated by the
phenyl groups in PhBP3. In fact, metal−metal distances for
related iridium complexes having “{Ir(H)(μ-H)}2” cores lie in
the range 2.79−2.70 Å.28
Complex 4 has a 32-valence-electron (ve) count, so that for
satisfying the 18-electron rule a formal rhodium−rhodium
double bond would be required. However, under the
perspective of two formally electron-deficient 16 ve centers it
is not necessary to postulate a metal−metal bond; the best
description of the bonding between the halves of the complex
corresponds to two delocalized 3c−2e M−H−M bonds, as
proposed by Braunstein for the complex [{(N2P)Ir(H)(μ-
H)}2]2+ (N2P = bis(oxazoline)phosphonite).28b
Figure 5. Structure (ORTEP at 50% level) of dicationic complex [2]2+.
Hydrogen atoms have been omitted, while only the ipso carbons of the
phenyl groups are shown for clarity.
corresponding to the complex [2](BF4)2, while selected bond
distances and angles are summarized in Table 2.
Table 2. Selected Bond Distances (Å) and Angles (deg) for
Complex [2]2+
Rh−P1
Rh−P2
Rh−P3
P1−Rh−N1
P2−Rh−N2
P3−Rh−N3
P1−Rh−P2
P1−Rh−P3
P2−Rh−P3
2.3392(8)
2.3317(7)
2.3225(8)
176.38(7)
178.48(7)
173.54(6)
86.59(3)
89.70(3)
90.50(3)
Rh−N1
Rh−N2
Rh−N3
N1−Rh−N2
N1−Rh−N3
N2−Rh−N3
P1−Rh−N2
P1−Rh−N3
2.109(3)
2.112(2)
2.123(3)
82.87(9)
84.07(9)
84.84(9)
93.51(6)
95.49(6)
In [(PhBP3)Rh(NCMe)3]2+, the rhodium atom shows the
typical octahedral environment of d6 metal complexes with the
tripodal ligand PhBP3 occupying three fac positions while three
acetonitrile molecules complete the coordination sphere.
Furthermore, exposure of the dihydride complex 3 to an
oxygen atmosphere gives an orange solid identified as the
dioxygen compound [{(PhBP3)Rh(μ-O2)}2].6 The reaction
was found to be fully reversible, and [{(PhBP3)Rh(μ-O2)}2]
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Figure 6. Structure (ORTEP at 50% level) of complex 4. Hydrogen atoms (except hydride ligands) have been omitted, and only the ipso carbons of
the phenyl groups are shown for clarity.
Table 3. Selected Bond Distances (Å) and Angles (deg) for
Complex 4
with BHT contained in the solvent. Figure 7 shows the
molecular structure of 5, while selected bond distances and
angles are collected in Table 4.
Rh−P1
Rh−P2
Rh−H2
2.4252(13)
2.2813(14)
1.87(2)
Rh−H2′
Rh−P3
Rh−H1
1.86(4)
2.2995(13)
1.59
Table 4. Selected Bond Distances (Å) and Angles (deg) for
Complex 5
P1−Rh−P2
P1−Rh−P3
P2−Rh−P3
90.38(5)
90.61(5)
83.97(5)
P1−Rh−H1
P2−Rh−H2
P3−Rh−H2′
171.8
174.6(13)
170.2(11)
Rh1−P1
Rh1−P2
Rh1−P3
Rh1−H1
2.3410(10)
2.3073(11)
2.3347(10)
1.87(4)
Rh2−P4
Rh2−P5
Rh2−P6
Rh2−H1
2.3682(10)
2.3080(9)
2.3137(10)
1.91(4)
transforms into [(PhBP3)Rh(H)2(NCMe)] (3) by reaction
with hydrogen in acetonitrile.
Rh1−H2
1.82(4)
Rh2−H2
1.86(3)
Initial experiments on the reaction of 1 with hydrogen in
tetrahydrofuran were disappointing, because a red product
could be isolated in variable yields depending apparently on the
reaction conditions. Once isolated, the red complex was
identified as the paramagnetic hydride [{(PhBP3)Rh}2(μ-H)3]
Rh1−H3
1.83(3)
Rh2−H3
1.92(3)
P1−Rh1−P2
P1−Rh1−P3
P2−Rh1−P3
P1−Rh1−H1
P2−Rh1−H2
P3−Rh1−H3
89.76(3)
89.58(3)
86.83(3)
165.9(11)
170.0(11)
173.3(12)
P4−Rh2−P5
P4−Rh2−P6
P5−Rh2−P6
P4−Rh2−H1
P5−Rh2−H2
P6−Rh2−H3
87.18(4)
91.10(4)
85.73(3)
168.7(11)
173.3(11)
167.5(10)
1
(5) (see below), which showed broad signals in the H NMR
and flat 31P{1H} NMR spectra. Nonetheless, monitoring the
reaction of 1 with hydrogen in d8-THF showed a clean
formation of the diamagnetic complex [(PhBP3)Rh-
(H)2(NCMe)] (3), which did not give 5 when air was allowed
to enter in the NMR tube. After discarding that traces of
oxygen were the responsible for this reaction, the radical trap
TEMPO (2,2,6,6-tetramethylpiperidin-1-yl)oxyl) was tested as
hydrogen abstractor, although without success. Finally, trans-
formation of complex 3 into 5 was achieved in the presence of
3,5-dimethyl-4-hydroxytoluene, similar to the case for 3,5-di-
tert-butyl-4-hydroxytoluene (BHT), the usual stabilizing agent
for THF. Therefore, complex 5 resulted from the reaction of 3
Complex 5 is formed by two “(PhBP3)Rh” subunits bridged
by three hydrogen atoms. The geometry around each rhodium
atom is almost octahedral, with angles close to 90° (see Table
4).
According to the neutral nature of the complex, it is a mixed-
valence Rh(II)/Rh(III) compound with an electron count of 31
ve: i.e., one electron less than for the tetrahydride complex 4, in
which there is not a metal−metal bond. Apart from the
delocalized 3c−2e bonds, the electron-counting rules require us
in this case to propose a metal−metal bond with a Rh−Rh
Figure 7. Structure (ORTEP at 50% level) of complex 5. Hydrogen atoms (except hydride ligands) have been omitted, and only the ipso carbons of
the phenyl groups are shown for clarity.
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bond order of 0.5 for 5. Accordingly, a relatively short
rhodium−rhodium bond distance (2.6488(6) Å) is observed.
This distance is quite similar to that found in the Rh(II)/
Rh(III) dicationic complex [{(MeCP3)Rh}2(μ-H)3]2+
(2.644(1) Å)29 and, as expected, longer that those found in
the related 30-ve Ir(II)/Ir(II) complex [{(Cp*)Ir}2(μ-H)3]+,
having a single iridium−iridium bond (averaged metal−metal
bond distances are 2.47 Å).30
Reactions of Complexes 1 and 3 with Electrophiles.
Addition of 1 mol equiv of HBF4 to an acetonitrile suspension
of 1 gave cleanly and quantitatively the cationic hydride
complex [(PhBP3)Rh(H)(NCMe)2]BF4 ([6]BF4), which was
isolated as a pale yellow solid (Scheme 2). Spectroscopic data
2H+ to H2 promoted by a rhodium compound, which provides
the two required electrons.
Quite different results were obtained when complex 1 was
protonated with acids having a coordinating anion, such as
benzoic or acetic acid. The kinetic products were found to be
the ethyl complexes [(PhBP3)Rh(η1-C2H5)(κ2-O2CR)] (R =
Ph (7), Me, (8)), which establish an equilibrium in solution
with the corresponding hydride complexes [(PhBP3)Rh(H)(κ2-
O2CR)] (R = Ph (9), Me (10)) and free ethylene (Scheme 3).
Scheme 3. Reactions of 1 and 3 with Carboxylic Acids
Scheme 2. Relationships among Complexes 1, 3, [2]2+, and
[6]+
In both cases, once the thermodynamic equilibria were reached,
1H NMR spectra showed a 45:55 ratio for 7:9 and 30:70 for
8:10. Accordingly, pure samples of 7 and 8 were obtained by
carrying out the reactions under an atmosphere of ethylene.
The ethyl group in 7 and 8 was clearly identified in the
1H{31P} NMR spectra from the sharp quartet and triplet at
chemical shifts of ca. δ 1.53 and 0.87 ppm, respectively. The
carbon signal from the coordinated carboxylate ligand was
of [6]+ agree with the proposed structure. Thus, the hydride
resonance was located at δ −7.09 ppm in the 1H NMR
A
M
spectrum as a doublet of pseudoquartets (JH,P = 187.2 Hz, JH,P
= 7.8 Hz and JH,Rh = 7.8 Hz), while the ν(Rh−H) stretching
was found at 1989 cm−1 in the IR spectrum. The inequivalent
phosphorus nuclei produce two signals in a 2:1 ratio (at δ 43.8
(dd, PM) and 1.3 (dt, PA) ppm) in the 31P{1H} NMR spectrum,
corresponding to an AM2X spin system (X = 103Rh). According
to its ionic nature, complex [6]BF4 behaves as a 1:1 electrolyte
in acetone solutions. The reaction was found to be fully
reversible, and thus complex [6]+ regenerates 1 upon saturation
of the acetonitrile solution with ethylene and addition of 1 mol
equiv of KOH (1H NMR evidence) (Scheme 2).
1
localized from H,13C-hmbc experiments at δ 179.9 and 185.9
ppm, respectively. The 31P{1H} NMR spectra show the typical
pattern for an AM2X spin system, according to the Cs symmetry
of the complexes with the carboxylate as chelating ligand. The
phosphorus trans to the ethyl group (PA) resonates, in both
cases, at high field (δ 2.9 and 3.4 ppm, respectively) and the
low JP,Rh value for this signal (around 66 Hz) is noteworthy in
comparison to those for the phosphorus trans to the acetate
ligands (PM, around 130 Hz). The related hydride derivatives 9
and 10 were cleanly prepared from the reactions of the
dihydride complex 3 with the corresponding acids. Analytical
and spectroscopic data for both agree with the proposed
formulation (see Experimental Section).
Complex [6]+ also results from the protonation of the
dihydride [(PhBP3)Rh(H)2(NCMe)] (3) with HBF4 in the
presence of acetonitrile. Moreover, addition of 2 mol equiv of
HBF4 to solutions of 3 in C6D6 caused the slow crystallization
of a white crystalline solid characterized as [(PhBP3)Rh-
(NCMe)3] ([2](BF4)2) along with hydrogen release (Scheme
2).31 As expected, a further addition of 1 mol equiv of HBF4
and acetonitrile to CD2Cl2 solutions of [6]+ gave rise to the
clean formation of [2]2+ along with hydrogen evolution.31
Hydride protonation reactions have been extensively used for
the preparation of dihydrogen complexes of the type [LnM(η2-
H2)], mainly for complexes of group 8 transition metals.32
However, such a complex was not isolated in our case, and H2
elimination with coordination of acetonitrile seems to be the
driving force for the protonation reaction. Overall, the sequence
1 → [6]+ → [2]2+ constitutes an example for the reduction of
Formation of the ethyl derivatives [(PhBP3)Rh(η1-C2H5)(κ2-
O2CR)] (R = Ph (7), Me (8)) requires some comments.
Starting from 1, a first step in which the acetonitrile ligand is
replaced by the carboxylic acid to form adducts of type A
(Scheme 4) can be assumed. At this point, two paths can be
proposed for the next step of the reaction. Path i involves
protonation at the rhodium(I) center by the acidic HO2C−
proton (to give B) followed by a β-migratory insertion to give
the ethyl intermediate C and coordination of the second
oxygen atom from the carboxylate to result in the complexes 7
and 8. Alternatively, path ii would start with the direct attack of
the proton to the coordinated ethylene to produce the ethyl
complex C followed by coordination of the free oxygen atom.
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Scheme 4. Alternative Paths for the Synthesis of Complexes 7−10
This path has been suggested in some instances.33 However,
the most commonly accepted path for the reactions of
complexes of late transition metals with acids is the protonation
of the metal to give hydride derivatives,34 and in less extension,
to a heteroatom bonded to the metal if it contains free electron
pairs: for example, an acac ligand.35 In our case, this third
possibility would produce the rhodium(I) intermediate
[(PhBP2PH)Rh(C2H4)(κ1-O2CR)], as may occur in the
reaction of 1 with the electrophile Me+ followed by ethylene
evolution (see below). However, this sequence with the
electrophile H+ would result in the product [(PhBP2PH)Rh-
(κ2-O2CR)], which is not the case, since the products of the
reaction are complexes 7−10. Nonetheless, which site is
protonated first, metal (path i) or carbon (path ii), is a difficult
question to answer.36 We believe that protonation at the carbon
is the most probable event (path ii), due to the high electronic
density at the ethylene coming from the strong π back-
donation.
scopic and analytical data of 11 agree with the proposed
formulation (see the Experimental Section). Thus, the most
relevant spectroscopic data are two resonances in the 31P{1H}
NMR spectrum; the doublet corresponds to two phosphorus
nuclei bound to rhodium, while the singlet at low field (δ 74.8
ppm) is due to the quaternization of the third phosphorus
atom. Accordingly, the signals of the −CH2 and −CH3 groups
bonded to this atom (PM) are also shifted to low field at δ 4.06
(JH,P = 12.4 Hz) and 2.81 (JH,P = 12.9 Hz) ppm, respectively, in
1
the H NMR spectrum.
Finally, it should be indicated that chloride should be avoided
in the reactions involving the aforementioned cationic
complexes, since it very easily replaces the labile acetonitrile
ligands. As a way of example, the cationic complex [(PhBP3)-
Rh(H)(NCMe)2]BF4 ([6]BF4) transforms into the neutral
[{(PhBP3)Rh(H)(μ-Cl}2]19 by reaction with PPNCl (PPN =
bis(triphenylphosphoranylidene)ammonium), while the dica-
tionic [(PhBP3)Rh(NCMe)3]2+ ([2]2+) gave the dinuclear
triply bridged complex [{(PhBP3)Rh}2(μ-Cl)3]+ ([12]+) in the
presence of PNNCl and eventually [(PhBP3)Rh(Cl)2].
Complex [12]+ was also obtained by reacting [(PhBP3)Rh-
(Cl)2]19 with 0.5 mol equiv of methyl triflate (MeOTf) in a
reaction in which one chloride ligand is eliminated as MeCl
(Scheme 6) (see the Experimental Section). Notice that the use
of MeOTf to abstract terminal halide ligands represents a clean
reaction that usually renders halide-bridged complexes,37 as an
alternative to the use of silver salts.
Starting from the ethyl complexes (7 and 8), a β-elimination
reaction to the hydride ethylene complex B (through the
intermediate C) provides an easy path for ethylene evolution
and formation of the hydride complexes 9 and 10. In the
reverse sense, starting from the hydride derivatives [(PhBP3)-
Rh(H)(κ2-O2CR)] (R = Ph (9), Me (10)), the thermodynamic
equilibria are again established on dissolving the complexes in
C6D6 saturated with ethylene. This reversibility seems to
suggest that the hydride ethylene compound B (Scheme 4) is a
key common intermediate.
According to the C3 symmetry of complex [12]+, a sharp
doublet at δ 33.5 ppm was found in the 31P{1H} NMR
spectrum. In addition, the nuclearity of the complex has been
confirmed from the measurement of the coefficient of diffusion
by NMR methods. Thus, DOSY experiments on [12]+ gave a
value of D = 4.37 × 10−10 m2/s, similar to those reported for
the related dinuclear complexes [{(PhBP3)Rh(μ-O2)}2] and
[{(PhBP3)Rh}2(μ-O2)(μ-OOH)]+ and quite lower than those
observed for mononuclear complexes.6 Finally, preliminary X-
ray diffraction studies on a crystal of low quality (see the
Supporting Information) confirmed the proposed structure,
being similar to that reported for the tricationic complexes of
the type [{(MeCP3)Rh}2(μ-Cl)3]3+.38
Other electrophiles such as Me+ were also tested as suitable
for C−C and/or P−C bond formation reactions. In this sense,
complex 1 reacted almost immediately with MeOTf to give a
complex alkylated at the phosphorus atom (P-methyl),
[{PhB(PMe)P2}Rh(κ2-O2SOCF3)] (11) (Scheme 5). Spectro-
Scheme 5. Synthesis of Complex 11 Containing an Alkylated
Phosphorus Atom
SUMMARY AND CONCLUSIONS
■
In summary, we have shown that a chemistry based on the
“(PhBP3)Rh” scaffold is easily accessible through the rhodium-
(I) complex [(PhBP3)Rh(CH2CH2)(NCMe)] (1) with the
labile ligands ethylene and acetonitrile. Reactivity studies on 1
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Scheme 6. Synthesis of Complex [12]+
recorded on Bruker AV 300 and AV 400 spectrometers operating at
300.13 and 400.13 MHz, respectively, for 1H. Chemical shifts are
reported in ppm and referenced to SiMe4, using the internal signal of
the deuterated solvent as reference (1H, 13C) and external H3PO4
(31P). IR spectra in solution were recorded with a Nicolet 550
spectrophotometer using NaCl cells, while IR spectra of solid samples
were recorded with a Perkin-Elmer 100 FT-IR spectrometer (4000−
400 cm−1) equipped with an ATR (attenuated total reflectance)
accessory. Conductivities were measured in acetone solutions using a
Philips PW 9501/01 conductimeter. Electrochemical experiments were
performed by means of an EG&G Research Model 273 potentiostat/
galvanostat. A three-electrode glass cell consisting of a platinum-disk
working electrode, a platinum-wire auxiliary electrode, and a KCl
saturated calomel reference (SCE) electrode was employed. Linear
voltamperometry was performed by using a rotating platinum
electrode (RDE) as the working electrode. The supporting electrolyte
solution (NBu4PF6, 0.1 M) was scanned over the solvent window to
ensure the absence of electroactive impurity curves. A concentration of
complex 1 of about 5 × 10−4 M was employed in all the
measurements.
have revealed it to be a very reactive species, easily oxidized by
chemical oxidants and suitable for a fast reaction with
hydrogen, affording the rhodium(III) complexes [(PhBP3)Rh-
(NCMe)3]2+ ([2]2+) and [(PhBP3)Rh(H)2(NCMe)] (3),
respectively. The neutral dihydride complex 3 was found to
be stable in solution but undergoes slow dissociation of
acetonitrile and dimerizes through the formation of hydrogen
bridges to [{(PhBP3)Rh(H)(μ-H)}2] (4). Complex 3 can be
also prepared in other solvents such as THF if free of stabilizing
agents such as BHT. Otherwise, hydrogen abstraction to the
red paramagnetic complex [{(PhBP3)Rh}2(μ-H)3] (5) takes
place. Hydride ligands in 3 possess a basic character, and they
are sequentially protonated (and removed as hydrogen) to
eventually form [2]2+ through the cationic monohydride
intermediate [(PhBP3)Rh(H)(NCMe)2]+ ([6]+). The overall
sequence 1 → [6]+ → [2]2+ constitutes an example for the
reduction of 2 H+ to H2 promoted by a rhodium compound,
which provides the required electrons. On the other hand, the
nucleophilic nature of complex 1 has been demonstrated from
its reactions with several Brønsted acids. Protonation of 1 with
HBF4 in acetonitrile is accomplished by ethylene evolution and
formation of the monohydride [6]+. Similar reactions with
carboxylic acids, such as benzoic or acetic acid, retain the
ethylene ligand as an ethyl group in complexes of the type
[(PhBP3)Rh(η1-C2H5)(κ2-O2CR)] (7 and 8). Most probably
these reactions occur by a direct attack of the nucleophile H+ at
the coordinated ethylene in 1, due to the high electronic
density at the ethylene coming from the strong π back-
donation. The ethyl complexes 7 and 8 establish an equilibrium
with the corresponding hydride compounds [(PhBP3)Rh(H)-
(κ2-O2CR)] (9 and 10) with ethylene loss. Both type of
complexes are related by a β-hydrogen elimination (from 7 and
8 to 9 and 10) and hydride insertion in the opposite way,
requiring the unobserved hydride complex [(PhBP3)Rh(H)-
(C2H4)(κ1-O2CR)] as a common intermediate. In other
instances, P−C bond formation reactions take place, as revealed
in the reactions of complex 1 with methyl triflate to give
[{PhB(PMe)P2}Rh(κ2-O2SOCF3)]. Further studies to develop
the potential of the “(PhBP3)Rh” scaffold in stoichiometric and
catalytic reactions are currently in progress.
Synthesis of the Complexes. [(PhBP3)Rh(CH2CH2)(NCMe)]
(1). A Schlenk tube was charged with solid [{Rh(μ-Cl)-
(C2H4)2}2] (120.0 mg, 0.31 mmol) and solid [Li(tmen)][PhB-
(CH2PPh2)3] (499.0 mg, 0.62 mmol). Addition of deoxy-
genated acetonitrile (3 mL) produced the dissolution of the
starting materials, while an orange crystalline solid precipitated
almost immediately. After the mixture was stirred for 20 min,
the solid was separated by decantation, washed with cold
acetonitrile (3 × 3 mL), and vacuum-dried. In this way complex
1 crystallizes with two molecules of acetonitrile. Yield: 471.9
mg (81%). 1H NMR (400 MHz, CD2Cl2, −50 °C): δ 8.29 (br s,
2H, BPho), 7.70−6.60 (33H, Ph), 2.48 (d, J = 6.6 Hz, 2H,
CH2CH2), 1.65 (δA, 2H) and 1.14 (δB, 2H, JAB = 14.3 Hz,
CH2PA), 1.48 (br, 2H, CH2CH2), 1.38 (s, 3H, NCCH3), 1.15
(br, 2H, CH2PM). 31P{1H} NMR (162 MHz, CD2Cl2, −50 °C):
δ 43.8 (dt, JP, Rh = 124, JP,P = 34 Hz, 1P, PM), 12.5 (dd, JP,Rh
=
110, JP,P = 34 Hz, 2P, PA). 13C{1H} NMR (100 MHz, CD2Cl2,
1
1
−50 °C, from H,13C-hsqc and H,13C-hmbc spectra): δ 119.3
(NCCH3), 34.1 (CH2CH2), 16.7 (CH2PM), 13.4 (CH2PA),
2.8 (NCCH3). MS (FAB+): m/z (%) 788 (100) [(PhBP3)Rh]+.
Anal. Calcd for C49H48NBP3Rh·2NCCH3 (939.68): C 67.74; H,
5.79; N, 4.47. Found: C, 67.97; H, 5.65; N, 4.42.
[(PhBP3)Rh(NCMe)3](PF6)2 ([2](PF6)2). To a solution of 1 (66.7 mg,
0.071 mmol) in dichloromethane/acetonitrile (4/0.5 mL) was added
[Fe(C5H5)2]PF6 (47.1 mg, 0.142 mmol). The mixture was stirred for 1
h and then evaporated to dryness. The residue was washed with
diethyl ether (3 × 5 mL) in order to remove [Fe(C5H5)2] and then
dissolved in dichloromethane (2 mL) and carefully layered with
diethyl ether (10 mL). The white crystalline solid that deposited in 3
days was separated by decantation, washed with cold diethyl ether, and
vacuum-dried. Yield: 72.8 mg (85%).
EXPERIMENTAL SECTION
■
Starting Materials and Physical Methods. All the operations
were carried out under an argon atmosphere using standard Schlenk
techniques. Solvents were dried and distilled under argon before use
by standard methods.39 The complexes [{Rh(μ-Cl)(C2H4)}2],40
[(PhBP3)Rh(Cl)2],19 and [Li(tmen)][PhB(CH2PPh2)3]8 were pre-
pared according to literature methods. Other reagents were
commercially available and were used as received. Elemental analyses
were performed using a Perkin-Elmer 2400 microanalyzer. FAB+ mass
spectra were recorded in a VG Autospec double-focusing mass
spectrometer, and 3-nitrobenzyl alcohol (NBA) was used as matrix.
MALDI-TOF+ mass spectra were obtained on a Bruker Microflex mass
spectrometer using DCTB (trans-2-[3-(4-tert-butylphenyl)-2-methyl-
2-propenylidene]malononitrile) as matrix. NMR spectra were
Alternatively, complex [(PhBP3)Rh(NCMe)3](BF4)2 can be pre-
pared from the addition of HBF4·Et2O (12.0 μL, 0.086 mmol) and
acetonitrile (4.5 μL, 0.086 mmol) to a dichloromethane solution of 1
(40.4 mg, 0.043 mmol in 3 mL). The white microcrystals that
deposited in 2 days were separated by decantation, washed with
1
hexane (2 × 3 mL) and vacuum-dried. Yield: 47.6 mg (93%). H
NMR (300 MHz, CD2Cl2, 25 °C): δ 7.59 (d, J = 7.1 Hz, 2H, BPho),
7.47 (t, JH,P = 7.2 Hz, J = 7.1 Hz, 18H, Ph2o+pP), 7.38 (t, J = 7.2 Hz,
m
2H, BPhm), 7.31 (t, J = 7.4 Hz, 12H, Ph2 P), 7.22 (t, J = 7.3 Hz, 1H,
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BPhp), 2.29 (s, 9H, NCCH3), 1.77 (br d, JH,P = 8.6 Hz, 6H, CH2P).
31P{1H} NMR (121 MHz, CD2Cl2, 25 °C): δ 32.3 (d, JP,Rh = 103 Hz,
Hz, 2P, PM), 1.3 (dt, JP,Rh = 73, JP,P = 21 Hz, 1P, PA). MS (MALDI-
TOF+): m/z (%) 789 (100) [(PhBP3)Rh(H)]+. Anal. Calcd for
C49H48N2B2F4P3Rh (958.36): C, 61.41; H, 5.05; N, 2.92. Found: C,
61.16; H, 5.15; N, 2.83. ΛM (5.0 × 10−4 in acetone) = 91.5 S mol−1
cm−1.
−
3P), −144.2 (sept, JP,F = 711 Hz, 1P, PF6 ). MS (MALDI-TOF+): m/z
(%) 788 (66) [(PhBP3)Rh]+. Anal. Calcd for C51H50BF12N3P5Rh
(1201.53): C, 50.98; H, 4.19; N, 3.50. Found: C, 50.57; H, 4.26; N,
3.53. ΛM (5.0 × 10−4 in acetone) = 129.5 S mol−1 cm−1.
[(PhBP3)Rh(H)2(NCMe)] (3). Exposure of solutions of 1 in
acetonitrile/toluene (1/5) to an atmosphere of hydrogen causes an
immediate discoloration. 1H NMR of an aliquot indicated the
quantitative transformation of 1 into the dihydride complex 3
previously reported.19 From these solutions, complex 3 was isolated
after concentration of the solution and precipitation with hexane.
Isolated yield: 80%.
Reaction of 1 with H2 in d8-THF. An NMR tube was charged with
complex 1 (10.3 mg, 0.011 mmol) and then dissolved in
deoxygenated, free of BHT d8-THF (0.5 mL). Exposure of the
solution to hydrogen for 5 min caused discoloration of the solution,
and the clean formation of 3 was observed. Selected spectroscopic data
[(PhBP3)Rh(η1-C2H5)(κ2-O2CPh)] (7). A Schlenk tube was charged
with solid 1 (75.2 mg, 0.080 mmol) and benzoic acid (9.8 mg, 0.080
mmol), and then toluene (3 mL) was added. The resulting suspension
was stirred for 2 h under an atmosphere of ethylene. Hexane (10 mL)
was added to the pale yellow suspension to complete the precipitation
of the solid, which was filtered, washed with cold hexane (2 × 5 mL),
and vacuum-dried. Yield: 56 mg (75%). IR (solid): ν(CO2)/cm−1
1513 (m), 1430 (s). 1H NMR (300 MHz, CD2Cl2, −40 °C): δ 7.98 (d,
J = 8.6 Hz, 2H, O2CPho), 7.92 (m, 4H, Ph2o1PM), 7.68 (d, J = 6.9 Hz,
2H, BPho), 7.58 (t, J = 8.6 Hz, 1H, O2CPhp), 7.48 (td, J = 11.0, 7.6 Hz,
2H, O2CPhm), 7.33 (m, 8H, BPhm + Ph2m1PM + Ph2p2PM), 7.14 (m,
o
p
5H, BPhp + Ph2m2PM), 6.99 (m, 12H, Ph2 PA + Ph2o2PM + Ph2 PA +
Ph2p1PM), 6.71 (t, J = 7.0 Hz, 4H, Ph2 PA), 1.76 (br, δA, 2H) and 1.49
m
(br, δB, 2H; CH2PM), 1.54 (m, 2H, CH2CH3), 1.14 (d, JH,P = 11.5 Hz,
2H, CH2PA), 0.87 (dt, JH,P =11.1 Hz, J = 7.4 Hz, 3H, CH2CH3).
31P{1H} NMR (121 MHz, CD2Cl2, −40 °C): δ 36.7 (dd, JP,Rh = 130,
JP,P = 23 Hz, 2P, PM), 2.9 (dt, JP,Rh = 67, JP,P = 23 Hz, PA). 13C{1H}
NMR (75 MHz, CD2Cl2, −40 °C): δ 179.9 (br s, O2CPh), 32.5 (br d,
JC,P = 91 Hz, CH2CH3), 15.2 (d, JC,P = 4 Hz, CH2CH3). MS (MALDI-
TOF+): m/z (%) 909 (5) [(PhBP3)Rh(O2CPh)]+, 789 (100)
[(PhBP3)Rh(H)]+. Anal. Calcd for C54H51BO2P3Rh (938.22): C,
69.10; H, 5.48. Found: C, 68.94; H, 5.67.
are as follows. 1H NMR (300 MHz, d8-THF, 25 °C): δ 7.77 (t, JH,P
=
7.9 Hz, J = 7.9 Hz, 4H, Ph2o1PA), 7,73 (ddd, JH,P = 11.5 Hz, J = 8.9 Hz,
o
J = 1.7 Hz, 4H, Ph2 PM), 7.56 (d, J = 7.4 Hz, 2H, BPho), 7.15 (J = 7.4
p
Hz, 2H, BPhm), 7.05 (m, 12H, Ph2o2+m2+p2PA, Ph2 PM), 6.96 (t, J = 6.9
m
Hz, 4H, Ph2m1PA), 6.94 (td, J = 8.5 Hz, J = 1.7 Hz, 6H, Ph2 PM
+
Ph2p1PA), 6.77 (t, J = 7.5 Hz, 1H, BPhp), 1.64 (s, 3H, NCCH3), 1.71
(δA, 2H) and 1.30 (δB, 2H, JAB = 12 Hz, CH2PA), 1.20 (d, JH,P = 13.7
Hz, 2H, CH2PM), −7.66 (ddd, JH,P = 158.3 Hz, JH,P = 9.7 Hz, JH,Rh
=
A
M
14.2 Hz, 1H, Rh−H). 31P{1H} NMR (121 MHz, d8-THF, 25 °C) δ
53.7 (dt, JP,Rh = 130 Hz, JP,P = 28 Hz, 1P, PM), 22.4 (dd, JP,Rh = 79 Hz,
JP,P = 28 Hz, 2P, PA). 13C{1H} NMR (100 MHz, d8-THF, 25 °C, from
1H,13C-hsqc and 1H,13C-hmbc spectra): δ 119.8 (NCCH3), 16.2
(CH2PA), 15.7 (CH2PM), 1.18 (NCCH3).
[(PhBP3)Rh(η1-C2H5)(κ2-O2CMe)] (8). To a suspension of 1 (110.0
mg, 0.117 mmol) in toluene (2 mL) was added acetic acid (6.7 μL,
0.117 mmol). The resulting suspension was stirred for 2 h under an
ethylene atmosphere, and the product was isolated as described above
for 7. Yield: 67.0 mg (65%). IR (solid): ν(CO2)/cm−1 1521 (m), 1462
(s). 1H NMR (400 MHz, CD2Cl2, −40 °C): δ 7.72 (m, 4H, Ph2o1PM),
7.62 (d, J = 7.0 Hz, 2H, BPho), 7.41 (t, J = 7.3 Hz 2H, Ph2p1PM), 7.31
(t, J = 7.4 Hz, 2H, BPhm), 7.24 (m, 6H, Ph2m1PM + Ph2p2PM), 7.11 (t, J
[{(PhBP3)Rh(H)(μ-H)}2] (4). A solution of [(PhBP3)Rh-
(H)2(NCMe)] (3; 15.0 mg, 0.018 mmol) in C6D6 (0.5 mL) was
maintained undisturbed under an atmosphere of hydrogen. Orange
microcrystals deposited in 1 week. The crystals were separated by
decantation, washed with pentane (2 × 2 mL), and vacuum-dried.
Yield: 10.7 mg (75%). IR (solid): ν(Rh−H)/cm−1 2007 (m). Anal.
Calcd for C90H86B2P6Rh2 (1580.92): C, 68.38; H, 5.48. Found: C,
68.49; H, 5.63.
o
p
= 7.0 Hz, 1H, BPhp), 7.07 (m, 10H, Ph2m2PM + Ph2 PA + Ph2 PA), 6.88
(m, 8H, Ph2 PA + Ph2o2PM), 2.03 (s, 3H, O2CCH3), 1.67 (br, δA, 2H)
m
and 1.40 (br, δB, 2H; CH2PM), 1.52 (m, 2H, CH2CH3), 1.33 (d, JH,P
=
11.6 Hz, 2H, CH2PA), 0.87 (m, 3H, CH2CH3). 31P{1H} NMR (162
MHz, CD2Cl2, −40 °C) δ 36.4 (dd, JP,Rh = 131 Hz, JP,P = 23 Hz, 2P,
PM), 3.4 (dt, JP,Rh = 65 Hz, JP,P = 23 Hz, 1P, PA). 13C{1H} NMR (101
[{(PhBP3)Rh}2(μ-H)3] (5). A solution of 1 (56.4 mg, 0.060 mmol) in
tetrahydrofuran (3 mL) that contained traces of BHT was kept under
an atmosphere of hydrogen overnight, and the mixture developed a
MHz, CD2Cl2, −40 °C, from H,13C-hsqc and H,13C-hmbc spectra):
δ 185.9 (O2CCH3), 31.1 (CH2CH3), 25.3 (O2CCH3), 14.8
(CH2CH3). MS (MALDI-TOF+): m/z (%) 847 (47) [(PhBP3)Rh-
(O2CCH3)]+, 789 (100) [(PhBP3)Rh(H)]+. Anal. Calcd for
C49H49BO2P3Rh (876.55): C, 67.14; H, 5.63. Found: C, 67.17; H,
5.57.
1
1
1
red color. The reaction was monitored by H NMR, while additional
volumes of THF were added to drive the reaction to completion in 2
days. The resulting red solution was evaporated to dryness, the residue
was dissolved in toluene (3 mL), and this solution was filtered through
Celite and carefully layered with hexane (10 mL). The red crystalline
solid that deposited in 3 days was separated by decantation, washed
with hexane (2 × 3 mL), and vacuum-dried. Yield: 33.2 mg (70%). IR
(CH2Cl2): ν(Rh−H)/cm−1 1636 (m). 1H NMR (300 MHz, C6D6, 25
°C): δ 11.57 (br s), 8.34−6.33 (br), 7.56 (d, J = 7.1 Hz, 2H, BPho),
7.37 (t, J = 7.1 Hz, 2H, BPhm), 7.28 (t, J = 7.1 Hz, 1H, BPhp), 3.69
(br), 1.72 (br). MS (MALDI-TOF+): m/z (%) 1579 (100) [M]+, 789
(81) [(PhBP3)Rh(H)]+. Anal. Calcd for C90H85B2P6Rh2 (1579.92): C,
68.42; H, 5.42. Found: C, 68.16; H, 5.20.
[(PhBP3)Rh(H)(κ2-O2CMe)] (10). Acetic acid (5.2 μL, 0.090 mmol)
was added to a solution of [(PhBP3)Rh(H)2(NCMe)] (3) (75.0 mg,
0.090 mmol) in toluene (3 mL). After it was stirred for 20 min, the
solution was layered with hexane (10 mL) and left to stand for 1 day.
The pale yellow solid that deposited was washed with hexane (3 × 3
mL) and vacuum-dried. Yield: 61.4 mg (80%). IR (solid): ν(Rh−H)/
cm−1 1967 (m), ν(CO2)/cm−1 1519 (m), 1457 (s). H NMR (300
1
MHz, C6D6, 25 °C): δ 8.06 (d, J = 7.2 Hz, 2H, BPho), 7.84 (m, 4H,
Ph2o1PM), 7.70 (m, 6H, Ph2o2PM + BPhm), 7.43 (br t, J = 7.3 Hz, 1H,
[(PhBP3)Rh(H)(NCMe)2]BF4 ([6]BF4). HBF4·Et2O (11.7 μL, 0.085
mmol) and acetonitrile (10.0 μL, 0.192 mmol) were added to a
solution of 1 (80.0 mg, 0.085 mmol) in dichloromethane (4 mL) at
−50 °C. The resulting yellow solution was warmed to room
temperature, and then it was concentrated to ca. 2 mL and carefully
layered with hexane (8 mL) to give a yellow solid. The product was
filtered off, washed with hexane (2 × 5 mL), and vacuum-dried. Yield:
61.1 mg (75%). IR (CH2Cl2): ν(Rh−H)/cm−1 1989 (m), ν(NC)/
cm−1 2304 (m). 1H NMR (400 MHz, CD2Cl2, 25 °C): δ 7.65 (m, 6H,
BPhp), 7.34 (br t, J = 8.6 Hz, 4H, Ph2 PA), 6.89 (m, 8H, Ph2m1PM
+
o
p
m
Ph2p1PM + Ph2 PA), 6.80 (br t, J = 7.5 Hz, 4H, Ph2 PA), 6.67 (m, 6H,
Ph2m2PM + Ph2p2PM), 1.92 (s, 3H, O2CCH3), 1.80 (br d, J = 12.3 Hz,
2H, CH2PM), 1.69 (m, 4H, CH2PM), −4.97 (ddd, JH,P = 193.2 Hz,
B
JH,Rh = 11.7 Hz, JH,P = 9.0 Hz, 1H, Rh−H). 31P{1H} NMR (121 MHz,
A
C6D6, 25 °C) δ 51.6 (dd, JP,Rh = 123 Hz, JP,P = 20 Hz, 2P, PM), 8.2 (dt,
JP,Rh = 74 Hz, JP−P = 20 Hz, 1P, PA). 13C{1H} NMR (75 MHz, C6D6,
25 °C, from 1H,13C-hsqc and 1H,13C-hmbc spectra): δ 185.3
(O2CCH3), 24.3 (O2CCH3). MS (MALDI-TOF+): m/z (%) 789
(100) [(PhBP3)Rh(H)]+. Anal. Calcd for C47H45BO2P3Rh (848.50):
C, 66.53; H, 5.35. Found: C, 65.58; H, 5.25.
o
Ph2om1PM + BPho), 7.56 (t, J = 8.6 Hz, 4H, Ph2 PA), 7.35 (m, 9H,
Ph2 PA + Ph2p1PM + BPhm+p), 7.22 (m, 4H, Ph2m2PM), 7.07 (m, 12H,
p
Ph2o2PM + Ph2m1PM + Ph2 PA + Ph2p2PM), 2.01 (s, 6H, NCCH3), 1.72
(d, JH,P = 11.7 Hz, 2H, CH2PA), 1.56 (m, 4H, CH2PM), −7.09 (dq,
[{PhB(PMe)P2}Rh(κ2-O2SOCF3)] (11). Addition of MeO3SCF3
(MeOTf, 2.5 μL, 0.023 mmol) to a solution of 1 (21.6 mg, 0.023
mmol) in CD2Cl2 (0.5 mL) caused the almost immediate discoloration
JH,P = 187.2 Hz, JH,P = 7.8 Hz, JH,Rh = 7.8 Hz, 1H, Rh−H). 31P{1H}
NMR (162 MHz, CD2Cl2, 25 °C): δ 43.8 (dd, JP,Rh = 114, JP,P = 21
A
M
2903
dx.doi.org/10.1021/om201129j | Organometallics 2012, 31, 2895−2906

Organometallics
Article
Table 5. Selected Crystal, Measurement, and Refinement Data for Compounds 1·2NCCH3, [2](BF4)2·3C6D6, 4·2C6H6, and
5·C7H8·C6H14
1·2NCCH3
[2](BF4)2·3C6D6
4·2C6H6
5·C7H8·C6H14
formula
formula wt
dolor
C49H48BNP3Rh·2(C2H3N)
939.62
C51H50B3F8N3P3Rh·3(C6D6)
C90H86B2P6Rh2·2(C6H6)
1737.06
C90H85B2P6Rh2·C7H8·C6H14
1758.15
red
1337.55
colorless
triclinic
orange
orange
cryst syst
space group
a (Å)
monoclinic
P21/n
triclinic
triclinic
P1
̅
P1
̅
P1
̅
15.7452(16)
13.4974(14)
21.886(2)
90
12.9713(10)
13.0463(10)
19.7923(15)
80.6362(10)
76.7235(10)
79.7700(10)
3182.0(4)
2
12.435(2)
12.972(2)
14.316(3)
91.630(2)
100.044(2)
114.017(2)
2064.4(6)
1
13.276(4)
13.764(4)
26.816(8)
99.910(4)
94.923(4)
116.112(6)
4261(2)
2
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
93.482(2)
90
4642.6(8)
4
Z
F(000)
ρcalcd (g cm−3
1952
1360
900
1830
)
1.344
1.396
1.397
1.370
μ (mm−1
)
0.511
0.411
0.566
0.549
cryst size (mm)
0.09 × 0.06 × 0.05
100(2)
0.20 × 0.19 × 0.14
100(2)
0.10 × 0.08 × 0.03
100(2)
0.20 × 0.15 × 0.08
100(2)
temp (K)
θ limits [°]
25.25
27.00
25.20
27.00
no. of collected rflns
no. of unique rflns (Rint)
no. of rflns with I > 2σ(I)
no. of params/restraints
R1 (on F, I > 2σ(I))
wR2 (on F2, all data)
25 414
37 706
15 838
49 931
8380 (0.1011)
5673
13 826 (0.0442)
10 289
7382 (0.0634)
5082
18 459 (0.0503)
15 482
548/10
747/0
509/2
1012/33
0.0481
0.0751
0.0438
0.0512
0.1433
0.1111
0.1221
0.1152
max/min Δρ (e Å−3
)
0.659/-0.711
1.060
0.846/−0.568
1.025
0.790/−1.159
0.985
0.981/−0.711
1.091
goodness of fit
of the solution from orange to pale yellow. NMR data indicated the
intermediates were not detected. A further addition of PPNCl (3.0 mg,
0.005 mmol) caused the complete transformation of [{(PhBP3)-
Rh}2(μ-Cl)3]+ into the neutral [(PhBP3)Rh(Cl)2] according to the
single doublet at δ 52.2 ppm (JP,Rh = 109 Hz) in the 31P{1H} NMR
spectrum, which does not evolve further to anionic species of the type
[(PhBP3)Rh(Cl)3]− in the presence of an excess of PPNCl.
formation of complex 12 in 80% yield. Selected spectroscopic data are
1
as follows. H NMR (300 MHz, CD2Cl2, 25 °C): δ 7.9−6.9 (35H,
Ph), 4.06 (d, JH,P =12.4 Hz, 2H, CH2PM), 2.81 (d, JH,P =12.9 Hz, 3H,
CH3PM), 1.55 (br, 4H, CH2PA). 31P{1H} NMR (121 MHz, CD2Cl2,
25 °C): δ 74.8 (s, 1P, PM), 34.1 (d, JP,Rh = 110, 2P, PA).
DFT Geometry Optimizations. The computational method used
was density functional theory (DFT) with the B3LYP exchange-
correlation functional,41 using the Gaussian 0942 program package.
The basis sets used for the full optimization of the structure were a
LanL2DZ effective core potential for the rhodium atom and 6-
31G(d,p) for the remaining atoms.
[{(PhBP3)Rh}2(μ-Cl)3]OTf ([12]OTf). To a solution of [(PhBP3)Rh-
(Cl)2] (50.3 mg, 0.059 mmol) in dichloromethane (2 mL) was added
MeOTf (3.2 μL, 0.029 mmol). After it was stirred for 6 h at room
temperature, the orange solution was carefully layered with diethyl
ether (10 mL) and maintained undisturbed for 2 days. The yellow-
orange microcrystals that deposited were decanted, washed with
diethyl ether (2 × 3 mL), and vacuum-dried. Yield: 40.0 mg (75%). 1H
NMR (300 MHz, C6D6, 25 °C): δ 7.64 (d, J = 7.3 Hz, 4H, BPho), 7.32
X-ray Diffraction Studies on 1·2NCCH3, [2](BF4)2·3C6D6,
4·2C6H6, and 5·C7H8·C6H14. Selected crystallographic data for these
complexes can be found in Table 5. Intensity measurements were
collected with a Smart Apex diffractometer, with graphite-mono-
chromated Mo Kα radiation. A semiempirical absorption correction
was applied to each data set, with the multiscan43 methods. All non-
hydrogen atoms were refined with anisotropic displacement
parameters, except those atoms involved in disorders, which were
refined with isotropic displacement parameters: a phenyl ring and an
acetonitrile solvent molecules in 1·2NCCH3, one benzene solvent
molecule in [2](BF4)2·3C6D6, and one hexane solvent molecule in
5·C7H8·C6H14. The hydrogen atoms were placed at calculated
positions and were refined isotropically in riding mode, with the
exception of the hydride ligands, which were found on the difference
Fourier maps and refined with some constraints. The structures were
solved by the Patterson method and refined by full-matrix least squares
with the program SHELX9744 in the WINGX45 package.
o
(m, 24H, Ph2 P), 7.30 (t, J = 7.4 Hz, 4H, BPhm), 7.17 (t, J = 7.6 Hz,
p
m
12H, Ph2 P), 7.07 (m, 26H, Ph2 P + BPhp), 1.45 (br d, JH,P = 7.0 Hz,
12H, CH2P). 31P{1H} NMR (121 MHz, CD2Cl2, 25 °C): δ 33.5 (d,
JP,Rh = 110 Hz, 6P). MS (MALDI-TOF+): m/z (%) 823 (100)
[(PhBP3)Rh(Cl)]+. Anal. Calcd for C91H82B2Cl3F3O3P6SRh2·2CH2Cl2
(2002.18): C, 55.79; H, 4.33; S, 1.60. Found: C, 55.69; H, 4.20; S,
1.49.
Reaction of [(PhBP3)Rh(H)(NCMe)2]BF4 ([6]BF4) with PPNCl.
HBF4·Et2O (2.0 μL, 0.014 mmol) and acetonitrile (2 μL) were
added to a solution of 1 (13.1 mg, 0.014 mmol) in CD2Cl2 at −50 °C.
The quantitative formation of [6]+ was observed by 1H NMR.
Addition of PPNCl (8.0 mg, 0.014 mmol) at room temperature causes
the replacement of acetonitrile and formation of the cis and trans
isomers of [{(PhBP3)Rh(H)(μ-Cl)}2], identified by comparison of the
NMR data with those previously reported.19
Reaction of [(PhBP3)Rh(NCMe)3](PF6)2 ([2](PF6)2) with PPNCl.
PPNCl (8.6 mg, 0.015 mmol) was added to a solution of
[(PhBP3)Rh(NCMe)3](PF6)2 ([2](PF6)2; 12.0 mg, 0.010 mmol) in
CD2Cl2 (0.5 mL). After the mixture was stirred for 10 min, the
ASSOCIATED CONTENT
■
S
* Supporting Information
Figures, tables, text, and CIF files giving full ORTEP diagrams,
selected spectroscopic data, DOSY experiments, atomic
31P{1H} NMR spectrum showed a sole doublet at δ 33.5 ppm (JP,Rh
=
110 Hz) corresponding to [{(PhBP3)Rh}2(μ-Cl)3]+, while other
2904
dx.doi.org/10.1021/om201129j | Organometallics 2012, 31, 2895−2906

Organometallics
Article
(10) (a) Brown, S. D.; Mehn, M. P.; Peters, J. C. J. Am. Chem. Soc.
2005, 127, 13146−13147. (b) Brown, S. D.; Peters, J. C. J. Am. Chem.
Soc. 2004, 126, 4538−4539.
coordinates for 1′, complete ref 41, and details of the X-ray
crystal structures of 1·2NCCH3, [2](BF4)2·3C6D6, 4·2C6H6,
and 5·C7H8·C6H14. This material is available free of charge via
the Internet at http://pubs.acs.org.
(11) Thomas, C. M.; Peters, J. C. Angew. Chem., Int. Ed. 2006, 45,
776−780.
(12) Turculet, L.; Feldman, J. D.; Tilley, T. D. Organometallics 2004,
23, 2488−2502.
AUTHOR INFORMATION
■
(13) (a) Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2004, 126,
6252−6254. (b) Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2003,
125, 10782−10783.
Corresponding Author
*E-mail: mciriano@unizar.es (M.A.C.); ctejel@unizar.es
(C.T.).
(14) Lu, C. C.; Peters, J. C. Inorg. Chem. 2006, 45, 8597−8607.
(15) Thomas, C. M.; Mankad, N. P.; Peters, J. C. J. Am. Chem. Soc.
2006, 128, 4956−4957.
Notes
The authors declare no competing financial interest.
(16) (a) Camerano, J. A.; Casado, M. A.; Ciriano, M. A.; Tejel, C.;
Oro, L. A. Chem. Eur. J. 2008, 14, 1897−1905. (b) Casado, M. A.;
Hack, V.; Camerano, J. A.; Ciriano, M. A.; Tejel, C.; Oro, L. A. Inorg.
Chem. 2005, 44, 9122−9124.
(17) Daida, E. J.; Peters, J. C. Inorg. Chem. 2004, 43, 7474−7485.
(18) Betley, T. A.; Peters, J. C. Inorg. Chem. 2003, 42, 5074−5084.
ACKNOWLEDGMENTS
■
This research was supported by the MICINN/FEDER
(Projects CTQ2008-03860 and CTQ2011-22516, Spain) and
Gobierno de Aragon
thank Gobierno de Aragon
́
/FSE (Group E70, Spain). A.M.G. and S.J.
for a fellowship.
(19) Jimen
́ ́
ez, S.; Lopez, J. A.; Ciriano, M. A.; Tejel, C.; Martínez, A.;
́
San
́
chez-Delgado, R. A. Organometallics 2009, 28, 3193−3202.
(20) Walker, J. M.; Cox, A. M.; Wang, R.; Spivak, G. J.
Organometallics 2010, 29, 6121−6124.
DEDICATION
■
(21) Tejel, C.; Ciriano, M. A.; Passarelli, V. Chem. Eur. J. 2011, 17,
91−95.
This article is dedicated to the memory of Prof. F. Gordon A.
Stone, a pioneer that devoted his whole life to chemistry and
lent a hand to some of us to take the first steps in
organometallic chemistry.
́
(22) Nicasio, M. C.; Paneque, M.; Perez, P. J.; Pizzano, A.; Poveda,
M. L.; Rey, L.; Sirol, S.; Taboada, S.; Trujillo, M.; Monge, A.; Ruiz, C.;
Carmona, E. Inorg. Chem. 2000, 39, 180−188.
(23) Paneque, M.; Perez, P. J.; Pizzano, A.; Poveda, M. L.; Taboada,
́
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